--- a/VERSION.libentt.txt	Sat Oct 02 08:30:22 2021 +0200
+++ b/VERSION.libentt.txt	Sat Oct 02 08:45:16 2021 +0200
@@ -1,1 +1,1 @@
-3.1.1
+3.8.1

--- a/libentt/entt/entt.hpp	Sat Oct 02 08:30:22 2021 +0200
+++ b/libentt/entt/entt.hpp	Sat Oct 02 08:45:16 2021 +0200
@@ -1,8 +1,21 @@
+// #include "config/version.h"
+#ifndef ENTT_CONFIG_VERSION_H
+#define ENTT_CONFIG_VERSION_H
+
+
+#define ENTT_VERSION_MAJOR 3
+#define ENTT_VERSION_MINOR 8
+#define ENTT_VERSION_PATCH 1
+
+
+#endif
+
 // #include "core/algorithm.hpp"
 #ifndef ENTT_CORE_ALGORITHM_HPP
 #define ENTT_CORE_ALGORITHM_HPP
 
 
+#include <vector>
 #include <utility>
 #include <iterator>
 #include <algorithm>
@@ -12,54 +25,93 @@
 #define ENTT_CORE_UTILITY_HPP
 
 
+#include <utility>
 // #include "../config/config.h"
 #ifndef ENTT_CONFIG_CONFIG_H
 #define ENTT_CONFIG_CONFIG_H
 
 
-#ifndef ENTT_NOEXCEPT
-#define ENTT_NOEXCEPT noexcept
-#endif // ENTT_NOEXCEPT
-
-
-#ifndef ENTT_HS_SUFFIX
-#define ENTT_HS_SUFFIX _hs
-#endif // ENTT_HS_SUFFIX
-
-
-#ifndef ENTT_HWS_SUFFIX
-#define ENTT_HWS_SUFFIX _hws
-#endif // ENTT_HWS_SUFFIX
-
-
-#ifndef ENTT_NO_ATOMIC
-#include <atomic>
-#define ENTT_MAYBE_ATOMIC(Type) std::atomic<Type>
-#else // ENTT_NO_ATOMIC
-#define ENTT_MAYBE_ATOMIC(Type) Type
-#endif // ENTT_NO_ATOMIC
+#if defined(__cpp_exceptions) && !defined(ENTT_NOEXCEPTION)
+#   define ENTT_NOEXCEPT noexcept
+#   define ENTT_THROW throw
+#   define ENTT_TRY try
+#   define ENTT_CATCH catch(...)
+#else
+#   define ENTT_NOEXCEPT
+#   define ENTT_THROW
+#   define ENTT_TRY if(true)
+#   define ENTT_CATCH if(false)
+#endif
+
+
+#if defined(__cpp_lib_launder) && __cpp_lib_launder >= 201606L
+#   include <new>
+#   define ENTT_LAUNDER(expr) std::launder(expr)
+#else
+#   define ENTT_LAUNDER(expr) expr
+#endif
+
+
+#ifndef ENTT_USE_ATOMIC
+#   define ENTT_MAYBE_ATOMIC(Type) Type
+#else
+#   include <atomic>
+#   define ENTT_MAYBE_ATOMIC(Type) std::atomic<Type>
+#endif
 
 
 #ifndef ENTT_ID_TYPE
-#include <cstdint>
-#define ENTT_ID_TYPE std::uint32_t
-#endif // ENTT_ID_TYPE
-
-
-#ifndef ENTT_PAGE_SIZE
-#define ENTT_PAGE_SIZE 32768
-#endif // ENTT_PAGE_SIZE
-
-
-#ifndef ENTT_DISABLE_ASSERT
-#include <cassert>
-#define ENTT_ASSERT(condition) assert(condition)
-#else // ENTT_DISABLE_ASSERT
-#define ENTT_ASSERT(...) ((void)0)
-#endif // ENTT_DISABLE_ASSERT
-
-
-#endif // ENTT_CONFIG_CONFIG_H
+#   include <cstdint>
+#   define ENTT_ID_TYPE std::uint32_t
+#endif
+
+
+#ifdef ENTT_SPARSE_PAGE
+	static_assert(ENTT_SPARSE_PAGE && ((ENTT_SPARSE_PAGE & (ENTT_SPARSE_PAGE - 1)) == 0), "ENTT_SPARSE_PAGE must be a power of two");
+#else
+#   define ENTT_SPARSE_PAGE 4096
+#endif
+
+
+#ifdef ENTT_PACKED_PAGE
+static_assert(ENTT_PACKED_PAGE && ((ENTT_PACKED_PAGE & (ENTT_PACKED_PAGE - 1)) == 0), "ENTT_PACKED_PAGE must be a power of two");
+#else
+#   define ENTT_PACKED_PAGE 1024
+#endif
+
+
+#ifdef ENTT_DISABLE_ASSERT
+#   undef ENTT_ASSERT
+#   define ENTT_ASSERT(...) (void(0))
+#elif !defined ENTT_ASSERT
+#   include <cassert>
+#   define ENTT_ASSERT(condition, ...) assert(condition)
+#endif
+
+
+#ifdef ENTT_NO_ETO
+#   include <type_traits>
+#   define ENTT_IGNORE_IF_EMPTY std::false_type
+#else
+#   include <type_traits>
+#   define ENTT_IGNORE_IF_EMPTY std::true_type
+#endif
+
+
+#ifndef ENTT_STANDARD_CPP
+#    if defined __clang__ || defined __GNUC__
+#       define ENTT_PRETTY_FUNCTION __PRETTY_FUNCTION__
+#       define ENTT_PRETTY_FUNCTION_PREFIX '='
+#       define ENTT_PRETTY_FUNCTION_SUFFIX ']'
+#    elif defined _MSC_VER
+#       define ENTT_PRETTY_FUNCTION __FUNCSIG__
+#       define ENTT_PRETTY_FUNCTION_PREFIX '<'
+#       define ENTT_PRETTY_FUNCTION_SUFFIX '>'
+#   endif
+#endif
+
+
+#endif
 
 
 
@@ -75,37 +127,92 @@
      * @return The submitted value as-is.
      */
     template<class Type>
-    constexpr Type && operator()(Type &&value) const ENTT_NOEXCEPT {
+    [[nodiscard]] constexpr Type && operator()(Type &&value) const ENTT_NOEXCEPT {
         return std::forward<Type>(value);
     }
 };
 
 
 /**
- * @brief Constant utility to disambiguate overloaded member functions.
- * @tparam Type Function type of the desired overload.
- * @tparam Class Type of class to which the member functions belong.
- * @param member A valid pointer to a member function.
- * @return Pointer to the member function.
+ * @brief Constant utility to disambiguate overloaded members of a class.
+ * @tparam Type Type of the desired overload.
+ * @tparam Class Type of class to which the member belongs.
+ * @param member A valid pointer to a member.
+ * @return Pointer to the member.
  */
 template<typename Type, typename Class>
-constexpr auto overload(Type Class:: *member) ENTT_NOEXCEPT { return member; }
+[[nodiscard]] constexpr auto overload(Type Class:: *member) ENTT_NOEXCEPT { return member; }
 
 
 /**
  * @brief Constant utility to disambiguate overloaded functions.
- * @tparam Type Function type of the desired overload.
+ * @tparam Func Function type of the desired overload.
  * @param func A valid pointer to a function.
  * @return Pointer to the function.
  */
-template<typename Type>
-constexpr auto overload(Type *func) ENTT_NOEXCEPT { return func; }
-
-
-}
-
-
-#endif // ENTT_CORE_UTILITY_HPP
+template<typename Func>
+[[nodiscard]] constexpr auto overload(Func *func) ENTT_NOEXCEPT { return func; }
+
+
+/**
+ * @brief Helper type for visitors.
+ * @tparam Func Types of function objects.
+ */
+template<class... Func>
+struct overloaded: Func... {
+    using Func::operator()...;
+};
+
+
+/**
+ * @brief Deduction guide.
+ * @tparam Func Types of function objects.
+ */
+template<class... Func>
+overloaded(Func...)
+-> overloaded<Func...>;
+
+
+/**
+ * @brief Basic implementation of a y-combinator.
+ * @tparam Func Type of a potentially recursive function.
+ */
+template<class Func>
+struct y_combinator {
+    /**
+     * @brief Constructs a y-combinator from a given function.
+     * @param recursive A potentially recursive function.
+     */
+    y_combinator(Func recursive):
+        func{std::move(recursive)}
+    {}
+
+    /**
+     * @brief Invokes a y-combinator and therefore its underlying function.
+     * @tparam Args Types of arguments to use to invoke the underlying function.
+     * @param args Parameters to use to invoke the underlying function.
+     * @return Return value of the underlying function, if any.
+     */
+    template <class... Args>
+    decltype(auto) operator()(Args &&... args) const {
+        return func(*this, std::forward<Args>(args)...);
+    }
+
+    /*! @copydoc operator()() */
+    template <class... Args>
+    decltype(auto) operator()(Args &&... args) {
+        return func(*this, std::forward<Args>(args)...);
+    }
+
+private:
+    Func func;
+};
+
+
+}
+
+
+#endif
 
 
 
@@ -157,7 +264,7 @@
     template<typename It, typename Compare = std::less<>>
     void operator()(It first, It last, Compare compare = Compare{}) const {
         if(first < last) {
-            for(auto it = first+1; it < last; ++it) {
+            for(auto it = first + 1; it < last; ++it) {
                 auto value = std::move(*it);
                 auto pre = it;
 
@@ -179,7 +286,7 @@
  */
 template<std::size_t Bit, std::size_t N>
 struct radix_sort {
-    static_assert((N % Bit) == 0);
+    static_assert((N % Bit) == 0, "The maximum number of bits to sort must be a multiple of the number of bits processed per pass");
 
     /**
      * @brief Sorts the elements in a range.
@@ -210,17 +317,17 @@
                 std::size_t index[buckets]{};
                 std::size_t count[buckets]{};
 
-                std::for_each(from, to, [&getter, &count, start](const value_type &item) {
-                    ++count[(getter(item) >> start) & mask];
-                });
-
-                std::for_each(std::next(std::begin(index)), std::end(index), [index = std::begin(index), count = std::begin(count)](auto &item) mutable {
-                    item = *(index++) + *(count++);
-                });
-
-                std::for_each(from, to, [&getter, &out, &index, start](value_type &item) {
-                    out[index[(getter(item) >> start) & mask]++] = std::move(item);
-                });
+                for(auto it = from; it != to; ++it) {
+                    ++count[(getter(*it) >> start) & mask];
+                }
+
+                for(std::size_t pos{}, end = buckets - 1u; pos < end; ++pos) {
+                    index[pos + 1u] = index[pos] + count[pos];
+                }
+
+                for(auto it = from; it != to; ++it) {
+                    out[index[(getter(*it) >> start) & mask]++] = std::move(*it);
+                }
             };
 
             for(std::size_t pass = 0; pass < (passes & ~1); pass += 2) {
@@ -240,59 +347,298 @@
 }
 
 
-#endif // ENTT_CORE_ALGORITHM_HPP
-
-// #include "core/family.hpp"
-#ifndef ENTT_CORE_FAMILY_HPP
-#define ENTT_CORE_FAMILY_HPP
-
-
-#include <type_traits>
-// #include "../config/config.h"
-
-
-
-namespace entt {
-
-
-/**
- * @brief Dynamic identifier generator.
- *
- * Utility class template that can be used to assign unique identifiers to types
- * at runtime. Use different specializations to create separate sets of
- * identifiers.
- */
-template<typename...>
-class family {
-    inline static ENTT_MAYBE_ATOMIC(ENTT_ID_TYPE) identifier;
-
-    template<typename...>
-    // clang (since version 9) started to complain if auto is used instead of ENTT_ID_TYPE
-    inline static const ENTT_ID_TYPE inner = identifier++;
-
-public:
-    /*! @brief Unsigned integer type. */
-    using family_type = ENTT_ID_TYPE;
-
-    /*! @brief Statically generated unique identifier for the given type. */
-    template<typename... Type>
-    // at the time I'm writing, clang crashes during compilation if auto is used instead of family_type
-    inline static const family_type type = inner<std::decay_t<Type>...>;
-};
-
-
-}
-
-
-#endif // ENTT_CORE_FAMILY_HPP
-
-// #include "core/hashed_string.hpp"
+#endif
+
+// #include "core/any.hpp"
+#ifndef ENTT_CORE_ANY_HPP
+#define ENTT_CORE_ANY_HPP
+
+
+#include <cstddef>
+#include <functional>
+#include <memory>
+#include <type_traits>
+#include <utility>
+// #include "../config/config.h"
+
+// #include "../core/utility.hpp"
+#ifndef ENTT_CORE_UTILITY_HPP
+#define ENTT_CORE_UTILITY_HPP
+
+
+#include <utility>
+// #include "../config/config.h"
+#ifndef ENTT_CONFIG_CONFIG_H
+#define ENTT_CONFIG_CONFIG_H
+
+
+#if defined(__cpp_exceptions) && !defined(ENTT_NOEXCEPTION)
+#   define ENTT_NOEXCEPT noexcept
+#   define ENTT_THROW throw
+#   define ENTT_TRY try
+#   define ENTT_CATCH catch(...)
+#else
+#   define ENTT_NOEXCEPT
+#   define ENTT_THROW
+#   define ENTT_TRY if(true)
+#   define ENTT_CATCH if(false)
+#endif
+
+
+#if defined(__cpp_lib_launder) && __cpp_lib_launder >= 201606L
+#   include <new>
+#   define ENTT_LAUNDER(expr) std::launder(expr)
+#else
+#   define ENTT_LAUNDER(expr) expr
+#endif
+
+
+#ifndef ENTT_USE_ATOMIC
+#   define ENTT_MAYBE_ATOMIC(Type) Type
+#else
+#   include <atomic>
+#   define ENTT_MAYBE_ATOMIC(Type) std::atomic<Type>
+#endif
+
+
+#ifndef ENTT_ID_TYPE
+#   include <cstdint>
+#   define ENTT_ID_TYPE std::uint32_t
+#endif
+
+
+#ifdef ENTT_SPARSE_PAGE
+	static_assert(ENTT_SPARSE_PAGE && ((ENTT_SPARSE_PAGE & (ENTT_SPARSE_PAGE - 1)) == 0), "ENTT_SPARSE_PAGE must be a power of two");
+#else
+#   define ENTT_SPARSE_PAGE 4096
+#endif
+
+
+#ifdef ENTT_PACKED_PAGE
+static_assert(ENTT_PACKED_PAGE && ((ENTT_PACKED_PAGE & (ENTT_PACKED_PAGE - 1)) == 0), "ENTT_PACKED_PAGE must be a power of two");
+#else
+#   define ENTT_PACKED_PAGE 1024
+#endif
+
+
+#ifdef ENTT_DISABLE_ASSERT
+#   undef ENTT_ASSERT
+#   define ENTT_ASSERT(...) (void(0))
+#elif !defined ENTT_ASSERT
+#   include <cassert>
+#   define ENTT_ASSERT(condition, ...) assert(condition)
+#endif
+
+
+#ifdef ENTT_NO_ETO
+#   include <type_traits>
+#   define ENTT_IGNORE_IF_EMPTY std::false_type
+#else
+#   include <type_traits>
+#   define ENTT_IGNORE_IF_EMPTY std::true_type
+#endif
+
+
+#ifndef ENTT_STANDARD_CPP
+#    if defined __clang__ || defined __GNUC__
+#       define ENTT_PRETTY_FUNCTION __PRETTY_FUNCTION__
+#       define ENTT_PRETTY_FUNCTION_PREFIX '='
+#       define ENTT_PRETTY_FUNCTION_SUFFIX ']'
+#    elif defined _MSC_VER
+#       define ENTT_PRETTY_FUNCTION __FUNCSIG__
+#       define ENTT_PRETTY_FUNCTION_PREFIX '<'
+#       define ENTT_PRETTY_FUNCTION_SUFFIX '>'
+#   endif
+#endif
+
+
+#endif
+
+
+
+namespace entt {
+
+
+/*! @brief Identity function object (waiting for C++20). */
+struct identity {
+    /**
+     * @brief Returns its argument unchanged.
+     * @tparam Type Type of the argument.
+     * @param value The actual argument.
+     * @return The submitted value as-is.
+     */
+    template<class Type>
+    [[nodiscard]] constexpr Type && operator()(Type &&value) const ENTT_NOEXCEPT {
+        return std::forward<Type>(value);
+    }
+};
+
+
+/**
+ * @brief Constant utility to disambiguate overloaded members of a class.
+ * @tparam Type Type of the desired overload.
+ * @tparam Class Type of class to which the member belongs.
+ * @param member A valid pointer to a member.
+ * @return Pointer to the member.
+ */
+template<typename Type, typename Class>
+[[nodiscard]] constexpr auto overload(Type Class:: *member) ENTT_NOEXCEPT { return member; }
+
+
+/**
+ * @brief Constant utility to disambiguate overloaded functions.
+ * @tparam Func Function type of the desired overload.
+ * @param func A valid pointer to a function.
+ * @return Pointer to the function.
+ */
+template<typename Func>
+[[nodiscard]] constexpr auto overload(Func *func) ENTT_NOEXCEPT { return func; }
+
+
+/**
+ * @brief Helper type for visitors.
+ * @tparam Func Types of function objects.
+ */
+template<class... Func>
+struct overloaded: Func... {
+    using Func::operator()...;
+};
+
+
+/**
+ * @brief Deduction guide.
+ * @tparam Func Types of function objects.
+ */
+template<class... Func>
+overloaded(Func...)
+-> overloaded<Func...>;
+
+
+/**
+ * @brief Basic implementation of a y-combinator.
+ * @tparam Func Type of a potentially recursive function.
+ */
+template<class Func>
+struct y_combinator {
+    /**
+     * @brief Constructs a y-combinator from a given function.
+     * @param recursive A potentially recursive function.
+     */
+    y_combinator(Func recursive):
+        func{std::move(recursive)}
+    {}
+
+    /**
+     * @brief Invokes a y-combinator and therefore its underlying function.
+     * @tparam Args Types of arguments to use to invoke the underlying function.
+     * @param args Parameters to use to invoke the underlying function.
+     * @return Return value of the underlying function, if any.
+     */
+    template <class... Args>
+    decltype(auto) operator()(Args &&... args) const {
+        return func(*this, std::forward<Args>(args)...);
+    }
+
+    /*! @copydoc operator()() */
+    template <class... Args>
+    decltype(auto) operator()(Args &&... args) {
+        return func(*this, std::forward<Args>(args)...);
+    }
+
+private:
+    Func func;
+};
+
+
+}
+
+
+#endif
+
+// #include "fwd.hpp"
+#ifndef ENTT_CORE_FWD_HPP
+#define ENTT_CORE_FWD_HPP
+
+
+#include <type_traits>
+// #include "../config/config.h"
+
+
+
+namespace entt {
+
+
+template<std::size_t Len = sizeof(double[2]), std::size_t = alignof(typename std::aligned_storage_t<Len + !Len>)>
+class basic_any;
+
+
+/*! @brief Alias declaration for type identifiers. */
+using id_type = ENTT_ID_TYPE;
+
+
+/*! @brief Alias declaration for the most common use case. */
+using any = basic_any<>;
+
+
+}
+
+
+#endif
+
+// #include "type_info.hpp"
+#ifndef ENTT_CORE_TYPE_INFO_HPP
+#define ENTT_CORE_TYPE_INFO_HPP
+
+
+#include <string_view>
+#include <type_traits>
+// #include "../config/config.h"
+
+// #include "../core/attribute.h"
+#ifndef ENTT_CORE_ATTRIBUTE_H
+#define ENTT_CORE_ATTRIBUTE_H
+
+
+#ifndef ENTT_EXPORT
+#   if defined _WIN32 || defined __CYGWIN__ || defined _MSC_VER
+#       define ENTT_EXPORT __declspec(dllexport)
+#       define ENTT_IMPORT __declspec(dllimport)
+#       define ENTT_HIDDEN
+#   elif defined __GNUC__ && __GNUC__ >= 4
+#       define ENTT_EXPORT __attribute__((visibility("default")))
+#       define ENTT_IMPORT __attribute__((visibility("default")))
+#       define ENTT_HIDDEN __attribute__((visibility("hidden")))
+#   else /* Unsupported compiler */
+#       define ENTT_EXPORT
+#       define ENTT_IMPORT
+#       define ENTT_HIDDEN
+#   endif
+#endif
+
+
+#ifndef ENTT_API
+#   if defined ENTT_API_EXPORT
+#       define ENTT_API ENTT_EXPORT
+#   elif defined ENTT_API_IMPORT
+#       define ENTT_API ENTT_IMPORT
+#   else /* No API */
+#       define ENTT_API
+#   endif
+#endif
+
+
+#endif
+
+// #include "hashed_string.hpp"
 #ifndef ENTT_CORE_HASHED_STRING_HPP
 #define ENTT_CORE_HASHED_STRING_HPP
 
 
 #include <cstddef>
-// #include "../config/config.h"
+#include <cstdint>
+// #include "../config/config.h"
+
+// #include "fwd.hpp"
 
 
 
@@ -314,6 +660,7 @@
 
 template<>
 struct fnv1a_traits<std::uint32_t> {
+    using type = std::uint32_t;
     static constexpr std::uint32_t offset = 2166136261;
     static constexpr std::uint32_t prime = 16777619;
 };
@@ -321,6 +668,7 @@
 
 template<>
 struct fnv1a_traits<std::uint64_t> {
+    using type = std::uint64_t;
     static constexpr std::uint64_t offset = 14695981039346656037ull;
     static constexpr std::uint64_t prime = 1099511628211ull;
 };
@@ -331,7 +679,7 @@
 
 /**
  * Internal details not to be documented.
- * @endcond TURN_OFF_DOXYGEN
+ * @endcond
  */
 
 
@@ -348,7 +696,7 @@
  */
 template<typename Char>
 class basic_hashed_string {
-    using traits_type = internal::fnv1a_traits<ENTT_ID_TYPE>;
+    using traits_type = internal::fnv1a_traits<id_type>;
 
     struct const_wrapper {
         // non-explicit constructor on purpose
@@ -357,15 +705,33 @@
     };
 
     // Fowler–Noll–Vo hash function v. 1a - the good
-    static constexpr ENTT_ID_TYPE helper(ENTT_ID_TYPE partial, const Char *curr) ENTT_NOEXCEPT {
-        return curr[0] == 0 ? partial : helper((partial^curr[0])*traits_type::prime, curr+1);
+    [[nodiscard]] static constexpr id_type helper(const Char *curr) ENTT_NOEXCEPT {
+        auto value = traits_type::offset;
+
+        while(*curr != 0) {
+            value = (value ^ static_cast<traits_type::type>(*(curr++))) * traits_type::prime;
+        }
+
+        return value;
     }
 
 public:
     /*! @brief Character type. */
     using value_type = Char;
     /*! @brief Unsigned integer type. */
-    using hash_type = ENTT_ID_TYPE;
+    using hash_type = id_type;
+
+    /**
+     * @brief Returns directly the numeric representation of a string view.
+     * @param str Human-readable identifer.
+     * @param size Length of the string to hash.
+     * @return The numeric representation of the string.
+     */
+    [[nodiscard]] static constexpr hash_type value(const value_type *str, std::size_t size) ENTT_NOEXCEPT {
+        id_type partial{traits_type::offset};
+        while(size--) { partial = (partial^(str++)[0])*traits_type::prime; }
+        return partial;
+    }
 
     /**
      * @brief Returns directly the numeric representation of a string.
@@ -383,8 +749,8 @@
      * @return The numeric representation of the string.
      */
     template<std::size_t N>
-    static constexpr hash_type to_value(const value_type (&str)[N]) ENTT_NOEXCEPT {
-        return helper(traits_type::offset, str);
+    [[nodiscard]] static constexpr hash_type value(const value_type (&str)[N]) ENTT_NOEXCEPT {
+        return helper(str);
     }
 
     /**
@@ -392,20 +758,8 @@
      * @param wrapper Helps achieving the purpose by relying on overloading.
      * @return The numeric representation of the string.
      */
-    static hash_type to_value(const_wrapper wrapper) ENTT_NOEXCEPT {
-        return helper(traits_type::offset, wrapper.str);
-    }
-
-    /**
-     * @brief Returns directly the numeric representation of a string view.
-     * @param str Human-readable identifer.
-     * @param size Length of the string to hash.
-     * @return The numeric representation of the string.
-     */
-    static hash_type to_value(const value_type *str, std::size_t size) ENTT_NOEXCEPT {
-        ENTT_ID_TYPE partial{traits_type::offset};
-        while(size--) { partial = (partial^(str++)[0])*traits_type::prime; }
-        return partial;
+    [[nodiscard]] static hash_type value(const_wrapper wrapper) ENTT_NOEXCEPT {
+        return helper(wrapper.str);
     }
 
     /*! @brief Constructs an empty hashed string. */
@@ -429,7 +783,7 @@
      */
     template<std::size_t N>
     constexpr basic_hashed_string(const value_type (&curr)[N]) ENTT_NOEXCEPT
-        : str{curr}, hash{helper(traits_type::offset, curr)}
+        : str{curr}, hash{helper(curr)}
     {}
 
     /**
@@ -438,14 +792,14 @@
      * @param wrapper Helps achieving the purpose by relying on overloading.
      */
     explicit constexpr basic_hashed_string(const_wrapper wrapper) ENTT_NOEXCEPT
-        : str{wrapper.str}, hash{helper(traits_type::offset, wrapper.str)}
+        : str{wrapper.str}, hash{helper(wrapper.str)}
     {}
 
     /**
      * @brief Returns the human-readable representation of a hashed string.
      * @return The string used to initialize the instance.
      */
-    constexpr const value_type * data() const ENTT_NOEXCEPT {
+    [[nodiscard]] constexpr const value_type * data() const ENTT_NOEXCEPT {
         return str;
     }
 
@@ -453,25 +807,25 @@
      * @brief Returns the numeric representation of a hashed string.
      * @return The numeric representation of the instance.
      */
-    constexpr hash_type value() const ENTT_NOEXCEPT {
+    [[nodiscard]] constexpr hash_type value() const ENTT_NOEXCEPT {
         return hash;
     }
 
-    /**
-     * @brief Returns the human-readable representation of a hashed string.
-     * @return The string used to initialize the instance.
-     */
-    constexpr operator const value_type *() const ENTT_NOEXCEPT { return str; }
-
-    /*! @copydoc value */
-    constexpr operator hash_type() const ENTT_NOEXCEPT { return hash; }
+    /*! @copydoc data */
+    [[nodiscard]] constexpr operator const value_type *() const ENTT_NOEXCEPT { return data(); }
+
+    /**
+     * @brief Returns the numeric representation of a hashed string.
+     * @return The numeric representation of the instance.
+     */
+    [[nodiscard]] constexpr operator hash_type() const ENTT_NOEXCEPT { return value(); }
 
     /**
      * @brief Compares two hashed strings.
      * @param other Hashed string with which to compare.
      * @return True if the two hashed strings are identical, false otherwise.
      */
-    constexpr bool operator==(const basic_hashed_string &other) const ENTT_NOEXCEPT {
+    [[nodiscard]] constexpr bool operator==(const basic_hashed_string &other) const ENTT_NOEXCEPT {
         return hash == other.hash;
     }
 
@@ -492,7 +846,7 @@
  * @param str Human-readable identifer.
  */
 template<typename Char, std::size_t N>
-basic_hashed_string(const Char (&str)[N]) ENTT_NOEXCEPT
+basic_hashed_string(const Char (&str)[N])
 -> basic_hashed_string<Char>;
 
 
@@ -504,7 +858,7 @@
  * @return True if the two hashed strings are identical, false otherwise.
  */
 template<typename Char>
-constexpr bool operator!=(const basic_hashed_string<Char> &lhs, const basic_hashed_string<Char> &rhs) ENTT_NOEXCEPT {
+[[nodiscard]] constexpr bool operator!=(const basic_hashed_string<Char> &lhs, const basic_hashed_string<Char> &rhs) ENTT_NOEXCEPT {
     return !(lhs == rhs);
 }
 
@@ -517,7 +871,7 @@
 using hashed_wstring = basic_hashed_string<wchar_t>;
 
 
-}
+inline namespace literals {
 
 
 /**
@@ -525,7 +879,7 @@
  * @param str The literal without its suffix.
  * @return A properly initialized hashed string.
  */
-constexpr entt::hashed_string operator"" ENTT_HS_SUFFIX(const char *str, std::size_t) ENTT_NOEXCEPT {
+[[nodiscard]] constexpr entt::hashed_string operator"" _hs(const char *str, std::size_t) ENTT_NOEXCEPT {
     return entt::hashed_string{str};
 }
 
@@ -535,22 +889,1773 @@
  * @param str The literal without its suffix.
  * @return A properly initialized hashed wstring.
  */
-constexpr entt::hashed_wstring operator"" ENTT_HWS_SUFFIX(const wchar_t *str, std::size_t) ENTT_NOEXCEPT {
+[[nodiscard]] constexpr entt::hashed_wstring operator"" _hws(const wchar_t *str, std::size_t) ENTT_NOEXCEPT {
     return entt::hashed_wstring{str};
 }
 
 
-#endif // ENTT_CORE_HASHED_STRING_HPP
+}
+
+
+}
+
+
+#endif
+
+// #include "fwd.hpp"
+
+
+
+namespace entt {
+
+
+/**
+ * @cond TURN_OFF_DOXYGEN
+ * Internal details not to be documented.
+ */
+
+
+namespace internal {
+
+
+struct ENTT_API type_seq final {
+    [[nodiscard]] static id_type next() ENTT_NOEXCEPT {
+        static ENTT_MAYBE_ATOMIC(id_type) value{};
+        return value++;
+    }
+};
+
+
+template<typename Type>
+[[nodiscard]] constexpr auto stripped_type_name() ENTT_NOEXCEPT {
+#if defined ENTT_PRETTY_FUNCTION
+    std::string_view pretty_function{ENTT_PRETTY_FUNCTION};
+    auto first = pretty_function.find_first_not_of(' ', pretty_function.find_first_of(ENTT_PRETTY_FUNCTION_PREFIX)+1);
+    auto value = pretty_function.substr(first, pretty_function.find_last_of(ENTT_PRETTY_FUNCTION_SUFFIX) - first);
+    return value;
+#else
+    return std::string_view{""};
+#endif
+}
+
+
+template<typename Type, auto = stripped_type_name<Type>().find_first_of('.')>
+[[nodiscard]] static constexpr std::string_view type_name(int) ENTT_NOEXCEPT {
+    constexpr auto value = stripped_type_name<Type>();
+    return value;
+}
+
+
+template<typename Type>
+[[nodiscard]] static std::string_view type_name(char) ENTT_NOEXCEPT {
+    static const auto value = stripped_type_name<Type>();
+    return value;
+}
+
+
+template<typename Type, auto = stripped_type_name<Type>().find_first_of('.')>
+[[nodiscard]] static constexpr id_type type_hash(int) ENTT_NOEXCEPT {
+    constexpr auto stripped = stripped_type_name<Type>();
+    constexpr auto value = hashed_string::value(stripped.data(), stripped.size());
+    return value;
+}
+
+
+template<typename Type>
+[[nodiscard]] static id_type type_hash(char) ENTT_NOEXCEPT {
+    static const auto value = [](const auto stripped) {
+        return hashed_string::value(stripped.data(), stripped.size());
+    }(stripped_type_name<Type>());
+    return value;
+}
+
+
+}
+
+
+/**
+ * Internal details not to be documented.
+ * @endcond
+ */
+
+
+/**
+ * @brief Type sequential identifier.
+ * @tparam Type Type for which to generate a sequential identifier.
+ */
+template<typename Type, typename = void>
+struct ENTT_API type_seq final {
+    /**
+     * @brief Returns the sequential identifier of a given type.
+     * @return The sequential identifier of a given type.
+     */
+    [[nodiscard]] static id_type value() ENTT_NOEXCEPT {
+        static const id_type value = internal::type_seq::next();
+        return value;
+    }
+
+    /*! @copydoc value */
+    [[nodiscard]] constexpr operator id_type() const ENTT_NOEXCEPT { return value(); }
+};
+
+
+/**
+ * @brief Type hash.
+ * @tparam Type Type for which to generate a hash value.
+ */
+template<typename Type, typename = void>
+struct type_hash final {
+    /**
+     * @brief Returns the numeric representation of a given type.
+     * @return The numeric representation of the given type.
+     */
+#if defined ENTT_PRETTY_FUNCTION
+    [[nodiscard]] static constexpr id_type value() ENTT_NOEXCEPT {
+        return internal::type_hash<Type>(0);
+#else
+    [[nodiscard]] static constexpr id_type value() ENTT_NOEXCEPT {
+        return type_seq<Type>::value();
+#endif
+    }
+
+    /*! @copydoc value */
+    [[nodiscard]] constexpr operator id_type() const ENTT_NOEXCEPT { return value(); }
+};
+
+
+/**
+ * @brief Type name.
+ * @tparam Type Type for which to generate a name.
+ */
+template<typename Type, typename = void>
+struct type_name final {
+    /**
+     * @brief Returns the name of a given type.
+     * @return The name of the given type.
+     */
+    [[nodiscard]] static constexpr std::string_view value() ENTT_NOEXCEPT {
+        return internal::type_name<Type>(0);
+    }
+
+    /*! @copydoc value */
+    [[nodiscard]] constexpr operator std::string_view() const ENTT_NOEXCEPT { return value(); }
+};
+
+
+/*! @brief Implementation specific information about a type. */
+class type_info final {
+    template<typename>
+    friend type_info type_id() ENTT_NOEXCEPT;
+
+    type_info(id_type seq_v, id_type hash_v, std::string_view name_v) ENTT_NOEXCEPT
+        : seq_value{seq_v},
+          hash_value{hash_v},
+          name_value{name_v}
+    {}
+
+public:
+    /*! @brief Default constructor. */
+    type_info() ENTT_NOEXCEPT
+        : type_info({}, {}, {})
+    {}
+
+    /*! @brief Default copy constructor. */
+    type_info(const type_info &) ENTT_NOEXCEPT = default;
+    /*! @brief Default move constructor. */
+    type_info(type_info &&) ENTT_NOEXCEPT = default;
+
+    /**
+     * @brief Default copy assignment operator.
+     * @return This type info object.
+     */
+    type_info & operator=(const type_info &) ENTT_NOEXCEPT = default;
+
+    /**
+     * @brief Default move assignment operator.
+     * @return This type info object.
+     */
+    type_info & operator=(type_info &&) ENTT_NOEXCEPT = default;
+
+    /**
+     * @brief Checks if a type info object is properly initialized.
+     * @return True if the object is properly initialized, false otherwise.
+     */
+    [[nodiscard]] explicit operator bool() const ENTT_NOEXCEPT {
+        return name_value.data() != nullptr;
+    }
+
+    /**
+     * @brief Type sequential identifier.
+     * @return Type sequential identifier.
+     */
+    [[nodiscard]] id_type seq() const ENTT_NOEXCEPT {
+        return seq_value;
+    }
+
+    /**
+     * @brief Type hash.
+     * @return Type hash.
+     */
+    [[nodiscard]] id_type hash() const ENTT_NOEXCEPT {
+        return hash_value;
+    }
+
+    /**
+     * @brief Type name.
+     * @return Type name.
+     */
+    [[nodiscard]] std::string_view name() const ENTT_NOEXCEPT {
+        return name_value;
+    }
+
+    /**
+     * @brief Compares the contents of two type info objects.
+     * @param other Object with which to compare.
+     * @return False if the two contents differ, true otherwise.
+     */
+    [[nodiscard]] bool operator==(const type_info &other) const ENTT_NOEXCEPT {
+        return hash_value == other.hash_value;
+    }
+
+private:
+    id_type seq_value;
+    id_type hash_value;
+    std::string_view name_value;
+};
+
+
+/**
+ * @brief Compares the contents of two type info objects.
+ * @param lhs A type info object.
+ * @param rhs A type info object.
+ * @return True if the two contents differ, false otherwise.
+ */
+[[nodiscard]] inline bool operator!=(const type_info &lhs, const type_info &rhs) ENTT_NOEXCEPT {
+    return !(lhs == rhs);
+}
+
+
+/**
+ * @brief Returns the type info object for a given type.
+ * @tparam Type Type for which to generate a type info object.
+ * @return The type info object for the given type.
+ */
+template<typename Type>
+[[nodiscard]] type_info type_id() ENTT_NOEXCEPT {
+    return type_info{
+        type_seq<std::remove_cv_t<std::remove_reference_t<Type>>>::value(),
+        type_hash<std::remove_cv_t<std::remove_reference_t<Type>>>::value(),
+        type_name<std::remove_cv_t<std::remove_reference_t<Type>>>::value()
+    };
+}
+
+
+}
+
+
+#endif
+
+// #include "type_traits.hpp"
+#ifndef ENTT_CORE_TYPE_TRAITS_HPP
+#define ENTT_CORE_TYPE_TRAITS_HPP
+
+
+#include <cstddef>
+#include <iterator>
+#include <type_traits>
+#include <utility>
+// #include "../config/config.h"
+
+// #include "fwd.hpp"
+
+
+
+namespace entt {
+
+
+/**
+ * @brief Utility class to disambiguate overloaded functions.
+ * @tparam N Number of choices available.
+ */
+template<std::size_t N>
+struct choice_t
+    // Unfortunately, doxygen cannot parse such a construct.
+    /*! @cond TURN_OFF_DOXYGEN */
+    : choice_t<N-1>
+    /*! @endcond */
+{};
+
+
+/*! @copybrief choice_t */
+template<>
+struct choice_t<0> {};
+
+
+/**
+ * @brief Variable template for the choice trick.
+ * @tparam N Number of choices available.
+ */
+template<std::size_t N>
+inline constexpr choice_t<N> choice{};
+
+
+/**
+ * @brief Identity type trait.
+ *
+ * Useful to establish non-deduced contexts in template argument deduction
+ * (waiting for C++20) or to provide types through function arguments.
+ *
+ * @tparam Type A type.
+ */
+template<typename Type>
+struct type_identity {
+    /*! @brief Identity type. */
+    using type = Type;
+};
+
+
+/**
+ * @brief Helper type.
+ * @tparam Type A type.
+ */
+template<typename Type>
+using type_identity_t = typename type_identity<Type>::type;
+
+
+/**
+ * @brief A type-only `sizeof` wrapper that returns 0 where `sizeof` complains.
+ * @tparam Type The type of which to return the size.
+ * @tparam The size of the type if `sizeof` accepts it, 0 otherwise.
+ */
+template<typename Type, typename = void>
+struct size_of: std::integral_constant<std::size_t, 0u> {};
+
+
+/*! @copydoc size_of */
+template<typename Type>
+struct size_of<Type, std::void_t<decltype(sizeof(Type))>>
+    : std::integral_constant<std::size_t, sizeof(Type)>
+{};
+
+
+/**
+ * @brief Helper variable template.
+ * @tparam Type The type of which to return the size.
+ */
+template<class Type>
+inline constexpr std::size_t size_of_v = size_of<Type>::value;
+
+
+/**
+ * @brief Using declaration to be used to _repeat_ the same type a number of
+ * times equal to the size of a given parameter pack.
+ * @tparam Type A type to repeat.
+ */
+template<typename Type, typename>
+using unpack_as_t = Type;
+
+
+/**
+ * @brief Helper variable template to be used to _repeat_ the same value a
+ * number of times equal to the size of a given parameter pack.
+ * @tparam Value A value to repeat.
+ */
+template<auto Value, typename>
+inline constexpr auto unpack_as_v = Value;
+
+
+/**
+ * @brief Wraps a static constant.
+ * @tparam Value A static constant.
+ */
+template<auto Value>
+using integral_constant = std::integral_constant<decltype(Value), Value>;
+
+
+/**
+ * @brief Alias template to facilitate the creation of named values.
+ * @tparam Value A constant value at least convertible to `id_type`.
+ */
+template<id_type Value>
+using tag = integral_constant<Value>;
+
+
+/**
+ * @brief A class to use to push around lists of types, nothing more.
+ * @tparam Type Types provided by the type list.
+ */
+template<typename... Type>
+struct type_list {
+    /*! @brief Type list type. */
+    using type = type_list;
+    /*! @brief Compile-time number of elements in the type list. */
+    static constexpr auto size = sizeof...(Type);
+};
+
+
+/*! @brief Primary template isn't defined on purpose. */
+template<std::size_t, typename>
+struct type_list_element;
+
+
+/**
+ * @brief Provides compile-time indexed access to the types of a type list.
+ * @tparam Index Index of the type to return.
+ * @tparam Type First type provided by the type list.
+ * @tparam Other Other types provided by the type list.
+ */
+template<std::size_t Index, typename Type, typename... Other>
+struct type_list_element<Index, type_list<Type, Other...>>
+    : type_list_element<Index - 1u, type_list<Other...>>
+{};
+
+
+/**
+ * @brief Provides compile-time indexed access to the types of a type list.
+ * @tparam Type First type provided by the type list.
+ * @tparam Other Other types provided by the type list.
+ */
+template<typename Type, typename... Other>
+struct type_list_element<0u, type_list<Type, Other...>> {
+    /*! @brief Searched type. */
+    using type = Type;
+};
+
+
+/**
+ * @brief Helper type.
+ * @tparam Index Index of the type to return.
+ * @tparam List Type list to search into.
+ */
+template<std::size_t Index, typename List>
+using type_list_element_t = typename type_list_element<Index, List>::type;
+
+
+/**
+ * @brief Concatenates multiple type lists.
+ * @tparam Type Types provided by the first type list.
+ * @tparam Other Types provided by the second type list.
+ * @return A type list composed by the types of both the type lists.
+ */
+template<typename... Type, typename... Other>
+constexpr type_list<Type..., Other...> operator+(type_list<Type...>, type_list<Other...>) { return {}; }
+
+
+/*! @brief Primary template isn't defined on purpose. */
+template<typename...>
+struct type_list_cat;
+
+
+/*! @brief Concatenates multiple type lists. */
+template<>
+struct type_list_cat<> {
+    /*! @brief A type list composed by the types of all the type lists. */
+    using type = type_list<>;
+};
+
+
+/**
+ * @brief Concatenates multiple type lists.
+ * @tparam Type Types provided by the first type list.
+ * @tparam Other Types provided by the second type list.
+ * @tparam List Other type lists, if any.
+ */
+template<typename... Type, typename... Other, typename... List>
+struct type_list_cat<type_list<Type...>, type_list<Other...>, List...> {
+    /*! @brief A type list composed by the types of all the type lists. */
+    using type = typename type_list_cat<type_list<Type..., Other...>, List...>::type;
+};
+
+
+/**
+ * @brief Concatenates multiple type lists.
+ * @tparam Type Types provided by the type list.
+ */
+template<typename... Type>
+struct type_list_cat<type_list<Type...>> {
+    /*! @brief A type list composed by the types of all the type lists. */
+    using type = type_list<Type...>;
+};
+
+
+/**
+ * @brief Helper type.
+ * @tparam List Type lists to concatenate.
+ */
+template<typename... List>
+using type_list_cat_t = typename type_list_cat<List...>::type;
+
+
+/*! @brief Primary template isn't defined on purpose. */
+template<typename>
+struct type_list_unique;
+
+
+/**
+ * @brief Removes duplicates types from a type list.
+ * @tparam Type One of the types provided by the given type list.
+ * @tparam Other The other types provided by the given type list.
+ */
+template<typename Type, typename... Other>
+struct type_list_unique<type_list<Type, Other...>> {
+    /*! @brief A type list without duplicate types. */
+    using type = std::conditional_t<
+        std::disjunction_v<std::is_same<Type, Other>...>,
+        typename type_list_unique<type_list<Other...>>::type,
+        type_list_cat_t<type_list<Type>, typename type_list_unique<type_list<Other...>>::type>
+    >;
+};
+
+
+/*! @brief Removes duplicates types from a type list. */
+template<>
+struct type_list_unique<type_list<>> {
+    /*! @brief A type list without duplicate types. */
+    using type = type_list<>;
+};
+
+
+/**
+ * @brief Helper type.
+ * @tparam Type A type list.
+ */
+template<typename Type>
+using type_list_unique_t = typename type_list_unique<Type>::type;
+
+
+/**
+ * @brief Provides the member constant `value` to true if a type list contains a
+ * given type, false otherwise.
+ * @tparam List Type list.
+ * @tparam Type Type to look for.
+ */
+template<typename List, typename Type>
+struct type_list_contains;
+
+
+/**
+ * @copybrief type_list_contains
+ * @tparam Type Types provided by the type list.
+ * @tparam Other Type to look for.
+ */
+template<typename... Type, typename Other>
+struct type_list_contains<type_list<Type...>, Other>: std::disjunction<std::is_same<Type, Other>...> {};
+
+
+/**
+ * @brief Helper variable template.
+ * @tparam List Type list.
+ * @tparam Type Type to look for.
+ */
+template<class List, typename Type>
+inline constexpr bool type_list_contains_v = type_list_contains<List, Type>::value;
+
+
+/*! @brief Primary template isn't defined on purpose. */
+template<typename...>
+struct type_list_diff;
+
+
+/**
+ * @brief Computes the difference between two type lists.
+ * @tparam Type Types provided by the first type list.
+ * @tparam Other Types provided by the second type list.
+ */
+template<typename... Type, typename... Other>
+struct type_list_diff<type_list<Type...>, type_list<Other...>> {
+    /*! @brief A type list that is the difference between the two type lists. */
+    using type = type_list_cat_t<std::conditional_t<type_list_contains_v<type_list<Other...>, Type>, type_list<>, type_list<Type>>...>;
+};
+
+
+/**
+ * @brief Helper type.
+ * @tparam List Type lists between which to compute the difference.
+ */
+template<typename... List>
+using type_list_diff_t = typename type_list_diff<List...>::type;
+
+
+/**
+ * @brief A class to use to push around lists of constant values, nothing more.
+ * @tparam Value Values provided by the value list.
+ */
+template<auto... Value>
+struct value_list {
+    /*! @brief Value list type. */
+    using type = value_list;
+    /*! @brief Compile-time number of elements in the value list. */
+    static constexpr auto size = sizeof...(Value);
+};
+
+
+/*! @brief Primary template isn't defined on purpose. */
+template<std::size_t, typename>
+struct value_list_element;
+
+
+/**
+ * @brief Provides compile-time indexed access to the values of a value list.
+ * @tparam Index Index of the value to return.
+ * @tparam Value First value provided by the value list.
+ * @tparam Other Other values provided by the value list.
+ */
+template<std::size_t Index, auto Value, auto... Other>
+struct value_list_element<Index, value_list<Value, Other...>>
+    : value_list_element<Index - 1u, value_list<Other...>>
+{};
+
+
+/**
+ * @brief Provides compile-time indexed access to the types of a type list.
+ * @tparam Value First value provided by the value list.
+ * @tparam Other Other values provided by the value list.
+ */
+template<auto Value, auto... Other>
+struct value_list_element<0u, value_list<Value, Other...>> {
+    /*! @brief Searched value. */
+    static constexpr auto value = Value;
+};
+
+
+/**
+ * @brief Helper type.
+ * @tparam Index Index of the value to return.
+ * @tparam List Value list to search into.
+ */
+template<std::size_t Index, typename List>
+inline constexpr auto value_list_element_v = value_list_element<Index, List>::value;
+
+
+/**
+ * @brief Concatenates multiple value lists.
+ * @tparam Value Values provided by the first value list.
+ * @tparam Other Values provided by the second value list.
+ * @return A value list composed by the values of both the value lists.
+ */
+template<auto... Value, auto... Other>
+constexpr value_list<Value..., Other...> operator+(value_list<Value...>, value_list<Other...>) { return {}; }
+
+
+/*! @brief Primary template isn't defined on purpose. */
+template<typename...>
+struct value_list_cat;
+
+
+/*! @brief Concatenates multiple value lists. */
+template<>
+struct value_list_cat<> {
+    /*! @brief A value list composed by the values of all the value lists. */
+    using type = value_list<>;
+};
+
+
+/**
+ * @brief Concatenates multiple value lists.
+ * @tparam Value Values provided by the first value list.
+ * @tparam Other Values provided by the second value list.
+ * @tparam List Other value lists, if any.
+ */
+template<auto... Value, auto... Other, typename... List>
+struct value_list_cat<value_list<Value...>, value_list<Other...>, List...> {
+    /*! @brief A value list composed by the values of all the value lists. */
+    using type = typename value_list_cat<value_list<Value..., Other...>, List...>::type;
+};
+
+
+/**
+ * @brief Concatenates multiple value lists.
+ * @tparam Value Values provided by the value list.
+ */
+template<auto... Value>
+struct value_list_cat<value_list<Value...>> {
+    /*! @brief A value list composed by the values of all the value lists. */
+    using type = value_list<Value...>;
+};
+
+
+/**
+ * @brief Helper type.
+ * @tparam List Value lists to concatenate.
+ */
+template<typename... List>
+using value_list_cat_t = typename value_list_cat<List...>::type;
+
+
+/*! @brief Same as std::is_invocable, but with tuples. */
+template<typename, typename>
+struct is_applicable: std::false_type {};
+
+
+/**
+ * @copybrief is_applicable
+ * @tparam Func A valid function type.
+ * @tparam Tuple Tuple-like type.
+ * @tparam Args The list of arguments to use to probe the function type.
+ */
+template<typename Func, template<typename...> class Tuple, typename... Args>
+struct is_applicable<Func, Tuple<Args...>>: std::is_invocable<Func, Args...> {};
+
+
+/**
+ * @copybrief is_applicable
+ * @tparam Func A valid function type.
+ * @tparam Tuple Tuple-like type.
+ * @tparam Args The list of arguments to use to probe the function type.
+ */
+template<typename Func, template<typename...> class Tuple, typename... Args>
+struct is_applicable<Func, const Tuple<Args...>>: std::is_invocable<Func, Args...> {};
+
+
+/**
+ * @brief Helper variable template.
+ * @tparam Func A valid function type.
+ * @tparam Args The list of arguments to use to probe the function type.
+ */
+template<typename Func, typename Args>
+inline constexpr bool is_applicable_v = is_applicable<Func, Args>::value;
+
+
+/*! @brief Same as std::is_invocable_r, but with tuples for arguments. */
+template<typename, typename, typename>
+struct is_applicable_r: std::false_type {};
+
+
+/**
+ * @copybrief is_applicable_r
+ * @tparam Ret The type to which the return type of the function should be
+ * convertible.
+ * @tparam Func A valid function type.
+ * @tparam Args The list of arguments to use to probe the function type.
+ */
+template<typename Ret, typename Func, typename... Args>
+struct is_applicable_r<Ret, Func, std::tuple<Args...>>: std::is_invocable_r<Ret, Func, Args...> {};
+
+
+/**
+ * @brief Helper variable template.
+ * @tparam Ret The type to which the return type of the function should be
+ * convertible.
+ * @tparam Func A valid function type.
+ * @tparam Args The list of arguments to use to probe the function type.
+ */
+template<typename Ret, typename Func, typename Args>
+inline constexpr bool is_applicable_r_v = is_applicable_r<Ret, Func, Args>::value;
+
+
+/**
+ * @brief Provides the member constant `value` to true if a given type is
+ * complete, false otherwise.
+ * @tparam Type The type to test.
+ */
+template<typename Type, typename = void>
+struct is_complete: std::false_type {};
+
+
+/*! @copydoc is_complete */
+template<typename Type>
+struct is_complete<Type, std::void_t<decltype(sizeof(Type))>>: std::true_type {};
+
+
+/**
+ * @brief Helper variable template.
+ * @tparam Type The type to test.
+ */
+template<typename Type>
+inline constexpr bool is_complete_v = is_complete<Type>::value;
+
+
+/**
+ * @brief Provides the member constant `value` to true if a given type is an
+ * iterator, false otherwise.
+ * @tparam Type The type to test.
+ */
+template<typename Type, typename = void>
+struct is_iterator: std::false_type {};
+
+
+/*! @copydoc is_iterator */
+template<typename Type>
+struct is_iterator<Type, std::void_t<typename std::iterator_traits<Type>::iterator_category>>
+    : std::true_type
+{};
+
+
+/**
+ * @brief Helper variable template.
+ * @tparam Type The type to test.
+ */
+template<typename Type>
+inline constexpr bool is_iterator_v = is_iterator<Type>::value;
+
+
+/**
+ * @brief Provides the member constant `value` to true if a given type is of the
+ * required iterator type, false otherwise.
+ * @tparam Type The type to test.
+ * @tparam It Required iterator type.
+ */
+template<typename Type, typename It, typename = void>
+struct is_iterator_type: std::false_type {};
+
+
+/*! @copydoc is_iterator_type */
+template<typename Type, typename It>
+struct is_iterator_type<Type, It, std::enable_if_t<is_iterator_v<Type> && std::is_same_v<Type, It>>>
+    : std::true_type
+{};
+
+
+/*! @copydoc is_iterator_type */
+template<typename Type, typename It>
+struct is_iterator_type<Type, It, std::enable_if_t<!std::is_same_v<Type, It>, std::void_t<typename It::iterator_type>>>
+    : is_iterator_type<Type, typename It::iterator_type>
+{};
+
+
+/**
+ * @brief Helper variable template.
+ * @tparam Type The type to test.
+ * @tparam It Required iterator type.
+ */
+template<typename Type, typename It>
+inline constexpr bool is_iterator_type_v = is_iterator_type<Type, It>::value;
+
+
+/**
+ * @cond TURN_OFF_DOXYGEN
+ * Internal details not to be documented.
+ */
+
+
+namespace internal {
+
+
+template<typename>
+[[nodiscard]] constexpr bool is_equality_comparable(...) { return false; }
+
+
+template<typename Type>
+[[nodiscard]] constexpr auto is_equality_comparable(choice_t<0>)
+-> decltype(std::declval<Type>() == std::declval<Type>()) { return true; }
+
+
+template<typename Type>
+[[nodiscard]] constexpr auto is_equality_comparable(choice_t<1>)
+-> decltype(std::declval<typename Type::value_type>(), std::declval<Type>() == std::declval<Type>()) {
+    if constexpr(is_iterator_v<Type>) {
+        return true;
+    } else if constexpr(std::is_same_v<typename Type::value_type, Type>) {
+        return is_equality_comparable<Type>(choice<0>);
+    } else {
+        return is_equality_comparable<typename Type::value_type>(choice<2>);
+    }
+}
+
+
+template<typename Type>
+[[nodiscard]] constexpr auto is_equality_comparable(choice_t<2>)
+-> decltype(std::declval<typename Type::mapped_type>(), std::declval<Type>() == std::declval<Type>()) {
+    return is_equality_comparable<typename Type::key_type>(choice<2>) && is_equality_comparable<typename Type::mapped_type>(choice<2>);
+}
+
+
+}
+
+
+/**
+ * Internal details not to be documented.
+ * @endcond
+ */
+
+
+/**
+ * @brief Provides the member constant `value` to true if a given type is
+ * equality comparable, false otherwise.
+ * @tparam Type The type to test.
+ */
+template<typename Type, typename = void>
+struct is_equality_comparable: std::bool_constant<internal::is_equality_comparable<Type>(choice<2>)> {};
+
+
+/**
+ * @brief Helper variable template.
+ * @tparam Type The type to test.
+ */
+template<class Type>
+inline constexpr bool is_equality_comparable_v = is_equality_comparable<Type>::value;
+
+
+/**
+ * @brief Transcribes the constness of a type to another type.
+ * @tparam To The type to which to transcribe the constness.
+ * @tparam From The type from which to transcribe the constness.
+ */
+template<typename To, typename From>
+struct constness_as {
+    /*! @brief The type resulting from the transcription of the constness. */
+    using type = std::remove_const_t<To>;
+};
+
+
+/*! @copydoc constness_as */
+template<typename To, typename From>
+struct constness_as<To, const From> {
+    /*! @brief The type resulting from the transcription of the constness. */
+    using type = std::add_const_t<To>;
+};
+
+
+/**
+ * @brief Alias template to facilitate the transcription of the constness.
+ * @tparam To The type to which to transcribe the constness.
+ * @tparam From The type from which to transcribe the constness.
+ */
+template<typename To, typename From>
+using constness_as_t = typename constness_as<To, From>::type;
+
+
+/**
+ * @brief Extracts the class of a non-static member object or function.
+ * @tparam Member A pointer to a non-static member object or function.
+ */
+template<typename Member>
+class member_class {
+    static_assert(std::is_member_pointer_v<Member>, "Invalid pointer type to non-static member object or function");
+
+    template<typename Class, typename Ret, typename... Args>
+    static Class * clazz(Ret(Class:: *)(Args...));
+
+    template<typename Class, typename Ret, typename... Args>
+    static Class * clazz(Ret(Class:: *)(Args...) const);
+
+    template<typename Class, typename Type>
+    static Class * clazz(Type Class:: *);
+
+public:
+    /*! @brief The class of the given non-static member object or function. */
+    using type = std::remove_pointer_t<decltype(clazz(std::declval<Member>()))>;
+};
+
+
+/**
+ * @brief Helper type.
+ * @tparam Member A pointer to a non-static member object or function.
+ */
+template<typename Member>
+using member_class_t = typename member_class<Member>::type;
+
+
+}
+
+
+#endif
+
+
+
+namespace entt {
+
+
+/**
+ * @brief A SBO friendly, type-safe container for single values of any type.
+ * @tparam Len Size of the storage reserved for the small buffer optimization.
+ * @tparam Align Optional alignment requirement.
+ */
+template<std::size_t Len, std::size_t Align>
+class basic_any {
+    enum class operation: std::uint8_t { COPY, MOVE, DTOR, COMP, ADDR, CADDR, TYPE };
+    enum class policy: std::uint8_t { OWNER, REF, CREF };
+
+    using storage_type = std::aligned_storage_t<Len + !Len, Align>;
+    using vtable_type = const void *(const operation, const basic_any &, void *);
+
+    template<typename Type>
+    static constexpr bool in_situ = Len && alignof(Type) <= alignof(storage_type) && sizeof(Type) <= sizeof(storage_type) && std::is_nothrow_move_constructible_v<Type>;
+
+    template<typename Type>
+    [[nodiscard]] static constexpr policy type_to_policy() {
+        if constexpr(std::is_lvalue_reference_v<Type>) {
+            if constexpr(std::is_const_v<std::remove_reference_t<Type>>) {
+                return policy::CREF;
+            } else {
+                return policy::REF;
+            }
+        } else {
+            return policy::OWNER;
+        }
+    }
+
+    template<typename Type>
+    [[nodiscard]] static bool compare(const void *lhs, const void *rhs) {
+        if constexpr(!std::is_function_v<Type> && is_equality_comparable_v<Type>) {
+            return *static_cast<const Type *>(lhs) == *static_cast<const Type *>(rhs);
+        } else {
+            return lhs == rhs;
+        }
+    }
+
+    template<typename Type>
+    static const void * basic_vtable([[maybe_unused]] const operation op, [[maybe_unused]] const basic_any &from, [[maybe_unused]] void *to) {
+        static_assert(std::is_same_v<std::remove_reference_t<std::remove_const_t<Type>>, Type>, "Invalid type");
+
+        if constexpr(!std::is_void_v<Type>) {
+            const Type *instance = (in_situ<Type> && from.mode == policy::OWNER)
+                ? ENTT_LAUNDER(reinterpret_cast<const Type *>(&from.storage))
+                : static_cast<const Type *>(from.instance);
+
+            switch(op) {
+            case operation::COPY:
+                if constexpr(std::is_copy_constructible_v<Type>) {
+                    static_cast<basic_any *>(to)->emplace<Type>(*instance);
+                }
+                break;
+            case operation::MOVE:
+                if constexpr(in_situ<Type>) {
+                    if(from.mode == policy::OWNER) {
+                        return new (&static_cast<basic_any *>(to)->storage) Type{std::move(*const_cast<Type *>(instance))};
+                    }
+                }
+
+                return (static_cast<basic_any *>(to)->instance = std::exchange(const_cast<basic_any &>(from).instance, nullptr));
+            case operation::DTOR:
+                if(from.mode == policy::OWNER) {
+                    if constexpr(in_situ<Type>) {
+                        instance->~Type();
+                    } else if constexpr(std::is_array_v<Type>) {
+                        delete[] instance;
+                    } else {
+                        delete instance;
+                    }
+                }
+                break;
+            case operation::COMP:
+                return compare<Type>(instance, (*static_cast<const basic_any **>(to))->data()) ? to : nullptr;
+            case operation::ADDR:
+                if(from.mode == policy::CREF) {
+                    return nullptr;
+                }
+                [[fallthrough]];
+            case operation::CADDR:
+                return instance;
+            case operation::TYPE:
+                *static_cast<type_info *>(to) = type_id<Type>();
+                break;
+            }
+        }
+
+        return nullptr;
+    }
+
+    template<typename Type, typename... Args>
+    void initialize([[maybe_unused]] Args &&... args) {
+        if constexpr(!std::is_void_v<Type>) {
+            if constexpr(std::is_lvalue_reference_v<Type>) {
+                static_assert(sizeof...(Args) == 1u && (std::is_lvalue_reference_v<Args> && ...), "Invalid arguments");
+                instance = (std::addressof(args), ...);
+            } else if constexpr(in_situ<Type>) {
+                if constexpr(sizeof...(Args) != 0u && std::is_aggregate_v<Type>) {
+                    new (&storage) Type{std::forward<Args>(args)...};
+                } else {
+                    new (&storage) Type(std::forward<Args>(args)...);
+                }
+            } else {
+                if constexpr(sizeof...(Args) != 0u && std::is_aggregate_v<Type>) {
+                    instance = new Type{std::forward<Args>(args)...};
+                } else {
+                    instance = new Type(std::forward<Args>(args)...);
+                }
+            }
+        }
+    }
+
+    basic_any(const basic_any &other, const policy pol) ENTT_NOEXCEPT
+        : instance{other.data()},
+          vtable{other.vtable},
+          mode{pol}
+    {}
+
+public:
+    /*! @brief Size of the internal storage. */
+    static constexpr auto length = Len;
+    /*! @brief Alignment requirement. */
+    static constexpr auto alignment = Align;
+
+    /*! @brief Default constructor. */
+    basic_any() ENTT_NOEXCEPT
+        : instance{},
+          vtable{&basic_vtable<void>},
+          mode{policy::OWNER}
+    {}
+
+    /**
+     * @brief Constructs a wrapper by directly initializing the new object.
+     * @tparam Type Type of object to use to initialize the wrapper.
+     * @tparam Args Types of arguments to use to construct the new instance.
+     * @param args Parameters to use to construct the instance.
+     */
+    template<typename Type, typename... Args>
+    explicit basic_any(std::in_place_type_t<Type>, Args &&... args)
+        : instance{},
+          vtable{&basic_vtable<std::remove_const_t<std::remove_reference_t<Type>>>},
+          mode{type_to_policy<Type>()}
+    {
+        initialize<Type>(std::forward<Args>(args)...);
+    }
+
+    /**
+     * @brief Constructs a wrapper that holds an unmanaged object.
+     * @tparam Type Type of object to use to initialize the wrapper.
+     * @param value An instance of an object to use to initialize the wrapper.
+     */
+    template<typename Type>
+    basic_any(std::reference_wrapper<Type> value) ENTT_NOEXCEPT
+        : basic_any{}
+    {
+        // invokes deprecated assignment operator (and avoids issues with vs2017)
+        *this = value;
+    }
+
+    /**
+     * @brief Constructs a wrapper from a given value.
+     * @tparam Type Type of object to use to initialize the wrapper.
+     * @param value An instance of an object to use to initialize the wrapper.
+     */
+    template<typename Type, typename = std::enable_if_t<!std::is_same_v<std::decay_t<Type>, basic_any>>>
+    basic_any(Type &&value)
+        : instance{},
+          vtable{&basic_vtable<std::decay_t<Type>>},
+          mode{policy::OWNER}
+    {
+        initialize<std::decay_t<Type>>(std::forward<Type>(value));
+    }
+
+    /**
+     * @brief Copy constructor.
+     * @param other The instance to copy from.
+     */
+    basic_any(const basic_any &other)
+        : instance{},
+          vtable{&basic_vtable<void>},
+          mode{policy::OWNER}
+    {
+        other.vtable(operation::COPY, other, this);
+    }
+
+    /**
+     * @brief Move constructor.
+     * @param other The instance to move from.
+     */
+    basic_any(basic_any &&other) ENTT_NOEXCEPT
+        : instance{},
+          vtable{other.vtable},
+          mode{other.mode}
+    {
+        vtable(operation::MOVE, other, this);
+    }
+
+    /*! @brief Frees the internal storage, whatever it means. */
+    ~basic_any() {
+        vtable(operation::DTOR, *this, nullptr);
+    }
+
+    /**
+     * @brief Copy assignment operator.
+     * @param other The instance to copy from.
+     * @return This any object.
+     */
+    basic_any & operator=(const basic_any &other) {
+        reset();
+        other.vtable(operation::COPY, other, this);
+        return *this;
+    }
+
+    /**
+     * @brief Move assignment operator.
+     * @param other The instance to move from.
+     * @return This any object.
+     */
+    basic_any & operator=(basic_any &&other) ENTT_NOEXCEPT {
+        std::exchange(vtable, other.vtable)(operation::DTOR, *this, nullptr);
+        other.vtable(operation::MOVE, other, this);
+        mode = other.mode;
+        return *this;
+    }
+
+    /**
+     * @brief Value assignment operator.
+     * @tparam Type Type of object to use to initialize the wrapper.
+     * @param value An instance of an object to use to initialize the wrapper.
+     * @return This any object.
+     */
+    template<typename Type>
+    [[deprecated("Use std::in_place_type<T &>, entt::make_any<T &>, emplace<Type &> or forward_as_any instead")]]
+    basic_any & operator=(std::reference_wrapper<Type> value) ENTT_NOEXCEPT {
+        emplace<Type &>(value.get());
+        return *this;
+    }
+
+    /**
+     * @brief Value assignment operator.
+     * @tparam Type Type of object to use to initialize the wrapper.
+     * @param value An instance of an object to use to initialize the wrapper.
+     * @return This any object.
+     */
+    template<typename Type>
+    std::enable_if_t<!std::is_same_v<std::decay_t<Type>, basic_any>, basic_any &>
+    operator=(Type &&value) {
+        emplace<std::decay_t<Type>>(std::forward<Type>(value));
+        return *this;
+    }
+
+    /**
+     * @brief Returns the type of the contained object.
+     * @return The type of the contained object, if any.
+     */
+    [[nodiscard]] type_info type() const ENTT_NOEXCEPT {
+        type_info info{};
+        vtable(operation::TYPE, *this, &info);
+        return info;
+    }
+
+    /**
+     * @brief Returns an opaque pointer to the contained instance.
+     * @return An opaque pointer the contained instance, if any.
+     */
+    [[nodiscard]] const void * data() const ENTT_NOEXCEPT {
+        return vtable(operation::CADDR, *this, nullptr);
+    }
+
+    /*! @copydoc data */
+    [[nodiscard]] void * data() ENTT_NOEXCEPT {
+        return const_cast<void *>(vtable(operation::ADDR, *this, nullptr));
+    }
+
+    /**
+     * @brief Replaces the contained object by creating a new instance directly.
+     * @tparam Type Type of object to use to initialize the wrapper.
+     * @tparam Args Types of arguments to use to construct the new instance.
+     * @param args Parameters to use to construct the instance.
+     */
+    template<typename Type, typename... Args>
+    void emplace(Args &&... args) {
+        std::exchange(vtable, &basic_vtable<std::remove_const_t<std::remove_reference_t<Type>>>)(operation::DTOR, *this, nullptr);
+        mode = type_to_policy<Type>();
+        initialize<Type>(std::forward<Args>(args)...);
+    }
+
+    /*! @brief Destroys contained object */
+    void reset() {
+        std::exchange(vtable, &basic_vtable<void>)(operation::DTOR, *this, nullptr);
+        mode = policy::OWNER;
+    }
+
+    /**
+     * @brief Returns false if a wrapper is empty, true otherwise.
+     * @return False if the wrapper is empty, true otherwise.
+     */
+    [[nodiscard]] explicit operator bool() const ENTT_NOEXCEPT {
+        return !(vtable(operation::CADDR, *this, nullptr) == nullptr);
+    }
+
+    /**
+     * @brief Checks if two wrappers differ in their content.
+     * @param other Wrapper with which to compare.
+     * @return False if the two objects differ in their content, true otherwise.
+     */
+    bool operator==(const basic_any &other) const ENTT_NOEXCEPT {
+        const basic_any *trampoline = &other;
+        return type() == other.type() && (vtable(operation::COMP, *this, &trampoline) || !other.data());
+    }
+
+    /**
+     * @brief Aliasing constructor.
+     * @return A wrapper that shares a reference to an unmanaged object.
+     */
+    [[nodiscard]] basic_any as_ref() ENTT_NOEXCEPT {
+        return basic_any{*this, (mode == policy::CREF ? policy::CREF : policy::REF)};
+    }
+
+    /*! @copydoc as_ref */
+    [[nodiscard]] basic_any as_ref() const ENTT_NOEXCEPT {
+        return basic_any{*this, policy::CREF};
+    }
+
+    /**
+     * @brief Returns true if a wrapper owns its object, false otherwise.
+     * @return True if the wrapper owns its object, false otherwise.
+     */
+    [[nodiscard]] bool owner() const ENTT_NOEXCEPT {
+        return (mode == policy::OWNER);
+    }
+
+private:
+    union { const void *instance; storage_type storage; };
+    vtable_type *vtable;
+    policy mode;
+};
+
+
+/**
+ * @brief Checks if two wrappers differ in their content.
+ * @tparam Len Size of the storage reserved for the small buffer optimization.
+ * @tparam Align Alignment requirement.
+ * @param lhs A wrapper, either empty or not.
+ * @param rhs A wrapper, either empty or not.
+ * @return True if the two wrappers differ in their content, false otherwise.
+ */
+template<std::size_t Len, std::size_t Align>
+[[nodiscard]] inline bool operator!=(const basic_any<Len, Align> &lhs, const basic_any<Len, Align> &rhs) ENTT_NOEXCEPT {
+    return !(lhs == rhs);
+}
+
+
+/**
+ * @brief Performs type-safe access to the contained object.
+ * @tparam Type Type to which conversion is required.
+ * @tparam Len Size of the storage reserved for the small buffer optimization.
+ * @tparam Align Alignment requirement.
+ * @param data Target any object.
+ * @return The element converted to the requested type.
+ */
+template<typename Type, std::size_t Len, std::size_t Align>
+Type any_cast(const basic_any<Len, Align> &data) ENTT_NOEXCEPT {
+    const auto * const instance = any_cast<std::remove_reference_t<Type>>(&data);
+    ENTT_ASSERT(instance, "Invalid instance");
+    return static_cast<Type>(*instance);
+}
+
+
+/*! @copydoc any_cast */
+template<typename Type, std::size_t Len, std::size_t Align>
+Type any_cast(basic_any<Len, Align> &data) ENTT_NOEXCEPT {
+    // forces const on non-reference types to make them work also with wrappers for const references
+    auto * const instance = any_cast<std::remove_reference_t<const Type>>(&data);
+    ENTT_ASSERT(instance, "Invalid instance");
+    return static_cast<Type>(*instance);
+}
+
+
+/*! @copydoc any_cast */
+template<typename Type, std::size_t Len, std::size_t Align>
+Type any_cast(basic_any<Len, Align> &&data) ENTT_NOEXCEPT {
+    // forces const on non-reference types to make them work also with wrappers for const references
+    auto * const instance = any_cast<std::remove_reference_t<const Type>>(&data);
+    ENTT_ASSERT(instance, "Invalid instance");
+    return static_cast<Type>(std::move(*instance));
+}
+
+
+/*! @copydoc any_cast */
+template<typename Type, std::size_t Len, std::size_t Align>
+const Type * any_cast(const basic_any<Len, Align> *data) ENTT_NOEXCEPT {
+    return (data->type() == type_id<Type>() ? static_cast<const Type *>(data->data()) : nullptr);
+}
+
+
+/*! @copydoc any_cast */
+template<typename Type, std::size_t Len, std::size_t Align>
+Type * any_cast(basic_any<Len, Align> *data) ENTT_NOEXCEPT {
+    // last attempt to make wrappers for const references return their values
+    return (data->type() == type_id<Type>() ? static_cast<Type *>(static_cast<constness_as_t<basic_any<Len, Align>, Type> *>(data)->data()) : nullptr);
+}
+
+
+/**
+ * @brief Constructs a wrapper from a given type, passing it all arguments.
+ * @tparam Type Type of object to use to initialize the wrapper.
+ * @tparam Len Size of the storage reserved for the small buffer optimization.
+ * @tparam Align Optional alignment requirement.
+ * @tparam Args Types of arguments to use to construct the new instance.
+ * @param args Parameters to use to construct the instance.
+ * @return A properly initialized wrapper for an object of the given type.
+ */
+template<typename Type, std::size_t Len = basic_any<>::length, std::size_t Align = basic_any<Len>::alignment, typename... Args>
+basic_any<Len, Align> make_any(Args &&... args) {
+    return basic_any<Len, Align>{std::in_place_type<Type>, std::forward<Args>(args)...};
+}
+
+
+/**
+ * @brief Forwards its argument and avoids copies for lvalue references.
+ * @tparam Len Size of the storage reserved for the small buffer optimization.
+ * @tparam Align Optional alignment requirement.
+ * @tparam Type Type of argument to use to construct the new instance.
+ * @param value Parameter to use to construct the instance.
+ * @return A properly initialized and not necessarily owning wrapper.
+ */
+template<std::size_t Len = basic_any<>::length, std::size_t Align = basic_any<Len>::alignment, typename Type>
+basic_any<Len, Align> forward_as_any(Type &&value) {
+    return basic_any<Len, Align>{std::in_place_type<std::conditional_t<std::is_rvalue_reference_v<Type>, std::decay_t<Type>, Type>>, std::forward<Type>(value)};
+}
+
+
+}
+
+
+#endif
+
+// #include "core/attribute.h"
+#ifndef ENTT_CORE_ATTRIBUTE_H
+#define ENTT_CORE_ATTRIBUTE_H
+
+
+#ifndef ENTT_EXPORT
+#   if defined _WIN32 || defined __CYGWIN__ || defined _MSC_VER
+#       define ENTT_EXPORT __declspec(dllexport)
+#       define ENTT_IMPORT __declspec(dllimport)
+#       define ENTT_HIDDEN
+#   elif defined __GNUC__ && __GNUC__ >= 4
+#       define ENTT_EXPORT __attribute__((visibility("default")))
+#       define ENTT_IMPORT __attribute__((visibility("default")))
+#       define ENTT_HIDDEN __attribute__((visibility("hidden")))
+#   else /* Unsupported compiler */
+#       define ENTT_EXPORT
+#       define ENTT_IMPORT
+#       define ENTT_HIDDEN
+#   endif
+#endif
+
+
+#ifndef ENTT_API
+#   if defined ENTT_API_EXPORT
+#       define ENTT_API ENTT_EXPORT
+#   elif defined ENTT_API_IMPORT
+#       define ENTT_API ENTT_IMPORT
+#   else /* No API */
+#       define ENTT_API
+#   endif
+#endif
+
+
+#endif
+
+// #include "core/family.hpp"
+#ifndef ENTT_CORE_FAMILY_HPP
+#define ENTT_CORE_FAMILY_HPP
+
+
+// #include "../config/config.h"
+
+// #include "fwd.hpp"
+
+
+
+namespace entt {
+
+
+/**
+ * @brief Dynamic identifier generator.
+ *
+ * Utility class template that can be used to assign unique identifiers to types
+ * at runtime. Use different specializations to create separate sets of
+ * identifiers.
+ */
+template<typename...>
+class family {
+    inline static ENTT_MAYBE_ATOMIC(id_type) identifier{};
+
+public:
+    /*! @brief Unsigned integer type. */
+    using family_type = id_type;
+
+    /*! @brief Statically generated unique identifier for the given type. */
+    template<typename... Type>
+    // at the time I'm writing, clang crashes during compilation if auto is used instead of family_type
+    inline static const family_type type = identifier++;
+};
+
+
+}
+
+
+#endif
+
+// #include "core/hashed_string.hpp"
+#ifndef ENTT_CORE_HASHED_STRING_HPP
+#define ENTT_CORE_HASHED_STRING_HPP
+
+
+#include <cstddef>
+#include <cstdint>
+// #include "../config/config.h"
+
+// #include "fwd.hpp"
+
+
+
+namespace entt {
+
+
+/**
+ * @cond TURN_OFF_DOXYGEN
+ * Internal details not to be documented.
+ */
+
+
+namespace internal {
+
+
+template<typename>
+struct fnv1a_traits;
+
+
+template<>
+struct fnv1a_traits<std::uint32_t> {
+    using type = std::uint32_t;
+    static constexpr std::uint32_t offset = 2166136261;
+    static constexpr std::uint32_t prime = 16777619;
+};
+
+
+template<>
+struct fnv1a_traits<std::uint64_t> {
+    using type = std::uint64_t;
+    static constexpr std::uint64_t offset = 14695981039346656037ull;
+    static constexpr std::uint64_t prime = 1099511628211ull;
+};
+
+
+}
+
+
+/**
+ * Internal details not to be documented.
+ * @endcond
+ */
+
+
+/**
+ * @brief Zero overhead unique identifier.
+ *
+ * A hashed string is a compile-time tool that allows users to use
+ * human-readable identifers in the codebase while using their numeric
+ * counterparts at runtime.<br/>
+ * Because of that, a hashed string can also be used in constant expressions if
+ * required.
+ *
+ * @tparam Char Character type.
+ */
+template<typename Char>
+class basic_hashed_string {
+    using traits_type = internal::fnv1a_traits<id_type>;
+
+    struct const_wrapper {
+        // non-explicit constructor on purpose
+        constexpr const_wrapper(const Char *curr) ENTT_NOEXCEPT: str{curr} {}
+        const Char *str;
+    };
+
+    // Fowler–Noll–Vo hash function v. 1a - the good
+    [[nodiscard]] static constexpr id_type helper(const Char *curr) ENTT_NOEXCEPT {
+        auto value = traits_type::offset;
+
+        while(*curr != 0) {
+            value = (value ^ static_cast<traits_type::type>(*(curr++))) * traits_type::prime;
+        }
+
+        return value;
+    }
+
+public:
+    /*! @brief Character type. */
+    using value_type = Char;
+    /*! @brief Unsigned integer type. */
+    using hash_type = id_type;
+
+    /**
+     * @brief Returns directly the numeric representation of a string view.
+     * @param str Human-readable identifer.
+     * @param size Length of the string to hash.
+     * @return The numeric representation of the string.
+     */
+    [[nodiscard]] static constexpr hash_type value(const value_type *str, std::size_t size) ENTT_NOEXCEPT {
+        id_type partial{traits_type::offset};
+        while(size--) { partial = (partial^(str++)[0])*traits_type::prime; }
+        return partial;
+    }
+
+    /**
+     * @brief Returns directly the numeric representation of a string.
+     *
+     * Forcing template resolution avoids implicit conversions. An
+     * human-readable identifier can be anything but a plain, old bunch of
+     * characters.<br/>
+     * Example of use:
+     * @code{.cpp}
+     * const auto value = basic_hashed_string<char>::to_value("my.png");
+     * @endcode
+     *
+     * @tparam N Number of characters of the identifier.
+     * @param str Human-readable identifer.
+     * @return The numeric representation of the string.
+     */
+    template<std::size_t N>
+    [[nodiscard]] static constexpr hash_type value(const value_type (&str)[N]) ENTT_NOEXCEPT {
+        return helper(str);
+    }
+
+    /**
+     * @brief Returns directly the numeric representation of a string.
+     * @param wrapper Helps achieving the purpose by relying on overloading.
+     * @return The numeric representation of the string.
+     */
+    [[nodiscard]] static hash_type value(const_wrapper wrapper) ENTT_NOEXCEPT {
+        return helper(wrapper.str);
+    }
+
+    /*! @brief Constructs an empty hashed string. */
+    constexpr basic_hashed_string() ENTT_NOEXCEPT
+        : str{nullptr}, hash{}
+    {}
+
+    /**
+     * @brief Constructs a hashed string from an array of const characters.
+     *
+     * Forcing template resolution avoids implicit conversions. An
+     * human-readable identifier can be anything but a plain, old bunch of
+     * characters.<br/>
+     * Example of use:
+     * @code{.cpp}
+     * basic_hashed_string<char> hs{"my.png"};
+     * @endcode
+     *
+     * @tparam N Number of characters of the identifier.
+     * @param curr Human-readable identifer.
+     */
+    template<std::size_t N>
+    constexpr basic_hashed_string(const value_type (&curr)[N]) ENTT_NOEXCEPT
+        : str{curr}, hash{helper(curr)}
+    {}
+
+    /**
+     * @brief Explicit constructor on purpose to avoid constructing a hashed
+     * string directly from a `const value_type *`.
+     * @param wrapper Helps achieving the purpose by relying on overloading.
+     */
+    explicit constexpr basic_hashed_string(const_wrapper wrapper) ENTT_NOEXCEPT
+        : str{wrapper.str}, hash{helper(wrapper.str)}
+    {}
+
+    /**
+     * @brief Returns the human-readable representation of a hashed string.
+     * @return The string used to initialize the instance.
+     */
+    [[nodiscard]] constexpr const value_type * data() const ENTT_NOEXCEPT {
+        return str;
+    }
+
+    /**
+     * @brief Returns the numeric representation of a hashed string.
+     * @return The numeric representation of the instance.
+     */
+    [[nodiscard]] constexpr hash_type value() const ENTT_NOEXCEPT {
+        return hash;
+    }
+
+    /*! @copydoc data */
+    [[nodiscard]] constexpr operator const value_type *() const ENTT_NOEXCEPT { return data(); }
+
+    /**
+     * @brief Returns the numeric representation of a hashed string.
+     * @return The numeric representation of the instance.
+     */
+    [[nodiscard]] constexpr operator hash_type() const ENTT_NOEXCEPT { return value(); }
+
+    /**
+     * @brief Compares two hashed strings.
+     * @param other Hashed string with which to compare.
+     * @return True if the two hashed strings are identical, false otherwise.
+     */
+    [[nodiscard]] constexpr bool operator==(const basic_hashed_string &other) const ENTT_NOEXCEPT {
+        return hash == other.hash;
+    }
+
+private:
+    const value_type *str;
+    hash_type hash;
+};
+
+
+/**
+ * @brief Deduction guide.
+ *
+ * It allows to deduce the character type of the hashed string directly from a
+ * human-readable identifer provided to the constructor.
+ *
+ * @tparam Char Character type.
+ * @tparam N Number of characters of the identifier.
+ * @param str Human-readable identifer.
+ */
+template<typename Char, std::size_t N>
+basic_hashed_string(const Char (&str)[N])
+-> basic_hashed_string<Char>;
+
+
+/**
+ * @brief Compares two hashed strings.
+ * @tparam Char Character type.
+ * @param lhs A valid hashed string.
+ * @param rhs A valid hashed string.
+ * @return True if the two hashed strings are identical, false otherwise.
+ */
+template<typename Char>
+[[nodiscard]] constexpr bool operator!=(const basic_hashed_string<Char> &lhs, const basic_hashed_string<Char> &rhs) ENTT_NOEXCEPT {
+    return !(lhs == rhs);
+}
+
+
+/*! @brief Aliases for common character types. */
+using hashed_string = basic_hashed_string<char>;
+
+
+/*! @brief Aliases for common character types. */
+using hashed_wstring = basic_hashed_string<wchar_t>;
+
+
+inline namespace literals {
+
+
+/**
+ * @brief User defined literal for hashed strings.
+ * @param str The literal without its suffix.
+ * @return A properly initialized hashed string.
+ */
+[[nodiscard]] constexpr entt::hashed_string operator"" _hs(const char *str, std::size_t) ENTT_NOEXCEPT {
+    return entt::hashed_string{str};
+}
+
+
+/**
+ * @brief User defined literal for hashed wstrings.
+ * @param str The literal without its suffix.
+ * @return A properly initialized hashed wstring.
+ */
+[[nodiscard]] constexpr entt::hashed_wstring operator"" _hws(const wchar_t *str, std::size_t) ENTT_NOEXCEPT {
+    return entt::hashed_wstring{str};
+}
+
+
+}
+
+
+}
+
+
+#endif
 
 // #include "core/ident.hpp"
 #ifndef ENTT_CORE_IDENT_HPP
 #define ENTT_CORE_IDENT_HPP
 
 
-#include <tuple>
-#include <utility>
-#include <type_traits>
-// #include "../config/config.h"
+#include <cstddef>
+#include <utility>
+#include <type_traits>
+// #include "../config/config.h"
+
+// #include "fwd.hpp"
+
+// #include "type_traits.hpp"
 
 
 
@@ -586,36 +2691,35 @@
  */
 template<typename... Types>
 class identifier {
-    using tuple_type = std::tuple<std::decay_t<Types>...>;
-
-    template<typename Type, std::size_t... Indexes>
-    static constexpr ENTT_ID_TYPE get(std::index_sequence<Indexes...>) ENTT_NOEXCEPT {
-        static_assert(std::disjunction_v<std::is_same<Type, Types>...>);
-        return (0 + ... + (std::is_same_v<Type, std::tuple_element_t<Indexes, tuple_type>> ? ENTT_ID_TYPE(Indexes) : ENTT_ID_TYPE{}));
+    template<typename Type, std::size_t... Index>
+    [[nodiscard]] static constexpr id_type get(std::index_sequence<Index...>) {
+        static_assert(std::disjunction_v<std::is_same<Type, Types>...>, "Invalid type");
+        return (0 + ... + (std::is_same_v<Type, type_list_element_t<Index, type_list<std::decay_t<Types>...>>> ? id_type{Index} : id_type{}));
     }
 
 public:
     /*! @brief Unsigned integer type. */
-    using identifier_type = ENTT_ID_TYPE;
+    using identifier_type = id_type;
 
     /*! @brief Statically generated unique identifier for the given type. */
     template<typename Type>
-    static constexpr identifier_type type = get<std::decay_t<Type>>(std::make_index_sequence<sizeof...(Types)>{});
-};
-
-
-}
-
-
-#endif // ENTT_CORE_IDENT_HPP
+    static constexpr identifier_type type = get<std::decay_t<Type>>(std::index_sequence_for<Types...>{});
+};
+
+
+}
+
+
+#endif
 
 // #include "core/monostate.hpp"
 #ifndef ENTT_CORE_MONOSTATE_HPP
 #define ENTT_CORE_MONOSTATE_HPP
 
 
-#include <cassert>
-// #include "../config/config.h"
+// #include "../config/config.h"
+
+// #include "fwd.hpp"
 
 
 
@@ -633,7 +2737,7 @@
  * both during an assignment and when they try to read back their data.
  * Otherwise, they can incur in unexpected results.
  */
-template<ENTT_ID_TYPE>
+template<id_type>
 struct monostate {
     /**
      * @brief Assigns a value of a specific type to a given key.
@@ -665,77 +2769,1217 @@
  * @brief Helper variable template.
  * @tparam Value Value used to differentiate between different variables.
  */
-template<ENTT_ID_TYPE Value>
+template<id_type Value>
 inline monostate<Value> monostate_v = {};
 
 
 }
 
 
-#endif // ENTT_CORE_MONOSTATE_HPP
+#endif
+
+// #include "core/type_info.hpp"
+#ifndef ENTT_CORE_TYPE_INFO_HPP
+#define ENTT_CORE_TYPE_INFO_HPP
+
+
+#include <string_view>
+#include <type_traits>
+// #include "../config/config.h"
+
+// #include "../core/attribute.h"
+
+// #include "hashed_string.hpp"
+
+// #include "fwd.hpp"
+
+
+
+namespace entt {
+
+
+/**
+ * @cond TURN_OFF_DOXYGEN
+ * Internal details not to be documented.
+ */
+
+
+namespace internal {
+
+
+struct ENTT_API type_seq final {
+    [[nodiscard]] static id_type next() ENTT_NOEXCEPT {
+        static ENTT_MAYBE_ATOMIC(id_type) value{};
+        return value++;
+    }
+};
+
+
+template<typename Type>
+[[nodiscard]] constexpr auto stripped_type_name() ENTT_NOEXCEPT {
+#if defined ENTT_PRETTY_FUNCTION
+    std::string_view pretty_function{ENTT_PRETTY_FUNCTION};
+    auto first = pretty_function.find_first_not_of(' ', pretty_function.find_first_of(ENTT_PRETTY_FUNCTION_PREFIX)+1);
+    auto value = pretty_function.substr(first, pretty_function.find_last_of(ENTT_PRETTY_FUNCTION_SUFFIX) - first);
+    return value;
+#else
+    return std::string_view{""};
+#endif
+}
+
+
+template<typename Type, auto = stripped_type_name<Type>().find_first_of('.')>
+[[nodiscard]] static constexpr std::string_view type_name(int) ENTT_NOEXCEPT {
+    constexpr auto value = stripped_type_name<Type>();
+    return value;
+}
+
+
+template<typename Type>
+[[nodiscard]] static std::string_view type_name(char) ENTT_NOEXCEPT {
+    static const auto value = stripped_type_name<Type>();
+    return value;
+}
+
+
+template<typename Type, auto = stripped_type_name<Type>().find_first_of('.')>
+[[nodiscard]] static constexpr id_type type_hash(int) ENTT_NOEXCEPT {
+    constexpr auto stripped = stripped_type_name<Type>();
+    constexpr auto value = hashed_string::value(stripped.data(), stripped.size());
+    return value;
+}
+
+
+template<typename Type>
+[[nodiscard]] static id_type type_hash(char) ENTT_NOEXCEPT {
+    static const auto value = [](const auto stripped) {
+        return hashed_string::value(stripped.data(), stripped.size());
+    }(stripped_type_name<Type>());
+    return value;
+}
+
+
+}
+
+
+/**
+ * Internal details not to be documented.
+ * @endcond
+ */
+
+
+/**
+ * @brief Type sequential identifier.
+ * @tparam Type Type for which to generate a sequential identifier.
+ */
+template<typename Type, typename = void>
+struct ENTT_API type_seq final {
+    /**
+     * @brief Returns the sequential identifier of a given type.
+     * @return The sequential identifier of a given type.
+     */
+    [[nodiscard]] static id_type value() ENTT_NOEXCEPT {
+        static const id_type value = internal::type_seq::next();
+        return value;
+    }
+
+    /*! @copydoc value */
+    [[nodiscard]] constexpr operator id_type() const ENTT_NOEXCEPT { return value(); }
+};
+
+
+/**
+ * @brief Type hash.
+ * @tparam Type Type for which to generate a hash value.
+ */
+template<typename Type, typename = void>
+struct type_hash final {
+    /**
+     * @brief Returns the numeric representation of a given type.
+     * @return The numeric representation of the given type.
+     */
+#if defined ENTT_PRETTY_FUNCTION
+    [[nodiscard]] static constexpr id_type value() ENTT_NOEXCEPT {
+        return internal::type_hash<Type>(0);
+#else
+    [[nodiscard]] static constexpr id_type value() ENTT_NOEXCEPT {
+        return type_seq<Type>::value();
+#endif
+    }
+
+    /*! @copydoc value */
+    [[nodiscard]] constexpr operator id_type() const ENTT_NOEXCEPT { return value(); }
+};
+
+
+/**
+ * @brief Type name.
+ * @tparam Type Type for which to generate a name.
+ */
+template<typename Type, typename = void>
+struct type_name final {
+    /**
+     * @brief Returns the name of a given type.
+     * @return The name of the given type.
+     */
+    [[nodiscard]] static constexpr std::string_view value() ENTT_NOEXCEPT {
+        return internal::type_name<Type>(0);
+    }
+
+    /*! @copydoc value */
+    [[nodiscard]] constexpr operator std::string_view() const ENTT_NOEXCEPT { return value(); }
+};
+
+
+/*! @brief Implementation specific information about a type. */
+class type_info final {
+    template<typename>
+    friend type_info type_id() ENTT_NOEXCEPT;
+
+    type_info(id_type seq_v, id_type hash_v, std::string_view name_v) ENTT_NOEXCEPT
+        : seq_value{seq_v},
+          hash_value{hash_v},
+          name_value{name_v}
+    {}
+
+public:
+    /*! @brief Default constructor. */
+    type_info() ENTT_NOEXCEPT
+        : type_info({}, {}, {})
+    {}
+
+    /*! @brief Default copy constructor. */
+    type_info(const type_info &) ENTT_NOEXCEPT = default;
+    /*! @brief Default move constructor. */
+    type_info(type_info &&) ENTT_NOEXCEPT = default;
+
+    /**
+     * @brief Default copy assignment operator.
+     * @return This type info object.
+     */
+    type_info & operator=(const type_info &) ENTT_NOEXCEPT = default;
+
+    /**
+     * @brief Default move assignment operator.
+     * @return This type info object.
+     */
+    type_info & operator=(type_info &&) ENTT_NOEXCEPT = default;
+
+    /**
+     * @brief Checks if a type info object is properly initialized.
+     * @return True if the object is properly initialized, false otherwise.
+     */
+    [[nodiscard]] explicit operator bool() const ENTT_NOEXCEPT {
+        return name_value.data() != nullptr;
+    }
+
+    /**
+     * @brief Type sequential identifier.
+     * @return Type sequential identifier.
+     */
+    [[nodiscard]] id_type seq() const ENTT_NOEXCEPT {
+        return seq_value;
+    }
+
+    /**
+     * @brief Type hash.
+     * @return Type hash.
+     */
+    [[nodiscard]] id_type hash() const ENTT_NOEXCEPT {
+        return hash_value;
+    }
+
+    /**
+     * @brief Type name.
+     * @return Type name.
+     */
+    [[nodiscard]] std::string_view name() const ENTT_NOEXCEPT {
+        return name_value;
+    }
+
+    /**
+     * @brief Compares the contents of two type info objects.
+     * @param other Object with which to compare.
+     * @return False if the two contents differ, true otherwise.
+     */
+    [[nodiscard]] bool operator==(const type_info &other) const ENTT_NOEXCEPT {
+        return hash_value == other.hash_value;
+    }
+
+private:
+    id_type seq_value;
+    id_type hash_value;
+    std::string_view name_value;
+};
+
+
+/**
+ * @brief Compares the contents of two type info objects.
+ * @param lhs A type info object.
+ * @param rhs A type info object.
+ * @return True if the two contents differ, false otherwise.
+ */
+[[nodiscard]] inline bool operator!=(const type_info &lhs, const type_info &rhs) ENTT_NOEXCEPT {
+    return !(lhs == rhs);
+}
+
+
+/**
+ * @brief Returns the type info object for a given type.
+ * @tparam Type Type for which to generate a type info object.
+ * @return The type info object for the given type.
+ */
+template<typename Type>
+[[nodiscard]] type_info type_id() ENTT_NOEXCEPT {
+    return type_info{
+        type_seq<std::remove_cv_t<std::remove_reference_t<Type>>>::value(),
+        type_hash<std::remove_cv_t<std::remove_reference_t<Type>>>::value(),
+        type_name<std::remove_cv_t<std::remove_reference_t<Type>>>::value()
+    };
+}
+
+
+}
+
+
+#endif
 
 // #include "core/type_traits.hpp"
 #ifndef ENTT_CORE_TYPE_TRAITS_HPP
 #define ENTT_CORE_TYPE_TRAITS_HPP
 
 
-#include <type_traits>
-// #include "../config/config.h"
-
-// #include "../core/hashed_string.hpp"
-#ifndef ENTT_CORE_HASHED_STRING_HPP
-#define ENTT_CORE_HASHED_STRING_HPP
-
-
-#include <cstddef>
+#include <cstddef>
+#include <iterator>
+#include <type_traits>
+#include <utility>
+// #include "../config/config.h"
+
+// #include "fwd.hpp"
+
+
+
+namespace entt {
+
+
+/**
+ * @brief Utility class to disambiguate overloaded functions.
+ * @tparam N Number of choices available.
+ */
+template<std::size_t N>
+struct choice_t
+    // Unfortunately, doxygen cannot parse such a construct.
+    /*! @cond TURN_OFF_DOXYGEN */
+    : choice_t<N-1>
+    /*! @endcond */
+{};
+
+
+/*! @copybrief choice_t */
+template<>
+struct choice_t<0> {};
+
+
+/**
+ * @brief Variable template for the choice trick.
+ * @tparam N Number of choices available.
+ */
+template<std::size_t N>
+inline constexpr choice_t<N> choice{};
+
+
+/**
+ * @brief Identity type trait.
+ *
+ * Useful to establish non-deduced contexts in template argument deduction
+ * (waiting for C++20) or to provide types through function arguments.
+ *
+ * @tparam Type A type.
+ */
+template<typename Type>
+struct type_identity {
+    /*! @brief Identity type. */
+    using type = Type;
+};
+
+
+/**
+ * @brief Helper type.
+ * @tparam Type A type.
+ */
+template<typename Type>
+using type_identity_t = typename type_identity<Type>::type;
+
+
+/**
+ * @brief A type-only `sizeof` wrapper that returns 0 where `sizeof` complains.
+ * @tparam Type The type of which to return the size.
+ * @tparam The size of the type if `sizeof` accepts it, 0 otherwise.
+ */
+template<typename Type, typename = void>
+struct size_of: std::integral_constant<std::size_t, 0u> {};
+
+
+/*! @copydoc size_of */
+template<typename Type>
+struct size_of<Type, std::void_t<decltype(sizeof(Type))>>
+    : std::integral_constant<std::size_t, sizeof(Type)>
+{};
+
+
+/**
+ * @brief Helper variable template.
+ * @tparam Type The type of which to return the size.
+ */
+template<class Type>
+inline constexpr std::size_t size_of_v = size_of<Type>::value;
+
+
+/**
+ * @brief Using declaration to be used to _repeat_ the same type a number of
+ * times equal to the size of a given parameter pack.
+ * @tparam Type A type to repeat.
+ */
+template<typename Type, typename>
+using unpack_as_t = Type;
+
+
+/**
+ * @brief Helper variable template to be used to _repeat_ the same value a
+ * number of times equal to the size of a given parameter pack.
+ * @tparam Value A value to repeat.
+ */
+template<auto Value, typename>
+inline constexpr auto unpack_as_v = Value;
+
+
+/**
+ * @brief Wraps a static constant.
+ * @tparam Value A static constant.
+ */
+template<auto Value>
+using integral_constant = std::integral_constant<decltype(Value), Value>;
+
+
+/**
+ * @brief Alias template to facilitate the creation of named values.
+ * @tparam Value A constant value at least convertible to `id_type`.
+ */
+template<id_type Value>
+using tag = integral_constant<Value>;
+
+
+/**
+ * @brief A class to use to push around lists of types, nothing more.
+ * @tparam Type Types provided by the type list.
+ */
+template<typename... Type>
+struct type_list {
+    /*! @brief Type list type. */
+    using type = type_list;
+    /*! @brief Compile-time number of elements in the type list. */
+    static constexpr auto size = sizeof...(Type);
+};
+
+
+/*! @brief Primary template isn't defined on purpose. */
+template<std::size_t, typename>
+struct type_list_element;
+
+
+/**
+ * @brief Provides compile-time indexed access to the types of a type list.
+ * @tparam Index Index of the type to return.
+ * @tparam Type First type provided by the type list.
+ * @tparam Other Other types provided by the type list.
+ */
+template<std::size_t Index, typename Type, typename... Other>
+struct type_list_element<Index, type_list<Type, Other...>>
+    : type_list_element<Index - 1u, type_list<Other...>>
+{};
+
+
+/**
+ * @brief Provides compile-time indexed access to the types of a type list.
+ * @tparam Type First type provided by the type list.
+ * @tparam Other Other types provided by the type list.
+ */
+template<typename Type, typename... Other>
+struct type_list_element<0u, type_list<Type, Other...>> {
+    /*! @brief Searched type. */
+    using type = Type;
+};
+
+
+/**
+ * @brief Helper type.
+ * @tparam Index Index of the type to return.
+ * @tparam List Type list to search into.
+ */
+template<std::size_t Index, typename List>
+using type_list_element_t = typename type_list_element<Index, List>::type;
+
+
+/**
+ * @brief Concatenates multiple type lists.
+ * @tparam Type Types provided by the first type list.
+ * @tparam Other Types provided by the second type list.
+ * @return A type list composed by the types of both the type lists.
+ */
+template<typename... Type, typename... Other>
+constexpr type_list<Type..., Other...> operator+(type_list<Type...>, type_list<Other...>) { return {}; }
+
+
+/*! @brief Primary template isn't defined on purpose. */
+template<typename...>
+struct type_list_cat;
+
+
+/*! @brief Concatenates multiple type lists. */
+template<>
+struct type_list_cat<> {
+    /*! @brief A type list composed by the types of all the type lists. */
+    using type = type_list<>;
+};
+
+
+/**
+ * @brief Concatenates multiple type lists.
+ * @tparam Type Types provided by the first type list.
+ * @tparam Other Types provided by the second type list.
+ * @tparam List Other type lists, if any.
+ */
+template<typename... Type, typename... Other, typename... List>
+struct type_list_cat<type_list<Type...>, type_list<Other...>, List...> {
+    /*! @brief A type list composed by the types of all the type lists. */
+    using type = typename type_list_cat<type_list<Type..., Other...>, List...>::type;
+};
+
+
+/**
+ * @brief Concatenates multiple type lists.
+ * @tparam Type Types provided by the type list.
+ */
+template<typename... Type>
+struct type_list_cat<type_list<Type...>> {
+    /*! @brief A type list composed by the types of all the type lists. */
+    using type = type_list<Type...>;
+};
+
+
+/**
+ * @brief Helper type.
+ * @tparam List Type lists to concatenate.
+ */
+template<typename... List>
+using type_list_cat_t = typename type_list_cat<List...>::type;
+
+
+/*! @brief Primary template isn't defined on purpose. */
+template<typename>
+struct type_list_unique;
+
+
+/**
+ * @brief Removes duplicates types from a type list.
+ * @tparam Type One of the types provided by the given type list.
+ * @tparam Other The other types provided by the given type list.
+ */
+template<typename Type, typename... Other>
+struct type_list_unique<type_list<Type, Other...>> {
+    /*! @brief A type list without duplicate types. */
+    using type = std::conditional_t<
+        std::disjunction_v<std::is_same<Type, Other>...>,
+        typename type_list_unique<type_list<Other...>>::type,
+        type_list_cat_t<type_list<Type>, typename type_list_unique<type_list<Other...>>::type>
+    >;
+};
+
+
+/*! @brief Removes duplicates types from a type list. */
+template<>
+struct type_list_unique<type_list<>> {
+    /*! @brief A type list without duplicate types. */
+    using type = type_list<>;
+};
+
+
+/**
+ * @brief Helper type.
+ * @tparam Type A type list.
+ */
+template<typename Type>
+using type_list_unique_t = typename type_list_unique<Type>::type;
+
+
+/**
+ * @brief Provides the member constant `value` to true if a type list contains a
+ * given type, false otherwise.
+ * @tparam List Type list.
+ * @tparam Type Type to look for.
+ */
+template<typename List, typename Type>
+struct type_list_contains;
+
+
+/**
+ * @copybrief type_list_contains
+ * @tparam Type Types provided by the type list.
+ * @tparam Other Type to look for.
+ */
+template<typename... Type, typename Other>
+struct type_list_contains<type_list<Type...>, Other>: std::disjunction<std::is_same<Type, Other>...> {};
+
+
+/**
+ * @brief Helper variable template.
+ * @tparam List Type list.
+ * @tparam Type Type to look for.
+ */
+template<class List, typename Type>
+inline constexpr bool type_list_contains_v = type_list_contains<List, Type>::value;
+
+
+/*! @brief Primary template isn't defined on purpose. */
+template<typename...>
+struct type_list_diff;
+
+
+/**
+ * @brief Computes the difference between two type lists.
+ * @tparam Type Types provided by the first type list.
+ * @tparam Other Types provided by the second type list.
+ */
+template<typename... Type, typename... Other>
+struct type_list_diff<type_list<Type...>, type_list<Other...>> {
+    /*! @brief A type list that is the difference between the two type lists. */
+    using type = type_list_cat_t<std::conditional_t<type_list_contains_v<type_list<Other...>, Type>, type_list<>, type_list<Type>>...>;
+};
+
+
+/**
+ * @brief Helper type.
+ * @tparam List Type lists between which to compute the difference.
+ */
+template<typename... List>
+using type_list_diff_t = typename type_list_diff<List...>::type;
+
+
+/**
+ * @brief A class to use to push around lists of constant values, nothing more.
+ * @tparam Value Values provided by the value list.
+ */
+template<auto... Value>
+struct value_list {
+    /*! @brief Value list type. */
+    using type = value_list;
+    /*! @brief Compile-time number of elements in the value list. */
+    static constexpr auto size = sizeof...(Value);
+};
+
+
+/*! @brief Primary template isn't defined on purpose. */
+template<std::size_t, typename>
+struct value_list_element;
+
+
+/**
+ * @brief Provides compile-time indexed access to the values of a value list.
+ * @tparam Index Index of the value to return.
+ * @tparam Value First value provided by the value list.
+ * @tparam Other Other values provided by the value list.
+ */
+template<std::size_t Index, auto Value, auto... Other>
+struct value_list_element<Index, value_list<Value, Other...>>
+    : value_list_element<Index - 1u, value_list<Other...>>
+{};
+
+
+/**
+ * @brief Provides compile-time indexed access to the types of a type list.
+ * @tparam Value First value provided by the value list.
+ * @tparam Other Other values provided by the value list.
+ */
+template<auto Value, auto... Other>
+struct value_list_element<0u, value_list<Value, Other...>> {
+    /*! @brief Searched value. */
+    static constexpr auto value = Value;
+};
+
+
+/**
+ * @brief Helper type.
+ * @tparam Index Index of the value to return.
+ * @tparam List Value list to search into.
+ */
+template<std::size_t Index, typename List>
+inline constexpr auto value_list_element_v = value_list_element<Index, List>::value;
+
+
+/**
+ * @brief Concatenates multiple value lists.
+ * @tparam Value Values provided by the first value list.
+ * @tparam Other Values provided by the second value list.
+ * @return A value list composed by the values of both the value lists.
+ */
+template<auto... Value, auto... Other>
+constexpr value_list<Value..., Other...> operator+(value_list<Value...>, value_list<Other...>) { return {}; }
+
+
+/*! @brief Primary template isn't defined on purpose. */
+template<typename...>
+struct value_list_cat;
+
+
+/*! @brief Concatenates multiple value lists. */
+template<>
+struct value_list_cat<> {
+    /*! @brief A value list composed by the values of all the value lists. */
+    using type = value_list<>;
+};
+
+
+/**
+ * @brief Concatenates multiple value lists.
+ * @tparam Value Values provided by the first value list.
+ * @tparam Other Values provided by the second value list.
+ * @tparam List Other value lists, if any.
+ */
+template<auto... Value, auto... Other, typename... List>
+struct value_list_cat<value_list<Value...>, value_list<Other...>, List...> {
+    /*! @brief A value list composed by the values of all the value lists. */
+    using type = typename value_list_cat<value_list<Value..., Other...>, List...>::type;
+};
+
+
+/**
+ * @brief Concatenates multiple value lists.
+ * @tparam Value Values provided by the value list.
+ */
+template<auto... Value>
+struct value_list_cat<value_list<Value...>> {
+    /*! @brief A value list composed by the values of all the value lists. */
+    using type = value_list<Value...>;
+};
+
+
+/**
+ * @brief Helper type.
+ * @tparam List Value lists to concatenate.
+ */
+template<typename... List>
+using value_list_cat_t = typename value_list_cat<List...>::type;
+
+
+/*! @brief Same as std::is_invocable, but with tuples. */
+template<typename, typename>
+struct is_applicable: std::false_type {};
+
+
+/**
+ * @copybrief is_applicable
+ * @tparam Func A valid function type.
+ * @tparam Tuple Tuple-like type.
+ * @tparam Args The list of arguments to use to probe the function type.
+ */
+template<typename Func, template<typename...> class Tuple, typename... Args>
+struct is_applicable<Func, Tuple<Args...>>: std::is_invocable<Func, Args...> {};
+
+
+/**
+ * @copybrief is_applicable
+ * @tparam Func A valid function type.
+ * @tparam Tuple Tuple-like type.
+ * @tparam Args The list of arguments to use to probe the function type.
+ */
+template<typename Func, template<typename...> class Tuple, typename... Args>
+struct is_applicable<Func, const Tuple<Args...>>: std::is_invocable<Func, Args...> {};
+
+
+/**
+ * @brief Helper variable template.
+ * @tparam Func A valid function type.
+ * @tparam Args The list of arguments to use to probe the function type.
+ */
+template<typename Func, typename Args>
+inline constexpr bool is_applicable_v = is_applicable<Func, Args>::value;
+
+
+/*! @brief Same as std::is_invocable_r, but with tuples for arguments. */
+template<typename, typename, typename>
+struct is_applicable_r: std::false_type {};
+
+
+/**
+ * @copybrief is_applicable_r
+ * @tparam Ret The type to which the return type of the function should be
+ * convertible.
+ * @tparam Func A valid function type.
+ * @tparam Args The list of arguments to use to probe the function type.
+ */
+template<typename Ret, typename Func, typename... Args>
+struct is_applicable_r<Ret, Func, std::tuple<Args...>>: std::is_invocable_r<Ret, Func, Args...> {};
+
+
+/**
+ * @brief Helper variable template.
+ * @tparam Ret The type to which the return type of the function should be
+ * convertible.
+ * @tparam Func A valid function type.
+ * @tparam Args The list of arguments to use to probe the function type.
+ */
+template<typename Ret, typename Func, typename Args>
+inline constexpr bool is_applicable_r_v = is_applicable_r<Ret, Func, Args>::value;
+
+
+/**
+ * @brief Provides the member constant `value` to true if a given type is
+ * complete, false otherwise.
+ * @tparam Type The type to test.
+ */
+template<typename Type, typename = void>
+struct is_complete: std::false_type {};
+
+
+/*! @copydoc is_complete */
+template<typename Type>
+struct is_complete<Type, std::void_t<decltype(sizeof(Type))>>: std::true_type {};
+
+
+/**
+ * @brief Helper variable template.
+ * @tparam Type The type to test.
+ */
+template<typename Type>
+inline constexpr bool is_complete_v = is_complete<Type>::value;
+
+
+/**
+ * @brief Provides the member constant `value` to true if a given type is an
+ * iterator, false otherwise.
+ * @tparam Type The type to test.
+ */
+template<typename Type, typename = void>
+struct is_iterator: std::false_type {};
+
+
+/*! @copydoc is_iterator */
+template<typename Type>
+struct is_iterator<Type, std::void_t<typename std::iterator_traits<Type>::iterator_category>>
+    : std::true_type
+{};
+
+
+/**
+ * @brief Helper variable template.
+ * @tparam Type The type to test.
+ */
+template<typename Type>
+inline constexpr bool is_iterator_v = is_iterator<Type>::value;
+
+
+/**
+ * @brief Provides the member constant `value` to true if a given type is of the
+ * required iterator type, false otherwise.
+ * @tparam Type The type to test.
+ * @tparam It Required iterator type.
+ */
+template<typename Type, typename It, typename = void>
+struct is_iterator_type: std::false_type {};
+
+
+/*! @copydoc is_iterator_type */
+template<typename Type, typename It>
+struct is_iterator_type<Type, It, std::enable_if_t<is_iterator_v<Type> && std::is_same_v<Type, It>>>
+    : std::true_type
+{};
+
+
+/*! @copydoc is_iterator_type */
+template<typename Type, typename It>
+struct is_iterator_type<Type, It, std::enable_if_t<!std::is_same_v<Type, It>, std::void_t<typename It::iterator_type>>>
+    : is_iterator_type<Type, typename It::iterator_type>
+{};
+
+
+/**
+ * @brief Helper variable template.
+ * @tparam Type The type to test.
+ * @tparam It Required iterator type.
+ */
+template<typename Type, typename It>
+inline constexpr bool is_iterator_type_v = is_iterator_type<Type, It>::value;
+
+
+/**
+ * @cond TURN_OFF_DOXYGEN
+ * Internal details not to be documented.
+ */
+
+
+namespace internal {
+
+
+template<typename>
+[[nodiscard]] constexpr bool is_equality_comparable(...) { return false; }
+
+
+template<typename Type>
+[[nodiscard]] constexpr auto is_equality_comparable(choice_t<0>)
+-> decltype(std::declval<Type>() == std::declval<Type>()) { return true; }
+
+
+template<typename Type>
+[[nodiscard]] constexpr auto is_equality_comparable(choice_t<1>)
+-> decltype(std::declval<typename Type::value_type>(), std::declval<Type>() == std::declval<Type>()) {
+    if constexpr(is_iterator_v<Type>) {
+        return true;
+    } else if constexpr(std::is_same_v<typename Type::value_type, Type>) {
+        return is_equality_comparable<Type>(choice<0>);
+    } else {
+        return is_equality_comparable<typename Type::value_type>(choice<2>);
+    }
+}
+
+
+template<typename Type>
+[[nodiscard]] constexpr auto is_equality_comparable(choice_t<2>)
+-> decltype(std::declval<typename Type::mapped_type>(), std::declval<Type>() == std::declval<Type>()) {
+    return is_equality_comparable<typename Type::key_type>(choice<2>) && is_equality_comparable<typename Type::mapped_type>(choice<2>);
+}
+
+
+}
+
+
+/**
+ * Internal details not to be documented.
+ * @endcond
+ */
+
+
+/**
+ * @brief Provides the member constant `value` to true if a given type is
+ * equality comparable, false otherwise.
+ * @tparam Type The type to test.
+ */
+template<typename Type, typename = void>
+struct is_equality_comparable: std::bool_constant<internal::is_equality_comparable<Type>(choice<2>)> {};
+
+
+/**
+ * @brief Helper variable template.
+ * @tparam Type The type to test.
+ */
+template<class Type>
+inline constexpr bool is_equality_comparable_v = is_equality_comparable<Type>::value;
+
+
+/**
+ * @brief Transcribes the constness of a type to another type.
+ * @tparam To The type to which to transcribe the constness.
+ * @tparam From The type from which to transcribe the constness.
+ */
+template<typename To, typename From>
+struct constness_as {
+    /*! @brief The type resulting from the transcription of the constness. */
+    using type = std::remove_const_t<To>;
+};
+
+
+/*! @copydoc constness_as */
+template<typename To, typename From>
+struct constness_as<To, const From> {
+    /*! @brief The type resulting from the transcription of the constness. */
+    using type = std::add_const_t<To>;
+};
+
+
+/**
+ * @brief Alias template to facilitate the transcription of the constness.
+ * @tparam To The type to which to transcribe the constness.
+ * @tparam From The type from which to transcribe the constness.
+ */
+template<typename To, typename From>
+using constness_as_t = typename constness_as<To, From>::type;
+
+
+/**
+ * @brief Extracts the class of a non-static member object or function.
+ * @tparam Member A pointer to a non-static member object or function.
+ */
+template<typename Member>
+class member_class {
+    static_assert(std::is_member_pointer_v<Member>, "Invalid pointer type to non-static member object or function");
+
+    template<typename Class, typename Ret, typename... Args>
+    static Class * clazz(Ret(Class:: *)(Args...));
+
+    template<typename Class, typename Ret, typename... Args>
+    static Class * clazz(Ret(Class:: *)(Args...) const);
+
+    template<typename Class, typename Type>
+    static Class * clazz(Type Class:: *);
+
+public:
+    /*! @brief The class of the given non-static member object or function. */
+    using type = std::remove_pointer_t<decltype(clazz(std::declval<Member>()))>;
+};
+
+
+/**
+ * @brief Helper type.
+ * @tparam Member A pointer to a non-static member object or function.
+ */
+template<typename Member>
+using member_class_t = typename member_class<Member>::type;
+
+
+}
+
+
+#endif
+
+// #include "core/utility.hpp"
+#ifndef ENTT_CORE_UTILITY_HPP
+#define ENTT_CORE_UTILITY_HPP
+
+
+#include <utility>
+// #include "../config/config.h"
+
+
+
+namespace entt {
+
+
+/*! @brief Identity function object (waiting for C++20). */
+struct identity {
+    /**
+     * @brief Returns its argument unchanged.
+     * @tparam Type Type of the argument.
+     * @param value The actual argument.
+     * @return The submitted value as-is.
+     */
+    template<class Type>
+    [[nodiscard]] constexpr Type && operator()(Type &&value) const ENTT_NOEXCEPT {
+        return std::forward<Type>(value);
+    }
+};
+
+
+/**
+ * @brief Constant utility to disambiguate overloaded members of a class.
+ * @tparam Type Type of the desired overload.
+ * @tparam Class Type of class to which the member belongs.
+ * @param member A valid pointer to a member.
+ * @return Pointer to the member.
+ */
+template<typename Type, typename Class>
+[[nodiscard]] constexpr auto overload(Type Class:: *member) ENTT_NOEXCEPT { return member; }
+
+
+/**
+ * @brief Constant utility to disambiguate overloaded functions.
+ * @tparam Func Function type of the desired overload.
+ * @param func A valid pointer to a function.
+ * @return Pointer to the function.
+ */
+template<typename Func>
+[[nodiscard]] constexpr auto overload(Func *func) ENTT_NOEXCEPT { return func; }
+
+
+/**
+ * @brief Helper type for visitors.
+ * @tparam Func Types of function objects.
+ */
+template<class... Func>
+struct overloaded: Func... {
+    using Func::operator()...;
+};
+
+
+/**
+ * @brief Deduction guide.
+ * @tparam Func Types of function objects.
+ */
+template<class... Func>
+overloaded(Func...)
+-> overloaded<Func...>;
+
+
+/**
+ * @brief Basic implementation of a y-combinator.
+ * @tparam Func Type of a potentially recursive function.
+ */
+template<class Func>
+struct y_combinator {
+    /**
+     * @brief Constructs a y-combinator from a given function.
+     * @param recursive A potentially recursive function.
+     */
+    y_combinator(Func recursive):
+        func{std::move(recursive)}
+    {}
+
+    /**
+     * @brief Invokes a y-combinator and therefore its underlying function.
+     * @tparam Args Types of arguments to use to invoke the underlying function.
+     * @param args Parameters to use to invoke the underlying function.
+     * @return Return value of the underlying function, if any.
+     */
+    template <class... Args>
+    decltype(auto) operator()(Args &&... args) const {
+        return func(*this, std::forward<Args>(args)...);
+    }
+
+    /*! @copydoc operator()() */
+    template <class... Args>
+    decltype(auto) operator()(Args &&... args) {
+        return func(*this, std::forward<Args>(args)...);
+    }
+
+private:
+    Func func;
+};
+
+
+}
+
+
+#endif
+
+// #include "entity/component.hpp"
+#ifndef ENTT_ENTITY_COMPONENT_HPP
+#define ENTT_ENTITY_COMPONENT_HPP
+
+
+#include <type_traits>
 // #include "../config/config.h"
 #ifndef ENTT_CONFIG_CONFIG_H
 #define ENTT_CONFIG_CONFIG_H
 
 
-#ifndef ENTT_NOEXCEPT
-#define ENTT_NOEXCEPT noexcept
-#endif // ENTT_NOEXCEPT
-
-
-#ifndef ENTT_HS_SUFFIX
-#define ENTT_HS_SUFFIX _hs
-#endif // ENTT_HS_SUFFIX
-
-
-#ifndef ENTT_HWS_SUFFIX
-#define ENTT_HWS_SUFFIX _hws
-#endif // ENTT_HWS_SUFFIX
-
-
-#ifndef ENTT_NO_ATOMIC
-#include <atomic>
-#define ENTT_MAYBE_ATOMIC(Type) std::atomic<Type>
-#else // ENTT_NO_ATOMIC
-#define ENTT_MAYBE_ATOMIC(Type) Type
-#endif // ENTT_NO_ATOMIC
+#if defined(__cpp_exceptions) && !defined(ENTT_NOEXCEPTION)
+#   define ENTT_NOEXCEPT noexcept
+#   define ENTT_THROW throw
+#   define ENTT_TRY try
+#   define ENTT_CATCH catch(...)
+#else
+#   define ENTT_NOEXCEPT
+#   define ENTT_THROW
+#   define ENTT_TRY if(true)
+#   define ENTT_CATCH if(false)
+#endif
+
+
+#if defined(__cpp_lib_launder) && __cpp_lib_launder >= 201606L
+#   include <new>
+#   define ENTT_LAUNDER(expr) std::launder(expr)
+#else
+#   define ENTT_LAUNDER(expr) expr
+#endif
+
+
+#ifndef ENTT_USE_ATOMIC
+#   define ENTT_MAYBE_ATOMIC(Type) Type
+#else
+#   include <atomic>
+#   define ENTT_MAYBE_ATOMIC(Type) std::atomic<Type>
+#endif
 
 
 #ifndef ENTT_ID_TYPE
+#   include <cstdint>
+#   define ENTT_ID_TYPE std::uint32_t
+#endif
+
+
+#ifdef ENTT_SPARSE_PAGE
+	static_assert(ENTT_SPARSE_PAGE && ((ENTT_SPARSE_PAGE & (ENTT_SPARSE_PAGE - 1)) == 0), "ENTT_SPARSE_PAGE must be a power of two");
+#else
+#   define ENTT_SPARSE_PAGE 4096
+#endif
+
+
+#ifdef ENTT_PACKED_PAGE
+static_assert(ENTT_PACKED_PAGE && ((ENTT_PACKED_PAGE & (ENTT_PACKED_PAGE - 1)) == 0), "ENTT_PACKED_PAGE must be a power of two");
+#else
+#   define ENTT_PACKED_PAGE 1024
+#endif
+
+
+#ifdef ENTT_DISABLE_ASSERT
+#   undef ENTT_ASSERT
+#   define ENTT_ASSERT(...) (void(0))
+#elif !defined ENTT_ASSERT
+#   include <cassert>
+#   define ENTT_ASSERT(condition, ...) assert(condition)
+#endif
+
+
+#ifdef ENTT_NO_ETO
+#   include <type_traits>
+#   define ENTT_IGNORE_IF_EMPTY std::false_type
+#else
+#   include <type_traits>
+#   define ENTT_IGNORE_IF_EMPTY std::true_type
+#endif
+
+
+#ifndef ENTT_STANDARD_CPP
+#    if defined __clang__ || defined __GNUC__
+#       define ENTT_PRETTY_FUNCTION __PRETTY_FUNCTION__
+#       define ENTT_PRETTY_FUNCTION_PREFIX '='
+#       define ENTT_PRETTY_FUNCTION_SUFFIX ']'
+#    elif defined _MSC_VER
+#       define ENTT_PRETTY_FUNCTION __FUNCSIG__
+#       define ENTT_PRETTY_FUNCTION_PREFIX '<'
+#       define ENTT_PRETTY_FUNCTION_SUFFIX '>'
+#   endif
+#endif
+
+
+#endif
+
+
+
+namespace entt {
+
+
+/*! @brief Commonly used default traits for all types. */
+struct basic_component_traits {
+    /*! @brief Pointer stability, default is `std::false_type`. */
+    using in_place_delete = std::false_type;
+    /*! @brief Empty type optimization, default is `ENTT_IGNORE_IF_EMPTY`. */
+    using ignore_if_empty = ENTT_IGNORE_IF_EMPTY;
+};
+
+
+/**
+ * @brief Common way to access various properties of components.
+ * @tparam Type Type of component.
+ */
+template<typename Type, typename = void>
+struct component_traits: basic_component_traits {
+    static_assert(std::is_same_v<std::decay_t<Type>, Type>, "Unsupported type");
+};
+
+
+}
+
+
+#endif
+
+// #include "entity/entity.hpp"
+#ifndef ENTT_ENTITY_ENTITY_HPP
+#define ENTT_ENTITY_ENTITY_HPP
+
+
+#include <cstddef>
 #include <cstdint>
-#define ENTT_ID_TYPE std::uint32_t
-#endif // ENTT_ID_TYPE
-
-
-#ifndef ENTT_PAGE_SIZE
-#define ENTT_PAGE_SIZE 32768
-#endif // ENTT_PAGE_SIZE
-
-
-#ifndef ENTT_DISABLE_ASSERT
-#include <cassert>
-#define ENTT_ASSERT(condition) assert(condition)
-#else // ENTT_DISABLE_ASSERT
-#define ENTT_ASSERT(...) ((void)0)
-#endif // ENTT_DISABLE_ASSERT
-
-
-#endif // ENTT_CONFIG_CONFIG_H
+#include <type_traits>
+// #include "../config/config.h"
 
 
 
@@ -751,271 +3995,3240 @@
 namespace internal {
 
 
-template<typename>
-struct fnv1a_traits;
-
-
-template<>
-struct fnv1a_traits<std::uint32_t> {
-    static constexpr std::uint32_t offset = 2166136261;
-    static constexpr std::uint32_t prime = 16777619;
-};
-
-
-template<>
-struct fnv1a_traits<std::uint64_t> {
-    static constexpr std::uint64_t offset = 14695981039346656037ull;
-    static constexpr std::uint64_t prime = 1099511628211ull;
-};
-
-
-}
-
-
-/**
- * Internal details not to be documented.
- * @endcond TURN_OFF_DOXYGEN
- */
-
-
-/**
- * @brief Zero overhead unique identifier.
- *
- * A hashed string is a compile-time tool that allows users to use
- * human-readable identifers in the codebase while using their numeric
- * counterparts at runtime.<br/>
- * Because of that, a hashed string can also be used in constant expressions if
- * required.
- *
- * @tparam Char Character type.
- */
-template<typename Char>
-class basic_hashed_string {
-    using traits_type = internal::fnv1a_traits<ENTT_ID_TYPE>;
-
-    struct const_wrapper {
-        // non-explicit constructor on purpose
-        constexpr const_wrapper(const Char *curr) ENTT_NOEXCEPT: str{curr} {}
-        const Char *str;
-    };
-
-    // Fowler–Noll–Vo hash function v. 1a - the good
-    static constexpr ENTT_ID_TYPE helper(ENTT_ID_TYPE partial, const Char *curr) ENTT_NOEXCEPT {
-        return curr[0] == 0 ? partial : helper((partial^curr[0])*traits_type::prime, curr+1);
-    }
-
-public:
-    /*! @brief Character type. */
-    using value_type = Char;
-    /*! @brief Unsigned integer type. */
-    using hash_type = ENTT_ID_TYPE;
-
-    /**
-     * @brief Returns directly the numeric representation of a string.
-     *
-     * Forcing template resolution avoids implicit conversions. An
-     * human-readable identifier can be anything but a plain, old bunch of
-     * characters.<br/>
-     * Example of use:
-     * @code{.cpp}
-     * const auto value = basic_hashed_string<char>::to_value("my.png");
-     * @endcode
-     *
-     * @tparam N Number of characters of the identifier.
-     * @param str Human-readable identifer.
-     * @return The numeric representation of the string.
-     */
-    template<std::size_t N>
-    static constexpr hash_type to_value(const value_type (&str)[N]) ENTT_NOEXCEPT {
-        return helper(traits_type::offset, str);
-    }
-
-    /**
-     * @brief Returns directly the numeric representation of a string.
-     * @param wrapper Helps achieving the purpose by relying on overloading.
-     * @return The numeric representation of the string.
-     */
-    static hash_type to_value(const_wrapper wrapper) ENTT_NOEXCEPT {
-        return helper(traits_type::offset, wrapper.str);
-    }
-
-    /**
-     * @brief Returns directly the numeric representation of a string view.
-     * @param str Human-readable identifer.
-     * @param size Length of the string to hash.
-     * @return The numeric representation of the string.
-     */
-    static hash_type to_value(const value_type *str, std::size_t size) ENTT_NOEXCEPT {
-        ENTT_ID_TYPE partial{traits_type::offset};
-        while(size--) { partial = (partial^(str++)[0])*traits_type::prime; }
-        return partial;
-    }
-
-    /*! @brief Constructs an empty hashed string. */
-    constexpr basic_hashed_string() ENTT_NOEXCEPT
-        : str{nullptr}, hash{}
-    {}
-
-    /**
-     * @brief Constructs a hashed string from an array of const characters.
-     *
-     * Forcing template resolution avoids implicit conversions. An
-     * human-readable identifier can be anything but a plain, old bunch of
-     * characters.<br/>
-     * Example of use:
-     * @code{.cpp}
-     * basic_hashed_string<char> hs{"my.png"};
-     * @endcode
-     *
-     * @tparam N Number of characters of the identifier.
-     * @param curr Human-readable identifer.
-     */
-    template<std::size_t N>
-    constexpr basic_hashed_string(const value_type (&curr)[N]) ENTT_NOEXCEPT
-        : str{curr}, hash{helper(traits_type::offset, curr)}
-    {}
-
-    /**
-     * @brief Explicit constructor on purpose to avoid constructing a hashed
-     * string directly from a `const value_type *`.
-     * @param wrapper Helps achieving the purpose by relying on overloading.
-     */
-    explicit constexpr basic_hashed_string(const_wrapper wrapper) ENTT_NOEXCEPT
-        : str{wrapper.str}, hash{helper(traits_type::offset, wrapper.str)}
-    {}
-
-    /**
-     * @brief Returns the human-readable representation of a hashed string.
-     * @return The string used to initialize the instance.
-     */
-    constexpr const value_type * data() const ENTT_NOEXCEPT {
-        return str;
-    }
-
-    /**
-     * @brief Returns the numeric representation of a hashed string.
-     * @return The numeric representation of the instance.
-     */
-    constexpr hash_type value() const ENTT_NOEXCEPT {
-        return hash;
-    }
-
-    /**
-     * @brief Returns the human-readable representation of a hashed string.
-     * @return The string used to initialize the instance.
-     */
-    constexpr operator const value_type *() const ENTT_NOEXCEPT { return str; }
-
-    /*! @copydoc value */
-    constexpr operator hash_type() const ENTT_NOEXCEPT { return hash; }
-
-    /**
-     * @brief Compares two hashed strings.
-     * @param other Hashed string with which to compare.
-     * @return True if the two hashed strings are identical, false otherwise.
-     */
-    constexpr bool operator==(const basic_hashed_string &other) const ENTT_NOEXCEPT {
-        return hash == other.hash;
-    }
-
-private:
-    const value_type *str;
-    hash_type hash;
+template<typename, typename = void>
+struct entt_traits;
+
+
+template<typename Type>
+struct entt_traits<Type, std::enable_if_t<std::is_enum_v<Type>>>
+    : entt_traits<std::underlying_type_t<Type>>
+{};
+
+
+template<typename Type>
+struct entt_traits<Type, std::enable_if_t<std::is_class_v<Type>>>
+    : entt_traits<typename Type::entity_type>
+{};
+
+
+template<>
+struct entt_traits<std::uint32_t> {
+    using entity_type = std::uint32_t;
+    using version_type = std::uint16_t;
+    using difference_type = std::int64_t;
+
+    static constexpr entity_type entity_mask = 0xFFFFF;
+    static constexpr entity_type version_mask = 0xFFF;
+    static constexpr std::size_t entity_shift = 20u;
+};
+
+
+template<>
+struct entt_traits<std::uint64_t> {
+    using entity_type = std::uint64_t;
+    using version_type = std::uint32_t;
+    using difference_type = std::int64_t;
+
+    static constexpr entity_type entity_mask = 0xFFFFFFFF;
+    static constexpr entity_type version_mask = 0xFFFFFFFF;
+    static constexpr std::size_t entity_shift = 32u;
+};
+
+
+}
+
+
+/**
+ * Internal details not to be documented.
+ * @endcond
+ */
+
+
+/**
+ * @brief Entity traits.
+ * @tparam Type Type of identifier.
+ */
+template<typename Type>
+class entt_traits: private internal::entt_traits<Type> {
+    using traits_type = internal::entt_traits<Type>;
+
+public:
+    /*! @brief Value type. */
+    using value_type = Type;
+    /*! @brief Underlying entity type. */
+    using entity_type = typename traits_type::entity_type;
+    /*! @brief Underlying version type. */
+    using version_type = typename traits_type::version_type;
+    /*! @brief Difference type. */
+    using difference_type = typename traits_type::difference_type;
+
+    /**
+     * @brief Converts an entity to its underlying type.
+     * @param value The value to convert.
+     * @return The integral representation of the given value.
+     */
+    [[nodiscard]] static constexpr entity_type to_integral(const value_type value) ENTT_NOEXCEPT {
+        return static_cast<entity_type>(value);
+    }
+
+    /**
+     * @brief Returns the entity part once converted to the underlying type.
+     * @param value The value to convert.
+     * @return The integral representation of the entity part.
+     */
+    [[nodiscard]] static constexpr entity_type to_entity(const value_type value) ENTT_NOEXCEPT {
+        return (to_integral(value) & traits_type::entity_mask);
+    }
+
+    /**
+     * @brief Returns the version part once converted to the underlying type.
+     * @param value The value to convert.
+     * @return The integral representation of the version part.
+     */
+    [[nodiscard]] static constexpr version_type to_version(const value_type value) ENTT_NOEXCEPT {
+        constexpr auto mask = (traits_type::version_mask << traits_type::entity_shift);
+        return ((to_integral(value) & mask) >> traits_type::entity_shift);
+    }
+
+    /**
+     * @brief Constructs an identifier from its parts.
+     *
+     * If the version part is not provided, a tombstone is returned.<br/>
+     * If the entity part is not provided, a null identifier is returned.
+     *
+     * @param entity The entity part of the identifier.
+     * @param version The version part of the identifier.
+     * @return A properly constructed identifier.
+     */
+    [[nodiscard]] static constexpr value_type construct(const entity_type entity = traits_type::entity_mask, const version_type version = traits_type::version_mask) ENTT_NOEXCEPT {
+        return value_type{(entity & traits_type::entity_mask) | (static_cast<entity_type>(version) << traits_type::entity_shift)};
+    }
+};
+
+
+/**
+ * @brief Converts an entity to its underlying type.
+ * @tparam Entity The value type.
+ * @param entity The value to convert.
+ * @return The integral representation of the given value.
+ */
+template<typename Entity>
+[[nodiscard]] constexpr auto to_integral(const Entity entity) ENTT_NOEXCEPT {
+    return entt_traits<Entity>::to_integral(entity);
+}
+
+
+/*! @brief Null object for all entity identifiers.  */
+struct null_t {
+    /**
+     * @brief Converts the null object to identifiers of any type.
+     * @tparam Entity Type of entity identifier.
+     * @return The null representation for the given type.
+     */
+    template<typename Entity>
+    [[nodiscard]] constexpr operator Entity() const ENTT_NOEXCEPT {
+        return entt_traits<Entity>::construct();
+    }
+
+    /**
+     * @brief Compares two null objects.
+     * @param other A null object.
+     * @return True in all cases.
+     */
+    [[nodiscard]] constexpr bool operator==([[maybe_unused]] const null_t other) const ENTT_NOEXCEPT {
+        return true;
+    }
+
+    /**
+     * @brief Compares two null objects.
+     * @param other A null object.
+     * @return False in all cases.
+     */
+    [[nodiscard]] constexpr bool operator!=([[maybe_unused]] const null_t other) const ENTT_NOEXCEPT {
+        return false;
+    }
+
+    /**
+     * @brief Compares a null object and an entity identifier of any type.
+     * @tparam Entity Type of entity identifier.
+     * @param entity Entity identifier with which to compare.
+     * @return False if the two elements differ, true otherwise.
+     */
+    template<typename Entity>
+    [[nodiscard]] constexpr bool operator==(const Entity entity) const ENTT_NOEXCEPT {
+        return entt_traits<Entity>::to_entity(entity) == entt_traits<Entity>::to_entity(*this);
+    }
+
+    /**
+     * @brief Compares a null object and an entity identifier of any type.
+     * @tparam Entity Type of entity identifier.
+     * @param entity Entity identifier with which to compare.
+     * @return True if the two elements differ, false otherwise.
+     */
+    template<typename Entity>
+    [[nodiscard]] constexpr bool operator!=(const Entity entity) const ENTT_NOEXCEPT {
+        return !(entity == *this);
+    }
+
+    /**
+     * @brief Creates a null object from an entity identifier of any type.
+     * @tparam Entity Type of entity identifier.
+     * @param entity Entity identifier to turn into a null object.
+     * @return The null representation for the given identifier.
+     */
+    template<typename Entity>
+    [[nodiscard]] constexpr Entity operator|(const Entity entity) const ENTT_NOEXCEPT {
+        return entt_traits<Entity>::construct(entt_traits<Entity>::to_entity(*this), entt_traits<Entity>::to_version(entity));
+    }
+};
+
+
+/**
+ * @brief Compares a null object and an entity identifier of any type.
+ * @tparam Entity Type of entity identifier.
+ * @param entity Entity identifier with which to compare.
+ * @param other A null object yet to be converted.
+ * @return False if the two elements differ, true otherwise.
+ */
+template<typename Entity>
+[[nodiscard]] constexpr bool operator==(const Entity entity, const null_t other) ENTT_NOEXCEPT {
+    return other.operator==(entity);
+}
+
+
+/**
+ * @brief Compares a null object and an entity identifier of any type.
+ * @tparam Entity Type of entity identifier.
+ * @param entity Entity identifier with which to compare.
+ * @param other A null object yet to be converted.
+ * @return True if the two elements differ, false otherwise.
+ */
+template<typename Entity>
+[[nodiscard]] constexpr bool operator!=(const Entity entity, const null_t other) ENTT_NOEXCEPT {
+    return !(other == entity);
+}
+
+
+/*! @brief Tombstone object for all entity identifiers.  */
+struct tombstone_t {
+    /**
+     * @brief Converts the tombstone object to identifiers of any type.
+     * @tparam Entity Type of entity identifier.
+     * @return The tombstone representation for the given type.
+     */
+    template<typename Entity>
+    [[nodiscard]] constexpr operator Entity() const ENTT_NOEXCEPT {
+        return entt_traits<Entity>::construct();
+    }
+
+    /**
+     * @brief Compares two tombstone objects.
+     * @param other A tombstone object.
+     * @return True in all cases.
+     */
+    [[nodiscard]] constexpr bool operator==([[maybe_unused]] const tombstone_t other) const ENTT_NOEXCEPT {
+        return true;
+    }
+
+    /**
+     * @brief Compares two tombstone objects.
+     * @param other A tombstone object.
+     * @return False in all cases.
+     */
+    [[nodiscard]] constexpr bool operator!=([[maybe_unused]] const tombstone_t other) const ENTT_NOEXCEPT {
+        return false;
+    }
+
+    /**
+     * @brief Compares a tombstone object and an entity identifier of any type.
+     * @tparam Entity Type of entity identifier.
+     * @param entity Entity identifier with which to compare.
+     * @return False if the two elements differ, true otherwise.
+     */
+    template<typename Entity>
+    [[nodiscard]] constexpr bool operator==(const Entity entity) const ENTT_NOEXCEPT {
+        return entt_traits<Entity>::to_version(entity) == entt_traits<Entity>::to_version(*this);
+    }
+
+    /**
+     * @brief Compares a tombstone object and an entity identifier of any type.
+     * @tparam Entity Type of entity identifier.
+     * @param entity Entity identifier with which to compare.
+     * @return True if the two elements differ, false otherwise.
+     */
+    template<typename Entity>
+    [[nodiscard]] constexpr bool operator!=(const Entity entity) const ENTT_NOEXCEPT {
+        return !(entity == *this);
+    }
+
+    /**
+     * @brief Creates a tombstone object from an entity identifier of any type.
+     * @tparam Entity Type of entity identifier.
+     * @param entity Entity identifier to turn into a tombstone object.
+     * @return The tombstone representation for the given identifier.
+     */
+    template<typename Entity>
+    [[nodiscard]] constexpr Entity operator|(const Entity entity) const ENTT_NOEXCEPT {
+        return entt_traits<Entity>::construct(entt_traits<Entity>::to_entity(entity));
+    }
+};
+
+
+/**
+ * @brief Compares a tombstone object and an entity identifier of any type.
+ * @tparam Entity Type of entity identifier.
+ * @param entity Entity identifier with which to compare.
+ * @param other A tombstone object yet to be converted.
+ * @return False if the two elements differ, true otherwise.
+ */
+template<typename Entity>
+[[nodiscard]] constexpr bool operator==(const Entity entity, const tombstone_t other) ENTT_NOEXCEPT {
+    return other.operator==(entity);
+}
+
+
+/**
+ * @brief Compares a tombstone object and an entity identifier of any type.
+ * @tparam Entity Type of entity identifier.
+ * @param entity Entity identifier with which to compare.
+ * @param other A tombstone object yet to be converted.
+ * @return True if the two elements differ, false otherwise.
+ */
+template<typename Entity>
+[[nodiscard]] constexpr bool operator!=(const Entity entity, const tombstone_t other) ENTT_NOEXCEPT {
+    return !(other == entity);
+}
+
+
+/**
+ * @brief Compile-time constant for null entities.
+ *
+ * There exist implicit conversions from this variable to entity identifiers of
+ * any allowed type. Similarly, there exist comparision operators between the
+ * null entity and any other entity identifier.
+ */
+inline constexpr null_t null{};
+
+
+/**
+ * @brief Compile-time constant for tombstone entities.
+ *
+ * There exist implicit conversions from this variable to entity identifiers of
+ * any allowed type. Similarly, there exist comparision operators between the
+ * tombstone entity and any other entity identifier.
+ */
+inline constexpr tombstone_t tombstone{};
+
+
+}
+
+
+#endif
+
+// #include "entity/group.hpp"
+#ifndef ENTT_ENTITY_GROUP_HPP
+#define ENTT_ENTITY_GROUP_HPP
+
+
+#include <tuple>
+#include <utility>
+#include <type_traits>
+// #include "../config/config.h"
+
+// #include "../core/type_traits.hpp"
+#ifndef ENTT_CORE_TYPE_TRAITS_HPP
+#define ENTT_CORE_TYPE_TRAITS_HPP
+
+
+#include <cstddef>
+#include <iterator>
+#include <type_traits>
+#include <utility>
+// #include "../config/config.h"
+#ifndef ENTT_CONFIG_CONFIG_H
+#define ENTT_CONFIG_CONFIG_H
+
+
+#if defined(__cpp_exceptions) && !defined(ENTT_NOEXCEPTION)
+#   define ENTT_NOEXCEPT noexcept
+#   define ENTT_THROW throw
+#   define ENTT_TRY try
+#   define ENTT_CATCH catch(...)
+#else
+#   define ENTT_NOEXCEPT
+#   define ENTT_THROW
+#   define ENTT_TRY if(true)
+#   define ENTT_CATCH if(false)
+#endif
+
+
+#if defined(__cpp_lib_launder) && __cpp_lib_launder >= 201606L
+#   include <new>
+#   define ENTT_LAUNDER(expr) std::launder(expr)
+#else
+#   define ENTT_LAUNDER(expr) expr
+#endif
+
+
+#ifndef ENTT_USE_ATOMIC
+#   define ENTT_MAYBE_ATOMIC(Type) Type
+#else
+#   include <atomic>
+#   define ENTT_MAYBE_ATOMIC(Type) std::atomic<Type>
+#endif
+
+
+#ifndef ENTT_ID_TYPE
+#   include <cstdint>
+#   define ENTT_ID_TYPE std::uint32_t
+#endif
+
+
+#ifdef ENTT_SPARSE_PAGE
+	static_assert(ENTT_SPARSE_PAGE && ((ENTT_SPARSE_PAGE & (ENTT_SPARSE_PAGE - 1)) == 0), "ENTT_SPARSE_PAGE must be a power of two");
+#else
+#   define ENTT_SPARSE_PAGE 4096
+#endif
+
+
+#ifdef ENTT_PACKED_PAGE
+static_assert(ENTT_PACKED_PAGE && ((ENTT_PACKED_PAGE & (ENTT_PACKED_PAGE - 1)) == 0), "ENTT_PACKED_PAGE must be a power of two");
+#else
+#   define ENTT_PACKED_PAGE 1024
+#endif
+
+
+#ifdef ENTT_DISABLE_ASSERT
+#   undef ENTT_ASSERT
+#   define ENTT_ASSERT(...) (void(0))
+#elif !defined ENTT_ASSERT
+#   include <cassert>
+#   define ENTT_ASSERT(condition, ...) assert(condition)
+#endif
+
+
+#ifdef ENTT_NO_ETO
+#   include <type_traits>
+#   define ENTT_IGNORE_IF_EMPTY std::false_type
+#else
+#   include <type_traits>
+#   define ENTT_IGNORE_IF_EMPTY std::true_type
+#endif
+
+
+#ifndef ENTT_STANDARD_CPP
+#    if defined __clang__ || defined __GNUC__
+#       define ENTT_PRETTY_FUNCTION __PRETTY_FUNCTION__
+#       define ENTT_PRETTY_FUNCTION_PREFIX '='
+#       define ENTT_PRETTY_FUNCTION_SUFFIX ']'
+#    elif defined _MSC_VER
+#       define ENTT_PRETTY_FUNCTION __FUNCSIG__
+#       define ENTT_PRETTY_FUNCTION_PREFIX '<'
+#       define ENTT_PRETTY_FUNCTION_SUFFIX '>'
+#   endif
+#endif
+
+
+#endif
+
+// #include "fwd.hpp"
+#ifndef ENTT_CORE_FWD_HPP
+#define ENTT_CORE_FWD_HPP
+
+
+#include <type_traits>
+// #include "../config/config.h"
+
+
+
+namespace entt {
+
+
+template<std::size_t Len = sizeof(double[2]), std::size_t = alignof(typename std::aligned_storage_t<Len + !Len>)>
+class basic_any;
+
+
+/*! @brief Alias declaration for type identifiers. */
+using id_type = ENTT_ID_TYPE;
+
+
+/*! @brief Alias declaration for the most common use case. */
+using any = basic_any<>;
+
+
+}
+
+
+#endif
+
+
+
+namespace entt {
+
+
+/**
+ * @brief Utility class to disambiguate overloaded functions.
+ * @tparam N Number of choices available.
+ */
+template<std::size_t N>
+struct choice_t
+    // Unfortunately, doxygen cannot parse such a construct.
+    /*! @cond TURN_OFF_DOXYGEN */
+    : choice_t<N-1>
+    /*! @endcond */
+{};
+
+
+/*! @copybrief choice_t */
+template<>
+struct choice_t<0> {};
+
+
+/**
+ * @brief Variable template for the choice trick.
+ * @tparam N Number of choices available.
+ */
+template<std::size_t N>
+inline constexpr choice_t<N> choice{};
+
+
+/**
+ * @brief Identity type trait.
+ *
+ * Useful to establish non-deduced contexts in template argument deduction
+ * (waiting for C++20) or to provide types through function arguments.
+ *
+ * @tparam Type A type.
+ */
+template<typename Type>
+struct type_identity {
+    /*! @brief Identity type. */
+    using type = Type;
+};
+
+
+/**
+ * @brief Helper type.
+ * @tparam Type A type.
+ */
+template<typename Type>
+using type_identity_t = typename type_identity<Type>::type;
+
+
+/**
+ * @brief A type-only `sizeof` wrapper that returns 0 where `sizeof` complains.
+ * @tparam Type The type of which to return the size.
+ * @tparam The size of the type if `sizeof` accepts it, 0 otherwise.
+ */
+template<typename Type, typename = void>
+struct size_of: std::integral_constant<std::size_t, 0u> {};
+
+
+/*! @copydoc size_of */
+template<typename Type>
+struct size_of<Type, std::void_t<decltype(sizeof(Type))>>
+    : std::integral_constant<std::size_t, sizeof(Type)>
+{};
+
+
+/**
+ * @brief Helper variable template.
+ * @tparam Type The type of which to return the size.
+ */
+template<class Type>
+inline constexpr std::size_t size_of_v = size_of<Type>::value;
+
+
+/**
+ * @brief Using declaration to be used to _repeat_ the same type a number of
+ * times equal to the size of a given parameter pack.
+ * @tparam Type A type to repeat.
+ */
+template<typename Type, typename>
+using unpack_as_t = Type;
+
+
+/**
+ * @brief Helper variable template to be used to _repeat_ the same value a
+ * number of times equal to the size of a given parameter pack.
+ * @tparam Value A value to repeat.
+ */
+template<auto Value, typename>
+inline constexpr auto unpack_as_v = Value;
+
+
+/**
+ * @brief Wraps a static constant.
+ * @tparam Value A static constant.
+ */
+template<auto Value>
+using integral_constant = std::integral_constant<decltype(Value), Value>;
+
+
+/**
+ * @brief Alias template to facilitate the creation of named values.
+ * @tparam Value A constant value at least convertible to `id_type`.
+ */
+template<id_type Value>
+using tag = integral_constant<Value>;
+
+
+/**
+ * @brief A class to use to push around lists of types, nothing more.
+ * @tparam Type Types provided by the type list.
+ */
+template<typename... Type>
+struct type_list {
+    /*! @brief Type list type. */
+    using type = type_list;
+    /*! @brief Compile-time number of elements in the type list. */
+    static constexpr auto size = sizeof...(Type);
+};
+
+
+/*! @brief Primary template isn't defined on purpose. */
+template<std::size_t, typename>
+struct type_list_element;
+
+
+/**
+ * @brief Provides compile-time indexed access to the types of a type list.
+ * @tparam Index Index of the type to return.
+ * @tparam Type First type provided by the type list.
+ * @tparam Other Other types provided by the type list.
+ */
+template<std::size_t Index, typename Type, typename... Other>
+struct type_list_element<Index, type_list<Type, Other...>>
+    : type_list_element<Index - 1u, type_list<Other...>>
+{};
+
+
+/**
+ * @brief Provides compile-time indexed access to the types of a type list.
+ * @tparam Type First type provided by the type list.
+ * @tparam Other Other types provided by the type list.
+ */
+template<typename Type, typename... Other>
+struct type_list_element<0u, type_list<Type, Other...>> {
+    /*! @brief Searched type. */
+    using type = Type;
+};
+
+
+/**
+ * @brief Helper type.
+ * @tparam Index Index of the type to return.
+ * @tparam List Type list to search into.
+ */
+template<std::size_t Index, typename List>
+using type_list_element_t = typename type_list_element<Index, List>::type;
+
+
+/**
+ * @brief Concatenates multiple type lists.
+ * @tparam Type Types provided by the first type list.
+ * @tparam Other Types provided by the second type list.
+ * @return A type list composed by the types of both the type lists.
+ */
+template<typename... Type, typename... Other>
+constexpr type_list<Type..., Other...> operator+(type_list<Type...>, type_list<Other...>) { return {}; }
+
+
+/*! @brief Primary template isn't defined on purpose. */
+template<typename...>
+struct type_list_cat;
+
+
+/*! @brief Concatenates multiple type lists. */
+template<>
+struct type_list_cat<> {
+    /*! @brief A type list composed by the types of all the type lists. */
+    using type = type_list<>;
+};
+
+
+/**
+ * @brief Concatenates multiple type lists.
+ * @tparam Type Types provided by the first type list.
+ * @tparam Other Types provided by the second type list.
+ * @tparam List Other type lists, if any.
+ */
+template<typename... Type, typename... Other, typename... List>
+struct type_list_cat<type_list<Type...>, type_list<Other...>, List...> {
+    /*! @brief A type list composed by the types of all the type lists. */
+    using type = typename type_list_cat<type_list<Type..., Other...>, List...>::type;
+};
+
+
+/**
+ * @brief Concatenates multiple type lists.
+ * @tparam Type Types provided by the type list.
+ */
+template<typename... Type>
+struct type_list_cat<type_list<Type...>> {
+    /*! @brief A type list composed by the types of all the type lists. */
+    using type = type_list<Type...>;
+};
+
+
+/**
+ * @brief Helper type.
+ * @tparam List Type lists to concatenate.
+ */
+template<typename... List>
+using type_list_cat_t = typename type_list_cat<List...>::type;
+
+
+/*! @brief Primary template isn't defined on purpose. */
+template<typename>
+struct type_list_unique;
+
+
+/**
+ * @brief Removes duplicates types from a type list.
+ * @tparam Type One of the types provided by the given type list.
+ * @tparam Other The other types provided by the given type list.
+ */
+template<typename Type, typename... Other>
+struct type_list_unique<type_list<Type, Other...>> {
+    /*! @brief A type list without duplicate types. */
+    using type = std::conditional_t<
+        std::disjunction_v<std::is_same<Type, Other>...>,
+        typename type_list_unique<type_list<Other...>>::type,
+        type_list_cat_t<type_list<Type>, typename type_list_unique<type_list<Other...>>::type>
+    >;
+};
+
+
+/*! @brief Removes duplicates types from a type list. */
+template<>
+struct type_list_unique<type_list<>> {
+    /*! @brief A type list without duplicate types. */
+    using type = type_list<>;
+};
+
+
+/**
+ * @brief Helper type.
+ * @tparam Type A type list.
+ */
+template<typename Type>
+using type_list_unique_t = typename type_list_unique<Type>::type;
+
+
+/**
+ * @brief Provides the member constant `value` to true if a type list contains a
+ * given type, false otherwise.
+ * @tparam List Type list.
+ * @tparam Type Type to look for.
+ */
+template<typename List, typename Type>
+struct type_list_contains;
+
+
+/**
+ * @copybrief type_list_contains
+ * @tparam Type Types provided by the type list.
+ * @tparam Other Type to look for.
+ */
+template<typename... Type, typename Other>
+struct type_list_contains<type_list<Type...>, Other>: std::disjunction<std::is_same<Type, Other>...> {};
+
+
+/**
+ * @brief Helper variable template.
+ * @tparam List Type list.
+ * @tparam Type Type to look for.
+ */
+template<class List, typename Type>
+inline constexpr bool type_list_contains_v = type_list_contains<List, Type>::value;
+
+
+/*! @brief Primary template isn't defined on purpose. */
+template<typename...>
+struct type_list_diff;
+
+
+/**
+ * @brief Computes the difference between two type lists.
+ * @tparam Type Types provided by the first type list.
+ * @tparam Other Types provided by the second type list.
+ */
+template<typename... Type, typename... Other>
+struct type_list_diff<type_list<Type...>, type_list<Other...>> {
+    /*! @brief A type list that is the difference between the two type lists. */
+    using type = type_list_cat_t<std::conditional_t<type_list_contains_v<type_list<Other...>, Type>, type_list<>, type_list<Type>>...>;
+};
+
+
+/**
+ * @brief Helper type.
+ * @tparam List Type lists between which to compute the difference.
+ */
+template<typename... List>
+using type_list_diff_t = typename type_list_diff<List...>::type;
+
+
+/**
+ * @brief A class to use to push around lists of constant values, nothing more.
+ * @tparam Value Values provided by the value list.
+ */
+template<auto... Value>
+struct value_list {
+    /*! @brief Value list type. */
+    using type = value_list;
+    /*! @brief Compile-time number of elements in the value list. */
+    static constexpr auto size = sizeof...(Value);
+};
+
+
+/*! @brief Primary template isn't defined on purpose. */
+template<std::size_t, typename>
+struct value_list_element;
+
+
+/**
+ * @brief Provides compile-time indexed access to the values of a value list.
+ * @tparam Index Index of the value to return.
+ * @tparam Value First value provided by the value list.
+ * @tparam Other Other values provided by the value list.
+ */
+template<std::size_t Index, auto Value, auto... Other>
+struct value_list_element<Index, value_list<Value, Other...>>
+    : value_list_element<Index - 1u, value_list<Other...>>
+{};
+
+
+/**
+ * @brief Provides compile-time indexed access to the types of a type list.
+ * @tparam Value First value provided by the value list.
+ * @tparam Other Other values provided by the value list.
+ */
+template<auto Value, auto... Other>
+struct value_list_element<0u, value_list<Value, Other...>> {
+    /*! @brief Searched value. */
+    static constexpr auto value = Value;
+};
+
+
+/**
+ * @brief Helper type.
+ * @tparam Index Index of the value to return.
+ * @tparam List Value list to search into.
+ */
+template<std::size_t Index, typename List>
+inline constexpr auto value_list_element_v = value_list_element<Index, List>::value;
+
+
+/**
+ * @brief Concatenates multiple value lists.
+ * @tparam Value Values provided by the first value list.
+ * @tparam Other Values provided by the second value list.
+ * @return A value list composed by the values of both the value lists.
+ */
+template<auto... Value, auto... Other>
+constexpr value_list<Value..., Other...> operator+(value_list<Value...>, value_list<Other...>) { return {}; }
+
+
+/*! @brief Primary template isn't defined on purpose. */
+template<typename...>
+struct value_list_cat;
+
+
+/*! @brief Concatenates multiple value lists. */
+template<>
+struct value_list_cat<> {
+    /*! @brief A value list composed by the values of all the value lists. */
+    using type = value_list<>;
+};
+
+
+/**
+ * @brief Concatenates multiple value lists.
+ * @tparam Value Values provided by the first value list.
+ * @tparam Other Values provided by the second value list.
+ * @tparam List Other value lists, if any.
+ */
+template<auto... Value, auto... Other, typename... List>
+struct value_list_cat<value_list<Value...>, value_list<Other...>, List...> {
+    /*! @brief A value list composed by the values of all the value lists. */
+    using type = typename value_list_cat<value_list<Value..., Other...>, List...>::type;
+};
+
+
+/**
+ * @brief Concatenates multiple value lists.
+ * @tparam Value Values provided by the value list.
+ */
+template<auto... Value>
+struct value_list_cat<value_list<Value...>> {
+    /*! @brief A value list composed by the values of all the value lists. */
+    using type = value_list<Value...>;
+};
+
+
+/**
+ * @brief Helper type.
+ * @tparam List Value lists to concatenate.
+ */
+template<typename... List>
+using value_list_cat_t = typename value_list_cat<List...>::type;
+
+
+/*! @brief Same as std::is_invocable, but with tuples. */
+template<typename, typename>
+struct is_applicable: std::false_type {};
+
+
+/**
+ * @copybrief is_applicable
+ * @tparam Func A valid function type.
+ * @tparam Tuple Tuple-like type.
+ * @tparam Args The list of arguments to use to probe the function type.
+ */
+template<typename Func, template<typename...> class Tuple, typename... Args>
+struct is_applicable<Func, Tuple<Args...>>: std::is_invocable<Func, Args...> {};
+
+
+/**
+ * @copybrief is_applicable
+ * @tparam Func A valid function type.
+ * @tparam Tuple Tuple-like type.
+ * @tparam Args The list of arguments to use to probe the function type.
+ */
+template<typename Func, template<typename...> class Tuple, typename... Args>
+struct is_applicable<Func, const Tuple<Args...>>: std::is_invocable<Func, Args...> {};
+
+
+/**
+ * @brief Helper variable template.
+ * @tparam Func A valid function type.
+ * @tparam Args The list of arguments to use to probe the function type.
+ */
+template<typename Func, typename Args>
+inline constexpr bool is_applicable_v = is_applicable<Func, Args>::value;
+
+
+/*! @brief Same as std::is_invocable_r, but with tuples for arguments. */
+template<typename, typename, typename>
+struct is_applicable_r: std::false_type {};
+
+
+/**
+ * @copybrief is_applicable_r
+ * @tparam Ret The type to which the return type of the function should be
+ * convertible.
+ * @tparam Func A valid function type.
+ * @tparam Args The list of arguments to use to probe the function type.
+ */
+template<typename Ret, typename Func, typename... Args>
+struct is_applicable_r<Ret, Func, std::tuple<Args...>>: std::is_invocable_r<Ret, Func, Args...> {};
+
+
+/**
+ * @brief Helper variable template.
+ * @tparam Ret The type to which the return type of the function should be
+ * convertible.
+ * @tparam Func A valid function type.
+ * @tparam Args The list of arguments to use to probe the function type.
+ */
+template<typename Ret, typename Func, typename Args>
+inline constexpr bool is_applicable_r_v = is_applicable_r<Ret, Func, Args>::value;
+
+
+/**
+ * @brief Provides the member constant `value` to true if a given type is
+ * complete, false otherwise.
+ * @tparam Type The type to test.
+ */
+template<typename Type, typename = void>
+struct is_complete: std::false_type {};
+
+
+/*! @copydoc is_complete */
+template<typename Type>
+struct is_complete<Type, std::void_t<decltype(sizeof(Type))>>: std::true_type {};
+
+
+/**
+ * @brief Helper variable template.
+ * @tparam Type The type to test.
+ */
+template<typename Type>
+inline constexpr bool is_complete_v = is_complete<Type>::value;
+
+
+/**
+ * @brief Provides the member constant `value` to true if a given type is an
+ * iterator, false otherwise.
+ * @tparam Type The type to test.
+ */
+template<typename Type, typename = void>
+struct is_iterator: std::false_type {};
+
+
+/*! @copydoc is_iterator */
+template<typename Type>
+struct is_iterator<Type, std::void_t<typename std::iterator_traits<Type>::iterator_category>>
+    : std::true_type
+{};
+
+
+/**
+ * @brief Helper variable template.
+ * @tparam Type The type to test.
+ */
+template<typename Type>
+inline constexpr bool is_iterator_v = is_iterator<Type>::value;
+
+
+/**
+ * @brief Provides the member constant `value` to true if a given type is of the
+ * required iterator type, false otherwise.
+ * @tparam Type The type to test.
+ * @tparam It Required iterator type.
+ */
+template<typename Type, typename It, typename = void>
+struct is_iterator_type: std::false_type {};
+
+
+/*! @copydoc is_iterator_type */
+template<typename Type, typename It>
+struct is_iterator_type<Type, It, std::enable_if_t<is_iterator_v<Type> && std::is_same_v<Type, It>>>
+    : std::true_type
+{};
+
+
+/*! @copydoc is_iterator_type */
+template<typename Type, typename It>
+struct is_iterator_type<Type, It, std::enable_if_t<!std::is_same_v<Type, It>, std::void_t<typename It::iterator_type>>>
+    : is_iterator_type<Type, typename It::iterator_type>
+{};
+
+
+/**
+ * @brief Helper variable template.
+ * @tparam Type The type to test.
+ * @tparam It Required iterator type.
+ */
+template<typename Type, typename It>
+inline constexpr bool is_iterator_type_v = is_iterator_type<Type, It>::value;
+
+
+/**
+ * @cond TURN_OFF_DOXYGEN
+ * Internal details not to be documented.
+ */
+
+
+namespace internal {
+
+
+template<typename>
+[[nodiscard]] constexpr bool is_equality_comparable(...) { return false; }
+
+
+template<typename Type>
+[[nodiscard]] constexpr auto is_equality_comparable(choice_t<0>)
+-> decltype(std::declval<Type>() == std::declval<Type>()) { return true; }
+
+
+template<typename Type>
+[[nodiscard]] constexpr auto is_equality_comparable(choice_t<1>)
+-> decltype(std::declval<typename Type::value_type>(), std::declval<Type>() == std::declval<Type>()) {
+    if constexpr(is_iterator_v<Type>) {
+        return true;
+    } else if constexpr(std::is_same_v<typename Type::value_type, Type>) {
+        return is_equality_comparable<Type>(choice<0>);
+    } else {
+        return is_equality_comparable<typename Type::value_type>(choice<2>);
+    }
+}
+
+
+template<typename Type>
+[[nodiscard]] constexpr auto is_equality_comparable(choice_t<2>)
+-> decltype(std::declval<typename Type::mapped_type>(), std::declval<Type>() == std::declval<Type>()) {
+    return is_equality_comparable<typename Type::key_type>(choice<2>) && is_equality_comparable<typename Type::mapped_type>(choice<2>);
+}
+
+
+}
+
+
+/**
+ * Internal details not to be documented.
+ * @endcond
+ */
+
+
+/**
+ * @brief Provides the member constant `value` to true if a given type is
+ * equality comparable, false otherwise.
+ * @tparam Type The type to test.
+ */
+template<typename Type, typename = void>
+struct is_equality_comparable: std::bool_constant<internal::is_equality_comparable<Type>(choice<2>)> {};
+
+
+/**
+ * @brief Helper variable template.
+ * @tparam Type The type to test.
+ */
+template<class Type>
+inline constexpr bool is_equality_comparable_v = is_equality_comparable<Type>::value;
+
+
+/**
+ * @brief Transcribes the constness of a type to another type.
+ * @tparam To The type to which to transcribe the constness.
+ * @tparam From The type from which to transcribe the constness.
+ */
+template<typename To, typename From>
+struct constness_as {
+    /*! @brief The type resulting from the transcription of the constness. */
+    using type = std::remove_const_t<To>;
+};
+
+
+/*! @copydoc constness_as */
+template<typename To, typename From>
+struct constness_as<To, const From> {
+    /*! @brief The type resulting from the transcription of the constness. */
+    using type = std::add_const_t<To>;
+};
+
+
+/**
+ * @brief Alias template to facilitate the transcription of the constness.
+ * @tparam To The type to which to transcribe the constness.
+ * @tparam From The type from which to transcribe the constness.
+ */
+template<typename To, typename From>
+using constness_as_t = typename constness_as<To, From>::type;
+
+
+/**
+ * @brief Extracts the class of a non-static member object or function.
+ * @tparam Member A pointer to a non-static member object or function.
+ */
+template<typename Member>
+class member_class {
+    static_assert(std::is_member_pointer_v<Member>, "Invalid pointer type to non-static member object or function");
+
+    template<typename Class, typename Ret, typename... Args>
+    static Class * clazz(Ret(Class:: *)(Args...));
+
+    template<typename Class, typename Ret, typename... Args>
+    static Class * clazz(Ret(Class:: *)(Args...) const);
+
+    template<typename Class, typename Type>
+    static Class * clazz(Type Class:: *);
+
+public:
+    /*! @brief The class of the given non-static member object or function. */
+    using type = std::remove_pointer_t<decltype(clazz(std::declval<Member>()))>;
+};
+
+
+/**
+ * @brief Helper type.
+ * @tparam Member A pointer to a non-static member object or function.
+ */
+template<typename Member>
+using member_class_t = typename member_class<Member>::type;
+
+
+}
+
+
+#endif
+
+// #include "entity.hpp"
+#ifndef ENTT_ENTITY_ENTITY_HPP
+#define ENTT_ENTITY_ENTITY_HPP
+
+
+#include <cstddef>
+#include <cstdint>
+#include <type_traits>
+// #include "../config/config.h"
+
+
+
+namespace entt {
+
+
+/**
+ * @cond TURN_OFF_DOXYGEN
+ * Internal details not to be documented.
+ */
+
+
+namespace internal {
+
+
+template<typename, typename = void>
+struct entt_traits;
+
+
+template<typename Type>
+struct entt_traits<Type, std::enable_if_t<std::is_enum_v<Type>>>
+    : entt_traits<std::underlying_type_t<Type>>
+{};
+
+
+template<typename Type>
+struct entt_traits<Type, std::enable_if_t<std::is_class_v<Type>>>
+    : entt_traits<typename Type::entity_type>
+{};
+
+
+template<>
+struct entt_traits<std::uint32_t> {
+    using entity_type = std::uint32_t;
+    using version_type = std::uint16_t;
+    using difference_type = std::int64_t;
+
+    static constexpr entity_type entity_mask = 0xFFFFF;
+    static constexpr entity_type version_mask = 0xFFF;
+    static constexpr std::size_t entity_shift = 20u;
+};
+
+
+template<>
+struct entt_traits<std::uint64_t> {
+    using entity_type = std::uint64_t;
+    using version_type = std::uint32_t;
+    using difference_type = std::int64_t;
+
+    static constexpr entity_type entity_mask = 0xFFFFFFFF;
+    static constexpr entity_type version_mask = 0xFFFFFFFF;
+    static constexpr std::size_t entity_shift = 32u;
+};
+
+
+}
+
+
+/**
+ * Internal details not to be documented.
+ * @endcond
+ */
+
+
+/**
+ * @brief Entity traits.
+ * @tparam Type Type of identifier.
+ */
+template<typename Type>
+class entt_traits: private internal::entt_traits<Type> {
+    using traits_type = internal::entt_traits<Type>;
+
+public:
+    /*! @brief Value type. */
+    using value_type = Type;
+    /*! @brief Underlying entity type. */
+    using entity_type = typename traits_type::entity_type;
+    /*! @brief Underlying version type. */
+    using version_type = typename traits_type::version_type;
+    /*! @brief Difference type. */
+    using difference_type = typename traits_type::difference_type;
+
+    /**
+     * @brief Converts an entity to its underlying type.
+     * @param value The value to convert.
+     * @return The integral representation of the given value.
+     */
+    [[nodiscard]] static constexpr entity_type to_integral(const value_type value) ENTT_NOEXCEPT {
+        return static_cast<entity_type>(value);
+    }
+
+    /**
+     * @brief Returns the entity part once converted to the underlying type.
+     * @param value The value to convert.
+     * @return The integral representation of the entity part.
+     */
+    [[nodiscard]] static constexpr entity_type to_entity(const value_type value) ENTT_NOEXCEPT {
+        return (to_integral(value) & traits_type::entity_mask);
+    }
+
+    /**
+     * @brief Returns the version part once converted to the underlying type.
+     * @param value The value to convert.
+     * @return The integral representation of the version part.
+     */
+    [[nodiscard]] static constexpr version_type to_version(const value_type value) ENTT_NOEXCEPT {
+        constexpr auto mask = (traits_type::version_mask << traits_type::entity_shift);
+        return ((to_integral(value) & mask) >> traits_type::entity_shift);
+    }
+
+    /**
+     * @brief Constructs an identifier from its parts.
+     *
+     * If the version part is not provided, a tombstone is returned.<br/>
+     * If the entity part is not provided, a null identifier is returned.
+     *
+     * @param entity The entity part of the identifier.
+     * @param version The version part of the identifier.
+     * @return A properly constructed identifier.
+     */
+    [[nodiscard]] static constexpr value_type construct(const entity_type entity = traits_type::entity_mask, const version_type version = traits_type::version_mask) ENTT_NOEXCEPT {
+        return value_type{(entity & traits_type::entity_mask) | (static_cast<entity_type>(version) << traits_type::entity_shift)};
+    }
+};
+
+
+/**
+ * @brief Converts an entity to its underlying type.
+ * @tparam Entity The value type.
+ * @param entity The value to convert.
+ * @return The integral representation of the given value.
+ */
+template<typename Entity>
+[[nodiscard]] constexpr auto to_integral(const Entity entity) ENTT_NOEXCEPT {
+    return entt_traits<Entity>::to_integral(entity);
+}
+
+
+/*! @brief Null object for all entity identifiers.  */
+struct null_t {
+    /**
+     * @brief Converts the null object to identifiers of any type.
+     * @tparam Entity Type of entity identifier.
+     * @return The null representation for the given type.
+     */
+    template<typename Entity>
+    [[nodiscard]] constexpr operator Entity() const ENTT_NOEXCEPT {
+        return entt_traits<Entity>::construct();
+    }
+
+    /**
+     * @brief Compares two null objects.
+     * @param other A null object.
+     * @return True in all cases.
+     */
+    [[nodiscard]] constexpr bool operator==([[maybe_unused]] const null_t other) const ENTT_NOEXCEPT {
+        return true;
+    }
+
+    /**
+     * @brief Compares two null objects.
+     * @param other A null object.
+     * @return False in all cases.
+     */
+    [[nodiscard]] constexpr bool operator!=([[maybe_unused]] const null_t other) const ENTT_NOEXCEPT {
+        return false;
+    }
+
+    /**
+     * @brief Compares a null object and an entity identifier of any type.
+     * @tparam Entity Type of entity identifier.
+     * @param entity Entity identifier with which to compare.
+     * @return False if the two elements differ, true otherwise.
+     */
+    template<typename Entity>
+    [[nodiscard]] constexpr bool operator==(const Entity entity) const ENTT_NOEXCEPT {
+        return entt_traits<Entity>::to_entity(entity) == entt_traits<Entity>::to_entity(*this);
+    }
+
+    /**
+     * @brief Compares a null object and an entity identifier of any type.
+     * @tparam Entity Type of entity identifier.
+     * @param entity Entity identifier with which to compare.
+     * @return True if the two elements differ, false otherwise.
+     */
+    template<typename Entity>
+    [[nodiscard]] constexpr bool operator!=(const Entity entity) const ENTT_NOEXCEPT {
+        return !(entity == *this);
+    }
+
+    /**
+     * @brief Creates a null object from an entity identifier of any type.
+     * @tparam Entity Type of entity identifier.
+     * @param entity Entity identifier to turn into a null object.
+     * @return The null representation for the given identifier.
+     */
+    template<typename Entity>
+    [[nodiscard]] constexpr Entity operator|(const Entity entity) const ENTT_NOEXCEPT {
+        return entt_traits<Entity>::construct(entt_traits<Entity>::to_entity(*this), entt_traits<Entity>::to_version(entity));
+    }
+};
+
+
+/**
+ * @brief Compares a null object and an entity identifier of any type.
+ * @tparam Entity Type of entity identifier.
+ * @param entity Entity identifier with which to compare.
+ * @param other A null object yet to be converted.
+ * @return False if the two elements differ, true otherwise.
+ */
+template<typename Entity>
+[[nodiscard]] constexpr bool operator==(const Entity entity, const null_t other) ENTT_NOEXCEPT {
+    return other.operator==(entity);
+}
+
+
+/**
+ * @brief Compares a null object and an entity identifier of any type.
+ * @tparam Entity Type of entity identifier.
+ * @param entity Entity identifier with which to compare.
+ * @param other A null object yet to be converted.
+ * @return True if the two elements differ, false otherwise.
+ */
+template<typename Entity>
+[[nodiscard]] constexpr bool operator!=(const Entity entity, const null_t other) ENTT_NOEXCEPT {
+    return !(other == entity);
+}
+
+
+/*! @brief Tombstone object for all entity identifiers.  */
+struct tombstone_t {
+    /**
+     * @brief Converts the tombstone object to identifiers of any type.
+     * @tparam Entity Type of entity identifier.
+     * @return The tombstone representation for the given type.
+     */
+    template<typename Entity>
+    [[nodiscard]] constexpr operator Entity() const ENTT_NOEXCEPT {
+        return entt_traits<Entity>::construct();
+    }
+
+    /**
+     * @brief Compares two tombstone objects.
+     * @param other A tombstone object.
+     * @return True in all cases.
+     */
+    [[nodiscard]] constexpr bool operator==([[maybe_unused]] const tombstone_t other) const ENTT_NOEXCEPT {
+        return true;
+    }
+
+    /**
+     * @brief Compares two tombstone objects.
+     * @param other A tombstone object.
+     * @return False in all cases.
+     */
+    [[nodiscard]] constexpr bool operator!=([[maybe_unused]] const tombstone_t other) const ENTT_NOEXCEPT {
+        return false;
+    }
+
+    /**
+     * @brief Compares a tombstone object and an entity identifier of any type.
+     * @tparam Entity Type of entity identifier.
+     * @param entity Entity identifier with which to compare.
+     * @return False if the two elements differ, true otherwise.
+     */
+    template<typename Entity>
+    [[nodiscard]] constexpr bool operator==(const Entity entity) const ENTT_NOEXCEPT {
+        return entt_traits<Entity>::to_version(entity) == entt_traits<Entity>::to_version(*this);
+    }
+
+    /**
+     * @brief Compares a tombstone object and an entity identifier of any type.
+     * @tparam Entity Type of entity identifier.
+     * @param entity Entity identifier with which to compare.
+     * @return True if the two elements differ, false otherwise.
+     */
+    template<typename Entity>
+    [[nodiscard]] constexpr bool operator!=(const Entity entity) const ENTT_NOEXCEPT {
+        return !(entity == *this);
+    }
+
+    /**
+     * @brief Creates a tombstone object from an entity identifier of any type.
+     * @tparam Entity Type of entity identifier.
+     * @param entity Entity identifier to turn into a tombstone object.
+     * @return The tombstone representation for the given identifier.
+     */
+    template<typename Entity>
+    [[nodiscard]] constexpr Entity operator|(const Entity entity) const ENTT_NOEXCEPT {
+        return entt_traits<Entity>::construct(entt_traits<Entity>::to_entity(entity));
+    }
+};
+
+
+/**
+ * @brief Compares a tombstone object and an entity identifier of any type.
+ * @tparam Entity Type of entity identifier.
+ * @param entity Entity identifier with which to compare.
+ * @param other A tombstone object yet to be converted.
+ * @return False if the two elements differ, true otherwise.
+ */
+template<typename Entity>
+[[nodiscard]] constexpr bool operator==(const Entity entity, const tombstone_t other) ENTT_NOEXCEPT {
+    return other.operator==(entity);
+}
+
+
+/**
+ * @brief Compares a tombstone object and an entity identifier of any type.
+ * @tparam Entity Type of entity identifier.
+ * @param entity Entity identifier with which to compare.
+ * @param other A tombstone object yet to be converted.
+ * @return True if the two elements differ, false otherwise.
+ */
+template<typename Entity>
+[[nodiscard]] constexpr bool operator!=(const Entity entity, const tombstone_t other) ENTT_NOEXCEPT {
+    return !(other == entity);
+}
+
+
+/**
+ * @brief Compile-time constant for null entities.
+ *
+ * There exist implicit conversions from this variable to entity identifiers of
+ * any allowed type. Similarly, there exist comparision operators between the
+ * null entity and any other entity identifier.
+ */
+inline constexpr null_t null{};
+
+
+/**
+ * @brief Compile-time constant for tombstone entities.
+ *
+ * There exist implicit conversions from this variable to entity identifiers of
+ * any allowed type. Similarly, there exist comparision operators between the
+ * tombstone entity and any other entity identifier.
+ */
+inline constexpr tombstone_t tombstone{};
+
+
+}
+
+
+#endif
+
+// #include "fwd.hpp"
+#ifndef ENTT_ENTITY_FWD_HPP
+#define ENTT_ENTITY_FWD_HPP
+
+
+#include <memory>
+// #include "../core/fwd.hpp"
+#ifndef ENTT_CORE_FWD_HPP
+#define ENTT_CORE_FWD_HPP
+
+
+#include <type_traits>
+// #include "../config/config.h"
+
+
+
+namespace entt {
+
+
+template<std::size_t Len = sizeof(double[2]), std::size_t = alignof(typename std::aligned_storage_t<Len + !Len>)>
+class basic_any;
+
+
+/*! @brief Alias declaration for type identifiers. */
+using id_type = ENTT_ID_TYPE;
+
+
+/*! @brief Alias declaration for the most common use case. */
+using any = basic_any<>;
+
+
+}
+
+
+#endif
+
+
+
+namespace entt {
+
+
+template<typename Entity, typename = std::allocator<Entity>>
+class basic_sparse_set;
+
+
+template<typename, typename Type, typename = std::allocator<Type>>
+struct basic_storage;
+
+
+template<typename>
+class basic_registry;
+
+
+template<typename...>
+struct basic_view;
+
+
+template<typename>
+class basic_runtime_view;
+
+
+template<typename...>
+class basic_group;
+
+
+template<typename>
+class basic_observer;
+
+
+template<typename>
+class basic_organizer;
+
+
+template<typename, typename...>
+struct basic_handle;
+
+
+template<typename>
+class basic_snapshot;
+
+
+template<typename>
+class basic_snapshot_loader;
+
+
+template<typename>
+class basic_continuous_loader;
+
+
+/*! @brief Default entity identifier. */
+enum class entity: id_type {};
+
+
+/*! @brief Alias declaration for the most common use case. */
+using sparse_set = basic_sparse_set<entity>;
+
+
+/**
+ * @brief Alias declaration for the most common use case.
+ * @tparam Args Other template parameters.
+ */
+template<typename... Args>
+using storage = basic_storage<entity, Args...>;
+
+
+/*! @brief Alias declaration for the most common use case. */
+using registry = basic_registry<entity>;
+
+
+/*! @brief Alias declaration for the most common use case. */
+using observer = basic_observer<entity>;
+
+
+/*! @brief Alias declaration for the most common use case. */
+using organizer = basic_organizer<entity>;
+
+
+/*! @brief Alias declaration for the most common use case. */
+using handle = basic_handle<entity>;
+
+
+/*! @brief Alias declaration for the most common use case. */
+using const_handle = basic_handle<const entity>;
+
+
+/**
+ * @brief Alias declaration for the most common use case.
+ * @tparam Args Other template parameters.
+ */
+template<typename... Args>
+using handle_view = basic_handle<entity, Args...>;
+
+
+/**
+ * @brief Alias declaration for the most common use case.
+ * @tparam Args Other template parameters.
+ */
+template<typename... Args>
+using const_handle_view = basic_handle<const entity, Args...>;
+
+
+/*! @brief Alias declaration for the most common use case. */
+using snapshot = basic_snapshot<entity>;
+
+
+/*! @brief Alias declaration for the most common use case. */
+using snapshot_loader = basic_snapshot_loader<entity>;
+
+
+/*! @brief Alias declaration for the most common use case. */
+using continuous_loader = basic_continuous_loader<entity>;
+
+
+/**
+ * @brief Alias declaration for the most common use case.
+ * @tparam Args Other template parameters.
+ */
+template<typename... Args>
+using view = basic_view<entity, Args...>;
+
+
+/*! @brief Alias declaration for the most common use case. */
+using runtime_view = basic_runtime_view<entity>;
+
+
+/**
+ * @brief Alias declaration for the most common use case.
+ * @tparam Args Other template parameters.
+ */
+template<typename... Args>
+using group = basic_group<entity, Args...>;
+
+
+}
+
+
+#endif
+
+// #include "sparse_set.hpp"
+#ifndef ENTT_ENTITY_SPARSE_SET_HPP
+#define ENTT_ENTITY_SPARSE_SET_HPP
+
+
+#include <cstddef>
+#include <iterator>
+#include <memory>
+#include <type_traits>
+#include <utility>
+// #include "../config/config.h"
+
+// #include "../core/algorithm.hpp"
+#ifndef ENTT_CORE_ALGORITHM_HPP
+#define ENTT_CORE_ALGORITHM_HPP
+
+
+#include <vector>
+#include <utility>
+#include <iterator>
+#include <algorithm>
+#include <functional>
+// #include "utility.hpp"
+#ifndef ENTT_CORE_UTILITY_HPP
+#define ENTT_CORE_UTILITY_HPP
+
+
+#include <utility>
+// #include "../config/config.h"
+
+
+
+namespace entt {
+
+
+/*! @brief Identity function object (waiting for C++20). */
+struct identity {
+    /**
+     * @brief Returns its argument unchanged.
+     * @tparam Type Type of the argument.
+     * @param value The actual argument.
+     * @return The submitted value as-is.
+     */
+    template<class Type>
+    [[nodiscard]] constexpr Type && operator()(Type &&value) const ENTT_NOEXCEPT {
+        return std::forward<Type>(value);
+    }
+};
+
+
+/**
+ * @brief Constant utility to disambiguate overloaded members of a class.
+ * @tparam Type Type of the desired overload.
+ * @tparam Class Type of class to which the member belongs.
+ * @param member A valid pointer to a member.
+ * @return Pointer to the member.
+ */
+template<typename Type, typename Class>
+[[nodiscard]] constexpr auto overload(Type Class:: *member) ENTT_NOEXCEPT { return member; }
+
+
+/**
+ * @brief Constant utility to disambiguate overloaded functions.
+ * @tparam Func Function type of the desired overload.
+ * @param func A valid pointer to a function.
+ * @return Pointer to the function.
+ */
+template<typename Func>
+[[nodiscard]] constexpr auto overload(Func *func) ENTT_NOEXCEPT { return func; }
+
+
+/**
+ * @brief Helper type for visitors.
+ * @tparam Func Types of function objects.
+ */
+template<class... Func>
+struct overloaded: Func... {
+    using Func::operator()...;
 };
 
 
 /**
  * @brief Deduction guide.
- *
- * It allows to deduce the character type of the hashed string directly from a
- * human-readable identifer provided to the constructor.
- *
- * @tparam Char Character type.
- * @tparam N Number of characters of the identifier.
- * @param str Human-readable identifer.
- */
-template<typename Char, std::size_t N>
-basic_hashed_string(const Char (&str)[N]) ENTT_NOEXCEPT
--> basic_hashed_string<Char>;
-
-
-/**
- * @brief Compares two hashed strings.
- * @tparam Char Character type.
- * @param lhs A valid hashed string.
- * @param rhs A valid hashed string.
- * @return True if the two hashed strings are identical, false otherwise.
- */
-template<typename Char>
-constexpr bool operator!=(const basic_hashed_string<Char> &lhs, const basic_hashed_string<Char> &rhs) ENTT_NOEXCEPT {
-    return !(lhs == rhs);
-}
-
-
-/*! @brief Aliases for common character types. */
-using hashed_string = basic_hashed_string<char>;
-
-
-/*! @brief Aliases for common character types. */
-using hashed_wstring = basic_hashed_string<wchar_t>;
-
-
-}
-
-
-/**
- * @brief User defined literal for hashed strings.
- * @param str The literal without its suffix.
- * @return A properly initialized hashed string.
- */
-constexpr entt::hashed_string operator"" ENTT_HS_SUFFIX(const char *str, std::size_t) ENTT_NOEXCEPT {
-    return entt::hashed_string{str};
-}
-
-
-/**
- * @brief User defined literal for hashed wstrings.
- * @param str The literal without its suffix.
- * @return A properly initialized hashed wstring.
- */
-constexpr entt::hashed_wstring operator"" ENTT_HWS_SUFFIX(const wchar_t *str, std::size_t) ENTT_NOEXCEPT {
-    return entt::hashed_wstring{str};
-}
-
-
-#endif // ENTT_CORE_HASHED_STRING_HPP
-
-
-
-namespace entt {
-
-
-/*! @brief A class to use to push around lists of types, nothing more. */
-template<typename...>
-struct type_list {};
-
-
-/*! @brief Primary template isn't defined on purpose. */
-template<typename>
-struct type_list_size;
-
-
-/**
- * @brief Compile-time number of elements in a type list.
+ * @tparam Func Types of function objects.
+ */
+template<class... Func>
+overloaded(Func...)
+-> overloaded<Func...>;
+
+
+/**
+ * @brief Basic implementation of a y-combinator.
+ * @tparam Func Type of a potentially recursive function.
+ */
+template<class Func>
+struct y_combinator {
+    /**
+     * @brief Constructs a y-combinator from a given function.
+     * @param recursive A potentially recursive function.
+     */
+    y_combinator(Func recursive):
+        func{std::move(recursive)}
+    {}
+
+    /**
+     * @brief Invokes a y-combinator and therefore its underlying function.
+     * @tparam Args Types of arguments to use to invoke the underlying function.
+     * @param args Parameters to use to invoke the underlying function.
+     * @return Return value of the underlying function, if any.
+     */
+    template <class... Args>
+    decltype(auto) operator()(Args &&... args) const {
+        return func(*this, std::forward<Args>(args)...);
+    }
+
+    /*! @copydoc operator()() */
+    template <class... Args>
+    decltype(auto) operator()(Args &&... args) {
+        return func(*this, std::forward<Args>(args)...);
+    }
+
+private:
+    Func func;
+};
+
+
+}
+
+
+#endif
+
+
+
+namespace entt {
+
+
+/**
+ * @brief Function object to wrap `std::sort` in a class type.
+ *
+ * Unfortunately, `std::sort` cannot be passed as template argument to a class
+ * template or a function template.<br/>
+ * This class fills the gap by wrapping some flavors of `std::sort` in a
+ * function object.
+ */
+struct std_sort {
+    /**
+     * @brief Sorts the elements in a range.
+     *
+     * Sorts the elements in a range using the given binary comparison function.
+     *
+     * @tparam It Type of random access iterator.
+     * @tparam Compare Type of comparison function object.
+     * @tparam Args Types of arguments to forward to the sort function.
+     * @param first An iterator to the first element of the range to sort.
+     * @param last An iterator past the last element of the range to sort.
+     * @param compare A valid comparison function object.
+     * @param args Arguments to forward to the sort function, if any.
+     */
+    template<typename It, typename Compare = std::less<>, typename... Args>
+    void operator()(It first, It last, Compare compare = Compare{}, Args &&... args) const {
+        std::sort(std::forward<Args>(args)..., std::move(first), std::move(last), std::move(compare));
+    }
+};
+
+
+/*! @brief Function object for performing insertion sort. */
+struct insertion_sort {
+    /**
+     * @brief Sorts the elements in a range.
+     *
+     * Sorts the elements in a range using the given binary comparison function.
+     *
+     * @tparam It Type of random access iterator.
+     * @tparam Compare Type of comparison function object.
+     * @param first An iterator to the first element of the range to sort.
+     * @param last An iterator past the last element of the range to sort.
+     * @param compare A valid comparison function object.
+     */
+    template<typename It, typename Compare = std::less<>>
+    void operator()(It first, It last, Compare compare = Compare{}) const {
+        if(first < last) {
+            for(auto it = first + 1; it < last; ++it) {
+                auto value = std::move(*it);
+                auto pre = it;
+
+                for(; pre > first && compare(value, *(pre-1)); --pre) {
+                    *pre = std::move(*(pre-1));
+                }
+
+                *pre = std::move(value);
+            }
+        }
+    }
+};
+
+
+/**
+ * @brief Function object for performing LSD radix sort.
+ * @tparam Bit Number of bits processed per pass.
+ * @tparam N Maximum number of bits to sort.
+ */
+template<std::size_t Bit, std::size_t N>
+struct radix_sort {
+    static_assert((N % Bit) == 0, "The maximum number of bits to sort must be a multiple of the number of bits processed per pass");
+
+    /**
+     * @brief Sorts the elements in a range.
+     *
+     * Sorts the elements in a range using the given _getter_ to access the
+     * actual data to be sorted.
+     *
+     * This implementation is inspired by the online book
+     * [Physically Based Rendering](http://www.pbr-book.org/3ed-2018/Primitives_and_Intersection_Acceleration/Bounding_Volume_Hierarchies.html#RadixSort).
+     *
+     * @tparam It Type of random access iterator.
+     * @tparam Getter Type of _getter_ function object.
+     * @param first An iterator to the first element of the range to sort.
+     * @param last An iterator past the last element of the range to sort.
+     * @param getter A valid _getter_ function object.
+     */
+    template<typename It, typename Getter = identity>
+    void operator()(It first, It last, Getter getter = Getter{}) const {
+        if(first < last) {
+            static constexpr auto mask = (1 << Bit) - 1;
+            static constexpr auto buckets = 1 << Bit;
+            static constexpr auto passes = N / Bit;
+
+            using value_type = typename std::iterator_traits<It>::value_type;
+            std::vector<value_type> aux(std::distance(first, last));
+
+            auto part = [getter = std::move(getter)](auto from, auto to, auto out, auto start) {
+                std::size_t index[buckets]{};
+                std::size_t count[buckets]{};
+
+                for(auto it = from; it != to; ++it) {
+                    ++count[(getter(*it) >> start) & mask];
+                }
+
+                for(std::size_t pos{}, end = buckets - 1u; pos < end; ++pos) {
+                    index[pos + 1u] = index[pos] + count[pos];
+                }
+
+                for(auto it = from; it != to; ++it) {
+                    out[index[(getter(*it) >> start) & mask]++] = std::move(*it);
+                }
+            };
+
+            for(std::size_t pass = 0; pass < (passes & ~1); pass += 2) {
+                part(first, last, aux.begin(), pass * Bit);
+                part(aux.begin(), aux.end(), first, (pass + 1) * Bit);
+            }
+
+            if constexpr(passes & 1) {
+                part(first, last, aux.begin(), (passes - 1) * Bit);
+                std::move(aux.begin(), aux.end(), first);
+            }
+        }
+    }
+};
+
+
+}
+
+
+#endif
+
+// #include "../core/fwd.hpp"
+
+// #include "entity.hpp"
+
+// #include "fwd.hpp"
+
+
+
+namespace entt {
+
+
+/*! @brief Sparse set deletion policy. */
+enum class deletion_policy: std::uint8_t {
+    /*! @brief Swap-and-pop deletion policy. */
+    swap_and_pop = 0u,
+    /*! @brief In-place deletion policy. */
+    in_place = 1u
+};
+
+
+/**
+ * @brief Basic sparse set implementation.
+ *
+ * Sparse set or packed array or whatever is the name users give it.<br/>
+ * Two arrays: an _external_ one and an _internal_ one; a _sparse_ one and a
+ * _packed_ one; one used for direct access through contiguous memory, the other
+ * one used to get the data through an extra level of indirection.<br/>
+ * This is largely used by the registry to offer users the fastest access ever
+ * to the components. Views and groups in general are almost entirely designed
+ * around sparse sets.
+ *
+ * This type of data structure is widely documented in the literature and on the
+ * web. This is nothing more than a customized implementation suitable for the
+ * purpose of the framework.
+ *
+ * @note
+ * Internal data structures arrange elements to maximize performance. There are
+ * no guarantees that entities are returned in the insertion order when iterate
+ * a sparse set. Do not make assumption on the order in any case.
+ *
+ * @tparam Entity A valid entity type (see entt_traits for more details).
+ * @tparam Allocator Type of allocator used to manage memory and elements.
+ */
+template<typename Entity, typename Allocator>
+class basic_sparse_set {
+    static constexpr auto growth_factor = 1.5;
+    static constexpr auto sparse_page = ENTT_SPARSE_PAGE;
+
+    using traits_type = entt_traits<Entity>;
+
+    using alloc_traits = typename std::allocator_traits<Allocator>::template rebind_traits<Entity>;
+    using alloc_pointer = typename alloc_traits::pointer;
+    using alloc_const_pointer = typename alloc_traits::const_pointer;
+
+    using bucket_alloc_traits = typename std::allocator_traits<Allocator>::template rebind_traits<alloc_pointer>;
+    using bucket_alloc_pointer = typename bucket_alloc_traits::pointer;
+
+    static_assert(alloc_traits::propagate_on_container_move_assignment::value);
+    static_assert(bucket_alloc_traits::propagate_on_container_move_assignment::value);
+
+    struct sparse_set_iterator final {
+        using difference_type = typename traits_type::difference_type;
+        using value_type = Entity;
+        using pointer = const value_type *;
+        using reference = const value_type &;
+        using iterator_category = std::random_access_iterator_tag;
+
+        sparse_set_iterator() ENTT_NOEXCEPT = default;
+
+        sparse_set_iterator(const alloc_const_pointer *ref, const difference_type idx) ENTT_NOEXCEPT
+            : packed{ref},
+              index{idx}
+        {}
+
+        sparse_set_iterator & operator++() ENTT_NOEXCEPT {
+            return --index, *this;
+        }
+
+        sparse_set_iterator operator++(int) ENTT_NOEXCEPT {
+            iterator orig = *this;
+            return ++(*this), orig;
+        }
+
+        sparse_set_iterator & operator--() ENTT_NOEXCEPT {
+            return ++index, *this;
+        }
+
+        sparse_set_iterator operator--(int) ENTT_NOEXCEPT {
+            sparse_set_iterator orig = *this;
+            return operator--(), orig;
+        }
+
+        sparse_set_iterator & operator+=(const difference_type value) ENTT_NOEXCEPT {
+            index -= value;
+            return *this;
+        }
+
+        sparse_set_iterator operator+(const difference_type value) const ENTT_NOEXCEPT {
+            sparse_set_iterator copy = *this;
+            return (copy += value);
+        }
+
+        sparse_set_iterator & operator-=(const difference_type value) ENTT_NOEXCEPT {
+            return (*this += -value);
+        }
+
+        sparse_set_iterator operator-(const difference_type value) const ENTT_NOEXCEPT {
+            return (*this + -value);
+        }
+
+        difference_type operator-(const sparse_set_iterator &other) const ENTT_NOEXCEPT {
+            return other.index - index;
+        }
+
+        [[nodiscard]] reference operator[](const difference_type value) const {
+            const auto pos = size_type(index-value-1u);
+            return (*packed)[pos];
+        }
+
+        [[nodiscard]] bool operator==(const sparse_set_iterator &other) const ENTT_NOEXCEPT {
+            return other.index == index;
+        }
+
+        [[nodiscard]] bool operator!=(const sparse_set_iterator &other) const ENTT_NOEXCEPT {
+            return !(*this == other);
+        }
+
+        [[nodiscard]] bool operator<(const sparse_set_iterator &other) const ENTT_NOEXCEPT {
+            return index > other.index;
+        }
+
+        [[nodiscard]] bool operator>(const sparse_set_iterator &other) const ENTT_NOEXCEPT {
+            return index < other.index;
+        }
+
+        [[nodiscard]] bool operator<=(const sparse_set_iterator &other) const ENTT_NOEXCEPT {
+            return !(*this > other);
+        }
+
+        [[nodiscard]] bool operator>=(const sparse_set_iterator &other) const ENTT_NOEXCEPT {
+            return !(*this < other);
+        }
+
+        [[nodiscard]] pointer operator->() const {
+            const auto pos = size_type(index-1u);
+            return std::addressof((*packed)[pos]);
+        }
+
+        [[nodiscard]] reference operator*() const {
+            return *operator->();
+        }
+
+    private:
+        const alloc_const_pointer *packed;
+        difference_type index;
+    };
+
+    [[nodiscard]] static auto page(const Entity entt) ENTT_NOEXCEPT {
+        return static_cast<size_type>(traits_type::to_entity(entt) / sparse_page);
+    }
+
+    [[nodiscard]] static auto offset(const Entity entt) ENTT_NOEXCEPT {
+        return static_cast<size_type>(traits_type::to_entity(entt) & (sparse_page - 1));
+    }
+
+    [[nodiscard]] auto assure_page(const std::size_t idx) {
+        if(!(idx < bucket)) {
+            const size_type sz = idx + 1u;
+            const auto mem = bucket_alloc_traits::allocate(bucket_allocator, sz);
+
+            std::uninitialized_value_construct(mem + bucket, mem + sz);
+            std::uninitialized_copy(sparse, sparse + bucket, mem);
+
+            std::destroy(sparse, sparse + bucket);
+            bucket_alloc_traits::deallocate(bucket_allocator, sparse, bucket);
+
+            sparse = mem;
+            bucket = sz;
+        }
+
+        if(!sparse[idx]) {
+            sparse[idx] = alloc_traits::allocate(allocator, sparse_page);
+            std::uninitialized_fill(sparse[idx], sparse[idx] + sparse_page, null);
+        }
+
+        return sparse[idx];
+    }
+
+    void resize_packed(const std::size_t req) {
+        ENTT_ASSERT((req != reserved) && !(req < count), "Invalid request");
+        const auto mem = alloc_traits::allocate(allocator, req);
+
+        std::uninitialized_copy(packed, packed + count, mem);
+        std::uninitialized_fill(mem + count, mem + req, tombstone);
+
+        std::destroy(packed, packed + reserved);
+        alloc_traits::deallocate(allocator, packed, reserved);
+
+        packed = mem;
+        reserved = req;
+    }
+
+    void release_memory() {
+        if(packed) {
+            for(size_type pos{}; pos < bucket; ++pos) {
+                if(sparse[pos]) {
+                    std::destroy(sparse[pos], sparse[pos] + sparse_page);
+                    alloc_traits::deallocate(allocator, sparse[pos], sparse_page);
+                }
+            }
+
+            std::destroy(packed, packed + reserved);
+            std::destroy(sparse, sparse + bucket);
+            alloc_traits::deallocate(allocator, packed, reserved);
+            bucket_alloc_traits::deallocate(bucket_allocator, sparse, bucket);
+        }
+    }
+
+protected:
+    /**
+     * @brief Swaps two entities in the internal packed array.
+     * @param lhs A valid position of an entity within storage.
+     * @param rhs A valid position of an entity within storage.
+     */
+    virtual void swap_at([[maybe_unused]] const std::size_t lhs, [[maybe_unused]] const std::size_t rhs) {}
+
+    /**
+     * @brief Moves an entity in the internal packed array.
+     * @param from A valid position of an entity within storage.
+     * @param to A valid position of an entity within storage.
+     */
+    virtual void move_and_pop([[maybe_unused]] const std::size_t from, [[maybe_unused]] const std::size_t to) {}
+
+    /**
+     * @brief Attempts to erase an entity from the internal packed array.
+     * @param entt A valid entity identifier.
+     * @param ud Optional user data that are forwarded as-is to derived classes.
+     */
+    virtual void swap_and_pop(const Entity entt, [[maybe_unused]] void *ud) {
+        auto &ref = sparse[page(entt)][offset(entt)];
+        const auto pos = static_cast<size_type>(traits_type::to_entity(ref));
+        ENTT_ASSERT(packed[pos] == entt, "Invalid entity identifier");
+        auto &last = packed[--count];
+
+        packed[pos] = last;
+        sparse[page(last)][offset(last)] = ref;
+        // lazy self-assignment guard
+        ref = null;
+        // unnecessary but it helps to detect nasty bugs
+        ENTT_ASSERT((last = tombstone, true), "");
+    }
+
+    /**
+     * @brief Attempts to erase an entity from the internal packed array.
+     * @param entt A valid entity identifier.
+     * @param ud Optional user data that are forwarded as-is to derived classes.
+     */
+    virtual void in_place_pop(const Entity entt, [[maybe_unused]] void *ud) {
+        auto &ref = sparse[page(entt)][offset(entt)];
+        const auto pos = static_cast<size_type>(traits_type::to_entity(ref));
+        ENTT_ASSERT(packed[pos] == entt, "Invalid entity identifier");
+
+        packed[pos] = std::exchange(free_list, traits_type::construct(static_cast<typename traits_type::entity_type>(pos)));
+        // lazy self-assignment guard
+        ref = null;
+    }
+
+public:
+    /*! @brief Allocator type. */
+    using allocator_type = typename alloc_traits::allocator_type;
+    /*! @brief Underlying entity identifier. */
+    using entity_type = Entity;
+    /*! @brief Unsigned integer type. */
+    using size_type = std::size_t;
+    /*! @brief Pointer type to contained entities. */
+    using pointer = alloc_const_pointer;
+    /*! @brief Random access iterator type. */
+    using iterator = sparse_set_iterator;
+    /*! @brief Reverse iterator type. */
+    using reverse_iterator = std::reverse_iterator<iterator>;
+
+    /**
+     * @brief Constructs an empty container with the given policy and allocator.
+     * @param pol Type of deletion policy.
+     * @param alloc Allocator to use (possibly default-constructed).
+     */
+    explicit basic_sparse_set(deletion_policy pol, const allocator_type &alloc = {})
+        : allocator{alloc},
+          bucket_allocator{alloc},
+          sparse{bucket_alloc_traits::allocate(bucket_allocator, 0u)},
+          packed{alloc_traits::allocate(allocator, 0u)},
+          bucket{0u},
+          count{0u},
+          reserved{0u},
+          free_list{tombstone},
+          mode{pol}
+    {}
+
+    /**
+     * @brief Constructs an empty container with the given allocator.
+     * @param alloc Allocator to use (possibly default-constructed).
+     */
+    explicit basic_sparse_set(const allocator_type &alloc = {})
+        : basic_sparse_set{deletion_policy::swap_and_pop, alloc}
+    {}
+
+    /**
+     * @brief Move constructor.
+     * @param other The instance to move from.
+     */
+    basic_sparse_set(basic_sparse_set &&other) ENTT_NOEXCEPT
+        : allocator{std::move(other.allocator)},
+          bucket_allocator{std::move(other.bucket_allocator)},
+          sparse{std::exchange(other.sparse, bucket_alloc_pointer{})},
+          packed{std::exchange(other.packed, alloc_pointer{})},
+          bucket{std::exchange(other.bucket, 0u)},
+          count{std::exchange(other.count, 0u)},
+          reserved{std::exchange(other.reserved, 0u)},
+          free_list{std::exchange(other.free_list, tombstone)},
+          mode{other.mode}
+    {}
+
+    /*! @brief Default destructor. */
+    virtual ~basic_sparse_set() {
+        release_memory();
+    }
+
+    /**
+     * @brief Move assignment operator.
+     * @param other The instance to move from.
+     * @return This sparse set.
+     */
+    basic_sparse_set & operator=(basic_sparse_set &&other) ENTT_NOEXCEPT {
+        release_memory();
+
+        allocator = std::move(other.allocator);
+        bucket_allocator = std::move(other.bucket_allocator);
+        sparse = std::exchange(other.sparse, bucket_alloc_pointer{});
+        packed = std::exchange(other.packed, alloc_pointer{});
+        bucket = std::exchange(other.bucket, 0u);
+        count = std::exchange(other.count, 0u);
+        reserved = std::exchange(other.reserved, 0u);
+        free_list = std::exchange(other.free_list, tombstone);
+        mode = other.mode;
+
+        return *this;
+    }
+
+    /**
+     * @brief Returns the deletion policy of a sparse set.
+     * @return The deletion policy of the sparse set.
+     */
+    [[nodiscard]] deletion_policy policy() const ENTT_NOEXCEPT {
+        return mode;
+    }
+
+    /**
+     * @brief Returns the next slot available for insertion.
+     * @return The next slot available for insertion.
+     */
+    [[nodiscard]] size_type slot() const ENTT_NOEXCEPT {
+        return free_list == null ? count : static_cast<size_type>(traits_type::to_entity(free_list));
+    }
+
+    /**
+     * @brief Increases the capacity of a sparse set.
+     *
+     * If the new capacity is greater than the current capacity, new storage is
+     * allocated, otherwise the method does nothing.
+     *
+     * @param cap Desired capacity.
+     */
+    void reserve(const size_type cap) {
+        if(cap > reserved) {
+            resize_packed(cap);
+        }
+    }
+
+    /**
+     * @brief Returns the number of elements that a sparse set has currently
+     * allocated space for.
+     * @return Capacity of the sparse set.
+     */
+    [[nodiscard]] size_type capacity() const ENTT_NOEXCEPT {
+        return reserved;
+    }
+
+    /*! @brief Requests the removal of unused capacity. */
+    void shrink_to_fit() {
+        if(count < reserved) {
+            resize_packed(count);
+        }
+    }
+
+    /**
+     * @brief Returns the extent of a sparse set.
+     *
+     * The extent of a sparse set is also the size of the internal sparse array.
+     * There is no guarantee that the internal packed array has the same size.
+     * Usually the size of the internal sparse array is equal or greater than
+     * the one of the internal packed array.
+     *
+     * @return Extent of the sparse set.
+     */
+    [[nodiscard]] size_type extent() const ENTT_NOEXCEPT {
+        return bucket * sparse_page;
+    }
+
+    /**
+     * @brief Returns the number of elements in a sparse set.
+     *
+     * The number of elements is also the size of the internal packed array.
+     * There is no guarantee that the internal sparse array has the same size.
+     * Usually the size of the internal sparse array is equal or greater than
+     * the one of the internal packed array.
+     *
+     * @return Number of elements.
+     */
+    [[nodiscard]] size_type size() const ENTT_NOEXCEPT {
+        return count;
+    }
+
+    /**
+     * @brief Checks whether a sparse set is empty.
+     * @return True if the sparse set is empty, false otherwise.
+     */
+    [[nodiscard]] bool empty() const ENTT_NOEXCEPT {
+        return (count == size_type{});
+    }
+
+    /**
+     * @brief Direct access to the internal packed array.
+     * @return A pointer to the internal packed array.
+     */
+    [[nodiscard]] pointer data() const ENTT_NOEXCEPT {
+        return packed;
+    }
+
+    /**
+     * @brief Returns an iterator to the beginning.
+     *
+     * The returned iterator points to the first entity of the internal packed
+     * array. If the sparse set is empty, the returned iterator will be equal to
+     * `end()`.
+     *
+     * @return An iterator to the first entity of the internal packed array.
+     */
+    [[nodiscard]] iterator begin() const ENTT_NOEXCEPT {
+        return iterator{std::addressof(packed), static_cast<typename traits_type::difference_type>(count)};
+    }
+
+    /**
+     * @brief Returns an iterator to the end.
+     *
+     * The returned iterator points to the element following the last entity in
+     * the internal packed array. Attempting to dereference the returned
+     * iterator results in undefined behavior.
+     *
+     * @return An iterator to the element following the last entity of the
+     * internal packed array.
+     */
+    [[nodiscard]] iterator end() const ENTT_NOEXCEPT {
+        return iterator{std::addressof(packed), {}};
+    }
+
+    /**
+     * @brief Returns a reverse iterator to the beginning.
+     *
+     * The returned iterator points to the first entity of the reversed internal
+     * packed array. If the sparse set is empty, the returned iterator will be
+     * equal to `rend()`.
+     *
+     * @return An iterator to the first entity of the reversed internal packed
+     * array.
+     */
+    [[nodiscard]] reverse_iterator rbegin() const ENTT_NOEXCEPT {
+        return std::make_reverse_iterator(end());
+    }
+
+    /**
+     * @brief Returns a reverse iterator to the end.
+     *
+     * The returned iterator points to the element following the last entity in
+     * the reversed internal packed array. Attempting to dereference the
+     * returned iterator results in undefined behavior.
+     *
+     * @return An iterator to the element following the last entity of the
+     * reversed internal packed array.
+     */
+    [[nodiscard]] reverse_iterator rend() const ENTT_NOEXCEPT {
+        return std::make_reverse_iterator(begin());
+    }
+
+    /**
+     * @brief Finds an entity.
+     * @param entt A valid entity identifier.
+     * @return An iterator to the given entity if it's found, past the end
+     * iterator otherwise.
+     */
+    [[nodiscard]] iterator find(const entity_type entt) const ENTT_NOEXCEPT {
+        return contains(entt) ? --(end() - index(entt)) : end();
+    }
+
+    /**
+     * @brief Checks if a sparse set contains an entity.
+     * @param entt A valid entity identifier.
+     * @return True if the sparse set contains the entity, false otherwise.
+     */
+    [[nodiscard]] bool contains(const entity_type entt) const ENTT_NOEXCEPT {
+        ENTT_ASSERT(entt != tombstone && entt != null, "Invalid entity");
+        const auto curr = page(entt);
+        // testing versions permits to avoid accessing the packed array
+        return (curr < bucket && sparse[curr] && sparse[curr][offset(entt)] != null);
+    }
+
+    /**
+     * @brief Returns the position of an entity in a sparse set.
+     *
+     * @warning
+     * Attempting to get the position of an entity that doesn't belong to the
+     * sparse set results in undefined behavior.
+     *
+     * @param entt A valid entity identifier.
+     * @return The position of the entity in the sparse set.
+     */
+    [[nodiscard]] size_type index(const entity_type entt) const ENTT_NOEXCEPT {
+        ENTT_ASSERT(contains(entt), "Set does not contain entity");
+        return static_cast<size_type>(traits_type::to_entity(sparse[page(entt)][offset(entt)]));
+    }
+
+    /**
+     * @brief Returns the entity at specified location, with bounds checking.
+     * @param pos The position for which to return the entity.
+     * @return The entity at specified location if any, a null entity otherwise.
+     */
+    [[nodiscard]] entity_type at(const size_type pos) const ENTT_NOEXCEPT {
+        return pos < count ? packed[pos] : null;
+    }
+
+    /**
+     * @brief Returns the entity at specified location, without bounds checking.
+     * @param pos The position for which to return the entity.
+     * @return The entity at specified location.
+     */
+    [[nodiscard]] entity_type operator[](const size_type pos) const ENTT_NOEXCEPT {
+        ENTT_ASSERT(pos < count, "Position is out of bounds");
+        return packed[pos];
+    }
+
+    /**
+     * @brief Appends an entity to a sparse set.
+     *
+     * @warning
+     * Attempting to assign an entity that already belongs to the sparse set
+     * results in undefined behavior.
+     *
+     * @param entt A valid entity identifier.
+     * @return The slot used for insertion.
+     */
+    size_type emplace_back(const entity_type entt) {
+        ENTT_ASSERT(!contains(entt), "Set already contains entity");
+
+        if(count == reserved) {
+            const size_type sz = static_cast<size_type>(reserved * growth_factor);
+            resize_packed(sz + !(sz > reserved));
+        }
+
+        assure_page(page(entt))[offset(entt)] = traits_type::construct(static_cast<typename traits_type::entity_type>(count));
+        packed[count] = entt;
+        return count++;
+    }
+
+    /**
+     * @brief Assigns an entity to a sparse set.
+     *
+     * @warning
+     * Attempting to assign an entity that already belongs to the sparse set
+     * results in undefined behavior.
+     *
+     * @param entt A valid entity identifier.
+     * @return The slot used for insertion.
+     */
+    size_type emplace(const entity_type entt) {
+        if(free_list == null) {
+            return emplace_back(entt);
+        } else {
+            ENTT_ASSERT(!contains(entt), "Set already contains entity");
+            const auto pos = static_cast<size_type>(traits_type::to_entity(free_list));
+            assure_page(page(entt))[offset(entt)] = traits_type::construct(static_cast<typename traits_type::entity_type>(pos));
+            free_list = std::exchange(packed[pos], entt);
+            return pos;
+        }
+    }
+
+    /**
+     * @brief Assigns one or more entities to a sparse set.
+     *
+     * @warning
+     * Attempting to assign an entity that already belongs to the sparse set
+     * results in undefined behavior.
+     *
+     * @tparam It Type of input iterator.
+     * @param first An iterator to the first element of the range of entities.
+     * @param last An iterator past the last element of the range of entities.
+     */
+    template<typename It>
+    void insert(It first, It last) {
+        reserve(count + std::distance(first, last));
+
+        for(; first != last; ++first) {
+            const auto entt = *first;
+            ENTT_ASSERT(!contains(entt), "Set already contains entity");
+            assure_page(page(entt))[offset(entt)] = traits_type::construct(static_cast<typename traits_type::entity_type>(count));
+            packed[count++] = entt;
+        }
+    }
+
+    /**
+     * @brief Erases an entity from a sparse set.
+     *
+     * @warning
+     * Attempting to erase an entity that doesn't belong to the sparse set
+     * results in undefined behavior.
+     *
+     * @param entt A valid entity identifier.
+     * @param ud Optional user data that are forwarded as-is to derived classes.
+     */
+    void erase(const entity_type entt, void *ud = nullptr) {
+        ENTT_ASSERT(contains(entt), "Set does not contain entity");
+        (mode == deletion_policy::in_place) ? in_place_pop(entt, ud) : swap_and_pop(entt, ud);
+    }
+
+    /**
+     * @brief Erases entities from a set.
+     *
+     * @sa erase
+     *
+     * @tparam It Type of input iterator.
+     * @param first An iterator to the first element of the range of entities.
+     * @param last An iterator past the last element of the range of entities.
+     * @param ud Optional user data that are forwarded as-is to derived classes.
+     */
+    template<typename It>
+    void erase(It first, It last, void *ud = nullptr) {
+        for(; first != last; ++first) {
+            erase(*first, ud);
+        }
+    }
+
+    /**
+     * @brief Removes an entity from a sparse set if it exists.
+     * @param entt A valid entity identifier.
+     * @param ud Optional user data that are forwarded as-is to derived classes.
+     * @return True if the entity is actually removed, false otherwise.
+     */
+    bool remove(const entity_type entt, void *ud = nullptr) {
+        return contains(entt) && (erase(entt, ud), true);
+    }
+
+    /**
+     * @brief Removes entities from a sparse set if they exist.
+     * @tparam It Type of input iterator.
+     * @param first An iterator to the first element of the range of entities.
+     * @param last An iterator past the last element of the range of entities.
+     * @param ud Optional user data that are forwarded as-is to derived classes.
+     * @return The number of entities actually removed.
+     */
+    template<typename It>
+    size_type remove(It first, It last, void *ud = nullptr) {
+        size_type found{};
+
+        for(; first != last; ++first) {
+            found += remove(*first, ud);
+        }
+
+        return found;
+    }
+
+    /*! @brief Removes all tombstones from the packed array of a sparse set. */
+    void compact() {
+        size_type next = count;
+        for(; next && packed[next - 1u] == tombstone; --next);
+
+        for(auto *it = &free_list; *it != null && next; it = std::addressof(packed[traits_type::to_entity(*it)])) {
+            if(const size_type pos = traits_type::to_entity(*it); pos < next) {
+                --next;
+                move_and_pop(next, pos);
+                std::swap(packed[next], packed[pos]);
+                sparse[page(packed[pos])][offset(packed[pos])] = traits_type::construct(static_cast<const typename traits_type::entity_type>(pos));
+                *it = traits_type::construct(static_cast<typename traits_type::entity_type>(next));
+                for(; next && packed[next - 1u] == tombstone; --next);
+            }
+        }
+
+        free_list = tombstone;
+        count = next;
+    }
+
+    /**
+     * @copybrief swap_at
+     *
+     * For what it's worth, this function affects both the internal sparse array
+     * and the internal packed array. Users should not care of that anyway.
+     *
+     * @warning
+     * Attempting to swap entities that don't belong to the sparse set results
+     * in undefined behavior.
+     *
+     * @param lhs A valid entity identifier.
+     * @param rhs A valid entity identifier.
+     */
+    void swap(const entity_type lhs, const entity_type rhs) {
+        ENTT_ASSERT(contains(lhs), "Set does not contain entity");
+        ENTT_ASSERT(contains(rhs), "Set does not contain entity");
+
+        auto &entt = sparse[page(lhs)][offset(lhs)];
+        auto &other = sparse[page(rhs)][offset(rhs)];
+
+        const auto from = static_cast<size_type>(traits_type::to_entity(entt));
+        const auto to = static_cast<size_type>(traits_type::to_entity(other));
+
+        // basic no-leak guarantee (with invalid state) if swapping throws
+        swap_at(from, to);
+        std::swap(entt, other);
+        std::swap(packed[from], packed[to]);
+    }
+
+    /**
+     * @brief Sort the first count elements according to the given comparison
+     * function.
+     *
+     * The comparison function object must return `true` if the first element
+     * is _less_ than the second one, `false` otherwise. The signature of the
+     * comparison function should be equivalent to the following:
+     *
+     * @code{.cpp}
+     * bool(const Entity, const Entity);
+     * @endcode
+     *
+     * Moreover, the comparison function object shall induce a
+     * _strict weak ordering_ on the values.
+     *
+     * The sort function object must offer a member function template
+     * `operator()` that accepts three arguments:
+     *
+     * * An iterator to the first element of the range to sort.
+     * * An iterator past the last element of the range to sort.
+     * * A comparison function to use to compare the elements.
+     *
+     * @tparam Compare Type of comparison function object.
+     * @tparam Sort Type of sort function object.
+     * @tparam Args Types of arguments to forward to the sort function object.
+     * @param length Number of elements to sort.
+     * @param compare A valid comparison function object.
+     * @param algo A valid sort function object.
+     * @param args Arguments to forward to the sort function object, if any.
+     */
+    template<typename Compare, typename Sort = std_sort, typename... Args>
+    void sort_n(const size_type length, Compare compare, Sort algo = Sort{}, Args &&... args) {
+        // basic no-leak guarantee (with invalid state) if sorting throws
+        ENTT_ASSERT(!(length > count), "Length exceeds the number of elements");
+        compact();
+
+        algo(std::make_reverse_iterator(packed + length), std::make_reverse_iterator(packed), std::move(compare), std::forward<Args>(args)...);
+
+        for(size_type pos{}; pos < length; ++pos) {
+            auto curr = pos;
+            auto next = index(packed[curr]);
+
+            while(curr != next) {
+                const auto idx = index(packed[next]);
+                const auto entt = packed[curr];
+
+                swap_at(next, idx);
+                sparse[page(entt)][offset(entt)] = traits_type::construct(static_cast<typename traits_type::entity_type>(curr));
+                curr = std::exchange(next, idx);
+            }
+        }
+    }
+
+    /**
+     * @brief Sort all elements according to the given comparison function.
+     *
+     * @sa sort_n
+     *
+     * @tparam Compare Type of comparison function object.
+     * @tparam Sort Type of sort function object.
+     * @tparam Args Types of arguments to forward to the sort function object.
+     * @param compare A valid comparison function object.
+     * @param algo A valid sort function object.
+     * @param args Arguments to forward to the sort function object, if any.
+     */
+    template<typename Compare, typename Sort = std_sort, typename... Args>
+    void sort(Compare compare, Sort algo = Sort{}, Args &&... args) {
+        sort_n(count, std::move(compare), std::move(algo), std::forward<Args>(args)...);
+    }
+
+    /**
+     * @brief Sort entities according to their order in another sparse set.
+     *
+     * Entities that are part of both the sparse sets are ordered internally
+     * according to the order they have in `other`. All the other entities goes
+     * to the end of the list and there are no guarantees on their order.<br/>
+     * In other terms, this function can be used to impose the same order on two
+     * sets by using one of them as a master and the other one as a slave.
+     *
+     * Iterating the sparse set with a couple of iterators returns elements in
+     * the expected order after a call to `respect`. See `begin` and `end` for
+     * more details.
+     *
+     * @param other The sparse sets that imposes the order of the entities.
+     */
+    void respect(const basic_sparse_set &other) {
+        compact();
+
+        const auto to = other.end();
+        auto from = other.begin();
+
+        for(size_type pos = count - 1; pos && from != to; ++from) {
+            if(contains(*from)) {
+                if(*from != packed[pos]) {
+                    // basic no-leak guarantee (with invalid state) if swapping throws
+                    swap(packed[pos], *from);
+                }
+
+                --pos;
+            }
+        }
+    }
+
+    /**
+     * @brief Clears a sparse set.
+     * @param ud Optional user data that are forwarded as-is to derived classes.
+     */
+    void clear(void *ud = nullptr) {
+        for(auto &&entity: *this) {
+            if(entity != tombstone) {
+                in_place_pop(entity, ud);
+            }
+        }
+
+        free_list = tombstone;
+        count = 0u;
+    }
+
+private:
+    typename alloc_traits::allocator_type allocator;
+    typename bucket_alloc_traits::allocator_type bucket_allocator;
+    bucket_alloc_pointer sparse;
+    alloc_pointer packed;
+    std::size_t bucket;
+    std::size_t count;
+    std::size_t reserved;
+    entity_type free_list;
+    deletion_policy mode;
+};
+
+
+}
+
+
+#endif
+
+// #include "storage.hpp"
+#ifndef ENTT_ENTITY_STORAGE_HPP
+#define ENTT_ENTITY_STORAGE_HPP
+
+
+#include <cstddef>
+#include <iterator>
+#include <memory>
+#include <tuple>
+#include <type_traits>
+#include <utility>
+// #include "../config/config.h"
+
+// #include "../core/algorithm.hpp"
+
+// #include "../core/fwd.hpp"
+
+// #include "../core/type_traits.hpp"
+
+// #include "../signal/sigh.hpp"
+#ifndef ENTT_SIGNAL_SIGH_HPP
+#define ENTT_SIGNAL_SIGH_HPP
+
+
+#include <vector>
+#include <utility>
+#include <iterator>
+#include <algorithm>
+#include <functional>
+#include <type_traits>
+// #include "../config/config.h"
+#ifndef ENTT_CONFIG_CONFIG_H
+#define ENTT_CONFIG_CONFIG_H
+
+
+#if defined(__cpp_exceptions) && !defined(ENTT_NOEXCEPTION)
+#   define ENTT_NOEXCEPT noexcept
+#   define ENTT_THROW throw
+#   define ENTT_TRY try
+#   define ENTT_CATCH catch(...)
+#else
+#   define ENTT_NOEXCEPT
+#   define ENTT_THROW
+#   define ENTT_TRY if(true)
+#   define ENTT_CATCH if(false)
+#endif
+
+
+#if defined(__cpp_lib_launder) && __cpp_lib_launder >= 201606L
+#   include <new>
+#   define ENTT_LAUNDER(expr) std::launder(expr)
+#else
+#   define ENTT_LAUNDER(expr) expr
+#endif
+
+
+#ifndef ENTT_USE_ATOMIC
+#   define ENTT_MAYBE_ATOMIC(Type) Type
+#else
+#   include <atomic>
+#   define ENTT_MAYBE_ATOMIC(Type) std::atomic<Type>
+#endif
+
+
+#ifndef ENTT_ID_TYPE
+#   include <cstdint>
+#   define ENTT_ID_TYPE std::uint32_t
+#endif
+
+
+#ifdef ENTT_SPARSE_PAGE
+	static_assert(ENTT_SPARSE_PAGE && ((ENTT_SPARSE_PAGE & (ENTT_SPARSE_PAGE - 1)) == 0), "ENTT_SPARSE_PAGE must be a power of two");
+#else
+#   define ENTT_SPARSE_PAGE 4096
+#endif
+
+
+#ifdef ENTT_PACKED_PAGE
+static_assert(ENTT_PACKED_PAGE && ((ENTT_PACKED_PAGE & (ENTT_PACKED_PAGE - 1)) == 0), "ENTT_PACKED_PAGE must be a power of two");
+#else
+#   define ENTT_PACKED_PAGE 1024
+#endif
+
+
+#ifdef ENTT_DISABLE_ASSERT
+#   undef ENTT_ASSERT
+#   define ENTT_ASSERT(...) (void(0))
+#elif !defined ENTT_ASSERT
+#   include <cassert>
+#   define ENTT_ASSERT(condition, ...) assert(condition)
+#endif
+
+
+#ifdef ENTT_NO_ETO
+#   include <type_traits>
+#   define ENTT_IGNORE_IF_EMPTY std::false_type
+#else
+#   include <type_traits>
+#   define ENTT_IGNORE_IF_EMPTY std::true_type
+#endif
+
+
+#ifndef ENTT_STANDARD_CPP
+#    if defined __clang__ || defined __GNUC__
+#       define ENTT_PRETTY_FUNCTION __PRETTY_FUNCTION__
+#       define ENTT_PRETTY_FUNCTION_PREFIX '='
+#       define ENTT_PRETTY_FUNCTION_SUFFIX ']'
+#    elif defined _MSC_VER
+#       define ENTT_PRETTY_FUNCTION __FUNCSIG__
+#       define ENTT_PRETTY_FUNCTION_PREFIX '<'
+#       define ENTT_PRETTY_FUNCTION_SUFFIX '>'
+#   endif
+#endif
+
+
+#endif
+
+// #include "delegate.hpp"
+#ifndef ENTT_SIGNAL_DELEGATE_HPP
+#define ENTT_SIGNAL_DELEGATE_HPP
+
+
+#include <tuple>
+#include <cstddef>
+#include <utility>
+#include <functional>
+#include <type_traits>
+// #include "../core/type_traits.hpp"
+#ifndef ENTT_CORE_TYPE_TRAITS_HPP
+#define ENTT_CORE_TYPE_TRAITS_HPP
+
+
+#include <cstddef>
+#include <iterator>
+#include <type_traits>
+#include <utility>
+// #include "../config/config.h"
+#ifndef ENTT_CONFIG_CONFIG_H
+#define ENTT_CONFIG_CONFIG_H
+
+
+#if defined(__cpp_exceptions) && !defined(ENTT_NOEXCEPTION)
+#   define ENTT_NOEXCEPT noexcept
+#   define ENTT_THROW throw
+#   define ENTT_TRY try
+#   define ENTT_CATCH catch(...)
+#else
+#   define ENTT_NOEXCEPT
+#   define ENTT_THROW
+#   define ENTT_TRY if(true)
+#   define ENTT_CATCH if(false)
+#endif
+
+
+#if defined(__cpp_lib_launder) && __cpp_lib_launder >= 201606L
+#   include <new>
+#   define ENTT_LAUNDER(expr) std::launder(expr)
+#else
+#   define ENTT_LAUNDER(expr) expr
+#endif
+
+
+#ifndef ENTT_USE_ATOMIC
+#   define ENTT_MAYBE_ATOMIC(Type) Type
+#else
+#   include <atomic>
+#   define ENTT_MAYBE_ATOMIC(Type) std::atomic<Type>
+#endif
+
+
+#ifndef ENTT_ID_TYPE
+#   include <cstdint>
+#   define ENTT_ID_TYPE std::uint32_t
+#endif
+
+
+#ifdef ENTT_SPARSE_PAGE
+	static_assert(ENTT_SPARSE_PAGE && ((ENTT_SPARSE_PAGE & (ENTT_SPARSE_PAGE - 1)) == 0), "ENTT_SPARSE_PAGE must be a power of two");
+#else
+#   define ENTT_SPARSE_PAGE 4096
+#endif
+
+
+#ifdef ENTT_PACKED_PAGE
+static_assert(ENTT_PACKED_PAGE && ((ENTT_PACKED_PAGE & (ENTT_PACKED_PAGE - 1)) == 0), "ENTT_PACKED_PAGE must be a power of two");
+#else
+#   define ENTT_PACKED_PAGE 1024
+#endif
+
+
+#ifdef ENTT_DISABLE_ASSERT
+#   undef ENTT_ASSERT
+#   define ENTT_ASSERT(...) (void(0))
+#elif !defined ENTT_ASSERT
+#   include <cassert>
+#   define ENTT_ASSERT(condition, ...) assert(condition)
+#endif
+
+
+#ifdef ENTT_NO_ETO
+#   include <type_traits>
+#   define ENTT_IGNORE_IF_EMPTY std::false_type
+#else
+#   include <type_traits>
+#   define ENTT_IGNORE_IF_EMPTY std::true_type
+#endif
+
+
+#ifndef ENTT_STANDARD_CPP
+#    if defined __clang__ || defined __GNUC__
+#       define ENTT_PRETTY_FUNCTION __PRETTY_FUNCTION__
+#       define ENTT_PRETTY_FUNCTION_PREFIX '='
+#       define ENTT_PRETTY_FUNCTION_SUFFIX ']'
+#    elif defined _MSC_VER
+#       define ENTT_PRETTY_FUNCTION __FUNCSIG__
+#       define ENTT_PRETTY_FUNCTION_PREFIX '<'
+#       define ENTT_PRETTY_FUNCTION_SUFFIX '>'
+#   endif
+#endif
+
+
+#endif
+
+// #include "fwd.hpp"
+#ifndef ENTT_CORE_FWD_HPP
+#define ENTT_CORE_FWD_HPP
+
+
+#include <type_traits>
+// #include "../config/config.h"
+
+
+
+namespace entt {
+
+
+template<std::size_t Len = sizeof(double[2]), std::size_t = alignof(typename std::aligned_storage_t<Len + !Len>)>
+class basic_any;
+
+
+/*! @brief Alias declaration for type identifiers. */
+using id_type = ENTT_ID_TYPE;
+
+
+/*! @brief Alias declaration for the most common use case. */
+using any = basic_any<>;
+
+
+}
+
+
+#endif
+
+
+
+namespace entt {
+
+
+/**
+ * @brief Utility class to disambiguate overloaded functions.
+ * @tparam N Number of choices available.
+ */
+template<std::size_t N>
+struct choice_t
+    // Unfortunately, doxygen cannot parse such a construct.
+    /*! @cond TURN_OFF_DOXYGEN */
+    : choice_t<N-1>
+    /*! @endcond */
+{};
+
+
+/*! @copybrief choice_t */
+template<>
+struct choice_t<0> {};
+
+
+/**
+ * @brief Variable template for the choice trick.
+ * @tparam N Number of choices available.
+ */
+template<std::size_t N>
+inline constexpr choice_t<N> choice{};
+
+
+/**
+ * @brief Identity type trait.
+ *
+ * Useful to establish non-deduced contexts in template argument deduction
+ * (waiting for C++20) or to provide types through function arguments.
+ *
+ * @tparam Type A type.
+ */
+template<typename Type>
+struct type_identity {
+    /*! @brief Identity type. */
+    using type = Type;
+};
+
+
+/**
+ * @brief Helper type.
+ * @tparam Type A type.
+ */
+template<typename Type>
+using type_identity_t = typename type_identity<Type>::type;
+
+
+/**
+ * @brief A type-only `sizeof` wrapper that returns 0 where `sizeof` complains.
+ * @tparam Type The type of which to return the size.
+ * @tparam The size of the type if `sizeof` accepts it, 0 otherwise.
+ */
+template<typename Type, typename = void>
+struct size_of: std::integral_constant<std::size_t, 0u> {};
+
+
+/*! @copydoc size_of */
+template<typename Type>
+struct size_of<Type, std::void_t<decltype(sizeof(Type))>>
+    : std::integral_constant<std::size_t, sizeof(Type)>
+{};
+
+
+/**
+ * @brief Helper variable template.
+ * @tparam Type The type of which to return the size.
+ */
+template<class Type>
+inline constexpr std::size_t size_of_v = size_of<Type>::value;
+
+
+/**
+ * @brief Using declaration to be used to _repeat_ the same type a number of
+ * times equal to the size of a given parameter pack.
+ * @tparam Type A type to repeat.
+ */
+template<typename Type, typename>
+using unpack_as_t = Type;
+
+
+/**
+ * @brief Helper variable template to be used to _repeat_ the same value a
+ * number of times equal to the size of a given parameter pack.
+ * @tparam Value A value to repeat.
+ */
+template<auto Value, typename>
+inline constexpr auto unpack_as_v = Value;
+
+
+/**
+ * @brief Wraps a static constant.
+ * @tparam Value A static constant.
+ */
+template<auto Value>
+using integral_constant = std::integral_constant<decltype(Value), Value>;
+
+
+/**
+ * @brief Alias template to facilitate the creation of named values.
+ * @tparam Value A constant value at least convertible to `id_type`.
+ */
+template<id_type Value>
+using tag = integral_constant<Value>;
+
+
+/**
+ * @brief A class to use to push around lists of types, nothing more.
  * @tparam Type Types provided by the type list.
  */
 template<typename... Type>
-struct type_list_size<type_list<Type...>>
-        : std::integral_constant<std::size_t, sizeof...(Type)>
-{};
-
-
-/**
- * @brief Helper variable template.
- * @tparam List Type list.
- */
-template<class List>
-constexpr auto type_list_size_v = type_list_size<List>::value;
+struct type_list {
+    /*! @brief Type list type. */
+    using type = type_list;
+    /*! @brief Compile-time number of elements in the type list. */
+    static constexpr auto size = sizeof...(Type);
+};
+
+
+/*! @brief Primary template isn't defined on purpose. */
+template<std::size_t, typename>
+struct type_list_element;
+
+
+/**
+ * @brief Provides compile-time indexed access to the types of a type list.
+ * @tparam Index Index of the type to return.
+ * @tparam Type First type provided by the type list.
+ * @tparam Other Other types provided by the type list.
+ */
+template<std::size_t Index, typename Type, typename... Other>
+struct type_list_element<Index, type_list<Type, Other...>>
+    : type_list_element<Index - 1u, type_list<Other...>>
+{};
+
+
+/**
+ * @brief Provides compile-time indexed access to the types of a type list.
+ * @tparam Type First type provided by the type list.
+ * @tparam Other Other types provided by the type list.
+ */
+template<typename Type, typename... Other>
+struct type_list_element<0u, type_list<Type, Other...>> {
+    /*! @brief Searched type. */
+    using type = Type;
+};
+
+
+/**
+ * @brief Helper type.
+ * @tparam Index Index of the type to return.
+ * @tparam List Type list to search into.
+ */
+template<std::size_t Index, typename List>
+using type_list_element_t = typename type_list_element<Index, List>::type;
+
+
+/**
+ * @brief Concatenates multiple type lists.
+ * @tparam Type Types provided by the first type list.
+ * @tparam Other Types provided by the second type list.
+ * @return A type list composed by the types of both the type lists.
+ */
+template<typename... Type, typename... Other>
+constexpr type_list<Type..., Other...> operator+(type_list<Type...>, type_list<Other...>) { return {}; }
 
 
 /*! @brief Primary template isn't defined on purpose. */
@@ -1100,342 +7313,3547 @@
 using type_list_unique_t = typename type_list_unique<Type>::type;
 
 
-/*! @brief Traits class used mainly to push things across boundaries. */
-template<typename>
-struct named_type_traits;
-
-
-/**
- * @brief Specialization used to get rid of constness.
- * @tparam Type Named type.
- */
-template<typename Type>
-struct named_type_traits<const Type>
-        : named_type_traits<Type>
-{};
-
-
-/**
- * @brief Helper type.
- * @tparam Type Potentially named type.
- */
-template<typename Type>
-using named_type_traits_t = typename named_type_traits<Type>::type;
-
-
-/**
- * @brief Provides the member constant `value` to true if a given type has a
- * name. In all other cases, `value` is false.
- */
-template<typename, typename = std::void_t<>>
-struct is_named_type: std::false_type {};
-
-
-/**
- * @brief Provides the member constant `value` to true if a given type has a
- * name. In all other cases, `value` is false.
- * @tparam Type Potentially named type.
- */
-template<typename Type>
-struct is_named_type<Type, std::void_t<named_type_traits_t<std::decay_t<Type>>>>: std::true_type {};
-
-
-/**
- * @brief Helper variable template.
- * @tparam Type Potentially named type.
+/**
+ * @brief Provides the member constant `value` to true if a type list contains a
+ * given type, false otherwise.
+ * @tparam List Type list.
+ * @tparam Type Type to look for.
+ */
+template<typename List, typename Type>
+struct type_list_contains;
+
+
+/**
+ * @copybrief type_list_contains
+ * @tparam Type Types provided by the type list.
+ * @tparam Other Type to look for.
+ */
+template<typename... Type, typename Other>
+struct type_list_contains<type_list<Type...>, Other>: std::disjunction<std::is_same<Type, Other>...> {};
+
+
+/**
+ * @brief Helper variable template.
+ * @tparam List Type list.
+ * @tparam Type Type to look for.
+ */
+template<class List, typename Type>
+inline constexpr bool type_list_contains_v = type_list_contains<List, Type>::value;
+
+
+/*! @brief Primary template isn't defined on purpose. */
+template<typename...>
+struct type_list_diff;
+
+
+/**
+ * @brief Computes the difference between two type lists.
+ * @tparam Type Types provided by the first type list.
+ * @tparam Other Types provided by the second type list.
+ */
+template<typename... Type, typename... Other>
+struct type_list_diff<type_list<Type...>, type_list<Other...>> {
+    /*! @brief A type list that is the difference between the two type lists. */
+    using type = type_list_cat_t<std::conditional_t<type_list_contains_v<type_list<Other...>, Type>, type_list<>, type_list<Type>>...>;
+};
+
+
+/**
+ * @brief Helper type.
+ * @tparam List Type lists between which to compute the difference.
+ */
+template<typename... List>
+using type_list_diff_t = typename type_list_diff<List...>::type;
+
+
+/**
+ * @brief A class to use to push around lists of constant values, nothing more.
+ * @tparam Value Values provided by the value list.
+ */
+template<auto... Value>
+struct value_list {
+    /*! @brief Value list type. */
+    using type = value_list;
+    /*! @brief Compile-time number of elements in the value list. */
+    static constexpr auto size = sizeof...(Value);
+};
+
+
+/*! @brief Primary template isn't defined on purpose. */
+template<std::size_t, typename>
+struct value_list_element;
+
+
+/**
+ * @brief Provides compile-time indexed access to the values of a value list.
+ * @tparam Index Index of the value to return.
+ * @tparam Value First value provided by the value list.
+ * @tparam Other Other values provided by the value list.
+ */
+template<std::size_t Index, auto Value, auto... Other>
+struct value_list_element<Index, value_list<Value, Other...>>
+    : value_list_element<Index - 1u, value_list<Other...>>
+{};
+
+
+/**
+ * @brief Provides compile-time indexed access to the types of a type list.
+ * @tparam Value First value provided by the value list.
+ * @tparam Other Other values provided by the value list.
+ */
+template<auto Value, auto... Other>
+struct value_list_element<0u, value_list<Value, Other...>> {
+    /*! @brief Searched value. */
+    static constexpr auto value = Value;
+};
+
+
+/**
+ * @brief Helper type.
+ * @tparam Index Index of the value to return.
+ * @tparam List Value list to search into.
+ */
+template<std::size_t Index, typename List>
+inline constexpr auto value_list_element_v = value_list_element<Index, List>::value;
+
+
+/**
+ * @brief Concatenates multiple value lists.
+ * @tparam Value Values provided by the first value list.
+ * @tparam Other Values provided by the second value list.
+ * @return A value list composed by the values of both the value lists.
+ */
+template<auto... Value, auto... Other>
+constexpr value_list<Value..., Other...> operator+(value_list<Value...>, value_list<Other...>) { return {}; }
+
+
+/*! @brief Primary template isn't defined on purpose. */
+template<typename...>
+struct value_list_cat;
+
+
+/*! @brief Concatenates multiple value lists. */
+template<>
+struct value_list_cat<> {
+    /*! @brief A value list composed by the values of all the value lists. */
+    using type = value_list<>;
+};
+
+
+/**
+ * @brief Concatenates multiple value lists.
+ * @tparam Value Values provided by the first value list.
+ * @tparam Other Values provided by the second value list.
+ * @tparam List Other value lists, if any.
+ */
+template<auto... Value, auto... Other, typename... List>
+struct value_list_cat<value_list<Value...>, value_list<Other...>, List...> {
+    /*! @brief A value list composed by the values of all the value lists. */
+    using type = typename value_list_cat<value_list<Value..., Other...>, List...>::type;
+};
+
+
+/**
+ * @brief Concatenates multiple value lists.
+ * @tparam Value Values provided by the value list.
+ */
+template<auto... Value>
+struct value_list_cat<value_list<Value...>> {
+    /*! @brief A value list composed by the values of all the value lists. */
+    using type = value_list<Value...>;
+};
+
+
+/**
+ * @brief Helper type.
+ * @tparam List Value lists to concatenate.
+ */
+template<typename... List>
+using value_list_cat_t = typename value_list_cat<List...>::type;
+
+
+/*! @brief Same as std::is_invocable, but with tuples. */
+template<typename, typename>
+struct is_applicable: std::false_type {};
+
+
+/**
+ * @copybrief is_applicable
+ * @tparam Func A valid function type.
+ * @tparam Tuple Tuple-like type.
+ * @tparam Args The list of arguments to use to probe the function type.
+ */
+template<typename Func, template<typename...> class Tuple, typename... Args>
+struct is_applicable<Func, Tuple<Args...>>: std::is_invocable<Func, Args...> {};
+
+
+/**
+ * @copybrief is_applicable
+ * @tparam Func A valid function type.
+ * @tparam Tuple Tuple-like type.
+ * @tparam Args The list of arguments to use to probe the function type.
+ */
+template<typename Func, template<typename...> class Tuple, typename... Args>
+struct is_applicable<Func, const Tuple<Args...>>: std::is_invocable<Func, Args...> {};
+
+
+/**
+ * @brief Helper variable template.
+ * @tparam Func A valid function type.
+ * @tparam Args The list of arguments to use to probe the function type.
+ */
+template<typename Func, typename Args>
+inline constexpr bool is_applicable_v = is_applicable<Func, Args>::value;
+
+
+/*! @brief Same as std::is_invocable_r, but with tuples for arguments. */
+template<typename, typename, typename>
+struct is_applicable_r: std::false_type {};
+
+
+/**
+ * @copybrief is_applicable_r
+ * @tparam Ret The type to which the return type of the function should be
+ * convertible.
+ * @tparam Func A valid function type.
+ * @tparam Args The list of arguments to use to probe the function type.
+ */
+template<typename Ret, typename Func, typename... Args>
+struct is_applicable_r<Ret, Func, std::tuple<Args...>>: std::is_invocable_r<Ret, Func, Args...> {};
+
+
+/**
+ * @brief Helper variable template.
+ * @tparam Ret The type to which the return type of the function should be
+ * convertible.
+ * @tparam Func A valid function type.
+ * @tparam Args The list of arguments to use to probe the function type.
+ */
+template<typename Ret, typename Func, typename Args>
+inline constexpr bool is_applicable_r_v = is_applicable_r<Ret, Func, Args>::value;
+
+
+/**
+ * @brief Provides the member constant `value` to true if a given type is
+ * complete, false otherwise.
+ * @tparam Type The type to test.
+ */
+template<typename Type, typename = void>
+struct is_complete: std::false_type {};
+
+
+/*! @copydoc is_complete */
+template<typename Type>
+struct is_complete<Type, std::void_t<decltype(sizeof(Type))>>: std::true_type {};
+
+
+/**
+ * @brief Helper variable template.
+ * @tparam Type The type to test.
+ */
+template<typename Type>
+inline constexpr bool is_complete_v = is_complete<Type>::value;
+
+
+/**
+ * @brief Provides the member constant `value` to true if a given type is an
+ * iterator, false otherwise.
+ * @tparam Type The type to test.
+ */
+template<typename Type, typename = void>
+struct is_iterator: std::false_type {};
+
+
+/*! @copydoc is_iterator */
+template<typename Type>
+struct is_iterator<Type, std::void_t<typename std::iterator_traits<Type>::iterator_category>>
+    : std::true_type
+{};
+
+
+/**
+ * @brief Helper variable template.
+ * @tparam Type The type to test.
+ */
+template<typename Type>
+inline constexpr bool is_iterator_v = is_iterator<Type>::value;
+
+
+/**
+ * @brief Provides the member constant `value` to true if a given type is of the
+ * required iterator type, false otherwise.
+ * @tparam Type The type to test.
+ * @tparam It Required iterator type.
+ */
+template<typename Type, typename It, typename = void>
+struct is_iterator_type: std::false_type {};
+
+
+/*! @copydoc is_iterator_type */
+template<typename Type, typename It>
+struct is_iterator_type<Type, It, std::enable_if_t<is_iterator_v<Type> && std::is_same_v<Type, It>>>
+    : std::true_type
+{};
+
+
+/*! @copydoc is_iterator_type */
+template<typename Type, typename It>
+struct is_iterator_type<Type, It, std::enable_if_t<!std::is_same_v<Type, It>, std::void_t<typename It::iterator_type>>>
+    : is_iterator_type<Type, typename It::iterator_type>
+{};
+
+
+/**
+ * @brief Helper variable template.
+ * @tparam Type The type to test.
+ * @tparam It Required iterator type.
+ */
+template<typename Type, typename It>
+inline constexpr bool is_iterator_type_v = is_iterator_type<Type, It>::value;
+
+
+/**
+ * @cond TURN_OFF_DOXYGEN
+ * Internal details not to be documented.
+ */
+
+
+namespace internal {
+
+
+template<typename>
+[[nodiscard]] constexpr bool is_equality_comparable(...) { return false; }
+
+
+template<typename Type>
+[[nodiscard]] constexpr auto is_equality_comparable(choice_t<0>)
+-> decltype(std::declval<Type>() == std::declval<Type>()) { return true; }
+
+
+template<typename Type>
+[[nodiscard]] constexpr auto is_equality_comparable(choice_t<1>)
+-> decltype(std::declval<typename Type::value_type>(), std::declval<Type>() == std::declval<Type>()) {
+    if constexpr(is_iterator_v<Type>) {
+        return true;
+    } else if constexpr(std::is_same_v<typename Type::value_type, Type>) {
+        return is_equality_comparable<Type>(choice<0>);
+    } else {
+        return is_equality_comparable<typename Type::value_type>(choice<2>);
+    }
+}
+
+
+template<typename Type>
+[[nodiscard]] constexpr auto is_equality_comparable(choice_t<2>)
+-> decltype(std::declval<typename Type::mapped_type>(), std::declval<Type>() == std::declval<Type>()) {
+    return is_equality_comparable<typename Type::key_type>(choice<2>) && is_equality_comparable<typename Type::mapped_type>(choice<2>);
+}
+
+
+}
+
+
+/**
+ * Internal details not to be documented.
+ * @endcond
+ */
+
+
+/**
+ * @brief Provides the member constant `value` to true if a given type is
+ * equality comparable, false otherwise.
+ * @tparam Type The type to test.
+ */
+template<typename Type, typename = void>
+struct is_equality_comparable: std::bool_constant<internal::is_equality_comparable<Type>(choice<2>)> {};
+
+
+/**
+ * @brief Helper variable template.
+ * @tparam Type The type to test.
  */
 template<class Type>
-constexpr auto is_named_type_v = is_named_type<Type>::value;
-
-
-/**
- * @brief Defines an enum class to use for opaque identifiers and a dedicate
- * `to_integer` function to convert the identifiers to their underlying type.
- * @param clazz The name to use for the enum class.
- * @param type The underlying type for the enum class.
- */
-#define ENTT_OPAQUE_TYPE(clazz, type)\
-    enum class clazz: type {};\
-    constexpr auto to_integer(const clazz id) ENTT_NOEXCEPT {\
-        return std::underlying_type_t<clazz>(id);\
-    }
-
-
-}
-
-
-/**
- * @brief Utility macro to deal with an issue of MSVC.
- *
- * See _msvc-doesnt-expand-va-args-correctly_ on SO for all the details.
- *
- * @param args Argument to expand.
- */
-#define ENTT_EXPAND(args) args
-
-
-/**
- * @brief Makes an already existing type a named type.
- *
- * The current definition contains a workaround for Clang 6 because it fails to
- * deduce correctly the type to use to specialize the class template.<br/>
- * With a compiler that fully supports C++17 and works fine with deduction
- * guides, the following should be fine instead:
- *
- * @code{.cpp}
- * std::integral_constant<ENTT_ID_TYPE, entt::basic_hashed_string{#type}>
- * @endcode
- *
- * In order to support even sligthly older compilers, I prefer to stick to the
- * implementation below.
- *
- * @param type Type to assign a name to.
- */
-#define ENTT_NAMED_TYPE(type)\
-    template<>\
-    struct entt::named_type_traits<type>\
-        : std::integral_constant<ENTT_ID_TYPE, entt::basic_hashed_string<std::remove_cv_t<std::remove_pointer_t<std::decay_t<decltype(#type)>>>>{#type}>\
-    {\
-        static_assert(std::is_same_v<std::decay_t<type>, type>);\
-    };
-
-
-/**
- * @brief Defines a named type (to use for structs).
- * @param clazz Name of the type to define.
- * @param body Body of the type to define.
- */
-#define ENTT_NAMED_STRUCT_ONLY(clazz, body)\
-    struct clazz body;\
-    ENTT_NAMED_TYPE(clazz)
-
-
-/**
- * @brief Defines a named type (to use for structs).
- * @param ns Namespace where to define the named type.
- * @param clazz Name of the type to define.
- * @param body Body of the type to define.
- */
-#define ENTT_NAMED_STRUCT_WITH_NAMESPACE(ns, clazz, body)\
-    namespace ns { struct clazz body; }\
-    ENTT_NAMED_TYPE(ns::clazz)
-
-
-/*! @brief Utility function to simulate macro overloading. */
-#define ENTT_NAMED_STRUCT_OVERLOAD(_1, _2, _3, FUNC, ...) FUNC
-/*! @brief Defines a named type (to use for structs). */
-#define ENTT_NAMED_STRUCT(...) ENTT_EXPAND(ENTT_NAMED_STRUCT_OVERLOAD(__VA_ARGS__, ENTT_NAMED_STRUCT_WITH_NAMESPACE, ENTT_NAMED_STRUCT_ONLY,)(__VA_ARGS__))
-
-
-/**
- * @brief Defines a named type (to use for classes).
- * @param clazz Name of the type to define.
- * @param body Body of the type to define.
- */
-#define ENTT_NAMED_CLASS_ONLY(clazz, body)\
-    class clazz body;\
-    ENTT_NAMED_TYPE(clazz)
-
-
-/**
- * @brief Defines a named type (to use for classes).
- * @param ns Namespace where to define the named type.
- * @param clazz Name of the type to define.
- * @param body Body of the type to define.
- */
-#define ENTT_NAMED_CLASS_WITH_NAMESPACE(ns, clazz, body)\
-    namespace ns { class clazz body; }\
-    ENTT_NAMED_TYPE(ns::clazz)
-
-
-/*! @brief Utility function to simulate macro overloading. */
-#define ENTT_NAMED_CLASS_MACRO(_1, _2, _3, FUNC, ...) FUNC
-/*! @brief Defines a named type (to use for classes). */
-#define ENTT_NAMED_CLASS(...) ENTT_EXPAND(ENTT_NAMED_CLASS_MACRO(__VA_ARGS__, ENTT_NAMED_CLASS_WITH_NAMESPACE, ENTT_NAMED_CLASS_ONLY,)(__VA_ARGS__))
-
-
-#endif // ENTT_CORE_TYPE_TRAITS_HPP
-
-// #include "core/utility.hpp"
-
-// #include "entity/actor.hpp"
-#ifndef ENTT_ENTITY_ACTOR_HPP
-#define ENTT_ENTITY_ACTOR_HPP
-
-
-#include <cassert>
-#include <utility>
-#include <type_traits>
-// #include "../config/config.h"
-#ifndef ENTT_CONFIG_CONFIG_H
-#define ENTT_CONFIG_CONFIG_H
-
-
-#ifndef ENTT_NOEXCEPT
-#define ENTT_NOEXCEPT noexcept
-#endif // ENTT_NOEXCEPT
-
-
-#ifndef ENTT_HS_SUFFIX
-#define ENTT_HS_SUFFIX _hs
-#endif // ENTT_HS_SUFFIX
-
-
-#ifndef ENTT_HWS_SUFFIX
-#define ENTT_HWS_SUFFIX _hws
-#endif // ENTT_HWS_SUFFIX
-
-
-#ifndef ENTT_NO_ATOMIC
-#include <atomic>
-#define ENTT_MAYBE_ATOMIC(Type) std::atomic<Type>
-#else // ENTT_NO_ATOMIC
-#define ENTT_MAYBE_ATOMIC(Type) Type
-#endif // ENTT_NO_ATOMIC
-
-
-#ifndef ENTT_ID_TYPE
-#include <cstdint>
-#define ENTT_ID_TYPE std::uint32_t
-#endif // ENTT_ID_TYPE
-
-
-#ifndef ENTT_PAGE_SIZE
-#define ENTT_PAGE_SIZE 32768
-#endif // ENTT_PAGE_SIZE
-
-
-#ifndef ENTT_DISABLE_ASSERT
-#include <cassert>
-#define ENTT_ASSERT(condition) assert(condition)
-#else // ENTT_DISABLE_ASSERT
-#define ENTT_ASSERT(...) ((void)0)
-#endif // ENTT_DISABLE_ASSERT
-
-
-#endif // ENTT_CONFIG_CONFIG_H
+inline constexpr bool is_equality_comparable_v = is_equality_comparable<Type>::value;
+
+
+/**
+ * @brief Transcribes the constness of a type to another type.
+ * @tparam To The type to which to transcribe the constness.
+ * @tparam From The type from which to transcribe the constness.
+ */
+template<typename To, typename From>
+struct constness_as {
+    /*! @brief The type resulting from the transcription of the constness. */
+    using type = std::remove_const_t<To>;
+};
+
+
+/*! @copydoc constness_as */
+template<typename To, typename From>
+struct constness_as<To, const From> {
+    /*! @brief The type resulting from the transcription of the constness. */
+    using type = std::add_const_t<To>;
+};
+
+
+/**
+ * @brief Alias template to facilitate the transcription of the constness.
+ * @tparam To The type to which to transcribe the constness.
+ * @tparam From The type from which to transcribe the constness.
+ */
+template<typename To, typename From>
+using constness_as_t = typename constness_as<To, From>::type;
+
+
+/**
+ * @brief Extracts the class of a non-static member object or function.
+ * @tparam Member A pointer to a non-static member object or function.
+ */
+template<typename Member>
+class member_class {
+    static_assert(std::is_member_pointer_v<Member>, "Invalid pointer type to non-static member object or function");
+
+    template<typename Class, typename Ret, typename... Args>
+    static Class * clazz(Ret(Class:: *)(Args...));
+
+    template<typename Class, typename Ret, typename... Args>
+    static Class * clazz(Ret(Class:: *)(Args...) const);
+
+    template<typename Class, typename Type>
+    static Class * clazz(Type Class:: *);
+
+public:
+    /*! @brief The class of the given non-static member object or function. */
+    using type = std::remove_pointer_t<decltype(clazz(std::declval<Member>()))>;
+};
+
+
+/**
+ * @brief Helper type.
+ * @tparam Member A pointer to a non-static member object or function.
+ */
+template<typename Member>
+using member_class_t = typename member_class<Member>::type;
+
+
+}
+
+
+#endif
+
+// #include "../config/config.h"
+
+
+
+namespace entt {
+
+
+/**
+ * @cond TURN_OFF_DOXYGEN
+ * Internal details not to be documented.
+ */
+
+
+namespace internal {
+
+
+template<typename Ret, typename... Args>
+auto function_pointer(Ret(*)(Args...)) -> Ret(*)(Args...);
+
+
+template<typename Ret, typename Type, typename... Args, typename Other>
+auto function_pointer(Ret(*)(Type, Args...), Other &&) -> Ret(*)(Args...);
+
+
+template<typename Class, typename Ret, typename... Args, typename... Other>
+auto function_pointer(Ret(Class:: *)(Args...), Other &&...) -> Ret(*)(Args...);
+
+
+template<typename Class, typename Ret, typename... Args, typename... Other>
+auto function_pointer(Ret(Class:: *)(Args...) const, Other &&...) -> Ret(*)(Args...);
+
+
+template<typename Class, typename Type, typename... Other>
+auto function_pointer(Type Class:: *, Other &&...) -> Type(*)();
+
+
+template<typename... Type>
+using function_pointer_t = decltype(internal::function_pointer(std::declval<Type>()...));
+
+
+template<typename... Class, typename Ret, typename... Args>
+[[nodiscard]] constexpr auto index_sequence_for(Ret(*)(Args...)) {
+    return std::index_sequence_for<Class..., Args...>{};
+}
+
+
+}
+
+
+/**
+ * Internal details not to be documented.
+ * @endcond
+ */
+
+
+/*! @brief Used to wrap a function or a member of a specified type. */
+template<auto>
+struct connect_arg_t {};
+
+
+/*! @brief Constant of type connect_arg_t used to disambiguate calls. */
+template<auto Func>
+inline constexpr connect_arg_t<Func> connect_arg{};
+
+
+/**
+ * @brief Basic delegate implementation.
+ *
+ * Primary template isn't defined on purpose. All the specializations give a
+ * compile-time error unless the template parameter is a function type.
+ */
+template<typename>
+class delegate;
+
+
+/**
+ * @brief Utility class to use to send around functions and members.
+ *
+ * Unmanaged delegate for function pointers and members. Users of this class are
+ * in charge of disconnecting instances before deleting them.
+ *
+ * A delegate can be used as a general purpose invoker without memory overhead
+ * for free functions possibly with payloads and bound or unbound members.
+ *
+ * @tparam Ret Return type of a function type.
+ * @tparam Args Types of arguments of a function type.
+ */
+template<typename Ret, typename... Args>
+class delegate<Ret(Args...)> {
+    template<auto Candidate, std::size_t... Index>
+    [[nodiscard]] auto wrap(std::index_sequence<Index...>) ENTT_NOEXCEPT {
+        return [](const void *, Args... args) -> Ret {
+            [[maybe_unused]] const auto arguments = std::forward_as_tuple(std::forward<Args>(args)...);
+            return static_cast<Ret>(std::invoke(Candidate, std::forward<type_list_element_t<Index, type_list<Args...>>>(std::get<Index>(arguments))...));
+        };
+    }
+
+    template<auto Candidate, typename Type, std::size_t... Index>
+    [[nodiscard]] auto wrap(Type &, std::index_sequence<Index...>) ENTT_NOEXCEPT {
+        return [](const void *payload, Args... args) -> Ret {
+            [[maybe_unused]] const auto arguments = std::forward_as_tuple(std::forward<Args>(args)...);
+            Type *curr = static_cast<Type *>(const_cast<constness_as_t<void, Type> *>(payload));
+            return static_cast<Ret>(std::invoke(Candidate, *curr, std::forward<type_list_element_t<Index, type_list<Args...>>>(std::get<Index>(arguments))...));
+        };
+    }
+
+    template<auto Candidate, typename Type, std::size_t... Index>
+    [[nodiscard]] auto wrap(Type *, std::index_sequence<Index...>) ENTT_NOEXCEPT {
+        return [](const void *payload, Args... args) -> Ret {
+            [[maybe_unused]] const auto arguments = std::forward_as_tuple(std::forward<Args>(args)...);
+            Type *curr = static_cast<Type *>(const_cast<constness_as_t<void, Type> *>(payload));
+            return static_cast<Ret>(std::invoke(Candidate, curr, std::forward<type_list_element_t<Index, type_list<Args...>>>(std::get<Index>(arguments))...));
+        };
+    }
+
+public:
+    /*! @brief Function type of the contained target. */
+    using function_type = Ret(const void *, Args...);
+    /*! @brief Function type of the delegate. */
+    using type = Ret(Args...);
+    /*! @brief Return type of the delegate. */
+    using result_type = Ret;
+
+    /*! @brief Default constructor. */
+    delegate() ENTT_NOEXCEPT
+        : fn{nullptr}, data{nullptr}
+    {}
+
+    /**
+     * @brief Constructs a delegate and connects a free function or an unbound
+     * member.
+     * @tparam Candidate Function or member to connect to the delegate.
+     */
+    template<auto Candidate>
+    delegate(connect_arg_t<Candidate>) ENTT_NOEXCEPT {
+        connect<Candidate>();
+    }
+
+    /**
+     * @brief Constructs a delegate and connects a free function with payload or
+     * a bound member.
+     * @tparam Candidate Function or member to connect to the delegate.
+     * @tparam Type Type of class or type of payload.
+     * @param value_or_instance A valid object that fits the purpose.
+     */
+    template<auto Candidate, typename Type>
+    delegate(connect_arg_t<Candidate>, Type &&value_or_instance) ENTT_NOEXCEPT {
+        connect<Candidate>(std::forward<Type>(value_or_instance));
+    }
+
+    /**
+     * @brief Constructs a delegate and connects an user defined function with
+     * optional payload.
+     * @param function Function to connect to the delegate.
+     * @param payload User defined arbitrary data.
+     */
+    delegate(function_type *function, const void *payload = nullptr) ENTT_NOEXCEPT {
+        connect(function, payload);
+    }
+
+    /**
+     * @brief Connects a free function or an unbound member to a delegate.
+     * @tparam Candidate Function or member to connect to the delegate.
+     */
+    template<auto Candidate>
+    void connect() ENTT_NOEXCEPT {
+        data = nullptr;
+
+        if constexpr(std::is_invocable_r_v<Ret, decltype(Candidate), Args...>) {
+            fn = [](const void *, Args... args) -> Ret {
+                return Ret(std::invoke(Candidate, std::forward<Args>(args)...));
+            };
+        } else if constexpr(std::is_member_pointer_v<decltype(Candidate)>) {
+            fn = wrap<Candidate>(internal::index_sequence_for<type_list_element_t<0, type_list<Args...>>>(internal::function_pointer_t<decltype(Candidate)>{}));
+        } else {
+            fn = wrap<Candidate>(internal::index_sequence_for(internal::function_pointer_t<decltype(Candidate)>{}));
+        }
+    }
+
+    /**
+     * @brief Connects a free function with payload or a bound member to a
+     * delegate.
+     *
+     * The delegate isn't responsible for the connected object or the payload.
+     * Users must always guarantee that the lifetime of the instance overcomes
+     * the one of the delegate.<br/>
+     * When used to connect a free function with payload, its signature must be
+     * such that the instance is the first argument before the ones used to
+     * define the delegate itself.
+     *
+     * @tparam Candidate Function or member to connect to the delegate.
+     * @tparam Type Type of class or type of payload.
+     * @param value_or_instance A valid reference that fits the purpose.
+     */
+    template<auto Candidate, typename Type>
+    void connect(Type &value_or_instance) ENTT_NOEXCEPT {
+        data = &value_or_instance;
+
+        if constexpr(std::is_invocable_r_v<Ret, decltype(Candidate), Type &, Args...>) {
+            fn = [](const void *payload, Args... args) -> Ret {
+                Type *curr = static_cast<Type *>(const_cast<constness_as_t<void, Type> *>(payload));
+                return Ret(std::invoke(Candidate, *curr, std::forward<Args>(args)...));
+            };
+        } else {
+            fn = wrap<Candidate>(value_or_instance, internal::index_sequence_for(internal::function_pointer_t<decltype(Candidate), Type>{}));
+        }
+    }
+
+    /**
+     * @brief Connects a free function with payload or a bound member to a
+     * delegate.
+     *
+     * @sa connect(Type &)
+     *
+     * @tparam Candidate Function or member to connect to the delegate.
+     * @tparam Type Type of class or type of payload.
+     * @param value_or_instance A valid pointer that fits the purpose.
+     */
+    template<auto Candidate, typename Type>
+    void connect(Type *value_or_instance) ENTT_NOEXCEPT {
+        data = value_or_instance;
+
+        if constexpr(std::is_invocable_r_v<Ret, decltype(Candidate), Type *, Args...>) {
+            fn = [](const void *payload, Args... args) -> Ret {
+                Type *curr = static_cast<Type *>(const_cast<constness_as_t<void, Type> *>(payload));
+                return Ret(std::invoke(Candidate, curr, std::forward<Args>(args)...));
+            };
+        } else {
+            fn = wrap<Candidate>(value_or_instance, internal::index_sequence_for(internal::function_pointer_t<decltype(Candidate), Type>{}));
+        }
+    }
+
+    /**
+     * @brief Connects an user defined function with optional payload to a
+     * delegate.
+     *
+     * The delegate isn't responsible for the connected object or the payload.
+     * Users must always guarantee that the lifetime of an instance overcomes
+     * the one of the delegate.<br/>
+     * The payload is returned as the first argument to the target function in
+     * all cases.
+     *
+     * @param function Function to connect to the delegate.
+     * @param payload User defined arbitrary data.
+     */
+    void connect(function_type *function, const void *payload = nullptr) ENTT_NOEXCEPT {
+        fn = function;
+        data = payload;
+    }
+
+    /**
+     * @brief Resets a delegate.
+     *
+     * After a reset, a delegate cannot be invoked anymore.
+     */
+    void reset() ENTT_NOEXCEPT {
+        fn = nullptr;
+        data = nullptr;
+    }
+
+    /**
+     * @brief Returns the instance or the payload linked to a delegate, if any.
+     * @return An opaque pointer to the underlying data.
+     */
+    [[nodiscard]] const void * instance() const ENTT_NOEXCEPT {
+        return data;
+    }
+
+    /**
+     * @brief Triggers a delegate.
+     *
+     * The delegate invokes the underlying function and returns the result.
+     *
+     * @warning
+     * Attempting to trigger an invalid delegate results in undefined
+     * behavior.
+     *
+     * @param args Arguments to use to invoke the underlying function.
+     * @return The value returned by the underlying function.
+     */
+    Ret operator()(Args... args) const {
+        ENTT_ASSERT(static_cast<bool>(*this), "Uninitialized delegate");
+        return fn(data, std::forward<Args>(args)...);
+    }
+
+    /**
+     * @brief Checks whether a delegate actually stores a listener.
+     * @return False if the delegate is empty, true otherwise.
+     */
+    [[nodiscard]] explicit operator bool() const ENTT_NOEXCEPT {
+        // no need to test also data
+        return !(fn == nullptr);
+    }
+
+    /**
+     * @brief Compares the contents of two delegates.
+     * @param other Delegate with which to compare.
+     * @return False if the two contents differ, true otherwise.
+     */
+    [[nodiscard]] bool operator==(const delegate<Ret(Args...)> &other) const ENTT_NOEXCEPT {
+        return fn == other.fn && data == other.data;
+    }
+
+private:
+    function_type *fn;
+    const void *data;
+};
+
+
+/**
+ * @brief Compares the contents of two delegates.
+ * @tparam Ret Return type of a function type.
+ * @tparam Args Types of arguments of a function type.
+ * @param lhs A valid delegate object.
+ * @param rhs A valid delegate object.
+ * @return True if the two contents differ, false otherwise.
+ */
+template<typename Ret, typename... Args>
+[[nodiscard]] bool operator!=(const delegate<Ret(Args...)> &lhs, const delegate<Ret(Args...)> &rhs) ENTT_NOEXCEPT {
+    return !(lhs == rhs);
+}
+
+
+/**
+ * @brief Deduction guide.
+ * @tparam Candidate Function or member to connect to the delegate.
+ */
+template<auto Candidate>
+delegate(connect_arg_t<Candidate>)
+-> delegate<std::remove_pointer_t<internal::function_pointer_t<decltype(Candidate)>>>;
+
+
+/**
+ * @brief Deduction guide.
+ * @tparam Candidate Function or member to connect to the delegate.
+ * @tparam Type Type of class or type of payload.
+ */
+template<auto Candidate, typename Type>
+delegate(connect_arg_t<Candidate>, Type &&)
+-> delegate<std::remove_pointer_t<internal::function_pointer_t<decltype(Candidate), Type>>>;
+
+
+/**
+ * @brief Deduction guide.
+ * @tparam Ret Return type of a function type.
+ * @tparam Args Types of arguments of a function type.
+ */
+template<typename Ret, typename... Args>
+delegate(Ret(*)(const void *, Args...), const void * = nullptr)
+-> delegate<Ret(Args...)>;
+
+
+}
+
+
+#endif
+
+// #include "fwd.hpp"
+#ifndef ENTT_SIGNAL_FWD_HPP
+#define ENTT_SIGNAL_FWD_HPP
+
+
+namespace entt {
+
+
+template<typename>
+class delegate;
+
+
+class dispatcher;
+
+
+template<typename>
+class emitter;
+
+
+class connection;
+
+
+struct scoped_connection;
+
+
+template<typename>
+class sink;
+
+
+template<typename>
+class sigh;
+
+
+}
+
+
+#endif
+
+
+
+namespace entt {
+
+
+/**
+ * @brief Sink class.
+ *
+ * Primary template isn't defined on purpose. All the specializations give a
+ * compile-time error unless the template parameter is a function type.
+ *
+ * @tparam Function A valid function type.
+ */
+template<typename Function>
+class sink;
+
+
+/**
+ * @brief Unmanaged signal handler.
+ *
+ * Primary template isn't defined on purpose. All the specializations give a
+ * compile-time error unless the template parameter is a function type.
+ *
+ * @tparam Function A valid function type.
+ */
+template<typename Function>
+class sigh;
+
+
+/**
+ * @brief Unmanaged signal handler.
+ *
+ * It works directly with references to classes and pointers to member functions
+ * as well as pointers to free functions. Users of this class are in charge of
+ * disconnecting instances before deleting them.
+ *
+ * This class serves mainly two purposes:
+ *
+ * * Creating signals to use later to notify a bunch of listeners.
+ * * Collecting results from a set of functions like in a voting system.
+ *
+ * @tparam Ret Return type of a function type.
+ * @tparam Args Types of arguments of a function type.
+ */
+template<typename Ret, typename... Args>
+class sigh<Ret(Args...)> {
+    /*! @brief A sink is allowed to modify a signal. */
+    friend class sink<Ret(Args...)>;
+
+public:
+    /*! @brief Unsigned integer type. */
+    using size_type = std::size_t;
+    /*! @brief Sink type. */
+    using sink_type = sink<Ret(Args...)>;
+
+    /**
+     * @brief Instance type when it comes to connecting member functions.
+     * @tparam Class Type of class to which the member function belongs.
+     */
+    template<typename Class>
+    using instance_type = Class *;
+
+    /**
+     * @brief Number of listeners connected to the signal.
+     * @return Number of listeners currently connected.
+     */
+    [[nodiscard]] size_type size() const ENTT_NOEXCEPT {
+        return calls.size();
+    }
+
+    /**
+     * @brief Returns false if at least a listener is connected to the signal.
+     * @return True if the signal has no listeners connected, false otherwise.
+     */
+    [[nodiscard]] bool empty() const ENTT_NOEXCEPT {
+        return calls.empty();
+    }
+
+    /**
+     * @brief Triggers a signal.
+     *
+     * All the listeners are notified. Order isn't guaranteed.
+     *
+     * @param args Arguments to use to invoke listeners.
+     */
+    void publish(Args... args) const {
+        for(auto &&call: std::as_const(calls)) {
+            call(args...);
+        }
+    }
+
+    /**
+     * @brief Collects return values from the listeners.
+     *
+     * The collector must expose a call operator with the following properties:
+     *
+     * * The return type is either `void` or such that it's convertible to
+     *   `bool`. In the second case, a true value will stop the iteration.
+     * * The list of parameters is empty if `Ret` is `void`, otherwise it
+     *   contains a single element such that `Ret` is convertible to it.
+     *
+     * @tparam Func Type of collector to use, if any.
+     * @param func A valid function object.
+     * @param args Arguments to use to invoke listeners.
+     */
+    template<typename Func>
+    void collect(Func func, Args... args) const {
+        for(auto &&call: calls) {
+            if constexpr(std::is_void_v<Ret>) {
+                if constexpr(std::is_invocable_r_v<bool, Func>) {
+                    call(args...);
+                    if(func()) { break; }
+                } else {
+                    call(args...);
+                    func();
+                }
+            } else {
+                if constexpr(std::is_invocable_r_v<bool, Func, Ret>) {
+                    if(func(call(args...))) { break; }
+                } else {
+                    func(call(args...));
+                }
+            }
+        }
+    }
+
+private:
+    std::vector<delegate<Ret(Args...)>> calls;
+};
+
+
+/**
+ * @brief Connection class.
+ *
+ * Opaque object the aim of which is to allow users to release an already
+ * estabilished connection without having to keep a reference to the signal or
+ * the sink that generated it.
+ */
+class connection {
+    /*! @brief A sink is allowed to create connection objects. */
+    template<typename>
+    friend class sink;
+
+    connection(delegate<void(void *)> fn, void *ref)
+        : disconnect{fn}, signal{ref}
+    {}
+
+public:
+    /*! @brief Default constructor. */
+    connection() = default;
+
+    /**
+     * @brief Checks whether a connection is properly initialized.
+     * @return True if the connection is properly initialized, false otherwise.
+     */
+    [[nodiscard]] explicit operator bool() const ENTT_NOEXCEPT {
+        return static_cast<bool>(disconnect);
+    }
+
+    /*! @brief Breaks the connection. */
+    void release() {
+        if(disconnect) {
+            disconnect(signal);
+            disconnect.reset();
+        }
+    }
+
+private:
+    delegate<void(void *)> disconnect;
+    void *signal{};
+};
+
+
+/**
+ * @brief Scoped connection class.
+ *
+ * Opaque object the aim of which is to allow users to release an already
+ * estabilished connection without having to keep a reference to the signal or
+ * the sink that generated it.<br/>
+ * A scoped connection automatically breaks the link between the two objects
+ * when it goes out of scope.
+ */
+struct scoped_connection {
+    /*! @brief Default constructor. */
+    scoped_connection() = default;
+
+    /**
+     * @brief Constructs a scoped connection from a basic connection.
+     * @param other A valid connection object.
+     */
+    scoped_connection(const connection &other)
+        : conn{other}
+    {}
+
+    /*! @brief Default copy constructor, deleted on purpose. */
+    scoped_connection(const scoped_connection &) = delete;
+
+    /*! @brief Automatically breaks the link on destruction. */
+    ~scoped_connection() {
+        conn.release();
+    }
+
+    /**
+     * @brief Default copy assignment operator, deleted on purpose.
+     * @return This scoped connection.
+     */
+    scoped_connection & operator=(const scoped_connection &) = delete;
+
+    /**
+     * @brief Acquires a connection.
+     * @param other The connection object to acquire.
+     * @return This scoped connection.
+     */
+    scoped_connection & operator=(connection other) {
+        conn = std::move(other);
+        return *this;
+    }
+
+    /**
+     * @brief Checks whether a scoped connection is properly initialized.
+     * @return True if the connection is properly initialized, false otherwise.
+     */
+    [[nodiscard]] explicit operator bool() const ENTT_NOEXCEPT {
+        return static_cast<bool>(conn);
+    }
+
+    /*! @brief Breaks the connection. */
+    void release() {
+        conn.release();
+    }
+
+private:
+    connection conn;
+};
+
+
+/**
+ * @brief Sink class.
+ *
+ * A sink is used to connect listeners to signals and to disconnect them.<br/>
+ * The function type for a listener is the one of the signal to which it
+ * belongs.
+ *
+ * The clear separation between a signal and a sink permits to store the former
+ * as private data member without exposing the publish functionality to the
+ * users of the class.
+ *
+ * @warning
+ * Lifetime of a sink must not overcome that of the signal to which it refers.
+ * In any other case, attempting to use a sink results in undefined behavior.
+ *
+ * @tparam Ret Return type of a function type.
+ * @tparam Args Types of arguments of a function type.
+ */
+template<typename Ret, typename... Args>
+class sink<Ret(Args...)> {
+    using signal_type = sigh<Ret(Args...)>;
+    using difference_type = typename std::iterator_traits<typename decltype(signal_type::calls)::iterator>::difference_type;
+
+    template<auto Candidate, typename Type>
+    static void release(Type value_or_instance, void *signal) {
+        sink{*static_cast<signal_type *>(signal)}.disconnect<Candidate>(value_or_instance);
+    }
+
+    template<auto Candidate>
+    static void release(void *signal) {
+        sink{*static_cast<signal_type *>(signal)}.disconnect<Candidate>();
+    }
+
+public:
+    /**
+     * @brief Constructs a sink that is allowed to modify a given signal.
+     * @param ref A valid reference to a signal object.
+     */
+    sink(sigh<Ret(Args...)> &ref) ENTT_NOEXCEPT
+        : offset{},
+          signal{&ref}
+    {}
+
+    /**
+     * @brief Returns false if at least a listener is connected to the sink.
+     * @return True if the sink has no listeners connected, false otherwise.
+     */
+    [[nodiscard]] bool empty() const ENTT_NOEXCEPT {
+        return signal->calls.empty();
+    }
+
+    /**
+     * @brief Returns a sink that connects before a given free function or an
+     * unbound member.
+     * @tparam Function A valid free function pointer.
+     * @return A properly initialized sink object.
+     */
+    template<auto Function>
+    [[nodiscard]] sink before() {
+        delegate<Ret(Args...)> call{};
+        call.template connect<Function>();
+
+        const auto &calls = signal->calls;
+        const auto it = std::find(calls.cbegin(), calls.cend(), std::move(call));
+
+        sink other{*this};
+        other.offset = std::distance(it, calls.cend());
+        return other;
+    }
+
+    /**
+     * @brief Returns a sink that connects before a free function with payload
+     * or a bound member.
+     * @tparam Candidate Member or free function to look for.
+     * @tparam Type Type of class or type of payload.
+     * @param value_or_instance A valid object that fits the purpose.
+     * @return A properly initialized sink object.
+     */
+    template<auto Candidate, typename Type>
+    [[nodiscard]] sink before(Type &&value_or_instance) {
+        delegate<Ret(Args...)> call{};
+        call.template connect<Candidate>(value_or_instance);
+
+        const auto &calls = signal->calls;
+        const auto it = std::find(calls.cbegin(), calls.cend(), std::move(call));
+
+        sink other{*this};
+        other.offset = std::distance(it, calls.cend());
+        return other;
+    }
+
+    /**
+     * @brief Returns a sink that connects before a given instance or specific
+     * payload.
+     * @tparam Type Type of class or type of payload.
+     * @param value_or_instance A valid object that fits the purpose.
+     * @return A properly initialized sink object.
+     */
+    template<typename Type>
+    [[nodiscard]] sink before(Type &value_or_instance) {
+        return before(&value_or_instance);
+    }
+
+    /**
+     * @brief Returns a sink that connects before a given instance or specific
+     * payload.
+     * @tparam Type Type of class or type of payload.
+     * @param value_or_instance A valid pointer that fits the purpose.
+     * @return A properly initialized sink object.
+     */
+    template<typename Type>
+    [[nodiscard]] sink before(Type *value_or_instance) {
+        sink other{*this};
+
+        if(value_or_instance) {
+            const auto &calls = signal->calls;
+            const auto it = std::find_if(calls.cbegin(), calls.cend(), [value_or_instance](const auto &delegate) {
+                return delegate.instance() == value_or_instance;
+            });
+
+            other.offset = std::distance(it, calls.cend());
+        }
+
+        return other;
+    }
+
+    /**
+     * @brief Returns a sink that connects before anything else.
+     * @return A properly initialized sink object.
+     */
+    [[nodiscard]] sink before() {
+        sink other{*this};
+        other.offset = signal->calls.size();
+        return other;
+    }
+
+    /**
+     * @brief Connects a free function or an unbound member to a signal.
+     *
+     * The signal handler performs checks to avoid multiple connections for the
+     * same function.
+     *
+     * @tparam Candidate Function or member to connect to the signal.
+     * @return A properly initialized connection object.
+     */
+    template<auto Candidate>
+    connection connect() {
+        disconnect<Candidate>();
+
+        delegate<Ret(Args...)> call{};
+        call.template connect<Candidate>();
+        signal->calls.insert(signal->calls.end() - offset, std::move(call));
+
+        delegate<void(void *)> conn{};
+        conn.template connect<&release<Candidate>>();
+        return { std::move(conn), signal };
+    }
+
+    /**
+     * @brief Connects a free function with payload or a bound member to a
+     * signal.
+     *
+     * The signal isn't responsible for the connected object or the payload.
+     * Users must always guarantee that the lifetime of the instance overcomes
+     * the one of the signal. On the other side, the signal handler performs
+     * checks to avoid multiple connections for the same function.<br/>
+     * When used to connect a free function with payload, its signature must be
+     * such that the instance is the first argument before the ones used to
+     * define the signal itself.
+     *
+     * @tparam Candidate Function or member to connect to the signal.
+     * @tparam Type Type of class or type of payload.
+     * @param value_or_instance A valid object that fits the purpose.
+     * @return A properly initialized connection object.
+     */
+    template<auto Candidate, typename Type>
+    connection connect(Type &&value_or_instance) {
+        disconnect<Candidate>(value_or_instance);
+
+        delegate<Ret(Args...)> call{};
+        call.template connect<Candidate>(value_or_instance);
+        signal->calls.insert(signal->calls.end() - offset, std::move(call));
+
+        delegate<void(void *)> conn{};
+        conn.template connect<&release<Candidate, Type>>(value_or_instance);
+        return { std::move(conn), signal };
+    }
+
+    /**
+     * @brief Disconnects a free function or an unbound member from a signal.
+     * @tparam Candidate Function or member to disconnect from the signal.
+     */
+    template<auto Candidate>
+    void disconnect() {
+        auto &calls = signal->calls;
+        delegate<Ret(Args...)> call{};
+        call.template connect<Candidate>();
+        calls.erase(std::remove(calls.begin(), calls.end(), std::move(call)), calls.end());
+    }
+
+    /**
+     * @brief Disconnects a free function with payload or a bound member from a
+     * signal.
+     * @tparam Candidate Function or member to disconnect from the signal.
+     * @tparam Type Type of class or type of payload.
+     * @param value_or_instance A valid object that fits the purpose.
+     */
+    template<auto Candidate, typename Type>
+    void disconnect(Type &&value_or_instance) {
+        auto &calls = signal->calls;
+        delegate<Ret(Args...)> call{};
+        call.template connect<Candidate>(value_or_instance);
+        calls.erase(std::remove(calls.begin(), calls.end(), std::move(call)), calls.end());
+    }
+
+    /**
+     * @brief Disconnects free functions with payload or bound members from a
+     * signal.
+     * @tparam Type Type of class or type of payload.
+     * @param value_or_instance A valid object that fits the purpose.
+     */
+    template<typename Type>
+    void disconnect(Type &value_or_instance) {
+        disconnect(&value_or_instance);
+    }
+
+    /**
+     * @brief Disconnects free functions with payload or bound members from a
+     * signal.
+     * @tparam Type Type of class or type of payload.
+     * @param value_or_instance A valid object that fits the purpose.
+     */
+    template<typename Type>
+    void disconnect(Type *value_or_instance) {
+        if(value_or_instance) {
+            auto &calls = signal->calls;
+            calls.erase(std::remove_if(calls.begin(), calls.end(), [value_or_instance](const auto &delegate) {
+                return delegate.instance() == value_or_instance;
+            }), calls.end());
+        }
+    }
+
+    /*! @brief Disconnects all the listeners from a signal. */
+    void disconnect() {
+        signal->calls.clear();
+    }
+
+private:
+    difference_type offset;
+    signal_type *signal;
+};
+
+
+/**
+ * @brief Deduction guide.
+ *
+ * It allows to deduce the function type of a sink directly from the signal it
+ * refers to.
+ *
+ * @tparam Ret Return type of a function type.
+ * @tparam Args Types of arguments of a function type.
+ */
+template<typename Ret, typename... Args>
+sink(sigh<Ret(Args...)> &)
+-> sink<Ret(Args...)>;
+
+
+}
+
+
+#endif
+
+// #include "component.hpp"
+#ifndef ENTT_ENTITY_COMPONENT_HPP
+#define ENTT_ENTITY_COMPONENT_HPP
+
+
+#include <type_traits>
+// #include "../config/config.h"
+
+
+
+namespace entt {
+
+
+/*! @brief Commonly used default traits for all types. */
+struct basic_component_traits {
+    /*! @brief Pointer stability, default is `std::false_type`. */
+    using in_place_delete = std::false_type;
+    /*! @brief Empty type optimization, default is `ENTT_IGNORE_IF_EMPTY`. */
+    using ignore_if_empty = ENTT_IGNORE_IF_EMPTY;
+};
+
+
+/**
+ * @brief Common way to access various properties of components.
+ * @tparam Type Type of component.
+ */
+template<typename Type, typename = void>
+struct component_traits: basic_component_traits {
+    static_assert(std::is_same_v<std::decay_t<Type>, Type>, "Unsupported type");
+};
+
+
+}
+
+
+#endif
+
+// #include "entity.hpp"
+
+// #include "fwd.hpp"
+
+// #include "sparse_set.hpp"
+
+
+
+namespace entt {
+
+
+/**
+ * @brief Basic storage implementation.
+ *
+ * This class is a refinement of a sparse set that associates an object to an
+ * entity. The main purpose of this class is to extend sparse sets to store
+ * components in a registry. It guarantees fast access both to the elements and
+ * to the entities.
+ *
+ * @note
+ * Entities and objects have the same order.
+ *
+ * @note
+ * Internal data structures arrange elements to maximize performance. There are
+ * no guarantees that objects are returned in the insertion order when iterate
+ * a storage. Do not make assumption on the order in any case.
+ *
+ * @warning
+ * Empty types aren't explicitly instantiated. Therefore, many of the functions
+ * normally available for non-empty types will not be available for empty ones.
+ *
+ * @sa sparse_set<Entity>
+ *
+ * @tparam Entity A valid entity type (see entt_traits for more details).
+ * @tparam Type Type of objects assigned to the entities.
+ * @tparam Allocator Type of allocator used to manage memory and elements.
+ */
+template<typename Entity, typename Type, typename Allocator, typename = void>
+class basic_storage_impl: public basic_sparse_set<Entity, typename std::allocator_traits<Allocator>::template rebind_alloc<Entity>> {
+    static constexpr auto packed_page = ENTT_PACKED_PAGE;
+
+    using comp_traits = component_traits<Type>;
+
+    using underlying_type = basic_sparse_set<Entity, typename std::allocator_traits<Allocator>::template rebind_alloc<Entity>>;
+    using difference_type = typename entt_traits<Entity>::difference_type;
+
+    using alloc_traits = typename std::allocator_traits<Allocator>::template rebind_traits<Type>;
+    using alloc_pointer = typename alloc_traits::pointer;
+    using alloc_const_pointer = typename alloc_traits::const_pointer;
+
+    using bucket_alloc_traits = typename std::allocator_traits<Allocator>::template rebind_traits<alloc_pointer>;
+    using bucket_alloc_pointer = typename bucket_alloc_traits::pointer;
+
+    using bucket_alloc_const_type = typename std::allocator_traits<Allocator>::template rebind_alloc<alloc_const_pointer>;
+    using bucket_alloc_const_pointer = typename std::allocator_traits<bucket_alloc_const_type>::const_pointer;
+
+    static_assert(alloc_traits::propagate_on_container_move_assignment::value);
+    static_assert(bucket_alloc_traits::propagate_on_container_move_assignment::value);
+
+    template<typename Value>
+    struct storage_iterator final {
+        using difference_type = typename basic_storage_impl::difference_type;
+        using value_type = Value;
+        using pointer = value_type *;
+        using reference = value_type &;
+        using iterator_category = std::random_access_iterator_tag;
+
+        storage_iterator() ENTT_NOEXCEPT = default;
+
+        storage_iterator(bucket_alloc_pointer const *ref, const typename basic_storage_impl::difference_type idx) ENTT_NOEXCEPT
+            : packed{ref},
+              index{idx}
+        {}
+
+        storage_iterator & operator++() ENTT_NOEXCEPT {
+            return --index, *this;
+        }
+
+        storage_iterator operator++(int) ENTT_NOEXCEPT {
+            storage_iterator orig = *this;
+            return ++(*this), orig;
+        }
+
+        storage_iterator & operator--() ENTT_NOEXCEPT {
+            return ++index, *this;
+        }
+
+        storage_iterator operator--(int) ENTT_NOEXCEPT {
+            storage_iterator orig = *this;
+            return operator--(), orig;
+        }
+
+        storage_iterator & operator+=(const difference_type value) ENTT_NOEXCEPT {
+            index -= value;
+            return *this;
+        }
+
+        storage_iterator operator+(const difference_type value) const ENTT_NOEXCEPT {
+            storage_iterator copy = *this;
+            return (copy += value);
+        }
+
+        storage_iterator & operator-=(const difference_type value) ENTT_NOEXCEPT {
+            return (*this += -value);
+        }
+
+        storage_iterator operator-(const difference_type value) const ENTT_NOEXCEPT {
+            return (*this + -value);
+        }
+
+        difference_type operator-(const storage_iterator &other) const ENTT_NOEXCEPT {
+            return other.index - index;
+        }
+
+        [[nodiscard]] reference operator[](const difference_type value) const ENTT_NOEXCEPT {
+            const auto pos = size_type(index-value-1);
+            return (*packed)[page(pos)][offset(pos)];
+        }
+
+        [[nodiscard]] bool operator==(const storage_iterator &other) const ENTT_NOEXCEPT {
+            return other.index == index;
+        }
+
+        [[nodiscard]] bool operator!=(const storage_iterator &other) const ENTT_NOEXCEPT {
+            return !(*this == other);
+        }
+
+        [[nodiscard]] bool operator<(const storage_iterator &other) const ENTT_NOEXCEPT {
+            return index > other.index;
+        }
+
+        [[nodiscard]] bool operator>(const storage_iterator &other) const ENTT_NOEXCEPT {
+            return index < other.index;
+        }
+
+        [[nodiscard]] bool operator<=(const storage_iterator &other) const ENTT_NOEXCEPT {
+            return !(*this > other);
+        }
+
+        [[nodiscard]] bool operator>=(const storage_iterator &other) const ENTT_NOEXCEPT {
+            return !(*this < other);
+        }
+
+        [[nodiscard]] pointer operator->() const ENTT_NOEXCEPT {
+            const auto pos = size_type(index-1u);
+            return std::addressof((*packed)[page(pos)][offset(pos)]);
+        }
+
+        [[nodiscard]] reference operator*() const ENTT_NOEXCEPT {
+            return *operator->();
+        }
+
+    private:
+        bucket_alloc_pointer const *packed;
+        difference_type index;
+    };
+
+    [[nodiscard]] static auto page(const std::size_t pos) ENTT_NOEXCEPT {
+        return pos / packed_page;
+    }
+
+    [[nodiscard]] static auto offset(const std::size_t pos) ENTT_NOEXCEPT {
+        return pos & (packed_page - 1);
+    }
+
+    void release_memory() {
+        if(packed) {
+            // no-throw stable erase iteration
+            underlying_type::clear();
+
+            for(size_type pos{}; pos < bucket; ++pos) {
+                alloc_traits::deallocate(allocator, packed[pos], packed_page);
+                bucket_alloc_traits::destroy(bucket_allocator, std::addressof(packed[pos]));
+            }
+
+            bucket_alloc_traits::deallocate(bucket_allocator, packed, bucket);
+        }
+    }
+
+    void assure_at_least(const std::size_t last) {
+        if(const auto idx = page(last - 1u); !(idx < bucket)) {
+            const size_type sz = idx + 1u;
+            const auto mem = bucket_alloc_traits::allocate(bucket_allocator, sz);
+            std::uninitialized_copy(packed, packed + bucket, mem);
+            size_type pos{};
+
+            ENTT_TRY {
+                for(pos = bucket; pos < sz; ++pos) {
+                    auto pg = alloc_traits::allocate(allocator, packed_page);
+                    bucket_alloc_traits::construct(bucket_allocator, std::addressof(mem[pos]), pg);
+                }
+            } ENTT_CATCH {
+                for(auto next = bucket; next < pos; ++next) {
+                    alloc_traits::deallocate(allocator, mem[next], packed_page);
+                }
+
+                std::destroy(mem, mem + pos);
+                bucket_alloc_traits::deallocate(bucket_allocator, mem, sz);
+                ENTT_THROW;
+            }
+
+            std::destroy(packed, packed + bucket);
+            bucket_alloc_traits::deallocate(bucket_allocator, packed, bucket);
+
+            packed = mem;
+            bucket = sz;
+        }
+    }
+
+    void release_unused_pages() {
+        if(const auto length = underlying_type::size() / packed_page; length < bucket) {
+            const auto mem = bucket_alloc_traits::allocate(bucket_allocator, length);
+            std::uninitialized_copy(packed, packed + length, mem);
+
+            for(auto pos = length; pos < bucket; ++pos) {
+                alloc_traits::deallocate(allocator, packed[pos], packed_page);
+                bucket_alloc_traits::destroy(bucket_allocator, std::addressof(packed[pos]));
+            }
+
+            bucket_alloc_traits::deallocate(bucket_allocator, packed, bucket);
+
+            packed = mem;
+            bucket = length;
+        }
+    }
+
+    template<typename... Args>
+    auto & push_at(const std::size_t pos, Args &&... args) {
+        ENTT_ASSERT(pos < (bucket * packed_page), "Out of bounds index");
+        auto *instance = std::addressof(packed[page(pos)][offset(pos)]);
+
+        if constexpr(std::is_aggregate_v<value_type>) {
+            alloc_traits::construct(allocator, instance, Type{std::forward<Args>(args)...});
+        } else {
+            alloc_traits::construct(allocator, instance, std::forward<Args>(args)...);
+        }
+
+        return *instance;
+    }
+
+    void pop_at(const std::size_t pos) {
+        alloc_traits::destroy(allocator, std::addressof(packed[page(pos)][offset(pos)]));
+    }
+
+protected:
+    /*! @copydoc basic_sparse_set::swap_at */
+    void swap_at(const std::size_t lhs, const std::size_t rhs) final {
+        std::swap(packed[page(lhs)][offset(lhs)], packed[page(rhs)][offset(rhs)]);
+    }
+
+    /*! @copydoc basic_sparse_set::move_and_pop */
+    void move_and_pop(const std::size_t from, const std::size_t to) final {
+        push_at(to, std::move(packed[page(from)][offset(from)]));
+        pop_at(from);
+    }
+
+    /*! @copydoc basic_sparse_set::swap_and_pop */
+    void swap_and_pop(const Entity entt, void *ud) override {
+        const auto pos = underlying_type::index(entt);
+        const auto last = underlying_type::size() - 1u;
+        auto &&elem = packed[page(pos)][offset(pos)];
+
+        // support for nosy destructors
+        [[maybe_unused]] auto unused = std::move(elem);
+        elem = std::move(packed[page(last)][offset(last)]);
+        pop_at(last);
+
+        underlying_type::swap_and_pop(entt, ud);
+    }
+
+    /*! @copydoc basic_sparse_set::in_place_pop */
+    void in_place_pop(const Entity entt, void *ud) override {
+        const auto pos = underlying_type::index(entt);
+        underlying_type::in_place_pop(entt, ud);
+        // support for nosy destructors
+        pop_at(pos);
+    }
+
+public:
+    /*! @brief Allocator type. */
+    using allocator_type = typename alloc_traits::allocator_type;
+    /*! @brief Type of the objects assigned to entities. */
+    using value_type = Type;
+    /*! @brief Underlying entity identifier. */
+    using entity_type = Entity;
+    /*! @brief Unsigned integer type. */
+    using size_type = std::size_t;
+    /*! @brief Pointer type to contained elements. */
+    using pointer = bucket_alloc_pointer;
+    /*! @brief Constant pointer type to contained elements. */
+    using const_pointer = bucket_alloc_const_pointer;
+    /*! @brief Random access iterator type. */
+    using iterator = storage_iterator<value_type>;
+    /*! @brief Constant random access iterator type. */
+    using const_iterator = storage_iterator<const value_type>;
+    /*! @brief Reverse iterator type. */
+    using reverse_iterator = std::reverse_iterator<iterator>;
+    /*! @brief Constant reverse iterator type. */
+    using const_reverse_iterator = std::reverse_iterator<const_iterator>;
+
+    /**
+     * @brief Default constructor.
+     * @param alloc Allocator to use (possibly default-constructed).
+     */
+    explicit basic_storage_impl(const allocator_type &alloc = {})
+        : underlying_type{deletion_policy{comp_traits::in_place_delete::value}, alloc},
+          allocator{alloc},
+          bucket_allocator{alloc},
+          packed{bucket_alloc_traits::allocate(bucket_allocator, 0u)},
+          bucket{}
+    {}
+
+    /**
+     * @brief Move constructor.
+     * @param other The instance to move from.
+     */
+    basic_storage_impl(basic_storage_impl &&other) ENTT_NOEXCEPT
+        : underlying_type{std::move(other)},
+          allocator{std::move(other.allocator)},
+          bucket_allocator{std::move(other.bucket_allocator)},
+          packed{std::exchange(other.packed, bucket_alloc_pointer{})},
+          bucket{std::exchange(other.bucket, 0u)}
+    {}
+
+    /*! @brief Default destructor. */
+    ~basic_storage_impl() override {
+        release_memory();
+    }
+
+    /**
+     * @brief Move assignment operator.
+     * @param other The instance to move from.
+     * @return This sparse set.
+     */
+    basic_storage_impl & operator=(basic_storage_impl &&other) ENTT_NOEXCEPT {
+        release_memory();
+
+        underlying_type::operator=(std::move(other));
+
+        allocator = std::move(other.allocator);
+        bucket_allocator = std::move(other.bucket_allocator);
+        packed = std::exchange(other.packed, bucket_alloc_pointer{});
+        bucket = std::exchange(other.bucket, 0u);
+
+        return *this;
+    }
+
+    /**
+     * @brief Increases the capacity of a storage.
+     *
+     * If the new capacity is greater than the current capacity, new storage is
+     * allocated, otherwise the method does nothing.
+     *
+     * @param cap Desired capacity.
+     */
+    void reserve(const size_type cap) {
+        underlying_type::reserve(cap);
+
+        if(cap > underlying_type::size()) {
+            assure_at_least(cap);
+        }
+    }
+
+    /**
+     * @brief Returns the number of elements that a storage has currently
+     * allocated space for.
+     * @return Capacity of the storage.
+     */
+    [[nodiscard]] size_type capacity() const ENTT_NOEXCEPT {
+        return bucket * packed_page;
+    }
+
+    /*! @brief Requests the removal of unused capacity. */
+    void shrink_to_fit() {
+        underlying_type::shrink_to_fit();
+        release_unused_pages();
+    }
+
+    /**
+     * @brief Direct access to the array of objects.
+     * @return A pointer to the array of objects.
+     */
+    [[nodiscard]] const_pointer raw() const ENTT_NOEXCEPT {
+        return packed;
+    }
+
+    /*! @copydoc raw */
+    [[nodiscard]] pointer raw() ENTT_NOEXCEPT {
+        return packed;
+    }
+
+    /**
+     * @brief Returns an iterator to the beginning.
+     *
+     * The returned iterator points to the first instance of the internal array.
+     * If the storage is empty, the returned iterator will be equal to `end()`.
+     *
+     * @return An iterator to the first instance of the internal array.
+     */
+    [[nodiscard]] const_iterator cbegin() const ENTT_NOEXCEPT {
+        const difference_type pos = underlying_type::size();
+        return const_iterator{std::addressof(packed), pos};
+    }
+
+    /*! @copydoc cbegin */
+    [[nodiscard]] const_iterator begin() const ENTT_NOEXCEPT {
+        return cbegin();
+    }
+
+    /*! @copydoc begin */
+    [[nodiscard]] iterator begin() ENTT_NOEXCEPT {
+        const difference_type pos = underlying_type::size();
+        return iterator{std::addressof(packed), pos};
+    }
+
+    /**
+     * @brief Returns an iterator to the end.
+     *
+     * The returned iterator points to the element following the last instance
+     * of the internal array. Attempting to dereference the returned iterator
+     * results in undefined behavior.
+     *
+     * @return An iterator to the element following the last instance of the
+     * internal array.
+     */
+    [[nodiscard]] const_iterator cend() const ENTT_NOEXCEPT {
+        return const_iterator{std::addressof(packed), {}};
+    }
+
+    /*! @copydoc cend */
+    [[nodiscard]] const_iterator end() const ENTT_NOEXCEPT {
+        return cend();
+    }
+
+    /*! @copydoc end */
+    [[nodiscard]] iterator end() ENTT_NOEXCEPT {
+        return iterator{std::addressof(packed), {}};
+    }
+
+    /**
+     * @brief Returns a reverse iterator to the beginning.
+     *
+     * The returned iterator points to the first instance of the reversed
+     * internal array. If the storage is empty, the returned iterator will be
+     * equal to `rend()`.
+     *
+     * @return An iterator to the first instance of the reversed internal array.
+     */
+    [[nodiscard]] const_reverse_iterator crbegin() const ENTT_NOEXCEPT {
+        return std::make_reverse_iterator(cend());
+    }
+
+    /*! @copydoc crbegin */
+    [[nodiscard]] const_reverse_iterator rbegin() const ENTT_NOEXCEPT {
+        return crbegin();
+    }
+
+    /*! @copydoc rbegin */
+    [[nodiscard]] reverse_iterator rbegin() ENTT_NOEXCEPT {
+        return std::make_reverse_iterator(end());
+    }
+
+    /**
+     * @brief Returns a reverse iterator to the end.
+     *
+     * The returned iterator points to the element following the last instance
+     * of the reversed internal array. Attempting to dereference the returned
+     * iterator results in undefined behavior.
+     *
+     * @return An iterator to the element following the last instance of the
+     * reversed internal array.
+     */
+    [[nodiscard]] const_reverse_iterator crend() const ENTT_NOEXCEPT {
+        return std::make_reverse_iterator(cbegin());
+    }
+
+    /*! @copydoc crend */
+    [[nodiscard]] const_reverse_iterator rend() const ENTT_NOEXCEPT {
+        return crend();
+    }
+
+    /*! @copydoc rend */
+    [[nodiscard]] reverse_iterator rend() ENTT_NOEXCEPT {
+        return std::make_reverse_iterator(begin());
+    }
+
+    /**
+     * @brief Returns the object assigned to an entity.
+     *
+     * @warning
+     * Attempting to use an entity that doesn't belong to the storage results in
+     * undefined behavior.
+     *
+     * @param entt A valid entity identifier.
+     * @return The object assigned to the entity.
+     */
+    [[nodiscard]] const value_type & get(const entity_type entt) const ENTT_NOEXCEPT {
+        const auto idx = underlying_type::index(entt);
+        return packed[page(idx)][offset(idx)];
+    }
+
+    /*! @copydoc get */
+    [[nodiscard]] value_type & get(const entity_type entt) ENTT_NOEXCEPT {
+        return const_cast<value_type &>(std::as_const(*this).get(entt));
+    }
+
+    /**
+     * @brief Assigns an entity to a storage and constructs its object.
+     *
+     * This version accept both types that can be constructed in place directly
+     * and types like aggregates that do not work well with a placement new as
+     * performed usually under the hood during an _emplace back_.
+     *
+     * @warning
+     * Attempting to use an entity that already belongs to the storage results
+     * in undefined behavior.
+     *
+     * @tparam Args Types of arguments to use to construct the object.
+     * @param entt A valid entity identifier.
+     * @param args Parameters to use to construct an object for the entity.
+     * @return A reference to the newly created object.
+     */
+    template<typename... Args>
+    value_type & emplace(const entity_type entt, Args &&... args) {
+        const auto pos = underlying_type::slot();
+        assure_at_least(pos + 1u);
+
+        auto &value = push_at(pos, std::forward<Args>(args)...);
+
+        ENTT_TRY {
+            [[maybe_unused]] const auto curr = underlying_type::emplace(entt);
+            ENTT_ASSERT(pos == curr, "Misplaced component");
+        } ENTT_CATCH {
+            pop_at(pos);
+            ENTT_THROW;
+        }
+
+        return value;
+    }
+
+    /**
+     * @brief Updates the instance assigned to a given entity in-place.
+     * @tparam Func Types of the function objects to invoke.
+     * @param entt A valid entity identifier.
+     * @param func Valid function objects.
+     * @return A reference to the updated instance.
+     */
+    template<typename... Func>
+    decltype(auto) patch(const entity_type entt, Func &&... func) {
+        const auto idx = underlying_type::index(entt);
+        auto &&elem = packed[page(idx)][offset(idx)];
+        (std::forward<Func>(func)(elem), ...);
+        return elem;
+    }
+
+    /**
+     * @brief Assigns one or more entities to a storage and constructs their
+     * objects from a given instance.
+     *
+     * @warning
+     * Attempting to assign an entity that already belongs to the storage
+     * results in undefined behavior.
+     *
+     * @tparam It Type of input iterator.
+     * @param first An iterator to the first element of the range of entities.
+     * @param last An iterator past the last element of the range of entities.
+     * @param value An instance of the object to construct.
+     */
+    template<typename It>
+    void insert(It first, It last, const value_type &value = {}) {
+        const auto cap = underlying_type::size() + std::distance(first, last);
+        underlying_type::reserve(cap);
+        assure_at_least(cap);
+
+        for(; first != last; ++first) {
+            push_at(underlying_type::size(), value);
+
+            ENTT_TRY {
+                underlying_type::emplace_back(*first);
+            } ENTT_CATCH {
+                pop_at(underlying_type::size());
+                ENTT_THROW;
+            }
+        }
+    }
+
+    /**
+     * @brief Assigns one or more entities to a storage and constructs their
+     * objects from a given range.
+     *
+     * @sa construct
+     *
+     * @tparam EIt Type of input iterator.
+     * @tparam CIt Type of input iterator.
+     * @param first An iterator to the first element of the range of entities.
+     * @param last An iterator past the last element of the range of entities.
+     * @param from An iterator to the first element of the range of objects.
+     */
+    template<typename EIt, typename CIt, typename = std::enable_if_t<std::is_same_v<std::decay_t<typename std::iterator_traits<CIt>::value_type>, value_type>>>
+    void insert(EIt first, EIt last, CIt from) {
+        const auto cap = underlying_type::size() + std::distance(first, last);
+        underlying_type::reserve(cap);
+        assure_at_least(cap);
+
+        for(; first != last; ++first, ++from) {
+            push_at(underlying_type::size(), *from);
+
+            ENTT_TRY {
+                underlying_type::emplace_back(*first);
+            } ENTT_CATCH {
+                pop_at(underlying_type::size());
+                ENTT_THROW;
+            }
+        }
+    }
+
+    /**
+     * @brief Sort elements according to the given comparison function.
+     *
+     * The comparison function object must return `true` if the first element
+     * is _less_ than the second one, `false` otherwise. The signature of the
+     * comparison function should be equivalent to one of the following:
+     *
+     * @code{.cpp}
+     * bool(const Entity, const Entity);
+     * bool(const Type &, const Type &);
+     * @endcode
+     *
+     * Moreover, the comparison function object shall induce a
+     * _strict weak ordering_ on the values.
+     *
+     * The sort function oject must offer a member function template
+     * `operator()` that accepts three arguments:
+     *
+     * * An iterator to the first element of the range to sort.
+     * * An iterator past the last element of the range to sort.
+     * * A comparison function to use to compare the elements.
+     *
+     * @warning
+     * Empty types are never instantiated. Therefore, only comparison function
+     * objects that require to return entities rather than components are
+     * accepted.
+     *
+     * @tparam Compare Type of comparison function object.
+     * @tparam Sort Type of sort function object.
+     * @tparam Args Types of arguments to forward to the sort function object.
+     * @param length Number of elements to sort.
+     * @param compare A valid comparison function object.
+     * @param algo A valid sort function object.
+     * @param args Arguments to forward to the sort function object, if any.
+     */
+    template<typename Compare, typename Sort = std_sort, typename... Args>
+    void sort_n(const size_type length, Compare compare, Sort algo = Sort{}, Args &&... args) {
+        if constexpr(std::is_invocable_v<Compare, const value_type &, const value_type &>) {
+            underlying_type::sort_n(length, [this, compare = std::move(compare)](const auto lhs, const auto rhs) {
+                const auto ilhs = underlying_type::index(lhs), irhs = underlying_type::index(rhs);
+                return compare(std::as_const(packed[page(ilhs)][offset(ilhs)]), std::as_const(packed[page(irhs)][offset(irhs)]));
+            }, std::move(algo), std::forward<Args>(args)...);
+        } else {
+            underlying_type::sort_n(length, std::move(compare), std::move(algo), std::forward<Args>(args)...);
+        }
+    }
+
+    /**
+     * @brief Sort all elements according to the given comparison function.
+     *
+     * @sa sort_n
+     *
+     * @tparam Compare Type of comparison function object.
+     * @tparam Sort Type of sort function object.
+     * @tparam Args Types of arguments to forward to the sort function object.
+     * @param compare A valid comparison function object.
+     * @param algo A valid sort function object.
+     * @param args Arguments to forward to the sort function object, if any.
+     */
+    template<typename Compare, typename Sort = std_sort, typename... Args>
+    void sort(Compare compare, Sort algo = Sort{}, Args &&... args) {
+        sort_n(underlying_type::size(), std::move(compare), std::move(algo), std::forward<Args>(args)...);
+    }
+
+private:
+    typename alloc_traits::allocator_type allocator;
+    typename bucket_alloc_traits::allocator_type bucket_allocator;
+    bucket_alloc_pointer packed;
+    size_type bucket;
+};
+
+
+/*! @copydoc basic_storage_impl */
+template<typename Entity, typename Type, typename Allocator>
+class basic_storage_impl<Entity, Type, Allocator, std::enable_if_t<component_traits<Type>::ignore_if_empty::value && std::is_empty_v<Type>>>
+    : public basic_sparse_set<Entity, typename std::allocator_traits<Allocator>::template rebind_alloc<Entity>>
+{
+    using comp_traits = component_traits<Type>;
+    using underlying_type = basic_sparse_set<Entity, typename std::allocator_traits<Allocator>::template rebind_alloc<Entity>>;
+    using alloc_traits = typename std::allocator_traits<Allocator>::template rebind_traits<Type>;
+
+public:
+    /*! @brief Allocator type. */
+    using allocator_type = typename alloc_traits::allocator_type;
+    /*! @brief Type of the objects assigned to entities. */
+    using value_type = Type;
+    /*! @brief Underlying entity identifier. */
+    using entity_type = Entity;
+    /*! @brief Unsigned integer type. */
+    using size_type = std::size_t;
+
+    /**
+     * @brief Default constructor.
+     * @param alloc Allocator to use (possibly default-constructed).
+     */
+    explicit basic_storage_impl(const allocator_type &alloc = {})
+        : underlying_type{deletion_policy{comp_traits::in_place_delete::value}, alloc}
+    {}
+
+    /**
+     * @brief Fake get function.
+     *
+     * @warning
+     * Attempting to use an entity that doesn't belong to the storage results in
+     * undefined behavior.
+     *
+     * @param entt A valid entity identifier.
+     */
+    void get([[maybe_unused]] const entity_type entt) const ENTT_NOEXCEPT {
+        ENTT_ASSERT(underlying_type::contains(entt), "Storage does not contain entity");
+    }
+
+    /**
+     * @brief Assigns an entity to a storage and constructs its object.
+     *
+     * @warning
+     * Attempting to use an entity that already belongs to the storage results
+     * in undefined behavior.
+     *
+     * @tparam Args Types of arguments to use to construct the object.
+     * @param entt A valid entity identifier.
+     * @param args Parameters to use to construct an object for the entity.
+     */
+    template<typename... Args>
+    void emplace(const entity_type entt, Args &&... args) {
+        [[maybe_unused]] value_type instance{std::forward<Args>(args)...};
+        underlying_type::emplace(entt);
+    }
+
+    /**
+    * @brief Updates the instance assigned to a given entity in-place.
+    * @tparam Func Types of the function objects to invoke.
+    * @param entt A valid entity identifier.
+    * @param func Valid function objects.
+    */
+    template<typename... Func>
+    void patch([[maybe_unused]] const entity_type entt, Func &&... func) {
+        ENTT_ASSERT(underlying_type::contains(entt), "Storage does not contain entity");
+        (std::forward<Func>(func)(), ...);
+    }
+
+    /**
+     * @brief Assigns one or more entities to a storage.
+     *
+     * @warning
+     * Attempting to assign an entity that already belongs to the storage
+     * results in undefined behavior.
+     *
+     * @tparam It Type of input iterator.
+     * @param first An iterator to the first element of the range of entities.
+     * @param last An iterator past the last element of the range of entities.
+     */
+    template<typename It>
+    void insert(It first, It last, const value_type & = {}) {
+        underlying_type::insert(first, last);
+    }
+};
+
+
+/**
+ * @brief Mixin type to use to wrap basic storage classes.
+ * @tparam Type The type of the underlying storage.
+ */
+template<typename Type>
+struct storage_adapter_mixin: Type {
+    static_assert(std::is_same_v<typename Type::value_type, std::decay_t<typename Type::value_type>>, "Invalid object type");
+
+    /*! @brief Type of the objects assigned to entities. */
+    using value_type = typename Type::value_type;
+    /*! @brief Underlying entity identifier. */
+    using entity_type = typename Type::entity_type;
+
+    /*! @brief Inherited constructors. */
+    using Type::Type;
+
+    /**
+     * @brief Assigns entities to a storage.
+     * @tparam Args Types of arguments to use to construct the object.
+     * @param entt A valid entity identifier.
+     * @param args Parameters to use to initialize the object.
+     * @return A reference to the newly created object.
+     */
+    template<typename... Args>
+    decltype(auto) emplace(basic_registry<entity_type> &, const entity_type entt, Args &&... args) {
+        return Type::emplace(entt, std::forward<Args>(args)...);
+    }
+
+    /**
+     * @brief Assigns entities to a storage.
+     * @tparam It Type of input iterator.
+     * @tparam Args Types of arguments to use to construct the objects assigned
+     * to the entities.
+     * @param first An iterator to the first element of the range of entities.
+     * @param last An iterator past the last element of the range of entities.
+     * @param args Parameters to use to initialize the objects assigned to the
+     * entities.
+     */
+    template<typename It, typename... Args>
+    void insert(basic_registry<entity_type> &, It first, It last, Args &&... args) {
+        Type::insert(first, last, std::forward<Args>(args)...);
+    }
+
+    /**
+     * @brief Patches the given instance for an entity.
+     * @tparam Func Types of the function objects to invoke.
+     * @param entt A valid entity identifier.
+     * @param func Valid function objects.
+     * @return A reference to the patched instance.
+     */
+    template<typename... Func>
+    decltype(auto) patch(basic_registry<entity_type> &, const entity_type entt, Func &&... func) {
+        return Type::patch(entt, std::forward<Func>(func)...);
+    }
+};
+
+
+/**
+ * @brief Mixin type to use to add signal support to storage types.
+ * @tparam Type The type of the underlying storage.
+ */
+template<typename Type>
+class sigh_storage_mixin final: public Type {
+    /*! @copydoc basic_sparse_set::swap_and_pop */
+    void swap_and_pop(const typename Type::entity_type entt, void *ud) final {
+        ENTT_ASSERT(ud != nullptr, "Invalid pointer to registry");
+        destruction.publish(*static_cast<basic_registry<typename Type::entity_type> *>(ud), entt);
+        Type::swap_and_pop(entt, ud);
+    }
+
+    /*! @copydoc basic_sparse_set::in_place_pop */
+    void in_place_pop(const typename Type::entity_type entt, void *ud) final {
+        ENTT_ASSERT(ud != nullptr, "Invalid pointer to registry");
+        destruction.publish(*static_cast<basic_registry<typename Type::entity_type> *>(ud), entt);
+        Type::in_place_pop(entt, ud);
+    }
+
+public:
+    /*! @brief Underlying value type. */
+    using value_type = typename Type::value_type;
+    /*! @brief Underlying entity identifier. */
+    using entity_type = typename Type::entity_type;
+
+    /*! @brief Inherited constructors. */
+    using Type::Type;
+
+    /**
+     * @brief Returns a sink object.
+     *
+     * The sink returned by this function can be used to receive notifications
+     * whenever a new instance is created and assigned to an entity.<br/>
+     * The function type for a listener is equivalent to:
+     *
+     * @code{.cpp}
+     * void(basic_registry<entity_type> &, entity_type);
+     * @endcode
+     *
+     * Listeners are invoked **after** the object has been assigned to the
+     * entity.
+     *
+     * @sa sink
+     *
+     * @return A temporary sink object.
+     */
+    [[nodiscard]] auto on_construct() ENTT_NOEXCEPT {
+        return sink{construction};
+    }
+
+    /**
+     * @brief Returns a sink object.
+     *
+     * The sink returned by this function can be used to receive notifications
+     * whenever an instance is explicitly updated.<br/>
+     * The function type for a listener is equivalent to:
+     *
+     * @code{.cpp}
+     * void(basic_registry<entity_type> &, entity_type);
+     * @endcode
+     *
+     * Listeners are invoked **after** the object has been updated.
+     *
+     * @sa sink
+     *
+     * @return A temporary sink object.
+     */
+    [[nodiscard]] auto on_update() ENTT_NOEXCEPT {
+        return sink{update};
+    }
+
+    /**
+     * @brief Returns a sink object.
+     *
+     * The sink returned by this function can be used to receive notifications
+     * whenever an instance is removed from an entity and thus destroyed.<br/>
+     * The function type for a listener is equivalent to:
+     *
+     * @code{.cpp}
+     * void(basic_registry<entity_type> &, entity_type);
+     * @endcode
+     *
+     * Listeners are invoked **before** the object has been removed from the
+     * entity.
+     *
+     * @sa sink
+     *
+     * @return A temporary sink object.
+     */
+    [[nodiscard]] auto on_destroy() ENTT_NOEXCEPT {
+        return sink{destruction};
+    }
+
+    /**
+     * @brief Assigns entities to a storage.
+     * @tparam Args Types of arguments to use to construct the object.
+     * @param owner The registry that issued the request.
+     * @param entt A valid entity identifier.
+     * @param args Parameters to use to initialize the object.
+     * @return A reference to the newly created object.
+     */
+    template<typename... Args>
+    decltype(auto) emplace(basic_registry<entity_type> &owner, const entity_type entt, Args &&... args) {
+        Type::emplace(entt, std::forward<Args>(args)...);
+        construction.publish(owner, entt);
+        return this->get(entt);
+    }
+
+    /**
+     * @brief Assigns entities to a storage.
+     * @tparam It Type of input iterator.
+     * @tparam Args Types of arguments to use to construct the objects assigned
+     * to the entities.
+     * @param owner The registry that issued the request.
+     * @param first An iterator to the first element of the range of entities.
+     * @param last An iterator past the last element of the range of entities.
+     * @param args Parameters to use to initialize the objects assigned to the
+     * entities.
+     */
+    template<typename It, typename... Args>
+    void insert(basic_registry<entity_type> &owner, It first, It last, Args &&... args) {
+        Type::insert(first, last, std::forward<Args>(args)...);
+
+        if(!construction.empty()) {
+            for(; first != last; ++first) {
+                construction.publish(owner, *first);
+            }
+        }
+    }
+
+    /**
+     * @brief Patches the given instance for an entity.
+     * @tparam Func Types of the function objects to invoke.
+     * @param owner The registry that issued the request.
+     * @param entt A valid entity identifier.
+     * @param func Valid function objects.
+     * @return A reference to the patched instance.
+     */
+    template<typename... Func>
+    decltype(auto) patch(basic_registry<entity_type> &owner, const entity_type entt, Func &&... func) {
+        Type::patch(entt, std::forward<Func>(func)...);
+        update.publish(owner, entt);
+        return this->get(entt);
+    }
+
+private:
+    sigh<void(basic_registry<entity_type> &, const entity_type)> construction{};
+    sigh<void(basic_registry<entity_type> &, const entity_type)> destruction{};
+    sigh<void(basic_registry<entity_type> &, const entity_type)> update{};
+};
+
+
+/**
+ * @brief Storage implementation dispatcher.
+ * @tparam Entity A valid entity type (see entt_traits for more details).
+ * @tparam Type Type of objects assigned to the entities.
+ * @tparam Allocator Type of allocator used to manage memory and elements.
+ */
+template<typename Entity, typename Type, typename Allocator>
+struct basic_storage: basic_storage_impl<Entity, Type, Allocator> {
+    using basic_storage_impl<Entity, Type, Allocator>::basic_storage_impl;
+};
+
+
+/**
+ * @brief Provides a common way to access certain properties of storage types.
+ * @tparam Entity A valid entity type (see entt_traits for more details).
+ * @tparam Type Type of objects managed by the storage class.
+ */
+template<typename Entity, typename Type, typename = void>
+struct storage_traits {
+    /*! @brief Resulting type after component-to-storage conversion. */
+    using storage_type = sigh_storage_mixin<basic_storage<Entity, Type>>;
+};
+
+
+/**
+ * @brief Gets the element assigned to an entity from a storage, if any.
+ * @tparam Type Storage type.
+ * @param container A valid instance of a storage class.
+ * @param entt A valid entity identifier.
+ * @return A possibly empty tuple containing the requested element.
+ */
+template<typename Type>
+[[nodiscard]] auto get_as_tuple([[maybe_unused]] Type &container, [[maybe_unused]] const typename Type::entity_type entt) {
+    static_assert(std::is_same_v<std::remove_const_t<Type>, typename storage_traits<typename Type::entity_type, typename Type::value_type>::storage_type>, "Invalid storage");
+
+    if constexpr(std::is_void_v<decltype(container.get({}))>) {
+        return std::make_tuple();
+    } else {
+        return std::forward_as_tuple(container.get(entt));
+    }
+}
+
+
+}
+
+
+#endif
+
+// #include "utility.hpp"
+#ifndef ENTT_ENTITY_UTILITY_HPP
+#define ENTT_ENTITY_UTILITY_HPP
+
+
+// #include "../core/type_traits.hpp"
+
+
+
+namespace entt {
+
+
+/**
+ * @brief Alias for exclusion lists.
+ * @tparam Type List of types.
+ */
+template<typename... Type>
+struct exclude_t: type_list<Type...> {};
+
+
+/**
+ * @brief Variable template for exclusion lists.
+ * @tparam Type List of types.
+ */
+template<typename... Type>
+inline constexpr exclude_t<Type...> exclude{};
+
+
+/**
+ * @brief Alias for lists of observed components.
+ * @tparam Type List of types.
+ */
+template<typename... Type>
+struct get_t: type_list<Type...>{};
+
+
+/**
+ * @brief Variable template for lists of observed components.
+ * @tparam Type List of types.
+ */
+template<typename... Type>
+inline constexpr get_t<Type...> get{};
+
+
+}
+
+
+#endif
+
+
+
+namespace entt {
+
+
+/**
+ * @brief Group.
+ *
+ * Primary template isn't defined on purpose. All the specializations give a
+ * compile-time error, but for a few reasonable cases.
+ */
+template<typename...>
+class basic_group;
+
+
+/**
+ * @brief Non-owning group.
+ *
+ * A non-owning group returns all entities and only the entities that have at
+ * least the given components. Moreover, it's guaranteed that the entity list
+ * is tightly packed in memory for fast iterations.
+ *
+ * @b Important
+ *
+ * Iterators aren't invalidated if:
+ *
+ * * New instances of the given components are created and assigned to entities.
+ * * The entity currently pointed is modified (as an example, if one of the
+ *   given components is removed from the entity to which the iterator points).
+ * * The entity currently pointed is destroyed.
+ *
+ * In all other cases, modifying the pools iterated by the group in any way
+ * invalidates all the iterators and using them results in undefined behavior.
+ *
+ * @note
+ * Groups share references to the underlying data structures of the registry
+ * that generated them. Therefore any change to the entities and to the
+ * components made by means of the registry are immediately reflected by all the
+ * groups.<br/>
+ * Moreover, sorting a non-owning group affects all the instances of the same
+ * group (it means that users don't have to call `sort` on each instance to sort
+ * all of them because they _share_ entities and components).
+ *
+ * @warning
+ * Lifetime of a group must not overcome that of the registry that generated it.
+ * In any other case, attempting to use a group results in undefined behavior.
+ *
+ * @tparam Entity A valid entity type (see entt_traits for more details).
+ * @tparam Exclude Types of components used to filter the group.
+ * @tparam Get Type of components observed by the group.
+ */
+template<typename Entity, typename... Exclude, typename... Get>
+class basic_group<Entity, exclude_t<Exclude...>, get_t<Get...>> final {
+    /*! @brief A registry is allowed to create groups. */
+    friend class basic_registry<Entity>;
+
+    using basic_common_type = basic_sparse_set<Entity>;
+
+    template<typename Component>
+    using storage_type = constness_as_t<typename storage_traits<Entity, std::remove_const_t<Component>>::storage_type, Component>;
+
+    class iterable final {
+        template<typename It>
+        struct iterable_iterator final {
+            using difference_type = std::ptrdiff_t;
+            using value_type = decltype(std::tuple_cat(std::tuple<Entity>{}, std::declval<basic_group>().get({})));
+            using pointer = void;
+            using reference = value_type;
+            using iterator_category = std::input_iterator_tag;
+
+            template<typename... Args>
+            iterable_iterator(It from, const std::tuple<storage_type<Get> *...> &args) ENTT_NOEXCEPT
+                : it{from},
+                  pools{args}
+            {}
+
+            iterable_iterator & operator++() ENTT_NOEXCEPT {
+                return ++it, *this;
+            }
+
+            iterable_iterator operator++(int) ENTT_NOEXCEPT {
+                iterable_iterator orig = *this;
+                return ++(*this), orig;
+            }
+
+            [[nodiscard]] reference operator*() const ENTT_NOEXCEPT {
+                const auto entt = *it;
+                return std::tuple_cat(std::make_tuple(entt), get_as_tuple(*std::get<storage_type<Get> *>(pools), entt)...);
+            }
+
+            [[nodiscard]] bool operator==(const iterable_iterator &other) const ENTT_NOEXCEPT {
+                return other.it == it;
+            }
+
+            [[nodiscard]] bool operator!=(const iterable_iterator &other) const ENTT_NOEXCEPT {
+                return !(*this == other);
+            }
+
+        private:
+            It it;
+            std::tuple<storage_type<Get> *...> pools;
+        };
+
+    public:
+        using iterator = iterable_iterator<typename basic_common_type::iterator>;
+        using reverse_iterator = iterable_iterator<typename basic_common_type::reverse_iterator>;
+
+        iterable(basic_common_type * const ref, const std::tuple<storage_type<Get> *...> &cpools)
+            : handler{ref},
+              pools{cpools}
+        {}
+
+        [[nodiscard]] iterator begin() const ENTT_NOEXCEPT {
+            return handler ? iterator{handler->begin(), pools} : iterator{{}, pools};
+        }
+
+        [[nodiscard]] iterator end() const ENTT_NOEXCEPT {
+            return handler ? iterator{handler->end(), pools} : iterator{{}, pools};
+        }
+
+        [[nodiscard]] reverse_iterator rbegin() const ENTT_NOEXCEPT {
+            return handler ? reverse_iterator{handler->rbegin(), pools} : reverse_iterator{{}, pools};
+        }
+
+        [[nodiscard]] reverse_iterator rend() const ENTT_NOEXCEPT {
+            return handler ? reverse_iterator{handler->rend(), pools} : reverse_iterator{{}, pools};
+        }
+
+    private:
+        basic_common_type * const handler;
+        const std::tuple<storage_type<Get> *...> pools;
+    };
+
+    basic_group(basic_common_type &ref, storage_type<Get> &... gpool) ENTT_NOEXCEPT
+        : handler{&ref},
+          pools{&gpool...}
+    {}
+
+public:
+    /*! @brief Underlying entity identifier. */
+    using entity_type = Entity;
+    /*! @brief Unsigned integer type. */
+    using size_type = std::size_t;
+    /*! @brief Random access iterator type. */
+    using iterator = typename basic_common_type::iterator;
+    /*! @brief Reversed iterator type. */
+    using reverse_iterator = typename basic_common_type::reverse_iterator;
+    /*! @brief Iterable group type. */
+    using iterable_group = iterable;
+
+    /*! @brief Default constructor to use to create empty, invalid groups. */
+    basic_group() ENTT_NOEXCEPT
+        : handler{}
+    {}
+
+    /**
+     * @brief Returns the number of entities that have the given components.
+     * @return Number of entities that have the given components.
+     */
+    [[nodiscard]] size_type size() const ENTT_NOEXCEPT {
+        return *this ? handler->size() : size_type{};
+    }
+
+    /**
+     * @brief Returns the number of elements that a group has currently
+     * allocated space for.
+     * @return Capacity of the group.
+     */
+    [[nodiscard]] size_type capacity() const ENTT_NOEXCEPT {
+        return *this ? handler->capacity() : size_type{};
+    }
+
+    /*! @brief Requests the removal of unused capacity. */
+    void shrink_to_fit() {
+        if(*this) {
+            handler->shrink_to_fit();
+        }
+    }
+
+    /**
+     * @brief Checks whether a group is empty.
+     * @return True if the group is empty, false otherwise.
+     */
+    [[nodiscard]] bool empty() const ENTT_NOEXCEPT {
+        return !*this || handler->empty();
+    }
+
+    /**
+     * @brief Direct access to the list of entities.
+     *
+     * The returned pointer is such that range `[data(), data() + size())` is
+     * always a valid range, even if the container is empty.
+     *
+     * @return A pointer to the array of entities.
+     */
+    [[nodiscard]] auto data() const ENTT_NOEXCEPT {
+        return *this ? handler->data() : nullptr;
+    }
+
+    /**
+     * @brief Returns an iterator to the first entity of the group.
+     *
+     * The returned iterator points to the first entity of the group. If the
+     * group is empty, the returned iterator will be equal to `end()`.
+     *
+     * @return An iterator to the first entity of the group.
+     */
+    [[nodiscard]] iterator begin() const ENTT_NOEXCEPT {
+        return *this ? handler->begin() : iterator{};
+    }
+
+    /**
+     * @brief Returns an iterator that is past the last entity of the group.
+     *
+     * The returned iterator points to the entity following the last entity of
+     * the group. Attempting to dereference the returned iterator results in
+     * undefined behavior.
+     *
+     * @return An iterator to the entity following the last entity of the
+     * group.
+     */
+    [[nodiscard]] iterator end() const ENTT_NOEXCEPT {
+        return *this ? handler->end() : iterator{};
+    }
+
+    /**
+     * @brief Returns an iterator to the first entity of the reversed group.
+     *
+     * The returned iterator points to the first entity of the reversed group.
+     * If the group is empty, the returned iterator will be equal to `rend()`.
+     *
+     * @return An iterator to the first entity of the reversed group.
+     */
+    [[nodiscard]] reverse_iterator rbegin() const ENTT_NOEXCEPT {
+        return *this ? handler->rbegin() : reverse_iterator{};
+    }
+
+    /**
+     * @brief Returns an iterator that is past the last entity of the reversed
+     * group.
+     *
+     * The returned iterator points to the entity following the last entity of
+     * the reversed group. Attempting to dereference the returned iterator
+     * results in undefined behavior.
+     *
+     * @return An iterator to the entity following the last entity of the
+     * reversed group.
+     */
+    [[nodiscard]] reverse_iterator rend() const ENTT_NOEXCEPT {
+        return *this ? handler->rend() : reverse_iterator{};
+    }
+
+    /**
+     * @brief Returns the first entity of the group, if any.
+     * @return The first entity of the group if one exists, the null entity
+     * otherwise.
+     */
+    [[nodiscard]] entity_type front() const {
+        const auto it = begin();
+        return it != end() ? *it : null;
+    }
+
+    /**
+     * @brief Returns the last entity of the group, if any.
+     * @return The last entity of the group if one exists, the null entity
+     * otherwise.
+     */
+    [[nodiscard]] entity_type back() const {
+        const auto it = rbegin();
+        return it != rend() ? *it : null;
+    }
+
+    /**
+     * @brief Finds an entity.
+     * @param entt A valid entity identifier.
+     * @return An iterator to the given entity if it's found, past the end
+     * iterator otherwise.
+     */
+    [[nodiscard]] iterator find(const entity_type entt) const {
+        const auto it = *this ? handler->find(entt) : iterator{};
+        return it != end() && *it == entt ? it : end();
+    }
+
+    /**
+     * @brief Returns the identifier that occupies the given position.
+     * @param pos Position of the element to return.
+     * @return The identifier that occupies the given position.
+     */
+    [[nodiscard]] entity_type operator[](const size_type pos) const {
+        return begin()[pos];
+    }
+
+    /**
+     * @brief Checks if a group is properly initialized.
+     * @return True if the group is properly initialized, false otherwise.
+     */
+    [[nodiscard]] explicit operator bool() const ENTT_NOEXCEPT {
+        return handler != nullptr;
+    }
+
+    /**
+     * @brief Checks if a group contains an entity.
+     * @param entt A valid entity identifier.
+     * @return True if the group contains the given entity, false otherwise.
+     */
+    [[nodiscard]] bool contains(const entity_type entt) const {
+        return *this && handler->contains(entt);
+    }
+
+    /**
+     * @brief Returns the components assigned to the given entity.
+     *
+     * Prefer this function instead of `registry::get` during iterations. It has
+     * far better performance than its counterpart.
+     *
+     * @warning
+     * Attempting to use an invalid component type results in a compilation
+     * error. Attempting to use an entity that doesn't belong to the group
+     * results in undefined behavior.
+     *
+     * @tparam Component Types of components to get.
+     * @param entt A valid entity identifier.
+     * @return The components assigned to the entity.
+     */
+    template<typename... Component>
+    [[nodiscard]] decltype(auto) get(const entity_type entt) const {
+        ENTT_ASSERT(contains(entt), "Group does not contain entity");
+
+        if constexpr(sizeof...(Component) == 0) {
+            return std::tuple_cat(get_as_tuple(*std::get<storage_type<Get> *>(pools), entt)...);
+        } else if constexpr(sizeof...(Component) == 1) {
+            return (std::get<storage_type<Component> *>(pools)->get(entt), ...);
+        } else {
+            return std::tuple_cat(get_as_tuple(*std::get<storage_type<Component> *>(pools), entt)...);
+        }
+    }
+
+    /**
+     * @brief Iterates entities and components and applies the given function
+     * object to them.
+     *
+     * The function object is invoked for each entity. It is provided with the
+     * entity itself and a set of references to non-empty components. The
+     * _constness_ of the components is as requested.<br/>
+     * The signature of the function must be equivalent to one of the following
+     * forms:
+     *
+     * @code{.cpp}
+     * void(const entity_type, Type &...);
+     * void(Type &...);
+     * @endcode
+     *
+     * @note
+     * Empty types aren't explicitly instantiated and therefore they are never
+     * returned during iterations.
+     *
+     * @tparam Func Type of the function object to invoke.
+     * @param func A valid function object.
+     */
+    template<typename Func>
+    void each(Func func) const {
+        for(const auto entt: *this) {
+            if constexpr(is_applicable_v<Func, decltype(std::tuple_cat(std::tuple<entity_type>{}, std::declval<basic_group>().get({})))>) {
+                std::apply(func, std::tuple_cat(std::make_tuple(entt), get(entt)));
+            } else {
+                std::apply(func, get(entt));
+            }
+        }
+    }
+
+    /**
+     * @brief Returns an iterable object to use to _visit_ the group.
+     *
+     * The iterable object returns tuples that contain the current entity and a
+     * set of references to its non-empty components. The _constness_ of the
+     * components is as requested.
+     *
+     * @note
+     * Empty types aren't explicitly instantiated and therefore they are never
+     * returned during iterations.
+     *
+     * @return An iterable object to use to _visit_ the group.
+     */
+    [[nodiscard]] iterable_group each() const ENTT_NOEXCEPT {
+        return iterable_group{handler, pools};
+    }
+
+    /**
+     * @brief Sort a group according to the given comparison function.
+     *
+     * Sort the group so that iterating it with a couple of iterators returns
+     * entities and components in the expected order. See `begin` and `end` for
+     * more details.
+     *
+     * The comparison function object must return `true` if the first element
+     * is _less_ than the second one, `false` otherwise. The signature of the
+     * comparison function should be equivalent to one of the following:
+     *
+     * @code{.cpp}
+     * bool(std::tuple<Component &...>, std::tuple<Component &...>);
+     * bool(const Component &..., const Component &...);
+     * bool(const Entity, const Entity);
+     * @endcode
+     *
+     * Where `Component` are such that they are iterated by the group.<br/>
+     * Moreover, the comparison function object shall induce a
+     * _strict weak ordering_ on the values.
+     *
+     * The sort function oject must offer a member function template
+     * `operator()` that accepts three arguments:
+     *
+     * * An iterator to the first element of the range to sort.
+     * * An iterator past the last element of the range to sort.
+     * * A comparison function to use to compare the elements.
+     *
+     * @tparam Component Optional types of components to compare.
+     * @tparam Compare Type of comparison function object.
+     * @tparam Sort Type of sort function object.
+     * @tparam Args Types of arguments to forward to the sort function object.
+     * @param compare A valid comparison function object.
+     * @param algo A valid sort function object.
+     * @param args Arguments to forward to the sort function object, if any.
+     */
+    template<typename... Component, typename Compare, typename Sort = std_sort, typename... Args>
+    void sort(Compare compare, Sort algo = Sort{}, Args &&... args) {
+        if(*this) {
+            if constexpr(sizeof...(Component) == 0) {
+                static_assert(std::is_invocable_v<Compare, const entity_type, const entity_type>, "Invalid comparison function");
+                handler->sort(std::move(compare), std::move(algo), std::forward<Args>(args)...);
+            }  else if constexpr(sizeof...(Component) == 1) {
+                handler->sort([this, compare = std::move(compare)](const entity_type lhs, const entity_type rhs) {
+                    return compare((std::get<storage_type<Component> *>(pools)->get(lhs), ...), (std::get<storage_type<Component> *>(pools)->get(rhs), ...));
+                }, std::move(algo), std::forward<Args>(args)...);
+            } else {
+                handler->sort([this, compare = std::move(compare)](const entity_type lhs, const entity_type rhs) {
+                    return compare(std::forward_as_tuple(std::get<storage_type<Component> *>(pools)->get(lhs)...), std::forward_as_tuple(std::get<storage_type<Component> *>(pools)->get(rhs)...));
+                }, std::move(algo), std::forward<Args>(args)...);
+            }
+        }
+    }
+
+    /**
+     * @brief Sort the shared pool of entities according to the given component.
+     *
+     * Non-owning groups of the same type share with the registry a pool of
+     * entities with its own order that doesn't depend on the order of any pool
+     * of components. Users can order the underlying data structure so that it
+     * respects the order of the pool of the given component.
+     *
+     * @note
+     * The shared pool of entities and thus its order is affected by the changes
+     * to each and every pool that it tracks. Therefore changes to those pools
+     * can quickly ruin the order imposed to the pool of entities shared between
+     * the non-owning groups.
+     *
+     * @tparam Component Type of component to use to impose the order.
+     */
+    template<typename Component>
+    void sort() const {
+        if(*this) {
+            handler->respect(*std::get<storage_type<Component> *>(pools));
+        }
+    }
+
+private:
+    basic_common_type * const handler;
+    const std::tuple<storage_type<Get> *...> pools;
+};
+
+
+/**
+ * @brief Owning group.
+ *
+ * Owning groups return all entities and only the entities that have at least
+ * the given components. Moreover:
+ *
+ * * It's guaranteed that the entity list is tightly packed in memory for fast
+ *   iterations.
+ * * It's guaranteed that the lists of owned components are tightly packed in
+ *   memory for even faster iterations and to allow direct access.
+ * * They stay true to the order of the owned components and all instances have
+ *   the same order in memory.
+ *
+ * The more types of components are owned by a group, the faster it is to
+ * iterate them.
+ *
+ * @b Important
+ *
+ * Iterators aren't invalidated if:
+ *
+ * * New instances of the given components are created and assigned to entities.
+ * * The entity currently pointed is modified (as an example, if one of the
+ *   given components is removed from the entity to which the iterator points).
+ * * The entity currently pointed is destroyed.
+ *
+ * In all other cases, modifying the pools iterated by the group in any way
+ * invalidates all the iterators and using them results in undefined behavior.
+ *
+ * @note
+ * Groups share references to the underlying data structures of the registry
+ * that generated them. Therefore any change to the entities and to the
+ * components made by means of the registry are immediately reflected by all the
+ * groups.
+ * Moreover, sorting an owning group affects all the instance of the same group
+ * (it means that users don't have to call `sort` on each instance to sort all
+ * of them because they share the underlying data structure).
+ *
+ * @warning
+ * Lifetime of a group must not overcome that of the registry that generated it.
+ * In any other case, attempting to use a group results in undefined behavior.
+ *
+ * @tparam Entity A valid entity type (see entt_traits for more details).
+ * @tparam Exclude Types of components used to filter the group.
+ * @tparam Get Types of components observed by the group.
+ * @tparam Owned Types of components owned by the group.
+ */
+template<typename Entity, typename... Exclude, typename... Get, typename... Owned>
+class basic_group<Entity, exclude_t<Exclude...>, get_t<Get...>, Owned...> final {
+    /*! @brief A registry is allowed to create groups. */
+    friend class basic_registry<Entity>;
+
+    using basic_common_type = basic_sparse_set<Entity>;
+
+    template<typename Component>
+    using storage_type = constness_as_t<typename storage_traits<Entity, std::remove_const_t<Component>>::storage_type, Component>;
+
+    class iterable final {
+        template<typename, typename>
+        struct iterable_iterator;
+
+        template<typename It, typename... OIt>
+        struct iterable_iterator<It, type_list<OIt...>> final {
+            using difference_type = std::ptrdiff_t;
+            using value_type = decltype(std::tuple_cat(std::tuple<Entity>{}, std::declval<basic_group>().get({})));
+            using pointer = void;
+            using reference = value_type;
+            using iterator_category = std::input_iterator_tag;
+
+            template<typename... Other>
+            iterable_iterator(It from, const std::tuple<Other...> &other, const std::tuple<storage_type<Get> *...> &cpools) ENTT_NOEXCEPT
+                : it{from},
+                  owned{std::get<OIt>(other)...},
+                  get{cpools}
+            {}
+
+            iterable_iterator & operator++() ENTT_NOEXCEPT {
+                return ++it, (++std::get<OIt>(owned), ...), *this;
+            }
+
+            iterable_iterator operator++(int) ENTT_NOEXCEPT {
+                iterable_iterator orig = *this;
+                return ++(*this), orig;
+            }
+
+            [[nodiscard]] reference operator*() const ENTT_NOEXCEPT {
+                return std::tuple_cat(
+                    std::make_tuple(*it),
+                    std::forward_as_tuple(*std::get<OIt>(owned)...),
+                    get_as_tuple(*std::get<storage_type<Get> *>(get), *it)...
+                );
+            }
+
+            [[nodiscard]] bool operator==(const iterable_iterator &other) const ENTT_NOEXCEPT {
+                return other.it == it;
+            }
+
+            [[nodiscard]] bool operator!=(const iterable_iterator &other) const ENTT_NOEXCEPT {
+                return !(*this == other);
+            }
+
+        private:
+            It it;
+            std::tuple<OIt...> owned;
+            std::tuple<storage_type<Get> *...> get;
+        };
+
+    public:
+        using iterator = iterable_iterator<
+            typename basic_common_type::iterator,
+            type_list_cat_t<std::conditional_t<std::is_void_v<decltype(std::declval<storage_type<Owned>>().get({}))>, type_list<>, type_list<decltype(std::declval<storage_type<Owned>>().end())>>...>
+        >;
+        using reverse_iterator = iterable_iterator<
+            typename basic_common_type::reverse_iterator,
+            type_list_cat_t<std::conditional_t<std::is_void_v<decltype(std::declval<storage_type<Owned>>().get({}))>, type_list<>, type_list<decltype(std::declval<storage_type<Owned>>().rbegin())>>...>
+        >;
+
+        iterable(std::tuple<storage_type<Owned> *..., storage_type<Get> *...> cpools, const std::size_t * const extent)
+            : pools{cpools},
+              length{extent}
+        {}
+
+        [[nodiscard]] iterator begin() const ENTT_NOEXCEPT {
+            return length ? iterator{
+                std::get<0>(pools)->basic_common_type::end() - *length,
+                std::make_tuple((std::get<storage_type<Owned> *>(pools)->end() - *length)...),
+                std::make_tuple(std::get<storage_type<Get> *>(pools)...)
+            } : iterator{{}, std::make_tuple(decltype(std::get<storage_type<Owned> *>(pools)->end()){}...), std::make_tuple(std::get<storage_type<Get> *>(pools)...)};
+        }
+
+        [[nodiscard]] iterator end() const ENTT_NOEXCEPT {
+            return length ? iterator{
+                std::get<0>(pools)->basic_common_type::end(),
+                std::make_tuple((std::get<storage_type<Owned> *>(pools)->end())...),
+                std::make_tuple(std::get<storage_type<Get> *>(pools)...)
+            } : iterator{{}, std::make_tuple(decltype(std::get<storage_type<Owned> *>(pools)->end()){}...), std::make_tuple(std::get<storage_type<Get> *>(pools)...)};
+        }
+
+        [[nodiscard]] reverse_iterator rbegin() const ENTT_NOEXCEPT {
+            return length ? reverse_iterator{
+                std::get<0>(pools)->basic_common_type::rbegin(),
+                std::make_tuple((std::get<storage_type<Owned> *>(pools)->rbegin())...),
+                std::make_tuple(std::get<storage_type<Get> *>(pools)...)
+            } : reverse_iterator{{}, std::make_tuple(decltype(std::get<storage_type<Owned> *>(pools)->rbegin()){}...), std::make_tuple(std::get<storage_type<Get> *>(pools)...)};
+        }
+
+        [[nodiscard]] reverse_iterator rend() const ENTT_NOEXCEPT {
+            return length ? reverse_iterator{
+                std::get<0>(pools)->basic_common_type::rbegin() + *length,
+                std::make_tuple((std::get<storage_type<Owned> *>(pools)->rbegin() + *length)...),
+                std::make_tuple(std::get<storage_type<Get> *>(pools)...)
+            } : reverse_iterator{{}, std::make_tuple(decltype(std::get<storage_type<Owned> *>(pools)->rbegin()){}...), std::make_tuple(std::get<storage_type<Get> *>(pools)...)};
+        }
+
+    private:
+        const std::tuple<storage_type<Owned> *..., storage_type<Get> *...> pools;
+        const std::size_t * const length;
+    };
+
+    basic_group(const std::size_t &extent, storage_type<Owned> &... opool, storage_type<Get> &... gpool) ENTT_NOEXCEPT
+        : pools{&opool..., &gpool...},
+          length{&extent}
+    {}
+
+public:
+    /*! @brief Underlying entity identifier. */
+    using entity_type = Entity;
+    /*! @brief Unsigned integer type. */
+    using size_type = std::size_t;
+    /*! @brief Random access iterator type. */
+    using iterator = typename basic_common_type::iterator;
+    /*! @brief Reversed iterator type. */
+    using reverse_iterator = typename basic_common_type::reverse_iterator;
+    /*! @brief Iterable group type. */
+    using iterable_group = iterable;
+
+    /*! @brief Default constructor to use to create empty, invalid groups. */
+    basic_group() ENTT_NOEXCEPT
+        : length{}
+    {}
+
+    /**
+     * @brief Returns the number of entities that have the given components.
+     * @return Number of entities that have the given components.
+     */
+    [[nodiscard]] size_type size() const ENTT_NOEXCEPT {
+        return *this ? *length : size_type{};
+    }
+
+    /**
+     * @brief Checks whether a group is empty.
+     * @return True if the group is empty, false otherwise.
+     */
+    [[nodiscard]] bool empty() const ENTT_NOEXCEPT {
+        return !*this || !*length;
+    }
+
+    /**
+     * @brief Direct access to the raw representation offered by the storage.
+     *
+     * @warning
+     * This function is only available for owned types.
+     *
+     * @tparam Component Type of component in which one is interested.
+     * @return A pointer to the array of components.
+     */
+    template<typename Component>
+    [[nodiscard]] auto raw() const ENTT_NOEXCEPT {
+        static_assert((std::is_same_v<Component, Owned> || ...), "Non-owned type");
+        auto *cpool = std::get<storage_type<Component> *>(pools);
+        return cpool ? cpool->raw() : decltype(cpool->raw()){};
+    }
+
+    /**
+     * @brief Direct access to the list of entities.
+     *
+     * The returned pointer is such that range `[data(), data() + size())` is
+     * always a valid range, even if the container is empty.
+     *
+     * @return A pointer to the array of entities.
+     */
+    [[nodiscard]] auto data() const ENTT_NOEXCEPT {
+        return *this ? std::get<0>(pools)->data() : nullptr;
+    }
+
+    /**
+     * @brief Returns an iterator to the first entity of the group.
+     *
+     * The returned iterator points to the first entity of the group. If the
+     * group is empty, the returned iterator will be equal to `end()`.
+     *
+     * @return An iterator to the first entity of the group.
+     */
+    [[nodiscard]] iterator begin() const ENTT_NOEXCEPT {
+        return *this ? (std::get<0>(pools)->basic_common_type::end() - *length) : iterator{};
+    }
+
+    /**
+     * @brief Returns an iterator that is past the last entity of the group.
+     *
+     * The returned iterator points to the entity following the last entity of
+     * the group. Attempting to dereference the returned iterator results in
+     * undefined behavior.
+     *
+     * @return An iterator to the entity following the last entity of the
+     * group.
+     */
+    [[nodiscard]] iterator end() const ENTT_NOEXCEPT {
+        return *this ? std::get<0>(pools)->basic_common_type::end() : iterator{};
+    }
+
+    /**
+     * @brief Returns an iterator to the first entity of the reversed group.
+     *
+     * The returned iterator points to the first entity of the reversed group.
+     * If the group is empty, the returned iterator will be equal to `rend()`.
+     *
+     * @return An iterator to the first entity of the reversed group.
+     */
+    [[nodiscard]] reverse_iterator rbegin() const ENTT_NOEXCEPT {
+        return *this ? std::get<0>(pools)->basic_common_type::rbegin() : reverse_iterator{};
+    }
+
+    /**
+     * @brief Returns an iterator that is past the last entity of the reversed
+     * group.
+     *
+     * The returned iterator points to the entity following the last entity of
+     * the reversed group. Attempting to dereference the returned iterator
+     * results in undefined behavior.
+     *
+     * @return An iterator to the entity following the last entity of the
+     * reversed group.
+     */
+    [[nodiscard]] reverse_iterator rend() const ENTT_NOEXCEPT {
+        return *this ? (std::get<0>(pools)->basic_common_type::rbegin() + *length) : reverse_iterator{};
+    }
+
+    /**
+     * @brief Returns the first entity of the group, if any.
+     * @return The first entity of the group if one exists, the null entity
+     * otherwise.
+     */
+    [[nodiscard]] entity_type front() const {
+        const auto it = begin();
+        return it != end() ? *it : null;
+    }
+
+    /**
+     * @brief Returns the last entity of the group, if any.
+     * @return The last entity of the group if one exists, the null entity
+     * otherwise.
+     */
+    [[nodiscard]] entity_type back() const {
+        const auto it = rbegin();
+        return it != rend() ? *it : null;
+    }
+
+    /**
+     * @brief Finds an entity.
+     * @param entt A valid entity identifier.
+     * @return An iterator to the given entity if it's found, past the end
+     * iterator otherwise.
+     */
+    [[nodiscard]] iterator find(const entity_type entt) const {
+        const auto it = *this ? std::get<0>(pools)->find(entt) : iterator{};
+        return it != end() && it >= begin() && *it == entt ? it : end();
+    }
+
+    /**
+     * @brief Returns the identifier that occupies the given position.
+     * @param pos Position of the element to return.
+     * @return The identifier that occupies the given position.
+     */
+    [[nodiscard]] entity_type operator[](const size_type pos) const {
+        return begin()[pos];
+    }
+
+    /**
+     * @brief Checks if a group is properly initialized.
+     * @return True if the group is properly initialized, false otherwise.
+     */
+    [[nodiscard]] explicit operator bool() const ENTT_NOEXCEPT {
+        return length != nullptr;
+    }
+
+    /**
+     * @brief Checks if a group contains an entity.
+     * @param entt A valid entity identifier.
+     * @return True if the group contains the given entity, false otherwise.
+     */
+    [[nodiscard]] bool contains(const entity_type entt) const {
+        return *this && std::get<0>(pools)->contains(entt) && (std::get<0>(pools)->index(entt) < (*length));
+    }
+
+    /**
+     * @brief Returns the components assigned to the given entity.
+     *
+     * Prefer this function instead of `registry::get` during iterations. It has
+     * far better performance than its counterpart.
+     *
+     * @warning
+     * Attempting to use an invalid component type results in a compilation
+     * error. Attempting to use an entity that doesn't belong to the group
+     * results in undefined behavior.
+     *
+     * @tparam Component Types of components to get.
+     * @param entt A valid entity identifier.
+     * @return The components assigned to the entity.
+     */
+    template<typename... Component>
+    [[nodiscard]] decltype(auto) get(const entity_type entt) const {
+        ENTT_ASSERT(contains(entt), "Group does not contain entity");
+
+        if constexpr(sizeof...(Component) == 0) {
+            return std::tuple_cat(get_as_tuple(*std::get<storage_type<Owned> *>(pools), entt)..., get_as_tuple(*std::get<storage_type<Get> *>(pools), entt)...);
+        } else if constexpr(sizeof...(Component) == 1) {
+            return (std::get<storage_type<Component> *>(pools)->get(entt), ...);
+        } else {
+            return std::tuple_cat(get_as_tuple(*std::get<storage_type<Component> *>(pools), entt)...);
+        }
+    }
+
+    /**
+     * @brief Iterates entities and components and applies the given function
+     * object to them.
+     *
+     * The function object is invoked for each entity. It is provided with the
+     * entity itself and a set of references to non-empty components. The
+     * _constness_ of the components is as requested.<br/>
+     * The signature of the function must be equivalent to one of the following
+     * forms:
+     *
+     * @code{.cpp}
+     * void(const entity_type, Type &...);
+     * void(Type &...);
+     * @endcode
+     *
+     * @note
+     * Empty types aren't explicitly instantiated and therefore they are never
+     * returned during iterations.
+     *
+     * @tparam Func Type of the function object to invoke.
+     * @param func A valid function object.
+     */
+    template<typename Func>
+    void each(Func func) const {
+        for(auto args: each()) {
+            if constexpr(is_applicable_v<Func, decltype(std::tuple_cat(std::tuple<entity_type>{}, std::declval<basic_group>().get({})))>) {
+                std::apply(func, args);
+            } else {
+                std::apply([&func](auto, auto &&... less) { func(std::forward<decltype(less)>(less)...); }, args);
+            }
+        }
+    }
+
+    /**
+     * @brief Returns an iterable object to use to _visit_ the group.
+     *
+     * The iterable object returns tuples that contain the current entity and a
+     * set of references to its non-empty components. The _constness_ of the
+     * components is as requested.
+     *
+     * @note
+     * Empty types aren't explicitly instantiated and therefore they are never
+     * returned during iterations.
+     *
+     * @return An iterable object to use to _visit_ the group.
+     */
+    [[nodiscard]] iterable_group each() const ENTT_NOEXCEPT {
+        return iterable_group{pools, length};
+    }
+
+    /**
+     * @brief Sort a group according to the given comparison function.
+     *
+     * Sort the group so that iterating it with a couple of iterators returns
+     * entities and components in the expected order. See `begin` and `end` for
+     * more details.
+     *
+     * The comparison function object must return `true` if the first element
+     * is _less_ than the second one, `false` otherwise. The signature of the
+     * comparison function should be equivalent to one of the following:
+     *
+     * @code{.cpp}
+     * bool(std::tuple<Component &...>, std::tuple<Component &...>);
+     * bool(const Component &, const Component &);
+     * bool(const Entity, const Entity);
+     * @endcode
+     *
+     * Where `Component` are either owned types or not but still such that they
+     * are iterated by the group.<br/>
+     * Moreover, the comparison function object shall induce a
+     * _strict weak ordering_ on the values.
+     *
+     * The sort function oject must offer a member function template
+     * `operator()` that accepts three arguments:
+     *
+     * * An iterator to the first element of the range to sort.
+     * * An iterator past the last element of the range to sort.
+     * * A comparison function to use to compare the elements.
+     *
+     * @tparam Component Optional types of components to compare.
+     * @tparam Compare Type of comparison function object.
+     * @tparam Sort Type of sort function object.
+     * @tparam Args Types of arguments to forward to the sort function object.
+     * @param compare A valid comparison function object.
+     * @param algo A valid sort function object.
+     * @param args Arguments to forward to the sort function object, if any.
+     */
+    template<typename... Component, typename Compare, typename Sort = std_sort, typename... Args>
+    void sort(Compare compare, Sort algo = Sort{}, Args &&... args) const {
+        auto *cpool = std::get<0>(pools);
+
+        if constexpr(sizeof...(Component) == 0) {
+            static_assert(std::is_invocable_v<Compare, const entity_type, const entity_type>, "Invalid comparison function");
+            cpool->sort_n(*length, std::move(compare), std::move(algo), std::forward<Args>(args)...);
+        } else if constexpr(sizeof...(Component) == 1) {
+            cpool->sort_n(*length, [this, compare = std::move(compare)](const entity_type lhs, const entity_type rhs) {
+                return compare((std::get<storage_type<Component> *>(pools)->get(lhs), ...), (std::get<storage_type<Component> *>(pools)->get(rhs), ...));
+            }, std::move(algo), std::forward<Args>(args)...);
+        } else {
+            cpool->sort_n(*length, [this, compare = std::move(compare)](const entity_type lhs, const entity_type rhs) {
+                return compare(std::forward_as_tuple(std::get<storage_type<Component> *>(pools)->get(lhs)...), std::forward_as_tuple(std::get<storage_type<Component> *>(pools)->get(rhs)...));
+            }, std::move(algo), std::forward<Args>(args)...);
+        }
+
+        [this](auto *head, auto *... other) {
+            for(auto next = *length; next; --next) {
+                const auto pos = next - 1;
+                [[maybe_unused]] const auto entt = head->data()[pos];
+                (other->swap(other->data()[pos], entt), ...);
+            }
+        }(std::get<storage_type<Owned> *>(pools)...);
+    }
+
+private:
+    const std::tuple<storage_type<Owned> *..., storage_type<Get> *...> pools;
+    const size_type * const length;
+};
+
+
+}
+
+
+#endif
+
+// #include "entity/handle.hpp"
+#ifndef ENTT_ENTITY_HANDLE_HPP
+#define ENTT_ENTITY_HANDLE_HPP
+
+
+#include <tuple>
+#include <type_traits>
+#include <utility>
+// #include "../config/config.h"
+
+// #include "../core/type_traits.hpp"
+
+// #include "fwd.hpp"
 
 // #include "registry.hpp"
 #ifndef ENTT_ENTITY_REGISTRY_HPP
 #define ENTT_ENTITY_REGISTRY_HPP
 
 
-#include <tuple>
-#include <vector>
-#include <memory>
-#include <utility>
+#include <algorithm>
 #include <cstddef>
 #include <iterator>
-#include <algorithm>
-#include <type_traits>
-// #include "../config/config.h"
-
-// #include "../core/family.hpp"
-#ifndef ENTT_CORE_FAMILY_HPP
-#define ENTT_CORE_FAMILY_HPP
-
-
-#include <type_traits>
+#include <memory>
+#include <tuple>
+#include <type_traits>
+#include <utility>
+#include <vector>
+// #include "../config/config.h"
+
+// #include "../core/algorithm.hpp"
+
+// #include "../core/any.hpp"
+#ifndef ENTT_CORE_ANY_HPP
+#define ENTT_CORE_ANY_HPP
+
+
+#include <cstddef>
+#include <functional>
+#include <memory>
+#include <type_traits>
+#include <utility>
+// #include "../config/config.h"
+
+// #include "../core/utility.hpp"
+#ifndef ENTT_CORE_UTILITY_HPP
+#define ENTT_CORE_UTILITY_HPP
+
+
+#include <utility>
 // #include "../config/config.h"
 #ifndef ENTT_CONFIG_CONFIG_H
 #define ENTT_CONFIG_CONFIG_H
 
 
-#ifndef ENTT_NOEXCEPT
-#define ENTT_NOEXCEPT noexcept
-#endif // ENTT_NOEXCEPT
-
-
-#ifndef ENTT_HS_SUFFIX
-#define ENTT_HS_SUFFIX _hs
-#endif // ENTT_HS_SUFFIX
-
-
-#ifndef ENTT_HWS_SUFFIX
-#define ENTT_HWS_SUFFIX _hws
-#endif // ENTT_HWS_SUFFIX
-
-
-#ifndef ENTT_NO_ATOMIC
-#include <atomic>
-#define ENTT_MAYBE_ATOMIC(Type) std::atomic<Type>
-#else // ENTT_NO_ATOMIC
-#define ENTT_MAYBE_ATOMIC(Type) Type
-#endif // ENTT_NO_ATOMIC
+#if defined(__cpp_exceptions) && !defined(ENTT_NOEXCEPTION)
+#   define ENTT_NOEXCEPT noexcept
+#   define ENTT_THROW throw
+#   define ENTT_TRY try
+#   define ENTT_CATCH catch(...)
+#else
+#   define ENTT_NOEXCEPT
+#   define ENTT_THROW
+#   define ENTT_TRY if(true)
+#   define ENTT_CATCH if(false)
+#endif
+
+
+#if defined(__cpp_lib_launder) && __cpp_lib_launder >= 201606L
+#   include <new>
+#   define ENTT_LAUNDER(expr) std::launder(expr)
+#else
+#   define ENTT_LAUNDER(expr) expr
+#endif
+
+
+#ifndef ENTT_USE_ATOMIC
+#   define ENTT_MAYBE_ATOMIC(Type) Type
+#else
+#   include <atomic>
+#   define ENTT_MAYBE_ATOMIC(Type) std::atomic<Type>
+#endif
 
 
 #ifndef ENTT_ID_TYPE
-#include <cstdint>
-#define ENTT_ID_TYPE std::uint32_t
-#endif // ENTT_ID_TYPE
-
-
-#ifndef ENTT_PAGE_SIZE
-#define ENTT_PAGE_SIZE 32768
-#endif // ENTT_PAGE_SIZE
-
-
-#ifndef ENTT_DISABLE_ASSERT
-#include <cassert>
-#define ENTT_ASSERT(condition) assert(condition)
-#else // ENTT_DISABLE_ASSERT
-#define ENTT_ASSERT(...) ((void)0)
-#endif // ENTT_DISABLE_ASSERT
-
-
-#endif // ENTT_CONFIG_CONFIG_H
-
-
-
-namespace entt {
-
-
-/**
- * @brief Dynamic identifier generator.
- *
- * Utility class template that can be used to assign unique identifiers to types
- * at runtime. Use different specializations to create separate sets of
- * identifiers.
- */
-template<typename...>
-class family {
-    inline static ENTT_MAYBE_ATOMIC(ENTT_ID_TYPE) identifier;
-
-    template<typename...>
-    // clang (since version 9) started to complain if auto is used instead of ENTT_ID_TYPE
-    inline static const ENTT_ID_TYPE inner = identifier++;
-
-public:
-    /*! @brief Unsigned integer type. */
-    using family_type = ENTT_ID_TYPE;
-
-    /*! @brief Statically generated unique identifier for the given type. */
-    template<typename... Type>
-    // at the time I'm writing, clang crashes during compilation if auto is used instead of family_type
-    inline static const family_type type = inner<std::decay_t<Type>...>;
-};
-
-
-}
-
-
-#endif // ENTT_CORE_FAMILY_HPP
-
-// #include "../core/algorithm.hpp"
-#ifndef ENTT_CORE_ALGORITHM_HPP
-#define ENTT_CORE_ALGORITHM_HPP
-
-
-#include <utility>
-#include <iterator>
-#include <algorithm>
-#include <functional>
-// #include "utility.hpp"
-#ifndef ENTT_CORE_UTILITY_HPP
-#define ENTT_CORE_UTILITY_HPP
-
-
-// #include "../config/config.h"
+#   include <cstdint>
+#   define ENTT_ID_TYPE std::uint32_t
+#endif
+
+
+#ifdef ENTT_SPARSE_PAGE
+	static_assert(ENTT_SPARSE_PAGE && ((ENTT_SPARSE_PAGE & (ENTT_SPARSE_PAGE - 1)) == 0), "ENTT_SPARSE_PAGE must be a power of two");
+#else
+#   define ENTT_SPARSE_PAGE 4096
+#endif
+
+
+#ifdef ENTT_PACKED_PAGE
+static_assert(ENTT_PACKED_PAGE && ((ENTT_PACKED_PAGE & (ENTT_PACKED_PAGE - 1)) == 0), "ENTT_PACKED_PAGE must be a power of two");
+#else
+#   define ENTT_PACKED_PAGE 1024
+#endif
+
+
+#ifdef ENTT_DISABLE_ASSERT
+#   undef ENTT_ASSERT
+#   define ENTT_ASSERT(...) (void(0))
+#elif !defined ENTT_ASSERT
+#   include <cassert>
+#   define ENTT_ASSERT(condition, ...) assert(condition)
+#endif
+
+
+#ifdef ENTT_NO_ETO
+#   include <type_traits>
+#   define ENTT_IGNORE_IF_EMPTY std::false_type
+#else
+#   include <type_traits>
+#   define ENTT_IGNORE_IF_EMPTY std::true_type
+#endif
+
+
+#ifndef ENTT_STANDARD_CPP
+#    if defined __clang__ || defined __GNUC__
+#       define ENTT_PRETTY_FUNCTION __PRETTY_FUNCTION__
+#       define ENTT_PRETTY_FUNCTION_PREFIX '='
+#       define ENTT_PRETTY_FUNCTION_SUFFIX ']'
+#    elif defined _MSC_VER
+#       define ENTT_PRETTY_FUNCTION __FUNCSIG__
+#       define ENTT_PRETTY_FUNCTION_PREFIX '<'
+#       define ENTT_PRETTY_FUNCTION_SUFFIX '>'
+#   endif
+#endif
+
+
+#endif
 
 
 
@@ -1451,235 +10869,149 @@
      * @return The submitted value as-is.
      */
     template<class Type>
-    constexpr Type && operator()(Type &&value) const ENTT_NOEXCEPT {
+    [[nodiscard]] constexpr Type && operator()(Type &&value) const ENTT_NOEXCEPT {
         return std::forward<Type>(value);
     }
 };
 
 
 /**
- * @brief Constant utility to disambiguate overloaded member functions.
- * @tparam Type Function type of the desired overload.
- * @tparam Class Type of class to which the member functions belong.
- * @param member A valid pointer to a member function.
- * @return Pointer to the member function.
+ * @brief Constant utility to disambiguate overloaded members of a class.
+ * @tparam Type Type of the desired overload.
+ * @tparam Class Type of class to which the member belongs.
+ * @param member A valid pointer to a member.
+ * @return Pointer to the member.
  */
 template<typename Type, typename Class>
-constexpr auto overload(Type Class:: *member) ENTT_NOEXCEPT { return member; }
+[[nodiscard]] constexpr auto overload(Type Class:: *member) ENTT_NOEXCEPT { return member; }
 
 
 /**
  * @brief Constant utility to disambiguate overloaded functions.
- * @tparam Type Function type of the desired overload.
+ * @tparam Func Function type of the desired overload.
  * @param func A valid pointer to a function.
  * @return Pointer to the function.
  */
-template<typename Type>
-constexpr auto overload(Type *func) ENTT_NOEXCEPT { return func; }
-
-
-}
-
-
-#endif // ENTT_CORE_UTILITY_HPP
-
-
-
-namespace entt {
-
-
-/**
- * @brief Function object to wrap `std::sort` in a class type.
- *
- * Unfortunately, `std::sort` cannot be passed as template argument to a class
- * template or a function template.<br/>
- * This class fills the gap by wrapping some flavors of `std::sort` in a
- * function object.
- */
-struct std_sort {
-    /**
-     * @brief Sorts the elements in a range.
-     *
-     * Sorts the elements in a range using the given binary comparison function.
-     *
-     * @tparam It Type of random access iterator.
-     * @tparam Compare Type of comparison function object.
-     * @tparam Args Types of arguments to forward to the sort function.
-     * @param first An iterator to the first element of the range to sort.
-     * @param last An iterator past the last element of the range to sort.
-     * @param compare A valid comparison function object.
-     * @param args Arguments to forward to the sort function, if any.
-     */
-    template<typename It, typename Compare = std::less<>, typename... Args>
-    void operator()(It first, It last, Compare compare = Compare{}, Args &&... args) const {
-        std::sort(std::forward<Args>(args)..., std::move(first), std::move(last), std::move(compare));
-    }
-};
-
-
-/*! @brief Function object for performing insertion sort. */
-struct insertion_sort {
-    /**
-     * @brief Sorts the elements in a range.
-     *
-     * Sorts the elements in a range using the given binary comparison function.
-     *
-     * @tparam It Type of random access iterator.
-     * @tparam Compare Type of comparison function object.
-     * @param first An iterator to the first element of the range to sort.
-     * @param last An iterator past the last element of the range to sort.
-     * @param compare A valid comparison function object.
-     */
-    template<typename It, typename Compare = std::less<>>
-    void operator()(It first, It last, Compare compare = Compare{}) const {
-        if(first < last) {
-            for(auto it = first+1; it < last; ++it) {
-                auto value = std::move(*it);
-                auto pre = it;
-
-                for(; pre > first && compare(value, *(pre-1)); --pre) {
-                    *pre = std::move(*(pre-1));
-                }
-
-                *pre = std::move(value);
-            }
-        }
-    }
-};
-
-
-/**
- * @brief Function object for performing LSD radix sort.
- * @tparam Bit Number of bits processed per pass.
- * @tparam N Maximum number of bits to sort.
- */
-template<std::size_t Bit, std::size_t N>
-struct radix_sort {
-    static_assert((N % Bit) == 0);
-
-    /**
-     * @brief Sorts the elements in a range.
-     *
-     * Sorts the elements in a range using the given _getter_ to access the
-     * actual data to be sorted.
-     *
-     * This implementation is inspired by the online book
-     * [Physically Based Rendering](http://www.pbr-book.org/3ed-2018/Primitives_and_Intersection_Acceleration/Bounding_Volume_Hierarchies.html#RadixSort).
-     *
-     * @tparam It Type of random access iterator.
-     * @tparam Getter Type of _getter_ function object.
-     * @param first An iterator to the first element of the range to sort.
-     * @param last An iterator past the last element of the range to sort.
-     * @param getter A valid _getter_ function object.
-     */
-    template<typename It, typename Getter = identity>
-    void operator()(It first, It last, Getter getter = Getter{}) const {
-        if(first < last) {
-            static constexpr auto mask = (1 << Bit) - 1;
-            static constexpr auto buckets = 1 << Bit;
-            static constexpr auto passes = N / Bit;
-
-            using value_type = typename std::iterator_traits<It>::value_type;
-            std::vector<value_type> aux(std::distance(first, last));
-
-            auto part = [getter = std::move(getter)](auto from, auto to, auto out, auto start) {
-                std::size_t index[buckets]{};
-                std::size_t count[buckets]{};
-
-                std::for_each(from, to, [&getter, &count, start](const value_type &item) {
-                    ++count[(getter(item) >> start) & mask];
-                });
-
-                std::for_each(std::next(std::begin(index)), std::end(index), [index = std::begin(index), count = std::begin(count)](auto &item) mutable {
-                    item = *(index++) + *(count++);
-                });
-
-                std::for_each(from, to, [&getter, &out, &index, start](value_type &item) {
-                    out[index[(getter(item) >> start) & mask]++] = std::move(item);
-                });
-            };
-
-            for(std::size_t pass = 0; pass < (passes & ~1); pass += 2) {
-                part(first, last, aux.begin(), pass * Bit);
-                part(aux.begin(), aux.end(), first, (pass + 1) * Bit);
-            }
-
-            if constexpr(passes & 1) {
-                part(first, last, aux.begin(), (passes - 1) * Bit);
-                std::move(aux.begin(), aux.end(), first);
-            }
-        }
-    }
-};
-
-
-}
-
-
-#endif // ENTT_CORE_ALGORITHM_HPP
-
-// #include "../core/type_traits.hpp"
-#ifndef ENTT_CORE_TYPE_TRAITS_HPP
-#define ENTT_CORE_TYPE_TRAITS_HPP
-
-
-#include <type_traits>
-// #include "../config/config.h"
-
-// #include "../core/hashed_string.hpp"
+template<typename Func>
+[[nodiscard]] constexpr auto overload(Func *func) ENTT_NOEXCEPT { return func; }
+
+
+/**
+ * @brief Helper type for visitors.
+ * @tparam Func Types of function objects.
+ */
+template<class... Func>
+struct overloaded: Func... {
+    using Func::operator()...;
+};
+
+
+/**
+ * @brief Deduction guide.
+ * @tparam Func Types of function objects.
+ */
+template<class... Func>
+overloaded(Func...)
+-> overloaded<Func...>;
+
+
+/**
+ * @brief Basic implementation of a y-combinator.
+ * @tparam Func Type of a potentially recursive function.
+ */
+template<class Func>
+struct y_combinator {
+    /**
+     * @brief Constructs a y-combinator from a given function.
+     * @param recursive A potentially recursive function.
+     */
+    y_combinator(Func recursive):
+        func{std::move(recursive)}
+    {}
+
+    /**
+     * @brief Invokes a y-combinator and therefore its underlying function.
+     * @tparam Args Types of arguments to use to invoke the underlying function.
+     * @param args Parameters to use to invoke the underlying function.
+     * @return Return value of the underlying function, if any.
+     */
+    template <class... Args>
+    decltype(auto) operator()(Args &&... args) const {
+        return func(*this, std::forward<Args>(args)...);
+    }
+
+    /*! @copydoc operator()() */
+    template <class... Args>
+    decltype(auto) operator()(Args &&... args) {
+        return func(*this, std::forward<Args>(args)...);
+    }
+
+private:
+    Func func;
+};
+
+
+}
+
+
+#endif
+
+// #include "fwd.hpp"
+
+// #include "type_info.hpp"
+#ifndef ENTT_CORE_TYPE_INFO_HPP
+#define ENTT_CORE_TYPE_INFO_HPP
+
+
+#include <string_view>
+#include <type_traits>
+// #include "../config/config.h"
+
+// #include "../core/attribute.h"
+#ifndef ENTT_CORE_ATTRIBUTE_H
+#define ENTT_CORE_ATTRIBUTE_H
+
+
+#ifndef ENTT_EXPORT
+#   if defined _WIN32 || defined __CYGWIN__ || defined _MSC_VER
+#       define ENTT_EXPORT __declspec(dllexport)
+#       define ENTT_IMPORT __declspec(dllimport)
+#       define ENTT_HIDDEN
+#   elif defined __GNUC__ && __GNUC__ >= 4
+#       define ENTT_EXPORT __attribute__((visibility("default")))
+#       define ENTT_IMPORT __attribute__((visibility("default")))
+#       define ENTT_HIDDEN __attribute__((visibility("hidden")))
+#   else /* Unsupported compiler */
+#       define ENTT_EXPORT
+#       define ENTT_IMPORT
+#       define ENTT_HIDDEN
+#   endif
+#endif
+
+
+#ifndef ENTT_API
+#   if defined ENTT_API_EXPORT
+#       define ENTT_API ENTT_EXPORT
+#   elif defined ENTT_API_IMPORT
+#       define ENTT_API ENTT_IMPORT
+#   else /* No API */
+#       define ENTT_API
+#   endif
+#endif
+
+
+#endif
+
+// #include "hashed_string.hpp"
 #ifndef ENTT_CORE_HASHED_STRING_HPP
 #define ENTT_CORE_HASHED_STRING_HPP
 
 
 #include <cstddef>
-// #include "../config/config.h"
-#ifndef ENTT_CONFIG_CONFIG_H
-#define ENTT_CONFIG_CONFIG_H
-
-
-#ifndef ENTT_NOEXCEPT
-#define ENTT_NOEXCEPT noexcept
-#endif // ENTT_NOEXCEPT
-
-
-#ifndef ENTT_HS_SUFFIX
-#define ENTT_HS_SUFFIX _hs
-#endif // ENTT_HS_SUFFIX
-
-
-#ifndef ENTT_HWS_SUFFIX
-#define ENTT_HWS_SUFFIX _hws
-#endif // ENTT_HWS_SUFFIX
-
-
-#ifndef ENTT_NO_ATOMIC
-#include <atomic>
-#define ENTT_MAYBE_ATOMIC(Type) std::atomic<Type>
-#else // ENTT_NO_ATOMIC
-#define ENTT_MAYBE_ATOMIC(Type) Type
-#endif // ENTT_NO_ATOMIC
-
-
-#ifndef ENTT_ID_TYPE
 #include <cstdint>
-#define ENTT_ID_TYPE std::uint32_t
-#endif // ENTT_ID_TYPE
-
-
-#ifndef ENTT_PAGE_SIZE
-#define ENTT_PAGE_SIZE 32768
-#endif // ENTT_PAGE_SIZE
-
-
-#ifndef ENTT_DISABLE_ASSERT
-#include <cassert>
-#define ENTT_ASSERT(condition) assert(condition)
-#else // ENTT_DISABLE_ASSERT
-#define ENTT_ASSERT(...) ((void)0)
-#endif // ENTT_DISABLE_ASSERT
-
-
-#endif // ENTT_CONFIG_CONFIG_H
+// #include "../config/config.h"
+
+// #include "fwd.hpp"
 
 
 
@@ -1701,6 +11033,7 @@
 
 template<>
 struct fnv1a_traits<std::uint32_t> {
+    using type = std::uint32_t;
     static constexpr std::uint32_t offset = 2166136261;
     static constexpr std::uint32_t prime = 16777619;
 };
@@ -1708,6 +11041,7 @@
 
 template<>
 struct fnv1a_traits<std::uint64_t> {
+    using type = std::uint64_t;
     static constexpr std::uint64_t offset = 14695981039346656037ull;
     static constexpr std::uint64_t prime = 1099511628211ull;
 };
@@ -1718,7 +11052,7 @@
 
 /**
  * Internal details not to be documented.
- * @endcond TURN_OFF_DOXYGEN
+ * @endcond
  */
 
 
@@ -1735,7 +11069,7 @@
  */
 template<typename Char>
 class basic_hashed_string {
-    using traits_type = internal::fnv1a_traits<ENTT_ID_TYPE>;
+    using traits_type = internal::fnv1a_traits<id_type>;
 
     struct const_wrapper {
         // non-explicit constructor on purpose
@@ -1744,15 +11078,33 @@
     };
 
     // Fowler–Noll–Vo hash function v. 1a - the good
-    static constexpr ENTT_ID_TYPE helper(ENTT_ID_TYPE partial, const Char *curr) ENTT_NOEXCEPT {
-        return curr[0] == 0 ? partial : helper((partial^curr[0])*traits_type::prime, curr+1);
+    [[nodiscard]] static constexpr id_type helper(const Char *curr) ENTT_NOEXCEPT {
+        auto value = traits_type::offset;
+
+        while(*curr != 0) {
+            value = (value ^ static_cast<traits_type::type>(*(curr++))) * traits_type::prime;
+        }
+
+        return value;
     }
 
 public:
     /*! @brief Character type. */
     using value_type = Char;
     /*! @brief Unsigned integer type. */
-    using hash_type = ENTT_ID_TYPE;
+    using hash_type = id_type;
+
+    /**
+     * @brief Returns directly the numeric representation of a string view.
+     * @param str Human-readable identifer.
+     * @param size Length of the string to hash.
+     * @return The numeric representation of the string.
+     */
+    [[nodiscard]] static constexpr hash_type value(const value_type *str, std::size_t size) ENTT_NOEXCEPT {
+        id_type partial{traits_type::offset};
+        while(size--) { partial = (partial^(str++)[0])*traits_type::prime; }
+        return partial;
+    }
 
     /**
      * @brief Returns directly the numeric representation of a string.
@@ -1770,8 +11122,8 @@
      * @return The numeric representation of the string.
      */
     template<std::size_t N>
-    static constexpr hash_type to_value(const value_type (&str)[N]) ENTT_NOEXCEPT {
-        return helper(traits_type::offset, str);
+    [[nodiscard]] static constexpr hash_type value(const value_type (&str)[N]) ENTT_NOEXCEPT {
+        return helper(str);
     }
 
     /**
@@ -1779,20 +11131,8 @@
      * @param wrapper Helps achieving the purpose by relying on overloading.
      * @return The numeric representation of the string.
      */
-    static hash_type to_value(const_wrapper wrapper) ENTT_NOEXCEPT {
-        return helper(traits_type::offset, wrapper.str);
-    }
-
-    /**
-     * @brief Returns directly the numeric representation of a string view.
-     * @param str Human-readable identifer.
-     * @param size Length of the string to hash.
-     * @return The numeric representation of the string.
-     */
-    static hash_type to_value(const value_type *str, std::size_t size) ENTT_NOEXCEPT {
-        ENTT_ID_TYPE partial{traits_type::offset};
-        while(size--) { partial = (partial^(str++)[0])*traits_type::prime; }
-        return partial;
+    [[nodiscard]] static hash_type value(const_wrapper wrapper) ENTT_NOEXCEPT {
+        return helper(wrapper.str);
     }
 
     /*! @brief Constructs an empty hashed string. */
@@ -1816,7 +11156,7 @@
      */
     template<std::size_t N>
     constexpr basic_hashed_string(const value_type (&curr)[N]) ENTT_NOEXCEPT
-        : str{curr}, hash{helper(traits_type::offset, curr)}
+        : str{curr}, hash{helper(curr)}
     {}
 
     /**
@@ -1825,14 +11165,14 @@
      * @param wrapper Helps achieving the purpose by relying on overloading.
      */
     explicit constexpr basic_hashed_string(const_wrapper wrapper) ENTT_NOEXCEPT
-        : str{wrapper.str}, hash{helper(traits_type::offset, wrapper.str)}
+        : str{wrapper.str}, hash{helper(wrapper.str)}
     {}
 
     /**
      * @brief Returns the human-readable representation of a hashed string.
      * @return The string used to initialize the instance.
      */
-    constexpr const value_type * data() const ENTT_NOEXCEPT {
+    [[nodiscard]] constexpr const value_type * data() const ENTT_NOEXCEPT {
         return str;
     }
 
@@ -1840,25 +11180,25 @@
      * @brief Returns the numeric representation of a hashed string.
      * @return The numeric representation of the instance.
      */
-    constexpr hash_type value() const ENTT_NOEXCEPT {
+    [[nodiscard]] constexpr hash_type value() const ENTT_NOEXCEPT {
         return hash;
     }
 
-    /**
-     * @brief Returns the human-readable representation of a hashed string.
-     * @return The string used to initialize the instance.
-     */
-    constexpr operator const value_type *() const ENTT_NOEXCEPT { return str; }
-
-    /*! @copydoc value */
-    constexpr operator hash_type() const ENTT_NOEXCEPT { return hash; }
+    /*! @copydoc data */
+    [[nodiscard]] constexpr operator const value_type *() const ENTT_NOEXCEPT { return data(); }
+
+    /**
+     * @brief Returns the numeric representation of a hashed string.
+     * @return The numeric representation of the instance.
+     */
+    [[nodiscard]] constexpr operator hash_type() const ENTT_NOEXCEPT { return value(); }
 
     /**
      * @brief Compares two hashed strings.
      * @param other Hashed string with which to compare.
      * @return True if the two hashed strings are identical, false otherwise.
      */
-    constexpr bool operator==(const basic_hashed_string &other) const ENTT_NOEXCEPT {
+    [[nodiscard]] constexpr bool operator==(const basic_hashed_string &other) const ENTT_NOEXCEPT {
         return hash == other.hash;
     }
 
@@ -1879,7 +11219,7 @@
  * @param str Human-readable identifer.
  */
 template<typename Char, std::size_t N>
-basic_hashed_string(const Char (&str)[N]) ENTT_NOEXCEPT
+basic_hashed_string(const Char (&str)[N])
 -> basic_hashed_string<Char>;
 
 
@@ -1891,7 +11231,7 @@
  * @return True if the two hashed strings are identical, false otherwise.
  */
 template<typename Char>
-constexpr bool operator!=(const basic_hashed_string<Char> &lhs, const basic_hashed_string<Char> &rhs) ENTT_NOEXCEPT {
+[[nodiscard]] constexpr bool operator!=(const basic_hashed_string<Char> &lhs, const basic_hashed_string<Char> &rhs) ENTT_NOEXCEPT {
     return !(lhs == rhs);
 }
 
@@ -1904,7 +11244,7 @@
 using hashed_wstring = basic_hashed_string<wchar_t>;
 
 
-}
+inline namespace literals {
 
 
 /**
@@ -1912,7 +11252,7 @@
  * @param str The literal without its suffix.
  * @return A properly initialized hashed string.
  */
-constexpr entt::hashed_string operator"" ENTT_HS_SUFFIX(const char *str, std::size_t) ENTT_NOEXCEPT {
+[[nodiscard]] constexpr entt::hashed_string operator"" _hs(const char *str, std::size_t) ENTT_NOEXCEPT {
     return entt::hashed_string{str};
 }
 
@@ -1922,44 +11262,5253 @@
  * @param str The literal without its suffix.
  * @return A properly initialized hashed wstring.
  */
-constexpr entt::hashed_wstring operator"" ENTT_HWS_SUFFIX(const wchar_t *str, std::size_t) ENTT_NOEXCEPT {
+[[nodiscard]] constexpr entt::hashed_wstring operator"" _hws(const wchar_t *str, std::size_t) ENTT_NOEXCEPT {
     return entt::hashed_wstring{str};
 }
 
 
-#endif // ENTT_CORE_HASHED_STRING_HPP
-
-
-
-namespace entt {
-
-
-/*! @brief A class to use to push around lists of types, nothing more. */
-template<typename...>
-struct type_list {};
-
-
-/*! @brief Primary template isn't defined on purpose. */
-template<typename>
-struct type_list_size;
-
-
-/**
- * @brief Compile-time number of elements in a type list.
+}
+
+
+}
+
+
+#endif
+
+// #include "fwd.hpp"
+
+
+
+namespace entt {
+
+
+/**
+ * @cond TURN_OFF_DOXYGEN
+ * Internal details not to be documented.
+ */
+
+
+namespace internal {
+
+
+struct ENTT_API type_seq final {
+    [[nodiscard]] static id_type next() ENTT_NOEXCEPT {
+        static ENTT_MAYBE_ATOMIC(id_type) value{};
+        return value++;
+    }
+};
+
+
+template<typename Type>
+[[nodiscard]] constexpr auto stripped_type_name() ENTT_NOEXCEPT {
+#if defined ENTT_PRETTY_FUNCTION
+    std::string_view pretty_function{ENTT_PRETTY_FUNCTION};
+    auto first = pretty_function.find_first_not_of(' ', pretty_function.find_first_of(ENTT_PRETTY_FUNCTION_PREFIX)+1);
+    auto value = pretty_function.substr(first, pretty_function.find_last_of(ENTT_PRETTY_FUNCTION_SUFFIX) - first);
+    return value;
+#else
+    return std::string_view{""};
+#endif
+}
+
+
+template<typename Type, auto = stripped_type_name<Type>().find_first_of('.')>
+[[nodiscard]] static constexpr std::string_view type_name(int) ENTT_NOEXCEPT {
+    constexpr auto value = stripped_type_name<Type>();
+    return value;
+}
+
+
+template<typename Type>
+[[nodiscard]] static std::string_view type_name(char) ENTT_NOEXCEPT {
+    static const auto value = stripped_type_name<Type>();
+    return value;
+}
+
+
+template<typename Type, auto = stripped_type_name<Type>().find_first_of('.')>
+[[nodiscard]] static constexpr id_type type_hash(int) ENTT_NOEXCEPT {
+    constexpr auto stripped = stripped_type_name<Type>();
+    constexpr auto value = hashed_string::value(stripped.data(), stripped.size());
+    return value;
+}
+
+
+template<typename Type>
+[[nodiscard]] static id_type type_hash(char) ENTT_NOEXCEPT {
+    static const auto value = [](const auto stripped) {
+        return hashed_string::value(stripped.data(), stripped.size());
+    }(stripped_type_name<Type>());
+    return value;
+}
+
+
+}
+
+
+/**
+ * Internal details not to be documented.
+ * @endcond
+ */
+
+
+/**
+ * @brief Type sequential identifier.
+ * @tparam Type Type for which to generate a sequential identifier.
+ */
+template<typename Type, typename = void>
+struct ENTT_API type_seq final {
+    /**
+     * @brief Returns the sequential identifier of a given type.
+     * @return The sequential identifier of a given type.
+     */
+    [[nodiscard]] static id_type value() ENTT_NOEXCEPT {
+        static const id_type value = internal::type_seq::next();
+        return value;
+    }
+
+    /*! @copydoc value */
+    [[nodiscard]] constexpr operator id_type() const ENTT_NOEXCEPT { return value(); }
+};
+
+
+/**
+ * @brief Type hash.
+ * @tparam Type Type for which to generate a hash value.
+ */
+template<typename Type, typename = void>
+struct type_hash final {
+    /**
+     * @brief Returns the numeric representation of a given type.
+     * @return The numeric representation of the given type.
+     */
+#if defined ENTT_PRETTY_FUNCTION
+    [[nodiscard]] static constexpr id_type value() ENTT_NOEXCEPT {
+        return internal::type_hash<Type>(0);
+#else
+    [[nodiscard]] static constexpr id_type value() ENTT_NOEXCEPT {
+        return type_seq<Type>::value();
+#endif
+    }
+
+    /*! @copydoc value */
+    [[nodiscard]] constexpr operator id_type() const ENTT_NOEXCEPT { return value(); }
+};
+
+
+/**
+ * @brief Type name.
+ * @tparam Type Type for which to generate a name.
+ */
+template<typename Type, typename = void>
+struct type_name final {
+    /**
+     * @brief Returns the name of a given type.
+     * @return The name of the given type.
+     */
+    [[nodiscard]] static constexpr std::string_view value() ENTT_NOEXCEPT {
+        return internal::type_name<Type>(0);
+    }
+
+    /*! @copydoc value */
+    [[nodiscard]] constexpr operator std::string_view() const ENTT_NOEXCEPT { return value(); }
+};
+
+
+/*! @brief Implementation specific information about a type. */
+class type_info final {
+    template<typename>
+    friend type_info type_id() ENTT_NOEXCEPT;
+
+    type_info(id_type seq_v, id_type hash_v, std::string_view name_v) ENTT_NOEXCEPT
+        : seq_value{seq_v},
+          hash_value{hash_v},
+          name_value{name_v}
+    {}
+
+public:
+    /*! @brief Default constructor. */
+    type_info() ENTT_NOEXCEPT
+        : type_info({}, {}, {})
+    {}
+
+    /*! @brief Default copy constructor. */
+    type_info(const type_info &) ENTT_NOEXCEPT = default;
+    /*! @brief Default move constructor. */
+    type_info(type_info &&) ENTT_NOEXCEPT = default;
+
+    /**
+     * @brief Default copy assignment operator.
+     * @return This type info object.
+     */
+    type_info & operator=(const type_info &) ENTT_NOEXCEPT = default;
+
+    /**
+     * @brief Default move assignment operator.
+     * @return This type info object.
+     */
+    type_info & operator=(type_info &&) ENTT_NOEXCEPT = default;
+
+    /**
+     * @brief Checks if a type info object is properly initialized.
+     * @return True if the object is properly initialized, false otherwise.
+     */
+    [[nodiscard]] explicit operator bool() const ENTT_NOEXCEPT {
+        return name_value.data() != nullptr;
+    }
+
+    /**
+     * @brief Type sequential identifier.
+     * @return Type sequential identifier.
+     */
+    [[nodiscard]] id_type seq() const ENTT_NOEXCEPT {
+        return seq_value;
+    }
+
+    /**
+     * @brief Type hash.
+     * @return Type hash.
+     */
+    [[nodiscard]] id_type hash() const ENTT_NOEXCEPT {
+        return hash_value;
+    }
+
+    /**
+     * @brief Type name.
+     * @return Type name.
+     */
+    [[nodiscard]] std::string_view name() const ENTT_NOEXCEPT {
+        return name_value;
+    }
+
+    /**
+     * @brief Compares the contents of two type info objects.
+     * @param other Object with which to compare.
+     * @return False if the two contents differ, true otherwise.
+     */
+    [[nodiscard]] bool operator==(const type_info &other) const ENTT_NOEXCEPT {
+        return hash_value == other.hash_value;
+    }
+
+private:
+    id_type seq_value;
+    id_type hash_value;
+    std::string_view name_value;
+};
+
+
+/**
+ * @brief Compares the contents of two type info objects.
+ * @param lhs A type info object.
+ * @param rhs A type info object.
+ * @return True if the two contents differ, false otherwise.
+ */
+[[nodiscard]] inline bool operator!=(const type_info &lhs, const type_info &rhs) ENTT_NOEXCEPT {
+    return !(lhs == rhs);
+}
+
+
+/**
+ * @brief Returns the type info object for a given type.
+ * @tparam Type Type for which to generate a type info object.
+ * @return The type info object for the given type.
+ */
+template<typename Type>
+[[nodiscard]] type_info type_id() ENTT_NOEXCEPT {
+    return type_info{
+        type_seq<std::remove_cv_t<std::remove_reference_t<Type>>>::value(),
+        type_hash<std::remove_cv_t<std::remove_reference_t<Type>>>::value(),
+        type_name<std::remove_cv_t<std::remove_reference_t<Type>>>::value()
+    };
+}
+
+
+}
+
+
+#endif
+
+// #include "type_traits.hpp"
+#ifndef ENTT_CORE_TYPE_TRAITS_HPP
+#define ENTT_CORE_TYPE_TRAITS_HPP
+
+
+#include <cstddef>
+#include <iterator>
+#include <type_traits>
+#include <utility>
+// #include "../config/config.h"
+
+// #include "fwd.hpp"
+
+
+
+namespace entt {
+
+
+/**
+ * @brief Utility class to disambiguate overloaded functions.
+ * @tparam N Number of choices available.
+ */
+template<std::size_t N>
+struct choice_t
+    // Unfortunately, doxygen cannot parse such a construct.
+    /*! @cond TURN_OFF_DOXYGEN */
+    : choice_t<N-1>
+    /*! @endcond */
+{};
+
+
+/*! @copybrief choice_t */
+template<>
+struct choice_t<0> {};
+
+
+/**
+ * @brief Variable template for the choice trick.
+ * @tparam N Number of choices available.
+ */
+template<std::size_t N>
+inline constexpr choice_t<N> choice{};
+
+
+/**
+ * @brief Identity type trait.
+ *
+ * Useful to establish non-deduced contexts in template argument deduction
+ * (waiting for C++20) or to provide types through function arguments.
+ *
+ * @tparam Type A type.
+ */
+template<typename Type>
+struct type_identity {
+    /*! @brief Identity type. */
+    using type = Type;
+};
+
+
+/**
+ * @brief Helper type.
+ * @tparam Type A type.
+ */
+template<typename Type>
+using type_identity_t = typename type_identity<Type>::type;
+
+
+/**
+ * @brief A type-only `sizeof` wrapper that returns 0 where `sizeof` complains.
+ * @tparam Type The type of which to return the size.
+ * @tparam The size of the type if `sizeof` accepts it, 0 otherwise.
+ */
+template<typename Type, typename = void>
+struct size_of: std::integral_constant<std::size_t, 0u> {};
+
+
+/*! @copydoc size_of */
+template<typename Type>
+struct size_of<Type, std::void_t<decltype(sizeof(Type))>>
+    : std::integral_constant<std::size_t, sizeof(Type)>
+{};
+
+
+/**
+ * @brief Helper variable template.
+ * @tparam Type The type of which to return the size.
+ */
+template<class Type>
+inline constexpr std::size_t size_of_v = size_of<Type>::value;
+
+
+/**
+ * @brief Using declaration to be used to _repeat_ the same type a number of
+ * times equal to the size of a given parameter pack.
+ * @tparam Type A type to repeat.
+ */
+template<typename Type, typename>
+using unpack_as_t = Type;
+
+
+/**
+ * @brief Helper variable template to be used to _repeat_ the same value a
+ * number of times equal to the size of a given parameter pack.
+ * @tparam Value A value to repeat.
+ */
+template<auto Value, typename>
+inline constexpr auto unpack_as_v = Value;
+
+
+/**
+ * @brief Wraps a static constant.
+ * @tparam Value A static constant.
+ */
+template<auto Value>
+using integral_constant = std::integral_constant<decltype(Value), Value>;
+
+
+/**
+ * @brief Alias template to facilitate the creation of named values.
+ * @tparam Value A constant value at least convertible to `id_type`.
+ */
+template<id_type Value>
+using tag = integral_constant<Value>;
+
+
+/**
+ * @brief A class to use to push around lists of types, nothing more.
  * @tparam Type Types provided by the type list.
  */
 template<typename... Type>
-struct type_list_size<type_list<Type...>>
-        : std::integral_constant<std::size_t, sizeof...(Type)>
-{};
+struct type_list {
+    /*! @brief Type list type. */
+    using type = type_list;
+    /*! @brief Compile-time number of elements in the type list. */
+    static constexpr auto size = sizeof...(Type);
+};
+
+
+/*! @brief Primary template isn't defined on purpose. */
+template<std::size_t, typename>
+struct type_list_element;
+
+
+/**
+ * @brief Provides compile-time indexed access to the types of a type list.
+ * @tparam Index Index of the type to return.
+ * @tparam Type First type provided by the type list.
+ * @tparam Other Other types provided by the type list.
+ */
+template<std::size_t Index, typename Type, typename... Other>
+struct type_list_element<Index, type_list<Type, Other...>>
+    : type_list_element<Index - 1u, type_list<Other...>>
+{};
+
+
+/**
+ * @brief Provides compile-time indexed access to the types of a type list.
+ * @tparam Type First type provided by the type list.
+ * @tparam Other Other types provided by the type list.
+ */
+template<typename Type, typename... Other>
+struct type_list_element<0u, type_list<Type, Other...>> {
+    /*! @brief Searched type. */
+    using type = Type;
+};
+
+
+/**
+ * @brief Helper type.
+ * @tparam Index Index of the type to return.
+ * @tparam List Type list to search into.
+ */
+template<std::size_t Index, typename List>
+using type_list_element_t = typename type_list_element<Index, List>::type;
+
+
+/**
+ * @brief Concatenates multiple type lists.
+ * @tparam Type Types provided by the first type list.
+ * @tparam Other Types provided by the second type list.
+ * @return A type list composed by the types of both the type lists.
+ */
+template<typename... Type, typename... Other>
+constexpr type_list<Type..., Other...> operator+(type_list<Type...>, type_list<Other...>) { return {}; }
+
+
+/*! @brief Primary template isn't defined on purpose. */
+template<typename...>
+struct type_list_cat;
+
+
+/*! @brief Concatenates multiple type lists. */
+template<>
+struct type_list_cat<> {
+    /*! @brief A type list composed by the types of all the type lists. */
+    using type = type_list<>;
+};
+
+
+/**
+ * @brief Concatenates multiple type lists.
+ * @tparam Type Types provided by the first type list.
+ * @tparam Other Types provided by the second type list.
+ * @tparam List Other type lists, if any.
+ */
+template<typename... Type, typename... Other, typename... List>
+struct type_list_cat<type_list<Type...>, type_list<Other...>, List...> {
+    /*! @brief A type list composed by the types of all the type lists. */
+    using type = typename type_list_cat<type_list<Type..., Other...>, List...>::type;
+};
+
+
+/**
+ * @brief Concatenates multiple type lists.
+ * @tparam Type Types provided by the type list.
+ */
+template<typename... Type>
+struct type_list_cat<type_list<Type...>> {
+    /*! @brief A type list composed by the types of all the type lists. */
+    using type = type_list<Type...>;
+};
+
+
+/**
+ * @brief Helper type.
+ * @tparam List Type lists to concatenate.
+ */
+template<typename... List>
+using type_list_cat_t = typename type_list_cat<List...>::type;
+
+
+/*! @brief Primary template isn't defined on purpose. */
+template<typename>
+struct type_list_unique;
+
+
+/**
+ * @brief Removes duplicates types from a type list.
+ * @tparam Type One of the types provided by the given type list.
+ * @tparam Other The other types provided by the given type list.
+ */
+template<typename Type, typename... Other>
+struct type_list_unique<type_list<Type, Other...>> {
+    /*! @brief A type list without duplicate types. */
+    using type = std::conditional_t<
+        std::disjunction_v<std::is_same<Type, Other>...>,
+        typename type_list_unique<type_list<Other...>>::type,
+        type_list_cat_t<type_list<Type>, typename type_list_unique<type_list<Other...>>::type>
+    >;
+};
+
+
+/*! @brief Removes duplicates types from a type list. */
+template<>
+struct type_list_unique<type_list<>> {
+    /*! @brief A type list without duplicate types. */
+    using type = type_list<>;
+};
+
+
+/**
+ * @brief Helper type.
+ * @tparam Type A type list.
+ */
+template<typename Type>
+using type_list_unique_t = typename type_list_unique<Type>::type;
+
+
+/**
+ * @brief Provides the member constant `value` to true if a type list contains a
+ * given type, false otherwise.
+ * @tparam List Type list.
+ * @tparam Type Type to look for.
+ */
+template<typename List, typename Type>
+struct type_list_contains;
+
+
+/**
+ * @copybrief type_list_contains
+ * @tparam Type Types provided by the type list.
+ * @tparam Other Type to look for.
+ */
+template<typename... Type, typename Other>
+struct type_list_contains<type_list<Type...>, Other>: std::disjunction<std::is_same<Type, Other>...> {};
 
 
 /**
  * @brief Helper variable template.
  * @tparam List Type list.
- */
-template<class List>
-constexpr auto type_list_size_v = type_list_size<List>::value;
+ * @tparam Type Type to look for.
+ */
+template<class List, typename Type>
+inline constexpr bool type_list_contains_v = type_list_contains<List, Type>::value;
+
+
+/*! @brief Primary template isn't defined on purpose. */
+template<typename...>
+struct type_list_diff;
+
+
+/**
+ * @brief Computes the difference between two type lists.
+ * @tparam Type Types provided by the first type list.
+ * @tparam Other Types provided by the second type list.
+ */
+template<typename... Type, typename... Other>
+struct type_list_diff<type_list<Type...>, type_list<Other...>> {
+    /*! @brief A type list that is the difference between the two type lists. */
+    using type = type_list_cat_t<std::conditional_t<type_list_contains_v<type_list<Other...>, Type>, type_list<>, type_list<Type>>...>;
+};
+
+
+/**
+ * @brief Helper type.
+ * @tparam List Type lists between which to compute the difference.
+ */
+template<typename... List>
+using type_list_diff_t = typename type_list_diff<List...>::type;
+
+
+/**
+ * @brief A class to use to push around lists of constant values, nothing more.
+ * @tparam Value Values provided by the value list.
+ */
+template<auto... Value>
+struct value_list {
+    /*! @brief Value list type. */
+    using type = value_list;
+    /*! @brief Compile-time number of elements in the value list. */
+    static constexpr auto size = sizeof...(Value);
+};
+
+
+/*! @brief Primary template isn't defined on purpose. */
+template<std::size_t, typename>
+struct value_list_element;
+
+
+/**
+ * @brief Provides compile-time indexed access to the values of a value list.
+ * @tparam Index Index of the value to return.
+ * @tparam Value First value provided by the value list.
+ * @tparam Other Other values provided by the value list.
+ */
+template<std::size_t Index, auto Value, auto... Other>
+struct value_list_element<Index, value_list<Value, Other...>>
+    : value_list_element<Index - 1u, value_list<Other...>>
+{};
+
+
+/**
+ * @brief Provides compile-time indexed access to the types of a type list.
+ * @tparam Value First value provided by the value list.
+ * @tparam Other Other values provided by the value list.
+ */
+template<auto Value, auto... Other>
+struct value_list_element<0u, value_list<Value, Other...>> {
+    /*! @brief Searched value. */
+    static constexpr auto value = Value;
+};
+
+
+/**
+ * @brief Helper type.
+ * @tparam Index Index of the value to return.
+ * @tparam List Value list to search into.
+ */
+template<std::size_t Index, typename List>
+inline constexpr auto value_list_element_v = value_list_element<Index, List>::value;
+
+
+/**
+ * @brief Concatenates multiple value lists.
+ * @tparam Value Values provided by the first value list.
+ * @tparam Other Values provided by the second value list.
+ * @return A value list composed by the values of both the value lists.
+ */
+template<auto... Value, auto... Other>
+constexpr value_list<Value..., Other...> operator+(value_list<Value...>, value_list<Other...>) { return {}; }
+
+
+/*! @brief Primary template isn't defined on purpose. */
+template<typename...>
+struct value_list_cat;
+
+
+/*! @brief Concatenates multiple value lists. */
+template<>
+struct value_list_cat<> {
+    /*! @brief A value list composed by the values of all the value lists. */
+    using type = value_list<>;
+};
+
+
+/**
+ * @brief Concatenates multiple value lists.
+ * @tparam Value Values provided by the first value list.
+ * @tparam Other Values provided by the second value list.
+ * @tparam List Other value lists, if any.
+ */
+template<auto... Value, auto... Other, typename... List>
+struct value_list_cat<value_list<Value...>, value_list<Other...>, List...> {
+    /*! @brief A value list composed by the values of all the value lists. */
+    using type = typename value_list_cat<value_list<Value..., Other...>, List...>::type;
+};
+
+
+/**
+ * @brief Concatenates multiple value lists.
+ * @tparam Value Values provided by the value list.
+ */
+template<auto... Value>
+struct value_list_cat<value_list<Value...>> {
+    /*! @brief A value list composed by the values of all the value lists. */
+    using type = value_list<Value...>;
+};
+
+
+/**
+ * @brief Helper type.
+ * @tparam List Value lists to concatenate.
+ */
+template<typename... List>
+using value_list_cat_t = typename value_list_cat<List...>::type;
+
+
+/*! @brief Same as std::is_invocable, but with tuples. */
+template<typename, typename>
+struct is_applicable: std::false_type {};
+
+
+/**
+ * @copybrief is_applicable
+ * @tparam Func A valid function type.
+ * @tparam Tuple Tuple-like type.
+ * @tparam Args The list of arguments to use to probe the function type.
+ */
+template<typename Func, template<typename...> class Tuple, typename... Args>
+struct is_applicable<Func, Tuple<Args...>>: std::is_invocable<Func, Args...> {};
+
+
+/**
+ * @copybrief is_applicable
+ * @tparam Func A valid function type.
+ * @tparam Tuple Tuple-like type.
+ * @tparam Args The list of arguments to use to probe the function type.
+ */
+template<typename Func, template<typename...> class Tuple, typename... Args>
+struct is_applicable<Func, const Tuple<Args...>>: std::is_invocable<Func, Args...> {};
+
+
+/**
+ * @brief Helper variable template.
+ * @tparam Func A valid function type.
+ * @tparam Args The list of arguments to use to probe the function type.
+ */
+template<typename Func, typename Args>
+inline constexpr bool is_applicable_v = is_applicable<Func, Args>::value;
+
+
+/*! @brief Same as std::is_invocable_r, but with tuples for arguments. */
+template<typename, typename, typename>
+struct is_applicable_r: std::false_type {};
+
+
+/**
+ * @copybrief is_applicable_r
+ * @tparam Ret The type to which the return type of the function should be
+ * convertible.
+ * @tparam Func A valid function type.
+ * @tparam Args The list of arguments to use to probe the function type.
+ */
+template<typename Ret, typename Func, typename... Args>
+struct is_applicable_r<Ret, Func, std::tuple<Args...>>: std::is_invocable_r<Ret, Func, Args...> {};
+
+
+/**
+ * @brief Helper variable template.
+ * @tparam Ret The type to which the return type of the function should be
+ * convertible.
+ * @tparam Func A valid function type.
+ * @tparam Args The list of arguments to use to probe the function type.
+ */
+template<typename Ret, typename Func, typename Args>
+inline constexpr bool is_applicable_r_v = is_applicable_r<Ret, Func, Args>::value;
+
+
+/**
+ * @brief Provides the member constant `value` to true if a given type is
+ * complete, false otherwise.
+ * @tparam Type The type to test.
+ */
+template<typename Type, typename = void>
+struct is_complete: std::false_type {};
+
+
+/*! @copydoc is_complete */
+template<typename Type>
+struct is_complete<Type, std::void_t<decltype(sizeof(Type))>>: std::true_type {};
+
+
+/**
+ * @brief Helper variable template.
+ * @tparam Type The type to test.
+ */
+template<typename Type>
+inline constexpr bool is_complete_v = is_complete<Type>::value;
+
+
+/**
+ * @brief Provides the member constant `value` to true if a given type is an
+ * iterator, false otherwise.
+ * @tparam Type The type to test.
+ */
+template<typename Type, typename = void>
+struct is_iterator: std::false_type {};
+
+
+/*! @copydoc is_iterator */
+template<typename Type>
+struct is_iterator<Type, std::void_t<typename std::iterator_traits<Type>::iterator_category>>
+    : std::true_type
+{};
+
+
+/**
+ * @brief Helper variable template.
+ * @tparam Type The type to test.
+ */
+template<typename Type>
+inline constexpr bool is_iterator_v = is_iterator<Type>::value;
+
+
+/**
+ * @brief Provides the member constant `value` to true if a given type is of the
+ * required iterator type, false otherwise.
+ * @tparam Type The type to test.
+ * @tparam It Required iterator type.
+ */
+template<typename Type, typename It, typename = void>
+struct is_iterator_type: std::false_type {};
+
+
+/*! @copydoc is_iterator_type */
+template<typename Type, typename It>
+struct is_iterator_type<Type, It, std::enable_if_t<is_iterator_v<Type> && std::is_same_v<Type, It>>>
+    : std::true_type
+{};
+
+
+/*! @copydoc is_iterator_type */
+template<typename Type, typename It>
+struct is_iterator_type<Type, It, std::enable_if_t<!std::is_same_v<Type, It>, std::void_t<typename It::iterator_type>>>
+    : is_iterator_type<Type, typename It::iterator_type>
+{};
+
+
+/**
+ * @brief Helper variable template.
+ * @tparam Type The type to test.
+ * @tparam It Required iterator type.
+ */
+template<typename Type, typename It>
+inline constexpr bool is_iterator_type_v = is_iterator_type<Type, It>::value;
+
+
+/**
+ * @cond TURN_OFF_DOXYGEN
+ * Internal details not to be documented.
+ */
+
+
+namespace internal {
+
+
+template<typename>
+[[nodiscard]] constexpr bool is_equality_comparable(...) { return false; }
+
+
+template<typename Type>
+[[nodiscard]] constexpr auto is_equality_comparable(choice_t<0>)
+-> decltype(std::declval<Type>() == std::declval<Type>()) { return true; }
+
+
+template<typename Type>
+[[nodiscard]] constexpr auto is_equality_comparable(choice_t<1>)
+-> decltype(std::declval<typename Type::value_type>(), std::declval<Type>() == std::declval<Type>()) {
+    if constexpr(is_iterator_v<Type>) {
+        return true;
+    } else if constexpr(std::is_same_v<typename Type::value_type, Type>) {
+        return is_equality_comparable<Type>(choice<0>);
+    } else {
+        return is_equality_comparable<typename Type::value_type>(choice<2>);
+    }
+}
+
+
+template<typename Type>
+[[nodiscard]] constexpr auto is_equality_comparable(choice_t<2>)
+-> decltype(std::declval<typename Type::mapped_type>(), std::declval<Type>() == std::declval<Type>()) {
+    return is_equality_comparable<typename Type::key_type>(choice<2>) && is_equality_comparable<typename Type::mapped_type>(choice<2>);
+}
+
+
+}
+
+
+/**
+ * Internal details not to be documented.
+ * @endcond
+ */
+
+
+/**
+ * @brief Provides the member constant `value` to true if a given type is
+ * equality comparable, false otherwise.
+ * @tparam Type The type to test.
+ */
+template<typename Type, typename = void>
+struct is_equality_comparable: std::bool_constant<internal::is_equality_comparable<Type>(choice<2>)> {};
+
+
+/**
+ * @brief Helper variable template.
+ * @tparam Type The type to test.
+ */
+template<class Type>
+inline constexpr bool is_equality_comparable_v = is_equality_comparable<Type>::value;
+
+
+/**
+ * @brief Transcribes the constness of a type to another type.
+ * @tparam To The type to which to transcribe the constness.
+ * @tparam From The type from which to transcribe the constness.
+ */
+template<typename To, typename From>
+struct constness_as {
+    /*! @brief The type resulting from the transcription of the constness. */
+    using type = std::remove_const_t<To>;
+};
+
+
+/*! @copydoc constness_as */
+template<typename To, typename From>
+struct constness_as<To, const From> {
+    /*! @brief The type resulting from the transcription of the constness. */
+    using type = std::add_const_t<To>;
+};
+
+
+/**
+ * @brief Alias template to facilitate the transcription of the constness.
+ * @tparam To The type to which to transcribe the constness.
+ * @tparam From The type from which to transcribe the constness.
+ */
+template<typename To, typename From>
+using constness_as_t = typename constness_as<To, From>::type;
+
+
+/**
+ * @brief Extracts the class of a non-static member object or function.
+ * @tparam Member A pointer to a non-static member object or function.
+ */
+template<typename Member>
+class member_class {
+    static_assert(std::is_member_pointer_v<Member>, "Invalid pointer type to non-static member object or function");
+
+    template<typename Class, typename Ret, typename... Args>
+    static Class * clazz(Ret(Class:: *)(Args...));
+
+    template<typename Class, typename Ret, typename... Args>
+    static Class * clazz(Ret(Class:: *)(Args...) const);
+
+    template<typename Class, typename Type>
+    static Class * clazz(Type Class:: *);
+
+public:
+    /*! @brief The class of the given non-static member object or function. */
+    using type = std::remove_pointer_t<decltype(clazz(std::declval<Member>()))>;
+};
+
+
+/**
+ * @brief Helper type.
+ * @tparam Member A pointer to a non-static member object or function.
+ */
+template<typename Member>
+using member_class_t = typename member_class<Member>::type;
+
+
+}
+
+
+#endif
+
+
+
+namespace entt {
+
+
+/**
+ * @brief A SBO friendly, type-safe container for single values of any type.
+ * @tparam Len Size of the storage reserved for the small buffer optimization.
+ * @tparam Align Optional alignment requirement.
+ */
+template<std::size_t Len, std::size_t Align>
+class basic_any {
+    enum class operation: std::uint8_t { COPY, MOVE, DTOR, COMP, ADDR, CADDR, TYPE };
+    enum class policy: std::uint8_t { OWNER, REF, CREF };
+
+    using storage_type = std::aligned_storage_t<Len + !Len, Align>;
+    using vtable_type = const void *(const operation, const basic_any &, void *);
+
+    template<typename Type>
+    static constexpr bool in_situ = Len && alignof(Type) <= alignof(storage_type) && sizeof(Type) <= sizeof(storage_type) && std::is_nothrow_move_constructible_v<Type>;
+
+    template<typename Type>
+    [[nodiscard]] static constexpr policy type_to_policy() {
+        if constexpr(std::is_lvalue_reference_v<Type>) {
+            if constexpr(std::is_const_v<std::remove_reference_t<Type>>) {
+                return policy::CREF;
+            } else {
+                return policy::REF;
+            }
+        } else {
+            return policy::OWNER;
+        }
+    }
+
+    template<typename Type>
+    [[nodiscard]] static bool compare(const void *lhs, const void *rhs) {
+        if constexpr(!std::is_function_v<Type> && is_equality_comparable_v<Type>) {
+            return *static_cast<const Type *>(lhs) == *static_cast<const Type *>(rhs);
+        } else {
+            return lhs == rhs;
+        }
+    }
+
+    template<typename Type>
+    static const void * basic_vtable([[maybe_unused]] const operation op, [[maybe_unused]] const basic_any &from, [[maybe_unused]] void *to) {
+        static_assert(std::is_same_v<std::remove_reference_t<std::remove_const_t<Type>>, Type>, "Invalid type");
+
+        if constexpr(!std::is_void_v<Type>) {
+            const Type *instance = (in_situ<Type> && from.mode == policy::OWNER)
+                ? ENTT_LAUNDER(reinterpret_cast<const Type *>(&from.storage))
+                : static_cast<const Type *>(from.instance);
+
+            switch(op) {
+            case operation::COPY:
+                if constexpr(std::is_copy_constructible_v<Type>) {
+                    static_cast<basic_any *>(to)->emplace<Type>(*instance);
+                }
+                break;
+            case operation::MOVE:
+                if constexpr(in_situ<Type>) {
+                    if(from.mode == policy::OWNER) {
+                        return new (&static_cast<basic_any *>(to)->storage) Type{std::move(*const_cast<Type *>(instance))};
+                    }
+                }
+
+                return (static_cast<basic_any *>(to)->instance = std::exchange(const_cast<basic_any &>(from).instance, nullptr));
+            case operation::DTOR:
+                if(from.mode == policy::OWNER) {
+                    if constexpr(in_situ<Type>) {
+                        instance->~Type();
+                    } else if constexpr(std::is_array_v<Type>) {
+                        delete[] instance;
+                    } else {
+                        delete instance;
+                    }
+                }
+                break;
+            case operation::COMP:
+                return compare<Type>(instance, (*static_cast<const basic_any **>(to))->data()) ? to : nullptr;
+            case operation::ADDR:
+                if(from.mode == policy::CREF) {
+                    return nullptr;
+                }
+                [[fallthrough]];
+            case operation::CADDR:
+                return instance;
+            case operation::TYPE:
+                *static_cast<type_info *>(to) = type_id<Type>();
+                break;
+            }
+        }
+
+        return nullptr;
+    }
+
+    template<typename Type, typename... Args>
+    void initialize([[maybe_unused]] Args &&... args) {
+        if constexpr(!std::is_void_v<Type>) {
+            if constexpr(std::is_lvalue_reference_v<Type>) {
+                static_assert(sizeof...(Args) == 1u && (std::is_lvalue_reference_v<Args> && ...), "Invalid arguments");
+                instance = (std::addressof(args), ...);
+            } else if constexpr(in_situ<Type>) {
+                if constexpr(sizeof...(Args) != 0u && std::is_aggregate_v<Type>) {
+                    new (&storage) Type{std::forward<Args>(args)...};
+                } else {
+                    new (&storage) Type(std::forward<Args>(args)...);
+                }
+            } else {
+                if constexpr(sizeof...(Args) != 0u && std::is_aggregate_v<Type>) {
+                    instance = new Type{std::forward<Args>(args)...};
+                } else {
+                    instance = new Type(std::forward<Args>(args)...);
+                }
+            }
+        }
+    }
+
+    basic_any(const basic_any &other, const policy pol) ENTT_NOEXCEPT
+        : instance{other.data()},
+          vtable{other.vtable},
+          mode{pol}
+    {}
+
+public:
+    /*! @brief Size of the internal storage. */
+    static constexpr auto length = Len;
+    /*! @brief Alignment requirement. */
+    static constexpr auto alignment = Align;
+
+    /*! @brief Default constructor. */
+    basic_any() ENTT_NOEXCEPT
+        : instance{},
+          vtable{&basic_vtable<void>},
+          mode{policy::OWNER}
+    {}
+
+    /**
+     * @brief Constructs a wrapper by directly initializing the new object.
+     * @tparam Type Type of object to use to initialize the wrapper.
+     * @tparam Args Types of arguments to use to construct the new instance.
+     * @param args Parameters to use to construct the instance.
+     */
+    template<typename Type, typename... Args>
+    explicit basic_any(std::in_place_type_t<Type>, Args &&... args)
+        : instance{},
+          vtable{&basic_vtable<std::remove_const_t<std::remove_reference_t<Type>>>},
+          mode{type_to_policy<Type>()}
+    {
+        initialize<Type>(std::forward<Args>(args)...);
+    }
+
+    /**
+     * @brief Constructs a wrapper that holds an unmanaged object.
+     * @tparam Type Type of object to use to initialize the wrapper.
+     * @param value An instance of an object to use to initialize the wrapper.
+     */
+    template<typename Type>
+    basic_any(std::reference_wrapper<Type> value) ENTT_NOEXCEPT
+        : basic_any{}
+    {
+        // invokes deprecated assignment operator (and avoids issues with vs2017)
+        *this = value;
+    }
+
+    /**
+     * @brief Constructs a wrapper from a given value.
+     * @tparam Type Type of object to use to initialize the wrapper.
+     * @param value An instance of an object to use to initialize the wrapper.
+     */
+    template<typename Type, typename = std::enable_if_t<!std::is_same_v<std::decay_t<Type>, basic_any>>>
+    basic_any(Type &&value)
+        : instance{},
+          vtable{&basic_vtable<std::decay_t<Type>>},
+          mode{policy::OWNER}
+    {
+        initialize<std::decay_t<Type>>(std::forward<Type>(value));
+    }
+
+    /**
+     * @brief Copy constructor.
+     * @param other The instance to copy from.
+     */
+    basic_any(const basic_any &other)
+        : instance{},
+          vtable{&basic_vtable<void>},
+          mode{policy::OWNER}
+    {
+        other.vtable(operation::COPY, other, this);
+    }
+
+    /**
+     * @brief Move constructor.
+     * @param other The instance to move from.
+     */
+    basic_any(basic_any &&other) ENTT_NOEXCEPT
+        : instance{},
+          vtable{other.vtable},
+          mode{other.mode}
+    {
+        vtable(operation::MOVE, other, this);
+    }
+
+    /*! @brief Frees the internal storage, whatever it means. */
+    ~basic_any() {
+        vtable(operation::DTOR, *this, nullptr);
+    }
+
+    /**
+     * @brief Copy assignment operator.
+     * @param other The instance to copy from.
+     * @return This any object.
+     */
+    basic_any & operator=(const basic_any &other) {
+        reset();
+        other.vtable(operation::COPY, other, this);
+        return *this;
+    }
+
+    /**
+     * @brief Move assignment operator.
+     * @param other The instance to move from.
+     * @return This any object.
+     */
+    basic_any & operator=(basic_any &&other) ENTT_NOEXCEPT {
+        std::exchange(vtable, other.vtable)(operation::DTOR, *this, nullptr);
+        other.vtable(operation::MOVE, other, this);
+        mode = other.mode;
+        return *this;
+    }
+
+    /**
+     * @brief Value assignment operator.
+     * @tparam Type Type of object to use to initialize the wrapper.
+     * @param value An instance of an object to use to initialize the wrapper.
+     * @return This any object.
+     */
+    template<typename Type>
+    [[deprecated("Use std::in_place_type<T &>, entt::make_any<T &>, emplace<Type &> or forward_as_any instead")]]
+    basic_any & operator=(std::reference_wrapper<Type> value) ENTT_NOEXCEPT {
+        emplace<Type &>(value.get());
+        return *this;
+    }
+
+    /**
+     * @brief Value assignment operator.
+     * @tparam Type Type of object to use to initialize the wrapper.
+     * @param value An instance of an object to use to initialize the wrapper.
+     * @return This any object.
+     */
+    template<typename Type>
+    std::enable_if_t<!std::is_same_v<std::decay_t<Type>, basic_any>, basic_any &>
+    operator=(Type &&value) {
+        emplace<std::decay_t<Type>>(std::forward<Type>(value));
+        return *this;
+    }
+
+    /**
+     * @brief Returns the type of the contained object.
+     * @return The type of the contained object, if any.
+     */
+    [[nodiscard]] type_info type() const ENTT_NOEXCEPT {
+        type_info info{};
+        vtable(operation::TYPE, *this, &info);
+        return info;
+    }
+
+    /**
+     * @brief Returns an opaque pointer to the contained instance.
+     * @return An opaque pointer the contained instance, if any.
+     */
+    [[nodiscard]] const void * data() const ENTT_NOEXCEPT {
+        return vtable(operation::CADDR, *this, nullptr);
+    }
+
+    /*! @copydoc data */
+    [[nodiscard]] void * data() ENTT_NOEXCEPT {
+        return const_cast<void *>(vtable(operation::ADDR, *this, nullptr));
+    }
+
+    /**
+     * @brief Replaces the contained object by creating a new instance directly.
+     * @tparam Type Type of object to use to initialize the wrapper.
+     * @tparam Args Types of arguments to use to construct the new instance.
+     * @param args Parameters to use to construct the instance.
+     */
+    template<typename Type, typename... Args>
+    void emplace(Args &&... args) {
+        std::exchange(vtable, &basic_vtable<std::remove_const_t<std::remove_reference_t<Type>>>)(operation::DTOR, *this, nullptr);
+        mode = type_to_policy<Type>();
+        initialize<Type>(std::forward<Args>(args)...);
+    }
+
+    /*! @brief Destroys contained object */
+    void reset() {
+        std::exchange(vtable, &basic_vtable<void>)(operation::DTOR, *this, nullptr);
+        mode = policy::OWNER;
+    }
+
+    /**
+     * @brief Returns false if a wrapper is empty, true otherwise.
+     * @return False if the wrapper is empty, true otherwise.
+     */
+    [[nodiscard]] explicit operator bool() const ENTT_NOEXCEPT {
+        return !(vtable(operation::CADDR, *this, nullptr) == nullptr);
+    }
+
+    /**
+     * @brief Checks if two wrappers differ in their content.
+     * @param other Wrapper with which to compare.
+     * @return False if the two objects differ in their content, true otherwise.
+     */
+    bool operator==(const basic_any &other) const ENTT_NOEXCEPT {
+        const basic_any *trampoline = &other;
+        return type() == other.type() && (vtable(operation::COMP, *this, &trampoline) || !other.data());
+    }
+
+    /**
+     * @brief Aliasing constructor.
+     * @return A wrapper that shares a reference to an unmanaged object.
+     */
+    [[nodiscard]] basic_any as_ref() ENTT_NOEXCEPT {
+        return basic_any{*this, (mode == policy::CREF ? policy::CREF : policy::REF)};
+    }
+
+    /*! @copydoc as_ref */
+    [[nodiscard]] basic_any as_ref() const ENTT_NOEXCEPT {
+        return basic_any{*this, policy::CREF};
+    }
+
+    /**
+     * @brief Returns true if a wrapper owns its object, false otherwise.
+     * @return True if the wrapper owns its object, false otherwise.
+     */
+    [[nodiscard]] bool owner() const ENTT_NOEXCEPT {
+        return (mode == policy::OWNER);
+    }
+
+private:
+    union { const void *instance; storage_type storage; };
+    vtable_type *vtable;
+    policy mode;
+};
+
+
+/**
+ * @brief Checks if two wrappers differ in their content.
+ * @tparam Len Size of the storage reserved for the small buffer optimization.
+ * @tparam Align Alignment requirement.
+ * @param lhs A wrapper, either empty or not.
+ * @param rhs A wrapper, either empty or not.
+ * @return True if the two wrappers differ in their content, false otherwise.
+ */
+template<std::size_t Len, std::size_t Align>
+[[nodiscard]] inline bool operator!=(const basic_any<Len, Align> &lhs, const basic_any<Len, Align> &rhs) ENTT_NOEXCEPT {
+    return !(lhs == rhs);
+}
+
+
+/**
+ * @brief Performs type-safe access to the contained object.
+ * @tparam Type Type to which conversion is required.
+ * @tparam Len Size of the storage reserved for the small buffer optimization.
+ * @tparam Align Alignment requirement.
+ * @param data Target any object.
+ * @return The element converted to the requested type.
+ */
+template<typename Type, std::size_t Len, std::size_t Align>
+Type any_cast(const basic_any<Len, Align> &data) ENTT_NOEXCEPT {
+    const auto * const instance = any_cast<std::remove_reference_t<Type>>(&data);
+    ENTT_ASSERT(instance, "Invalid instance");
+    return static_cast<Type>(*instance);
+}
+
+
+/*! @copydoc any_cast */
+template<typename Type, std::size_t Len, std::size_t Align>
+Type any_cast(basic_any<Len, Align> &data) ENTT_NOEXCEPT {
+    // forces const on non-reference types to make them work also with wrappers for const references
+    auto * const instance = any_cast<std::remove_reference_t<const Type>>(&data);
+    ENTT_ASSERT(instance, "Invalid instance");
+    return static_cast<Type>(*instance);
+}
+
+
+/*! @copydoc any_cast */
+template<typename Type, std::size_t Len, std::size_t Align>
+Type any_cast(basic_any<Len, Align> &&data) ENTT_NOEXCEPT {
+    // forces const on non-reference types to make them work also with wrappers for const references
+    auto * const instance = any_cast<std::remove_reference_t<const Type>>(&data);
+    ENTT_ASSERT(instance, "Invalid instance");
+    return static_cast<Type>(std::move(*instance));
+}
+
+
+/*! @copydoc any_cast */
+template<typename Type, std::size_t Len, std::size_t Align>
+const Type * any_cast(const basic_any<Len, Align> *data) ENTT_NOEXCEPT {
+    return (data->type() == type_id<Type>() ? static_cast<const Type *>(data->data()) : nullptr);
+}
+
+
+/*! @copydoc any_cast */
+template<typename Type, std::size_t Len, std::size_t Align>
+Type * any_cast(basic_any<Len, Align> *data) ENTT_NOEXCEPT {
+    // last attempt to make wrappers for const references return their values
+    return (data->type() == type_id<Type>() ? static_cast<Type *>(static_cast<constness_as_t<basic_any<Len, Align>, Type> *>(data)->data()) : nullptr);
+}
+
+
+/**
+ * @brief Constructs a wrapper from a given type, passing it all arguments.
+ * @tparam Type Type of object to use to initialize the wrapper.
+ * @tparam Len Size of the storage reserved for the small buffer optimization.
+ * @tparam Align Optional alignment requirement.
+ * @tparam Args Types of arguments to use to construct the new instance.
+ * @param args Parameters to use to construct the instance.
+ * @return A properly initialized wrapper for an object of the given type.
+ */
+template<typename Type, std::size_t Len = basic_any<>::length, std::size_t Align = basic_any<Len>::alignment, typename... Args>
+basic_any<Len, Align> make_any(Args &&... args) {
+    return basic_any<Len, Align>{std::in_place_type<Type>, std::forward<Args>(args)...};
+}
+
+
+/**
+ * @brief Forwards its argument and avoids copies for lvalue references.
+ * @tparam Len Size of the storage reserved for the small buffer optimization.
+ * @tparam Align Optional alignment requirement.
+ * @tparam Type Type of argument to use to construct the new instance.
+ * @param value Parameter to use to construct the instance.
+ * @return A properly initialized and not necessarily owning wrapper.
+ */
+template<std::size_t Len = basic_any<>::length, std::size_t Align = basic_any<Len>::alignment, typename Type>
+basic_any<Len, Align> forward_as_any(Type &&value) {
+    return basic_any<Len, Align>{std::in_place_type<std::conditional_t<std::is_rvalue_reference_v<Type>, std::decay_t<Type>, Type>>, std::forward<Type>(value)};
+}
+
+
+}
+
+
+#endif
+
+// #include "../core/fwd.hpp"
+
+// #include "../core/type_info.hpp"
+#ifndef ENTT_CORE_TYPE_INFO_HPP
+#define ENTT_CORE_TYPE_INFO_HPP
+
+
+#include <string_view>
+#include <type_traits>
+// #include "../config/config.h"
+
+// #include "../core/attribute.h"
+
+// #include "hashed_string.hpp"
+
+// #include "fwd.hpp"
+
+
+
+namespace entt {
+
+
+/**
+ * @cond TURN_OFF_DOXYGEN
+ * Internal details not to be documented.
+ */
+
+
+namespace internal {
+
+
+struct ENTT_API type_seq final {
+    [[nodiscard]] static id_type next() ENTT_NOEXCEPT {
+        static ENTT_MAYBE_ATOMIC(id_type) value{};
+        return value++;
+    }
+};
+
+
+template<typename Type>
+[[nodiscard]] constexpr auto stripped_type_name() ENTT_NOEXCEPT {
+#if defined ENTT_PRETTY_FUNCTION
+    std::string_view pretty_function{ENTT_PRETTY_FUNCTION};
+    auto first = pretty_function.find_first_not_of(' ', pretty_function.find_first_of(ENTT_PRETTY_FUNCTION_PREFIX)+1);
+    auto value = pretty_function.substr(first, pretty_function.find_last_of(ENTT_PRETTY_FUNCTION_SUFFIX) - first);
+    return value;
+#else
+    return std::string_view{""};
+#endif
+}
+
+
+template<typename Type, auto = stripped_type_name<Type>().find_first_of('.')>
+[[nodiscard]] static constexpr std::string_view type_name(int) ENTT_NOEXCEPT {
+    constexpr auto value = stripped_type_name<Type>();
+    return value;
+}
+
+
+template<typename Type>
+[[nodiscard]] static std::string_view type_name(char) ENTT_NOEXCEPT {
+    static const auto value = stripped_type_name<Type>();
+    return value;
+}
+
+
+template<typename Type, auto = stripped_type_name<Type>().find_first_of('.')>
+[[nodiscard]] static constexpr id_type type_hash(int) ENTT_NOEXCEPT {
+    constexpr auto stripped = stripped_type_name<Type>();
+    constexpr auto value = hashed_string::value(stripped.data(), stripped.size());
+    return value;
+}
+
+
+template<typename Type>
+[[nodiscard]] static id_type type_hash(char) ENTT_NOEXCEPT {
+    static const auto value = [](const auto stripped) {
+        return hashed_string::value(stripped.data(), stripped.size());
+    }(stripped_type_name<Type>());
+    return value;
+}
+
+
+}
+
+
+/**
+ * Internal details not to be documented.
+ * @endcond
+ */
+
+
+/**
+ * @brief Type sequential identifier.
+ * @tparam Type Type for which to generate a sequential identifier.
+ */
+template<typename Type, typename = void>
+struct ENTT_API type_seq final {
+    /**
+     * @brief Returns the sequential identifier of a given type.
+     * @return The sequential identifier of a given type.
+     */
+    [[nodiscard]] static id_type value() ENTT_NOEXCEPT {
+        static const id_type value = internal::type_seq::next();
+        return value;
+    }
+
+    /*! @copydoc value */
+    [[nodiscard]] constexpr operator id_type() const ENTT_NOEXCEPT { return value(); }
+};
+
+
+/**
+ * @brief Type hash.
+ * @tparam Type Type for which to generate a hash value.
+ */
+template<typename Type, typename = void>
+struct type_hash final {
+    /**
+     * @brief Returns the numeric representation of a given type.
+     * @return The numeric representation of the given type.
+     */
+#if defined ENTT_PRETTY_FUNCTION
+    [[nodiscard]] static constexpr id_type value() ENTT_NOEXCEPT {
+        return internal::type_hash<Type>(0);
+#else
+    [[nodiscard]] static constexpr id_type value() ENTT_NOEXCEPT {
+        return type_seq<Type>::value();
+#endif
+    }
+
+    /*! @copydoc value */
+    [[nodiscard]] constexpr operator id_type() const ENTT_NOEXCEPT { return value(); }
+};
+
+
+/**
+ * @brief Type name.
+ * @tparam Type Type for which to generate a name.
+ */
+template<typename Type, typename = void>
+struct type_name final {
+    /**
+     * @brief Returns the name of a given type.
+     * @return The name of the given type.
+     */
+    [[nodiscard]] static constexpr std::string_view value() ENTT_NOEXCEPT {
+        return internal::type_name<Type>(0);
+    }
+
+    /*! @copydoc value */
+    [[nodiscard]] constexpr operator std::string_view() const ENTT_NOEXCEPT { return value(); }
+};
+
+
+/*! @brief Implementation specific information about a type. */
+class type_info final {
+    template<typename>
+    friend type_info type_id() ENTT_NOEXCEPT;
+
+    type_info(id_type seq_v, id_type hash_v, std::string_view name_v) ENTT_NOEXCEPT
+        : seq_value{seq_v},
+          hash_value{hash_v},
+          name_value{name_v}
+    {}
+
+public:
+    /*! @brief Default constructor. */
+    type_info() ENTT_NOEXCEPT
+        : type_info({}, {}, {})
+    {}
+
+    /*! @brief Default copy constructor. */
+    type_info(const type_info &) ENTT_NOEXCEPT = default;
+    /*! @brief Default move constructor. */
+    type_info(type_info &&) ENTT_NOEXCEPT = default;
+
+    /**
+     * @brief Default copy assignment operator.
+     * @return This type info object.
+     */
+    type_info & operator=(const type_info &) ENTT_NOEXCEPT = default;
+
+    /**
+     * @brief Default move assignment operator.
+     * @return This type info object.
+     */
+    type_info & operator=(type_info &&) ENTT_NOEXCEPT = default;
+
+    /**
+     * @brief Checks if a type info object is properly initialized.
+     * @return True if the object is properly initialized, false otherwise.
+     */
+    [[nodiscard]] explicit operator bool() const ENTT_NOEXCEPT {
+        return name_value.data() != nullptr;
+    }
+
+    /**
+     * @brief Type sequential identifier.
+     * @return Type sequential identifier.
+     */
+    [[nodiscard]] id_type seq() const ENTT_NOEXCEPT {
+        return seq_value;
+    }
+
+    /**
+     * @brief Type hash.
+     * @return Type hash.
+     */
+    [[nodiscard]] id_type hash() const ENTT_NOEXCEPT {
+        return hash_value;
+    }
+
+    /**
+     * @brief Type name.
+     * @return Type name.
+     */
+    [[nodiscard]] std::string_view name() const ENTT_NOEXCEPT {
+        return name_value;
+    }
+
+    /**
+     * @brief Compares the contents of two type info objects.
+     * @param other Object with which to compare.
+     * @return False if the two contents differ, true otherwise.
+     */
+    [[nodiscard]] bool operator==(const type_info &other) const ENTT_NOEXCEPT {
+        return hash_value == other.hash_value;
+    }
+
+private:
+    id_type seq_value;
+    id_type hash_value;
+    std::string_view name_value;
+};
+
+
+/**
+ * @brief Compares the contents of two type info objects.
+ * @param lhs A type info object.
+ * @param rhs A type info object.
+ * @return True if the two contents differ, false otherwise.
+ */
+[[nodiscard]] inline bool operator!=(const type_info &lhs, const type_info &rhs) ENTT_NOEXCEPT {
+    return !(lhs == rhs);
+}
+
+
+/**
+ * @brief Returns the type info object for a given type.
+ * @tparam Type Type for which to generate a type info object.
+ * @return The type info object for the given type.
+ */
+template<typename Type>
+[[nodiscard]] type_info type_id() ENTT_NOEXCEPT {
+    return type_info{
+        type_seq<std::remove_cv_t<std::remove_reference_t<Type>>>::value(),
+        type_hash<std::remove_cv_t<std::remove_reference_t<Type>>>::value(),
+        type_name<std::remove_cv_t<std::remove_reference_t<Type>>>::value()
+    };
+}
+
+
+}
+
+
+#endif
+
+// #include "../core/type_traits.hpp"
+
+// #include "component.hpp"
+
+// #include "entity.hpp"
+
+// #include "fwd.hpp"
+
+// #include "group.hpp"
+#ifndef ENTT_ENTITY_GROUP_HPP
+#define ENTT_ENTITY_GROUP_HPP
+
+
+#include <tuple>
+#include <utility>
+#include <type_traits>
+// #include "../config/config.h"
+
+// #include "../core/type_traits.hpp"
+
+// #include "entity.hpp"
+
+// #include "fwd.hpp"
+
+// #include "sparse_set.hpp"
+
+// #include "storage.hpp"
+
+// #include "utility.hpp"
+
+
+
+namespace entt {
+
+
+/**
+ * @brief Group.
+ *
+ * Primary template isn't defined on purpose. All the specializations give a
+ * compile-time error, but for a few reasonable cases.
+ */
+template<typename...>
+class basic_group;
+
+
+/**
+ * @brief Non-owning group.
+ *
+ * A non-owning group returns all entities and only the entities that have at
+ * least the given components. Moreover, it's guaranteed that the entity list
+ * is tightly packed in memory for fast iterations.
+ *
+ * @b Important
+ *
+ * Iterators aren't invalidated if:
+ *
+ * * New instances of the given components are created and assigned to entities.
+ * * The entity currently pointed is modified (as an example, if one of the
+ *   given components is removed from the entity to which the iterator points).
+ * * The entity currently pointed is destroyed.
+ *
+ * In all other cases, modifying the pools iterated by the group in any way
+ * invalidates all the iterators and using them results in undefined behavior.
+ *
+ * @note
+ * Groups share references to the underlying data structures of the registry
+ * that generated them. Therefore any change to the entities and to the
+ * components made by means of the registry are immediately reflected by all the
+ * groups.<br/>
+ * Moreover, sorting a non-owning group affects all the instances of the same
+ * group (it means that users don't have to call `sort` on each instance to sort
+ * all of them because they _share_ entities and components).
+ *
+ * @warning
+ * Lifetime of a group must not overcome that of the registry that generated it.
+ * In any other case, attempting to use a group results in undefined behavior.
+ *
+ * @tparam Entity A valid entity type (see entt_traits for more details).
+ * @tparam Exclude Types of components used to filter the group.
+ * @tparam Get Type of components observed by the group.
+ */
+template<typename Entity, typename... Exclude, typename... Get>
+class basic_group<Entity, exclude_t<Exclude...>, get_t<Get...>> final {
+    /*! @brief A registry is allowed to create groups. */
+    friend class basic_registry<Entity>;
+
+    using basic_common_type = basic_sparse_set<Entity>;
+
+    template<typename Component>
+    using storage_type = constness_as_t<typename storage_traits<Entity, std::remove_const_t<Component>>::storage_type, Component>;
+
+    class iterable final {
+        template<typename It>
+        struct iterable_iterator final {
+            using difference_type = std::ptrdiff_t;
+            using value_type = decltype(std::tuple_cat(std::tuple<Entity>{}, std::declval<basic_group>().get({})));
+            using pointer = void;
+            using reference = value_type;
+            using iterator_category = std::input_iterator_tag;
+
+            template<typename... Args>
+            iterable_iterator(It from, const std::tuple<storage_type<Get> *...> &args) ENTT_NOEXCEPT
+                : it{from},
+                  pools{args}
+            {}
+
+            iterable_iterator & operator++() ENTT_NOEXCEPT {
+                return ++it, *this;
+            }
+
+            iterable_iterator operator++(int) ENTT_NOEXCEPT {
+                iterable_iterator orig = *this;
+                return ++(*this), orig;
+            }
+
+            [[nodiscard]] reference operator*() const ENTT_NOEXCEPT {
+                const auto entt = *it;
+                return std::tuple_cat(std::make_tuple(entt), get_as_tuple(*std::get<storage_type<Get> *>(pools), entt)...);
+            }
+
+            [[nodiscard]] bool operator==(const iterable_iterator &other) const ENTT_NOEXCEPT {
+                return other.it == it;
+            }
+
+            [[nodiscard]] bool operator!=(const iterable_iterator &other) const ENTT_NOEXCEPT {
+                return !(*this == other);
+            }
+
+        private:
+            It it;
+            std::tuple<storage_type<Get> *...> pools;
+        };
+
+    public:
+        using iterator = iterable_iterator<typename basic_common_type::iterator>;
+        using reverse_iterator = iterable_iterator<typename basic_common_type::reverse_iterator>;
+
+        iterable(basic_common_type * const ref, const std::tuple<storage_type<Get> *...> &cpools)
+            : handler{ref},
+              pools{cpools}
+        {}
+
+        [[nodiscard]] iterator begin() const ENTT_NOEXCEPT {
+            return handler ? iterator{handler->begin(), pools} : iterator{{}, pools};
+        }
+
+        [[nodiscard]] iterator end() const ENTT_NOEXCEPT {
+            return handler ? iterator{handler->end(), pools} : iterator{{}, pools};
+        }
+
+        [[nodiscard]] reverse_iterator rbegin() const ENTT_NOEXCEPT {
+            return handler ? reverse_iterator{handler->rbegin(), pools} : reverse_iterator{{}, pools};
+        }
+
+        [[nodiscard]] reverse_iterator rend() const ENTT_NOEXCEPT {
+            return handler ? reverse_iterator{handler->rend(), pools} : reverse_iterator{{}, pools};
+        }
+
+    private:
+        basic_common_type * const handler;
+        const std::tuple<storage_type<Get> *...> pools;
+    };
+
+    basic_group(basic_common_type &ref, storage_type<Get> &... gpool) ENTT_NOEXCEPT
+        : handler{&ref},
+          pools{&gpool...}
+    {}
+
+public:
+    /*! @brief Underlying entity identifier. */
+    using entity_type = Entity;
+    /*! @brief Unsigned integer type. */
+    using size_type = std::size_t;
+    /*! @brief Random access iterator type. */
+    using iterator = typename basic_common_type::iterator;
+    /*! @brief Reversed iterator type. */
+    using reverse_iterator = typename basic_common_type::reverse_iterator;
+    /*! @brief Iterable group type. */
+    using iterable_group = iterable;
+
+    /*! @brief Default constructor to use to create empty, invalid groups. */
+    basic_group() ENTT_NOEXCEPT
+        : handler{}
+    {}
+
+    /**
+     * @brief Returns the number of entities that have the given components.
+     * @return Number of entities that have the given components.
+     */
+    [[nodiscard]] size_type size() const ENTT_NOEXCEPT {
+        return *this ? handler->size() : size_type{};
+    }
+
+    /**
+     * @brief Returns the number of elements that a group has currently
+     * allocated space for.
+     * @return Capacity of the group.
+     */
+    [[nodiscard]] size_type capacity() const ENTT_NOEXCEPT {
+        return *this ? handler->capacity() : size_type{};
+    }
+
+    /*! @brief Requests the removal of unused capacity. */
+    void shrink_to_fit() {
+        if(*this) {
+            handler->shrink_to_fit();
+        }
+    }
+
+    /**
+     * @brief Checks whether a group is empty.
+     * @return True if the group is empty, false otherwise.
+     */
+    [[nodiscard]] bool empty() const ENTT_NOEXCEPT {
+        return !*this || handler->empty();
+    }
+
+    /**
+     * @brief Direct access to the list of entities.
+     *
+     * The returned pointer is such that range `[data(), data() + size())` is
+     * always a valid range, even if the container is empty.
+     *
+     * @return A pointer to the array of entities.
+     */
+    [[nodiscard]] auto data() const ENTT_NOEXCEPT {
+        return *this ? handler->data() : nullptr;
+    }
+
+    /**
+     * @brief Returns an iterator to the first entity of the group.
+     *
+     * The returned iterator points to the first entity of the group. If the
+     * group is empty, the returned iterator will be equal to `end()`.
+     *
+     * @return An iterator to the first entity of the group.
+     */
+    [[nodiscard]] iterator begin() const ENTT_NOEXCEPT {
+        return *this ? handler->begin() : iterator{};
+    }
+
+    /**
+     * @brief Returns an iterator that is past the last entity of the group.
+     *
+     * The returned iterator points to the entity following the last entity of
+     * the group. Attempting to dereference the returned iterator results in
+     * undefined behavior.
+     *
+     * @return An iterator to the entity following the last entity of the
+     * group.
+     */
+    [[nodiscard]] iterator end() const ENTT_NOEXCEPT {
+        return *this ? handler->end() : iterator{};
+    }
+
+    /**
+     * @brief Returns an iterator to the first entity of the reversed group.
+     *
+     * The returned iterator points to the first entity of the reversed group.
+     * If the group is empty, the returned iterator will be equal to `rend()`.
+     *
+     * @return An iterator to the first entity of the reversed group.
+     */
+    [[nodiscard]] reverse_iterator rbegin() const ENTT_NOEXCEPT {
+        return *this ? handler->rbegin() : reverse_iterator{};
+    }
+
+    /**
+     * @brief Returns an iterator that is past the last entity of the reversed
+     * group.
+     *
+     * The returned iterator points to the entity following the last entity of
+     * the reversed group. Attempting to dereference the returned iterator
+     * results in undefined behavior.
+     *
+     * @return An iterator to the entity following the last entity of the
+     * reversed group.
+     */
+    [[nodiscard]] reverse_iterator rend() const ENTT_NOEXCEPT {
+        return *this ? handler->rend() : reverse_iterator{};
+    }
+
+    /**
+     * @brief Returns the first entity of the group, if any.
+     * @return The first entity of the group if one exists, the null entity
+     * otherwise.
+     */
+    [[nodiscard]] entity_type front() const {
+        const auto it = begin();
+        return it != end() ? *it : null;
+    }
+
+    /**
+     * @brief Returns the last entity of the group, if any.
+     * @return The last entity of the group if one exists, the null entity
+     * otherwise.
+     */
+    [[nodiscard]] entity_type back() const {
+        const auto it = rbegin();
+        return it != rend() ? *it : null;
+    }
+
+    /**
+     * @brief Finds an entity.
+     * @param entt A valid entity identifier.
+     * @return An iterator to the given entity if it's found, past the end
+     * iterator otherwise.
+     */
+    [[nodiscard]] iterator find(const entity_type entt) const {
+        const auto it = *this ? handler->find(entt) : iterator{};
+        return it != end() && *it == entt ? it : end();
+    }
+
+    /**
+     * @brief Returns the identifier that occupies the given position.
+     * @param pos Position of the element to return.
+     * @return The identifier that occupies the given position.
+     */
+    [[nodiscard]] entity_type operator[](const size_type pos) const {
+        return begin()[pos];
+    }
+
+    /**
+     * @brief Checks if a group is properly initialized.
+     * @return True if the group is properly initialized, false otherwise.
+     */
+    [[nodiscard]] explicit operator bool() const ENTT_NOEXCEPT {
+        return handler != nullptr;
+    }
+
+    /**
+     * @brief Checks if a group contains an entity.
+     * @param entt A valid entity identifier.
+     * @return True if the group contains the given entity, false otherwise.
+     */
+    [[nodiscard]] bool contains(const entity_type entt) const {
+        return *this && handler->contains(entt);
+    }
+
+    /**
+     * @brief Returns the components assigned to the given entity.
+     *
+     * Prefer this function instead of `registry::get` during iterations. It has
+     * far better performance than its counterpart.
+     *
+     * @warning
+     * Attempting to use an invalid component type results in a compilation
+     * error. Attempting to use an entity that doesn't belong to the group
+     * results in undefined behavior.
+     *
+     * @tparam Component Types of components to get.
+     * @param entt A valid entity identifier.
+     * @return The components assigned to the entity.
+     */
+    template<typename... Component>
+    [[nodiscard]] decltype(auto) get(const entity_type entt) const {
+        ENTT_ASSERT(contains(entt), "Group does not contain entity");
+
+        if constexpr(sizeof...(Component) == 0) {
+            return std::tuple_cat(get_as_tuple(*std::get<storage_type<Get> *>(pools), entt)...);
+        } else if constexpr(sizeof...(Component) == 1) {
+            return (std::get<storage_type<Component> *>(pools)->get(entt), ...);
+        } else {
+            return std::tuple_cat(get_as_tuple(*std::get<storage_type<Component> *>(pools), entt)...);
+        }
+    }
+
+    /**
+     * @brief Iterates entities and components and applies the given function
+     * object to them.
+     *
+     * The function object is invoked for each entity. It is provided with the
+     * entity itself and a set of references to non-empty components. The
+     * _constness_ of the components is as requested.<br/>
+     * The signature of the function must be equivalent to one of the following
+     * forms:
+     *
+     * @code{.cpp}
+     * void(const entity_type, Type &...);
+     * void(Type &...);
+     * @endcode
+     *
+     * @note
+     * Empty types aren't explicitly instantiated and therefore they are never
+     * returned during iterations.
+     *
+     * @tparam Func Type of the function object to invoke.
+     * @param func A valid function object.
+     */
+    template<typename Func>
+    void each(Func func) const {
+        for(const auto entt: *this) {
+            if constexpr(is_applicable_v<Func, decltype(std::tuple_cat(std::tuple<entity_type>{}, std::declval<basic_group>().get({})))>) {
+                std::apply(func, std::tuple_cat(std::make_tuple(entt), get(entt)));
+            } else {
+                std::apply(func, get(entt));
+            }
+        }
+    }
+
+    /**
+     * @brief Returns an iterable object to use to _visit_ the group.
+     *
+     * The iterable object returns tuples that contain the current entity and a
+     * set of references to its non-empty components. The _constness_ of the
+     * components is as requested.
+     *
+     * @note
+     * Empty types aren't explicitly instantiated and therefore they are never
+     * returned during iterations.
+     *
+     * @return An iterable object to use to _visit_ the group.
+     */
+    [[nodiscard]] iterable_group each() const ENTT_NOEXCEPT {
+        return iterable_group{handler, pools};
+    }
+
+    /**
+     * @brief Sort a group according to the given comparison function.
+     *
+     * Sort the group so that iterating it with a couple of iterators returns
+     * entities and components in the expected order. See `begin` and `end` for
+     * more details.
+     *
+     * The comparison function object must return `true` if the first element
+     * is _less_ than the second one, `false` otherwise. The signature of the
+     * comparison function should be equivalent to one of the following:
+     *
+     * @code{.cpp}
+     * bool(std::tuple<Component &...>, std::tuple<Component &...>);
+     * bool(const Component &..., const Component &...);
+     * bool(const Entity, const Entity);
+     * @endcode
+     *
+     * Where `Component` are such that they are iterated by the group.<br/>
+     * Moreover, the comparison function object shall induce a
+     * _strict weak ordering_ on the values.
+     *
+     * The sort function oject must offer a member function template
+     * `operator()` that accepts three arguments:
+     *
+     * * An iterator to the first element of the range to sort.
+     * * An iterator past the last element of the range to sort.
+     * * A comparison function to use to compare the elements.
+     *
+     * @tparam Component Optional types of components to compare.
+     * @tparam Compare Type of comparison function object.
+     * @tparam Sort Type of sort function object.
+     * @tparam Args Types of arguments to forward to the sort function object.
+     * @param compare A valid comparison function object.
+     * @param algo A valid sort function object.
+     * @param args Arguments to forward to the sort function object, if any.
+     */
+    template<typename... Component, typename Compare, typename Sort = std_sort, typename... Args>
+    void sort(Compare compare, Sort algo = Sort{}, Args &&... args) {
+        if(*this) {
+            if constexpr(sizeof...(Component) == 0) {
+                static_assert(std::is_invocable_v<Compare, const entity_type, const entity_type>, "Invalid comparison function");
+                handler->sort(std::move(compare), std::move(algo), std::forward<Args>(args)...);
+            }  else if constexpr(sizeof...(Component) == 1) {
+                handler->sort([this, compare = std::move(compare)](const entity_type lhs, const entity_type rhs) {
+                    return compare((std::get<storage_type<Component> *>(pools)->get(lhs), ...), (std::get<storage_type<Component> *>(pools)->get(rhs), ...));
+                }, std::move(algo), std::forward<Args>(args)...);
+            } else {
+                handler->sort([this, compare = std::move(compare)](const entity_type lhs, const entity_type rhs) {
+                    return compare(std::forward_as_tuple(std::get<storage_type<Component> *>(pools)->get(lhs)...), std::forward_as_tuple(std::get<storage_type<Component> *>(pools)->get(rhs)...));
+                }, std::move(algo), std::forward<Args>(args)...);
+            }
+        }
+    }
+
+    /**
+     * @brief Sort the shared pool of entities according to the given component.
+     *
+     * Non-owning groups of the same type share with the registry a pool of
+     * entities with its own order that doesn't depend on the order of any pool
+     * of components. Users can order the underlying data structure so that it
+     * respects the order of the pool of the given component.
+     *
+     * @note
+     * The shared pool of entities and thus its order is affected by the changes
+     * to each and every pool that it tracks. Therefore changes to those pools
+     * can quickly ruin the order imposed to the pool of entities shared between
+     * the non-owning groups.
+     *
+     * @tparam Component Type of component to use to impose the order.
+     */
+    template<typename Component>
+    void sort() const {
+        if(*this) {
+            handler->respect(*std::get<storage_type<Component> *>(pools));
+        }
+    }
+
+private:
+    basic_common_type * const handler;
+    const std::tuple<storage_type<Get> *...> pools;
+};
+
+
+/**
+ * @brief Owning group.
+ *
+ * Owning groups return all entities and only the entities that have at least
+ * the given components. Moreover:
+ *
+ * * It's guaranteed that the entity list is tightly packed in memory for fast
+ *   iterations.
+ * * It's guaranteed that the lists of owned components are tightly packed in
+ *   memory for even faster iterations and to allow direct access.
+ * * They stay true to the order of the owned components and all instances have
+ *   the same order in memory.
+ *
+ * The more types of components are owned by a group, the faster it is to
+ * iterate them.
+ *
+ * @b Important
+ *
+ * Iterators aren't invalidated if:
+ *
+ * * New instances of the given components are created and assigned to entities.
+ * * The entity currently pointed is modified (as an example, if one of the
+ *   given components is removed from the entity to which the iterator points).
+ * * The entity currently pointed is destroyed.
+ *
+ * In all other cases, modifying the pools iterated by the group in any way
+ * invalidates all the iterators and using them results in undefined behavior.
+ *
+ * @note
+ * Groups share references to the underlying data structures of the registry
+ * that generated them. Therefore any change to the entities and to the
+ * components made by means of the registry are immediately reflected by all the
+ * groups.
+ * Moreover, sorting an owning group affects all the instance of the same group
+ * (it means that users don't have to call `sort` on each instance to sort all
+ * of them because they share the underlying data structure).
+ *
+ * @warning
+ * Lifetime of a group must not overcome that of the registry that generated it.
+ * In any other case, attempting to use a group results in undefined behavior.
+ *
+ * @tparam Entity A valid entity type (see entt_traits for more details).
+ * @tparam Exclude Types of components used to filter the group.
+ * @tparam Get Types of components observed by the group.
+ * @tparam Owned Types of components owned by the group.
+ */
+template<typename Entity, typename... Exclude, typename... Get, typename... Owned>
+class basic_group<Entity, exclude_t<Exclude...>, get_t<Get...>, Owned...> final {
+    /*! @brief A registry is allowed to create groups. */
+    friend class basic_registry<Entity>;
+
+    using basic_common_type = basic_sparse_set<Entity>;
+
+    template<typename Component>
+    using storage_type = constness_as_t<typename storage_traits<Entity, std::remove_const_t<Component>>::storage_type, Component>;
+
+    class iterable final {
+        template<typename, typename>
+        struct iterable_iterator;
+
+        template<typename It, typename... OIt>
+        struct iterable_iterator<It, type_list<OIt...>> final {
+            using difference_type = std::ptrdiff_t;
+            using value_type = decltype(std::tuple_cat(std::tuple<Entity>{}, std::declval<basic_group>().get({})));
+            using pointer = void;
+            using reference = value_type;
+            using iterator_category = std::input_iterator_tag;
+
+            template<typename... Other>
+            iterable_iterator(It from, const std::tuple<Other...> &other, const std::tuple<storage_type<Get> *...> &cpools) ENTT_NOEXCEPT
+                : it{from},
+                  owned{std::get<OIt>(other)...},
+                  get{cpools}
+            {}
+
+            iterable_iterator & operator++() ENTT_NOEXCEPT {
+                return ++it, (++std::get<OIt>(owned), ...), *this;
+            }
+
+            iterable_iterator operator++(int) ENTT_NOEXCEPT {
+                iterable_iterator orig = *this;
+                return ++(*this), orig;
+            }
+
+            [[nodiscard]] reference operator*() const ENTT_NOEXCEPT {
+                return std::tuple_cat(
+                    std::make_tuple(*it),
+                    std::forward_as_tuple(*std::get<OIt>(owned)...),
+                    get_as_tuple(*std::get<storage_type<Get> *>(get), *it)...
+                );
+            }
+
+            [[nodiscard]] bool operator==(const iterable_iterator &other) const ENTT_NOEXCEPT {
+                return other.it == it;
+            }
+
+            [[nodiscard]] bool operator!=(const iterable_iterator &other) const ENTT_NOEXCEPT {
+                return !(*this == other);
+            }
+
+        private:
+            It it;
+            std::tuple<OIt...> owned;
+            std::tuple<storage_type<Get> *...> get;
+        };
+
+    public:
+        using iterator = iterable_iterator<
+            typename basic_common_type::iterator,
+            type_list_cat_t<std::conditional_t<std::is_void_v<decltype(std::declval<storage_type<Owned>>().get({}))>, type_list<>, type_list<decltype(std::declval<storage_type<Owned>>().end())>>...>
+        >;
+        using reverse_iterator = iterable_iterator<
+            typename basic_common_type::reverse_iterator,
+            type_list_cat_t<std::conditional_t<std::is_void_v<decltype(std::declval<storage_type<Owned>>().get({}))>, type_list<>, type_list<decltype(std::declval<storage_type<Owned>>().rbegin())>>...>
+        >;
+
+        iterable(std::tuple<storage_type<Owned> *..., storage_type<Get> *...> cpools, const std::size_t * const extent)
+            : pools{cpools},
+              length{extent}
+        {}
+
+        [[nodiscard]] iterator begin() const ENTT_NOEXCEPT {
+            return length ? iterator{
+                std::get<0>(pools)->basic_common_type::end() - *length,
+                std::make_tuple((std::get<storage_type<Owned> *>(pools)->end() - *length)...),
+                std::make_tuple(std::get<storage_type<Get> *>(pools)...)
+            } : iterator{{}, std::make_tuple(decltype(std::get<storage_type<Owned> *>(pools)->end()){}...), std::make_tuple(std::get<storage_type<Get> *>(pools)...)};
+        }
+
+        [[nodiscard]] iterator end() const ENTT_NOEXCEPT {
+            return length ? iterator{
+                std::get<0>(pools)->basic_common_type::end(),
+                std::make_tuple((std::get<storage_type<Owned> *>(pools)->end())...),
+                std::make_tuple(std::get<storage_type<Get> *>(pools)...)
+            } : iterator{{}, std::make_tuple(decltype(std::get<storage_type<Owned> *>(pools)->end()){}...), std::make_tuple(std::get<storage_type<Get> *>(pools)...)};
+        }
+
+        [[nodiscard]] reverse_iterator rbegin() const ENTT_NOEXCEPT {
+            return length ? reverse_iterator{
+                std::get<0>(pools)->basic_common_type::rbegin(),
+                std::make_tuple((std::get<storage_type<Owned> *>(pools)->rbegin())...),
+                std::make_tuple(std::get<storage_type<Get> *>(pools)...)
+            } : reverse_iterator{{}, std::make_tuple(decltype(std::get<storage_type<Owned> *>(pools)->rbegin()){}...), std::make_tuple(std::get<storage_type<Get> *>(pools)...)};
+        }
+
+        [[nodiscard]] reverse_iterator rend() const ENTT_NOEXCEPT {
+            return length ? reverse_iterator{
+                std::get<0>(pools)->basic_common_type::rbegin() + *length,
+                std::make_tuple((std::get<storage_type<Owned> *>(pools)->rbegin() + *length)...),
+                std::make_tuple(std::get<storage_type<Get> *>(pools)...)
+            } : reverse_iterator{{}, std::make_tuple(decltype(std::get<storage_type<Owned> *>(pools)->rbegin()){}...), std::make_tuple(std::get<storage_type<Get> *>(pools)...)};
+        }
+
+    private:
+        const std::tuple<storage_type<Owned> *..., storage_type<Get> *...> pools;
+        const std::size_t * const length;
+    };
+
+    basic_group(const std::size_t &extent, storage_type<Owned> &... opool, storage_type<Get> &... gpool) ENTT_NOEXCEPT
+        : pools{&opool..., &gpool...},
+          length{&extent}
+    {}
+
+public:
+    /*! @brief Underlying entity identifier. */
+    using entity_type = Entity;
+    /*! @brief Unsigned integer type. */
+    using size_type = std::size_t;
+    /*! @brief Random access iterator type. */
+    using iterator = typename basic_common_type::iterator;
+    /*! @brief Reversed iterator type. */
+    using reverse_iterator = typename basic_common_type::reverse_iterator;
+    /*! @brief Iterable group type. */
+    using iterable_group = iterable;
+
+    /*! @brief Default constructor to use to create empty, invalid groups. */
+    basic_group() ENTT_NOEXCEPT
+        : length{}
+    {}
+
+    /**
+     * @brief Returns the number of entities that have the given components.
+     * @return Number of entities that have the given components.
+     */
+    [[nodiscard]] size_type size() const ENTT_NOEXCEPT {
+        return *this ? *length : size_type{};
+    }
+
+    /**
+     * @brief Checks whether a group is empty.
+     * @return True if the group is empty, false otherwise.
+     */
+    [[nodiscard]] bool empty() const ENTT_NOEXCEPT {
+        return !*this || !*length;
+    }
+
+    /**
+     * @brief Direct access to the raw representation offered by the storage.
+     *
+     * @warning
+     * This function is only available for owned types.
+     *
+     * @tparam Component Type of component in which one is interested.
+     * @return A pointer to the array of components.
+     */
+    template<typename Component>
+    [[nodiscard]] auto raw() const ENTT_NOEXCEPT {
+        static_assert((std::is_same_v<Component, Owned> || ...), "Non-owned type");
+        auto *cpool = std::get<storage_type<Component> *>(pools);
+        return cpool ? cpool->raw() : decltype(cpool->raw()){};
+    }
+
+    /**
+     * @brief Direct access to the list of entities.
+     *
+     * The returned pointer is such that range `[data(), data() + size())` is
+     * always a valid range, even if the container is empty.
+     *
+     * @return A pointer to the array of entities.
+     */
+    [[nodiscard]] auto data() const ENTT_NOEXCEPT {
+        return *this ? std::get<0>(pools)->data() : nullptr;
+    }
+
+    /**
+     * @brief Returns an iterator to the first entity of the group.
+     *
+     * The returned iterator points to the first entity of the group. If the
+     * group is empty, the returned iterator will be equal to `end()`.
+     *
+     * @return An iterator to the first entity of the group.
+     */
+    [[nodiscard]] iterator begin() const ENTT_NOEXCEPT {
+        return *this ? (std::get<0>(pools)->basic_common_type::end() - *length) : iterator{};
+    }
+
+    /**
+     * @brief Returns an iterator that is past the last entity of the group.
+     *
+     * The returned iterator points to the entity following the last entity of
+     * the group. Attempting to dereference the returned iterator results in
+     * undefined behavior.
+     *
+     * @return An iterator to the entity following the last entity of the
+     * group.
+     */
+    [[nodiscard]] iterator end() const ENTT_NOEXCEPT {
+        return *this ? std::get<0>(pools)->basic_common_type::end() : iterator{};
+    }
+
+    /**
+     * @brief Returns an iterator to the first entity of the reversed group.
+     *
+     * The returned iterator points to the first entity of the reversed group.
+     * If the group is empty, the returned iterator will be equal to `rend()`.
+     *
+     * @return An iterator to the first entity of the reversed group.
+     */
+    [[nodiscard]] reverse_iterator rbegin() const ENTT_NOEXCEPT {
+        return *this ? std::get<0>(pools)->basic_common_type::rbegin() : reverse_iterator{};
+    }
+
+    /**
+     * @brief Returns an iterator that is past the last entity of the reversed
+     * group.
+     *
+     * The returned iterator points to the entity following the last entity of
+     * the reversed group. Attempting to dereference the returned iterator
+     * results in undefined behavior.
+     *
+     * @return An iterator to the entity following the last entity of the
+     * reversed group.
+     */
+    [[nodiscard]] reverse_iterator rend() const ENTT_NOEXCEPT {
+        return *this ? (std::get<0>(pools)->basic_common_type::rbegin() + *length) : reverse_iterator{};
+    }
+
+    /**
+     * @brief Returns the first entity of the group, if any.
+     * @return The first entity of the group if one exists, the null entity
+     * otherwise.
+     */
+    [[nodiscard]] entity_type front() const {
+        const auto it = begin();
+        return it != end() ? *it : null;
+    }
+
+    /**
+     * @brief Returns the last entity of the group, if any.
+     * @return The last entity of the group if one exists, the null entity
+     * otherwise.
+     */
+    [[nodiscard]] entity_type back() const {
+        const auto it = rbegin();
+        return it != rend() ? *it : null;
+    }
+
+    /**
+     * @brief Finds an entity.
+     * @param entt A valid entity identifier.
+     * @return An iterator to the given entity if it's found, past the end
+     * iterator otherwise.
+     */
+    [[nodiscard]] iterator find(const entity_type entt) const {
+        const auto it = *this ? std::get<0>(pools)->find(entt) : iterator{};
+        return it != end() && it >= begin() && *it == entt ? it : end();
+    }
+
+    /**
+     * @brief Returns the identifier that occupies the given position.
+     * @param pos Position of the element to return.
+     * @return The identifier that occupies the given position.
+     */
+    [[nodiscard]] entity_type operator[](const size_type pos) const {
+        return begin()[pos];
+    }
+
+    /**
+     * @brief Checks if a group is properly initialized.
+     * @return True if the group is properly initialized, false otherwise.
+     */
+    [[nodiscard]] explicit operator bool() const ENTT_NOEXCEPT {
+        return length != nullptr;
+    }
+
+    /**
+     * @brief Checks if a group contains an entity.
+     * @param entt A valid entity identifier.
+     * @return True if the group contains the given entity, false otherwise.
+     */
+    [[nodiscard]] bool contains(const entity_type entt) const {
+        return *this && std::get<0>(pools)->contains(entt) && (std::get<0>(pools)->index(entt) < (*length));
+    }
+
+    /**
+     * @brief Returns the components assigned to the given entity.
+     *
+     * Prefer this function instead of `registry::get` during iterations. It has
+     * far better performance than its counterpart.
+     *
+     * @warning
+     * Attempting to use an invalid component type results in a compilation
+     * error. Attempting to use an entity that doesn't belong to the group
+     * results in undefined behavior.
+     *
+     * @tparam Component Types of components to get.
+     * @param entt A valid entity identifier.
+     * @return The components assigned to the entity.
+     */
+    template<typename... Component>
+    [[nodiscard]] decltype(auto) get(const entity_type entt) const {
+        ENTT_ASSERT(contains(entt), "Group does not contain entity");
+
+        if constexpr(sizeof...(Component) == 0) {
+            return std::tuple_cat(get_as_tuple(*std::get<storage_type<Owned> *>(pools), entt)..., get_as_tuple(*std::get<storage_type<Get> *>(pools), entt)...);
+        } else if constexpr(sizeof...(Component) == 1) {
+            return (std::get<storage_type<Component> *>(pools)->get(entt), ...);
+        } else {
+            return std::tuple_cat(get_as_tuple(*std::get<storage_type<Component> *>(pools), entt)...);
+        }
+    }
+
+    /**
+     * @brief Iterates entities and components and applies the given function
+     * object to them.
+     *
+     * The function object is invoked for each entity. It is provided with the
+     * entity itself and a set of references to non-empty components. The
+     * _constness_ of the components is as requested.<br/>
+     * The signature of the function must be equivalent to one of the following
+     * forms:
+     *
+     * @code{.cpp}
+     * void(const entity_type, Type &...);
+     * void(Type &...);
+     * @endcode
+     *
+     * @note
+     * Empty types aren't explicitly instantiated and therefore they are never
+     * returned during iterations.
+     *
+     * @tparam Func Type of the function object to invoke.
+     * @param func A valid function object.
+     */
+    template<typename Func>
+    void each(Func func) const {
+        for(auto args: each()) {
+            if constexpr(is_applicable_v<Func, decltype(std::tuple_cat(std::tuple<entity_type>{}, std::declval<basic_group>().get({})))>) {
+                std::apply(func, args);
+            } else {
+                std::apply([&func](auto, auto &&... less) { func(std::forward<decltype(less)>(less)...); }, args);
+            }
+        }
+    }
+
+    /**
+     * @brief Returns an iterable object to use to _visit_ the group.
+     *
+     * The iterable object returns tuples that contain the current entity and a
+     * set of references to its non-empty components. The _constness_ of the
+     * components is as requested.
+     *
+     * @note
+     * Empty types aren't explicitly instantiated and therefore they are never
+     * returned during iterations.
+     *
+     * @return An iterable object to use to _visit_ the group.
+     */
+    [[nodiscard]] iterable_group each() const ENTT_NOEXCEPT {
+        return iterable_group{pools, length};
+    }
+
+    /**
+     * @brief Sort a group according to the given comparison function.
+     *
+     * Sort the group so that iterating it with a couple of iterators returns
+     * entities and components in the expected order. See `begin` and `end` for
+     * more details.
+     *
+     * The comparison function object must return `true` if the first element
+     * is _less_ than the second one, `false` otherwise. The signature of the
+     * comparison function should be equivalent to one of the following:
+     *
+     * @code{.cpp}
+     * bool(std::tuple<Component &...>, std::tuple<Component &...>);
+     * bool(const Component &, const Component &);
+     * bool(const Entity, const Entity);
+     * @endcode
+     *
+     * Where `Component` are either owned types or not but still such that they
+     * are iterated by the group.<br/>
+     * Moreover, the comparison function object shall induce a
+     * _strict weak ordering_ on the values.
+     *
+     * The sort function oject must offer a member function template
+     * `operator()` that accepts three arguments:
+     *
+     * * An iterator to the first element of the range to sort.
+     * * An iterator past the last element of the range to sort.
+     * * A comparison function to use to compare the elements.
+     *
+     * @tparam Component Optional types of components to compare.
+     * @tparam Compare Type of comparison function object.
+     * @tparam Sort Type of sort function object.
+     * @tparam Args Types of arguments to forward to the sort function object.
+     * @param compare A valid comparison function object.
+     * @param algo A valid sort function object.
+     * @param args Arguments to forward to the sort function object, if any.
+     */
+    template<typename... Component, typename Compare, typename Sort = std_sort, typename... Args>
+    void sort(Compare compare, Sort algo = Sort{}, Args &&... args) const {
+        auto *cpool = std::get<0>(pools);
+
+        if constexpr(sizeof...(Component) == 0) {
+            static_assert(std::is_invocable_v<Compare, const entity_type, const entity_type>, "Invalid comparison function");
+            cpool->sort_n(*length, std::move(compare), std::move(algo), std::forward<Args>(args)...);
+        } else if constexpr(sizeof...(Component) == 1) {
+            cpool->sort_n(*length, [this, compare = std::move(compare)](const entity_type lhs, const entity_type rhs) {
+                return compare((std::get<storage_type<Component> *>(pools)->get(lhs), ...), (std::get<storage_type<Component> *>(pools)->get(rhs), ...));
+            }, std::move(algo), std::forward<Args>(args)...);
+        } else {
+            cpool->sort_n(*length, [this, compare = std::move(compare)](const entity_type lhs, const entity_type rhs) {
+                return compare(std::forward_as_tuple(std::get<storage_type<Component> *>(pools)->get(lhs)...), std::forward_as_tuple(std::get<storage_type<Component> *>(pools)->get(rhs)...));
+            }, std::move(algo), std::forward<Args>(args)...);
+        }
+
+        [this](auto *head, auto *... other) {
+            for(auto next = *length; next; --next) {
+                const auto pos = next - 1;
+                [[maybe_unused]] const auto entt = head->data()[pos];
+                (other->swap(other->data()[pos], entt), ...);
+            }
+        }(std::get<storage_type<Owned> *>(pools)...);
+    }
+
+private:
+    const std::tuple<storage_type<Owned> *..., storage_type<Get> *...> pools;
+    const size_type * const length;
+};
+
+
+}
+
+
+#endif
+
+// #include "poly_storage.hpp"
+#ifndef ENTT_ENTITY_POLY_STORAGE_HPP
+#define ENTT_ENTITY_POLY_STORAGE_HPP
+
+
+#include <cstddef>
+#include <tuple>
+// #include "../core/type_info.hpp"
+
+// #include "../core/type_traits.hpp"
+
+// #include "../poly/poly.hpp"
+#ifndef ENTT_POLY_POLY_HPP
+#define ENTT_POLY_POLY_HPP
+
+
+#include <cstddef>
+#include <functional>
+#include <tuple>
+#include <type_traits>
+#include <utility>
+// #include "../config/config.h"
+#ifndef ENTT_CONFIG_CONFIG_H
+#define ENTT_CONFIG_CONFIG_H
+
+
+#if defined(__cpp_exceptions) && !defined(ENTT_NOEXCEPTION)
+#   define ENTT_NOEXCEPT noexcept
+#   define ENTT_THROW throw
+#   define ENTT_TRY try
+#   define ENTT_CATCH catch(...)
+#else
+#   define ENTT_NOEXCEPT
+#   define ENTT_THROW
+#   define ENTT_TRY if(true)
+#   define ENTT_CATCH if(false)
+#endif
+
+
+#if defined(__cpp_lib_launder) && __cpp_lib_launder >= 201606L
+#   include <new>
+#   define ENTT_LAUNDER(expr) std::launder(expr)
+#else
+#   define ENTT_LAUNDER(expr) expr
+#endif
+
+
+#ifndef ENTT_USE_ATOMIC
+#   define ENTT_MAYBE_ATOMIC(Type) Type
+#else
+#   include <atomic>
+#   define ENTT_MAYBE_ATOMIC(Type) std::atomic<Type>
+#endif
+
+
+#ifndef ENTT_ID_TYPE
+#   include <cstdint>
+#   define ENTT_ID_TYPE std::uint32_t
+#endif
+
+
+#ifdef ENTT_SPARSE_PAGE
+	static_assert(ENTT_SPARSE_PAGE && ((ENTT_SPARSE_PAGE & (ENTT_SPARSE_PAGE - 1)) == 0), "ENTT_SPARSE_PAGE must be a power of two");
+#else
+#   define ENTT_SPARSE_PAGE 4096
+#endif
+
+
+#ifdef ENTT_PACKED_PAGE
+static_assert(ENTT_PACKED_PAGE && ((ENTT_PACKED_PAGE & (ENTT_PACKED_PAGE - 1)) == 0), "ENTT_PACKED_PAGE must be a power of two");
+#else
+#   define ENTT_PACKED_PAGE 1024
+#endif
+
+
+#ifdef ENTT_DISABLE_ASSERT
+#   undef ENTT_ASSERT
+#   define ENTT_ASSERT(...) (void(0))
+#elif !defined ENTT_ASSERT
+#   include <cassert>
+#   define ENTT_ASSERT(condition, ...) assert(condition)
+#endif
+
+
+#ifdef ENTT_NO_ETO
+#   include <type_traits>
+#   define ENTT_IGNORE_IF_EMPTY std::false_type
+#else
+#   include <type_traits>
+#   define ENTT_IGNORE_IF_EMPTY std::true_type
+#endif
+
+
+#ifndef ENTT_STANDARD_CPP
+#    if defined __clang__ || defined __GNUC__
+#       define ENTT_PRETTY_FUNCTION __PRETTY_FUNCTION__
+#       define ENTT_PRETTY_FUNCTION_PREFIX '='
+#       define ENTT_PRETTY_FUNCTION_SUFFIX ']'
+#    elif defined _MSC_VER
+#       define ENTT_PRETTY_FUNCTION __FUNCSIG__
+#       define ENTT_PRETTY_FUNCTION_PREFIX '<'
+#       define ENTT_PRETTY_FUNCTION_SUFFIX '>'
+#   endif
+#endif
+
+
+#endif
+
+// #include "../core/any.hpp"
+#ifndef ENTT_CORE_ANY_HPP
+#define ENTT_CORE_ANY_HPP
+
+
+#include <cstddef>
+#include <functional>
+#include <memory>
+#include <type_traits>
+#include <utility>
+// #include "../config/config.h"
+#ifndef ENTT_CONFIG_CONFIG_H
+#define ENTT_CONFIG_CONFIG_H
+
+
+#if defined(__cpp_exceptions) && !defined(ENTT_NOEXCEPTION)
+#   define ENTT_NOEXCEPT noexcept
+#   define ENTT_THROW throw
+#   define ENTT_TRY try
+#   define ENTT_CATCH catch(...)
+#else
+#   define ENTT_NOEXCEPT
+#   define ENTT_THROW
+#   define ENTT_TRY if(true)
+#   define ENTT_CATCH if(false)
+#endif
+
+
+#if defined(__cpp_lib_launder) && __cpp_lib_launder >= 201606L
+#   include <new>
+#   define ENTT_LAUNDER(expr) std::launder(expr)
+#else
+#   define ENTT_LAUNDER(expr) expr
+#endif
+
+
+#ifndef ENTT_USE_ATOMIC
+#   define ENTT_MAYBE_ATOMIC(Type) Type
+#else
+#   include <atomic>
+#   define ENTT_MAYBE_ATOMIC(Type) std::atomic<Type>
+#endif
+
+
+#ifndef ENTT_ID_TYPE
+#   include <cstdint>
+#   define ENTT_ID_TYPE std::uint32_t
+#endif
+
+
+#ifdef ENTT_SPARSE_PAGE
+	static_assert(ENTT_SPARSE_PAGE && ((ENTT_SPARSE_PAGE & (ENTT_SPARSE_PAGE - 1)) == 0), "ENTT_SPARSE_PAGE must be a power of two");
+#else
+#   define ENTT_SPARSE_PAGE 4096
+#endif
+
+
+#ifdef ENTT_PACKED_PAGE
+static_assert(ENTT_PACKED_PAGE && ((ENTT_PACKED_PAGE & (ENTT_PACKED_PAGE - 1)) == 0), "ENTT_PACKED_PAGE must be a power of two");
+#else
+#   define ENTT_PACKED_PAGE 1024
+#endif
+
+
+#ifdef ENTT_DISABLE_ASSERT
+#   undef ENTT_ASSERT
+#   define ENTT_ASSERT(...) (void(0))
+#elif !defined ENTT_ASSERT
+#   include <cassert>
+#   define ENTT_ASSERT(condition, ...) assert(condition)
+#endif
+
+
+#ifdef ENTT_NO_ETO
+#   include <type_traits>
+#   define ENTT_IGNORE_IF_EMPTY std::false_type
+#else
+#   include <type_traits>
+#   define ENTT_IGNORE_IF_EMPTY std::true_type
+#endif
+
+
+#ifndef ENTT_STANDARD_CPP
+#    if defined __clang__ || defined __GNUC__
+#       define ENTT_PRETTY_FUNCTION __PRETTY_FUNCTION__
+#       define ENTT_PRETTY_FUNCTION_PREFIX '='
+#       define ENTT_PRETTY_FUNCTION_SUFFIX ']'
+#    elif defined _MSC_VER
+#       define ENTT_PRETTY_FUNCTION __FUNCSIG__
+#       define ENTT_PRETTY_FUNCTION_PREFIX '<'
+#       define ENTT_PRETTY_FUNCTION_SUFFIX '>'
+#   endif
+#endif
+
+
+#endif
+
+// #include "../core/utility.hpp"
+#ifndef ENTT_CORE_UTILITY_HPP
+#define ENTT_CORE_UTILITY_HPP
+
+
+#include <utility>
+// #include "../config/config.h"
+#ifndef ENTT_CONFIG_CONFIG_H
+#define ENTT_CONFIG_CONFIG_H
+
+
+#if defined(__cpp_exceptions) && !defined(ENTT_NOEXCEPTION)
+#   define ENTT_NOEXCEPT noexcept
+#   define ENTT_THROW throw
+#   define ENTT_TRY try
+#   define ENTT_CATCH catch(...)
+#else
+#   define ENTT_NOEXCEPT
+#   define ENTT_THROW
+#   define ENTT_TRY if(true)
+#   define ENTT_CATCH if(false)
+#endif
+
+
+#if defined(__cpp_lib_launder) && __cpp_lib_launder >= 201606L
+#   include <new>
+#   define ENTT_LAUNDER(expr) std::launder(expr)
+#else
+#   define ENTT_LAUNDER(expr) expr
+#endif
+
+
+#ifndef ENTT_USE_ATOMIC
+#   define ENTT_MAYBE_ATOMIC(Type) Type
+#else
+#   include <atomic>
+#   define ENTT_MAYBE_ATOMIC(Type) std::atomic<Type>
+#endif
+
+
+#ifndef ENTT_ID_TYPE
+#   include <cstdint>
+#   define ENTT_ID_TYPE std::uint32_t
+#endif
+
+
+#ifdef ENTT_SPARSE_PAGE
+	static_assert(ENTT_SPARSE_PAGE && ((ENTT_SPARSE_PAGE & (ENTT_SPARSE_PAGE - 1)) == 0), "ENTT_SPARSE_PAGE must be a power of two");
+#else
+#   define ENTT_SPARSE_PAGE 4096
+#endif
+
+
+#ifdef ENTT_PACKED_PAGE
+static_assert(ENTT_PACKED_PAGE && ((ENTT_PACKED_PAGE & (ENTT_PACKED_PAGE - 1)) == 0), "ENTT_PACKED_PAGE must be a power of two");
+#else
+#   define ENTT_PACKED_PAGE 1024
+#endif
+
+
+#ifdef ENTT_DISABLE_ASSERT
+#   undef ENTT_ASSERT
+#   define ENTT_ASSERT(...) (void(0))
+#elif !defined ENTT_ASSERT
+#   include <cassert>
+#   define ENTT_ASSERT(condition, ...) assert(condition)
+#endif
+
+
+#ifdef ENTT_NO_ETO
+#   include <type_traits>
+#   define ENTT_IGNORE_IF_EMPTY std::false_type
+#else
+#   include <type_traits>
+#   define ENTT_IGNORE_IF_EMPTY std::true_type
+#endif
+
+
+#ifndef ENTT_STANDARD_CPP
+#    if defined __clang__ || defined __GNUC__
+#       define ENTT_PRETTY_FUNCTION __PRETTY_FUNCTION__
+#       define ENTT_PRETTY_FUNCTION_PREFIX '='
+#       define ENTT_PRETTY_FUNCTION_SUFFIX ']'
+#    elif defined _MSC_VER
+#       define ENTT_PRETTY_FUNCTION __FUNCSIG__
+#       define ENTT_PRETTY_FUNCTION_PREFIX '<'
+#       define ENTT_PRETTY_FUNCTION_SUFFIX '>'
+#   endif
+#endif
+
+
+#endif
+
+
+
+namespace entt {
+
+
+/*! @brief Identity function object (waiting for C++20). */
+struct identity {
+    /**
+     * @brief Returns its argument unchanged.
+     * @tparam Type Type of the argument.
+     * @param value The actual argument.
+     * @return The submitted value as-is.
+     */
+    template<class Type>
+    [[nodiscard]] constexpr Type && operator()(Type &&value) const ENTT_NOEXCEPT {
+        return std::forward<Type>(value);
+    }
+};
+
+
+/**
+ * @brief Constant utility to disambiguate overloaded members of a class.
+ * @tparam Type Type of the desired overload.
+ * @tparam Class Type of class to which the member belongs.
+ * @param member A valid pointer to a member.
+ * @return Pointer to the member.
+ */
+template<typename Type, typename Class>
+[[nodiscard]] constexpr auto overload(Type Class:: *member) ENTT_NOEXCEPT { return member; }
+
+
+/**
+ * @brief Constant utility to disambiguate overloaded functions.
+ * @tparam Func Function type of the desired overload.
+ * @param func A valid pointer to a function.
+ * @return Pointer to the function.
+ */
+template<typename Func>
+[[nodiscard]] constexpr auto overload(Func *func) ENTT_NOEXCEPT { return func; }
+
+
+/**
+ * @brief Helper type for visitors.
+ * @tparam Func Types of function objects.
+ */
+template<class... Func>
+struct overloaded: Func... {
+    using Func::operator()...;
+};
+
+
+/**
+ * @brief Deduction guide.
+ * @tparam Func Types of function objects.
+ */
+template<class... Func>
+overloaded(Func...)
+-> overloaded<Func...>;
+
+
+/**
+ * @brief Basic implementation of a y-combinator.
+ * @tparam Func Type of a potentially recursive function.
+ */
+template<class Func>
+struct y_combinator {
+    /**
+     * @brief Constructs a y-combinator from a given function.
+     * @param recursive A potentially recursive function.
+     */
+    y_combinator(Func recursive):
+        func{std::move(recursive)}
+    {}
+
+    /**
+     * @brief Invokes a y-combinator and therefore its underlying function.
+     * @tparam Args Types of arguments to use to invoke the underlying function.
+     * @param args Parameters to use to invoke the underlying function.
+     * @return Return value of the underlying function, if any.
+     */
+    template <class... Args>
+    decltype(auto) operator()(Args &&... args) const {
+        return func(*this, std::forward<Args>(args)...);
+    }
+
+    /*! @copydoc operator()() */
+    template <class... Args>
+    decltype(auto) operator()(Args &&... args) {
+        return func(*this, std::forward<Args>(args)...);
+    }
+
+private:
+    Func func;
+};
+
+
+}
+
+
+#endif
+
+// #include "fwd.hpp"
+#ifndef ENTT_CORE_FWD_HPP
+#define ENTT_CORE_FWD_HPP
+
+
+#include <type_traits>
+// #include "../config/config.h"
+
+
+
+namespace entt {
+
+
+template<std::size_t Len = sizeof(double[2]), std::size_t = alignof(typename std::aligned_storage_t<Len + !Len>)>
+class basic_any;
+
+
+/*! @brief Alias declaration for type identifiers. */
+using id_type = ENTT_ID_TYPE;
+
+
+/*! @brief Alias declaration for the most common use case. */
+using any = basic_any<>;
+
+
+}
+
+
+#endif
+
+// #include "type_info.hpp"
+#ifndef ENTT_CORE_TYPE_INFO_HPP
+#define ENTT_CORE_TYPE_INFO_HPP
+
+
+#include <string_view>
+#include <type_traits>
+// #include "../config/config.h"
+
+// #include "../core/attribute.h"
+#ifndef ENTT_CORE_ATTRIBUTE_H
+#define ENTT_CORE_ATTRIBUTE_H
+
+
+#ifndef ENTT_EXPORT
+#   if defined _WIN32 || defined __CYGWIN__ || defined _MSC_VER
+#       define ENTT_EXPORT __declspec(dllexport)
+#       define ENTT_IMPORT __declspec(dllimport)
+#       define ENTT_HIDDEN
+#   elif defined __GNUC__ && __GNUC__ >= 4
+#       define ENTT_EXPORT __attribute__((visibility("default")))
+#       define ENTT_IMPORT __attribute__((visibility("default")))
+#       define ENTT_HIDDEN __attribute__((visibility("hidden")))
+#   else /* Unsupported compiler */
+#       define ENTT_EXPORT
+#       define ENTT_IMPORT
+#       define ENTT_HIDDEN
+#   endif
+#endif
+
+
+#ifndef ENTT_API
+#   if defined ENTT_API_EXPORT
+#       define ENTT_API ENTT_EXPORT
+#   elif defined ENTT_API_IMPORT
+#       define ENTT_API ENTT_IMPORT
+#   else /* No API */
+#       define ENTT_API
+#   endif
+#endif
+
+
+#endif
+
+// #include "hashed_string.hpp"
+#ifndef ENTT_CORE_HASHED_STRING_HPP
+#define ENTT_CORE_HASHED_STRING_HPP
+
+
+#include <cstddef>
+#include <cstdint>
+// #include "../config/config.h"
+
+// #include "fwd.hpp"
+
+
+
+namespace entt {
+
+
+/**
+ * @cond TURN_OFF_DOXYGEN
+ * Internal details not to be documented.
+ */
+
+
+namespace internal {
+
+
+template<typename>
+struct fnv1a_traits;
+
+
+template<>
+struct fnv1a_traits<std::uint32_t> {
+    using type = std::uint32_t;
+    static constexpr std::uint32_t offset = 2166136261;
+    static constexpr std::uint32_t prime = 16777619;
+};
+
+
+template<>
+struct fnv1a_traits<std::uint64_t> {
+    using type = std::uint64_t;
+    static constexpr std::uint64_t offset = 14695981039346656037ull;
+    static constexpr std::uint64_t prime = 1099511628211ull;
+};
+
+
+}
+
+
+/**
+ * Internal details not to be documented.
+ * @endcond
+ */
+
+
+/**
+ * @brief Zero overhead unique identifier.
+ *
+ * A hashed string is a compile-time tool that allows users to use
+ * human-readable identifers in the codebase while using their numeric
+ * counterparts at runtime.<br/>
+ * Because of that, a hashed string can also be used in constant expressions if
+ * required.
+ *
+ * @tparam Char Character type.
+ */
+template<typename Char>
+class basic_hashed_string {
+    using traits_type = internal::fnv1a_traits<id_type>;
+
+    struct const_wrapper {
+        // non-explicit constructor on purpose
+        constexpr const_wrapper(const Char *curr) ENTT_NOEXCEPT: str{curr} {}
+        const Char *str;
+    };
+
+    // Fowler–Noll–Vo hash function v. 1a - the good
+    [[nodiscard]] static constexpr id_type helper(const Char *curr) ENTT_NOEXCEPT {
+        auto value = traits_type::offset;
+
+        while(*curr != 0) {
+            value = (value ^ static_cast<traits_type::type>(*(curr++))) * traits_type::prime;
+        }
+
+        return value;
+    }
+
+public:
+    /*! @brief Character type. */
+    using value_type = Char;
+    /*! @brief Unsigned integer type. */
+    using hash_type = id_type;
+
+    /**
+     * @brief Returns directly the numeric representation of a string view.
+     * @param str Human-readable identifer.
+     * @param size Length of the string to hash.
+     * @return The numeric representation of the string.
+     */
+    [[nodiscard]] static constexpr hash_type value(const value_type *str, std::size_t size) ENTT_NOEXCEPT {
+        id_type partial{traits_type::offset};
+        while(size--) { partial = (partial^(str++)[0])*traits_type::prime; }
+        return partial;
+    }
+
+    /**
+     * @brief Returns directly the numeric representation of a string.
+     *
+     * Forcing template resolution avoids implicit conversions. An
+     * human-readable identifier can be anything but a plain, old bunch of
+     * characters.<br/>
+     * Example of use:
+     * @code{.cpp}
+     * const auto value = basic_hashed_string<char>::to_value("my.png");
+     * @endcode
+     *
+     * @tparam N Number of characters of the identifier.
+     * @param str Human-readable identifer.
+     * @return The numeric representation of the string.
+     */
+    template<std::size_t N>
+    [[nodiscard]] static constexpr hash_type value(const value_type (&str)[N]) ENTT_NOEXCEPT {
+        return helper(str);
+    }
+
+    /**
+     * @brief Returns directly the numeric representation of a string.
+     * @param wrapper Helps achieving the purpose by relying on overloading.
+     * @return The numeric representation of the string.
+     */
+    [[nodiscard]] static hash_type value(const_wrapper wrapper) ENTT_NOEXCEPT {
+        return helper(wrapper.str);
+    }
+
+    /*! @brief Constructs an empty hashed string. */
+    constexpr basic_hashed_string() ENTT_NOEXCEPT
+        : str{nullptr}, hash{}
+    {}
+
+    /**
+     * @brief Constructs a hashed string from an array of const characters.
+     *
+     * Forcing template resolution avoids implicit conversions. An
+     * human-readable identifier can be anything but a plain, old bunch of
+     * characters.<br/>
+     * Example of use:
+     * @code{.cpp}
+     * basic_hashed_string<char> hs{"my.png"};
+     * @endcode
+     *
+     * @tparam N Number of characters of the identifier.
+     * @param curr Human-readable identifer.
+     */
+    template<std::size_t N>
+    constexpr basic_hashed_string(const value_type (&curr)[N]) ENTT_NOEXCEPT
+        : str{curr}, hash{helper(curr)}
+    {}
+
+    /**
+     * @brief Explicit constructor on purpose to avoid constructing a hashed
+     * string directly from a `const value_type *`.
+     * @param wrapper Helps achieving the purpose by relying on overloading.
+     */
+    explicit constexpr basic_hashed_string(const_wrapper wrapper) ENTT_NOEXCEPT
+        : str{wrapper.str}, hash{helper(wrapper.str)}
+    {}
+
+    /**
+     * @brief Returns the human-readable representation of a hashed string.
+     * @return The string used to initialize the instance.
+     */
+    [[nodiscard]] constexpr const value_type * data() const ENTT_NOEXCEPT {
+        return str;
+    }
+
+    /**
+     * @brief Returns the numeric representation of a hashed string.
+     * @return The numeric representation of the instance.
+     */
+    [[nodiscard]] constexpr hash_type value() const ENTT_NOEXCEPT {
+        return hash;
+    }
+
+    /*! @copydoc data */
+    [[nodiscard]] constexpr operator const value_type *() const ENTT_NOEXCEPT { return data(); }
+
+    /**
+     * @brief Returns the numeric representation of a hashed string.
+     * @return The numeric representation of the instance.
+     */
+    [[nodiscard]] constexpr operator hash_type() const ENTT_NOEXCEPT { return value(); }
+
+    /**
+     * @brief Compares two hashed strings.
+     * @param other Hashed string with which to compare.
+     * @return True if the two hashed strings are identical, false otherwise.
+     */
+    [[nodiscard]] constexpr bool operator==(const basic_hashed_string &other) const ENTT_NOEXCEPT {
+        return hash == other.hash;
+    }
+
+private:
+    const value_type *str;
+    hash_type hash;
+};
+
+
+/**
+ * @brief Deduction guide.
+ *
+ * It allows to deduce the character type of the hashed string directly from a
+ * human-readable identifer provided to the constructor.
+ *
+ * @tparam Char Character type.
+ * @tparam N Number of characters of the identifier.
+ * @param str Human-readable identifer.
+ */
+template<typename Char, std::size_t N>
+basic_hashed_string(const Char (&str)[N])
+-> basic_hashed_string<Char>;
+
+
+/**
+ * @brief Compares two hashed strings.
+ * @tparam Char Character type.
+ * @param lhs A valid hashed string.
+ * @param rhs A valid hashed string.
+ * @return True if the two hashed strings are identical, false otherwise.
+ */
+template<typename Char>
+[[nodiscard]] constexpr bool operator!=(const basic_hashed_string<Char> &lhs, const basic_hashed_string<Char> &rhs) ENTT_NOEXCEPT {
+    return !(lhs == rhs);
+}
+
+
+/*! @brief Aliases for common character types. */
+using hashed_string = basic_hashed_string<char>;
+
+
+/*! @brief Aliases for common character types. */
+using hashed_wstring = basic_hashed_string<wchar_t>;
+
+
+inline namespace literals {
+
+
+/**
+ * @brief User defined literal for hashed strings.
+ * @param str The literal without its suffix.
+ * @return A properly initialized hashed string.
+ */
+[[nodiscard]] constexpr entt::hashed_string operator"" _hs(const char *str, std::size_t) ENTT_NOEXCEPT {
+    return entt::hashed_string{str};
+}
+
+
+/**
+ * @brief User defined literal for hashed wstrings.
+ * @param str The literal without its suffix.
+ * @return A properly initialized hashed wstring.
+ */
+[[nodiscard]] constexpr entt::hashed_wstring operator"" _hws(const wchar_t *str, std::size_t) ENTT_NOEXCEPT {
+    return entt::hashed_wstring{str};
+}
+
+
+}
+
+
+}
+
+
+#endif
+
+// #include "fwd.hpp"
+
+
+
+namespace entt {
+
+
+/**
+ * @cond TURN_OFF_DOXYGEN
+ * Internal details not to be documented.
+ */
+
+
+namespace internal {
+
+
+struct ENTT_API type_seq final {
+    [[nodiscard]] static id_type next() ENTT_NOEXCEPT {
+        static ENTT_MAYBE_ATOMIC(id_type) value{};
+        return value++;
+    }
+};
+
+
+template<typename Type>
+[[nodiscard]] constexpr auto stripped_type_name() ENTT_NOEXCEPT {
+#if defined ENTT_PRETTY_FUNCTION
+    std::string_view pretty_function{ENTT_PRETTY_FUNCTION};
+    auto first = pretty_function.find_first_not_of(' ', pretty_function.find_first_of(ENTT_PRETTY_FUNCTION_PREFIX)+1);
+    auto value = pretty_function.substr(first, pretty_function.find_last_of(ENTT_PRETTY_FUNCTION_SUFFIX) - first);
+    return value;
+#else
+    return std::string_view{""};
+#endif
+}
+
+
+template<typename Type, auto = stripped_type_name<Type>().find_first_of('.')>
+[[nodiscard]] static constexpr std::string_view type_name(int) ENTT_NOEXCEPT {
+    constexpr auto value = stripped_type_name<Type>();
+    return value;
+}
+
+
+template<typename Type>
+[[nodiscard]] static std::string_view type_name(char) ENTT_NOEXCEPT {
+    static const auto value = stripped_type_name<Type>();
+    return value;
+}
+
+
+template<typename Type, auto = stripped_type_name<Type>().find_first_of('.')>
+[[nodiscard]] static constexpr id_type type_hash(int) ENTT_NOEXCEPT {
+    constexpr auto stripped = stripped_type_name<Type>();
+    constexpr auto value = hashed_string::value(stripped.data(), stripped.size());
+    return value;
+}
+
+
+template<typename Type>
+[[nodiscard]] static id_type type_hash(char) ENTT_NOEXCEPT {
+    static const auto value = [](const auto stripped) {
+        return hashed_string::value(stripped.data(), stripped.size());
+    }(stripped_type_name<Type>());
+    return value;
+}
+
+
+}
+
+
+/**
+ * Internal details not to be documented.
+ * @endcond
+ */
+
+
+/**
+ * @brief Type sequential identifier.
+ * @tparam Type Type for which to generate a sequential identifier.
+ */
+template<typename Type, typename = void>
+struct ENTT_API type_seq final {
+    /**
+     * @brief Returns the sequential identifier of a given type.
+     * @return The sequential identifier of a given type.
+     */
+    [[nodiscard]] static id_type value() ENTT_NOEXCEPT {
+        static const id_type value = internal::type_seq::next();
+        return value;
+    }
+
+    /*! @copydoc value */
+    [[nodiscard]] constexpr operator id_type() const ENTT_NOEXCEPT { return value(); }
+};
+
+
+/**
+ * @brief Type hash.
+ * @tparam Type Type for which to generate a hash value.
+ */
+template<typename Type, typename = void>
+struct type_hash final {
+    /**
+     * @brief Returns the numeric representation of a given type.
+     * @return The numeric representation of the given type.
+     */
+#if defined ENTT_PRETTY_FUNCTION
+    [[nodiscard]] static constexpr id_type value() ENTT_NOEXCEPT {
+        return internal::type_hash<Type>(0);
+#else
+    [[nodiscard]] static constexpr id_type value() ENTT_NOEXCEPT {
+        return type_seq<Type>::value();
+#endif
+    }
+
+    /*! @copydoc value */
+    [[nodiscard]] constexpr operator id_type() const ENTT_NOEXCEPT { return value(); }
+};
+
+
+/**
+ * @brief Type name.
+ * @tparam Type Type for which to generate a name.
+ */
+template<typename Type, typename = void>
+struct type_name final {
+    /**
+     * @brief Returns the name of a given type.
+     * @return The name of the given type.
+     */
+    [[nodiscard]] static constexpr std::string_view value() ENTT_NOEXCEPT {
+        return internal::type_name<Type>(0);
+    }
+
+    /*! @copydoc value */
+    [[nodiscard]] constexpr operator std::string_view() const ENTT_NOEXCEPT { return value(); }
+};
+
+
+/*! @brief Implementation specific information about a type. */
+class type_info final {
+    template<typename>
+    friend type_info type_id() ENTT_NOEXCEPT;
+
+    type_info(id_type seq_v, id_type hash_v, std::string_view name_v) ENTT_NOEXCEPT
+        : seq_value{seq_v},
+          hash_value{hash_v},
+          name_value{name_v}
+    {}
+
+public:
+    /*! @brief Default constructor. */
+    type_info() ENTT_NOEXCEPT
+        : type_info({}, {}, {})
+    {}
+
+    /*! @brief Default copy constructor. */
+    type_info(const type_info &) ENTT_NOEXCEPT = default;
+    /*! @brief Default move constructor. */
+    type_info(type_info &&) ENTT_NOEXCEPT = default;
+
+    /**
+     * @brief Default copy assignment operator.
+     * @return This type info object.
+     */
+    type_info & operator=(const type_info &) ENTT_NOEXCEPT = default;
+
+    /**
+     * @brief Default move assignment operator.
+     * @return This type info object.
+     */
+    type_info & operator=(type_info &&) ENTT_NOEXCEPT = default;
+
+    /**
+     * @brief Checks if a type info object is properly initialized.
+     * @return True if the object is properly initialized, false otherwise.
+     */
+    [[nodiscard]] explicit operator bool() const ENTT_NOEXCEPT {
+        return name_value.data() != nullptr;
+    }
+
+    /**
+     * @brief Type sequential identifier.
+     * @return Type sequential identifier.
+     */
+    [[nodiscard]] id_type seq() const ENTT_NOEXCEPT {
+        return seq_value;
+    }
+
+    /**
+     * @brief Type hash.
+     * @return Type hash.
+     */
+    [[nodiscard]] id_type hash() const ENTT_NOEXCEPT {
+        return hash_value;
+    }
+
+    /**
+     * @brief Type name.
+     * @return Type name.
+     */
+    [[nodiscard]] std::string_view name() const ENTT_NOEXCEPT {
+        return name_value;
+    }
+
+    /**
+     * @brief Compares the contents of two type info objects.
+     * @param other Object with which to compare.
+     * @return False if the two contents differ, true otherwise.
+     */
+    [[nodiscard]] bool operator==(const type_info &other) const ENTT_NOEXCEPT {
+        return hash_value == other.hash_value;
+    }
+
+private:
+    id_type seq_value;
+    id_type hash_value;
+    std::string_view name_value;
+};
+
+
+/**
+ * @brief Compares the contents of two type info objects.
+ * @param lhs A type info object.
+ * @param rhs A type info object.
+ * @return True if the two contents differ, false otherwise.
+ */
+[[nodiscard]] inline bool operator!=(const type_info &lhs, const type_info &rhs) ENTT_NOEXCEPT {
+    return !(lhs == rhs);
+}
+
+
+/**
+ * @brief Returns the type info object for a given type.
+ * @tparam Type Type for which to generate a type info object.
+ * @return The type info object for the given type.
+ */
+template<typename Type>
+[[nodiscard]] type_info type_id() ENTT_NOEXCEPT {
+    return type_info{
+        type_seq<std::remove_cv_t<std::remove_reference_t<Type>>>::value(),
+        type_hash<std::remove_cv_t<std::remove_reference_t<Type>>>::value(),
+        type_name<std::remove_cv_t<std::remove_reference_t<Type>>>::value()
+    };
+}
+
+
+}
+
+
+#endif
+
+// #include "type_traits.hpp"
+#ifndef ENTT_CORE_TYPE_TRAITS_HPP
+#define ENTT_CORE_TYPE_TRAITS_HPP
+
+
+#include <cstddef>
+#include <iterator>
+#include <type_traits>
+#include <utility>
+// #include "../config/config.h"
+
+// #include "fwd.hpp"
+
+
+
+namespace entt {
+
+
+/**
+ * @brief Utility class to disambiguate overloaded functions.
+ * @tparam N Number of choices available.
+ */
+template<std::size_t N>
+struct choice_t
+    // Unfortunately, doxygen cannot parse such a construct.
+    /*! @cond TURN_OFF_DOXYGEN */
+    : choice_t<N-1>
+    /*! @endcond */
+{};
+
+
+/*! @copybrief choice_t */
+template<>
+struct choice_t<0> {};
+
+
+/**
+ * @brief Variable template for the choice trick.
+ * @tparam N Number of choices available.
+ */
+template<std::size_t N>
+inline constexpr choice_t<N> choice{};
+
+
+/**
+ * @brief Identity type trait.
+ *
+ * Useful to establish non-deduced contexts in template argument deduction
+ * (waiting for C++20) or to provide types through function arguments.
+ *
+ * @tparam Type A type.
+ */
+template<typename Type>
+struct type_identity {
+    /*! @brief Identity type. */
+    using type = Type;
+};
+
+
+/**
+ * @brief Helper type.
+ * @tparam Type A type.
+ */
+template<typename Type>
+using type_identity_t = typename type_identity<Type>::type;
+
+
+/**
+ * @brief A type-only `sizeof` wrapper that returns 0 where `sizeof` complains.
+ * @tparam Type The type of which to return the size.
+ * @tparam The size of the type if `sizeof` accepts it, 0 otherwise.
+ */
+template<typename Type, typename = void>
+struct size_of: std::integral_constant<std::size_t, 0u> {};
+
+
+/*! @copydoc size_of */
+template<typename Type>
+struct size_of<Type, std::void_t<decltype(sizeof(Type))>>
+    : std::integral_constant<std::size_t, sizeof(Type)>
+{};
+
+
+/**
+ * @brief Helper variable template.
+ * @tparam Type The type of which to return the size.
+ */
+template<class Type>
+inline constexpr std::size_t size_of_v = size_of<Type>::value;
+
+
+/**
+ * @brief Using declaration to be used to _repeat_ the same type a number of
+ * times equal to the size of a given parameter pack.
+ * @tparam Type A type to repeat.
+ */
+template<typename Type, typename>
+using unpack_as_t = Type;
+
+
+/**
+ * @brief Helper variable template to be used to _repeat_ the same value a
+ * number of times equal to the size of a given parameter pack.
+ * @tparam Value A value to repeat.
+ */
+template<auto Value, typename>
+inline constexpr auto unpack_as_v = Value;
+
+
+/**
+ * @brief Wraps a static constant.
+ * @tparam Value A static constant.
+ */
+template<auto Value>
+using integral_constant = std::integral_constant<decltype(Value), Value>;
+
+
+/**
+ * @brief Alias template to facilitate the creation of named values.
+ * @tparam Value A constant value at least convertible to `id_type`.
+ */
+template<id_type Value>
+using tag = integral_constant<Value>;
+
+
+/**
+ * @brief A class to use to push around lists of types, nothing more.
+ * @tparam Type Types provided by the type list.
+ */
+template<typename... Type>
+struct type_list {
+    /*! @brief Type list type. */
+    using type = type_list;
+    /*! @brief Compile-time number of elements in the type list. */
+    static constexpr auto size = sizeof...(Type);
+};
+
+
+/*! @brief Primary template isn't defined on purpose. */
+template<std::size_t, typename>
+struct type_list_element;
+
+
+/**
+ * @brief Provides compile-time indexed access to the types of a type list.
+ * @tparam Index Index of the type to return.
+ * @tparam Type First type provided by the type list.
+ * @tparam Other Other types provided by the type list.
+ */
+template<std::size_t Index, typename Type, typename... Other>
+struct type_list_element<Index, type_list<Type, Other...>>
+    : type_list_element<Index - 1u, type_list<Other...>>
+{};
+
+
+/**
+ * @brief Provides compile-time indexed access to the types of a type list.
+ * @tparam Type First type provided by the type list.
+ * @tparam Other Other types provided by the type list.
+ */
+template<typename Type, typename... Other>
+struct type_list_element<0u, type_list<Type, Other...>> {
+    /*! @brief Searched type. */
+    using type = Type;
+};
+
+
+/**
+ * @brief Helper type.
+ * @tparam Index Index of the type to return.
+ * @tparam List Type list to search into.
+ */
+template<std::size_t Index, typename List>
+using type_list_element_t = typename type_list_element<Index, List>::type;
+
+
+/**
+ * @brief Concatenates multiple type lists.
+ * @tparam Type Types provided by the first type list.
+ * @tparam Other Types provided by the second type list.
+ * @return A type list composed by the types of both the type lists.
+ */
+template<typename... Type, typename... Other>
+constexpr type_list<Type..., Other...> operator+(type_list<Type...>, type_list<Other...>) { return {}; }
+
+
+/*! @brief Primary template isn't defined on purpose. */
+template<typename...>
+struct type_list_cat;
+
+
+/*! @brief Concatenates multiple type lists. */
+template<>
+struct type_list_cat<> {
+    /*! @brief A type list composed by the types of all the type lists. */
+    using type = type_list<>;
+};
+
+
+/**
+ * @brief Concatenates multiple type lists.
+ * @tparam Type Types provided by the first type list.
+ * @tparam Other Types provided by the second type list.
+ * @tparam List Other type lists, if any.
+ */
+template<typename... Type, typename... Other, typename... List>
+struct type_list_cat<type_list<Type...>, type_list<Other...>, List...> {
+    /*! @brief A type list composed by the types of all the type lists. */
+    using type = typename type_list_cat<type_list<Type..., Other...>, List...>::type;
+};
+
+
+/**
+ * @brief Concatenates multiple type lists.
+ * @tparam Type Types provided by the type list.
+ */
+template<typename... Type>
+struct type_list_cat<type_list<Type...>> {
+    /*! @brief A type list composed by the types of all the type lists. */
+    using type = type_list<Type...>;
+};
+
+
+/**
+ * @brief Helper type.
+ * @tparam List Type lists to concatenate.
+ */
+template<typename... List>
+using type_list_cat_t = typename type_list_cat<List...>::type;
+
+
+/*! @brief Primary template isn't defined on purpose. */
+template<typename>
+struct type_list_unique;
+
+
+/**
+ * @brief Removes duplicates types from a type list.
+ * @tparam Type One of the types provided by the given type list.
+ * @tparam Other The other types provided by the given type list.
+ */
+template<typename Type, typename... Other>
+struct type_list_unique<type_list<Type, Other...>> {
+    /*! @brief A type list without duplicate types. */
+    using type = std::conditional_t<
+        std::disjunction_v<std::is_same<Type, Other>...>,
+        typename type_list_unique<type_list<Other...>>::type,
+        type_list_cat_t<type_list<Type>, typename type_list_unique<type_list<Other...>>::type>
+    >;
+};
+
+
+/*! @brief Removes duplicates types from a type list. */
+template<>
+struct type_list_unique<type_list<>> {
+    /*! @brief A type list without duplicate types. */
+    using type = type_list<>;
+};
+
+
+/**
+ * @brief Helper type.
+ * @tparam Type A type list.
+ */
+template<typename Type>
+using type_list_unique_t = typename type_list_unique<Type>::type;
+
+
+/**
+ * @brief Provides the member constant `value` to true if a type list contains a
+ * given type, false otherwise.
+ * @tparam List Type list.
+ * @tparam Type Type to look for.
+ */
+template<typename List, typename Type>
+struct type_list_contains;
+
+
+/**
+ * @copybrief type_list_contains
+ * @tparam Type Types provided by the type list.
+ * @tparam Other Type to look for.
+ */
+template<typename... Type, typename Other>
+struct type_list_contains<type_list<Type...>, Other>: std::disjunction<std::is_same<Type, Other>...> {};
+
+
+/**
+ * @brief Helper variable template.
+ * @tparam List Type list.
+ * @tparam Type Type to look for.
+ */
+template<class List, typename Type>
+inline constexpr bool type_list_contains_v = type_list_contains<List, Type>::value;
+
+
+/*! @brief Primary template isn't defined on purpose. */
+template<typename...>
+struct type_list_diff;
+
+
+/**
+ * @brief Computes the difference between two type lists.
+ * @tparam Type Types provided by the first type list.
+ * @tparam Other Types provided by the second type list.
+ */
+template<typename... Type, typename... Other>
+struct type_list_diff<type_list<Type...>, type_list<Other...>> {
+    /*! @brief A type list that is the difference between the two type lists. */
+    using type = type_list_cat_t<std::conditional_t<type_list_contains_v<type_list<Other...>, Type>, type_list<>, type_list<Type>>...>;
+};
+
+
+/**
+ * @brief Helper type.
+ * @tparam List Type lists between which to compute the difference.
+ */
+template<typename... List>
+using type_list_diff_t = typename type_list_diff<List...>::type;
+
+
+/**
+ * @brief A class to use to push around lists of constant values, nothing more.
+ * @tparam Value Values provided by the value list.
+ */
+template<auto... Value>
+struct value_list {
+    /*! @brief Value list type. */
+    using type = value_list;
+    /*! @brief Compile-time number of elements in the value list. */
+    static constexpr auto size = sizeof...(Value);
+};
+
+
+/*! @brief Primary template isn't defined on purpose. */
+template<std::size_t, typename>
+struct value_list_element;
+
+
+/**
+ * @brief Provides compile-time indexed access to the values of a value list.
+ * @tparam Index Index of the value to return.
+ * @tparam Value First value provided by the value list.
+ * @tparam Other Other values provided by the value list.
+ */
+template<std::size_t Index, auto Value, auto... Other>
+struct value_list_element<Index, value_list<Value, Other...>>
+    : value_list_element<Index - 1u, value_list<Other...>>
+{};
+
+
+/**
+ * @brief Provides compile-time indexed access to the types of a type list.
+ * @tparam Value First value provided by the value list.
+ * @tparam Other Other values provided by the value list.
+ */
+template<auto Value, auto... Other>
+struct value_list_element<0u, value_list<Value, Other...>> {
+    /*! @brief Searched value. */
+    static constexpr auto value = Value;
+};
+
+
+/**
+ * @brief Helper type.
+ * @tparam Index Index of the value to return.
+ * @tparam List Value list to search into.
+ */
+template<std::size_t Index, typename List>
+inline constexpr auto value_list_element_v = value_list_element<Index, List>::value;
+
+
+/**
+ * @brief Concatenates multiple value lists.
+ * @tparam Value Values provided by the first value list.
+ * @tparam Other Values provided by the second value list.
+ * @return A value list composed by the values of both the value lists.
+ */
+template<auto... Value, auto... Other>
+constexpr value_list<Value..., Other...> operator+(value_list<Value...>, value_list<Other...>) { return {}; }
+
+
+/*! @brief Primary template isn't defined on purpose. */
+template<typename...>
+struct value_list_cat;
+
+
+/*! @brief Concatenates multiple value lists. */
+template<>
+struct value_list_cat<> {
+    /*! @brief A value list composed by the values of all the value lists. */
+    using type = value_list<>;
+};
+
+
+/**
+ * @brief Concatenates multiple value lists.
+ * @tparam Value Values provided by the first value list.
+ * @tparam Other Values provided by the second value list.
+ * @tparam List Other value lists, if any.
+ */
+template<auto... Value, auto... Other, typename... List>
+struct value_list_cat<value_list<Value...>, value_list<Other...>, List...> {
+    /*! @brief A value list composed by the values of all the value lists. */
+    using type = typename value_list_cat<value_list<Value..., Other...>, List...>::type;
+};
+
+
+/**
+ * @brief Concatenates multiple value lists.
+ * @tparam Value Values provided by the value list.
+ */
+template<auto... Value>
+struct value_list_cat<value_list<Value...>> {
+    /*! @brief A value list composed by the values of all the value lists. */
+    using type = value_list<Value...>;
+};
+
+
+/**
+ * @brief Helper type.
+ * @tparam List Value lists to concatenate.
+ */
+template<typename... List>
+using value_list_cat_t = typename value_list_cat<List...>::type;
+
+
+/*! @brief Same as std::is_invocable, but with tuples. */
+template<typename, typename>
+struct is_applicable: std::false_type {};
+
+
+/**
+ * @copybrief is_applicable
+ * @tparam Func A valid function type.
+ * @tparam Tuple Tuple-like type.
+ * @tparam Args The list of arguments to use to probe the function type.
+ */
+template<typename Func, template<typename...> class Tuple, typename... Args>
+struct is_applicable<Func, Tuple<Args...>>: std::is_invocable<Func, Args...> {};
+
+
+/**
+ * @copybrief is_applicable
+ * @tparam Func A valid function type.
+ * @tparam Tuple Tuple-like type.
+ * @tparam Args The list of arguments to use to probe the function type.
+ */
+template<typename Func, template<typename...> class Tuple, typename... Args>
+struct is_applicable<Func, const Tuple<Args...>>: std::is_invocable<Func, Args...> {};
+
+
+/**
+ * @brief Helper variable template.
+ * @tparam Func A valid function type.
+ * @tparam Args The list of arguments to use to probe the function type.
+ */
+template<typename Func, typename Args>
+inline constexpr bool is_applicable_v = is_applicable<Func, Args>::value;
+
+
+/*! @brief Same as std::is_invocable_r, but with tuples for arguments. */
+template<typename, typename, typename>
+struct is_applicable_r: std::false_type {};
+
+
+/**
+ * @copybrief is_applicable_r
+ * @tparam Ret The type to which the return type of the function should be
+ * convertible.
+ * @tparam Func A valid function type.
+ * @tparam Args The list of arguments to use to probe the function type.
+ */
+template<typename Ret, typename Func, typename... Args>
+struct is_applicable_r<Ret, Func, std::tuple<Args...>>: std::is_invocable_r<Ret, Func, Args...> {};
+
+
+/**
+ * @brief Helper variable template.
+ * @tparam Ret The type to which the return type of the function should be
+ * convertible.
+ * @tparam Func A valid function type.
+ * @tparam Args The list of arguments to use to probe the function type.
+ */
+template<typename Ret, typename Func, typename Args>
+inline constexpr bool is_applicable_r_v = is_applicable_r<Ret, Func, Args>::value;
+
+
+/**
+ * @brief Provides the member constant `value` to true if a given type is
+ * complete, false otherwise.
+ * @tparam Type The type to test.
+ */
+template<typename Type, typename = void>
+struct is_complete: std::false_type {};
+
+
+/*! @copydoc is_complete */
+template<typename Type>
+struct is_complete<Type, std::void_t<decltype(sizeof(Type))>>: std::true_type {};
+
+
+/**
+ * @brief Helper variable template.
+ * @tparam Type The type to test.
+ */
+template<typename Type>
+inline constexpr bool is_complete_v = is_complete<Type>::value;
+
+
+/**
+ * @brief Provides the member constant `value` to true if a given type is an
+ * iterator, false otherwise.
+ * @tparam Type The type to test.
+ */
+template<typename Type, typename = void>
+struct is_iterator: std::false_type {};
+
+
+/*! @copydoc is_iterator */
+template<typename Type>
+struct is_iterator<Type, std::void_t<typename std::iterator_traits<Type>::iterator_category>>
+    : std::true_type
+{};
+
+
+/**
+ * @brief Helper variable template.
+ * @tparam Type The type to test.
+ */
+template<typename Type>
+inline constexpr bool is_iterator_v = is_iterator<Type>::value;
+
+
+/**
+ * @brief Provides the member constant `value` to true if a given type is of the
+ * required iterator type, false otherwise.
+ * @tparam Type The type to test.
+ * @tparam It Required iterator type.
+ */
+template<typename Type, typename It, typename = void>
+struct is_iterator_type: std::false_type {};
+
+
+/*! @copydoc is_iterator_type */
+template<typename Type, typename It>
+struct is_iterator_type<Type, It, std::enable_if_t<is_iterator_v<Type> && std::is_same_v<Type, It>>>
+    : std::true_type
+{};
+
+
+/*! @copydoc is_iterator_type */
+template<typename Type, typename It>
+struct is_iterator_type<Type, It, std::enable_if_t<!std::is_same_v<Type, It>, std::void_t<typename It::iterator_type>>>
+    : is_iterator_type<Type, typename It::iterator_type>
+{};
+
+
+/**
+ * @brief Helper variable template.
+ * @tparam Type The type to test.
+ * @tparam It Required iterator type.
+ */
+template<typename Type, typename It>
+inline constexpr bool is_iterator_type_v = is_iterator_type<Type, It>::value;
+
+
+/**
+ * @cond TURN_OFF_DOXYGEN
+ * Internal details not to be documented.
+ */
+
+
+namespace internal {
+
+
+template<typename>
+[[nodiscard]] constexpr bool is_equality_comparable(...) { return false; }
+
+
+template<typename Type>
+[[nodiscard]] constexpr auto is_equality_comparable(choice_t<0>)
+-> decltype(std::declval<Type>() == std::declval<Type>()) { return true; }
+
+
+template<typename Type>
+[[nodiscard]] constexpr auto is_equality_comparable(choice_t<1>)
+-> decltype(std::declval<typename Type::value_type>(), std::declval<Type>() == std::declval<Type>()) {
+    if constexpr(is_iterator_v<Type>) {
+        return true;
+    } else if constexpr(std::is_same_v<typename Type::value_type, Type>) {
+        return is_equality_comparable<Type>(choice<0>);
+    } else {
+        return is_equality_comparable<typename Type::value_type>(choice<2>);
+    }
+}
+
+
+template<typename Type>
+[[nodiscard]] constexpr auto is_equality_comparable(choice_t<2>)
+-> decltype(std::declval<typename Type::mapped_type>(), std::declval<Type>() == std::declval<Type>()) {
+    return is_equality_comparable<typename Type::key_type>(choice<2>) && is_equality_comparable<typename Type::mapped_type>(choice<2>);
+}
+
+
+}
+
+
+/**
+ * Internal details not to be documented.
+ * @endcond
+ */
+
+
+/**
+ * @brief Provides the member constant `value` to true if a given type is
+ * equality comparable, false otherwise.
+ * @tparam Type The type to test.
+ */
+template<typename Type, typename = void>
+struct is_equality_comparable: std::bool_constant<internal::is_equality_comparable<Type>(choice<2>)> {};
+
+
+/**
+ * @brief Helper variable template.
+ * @tparam Type The type to test.
+ */
+template<class Type>
+inline constexpr bool is_equality_comparable_v = is_equality_comparable<Type>::value;
+
+
+/**
+ * @brief Transcribes the constness of a type to another type.
+ * @tparam To The type to which to transcribe the constness.
+ * @tparam From The type from which to transcribe the constness.
+ */
+template<typename To, typename From>
+struct constness_as {
+    /*! @brief The type resulting from the transcription of the constness. */
+    using type = std::remove_const_t<To>;
+};
+
+
+/*! @copydoc constness_as */
+template<typename To, typename From>
+struct constness_as<To, const From> {
+    /*! @brief The type resulting from the transcription of the constness. */
+    using type = std::add_const_t<To>;
+};
+
+
+/**
+ * @brief Alias template to facilitate the transcription of the constness.
+ * @tparam To The type to which to transcribe the constness.
+ * @tparam From The type from which to transcribe the constness.
+ */
+template<typename To, typename From>
+using constness_as_t = typename constness_as<To, From>::type;
+
+
+/**
+ * @brief Extracts the class of a non-static member object or function.
+ * @tparam Member A pointer to a non-static member object or function.
+ */
+template<typename Member>
+class member_class {
+    static_assert(std::is_member_pointer_v<Member>, "Invalid pointer type to non-static member object or function");
+
+    template<typename Class, typename Ret, typename... Args>
+    static Class * clazz(Ret(Class:: *)(Args...));
+
+    template<typename Class, typename Ret, typename... Args>
+    static Class * clazz(Ret(Class:: *)(Args...) const);
+
+    template<typename Class, typename Type>
+    static Class * clazz(Type Class:: *);
+
+public:
+    /*! @brief The class of the given non-static member object or function. */
+    using type = std::remove_pointer_t<decltype(clazz(std::declval<Member>()))>;
+};
+
+
+/**
+ * @brief Helper type.
+ * @tparam Member A pointer to a non-static member object or function.
+ */
+template<typename Member>
+using member_class_t = typename member_class<Member>::type;
+
+
+}
+
+
+#endif
+
+
+
+namespace entt {
+
+
+/**
+ * @brief A SBO friendly, type-safe container for single values of any type.
+ * @tparam Len Size of the storage reserved for the small buffer optimization.
+ * @tparam Align Optional alignment requirement.
+ */
+template<std::size_t Len, std::size_t Align>
+class basic_any {
+    enum class operation: std::uint8_t { COPY, MOVE, DTOR, COMP, ADDR, CADDR, TYPE };
+    enum class policy: std::uint8_t { OWNER, REF, CREF };
+
+    using storage_type = std::aligned_storage_t<Len + !Len, Align>;
+    using vtable_type = const void *(const operation, const basic_any &, void *);
+
+    template<typename Type>
+    static constexpr bool in_situ = Len && alignof(Type) <= alignof(storage_type) && sizeof(Type) <= sizeof(storage_type) && std::is_nothrow_move_constructible_v<Type>;
+
+    template<typename Type>
+    [[nodiscard]] static constexpr policy type_to_policy() {
+        if constexpr(std::is_lvalue_reference_v<Type>) {
+            if constexpr(std::is_const_v<std::remove_reference_t<Type>>) {
+                return policy::CREF;
+            } else {
+                return policy::REF;
+            }
+        } else {
+            return policy::OWNER;
+        }
+    }
+
+    template<typename Type>
+    [[nodiscard]] static bool compare(const void *lhs, const void *rhs) {
+        if constexpr(!std::is_function_v<Type> && is_equality_comparable_v<Type>) {
+            return *static_cast<const Type *>(lhs) == *static_cast<const Type *>(rhs);
+        } else {
+            return lhs == rhs;
+        }
+    }
+
+    template<typename Type>
+    static const void * basic_vtable([[maybe_unused]] const operation op, [[maybe_unused]] const basic_any &from, [[maybe_unused]] void *to) {
+        static_assert(std::is_same_v<std::remove_reference_t<std::remove_const_t<Type>>, Type>, "Invalid type");
+
+        if constexpr(!std::is_void_v<Type>) {
+            const Type *instance = (in_situ<Type> && from.mode == policy::OWNER)
+                ? ENTT_LAUNDER(reinterpret_cast<const Type *>(&from.storage))
+                : static_cast<const Type *>(from.instance);
+
+            switch(op) {
+            case operation::COPY:
+                if constexpr(std::is_copy_constructible_v<Type>) {
+                    static_cast<basic_any *>(to)->emplace<Type>(*instance);
+                }
+                break;
+            case operation::MOVE:
+                if constexpr(in_situ<Type>) {
+                    if(from.mode == policy::OWNER) {
+                        return new (&static_cast<basic_any *>(to)->storage) Type{std::move(*const_cast<Type *>(instance))};
+                    }
+                }
+
+                return (static_cast<basic_any *>(to)->instance = std::exchange(const_cast<basic_any &>(from).instance, nullptr));
+            case operation::DTOR:
+                if(from.mode == policy::OWNER) {
+                    if constexpr(in_situ<Type>) {
+                        instance->~Type();
+                    } else if constexpr(std::is_array_v<Type>) {
+                        delete[] instance;
+                    } else {
+                        delete instance;
+                    }
+                }
+                break;
+            case operation::COMP:
+                return compare<Type>(instance, (*static_cast<const basic_any **>(to))->data()) ? to : nullptr;
+            case operation::ADDR:
+                if(from.mode == policy::CREF) {
+                    return nullptr;
+                }
+                [[fallthrough]];
+            case operation::CADDR:
+                return instance;
+            case operation::TYPE:
+                *static_cast<type_info *>(to) = type_id<Type>();
+                break;
+            }
+        }
+
+        return nullptr;
+    }
+
+    template<typename Type, typename... Args>
+    void initialize([[maybe_unused]] Args &&... args) {
+        if constexpr(!std::is_void_v<Type>) {
+            if constexpr(std::is_lvalue_reference_v<Type>) {
+                static_assert(sizeof...(Args) == 1u && (std::is_lvalue_reference_v<Args> && ...), "Invalid arguments");
+                instance = (std::addressof(args), ...);
+            } else if constexpr(in_situ<Type>) {
+                if constexpr(sizeof...(Args) != 0u && std::is_aggregate_v<Type>) {
+                    new (&storage) Type{std::forward<Args>(args)...};
+                } else {
+                    new (&storage) Type(std::forward<Args>(args)...);
+                }
+            } else {
+                if constexpr(sizeof...(Args) != 0u && std::is_aggregate_v<Type>) {
+                    instance = new Type{std::forward<Args>(args)...};
+                } else {
+                    instance = new Type(std::forward<Args>(args)...);
+                }
+            }
+        }
+    }
+
+    basic_any(const basic_any &other, const policy pol) ENTT_NOEXCEPT
+        : instance{other.data()},
+          vtable{other.vtable},
+          mode{pol}
+    {}
+
+public:
+    /*! @brief Size of the internal storage. */
+    static constexpr auto length = Len;
+    /*! @brief Alignment requirement. */
+    static constexpr auto alignment = Align;
+
+    /*! @brief Default constructor. */
+    basic_any() ENTT_NOEXCEPT
+        : instance{},
+          vtable{&basic_vtable<void>},
+          mode{policy::OWNER}
+    {}
+
+    /**
+     * @brief Constructs a wrapper by directly initializing the new object.
+     * @tparam Type Type of object to use to initialize the wrapper.
+     * @tparam Args Types of arguments to use to construct the new instance.
+     * @param args Parameters to use to construct the instance.
+     */
+    template<typename Type, typename... Args>
+    explicit basic_any(std::in_place_type_t<Type>, Args &&... args)
+        : instance{},
+          vtable{&basic_vtable<std::remove_const_t<std::remove_reference_t<Type>>>},
+          mode{type_to_policy<Type>()}
+    {
+        initialize<Type>(std::forward<Args>(args)...);
+    }
+
+    /**
+     * @brief Constructs a wrapper that holds an unmanaged object.
+     * @tparam Type Type of object to use to initialize the wrapper.
+     * @param value An instance of an object to use to initialize the wrapper.
+     */
+    template<typename Type>
+    basic_any(std::reference_wrapper<Type> value) ENTT_NOEXCEPT
+        : basic_any{}
+    {
+        // invokes deprecated assignment operator (and avoids issues with vs2017)
+        *this = value;
+    }
+
+    /**
+     * @brief Constructs a wrapper from a given value.
+     * @tparam Type Type of object to use to initialize the wrapper.
+     * @param value An instance of an object to use to initialize the wrapper.
+     */
+    template<typename Type, typename = std::enable_if_t<!std::is_same_v<std::decay_t<Type>, basic_any>>>
+    basic_any(Type &&value)
+        : instance{},
+          vtable{&basic_vtable<std::decay_t<Type>>},
+          mode{policy::OWNER}
+    {
+        initialize<std::decay_t<Type>>(std::forward<Type>(value));
+    }
+
+    /**
+     * @brief Copy constructor.
+     * @param other The instance to copy from.
+     */
+    basic_any(const basic_any &other)
+        : instance{},
+          vtable{&basic_vtable<void>},
+          mode{policy::OWNER}
+    {
+        other.vtable(operation::COPY, other, this);
+    }
+
+    /**
+     * @brief Move constructor.
+     * @param other The instance to move from.
+     */
+    basic_any(basic_any &&other) ENTT_NOEXCEPT
+        : instance{},
+          vtable{other.vtable},
+          mode{other.mode}
+    {
+        vtable(operation::MOVE, other, this);
+    }
+
+    /*! @brief Frees the internal storage, whatever it means. */
+    ~basic_any() {
+        vtable(operation::DTOR, *this, nullptr);
+    }
+
+    /**
+     * @brief Copy assignment operator.
+     * @param other The instance to copy from.
+     * @return This any object.
+     */
+    basic_any & operator=(const basic_any &other) {
+        reset();
+        other.vtable(operation::COPY, other, this);
+        return *this;
+    }
+
+    /**
+     * @brief Move assignment operator.
+     * @param other The instance to move from.
+     * @return This any object.
+     */
+    basic_any & operator=(basic_any &&other) ENTT_NOEXCEPT {
+        std::exchange(vtable, other.vtable)(operation::DTOR, *this, nullptr);
+        other.vtable(operation::MOVE, other, this);
+        mode = other.mode;
+        return *this;
+    }
+
+    /**
+     * @brief Value assignment operator.
+     * @tparam Type Type of object to use to initialize the wrapper.
+     * @param value An instance of an object to use to initialize the wrapper.
+     * @return This any object.
+     */
+    template<typename Type>
+    [[deprecated("Use std::in_place_type<T &>, entt::make_any<T &>, emplace<Type &> or forward_as_any instead")]]
+    basic_any & operator=(std::reference_wrapper<Type> value) ENTT_NOEXCEPT {
+        emplace<Type &>(value.get());
+        return *this;
+    }
+
+    /**
+     * @brief Value assignment operator.
+     * @tparam Type Type of object to use to initialize the wrapper.
+     * @param value An instance of an object to use to initialize the wrapper.
+     * @return This any object.
+     */
+    template<typename Type>
+    std::enable_if_t<!std::is_same_v<std::decay_t<Type>, basic_any>, basic_any &>
+    operator=(Type &&value) {
+        emplace<std::decay_t<Type>>(std::forward<Type>(value));
+        return *this;
+    }
+
+    /**
+     * @brief Returns the type of the contained object.
+     * @return The type of the contained object, if any.
+     */
+    [[nodiscard]] type_info type() const ENTT_NOEXCEPT {
+        type_info info{};
+        vtable(operation::TYPE, *this, &info);
+        return info;
+    }
+
+    /**
+     * @brief Returns an opaque pointer to the contained instance.
+     * @return An opaque pointer the contained instance, if any.
+     */
+    [[nodiscard]] const void * data() const ENTT_NOEXCEPT {
+        return vtable(operation::CADDR, *this, nullptr);
+    }
+
+    /*! @copydoc data */
+    [[nodiscard]] void * data() ENTT_NOEXCEPT {
+        return const_cast<void *>(vtable(operation::ADDR, *this, nullptr));
+    }
+
+    /**
+     * @brief Replaces the contained object by creating a new instance directly.
+     * @tparam Type Type of object to use to initialize the wrapper.
+     * @tparam Args Types of arguments to use to construct the new instance.
+     * @param args Parameters to use to construct the instance.
+     */
+    template<typename Type, typename... Args>
+    void emplace(Args &&... args) {
+        std::exchange(vtable, &basic_vtable<std::remove_const_t<std::remove_reference_t<Type>>>)(operation::DTOR, *this, nullptr);
+        mode = type_to_policy<Type>();
+        initialize<Type>(std::forward<Args>(args)...);
+    }
+
+    /*! @brief Destroys contained object */
+    void reset() {
+        std::exchange(vtable, &basic_vtable<void>)(operation::DTOR, *this, nullptr);
+        mode = policy::OWNER;
+    }
+
+    /**
+     * @brief Returns false if a wrapper is empty, true otherwise.
+     * @return False if the wrapper is empty, true otherwise.
+     */
+    [[nodiscard]] explicit operator bool() const ENTT_NOEXCEPT {
+        return !(vtable(operation::CADDR, *this, nullptr) == nullptr);
+    }
+
+    /**
+     * @brief Checks if two wrappers differ in their content.
+     * @param other Wrapper with which to compare.
+     * @return False if the two objects differ in their content, true otherwise.
+     */
+    bool operator==(const basic_any &other) const ENTT_NOEXCEPT {
+        const basic_any *trampoline = &other;
+        return type() == other.type() && (vtable(operation::COMP, *this, &trampoline) || !other.data());
+    }
+
+    /**
+     * @brief Aliasing constructor.
+     * @return A wrapper that shares a reference to an unmanaged object.
+     */
+    [[nodiscard]] basic_any as_ref() ENTT_NOEXCEPT {
+        return basic_any{*this, (mode == policy::CREF ? policy::CREF : policy::REF)};
+    }
+
+    /*! @copydoc as_ref */
+    [[nodiscard]] basic_any as_ref() const ENTT_NOEXCEPT {
+        return basic_any{*this, policy::CREF};
+    }
+
+    /**
+     * @brief Returns true if a wrapper owns its object, false otherwise.
+     * @return True if the wrapper owns its object, false otherwise.
+     */
+    [[nodiscard]] bool owner() const ENTT_NOEXCEPT {
+        return (mode == policy::OWNER);
+    }
+
+private:
+    union { const void *instance; storage_type storage; };
+    vtable_type *vtable;
+    policy mode;
+};
+
+
+/**
+ * @brief Checks if two wrappers differ in their content.
+ * @tparam Len Size of the storage reserved for the small buffer optimization.
+ * @tparam Align Alignment requirement.
+ * @param lhs A wrapper, either empty or not.
+ * @param rhs A wrapper, either empty or not.
+ * @return True if the two wrappers differ in their content, false otherwise.
+ */
+template<std::size_t Len, std::size_t Align>
+[[nodiscard]] inline bool operator!=(const basic_any<Len, Align> &lhs, const basic_any<Len, Align> &rhs) ENTT_NOEXCEPT {
+    return !(lhs == rhs);
+}
+
+
+/**
+ * @brief Performs type-safe access to the contained object.
+ * @tparam Type Type to which conversion is required.
+ * @tparam Len Size of the storage reserved for the small buffer optimization.
+ * @tparam Align Alignment requirement.
+ * @param data Target any object.
+ * @return The element converted to the requested type.
+ */
+template<typename Type, std::size_t Len, std::size_t Align>
+Type any_cast(const basic_any<Len, Align> &data) ENTT_NOEXCEPT {
+    const auto * const instance = any_cast<std::remove_reference_t<Type>>(&data);
+    ENTT_ASSERT(instance, "Invalid instance");
+    return static_cast<Type>(*instance);
+}
+
+
+/*! @copydoc any_cast */
+template<typename Type, std::size_t Len, std::size_t Align>
+Type any_cast(basic_any<Len, Align> &data) ENTT_NOEXCEPT {
+    // forces const on non-reference types to make them work also with wrappers for const references
+    auto * const instance = any_cast<std::remove_reference_t<const Type>>(&data);
+    ENTT_ASSERT(instance, "Invalid instance");
+    return static_cast<Type>(*instance);
+}
+
+
+/*! @copydoc any_cast */
+template<typename Type, std::size_t Len, std::size_t Align>
+Type any_cast(basic_any<Len, Align> &&data) ENTT_NOEXCEPT {
+    // forces const on non-reference types to make them work also with wrappers for const references
+    auto * const instance = any_cast<std::remove_reference_t<const Type>>(&data);
+    ENTT_ASSERT(instance, "Invalid instance");
+    return static_cast<Type>(std::move(*instance));
+}
+
+
+/*! @copydoc any_cast */
+template<typename Type, std::size_t Len, std::size_t Align>
+const Type * any_cast(const basic_any<Len, Align> *data) ENTT_NOEXCEPT {
+    return (data->type() == type_id<Type>() ? static_cast<const Type *>(data->data()) : nullptr);
+}
+
+
+/*! @copydoc any_cast */
+template<typename Type, std::size_t Len, std::size_t Align>
+Type * any_cast(basic_any<Len, Align> *data) ENTT_NOEXCEPT {
+    // last attempt to make wrappers for const references return their values
+    return (data->type() == type_id<Type>() ? static_cast<Type *>(static_cast<constness_as_t<basic_any<Len, Align>, Type> *>(data)->data()) : nullptr);
+}
+
+
+/**
+ * @brief Constructs a wrapper from a given type, passing it all arguments.
+ * @tparam Type Type of object to use to initialize the wrapper.
+ * @tparam Len Size of the storage reserved for the small buffer optimization.
+ * @tparam Align Optional alignment requirement.
+ * @tparam Args Types of arguments to use to construct the new instance.
+ * @param args Parameters to use to construct the instance.
+ * @return A properly initialized wrapper for an object of the given type.
+ */
+template<typename Type, std::size_t Len = basic_any<>::length, std::size_t Align = basic_any<Len>::alignment, typename... Args>
+basic_any<Len, Align> make_any(Args &&... args) {
+    return basic_any<Len, Align>{std::in_place_type<Type>, std::forward<Args>(args)...};
+}
+
+
+/**
+ * @brief Forwards its argument and avoids copies for lvalue references.
+ * @tparam Len Size of the storage reserved for the small buffer optimization.
+ * @tparam Align Optional alignment requirement.
+ * @tparam Type Type of argument to use to construct the new instance.
+ * @param value Parameter to use to construct the instance.
+ * @return A properly initialized and not necessarily owning wrapper.
+ */
+template<std::size_t Len = basic_any<>::length, std::size_t Align = basic_any<Len>::alignment, typename Type>
+basic_any<Len, Align> forward_as_any(Type &&value) {
+    return basic_any<Len, Align>{std::in_place_type<std::conditional_t<std::is_rvalue_reference_v<Type>, std::decay_t<Type>, Type>>, std::forward<Type>(value)};
+}
+
+
+}
+
+
+#endif
+
+// #include "../core/type_info.hpp"
+#ifndef ENTT_CORE_TYPE_INFO_HPP
+#define ENTT_CORE_TYPE_INFO_HPP
+
+
+#include <string_view>
+#include <type_traits>
+// #include "../config/config.h"
+
+// #include "../core/attribute.h"
+
+// #include "hashed_string.hpp"
+
+// #include "fwd.hpp"
+
+
+
+namespace entt {
+
+
+/**
+ * @cond TURN_OFF_DOXYGEN
+ * Internal details not to be documented.
+ */
+
+
+namespace internal {
+
+
+struct ENTT_API type_seq final {
+    [[nodiscard]] static id_type next() ENTT_NOEXCEPT {
+        static ENTT_MAYBE_ATOMIC(id_type) value{};
+        return value++;
+    }
+};
+
+
+template<typename Type>
+[[nodiscard]] constexpr auto stripped_type_name() ENTT_NOEXCEPT {
+#if defined ENTT_PRETTY_FUNCTION
+    std::string_view pretty_function{ENTT_PRETTY_FUNCTION};
+    auto first = pretty_function.find_first_not_of(' ', pretty_function.find_first_of(ENTT_PRETTY_FUNCTION_PREFIX)+1);
+    auto value = pretty_function.substr(first, pretty_function.find_last_of(ENTT_PRETTY_FUNCTION_SUFFIX) - first);
+    return value;
+#else
+    return std::string_view{""};
+#endif
+}
+
+
+template<typename Type, auto = stripped_type_name<Type>().find_first_of('.')>
+[[nodiscard]] static constexpr std::string_view type_name(int) ENTT_NOEXCEPT {
+    constexpr auto value = stripped_type_name<Type>();
+    return value;
+}
+
+
+template<typename Type>
+[[nodiscard]] static std::string_view type_name(char) ENTT_NOEXCEPT {
+    static const auto value = stripped_type_name<Type>();
+    return value;
+}
+
+
+template<typename Type, auto = stripped_type_name<Type>().find_first_of('.')>
+[[nodiscard]] static constexpr id_type type_hash(int) ENTT_NOEXCEPT {
+    constexpr auto stripped = stripped_type_name<Type>();
+    constexpr auto value = hashed_string::value(stripped.data(), stripped.size());
+    return value;
+}
+
+
+template<typename Type>
+[[nodiscard]] static id_type type_hash(char) ENTT_NOEXCEPT {
+    static const auto value = [](const auto stripped) {
+        return hashed_string::value(stripped.data(), stripped.size());
+    }(stripped_type_name<Type>());
+    return value;
+}
+
+
+}
+
+
+/**
+ * Internal details not to be documented.
+ * @endcond
+ */
+
+
+/**
+ * @brief Type sequential identifier.
+ * @tparam Type Type for which to generate a sequential identifier.
+ */
+template<typename Type, typename = void>
+struct ENTT_API type_seq final {
+    /**
+     * @brief Returns the sequential identifier of a given type.
+     * @return The sequential identifier of a given type.
+     */
+    [[nodiscard]] static id_type value() ENTT_NOEXCEPT {
+        static const id_type value = internal::type_seq::next();
+        return value;
+    }
+
+    /*! @copydoc value */
+    [[nodiscard]] constexpr operator id_type() const ENTT_NOEXCEPT { return value(); }
+};
+
+
+/**
+ * @brief Type hash.
+ * @tparam Type Type for which to generate a hash value.
+ */
+template<typename Type, typename = void>
+struct type_hash final {
+    /**
+     * @brief Returns the numeric representation of a given type.
+     * @return The numeric representation of the given type.
+     */
+#if defined ENTT_PRETTY_FUNCTION
+    [[nodiscard]] static constexpr id_type value() ENTT_NOEXCEPT {
+        return internal::type_hash<Type>(0);
+#else
+    [[nodiscard]] static constexpr id_type value() ENTT_NOEXCEPT {
+        return type_seq<Type>::value();
+#endif
+    }
+
+    /*! @copydoc value */
+    [[nodiscard]] constexpr operator id_type() const ENTT_NOEXCEPT { return value(); }
+};
+
+
+/**
+ * @brief Type name.
+ * @tparam Type Type for which to generate a name.
+ */
+template<typename Type, typename = void>
+struct type_name final {
+    /**
+     * @brief Returns the name of a given type.
+     * @return The name of the given type.
+     */
+    [[nodiscard]] static constexpr std::string_view value() ENTT_NOEXCEPT {
+        return internal::type_name<Type>(0);
+    }
+
+    /*! @copydoc value */
+    [[nodiscard]] constexpr operator std::string_view() const ENTT_NOEXCEPT { return value(); }
+};
+
+
+/*! @brief Implementation specific information about a type. */
+class type_info final {
+    template<typename>
+    friend type_info type_id() ENTT_NOEXCEPT;
+
+    type_info(id_type seq_v, id_type hash_v, std::string_view name_v) ENTT_NOEXCEPT
+        : seq_value{seq_v},
+          hash_value{hash_v},
+          name_value{name_v}
+    {}
+
+public:
+    /*! @brief Default constructor. */
+    type_info() ENTT_NOEXCEPT
+        : type_info({}, {}, {})
+    {}
+
+    /*! @brief Default copy constructor. */
+    type_info(const type_info &) ENTT_NOEXCEPT = default;
+    /*! @brief Default move constructor. */
+    type_info(type_info &&) ENTT_NOEXCEPT = default;
+
+    /**
+     * @brief Default copy assignment operator.
+     * @return This type info object.
+     */
+    type_info & operator=(const type_info &) ENTT_NOEXCEPT = default;
+
+    /**
+     * @brief Default move assignment operator.
+     * @return This type info object.
+     */
+    type_info & operator=(type_info &&) ENTT_NOEXCEPT = default;
+
+    /**
+     * @brief Checks if a type info object is properly initialized.
+     * @return True if the object is properly initialized, false otherwise.
+     */
+    [[nodiscard]] explicit operator bool() const ENTT_NOEXCEPT {
+        return name_value.data() != nullptr;
+    }
+
+    /**
+     * @brief Type sequential identifier.
+     * @return Type sequential identifier.
+     */
+    [[nodiscard]] id_type seq() const ENTT_NOEXCEPT {
+        return seq_value;
+    }
+
+    /**
+     * @brief Type hash.
+     * @return Type hash.
+     */
+    [[nodiscard]] id_type hash() const ENTT_NOEXCEPT {
+        return hash_value;
+    }
+
+    /**
+     * @brief Type name.
+     * @return Type name.
+     */
+    [[nodiscard]] std::string_view name() const ENTT_NOEXCEPT {
+        return name_value;
+    }
+
+    /**
+     * @brief Compares the contents of two type info objects.
+     * @param other Object with which to compare.
+     * @return False if the two contents differ, true otherwise.
+     */
+    [[nodiscard]] bool operator==(const type_info &other) const ENTT_NOEXCEPT {
+        return hash_value == other.hash_value;
+    }
+
+private:
+    id_type seq_value;
+    id_type hash_value;
+    std::string_view name_value;
+};
+
+
+/**
+ * @brief Compares the contents of two type info objects.
+ * @param lhs A type info object.
+ * @param rhs A type info object.
+ * @return True if the two contents differ, false otherwise.
+ */
+[[nodiscard]] inline bool operator!=(const type_info &lhs, const type_info &rhs) ENTT_NOEXCEPT {
+    return !(lhs == rhs);
+}
+
+
+/**
+ * @brief Returns the type info object for a given type.
+ * @tparam Type Type for which to generate a type info object.
+ * @return The type info object for the given type.
+ */
+template<typename Type>
+[[nodiscard]] type_info type_id() ENTT_NOEXCEPT {
+    return type_info{
+        type_seq<std::remove_cv_t<std::remove_reference_t<Type>>>::value(),
+        type_hash<std::remove_cv_t<std::remove_reference_t<Type>>>::value(),
+        type_name<std::remove_cv_t<std::remove_reference_t<Type>>>::value()
+    };
+}
+
+
+}
+
+
+#endif
+
+// #include "../core/type_traits.hpp"
+#ifndef ENTT_CORE_TYPE_TRAITS_HPP
+#define ENTT_CORE_TYPE_TRAITS_HPP
+
+
+#include <cstddef>
+#include <iterator>
+#include <type_traits>
+#include <utility>
+// #include "../config/config.h"
+
+// #include "fwd.hpp"
+
+
+
+namespace entt {
+
+
+/**
+ * @brief Utility class to disambiguate overloaded functions.
+ * @tparam N Number of choices available.
+ */
+template<std::size_t N>
+struct choice_t
+    // Unfortunately, doxygen cannot parse such a construct.
+    /*! @cond TURN_OFF_DOXYGEN */
+    : choice_t<N-1>
+    /*! @endcond */
+{};
+
+
+/*! @copybrief choice_t */
+template<>
+struct choice_t<0> {};
+
+
+/**
+ * @brief Variable template for the choice trick.
+ * @tparam N Number of choices available.
+ */
+template<std::size_t N>
+inline constexpr choice_t<N> choice{};
+
+
+/**
+ * @brief Identity type trait.
+ *
+ * Useful to establish non-deduced contexts in template argument deduction
+ * (waiting for C++20) or to provide types through function arguments.
+ *
+ * @tparam Type A type.
+ */
+template<typename Type>
+struct type_identity {
+    /*! @brief Identity type. */
+    using type = Type;
+};
+
+
+/**
+ * @brief Helper type.
+ * @tparam Type A type.
+ */
+template<typename Type>
+using type_identity_t = typename type_identity<Type>::type;
+
+
+/**
+ * @brief A type-only `sizeof` wrapper that returns 0 where `sizeof` complains.
+ * @tparam Type The type of which to return the size.
+ * @tparam The size of the type if `sizeof` accepts it, 0 otherwise.
+ */
+template<typename Type, typename = void>
+struct size_of: std::integral_constant<std::size_t, 0u> {};
+
+
+/*! @copydoc size_of */
+template<typename Type>
+struct size_of<Type, std::void_t<decltype(sizeof(Type))>>
+    : std::integral_constant<std::size_t, sizeof(Type)>
+{};
+
+
+/**
+ * @brief Helper variable template.
+ * @tparam Type The type of which to return the size.
+ */
+template<class Type>
+inline constexpr std::size_t size_of_v = size_of<Type>::value;
+
+
+/**
+ * @brief Using declaration to be used to _repeat_ the same type a number of
+ * times equal to the size of a given parameter pack.
+ * @tparam Type A type to repeat.
+ */
+template<typename Type, typename>
+using unpack_as_t = Type;
+
+
+/**
+ * @brief Helper variable template to be used to _repeat_ the same value a
+ * number of times equal to the size of a given parameter pack.
+ * @tparam Value A value to repeat.
+ */
+template<auto Value, typename>
+inline constexpr auto unpack_as_v = Value;
+
+
+/**
+ * @brief Wraps a static constant.
+ * @tparam Value A static constant.
+ */
+template<auto Value>
+using integral_constant = std::integral_constant<decltype(Value), Value>;
+
+
+/**
+ * @brief Alias template to facilitate the creation of named values.
+ * @tparam Value A constant value at least convertible to `id_type`.
+ */
+template<id_type Value>
+using tag = integral_constant<Value>;
+
+
+/**
+ * @brief A class to use to push around lists of types, nothing more.
+ * @tparam Type Types provided by the type list.
+ */
+template<typename... Type>
+struct type_list {
+    /*! @brief Type list type. */
+    using type = type_list;
+    /*! @brief Compile-time number of elements in the type list. */
+    static constexpr auto size = sizeof...(Type);
+};
+
+
+/*! @brief Primary template isn't defined on purpose. */
+template<std::size_t, typename>
+struct type_list_element;
+
+
+/**
+ * @brief Provides compile-time indexed access to the types of a type list.
+ * @tparam Index Index of the type to return.
+ * @tparam Type First type provided by the type list.
+ * @tparam Other Other types provided by the type list.
+ */
+template<std::size_t Index, typename Type, typename... Other>
+struct type_list_element<Index, type_list<Type, Other...>>
+    : type_list_element<Index - 1u, type_list<Other...>>
+{};
+
+
+/**
+ * @brief Provides compile-time indexed access to the types of a type list.
+ * @tparam Type First type provided by the type list.
+ * @tparam Other Other types provided by the type list.
+ */
+template<typename Type, typename... Other>
+struct type_list_element<0u, type_list<Type, Other...>> {
+    /*! @brief Searched type. */
+    using type = Type;
+};
+
+
+/**
+ * @brief Helper type.
+ * @tparam Index Index of the type to return.
+ * @tparam List Type list to search into.
+ */
+template<std::size_t Index, typename List>
+using type_list_element_t = typename type_list_element<Index, List>::type;
+
+
+/**
+ * @brief Concatenates multiple type lists.
+ * @tparam Type Types provided by the first type list.
+ * @tparam Other Types provided by the second type list.
+ * @return A type list composed by the types of both the type lists.
+ */
+template<typename... Type, typename... Other>
+constexpr type_list<Type..., Other...> operator+(type_list<Type...>, type_list<Other...>) { return {}; }
 
 
 /*! @brief Primary template isn't defined on purpose. */
@@ -2044,225 +16593,303 @@
 using type_list_unique_t = typename type_list_unique<Type>::type;
 
 
-/*! @brief Traits class used mainly to push things across boundaries. */
-template<typename>
-struct named_type_traits;
-
-
-/**
- * @brief Specialization used to get rid of constness.
- * @tparam Type Named type.
- */
-template<typename Type>
-struct named_type_traits<const Type>
-        : named_type_traits<Type>
-{};
-
-
-/**
- * @brief Helper type.
- * @tparam Type Potentially named type.
- */
-template<typename Type>
-using named_type_traits_t = typename named_type_traits<Type>::type;
-
-
-/**
- * @brief Provides the member constant `value` to true if a given type has a
- * name. In all other cases, `value` is false.
- */
-template<typename, typename = std::void_t<>>
-struct is_named_type: std::false_type {};
-
-
-/**
- * @brief Provides the member constant `value` to true if a given type has a
- * name. In all other cases, `value` is false.
- * @tparam Type Potentially named type.
- */
-template<typename Type>
-struct is_named_type<Type, std::void_t<named_type_traits_t<std::decay_t<Type>>>>: std::true_type {};
-
-
-/**
- * @brief Helper variable template.
- * @tparam Type Potentially named type.
- */
-template<class Type>
-constexpr auto is_named_type_v = is_named_type<Type>::value;
-
-
-/**
- * @brief Defines an enum class to use for opaque identifiers and a dedicate
- * `to_integer` function to convert the identifiers to their underlying type.
- * @param clazz The name to use for the enum class.
- * @param type The underlying type for the enum class.
- */
-#define ENTT_OPAQUE_TYPE(clazz, type)\
-    enum class clazz: type {};\
-    constexpr auto to_integer(const clazz id) ENTT_NOEXCEPT {\
-        return std::underlying_type_t<clazz>(id);\
-    }
-
-
-}
-
-
-/**
- * @brief Utility macro to deal with an issue of MSVC.
- *
- * See _msvc-doesnt-expand-va-args-correctly_ on SO for all the details.
- *
- * @param args Argument to expand.
- */
-#define ENTT_EXPAND(args) args
-
-
-/**
- * @brief Makes an already existing type a named type.
- *
- * The current definition contains a workaround for Clang 6 because it fails to
- * deduce correctly the type to use to specialize the class template.<br/>
- * With a compiler that fully supports C++17 and works fine with deduction
- * guides, the following should be fine instead:
- *
- * @code{.cpp}
- * std::integral_constant<ENTT_ID_TYPE, entt::basic_hashed_string{#type}>
- * @endcode
- *
- * In order to support even sligthly older compilers, I prefer to stick to the
- * implementation below.
- *
- * @param type Type to assign a name to.
- */
-#define ENTT_NAMED_TYPE(type)\
-    template<>\
-    struct entt::named_type_traits<type>\
-        : std::integral_constant<ENTT_ID_TYPE, entt::basic_hashed_string<std::remove_cv_t<std::remove_pointer_t<std::decay_t<decltype(#type)>>>>{#type}>\
-    {\
-        static_assert(std::is_same_v<std::decay_t<type>, type>);\
-    };
-
-
-/**
- * @brief Defines a named type (to use for structs).
- * @param clazz Name of the type to define.
- * @param body Body of the type to define.
- */
-#define ENTT_NAMED_STRUCT_ONLY(clazz, body)\
-    struct clazz body;\
-    ENTT_NAMED_TYPE(clazz)
-
-
-/**
- * @brief Defines a named type (to use for structs).
- * @param ns Namespace where to define the named type.
- * @param clazz Name of the type to define.
- * @param body Body of the type to define.
- */
-#define ENTT_NAMED_STRUCT_WITH_NAMESPACE(ns, clazz, body)\
-    namespace ns { struct clazz body; }\
-    ENTT_NAMED_TYPE(ns::clazz)
-
-
-/*! @brief Utility function to simulate macro overloading. */
-#define ENTT_NAMED_STRUCT_OVERLOAD(_1, _2, _3, FUNC, ...) FUNC
-/*! @brief Defines a named type (to use for structs). */
-#define ENTT_NAMED_STRUCT(...) ENTT_EXPAND(ENTT_NAMED_STRUCT_OVERLOAD(__VA_ARGS__, ENTT_NAMED_STRUCT_WITH_NAMESPACE, ENTT_NAMED_STRUCT_ONLY,)(__VA_ARGS__))
-
-
-/**
- * @brief Defines a named type (to use for classes).
- * @param clazz Name of the type to define.
- * @param body Body of the type to define.
- */
-#define ENTT_NAMED_CLASS_ONLY(clazz, body)\
-    class clazz body;\
-    ENTT_NAMED_TYPE(clazz)
-
-
-/**
- * @brief Defines a named type (to use for classes).
- * @param ns Namespace where to define the named type.
- * @param clazz Name of the type to define.
- * @param body Body of the type to define.
- */
-#define ENTT_NAMED_CLASS_WITH_NAMESPACE(ns, clazz, body)\
-    namespace ns { class clazz body; }\
-    ENTT_NAMED_TYPE(ns::clazz)
-
-
-/*! @brief Utility function to simulate macro overloading. */
-#define ENTT_NAMED_CLASS_MACRO(_1, _2, _3, FUNC, ...) FUNC
-/*! @brief Defines a named type (to use for classes). */
-#define ENTT_NAMED_CLASS(...) ENTT_EXPAND(ENTT_NAMED_CLASS_MACRO(__VA_ARGS__, ENTT_NAMED_CLASS_WITH_NAMESPACE, ENTT_NAMED_CLASS_ONLY,)(__VA_ARGS__))
-
-
-#endif // ENTT_CORE_TYPE_TRAITS_HPP
-
-// #include "../signal/delegate.hpp"
-#ifndef ENTT_SIGNAL_DELEGATE_HPP
-#define ENTT_SIGNAL_DELEGATE_HPP
-
-
-#include <tuple>
-#include <cstring>
-#include <utility>
-#include <algorithm>
-#include <functional>
-#include <type_traits>
-// #include "../config/config.h"
-#ifndef ENTT_CONFIG_CONFIG_H
-#define ENTT_CONFIG_CONFIG_H
-
-
-#ifndef ENTT_NOEXCEPT
-#define ENTT_NOEXCEPT noexcept
-#endif // ENTT_NOEXCEPT
-
-
-#ifndef ENTT_HS_SUFFIX
-#define ENTT_HS_SUFFIX _hs
-#endif // ENTT_HS_SUFFIX
-
-
-#ifndef ENTT_HWS_SUFFIX
-#define ENTT_HWS_SUFFIX _hws
-#endif // ENTT_HWS_SUFFIX
-
-
-#ifndef ENTT_NO_ATOMIC
-#include <atomic>
-#define ENTT_MAYBE_ATOMIC(Type) std::atomic<Type>
-#else // ENTT_NO_ATOMIC
-#define ENTT_MAYBE_ATOMIC(Type) Type
-#endif // ENTT_NO_ATOMIC
-
-
-#ifndef ENTT_ID_TYPE
-#include <cstdint>
-#define ENTT_ID_TYPE std::uint32_t
-#endif // ENTT_ID_TYPE
-
-
-#ifndef ENTT_PAGE_SIZE
-#define ENTT_PAGE_SIZE 32768
-#endif // ENTT_PAGE_SIZE
-
-
-#ifndef ENTT_DISABLE_ASSERT
-#include <cassert>
-#define ENTT_ASSERT(condition) assert(condition)
-#else // ENTT_DISABLE_ASSERT
-#define ENTT_ASSERT(...) ((void)0)
-#endif // ENTT_DISABLE_ASSERT
-
-
-#endif // ENTT_CONFIG_CONFIG_H
-
-
-
-namespace entt {
+/**
+ * @brief Provides the member constant `value` to true if a type list contains a
+ * given type, false otherwise.
+ * @tparam List Type list.
+ * @tparam Type Type to look for.
+ */
+template<typename List, typename Type>
+struct type_list_contains;
+
+
+/**
+ * @copybrief type_list_contains
+ * @tparam Type Types provided by the type list.
+ * @tparam Other Type to look for.
+ */
+template<typename... Type, typename Other>
+struct type_list_contains<type_list<Type...>, Other>: std::disjunction<std::is_same<Type, Other>...> {};
+
+
+/**
+ * @brief Helper variable template.
+ * @tparam List Type list.
+ * @tparam Type Type to look for.
+ */
+template<class List, typename Type>
+inline constexpr bool type_list_contains_v = type_list_contains<List, Type>::value;
+
+
+/*! @brief Primary template isn't defined on purpose. */
+template<typename...>
+struct type_list_diff;
+
+
+/**
+ * @brief Computes the difference between two type lists.
+ * @tparam Type Types provided by the first type list.
+ * @tparam Other Types provided by the second type list.
+ */
+template<typename... Type, typename... Other>
+struct type_list_diff<type_list<Type...>, type_list<Other...>> {
+    /*! @brief A type list that is the difference between the two type lists. */
+    using type = type_list_cat_t<std::conditional_t<type_list_contains_v<type_list<Other...>, Type>, type_list<>, type_list<Type>>...>;
+};
+
+
+/**
+ * @brief Helper type.
+ * @tparam List Type lists between which to compute the difference.
+ */
+template<typename... List>
+using type_list_diff_t = typename type_list_diff<List...>::type;
+
+
+/**
+ * @brief A class to use to push around lists of constant values, nothing more.
+ * @tparam Value Values provided by the value list.
+ */
+template<auto... Value>
+struct value_list {
+    /*! @brief Value list type. */
+    using type = value_list;
+    /*! @brief Compile-time number of elements in the value list. */
+    static constexpr auto size = sizeof...(Value);
+};
+
+
+/*! @brief Primary template isn't defined on purpose. */
+template<std::size_t, typename>
+struct value_list_element;
+
+
+/**
+ * @brief Provides compile-time indexed access to the values of a value list.
+ * @tparam Index Index of the value to return.
+ * @tparam Value First value provided by the value list.
+ * @tparam Other Other values provided by the value list.
+ */
+template<std::size_t Index, auto Value, auto... Other>
+struct value_list_element<Index, value_list<Value, Other...>>
+    : value_list_element<Index - 1u, value_list<Other...>>
+{};
+
+
+/**
+ * @brief Provides compile-time indexed access to the types of a type list.
+ * @tparam Value First value provided by the value list.
+ * @tparam Other Other values provided by the value list.
+ */
+template<auto Value, auto... Other>
+struct value_list_element<0u, value_list<Value, Other...>> {
+    /*! @brief Searched value. */
+    static constexpr auto value = Value;
+};
+
+
+/**
+ * @brief Helper type.
+ * @tparam Index Index of the value to return.
+ * @tparam List Value list to search into.
+ */
+template<std::size_t Index, typename List>
+inline constexpr auto value_list_element_v = value_list_element<Index, List>::value;
+
+
+/**
+ * @brief Concatenates multiple value lists.
+ * @tparam Value Values provided by the first value list.
+ * @tparam Other Values provided by the second value list.
+ * @return A value list composed by the values of both the value lists.
+ */
+template<auto... Value, auto... Other>
+constexpr value_list<Value..., Other...> operator+(value_list<Value...>, value_list<Other...>) { return {}; }
+
+
+/*! @brief Primary template isn't defined on purpose. */
+template<typename...>
+struct value_list_cat;
+
+
+/*! @brief Concatenates multiple value lists. */
+template<>
+struct value_list_cat<> {
+    /*! @brief A value list composed by the values of all the value lists. */
+    using type = value_list<>;
+};
+
+
+/**
+ * @brief Concatenates multiple value lists.
+ * @tparam Value Values provided by the first value list.
+ * @tparam Other Values provided by the second value list.
+ * @tparam List Other value lists, if any.
+ */
+template<auto... Value, auto... Other, typename... List>
+struct value_list_cat<value_list<Value...>, value_list<Other...>, List...> {
+    /*! @brief A value list composed by the values of all the value lists. */
+    using type = typename value_list_cat<value_list<Value..., Other...>, List...>::type;
+};
+
+
+/**
+ * @brief Concatenates multiple value lists.
+ * @tparam Value Values provided by the value list.
+ */
+template<auto... Value>
+struct value_list_cat<value_list<Value...>> {
+    /*! @brief A value list composed by the values of all the value lists. */
+    using type = value_list<Value...>;
+};
+
+
+/**
+ * @brief Helper type.
+ * @tparam List Value lists to concatenate.
+ */
+template<typename... List>
+using value_list_cat_t = typename value_list_cat<List...>::type;
+
+
+/*! @brief Same as std::is_invocable, but with tuples. */
+template<typename, typename>
+struct is_applicable: std::false_type {};
+
+
+/**
+ * @copybrief is_applicable
+ * @tparam Func A valid function type.
+ * @tparam Tuple Tuple-like type.
+ * @tparam Args The list of arguments to use to probe the function type.
+ */
+template<typename Func, template<typename...> class Tuple, typename... Args>
+struct is_applicable<Func, Tuple<Args...>>: std::is_invocable<Func, Args...> {};
+
+
+/**
+ * @copybrief is_applicable
+ * @tparam Func A valid function type.
+ * @tparam Tuple Tuple-like type.
+ * @tparam Args The list of arguments to use to probe the function type.
+ */
+template<typename Func, template<typename...> class Tuple, typename... Args>
+struct is_applicable<Func, const Tuple<Args...>>: std::is_invocable<Func, Args...> {};
+
+
+/**
+ * @brief Helper variable template.
+ * @tparam Func A valid function type.
+ * @tparam Args The list of arguments to use to probe the function type.
+ */
+template<typename Func, typename Args>
+inline constexpr bool is_applicable_v = is_applicable<Func, Args>::value;
+
+
+/*! @brief Same as std::is_invocable_r, but with tuples for arguments. */
+template<typename, typename, typename>
+struct is_applicable_r: std::false_type {};
+
+
+/**
+ * @copybrief is_applicable_r
+ * @tparam Ret The type to which the return type of the function should be
+ * convertible.
+ * @tparam Func A valid function type.
+ * @tparam Args The list of arguments to use to probe the function type.
+ */
+template<typename Ret, typename Func, typename... Args>
+struct is_applicable_r<Ret, Func, std::tuple<Args...>>: std::is_invocable_r<Ret, Func, Args...> {};
+
+
+/**
+ * @brief Helper variable template.
+ * @tparam Ret The type to which the return type of the function should be
+ * convertible.
+ * @tparam Func A valid function type.
+ * @tparam Args The list of arguments to use to probe the function type.
+ */
+template<typename Ret, typename Func, typename Args>
+inline constexpr bool is_applicable_r_v = is_applicable_r<Ret, Func, Args>::value;
+
+
+/**
+ * @brief Provides the member constant `value` to true if a given type is
+ * complete, false otherwise.
+ * @tparam Type The type to test.
+ */
+template<typename Type, typename = void>
+struct is_complete: std::false_type {};
+
+
+/*! @copydoc is_complete */
+template<typename Type>
+struct is_complete<Type, std::void_t<decltype(sizeof(Type))>>: std::true_type {};
+
+
+/**
+ * @brief Helper variable template.
+ * @tparam Type The type to test.
+ */
+template<typename Type>
+inline constexpr bool is_complete_v = is_complete<Type>::value;
+
+
+/**
+ * @brief Provides the member constant `value` to true if a given type is an
+ * iterator, false otherwise.
+ * @tparam Type The type to test.
+ */
+template<typename Type, typename = void>
+struct is_iterator: std::false_type {};
+
+
+/*! @copydoc is_iterator */
+template<typename Type>
+struct is_iterator<Type, std::void_t<typename std::iterator_traits<Type>::iterator_category>>
+    : std::true_type
+{};
+
+
+/**
+ * @brief Helper variable template.
+ * @tparam Type The type to test.
+ */
+template<typename Type>
+inline constexpr bool is_iterator_v = is_iterator<Type>::value;
+
+
+/**
+ * @brief Provides the member constant `value` to true if a given type is of the
+ * required iterator type, false otherwise.
+ * @tparam Type The type to test.
+ * @tparam It Required iterator type.
+ */
+template<typename Type, typename It, typename = void>
+struct is_iterator_type: std::false_type {};
+
+
+/*! @copydoc is_iterator_type */
+template<typename Type, typename It>
+struct is_iterator_type<Type, It, std::enable_if_t<is_iterator_v<Type> && std::is_same_v<Type, It>>>
+    : std::true_type
+{};
+
+
+/*! @copydoc is_iterator_type */
+template<typename Type, typename It>
+struct is_iterator_type<Type, It, std::enable_if_t<!std::is_same_v<Type, It>, std::void_t<typename It::iterator_type>>>
+    : is_iterator_type<Type, typename It::iterator_type>
+{};
+
+
+/**
+ * @brief Helper variable template.
+ * @tparam Type The type to test.
+ * @tparam It Required iterator type.
+ */
+template<typename Type, typename It>
+inline constexpr bool is_iterator_type_v = is_iterator_type<Type, It>::value;
 
 
 /**
@@ -2274,766 +16901,534 @@
 namespace internal {
 
 
-template<typename Ret, typename... Args>
-auto to_function_pointer(Ret(*)(Args...)) -> Ret(*)(Args...);
-
-
-template<typename Ret, typename... Args, typename Type, typename Payload, typename = std::enable_if_t<std::is_convertible_v<Payload &, Type &>>>
-auto to_function_pointer(Ret(*)(Type &, Args...), Payload &) -> Ret(*)(Args...);
-
-
-template<typename Class, typename Ret, typename... Args>
-auto to_function_pointer(Ret(Class:: *)(Args...), const Class &) -> Ret(*)(Args...);
-
-
-template<typename Class, typename Ret, typename... Args>
-auto to_function_pointer(Ret(Class:: *)(Args...) const, const Class &) -> Ret(*)(Args...);
-
-
-template<typename Class, typename Type>
-auto to_function_pointer(Type Class:: *, const Class &) -> Type(*)();
-
-
-template<typename>
-struct function_extent;
-
-
-template<typename Ret, typename... Args>
-struct function_extent<Ret(*)(Args...)> {
-    static constexpr auto value = sizeof...(Args);
-};
-
-
-template<typename Func>
-constexpr auto function_extent_v = function_extent<Func>::value;
-
-
-}
-
-
-/**
- * Internal details not to be documented.
- * @endcond TURN_OFF_DOXYGEN
- */
-
-
-/*! @brief Used to wrap a function or a member of a specified type. */
-template<auto>
-struct connect_arg_t {};
-
-
-/*! @brief Constant of type connect_arg_t used to disambiguate calls. */
-template<auto Func>
-constexpr connect_arg_t<Func> connect_arg{};
-
-
-/**
- * @brief Basic delegate implementation.
- *
- * Primary template isn't defined on purpose. All the specializations give a
- * compile-time error unless the template parameter is a function type.
- */
-template<typename>
-class delegate;
-
-
-/**
- * @brief Utility class to use to send around functions and members.
- *
- * Unmanaged delegate for function pointers and members. Users of this class are
- * in charge of disconnecting instances before deleting them.
- *
- * A delegate can be used as general purpose invoker with no memory overhead for
- * free functions (with or without payload) and members provided along with an
- * instance on which to invoke them.
- *
- * @tparam Ret Return type of a function type.
- * @tparam Args Types of arguments of a function type.
- */
-template<typename Ret, typename... Args>
-class delegate<Ret(Args...)> {
-    using proto_fn_type = Ret(const void *, std::tuple<Args &&...>);
-
-    template<auto Function, std::size_t... Index>
-    void connect(std::index_sequence<Index...>) ENTT_NOEXCEPT {
-        static_assert(std::is_invocable_r_v<Ret, decltype(Function), std::tuple_element_t<Index, std::tuple<Args...>>...>);
-        data = nullptr;
-
-        fn = [](const void *, std::tuple<Args &&...> args) -> Ret {
-            // Ret(...) makes void(...) eat the return values to avoid errors
-            return Ret(std::invoke(Function, std::forward<std::tuple_element_t<Index, std::tuple<Args...>>>(std::get<Index>(args))...));
-        };
-    }
-
-    template<auto Candidate, typename Type, std::size_t... Index>
-    void connect(Type &value_or_instance, std::index_sequence<Index...>) ENTT_NOEXCEPT {
-        static_assert(std::is_invocable_r_v<Ret, decltype(Candidate), Type &, std::tuple_element_t<Index, std::tuple<Args...>>...>);
-        data = &value_or_instance;
-
-        fn = [](const void *payload, std::tuple<Args &&...> args) -> Ret {
-            Type *curr = nullptr;
-
-            if constexpr(std::is_const_v<Type>) {
-                curr = static_cast<Type *>(payload);
-            } else {
-                curr = static_cast<Type *>(const_cast<void *>(payload));
-            }
-
-            // Ret(...) makes void(...) eat the return values to avoid errors
-            return Ret(std::invoke(Candidate, *curr, std::forward<std::tuple_element_t<Index, std::tuple<Args...>>>(std::get<Index>(args))...));
-        };
-    }
-
-public:
-    /*! @brief Function type of the delegate. */
-    using function_type = Ret(Args...);
-
-    /*! @brief Default constructor. */
-    delegate() ENTT_NOEXCEPT
-        : fn{nullptr}, data{nullptr}
-    {}
-
-    /**
-     * @brief Constructs a delegate and connects a free function to it.
-     * @tparam Function A valid free function pointer.
-     */
-    template<auto Function>
-    delegate(connect_arg_t<Function>) ENTT_NOEXCEPT
-        : delegate{}
-    {
-        connect<Function>();
-    }
-
-    /**
-     * @brief Constructs a delegate and connects a member for a given instance
-     * or a free function with payload.
-     * @tparam Candidate Member or free function to connect to the delegate.
-     * @tparam Type Type of class or type of payload.
-     * @param value_or_instance A valid reference that fits the purpose.
-     */
-    template<auto Candidate, typename Type>
-    delegate(connect_arg_t<Candidate>, Type &value_or_instance) ENTT_NOEXCEPT
-        : delegate{}
+template<typename>
+[[nodiscard]] constexpr bool is_equality_comparable(...) { return false; }
+
+
+template<typename Type>
+[[nodiscard]] constexpr auto is_equality_comparable(choice_t<0>)
+-> decltype(std::declval<Type>() == std::declval<Type>()) { return true; }
+
+
+template<typename Type>
+[[nodiscard]] constexpr auto is_equality_comparable(choice_t<1>)
+-> decltype(std::declval<typename Type::value_type>(), std::declval<Type>() == std::declval<Type>()) {
+    if constexpr(is_iterator_v<Type>) {
+        return true;
+    } else if constexpr(std::is_same_v<typename Type::value_type, Type>) {
+        return is_equality_comparable<Type>(choice<0>);
+    } else {
+        return is_equality_comparable<typename Type::value_type>(choice<2>);
+    }
+}
+
+
+template<typename Type>
+[[nodiscard]] constexpr auto is_equality_comparable(choice_t<2>)
+-> decltype(std::declval<typename Type::mapped_type>(), std::declval<Type>() == std::declval<Type>()) {
+    return is_equality_comparable<typename Type::key_type>(choice<2>) && is_equality_comparable<typename Type::mapped_type>(choice<2>);
+}
+
+
+}
+
+
+/**
+ * Internal details not to be documented.
+ * @endcond
+ */
+
+
+/**
+ * @brief Provides the member constant `value` to true if a given type is
+ * equality comparable, false otherwise.
+ * @tparam Type The type to test.
+ */
+template<typename Type, typename = void>
+struct is_equality_comparable: std::bool_constant<internal::is_equality_comparable<Type>(choice<2>)> {};
+
+
+/**
+ * @brief Helper variable template.
+ * @tparam Type The type to test.
+ */
+template<class Type>
+inline constexpr bool is_equality_comparable_v = is_equality_comparable<Type>::value;
+
+
+/**
+ * @brief Transcribes the constness of a type to another type.
+ * @tparam To The type to which to transcribe the constness.
+ * @tparam From The type from which to transcribe the constness.
+ */
+template<typename To, typename From>
+struct constness_as {
+    /*! @brief The type resulting from the transcription of the constness. */
+    using type = std::remove_const_t<To>;
+};
+
+
+/*! @copydoc constness_as */
+template<typename To, typename From>
+struct constness_as<To, const From> {
+    /*! @brief The type resulting from the transcription of the constness. */
+    using type = std::add_const_t<To>;
+};
+
+
+/**
+ * @brief Alias template to facilitate the transcription of the constness.
+ * @tparam To The type to which to transcribe the constness.
+ * @tparam From The type from which to transcribe the constness.
+ */
+template<typename To, typename From>
+using constness_as_t = typename constness_as<To, From>::type;
+
+
+/**
+ * @brief Extracts the class of a non-static member object or function.
+ * @tparam Member A pointer to a non-static member object or function.
+ */
+template<typename Member>
+class member_class {
+    static_assert(std::is_member_pointer_v<Member>, "Invalid pointer type to non-static member object or function");
+
+    template<typename Class, typename Ret, typename... Args>
+    static Class * clazz(Ret(Class:: *)(Args...));
+
+    template<typename Class, typename Ret, typename... Args>
+    static Class * clazz(Ret(Class:: *)(Args...) const);
+
+    template<typename Class, typename Type>
+    static Class * clazz(Type Class:: *);
+
+public:
+    /*! @brief The class of the given non-static member object or function. */
+    using type = std::remove_pointer_t<decltype(clazz(std::declval<Member>()))>;
+};
+
+
+/**
+ * @brief Helper type.
+ * @tparam Member A pointer to a non-static member object or function.
+ */
+template<typename Member>
+using member_class_t = typename member_class<Member>::type;
+
+
+}
+
+
+#endif
+
+// #include "fwd.hpp"
+#ifndef ENTT_POLY_FWD_HPP
+#define ENTT_POLY_FWD_HPP
+
+
+#include <type_traits>
+
+
+namespace entt {
+
+
+template<typename, std::size_t Len, std::size_t = alignof(typename std::aligned_storage_t<Len + !Len>)>
+class basic_poly;
+
+
+/**
+ * @brief Alias declaration for the most common use case.
+ * @tparam Concept Concept descriptor.
+ */
+template<typename Concept>
+using poly = basic_poly<Concept, sizeof(double[2])>;
+
+
+}
+
+
+#endif
+
+
+
+namespace entt {
+
+
+/*! @brief Inspector class used to infer the type of the virtual table. */
+struct poly_inspector {
+    /**
+     * @brief Generic conversion operator (definition only).
+     * @tparam Type Type to which conversion is requested.
+     */
+    template <class Type>
+    operator Type &&() const;
+
+    /**
+     * @brief Dummy invocation function (definition only).
+     * @tparam Member Index of the function to invoke.
+     * @tparam Args Types of arguments to pass to the function.
+     * @param args The arguments to pass to the function.
+     * @return A poly inspector convertible to any type.
+     */
+    template<auto Member, typename... Args>
+    poly_inspector invoke(Args &&... args) const;
+
+    /*! @copydoc invoke */
+    template<auto Member, typename... Args>
+    poly_inspector invoke(Args &&... args);
+};
+
+
+/**
+ * @brief Static virtual table factory.
+ * @tparam Concept Concept descriptor.
+ * @tparam Len Size of the storage reserved for the small buffer optimization.
+ * @tparam Align Alignment requirement.
+ */
+template<typename Concept, std::size_t Len, std::size_t Align>
+class poly_vtable {
+    using inspector = typename Concept::template type<poly_inspector>;
+
+    template<typename Ret, typename... Args>
+    static auto vtable_entry(Ret(*)(inspector &, Args...)) -> Ret(*)(basic_any<Len, Align> &, Args...);
+
+    template<typename Ret, typename... Args>
+    static auto vtable_entry(Ret(*)(const inspector &, Args...)) -> Ret(*)(const basic_any<Len, Align> &, Args...);
+
+    template<typename Ret, typename... Args>
+    static auto vtable_entry(Ret(*)(Args...)) -> Ret(*)(const basic_any<Len, Align> &, Args...);
+
+    template<typename Ret, typename... Args>
+    static auto vtable_entry(Ret(inspector:: *)(Args...)) -> Ret(*)(basic_any<Len, Align> &, Args...);
+
+    template<typename Ret, typename... Args>
+    static auto vtable_entry(Ret(inspector:: *)(Args...) const) -> Ret(*)(const basic_any<Len, Align> &, Args...);
+
+    template<auto... Candidate>
+    static auto make_vtable(value_list<Candidate...>)
+    -> decltype(std::make_tuple(vtable_entry(Candidate)...));
+
+    template<typename... Func>
+    [[nodiscard]] static constexpr auto make_vtable(type_list<Func...>)  {
+        if constexpr(sizeof...(Func) == 0) {
+            return decltype(make_vtable(typename Concept::template impl<inspector>{})){};
+        } else if constexpr((std::is_function_v<Func> && ...)) {
+            return decltype(std::make_tuple(vtable_entry(std::declval<Func inspector:: *>())...)){};
+        }
+    }
+
+    template<typename Type, auto Candidate, typename Ret, typename Any, typename... Args>
+    static void fill_vtable_entry(Ret(* &entry)(Any &, Args...)) {
+        if constexpr(std::is_invocable_r_v<Ret, decltype(Candidate), Args...>) {
+            entry = +[](Any &, Args... args) -> Ret {
+                return std::invoke(Candidate, std::forward<Args>(args)...);
+            };
+        } else {
+            entry = +[](Any &instance, Args... args) -> Ret {
+                return static_cast<Ret>(std::invoke(Candidate, any_cast<constness_as_t<Type, Any> &>(instance), std::forward<Args>(args)...));
+            };
+        }
+    }
+
+    template<typename Type, auto... Index>
+    [[nodiscard]] static auto fill_vtable(std::index_sequence<Index...>) {
+        type impl{};
+        (fill_vtable_entry<Type, value_list_element_v<Index, typename Concept::template impl<Type>>>(std::get<Index>(impl)), ...);
+        return impl;
+    }
+
+public:
+    /*! @brief Virtual table type. */
+    using type = decltype(make_vtable(Concept{}));
+
+    /**
+     * @brief Returns a static virtual table for a specific concept and type.
+     * @tparam Type The type for which to generate the virtual table.
+     * @return A static virtual table for the given concept and type.
+     */
+    template<typename Type>
+    [[nodiscard]] static const auto * instance() {
+        static_assert(std::is_same_v<Type, std::decay_t<Type>>, "Type differs from its decayed form");
+        static const auto vtable = fill_vtable<Type>(std::make_index_sequence<Concept::template impl<Type>::size>{});
+        return &vtable;
+    }
+};
+
+
+/**
+ * @brief Poly base class used to inject functionalities into concepts.
+ * @tparam Poly The outermost poly class.
+ */
+template<typename Poly>
+struct poly_base {
+    /**
+     * @brief Invokes a function from the static virtual table.
+     * @tparam Member Index of the function to invoke.
+     * @tparam Args Types of arguments to pass to the function.
+     * @param self A reference to the poly object that made the call.
+     * @param args The arguments to pass to the function.
+     * @return The return value of the invoked function, if any.
+     */
+    template<auto Member, typename... Args>
+    [[nodiscard]] decltype(auto) invoke(const poly_base &self, Args &&... args) const {
+        const auto &poly = static_cast<const Poly &>(self);
+        return std::get<Member>(*poly.vtable)(poly.storage, std::forward<Args>(args)...);
+    }
+
+    /*! @copydoc invoke */
+    template<auto Member, typename... Args>
+    [[nodiscard]] decltype(auto) invoke(poly_base &self, Args &&... args) {
+        auto &poly = static_cast<Poly &>(self);
+        return std::get<Member>(*poly.vtable)(poly.storage, std::forward<Args>(args)...);
+    }
+};
+
+
+/**
+ * @brief Shortcut for calling `poly_base<Type>::invoke`.
+ * @tparam Member Index of the function to invoke.
+ * @tparam Poly A fully defined poly object.
+ * @tparam Args Types of arguments to pass to the function.
+ * @param self A reference to the poly object that made the call.
+ * @param args The arguments to pass to the function.
+ * @return The return value of the invoked function, if any.
+ */
+template<auto Member, typename Poly, typename... Args>
+decltype(auto) poly_call(Poly &&self, Args &&... args) {
+    return std::forward<Poly>(self).template invoke<Member>(self, std::forward<Args>(args)...);
+}
+
+
+/**
+ * @brief Static polymorphism made simple and within everyone's reach.
+ *
+ * Static polymorphism is a very powerful tool in C++, albeit sometimes
+ * cumbersome to obtain.<br/>
+ * This class aims to make it simple and easy to use.
+ *
+ * @note
+ * Both deduced and defined static virtual tables are supported.<br/>
+ * Moreover, the `poly` class template also works with unmanaged objects.
+ *
+ * @tparam Concept Concept descriptor.
+ * @tparam Len Size of the storage reserved for the small buffer optimization.
+ * @tparam Align Optional alignment requirement.
+ */
+template<typename Concept, std::size_t Len, std::size_t Align>
+class basic_poly: private Concept::template type<poly_base<basic_poly<Concept, Len, Align>>> {
+    /*! @brief A poly base is allowed to snoop into a poly object. */
+    friend struct poly_base<basic_poly>;
+
+    using vtable_type = typename poly_vtable<Concept, Len, Align>::type;
+
+public:
+    /*! @brief Concept type. */
+    using concept_type = typename Concept::template type<poly_base<basic_poly>>;
+
+    /*! @brief Default constructor. */
+    basic_poly() ENTT_NOEXCEPT
+        : storage{},
+          vtable{}
+    {}
+
+    /**
+     * @brief Constructs a poly by directly initializing the new object.
+     * @tparam Type Type of object to use to initialize the poly.
+     * @tparam Args Types of arguments to use to construct the new instance.
+     * @param args Parameters to use to construct the instance.
+     */
+    template<typename Type, typename... Args>
+    explicit basic_poly(std::in_place_type_t<Type>, Args &&... args)
+        : storage{std::in_place_type<Type>, std::forward<Args>(args)...},
+          vtable{poly_vtable<Concept, Len, Align>::template instance<std::remove_const_t<std::remove_reference_t<Type>>>()}
+    {}
+
+    /**
+     * @brief Constructs a poly from a given value.
+     * @tparam Type Type of object to use to initialize the poly.
+     * @param value An instance of an object to use to initialize the poly.
+     */
+    template<typename Type, typename = std::enable_if_t<!std::is_same_v<std::remove_cv_t<std::remove_reference_t<Type>>, basic_poly>>>
+    basic_poly(Type &&value) ENTT_NOEXCEPT
+        : basic_poly{std::in_place_type<std::remove_cv_t<std::remove_reference_t<Type>>>, std::forward<Type>(value)}
+    {}
+
+    /**
+     * @brief Copy constructor.
+     * @param other The instance to copy from.
+     */
+    basic_poly(const basic_poly &other) = default;
+
+    /**
+     * @brief Move constructor.
+     * @param other The instance to move from.
+     */
+    basic_poly(basic_poly &&other) ENTT_NOEXCEPT
+        : basic_poly{}
     {
-        connect<Candidate>(value_or_instance);
-    }
-
-    /**
-     * @brief Connects a free function to a delegate.
-     * @tparam Function A valid free function pointer.
-     */
-    template<auto Function>
-    void connect() ENTT_NOEXCEPT {
-        constexpr auto extent = internal::function_extent_v<decltype(internal::to_function_pointer(std::declval<decltype(Function)>()))>;
-        connect<Function>(std::make_index_sequence<extent>{});
-    }
-
-    /**
-     * @brief Connects a member function for a given instance or a free function
-     * with payload to a delegate.
-     *
-     * The delegate isn't responsible for the connected object or the payload.
-     * Users must always guarantee that the lifetime of the instance overcomes
-     * the one  of the delegate.<br/>
-     * When used to connect a free function with payload, its signature must be
-     * such that the instance is the first argument before the ones used to
-     * define the delegate itself.
-     *
-     * @tparam Candidate Member or free function to connect to the delegate.
-     * @tparam Type Type of class or type of payload.
-     * @param value_or_instance A valid reference that fits the purpose.
-     */
-    template<auto Candidate, typename Type>
-    void connect(Type &value_or_instance) ENTT_NOEXCEPT {
-        constexpr auto extent = internal::function_extent_v<decltype(internal::to_function_pointer(std::declval<decltype(Candidate)>(), value_or_instance))>;
-        connect<Candidate>(value_or_instance, std::make_index_sequence<extent>{});
-    }
-
-    /**
-     * @brief Resets a delegate.
-     *
-     * After a reset, a delegate cannot be invoked anymore.
-     */
-    void reset() ENTT_NOEXCEPT {
-        fn = nullptr;
-        data = nullptr;
-    }
-
-    /**
-     * @brief Returns the instance or the payload linked to a delegate, if any.
-     * @return An opaque pointer to the underlying data.
-     */
-    const void * instance() const ENTT_NOEXCEPT {
-        return data;
-    }
-
-    /**
-     * @brief Triggers a delegate.
-     *
-     * The delegate invokes the underlying function and returns the result.
-     *
-     * @warning
-     * Attempting to trigger an invalid delegate results in undefined
-     * behavior.<br/>
-     * An assertion will abort the execution at runtime in debug mode if the
-     * delegate has not yet been set.
-     *
-     * @param args Arguments to use to invoke the underlying function.
-     * @return The value returned by the underlying function.
-     */
-    Ret operator()(Args... args) const {
-        ENTT_ASSERT(fn);
-        return fn(data, std::forward_as_tuple(std::forward<Args>(args)...));
-    }
-
-    /**
-     * @brief Checks whether a delegate actually stores a listener.
-     * @return False if the delegate is empty, true otherwise.
-     */
-    explicit operator bool() const ENTT_NOEXCEPT {
-        // no need to test also data
-        return fn;
-    }
-
-    /**
-     * @brief Compares the contents of two delegates.
-     * @param other Delegate with which to compare.
-     * @return False if the two contents differ, true otherwise.
-     */
-    bool operator==(const delegate<Ret(Args...)> &other) const ENTT_NOEXCEPT {
-        return fn == other.fn && data == other.data;
-    }
-
-private:
-    proto_fn_type *fn;
-    const void *data;
-};
-
-
-/**
- * @brief Compares the contents of two delegates.
- * @tparam Ret Return type of a function type.
- * @tparam Args Types of arguments of a function type.
- * @param lhs A valid delegate object.
- * @param rhs A valid delegate object.
- * @return True if the two contents differ, false otherwise.
- */
-template<typename Ret, typename... Args>
-bool operator!=(const delegate<Ret(Args...)> &lhs, const delegate<Ret(Args...)> &rhs) ENTT_NOEXCEPT {
-    return !(lhs == rhs);
-}
-
-
-/**
- * @brief Deduction guide.
- *
- * It allows to deduce the function type of the delegate directly from a
- * function provided to the constructor.
- *
- * @tparam Function A valid free function pointer.
- */
-template<auto Function>
-delegate(connect_arg_t<Function>) ENTT_NOEXCEPT
--> delegate<std::remove_pointer_t<decltype(internal::to_function_pointer(Function))>>;
-
-
-/**
- * @brief Deduction guide.
- *
- * It allows to deduce the function type of the delegate directly from a member
- * or a free function with payload provided to the constructor.
- *
- * @param value_or_instance A valid reference that fits the purpose.
- * @tparam Candidate Member or free function to connect to the delegate.
- * @tparam Type Type of class or type of payload.
- */
-template<auto Candidate, typename Type>
-delegate(connect_arg_t<Candidate>, Type &value_or_instance) ENTT_NOEXCEPT
--> delegate<std::remove_pointer_t<decltype(internal::to_function_pointer(Candidate, value_or_instance))>>;
-
-
-}
-
-
-#endif // ENTT_SIGNAL_DELEGATE_HPP
-
-// #include "../signal/sigh.hpp"
-#ifndef ENTT_SIGNAL_SIGH_HPP
-#define ENTT_SIGNAL_SIGH_HPP
-
-
-#include <algorithm>
-#include <utility>
-#include <vector>
-#include <functional>
-#include <type_traits>
-// #include "../config/config.h"
-
-// #include "delegate.hpp"
-
-// #include "fwd.hpp"
-#ifndef ENTT_SIGNAL_FWD_HPP
-#define ENTT_SIGNAL_FWD_HPP
-
-
-// #include "../config/config.h"
-
-
-
-namespace entt {
-
-
-/*! @class delegate */
-template<typename>
-class delegate;
-
-/*! @class sink */
-template<typename>
-class sink;
-
-/*! @class sigh */
-template<typename>
-class sigh;
-
-
-}
-
-
-#endif // ENTT_SIGNAL_FWD_HPP
-
-
-
-namespace entt {
-
-
-/**
- * @brief Sink class.
- *
- * Primary template isn't defined on purpose. All the specializations give a
- * compile-time error unless the template parameter is a function type.
- *
- * @tparam Function A valid function type.
- */
-template<typename Function>
-class sink;
-
-
-/**
- * @brief Unmanaged signal handler.
- *
- * Primary template isn't defined on purpose. All the specializations give a
- * compile-time error unless the template parameter is a function type.
- *
- * @tparam Function A valid function type.
- */
-template<typename Function>
-class sigh;
-
-
-/**
- * @brief Unmanaged signal handler.
- *
- * It works directly with references to classes and pointers to member functions
- * as well as pointers to free functions. Users of this class are in charge of
- * disconnecting instances before deleting them.
- *
- * This class serves mainly two purposes:
- *
- * * Creating signals to use later to notify a bunch of listeners.
- * * Collecting results from a set of functions like in a voting system.
- *
- * @tparam Ret Return type of a function type.
- * @tparam Args Types of arguments of a function type.
- */
-template<typename Ret, typename... Args>
-class sigh<Ret(Args...)> {
-    /*! @brief A sink is allowed to modify a signal. */
-    friend class sink<Ret(Args...)>;
-
-public:
-    /*! @brief Unsigned integer type. */
-    using size_type = typename std::vector<delegate<Ret(Args...)>>::size_type;
-    /*! @brief Sink type. */
-    using sink_type = entt::sink<Ret(Args...)>;
-
-    /**
-     * @brief Instance type when it comes to connecting member functions.
-     * @tparam Class Type of class to which the member function belongs.
-     */
-    template<typename Class>
-    using instance_type = Class *;
-
-    /**
-     * @brief Number of listeners connected to the signal.
-     * @return Number of listeners currently connected.
-     */
-    size_type size() const ENTT_NOEXCEPT {
-        return calls.size();
-    }
-
-    /**
-     * @brief Returns false if at least a listener is connected to the signal.
-     * @return True if the signal has no listeners connected, false otherwise.
-     */
-    bool empty() const ENTT_NOEXCEPT {
-        return calls.empty();
-    }
-
-    /**
-     * @brief Triggers a signal.
-     *
-     * All the listeners are notified. Order isn't guaranteed.
-     *
-     * @param args Arguments to use to invoke listeners.
-     */
-    void publish(Args... args) const {
-        for(auto pos = calls.size(); pos; --pos) {
-            calls[pos-1](args...);
-        }
-    }
-
-    /**
-     * @brief Collects return values from the listeners.
-     *
-     * The collector must expose a call operator with the following properties:
-     *
-     * * The return type is either `void` or such that it's convertible to
-     *   `bool`. In the second case, a true value will stop the iteration.
-     * * The list of parameters is empty if `Ret` is `void`, otherwise it
-     *   contains a single element such that `Ret` is convertible to it.
-     *
-     * @tparam Func Type of collector to use, if any.
-     * @param func A valid function object.
-     * @param args Arguments to use to invoke listeners.
-     */
-    template<typename Func>
-    void collect(Func func, Args... args) const {
-        bool stop = false;
-
-        for(auto pos = calls.size(); pos && !stop; --pos) {
-            if constexpr(std::is_void_v<Ret>) {
-                if constexpr(std::is_invocable_r_v<bool, Func>) {
-                    calls[pos-1](args...);
-                    stop = func();
-                } else {
-                    calls[pos-1](args...);
-                    func();
-                }
-            } else {
-                if constexpr(std::is_invocable_r_v<bool, Func, Ret>) {
-                    stop = func(calls[pos-1](args...));
-                } else {
-                    func(calls[pos-1](args...));
-                }
-            }
-        }
-    }
-
-private:
-    std::vector<delegate<Ret(Args...)>> calls;
-};
-
-
-/**
- * @brief Connection class.
- *
- * Opaque object the aim of which is to allow users to release an already
- * estabilished connection without having to keep a reference to the signal or
- * the sink that generated it.
- */
-class connection {
-    /*! @brief A sink is allowed to create connection objects. */
-    template<typename>
-    friend class sink;
-
-    connection(delegate<void(void *)> fn, void *ref)
-        : disconnect{fn}, signal{ref}
-    {}
-
-public:
-    /*! Default constructor. */
-    connection() = default;
-
-    /*! @brief Default copy constructor. */
-    connection(const connection &) = default;
-
-    /**
-     * @brief Default move constructor.
-     * @param other The instance to move from.
-     */
-    connection(connection &&other)
-        : connection{}
-    {
-        std::swap(disconnect, other.disconnect);
-        std::swap(signal, other.signal);
-    }
-
-    /*! @brief Default copy assignment operator. @return This connection. */
-    connection & operator=(const connection &) = default;
-
-    /**
-     * @brief Default move assignment operator.
-     * @param other The instance to move from.
-     * @return This connection.
-     */
-    connection & operator=(connection &&other) {
-        if(this != &other) {
-            auto tmp{std::move(other)};
-            disconnect = tmp.disconnect;
-            signal = tmp.signal;
-        }
-
-        return *this;
-    }
-
-    /**
-     * @brief Checks whether a connection is properly initialized.
-     * @return True if the connection is properly initialized, false otherwise.
-     */
-    explicit operator bool() const ENTT_NOEXCEPT {
-        return static_cast<bool>(disconnect);
-    }
-
-    /*! @brief Breaks the connection. */
-    void release() {
-        if(disconnect) {
-            disconnect(signal);
-            disconnect.reset();
-        }
-    }
-
-private:
-    delegate<void(void *)> disconnect;
-    void *signal{};
-};
-
-
-/**
- * @brief Scoped connection class.
- *
- * Opaque object the aim of which is to allow users to release an already
- * estabilished connection without having to keep a reference to the signal or
- * the sink that generated it.<br/>
- * A scoped connection automatically breaks the link between the two objects
- * when it goes out of scope.
- */
-struct scoped_connection: private connection {
-    /*! Default constructor. */
-    scoped_connection() = default;
-
-    /**
-     * @brief Constructs a scoped connection from a basic connection.
-     * @param conn A valid connection object.
-     */
-    scoped_connection(const connection &conn)
-        : connection{conn}
-    {}
-
-    /*! @brief Default copy constructor, deleted on purpose. */
-    scoped_connection(const scoped_connection &) = delete;
-
-    /*! @brief Default move constructor. */
-    scoped_connection(scoped_connection &&) = default;
-
-    /*! @brief Automatically breaks the link on destruction. */
-    ~scoped_connection() {
-        connection::release();
-    }
-
-    /**
-     * @brief Default copy assignment operator, deleted on purpose.
-     * @return This scoped connection.
-     */
-    scoped_connection & operator=(const scoped_connection &) = delete;
-
-    /**
-     * @brief Default move assignment operator.
-     * @return This scoped connection.
-     */
-    scoped_connection & operator=(scoped_connection &&) = default;
-
-    /**
-     * @brief Copies a connection.
-     * @param other The connection object to copy.
-     * @return This scoped connection.
-     */
-    scoped_connection & operator=(const connection &other) {
-        static_cast<connection &>(*this) = other;
-        return *this;
-    }
-
-    /**
-     * @brief Moves a connection.
-     * @param other The connection object to move.
-     * @return This scoped connection.
-     */
-    scoped_connection & operator=(connection &&other) {
-        static_cast<connection &>(*this) = std::move(other);
-        return *this;
-    }
-
-    using connection::operator bool;
-    using connection::release;
-};
-
-
-/**
- * @brief Sink class.
- *
- * A sink is used to connect listeners to signals and to disconnect them.<br/>
- * The function type for a listener is the one of the signal to which it
- * belongs.
- *
- * The clear separation between a signal and a sink permits to store the former
- * as private data member without exposing the publish functionality to the
- * users of the class.
- *
- * @tparam Ret Return type of a function type.
- * @tparam Args Types of arguments of a function type.
- */
-template<typename Ret, typename... Args>
-class sink<Ret(Args...)> {
-    using signal_type = sigh<Ret(Args...)>;
-
-    template<auto Candidate, typename Type>
-    static void release(Type &value_or_instance, void *signal) {
-        sink{*static_cast<signal_type *>(signal)}.disconnect<Candidate>(value_or_instance);
-    }
-
-    template<auto Function>
-    static void release(void *signal) {
-        sink{*static_cast<signal_type *>(signal)}.disconnect<Function>();
-    }
-
-public:
-    /**
-     * @brief Constructs a sink that is allowed to modify a given signal.
-     * @param ref A valid reference to a signal object.
-     */
-    sink(sigh<Ret(Args...)> &ref) ENTT_NOEXCEPT
-        : signal{&ref}
-    {}
-
-    /**
-     * @brief Returns false if at least a listener is connected to the sink.
-     * @return True if the sink has no listeners connected, false otherwise.
-     */
-    bool empty() const ENTT_NOEXCEPT {
-        return signal->calls.empty();
-    }
-
-    /**
-     * @brief Connects a free function to a signal.
-     *
-     * The signal handler performs checks to avoid multiple connections for free
-     * functions.
-     *
-     * @tparam Function A valid free function pointer.
-     * @return A properly initialized connection object.
-     */
-    template<auto Function>
-    connection connect() {
-        disconnect<Function>();
-        delegate<void(void *)> conn{};
-        conn.template connect<&release<Function>>();
-        signal->calls.emplace_back(delegate<Ret(Args...)>{connect_arg<Function>});
-        return { std::move(conn), signal };
-    }
-
-    /**
-     * @brief Connects a member function or a free function with payload to a
-     * signal.
-     *
-     * The signal isn't responsible for the connected object or the payload.
-     * Users must always guarantee that the lifetime of the instance overcomes
-     * the one  of the delegate. On the other side, the signal handler performs
-     * checks to avoid multiple connections for the same function.<br/>
-     * When used to connect a free function with payload, its signature must be
-     * such that the instance is the first argument before the ones used to
-     * define the delegate itself.
-     *
-     * @tparam Candidate Member or free function to connect to the signal.
-     * @tparam Type Type of class or type of payload.
-     * @param value_or_instance A valid reference that fits the purpose.
-     * @return A properly initialized connection object.
-     */
-    template<auto Candidate, typename Type>
-    connection connect(Type &value_or_instance) {
-        disconnect<Candidate>(value_or_instance);
-        delegate<void(void *)> conn{};
-        conn.template connect<&sink::release<Candidate, Type>>(value_or_instance);
-        signal->calls.emplace_back(delegate<Ret(Args...)>{connect_arg<Candidate>, value_or_instance});
-        return { std::move(conn), signal };
-    }
-
-    /**
-     * @brief Disconnects a free function from a signal.
-     * @tparam Function A valid free function pointer.
-     */
-    template<auto Function>
-    void disconnect() {
-        auto &calls = signal->calls;
-        delegate<Ret(Args...)> delegate{};
-        delegate.template connect<Function>();
-        calls.erase(std::remove(calls.begin(), calls.end(), delegate), calls.end());
-    }
-
-    /**
-     * @brief Disconnects a member function or a free function with payload from
-     * a signal.
-     * @tparam Candidate Member or free function to disconnect from the signal.
-     * @tparam Type Type of class or type of payload.
-     * @param value_or_instance A valid reference that fits the purpose.
-     */
-    template<auto Candidate, typename Type>
-    void disconnect(Type &value_or_instance) {
-        auto &calls = signal->calls;
-        delegate<Ret(Args...)> delegate{};
-        delegate.template connect<Candidate>(value_or_instance);
-        calls.erase(std::remove(calls.begin(), calls.end(), delegate), calls.end());
-    }
-
-    /**
-     * @brief Disconnects member functions or free functions based on an
-     * instance or specific payload.
-     * @tparam Type Type of class or type of payload.
-     * @param value_or_instance A valid reference that fits the purpose.
-     */
-    template<typename Type>
-    void disconnect(const Type &value_or_instance) {
-        auto &calls = signal->calls;
-        calls.erase(std::remove_if(calls.begin(), calls.end(), [&value_or_instance](const auto &delegate) {
-            return delegate.instance() == &value_or_instance;
-        }), calls.end());
-    }
-
-    /*! @brief Disconnects all the listeners from a signal. */
-    void disconnect() {
-        signal->calls.clear();
-    }
-
-private:
-    signal_type *signal;
-};
-
-
-/**
- * @brief Deduction guide.
- *
- * It allows to deduce the function type of a sink directly from the signal it
- * refers to.
- *
- * @tparam Ret Return type of a function type.
- * @tparam Args Types of arguments of a function type.
- */
-template<typename Ret, typename... Args>
-sink(sigh<Ret(Args...)> &) ENTT_NOEXCEPT -> sink<Ret(Args...)>;
-
-
-}
-
-
-#endif // ENTT_SIGNAL_SIGH_HPP
+        swap(*this, other);
+    }
+
+    /**
+     * @brief Assignment operator.
+     * @param other The instance to assign from.
+     * @return This poly object.
+     */
+    basic_poly & operator=(basic_poly other) {
+        swap(other, *this);
+        return *this;
+    }
+
+    /**
+     * @brief Returns the type of the contained object.
+     * @return The type of the contained object, if any.
+     */
+    [[nodiscard]] type_info type() const ENTT_NOEXCEPT {
+        return storage.type();
+    }
+
+    /**
+     * @brief Returns an opaque pointer to the contained instance.
+     * @return An opaque pointer the contained instance, if any.
+     */
+    [[nodiscard]] const void * data() const ENTT_NOEXCEPT {
+        return storage.data();
+    }
+
+    /*! @copydoc data */
+    [[nodiscard]] void * data() ENTT_NOEXCEPT {
+        return storage.data();
+    }
+
+    /**
+     * @brief Replaces the contained object by creating a new instance directly.
+     * @tparam Type Type of object to use to initialize the poly.
+     * @tparam Args Types of arguments to use to construct the new instance.
+     * @param args Parameters to use to construct the instance.
+     */
+    template<typename Type, typename... Args>
+    void emplace(Args &&... args) {
+        *this = basic_poly{std::in_place_type<Type>, std::forward<Args>(args)...};
+    }
+
+    /*! @brief Destroys contained object */
+    void reset() {
+        *this = basic_poly{};
+    }
+
+    /**
+     * @brief Returns false if a poly is empty, true otherwise.
+     * @return False if the poly is empty, true otherwise.
+     */
+    [[nodiscard]] explicit operator bool() const ENTT_NOEXCEPT {
+        return !(vtable == nullptr);
+    }
+
+    /**
+     * @brief Returns a pointer to the underlying concept.
+     * @return A pointer to the underlying concept.
+     */
+    [[nodiscard]] concept_type * operator->() ENTT_NOEXCEPT {
+        return this;
+    }
+
+    /*! @copydoc operator-> */
+    [[nodiscard]] const concept_type * operator->() const ENTT_NOEXCEPT {
+        return this;
+    }
+
+    /**
+     * @brief Swaps two poly objects.
+     * @param lhs A valid poly object.
+     * @param rhs A valid poly object.
+     */
+    friend void swap(basic_poly &lhs, basic_poly &rhs) {
+        using std::swap;
+        swap(lhs.storage, rhs.storage);
+        swap(lhs.vtable, rhs.vtable);
+    }
+
+    /**
+     * @brief Aliasing constructor.
+     * @return A poly that shares a reference to an unmanaged object.
+     */
+    [[nodiscard]] basic_poly as_ref() ENTT_NOEXCEPT {
+        basic_poly ref = std::as_const(*this).as_ref();
+        ref.storage = storage.as_ref();
+        return ref;
+    }
+
+    /*! @copydoc as_ref */
+    [[nodiscard]] basic_poly as_ref() const ENTT_NOEXCEPT {
+        basic_poly ref{};
+        ref.storage = storage.as_ref();
+        ref.vtable = vtable;
+        return ref;
+    }
+
+private:
+    basic_any<Len, Align> storage;
+    const vtable_type *vtable;
+};
+
+
+}
+
+
+#endif
+
+// #include "fwd.hpp"
+
+
+
+namespace entt {
+
+
+/**
+ * @brief Basic poly storage implementation.
+ * @tparam Entity A valid entity type (see entt_traits for more details).
+ */
+template<typename Entity>
+struct Storage: type_list<type_info() const ENTT_NOEXCEPT> {
+    /**
+     * @brief Concept definition.
+     * @tparam Base Opaque base class from which to inherit.
+     */
+    template<typename Base>
+    struct type: Base {
+        /**
+         * @brief Returns a type info for the contained objects.
+         * @return The type info for the contained objects.
+         */
+        type_info value_type() const ENTT_NOEXCEPT {
+            return poly_call<0>(*this);
+        }
+    };
+
+    /**
+     * @brief Concept implementation.
+     * @tparam Type Type for which to generate an implementation.
+     */
+    template<typename Type>
+    using impl = value_list<&type_id<typename Type::value_type>>;
+};
+
+
+/**
+ * @brief Defines the poly storage type associate with a given entity type.
+ * @tparam Entity A valid entity type (see entt_traits for more details).
+ */
+template<typename Entity, typename = void>
+struct poly_storage_traits {
+    /*! @brief Poly storage type for the given entity type. */
+    using storage_type = poly<Storage<Entity>>;
+};
+
+
+}
+
+
+#endif
 
 // #include "runtime_view.hpp"
 #ifndef ENTT_ENTITY_RUNTIME_VIEW_HPP
@@ -3041,897 +17436,15 @@
 
 
 #include <iterator>
-#include <cassert>
 #include <vector>
 #include <utility>
 #include <algorithm>
 #include <type_traits>
 // #include "../config/config.h"
 
+// #include "entity.hpp"
+
 // #include "sparse_set.hpp"
-#ifndef ENTT_ENTITY_SPARSE_SET_HPP
-#define ENTT_ENTITY_SPARSE_SET_HPP
-
-
-#include <algorithm>
-#include <iterator>
-#include <utility>
-#include <vector>
-#include <memory>
-#include <cstddef>
-#include <numeric>
-#include <type_traits>
-// #include "../config/config.h"
-
-// #include "../core/algorithm.hpp"
-
-// #include "entity.hpp"
-#ifndef ENTT_ENTITY_ENTITY_HPP
-#define ENTT_ENTITY_ENTITY_HPP
-
-
-#include <cstdint>
-#include <type_traits>
-// #include "../config/config.h"
-
-
-
-namespace entt {
-
-
-/**
- * @brief Entity traits.
- *
- * Primary template isn't defined on purpose. All the specializations give a
- * compile-time error unless the template parameter is an accepted entity type.
- */
-template<typename>
-struct entt_traits;
-
-
-/**
- * @brief Entity traits for a 16 bits entity identifier.
- *
- * A 16 bits entity identifier guarantees:
- *
- * * 12 bits for the entity number (up to 4k entities).
- * * 4 bit for the version (resets in [0-15]).
- */
-template<>
-struct entt_traits<std::uint16_t> {
-    /*! @brief Underlying entity type. */
-    using entity_type = std::uint16_t;
-    /*! @brief Underlying version type. */
-    using version_type = std::uint8_t;
-    /*! @brief Difference type. */
-    using difference_type = std::int32_t;
-
-    /*! @brief Mask to use to get the entity number out of an identifier. */
-    static constexpr std::uint16_t entity_mask = 0xFFF;
-    /*! @brief Mask to use to get the version out of an identifier. */
-    static constexpr std::uint16_t version_mask = 0xF;
-    /*! @brief Extent of the entity number within an identifier. */
-    static constexpr auto entity_shift = 12;
-};
-
-
-/**
- * @brief Entity traits for a 32 bits entity identifier.
- *
- * A 32 bits entity identifier guarantees:
- *
- * * 20 bits for the entity number (suitable for almost all the games).
- * * 12 bit for the version (resets in [0-4095]).
- */
-template<>
-struct entt_traits<std::uint32_t> {
-    /*! @brief Underlying entity type. */
-    using entity_type = std::uint32_t;
-    /*! @brief Underlying version type. */
-    using version_type = std::uint16_t;
-    /*! @brief Difference type. */
-    using difference_type = std::int64_t;
-
-    /*! @brief Mask to use to get the entity number out of an identifier. */
-    static constexpr std::uint32_t entity_mask = 0xFFFFF;
-    /*! @brief Mask to use to get the version out of an identifier. */
-    static constexpr std::uint32_t version_mask = 0xFFF;
-    /*! @brief Extent of the entity number within an identifier. */
-    static constexpr auto entity_shift = 20;
-};
-
-
-/**
- * @brief Entity traits for a 64 bits entity identifier.
- *
- * A 64 bits entity identifier guarantees:
- *
- * * 32 bits for the entity number (an indecently large number).
- * * 32 bit for the version (an indecently large number).
- */
-template<>
-struct entt_traits<std::uint64_t> {
-    /*! @brief Underlying entity type. */
-    using entity_type = std::uint64_t;
-    /*! @brief Underlying version type. */
-    using version_type = std::uint32_t;
-    /*! @brief Difference type. */
-    using difference_type = std::int64_t;
-
-    /*! @brief Mask to use to get the entity number out of an identifier. */
-    static constexpr std::uint64_t entity_mask = 0xFFFFFFFF;
-    /*! @brief Mask to use to get the version out of an identifier. */
-    static constexpr std::uint64_t version_mask = 0xFFFFFFFF;
-    /*! @brief Extent of the entity number within an identifier. */
-    static constexpr auto entity_shift = 32;
-};
-
-
-/**
- * @cond TURN_OFF_DOXYGEN
- * Internal details not to be documented.
- */
-
-
-namespace internal {
-
-
-class null {
-    template<typename Entity>
-    using traits_type = entt_traits<std::underlying_type_t<Entity>>;
-
-public:
-    template<typename Entity>
-    constexpr operator Entity() const ENTT_NOEXCEPT {
-        return Entity{traits_type<Entity>::entity_mask};
-    }
-
-    constexpr bool operator==(null) const ENTT_NOEXCEPT {
-        return true;
-    }
-
-    constexpr bool operator!=(null) const ENTT_NOEXCEPT {
-        return false;
-    }
-
-    template<typename Entity>
-    constexpr bool operator==(const Entity entity) const ENTT_NOEXCEPT {
-        return (to_integer(entity) & traits_type<Entity>::entity_mask) == to_integer(static_cast<Entity>(*this));
-    }
-
-    template<typename Entity>
-    constexpr bool operator!=(const Entity entity) const ENTT_NOEXCEPT {
-        return !(entity == *this);
-    }
-};
-
-
-template<typename Entity>
-constexpr bool operator==(const Entity entity, null other) ENTT_NOEXCEPT {
-    return other == entity;
-}
-
-
-template<typename Entity>
-constexpr bool operator!=(const Entity entity, null other) ENTT_NOEXCEPT {
-    return other != entity;
-}
-
-
-}
-
-
-/**
- * Internal details not to be documented.
- * @endcond TURN_OFF_DOXYGEN
- */
-
-
-/**
- * @brief Compile-time constant for null entities.
- *
- * There exist implicit conversions from this variable to entity identifiers of
- * any allowed type. Similarly, there exist comparision operators between the
- * null entity and any other entity identifier.
- */
-constexpr auto null = internal::null{};
-
-
-}
-
-
-#endif // ENTT_ENTITY_ENTITY_HPP
-
-// #include "fwd.hpp"
-#ifndef ENTT_ENTITY_FWD_HPP
-#define ENTT_ENTITY_FWD_HPP
-
-
-#include <cstdint>
-// #include "../config/config.h"
-
-// #include "../core/type_traits.hpp"
-
-
-
-namespace entt {
-
-/*! @class basic_registry */
-template <typename>
-class basic_registry;
-
-/*! @class basic_view */
-template<typename, typename...>
-class basic_view;
-
-/*! @class basic_runtime_view */
-template<typename>
-class basic_runtime_view;
-
-/*! @class basic_group */
-template<typename...>
-class basic_group;
-
-/*! @class basic_observer */
-template<typename>
-class basic_observer;
-
-/*! @class basic_actor */
-template <typename>
-struct basic_actor;
-
-/*! @class basic_prototype */
-template<typename>
-class basic_prototype;
-
-/*! @class basic_snapshot */
-template<typename>
-class basic_snapshot;
-
-/*! @class basic_snapshot_loader */
-template<typename>
-class basic_snapshot_loader;
-
-/*! @class basic_continuous_loader */
-template<typename>
-class basic_continuous_loader;
-
-/*! @brief Alias declaration for the most common use case. */
-ENTT_OPAQUE_TYPE(entity, ENTT_ID_TYPE)
-
-/*! @brief Alias declaration for the most common use case. */
-ENTT_OPAQUE_TYPE(component, ENTT_ID_TYPE)
-
-/*! @brief Alias declaration for the most common use case. */
-using registry = basic_registry<entity>;
-
-/*! @brief Alias declaration for the most common use case. */
-using observer = basic_observer<entity>;
-
-/*! @brief Alias declaration for the most common use case. */
-using actor = basic_actor<entity>;
-
-/*! @brief Alias declaration for the most common use case. */
-using prototype = basic_prototype<entity>;
-
-/*! @brief Alias declaration for the most common use case. */
-using snapshot = basic_snapshot<entity>;
-
-/*! @brief Alias declaration for the most common use case. */
-using snapshot_loader = basic_snapshot_loader<entity>;
-
-/*! @brief Alias declaration for the most common use case. */
-using continuous_loader = basic_continuous_loader<entity>;
-
-/**
- * @brief Alias declaration for the most common use case.
- * @tparam Component Types of components iterated by the view.
- */
-template<typename... Types>
-using view = basic_view<entity, Types...>;
-
-/*! @brief Alias declaration for the most common use case. */
-using runtime_view = basic_runtime_view<entity>;
-
-/**
- * @brief Alias declaration for the most common use case.
- * @tparam Types Types of components iterated by the group.
- */
-template<typename... Types>
-using group = basic_group<entity, Types...>;
-
-
-}
-
-
-#endif // ENTT_ENTITY_FWD_HPP
-
-
-
-namespace entt {
-
-
-/**
- * @brief Basic sparse set implementation.
- *
- * Sparse set or packed array or whatever is the name users give it.<br/>
- * Two arrays: an _external_ one and an _internal_ one; a _sparse_ one and a
- * _packed_ one; one used for direct access through contiguous memory, the other
- * one used to get the data through an extra level of indirection.<br/>
- * This is largely used by the registry to offer users the fastest access ever
- * to the components. Views and groups in general are almost entirely designed
- * around sparse sets.
- *
- * This type of data structure is widely documented in the literature and on the
- * web. This is nothing more than a customized implementation suitable for the
- * purpose of the framework.
- *
- * @note
- * There are no guarantees that entities are returned in the insertion order
- * when iterate a sparse set. Do not make assumption on the order in any case.
- *
- * @note
- * Internal data structures arrange elements to maximize performance. Because of
- * that, there are no guarantees that elements have the expected order when
- * iterate directly the internal packed array (see `data` and `size` member
- * functions for that). Use `begin` and `end` instead.
- *
- * @tparam Entity A valid entity type (see entt_traits for more details).
- */
-template<typename Entity>
-class sparse_set {
-    using traits_type = entt_traits<std::underlying_type_t<Entity>>;
-
-    static_assert(ENTT_PAGE_SIZE && ((ENTT_PAGE_SIZE & (ENTT_PAGE_SIZE - 1)) == 0));
-    static constexpr auto entt_per_page = ENTT_PAGE_SIZE / sizeof(typename traits_type::entity_type);
-
-    class iterator {
-        friend class sparse_set<Entity>;
-
-        using direct_type = const std::vector<Entity>;
-        using index_type = typename traits_type::difference_type;
-
-        iterator(direct_type *ref, const index_type idx) ENTT_NOEXCEPT
-            : direct{ref}, index{idx}
-        {}
-
-    public:
-        using difference_type = index_type;
-        using value_type = Entity;
-        using pointer = const value_type *;
-        using reference = const value_type &;
-        using iterator_category = std::random_access_iterator_tag;
-
-        iterator() ENTT_NOEXCEPT = default;
-
-        iterator & operator++() ENTT_NOEXCEPT {
-            return --index, *this;
-        }
-
-        iterator operator++(int) ENTT_NOEXCEPT {
-            iterator orig = *this;
-            return ++(*this), orig;
-        }
-
-        iterator & operator--() ENTT_NOEXCEPT {
-            return ++index, *this;
-        }
-
-        iterator operator--(int) ENTT_NOEXCEPT {
-            iterator orig = *this;
-            return --(*this), orig;
-        }
-
-        iterator & operator+=(const difference_type value) ENTT_NOEXCEPT {
-            index -= value;
-            return *this;
-        }
-
-        iterator operator+(const difference_type value) const ENTT_NOEXCEPT {
-            return iterator{direct, index-value};
-        }
-
-        iterator & operator-=(const difference_type value) ENTT_NOEXCEPT {
-            return (*this += -value);
-        }
-
-        iterator operator-(const difference_type value) const ENTT_NOEXCEPT {
-            return (*this + -value);
-        }
-
-        difference_type operator-(const iterator &other) const ENTT_NOEXCEPT {
-            return other.index - index;
-        }
-
-        reference operator[](const difference_type value) const ENTT_NOEXCEPT {
-            const auto pos = size_type(index-value-1);
-            return (*direct)[pos];
-        }
-
-        bool operator==(const iterator &other) const ENTT_NOEXCEPT {
-            return other.index == index;
-        }
-
-        bool operator!=(const iterator &other) const ENTT_NOEXCEPT {
-            return !(*this == other);
-        }
-
-        bool operator<(const iterator &other) const ENTT_NOEXCEPT {
-            return index > other.index;
-        }
-
-        bool operator>(const iterator &other) const ENTT_NOEXCEPT {
-            return index < other.index;
-        }
-
-        bool operator<=(const iterator &other) const ENTT_NOEXCEPT {
-            return !(*this > other);
-        }
-
-        bool operator>=(const iterator &other) const ENTT_NOEXCEPT {
-            return !(*this < other);
-        }
-
-        pointer operator->() const ENTT_NOEXCEPT {
-            const auto pos = size_type(index-1);
-            return &(*direct)[pos];
-        }
-
-        reference operator*() const ENTT_NOEXCEPT {
-            return *operator->();
-        }
-
-    private:
-        direct_type *direct;
-        index_type index;
-    };
-
-    void assure(const std::size_t page) {
-        if(!(page < reverse.size())) {
-            reverse.resize(page+1);
-        }
-
-        if(!reverse[page]) {
-            reverse[page] = std::make_unique<entity_type[]>(entt_per_page);
-            // null is safe in all cases for our purposes
-            std::fill_n(reverse[page].get(), entt_per_page, null);
-        }
-    }
-
-    auto map(const Entity entt) const ENTT_NOEXCEPT {
-        const auto identifier = to_integer(entt) & traits_type::entity_mask;
-        const auto page = size_type(identifier / entt_per_page);
-        const auto offset = size_type(identifier & (entt_per_page - 1));
-        return std::make_pair(page, offset);
-    }
-
-public:
-    /*! @brief Underlying entity identifier. */
-    using entity_type = Entity;
-    /*! @brief Unsigned integer type. */
-    using size_type = std::size_t;
-    /*! @brief Random access iterator type. */
-    using iterator_type = iterator;
-
-    /*! @brief Default constructor. */
-    sparse_set() = default;
-
-    /**
-     * @brief Copy constructor.
-     * @param other The instance to copy from.
-     */
-    sparse_set(const sparse_set &other)
-        : reverse{},
-          direct{other.direct}
-    {
-        for(size_type pos{}, last = other.reverse.size(); pos < last; ++pos) {
-            if(other.reverse[pos]) {
-                assure(pos);
-                std::copy_n(other.reverse[pos].get(), entt_per_page, reverse[pos].get());
-            }
-        }
-    }
-
-    /*! @brief Default move constructor. */
-    sparse_set(sparse_set &&) = default;
-
-    /*! @brief Default destructor. */
-    virtual ~sparse_set() ENTT_NOEXCEPT = default;
-
-    /**
-     * @brief Copy assignment operator.
-     * @param other The instance to copy from.
-     * @return This sparse set.
-     */
-    sparse_set & operator=(const sparse_set &other) {
-        if(&other != this) {
-            auto tmp{other};
-            *this = std::move(tmp);
-        }
-
-        return *this;
-    }
-
-    /*! @brief Default move assignment operator. @return This sparse set. */
-    sparse_set & operator=(sparse_set &&) = default;
-
-    /**
-     * @brief Increases the capacity of a sparse set.
-     *
-     * If the new capacity is greater than the current capacity, new storage is
-     * allocated, otherwise the method does nothing.
-     *
-     * @param cap Desired capacity.
-     */
-    void reserve(const size_type cap) {
-        direct.reserve(cap);
-    }
-
-    /**
-     * @brief Returns the number of elements that a sparse set has currently
-     * allocated space for.
-     * @return Capacity of the sparse set.
-     */
-    size_type capacity() const ENTT_NOEXCEPT {
-        return direct.capacity();
-    }
-
-    /*! @brief Requests the removal of unused capacity. */
-    void shrink_to_fit() {
-        // conservative approach
-        if(direct.empty()) {
-            reverse.clear();
-        }
-
-        reverse.shrink_to_fit();
-        direct.shrink_to_fit();
-    }
-
-    /**
-     * @brief Returns the extent of a sparse set.
-     *
-     * The extent of a sparse set is also the size of the internal sparse array.
-     * There is no guarantee that the internal packed array has the same size.
-     * Usually the size of the internal sparse array is equal or greater than
-     * the one of the internal packed array.
-     *
-     * @return Extent of the sparse set.
-     */
-    size_type extent() const ENTT_NOEXCEPT {
-        return reverse.size() * entt_per_page;
-    }
-
-    /**
-     * @brief Returns the number of elements in a sparse set.
-     *
-     * The number of elements is also the size of the internal packed array.
-     * There is no guarantee that the internal sparse array has the same size.
-     * Usually the size of the internal sparse array is equal or greater than
-     * the one of the internal packed array.
-     *
-     * @return Number of elements.
-     */
-    size_type size() const ENTT_NOEXCEPT {
-        return direct.size();
-    }
-
-    /**
-     * @brief Checks whether a sparse set is empty.
-     * @return True if the sparse set is empty, false otherwise.
-     */
-    bool empty() const ENTT_NOEXCEPT {
-        return direct.empty();
-    }
-
-    /**
-     * @brief Direct access to the internal packed array.
-     *
-     * The returned pointer is such that range `[data(), data() + size()]` is
-     * always a valid range, even if the container is empty.
-     *
-     * @note
-     * There are no guarantees on the order, even though `respect` has been
-     * previously invoked. Internal data structures arrange elements to maximize
-     * performance. Accessing them directly gives a performance boost but less
-     * guarantees. Use `begin` and `end` if you want to iterate the sparse set
-     * in the expected order.
-     *
-     * @return A pointer to the internal packed array.
-     */
-    const entity_type * data() const ENTT_NOEXCEPT {
-        return direct.data();
-    }
-
-    /**
-     * @brief Returns an iterator to the beginning.
-     *
-     * The returned iterator points to the first entity of the internal packed
-     * array. If the sparse set is empty, the returned iterator will be equal to
-     * `end()`.
-     *
-     * @note
-     * Random access iterators stay true to the order imposed by a call to
-     * `respect`.
-     *
-     * @return An iterator to the first entity of the internal packed array.
-     */
-    iterator_type begin() const ENTT_NOEXCEPT {
-        const typename traits_type::difference_type pos = direct.size();
-        return iterator_type{&direct, pos};
-    }
-
-    /**
-     * @brief Returns an iterator to the end.
-     *
-     * The returned iterator points to the element following the last entity in
-     * the internal packed array. Attempting to dereference the returned
-     * iterator results in undefined behavior.
-     *
-     * @note
-     * Random access iterators stay true to the order imposed by a call to
-     * `respect`.
-     *
-     * @return An iterator to the element following the last entity of the
-     * internal packed array.
-     */
-    iterator_type end() const ENTT_NOEXCEPT {
-        return iterator_type{&direct, {}};
-    }
-
-    /**
-     * @brief Finds an entity.
-     * @param entt A valid entity identifier.
-     * @return An iterator to the given entity if it's found, past the end
-     * iterator otherwise.
-     */
-    iterator_type find(const entity_type entt) const ENTT_NOEXCEPT {
-        return has(entt) ? --(end() - index(entt)) : end();
-    }
-
-    /**
-     * @brief Checks if a sparse set contains an entity.
-     * @param entt A valid entity identifier.
-     * @return True if the sparse set contains the entity, false otherwise.
-     */
-    bool has(const entity_type entt) const ENTT_NOEXCEPT {
-        auto [page, offset] = map(entt);
-        // testing against null permits to avoid accessing the direct vector
-        return (page < reverse.size() && reverse[page] && reverse[page][offset] != null);
-    }
-
-    /**
-     * @brief Returns the position of an entity in a sparse set.
-     *
-     * @warning
-     * Attempting to get the position of an entity that doesn't belong to the
-     * sparse set results in undefined behavior.<br/>
-     * An assertion will abort the execution at runtime in debug mode if the
-     * sparse set doesn't contain the given entity.
-     *
-     * @param entt A valid entity identifier.
-     * @return The position of the entity in the sparse set.
-     */
-    size_type index(const entity_type entt) const ENTT_NOEXCEPT {
-        ENTT_ASSERT(has(entt));
-        auto [page, offset] = map(entt);
-        return size_type(reverse[page][offset]);
-    }
-
-    /**
-     * @brief Assigns an entity to a sparse set.
-     *
-     * @warning
-     * Attempting to assign an entity that already belongs to the sparse set
-     * results in undefined behavior.<br/>
-     * An assertion will abort the execution at runtime in debug mode if the
-     * sparse set already contains the given entity.
-     *
-     * @param entt A valid entity identifier.
-     */
-    void construct(const entity_type entt) {
-        ENTT_ASSERT(!has(entt));
-        auto [page, offset] = map(entt);
-        assure(page);
-        reverse[page][offset] = entity_type(direct.size());
-        direct.push_back(entt);
-    }
-
-    /**
-     * @brief Assigns one or more entities to a sparse set.
-     *
-     * @warning
-     * Attempting to assign an entity that already belongs to the sparse set
-     * results in undefined behavior.<br/>
-     * An assertion will abort the execution at runtime in debug mode if the
-     * sparse set already contains the given entity.
-     *
-     * @tparam It Type of forward iterator.
-     * @param first An iterator to the first element of the range of entities.
-     * @param last An iterator past the last element of the range of entities.
-     */
-    template<typename It>
-    void batch(It first, It last) {
-        std::for_each(std::make_reverse_iterator(last), std::make_reverse_iterator(first), [this, next = direct.size()](const auto entt) mutable {
-            ENTT_ASSERT(!has(entt));
-            auto [page, offset] = map(entt);
-            assure(page);
-            reverse[page][offset] = entity_type(next++);
-        });
-
-        direct.insert(direct.end(), first, last);
-    }
-
-    /**
-     * @brief Removes an entity from a sparse set.
-     *
-     * @warning
-     * Attempting to remove an entity that doesn't belong to the sparse set
-     * results in undefined behavior.<br/>
-     * An assertion will abort the execution at runtime in debug mode if the
-     * sparse set doesn't contain the given entity.
-     *
-     * @param entt A valid entity identifier.
-     */
-    void destroy(const entity_type entt) {
-        ENTT_ASSERT(has(entt));
-        auto [from_page, from_offset] = map(entt);
-        auto [to_page, to_offset] = map(direct.back());
-        direct[size_type(reverse[from_page][from_offset])] = entity_type(direct.back());
-        reverse[to_page][to_offset] = reverse[from_page][from_offset];
-        reverse[from_page][from_offset] = null;
-        direct.pop_back();
-    }
-
-    /**
-     * @brief Swaps two entities in the internal packed array.
-     *
-     * For what it's worth, this function affects both the internal sparse array
-     * and the internal packed array. Users should not care of that anyway.
-     *
-     * @warning
-     * Attempting to swap entities that don't belong to the sparse set results
-     * in undefined behavior.<br/>
-     * An assertion will abort the execution at runtime in debug mode if the
-     * sparse set doesn't contain the given entities.
-     *
-     * @param lhs A valid position within the sparse set.
-     * @param rhs A valid position within the sparse set.
-     */
-    virtual void swap(const size_type lhs, const size_type rhs) ENTT_NOEXCEPT {
-        ENTT_ASSERT(lhs < direct.size());
-        ENTT_ASSERT(rhs < direct.size());
-        auto [src_page, src_offset] = map(direct[lhs]);
-        auto [dst_page, dst_offset] = map(direct[rhs]);
-        std::swap(reverse[src_page][src_offset], reverse[dst_page][dst_offset]);
-        std::swap(direct[lhs], direct[rhs]);
-    }
-
-    /**
-     * @brief Sort elements according to the given comparison function.
-     *
-     * Sort the elements so that iterating the range with a couple of iterators
-     * returns them in the expected order. See `begin` and `end` for more
-     * details.
-     *
-     * The comparison function object must return `true` if the first element
-     * is _less_ than the second one, `false` otherwise. The signature of the
-     * comparison function should be equivalent to the following:
-     *
-     * @code{.cpp}
-     * bool(const Entity, const Entity);
-     * @endcode
-     *
-     * Moreover, the comparison function object shall induce a
-     * _strict weak ordering_ on the values.
-     *
-     * The sort function oject must offer a member function template
-     * `operator()` that accepts three arguments:
-     *
-     * * An iterator to the first element of the range to sort.
-     * * An iterator past the last element of the range to sort.
-     * * A comparison function to use to compare the elements.
-     *
-     * The comparison function object received by the sort function object
-     * hasn't necessarily the type of the one passed along with the other
-     * parameters to this member function.
-     *
-     * @note
-     * Attempting to iterate elements using a raw pointer returned by a call to
-     * either `data` or `raw` gives no guarantees on the order, even though
-     * `sort` has been invoked.
-     *
-     * @tparam Compare Type of comparison function object.
-     * @tparam Sort Type of sort function object.
-     * @tparam Args Types of arguments to forward to the sort function object.
-     * @param first An iterator to the first element of the range to sort.
-     * @param last An iterator past the last element of the range to sort.
-     * @param compare A valid comparison function object.
-     * @param algo A valid sort function object.
-     * @param args Arguments to forward to the sort function object, if any.
-     */
-    template<typename Compare, typename Sort = std_sort, typename... Args>
-    void sort(iterator_type first, iterator_type last, Compare compare, Sort algo = Sort{}, Args &&... args) {
-        ENTT_ASSERT(!(first > last));
-
-        std::vector<size_type> copy(last - first);
-        const auto offset = std::distance(last, end());
-        std::iota(copy.begin(), copy.end(), size_type{});
-
-        algo(copy.rbegin(), copy.rend(), [this, offset, compare = std::move(compare)](const auto lhs, const auto rhs) {
-            return compare(std::as_const(direct[lhs+offset]), std::as_const(direct[rhs+offset]));
-        }, std::forward<Args>(args)...);
-
-        for(size_type pos{}, length = copy.size(); pos < length; ++pos) {
-            auto curr = pos;
-            auto next = copy[curr];
-
-            while(curr != next) {
-                swap(copy[curr] + offset, copy[next] + offset);
-                copy[curr] = curr;
-                curr = next;
-                next = copy[curr];
-            }
-        }
-    }
-
-    /**
-     * @brief Sort entities according to their order in another sparse set.
-     *
-     * Entities that are part of both the sparse sets are ordered internally
-     * according to the order they have in `other`. All the other entities goes
-     * to the end of the list and there are no guarantees on their order.<br/>
-     * In other terms, this function can be used to impose the same order on two
-     * sets by using one of them as a master and the other one as a slave.
-     *
-     * Iterating the sparse set with a couple of iterators returns elements in
-     * the expected order after a call to `respect`. See `begin` and `end` for
-     * more details.
-     *
-     * @note
-     * Attempting to iterate elements using the raw pointer returned by `data`
-     * gives no guarantees on the order, even though `respect` has been invoked.
-     *
-     * @param other The sparse sets that imposes the order of the entities.
-     */
-    void respect(const sparse_set &other) ENTT_NOEXCEPT {
-        const auto to = other.end();
-        auto from = other.begin();
-
-        size_type pos = direct.size() - 1;
-
-        while(pos && from != to) {
-            if(has(*from)) {
-                if(*from != direct[pos]) {
-                    swap(pos, index(*from));
-                }
-
-                --pos;
-            }
-
-            ++from;
-        }
-    }
-
-    /**
-     * @brief Resets a sparse set.
-     */
-    void reset() {
-        reverse.clear();
-        direct.clear();
-    }
-
-private:
-    std::vector<std::unique_ptr<entity_type[]>> reverse;
-    std::vector<entity_type> direct;
-};
-
-
-}
-
-
-#endif // ENTT_ENTITY_SPARSE_SET_HPP
-
-// #include "entity.hpp"
 
 // #include "fwd.hpp"
 
@@ -3973,120 +17486,130 @@
  * have a valid reference and won't be updated accordingly).
  *
  * @warning
- * Lifetime of a view must overcome the one of the registry that generated it.
+ * Lifetime of a view must not overcome that of the registry that generated it.
  * In any other case, attempting to use a view results in undefined behavior.
  *
  * @tparam Entity A valid entity type (see entt_traits for more details).
  */
 template<typename Entity>
-class basic_runtime_view {
-    /*! @brief A registry is allowed to create views. */
-    friend class basic_registry<Entity>;
-
-    using underlying_iterator_type = typename sparse_set<Entity>::iterator_type;
-    using traits_type = entt_traits<std::underlying_type_t<Entity>>;
-
-    class iterator {
-        friend class basic_runtime_view<Entity>;
-
-        iterator(underlying_iterator_type first, underlying_iterator_type last, const sparse_set<Entity> * const *others, const sparse_set<Entity> * const *length) ENTT_NOEXCEPT
-            : begin{first},
-              end{last},
-              from{others},
-              to{length}
-        {
-            if(begin != end && !valid()) {
-                ++(*this);
-            }
-        }
-
-        bool valid() const ENTT_NOEXCEPT {
-            return std::all_of(from, to, [entt = *begin](const auto *view) {
-                return view->has(entt);
-            });
+class basic_runtime_view final {
+    using basic_common_type = basic_sparse_set<Entity>;
+    using underlying_iterator = typename basic_common_type::iterator;
+
+    class view_iterator final {
+        [[nodiscard]] bool valid() const {
+            const auto entt = *it;
+
+            return (!stable_storage || (entt != tombstone))
+                && std::all_of(pools->begin()++, pools->end(), [entt](const auto *curr) { return curr->contains(entt); })
+                && std::none_of(filter->cbegin(), filter->cend(), [entt](const auto *curr) { return curr && curr->contains(entt); });
         }
 
     public:
-        using difference_type = typename underlying_iterator_type::difference_type;
-        using value_type = typename underlying_iterator_type::value_type;
-        using pointer = typename underlying_iterator_type::pointer;
-        using reference = typename underlying_iterator_type::reference;
-        using iterator_category = std::forward_iterator_tag;
-
-        iterator() ENTT_NOEXCEPT = default;
-
-        iterator & operator++() ENTT_NOEXCEPT {
-            return (++begin != end && !valid()) ? ++(*this) : *this;
-        }
-
-        iterator operator++(int) ENTT_NOEXCEPT {
-            iterator orig = *this;
+        using difference_type = typename underlying_iterator::difference_type;
+        using value_type = typename underlying_iterator::value_type;
+        using pointer = typename underlying_iterator::pointer;
+        using reference = typename underlying_iterator::reference;
+        using iterator_category = std::bidirectional_iterator_tag;
+
+        view_iterator() ENTT_NOEXCEPT = default;
+
+        view_iterator(const std::vector<const basic_common_type *> &cpools, const std::vector<const basic_common_type *> &ignore, underlying_iterator curr) ENTT_NOEXCEPT
+            : pools{&cpools},
+              filter{&ignore},
+              it{curr},
+              stable_storage{std::any_of(pools->cbegin(), pools->cend(), [](const basic_common_type *cpool) { return (cpool->policy() == deletion_policy::in_place); })}
+        {
+            if(it != (*pools)[0]->end() && !valid()) {
+                ++(*this);
+            }
+        }
+
+        view_iterator & operator++() {
+            while(++it != (*pools)[0]->end() && !valid());
+            return *this;
+        }
+
+        view_iterator operator++(int) {
+            view_iterator orig = *this;
             return ++(*this), orig;
         }
 
-        bool operator==(const iterator &other) const ENTT_NOEXCEPT {
-            return other.begin == begin;
-        }
-
-        bool operator!=(const iterator &other) const ENTT_NOEXCEPT {
+        view_iterator & operator--() ENTT_NOEXCEPT {
+            while(--it != (*pools)[0]->begin() && !valid());
+            return *this;
+        }
+
+        view_iterator operator--(int) ENTT_NOEXCEPT {
+            view_iterator orig = *this;
+            return operator--(), orig;
+        }
+
+        [[nodiscard]] bool operator==(const view_iterator &other) const ENTT_NOEXCEPT {
+            return other.it == it;
+        }
+
+        [[nodiscard]] bool operator!=(const view_iterator &other) const ENTT_NOEXCEPT {
             return !(*this == other);
         }
 
-        pointer operator->() const ENTT_NOEXCEPT {
-            return begin.operator->();
-        }
-
-        reference operator*() const ENTT_NOEXCEPT {
+        [[nodiscard]] pointer operator->() const {
+            return it.operator->();
+        }
+
+        [[nodiscard]] reference operator*() const {
             return *operator->();
         }
 
     private:
-        underlying_iterator_type begin;
-        underlying_iterator_type end;
-        const sparse_set<Entity> * const *from;
-        const sparse_set<Entity> * const *to;
-    };
-
-    basic_runtime_view(std::vector<const sparse_set<Entity> *> others) ENTT_NOEXCEPT
-        : pools{std::move(others)}
-    {
-        const auto it = std::min_element(pools.begin(), pools.end(), [](const auto *lhs, const auto *rhs) {
-            return (!lhs && rhs) || (lhs && rhs && lhs->size() < rhs->size());
-        });
-
-        // brings the best candidate (if any) on front of the vector
-        std::rotate(pools.begin(), it, pools.end());
-    }
-
-    bool valid() const ENTT_NOEXCEPT {
+        const std::vector<const basic_common_type *> *pools;
+        const std::vector<const basic_common_type *> *filter;
+        underlying_iterator it;
+        bool stable_storage;
+    };
+
+    [[nodiscard]] bool valid() const {
         return !pools.empty() && pools.front();
     }
 
 public:
     /*! @brief Underlying entity identifier. */
-    using entity_type = typename sparse_set<Entity>::entity_type;
+    using entity_type = Entity;
     /*! @brief Unsigned integer type. */
-    using size_type = typename sparse_set<Entity>::size_type;
-    /*! @brief Input iterator type. */
-    using iterator_type = iterator;
-
-    /**
-     * @brief Estimates the number of entities that have the given components.
-     * @return Estimated number of entities that have the given components.
-     */
-    size_type size() const ENTT_NOEXCEPT {
+    using size_type = std::size_t;
+    /*! @brief Bidirectional iterator type. */
+    using iterator = view_iterator;
+
+    /*! @brief Default constructor to use to create empty, invalid views. */
+    basic_runtime_view() ENTT_NOEXCEPT
+        : pools{},
+          filter{}
+    {}
+
+    /**
+     * @brief Constructs a runtime view from a set of storage classes.
+     * @param cpools The storage for the types to iterate.
+     * @param epools The storage for the types used to filter the view.
+     */
+    basic_runtime_view(std::vector<const basic_common_type *> cpools, std::vector<const basic_common_type *> epools) ENTT_NOEXCEPT
+        : pools{std::move(cpools)},
+          filter{std::move(epools)}
+    {
+        // brings the best candidate (if any) on front of the vector
+        std::rotate(pools.begin(), std::min_element(pools.begin(), pools.end(), [](const auto *lhs, const auto *rhs) {
+            return (!lhs && rhs) || (lhs && rhs && lhs->size() < rhs->size());
+        }), pools.end());
+    }
+
+    /**
+     * @brief Estimates the number of entities iterated by the view.
+     * @return Estimated number of entities iterated by the view.
+     */
+    [[nodiscard]] size_type size_hint() const {
         return valid() ? pools.front()->size() : size_type{};
     }
 
     /**
-     * @brief Checks if the view is definitely empty.
-     * @return True if the view is definitely empty, false otherwise.
-     */
-    bool empty() const ENTT_NOEXCEPT {
-        return !valid() || pools.front()->empty();
-    }
-
-    /**
      * @brief Returns an iterator to the first entity that has the given
      * components.
      *
@@ -4094,22 +17617,10 @@
      * components. If the view is empty, the returned iterator will be equal to
      * `end()`.
      *
-     * @note
-     * Input iterators stay true to the order imposed to the underlying data
-     * structures.
-     *
      * @return An iterator to the first entity that has the given components.
      */
-    iterator_type begin() const ENTT_NOEXCEPT {
-        iterator_type it{};
-
-        if(valid()) {
-            const auto &pool = *pools.front();
-            const auto * const *data = pools.data();
-            it = { pool.begin(), pool.end(), data + 1, data + pools.size() };
-        }
-
-        return it;
+    [[nodiscard]] iterator begin() const {
+        return valid() ? iterator{pools, filter, pools[0]->begin()} : iterator{};
     }
 
     /**
@@ -4120,22 +17631,11 @@
      * has the given components. Attempting to dereference the returned iterator
      * results in undefined behavior.
      *
-     * @note
-     * Input iterators stay true to the order imposed to the underlying data
-     * structures.
-     *
      * @return An iterator to the entity following the last entity that has the
      * given components.
      */
-    iterator_type end() const ENTT_NOEXCEPT {
-        iterator_type it{};
-
-        if(valid()) {
-            const auto &pool = *pools.front();
-            it = { pool.end(), pool.end(), nullptr, nullptr };
-        }
-
-        return it;
+    [[nodiscard]] iterator end() const {
+        return valid() ? iterator{pools, filter, pools[0]->end()} : iterator{};
     }
 
     /**
@@ -4143,10 +17643,9 @@
      * @param entt A valid entity identifier.
      * @return True if the view contains the given entity, false otherwise.
      */
-    bool contains(const entity_type entt) const ENTT_NOEXCEPT {
-        return valid() && std::all_of(pools.cbegin(), pools.cend(), [entt](const auto *view) {
-            return view->find(entt) != view->end();
-        });
+    [[nodiscard]] bool contains(const entity_type entt) const {
+        return valid() && std::all_of(pools.cbegin(), pools.cend(), [entt](const auto *curr) { return curr->contains(entt); })
+                && std::none_of(filter.cbegin(), filter.cend(), [entt](const auto *curr) { return curr && curr->contains(entt); });
     }
 
     /**
@@ -4166,38 +17665,6884 @@
      */
     template<typename Func>
     void each(Func func) const {
-        std::for_each(begin(), end(), func);
-    }
-
-private:
-    std::vector<const sparse_set<Entity> *> pools;
-};
-
-
-}
-
-
-#endif // ENTT_ENTITY_RUNTIME_VIEW_HPP
+        for(const auto entity: *this) {
+            func(entity);
+        }
+    }
+
+private:
+    std::vector<const basic_common_type *> pools;
+    std::vector<const basic_common_type *> filter;
+};
+
+
+}
+
+
+#endif
+
+// #include "sparse_set.hpp"
+
+// #include "storage.hpp"
+
+// #include "utility.hpp"
+
+// #include "view.hpp"
+#ifndef ENTT_ENTITY_VIEW_HPP
+#define ENTT_ENTITY_VIEW_HPP
+
+
+#include <iterator>
+#include <array>
+#include <tuple>
+#include <utility>
+#include <algorithm>
+#include <type_traits>
+// #include "../config/config.h"
+
+// #include "../core/type_traits.hpp"
+
+// #include "component.hpp"
+
+// #include "entity.hpp"
+
+// #include "fwd.hpp"
 
 // #include "sparse_set.hpp"
 
-// #include "snapshot.hpp"
+// #include "storage.hpp"
+
+// #include "utility.hpp"
+
+
+
+namespace entt {
+
+
+/**
+ * @cond TURN_OFF_DOXYGEN
+ * Internal details not to be documented.
+ */
+
+
+namespace internal {
+
+
+template<typename Policy, typename It, std::size_t AllOf, std::size_t NoneOf>
+class view_iterator final {
+    using basic_common_type = basic_sparse_set<typename std::iterator_traits<It>::value_type>;
+
+    [[nodiscard]] bool valid() const {
+        const auto entt = *it;
+        return Policy::accept(entt)
+            && std::apply([entt](const auto *... curr) { return (curr->contains(entt) && ...); }, pools)
+            && std::apply([entt](const auto *... curr) { return (!curr->contains(entt) && ...); }, filter);
+    }
+
+public:
+    using iterator_type = It;
+    using difference_type = typename std::iterator_traits<It>::difference_type;
+    using value_type = typename std::iterator_traits<It>::value_type;
+    using pointer = typename std::iterator_traits<It>::pointer;
+    using reference = typename std::iterator_traits<It>::reference;
+    using iterator_category = std::bidirectional_iterator_tag;
+
+    view_iterator() ENTT_NOEXCEPT
+        : first{},
+          last{},
+          it{},
+          pools{},
+          filter{}
+    {}
+
+    view_iterator(It from, It to, It curr, std::array<const basic_common_type *, AllOf> all_of, std::array<const basic_common_type *, NoneOf> none_of) ENTT_NOEXCEPT
+        : first{from},
+          last{to},
+          it{curr},
+          pools{all_of},
+          filter{none_of}
+    {
+        if(it != last && !valid()) {
+            ++(*this);
+        }
+    }
+
+    view_iterator & operator++() ENTT_NOEXCEPT {
+        while(++it != last && !valid());
+        return *this;
+    }
+
+    view_iterator operator++(int) ENTT_NOEXCEPT {
+        view_iterator orig = *this;
+        return ++(*this), orig;
+    }
+
+    view_iterator & operator--() ENTT_NOEXCEPT {
+        while(--it != first && !valid());
+        return *this;
+    }
+
+    view_iterator operator--(int) ENTT_NOEXCEPT {
+        view_iterator orig = *this;
+        return operator--(), orig;
+    }
+
+    [[nodiscard]] bool operator==(const view_iterator &other) const ENTT_NOEXCEPT {
+        return other.it == it;
+    }
+
+    [[nodiscard]] bool operator!=(const view_iterator &other) const ENTT_NOEXCEPT {
+        return !(*this == other);
+    }
+
+    [[nodiscard]] pointer operator->() const {
+        return &*it;
+    }
+
+    [[nodiscard]] reference operator*() const {
+        return *operator->();
+    }
+
+private:
+    It first;
+    It last;
+    It it;
+    std::array<const basic_common_type *, AllOf> pools;
+    std::array<const basic_common_type *, NoneOf> filter;
+};
+
+
+}
+
+
+/**
+ * Internal details not to be documented.
+ * @endcond
+ */
+
+
+/*! @brief Stable storage policy, aimed at pointer stability. */
+struct stable_storage_policy {
+    /**
+    * @cond TURN_OFF_DOXYGEN
+    * Internal details not to be documented.
+    */
+    template<typename Entity>
+    [[nodiscard]] static constexpr bool accept(const Entity entity) ENTT_NOEXCEPT {
+        return entity != tombstone;
+    }
+    /**
+    * Internal details not to be documented.
+    * @endcond
+    */
+};
+
+
+/*! @brief Packed storage policy, aimed at faster linear iteration. */
+struct packed_storage_policy {
+    /**
+    * @cond TURN_OFF_DOXYGEN
+    * Internal details not to be documented.
+    */
+    template<typename Entity>
+    [[nodiscard]] static constexpr bool accept(const Entity) ENTT_NOEXCEPT {
+        return true;
+    }
+    /**
+    * Internal details not to be documented.
+    * @endcond
+    */
+};
+
+
+/**
+ * @brief View implementation.
+ *
+ * Primary template isn't defined on purpose. All the specializations give a
+ * compile-time error, but for a few reasonable cases.
+ */
+template<typename...>
+class basic_view_impl;
+
+
+/*! @brief View implementation dispatcher. */
+template<typename...>
+struct basic_view;
+
+
+/**
+ * @brief Multi component view.
+ *
+ * Multi component views iterate over those entities that have at least all the
+ * given components in their bags. During initialization, a multi component view
+ * looks at the number of entities available for each component and uses the
+ * smallest set in order to get a performance boost when iterate.
+ *
+ * @b Important
+ *
+ * Iterators aren't invalidated if:
+ *
+ * * New instances of the given components are created and assigned to entities.
+ * * The entity currently pointed is modified (as an example, if one of the
+ *   given components is removed from the entity to which the iterator points).
+ * * The entity currently pointed is destroyed.
+ *
+ * In all other cases, modifying the pools iterated by the view in any way
+ * invalidates all the iterators and using them results in undefined behavior.
+ *
+ * @note
+ * Views share references to the underlying data structures of the registry that
+ * generated them. Therefore any change to the entities and to the components
+ * made by means of the registry are immediately reflected by views.
+ *
+ * @warning
+ * Lifetime of a view must not overcome that of the registry that generated it.
+ * In any other case, attempting to use a view results in undefined behavior.
+ *
+ * @tparam Policy Common (stricter) storage policy.
+ * @tparam Entity A valid entity type (see entt_traits for more details).
+ * @tparam Exclude Types of components used to filter the view.
+ * @tparam Component Types of components iterated by the view.
+ */
+template<typename Policy, typename Entity, typename... Exclude, typename... Component>
+class basic_view_impl<Policy, Entity, exclude_t<Exclude...>, Component...> {
+    using basic_common_type = basic_sparse_set<Entity>;
+
+    template<typename Comp>
+    using storage_type = constness_as_t<typename storage_traits<Entity, std::remove_const_t<Comp>>::storage_type, Comp>;
+
+    class iterable final {
+        template<typename It>
+        struct iterable_iterator final {
+            using difference_type = std::ptrdiff_t;
+            using value_type = decltype(std::tuple_cat(std::tuple<Entity>{}, std::declval<basic_view_impl>().get({})));
+            using pointer = void;
+            using reference = value_type;
+            using iterator_category = std::input_iterator_tag;
+
+            iterable_iterator(It from, const basic_view_impl *parent) ENTT_NOEXCEPT
+                : it{from},
+                  view{parent}
+            {}
+
+            iterable_iterator & operator++() ENTT_NOEXCEPT {
+                return ++it, *this;
+            }
+
+            iterable_iterator operator++(int) ENTT_NOEXCEPT {
+                iterable_iterator orig = *this;
+                return ++(*this), orig;
+            }
+
+            [[nodiscard]] reference operator*() const ENTT_NOEXCEPT {
+                return std::tuple_cat(std::make_tuple(*it), view->get(*it));
+            }
+
+            [[nodiscard]] bool operator==(const iterable_iterator &other) const ENTT_NOEXCEPT {
+                return other.it == it;
+            }
+
+            [[nodiscard]] bool operator!=(const iterable_iterator &other) const ENTT_NOEXCEPT {
+                return !(*this == other);
+            }
+
+        private:
+            It it;
+            const basic_view_impl *view;
+        };
+
+    public:
+        using iterator = iterable_iterator<internal::view_iterator<Policy, typename basic_common_type::iterator, sizeof...(Component) - 1u, sizeof...(Exclude)>>;
+        using reverse_iterator = iterable_iterator<internal::view_iterator<Policy, typename basic_common_type::reverse_iterator, sizeof...(Component) - 1u, sizeof...(Exclude)>>;
+
+        iterable(const basic_view_impl &parent)
+            : view{parent}
+        {}
+
+        [[nodiscard]] iterator begin() const ENTT_NOEXCEPT {
+            return { view.begin(), &view };
+        }
+
+        [[nodiscard]] iterator end() const ENTT_NOEXCEPT {
+            return { view.end(), &view };
+        }
+
+        [[nodiscard]] reverse_iterator rbegin() const ENTT_NOEXCEPT {
+            return { view.rbegin(), &view };
+        }
+
+        [[nodiscard]] reverse_iterator rend() const ENTT_NOEXCEPT {
+            return { view.rend(), &view };
+        }
+
+    private:
+        const basic_view_impl view;
+    };
+
+    [[nodiscard]] const auto * candidate() const ENTT_NOEXCEPT {
+        return (std::min)({ static_cast<const basic_common_type *>(std::get<storage_type<Component> *>(pools))... }, [](const auto *lhs, const auto *rhs) {
+            return lhs->size() < rhs->size();
+        });
+    }
+
+    [[nodiscard]] auto pools_to_unchecked_array() const ENTT_NOEXCEPT {
+        std::size_t pos{};
+        std::array<const basic_common_type *, sizeof...(Component) - 1u> other{};
+        (static_cast<void>(std::get<storage_type<Component> *>(pools) == view ? void() : void(other[pos++] = std::get<storage_type<Component> *>(pools))), ...);
+        return other;
+    }
+
+    [[nodiscard]] auto filter_to_array() const ENTT_NOEXCEPT {
+        return std::array<const basic_common_type *, sizeof...(Exclude)>{std::get<const storage_type<Exclude> *>(filter)...};
+    }
+
+    template<typename Comp, typename It>
+    [[nodiscard]] auto dispatch_get([[maybe_unused]] It &it, [[maybe_unused]] const Entity entt) const {
+        if constexpr(std::is_same_v<typename std::iterator_traits<It>::value_type, typename storage_type<Comp>::value_type>) {
+            return std::forward_as_tuple(*it);
+        } else {
+            return get_as_tuple(*std::get<storage_type<Comp> *>(pools), entt);
+        }
+    }
+
+    template<typename Comp, typename Func>
+    void traverse(Func func) const {
+        if constexpr(std::is_void_v<decltype(std::get<storage_type<Comp> *>(pools)->get({}))>) {
+            for(const auto entt: static_cast<const basic_common_type &>(*std::get<storage_type<Comp> *>(pools))) {
+                if(Policy::accept(entt) && ((std::is_same_v<Comp, Component> || std::get<storage_type<Component> *>(pools)->contains(entt)) && ...)
+                    && (!std::get<const storage_type<Exclude> *>(filter)->contains(entt) && ...))
+                {
+                    if constexpr(is_applicable_v<Func, decltype(std::tuple_cat(std::tuple<entity_type>{}, std::declval<basic_view_impl>().get({})))>) {
+                        std::apply(func, std::tuple_cat(std::make_tuple(entt), get(entt)));
+                    } else {
+                        std::apply(func, get(entt));
+                    }
+                }
+            }
+        } else {
+            auto it = std::get<storage_type<Comp> *>(pools)->begin();
+
+            for(const auto entt: static_cast<const basic_common_type &>(*std::get<storage_type<Comp> *>(pools))) {
+                if(Policy::accept(entt) && ((std::is_same_v<Comp, Component> || std::get<storage_type<Component> *>(pools)->contains(entt)) && ...)
+                    && (!std::get<const storage_type<Exclude> *>(filter)->contains(entt) && ...))
+                {
+                    if constexpr(is_applicable_v<Func, decltype(std::tuple_cat(std::tuple<entity_type>{}, std::declval<basic_view_impl>().get({})))>) {
+                        std::apply(func, std::tuple_cat(std::make_tuple(entt), dispatch_get<Component>(it, entt)...));
+                    } else {
+                        std::apply(func, std::tuple_cat(dispatch_get<Component>(it, entt)...));
+                    }
+                }
+
+                ++it;
+            }
+        }
+    }
+
+public:
+    /*! @brief Underlying entity identifier. */
+    using entity_type = Entity;
+    /*! @brief Unsigned integer type. */
+    using size_type = std::size_t;
+    /*! @brief Bidirectional iterator type. */
+    using iterator = internal::view_iterator<Policy, typename basic_common_type::iterator, sizeof...(Component) - 1u, sizeof...(Exclude)>;
+    /*! @brief Reverse iterator type. */
+    using reverse_iterator = internal::view_iterator<Policy, typename basic_common_type::reverse_iterator, sizeof...(Component) - 1u, sizeof...(Exclude)>;
+    /*! @brief Iterable view type. */
+    using iterable_view = iterable;
+
+    /*! @brief Default constructor to use to create empty, invalid views. */
+    basic_view_impl() ENTT_NOEXCEPT
+        : view{}
+    {}
+
+    /**
+     * @brief Constructs a multi-type view from a set of storage classes.
+     * @param component The storage for the types to iterate.
+     * @param epool The storage for the types used to filter the view.
+     */
+    basic_view_impl(storage_type<Component> &... component, const storage_type<Exclude> &... epool) ENTT_NOEXCEPT
+        : pools{&component...},
+          filter{&epool...},
+          view{candidate()}
+    {}
+
+    /**
+     * @brief Forces the type to use to drive iterations.
+     * @tparam Comp Type of component to use to drive the iteration.
+     */
+    template<typename Comp>
+    void use() const ENTT_NOEXCEPT {
+        view = std::get<storage_type<Comp> *>(pools);
+    }
+
+    /**
+     * @brief Estimates the number of entities iterated by the view.
+     * @return Estimated number of entities iterated by the view.
+     */
+    [[nodiscard]] size_type size_hint() const ENTT_NOEXCEPT {
+        return view->size();
+    }
+
+    /**
+     * @brief Returns an iterator to the first entity of the view.
+     *
+     * The returned iterator points to the first entity of the view. If the view
+     * is empty, the returned iterator will be equal to `end()`.
+     *
+     * @return An iterator to the first entity of the view.
+     */
+    [[nodiscard]] iterator begin() const {
+        return iterator{view->begin(), view->end(), view->begin(), pools_to_unchecked_array(), filter_to_array()};
+    }
+
+    /**
+     * @brief Returns an iterator that is past the last entity of the view.
+     *
+     * The returned iterator points to the entity following the last entity of
+     * the view. Attempting to dereference the returned iterator results in
+     * undefined behavior.
+     *
+     * @return An iterator to the entity following the last entity of the view.
+     */
+    [[nodiscard]] iterator end() const {
+        return iterator{view->begin(), view->end(), view->end(), pools_to_unchecked_array(), filter_to_array()};
+    }
+
+    /**
+     * @brief Returns an iterator to the first entity of the reversed view.
+     *
+     * The returned iterator points to the first entity of the reversed view. If
+     * the view is empty, the returned iterator will be equal to `rend()`.
+     *
+     * @return An iterator to the first entity of the reversed view.
+     */
+    [[nodiscard]] reverse_iterator rbegin() const {
+        return reverse_iterator{view->rbegin(), view->rend(), view->rbegin(), pools_to_unchecked_array(), filter_to_array()};
+    }
+
+    /**
+     * @brief Returns an iterator that is past the last entity of the reversed
+     * view.
+     *
+     * The returned iterator points to the entity following the last entity of
+     * the reversed view. Attempting to dereference the returned iterator
+     * results in undefined behavior.
+     *
+     * @return An iterator to the entity following the last entity of the
+     * reversed view.
+     */
+    [[nodiscard]] reverse_iterator rend() const {
+        return reverse_iterator{view->rbegin(), view->rend(), view->rend(), pools_to_unchecked_array(), filter_to_array()};
+    }
+
+    /**
+     * @brief Returns the first entity of the view, if any.
+     * @return The first entity of the view if one exists, the null entity
+     * otherwise.
+     */
+    [[nodiscard]] entity_type front() const {
+        const auto it = begin();
+        return it != end() ? *it : null;
+    }
+
+    /**
+     * @brief Returns the last entity of the view, if any.
+     * @return The last entity of the view if one exists, the null entity
+     * otherwise.
+     */
+    [[nodiscard]] entity_type back() const {
+        const auto it = rbegin();
+        return it != rend() ? *it : null;
+    }
+
+    /**
+     * @brief Finds an entity.
+     * @param entt A valid entity identifier.
+     * @return An iterator to the given entity if it's found, past the end
+     * iterator otherwise.
+     */
+    [[nodiscard]] iterator find(const entity_type entt) const {
+        const auto it = iterator{view->begin(), view->end(), view->find(entt), pools_to_unchecked_array(), filter_to_array()};
+        return (it != end() && *it == entt) ? it : end();
+    }
+
+    /**
+     * @brief Checks if a view is properly initialized.
+     * @return True if the view is properly initialized, false otherwise.
+     */
+    [[nodiscard]] explicit operator bool() const ENTT_NOEXCEPT {
+        return view != nullptr;
+    }
+
+    /**
+     * @brief Checks if a view contains an entity.
+     * @param entt A valid entity identifier.
+     * @return True if the view contains the given entity, false otherwise.
+     */
+    [[nodiscard]] bool contains(const entity_type entt) const {
+        return (std::get<storage_type<Component> *>(pools)->contains(entt) && ...) && (!std::get<const storage_type<Exclude> *>(filter)->contains(entt) && ...);
+    }
+
+    /**
+     * @brief Returns the components assigned to the given entity.
+     *
+     * Prefer this function instead of `registry::get` during iterations. It has
+     * far better performance than its counterpart.
+     *
+     * @warning
+     * Attempting to use an invalid component type results in a compilation
+     * error. Attempting to use an entity that doesn't belong to the view
+     * results in undefined behavior.
+     *
+     * @tparam Comp Types of components to get.
+     * @param entt A valid entity identifier.
+     * @return The components assigned to the entity.
+     */
+    template<typename... Comp>
+    [[nodiscard]] decltype(auto) get([[maybe_unused]] const entity_type entt) const {
+        ENTT_ASSERT(contains(entt), "View does not contain entity");
+
+        if constexpr(sizeof...(Comp) == 0) {
+            return std::tuple_cat(get_as_tuple(*std::get<storage_type<Component> *>(pools), entt)...);
+        } else if constexpr(sizeof...(Comp) == 1) {
+            return (std::get<storage_type<Comp> *>(pools)->get(entt), ...);
+        } else {
+            return std::tuple_cat(get_as_tuple(*std::get<storage_type<Comp> *>(pools), entt)...);
+        }
+    }
+
+    /**
+     * @brief Iterates entities and components and applies the given function
+     * object to them.
+     *
+     * The function object is invoked for each entity. It is provided with the
+     * entity itself and a set of references to non-empty components. The
+     * _constness_ of the components is as requested.<br/>
+     * The signature of the function must be equivalent to one of the following
+     * forms:
+     *
+     * @code{.cpp}
+     * void(const entity_type, Type &...);
+     * void(Type &...);
+     * @endcode
+     *
+     * @note
+     * Empty types aren't explicitly instantiated and therefore they are never
+     * returned during iterations.
+     *
+     * @tparam Func Type of the function object to invoke.
+     * @param func A valid function object.
+     */
+    template<typename Func>
+    void each(Func func) const {
+        ((std::get<storage_type<Component> *>(pools) == view ? traverse<Component>(std::move(func)) : void()), ...);
+    }
+
+    /**
+     * @brief Iterates entities and components and applies the given function
+     * object to them.
+     *
+     * The pool of the suggested component is used to lead the iterations. The
+     * returned entities will therefore respect the order of the pool associated
+     * with that type.
+     *
+     * @sa each
+     *
+     * @tparam Comp Type of component to use to drive the iteration.
+     * @tparam Func Type of the function object to invoke.
+     * @param func A valid function object.
+     */
+    template<typename Comp, typename Func>
+    void each(Func func) const {
+        use<Comp>();
+        traverse<Comp>(std::move(func));
+    }
+
+    /**
+     * @brief Returns an iterable object to use to _visit_ the view.
+     *
+     * The iterable object returns tuples that contain the current entity and a
+     * set of references to its non-empty components. The _constness_ of the
+     * components is as requested.
+     *
+     * @note
+     * Empty types aren't explicitly instantiated and therefore they are never
+     * returned during iterations.
+     *
+     * @return An iterable object to use to _visit_ the view.
+     */
+    [[nodiscard]] iterable_view each() const ENTT_NOEXCEPT {
+        return iterable_view{*this};
+    }
+
+    /**
+     * @brief Returns an iterable object to use to _visit_ the view.
+     *
+     * The pool of the suggested component is used to lead the iterations. The
+     * returned elements will therefore respect the order of the pool associated
+     * with that type.
+     *
+     * @sa each
+     *
+     * @tparam Comp Type of component to use to drive the iteration.
+     * @return An iterable object to use to _visit_ the view.
+     */
+    template<typename Comp>
+    [[nodiscard]] iterable_view each() const ENTT_NOEXCEPT {
+        use<Comp>();
+        return iterable_view{*this};
+    }
+
+    /**
+     * @brief Combines two views in a _more specific_ one (friend function).
+     * @tparam Id A valid entity type (see entt_traits for more details).
+     * @tparam ELhs Filter list of the first view.
+     * @tparam CLhs Component list of the first view.
+     * @tparam ERhs Filter list of the second view.
+     * @tparam CRhs Component list of the second view.
+     * @return A more specific view.
+     */
+    template<typename Id, typename... ELhs, typename... CLhs, typename... ERhs, typename... CRhs>
+    friend auto operator|(const basic_view<Id, exclude_t<ELhs...>, CLhs...> &, const basic_view<Id, exclude_t<ERhs...>, CRhs...> &);
+
+private:
+    const std::tuple<storage_type<Component> *...> pools;
+    const std::tuple<const storage_type<Exclude> *...> filter;
+    mutable const basic_common_type *view;
+};
+
+
+/**
+ * @brief Single component view specialization.
+ *
+ * Single component views are specialized in order to get a boost in terms of
+ * performance. This kind of views can access the underlying data structure
+ * directly and avoid superfluous checks.
+ *
+ * @b Important
+ *
+ * Iterators aren't invalidated if:
+ *
+ * * New instances of the given component are created and assigned to entities.
+ * * The entity currently pointed is modified (as an example, the given
+ *   component is removed from the entity to which the iterator points).
+ * * The entity currently pointed is destroyed.
+ *
+ * In all other cases, modifying the pool iterated by the view in any way
+ * invalidates all the iterators and using them results in undefined behavior.
+ *
+ * @note
+ * Views share a reference to the underlying data structure of the registry that
+ * generated them. Therefore any change to the entities and to the components
+ * made by means of the registry are immediately reflected by views.
+ *
+ * @warning
+ * Lifetime of a view must not overcome that of the registry that generated it.
+ * In any other case, attempting to use a view results in undefined behavior.
+ *
+ * @tparam Entity A valid entity type (see entt_traits for more details).
+ * @tparam Component Type of component iterated by the view.
+ */
+template<typename Entity, typename Component>
+class basic_view_impl<packed_storage_policy, Entity, exclude_t<>, Component> {
+    using basic_common_type = basic_sparse_set<Entity>;
+    using storage_type = constness_as_t<typename storage_traits<Entity, std::remove_const_t<Component>>::storage_type, Component>;
+
+    class iterable final {
+        template<typename... It>
+        struct iterable_iterator final {
+            using difference_type = std::ptrdiff_t;
+            using value_type = decltype(std::tuple_cat(std::tuple<Entity>{}, std::declval<basic_view_impl>().get({})));
+            using pointer = void;
+            using reference = value_type;
+            using iterator_category = std::input_iterator_tag;
+
+            template<typename... Discard>
+            iterable_iterator(It... from, Discard...) ENTT_NOEXCEPT
+                : it{from...}
+            {}
+
+            iterable_iterator & operator++() ENTT_NOEXCEPT {
+                return (++std::get<It>(it), ...), *this;
+            }
+
+            iterable_iterator operator++(int) ENTT_NOEXCEPT {
+                iterable_iterator orig = *this;
+                return ++(*this), orig;
+            }
+
+            [[nodiscard]] reference operator*() const ENTT_NOEXCEPT {
+                return { *std::get<It>(it)... };
+            }
+
+            [[nodiscard]] bool operator==(const iterable_iterator &other) const ENTT_NOEXCEPT {
+                return std::get<0>(other.it) == std::get<0>(it);
+            }
+
+            [[nodiscard]] bool operator!=(const iterable_iterator &other) const ENTT_NOEXCEPT {
+                return !(*this == other);
+            }
+
+        private:
+            std::tuple<It...> it;
+        };
+
+    public:
+        using iterator = std::conditional_t<
+            std::is_void_v<decltype(std::declval<storage_type>().get({}))>,
+            iterable_iterator<typename basic_common_type::iterator>,
+            iterable_iterator<typename basic_common_type::iterator, decltype(std::declval<storage_type>().begin())>
+        >;
+        using reverse_iterator = std::conditional_t<
+            std::is_void_v<decltype(std::declval<storage_type>().get({}))>,
+            iterable_iterator<typename basic_common_type::reverse_iterator>,
+            iterable_iterator<typename basic_common_type::reverse_iterator, decltype(std::declval<storage_type>().rbegin())>
+        >;
+
+        iterable(storage_type &ref)
+            : pool{&ref}
+        {}
+
+        [[nodiscard]] iterator begin() const ENTT_NOEXCEPT {
+            return iterator{pool->basic_common_type::begin(), pool->begin()};
+        }
+
+        [[nodiscard]] iterator end() const ENTT_NOEXCEPT {
+            return iterator{pool->basic_common_type::end(), pool->end()};
+        }
+
+        [[nodiscard]] reverse_iterator rbegin() const ENTT_NOEXCEPT {
+            return reverse_iterator{pool->basic_common_type::rbegin(), pool->rbegin()};
+        }
+
+        [[nodiscard]] reverse_iterator rend() const ENTT_NOEXCEPT {
+            return reverse_iterator{pool->basic_common_type::rend(), pool->rend()};
+        }
+
+    private:
+        storage_type * const pool;
+    };
+
+public:
+    /*! @brief Underlying entity identifier. */
+    using entity_type = Entity;
+    /*! @brief Unsigned integer type. */
+    using size_type = std::size_t;
+    /*! @brief Random access iterator type. */
+    using iterator = typename basic_common_type::iterator;
+    /*! @brief Reversed iterator type. */
+    using reverse_iterator = typename basic_common_type::reverse_iterator;
+    /*! @brief Iterable view type. */
+    using iterable_view = iterable;
+
+    /*! @brief Default constructor to use to create empty, invalid views. */
+    basic_view_impl() ENTT_NOEXCEPT
+        : pools{},
+          filter{}
+    {}
+
+    /**
+     * @brief Constructs a single-type view from a storage class.
+     * @param ref The storage for the type to iterate.
+     */
+    basic_view_impl(storage_type &ref) ENTT_NOEXCEPT
+        : pools{&ref},
+          filter{}
+    {}
+
+    /**
+     * @brief Returns the number of entities that have the given component.
+     * @return Number of entities that have the given component.
+     */
+    [[nodiscard]] size_type size() const ENTT_NOEXCEPT {
+        return std::get<0>(pools)->size();
+    }
+
+    /**
+     * @brief Checks whether a view is empty.
+     * @return True if the view is empty, false otherwise.
+     */
+    [[nodiscard]] bool empty() const ENTT_NOEXCEPT {
+        return std::get<0>(pools)->empty();
+    }
+
+    /**
+     * @brief Direct access to the raw representation offered by the storage.
+     * @return A pointer to the array of components.
+     */
+    [[nodiscard]] auto raw() const ENTT_NOEXCEPT {
+        return std::get<0>(pools)->raw();
+    }
+
+    /**
+     * @brief Direct access to the list of entities.
+     *
+     * The returned pointer is such that range `[data(), data() + size())` is
+     * always a valid range, even if the container is empty.
+     *
+     * @return A pointer to the array of entities.
+     */
+    [[nodiscard]] auto data() const ENTT_NOEXCEPT {
+        return std::get<0>(pools)->data();
+    }
+
+    /**
+     * @brief Returns an iterator to the first entity of the view.
+     *
+     * The returned iterator points to the first entity of the view. If the view
+     * is empty, the returned iterator will be equal to `end()`.
+     *
+     * @return An iterator to the first entity of the view.
+     */
+    [[nodiscard]] iterator begin() const ENTT_NOEXCEPT {
+        return std::get<0>(pools)->basic_common_type::begin();
+    }
+
+    /**
+     * @brief Returns an iterator that is past the last entity of the view.
+     *
+     * The returned iterator points to the entity following the last entity of
+     * the view. Attempting to dereference the returned iterator results in
+     * undefined behavior.
+     *
+     * @return An iterator to the entity following the last entity of the view.
+     */
+    [[nodiscard]] iterator end() const ENTT_NOEXCEPT {
+        return std::get<0>(pools)->basic_common_type::end();
+    }
+
+    /**
+     * @brief Returns an iterator to the first entity of the reversed view.
+     *
+     * The returned iterator points to the first entity of the reversed view. If
+     * the view is empty, the returned iterator will be equal to `rend()`.
+     *
+     * @return An iterator to the first entity of the reversed view.
+     */
+    [[nodiscard]] reverse_iterator rbegin() const ENTT_NOEXCEPT {
+        return std::get<0>(pools)->basic_common_type::rbegin();
+    }
+
+    /**
+     * @brief Returns an iterator that is past the last entity of the reversed
+     * view.
+     *
+     * The returned iterator points to the entity following the last entity of
+     * the reversed view. Attempting to dereference the returned iterator
+     * results in undefined behavior.
+     *
+     * @return An iterator to the entity following the last entity of the
+     * reversed view.
+     */
+    [[nodiscard]] reverse_iterator rend() const ENTT_NOEXCEPT {
+        return std::get<0>(pools)->basic_common_type::rend();
+    }
+
+    /**
+     * @brief Returns the first entity of the view, if any.
+     * @return The first entity of the view if one exists, the null entity
+     * otherwise.
+     */
+    [[nodiscard]] entity_type front() const {
+        const auto it = begin();
+        return it != end() ? *it : null;
+    }
+
+    /**
+     * @brief Returns the last entity of the view, if any.
+     * @return The last entity of the view if one exists, the null entity
+     * otherwise.
+     */
+    [[nodiscard]] entity_type back() const {
+        const auto it = rbegin();
+        return it != rend() ? *it : null;
+    }
+
+    /**
+     * @brief Finds an entity.
+     * @param entt A valid entity identifier.
+     * @return An iterator to the given entity if it's found, past the end
+     * iterator otherwise.
+     */
+    [[nodiscard]] iterator find(const entity_type entt) const {
+        const auto it = std::get<0>(pools)->find(entt);
+        return it != end() && *it == entt ? it : end();
+    }
+
+    /**
+     * @brief Returns the identifier that occupies the given position.
+     * @param pos Position of the element to return.
+     * @return The identifier that occupies the given position.
+     */
+    [[nodiscard]] entity_type operator[](const size_type pos) const {
+        return begin()[pos];
+    }
+
+    /**
+     * @brief Checks if a view is properly initialized.
+     * @return True if the view is properly initialized, false otherwise.
+     */
+    [[nodiscard]] explicit operator bool() const ENTT_NOEXCEPT {
+        return std::get<0>(pools) != nullptr;
+    }
+
+    /**
+     * @brief Checks if a view contains an entity.
+     * @param entt A valid entity identifier.
+     * @return True if the view contains the given entity, false otherwise.
+     */
+    [[nodiscard]] bool contains(const entity_type entt) const {
+        return std::get<0>(pools)->contains(entt);
+    }
+
+    /**
+     * @brief Returns the component assigned to the given entity.
+     *
+     * Prefer this function instead of `registry::get` during iterations. It has
+     * far better performance than its counterpart.
+     *
+     * @warning
+     * Attempting to use an invalid component type results in a compilation
+     * error. Attempting to use an entity that doesn't belong to the view
+     * results in undefined behavior.
+     *
+     * @tparam Comp Types of components to get.
+     * @param entt A valid entity identifier.
+     * @return The component assigned to the entity.
+     */
+    template<typename... Comp>
+    [[nodiscard]] decltype(auto) get(const entity_type entt) const {
+        ENTT_ASSERT(contains(entt), "View does not contain entity");
+
+        if constexpr(sizeof...(Comp) == 0) {
+            return get_as_tuple(*std::get<0>(pools), entt);
+        } else {
+            static_assert(std::is_same_v<Comp..., Component>, "Invalid component type");
+            return std::get<0>(pools)->get(entt);
+        }
+    }
+
+    /**
+     * @brief Iterates entities and components and applies the given function
+     * object to them.
+     *
+     * The function object is invoked for each entity. It is provided with the
+     * entity itself and a reference to the component if it's a non-empty one.
+     * The _constness_ of the component is as requested.<br/>
+     * The signature of the function must be equivalent to one of the following
+     * forms:
+     *
+     * @code{.cpp}
+     * void(const entity_type, Component &);
+     * void(Component &);
+     * @endcode
+     *
+     * @note
+     * Empty types aren't explicitly instantiated and therefore they are never
+     * returned during iterations.
+     *
+     * @tparam Func Type of the function object to invoke.
+     * @param func A valid function object.
+     */
+    template<typename Func>
+    void each(Func func) const {
+        if constexpr(std::is_void_v<decltype(std::get<0>(pools)->get({}))>) {
+            if constexpr(std::is_invocable_v<Func>) {
+                for(auto pos = size(); pos; --pos) {
+                    func();
+                }
+            } else {
+                for(auto entity: *this) {
+                    func(entity);
+                }
+            }
+        } else {
+            if constexpr(is_applicable_v<Func, decltype(*each().begin())>) {
+                for(const auto pack: each()) {
+                    std::apply(func, pack);
+                }
+            } else {
+                for(auto &&component: *std::get<0>(pools)) {
+                    func(component);
+                }
+            }
+        }
+    }
+
+    /**
+     * @brief Returns an iterable object to use to _visit_ the view.
+     *
+     * The iterable object returns tuples that contain the current entity and a
+     * reference to its component if it's a non-empty one. The _constness_ of
+     * the component is as requested.
+     *
+     * @note
+     * Empty types aren't explicitly instantiated and therefore they are never
+     * returned during iterations.
+     *
+     * @return An iterable object to use to _visit_ the view.
+     */
+    [[nodiscard]] iterable_view each() const ENTT_NOEXCEPT {
+        return iterable_view{*std::get<0>(pools)};
+    }
+
+    /**
+     * @brief Combines two views in a _more specific_ one (friend function).
+     * @tparam Id A valid entity type (see entt_traits for more details).
+     * @tparam ELhs Filter list of the first view.
+     * @tparam CLhs Component list of the first view.
+     * @tparam ERhs Filter list of the second view.
+     * @tparam CRhs Component list of the second view.
+     * @return A more specific view.
+     */
+    template<typename Id, typename... ELhs, typename... CLhs, typename... ERhs, typename... CRhs>
+    friend auto operator|(const basic_view<Id, exclude_t<ELhs...>, CLhs...> &, const basic_view<Id, exclude_t<ERhs...>, CRhs...> &);
+
+private:
+    const std::tuple<storage_type *> pools;
+    const std::tuple<> filter;
+};
+
+
+/**
+ * @brief View implementation dispatcher.
+ * @tparam Entity A valid entity type (see entt_traits for more details).
+ * @tparam Exclude Types of components used to filter the view.
+ * @tparam Component Types of components iterated by the view.
+ */
+template<typename Entity, typename... Exclude, typename... Component>
+struct basic_view<Entity, exclude_t<Exclude...>, Component...>
+    : basic_view_impl<std::conditional_t<std::disjunction_v<typename component_traits<std::remove_const_t<Component>>::in_place_delete...>, stable_storage_policy, packed_storage_policy>, Entity, exclude_t<Exclude...>, Component...>
+{
+    /*! @brief Most restrictive storage policy of all component types. */
+    using storage_policy = std::conditional_t<std::disjunction_v<typename component_traits<std::remove_const_t<Component>>::in_place_delete...>, stable_storage_policy, packed_storage_policy>;
+    using basic_view_impl<storage_policy, Entity, exclude_t<Exclude...>, Component...>::basic_view_impl;
+};
+
+
+/**
+ * @brief Deduction guide.
+ * @tparam Storage Type of storage classes used to create the view.
+ * @param storage The storage for the types to iterate.
+ */
+template<typename... Storage>
+basic_view(Storage &... storage)
+-> basic_view<std::common_type_t<typename Storage::entity_type...>, entt::exclude_t<>, constness_as_t<typename Storage::value_type, Storage>...>;
+
+
+/**
+ * @brief Combines two views in a _more specific_ one.
+ * @tparam Entity A valid entity type (see entt_traits for more details).
+ * @tparam ELhs Filter list of the first view.
+ * @tparam CLhs Component list of the first view.
+ * @tparam ERhs Filter list of the second view.
+ * @tparam CRhs Component list of the second view.
+ * @param lhs A valid reference to the first view.
+ * @param rhs A valid reference to the second view.
+ * @return A more specific view.
+ */
+template<typename Entity, typename... ELhs, typename... CLhs, typename... ERhs, typename... CRhs>
+[[nodiscard]] auto operator|(const basic_view<Entity, exclude_t<ELhs...>, CLhs...> &lhs, const basic_view<Entity, exclude_t<ERhs...>, CRhs...> &rhs) {
+    using view_type = basic_view<Entity, exclude_t<ELhs..., ERhs...>, CLhs..., CRhs...>;
+    return std::apply([](auto *... storage) { return view_type{*storage...}; }, std::tuple_cat(lhs.pools, rhs.pools, lhs.filter, rhs.filter));
+}
+
+
+}
+
+
+#endif
+
+
+
+namespace entt {
+
+
+/**
+ * @brief Fast and reliable entity-component system.
+ *
+ * The registry is the core class of the entity-component framework.<br/>
+ * It stores entities and arranges pools of components on a per request basis.
+ * By means of a registry, users can manage entities and components, then create
+ * views or groups to iterate them.
+ *
+ * @tparam Entity A valid entity type (see entt_traits for more details).
+ */
+template<typename Entity>
+class basic_registry {
+    using traits_type = entt_traits<Entity>;
+    using poly_storage_type = typename poly_storage_traits<Entity>::storage_type;
+    using basic_common_type = basic_sparse_set<Entity>;
+
+    template<typename Component>
+    using storage_type = constness_as_t<typename storage_traits<Entity, std::remove_const_t<Component>>::storage_type, Component>;
+
+    struct pool_data {
+        poly_storage_type poly;
+        std::unique_ptr<basic_common_type> pool{};
+    };
+
+    template<typename...>
+    struct group_handler;
+
+    template<typename... Exclude, typename... Get, typename... Owned>
+    struct group_handler<exclude_t<Exclude...>, get_t<Get...>, Owned...> {
+        static_assert(!std::disjunction_v<typename component_traits<Owned>::in_place_delete...>, "Groups do not support in-place delete");
+        static_assert(std::conjunction_v<std::is_same<Owned, std::remove_const_t<Owned>>..., std::is_same<Get, std::remove_const_t<Get>>..., std::is_same<Exclude, std::remove_const_t<Exclude>>...>, "One or more component types are invalid");
+        std::conditional_t<sizeof...(Owned) == 0, basic_common_type, std::size_t> current{};
+
+        template<typename Component>
+        void maybe_valid_if(basic_registry &owner, const Entity entt) {
+            [[maybe_unused]] const auto cpools = std::make_tuple(owner.assure<Owned>()...);
+
+            const auto is_valid = ((std::is_same_v<Component, Owned> || std::get<storage_type<Owned> *>(cpools)->contains(entt)) && ...)
+                    && ((std::is_same_v<Component, Get> || owner.assure<Get>()->contains(entt)) && ...)
+                    && ((std::is_same_v<Component, Exclude> || !owner.assure<Exclude>()->contains(entt)) && ...);
+
+            if constexpr(sizeof...(Owned) == 0) {
+                if(is_valid && !current.contains(entt)) {
+                    current.emplace(entt);
+                }
+            } else {
+                if(is_valid && !(std::get<0>(cpools)->index(entt) < current)) {
+                    const auto pos = current++;
+                    (std::get<storage_type<Owned> *>(cpools)->swap(std::get<storage_type<Owned> *>(cpools)->data()[pos], entt), ...);
+                }
+            }
+        }
+
+        void discard_if([[maybe_unused]] basic_registry &owner, const Entity entt) {
+            if constexpr(sizeof...(Owned) == 0) {
+                current.remove(entt);
+            } else {
+                if(const auto cpools = std::make_tuple(owner.assure<Owned>()...); std::get<0>(cpools)->contains(entt) && (std::get<0>(cpools)->index(entt) < current)) {
+                    const auto pos = --current;
+                    (std::get<storage_type<Owned> *>(cpools)->swap(std::get<storage_type<Owned> *>(cpools)->data()[pos], entt), ...);
+                }
+            }
+        }
+    };
+
+    struct group_data {
+        std::size_t size;
+        std::unique_ptr<void, void(*)(void *)> group;
+        bool (* owned)(const id_type) ENTT_NOEXCEPT;
+        bool (* get)(const id_type) ENTT_NOEXCEPT;
+        bool (* exclude)(const id_type) ENTT_NOEXCEPT;
+    };
+
+    template<typename Component>
+    [[nodiscard]] storage_type<Component> * assure() const {
+        static_assert(std::is_same_v<Component, std::decay_t<Component>>, "Non-decayed types not allowed");
+        const auto index = type_seq<Component>::value();
+
+        if(!(index < pools.size())) {
+            pools.resize(size_type(index)+1u);
+        }
+
+        if(auto &&pdata = pools[index]; !pdata.pool) {
+            pdata.pool.reset(new storage_type<Component>());
+            pdata.poly.template emplace<storage_type<Component> &>(*static_cast<storage_type<Component> *>(pdata.pool.get()));
+        }
+
+        return static_cast<storage_type<Component> *>(pools[index].pool.get());
+    }
+
+    template<typename Component>
+    [[nodiscard]] const storage_type<Component> * pool_if_exists() const ENTT_NOEXCEPT {
+        static_assert(std::is_same_v<Component, std::decay_t<Component>>, "Non-decayed types not allowed");
+        const auto index = type_seq<Component>::value();
+        return (!(index < pools.size()) || !pools[index].pool) ? nullptr : static_cast<const storage_type<Component> *>(pools[index].pool.get());
+    }
+
+    auto generate_identifier(const std::size_t pos) ENTT_NOEXCEPT {
+        ENTT_ASSERT(pos < traits_type::to_integral(null), "No entities available");
+        return traits_type::construct(static_cast<typename traits_type::entity_type>(pos), {});
+    }
+
+    auto recycle_identifier() ENTT_NOEXCEPT {
+        ENTT_ASSERT(free_list != null, "No entities available");
+        const auto curr = traits_type::to_entity(free_list);
+        free_list = (tombstone | entities[curr]);
+        return (entities[curr] = traits_type::construct(curr, traits_type::to_version(entities[curr])));
+    }
+
+    auto release_entity(const Entity entity, const typename traits_type::version_type version) {
+        const typename traits_type::version_type vers = version + (version == traits_type::to_version(tombstone));
+        entities[traits_type::to_entity(entity)] = traits_type::construct(traits_type::to_entity(free_list), vers);
+        free_list = (tombstone | entity);
+        return vers;
+    }
+
+public:
+    /*! @brief Underlying entity identifier. */
+    using entity_type = Entity;
+    /*! @brief Underlying version type. */
+    using version_type = typename traits_type::version_type;
+    /*! @brief Unsigned integer type. */
+    using size_type = std::size_t;
+    /*! @brief Poly storage type. */
+    using poly_storage = typename poly_storage_traits<Entity>::storage_type;
+
+    /**
+     * @brief Returns the entity identifier without the version.
+     * @param entity An entity identifier, either valid or not.
+     * @return The entity identifier without the version.
+     */
+    [[nodiscard]] static entity_type entity(const entity_type entity) ENTT_NOEXCEPT {
+        return traits_type::construct(traits_type::to_entity(entity), {});
+    }
+
+    /**
+     * @brief Returns the version stored along with an entity identifier.
+     * @param entity An entity identifier, either valid or not.
+     * @return The version stored along with the given entity identifier.
+     */
+    [[nodiscard]] static version_type version(const entity_type entity) ENTT_NOEXCEPT {
+        return traits_type::to_version(entity);
+    }
+
+    /*! @brief Default constructor. */
+    basic_registry() = default;
+
+    /*! @brief Default move constructor. */
+    basic_registry(basic_registry &&) = default;
+
+    /*! @brief Default move assignment operator. @return This registry. */
+    basic_registry & operator=(basic_registry &&) = default;
+
+    /**
+     * @brief Prepares a pool for the given type if required.
+     * @tparam Component Type of component for which to prepare a pool.
+     */
+    template<typename Component>
+    void prepare() {
+        // suppress the warning due to the [[nodiscard]] attribute
+        static_cast<void>(assure<Component>());
+    }
+
+    /**
+     * @brief Returns a poly storage for a given type.
+     * @param info The type for which to return a poly storage.
+     * @return A valid poly storage if a pool for the given type exists, an
+     * empty and thus invalid element otherwise.
+     */
+    poly_storage & storage(const type_info info) {
+        ENTT_ASSERT(info.seq() < pools.size() && pools[info.seq()].poly, "Storage not available");
+        return pools[info.seq()].poly;
+    }
+
+    /*! @copydoc storage */
+    const poly_storage & storage(const type_info info) const {
+        ENTT_ASSERT(info.seq() < pools.size() && pools[info.seq()].poly, "Storage not available");
+        return pools[info.seq()].poly;
+    }
+
+    /**
+     * @brief Returns the number of existing components of the given type.
+     * @tparam Component Type of component of which to return the size.
+     * @return Number of existing components of the given type.
+     */
+    template<typename Component>
+    [[nodiscard]] size_type size() const {
+        const auto *cpool = pool_if_exists<Component>();
+        return cpool ? cpool->size() : size_type{};
+    }
+
+    /**
+     * @brief Returns the number of entities created so far.
+     * @return Number of entities created so far.
+     */
+    [[nodiscard]] size_type size() const ENTT_NOEXCEPT {
+        return entities.size();
+    }
+
+    /**
+     * @brief Returns the number of entities still in use.
+     * @return Number of entities still in use.
+     */
+    [[nodiscard]] size_type alive() const {
+        auto sz = entities.size();
+
+        for(auto curr = free_list; curr != null; --sz) {
+            curr = entities[traits_type::to_entity(curr)];
+        }
+
+        return sz;
+    }
+
+    /**
+     * @brief Increases the capacity of the registry or of the pools for the
+     * given components.
+     *
+     * If no components are specified, the capacity of the registry is
+     * increased, that is the number of entities it contains. Otherwise the
+     * capacity of the pools for the given components is increased.<br/>
+     * In both cases, if the new capacity is greater than the current capacity,
+     * new storage is allocated, otherwise the method does nothing.
+     *
+     * @tparam Component Types of components for which to reserve storage.
+     * @param cap Desired capacity.
+     */
+    template<typename... Component>
+    void reserve(const size_type cap) {
+        if constexpr(sizeof...(Component) == 0) {
+            entities.reserve(cap);
+        } else {
+            (assure<Component>()->reserve(cap), ...);
+        }
+    }
+
+    /**
+     * @brief Reserves enough space to store `count` pools.
+     * @param count Number of pools to reserve space for.
+     */
+    [[deprecated("No longer supported")]]
+    void reserve_pools(const size_t count) {
+        pools.reserve(count);
+    }
+
+    /**
+     * @brief Returns the capacity of the pool for the given component.
+     * @tparam Component Type of component in which one is interested.
+     * @return Capacity of the pool of the given component.
+     */
+    template<typename Component>
+    [[nodiscard]] size_type capacity() const {
+        const auto *cpool = pool_if_exists<Component>();
+        return cpool ? cpool->capacity() : size_type{};
+    }
+
+    /**
+     * @brief Returns the number of entities that a registry has currently
+     * allocated space for.
+     * @return Capacity of the registry.
+     */
+    [[nodiscard]] size_type capacity() const ENTT_NOEXCEPT {
+        return entities.capacity();
+    }
+
+    /**
+     * @brief Requests the removal of unused capacity for the given components.
+     * @tparam Component Types of components for which to reclaim unused
+     * capacity.
+     */
+    template<typename... Component>
+    void shrink_to_fit() {
+        (assure<Component>()->shrink_to_fit(), ...);
+    }
+
+    /**
+     * @brief Checks whether the registry or the pools of the given components
+     * are empty.
+     *
+     * A registry is considered empty when it doesn't contain entities that are
+     * still in use.
+     *
+     * @tparam Component Types of components in which one is interested.
+     * @return True if the registry or the pools of the given components are
+     * empty, false otherwise.
+     */
+    template<typename... Component>
+    [[nodiscard]] bool empty() const {
+        if constexpr(sizeof...(Component) == 0) {
+            return !alive();
+        } else {
+            return [](const auto *... cpool) { return ((!cpool || cpool->empty()) && ...); }(pool_if_exists<Component>()...);
+        }
+    }
+
+    /**
+     * @brief Direct access to the list of entities of a registry.
+     *
+     * The returned pointer is such that range `[data(), data() + size())` is
+     * always a valid range, even if the container is empty.
+     *
+     * @warning
+     * This list contains both valid and destroyed entities and isn't suitable
+     * for direct use.
+     *
+     * @return A pointer to the array of entities.
+     */
+    [[nodiscard]] const entity_type * data() const ENTT_NOEXCEPT {
+        return entities.data();
+    }
+
+    /**
+     * @brief Returns the head of the list of released entities.
+     *
+     * This function is intended for use in conjunction with `assign`.<br/>
+     * The returned entity has an invalid identifier in all cases.
+     *
+     * @return The head of the list of released entities.
+     */
+    [[nodiscard]] entity_type released() const ENTT_NOEXCEPT {
+        return free_list;
+    }
+
+    /*! @copydoc released */
+    [[deprecated("Use ::released instead")]]
+    [[nodiscard]] entity_type destroyed() const ENTT_NOEXCEPT {
+        return released();
+    }
+
+    /**
+     * @brief Checks if an entity identifier refers to a valid entity.
+     * @param entity An entity identifier, either valid or not.
+     * @return True if the identifier is valid, false otherwise.
+     */
+    [[nodiscard]] bool valid(const entity_type entity) const {
+        const auto pos = size_type(traits_type::to_entity(entity));
+        return (pos < entities.size() && entities[pos] == entity);
+    }
+
+    /**
+     * @brief Returns the actual version for an entity identifier.
+     *
+     * @warning
+     * Attempting to use an entity that doesn't belong to the registry results
+     * in undefined behavior. An entity belongs to the registry even if it has
+     * been previously destroyed and/or recycled.
+     *
+     * @param entity A valid entity identifier.
+     * @return Actual version for the given entity identifier.
+     */
+    [[nodiscard]] version_type current(const entity_type entity) const {
+        const auto pos = size_type(traits_type::to_entity(entity));
+        ENTT_ASSERT(pos < entities.size(), "Entity does not exist");
+        return version(entities[pos]);
+    }
+
+    /**
+     * @brief Creates a new entity and returns it.
+     *
+     * There are two kinds of possible entity identifiers:
+     *
+     * * Newly created ones in case no entities have been previously destroyed.
+     * * Recycled ones with updated versions.
+     *
+     * @return A valid entity identifier.
+     */
+    [[nodiscard]] entity_type create() {
+        return (free_list == null) ? entities.emplace_back(generate_identifier(entities.size())) : recycle_identifier();
+    }
+
+    /**
+     * @brief Creates a new entity and returns it.
+     *
+     * @sa create
+     *
+     * If the requested entity isn't in use, the suggested identifier is created
+     * and returned. Otherwise, a new identifier is generated.
+     *
+     * @param hint Required entity identifier.
+     * @return A valid entity identifier.
+     */
+    [[nodiscard]] entity_type create(const entity_type hint) {
+        const auto length = entities.size();
+
+        if(hint == null || hint == tombstone) {
+            return create();
+        } else if(const auto req = traits_type::to_entity(hint); !(req < length)) {
+            entities.resize(size_type(req) + 1u, null);
+
+            for(auto pos = length; pos < req; ++pos) {
+                release_entity(generate_identifier(pos), {});
+            }
+
+            return (entities[req] = hint);
+        } else if(const auto curr = traits_type::to_entity(entities[req]); req == curr) {
+            return create();
+        } else {
+            auto *it = &free_list;
+            for(; traits_type::to_entity(*it) != req; it = &entities[traits_type::to_entity(*it)]);
+            *it = traits_type::construct(curr, traits_type::to_version(*it));
+            return (entities[req] = hint);
+        }
+    }
+
+    /**
+     * @brief Assigns each element in a range an entity.
+     *
+     * @sa create
+     *
+     * @tparam It Type of forward iterator.
+     * @param first An iterator to the first element of the range to generate.
+     * @param last An iterator past the last element of the range to generate.
+     */
+    template<typename It>
+    void create(It first, It last) {
+        for(; free_list != null && first != last; ++first) {
+            *first = recycle_identifier();
+        }
+
+        const auto length = entities.size();
+        entities.resize(length + std::distance(first, last), null);
+
+        for(auto pos = length; first != last; ++first, ++pos) {
+            *first = entities[pos] = generate_identifier(pos);
+        }
+    }
+
+    /**
+     * @brief Assigns entities to an empty registry.
+     *
+     * This function is intended for use in conjunction with `data`, `size` and
+     * `destroyed`.<br/>
+     * Don't try to inject ranges of randomly generated entities nor the _wrong_
+     * head for the list of destroyed entities. There is no guarantee that a
+     * registry will continue to work properly in this case.
+     *
+     * @warning
+     * There must be no entities still alive for this to work properly.
+     *
+     * @tparam It Type of input iterator.
+     * @param first An iterator to the first element of the range of entities.
+     * @param last An iterator past the last element of the range of entities.
+     * @param destroyed The head of the list of destroyed entities.
+     */
+    template<typename It>
+    void assign(It first, It last, const entity_type destroyed) {
+        ENTT_ASSERT(!alive(), "Entities still alive");
+        entities.assign(first, last);
+        free_list = destroyed;
+    }
+
+    /**
+     * @brief Releases an entity identifier.
+     *
+     * The version is updated and the identifier can be recycled at any time.
+     *
+     * @warning
+     * Attempting to use an invalid entity results in undefined behavior.
+     *
+     * @param entity A valid entity identifier.
+     * @return The version of the recycled entity.
+     */
+    version_type release(const entity_type entity) {
+        return release(entity, version(entity) + 1u);
+    }
+
+    /**
+     * @brief Releases an entity identifier.
+     *
+     * The suggested version or the valid version closest to the suggested one
+     * is used instead of the implicitly generated version.
+     *
+     * @sa release
+     *
+     * @param entity A valid entity identifier.
+     * @param version A desired version upon destruction.
+     * @return The version actually assigned to the entity.
+     */
+    version_type release(const entity_type entity, const version_type version) {
+        ENTT_ASSERT(orphan(entity), "Non-orphan entity");
+        return release_entity(entity, version);
+    }
+
+    /**
+     * @brief Releases all entity identifiers in a range.
+     *
+     * @sa release
+     *
+     * @tparam It Type of input iterator.
+     * @param first An iterator to the first element of the range of entities.
+     * @param last An iterator past the last element of the range of entities.
+     */
+    template<typename It>
+    void release(It first, It last) {
+        for(; first != last; ++first) {
+            release(*first, version(*first) + 1u);
+        }
+    }
+
+    /**
+     * @brief Destroys an entity and releases its identifier.
+     *
+     * The version is updated and the identifier can be recycled at any time.
+     *
+     * @warning
+     * Adding or removing components to an entity that is being destroyed can
+     * result in undefined behavior.
+     *
+     * @warning
+     * Attempting to use an invalid entity results in undefined behavior.
+     *
+     * @param entity A valid entity identifier.
+     * @return The version of the recycled entity.
+     */
+    version_type destroy(const entity_type entity) {
+        return destroy(entity, version(entity) + 1u);
+    }
+
+    /**
+     * @brief Destroys an entity and releases its identifier.
+     *
+     * The suggested version or the valid version closest to the suggested one
+     * is used instead of the implicitly generated version.
+     *
+     * @sa destroy
+     *
+     * @param entity A valid entity identifier.
+     * @param version A desired version upon destruction.
+     * @return The version actually assigned to the entity.
+     */
+    version_type destroy(const entity_type entity, const version_type version) {
+        ENTT_ASSERT(valid(entity), "Invalid entity");
+
+        for(auto &&pdata: pools) {
+            pdata.pool && pdata.pool->remove(entity, this);
+        }
+
+        return release_entity(entity, version);
+    }
+
+    /**
+     * @brief Destroys all entities in a range and releases their identifiers.
+     *
+     * @sa destroy
+     *
+     * @tparam It Type of input iterator.
+     * @param first An iterator to the first element of the range of entities.
+     * @param last An iterator past the last element of the range of entities.
+     */
+    template<typename It>
+    void destroy(It first, It last) {
+        if constexpr(is_iterator_type_v<typename basic_common_type::iterator, It>) {
+            for(; first != last; ++first) {
+                destroy(*first, version(*first) + 1u);
+            }
+        } else {
+            for(auto &&pdata: pools) {
+                pdata.pool && pdata.pool->remove(first, last, this);
+            }
+
+            release(first, last);
+        }
+    }
+
+    /**
+     * @brief Assigns the given component to an entity.
+     *
+     * A new instance of the given component is created and initialized with the
+     * arguments provided (the component must have a proper constructor or be of
+     * aggregate type). Then the component is assigned to the given entity.
+     *
+     * @warning
+     * Attempting to use an invalid entity or to assign a component to an entity
+     * that already owns it results in undefined behavior.
+     *
+     * @tparam Component Type of component to create.
+     * @tparam Args Types of arguments to use to construct the component.
+     * @param entity A valid entity identifier.
+     * @param args Parameters to use to initialize the component.
+     * @return A reference to the newly created component.
+     */
+    template<typename Component, typename... Args>
+    decltype(auto) emplace(const entity_type entity, Args &&... args) {
+        ENTT_ASSERT(valid(entity), "Invalid entity");
+        return assure<Component>()->emplace(*this, entity, std::forward<Args>(args)...);
+    }
+
+    /**
+     * @brief Assigns each entity in a range the given component.
+     *
+     * @sa emplace
+     *
+     * @tparam Component Type of component to create.
+     * @tparam It Type of input iterator.
+     * @param first An iterator to the first element of the range of entities.
+     * @param last An iterator past the last element of the range of entities.
+     * @param value An instance of the component to assign.
+     */
+    template<typename Component, typename It>
+    void insert(It first, It last, const Component &value = {}) {
+        ENTT_ASSERT(std::all_of(first, last, [this](const auto entity) { return valid(entity); }), "Invalid entity");
+        assure<Component>()->insert(*this, first, last, value);
+    }
+
+    /**
+     * @brief Assigns each entity in a range the given components.
+     *
+     * @sa emplace
+     *
+     * @tparam Component Type of component to create.
+     * @tparam EIt Type of input iterator.
+     * @tparam CIt Type of input iterator.
+     * @param first An iterator to the first element of the range of entities.
+     * @param last An iterator past the last element of the range of entities.
+     * @param from An iterator to the first element of the range of components.
+     */
+    template<typename Component, typename EIt, typename CIt, typename = std::enable_if_t<std::is_same_v<std::decay_t<typename std::iterator_traits<CIt>::value_type>, Component>>>
+    void insert(EIt first, EIt last, CIt from) {
+        static_assert(std::is_constructible_v<Component, typename std::iterator_traits<CIt>::value_type>, "Invalid value type");
+        ENTT_ASSERT(std::all_of(first, last, [this](const auto entity) { return valid(entity); }), "Invalid entity");
+        assure<Component>()->insert(*this, first, last, from);
+    }
+
+    /**
+     * @brief Assigns or replaces the given component for an entity.
+     *
+     * Equivalent to the following snippet (pseudocode):
+     *
+     * @code{.cpp}
+     * auto &component = registry.all_of<Component>(entity) ? registry.replace<Component>(entity, args...) : registry.emplace<Component>(entity, args...);
+     * @endcode
+     *
+     * Prefer this function anyway because it has slightly better performance.
+     *
+     * @warning
+     * Attempting to use an invalid entity results in undefined behavior.
+     *
+     * @tparam Component Type of component to assign or replace.
+     * @tparam Args Types of arguments to use to construct the component.
+     * @param entity A valid entity identifier.
+     * @param args Parameters to use to initialize the component.
+     * @return A reference to the newly created component.
+     */
+    template<typename Component, typename... Args>
+    decltype(auto) emplace_or_replace(const entity_type entity, Args &&... args) {
+        ENTT_ASSERT(valid(entity), "Invalid entity");
+        auto *cpool = assure<Component>();
+
+        return cpool->contains(entity)
+            ? cpool->patch(*this, entity, [&args...](auto &... curr) { ((curr = Component{std::forward<Args>(args)...}), ...); })
+            : cpool->emplace(*this, entity, std::forward<Args>(args)...);
+    }
+
+    /**
+     * @brief Patches the given component for an entity.
+     *
+     * The signature of the functions should be equivalent to the following:
+     *
+     * @code{.cpp}
+     * void(Component &);
+     * @endcode
+     *
+     * @note
+     * Empty types aren't explicitly instantiated and therefore they are never
+     * returned. However, this function can be used to trigger an update signal
+     * for them.
+     *
+     * @warning
+     * Attempting to use an invalid entity or to patch a component of an entity
+     * that doesn't own it results in undefined behavior.
+     *
+     * @tparam Component Type of component to patch.
+     * @tparam Func Types of the function objects to invoke.
+     * @param entity A valid entity identifier.
+     * @param func Valid function objects.
+     * @return A reference to the patched component.
+     */
+    template<typename Component, typename... Func>
+    decltype(auto) patch(const entity_type entity, Func &&... func) {
+        ENTT_ASSERT(valid(entity), "Invalid entity");
+        return assure<Component>()->patch(*this, entity, std::forward<Func>(func)...);
+    }
+
+    /**
+     * @brief Replaces the given component for an entity.
+     *
+     * A new instance of the given component is created and initialized with the
+     * arguments provided (the component must have a proper constructor or be of
+     * aggregate type). Then the component is assigned to the given entity.
+     *
+     * @warning
+     * Attempting to use an invalid entity or to replace a component of an
+     * entity that doesn't own it results in undefined behavior.
+     *
+     * @tparam Component Type of component to replace.
+     * @tparam Args Types of arguments to use to construct the component.
+     * @param entity A valid entity identifier.
+     * @param args Parameters to use to initialize the component.
+     * @return A reference to the component being replaced.
+     */
+    template<typename Component, typename... Args>
+    decltype(auto) replace(const entity_type entity, Args &&... args) {
+        return assure<Component>()->patch(*this, entity, [&args...](auto &... curr) { ((curr = Component{std::forward<Args>(args)...}), ...); });
+    }
+
+    /**
+     * @brief Removes the given components from an entity.
+     *
+     * @warning
+     * Attempting to use an invalid entity results in undefined behavior.
+     *
+     * @tparam Component Types of components to remove.
+     * @param entity A valid entity identifier.
+     * @return The number of components actually removed.
+     */
+    template<typename... Component>
+    size_type remove(const entity_type entity) {
+        ENTT_ASSERT(valid(entity), "Invalid entity");
+        static_assert(sizeof...(Component) > 0, "Provide one or more component types");
+        return (assure<Component>()->remove(entity, this) + ... + size_type{});
+    }
+
+    /**
+     * @brief Removes the given components from all the entities in a range.
+     *
+     * @sa remove
+     *
+     * @tparam Component Types of components to remove.
+     * @tparam It Type of input iterator.
+     * @param first An iterator to the first element of the range of entities.
+     * @param last An iterator past the last element of the range of entities.
+     * @return The number of components actually removed.
+     */
+    template<typename... Component, typename It>
+    size_type remove(It first, It last) {
+        static_assert(sizeof...(Component) > 0, "Provide one or more component types");
+        const auto cpools = std::make_tuple(assure<Component>()...);
+        size_type count{};
+
+        for(; first != last; ++first) {
+            const auto entity = *first;
+            ENTT_ASSERT(valid(entity), "Invalid entity");
+            count += (std::get<storage_type<Component> *>(cpools)->remove(entity, this) + ...);
+        }
+
+        return count;
+    }
+
+    /**
+     * @brief Erases the given components from an entity.
+     *
+     * @warning
+     * Attempting to use an invalid entity or to erase a component from an
+     * entity that doesn't own it results in undefined behavior.
+     *
+     * @tparam Component Types of components to erase.
+     * @param entity A valid entity identifier.
+     */
+    template<typename... Component>
+    void erase(const entity_type entity) {
+        ENTT_ASSERT(valid(entity), "Invalid entity");
+        static_assert(sizeof...(Component) > 0, "Provide one or more component types");
+        (assure<Component>()->erase(entity, this), ...);
+    }
+
+    /**
+     * @brief Erases the given components from all the entities in a range.
+     *
+     * @sa erase
+     *
+     * @tparam Component Types of components to erase.
+     * @tparam It Type of input iterator.
+     * @param first An iterator to the first element of the range of entities.
+     * @param last An iterator past the last element of the range of entities.
+     */
+    template<typename... Component, typename It>
+    void erase(It first, It last) {
+        static_assert(sizeof...(Component) > 0, "Provide one or more component types");
+        const auto cpools = std::make_tuple(assure<Component>()...);
+
+        for(; first != last; ++first) {
+            const auto entity = *first;
+            ENTT_ASSERT(valid(entity), "Invalid entity");
+            (std::get<storage_type<Component> *>(cpools)->erase(entity, this), ...);
+        }
+    }
+
+    /**
+     * @brief Removes all tombstones from a registry or only the pools for the
+     * given components.
+     * @tparam Component Types of components for which to clear all tombstones.
+     */
+    template<typename... Component>
+    void compact() {
+        if constexpr(sizeof...(Component) == 0) {
+            for(auto &&pdata: pools) {
+                pdata.pool && (pdata.pool->compact(), true);
+            }
+        } else {
+            (assure<Component>()->compact(), ...);
+        }
+    }
+
+    /*! @copydoc remove */
+    template<typename... Component>
+    [[deprecated("Use ::remove instead")]]
+    size_type remove_if_exists(const entity_type entity) {
+        return remove<Component...>(entity);
+    }
+
+    /**
+     * @brief Removes all the components from an entity and makes it orphaned.
+     *
+     * @warning
+     * In case there are listeners that observe the destruction of components
+     * and assign other components to the entity in their bodies, the result of
+     * invoking this function may not be as expected. In the worst case, it
+     * could lead to undefined behavior.
+     *
+     * @warning
+     * Attempting to use an invalid entity results in undefined behavior.
+     *
+     * @param entity A valid entity identifier.
+     */
+    [[deprecated("Use ::destroy(entity)/::create(entity) instead")]]
+    void remove_all(const entity_type entity) {
+        ENTT_ASSERT(valid(entity), "Invalid entity");
+
+        for(auto &&pdata: pools) {
+            pdata.pool && pdata.pool->remove(entity, this);
+        }
+    }
+
+    /**
+     * @brief Checks if an entity has all the given components.
+     *
+     * @warning
+     * Attempting to use an invalid entity results in undefined behavior.
+     *
+     * @tparam Component Components for which to perform the check.
+     * @param entity A valid entity identifier.
+     * @return True if the entity has all the components, false otherwise.
+     */
+    template<typename... Component>
+    [[nodiscard]] bool all_of(const entity_type entity) const {
+        ENTT_ASSERT(valid(entity), "Invalid entity");
+        return [entity](const auto *... cpool) { return ((cpool && cpool->contains(entity)) && ...); }(pool_if_exists<Component>()...);
+    }
+
+    /**
+     * @brief Checks if an entity has at least one of the given components.
+     *
+     * @warning
+     * Attempting to use an invalid entity results in undefined behavior.
+     *
+     * @tparam Component Components for which to perform the check.
+     * @param entity A valid entity identifier.
+     * @return True if the entity has at least one of the given components,
+     * false otherwise.
+     */
+    template<typename... Component>
+    [[nodiscard]] bool any_of(const entity_type entity) const {
+        ENTT_ASSERT(valid(entity), "Invalid entity");
+        return [entity](const auto *... cpool) { return !((!cpool || !cpool->contains(entity)) && ...); }(pool_if_exists<Component>()...);
+    }
+
+    /**
+     * @brief Returns references to the given components for an entity.
+     *
+     * @warning
+     * Attempting to use an invalid entity or to get a component from an entity
+     * that doesn't own it results in undefined behavior.
+     *
+     * @tparam Component Types of components to get.
+     * @param entity A valid entity identifier.
+     * @return References to the components owned by the entity.
+     */
+    template<typename... Component>
+    [[nodiscard]] decltype(auto) get([[maybe_unused]] const entity_type entity) const {
+        ENTT_ASSERT(valid(entity), "Invalid entity");
+
+        if constexpr(sizeof...(Component) == 1) {
+            const auto *cpool = pool_if_exists<std::remove_const_t<Component>...>();
+            ENTT_ASSERT(cpool, "Storage not available");
+            return cpool->get(entity);
+        } else {
+            return std::forward_as_tuple(get<Component>(entity)...);
+        }
+    }
+
+    /*! @copydoc get */
+    template<typename... Component>
+    [[nodiscard]] decltype(auto) get([[maybe_unused]] const entity_type entity) {
+        ENTT_ASSERT(valid(entity), "Invalid entity");
+
+        if constexpr(sizeof...(Component) == 1) {
+            return (const_cast<Component &>(assure<std::remove_const_t<Component>>()->get(entity)), ...);
+        } else {
+            return std::forward_as_tuple(get<Component>(entity)...);
+        }
+    }
+
+    /**
+     * @brief Returns a reference to the given component for an entity.
+     *
+     * In case the entity doesn't own the component, the parameters provided are
+     * used to construct it.<br/>
+     * Equivalent to the following snippet (pseudocode):
+     *
+     * @code{.cpp}
+     * auto &component = registry.all_of<Component>(entity) ? registry.get<Component>(entity) : registry.emplace<Component>(entity, args...);
+     * @endcode
+     *
+     * Prefer this function anyway because it has slightly better performance.
+     *
+     * @warning
+     * Attempting to use an invalid entity results in undefined behavior.
+     *
+     * @tparam Component Type of component to get.
+     * @tparam Args Types of arguments to use to construct the component.
+     * @param entity A valid entity identifier.
+     * @param args Parameters to use to initialize the component.
+     * @return Reference to the component owned by the entity.
+     */
+    template<typename Component, typename... Args>
+    [[nodiscard]] decltype(auto) get_or_emplace(const entity_type entity, Args &&... args) {
+        ENTT_ASSERT(valid(entity), "Invalid entity");
+        auto *cpool = assure<Component>();
+        return cpool->contains(entity) ? cpool->get(entity) : cpool->emplace(*this, entity, std::forward<Args>(args)...);
+    }
+
+    /**
+     * @brief Returns pointers to the given components for an entity.
+     *
+     * @warning
+     * Attempting to use an invalid entity results in undefined behavior.
+     *
+     * @note
+     * The registry retains ownership of the pointed-to components.
+     *
+     * @tparam Component Types of components to get.
+     * @param entity A valid entity identifier.
+     * @return Pointers to the components owned by the entity.
+     */
+    template<typename... Component>
+    [[nodiscard]] auto try_get([[maybe_unused]] const entity_type entity) const {
+        ENTT_ASSERT(valid(entity), "Invalid entity");
+
+        if constexpr(sizeof...(Component) == 1) {
+            const auto *cpool = pool_if_exists<std::remove_const_t<Component>...>();
+            return (cpool && cpool->contains(entity)) ? &cpool->get(entity) : nullptr;
+        } else {
+            return std::make_tuple(try_get<Component>(entity)...);
+        }
+    }
+
+    /*! @copydoc try_get */
+    template<typename... Component>
+    [[nodiscard]] auto try_get([[maybe_unused]] const entity_type entity) {
+        ENTT_ASSERT(valid(entity), "Invalid entity");
+
+        if constexpr(sizeof...(Component) == 1) {
+            return (const_cast<Component *>(std::as_const(*this).template try_get<Component>(entity)), ...);
+        } else {
+            return std::make_tuple(try_get<Component>(entity)...);
+        }
+    }
+
+    /**
+     * @brief Clears a whole registry or the pools for the given components.
+     * @tparam Component Types of components to remove from their entities.
+     */
+    template<typename... Component>
+    void clear() {
+        if constexpr(sizeof...(Component) == 0) {
+            for(auto &&pdata: pools) {
+                pdata.pool && (pdata.pool->clear(this), true);
+            }
+
+            each([this](const auto entity) { release_entity(entity, version(entity) + 1u); });
+        } else {
+            (assure<Component>()->clear(this), ...);
+        }
+    }
+
+    /**
+     * @brief Iterates all the entities that are still in use.
+     *
+     * The function object is invoked for each entity that is still in use.<br/>
+     * The signature of the function should be equivalent to the following:
+     *
+     * @code{.cpp}
+     * void(const Entity);
+     * @endcode
+     *
+     * This function is fairly slow and should not be used frequently. However,
+     * it's useful for iterating all the entities still in use, regardless of
+     * their components.
+     *
+     * @tparam Func Type of the function object to invoke.
+     * @param func A valid function object.
+     */
+    template<typename Func>
+    void each(Func func) const {
+        if(free_list == null) {
+            for(auto pos = entities.size(); pos; --pos) {
+                func(entities[pos-1]);
+            }
+        } else {
+            for(auto pos = entities.size(); pos; --pos) {
+                if(const auto entity = entities[pos - 1]; traits_type::to_entity(entity) == (pos - 1)) {
+                    func(entity);
+                }
+            }
+        }
+    }
+
+    /**
+     * @brief Checks if an entity has components assigned.
+     * @param entity A valid entity identifier.
+     * @return True if the entity has no components assigned, false otherwise.
+     */
+    [[nodiscard]] bool orphan(const entity_type entity) const {
+        ENTT_ASSERT(valid(entity), "Invalid entity");
+        return std::none_of(pools.cbegin(), pools.cend(), [entity](auto &&pdata) { return pdata.pool && pdata.pool->contains(entity); });
+    }
+
+    /**
+     * @brief Iterates orphans and applies them the given function object.
+     *
+     * The function object is invoked for each entity that is still in use and
+     * has no components assigned.<br/>
+     * The signature of the function should be equivalent to the following:
+     *
+     * @code{.cpp}
+     * void(const Entity);
+     * @endcode
+     *
+     * This function can be very slow and should not be used frequently.
+     *
+     * @tparam Func Type of the function object to invoke.
+     * @param func A valid function object.
+     */
+    template<typename Func>
+    void orphans(Func func) const {
+        each([this, &func](const auto entity) {
+            if(orphan(entity)) {
+                func(entity);
+            }
+        });
+    }
+
+    /**
+     * @brief Returns a sink object for the given component.
+     *
+     * The sink returned by this function can be used to receive notifications
+     * whenever a new instance of the given component is created and assigned to
+     * an entity.<br/>
+     * The function type for a listener is equivalent to:
+     *
+     * @code{.cpp}
+     * void(basic_registry<Entity> &, Entity);
+     * @endcode
+     *
+     * Listeners are invoked **after** the component has been assigned to the
+     * entity.
+     *
+     * @sa sink
+     *
+     * @tparam Component Type of component of which to get the sink.
+     * @return A temporary sink object.
+     */
+    template<typename Component>
+    [[nodiscard]] auto on_construct() {
+        return assure<Component>()->on_construct();
+    }
+
+    /**
+     * @brief Returns a sink object for the given component.
+     *
+     * The sink returned by this function can be used to receive notifications
+     * whenever an instance of the given component is explicitly updated.<br/>
+     * The function type for a listener is equivalent to:
+     *
+     * @code{.cpp}
+     * void(basic_registry<Entity> &, Entity);
+     * @endcode
+     *
+     * Listeners are invoked **after** the component has been updated.
+     *
+     * @sa sink
+     *
+     * @tparam Component Type of component of which to get the sink.
+     * @return A temporary sink object.
+     */
+    template<typename Component>
+    [[nodiscard]] auto on_update() {
+        return assure<Component>()->on_update();
+    }
+
+    /**
+     * @brief Returns a sink object for the given component.
+     *
+     * The sink returned by this function can be used to receive notifications
+     * whenever an instance of the given component is removed from an entity and
+     * thus destroyed.<br/>
+     * The function type for a listener is equivalent to:
+     *
+     * @code{.cpp}
+     * void(basic_registry<Entity> &, Entity);
+     * @endcode
+     *
+     * Listeners are invoked **before** the component has been removed from the
+     * entity.
+     *
+     * @sa sink
+     *
+     * @tparam Component Type of component of which to get the sink.
+     * @return A temporary sink object.
+     */
+    template<typename Component>
+    [[nodiscard]] auto on_destroy() {
+        return assure<Component>()->on_destroy();
+    }
+
+    /**
+     * @brief Returns a view for the given components.
+     *
+     * This kind of objects are created on the fly and share with the registry
+     * its internal data structures.<br/>
+     * Feel free to discard a view after the use. Creating and destroying a view
+     * is an incredibly cheap operation because they do not require any type of
+     * initialization.<br/>
+     * As a rule of thumb, storing a view should never be an option.
+     *
+     * Views do their best to iterate the smallest set of candidate entities.
+     * In particular:
+     *
+     * * Single component views are incredibly fast and iterate a packed array
+     *   of entities, all of which has the given component.
+     * * Multi component views look at the number of entities available for each
+     *   component and pick up a reference to the smallest set of candidates to
+     *   test for the given components.
+     *
+     * Views in no way affect the functionalities of the registry nor those of
+     * the underlying pools.
+     *
+     * @note
+     * Multi component views are pretty fast. However their performance tend to
+     * degenerate when the number of components to iterate grows up and the most
+     * of the entities have all the given components.<br/>
+     * To get a performance boost, consider using a group instead.
+     *
+     * @tparam Component Type of components used to construct the view.
+     * @tparam Exclude Types of components used to filter the view.
+     * @return A newly created view.
+     */
+    template<typename... Component, typename... Exclude>
+    [[nodiscard]] basic_view<Entity, exclude_t<Exclude...>, std::add_const_t<Component>...> view(exclude_t<Exclude...> = {}) const {
+        static_assert(sizeof...(Component) > 0, "Exclusion-only views are not supported");
+        return { *assure<std::remove_const_t<Component>>()..., *assure<Exclude>()... };
+    }
+
+    /*! @copydoc view */
+    template<typename... Component, typename... Exclude>
+    [[nodiscard]] basic_view<Entity, exclude_t<Exclude...>, Component...> view(exclude_t<Exclude...> = {}) {
+        static_assert(sizeof...(Component) > 0, "Exclusion-only views are not supported");
+        return { *assure<std::remove_const_t<Component>>()..., *assure<Exclude>()... };
+    }
+
+    /**
+     * @brief Returns a runtime view for the given components.
+     *
+     * This kind of objects are created on the fly and share with the registry
+     * its internal data structures.<br/>
+     * Users should throw away the view after use. Fortunately, creating and
+     * destroying a runtime view is an incredibly cheap operation because they
+     * do not require any type of initialization.<br/>
+     * As a rule of thumb, storing a view should never be an option.
+     *
+     * Runtime views are to be used when users want to construct a view from
+     * some external inputs and don't know at compile-time what are the required
+     * components.
+     *
+     * @tparam ItComp Type of input iterator for the components to use to
+     * construct the view.
+     * @tparam ItExcl Type of input iterator for the components to use to filter
+     * the view.
+     * @param first An iterator to the first element of the range of components
+     * to use to construct the view.
+     * @param last An iterator past the last element of the range of components
+     * to use to construct the view.
+     * @param from An iterator to the first element of the range of components
+     * to use to filter the view.
+     * @param to An iterator past the last element of the range of components to
+     * use to filter the view.
+     * @return A newly created runtime view.
+     */
+    template<typename ItComp, typename ItExcl = id_type *>
+    [[nodiscard]] basic_runtime_view<Entity> runtime_view(ItComp first, ItComp last, ItExcl from = {}, ItExcl to = {}) const {
+        std::vector<const basic_common_type *> component(std::distance(first, last));
+        std::vector<const basic_common_type *> filter(std::distance(from, to));
+
+        std::transform(first, last, component.begin(), [this](const auto ctype) {
+            const auto it = std::find_if(pools.cbegin(), pools.cend(), [ctype](auto &&pdata) { return pdata.poly && pdata.poly->value_type().hash() == ctype; });
+            return it == pools.cend() ? nullptr : it->pool.get();
+        });
+
+        std::transform(from, to, filter.begin(), [this](const auto ctype) {
+            const auto it = std::find_if(pools.cbegin(), pools.cend(), [ctype](auto &&pdata) { return pdata.poly && pdata.poly->value_type().hash() == ctype; });
+            return it == pools.cend() ? nullptr : it->pool.get();
+        });
+
+        return { std::move(component), std::move(filter) };
+    }
+
+    /**
+     * @brief Returns a group for the given components.
+     *
+     * This kind of objects are created on the fly and share with the registry
+     * its internal data structures.<br/>
+     * Feel free to discard a group after the use. Creating and destroying a
+     * group is an incredibly cheap operation because they do not require any
+     * type of initialization, but for the first time they are requested.<br/>
+     * As a rule of thumb, storing a group should never be an option.
+     *
+     * Groups support exclusion lists and can own types of components. The more
+     * types are owned by a group, the faster it is to iterate entities and
+     * components.<br/>
+     * However, groups also affect some features of the registry such as the
+     * creation and destruction of components, which will consequently be
+     * slightly slower (nothing that can be noticed in most cases).
+     *
+     * @note
+     * Pools of components that are owned by a group cannot be sorted anymore.
+     * The group takes the ownership of the pools and arrange components so as
+     * to iterate them as fast as possible.
+     *
+     * @tparam Owned Types of components owned by the group.
+     * @tparam Get Types of components observed by the group.
+     * @tparam Exclude Types of components used to filter the group.
+     * @return A newly created group.
+     */
+    template<typename... Owned, typename... Get, typename... Exclude>
+    [[nodiscard]] basic_group<Entity, exclude_t<Exclude...>, get_t<Get...>, Owned...> group(get_t<Get...>, exclude_t<Exclude...> = {}) {
+        static_assert(sizeof...(Owned) + sizeof...(Get) > 0, "Exclusion-only groups are not supported");
+        static_assert(sizeof...(Owned) + sizeof...(Get) + sizeof...(Exclude) > 1, "Single component groups are not allowed");
+
+        using handler_type = group_handler<exclude_t<Exclude...>, get_t<std::remove_const_t<Get>...>, std::remove_const_t<Owned>...>;
+
+        const auto cpools = std::make_tuple(assure<std::remove_const_t<Owned>>()..., assure<std::remove_const_t<Get>>()...);
+        constexpr auto size = sizeof...(Owned) + sizeof...(Get) + sizeof...(Exclude);
+        handler_type *handler = nullptr;
+
+        if(auto it = std::find_if(groups.cbegin(), groups.cend(), [size](const auto &gdata) {
+            return gdata.size == size
+                && (gdata.owned(type_hash<std::remove_const_t<Owned>>::value()) && ...)
+                && (gdata.get(type_hash<std::remove_const_t<Get>>::value()) && ...)
+                && (gdata.exclude(type_hash<Exclude>::value()) && ...);
+        }); it != groups.cend())
+        {
+            handler = static_cast<handler_type *>(it->group.get());
+        }
+
+        if(!handler) {
+            group_data candidate = {
+                size,
+                { new handler_type{}, [](void *instance) { delete static_cast<handler_type *>(instance); } },
+                []([[maybe_unused]] const id_type ctype) ENTT_NOEXCEPT { return ((ctype == type_hash<std::remove_const_t<Owned>>::value()) || ...); },
+                []([[maybe_unused]] const id_type ctype) ENTT_NOEXCEPT { return ((ctype == type_hash<std::remove_const_t<Get>>::value()) || ...); },
+                []([[maybe_unused]] const id_type ctype) ENTT_NOEXCEPT { return ((ctype == type_hash<Exclude>::value()) || ...); },
+            };
+
+            handler = static_cast<handler_type *>(candidate.group.get());
+
+            const void *maybe_valid_if = nullptr;
+            const void *discard_if = nullptr;
+
+            if constexpr(sizeof...(Owned) == 0) {
+                groups.push_back(std::move(candidate));
+            } else {
+                ENTT_ASSERT(std::all_of(groups.cbegin(), groups.cend(), [size](const auto &gdata) {
+                    const auto overlapping = (0u + ... + gdata.owned(type_hash<std::remove_const_t<Owned>>::value()));
+                    const auto sz = overlapping + (0u + ... + gdata.get(type_hash<std::remove_const_t<Get>>::value())) + (0u + ... + gdata.exclude(type_hash<Exclude>::value()));
+                    return !overlapping || ((sz == size) || (sz == gdata.size));
+                }), "Conflicting groups");
+
+                const auto next = std::find_if_not(groups.cbegin(), groups.cend(), [size](const auto &gdata) {
+                    return !(0u + ... + gdata.owned(type_hash<std::remove_const_t<Owned>>::value())) || (size > gdata.size);
+                });
+
+                const auto prev = std::find_if(std::make_reverse_iterator(next), groups.crend(), [](const auto &gdata) {
+                    return (0u + ... + gdata.owned(type_hash<std::remove_const_t<Owned>>::value()));
+                });
+
+                maybe_valid_if = (next == groups.cend() ? maybe_valid_if : next->group.get());
+                discard_if = (prev == groups.crend() ? discard_if : prev->group.get());
+                groups.insert(next, std::move(candidate));
+            }
+
+            (on_construct<std::remove_const_t<Owned>>().before(maybe_valid_if).template connect<&handler_type::template maybe_valid_if<std::remove_const_t<Owned>>>(*handler), ...);
+            (on_construct<std::remove_const_t<Get>>().before(maybe_valid_if).template connect<&handler_type::template maybe_valid_if<std::remove_const_t<Get>>>(*handler), ...);
+            (on_destroy<Exclude>().before(maybe_valid_if).template connect<&handler_type::template maybe_valid_if<Exclude>>(*handler), ...);
+
+            (on_destroy<std::remove_const_t<Owned>>().before(discard_if).template connect<&handler_type::discard_if>(*handler), ...);
+            (on_destroy<std::remove_const_t<Get>>().before(discard_if).template connect<&handler_type::discard_if>(*handler), ...);
+            (on_construct<Exclude>().before(discard_if).template connect<&handler_type::discard_if>(*handler), ...);
+
+            if constexpr(sizeof...(Owned) == 0) {
+                for(const auto entity: view<Owned..., Get...>(exclude<Exclude...>)) {
+                    handler->current.emplace(entity);
+                }
+            } else {
+                // we cannot iterate backwards because we want to leave behind valid entities in case of owned types
+                for(auto *first = std::get<0>(cpools)->data(), *last = first + std::get<0>(cpools)->size(); first != last; ++first) {
+                    handler->template maybe_valid_if<type_list_element_t<0, type_list<std::remove_const_t<Owned>...>>>(*this, *first);
+                }
+            }
+        }
+
+        return { handler->current, *std::get<storage_type<std::remove_const_t<Owned>> *>(cpools)..., *std::get<storage_type<std::remove_const_t<Get>> *>(cpools)... };
+    }
+
+    /**
+     * @brief Returns a group for the given components.
+     *
+     * @sa group
+     *
+     * @tparam Owned Types of components owned by the group.
+     * @tparam Get Types of components observed by the group.
+     * @tparam Exclude Types of components used to filter the group.
+     * @return A newly created group.
+     */
+    template<typename... Owned, typename... Get, typename... Exclude>
+    [[nodiscard]] basic_group<Entity, exclude_t<Exclude...>, get_t<std::add_const_t<Get>...>, std::add_const_t<Owned>...> group_if_exists(get_t<Get...>, exclude_t<Exclude...> = {}) const {
+        if(auto it = std::find_if(groups.cbegin(), groups.cend(), [](const auto &gdata) {
+            return gdata.size == (sizeof...(Owned) + sizeof...(Get) + sizeof...(Exclude))
+                && (gdata.owned(type_hash<std::remove_const_t<Owned>>::value()) && ...)
+                && (gdata.get(type_hash<std::remove_const_t<Get>>::value()) && ...)
+                && (gdata.exclude(type_hash<Exclude>::value()) && ...);
+            }); it == groups.cend())
+        {
+            return {};
+        } else {
+            using handler_type = group_handler<exclude_t<Exclude...>, get_t<std::remove_const_t<Get>...>, std::remove_const_t<Owned>...>;
+            return { static_cast<handler_type *>(it->group.get())->current, *pool_if_exists<std::remove_const_t<Owned>>()... , *pool_if_exists<std::remove_const_t<Get>>()... };
+        }
+    }
+
+    /**
+     * @brief Returns a group for the given components.
+     *
+     * @sa group
+     *
+     * @tparam Owned Types of components owned by the group.
+     * @tparam Exclude Types of components used to filter the group.
+     * @return A newly created group.
+     */
+    template<typename... Owned, typename... Exclude>
+    [[nodiscard]] basic_group<Entity, exclude_t<Exclude...>, get_t<>, Owned...> group(exclude_t<Exclude...> = {}) {
+        return group<Owned...>(get_t<>{}, exclude<Exclude...>);
+    }
+
+    /**
+     * @brief Returns a group for the given components.
+     *
+     * @sa group_if_exists
+     *
+     * @tparam Owned Types of components owned by the group.
+     * @tparam Exclude Types of components used to filter the group.
+     * @return A newly created group.
+     */
+    template<typename... Owned, typename... Exclude>
+    [[nodiscard]] basic_group<Entity, exclude_t<Exclude...>, get_t<>, std::add_const_t<Owned>...> group_if_exists(exclude_t<Exclude...> = {}) const {
+        return group_if_exists<std::add_const_t<Owned>...>(get_t<>{}, exclude<Exclude...>);
+    }
+
+    /**
+     * @brief Checks whether the given components belong to any group.
+     * @tparam Component Types of components in which one is interested.
+     * @return True if the pools of the given components are sortable, false
+     * otherwise.
+     */
+    template<typename... Component>
+    [[nodiscard]] bool sortable() const {
+        return std::none_of(groups.cbegin(), groups.cend(), [](auto &&gdata) { return (gdata.owned(type_hash<std::remove_const_t<Component>>::value()) || ...); });
+    }
+
+    /**
+     * @brief Checks whether a group can be sorted.
+     * @tparam Owned Types of components owned by the group.
+     * @tparam Get Types of components observed by the group.
+     * @tparam Exclude Types of components used to filter the group.
+     * @return True if the group can be sorted, false otherwise.
+     */
+    template<typename... Owned, typename... Get, typename... Exclude>
+    [[nodiscard]] bool sortable(const basic_group<Entity, exclude_t<Exclude...>, get_t<Get...>, Owned...> &) ENTT_NOEXCEPT {
+        constexpr auto size = sizeof...(Owned) + sizeof...(Get) + sizeof...(Exclude);
+        return std::find_if(groups.cbegin(), groups.cend(), [size](const auto &gdata) {
+            return (0u + ... + gdata.owned(type_hash<std::remove_const_t<Owned>>::value())) && (size < gdata.size);
+        }) == groups.cend();
+    }
+
+    /**
+     * @brief Sorts the pool of entities for the given component.
+     *
+     * The order of the elements in a pool is highly affected by assignments
+     * of components to entities and deletions. Components are arranged to
+     * maximize the performance during iterations and users should not make any
+     * assumption on the order.<br/>
+     * This function can be used to impose an order to the elements in the pool
+     * of the given component. The order is kept valid until a component of the
+     * given type is assigned or removed from an entity.
+     *
+     * The comparison function object must return `true` if the first element
+     * is _less_ than the second one, `false` otherwise. The signature of the
+     * comparison function should be equivalent to one of the following:
+     *
+     * @code{.cpp}
+     * bool(const Entity, const Entity);
+     * bool(const Component &, const Component &);
+     * @endcode
+     *
+     * Moreover, the comparison function object shall induce a
+     * _strict weak ordering_ on the values.
+     *
+     * The sort function oject must offer a member function template
+     * `operator()` that accepts three arguments:
+     *
+     * * An iterator to the first element of the range to sort.
+     * * An iterator past the last element of the range to sort.
+     * * A comparison function to use to compare the elements.
+     *
+     * The comparison funtion object received by the sort function object hasn't
+     * necessarily the type of the one passed along with the other parameters to
+     * this member function.
+     *
+     * @warning
+     * Pools of components owned by a group cannot be sorted.
+     *
+     * @tparam Component Type of components to sort.
+     * @tparam Compare Type of comparison function object.
+     * @tparam Sort Type of sort function object.
+     * @tparam Args Types of arguments to forward to the sort function object.
+     * @param compare A valid comparison function object.
+     * @param algo A valid sort function object.
+     * @param args Arguments to forward to the sort function object, if any.
+     */
+    template<typename Component, typename Compare, typename Sort = std_sort, typename... Args>
+    void sort(Compare compare, Sort algo = Sort{}, Args &&... args) {
+        ENTT_ASSERT(sortable<Component>(), "Cannot sort owned storage");
+        assure<Component>()->sort(std::move(compare), std::move(algo), std::forward<Args>(args)...);
+    }
+
+    /**
+     * @brief Sorts two pools of components in the same way.
+     *
+     * The order of the elements in a pool is highly affected by assignments
+     * of components to entities and deletions. Components are arranged to
+     * maximize the performance during iterations and users should not make any
+     * assumption on the order.
+     *
+     * It happens that different pools of components must be sorted the same way
+     * because of runtime and/or performance constraints. This function can be
+     * used to order a pool of components according to the order between the
+     * entities in another pool of components.
+     *
+     * @b How @b it @b works
+     *
+     * Being `A` and `B` the two sets where `B` is the master (the one the order
+     * of which rules) and `A` is the slave (the one to sort), after a call to
+     * this function an iterator for `A` will return the entities according to
+     * the following rules:
+     *
+     * * All the entities in `A` that are also in `B` are returned first
+     *   according to the order they have in `B`.
+     * * All the entities in `A` that are not in `B` are returned in no
+     *   particular order after all the other entities.
+     *
+     * Any subsequent change to `B` won't affect the order in `A`.
+     *
+     * @warning
+     * Pools of components owned by a group cannot be sorted.
+     *
+     * @tparam To Type of components to sort.
+     * @tparam From Type of components to use to sort.
+     */
+    template<typename To, typename From>
+    void sort() {
+        ENTT_ASSERT(sortable<To>(), "Cannot sort owned storage");
+        assure<To>()->respect(*assure<From>());
+    }
+
+    /**
+     * @brief Visits an entity and returns the type info for its components.
+     *
+     * The signature of the function should be equivalent to the following:
+     *
+     * @code{.cpp}
+     * void(const type_info);
+     * @endcode
+     *
+     * Returned identifiers are those of the components owned by the entity.
+     *
+     * @sa type_info
+     *
+     * @warning
+     * It's not specified whether a component attached to or removed from the
+     * given entity during the visit is returned or not to the caller.
+     *
+     * @tparam Func Type of the function object to invoke.
+     * @param entity A valid entity identifier.
+     * @param func A valid function object.
+     */
+    template<typename Func>
+    void visit(entity_type entity, Func func) const {
+        for(auto pos = pools.size(); pos; --pos) {
+            if(const auto &pdata = pools[pos-1]; pdata.pool && pdata.pool->contains(entity)) {
+                func(pdata.poly->value_type());
+            }
+        }
+    }
+
+    /**
+     * @brief Visits a registry and returns the type info for its components.
+     *
+     * The signature of the function should be equivalent to the following:
+     *
+     * @code{.cpp}
+     * void(const type_info);
+     * @endcode
+     *
+     * Returned identifiers are those of the components managed by the registry.
+     *
+     * @sa type_info
+     *
+     * @warning
+     * It's not specified whether a component for which a pool is created during
+     * the visit is returned or not to the caller.
+     *
+     * @tparam Func Type of the function object to invoke.
+     * @param func A valid function object.
+     */
+    template<typename Func>
+    void visit(Func func) const {
+        for(auto pos = pools.size(); pos; --pos) {
+            if(const auto &pdata = pools[pos-1]; pdata.pool) {
+                func(pdata.poly->value_type());
+            }
+        }
+    }
+
+    /**
+     * @brief Binds an object to the context of the registry.
+     *
+     * If the value already exists it is overwritten, otherwise a new instance
+     * of the given type is created and initialized with the arguments provided.
+     *
+     * @tparam Type Type of object to set.
+     * @tparam Args Types of arguments to use to construct the object.
+     * @param args Parameters to use to initialize the value.
+     * @return A reference to the newly created object.
+     */
+    template<typename Type, typename... Args>
+    Type & set(Args &&... args) {
+        unset<Type>();
+        vars.emplace_back(std::in_place_type<Type>, std::forward<Args>(args)...);
+        return any_cast<Type &>(vars.back());
+    }
+
+    /**
+     * @brief Unsets a context variable if it exists.
+     * @tparam Type Type of object to set.
+     */
+    template<typename Type>
+    void unset() {
+        vars.erase(std::remove_if(vars.begin(), vars.end(), [type = type_id<Type>()](auto &&var) { return var.type() == type; }), vars.end());
+    }
+
+    /**
+     * @brief Binds an object to the context of the registry.
+     *
+     * In case the context doesn't contain the given object, the parameters
+     * provided are used to construct it.
+     *
+     * @tparam Type Type of object to set.
+     * @tparam Args Types of arguments to use to construct the object.
+     * @param args Parameters to use to initialize the object.
+     * @return A reference to the object in the context of the registry.
+     */
+    template<typename Type, typename... Args>
+    [[nodiscard]] Type & ctx_or_set(Args &&... args) {
+        auto *value = try_ctx<Type>();
+        return value ? *value : set<Type>(std::forward<Args>(args)...);
+    }
+
+    /**
+     * @brief Returns a pointer to an object in the context of the registry.
+     * @tparam Type Type of object to get.
+     * @return A pointer to the object if it exists in the context of the
+     * registry, a null pointer otherwise.
+     */
+    template<typename Type>
+    [[nodiscard]] std::add_const_t<Type> * try_ctx() const {
+        auto it = std::find_if(vars.cbegin(), vars.cend(), [type = type_id<Type>()](auto &&var) { return var.type() == type; });
+        return it == vars.cend() ? nullptr : any_cast<std::add_const_t<Type>>(&*it);
+    }
+
+    /*! @copydoc try_ctx */
+    template<typename Type>
+    [[nodiscard]] Type * try_ctx() {
+        auto it = std::find_if(vars.begin(), vars.end(), [type = type_id<Type>()](auto &&var) { return var.type() == type; });
+        return it == vars.end() ? nullptr : any_cast<Type>(&*it);
+    }
+
+    /**
+     * @brief Returns a reference to an object in the context of the registry.
+     *
+     * @warning
+     * Attempting to get a context variable that doesn't exist results in
+     * undefined behavior.
+     *
+     * @tparam Type Type of object to get.
+     * @return A valid reference to the object in the context of the registry.
+     */
+    template<typename Type>
+    [[nodiscard]] std::add_const_t<Type> & ctx() const {
+        auto it = std::find_if(vars.cbegin(), vars.cend(), [type = type_id<Type>()](auto &&var) { return var.type() == type; });
+        ENTT_ASSERT(it != vars.cend(), "Invalid instance");
+        return any_cast<std::add_const_t<Type> &>(*it);
+    }
+
+    /*! @copydoc ctx */
+    template<typename Type>
+    [[nodiscard]] Type & ctx() {
+        auto it = std::find_if(vars.begin(), vars.end(), [type = type_id<Type>()](auto &&var) { return var.type() == type; });
+        ENTT_ASSERT(it != vars.end(), "Invalid instance");
+        return any_cast<Type &>(*it);
+    }
+
+    /**
+     * @brief Visits a registry and returns the type info for its context
+     * variables.
+     *
+     * The signature of the function should be equivalent to the following:
+     *
+     * @code{.cpp}
+     * void(const type_info);
+     * @endcode
+     *
+     * Returned identifiers are those of the context variables currently set.
+     *
+     * @sa type_info
+     *
+     * @warning
+     * It's not specified whether a context variable created during the visit is
+     * returned or not to the caller.
+     *
+     * @tparam Func Type of the function object to invoke.
+     * @param func A valid function object.
+     */
+    template<typename Func>
+    void ctx(Func func) const {
+        for(auto pos = vars.size(); pos; --pos) {
+            func(vars[pos-1].type());
+        }
+    }
+
+private:
+    std::vector<basic_any<0u>> vars{};
+    mutable std::vector<pool_data> pools{};
+    std::vector<group_data> groups{};
+    std::vector<entity_type> entities{};
+    entity_type free_list{tombstone};
+};
+
+
+}
+
+
+#endif
+
+
+
+namespace entt {
+
+
+/**
+ * @brief Non-owning handle to an entity.
+ *
+ * Tiny wrapper around a registry and an entity.
+ *
+ * @tparam Entity A valid entity type (see entt_traits for more details).
+ * @tparam Type Types to which to restrict the scope of a handle.
+ */
+template<typename Entity, typename... Type>
+struct basic_handle {
+    /*! @brief Type of registry accepted by the handle. */
+    using registry_type = constness_as_t<basic_registry<std::remove_const_t<Entity>>, Entity>;
+    /*! @brief Underlying entity identifier. */
+    using entity_type = typename registry_type::entity_type;
+    /*! @brief Underlying version type. */
+    using version_type = typename registry_type::version_type;
+    /*! @brief Unsigned integer type. */
+    using size_type = typename registry_type::size_type;
+
+    /*! @brief Constructs an invalid handle. */
+    basic_handle() ENTT_NOEXCEPT
+        : reg{}, entt{null}
+    {}
+
+    /**
+     * @brief Constructs a handle from a given registry and entity.
+     * @param ref An instance of the registry class.
+     * @param value An entity identifier.
+     */
+    basic_handle(registry_type &ref, entity_type value) ENTT_NOEXCEPT
+        : reg{&ref}, entt{value}
+    {}
+
+    /**
+     * @brief Compares two handles.
+     * @tparam Args Template parameters of the handle with which to compare.
+     * @param other Handle with which to compare.
+     * @return True if both handles refer to the same registry and the same
+     * entity, false otherwise.
+     */
+    template<typename... Args>
+    [[nodiscard]] bool operator==(const basic_handle<Args...> &other) const ENTT_NOEXCEPT {
+        return reg == other.registry() && entt == other.entity();
+    }
+
+    /**
+     * @brief Constructs a const handle from a non-const one.
+     * @tparam Other A valid entity type (see entt_traits for more details).
+     * @tparam Args Scope of the handle to construct.
+     * @return A const handle referring to the same registry and the same
+     * entity.
+     */
+    template<typename Other, typename... Args>
+    operator basic_handle<Other, Args...>() const ENTT_NOEXCEPT {
+        static_assert(
+            (std::is_same_v<Other, Entity> || std::is_same_v<std::remove_const_t<Other>, Entity>)
+            && (sizeof...(Type) == 0 || ((sizeof...(Args) != 0 && sizeof...(Args) <= sizeof...(Type)) && ... && (type_list_contains_v<type_list<Type...>, Args>))),
+            "Invalid conversion between different handles"
+        );
+
+        return reg ? basic_handle<Other, Args...>{*reg, entt} : basic_handle<Other, Args...>{};
+    }
+
+    /**
+     * @brief Converts a handle to its underlying entity.
+     * @return An entity identifier.
+     */
+    [[nodiscard]] operator entity_type() const ENTT_NOEXCEPT {
+        return entity();
+    }
+
+    /**
+     * @brief Checks if a handle refers to non-null registry pointer and entity.
+     * @return True if the handle refers to non-null registry and entity, false otherwise.
+     */
+    [[nodiscard]] explicit operator bool() const ENTT_NOEXCEPT {
+        return reg && reg->valid(entt);
+    }
+
+    /**
+     * @brief Checks if a handle refers to a valid entity or not.
+     * @return True if the handle refers to a valid entity, false otherwise.
+     */
+    [[nodiscard]] bool valid() const {
+        return reg->valid(entt);
+    }
+
+    /**
+     * @brief Returns a pointer to the underlying registry, if any.
+     * @return A pointer to the underlying registry, if any.
+     */
+    [[nodiscard]] registry_type * registry() const ENTT_NOEXCEPT {
+        return reg;
+    }
+
+    /**
+     * @brief Returns the entity associated with a handle.
+     * @return The entity associated with the handle.
+     */
+    [[nodiscard]] entity_type entity() const ENTT_NOEXCEPT {
+        return entt;
+    }
+
+    /**
+     * @brief Destroys the entity associated with a handle.
+     * @sa basic_registry::destroy
+     */
+    void destroy() {
+        reg->destroy(entt);
+    }
+
+    /**
+     * @brief Destroys the entity associated with a handle.
+     * @sa basic_registry::destroy
+     * @param version A desired version upon destruction.
+     */
+    void destroy(const version_type version) {
+        reg->destroy(entt, version);
+    }
+
+    /**
+     * @brief Assigns the given component to a handle.
+     * @sa basic_registry::emplace
+     * @tparam Component Type of component to create.
+     * @tparam Args Types of arguments to use to construct the component.
+     * @param args Parameters to use to initialize the component.
+     * @return A reference to the newly created component.
+     */
+    template<typename Component, typename... Args>
+    decltype(auto) emplace(Args &&... args) const {
+        static_assert(((sizeof...(Type) == 0) || ... || std::is_same_v<Component, Type>), "Invalid type");
+        return reg->template emplace<Component>(entt, std::forward<Args>(args)...);
+    }
+
+    /**
+     * @brief Assigns or replaces the given component for a handle.
+     * @sa basic_registry::emplace_or_replace
+     * @tparam Component Type of component to assign or replace.
+     * @tparam Args Types of arguments to use to construct the component.
+     * @param args Parameters to use to initialize the component.
+     * @return A reference to the newly created component.
+     */
+    template<typename Component, typename... Args>
+    decltype(auto) emplace_or_replace(Args &&... args) const {
+        static_assert(((sizeof...(Type) == 0) || ... || std::is_same_v<Component, Type>), "Invalid type");
+        return reg->template emplace_or_replace<Component>(entt, std::forward<Args>(args)...);
+    }
+
+    /**
+     * @brief Patches the given component for a handle.
+     * @sa basic_registry::patch
+     * @tparam Component Type of component to patch.
+     * @tparam Func Types of the function objects to invoke.
+     * @param func Valid function objects.
+     * @return A reference to the patched component.
+     */
+    template<typename Component, typename... Func>
+    decltype(auto) patch(Func &&... func) const {
+        static_assert(((sizeof...(Type) == 0) || ... || std::is_same_v<Component, Type>), "Invalid type");
+        return reg->template patch<Component>(entt, std::forward<Func>(func)...);
+    }
+
+    /**
+     * @brief Replaces the given component for a handle.
+     * @sa basic_registry::replace
+     * @tparam Component Type of component to replace.
+     * @tparam Args Types of arguments to use to construct the component.
+     * @param args Parameters to use to initialize the component.
+     * @return A reference to the component being replaced.
+     */
+    template<typename Component, typename... Args>
+    decltype(auto) replace(Args &&... args) const {
+        static_assert(((sizeof...(Type) == 0) || ... || std::is_same_v<Component, Type>), "Invalid type");
+        return reg->template replace<Component>(entt, std::forward<Args>(args)...);
+    }
+
+    /**
+     * @brief Removes the given components from a handle.
+     * @sa basic_registry::remove
+     * @tparam Component Types of components to remove.
+     * @return The number of components actually removed.
+     */
+    template<typename... Component>
+    size_type remove() const {
+        static_assert(sizeof...(Type) == 0 || (type_list_contains_v<type_list<Type...>, Component> && ...), "Invalid type");
+        return reg->template remove<Component...>(entt);
+    }
+
+    /**
+     * @brief Erases the given components from a handle.
+     * @sa basic_registry::erase
+     * @tparam Component Types of components to erase.
+     */
+    template<typename... Component>
+    void erase() const {
+        static_assert(sizeof...(Type) == 0 || (type_list_contains_v<type_list<Type...>, Component> && ...), "Invalid type");
+        reg->template erase<Component...>(entt);
+    }
+
+    /*! @copydoc remove */
+    template<typename... Component>
+    [[deprecated("Use ::remove instead")]]
+    size_type remove_if_exists() const {
+        return remove<Component...>();
+    }
+
+    /**
+     * @brief Removes all the components from a handle and makes it orphaned.
+     * @sa basic_registry::remove_all
+     */
+    [[deprecated("No longer supported")]]
+    void remove_all() const {
+        static_assert(sizeof...(Type) == 0, "Invalid operation");
+        reg->remove_all(entt);
+    }
+
+    /**
+     * @brief Checks if a handle has all the given components.
+     * @sa basic_registry::all_of
+     * @tparam Component Components for which to perform the check.
+     * @return True if the handle has all the components, false otherwise.
+     */
+    template<typename... Component>
+    [[nodiscard]] decltype(auto) all_of() const {
+        return reg->template all_of<Component...>(entt);
+    }
+
+    /**
+     * @brief Checks if a handle has at least one of the given components.
+     * @sa basic_registry::any_of
+     * @tparam Component Components for which to perform the check.
+     * @return True if the handle has at least one of the given components,
+     * false otherwise.
+     */
+    template<typename... Component>
+    [[nodiscard]] decltype(auto) any_of() const {
+        return reg->template any_of<Component...>(entt);
+    }
+
+    /**
+     * @brief Returns references to the given components for a handle.
+     * @sa basic_registry::get
+     * @tparam Component Types of components to get.
+     * @return References to the components owned by the handle.
+     */
+    template<typename... Component>
+    [[nodiscard]] decltype(auto) get() const {
+        static_assert(sizeof...(Type) == 0 || (type_list_contains_v<type_list<Type...>, Component> && ...), "Invalid type");
+        return reg->template get<Component...>(entt);
+    }
+
+    /**
+     * @brief Returns a reference to the given component for a handle.
+     * @sa basic_registry::get_or_emplace
+     * @tparam Component Type of component to get.
+     * @tparam Args Types of arguments to use to construct the component.
+     * @param args Parameters to use to initialize the component.
+     * @return Reference to the component owned by the handle.
+     */
+    template<typename Component, typename... Args>
+    [[nodiscard]] decltype(auto) get_or_emplace(Args &&... args) const {
+        static_assert(((sizeof...(Type) == 0) || ... || std::is_same_v<Component, Type>), "Invalid type");
+        return reg->template get_or_emplace<Component>(entt, std::forward<Args>(args)...);
+    }
+
+    /**
+     * @brief Returns pointers to the given components for a handle.
+     * @sa basic_registry::try_get
+     * @tparam Component Types of components to get.
+     * @return Pointers to the components owned by the handle.
+     */
+    template<typename... Component>
+    [[nodiscard]] auto try_get() const {
+        static_assert(sizeof...(Type) == 0 || (type_list_contains_v<type_list<Type...>, Component> && ...), "Invalid type");
+        return reg->template try_get<Component...>(entt);
+    }
+
+    /**
+     * @brief Checks if a handle has components assigned.
+     * @return True if the handle has no components assigned, false otherwise.
+     */
+    [[nodiscard]] bool orphan() const {
+        return reg->orphan(entt);
+    }
+
+    /**
+     * @brief Visits a handle and returns the types for its components.
+     * @sa basic_registry::visit
+     * @tparam Func Type of the function object to invoke.
+     * @param func A valid function object.
+     */
+    template<typename Func>
+    void visit(Func &&func) const {
+        reg->visit(entt, std::forward<Func>(func));
+    }
+
+private:
+    registry_type *reg;
+    entity_type entt;
+};
+
+
+/**
+ * @brief Compares two handles.
+ * @tparam Type A valid entity type (see entt_traits for more details).
+ * @tparam Other A valid entity type (see entt_traits for more details).
+ * @param lhs A valid handle.
+ * @param rhs A valid handle.
+ * @return False if both handles refer to the same registry and the same
+ * entity, true otherwise.
+ */
+template<typename Type, typename Other>
+bool operator!=(const basic_handle<Type> &lhs, const basic_handle<Other> &rhs) ENTT_NOEXCEPT {
+    return !(lhs == rhs);
+}
+
+
+/**
+ * @brief Deduction guide.
+ * @tparam Entity A valid entity type (see entt_traits for more details).
+ */
+template<typename Entity>
+basic_handle(basic_registry<Entity> &, Entity)
+-> basic_handle<Entity>;
+
+
+/**
+ * @brief Deduction guide.
+ * @tparam Entity A valid entity type (see entt_traits for more details).
+ */
+template<typename Entity>
+basic_handle(const basic_registry<Entity> &, Entity)
+-> basic_handle<const Entity>;
+
+
+}
+
+
+#endif
+
+// #include "entity/helper.hpp"
+#ifndef ENTT_ENTITY_HELPER_HPP
+#define ENTT_ENTITY_HELPER_HPP
+
+
+#include <type_traits>
+// #include "../config/config.h"
+
+// #include "../core/fwd.hpp"
+
+// #include "../core/type_traits.hpp"
+
+// #include "../signal/delegate.hpp"
+#ifndef ENTT_SIGNAL_DELEGATE_HPP
+#define ENTT_SIGNAL_DELEGATE_HPP
+
+
+#include <tuple>
+#include <cstddef>
+#include <utility>
+#include <functional>
+#include <type_traits>
+// #include "../core/type_traits.hpp"
+
+// #include "../config/config.h"
+
+
+
+namespace entt {
+
+
+/**
+ * @cond TURN_OFF_DOXYGEN
+ * Internal details not to be documented.
+ */
+
+
+namespace internal {
+
+
+template<typename Ret, typename... Args>
+auto function_pointer(Ret(*)(Args...)) -> Ret(*)(Args...);
+
+
+template<typename Ret, typename Type, typename... Args, typename Other>
+auto function_pointer(Ret(*)(Type, Args...), Other &&) -> Ret(*)(Args...);
+
+
+template<typename Class, typename Ret, typename... Args, typename... Other>
+auto function_pointer(Ret(Class:: *)(Args...), Other &&...) -> Ret(*)(Args...);
+
+
+template<typename Class, typename Ret, typename... Args, typename... Other>
+auto function_pointer(Ret(Class:: *)(Args...) const, Other &&...) -> Ret(*)(Args...);
+
+
+template<typename Class, typename Type, typename... Other>
+auto function_pointer(Type Class:: *, Other &&...) -> Type(*)();
+
+
+template<typename... Type>
+using function_pointer_t = decltype(internal::function_pointer(std::declval<Type>()...));
+
+
+template<typename... Class, typename Ret, typename... Args>
+[[nodiscard]] constexpr auto index_sequence_for(Ret(*)(Args...)) {
+    return std::index_sequence_for<Class..., Args...>{};
+}
+
+
+}
+
+
+/**
+ * Internal details not to be documented.
+ * @endcond
+ */
+
+
+/*! @brief Used to wrap a function or a member of a specified type. */
+template<auto>
+struct connect_arg_t {};
+
+
+/*! @brief Constant of type connect_arg_t used to disambiguate calls. */
+template<auto Func>
+inline constexpr connect_arg_t<Func> connect_arg{};
+
+
+/**
+ * @brief Basic delegate implementation.
+ *
+ * Primary template isn't defined on purpose. All the specializations give a
+ * compile-time error unless the template parameter is a function type.
+ */
+template<typename>
+class delegate;
+
+
+/**
+ * @brief Utility class to use to send around functions and members.
+ *
+ * Unmanaged delegate for function pointers and members. Users of this class are
+ * in charge of disconnecting instances before deleting them.
+ *
+ * A delegate can be used as a general purpose invoker without memory overhead
+ * for free functions possibly with payloads and bound or unbound members.
+ *
+ * @tparam Ret Return type of a function type.
+ * @tparam Args Types of arguments of a function type.
+ */
+template<typename Ret, typename... Args>
+class delegate<Ret(Args...)> {
+    template<auto Candidate, std::size_t... Index>
+    [[nodiscard]] auto wrap(std::index_sequence<Index...>) ENTT_NOEXCEPT {
+        return [](const void *, Args... args) -> Ret {
+            [[maybe_unused]] const auto arguments = std::forward_as_tuple(std::forward<Args>(args)...);
+            return static_cast<Ret>(std::invoke(Candidate, std::forward<type_list_element_t<Index, type_list<Args...>>>(std::get<Index>(arguments))...));
+        };
+    }
+
+    template<auto Candidate, typename Type, std::size_t... Index>
+    [[nodiscard]] auto wrap(Type &, std::index_sequence<Index...>) ENTT_NOEXCEPT {
+        return [](const void *payload, Args... args) -> Ret {
+            [[maybe_unused]] const auto arguments = std::forward_as_tuple(std::forward<Args>(args)...);
+            Type *curr = static_cast<Type *>(const_cast<constness_as_t<void, Type> *>(payload));
+            return static_cast<Ret>(std::invoke(Candidate, *curr, std::forward<type_list_element_t<Index, type_list<Args...>>>(std::get<Index>(arguments))...));
+        };
+    }
+
+    template<auto Candidate, typename Type, std::size_t... Index>
+    [[nodiscard]] auto wrap(Type *, std::index_sequence<Index...>) ENTT_NOEXCEPT {
+        return [](const void *payload, Args... args) -> Ret {
+            [[maybe_unused]] const auto arguments = std::forward_as_tuple(std::forward<Args>(args)...);
+            Type *curr = static_cast<Type *>(const_cast<constness_as_t<void, Type> *>(payload));
+            return static_cast<Ret>(std::invoke(Candidate, curr, std::forward<type_list_element_t<Index, type_list<Args...>>>(std::get<Index>(arguments))...));
+        };
+    }
+
+public:
+    /*! @brief Function type of the contained target. */
+    using function_type = Ret(const void *, Args...);
+    /*! @brief Function type of the delegate. */
+    using type = Ret(Args...);
+    /*! @brief Return type of the delegate. */
+    using result_type = Ret;
+
+    /*! @brief Default constructor. */
+    delegate() ENTT_NOEXCEPT
+        : fn{nullptr}, data{nullptr}
+    {}
+
+    /**
+     * @brief Constructs a delegate and connects a free function or an unbound
+     * member.
+     * @tparam Candidate Function or member to connect to the delegate.
+     */
+    template<auto Candidate>
+    delegate(connect_arg_t<Candidate>) ENTT_NOEXCEPT {
+        connect<Candidate>();
+    }
+
+    /**
+     * @brief Constructs a delegate and connects a free function with payload or
+     * a bound member.
+     * @tparam Candidate Function or member to connect to the delegate.
+     * @tparam Type Type of class or type of payload.
+     * @param value_or_instance A valid object that fits the purpose.
+     */
+    template<auto Candidate, typename Type>
+    delegate(connect_arg_t<Candidate>, Type &&value_or_instance) ENTT_NOEXCEPT {
+        connect<Candidate>(std::forward<Type>(value_or_instance));
+    }
+
+    /**
+     * @brief Constructs a delegate and connects an user defined function with
+     * optional payload.
+     * @param function Function to connect to the delegate.
+     * @param payload User defined arbitrary data.
+     */
+    delegate(function_type *function, const void *payload = nullptr) ENTT_NOEXCEPT {
+        connect(function, payload);
+    }
+
+    /**
+     * @brief Connects a free function or an unbound member to a delegate.
+     * @tparam Candidate Function or member to connect to the delegate.
+     */
+    template<auto Candidate>
+    void connect() ENTT_NOEXCEPT {
+        data = nullptr;
+
+        if constexpr(std::is_invocable_r_v<Ret, decltype(Candidate), Args...>) {
+            fn = [](const void *, Args... args) -> Ret {
+                return Ret(std::invoke(Candidate, std::forward<Args>(args)...));
+            };
+        } else if constexpr(std::is_member_pointer_v<decltype(Candidate)>) {
+            fn = wrap<Candidate>(internal::index_sequence_for<type_list_element_t<0, type_list<Args...>>>(internal::function_pointer_t<decltype(Candidate)>{}));
+        } else {
+            fn = wrap<Candidate>(internal::index_sequence_for(internal::function_pointer_t<decltype(Candidate)>{}));
+        }
+    }
+
+    /**
+     * @brief Connects a free function with payload or a bound member to a
+     * delegate.
+     *
+     * The delegate isn't responsible for the connected object or the payload.
+     * Users must always guarantee that the lifetime of the instance overcomes
+     * the one of the delegate.<br/>
+     * When used to connect a free function with payload, its signature must be
+     * such that the instance is the first argument before the ones used to
+     * define the delegate itself.
+     *
+     * @tparam Candidate Function or member to connect to the delegate.
+     * @tparam Type Type of class or type of payload.
+     * @param value_or_instance A valid reference that fits the purpose.
+     */
+    template<auto Candidate, typename Type>
+    void connect(Type &value_or_instance) ENTT_NOEXCEPT {
+        data = &value_or_instance;
+
+        if constexpr(std::is_invocable_r_v<Ret, decltype(Candidate), Type &, Args...>) {
+            fn = [](const void *payload, Args... args) -> Ret {
+                Type *curr = static_cast<Type *>(const_cast<constness_as_t<void, Type> *>(payload));
+                return Ret(std::invoke(Candidate, *curr, std::forward<Args>(args)...));
+            };
+        } else {
+            fn = wrap<Candidate>(value_or_instance, internal::index_sequence_for(internal::function_pointer_t<decltype(Candidate), Type>{}));
+        }
+    }
+
+    /**
+     * @brief Connects a free function with payload or a bound member to a
+     * delegate.
+     *
+     * @sa connect(Type &)
+     *
+     * @tparam Candidate Function or member to connect to the delegate.
+     * @tparam Type Type of class or type of payload.
+     * @param value_or_instance A valid pointer that fits the purpose.
+     */
+    template<auto Candidate, typename Type>
+    void connect(Type *value_or_instance) ENTT_NOEXCEPT {
+        data = value_or_instance;
+
+        if constexpr(std::is_invocable_r_v<Ret, decltype(Candidate), Type *, Args...>) {
+            fn = [](const void *payload, Args... args) -> Ret {
+                Type *curr = static_cast<Type *>(const_cast<constness_as_t<void, Type> *>(payload));
+                return Ret(std::invoke(Candidate, curr, std::forward<Args>(args)...));
+            };
+        } else {
+            fn = wrap<Candidate>(value_or_instance, internal::index_sequence_for(internal::function_pointer_t<decltype(Candidate), Type>{}));
+        }
+    }
+
+    /**
+     * @brief Connects an user defined function with optional payload to a
+     * delegate.
+     *
+     * The delegate isn't responsible for the connected object or the payload.
+     * Users must always guarantee that the lifetime of an instance overcomes
+     * the one of the delegate.<br/>
+     * The payload is returned as the first argument to the target function in
+     * all cases.
+     *
+     * @param function Function to connect to the delegate.
+     * @param payload User defined arbitrary data.
+     */
+    void connect(function_type *function, const void *payload = nullptr) ENTT_NOEXCEPT {
+        fn = function;
+        data = payload;
+    }
+
+    /**
+     * @brief Resets a delegate.
+     *
+     * After a reset, a delegate cannot be invoked anymore.
+     */
+    void reset() ENTT_NOEXCEPT {
+        fn = nullptr;
+        data = nullptr;
+    }
+
+    /**
+     * @brief Returns the instance or the payload linked to a delegate, if any.
+     * @return An opaque pointer to the underlying data.
+     */
+    [[nodiscard]] const void * instance() const ENTT_NOEXCEPT {
+        return data;
+    }
+
+    /**
+     * @brief Triggers a delegate.
+     *
+     * The delegate invokes the underlying function and returns the result.
+     *
+     * @warning
+     * Attempting to trigger an invalid delegate results in undefined
+     * behavior.
+     *
+     * @param args Arguments to use to invoke the underlying function.
+     * @return The value returned by the underlying function.
+     */
+    Ret operator()(Args... args) const {
+        ENTT_ASSERT(static_cast<bool>(*this), "Uninitialized delegate");
+        return fn(data, std::forward<Args>(args)...);
+    }
+
+    /**
+     * @brief Checks whether a delegate actually stores a listener.
+     * @return False if the delegate is empty, true otherwise.
+     */
+    [[nodiscard]] explicit operator bool() const ENTT_NOEXCEPT {
+        // no need to test also data
+        return !(fn == nullptr);
+    }
+
+    /**
+     * @brief Compares the contents of two delegates.
+     * @param other Delegate with which to compare.
+     * @return False if the two contents differ, true otherwise.
+     */
+    [[nodiscard]] bool operator==(const delegate<Ret(Args...)> &other) const ENTT_NOEXCEPT {
+        return fn == other.fn && data == other.data;
+    }
+
+private:
+    function_type *fn;
+    const void *data;
+};
+
+
+/**
+ * @brief Compares the contents of two delegates.
+ * @tparam Ret Return type of a function type.
+ * @tparam Args Types of arguments of a function type.
+ * @param lhs A valid delegate object.
+ * @param rhs A valid delegate object.
+ * @return True if the two contents differ, false otherwise.
+ */
+template<typename Ret, typename... Args>
+[[nodiscard]] bool operator!=(const delegate<Ret(Args...)> &lhs, const delegate<Ret(Args...)> &rhs) ENTT_NOEXCEPT {
+    return !(lhs == rhs);
+}
+
+
+/**
+ * @brief Deduction guide.
+ * @tparam Candidate Function or member to connect to the delegate.
+ */
+template<auto Candidate>
+delegate(connect_arg_t<Candidate>)
+-> delegate<std::remove_pointer_t<internal::function_pointer_t<decltype(Candidate)>>>;
+
+
+/**
+ * @brief Deduction guide.
+ * @tparam Candidate Function or member to connect to the delegate.
+ * @tparam Type Type of class or type of payload.
+ */
+template<auto Candidate, typename Type>
+delegate(connect_arg_t<Candidate>, Type &&)
+-> delegate<std::remove_pointer_t<internal::function_pointer_t<decltype(Candidate), Type>>>;
+
+
+/**
+ * @brief Deduction guide.
+ * @tparam Ret Return type of a function type.
+ * @tparam Args Types of arguments of a function type.
+ */
+template<typename Ret, typename... Args>
+delegate(Ret(*)(const void *, Args...), const void * = nullptr)
+-> delegate<Ret(Args...)>;
+
+
+}
+
+
+#endif
+
+// #include "registry.hpp"
+
+// #include "fwd.hpp"
+
+
+
+namespace entt {
+
+
+/**
+ * @brief Converts a registry to a view.
+ * @tparam Entity A valid entity type (see entt_traits for more details).
+ */
+template<typename Entity>
+struct as_view {
+    /*! @brief Underlying entity identifier. */
+    using entity_type = std::remove_const_t<Entity>;
+    /*! @brief Type of registry to convert. */
+    using registry_type = constness_as_t<basic_registry<entity_type>, Entity>;
+
+    /**
+     * @brief Constructs a converter for a given registry.
+     * @param source A valid reference to a registry.
+     */
+    as_view(registry_type &source) ENTT_NOEXCEPT: reg{source} {}
+
+    /**
+     * @brief Conversion function from a registry to a view.
+     * @tparam Exclude Types of components used to filter the view.
+     * @tparam Component Type of components used to construct the view.
+     * @return A newly created view.
+     */
+    template<typename Exclude, typename... Component>
+    operator basic_view<entity_type, Exclude, Component...>() const {
+        return reg.template view<Component...>(Exclude{});
+    }
+
+private:
+    registry_type &reg;
+};
+
+
+/**
+ * @brief Deduction guide.
+ * @tparam Entity A valid entity type (see entt_traits for more details).
+ */
+template<typename Entity>
+as_view(basic_registry<Entity> &) -> as_view<Entity>;
+
+
+/**
+ * @brief Deduction guide.
+ * @tparam Entity A valid entity type (see entt_traits for more details).
+ */
+template<typename Entity>
+as_view(const basic_registry<Entity> &) -> as_view<const Entity>;
+
+
+/**
+ * @brief Converts a registry to a group.
+ * @tparam Entity A valid entity type (see entt_traits for more details).
+ */
+template<typename Entity>
+struct as_group {
+    /*! @brief Underlying entity identifier. */
+    using entity_type = std::remove_const_t<Entity>;
+    /*! @brief Type of registry to convert. */
+    using registry_type = constness_as_t<basic_registry<entity_type>, Entity>;
+
+    /**
+     * @brief Constructs a converter for a given registry.
+     * @param source A valid reference to a registry.
+     */
+    as_group(registry_type &source) ENTT_NOEXCEPT: reg{source} {}
+
+    /**
+     * @brief Conversion function from a registry to a group.
+     * @tparam Exclude Types of components used to filter the group.
+     * @tparam Get Types of components observed by the group.
+     * @tparam Owned Types of components owned by the group.
+     * @return A newly created group.
+     */
+    template<typename Exclude, typename Get, typename... Owned>
+    operator basic_group<entity_type, Exclude, Get, Owned...>() const {
+        if constexpr(std::is_const_v<registry_type>) {
+            return reg.template group_if_exists<Owned...>(Get{}, Exclude{});
+        } else {
+            return reg.template group<Owned...>(Get{}, Exclude{});
+        }
+    }
+
+private:
+    registry_type &reg;
+};
+
+
+/**
+ * @brief Deduction guide.
+ * @tparam Entity A valid entity type (see entt_traits for more details).
+ */
+template<typename Entity>
+as_group(basic_registry<Entity> &) -> as_group<Entity>;
+
+
+/**
+ * @brief Deduction guide.
+ * @tparam Entity A valid entity type (see entt_traits for more details).
+ */
+template<typename Entity>
+as_group(const basic_registry<Entity> &) -> as_group<const Entity>;
+
+
+
+/**
+ * @brief Helper to create a listener that directly invokes a member function.
+ * @tparam Member Member function to invoke on a component of the given type.
+ * @tparam Entity A valid entity type (see entt_traits for more details).
+ * @param reg A registry that contains the given entity and its components.
+ * @param entt Entity from which to get the component.
+ */
+template<auto Member, typename Entity = entity>
+void invoke(basic_registry<Entity> &reg, const Entity entt) {
+    static_assert(std::is_member_function_pointer_v<decltype(Member)>, "Invalid pointer to non-static member function");
+    delegate<void(basic_registry<Entity> &, const Entity)> func;
+    func.template connect<Member>(reg.template get<member_class_t<decltype(Member)>>(entt));
+    func(reg, entt);
+}
+
+
+/**
+ * @brief Returns the entity associated with a given component.
+ *
+ * @warning
+ * Currently, this function only works correctly with the default pool as it
+ * makes assumptions about how the components are laid out.
+ *
+ * @tparam Entity A valid entity type (see entt_traits for more details).
+ * @tparam Component Type of component.
+ * @param reg A registry that contains the given entity and its components.
+ * @param instance A valid component instance.
+ * @return The entity associated with the given component.
+ */
+template<typename Entity, typename Component>
+Entity to_entity(const basic_registry<Entity> &reg, const Component &instance) {
+    const auto view = reg.template view<const Component>();
+    const auto *addr = std::addressof(instance);
+
+    for(auto it = view.rbegin(), last = view.rend(); it < last; it += ENTT_PACKED_PAGE) {
+        if(const auto dist = (addr - std::addressof(view.template get<const Component>(*it))); dist >= 0 && dist < ENTT_PACKED_PAGE) {
+            return *(it + dist);
+        }
+    }
+
+    return entt::null;
+}
+
+
+}
+
+
+#endif
+
+// #include "entity/observer.hpp"
+#ifndef ENTT_ENTITY_OBSERVER_HPP
+#define ENTT_ENTITY_OBSERVER_HPP
+
+
+#include <limits>
+#include <cstddef>
+#include <cstdint>
+#include <utility>
+#include <type_traits>
+// #include "../config/config.h"
+
+// #include "../core/type_traits.hpp"
+
+// #include "../signal/delegate.hpp"
+
+// #include "registry.hpp"
+
+// #include "storage.hpp"
+
+// #include "utility.hpp"
+
+// #include "entity.hpp"
+
+// #include "fwd.hpp"
+
+
+
+namespace entt {
+
+
+/*! @brief Grouping matcher. */
+template<typename...>
+struct matcher {};
+
+
+/**
+ * @brief Collector.
+ *
+ * Primary template isn't defined on purpose. All the specializations give a
+ * compile-time error, but for a few reasonable cases.
+ */
+template<typename...>
+struct basic_collector;
+
+
+/**
+ * @brief Collector.
+ *
+ * A collector contains a set of rules (literally, matchers) to use to track
+ * entities.<br/>
+ * Its main purpose is to generate a descriptor that allows an observer to know
+ * how to connect to a registry.
+ */
+template<>
+struct basic_collector<> {
+    /**
+     * @brief Adds a grouping matcher to the collector.
+     * @tparam AllOf Types of components tracked by the matcher.
+     * @tparam NoneOf Types of components used to filter out entities.
+     * @return The updated collector.
+     */
+    template<typename... AllOf, typename... NoneOf>
+    static constexpr auto group(exclude_t<NoneOf...> = {}) ENTT_NOEXCEPT {
+        return basic_collector<matcher<type_list<>, type_list<>, type_list<NoneOf...>, AllOf...>>{};
+    }
+
+    /**
+     * @brief Adds an observing matcher to the collector.
+     * @tparam AnyOf Type of component for which changes should be detected.
+     * @return The updated collector.
+     */
+    template<typename AnyOf>
+    static constexpr auto update() ENTT_NOEXCEPT {
+        return basic_collector<matcher<type_list<>, type_list<>, AnyOf>>{};
+    }
+};
+
+/**
+ * @brief Collector.
+ * @copydetails basic_collector<>
+ * @tparam Reject Untracked types used to filter out entities.
+ * @tparam Require Untracked types required by the matcher.
+ * @tparam Rule Specific details of the current matcher.
+ * @tparam Other Other matchers.
+ */
+template<typename... Reject, typename... Require, typename... Rule, typename... Other>
+struct basic_collector<matcher<type_list<Reject...>, type_list<Require...>, Rule...>, Other...> {
+    /*! @brief Current matcher. */
+    using current_type = matcher<type_list<Reject...>, type_list<Require...>, Rule...>;
+
+    /**
+     * @brief Adds a grouping matcher to the collector.
+     * @tparam AllOf Types of components tracked by the matcher.
+     * @tparam NoneOf Types of components used to filter out entities.
+     * @return The updated collector.
+     */
+    template<typename... AllOf, typename... NoneOf>
+    static constexpr auto group(exclude_t<NoneOf...> = {}) ENTT_NOEXCEPT {
+        return basic_collector<matcher<type_list<>, type_list<>, type_list<NoneOf...>, AllOf...>, current_type, Other...>{};
+    }
+
+    /**
+     * @brief Adds an observing matcher to the collector.
+     * @tparam AnyOf Type of component for which changes should be detected.
+     * @return The updated collector.
+     */
+    template<typename AnyOf>
+    static constexpr auto update() ENTT_NOEXCEPT {
+        return basic_collector<matcher<type_list<>, type_list<>, AnyOf>, current_type, Other...>{};
+    }
+
+    /**
+     * @brief Updates the filter of the last added matcher.
+     * @tparam AllOf Types of components required by the matcher.
+     * @tparam NoneOf Types of components used to filter out entities.
+     * @return The updated collector.
+     */
+    template<typename... AllOf, typename... NoneOf>
+    static constexpr auto where(exclude_t<NoneOf...> = {}) ENTT_NOEXCEPT {
+        using extended_type = matcher<type_list<Reject..., NoneOf...>, type_list<Require..., AllOf...>, Rule...>;
+        return basic_collector<extended_type, Other...>{};
+    }
+};
+
+
+/*! @brief Variable template used to ease the definition of collectors. */
+inline constexpr basic_collector<> collector{};
+
+
+/**
+ * @brief Observer.
+ *
+ * An observer returns all the entities and only the entities that fit the
+ * requirements of at least one matcher. Moreover, it's guaranteed that the
+ * entity list is tightly packed in memory for fast iterations.<br/>
+ * In general, observers don't stay true to the order of any set of components.
+ *
+ * Observers work mainly with two types of matchers, provided through a
+ * collector:
+ *
+ * * Observing matcher: an observer will return at least all the living entities
+ *   for which one or more of the given components have been updated and not yet
+ *   destroyed.
+ * * Grouping matcher: an observer will return at least all the living entities
+ *   that would have entered the given group if it existed and that would have
+ *   not yet left it.
+ *
+ * If an entity respects the requirements of multiple matchers, it will be
+ * returned once and only once by the observer in any case.
+ *
+ * Matchers support also filtering by means of a _where_ clause that accepts
+ * both a list of types and an exclusion list.<br/>
+ * Whenever a matcher finds that an entity matches its requirements, the
+ * condition of the filter is verified before to register the entity itself.
+ * Moreover, a registered entity isn't returned by the observer if the condition
+ * set by the filter is broken in the meantime.
+ *
+ * @b Important
+ *
+ * Iterators aren't invalidated if:
+ *
+ * * New instances of the given components are created and assigned to entities.
+ * * The entity currently pointed is modified (as an example, if one of the
+ *   given components is removed from the entity to which the iterator points).
+ * * The entity currently pointed is destroyed.
+ *
+ * In all the other cases, modifying the pools of the given components in any
+ * way invalidates all the iterators and using them results in undefined
+ * behavior.
+ *
+ * @warning
+ * Lifetime of an observer doesn't necessarily have to overcome that of the
+ * registry to which it is connected. However, the observer must be disconnected
+ * from the registry before being destroyed to avoid crashes due to dangling
+ * pointers.
+ *
+ * @tparam Entity A valid entity type (see entt_traits for more details).
+ */
+template<typename Entity>
+class basic_observer {
+    using payload_type = std::uint32_t;
+
+    template<typename>
+    struct matcher_handler;
+
+    template<typename... Reject, typename... Require, typename AnyOf>
+    struct matcher_handler<matcher<type_list<Reject...>, type_list<Require...>, AnyOf>> {
+        template<std::size_t Index>
+        static void maybe_valid_if(basic_observer &obs, basic_registry<Entity> &reg, const Entity entt) {
+            if(reg.template all_of<Require...>(entt) && !reg.template any_of<Reject...>(entt)) {
+                if(!obs.storage.contains(entt)) {
+                    obs.storage.emplace(entt);
+                }
+
+                obs.storage.get(entt) |= (1 << Index);
+            }
+        }
+
+        template<std::size_t Index>
+        static void discard_if(basic_observer &obs, basic_registry<Entity> &, const Entity entt) {
+            if(obs.storage.contains(entt) && !(obs.storage.get(entt) &= (~(1 << Index)))) {
+                obs.storage.erase(entt);
+            }
+        }
+
+        template<std::size_t Index>
+        static void connect(basic_observer &obs, basic_registry<Entity> &reg) {
+            (reg.template on_destroy<Require>().template connect<&discard_if<Index>>(obs), ...);
+            (reg.template on_construct<Reject>().template connect<&discard_if<Index>>(obs), ...);
+            reg.template on_update<AnyOf>().template connect<&maybe_valid_if<Index>>(obs);
+            reg.template on_destroy<AnyOf>().template connect<&discard_if<Index>>(obs);
+        }
+
+        static void disconnect(basic_observer &obs, basic_registry<Entity> &reg) {
+            (reg.template on_destroy<Require>().disconnect(obs), ...);
+            (reg.template on_construct<Reject>().disconnect(obs), ...);
+            reg.template on_update<AnyOf>().disconnect(obs);
+            reg.template on_destroy<AnyOf>().disconnect(obs);
+        }
+    };
+
+    template<typename... Reject, typename... Require, typename... NoneOf, typename... AllOf>
+    struct matcher_handler<matcher<type_list<Reject...>, type_list<Require...>, type_list<NoneOf...>, AllOf...>> {
+        template<std::size_t Index, typename... Ignore>
+        static void maybe_valid_if(basic_observer &obs, basic_registry<Entity> &reg, const Entity entt) {
+            if([&reg, entt]() {
+                if constexpr(sizeof...(Ignore) == 0) {
+                    return reg.template all_of<AllOf..., Require...>(entt) && !reg.template any_of<NoneOf..., Reject...>(entt);
+                } else {
+                    return reg.template all_of<AllOf..., Require...>(entt) && ((std::is_same_v<Ignore..., NoneOf> || !reg.template any_of<NoneOf>(entt)) && ...) && !reg.template any_of<Reject...>(entt);
+                }
+            }())
+            {
+                if(!obs.storage.contains(entt)) {
+                    obs.storage.emplace(entt);
+                }
+
+                obs.storage.get(entt) |= (1 << Index);
+            }
+        }
+
+        template<std::size_t Index>
+        static void discard_if(basic_observer &obs, basic_registry<Entity> &, const Entity entt) {
+            if(obs.storage.contains(entt) && !(obs.storage.get(entt) &= (~(1 << Index)))) {
+                obs.storage.erase(entt);
+            }
+        }
+
+        template<std::size_t Index>
+        static void connect(basic_observer &obs, basic_registry<Entity> &reg) {
+            (reg.template on_destroy<Require>().template connect<&discard_if<Index>>(obs), ...);
+            (reg.template on_construct<Reject>().template connect<&discard_if<Index>>(obs), ...);
+            (reg.template on_construct<AllOf>().template connect<&maybe_valid_if<Index>>(obs), ...);
+            (reg.template on_destroy<NoneOf>().template connect<&maybe_valid_if<Index, NoneOf>>(obs), ...);
+            (reg.template on_destroy<AllOf>().template connect<&discard_if<Index>>(obs), ...);
+            (reg.template on_construct<NoneOf>().template connect<&discard_if<Index>>(obs), ...);
+        }
+
+        static void disconnect(basic_observer &obs, basic_registry<Entity> &reg) {
+            (reg.template on_destroy<Require>().disconnect(obs), ...);
+            (reg.template on_construct<Reject>().disconnect(obs), ...);
+            (reg.template on_construct<AllOf>().disconnect(obs), ...);
+            (reg.template on_destroy<NoneOf>().disconnect(obs), ...);
+            (reg.template on_destroy<AllOf>().disconnect(obs), ...);
+            (reg.template on_construct<NoneOf>().disconnect(obs), ...);
+        }
+    };
+
+    template<typename... Matcher>
+    static void disconnect(basic_registry<Entity> &reg, basic_observer &obs) {
+        (matcher_handler<Matcher>::disconnect(obs, reg), ...);
+    }
+
+    template<typename... Matcher, std::size_t... Index>
+    void connect(basic_registry<Entity> &reg, std::index_sequence<Index...>) {
+        static_assert(sizeof...(Matcher) < std::numeric_limits<payload_type>::digits, "Too many matchers");
+        (matcher_handler<Matcher>::template connect<Index>(*this, reg), ...);
+        release.template connect<&basic_observer::disconnect<Matcher...>>(reg);
+    }
+
+public:
+    /*! @brief Underlying entity identifier. */
+    using entity_type = Entity;
+    /*! @brief Unsigned integer type. */
+    using size_type = std::size_t;
+    /*! @brief Random access iterator type. */
+    using iterator = typename basic_sparse_set<Entity>::iterator;
+
+    /*! @brief Default constructor. */
+    basic_observer()
+        : release{},
+          storage{}
+    {}
+
+    /*! @brief Default copy constructor, deleted on purpose. */
+    basic_observer(const basic_observer &) = delete;
+    /*! @brief Default move constructor, deleted on purpose. */
+    basic_observer(basic_observer &&) = delete;
+
+    /**
+     * @brief Creates an observer and connects it to a given registry.
+     * @tparam Matcher Types of matchers to use to initialize the observer.
+     * @param reg A valid reference to a registry.
+     */
+    template<typename... Matcher>
+    basic_observer(basic_registry<entity_type> &reg, basic_collector<Matcher...>)
+        : basic_observer{}
+    {
+        connect<Matcher...>(reg, std::index_sequence_for<Matcher...>{});
+    }
+
+    /*! @brief Default destructor. */
+    ~basic_observer() = default;
+
+    /**
+     * @brief Default copy assignment operator, deleted on purpose.
+     * @return This observer.
+     */
+    basic_observer & operator=(const basic_observer &) = delete;
+
+    /**
+     * @brief Default move assignment operator, deleted on purpose.
+     * @return This observer.
+     */
+    basic_observer & operator=(basic_observer &&) = delete;
+
+    /**
+     * @brief Connects an observer to a given registry.
+     * @tparam Matcher Types of matchers to use to initialize the observer.
+     * @param reg A valid reference to a registry.
+     */
+    template<typename... Matcher>
+    void connect(basic_registry<entity_type> &reg, basic_collector<Matcher...>) {
+        disconnect();
+        connect<Matcher...>(reg, std::index_sequence_for<Matcher...>{});
+        storage.clear();
+    }
+
+    /*! @brief Disconnects an observer from the registry it keeps track of. */
+    void disconnect() {
+        if(release) {
+            release(*this);
+            release.reset();
+        }
+    }
+
+    /**
+     * @brief Returns the number of elements in an observer.
+     * @return Number of elements.
+     */
+    [[nodiscard]] size_type size() const ENTT_NOEXCEPT {
+        return storage.size();
+    }
+
+    /**
+     * @brief Checks whether an observer is empty.
+     * @return True if the observer is empty, false otherwise.
+     */
+    [[nodiscard]] bool empty() const ENTT_NOEXCEPT {
+        return storage.empty();
+    }
+
+    /**
+     * @brief Direct access to the list of entities of the observer.
+     *
+     * The returned pointer is such that range `[data(), data() + size())` is
+     * always a valid range, even if the container is empty.
+     *
+     * @note
+     * Entities are in the reverse order as returned by the `begin`/`end`
+     * iterators.
+     *
+     * @return A pointer to the array of entities.
+     */
+    [[nodiscard]] const entity_type * data() const ENTT_NOEXCEPT {
+        return storage.data();
+    }
+
+    /**
+     * @brief Returns an iterator to the first entity of the observer.
+     *
+     * The returned iterator points to the first entity of the observer. If the
+     * container is empty, the returned iterator will be equal to `end()`.
+     *
+     * @return An iterator to the first entity of the observer.
+     */
+    [[nodiscard]] iterator begin() const ENTT_NOEXCEPT {
+        return storage.basic_sparse_set<entity_type>::begin();
+    }
+
+    /**
+     * @brief Returns an iterator that is past the last entity of the observer.
+     *
+     * The returned iterator points to the entity following the last entity of
+     * the observer. Attempting to dereference the returned iterator results in
+     * undefined behavior.
+     *
+     * @return An iterator to the entity following the last entity of the
+     * observer.
+     */
+    [[nodiscard]] iterator end() const ENTT_NOEXCEPT {
+        return storage.basic_sparse_set<entity_type>::end();
+    }
+
+    /*! @brief Clears the underlying container. */
+    void clear() ENTT_NOEXCEPT {
+        storage.clear();
+    }
+
+    /**
+     * @brief Iterates entities and applies the given function object to them.
+     *
+     * The function object is invoked for each entity.<br/>
+     * The signature of the function must be equivalent to the following form:
+     *
+     * @code{.cpp}
+     * void(const entity_type);
+     * @endcode
+     *
+     * @tparam Func Type of the function object to invoke.
+     * @param func A valid function object.
+     */
+    template<typename Func>
+    void each(Func func) const {
+        for(const auto entity: *this) {
+            func(entity);
+        }
+    }
+
+    /**
+     * @brief Iterates entities and applies the given function object to them,
+     * then clears the observer.
+     *
+     * @sa each
+     *
+     * @tparam Func Type of the function object to invoke.
+     * @param func A valid function object.
+     */
+    template<typename Func>
+    void each(Func func) {
+        std::as_const(*this).each(std::move(func));
+        clear();
+    }
+
+private:
+    delegate<void(basic_observer &)> release;
+    basic_storage<entity_type, payload_type> storage;
+};
+
+
+}
+
+
+#endif
+
+// #include "entity/organizer.hpp"
+#ifndef ENTT_ENTITY_ORGANIZER_HPP
+#define ENTT_ENTITY_ORGANIZER_HPP
+
+
+#include <cstddef>
+#include <algorithm>
+#include <type_traits>
+#include <unordered_map>
+#include <utility>
+#include <vector>
+// #include "../core/type_info.hpp"
+
+// #include "../core/type_traits.hpp"
+
+// #include "fwd.hpp"
+
+// #include "helper.hpp"
+#ifndef ENTT_ENTITY_HELPER_HPP
+#define ENTT_ENTITY_HELPER_HPP
+
+
+#include <type_traits>
+// #include "../config/config.h"
+
+// #include "../core/fwd.hpp"
+
+// #include "../core/type_traits.hpp"
+
+// #include "../signal/delegate.hpp"
+
+// #include "registry.hpp"
+
+// #include "fwd.hpp"
+
+
+
+namespace entt {
+
+
+/**
+ * @brief Converts a registry to a view.
+ * @tparam Entity A valid entity type (see entt_traits for more details).
+ */
+template<typename Entity>
+struct as_view {
+    /*! @brief Underlying entity identifier. */
+    using entity_type = std::remove_const_t<Entity>;
+    /*! @brief Type of registry to convert. */
+    using registry_type = constness_as_t<basic_registry<entity_type>, Entity>;
+
+    /**
+     * @brief Constructs a converter for a given registry.
+     * @param source A valid reference to a registry.
+     */
+    as_view(registry_type &source) ENTT_NOEXCEPT: reg{source} {}
+
+    /**
+     * @brief Conversion function from a registry to a view.
+     * @tparam Exclude Types of components used to filter the view.
+     * @tparam Component Type of components used to construct the view.
+     * @return A newly created view.
+     */
+    template<typename Exclude, typename... Component>
+    operator basic_view<entity_type, Exclude, Component...>() const {
+        return reg.template view<Component...>(Exclude{});
+    }
+
+private:
+    registry_type &reg;
+};
+
+
+/**
+ * @brief Deduction guide.
+ * @tparam Entity A valid entity type (see entt_traits for more details).
+ */
+template<typename Entity>
+as_view(basic_registry<Entity> &) -> as_view<Entity>;
+
+
+/**
+ * @brief Deduction guide.
+ * @tparam Entity A valid entity type (see entt_traits for more details).
+ */
+template<typename Entity>
+as_view(const basic_registry<Entity> &) -> as_view<const Entity>;
+
+
+/**
+ * @brief Converts a registry to a group.
+ * @tparam Entity A valid entity type (see entt_traits for more details).
+ */
+template<typename Entity>
+struct as_group {
+    /*! @brief Underlying entity identifier. */
+    using entity_type = std::remove_const_t<Entity>;
+    /*! @brief Type of registry to convert. */
+    using registry_type = constness_as_t<basic_registry<entity_type>, Entity>;
+
+    /**
+     * @brief Constructs a converter for a given registry.
+     * @param source A valid reference to a registry.
+     */
+    as_group(registry_type &source) ENTT_NOEXCEPT: reg{source} {}
+
+    /**
+     * @brief Conversion function from a registry to a group.
+     * @tparam Exclude Types of components used to filter the group.
+     * @tparam Get Types of components observed by the group.
+     * @tparam Owned Types of components owned by the group.
+     * @return A newly created group.
+     */
+    template<typename Exclude, typename Get, typename... Owned>
+    operator basic_group<entity_type, Exclude, Get, Owned...>() const {
+        if constexpr(std::is_const_v<registry_type>) {
+            return reg.template group_if_exists<Owned...>(Get{}, Exclude{});
+        } else {
+            return reg.template group<Owned...>(Get{}, Exclude{});
+        }
+    }
+
+private:
+    registry_type &reg;
+};
+
+
+/**
+ * @brief Deduction guide.
+ * @tparam Entity A valid entity type (see entt_traits for more details).
+ */
+template<typename Entity>
+as_group(basic_registry<Entity> &) -> as_group<Entity>;
+
+
+/**
+ * @brief Deduction guide.
+ * @tparam Entity A valid entity type (see entt_traits for more details).
+ */
+template<typename Entity>
+as_group(const basic_registry<Entity> &) -> as_group<const Entity>;
+
+
+
+/**
+ * @brief Helper to create a listener that directly invokes a member function.
+ * @tparam Member Member function to invoke on a component of the given type.
+ * @tparam Entity A valid entity type (see entt_traits for more details).
+ * @param reg A registry that contains the given entity and its components.
+ * @param entt Entity from which to get the component.
+ */
+template<auto Member, typename Entity = entity>
+void invoke(basic_registry<Entity> &reg, const Entity entt) {
+    static_assert(std::is_member_function_pointer_v<decltype(Member)>, "Invalid pointer to non-static member function");
+    delegate<void(basic_registry<Entity> &, const Entity)> func;
+    func.template connect<Member>(reg.template get<member_class_t<decltype(Member)>>(entt));
+    func(reg, entt);
+}
+
+
+/**
+ * @brief Returns the entity associated with a given component.
+ *
+ * @warning
+ * Currently, this function only works correctly with the default pool as it
+ * makes assumptions about how the components are laid out.
+ *
+ * @tparam Entity A valid entity type (see entt_traits for more details).
+ * @tparam Component Type of component.
+ * @param reg A registry that contains the given entity and its components.
+ * @param instance A valid component instance.
+ * @return The entity associated with the given component.
+ */
+template<typename Entity, typename Component>
+Entity to_entity(const basic_registry<Entity> &reg, const Component &instance) {
+    const auto view = reg.template view<const Component>();
+    const auto *addr = std::addressof(instance);
+
+    for(auto it = view.rbegin(), last = view.rend(); it < last; it += ENTT_PACKED_PAGE) {
+        if(const auto dist = (addr - std::addressof(view.template get<const Component>(*it))); dist >= 0 && dist < ENTT_PACKED_PAGE) {
+            return *(it + dist);
+        }
+    }
+
+    return entt::null;
+}
+
+
+}
+
+
+#endif
+
+
+
+namespace entt {
+
+
+/**
+ * @cond TURN_OFF_DOXYGEN
+ * Internal details not to be documented.
+ */
+
+
+namespace internal {
+
+
+template<typename>
+struct is_view: std::false_type {};
+
+template<typename Entity, typename... Exclude, typename... Component>
+struct is_view<basic_view<Entity, exclude_t<Exclude...>, Component...>>: std::true_type {};
+
+template<typename Type>
+inline constexpr bool is_view_v = is_view<Type>::value;
+
+
+template<typename Type, typename Override>
+struct unpack_type {
+    using ro = std::conditional_t<
+        type_list_contains_v<Override, std::add_const_t<Type>> || (std::is_const_v<Type> && !type_list_contains_v<Override, std::remove_const_t<Type>>),
+        type_list<std::remove_const_t<Type>>,
+        type_list<>
+    >;
+
+    using rw = std::conditional_t<
+        type_list_contains_v<Override, std::remove_const_t<Type>> || (!std::is_const_v<Type> && !type_list_contains_v<Override, std::add_const_t<Type>>),
+        type_list<Type>,
+        type_list<>
+    >;
+};
+
+template<typename Entity, typename... Override>
+struct unpack_type<basic_registry<Entity>, type_list<Override...>> {
+    using ro = type_list<>;
+    using rw = type_list<>;
+};
+
+template<typename Entity, typename... Override>
+struct unpack_type<const basic_registry<Entity>, type_list<Override...>>
+    : unpack_type<basic_registry<Entity>, type_list<Override...>>
+{};
+
+template<typename Entity, typename... Exclude, typename... Component, typename... Override>
+struct unpack_type<basic_view<Entity, exclude_t<Exclude...>, Component...>, type_list<Override...>> {
+    using ro = type_list_cat_t<type_list<Exclude...>, typename unpack_type<Component, type_list<Override...>>::ro...>;
+    using rw = type_list_cat_t<typename unpack_type<Component, type_list<Override...>>::rw...>;
+};
+
+template<typename Entity, typename... Exclude, typename... Component, typename... Override>
+struct unpack_type<const basic_view<Entity, exclude_t<Exclude...>, Component...>, type_list<Override...>>
+    : unpack_type<basic_view<Entity, exclude_t<Exclude...>, Component...>, type_list<Override...>>
+{};
+
+
+template<typename, typename>
+struct resource;
+
+template<typename... Args, typename... Req>
+struct resource<type_list<Args...>, type_list<Req...>> {
+    using args = type_list<std::remove_const_t<Args>...>;
+    using ro = type_list_cat_t<typename unpack_type<Args, type_list<Req...>>::ro..., typename unpack_type<Req, type_list<>>::ro...>;
+    using rw = type_list_cat_t<typename unpack_type<Args, type_list<Req...>>::rw..., typename unpack_type<Req, type_list<>>::rw...>;
+};
+
+
+template<typename... Req, typename Ret, typename... Args>
+resource<type_list<std::remove_reference_t<Args>...>, type_list<Req...>> free_function_to_resource(Ret(*)(Args...));
+
+template<typename... Req, typename Ret, typename Type, typename... Args>
+resource<type_list<std::remove_reference_t<Args>...>, type_list<Req...>> constrained_function_to_resource(Ret(*)(Type &, Args...));
+
+template<typename... Req, typename Ret, typename Class, typename... Args>
+resource<type_list<std::remove_reference_t<Args>...>, type_list<Req...>> constrained_function_to_resource(Ret(Class:: *)(Args...));
+
+template<typename... Req, typename Ret, typename Class, typename... Args>
+resource<type_list<std::remove_reference_t<Args>...>, type_list<Req...>> constrained_function_to_resource(Ret(Class:: *)(Args...) const);
+
+template<typename... Req>
+resource<type_list<>, type_list<Req...>> to_resource();
+
+
+}
+
+
+/**
+ * Internal details not to be documented.
+ * @endcond
+ */
+
+
+/**
+ * @brief Utility class for creating a static task graph.
+ *
+ * This class offers minimal support (but sufficient in many cases) for creating
+ * an execution graph from functions and their requirements on resources.<br/>
+ * Note that the resulting tasks aren't executed in any case. This isn't the
+ * goal of the tool. Instead, they are returned to the user in the form of a
+ * graph that allows for safe execution.
+ *
+ * @tparam Entity A valid entity type (see entt_traits for more details).
+ */
+template<typename Entity>
+class basic_organizer final {
+    using callback_type = void(const void *, entt::basic_registry<Entity> &);
+    using prepare_type = void(entt::basic_registry<Entity> &);
+    using dependency_type = std::size_t(const bool, type_info *, const std::size_t);
+
+    struct vertex_data final {
+        std::size_t ro_count{};
+        std::size_t rw_count{};
+        const char *name{};
+        const void *payload{};
+        callback_type *callback{};
+        dependency_type *dependency;
+        prepare_type *prepare{};
+        type_info info{};
+    };
+
+    template<typename Type>
+    [[nodiscard]] static decltype(auto) extract(basic_registry<Entity> &reg) {
+        if constexpr(std::is_same_v<Type, basic_registry<Entity>>) {
+            return reg;
+        } else if constexpr(internal::is_view_v<Type>) {
+            return as_view{reg};
+        } else {
+            return reg.template ctx_or_set<std::remove_reference_t<Type>>();
+        }
+    }
+
+    template<typename... Args>
+    [[nodiscard]] static auto to_args(basic_registry<Entity> &reg, type_list<Args...>) {
+        return std::tuple<decltype(extract<Args>(reg))...>(extract<Args>(reg)...);
+    }
+
+    template<typename... Type>
+    static std::size_t fill_dependencies(type_list<Type...>, [[maybe_unused]] type_info *buffer, [[maybe_unused]] const std::size_t count) {
+        if constexpr(sizeof...(Type) == 0u) {
+            return {};
+        } else {
+            type_info info[sizeof...(Type)]{type_id<Type>()...};
+            const auto length = (std::min)(count, sizeof...(Type));
+            std::copy_n(info, length, buffer);
+            return length;
+        }
+    }
+
+    template<typename... RO, typename... RW>
+    void track_dependencies(std::size_t index, const bool requires_registry, type_list<RO...>, type_list<RW...>) {
+        dependencies[type_hash<basic_registry<Entity>>::value()].emplace_back(index, requires_registry || (sizeof...(RO) + sizeof...(RW) == 0u));
+        (dependencies[type_hash<RO>::value()].emplace_back(index, false), ...);
+        (dependencies[type_hash<RW>::value()].emplace_back(index, true), ...);
+    }
+
+    [[nodiscard]] std::vector<bool> adjacency_matrix() {
+        const auto length = vertices.size();
+        std::vector<bool> edges(length * length, false);
+
+        // creates the ajacency matrix
+        for(const auto &deps: dependencies) {
+            const auto last = deps.second.cend();
+            auto it = deps.second.cbegin();
+
+            while(it != last) {
+                if(it->second) {
+                    // rw item
+                    if(auto curr = it++; it != last) {
+                        if(it->second) {
+                            edges[curr->first * length + it->first] = true;
+                        } else {
+                            if(const auto next = std::find_if(it, last, [](const auto &elem) { return elem.second; }); next != last) {
+                                for(; it != next; ++it) {
+                                    edges[curr->first * length + it->first] = true;
+                                    edges[it->first * length + next->first] = true;
+                                }
+                            } else {
+                                for(; it != next; ++it) {
+                                    edges[curr->first * length + it->first] = true;
+                                }
+                            }
+                        }
+                    }
+                } else {
+                    // ro item, possibly only on first iteration
+                    if(const auto next = std::find_if(it, last, [](const auto &elem) { return elem.second; }); next != last) {
+                        for(; it != next; ++it) {
+                            edges[it->first * length + next->first] = true;
+                        }
+                    } else {
+                        it = last;
+                    }
+                }
+            }
+        }
+
+        // computes the transitive closure
+        for(std::size_t vk{}; vk < length; ++vk) {
+            for(std::size_t vi{}; vi < length; ++vi) {
+                for(std::size_t vj{}; vj < length; ++vj) {
+                    edges[vi * length + vj] = edges[vi * length + vj] || (edges[vi * length + vk] && edges[vk * length + vj]);
+                }
+            }
+        }
+
+        // applies the transitive reduction
+        for(std::size_t vert{}; vert < length; ++vert) {
+            edges[vert * length + vert] = false;
+        }
+
+        for(std::size_t vj{}; vj < length; ++vj) {
+            for(std::size_t vi{}; vi < length; ++vi) {
+                if(edges[vi * length + vj]) {
+                    for(std::size_t vk{}; vk < length; ++vk) {
+                        if(edges[vj * length + vk]) {
+                            edges[vi * length + vk] = false;
+                        }
+                    }
+                }
+            }
+        }
+
+        return edges;
+    }
+
+public:
+    /*! @brief Underlying entity identifier. */
+    using entity_type = Entity;
+    /*! @brief Unsigned integer type. */
+    using size_type = std::size_t;
+    /*! @brief Raw task function type. */
+    using function_type = callback_type;
+
+    /*! @brief Vertex type of a task graph defined as an adjacency list. */
+    struct vertex {
+        /**
+         * @brief Constructs a vertex of the task graph.
+         * @param vtype True if the vertex is a top-level one, false otherwise.
+         * @param data The data associated with the vertex.
+         * @param edges The indices of the children in the adjacency list.
+         */
+        vertex(const bool vtype, vertex_data data, std::vector<std::size_t> edges)
+            : is_top_level{vtype},
+              node{std::move(data)},
+              reachable{std::move(edges)}
+        {}
+
+        /**
+         * @brief Fills a buffer with the type info objects for the writable
+         * resources of a vertex.
+         * @param buffer A buffer pre-allocated by the user.
+         * @param length The length of the user-supplied buffer.
+         * @return The number of type info objects written to the buffer.
+         */
+        size_type ro_dependency(type_info *buffer, const std::size_t length) const ENTT_NOEXCEPT {
+            return node.dependency(false, buffer, length);
+        }
+
+        /**
+         * @brief Fills a buffer with the type info objects for the read-only
+         * resources of a vertex.
+         * @param buffer A buffer pre-allocated by the user.
+         * @param length The length of the user-supplied buffer.
+         * @return The number of type info objects written to the buffer.
+         */
+        size_type rw_dependency(type_info *buffer, const std::size_t length) const ENTT_NOEXCEPT {
+            return node.dependency(true, buffer, length);
+        }
+
+        /**
+         * @brief Returns the number of read-only resources of a vertex.
+         * @return The number of read-only resources of the vertex.
+         */
+        size_type ro_count() const ENTT_NOEXCEPT {
+            return node.ro_count;
+        }
+
+        /**
+         * @brief Returns the number of writable resources of a vertex.
+         * @return The number of writable resources of the vertex.
+         */
+        size_type rw_count() const ENTT_NOEXCEPT {
+            return node.rw_count;
+        }
+
+        /**
+         * @brief Checks if a vertex is also a top-level one.
+         * @return True if the vertex is a top-level one, false otherwise.
+         */
+        bool top_level() const ENTT_NOEXCEPT {
+            return is_top_level;
+        }
+
+        /**
+         * @brief Returns a type info object associated with a vertex.
+         * @return A properly initialized type info object.
+         */
+        type_info info() const ENTT_NOEXCEPT {
+            return node.info;
+        }
+
+        /**
+         * @brief Returns a user defined name associated with a vertex, if any.
+         * @return The user defined name associated with the vertex, if any.
+         */
+        const char * name() const ENTT_NOEXCEPT {
+            return node.name;
+        }
+
+        /**
+         * @brief Returns the function associated with a vertex.
+         * @return The function associated with the vertex.
+         */
+        function_type * callback() const ENTT_NOEXCEPT {
+            return node.callback;
+        }
+
+        /**
+         * @brief Returns the payload associated with a vertex, if any.
+         * @return The payload associated with the vertex, if any.
+         */
+        const void * data() const ENTT_NOEXCEPT {
+            return node.payload;
+        }
+
+        /**
+         * @brief Returns the list of nodes reachable from a given vertex.
+         * @return The list of nodes reachable from the vertex.
+         */
+        const std::vector<std::size_t> & children() const ENTT_NOEXCEPT {
+            return reachable;
+        }
+
+        /**
+         * @brief Prepares a registry and assures that all required resources
+         * are properly instantiated before using them.
+         * @param reg A valid registry.
+         */
+        void prepare(basic_registry<entity_type> &reg) const {
+            node.prepare ? node.prepare(reg) : void();
+        }
+
+    private:
+        bool is_top_level;
+        vertex_data node;
+        std::vector<std::size_t> reachable;
+    };
+
+    /**
+     * @brief Adds a free function to the task list.
+     * @tparam Candidate Function to add to the task list.
+     * @tparam Req Additional requirements and/or override resource access mode.
+     * @param name Optional name to associate with the task.
+     */
+    template<auto Candidate, typename... Req>
+    void emplace(const char *name = nullptr) {
+        using resource_type = decltype(internal::free_function_to_resource<Req...>(Candidate));
+        constexpr auto requires_registry = type_list_contains_v<typename resource_type::args, basic_registry<entity_type>>;
+
+        callback_type *callback = +[](const void *, basic_registry<entity_type> &reg) {
+            std::apply(Candidate, to_args(reg, typename resource_type::args{}));
+        };
+
+        track_dependencies(vertices.size(), requires_registry, typename resource_type::ro{}, typename resource_type::rw{});
+
+        vertices.push_back({
+            resource_type::ro::size,
+            resource_type::rw::size,
+            name,
+            nullptr,
+            callback,
+            +[](const bool rw, type_info *buffer, const std::size_t length) { return rw ? fill_dependencies(typename resource_type::rw{}, buffer, length) : fill_dependencies(typename resource_type::ro{}, buffer, length); },
+            +[](basic_registry<entity_type> &reg) { void(to_args(reg, typename resource_type::args{})); },
+            type_id<std::integral_constant<decltype(Candidate), Candidate>>()
+        });
+    }
+
+    /**
+     * @brief Adds a free function with payload or a member function with an
+     * instance to the task list.
+     * @tparam Candidate Function or member to add to the task list.
+     * @tparam Req Additional requirements and/or override resource access mode.
+     * @tparam Type Type of class or type of payload.
+     * @param value_or_instance A valid object that fits the purpose.
+     * @param name Optional name to associate with the task.
+     */
+    template<auto Candidate, typename... Req, typename Type>
+    void emplace(Type &value_or_instance, const char *name = nullptr) {
+        using resource_type = decltype(internal::constrained_function_to_resource<Req...>(Candidate));
+        constexpr auto requires_registry = type_list_contains_v<typename resource_type::args, basic_registry<entity_type>>;
+
+        callback_type *callback = +[](const void *payload, basic_registry<entity_type> &reg) {
+            Type *curr = static_cast<Type *>(const_cast<constness_as_t<void, Type> *>(payload));
+            std::apply(Candidate, std::tuple_cat(std::forward_as_tuple(*curr), to_args(reg, typename resource_type::args{})));
+        };
+
+        track_dependencies(vertices.size(), requires_registry, typename resource_type::ro{}, typename resource_type::rw{});
+
+        vertices.push_back({
+            resource_type::ro::size,
+            resource_type::rw::size,
+            name,
+            &value_or_instance,
+            callback,
+            +[](const bool rw, type_info *buffer, const std::size_t length) {
+                return rw ? fill_dependencies(typename resource_type::rw{}, buffer, length) : fill_dependencies(typename resource_type::ro{}, buffer, length);
+            },
+            +[](basic_registry<entity_type> &reg) {
+                void(to_args(reg, typename resource_type::args{}));
+            },
+            type_id<std::integral_constant<decltype(Candidate), Candidate>>()
+        });
+    }
+
+    /**
+     * @brief Adds an user defined function with optional payload to the task
+     * list.
+     * @tparam Req Additional requirements and/or override resource access mode.
+     * @param func Function to add to the task list.
+     * @param payload User defined arbitrary data.
+     * @param name Optional name to associate with the task.
+     */
+    template<typename... Req>
+    void emplace(function_type *func, const void *payload = nullptr, const char *name = nullptr) {
+        using resource_type = internal::resource<type_list<>, type_list<Req...>>;
+        track_dependencies(vertices.size(), true, typename resource_type::ro{}, typename resource_type::rw{});
+
+        vertices.push_back({
+            resource_type::ro::size,
+            resource_type::rw::size,
+            name,
+            payload,
+            func,
+            +[](const bool rw, type_info *buffer, const std::size_t length) {
+                return rw ? fill_dependencies(typename resource_type::rw{}, buffer, length) : fill_dependencies(typename resource_type::ro{}, buffer, length);
+            },
+            nullptr,
+            type_info{}
+        });
+    }
+
+    /**
+     * @brief Generates a task graph for the current content.
+     * @return The adjacency list of the task graph.
+     */
+    std::vector<vertex> graph() {
+        const auto edges = adjacency_matrix();
+
+        // creates the adjacency list
+        std::vector<vertex> adjacency_list{};
+        adjacency_list.reserve(vertices.size());
+
+        for(std::size_t col{}, length = vertices.size(); col < length; ++col) {
+            std::vector<std::size_t> reachable{};
+            const auto row = col * length;
+            bool is_top_level = true;
+
+            for(std::size_t next{}; next < length; ++next) {
+                if(edges[row + next]) {
+                    reachable.push_back(next);
+                }
+            }
+
+            for(std::size_t next{}; next < length && is_top_level; ++next) {
+                is_top_level = !edges[next * length + col];
+            }
+
+            adjacency_list.emplace_back(is_top_level, vertices[col], std::move(reachable));
+        }
+
+        return adjacency_list;
+    }
+
+    /*! @brief Erases all elements from a container. */
+    void clear() {
+        dependencies.clear();
+        vertices.clear();
+    }
+
+private:
+    std::unordered_map<entt::id_type, std::vector<std::pair<std::size_t, bool>>> dependencies;
+    std::vector<vertex_data> vertices;
+};
+
+
+}
+
+
+#endif
+
+// #include "entity/poly_storage.hpp"
+#ifndef ENTT_ENTITY_POLY_STORAGE_HPP
+#define ENTT_ENTITY_POLY_STORAGE_HPP
+
+
+#include <cstddef>
+#include <tuple>
+// #include "../core/type_info.hpp"
+
+// #include "../core/type_traits.hpp"
+
+// #include "../poly/poly.hpp"
+
+// #include "fwd.hpp"
+
+
+
+namespace entt {
+
+
+/**
+ * @brief Basic poly storage implementation.
+ * @tparam Entity A valid entity type (see entt_traits for more details).
+ */
+template<typename Entity>
+struct Storage: type_list<type_info() const ENTT_NOEXCEPT> {
+    /**
+     * @brief Concept definition.
+     * @tparam Base Opaque base class from which to inherit.
+     */
+    template<typename Base>
+    struct type: Base {
+        /**
+         * @brief Returns a type info for the contained objects.
+         * @return The type info for the contained objects.
+         */
+        type_info value_type() const ENTT_NOEXCEPT {
+            return poly_call<0>(*this);
+        }
+    };
+
+    /**
+     * @brief Concept implementation.
+     * @tparam Type Type for which to generate an implementation.
+     */
+    template<typename Type>
+    using impl = value_list<&type_id<typename Type::value_type>>;
+};
+
+
+/**
+ * @brief Defines the poly storage type associate with a given entity type.
+ * @tparam Entity A valid entity type (see entt_traits for more details).
+ */
+template<typename Entity, typename = void>
+struct poly_storage_traits {
+    /*! @brief Poly storage type for the given entity type. */
+    using storage_type = poly<Storage<Entity>>;
+};
+
+
+}
+
+
+#endif
+
+// #include "entity/registry.hpp"
+#ifndef ENTT_ENTITY_REGISTRY_HPP
+#define ENTT_ENTITY_REGISTRY_HPP
+
+
+#include <algorithm>
+#include <cstddef>
+#include <iterator>
+#include <memory>
+#include <tuple>
+#include <type_traits>
+#include <utility>
+#include <vector>
+// #include "../config/config.h"
+
+// #include "../core/algorithm.hpp"
+
+// #include "../core/any.hpp"
+
+// #include "../core/fwd.hpp"
+
+// #include "../core/type_info.hpp"
+
+// #include "../core/type_traits.hpp"
+
+// #include "component.hpp"
+
+// #include "entity.hpp"
+
+// #include "fwd.hpp"
+
+// #include "group.hpp"
+
+// #include "poly_storage.hpp"
+
+// #include "runtime_view.hpp"
+
+// #include "sparse_set.hpp"
+
+// #include "storage.hpp"
+
+// #include "utility.hpp"
+
+// #include "view.hpp"
+
+
+
+namespace entt {
+
+
+/**
+ * @brief Fast and reliable entity-component system.
+ *
+ * The registry is the core class of the entity-component framework.<br/>
+ * It stores entities and arranges pools of components on a per request basis.
+ * By means of a registry, users can manage entities and components, then create
+ * views or groups to iterate them.
+ *
+ * @tparam Entity A valid entity type (see entt_traits for more details).
+ */
+template<typename Entity>
+class basic_registry {
+    using traits_type = entt_traits<Entity>;
+    using poly_storage_type = typename poly_storage_traits<Entity>::storage_type;
+    using basic_common_type = basic_sparse_set<Entity>;
+
+    template<typename Component>
+    using storage_type = constness_as_t<typename storage_traits<Entity, std::remove_const_t<Component>>::storage_type, Component>;
+
+    struct pool_data {
+        poly_storage_type poly;
+        std::unique_ptr<basic_common_type> pool{};
+    };
+
+    template<typename...>
+    struct group_handler;
+
+    template<typename... Exclude, typename... Get, typename... Owned>
+    struct group_handler<exclude_t<Exclude...>, get_t<Get...>, Owned...> {
+        static_assert(!std::disjunction_v<typename component_traits<Owned>::in_place_delete...>, "Groups do not support in-place delete");
+        static_assert(std::conjunction_v<std::is_same<Owned, std::remove_const_t<Owned>>..., std::is_same<Get, std::remove_const_t<Get>>..., std::is_same<Exclude, std::remove_const_t<Exclude>>...>, "One or more component types are invalid");
+        std::conditional_t<sizeof...(Owned) == 0, basic_common_type, std::size_t> current{};
+
+        template<typename Component>
+        void maybe_valid_if(basic_registry &owner, const Entity entt) {
+            [[maybe_unused]] const auto cpools = std::make_tuple(owner.assure<Owned>()...);
+
+            const auto is_valid = ((std::is_same_v<Component, Owned> || std::get<storage_type<Owned> *>(cpools)->contains(entt)) && ...)
+                    && ((std::is_same_v<Component, Get> || owner.assure<Get>()->contains(entt)) && ...)
+                    && ((std::is_same_v<Component, Exclude> || !owner.assure<Exclude>()->contains(entt)) && ...);
+
+            if constexpr(sizeof...(Owned) == 0) {
+                if(is_valid && !current.contains(entt)) {
+                    current.emplace(entt);
+                }
+            } else {
+                if(is_valid && !(std::get<0>(cpools)->index(entt) < current)) {
+                    const auto pos = current++;
+                    (std::get<storage_type<Owned> *>(cpools)->swap(std::get<storage_type<Owned> *>(cpools)->data()[pos], entt), ...);
+                }
+            }
+        }
+
+        void discard_if([[maybe_unused]] basic_registry &owner, const Entity entt) {
+            if constexpr(sizeof...(Owned) == 0) {
+                current.remove(entt);
+            } else {
+                if(const auto cpools = std::make_tuple(owner.assure<Owned>()...); std::get<0>(cpools)->contains(entt) && (std::get<0>(cpools)->index(entt) < current)) {
+                    const auto pos = --current;
+                    (std::get<storage_type<Owned> *>(cpools)->swap(std::get<storage_type<Owned> *>(cpools)->data()[pos], entt), ...);
+                }
+            }
+        }
+    };
+
+    struct group_data {
+        std::size_t size;
+        std::unique_ptr<void, void(*)(void *)> group;
+        bool (* owned)(const id_type) ENTT_NOEXCEPT;
+        bool (* get)(const id_type) ENTT_NOEXCEPT;
+        bool (* exclude)(const id_type) ENTT_NOEXCEPT;
+    };
+
+    template<typename Component>
+    [[nodiscard]] storage_type<Component> * assure() const {
+        static_assert(std::is_same_v<Component, std::decay_t<Component>>, "Non-decayed types not allowed");
+        const auto index = type_seq<Component>::value();
+
+        if(!(index < pools.size())) {
+            pools.resize(size_type(index)+1u);
+        }
+
+        if(auto &&pdata = pools[index]; !pdata.pool) {
+            pdata.pool.reset(new storage_type<Component>());
+            pdata.poly.template emplace<storage_type<Component> &>(*static_cast<storage_type<Component> *>(pdata.pool.get()));
+        }
+
+        return static_cast<storage_type<Component> *>(pools[index].pool.get());
+    }
+
+    template<typename Component>
+    [[nodiscard]] const storage_type<Component> * pool_if_exists() const ENTT_NOEXCEPT {
+        static_assert(std::is_same_v<Component, std::decay_t<Component>>, "Non-decayed types not allowed");
+        const auto index = type_seq<Component>::value();
+        return (!(index < pools.size()) || !pools[index].pool) ? nullptr : static_cast<const storage_type<Component> *>(pools[index].pool.get());
+    }
+
+    auto generate_identifier(const std::size_t pos) ENTT_NOEXCEPT {
+        ENTT_ASSERT(pos < traits_type::to_integral(null), "No entities available");
+        return traits_type::construct(static_cast<typename traits_type::entity_type>(pos), {});
+    }
+
+    auto recycle_identifier() ENTT_NOEXCEPT {
+        ENTT_ASSERT(free_list != null, "No entities available");
+        const auto curr = traits_type::to_entity(free_list);
+        free_list = (tombstone | entities[curr]);
+        return (entities[curr] = traits_type::construct(curr, traits_type::to_version(entities[curr])));
+    }
+
+    auto release_entity(const Entity entity, const typename traits_type::version_type version) {
+        const typename traits_type::version_type vers = version + (version == traits_type::to_version(tombstone));
+        entities[traits_type::to_entity(entity)] = traits_type::construct(traits_type::to_entity(free_list), vers);
+        free_list = (tombstone | entity);
+        return vers;
+    }
+
+public:
+    /*! @brief Underlying entity identifier. */
+    using entity_type = Entity;
+    /*! @brief Underlying version type. */
+    using version_type = typename traits_type::version_type;
+    /*! @brief Unsigned integer type. */
+    using size_type = std::size_t;
+    /*! @brief Poly storage type. */
+    using poly_storage = typename poly_storage_traits<Entity>::storage_type;
+
+    /**
+     * @brief Returns the entity identifier without the version.
+     * @param entity An entity identifier, either valid or not.
+     * @return The entity identifier without the version.
+     */
+    [[nodiscard]] static entity_type entity(const entity_type entity) ENTT_NOEXCEPT {
+        return traits_type::construct(traits_type::to_entity(entity), {});
+    }
+
+    /**
+     * @brief Returns the version stored along with an entity identifier.
+     * @param entity An entity identifier, either valid or not.
+     * @return The version stored along with the given entity identifier.
+     */
+    [[nodiscard]] static version_type version(const entity_type entity) ENTT_NOEXCEPT {
+        return traits_type::to_version(entity);
+    }
+
+    /*! @brief Default constructor. */
+    basic_registry() = default;
+
+    /*! @brief Default move constructor. */
+    basic_registry(basic_registry &&) = default;
+
+    /*! @brief Default move assignment operator. @return This registry. */
+    basic_registry & operator=(basic_registry &&) = default;
+
+    /**
+     * @brief Prepares a pool for the given type if required.
+     * @tparam Component Type of component for which to prepare a pool.
+     */
+    template<typename Component>
+    void prepare() {
+        // suppress the warning due to the [[nodiscard]] attribute
+        static_cast<void>(assure<Component>());
+    }
+
+    /**
+     * @brief Returns a poly storage for a given type.
+     * @param info The type for which to return a poly storage.
+     * @return A valid poly storage if a pool for the given type exists, an
+     * empty and thus invalid element otherwise.
+     */
+    poly_storage & storage(const type_info info) {
+        ENTT_ASSERT(info.seq() < pools.size() && pools[info.seq()].poly, "Storage not available");
+        return pools[info.seq()].poly;
+    }
+
+    /*! @copydoc storage */
+    const poly_storage & storage(const type_info info) const {
+        ENTT_ASSERT(info.seq() < pools.size() && pools[info.seq()].poly, "Storage not available");
+        return pools[info.seq()].poly;
+    }
+
+    /**
+     * @brief Returns the number of existing components of the given type.
+     * @tparam Component Type of component of which to return the size.
+     * @return Number of existing components of the given type.
+     */
+    template<typename Component>
+    [[nodiscard]] size_type size() const {
+        const auto *cpool = pool_if_exists<Component>();
+        return cpool ? cpool->size() : size_type{};
+    }
+
+    /**
+     * @brief Returns the number of entities created so far.
+     * @return Number of entities created so far.
+     */
+    [[nodiscard]] size_type size() const ENTT_NOEXCEPT {
+        return entities.size();
+    }
+
+    /**
+     * @brief Returns the number of entities still in use.
+     * @return Number of entities still in use.
+     */
+    [[nodiscard]] size_type alive() const {
+        auto sz = entities.size();
+
+        for(auto curr = free_list; curr != null; --sz) {
+            curr = entities[traits_type::to_entity(curr)];
+        }
+
+        return sz;
+    }
+
+    /**
+     * @brief Increases the capacity of the registry or of the pools for the
+     * given components.
+     *
+     * If no components are specified, the capacity of the registry is
+     * increased, that is the number of entities it contains. Otherwise the
+     * capacity of the pools for the given components is increased.<br/>
+     * In both cases, if the new capacity is greater than the current capacity,
+     * new storage is allocated, otherwise the method does nothing.
+     *
+     * @tparam Component Types of components for which to reserve storage.
+     * @param cap Desired capacity.
+     */
+    template<typename... Component>
+    void reserve(const size_type cap) {
+        if constexpr(sizeof...(Component) == 0) {
+            entities.reserve(cap);
+        } else {
+            (assure<Component>()->reserve(cap), ...);
+        }
+    }
+
+    /**
+     * @brief Reserves enough space to store `count` pools.
+     * @param count Number of pools to reserve space for.
+     */
+    [[deprecated("No longer supported")]]
+    void reserve_pools(const size_t count) {
+        pools.reserve(count);
+    }
+
+    /**
+     * @brief Returns the capacity of the pool for the given component.
+     * @tparam Component Type of component in which one is interested.
+     * @return Capacity of the pool of the given component.
+     */
+    template<typename Component>
+    [[nodiscard]] size_type capacity() const {
+        const auto *cpool = pool_if_exists<Component>();
+        return cpool ? cpool->capacity() : size_type{};
+    }
+
+    /**
+     * @brief Returns the number of entities that a registry has currently
+     * allocated space for.
+     * @return Capacity of the registry.
+     */
+    [[nodiscard]] size_type capacity() const ENTT_NOEXCEPT {
+        return entities.capacity();
+    }
+
+    /**
+     * @brief Requests the removal of unused capacity for the given components.
+     * @tparam Component Types of components for which to reclaim unused
+     * capacity.
+     */
+    template<typename... Component>
+    void shrink_to_fit() {
+        (assure<Component>()->shrink_to_fit(), ...);
+    }
+
+    /**
+     * @brief Checks whether the registry or the pools of the given components
+     * are empty.
+     *
+     * A registry is considered empty when it doesn't contain entities that are
+     * still in use.
+     *
+     * @tparam Component Types of components in which one is interested.
+     * @return True if the registry or the pools of the given components are
+     * empty, false otherwise.
+     */
+    template<typename... Component>
+    [[nodiscard]] bool empty() const {
+        if constexpr(sizeof...(Component) == 0) {
+            return !alive();
+        } else {
+            return [](const auto *... cpool) { return ((!cpool || cpool->empty()) && ...); }(pool_if_exists<Component>()...);
+        }
+    }
+
+    /**
+     * @brief Direct access to the list of entities of a registry.
+     *
+     * The returned pointer is such that range `[data(), data() + size())` is
+     * always a valid range, even if the container is empty.
+     *
+     * @warning
+     * This list contains both valid and destroyed entities and isn't suitable
+     * for direct use.
+     *
+     * @return A pointer to the array of entities.
+     */
+    [[nodiscard]] const entity_type * data() const ENTT_NOEXCEPT {
+        return entities.data();
+    }
+
+    /**
+     * @brief Returns the head of the list of released entities.
+     *
+     * This function is intended for use in conjunction with `assign`.<br/>
+     * The returned entity has an invalid identifier in all cases.
+     *
+     * @return The head of the list of released entities.
+     */
+    [[nodiscard]] entity_type released() const ENTT_NOEXCEPT {
+        return free_list;
+    }
+
+    /*! @copydoc released */
+    [[deprecated("Use ::released instead")]]
+    [[nodiscard]] entity_type destroyed() const ENTT_NOEXCEPT {
+        return released();
+    }
+
+    /**
+     * @brief Checks if an entity identifier refers to a valid entity.
+     * @param entity An entity identifier, either valid or not.
+     * @return True if the identifier is valid, false otherwise.
+     */
+    [[nodiscard]] bool valid(const entity_type entity) const {
+        const auto pos = size_type(traits_type::to_entity(entity));
+        return (pos < entities.size() && entities[pos] == entity);
+    }
+
+    /**
+     * @brief Returns the actual version for an entity identifier.
+     *
+     * @warning
+     * Attempting to use an entity that doesn't belong to the registry results
+     * in undefined behavior. An entity belongs to the registry even if it has
+     * been previously destroyed and/or recycled.
+     *
+     * @param entity A valid entity identifier.
+     * @return Actual version for the given entity identifier.
+     */
+    [[nodiscard]] version_type current(const entity_type entity) const {
+        const auto pos = size_type(traits_type::to_entity(entity));
+        ENTT_ASSERT(pos < entities.size(), "Entity does not exist");
+        return version(entities[pos]);
+    }
+
+    /**
+     * @brief Creates a new entity and returns it.
+     *
+     * There are two kinds of possible entity identifiers:
+     *
+     * * Newly created ones in case no entities have been previously destroyed.
+     * * Recycled ones with updated versions.
+     *
+     * @return A valid entity identifier.
+     */
+    [[nodiscard]] entity_type create() {
+        return (free_list == null) ? entities.emplace_back(generate_identifier(entities.size())) : recycle_identifier();
+    }
+
+    /**
+     * @brief Creates a new entity and returns it.
+     *
+     * @sa create
+     *
+     * If the requested entity isn't in use, the suggested identifier is created
+     * and returned. Otherwise, a new identifier is generated.
+     *
+     * @param hint Required entity identifier.
+     * @return A valid entity identifier.
+     */
+    [[nodiscard]] entity_type create(const entity_type hint) {
+        const auto length = entities.size();
+
+        if(hint == null || hint == tombstone) {
+            return create();
+        } else if(const auto req = traits_type::to_entity(hint); !(req < length)) {
+            entities.resize(size_type(req) + 1u, null);
+
+            for(auto pos = length; pos < req; ++pos) {
+                release_entity(generate_identifier(pos), {});
+            }
+
+            return (entities[req] = hint);
+        } else if(const auto curr = traits_type::to_entity(entities[req]); req == curr) {
+            return create();
+        } else {
+            auto *it = &free_list;
+            for(; traits_type::to_entity(*it) != req; it = &entities[traits_type::to_entity(*it)]);
+            *it = traits_type::construct(curr, traits_type::to_version(*it));
+            return (entities[req] = hint);
+        }
+    }
+
+    /**
+     * @brief Assigns each element in a range an entity.
+     *
+     * @sa create
+     *
+     * @tparam It Type of forward iterator.
+     * @param first An iterator to the first element of the range to generate.
+     * @param last An iterator past the last element of the range to generate.
+     */
+    template<typename It>
+    void create(It first, It last) {
+        for(; free_list != null && first != last; ++first) {
+            *first = recycle_identifier();
+        }
+
+        const auto length = entities.size();
+        entities.resize(length + std::distance(first, last), null);
+
+        for(auto pos = length; first != last; ++first, ++pos) {
+            *first = entities[pos] = generate_identifier(pos);
+        }
+    }
+
+    /**
+     * @brief Assigns entities to an empty registry.
+     *
+     * This function is intended for use in conjunction with `data`, `size` and
+     * `destroyed`.<br/>
+     * Don't try to inject ranges of randomly generated entities nor the _wrong_
+     * head for the list of destroyed entities. There is no guarantee that a
+     * registry will continue to work properly in this case.
+     *
+     * @warning
+     * There must be no entities still alive for this to work properly.
+     *
+     * @tparam It Type of input iterator.
+     * @param first An iterator to the first element of the range of entities.
+     * @param last An iterator past the last element of the range of entities.
+     * @param destroyed The head of the list of destroyed entities.
+     */
+    template<typename It>
+    void assign(It first, It last, const entity_type destroyed) {
+        ENTT_ASSERT(!alive(), "Entities still alive");
+        entities.assign(first, last);
+        free_list = destroyed;
+    }
+
+    /**
+     * @brief Releases an entity identifier.
+     *
+     * The version is updated and the identifier can be recycled at any time.
+     *
+     * @warning
+     * Attempting to use an invalid entity results in undefined behavior.
+     *
+     * @param entity A valid entity identifier.
+     * @return The version of the recycled entity.
+     */
+    version_type release(const entity_type entity) {
+        return release(entity, version(entity) + 1u);
+    }
+
+    /**
+     * @brief Releases an entity identifier.
+     *
+     * The suggested version or the valid version closest to the suggested one
+     * is used instead of the implicitly generated version.
+     *
+     * @sa release
+     *
+     * @param entity A valid entity identifier.
+     * @param version A desired version upon destruction.
+     * @return The version actually assigned to the entity.
+     */
+    version_type release(const entity_type entity, const version_type version) {
+        ENTT_ASSERT(orphan(entity), "Non-orphan entity");
+        return release_entity(entity, version);
+    }
+
+    /**
+     * @brief Releases all entity identifiers in a range.
+     *
+     * @sa release
+     *
+     * @tparam It Type of input iterator.
+     * @param first An iterator to the first element of the range of entities.
+     * @param last An iterator past the last element of the range of entities.
+     */
+    template<typename It>
+    void release(It first, It last) {
+        for(; first != last; ++first) {
+            release(*first, version(*first) + 1u);
+        }
+    }
+
+    /**
+     * @brief Destroys an entity and releases its identifier.
+     *
+     * The version is updated and the identifier can be recycled at any time.
+     *
+     * @warning
+     * Adding or removing components to an entity that is being destroyed can
+     * result in undefined behavior.
+     *
+     * @warning
+     * Attempting to use an invalid entity results in undefined behavior.
+     *
+     * @param entity A valid entity identifier.
+     * @return The version of the recycled entity.
+     */
+    version_type destroy(const entity_type entity) {
+        return destroy(entity, version(entity) + 1u);
+    }
+
+    /**
+     * @brief Destroys an entity and releases its identifier.
+     *
+     * The suggested version or the valid version closest to the suggested one
+     * is used instead of the implicitly generated version.
+     *
+     * @sa destroy
+     *
+     * @param entity A valid entity identifier.
+     * @param version A desired version upon destruction.
+     * @return The version actually assigned to the entity.
+     */
+    version_type destroy(const entity_type entity, const version_type version) {
+        ENTT_ASSERT(valid(entity), "Invalid entity");
+
+        for(auto &&pdata: pools) {
+            pdata.pool && pdata.pool->remove(entity, this);
+        }
+
+        return release_entity(entity, version);
+    }
+
+    /**
+     * @brief Destroys all entities in a range and releases their identifiers.
+     *
+     * @sa destroy
+     *
+     * @tparam It Type of input iterator.
+     * @param first An iterator to the first element of the range of entities.
+     * @param last An iterator past the last element of the range of entities.
+     */
+    template<typename It>
+    void destroy(It first, It last) {
+        if constexpr(is_iterator_type_v<typename basic_common_type::iterator, It>) {
+            for(; first != last; ++first) {
+                destroy(*first, version(*first) + 1u);
+            }
+        } else {
+            for(auto &&pdata: pools) {
+                pdata.pool && pdata.pool->remove(first, last, this);
+            }
+
+            release(first, last);
+        }
+    }
+
+    /**
+     * @brief Assigns the given component to an entity.
+     *
+     * A new instance of the given component is created and initialized with the
+     * arguments provided (the component must have a proper constructor or be of
+     * aggregate type). Then the component is assigned to the given entity.
+     *
+     * @warning
+     * Attempting to use an invalid entity or to assign a component to an entity
+     * that already owns it results in undefined behavior.
+     *
+     * @tparam Component Type of component to create.
+     * @tparam Args Types of arguments to use to construct the component.
+     * @param entity A valid entity identifier.
+     * @param args Parameters to use to initialize the component.
+     * @return A reference to the newly created component.
+     */
+    template<typename Component, typename... Args>
+    decltype(auto) emplace(const entity_type entity, Args &&... args) {
+        ENTT_ASSERT(valid(entity), "Invalid entity");
+        return assure<Component>()->emplace(*this, entity, std::forward<Args>(args)...);
+    }
+
+    /**
+     * @brief Assigns each entity in a range the given component.
+     *
+     * @sa emplace
+     *
+     * @tparam Component Type of component to create.
+     * @tparam It Type of input iterator.
+     * @param first An iterator to the first element of the range of entities.
+     * @param last An iterator past the last element of the range of entities.
+     * @param value An instance of the component to assign.
+     */
+    template<typename Component, typename It>
+    void insert(It first, It last, const Component &value = {}) {
+        ENTT_ASSERT(std::all_of(first, last, [this](const auto entity) { return valid(entity); }), "Invalid entity");
+        assure<Component>()->insert(*this, first, last, value);
+    }
+
+    /**
+     * @brief Assigns each entity in a range the given components.
+     *
+     * @sa emplace
+     *
+     * @tparam Component Type of component to create.
+     * @tparam EIt Type of input iterator.
+     * @tparam CIt Type of input iterator.
+     * @param first An iterator to the first element of the range of entities.
+     * @param last An iterator past the last element of the range of entities.
+     * @param from An iterator to the first element of the range of components.
+     */
+    template<typename Component, typename EIt, typename CIt, typename = std::enable_if_t<std::is_same_v<std::decay_t<typename std::iterator_traits<CIt>::value_type>, Component>>>
+    void insert(EIt first, EIt last, CIt from) {
+        static_assert(std::is_constructible_v<Component, typename std::iterator_traits<CIt>::value_type>, "Invalid value type");
+        ENTT_ASSERT(std::all_of(first, last, [this](const auto entity) { return valid(entity); }), "Invalid entity");
+        assure<Component>()->insert(*this, first, last, from);
+    }
+
+    /**
+     * @brief Assigns or replaces the given component for an entity.
+     *
+     * Equivalent to the following snippet (pseudocode):
+     *
+     * @code{.cpp}
+     * auto &component = registry.all_of<Component>(entity) ? registry.replace<Component>(entity, args...) : registry.emplace<Component>(entity, args...);
+     * @endcode
+     *
+     * Prefer this function anyway because it has slightly better performance.
+     *
+     * @warning
+     * Attempting to use an invalid entity results in undefined behavior.
+     *
+     * @tparam Component Type of component to assign or replace.
+     * @tparam Args Types of arguments to use to construct the component.
+     * @param entity A valid entity identifier.
+     * @param args Parameters to use to initialize the component.
+     * @return A reference to the newly created component.
+     */
+    template<typename Component, typename... Args>
+    decltype(auto) emplace_or_replace(const entity_type entity, Args &&... args) {
+        ENTT_ASSERT(valid(entity), "Invalid entity");
+        auto *cpool = assure<Component>();
+
+        return cpool->contains(entity)
+            ? cpool->patch(*this, entity, [&args...](auto &... curr) { ((curr = Component{std::forward<Args>(args)...}), ...); })
+            : cpool->emplace(*this, entity, std::forward<Args>(args)...);
+    }
+
+    /**
+     * @brief Patches the given component for an entity.
+     *
+     * The signature of the functions should be equivalent to the following:
+     *
+     * @code{.cpp}
+     * void(Component &);
+     * @endcode
+     *
+     * @note
+     * Empty types aren't explicitly instantiated and therefore they are never
+     * returned. However, this function can be used to trigger an update signal
+     * for them.
+     *
+     * @warning
+     * Attempting to use an invalid entity or to patch a component of an entity
+     * that doesn't own it results in undefined behavior.
+     *
+     * @tparam Component Type of component to patch.
+     * @tparam Func Types of the function objects to invoke.
+     * @param entity A valid entity identifier.
+     * @param func Valid function objects.
+     * @return A reference to the patched component.
+     */
+    template<typename Component, typename... Func>
+    decltype(auto) patch(const entity_type entity, Func &&... func) {
+        ENTT_ASSERT(valid(entity), "Invalid entity");
+        return assure<Component>()->patch(*this, entity, std::forward<Func>(func)...);
+    }
+
+    /**
+     * @brief Replaces the given component for an entity.
+     *
+     * A new instance of the given component is created and initialized with the
+     * arguments provided (the component must have a proper constructor or be of
+     * aggregate type). Then the component is assigned to the given entity.
+     *
+     * @warning
+     * Attempting to use an invalid entity or to replace a component of an
+     * entity that doesn't own it results in undefined behavior.
+     *
+     * @tparam Component Type of component to replace.
+     * @tparam Args Types of arguments to use to construct the component.
+     * @param entity A valid entity identifier.
+     * @param args Parameters to use to initialize the component.
+     * @return A reference to the component being replaced.
+     */
+    template<typename Component, typename... Args>
+    decltype(auto) replace(const entity_type entity, Args &&... args) {
+        return assure<Component>()->patch(*this, entity, [&args...](auto &... curr) { ((curr = Component{std::forward<Args>(args)...}), ...); });
+    }
+
+    /**
+     * @brief Removes the given components from an entity.
+     *
+     * @warning
+     * Attempting to use an invalid entity results in undefined behavior.
+     *
+     * @tparam Component Types of components to remove.
+     * @param entity A valid entity identifier.
+     * @return The number of components actually removed.
+     */
+    template<typename... Component>
+    size_type remove(const entity_type entity) {
+        ENTT_ASSERT(valid(entity), "Invalid entity");
+        static_assert(sizeof...(Component) > 0, "Provide one or more component types");
+        return (assure<Component>()->remove(entity, this) + ... + size_type{});
+    }
+
+    /**
+     * @brief Removes the given components from all the entities in a range.
+     *
+     * @sa remove
+     *
+     * @tparam Component Types of components to remove.
+     * @tparam It Type of input iterator.
+     * @param first An iterator to the first element of the range of entities.
+     * @param last An iterator past the last element of the range of entities.
+     * @return The number of components actually removed.
+     */
+    template<typename... Component, typename It>
+    size_type remove(It first, It last) {
+        static_assert(sizeof...(Component) > 0, "Provide one or more component types");
+        const auto cpools = std::make_tuple(assure<Component>()...);
+        size_type count{};
+
+        for(; first != last; ++first) {
+            const auto entity = *first;
+            ENTT_ASSERT(valid(entity), "Invalid entity");
+            count += (std::get<storage_type<Component> *>(cpools)->remove(entity, this) + ...);
+        }
+
+        return count;
+    }
+
+    /**
+     * @brief Erases the given components from an entity.
+     *
+     * @warning
+     * Attempting to use an invalid entity or to erase a component from an
+     * entity that doesn't own it results in undefined behavior.
+     *
+     * @tparam Component Types of components to erase.
+     * @param entity A valid entity identifier.
+     */
+    template<typename... Component>
+    void erase(const entity_type entity) {
+        ENTT_ASSERT(valid(entity), "Invalid entity");
+        static_assert(sizeof...(Component) > 0, "Provide one or more component types");
+        (assure<Component>()->erase(entity, this), ...);
+    }
+
+    /**
+     * @brief Erases the given components from all the entities in a range.
+     *
+     * @sa erase
+     *
+     * @tparam Component Types of components to erase.
+     * @tparam It Type of input iterator.
+     * @param first An iterator to the first element of the range of entities.
+     * @param last An iterator past the last element of the range of entities.
+     */
+    template<typename... Component, typename It>
+    void erase(It first, It last) {
+        static_assert(sizeof...(Component) > 0, "Provide one or more component types");
+        const auto cpools = std::make_tuple(assure<Component>()...);
+
+        for(; first != last; ++first) {
+            const auto entity = *first;
+            ENTT_ASSERT(valid(entity), "Invalid entity");
+            (std::get<storage_type<Component> *>(cpools)->erase(entity, this), ...);
+        }
+    }
+
+    /**
+     * @brief Removes all tombstones from a registry or only the pools for the
+     * given components.
+     * @tparam Component Types of components for which to clear all tombstones.
+     */
+    template<typename... Component>
+    void compact() {
+        if constexpr(sizeof...(Component) == 0) {
+            for(auto &&pdata: pools) {
+                pdata.pool && (pdata.pool->compact(), true);
+            }
+        } else {
+            (assure<Component>()->compact(), ...);
+        }
+    }
+
+    /*! @copydoc remove */
+    template<typename... Component>
+    [[deprecated("Use ::remove instead")]]
+    size_type remove_if_exists(const entity_type entity) {
+        return remove<Component...>(entity);
+    }
+
+    /**
+     * @brief Removes all the components from an entity and makes it orphaned.
+     *
+     * @warning
+     * In case there are listeners that observe the destruction of components
+     * and assign other components to the entity in their bodies, the result of
+     * invoking this function may not be as expected. In the worst case, it
+     * could lead to undefined behavior.
+     *
+     * @warning
+     * Attempting to use an invalid entity results in undefined behavior.
+     *
+     * @param entity A valid entity identifier.
+     */
+    [[deprecated("Use ::destroy(entity)/::create(entity) instead")]]
+    void remove_all(const entity_type entity) {
+        ENTT_ASSERT(valid(entity), "Invalid entity");
+
+        for(auto &&pdata: pools) {
+            pdata.pool && pdata.pool->remove(entity, this);
+        }
+    }
+
+    /**
+     * @brief Checks if an entity has all the given components.
+     *
+     * @warning
+     * Attempting to use an invalid entity results in undefined behavior.
+     *
+     * @tparam Component Components for which to perform the check.
+     * @param entity A valid entity identifier.
+     * @return True if the entity has all the components, false otherwise.
+     */
+    template<typename... Component>
+    [[nodiscard]] bool all_of(const entity_type entity) const {
+        ENTT_ASSERT(valid(entity), "Invalid entity");
+        return [entity](const auto *... cpool) { return ((cpool && cpool->contains(entity)) && ...); }(pool_if_exists<Component>()...);
+    }
+
+    /**
+     * @brief Checks if an entity has at least one of the given components.
+     *
+     * @warning
+     * Attempting to use an invalid entity results in undefined behavior.
+     *
+     * @tparam Component Components for which to perform the check.
+     * @param entity A valid entity identifier.
+     * @return True if the entity has at least one of the given components,
+     * false otherwise.
+     */
+    template<typename... Component>
+    [[nodiscard]] bool any_of(const entity_type entity) const {
+        ENTT_ASSERT(valid(entity), "Invalid entity");
+        return [entity](const auto *... cpool) { return !((!cpool || !cpool->contains(entity)) && ...); }(pool_if_exists<Component>()...);
+    }
+
+    /**
+     * @brief Returns references to the given components for an entity.
+     *
+     * @warning
+     * Attempting to use an invalid entity or to get a component from an entity
+     * that doesn't own it results in undefined behavior.
+     *
+     * @tparam Component Types of components to get.
+     * @param entity A valid entity identifier.
+     * @return References to the components owned by the entity.
+     */
+    template<typename... Component>
+    [[nodiscard]] decltype(auto) get([[maybe_unused]] const entity_type entity) const {
+        ENTT_ASSERT(valid(entity), "Invalid entity");
+
+        if constexpr(sizeof...(Component) == 1) {
+            const auto *cpool = pool_if_exists<std::remove_const_t<Component>...>();
+            ENTT_ASSERT(cpool, "Storage not available");
+            return cpool->get(entity);
+        } else {
+            return std::forward_as_tuple(get<Component>(entity)...);
+        }
+    }
+
+    /*! @copydoc get */
+    template<typename... Component>
+    [[nodiscard]] decltype(auto) get([[maybe_unused]] const entity_type entity) {
+        ENTT_ASSERT(valid(entity), "Invalid entity");
+
+        if constexpr(sizeof...(Component) == 1) {
+            return (const_cast<Component &>(assure<std::remove_const_t<Component>>()->get(entity)), ...);
+        } else {
+            return std::forward_as_tuple(get<Component>(entity)...);
+        }
+    }
+
+    /**
+     * @brief Returns a reference to the given component for an entity.
+     *
+     * In case the entity doesn't own the component, the parameters provided are
+     * used to construct it.<br/>
+     * Equivalent to the following snippet (pseudocode):
+     *
+     * @code{.cpp}
+     * auto &component = registry.all_of<Component>(entity) ? registry.get<Component>(entity) : registry.emplace<Component>(entity, args...);
+     * @endcode
+     *
+     * Prefer this function anyway because it has slightly better performance.
+     *
+     * @warning
+     * Attempting to use an invalid entity results in undefined behavior.
+     *
+     * @tparam Component Type of component to get.
+     * @tparam Args Types of arguments to use to construct the component.
+     * @param entity A valid entity identifier.
+     * @param args Parameters to use to initialize the component.
+     * @return Reference to the component owned by the entity.
+     */
+    template<typename Component, typename... Args>
+    [[nodiscard]] decltype(auto) get_or_emplace(const entity_type entity, Args &&... args) {
+        ENTT_ASSERT(valid(entity), "Invalid entity");
+        auto *cpool = assure<Component>();
+        return cpool->contains(entity) ? cpool->get(entity) : cpool->emplace(*this, entity, std::forward<Args>(args)...);
+    }
+
+    /**
+     * @brief Returns pointers to the given components for an entity.
+     *
+     * @warning
+     * Attempting to use an invalid entity results in undefined behavior.
+     *
+     * @note
+     * The registry retains ownership of the pointed-to components.
+     *
+     * @tparam Component Types of components to get.
+     * @param entity A valid entity identifier.
+     * @return Pointers to the components owned by the entity.
+     */
+    template<typename... Component>
+    [[nodiscard]] auto try_get([[maybe_unused]] const entity_type entity) const {
+        ENTT_ASSERT(valid(entity), "Invalid entity");
+
+        if constexpr(sizeof...(Component) == 1) {
+            const auto *cpool = pool_if_exists<std::remove_const_t<Component>...>();
+            return (cpool && cpool->contains(entity)) ? &cpool->get(entity) : nullptr;
+        } else {
+            return std::make_tuple(try_get<Component>(entity)...);
+        }
+    }
+
+    /*! @copydoc try_get */
+    template<typename... Component>
+    [[nodiscard]] auto try_get([[maybe_unused]] const entity_type entity) {
+        ENTT_ASSERT(valid(entity), "Invalid entity");
+
+        if constexpr(sizeof...(Component) == 1) {
+            return (const_cast<Component *>(std::as_const(*this).template try_get<Component>(entity)), ...);
+        } else {
+            return std::make_tuple(try_get<Component>(entity)...);
+        }
+    }
+
+    /**
+     * @brief Clears a whole registry or the pools for the given components.
+     * @tparam Component Types of components to remove from their entities.
+     */
+    template<typename... Component>
+    void clear() {
+        if constexpr(sizeof...(Component) == 0) {
+            for(auto &&pdata: pools) {
+                pdata.pool && (pdata.pool->clear(this), true);
+            }
+
+            each([this](const auto entity) { release_entity(entity, version(entity) + 1u); });
+        } else {
+            (assure<Component>()->clear(this), ...);
+        }
+    }
+
+    /**
+     * @brief Iterates all the entities that are still in use.
+     *
+     * The function object is invoked for each entity that is still in use.<br/>
+     * The signature of the function should be equivalent to the following:
+     *
+     * @code{.cpp}
+     * void(const Entity);
+     * @endcode
+     *
+     * This function is fairly slow and should not be used frequently. However,
+     * it's useful for iterating all the entities still in use, regardless of
+     * their components.
+     *
+     * @tparam Func Type of the function object to invoke.
+     * @param func A valid function object.
+     */
+    template<typename Func>
+    void each(Func func) const {
+        if(free_list == null) {
+            for(auto pos = entities.size(); pos; --pos) {
+                func(entities[pos-1]);
+            }
+        } else {
+            for(auto pos = entities.size(); pos; --pos) {
+                if(const auto entity = entities[pos - 1]; traits_type::to_entity(entity) == (pos - 1)) {
+                    func(entity);
+                }
+            }
+        }
+    }
+
+    /**
+     * @brief Checks if an entity has components assigned.
+     * @param entity A valid entity identifier.
+     * @return True if the entity has no components assigned, false otherwise.
+     */
+    [[nodiscard]] bool orphan(const entity_type entity) const {
+        ENTT_ASSERT(valid(entity), "Invalid entity");
+        return std::none_of(pools.cbegin(), pools.cend(), [entity](auto &&pdata) { return pdata.pool && pdata.pool->contains(entity); });
+    }
+
+    /**
+     * @brief Iterates orphans and applies them the given function object.
+     *
+     * The function object is invoked for each entity that is still in use and
+     * has no components assigned.<br/>
+     * The signature of the function should be equivalent to the following:
+     *
+     * @code{.cpp}
+     * void(const Entity);
+     * @endcode
+     *
+     * This function can be very slow and should not be used frequently.
+     *
+     * @tparam Func Type of the function object to invoke.
+     * @param func A valid function object.
+     */
+    template<typename Func>
+    void orphans(Func func) const {
+        each([this, &func](const auto entity) {
+            if(orphan(entity)) {
+                func(entity);
+            }
+        });
+    }
+
+    /**
+     * @brief Returns a sink object for the given component.
+     *
+     * The sink returned by this function can be used to receive notifications
+     * whenever a new instance of the given component is created and assigned to
+     * an entity.<br/>
+     * The function type for a listener is equivalent to:
+     *
+     * @code{.cpp}
+     * void(basic_registry<Entity> &, Entity);
+     * @endcode
+     *
+     * Listeners are invoked **after** the component has been assigned to the
+     * entity.
+     *
+     * @sa sink
+     *
+     * @tparam Component Type of component of which to get the sink.
+     * @return A temporary sink object.
+     */
+    template<typename Component>
+    [[nodiscard]] auto on_construct() {
+        return assure<Component>()->on_construct();
+    }
+
+    /**
+     * @brief Returns a sink object for the given component.
+     *
+     * The sink returned by this function can be used to receive notifications
+     * whenever an instance of the given component is explicitly updated.<br/>
+     * The function type for a listener is equivalent to:
+     *
+     * @code{.cpp}
+     * void(basic_registry<Entity> &, Entity);
+     * @endcode
+     *
+     * Listeners are invoked **after** the component has been updated.
+     *
+     * @sa sink
+     *
+     * @tparam Component Type of component of which to get the sink.
+     * @return A temporary sink object.
+     */
+    template<typename Component>
+    [[nodiscard]] auto on_update() {
+        return assure<Component>()->on_update();
+    }
+
+    /**
+     * @brief Returns a sink object for the given component.
+     *
+     * The sink returned by this function can be used to receive notifications
+     * whenever an instance of the given component is removed from an entity and
+     * thus destroyed.<br/>
+     * The function type for a listener is equivalent to:
+     *
+     * @code{.cpp}
+     * void(basic_registry<Entity> &, Entity);
+     * @endcode
+     *
+     * Listeners are invoked **before** the component has been removed from the
+     * entity.
+     *
+     * @sa sink
+     *
+     * @tparam Component Type of component of which to get the sink.
+     * @return A temporary sink object.
+     */
+    template<typename Component>
+    [[nodiscard]] auto on_destroy() {
+        return assure<Component>()->on_destroy();
+    }
+
+    /**
+     * @brief Returns a view for the given components.
+     *
+     * This kind of objects are created on the fly and share with the registry
+     * its internal data structures.<br/>
+     * Feel free to discard a view after the use. Creating and destroying a view
+     * is an incredibly cheap operation because they do not require any type of
+     * initialization.<br/>
+     * As a rule of thumb, storing a view should never be an option.
+     *
+     * Views do their best to iterate the smallest set of candidate entities.
+     * In particular:
+     *
+     * * Single component views are incredibly fast and iterate a packed array
+     *   of entities, all of which has the given component.
+     * * Multi component views look at the number of entities available for each
+     *   component and pick up a reference to the smallest set of candidates to
+     *   test for the given components.
+     *
+     * Views in no way affect the functionalities of the registry nor those of
+     * the underlying pools.
+     *
+     * @note
+     * Multi component views are pretty fast. However their performance tend to
+     * degenerate when the number of components to iterate grows up and the most
+     * of the entities have all the given components.<br/>
+     * To get a performance boost, consider using a group instead.
+     *
+     * @tparam Component Type of components used to construct the view.
+     * @tparam Exclude Types of components used to filter the view.
+     * @return A newly created view.
+     */
+    template<typename... Component, typename... Exclude>
+    [[nodiscard]] basic_view<Entity, exclude_t<Exclude...>, std::add_const_t<Component>...> view(exclude_t<Exclude...> = {}) const {
+        static_assert(sizeof...(Component) > 0, "Exclusion-only views are not supported");
+        return { *assure<std::remove_const_t<Component>>()..., *assure<Exclude>()... };
+    }
+
+    /*! @copydoc view */
+    template<typename... Component, typename... Exclude>
+    [[nodiscard]] basic_view<Entity, exclude_t<Exclude...>, Component...> view(exclude_t<Exclude...> = {}) {
+        static_assert(sizeof...(Component) > 0, "Exclusion-only views are not supported");
+        return { *assure<std::remove_const_t<Component>>()..., *assure<Exclude>()... };
+    }
+
+    /**
+     * @brief Returns a runtime view for the given components.
+     *
+     * This kind of objects are created on the fly and share with the registry
+     * its internal data structures.<br/>
+     * Users should throw away the view after use. Fortunately, creating and
+     * destroying a runtime view is an incredibly cheap operation because they
+     * do not require any type of initialization.<br/>
+     * As a rule of thumb, storing a view should never be an option.
+     *
+     * Runtime views are to be used when users want to construct a view from
+     * some external inputs and don't know at compile-time what are the required
+     * components.
+     *
+     * @tparam ItComp Type of input iterator for the components to use to
+     * construct the view.
+     * @tparam ItExcl Type of input iterator for the components to use to filter
+     * the view.
+     * @param first An iterator to the first element of the range of components
+     * to use to construct the view.
+     * @param last An iterator past the last element of the range of components
+     * to use to construct the view.
+     * @param from An iterator to the first element of the range of components
+     * to use to filter the view.
+     * @param to An iterator past the last element of the range of components to
+     * use to filter the view.
+     * @return A newly created runtime view.
+     */
+    template<typename ItComp, typename ItExcl = id_type *>
+    [[nodiscard]] basic_runtime_view<Entity> runtime_view(ItComp first, ItComp last, ItExcl from = {}, ItExcl to = {}) const {
+        std::vector<const basic_common_type *> component(std::distance(first, last));
+        std::vector<const basic_common_type *> filter(std::distance(from, to));
+
+        std::transform(first, last, component.begin(), [this](const auto ctype) {
+            const auto it = std::find_if(pools.cbegin(), pools.cend(), [ctype](auto &&pdata) { return pdata.poly && pdata.poly->value_type().hash() == ctype; });
+            return it == pools.cend() ? nullptr : it->pool.get();
+        });
+
+        std::transform(from, to, filter.begin(), [this](const auto ctype) {
+            const auto it = std::find_if(pools.cbegin(), pools.cend(), [ctype](auto &&pdata) { return pdata.poly && pdata.poly->value_type().hash() == ctype; });
+            return it == pools.cend() ? nullptr : it->pool.get();
+        });
+
+        return { std::move(component), std::move(filter) };
+    }
+
+    /**
+     * @brief Returns a group for the given components.
+     *
+     * This kind of objects are created on the fly and share with the registry
+     * its internal data structures.<br/>
+     * Feel free to discard a group after the use. Creating and destroying a
+     * group is an incredibly cheap operation because they do not require any
+     * type of initialization, but for the first time they are requested.<br/>
+     * As a rule of thumb, storing a group should never be an option.
+     *
+     * Groups support exclusion lists and can own types of components. The more
+     * types are owned by a group, the faster it is to iterate entities and
+     * components.<br/>
+     * However, groups also affect some features of the registry such as the
+     * creation and destruction of components, which will consequently be
+     * slightly slower (nothing that can be noticed in most cases).
+     *
+     * @note
+     * Pools of components that are owned by a group cannot be sorted anymore.
+     * The group takes the ownership of the pools and arrange components so as
+     * to iterate them as fast as possible.
+     *
+     * @tparam Owned Types of components owned by the group.
+     * @tparam Get Types of components observed by the group.
+     * @tparam Exclude Types of components used to filter the group.
+     * @return A newly created group.
+     */
+    template<typename... Owned, typename... Get, typename... Exclude>
+    [[nodiscard]] basic_group<Entity, exclude_t<Exclude...>, get_t<Get...>, Owned...> group(get_t<Get...>, exclude_t<Exclude...> = {}) {
+        static_assert(sizeof...(Owned) + sizeof...(Get) > 0, "Exclusion-only groups are not supported");
+        static_assert(sizeof...(Owned) + sizeof...(Get) + sizeof...(Exclude) > 1, "Single component groups are not allowed");
+
+        using handler_type = group_handler<exclude_t<Exclude...>, get_t<std::remove_const_t<Get>...>, std::remove_const_t<Owned>...>;
+
+        const auto cpools = std::make_tuple(assure<std::remove_const_t<Owned>>()..., assure<std::remove_const_t<Get>>()...);
+        constexpr auto size = sizeof...(Owned) + sizeof...(Get) + sizeof...(Exclude);
+        handler_type *handler = nullptr;
+
+        if(auto it = std::find_if(groups.cbegin(), groups.cend(), [size](const auto &gdata) {
+            return gdata.size == size
+                && (gdata.owned(type_hash<std::remove_const_t<Owned>>::value()) && ...)
+                && (gdata.get(type_hash<std::remove_const_t<Get>>::value()) && ...)
+                && (gdata.exclude(type_hash<Exclude>::value()) && ...);
+        }); it != groups.cend())
+        {
+            handler = static_cast<handler_type *>(it->group.get());
+        }
+
+        if(!handler) {
+            group_data candidate = {
+                size,
+                { new handler_type{}, [](void *instance) { delete static_cast<handler_type *>(instance); } },
+                []([[maybe_unused]] const id_type ctype) ENTT_NOEXCEPT { return ((ctype == type_hash<std::remove_const_t<Owned>>::value()) || ...); },
+                []([[maybe_unused]] const id_type ctype) ENTT_NOEXCEPT { return ((ctype == type_hash<std::remove_const_t<Get>>::value()) || ...); },
+                []([[maybe_unused]] const id_type ctype) ENTT_NOEXCEPT { return ((ctype == type_hash<Exclude>::value()) || ...); },
+            };
+
+            handler = static_cast<handler_type *>(candidate.group.get());
+
+            const void *maybe_valid_if = nullptr;
+            const void *discard_if = nullptr;
+
+            if constexpr(sizeof...(Owned) == 0) {
+                groups.push_back(std::move(candidate));
+            } else {
+                ENTT_ASSERT(std::all_of(groups.cbegin(), groups.cend(), [size](const auto &gdata) {
+                    const auto overlapping = (0u + ... + gdata.owned(type_hash<std::remove_const_t<Owned>>::value()));
+                    const auto sz = overlapping + (0u + ... + gdata.get(type_hash<std::remove_const_t<Get>>::value())) + (0u + ... + gdata.exclude(type_hash<Exclude>::value()));
+                    return !overlapping || ((sz == size) || (sz == gdata.size));
+                }), "Conflicting groups");
+
+                const auto next = std::find_if_not(groups.cbegin(), groups.cend(), [size](const auto &gdata) {
+                    return !(0u + ... + gdata.owned(type_hash<std::remove_const_t<Owned>>::value())) || (size > gdata.size);
+                });
+
+                const auto prev = std::find_if(std::make_reverse_iterator(next), groups.crend(), [](const auto &gdata) {
+                    return (0u + ... + gdata.owned(type_hash<std::remove_const_t<Owned>>::value()));
+                });
+
+                maybe_valid_if = (next == groups.cend() ? maybe_valid_if : next->group.get());
+                discard_if = (prev == groups.crend() ? discard_if : prev->group.get());
+                groups.insert(next, std::move(candidate));
+            }
+
+            (on_construct<std::remove_const_t<Owned>>().before(maybe_valid_if).template connect<&handler_type::template maybe_valid_if<std::remove_const_t<Owned>>>(*handler), ...);
+            (on_construct<std::remove_const_t<Get>>().before(maybe_valid_if).template connect<&handler_type::template maybe_valid_if<std::remove_const_t<Get>>>(*handler), ...);
+            (on_destroy<Exclude>().before(maybe_valid_if).template connect<&handler_type::template maybe_valid_if<Exclude>>(*handler), ...);
+
+            (on_destroy<std::remove_const_t<Owned>>().before(discard_if).template connect<&handler_type::discard_if>(*handler), ...);
+            (on_destroy<std::remove_const_t<Get>>().before(discard_if).template connect<&handler_type::discard_if>(*handler), ...);
+            (on_construct<Exclude>().before(discard_if).template connect<&handler_type::discard_if>(*handler), ...);
+
+            if constexpr(sizeof...(Owned) == 0) {
+                for(const auto entity: view<Owned..., Get...>(exclude<Exclude...>)) {
+                    handler->current.emplace(entity);
+                }
+            } else {
+                // we cannot iterate backwards because we want to leave behind valid entities in case of owned types
+                for(auto *first = std::get<0>(cpools)->data(), *last = first + std::get<0>(cpools)->size(); first != last; ++first) {
+                    handler->template maybe_valid_if<type_list_element_t<0, type_list<std::remove_const_t<Owned>...>>>(*this, *first);
+                }
+            }
+        }
+
+        return { handler->current, *std::get<storage_type<std::remove_const_t<Owned>> *>(cpools)..., *std::get<storage_type<std::remove_const_t<Get>> *>(cpools)... };
+    }
+
+    /**
+     * @brief Returns a group for the given components.
+     *
+     * @sa group
+     *
+     * @tparam Owned Types of components owned by the group.
+     * @tparam Get Types of components observed by the group.
+     * @tparam Exclude Types of components used to filter the group.
+     * @return A newly created group.
+     */
+    template<typename... Owned, typename... Get, typename... Exclude>
+    [[nodiscard]] basic_group<Entity, exclude_t<Exclude...>, get_t<std::add_const_t<Get>...>, std::add_const_t<Owned>...> group_if_exists(get_t<Get...>, exclude_t<Exclude...> = {}) const {
+        if(auto it = std::find_if(groups.cbegin(), groups.cend(), [](const auto &gdata) {
+            return gdata.size == (sizeof...(Owned) + sizeof...(Get) + sizeof...(Exclude))
+                && (gdata.owned(type_hash<std::remove_const_t<Owned>>::value()) && ...)
+                && (gdata.get(type_hash<std::remove_const_t<Get>>::value()) && ...)
+                && (gdata.exclude(type_hash<Exclude>::value()) && ...);
+            }); it == groups.cend())
+        {
+            return {};
+        } else {
+            using handler_type = group_handler<exclude_t<Exclude...>, get_t<std::remove_const_t<Get>...>, std::remove_const_t<Owned>...>;
+            return { static_cast<handler_type *>(it->group.get())->current, *pool_if_exists<std::remove_const_t<Owned>>()... , *pool_if_exists<std::remove_const_t<Get>>()... };
+        }
+    }
+
+    /**
+     * @brief Returns a group for the given components.
+     *
+     * @sa group
+     *
+     * @tparam Owned Types of components owned by the group.
+     * @tparam Exclude Types of components used to filter the group.
+     * @return A newly created group.
+     */
+    template<typename... Owned, typename... Exclude>
+    [[nodiscard]] basic_group<Entity, exclude_t<Exclude...>, get_t<>, Owned...> group(exclude_t<Exclude...> = {}) {
+        return group<Owned...>(get_t<>{}, exclude<Exclude...>);
+    }
+
+    /**
+     * @brief Returns a group for the given components.
+     *
+     * @sa group_if_exists
+     *
+     * @tparam Owned Types of components owned by the group.
+     * @tparam Exclude Types of components used to filter the group.
+     * @return A newly created group.
+     */
+    template<typename... Owned, typename... Exclude>
+    [[nodiscard]] basic_group<Entity, exclude_t<Exclude...>, get_t<>, std::add_const_t<Owned>...> group_if_exists(exclude_t<Exclude...> = {}) const {
+        return group_if_exists<std::add_const_t<Owned>...>(get_t<>{}, exclude<Exclude...>);
+    }
+
+    /**
+     * @brief Checks whether the given components belong to any group.
+     * @tparam Component Types of components in which one is interested.
+     * @return True if the pools of the given components are sortable, false
+     * otherwise.
+     */
+    template<typename... Component>
+    [[nodiscard]] bool sortable() const {
+        return std::none_of(groups.cbegin(), groups.cend(), [](auto &&gdata) { return (gdata.owned(type_hash<std::remove_const_t<Component>>::value()) || ...); });
+    }
+
+    /**
+     * @brief Checks whether a group can be sorted.
+     * @tparam Owned Types of components owned by the group.
+     * @tparam Get Types of components observed by the group.
+     * @tparam Exclude Types of components used to filter the group.
+     * @return True if the group can be sorted, false otherwise.
+     */
+    template<typename... Owned, typename... Get, typename... Exclude>
+    [[nodiscard]] bool sortable(const basic_group<Entity, exclude_t<Exclude...>, get_t<Get...>, Owned...> &) ENTT_NOEXCEPT {
+        constexpr auto size = sizeof...(Owned) + sizeof...(Get) + sizeof...(Exclude);
+        return std::find_if(groups.cbegin(), groups.cend(), [size](const auto &gdata) {
+            return (0u + ... + gdata.owned(type_hash<std::remove_const_t<Owned>>::value())) && (size < gdata.size);
+        }) == groups.cend();
+    }
+
+    /**
+     * @brief Sorts the pool of entities for the given component.
+     *
+     * The order of the elements in a pool is highly affected by assignments
+     * of components to entities and deletions. Components are arranged to
+     * maximize the performance during iterations and users should not make any
+     * assumption on the order.<br/>
+     * This function can be used to impose an order to the elements in the pool
+     * of the given component. The order is kept valid until a component of the
+     * given type is assigned or removed from an entity.
+     *
+     * The comparison function object must return `true` if the first element
+     * is _less_ than the second one, `false` otherwise. The signature of the
+     * comparison function should be equivalent to one of the following:
+     *
+     * @code{.cpp}
+     * bool(const Entity, const Entity);
+     * bool(const Component &, const Component &);
+     * @endcode
+     *
+     * Moreover, the comparison function object shall induce a
+     * _strict weak ordering_ on the values.
+     *
+     * The sort function oject must offer a member function template
+     * `operator()` that accepts three arguments:
+     *
+     * * An iterator to the first element of the range to sort.
+     * * An iterator past the last element of the range to sort.
+     * * A comparison function to use to compare the elements.
+     *
+     * The comparison funtion object received by the sort function object hasn't
+     * necessarily the type of the one passed along with the other parameters to
+     * this member function.
+     *
+     * @warning
+     * Pools of components owned by a group cannot be sorted.
+     *
+     * @tparam Component Type of components to sort.
+     * @tparam Compare Type of comparison function object.
+     * @tparam Sort Type of sort function object.
+     * @tparam Args Types of arguments to forward to the sort function object.
+     * @param compare A valid comparison function object.
+     * @param algo A valid sort function object.
+     * @param args Arguments to forward to the sort function object, if any.
+     */
+    template<typename Component, typename Compare, typename Sort = std_sort, typename... Args>
+    void sort(Compare compare, Sort algo = Sort{}, Args &&... args) {
+        ENTT_ASSERT(sortable<Component>(), "Cannot sort owned storage");
+        assure<Component>()->sort(std::move(compare), std::move(algo), std::forward<Args>(args)...);
+    }
+
+    /**
+     * @brief Sorts two pools of components in the same way.
+     *
+     * The order of the elements in a pool is highly affected by assignments
+     * of components to entities and deletions. Components are arranged to
+     * maximize the performance during iterations and users should not make any
+     * assumption on the order.
+     *
+     * It happens that different pools of components must be sorted the same way
+     * because of runtime and/or performance constraints. This function can be
+     * used to order a pool of components according to the order between the
+     * entities in another pool of components.
+     *
+     * @b How @b it @b works
+     *
+     * Being `A` and `B` the two sets where `B` is the master (the one the order
+     * of which rules) and `A` is the slave (the one to sort), after a call to
+     * this function an iterator for `A` will return the entities according to
+     * the following rules:
+     *
+     * * All the entities in `A` that are also in `B` are returned first
+     *   according to the order they have in `B`.
+     * * All the entities in `A` that are not in `B` are returned in no
+     *   particular order after all the other entities.
+     *
+     * Any subsequent change to `B` won't affect the order in `A`.
+     *
+     * @warning
+     * Pools of components owned by a group cannot be sorted.
+     *
+     * @tparam To Type of components to sort.
+     * @tparam From Type of components to use to sort.
+     */
+    template<typename To, typename From>
+    void sort() {
+        ENTT_ASSERT(sortable<To>(), "Cannot sort owned storage");
+        assure<To>()->respect(*assure<From>());
+    }
+
+    /**
+     * @brief Visits an entity and returns the type info for its components.
+     *
+     * The signature of the function should be equivalent to the following:
+     *
+     * @code{.cpp}
+     * void(const type_info);
+     * @endcode
+     *
+     * Returned identifiers are those of the components owned by the entity.
+     *
+     * @sa type_info
+     *
+     * @warning
+     * It's not specified whether a component attached to or removed from the
+     * given entity during the visit is returned or not to the caller.
+     *
+     * @tparam Func Type of the function object to invoke.
+     * @param entity A valid entity identifier.
+     * @param func A valid function object.
+     */
+    template<typename Func>
+    void visit(entity_type entity, Func func) const {
+        for(auto pos = pools.size(); pos; --pos) {
+            if(const auto &pdata = pools[pos-1]; pdata.pool && pdata.pool->contains(entity)) {
+                func(pdata.poly->value_type());
+            }
+        }
+    }
+
+    /**
+     * @brief Visits a registry and returns the type info for its components.
+     *
+     * The signature of the function should be equivalent to the following:
+     *
+     * @code{.cpp}
+     * void(const type_info);
+     * @endcode
+     *
+     * Returned identifiers are those of the components managed by the registry.
+     *
+     * @sa type_info
+     *
+     * @warning
+     * It's not specified whether a component for which a pool is created during
+     * the visit is returned or not to the caller.
+     *
+     * @tparam Func Type of the function object to invoke.
+     * @param func A valid function object.
+     */
+    template<typename Func>
+    void visit(Func func) const {
+        for(auto pos = pools.size(); pos; --pos) {
+            if(const auto &pdata = pools[pos-1]; pdata.pool) {
+                func(pdata.poly->value_type());
+            }
+        }
+    }
+
+    /**
+     * @brief Binds an object to the context of the registry.
+     *
+     * If the value already exists it is overwritten, otherwise a new instance
+     * of the given type is created and initialized with the arguments provided.
+     *
+     * @tparam Type Type of object to set.
+     * @tparam Args Types of arguments to use to construct the object.
+     * @param args Parameters to use to initialize the value.
+     * @return A reference to the newly created object.
+     */
+    template<typename Type, typename... Args>
+    Type & set(Args &&... args) {
+        unset<Type>();
+        vars.emplace_back(std::in_place_type<Type>, std::forward<Args>(args)...);
+        return any_cast<Type &>(vars.back());
+    }
+
+    /**
+     * @brief Unsets a context variable if it exists.
+     * @tparam Type Type of object to set.
+     */
+    template<typename Type>
+    void unset() {
+        vars.erase(std::remove_if(vars.begin(), vars.end(), [type = type_id<Type>()](auto &&var) { return var.type() == type; }), vars.end());
+    }
+
+    /**
+     * @brief Binds an object to the context of the registry.
+     *
+     * In case the context doesn't contain the given object, the parameters
+     * provided are used to construct it.
+     *
+     * @tparam Type Type of object to set.
+     * @tparam Args Types of arguments to use to construct the object.
+     * @param args Parameters to use to initialize the object.
+     * @return A reference to the object in the context of the registry.
+     */
+    template<typename Type, typename... Args>
+    [[nodiscard]] Type & ctx_or_set(Args &&... args) {
+        auto *value = try_ctx<Type>();
+        return value ? *value : set<Type>(std::forward<Args>(args)...);
+    }
+
+    /**
+     * @brief Returns a pointer to an object in the context of the registry.
+     * @tparam Type Type of object to get.
+     * @return A pointer to the object if it exists in the context of the
+     * registry, a null pointer otherwise.
+     */
+    template<typename Type>
+    [[nodiscard]] std::add_const_t<Type> * try_ctx() const {
+        auto it = std::find_if(vars.cbegin(), vars.cend(), [type = type_id<Type>()](auto &&var) { return var.type() == type; });
+        return it == vars.cend() ? nullptr : any_cast<std::add_const_t<Type>>(&*it);
+    }
+
+    /*! @copydoc try_ctx */
+    template<typename Type>
+    [[nodiscard]] Type * try_ctx() {
+        auto it = std::find_if(vars.begin(), vars.end(), [type = type_id<Type>()](auto &&var) { return var.type() == type; });
+        return it == vars.end() ? nullptr : any_cast<Type>(&*it);
+    }
+
+    /**
+     * @brief Returns a reference to an object in the context of the registry.
+     *
+     * @warning
+     * Attempting to get a context variable that doesn't exist results in
+     * undefined behavior.
+     *
+     * @tparam Type Type of object to get.
+     * @return A valid reference to the object in the context of the registry.
+     */
+    template<typename Type>
+    [[nodiscard]] std::add_const_t<Type> & ctx() const {
+        auto it = std::find_if(vars.cbegin(), vars.cend(), [type = type_id<Type>()](auto &&var) { return var.type() == type; });
+        ENTT_ASSERT(it != vars.cend(), "Invalid instance");
+        return any_cast<std::add_const_t<Type> &>(*it);
+    }
+
+    /*! @copydoc ctx */
+    template<typename Type>
+    [[nodiscard]] Type & ctx() {
+        auto it = std::find_if(vars.begin(), vars.end(), [type = type_id<Type>()](auto &&var) { return var.type() == type; });
+        ENTT_ASSERT(it != vars.end(), "Invalid instance");
+        return any_cast<Type &>(*it);
+    }
+
+    /**
+     * @brief Visits a registry and returns the type info for its context
+     * variables.
+     *
+     * The signature of the function should be equivalent to the following:
+     *
+     * @code{.cpp}
+     * void(const type_info);
+     * @endcode
+     *
+     * Returned identifiers are those of the context variables currently set.
+     *
+     * @sa type_info
+     *
+     * @warning
+     * It's not specified whether a context variable created during the visit is
+     * returned or not to the caller.
+     *
+     * @tparam Func Type of the function object to invoke.
+     * @param func A valid function object.
+     */
+    template<typename Func>
+    void ctx(Func func) const {
+        for(auto pos = vars.size(); pos; --pos) {
+            func(vars[pos-1].type());
+        }
+    }
+
+private:
+    std::vector<basic_any<0u>> vars{};
+    mutable std::vector<pool_data> pools{};
+    std::vector<group_data> groups{};
+    std::vector<entity_type> entities{};
+    entity_type free_list{tombstone};
+};
+
+
+}
+
+
+#endif
+
+// #include "entity/runtime_view.hpp"
+#ifndef ENTT_ENTITY_RUNTIME_VIEW_HPP
+#define ENTT_ENTITY_RUNTIME_VIEW_HPP
+
+
+#include <iterator>
+#include <vector>
+#include <utility>
+#include <algorithm>
+#include <type_traits>
+// #include "../config/config.h"
+
+// #include "entity.hpp"
+
+// #include "sparse_set.hpp"
+
+// #include "fwd.hpp"
+
+
+
+namespace entt {
+
+
+/**
+ * @brief Runtime view.
+ *
+ * Runtime views iterate over those entities that have at least all the given
+ * components in their bags. During initialization, a runtime view looks at the
+ * number of entities available for each component and picks up a reference to
+ * the smallest set of candidate entities in order to get a performance boost
+ * when iterate.<br/>
+ * Order of elements during iterations are highly dependent on the order of the
+ * underlying data structures. See sparse_set and its specializations for more
+ * details.
+ *
+ * @b Important
+ *
+ * Iterators aren't invalidated if:
+ *
+ * * New instances of the given components are created and assigned to entities.
+ * * The entity currently pointed is modified (as an example, if one of the
+ *   given components is removed from the entity to which the iterator points).
+ * * The entity currently pointed is destroyed.
+ *
+ * In all the other cases, modifying the pools of the given components in any
+ * way invalidates all the iterators and using them results in undefined
+ * behavior.
+ *
+ * @note
+ * Views share references to the underlying data structures of the registry that
+ * generated them. Therefore any change to the entities and to the components
+ * made by means of the registry are immediately reflected by the views, unless
+ * a pool was missing when the view was built (in this case, the view won't
+ * have a valid reference and won't be updated accordingly).
+ *
+ * @warning
+ * Lifetime of a view must not overcome that of the registry that generated it.
+ * In any other case, attempting to use a view results in undefined behavior.
+ *
+ * @tparam Entity A valid entity type (see entt_traits for more details).
+ */
+template<typename Entity>
+class basic_runtime_view final {
+    using basic_common_type = basic_sparse_set<Entity>;
+    using underlying_iterator = typename basic_common_type::iterator;
+
+    class view_iterator final {
+        [[nodiscard]] bool valid() const {
+            const auto entt = *it;
+
+            return (!stable_storage || (entt != tombstone))
+                && std::all_of(pools->begin()++, pools->end(), [entt](const auto *curr) { return curr->contains(entt); })
+                && std::none_of(filter->cbegin(), filter->cend(), [entt](const auto *curr) { return curr && curr->contains(entt); });
+        }
+
+    public:
+        using difference_type = typename underlying_iterator::difference_type;
+        using value_type = typename underlying_iterator::value_type;
+        using pointer = typename underlying_iterator::pointer;
+        using reference = typename underlying_iterator::reference;
+        using iterator_category = std::bidirectional_iterator_tag;
+
+        view_iterator() ENTT_NOEXCEPT = default;
+
+        view_iterator(const std::vector<const basic_common_type *> &cpools, const std::vector<const basic_common_type *> &ignore, underlying_iterator curr) ENTT_NOEXCEPT
+            : pools{&cpools},
+              filter{&ignore},
+              it{curr},
+              stable_storage{std::any_of(pools->cbegin(), pools->cend(), [](const basic_common_type *cpool) { return (cpool->policy() == deletion_policy::in_place); })}
+        {
+            if(it != (*pools)[0]->end() && !valid()) {
+                ++(*this);
+            }
+        }
+
+        view_iterator & operator++() {
+            while(++it != (*pools)[0]->end() && !valid());
+            return *this;
+        }
+
+        view_iterator operator++(int) {
+            view_iterator orig = *this;
+            return ++(*this), orig;
+        }
+
+        view_iterator & operator--() ENTT_NOEXCEPT {
+            while(--it != (*pools)[0]->begin() && !valid());
+            return *this;
+        }
+
+        view_iterator operator--(int) ENTT_NOEXCEPT {
+            view_iterator orig = *this;
+            return operator--(), orig;
+        }
+
+        [[nodiscard]] bool operator==(const view_iterator &other) const ENTT_NOEXCEPT {
+            return other.it == it;
+        }
+
+        [[nodiscard]] bool operator!=(const view_iterator &other) const ENTT_NOEXCEPT {
+            return !(*this == other);
+        }
+
+        [[nodiscard]] pointer operator->() const {
+            return it.operator->();
+        }
+
+        [[nodiscard]] reference operator*() const {
+            return *operator->();
+        }
+
+    private:
+        const std::vector<const basic_common_type *> *pools;
+        const std::vector<const basic_common_type *> *filter;
+        underlying_iterator it;
+        bool stable_storage;
+    };
+
+    [[nodiscard]] bool valid() const {
+        return !pools.empty() && pools.front();
+    }
+
+public:
+    /*! @brief Underlying entity identifier. */
+    using entity_type = Entity;
+    /*! @brief Unsigned integer type. */
+    using size_type = std::size_t;
+    /*! @brief Bidirectional iterator type. */
+    using iterator = view_iterator;
+
+    /*! @brief Default constructor to use to create empty, invalid views. */
+    basic_runtime_view() ENTT_NOEXCEPT
+        : pools{},
+          filter{}
+    {}
+
+    /**
+     * @brief Constructs a runtime view from a set of storage classes.
+     * @param cpools The storage for the types to iterate.
+     * @param epools The storage for the types used to filter the view.
+     */
+    basic_runtime_view(std::vector<const basic_common_type *> cpools, std::vector<const basic_common_type *> epools) ENTT_NOEXCEPT
+        : pools{std::move(cpools)},
+          filter{std::move(epools)}
+    {
+        // brings the best candidate (if any) on front of the vector
+        std::rotate(pools.begin(), std::min_element(pools.begin(), pools.end(), [](const auto *lhs, const auto *rhs) {
+            return (!lhs && rhs) || (lhs && rhs && lhs->size() < rhs->size());
+        }), pools.end());
+    }
+
+    /**
+     * @brief Estimates the number of entities iterated by the view.
+     * @return Estimated number of entities iterated by the view.
+     */
+    [[nodiscard]] size_type size_hint() const {
+        return valid() ? pools.front()->size() : size_type{};
+    }
+
+    /**
+     * @brief Returns an iterator to the first entity that has the given
+     * components.
+     *
+     * The returned iterator points to the first entity that has the given
+     * components. If the view is empty, the returned iterator will be equal to
+     * `end()`.
+     *
+     * @return An iterator to the first entity that has the given components.
+     */
+    [[nodiscard]] iterator begin() const {
+        return valid() ? iterator{pools, filter, pools[0]->begin()} : iterator{};
+    }
+
+    /**
+     * @brief Returns an iterator that is past the last entity that has the
+     * given components.
+     *
+     * The returned iterator points to the entity following the last entity that
+     * has the given components. Attempting to dereference the returned iterator
+     * results in undefined behavior.
+     *
+     * @return An iterator to the entity following the last entity that has the
+     * given components.
+     */
+    [[nodiscard]] iterator end() const {
+        return valid() ? iterator{pools, filter, pools[0]->end()} : iterator{};
+    }
+
+    /**
+     * @brief Checks if a view contains an entity.
+     * @param entt A valid entity identifier.
+     * @return True if the view contains the given entity, false otherwise.
+     */
+    [[nodiscard]] bool contains(const entity_type entt) const {
+        return valid() && std::all_of(pools.cbegin(), pools.cend(), [entt](const auto *curr) { return curr->contains(entt); })
+                && std::none_of(filter.cbegin(), filter.cend(), [entt](const auto *curr) { return curr && curr->contains(entt); });
+    }
+
+    /**
+     * @brief Iterates entities and applies the given function object to them.
+     *
+     * The function object is invoked for each entity. It is provided only with
+     * the entity itself. To get the components, users can use the registry with
+     * which the view was built.<br/>
+     * The signature of the function should be equivalent to the following:
+     *
+     * @code{.cpp}
+     * void(const entity_type);
+     * @endcode
+     *
+     * @tparam Func Type of the function object to invoke.
+     * @param func A valid function object.
+     */
+    template<typename Func>
+    void each(Func func) const {
+        for(const auto entity: *this) {
+            func(entity);
+        }
+    }
+
+private:
+    std::vector<const basic_common_type *> pools;
+    std::vector<const basic_common_type *> filter;
+};
+
+
+}
+
+
+#endif
+
+// #include "entity/snapshot.hpp"
 #ifndef ENTT_ENTITY_SNAPSHOT_HPP
 #define ENTT_ENTITY_SNAPSHOT_HPP
 
 
 #include <array>
 #include <cstddef>
-#include <utility>
 #include <iterator>
+#include <tuple>
 #include <type_traits>
 #include <unordered_map>
-// #include "../config/config.h"
+#include <utility>
+#include <vector>
+// #include "../config/config.h"
+
+// #include "../core/type_traits.hpp"
 
 // #include "entity.hpp"
 
 // #include "fwd.hpp"
 
+// #include "registry.hpp"
+
 
 
 namespace entt {
@@ -4215,49 +24560,47 @@
  */
 template<typename Entity>
 class basic_snapshot {
-    /*! @brief A registry is allowed to create snapshots. */
-    friend class basic_registry<Entity>;
-
-    using follow_fn_type = Entity(const basic_registry<Entity> &, const Entity);
-    using traits_type = entt_traits<std::underlying_type_t<Entity>>;
-
-    basic_snapshot(const basic_registry<Entity> *source, Entity init, follow_fn_type *fn) ENTT_NOEXCEPT
-        : reg{source},
-          seed{init},
-          follow{fn}
-    {}
+    using traits_type = entt_traits<Entity>;
 
     template<typename Component, typename Archive, typename It>
     void get(Archive &archive, std::size_t sz, It first, It last) const {
+        const auto view = reg->template view<std::add_const_t<Component>>();
         archive(typename traits_type::entity_type(sz));
 
         while(first != last) {
             const auto entt = *(first++);
 
-            if(reg->template has<Component>(entt)) {
-                if constexpr(std::is_empty_v<Component>) {
-                    archive(entt);
-                } else {
-                    archive(entt, reg->template get<Component>(entt));
-                }
-            }
-        }
-    }
-
-    template<typename... Component, typename Archive, typename It, std::size_t... Indexes>
-    void component(Archive &archive, It first, It last, std::index_sequence<Indexes...>) const {
-        std::array<std::size_t, sizeof...(Indexes)> size{};
+            if(reg->template all_of<Component>(entt)) {
+                std::apply(archive, std::tuple_cat(std::make_tuple(entt), view.get(entt)));
+            }
+        }
+    }
+
+    template<typename... Component, typename Archive, typename It, std::size_t... Index>
+    void component(Archive &archive, It first, It last, std::index_sequence<Index...>) const {
+        std::array<std::size_t, sizeof...(Index)> size{};
         auto begin = first;
 
         while(begin != last) {
             const auto entt = *(begin++);
-            ((reg->template has<Component>(entt) ? ++size[Indexes] : size[Indexes]), ...);
-        }
-
-        (get<Component>(archive, size[Indexes], first, last), ...);
-    }
-
-public:
+            ((reg->template all_of<Component>(entt) ? ++size[Index] : 0u), ...);
+        }
+
+        (get<Component>(archive, size[Index], first, last), ...);
+    }
+
+public:
+    /*! @brief Underlying entity identifier. */
+    using entity_type = Entity;
+
+    /**
+     * @brief Constructs an instance that is bound to a given registry.
+     * @param source A valid reference to a registry.
+     */
+    basic_snapshot(const basic_registry<entity_type> &source) ENTT_NOEXCEPT
+        : reg{&source}
+    {}
+
     /*! @brief Default move constructor. */
     basic_snapshot(basic_snapshot &&) = default;
 
@@ -4265,10 +24608,10 @@
     basic_snapshot & operator=(basic_snapshot &&) = default;
 
     /**
-     * @brief Puts aside all the entities that are still in use.
+     * @brief Puts aside all the entities from the underlying registry.
      *
      * Entities are serialized along with their versions. Destroyed entities are
-     * not taken in consideration by this function.
+     * taken in consideration as well by this function.
      *
      * @tparam Archive Type of output archive.
      * @param archive A valid reference to an output archive.
@@ -4276,35 +24619,15 @@
      */
     template<typename Archive>
     const basic_snapshot & entities(Archive &archive) const {
-        archive(typename traits_type::entity_type(reg->alive()));
-        reg->each([&archive](const auto entt) { archive(entt); });
-        return *this;
-    }
-
-    /**
-     * @brief Puts aside destroyed entities.
-     *
-     * Entities are serialized along with their versions. Entities that are
-     * still in use are not taken in consideration by this function.
-     *
-     * @tparam Archive Type of output archive.
-     * @param archive A valid reference to an output archive.
-     * @return An object of this type to continue creating the snapshot.
-     */
-    template<typename Archive>
-    const basic_snapshot & destroyed(Archive &archive) const {
-        auto size = reg->size() - reg->alive();
-        archive(typename traits_type::entity_type(size));
-
-        if(size) {
-            auto curr = seed;
-            archive(curr);
-
-            for(--size; size; --size) {
-                curr = follow(*reg, curr);
-                archive(curr);
-            }
-        }
+        const auto sz = reg->size();
+
+        archive(typename traits_type::entity_type(sz));
+
+        for(auto first = reg->data(), last = first + sz; first != last; ++first) {
+            archive(*first);
+        }
+
+        archive(reg->released());
 
         return *this;
     }
@@ -4322,26 +24645,14 @@
      */
     template<typename... Component, typename Archive>
     const basic_snapshot & component(Archive &archive) const {
-        if constexpr(sizeof...(Component) == 1) {
-            const auto sz = reg->template size<Component...>();
-            const auto *entities = reg->template data<Component...>();
-
-            archive(typename traits_type::entity_type(sz));
-
-            for(std::remove_const_t<decltype(sz)> pos{}; pos < sz; ++pos) {
-                const auto entt = entities[pos];
-
-                if constexpr(std::is_empty_v<Component...>) {
-                    archive(entt);
-                } else {
-                    archive(entt, reg->template get<Component...>(entt));
-                }
-            };
+        if constexpr(sizeof...(Component) == 1u) {
+            const auto view = reg->template view<const Component...>();
+            (component<Component>(archive, view.data(), view.data() + view.size()), ...);
+            return *this;
         } else {
             (component<Component>(archive), ...);
-        }
-
-        return *this;
+            return *this;
+        }
     }
 
     /**
@@ -4360,14 +24671,12 @@
      */
     template<typename... Component, typename Archive, typename It>
     const basic_snapshot & component(Archive &archive, It first, It last) const {
-        component<Component...>(archive, first, last, std::make_index_sequence<sizeof...(Component)>{});
-        return *this;
-    }
-
-private:
-    const basic_registry<Entity> *reg;
-    const Entity seed;
-    follow_fn_type *follow;
+        component<Component...>(archive, first, last, std::index_sequence_for<Component...>{});
+        return *this;
+    }
+
+private:
+    const basic_registry<entity_type> *reg;
 };
 
 
@@ -4383,55 +24692,49 @@
  */
 template<typename Entity>
 class basic_snapshot_loader {
-    /*! @brief A registry is allowed to create snapshot loaders. */
-    friend class basic_registry<Entity>;
-
-    using force_fn_type = void(basic_registry<Entity> &, const Entity, const bool);
-    using traits_type = entt_traits<std::underlying_type_t<Entity>>;
-
-    basic_snapshot_loader(basic_registry<Entity> *source, force_fn_type *fn) ENTT_NOEXCEPT
-        : reg{source},
-          force{fn}
-    {
-        // to restore a snapshot as a whole requires a clean registry
-        ENTT_ASSERT(reg->empty());
-    }
-
-    template<typename Archive>
-    void assure(Archive &archive, bool discard) const {
+    using traits_type = entt_traits<Entity>;
+
+    template<typename Type, typename Archive>
+    void assign(Archive &archive) const {
         typename traits_type::entity_type length{};
         archive(length);
 
-        while(length--) {
-            Entity entt{};
-            archive(entt);
-            force(*reg, entt, discard);
-        }
-    }
-
-    template<typename Type, typename Archive, typename... Args>
-    void assign(Archive &archive, Args... args) const {
-        typename traits_type::entity_type length{};
-        archive(length);
-
-        while(length--) {
-            static constexpr auto discard = false;
-            Entity entt{};
-
-            if constexpr(std::is_empty_v<Type>) {
+        entity_type entt{};
+
+        if constexpr(std::tuple_size_v<decltype(reg->template view<Type>().get({}))> == 0) {
+            while(length--) {
                 archive(entt);
-                force(*reg, entt, discard);
-                reg->template assign<Type>(args..., entt);
-            } else {
-                Type instance{};
+                const auto entity = reg->valid(entt) ? entt : reg->create(entt);
+                ENTT_ASSERT(entity == entt, "Entity not available for use");
+                reg->template emplace<Type>(entity);
+            }
+        } else {
+            Type instance{};
+
+            while(length--) {
                 archive(entt, instance);
-                force(*reg, entt, discard);
-                reg->template assign<Type>(args..., entt, std::as_const(instance));
-            }
-        }
-    }
-
-public:
+                const auto entity = reg->valid(entt) ? entt : reg->create(entt);
+                ENTT_ASSERT(entity == entt, "Entity not available for use");
+                reg->template emplace<Type>(entity, std::move(instance));
+            }
+        }
+    }
+
+public:
+    /*! @brief Underlying entity identifier. */
+    using entity_type = Entity;
+
+    /**
+     * @brief Constructs an instance that is bound to a given registry.
+     * @param source A valid reference to a registry.
+     */
+    basic_snapshot_loader(basic_registry<entity_type> &source) ENTT_NOEXCEPT
+        : reg{&source}
+    {
+        // restoring a snapshot as a whole requires a clean registry
+        ENTT_ASSERT(reg->empty(), "Registry must be empty");
+    }
+
     /*! @brief Default move constructor. */
     basic_snapshot_loader(basic_snapshot_loader &&) = default;
 
@@ -4450,25 +24753,20 @@
      */
     template<typename Archive>
     const basic_snapshot_loader & entities(Archive &archive) const {
-        static constexpr auto discard = false;
-        assure(archive, discard);
-        return *this;
-    }
-
-    /**
-     * @brief Restores entities that were destroyed during serialization.
-     *
-     * This function restores the entities that were destroyed during
-     * serialization and gives them the versions they originally had.
-     *
-     * @tparam Archive Type of input archive.
-     * @param archive A valid reference to an input archive.
-     * @return A valid loader to continue restoring data.
-     */
-    template<typename Archive>
-    const basic_snapshot_loader & destroyed(Archive &archive) const {
-        static constexpr auto discard = true;
-        assure(archive, discard);
+        typename traits_type::entity_type length{};
+
+        archive(length);
+        std::vector<entity_type> all(length);
+
+        for(decltype(length) pos{}; pos < length; ++pos) {
+            archive(all[pos]);
+        }
+
+        entity_type destroyed;
+        archive(destroyed);
+
+        reg->assign(all.cbegin(), all.cend(), destroyed);
+
         return *this;
     }
 
@@ -4503,15 +24801,14 @@
      */
     const basic_snapshot_loader & orphans() const {
         reg->orphans([this](const auto entt) {
-            reg->destroy(entt);
+            reg->release(entt);
         });
 
         return *this;
     }
 
 private:
-    basic_registry<Entity> *reg;
-    force_fn_type *force;
+    basic_registry<entity_type> *reg;
 };
 
 
@@ -4519,7 +24816,7 @@
  * @brief Utility class for _continuous loading_.
  *
  * A _continuous loader_ is designed to load data from a source registry to a
- * (possibly) non-empty destination. The loader can accomodate in a registry
+ * (possibly) non-empty destination. The loader can accommodate in a registry
  * more than one snapshot in a sort of _continuous loading_ that updates the
  * destination one step at a time.<br/>
  * Identifiers that entities originally had are not transferred to the target.
@@ -4533,12 +24830,10 @@
  */
 template<typename Entity>
 class basic_continuous_loader {
-    using traits_type = entt_traits<std::underlying_type_t<Entity>>;
+    using traits_type = entt_traits<Entity>;
 
     void destroy(Entity entt) {
-        const auto it = remloc.find(entt);
-
-        if(it == remloc.cend()) {
+        if(const auto it = remloc.find(entt); it == remloc.cend()) {
             const auto local = reg->create();
             remloc.emplace(entt, std::make_pair(local, true));
             reg->destroy(local);
@@ -4552,45 +24847,68 @@
             const auto local = reg->create();
             remloc.emplace(entt, std::make_pair(local, true));
         } else {
-            remloc[entt].first = reg->valid(remloc[entt].first) ? remloc[entt].first : reg->create();
+            if(!reg->valid(remloc[entt].first)) {
+                remloc[entt].first = reg->create();
+            }
+
             // set the dirty flag
             remloc[entt].second = true;
         }
     }
 
+    template<typename Container>
+    auto update(int, Container &container)
+    -> decltype(typename Container::mapped_type{}, void()) {
+        // map like container
+        Container other;
+
+        for(auto &&pair: container) {
+            using first_type = std::remove_const_t<typename std::decay_t<decltype(pair)>::first_type>;
+            using second_type = typename std::decay_t<decltype(pair)>::second_type;
+
+            if constexpr(std::is_same_v<first_type, entity_type> && std::is_same_v<second_type, entity_type>) {
+                other.emplace(map(pair.first), map(pair.second));
+            } else if constexpr(std::is_same_v<first_type, entity_type>) {
+                other.emplace(map(pair.first), std::move(pair.second));
+            } else {
+                static_assert(std::is_same_v<second_type, entity_type>, "Neither the key nor the value are of entity type");
+                other.emplace(std::move(pair.first), map(pair.second));
+            }
+        }
+
+        std::swap(container, other);
+    }
+
+    template<typename Container>
+    auto update(char, Container &container)
+    -> decltype(typename Container::value_type{}, void()) {
+        // vector like container
+        static_assert(std::is_same_v<typename Container::value_type, entity_type>, "Invalid value type");
+
+        for(auto &&entt: container) {
+            entt = map(entt);
+        }
+    }
+
     template<typename Other, typename Type, typename Member>
-    void update(Other &instance, Member Type:: *member) {
+    void update([[maybe_unused]] Other &instance, [[maybe_unused]] Member Type:: *member) {
         if constexpr(!std::is_same_v<Other, Type>) {
             return;
-        } else if constexpr(std::is_same_v<Member, Entity>) {
+        } else if constexpr(std::is_same_v<Member, entity_type>) {
             instance.*member = map(instance.*member);
         } else {
             // maybe a container? let's try...
-            for(auto &entt: instance.*member) {
-                entt = map(entt);
-            }
-        }
-    }
-
-    template<typename Archive>
-    void assure(Archive &archive, void(basic_continuous_loader:: *member)(Entity)) {
-        typename traits_type::entity_type length{};
-        archive(length);
-
-        while(length--) {
-            Entity entt{};
-            archive(entt);
-            (this->*member)(entt);
+            update(0, instance.*member);
         }
     }
 
     template<typename Component>
-    void reset() {
+    void remove_if_exists() {
         for(auto &&ref: remloc) {
             const auto local = ref.second.first;
 
             if(reg->valid(local)) {
-                reg->template reset<Component>(local);
+                reg->template remove<Component>(local);
             }
         }
     }
@@ -4600,19 +24918,22 @@
         typename traits_type::entity_type length{};
         archive(length);
 
-        while(length--) {
-            Entity entt{};
-
-            if constexpr(std::is_empty_v<Other>) {
+        entity_type entt{};
+
+        if constexpr(std::tuple_size_v<decltype(reg->template view<Other>().get({}))> == 0) {
+            while(length--) {
                 archive(entt);
                 restore(entt);
-                reg->template assign_or_replace<Other>(map(entt));
-            } else {
-                Other instance{};
+                reg->template emplace_or_replace<Other>(map(entt));
+            }
+        } else {
+            Other instance{};
+
+            while(length--) {
                 archive(entt, instance);
                 (update(instance, member), ...);
                 restore(entt);
-                reg->template assign_or_replace<Other>(map(entt), std::as_const(instance));
+                reg->template emplace_or_replace<Other>(map(entt), std::move(instance));
             }
         }
     }
@@ -4622,7 +24943,7 @@
     using entity_type = Entity;
 
     /**
-     * @brief Constructs a loader that is bound to a given registry.
+     * @brief Constructs an instance that is bound to a given registry.
      * @param source A valid reference to a registry.
      */
     basic_continuous_loader(basic_registry<entity_type> &source) ENTT_NOEXCEPT
@@ -4647,23 +24968,24 @@
      */
     template<typename Archive>
     basic_continuous_loader & entities(Archive &archive) {
-        assure(archive, &basic_continuous_loader::restore);
-        return *this;
-    }
-
-    /**
-     * @brief Restores entities that were destroyed during serialization.
-     *
-     * This function restores the entities that were destroyed during
-     * serialization and creates local counterparts for them if required.
-     *
-     * @tparam Archive Type of input archive.
-     * @param archive A valid reference to an input archive.
-     * @return A non-const reference to this loader.
-     */
-    template<typename Archive>
-    basic_continuous_loader & destroyed(Archive &archive) {
-        assure(archive, &basic_continuous_loader::destroy);
+        typename traits_type::entity_type length{};
+        entity_type entt{};
+
+        archive(length);
+
+        for(decltype(length) pos{}; pos < length; ++pos) {
+            archive(entt);
+
+            if(const auto entity = traits_type::to_entity(entt); entity == pos) {
+                restore(entt);
+            } else {
+                destroy(entt);
+            }
+        }
+
+        // discards the head of the list of destroyed entities
+        archive(entt);
+
         return *this;
     }
 
@@ -4688,7 +25010,7 @@
      */
     template<typename... Component, typename Archive, typename... Type, typename... Member>
     basic_continuous_loader & component(Archive &archive, Member Type:: *... member) {
-        (reset<Component>(), ...);
+        (remove_if_exists<Component>(), ...);
         (assign<Component>(archive, member...), ...);
         return *this;
     }
@@ -4735,7 +25057,7 @@
      */
     basic_continuous_loader & orphans() {
         reg->orphans([this](const auto entt) {
-            reg->destroy(entt);
+            reg->release(entt);
         });
 
         return *this;
@@ -4746,7 +25068,7 @@
      * @param entt An entity identifier.
      * @return True if `entity` is managed by the loader, false otherwise.
      */
-    bool has(entity_type entt) const ENTT_NOEXCEPT {
+    [[nodiscard]] bool contains(entity_type entt) const ENTT_NOEXCEPT {
         return (remloc.find(entt) != remloc.cend());
     }
 
@@ -4755,7 +25077,7 @@
      * @param entt An entity identifier.
      * @return The local identifier if any, the null entity otherwise.
      */
-    entity_type map(entity_type entt) const ENTT_NOEXCEPT {
+    [[nodiscard]] entity_type map(entity_type entt) const ENTT_NOEXCEPT {
         const auto it = remloc.find(entt);
         entity_type other = null;
 
@@ -4775,28 +25097,914 @@
 }
 
 
-#endif // ENTT_ENTITY_SNAPSHOT_HPP
-
-// #include "storage.hpp"
-#ifndef ENTT_ENTITY_STORAGE_HPP
-#define ENTT_ENTITY_STORAGE_HPP
-
-
-#include <algorithm>
+#endif
+
+// #include "entity/sparse_set.hpp"
+#ifndef ENTT_ENTITY_SPARSE_SET_HPP
+#define ENTT_ENTITY_SPARSE_SET_HPP
+
+
+#include <cstddef>
 #include <iterator>
-#include <numeric>
-#include <utility>
-#include <vector>
-#include <cstddef>
-#include <type_traits>
+#include <memory>
+#include <type_traits>
+#include <utility>
 // #include "../config/config.h"
 
 // #include "../core/algorithm.hpp"
 
-// #include "sparse_set.hpp"
+// #include "../core/fwd.hpp"
 
 // #include "entity.hpp"
 
+// #include "fwd.hpp"
+
+
+
+namespace entt {
+
+
+/*! @brief Sparse set deletion policy. */
+enum class deletion_policy: std::uint8_t {
+    /*! @brief Swap-and-pop deletion policy. */
+    swap_and_pop = 0u,
+    /*! @brief In-place deletion policy. */
+    in_place = 1u
+};
+
+
+/**
+ * @brief Basic sparse set implementation.
+ *
+ * Sparse set or packed array or whatever is the name users give it.<br/>
+ * Two arrays: an _external_ one and an _internal_ one; a _sparse_ one and a
+ * _packed_ one; one used for direct access through contiguous memory, the other
+ * one used to get the data through an extra level of indirection.<br/>
+ * This is largely used by the registry to offer users the fastest access ever
+ * to the components. Views and groups in general are almost entirely designed
+ * around sparse sets.
+ *
+ * This type of data structure is widely documented in the literature and on the
+ * web. This is nothing more than a customized implementation suitable for the
+ * purpose of the framework.
+ *
+ * @note
+ * Internal data structures arrange elements to maximize performance. There are
+ * no guarantees that entities are returned in the insertion order when iterate
+ * a sparse set. Do not make assumption on the order in any case.
+ *
+ * @tparam Entity A valid entity type (see entt_traits for more details).
+ * @tparam Allocator Type of allocator used to manage memory and elements.
+ */
+template<typename Entity, typename Allocator>
+class basic_sparse_set {
+    static constexpr auto growth_factor = 1.5;
+    static constexpr auto sparse_page = ENTT_SPARSE_PAGE;
+
+    using traits_type = entt_traits<Entity>;
+
+    using alloc_traits = typename std::allocator_traits<Allocator>::template rebind_traits<Entity>;
+    using alloc_pointer = typename alloc_traits::pointer;
+    using alloc_const_pointer = typename alloc_traits::const_pointer;
+
+    using bucket_alloc_traits = typename std::allocator_traits<Allocator>::template rebind_traits<alloc_pointer>;
+    using bucket_alloc_pointer = typename bucket_alloc_traits::pointer;
+
+    static_assert(alloc_traits::propagate_on_container_move_assignment::value);
+    static_assert(bucket_alloc_traits::propagate_on_container_move_assignment::value);
+
+    struct sparse_set_iterator final {
+        using difference_type = typename traits_type::difference_type;
+        using value_type = Entity;
+        using pointer = const value_type *;
+        using reference = const value_type &;
+        using iterator_category = std::random_access_iterator_tag;
+
+        sparse_set_iterator() ENTT_NOEXCEPT = default;
+
+        sparse_set_iterator(const alloc_const_pointer *ref, const difference_type idx) ENTT_NOEXCEPT
+            : packed{ref},
+              index{idx}
+        {}
+
+        sparse_set_iterator & operator++() ENTT_NOEXCEPT {
+            return --index, *this;
+        }
+
+        sparse_set_iterator operator++(int) ENTT_NOEXCEPT {
+            iterator orig = *this;
+            return ++(*this), orig;
+        }
+
+        sparse_set_iterator & operator--() ENTT_NOEXCEPT {
+            return ++index, *this;
+        }
+
+        sparse_set_iterator operator--(int) ENTT_NOEXCEPT {
+            sparse_set_iterator orig = *this;
+            return operator--(), orig;
+        }
+
+        sparse_set_iterator & operator+=(const difference_type value) ENTT_NOEXCEPT {
+            index -= value;
+            return *this;
+        }
+
+        sparse_set_iterator operator+(const difference_type value) const ENTT_NOEXCEPT {
+            sparse_set_iterator copy = *this;
+            return (copy += value);
+        }
+
+        sparse_set_iterator & operator-=(const difference_type value) ENTT_NOEXCEPT {
+            return (*this += -value);
+        }
+
+        sparse_set_iterator operator-(const difference_type value) const ENTT_NOEXCEPT {
+            return (*this + -value);
+        }
+
+        difference_type operator-(const sparse_set_iterator &other) const ENTT_NOEXCEPT {
+            return other.index - index;
+        }
+
+        [[nodiscard]] reference operator[](const difference_type value) const {
+            const auto pos = size_type(index-value-1u);
+            return (*packed)[pos];
+        }
+
+        [[nodiscard]] bool operator==(const sparse_set_iterator &other) const ENTT_NOEXCEPT {
+            return other.index == index;
+        }
+
+        [[nodiscard]] bool operator!=(const sparse_set_iterator &other) const ENTT_NOEXCEPT {
+            return !(*this == other);
+        }
+
+        [[nodiscard]] bool operator<(const sparse_set_iterator &other) const ENTT_NOEXCEPT {
+            return index > other.index;
+        }
+
+        [[nodiscard]] bool operator>(const sparse_set_iterator &other) const ENTT_NOEXCEPT {
+            return index < other.index;
+        }
+
+        [[nodiscard]] bool operator<=(const sparse_set_iterator &other) const ENTT_NOEXCEPT {
+            return !(*this > other);
+        }
+
+        [[nodiscard]] bool operator>=(const sparse_set_iterator &other) const ENTT_NOEXCEPT {
+            return !(*this < other);
+        }
+
+        [[nodiscard]] pointer operator->() const {
+            const auto pos = size_type(index-1u);
+            return std::addressof((*packed)[pos]);
+        }
+
+        [[nodiscard]] reference operator*() const {
+            return *operator->();
+        }
+
+    private:
+        const alloc_const_pointer *packed;
+        difference_type index;
+    };
+
+    [[nodiscard]] static auto page(const Entity entt) ENTT_NOEXCEPT {
+        return static_cast<size_type>(traits_type::to_entity(entt) / sparse_page);
+    }
+
+    [[nodiscard]] static auto offset(const Entity entt) ENTT_NOEXCEPT {
+        return static_cast<size_type>(traits_type::to_entity(entt) & (sparse_page - 1));
+    }
+
+    [[nodiscard]] auto assure_page(const std::size_t idx) {
+        if(!(idx < bucket)) {
+            const size_type sz = idx + 1u;
+            const auto mem = bucket_alloc_traits::allocate(bucket_allocator, sz);
+
+            std::uninitialized_value_construct(mem + bucket, mem + sz);
+            std::uninitialized_copy(sparse, sparse + bucket, mem);
+
+            std::destroy(sparse, sparse + bucket);
+            bucket_alloc_traits::deallocate(bucket_allocator, sparse, bucket);
+
+            sparse = mem;
+            bucket = sz;
+        }
+
+        if(!sparse[idx]) {
+            sparse[idx] = alloc_traits::allocate(allocator, sparse_page);
+            std::uninitialized_fill(sparse[idx], sparse[idx] + sparse_page, null);
+        }
+
+        return sparse[idx];
+    }
+
+    void resize_packed(const std::size_t req) {
+        ENTT_ASSERT((req != reserved) && !(req < count), "Invalid request");
+        const auto mem = alloc_traits::allocate(allocator, req);
+
+        std::uninitialized_copy(packed, packed + count, mem);
+        std::uninitialized_fill(mem + count, mem + req, tombstone);
+
+        std::destroy(packed, packed + reserved);
+        alloc_traits::deallocate(allocator, packed, reserved);
+
+        packed = mem;
+        reserved = req;
+    }
+
+    void release_memory() {
+        if(packed) {
+            for(size_type pos{}; pos < bucket; ++pos) {
+                if(sparse[pos]) {
+                    std::destroy(sparse[pos], sparse[pos] + sparse_page);
+                    alloc_traits::deallocate(allocator, sparse[pos], sparse_page);
+                }
+            }
+
+            std::destroy(packed, packed + reserved);
+            std::destroy(sparse, sparse + bucket);
+            alloc_traits::deallocate(allocator, packed, reserved);
+            bucket_alloc_traits::deallocate(bucket_allocator, sparse, bucket);
+        }
+    }
+
+protected:
+    /**
+     * @brief Swaps two entities in the internal packed array.
+     * @param lhs A valid position of an entity within storage.
+     * @param rhs A valid position of an entity within storage.
+     */
+    virtual void swap_at([[maybe_unused]] const std::size_t lhs, [[maybe_unused]] const std::size_t rhs) {}
+
+    /**
+     * @brief Moves an entity in the internal packed array.
+     * @param from A valid position of an entity within storage.
+     * @param to A valid position of an entity within storage.
+     */
+    virtual void move_and_pop([[maybe_unused]] const std::size_t from, [[maybe_unused]] const std::size_t to) {}
+
+    /**
+     * @brief Attempts to erase an entity from the internal packed array.
+     * @param entt A valid entity identifier.
+     * @param ud Optional user data that are forwarded as-is to derived classes.
+     */
+    virtual void swap_and_pop(const Entity entt, [[maybe_unused]] void *ud) {
+        auto &ref = sparse[page(entt)][offset(entt)];
+        const auto pos = static_cast<size_type>(traits_type::to_entity(ref));
+        ENTT_ASSERT(packed[pos] == entt, "Invalid entity identifier");
+        auto &last = packed[--count];
+
+        packed[pos] = last;
+        sparse[page(last)][offset(last)] = ref;
+        // lazy self-assignment guard
+        ref = null;
+        // unnecessary but it helps to detect nasty bugs
+        ENTT_ASSERT((last = tombstone, true), "");
+    }
+
+    /**
+     * @brief Attempts to erase an entity from the internal packed array.
+     * @param entt A valid entity identifier.
+     * @param ud Optional user data that are forwarded as-is to derived classes.
+     */
+    virtual void in_place_pop(const Entity entt, [[maybe_unused]] void *ud) {
+        auto &ref = sparse[page(entt)][offset(entt)];
+        const auto pos = static_cast<size_type>(traits_type::to_entity(ref));
+        ENTT_ASSERT(packed[pos] == entt, "Invalid entity identifier");
+
+        packed[pos] = std::exchange(free_list, traits_type::construct(static_cast<typename traits_type::entity_type>(pos)));
+        // lazy self-assignment guard
+        ref = null;
+    }
+
+public:
+    /*! @brief Allocator type. */
+    using allocator_type = typename alloc_traits::allocator_type;
+    /*! @brief Underlying entity identifier. */
+    using entity_type = Entity;
+    /*! @brief Unsigned integer type. */
+    using size_type = std::size_t;
+    /*! @brief Pointer type to contained entities. */
+    using pointer = alloc_const_pointer;
+    /*! @brief Random access iterator type. */
+    using iterator = sparse_set_iterator;
+    /*! @brief Reverse iterator type. */
+    using reverse_iterator = std::reverse_iterator<iterator>;
+
+    /**
+     * @brief Constructs an empty container with the given policy and allocator.
+     * @param pol Type of deletion policy.
+     * @param alloc Allocator to use (possibly default-constructed).
+     */
+    explicit basic_sparse_set(deletion_policy pol, const allocator_type &alloc = {})
+        : allocator{alloc},
+          bucket_allocator{alloc},
+          sparse{bucket_alloc_traits::allocate(bucket_allocator, 0u)},
+          packed{alloc_traits::allocate(allocator, 0u)},
+          bucket{0u},
+          count{0u},
+          reserved{0u},
+          free_list{tombstone},
+          mode{pol}
+    {}
+
+    /**
+     * @brief Constructs an empty container with the given allocator.
+     * @param alloc Allocator to use (possibly default-constructed).
+     */
+    explicit basic_sparse_set(const allocator_type &alloc = {})
+        : basic_sparse_set{deletion_policy::swap_and_pop, alloc}
+    {}
+
+    /**
+     * @brief Move constructor.
+     * @param other The instance to move from.
+     */
+    basic_sparse_set(basic_sparse_set &&other) ENTT_NOEXCEPT
+        : allocator{std::move(other.allocator)},
+          bucket_allocator{std::move(other.bucket_allocator)},
+          sparse{std::exchange(other.sparse, bucket_alloc_pointer{})},
+          packed{std::exchange(other.packed, alloc_pointer{})},
+          bucket{std::exchange(other.bucket, 0u)},
+          count{std::exchange(other.count, 0u)},
+          reserved{std::exchange(other.reserved, 0u)},
+          free_list{std::exchange(other.free_list, tombstone)},
+          mode{other.mode}
+    {}
+
+    /*! @brief Default destructor. */
+    virtual ~basic_sparse_set() {
+        release_memory();
+    }
+
+    /**
+     * @brief Move assignment operator.
+     * @param other The instance to move from.
+     * @return This sparse set.
+     */
+    basic_sparse_set & operator=(basic_sparse_set &&other) ENTT_NOEXCEPT {
+        release_memory();
+
+        allocator = std::move(other.allocator);
+        bucket_allocator = std::move(other.bucket_allocator);
+        sparse = std::exchange(other.sparse, bucket_alloc_pointer{});
+        packed = std::exchange(other.packed, alloc_pointer{});
+        bucket = std::exchange(other.bucket, 0u);
+        count = std::exchange(other.count, 0u);
+        reserved = std::exchange(other.reserved, 0u);
+        free_list = std::exchange(other.free_list, tombstone);
+        mode = other.mode;
+
+        return *this;
+    }
+
+    /**
+     * @brief Returns the deletion policy of a sparse set.
+     * @return The deletion policy of the sparse set.
+     */
+    [[nodiscard]] deletion_policy policy() const ENTT_NOEXCEPT {
+        return mode;
+    }
+
+    /**
+     * @brief Returns the next slot available for insertion.
+     * @return The next slot available for insertion.
+     */
+    [[nodiscard]] size_type slot() const ENTT_NOEXCEPT {
+        return free_list == null ? count : static_cast<size_type>(traits_type::to_entity(free_list));
+    }
+
+    /**
+     * @brief Increases the capacity of a sparse set.
+     *
+     * If the new capacity is greater than the current capacity, new storage is
+     * allocated, otherwise the method does nothing.
+     *
+     * @param cap Desired capacity.
+     */
+    void reserve(const size_type cap) {
+        if(cap > reserved) {
+            resize_packed(cap);
+        }
+    }
+
+    /**
+     * @brief Returns the number of elements that a sparse set has currently
+     * allocated space for.
+     * @return Capacity of the sparse set.
+     */
+    [[nodiscard]] size_type capacity() const ENTT_NOEXCEPT {
+        return reserved;
+    }
+
+    /*! @brief Requests the removal of unused capacity. */
+    void shrink_to_fit() {
+        if(count < reserved) {
+            resize_packed(count);
+        }
+    }
+
+    /**
+     * @brief Returns the extent of a sparse set.
+     *
+     * The extent of a sparse set is also the size of the internal sparse array.
+     * There is no guarantee that the internal packed array has the same size.
+     * Usually the size of the internal sparse array is equal or greater than
+     * the one of the internal packed array.
+     *
+     * @return Extent of the sparse set.
+     */
+    [[nodiscard]] size_type extent() const ENTT_NOEXCEPT {
+        return bucket * sparse_page;
+    }
+
+    /**
+     * @brief Returns the number of elements in a sparse set.
+     *
+     * The number of elements is also the size of the internal packed array.
+     * There is no guarantee that the internal sparse array has the same size.
+     * Usually the size of the internal sparse array is equal or greater than
+     * the one of the internal packed array.
+     *
+     * @return Number of elements.
+     */
+    [[nodiscard]] size_type size() const ENTT_NOEXCEPT {
+        return count;
+    }
+
+    /**
+     * @brief Checks whether a sparse set is empty.
+     * @return True if the sparse set is empty, false otherwise.
+     */
+    [[nodiscard]] bool empty() const ENTT_NOEXCEPT {
+        return (count == size_type{});
+    }
+
+    /**
+     * @brief Direct access to the internal packed array.
+     * @return A pointer to the internal packed array.
+     */
+    [[nodiscard]] pointer data() const ENTT_NOEXCEPT {
+        return packed;
+    }
+
+    /**
+     * @brief Returns an iterator to the beginning.
+     *
+     * The returned iterator points to the first entity of the internal packed
+     * array. If the sparse set is empty, the returned iterator will be equal to
+     * `end()`.
+     *
+     * @return An iterator to the first entity of the internal packed array.
+     */
+    [[nodiscard]] iterator begin() const ENTT_NOEXCEPT {
+        return iterator{std::addressof(packed), static_cast<typename traits_type::difference_type>(count)};
+    }
+
+    /**
+     * @brief Returns an iterator to the end.
+     *
+     * The returned iterator points to the element following the last entity in
+     * the internal packed array. Attempting to dereference the returned
+     * iterator results in undefined behavior.
+     *
+     * @return An iterator to the element following the last entity of the
+     * internal packed array.
+     */
+    [[nodiscard]] iterator end() const ENTT_NOEXCEPT {
+        return iterator{std::addressof(packed), {}};
+    }
+
+    /**
+     * @brief Returns a reverse iterator to the beginning.
+     *
+     * The returned iterator points to the first entity of the reversed internal
+     * packed array. If the sparse set is empty, the returned iterator will be
+     * equal to `rend()`.
+     *
+     * @return An iterator to the first entity of the reversed internal packed
+     * array.
+     */
+    [[nodiscard]] reverse_iterator rbegin() const ENTT_NOEXCEPT {
+        return std::make_reverse_iterator(end());
+    }
+
+    /**
+     * @brief Returns a reverse iterator to the end.
+     *
+     * The returned iterator points to the element following the last entity in
+     * the reversed internal packed array. Attempting to dereference the
+     * returned iterator results in undefined behavior.
+     *
+     * @return An iterator to the element following the last entity of the
+     * reversed internal packed array.
+     */
+    [[nodiscard]] reverse_iterator rend() const ENTT_NOEXCEPT {
+        return std::make_reverse_iterator(begin());
+    }
+
+    /**
+     * @brief Finds an entity.
+     * @param entt A valid entity identifier.
+     * @return An iterator to the given entity if it's found, past the end
+     * iterator otherwise.
+     */
+    [[nodiscard]] iterator find(const entity_type entt) const ENTT_NOEXCEPT {
+        return contains(entt) ? --(end() - index(entt)) : end();
+    }
+
+    /**
+     * @brief Checks if a sparse set contains an entity.
+     * @param entt A valid entity identifier.
+     * @return True if the sparse set contains the entity, false otherwise.
+     */
+    [[nodiscard]] bool contains(const entity_type entt) const ENTT_NOEXCEPT {
+        ENTT_ASSERT(entt != tombstone && entt != null, "Invalid entity");
+        const auto curr = page(entt);
+        // testing versions permits to avoid accessing the packed array
+        return (curr < bucket && sparse[curr] && sparse[curr][offset(entt)] != null);
+    }
+
+    /**
+     * @brief Returns the position of an entity in a sparse set.
+     *
+     * @warning
+     * Attempting to get the position of an entity that doesn't belong to the
+     * sparse set results in undefined behavior.
+     *
+     * @param entt A valid entity identifier.
+     * @return The position of the entity in the sparse set.
+     */
+    [[nodiscard]] size_type index(const entity_type entt) const ENTT_NOEXCEPT {
+        ENTT_ASSERT(contains(entt), "Set does not contain entity");
+        return static_cast<size_type>(traits_type::to_entity(sparse[page(entt)][offset(entt)]));
+    }
+
+    /**
+     * @brief Returns the entity at specified location, with bounds checking.
+     * @param pos The position for which to return the entity.
+     * @return The entity at specified location if any, a null entity otherwise.
+     */
+    [[nodiscard]] entity_type at(const size_type pos) const ENTT_NOEXCEPT {
+        return pos < count ? packed[pos] : null;
+    }
+
+    /**
+     * @brief Returns the entity at specified location, without bounds checking.
+     * @param pos The position for which to return the entity.
+     * @return The entity at specified location.
+     */
+    [[nodiscard]] entity_type operator[](const size_type pos) const ENTT_NOEXCEPT {
+        ENTT_ASSERT(pos < count, "Position is out of bounds");
+        return packed[pos];
+    }
+
+    /**
+     * @brief Appends an entity to a sparse set.
+     *
+     * @warning
+     * Attempting to assign an entity that already belongs to the sparse set
+     * results in undefined behavior.
+     *
+     * @param entt A valid entity identifier.
+     * @return The slot used for insertion.
+     */
+    size_type emplace_back(const entity_type entt) {
+        ENTT_ASSERT(!contains(entt), "Set already contains entity");
+
+        if(count == reserved) {
+            const size_type sz = static_cast<size_type>(reserved * growth_factor);
+            resize_packed(sz + !(sz > reserved));
+        }
+
+        assure_page(page(entt))[offset(entt)] = traits_type::construct(static_cast<typename traits_type::entity_type>(count));
+        packed[count] = entt;
+        return count++;
+    }
+
+    /**
+     * @brief Assigns an entity to a sparse set.
+     *
+     * @warning
+     * Attempting to assign an entity that already belongs to the sparse set
+     * results in undefined behavior.
+     *
+     * @param entt A valid entity identifier.
+     * @return The slot used for insertion.
+     */
+    size_type emplace(const entity_type entt) {
+        if(free_list == null) {
+            return emplace_back(entt);
+        } else {
+            ENTT_ASSERT(!contains(entt), "Set already contains entity");
+            const auto pos = static_cast<size_type>(traits_type::to_entity(free_list));
+            assure_page(page(entt))[offset(entt)] = traits_type::construct(static_cast<typename traits_type::entity_type>(pos));
+            free_list = std::exchange(packed[pos], entt);
+            return pos;
+        }
+    }
+
+    /**
+     * @brief Assigns one or more entities to a sparse set.
+     *
+     * @warning
+     * Attempting to assign an entity that already belongs to the sparse set
+     * results in undefined behavior.
+     *
+     * @tparam It Type of input iterator.
+     * @param first An iterator to the first element of the range of entities.
+     * @param last An iterator past the last element of the range of entities.
+     */
+    template<typename It>
+    void insert(It first, It last) {
+        reserve(count + std::distance(first, last));
+
+        for(; first != last; ++first) {
+            const auto entt = *first;
+            ENTT_ASSERT(!contains(entt), "Set already contains entity");
+            assure_page(page(entt))[offset(entt)] = traits_type::construct(static_cast<typename traits_type::entity_type>(count));
+            packed[count++] = entt;
+        }
+    }
+
+    /**
+     * @brief Erases an entity from a sparse set.
+     *
+     * @warning
+     * Attempting to erase an entity that doesn't belong to the sparse set
+     * results in undefined behavior.
+     *
+     * @param entt A valid entity identifier.
+     * @param ud Optional user data that are forwarded as-is to derived classes.
+     */
+    void erase(const entity_type entt, void *ud = nullptr) {
+        ENTT_ASSERT(contains(entt), "Set does not contain entity");
+        (mode == deletion_policy::in_place) ? in_place_pop(entt, ud) : swap_and_pop(entt, ud);
+    }
+
+    /**
+     * @brief Erases entities from a set.
+     *
+     * @sa erase
+     *
+     * @tparam It Type of input iterator.
+     * @param first An iterator to the first element of the range of entities.
+     * @param last An iterator past the last element of the range of entities.
+     * @param ud Optional user data that are forwarded as-is to derived classes.
+     */
+    template<typename It>
+    void erase(It first, It last, void *ud = nullptr) {
+        for(; first != last; ++first) {
+            erase(*first, ud);
+        }
+    }
+
+    /**
+     * @brief Removes an entity from a sparse set if it exists.
+     * @param entt A valid entity identifier.
+     * @param ud Optional user data that are forwarded as-is to derived classes.
+     * @return True if the entity is actually removed, false otherwise.
+     */
+    bool remove(const entity_type entt, void *ud = nullptr) {
+        return contains(entt) && (erase(entt, ud), true);
+    }
+
+    /**
+     * @brief Removes entities from a sparse set if they exist.
+     * @tparam It Type of input iterator.
+     * @param first An iterator to the first element of the range of entities.
+     * @param last An iterator past the last element of the range of entities.
+     * @param ud Optional user data that are forwarded as-is to derived classes.
+     * @return The number of entities actually removed.
+     */
+    template<typename It>
+    size_type remove(It first, It last, void *ud = nullptr) {
+        size_type found{};
+
+        for(; first != last; ++first) {
+            found += remove(*first, ud);
+        }
+
+        return found;
+    }
+
+    /*! @brief Removes all tombstones from the packed array of a sparse set. */
+    void compact() {
+        size_type next = count;
+        for(; next && packed[next - 1u] == tombstone; --next);
+
+        for(auto *it = &free_list; *it != null && next; it = std::addressof(packed[traits_type::to_entity(*it)])) {
+            if(const size_type pos = traits_type::to_entity(*it); pos < next) {
+                --next;
+                move_and_pop(next, pos);
+                std::swap(packed[next], packed[pos]);
+                sparse[page(packed[pos])][offset(packed[pos])] = traits_type::construct(static_cast<const typename traits_type::entity_type>(pos));
+                *it = traits_type::construct(static_cast<typename traits_type::entity_type>(next));
+                for(; next && packed[next - 1u] == tombstone; --next);
+            }
+        }
+
+        free_list = tombstone;
+        count = next;
+    }
+
+    /**
+     * @copybrief swap_at
+     *
+     * For what it's worth, this function affects both the internal sparse array
+     * and the internal packed array. Users should not care of that anyway.
+     *
+     * @warning
+     * Attempting to swap entities that don't belong to the sparse set results
+     * in undefined behavior.
+     *
+     * @param lhs A valid entity identifier.
+     * @param rhs A valid entity identifier.
+     */
+    void swap(const entity_type lhs, const entity_type rhs) {
+        ENTT_ASSERT(contains(lhs), "Set does not contain entity");
+        ENTT_ASSERT(contains(rhs), "Set does not contain entity");
+
+        auto &entt = sparse[page(lhs)][offset(lhs)];
+        auto &other = sparse[page(rhs)][offset(rhs)];
+
+        const auto from = static_cast<size_type>(traits_type::to_entity(entt));
+        const auto to = static_cast<size_type>(traits_type::to_entity(other));
+
+        // basic no-leak guarantee (with invalid state) if swapping throws
+        swap_at(from, to);
+        std::swap(entt, other);
+        std::swap(packed[from], packed[to]);
+    }
+
+    /**
+     * @brief Sort the first count elements according to the given comparison
+     * function.
+     *
+     * The comparison function object must return `true` if the first element
+     * is _less_ than the second one, `false` otherwise. The signature of the
+     * comparison function should be equivalent to the following:
+     *
+     * @code{.cpp}
+     * bool(const Entity, const Entity);
+     * @endcode
+     *
+     * Moreover, the comparison function object shall induce a
+     * _strict weak ordering_ on the values.
+     *
+     * The sort function object must offer a member function template
+     * `operator()` that accepts three arguments:
+     *
+     * * An iterator to the first element of the range to sort.
+     * * An iterator past the last element of the range to sort.
+     * * A comparison function to use to compare the elements.
+     *
+     * @tparam Compare Type of comparison function object.
+     * @tparam Sort Type of sort function object.
+     * @tparam Args Types of arguments to forward to the sort function object.
+     * @param length Number of elements to sort.
+     * @param compare A valid comparison function object.
+     * @param algo A valid sort function object.
+     * @param args Arguments to forward to the sort function object, if any.
+     */
+    template<typename Compare, typename Sort = std_sort, typename... Args>
+    void sort_n(const size_type length, Compare compare, Sort algo = Sort{}, Args &&... args) {
+        // basic no-leak guarantee (with invalid state) if sorting throws
+        ENTT_ASSERT(!(length > count), "Length exceeds the number of elements");
+        compact();
+
+        algo(std::make_reverse_iterator(packed + length), std::make_reverse_iterator(packed), std::move(compare), std::forward<Args>(args)...);
+
+        for(size_type pos{}; pos < length; ++pos) {
+            auto curr = pos;
+            auto next = index(packed[curr]);
+
+            while(curr != next) {
+                const auto idx = index(packed[next]);
+                const auto entt = packed[curr];
+
+                swap_at(next, idx);
+                sparse[page(entt)][offset(entt)] = traits_type::construct(static_cast<typename traits_type::entity_type>(curr));
+                curr = std::exchange(next, idx);
+            }
+        }
+    }
+
+    /**
+     * @brief Sort all elements according to the given comparison function.
+     *
+     * @sa sort_n
+     *
+     * @tparam Compare Type of comparison function object.
+     * @tparam Sort Type of sort function object.
+     * @tparam Args Types of arguments to forward to the sort function object.
+     * @param compare A valid comparison function object.
+     * @param algo A valid sort function object.
+     * @param args Arguments to forward to the sort function object, if any.
+     */
+    template<typename Compare, typename Sort = std_sort, typename... Args>
+    void sort(Compare compare, Sort algo = Sort{}, Args &&... args) {
+        sort_n(count, std::move(compare), std::move(algo), std::forward<Args>(args)...);
+    }
+
+    /**
+     * @brief Sort entities according to their order in another sparse set.
+     *
+     * Entities that are part of both the sparse sets are ordered internally
+     * according to the order they have in `other`. All the other entities goes
+     * to the end of the list and there are no guarantees on their order.<br/>
+     * In other terms, this function can be used to impose the same order on two
+     * sets by using one of them as a master and the other one as a slave.
+     *
+     * Iterating the sparse set with a couple of iterators returns elements in
+     * the expected order after a call to `respect`. See `begin` and `end` for
+     * more details.
+     *
+     * @param other The sparse sets that imposes the order of the entities.
+     */
+    void respect(const basic_sparse_set &other) {
+        compact();
+
+        const auto to = other.end();
+        auto from = other.begin();
+
+        for(size_type pos = count - 1; pos && from != to; ++from) {
+            if(contains(*from)) {
+                if(*from != packed[pos]) {
+                    // basic no-leak guarantee (with invalid state) if swapping throws
+                    swap(packed[pos], *from);
+                }
+
+                --pos;
+            }
+        }
+    }
+
+    /**
+     * @brief Clears a sparse set.
+     * @param ud Optional user data that are forwarded as-is to derived classes.
+     */
+    void clear(void *ud = nullptr) {
+        for(auto &&entity: *this) {
+            if(entity != tombstone) {
+                in_place_pop(entity, ud);
+            }
+        }
+
+        free_list = tombstone;
+        count = 0u;
+    }
+
+private:
+    typename alloc_traits::allocator_type allocator;
+    typename bucket_alloc_traits::allocator_type bucket_allocator;
+    bucket_alloc_pointer sparse;
+    alloc_pointer packed;
+    std::size_t bucket;
+    std::size_t count;
+    std::size_t reserved;
+    entity_type free_list;
+    deletion_policy mode;
+};
+
+
+}
+
+
+#endif
+
+// #include "entity/storage.hpp"
+#ifndef ENTT_ENTITY_STORAGE_HPP
+#define ENTT_ENTITY_STORAGE_HPP
+
+
+#include <cstddef>
+#include <iterator>
+#include <memory>
+#include <tuple>
+#include <type_traits>
+#include <utility>
+// #include "../config/config.h"
+
+// #include "../core/algorithm.hpp"
+
+// #include "../core/fwd.hpp"
+
+// #include "../core/type_traits.hpp"
+
+// #include "../signal/sigh.hpp"
+
+// #include "component.hpp"
+
+// #include "entity.hpp"
+
+// #include "fwd.hpp"
+
+// #include "sparse_set.hpp"
+
 
 
 namespace entt {
@@ -4811,144 +26019,332 @@
  * to the entities.
  *
  * @note
- * Entities and objects have the same order. It's guaranteed both in case of raw
- * access (either to entities or objects) and when using random or input access
- * iterators.
+ * Entities and objects have the same order.
  *
  * @note
- * Internal data structures arrange elements to maximize performance. Because of
- * that, there are no guarantees that elements have the expected order when
- * iterate directly the internal packed array (see `raw` and `size` member
- * functions for that). Use `begin` and `end` instead.
+ * Internal data structures arrange elements to maximize performance. There are
+ * no guarantees that objects are returned in the insertion order when iterate
+ * a storage. Do not make assumption on the order in any case.
  *
  * @warning
- * Empty types aren't explicitly instantiated. Temporary objects are returned in
- * place of the instances of the components and raw access isn't available for
- * them.
+ * Empty types aren't explicitly instantiated. Therefore, many of the functions
+ * normally available for non-empty types will not be available for empty ones.
  *
  * @sa sparse_set<Entity>
  *
  * @tparam Entity A valid entity type (see entt_traits for more details).
  * @tparam Type Type of objects assigned to the entities.
- */
-template<typename Entity, typename Type, typename = std::void_t<>>
-class basic_storage: public sparse_set<Entity> {
-    using underlying_type = sparse_set<Entity>;
-    using traits_type = entt_traits<std::underlying_type_t<Entity>>;
-
-    template<bool Const>
-    class iterator {
-        friend class basic_storage<Entity, Type>;
-
-        using instance_type = std::conditional_t<Const, const std::vector<Type>, std::vector<Type>>;
-        using index_type = typename traits_type::difference_type;
-
-        iterator(instance_type *ref, const index_type idx) ENTT_NOEXCEPT
-            : instances{ref}, index{idx}
-        {}
-
-    public:
-        using difference_type = index_type;
-        using value_type = Type;
-        using pointer = std::conditional_t<Const, const value_type *, value_type *>;
-        using reference = std::conditional_t<Const, const value_type &, value_type &>;
+ * @tparam Allocator Type of allocator used to manage memory and elements.
+ */
+template<typename Entity, typename Type, typename Allocator, typename = void>
+class basic_storage_impl: public basic_sparse_set<Entity, typename std::allocator_traits<Allocator>::template rebind_alloc<Entity>> {
+    static constexpr auto packed_page = ENTT_PACKED_PAGE;
+
+    using comp_traits = component_traits<Type>;
+
+    using underlying_type = basic_sparse_set<Entity, typename std::allocator_traits<Allocator>::template rebind_alloc<Entity>>;
+    using difference_type = typename entt_traits<Entity>::difference_type;
+
+    using alloc_traits = typename std::allocator_traits<Allocator>::template rebind_traits<Type>;
+    using alloc_pointer = typename alloc_traits::pointer;
+    using alloc_const_pointer = typename alloc_traits::const_pointer;
+
+    using bucket_alloc_traits = typename std::allocator_traits<Allocator>::template rebind_traits<alloc_pointer>;
+    using bucket_alloc_pointer = typename bucket_alloc_traits::pointer;
+
+    using bucket_alloc_const_type = typename std::allocator_traits<Allocator>::template rebind_alloc<alloc_const_pointer>;
+    using bucket_alloc_const_pointer = typename std::allocator_traits<bucket_alloc_const_type>::const_pointer;
+
+    static_assert(alloc_traits::propagate_on_container_move_assignment::value);
+    static_assert(bucket_alloc_traits::propagate_on_container_move_assignment::value);
+
+    template<typename Value>
+    struct storage_iterator final {
+        using difference_type = typename basic_storage_impl::difference_type;
+        using value_type = Value;
+        using pointer = value_type *;
+        using reference = value_type &;
         using iterator_category = std::random_access_iterator_tag;
 
-        iterator() ENTT_NOEXCEPT = default;
-
-        iterator & operator++() ENTT_NOEXCEPT {
+        storage_iterator() ENTT_NOEXCEPT = default;
+
+        storage_iterator(bucket_alloc_pointer const *ref, const typename basic_storage_impl::difference_type idx) ENTT_NOEXCEPT
+            : packed{ref},
+              index{idx}
+        {}
+
+        storage_iterator & operator++() ENTT_NOEXCEPT {
             return --index, *this;
         }
 
-        iterator operator++(int) ENTT_NOEXCEPT {
-            iterator orig = *this;
+        storage_iterator operator++(int) ENTT_NOEXCEPT {
+            storage_iterator orig = *this;
             return ++(*this), orig;
         }
 
-        iterator & operator--() ENTT_NOEXCEPT {
+        storage_iterator & operator--() ENTT_NOEXCEPT {
             return ++index, *this;
         }
 
-        iterator operator--(int) ENTT_NOEXCEPT {
-            iterator orig = *this;
-            return --(*this), orig;
-        }
-
-        iterator & operator+=(const difference_type value) ENTT_NOEXCEPT {
+        storage_iterator operator--(int) ENTT_NOEXCEPT {
+            storage_iterator orig = *this;
+            return operator--(), orig;
+        }
+
+        storage_iterator & operator+=(const difference_type value) ENTT_NOEXCEPT {
             index -= value;
             return *this;
         }
 
-        iterator operator+(const difference_type value) const ENTT_NOEXCEPT {
-            return iterator{instances, index-value};
-        }
-
-        iterator & operator-=(const difference_type value) ENTT_NOEXCEPT {
+        storage_iterator operator+(const difference_type value) const ENTT_NOEXCEPT {
+            storage_iterator copy = *this;
+            return (copy += value);
+        }
+
+        storage_iterator & operator-=(const difference_type value) ENTT_NOEXCEPT {
             return (*this += -value);
         }
 
-        iterator operator-(const difference_type value) const ENTT_NOEXCEPT {
+        storage_iterator operator-(const difference_type value) const ENTT_NOEXCEPT {
             return (*this + -value);
         }
 
-        difference_type operator-(const iterator &other) const ENTT_NOEXCEPT {
+        difference_type operator-(const storage_iterator &other) const ENTT_NOEXCEPT {
             return other.index - index;
         }
 
-        reference operator[](const difference_type value) const ENTT_NOEXCEPT {
+        [[nodiscard]] reference operator[](const difference_type value) const ENTT_NOEXCEPT {
             const auto pos = size_type(index-value-1);
-            return (*instances)[pos];
-        }
-
-        bool operator==(const iterator &other) const ENTT_NOEXCEPT {
+            return (*packed)[page(pos)][offset(pos)];
+        }
+
+        [[nodiscard]] bool operator==(const storage_iterator &other) const ENTT_NOEXCEPT {
             return other.index == index;
         }
 
-        bool operator!=(const iterator &other) const ENTT_NOEXCEPT {
+        [[nodiscard]] bool operator!=(const storage_iterator &other) const ENTT_NOEXCEPT {
             return !(*this == other);
         }
 
-        bool operator<(const iterator &other) const ENTT_NOEXCEPT {
+        [[nodiscard]] bool operator<(const storage_iterator &other) const ENTT_NOEXCEPT {
             return index > other.index;
         }
 
-        bool operator>(const iterator &other) const ENTT_NOEXCEPT {
+        [[nodiscard]] bool operator>(const storage_iterator &other) const ENTT_NOEXCEPT {
             return index < other.index;
         }
 
-        bool operator<=(const iterator &other) const ENTT_NOEXCEPT {
+        [[nodiscard]] bool operator<=(const storage_iterator &other) const ENTT_NOEXCEPT {
             return !(*this > other);
         }
 
-        bool operator>=(const iterator &other) const ENTT_NOEXCEPT {
+        [[nodiscard]] bool operator>=(const storage_iterator &other) const ENTT_NOEXCEPT {
             return !(*this < other);
         }
 
-        pointer operator->() const ENTT_NOEXCEPT {
-            const auto pos = size_type(index-1);
-            return &(*instances)[pos];
-        }
-
-        reference operator*() const ENTT_NOEXCEPT {
+        [[nodiscard]] pointer operator->() const ENTT_NOEXCEPT {
+            const auto pos = size_type(index-1u);
+            return std::addressof((*packed)[page(pos)][offset(pos)]);
+        }
+
+        [[nodiscard]] reference operator*() const ENTT_NOEXCEPT {
             return *operator->();
         }
 
     private:
-        instance_type *instances;
-        index_type index;
-    };
-
-public:
-    /*! @brief Type of the objects associated with the entities. */
-    using object_type = Type;
+        bucket_alloc_pointer const *packed;
+        difference_type index;
+    };
+
+    [[nodiscard]] static auto page(const std::size_t pos) ENTT_NOEXCEPT {
+        return pos / packed_page;
+    }
+
+    [[nodiscard]] static auto offset(const std::size_t pos) ENTT_NOEXCEPT {
+        return pos & (packed_page - 1);
+    }
+
+    void release_memory() {
+        if(packed) {
+            // no-throw stable erase iteration
+            underlying_type::clear();
+
+            for(size_type pos{}; pos < bucket; ++pos) {
+                alloc_traits::deallocate(allocator, packed[pos], packed_page);
+                bucket_alloc_traits::destroy(bucket_allocator, std::addressof(packed[pos]));
+            }
+
+            bucket_alloc_traits::deallocate(bucket_allocator, packed, bucket);
+        }
+    }
+
+    void assure_at_least(const std::size_t last) {
+        if(const auto idx = page(last - 1u); !(idx < bucket)) {
+            const size_type sz = idx + 1u;
+            const auto mem = bucket_alloc_traits::allocate(bucket_allocator, sz);
+            std::uninitialized_copy(packed, packed + bucket, mem);
+            size_type pos{};
+
+            ENTT_TRY {
+                for(pos = bucket; pos < sz; ++pos) {
+                    auto pg = alloc_traits::allocate(allocator, packed_page);
+                    bucket_alloc_traits::construct(bucket_allocator, std::addressof(mem[pos]), pg);
+                }
+            } ENTT_CATCH {
+                for(auto next = bucket; next < pos; ++next) {
+                    alloc_traits::deallocate(allocator, mem[next], packed_page);
+                }
+
+                std::destroy(mem, mem + pos);
+                bucket_alloc_traits::deallocate(bucket_allocator, mem, sz);
+                ENTT_THROW;
+            }
+
+            std::destroy(packed, packed + bucket);
+            bucket_alloc_traits::deallocate(bucket_allocator, packed, bucket);
+
+            packed = mem;
+            bucket = sz;
+        }
+    }
+
+    void release_unused_pages() {
+        if(const auto length = underlying_type::size() / packed_page; length < bucket) {
+            const auto mem = bucket_alloc_traits::allocate(bucket_allocator, length);
+            std::uninitialized_copy(packed, packed + length, mem);
+
+            for(auto pos = length; pos < bucket; ++pos) {
+                alloc_traits::deallocate(allocator, packed[pos], packed_page);
+                bucket_alloc_traits::destroy(bucket_allocator, std::addressof(packed[pos]));
+            }
+
+            bucket_alloc_traits::deallocate(bucket_allocator, packed, bucket);
+
+            packed = mem;
+            bucket = length;
+        }
+    }
+
+    template<typename... Args>
+    auto & push_at(const std::size_t pos, Args &&... args) {
+        ENTT_ASSERT(pos < (bucket * packed_page), "Out of bounds index");
+        auto *instance = std::addressof(packed[page(pos)][offset(pos)]);
+
+        if constexpr(std::is_aggregate_v<value_type>) {
+            alloc_traits::construct(allocator, instance, Type{std::forward<Args>(args)...});
+        } else {
+            alloc_traits::construct(allocator, instance, std::forward<Args>(args)...);
+        }
+
+        return *instance;
+    }
+
+    void pop_at(const std::size_t pos) {
+        alloc_traits::destroy(allocator, std::addressof(packed[page(pos)][offset(pos)]));
+    }
+
+protected:
+    /*! @copydoc basic_sparse_set::swap_at */
+    void swap_at(const std::size_t lhs, const std::size_t rhs) final {
+        std::swap(packed[page(lhs)][offset(lhs)], packed[page(rhs)][offset(rhs)]);
+    }
+
+    /*! @copydoc basic_sparse_set::move_and_pop */
+    void move_and_pop(const std::size_t from, const std::size_t to) final {
+        push_at(to, std::move(packed[page(from)][offset(from)]));
+        pop_at(from);
+    }
+
+    /*! @copydoc basic_sparse_set::swap_and_pop */
+    void swap_and_pop(const Entity entt, void *ud) override {
+        const auto pos = underlying_type::index(entt);
+        const auto last = underlying_type::size() - 1u;
+        auto &&elem = packed[page(pos)][offset(pos)];
+
+        // support for nosy destructors
+        [[maybe_unused]] auto unused = std::move(elem);
+        elem = std::move(packed[page(last)][offset(last)]);
+        pop_at(last);
+
+        underlying_type::swap_and_pop(entt, ud);
+    }
+
+    /*! @copydoc basic_sparse_set::in_place_pop */
+    void in_place_pop(const Entity entt, void *ud) override {
+        const auto pos = underlying_type::index(entt);
+        underlying_type::in_place_pop(entt, ud);
+        // support for nosy destructors
+        pop_at(pos);
+    }
+
+public:
+    /*! @brief Allocator type. */
+    using allocator_type = typename alloc_traits::allocator_type;
+    /*! @brief Type of the objects assigned to entities. */
+    using value_type = Type;
     /*! @brief Underlying entity identifier. */
-    using entity_type = typename underlying_type::entity_type;
+    using entity_type = Entity;
     /*! @brief Unsigned integer type. */
-    using size_type = typename underlying_type::size_type;
+    using size_type = std::size_t;
+    /*! @brief Pointer type to contained elements. */
+    using pointer = bucket_alloc_pointer;
+    /*! @brief Constant pointer type to contained elements. */
+    using const_pointer = bucket_alloc_const_pointer;
     /*! @brief Random access iterator type. */
-    using iterator_type = iterator<false>;
+    using iterator = storage_iterator<value_type>;
     /*! @brief Constant random access iterator type. */
-    using const_iterator_type = iterator<true>;
+    using const_iterator = storage_iterator<const value_type>;
+    /*! @brief Reverse iterator type. */
+    using reverse_iterator = std::reverse_iterator<iterator>;
+    /*! @brief Constant reverse iterator type. */
+    using const_reverse_iterator = std::reverse_iterator<const_iterator>;
+
+    /**
+     * @brief Default constructor.
+     * @param alloc Allocator to use (possibly default-constructed).
+     */
+    explicit basic_storage_impl(const allocator_type &alloc = {})
+        : underlying_type{deletion_policy{comp_traits::in_place_delete::value}, alloc},
+          allocator{alloc},
+          bucket_allocator{alloc},
+          packed{bucket_alloc_traits::allocate(bucket_allocator, 0u)},
+          bucket{}
+    {}
+
+    /**
+     * @brief Move constructor.
+     * @param other The instance to move from.
+     */
+    basic_storage_impl(basic_storage_impl &&other) ENTT_NOEXCEPT
+        : underlying_type{std::move(other)},
+          allocator{std::move(other.allocator)},
+          bucket_allocator{std::move(other.bucket_allocator)},
+          packed{std::exchange(other.packed, bucket_alloc_pointer{})},
+          bucket{std::exchange(other.bucket, 0u)}
+    {}
+
+    /*! @brief Default destructor. */
+    ~basic_storage_impl() override {
+        release_memory();
+    }
+
+    /**
+     * @brief Move assignment operator.
+     * @param other The instance to move from.
+     * @return This sparse set.
+     */
+    basic_storage_impl & operator=(basic_storage_impl &&other) ENTT_NOEXCEPT {
+        release_memory();
+
+        underlying_type::operator=(std::move(other));
+
+        allocator = std::move(other.allocator);
+        bucket_allocator = std::move(other.bucket_allocator);
+        packed = std::exchange(other.packed, bucket_alloc_pointer{});
+        bucket = std::exchange(other.bucket, 0u);
+
+        return *this;
+    }
 
     /**
      * @brief Increases the capacity of a storage.
@@ -4960,128 +26356,153 @@
      */
     void reserve(const size_type cap) {
         underlying_type::reserve(cap);
-        instances.reserve(cap);
+
+        if(cap > underlying_type::size()) {
+            assure_at_least(cap);
+        }
+    }
+
+    /**
+     * @brief Returns the number of elements that a storage has currently
+     * allocated space for.
+     * @return Capacity of the storage.
+     */
+    [[nodiscard]] size_type capacity() const ENTT_NOEXCEPT {
+        return bucket * packed_page;
     }
 
     /*! @brief Requests the removal of unused capacity. */
     void shrink_to_fit() {
         underlying_type::shrink_to_fit();
-        instances.shrink_to_fit();
+        release_unused_pages();
     }
 
     /**
      * @brief Direct access to the array of objects.
-     *
-     * The returned pointer is such that range `[raw(), raw() + size()]` is
-     * always a valid range, even if the container is empty.
-     *
-     * @note
-     * There are no guarantees on the order, even though either `sort` or
-     * `respect` has been previously invoked. Internal data structures arrange
-     * elements to maximize performance. Accessing them directly gives a
-     * performance boost but less guarantees. Use `begin` and `end` if you want
-     * to iterate the storage in the expected order.
-     *
      * @return A pointer to the array of objects.
      */
-    const object_type * raw() const ENTT_NOEXCEPT {
-        return instances.data();
+    [[nodiscard]] const_pointer raw() const ENTT_NOEXCEPT {
+        return packed;
     }
 
     /*! @copydoc raw */
-    object_type * raw() ENTT_NOEXCEPT {
-        return const_cast<object_type *>(std::as_const(*this).raw());
+    [[nodiscard]] pointer raw() ENTT_NOEXCEPT {
+        return packed;
     }
 
     /**
      * @brief Returns an iterator to the beginning.
      *
-     * The returned iterator points to the first instance of the given type. If
-     * the storage is empty, the returned iterator will be equal to `end()`.
-     *
-     * @note
-     * Random access iterators stay true to the order imposed by a call to
-     * either `sort` or `respect`.
-     *
-     * @return An iterator to the first instance of the given type.
-     */
-    const_iterator_type cbegin() const ENTT_NOEXCEPT {
-        const typename traits_type::difference_type pos = underlying_type::size();
-        return const_iterator_type{&instances, pos};
+     * The returned iterator points to the first instance of the internal array.
+     * If the storage is empty, the returned iterator will be equal to `end()`.
+     *
+     * @return An iterator to the first instance of the internal array.
+     */
+    [[nodiscard]] const_iterator cbegin() const ENTT_NOEXCEPT {
+        const difference_type pos = underlying_type::size();
+        return const_iterator{std::addressof(packed), pos};
     }
 
     /*! @copydoc cbegin */
-    const_iterator_type begin() const ENTT_NOEXCEPT {
+    [[nodiscard]] const_iterator begin() const ENTT_NOEXCEPT {
         return cbegin();
     }
 
     /*! @copydoc begin */
-    iterator_type begin() ENTT_NOEXCEPT {
-        const typename traits_type::difference_type pos = underlying_type::size();
-        return iterator_type{&instances, pos};
+    [[nodiscard]] iterator begin() ENTT_NOEXCEPT {
+        const difference_type pos = underlying_type::size();
+        return iterator{std::addressof(packed), pos};
     }
 
     /**
      * @brief Returns an iterator to the end.
      *
      * The returned iterator points to the element following the last instance
-     * of the given type. Attempting to dereference the returned iterator
+     * of the internal array. Attempting to dereference the returned iterator
      * results in undefined behavior.
      *
-     * @note
-     * Random access iterators stay true to the order imposed by a call to
-     * either `sort` or `respect`.
-     *
      * @return An iterator to the element following the last instance of the
-     * given type.
-     */
-    const_iterator_type cend() const ENTT_NOEXCEPT {
-        return const_iterator_type{&instances, {}};
+     * internal array.
+     */
+    [[nodiscard]] const_iterator cend() const ENTT_NOEXCEPT {
+        return const_iterator{std::addressof(packed), {}};
     }
 
     /*! @copydoc cend */
-    const_iterator_type end() const ENTT_NOEXCEPT {
+    [[nodiscard]] const_iterator end() const ENTT_NOEXCEPT {
         return cend();
     }
 
     /*! @copydoc end */
-    iterator_type end() ENTT_NOEXCEPT {
-        return iterator_type{&instances, {}};
-    }
-
-    /**
-     * @brief Returns the object associated with an entity.
+    [[nodiscard]] iterator end() ENTT_NOEXCEPT {
+        return iterator{std::addressof(packed), {}};
+    }
+
+    /**
+     * @brief Returns a reverse iterator to the beginning.
+     *
+     * The returned iterator points to the first instance of the reversed
+     * internal array. If the storage is empty, the returned iterator will be
+     * equal to `rend()`.
+     *
+     * @return An iterator to the first instance of the reversed internal array.
+     */
+    [[nodiscard]] const_reverse_iterator crbegin() const ENTT_NOEXCEPT {
+        return std::make_reverse_iterator(cend());
+    }
+
+    /*! @copydoc crbegin */
+    [[nodiscard]] const_reverse_iterator rbegin() const ENTT_NOEXCEPT {
+        return crbegin();
+    }
+
+    /*! @copydoc rbegin */
+    [[nodiscard]] reverse_iterator rbegin() ENTT_NOEXCEPT {
+        return std::make_reverse_iterator(end());
+    }
+
+    /**
+     * @brief Returns a reverse iterator to the end.
+     *
+     * The returned iterator points to the element following the last instance
+     * of the reversed internal array. Attempting to dereference the returned
+     * iterator results in undefined behavior.
+     *
+     * @return An iterator to the element following the last instance of the
+     * reversed internal array.
+     */
+    [[nodiscard]] const_reverse_iterator crend() const ENTT_NOEXCEPT {
+        return std::make_reverse_iterator(cbegin());
+    }
+
+    /*! @copydoc crend */
+    [[nodiscard]] const_reverse_iterator rend() const ENTT_NOEXCEPT {
+        return crend();
+    }
+
+    /*! @copydoc rend */
+    [[nodiscard]] reverse_iterator rend() ENTT_NOEXCEPT {
+        return std::make_reverse_iterator(begin());
+    }
+
+    /**
+     * @brief Returns the object assigned to an entity.
      *
      * @warning
      * Attempting to use an entity that doesn't belong to the storage results in
-     * undefined behavior.<br/>
-     * An assertion will abort the execution at runtime in debug mode if the
-     * storage doesn't contain the given entity.
-     *
-     * @param entt A valid entity identifier.
-     * @return The object associated with the entity.
-     */
-    const object_type & get(const entity_type entt) const ENTT_NOEXCEPT {
-        return instances[underlying_type::index(entt)];
+     * undefined behavior.
+     *
+     * @param entt A valid entity identifier.
+     * @return The object assigned to the entity.
+     */
+    [[nodiscard]] const value_type & get(const entity_type entt) const ENTT_NOEXCEPT {
+        const auto idx = underlying_type::index(entt);
+        return packed[page(idx)][offset(idx)];
     }
 
     /*! @copydoc get */
-    object_type & get(const entity_type entt) ENTT_NOEXCEPT {
-        return const_cast<object_type &>(std::as_const(*this).get(entt));
-    }
-
-    /**
-     * @brief Returns a pointer to the object associated with an entity, if any.
-     * @param entt A valid entity identifier.
-     * @return The object associated with the entity, if any.
-     */
-    const object_type * try_get(const entity_type entt) const ENTT_NOEXCEPT {
-        return underlying_type::has(entt) ? (instances.data() + underlying_type::index(entt)) : nullptr;
-    }
-
-    /*! @copydoc try_get */
-    object_type * try_get(const entity_type entt) ENTT_NOEXCEPT {
-        return const_cast<object_type *>(std::as_const(*this).try_get(entt));
+    [[nodiscard]] value_type & get(const entity_type entt) ENTT_NOEXCEPT {
+        return const_cast<value_type &>(std::as_const(*this).get(entt));
     }
 
     /**
@@ -5093,121 +26514,110 @@
      *
      * @warning
      * Attempting to use an entity that already belongs to the storage results
-     * in undefined behavior.<br/>
-     * An assertion will abort the execution at runtime in debug mode if the
-     * storage already contains the given entity.
+     * in undefined behavior.
      *
      * @tparam Args Types of arguments to use to construct the object.
      * @param entt A valid entity identifier.
      * @param args Parameters to use to construct an object for the entity.
-     * @return The object associated with the entity.
+     * @return A reference to the newly created object.
      */
     template<typename... Args>
-    object_type & construct(const entity_type entt, Args &&... args) {
-        if constexpr(std::is_aggregate_v<object_type>) {
-            instances.emplace_back(Type{std::forward<Args>(args)...});
-        } else {
-            instances.emplace_back(std::forward<Args>(args)...);
-        }
-
-        // entity goes after component in case constructor throws
-        underlying_type::construct(entt);
-        return instances.back();
-    }
-
-    /**
-     * @brief Assigns one or more entities to a storage and default constructs
-     * their objects.
-     *
-     * The object type must be at least move and default insertable.
+    value_type & emplace(const entity_type entt, Args &&... args) {
+        const auto pos = underlying_type::slot();
+        assure_at_least(pos + 1u);
+
+        auto &value = push_at(pos, std::forward<Args>(args)...);
+
+        ENTT_TRY {
+            [[maybe_unused]] const auto curr = underlying_type::emplace(entt);
+            ENTT_ASSERT(pos == curr, "Misplaced component");
+        } ENTT_CATCH {
+            pop_at(pos);
+            ENTT_THROW;
+        }
+
+        return value;
+    }
+
+    /**
+     * @brief Updates the instance assigned to a given entity in-place.
+     * @tparam Func Types of the function objects to invoke.
+     * @param entt A valid entity identifier.
+     * @param func Valid function objects.
+     * @return A reference to the updated instance.
+     */
+    template<typename... Func>
+    decltype(auto) patch(const entity_type entt, Func &&... func) {
+        const auto idx = underlying_type::index(entt);
+        auto &&elem = packed[page(idx)][offset(idx)];
+        (std::forward<Func>(func)(elem), ...);
+        return elem;
+    }
+
+    /**
+     * @brief Assigns one or more entities to a storage and constructs their
+     * objects from a given instance.
      *
      * @warning
      * Attempting to assign an entity that already belongs to the storage
-     * results in undefined behavior.<br/>
-     * An assertion will abort the execution at runtime in debug mode if the
-     * storage already contains the given entity.
-     *
-     * @tparam It Type of forward iterator.
+     * results in undefined behavior.
+     *
+     * @tparam It Type of input iterator.
      * @param first An iterator to the first element of the range of entities.
      * @param last An iterator past the last element of the range of entities.
-     * @return An iterator to the list of instances just created and sorted the
-     * same of the entities.
-     */
-    template<typename It>
-    iterator_type batch(It first, It last) {
-        instances.resize(instances.size() + std::distance(first, last));
-        // entity goes after component in case constructor throws
-        underlying_type::batch(first, last);
-        return begin();
-    }
-
-    /**
-     * @brief Assigns one or more entities to a storage and copy constructs
-     * their objects.
-     *
-     * The object type must be at least move and copy insertable.
-     *
-     * @sa batch
-     *
-     * @tparam It Type of forward iterator.
-     * @param first An iterator to the first element of the range of entities.
-     * @param last An iterator past the last element of the range of entities.
-     * @param value The value to initialize the new objects with.
-     * @return An iterator to the list of instances just created and sorted the
-     * same of the entities.
+     * @param value An instance of the object to construct.
      */
     template<typename It>
-    iterator_type batch(It first, It last, const object_type &value) {
-        instances.resize(instances.size() + std::distance(first, last), value);
-        // entity goes after component in case constructor throws
-        underlying_type::batch(first, last);
-        return begin();
-    }
-
-    /**
-     * @brief Removes an entity from a storage and destroys its object.
-     *
-     * @warning
-     * Attempting to use an entity that doesn't belong to the storage results in
-     * undefined behavior.<br/>
-     * An assertion will abort the execution at runtime in debug mode if the
-     * storage doesn't contain the given entity.
-     *
-     * @param entt A valid entity identifier.
-     */
-    void destroy(const entity_type entt) {
-        auto other = std::move(instances.back());
-        instances[underlying_type::index(entt)] = std::move(other);
-        instances.pop_back();
-        underlying_type::destroy(entt);
-    }
-
-    /**
-     * @brief Swaps entities and objects in the internal packed arrays.
-     *
-     * @warning
-     * Attempting to swap entities that don't belong to the sparse set results
-     * in undefined behavior.<br/>
-     * An assertion will abort the execution at runtime in debug mode if the
-     * sparse set doesn't contain the given entities.
-     *
-     * @param lhs A valid position within the sparse set.
-     * @param rhs A valid position within the sparse set.
-     */
-    void swap(const size_type lhs, const size_type rhs) ENTT_NOEXCEPT override {
-        ENTT_ASSERT(lhs < instances.size());
-        ENTT_ASSERT(rhs < instances.size());
-        std::swap(instances[lhs], instances[rhs]);
-        underlying_type::swap(lhs, rhs);
+    void insert(It first, It last, const value_type &value = {}) {
+        const auto cap = underlying_type::size() + std::distance(first, last);
+        underlying_type::reserve(cap);
+        assure_at_least(cap);
+
+        for(; first != last; ++first) {
+            push_at(underlying_type::size(), value);
+
+            ENTT_TRY {
+                underlying_type::emplace_back(*first);
+            } ENTT_CATCH {
+                pop_at(underlying_type::size());
+                ENTT_THROW;
+            }
+        }
+    }
+
+    /**
+     * @brief Assigns one or more entities to a storage and constructs their
+     * objects from a given range.
+     *
+     * @sa construct
+     *
+     * @tparam EIt Type of input iterator.
+     * @tparam CIt Type of input iterator.
+     * @param first An iterator to the first element of the range of entities.
+     * @param last An iterator past the last element of the range of entities.
+     * @param from An iterator to the first element of the range of objects.
+     */
+    template<typename EIt, typename CIt, typename = std::enable_if_t<std::is_same_v<std::decay_t<typename std::iterator_traits<CIt>::value_type>, value_type>>>
+    void insert(EIt first, EIt last, CIt from) {
+        const auto cap = underlying_type::size() + std::distance(first, last);
+        underlying_type::reserve(cap);
+        assure_at_least(cap);
+
+        for(; first != last; ++first, ++from) {
+            push_at(underlying_type::size(), *from);
+
+            ENTT_TRY {
+                underlying_type::emplace_back(*first);
+            } ENTT_CATCH {
+                pop_at(underlying_type::size());
+                ENTT_THROW;
+            }
+        }
     }
 
     /**
      * @brief Sort elements according to the given comparison function.
      *
-     * Sort the elements so that iterating the range with a couple of iterators
-     * returns them in the expected order. See `begin` and `end` for more
-     * details.
-     *
      * The comparison function object must return `true` if the first element
      * is _less_ than the second one, `false` otherwise. The signature of the
      * comparison function should be equivalent to one of the following:
@@ -5227,267 +26637,402 @@
      * * An iterator past the last element of the range to sort.
      * * A comparison function to use to compare the elements.
      *
-     * The comparison function object received by the sort function object
-     * hasn't necessarily the type of the one passed along with the other
-     * parameters to this member function.
-     *
-     * @note
-     * Attempting to iterate elements using a raw pointer returned by a call to
-     * either `data` or `raw` gives no guarantees on the order, even though
-     * `sort` has been invoked.
+     * @warning
+     * Empty types are never instantiated. Therefore, only comparison function
+     * objects that require to return entities rather than components are
+     * accepted.
      *
      * @tparam Compare Type of comparison function object.
      * @tparam Sort Type of sort function object.
      * @tparam Args Types of arguments to forward to the sort function object.
-     * @param first An iterator to the first element of the range to sort.
-     * @param last An iterator past the last element of the range to sort.
+     * @param length Number of elements to sort.
+     * @param compare A valid comparison function object.
+     * @param algo A valid sort function object.
+     * @param args Arguments to forward to the sort function object, if any.
+     */
+    template<typename Compare, typename Sort = std_sort, typename... Args>
+    void sort_n(const size_type length, Compare compare, Sort algo = Sort{}, Args &&... args) {
+        if constexpr(std::is_invocable_v<Compare, const value_type &, const value_type &>) {
+            underlying_type::sort_n(length, [this, compare = std::move(compare)](const auto lhs, const auto rhs) {
+                const auto ilhs = underlying_type::index(lhs), irhs = underlying_type::index(rhs);
+                return compare(std::as_const(packed[page(ilhs)][offset(ilhs)]), std::as_const(packed[page(irhs)][offset(irhs)]));
+            }, std::move(algo), std::forward<Args>(args)...);
+        } else {
+            underlying_type::sort_n(length, std::move(compare), std::move(algo), std::forward<Args>(args)...);
+        }
+    }
+
+    /**
+     * @brief Sort all elements according to the given comparison function.
+     *
+     * @sa sort_n
+     *
+     * @tparam Compare Type of comparison function object.
+     * @tparam Sort Type of sort function object.
+     * @tparam Args Types of arguments to forward to the sort function object.
      * @param compare A valid comparison function object.
      * @param algo A valid sort function object.
      * @param args Arguments to forward to the sort function object, if any.
      */
     template<typename Compare, typename Sort = std_sort, typename... Args>
-    void sort(iterator_type first, iterator_type last, Compare compare, Sort algo = Sort{}, Args &&... args) {
-        ENTT_ASSERT(!(first > last));
-
-        const auto from = underlying_type::begin() + std::distance(begin(), first);
-        const auto to = from + std::distance(first, last);
-
-        if constexpr(std::is_invocable_v<Compare, const object_type &, const object_type &>) {
-            static_assert(!std::is_empty_v<object_type>);
-
-            underlying_type::sort(from, to, [this, compare = std::move(compare)](const auto lhs, const auto rhs) {
-                return compare(std::as_const(instances[underlying_type::index(lhs)]), std::as_const(instances[underlying_type::index(rhs)]));
-            }, std::move(algo), std::forward<Args>(args)...);
-        } else {
-            underlying_type::sort(from, to, std::move(compare), std::move(algo), std::forward<Args>(args)...);
-        }
-    }
-
-    /*! @brief Resets a storage. */
-    void reset() {
-        underlying_type::reset();
-        instances.clear();
-    }
-
-private:
-    std::vector<object_type> instances;
-};
-
-
-/*! @copydoc basic_storage */
-template<typename Entity, typename Type>
-class basic_storage<Entity, Type, std::enable_if_t<std::is_empty_v<Type>>>: public sparse_set<Entity> {
-    using traits_type = entt_traits<std::underlying_type_t<Entity>>;
-    using underlying_type = sparse_set<Entity>;
-
-    class iterator {
-        friend class basic_storage<Entity, Type>;
-
-        using index_type = typename traits_type::difference_type;
-
-        iterator(const index_type idx) ENTT_NOEXCEPT
-            : index{idx}
-        {}
-
-    public:
-        using difference_type = index_type;
-        using value_type = Type;
-        using pointer = const value_type *;
-        using reference = value_type;
-        using iterator_category = std::input_iterator_tag;
-
-        iterator() ENTT_NOEXCEPT = default;
-
-        iterator & operator++() ENTT_NOEXCEPT {
-            return --index, *this;
-        }
-
-        iterator operator++(int) ENTT_NOEXCEPT {
-            iterator orig = *this;
-            return ++(*this), orig;
-        }
-
-        iterator & operator--() ENTT_NOEXCEPT {
-            return ++index, *this;
-        }
-
-        iterator operator--(int) ENTT_NOEXCEPT {
-            iterator orig = *this;
-            return --(*this), orig;
-        }
-
-        iterator & operator+=(const difference_type value) ENTT_NOEXCEPT {
-            index -= value;
-            return *this;
-        }
-
-        iterator operator+(const difference_type value) const ENTT_NOEXCEPT {
-            return iterator{index-value};
-        }
-
-        iterator & operator-=(const difference_type value) ENTT_NOEXCEPT {
-            return (*this += -value);
-        }
-
-        iterator operator-(const difference_type value) const ENTT_NOEXCEPT {
-            return (*this + -value);
-        }
-
-        difference_type operator-(const iterator &other) const ENTT_NOEXCEPT {
-            return other.index - index;
-        }
-
-        reference operator[](const difference_type) const ENTT_NOEXCEPT {
-            return {};
-        }
-
-        bool operator==(const iterator &other) const ENTT_NOEXCEPT {
-            return other.index == index;
-        }
-
-        bool operator!=(const iterator &other) const ENTT_NOEXCEPT {
-            return !(*this == other);
-        }
-
-        bool operator<(const iterator &other) const ENTT_NOEXCEPT {
-            return index > other.index;
-        }
-
-        bool operator>(const iterator &other) const ENTT_NOEXCEPT {
-            return index < other.index;
-        }
-
-        bool operator<=(const iterator &other) const ENTT_NOEXCEPT {
-            return !(*this > other);
-        }
-
-        bool operator>=(const iterator &other) const ENTT_NOEXCEPT {
-            return !(*this < other);
-        }
-
-        pointer operator->() const ENTT_NOEXCEPT {
-            return nullptr;
-        }
-
-        reference operator*() const ENTT_NOEXCEPT {
-            return {};
-        }
-
-    private:
-        index_type index;
-    };
-
-public:
-    /*! @brief Type of the objects associated with the entities. */
-    using object_type = Type;
+    void sort(Compare compare, Sort algo = Sort{}, Args &&... args) {
+        sort_n(underlying_type::size(), std::move(compare), std::move(algo), std::forward<Args>(args)...);
+    }
+
+private:
+    typename alloc_traits::allocator_type allocator;
+    typename bucket_alloc_traits::allocator_type bucket_allocator;
+    bucket_alloc_pointer packed;
+    size_type bucket;
+};
+
+
+/*! @copydoc basic_storage_impl */
+template<typename Entity, typename Type, typename Allocator>
+class basic_storage_impl<Entity, Type, Allocator, std::enable_if_t<component_traits<Type>::ignore_if_empty::value && std::is_empty_v<Type>>>
+    : public basic_sparse_set<Entity, typename std::allocator_traits<Allocator>::template rebind_alloc<Entity>>
+{
+    using comp_traits = component_traits<Type>;
+    using underlying_type = basic_sparse_set<Entity, typename std::allocator_traits<Allocator>::template rebind_alloc<Entity>>;
+    using alloc_traits = typename std::allocator_traits<Allocator>::template rebind_traits<Type>;
+
+public:
+    /*! @brief Allocator type. */
+    using allocator_type = typename alloc_traits::allocator_type;
+    /*! @brief Type of the objects assigned to entities. */
+    using value_type = Type;
     /*! @brief Underlying entity identifier. */
-    using entity_type = typename underlying_type::entity_type;
+    using entity_type = Entity;
     /*! @brief Unsigned integer type. */
-    using size_type = typename underlying_type::size_type;
-    /*! @brief Random access iterator type. */
-    using iterator_type = iterator;
-
-    /**
-     * @brief Returns an iterator to the beginning.
-     *
-     * The returned iterator points to the first instance of the given type. If
-     * the storage is empty, the returned iterator will be equal to `end()`.
-     *
-     * @note
-     * Input iterators stay true to the order imposed by a call to either `sort`
-     * or `respect`.
-     *
-     * @return An iterator to the first instance of the given type.
-     */
-    iterator_type cbegin() const ENTT_NOEXCEPT {
-        const typename traits_type::difference_type pos = underlying_type::size();
-        return iterator_type{pos};
-    }
-
-    /*! @copydoc cbegin */
-    iterator_type begin() const ENTT_NOEXCEPT {
-        return cbegin();
-    }
-
-    /**
-     * @brief Returns an iterator to the end.
-     *
-     * The returned iterator points to the element following the last instance
-     * of the given type. Attempting to dereference the returned iterator
-     * results in undefined behavior.
-     *
-     * @note
-     * Input iterators stay true to the order imposed by a call to either `sort`
-     * or `respect`.
-     *
-     * @return An iterator to the element following the last instance of the
-     * given type.
-     */
-    iterator_type cend() const ENTT_NOEXCEPT {
-        return iterator_type{};
-    }
-
-    /*! @copydoc cend */
-    iterator_type end() const ENTT_NOEXCEPT {
-        return cend();
-    }
-
-    /**
-     * @brief Returns the object associated with an entity.
-     *
-     * @note
-     * Empty types aren't explicitly instantiated. Therefore, this function
-     * always returns a temporary object.
+    using size_type = std::size_t;
+
+    /**
+     * @brief Default constructor.
+     * @param alloc Allocator to use (possibly default-constructed).
+     */
+    explicit basic_storage_impl(const allocator_type &alloc = {})
+        : underlying_type{deletion_policy{comp_traits::in_place_delete::value}, alloc}
+    {}
+
+    /**
+     * @brief Fake get function.
      *
      * @warning
      * Attempting to use an entity that doesn't belong to the storage results in
-     * undefined behavior.<br/>
-     * An assertion will abort the execution at runtime in debug mode if the
-     * storage doesn't contain the given entity.
-     *
-     * @param entt A valid entity identifier.
-     * @return The object associated with the entity.
-     */
-    object_type get([[maybe_unused]] const entity_type entt) const ENTT_NOEXCEPT {
-        ENTT_ASSERT(underlying_type::has(entt));
-        return {};
+     * undefined behavior.
+     *
+     * @param entt A valid entity identifier.
+     */
+    void get([[maybe_unused]] const entity_type entt) const ENTT_NOEXCEPT {
+        ENTT_ASSERT(underlying_type::contains(entt), "Storage does not contain entity");
+    }
+
+    /**
+     * @brief Assigns an entity to a storage and constructs its object.
+     *
+     * @warning
+     * Attempting to use an entity that already belongs to the storage results
+     * in undefined behavior.
+     *
+     * @tparam Args Types of arguments to use to construct the object.
+     * @param entt A valid entity identifier.
+     * @param args Parameters to use to construct an object for the entity.
+     */
+    template<typename... Args>
+    void emplace(const entity_type entt, Args &&... args) {
+        [[maybe_unused]] value_type instance{std::forward<Args>(args)...};
+        underlying_type::emplace(entt);
+    }
+
+    /**
+    * @brief Updates the instance assigned to a given entity in-place.
+    * @tparam Func Types of the function objects to invoke.
+    * @param entt A valid entity identifier.
+    * @param func Valid function objects.
+    */
+    template<typename... Func>
+    void patch([[maybe_unused]] const entity_type entt, Func &&... func) {
+        ENTT_ASSERT(underlying_type::contains(entt), "Storage does not contain entity");
+        (std::forward<Func>(func)(), ...);
     }
 
     /**
      * @brief Assigns one or more entities to a storage.
      *
-     * The object type must be at least default constructible.
-     *
      * @warning
      * Attempting to assign an entity that already belongs to the storage
-     * results in undefined behavior.<br/>
-     * An assertion will abort the execution at runtime in debug mode if the
-     * storage already contains the given entity.
-     *
-     * @tparam It Type of forward iterator.
+     * results in undefined behavior.
+     *
+     * @tparam It Type of input iterator.
      * @param first An iterator to the first element of the range of entities.
      * @param last An iterator past the last element of the range of entities.
-     * @return An iterator to the list of instances just created and sorted the
-     * same of the entities.
      */
     template<typename It>
-    iterator_type batch(It first, It last, const object_type & = {}) {
-        underlying_type::batch(first, last);
-        return begin();
-    }
-};
-
-/*! @copydoc basic_storage */
-template<typename Entity, typename Type>
-struct storage: basic_storage<Entity, Type> {};
-
-
-}
-
-
-#endif // ENTT_ENTITY_STORAGE_HPP
-
-// #include "utility.hpp"
+    void insert(It first, It last, const value_type & = {}) {
+        underlying_type::insert(first, last);
+    }
+};
+
+
+/**
+ * @brief Mixin type to use to wrap basic storage classes.
+ * @tparam Type The type of the underlying storage.
+ */
+template<typename Type>
+struct storage_adapter_mixin: Type {
+    static_assert(std::is_same_v<typename Type::value_type, std::decay_t<typename Type::value_type>>, "Invalid object type");
+
+    /*! @brief Type of the objects assigned to entities. */
+    using value_type = typename Type::value_type;
+    /*! @brief Underlying entity identifier. */
+    using entity_type = typename Type::entity_type;
+
+    /*! @brief Inherited constructors. */
+    using Type::Type;
+
+    /**
+     * @brief Assigns entities to a storage.
+     * @tparam Args Types of arguments to use to construct the object.
+     * @param entt A valid entity identifier.
+     * @param args Parameters to use to initialize the object.
+     * @return A reference to the newly created object.
+     */
+    template<typename... Args>
+    decltype(auto) emplace(basic_registry<entity_type> &, const entity_type entt, Args &&... args) {
+        return Type::emplace(entt, std::forward<Args>(args)...);
+    }
+
+    /**
+     * @brief Assigns entities to a storage.
+     * @tparam It Type of input iterator.
+     * @tparam Args Types of arguments to use to construct the objects assigned
+     * to the entities.
+     * @param first An iterator to the first element of the range of entities.
+     * @param last An iterator past the last element of the range of entities.
+     * @param args Parameters to use to initialize the objects assigned to the
+     * entities.
+     */
+    template<typename It, typename... Args>
+    void insert(basic_registry<entity_type> &, It first, It last, Args &&... args) {
+        Type::insert(first, last, std::forward<Args>(args)...);
+    }
+
+    /**
+     * @brief Patches the given instance for an entity.
+     * @tparam Func Types of the function objects to invoke.
+     * @param entt A valid entity identifier.
+     * @param func Valid function objects.
+     * @return A reference to the patched instance.
+     */
+    template<typename... Func>
+    decltype(auto) patch(basic_registry<entity_type> &, const entity_type entt, Func &&... func) {
+        return Type::patch(entt, std::forward<Func>(func)...);
+    }
+};
+
+
+/**
+ * @brief Mixin type to use to add signal support to storage types.
+ * @tparam Type The type of the underlying storage.
+ */
+template<typename Type>
+class sigh_storage_mixin final: public Type {
+    /*! @copydoc basic_sparse_set::swap_and_pop */
+    void swap_and_pop(const typename Type::entity_type entt, void *ud) final {
+        ENTT_ASSERT(ud != nullptr, "Invalid pointer to registry");
+        destruction.publish(*static_cast<basic_registry<typename Type::entity_type> *>(ud), entt);
+        Type::swap_and_pop(entt, ud);
+    }
+
+    /*! @copydoc basic_sparse_set::in_place_pop */
+    void in_place_pop(const typename Type::entity_type entt, void *ud) final {
+        ENTT_ASSERT(ud != nullptr, "Invalid pointer to registry");
+        destruction.publish(*static_cast<basic_registry<typename Type::entity_type> *>(ud), entt);
+        Type::in_place_pop(entt, ud);
+    }
+
+public:
+    /*! @brief Underlying value type. */
+    using value_type = typename Type::value_type;
+    /*! @brief Underlying entity identifier. */
+    using entity_type = typename Type::entity_type;
+
+    /*! @brief Inherited constructors. */
+    using Type::Type;
+
+    /**
+     * @brief Returns a sink object.
+     *
+     * The sink returned by this function can be used to receive notifications
+     * whenever a new instance is created and assigned to an entity.<br/>
+     * The function type for a listener is equivalent to:
+     *
+     * @code{.cpp}
+     * void(basic_registry<entity_type> &, entity_type);
+     * @endcode
+     *
+     * Listeners are invoked **after** the object has been assigned to the
+     * entity.
+     *
+     * @sa sink
+     *
+     * @return A temporary sink object.
+     */
+    [[nodiscard]] auto on_construct() ENTT_NOEXCEPT {
+        return sink{construction};
+    }
+
+    /**
+     * @brief Returns a sink object.
+     *
+     * The sink returned by this function can be used to receive notifications
+     * whenever an instance is explicitly updated.<br/>
+     * The function type for a listener is equivalent to:
+     *
+     * @code{.cpp}
+     * void(basic_registry<entity_type> &, entity_type);
+     * @endcode
+     *
+     * Listeners are invoked **after** the object has been updated.
+     *
+     * @sa sink
+     *
+     * @return A temporary sink object.
+     */
+    [[nodiscard]] auto on_update() ENTT_NOEXCEPT {
+        return sink{update};
+    }
+
+    /**
+     * @brief Returns a sink object.
+     *
+     * The sink returned by this function can be used to receive notifications
+     * whenever an instance is removed from an entity and thus destroyed.<br/>
+     * The function type for a listener is equivalent to:
+     *
+     * @code{.cpp}
+     * void(basic_registry<entity_type> &, entity_type);
+     * @endcode
+     *
+     * Listeners are invoked **before** the object has been removed from the
+     * entity.
+     *
+     * @sa sink
+     *
+     * @return A temporary sink object.
+     */
+    [[nodiscard]] auto on_destroy() ENTT_NOEXCEPT {
+        return sink{destruction};
+    }
+
+    /**
+     * @brief Assigns entities to a storage.
+     * @tparam Args Types of arguments to use to construct the object.
+     * @param owner The registry that issued the request.
+     * @param entt A valid entity identifier.
+     * @param args Parameters to use to initialize the object.
+     * @return A reference to the newly created object.
+     */
+    template<typename... Args>
+    decltype(auto) emplace(basic_registry<entity_type> &owner, const entity_type entt, Args &&... args) {
+        Type::emplace(entt, std::forward<Args>(args)...);
+        construction.publish(owner, entt);
+        return this->get(entt);
+    }
+
+    /**
+     * @brief Assigns entities to a storage.
+     * @tparam It Type of input iterator.
+     * @tparam Args Types of arguments to use to construct the objects assigned
+     * to the entities.
+     * @param owner The registry that issued the request.
+     * @param first An iterator to the first element of the range of entities.
+     * @param last An iterator past the last element of the range of entities.
+     * @param args Parameters to use to initialize the objects assigned to the
+     * entities.
+     */
+    template<typename It, typename... Args>
+    void insert(basic_registry<entity_type> &owner, It first, It last, Args &&... args) {
+        Type::insert(first, last, std::forward<Args>(args)...);
+
+        if(!construction.empty()) {
+            for(; first != last; ++first) {
+                construction.publish(owner, *first);
+            }
+        }
+    }
+
+    /**
+     * @brief Patches the given instance for an entity.
+     * @tparam Func Types of the function objects to invoke.
+     * @param owner The registry that issued the request.
+     * @param entt A valid entity identifier.
+     * @param func Valid function objects.
+     * @return A reference to the patched instance.
+     */
+    template<typename... Func>
+    decltype(auto) patch(basic_registry<entity_type> &owner, const entity_type entt, Func &&... func) {
+        Type::patch(entt, std::forward<Func>(func)...);
+        update.publish(owner, entt);
+        return this->get(entt);
+    }
+
+private:
+    sigh<void(basic_registry<entity_type> &, const entity_type)> construction{};
+    sigh<void(basic_registry<entity_type> &, const entity_type)> destruction{};
+    sigh<void(basic_registry<entity_type> &, const entity_type)> update{};
+};
+
+
+/**
+ * @brief Storage implementation dispatcher.
+ * @tparam Entity A valid entity type (see entt_traits for more details).
+ * @tparam Type Type of objects assigned to the entities.
+ * @tparam Allocator Type of allocator used to manage memory and elements.
+ */
+template<typename Entity, typename Type, typename Allocator>
+struct basic_storage: basic_storage_impl<Entity, Type, Allocator> {
+    using basic_storage_impl<Entity, Type, Allocator>::basic_storage_impl;
+};
+
+
+/**
+ * @brief Provides a common way to access certain properties of storage types.
+ * @tparam Entity A valid entity type (see entt_traits for more details).
+ * @tparam Type Type of objects managed by the storage class.
+ */
+template<typename Entity, typename Type, typename = void>
+struct storage_traits {
+    /*! @brief Resulting type after component-to-storage conversion. */
+    using storage_type = sigh_storage_mixin<basic_storage<Entity, Type>>;
+};
+
+
+/**
+ * @brief Gets the element assigned to an entity from a storage, if any.
+ * @tparam Type Storage type.
+ * @param container A valid instance of a storage class.
+ * @param entt A valid entity identifier.
+ * @return A possibly empty tuple containing the requested element.
+ */
+template<typename Type>
+[[nodiscard]] auto get_as_tuple([[maybe_unused]] Type &container, [[maybe_unused]] const typename Type::entity_type entt) {
+    static_assert(std::is_same_v<std::remove_const_t<Type>, typename storage_traits<typename Type::entity_type, typename Type::value_type>::storage_type>, "Invalid storage");
+
+    if constexpr(std::is_void_v<decltype(container.get({}))>) {
+        return std::make_tuple();
+    } else {
+        return std::forward_as_tuple(container.get(entt));
+    }
+}
+
+
+}
+
+
+#endif
+
+// #include "entity/utility.hpp"
 #ifndef ENTT_ENTITY_UTILITY_HPP
 #define ENTT_ENTITY_UTILITY_HPP
 
@@ -5512,7 +27057,7 @@
  * @tparam Type List of types.
  */
 template<typename... Type>
-constexpr exclude_t<Type...> exclude{};
+inline constexpr exclude_t<Type...> exclude{};
 
 
 /**
@@ -5528,886 +27073,15 @@
  * @tparam Type List of types.
  */
 template<typename... Type>
-constexpr get_t<Type...> get{};
-
-
-}
-
-
-#endif // ENTT_ENTITY_UTILITY_HPP
-
-// #include "entity.hpp"
-
-// #include "group.hpp"
-#ifndef ENTT_ENTITY_GROUP_HPP
-#define ENTT_ENTITY_GROUP_HPP
-
-
-#include <tuple>
-#include <utility>
-#include <type_traits>
-// #include "../config/config.h"
-
-// #include "../core/type_traits.hpp"
-
-// #include "sparse_set.hpp"
-
-// #include "storage.hpp"
-
-// #include "utility.hpp"
-
-// #include "fwd.hpp"
-
-
-
-namespace entt {
-
-
-/**
- * @brief Group.
- *
- * Primary template isn't defined on purpose. All the specializations give a
- * compile-time error, but for a few reasonable cases.
- */
-template<typename...>
-class basic_group;
-
-
-/**
- * @brief Non-owning group.
- *
- * A non-owning group returns all the entities and only the entities that have
- * at least the given components. Moreover, it's guaranteed that the entity list
- * is tightly packed in memory for fast iterations.<br/>
- * In general, non-owning groups don't stay true to the order of any set of
- * components unless users explicitly sort them.
- *
- * @b Important
- *
- * Iterators aren't invalidated if:
- *
- * * New instances of the given components are created and assigned to entities.
- * * The entity currently pointed is modified (as an example, if one of the
- *   given components is removed from the entity to which the iterator points).
- * * The entity currently pointed is destroyed.
- *
- * In all the other cases, modifying the pools of the given components in any
- * way invalidates all the iterators and using them results in undefined
- * behavior.
- *
- * @note
- * Groups share references to the underlying data structures of the registry
- * that generated them. Therefore any change to the entities and to the
- * components made by means of the registry are immediately reflected by all the
- * groups.<br/>
- * Moreover, sorting a non-owning group affects all the instance of the same
- * group (it means that users don't have to call `sort` on each instance to sort
- * all of them because they share the set of entities).
- *
- * @warning
- * Lifetime of a group must overcome the one of the registry that generated it.
- * In any other case, attempting to use a group results in undefined behavior.
- *
- * @tparam Entity A valid entity type (see entt_traits for more details).
- * @tparam Exclude Types of components used to filter the group.
- * @tparam Get Type of component observed by the group.
- * @tparam Other Other types of components observed by the group.
- */
-template<typename Entity, typename... Exclude, typename Get, typename... Other>
-class basic_group<Entity, exclude_t<Exclude...>, get_t<Get, Other...>> {
-    /*! @brief A registry is allowed to create groups. */
-    friend class basic_registry<Entity>;
-
-    template<typename Component>
-    using pool_type = std::conditional_t<std::is_const_v<Component>, const storage<Entity, std::remove_const_t<Component>>, storage<Entity, Component>>;
-
-    // we could use pool_type<Type> *..., but vs complains about it and refuses to compile for unknown reasons (most likely a bug)
-    basic_group(sparse_set<Entity> *ref, storage<Entity, std::remove_const_t<Get>> *get, storage<Entity, std::remove_const_t<Other>> *... other) ENTT_NOEXCEPT
-        : handler{ref},
-          pools{get, other...}
-    {}
-
-    template<typename Func, typename... Weak>
-    void traverse(Func func, type_list<Weak...>) const {
-        for(const auto entt: *handler) {
-            if constexpr(std::is_invocable_v<Func, decltype(get<Weak>({}))...>) {
-                func(std::get<pool_type<Weak> *>(pools)->get(entt)...);
-            } else {
-                func(entt, std::get<pool_type<Weak> *>(pools)->get(entt)...);
-            }
-        };
-    }
-
-public:
-    /*! @brief Underlying entity identifier. */
-    using entity_type = typename sparse_set<Entity>::entity_type;
-    /*! @brief Unsigned integer type. */
-    using size_type = typename sparse_set<Entity>::size_type;
-    /*! @brief Input iterator type. */
-    using iterator_type = typename sparse_set<Entity>::iterator_type;
-
-    /**
-     * @brief Returns the number of existing components of the given type.
-     * @tparam Component Type of component of which to return the size.
-     * @return Number of existing components of the given type.
-     */
-    template<typename Component>
-    size_type size() const ENTT_NOEXCEPT {
-        return std::get<pool_type<Component> *>(pools)->size();
-    }
-
-    /**
-     * @brief Returns the number of entities that have the given components.
-     * @return Number of entities that have the given components.
-     */
-    size_type size() const ENTT_NOEXCEPT {
-        return handler->size();
-    }
-
-    /**
-     * @brief Returns the number of elements that a group has currently
-     * allocated space for.
-     * @return Capacity of the group.
-     */
-    size_type capacity() const ENTT_NOEXCEPT {
-        return handler->capacity();
-    }
-
-    /*! @brief Requests the removal of unused capacity. */
-    void shrink_to_fit() {
-        handler->shrink_to_fit();
-    }
-
-    /**
-     * @brief Checks whether the pool of a given component is empty.
-     * @tparam Component Type of component in which one is interested.
-     * @return True if the pool of the given component is empty, false
-     * otherwise.
-     */
-    template<typename Component>
-    bool empty() const ENTT_NOEXCEPT {
-        return std::get<pool_type<Component> *>(pools)->empty();
-    }
-
-    /**
-     * @brief Checks whether the group is empty.
-     * @return True if the group is empty, false otherwise.
-     */
-    bool empty() const ENTT_NOEXCEPT {
-        return handler->empty();
-    }
-
-    /**
-     * @brief Direct access to the list of components of a given pool.
-     *
-     * The returned pointer is such that range
-     * `[raw<Component>(), raw<Component>() + size<Component>()]` is always a
-     * valid range, even if the container is empty.
-     *
-     * @note
-     * There are no guarantees on the order of the components. Use `begin` and
-     * `end` if you want to iterate the group in the expected order.
-     *
-     * @tparam Component Type of component in which one is interested.
-     * @return A pointer to the array of components.
-     */
-    template<typename Component>
-    Component * raw() const ENTT_NOEXCEPT {
-        return std::get<pool_type<Component> *>(pools)->raw();
-    }
-
-    /**
-     * @brief Direct access to the list of entities of a given pool.
-     *
-     * The returned pointer is such that range
-     * `[data<Component>(), data<Component>() + size<Component>()]` is always a
-     * valid range, even if the container is empty.
-     *
-     * @note
-     * There are no guarantees on the order of the entities. Use `begin` and
-     * `end` if you want to iterate the group in the expected order.
-     *
-     * @tparam Component Type of component in which one is interested.
-     * @return A pointer to the array of entities.
-     */
-    template<typename Component>
-    const entity_type * data() const ENTT_NOEXCEPT {
-        return std::get<pool_type<Component> *>(pools)->data();
-    }
-
-    /**
-     * @brief Direct access to the list of entities.
-     *
-     * The returned pointer is such that range `[data(), data() + size()]` is
-     * always a valid range, even if the container is empty.
-     *
-     * @note
-     * There are no guarantees on the order of the entities. Use `begin` and
-     * `end` if you want to iterate the group in the expected order.
-     *
-     * @return A pointer to the array of entities.
-     */
-    const entity_type * data() const ENTT_NOEXCEPT {
-        return handler->data();
-    }
-
-    /**
-     * @brief Returns an iterator to the first entity that has the given
-     * components.
-     *
-     * The returned iterator points to the first entity that has the given
-     * components. If the group is empty, the returned iterator will be equal to
-     * `end()`.
-     *
-     * @note
-     * Input iterators stay true to the order imposed to the underlying data
-     * structures.
-     *
-     * @return An iterator to the first entity that has the given components.
-     */
-    iterator_type begin() const ENTT_NOEXCEPT {
-        return handler->begin();
-    }
-
-    /**
-     * @brief Returns an iterator that is past the last entity that has the
-     * given components.
-     *
-     * The returned iterator points to the entity following the last entity that
-     * has the given components. Attempting to dereference the returned iterator
-     * results in undefined behavior.
-     *
-     * @note
-     * Input iterators stay true to the order imposed to the underlying data
-     * structures.
-     *
-     * @return An iterator to the entity following the last entity that has the
-     * given components.
-     */
-    iterator_type end() const ENTT_NOEXCEPT {
-        return handler->end();
-    }
-
-    /**
-     * @brief Finds an entity.
-     * @param entt A valid entity identifier.
-     * @return An iterator to the given entity if it's found, past the end
-     * iterator otherwise.
-     */
-    iterator_type find(const entity_type entt) const ENTT_NOEXCEPT {
-        const auto it = handler->find(entt);
-        return it != end() && *it == entt ? it : end();
-    }
-
-    /**
-     * @brief Returns the identifier that occupies the given position.
-     * @param pos Position of the element to return.
-     * @return The identifier that occupies the given position.
-     */
-    entity_type operator[](const size_type pos) const ENTT_NOEXCEPT {
-        return begin()[pos];
-    }
-
-    /**
-     * @brief Checks if a group contains an entity.
-     * @param entt A valid entity identifier.
-     * @return True if the group contains the given entity, false otherwise.
-     */
-    bool contains(const entity_type entt) const ENTT_NOEXCEPT {
-        return find(entt) != end();
-    }
-
-    /**
-     * @brief Returns the components assigned to the given entity.
-     *
-     * Prefer this function instead of `registry::get` during iterations. It has
-     * far better performance than its companion function.
-     *
-     * @warning
-     * Attempting to use an invalid component type results in a compilation
-     * error. Attempting to use an entity that doesn't belong to the group
-     * results in undefined behavior.<br/>
-     * An assertion will abort the execution at runtime in debug mode if the
-     * group doesn't contain the given entity.
-     *
-     * @tparam Component Types of components to get.
-     * @param entt A valid entity identifier.
-     * @return The components assigned to the entity.
-     */
-    template<typename... Component>
-    decltype(auto) get([[maybe_unused]] const entity_type entt) const ENTT_NOEXCEPT {
-        ENTT_ASSERT(contains(entt));
-
-        if constexpr(sizeof...(Component) == 1) {
-            return (std::get<pool_type<Component> *>(pools)->get(entt), ...);
-        } else {
-            return std::tuple<decltype(get<Component>(entt))...>{get<Component>(entt)...};
-        }
-    }
-
-    /**
-     * @brief Iterates entities and components and applies the given function
-     * object to them.
-     *
-     * The function object is invoked for each entity. It is provided with the
-     * entity itself and a set of references to all its components. The
-     * _constness_ of the components is as requested.<br/>
-     * The signature of the function must be equivalent to one of the following
-     * forms:
-     *
-     * @code{.cpp}
-     * void(const entity_type, Get &, Other &...);
-     * void(Get &, Other &...);
-     * @endcode
-     *
-     * @note
-     * Empty types aren't explicitly instantiated. Therefore, temporary objects
-     * are returned during iterations. They can be caught only by copy or with
-     * const references.
-     *
-     * @tparam Func Type of the function object to invoke.
-     * @param func A valid function object.
-     */
-    template<typename Func>
-    void each(Func func) const {
-        traverse(std::move(func), type_list<Get, Other...>{});
-    }
-
-    /**
-     * @brief Iterates entities and components and applies the given function
-     * object to them.
-     *
-     * The function object is invoked for each entity. It is provided with the
-     * entity itself and a set of references to non-empty components. The
-     * _constness_ of the components is as requested.<br/>
-     * The signature of the function must be equivalent to one of the following
-     * forms:
-     *
-     * @code{.cpp}
-     * void(const entity_type, Type &...);
-     * void(Type &...);
-     * @endcode
-     *
-     * @sa each
-     *
-     * @tparam Func Type of the function object to invoke.
-     * @param func A valid function object.
-     */
-    template<typename Func>
-    void less(Func func) const {
-        using get_type_list = std::conditional_t<std::is_empty_v<Get>, type_list<>, type_list<Get>>;
-        using other_type_list = type_list_cat_t<std::conditional_t<std::is_empty_v<Other>, type_list<>, type_list<Other>>...>;
-        traverse(std::move(func), type_list_cat_t<get_type_list, other_type_list>{});
-    }
-
-    /**
-     * @brief Sort a group according to the given comparison function.
-     *
-     * Sort the group so that iterating it with a couple of iterators returns
-     * entities and components in the expected order. See `begin` and `end` for
-     * more details.
-     *
-     * The comparison function object must return `true` if the first element
-     * is _less_ than the second one, `false` otherwise. The signature of the
-     * comparison function should be equivalent to one of the following:
-     *
-     * @code{.cpp}
-     * bool(std::tuple<Component &...>, std::tuple<Component &...>);
-     * bool(const Component &..., const Component &...);
-     * bool(const Entity, const Entity);
-     * @endcode
-     *
-     * Where `Component` are such that they are iterated by the group.<br/>
-     * Moreover, the comparison function object shall induce a
-     * _strict weak ordering_ on the values.
-     *
-     * The sort function oject must offer a member function template
-     * `operator()` that accepts three arguments:
-     *
-     * * An iterator to the first element of the range to sort.
-     * * An iterator past the last element of the range to sort.
-     * * A comparison function to use to compare the elements.
-     *
-     * The comparison function object received by the sort function object
-     * hasn't necessarily the type of the one passed along with the other
-     * parameters to this member function.
-     *
-     * @note
-     * Attempting to iterate elements using a raw pointer returned by a call to
-     * either `data` or `raw` gives no guarantees on the order, even though
-     * `sort` has been invoked.
-     *
-     * @tparam Component Optional types of components to compare.
-     * @tparam Compare Type of comparison function object.
-     * @tparam Sort Type of sort function object.
-     * @tparam Args Types of arguments to forward to the sort function object.
-     * @param compare A valid comparison function object.
-     * @param algo A valid sort function object.
-     * @param args Arguments to forward to the sort function object, if any.
-     */
-    template<typename... Component, typename Compare, typename Sort = std_sort, typename... Args>
-    void sort(Compare compare, Sort algo = Sort{}, Args &&... args) {
-        if constexpr(sizeof...(Component) == 0) {
-            static_assert(std::is_invocable_v<Compare, const entity_type, const entity_type>);
-            handler->sort(handler->begin(), handler->end(), std::move(compare), std::move(algo), std::forward<Args>(args)...);
-        } else {
-            handler->sort(handler->begin(), handler->end(), [this, compare = std::move(compare)](const entity_type lhs, const entity_type rhs) {
-                // useless this-> used to suppress a warning with clang
-                return compare(this->get<Component...>(lhs), this->get<Component...>(rhs));
-            }, std::move(algo), std::forward<Args>(args)...);
-        }
-    }
-
-    /**
-     * @brief Sort the shared pool of entities according to the given component.
-     *
-     * Non-owning groups of the same type share with the registry a pool of
-     * entities with  its own order that doesn't depend on the order of any pool
-     * of components. Users can order the underlying data structure so that it
-     * respects the order of the pool of the given component.
-     *
-     * @note
-     * The shared pool of entities and thus its order is affected by the changes
-     * to each and every pool that it tracks. Therefore changes to those pools
-     * can quickly ruin the order imposed to the pool of entities shared between
-     * the non-owning groups.
-     *
-     * @tparam Component Type of component to use to impose the order.
-     */
-    template<typename Component>
-    void sort() const {
-        handler->respect(*std::get<pool_type<Component> *>(pools));
-    }
-
-private:
-    sparse_set<entity_type> *handler;
-    const std::tuple<pool_type<Get> *, pool_type<Other> *...> pools;
-};
-
-
-/**
- * @brief Owning group.
- *
- * Owning groups return all the entities and only the entities that have at
- * least the given components. Moreover:
- *
- * * It's guaranteed that the entity list is tightly packed in memory for fast
- *   iterations.
- * * It's guaranteed that the lists of owned components are tightly packed in
- *   memory for even faster iterations and to allow direct access.
- * * They stay true to the order of the owned components and all the owned
- *   components have the same order in memory.
- *
- * The more types of components are owned by a group, the faster it is to
- * iterate them.
- *
- * @b Important
- *
- * Iterators aren't invalidated if:
- *
- * * New instances of the given components are created and assigned to entities.
- * * The entity currently pointed is modified (as an example, if one of the
- *   given components is removed from the entity to which the iterator points).
- * * The entity currently pointed is destroyed.
- *
- * In all the other cases, modifying the pools of the given components in any
- * way invalidates all the iterators and using them results in undefined
- * behavior.
- *
- * @note
- * Groups share references to the underlying data structures of the registry
- * that generated them. Therefore any change to the entities and to the
- * components made by means of the registry are immediately reflected by all the
- * groups.
- * Moreover, sorting an owning group affects all the instance of the same group
- * (it means that users don't have to call `sort` on each instance to sort all
- * of them because they share the underlying data structure).
- *
- * @warning
- * Lifetime of a group must overcome the one of the registry that generated it.
- * In any other case, attempting to use a group results in undefined behavior.
- *
- * @tparam Entity A valid entity type (see entt_traits for more details).
- * @tparam Exclude Types of components used to filter the group.
- * @tparam Get Types of components observed by the group.
- * @tparam Owned Type of component owned by the group.
- * @tparam Other Other types of components owned by the group.
- */
-template<typename Entity, typename... Exclude, typename... Get, typename Owned, typename... Other>
-class basic_group<Entity, exclude_t<Exclude...>, get_t<Get...>, Owned, Other...> {
-    /*! @brief A registry is allowed to create groups. */
-    friend class basic_registry<Entity>;
-
-    template<typename Component>
-    using pool_type = std::conditional_t<std::is_const_v<Component>, const storage<Entity, std::remove_const_t<Component>>, storage<Entity, Component>>;
-
-    template<typename Component>
-    using component_iterator_type = decltype(std::declval<pool_type<Component>>().begin());
-
-    // we could use pool_type<Type> *..., but vs complains about it and refuses to compile for unknown reasons (most likely a bug)
-    basic_group(const typename basic_registry<Entity>::size_type *sz, storage<Entity, std::remove_const_t<Owned>> *owned, storage<Entity, std::remove_const_t<Other>> *... other, storage<Entity, std::remove_const_t<Get>> *... get) ENTT_NOEXCEPT
-        : length{sz},
-          pools{owned, other..., get...}
-    {}
-
-    template<typename Func, typename... Strong, typename... Weak>
-    void traverse(Func func, type_list<Strong...>, type_list<Weak...>) const {
-        [[maybe_unused]] auto raw = std::make_tuple((std::get<pool_type<Strong> *>(pools)->end() - *length)...);
-        [[maybe_unused]] auto data = std::get<pool_type<Owned> *>(pools)->sparse_set<entity_type>::end() - *length;
-
-        for(auto next = *length; next; --next) {
-            if constexpr(std::is_invocable_v<Func, decltype(get<Strong>({}))..., decltype(get<Weak>({}))...>) {
-                if constexpr(sizeof...(Weak) == 0) {
-                    func(*(std::get<component_iterator_type<Strong>>(raw)++)...);
-                } else {
-                    const auto entt = *(data++);
-                    func(*(std::get<component_iterator_type<Strong>>(raw)++)..., std::get<pool_type<Weak> *>(pools)->get(entt)...);
-                }
-            } else {
-                const auto entt = *(data++);
-                func(entt, *(std::get<component_iterator_type<Strong>>(raw)++)..., std::get<pool_type<Weak> *>(pools)->get(entt)...);
-            }
-        }
-    }
-
-public:
-    /*! @brief Underlying entity identifier. */
-    using entity_type = typename sparse_set<Entity>::entity_type;
-    /*! @brief Unsigned integer type. */
-    using size_type = typename sparse_set<Entity>::size_type;
-    /*! @brief Input iterator type. */
-    using iterator_type = typename sparse_set<Entity>::iterator_type;
-
-    /**
-     * @brief Returns the number of existing components of the given type.
-     * @tparam Component Type of component of which to return the size.
-     * @return Number of existing components of the given type.
-     */
-    template<typename Component>
-    size_type size() const ENTT_NOEXCEPT {
-        return std::get<pool_type<Component> *>(pools)->size();
-    }
-
-    /**
-     * @brief Returns the number of entities that have the given components.
-     * @return Number of entities that have the given components.
-     */
-    size_type size() const ENTT_NOEXCEPT {
-        return *length;
-    }
-
-    /**
-     * @brief Checks whether the pool of a given component is empty.
-     * @tparam Component Type of component in which one is interested.
-     * @return True if the pool of the given component is empty, false
-     * otherwise.
-     */
-    template<typename Component>
-    bool empty() const ENTT_NOEXCEPT {
-        return std::get<pool_type<Component> *>(pools)->empty();
-    }
-
-    /**
-     * @brief Checks whether the group is empty.
-     * @return True if the group is empty, false otherwise.
-     */
-    bool empty() const ENTT_NOEXCEPT {
-        return !*length;
-    }
-
-    /**
-     * @brief Direct access to the list of components of a given pool.
-     *
-     * The returned pointer is such that range
-     * `[raw<Component>(), raw<Component>() + size<Component>()]` is always a
-     * valid range, even if the container is empty.<br/>
-     * Moreover, in case the group owns the given component, the range
-     * `[raw<Component>(), raw<Component>() + size()]` is such that it contains
-     * the instances that are part of the group itself.
-     *
-     * @note
-     * There are no guarantees on the order of the components. Use `begin` and
-     * `end` if you want to iterate the group in the expected order.
-     *
-     * @tparam Component Type of component in which one is interested.
-     * @return A pointer to the array of components.
-     */
-    template<typename Component>
-    Component * raw() const ENTT_NOEXCEPT {
-        return std::get<pool_type<Component> *>(pools)->raw();
-    }
-
-    /**
-     * @brief Direct access to the list of entities of a given pool.
-     *
-     * The returned pointer is such that range
-     * `[data<Component>(), data<Component>() + size<Component>()]` is always a
-     * valid range, even if the container is empty.<br/>
-     * Moreover, in case the group owns the given component, the range
-     * `[data<Component>(), data<Component>() + size()]` is such that it
-     * contains the entities that are part of the group itself.
-     *
-     * @note
-     * There are no guarantees on the order of the entities. Use `begin` and
-     * `end` if you want to iterate the group in the expected order.
-     *
-     * @tparam Component Type of component in which one is interested.
-     * @return A pointer to the array of entities.
-     */
-    template<typename Component>
-    const entity_type * data() const ENTT_NOEXCEPT {
-        return std::get<pool_type<Component> *>(pools)->data();
-    }
-
-    /**
-     * @brief Direct access to the list of entities.
-     *
-     * The returned pointer is such that range `[data(), data() + size()]` is
-     * always a valid range, even if the container is empty.
-     *
-     * @note
-     * There are no guarantees on the order of the entities. Use `begin` and
-     * `end` if you want to iterate the group in the expected order.
-     *
-     * @return A pointer to the array of entities.
-     */
-    const entity_type * data() const ENTT_NOEXCEPT {
-        return std::get<pool_type<Owned> *>(pools)->data();
-    }
-
-    /**
-     * @brief Returns an iterator to the first entity that has the given
-     * components.
-     *
-     * The returned iterator points to the first entity that has the given
-     * components. If the group is empty, the returned iterator will be equal to
-     * `end()`.
-     *
-     * @note
-     * Input iterators stay true to the order imposed to the underlying data
-     * structures.
-     *
-     * @return An iterator to the first entity that has the given components.
-     */
-    iterator_type begin() const ENTT_NOEXCEPT {
-        return std::get<pool_type<Owned> *>(pools)->sparse_set<entity_type>::end() - *length;
-    }
-
-    /**
-     * @brief Returns an iterator that is past the last entity that has the
-     * given components.
-     *
-     * The returned iterator points to the entity following the last entity that
-     * has the given components. Attempting to dereference the returned iterator
-     * results in undefined behavior.
-     *
-     * @note
-     * Input iterators stay true to the order imposed to the underlying data
-     * structures.
-     *
-     * @return An iterator to the entity following the last entity that has the
-     * given components.
-     */
-    iterator_type end() const ENTT_NOEXCEPT {
-        return std::get<pool_type<Owned> *>(pools)->sparse_set<entity_type>::end();
-    }
-
-    /**
-     * @brief Finds an entity.
-     * @param entt A valid entity identifier.
-     * @return An iterator to the given entity if it's found, past the end
-     * iterator otherwise.
-     */
-    iterator_type find(const entity_type entt) const ENTT_NOEXCEPT {
-        const auto it = std::get<pool_type<Owned> *>(pools)->find(entt);
-        return it != end() && it >= begin() && *it == entt ? it : end();
-    }
-
-    /**
-     * @brief Returns the identifier that occupies the given position.
-     * @param pos Position of the element to return.
-     * @return The identifier that occupies the given position.
-     */
-    entity_type operator[](const size_type pos) const ENTT_NOEXCEPT {
-        return begin()[pos];
-    }
-
-    /**
-     * @brief Checks if a group contains an entity.
-     * @param entt A valid entity identifier.
-     * @return True if the group contains the given entity, false otherwise.
-     */
-    bool contains(const entity_type entt) const ENTT_NOEXCEPT {
-        return find(entt) != end();
-    }
-
-    /**
-     * @brief Returns the components assigned to the given entity.
-     *
-     * Prefer this function instead of `registry::get` during iterations. It has
-     * far better performance than its companion function.
-     *
-     * @warning
-     * Attempting to use an invalid component type results in a compilation
-     * error. Attempting to use an entity that doesn't belong to the group
-     * results in undefined behavior.<br/>
-     * An assertion will abort the execution at runtime in debug mode if the
-     * group doesn't contain the given entity.
-     *
-     * @tparam Component Types of components to get.
-     * @param entt A valid entity identifier.
-     * @return The components assigned to the entity.
-     */
-    template<typename... Component>
-    decltype(auto) get([[maybe_unused]] const entity_type entt) const ENTT_NOEXCEPT {
-        ENTT_ASSERT(contains(entt));
-
-        if constexpr(sizeof...(Component) == 1) {
-            return (std::get<pool_type<Component> *>(pools)->get(entt), ...);
-        } else {
-            return std::tuple<decltype(get<Component>(entt))...>{get<Component>(entt)...};
-        }
-    }
-
-    /**
-     * @brief Iterates entities and components and applies the given function
-     * object to them.
-     *
-     * The function object is invoked for each entity. It is provided with the
-     * entity itself and a set of references to all its components. The
-     * _constness_ of the components is as requested.<br/>
-     * The signature of the function must be equivalent to one of the following
-     * forms:
-     *
-     * @code{.cpp}
-     * void(const entity_type, Owned &, Other &..., Get &...);
-     * void(Owned &, Other &..., Get &...);
-     * @endcode
-     *
-     * @note
-     * Empty types aren't explicitly instantiated. Therefore, temporary objects
-     * are returned during iterations. They can be caught only by copy or with
-     * const references.
-     *
-     * @tparam Func Type of the function object to invoke.
-     * @param func A valid function object.
-     */
-    template<typename Func>
-    void each(Func func) const {
-        traverse(std::move(func), type_list<Owned, Other...>{}, type_list<Get...>{});
-    }
-
-    /**
-     * @brief Iterates entities and components and applies the given function
-     * object to them.
-     *
-     * The function object is invoked for each entity. It is provided with the
-     * entity itself and a set of references to non-empty components. The
-     * _constness_ of the components is as requested.<br/>
-     * The signature of the function must be equivalent to one of the following
-     * forms:
-     *
-     * @code{.cpp}
-     * void(const entity_type, Type &...);
-     * void(Type &...);
-     * @endcode
-     *
-     * @sa each
-     *
-     * @tparam Func Type of the function object to invoke.
-     * @param func A valid function object.
-     */
-    template<typename Func>
-    void less(Func func) const {
-        using owned_type_list = std::conditional_t<std::is_empty_v<Owned>, type_list<>, type_list<Owned>>;
-        using other_type_list = type_list_cat_t<std::conditional_t<std::is_empty_v<Other>, type_list<>, type_list<Other>>...>;
-        using get_type_list = type_list_cat_t<std::conditional_t<std::is_empty_v<Get>, type_list<>, type_list<Get>>...>;
-        traverse(std::move(func), type_list_cat_t<owned_type_list, other_type_list>{}, get_type_list{});
-    }
-
-    /**
-     * @brief Sort a group according to the given comparison function.
-     *
-     * Sort the group so that iterating it with a couple of iterators returns
-     * entities and components in the expected order. See `begin` and `end` for
-     * more details.
-     *
-     * The comparison function object must return `true` if the first element
-     * is _less_ than the second one, `false` otherwise. The signature of the
-     * comparison function should be equivalent to one of the following:
-     *
-     * @code{.cpp}
-     * bool(std::tuple<Component &...>, std::tuple<Component &...>);
-     * bool(const Component &, const Component &);
-     * bool(const Entity, const Entity);
-     * @endcode
-     *
-     * Where `Component` are either owned types or not but still such that they
-     * are iterated by the group.<br/>
-     * Moreover, the comparison function object shall induce a
-     * _strict weak ordering_ on the values.
-     *
-     * The sort function oject must offer a member function template
-     * `operator()` that accepts three arguments:
-     *
-     * * An iterator to the first element of the range to sort.
-     * * An iterator past the last element of the range to sort.
-     * * A comparison function to use to compare the elements.
-     *
-     * The comparison function object received by the sort function object
-     * hasn't necessarily the type of the one passed along with the other
-     * parameters to this member function.
-     *
-     * @note
-     * Attempting to iterate elements using a raw pointer returned by a call to
-     * either `data` or `raw` gives no guarantees on the order, even though
-     * `sort` has been invoked.
-     *
-     * @tparam Component Optional types of components to compare.
-     * @tparam Compare Type of comparison function object.
-     * @tparam Sort Type of sort function object.
-     * @tparam Args Types of arguments to forward to the sort function object.
-     * @param compare A valid comparison function object.
-     * @param algo A valid sort function object.
-     * @param args Arguments to forward to the sort function object, if any.
-     */
-    template<typename... Component, typename Compare, typename Sort = std_sort, typename... Args>
-    void sort(Compare compare, Sort algo = Sort{}, Args &&... args) {
-        auto *cpool = std::get<pool_type<Owned> *>(pools);
-
-        if constexpr(sizeof...(Component) == 0) {
-            static_assert(std::is_invocable_v<Compare, const entity_type, const entity_type>);
-            cpool->sort(cpool->end()-*length, cpool->end(), std::move(compare), std::move(algo), std::forward<Args>(args)...);
-        } else {
-            cpool->sort(cpool->end()-*length, cpool->end(), [this, compare = std::move(compare)](const entity_type lhs, const entity_type rhs) {
-                // useless this-> used to suppress a warning with clang
-                return compare(this->get<Component...>(lhs), this->get<Component...>(rhs));
-            }, std::move(algo), std::forward<Args>(args)...);
-        }
-
-        for(auto next = *length; next; --next) {
-            ([next = next-1, curr = cpool->data()[next-1]](auto *cpool) {
-                const auto pos = cpool->index(curr);
-
-                if(pos != next) {
-                    cpool->swap(next, cpool->index(curr));
-                }
-            }(std::get<pool_type<Other> *>(pools)), ...);
-        }
-    }
-
-private:
-    const typename basic_registry<Entity>::size_type *length;
-    const std::tuple<pool_type<Owned> *, pool_type<Other> *..., pool_type<Get> *...> pools;
-};
-
-
-}
-
-
-#endif // ENTT_ENTITY_GROUP_HPP
-
-// #include "view.hpp"
+inline constexpr get_t<Type...> get{};
+
+
+}
+
+
+#endif
+
+// #include "entity/view.hpp"
 #ifndef ENTT_ENTITY_VIEW_HPP
 #define ENTT_ENTITY_VIEW_HPP
 
@@ -6422,17 +27096,172 @@
 
 // #include "../core/type_traits.hpp"
 
+// #include "component.hpp"
+
+// #include "entity.hpp"
+
+// #include "fwd.hpp"
+
 // #include "sparse_set.hpp"
 
 // #include "storage.hpp"
 
-// #include "entity.hpp"
-
-// #include "fwd.hpp"
-
-
-
-namespace entt {
+// #include "utility.hpp"
+
+
+
+namespace entt {
+
+
+/**
+ * @cond TURN_OFF_DOXYGEN
+ * Internal details not to be documented.
+ */
+
+
+namespace internal {
+
+
+template<typename Policy, typename It, std::size_t AllOf, std::size_t NoneOf>
+class view_iterator final {
+    using basic_common_type = basic_sparse_set<typename std::iterator_traits<It>::value_type>;
+
+    [[nodiscard]] bool valid() const {
+        const auto entt = *it;
+        return Policy::accept(entt)
+            && std::apply([entt](const auto *... curr) { return (curr->contains(entt) && ...); }, pools)
+            && std::apply([entt](const auto *... curr) { return (!curr->contains(entt) && ...); }, filter);
+    }
+
+public:
+    using iterator_type = It;
+    using difference_type = typename std::iterator_traits<It>::difference_type;
+    using value_type = typename std::iterator_traits<It>::value_type;
+    using pointer = typename std::iterator_traits<It>::pointer;
+    using reference = typename std::iterator_traits<It>::reference;
+    using iterator_category = std::bidirectional_iterator_tag;
+
+    view_iterator() ENTT_NOEXCEPT
+        : first{},
+          last{},
+          it{},
+          pools{},
+          filter{}
+    {}
+
+    view_iterator(It from, It to, It curr, std::array<const basic_common_type *, AllOf> all_of, std::array<const basic_common_type *, NoneOf> none_of) ENTT_NOEXCEPT
+        : first{from},
+          last{to},
+          it{curr},
+          pools{all_of},
+          filter{none_of}
+    {
+        if(it != last && !valid()) {
+            ++(*this);
+        }
+    }
+
+    view_iterator & operator++() ENTT_NOEXCEPT {
+        while(++it != last && !valid());
+        return *this;
+    }
+
+    view_iterator operator++(int) ENTT_NOEXCEPT {
+        view_iterator orig = *this;
+        return ++(*this), orig;
+    }
+
+    view_iterator & operator--() ENTT_NOEXCEPT {
+        while(--it != first && !valid());
+        return *this;
+    }
+
+    view_iterator operator--(int) ENTT_NOEXCEPT {
+        view_iterator orig = *this;
+        return operator--(), orig;
+    }
+
+    [[nodiscard]] bool operator==(const view_iterator &other) const ENTT_NOEXCEPT {
+        return other.it == it;
+    }
+
+    [[nodiscard]] bool operator!=(const view_iterator &other) const ENTT_NOEXCEPT {
+        return !(*this == other);
+    }
+
+    [[nodiscard]] pointer operator->() const {
+        return &*it;
+    }
+
+    [[nodiscard]] reference operator*() const {
+        return *operator->();
+    }
+
+private:
+    It first;
+    It last;
+    It it;
+    std::array<const basic_common_type *, AllOf> pools;
+    std::array<const basic_common_type *, NoneOf> filter;
+};
+
+
+}
+
+
+/**
+ * Internal details not to be documented.
+ * @endcond
+ */
+
+
+/*! @brief Stable storage policy, aimed at pointer stability. */
+struct stable_storage_policy {
+    /**
+    * @cond TURN_OFF_DOXYGEN
+    * Internal details not to be documented.
+    */
+    template<typename Entity>
+    [[nodiscard]] static constexpr bool accept(const Entity entity) ENTT_NOEXCEPT {
+        return entity != tombstone;
+    }
+    /**
+    * Internal details not to be documented.
+    * @endcond
+    */
+};
+
+
+/*! @brief Packed storage policy, aimed at faster linear iteration. */
+struct packed_storage_policy {
+    /**
+    * @cond TURN_OFF_DOXYGEN
+    * Internal details not to be documented.
+    */
+    template<typename Entity>
+    [[nodiscard]] static constexpr bool accept(const Entity) ENTT_NOEXCEPT {
+        return true;
+    }
+    /**
+    * Internal details not to be documented.
+    * @endcond
+    */
+};
+
+
+/**
+ * @brief View implementation.
+ *
+ * Primary template isn't defined on purpose. All the specializations give a
+ * compile-time error, but for a few reasonable cases.
+ */
+template<typename...>
+class basic_view_impl;
+
+
+/*! @brief View implementation dispatcher. */
+template<typename...>
+struct basic_view;
 
 
 /**
@@ -6440,12 +27269,8 @@
  *
  * Multi component views iterate over those entities that have at least all the
  * given components in their bags. During initialization, a multi component view
- * looks at the number of entities available for each component and picks up a
- * reference to the smallest set of candidate entities in order to get a
- * performance boost when iterate.<br/>
- * Order of elements during iterations are highly dependent on the order of the
- * underlying data structures. See sparse_set and its specializations for more
- * details.
+ * looks at the number of entities available for each component and uses the
+ * smallest set in order to get a performance boost when iterate.
  *
  * @b Important
  *
@@ -6456,9 +27281,8 @@
  *   given components is removed from the entity to which the iterator points).
  * * The entity currently pointed is destroyed.
  *
- * In all the other cases, modifying the pools of the given components in any
- * way invalidates all the iterators and using them results in undefined
- * behavior.
+ * In all other cases, modifying the pools iterated by the view in any way
+ * invalidates all the iterators and using them results in undefined behavior.
  *
  * @note
  * Views share references to the underlying data structures of the registry that
@@ -6466,265 +27290,263 @@
  * made by means of the registry are immediately reflected by views.
  *
  * @warning
- * Lifetime of a view must overcome the one of the registry that generated it.
+ * Lifetime of a view must not overcome that of the registry that generated it.
  * In any other case, attempting to use a view results in undefined behavior.
  *
- * @tparam Entity A valid entity type (see entt_traits for more details).
+ * @tparam Policy Common (stricter) storage policy.
+ * @tparam Entity A valid entity type (see entt_traits for more details).
+ * @tparam Exclude Types of components used to filter the view.
  * @tparam Component Types of components iterated by the view.
  */
-template<typename Entity, typename... Component>
-class basic_view {
-    static_assert(sizeof...(Component) > 1);
-
-    /*! @brief A registry is allowed to create views. */
-    friend class basic_registry<Entity>;
-
-    template<typename Comp>
-    using pool_type = std::conditional_t<std::is_const_v<Comp>, const storage<Entity, std::remove_const_t<Comp>>, storage<Entity, Comp>>;
+template<typename Policy, typename Entity, typename... Exclude, typename... Component>
+class basic_view_impl<Policy, Entity, exclude_t<Exclude...>, Component...> {
+    using basic_common_type = basic_sparse_set<Entity>;
 
     template<typename Comp>
-    using component_iterator_type = decltype(std::declval<pool_type<Comp>>().begin());
-
-    using underlying_iterator_type = typename sparse_set<Entity>::iterator_type;
-    using unchecked_type = std::array<const sparse_set<Entity> *, (sizeof...(Component) - 1)>;
-    using traits_type = entt_traits<std::underlying_type_t<Entity>>;
-
-    class iterator {
-        friend class basic_view<Entity, Component...>;
-
-        iterator(unchecked_type other, underlying_iterator_type first, underlying_iterator_type last) ENTT_NOEXCEPT
-            : unchecked{other},
-              begin{first},
-              end{last}
-        {
-            if(begin != end && !valid()) {
-                ++(*this);
-            }
-        }
-
-        bool valid() const ENTT_NOEXCEPT {
-            return std::all_of(unchecked.cbegin(), unchecked.cend(), [this](const sparse_set<Entity> *view) {
-                return view->has(*begin);
-            });
-        }
+    using storage_type = constness_as_t<typename storage_traits<Entity, std::remove_const_t<Comp>>::storage_type, Comp>;
+
+    class iterable final {
+        template<typename It>
+        struct iterable_iterator final {
+            using difference_type = std::ptrdiff_t;
+            using value_type = decltype(std::tuple_cat(std::tuple<Entity>{}, std::declval<basic_view_impl>().get({})));
+            using pointer = void;
+            using reference = value_type;
+            using iterator_category = std::input_iterator_tag;
+
+            iterable_iterator(It from, const basic_view_impl *parent) ENTT_NOEXCEPT
+                : it{from},
+                  view{parent}
+            {}
+
+            iterable_iterator & operator++() ENTT_NOEXCEPT {
+                return ++it, *this;
+            }
+
+            iterable_iterator operator++(int) ENTT_NOEXCEPT {
+                iterable_iterator orig = *this;
+                return ++(*this), orig;
+            }
+
+            [[nodiscard]] reference operator*() const ENTT_NOEXCEPT {
+                return std::tuple_cat(std::make_tuple(*it), view->get(*it));
+            }
+
+            [[nodiscard]] bool operator==(const iterable_iterator &other) const ENTT_NOEXCEPT {
+                return other.it == it;
+            }
+
+            [[nodiscard]] bool operator!=(const iterable_iterator &other) const ENTT_NOEXCEPT {
+                return !(*this == other);
+            }
+
+        private:
+            It it;
+            const basic_view_impl *view;
+        };
 
     public:
-        using difference_type = typename underlying_iterator_type::difference_type;
-        using value_type = typename underlying_iterator_type::value_type;
-        using pointer = typename underlying_iterator_type::pointer;
-        using reference = typename underlying_iterator_type::reference;
-        using iterator_category = std::forward_iterator_tag;
-
-        iterator() ENTT_NOEXCEPT = default;
-
-        iterator & operator++() ENTT_NOEXCEPT {
-            return (++begin != end && !valid()) ? ++(*this) : *this;
-        }
-
-        iterator operator++(int) ENTT_NOEXCEPT {
-            iterator orig = *this;
-            return ++(*this), orig;
-        }
-
-        bool operator==(const iterator &other) const ENTT_NOEXCEPT {
-            return other.begin == begin;
-        }
-
-        bool operator!=(const iterator &other) const ENTT_NOEXCEPT {
-            return !(*this == other);
-        }
-
-        pointer operator->() const ENTT_NOEXCEPT {
-            return begin.operator->();
-        }
-
-        reference operator*() const ENTT_NOEXCEPT {
-            return *operator->();
+        using iterator = iterable_iterator<internal::view_iterator<Policy, typename basic_common_type::iterator, sizeof...(Component) - 1u, sizeof...(Exclude)>>;
+        using reverse_iterator = iterable_iterator<internal::view_iterator<Policy, typename basic_common_type::reverse_iterator, sizeof...(Component) - 1u, sizeof...(Exclude)>>;
+
+        iterable(const basic_view_impl &parent)
+            : view{parent}
+        {}
+
+        [[nodiscard]] iterator begin() const ENTT_NOEXCEPT {
+            return { view.begin(), &view };
+        }
+
+        [[nodiscard]] iterator end() const ENTT_NOEXCEPT {
+            return { view.end(), &view };
+        }
+
+        [[nodiscard]] reverse_iterator rbegin() const ENTT_NOEXCEPT {
+            return { view.rbegin(), &view };
+        }
+
+        [[nodiscard]] reverse_iterator rend() const ENTT_NOEXCEPT {
+            return { view.rend(), &view };
         }
 
     private:
-        unchecked_type unchecked;
-        underlying_iterator_type begin;
-        underlying_iterator_type end;
-    };
-
-    // we could use pool_type<Component> *..., but vs complains about it and refuses to compile for unknown reasons (likely a bug)
-    basic_view(storage<Entity, std::remove_const_t<Component>> *... ref) ENTT_NOEXCEPT
-        : pools{ref...}
-    {}
-
-    const sparse_set<Entity> * candidate() const ENTT_NOEXCEPT {
-        return std::min({ static_cast<const sparse_set<Entity> *>(std::get<pool_type<Component> *>(pools))... }, [](const auto *lhs, const auto *rhs) {
+        const basic_view_impl view;
+    };
+
+    [[nodiscard]] const auto * candidate() const ENTT_NOEXCEPT {
+        return (std::min)({ static_cast<const basic_common_type *>(std::get<storage_type<Component> *>(pools))... }, [](const auto *lhs, const auto *rhs) {
             return lhs->size() < rhs->size();
         });
     }
 
-    unchecked_type unchecked(const sparse_set<Entity> *view) const ENTT_NOEXCEPT {
-        unchecked_type other{};
-        typename unchecked_type::size_type pos{};
-        ((std::get<pool_type<Component> *>(pools) == view ? nullptr : (other[pos++] = std::get<pool_type<Component> *>(pools))), ...);
+    [[nodiscard]] auto pools_to_unchecked_array() const ENTT_NOEXCEPT {
+        std::size_t pos{};
+        std::array<const basic_common_type *, sizeof...(Component) - 1u> other{};
+        (static_cast<void>(std::get<storage_type<Component> *>(pools) == view ? void() : void(other[pos++] = std::get<storage_type<Component> *>(pools))), ...);
         return other;
     }
 
-    template<typename Comp, typename Other>
-    decltype(auto) get([[maybe_unused]] component_iterator_type<Comp> it, [[maybe_unused]] pool_type<Other> *cpool, [[maybe_unused]] const Entity entt) const ENTT_NOEXCEPT {
-        if constexpr(std::is_same_v<Comp, Other>) {
-            return *it;
-        } else {
-            return cpool->get(entt);
-        }
-    }
-
-    template<typename Comp, typename Func, typename... Other, typename... Type>
-    void traverse(Func func, type_list<Other...>, type_list<Type...>) const {
-        const auto end = std::get<pool_type<Comp> *>(pools)->sparse_set<Entity>::end();
-        auto begin = std::get<pool_type<Comp> *>(pools)->sparse_set<Entity>::begin();
-
-        if constexpr(std::disjunction_v<std::is_same<Comp, Type>...>) {
-            std::for_each(begin, end, [this, raw = std::get<pool_type<Comp> *>(pools)->begin(), &func](const auto entity) mutable {
-                auto curr = raw++;
-
-                if((std::get<pool_type<Other> *>(pools)->has(entity) && ...)) {
-                    if constexpr(std::is_invocable_v<Func, decltype(get<Type>({}))...>) {
-                        func(get<Comp, Type>(curr, std::get<pool_type<Type> *>(pools), entity)...);
+    [[nodiscard]] auto filter_to_array() const ENTT_NOEXCEPT {
+        return std::array<const basic_common_type *, sizeof...(Exclude)>{std::get<const storage_type<Exclude> *>(filter)...};
+    }
+
+    template<typename Comp, typename It>
+    [[nodiscard]] auto dispatch_get([[maybe_unused]] It &it, [[maybe_unused]] const Entity entt) const {
+        if constexpr(std::is_same_v<typename std::iterator_traits<It>::value_type, typename storage_type<Comp>::value_type>) {
+            return std::forward_as_tuple(*it);
+        } else {
+            return get_as_tuple(*std::get<storage_type<Comp> *>(pools), entt);
+        }
+    }
+
+    template<typename Comp, typename Func>
+    void traverse(Func func) const {
+        if constexpr(std::is_void_v<decltype(std::get<storage_type<Comp> *>(pools)->get({}))>) {
+            for(const auto entt: static_cast<const basic_common_type &>(*std::get<storage_type<Comp> *>(pools))) {
+                if(Policy::accept(entt) && ((std::is_same_v<Comp, Component> || std::get<storage_type<Component> *>(pools)->contains(entt)) && ...)
+                    && (!std::get<const storage_type<Exclude> *>(filter)->contains(entt) && ...))
+                {
+                    if constexpr(is_applicable_v<Func, decltype(std::tuple_cat(std::tuple<entity_type>{}, std::declval<basic_view_impl>().get({})))>) {
+                        std::apply(func, std::tuple_cat(std::make_tuple(entt), get(entt)));
                     } else {
-                        func(entity, get<Comp, Type>(curr, std::get<pool_type<Type> *>(pools), entity)...);
+                        std::apply(func, get(entt));
                     }
                 }
-            });
-        } else {
-            std::for_each(begin, end, [this, &func](const auto entity) mutable {
-                if((std::get<pool_type<Other> *>(pools)->has(entity) && ...)) {
-                    if constexpr(std::is_invocable_v<Func, decltype(get<Type>({}))...>) {
-                        func(std::get<pool_type<Type> *>(pools)->get(entity)...);
+            }
+        } else {
+            auto it = std::get<storage_type<Comp> *>(pools)->begin();
+
+            for(const auto entt: static_cast<const basic_common_type &>(*std::get<storage_type<Comp> *>(pools))) {
+                if(Policy::accept(entt) && ((std::is_same_v<Comp, Component> || std::get<storage_type<Component> *>(pools)->contains(entt)) && ...)
+                    && (!std::get<const storage_type<Exclude> *>(filter)->contains(entt) && ...))
+                {
+                    if constexpr(is_applicable_v<Func, decltype(std::tuple_cat(std::tuple<entity_type>{}, std::declval<basic_view_impl>().get({})))>) {
+                        std::apply(func, std::tuple_cat(std::make_tuple(entt), dispatch_get<Component>(it, entt)...));
                     } else {
-                        func(entity, std::get<pool_type<Type> *>(pools)->get(entity)...);
+                        std::apply(func, std::tuple_cat(dispatch_get<Component>(it, entt)...));
                     }
                 }
-            });
+
+                ++it;
+            }
         }
     }
 
 public:
     /*! @brief Underlying entity identifier. */
-    using entity_type = typename sparse_set<Entity>::entity_type;
+    using entity_type = Entity;
     /*! @brief Unsigned integer type. */
-    using size_type = typename sparse_set<Entity>::size_type;
-    /*! @brief Input iterator type. */
-    using iterator_type = iterator;
-
-    /**
-     * @brief Returns the number of existing components of the given type.
-     * @tparam Comp Type of component of which to return the size.
-     * @return Number of existing components of the given type.
-     */
-    template<typename Comp>
-    size_type size() const ENTT_NOEXCEPT {
-        return std::get<pool_type<Comp> *>(pools)->size();
-    }
-
-    /**
-     * @brief Estimates the number of entities that have the given components.
-     * @return Estimated number of entities that have the given components.
-     */
-    size_type size() const ENTT_NOEXCEPT {
-        return std::min({ std::get<pool_type<Component> *>(pools)->size()... });
-    }
-
-    /**
-     * @brief Checks whether the pool of a given component is empty.
-     * @tparam Comp Type of component in which one is interested.
-     * @return True if the pool of the given component is empty, false
-     * otherwise.
-     */
-    template<typename Comp>
-    bool empty() const ENTT_NOEXCEPT {
-        return std::get<pool_type<Comp> *>(pools)->empty();
-    }
-
-    /**
-     * @brief Checks if the view is definitely empty.
-     * @return True if the view is definitely empty, false otherwise.
-     */
-    bool empty() const ENTT_NOEXCEPT {
-        return (std::get<pool_type<Component> *>(pools)->empty() || ...);
-    }
-
-    /**
-     * @brief Direct access to the list of components of a given pool.
-     *
-     * The returned pointer is such that range
-     * `[raw<Comp>(), raw<Comp>() + size<Comp>()]` is always a valid range, even
-     * if the container is empty.
-     *
-     * @note
-     * There are no guarantees on the order of the components. Use `begin` and
-     * `end` if you want to iterate the view in the expected order.
-     *
-     * @tparam Comp Type of component in which one is interested.
-     * @return A pointer to the array of components.
+    using size_type = std::size_t;
+    /*! @brief Bidirectional iterator type. */
+    using iterator = internal::view_iterator<Policy, typename basic_common_type::iterator, sizeof...(Component) - 1u, sizeof...(Exclude)>;
+    /*! @brief Reverse iterator type. */
+    using reverse_iterator = internal::view_iterator<Policy, typename basic_common_type::reverse_iterator, sizeof...(Component) - 1u, sizeof...(Exclude)>;
+    /*! @brief Iterable view type. */
+    using iterable_view = iterable;
+
+    /*! @brief Default constructor to use to create empty, invalid views. */
+    basic_view_impl() ENTT_NOEXCEPT
+        : view{}
+    {}
+
+    /**
+     * @brief Constructs a multi-type view from a set of storage classes.
+     * @param component The storage for the types to iterate.
+     * @param epool The storage for the types used to filter the view.
+     */
+    basic_view_impl(storage_type<Component> &... component, const storage_type<Exclude> &... epool) ENTT_NOEXCEPT
+        : pools{&component...},
+          filter{&epool...},
+          view{candidate()}
+    {}
+
+    /**
+     * @brief Forces the type to use to drive iterations.
+     * @tparam Comp Type of component to use to drive the iteration.
      */
     template<typename Comp>
-    Comp * raw() const ENTT_NOEXCEPT {
-        return std::get<pool_type<Comp> *>(pools)->raw();
-    }
-
-    /**
-     * @brief Direct access to the list of entities of a given pool.
-     *
-     * The returned pointer is such that range
-     * `[data<Comp>(), data<Comp>() + size<Comp>()]` is always a valid range,
-     * even if the container is empty.
-     *
-     * @note
-     * There are no guarantees on the order of the entities. Use `begin` and
-     * `end` if you want to iterate the view in the expected order.
-     *
-     * @tparam Comp Type of component in which one is interested.
-     * @return A pointer to the array of entities.
-     */
-    template<typename Comp>
-    const entity_type * data() const ENTT_NOEXCEPT {
-        return std::get<pool_type<Comp> *>(pools)->data();
-    }
-
-    /**
-     * @brief Returns an iterator to the first entity that has the given
-     * components.
-     *
-     * The returned iterator points to the first entity that has the given
-     * components. If the view is empty, the returned iterator will be equal to
-     * `end()`.
-     *
-     * @note
-     * Input iterators stay true to the order imposed to the underlying data
-     * structures.
-     *
-     * @return An iterator to the first entity that has the given components.
-     */
-    iterator_type begin() const ENTT_NOEXCEPT {
-        const auto *view = candidate();
-        return iterator_type{unchecked(view), view->begin(), view->end()};
-    }
-
-    /**
-     * @brief Returns an iterator that is past the last entity that has the
-     * given components.
-     *
-     * The returned iterator points to the entity following the last entity that
-     * has the given components. Attempting to dereference the returned iterator
+    void use() const ENTT_NOEXCEPT {
+        view = std::get<storage_type<Comp> *>(pools);
+    }
+
+    /**
+     * @brief Estimates the number of entities iterated by the view.
+     * @return Estimated number of entities iterated by the view.
+     */
+    [[nodiscard]] size_type size_hint() const ENTT_NOEXCEPT {
+        return view->size();
+    }
+
+    /**
+     * @brief Returns an iterator to the first entity of the view.
+     *
+     * The returned iterator points to the first entity of the view. If the view
+     * is empty, the returned iterator will be equal to `end()`.
+     *
+     * @return An iterator to the first entity of the view.
+     */
+    [[nodiscard]] iterator begin() const {
+        return iterator{view->begin(), view->end(), view->begin(), pools_to_unchecked_array(), filter_to_array()};
+    }
+
+    /**
+     * @brief Returns an iterator that is past the last entity of the view.
+     *
+     * The returned iterator points to the entity following the last entity of
+     * the view. Attempting to dereference the returned iterator results in
+     * undefined behavior.
+     *
+     * @return An iterator to the entity following the last entity of the view.
+     */
+    [[nodiscard]] iterator end() const {
+        return iterator{view->begin(), view->end(), view->end(), pools_to_unchecked_array(), filter_to_array()};
+    }
+
+    /**
+     * @brief Returns an iterator to the first entity of the reversed view.
+     *
+     * The returned iterator points to the first entity of the reversed view. If
+     * the view is empty, the returned iterator will be equal to `rend()`.
+     *
+     * @return An iterator to the first entity of the reversed view.
+     */
+    [[nodiscard]] reverse_iterator rbegin() const {
+        return reverse_iterator{view->rbegin(), view->rend(), view->rbegin(), pools_to_unchecked_array(), filter_to_array()};
+    }
+
+    /**
+     * @brief Returns an iterator that is past the last entity of the reversed
+     * view.
+     *
+     * The returned iterator points to the entity following the last entity of
+     * the reversed view. Attempting to dereference the returned iterator
      * results in undefined behavior.
      *
-     * @note
-     * Input iterators stay true to the order imposed to the underlying data
-     * structures.
-     *
-     * @return An iterator to the entity following the last entity that has the
-     * given components.
-     */
-    iterator_type end() const ENTT_NOEXCEPT {
-        const auto *view = candidate();
-        return iterator_type{unchecked(view), view->end(), view->end()};
+     * @return An iterator to the entity following the last entity of the
+     * reversed view.
+     */
+    [[nodiscard]] reverse_iterator rend() const {
+        return reverse_iterator{view->rbegin(), view->rend(), view->rend(), pools_to_unchecked_array(), filter_to_array()};
+    }
+
+    /**
+     * @brief Returns the first entity of the view, if any.
+     * @return The first entity of the view if one exists, the null entity
+     * otherwise.
+     */
+    [[nodiscard]] entity_type front() const {
+        const auto it = begin();
+        return it != end() ? *it : null;
+    }
+
+    /**
+     * @brief Returns the last entity of the view, if any.
+     * @return The last entity of the view if one exists, the null entity
+     * otherwise.
+     */
+    [[nodiscard]] entity_type back() const {
+        const auto it = rbegin();
+        return it != rend() ? *it : null;
     }
 
     /**
@@ -6733,138 +27555,54 @@
      * @return An iterator to the given entity if it's found, past the end
      * iterator otherwise.
      */
-    iterator_type find(const entity_type entt) const ENTT_NOEXCEPT {
-        const auto *view = candidate();
-        iterator_type it{unchecked(view), view->find(entt), view->end()};
+    [[nodiscard]] iterator find(const entity_type entt) const {
+        const auto it = iterator{view->begin(), view->end(), view->find(entt), pools_to_unchecked_array(), filter_to_array()};
         return (it != end() && *it == entt) ? it : end();
     }
 
     /**
+     * @brief Checks if a view is properly initialized.
+     * @return True if the view is properly initialized, false otherwise.
+     */
+    [[nodiscard]] explicit operator bool() const ENTT_NOEXCEPT {
+        return view != nullptr;
+    }
+
+    /**
      * @brief Checks if a view contains an entity.
      * @param entt A valid entity identifier.
      * @return True if the view contains the given entity, false otherwise.
      */
-    bool contains(const entity_type entt) const ENTT_NOEXCEPT {
-        return find(entt) != end();
+    [[nodiscard]] bool contains(const entity_type entt) const {
+        return (std::get<storage_type<Component> *>(pools)->contains(entt) && ...) && (!std::get<const storage_type<Exclude> *>(filter)->contains(entt) && ...);
     }
 
     /**
      * @brief Returns the components assigned to the given entity.
      *
      * Prefer this function instead of `registry::get` during iterations. It has
-     * far better performance than its companion function.
+     * far better performance than its counterpart.
      *
      * @warning
      * Attempting to use an invalid component type results in a compilation
      * error. Attempting to use an entity that doesn't belong to the view
-     * results in undefined behavior.<br/>
-     * An assertion will abort the execution at runtime in debug mode if the
-     * view doesn't contain the given entity.
+     * results in undefined behavior.
      *
      * @tparam Comp Types of components to get.
      * @param entt A valid entity identifier.
      * @return The components assigned to the entity.
      */
     template<typename... Comp>
-    decltype(auto) get([[maybe_unused]] const entity_type entt) const ENTT_NOEXCEPT {
-        ENTT_ASSERT(contains(entt));
-
-        if constexpr(sizeof...(Comp) == 1) {
-            return (std::get<pool_type<Comp> *>(pools)->get(entt), ...);
-        } else {
-            return std::tuple<decltype(get<Comp>(entt))...>{get<Comp>(entt)...};
-        }
-    }
-
-    /**
-     * @brief Iterates entities and components and applies the given function
-     * object to them.
-     *
-     * The function object is invoked for each entity. It is provided with the
-     * entity itself and a set of references to all its components. The
-     * _constness_ of the components is as requested.<br/>
-     * The signature of the function must be equivalent to one of the following
-     * forms:
-     *
-     * @code{.cpp}
-     * void(const entity_type, Component &...);
-     * void(Component &...);
-     * @endcode
-     *
-     * @note
-     * Empty types aren't explicitly instantiated. Therefore, temporary objects
-     * are returned during iterations. They can be caught only by copy or with
-     * const references.
-     *
-     * @tparam Func Type of the function object to invoke.
-     * @param func A valid function object.
-     */
-    template<typename Func>
-    void each(Func func) const {
-        const auto *view = candidate();
-        ((std::get<pool_type<Component> *>(pools) == view ? each<Component>(std::move(func)) : void()), ...);
-    }
-
-    /**
-     * @brief Iterates entities and components and applies the given function
-     * object to them.
-     *
-     * The function object is invoked for each entity. It is provided with the
-     * entity itself and a set of references to all its components. The
-     * _constness_ of the components is as requested.<br/>
-     * The signature of the function must be equivalent to one of the following
-     * forms:
-     *
-     * @code{.cpp}
-     * void(const entity_type, Component &...);
-     * void(Component &...);
-     * @endcode
-     *
-     * The pool of the suggested component is used to lead the iterations. The
-     * returned entities will therefore respect the order of the pool associated
-     * with that type.<br/>
-     * It is no longer guaranteed that the performance is the best possible, but
-     * there will be greater control over the order of iteration.
-     *
-     * @note
-     * Empty types aren't explicitly instantiated. Therefore, temporary objects
-     * are returned during iterations. They can be caught only by copy or with
-     * const references.
-     *
-     * @tparam Comp Type of component to use to enforce the iteration order.
-     * @tparam Func Type of the function object to invoke.
-     * @param func A valid function object.
-     */
-    template<typename Comp, typename Func>
-    void each(Func func) const {
-        using other_type = type_list_cat_t<std::conditional_t<std::is_same_v<Comp, Component>, type_list<>, type_list<Component>>...>;
-        traverse<Comp>(std::move(func), other_type{}, type_list<Component...>{});
-    }
-
-    /**
-     * @brief Iterates entities and components and applies the given function
-     * object to them.
-     *
-     * The function object is invoked for each entity. It is provided with the
-     * entity itself and a set of references to non-empty components. The
-     * _constness_ of the components is as requested.<br/>
-     * The signature of the function must be equivalent to one of the following
-     * forms:
-     *
-     * @code{.cpp}
-     * void(const entity_type, Type &...);
-     * void(Type &...);
-     * @endcode
-     *
-     * @sa each
-     *
-     * @tparam Func Type of the function object to invoke.
-     * @param func A valid function object.
-     */
-    template<typename Func>
-    void less(Func func) const {
-        const auto *view = candidate();
-        ((std::get<pool_type<Component> *>(pools) == view ? less<Component>(std::move(func)) : void()), ...);
+    [[nodiscard]] decltype(auto) get([[maybe_unused]] const entity_type entt) const {
+        ENTT_ASSERT(contains(entt), "View does not contain entity");
+
+        if constexpr(sizeof...(Comp) == 0) {
+            return std::tuple_cat(get_as_tuple(*std::get<storage_type<Component> *>(pools), entt)...);
+        } else if constexpr(sizeof...(Comp) == 1) {
+            return (std::get<storage_type<Comp> *>(pools)->get(entt), ...);
+        } else {
+            return std::tuple_cat(get_as_tuple(*std::get<storage_type<Comp> *>(pools), entt)...);
+        }
     }
 
     /**
@@ -6882,27 +27620,89 @@
      * void(Type &...);
      * @endcode
      *
+     * @note
+     * Empty types aren't explicitly instantiated and therefore they are never
+     * returned during iterations.
+     *
+     * @tparam Func Type of the function object to invoke.
+     * @param func A valid function object.
+     */
+    template<typename Func>
+    void each(Func func) const {
+        ((std::get<storage_type<Component> *>(pools) == view ? traverse<Component>(std::move(func)) : void()), ...);
+    }
+
+    /**
+     * @brief Iterates entities and components and applies the given function
+     * object to them.
+     *
      * The pool of the suggested component is used to lead the iterations. The
      * returned entities will therefore respect the order of the pool associated
-     * with that type.<br/>
-     * It is no longer guaranteed that the performance is the best possible, but
-     * there will be greater control over the order of iteration.
+     * with that type.
      *
      * @sa each
      *
-     * @tparam Comp Type of component to use to enforce the iteration order.
+     * @tparam Comp Type of component to use to drive the iteration.
      * @tparam Func Type of the function object to invoke.
      * @param func A valid function object.
      */
     template<typename Comp, typename Func>
-    void less(Func func) const {
-        using other_type = type_list_cat_t<std::conditional_t<std::is_same_v<Comp, Component>, type_list<>, type_list<Component>>...>;
-        using non_empty_type = type_list_cat_t<std::conditional_t<std::is_empty_v<Component>, type_list<>, type_list<Component>>...>;
-        traverse<Comp>(std::move(func), other_type{}, non_empty_type{});
-    }
-
-private:
-    const std::tuple<pool_type<Component> *...> pools;
+    void each(Func func) const {
+        use<Comp>();
+        traverse<Comp>(std::move(func));
+    }
+
+    /**
+     * @brief Returns an iterable object to use to _visit_ the view.
+     *
+     * The iterable object returns tuples that contain the current entity and a
+     * set of references to its non-empty components. The _constness_ of the
+     * components is as requested.
+     *
+     * @note
+     * Empty types aren't explicitly instantiated and therefore they are never
+     * returned during iterations.
+     *
+     * @return An iterable object to use to _visit_ the view.
+     */
+    [[nodiscard]] iterable_view each() const ENTT_NOEXCEPT {
+        return iterable_view{*this};
+    }
+
+    /**
+     * @brief Returns an iterable object to use to _visit_ the view.
+     *
+     * The pool of the suggested component is used to lead the iterations. The
+     * returned elements will therefore respect the order of the pool associated
+     * with that type.
+     *
+     * @sa each
+     *
+     * @tparam Comp Type of component to use to drive the iteration.
+     * @return An iterable object to use to _visit_ the view.
+     */
+    template<typename Comp>
+    [[nodiscard]] iterable_view each() const ENTT_NOEXCEPT {
+        use<Comp>();
+        return iterable_view{*this};
+    }
+
+    /**
+     * @brief Combines two views in a _more specific_ one (friend function).
+     * @tparam Id A valid entity type (see entt_traits for more details).
+     * @tparam ELhs Filter list of the first view.
+     * @tparam CLhs Component list of the first view.
+     * @tparam ERhs Filter list of the second view.
+     * @tparam CRhs Component list of the second view.
+     * @return A more specific view.
+     */
+    template<typename Id, typename... ELhs, typename... CLhs, typename... ERhs, typename... CRhs>
+    friend auto operator|(const basic_view<Id, exclude_t<ELhs...>, CLhs...> &, const basic_view<Id, exclude_t<ERhs...>, CRhs...> &);
+
+private:
+    const std::tuple<storage_type<Component> *...> pools;
+    const std::tuple<const storage_type<Exclude> *...> filter;
+    mutable const basic_common_type *view;
 };
 
 
@@ -6911,10 +27711,7 @@
  *
  * Single component views are specialized in order to get a boost in terms of
  * performance. This kind of views can access the underlying data structure
- * directly and avoid superfluous checks.<br/>
- * Order of elements during iterations are highly dependent on the order of the
- * underlying data structure. See sparse_set and its specializations for more
- * details.
+ * directly and avoid superfluous checks.
  *
  * @b Important
  *
@@ -6925,7 +27722,7 @@
  *   component is removed from the entity to which the iterator points).
  * * The entity currently pointed is destroyed.
  *
- * In all the other cases, modifying the pool of the given component in any way
+ * In all other cases, modifying the pool iterated by the view in any way
  * invalidates all the iterators and using them results in undefined behavior.
  *
  * @note
@@ -6934,116 +27731,225 @@
  * made by means of the registry are immediately reflected by views.
  *
  * @warning
- * Lifetime of a view must overcome the one of the registry that generated it.
+ * Lifetime of a view must not overcome that of the registry that generated it.
  * In any other case, attempting to use a view results in undefined behavior.
  *
  * @tparam Entity A valid entity type (see entt_traits for more details).
  * @tparam Component Type of component iterated by the view.
  */
 template<typename Entity, typename Component>
-class basic_view<Entity, Component> {
-    /*! @brief A registry is allowed to create views. */
-    friend class basic_registry<Entity>;
-
-    using pool_type = std::conditional_t<std::is_const_v<Component>, const storage<Entity, std::remove_const_t<Component>>, storage<Entity, Component>>;
-
-    basic_view(pool_type *ref) ENTT_NOEXCEPT
-        : pool{ref}
-    {}
-
-public:
-    /*! @brief Type of component iterated by the view. */
-    using raw_type = Component;
+class basic_view_impl<packed_storage_policy, Entity, exclude_t<>, Component> {
+    using basic_common_type = basic_sparse_set<Entity>;
+    using storage_type = constness_as_t<typename storage_traits<Entity, std::remove_const_t<Component>>::storage_type, Component>;
+
+    class iterable final {
+        template<typename... It>
+        struct iterable_iterator final {
+            using difference_type = std::ptrdiff_t;
+            using value_type = decltype(std::tuple_cat(std::tuple<Entity>{}, std::declval<basic_view_impl>().get({})));
+            using pointer = void;
+            using reference = value_type;
+            using iterator_category = std::input_iterator_tag;
+
+            template<typename... Discard>
+            iterable_iterator(It... from, Discard...) ENTT_NOEXCEPT
+                : it{from...}
+            {}
+
+            iterable_iterator & operator++() ENTT_NOEXCEPT {
+                return (++std::get<It>(it), ...), *this;
+            }
+
+            iterable_iterator operator++(int) ENTT_NOEXCEPT {
+                iterable_iterator orig = *this;
+                return ++(*this), orig;
+            }
+
+            [[nodiscard]] reference operator*() const ENTT_NOEXCEPT {
+                return { *std::get<It>(it)... };
+            }
+
+            [[nodiscard]] bool operator==(const iterable_iterator &other) const ENTT_NOEXCEPT {
+                return std::get<0>(other.it) == std::get<0>(it);
+            }
+
+            [[nodiscard]] bool operator!=(const iterable_iterator &other) const ENTT_NOEXCEPT {
+                return !(*this == other);
+            }
+
+        private:
+            std::tuple<It...> it;
+        };
+
+    public:
+        using iterator = std::conditional_t<
+            std::is_void_v<decltype(std::declval<storage_type>().get({}))>,
+            iterable_iterator<typename basic_common_type::iterator>,
+            iterable_iterator<typename basic_common_type::iterator, decltype(std::declval<storage_type>().begin())>
+        >;
+        using reverse_iterator = std::conditional_t<
+            std::is_void_v<decltype(std::declval<storage_type>().get({}))>,
+            iterable_iterator<typename basic_common_type::reverse_iterator>,
+            iterable_iterator<typename basic_common_type::reverse_iterator, decltype(std::declval<storage_type>().rbegin())>
+        >;
+
+        iterable(storage_type &ref)
+            : pool{&ref}
+        {}
+
+        [[nodiscard]] iterator begin() const ENTT_NOEXCEPT {
+            return iterator{pool->basic_common_type::begin(), pool->begin()};
+        }
+
+        [[nodiscard]] iterator end() const ENTT_NOEXCEPT {
+            return iterator{pool->basic_common_type::end(), pool->end()};
+        }
+
+        [[nodiscard]] reverse_iterator rbegin() const ENTT_NOEXCEPT {
+            return reverse_iterator{pool->basic_common_type::rbegin(), pool->rbegin()};
+        }
+
+        [[nodiscard]] reverse_iterator rend() const ENTT_NOEXCEPT {
+            return reverse_iterator{pool->basic_common_type::rend(), pool->rend()};
+        }
+
+    private:
+        storage_type * const pool;
+    };
+
+public:
     /*! @brief Underlying entity identifier. */
-    using entity_type = typename pool_type::entity_type;
+    using entity_type = Entity;
     /*! @brief Unsigned integer type. */
-    using size_type = typename pool_type::size_type;
-    /*! @brief Input iterator type. */
-    using iterator_type = typename sparse_set<Entity>::iterator_type;
+    using size_type = std::size_t;
+    /*! @brief Random access iterator type. */
+    using iterator = typename basic_common_type::iterator;
+    /*! @brief Reversed iterator type. */
+    using reverse_iterator = typename basic_common_type::reverse_iterator;
+    /*! @brief Iterable view type. */
+    using iterable_view = iterable;
+
+    /*! @brief Default constructor to use to create empty, invalid views. */
+    basic_view_impl() ENTT_NOEXCEPT
+        : pools{},
+          filter{}
+    {}
+
+    /**
+     * @brief Constructs a single-type view from a storage class.
+     * @param ref The storage for the type to iterate.
+     */
+    basic_view_impl(storage_type &ref) ENTT_NOEXCEPT
+        : pools{&ref},
+          filter{}
+    {}
 
     /**
      * @brief Returns the number of entities that have the given component.
      * @return Number of entities that have the given component.
      */
-    size_type size() const ENTT_NOEXCEPT {
-        return pool->size();
-    }
-
-    /**
-     * @brief Checks whether the view is empty.
+    [[nodiscard]] size_type size() const ENTT_NOEXCEPT {
+        return std::get<0>(pools)->size();
+    }
+
+    /**
+     * @brief Checks whether a view is empty.
      * @return True if the view is empty, false otherwise.
      */
-    bool empty() const ENTT_NOEXCEPT {
-        return pool->empty();
-    }
-
-    /**
-     * @brief Direct access to the list of components.
-     *
-     * The returned pointer is such that range `[raw(), raw() + size()]` is
-     * always a valid range, even if the container is empty.
-     *
-     * @note
-     * There are no guarantees on the order of the components. Use `begin` and
-     * `end` if you want to iterate the view in the expected order.
-     *
+    [[nodiscard]] bool empty() const ENTT_NOEXCEPT {
+        return std::get<0>(pools)->empty();
+    }
+
+    /**
+     * @brief Direct access to the raw representation offered by the storage.
      * @return A pointer to the array of components.
      */
-    raw_type * raw() const ENTT_NOEXCEPT {
-        return pool->raw();
+    [[nodiscard]] auto raw() const ENTT_NOEXCEPT {
+        return std::get<0>(pools)->raw();
     }
 
     /**
      * @brief Direct access to the list of entities.
      *
-     * The returned pointer is such that range `[data(), data() + size()]` is
+     * The returned pointer is such that range `[data(), data() + size())` is
      * always a valid range, even if the container is empty.
      *
-     * @note
-     * There are no guarantees on the order of the entities. Use `begin` and
-     * `end` if you want to iterate the view in the expected order.
-     *
      * @return A pointer to the array of entities.
      */
-    const entity_type * data() const ENTT_NOEXCEPT {
-        return pool->data();
-    }
-
-    /**
-     * @brief Returns an iterator to the first entity that has the given
-     * component.
-     *
-     * The returned iterator points to the first entity that has the given
-     * component. If the view is empty, the returned iterator will be equal to
-     * `end()`.
-     *
-     * @note
-     * Input iterators stay true to the order imposed to the underlying data
-     * structures.
-     *
-     * @return An iterator to the first entity that has the given component.
-     */
-    iterator_type begin() const ENTT_NOEXCEPT {
-        return pool->sparse_set<Entity>::begin();
-    }
-
-    /**
-     * @brief Returns an iterator that is past the last entity that has the
-     * given component.
-     *
-     * The returned iterator points to the entity following the last entity that
-     * has the given component. Attempting to dereference the returned iterator
+    [[nodiscard]] auto data() const ENTT_NOEXCEPT {
+        return std::get<0>(pools)->data();
+    }
+
+    /**
+     * @brief Returns an iterator to the first entity of the view.
+     *
+     * The returned iterator points to the first entity of the view. If the view
+     * is empty, the returned iterator will be equal to `end()`.
+     *
+     * @return An iterator to the first entity of the view.
+     */
+    [[nodiscard]] iterator begin() const ENTT_NOEXCEPT {
+        return std::get<0>(pools)->basic_common_type::begin();
+    }
+
+    /**
+     * @brief Returns an iterator that is past the last entity of the view.
+     *
+     * The returned iterator points to the entity following the last entity of
+     * the view. Attempting to dereference the returned iterator results in
+     * undefined behavior.
+     *
+     * @return An iterator to the entity following the last entity of the view.
+     */
+    [[nodiscard]] iterator end() const ENTT_NOEXCEPT {
+        return std::get<0>(pools)->basic_common_type::end();
+    }
+
+    /**
+     * @brief Returns an iterator to the first entity of the reversed view.
+     *
+     * The returned iterator points to the first entity of the reversed view. If
+     * the view is empty, the returned iterator will be equal to `rend()`.
+     *
+     * @return An iterator to the first entity of the reversed view.
+     */
+    [[nodiscard]] reverse_iterator rbegin() const ENTT_NOEXCEPT {
+        return std::get<0>(pools)->basic_common_type::rbegin();
+    }
+
+    /**
+     * @brief Returns an iterator that is past the last entity of the reversed
+     * view.
+     *
+     * The returned iterator points to the entity following the last entity of
+     * the reversed view. Attempting to dereference the returned iterator
      * results in undefined behavior.
      *
-     * @note
-     * Input iterators stay true to the order imposed to the underlying data
-     * structures.
-     *
-     * @return An iterator to the entity following the last entity that has the
-     * given component.
-     */
-    iterator_type end() const ENTT_NOEXCEPT {
-        return pool->sparse_set<Entity>::end();
+     * @return An iterator to the entity following the last entity of the
+     * reversed view.
+     */
+    [[nodiscard]] reverse_iterator rend() const ENTT_NOEXCEPT {
+        return std::get<0>(pools)->basic_common_type::rend();
+    }
+
+    /**
+     * @brief Returns the first entity of the view, if any.
+     * @return The first entity of the view if one exists, the null entity
+     * otherwise.
+     */
+    [[nodiscard]] entity_type front() const {
+        const auto it = begin();
+        return it != end() ? *it : null;
+    }
+
+    /**
+     * @brief Returns the last entity of the view, if any.
+     * @return The last entity of the view if one exists, the null entity
+     * otherwise.
+     */
+    [[nodiscard]] entity_type back() const {
+        const auto it = rbegin();
+        return it != rend() ? *it : null;
     }
 
     /**
@@ -7052,8 +27958,8 @@
      * @return An iterator to the given entity if it's found, past the end
      * iterator otherwise.
      */
-    iterator_type find(const entity_type entt) const ENTT_NOEXCEPT {
-        const auto it = pool->find(entt);
+    [[nodiscard]] iterator find(const entity_type entt) const {
+        const auto it = std::get<0>(pools)->find(entt);
         return it != end() && *it == entt ? it : end();
     }
 
@@ -7062,37 +27968,52 @@
      * @param pos Position of the element to return.
      * @return The identifier that occupies the given position.
      */
-    entity_type operator[](const size_type pos) const ENTT_NOEXCEPT {
+    [[nodiscard]] entity_type operator[](const size_type pos) const {
         return begin()[pos];
     }
 
     /**
+     * @brief Checks if a view is properly initialized.
+     * @return True if the view is properly initialized, false otherwise.
+     */
+    [[nodiscard]] explicit operator bool() const ENTT_NOEXCEPT {
+        return std::get<0>(pools) != nullptr;
+    }
+
+    /**
      * @brief Checks if a view contains an entity.
      * @param entt A valid entity identifier.
      * @return True if the view contains the given entity, false otherwise.
      */
-    bool contains(const entity_type entt) const ENTT_NOEXCEPT {
-        return find(entt) != end();
+    [[nodiscard]] bool contains(const entity_type entt) const {
+        return std::get<0>(pools)->contains(entt);
     }
 
     /**
      * @brief Returns the component assigned to the given entity.
      *
      * Prefer this function instead of `registry::get` during iterations. It has
-     * far better performance than its companion function.
-     *
-     * @warning
-     * Attempting to use an entity that doesn't belong to the view results in
-     * undefined behavior.<br/>
-     * An assertion will abort the execution at runtime in debug mode if the
-     * view doesn't contain the given entity.
-     *
+     * far better performance than its counterpart.
+     *
+     * @warning
+     * Attempting to use an invalid component type results in a compilation
+     * error. Attempting to use an entity that doesn't belong to the view
+     * results in undefined behavior.
+     *
+     * @tparam Comp Types of components to get.
      * @param entt A valid entity identifier.
      * @return The component assigned to the entity.
      */
-    decltype(auto) get(const entity_type entt) const ENTT_NOEXCEPT {
-        ENTT_ASSERT(contains(entt));
-        return pool->get(entt);
+    template<typename... Comp>
+    [[nodiscard]] decltype(auto) get(const entity_type entt) const {
+        ENTT_ASSERT(contains(entt), "View does not contain entity");
+
+        if constexpr(sizeof...(Comp) == 0) {
+            return get_as_tuple(*std::get<0>(pools), entt);
+        } else {
+            static_assert(std::is_same_v<Comp..., Component>, "Invalid component type");
+            return std::get<0>(pools)->get(entt);
+        }
     }
 
     /**
@@ -7100,8 +28021,8 @@
      * object to them.
      *
      * The function object is invoked for each entity. It is provided with the
-     * entity itself and a reference to its component. The _constness_ of the
-     * component is as requested.<br/>
+     * entity itself and a reference to the component if it's a non-empty one.
+     * The _constness_ of the component is as requested.<br/>
      * The signature of the function must be equivalent to one of the following
      * forms:
      *
@@ -7111,2689 +28032,120 @@
      * @endcode
      *
      * @note
-     * Empty types aren't explicitly instantiated. Therefore, temporary objects
-     * are returned during iterations. They can be caught only by copy or with
-     * const references.
-     *
-     * @tparam Func Type of the function object to invoke.
-     * @param func A valid function object.
-     */
-    template<typename Func>
-    void each(Func func) const {
-        if constexpr(std::is_invocable_v<Func, decltype(get({}))>) {
-            std::for_each(pool->begin(), pool->end(), std::move(func));
-        } else {
-            std::for_each(pool->sparse_set<Entity>::begin(), pool->sparse_set<Entity>::end(), [&func, raw = pool->begin()](const auto entt) mutable {
-                func(entt, *(raw++));
-            });
-        }
-    }
-
-    /**
-     * @brief Iterates entities and components and applies the given function
-     * object to them.
-     *
-     * The function object is invoked for each entity. It is provided with the
-     * entity itself and a reference to its component if it's a non-empty one.
-     * The _constness_ of the component is as requested.<br/>
-     * The signature of the function must be equivalent to one of the following
-     * forms in case the component isn't an empty one:
-     *
-     * @code{.cpp}
-     * void(const entity_type, Component &);
-     * void(Component &);
-     * @endcode
-     *
-     * In case the component is an empty one instead, the following forms are
-     * accepted:
-     *
-     * @code{.cpp}
-     * void(const entity_type);
-     * void();
-     * @endcode
-     *
-     * @sa each
-     *
-     * @tparam Func Type of the function object to invoke.
-     * @param func A valid function object.
-     */
-    template<typename Func>
-    void less(Func func) const {
-        if constexpr(std::is_empty_v<Component>) {
-            if constexpr(std::is_invocable_v<Func>) {
-                for(auto pos = pool->size(); pos; --pos) {
-                    func();
-                }
-            } else {
-                std::for_each(pool->sparse_set<Entity>::begin(), pool->sparse_set<Entity>::end(), std::move(func));
-            }
-        } else {
-            each(std::move(func));
-        }
-    }
-
-private:
-    pool_type *pool;
-};
-
-
-}
-
-
-#endif // ENTT_ENTITY_VIEW_HPP
-
-// #include "fwd.hpp"
-
-
-
-namespace entt {
-
-
-/**
- * @brief Fast and reliable entity-component system.
- *
- * The registry is the core class of the entity-component framework.<br/>
- * It stores entities and arranges pools of components on a per request basis.
- * By means of a registry, users can manage entities and components and thus
- * create views or groups to iterate them.
- *
- * @tparam Entity A valid entity type (see entt_traits for more details).
- */
-template<typename Entity>
-class basic_registry {
-    using context_family = family<struct internal_registry_context_family>;
-    using component_family = family<struct internal_registry_component_family>;
-    using traits_type = entt_traits<std::underlying_type_t<Entity>>;
-
-    struct group_type {
-        std::size_t owned{};
-    };
-
-    template<typename Component>
-    struct pool_handler: storage<Entity, Component> {
-        group_type *group{};
-
-        pool_handler() ENTT_NOEXCEPT = default;
-
-        pool_handler(const storage<Entity, Component> &other)
-            : storage<Entity, Component>{other}
-        {}
-
-        auto on_construct() ENTT_NOEXCEPT {
-            return sink{construction};
-        }
-
-        auto on_replace() ENTT_NOEXCEPT {
-            return sink{update};
-        }
-
-        auto on_destroy() ENTT_NOEXCEPT {
-            return sink{destruction};
-        }
-
-        template<typename... Args>
-        decltype(auto) assign(basic_registry &registry, const Entity entt, Args &&... args) {
-            if constexpr(std::is_empty_v<Component>) {
-                storage<Entity, Component>::construct(entt);
-                construction.publish(entt, registry, Component{});
-                return Component{std::forward<Args>(args)...};
-            } else {
-                auto &component = storage<Entity, Component>::construct(entt, std::forward<Args>(args)...);
-                construction.publish(entt, registry, component);
-                return component;
-            }
-        }
-
-        template<typename It, typename... Comp>
-        auto batch(basic_registry &registry, It first, It last, const Comp &... value) {
-            auto it = storage<Entity, Component>::batch(first, last, value...);
-
-            if(!construction.empty()) {
-                std::for_each(first, last, [this, &registry, it](const auto entt) mutable {
-                    construction.publish(entt, registry, *(it++));
-                });
-            }
-
-            return it;
-        }
-
-        void remove(basic_registry &registry, const Entity entt) {
-            destruction.publish(entt, registry);
-            storage<Entity, Component>::destroy(entt);
-        }
-
-        template<typename... Args>
-        decltype(auto) replace(basic_registry &registry, const Entity entt, Args &&... args) {
-            if constexpr(std::is_empty_v<Component>) {
-                ENTT_ASSERT((storage<Entity, Component>::has(entt)));
-                update.publish(entt, registry, Component{});
-                return Component{std::forward<Args>(args)...};
-            } else {
-                Component component{std::forward<Args>(args)...};
-                update.publish(entt, registry, component);
-                return (storage<Entity, Component>::get(entt) = std::move(component));
-            }
-        }
-
-    private:
-        using reference_type = std::conditional_t<std::is_empty_v<Component>, const Component &, Component &>;
-        sigh<void(const Entity, basic_registry &, reference_type)> construction{};
-        sigh<void(const Entity, basic_registry &, reference_type)> update{};
-        sigh<void(const Entity, basic_registry &)> destruction{};
-    };
-
-    template<typename Component>
-    using pool_type = pool_handler<std::decay_t<Component>>;
-
-    template<typename...>
-    struct group_handler;
-
-    template<typename... Exclude, typename... Get>
-    struct group_handler<exclude_t<Exclude...>, get_t<Get...>>: sparse_set<Entity> {
-        std::tuple<pool_type<Get> *..., pool_type<Exclude> *...> cpools{};
-
-        template<typename Component>
-        void maybe_valid_if(const Entity entt) {
-            if constexpr(std::disjunction_v<std::is_same<Get, Component>...>) {
-                if(((std::is_same_v<Component, Get> || std::get<pool_type<Get> *>(cpools)->has(entt)) && ...)
-                        && !(std::get<pool_type<Exclude> *>(cpools)->has(entt) || ...))
-                {
-                    this->construct(entt);
-                }
-            } else if constexpr(std::disjunction_v<std::is_same<Exclude, Component>...>) {
-                if((std::get<pool_type<Get> *>(cpools)->has(entt) && ...)
-                        && ((std::is_same_v<Exclude, Component> || !std::get<pool_type<Exclude> *>(cpools)->has(entt)) && ...)) {
-                    this->construct(entt);
-                }
-            }
-        }
-
-        void discard_if(const Entity entt) {
-            if(this->has(entt)) {
-                this->destroy(entt);
-            }
-        }
-    };
-
-    template<typename... Exclude, typename... Get, typename... Owned>
-    struct group_handler<exclude_t<Exclude...>, get_t<Get...>, Owned...>: group_type {
-        std::tuple<pool_type<Owned> *..., pool_type<Get> *..., pool_type<Exclude> *...> cpools{};
-
-        template<typename Component>
-        void maybe_valid_if(const Entity entt) {
-            if constexpr(std::disjunction_v<std::is_same<Owned, Component>..., std::is_same<Get, Component>...>) {
-                if(((std::is_same_v<Component, Owned> || std::get<pool_type<Owned> *>(cpools)->has(entt)) && ...)
-                        && ((std::is_same_v<Component, Get> || std::get<pool_type<Get> *>(cpools)->has(entt)) && ...)
-                        && !(std::get<pool_type<Exclude> *>(cpools)->has(entt) || ...))
-                {
-                    const auto pos = this->owned++;
-                    (std::get<pool_type<Owned> *>(cpools)->swap(std::get<pool_type<Owned> *>(cpools)->index(entt), pos), ...);
-                }
-            } else if constexpr(std::disjunction_v<std::is_same<Exclude, Component>...>) {
-                if((std::get<pool_type<Owned> *>(cpools)->has(entt) && ...)
-                        && (std::get<pool_type<Get> *>(cpools)->has(entt) && ...)
-                        && ((std::is_same_v<Exclude, Component> || !std::get<pool_type<Exclude> *>(cpools)->has(entt)) && ...))
-                {
-                    const auto pos = this->owned++;
-                    (std::get<pool_type<Owned> *>(cpools)->swap(std::get<pool_type<Owned> *>(cpools)->index(entt), pos), ...);
-                }
-            }
-        }
-
-        void discard_if(const Entity entt) {
-            if(std::get<0>(cpools)->has(entt) && std::get<0>(cpools)->index(entt) < this->owned) {
-                const auto pos = --this->owned;
-                (std::get<pool_type<Owned> *>(cpools)->swap(std::get<pool_type<Owned> *>(cpools)->index(entt), pos), ...);
-            }
-        }
-    };
-
-    struct pool_data {
-        std::unique_ptr<sparse_set<Entity>> pool;
-        void(* remove)(sparse_set<Entity> &, basic_registry &, const Entity);
-        std::unique_ptr<sparse_set<Entity>>(* clone)(const sparse_set<Entity> &);
-        void(* stomp)(const sparse_set<Entity> &, const Entity, basic_registry &, const Entity);
-        ENTT_ID_TYPE runtime_type;
-    };
-
-    struct group_data {
-        const std::size_t extent[3];
-        std::unique_ptr<void, void(*)(void *)> group;
-        bool(* const is_same)(const component *);
-    };
-
-    struct ctx_variable {
-        std::unique_ptr<void, void(*)(void *)> value;
-        ENTT_ID_TYPE runtime_type;
-    };
-
-    template<typename Type, typename Family>
-    static ENTT_ID_TYPE runtime_type() ENTT_NOEXCEPT {
-        if constexpr(is_named_type_v<Type>) {
-            return named_type_traits<Type>::value;
-        } else {
-            return Family::template type<Type>;
-        }
-    }
-
-    void release(const Entity entity) {
-        // lengthens the implicit list of destroyed entities
-        const auto entt = to_integer(entity) & traits_type::entity_mask;
-        const auto version = ((to_integer(entity) >> traits_type::entity_shift) + 1) << traits_type::entity_shift;
-        const auto node = to_integer(destroyed) | version;
-        entities[entt] = Entity{node};
-        destroyed = Entity{entt};
-    }
-
-    template<typename Component>
-    const pool_type<Component> * pool() const ENTT_NOEXCEPT {
-        const auto ctype = to_integer(type<Component>());
-
-        if constexpr(is_named_type_v<Component>) {
-            const auto it = std::find_if(pools.begin()+skip_family_pools, pools.end(), [ctype](const auto &candidate) {
-                return candidate.runtime_type == ctype;
-            });
-
-            return it == pools.cend() ? nullptr : static_cast<const pool_type<Component> *>(it->pool.get());
-        } else {
-            return ctype < skip_family_pools ? static_cast<const pool_type<Component> *>(pools[ctype].pool.get()) : nullptr;
-        }
-    }
-
-    template<typename Component>
-    pool_type<Component> * pool() ENTT_NOEXCEPT {
-        return const_cast<pool_type<Component> *>(std::as_const(*this).template pool<Component>());
-    }
-
-    template<typename Component>
-    pool_type<Component> * assure() {
-        const auto ctype = to_integer(type<Component>());
-        pool_data *pdata = nullptr;
-
-        if constexpr(is_named_type_v<Component>) {
-            const auto it = std::find_if(pools.begin()+skip_family_pools, pools.end(), [ctype](const auto &candidate) {
-                return candidate.runtime_type == ctype;
-            });
-
-            pdata = (it == pools.cend() ? &pools.emplace_back() : &(*it));
-        } else {
-            if(!(ctype < skip_family_pools)) {
-                pools.reserve(pools.size()+ctype-skip_family_pools+1);
-
-                while(!(ctype < skip_family_pools)) {
-                    pools.emplace(pools.begin()+(skip_family_pools++), pool_data{});
-                }
-            }
-
-            pdata = &pools[ctype];
-        }
-
-        if(!pdata->pool) {
-            pdata->runtime_type = ctype;
-            pdata->pool = std::make_unique<pool_type<Component>>();
-
-            pdata->remove = [](sparse_set<Entity> &cpool, basic_registry &registry, const Entity entt) {
-                static_cast<pool_type<Component> &>(cpool).remove(registry, entt);
-            };
-
-            if constexpr(std::is_copy_constructible_v<std::decay_t<Component>>) {
-                pdata->clone = [](const sparse_set<Entity> &cpool) -> std::unique_ptr<sparse_set<Entity>> {
-                    return std::make_unique<pool_type<Component>>(static_cast<const pool_type<Component> &>(cpool));
-                };
-
-                pdata->stomp = [](const sparse_set<Entity> &cpool, const Entity from, basic_registry &other, const Entity to) {
-                    other.assign_or_replace<Component>(to, static_cast<const pool_type<Component> &>(cpool).get(from));
-                };
-            } else {
-                pdata->clone = nullptr;
-                pdata->stomp = nullptr;
-            }
-        }
-
-        return static_cast<pool_type<Component> *>(pdata->pool.get());
-    }
-
-public:
-    /*! @brief Underlying entity identifier. */
-    using entity_type = Entity;
-    /*! @brief Underlying version type. */
-    using version_type = typename traits_type::version_type;
-    /*! @brief Unsigned integer type. */
-    using size_type = typename sparse_set<Entity>::size_type;
-
-    /*! @brief Default constructor. */
-    basic_registry() ENTT_NOEXCEPT = default;
-
-    /*! @brief Default move constructor. */
-    basic_registry(basic_registry &&) = default;
-
-    /*! @brief Default move assignment operator. @return This registry. */
-    basic_registry & operator=(basic_registry &&) = default;
-
-    /**
-     * @brief Returns the opaque identifier of a component.
-     *
-     * The given component doesn't need to be necessarily in use.<br/>
-     * Do not use this functionality to generate numeric identifiers for types
-     * at runtime. They aren't guaranteed to be stable between different runs.
-     *
-     * @tparam Component Type of component to query.
-     * @return Runtime the opaque identifier of the given type of component.
-     */
-    template<typename Component>
-    static component type() ENTT_NOEXCEPT {
-        return component{runtime_type<Component, component_family>()};
-    }
-
-    /**
-     * @brief Returns the number of existing components of the given type.
-     * @tparam Component Type of component of which to return the size.
-     * @return Number of existing components of the given type.
-     */
-    template<typename Component>
-    size_type size() const ENTT_NOEXCEPT {
-        const auto *cpool = pool<Component>();
-        return cpool ? cpool->size() : size_type{};
-    }
-
-    /**
-     * @brief Returns the number of entities created so far.
-     * @return Number of entities created so far.
-     */
-    size_type size() const ENTT_NOEXCEPT {
-        return entities.size();
-    }
-
-    /**
-     * @brief Returns the number of entities still in use.
-     * @return Number of entities still in use.
-     */
-    size_type alive() const ENTT_NOEXCEPT {
-        auto curr = destroyed;
-        size_type cnt{};
-
-        while(curr != null) {
-            curr = entities[to_integer(curr) & traits_type::entity_mask];
-            ++cnt;
-        }
-
-        return entities.size() - cnt;
-    }
-
-    /**
-     * @brief Increases the capacity of the pool for the given component.
-     *
-     * If the new capacity is greater than the current capacity, new storage is
-     * allocated, otherwise the method does nothing.
-     *
-     * @tparam Component Type of component for which to reserve storage.
-     * @param cap Desired capacity.
-     */
-    template<typename Component>
-    void reserve(const size_type cap) {
-        assure<Component>()->reserve(cap);
-    }
-
-    /**
-     * @brief Increases the capacity of a registry in terms of entities.
-     *
-     * If the new capacity is greater than the current capacity, new storage is
-     * allocated, otherwise the method does nothing.
-     *
-     * @param cap Desired capacity.
-     */
-    void reserve(const size_type cap) {
-        entities.reserve(cap);
-    }
-
-    /**
-     * @brief Returns the capacity of the pool for the given component.
-     * @tparam Component Type of component in which one is interested.
-     * @return Capacity of the pool of the given component.
-     */
-    template<typename Component>
-    size_type capacity() const ENTT_NOEXCEPT {
-        const auto *cpool = pool<Component>();
-        return cpool ? cpool->capacity() : size_type{};
-    }
-
-    /**
-     * @brief Returns the number of entities that a registry has currently
-     * allocated space for.
-     * @return Capacity of the registry.
-     */
-    size_type capacity() const ENTT_NOEXCEPT {
-        return entities.capacity();
-    }
-
-    /**
-     * @brief Requests the removal of unused capacity for a given component.
-     * @tparam Component Type of component for which to reclaim unused capacity.
-     */
-    template<typename Component>
-    void shrink_to_fit() {
-        assure<Component>()->shrink_to_fit();
-    }
-
-    /**
-     * @brief Checks whether the pool of a given component is empty.
-     * @tparam Component Type of component in which one is interested.
-     * @return True if the pool of the given component is empty, false
-     * otherwise.
-     */
-    template<typename Component>
-    bool empty() const ENTT_NOEXCEPT {
-        const auto *cpool = pool<Component>();
-        return cpool ? cpool->empty() : true;
-    }
-
-    /**
-     * @brief Checks if there exists at least an entity still in use.
-     * @return True if at least an entity is still in use, false otherwise.
-     */
-    bool empty() const ENTT_NOEXCEPT {
-        return !alive();
-    }
-
-    /**
-     * @brief Direct access to the list of components of a given pool.
-     *
-     * The returned pointer is such that range
-     * `[raw<Component>(), raw<Component>() + size<Component>()]` is always a
-     * valid range, even if the container is empty.
-     *
-     * There are no guarantees on the order of the components. Use a view if you
-     * want to iterate entities and components in the expected order.
-     *
-     * @note
-     * Empty components aren't explicitly instantiated. Only one instance of the
-     * given type is created. Therefore, this function always returns a pointer
-     * to that instance.
-     *
-     * @tparam Component Type of component in which one is interested.
-     * @return A pointer to the array of components of the given type.
-     */
-    template<typename Component>
-    const Component * raw() const ENTT_NOEXCEPT {
-        const auto *cpool = pool<Component>();
-        return cpool ? cpool->raw() : nullptr;
-    }
-
-    /*! @copydoc raw */
-    template<typename Component>
-    Component * raw() ENTT_NOEXCEPT {
-        return const_cast<Component *>(std::as_const(*this).template raw<Component>());
-    }
-
-    /**
-     * @brief Direct access to the list of entities of a given pool.
-     *
-     * The returned pointer is such that range
-     * `[data<Component>(), data<Component>() + size<Component>()]` is always a
-     * valid range, even if the container is empty.
-     *
-     * There are no guarantees on the order of the entities. Use a view if you
-     * want to iterate entities and components in the expected order.
-     *
-     * @tparam Component Type of component in which one is interested.
-     * @return A pointer to the array of entities.
-     */
-    template<typename Component>
-    const entity_type * data() const ENTT_NOEXCEPT {
-        const auto *cpool = pool<Component>();
-        return cpool ? cpool->data() : nullptr;
-    }
-
-    /**
-     * @brief Checks if an entity identifier refers to a valid entity.
-     * @param entity An entity identifier, either valid or not.
-     * @return True if the identifier is valid, false otherwise.
-     */
-    bool valid(const entity_type entity) const ENTT_NOEXCEPT {
-        const auto pos = size_type(to_integer(entity) & traits_type::entity_mask);
-        return (pos < entities.size() && entities[pos] == entity);
-    }
-
-    /**
-     * @brief Returns the entity identifier without the version.
-     * @param entity An entity identifier, either valid or not.
-     * @return The entity identifier without the version.
-     */
-    static entity_type entity(const entity_type entity) ENTT_NOEXCEPT {
-        return entity_type{to_integer(entity) & traits_type::entity_mask};
-    }
-
-    /**
-     * @brief Returns the version stored along with an entity identifier.
-     * @param entity An entity identifier, either valid or not.
-     * @return The version stored along with the given entity identifier.
-     */
-    static version_type version(const entity_type entity) ENTT_NOEXCEPT {
-        return version_type(to_integer(entity) >> traits_type::entity_shift);
-    }
-
-    /**
-     * @brief Returns the actual version for an entity identifier.
-     *
-     * @warning
-     * Attempting to use an entity that doesn't belong to the registry results
-     * in undefined behavior. An entity belongs to the registry even if it has
-     * been previously destroyed and/or recycled.<br/>
-     * An assertion will abort the execution at runtime in debug mode if the
-     * registry doesn't own the given entity.
-     *
-     * @param entity A valid entity identifier.
-     * @return Actual version for the given entity identifier.
-     */
-    version_type current(const entity_type entity) const ENTT_NOEXCEPT {
-        const auto pos = size_type(to_integer(entity) & traits_type::entity_mask);
-        ENTT_ASSERT(pos < entities.size());
-        return version_type(to_integer(entities[pos]) >> traits_type::entity_shift);
-    }
-
-    /**
-     * @brief Creates a new entity and returns it.
-     *
-     * There are two kinds of entity identifiers:
-     *
-     * * Newly created ones in case no entities have been previously destroyed.
-     * * Recycled ones with updated versions.
-     *
-     * Users should not care about the type of the returned entity identifier.
-     * In case entity identifers are stored around, the `valid` member
-     * function can be used to know if they are still valid or the entity has
-     * been destroyed and potentially recycled.<br/>
-     * The returned entity has assigned the given components, if any.
-     *
-     * The components must be at least default constructible. A compilation
-     * error will occur otherwhise.
-     *
-     * @tparam Component Types of components to assign to the entity.
-     * @return A valid entity identifier if the component list is empty, a tuple
-     * containing the entity identifier and the references to the components
-     * just created otherwise.
-     */
-    template<typename... Component>
-    auto create() {
-        entity_type entities[1]{};
-
-        if constexpr(sizeof...(Component) == 0) {
-            create<Component...>(std::begin(entities), std::end(entities));
-            return entities[0];
-        } else {
-            auto it = create<Component...>(std::begin(entities), std::end(entities));
-            return std::tuple<entity_type, decltype(assign<Component>(entities[0]))...>{entities[0], *std::get<typename pool_type<Component>::iterator_type>(it)...};
-        }
-    }
-
-    /**
-     * @brief Assigns each element in a range an entity.
-     *
-     * @sa create
-     *
-     * The components must be at least move and default insertable. A
-     * compilation error will occur otherwhise.
-     *
-     * @tparam Component Types of components to assign to the entity.
-     * @tparam It Type of forward iterator.
-     * @param first An iterator to the first element of the range to generate.
-     * @param last An iterator past the last element of the range to generate.
-     * @return No return value if the component list is empty, a tuple
-     * containing the iterators to the lists of components just created and
-     * sorted the same of the entities otherwise.
-     */
-    template<typename... Component, typename It>
-    auto create(It first, It last) {
-        static_assert(std::is_convertible_v<entity_type, typename std::iterator_traits<It>::value_type>);
-
-        std::generate(first, last, [this]() {
-            entity_type curr;
-
-            if(destroyed == null) {
-                curr = entities.emplace_back(entity_type(entities.size()));
-                // traits_type::entity_mask is reserved to allow for null identifiers
-                ENTT_ASSERT(to_integer(curr) < traits_type::entity_mask);
-            } else {
-                const auto entt = to_integer(destroyed);
-                const auto version = to_integer(entities[entt]) & (traits_type::version_mask << traits_type::entity_shift);
-                destroyed = entity_type{to_integer(entities[entt]) & traits_type::entity_mask};
-                curr = entity_type{entt | version};
-                entities[entt] = curr;
-            }
-
-            return curr;
-        });
-
-        if constexpr(sizeof...(Component) > 0) {
-            return std::make_tuple(assure<Component>()->batch(*this, first, last)...);
-        }
-    }
-
-    /**
-     * @brief Creates a new entity from a prototype entity.
-     *
-     * @sa create
-     *
-     * @tparam Component Types of components to copy.
-     * @tparam Exclude Types of components not to be copied.
-     * @param src A valid entity identifier to be copied.
-     * @param other The registry that owns the source entity.
-     * @return A valid entity identifier.
-     */
-    template<typename... Component, typename... Exclude>
-    auto create(entity_type src, basic_registry &other, exclude_t<Exclude...> = {}) {
-        entity_type entities[1]{};
-        create<Component...>(std::begin(entities), std::end(entities), src, other, exclude<Exclude...>);
-        return entities[0];
-    }
-
-    /**
-     * @brief Assigns each element in a range an entity from a prototype entity.
-     *
-     * @sa create
-     *
-     * @tparam Component Types of components to copy.
-     * @tparam Exclude Types of components not to be copied.
-     * @param first An iterator to the first element of the range to generate.
-     * @param last An iterator past the last element of the range to generate.
-     * @param src A valid entity identifier to be copied.
-     * @param other The registry that owns the source entity.
-     */
-    template<typename... Component, typename It, typename... Exclude>
-    void create(It first, It last, entity_type src, basic_registry &other, exclude_t<Exclude...> = {}) {
-        create(first, last);
-
-        if constexpr(sizeof...(Component) == 0) {
-            stomp<Component...>(first, last, src, other, exclude<Exclude...>);
-        } else {
-            static_assert(sizeof...(Component) == 0 || sizeof...(Exclude) == 0);
-            (assure<Component>()->batch(*this, first, last, other.get<Component>(src)), ...);
-        }
-    }
-
-    /**
-     * @brief Destroys an entity and lets the registry recycle the identifier.
-     *
-     * When an entity is destroyed, its version is updated and the identifier
-     * can be recycled at any time. In case entity identifers are stored around,
-     * the `valid` member function can be used to know if they are still valid
-     * or the entity has been destroyed and potentially recycled.
-     *
-     * @warning
-     * In case there are listeners that observe the destruction of components
-     * and assign other components to the entity in their bodies, the result of
-     * invoking this function may not be as expected. In the worst case, it
-     * could lead to undefined behavior. An assertion will abort the execution
-     * at runtime in debug mode if a violation is detected.
-     *
-     * @warning
-     * Attempting to use an invalid entity results in undefined behavior.<br/>
-     * An assertion will abort the execution at runtime in debug mode in case of
-     * invalid entity.
-     *
-     * @param entity A valid entity identifier.
-     */
-    void destroy(const entity_type entity) {
-        ENTT_ASSERT(valid(entity));
-
-        for(auto pos = pools.size(); pos; --pos) {
-            if(auto &pdata = pools[pos-1]; pdata.pool && pdata.pool->has(entity)) {
-                pdata.remove(*pdata.pool, *this, entity);
-            }
-        };
-
-        // just a way to protect users from listeners that attach components
-        ENTT_ASSERT(orphan(entity));
-        release(entity);
-    }
-
-    /**
-     * @brief Destroys all the entities in a range.
-     *
-     * @sa destroy
-     *
-     * @tparam It Type of input iterator.
-     * @param first An iterator to the first element of the range to generate.
-     * @param last An iterator past the last element of the range to generate.
-     */
-    template<typename It>
-    void destroy(It first, It last) {
-        // useless this-> used to suppress a warning with clang
-        std::for_each(first, last, [this](const auto entity) { this->destroy(entity); });
-    }
-
-    /**
-     * @brief Assigns the given component to an entity.
-     *
-     * A new instance of the given component is created and initialized with the
-     * arguments provided (the component must have a proper constructor or be of
-     * aggregate type). Then the component is assigned to the given entity.
-     *
-     * @warning
-     * Attempting to use an invalid entity or to assign a component to an entity
-     * that already owns it results in undefined behavior.<br/>
-     * An assertion will abort the execution at runtime in debug mode in case of
-     * invalid entity or if the entity already owns an instance of the given
-     * component.
-     *
-     * @tparam Component Type of component to create.
-     * @tparam Args Types of arguments to use to construct the component.
-     * @param entity A valid entity identifier.
-     * @param args Parameters to use to initialize the component.
-     * @return A reference to the newly created component.
-     */
-    template<typename Component, typename... Args>
-    decltype(auto) assign(const entity_type entity, [[maybe_unused]] Args &&... args) {
-        ENTT_ASSERT(valid(entity));
-        return assure<Component>()->assign(*this, entity, std::forward<Args>(args)...);
-    }
-
-    /**
-     * @brief Removes the given component from an entity.
-     *
-     * @warning
-     * Attempting to use an invalid entity or to remove a component from an
-     * entity that doesn't own it results in undefined behavior.<br/>
-     * An assertion will abort the execution at runtime in debug mode in case of
-     * invalid entity or if the entity doesn't own an instance of the given
-     * component.
-     *
-     * @tparam Component Type of component to remove.
-     * @param entity A valid entity identifier.
-     */
-    template<typename Component>
-    void remove(const entity_type entity) {
-        ENTT_ASSERT(valid(entity));
-        pool<Component>()->remove(*this, entity);
-    }
-
-    /**
-     * @brief Checks if an entity has all the given components.
-     *
-     * @warning
-     * Attempting to use an invalid entity results in undefined behavior.<br/>
-     * An assertion will abort the execution at runtime in debug mode in case of
-     * invalid entity.
-     *
-     * @tparam Component Components for which to perform the check.
-     * @param entity A valid entity identifier.
-     * @return True if the entity has all the components, false otherwise.
-     */
-    template<typename... Component>
-    bool has(const entity_type entity) const ENTT_NOEXCEPT {
-        ENTT_ASSERT(valid(entity));
-        [[maybe_unused]] const auto cpools = std::make_tuple(pool<Component>()...);
-        return ((std::get<const pool_type<Component> *>(cpools) ? std::get<const pool_type<Component> *>(cpools)->has(entity) : false) && ...);
-    }
-
-    /**
-     * @brief Returns references to the given components for an entity.
-     *
-     * @warning
-     * Attempting to use an invalid entity or to get a component from an entity
-     * that doesn't own it results in undefined behavior.<br/>
-     * An assertion will abort the execution at runtime in debug mode in case of
-     * invalid entity or if the entity doesn't own an instance of the given
-     * component.
-     *
-     * @tparam Component Types of components to get.
-     * @param entity A valid entity identifier.
-     * @return References to the components owned by the entity.
-     */
-    template<typename... Component>
-    decltype(auto) get([[maybe_unused]] const entity_type entity) const {
-        ENTT_ASSERT(valid(entity));
-
-        if constexpr(sizeof...(Component) == 1) {
-            return (pool<Component>()->get(entity), ...);
-        } else {
-            return std::tuple<decltype(get<Component>(entity))...>{get<Component>(entity)...};
-        }
-    }
-
-    /*! @copydoc get */
-    template<typename... Component>
-    decltype(auto) get([[maybe_unused]] const entity_type entity) ENTT_NOEXCEPT {
-        ENTT_ASSERT(valid(entity));
-
-        if constexpr(sizeof...(Component) == 1) {
-            return (pool<Component>()->get(entity), ...);
-        } else {
-            return std::tuple<decltype(get<Component>(entity))...>{get<Component>(entity)...};
-        }
-    }
-
-    /**
-     * @brief Returns a reference to the given component for an entity.
-     *
-     * In case the entity doesn't own the component, the parameters provided are
-     * used to construct it.<br/>
-     * Equivalent to the following snippet (pseudocode):
-     *
-     * @code{.cpp}
-     * auto &component = registry.has<Component>(entity) ? registry.get<Component>(entity) : registry.assign<Component>(entity, args...);
-     * @endcode
-     *
-     * Prefer this function anyway because it has slightly better performance.
-     *
-     * @warning
-     * Attempting to use an invalid entity results in undefined behavior.<br/>
-     * An assertion will abort the execution at runtime in debug mode in case of
-     * invalid entity.
-     *
-     * @tparam Component Type of component to get.
-     * @tparam Args Types of arguments to use to construct the component.
-     * @param entity A valid entity identifier.
-     * @param args Parameters to use to initialize the component.
-     * @return Reference to the component owned by the entity.
-     */
-    template<typename Component, typename... Args>
-    decltype(auto) get_or_assign(const entity_type entity, Args &&... args) ENTT_NOEXCEPT {
-        ENTT_ASSERT(valid(entity));
-        auto *cpool = assure<Component>();
-        return cpool->has(entity) ? cpool->get(entity) : cpool->assign(*this, entity, std::forward<Args>(args)...);
-    }
-
-    /**
-     * @brief Returns pointers to the given components for an entity.
-     *
-     * @warning
-     * Attempting to use an invalid entity results in undefined behavior.<br/>
-     * An assertion will abort the execution at runtime in debug mode in case of
-     * invalid entity.
-     *
-     * @tparam Component Types of components to get.
-     * @param entity A valid entity identifier.
-     * @return Pointers to the components owned by the entity.
-     */
-    template<typename... Component>
-    auto try_get([[maybe_unused]] const entity_type entity) const ENTT_NOEXCEPT {
-        ENTT_ASSERT(valid(entity));
-
-        if constexpr(sizeof...(Component) == 1) {
-            const auto cpools = std::make_tuple(pool<Component>()...);
-            return ((std::get<const pool_type<Component> *>(cpools) ? std::get<const pool_type<Component> *>(cpools)->try_get(entity) : nullptr), ...);
-        } else {
-            return std::tuple<std::add_const_t<Component> *...>{try_get<Component>(entity)...};
-        }
-    }
-
-    /*! @copydoc try_get */
-    template<typename... Component>
-    auto try_get([[maybe_unused]] const entity_type entity) ENTT_NOEXCEPT {
-        if constexpr(sizeof...(Component) == 1) {
-            return (const_cast<Component *>(std::as_const(*this).template try_get<Component>(entity)), ...);
-        } else {
-            return std::tuple<Component *...>{try_get<Component>(entity)...};
-        }
-    }
-
-    /**
-     * @brief Replaces the given component for an entity.
-     *
-     * A new instance of the given component is created and initialized with the
-     * arguments provided (the component must have a proper constructor or be of
-     * aggregate type). Then the component is assigned to the given entity.
-     *
-     * @warning
-     * Attempting to use an invalid entity or to replace a component of an
-     * entity that doesn't own it results in undefined behavior.<br/>
-     * An assertion will abort the execution at runtime in debug mode in case of
-     * invalid entity or if the entity doesn't own an instance of the given
-     * component.
-     *
-     * @tparam Component Type of component to replace.
-     * @tparam Args Types of arguments to use to construct the component.
-     * @param entity A valid entity identifier.
-     * @param args Parameters to use to initialize the component.
-     * @return A reference to the newly created component.
-     */
-    template<typename Component, typename... Args>
-    decltype(auto) replace(const entity_type entity, Args &&... args) {
-        ENTT_ASSERT(valid(entity));
-        return pool<Component>()->replace(*this, entity, std::forward<Args>(args)...);
-    }
-
-    /**
-     * @brief Assigns or replaces the given component for an entity.
-     *
-     * Equivalent to the following snippet (pseudocode):
-     *
-     * @code{.cpp}
-     * auto &component = registry.has<Component>(entity) ? registry.replace<Component>(entity, args...) : registry.assign<Component>(entity, args...);
-     * @endcode
-     *
-     * Prefer this function anyway because it has slightly better performance.
-     *
-     * @warning
-     * Attempting to use an invalid entity results in undefined behavior.<br/>
-     * An assertion will abort the execution at runtime in debug mode in case of
-     * invalid entity.
-     *
-     * @tparam Component Type of component to assign or replace.
-     * @tparam Args Types of arguments to use to construct the component.
-     * @param entity A valid entity identifier.
-     * @param args Parameters to use to initialize the component.
-     * @return A reference to the newly created component.
-     */
-    template<typename Component, typename... Args>
-    decltype(auto) assign_or_replace(const entity_type entity, Args &&... args) {
-        ENTT_ASSERT(valid(entity));
-        auto *cpool = assure<Component>();
-        return cpool->has(entity) ? cpool->replace(*this, entity, std::forward<Args>(args)...) : cpool->assign(*this, entity, std::forward<Args>(args)...);
-    }
-
-    /**
-     * @brief Returns a sink object for the given component.
-     *
-     * A sink is an opaque object used to connect listeners to components.<br/>
-     * The sink returned by this function can be used to receive notifications
-     * whenever a new instance of the given component is created and assigned to
-     * an entity.
-     *
-     * The function type for a listener is equivalent to:
-     *
-     * @code{.cpp}
-     * void(Entity, registry<Entity> &, Component &);
-     * @endcode
-     *
-     * Listeners are invoked **after** the component has been assigned to the
-     * entity. The order of invocation of the listeners isn't guaranteed.
-     *
-     * @note
-     * Empty types aren't explicitly instantiated. Therefore, temporary objects
-     * are returned through signals. They can be caught only by copy or with
-     * const references.
-     *
-     * @sa sink
-     *
-     * @tparam Component Type of component of which to get the sink.
-     * @return A temporary sink object.
-     */
-    template<typename Component>
-    auto on_construct() ENTT_NOEXCEPT {
-        return assure<Component>()->on_construct();
-    }
-
-    /**
-     * @brief Returns a sink object for the given component.
-     *
-     * A sink is an opaque object used to connect listeners to components.<br/>
-     * The sink returned by this function can be used to receive notifications
-     * whenever an instance of the given component is explicitly replaced.
-     *
-     * The function type for a listener is equivalent to:
-     *
-     * @code{.cpp}
-     * void(Entity, registry<Entity> &, Component &);
-     * @endcode
-     *
-     * Listeners are invoked **before** the component has been replaced. The
-     * order of invocation of the listeners isn't guaranteed.
-     *
-     * @note
-     * Empty types aren't explicitly instantiated. Therefore, temporary objects
-     * are returned through signals. They can be caught only by copy or with
-     * const references.
-     *
-     * @sa sink
-     *
-     * @tparam Component Type of component of which to get the sink.
-     * @return A temporary sink object.
-     */
-    template<typename Component>
-    auto on_replace() ENTT_NOEXCEPT {
-        return assure<Component>()->on_replace();
-    }
-
-    /**
-     * @brief Returns a sink object for the given component.
-     *
-     * A sink is an opaque object used to connect listeners to components.<br/>
-     * The sink returned by this function can be used to receive notifications
-     * whenever an instance of the given component is removed from an entity and
-     * thus destroyed.
-     *
-     * The function type for a listener is equivalent to:
-     *
-     * @code{.cpp}
-     * void(Entity, registry<Entity> &);
-     * @endcode
-     *
-     * Listeners are invoked **before** the component has been removed from the
-     * entity. The order of invocation of the listeners isn't guaranteed.
-     *
-     * @note
-     * Empty types aren't explicitly instantiated. Therefore, temporary objects
-     * are returned through signals. They can be caught only by copy or with
-     * const references.
-     *
-     * @sa sink
-     *
-     * @tparam Component Type of component of which to get the sink.
-     * @return A temporary sink object.
-     */
-    template<typename Component>
-    auto on_destroy() ENTT_NOEXCEPT {
-        return assure<Component>()->on_destroy();
-    }
-
-    /**
-     * @brief Sorts the pool of entities for the given component.
-     *
-     * The order of the elements in a pool is highly affected by assignments
-     * of components to entities and deletions. Components are arranged to
-     * maximize the performance during iterations and users should not make any
-     * assumption on the order.<br/>
-     * This function can be used to impose an order to the elements in the pool
-     * of the given component. The order is kept valid until a component of the
-     * given type is assigned or removed from an entity.
-     *
-     * The comparison function object must return `true` if the first element
-     * is _less_ than the second one, `false` otherwise. The signature of the
-     * comparison function should be equivalent to one of the following:
-     *
-     * @code{.cpp}
-     * bool(const Entity, const Entity);
-     * bool(const Component &, const Component &);
-     * @endcode
-     *
-     * Moreover, the comparison function object shall induce a
-     * _strict weak ordering_ on the values.
-     *
-     * The sort function oject must offer a member function template
-     * `operator()` that accepts three arguments:
-     *
-     * * An iterator to the first element of the range to sort.
-     * * An iterator past the last element of the range to sort.
-     * * A comparison function to use to compare the elements.
-     *
-     * The comparison funtion object received by the sort function object hasn't
-     * necessarily the type of the one passed along with the other parameters to
-     * this member function.
-     *
-     * @warning
-     * Pools of components owned by a group are only partially sorted.<br/>
-     * In other words, only the elements that aren't part of the group are
-     * sorted by this function. Use the `sort` member function of a group to
-     * sort the other half of the pool.
-     *
-     * @tparam Component Type of components to sort.
-     * @tparam Compare Type of comparison function object.
-     * @tparam Sort Type of sort function object.
-     * @tparam Args Types of arguments to forward to the sort function object.
-     * @param compare A valid comparison function object.
-     * @param algo A valid sort function object.
-     * @param args Arguments to forward to the sort function object, if any.
-     */
-    template<typename Component, typename Compare, typename Sort = std_sort, typename... Args>
-    void sort(Compare compare, Sort algo = Sort{}, Args &&... args) {
-        if(auto *cpool = assure<Component>(); cpool->group) {
-            const auto last = cpool->end() - cpool->group->owned;
-            cpool->sort(cpool->begin(), last, std::move(compare), std::move(algo), std::forward<Args>(args)...);
-        } else {
-            cpool->sort(cpool->begin(), cpool->end(), std::move(compare), std::move(algo), std::forward<Args>(args)...);
-        }
-    }
-
-    /**
-     * @brief Sorts two pools of components in the same way.
-     *
-     * The order of the elements in a pool is highly affected by assignments
-     * of components to entities and deletions. Components are arranged to
-     * maximize the performance during iterations and users should not make any
-     * assumption on the order.
-     *
-     * It happens that different pools of components must be sorted the same way
-     * because of runtime and/or performance constraints. This function can be
-     * used to order a pool of components according to the order between the
-     * entities in another pool of components.
-     *
-     * @b How @b it @b works
-     *
-     * Being `A` and `B` the two sets where `B` is the master (the one the order
-     * of which rules) and `A` is the slave (the one to sort), after a call to
-     * this function an iterator for `A` will return the entities according to
-     * the following rules:
-     *
-     * * All the entities in `A` that are also in `B` are returned first
-     *   according to the order they have in `B`.
-     * * All the entities in `A` that are not in `B` are returned in no
-     *   particular order after all the other entities.
-     *
-     * Any subsequent change to `B` won't affect the order in `A`.
-     *
-     * @warning
-     * Pools of components owned by a group cannot be sorted this way.<br/>
-     * An assertion will abort the execution at runtime in debug mode in case
-     * the pool is owned by a group.
-     *
-     * @tparam To Type of components to sort.
-     * @tparam From Type of components to use to sort.
-     */
-    template<typename To, typename From>
-    void sort() {
-        ENTT_ASSERT(!owned<To>());
-        assure<To>()->respect(*assure<From>());
-    }
-
-    /**
-     * @brief Resets the given component for an entity.
-     *
-     * If the entity has an instance of the component, this function removes the
-     * component from the entity. Otherwise it does nothing.
-     *
-     * @warning
-     * Attempting to use an invalid entity results in undefined behavior.<br/>
-     * An assertion will abort the execution at runtime in debug mode in case of
-     * invalid entity.
-     *
-     * @tparam Component Type of component to reset.
-     * @param entity A valid entity identifier.
-     */
-    template<typename Component>
-    void reset(const entity_type entity) {
-        ENTT_ASSERT(valid(entity));
-
-        if(auto *cpool = assure<Component>(); cpool->has(entity)) {
-            cpool->remove(*this, entity);
-        }
-    }
-
-    /**
-     * @brief Resets the pool of the given component.
-     *
-     * For each entity that has an instance of the given component, the
-     * component itself is removed and thus destroyed.
-     *
-     * @tparam Component Type of component whose pool must be reset.
-     */
-    template<typename Component>
-    void reset() {
-        if(auto *cpool = assure<Component>(); cpool->on_destroy().empty()) {
-            // no group set, otherwise the signal wouldn't be empty
-            cpool->reset();
-        } else {
-            for(const auto entity: static_cast<const sparse_set<entity_type> &>(*cpool)) {
-                cpool->remove(*this, entity);
-            }
-        }
-    }
-
-    /**
-     * @brief Resets a whole registry.
-     *
-     * Destroys all the entities. After a call to `reset`, all the entities
-     * still in use are recycled with a new version number. In case entity
-     * identifers are stored around, the `valid` member function can be used
-     * to know if they are still valid.
-     */
-    void reset() {
-        each([this](const auto entity) {
-            // useless this-> used to suppress a warning with clang
-            this->destroy(entity);
-        });
-    }
-
-    /**
-     * @brief Iterates all the entities that are still in use.
-     *
-     * The function object is invoked for each entity that is still in use.<br/>
-     * The signature of the function should be equivalent to the following:
-     *
-     * @code{.cpp}
-     * void(const Entity);
-     * @endcode
-     *
-     * This function is fairly slow and should not be used frequently. However,
-     * it's useful for iterating all the entities still in use, regardless of
-     * their components.
+     * Empty types aren't explicitly instantiated and therefore they are never
+     * returned during iterations.
      *
      * @tparam Func Type of the function object to invoke.
      * @param func A valid function object.
      */
     template<typename Func>
     void each(Func func) const {
-        static_assert(std::is_invocable_v<Func, entity_type>);
-
-        if(destroyed == null) {
-            for(auto pos = entities.size(); pos; --pos) {
-                func(entities[pos-1]);
-            }
-        } else {
-            for(auto pos = entities.size(); pos; --pos) {
-                const auto curr = entity_type(pos - 1);
-                const auto entity = entities[to_integer(curr)];
-                const auto entt = entity_type{to_integer(entity) & traits_type::entity_mask};
-
-                if(curr == entt) {
+        if constexpr(std::is_void_v<decltype(std::get<0>(pools)->get({}))>) {
+            if constexpr(std::is_invocable_v<Func>) {
+                for(auto pos = size(); pos; --pos) {
+                    func();
+                }
+            } else {
+                for(auto entity: *this) {
                     func(entity);
                 }
             }
-        }
-    }
-
-    /**
-     * @brief Checks if an entity has components assigned.
-     * @param entity A valid entity identifier.
-     * @return True if the entity has no components assigned, false otherwise.
-     */
-    bool orphan(const entity_type entity) const {
-        ENTT_ASSERT(valid(entity));
-        bool orphan = true;
-
-        for(std::size_t pos{}, last = pools.size(); pos < last && orphan; ++pos) {
-            const auto &pdata = pools[pos];
-            orphan = !(pdata.pool && pdata.pool->has(entity));
-        }
-
-        return orphan;
-    }
-
-    /**
-     * @brief Iterates orphans and applies them the given function object.
-     *
-     * The function object is invoked for each entity that is still in use and
-     * has no components assigned.<br/>
-     * The signature of the function should be equivalent to the following:
-     *
-     * @code{.cpp}
-     * void(const Entity);
-     * @endcode
-     *
-     * This function can be very slow and should not be used frequently.
-     *
-     * @tparam Func Type of the function object to invoke.
-     * @param func A valid function object.
-     */
-    template<typename Func>
-    void orphans(Func func) const {
-        static_assert(std::is_invocable_v<Func, entity_type>);
-
-        each([this, &func](const auto entity) {
-            if(orphan(entity)) {
-                func(entity);
-            }
-        });
-    }
-
-    /**
-     * @brief Returns a view for the given components.
-     *
-     * This kind of objects are created on the fly and share with the registry
-     * its internal data structures.<br/>
-     * Feel free to discard a view after the use. Creating and destroying a view
-     * is an incredibly cheap operation because they do not require any type of
-     * initialization.<br/>
-     * As a rule of thumb, storing a view should never be an option.
-     *
-     * Views do their best to iterate the smallest set of candidate entities.
-     * In particular:
-     *
-     * * Single component views are incredibly fast and iterate a packed array
-     *   of entities, all of which has the given component.
-     * * Multi component views look at the number of entities available for each
-     *   component and pick up a reference to the smallest set of candidates to
-     *   test for the given components.
-     *
-     * Views in no way affect the functionalities of the registry nor those of
-     * the underlying pools.
-     *
-     * @note
-     * Multi component views are pretty fast. However their performance tend to
-     * degenerate when the number of components to iterate grows up and the most
-     * of the entities have all the given components.<br/>
-     * To get a performance boost, consider using a group instead.
-     *
-     * @tparam Component Type of components used to construct the view.
-     * @return A newly created view.
-     */
-    template<typename... Component>
-    entt::basic_view<Entity, Component...> view() {
-        return { assure<Component>()... };
-    }
-
-    /*! @copydoc view */
-    template<typename... Component>
-    entt::basic_view<Entity, Component...> view() const {
-        static_assert(std::conjunction_v<std::is_const<Component>...>);
-        return const_cast<basic_registry *>(this)->view<Component...>();
-    }
-
-    /**
-     * @brief Checks whether a given component belongs to a group.
-     * @tparam Component Type of component in which one is interested.
-     * @return True if the component belongs to a group, false otherwise.
-     */
-    template<typename Component>
-    bool owned() const ENTT_NOEXCEPT {
-        const auto *cpool = pool<Component>();
-        return cpool && cpool->group;
-    }
-
-    /**
-     * @brief Returns a group for the given components.
-     *
-     * This kind of objects are created on the fly and share with the registry
-     * its internal data structures.<br/>
-     * Feel free to discard a group after the use. Creating and destroying a
-     * group is an incredibly cheap operation because they do not require any
-     * type of initialization, but for the first time they are requested.<br/>
-     * As a rule of thumb, storing a group should never be an option.
-     *
-     * Groups support exclusion lists and can own types of components. The more
-     * types are owned by a group, the faster it is to iterate entities and
-     * components.<br/>
-     * However, groups also affect some features of the registry such as the
-     * creation and destruction of components, which will consequently be
-     * slightly slower (nothing that can be noticed in most cases).
-     *
-     * @note
-     * Pools of components that are owned by a group cannot be sorted anymore.
-     * The group takes the ownership of the pools and arrange components so as
-     * to iterate them as fast as possible.
-     *
-     * @tparam Owned Types of components owned by the group.
-     * @tparam Get Types of components observed by the group.
-     * @tparam Exclude Types of components used to filter the group.
-     * @return A newly created group.
-     */
-    template<typename... Owned, typename... Get, typename... Exclude>
-    entt::basic_group<Entity, exclude_t<Exclude...>, get_t<Get...>, Owned...> group(get_t<Get...>, exclude_t<Exclude...> = {}) {
-        static_assert(sizeof...(Owned) + sizeof...(Get) > 0);
-        static_assert(sizeof...(Owned) + sizeof...(Get) + sizeof...(Exclude) > 1);
-
-        using handler_type = group_handler<exclude_t<Exclude...>, get_t<Get...>, Owned...>;
-
-        const std::size_t extent[] = { sizeof...(Owned), sizeof...(Get), sizeof...(Exclude) };
-        const component types[] = { type<Owned>()..., type<Get>()..., type<Exclude>()... };
-        handler_type *curr = nullptr;
-
-        if(auto it = std::find_if(groups.begin(), groups.end(), [&extent, &types](auto &&gdata) {
-            return std::equal(std::begin(extent), std::end(extent), gdata.extent) && gdata.is_same(types);
-        }); it != groups.cend())
-        {
-            curr = static_cast<handler_type *>(it->group.get());
-        }
-
-        if(!curr) {
-            groups.push_back(group_data{
-                { sizeof...(Owned), sizeof...(Get), sizeof...(Exclude) },
-                decltype(group_data::group){new handler_type{}, [](void *gptr) { delete static_cast<handler_type *>(gptr); }},
-                [](const component *other) {
-                    const component ctypes[] = { type<Owned>()..., type<Get>()..., type<Exclude>()... };
-                    return std::equal(std::begin(ctypes), std::end(ctypes), other);
-                }
-            });
-
-            curr = static_cast<handler_type *>(groups.back().group.get());
-
-            ((std::get<pool_type<Owned> *>(curr->cpools) = assure<Owned>()), ...);
-            ((std::get<pool_type<Get> *>(curr->cpools) = assure<Get>()), ...);
-            ((std::get<pool_type<Exclude> *>(curr->cpools) = assure<Exclude>()), ...);
-
-            ENTT_ASSERT((!std::get<pool_type<Owned> *>(curr->cpools)->group && ...));
-
-            ((std::get<pool_type<Owned> *>(curr->cpools)->group = curr), ...);
-            (std::get<pool_type<Owned> *>(curr->cpools)->on_construct().template connect<&handler_type::template maybe_valid_if<Owned>>(*curr), ...);
-            (std::get<pool_type<Owned> *>(curr->cpools)->on_destroy().template connect<&handler_type::discard_if>(*curr), ...);
-
-            (std::get<pool_type<Get> *>(curr->cpools)->on_construct().template connect<&handler_type::template maybe_valid_if<Get>>(*curr), ...);
-            (std::get<pool_type<Get> *>(curr->cpools)->on_destroy().template connect<&handler_type::discard_if>(*curr), ...);
-
-            (std::get<pool_type<Exclude> *>(curr->cpools)->on_destroy().template connect<&handler_type::template maybe_valid_if<Exclude>>(*curr), ...);
-            (std::get<pool_type<Exclude> *>(curr->cpools)->on_construct().template connect<&handler_type::discard_if>(*curr), ...);
-
-            const auto *cpool = std::min({
-                static_cast<sparse_set<Entity> *>(std::get<pool_type<Owned> *>(curr->cpools))...,
-                static_cast<sparse_set<Entity> *>(std::get<pool_type<Get> *>(curr->cpools))...
-            }, [](const auto *lhs, const auto *rhs) {
-                return lhs->size() < rhs->size();
-            });
-
-            // we cannot iterate backwards because we want to leave behind valid entities in case of owned types
-            std::for_each(cpool->data(), cpool->data() + cpool->size(), [curr](const auto entity) {
-                if((std::get<pool_type<Owned> *>(curr->cpools)->has(entity) && ...)
-                        && (std::get<pool_type<Get> *>(curr->cpools)->has(entity) && ...)
-                        && !(std::get<pool_type<Exclude> *>(curr->cpools)->has(entity) || ...))
-                {
-                    if constexpr(sizeof...(Owned) == 0) {
-                        curr->construct(entity);
-                    } else {
-                        const auto pos = curr->owned++;
-                        // useless this-> used to suppress a warning with clang
-                        (std::get<pool_type<Owned> *>(curr->cpools)->swap(std::get<pool_type<Owned> *>(curr->cpools)->index(entity), pos), ...);
-                    }
-                }
-            });
-        }
-
-        if constexpr(sizeof...(Owned) == 0) {
-            return { static_cast<sparse_set<Entity> *>(curr), std::get<pool_type<Get> *>(curr->cpools)... };
-        } else {
-            return { &curr->owned, std::get<pool_type<Owned> *>(curr->cpools)... , std::get<pool_type<Get> *>(curr->cpools)... };
-        }
-    }
-
-    /**
-     * @brief Returns a group for the given components.
-     *
-     * @sa group
-     *
-     * @tparam Owned Types of components owned by the group.
-     * @tparam Get Types of components observed by the group.
-     * @tparam Exclude Types of components used to filter the group.
-     * @return A newly created group.
-     */
-    template<typename... Owned, typename... Get, typename... Exclude>
-    entt::basic_group<Entity, exclude_t<Exclude...>, get_t<Get...>, Owned...> group(get_t<Get...>, exclude_t<Exclude...> = {}) const {
-        static_assert(std::conjunction_v<std::is_const<Owned>..., std::is_const<Get>...>);
-        return const_cast<basic_registry *>(this)->group<Owned...>(entt::get<Get...>, exclude<Exclude...>);
-    }
-
-    /**
-     * @brief Returns a group for the given components.
-     *
-     * @sa group
-     *
-     * @tparam Owned Types of components owned by the group.
-     * @tparam Exclude Types of components used to filter the group.
-     * @return A newly created group.
-     */
-    template<typename... Owned, typename... Exclude>
-    entt::basic_group<Entity, exclude_t<Exclude...>, get_t<>, Owned...> group(exclude_t<Exclude...> = {}) {
-        return group<Owned...>(entt::get<>, exclude<Exclude...>);
-    }
-
-    /**
-     * @brief Returns a group for the given components.
-     *
-     * @sa group
-     *
-     * @tparam Owned Types of components owned by the group.
-     * @tparam Exclude Types of components used to filter the group.
-     * @return A newly created group.
-     */
-    template<typename... Owned, typename... Exclude>
-    entt::basic_group<Entity, exclude_t<Exclude...>, get_t<>, Owned...> group(exclude_t<Exclude...> = {}) const {
-        static_assert(std::conjunction_v<std::is_const<Owned>...>);
-        return const_cast<basic_registry *>(this)->group<Owned...>(exclude<Exclude...>);
-    }
-
-    /**
-     * @brief Returns a runtime view for the given components.
-     *
-     * This kind of objects are created on the fly and share with the registry
-     * its internal data structures.<br/>
-     * Users should throw away the view after use. Fortunately, creating and
-     * destroying a runtime view is an incredibly cheap operation because they
-     * do not require any type of initialization.<br/>
-     * As a rule of thumb, storing a view should never be an option.
-     *
-     * Runtime views are to be used when users want to construct a view from
-     * some external inputs and don't know at compile-time what are the required
-     * components.<br/>
-     * This is particularly well suited to plugin systems and mods in general.
-     *
-     * @tparam It Type of input iterator.
-     * @param first An iterator to the first element of the range of components.
-     * @param last An iterator past the last element of the range of components.
-     * @return A newly created runtime view.
-     */
-    template<typename It>
-    entt::basic_runtime_view<Entity> runtime_view(It first, It last) const {
-        static_assert(std::is_same_v<typename std::iterator_traits<It>::value_type, component>);
-        std::vector<const sparse_set<Entity> *> set(std::distance(first, last));
-
-        std::transform(first, last, set.begin(), [this](const component ctype) {
-            auto it = std::find_if(pools.begin(), pools.end(), [ctype = to_integer(ctype)](const auto &pdata) {
-                return pdata.pool && pdata.runtime_type == ctype;
-            });
-
-            return it != pools.cend() && it->pool ? it->pool.get() : nullptr;
-        });
-
-        return { std::move(set) };
-    }
-
-    /**
-     * @brief Returns a full or partial copy of a registry.
-     *
-     * The components must be copyable for obvious reasons. The entities
-     * maintain their versions once copied.<br/>
-     * If no components are provided, the registry will try to clone all the
-     * existing pools. The ones for non-copyable types won't be cloned.
-     *
-     * This feature supports exclusion lists. The excluded types have higher
-     * priority than those indicated for cloning. An excluded type will never be
-     * cloned.
-     *
-     * @note
-     * There isn't an efficient way to know if all the entities are assigned at
-     * least one component once copied. Therefore, there may be orphans. It is
-     * up to the caller to clean up the registry if necessary.
+        } else {
+            if constexpr(is_applicable_v<Func, decltype(*each().begin())>) {
+                for(const auto pack: each()) {
+                    std::apply(func, pack);
+                }
+            } else {
+                for(auto &&component: *std::get<0>(pools)) {
+                    func(component);
+                }
+            }
+        }
+    }
+
+    /**
+     * @brief Returns an iterable object to use to _visit_ the view.
+     *
+     * The iterable object returns tuples that contain the current entity and a
+     * reference to its component if it's a non-empty one. The _constness_ of
+     * the component is as requested.
      *
      * @note
-     * Listeners and groups aren't copied. It is up to the caller to connect the
-     * listeners of interest to the new registry and to set up groups.
-     *
-     * @warning
-     * Attempting to clone components that aren't copyable results in unexpected
-     * behaviors.<br/>
-     * A static assertion will abort the compilation when the components
-     * provided aren't copy constructible. Otherwise, an assertion will abort
-     * the execution at runtime in debug mode in case one or more pools cannot
-     * be cloned.
-     *
-     * @tparam Component Types of components to clone.
-     * @tparam Exclude Types of components not to be cloned.
-     * @return A fresh copy of the registry.
-     */
-    template<typename... Component, typename... Exclude>
-    basic_registry clone(exclude_t<Exclude...> = {}) const {
-        static_assert(std::conjunction_v<std::is_copy_constructible<Component>...>);
-        basic_registry other;
-
-        other.pools.resize(pools.size());
-
-        for(auto pos = pools.size(); pos; --pos) {
-            const auto &pdata = pools[pos-1];
-            ENTT_ASSERT(!sizeof...(Component) || !pdata.pool || pdata.clone);
-
-            if(pdata.pool && pdata.clone
-                    && (!sizeof...(Component) || ... || (pdata.runtime_type == to_integer(type<Component>())))
-                    && ((pdata.runtime_type != to_integer(type<Exclude>())) && ...))
-            {
-                auto &curr = other.pools[pos-1];
-                curr.remove = pdata.remove;
-                curr.clone = pdata.clone;
-                curr.stomp = pdata.stomp;
-                curr.pool = pdata.clone ? pdata.clone(*pdata.pool) : nullptr;
-                curr.runtime_type = pdata.runtime_type;
-            }
-        }
-
-        other.skip_family_pools = skip_family_pools;
-        other.destroyed = destroyed;
-        other.entities = entities;
-
-        other.pools.erase(std::remove_if(other.pools.begin()+skip_family_pools, other.pools.end(), [](const auto &pdata) {
-            return !pdata.pool;
-        }), other.pools.end());
-
-        return other;
-    }
-
-    /**
-     * @brief Stomps an entity and its components.
-     *
-     * The components must be copyable for obvious reasons. The entities
-     * must be both valid.<br/>
-     * If no components are provided, the registry will try to copy all the
-     * existing types. The non-copyable ones will be ignored.
-     *
-     * This feature supports exclusion lists as an alternative to component
-     * lists. An excluded type will never be copied.
-     *
-     * @warning
-     * Attempting to copy components that aren't copyable results in unexpected
-     * behaviors.<br/>
-     * A static assertion will abort the compilation when the components
-     * provided aren't copy constructible. Otherwise, an assertion will abort
-     * the execution at runtime in debug mode in case one or more types cannot
-     * be copied.
-     *
-     * @warning
-     * Attempting to use invalid entities results in undefined behavior.<br/>
-     * An assertion will abort the execution at runtime in debug mode in case of
-     * invalid entities.
-     *
-     * @tparam Component Types of components to copy.
-     * @tparam Exclude Types of components not to be copied.
-     * @param dst A valid entity identifier to copy to.
-     * @param src A valid entity identifier to be copied.
-     * @param other The registry that owns the source entity.
-     */
-    template<typename... Component, typename... Exclude>
-    void stomp(const entity_type dst, const entity_type src, basic_registry &other, exclude_t<Exclude...> = {}) {
-        const entity_type entities[1]{dst};
-        stomp<Component...>(std::begin(entities), std::end(entities), src, other, exclude<Exclude...>);
-    }
-
-    /**
-     * @brief Stomps the entities in a range and their components.
-     *
-     * @sa stomp
-     *
-     * @tparam Component Types of components to copy.
-     * @tparam Exclude Types of components not to be copied.
-     * @param first An iterator to the first element of the range to stomp.
-     * @param last An iterator past the last element of the range to stomp.
-     * @param src A valid entity identifier to be copied.
-     * @param other The registry that owns the source entity.
-     */
-    template<typename... Component, typename It, typename... Exclude>
-    void stomp(It first, It last, const entity_type src, basic_registry &other, exclude_t<Exclude...> = {}) {
-        static_assert(sizeof...(Component) == 0 || sizeof...(Exclude) == 0);
-        static_assert(std::conjunction_v<std::is_copy_constructible<Component>...>);
-
-        for(auto pos = other.pools.size(); pos; --pos) {
-            const auto &pdata = other.pools[pos-1];
-            ENTT_ASSERT(!sizeof...(Component) || !pdata.pool || pdata.stomp);
-
-            if(pdata.pool && pdata.stomp
-                    && (!sizeof...(Component) || ... || (pdata.runtime_type == to_integer(type<Component>())))
-                    && ((pdata.runtime_type != to_integer(type<Exclude>())) && ...)
-                    && pdata.pool->has(src))
-            {
-                std::for_each(first, last, [this, &pdata, src](const auto entity) {
-                    pdata.stomp(*pdata.pool, src, *this, entity);
-                });
-            }
-        }
-    }
-
-    /**
-     * @brief Returns a temporary object to use to create snapshots.
-     *
-     * A snapshot is either a full or a partial dump of a registry.<br/>
-     * It can be used to save and restore its internal state or to keep two or
-     * more instances of this class in sync, as an example in a client-server
-     * architecture.
-     *
-     * @return A temporary object to use to take snasphosts.
-     */
-    entt::basic_snapshot<Entity> snapshot() const ENTT_NOEXCEPT {
-        using follow_fn_type = entity_type(const basic_registry &, const entity_type);
-
-        const auto head = to_integer(destroyed);
-        const entity_type seed = (destroyed == null) ? destroyed : entity_type{head | (to_integer(entities[head]) & (traits_type::version_mask << traits_type::entity_shift))};
-
-        follow_fn_type *follow = [](const basic_registry &reg, const entity_type entity) -> entity_type {
-            const auto &others = reg.entities;
-            const auto entt = to_integer(entity) & traits_type::entity_mask;
-            const auto curr = to_integer(others[entt]) & traits_type::entity_mask;
-            return entity_type{curr | (to_integer(others[curr]) & (traits_type::version_mask << traits_type::entity_shift))};
-        };
-
-        return { this, seed, follow };
-    }
-
-    /**
-     * @brief Returns a temporary object to use to load snapshots.
-     *
-     * A snapshot is either a full or a partial dump of a registry.<br/>
-     * It can be used to save and restore its internal state or to keep two or
-     * more instances of this class in sync, as an example in a client-server
-     * architecture.
-     *
-     * @note
-     * The loader returned by this function requires that the registry be empty.
-     * In case it isn't, all the data will be automatically deleted before to
-     * return.
-     *
-     * @return A temporary object to use to load snasphosts.
-     */
-    basic_snapshot_loader<Entity> loader() ENTT_NOEXCEPT {
-        using force_fn_type = void(basic_registry &, const entity_type, const bool);
-
-        force_fn_type *force = [](basic_registry &reg, const entity_type entity, const bool discard) {
-            const auto entt = to_integer(entity) & traits_type::entity_mask;
-            auto &others = reg.entities;
-
-            if(!(entt < others.size())) {
-                auto curr = others.size();
-                others.resize(entt + 1);
-
-                std::generate(others.data() + curr, others.data() + entt, [curr]() mutable {
-                    return entity_type(curr++);
-                });
-            }
-
-            others[entt] = entity;
-
-            if(discard) {
-                reg.destroy(entity);
-                const auto version = to_integer(entity) & (traits_type::version_mask << traits_type::entity_shift);
-                others[entt] = entity_type{(to_integer(others[entt]) & traits_type::entity_mask) | version};
-            }
-        };
-
-        reset();
-        entities.clear();
-        destroyed = null;
-
-        return { this, force };
-    }
-
-    /**
-     * @brief Binds an object to the context of the registry.
-     *
-     * If the value already exists it is overwritten, otherwise a new instance
-     * of the given type is created and initialized with the arguments provided.
-     *
-     * @tparam Type Type of object to set.
-     * @tparam Args Types of arguments to use to construct the object.
-     * @param args Parameters to use to initialize the value.
-     * @return A reference to the newly created object.
-     */
-    template<typename Type, typename... Args>
-    Type & set(Args &&... args) {
-        const auto ctype = runtime_type<Type, context_family>();
-        auto it = std::find_if(vars.begin(), vars.end(), [ctype](const auto &candidate) {
-            return candidate.runtime_type == ctype;
-        });
-
-        if(it == vars.cend()) {
-            vars.push_back({
-                decltype(ctx_variable::value){new Type{std::forward<Args>(args)...}, [](void *ptr) { delete static_cast<Type *>(ptr); }},
-                ctype
-            });
-
-            it = std::prev(vars.end());
-        } else {
-            it->value.reset(new Type{std::forward<Args>(args)...});
-        }
-
-        return *static_cast<Type *>(it->value.get());
-    }
-
-    /**
-     * @brief Unsets a context variable if it exists.
-     * @tparam Type Type of object to set.
-     */
-    template<typename Type>
-    void unset() {
-        vars.erase(std::remove_if(vars.begin(), vars.end(), [](auto &var) {
-            return var.runtime_type == runtime_type<Type, context_family>();
-        }), vars.end());
-    }
-
-    /**
-     * @brief Binds an object to the context of the registry.
-     *
-     * In case the context doesn't contain the given object, the parameters
-     * provided are used to construct it.
-     *
-     * @tparam Type Type of object to set.
-     * @tparam Args Types of arguments to use to construct the object.
-     * @param args Parameters to use to initialize the object.
-     * @return Reference to the object.
-     */
-    template<typename Type, typename... Args>
-    Type & ctx_or_set(Args &&... args) {
-        auto *type = try_ctx<Type>();
-        return type ? *type : set<Type>(std::forward<Args>(args)...);
-    }
-
-    /**
-     * @brief Returns a pointer to an object in the context of the registry.
-     * @tparam Type Type of object to get.
-     * @return A pointer to the object if it exists in the context of the
-     * registry, a null pointer otherwise.
-     */
-    template<typename Type>
-    const Type * try_ctx() const ENTT_NOEXCEPT {
-        const auto it = std::find_if(vars.begin(), vars.end(), [](const auto &var) {
-            return var.runtime_type == runtime_type<Type, context_family>();
-        });
-
-        return (it == vars.cend()) ? nullptr : static_cast<const Type *>(it->value.get());
-    }
-
-    /*! @copydoc try_ctx */
-    template<typename Type>
-    Type * try_ctx() ENTT_NOEXCEPT {
-        return const_cast<Type *>(std::as_const(*this).template try_ctx<Type>());
-    }
-
-    /**
-     * @brief Returns a reference to an object in the context of the registry.
-     *
-     * @warning
-     * Attempting to get a context variable that doesn't exist results in
-     * undefined behavior.<br/>
-     * An assertion will abort the execution at runtime in debug mode in case of
-     * invalid requests.
-     *
-     * @tparam Type Type of object to get.
-     * @return A valid reference to the object in the context of the registry.
-     */
-    template<typename Type>
-    const Type & ctx() const ENTT_NOEXCEPT {
-        const auto *instance = try_ctx<Type>();
-        ENTT_ASSERT(instance);
-        return *instance;
-    }
-
-    /*! @copydoc ctx */
-    template<typename Type>
-    Type & ctx() ENTT_NOEXCEPT {
-        return const_cast<Type &>(std::as_const(*this).template ctx<Type>());
-    }
-
-private:
-    std::size_t skip_family_pools{};
-    std::vector<pool_data> pools{};
-    std::vector<group_data> groups{};
-    std::vector<ctx_variable> vars{};
-    std::vector<entity_type> entities{};
-    entity_type destroyed{null};
-};
-
-
-}
-
-
-#endif // ENTT_ENTITY_REGISTRY_HPP
-
-// #include "entity.hpp"
-
-// #include "fwd.hpp"
-
-
-
-namespace entt {
-
-
-/**
- * @brief Dedicated to those who aren't confident with the
- * entity-component-system architecture.
- *
- * Tiny wrapper around a registry, for all those users that aren't confident
- * with entity-component-system architecture and prefer to iterate objects
- * directly.
- *
- * @tparam Entity A valid entity type (see entt_traits for more details).
- */
-template<typename Entity>
-struct basic_actor {
-    /*! @brief Type of registry used internally. */
-    using registry_type = basic_registry<Entity>;
-    /*! @brief Underlying entity identifier. */
-    using entity_type = Entity;
-
-    basic_actor() ENTT_NOEXCEPT
-        : entt{entt::null}, reg{nullptr}
-    {}
-
-    /**
-     * @brief Constructs an actor from a given registry.
-     * @param ref An instance of the registry class.
-     */
-    explicit basic_actor(registry_type &ref)
-        : entt{ref.create()}, reg{&ref}
-    {}
-
-    /**
-     * @brief Constructs an actor from a given entity.
-     * @param entity A valid entity identifier.
-     * @param ref An instance of the registry class.
-     */
-    explicit basic_actor(entity_type entity, registry_type &ref)
-        : entt{entity}, reg{&ref}
-    {
-        ENTT_ASSERT(ref.valid(entity));
-    }
-
-    /*! @brief Default destructor. */
-    virtual ~basic_actor() {
-        if(*this) {
-            reg->destroy(entt);
-        }
-    }
-
-    /**
-     * @brief Move constructor.
-     *
-     * After actor move construction, instances that have been moved from are
-     * placed in a valid but unspecified state. It's highly discouraged to
-     * continue using them.
-     *
-     * @param other The instance to move from.
-     */
-    basic_actor(basic_actor &&other)
-        : entt{other.entt}, reg{other.reg}
-    {
-        other.entt = null;
-    }
-
-    /**
-     * @brief Move assignment operator.
-     *
-     * After actor move assignment, instances that have been moved from are
-     * placed in a valid but unspecified state. It's highly discouraged to
-     * continue using them.
-     *
-     * @param other The instance to move from.
-     * @return This actor.
-     */
-    basic_actor & operator=(basic_actor &&other) {
-        if(this != &other) {
-            auto tmp{std::move(other)};
-            std::swap(reg, tmp.reg);
-            std::swap(entt, tmp.entt);
-        }
-
-        return *this;
-    }
-
-    /**
-     * @brief Assigns the given component to an actor.
-     *
-     * A new instance of the given component is created and initialized with the
-     * arguments provided (the component must have a proper constructor or be of
-     * aggregate type). Then the component is assigned to the actor.<br/>
-     * In case the actor already has a component of the given type, it's
-     * replaced with the new one.
-     *
-     * @tparam Component Type of the component to create.
-     * @tparam Args Types of arguments to use to construct the component.
-     * @param args Parameters to use to initialize the component.
-     * @return A reference to the newly created component.
-     */
-    template<typename Component, typename... Args>
-    decltype(auto) assign(Args &&... args) {
-        return reg->template assign_or_replace<Component>(entt, std::forward<Args>(args)...);
-    }
-
-    /**
-     * @brief Removes the given component from an actor.
-     * @tparam Component Type of the component to remove.
-     */
-    template<typename Component>
-    void remove() {
-        reg->template remove<Component>(entt);
-    }
-
-    /**
-     * @brief Checks if an actor has the given components.
-     * @tparam Component Components for which to perform the check.
-     * @return True if the actor has all the components, false otherwise.
-     */
-    template<typename... Component>
-    bool has() const ENTT_NOEXCEPT {
-        return (reg->template has<Component>(entt) && ...);
-    }
-
-    /**
-     * @brief Returns references to the given components for an actor.
-     * @tparam Component Types of components to get.
-     * @return References to the components owned by the actor.
-     */
-    template<typename... Component>
-    decltype(auto) get() const ENTT_NOEXCEPT {
-        return std::as_const(*reg).template get<Component...>(entt);
-    }
-
-    /*! @copydoc get */
-    template<typename... Component>
-    decltype(auto) get() ENTT_NOEXCEPT {
-        return reg->template get<Component...>(entt);
-    }
-
-    /**
-     * @brief Returns pointers to the given components for an actor.
-     * @tparam Component Types of components to get.
-     * @return Pointers to the components owned by the actor.
-     */
-    template<typename... Component>
-    auto try_get() const ENTT_NOEXCEPT {
-        return std::as_const(*reg).template try_get<Component...>(entt);
-    }
-
-    /*! @copydoc try_get */
-    template<typename... Component>
-    auto try_get() ENTT_NOEXCEPT {
-        return reg->template try_get<Component...>(entt);
-    }
-
-    /**
-     * @brief Returns a reference to the underlying registry.
-     * @return A reference to the underlying registry.
-     */
-    const registry_type & backend() const ENTT_NOEXCEPT {
-        return *reg;
-    }
-
-    /*! @copydoc backend */
-    registry_type & backend() ENTT_NOEXCEPT {
-        return const_cast<registry_type &>(std::as_const(*this).backend());
-    }
-
-    /**
-     * @brief Returns the entity associated with an actor.
-     * @return The entity associated with the actor.
-     */
-    entity_type entity() const ENTT_NOEXCEPT {
-        return entt;
-    }
-
-    /**
-     * @brief Checks if an actor refers to a valid entity or not.
-     * @return True if the actor refers to a valid entity, false otherwise.
-     */
-    explicit operator bool() const ENTT_NOEXCEPT {
-        return reg && reg->valid(entt);
-    }
-
-private:
-    entity_type entt;
-    registry_type *reg;
-};
-
-
-}
-
-
-#endif // ENTT_ENTITY_ACTOR_HPP
-
-// #include "entity/entity.hpp"
-
-// #include "entity/group.hpp"
-
-// #include "entity/helper.hpp"
-#ifndef ENTT_ENTITY_HELPER_HPP
-#define ENTT_ENTITY_HELPER_HPP
-
-
-#include <type_traits>
-// #include "../config/config.h"
-
-// #include "../signal/sigh.hpp"
-
-// #include "registry.hpp"
-
-
-
-namespace entt {
-
-
-/**
- * @brief Converts a registry to a view.
- * @tparam Const Constness of the accepted registry.
- * @tparam Entity A valid entity type (see entt_traits for more details).
- */
-template<bool Const, typename Entity>
-struct as_view {
-    /*! @brief Type of registry to convert. */
-    using registry_type = std::conditional_t<Const, const entt::basic_registry<Entity>, entt::basic_registry<Entity>>;
-
-    /**
-     * @brief Constructs a converter for a given registry.
-     * @param source A valid reference to a registry.
-     */
-    as_view(registry_type &source) ENTT_NOEXCEPT: reg{source} {}
-
-    /**
-     * @brief Conversion function from a registry to a view.
-     * @tparam Component Type of components used to construct the view.
-     * @return A newly created view.
-     */
-    template<typename... Component>
-    operator entt::basic_view<Entity, Component...>() const {
-        return reg.template view<Component...>();
-    }
-
-private:
-    registry_type &reg;
+     * Empty types aren't explicitly instantiated and therefore they are never
+     * returned during iterations.
+     *
+     * @return An iterable object to use to _visit_ the view.
+     */
+    [[nodiscard]] iterable_view each() const ENTT_NOEXCEPT {
+        return iterable_view{*std::get<0>(pools)};
+    }
+
+    /**
+     * @brief Combines two views in a _more specific_ one (friend function).
+     * @tparam Id A valid entity type (see entt_traits for more details).
+     * @tparam ELhs Filter list of the first view.
+     * @tparam CLhs Component list of the first view.
+     * @tparam ERhs Filter list of the second view.
+     * @tparam CRhs Component list of the second view.
+     * @return A more specific view.
+     */
+    template<typename Id, typename... ELhs, typename... CLhs, typename... ERhs, typename... CRhs>
+    friend auto operator|(const basic_view<Id, exclude_t<ELhs...>, CLhs...> &, const basic_view<Id, exclude_t<ERhs...>, CRhs...> &);
+
+private:
+    const std::tuple<storage_type *> pools;
+    const std::tuple<> filter;
+};
+
+
+/**
+ * @brief View implementation dispatcher.
+ * @tparam Entity A valid entity type (see entt_traits for more details).
+ * @tparam Exclude Types of components used to filter the view.
+ * @tparam Component Types of components iterated by the view.
+ */
+template<typename Entity, typename... Exclude, typename... Component>
+struct basic_view<Entity, exclude_t<Exclude...>, Component...>
+    : basic_view_impl<std::conditional_t<std::disjunction_v<typename component_traits<std::remove_const_t<Component>>::in_place_delete...>, stable_storage_policy, packed_storage_policy>, Entity, exclude_t<Exclude...>, Component...>
+{
+    /*! @brief Most restrictive storage policy of all component types. */
+    using storage_policy = std::conditional_t<std::disjunction_v<typename component_traits<std::remove_const_t<Component>>::in_place_delete...>, stable_storage_policy, packed_storage_policy>;
+    using basic_view_impl<storage_policy, Entity, exclude_t<Exclude...>, Component...>::basic_view_impl;
 };
 
 
 /**
  * @brief Deduction guide.
- *
- * It allows to deduce the constness of a registry directly from the instance
- * provided to the constructor.
- *
- * @tparam Entity A valid entity type (see entt_traits for more details).
- */
-template<typename Entity>
-as_view(basic_registry<Entity> &) ENTT_NOEXCEPT -> as_view<false, Entity>;
-
-
-/*! @copydoc as_view */
-template<typename Entity>
-as_view(const basic_registry<Entity> &) ENTT_NOEXCEPT -> as_view<true, Entity>;
-
-
-/**
- * @brief Converts a registry to a group.
- * @tparam Const Constness of the accepted registry.
- * @tparam Entity A valid entity type (see entt_traits for more details).
- */
-template<bool Const, typename Entity>
-struct as_group {
-    /*! @brief Type of registry to convert. */
-    using registry_type = std::conditional_t<Const, const entt::basic_registry<Entity>, entt::basic_registry<Entity>>;
-
-    /**
-     * @brief Constructs a converter for a given registry.
-     * @param source A valid reference to a registry.
-     */
-    as_group(registry_type &source) ENTT_NOEXCEPT: reg{source} {}
-
-    /**
-     * @brief Conversion function from a registry to a group.
-     * @tparam Exclude Types of components used to filter the group.
-     * @tparam Get Types of components observed by the group.
-     * @tparam Owned Types of components owned by the group.
-     * @return A newly created group.
-     */
-    template<typename Exclude, typename Get, typename... Owned>
-    operator entt::basic_group<Entity, Exclude, Get, Owned...>() const {
-        return reg.template group<Owned...>(Get{}, Exclude{});
-    }
-
-private:
-    registry_type &reg;
-};
-
-
-/**
- * @brief Deduction guide.
- *
- * It allows to deduce the constness of a registry directly from the instance
- * provided to the constructor.
- *
- * @tparam Entity A valid entity type (see entt_traits for more details).
- */
-template<typename Entity>
-as_group(basic_registry<Entity> &) ENTT_NOEXCEPT -> as_group<false, Entity>;
-
-
-/*! @copydoc as_group */
-template<typename Entity>
-as_group(const basic_registry<Entity> &) ENTT_NOEXCEPT -> as_group<true, Entity>;
-
-
-/**
- * @brief Alias template to ease the assignment of tags to entities.
- *
- * If used in combination with hashed strings, it simplifies the assignment of
- * tags to entities and the use of tags in general where a type would be
- * required otherwise.<br/>
- * As an example and where the user defined literal for hashed strings hasn't
- * been changed:
- * @code{.cpp}
- * entt::registry registry;
- * registry.assign<entt::tag<"enemy"_hs>>(entity);
- * @endcode
- *
- * @note
- * Tags are empty components and therefore candidates for the empty component
- * optimization.
- *
- * @tparam Value The numeric representation of an instance of hashed string.
- */
-template<ENTT_ID_TYPE Value>
-using tag = std::integral_constant<ENTT_ID_TYPE, Value>;
-
-
-}
-
-
-#endif // ENTT_ENTITY_HELPER_HPP
-
-// #include "entity/observer.hpp"
-#ifndef ENTT_ENTITY_OBSERVER_HPP
-#define ENTT_ENTITY_OBSERVER_HPP
-
-
-#include <limits>
-#include <cstddef>
-#include <cstdint>
-#include <utility>
-#include <algorithm>
-#include <type_traits>
-// #include "../config/config.h"
-
-// #include "../core/type_traits.hpp"
-
-// #include "registry.hpp"
-
-// #include "storage.hpp"
-
-// #include "entity.hpp"
-
-// #include "fwd.hpp"
-
-
-
-namespace entt {
-
-
-/*! @brief Grouping matcher. */
-template<typename...>
-struct matcher {};
-
-
-/**
- * @brief Collector.
- *
- * Primary template isn't defined on purpose. All the specializations give a
- * compile-time error, but for a few reasonable cases.
- */
-template<typename...>
-struct basic_collector;
-
-
-/**
- * @brief Collector.
- *
- * A collector contains a set of rules (literally, matchers) to use to track
- * entities.<br/>
- * Its main purpose is to generate a descriptor that allows an observer to know
- * how to connect to a registry.
- */
-template<>
-struct basic_collector<> {
-    /**
-     * @brief Adds a grouping matcher to the collector.
-     * @tparam AllOf Types of components tracked by the matcher.
-     * @tparam NoneOf Types of components used to filter out entities.
-     * @return The updated collector.
-     */
-    template<typename... AllOf, typename... NoneOf>
-    static constexpr auto group(exclude_t<NoneOf...> = {}) ENTT_NOEXCEPT {
-        return basic_collector<matcher<matcher<type_list<>, type_list<>>, type_list<NoneOf...>, type_list<AllOf...>>>{};
-    }
-
-    /**
-     * @brief Adds an observing matcher to the collector.
-     * @tparam AnyOf Type of component for which changes should be detected.
-     * @return The updated collector.
-     */
-    template<typename AnyOf>
-    static constexpr auto replace() ENTT_NOEXCEPT {
-        return basic_collector<matcher<matcher<type_list<>, type_list<>>, AnyOf>>{};
-    }
-};
-
-/**
- * @brief Collector.
- * @copydetails basic_collector<>
- * @tparam AnyOf Types of components for which changes should be detected.
- * @tparam Matcher Types of grouping matchers.
- */
-template<typename... Reject, typename... Require, typename... Rule, typename... Other>
-struct basic_collector<matcher<matcher<type_list<Reject...>, type_list<Require...>>, Rule...>, Other...> {
-    /**
-     * @brief Adds a grouping matcher to the collector.
-     * @tparam AllOf Types of components tracked by the matcher.
-     * @tparam NoneOf Types of components used to filter out entities.
-     * @return The updated collector.
-     */
-    template<typename... AllOf, typename... NoneOf>
-    static constexpr auto group(exclude_t<NoneOf...> = {}) ENTT_NOEXCEPT {
-        using first = matcher<matcher<type_list<Reject...>, type_list<Require...>>, Rule...>;
-        return basic_collector<first, Other..., matcher<matcher<type_list<>, type_list<>>, type_list<NoneOf...>, type_list<AllOf...>>>{};
-    }
-
-    /**
-     * @brief Adds an observing matcher to the collector.
-     * @tparam AnyOf Type of component for which changes should be detected.
-     * @return The updated collector.
-     */
-    template<typename AnyOf>
-    static constexpr auto replace() ENTT_NOEXCEPT {
-        using first = matcher<matcher<type_list<Reject...>, type_list<Require...>>, Rule...>;
-        return basic_collector<first, Other..., matcher<matcher<type_list<>, type_list<>>, AnyOf>>{};
-    }
-
-    /**
-     * @brief Updates the filter of the last added matcher.
-     * @tparam AllOf Types of components required by the matcher.
-     * @tparam NoneOf Types of components used to filter out entities.
-     * @return The updated collector.
-     */
-    template<typename... AllOf, typename... NoneOf>
-    static constexpr auto where(exclude_t<NoneOf...> = {}) ENTT_NOEXCEPT {
-        return basic_collector<matcher<matcher<type_list<Reject..., NoneOf...>, type_list<Require..., AllOf...>>, Rule...>, Other...>{};
-    }
-};
-
-
-/*! @brief Variable template used to ease the definition of collectors. */
-constexpr basic_collector<> collector{};
-
-
-/**
- * @brief Observer.
- *
- * An observer returns all the entities and only the entities that fit the
- * requirements of at least one matcher. Moreover, it's guaranteed that the
- * entity list is tightly packed in memory for fast iterations.<br/>
- * In general, observers don't stay true to the order of any set of components.
- *
- * Observers work mainly with two types of matchers, provided through a
- * collector:
- *
- * * Observing matcher: an observer will return at least all the living entities
- *   for which one or more of the given components have been explicitly
- *   replaced and not yet destroyed.
- * * Grouping matcher: an observer will return at least all the living entities
- *   that would have entered the given group if it existed and that would have
- *   not yet left it.
- *
- * If an entity respects the requirements of multiple matchers, it will be
- * returned once and only once by the observer in any case.
- *
- * Matchers support also filtering by means of a _where_ clause that accepts
- * both a list of types and an exclusion list.<br/>
- * Whenever a matcher finds that an entity matches its requirements, the
- * condition of the filter is verified before to register the entity itself.
- * Moreover, a registered entity isn't returned by the observer if the condition
- * set by the filter is broken in the meantime.
- *
- * @b Important
- *
- * Iterators aren't invalidated if:
- *
- * * New instances of the given components are created and assigned to entities.
- * * The entity currently pointed is modified (as an example, if one of the
- *   given components is removed from the entity to which the iterator points).
- * * The entity currently pointed is destroyed.
- *
- * In all the other cases, modifying the pools of the given components in any
- * way invalidates all the iterators and using them results in undefined
- * behavior.
- *
- * @warning
- * Lifetime of an observer doesn't necessarily have to overcome the one of the
- * registry to which it is connected. However, the observer must be disconnected
- * from the registry before being destroyed to avoid crashes due to dangling
- * pointers.
- *
- * @tparam Entity A valid entity type (see entt_traits for more details).
- */
-template<typename Entity>
-class basic_observer {
-    using payload_type = std::uint32_t;
-
-    template<typename>
-    struct matcher_handler;
-
-    template<typename... Reject, typename... Require, typename AnyOf>
-    struct matcher_handler<matcher<matcher<type_list<Reject...>, type_list<Require...>>, AnyOf>> {
-        template<std::size_t Index>
-        static void maybe_valid_if(basic_observer &obs, const Entity entt, const basic_registry<Entity> &reg) {
-            if(reg.template has<Require...>(entt) && !(reg.template has<Reject>(entt) || ...)) {
-                auto *comp = obs.view.try_get(entt);
-                (comp ? *comp : obs.view.construct(entt)) |= (1 << Index);
-            }
-        }
-
-        template<std::size_t Index>
-        static void discard_if(basic_observer &obs, const Entity entt) {
-            if(auto *value = obs.view.try_get(entt); value && !(*value &= (~(1 << Index)))) {
-                obs.view.destroy(entt);
-            }
-        }
-
-        template<std::size_t Index>
-        static void connect(basic_observer &obs, basic_registry<Entity> &reg) {
-            (reg.template on_destroy<Require>().template connect<&discard_if<Index>>(obs), ...);
-            (reg.template on_construct<Reject>().template connect<&discard_if<Index>>(obs), ...);
-            reg.template on_replace<AnyOf>().template connect<&maybe_valid_if<Index>>(obs);
-            reg.template on_destroy<AnyOf>().template connect<&discard_if<Index>>(obs);
-        }
-
-        static void disconnect(basic_observer &obs, basic_registry<Entity> &reg) {
-            (reg.template on_destroy<Require>().disconnect(obs), ...);
-            (reg.template on_construct<Reject>().disconnect(obs), ...);
-            reg.template on_replace<AnyOf>().disconnect(obs);
-            reg.template on_destroy<AnyOf>().disconnect(obs);
-        }
-    };
-
-    template<typename... Reject, typename... Require, typename... NoneOf, typename... AllOf>
-    struct matcher_handler<matcher<matcher<type_list<Reject...>, type_list<Require...>>, type_list<NoneOf...>, type_list<AllOf...>>> {
-        template<std::size_t Index>
-        static void maybe_valid_if(basic_observer &obs, const Entity entt, const basic_registry<Entity> &reg) {
-            if(reg.template has<AllOf...>(entt) && !(reg.template has<NoneOf>(entt) || ...)
-                    && reg.template has<Require...>(entt) && !(reg.template has<Reject>(entt) || ...))
-            {
-                auto *comp = obs.view.try_get(entt);
-                (comp ? *comp : obs.view.construct(entt)) |= (1 << Index);
-            }
-        }
-
-        template<std::size_t Index>
-        static void discard_if(basic_observer &obs, const Entity entt) {
-            if(auto *value = obs.view.try_get(entt); value && !(*value &= (~(1 << Index)))) {
-                obs.view.destroy(entt);
-            }
-        }
-
-        template<std::size_t Index>
-        static void connect(basic_observer &obs, basic_registry<Entity> &reg) {
-            (reg.template on_destroy<Require>().template connect<&discard_if<Index>>(obs), ...);
-            (reg.template on_construct<Reject>().template connect<&discard_if<Index>>(obs), ...);
-            (reg.template on_construct<AllOf>().template connect<&maybe_valid_if<Index>>(obs), ...);
-            (reg.template on_destroy<NoneOf>().template connect<&maybe_valid_if<Index>>(obs), ...);
-            (reg.template on_destroy<AllOf>().template connect<&discard_if<Index>>(obs), ...);
-            (reg.template on_construct<NoneOf>().template connect<&discard_if<Index>>(obs), ...);
-        }
-
-        static void disconnect(basic_observer &obs, basic_registry<Entity> &reg) {
-            (reg.template on_destroy<Require>().disconnect(obs), ...);
-            (reg.template on_construct<Reject>().disconnect(obs), ...);
-            (reg.template on_construct<AllOf>().disconnect(obs), ...);
-            (reg.template on_destroy<NoneOf>().disconnect(obs), ...);
-            (reg.template on_destroy<AllOf>().disconnect(obs), ...);
-            (reg.template on_construct<NoneOf>().disconnect(obs), ...);
-        }
-    };
-
-    template<typename... Matcher>
-    static void disconnect(basic_observer &obs, basic_registry<Entity> &reg) {
-        (matcher_handler<Matcher>::disconnect(obs, reg), ...);
-    }
-
-    template<typename... Matcher, std::size_t... Index>
-    void connect(basic_registry<Entity> &reg, std::index_sequence<Index...>) {
-        static_assert(sizeof...(Matcher) < std::numeric_limits<payload_type>::digits);
-        (matcher_handler<Matcher>::template connect<Index>(*this, reg), ...);
-        release = &basic_observer::disconnect<Matcher...>;
-    }
-
-public:
-    /*! @brief Underlying entity identifier. */
-    using entity_type = Entity;
-    /*! @brief Unsigned integer type. */
-    using size_type = typename sparse_set<Entity>::size_type;
-    /*! @brief Input iterator type. */
-    using iterator_type = typename sparse_set<Entity>::iterator_type;
-
-    /*! @brief Default constructor. */
-    basic_observer() ENTT_NOEXCEPT
-        : target{}, release{}, view{}
-    {}
-
-    /*! @brief Default copy constructor, deleted on purpose. */
-    basic_observer(const basic_observer &) = delete;
-    /*! @brief Default move constructor, deleted on purpose. */
-    basic_observer(basic_observer &&) = delete;
-
-    /**
-     * @brief Creates an observer and connects it to a given registry.
-     * @tparam Matcher Types of matchers to use to initialize the observer.
-     * @param reg A valid reference to a registry.
-     */
-    template<typename... Matcher>
-    basic_observer(basic_registry<entity_type> &reg, basic_collector<Matcher...>) ENTT_NOEXCEPT
-        : target{&reg},
-          release{},
-          view{}
-    {
-        connect<Matcher...>(reg, std::make_index_sequence<sizeof...(Matcher)>{});
-    }
-
-    /*! @brief Default destructor. */
-    ~basic_observer() = default;
-
-    /**
-     * @brief Default copy assignment operator, deleted on purpose.
-     * @return This observer.
-     */
-    basic_observer & operator=(const basic_observer &) = delete;
-
-    /**
-     * @brief Default move assignment operator, deleted on purpose.
-     * @return This observer.
-     */
-    basic_observer & operator=(basic_observer &&) = delete;
-
-    /**
-     * @brief Connects an observer to a given registry.
-     * @tparam Matcher Types of matchers to use to initialize the observer.
-     * @param reg A valid reference to a registry.
-     */
-    template<typename... Matcher>
-    void connect(basic_registry<entity_type> &reg, basic_collector<Matcher...>) {
-        disconnect();
-        connect<Matcher...>(reg, std::make_index_sequence<sizeof...(Matcher)>{});
-        target = &reg;
-        view.reset();
-    }
-
-    /*! @brief Disconnects an observer from the registry it keeps track of. */
-    void disconnect() {
-        if(release) {
-            release(*this, *target);
-            release = nullptr;
-        }
-    }
-
-    /**
-     * @brief Returns the number of elements in an observer.
-     * @return Number of elements.
-     */
-    size_type size() const ENTT_NOEXCEPT {
-        return view.size();
-    }
-
-    /**
-     * @brief Checks whether an observer is empty.
-     * @return True if the observer is empty, false otherwise.
-     */
-    bool empty() const ENTT_NOEXCEPT {
-        return view.empty();
-    }
-
-    /**
-     * @brief Direct access to the list of entities of the observer.
-     *
-     * The returned pointer is such that range `[data(), data() + size()]` is
-     * always a valid range, even if the container is empty.
-     *
-     * @note
-     * There are no guarantees on the order of the entities. Use `begin` and
-     * `end` if you want to iterate the observer in the expected order.
-     *
-     * @return A pointer to the array of entities.
-     */
-    const entity_type * data() const ENTT_NOEXCEPT {
-        return view.data();
-    }
-
-    /**
-     * @brief Returns an iterator to the first entity of the observer.
-     *
-     * The returned iterator points to the first entity of the observer. If the
-     * container is empty, the returned iterator will be equal to `end()`.
-     *
-     * @return An iterator to the first entity of the observer.
-     */
-    iterator_type begin() const ENTT_NOEXCEPT {
-        return view.sparse_set<entity_type>::begin();
-    }
-
-    /**
-     * @brief Returns an iterator that is past the last entity of the observer.
-     *
-     * The returned iterator points to the entity following the last entity of
-     * the observer. Attempting to dereference the returned iterator results in
-     * undefined behavior.
-     *
-     * @return An iterator to the entity following the last entity of the
-     * observer.
-     */
-    iterator_type end() const ENTT_NOEXCEPT {
-        return view.sparse_set<entity_type>::end();
-    }
-
-    /*! @brief Resets the underlying container. */
-    void clear() {
-        view.reset();
-    }
-
-    /**
-     * @brief Iterates entities and applies the given function object to them,
-     * then clears the observer.
-     *
-     * The function object is invoked for each entity.<br/>
-     * The signature of the function must be equivalent to the following form:
-     *
-     * @code{.cpp}
-     * void(const entity_type);
-     * @endcode
-     *
-     * @tparam Func Type of the function object to invoke.
-     * @param func A valid function object.
-     */
-    template<typename Func>
-    void each(Func func) const {
-        static_assert(std::is_invocable_v<Func, entity_type>);
-        std::for_each(begin(), end(), std::move(func));
-    }
-
-    /**
-     * @brief Iterates entities and applies the given function object to them,
-     * then clears the observer.
-     *
-     * The function object is invoked for each entity.<br/>
-     * The signature of the function must be equivalent to the following form:
-     *
-     * @code{.cpp}
-     * void(const entity_type);
-     * @endcode
-     *
-     * @tparam Func Type of the function object to invoke.
-     * @param func A valid function object.
-     */
-    template<typename Func>
-    void each(Func func) {
-        std::as_const(*this).each(std::move(func));
-        clear();
-    }
-
-private:
-    basic_registry<entity_type> *target;
-    void(* release)(basic_observer &, basic_registry<entity_type> &);
-    storage<entity_type, payload_type> view;
-};
-
-
-}
-
-
-#endif // ENTT_ENTITY_OBSERVER_HPP
-
-// #include "entity/registry.hpp"
-
-// #include "entity/runtime_view.hpp"
-
-// #include "entity/snapshot.hpp"
-
-// #include "entity/sparse_set.hpp"
-
-// #include "entity/storage.hpp"
-
-// #include "entity/utility.hpp"
-
-// #include "entity/view.hpp"
+ * @tparam Storage Type of storage classes used to create the view.
+ * @param storage The storage for the types to iterate.
+ */
+template<typename... Storage>
+basic_view(Storage &... storage)
+-> basic_view<std::common_type_t<typename Storage::entity_type...>, entt::exclude_t<>, constness_as_t<typename Storage::value_type, Storage>...>;
+
+
+/**
+ * @brief Combines two views in a _more specific_ one.
+ * @tparam Entity A valid entity type (see entt_traits for more details).
+ * @tparam ELhs Filter list of the first view.
+ * @tparam CLhs Component list of the first view.
+ * @tparam ERhs Filter list of the second view.
+ * @tparam CRhs Component list of the second view.
+ * @param lhs A valid reference to the first view.
+ * @param rhs A valid reference to the second view.
+ * @return A more specific view.
+ */
+template<typename Entity, typename... ELhs, typename... CLhs, typename... ERhs, typename... CRhs>
+[[nodiscard]] auto operator|(const basic_view<Entity, exclude_t<ELhs...>, CLhs...> &lhs, const basic_view<Entity, exclude_t<ERhs...>, CRhs...> &rhs) {
+    using view_type = basic_view<Entity, exclude_t<ELhs..., ERhs...>, CLhs..., CRhs...>;
+    return std::apply([](auto *... storage) { return view_type{*storage...}; }, std::tuple_cat(lhs.pools, rhs.pools, lhs.filter, rhs.filter));
+}
+
+
+}
+
+
+#endif
 
 // #include "locator/locator.hpp"
 #ifndef ENTT_LOCATOR_LOCATOR_HPP
@@ -9807,49 +28159,87 @@
 #define ENTT_CONFIG_CONFIG_H
 
 
-#ifndef ENTT_NOEXCEPT
-#define ENTT_NOEXCEPT noexcept
-#endif // ENTT_NOEXCEPT
-
-
-#ifndef ENTT_HS_SUFFIX
-#define ENTT_HS_SUFFIX _hs
-#endif // ENTT_HS_SUFFIX
-
-
-#ifndef ENTT_HWS_SUFFIX
-#define ENTT_HWS_SUFFIX _hws
-#endif // ENTT_HWS_SUFFIX
-
-
-#ifndef ENTT_NO_ATOMIC
-#include <atomic>
-#define ENTT_MAYBE_ATOMIC(Type) std::atomic<Type>
-#else // ENTT_NO_ATOMIC
-#define ENTT_MAYBE_ATOMIC(Type) Type
-#endif // ENTT_NO_ATOMIC
+#if defined(__cpp_exceptions) && !defined(ENTT_NOEXCEPTION)
+#   define ENTT_NOEXCEPT noexcept
+#   define ENTT_THROW throw
+#   define ENTT_TRY try
+#   define ENTT_CATCH catch(...)
+#else
+#   define ENTT_NOEXCEPT
+#   define ENTT_THROW
+#   define ENTT_TRY if(true)
+#   define ENTT_CATCH if(false)
+#endif
+
+
+#if defined(__cpp_lib_launder) && __cpp_lib_launder >= 201606L
+#   include <new>
+#   define ENTT_LAUNDER(expr) std::launder(expr)
+#else
+#   define ENTT_LAUNDER(expr) expr
+#endif
+
+
+#ifndef ENTT_USE_ATOMIC
+#   define ENTT_MAYBE_ATOMIC(Type) Type
+#else
+#   include <atomic>
+#   define ENTT_MAYBE_ATOMIC(Type) std::atomic<Type>
+#endif
 
 
 #ifndef ENTT_ID_TYPE
-#include <cstdint>
-#define ENTT_ID_TYPE std::uint32_t
-#endif // ENTT_ID_TYPE
-
-
-#ifndef ENTT_PAGE_SIZE
-#define ENTT_PAGE_SIZE 32768
-#endif // ENTT_PAGE_SIZE
-
-
-#ifndef ENTT_DISABLE_ASSERT
-#include <cassert>
-#define ENTT_ASSERT(condition) assert(condition)
-#else // ENTT_DISABLE_ASSERT
-#define ENTT_ASSERT(...) ((void)0)
-#endif // ENTT_DISABLE_ASSERT
-
-
-#endif // ENTT_CONFIG_CONFIG_H
+#   include <cstdint>
+#   define ENTT_ID_TYPE std::uint32_t
+#endif
+
+
+#ifdef ENTT_SPARSE_PAGE
+	static_assert(ENTT_SPARSE_PAGE && ((ENTT_SPARSE_PAGE & (ENTT_SPARSE_PAGE - 1)) == 0), "ENTT_SPARSE_PAGE must be a power of two");
+#else
+#   define ENTT_SPARSE_PAGE 4096
+#endif
+
+
+#ifdef ENTT_PACKED_PAGE
+static_assert(ENTT_PACKED_PAGE && ((ENTT_PACKED_PAGE & (ENTT_PACKED_PAGE - 1)) == 0), "ENTT_PACKED_PAGE must be a power of two");
+#else
+#   define ENTT_PACKED_PAGE 1024
+#endif
+
+
+#ifdef ENTT_DISABLE_ASSERT
+#   undef ENTT_ASSERT
+#   define ENTT_ASSERT(...) (void(0))
+#elif !defined ENTT_ASSERT
+#   include <cassert>
+#   define ENTT_ASSERT(condition, ...) assert(condition)
+#endif
+
+
+#ifdef ENTT_NO_ETO
+#   include <type_traits>
+#   define ENTT_IGNORE_IF_EMPTY std::false_type
+#else
+#   include <type_traits>
+#   define ENTT_IGNORE_IF_EMPTY std::true_type
+#endif
+
+
+#ifndef ENTT_STANDARD_CPP
+#    if defined __clang__ || defined __GNUC__
+#       define ENTT_PRETTY_FUNCTION __PRETTY_FUNCTION__
+#       define ENTT_PRETTY_FUNCTION_PREFIX '='
+#       define ENTT_PRETTY_FUNCTION_SUFFIX ']'
+#    elif defined _MSC_VER
+#       define ENTT_PRETTY_FUNCTION __FUNCSIG__
+#       define ENTT_PRETTY_FUNCTION_PREFIX '<'
+#       define ENTT_PRETTY_FUNCTION_SUFFIX '>'
+#   endif
+#endif
+
+
+#endif
 
 
 
@@ -9881,7 +28271,7 @@
      * @brief Tests if a valid service implementation is set.
      * @return True if the service is set, false otherwise.
      */
-    static bool empty() ENTT_NOEXCEPT {
+    [[nodiscard]] static bool empty() ENTT_NOEXCEPT {
         return !static_cast<bool>(service);
     }
 
@@ -9895,7 +28285,7 @@
      *
      * @return A reference to the service implementation currently set, if any.
      */
-    static std::weak_ptr<Service> get() ENTT_NOEXCEPT {
+    [[nodiscard]] static std::weak_ptr<Service> get() ENTT_NOEXCEPT {
         return service;
     }
 
@@ -9913,7 +28303,7 @@
      *
      * @return A reference to the service implementation currently set, if any.
      */
-    static Service & ref() ENTT_NOEXCEPT {
+    [[nodiscard]] static Service & ref() ENTT_NOEXCEPT {
         return *service;
     }
 
@@ -9933,7 +28323,7 @@
      * @param ptr Service to use to replace the current one.
      */
     static void set(std::shared_ptr<Service> ptr) {
-        ENTT_ASSERT(static_cast<bool>(ptr));
+        ENTT_ASSERT(static_cast<bool>(ptr), "Null service not allowed");
         service = std::move(ptr);
     }
 
@@ -9954,127 +28344,828 @@
 }
 
 
-#endif // ENTT_LOCATOR_LOCATOR_HPP
-
-// #include "meta/factory.hpp"
-#ifndef ENTT_META_FACTORY_HPP
-#define ENTT_META_FACTORY_HPP
-
-
-#include <tuple>
+#endif
+
+// #include "meta/adl_pointer.hpp"
+#ifndef ENTT_META_ADL_POINTER_HPP
+#define ENTT_META_ADL_POINTER_HPP
+
+
+namespace entt {
+
+
+/**
+ * @brief ADL based lookup function for dereferencing meta pointer-like types.
+ * @tparam Type Element type.
+ * @param value A pointer-like object.
+ * @return The value returned from the dereferenced pointer.
+ */
+template<typename Type>
+decltype(auto) dereference_meta_pointer_like(const Type &value) {
+    return *value;
+}
+
+
+/**
+ * @brief Fake ADL based lookup function for meta pointer-like types.
+ * @tparam Type Element type.
+ */
+template<typename Type>
+struct adl_meta_pointer_like {
+    /**
+     * @brief Uses the default ADL based lookup method to resolve the call.
+     * @param value A pointer-like object.
+     * @return The value returned from the dereferenced pointer.
+     */
+    static decltype(auto) dereference(const Type &value) {
+        return dereference_meta_pointer_like(value);
+    }
+};
+
+
+}
+
+
+#endif
+
+// #include "meta/container.hpp"
+#ifndef ENTT_META_CONTAINER_HPP
+#define ENTT_META_CONTAINER_HPP
+
+
 #include <array>
-#include <cstddef>
-#include <utility>
-#include <functional>
-#include <type_traits>
+#include <map>
+#include <set>
+#include <type_traits>
+#include <unordered_map>
+#include <unordered_set>
+#include <utility>
+#include <vector>
 // #include "../config/config.h"
 #ifndef ENTT_CONFIG_CONFIG_H
 #define ENTT_CONFIG_CONFIG_H
 
 
-#ifndef ENTT_NOEXCEPT
-#define ENTT_NOEXCEPT noexcept
-#endif // ENTT_NOEXCEPT
-
-
-#ifndef ENTT_HS_SUFFIX
-#define ENTT_HS_SUFFIX _hs
-#endif // ENTT_HS_SUFFIX
-
-
-#ifndef ENTT_HWS_SUFFIX
-#define ENTT_HWS_SUFFIX _hws
-#endif // ENTT_HWS_SUFFIX
-
-
-#ifndef ENTT_NO_ATOMIC
-#include <atomic>
-#define ENTT_MAYBE_ATOMIC(Type) std::atomic<Type>
-#else // ENTT_NO_ATOMIC
-#define ENTT_MAYBE_ATOMIC(Type) Type
-#endif // ENTT_NO_ATOMIC
+#if defined(__cpp_exceptions) && !defined(ENTT_NOEXCEPTION)
+#   define ENTT_NOEXCEPT noexcept
+#   define ENTT_THROW throw
+#   define ENTT_TRY try
+#   define ENTT_CATCH catch(...)
+#else
+#   define ENTT_NOEXCEPT
+#   define ENTT_THROW
+#   define ENTT_TRY if(true)
+#   define ENTT_CATCH if(false)
+#endif
+
+
+#if defined(__cpp_lib_launder) && __cpp_lib_launder >= 201606L
+#   include <new>
+#   define ENTT_LAUNDER(expr) std::launder(expr)
+#else
+#   define ENTT_LAUNDER(expr) expr
+#endif
+
+
+#ifndef ENTT_USE_ATOMIC
+#   define ENTT_MAYBE_ATOMIC(Type) Type
+#else
+#   include <atomic>
+#   define ENTT_MAYBE_ATOMIC(Type) std::atomic<Type>
+#endif
+
+
+#ifndef ENTT_ID_TYPE
+#   include <cstdint>
+#   define ENTT_ID_TYPE std::uint32_t
+#endif
+
+
+#ifdef ENTT_SPARSE_PAGE
+	static_assert(ENTT_SPARSE_PAGE && ((ENTT_SPARSE_PAGE & (ENTT_SPARSE_PAGE - 1)) == 0), "ENTT_SPARSE_PAGE must be a power of two");
+#else
+#   define ENTT_SPARSE_PAGE 4096
+#endif
+
+
+#ifdef ENTT_PACKED_PAGE
+static_assert(ENTT_PACKED_PAGE && ((ENTT_PACKED_PAGE & (ENTT_PACKED_PAGE - 1)) == 0), "ENTT_PACKED_PAGE must be a power of two");
+#else
+#   define ENTT_PACKED_PAGE 1024
+#endif
+
+
+#ifdef ENTT_DISABLE_ASSERT
+#   undef ENTT_ASSERT
+#   define ENTT_ASSERT(...) (void(0))
+#elif !defined ENTT_ASSERT
+#   include <cassert>
+#   define ENTT_ASSERT(condition, ...) assert(condition)
+#endif
+
+
+#ifdef ENTT_NO_ETO
+#   include <type_traits>
+#   define ENTT_IGNORE_IF_EMPTY std::false_type
+#else
+#   include <type_traits>
+#   define ENTT_IGNORE_IF_EMPTY std::true_type
+#endif
+
+
+#ifndef ENTT_STANDARD_CPP
+#    if defined __clang__ || defined __GNUC__
+#       define ENTT_PRETTY_FUNCTION __PRETTY_FUNCTION__
+#       define ENTT_PRETTY_FUNCTION_PREFIX '='
+#       define ENTT_PRETTY_FUNCTION_SUFFIX ']'
+#    elif defined _MSC_VER
+#       define ENTT_PRETTY_FUNCTION __FUNCSIG__
+#       define ENTT_PRETTY_FUNCTION_PREFIX '<'
+#       define ENTT_PRETTY_FUNCTION_SUFFIX '>'
+#   endif
+#endif
+
+
+#endif
+
+// #include "../core/type_traits.hpp"
+#ifndef ENTT_CORE_TYPE_TRAITS_HPP
+#define ENTT_CORE_TYPE_TRAITS_HPP
+
+
+#include <cstddef>
+#include <iterator>
+#include <type_traits>
+#include <utility>
+// #include "../config/config.h"
+#ifndef ENTT_CONFIG_CONFIG_H
+#define ENTT_CONFIG_CONFIG_H
+
+
+#if defined(__cpp_exceptions) && !defined(ENTT_NOEXCEPTION)
+#   define ENTT_NOEXCEPT noexcept
+#   define ENTT_THROW throw
+#   define ENTT_TRY try
+#   define ENTT_CATCH catch(...)
+#else
+#   define ENTT_NOEXCEPT
+#   define ENTT_THROW
+#   define ENTT_TRY if(true)
+#   define ENTT_CATCH if(false)
+#endif
+
+
+#if defined(__cpp_lib_launder) && __cpp_lib_launder >= 201606L
+#   include <new>
+#   define ENTT_LAUNDER(expr) std::launder(expr)
+#else
+#   define ENTT_LAUNDER(expr) expr
+#endif
+
+
+#ifndef ENTT_USE_ATOMIC
+#   define ENTT_MAYBE_ATOMIC(Type) Type
+#else
+#   include <atomic>
+#   define ENTT_MAYBE_ATOMIC(Type) std::atomic<Type>
+#endif
 
 
 #ifndef ENTT_ID_TYPE
-#include <cstdint>
-#define ENTT_ID_TYPE std::uint32_t
-#endif // ENTT_ID_TYPE
-
-
-#ifndef ENTT_PAGE_SIZE
-#define ENTT_PAGE_SIZE 32768
-#endif // ENTT_PAGE_SIZE
-
-
-#ifndef ENTT_DISABLE_ASSERT
-#include <cassert>
-#define ENTT_ASSERT(condition) assert(condition)
-#else // ENTT_DISABLE_ASSERT
-#define ENTT_ASSERT(...) ((void)0)
-#endif // ENTT_DISABLE_ASSERT
-
-
-#endif // ENTT_CONFIG_CONFIG_H
-
-// #include "policy.hpp"
-#ifndef ENTT_META_POLICY_HPP
-#define ENTT_META_POLICY_HPP
-
-
-namespace entt {
-
-
-/*! @brief Empty class type used to request the _as alias_ policy. */
-struct as_alias_t {};
-
-
-/*! @brief Disambiguation tag. */
-constexpr as_alias_t as_alias;
-
-
-/*! @brief Empty class type used to request the _as-is_ policy. */
-struct as_is_t {};
-
-
-/*! @brief Empty class type used to request the _as void_ policy. */
-struct as_void_t {};
-
-
-}
-
-
-#endif // ENTT_META_POLICY_HPP
-
-// #include "meta.hpp"
-#ifndef ENTT_META_META_HPP
-#define ENTT_META_META_HPP
-
-
-#include <array>
-#include <memory>
-#include <cstring>
-#include <cstddef>
-#include <utility>
-#include <type_traits>
-// #include "../config/config.h"
-
-// #include "policy.hpp"
-
-
-
-namespace entt {
-
-
-class meta_any;
-struct meta_handle;
-class meta_prop;
-class meta_base;
-class meta_conv;
-class meta_ctor;
-class meta_dtor;
-class meta_data;
-class meta_func;
-class meta_type;
+#   include <cstdint>
+#   define ENTT_ID_TYPE std::uint32_t
+#endif
+
+
+#ifdef ENTT_SPARSE_PAGE
+	static_assert(ENTT_SPARSE_PAGE && ((ENTT_SPARSE_PAGE & (ENTT_SPARSE_PAGE - 1)) == 0), "ENTT_SPARSE_PAGE must be a power of two");
+#else
+#   define ENTT_SPARSE_PAGE 4096
+#endif
+
+
+#ifdef ENTT_PACKED_PAGE
+static_assert(ENTT_PACKED_PAGE && ((ENTT_PACKED_PAGE & (ENTT_PACKED_PAGE - 1)) == 0), "ENTT_PACKED_PAGE must be a power of two");
+#else
+#   define ENTT_PACKED_PAGE 1024
+#endif
+
+
+#ifdef ENTT_DISABLE_ASSERT
+#   undef ENTT_ASSERT
+#   define ENTT_ASSERT(...) (void(0))
+#elif !defined ENTT_ASSERT
+#   include <cassert>
+#   define ENTT_ASSERT(condition, ...) assert(condition)
+#endif
+
+
+#ifdef ENTT_NO_ETO
+#   include <type_traits>
+#   define ENTT_IGNORE_IF_EMPTY std::false_type
+#else
+#   include <type_traits>
+#   define ENTT_IGNORE_IF_EMPTY std::true_type
+#endif
+
+
+#ifndef ENTT_STANDARD_CPP
+#    if defined __clang__ || defined __GNUC__
+#       define ENTT_PRETTY_FUNCTION __PRETTY_FUNCTION__
+#       define ENTT_PRETTY_FUNCTION_PREFIX '='
+#       define ENTT_PRETTY_FUNCTION_SUFFIX ']'
+#    elif defined _MSC_VER
+#       define ENTT_PRETTY_FUNCTION __FUNCSIG__
+#       define ENTT_PRETTY_FUNCTION_PREFIX '<'
+#       define ENTT_PRETTY_FUNCTION_SUFFIX '>'
+#   endif
+#endif
+
+
+#endif
+
+// #include "fwd.hpp"
+#ifndef ENTT_CORE_FWD_HPP
+#define ENTT_CORE_FWD_HPP
+
+
+#include <type_traits>
+// #include "../config/config.h"
+
+
+
+namespace entt {
+
+
+template<std::size_t Len = sizeof(double[2]), std::size_t = alignof(typename std::aligned_storage_t<Len + !Len>)>
+class basic_any;
+
+
+/*! @brief Alias declaration for type identifiers. */
+using id_type = ENTT_ID_TYPE;
+
+
+/*! @brief Alias declaration for the most common use case. */
+using any = basic_any<>;
+
+
+}
+
+
+#endif
+
+
+
+namespace entt {
+
+
+/**
+ * @brief Utility class to disambiguate overloaded functions.
+ * @tparam N Number of choices available.
+ */
+template<std::size_t N>
+struct choice_t
+    // Unfortunately, doxygen cannot parse such a construct.
+    /*! @cond TURN_OFF_DOXYGEN */
+    : choice_t<N-1>
+    /*! @endcond */
+{};
+
+
+/*! @copybrief choice_t */
+template<>
+struct choice_t<0> {};
+
+
+/**
+ * @brief Variable template for the choice trick.
+ * @tparam N Number of choices available.
+ */
+template<std::size_t N>
+inline constexpr choice_t<N> choice{};
+
+
+/**
+ * @brief Identity type trait.
+ *
+ * Useful to establish non-deduced contexts in template argument deduction
+ * (waiting for C++20) or to provide types through function arguments.
+ *
+ * @tparam Type A type.
+ */
+template<typename Type>
+struct type_identity {
+    /*! @brief Identity type. */
+    using type = Type;
+};
+
+
+/**
+ * @brief Helper type.
+ * @tparam Type A type.
+ */
+template<typename Type>
+using type_identity_t = typename type_identity<Type>::type;
+
+
+/**
+ * @brief A type-only `sizeof` wrapper that returns 0 where `sizeof` complains.
+ * @tparam Type The type of which to return the size.
+ * @tparam The size of the type if `sizeof` accepts it, 0 otherwise.
+ */
+template<typename Type, typename = void>
+struct size_of: std::integral_constant<std::size_t, 0u> {};
+
+
+/*! @copydoc size_of */
+template<typename Type>
+struct size_of<Type, std::void_t<decltype(sizeof(Type))>>
+    : std::integral_constant<std::size_t, sizeof(Type)>
+{};
+
+
+/**
+ * @brief Helper variable template.
+ * @tparam Type The type of which to return the size.
+ */
+template<class Type>
+inline constexpr std::size_t size_of_v = size_of<Type>::value;
+
+
+/**
+ * @brief Using declaration to be used to _repeat_ the same type a number of
+ * times equal to the size of a given parameter pack.
+ * @tparam Type A type to repeat.
+ */
+template<typename Type, typename>
+using unpack_as_t = Type;
+
+
+/**
+ * @brief Helper variable template to be used to _repeat_ the same value a
+ * number of times equal to the size of a given parameter pack.
+ * @tparam Value A value to repeat.
+ */
+template<auto Value, typename>
+inline constexpr auto unpack_as_v = Value;
+
+
+/**
+ * @brief Wraps a static constant.
+ * @tparam Value A static constant.
+ */
+template<auto Value>
+using integral_constant = std::integral_constant<decltype(Value), Value>;
+
+
+/**
+ * @brief Alias template to facilitate the creation of named values.
+ * @tparam Value A constant value at least convertible to `id_type`.
+ */
+template<id_type Value>
+using tag = integral_constant<Value>;
+
+
+/**
+ * @brief A class to use to push around lists of types, nothing more.
+ * @tparam Type Types provided by the type list.
+ */
+template<typename... Type>
+struct type_list {
+    /*! @brief Type list type. */
+    using type = type_list;
+    /*! @brief Compile-time number of elements in the type list. */
+    static constexpr auto size = sizeof...(Type);
+};
+
+
+/*! @brief Primary template isn't defined on purpose. */
+template<std::size_t, typename>
+struct type_list_element;
+
+
+/**
+ * @brief Provides compile-time indexed access to the types of a type list.
+ * @tparam Index Index of the type to return.
+ * @tparam Type First type provided by the type list.
+ * @tparam Other Other types provided by the type list.
+ */
+template<std::size_t Index, typename Type, typename... Other>
+struct type_list_element<Index, type_list<Type, Other...>>
+    : type_list_element<Index - 1u, type_list<Other...>>
+{};
+
+
+/**
+ * @brief Provides compile-time indexed access to the types of a type list.
+ * @tparam Type First type provided by the type list.
+ * @tparam Other Other types provided by the type list.
+ */
+template<typename Type, typename... Other>
+struct type_list_element<0u, type_list<Type, Other...>> {
+    /*! @brief Searched type. */
+    using type = Type;
+};
+
+
+/**
+ * @brief Helper type.
+ * @tparam Index Index of the type to return.
+ * @tparam List Type list to search into.
+ */
+template<std::size_t Index, typename List>
+using type_list_element_t = typename type_list_element<Index, List>::type;
+
+
+/**
+ * @brief Concatenates multiple type lists.
+ * @tparam Type Types provided by the first type list.
+ * @tparam Other Types provided by the second type list.
+ * @return A type list composed by the types of both the type lists.
+ */
+template<typename... Type, typename... Other>
+constexpr type_list<Type..., Other...> operator+(type_list<Type...>, type_list<Other...>) { return {}; }
+
+
+/*! @brief Primary template isn't defined on purpose. */
+template<typename...>
+struct type_list_cat;
+
+
+/*! @brief Concatenates multiple type lists. */
+template<>
+struct type_list_cat<> {
+    /*! @brief A type list composed by the types of all the type lists. */
+    using type = type_list<>;
+};
+
+
+/**
+ * @brief Concatenates multiple type lists.
+ * @tparam Type Types provided by the first type list.
+ * @tparam Other Types provided by the second type list.
+ * @tparam List Other type lists, if any.
+ */
+template<typename... Type, typename... Other, typename... List>
+struct type_list_cat<type_list<Type...>, type_list<Other...>, List...> {
+    /*! @brief A type list composed by the types of all the type lists. */
+    using type = typename type_list_cat<type_list<Type..., Other...>, List...>::type;
+};
+
+
+/**
+ * @brief Concatenates multiple type lists.
+ * @tparam Type Types provided by the type list.
+ */
+template<typename... Type>
+struct type_list_cat<type_list<Type...>> {
+    /*! @brief A type list composed by the types of all the type lists. */
+    using type = type_list<Type...>;
+};
+
+
+/**
+ * @brief Helper type.
+ * @tparam List Type lists to concatenate.
+ */
+template<typename... List>
+using type_list_cat_t = typename type_list_cat<List...>::type;
+
+
+/*! @brief Primary template isn't defined on purpose. */
+template<typename>
+struct type_list_unique;
+
+
+/**
+ * @brief Removes duplicates types from a type list.
+ * @tparam Type One of the types provided by the given type list.
+ * @tparam Other The other types provided by the given type list.
+ */
+template<typename Type, typename... Other>
+struct type_list_unique<type_list<Type, Other...>> {
+    /*! @brief A type list without duplicate types. */
+    using type = std::conditional_t<
+        std::disjunction_v<std::is_same<Type, Other>...>,
+        typename type_list_unique<type_list<Other...>>::type,
+        type_list_cat_t<type_list<Type>, typename type_list_unique<type_list<Other...>>::type>
+    >;
+};
+
+
+/*! @brief Removes duplicates types from a type list. */
+template<>
+struct type_list_unique<type_list<>> {
+    /*! @brief A type list without duplicate types. */
+    using type = type_list<>;
+};
+
+
+/**
+ * @brief Helper type.
+ * @tparam Type A type list.
+ */
+template<typename Type>
+using type_list_unique_t = typename type_list_unique<Type>::type;
+
+
+/**
+ * @brief Provides the member constant `value` to true if a type list contains a
+ * given type, false otherwise.
+ * @tparam List Type list.
+ * @tparam Type Type to look for.
+ */
+template<typename List, typename Type>
+struct type_list_contains;
+
+
+/**
+ * @copybrief type_list_contains
+ * @tparam Type Types provided by the type list.
+ * @tparam Other Type to look for.
+ */
+template<typename... Type, typename Other>
+struct type_list_contains<type_list<Type...>, Other>: std::disjunction<std::is_same<Type, Other>...> {};
+
+
+/**
+ * @brief Helper variable template.
+ * @tparam List Type list.
+ * @tparam Type Type to look for.
+ */
+template<class List, typename Type>
+inline constexpr bool type_list_contains_v = type_list_contains<List, Type>::value;
+
+
+/*! @brief Primary template isn't defined on purpose. */
+template<typename...>
+struct type_list_diff;
+
+
+/**
+ * @brief Computes the difference between two type lists.
+ * @tparam Type Types provided by the first type list.
+ * @tparam Other Types provided by the second type list.
+ */
+template<typename... Type, typename... Other>
+struct type_list_diff<type_list<Type...>, type_list<Other...>> {
+    /*! @brief A type list that is the difference between the two type lists. */
+    using type = type_list_cat_t<std::conditional_t<type_list_contains_v<type_list<Other...>, Type>, type_list<>, type_list<Type>>...>;
+};
+
+
+/**
+ * @brief Helper type.
+ * @tparam List Type lists between which to compute the difference.
+ */
+template<typename... List>
+using type_list_diff_t = typename type_list_diff<List...>::type;
+
+
+/**
+ * @brief A class to use to push around lists of constant values, nothing more.
+ * @tparam Value Values provided by the value list.
+ */
+template<auto... Value>
+struct value_list {
+    /*! @brief Value list type. */
+    using type = value_list;
+    /*! @brief Compile-time number of elements in the value list. */
+    static constexpr auto size = sizeof...(Value);
+};
+
+
+/*! @brief Primary template isn't defined on purpose. */
+template<std::size_t, typename>
+struct value_list_element;
+
+
+/**
+ * @brief Provides compile-time indexed access to the values of a value list.
+ * @tparam Index Index of the value to return.
+ * @tparam Value First value provided by the value list.
+ * @tparam Other Other values provided by the value list.
+ */
+template<std::size_t Index, auto Value, auto... Other>
+struct value_list_element<Index, value_list<Value, Other...>>
+    : value_list_element<Index - 1u, value_list<Other...>>
+{};
+
+
+/**
+ * @brief Provides compile-time indexed access to the types of a type list.
+ * @tparam Value First value provided by the value list.
+ * @tparam Other Other values provided by the value list.
+ */
+template<auto Value, auto... Other>
+struct value_list_element<0u, value_list<Value, Other...>> {
+    /*! @brief Searched value. */
+    static constexpr auto value = Value;
+};
+
+
+/**
+ * @brief Helper type.
+ * @tparam Index Index of the value to return.
+ * @tparam List Value list to search into.
+ */
+template<std::size_t Index, typename List>
+inline constexpr auto value_list_element_v = value_list_element<Index, List>::value;
+
+
+/**
+ * @brief Concatenates multiple value lists.
+ * @tparam Value Values provided by the first value list.
+ * @tparam Other Values provided by the second value list.
+ * @return A value list composed by the values of both the value lists.
+ */
+template<auto... Value, auto... Other>
+constexpr value_list<Value..., Other...> operator+(value_list<Value...>, value_list<Other...>) { return {}; }
+
+
+/*! @brief Primary template isn't defined on purpose. */
+template<typename...>
+struct value_list_cat;
+
+
+/*! @brief Concatenates multiple value lists. */
+template<>
+struct value_list_cat<> {
+    /*! @brief A value list composed by the values of all the value lists. */
+    using type = value_list<>;
+};
+
+
+/**
+ * @brief Concatenates multiple value lists.
+ * @tparam Value Values provided by the first value list.
+ * @tparam Other Values provided by the second value list.
+ * @tparam List Other value lists, if any.
+ */
+template<auto... Value, auto... Other, typename... List>
+struct value_list_cat<value_list<Value...>, value_list<Other...>, List...> {
+    /*! @brief A value list composed by the values of all the value lists. */
+    using type = typename value_list_cat<value_list<Value..., Other...>, List...>::type;
+};
+
+
+/**
+ * @brief Concatenates multiple value lists.
+ * @tparam Value Values provided by the value list.
+ */
+template<auto... Value>
+struct value_list_cat<value_list<Value...>> {
+    /*! @brief A value list composed by the values of all the value lists. */
+    using type = value_list<Value...>;
+};
+
+
+/**
+ * @brief Helper type.
+ * @tparam List Value lists to concatenate.
+ */
+template<typename... List>
+using value_list_cat_t = typename value_list_cat<List...>::type;
+
+
+/*! @brief Same as std::is_invocable, but with tuples. */
+template<typename, typename>
+struct is_applicable: std::false_type {};
+
+
+/**
+ * @copybrief is_applicable
+ * @tparam Func A valid function type.
+ * @tparam Tuple Tuple-like type.
+ * @tparam Args The list of arguments to use to probe the function type.
+ */
+template<typename Func, template<typename...> class Tuple, typename... Args>
+struct is_applicable<Func, Tuple<Args...>>: std::is_invocable<Func, Args...> {};
+
+
+/**
+ * @copybrief is_applicable
+ * @tparam Func A valid function type.
+ * @tparam Tuple Tuple-like type.
+ * @tparam Args The list of arguments to use to probe the function type.
+ */
+template<typename Func, template<typename...> class Tuple, typename... Args>
+struct is_applicable<Func, const Tuple<Args...>>: std::is_invocable<Func, Args...> {};
+
+
+/**
+ * @brief Helper variable template.
+ * @tparam Func A valid function type.
+ * @tparam Args The list of arguments to use to probe the function type.
+ */
+template<typename Func, typename Args>
+inline constexpr bool is_applicable_v = is_applicable<Func, Args>::value;
+
+
+/*! @brief Same as std::is_invocable_r, but with tuples for arguments. */
+template<typename, typename, typename>
+struct is_applicable_r: std::false_type {};
+
+
+/**
+ * @copybrief is_applicable_r
+ * @tparam Ret The type to which the return type of the function should be
+ * convertible.
+ * @tparam Func A valid function type.
+ * @tparam Args The list of arguments to use to probe the function type.
+ */
+template<typename Ret, typename Func, typename... Args>
+struct is_applicable_r<Ret, Func, std::tuple<Args...>>: std::is_invocable_r<Ret, Func, Args...> {};
+
+
+/**
+ * @brief Helper variable template.
+ * @tparam Ret The type to which the return type of the function should be
+ * convertible.
+ * @tparam Func A valid function type.
+ * @tparam Args The list of arguments to use to probe the function type.
+ */
+template<typename Ret, typename Func, typename Args>
+inline constexpr bool is_applicable_r_v = is_applicable_r<Ret, Func, Args>::value;
+
+
+/**
+ * @brief Provides the member constant `value` to true if a given type is
+ * complete, false otherwise.
+ * @tparam Type The type to test.
+ */
+template<typename Type, typename = void>
+struct is_complete: std::false_type {};
+
+
+/*! @copydoc is_complete */
+template<typename Type>
+struct is_complete<Type, std::void_t<decltype(sizeof(Type))>>: std::true_type {};
+
+
+/**
+ * @brief Helper variable template.
+ * @tparam Type The type to test.
+ */
+template<typename Type>
+inline constexpr bool is_complete_v = is_complete<Type>::value;
+
+
+/**
+ * @brief Provides the member constant `value` to true if a given type is an
+ * iterator, false otherwise.
+ * @tparam Type The type to test.
+ */
+template<typename Type, typename = void>
+struct is_iterator: std::false_type {};
+
+
+/*! @copydoc is_iterator */
+template<typename Type>
+struct is_iterator<Type, std::void_t<typename std::iterator_traits<Type>::iterator_category>>
+    : std::true_type
+{};
+
+
+/**
+ * @brief Helper variable template.
+ * @tparam Type The type to test.
+ */
+template<typename Type>
+inline constexpr bool is_iterator_v = is_iterator<Type>::value;
+
+
+/**
+ * @brief Provides the member constant `value` to true if a given type is of the
+ * required iterator type, false otherwise.
+ * @tparam Type The type to test.
+ * @tparam It Required iterator type.
+ */
+template<typename Type, typename It, typename = void>
+struct is_iterator_type: std::false_type {};
+
+
+/*! @copydoc is_iterator_type */
+template<typename Type, typename It>
+struct is_iterator_type<Type, It, std::enable_if_t<is_iterator_v<Type> && std::is_same_v<Type, It>>>
+    : std::true_type
+{};
+
+
+/*! @copydoc is_iterator_type */
+template<typename Type, typename It>
+struct is_iterator_type<Type, It, std::enable_if_t<!std::is_same_v<Type, It>, std::void_t<typename It::iterator_type>>>
+    : is_iterator_type<Type, typename It::iterator_type>
+{};
+
+
+/**
+ * @brief Helper variable template.
+ * @tparam Type The type to test.
+ * @tparam It Required iterator type.
+ */
+template<typename Type, typename It>
+inline constexpr bool is_iterator_type_v = is_iterator_type<Type, It>::value;
 
 
 /**
@@ -10086,2087 +29177,649 @@
 namespace internal {
 
 
-struct meta_type_node;
-
-
-struct meta_prop_node {
-    meta_prop_node * next;
-    meta_any(* const key)();
-    meta_any(* const value)();
-    meta_prop(* const meta)() ENTT_NOEXCEPT;
-};
-
-
-struct meta_base_node {
-    meta_base_node ** const underlying;
-    meta_type_node * const parent;
-    meta_base_node * next;
-    meta_type_node *(* const type)() ENTT_NOEXCEPT;
-    void *(* const cast)(void *) ENTT_NOEXCEPT;
-    meta_base(* const meta)() ENTT_NOEXCEPT;
-};
-
-
-struct meta_conv_node {
-    meta_conv_node ** const underlying;
-    meta_type_node * const parent;
-    meta_conv_node * next;
-    meta_type_node *(* const type)() ENTT_NOEXCEPT;
-    meta_any(* const conv)(const void *);
-    meta_conv(* const meta)() ENTT_NOEXCEPT;
-};
-
-
-struct meta_ctor_node {
-    using size_type = std::size_t;
-    meta_ctor_node ** const underlying;
-    meta_type_node * const parent;
-    meta_ctor_node * next;
-    meta_prop_node * prop;
-    const size_type size;
-    meta_type_node *(* const arg)(size_type) ENTT_NOEXCEPT;
-    meta_any(* const invoke)(meta_any * const);
-    meta_ctor(* const meta)() ENTT_NOEXCEPT;
-};
-
-
-struct meta_dtor_node {
-    meta_dtor_node ** const underlying;
-    meta_type_node * const parent;
-    bool(* const invoke)(meta_handle);
-    meta_dtor(* const meta)() ENTT_NOEXCEPT;
-};
-
-
-struct meta_data_node {
-    meta_data_node ** const underlying;
-    ENTT_ID_TYPE identifier;
-    meta_type_node * const parent;
-    meta_data_node * next;
-    meta_prop_node * prop;
-    const bool is_const;
-    const bool is_static;
-    meta_type_node *(* const type)() ENTT_NOEXCEPT;
-    bool(* const set)(meta_handle, meta_any, meta_any);
-    meta_any(* const get)(meta_handle, meta_any);
-    meta_data(* const meta)() ENTT_NOEXCEPT;
-};
-
-
-struct meta_func_node {
-    using size_type = std::size_t;
-    meta_func_node ** const underlying;
-    ENTT_ID_TYPE identifier;
-    meta_type_node * const parent;
-    meta_func_node * next;
-    meta_prop_node * prop;
-    const size_type size;
-    const bool is_const;
-    const bool is_static;
-    meta_type_node *(* const ret)() ENTT_NOEXCEPT;
-    meta_type_node *(* const arg)(size_type) ENTT_NOEXCEPT;
-    meta_any(* const invoke)(meta_handle, meta_any *);
-    meta_func(* const meta)() ENTT_NOEXCEPT;
-};
-
-
-struct meta_type_node {
-    using size_type = std::size_t;
-    ENTT_ID_TYPE identifier;
-    meta_type_node * next;
-    meta_prop_node * prop;
-    const bool is_void;
-    const bool is_integral;
-    const bool is_floating_point;
-    const bool is_array;
-    const bool is_enum;
-    const bool is_union;
-    const bool is_class;
-    const bool is_pointer;
-    const bool is_function;
-    const bool is_member_object_pointer;
-    const bool is_member_function_pointer;
-    const size_type extent;
-    meta_type(* const remove_pointer)() ENTT_NOEXCEPT;
-    meta_type(* const meta)() ENTT_NOEXCEPT;
-    meta_base_node *base{nullptr};
-    meta_conv_node *conv{nullptr};
-    meta_ctor_node *ctor{nullptr};
-    meta_dtor_node *dtor{nullptr};
-    meta_data_node *data{nullptr};
-    meta_func_node *func{nullptr};
-};
-
-
-template<typename...>
-struct meta_node {
-    inline static meta_type_node *type = nullptr;
-};
-
-
-template<typename Type>
-struct meta_node<Type> {
-    inline static meta_type_node *type = nullptr;
-
-    template<typename>
-    inline static meta_base_node *base = nullptr;
-
-    template<typename>
-    inline static meta_conv_node *conv = nullptr;
-
-    template<typename>
-    inline static meta_ctor_node *ctor = nullptr;
-
-    template<auto>
-    inline static meta_dtor_node *dtor = nullptr;
-
-    template<auto...>
-    inline static meta_data_node *data = nullptr;
-
-    template<auto>
-    inline static meta_func_node *func = nullptr;
-
-    inline static meta_type_node * resolve() ENTT_NOEXCEPT;
-};
-
-
-template<typename... Type>
-struct meta_info: meta_node<std::remove_cv_t<std::remove_reference_t<Type>>...> {};
-
-
-template<typename Op, typename Node>
-void iterate(Op op, const Node *curr) ENTT_NOEXCEPT {
-    while(curr) {
-        op(curr);
-        curr = curr->next;
-    }
-}
-
-
-template<auto Member, typename Op>
-void iterate(Op op, const meta_type_node *node) ENTT_NOEXCEPT {
-    if(node) {
-        auto *curr = node->base;
-        iterate(op, node->*Member);
-
-        while(curr) {
-            iterate<Member>(op, curr->type());
-            curr = curr->next;
-        }
-    }
-}
-
-
-template<typename Op, typename Node>
-auto find_if(Op op, const Node *curr) ENTT_NOEXCEPT {
-    while(curr && !op(curr)) {
-        curr = curr->next;
-    }
-
-    return curr;
-}
-
-
-template<auto Member, typename Op>
-auto find_if(Op op, const meta_type_node *node) ENTT_NOEXCEPT
--> decltype(find_if(op, node->*Member)) {
-    decltype(find_if(op, node->*Member)) ret = nullptr;
-
-    if(node) {
-        ret = find_if(op, node->*Member);
-        auto *curr = node->base;
-
-        while(curr && !ret) {
-            ret = find_if<Member>(op, curr->type());
-            curr = curr->next;
-        }
-    }
-
-    return ret;
-}
-
-
-template<typename Type>
-const Type * try_cast(const meta_type_node *node, void *instance) ENTT_NOEXCEPT {
-    const auto *type = meta_info<Type>::resolve();
-    void *ret = nullptr;
-
-    if(node == type) {
-        ret = instance;
+template<typename>
+[[nodiscard]] constexpr bool is_equality_comparable(...) { return false; }
+
+
+template<typename Type>
+[[nodiscard]] constexpr auto is_equality_comparable(choice_t<0>)
+-> decltype(std::declval<Type>() == std::declval<Type>()) { return true; }
+
+
+template<typename Type>
+[[nodiscard]] constexpr auto is_equality_comparable(choice_t<1>)
+-> decltype(std::declval<typename Type::value_type>(), std::declval<Type>() == std::declval<Type>()) {
+    if constexpr(is_iterator_v<Type>) {
+        return true;
+    } else if constexpr(std::is_same_v<typename Type::value_type, Type>) {
+        return is_equality_comparable<Type>(choice<0>);
     } else {
-        const auto *base = find_if<&meta_type_node::base>([type](auto *candidate) {
-            return candidate->type() == type;
-        }, node);
-
-        ret = base ? base->cast(instance) : nullptr;
-    }
-
-    return static_cast<const Type *>(ret);
-}
-
-
-template<auto Member>
-inline bool can_cast_or_convert(const meta_type_node *from, const meta_type_node *to) ENTT_NOEXCEPT {
-    return (from == to) || find_if<Member>([to](auto *node) {
-        return node->type() == to;
-    }, from);
-}
-
-
-template<typename... Args, std::size_t... Indexes>
-inline auto ctor(std::index_sequence<Indexes...>, const meta_type_node *node) ENTT_NOEXCEPT {
-    return internal::find_if([](auto *candidate) {
-        return candidate->size == sizeof...(Args) &&
-                (([](auto *from, auto *to) {
-                    return internal::can_cast_or_convert<&internal::meta_type_node::base>(from, to)
-                            || internal::can_cast_or_convert<&internal::meta_type_node::conv>(from, to);
-                }(internal::meta_info<Args>::resolve(), candidate->arg(Indexes))) && ...);
-    }, node->ctor);
-}
-
-
-}
-
-
-/**
- * Internal details not to be documented.
- * @endcond TURN_OFF_DOXYGEN
- */
-
-
-/**
- * @brief Meta any object.
- *
- * A meta any is an opaque container for single values of any type.
- *
- * This class uses a technique called small buffer optimization (SBO) to
- * completely eliminate the need to allocate memory, where possible.<br/>
- * From the user's point of view, nothing will change, but the elimination of
- * allocations will reduce the jumps in memory and therefore will avoid chasing
- * of pointers. This will greatly improve the use of the cache, thus increasing
- * the overall performance.
- */
-class meta_any {
-    /*! @brief A meta handle is allowed to _inherit_ from a meta any. */
-    friend struct meta_handle;
-
-    using storage_type = std::aligned_storage_t<sizeof(void *), alignof(void *)>;
-    using compare_fn_type = bool(const void *, const void *);
-    using copy_fn_type = void *(storage_type &, const void *);
-    using destroy_fn_type = void(void *);
-    using steal_fn_type = void *(storage_type &, void *, destroy_fn_type *);
-
-    template<typename Type, typename = std::void_t<>>
-    struct type_traits {
-        template<typename... Args>
-        static void * instance(storage_type &storage, Args &&... args) {
-            auto instance = std::make_unique<Type>(std::forward<Args>(args)...);
-            new (&storage) Type *{instance.get()};
-            return instance.release();
-        }
-
-        static void destroy(void *instance) {
-            auto *node = internal::meta_info<Type>::resolve();
-            auto *actual = static_cast<Type *>(instance);
-            [[maybe_unused]] const bool destroyed = node->meta().destroy(*actual);
-            ENTT_ASSERT(destroyed);
-            delete actual;
-        }
-
-        static void * copy(storage_type &storage, const void *other) {
-            auto instance = std::make_unique<Type>(*static_cast<const Type *>(other));
-            new (&storage) Type *{instance.get()};
-            return instance.release();
-        }
-
-        static void * steal(storage_type &to, void *from, destroy_fn_type *) {
-            auto *instance = static_cast<Type *>(from);
-            new (&to) Type *{instance};
-            return instance;
-        }
-    };
-
-    template<typename Type>
-    struct type_traits<Type, std::enable_if_t<sizeof(Type) <= sizeof(void *) && std::is_nothrow_move_constructible_v<Type>>> {
-        template<typename... Args>
-        static void * instance(storage_type &storage, Args &&... args) {
-            return new (&storage) Type{std::forward<Args>(args)...};
-        }
-
-        static void destroy(void *instance) {
-            auto *node = internal::meta_info<Type>::resolve();
-            auto *actual = static_cast<Type *>(instance);
-            [[maybe_unused]] const bool destroyed = node->meta().destroy(*actual);
-            ENTT_ASSERT(destroyed);
-            actual->~Type();
-        }
-
-        static void * copy(storage_type &storage, const void *instance) {
-            return new (&storage) Type{*static_cast<const Type *>(instance)};
-        }
-
-        static void * steal(storage_type &to, void *from, destroy_fn_type *destroy_fn) {
-            void *instance = new (&to) Type{std::move(*static_cast<Type *>(from))};
-            destroy_fn(from);
-            return instance;
-        }
-    };
-
-    template<typename Type>
-    static auto compare(int, const Type &lhs, const Type &rhs)
-    -> decltype(lhs == rhs, bool{}) {
-        return lhs == rhs;
-    }
-
-    template<typename Type>
-    static bool compare(char, const Type &lhs, const Type &rhs) {
-        return &lhs == &rhs;
-    }
-
-public:
-    /*! @brief Default constructor. */
-    meta_any() ENTT_NOEXCEPT
-        : storage{},
-          instance{nullptr},
-          node{nullptr},
-          destroy_fn{nullptr},
-          compare_fn{nullptr},
-          copy_fn{nullptr},
-          steal_fn{nullptr}
-    {}
-
-    /**
-     * @brief Constructs a meta any by directly initializing the new object.
-     * @tparam Type Type of object to use to initialize the container.
-     * @tparam Args Types of arguments to use to construct the new instance.
-     * @param args Parameters to use to construct the instance.
-     */
-    template<typename Type, typename... Args>
-    explicit meta_any(std::in_place_type_t<Type>, [[maybe_unused]] Args &&... args)
-        : meta_any{}
-    {
-        node = internal::meta_info<Type>::resolve();
-
-        if constexpr(!std::is_void_v<Type>) {
-            using traits_type = type_traits<std::remove_cv_t<std::remove_reference_t<Type>>>;
-            instance = traits_type::instance(storage, std::forward<Args>(args)...);
-            destroy_fn = &traits_type::destroy;
-            copy_fn = &traits_type::copy;
-            steal_fn = &traits_type::steal;
-
-            compare_fn = [](const void *lhs, const void *rhs) {
-                return compare(0, *static_cast<const Type *>(lhs), *static_cast<const Type *>(rhs));
-            };
-        }
-    }
-
-    /**
-     * @brief Constructs a meta any that holds an unmanaged object.
-     * @tparam Type Type of object to use to initialize the container.
-     * @param type An instance of an object to use to initialize the container.
-     */
-    template<typename Type>
-    explicit meta_any(as_alias_t, Type &type)
-        : meta_any{}
-    {
-        node = internal::meta_info<Type>::resolve();
-        instance = &type;
-
-        compare_fn = [](const void *lhs, const void *rhs) {
-            return compare(0, *static_cast<const Type *>(lhs), *static_cast<const Type *>(rhs));
-        };
-    }
-
-    /**
-     * @brief Constructs a meta any from a given value.
-     * @tparam Type Type of object to use to initialize the container.
-     * @param type An instance of an object to use to initialize the container.
-     */
-    template<typename Type, typename = std::enable_if_t<!std::is_same_v<std::remove_cv_t<std::remove_reference_t<Type>>, meta_any>>>
-    meta_any(Type &&type)
-        : meta_any{std::in_place_type<std::remove_cv_t<std::remove_reference_t<Type>>>, std::forward<Type>(type)}
-    {}
-
-    /**
-     * @brief Copy constructor.
-     * @param other The instance to copy from.
-     */
-    meta_any(const meta_any &other)
-        : meta_any{}
-    {
-        node = other.node;
-        instance = other.copy_fn ? other.copy_fn(storage, other.instance) : other.instance;
-        destroy_fn = other.destroy_fn;
-        compare_fn = other.compare_fn;
-        copy_fn = other.copy_fn;
-        steal_fn = other.steal_fn;
-    }
-
-    /**
-     * @brief Move constructor.
-     *
-     * After meta any move construction, instances that have been moved from
-     * are placed in a valid but unspecified state. It's highly discouraged to
-     * continue using them.
-     *
-     * @param other The instance to move from.
-     */
-    meta_any(meta_any &&other) ENTT_NOEXCEPT
-        : meta_any{}
-    {
-        swap(*this, other);
-    }
-
-    /*! @brief Frees the internal storage, whatever it means. */
-    ~meta_any() {
-        if(destroy_fn) {
-            destroy_fn(instance);
-        }
-    }
-
-    /**
-     * @brief Assignment operator.
-     * @tparam Type Type of object to use to initialize the container.
-     * @param type An instance of an object to use to initialize the container.
-     * @return This meta any object.
-     */
-    template<typename Type, typename = std::enable_if_t<!std::is_same_v<std::remove_cv_t<std::remove_reference_t<Type>>, meta_any>>>
-    meta_any & operator=(Type &&type) {
-        return (*this = meta_any{std::forward<Type>(type)});
-    }
-
-    /**
-     * @brief Copy assignment operator.
-     * @param other The instance to assign.
-     * @return This meta any object.
-     */
-    meta_any & operator=(const meta_any &other) {
-        return (*this = meta_any{other});
-    }
-
-    /**
-     * @brief Move assignment operator.
-     * @param other The instance to assign.
-     * @return This meta any object.
-     */
-    meta_any & operator=(meta_any &&other) ENTT_NOEXCEPT {
-        meta_any any{std::move(other)};
-        swap(any, *this);
-        return *this;
-    }
-
-    /**
-     * @brief Returns the meta type of the underlying object.
-     * @return The meta type of the underlying object, if any.
-     */
-    inline meta_type type() const ENTT_NOEXCEPT;
-
-    /**
-     * @brief Returns an opaque pointer to the contained instance.
-     * @return An opaque pointer the contained instance, if any.
-     */
-    const void * data() const ENTT_NOEXCEPT {
-        return instance;
-    }
-
-    /*! @copydoc data */
-    void * data() ENTT_NOEXCEPT {
-        return const_cast<void *>(std::as_const(*this).data());
-    }
-
-    /**
-     * @brief Tries to cast an instance to a given type.
-     * @tparam Type Type to which to cast the instance.
-     * @return A (possibly null) pointer to the contained instance.
-     */
-    template<typename Type>
-    const Type * try_cast() const ENTT_NOEXCEPT {
-        return internal::try_cast<Type>(node, instance);
-    }
-
-    /*! @copydoc try_cast */
-    template<typename Type>
-    Type * try_cast() ENTT_NOEXCEPT {
-        return const_cast<Type *>(std::as_const(*this).try_cast<Type>());
-    }
-
-    /**
-     * @brief Tries to cast an instance to a given type.
-     *
-     * The type of the instance must be such that the cast is possible.
-     *
-     * @warning
-     * Attempting to perform a cast that isn't viable results in undefined
-     * behavior.<br/>
-     * An assertion will abort the execution at runtime in debug mode in case
-     * the cast is not feasible.
-     *
-     * @tparam Type Type to which to cast the instance.
-     * @return A reference to the contained instance.
-     */
-    template<typename Type>
-    const Type & cast() const ENTT_NOEXCEPT {
-        auto *actual = try_cast<Type>();
-        ENTT_ASSERT(actual);
-        return *actual;
-    }
-
-    /*! @copydoc cast */
-    template<typename Type>
-    Type & cast() ENTT_NOEXCEPT {
-        return const_cast<Type &>(std::as_const(*this).cast<Type>());
-    }
-
-    /**
-     * @brief Tries to convert an instance to a given type and returns it.
-     * @tparam Type Type to which to convert the instance.
-     * @return A valid meta any object if the conversion is possible, an invalid
-     * one otherwise.
-     */
-    template<typename Type>
-    meta_any convert() const {
-        meta_any any{};
-
-        if(const auto *type = internal::meta_info<Type>::resolve(); node == type) {
-            any = *static_cast<const Type *>(instance);
-        } else {
-            const auto *conv = internal::find_if<&internal::meta_type_node::conv>([type](auto *other) {
-                return other->type() == type;
-            }, node);
-
-            if(conv) {
-                any = conv->conv(instance);
-            }
-        }
-
-        return any;
-    }
-
-    /**
-     * @brief Tries to convert an instance to a given type.
-     * @tparam Type Type to which to convert the instance.
-     * @return True if the conversion is possible, false otherwise.
-     */
-    template<typename Type>
-    bool convert() {
-        bool valid = (node == internal::meta_info<Type>::resolve());
-
-        if(!valid) {
-            if(auto any = std::as_const(*this).convert<Type>(); any) {
-                swap(any, *this);
-                valid = true;
-            }
-        }
-
-        return valid;
-    }
-
-    /**
-     * @brief Replaces the contained object by initializing a new instance
-     * directly.
-     * @tparam Type Type of object to use to initialize the container.
-     * @tparam Args Types of arguments to use to construct the new instance.
-     * @param args Parameters to use to construct the instance.
-     */
-    template<typename Type, typename... Args>
-    void emplace(Args&& ... args) {
-        *this = meta_any{std::in_place_type_t<Type>{}, std::forward<Args>(args)...};
-    }
-
-    /**
-     * @brief Returns false if a container is empty, true otherwise.
-     * @return False if the container is empty, true otherwise.
-     */
-    explicit operator bool() const ENTT_NOEXCEPT {
-        return node;
-    }
-
-    /**
-     * @brief Checks if two containers differ in their content.
-     * @param other Container with which to compare.
-     * @return False if the two containers differ in their content, true
-     * otherwise.
-     */
-    bool operator==(const meta_any &other) const ENTT_NOEXCEPT {
-        return node == other.node && ((!compare_fn && !other.compare_fn) || compare_fn(instance, other.instance));
-    }
-
-    /**
-     * @brief Swaps two meta any objects.
-     * @param lhs A valid meta any object.
-     * @param rhs A valid meta any object.
-     */
-    friend void swap(meta_any &lhs, meta_any &rhs) ENTT_NOEXCEPT {
-        if(lhs.steal_fn && rhs.steal_fn) {
-            storage_type buffer;
-            auto *temp = lhs.steal_fn(buffer, lhs.instance, lhs.destroy_fn);
-            lhs.instance = rhs.steal_fn(lhs.storage, rhs.instance, rhs.destroy_fn);
-            rhs.instance = lhs.steal_fn(rhs.storage, temp, lhs.destroy_fn);
-        } else if(lhs.steal_fn) {
-            lhs.instance = lhs.steal_fn(rhs.storage, lhs.instance, lhs.destroy_fn);
-            std::swap(rhs.instance, lhs.instance);
-        } else if(rhs.steal_fn) {
-            rhs.instance = rhs.steal_fn(lhs.storage, rhs.instance, rhs.destroy_fn);
-            std::swap(rhs.instance, lhs.instance);
-        } else {
-            std::swap(lhs.instance, rhs.instance);
-        }
-
-        std::swap(lhs.node, rhs.node);
-        std::swap(lhs.destroy_fn, rhs.destroy_fn);
-        std::swap(lhs.compare_fn, rhs.compare_fn);
-        std::swap(lhs.copy_fn, rhs.copy_fn);
-        std::swap(lhs.steal_fn, rhs.steal_fn);
-    }
-
-private:
-    storage_type storage;
-    void *instance;
-    internal::meta_type_node *node;
-    destroy_fn_type *destroy_fn;
-    compare_fn_type *compare_fn;
-    copy_fn_type *copy_fn;
-    steal_fn_type *steal_fn;
-};
-
-
-/**
- * @brief Meta handle object.
- *
- * A meta handle is an opaque pointer to an instance of any type.
- *
- * A handle doesn't perform copies and isn't responsible for the contained
- * object. It doesn't prolong the lifetime of the pointed instance. Users are
- * responsible for ensuring that the target object remains alive for the entire
- * interval of use of the handle.
- */
-struct meta_handle {
-    /*! @brief Default constructor. */
-    meta_handle() ENTT_NOEXCEPT
-        : node{nullptr},
-          instance{nullptr}
-    {}
-
-    /**
-     * @brief Constructs a meta handle from a meta any object.
-     * @param any A reference to an object to use to initialize the handle.
-     */
-    meta_handle(meta_any &any) ENTT_NOEXCEPT
-        : node{any.node},
-          instance{any.instance}
-    {}
-
-    /**
-     * @brief Constructs a meta handle from a given instance.
-     * @tparam Type Type of object to use to initialize the handle.
-     * @param obj A reference to an object to use to initialize the handle.
-     */
-    template<typename Type, typename = std::enable_if_t<!std::is_same_v<std::remove_cv_t<std::remove_reference_t<Type>>, meta_handle>>>
-    meta_handle(Type &obj) ENTT_NOEXCEPT
-        : node{internal::meta_info<Type>::resolve()},
-          instance{&obj}
-    {}
-
-    /**
-     * @brief Returns the meta type of the underlying object.
-     * @return The meta type of the underlying object, if any.
-     */
-    inline meta_type type() const ENTT_NOEXCEPT;
-
-    /**
-     * @brief Returns an opaque pointer to the contained instance.
-     * @return An opaque pointer the contained instance, if any.
-     */
-    const void * data() const ENTT_NOEXCEPT {
-        return instance;
-    }
-
-    /*! @copydoc data */
-    void * data() ENTT_NOEXCEPT {
-        return const_cast<void *>(std::as_const(*this).data());
-    }
-
-    /**
-     * @brief Tries to cast an instance to a given type.
-     * @tparam Type Type to which to cast the instance.
-     * @return A (possibly null) pointer to the underlying object.
-     */
-    template<typename Type>
-    const Type * data() const ENTT_NOEXCEPT {
-        return internal::try_cast<Type>(node, instance);
-    }
-
-    /*! @copydoc data */
-    template<typename Type>
-    Type * data() ENTT_NOEXCEPT {
-        return const_cast<Type *>(std::as_const(*this).data<Type>());
-    }
-
-    /**
-     * @brief Returns false if a handle is empty, true otherwise.
-     * @return False if the handle is empty, true otherwise.
-     */
-    explicit operator bool() const ENTT_NOEXCEPT {
-        return instance;
-    }
-
-private:
-    const internal::meta_type_node *node;
-    void *instance;
-};
-
-
-/**
- * @brief Checks if two containers differ in their content.
- * @param lhs A meta any object, either empty or not.
- * @param rhs A meta any object, either empty or not.
- * @return True if the two containers differ in their content, false otherwise.
- */
-inline bool operator!=(const meta_any &lhs, const meta_any &rhs) ENTT_NOEXCEPT {
-    return !(lhs == rhs);
-}
-
-
-/**
- * @brief Meta property object.
- *
- * A meta property is an opaque container for a key/value pair.<br/>
- * Properties are associated with any other meta object to enrich it.
- */
-class meta_prop {
-    /*! @brief A meta factory is allowed to create meta objects. */
-    template<typename> friend class meta_factory;
-
-    meta_prop(const internal::meta_prop_node *curr) ENTT_NOEXCEPT
-        : node{curr}
-    {}
-
-public:
-    /*! @brief Default constructor. */
-    meta_prop() ENTT_NOEXCEPT
-        : node{nullptr}
-    {}
-
-    /**
-     * @brief Returns the stored key.
-     * @return A meta any containing the key stored with the given property.
-     */
-    meta_any key() const ENTT_NOEXCEPT {
-        return node->key();
-    }
-
-    /**
-     * @brief Returns the stored value.
-     * @return A meta any containing the value stored with the given property.
-     */
-    meta_any value() const ENTT_NOEXCEPT {
-        return node->value();
-    }
-
-    /**
-     * @brief Returns true if a meta object is valid, false otherwise.
-     * @return True if the meta object is valid, false otherwise.
-     */
-    explicit operator bool() const ENTT_NOEXCEPT {
-        return node;
-    }
-
-    /**
-     * @brief Checks if two meta objects refer to the same node.
-     * @param other The meta object with which to compare.
-     * @return True if the two meta objects refer to the same node, false
-     * otherwise.
-     */
-    bool operator==(const meta_prop &other) const ENTT_NOEXCEPT {
-        return node == other.node;
-    }
-
-private:
-    const internal::meta_prop_node *node;
-};
-
-
-/**
- * @brief Checks if two meta objects refer to the same node.
- * @param lhs A meta object, either valid or not.
- * @param rhs A meta object, either valid or not.
- * @return True if the two meta objects refer to the same node, false otherwise.
- */
-inline bool operator!=(const meta_prop &lhs, const meta_prop &rhs) ENTT_NOEXCEPT {
-    return !(lhs == rhs);
-}
-
-
-/**
- * @brief Meta base object.
- *
- * A meta base is an opaque container for a base class to be used to walk
- * through hierarchies.
- */
-class meta_base {
-    /*! @brief A meta factory is allowed to create meta objects. */
-    template<typename> friend class meta_factory;
-
-    meta_base(const internal::meta_base_node *curr) ENTT_NOEXCEPT
-        : node{curr}
-    {}
-
-public:
-    /*! @brief Default constructor. */
-    meta_base() ENTT_NOEXCEPT
-        : node{nullptr}
-    {}
-
-    /**
-     * @brief Returns the meta type to which a meta base belongs.
-     * @return The meta type to which the meta base belongs.
-     */
-    inline meta_type parent() const ENTT_NOEXCEPT;
-
-    /**
-     * @brief Returns the meta type of a given meta base.
-     * @return The meta type of the meta base.
-     */
-    inline meta_type type() const ENTT_NOEXCEPT;
-
-    /**
-     * @brief Casts an instance from a parent type to a base type.
-     * @param instance The instance to cast.
-     * @return An opaque pointer to the base type.
-     */
-    void * cast(void *instance) const ENTT_NOEXCEPT {
-        return node->cast(instance);
-    }
-
-    /**
-     * @brief Returns true if a meta object is valid, false otherwise.
-     * @return True if the meta object is valid, false otherwise.
-     */
-    explicit operator bool() const ENTT_NOEXCEPT {
-        return node;
-    }
-
-    /**
-     * @brief Checks if two meta objects refer to the same node.
-     * @param other The meta object with which to compare.
-     * @return True if the two meta objects refer to the same node, false
-     * otherwise.
-     */
-    bool operator==(const meta_base &other) const ENTT_NOEXCEPT {
-        return node == other.node;
-    }
-
-private:
-    const internal::meta_base_node *node;
-};
-
-
-/**
- * @brief Checks if two meta objects refer to the same node.
- * @param lhs A meta object, either valid or not.
- * @param rhs A meta object, either valid or not.
- * @return True if the two meta objects refer to the same node, false otherwise.
- */
-inline bool operator!=(const meta_base &lhs, const meta_base &rhs) ENTT_NOEXCEPT {
-    return !(lhs == rhs);
-}
-
-
-/**
- * @brief Meta conversion function object.
- *
- * A meta conversion function is an opaque container for a conversion function
- * to be used to convert a given instance to another type.
- */
-class meta_conv {
-    /*! @brief A meta factory is allowed to create meta objects. */
-    template<typename> friend class meta_factory;
-
-    meta_conv(const internal::meta_conv_node *curr) ENTT_NOEXCEPT
-        : node{curr}
-    {}
-
-public:
-    /*! @brief Default constructor. */
-    meta_conv() ENTT_NOEXCEPT
-        : node{nullptr}
-    {}
-
-    /**
-     * @brief Returns the meta type to which a meta conversion function belongs.
-     * @return The meta type to which the meta conversion function belongs.
-     */
-    inline meta_type parent() const ENTT_NOEXCEPT;
-
-    /**
-     * @brief Returns the meta type of a given meta conversion function.
-     * @return The meta type of the meta conversion function.
-     */
-    inline meta_type type() const ENTT_NOEXCEPT;
-
-    /**
-     * @brief Converts an instance to a given type.
-     * @param instance The instance to convert.
-     * @return An opaque pointer to the instance to convert.
-     */
-    meta_any convert(const void *instance) const ENTT_NOEXCEPT {
-        return node->conv(instance);
-    }
-
-    /**
-     * @brief Returns true if a meta object is valid, false otherwise.
-     * @return True if the meta object is valid, false otherwise.
-     */
-    explicit operator bool() const ENTT_NOEXCEPT {
-        return node;
-    }
-
-    /**
-     * @brief Checks if two meta objects refer to the same node.
-     * @param other The meta object with which to compare.
-     * @return True if the two meta objects refer to the same node, false
-     * otherwise.
-     */
-    bool operator==(const meta_conv &other) const ENTT_NOEXCEPT {
-        return node == other.node;
-    }
-
-private:
-    const internal::meta_conv_node *node;
-};
-
-
-/**
- * @brief Checks if two meta objects refer to the same node.
- * @param lhs A meta object, either valid or not.
- * @param rhs A meta object, either valid or not.
- * @return True if the two meta objects refer to the same node, false otherwise.
- */
-inline bool operator!=(const meta_conv &lhs, const meta_conv &rhs) ENTT_NOEXCEPT {
-    return !(lhs == rhs);
-}
-
-
-/**
- * @brief Meta constructor object.
- *
- * A meta constructor is an opaque container for a function to be used to
- * construct instances of a given type.
- */
-class meta_ctor {
-    /*! @brief A meta factory is allowed to create meta objects. */
-    template<typename> friend class meta_factory;
-
-    meta_ctor(const internal::meta_ctor_node *curr) ENTT_NOEXCEPT
-        : node{curr}
-    {}
-
-public:
-    /*! @brief Unsigned integer type. */
-    using size_type = typename internal::meta_ctor_node::size_type;
-
-    /*! @brief Default constructor. */
-    meta_ctor() ENTT_NOEXCEPT
-        : node{nullptr}
-    {}
-
-    /**
-     * @brief Returns the meta type to which a meta constructor belongs.
-     * @return The meta type to which the meta constructor belongs.
-     */
-    inline meta_type parent() const ENTT_NOEXCEPT;
-
-    /**
-     * @brief Returns the number of arguments accepted by a meta constructor.
-     * @return The number of arguments accepted by the meta constructor.
-     */
-    size_type size() const ENTT_NOEXCEPT {
-        return node->size;
-    }
-
-    /**
-     * @brief Returns the meta type of the i-th argument of a meta constructor.
-     * @param index The index of the argument of which to return the meta type.
-     * @return The meta type of the i-th argument of a meta constructor, if any.
-     */
-    meta_type arg(size_type index) const ENTT_NOEXCEPT;
-
-    /**
-     * @brief Creates an instance of the underlying type, if possible.
-     *
-     * To create a valid instance, the types of the parameters must coincide
-     * exactly with those required by the underlying meta constructor.
-     * Otherwise, an empty and then invalid container is returned.
-     *
-     * @tparam Args Types of arguments to use to construct the instance.
-     * @param args Parameters to use to construct the instance.
-     * @return A meta any containing the new instance, if any.
-     */
-    template<typename... Args>
-    meta_any invoke(Args &&... args) const {
-        std::array<meta_any, sizeof...(Args)> arguments{{std::forward<Args>(args)...}};
-        meta_any any{};
-
-        if(sizeof...(Args) == size()) {
-            any = node->invoke(arguments.data());
-        }
-
-        return any;
-    }
-
-    /**
-     * @brief Iterates all the properties assigned to a meta constructor.
-     * @tparam Op Type of the function object to invoke.
-     * @param op A valid function object.
-     */
-    template<typename Op>
-    std::enable_if_t<std::is_invocable_v<Op, meta_prop>, void>
-    prop(Op op) const ENTT_NOEXCEPT {
-        internal::iterate([op = std::move(op)](auto *curr) {
-            op(curr->meta());
-        }, node->prop);
-    }
-
-    /**
-     * @brief Returns the property associated with a given key.
-     * @tparam Key Type of key to use to search for a property.
-     * @param key The key to use to search for a property.
-     * @return The property associated with the given key, if any.
-     */
-    template<typename Key>
-    std::enable_if_t<!std::is_invocable_v<Key, meta_prop>, meta_prop>
-    prop(Key &&key) const ENTT_NOEXCEPT {
-        const auto *curr = internal::find_if([key = meta_any{std::forward<Key>(key)}](auto *candidate) {
-            return candidate->key() == key;
-        }, node->prop);
-
-        return curr ? curr->meta() : meta_prop{};
-    }
-
-    /**
-     * @brief Returns true if a meta object is valid, false otherwise.
-     * @return True if the meta object is valid, false otherwise.
-     */
-    explicit operator bool() const ENTT_NOEXCEPT {
-        return node;
-    }
-
-    /**
-     * @brief Checks if two meta objects refer to the same node.
-     * @param other The meta object with which to compare.
-     * @return True if the two meta objects refer to the same node, false
-     * otherwise.
-     */
-    bool operator==(const meta_ctor &other) const ENTT_NOEXCEPT {
-        return node == other.node;
-    }
-
-private:
-    const internal::meta_ctor_node *node;
-};
-
-
-/**
- * @brief Checks if two meta objects refer to the same node.
- * @param lhs A meta object, either valid or not.
- * @param rhs A meta object, either valid or not.
- * @return True if the two meta objects refer to the same node, false otherwise.
- */
-inline bool operator!=(const meta_ctor &lhs, const meta_ctor &rhs) ENTT_NOEXCEPT {
-    return !(lhs == rhs);
-}
-
-
-/**
- * @brief Meta destructor object.
- *
- * A meta destructor is an opaque container for a function to be used to
- * destroy instances of a given type.
- */
-class meta_dtor {
-    /*! @brief A meta factory is allowed to create meta objects. */
-    template<typename> friend class meta_factory;
-
-    meta_dtor(const internal::meta_dtor_node *curr) ENTT_NOEXCEPT
-        : node{curr}
-    {}
-
-public:
-    /*! @brief Default constructor. */
-    meta_dtor() ENTT_NOEXCEPT
-        : node{nullptr}
-    {}
-
-    /**
-     * @brief Returns the meta type to which a meta destructor belongs.
-     * @return The meta type to which the meta destructor belongs.
-     */
-    inline meta_type parent() const ENTT_NOEXCEPT;
-
-    /**
-     * @brief Destroys an instance of the underlying type.
-     *
-     * It must be possible to cast the instance to the parent type of the meta
-     * destructor. Otherwise, invoking the meta destructor results in an
-     * undefined behavior.
-     *
-     * @param handle An opaque pointer to an instance of the underlying type.
+        return is_equality_comparable<typename Type::value_type>(choice<2>);
+    }
+}
+
+
+template<typename Type>
+[[nodiscard]] constexpr auto is_equality_comparable(choice_t<2>)
+-> decltype(std::declval<typename Type::mapped_type>(), std::declval<Type>() == std::declval<Type>()) {
+    return is_equality_comparable<typename Type::key_type>(choice<2>) && is_equality_comparable<typename Type::mapped_type>(choice<2>);
+}
+
+
+}
+
+
+/**
+ * Internal details not to be documented.
+ * @endcond
+ */
+
+
+/**
+ * @brief Provides the member constant `value` to true if a given type is
+ * equality comparable, false otherwise.
+ * @tparam Type The type to test.
+ */
+template<typename Type, typename = void>
+struct is_equality_comparable: std::bool_constant<internal::is_equality_comparable<Type>(choice<2>)> {};
+
+
+/**
+ * @brief Helper variable template.
+ * @tparam Type The type to test.
+ */
+template<class Type>
+inline constexpr bool is_equality_comparable_v = is_equality_comparable<Type>::value;
+
+
+/**
+ * @brief Transcribes the constness of a type to another type.
+ * @tparam To The type to which to transcribe the constness.
+ * @tparam From The type from which to transcribe the constness.
+ */
+template<typename To, typename From>
+struct constness_as {
+    /*! @brief The type resulting from the transcription of the constness. */
+    using type = std::remove_const_t<To>;
+};
+
+
+/*! @copydoc constness_as */
+template<typename To, typename From>
+struct constness_as<To, const From> {
+    /*! @brief The type resulting from the transcription of the constness. */
+    using type = std::add_const_t<To>;
+};
+
+
+/**
+ * @brief Alias template to facilitate the transcription of the constness.
+ * @tparam To The type to which to transcribe the constness.
+ * @tparam From The type from which to transcribe the constness.
+ */
+template<typename To, typename From>
+using constness_as_t = typename constness_as<To, From>::type;
+
+
+/**
+ * @brief Extracts the class of a non-static member object or function.
+ * @tparam Member A pointer to a non-static member object or function.
+ */
+template<typename Member>
+class member_class {
+    static_assert(std::is_member_pointer_v<Member>, "Invalid pointer type to non-static member object or function");
+
+    template<typename Class, typename Ret, typename... Args>
+    static Class * clazz(Ret(Class:: *)(Args...));
+
+    template<typename Class, typename Ret, typename... Args>
+    static Class * clazz(Ret(Class:: *)(Args...) const);
+
+    template<typename Class, typename Type>
+    static Class * clazz(Type Class:: *);
+
+public:
+    /*! @brief The class of the given non-static member object or function. */
+    using type = std::remove_pointer_t<decltype(clazz(std::declval<Member>()))>;
+};
+
+
+/**
+ * @brief Helper type.
+ * @tparam Member A pointer to a non-static member object or function.
+ */
+template<typename Member>
+using member_class_t = typename member_class<Member>::type;
+
+
+}
+
+
+#endif
+
+// #include "type_traits.hpp"
+#ifndef ENTT_META_TYPE_TRAITS_HPP
+#define ENTT_META_TYPE_TRAITS_HPP
+
+
+#include <type_traits>
+
+
+namespace entt {
+
+
+/**
+ * @brief Traits class template to be specialized to enable support for meta
+ * template information.
+ */
+template<typename>
+struct meta_template_traits;
+
+
+/**
+ * @brief Traits class template to be specialized to enable support for meta
+ * sequence containers.
+ */
+template<typename>
+struct meta_sequence_container_traits;
+
+
+/**
+ * @brief Traits class template to be specialized to enable support for meta
+ * associative containers.
+ */
+template<typename>
+struct meta_associative_container_traits;
+
+
+/**
+ * @brief Provides the member constant `value` to true if a meta associative
+ * container claims to wrap a key-only type, false otherwise.
+ * @tparam Type Potentially key-only meta associative container type.
+ */
+template<typename, typename = void>
+struct is_key_only_meta_associative_container: std::true_type {};
+
+
+/*! @copydoc is_key_only_meta_associative_container */
+template<typename Type>
+struct is_key_only_meta_associative_container<Type, std::void_t<typename meta_associative_container_traits<Type>::type::mapped_type>>
+    : std::false_type
+{};
+
+
+/**
+ * @brief Helper variable template.
+ * @tparam Type Potentially key-only meta associative container type.
+ */
+template<typename Type>
+inline constexpr auto is_key_only_meta_associative_container_v = is_key_only_meta_associative_container<Type>::value;
+
+
+/**
+ * @brief Provides the member constant `value` to true if a given type is a
+ * pointer-like type from the point of view of the meta system, false otherwise.
+ * @tparam Type Potentially pointer-like type.
+ */
+template<typename>
+struct is_meta_pointer_like: std::false_type {};
+
+
+/**
+ * @brief Partial specialization to ensure that const pointer-like types are
+ * also accepted.
+ * @tparam Type Potentially pointer-like type.
+ */
+template<typename Type>
+struct is_meta_pointer_like<const Type>: is_meta_pointer_like<Type> {};
+
+
+/**
+ * @brief Helper variable template.
+ * @tparam Type Potentially pointer-like type.
+ */
+template<typename Type>
+inline constexpr auto is_meta_pointer_like_v = is_meta_pointer_like<Type>::value;
+
+
+}
+
+
+#endif
+
+
+
+namespace entt {
+
+
+/**
+ * @brief Container traits.
+ * @tparam Container Type of the underlying container.
+ * @tparam Trait Traits associated with the underlying container.
+ */
+template<typename Container, template<typename> class... Trait>
+struct meta_container_traits: public Trait<Container>... {
+    /*! @brief Type of container. */
+    using type = Container;
+};
+
+
+/**
+ * @brief Basic STL-compatible container traits
+ * @tparam Container The type of the container.
+ */
+template<typename Container>
+struct basic_container {
+    /**
+     * @brief Returns the size of the given container.
+     * @param cont The container for which to return the size.
+     * @return The size of the given container.
+     */
+    [[nodiscard]] static typename Container::size_type size(const Container &cont) ENTT_NOEXCEPT {
+        return cont.size();
+    }
+
+    /**
+     * @brief Returns an iterator to the first element of the given container.
+     * @param cont The container for which to return the iterator.
+     * @return An iterator to the first element of the given container.
+     */
+    [[nodiscard]] static typename Container::iterator begin(Container &cont) {
+        return cont.begin();
+    }
+
+    /**
+     * @brief Returns an iterator to the first element of the given container.
+     * @param cont The container for which to return the iterator.
+     * @return An iterator to the first element of the given container.
+     */
+    [[nodiscard]] static typename Container::const_iterator cbegin(const Container &cont) {
+        return cont.begin();
+    }
+
+    /**
+     * @brief Returns an iterator past the last element of the given container.
+     * @param cont The container for which to return the iterator.
+     * @return An iterator past the last element of the given container.
+     */
+    [[nodiscard]] static typename Container::iterator end(Container &cont) {
+        return cont.end();
+    }
+
+    /**
+     * @brief Returns an iterator past the last element of the given container.
+     * @param cont The container for which to return the iterator.
+     * @return An iterator past the last element of the given container.
+     */
+    [[nodiscard]] static typename Container::const_iterator cend(const Container &cont) {
+        return cont.end();
+    }
+};
+
+
+/**
+ * @brief Basic STL-compatible associative container traits
+ * @tparam Container The type of the container.
+ */
+template<typename Container>
+struct basic_associative_container {
+    /**
+     * @brief Returns an iterator to the element with key equivalent to the
+     * given one, if any.
+     * @param cont The container in which to search for the element.
+     * @param key The key of the element to search.
+     * @return An iterator to the element with the given key, if any.
+     */
+    [[nodiscard]] static typename Container::iterator find(Container &cont, const typename Container::key_type &key) {
+        return cont.find(key);
+    }
+
+    /*! @copydoc find */
+    [[nodiscard]] static typename Container::const_iterator cfind(const Container &cont, const typename Container::key_type &key) {
+        return cont.find(key);
+    }
+};
+
+
+/**
+ * @brief Basic STL-compatible dynamic container traits
+ * @tparam Container The type of the container.
+ */
+template<typename Container>
+struct basic_dynamic_container {
+    /**
+     * @brief Clears the content of the given container.
+     * @param cont The container for which to clear the content.
      * @return True in case of success, false otherwise.
      */
-    bool invoke(meta_handle handle) const {
-        return node->invoke(handle);
-    }
-
-    /**
-     * @brief Returns true if a meta object is valid, false otherwise.
-     * @return True if the meta object is valid, false otherwise.
-     */
-    explicit operator bool() const ENTT_NOEXCEPT {
-        return node;
-    }
-
-    /**
-     * @brief Checks if two meta objects refer to the same node.
-     * @param other The meta object with which to compare.
-     * @return True if the two meta objects refer to the same node, false
-     * otherwise.
-     */
-    bool operator==(const meta_dtor &other) const ENTT_NOEXCEPT {
-        return node == other.node;
-    }
-
-private:
-    const internal::meta_dtor_node *node;
-};
-
-
-/**
- * @brief Checks if two meta objects refer to the same node.
- * @param lhs A meta object, either valid or not.
- * @param rhs A meta object, either valid or not.
- * @return True if the two meta objects refer to the same node, false otherwise.
- */
-inline bool operator!=(const meta_dtor &lhs, const meta_dtor &rhs) ENTT_NOEXCEPT {
-    return !(lhs == rhs);
-}
-
-
-/**
- * @brief Meta data object.
- *
- * A meta data is an opaque container for a data member associated with a given
- * type.
- */
-class meta_data {
-    /*! @brief A meta factory is allowed to create meta objects. */
-    template<typename> friend class meta_factory;
-
-    meta_data(const internal::meta_data_node *curr) ENTT_NOEXCEPT
-        : node{curr}
-    {}
-
-public:
-    /*! @brief Default constructor. */
-    meta_data() ENTT_NOEXCEPT
-        : node{nullptr}
-    {}
-
-    /**
-     * @brief Returns the identifier assigned to a given meta data.
-     * @return The identifier assigned to the meta data.
-     */
-    ENTT_ID_TYPE identifier() const ENTT_NOEXCEPT {
-        return node->identifier;
-    }
-
-    /**
-     * @brief Returns the meta type to which a meta data belongs.
-     * @return The meta type to which the meta data belongs.
-     */
-    inline meta_type parent() const ENTT_NOEXCEPT;
-
-    /**
-     * @brief Indicates whether a given meta data is constant or not.
-     * @return True if the meta data is constant, false otherwise.
-     */
-    bool is_const() const ENTT_NOEXCEPT {
-        return node->is_const;
-    }
-
-    /**
-     * @brief Indicates whether a given meta data is static or not.
-     *
-     * A static meta data is such that it can be accessed using a null pointer
-     * as an instance.
-     *
-     * @return True if the meta data is static, false otherwise.
-     */
-    bool is_static() const ENTT_NOEXCEPT {
-        return node->is_static;
-    }
-
-    /**
-     * @brief Returns the meta type of a given meta data.
-     * @return The meta type of the meta data.
-     */
-    inline meta_type type() const ENTT_NOEXCEPT;
-
-    /**
-     * @brief Sets the value of the variable enclosed by a given meta type.
-     *
-     * It must be possible to cast the instance to the parent type of the meta
-     * data. Otherwise, invoking the setter results in an undefined
-     * behavior.<br/>
-     * The type of the value must coincide exactly with that of the variable
-     * enclosed by the meta data. Otherwise, invoking the setter does nothing.
-     *
-     * @tparam Type Type of value to assign.
-     * @param handle An opaque pointer to an instance of the underlying type.
-     * @param value Parameter to use to set the underlying variable.
-     * @return True in case of success, false otherwise.
-     */
-    template<typename Type>
-    bool set(meta_handle handle, Type &&value) const {
-        return node->set(handle, meta_any{}, std::forward<Type>(value));
-    }
-
-    /**
-     * @brief Sets the i-th element of an array enclosed by a given meta type.
-     *
-     * It must be possible to cast the instance to the parent type of the meta
-     * data. Otherwise, invoking the setter results in an undefined
-     * behavior.<br/>
-     * The type of the value must coincide exactly with that of the array type
-     * enclosed by the meta data. Otherwise, invoking the setter does nothing.
-     *
-     * @tparam Type Type of value to assign.
-     * @param handle An opaque pointer to an instance of the underlying type.
-     * @param index Position of the underlying element to set.
-     * @param value Parameter to use to set the underlying element.
+    [[nodiscard]] static bool clear([[maybe_unused]] Container &cont) {
+        return cont.clear(), true;
+    }
+};
+
+
+/**
+ * @brief Basic STL-compatible dynamic associative container traits
+ * @tparam Container The type of the container.
+ */
+template<typename Container>
+struct basic_dynamic_associative_container {
+    /**
+     * @brief Removes the specified element from the given container.
+     * @param cont The container from which to remove the element.
+     * @param key The element to remove.
+     * @return A bool denoting whether the removal took place.
+     */
+    [[nodiscard]] static bool erase([[maybe_unused]] Container &cont, [[maybe_unused]] const typename Container::key_type &key) {
+        const auto sz = cont.size();
+        return cont.erase(key) != sz;
+    }
+};
+
+
+/**
+ * @brief Basic STL-compatible sequence container traits
+ * @tparam Container The type of the container.
+ */
+template<typename Container>
+struct basic_sequence_container {
+    /**
+     * @brief Returns a reference to the element at the specified location of
+     * the given container (no bounds checking is performed).
+     * @param cont The container from which to get the element.
+     * @param pos The position of the element to return.
+     * @return A reference to the requested element.
+     */
+    [[nodiscard]] static typename Container::reference get(Container &cont, typename Container::size_type pos) {
+        return cont[pos];
+    }
+
+    /*! @copydoc get */
+    [[nodiscard]] static typename Container::const_reference cget(const Container &cont, typename Container::size_type pos) {
+        return cont[pos];
+    }
+};
+
+
+/**
+ * @brief STL-compatible dynamic associative key-only container traits
+ * @tparam Container The type of the container.
+ */
+template<typename Container>
+struct dynamic_associative_key_only_container {
+    /**
+     * @brief Inserts an element into the given container.
+     * @param cont The container in which to insert the element.
+     * @param key The element to insert.
+     * @return A bool denoting whether the insertion took place.
+     */
+    [[nodiscard]] static bool insert([[maybe_unused]] Container &cont, [[maybe_unused]] const typename Container::key_type &key) {
+        return cont.insert(key).second;
+    }
+};
+
+
+/**
+ * @brief STL-compatible dynamic key-value associative container traits
+ * @tparam Container The type of the container.
+ */
+template<typename Container>
+struct dynamic_associative_key_value_container {
+    /**
+     * @brief Inserts an element (a key/value pair) into the given container.
+     * @param cont The container in which to insert the element.
+     * @param key The key of the element to insert.
+     * @param value The value of the element to insert.
+     * @return A bool denoting whether the insertion took place.
+     */
+    [[nodiscard]] static bool insert([[maybe_unused]] Container &cont, [[maybe_unused]] const typename Container::key_type &key, [[maybe_unused]] const typename Container::mapped_type &value) {
+        return cont.insert(std::make_pair(key, value)).second;
+    }
+};
+
+
+/**
+ * @brief STL-compatible dynamic sequence container traits
+ * @tparam Container The type of the container.
+ */
+template<typename Container>
+struct dynamic_sequence_container {
+    /**
+     * @brief Resizes the given container to contain the given number of
+     * elements.
+     * @param cont The container to resize.
+     * @param sz The new size of the container.
      * @return True in case of success, false otherwise.
      */
-    template<typename Type>
-    bool set(meta_handle handle, std::size_t index, Type &&value) const {
-        ENTT_ASSERT(index < node->type()->extent);
-        return node->set(handle, index, std::forward<Type>(value));
-    }
-
-    /**
-     * @brief Gets the value of the variable enclosed by a given meta type.
-     *
-     * It must be possible to cast the instance to the parent type of the meta
-     * data. Otherwise, invoking the getter results in an undefined behavior.
-     *
-     * @param handle An opaque pointer to an instance of the underlying type.
-     * @return A meta any containing the value of the underlying variable.
-     */
-    meta_any get(meta_handle handle) const ENTT_NOEXCEPT {
-        return node->get(handle, meta_any{});
-    }
-
-    /**
-     * @brief Gets the i-th element of an array enclosed by a given meta type.
-     *
-     * It must be possible to cast the instance to the parent type of the meta
-     * data. Otherwise, invoking the getter results in an undefined behavior.
-     *
-     * @param handle An opaque pointer to an instance of the underlying type.
-     * @param index Position of the underlying element to get.
-     * @return A meta any containing the value of the underlying element.
-     */
-    meta_any get(meta_handle handle, std::size_t index) const ENTT_NOEXCEPT {
-        ENTT_ASSERT(index < node->type()->extent);
-        return node->get(handle, index);
-    }
-
-    /**
-     * @brief Iterates all the properties assigned to a meta data.
-     * @tparam Op Type of the function object to invoke.
-     * @param op A valid function object.
-     */
-    template<typename Op>
-    std::enable_if_t<std::is_invocable_v<Op, meta_prop>, void>
-    prop(Op op) const ENTT_NOEXCEPT {
-        internal::iterate([op = std::move(op)](auto *curr) {
-            op(curr->meta());
-        }, node->prop);
-    }
-
-    /**
-     * @brief Returns the property associated with a given key.
-     * @tparam Key Type of key to use to search for a property.
-     * @param key The key to use to search for a property.
-     * @return The property associated with the given key, if any.
-     */
-    template<typename Key>
-    std::enable_if_t<!std::is_invocable_v<Key, meta_prop>, meta_prop>
-    prop(Key &&key) const ENTT_NOEXCEPT {
-        const auto *curr = internal::find_if([key = meta_any{std::forward<Key>(key)}](auto *candidate) {
-            return candidate->key() == key;
-        }, node->prop);
-
-        return curr ? curr->meta() : meta_prop{};
-    }
-
-    /**
-     * @brief Returns true if a meta object is valid, false otherwise.
-     * @return True if the meta object is valid, false otherwise.
-     */
-    explicit operator bool() const ENTT_NOEXCEPT {
-        return node;
-    }
-
-    /**
-     * @brief Checks if two meta objects refer to the same node.
-     * @param other The meta object with which to compare.
-     * @return True if the two meta objects refer to the same node, false
-     * otherwise.
-     */
-    bool operator==(const meta_data &other) const ENTT_NOEXCEPT {
-        return node == other.node;
-    }
-
-private:
-    const internal::meta_data_node *node;
-};
-
-
-/**
- * @brief Checks if two meta objects refer to the same node.
- * @param lhs A meta object, either valid or not.
- * @param rhs A meta object, either valid or not.
- * @return True if the two meta objects refer to the same node, false otherwise.
- */
-inline bool operator!=(const meta_data &lhs, const meta_data &rhs) ENTT_NOEXCEPT {
-    return !(lhs == rhs);
-}
-
-
-/**
- * @brief Meta function object.
- *
- * A meta function is an opaque container for a member function associated with
- * a given type.
- */
-class meta_func {
-    /*! @brief A meta factory is allowed to create meta objects. */
-    template<typename> friend class meta_factory;
-
-    meta_func(const internal::meta_func_node *curr) ENTT_NOEXCEPT
-        : node{curr}
-    {}
-
-public:
-    /*! @brief Unsigned integer type. */
-    using size_type = typename internal::meta_func_node::size_type;
-
-    /*! @brief Default constructor. */
-    meta_func() ENTT_NOEXCEPT
-        : node{nullptr}
-    {}
-
-    /**
-     * @brief Returns the identifier assigned to a given meta function.
-     * @return The identifier assigned to the meta function.
-     */
-    ENTT_ID_TYPE identifier() const ENTT_NOEXCEPT {
-        return node->identifier;
-    }
-
-    /**
-     * @brief Returns the meta type to which a meta function belongs.
-     * @return The meta type to which the meta function belongs.
-     */
-    inline meta_type parent() const ENTT_NOEXCEPT;
-
-    /**
-     * @brief Returns the number of arguments accepted by a meta function.
-     * @return The number of arguments accepted by the meta function.
-     */
-    size_type size() const ENTT_NOEXCEPT {
-        return node->size;
-    }
-
-    /**
-     * @brief Indicates whether a given meta function is constant or not.
-     * @return True if the meta function is constant, false otherwise.
-     */
-    bool is_const() const ENTT_NOEXCEPT {
-        return node->is_const;
-    }
-
-    /**
-     * @brief Indicates whether a given meta function is static or not.
-     *
-     * A static meta function is such that it can be invoked using a null
-     * pointer as an instance.
-     *
-     * @return True if the meta function is static, false otherwise.
-     */
-    bool is_static() const ENTT_NOEXCEPT {
-        return node->is_static;
-    }
-
-    /**
-     * @brief Returns the meta type of the return type of a meta function.
-     * @return The meta type of the return type of the meta function.
-     */
-    inline meta_type ret() const ENTT_NOEXCEPT;
-
-    /**
-     * @brief Returns the meta type of the i-th argument of a meta function.
-     * @param index The index of the argument of which to return the meta type.
-     * @return The meta type of the i-th argument of a meta function, if any.
-     */
-    inline meta_type arg(size_type index) const ENTT_NOEXCEPT;
-
-    /**
-     * @brief Invokes the underlying function, if possible.
-     *
-     * To invoke a meta function, the types of the parameters must coincide
-     * exactly with those required by the underlying function. Otherwise, an
-     * empty and then invalid container is returned.<br/>
-     * It must be possible to cast the instance to the parent type of the meta
-     * function. Otherwise, invoking the underlying function results in an
-     * undefined behavior.
-     *
-     * @tparam Args Types of arguments to use to invoke the function.
-     * @param handle An opaque pointer to an instance of the underlying type.
-     * @param args Parameters to use to invoke the function.
-     * @return A meta any containing the returned value, if any.
-     */
-    template<typename... Args>
-    meta_any invoke(meta_handle handle, Args &&... args) const {
-        std::array<meta_any, sizeof...(Args)> arguments{{std::forward<Args>(args)...}};
-        meta_any any{};
-
-        if(sizeof...(Args) == size()) {
-            any = node->invoke(handle, arguments.data());
-        }
-
-        return any;
-    }
-
-    /**
-     * @brief Iterates all the properties assigned to a meta function.
-     * @tparam Op Type of the function object to invoke.
-     * @param op A valid function object.
-     */
-    template<typename Op>
-    std::enable_if_t<std::is_invocable_v<Op, meta_prop>, void>
-    prop(Op op) const ENTT_NOEXCEPT {
-        internal::iterate([op = std::move(op)](auto *curr) {
-            op(curr->meta());
-        }, node->prop);
-    }
-
-    /**
-     * @brief Returns the property associated with a given key.
-     * @tparam Key Type of key to use to search for a property.
-     * @param key The key to use to search for a property.
-     * @return The property associated with the given key, if any.
-     */
-    template<typename Key>
-    std::enable_if_t<!std::is_invocable_v<Key, meta_prop>, meta_prop>
-    prop(Key &&key) const ENTT_NOEXCEPT {
-        const auto *curr = internal::find_if([key = meta_any{std::forward<Key>(key)}](auto *candidate) {
-            return candidate->key() == key;
-        }, node->prop);
-
-        return curr ? curr->meta() : meta_prop{};
-    }
-
-    /**
-     * @brief Returns true if a meta object is valid, false otherwise.
-     * @return True if the meta object is valid, false otherwise.
-     */
-    explicit operator bool() const ENTT_NOEXCEPT {
-        return node;
-    }
-
-    /**
-     * @brief Checks if two meta objects refer to the same node.
-     * @param other The meta object with which to compare.
-     * @return True if the two meta objects refer to the same node, false
-     * otherwise.
-     */
-    bool operator==(const meta_func &other) const ENTT_NOEXCEPT {
-        return node == other.node;
-    }
-
-private:
-    const internal::meta_func_node *node;
-};
-
-
-/**
- * @brief Checks if two meta objects refer to the same node.
- * @param lhs A meta object, either valid or not.
- * @param rhs A meta object, either valid or not.
- * @return True if the two meta objects refer to the same node, false otherwise.
- */
-inline bool operator!=(const meta_func &lhs, const meta_func &rhs) ENTT_NOEXCEPT {
-    return !(lhs == rhs);
-}
-
-
-/**
- * @brief Meta type object.
- *
- * A meta type is the starting point for accessing a reflected type, thus being
- * able to work through it on real objects.
- */
-class meta_type {
-    /*! @brief A meta node is allowed to create meta objects. */
-    template<typename...> friend struct internal::meta_node;
-
-    meta_type(const internal::meta_type_node *curr) ENTT_NOEXCEPT
-        : node{curr}
-    {}
-
-public:
-    /*! @brief Unsigned integer type. */
-    using size_type = typename internal::meta_type_node::size_type;
-
-    /*! @brief Default constructor. */
-    meta_type() ENTT_NOEXCEPT
-        : node{nullptr}
-    {}
-
-    /**
-     * @brief Returns the identifier assigned to a given meta type.
-     * @return The identifier assigned to the meta type.
-     */
-    ENTT_ID_TYPE identifier() const ENTT_NOEXCEPT {
-        return node->identifier;
-    }
-
-    /**
-     * @brief Indicates whether a given meta type refers to void or not.
-     * @return True if the underlying type is void, false otherwise.
-     */
-    bool is_void() const ENTT_NOEXCEPT {
-        return node->is_void;
-    }
-
-    /**
-     * @brief Indicates whether a given meta type refers to an integral type or
-     * not.
-     * @return True if the underlying type is an integral type, false otherwise.
-     */
-    bool is_integral() const ENTT_NOEXCEPT {
-        return node->is_integral;
-    }
-
-    /**
-     * @brief Indicates whether a given meta type refers to a floating-point
-     * type or not.
-     * @return True if the underlying type is a floating-point type, false
-     * otherwise.
-     */
-    bool is_floating_point() const ENTT_NOEXCEPT {
-        return node->is_floating_point;
-    }
-
-    /**
-     * @brief Indicates whether a given meta type refers to an array type or
-     * not.
-     * @return True if the underlying type is an array type, false otherwise.
-     */
-    bool is_array() const ENTT_NOEXCEPT {
-        return node->is_array;
-    }
-
-    /**
-     * @brief Indicates whether a given meta type refers to an enum or not.
-     * @return True if the underlying type is an enum, false otherwise.
-     */
-    bool is_enum() const ENTT_NOEXCEPT {
-        return node->is_enum;
-    }
-
-    /**
-     * @brief Indicates whether a given meta type refers to an union or not.
-     * @return True if the underlying type is an union, false otherwise.
-     */
-    bool is_union() const ENTT_NOEXCEPT {
-        return node->is_union;
-    }
-
-    /**
-     * @brief Indicates whether a given meta type refers to a class or not.
-     * @return True if the underlying type is a class, false otherwise.
-     */
-    bool is_class() const ENTT_NOEXCEPT {
-        return node->is_class;
-    }
-
-    /**
-     * @brief Indicates whether a given meta type refers to a pointer or not.
-     * @return True if the underlying type is a pointer, false otherwise.
-     */
-    bool is_pointer() const ENTT_NOEXCEPT {
-        return node->is_pointer;
-    }
-
-    /**
-     * @brief Indicates whether a given meta type refers to a function type or
-     * not.
-     * @return True if the underlying type is a function, false otherwise.
-     */
-    bool is_function() const ENTT_NOEXCEPT {
-        return node->is_function;
-    }
-
-    /**
-     * @brief Indicates whether a given meta type refers to a pointer to data
-     * member or not.
-     * @return True if the underlying type is a pointer to data member, false
-     * otherwise.
-     */
-    bool is_member_object_pointer() const ENTT_NOEXCEPT {
-        return node->is_member_object_pointer;
-    }
-
-    /**
-     * @brief Indicates whether a given meta type refers to a pointer to member
-     * function or not.
-     * @return True if the underlying type is a pointer to member function,
-     * false otherwise.
-     */
-    bool is_member_function_pointer() const ENTT_NOEXCEPT {
-        return node->is_member_function_pointer;
-    }
-
-    /**
-     * @brief If a given meta type refers to an array type, provides the number
-     * of elements of the array.
-     * @return The number of elements of the array if the underlying type is an
-     * array type, 0 otherwise.
-     */
-    size_type extent() const ENTT_NOEXCEPT {
-        return node->extent;
-    }
-
-    /**
-     * @brief Provides the meta type for which the pointer is defined.
-     * @return The meta type for which the pointer is defined or this meta type
-     * if it doesn't refer to a pointer type.
-     */
-    meta_type remove_pointer() const ENTT_NOEXCEPT {
-        return node->remove_pointer();
-    }
-
-    /**
-     * @brief Iterates all the meta base of a meta type.
-     *
-     * Iteratively returns **all** the base classes of the given type.
-     *
-     * @tparam Op Type of the function object to invoke.
-     * @param op A valid function object.
-     */
-    template<typename Op>
-    std::enable_if_t<std::is_invocable_v<Op, meta_base>, void>
-    base(Op op) const ENTT_NOEXCEPT {
-        internal::iterate<&internal::meta_type_node::base>([op = std::move(op)](auto *curr) {
-            op(curr->meta());
-        }, node);
-    }
-
-    /**
-     * @brief Returns the meta base associated with a given identifier.
-     *
-     * Searches recursively among **all** the base classes of the given type.
-     *
-     * @param identifier Unique identifier.
-     * @return The meta base associated with the given identifier, if any.
-     */
-    meta_base base(const ENTT_ID_TYPE identifier) const ENTT_NOEXCEPT {
-        const auto *curr = internal::find_if<&internal::meta_type_node::base>([identifier](auto *candidate) {
-            return candidate->type()->identifier == identifier;
-        }, node);
-
-        return curr ? curr->meta() : meta_base{};
-    }
-
-    /**
-     * @brief Iterates all the meta conversion functions of a meta type.
-     *
-     * Iteratively returns **all** the meta conversion functions of the given
-     * type.
-     *
-     * @tparam Op Type of the function object to invoke.
-     * @param op A valid function object.
-     */
-    template<typename Op>
-    void conv(Op op) const ENTT_NOEXCEPT {
-        internal::iterate<&internal::meta_type_node::conv>([op = std::move(op)](auto *curr) {
-            op(curr->meta());
-        }, node);
-    }
-
-    /**
-     * @brief Returns the meta conversion function associated with a given type.
-     *
-     * Searches recursively among **all** the conversion functions of the given
-     * type.
-     *
-     * @tparam Type The type to use to search for a meta conversion function.
-     * @return The meta conversion function associated with the given type, if
-     * any.
-     */
-    template<typename Type>
-    meta_conv conv() const ENTT_NOEXCEPT {
-        const auto *curr = internal::find_if<&internal::meta_type_node::conv>([type = internal::meta_info<Type>::resolve()](auto *candidate) {
-            return candidate->type() == type;
-        }, node);
-
-        return curr ? curr->meta() : meta_conv{};
-    }
-
-    /**
-     * @brief Iterates all the meta constructors of a meta type.
-     * @tparam Op Type of the function object to invoke.
-     * @param op A valid function object.
-     */
-    template<typename Op>
-    void ctor(Op op) const ENTT_NOEXCEPT {
-        internal::iterate([op = std::move(op)](auto *curr) {
-            op(curr->meta());
-        }, node->ctor);
-    }
-
-    /**
-     * @brief Returns the meta constructor that accepts a given list of types of
-     * arguments.
-     * @return The requested meta constructor, if any.
-     */
-    template<typename... Args>
-    meta_ctor ctor() const ENTT_NOEXCEPT {
-        const auto *curr = internal::ctor<Args...>(std::make_index_sequence<sizeof...(Args)>{}, node);
-        return curr ? curr->meta() : meta_ctor{};
-    }
-
-    /**
-     * @brief Returns the meta destructor associated with a given type.
-     * @return The meta destructor associated with the given type, if any.
-     */
-    meta_dtor dtor() const ENTT_NOEXCEPT {
-        return node->dtor ? node->dtor->meta() : meta_dtor{};
-    }
-
-    /**
-     * @brief Iterates all the meta data of a meta type.
-     *
-     * Iteratively returns **all** the meta data of the given type. This means
-     * that the meta data of the base classes will also be returned, if any.
-     *
-     * @tparam Op Type of the function object to invoke.
-     * @param op A valid function object.
-     */
-    template<typename Op>
-    std::enable_if_t<std::is_invocable_v<Op, meta_data>, void>
-    data(Op op) const ENTT_NOEXCEPT {
-        internal::iterate<&internal::meta_type_node::data>([op = std::move(op)](auto *curr) {
-            op(curr->meta());
-        }, node);
-    }
-
-    /**
-     * @brief Returns the meta data associated with a given identifier.
-     *
-     * Searches recursively among **all** the meta data of the given type. This
-     * means that the meta data of the base classes will also be inspected, if
-     * any.
-     *
-     * @param identifier Unique identifier.
-     * @return The meta data associated with the given identifier, if any.
-     */
-    meta_data data(const ENTT_ID_TYPE identifier) const ENTT_NOEXCEPT {
-        const auto *curr = internal::find_if<&internal::meta_type_node::data>([identifier](auto *candidate) {
-            return candidate->identifier == identifier;
-        }, node);
-
-        return curr ? curr->meta() : meta_data{};
-    }
-
-    /**
-     * @brief Iterates all the meta functions of a meta type.
-     *
-     * Iteratively returns **all** the meta functions of the given type. This
-     * means that the meta functions of the base classes will also be returned,
-     * if any.
-     *
-     * @tparam Op Type of the function object to invoke.
-     * @param op A valid function object.
-     */
-    template<typename Op>
-    std::enable_if_t<std::is_invocable_v<Op, meta_func>, void>
-    func(Op op) const ENTT_NOEXCEPT {
-        internal::iterate<&internal::meta_type_node::func>([op = std::move(op)](auto *curr) {
-            op(curr->meta());
-        }, node);
-    }
-
-    /**
-     * @brief Returns the meta function associated with a given identifier.
-     *
-     * Searches recursively among **all** the meta functions of the given type.
-     * This means that the meta functions of the base classes will also be
-     * inspected, if any.
-     *
-     * @param identifier Unique identifier.
-     * @return The meta function associated with the given identifier, if any.
-     */
-    meta_func func(const ENTT_ID_TYPE identifier) const ENTT_NOEXCEPT {
-        const auto *curr = internal::find_if<&internal::meta_type_node::func>([identifier](auto *candidate) {
-            return candidate->identifier == identifier;
-        }, node);
-
-        return curr ? curr->meta() : meta_func{};
-    }
-
-    /**
-     * @brief Creates an instance of the underlying type, if possible.
-     *
-     * To create a valid instance, the types of the parameters must coincide
-     * exactly with those required by the underlying meta constructor.
-     * Otherwise, an empty and then invalid container is returned.
-     *
-     * @tparam Args Types of arguments to use to construct the instance.
-     * @param args Parameters to use to construct the instance.
-     * @return A meta any containing the new instance, if any.
-     */
-    template<typename... Args>
-    meta_any construct(Args &&... args) const {
-        std::array<meta_any, sizeof...(Args)> arguments{{std::forward<Args>(args)...}};
-        meta_any any{};
-
-        internal::find_if<&internal::meta_type_node::ctor>([data = arguments.data(), &any](auto *curr) -> bool {
-            if(curr->size == sizeof...(args)) {
-                any = curr->invoke(data);
-            }
-
-            return static_cast<bool>(any);
-        }, node);
-
-        return any;
-    }
-
-    /**
-     * @brief Destroys an instance of the underlying type.
-     *
-     * It must be possible to cast the instance to the underlying type.
-     * Otherwise, invoking the meta destructor results in an undefined
-     * behavior.<br/>
-     * If no destructor has been set, this function returns true without doing
-     * anything.
-     *
-     * @param handle An opaque pointer to an instance of the underlying type.
-     * @return True in case of success, false otherwise.
-     */
-    bool destroy(meta_handle handle) const {
-        return (handle.type() == node->meta()) && (!node->dtor || node->dtor->invoke(handle));
-    }
-
-    /**
-     * @brief Iterates all the properties assigned to a meta type.
-     *
-     * Iteratively returns **all** the properties of the given type. This means
-     * that the properties of the base classes will also be returned, if any.
-     *
-     * @tparam Op Type of the function object to invoke.
-     * @param op A valid function object.
-     */
-    template<typename Op>
-    std::enable_if_t<std::is_invocable_v<Op, meta_prop>, void>
-    prop(Op op) const ENTT_NOEXCEPT {
-        internal::iterate<&internal::meta_type_node::prop>([op = std::move(op)](auto *curr) {
-            op(curr->meta());
-        }, node);
-    }
-
-    /**
-     * @brief Returns the property associated with a given key.
-     *
-     * Searches recursively among **all** the properties of the given type. This
-     * means that the properties of the base classes will also be inspected, if
-     * any.
-     *
-     * @tparam Key Type of key to use to search for a property.
-     * @param key The key to use to search for a property.
-     * @return The property associated with the given key, if any.
-     */
-    template<typename Key>
-    std::enable_if_t<!std::is_invocable_v<Key, meta_prop>, meta_prop>
-    prop(Key &&key) const ENTT_NOEXCEPT {
-        const auto *curr = internal::find_if<&internal::meta_type_node::prop>([key = meta_any{std::forward<Key>(key)}](auto *candidate) {
-            return candidate->key() == key;
-        }, node);
-
-        return curr ? curr->meta() : meta_prop{};
-    }
-
-    /**
-     * @brief Returns true if a meta object is valid, false otherwise.
-     * @return True if the meta object is valid, false otherwise.
-     */
-    explicit operator bool() const ENTT_NOEXCEPT {
-        return node;
-    }
-
-    /**
-     * @brief Checks if two meta objects refer to the same node.
-     * @param other The meta object with which to compare.
-     * @return True if the two meta objects refer to the same node, false
-     * otherwise.
-     */
-    bool operator==(const meta_type &other) const ENTT_NOEXCEPT {
-        return node == other.node;
-    }
-
-private:
-    const internal::meta_type_node *node;
-};
-
-
-/**
- * @brief Checks if two meta objects refer to the same node.
- * @param lhs A meta object, either valid or not.
- * @param rhs A meta object, either valid or not.
- * @return True if the two meta objects refer to the same node, false otherwise.
- */
-inline bool operator!=(const meta_type &lhs, const meta_type &rhs) ENTT_NOEXCEPT {
-    return !(lhs == rhs);
-}
-
-
-inline meta_type meta_any::type() const ENTT_NOEXCEPT {
-    return node ? node->meta() : meta_type{};
-}
-
-
-inline meta_type meta_handle::type() const ENTT_NOEXCEPT {
-    return node ? node->meta() : meta_type{};
-}
-
-
-inline meta_type meta_base::parent() const ENTT_NOEXCEPT {
-    return node->parent->meta();
-}
-
-
-inline meta_type meta_base::type() const ENTT_NOEXCEPT {
-    return node->type()->meta();
-}
-
-
-inline meta_type meta_conv::parent() const ENTT_NOEXCEPT {
-    return node->parent->meta();
-}
-
-
-inline meta_type meta_conv::type() const ENTT_NOEXCEPT {
-    return node->type()->meta();
-}
-
-
-inline meta_type meta_ctor::parent() const ENTT_NOEXCEPT {
-    return node->parent->meta();
-}
-
-
-inline meta_type meta_ctor::arg(size_type index) const ENTT_NOEXCEPT {
-    return index < size() ? node->arg(index)->meta() : meta_type{};
-}
-
-
-inline meta_type meta_dtor::parent() const ENTT_NOEXCEPT {
-    return node->parent->meta();
-}
-
-
-inline meta_type meta_data::parent() const ENTT_NOEXCEPT {
-    return node->parent->meta();
-}
-
-
-inline meta_type meta_data::type() const ENTT_NOEXCEPT {
-    return node->type()->meta();
-}
-
-
-inline meta_type meta_func::parent() const ENTT_NOEXCEPT {
-    return node->parent->meta();
-}
-
-
-inline meta_type meta_func::ret() const ENTT_NOEXCEPT {
-    return node->ret()->meta();
-}
-
-
-inline meta_type meta_func::arg(size_type index) const ENTT_NOEXCEPT {
-    return index < size() ? node->arg(index)->meta() : meta_type{};
-}
-
-
-/**
- * @cond TURN_OFF_DOXYGEN
- * Internal details not to be documented.
- */
-
-
-namespace internal {
-
-
-template<typename Type>
-inline meta_type_node * meta_node<Type>::resolve() ENTT_NOEXCEPT {
-    if(!type) {
-        static meta_type_node node{
-            {},
-            nullptr,
-            nullptr,
-            std::is_void_v<Type>,
-            std::is_integral_v<Type>,
-            std::is_floating_point_v<Type>,
-            std::is_array_v<Type>,
-            std::is_enum_v<Type>,
-            std::is_union_v<Type>,
-            std::is_class_v<Type>,
-            std::is_pointer_v<Type>,
-            std::is_function_v<Type>,
-            std::is_member_object_pointer_v<Type>,
-            std::is_member_function_pointer_v<Type>,
-            std::extent_v<Type>,
-            []() ENTT_NOEXCEPT -> meta_type {
-                return internal::meta_info<std::remove_pointer_t<Type>>::resolve();
-            },
-            []() ENTT_NOEXCEPT -> meta_type {
-                return &node;
-            }
-        };
-
-        type = &node;
-    }
-
-    return type;
-}
-
-
-}
-
-
-/**
- * Internal details not to be documented.
- * @endcond TURN_OFF_DOXYGEN
- */
-
-
-}
-
-
-#endif // ENTT_META_META_HPP
+    [[nodiscard]] static bool resize([[maybe_unused]] Container &cont, [[maybe_unused]] typename Container::size_type sz) {
+        return cont.resize(sz), true;
+    }
+
+    /**
+     * @brief Inserts an element at the specified location of the given
+     * container.
+     * @param cont The container into which to insert the element.
+     * @param it Iterator before which the element will be inserted.
+     * @param value Element value to insert.
+     * @return A pair consisting of an iterator to the inserted element (in case
+     * of success) and a bool denoting whether the insertion took place.
+     */
+    [[nodiscard]] static std::pair<typename Container::iterator, bool> insert([[maybe_unused]] Container &cont, [[maybe_unused]] typename Container::const_iterator it, [[maybe_unused]] const typename Container::value_type &value) {
+        return { cont.insert(it, value), true };
+    }
+
+    /**
+     * @brief Removes the element at the specified location from the given
+     * container.
+     * @param cont The container from which to remove the element.
+     * @param it Iterator to the element to remove.
+     * @return A pair consisting of an iterator following the last removed
+     * element (in case of success) and a bool denoting whether the insertion
+     * took place.
+     */
+    [[nodiscard]] static std::pair<typename Container::iterator, bool> erase([[maybe_unused]] Container &cont, [[maybe_unused]] typename Container::const_iterator it) {
+        return { cont.erase(it), true };
+    }
+};
+
+
+/**
+ * @brief STL-compatible fixed sequence container traits
+ * @tparam Container The type of the container.
+ */
+template<typename Container>
+struct fixed_sequence_container {
+    /**
+     * @brief Does nothing.
+     * @return False to indicate failure in all cases.
+     */
+    [[nodiscard]] static bool resize(const Container &, typename Container::size_type) {
+        return false;
+    }
+
+    /**
+     * @brief Does nothing.
+     * @return False to indicate failure in all cases.
+     */
+    [[nodiscard]] static bool clear(const Container &) {
+        return false;
+    }
+
+    /**
+     * @brief Does nothing.
+     * @return A pair consisting of an invalid iterator and a false value to
+     * indicate failure in all cases.
+     */
+    [[nodiscard]] static std::pair<typename Container::iterator, bool> insert(const Container &, typename Container::const_iterator, const typename Container::value_type &) {
+        return { {}, false };
+    }
+
+    /**
+     * @brief Does nothing.
+     * @return A pair consisting of an invalid iterator and a false value to
+     * indicate failure in all cases.
+     */
+    [[nodiscard]] static std::pair<typename Container::iterator, bool> erase(const Container &, typename Container::const_iterator) {
+        return { {}, false };
+    }
+};
+
+
+/**
+ * @brief Meta sequence container traits for `std::vector`s of any type.
+ * @tparam Type The type of elements.
+ * @tparam Args Other arguments.
+ */
+template<typename Type, typename... Args>
+struct meta_sequence_container_traits<std::vector<Type, Args...>>
+        : meta_container_traits<
+              std::vector<Type, Args...>,
+              basic_container,
+              basic_dynamic_container,
+              basic_sequence_container,
+              dynamic_sequence_container
+          >
+{};
+
+
+/**
+ * @brief Meta sequence container traits for `std::array`s of any type.
+ * @tparam Type The type of elements.
+ * @tparam N The number of elements.
+ */
+template<typename Type, auto N>
+struct meta_sequence_container_traits<std::array<Type, N>>
+        : meta_container_traits<
+              std::array<Type, N>,
+              basic_container,
+              basic_sequence_container,
+              fixed_sequence_container
+          >
+{};
+
+
+/**
+ * @brief Meta associative container traits for `std::map`s of any type.
+ * @tparam Key The key type of elements.
+ * @tparam Value The value type of elements.
+ * @tparam Args Other arguments.
+ */
+template<typename Key, typename Value, typename... Args>
+struct meta_associative_container_traits<std::map<Key, Value, Args...>>
+        : meta_container_traits<
+              std::map<Key, Value, Args...>,
+              basic_container,
+              basic_associative_container,
+              basic_dynamic_container,
+              basic_dynamic_associative_container,
+              dynamic_associative_key_value_container
+          >
+{};
+
+
+/**
+ * @brief Meta associative container traits for `std::unordered_map`s of any
+ * type.
+ * @tparam Key The key type of elements.
+ * @tparam Value The value type of elements.
+ * @tparam Args Other arguments.
+ */
+template<typename Key, typename Value, typename... Args>
+struct meta_associative_container_traits<std::unordered_map<Key, Value, Args...>>
+        : meta_container_traits<
+              std::unordered_map<Key, Value, Args...>,
+              basic_container,
+              basic_associative_container,
+              basic_dynamic_container,
+              basic_dynamic_associative_container,
+              dynamic_associative_key_value_container
+          >
+{};
+
+
+/**
+ * @brief Meta associative container traits for `std::set`s of any type.
+ * @tparam Key The type of elements.
+ * @tparam Args Other arguments.
+ */
+template<typename Key, typename... Args>
+struct meta_associative_container_traits<std::set<Key, Args...>>
+        : meta_container_traits<
+              std::set<Key, Args...>,
+              basic_container,
+              basic_associative_container,
+              basic_dynamic_container,
+              basic_dynamic_associative_container,
+              dynamic_associative_key_only_container
+          >
+{};
+
+
+/**
+ * @brief Meta associative container traits for `std::unordered_set`s of any
+ * type.
+ * @tparam Key The type of elements.
+ * @tparam Args Other arguments.
+ */
+template<typename Key, typename... Args>
+struct meta_associative_container_traits<std::unordered_set<Key, Args...>>
+        : meta_container_traits<
+              std::unordered_set<Key, Args...>,
+              basic_container,
+              basic_associative_container,
+              basic_dynamic_container,
+              basic_dynamic_associative_container,
+              dynamic_associative_key_only_container
+          >
+{};
+
+
+}
+
+
+#endif
+
+// #include "meta/ctx.hpp"
+#ifndef ENTT_META_CTX_HPP
+#define ENTT_META_CTX_HPP
+
+
+// #include "../core/attribute.h"
+#ifndef ENTT_CORE_ATTRIBUTE_H
+#define ENTT_CORE_ATTRIBUTE_H
+
+
+#ifndef ENTT_EXPORT
+#   if defined _WIN32 || defined __CYGWIN__ || defined _MSC_VER
+#       define ENTT_EXPORT __declspec(dllexport)
+#       define ENTT_IMPORT __declspec(dllimport)
+#       define ENTT_HIDDEN
+#   elif defined __GNUC__ && __GNUC__ >= 4
+#       define ENTT_EXPORT __attribute__((visibility("default")))
+#       define ENTT_IMPORT __attribute__((visibility("default")))
+#       define ENTT_HIDDEN __attribute__((visibility("hidden")))
+#   else /* Unsupported compiler */
+#       define ENTT_EXPORT
+#       define ENTT_IMPORT
+#       define ENTT_HIDDEN
+#   endif
+#endif
+
+
+#ifndef ENTT_API
+#   if defined ENTT_API_EXPORT
+#       define ENTT_API ENTT_EXPORT
+#   elif defined ENTT_API_IMPORT
+#       define ENTT_API ENTT_IMPORT
+#   else /* No API */
+#       define ENTT_API
+#   endif
+#endif
+
+
+#endif
+
+// #include "../config/config.h"
 
 
 
@@ -12182,2626 +29835,150 @@
 namespace internal {
 
 
-template<typename>
-struct meta_function_helper;
-
-
-template<typename Ret, typename... Args>
-struct meta_function_helper<Ret(Args...)> {
-    using return_type = std::remove_cv_t<std::remove_reference_t<Ret>>;
-    using args_type = std::tuple<std::remove_cv_t<std::remove_reference_t<Args>>...>;
-
-    static constexpr auto size = sizeof...(Args);
-    static constexpr auto is_const = false;
-
-    static auto arg(typename internal::meta_func_node::size_type index) ENTT_NOEXCEPT {
-        return std::array<meta_type_node *, sizeof...(Args)>{{meta_info<Args>::resolve()...}}[index];
-    }
-};
-
-
-template<typename Ret, typename... Args>
-struct meta_function_helper<Ret(Args...) const>: meta_function_helper<Ret(Args...)> {
-    static constexpr auto is_const = true;
-};
-
-
-template<typename Ret, typename... Args, typename Class>
-constexpr meta_function_helper<Ret(Args...)>
-to_meta_function_helper(Ret(Class:: *)(Args...));
-
-
-template<typename Ret, typename... Args, typename Class>
-constexpr meta_function_helper<Ret(Args...) const>
-to_meta_function_helper(Ret(Class:: *)(Args...) const);
-
-
-template<typename Ret, typename... Args>
-constexpr meta_function_helper<Ret(Args...)>
-to_meta_function_helper(Ret(*)(Args...));
-
-
-template<typename Candidate>
-using meta_function_helper_t = decltype(to_meta_function_helper(std::declval<Candidate>()));
-
-
-template<typename Type, typename... Args, std::size_t... Indexes>
-meta_any construct(meta_any * const args, std::index_sequence<Indexes...>) {
-    [[maybe_unused]] auto direct = std::make_tuple((args+Indexes)->try_cast<Args>()...);
-    meta_any any{};
-
-    if(((std::get<Indexes>(direct) || (args+Indexes)->convert<Args>()) && ...)) {
-        any = Type{(std::get<Indexes>(direct) ? *std::get<Indexes>(direct) : (args+Indexes)->cast<Args>())...};
-    }
-
-    return any;
-}
-
-
-template<bool Const, typename Type, auto Data>
-bool setter([[maybe_unused]] meta_handle handle, [[maybe_unused]] meta_any index, [[maybe_unused]] meta_any value) {
-    bool accepted = false;
-
-    if constexpr(!Const) {
-        if constexpr(std::is_function_v<std::remove_pointer_t<decltype(Data)>> || std::is_member_function_pointer_v<decltype(Data)>) {
-            using helper_type = meta_function_helper_t<decltype(Data)>;
-            using data_type = std::tuple_element_t<!std::is_member_function_pointer_v<decltype(Data)>, typename helper_type::args_type>;
-            static_assert(std::is_invocable_v<decltype(Data), Type &, data_type>);
-            auto *direct = value.try_cast<data_type>();
-            auto *clazz = handle.data<Type>();
-
-            if(clazz && (direct || value.convert<data_type>())) {
-                std::invoke(Data, *clazz, direct ? *direct : value.cast<data_type>());
-                accepted = true;
-            }
-        } else if constexpr(std::is_member_object_pointer_v<decltype(Data)>) {
-            using data_type = std::remove_cv_t<std::remove_reference_t<decltype(std::declval<Type>().*Data)>>;
-            static_assert(std::is_invocable_v<decltype(Data), Type *>);
-            auto *clazz = handle.data<Type>();
-
-            if constexpr(std::is_array_v<data_type>) {
-                using underlying_type = std::remove_extent_t<data_type>;
-                auto *direct = value.try_cast<underlying_type>();
-                auto *idx = index.try_cast<std::size_t>();
-
-                if(clazz && idx && (direct || value.convert<underlying_type>())) {
-                    std::invoke(Data, clazz)[*idx] = direct ? *direct : value.cast<underlying_type>();
-                    accepted = true;
-                }
-            } else {
-                auto *direct = value.try_cast<data_type>();
-
-                if(clazz && (direct || value.convert<data_type>())) {
-                    std::invoke(Data, clazz) = (direct ? *direct : value.cast<data_type>());
-                    accepted = true;
-                }
-            }
-        } else {
-            static_assert(std::is_pointer_v<decltype(Data)>);
-            using data_type = std::remove_cv_t<std::remove_reference_t<decltype(*Data)>>;
-
-            if constexpr(std::is_array_v<data_type>) {
-                using underlying_type = std::remove_extent_t<data_type>;
-                auto *direct = value.try_cast<underlying_type>();
-                auto *idx = index.try_cast<std::size_t>();
-
-                if(idx && (direct || value.convert<underlying_type>())) {
-                    (*Data)[*idx] = (direct ? *direct : value.cast<underlying_type>());
-                    accepted = true;
-                }
-            } else {
-                auto *direct = value.try_cast<data_type>();
-
-                if(direct || value.convert<data_type>()) {
-                    *Data = (direct ? *direct : value.cast<data_type>());
-                    accepted = true;
-                }
-            }
-        }
-    }
-
-    return accepted;
-}
-
-
-template<typename Type, auto Data, typename Policy>
-meta_any getter([[maybe_unused]] meta_handle handle, [[maybe_unused]] meta_any index) {
-    auto dispatch = [](auto &&value) {
-        if constexpr(std::is_same_v<Policy, as_void_t>) {
-            return meta_any{std::in_place_type<void>};
-        } else if constexpr(std::is_same_v<Policy, as_alias_t>) {
-            return meta_any{as_alias, std::forward<decltype(value)>(value)};
-        } else {
-            static_assert(std::is_same_v<Policy, as_is_t>);
-            return meta_any{std::forward<decltype(value)>(value)};
-        }
-    };
-
-    if constexpr(std::is_function_v<std::remove_pointer_t<decltype(Data)>> || std::is_member_function_pointer_v<decltype(Data)>) {
-        static_assert(std::is_invocable_v<decltype(Data), Type &>);
-        auto *clazz = handle.data<Type>();
-        return clazz ? dispatch(std::invoke(Data, *clazz)) : meta_any{};
-    } else if constexpr(std::is_member_object_pointer_v<decltype(Data)>) {
-        using data_type = std::remove_cv_t<std::remove_reference_t<decltype(std::declval<Type>().*Data)>>;
-        static_assert(std::is_invocable_v<decltype(Data), Type *>);
-        auto *clazz = handle.data<Type>();
-
-        if constexpr(std::is_array_v<data_type>) {
-            auto *idx = index.try_cast<std::size_t>();
-            return (clazz && idx) ? dispatch(std::invoke(Data, clazz)[*idx]) : meta_any{};
-        } else {
-            return clazz ? dispatch(std::invoke(Data, clazz)) : meta_any{};
-        }
-    } else {
-        static_assert(std::is_pointer_v<std::decay_t<decltype(Data)>>);
-
-        if constexpr(std::is_array_v<std::remove_pointer_t<decltype(Data)>>) {
-            auto *idx = index.try_cast<std::size_t>();
-            return idx ? dispatch((*Data)[*idx]) : meta_any{};
-        } else {
-            return dispatch(*Data);
-        }
-    }
-}
-
-
-template<typename Type, auto Candidate, typename Policy, std::size_t... Indexes>
-meta_any invoke([[maybe_unused]] meta_handle handle, meta_any *args, std::index_sequence<Indexes...>) {
-    using helper_type = meta_function_helper_t<decltype(Candidate)>;
-
-    auto dispatch = [](auto *... args) {
-        if constexpr(std::is_void_v<typename helper_type::return_type> || std::is_same_v<Policy, as_void_t>) {
-            std::invoke(Candidate, *args...);
-            return meta_any{std::in_place_type<void>};
-        } else if constexpr(std::is_same_v<Policy, as_alias_t>) {
-            return meta_any{as_alias, std::invoke(Candidate, *args...)};
-        } else {
-            static_assert(std::is_same_v<Policy, as_is_t>);
-            return meta_any{std::invoke(Candidate, *args...)};
-        }
-    };
-
-    [[maybe_unused]] const auto direct = std::make_tuple([](meta_any *any, auto *instance) {
-        using arg_type = std::remove_reference_t<decltype(*instance)>;
-
-        if(!instance && any->convert<arg_type>()) {
-            instance = any->try_cast<arg_type>();
-        }
-
-        return instance;
-    }(args+Indexes, (args+Indexes)->try_cast<std::tuple_element_t<Indexes, typename helper_type::args_type>>())...);
-
-    if constexpr(std::is_function_v<std::remove_pointer_t<decltype(Candidate)>>) {
-        return (std::get<Indexes>(direct) && ...) ? dispatch(std::get<Indexes>(direct)...) : meta_any{};
-    } else {
-        auto *clazz = handle.data<Type>();
-        return (clazz && (std::get<Indexes>(direct) && ...)) ? dispatch(clazz, std::get<Indexes>(direct)...) : meta_any{};
-    }
-}
-
-
-}
-
-
-/**
- * Internal details not to be documented.
- * @endcond TURN_OFF_DOXYGEN
- */
-
-
-/**
- * @brief A meta factory to be used for reflection purposes.
- *
- * A meta factory is an utility class used to reflect types, data and functions
- * of all sorts. This class ensures that the underlying web of types is built
- * correctly and performs some checks in debug mode to ensure that there are no
- * subtle errors at runtime.
- *
- * @tparam Type Reflected type for which the factory was created.
- */
-template<typename Type>
-class meta_factory {
-    static_assert(std::is_same_v<Type, std::decay_t<Type>>);
-
-    template<typename Node>
-    bool duplicate(const ENTT_ID_TYPE identifier, const Node *node) ENTT_NOEXCEPT {
-        return node ? node->identifier == identifier || duplicate(identifier, node->next) : false;
-    }
-
-    bool duplicate(const meta_any &key, const internal::meta_prop_node *node) ENTT_NOEXCEPT {
-        return node ? node->key() == key || duplicate(key, node->next) : false;
-    }
-
-    template<typename>
-    internal::meta_prop_node * properties() {
-        return nullptr;
-    }
-
-    template<typename Owner, typename Property, typename... Other>
-    internal::meta_prop_node * properties(Property &&property, Other &&... other) {
-        static std::remove_cv_t<std::remove_reference_t<Property>> prop{};
-
-        static internal::meta_prop_node node{
-            nullptr,
-            []() -> meta_any {
-                return std::as_const(std::get<0>(prop));
-            },
-            []() -> meta_any {
-                return std::as_const(std::get<1>(prop));
-            },
-            []() ENTT_NOEXCEPT -> meta_prop {
-                return &node;
-            }
-        };
-
-        prop = std::forward<Property>(property);
-        node.next = properties<Owner>(std::forward<Other>(other)...);
-        ENTT_ASSERT(!duplicate(meta_any{std::get<0>(prop)}, node.next));
-        return &node;
-    }
-
-    void unregister_prop(internal::meta_prop_node **prop) {
-        while(*prop) {
-            auto *node = *prop;
-            *prop = node->next;
-            node->next = nullptr;
-        }
-    }
-
-    void unregister_dtor() {
-        if(auto node = internal::meta_info<Type>::type->dtor; node) {
-            internal::meta_info<Type>::type->dtor = nullptr;
-            *node->underlying = nullptr;
-        }
-    }
-
-    template<auto Member>
-    auto unregister_all(int)
-    -> decltype((internal::meta_info<Type>::type->*Member)->prop, void()) {
-        while(internal::meta_info<Type>::type->*Member) {
-            auto node = internal::meta_info<Type>::type->*Member;
-            internal::meta_info<Type>::type->*Member = node->next;
-            unregister_prop(&node->prop);
-            node->next = nullptr;
-            *node->underlying = nullptr;
-        }
-    }
-
-    template<auto Member>
-    void unregister_all(char) {
-        while(internal::meta_info<Type>::type->*Member) {
-            auto node = internal::meta_info<Type>::type->*Member;
-            internal::meta_info<Type>::type->*Member = node->next;
-            node->next = nullptr;
-            *node->underlying = nullptr;
-        }
-    }
-
-    meta_factory() ENTT_NOEXCEPT = default;
-
-public:
-    /**
-     * @brief Extends a meta type by assigning it an identifier and properties.
-     * @tparam Property Types of properties to assign to the meta type.
-     * @param identifier Unique identifier.
-     * @param property Properties to assign to the meta type.
-     * @return A meta factory for the parent type.
-     */
-    template<typename... Property>
-    meta_factory type(const ENTT_ID_TYPE identifier, Property &&... property) ENTT_NOEXCEPT {
-        ENTT_ASSERT(!internal::meta_info<Type>::type);
-        auto *node = internal::meta_info<Type>::resolve();
-        node->identifier = identifier;
-        node->next = internal::meta_info<>::type;
-        node->prop = properties<Type>(std::forward<Property>(property)...);
-        ENTT_ASSERT(!duplicate(identifier, node->next));
-        internal::meta_info<Type>::type = node;
-        internal::meta_info<>::type = node;
-
-        return *this;
-    }
-
-    /**
-     * @brief Assigns a meta base to a meta type.
-     *
-     * A reflected base class must be a real base class of the reflected type.
-     *
-     * @tparam Base Type of the base class to assign to the meta type.
-     * @return A meta factory for the parent type.
-     */
-    template<typename Base>
-    meta_factory base() ENTT_NOEXCEPT {
-        static_assert(std::is_base_of_v<Base, Type>);
-        auto * const type = internal::meta_info<Type>::resolve();
-
-        static internal::meta_base_node node{
-            &internal::meta_info<Type>::template base<Base>,
-            type,
-            nullptr,
-            &internal::meta_info<Base>::resolve,
-            [](void *instance) ENTT_NOEXCEPT -> void * {
-                return static_cast<Base *>(static_cast<Type *>(instance));
-            },
-            []() ENTT_NOEXCEPT -> meta_base {
-                return &node;
-            }
-        };
-
-        node.next = type->base;
-        ENTT_ASSERT((!internal::meta_info<Type>::template base<Base>));
-        internal::meta_info<Type>::template base<Base> = &node;
-        type->base = &node;
-
-        return *this;
-    }
-
-    /**
-     * @brief Assigns a meta conversion function to a meta type.
-     *
-     * The given type must be such that an instance of the reflected type can be
-     * converted to it.
-     *
-     * @tparam To Type of the conversion function to assign to the meta type.
-     * @return A meta factory for the parent type.
-     */
-    template<typename To>
-    meta_factory conv() ENTT_NOEXCEPT {
-        static_assert(std::is_convertible_v<Type, To>);
-        auto * const type = internal::meta_info<Type>::resolve();
-
-        static internal::meta_conv_node node{
-            &internal::meta_info<Type>::template conv<To>,
-            type,
-            nullptr,
-            &internal::meta_info<To>::resolve,
-            [](const void *instance) -> meta_any {
-                return static_cast<To>(*static_cast<const Type *>(instance));
-            },
-            []() ENTT_NOEXCEPT -> meta_conv {
-                return &node;
-            }
-        };
-
-        node.next = type->conv;
-        ENTT_ASSERT((!internal::meta_info<Type>::template conv<To>));
-        internal::meta_info<Type>::template conv<To> = &node;
-        type->conv = &node;
-
-        return *this;
-    }
-
-    /**
-     * @brief Assigns a meta conversion function to a meta type.
-     *
-     * Conversion functions can be either free functions or member
-     * functions.<br/>
-     * In case of free functions, they must accept a const reference to an
-     * instance of the parent type as an argument. In case of member functions,
-     * they should have no arguments at all.
-     *
-     * @tparam Candidate The actual function to use for the conversion.
-     * @return A meta factory for the parent type.
-     */
-    template<auto Candidate>
-    meta_factory conv() ENTT_NOEXCEPT {
-        using conv_type = std::invoke_result_t<decltype(Candidate), Type &>;
-        auto * const type = internal::meta_info<Type>::resolve();
-
-        static internal::meta_conv_node node{
-            &internal::meta_info<Type>::template conv<conv_type>,
-            type,
-            nullptr,
-            &internal::meta_info<conv_type>::resolve,
-            [](const void *instance) -> meta_any {
-                return std::invoke(Candidate, *static_cast<const Type *>(instance));
-            },
-            []() ENTT_NOEXCEPT -> meta_conv {
-                return &node;
-            }
-        };
-
-        node.next = type->conv;
-        ENTT_ASSERT((!internal::meta_info<Type>::template conv<conv_type>));
-        internal::meta_info<Type>::template conv<conv_type> = &node;
-        type->conv = &node;
-
-        return *this;
-    }
-
-    /**
-     * @brief Assigns a meta constructor to a meta type.
-     *
-     * Free functions can be assigned to meta types in the role of constructors.
-     * All that is required is that they return an instance of the underlying
-     * type.<br/>
-     * From a client's point of view, nothing changes if a constructor of a meta
-     * type is a built-in one or a free function.
-     *
-     * @tparam Func The actual function to use as a constructor.
-     * @tparam Policy Optional policy (no policy set by default).
-     * @tparam Property Types of properties to assign to the meta data.
-     * @param property Properties to assign to the meta data.
-     * @return A meta factory for the parent type.
-     */
-    template<auto Func, typename Policy = as_is_t, typename... Property>
-    meta_factory ctor(Property &&... property) ENTT_NOEXCEPT {
-        using helper_type = internal::meta_function_helper_t<decltype(Func)>;
-        static_assert(std::is_same_v<typename helper_type::return_type, Type>);
-        auto * const type = internal::meta_info<Type>::resolve();
-
-        static internal::meta_ctor_node node{
-            &internal::meta_info<Type>::template ctor<typename helper_type::args_type>,
-            type,
-            nullptr,
-            nullptr,
-            helper_type::size,
-            &helper_type::arg,
-            [](meta_any * const any) {
-                return internal::invoke<Type, Func, Policy>({}, any, std::make_index_sequence<helper_type::size>{});
-            },
-            []() ENTT_NOEXCEPT -> meta_ctor {
-                return &node;
-            }
-        };
-
-        node.next = type->ctor;
-        node.prop = properties<typename helper_type::args_type>(std::forward<Property>(property)...);
-        ENTT_ASSERT((!internal::meta_info<Type>::template ctor<typename helper_type::args_type>));
-        internal::meta_info<Type>::template ctor<typename helper_type::args_type> = &node;
-        type->ctor = &node;
-
-        return *this;
-    }
-
-    /**
-     * @brief Assigns a meta constructor to a meta type.
-     *
-     * A meta constructor is uniquely identified by the types of its arguments
-     * and is such that there exists an actual constructor of the underlying
-     * type that can be invoked with parameters whose types are those given.
-     *
-     * @tparam Args Types of arguments to use to construct an instance.
-     * @tparam Property Types of properties to assign to the meta data.
-     * @param property Properties to assign to the meta data.
-     * @return A meta factory for the parent type.
-     */
-    template<typename... Args, typename... Property>
-    meta_factory ctor(Property &&... property) ENTT_NOEXCEPT {
-        using helper_type = internal::meta_function_helper_t<Type(*)(Args...)>;
-        auto * const type = internal::meta_info<Type>::resolve();
-
-        static internal::meta_ctor_node node{
-            &internal::meta_info<Type>::template ctor<typename helper_type::args_type>,
-            type,
-            nullptr,
-            nullptr,
-            helper_type::size,
-            &helper_type::arg,
-            [](meta_any * const any) {
-                return internal::construct<Type, std::remove_cv_t<std::remove_reference_t<Args>>...>(any, std::make_index_sequence<helper_type::size>{});
-            },
-            []() ENTT_NOEXCEPT -> meta_ctor {
-                return &node;
-            }
-        };
-
-        node.next = type->ctor;
-        node.prop = properties<typename helper_type::args_type>(std::forward<Property>(property)...);
-        ENTT_ASSERT((!internal::meta_info<Type>::template ctor<typename helper_type::args_type>));
-        internal::meta_info<Type>::template ctor<typename helper_type::args_type> = &node;
-        type->ctor = &node;
-
-        return *this;
-    }
-
-    /**
-     * @brief Assigns a meta destructor to a meta type.
-     *
-     * Free functions can be assigned to meta types in the role of destructors.
-     * The signature of the function should identical to the following:
-     *
-     * @code{.cpp}
-     * void(Type &);
-     * @endcode
-     *
-     * The purpose is to give users the ability to free up resources that
-     * require special treatment before an object is actually destroyed.
-     *
-     * @tparam Func The actual function to use as a destructor.
-     * @return A meta factory for the parent type.
-     */
-    template<auto Func>
-    meta_factory dtor() ENTT_NOEXCEPT {
-        static_assert(std::is_invocable_v<decltype(Func), Type &>);
-        auto * const type = internal::meta_info<Type>::resolve();
-
-        static internal::meta_dtor_node node{
-            &internal::meta_info<Type>::template dtor<Func>,
-            type,
-            [](meta_handle handle) {
-                return handle.type() == internal::meta_info<Type>::resolve()->meta()
-                        ? (std::invoke(Func, *handle.data<Type>()), true)
-                        : false;
-            },
-            []() ENTT_NOEXCEPT -> meta_dtor {
-                return &node;
-            }
-        };
-
-        ENTT_ASSERT(!internal::meta_info<Type>::type->dtor);
-        ENTT_ASSERT((!internal::meta_info<Type>::template dtor<Func>));
-        internal::meta_info<Type>::template dtor<Func> = &node;
-        internal::meta_info<Type>::type->dtor = &node;
-
-        return *this;
-    }
-
-    /**
-     * @brief Assigns a meta data to a meta type.
-     *
-     * Both data members and static and global variables, as well as constants
-     * of any kind, can be assigned to a meta type.<br/>
-     * From a client's point of view, all the variables associated with the
-     * reflected object will appear as if they were part of the type itself.
-     *
-     * @tparam Data The actual variable to attach to the meta type.
-     * @tparam Policy Optional policy (no policy set by default).
-     * @tparam Property Types of properties to assign to the meta data.
-     * @param identifier Unique identifier.
-     * @param property Properties to assign to the meta data.
-     * @return A meta factory for the parent type.
-     */
-    template<auto Data, typename Policy = as_is_t, typename... Property>
-    meta_factory data(const ENTT_ID_TYPE identifier, Property &&... property) ENTT_NOEXCEPT {
-        auto * const type = internal::meta_info<Type>::resolve();
-        internal::meta_data_node *curr = nullptr;
-
-        if constexpr(std::is_same_v<Type, decltype(Data)>) {
-            static_assert(std::is_same_v<Policy, as_is_t>);
-
-            static internal::meta_data_node node{
-                &internal::meta_info<Type>::template data<Data>,
-                {},
-                type,
-                nullptr,
-                nullptr,
-                true,
-                true,
-                &internal::meta_info<Type>::resolve,
-                [](meta_handle, meta_any, meta_any) { return false; },
-                [](meta_handle, meta_any) -> meta_any { return Data; },
-                []() ENTT_NOEXCEPT -> meta_data {
-                    return &node;
-                }
-            };
-
-            node.prop = properties<std::integral_constant<Type, Data>>(std::forward<Property>(property)...);
-            curr = &node;
-        } else if constexpr(std::is_member_object_pointer_v<decltype(Data)>) {
-            using data_type = std::remove_reference_t<decltype(std::declval<Type>().*Data)>;
-
-            static internal::meta_data_node node{
-                &internal::meta_info<Type>::template data<Data>,
-                {},
-                type,
-                nullptr,
-                nullptr,
-                std::is_const_v<data_type>,
-                !std::is_member_object_pointer_v<decltype(Data)>,
-                &internal::meta_info<data_type>::resolve,
-                &internal::setter<std::is_const_v<data_type>, Type, Data>,
-                &internal::getter<Type, Data, Policy>,
-                []() ENTT_NOEXCEPT -> meta_data {
-                    return &node;
-                }
-            };
-
-            node.prop = properties<std::integral_constant<decltype(Data), Data>>(std::forward<Property>(property)...);
-            curr = &node;
-        } else {
-            static_assert(std::is_pointer_v<std::decay_t<decltype(Data)>>);
-            using data_type = std::remove_pointer_t<std::decay_t<decltype(Data)>>;
-
-            static internal::meta_data_node node{
-                &internal::meta_info<Type>::template data<Data>,
-                {},
-                type,
-                nullptr,
-                nullptr,
-                std::is_const_v<data_type>,
-                !std::is_member_object_pointer_v<decltype(Data)>,
-                &internal::meta_info<data_type>::resolve,
-                &internal::setter<std::is_const_v<data_type>, Type, Data>,
-                &internal::getter<Type, Data, Policy>,
-                []() ENTT_NOEXCEPT -> meta_data {
-                    return &node;
-                }
-            };
-
-            node.prop = properties<std::integral_constant<decltype(Data), Data>>(std::forward<Property>(property)...);
-            curr = &node;
-        }
-
-        curr->identifier = identifier;
-        curr->next = type->data;
-        ENTT_ASSERT(!duplicate(identifier, curr->next));
-        ENTT_ASSERT((!internal::meta_info<Type>::template data<Data>));
-        internal::meta_info<Type>::template data<Data> = curr;
-        type->data = curr;
-
-        return *this;
-    }
-
-    /**
-     * @brief Assigns a meta data to a meta type by means of its setter and
-     * getter.
-     *
-     * Setters and getters can be either free functions, member functions or a
-     * mix of them.<br/>
-     * In case of free functions, setters and getters must accept a reference to
-     * an instance of the parent type as their first argument. A setter has then
-     * an extra argument of a type convertible to that of the parameter to
-     * set.<br/>
-     * In case of member functions, getters have no arguments at all, while
-     * setters has an argument of a type convertible to that of the parameter to
-     * set.
-     *
-     * @tparam Setter The actual function to use as a setter.
-     * @tparam Getter The actual function to use as a getter.
-     * @tparam Policy Optional policy (no policy set by default).
-     * @tparam Property Types of properties to assign to the meta data.
-     * @param identifier Unique identifier.
-     * @param property Properties to assign to the meta data.
-     * @return A meta factory for the parent type.
-     */
-    template<auto Setter, auto Getter, typename Policy = as_is_t, typename... Property>
-    meta_factory data(const ENTT_ID_TYPE identifier, Property &&... property) ENTT_NOEXCEPT {
-        using owner_type = std::tuple<std::integral_constant<decltype(Setter), Setter>, std::integral_constant<decltype(Getter), Getter>>;
-        using underlying_type = std::invoke_result_t<decltype(Getter), Type &>;
-        static_assert(std::is_invocable_v<decltype(Setter), Type &, underlying_type>);
-        auto * const type = internal::meta_info<Type>::resolve();
-
-        static internal::meta_data_node node{
-            &internal::meta_info<Type>::template data<Setter, Getter>,
-            {},
-            type,
-            nullptr,
-            nullptr,
-            false,
-            false,
-            &internal::meta_info<underlying_type>::resolve,
-            &internal::setter<false, Type, Setter>,
-            &internal::getter<Type, Getter, Policy>,
-            []() ENTT_NOEXCEPT -> meta_data {
-                return &node;
-            }
-        };
-
-        node.identifier = identifier;
-        node.next = type->data;
-        node.prop = properties<owner_type>(std::forward<Property>(property)...);
-        ENTT_ASSERT(!duplicate(identifier, node.next));
-        ENTT_ASSERT((!internal::meta_info<Type>::template data<Setter, Getter>));
-        internal::meta_info<Type>::template data<Setter, Getter> = &node;
-        type->data = &node;
-
-        return *this;
-    }
-
-    /**
-     * @brief Assigns a meta funcion to a meta type.
-     *
-     * Both member functions and free functions can be assigned to a meta
-     * type.<br/>
-     * From a client's point of view, all the functions associated with the
-     * reflected object will appear as if they were part of the type itself.
-     *
-     * @tparam Candidate The actual function to attach to the meta type.
-     * @tparam Policy Optional policy (no policy set by default).
-     * @tparam Property Types of properties to assign to the meta function.
-     * @param identifier Unique identifier.
-     * @param property Properties to assign to the meta function.
-     * @return A meta factory for the parent type.
-     */
-    template<auto Candidate, typename Policy = as_is_t, typename... Property>
-    meta_factory func(const ENTT_ID_TYPE identifier, Property &&... property) ENTT_NOEXCEPT {
-        using owner_type = std::integral_constant<decltype(Candidate), Candidate>;
-        using helper_type = internal::meta_function_helper_t<decltype(Candidate)>;
-        auto * const type = internal::meta_info<Type>::resolve();
-
-        static internal::meta_func_node node{
-            &internal::meta_info<Type>::template func<Candidate>,
-            {},
-            type,
-            nullptr,
-            nullptr,
-            helper_type::size,
-            helper_type::is_const,
-            !std::is_member_function_pointer_v<decltype(Candidate)>,
-            &internal::meta_info<std::conditional_t<std::is_same_v<Policy, as_void_t>, void, typename helper_type::return_type>>::resolve,
-            &helper_type::arg,
-            [](meta_handle handle, meta_any *any) {
-                return internal::invoke<Type, Candidate, Policy>(handle, any, std::make_index_sequence<helper_type::size>{});
-            },
-            []() ENTT_NOEXCEPT -> meta_func {
-                return &node;
-            }
-        };
-
-        node.identifier = identifier;
-        node.next = type->func;
-        node.prop = properties<owner_type>(std::forward<Property>(property)...);
-        ENTT_ASSERT(!duplicate(identifier, node.next));
-        ENTT_ASSERT((!internal::meta_info<Type>::template func<Candidate>));
-        internal::meta_info<Type>::template func<Candidate> = &node;
-        type->func = &node;
-
-        return *this;
-    }
-
-    /**
-     * @brief Unregisters a meta type and all its parts.
-     *
-     * This function unregisters a meta type and all its data members, member
-     * functions and properties, as well as its constructors, destructors and
-     * conversion functions if any.<br/>
-     * Base classes aren't unregistered but the link between the two types is
-     * removed.
-     *
-     * @return True if the meta type exists, false otherwise.
-     */
-    bool unregister() ENTT_NOEXCEPT {
-        const auto registered = internal::meta_info<Type>::type;
-
-        if(registered) {
-            if(auto *curr = internal::meta_info<>::type; curr == internal::meta_info<Type>::type) {
-                internal::meta_info<>::type = internal::meta_info<Type>::type->next;
-            } else {
-                while(curr && curr->next != internal::meta_info<Type>::type) {
-                    curr = curr->next;
-                }
-
-                if(curr) {
-                    curr->next = internal::meta_info<Type>::type->next;
-                }
-            }
-
-            unregister_prop(&internal::meta_info<Type>::type->prop);
-            unregister_all<&internal::meta_type_node::base>(0);
-            unregister_all<&internal::meta_type_node::conv>(0);
-            unregister_all<&internal::meta_type_node::ctor>(0);
-            unregister_all<&internal::meta_type_node::data>(0);
-            unregister_all<&internal::meta_type_node::func>(0);
-            unregister_dtor();
-
-            internal::meta_info<Type>::type->identifier = {};
-            internal::meta_info<Type>::type->next = nullptr;
-            internal::meta_info<Type>::type = nullptr;
-        }
-
-        return registered;
-    }
-};
-
-
-/**
- * @brief Utility function to use for reflection.
- *
- * This is the point from which everything starts.<br/>
- * By invoking this function with a type that is not yet reflected, a meta type
- * is created to which it will be possible to attach data and functions through
- * a dedicated factory.
- *
- * @tparam Type Type to reflect.
- * @tparam Property Types of properties to assign to the reflected type.
- * @param identifier Unique identifier.
- * @param property Properties to assign to the reflected type.
- * @return A meta factory for the given type.
- */
-template<typename Type, typename... Property>
-inline meta_factory<Type> reflect(const ENTT_ID_TYPE identifier, Property &&... property) ENTT_NOEXCEPT {
-    return meta_factory<Type>{}.type(identifier, std::forward<Property>(property)...);
-}
-
-
-/**
- * @brief Utility function to use for reflection.
- *
- * This is the point from which everything starts.<br/>
- * By invoking this function with a type that is not yet reflected, a meta type
- * is created to which it will be possible to attach data and functions through
- * a dedicated factory.
- *
- * @tparam Type Type to reflect.
- * @return A meta factory for the given type.
- */
-template<typename Type>
-inline meta_factory<Type> reflect() ENTT_NOEXCEPT {
-    return meta_factory<Type>{};
-}
-
-
-/**
- * @brief Utility function to unregister a type.
- *
- * This function unregisters a type and all its data members, member functions
- * and properties, as well as its constructors, destructors and conversion
- * functions if any.<br/>
- * Base classes aren't unregistered but the link between the two types is
- * removed.
- *
- * @tparam Type Type to unregister.
- * @return True if the type to unregister exists, false otherwise.
- */
-template<typename Type>
-inline bool unregister() ENTT_NOEXCEPT {
-    return meta_factory<Type>{}.unregister();
-}
-
-
-/**
- * @brief Returns the meta type associated with a given type.
- * @tparam Type Type to use to search for a meta type.
- * @return The meta type associated with the given type, if any.
- */
-template<typename Type>
-inline meta_type resolve() ENTT_NOEXCEPT {
-    return internal::meta_info<Type>::resolve()->meta();
-}
-
-
-/**
- * @brief Returns the meta type associated with a given identifier.
- * @param identifier Unique identifier.
- * @return The meta type associated with the given identifier, if any.
- */
-inline meta_type resolve(const ENTT_ID_TYPE identifier) ENTT_NOEXCEPT {
-    const auto *curr = internal::find_if([identifier](auto *node) {
-        return node->identifier == identifier;
-    }, internal::meta_info<>::type);
-
-    return curr ? curr->meta() : meta_type{};
-}
-
-
-/**
- * @brief Iterates all the reflected types.
- * @tparam Op Type of the function object to invoke.
- * @param op A valid function object.
- */
-template<typename Op>
-inline std::enable_if_t<std::is_invocable_v<Op, meta_type>, void>
-resolve(Op op) ENTT_NOEXCEPT {
-    internal::iterate([op = std::move(op)](auto *node) {
-        op(node->meta());
-    }, internal::meta_info<>::type);
-}
-
-
-}
-
-
-#endif // ENTT_META_FACTORY_HPP
-
-// #include "meta/meta.hpp"
-
-// #include "meta/policy.hpp"
-
-// #include "process/process.hpp"
-#ifndef ENTT_PROCESS_PROCESS_HPP
-#define ENTT_PROCESS_PROCESS_HPP
-
-
-#include <utility>
-#include <type_traits>
-// #include "../config/config.h"
-#ifndef ENTT_CONFIG_CONFIG_H
-#define ENTT_CONFIG_CONFIG_H
-
-
-#ifndef ENTT_NOEXCEPT
-#define ENTT_NOEXCEPT noexcept
-#endif // ENTT_NOEXCEPT
-
-
-#ifndef ENTT_HS_SUFFIX
-#define ENTT_HS_SUFFIX _hs
-#endif // ENTT_HS_SUFFIX
-
-
-#ifndef ENTT_HWS_SUFFIX
-#define ENTT_HWS_SUFFIX _hws
-#endif // ENTT_HWS_SUFFIX
-
-
-#ifndef ENTT_NO_ATOMIC
-#include <atomic>
-#define ENTT_MAYBE_ATOMIC(Type) std::atomic<Type>
-#else // ENTT_NO_ATOMIC
-#define ENTT_MAYBE_ATOMIC(Type) Type
-#endif // ENTT_NO_ATOMIC
-
-
-#ifndef ENTT_ID_TYPE
-#include <cstdint>
-#define ENTT_ID_TYPE std::uint32_t
-#endif // ENTT_ID_TYPE
-
-
-#ifndef ENTT_PAGE_SIZE
-#define ENTT_PAGE_SIZE 32768
-#endif // ENTT_PAGE_SIZE
-
-
-#ifndef ENTT_DISABLE_ASSERT
-#include <cassert>
-#define ENTT_ASSERT(condition) assert(condition)
-#else // ENTT_DISABLE_ASSERT
-#define ENTT_ASSERT(...) ((void)0)
-#endif // ENTT_DISABLE_ASSERT
-
-
-#endif // ENTT_CONFIG_CONFIG_H
-
-
-
-namespace entt {
-
-
-/**
- * @brief Base class for processes.
- *
- * This class stays true to the CRTP idiom. Derived classes must specify what's
- * the intended type for elapsed times.<br/>
- * A process should expose publicly the following member functions whether
- * required:
- *
- * * @code{.cpp}
- *   void update(Delta, void *);
- *   @endcode
- *
- *   It's invoked once per tick until a process is explicitly aborted or it
- *   terminates either with or without errors. Even though it's not mandatory to
- *   declare this member function, as a rule of thumb each process should at
- *   least define it to work properly. The `void *` parameter is an opaque
- *   pointer to user data (if any) forwarded directly to the process during an
- *   update.
- *
- * * @code{.cpp}
- *   void init();
- *   @endcode
- *
- *   It's invoked when the process joins the running queue of a scheduler. This
- *   happens as soon as it's attached to the scheduler if the process is a top
- *   level one, otherwise when it replaces its parent if the process is a
- *   continuation.
- *
- * * @code{.cpp}
- *   void succeeded();
- *   @endcode
- *
- *   It's invoked in case of success, immediately after an update and during the
- *   same tick.
- *
- * * @code{.cpp}
- *   void failed();
- *   @endcode
- *
- *   It's invoked in case of errors, immediately after an update and during the
- *   same tick.
- *
- * * @code{.cpp}
- *   void aborted();
- *   @endcode
- *
- *   It's invoked only if a process is explicitly aborted. There is no guarantee
- *   that it executes in the same tick, this depends solely on whether the
- *   process is aborted immediately or not.
- *
- * Derived classes can change the internal state of a process by invoking the
- * `succeed` and `fail` protected member functions and even pause or unpause the
- * process itself.
- *
- * @sa scheduler
- *
- * @tparam Derived Actual type of process that extends the class template.
- * @tparam Delta Type to use to provide elapsed time.
- */
-template<typename Derived, typename Delta>
-class process {
-    enum class state: unsigned int {
-        UNINITIALIZED = 0,
-        RUNNING,
-        PAUSED,
-        SUCCEEDED,
-        FAILED,
-        ABORTED,
-        FINISHED
-    };
-
-    template<state value>
-    using state_value_t = std::integral_constant<state, value>;
-
-    template<typename Target = Derived>
-    auto tick(int, state_value_t<state::UNINITIALIZED>)
-    -> decltype(std::declval<Target>().init()) {
-        static_cast<Target *>(this)->init();
-    }
-
-    template<typename Target = Derived>
-    auto tick(int, state_value_t<state::RUNNING>, Delta delta, void *data)
-    -> decltype(std::declval<Target>().update(delta, data)) {
-        static_cast<Target *>(this)->update(delta, data);
-    }
-
-    template<typename Target = Derived>
-    auto tick(int, state_value_t<state::SUCCEEDED>)
-    -> decltype(std::declval<Target>().succeeded()) {
-        static_cast<Target *>(this)->succeeded();
-    }
-
-    template<typename Target = Derived>
-    auto tick(int, state_value_t<state::FAILED>)
-    -> decltype(std::declval<Target>().failed()) {
-        static_cast<Target *>(this)->failed();
-    }
-
-    template<typename Target = Derived>
-    auto tick(int, state_value_t<state::ABORTED>)
-    -> decltype(std::declval<Target>().aborted()) {
-        static_cast<Target *>(this)->aborted();
-    }
-
-    template<state value, typename... Args>
-    void tick(char, state_value_t<value>, Args &&...) const ENTT_NOEXCEPT {}
-
-protected:
-    /**
-     * @brief Terminates a process with success if it's still alive.
-     *
-     * The function is idempotent and it does nothing if the process isn't
-     * alive.
-     */
-    void succeed() ENTT_NOEXCEPT {
-        if(alive()) {
-            current = state::SUCCEEDED;
-        }
-    }
-
-    /**
-     * @brief Terminates a process with errors if it's still alive.
-     *
-     * The function is idempotent and it does nothing if the process isn't
-     * alive.
-     */
-    void fail() ENTT_NOEXCEPT {
-        if(alive()) {
-            current = state::FAILED;
-        }
-    }
-
-    /**
-     * @brief Stops a process if it's in a running state.
-     *
-     * The function is idempotent and it does nothing if the process isn't
-     * running.
-     */
-    void pause() ENTT_NOEXCEPT {
-        if(current == state::RUNNING) {
-            current = state::PAUSED;
-        }
-    }
-
-    /**
-     * @brief Restarts a process if it's paused.
-     *
-     * The function is idempotent and it does nothing if the process isn't
-     * paused.
-     */
-    void unpause() ENTT_NOEXCEPT {
-        if(current  == state::PAUSED) {
-            current  = state::RUNNING;
-        }
-    }
-
-public:
-    /*! @brief Type used to provide elapsed time. */
-    using delta_type = Delta;
-
-    /*! @brief Default destructor. */
-    virtual ~process() ENTT_NOEXCEPT {
-        static_assert(std::is_base_of_v<process, Derived>);
-    }
-
-    /**
-     * @brief Aborts a process if it's still alive.
-     *
-     * The function is idempotent and it does nothing if the process isn't
-     * alive.
-     *
-     * @param immediately Requests an immediate operation.
-     */
-    void abort(const bool immediately = false) ENTT_NOEXCEPT {
-        if(alive()) {
-            current = state::ABORTED;
-
-            if(immediately) {
-                tick(0);
-            }
-        }
-    }
-
-    /**
-     * @brief Returns true if a process is either running or paused.
-     * @return True if the process is still alive, false otherwise.
-     */
-    bool alive() const ENTT_NOEXCEPT {
-        return current == state::RUNNING || current == state::PAUSED;
-    }
-
-    /**
-     * @brief Returns true if a process is already terminated.
-     * @return True if the process is terminated, false otherwise.
-     */
-    bool dead() const ENTT_NOEXCEPT {
-        return current == state::FINISHED;
-    }
-
-    /**
-     * @brief Returns true if a process is currently paused.
-     * @return True if the process is paused, false otherwise.
-     */
-    bool paused() const ENTT_NOEXCEPT {
-        return current == state::PAUSED;
-    }
-
-    /**
-     * @brief Returns true if a process terminated with errors.
-     * @return True if the process terminated with errors, false otherwise.
-     */
-    bool rejected() const ENTT_NOEXCEPT {
-        return stopped;
-    }
-
-    /**
-     * @brief Updates a process and its internal state if required.
-     * @param delta Elapsed time.
-     * @param data Optional data.
-     */
-    void tick(const Delta delta, void *data = nullptr) {
-        switch (current) {
-        case state::UNINITIALIZED:
-            tick(0, state_value_t<state::UNINITIALIZED>{});
-            current = state::RUNNING;
-            break;
-        case state::RUNNING:
-            tick(0, state_value_t<state::RUNNING>{}, delta, data);
-            break;
-        default:
-            // suppress warnings
-            break;
-        }
-
-        // if it's dead, it must be notified and removed immediately
-        switch(current) {
-        case state::SUCCEEDED:
-            tick(0, state_value_t<state::SUCCEEDED>{});
-            current = state::FINISHED;
-            break;
-        case state::FAILED:
-            tick(0, state_value_t<state::FAILED>{});
-            current = state::FINISHED;
-            stopped = true;
-            break;
-        case state::ABORTED:
-            tick(0, state_value_t<state::ABORTED>{});
-            current = state::FINISHED;
-            stopped = true;
-            break;
-        default:
-            // suppress warnings
-            break;
-        }
-    }
-
-private:
-    state current{state::UNINITIALIZED};
-    bool stopped{false};
-};
-
-
-/**
- * @brief Adaptor for lambdas and functors to turn them into processes.
- *
- * Lambdas and functors can't be used directly with a scheduler for they are not
- * properly defined processes with managed life cycles.<br/>
- * This class helps in filling the gap and turning lambdas and functors into
- * full featured processes usable by a scheduler.
- *
- * The signature of the function call operator should be equivalent to the
- * following:
- *
- * @code{.cpp}
- * void(Delta delta, void *data, auto succeed, auto fail);
- * @endcode
- *
- * Where:
- *
- * * `delta` is the elapsed time.
- * * `data` is an opaque pointer to user data if any, `nullptr` otherwise.
- * * `succeed` is a function to call when a process terminates with success.
- * * `fail` is a function to call when a process terminates with errors.
- *
- * The signature of the function call operator of both `succeed` and `fail`
- * is equivalent to the following:
- *
- * @code{.cpp}
- * void();
- * @endcode
- *
- * Usually users shouldn't worry about creating adaptors. A scheduler will
- * create them internally each and avery time a lambda or a functor is used as
- * a process.
- *
- * @sa process
- * @sa scheduler
- *
- * @tparam Func Actual type of process.
- * @tparam Delta Type to use to provide elapsed time.
- */
-template<typename Func, typename Delta>
-struct process_adaptor: process<process_adaptor<Func, Delta>, Delta>, private Func {
-    /**
-     * @brief Constructs a process adaptor from a lambda or a functor.
-     * @tparam Args Types of arguments to use to initialize the actual process.
-     * @param args Parameters to use to initialize the actual process.
-     */
-    template<typename... Args>
-    process_adaptor(Args &&... args)
-        : Func{std::forward<Args>(args)...}
-    {}
-
-    /**
-     * @brief Updates a process and its internal state if required.
-     * @param delta Elapsed time.
-     * @param data Optional data.
-     */
-    void update(const Delta delta, void *data) {
-        Func::operator()(delta, data, [this]() { this->succeed(); }, [this]() { this->fail(); });
-    }
-};
-
-
-}
-
-
-#endif // ENTT_PROCESS_PROCESS_HPP
-
-// #include "process/scheduler.hpp"
-#ifndef ENTT_PROCESS_SCHEDULER_HPP
-#define ENTT_PROCESS_SCHEDULER_HPP
-
-
-#include <vector>
-#include <memory>
-#include <utility>
-#include <algorithm>
-#include <type_traits>
-// #include "../config/config.h"
-
-// #include "process.hpp"
-
-
-
-namespace entt {
-
-
-/**
- * @brief Cooperative scheduler for processes.
- *
- * A cooperative scheduler runs processes and helps managing their life cycles.
- *
- * Each process is invoked once per tick. If a process terminates, it's
- * removed automatically from the scheduler and it's never invoked again.<br/>
- * A process can also have a child. In this case, the process is replaced with
- * its child when it terminates if it returns with success. In case of errors,
- * both the process and its child are discarded.
- *
- * Example of use (pseudocode):
- *
- * @code{.cpp}
- * scheduler.attach([](auto delta, void *, auto succeed, auto fail) {
- *     // code
- * }).then<my_process>(arguments...);
- * @endcode
- *
- * In order to invoke all scheduled processes, call the `update` member function
- * passing it the elapsed time to forward to the tasks.
- *
- * @sa process
- *
- * @tparam Delta Type to use to provide elapsed time.
- */
-template<typename Delta>
-class scheduler {
-    struct process_handler {
-        using instance_type = std::unique_ptr<void, void(*)(void *)>;
-        using update_fn_type = bool(process_handler &, Delta, void *);
-        using abort_fn_type = void(process_handler &, bool);
-        using next_type = std::unique_ptr<process_handler>;
-
-        instance_type instance;
-        update_fn_type *update;
-        abort_fn_type *abort;
-        next_type next;
-    };
-
-    struct continuation {
-        continuation(process_handler *ref)
-            : handler{ref}
-        {
-            ENTT_ASSERT(handler);
-        }
-
-        template<typename Proc, typename... Args>
-        continuation then(Args &&... args) {
-            static_assert(std::is_base_of_v<process<Proc, Delta>, Proc>);
-            auto proc = typename process_handler::instance_type{new Proc{std::forward<Args>(args)...}, &scheduler::deleter<Proc>};
-            handler->next.reset(new process_handler{std::move(proc), &scheduler::update<Proc>, &scheduler::abort<Proc>, nullptr});
-            handler = handler->next.get();
-            return *this;
-        }
-
-        template<typename Func>
-        continuation then(Func &&func) {
-            return then<process_adaptor<std::decay_t<Func>, Delta>>(std::forward<Func>(func));
-        }
-
-    private:
-        process_handler *handler;
-    };
-
-    template<typename Proc>
-    static bool update(process_handler &handler, const Delta delta, void *data) {
-        auto *process = static_cast<Proc *>(handler.instance.get());
-        process->tick(delta, data);
-
-        auto dead = process->dead();
-
-        if(dead) {
-            if(handler.next && !process->rejected()) {
-                handler = std::move(*handler.next);
-                // forces the process to exit the uninitialized state
-                dead = handler.update(handler, {}, nullptr);
-            } else {
-                handler.instance.reset();
-            }
-        }
-
-        return dead;
-    }
-
-    template<typename Proc>
-    static void abort(process_handler &handler, const bool immediately) {
-        static_cast<Proc *>(handler.instance.get())->abort(immediately);
-    }
-
-    template<typename Proc>
-    static void deleter(void *proc) {
-        delete static_cast<Proc *>(proc);
-    }
-
-public:
-    /*! @brief Unsigned integer type. */
-    using size_type = typename std::vector<process_handler>::size_type;
-
-    /*! @brief Default constructor. */
-    scheduler() ENTT_NOEXCEPT = default;
-
-    /*! @brief Default move constructor. */
-    scheduler(scheduler &&) = default;
-
-    /*! @brief Default move assignment operator. @return This scheduler. */
-    scheduler & operator=(scheduler &&) = default;
-
-    /**
-     * @brief Number of processes currently scheduled.
-     * @return Number of processes currently scheduled.
-     */
-    size_type size() const ENTT_NOEXCEPT {
-        return handlers.size();
-    }
-
-    /**
-     * @brief Returns true if at least a process is currently scheduled.
-     * @return True if there are scheduled processes, false otherwise.
-     */
-    bool empty() const ENTT_NOEXCEPT {
-        return handlers.empty();
-    }
-
-    /**
-     * @brief Discards all scheduled processes.
-     *
-     * Processes aren't aborted. They are discarded along with their children
-     * and never executed again.
-     */
-    void clear() {
-        handlers.clear();
-    }
-
-    /**
-     * @brief Schedules a process for the next tick.
-     *
-     * Returned value is an opaque object that can be used to attach a child to
-     * the given process. The child is automatically scheduled when the process
-     * terminates and only if the process returns with success.
-     *
-     * Example of use (pseudocode):
-     *
-     * @code{.cpp}
-     * // schedules a task in the form of a process class
-     * scheduler.attach<my_process>(arguments...)
-     * // appends a child in the form of a lambda function
-     * .then([](auto delta, void *, auto succeed, auto fail) {
-     *     // code
-     * })
-     * // appends a child in the form of another process class
-     * .then<my_other_process>();
-     * @endcode
-     *
-     * @tparam Proc Type of process to schedule.
-     * @tparam Args Types of arguments to use to initialize the process.
-     * @param args Parameters to use to initialize the process.
-     * @return An opaque object to use to concatenate processes.
-     */
-    template<typename Proc, typename... Args>
-    auto attach(Args &&... args) {
-        static_assert(std::is_base_of_v<process<Proc, Delta>, Proc>);
-        auto proc = typename process_handler::instance_type{new Proc{std::forward<Args>(args)...}, &scheduler::deleter<Proc>};
-        process_handler handler{std::move(proc), &scheduler::update<Proc>, &scheduler::abort<Proc>, nullptr};
-        // forces the process to exit the uninitialized state
-        handler.update(handler, {}, nullptr);
-        return continuation{&handlers.emplace_back(std::move(handler))};
-    }
-
-    /**
-     * @brief Schedules a process for the next tick.
-     *
-     * A process can be either a lambda or a functor. The scheduler wraps both
-     * of them in a process adaptor internally.<br/>
-     * The signature of the function call operator should be equivalent to the
-     * following:
-     *
-     * @code{.cpp}
-     * void(Delta delta, void *data, auto succeed, auto fail);
-     * @endcode
-     *
-     * Where:
-     *
-     * * `delta` is the elapsed time.
-     * * `data` is an opaque pointer to user data if any, `nullptr` otherwise.
-     * * `succeed` is a function to call when a process terminates with success.
-     * * `fail` is a function to call when a process terminates with errors.
-     *
-     * The signature of the function call operator of both `succeed` and `fail`
-     * is equivalent to the following:
-     *
-     * @code{.cpp}
-     * void();
-     * @endcode
-     *
-     * Returned value is an opaque object that can be used to attach a child to
-     * the given process. The child is automatically scheduled when the process
-     * terminates and only if the process returns with success.
-     *
-     * Example of use (pseudocode):
-     *
-     * @code{.cpp}
-     * // schedules a task in the form of a lambda function
-     * scheduler.attach([](auto delta, void *, auto succeed, auto fail) {
-     *     // code
-     * })
-     * // appends a child in the form of another lambda function
-     * .then([](auto delta, void *, auto succeed, auto fail) {
-     *     // code
-     * })
-     * // appends a child in the form of a process class
-     * .then<my_process>(arguments...);
-     * @endcode
-     *
-     * @sa process_adaptor
-     *
-     * @tparam Func Type of process to schedule.
-     * @param func Either a lambda or a functor to use as a process.
-     * @return An opaque object to use to concatenate processes.
-     */
-    template<typename Func>
-    auto attach(Func &&func) {
-        using Proc = process_adaptor<std::decay_t<Func>, Delta>;
-        return attach<Proc>(std::forward<Func>(func));
-    }
-
-    /**
-     * @brief Updates all scheduled processes.
-     *
-     * All scheduled processes are executed in no specific order.<br/>
-     * If a process terminates with success, it's replaced with its child, if
-     * any. Otherwise, if a process terminates with an error, it's removed along
-     * with its child.
-     *
-     * @param delta Elapsed time.
-     * @param data Optional data.
-     */
-    void update(const Delta delta, void *data = nullptr) {
-        bool clean = false;
-
-        for(auto pos = handlers.size(); pos; --pos) {
-            auto &handler = handlers[pos-1];
-            const bool dead = handler.update(handler, delta, data);
-            clean = clean || dead;
-        }
-
-        if(clean) {
-            handlers.erase(std::remove_if(handlers.begin(), handlers.end(), [](auto &handler) {
-                return !handler.instance;
-            }), handlers.end());
-        }
-    }
-
-    /**
-     * @brief Aborts all scheduled processes.
-     *
-     * Unless an immediate operation is requested, the abort is scheduled for
-     * the next tick. Processes won't be executed anymore in any case.<br/>
-     * Once a process is fully aborted and thus finished, it's discarded along
-     * with its child, if any.
-     *
-     * @param immediately Requests an immediate operation.
-     */
-    void abort(const bool immediately = false) {
-        decltype(handlers) exec;
-        exec.swap(handlers);
-
-        std::for_each(exec.begin(), exec.end(), [immediately](auto &handler) {
-            handler.abort(handler, immediately);
-        });
-
-        std::move(handlers.begin(), handlers.end(), std::back_inserter(exec));
-        handlers.swap(exec);
-    }
-
-private:
-    std::vector<process_handler> handlers{};
-};
-
-
-}
-
-
-#endif // ENTT_PROCESS_SCHEDULER_HPP
-
-// #include "resource/cache.hpp"
-#ifndef ENTT_RESOURCE_CACHE_HPP
-#define ENTT_RESOURCE_CACHE_HPP
-
-
-#include <memory>
-#include <utility>
-#include <type_traits>
-#include <unordered_map>
-// #include "../config/config.h"
-#ifndef ENTT_CONFIG_CONFIG_H
-#define ENTT_CONFIG_CONFIG_H
-
-
-#ifndef ENTT_NOEXCEPT
-#define ENTT_NOEXCEPT noexcept
-#endif // ENTT_NOEXCEPT
-
-
-#ifndef ENTT_HS_SUFFIX
-#define ENTT_HS_SUFFIX _hs
-#endif // ENTT_HS_SUFFIX
-
-
-#ifndef ENTT_HWS_SUFFIX
-#define ENTT_HWS_SUFFIX _hws
-#endif // ENTT_HWS_SUFFIX
-
-
-#ifndef ENTT_NO_ATOMIC
-#include <atomic>
-#define ENTT_MAYBE_ATOMIC(Type) std::atomic<Type>
-#else // ENTT_NO_ATOMIC
-#define ENTT_MAYBE_ATOMIC(Type) Type
-#endif // ENTT_NO_ATOMIC
-
-
-#ifndef ENTT_ID_TYPE
-#include <cstdint>
-#define ENTT_ID_TYPE std::uint32_t
-#endif // ENTT_ID_TYPE
-
-
-#ifndef ENTT_PAGE_SIZE
-#define ENTT_PAGE_SIZE 32768
-#endif // ENTT_PAGE_SIZE
-
-
-#ifndef ENTT_DISABLE_ASSERT
-#include <cassert>
-#define ENTT_ASSERT(condition) assert(condition)
-#else // ENTT_DISABLE_ASSERT
-#define ENTT_ASSERT(...) ((void)0)
-#endif // ENTT_DISABLE_ASSERT
-
-
-#endif // ENTT_CONFIG_CONFIG_H
-
-// #include "handle.hpp"
-#ifndef ENTT_RESOURCE_HANDLE_HPP
-#define ENTT_RESOURCE_HANDLE_HPP
-
-
-#include <memory>
-#include <utility>
-// #include "../config/config.h"
-
-// #include "fwd.hpp"
-#ifndef ENTT_RESOURCE_FWD_HPP
-#define ENTT_RESOURCE_FWD_HPP
-
-
-// #include "../config/config.h"
-
-
-
-namespace entt {
-
-
-/*! @class resource_cache */
-template<typename>
-class resource_cache;
-
-/*! @class resource_handle */
-template<typename>
-class resource_handle;
-
-/*! @class resource_loader */
-template<typename, typename>
-class resource_loader;
-
-
-}
-
-
-#endif // ENTT_RESOURCE_FWD_HPP
-
-
-
-namespace entt {
-
-
-/**
- * @brief Shared resource handle.
- *
- * A shared resource handle is a small class that wraps a resource and keeps it
- * alive even if it's deleted from the cache. It can be either copied or
- * moved. A handle shares a reference to the same resource with all the other
- * handles constructed for the same identifier.<br/>
- * As a rule of thumb, resources should never be copied nor moved. Handles are
- * the way to go to keep references to them.
- *
- * @tparam Resource Type of resource managed by a handle.
- */
-template<typename Resource>
-class resource_handle {
-    /*! @brief Resource handles are friends of their caches. */
-    friend class resource_cache<Resource>;
-
-    resource_handle(std::shared_ptr<Resource> res) ENTT_NOEXCEPT
-        : resource{std::move(res)}
-    {}
-
-public:
-    /*! @brief Default constructor. */
-    resource_handle() ENTT_NOEXCEPT = default;
-
-    /**
-     * @brief Gets a reference to the managed resource.
-     *
-     * @warning
-     * The behavior is undefined if the handle doesn't contain a resource.<br/>
-     * An assertion will abort the execution at runtime in debug mode if the
-     * handle is empty.
-     *
-     * @return A reference to the managed resource.
-     */
-    const Resource & get() const ENTT_NOEXCEPT {
-        ENTT_ASSERT(static_cast<bool>(resource));
-        return *resource;
-    }
-
-    /*! @copydoc get */
-    Resource & get() ENTT_NOEXCEPT {
-        return const_cast<Resource &>(std::as_const(*this).get());
-    }
-
-    /*! @copydoc get */
-    operator const Resource & () const ENTT_NOEXCEPT { return get(); }
-
-    /*! @copydoc get */
-    operator Resource & () ENTT_NOEXCEPT { return get(); }
-
-    /*! @copydoc get */
-    const Resource & operator *() const ENTT_NOEXCEPT { return get(); }
-
-    /*! @copydoc get */
-    Resource & operator *() ENTT_NOEXCEPT { return get(); }
-
-    /**
-     * @brief Gets a pointer to the managed resource.
-     *
-     * @warning
-     * The behavior is undefined if the handle doesn't contain a resource.<br/>
-     * An assertion will abort the execution at runtime in debug mode if the
-     * handle is empty.
-     *
-     * @return A pointer to the managed resource or `nullptr` if the handle
-     * contains no resource at all.
-     */
-    const Resource * operator->() const ENTT_NOEXCEPT {
-        ENTT_ASSERT(static_cast<bool>(resource));
-        return resource.get();
-    }
-
-    /*! @copydoc operator-> */
-    Resource * operator->() ENTT_NOEXCEPT {
-        return const_cast<Resource *>(std::as_const(*this).operator->());
-    }
-
-    /**
-     * @brief Returns true if a handle contains a resource, false otherwise.
-     * @return True if the handle contains a resource, false otherwise.
-     */
-    explicit operator bool() const { return static_cast<bool>(resource); }
-
-private:
-    std::shared_ptr<Resource> resource;
-};
-
-
-}
-
-
-#endif // ENTT_RESOURCE_HANDLE_HPP
-
-// #include "loader.hpp"
-#ifndef ENTT_RESOURCE_LOADER_HPP
-#define ENTT_RESOURCE_LOADER_HPP
-
-
-#include <memory>
-// #include "fwd.hpp"
-
-
-
-namespace entt {
-
-
-/**
- * @brief Base class for resource loaders.
- *
- * Resource loaders must inherit from this class and stay true to the CRTP
- * idiom. Moreover, a resource loader must expose a public, const member
- * function named `load` that accepts a variable number of arguments and returns
- * a shared pointer to the resource just created.<br/>
- * As an example:
- *
- * @code{.cpp}
- * struct my_resource {};
- *
- * struct my_loader: entt::resource_loader<my_loader, my_resource> {
- *     std::shared_ptr<my_resource> load(int) const {
- *         // use the integer value somehow
- *         return std::make_shared<my_resource>();
- *     }
- * };
- * @endcode
- *
- * In general, resource loaders should not have a state or retain data of any
- * type. They should let the cache manage their resources instead.
- *
- * @note
- * Base class and CRTP idiom aren't strictly required with the current
- * implementation. One could argue that a cache can easily work with loaders of
- * any type. However, future changes won't be breaking ones by forcing the use
- * of a base class today and that's why the model is already in its place.
- *
- * @tparam Loader Type of the derived class.
- * @tparam Resource Type of resource for which to use the loader.
- */
-template<typename Loader, typename Resource>
-class resource_loader {
-    /*! @brief Resource loaders are friends of their caches. */
-    friend class resource_cache<Resource>;
-
-    /**
-     * @brief Loads the resource and returns it.
-     * @tparam Args Types of arguments for the loader.
-     * @param args Arguments for the loader.
-     * @return The resource just loaded or an empty pointer in case of errors.
-     */
-    template<typename... Args>
-    std::shared_ptr<Resource> get(Args &&... args) const {
-        return static_cast<const Loader *>(this)->load(std::forward<Args>(args)...);
-    }
-};
-
-
-}
-
-
-#endif // ENTT_RESOURCE_LOADER_HPP
-
-// #include "fwd.hpp"
-
-
-
-namespace entt {
-
-
-/**
- * @brief Simple cache for resources of a given type.
- *
- * Minimal implementation of a cache for resources of a given type. It doesn't
- * offer much functionalities but it's suitable for small or medium sized
- * applications and can be freely inherited to add targeted functionalities for
- * large sized applications.
- *
- * @tparam Resource Type of resources managed by a cache.
- */
-template<typename Resource>
-class resource_cache {
-    using container_type = std::unordered_map<ENTT_ID_TYPE, std::shared_ptr<Resource>>;
-
-public:
-    /*! @brief Unsigned integer type. */
-    using size_type = typename container_type::size_type;
-    /*! @brief Type of resources managed by a cache. */
-    using resource_type = ENTT_ID_TYPE;
-
-    /*! @brief Default constructor. */
-    resource_cache() = default;
-
-    /*! @brief Default move constructor. */
-    resource_cache(resource_cache &&) = default;
-
-    /*! @brief Default move assignment operator. @return This cache. */
-    resource_cache & operator=(resource_cache &&) = default;
-
-    /**
-     * @brief Number of resources managed by a cache.
-     * @return Number of resources currently stored.
-     */
-    size_type size() const ENTT_NOEXCEPT {
-        return resources.size();
-    }
-
-    /**
-     * @brief Returns true if a cache contains no resources, false otherwise.
-     * @return True if the cache contains no resources, false otherwise.
-     */
-    bool empty() const ENTT_NOEXCEPT {
-        return resources.empty();
-    }
-
-    /**
-     * @brief Clears a cache and discards all its resources.
-     *
-     * Handles are not invalidated and the memory used by a resource isn't
-     * freed as long as at least a handle keeps the resource itself alive.
-     */
-    void clear() ENTT_NOEXCEPT {
-        resources.clear();
-    }
-
-    /**
-     * @brief Loads the resource that corresponds to a given identifier.
-     *
-     * In case an identifier isn't already present in the cache, it loads its
-     * resource and stores it aside for future uses. Arguments are forwarded
-     * directly to the loader in order to construct properly the requested
-     * resource.
-     *
-     * @note
-     * If the identifier is already present in the cache, this function does
-     * nothing and the arguments are simply discarded.
-     *
-     * @warning
-     * If the resource cannot be loaded correctly, the returned handle will be
-     * invalid and any use of it will result in undefined behavior.
-     *
-     * @tparam Loader Type of loader to use to load the resource if required.
-     * @tparam Args Types of arguments to use to load the resource if required.
-     * @param id Unique resource identifier.
-     * @param args Arguments to use to load the resource if required.
-     * @return A handle for the given resource.
-     */
-    template<typename Loader, typename... Args>
-    resource_handle<Resource> load(const resource_type id, Args &&... args) {
-        static_assert(std::is_base_of_v<resource_loader<Loader, Resource>, Loader>);
-        resource_handle<Resource> handle{};
-
-        if(auto it = resources.find(id); it == resources.cend()) {
-            if(auto resource = Loader{}.get(std::forward<Args>(args)...); resource) {
-                resources[id] = resource;
-                handle = std::move(resource);
-            }
-        } else {
-            handle = it->second;
-        }
-
-        return handle;
-    }
-
-    /**
-     * @brief Reloads a resource or loads it for the first time if not present.
-     *
-     * Equivalent to the following snippet (pseudocode):
-     *
-     * @code{.cpp}
-     * cache.discard(id);
-     * cache.load(id, args...);
-     * @endcode
-     *
-     * Arguments are forwarded directly to the loader in order to construct
-     * properly the requested resource.
-     *
-     * @warning
-     * If the resource cannot be loaded correctly, the returned handle will be
-     * invalid and any use of it will result in undefined behavior.
-     *
-     * @tparam Loader Type of loader to use to load the resource.
-     * @tparam Args Types of arguments to use to load the resource.
-     * @param id Unique resource identifier.
-     * @param args Arguments to use to load the resource.
-     * @return A handle for the given resource.
-     */
-    template<typename Loader, typename... Args>
-    resource_handle<Resource> reload(const resource_type id, Args &&... args) {
-        return (discard(id), load<Loader>(id, std::forward<Args>(args)...));
-    }
-
-    /**
-     * @brief Creates a temporary handle for a resource.
-     *
-     * Arguments are forwarded directly to the loader in order to construct
-     * properly the requested resource. The handle isn't stored aside and the
-     * cache isn't in charge of the lifetime of the resource itself.
-     *
-     * @tparam Loader Type of loader to use to load the resource.
-     * @tparam Args Types of arguments to use to load the resource.
-     * @param args Arguments to use to load the resource.
-     * @return A handle for the given resource.
-     */
-    template<typename Loader, typename... Args>
-    resource_handle<Resource> temp(Args &&... args) const {
-        return { Loader{}.get(std::forward<Args>(args)...) };
-    }
-
-    /**
-     * @brief Creates a handle for a given resource identifier.
-     *
-     * A resource handle can be in a either valid or invalid state. In other
-     * terms, a resource handle is properly initialized with a resource if the
-     * cache contains the resource itself. Otherwise the returned handle is
-     * uninitialized and accessing it results in undefined behavior.
-     *
-     * @sa resource_handle
-     *
-     * @param id Unique resource identifier.
-     * @return A handle for the given resource.
-     */
-    resource_handle<Resource> handle(const resource_type id) const {
-        auto it = resources.find(id);
-        return { it == resources.end() ? nullptr : it->second };
-    }
-
-    /**
-     * @brief Checks if a cache contains a given identifier.
-     * @param id Unique resource identifier.
-     * @return True if the cache contains the resource, false otherwise.
-     */
-    bool contains(const resource_type id) const ENTT_NOEXCEPT {
-        return (resources.find(id) != resources.cend());
-    }
-
-    /**
-     * @brief Discards the resource that corresponds to a given identifier.
-     *
-     * Handles are not invalidated and the memory used by the resource isn't
-     * freed as long as at least a handle keeps the resource itself alive.
-     *
-     * @param id Unique resource identifier.
-     */
-    void discard(const resource_type id) ENTT_NOEXCEPT {
-        if(auto it = resources.find(id); it != resources.end()) {
-            resources.erase(it);
-        }
-    }
-
-    /**
-     * @brief Iterates all resources.
-     *
-     * The function object is invoked for each element. It is provided with
-     * either the resource identifier, the resource handle or both of them.<br/>
-     * The signature of the function must be equivalent to one of the following
-     * forms:
-     *
-     * @code{.cpp}
-     * void(const resource_type);
-     * void(resource_handle<Resource>);
-     * void(const resource_type, resource_handle<Resource>);
-     * @endcode
-     *
-     * @tparam Func Type of the function object to invoke.
-     * @param func A valid function object.
-     */
-    template <typename Func>
-    void each(Func func) const {
-        auto begin = resources.begin();
-        auto end = resources.end();
-
-        while(begin != end) {
-            auto curr = begin++;
-
-            if constexpr(std::is_invocable_v<Func, resource_type>) {
-                func(curr->first);
-            } else if constexpr(std::is_invocable_v<Func, resource_handle<Resource>>) {
-                func(resource_handle{ curr->second });
-            } else {
-                func(curr->first, resource_handle{ curr->second });
-            }
-        }
-    }
-
-private:
-    container_type resources;
-};
-
-
-}
-
-
-#endif // ENTT_RESOURCE_CACHE_HPP
-
-// #include "resource/handle.hpp"
-
-// #include "resource/loader.hpp"
-
-// #include "signal/delegate.hpp"
-#ifndef ENTT_SIGNAL_DELEGATE_HPP
-#define ENTT_SIGNAL_DELEGATE_HPP
-
-
+struct meta_type_node;
+
+
+struct ENTT_API meta_context {
+    // we could use the lines below but VS2017 returns with an ICE if combined with ENTT_API despite the code being valid C++
+    //     inline static meta_type_node *local = nullptr;
+    //     inline static meta_type_node **global = &local;
+
+    [[nodiscard]] static meta_type_node * & local() ENTT_NOEXCEPT {
+        static meta_type_node *chain = nullptr;
+        return chain;
+    }
+
+    [[nodiscard]] static meta_type_node ** & global() ENTT_NOEXCEPT {
+        static meta_type_node **chain = &local();
+        return chain;
+    }
+};
+
+
+}
+
+
+/**
+ * Internal details not to be documented.
+ * @endcond
+ */
+
+
+/*! @brief Opaque container for a meta context. */
+struct meta_ctx {
+    /**
+     * @brief Binds the meta system to a given context.
+     * @param other A valid context to which to bind.
+     */
+    static void bind(meta_ctx other) ENTT_NOEXCEPT {
+        internal::meta_context::global() = other.ctx;
+    }
+
+private:
+    internal::meta_type_node **ctx{&internal::meta_context::local()};
+};
+
+
+}
+
+
+#endif
+
+// #include "meta/factory.hpp"
+#ifndef ENTT_META_FACTORY_HPP
+#define ENTT_META_FACTORY_HPP
+
+
+#include <cstddef>
 #include <tuple>
-#include <cstring>
-#include <utility>
-#include <algorithm>
-#include <functional>
-#include <type_traits>
-// #include "../config/config.h"
-#ifndef ENTT_CONFIG_CONFIG_H
-#define ENTT_CONFIG_CONFIG_H
-
-
-#ifndef ENTT_NOEXCEPT
-#define ENTT_NOEXCEPT noexcept
-#endif // ENTT_NOEXCEPT
-
-
-#ifndef ENTT_HS_SUFFIX
-#define ENTT_HS_SUFFIX _hs
-#endif // ENTT_HS_SUFFIX
-
-
-#ifndef ENTT_HWS_SUFFIX
-#define ENTT_HWS_SUFFIX _hws
-#endif // ENTT_HWS_SUFFIX
-
-
-#ifndef ENTT_NO_ATOMIC
-#include <atomic>
-#define ENTT_MAYBE_ATOMIC(Type) std::atomic<Type>
-#else // ENTT_NO_ATOMIC
-#define ENTT_MAYBE_ATOMIC(Type) Type
-#endif // ENTT_NO_ATOMIC
-
-
-#ifndef ENTT_ID_TYPE
-#include <cstdint>
-#define ENTT_ID_TYPE std::uint32_t
-#endif // ENTT_ID_TYPE
-
-
-#ifndef ENTT_PAGE_SIZE
-#define ENTT_PAGE_SIZE 32768
-#endif // ENTT_PAGE_SIZE
-
-
-#ifndef ENTT_DISABLE_ASSERT
-#include <cassert>
-#define ENTT_ASSERT(condition) assert(condition)
-#else // ENTT_DISABLE_ASSERT
-#define ENTT_ASSERT(...) ((void)0)
-#endif // ENTT_DISABLE_ASSERT
-
-
-#endif // ENTT_CONFIG_CONFIG_H
-
-
-
-namespace entt {
-
-
-/**
- * @cond TURN_OFF_DOXYGEN
- * Internal details not to be documented.
- */
-
-
-namespace internal {
-
-
-template<typename Ret, typename... Args>
-auto to_function_pointer(Ret(*)(Args...)) -> Ret(*)(Args...);
-
-
-template<typename Ret, typename... Args, typename Type, typename Payload, typename = std::enable_if_t<std::is_convertible_v<Payload &, Type &>>>
-auto to_function_pointer(Ret(*)(Type &, Args...), Payload &) -> Ret(*)(Args...);
-
-
-template<typename Class, typename Ret, typename... Args>
-auto to_function_pointer(Ret(Class:: *)(Args...), const Class &) -> Ret(*)(Args...);
-
-
-template<typename Class, typename Ret, typename... Args>
-auto to_function_pointer(Ret(Class:: *)(Args...) const, const Class &) -> Ret(*)(Args...);
-
-
-template<typename Class, typename Type>
-auto to_function_pointer(Type Class:: *, const Class &) -> Type(*)();
-
-
-template<typename>
-struct function_extent;
-
-
-template<typename Ret, typename... Args>
-struct function_extent<Ret(*)(Args...)> {
-    static constexpr auto value = sizeof...(Args);
-};
-
-
-template<typename Func>
-constexpr auto function_extent_v = function_extent<Func>::value;
-
-
-}
-
-
-/**
- * Internal details not to be documented.
- * @endcond TURN_OFF_DOXYGEN
- */
-
-
-/*! @brief Used to wrap a function or a member of a specified type. */
-template<auto>
-struct connect_arg_t {};
-
-
-/*! @brief Constant of type connect_arg_t used to disambiguate calls. */
-template<auto Func>
-constexpr connect_arg_t<Func> connect_arg{};
-
-
-/**
- * @brief Basic delegate implementation.
- *
- * Primary template isn't defined on purpose. All the specializations give a
- * compile-time error unless the template parameter is a function type.
- */
-template<typename>
-class delegate;
-
-
-/**
- * @brief Utility class to use to send around functions and members.
- *
- * Unmanaged delegate for function pointers and members. Users of this class are
- * in charge of disconnecting instances before deleting them.
- *
- * A delegate can be used as general purpose invoker with no memory overhead for
- * free functions (with or without payload) and members provided along with an
- * instance on which to invoke them.
- *
- * @tparam Ret Return type of a function type.
- * @tparam Args Types of arguments of a function type.
- */
-template<typename Ret, typename... Args>
-class delegate<Ret(Args...)> {
-    using proto_fn_type = Ret(const void *, std::tuple<Args &&...>);
-
-    template<auto Function, std::size_t... Index>
-    void connect(std::index_sequence<Index...>) ENTT_NOEXCEPT {
-        static_assert(std::is_invocable_r_v<Ret, decltype(Function), std::tuple_element_t<Index, std::tuple<Args...>>...>);
-        data = nullptr;
-
-        fn = [](const void *, std::tuple<Args &&...> args) -> Ret {
-            // Ret(...) makes void(...) eat the return values to avoid errors
-            return Ret(std::invoke(Function, std::forward<std::tuple_element_t<Index, std::tuple<Args...>>>(std::get<Index>(args))...));
-        };
-    }
-
-    template<auto Candidate, typename Type, std::size_t... Index>
-    void connect(Type &value_or_instance, std::index_sequence<Index...>) ENTT_NOEXCEPT {
-        static_assert(std::is_invocable_r_v<Ret, decltype(Candidate), Type &, std::tuple_element_t<Index, std::tuple<Args...>>...>);
-        data = &value_or_instance;
-
-        fn = [](const void *payload, std::tuple<Args &&...> args) -> Ret {
-            Type *curr = nullptr;
-
-            if constexpr(std::is_const_v<Type>) {
-                curr = static_cast<Type *>(payload);
-            } else {
-                curr = static_cast<Type *>(const_cast<void *>(payload));
-            }
-
-            // Ret(...) makes void(...) eat the return values to avoid errors
-            return Ret(std::invoke(Candidate, *curr, std::forward<std::tuple_element_t<Index, std::tuple<Args...>>>(std::get<Index>(args))...));
-        };
-    }
-
-public:
-    /*! @brief Function type of the delegate. */
-    using function_type = Ret(Args...);
-
-    /*! @brief Default constructor. */
-    delegate() ENTT_NOEXCEPT
-        : fn{nullptr}, data{nullptr}
-    {}
-
-    /**
-     * @brief Constructs a delegate and connects a free function to it.
-     * @tparam Function A valid free function pointer.
-     */
-    template<auto Function>
-    delegate(connect_arg_t<Function>) ENTT_NOEXCEPT
-        : delegate{}
-    {
-        connect<Function>();
-    }
-
-    /**
-     * @brief Constructs a delegate and connects a member for a given instance
-     * or a free function with payload.
-     * @tparam Candidate Member or free function to connect to the delegate.
-     * @tparam Type Type of class or type of payload.
-     * @param value_or_instance A valid reference that fits the purpose.
-     */
-    template<auto Candidate, typename Type>
-    delegate(connect_arg_t<Candidate>, Type &value_or_instance) ENTT_NOEXCEPT
-        : delegate{}
-    {
-        connect<Candidate>(value_or_instance);
-    }
-
-    /**
-     * @brief Connects a free function to a delegate.
-     * @tparam Function A valid free function pointer.
-     */
-    template<auto Function>
-    void connect() ENTT_NOEXCEPT {
-        constexpr auto extent = internal::function_extent_v<decltype(internal::to_function_pointer(std::declval<decltype(Function)>()))>;
-        connect<Function>(std::make_index_sequence<extent>{});
-    }
-
-    /**
-     * @brief Connects a member function for a given instance or a free function
-     * with payload to a delegate.
-     *
-     * The delegate isn't responsible for the connected object or the payload.
-     * Users must always guarantee that the lifetime of the instance overcomes
-     * the one  of the delegate.<br/>
-     * When used to connect a free function with payload, its signature must be
-     * such that the instance is the first argument before the ones used to
-     * define the delegate itself.
-     *
-     * @tparam Candidate Member or free function to connect to the delegate.
-     * @tparam Type Type of class or type of payload.
-     * @param value_or_instance A valid reference that fits the purpose.
-     */
-    template<auto Candidate, typename Type>
-    void connect(Type &value_or_instance) ENTT_NOEXCEPT {
-        constexpr auto extent = internal::function_extent_v<decltype(internal::to_function_pointer(std::declval<decltype(Candidate)>(), value_or_instance))>;
-        connect<Candidate>(value_or_instance, std::make_index_sequence<extent>{});
-    }
-
-    /**
-     * @brief Resets a delegate.
-     *
-     * After a reset, a delegate cannot be invoked anymore.
-     */
-    void reset() ENTT_NOEXCEPT {
-        fn = nullptr;
-        data = nullptr;
-    }
-
-    /**
-     * @brief Returns the instance or the payload linked to a delegate, if any.
-     * @return An opaque pointer to the underlying data.
-     */
-    const void * instance() const ENTT_NOEXCEPT {
-        return data;
-    }
-
-    /**
-     * @brief Triggers a delegate.
-     *
-     * The delegate invokes the underlying function and returns the result.
-     *
-     * @warning
-     * Attempting to trigger an invalid delegate results in undefined
-     * behavior.<br/>
-     * An assertion will abort the execution at runtime in debug mode if the
-     * delegate has not yet been set.
-     *
-     * @param args Arguments to use to invoke the underlying function.
-     * @return The value returned by the underlying function.
-     */
-    Ret operator()(Args... args) const {
-        ENTT_ASSERT(fn);
-        return fn(data, std::forward_as_tuple(std::forward<Args>(args)...));
-    }
-
-    /**
-     * @brief Checks whether a delegate actually stores a listener.
-     * @return False if the delegate is empty, true otherwise.
-     */
-    explicit operator bool() const ENTT_NOEXCEPT {
-        // no need to test also data
-        return fn;
-    }
-
-    /**
-     * @brief Compares the contents of two delegates.
-     * @param other Delegate with which to compare.
-     * @return False if the two contents differ, true otherwise.
-     */
-    bool operator==(const delegate<Ret(Args...)> &other) const ENTT_NOEXCEPT {
-        return fn == other.fn && data == other.data;
-    }
-
-private:
-    proto_fn_type *fn;
-    const void *data;
-};
-
-
-/**
- * @brief Compares the contents of two delegates.
- * @tparam Ret Return type of a function type.
- * @tparam Args Types of arguments of a function type.
- * @param lhs A valid delegate object.
- * @param rhs A valid delegate object.
- * @return True if the two contents differ, false otherwise.
- */
-template<typename Ret, typename... Args>
-bool operator!=(const delegate<Ret(Args...)> &lhs, const delegate<Ret(Args...)> &rhs) ENTT_NOEXCEPT {
-    return !(lhs == rhs);
-}
-
-
-/**
- * @brief Deduction guide.
- *
- * It allows to deduce the function type of the delegate directly from a
- * function provided to the constructor.
- *
- * @tparam Function A valid free function pointer.
- */
-template<auto Function>
-delegate(connect_arg_t<Function>) ENTT_NOEXCEPT
--> delegate<std::remove_pointer_t<decltype(internal::to_function_pointer(Function))>>;
-
-
-/**
- * @brief Deduction guide.
- *
- * It allows to deduce the function type of the delegate directly from a member
- * or a free function with payload provided to the constructor.
- *
- * @param value_or_instance A valid reference that fits the purpose.
- * @tparam Candidate Member or free function to connect to the delegate.
- * @tparam Type Type of class or type of payload.
- */
-template<auto Candidate, typename Type>
-delegate(connect_arg_t<Candidate>, Type &value_or_instance) ENTT_NOEXCEPT
--> delegate<std::remove_pointer_t<decltype(internal::to_function_pointer(Candidate, value_or_instance))>>;
-
-
-}
-
-
-#endif // ENTT_SIGNAL_DELEGATE_HPP
-
-// #include "signal/dispatcher.hpp"
-#ifndef ENTT_SIGNAL_DISPATCHER_HPP
-#define ENTT_SIGNAL_DISPATCHER_HPP
-
-
-#include <vector>
-#include <memory>
-#include <utility>
-#include <type_traits>
-// #include "../config/config.h"
-
-// #include "../core/family.hpp"
-#ifndef ENTT_CORE_FAMILY_HPP
-#define ENTT_CORE_FAMILY_HPP
-
-
-#include <type_traits>
-// #include "../config/config.h"
-#ifndef ENTT_CONFIG_CONFIG_H
-#define ENTT_CONFIG_CONFIG_H
-
-
-#ifndef ENTT_NOEXCEPT
-#define ENTT_NOEXCEPT noexcept
-#endif // ENTT_NOEXCEPT
-
-
-#ifndef ENTT_HS_SUFFIX
-#define ENTT_HS_SUFFIX _hs
-#endif // ENTT_HS_SUFFIX
-
-
-#ifndef ENTT_HWS_SUFFIX
-#define ENTT_HWS_SUFFIX _hws
-#endif // ENTT_HWS_SUFFIX
-
-
-#ifndef ENTT_NO_ATOMIC
-#include <atomic>
-#define ENTT_MAYBE_ATOMIC(Type) std::atomic<Type>
-#else // ENTT_NO_ATOMIC
-#define ENTT_MAYBE_ATOMIC(Type) Type
-#endif // ENTT_NO_ATOMIC
-
-
-#ifndef ENTT_ID_TYPE
-#include <cstdint>
-#define ENTT_ID_TYPE std::uint32_t
-#endif // ENTT_ID_TYPE
-
-
-#ifndef ENTT_PAGE_SIZE
-#define ENTT_PAGE_SIZE 32768
-#endif // ENTT_PAGE_SIZE
-
-
-#ifndef ENTT_DISABLE_ASSERT
-#include <cassert>
-#define ENTT_ASSERT(condition) assert(condition)
-#else // ENTT_DISABLE_ASSERT
-#define ENTT_ASSERT(...) ((void)0)
-#endif // ENTT_DISABLE_ASSERT
-
-
-#endif // ENTT_CONFIG_CONFIG_H
-
-
-
-namespace entt {
-
-
-/**
- * @brief Dynamic identifier generator.
- *
- * Utility class template that can be used to assign unique identifiers to types
- * at runtime. Use different specializations to create separate sets of
- * identifiers.
- */
-template<typename...>
-class family {
-    inline static ENTT_MAYBE_ATOMIC(ENTT_ID_TYPE) identifier;
-
-    template<typename...>
-    // clang (since version 9) started to complain if auto is used instead of ENTT_ID_TYPE
-    inline static const ENTT_ID_TYPE inner = identifier++;
-
-public:
-    /*! @brief Unsigned integer type. */
-    using family_type = ENTT_ID_TYPE;
-
-    /*! @brief Statically generated unique identifier for the given type. */
-    template<typename... Type>
-    // at the time I'm writing, clang crashes during compilation if auto is used instead of family_type
-    inline static const family_type type = inner<std::decay_t<Type>...>;
-};
-
-
-}
-
-
-#endif // ENTT_CORE_FAMILY_HPP
-
-// #include "../core/type_traits.hpp"
-#ifndef ENTT_CORE_TYPE_TRAITS_HPP
-#define ENTT_CORE_TYPE_TRAITS_HPP
-
-
-#include <type_traits>
-// #include "../config/config.h"
-
-// #include "../core/hashed_string.hpp"
+#include <type_traits>
+#include <utility>
+// #include "../config/config.h"
+
+// #include "../core/fwd.hpp"
+#ifndef ENTT_CORE_FWD_HPP
+#define ENTT_CORE_FWD_HPP
+
+
+#include <type_traits>
+// #include "../config/config.h"
+
+
+
+namespace entt {
+
+
+template<std::size_t Len = sizeof(double[2]), std::size_t = alignof(typename std::aligned_storage_t<Len + !Len>)>
+class basic_any;
+
+
+/*! @brief Alias declaration for type identifiers. */
+using id_type = ENTT_ID_TYPE;
+
+
+/*! @brief Alias declaration for the most common use case. */
+using any = basic_any<>;
+
+
+}
+
+
+#endif
+
+// #include "../core/type_info.hpp"
+#ifndef ENTT_CORE_TYPE_INFO_HPP
+#define ENTT_CORE_TYPE_INFO_HPP
+
+
+#include <string_view>
+#include <type_traits>
+// #include "../config/config.h"
+
+// #include "../core/attribute.h"
+#ifndef ENTT_CORE_ATTRIBUTE_H
+#define ENTT_CORE_ATTRIBUTE_H
+
+
+#ifndef ENTT_EXPORT
+#   if defined _WIN32 || defined __CYGWIN__ || defined _MSC_VER
+#       define ENTT_EXPORT __declspec(dllexport)
+#       define ENTT_IMPORT __declspec(dllimport)
+#       define ENTT_HIDDEN
+#   elif defined __GNUC__ && __GNUC__ >= 4
+#       define ENTT_EXPORT __attribute__((visibility("default")))
+#       define ENTT_IMPORT __attribute__((visibility("default")))
+#       define ENTT_HIDDEN __attribute__((visibility("hidden")))
+#   else /* Unsupported compiler */
+#       define ENTT_EXPORT
+#       define ENTT_IMPORT
+#       define ENTT_HIDDEN
+#   endif
+#endif
+
+
+#ifndef ENTT_API
+#   if defined ENTT_API_EXPORT
+#       define ENTT_API ENTT_EXPORT
+#   elif defined ENTT_API_IMPORT
+#       define ENTT_API ENTT_IMPORT
+#   else /* No API */
+#       define ENTT_API
+#   endif
+#endif
+
+
+#endif
+
+// #include "hashed_string.hpp"
 #ifndef ENTT_CORE_HASHED_STRING_HPP
 #define ENTT_CORE_HASHED_STRING_HPP
 
 
 #include <cstddef>
-// #include "../config/config.h"
-#ifndef ENTT_CONFIG_CONFIG_H
-#define ENTT_CONFIG_CONFIG_H
-
-
-#ifndef ENTT_NOEXCEPT
-#define ENTT_NOEXCEPT noexcept
-#endif // ENTT_NOEXCEPT
-
-
-#ifndef ENTT_HS_SUFFIX
-#define ENTT_HS_SUFFIX _hs
-#endif // ENTT_HS_SUFFIX
-
-
-#ifndef ENTT_HWS_SUFFIX
-#define ENTT_HWS_SUFFIX _hws
-#endif // ENTT_HWS_SUFFIX
-
-
-#ifndef ENTT_NO_ATOMIC
-#include <atomic>
-#define ENTT_MAYBE_ATOMIC(Type) std::atomic<Type>
-#else // ENTT_NO_ATOMIC
-#define ENTT_MAYBE_ATOMIC(Type) Type
-#endif // ENTT_NO_ATOMIC
-
-
-#ifndef ENTT_ID_TYPE
 #include <cstdint>
-#define ENTT_ID_TYPE std::uint32_t
-#endif // ENTT_ID_TYPE
-
-
-#ifndef ENTT_PAGE_SIZE
-#define ENTT_PAGE_SIZE 32768
-#endif // ENTT_PAGE_SIZE
-
-
-#ifndef ENTT_DISABLE_ASSERT
-#include <cassert>
-#define ENTT_ASSERT(condition) assert(condition)
-#else // ENTT_DISABLE_ASSERT
-#define ENTT_ASSERT(...) ((void)0)
-#endif // ENTT_DISABLE_ASSERT
-
-
-#endif // ENTT_CONFIG_CONFIG_H
+// #include "../config/config.h"
+
+// #include "fwd.hpp"
 
 
 
@@ -14823,6 +30000,7 @@
 
 template<>
 struct fnv1a_traits<std::uint32_t> {
+    using type = std::uint32_t;
     static constexpr std::uint32_t offset = 2166136261;
     static constexpr std::uint32_t prime = 16777619;
 };
@@ -14830,6 +30008,7 @@
 
 template<>
 struct fnv1a_traits<std::uint64_t> {
+    using type = std::uint64_t;
     static constexpr std::uint64_t offset = 14695981039346656037ull;
     static constexpr std::uint64_t prime = 1099511628211ull;
 };
@@ -14840,7 +30019,7 @@
 
 /**
  * Internal details not to be documented.
- * @endcond TURN_OFF_DOXYGEN
+ * @endcond
  */
 
 
@@ -14857,7 +30036,7 @@
  */
 template<typename Char>
 class basic_hashed_string {
-    using traits_type = internal::fnv1a_traits<ENTT_ID_TYPE>;
+    using traits_type = internal::fnv1a_traits<id_type>;
 
     struct const_wrapper {
         // non-explicit constructor on purpose
@@ -14866,15 +30045,33 @@
     };
 
     // Fowler–Noll–Vo hash function v. 1a - the good
-    static constexpr ENTT_ID_TYPE helper(ENTT_ID_TYPE partial, const Char *curr) ENTT_NOEXCEPT {
-        return curr[0] == 0 ? partial : helper((partial^curr[0])*traits_type::prime, curr+1);
+    [[nodiscard]] static constexpr id_type helper(const Char *curr) ENTT_NOEXCEPT {
+        auto value = traits_type::offset;
+
+        while(*curr != 0) {
+            value = (value ^ static_cast<traits_type::type>(*(curr++))) * traits_type::prime;
+        }
+
+        return value;
     }
 
 public:
     /*! @brief Character type. */
     using value_type = Char;
     /*! @brief Unsigned integer type. */
-    using hash_type = ENTT_ID_TYPE;
+    using hash_type = id_type;
+
+    /**
+     * @brief Returns directly the numeric representation of a string view.
+     * @param str Human-readable identifer.
+     * @param size Length of the string to hash.
+     * @return The numeric representation of the string.
+     */
+    [[nodiscard]] static constexpr hash_type value(const value_type *str, std::size_t size) ENTT_NOEXCEPT {
+        id_type partial{traits_type::offset};
+        while(size--) { partial = (partial^(str++)[0])*traits_type::prime; }
+        return partial;
+    }
 
     /**
      * @brief Returns directly the numeric representation of a string.
@@ -14892,8 +30089,8 @@
      * @return The numeric representation of the string.
      */
     template<std::size_t N>
-    static constexpr hash_type to_value(const value_type (&str)[N]) ENTT_NOEXCEPT {
-        return helper(traits_type::offset, str);
+    [[nodiscard]] static constexpr hash_type value(const value_type (&str)[N]) ENTT_NOEXCEPT {
+        return helper(str);
     }
 
     /**
@@ -14901,20 +30098,8 @@
      * @param wrapper Helps achieving the purpose by relying on overloading.
      * @return The numeric representation of the string.
      */
-    static hash_type to_value(const_wrapper wrapper) ENTT_NOEXCEPT {
-        return helper(traits_type::offset, wrapper.str);
-    }
-
-    /**
-     * @brief Returns directly the numeric representation of a string view.
-     * @param str Human-readable identifer.
-     * @param size Length of the string to hash.
-     * @return The numeric representation of the string.
-     */
-    static hash_type to_value(const value_type *str, std::size_t size) ENTT_NOEXCEPT {
-        ENTT_ID_TYPE partial{traits_type::offset};
-        while(size--) { partial = (partial^(str++)[0])*traits_type::prime; }
-        return partial;
+    [[nodiscard]] static hash_type value(const_wrapper wrapper) ENTT_NOEXCEPT {
+        return helper(wrapper.str);
     }
 
     /*! @brief Constructs an empty hashed string. */
@@ -14938,7 +30123,7 @@
      */
     template<std::size_t N>
     constexpr basic_hashed_string(const value_type (&curr)[N]) ENTT_NOEXCEPT
-        : str{curr}, hash{helper(traits_type::offset, curr)}
+        : str{curr}, hash{helper(curr)}
     {}
 
     /**
@@ -14947,14 +30132,14 @@
      * @param wrapper Helps achieving the purpose by relying on overloading.
      */
     explicit constexpr basic_hashed_string(const_wrapper wrapper) ENTT_NOEXCEPT
-        : str{wrapper.str}, hash{helper(traits_type::offset, wrapper.str)}
+        : str{wrapper.str}, hash{helper(wrapper.str)}
     {}
 
     /**
      * @brief Returns the human-readable representation of a hashed string.
      * @return The string used to initialize the instance.
      */
-    constexpr const value_type * data() const ENTT_NOEXCEPT {
+    [[nodiscard]] constexpr const value_type * data() const ENTT_NOEXCEPT {
         return str;
     }
 
@@ -14962,25 +30147,25 @@
      * @brief Returns the numeric representation of a hashed string.
      * @return The numeric representation of the instance.
      */
-    constexpr hash_type value() const ENTT_NOEXCEPT {
+    [[nodiscard]] constexpr hash_type value() const ENTT_NOEXCEPT {
         return hash;
     }
 
-    /**
-     * @brief Returns the human-readable representation of a hashed string.
-     * @return The string used to initialize the instance.
-     */
-    constexpr operator const value_type *() const ENTT_NOEXCEPT { return str; }
-
-    /*! @copydoc value */
-    constexpr operator hash_type() const ENTT_NOEXCEPT { return hash; }
+    /*! @copydoc data */
+    [[nodiscard]] constexpr operator const value_type *() const ENTT_NOEXCEPT { return data(); }
+
+    /**
+     * @brief Returns the numeric representation of a hashed string.
+     * @return The numeric representation of the instance.
+     */
+    [[nodiscard]] constexpr operator hash_type() const ENTT_NOEXCEPT { return value(); }
 
     /**
      * @brief Compares two hashed strings.
      * @param other Hashed string with which to compare.
      * @return True if the two hashed strings are identical, false otherwise.
      */
-    constexpr bool operator==(const basic_hashed_string &other) const ENTT_NOEXCEPT {
+    [[nodiscard]] constexpr bool operator==(const basic_hashed_string &other) const ENTT_NOEXCEPT {
         return hash == other.hash;
     }
 
@@ -15001,7 +30186,7 @@
  * @param str Human-readable identifer.
  */
 template<typename Char, std::size_t N>
-basic_hashed_string(const Char (&str)[N]) ENTT_NOEXCEPT
+basic_hashed_string(const Char (&str)[N])
 -> basic_hashed_string<Char>;
 
 
@@ -15013,7 +30198,7 @@
  * @return True if the two hashed strings are identical, false otherwise.
  */
 template<typename Char>
-constexpr bool operator!=(const basic_hashed_string<Char> &lhs, const basic_hashed_string<Char> &rhs) ENTT_NOEXCEPT {
+[[nodiscard]] constexpr bool operator!=(const basic_hashed_string<Char> &lhs, const basic_hashed_string<Char> &rhs) ENTT_NOEXCEPT {
     return !(lhs == rhs);
 }
 
@@ -15026,7 +30211,7 @@
 using hashed_wstring = basic_hashed_string<wchar_t>;
 
 
-}
+inline namespace literals {
 
 
 /**
@@ -15034,7 +30219,7 @@
  * @param str The literal without its suffix.
  * @return A properly initialized hashed string.
  */
-constexpr entt::hashed_string operator"" ENTT_HS_SUFFIX(const char *str, std::size_t) ENTT_NOEXCEPT {
+[[nodiscard]] constexpr entt::hashed_string operator"" _hs(const char *str, std::size_t) ENTT_NOEXCEPT {
     return entt::hashed_string{str};
 }
 
@@ -15044,44 +30229,945 @@
  * @param str The literal without its suffix.
  * @return A properly initialized hashed wstring.
  */
-constexpr entt::hashed_wstring operator"" ENTT_HWS_SUFFIX(const wchar_t *str, std::size_t) ENTT_NOEXCEPT {
+[[nodiscard]] constexpr entt::hashed_wstring operator"" _hws(const wchar_t *str, std::size_t) ENTT_NOEXCEPT {
     return entt::hashed_wstring{str};
 }
 
 
-#endif // ENTT_CORE_HASHED_STRING_HPP
-
-
-
-namespace entt {
-
-
-/*! @brief A class to use to push around lists of types, nothing more. */
-template<typename...>
-struct type_list {};
-
-
-/*! @brief Primary template isn't defined on purpose. */
-template<typename>
-struct type_list_size;
-
-
-/**
- * @brief Compile-time number of elements in a type list.
+}
+
+
+}
+
+
+#endif
+
+// #include "fwd.hpp"
+
+
+
+namespace entt {
+
+
+/**
+ * @cond TURN_OFF_DOXYGEN
+ * Internal details not to be documented.
+ */
+
+
+namespace internal {
+
+
+struct ENTT_API type_seq final {
+    [[nodiscard]] static id_type next() ENTT_NOEXCEPT {
+        static ENTT_MAYBE_ATOMIC(id_type) value{};
+        return value++;
+    }
+};
+
+
+template<typename Type>
+[[nodiscard]] constexpr auto stripped_type_name() ENTT_NOEXCEPT {
+#if defined ENTT_PRETTY_FUNCTION
+    std::string_view pretty_function{ENTT_PRETTY_FUNCTION};
+    auto first = pretty_function.find_first_not_of(' ', pretty_function.find_first_of(ENTT_PRETTY_FUNCTION_PREFIX)+1);
+    auto value = pretty_function.substr(first, pretty_function.find_last_of(ENTT_PRETTY_FUNCTION_SUFFIX) - first);
+    return value;
+#else
+    return std::string_view{""};
+#endif
+}
+
+
+template<typename Type, auto = stripped_type_name<Type>().find_first_of('.')>
+[[nodiscard]] static constexpr std::string_view type_name(int) ENTT_NOEXCEPT {
+    constexpr auto value = stripped_type_name<Type>();
+    return value;
+}
+
+
+template<typename Type>
+[[nodiscard]] static std::string_view type_name(char) ENTT_NOEXCEPT {
+    static const auto value = stripped_type_name<Type>();
+    return value;
+}
+
+
+template<typename Type, auto = stripped_type_name<Type>().find_first_of('.')>
+[[nodiscard]] static constexpr id_type type_hash(int) ENTT_NOEXCEPT {
+    constexpr auto stripped = stripped_type_name<Type>();
+    constexpr auto value = hashed_string::value(stripped.data(), stripped.size());
+    return value;
+}
+
+
+template<typename Type>
+[[nodiscard]] static id_type type_hash(char) ENTT_NOEXCEPT {
+    static const auto value = [](const auto stripped) {
+        return hashed_string::value(stripped.data(), stripped.size());
+    }(stripped_type_name<Type>());
+    return value;
+}
+
+
+}
+
+
+/**
+ * Internal details not to be documented.
+ * @endcond
+ */
+
+
+/**
+ * @brief Type sequential identifier.
+ * @tparam Type Type for which to generate a sequential identifier.
+ */
+template<typename Type, typename = void>
+struct ENTT_API type_seq final {
+    /**
+     * @brief Returns the sequential identifier of a given type.
+     * @return The sequential identifier of a given type.
+     */
+    [[nodiscard]] static id_type value() ENTT_NOEXCEPT {
+        static const id_type value = internal::type_seq::next();
+        return value;
+    }
+
+    /*! @copydoc value */
+    [[nodiscard]] constexpr operator id_type() const ENTT_NOEXCEPT { return value(); }
+};
+
+
+/**
+ * @brief Type hash.
+ * @tparam Type Type for which to generate a hash value.
+ */
+template<typename Type, typename = void>
+struct type_hash final {
+    /**
+     * @brief Returns the numeric representation of a given type.
+     * @return The numeric representation of the given type.
+     */
+#if defined ENTT_PRETTY_FUNCTION
+    [[nodiscard]] static constexpr id_type value() ENTT_NOEXCEPT {
+        return internal::type_hash<Type>(0);
+#else
+    [[nodiscard]] static constexpr id_type value() ENTT_NOEXCEPT {
+        return type_seq<Type>::value();
+#endif
+    }
+
+    /*! @copydoc value */
+    [[nodiscard]] constexpr operator id_type() const ENTT_NOEXCEPT { return value(); }
+};
+
+
+/**
+ * @brief Type name.
+ * @tparam Type Type for which to generate a name.
+ */
+template<typename Type, typename = void>
+struct type_name final {
+    /**
+     * @brief Returns the name of a given type.
+     * @return The name of the given type.
+     */
+    [[nodiscard]] static constexpr std::string_view value() ENTT_NOEXCEPT {
+        return internal::type_name<Type>(0);
+    }
+
+    /*! @copydoc value */
+    [[nodiscard]] constexpr operator std::string_view() const ENTT_NOEXCEPT { return value(); }
+};
+
+
+/*! @brief Implementation specific information about a type. */
+class type_info final {
+    template<typename>
+    friend type_info type_id() ENTT_NOEXCEPT;
+
+    type_info(id_type seq_v, id_type hash_v, std::string_view name_v) ENTT_NOEXCEPT
+        : seq_value{seq_v},
+          hash_value{hash_v},
+          name_value{name_v}
+    {}
+
+public:
+    /*! @brief Default constructor. */
+    type_info() ENTT_NOEXCEPT
+        : type_info({}, {}, {})
+    {}
+
+    /*! @brief Default copy constructor. */
+    type_info(const type_info &) ENTT_NOEXCEPT = default;
+    /*! @brief Default move constructor. */
+    type_info(type_info &&) ENTT_NOEXCEPT = default;
+
+    /**
+     * @brief Default copy assignment operator.
+     * @return This type info object.
+     */
+    type_info & operator=(const type_info &) ENTT_NOEXCEPT = default;
+
+    /**
+     * @brief Default move assignment operator.
+     * @return This type info object.
+     */
+    type_info & operator=(type_info &&) ENTT_NOEXCEPT = default;
+
+    /**
+     * @brief Checks if a type info object is properly initialized.
+     * @return True if the object is properly initialized, false otherwise.
+     */
+    [[nodiscard]] explicit operator bool() const ENTT_NOEXCEPT {
+        return name_value.data() != nullptr;
+    }
+
+    /**
+     * @brief Type sequential identifier.
+     * @return Type sequential identifier.
+     */
+    [[nodiscard]] id_type seq() const ENTT_NOEXCEPT {
+        return seq_value;
+    }
+
+    /**
+     * @brief Type hash.
+     * @return Type hash.
+     */
+    [[nodiscard]] id_type hash() const ENTT_NOEXCEPT {
+        return hash_value;
+    }
+
+    /**
+     * @brief Type name.
+     * @return Type name.
+     */
+    [[nodiscard]] std::string_view name() const ENTT_NOEXCEPT {
+        return name_value;
+    }
+
+    /**
+     * @brief Compares the contents of two type info objects.
+     * @param other Object with which to compare.
+     * @return False if the two contents differ, true otherwise.
+     */
+    [[nodiscard]] bool operator==(const type_info &other) const ENTT_NOEXCEPT {
+        return hash_value == other.hash_value;
+    }
+
+private:
+    id_type seq_value;
+    id_type hash_value;
+    std::string_view name_value;
+};
+
+
+/**
+ * @brief Compares the contents of two type info objects.
+ * @param lhs A type info object.
+ * @param rhs A type info object.
+ * @return True if the two contents differ, false otherwise.
+ */
+[[nodiscard]] inline bool operator!=(const type_info &lhs, const type_info &rhs) ENTT_NOEXCEPT {
+    return !(lhs == rhs);
+}
+
+
+/**
+ * @brief Returns the type info object for a given type.
+ * @tparam Type Type for which to generate a type info object.
+ * @return The type info object for the given type.
+ */
+template<typename Type>
+[[nodiscard]] type_info type_id() ENTT_NOEXCEPT {
+    return type_info{
+        type_seq<std::remove_cv_t<std::remove_reference_t<Type>>>::value(),
+        type_hash<std::remove_cv_t<std::remove_reference_t<Type>>>::value(),
+        type_name<std::remove_cv_t<std::remove_reference_t<Type>>>::value()
+    };
+}
+
+
+}
+
+
+#endif
+
+// #include "../core/type_traits.hpp"
+
+// #include "meta.hpp"
+#ifndef ENTT_META_META_HPP
+#define ENTT_META_META_HPP
+
+
+#include <cstddef>
+#include <functional>
+#include <iterator>
+#include <memory>
+#include <type_traits>
+#include <utility>
+// #include "../config/config.h"
+
+// #include "../core/any.hpp"
+#ifndef ENTT_CORE_ANY_HPP
+#define ENTT_CORE_ANY_HPP
+
+
+#include <cstddef>
+#include <functional>
+#include <memory>
+#include <type_traits>
+#include <utility>
+// #include "../config/config.h"
+
+// #include "../core/utility.hpp"
+#ifndef ENTT_CORE_UTILITY_HPP
+#define ENTT_CORE_UTILITY_HPP
+
+
+#include <utility>
+// #include "../config/config.h"
+#ifndef ENTT_CONFIG_CONFIG_H
+#define ENTT_CONFIG_CONFIG_H
+
+
+#if defined(__cpp_exceptions) && !defined(ENTT_NOEXCEPTION)
+#   define ENTT_NOEXCEPT noexcept
+#   define ENTT_THROW throw
+#   define ENTT_TRY try
+#   define ENTT_CATCH catch(...)
+#else
+#   define ENTT_NOEXCEPT
+#   define ENTT_THROW
+#   define ENTT_TRY if(true)
+#   define ENTT_CATCH if(false)
+#endif
+
+
+#if defined(__cpp_lib_launder) && __cpp_lib_launder >= 201606L
+#   include <new>
+#   define ENTT_LAUNDER(expr) std::launder(expr)
+#else
+#   define ENTT_LAUNDER(expr) expr
+#endif
+
+
+#ifndef ENTT_USE_ATOMIC
+#   define ENTT_MAYBE_ATOMIC(Type) Type
+#else
+#   include <atomic>
+#   define ENTT_MAYBE_ATOMIC(Type) std::atomic<Type>
+#endif
+
+
+#ifndef ENTT_ID_TYPE
+#   include <cstdint>
+#   define ENTT_ID_TYPE std::uint32_t
+#endif
+
+
+#ifdef ENTT_SPARSE_PAGE
+	static_assert(ENTT_SPARSE_PAGE && ((ENTT_SPARSE_PAGE & (ENTT_SPARSE_PAGE - 1)) == 0), "ENTT_SPARSE_PAGE must be a power of two");
+#else
+#   define ENTT_SPARSE_PAGE 4096
+#endif
+
+
+#ifdef ENTT_PACKED_PAGE
+static_assert(ENTT_PACKED_PAGE && ((ENTT_PACKED_PAGE & (ENTT_PACKED_PAGE - 1)) == 0), "ENTT_PACKED_PAGE must be a power of two");
+#else
+#   define ENTT_PACKED_PAGE 1024
+#endif
+
+
+#ifdef ENTT_DISABLE_ASSERT
+#   undef ENTT_ASSERT
+#   define ENTT_ASSERT(...) (void(0))
+#elif !defined ENTT_ASSERT
+#   include <cassert>
+#   define ENTT_ASSERT(condition, ...) assert(condition)
+#endif
+
+
+#ifdef ENTT_NO_ETO
+#   include <type_traits>
+#   define ENTT_IGNORE_IF_EMPTY std::false_type
+#else
+#   include <type_traits>
+#   define ENTT_IGNORE_IF_EMPTY std::true_type
+#endif
+
+
+#ifndef ENTT_STANDARD_CPP
+#    if defined __clang__ || defined __GNUC__
+#       define ENTT_PRETTY_FUNCTION __PRETTY_FUNCTION__
+#       define ENTT_PRETTY_FUNCTION_PREFIX '='
+#       define ENTT_PRETTY_FUNCTION_SUFFIX ']'
+#    elif defined _MSC_VER
+#       define ENTT_PRETTY_FUNCTION __FUNCSIG__
+#       define ENTT_PRETTY_FUNCTION_PREFIX '<'
+#       define ENTT_PRETTY_FUNCTION_SUFFIX '>'
+#   endif
+#endif
+
+
+#endif
+
+
+
+namespace entt {
+
+
+/*! @brief Identity function object (waiting for C++20). */
+struct identity {
+    /**
+     * @brief Returns its argument unchanged.
+     * @tparam Type Type of the argument.
+     * @param value The actual argument.
+     * @return The submitted value as-is.
+     */
+    template<class Type>
+    [[nodiscard]] constexpr Type && operator()(Type &&value) const ENTT_NOEXCEPT {
+        return std::forward<Type>(value);
+    }
+};
+
+
+/**
+ * @brief Constant utility to disambiguate overloaded members of a class.
+ * @tparam Type Type of the desired overload.
+ * @tparam Class Type of class to which the member belongs.
+ * @param member A valid pointer to a member.
+ * @return Pointer to the member.
+ */
+template<typename Type, typename Class>
+[[nodiscard]] constexpr auto overload(Type Class:: *member) ENTT_NOEXCEPT { return member; }
+
+
+/**
+ * @brief Constant utility to disambiguate overloaded functions.
+ * @tparam Func Function type of the desired overload.
+ * @param func A valid pointer to a function.
+ * @return Pointer to the function.
+ */
+template<typename Func>
+[[nodiscard]] constexpr auto overload(Func *func) ENTT_NOEXCEPT { return func; }
+
+
+/**
+ * @brief Helper type for visitors.
+ * @tparam Func Types of function objects.
+ */
+template<class... Func>
+struct overloaded: Func... {
+    using Func::operator()...;
+};
+
+
+/**
+ * @brief Deduction guide.
+ * @tparam Func Types of function objects.
+ */
+template<class... Func>
+overloaded(Func...)
+-> overloaded<Func...>;
+
+
+/**
+ * @brief Basic implementation of a y-combinator.
+ * @tparam Func Type of a potentially recursive function.
+ */
+template<class Func>
+struct y_combinator {
+    /**
+     * @brief Constructs a y-combinator from a given function.
+     * @param recursive A potentially recursive function.
+     */
+    y_combinator(Func recursive):
+        func{std::move(recursive)}
+    {}
+
+    /**
+     * @brief Invokes a y-combinator and therefore its underlying function.
+     * @tparam Args Types of arguments to use to invoke the underlying function.
+     * @param args Parameters to use to invoke the underlying function.
+     * @return Return value of the underlying function, if any.
+     */
+    template <class... Args>
+    decltype(auto) operator()(Args &&... args) const {
+        return func(*this, std::forward<Args>(args)...);
+    }
+
+    /*! @copydoc operator()() */
+    template <class... Args>
+    decltype(auto) operator()(Args &&... args) {
+        return func(*this, std::forward<Args>(args)...);
+    }
+
+private:
+    Func func;
+};
+
+
+}
+
+
+#endif
+
+// #include "fwd.hpp"
+
+// #include "type_info.hpp"
+#ifndef ENTT_CORE_TYPE_INFO_HPP
+#define ENTT_CORE_TYPE_INFO_HPP
+
+
+#include <string_view>
+#include <type_traits>
+// #include "../config/config.h"
+
+// #include "../core/attribute.h"
+
+// #include "hashed_string.hpp"
+
+// #include "fwd.hpp"
+
+
+
+namespace entt {
+
+
+/**
+ * @cond TURN_OFF_DOXYGEN
+ * Internal details not to be documented.
+ */
+
+
+namespace internal {
+
+
+struct ENTT_API type_seq final {
+    [[nodiscard]] static id_type next() ENTT_NOEXCEPT {
+        static ENTT_MAYBE_ATOMIC(id_type) value{};
+        return value++;
+    }
+};
+
+
+template<typename Type>
+[[nodiscard]] constexpr auto stripped_type_name() ENTT_NOEXCEPT {
+#if defined ENTT_PRETTY_FUNCTION
+    std::string_view pretty_function{ENTT_PRETTY_FUNCTION};
+    auto first = pretty_function.find_first_not_of(' ', pretty_function.find_first_of(ENTT_PRETTY_FUNCTION_PREFIX)+1);
+    auto value = pretty_function.substr(first, pretty_function.find_last_of(ENTT_PRETTY_FUNCTION_SUFFIX) - first);
+    return value;
+#else
+    return std::string_view{""};
+#endif
+}
+
+
+template<typename Type, auto = stripped_type_name<Type>().find_first_of('.')>
+[[nodiscard]] static constexpr std::string_view type_name(int) ENTT_NOEXCEPT {
+    constexpr auto value = stripped_type_name<Type>();
+    return value;
+}
+
+
+template<typename Type>
+[[nodiscard]] static std::string_view type_name(char) ENTT_NOEXCEPT {
+    static const auto value = stripped_type_name<Type>();
+    return value;
+}
+
+
+template<typename Type, auto = stripped_type_name<Type>().find_first_of('.')>
+[[nodiscard]] static constexpr id_type type_hash(int) ENTT_NOEXCEPT {
+    constexpr auto stripped = stripped_type_name<Type>();
+    constexpr auto value = hashed_string::value(stripped.data(), stripped.size());
+    return value;
+}
+
+
+template<typename Type>
+[[nodiscard]] static id_type type_hash(char) ENTT_NOEXCEPT {
+    static const auto value = [](const auto stripped) {
+        return hashed_string::value(stripped.data(), stripped.size());
+    }(stripped_type_name<Type>());
+    return value;
+}
+
+
+}
+
+
+/**
+ * Internal details not to be documented.
+ * @endcond
+ */
+
+
+/**
+ * @brief Type sequential identifier.
+ * @tparam Type Type for which to generate a sequential identifier.
+ */
+template<typename Type, typename = void>
+struct ENTT_API type_seq final {
+    /**
+     * @brief Returns the sequential identifier of a given type.
+     * @return The sequential identifier of a given type.
+     */
+    [[nodiscard]] static id_type value() ENTT_NOEXCEPT {
+        static const id_type value = internal::type_seq::next();
+        return value;
+    }
+
+    /*! @copydoc value */
+    [[nodiscard]] constexpr operator id_type() const ENTT_NOEXCEPT { return value(); }
+};
+
+
+/**
+ * @brief Type hash.
+ * @tparam Type Type for which to generate a hash value.
+ */
+template<typename Type, typename = void>
+struct type_hash final {
+    /**
+     * @brief Returns the numeric representation of a given type.
+     * @return The numeric representation of the given type.
+     */
+#if defined ENTT_PRETTY_FUNCTION
+    [[nodiscard]] static constexpr id_type value() ENTT_NOEXCEPT {
+        return internal::type_hash<Type>(0);
+#else
+    [[nodiscard]] static constexpr id_type value() ENTT_NOEXCEPT {
+        return type_seq<Type>::value();
+#endif
+    }
+
+    /*! @copydoc value */
+    [[nodiscard]] constexpr operator id_type() const ENTT_NOEXCEPT { return value(); }
+};
+
+
+/**
+ * @brief Type name.
+ * @tparam Type Type for which to generate a name.
+ */
+template<typename Type, typename = void>
+struct type_name final {
+    /**
+     * @brief Returns the name of a given type.
+     * @return The name of the given type.
+     */
+    [[nodiscard]] static constexpr std::string_view value() ENTT_NOEXCEPT {
+        return internal::type_name<Type>(0);
+    }
+
+    /*! @copydoc value */
+    [[nodiscard]] constexpr operator std::string_view() const ENTT_NOEXCEPT { return value(); }
+};
+
+
+/*! @brief Implementation specific information about a type. */
+class type_info final {
+    template<typename>
+    friend type_info type_id() ENTT_NOEXCEPT;
+
+    type_info(id_type seq_v, id_type hash_v, std::string_view name_v) ENTT_NOEXCEPT
+        : seq_value{seq_v},
+          hash_value{hash_v},
+          name_value{name_v}
+    {}
+
+public:
+    /*! @brief Default constructor. */
+    type_info() ENTT_NOEXCEPT
+        : type_info({}, {}, {})
+    {}
+
+    /*! @brief Default copy constructor. */
+    type_info(const type_info &) ENTT_NOEXCEPT = default;
+    /*! @brief Default move constructor. */
+    type_info(type_info &&) ENTT_NOEXCEPT = default;
+
+    /**
+     * @brief Default copy assignment operator.
+     * @return This type info object.
+     */
+    type_info & operator=(const type_info &) ENTT_NOEXCEPT = default;
+
+    /**
+     * @brief Default move assignment operator.
+     * @return This type info object.
+     */
+    type_info & operator=(type_info &&) ENTT_NOEXCEPT = default;
+
+    /**
+     * @brief Checks if a type info object is properly initialized.
+     * @return True if the object is properly initialized, false otherwise.
+     */
+    [[nodiscard]] explicit operator bool() const ENTT_NOEXCEPT {
+        return name_value.data() != nullptr;
+    }
+
+    /**
+     * @brief Type sequential identifier.
+     * @return Type sequential identifier.
+     */
+    [[nodiscard]] id_type seq() const ENTT_NOEXCEPT {
+        return seq_value;
+    }
+
+    /**
+     * @brief Type hash.
+     * @return Type hash.
+     */
+    [[nodiscard]] id_type hash() const ENTT_NOEXCEPT {
+        return hash_value;
+    }
+
+    /**
+     * @brief Type name.
+     * @return Type name.
+     */
+    [[nodiscard]] std::string_view name() const ENTT_NOEXCEPT {
+        return name_value;
+    }
+
+    /**
+     * @brief Compares the contents of two type info objects.
+     * @param other Object with which to compare.
+     * @return False if the two contents differ, true otherwise.
+     */
+    [[nodiscard]] bool operator==(const type_info &other) const ENTT_NOEXCEPT {
+        return hash_value == other.hash_value;
+    }
+
+private:
+    id_type seq_value;
+    id_type hash_value;
+    std::string_view name_value;
+};
+
+
+/**
+ * @brief Compares the contents of two type info objects.
+ * @param lhs A type info object.
+ * @param rhs A type info object.
+ * @return True if the two contents differ, false otherwise.
+ */
+[[nodiscard]] inline bool operator!=(const type_info &lhs, const type_info &rhs) ENTT_NOEXCEPT {
+    return !(lhs == rhs);
+}
+
+
+/**
+ * @brief Returns the type info object for a given type.
+ * @tparam Type Type for which to generate a type info object.
+ * @return The type info object for the given type.
+ */
+template<typename Type>
+[[nodiscard]] type_info type_id() ENTT_NOEXCEPT {
+    return type_info{
+        type_seq<std::remove_cv_t<std::remove_reference_t<Type>>>::value(),
+        type_hash<std::remove_cv_t<std::remove_reference_t<Type>>>::value(),
+        type_name<std::remove_cv_t<std::remove_reference_t<Type>>>::value()
+    };
+}
+
+
+}
+
+
+#endif
+
+// #include "type_traits.hpp"
+#ifndef ENTT_CORE_TYPE_TRAITS_HPP
+#define ENTT_CORE_TYPE_TRAITS_HPP
+
+
+#include <cstddef>
+#include <iterator>
+#include <type_traits>
+#include <utility>
+// #include "../config/config.h"
+
+// #include "fwd.hpp"
+
+
+
+namespace entt {
+
+
+/**
+ * @brief Utility class to disambiguate overloaded functions.
+ * @tparam N Number of choices available.
+ */
+template<std::size_t N>
+struct choice_t
+    // Unfortunately, doxygen cannot parse such a construct.
+    /*! @cond TURN_OFF_DOXYGEN */
+    : choice_t<N-1>
+    /*! @endcond */
+{};
+
+
+/*! @copybrief choice_t */
+template<>
+struct choice_t<0> {};
+
+
+/**
+ * @brief Variable template for the choice trick.
+ * @tparam N Number of choices available.
+ */
+template<std::size_t N>
+inline constexpr choice_t<N> choice{};
+
+
+/**
+ * @brief Identity type trait.
+ *
+ * Useful to establish non-deduced contexts in template argument deduction
+ * (waiting for C++20) or to provide types through function arguments.
+ *
+ * @tparam Type A type.
+ */
+template<typename Type>
+struct type_identity {
+    /*! @brief Identity type. */
+    using type = Type;
+};
+
+
+/**
+ * @brief Helper type.
+ * @tparam Type A type.
+ */
+template<typename Type>
+using type_identity_t = typename type_identity<Type>::type;
+
+
+/**
+ * @brief A type-only `sizeof` wrapper that returns 0 where `sizeof` complains.
+ * @tparam Type The type of which to return the size.
+ * @tparam The size of the type if `sizeof` accepts it, 0 otherwise.
+ */
+template<typename Type, typename = void>
+struct size_of: std::integral_constant<std::size_t, 0u> {};
+
+
+/*! @copydoc size_of */
+template<typename Type>
+struct size_of<Type, std::void_t<decltype(sizeof(Type))>>
+    : std::integral_constant<std::size_t, sizeof(Type)>
+{};
+
+
+/**
+ * @brief Helper variable template.
+ * @tparam Type The type of which to return the size.
+ */
+template<class Type>
+inline constexpr std::size_t size_of_v = size_of<Type>::value;
+
+
+/**
+ * @brief Using declaration to be used to _repeat_ the same type a number of
+ * times equal to the size of a given parameter pack.
+ * @tparam Type A type to repeat.
+ */
+template<typename Type, typename>
+using unpack_as_t = Type;
+
+
+/**
+ * @brief Helper variable template to be used to _repeat_ the same value a
+ * number of times equal to the size of a given parameter pack.
+ * @tparam Value A value to repeat.
+ */
+template<auto Value, typename>
+inline constexpr auto unpack_as_v = Value;
+
+
+/**
+ * @brief Wraps a static constant.
+ * @tparam Value A static constant.
+ */
+template<auto Value>
+using integral_constant = std::integral_constant<decltype(Value), Value>;
+
+
+/**
+ * @brief Alias template to facilitate the creation of named values.
+ * @tparam Value A constant value at least convertible to `id_type`.
+ */
+template<id_type Value>
+using tag = integral_constant<Value>;
+
+
+/**
+ * @brief A class to use to push around lists of types, nothing more.
  * @tparam Type Types provided by the type list.
  */
 template<typename... Type>
-struct type_list_size<type_list<Type...>>
-        : std::integral_constant<std::size_t, sizeof...(Type)>
-{};
-
-
-/**
- * @brief Helper variable template.
- * @tparam List Type list.
- */
-template<class List>
-constexpr auto type_list_size_v = type_list_size<List>::value;
+struct type_list {
+    /*! @brief Type list type. */
+    using type = type_list;
+    /*! @brief Compile-time number of elements in the type list. */
+    static constexpr auto size = sizeof...(Type);
+};
+
+
+/*! @brief Primary template isn't defined on purpose. */
+template<std::size_t, typename>
+struct type_list_element;
+
+
+/**
+ * @brief Provides compile-time indexed access to the types of a type list.
+ * @tparam Index Index of the type to return.
+ * @tparam Type First type provided by the type list.
+ * @tparam Other Other types provided by the type list.
+ */
+template<std::size_t Index, typename Type, typename... Other>
+struct type_list_element<Index, type_list<Type, Other...>>
+    : type_list_element<Index - 1u, type_list<Other...>>
+{};
+
+
+/**
+ * @brief Provides compile-time indexed access to the types of a type list.
+ * @tparam Type First type provided by the type list.
+ * @tparam Other Other types provided by the type list.
+ */
+template<typename Type, typename... Other>
+struct type_list_element<0u, type_list<Type, Other...>> {
+    /*! @brief Searched type. */
+    using type = Type;
+};
+
+
+/**
+ * @brief Helper type.
+ * @tparam Index Index of the type to return.
+ * @tparam List Type list to search into.
+ */
+template<std::size_t Index, typename List>
+using type_list_element_t = typename type_list_element<Index, List>::type;
+
+
+/**
+ * @brief Concatenates multiple type lists.
+ * @tparam Type Types provided by the first type list.
+ * @tparam Other Types provided by the second type list.
+ * @return A type list composed by the types of both the type lists.
+ */
+template<typename... Type, typename... Other>
+constexpr type_list<Type..., Other...> operator+(type_list<Type...>, type_list<Other...>) { return {}; }
 
 
 /*! @brief Primary template isn't defined on purpose. */
@@ -15166,197 +31252,15764 @@
 using type_list_unique_t = typename type_list_unique<Type>::type;
 
 
-/*! @brief Traits class used mainly to push things across boundaries. */
-template<typename>
-struct named_type_traits;
-
-
-/**
- * @brief Specialization used to get rid of constness.
- * @tparam Type Named type.
- */
-template<typename Type>
-struct named_type_traits<const Type>
-        : named_type_traits<Type>
-{};
-
-
-/**
- * @brief Helper type.
- * @tparam Type Potentially named type.
- */
-template<typename Type>
-using named_type_traits_t = typename named_type_traits<Type>::type;
-
-
-/**
- * @brief Provides the member constant `value` to true if a given type has a
- * name. In all other cases, `value` is false.
- */
-template<typename, typename = std::void_t<>>
-struct is_named_type: std::false_type {};
-
-
-/**
- * @brief Provides the member constant `value` to true if a given type has a
- * name. In all other cases, `value` is false.
- * @tparam Type Potentially named type.
- */
-template<typename Type>
-struct is_named_type<Type, std::void_t<named_type_traits_t<std::decay_t<Type>>>>: std::true_type {};
-
-
-/**
- * @brief Helper variable template.
- * @tparam Type Potentially named type.
+/**
+ * @brief Provides the member constant `value` to true if a type list contains a
+ * given type, false otherwise.
+ * @tparam List Type list.
+ * @tparam Type Type to look for.
+ */
+template<typename List, typename Type>
+struct type_list_contains;
+
+
+/**
+ * @copybrief type_list_contains
+ * @tparam Type Types provided by the type list.
+ * @tparam Other Type to look for.
+ */
+template<typename... Type, typename Other>
+struct type_list_contains<type_list<Type...>, Other>: std::disjunction<std::is_same<Type, Other>...> {};
+
+
+/**
+ * @brief Helper variable template.
+ * @tparam List Type list.
+ * @tparam Type Type to look for.
+ */
+template<class List, typename Type>
+inline constexpr bool type_list_contains_v = type_list_contains<List, Type>::value;
+
+
+/*! @brief Primary template isn't defined on purpose. */
+template<typename...>
+struct type_list_diff;
+
+
+/**
+ * @brief Computes the difference between two type lists.
+ * @tparam Type Types provided by the first type list.
+ * @tparam Other Types provided by the second type list.
+ */
+template<typename... Type, typename... Other>
+struct type_list_diff<type_list<Type...>, type_list<Other...>> {
+    /*! @brief A type list that is the difference between the two type lists. */
+    using type = type_list_cat_t<std::conditional_t<type_list_contains_v<type_list<Other...>, Type>, type_list<>, type_list<Type>>...>;
+};
+
+
+/**
+ * @brief Helper type.
+ * @tparam List Type lists between which to compute the difference.
+ */
+template<typename... List>
+using type_list_diff_t = typename type_list_diff<List...>::type;
+
+
+/**
+ * @brief A class to use to push around lists of constant values, nothing more.
+ * @tparam Value Values provided by the value list.
+ */
+template<auto... Value>
+struct value_list {
+    /*! @brief Value list type. */
+    using type = value_list;
+    /*! @brief Compile-time number of elements in the value list. */
+    static constexpr auto size = sizeof...(Value);
+};
+
+
+/*! @brief Primary template isn't defined on purpose. */
+template<std::size_t, typename>
+struct value_list_element;
+
+
+/**
+ * @brief Provides compile-time indexed access to the values of a value list.
+ * @tparam Index Index of the value to return.
+ * @tparam Value First value provided by the value list.
+ * @tparam Other Other values provided by the value list.
+ */
+template<std::size_t Index, auto Value, auto... Other>
+struct value_list_element<Index, value_list<Value, Other...>>
+    : value_list_element<Index - 1u, value_list<Other...>>
+{};
+
+
+/**
+ * @brief Provides compile-time indexed access to the types of a type list.
+ * @tparam Value First value provided by the value list.
+ * @tparam Other Other values provided by the value list.
+ */
+template<auto Value, auto... Other>
+struct value_list_element<0u, value_list<Value, Other...>> {
+    /*! @brief Searched value. */
+    static constexpr auto value = Value;
+};
+
+
+/**
+ * @brief Helper type.
+ * @tparam Index Index of the value to return.
+ * @tparam List Value list to search into.
+ */
+template<std::size_t Index, typename List>
+inline constexpr auto value_list_element_v = value_list_element<Index, List>::value;
+
+
+/**
+ * @brief Concatenates multiple value lists.
+ * @tparam Value Values provided by the first value list.
+ * @tparam Other Values provided by the second value list.
+ * @return A value list composed by the values of both the value lists.
+ */
+template<auto... Value, auto... Other>
+constexpr value_list<Value..., Other...> operator+(value_list<Value...>, value_list<Other...>) { return {}; }
+
+
+/*! @brief Primary template isn't defined on purpose. */
+template<typename...>
+struct value_list_cat;
+
+
+/*! @brief Concatenates multiple value lists. */
+template<>
+struct value_list_cat<> {
+    /*! @brief A value list composed by the values of all the value lists. */
+    using type = value_list<>;
+};
+
+
+/**
+ * @brief Concatenates multiple value lists.
+ * @tparam Value Values provided by the first value list.
+ * @tparam Other Values provided by the second value list.
+ * @tparam List Other value lists, if any.
+ */
+template<auto... Value, auto... Other, typename... List>
+struct value_list_cat<value_list<Value...>, value_list<Other...>, List...> {
+    /*! @brief A value list composed by the values of all the value lists. */
+    using type = typename value_list_cat<value_list<Value..., Other...>, List...>::type;
+};
+
+
+/**
+ * @brief Concatenates multiple value lists.
+ * @tparam Value Values provided by the value list.
+ */
+template<auto... Value>
+struct value_list_cat<value_list<Value...>> {
+    /*! @brief A value list composed by the values of all the value lists. */
+    using type = value_list<Value...>;
+};
+
+
+/**
+ * @brief Helper type.
+ * @tparam List Value lists to concatenate.
+ */
+template<typename... List>
+using value_list_cat_t = typename value_list_cat<List...>::type;
+
+
+/*! @brief Same as std::is_invocable, but with tuples. */
+template<typename, typename>
+struct is_applicable: std::false_type {};
+
+
+/**
+ * @copybrief is_applicable
+ * @tparam Func A valid function type.
+ * @tparam Tuple Tuple-like type.
+ * @tparam Args The list of arguments to use to probe the function type.
+ */
+template<typename Func, template<typename...> class Tuple, typename... Args>
+struct is_applicable<Func, Tuple<Args...>>: std::is_invocable<Func, Args...> {};
+
+
+/**
+ * @copybrief is_applicable
+ * @tparam Func A valid function type.
+ * @tparam Tuple Tuple-like type.
+ * @tparam Args The list of arguments to use to probe the function type.
+ */
+template<typename Func, template<typename...> class Tuple, typename... Args>
+struct is_applicable<Func, const Tuple<Args...>>: std::is_invocable<Func, Args...> {};
+
+
+/**
+ * @brief Helper variable template.
+ * @tparam Func A valid function type.
+ * @tparam Args The list of arguments to use to probe the function type.
+ */
+template<typename Func, typename Args>
+inline constexpr bool is_applicable_v = is_applicable<Func, Args>::value;
+
+
+/*! @brief Same as std::is_invocable_r, but with tuples for arguments. */
+template<typename, typename, typename>
+struct is_applicable_r: std::false_type {};
+
+
+/**
+ * @copybrief is_applicable_r
+ * @tparam Ret The type to which the return type of the function should be
+ * convertible.
+ * @tparam Func A valid function type.
+ * @tparam Args The list of arguments to use to probe the function type.
+ */
+template<typename Ret, typename Func, typename... Args>
+struct is_applicable_r<Ret, Func, std::tuple<Args...>>: std::is_invocable_r<Ret, Func, Args...> {};
+
+
+/**
+ * @brief Helper variable template.
+ * @tparam Ret The type to which the return type of the function should be
+ * convertible.
+ * @tparam Func A valid function type.
+ * @tparam Args The list of arguments to use to probe the function type.
+ */
+template<typename Ret, typename Func, typename Args>
+inline constexpr bool is_applicable_r_v = is_applicable_r<Ret, Func, Args>::value;
+
+
+/**
+ * @brief Provides the member constant `value` to true if a given type is
+ * complete, false otherwise.
+ * @tparam Type The type to test.
+ */
+template<typename Type, typename = void>
+struct is_complete: std::false_type {};
+
+
+/*! @copydoc is_complete */
+template<typename Type>
+struct is_complete<Type, std::void_t<decltype(sizeof(Type))>>: std::true_type {};
+
+
+/**
+ * @brief Helper variable template.
+ * @tparam Type The type to test.
+ */
+template<typename Type>
+inline constexpr bool is_complete_v = is_complete<Type>::value;
+
+
+/**
+ * @brief Provides the member constant `value` to true if a given type is an
+ * iterator, false otherwise.
+ * @tparam Type The type to test.
+ */
+template<typename Type, typename = void>
+struct is_iterator: std::false_type {};
+
+
+/*! @copydoc is_iterator */
+template<typename Type>
+struct is_iterator<Type, std::void_t<typename std::iterator_traits<Type>::iterator_category>>
+    : std::true_type
+{};
+
+
+/**
+ * @brief Helper variable template.
+ * @tparam Type The type to test.
+ */
+template<typename Type>
+inline constexpr bool is_iterator_v = is_iterator<Type>::value;
+
+
+/**
+ * @brief Provides the member constant `value` to true if a given type is of the
+ * required iterator type, false otherwise.
+ * @tparam Type The type to test.
+ * @tparam It Required iterator type.
+ */
+template<typename Type, typename It, typename = void>
+struct is_iterator_type: std::false_type {};
+
+
+/*! @copydoc is_iterator_type */
+template<typename Type, typename It>
+struct is_iterator_type<Type, It, std::enable_if_t<is_iterator_v<Type> && std::is_same_v<Type, It>>>
+    : std::true_type
+{};
+
+
+/*! @copydoc is_iterator_type */
+template<typename Type, typename It>
+struct is_iterator_type<Type, It, std::enable_if_t<!std::is_same_v<Type, It>, std::void_t<typename It::iterator_type>>>
+    : is_iterator_type<Type, typename It::iterator_type>
+{};
+
+
+/**
+ * @brief Helper variable template.
+ * @tparam Type The type to test.
+ * @tparam It Required iterator type.
+ */
+template<typename Type, typename It>
+inline constexpr bool is_iterator_type_v = is_iterator_type<Type, It>::value;
+
+
+/**
+ * @cond TURN_OFF_DOXYGEN
+ * Internal details not to be documented.
+ */
+
+
+namespace internal {
+
+
+template<typename>
+[[nodiscard]] constexpr bool is_equality_comparable(...) { return false; }
+
+
+template<typename Type>
+[[nodiscard]] constexpr auto is_equality_comparable(choice_t<0>)
+-> decltype(std::declval<Type>() == std::declval<Type>()) { return true; }
+
+
+template<typename Type>
+[[nodiscard]] constexpr auto is_equality_comparable(choice_t<1>)
+-> decltype(std::declval<typename Type::value_type>(), std::declval<Type>() == std::declval<Type>()) {
+    if constexpr(is_iterator_v<Type>) {
+        return true;
+    } else if constexpr(std::is_same_v<typename Type::value_type, Type>) {
+        return is_equality_comparable<Type>(choice<0>);
+    } else {
+        return is_equality_comparable<typename Type::value_type>(choice<2>);
+    }
+}
+
+
+template<typename Type>
+[[nodiscard]] constexpr auto is_equality_comparable(choice_t<2>)
+-> decltype(std::declval<typename Type::mapped_type>(), std::declval<Type>() == std::declval<Type>()) {
+    return is_equality_comparable<typename Type::key_type>(choice<2>) && is_equality_comparable<typename Type::mapped_type>(choice<2>);
+}
+
+
+}
+
+
+/**
+ * Internal details not to be documented.
+ * @endcond
+ */
+
+
+/**
+ * @brief Provides the member constant `value` to true if a given type is
+ * equality comparable, false otherwise.
+ * @tparam Type The type to test.
+ */
+template<typename Type, typename = void>
+struct is_equality_comparable: std::bool_constant<internal::is_equality_comparable<Type>(choice<2>)> {};
+
+
+/**
+ * @brief Helper variable template.
+ * @tparam Type The type to test.
  */
 template<class Type>
-constexpr auto is_named_type_v = is_named_type<Type>::value;
-
-
-/**
- * @brief Defines an enum class to use for opaque identifiers and a dedicate
- * `to_integer` function to convert the identifiers to their underlying type.
- * @param clazz The name to use for the enum class.
- * @param type The underlying type for the enum class.
- */
-#define ENTT_OPAQUE_TYPE(clazz, type)\
-    enum class clazz: type {};\
-    constexpr auto to_integer(const clazz id) ENTT_NOEXCEPT {\
-        return std::underlying_type_t<clazz>(id);\
-    }
-
-
-}
-
-
-/**
- * @brief Utility macro to deal with an issue of MSVC.
- *
- * See _msvc-doesnt-expand-va-args-correctly_ on SO for all the details.
- *
- * @param args Argument to expand.
- */
-#define ENTT_EXPAND(args) args
-
-
-/**
- * @brief Makes an already existing type a named type.
- *
- * The current definition contains a workaround for Clang 6 because it fails to
- * deduce correctly the type to use to specialize the class template.<br/>
- * With a compiler that fully supports C++17 and works fine with deduction
- * guides, the following should be fine instead:
+inline constexpr bool is_equality_comparable_v = is_equality_comparable<Type>::value;
+
+
+/**
+ * @brief Transcribes the constness of a type to another type.
+ * @tparam To The type to which to transcribe the constness.
+ * @tparam From The type from which to transcribe the constness.
+ */
+template<typename To, typename From>
+struct constness_as {
+    /*! @brief The type resulting from the transcription of the constness. */
+    using type = std::remove_const_t<To>;
+};
+
+
+/*! @copydoc constness_as */
+template<typename To, typename From>
+struct constness_as<To, const From> {
+    /*! @brief The type resulting from the transcription of the constness. */
+    using type = std::add_const_t<To>;
+};
+
+
+/**
+ * @brief Alias template to facilitate the transcription of the constness.
+ * @tparam To The type to which to transcribe the constness.
+ * @tparam From The type from which to transcribe the constness.
+ */
+template<typename To, typename From>
+using constness_as_t = typename constness_as<To, From>::type;
+
+
+/**
+ * @brief Extracts the class of a non-static member object or function.
+ * @tparam Member A pointer to a non-static member object or function.
+ */
+template<typename Member>
+class member_class {
+    static_assert(std::is_member_pointer_v<Member>, "Invalid pointer type to non-static member object or function");
+
+    template<typename Class, typename Ret, typename... Args>
+    static Class * clazz(Ret(Class:: *)(Args...));
+
+    template<typename Class, typename Ret, typename... Args>
+    static Class * clazz(Ret(Class:: *)(Args...) const);
+
+    template<typename Class, typename Type>
+    static Class * clazz(Type Class:: *);
+
+public:
+    /*! @brief The class of the given non-static member object or function. */
+    using type = std::remove_pointer_t<decltype(clazz(std::declval<Member>()))>;
+};
+
+
+/**
+ * @brief Helper type.
+ * @tparam Member A pointer to a non-static member object or function.
+ */
+template<typename Member>
+using member_class_t = typename member_class<Member>::type;
+
+
+}
+
+
+#endif
+
+
+
+namespace entt {
+
+
+/**
+ * @brief A SBO friendly, type-safe container for single values of any type.
+ * @tparam Len Size of the storage reserved for the small buffer optimization.
+ * @tparam Align Optional alignment requirement.
+ */
+template<std::size_t Len, std::size_t Align>
+class basic_any {
+    enum class operation: std::uint8_t { COPY, MOVE, DTOR, COMP, ADDR, CADDR, TYPE };
+    enum class policy: std::uint8_t { OWNER, REF, CREF };
+
+    using storage_type = std::aligned_storage_t<Len + !Len, Align>;
+    using vtable_type = const void *(const operation, const basic_any &, void *);
+
+    template<typename Type>
+    static constexpr bool in_situ = Len && alignof(Type) <= alignof(storage_type) && sizeof(Type) <= sizeof(storage_type) && std::is_nothrow_move_constructible_v<Type>;
+
+    template<typename Type>
+    [[nodiscard]] static constexpr policy type_to_policy() {
+        if constexpr(std::is_lvalue_reference_v<Type>) {
+            if constexpr(std::is_const_v<std::remove_reference_t<Type>>) {
+                return policy::CREF;
+            } else {
+                return policy::REF;
+            }
+        } else {
+            return policy::OWNER;
+        }
+    }
+
+    template<typename Type>
+    [[nodiscard]] static bool compare(const void *lhs, const void *rhs) {
+        if constexpr(!std::is_function_v<Type> && is_equality_comparable_v<Type>) {
+            return *static_cast<const Type *>(lhs) == *static_cast<const Type *>(rhs);
+        } else {
+            return lhs == rhs;
+        }
+    }
+
+    template<typename Type>
+    static const void * basic_vtable([[maybe_unused]] const operation op, [[maybe_unused]] const basic_any &from, [[maybe_unused]] void *to) {
+        static_assert(std::is_same_v<std::remove_reference_t<std::remove_const_t<Type>>, Type>, "Invalid type");
+
+        if constexpr(!std::is_void_v<Type>) {
+            const Type *instance = (in_situ<Type> && from.mode == policy::OWNER)
+                ? ENTT_LAUNDER(reinterpret_cast<const Type *>(&from.storage))
+                : static_cast<const Type *>(from.instance);
+
+            switch(op) {
+            case operation::COPY:
+                if constexpr(std::is_copy_constructible_v<Type>) {
+                    static_cast<basic_any *>(to)->emplace<Type>(*instance);
+                }
+                break;
+            case operation::MOVE:
+                if constexpr(in_situ<Type>) {
+                    if(from.mode == policy::OWNER) {
+                        return new (&static_cast<basic_any *>(to)->storage) Type{std::move(*const_cast<Type *>(instance))};
+                    }
+                }
+
+                return (static_cast<basic_any *>(to)->instance = std::exchange(const_cast<basic_any &>(from).instance, nullptr));
+            case operation::DTOR:
+                if(from.mode == policy::OWNER) {
+                    if constexpr(in_situ<Type>) {
+                        instance->~Type();
+                    } else if constexpr(std::is_array_v<Type>) {
+                        delete[] instance;
+                    } else {
+                        delete instance;
+                    }
+                }
+                break;
+            case operation::COMP:
+                return compare<Type>(instance, (*static_cast<const basic_any **>(to))->data()) ? to : nullptr;
+            case operation::ADDR:
+                if(from.mode == policy::CREF) {
+                    return nullptr;
+                }
+                [[fallthrough]];
+            case operation::CADDR:
+                return instance;
+            case operation::TYPE:
+                *static_cast<type_info *>(to) = type_id<Type>();
+                break;
+            }
+        }
+
+        return nullptr;
+    }
+
+    template<typename Type, typename... Args>
+    void initialize([[maybe_unused]] Args &&... args) {
+        if constexpr(!std::is_void_v<Type>) {
+            if constexpr(std::is_lvalue_reference_v<Type>) {
+                static_assert(sizeof...(Args) == 1u && (std::is_lvalue_reference_v<Args> && ...), "Invalid arguments");
+                instance = (std::addressof(args), ...);
+            } else if constexpr(in_situ<Type>) {
+                if constexpr(sizeof...(Args) != 0u && std::is_aggregate_v<Type>) {
+                    new (&storage) Type{std::forward<Args>(args)...};
+                } else {
+                    new (&storage) Type(std::forward<Args>(args)...);
+                }
+            } else {
+                if constexpr(sizeof...(Args) != 0u && std::is_aggregate_v<Type>) {
+                    instance = new Type{std::forward<Args>(args)...};
+                } else {
+                    instance = new Type(std::forward<Args>(args)...);
+                }
+            }
+        }
+    }
+
+    basic_any(const basic_any &other, const policy pol) ENTT_NOEXCEPT
+        : instance{other.data()},
+          vtable{other.vtable},
+          mode{pol}
+    {}
+
+public:
+    /*! @brief Size of the internal storage. */
+    static constexpr auto length = Len;
+    /*! @brief Alignment requirement. */
+    static constexpr auto alignment = Align;
+
+    /*! @brief Default constructor. */
+    basic_any() ENTT_NOEXCEPT
+        : instance{},
+          vtable{&basic_vtable<void>},
+          mode{policy::OWNER}
+    {}
+
+    /**
+     * @brief Constructs a wrapper by directly initializing the new object.
+     * @tparam Type Type of object to use to initialize the wrapper.
+     * @tparam Args Types of arguments to use to construct the new instance.
+     * @param args Parameters to use to construct the instance.
+     */
+    template<typename Type, typename... Args>
+    explicit basic_any(std::in_place_type_t<Type>, Args &&... args)
+        : instance{},
+          vtable{&basic_vtable<std::remove_const_t<std::remove_reference_t<Type>>>},
+          mode{type_to_policy<Type>()}
+    {
+        initialize<Type>(std::forward<Args>(args)...);
+    }
+
+    /**
+     * @brief Constructs a wrapper that holds an unmanaged object.
+     * @tparam Type Type of object to use to initialize the wrapper.
+     * @param value An instance of an object to use to initialize the wrapper.
+     */
+    template<typename Type>
+    basic_any(std::reference_wrapper<Type> value) ENTT_NOEXCEPT
+        : basic_any{}
+    {
+        // invokes deprecated assignment operator (and avoids issues with vs2017)
+        *this = value;
+    }
+
+    /**
+     * @brief Constructs a wrapper from a given value.
+     * @tparam Type Type of object to use to initialize the wrapper.
+     * @param value An instance of an object to use to initialize the wrapper.
+     */
+    template<typename Type, typename = std::enable_if_t<!std::is_same_v<std::decay_t<Type>, basic_any>>>
+    basic_any(Type &&value)
+        : instance{},
+          vtable{&basic_vtable<std::decay_t<Type>>},
+          mode{policy::OWNER}
+    {
+        initialize<std::decay_t<Type>>(std::forward<Type>(value));
+    }
+
+    /**
+     * @brief Copy constructor.
+     * @param other The instance to copy from.
+     */
+    basic_any(const basic_any &other)
+        : instance{},
+          vtable{&basic_vtable<void>},
+          mode{policy::OWNER}
+    {
+        other.vtable(operation::COPY, other, this);
+    }
+
+    /**
+     * @brief Move constructor.
+     * @param other The instance to move from.
+     */
+    basic_any(basic_any &&other) ENTT_NOEXCEPT
+        : instance{},
+          vtable{other.vtable},
+          mode{other.mode}
+    {
+        vtable(operation::MOVE, other, this);
+    }
+
+    /*! @brief Frees the internal storage, whatever it means. */
+    ~basic_any() {
+        vtable(operation::DTOR, *this, nullptr);
+    }
+
+    /**
+     * @brief Copy assignment operator.
+     * @param other The instance to copy from.
+     * @return This any object.
+     */
+    basic_any & operator=(const basic_any &other) {
+        reset();
+        other.vtable(operation::COPY, other, this);
+        return *this;
+    }
+
+    /**
+     * @brief Move assignment operator.
+     * @param other The instance to move from.
+     * @return This any object.
+     */
+    basic_any & operator=(basic_any &&other) ENTT_NOEXCEPT {
+        std::exchange(vtable, other.vtable)(operation::DTOR, *this, nullptr);
+        other.vtable(operation::MOVE, other, this);
+        mode = other.mode;
+        return *this;
+    }
+
+    /**
+     * @brief Value assignment operator.
+     * @tparam Type Type of object to use to initialize the wrapper.
+     * @param value An instance of an object to use to initialize the wrapper.
+     * @return This any object.
+     */
+    template<typename Type>
+    [[deprecated("Use std::in_place_type<T &>, entt::make_any<T &>, emplace<Type &> or forward_as_any instead")]]
+    basic_any & operator=(std::reference_wrapper<Type> value) ENTT_NOEXCEPT {
+        emplace<Type &>(value.get());
+        return *this;
+    }
+
+    /**
+     * @brief Value assignment operator.
+     * @tparam Type Type of object to use to initialize the wrapper.
+     * @param value An instance of an object to use to initialize the wrapper.
+     * @return This any object.
+     */
+    template<typename Type>
+    std::enable_if_t<!std::is_same_v<std::decay_t<Type>, basic_any>, basic_any &>
+    operator=(Type &&value) {
+        emplace<std::decay_t<Type>>(std::forward<Type>(value));
+        return *this;
+    }
+
+    /**
+     * @brief Returns the type of the contained object.
+     * @return The type of the contained object, if any.
+     */
+    [[nodiscard]] type_info type() const ENTT_NOEXCEPT {
+        type_info info{};
+        vtable(operation::TYPE, *this, &info);
+        return info;
+    }
+
+    /**
+     * @brief Returns an opaque pointer to the contained instance.
+     * @return An opaque pointer the contained instance, if any.
+     */
+    [[nodiscard]] const void * data() const ENTT_NOEXCEPT {
+        return vtable(operation::CADDR, *this, nullptr);
+    }
+
+    /*! @copydoc data */
+    [[nodiscard]] void * data() ENTT_NOEXCEPT {
+        return const_cast<void *>(vtable(operation::ADDR, *this, nullptr));
+    }
+
+    /**
+     * @brief Replaces the contained object by creating a new instance directly.
+     * @tparam Type Type of object to use to initialize the wrapper.
+     * @tparam Args Types of arguments to use to construct the new instance.
+     * @param args Parameters to use to construct the instance.
+     */
+    template<typename Type, typename... Args>
+    void emplace(Args &&... args) {
+        std::exchange(vtable, &basic_vtable<std::remove_const_t<std::remove_reference_t<Type>>>)(operation::DTOR, *this, nullptr);
+        mode = type_to_policy<Type>();
+        initialize<Type>(std::forward<Args>(args)...);
+    }
+
+    /*! @brief Destroys contained object */
+    void reset() {
+        std::exchange(vtable, &basic_vtable<void>)(operation::DTOR, *this, nullptr);
+        mode = policy::OWNER;
+    }
+
+    /**
+     * @brief Returns false if a wrapper is empty, true otherwise.
+     * @return False if the wrapper is empty, true otherwise.
+     */
+    [[nodiscard]] explicit operator bool() const ENTT_NOEXCEPT {
+        return !(vtable(operation::CADDR, *this, nullptr) == nullptr);
+    }
+
+    /**
+     * @brief Checks if two wrappers differ in their content.
+     * @param other Wrapper with which to compare.
+     * @return False if the two objects differ in their content, true otherwise.
+     */
+    bool operator==(const basic_any &other) const ENTT_NOEXCEPT {
+        const basic_any *trampoline = &other;
+        return type() == other.type() && (vtable(operation::COMP, *this, &trampoline) || !other.data());
+    }
+
+    /**
+     * @brief Aliasing constructor.
+     * @return A wrapper that shares a reference to an unmanaged object.
+     */
+    [[nodiscard]] basic_any as_ref() ENTT_NOEXCEPT {
+        return basic_any{*this, (mode == policy::CREF ? policy::CREF : policy::REF)};
+    }
+
+    /*! @copydoc as_ref */
+    [[nodiscard]] basic_any as_ref() const ENTT_NOEXCEPT {
+        return basic_any{*this, policy::CREF};
+    }
+
+    /**
+     * @brief Returns true if a wrapper owns its object, false otherwise.
+     * @return True if the wrapper owns its object, false otherwise.
+     */
+    [[nodiscard]] bool owner() const ENTT_NOEXCEPT {
+        return (mode == policy::OWNER);
+    }
+
+private:
+    union { const void *instance; storage_type storage; };
+    vtable_type *vtable;
+    policy mode;
+};
+
+
+/**
+ * @brief Checks if two wrappers differ in their content.
+ * @tparam Len Size of the storage reserved for the small buffer optimization.
+ * @tparam Align Alignment requirement.
+ * @param lhs A wrapper, either empty or not.
+ * @param rhs A wrapper, either empty or not.
+ * @return True if the two wrappers differ in their content, false otherwise.
+ */
+template<std::size_t Len, std::size_t Align>
+[[nodiscard]] inline bool operator!=(const basic_any<Len, Align> &lhs, const basic_any<Len, Align> &rhs) ENTT_NOEXCEPT {
+    return !(lhs == rhs);
+}
+
+
+/**
+ * @brief Performs type-safe access to the contained object.
+ * @tparam Type Type to which conversion is required.
+ * @tparam Len Size of the storage reserved for the small buffer optimization.
+ * @tparam Align Alignment requirement.
+ * @param data Target any object.
+ * @return The element converted to the requested type.
+ */
+template<typename Type, std::size_t Len, std::size_t Align>
+Type any_cast(const basic_any<Len, Align> &data) ENTT_NOEXCEPT {
+    const auto * const instance = any_cast<std::remove_reference_t<Type>>(&data);
+    ENTT_ASSERT(instance, "Invalid instance");
+    return static_cast<Type>(*instance);
+}
+
+
+/*! @copydoc any_cast */
+template<typename Type, std::size_t Len, std::size_t Align>
+Type any_cast(basic_any<Len, Align> &data) ENTT_NOEXCEPT {
+    // forces const on non-reference types to make them work also with wrappers for const references
+    auto * const instance = any_cast<std::remove_reference_t<const Type>>(&data);
+    ENTT_ASSERT(instance, "Invalid instance");
+    return static_cast<Type>(*instance);
+}
+
+
+/*! @copydoc any_cast */
+template<typename Type, std::size_t Len, std::size_t Align>
+Type any_cast(basic_any<Len, Align> &&data) ENTT_NOEXCEPT {
+    // forces const on non-reference types to make them work also with wrappers for const references
+    auto * const instance = any_cast<std::remove_reference_t<const Type>>(&data);
+    ENTT_ASSERT(instance, "Invalid instance");
+    return static_cast<Type>(std::move(*instance));
+}
+
+
+/*! @copydoc any_cast */
+template<typename Type, std::size_t Len, std::size_t Align>
+const Type * any_cast(const basic_any<Len, Align> *data) ENTT_NOEXCEPT {
+    return (data->type() == type_id<Type>() ? static_cast<const Type *>(data->data()) : nullptr);
+}
+
+
+/*! @copydoc any_cast */
+template<typename Type, std::size_t Len, std::size_t Align>
+Type * any_cast(basic_any<Len, Align> *data) ENTT_NOEXCEPT {
+    // last attempt to make wrappers for const references return their values
+    return (data->type() == type_id<Type>() ? static_cast<Type *>(static_cast<constness_as_t<basic_any<Len, Align>, Type> *>(data)->data()) : nullptr);
+}
+
+
+/**
+ * @brief Constructs a wrapper from a given type, passing it all arguments.
+ * @tparam Type Type of object to use to initialize the wrapper.
+ * @tparam Len Size of the storage reserved for the small buffer optimization.
+ * @tparam Align Optional alignment requirement.
+ * @tparam Args Types of arguments to use to construct the new instance.
+ * @param args Parameters to use to construct the instance.
+ * @return A properly initialized wrapper for an object of the given type.
+ */
+template<typename Type, std::size_t Len = basic_any<>::length, std::size_t Align = basic_any<Len>::alignment, typename... Args>
+basic_any<Len, Align> make_any(Args &&... args) {
+    return basic_any<Len, Align>{std::in_place_type<Type>, std::forward<Args>(args)...};
+}
+
+
+/**
+ * @brief Forwards its argument and avoids copies for lvalue references.
+ * @tparam Len Size of the storage reserved for the small buffer optimization.
+ * @tparam Align Optional alignment requirement.
+ * @tparam Type Type of argument to use to construct the new instance.
+ * @param value Parameter to use to construct the instance.
+ * @return A properly initialized and not necessarily owning wrapper.
+ */
+template<std::size_t Len = basic_any<>::length, std::size_t Align = basic_any<Len>::alignment, typename Type>
+basic_any<Len, Align> forward_as_any(Type &&value) {
+    return basic_any<Len, Align>{std::in_place_type<std::conditional_t<std::is_rvalue_reference_v<Type>, std::decay_t<Type>, Type>>, std::forward<Type>(value)};
+}
+
+
+}
+
+
+#endif
+
+// #include "../core/fwd.hpp"
+
+// #include "../core/utility.hpp"
+#ifndef ENTT_CORE_UTILITY_HPP
+#define ENTT_CORE_UTILITY_HPP
+
+
+#include <utility>
+// #include "../config/config.h"
+
+
+
+namespace entt {
+
+
+/*! @brief Identity function object (waiting for C++20). */
+struct identity {
+    /**
+     * @brief Returns its argument unchanged.
+     * @tparam Type Type of the argument.
+     * @param value The actual argument.
+     * @return The submitted value as-is.
+     */
+    template<class Type>
+    [[nodiscard]] constexpr Type && operator()(Type &&value) const ENTT_NOEXCEPT {
+        return std::forward<Type>(value);
+    }
+};
+
+
+/**
+ * @brief Constant utility to disambiguate overloaded members of a class.
+ * @tparam Type Type of the desired overload.
+ * @tparam Class Type of class to which the member belongs.
+ * @param member A valid pointer to a member.
+ * @return Pointer to the member.
+ */
+template<typename Type, typename Class>
+[[nodiscard]] constexpr auto overload(Type Class:: *member) ENTT_NOEXCEPT { return member; }
+
+
+/**
+ * @brief Constant utility to disambiguate overloaded functions.
+ * @tparam Func Function type of the desired overload.
+ * @param func A valid pointer to a function.
+ * @return Pointer to the function.
+ */
+template<typename Func>
+[[nodiscard]] constexpr auto overload(Func *func) ENTT_NOEXCEPT { return func; }
+
+
+/**
+ * @brief Helper type for visitors.
+ * @tparam Func Types of function objects.
+ */
+template<class... Func>
+struct overloaded: Func... {
+    using Func::operator()...;
+};
+
+
+/**
+ * @brief Deduction guide.
+ * @tparam Func Types of function objects.
+ */
+template<class... Func>
+overloaded(Func...)
+-> overloaded<Func...>;
+
+
+/**
+ * @brief Basic implementation of a y-combinator.
+ * @tparam Func Type of a potentially recursive function.
+ */
+template<class Func>
+struct y_combinator {
+    /**
+     * @brief Constructs a y-combinator from a given function.
+     * @param recursive A potentially recursive function.
+     */
+    y_combinator(Func recursive):
+        func{std::move(recursive)}
+    {}
+
+    /**
+     * @brief Invokes a y-combinator and therefore its underlying function.
+     * @tparam Args Types of arguments to use to invoke the underlying function.
+     * @param args Parameters to use to invoke the underlying function.
+     * @return Return value of the underlying function, if any.
+     */
+    template <class... Args>
+    decltype(auto) operator()(Args &&... args) const {
+        return func(*this, std::forward<Args>(args)...);
+    }
+
+    /*! @copydoc operator()() */
+    template <class... Args>
+    decltype(auto) operator()(Args &&... args) {
+        return func(*this, std::forward<Args>(args)...);
+    }
+
+private:
+    Func func;
+};
+
+
+}
+
+
+#endif
+
+// #include "../core/type_info.hpp"
+
+// #include "../core/type_traits.hpp"
+
+// #include "adl_pointer.hpp"
+#ifndef ENTT_META_ADL_POINTER_HPP
+#define ENTT_META_ADL_POINTER_HPP
+
+
+namespace entt {
+
+
+/**
+ * @brief ADL based lookup function for dereferencing meta pointer-like types.
+ * @tparam Type Element type.
+ * @param value A pointer-like object.
+ * @return The value returned from the dereferenced pointer.
+ */
+template<typename Type>
+decltype(auto) dereference_meta_pointer_like(const Type &value) {
+    return *value;
+}
+
+
+/**
+ * @brief Fake ADL based lookup function for meta pointer-like types.
+ * @tparam Type Element type.
+ */
+template<typename Type>
+struct adl_meta_pointer_like {
+    /**
+     * @brief Uses the default ADL based lookup method to resolve the call.
+     * @param value A pointer-like object.
+     * @return The value returned from the dereferenced pointer.
+     */
+    static decltype(auto) dereference(const Type &value) {
+        return dereference_meta_pointer_like(value);
+    }
+};
+
+
+}
+
+
+#endif
+
+// #include "ctx.hpp"
+#ifndef ENTT_META_CTX_HPP
+#define ENTT_META_CTX_HPP
+
+
+// #include "../core/attribute.h"
+
+// #include "../config/config.h"
+
+
+
+namespace entt {
+
+
+/**
+ * @cond TURN_OFF_DOXYGEN
+ * Internal details not to be documented.
+ */
+
+
+namespace internal {
+
+
+struct meta_type_node;
+
+
+struct ENTT_API meta_context {
+    // we could use the lines below but VS2017 returns with an ICE if combined with ENTT_API despite the code being valid C++
+    //     inline static meta_type_node *local = nullptr;
+    //     inline static meta_type_node **global = &local;
+
+    [[nodiscard]] static meta_type_node * & local() ENTT_NOEXCEPT {
+        static meta_type_node *chain = nullptr;
+        return chain;
+    }
+
+    [[nodiscard]] static meta_type_node ** & global() ENTT_NOEXCEPT {
+        static meta_type_node **chain = &local();
+        return chain;
+    }
+};
+
+
+}
+
+
+/**
+ * Internal details not to be documented.
+ * @endcond
+ */
+
+
+/*! @brief Opaque container for a meta context. */
+struct meta_ctx {
+    /**
+     * @brief Binds the meta system to a given context.
+     * @param other A valid context to which to bind.
+     */
+    static void bind(meta_ctx other) ENTT_NOEXCEPT {
+        internal::meta_context::global() = other.ctx;
+    }
+
+private:
+    internal::meta_type_node **ctx{&internal::meta_context::local()};
+};
+
+
+}
+
+
+#endif
+
+// #include "node.hpp"
+#ifndef ENTT_META_NODE_HPP
+#define ENTT_META_NODE_HPP
+
+
+#include <cstddef>
+#include <type_traits>
+#include <utility>
+// #include "../config/config.h"
+
+// #include "../core/attribute.h"
+
+// #include "../core/fwd.hpp"
+
+// #include "../core/type_info.hpp"
+
+// #include "../core/type_traits.hpp"
+
+// #include "type_traits.hpp"
+
+
+
+namespace entt {
+
+
+class meta_any;
+class meta_type;
+struct meta_handle;
+
+
+/**
+ * @cond TURN_OFF_DOXYGEN
+ * Internal details not to be documented.
+ */
+
+
+namespace internal {
+
+
+struct meta_type_node;
+
+
+struct meta_prop_node {
+    meta_prop_node * next;
+    const meta_any &id;
+    meta_any &value;
+};
+
+
+struct meta_base_node {
+    meta_type_node * const parent;
+    meta_base_node * next;
+    meta_type_node *(* const type)() ENTT_NOEXCEPT;
+    const void *(* const cast)(const void *) ENTT_NOEXCEPT;
+};
+
+
+struct meta_conv_node {
+    meta_type_node * const parent;
+    meta_conv_node * next;
+    meta_type_node *(* const type)() ENTT_NOEXCEPT;
+    meta_any(* const conv)(const void *);
+};
+
+
+struct meta_ctor_node {
+    using size_type = std::size_t;
+    meta_type_node * const parent;
+    meta_ctor_node * next;
+    meta_prop_node * prop;
+    const size_type arity;
+    meta_type(* const arg)(const size_type) ENTT_NOEXCEPT;
+    meta_any(* const invoke)(meta_any * const);
+};
+
+
+struct meta_data_node {
+    id_type id;
+    meta_type_node * const parent;
+    meta_data_node * next;
+    meta_prop_node * prop;
+    const bool is_const;
+    const bool is_static;
+    meta_type_node *(* const type)() ENTT_NOEXCEPT;
+    bool(* const set)(meta_handle, meta_any);
+    meta_any(* const get)(meta_handle);
+};
+
+
+struct meta_func_node {
+    using size_type = std::size_t;
+    id_type id;
+    meta_type_node * const parent;
+    meta_func_node * next;
+    meta_prop_node * prop;
+    const size_type arity;
+    const bool is_const;
+    const bool is_static;
+    meta_type_node *(* const ret)() ENTT_NOEXCEPT;
+    meta_type(* const arg)(const size_type) ENTT_NOEXCEPT;
+    meta_any(* const invoke)(meta_handle, meta_any *);
+};
+
+
+struct meta_template_info {
+    using size_type = std::size_t;
+    const bool is_template_specialization;
+    const size_type arity;
+    meta_type_node *(* const type)() ENTT_NOEXCEPT;
+    meta_type_node *(* const arg)(const size_type) ENTT_NOEXCEPT;
+};
+
+
+struct meta_type_node {
+    using size_type = std::size_t;
+    const type_info info;
+    id_type id;
+    meta_type_node * next;
+    meta_prop_node * prop;
+    const size_type size_of;
+    const bool is_void;
+    const bool is_integral;
+    const bool is_floating_point;
+    const bool is_array;
+    const bool is_enum;
+    const bool is_union;
+    const bool is_class;
+    const bool is_pointer;
+    const bool is_function_pointer;
+    const bool is_member_object_pointer;
+    const bool is_member_function_pointer;
+    const bool is_pointer_like;
+    const bool is_sequence_container;
+    const bool is_associative_container;
+    const meta_template_info template_info;
+    const size_type rank;
+    size_type(* const extent)(const size_type) ENTT_NOEXCEPT ;
+    meta_type_node *(* const remove_pointer)() ENTT_NOEXCEPT;
+    meta_type_node *(* const remove_extent)() ENTT_NOEXCEPT;
+    meta_ctor_node * const def_ctor;
+    meta_ctor_node *ctor{nullptr};
+    meta_base_node *base{nullptr};
+    meta_conv_node *conv{nullptr};
+    meta_data_node *data{nullptr};
+    meta_func_node *func{nullptr};
+    void(* dtor)(void *){nullptr};
+};
+
+
+template<auto Member, typename Op, typename Node>
+auto meta_visit(const Op &op, const Node *node)
+-> std::decay_t<decltype(node->*Member)> {
+    for(auto *curr = node->*Member; curr; curr = curr->next) {
+        if(op(curr)) {
+            return curr;
+        }
+    }
+
+    if constexpr(std::is_same_v<Node, meta_type_node>) {
+        for(auto *curr = node->base; curr; curr = curr->next) {
+            if(auto *ret = meta_visit<Member>(op, curr->type()); ret) {
+                return ret;
+            }
+        }
+    }
+
+    return nullptr;
+}
+
+
+template<typename... Args>
+meta_type_node * meta_arg_node(type_list<Args...>, const std::size_t index) ENTT_NOEXCEPT;
+
+
+template<typename Type>
+class ENTT_API meta_node {
+    static_assert(std::is_same_v<Type, std::remove_cv_t<std::remove_reference_t<Type>>>, "Invalid type");
+
+    template<std::size_t... Index>
+    [[nodiscard]] static auto extent(const meta_type_node::size_type dim, std::index_sequence<Index...>) ENTT_NOEXCEPT {
+        meta_type_node::size_type ext{};
+        ((ext = (dim == Index ? std::extent_v<Type, Index> : ext)), ...);
+        return ext;
+    }
+
+    [[nodiscard]] static meta_ctor_node * meta_default_constructor([[maybe_unused]] meta_type_node *type) ENTT_NOEXCEPT {
+        if constexpr(std::is_default_constructible_v<Type>) {
+            static meta_ctor_node node{
+                type,
+                nullptr,
+                nullptr,
+                0u,
+                nullptr,
+                [](meta_any * const) { return meta_any{std::in_place_type<Type>}; }
+            };
+
+            return &node;
+        } else {
+            return nullptr;
+        }
+    }
+
+    [[nodiscard]] static meta_template_info meta_template_descriptor() ENTT_NOEXCEPT {
+        if constexpr(is_complete_v<meta_template_traits<Type>>) {
+            return {
+                true,
+                meta_template_traits<Type>::args_type::size,
+                &meta_node<typename meta_template_traits<Type>::class_type>::resolve,
+                [](const std::size_t index) ENTT_NOEXCEPT {
+                    return meta_arg_node(typename meta_template_traits<Type>::args_type{}, index);
+                }
+            };
+        } else {
+            return { false, 0u, nullptr, nullptr };
+        }
+    }
+
+public:
+    [[nodiscard]] static meta_type_node * resolve() ENTT_NOEXCEPT {
+        static meta_type_node node{
+            type_id<Type>(),
+            {},
+            nullptr,
+            nullptr,
+            size_of_v<Type>,
+            std::is_void_v<Type>,
+            std::is_integral_v<Type>,
+            std::is_floating_point_v<Type>,
+            std::is_array_v<Type>,
+            std::is_enum_v<Type>,
+            std::is_union_v<Type>,
+            std::is_class_v<Type>,
+            std::is_pointer_v<Type>,
+            std::is_pointer_v<Type> && std::is_function_v<std::remove_pointer_t<Type>>,
+            std::is_member_object_pointer_v<Type>,
+            std::is_member_function_pointer_v<Type>,
+            is_meta_pointer_like_v<Type>,
+            is_complete_v<meta_sequence_container_traits<Type>>,
+            is_complete_v<meta_associative_container_traits<Type>>,
+            meta_template_descriptor(),
+            std::rank_v<Type>,
+            [](meta_type_node::size_type dim) ENTT_NOEXCEPT { return extent(dim, std::make_index_sequence<std::rank_v<Type>>{}); },
+            &meta_node<std::remove_cv_t<std::remove_reference_t<std::remove_pointer_t<Type>>>>::resolve,
+            &meta_node<std::remove_cv_t<std::remove_reference_t<std::remove_extent_t<Type>>>>::resolve,
+            meta_default_constructor(&node),
+            meta_default_constructor(&node)
+        };
+
+        return &node;
+    }
+};
+
+
+template<typename Type>
+struct meta_info: meta_node<std::remove_cv_t<std::remove_reference_t<Type>>> {};
+
+
+template<typename... Args>
+meta_type_node * meta_arg_node(type_list<Args...>, const std::size_t index) ENTT_NOEXCEPT {
+    meta_type_node *args[sizeof...(Args) + 1u]{nullptr, internal::meta_info<Args>::resolve()...};
+    return args[index + 1u];
+}
+
+
+}
+
+
+/**
+ * Internal details not to be documented.
+ * @endcond
+ */
+
+
+}
+
+
+#endif
+
+// #include "range.hpp"
+#ifndef ENTT_META_RANGE_HPP
+#define ENTT_META_RANGE_HPP
+
+
+#include <cstddef>
+#include <iterator>
+
+
+namespace entt {
+
+
+/**
+ * @brief Iterable range to use to iterate all types of meta objects.
+ * @tparam Type Type of meta objects returned.
+ * @tparam Node Type of meta nodes iterated.
+ */
+template<typename Type, typename Node = typename Type::node_type>
+class meta_range {
+    struct range_iterator {
+        using difference_type = std::ptrdiff_t;
+        using value_type = Type;
+        using pointer = void;
+        using reference = value_type;
+        using iterator_category = std::input_iterator_tag;
+        using node_type = Node;
+
+        range_iterator() ENTT_NOEXCEPT = default;
+
+        range_iterator(node_type *head) ENTT_NOEXCEPT
+            : it{head}
+        {}
+
+        range_iterator & operator++() ENTT_NOEXCEPT {
+            return (it = it->next), *this;
+        }
+
+        range_iterator operator++(int) ENTT_NOEXCEPT {
+            range_iterator orig = *this;
+            return ++(*this), orig;
+        }
+
+        [[nodiscard]] reference operator*() const ENTT_NOEXCEPT {
+            return it;
+        }
+
+        [[nodiscard]] bool operator==(const range_iterator &other) const ENTT_NOEXCEPT {
+            return other.it == it;
+        }
+
+        [[nodiscard]] bool operator!=(const range_iterator &other) const ENTT_NOEXCEPT {
+            return !(*this == other);
+        }
+
+    private:
+        node_type *it{};
+    };
+
+public:
+    /*! @brief Node type. */
+    using node_type = Node;
+    /*! @brief Input iterator type. */
+    using iterator = range_iterator;
+
+    /*! @brief Default constructor. */
+    meta_range() ENTT_NOEXCEPT = default;
+
+    /**
+     * @brief Constructs a meta range from a given node.
+     * @param head The underlying node with which to construct the range.
+     */
+    meta_range(node_type *head)
+        : node{head}
+    {}
+
+    /**
+     * @brief Returns an iterator to the beginning.
+     * @return An iterator to the first meta object of the range.
+     */
+    [[nodiscard]] iterator begin() const ENTT_NOEXCEPT {
+        return iterator{node};
+    }
+
+    /**
+     * @brief Returns an iterator to the end.
+     * @return An iterator to the element following the last meta object of the
+     * range.
+     */
+    [[nodiscard]] iterator end() const ENTT_NOEXCEPT {
+        return iterator{};
+    }
+
+private:
+    node_type *node{nullptr};
+};
+
+
+}
+
+
+#endif
+
+// #include "type_traits.hpp"
+
+
+
+namespace entt {
+
+
+class meta_any;
+class meta_type;
+
+
+/*! @brief Proxy object for sequence containers. */
+class meta_sequence_container {
+    template<typename>
+    struct meta_sequence_container_proxy;
+
+    class meta_iterator;
+
+public:
+    /*! @brief Unsigned integer type. */
+    using size_type = std::size_t;
+    /*! @brief Meta iterator type. */
+    using iterator = meta_iterator;
+
+    /*! @brief Default constructor. */
+    meta_sequence_container() ENTT_NOEXCEPT = default;
+
+    /**
+     * @brief Construct a proxy object for sequence containers.
+     * @tparam Type Type of container to wrap.
+     * @param instance The container to wrap.
+     */
+    template<typename Type>
+    meta_sequence_container(std::in_place_type_t<Type>, any instance) ENTT_NOEXCEPT
+        : value_type_fn{&meta_sequence_container_proxy<Type>::value_type},
+          size_fn{&meta_sequence_container_proxy<Type>::size},
+          resize_fn{&meta_sequence_container_proxy<Type>::resize},
+          clear_fn{&meta_sequence_container_proxy<Type>::clear},
+          begin_fn{&meta_sequence_container_proxy<Type>::begin},
+          end_fn{&meta_sequence_container_proxy<Type>::end},
+          insert_fn{&meta_sequence_container_proxy<Type>::insert},
+          erase_fn{&meta_sequence_container_proxy<Type>::erase},
+          get_fn{&meta_sequence_container_proxy<Type>::get},
+          storage{std::move(instance)}
+    {}
+
+    [[nodiscard]] inline meta_type value_type() const ENTT_NOEXCEPT;
+    [[nodiscard]] inline size_type size() const ENTT_NOEXCEPT;
+    inline bool resize(size_type);
+    inline bool clear();
+    [[nodiscard]] inline iterator begin();
+    [[nodiscard]] inline iterator end();
+    inline std::pair<iterator, bool> insert(iterator, meta_any);
+    inline std::pair<iterator, bool> erase(iterator);
+    [[nodiscard]] inline meta_any operator[](size_type);
+    [[nodiscard]] inline explicit operator bool() const ENTT_NOEXCEPT;
+
+private:
+    meta_type(* value_type_fn)() ENTT_NOEXCEPT = nullptr;
+    size_type(* size_fn)(const any &) ENTT_NOEXCEPT = nullptr;
+    bool(* resize_fn)(any &, size_type) = nullptr;
+    bool(* clear_fn)(any &) = nullptr;
+    iterator(* begin_fn)(any &) = nullptr;
+    iterator(* end_fn)(any &) = nullptr;
+    std::pair<iterator, bool>(* insert_fn)(any &, iterator, meta_any &) = nullptr;
+    std::pair<iterator, bool>(* erase_fn)(any &, iterator) = nullptr;
+    meta_any(* get_fn)(any &, size_type) = nullptr;
+    any storage{};
+};
+
+
+/*! @brief Proxy object for associative containers. */
+class meta_associative_container {
+    template<typename>
+    struct meta_associative_container_proxy;
+
+    class meta_iterator;
+
+public:
+    /*! @brief Unsigned integer type. */
+    using size_type = std::size_t;
+    /*! @brief Meta iterator type. */
+    using iterator = meta_iterator;
+
+    /*! @brief Default constructor. */
+    meta_associative_container() ENTT_NOEXCEPT = default;
+
+    /**
+     * @brief Construct a proxy object for associative containers.
+     * @tparam Type Type of container to wrap.
+     * @param instance The container to wrap.
+     */
+    template<typename Type>
+    meta_associative_container(std::in_place_type_t<Type>, any instance) ENTT_NOEXCEPT
+        : key_only_container{is_key_only_meta_associative_container_v<Type>},
+          key_type_fn{&meta_associative_container_proxy<Type>::key_type},
+          mapped_type_fn{&meta_associative_container_proxy<Type>::mapped_type},
+          value_type_fn{&meta_associative_container_proxy<Type>::value_type},
+          size_fn{&meta_associative_container_proxy<Type>::size},
+          clear_fn{&meta_associative_container_proxy<Type>::clear},
+          begin_fn{&meta_associative_container_proxy<Type>::begin},
+          end_fn{&meta_associative_container_proxy<Type>::end},
+          insert_fn{&meta_associative_container_proxy<Type>::insert},
+          erase_fn{&meta_associative_container_proxy<Type>::erase},
+          find_fn{&meta_associative_container_proxy<Type>::find},
+          storage{std::move(instance)}
+    {}
+
+    [[nodiscard]] inline bool key_only() const ENTT_NOEXCEPT;
+    [[nodiscard]] inline meta_type key_type() const ENTT_NOEXCEPT;
+    [[nodiscard]] inline meta_type mapped_type() const ENTT_NOEXCEPT;
+    [[nodiscard]] inline meta_type value_type() const ENTT_NOEXCEPT;
+    [[nodiscard]] inline size_type size() const ENTT_NOEXCEPT;
+    inline bool clear();
+    [[nodiscard]] inline iterator begin();
+    [[nodiscard]] inline iterator end();
+    inline bool insert(meta_any, meta_any);
+    inline bool erase(meta_any);
+    [[nodiscard]] inline iterator find(meta_any);
+    [[nodiscard]] inline explicit operator bool() const ENTT_NOEXCEPT;
+
+private:
+    bool key_only_container{};
+    meta_type(* key_type_fn)() ENTT_NOEXCEPT = nullptr;
+    meta_type(* mapped_type_fn)() ENTT_NOEXCEPT = nullptr;
+    meta_type(* value_type_fn)() ENTT_NOEXCEPT = nullptr;
+    size_type(* size_fn)(const any &) ENTT_NOEXCEPT = nullptr;
+    bool(* clear_fn)(any &) = nullptr;
+    iterator(* begin_fn)(any &) = nullptr;
+    iterator(* end_fn)(any &) = nullptr;
+    bool(* insert_fn)(any &, meta_any &, meta_any &) = nullptr;
+    bool(* erase_fn)(any &, meta_any &) = nullptr;
+    iterator(* find_fn)(any &, meta_any &) = nullptr;
+    any storage{};
+};
+
+
+/*! @brief Opaque wrapper for values of any type. */
+class meta_any {
+    enum class operation { DTOR, DEREF, SEQ, ASSOC };
+
+    using vtable_type = void(const operation, const any &, void *);
+
+    template<typename Type>
+    static void basic_vtable([[maybe_unused]] const operation op, [[maybe_unused]] const any &from, [[maybe_unused]] void *to) {
+        static_assert(std::is_same_v<std::remove_reference_t<std::remove_const_t<Type>>, Type>, "Invalid type");
+
+        if constexpr(!std::is_void_v<Type>) {
+            switch(op) {
+            case operation::DTOR:
+                if(auto *curr = static_cast<internal::meta_type_node *>(to); curr->dtor && from.owner()) {
+                    curr->dtor(const_cast<any &>(from).data());
+                }
+                break;
+            case operation::DEREF:
+                if constexpr(is_meta_pointer_like_v<Type>) {
+                    using element_type = std::remove_const_t<typename std::pointer_traits<Type>::element_type>;
+
+                    if constexpr(std::is_function_v<element_type>) {
+                        *static_cast<meta_any *>(to) = any_cast<Type>(from);
+                    } else if constexpr(!std::is_same_v<std::remove_const_t<typename std::pointer_traits<Type>::element_type>, void>) {
+                        using in_place_type = decltype(adl_meta_pointer_like<Type>::dereference(any_cast<const Type &>(from)));
+                        static_cast<meta_any *>(to)->emplace<in_place_type>(adl_meta_pointer_like<Type>::dereference(any_cast<const Type &>(from)));
+                    }
+                }
+                break;
+            case operation::SEQ:
+                if constexpr(is_complete_v<meta_sequence_container_traits<Type>>) {
+                    *static_cast<meta_sequence_container *>(to) = { std::in_place_type<Type>, std::move(const_cast<any &>(from)) };
+                }
+                break;
+            case operation::ASSOC:
+                if constexpr(is_complete_v<meta_associative_container_traits<Type>>) {
+                    *static_cast<meta_associative_container *>(to) = { std::in_place_type<Type>, std::move(const_cast<any &>(from)) };
+                }
+                break;
+            }
+        }
+    }
+
+    meta_any(const meta_any &other, any ref) ENTT_NOEXCEPT
+        : storage{std::move(ref)},
+          node{storage ? other.node : nullptr},
+          vtable{storage ? other.vtable : &basic_vtable<void>}
+    {}
+
+public:
+    /*! @brief Default constructor. */
+    meta_any() ENTT_NOEXCEPT
+        : storage{},
+          node{},
+          vtable{&basic_vtable<void>}
+    {}
+
+    /**
+     * @brief Constructs a wrapper by directly initializing the new object.
+     * @tparam Type Type of object to use to initialize the wrapper.
+     * @tparam Args Types of arguments to use to construct the new instance.
+     * @param args Parameters to use to construct the instance.
+     */
+    template<typename Type, typename... Args>
+    explicit meta_any(std::in_place_type_t<Type>, Args &&... args)
+        : storage{std::in_place_type<Type>, std::forward<Args>(args)...},
+          node{internal::meta_info<Type>::resolve()},
+          vtable{&basic_vtable<std::remove_const_t<std::remove_reference_t<Type>>>}
+    {}
+
+    /**
+     * @brief Constructs a wrapper that holds an unmanaged object.
+     * @tparam Type Type of object to use to initialize the wrapper.
+     * @param value An instance of an object to use to initialize the wrapper.
+     */
+    template<typename Type>
+    meta_any(std::reference_wrapper<Type> value)
+        : meta_any{}
+    {
+        // invokes deprecated assignment operator (and avoids issues with vs2017)
+        *this = value;
+    }
+
+    /**
+     * @brief Constructs a wrapper from a given value.
+     * @tparam Type Type of object to use to initialize the wrapper.
+     * @param value An instance of an object to use to initialize the wrapper.
+     */
+    template<typename Type, typename = std::enable_if_t<!std::is_same_v<std::decay_t<Type>, meta_any>>>
+    meta_any(Type &&value)
+        : storage{std::forward<Type>(value)},
+          node{internal::meta_info<std::decay_t<Type>>::resolve()},
+          vtable{&basic_vtable<std::decay_t<Type>>}
+    {}
+
+    /**
+     * @brief Copy constructor.
+     * @param other The instance to copy from.
+     */
+    meta_any(const meta_any &other) = default;
+
+    /**
+     * @brief Move constructor.
+     * @param other The instance to move from.
+     */
+    meta_any(meta_any &&other) ENTT_NOEXCEPT
+        : storage{std::move(other.storage)},
+          node{std::exchange(other.node, nullptr)},
+          vtable{std::exchange(other.vtable, &basic_vtable<void>)}
+    {}
+
+    /*! @brief Frees the internal storage, whatever it means. */
+    ~meta_any() {
+        vtable(operation::DTOR, storage, node);
+    }
+
+    /**
+     * @brief Copy assignment operator.
+     * @param other The instance to copy from.
+     * @return This meta any object.
+     */
+    meta_any & operator=(const meta_any &other) {
+        std::exchange(vtable, other.vtable)(operation::DTOR, storage, node);
+        storage = other.storage;
+        node = other.node;
+        return *this;
+    }
+
+    /**
+     * @brief Move assignment operator.
+     * @param other The instance to move from.
+     * @return This meta any object.
+     */
+    meta_any & operator=(meta_any &&other) ENTT_NOEXCEPT {
+        std::exchange(vtable, std::exchange(other.vtable, &basic_vtable<void>))(operation::DTOR, storage, node);
+        storage = std::move(other.storage);
+        node = std::exchange(other.node, nullptr);
+        return *this;
+    }
+
+    /**
+     * @brief Value assignment operator.
+     * @tparam Type Type of object to use to initialize the wrapper.
+     * @param value An instance of an object to use to initialize the wrapper.
+     * @return This meta any object.
+     */
+    template<typename Type>
+    [[deprecated("Use std::in_place_type<T &>, entt::make_meta<T &>, emplace<Type &> or forward_as_meta instead")]]
+    meta_any & operator=(std::reference_wrapper<Type> value) {
+        emplace<Type &>(value.get());
+        return *this;
+    }
+
+    /**
+     * @brief Value assignment operator.
+     * @tparam Type Type of object to use to initialize the wrapper.
+     * @param value An instance of an object to use to initialize the wrapper.
+     * @return This meta any object.
+     */
+    template<typename Type>
+    std::enable_if_t<!std::is_same_v<std::decay_t<Type>, meta_any>, meta_any &>
+    operator=(Type &&value) {
+        emplace<std::decay_t<Type>>(std::forward<Type>(value));
+        return *this;
+    }
+
+    /**
+     * @brief Returns the type of the underlying object.
+     * @return The type of the underlying object, if any.
+     */
+    [[nodiscard]] inline meta_type type() const ENTT_NOEXCEPT;
+
+    /**
+     * @brief Returns an opaque pointer to the contained instance.
+     * @return An opaque pointer the contained instance, if any.
+     */
+    [[nodiscard]] const void * data() const ENTT_NOEXCEPT {
+        return storage.data();
+    }
+
+    /*! @copydoc data */
+    [[nodiscard]] void * data() ENTT_NOEXCEPT {
+        return storage.data();
+    }
+
+    /**
+     * @brief Invokes the underlying function, if possible.
+     *
+     * @sa meta_func::invoke
+     *
+     * @tparam Args Types of arguments to use to invoke the function.
+     * @param id Unique identifier.
+     * @param args Parameters to use to invoke the function.
+     * @return A wrapper containing the returned value, if any.
+     */
+    template<typename... Args>
+    meta_any invoke(const id_type id, Args &&... args) const;
+
+    /*! @copydoc invoke */
+    template<typename... Args>
+    meta_any invoke(const id_type id, Args &&... args);
+
+    /**
+     * @brief Sets the value of a given variable.
+     *
+     * The type of the value must be such that a cast or conversion to the type
+     * of the variable is possible. Otherwise, invoking the setter does nothing.
+     *
+     * @tparam Type Type of value to assign.
+     * @param id Unique identifier.
+     * @param value Parameter to use to set the underlying variable.
+     * @return True in case of success, false otherwise.
+     */
+    template<typename Type>
+    bool set(const id_type id, Type &&value);
+
+    /**
+     * @brief Gets the value of a given variable.
+     * @param id Unique identifier.
+     * @return A wrapper containing the value of the underlying variable.
+     */
+    [[nodiscard]] meta_any get(const id_type id) const;
+
+    /*! @copydoc get */
+    [[nodiscard]] meta_any get(const id_type id);
+
+    /**
+     * @brief Tries to cast an instance to a given type.
+     * @tparam Type Type to which to cast the instance.
+     * @return A (possibly null) pointer to the contained instance.
+     */
+    template<typename Type>
+    [[nodiscard]] const Type * try_cast() const {
+        if(node) {
+            if(const auto info = type_id<Type>(); node->info == info) {
+                return any_cast<Type>(&storage);
+            } else if(const auto *base = internal::meta_visit<&internal::meta_type_node::base>([info](const auto *curr) { return curr->type()->info == info; }, node); base) {
+                return static_cast<const Type *>(base->cast(storage.data()));
+            }
+        }
+
+        return nullptr;
+    }
+
+    /*! @copydoc try_cast */
+    template<typename Type>
+    [[nodiscard]] Type * try_cast() {
+        if(node) {
+            if(const auto info = type_id<Type>(); node->info == info) {
+                return any_cast<Type>(&storage);
+            } else if(const auto *base = internal::meta_visit<&internal::meta_type_node::base>([info](const auto *curr) { return curr->type()->info == info; }, node); base) {
+                return static_cast<Type *>(const_cast<constness_as_t<void, Type> *>(base->cast(static_cast<constness_as_t<any, Type> &>(storage).data())));
+            }
+        }
+
+        return nullptr;
+    }
+
+    /**
+     * @brief Tries to cast an instance to a given type.
+     *
+     * The type of the instance must be such that the cast is possible.
+     *
+     * @warning
+     * Attempting to perform an invalid cast results in undefined behavior.
+     *
+     * @tparam Type Type to which to cast the instance.
+     * @return A reference to the contained instance.
+     */
+    template<typename Type>
+    [[nodiscard]] Type cast() const {
+        auto * const instance = try_cast<std::remove_reference_t<Type>>();
+        ENTT_ASSERT(instance, "Invalid instance");
+        return static_cast<Type>(*instance);
+    }
+
+    /*! @copydoc cast */
+    template<typename Type>
+    [[nodiscard]] Type cast() {
+        // forces const on non-reference types to make them work also with wrappers for const references
+        auto * const instance = try_cast<std::remove_reference_t<const Type>>();
+        ENTT_ASSERT(instance, "Invalid instance");
+        return static_cast<Type>(*instance);
+    }
+
+    /**
+     * @brief Converts an object in such a way that a given cast becomes viable.
+     * @tparam Type Type to which the cast is requested.
+     * @return A valid meta any object if there exists a viable conversion, an
+     * invalid one otherwise.
+     */
+    template<typename Type>
+    [[nodiscard]] meta_any allow_cast() const {
+        if(try_cast<std::remove_reference_t<Type>>() != nullptr) {
+            return as_ref();
+        } else if(node) {
+            if(const auto * const conv = internal::meta_visit<&internal::meta_type_node::conv>([info = type_id<Type>()](const auto *curr) { return curr->type()->info == info; }, node); conv) {
+                return conv->conv(storage.data());
+            }
+        }
+
+        return {};
+    }
+
+    /**
+     * @brief Converts an object in such a way that a given cast becomes viable.
+     * @tparam Type Type to which the cast is requested.
+     * @return True if there exists a viable conversion, false otherwise.
+     */
+    template<typename Type>
+    bool allow_cast() {
+        // forces const on non-reference types to make them work also with wrappers for const references
+        if(try_cast<std::remove_reference_t<const Type>>() != nullptr) {
+            return true;
+        } else if(node) {
+            if(const auto * const conv = internal::meta_visit<&internal::meta_type_node::conv>([info = type_id<Type>()](const auto *curr) { return curr->type()->info == info; }, node); conv) {
+                *this = conv->conv(std::as_const(storage).data());
+                return true;
+            }
+        }
+
+        return false;
+    }
+
+    /**
+     * @brief Replaces the contained object by creating a new instance directly.
+     * @tparam Type Type of object to use to initialize the wrapper.
+     * @tparam Args Types of arguments to use to construct the new instance.
+     * @param args Parameters to use to construct the instance.
+     */
+    template<typename Type, typename... Args>
+    void emplace(Args &&... args) {
+        std::exchange(vtable, &basic_vtable<std::remove_const_t<std::remove_reference_t<Type>>>)(operation::DTOR, storage, node);
+        storage.emplace<Type>(std::forward<Args>(args)...);
+        node = internal::meta_info<Type>::resolve();
+    }
+
+    /*! @brief Destroys contained object */
+    void reset() {
+        std::exchange(vtable, &basic_vtable<void>)(operation::DTOR, storage, node);
+        storage.reset();
+        node = nullptr;
+    }
+
+    /**
+     * @brief Returns a sequence container proxy.
+     * @return A sequence container proxy for the underlying object.
+     */
+    [[nodiscard]] meta_sequence_container as_sequence_container() ENTT_NOEXCEPT {
+        meta_sequence_container proxy;
+        vtable(operation::SEQ, storage.as_ref(), &proxy);
+        return proxy;
+    }
+
+    /*! @copydoc as_sequence_container */
+    [[nodiscard]] meta_sequence_container as_sequence_container() const ENTT_NOEXCEPT {
+        meta_sequence_container proxy;
+        vtable(operation::SEQ, storage.as_ref(), &proxy);
+        return proxy;
+    }
+
+    /**
+     * @brief Returns an associative container proxy.
+     * @return An associative container proxy for the underlying object.
+     */
+    [[nodiscard]] meta_associative_container as_associative_container() ENTT_NOEXCEPT {
+        meta_associative_container proxy;
+        vtable(operation::ASSOC, storage.as_ref(), &proxy);
+        return proxy;
+    }
+
+    /*! @copydoc as_associative_container */
+    [[nodiscard]] meta_associative_container as_associative_container() const ENTT_NOEXCEPT {
+        meta_associative_container proxy;
+        vtable(operation::ASSOC, storage.as_ref(), &proxy);
+        return proxy;
+    }
+
+    /**
+     * @brief Indirection operator for dereferencing opaque objects.
+     * @return A wrapper that shares a reference to an unmanaged object if the
+     * wrapped element is dereferenceable, an invalid meta any otherwise.
+     */
+    [[nodiscard]] meta_any operator*() const ENTT_NOEXCEPT {
+        meta_any ret{};
+        vtable(operation::DEREF, storage, &ret);
+        return ret;
+    }
+
+    /**
+     * @brief Returns false if a wrapper is invalid, true otherwise.
+     * @return False if the wrapper is invalid, true otherwise.
+     */
+    [[nodiscard]] explicit operator bool() const ENTT_NOEXCEPT {
+        return !(node == nullptr);
+    }
+
+    /**
+     * @brief Checks if two wrappers differ in their content.
+     * @param other Wrapper with which to compare.
+     * @return False if the two objects differ in their content, true otherwise.
+     */
+    [[nodiscard]] bool operator==(const meta_any &other) const {
+        return (!node && !other.node) || (node && other.node && node->info == other.node->info && storage == other.storage);
+    }
+
+    /**
+     * @brief Aliasing constructor.
+     * @return A wrapper that shares a reference to an unmanaged object.
+     */
+    [[nodiscard]] meta_any as_ref() ENTT_NOEXCEPT {
+        return meta_any{*this, storage.as_ref()};
+    }
+
+    /*! @copydoc as_ref */
+    [[nodiscard]] meta_any as_ref() const ENTT_NOEXCEPT {
+        return meta_any{*this, storage.as_ref()};
+    }
+
+private:
+    any storage;
+    internal::meta_type_node *node;
+    vtable_type *vtable;
+};
+
+
+/**
+ * @brief Checks if two wrappers differ in their content.
+ * @param lhs A wrapper, either empty or not.
+ * @param rhs A wrapper, either empty or not.
+ * @return True if the two wrappers differ in their content, false otherwise.
+ */
+[[nodiscard]] inline bool operator!=(const meta_any &lhs, const meta_any &rhs) ENTT_NOEXCEPT {
+    return !(lhs == rhs);
+}
+
+
+/**
+ * @brief Constructs a wrapper from a given type, passing it all arguments.
+ * @tparam Type Type of object to use to initialize the wrapper.
+ * @tparam Args Types of arguments to use to construct the new instance.
+ * @param args Parameters to use to construct the instance.
+ * @return A properly initialized wrapper for an object of the given type.
+ */
+template<typename Type, typename... Args>
+meta_any make_meta(Args &&... args) {
+    return meta_any{std::in_place_type<Type>, std::forward<Args>(args)...};
+}
+
+
+/**
+ * @brief Forwards its argument and avoids copies for lvalue references.
+ * @tparam Type Type of argument to use to construct the new instance.
+ * @param value Parameter to use to construct the instance.
+ * @return A properly initialized and not necessarily owning wrapper.
+ */
+template<typename Type>
+meta_any forward_as_meta(Type &&value) {
+    return meta_any{std::in_place_type<std::conditional_t<std::is_rvalue_reference_v<Type>, std::decay_t<Type>, Type>>, std::forward<Type>(value)};
+}
+
+
+/**
+ * @brief Opaque pointers to instances of any type.
+ *
+ * A handle doesn't perform copies and isn't responsible for the contained
+ * object. It doesn't prolong the lifetime of the pointed instance.<br/>
+ * Handles are used to generate references to actual objects when needed.
+ */
+struct meta_handle {
+    /*! @brief Default constructor. */
+    meta_handle() = default;
+
+
+    /*! @brief Default copy constructor, deleted on purpose. */
+    meta_handle(const meta_handle &) = delete;
+
+    /*! @brief Default move constructor. */
+    meta_handle(meta_handle &&) = default;
+
+    /**
+     * @brief Default copy assignment operator, deleted on purpose.
+     * @return This meta handle.
+     */
+    meta_handle & operator=(const meta_handle &) = delete;
+
+    /**
+     * @brief Default move assignment operator.
+     * @return This meta handle.
+     */
+    meta_handle & operator=(meta_handle &&) = default;
+
+    /**
+     * @brief Creates a handle that points to an unmanaged object.
+     * @tparam Type Type of object to use to initialize the handle.
+     * @param value An instance of an object to use to initialize the handle.
+     */
+    template<typename Type, typename = std::enable_if_t<!std::is_same_v<std::decay_t<Type>, meta_handle>>>
+    meta_handle(Type &value) ENTT_NOEXCEPT
+        : meta_handle{}
+    {
+        if constexpr(std::is_same_v<std::decay_t<Type>, meta_any>) {
+            any = value.as_ref();
+        } else {
+            any.emplace<Type &>(value);
+        }
+    }
+
+    /**
+     * @brief Returns false if a handle is invalid, true otherwise.
+     * @return False if the handle is invalid, true otherwise.
+     */
+    [[nodiscard]] explicit operator bool() const ENTT_NOEXCEPT {
+        return static_cast<bool>(any);
+    }
+
+    /**
+     * @brief Access operator for accessing the contained opaque object.
+     * @return A wrapper that shares a reference to an unmanaged object.
+     */
+    [[nodiscard]] meta_any * operator->() {
+        return &any;
+    }
+
+    /*! @copydoc operator-> */
+    [[nodiscard]] const meta_any * operator->() const {
+        return &any;
+    }
+
+private:
+    meta_any any;
+};
+
+
+/*! @brief Opaque wrapper for properties of any type. */
+struct meta_prop {
+    /*! @brief Node type. */
+    using node_type = internal::meta_prop_node;
+
+    /**
+     * @brief Constructs an instance from a given node.
+     * @param curr The underlying node with which to construct the instance.
+     */
+    meta_prop(const node_type *curr = nullptr) ENTT_NOEXCEPT
+        : node{curr}
+    {}
+
+    /**
+     * @brief Returns the stored key as a const reference.
+     * @return A wrapper containing the key stored with the property.
+     */
+    [[nodiscard]] meta_any key() const {
+        return node->id.as_ref();
+    }
+
+    /**
+     * @brief Returns the stored value by copy.
+     * @return A wrapper containing the value stored with the property.
+     */
+    [[nodiscard]] meta_any value() const {
+        return node->value;
+    }
+
+    /**
+     * @brief Returns true if an object is valid, false otherwise.
+     * @return True if the object is valid, false otherwise.
+     */
+    [[nodiscard]] explicit operator bool() const ENTT_NOEXCEPT {
+        return !(node == nullptr);
+    }
+
+private:
+    const node_type *node;
+};
+
+
+/*! @brief Opaque wrapper for constructors. */
+struct meta_ctor {
+    /*! @brief Node type. */
+    using node_type = internal::meta_ctor_node;
+    /*! @brief Unsigned integer type. */
+    using size_type = typename node_type::size_type;
+
+    /*! @copydoc meta_prop::meta_prop */
+    meta_ctor(const node_type *curr = nullptr) ENTT_NOEXCEPT
+        : node{curr}
+    {}
+
+    /**
+     * @brief Returns the type to which an object belongs.
+     * @return The type to which the object belongs.
+     */
+    [[nodiscard]] inline meta_type parent() const ENTT_NOEXCEPT;
+
+    /**
+     * @brief Returns the number of arguments accepted by a constructor.
+     * @return The number of arguments accepted by the constructor.
+     */
+    [[nodiscard]] size_type arity() const ENTT_NOEXCEPT {
+        return node->arity;
+    }
+
+    /**
+     * @brief Returns the type of the i-th argument of a constructor.
+     * @param index Index of the argument of which to return the type.
+     * @return The type of the i-th argument of a constructor.
+     */
+    [[nodiscard]] meta_type arg(size_type index) const ENTT_NOEXCEPT;
+
+    /**
+     * @brief Creates an instance of the underlying type, if possible.
+     *
+     * Parameters must be such that a cast or conversion to the required types
+     * is possible. Otherwise, an empty and thus invalid wrapper is returned.
+     *
+     * @param args Parameters to use to construct the instance.
+     * @param sz Number of parameters to use to construct the instance.
+     * @return A wrapper containing the new instance, if any.
+     */
+    [[nodiscard]] meta_any invoke(meta_any * const args, const size_type sz) const {
+        return sz == arity() ? node->invoke(args) : meta_any{};
+    }
+
+    /**
+     * @copybrief invoke
+     *
+     * @sa invoke
+     *
+     * @tparam Args Types of arguments to use to construct the instance.
+     * @param args Parameters to use to construct the instance.
+     * @return A wrapper containing the new instance, if any.
+     */
+    template<typename... Args>
+    [[nodiscard]] meta_any invoke([[maybe_unused]] Args &&... args) const {
+        meta_any arguments[sizeof...(Args) + 1u]{std::forward<Args>(args)...};
+        return invoke(arguments, sizeof...(Args));
+    }
+
+    /**
+     * @brief Returns a range to use to visit all properties.
+     * @return An iterable range to use to visit all properties.
+     */
+    [[nodiscard]] meta_range<meta_prop> prop() const ENTT_NOEXCEPT {
+        return node->prop;
+    }
+
+    /**
+     * @brief Returns the property associated with a given key.
+     * @param key The key to use to search for a property.
+     * @return The property associated with the given key, if any.
+     */
+    [[nodiscard]] meta_prop prop(meta_any key) const {
+        return internal::meta_visit<&node_type::prop>([&key](const auto *curr) { return curr->id == key; }, node);
+    }
+
+    /**
+     * @brief Returns true if an object is valid, false otherwise.
+     * @return True if the object is valid, false otherwise.
+     */
+    [[nodiscard]] explicit operator bool() const ENTT_NOEXCEPT {
+        return !(node == nullptr);
+    }
+
+private:
+    const node_type *node;
+};
+
+
+/*! @brief Opaque wrapper for data members. */
+struct meta_data {
+    /*! @brief Node type. */
+    using node_type = internal::meta_data_node;
+
+    /*! @copydoc meta_prop::meta_prop */
+    meta_data(const node_type *curr = nullptr) ENTT_NOEXCEPT
+        : node{curr}
+    {}
+
+    /*! @copydoc meta_type::id */
+    [[nodiscard]] id_type id() const ENTT_NOEXCEPT {
+        return node->id;
+    }
+
+    /*! @copydoc meta_ctor::parent */
+    [[nodiscard]] inline meta_type parent() const ENTT_NOEXCEPT;
+
+    /**
+     * @brief Indicates whether a data member is constant or not.
+     * @return True if the data member is constant, false otherwise.
+     */
+    [[nodiscard]] bool is_const() const ENTT_NOEXCEPT {
+        return node->is_const;
+    }
+
+    /**
+     * @brief Indicates whether a data member is static or not.
+     * @return True if the data member is static, false otherwise.
+     */
+    [[nodiscard]] bool is_static() const ENTT_NOEXCEPT {
+        return node->is_static;
+    }
+
+    /*! @copydoc meta_any::type */
+    [[nodiscard]] inline meta_type type() const ENTT_NOEXCEPT;
+
+    /**
+     * @brief Sets the value of a given variable.
+     *
+     * It must be possible to cast the instance to the parent type of the data
+     * member. Otherwise, invoking the setter results in an undefined
+     * behavior.<br/>
+     * The type of the value must be such that a cast or conversion to the type
+     * of the variable is possible. Otherwise, invoking the setter does nothing.
+     *
+     * @tparam Type Type of value to assign.
+     * @param instance An opaque instance of the underlying type.
+     * @param value Parameter to use to set the underlying variable.
+     * @return True in case of success, false otherwise.
+     */
+    template<typename Type>
+    bool set(meta_handle instance, Type &&value) const {
+        return node->set && node->set(std::move(instance), std::forward<Type>(value));
+    }
+
+    /**
+     * @brief Gets the value of a given variable.
+     *
+     * It must be possible to cast the instance to the parent type of the data
+     * member. Otherwise, invoking the getter results in an undefined behavior.
+     *
+     * @param instance An opaque instance of the underlying type.
+     * @return A wrapper containing the value of the underlying variable.
+     */
+    [[nodiscard]] meta_any get(meta_handle instance) const {
+        return node->get(std::move(instance));
+    }
+
+    /*! @copydoc meta_ctor::prop */
+    [[nodiscard]] meta_range<meta_prop> prop() const ENTT_NOEXCEPT {
+        return node->prop;
+    }
+
+    /**
+     * @brief Returns the property associated with a given key.
+     * @param key The key to use to search for a property.
+     * @return The property associated with the given key, if any.
+     */
+    [[nodiscard]] meta_prop prop(meta_any key) const {
+        return internal::meta_visit<&node_type::prop>([&key](const auto *curr) { return curr->id == key; }, node);
+    }
+
+    /**
+     * @brief Returns true if an object is valid, false otherwise.
+     * @return True if the object is valid, false otherwise.
+     */
+    [[nodiscard]] explicit operator bool() const ENTT_NOEXCEPT {
+        return !(node == nullptr);
+    }
+
+private:
+    const node_type *node;
+};
+
+
+/*! @brief Opaque wrapper for member functions. */
+struct meta_func {
+    /*! @brief Node type. */
+    using node_type = internal::meta_func_node;
+    /*! @brief Unsigned integer type. */
+    using size_type = typename node_type::size_type;
+
+    /*! @copydoc meta_prop::meta_prop */
+    meta_func(const node_type *curr = nullptr) ENTT_NOEXCEPT
+        : node{curr}
+    {}
+
+    /*! @copydoc meta_type::id */
+    [[nodiscard]] id_type id() const ENTT_NOEXCEPT {
+        return node->id;
+    }
+
+    /*! @copydoc meta_ctor::parent */
+    [[nodiscard]] inline meta_type parent() const ENTT_NOEXCEPT;
+
+    /**
+     * @brief Returns the number of arguments accepted by a member function.
+     * @return The number of arguments accepted by the member function.
+     */
+    [[nodiscard]] size_type arity() const ENTT_NOEXCEPT {
+        return node->arity;
+    }
+
+    /**
+     * @brief Indicates whether a member function is constant or not.
+     * @return True if the member function is constant, false otherwise.
+     */
+    [[nodiscard]] bool is_const() const ENTT_NOEXCEPT {
+        return node->is_const;
+    }
+
+    /**
+     * @brief Indicates whether a member function is static or not.
+     * @return True if the member function is static, false otherwise.
+     */
+    [[nodiscard]] bool is_static() const ENTT_NOEXCEPT {
+        return node->is_static;
+    }
+
+    /**
+     * @brief Returns the return type of a member function.
+     * @return The return type of the member function.
+     */
+    [[nodiscard]] inline meta_type ret() const ENTT_NOEXCEPT;
+
+    /**
+     * @brief Returns the type of the i-th argument of a member function.
+     * @param index Index of the argument of which to return the type.
+     * @return The type of the i-th argument of a member function.
+     */
+    [[nodiscard]] inline meta_type arg(size_type index) const ENTT_NOEXCEPT;
+
+    /**
+     * @brief Invokes the underlying function, if possible.
+     *
+     * To invoke a member function, the parameters must be such that a cast or
+     * conversion to the required types is possible. Otherwise, an empty and
+     * thus invalid wrapper is returned.<br/>
+     * It must be possible to cast the instance to the parent type of the member
+     * function. Otherwise, invoking the underlying function results in an
+     * undefined behavior.
+     *
+     * @param instance An opaque instance of the underlying type.
+     * @param args Parameters to use to invoke the function.
+     * @param sz Number of parameters to use to invoke the function.
+     * @return A wrapper containing the returned value, if any.
+     */
+    meta_any invoke(meta_handle instance, meta_any * const args, const size_type sz) const {
+        return sz == arity() ? node->invoke(std::move(instance), args) : meta_any{};
+    }
+
+    /**
+     * @copybrief invoke
+     *
+     * @sa invoke
+     *
+     * @tparam Args Types of arguments to use to invoke the function.
+     * @param instance An opaque instance of the underlying type.
+     * @param args Parameters to use to invoke the function.
+     * @return A wrapper containing the new instance, if any.
+     */
+    template<typename... Args>
+    meta_any invoke(meta_handle instance, Args &&... args) const {
+        meta_any arguments[sizeof...(Args) + 1u]{std::forward<Args>(args)...};
+        return invoke(std::move(instance), arguments, sizeof...(Args));
+    }
+
+    /*! @copydoc meta_ctor::prop */
+    [[nodiscard]] meta_range<meta_prop> prop() const ENTT_NOEXCEPT {
+        return node->prop;
+    }
+
+    /**
+     * @brief Returns the property associated with a given key.
+     * @param key The key to use to search for a property.
+     * @return The property associated with the given key, if any.
+     */
+    [[nodiscard]] meta_prop prop(meta_any key) const {
+        return internal::meta_visit<&node_type::prop>([&key](const auto *curr) { return curr->id == key; }, node);
+    }
+
+    /**
+     * @brief Returns true if an object is valid, false otherwise.
+     * @return True if the object is valid, false otherwise.
+     */
+    [[nodiscard]] explicit operator bool() const ENTT_NOEXCEPT {
+        return !(node == nullptr);
+    }
+
+private:
+    const node_type *node;
+};
+
+
+/*! @brief Opaque wrapper for types. */
+class meta_type {
+    static bool can_cast_or_convert(const internal::meta_type_node *type, const type_info info) ENTT_NOEXCEPT {
+        if(type->info == info) {
+            return true;
+        }
+
+        for(const auto *curr = type->conv; curr; curr = curr->next) {
+            if(curr->type()->info == info) {
+                return true;
+            }
+        }
+
+        for(const auto *curr = type->base; curr; curr = curr->next) {
+            if(auto *target = curr->type(); can_cast_or_convert(target, info)) {
+                return true;
+            }
+        }
+
+        return false;
+    }
+
+    template<typename... Args, auto... Index>
+    [[nodiscard]] static const internal::meta_ctor_node * ctor(const internal::meta_ctor_node *curr, std::index_sequence<Index...>) {
+        for(; curr; curr = curr->next) {
+            if(curr->arity == sizeof...(Args) && (can_cast_or_convert(internal::meta_info<Args>::resolve(), curr->arg(Index).info()) && ...)) {
+                return curr;
+            }
+        }
+
+        return nullptr;
+    }
+
+    template<auto... Member, typename Node>
+    void unregister_all(Node **curr) {
+        while(*curr) {
+            (unregister_all(&((*curr)->*Member)), ...);
+            *curr = std::exchange((*curr)->next, nullptr);
+        }
+    }
+
+public:
+    /*! @brief Node type. */
+    using node_type = internal::meta_type_node;
+    /*! @brief Node type. */
+    using base_node_type = internal::meta_base_node;
+    /*! @brief Unsigned integer type. */
+    using size_type = typename node_type::size_type;
+
+    /*! @copydoc meta_prop::meta_prop */
+    meta_type(node_type *curr = nullptr) ENTT_NOEXCEPT
+        : node{curr}
+    {}
+
+    /**
+     * @brief Constructs an instance from a given base node.
+     * @param curr The base node with which to construct the instance.
+     */
+    meta_type(base_node_type *curr) ENTT_NOEXCEPT
+        : node{curr ? curr->type() : nullptr}
+    {}
+
+    /**
+     * @brief Returns the type info object of the underlying type.
+     * @return The type info object of the underlying type.
+     */
+    [[nodiscard]] type_info info() const ENTT_NOEXCEPT {
+        return node->info;
+    }
+
+    /**
+     * @brief Returns the identifier assigned to a type.
+     * @return The identifier assigned to the type.
+     */
+    [[nodiscard]] id_type id() const ENTT_NOEXCEPT {
+        return node->id;
+    }
+
+    /**
+     * @brief Returns the size of the underlying type if known.
+     * @return The size of the underlying type if known, 0 otherwise.
+     */
+    [[nodiscard]] size_type size_of() const ENTT_NOEXCEPT {
+        return node->size_of;
+    }
+
+    /**
+     * @brief Checks whether a type refers to void or not.
+     * @return True if the underlying type is void, false otherwise.
+     */
+    [[nodiscard]] bool is_void() const ENTT_NOEXCEPT {
+        return node->is_void;
+    }
+
+    /**
+     * @brief Checks whether a type refers to an integral type or not.
+     * @return True if the underlying type is an integral type, false otherwise.
+     */
+    [[nodiscard]] bool is_integral() const ENTT_NOEXCEPT {
+        return node->is_integral;
+    }
+
+    /**
+     * @brief Checks whether a type refers to a floating-point type or not.
+     * @return True if the underlying type is a floating-point type, false
+     * otherwise.
+     */
+    [[nodiscard]] bool is_floating_point() const ENTT_NOEXCEPT {
+        return node->is_floating_point;
+    }
+
+    /**
+     * @brief Checks whether a type refers to an array type or not.
+     * @return True if the underlying type is an array type, false otherwise.
+     */
+    [[nodiscard]] bool is_array() const ENTT_NOEXCEPT {
+        return node->is_array;
+    }
+
+    /**
+     * @brief Checks whether a type refers to an enum or not.
+     * @return True if the underlying type is an enum, false otherwise.
+     */
+    [[nodiscard]] bool is_enum() const ENTT_NOEXCEPT {
+        return node->is_enum;
+    }
+
+    /**
+     * @brief Checks whether a type refers to an union or not.
+     * @return True if the underlying type is an union, false otherwise.
+     */
+    [[nodiscard]] bool is_union() const ENTT_NOEXCEPT {
+        return node->is_union;
+    }
+
+    /**
+     * @brief Checks whether a type refers to a class or not.
+     * @return True if the underlying type is a class, false otherwise.
+     */
+    [[nodiscard]] bool is_class() const ENTT_NOEXCEPT {
+        return node->is_class;
+    }
+
+    /**
+     * @brief Checks whether a type refers to a pointer or not.
+     * @return True if the underlying type is a pointer, false otherwise.
+     */
+    [[nodiscard]] bool is_pointer() const ENTT_NOEXCEPT {
+        return node->is_pointer;
+    }
+
+    /**
+     * @brief Checks whether a type refers to a function pointer or not.
+     * @return True if the underlying type is a function pointer, false
+     * otherwise.
+     */
+    [[nodiscard]] bool is_function_pointer() const ENTT_NOEXCEPT {
+        return node->is_function_pointer;
+    }
+
+    /**
+     * @brief Checks whether a type refers to a pointer to data member or not.
+     * @return True if the underlying type is a pointer to data member, false
+     * otherwise.
+     */
+    [[nodiscard]] bool is_member_object_pointer() const ENTT_NOEXCEPT {
+        return node->is_member_object_pointer;
+    }
+
+    /**
+     * @brief Checks whether a type refers to a pointer to member function or
+     * not.
+     * @return True if the underlying type is a pointer to member function,
+     * false otherwise.
+     */
+    [[nodiscard]] bool is_member_function_pointer() const ENTT_NOEXCEPT {
+        return node->is_member_function_pointer;
+    }
+
+    /**
+     * @brief Checks whether a type is a pointer-like type or not.
+     * @return True if the underlying type is a pointer-like one, false
+     * otherwise.
+     */
+    [[nodiscard]] bool is_pointer_like() const ENTT_NOEXCEPT {
+        return node->is_pointer_like;
+    }
+
+    /**
+     * @brief Checks whether a type refers to a sequence container or not.
+     * @return True if the type is a sequence container, false otherwise.
+     */
+    [[nodiscard]] bool is_sequence_container() const ENTT_NOEXCEPT {
+        return node->is_sequence_container;
+    }
+
+    /**
+     * @brief Checks whether a type refers to an associative container or not.
+     * @return True if the type is an associative container, false otherwise.
+     */
+    [[nodiscard]] bool is_associative_container() const ENTT_NOEXCEPT {
+        return node->is_associative_container;
+    }
+
+    /**
+     * @brief Checks whether a type refers to a recognized class template
+     * specialization or not.
+     * @return True if the type is a recognized class template specialization,
+     * false otherwise.
+     */
+    [[nodiscard]] bool is_template_specialization() const ENTT_NOEXCEPT {
+        return node->template_info.is_template_specialization;
+    }
+
+    /**
+     * @brief Returns the number of template arguments, if any.
+     * @return The number of template arguments, if any.
+     */
+    [[nodiscard]] size_type template_arity() const ENTT_NOEXCEPT {
+        return node->template_info.arity;
+    }
+
+    /**
+     * @brief Returns a tag for the class template of the underlying type.
+     *
+     * @sa meta_class_template_tag
+     *
+     * @return The tag for the class template of the underlying type.
+     */
+    [[nodiscard]] inline meta_type template_type() const ENTT_NOEXCEPT {
+        return is_template_specialization() ?  node->template_info.type() : meta_type{};
+    }
+
+    /**
+     * @brief Returns the type of the i-th template argument of a type.
+     * @param index Index of the template argument of which to return the type.
+     * @return The type of the i-th template argument of a type.
+     */
+    [[nodiscard]] inline meta_type template_arg(size_type index) const ENTT_NOEXCEPT {
+        return index < template_arity() ? node->template_info.arg(index) : meta_type{};
+    }
+
+    /**
+     * @brief Provides the number of dimensions of an array type.
+     * @return The number of dimensions in case of array types, 0 otherwise.
+     */
+    [[nodiscard]] size_type rank() const ENTT_NOEXCEPT {
+        return node->rank;
+    }
+
+    /**
+     * @brief The number of elements along the given dimension of an array type.
+     * @param dim The dimension of which to return the number of elements.
+     * @return The number of elements along the given dimension in case of array
+     * types, 0 otherwise.
+     */
+    [[nodiscard]] size_type extent(size_type dim = {}) const ENTT_NOEXCEPT {
+        return node->extent(dim);
+    }
+
+    /**
+     * @brief Provides the type for which the pointer is defined.
+     * @return The type for which the pointer is defined or this type if it
+     * doesn't refer to a pointer type.
+     */
+    [[nodiscard]] meta_type remove_pointer() const ENTT_NOEXCEPT {
+        return node->remove_pointer();
+    }
+
+    /**
+     * @brief Provides the type for which the array is defined.
+     * @return The type for which the array is defined or this type if it
+     * doesn't refer to an array type.
+     */
+    [[nodiscard]] meta_type remove_extent() const ENTT_NOEXCEPT {
+        return node->remove_extent();
+    }
+
+    /**
+     * @brief Returns a range to use to visit top-level base meta types.
+     * @return An iterable range to use to visit top-level base meta types.
+     */
+    [[nodiscard]] meta_range<meta_type, internal::meta_base_node> base() const ENTT_NOEXCEPT {
+        return node->base;
+    }
+
+    /**
+     * @brief Returns the base meta type associated with a given identifier.
+     * @param id Unique identifier.
+     * @return The base meta type associated with the given identifier, if any.
+     */
+    [[nodiscard]] meta_type base(const id_type id) const {
+        return internal::meta_visit<&node_type::base>([id](const auto *curr) { return curr->type()->id == id; }, node);
+    }
+
+    /**
+     * @brief Returns a range to use to visit top-level constructors.
+     * @return An iterable range to use to visit top-level constructors.
+     */
+    [[nodiscard]] meta_range<meta_ctor> ctor() const ENTT_NOEXCEPT {
+        return node->ctor;
+    }
+
+    /**
+     * @brief Returns a constructor for a given list of types of arguments.
+     * @tparam Args Constructor arguments.
+     * @return The requested constructor, if any.
+     */
+    template<typename... Args>
+    [[nodiscard]] meta_ctor ctor() const {
+        return ctor<Args...>(node->ctor, std::make_index_sequence<sizeof...(Args)>{});
+    }
+
+    /**
+     * @brief Returns a range to use to visit top-level data.
+     * @return An iterable range to use to visit top-level data.
+     */
+    [[nodiscard]] meta_range<meta_data> data() const ENTT_NOEXCEPT {
+        return node->data;
+    }
+
+    /**
+     * @brief Returns the data associated with a given identifier.
+     *
+     * The data of the base classes will also be visited, if any.
+     *
+     * @param id Unique identifier.
+     * @return The data associated with the given identifier, if any.
+     */
+    [[nodiscard]] meta_data data(const id_type id) const {
+        return internal::meta_visit<&node_type::data>([id](const auto *curr) { return curr->id == id; }, node);
+    }
+
+    /**
+     * @brief Returns a range to use to visit top-level functions.
+     * @return An iterable range to use to visit top-level functions.
+     */
+    [[nodiscard]] meta_range<meta_func> func() const ENTT_NOEXCEPT {
+        return node->func;
+    }
+
+    /**
+     * @brief Returns the function associated with a given identifier.
+     *
+     * The functions of the base classes will also be visited, if any.<br/>
+     * In the case of overloaded functions, the first one with the required
+     * identifier will be returned.
+     *
+     * @param id Unique identifier.
+     * @return The function associated with the given identifier, if any.
+     */
+    [[nodiscard]] meta_func func(const id_type id) const {
+        return internal::meta_visit<&node_type::func>([id](const auto *curr) { return curr->id == id; }, node);
+    }
+
+    /**
+     * @brief Creates an instance of the underlying type, if possible.
+     *
+     * Parameters must be such that a cast or conversion to the required types
+     * is possible. Otherwise, an empty and thus invalid wrapper is returned.
+     *
+     * @param args Parameters to use to construct the instance.
+     * @param sz Number of parameters to use to construct the instance.
+     * @return A wrapper containing the new instance, if any.
+     */
+    [[nodiscard]] meta_any construct(meta_any * const args, const size_type sz) const {
+        meta_any ret{};
+        internal::meta_visit<&node_type::ctor>([args, sz, &ret](const auto *curr) { return (curr->arity == sz) && (ret = curr->invoke(args)); }, node);
+        return ret;
+    }
+
+    /**
+     * @copybrief construct
+     *
+     * @sa construct
+     *
+     * @tparam Args Types of arguments to use to construct the instance.
+     * @param args Parameters to use to construct the instance.
+     * @return A wrapper containing the new instance, if any.
+     */
+    template<typename... Args>
+    [[nodiscard]] meta_any construct(Args &&... args) const {
+        meta_any arguments[sizeof...(Args) + 1u]{std::forward<Args>(args)...};
+        return construct(arguments, sizeof...(Args));
+    }
+
+    /**
+     * @brief Invokes a function given an identifier, if possible.
+     *
+     * It must be possible to cast the instance to the parent type of the member
+     * function. Otherwise, invoking the underlying function results in an
+     * undefined behavior.
+     *
+     * @sa meta_func::invoke
+     *
+     * @param id Unique identifier.
+     * @param instance An opaque instance of the underlying type.
+     * @param args Parameters to use to invoke the function.
+     * @param sz Number of parameters to use to invoke the function.
+     * @return A wrapper containing the returned value, if any.
+     */
+    meta_any invoke(const id_type id, meta_handle instance, meta_any * const args, const size_type sz) const {
+        const internal::meta_func_node* candidate{};
+        size_type extent{sz + 1u};
+        bool ambiguous{};
+
+        for(auto *it = internal::meta_visit<&node_type::func>([id, sz](const auto *curr) { return curr->id == id && curr->arity == sz; }, node); it && it->id == id && it->arity == sz; it = it->next) {
+            size_type direct{};
+            size_type ext{};
+
+            for(size_type next{}; next < sz && next == (direct + ext); ++next) {
+                const auto type = args[next].type();
+                const auto req = it->arg(next).info();
+                type.info() == req ? ++direct : (ext += can_cast_or_convert(type.node, req));
+            }
+
+            if((direct + ext) == sz) {
+                if(ext < extent) {
+                    candidate = it;
+                    extent = ext;
+                    ambiguous = false;
+                } else if(ext == extent) {
+                    ambiguous = true;
+                }
+            }
+        }
+
+        return (candidate && !ambiguous) ? candidate->invoke(std::move(instance), args) : meta_any{};
+    }
+
+    /**
+     * @copybrief invoke
+     *
+     * @sa invoke
+     *
+     * @param id Unique identifier.
+     * @tparam Args Types of arguments to use to invoke the function.
+     * @param instance An opaque instance of the underlying type.
+     * @param args Parameters to use to invoke the function.
+     * @return A wrapper containing the new instance, if any.
+     */
+    template<typename... Args>
+    meta_any invoke(const id_type id, meta_handle instance, Args &&... args) const {
+        meta_any arguments[sizeof...(Args) + 1u]{std::forward<Args>(args)...};
+        return invoke(id, std::move(instance), arguments, sizeof...(Args));
+    }
+
+    /**
+     * @brief Sets the value of a given variable.
+     *
+     * It must be possible to cast the instance to the parent type of the data
+     * member. Otherwise, invoking the setter results in an undefined
+     * behavior.<br/>
+     * The type of the value must be such that a cast or conversion to the type
+     * of the variable is possible. Otherwise, invoking the setter does nothing.
+     *
+     * @tparam Type Type of value to assign.
+     * @param id Unique identifier.
+     * @param instance An opaque instance of the underlying type.
+     * @param value Parameter to use to set the underlying variable.
+     * @return True in case of success, false otherwise.
+     */
+    template<typename Type>
+    bool set(const id_type id, meta_handle instance, Type &&value) const {
+        const auto candidate = data(id);
+        return candidate && candidate.set(std::move(instance), std::forward<Type>(value));
+    }
+
+    /**
+     * @brief Gets the value of a given variable.
+     *
+     * It must be possible to cast the instance to the parent type of the data
+     * member. Otherwise, invoking the getter results in an undefined behavior.
+     *
+     * @param id Unique identifier.
+     * @param instance An opaque instance of the underlying type.
+     * @return A wrapper containing the value of the underlying variable.
+     */
+    [[nodiscard]] meta_any get(const id_type id, meta_handle instance) const {
+        const auto candidate = data(id);
+        return candidate ? candidate.get(std::move(instance)) : meta_any{};
+    }
+
+    /**
+     * @brief Returns a range to use to visit top-level properties.
+     * @return An iterable range to use to visit top-level properties.
+     */
+    [[nodiscard]] meta_range<meta_prop> prop() const ENTT_NOEXCEPT {
+        return node->prop;
+    }
+
+    /**
+     * @brief Returns the property associated with a given key.
+     *
+     * Properties of the base classes will also be visited, if any.
+     *
+     * @param key The key to use to search for a property.
+     * @return The property associated with the given key, if any.
+     */
+    [[nodiscard]] meta_prop prop(meta_any key) const {
+        return internal::meta_visit<&node_type::prop>([&key](const auto *curr) { return curr->id == key; }, node);
+    }
+
+    /**
+     * @brief Returns true if an object is valid, false otherwise.
+     * @return True if the object is valid, false otherwise.
+     */
+    [[nodiscard]] explicit operator bool() const ENTT_NOEXCEPT {
+        return !(node == nullptr);
+    }
+
+    /**
+     * @brief Checks if two objects refer to the same type.
+     * @param other The object with which to compare.
+     * @return True if the objects refer to the same type, false otherwise.
+     */
+    [[nodiscard]] bool operator==(const meta_type &other) const ENTT_NOEXCEPT {
+        return (!node && !other.node) || (node && other.node && node->info == other.node->info);
+    }
+
+    /**
+     * @brief Resets a type and all its parts.
+     *
+     * This function resets a type and all its data members, member functions
+     * and properties, as well as its constructors, destructors and conversion
+     * functions if any.<br/>
+     * Base classes aren't reset but the link between the two types is removed.
+     *
+     * The type is also removed from the list of searchable types.
+     */
+    void reset() ENTT_NOEXCEPT {
+        for(auto** it = internal::meta_context::global(); *it; it = &(*it)->next) {
+            if(*it == node) {
+                *it = (*it)->next;
+                break;
+            }
+        }
+
+        unregister_all(&node->prop);
+        unregister_all(&node->base);
+        unregister_all(&node->conv);
+        unregister_all<&internal::meta_ctor_node::prop>(&node->ctor);
+        unregister_all<&internal::meta_data_node::prop>(&node->data);
+        unregister_all<&internal::meta_func_node::prop>(&node->func);
+
+        node->id = {};
+        node->ctor = node->def_ctor;
+        node->dtor = nullptr;
+    }
+
+private:
+    node_type *node;
+};
+
+
+/**
+ * @brief Checks if two objects refer to the same type.
+ * @param lhs An object, either valid or not.
+ * @param rhs An object, either valid or not.
+ * @return False if the objects refer to the same node, true otherwise.
+ */
+[[nodiscard]] inline bool operator!=(const meta_type &lhs, const meta_type &rhs) ENTT_NOEXCEPT {
+    return !(lhs == rhs);
+}
+
+
+[[nodiscard]] inline meta_type meta_any::type() const ENTT_NOEXCEPT {
+    return node;
+}
+
+
+template<typename... Args>
+meta_any meta_any::invoke(const id_type id, Args &&... args) const {
+    return type().invoke(id, *this, std::forward<Args>(args)...);
+}
+
+
+template<typename... Args>
+meta_any meta_any::invoke(const id_type id, Args &&... args) {
+    return type().invoke(id, *this, std::forward<Args>(args)...);
+}
+
+
+template<typename Type>
+bool meta_any::set(const id_type id, Type &&value) {
+    return type().set(id, *this, std::forward<Type>(value));
+}
+
+
+[[nodiscard]] inline meta_any meta_any::get(const id_type id) const {
+    return type().get(id, *this);
+}
+
+
+[[nodiscard]] inline meta_any meta_any::get(const id_type id) {
+    return type().get(id, *this);
+}
+
+
+[[nodiscard]] inline meta_type meta_ctor::parent() const ENTT_NOEXCEPT {
+    return node->parent;
+}
+
+
+[[nodiscard]] inline meta_type meta_ctor::arg(size_type index) const ENTT_NOEXCEPT {
+    return index < arity() ? node->arg(index) : meta_type{};
+}
+
+
+[[nodiscard]] inline meta_type meta_data::parent() const ENTT_NOEXCEPT {
+    return node->parent;
+}
+
+
+[[nodiscard]] inline meta_type meta_data::type() const ENTT_NOEXCEPT {
+    return node->type();
+}
+
+
+[[nodiscard]] inline meta_type meta_func::parent() const ENTT_NOEXCEPT {
+    return node->parent;
+}
+
+
+[[nodiscard]] inline meta_type meta_func::ret() const ENTT_NOEXCEPT {
+    return node->ret();
+}
+
+
+[[nodiscard]] inline meta_type meta_func::arg(size_type index) const ENTT_NOEXCEPT {
+    return index < arity() ? node->arg(index) : meta_type{};
+}
+
+
+/*! @brief Opaque iterator for sequence containers. */
+class meta_sequence_container::meta_iterator {
+    /*! @brief A sequence container can access the underlying iterator. */
+    friend class meta_sequence_container;
+
+    enum class operation { INCR, DEREF };
+
+    using vtable_type = void(const operation, const any &, void *);
+
+    template<typename It>
+    static void basic_vtable(const operation op, const any &from, void *to) {
+        switch(op) {
+        case operation::INCR:
+            ++any_cast<It &>(const_cast<any &>(from));
+            break;
+        case operation::DEREF:
+            static_cast<meta_any *>(to)->emplace<typename std::iterator_traits<It>::reference>(*any_cast<const It &>(from));
+            break;
+        }
+    }
+
+public:
+    /*! @brief Signed integer type. */
+    using difference_type = std::ptrdiff_t;
+    /*! @brief Type of elements returned by the iterator. */
+    using value_type = meta_any;
+    /*! @brief Pointer type, `void` on purpose. */
+    using pointer = void;
+    /*! @brief Reference type, it is **not** an actual reference. */
+    using reference = value_type;
+    /*! @brief Iterator category. */
+    using iterator_category = std::input_iterator_tag;
+
+    /*! @brief Default constructor. */
+    meta_iterator() ENTT_NOEXCEPT = default;
+
+    /**
+     * @brief Constructs a meta iterator from a given iterator.
+     * @tparam It Type of actual iterator with which to build the meta iterator.
+     * @param iter The actual iterator with which to build the meta iterator.
+     */
+    template<typename It>
+    meta_iterator(It iter)
+        : vtable{&basic_vtable<It>},
+          handle{std::move(iter)}
+    {}
+
+    /*! @brief Pre-increment operator. @return This iterator. */
+    meta_iterator & operator++() ENTT_NOEXCEPT {
+        return vtable(operation::INCR, handle, nullptr), *this;
+    }
+
+    /*! @brief Post-increment operator. @return This iterator. */
+    meta_iterator operator++(int) ENTT_NOEXCEPT {
+        meta_iterator orig = *this;
+        return ++(*this), orig;
+    }
+
+    /**
+     * @brief Checks if two iterators refer to the same element.
+     * @param other The iterator with which to compare.
+     * @return True if the iterators refer to the same element, false otherwise.
+     */
+    [[nodiscard]] bool operator==(const meta_iterator &other) const ENTT_NOEXCEPT {
+        return handle == other.handle;
+    }
+
+    /**
+     * @brief Checks if two iterators refer to the same element.
+     * @param other The iterator with which to compare.
+     * @return False if the iterators refer to the same element, true otherwise.
+     */
+    [[nodiscard]] bool operator!=(const meta_iterator &other) const ENTT_NOEXCEPT {
+        return !(*this == other);
+    }
+
+    /**
+     * @brief Indirection operator.
+     * @return The element to which the iterator points.
+     */
+    [[nodiscard]] reference operator*() const {
+        meta_any other;
+        vtable(operation::DEREF, handle, &other);
+        return other;
+    }
+
+    /**
+     * @brief Returns false if an iterator is invalid, true otherwise.
+     * @return False if the iterator is invalid, true otherwise.
+     */
+    [[nodiscard]] explicit operator bool() const ENTT_NOEXCEPT {
+        return static_cast<bool>(handle);
+    }
+
+private:
+    vtable_type *vtable{};
+    any handle{};
+};
+
+
+template<typename Type>
+struct meta_sequence_container::meta_sequence_container_proxy {
+    using traits_type = meta_sequence_container_traits<Type>;
+
+    [[nodiscard]] static meta_type value_type() ENTT_NOEXCEPT {
+        return internal::meta_info<typename Type::value_type>::resolve();
+    }
+
+    [[nodiscard]] static size_type size(const any &container) ENTT_NOEXCEPT {
+        return traits_type::size(any_cast<const Type &>(container));
+    }
+
+    [[nodiscard]] static bool resize(any &container, size_type sz) {
+        auto * const cont = any_cast<Type>(&container);
+        return cont && traits_type::resize(*cont, sz);
+    }
+
+    [[nodiscard]] static bool clear(any &container) {
+        auto * const cont = any_cast<Type>(&container);
+        return cont && traits_type::clear(*cont);
+    }
+
+    [[nodiscard]] static iterator begin(any &container) {
+        if(auto * const cont = any_cast<Type>(&container); cont) {
+            return iterator{traits_type::begin(*cont)};
+        }
+
+        return iterator{traits_type::cbegin(any_cast<const Type &>(container))};
+    }
+
+    [[nodiscard]] static iterator end(any &container) {
+        if(auto * const cont = any_cast<Type>(&container); cont) {
+            return iterator{traits_type::end(*cont)};
+        }
+
+        return iterator{traits_type::cend(any_cast<const Type &>(container))};
+    }
+
+    [[nodiscard]] static std::pair<iterator, bool> insert(any &container, iterator it, meta_any &value) {
+        if(auto * const cont = any_cast<Type>(&container); cont) {
+            // this abomination is necessary because only on macos value_type and const_reference are different types for std::vector<bool>
+            if(value.allow_cast<typename Type::const_reference>() || value.allow_cast<typename Type::value_type>()) {
+                const auto *element = value.try_cast<std::remove_reference_t<typename Type::const_reference>>();
+                auto ret = traits_type::insert(*cont, any_cast<const typename Type::iterator &>(it.handle), element ? *element : value.cast<typename Type::value_type>());
+                return { iterator{std::move(ret.first)}, ret.second };
+            }
+        }
+
+        return {};
+    }
+
+    [[nodiscard]] static std::pair<iterator, bool> erase(any &container, iterator it) {
+        if(auto * const cont = any_cast<Type>(&container); cont) {
+            auto ret = traits_type::erase(*cont, any_cast<const typename Type::iterator &>(it.handle));
+            return { iterator{std::move(ret.first)}, ret.second };
+        }
+
+        return {};
+    }
+
+    [[nodiscard]] static meta_any get(any &container, size_type pos) {
+        if(auto * const cont = any_cast<Type>(&container); cont) {
+            return meta_any{std::in_place_type<typename Type::reference>, traits_type::get(*cont, pos)};
+        }
+
+        return meta_any{std::in_place_type<typename Type::const_reference>, traits_type::cget(any_cast<const Type &>(container), pos)};
+    }
+};
+
+
+/**
+ * @brief Returns the meta value type of a container.
+ * @return The meta value type of the container.
+ */
+[[nodiscard]] inline meta_type meta_sequence_container::value_type() const ENTT_NOEXCEPT {
+    return value_type_fn();
+}
+
+
+/**
+ * @brief Returns the size of a container.
+ * @return The size of the container.
+ */
+[[nodiscard]] inline meta_sequence_container::size_type meta_sequence_container::size() const ENTT_NOEXCEPT {
+    return size_fn(storage);
+}
+
+
+/**
+ * @brief Resizes a container to contain a given number of elements.
+ * @param sz The new size of the container.
+ * @return True in case of success, false otherwise.
+ */
+inline bool meta_sequence_container::resize(size_type sz) {
+    return resize_fn(storage, sz);
+}
+
+
+/**
+ * @brief Clears the content of a container.
+ * @return True in case of success, false otherwise.
+ */
+inline bool meta_sequence_container::clear() {
+    return clear_fn(storage);
+}
+
+
+/**
+ * @brief Returns an iterator to the first element of a container.
+ * @return An iterator to the first element of the container.
+ */
+[[nodiscard]] inline meta_sequence_container::iterator meta_sequence_container::begin() {
+    return begin_fn(storage);
+}
+
+
+/**
+ * @brief Returns an iterator that is past the last element of a container.
+ * @return An iterator that is past the last element of the container.
+ */
+[[nodiscard]] inline meta_sequence_container::iterator meta_sequence_container::end() {
+    return end_fn(storage);
+}
+
+
+/**
+ * @brief Inserts an element at a specified location of a container.
+ * @param it Iterator before which the element will be inserted.
+ * @param value Element value to insert.
+ * @return A pair consisting of an iterator to the inserted element (in case of
+ * success) and a bool denoting whether the insertion took place.
+ */
+inline std::pair<meta_sequence_container::iterator, bool> meta_sequence_container::insert(iterator it, meta_any value) {
+    return insert_fn(storage, it, value);
+}
+
+
+/**
+ * @brief Removes a given element from a container.
+ * @param it Iterator to the element to remove.
+ * @return A pair consisting of an iterator following the last removed element
+ * (in case of success) and a bool denoting whether the insertion took place.
+ */
+inline std::pair<meta_sequence_container::iterator, bool> meta_sequence_container::erase(iterator it) {
+    return erase_fn(storage, it);
+}
+
+
+/**
+ * @brief Returns a reference to the element at a given location of a container
+ * (no bounds checking is performed).
+ * @param pos The position of the element to return.
+ * @return A reference to the requested element properly wrapped.
+ */
+[[nodiscard]] inline meta_any meta_sequence_container::operator[](size_type pos) {
+    return get_fn(storage, pos);
+}
+
+
+/**
+ * @brief Returns false if a proxy is invalid, true otherwise.
+ * @return False if the proxy is invalid, true otherwise.
+ */
+[[nodiscard]] inline meta_sequence_container::operator bool() const ENTT_NOEXCEPT {
+    return static_cast<bool>(storage);
+}
+
+
+/*! @brief Opaque iterator for associative containers. */
+class meta_associative_container::meta_iterator {
+    enum operation { INCR, DEREF };
+
+    using vtable_type = void(const operation, const any &, void *);
+
+    template<bool KeyOnly, typename It>
+    static void basic_vtable(const operation op, const any &from, void *to) {
+        switch(op) {
+        case operation::INCR:
+            ++any_cast<It &>(const_cast<any &>(from));
+            break;
+        case operation::DEREF:
+            const auto &it = any_cast<const It &>(from);
+            if constexpr(KeyOnly) {
+                static_cast<std::pair<meta_any, meta_any> *>(to)->first.emplace<decltype(*it)>(*it);
+            } else {
+                static_cast<std::pair<meta_any, meta_any> *>(to)->first.emplace<decltype((it->first))>(it->first);
+                static_cast<std::pair<meta_any, meta_any> *>(to)->second.emplace<decltype((it->second))>(it->second);
+            }
+            break;
+        }
+    }
+
+public:
+    /*! @brief Signed integer type. */
+    using difference_type = std::ptrdiff_t;
+    /*! @brief Type of elements returned by the iterator. */
+    using value_type = std::pair<meta_any, meta_any>;
+    /*! @brief Pointer type, `void` on purpose. */
+    using pointer = void;
+    /*! @brief Reference type, it is **not** an actual reference. */
+    using reference = value_type;
+    /*! @brief Iterator category. */
+    using iterator_category = std::input_iterator_tag;
+
+    /*! @brief Default constructor. */
+    meta_iterator() ENTT_NOEXCEPT = default;
+
+    /**
+     * @brief Constructs an meta iterator from a given iterator.
+     * @tparam KeyOnly True if the container is also key-only, false otherwise.
+     * @tparam It Type of actual iterator with which to build the meta iterator.
+     * @param iter The actual iterator with which to build the meta iterator.
+     */
+    template<bool KeyOnly, typename It>
+    meta_iterator(std::integral_constant<bool, KeyOnly>, It iter)
+        : vtable{&basic_vtable<KeyOnly, It>},
+          handle{std::move(iter)}
+    {}
+
+    /*! @brief Pre-increment operator. @return This iterator. */
+    meta_iterator & operator++() ENTT_NOEXCEPT {
+        return vtable(operation::INCR, handle, nullptr), *this;
+    }
+
+    /*! @brief Post-increment operator. @return This iterator. */
+    meta_iterator operator++(int) ENTT_NOEXCEPT {
+        meta_iterator orig = *this;
+        return ++(*this), orig;
+    }
+
+    /**
+     * @brief Checks if two iterators refer to the same element.
+     * @param other The iterator with which to compare.
+     * @return True if the iterators refer to the same element, false otherwise.
+     */
+    [[nodiscard]] bool operator==(const meta_iterator &other) const ENTT_NOEXCEPT {
+        return handle == other.handle;
+    }
+
+    /**
+     * @brief Checks if two iterators refer to the same element.
+     * @param other The iterator with which to compare.
+     * @return False if the iterators refer to the same element, true otherwise.
+     */
+    [[nodiscard]] bool operator!=(const meta_iterator &other) const ENTT_NOEXCEPT {
+        return !(*this == other);
+    }
+
+    /**
+     * @brief Indirection operator.
+     * @return The element to which the iterator points.
+     */
+    [[nodiscard]] reference operator*() const {
+        reference other;
+        vtable(operation::DEREF, handle, &other);
+        return other;
+    }
+
+    /**
+     * @brief Returns false if an iterator is invalid, true otherwise.
+     * @return False if the iterator is invalid, true otherwise.
+     */
+    [[nodiscard]] explicit operator bool() const ENTT_NOEXCEPT {
+        return static_cast<bool>(handle);
+    }
+
+private:
+    vtable_type *vtable{};
+    any handle{};
+};
+
+
+template<typename Type>
+struct meta_associative_container::meta_associative_container_proxy {
+    using traits_type = meta_associative_container_traits<Type>;
+
+    [[nodiscard]] static meta_type key_type() ENTT_NOEXCEPT {
+        return internal::meta_info<typename Type::key_type>::resolve();
+    }
+
+    [[nodiscard]] static meta_type mapped_type() ENTT_NOEXCEPT {
+        if constexpr(is_key_only_meta_associative_container_v<Type>) {
+            return meta_type{};
+        } else {
+            return internal::meta_info<typename Type::mapped_type>::resolve();
+        }
+    }
+
+    [[nodiscard]] static meta_type value_type() ENTT_NOEXCEPT {
+        return internal::meta_info<typename Type::value_type>::resolve();
+    }
+
+    [[nodiscard]] static size_type size(const any &container) ENTT_NOEXCEPT {
+        return traits_type::size(any_cast<const Type &>(container));
+    }
+
+    [[nodiscard]] static bool clear(any &container) {
+        auto * const cont = any_cast<Type>(&container);
+        return cont && traits_type::clear(*cont);
+    }
+
+    [[nodiscard]] static iterator begin(any &container) {
+        if(auto * const cont = any_cast<Type>(&container); cont) {
+            return iterator{is_key_only_meta_associative_container<Type>{}, traits_type::begin(*cont)};
+        }
+
+        return iterator{is_key_only_meta_associative_container<Type>{}, traits_type::cbegin(any_cast<const Type &>(container))};
+    }
+
+    [[nodiscard]] static iterator end(any &container) {
+        if(auto * const cont = any_cast<Type>(&container); cont) {
+            return iterator{is_key_only_meta_associative_container<Type>{}, traits_type::end(*cont)};
+        }
+
+        return iterator{is_key_only_meta_associative_container<Type>{}, traits_type::cend(any_cast<const Type &>(container))};
+    }
+
+    [[nodiscard]] static bool insert(any &container, meta_any &key, meta_any &value) {
+        if(auto * const cont = any_cast<Type>(&container); cont && key.allow_cast<const typename Type::key_type &>()) {
+            if constexpr(is_key_only_meta_associative_container_v<Type>) {
+                return traits_type::insert(*cont, key.cast<const typename Type::key_type &>());
+            } else {
+                return value.allow_cast<const typename Type::mapped_type &>()
+                    && traits_type::insert(*cont, key.cast<const typename Type::key_type &>(), value.cast<const typename Type::mapped_type &>());
+            }
+        }
+
+        return false;
+    }
+
+    [[nodiscard]] static bool erase(any &container, meta_any &key) {
+        if(auto * const cont = any_cast<Type>(&container); cont && key.allow_cast<const typename Type::key_type &>()) {
+            return traits_type::erase(*cont, key.cast<const typename Type::key_type &>());
+        }
+
+        return false;
+    }
+
+    [[nodiscard]] static iterator find(any &container, meta_any &key) {
+        if(key.allow_cast<const typename Type::key_type &>()) {
+            if(auto * const cont = any_cast<Type>(&container); cont) {
+                return iterator{is_key_only_meta_associative_container<Type>{}, traits_type::find(*cont, key.cast<const typename Type::key_type &>())};
+            }
+
+            return iterator{is_key_only_meta_associative_container<Type>{}, traits_type::cfind(any_cast<const Type &>(container), key.cast<const typename Type::key_type &>())};
+        }
+
+        return {};
+    }
+};
+
+
+/**
+ * @brief Returns true if a container is also key-only, false otherwise.
+ * @return True if the associative container is also key-only, false otherwise.
+ */
+[[nodiscard]] inline bool meta_associative_container::key_only() const ENTT_NOEXCEPT {
+    return key_only_container;
+}
+
+
+/**
+ * @brief Returns the meta key type of a container.
+ * @return The meta key type of the a container.
+ */
+[[nodiscard]] inline meta_type meta_associative_container::key_type() const ENTT_NOEXCEPT {
+    return key_type_fn();
+}
+
+
+/**
+ * @brief Returns the meta mapped type of a container.
+ * @return The meta mapped type of the a container.
+ */
+[[nodiscard]] inline meta_type meta_associative_container::mapped_type() const ENTT_NOEXCEPT {
+    return mapped_type_fn();
+}
+
+
+/*! @copydoc meta_sequence_container::value_type */
+[[nodiscard]] inline meta_type meta_associative_container::value_type() const ENTT_NOEXCEPT {
+    return value_type_fn();
+}
+
+
+/*! @copydoc meta_sequence_container::size */
+[[nodiscard]] inline meta_associative_container::size_type meta_associative_container::size() const ENTT_NOEXCEPT {
+    return size_fn(storage);
+}
+
+
+/*! @copydoc meta_sequence_container::clear */
+inline bool meta_associative_container::clear() {
+    return clear_fn(storage);
+}
+
+
+/*! @copydoc meta_sequence_container::begin */
+[[nodiscard]] inline meta_associative_container::iterator meta_associative_container::begin() {
+    return begin_fn(storage);
+}
+
+
+/*! @copydoc meta_sequence_container::end */
+[[nodiscard]] inline meta_associative_container::iterator meta_associative_container::end() {
+    return end_fn(storage);
+}
+
+
+/**
+ * @brief Inserts an element (a key/value pair) into a container.
+ * @param key The key of the element to insert.
+ * @param value The value of the element to insert.
+ * @return A bool denoting whether the insertion took place.
+ */
+inline bool meta_associative_container::insert(meta_any key, meta_any value = {}) {
+    return insert_fn(storage, key, value);
+}
+
+
+/**
+ * @brief Removes the specified element from a container.
+ * @param key The key of the element to remove.
+ * @return A bool denoting whether the removal took place.
+ */
+inline bool meta_associative_container::erase(meta_any key) {
+    return erase_fn(storage, key);
+}
+
+
+/**
+ * @brief Returns an iterator to the element with a given key, if any.
+ * @param key The key of the element to search.
+ * @return An iterator to the element with the given key, if any.
+ */
+[[nodiscard]] inline meta_associative_container::iterator meta_associative_container::find(meta_any key) {
+    return find_fn(storage, key);
+}
+
+
+/**
+ * @brief Returns false if a proxy is invalid, true otherwise.
+ * @return False if the proxy is invalid, true otherwise.
+ */
+[[nodiscard]] inline meta_associative_container::operator bool() const ENTT_NOEXCEPT {
+    return static_cast<bool>(storage);
+}
+
+
+}
+
+
+#endif
+
+// #include "node.hpp"
+
+// #include "policy.hpp"
+#ifndef ENTT_META_POLICY_HPP
+#define ENTT_META_POLICY_HPP
+
+
+namespace entt {
+
+
+/*! @brief Empty class type used to request the _as ref_ policy. */
+struct as_ref_t {};
+
+
+/*! @brief Empty class type used to request the _as cref_ policy. */
+struct as_cref_t {};
+
+
+/*! @brief Empty class type used to request the _as-is_ policy. */
+struct as_is_t {};
+
+
+/*! @brief Empty class type used to request the _as void_ policy. */
+struct as_void_t {};
+
+
+}
+
+
+#endif
+
+// #include "utility.hpp"
+#ifndef ENTT_META_UTILITY_HPP
+#define ENTT_META_UTILITY_HPP
+
+
+#include <cstddef>
+#include <functional>
+#include <type_traits>
+#include <utility>
+// #include "../config/config.h"
+
+// #include "../core/type_traits.hpp"
+
+// #include "meta.hpp"
+
+// #include "node.hpp"
+
+// #include "policy.hpp"
+
+
+
+namespace entt {
+
+
+/*! @brief Primary template isn't defined on purpose. */
+template<typename, typename>
+struct meta_function_descriptor;
+
+
+/**
+ * @brief Meta function descriptor.
+ * @tparam Type Reflected type to which the meta function is associated.
+ * @tparam Ret Function return type.
+ * @tparam Class Actual owner of the member function.
+ * @tparam Args Function arguments.
+ */
+template<typename Type, typename Ret, typename Class, typename... Args>
+struct meta_function_descriptor<Type, Ret(Class:: *)(Args...) const> {
+    /*! @brief Meta function return type. */
+    using return_type = Ret;
+    /*! @brief Meta function arguments. */
+    using args_type = std::conditional_t<std::is_same_v<Type, Class>, type_list<Args...>, type_list<const Class &, Args...>>;
+
+    /*! @brief True if the meta function is const, false otherwise. */
+    static constexpr auto is_const = true;
+    /*! @brief True if the meta function is static, false otherwise. */
+    static constexpr auto is_static = !std::is_same_v<Type, Class>;
+};
+
+
+/**
+ * @brief Meta function descriptor.
+ * @tparam Type Reflected type to which the meta function is associated.
+ * @tparam Ret Function return type.
+ * @tparam Class Actual owner of the member function.
+ * @tparam Args Function arguments.
+ */
+template<typename Type, typename Ret, typename Class, typename... Args>
+struct meta_function_descriptor<Type, Ret(Class:: *)(Args...)> {
+    /*! @brief Meta function return type. */
+    using return_type = Ret;
+    /*! @brief Meta function arguments. */
+    using args_type = std::conditional_t<std::is_same_v<Type, Class>, type_list<Args...>, type_list<Class &, Args...>>;
+
+    /*! @brief True if the meta function is const, false otherwise. */
+    static constexpr auto is_const = false;
+    /*! @brief True if the meta function is static, false otherwise. */
+    static constexpr auto is_static = !std::is_same_v<Type, Class>;
+};
+
+
+/**
+ * @brief Meta function descriptor.
+ * @tparam Type Reflected type to which the meta function is associated.
+ * @tparam Ret Function return type.
+ * @tparam Args Function arguments.
+ */
+template<typename Type, typename Ret, typename... Args>
+struct meta_function_descriptor<Type, Ret(*)(Args...)> {
+    /*! @brief Meta function return type. */
+    using return_type = Ret;
+    /*! @brief Meta function arguments. */
+    using args_type = type_list<Args...>;
+
+    /*! @brief True if the meta function is const, false otherwise. */
+    static constexpr auto is_const = false;
+    /*! @brief True if the meta function is static, false otherwise. */
+    static constexpr auto is_static = true;
+};
+
+
+/**
+ * @brief Meta function helper.
+ *
+ * Converts a function type to be associated with a reflected type into its meta
+ * function descriptor.
+ *
+ * @tparam Type Reflected type to which the meta function is associated.
+ * @tparam Candidate The actual function to associate with the reflected type.
+ */
+template<typename Type, typename Candidate>
+class meta_function_helper {
+    template<typename Ret, typename... Args, typename Class>
+    static constexpr meta_function_descriptor<Type, Ret(Class:: *)(Args...) const> get_rid_of_noexcept(Ret(Class:: *)(Args...) const);
+
+    template<typename Ret, typename... Args, typename Class>
+    static constexpr meta_function_descriptor<Type, Ret(Class:: *)(Args...)> get_rid_of_noexcept(Ret(Class:: *)(Args...));
+
+    template<typename Ret, typename... Args>
+    static constexpr meta_function_descriptor<Type, Ret(*)(Args...)> get_rid_of_noexcept(Ret(*)(Args...));
+
+public:
+    /*! @brief The meta function descriptor of the given function. */
+    using type = decltype(get_rid_of_noexcept(std::declval<Candidate>()));
+};
+
+
+/**
+ * @brief Helper type.
+ * @tparam Type Reflected type to which the meta function is associated.
+ * @tparam Candidate The actual function to associate with the reflected type.
+ */
+template<typename Type, typename Candidate>
+using meta_function_helper_t = typename meta_function_helper<Type, Candidate>::type;
+
+
+/**
+ * @brief Returns the meta type of the i-th element of a list of arguments.
+ * @tparam Args Actual types of arguments.
+ * @return The meta type of the i-th element of the list of arguments.
+ */
+template<typename... Args>
+[[nodiscard]] static meta_type meta_arg(type_list<Args...>, const std::size_t index) ENTT_NOEXCEPT {
+    return internal::meta_arg_node(type_list<Args...>{}, index);
+}
+
+
+/**
+ * @brief Constructs an instance given a list of erased parameters, if possible.
+ * @tparam Type Actual type of the instance to construct.
+ * @tparam Args Types of arguments expected.
+ * @tparam Index Indexes to use to extract erased arguments from their list.
+ * @param args Parameters to use to construct the instance.
+ * @return A meta any containing the new instance, if any.
+ */
+template<typename Type, typename... Args, std::size_t... Index>
+[[nodiscard]] meta_any meta_construct(meta_any * const args, std::index_sequence<Index...>) {
+    if(((args+Index)->allow_cast<Args>() && ...)) {
+        return Type{(args+Index)->cast<Args>()...};
+    }
+
+    return {};
+}
+
+
+/**
+ * @brief Sets the value of a given variable.
+ * @tparam Type Reflected type to which the variable is associated.
+ * @tparam Data The actual variable to set.
+ * @param instance An opaque instance of the underlying type, if required.
+ * @param value Parameter to use to set the variable.
+ * @return True in case of success, false otherwise.
+ */
+template<typename Type, auto Data>
+[[nodiscard]] bool meta_setter([[maybe_unused]] meta_handle instance, [[maybe_unused]] meta_any value) {
+    if constexpr(!std::is_same_v<decltype(Data), Type> && !std::is_same_v<decltype(Data), std::nullptr_t>) {
+        if constexpr(std::is_function_v<std::remove_reference_t<std::remove_pointer_t<decltype(Data)>>>) {
+            using data_type = type_list_element_t<1u, typename meta_function_helper_t<Type, decltype(Data)>::args_type>;
+
+            if(auto * const clazz = instance->try_cast<Type>(); clazz && value.allow_cast<data_type>()) {
+                Data(*clazz, value.cast<data_type>());
+                return true;
+            }
+        } else if constexpr(std::is_member_function_pointer_v<decltype(Data)>) {
+            using data_type = type_list_element_t<0u, typename meta_function_helper_t<Type, decltype(Data)>::args_type>;
+
+            if(auto * const clazz = instance->try_cast<Type>(); clazz && value.allow_cast<data_type>()) {
+                (clazz->*Data)(value.cast<data_type>());
+                return true;
+            }
+        } else if constexpr(std::is_member_object_pointer_v<decltype(Data)>) {
+            using data_type = std::remove_reference_t<decltype(std::declval<Type>().*Data)>;
+
+            if constexpr(!std::is_array_v<data_type> && !std::is_const_v<data_type>) {
+                if(auto * const clazz = instance->try_cast<Type>(); clazz && value.allow_cast<data_type>()) {
+                    clazz->*Data = value.cast<data_type>();
+                    return true;
+                }
+            }
+        } else {
+            using data_type = std::remove_reference_t<decltype(*Data)>;
+
+            if constexpr(!std::is_array_v<data_type> && !std::is_const_v<data_type>) {
+                if(value.allow_cast<data_type>()) {
+                    *Data = value.cast<data_type>();
+                    return true;
+                }
+            }
+        }
+    }
+
+    return false;
+}
+
+
+/**
+ * @brief Wraps a value depending on the given policy.
+ * @tparam Policy Optional policy (no policy set by default).
+ * @tparam Type Type of value to wrap.
+ * @param value Value to wrap.
+ * @return A meta any containing the returned value.
+ */
+template<typename Policy = as_is_t, typename Type>
+meta_any meta_dispatch(Type &&value) {
+    if constexpr(std::is_same_v<Policy, as_void_t>) {
+        return meta_any{std::in_place_type<void>, std::forward<Type>(value)};
+    } else if constexpr(std::is_same_v<Policy, as_ref_t>) {
+        return meta_any{std::in_place_type<Type>, std::forward<Type>(value)};
+    } else if constexpr(std::is_same_v<Policy, as_cref_t>) {
+        static_assert(std::is_lvalue_reference_v<Type>, "Invalid type");
+        return meta_any{std::in_place_type<const std::remove_reference_t<Type> &>, std::as_const(value)};
+    } else {
+        static_assert(std::is_same_v<Policy, as_is_t>, "Policy not supported");
+        return meta_any{std::forward<Type>(value)};
+    }
+}
+
+
+/**
+ * @brief Gets the value of a given variable.
+ * @tparam Type Reflected type to which the variable is associated.
+ * @tparam Data The actual variable to get.
+ * @tparam Policy Optional policy (no policy set by default).
+ * @param instance An opaque instance of the underlying type, if required.
+ * @return A meta any containing the value of the underlying variable.
+ */
+template<typename Type, auto Data, typename Policy = as_is_t>
+[[nodiscard]] meta_any meta_getter([[maybe_unused]] meta_handle instance) {
+    if constexpr(std::is_function_v<std::remove_reference_t<std::remove_pointer_t<decltype(Data)>>>) {
+        auto * const clazz = instance->try_cast<std::conditional_t<std::is_invocable_v<decltype(Data), const Type &>, const Type, Type>>();
+        return clazz ? meta_dispatch<Policy>(Data(*clazz)) : meta_any{};
+    } else if constexpr(std::is_member_function_pointer_v<decltype(Data)>) {
+        auto * const clazz = instance->try_cast<std::conditional_t<std::is_invocable_v<decltype(Data), const Type &>, const Type, Type>>();
+        return clazz ? meta_dispatch<Policy>((clazz->*Data)()) : meta_any{};
+    } else if constexpr(std::is_member_object_pointer_v<decltype(Data)>) {
+        if constexpr(!std::is_array_v<std::remove_cv_t<std::remove_reference_t<decltype(std::declval<Type>().*Data)>>>) {
+            if(auto * clazz = instance->try_cast<Type>(); clazz) {
+                return meta_dispatch<Policy>(clazz->*Data);
+            } else if(auto * fallback = instance->try_cast<const Type>(); fallback) {
+                return meta_dispatch<Policy>(fallback->*Data);
+            }
+        }
+
+        return meta_any{};
+    } else if constexpr(std::is_pointer_v<decltype(Data)>) {
+        if constexpr(std::is_array_v<std::remove_pointer_t<decltype(Data)>>) {
+            return meta_any{};
+        } else {
+            return meta_dispatch<Policy>(*Data);
+        }
+    } else {
+        return meta_dispatch<Policy>(Data);
+    }
+}
+
+
+/**
+ * @brief Invokes a function given a list of erased parameters, if possible.
+ * @tparam Type Reflected type to which the function is associated.
+ * @tparam Candidate The actual function to invoke.
+ * @tparam Policy Optional policy (no policy set by default).
+ * @tparam Index Indexes to use to extract erased arguments from their list.
+ * @param instance An opaque instance of the underlying type, if required.
+ * @param args Parameters to use to invoke the function.
+ * @return A meta any containing the returned value, if any.
+ */
+template<typename Type, auto Candidate, typename Policy = as_is_t, std::size_t... Index>
+[[nodiscard]] std::enable_if_t<!std::is_invocable_v<decltype(Candidate)>, meta_any> meta_invoke([[maybe_unused]] meta_handle instance, meta_any *args, std::index_sequence<Index...>) {
+    using descriptor = meta_function_helper_t<Type, decltype(Candidate)>;
+
+    const auto invoke = [](auto &&maybe_clazz, auto &&... other) {
+        if constexpr(std::is_member_function_pointer_v<decltype(Candidate)>) {
+            if constexpr(std::is_void_v<typename descriptor::return_type>) {
+                (std::forward<decltype(maybe_clazz)>(maybe_clazz).*Candidate)(std::forward<decltype(other)>(other)...);
+                return meta_any{std::in_place_type<void>};
+            } else {
+                return meta_dispatch<Policy>((std::forward<decltype(maybe_clazz)>(maybe_clazz).*Candidate)(std::forward<decltype(other)>(other)...));
+            }
+        } else {
+            if constexpr(std::is_void_v<typename descriptor::return_type>) {
+                Candidate(std::forward<decltype(maybe_clazz)>(maybe_clazz), std::forward<decltype(other)>(other)...);
+                return meta_any{std::in_place_type<void>};
+            } else {
+                return meta_dispatch<Policy>(Candidate(std::forward<decltype(maybe_clazz)>(maybe_clazz), std::forward<decltype(other)>(other)...));
+            }
+        }
+    };
+
+    if constexpr(std::is_invocable_v<decltype(Candidate), const Type &, type_list_element_t<Index, typename descriptor::args_type>...>) {
+        if(const auto * const clazz = instance->try_cast<const Type>(); clazz && ((args+Index)->allow_cast<type_list_element_t<Index, typename descriptor::args_type>>() && ...)) {
+            return invoke(*clazz, (args+Index)->cast<type_list_element_t<Index, typename descriptor::args_type>>()...);
+        }
+    } else if constexpr(std::is_invocable_v<decltype(Candidate), Type &, type_list_element_t<Index, typename descriptor::args_type>...>) {
+        if(auto * const clazz = instance->try_cast<Type>(); clazz && ((args+Index)->allow_cast<type_list_element_t<Index, typename descriptor::args_type>>() && ...)) {
+            return invoke(*clazz, (args+Index)->cast<type_list_element_t<Index, typename descriptor::args_type>>()...);
+        }
+    } else {
+        if(((args+Index)->allow_cast<type_list_element_t<Index, typename descriptor::args_type>>() && ...)) {
+            return invoke((args+Index)->cast<type_list_element_t<Index, typename descriptor::args_type>>()...);
+        }
+    }
+
+    return meta_any{};
+}
+
+
+/**
+ * @brief Invokes a function given a list of erased parameters, if possible.
+ * @tparam Type Reflected type to which the function is associated.
+ * @tparam Candidate The actual function to invoke.
+ * @tparam Policy Optional policy (no policy set by default).
+ * @tparam Index Indexes to use to extract erased arguments from their list.
+ * @return A meta any containing the returned value, if any.
+ */
+template<typename Type, auto Candidate, typename Policy = as_is_t, std::size_t... Index>
+[[nodiscard]] std::enable_if_t<std::is_invocable_v<decltype(Candidate)>, meta_any> meta_invoke(meta_handle, meta_any *, std::index_sequence<Index...>) {
+    if constexpr(std::is_void_v<decltype(Candidate())>) {
+        Candidate();
+        return meta_any{std::in_place_type<void>};
+    } else {
+        return meta_dispatch<Policy>(Candidate());
+    }
+}
+
+
+}
+
+
+#endif
+
+
+
+namespace entt {
+
+
+/**
+ * @cond TURN_OFF_DOXYGEN
+ * Internal details not to be documented.
+ */
+
+
+namespace internal {
+
+
+template<typename Node>
+[[nodiscard]] bool find_if(const Node *candidate, const Node *node) ENTT_NOEXCEPT {
+    return node && (node == candidate || find_if(candidate, node->next));
+}
+
+
+template<typename Id, typename Node>
+[[nodiscard]] bool find_if_not(const Id id, Node *node, const Node *owner) ENTT_NOEXCEPT {
+    if constexpr(std::is_pointer_v<Id>) {
+        return node && ((*node->id == *id && node != owner) || find_if_not(id, node->next, owner));
+    } else {
+        return node && ((node->id == id && node != owner) || find_if_not(id, node->next, owner));
+    }
+}
+
+
+}
+
+
+/**
+ * Internal details not to be documented.
+ * @endcond
+ */
+
+
+/**
+ * @brief Meta factory to be used for reflection purposes.
+ *
+ * The meta factory is an utility class used to reflect types, data members and
+ * functions of all sorts. This class ensures that the underlying web of types
+ * is built correctly and performs some checks in debug mode to ensure that
+ * there are no subtle errors at runtime.
+ */
+template<typename...>
+struct meta_factory;
+
+
+/**
+ * @brief Extended meta factory to be used for reflection purposes.
+ * @tparam Type Reflected type for which the factory was created.
+ * @tparam Spec Property specialization pack used to disambiguate overloads.
+ */
+template<typename Type, typename... Spec>
+struct meta_factory<Type, Spec...>: public meta_factory<Type> {
+private:
+    template<std::size_t Step = 0, std::size_t... Index, typename... Property, typename... Other>
+    void unpack(std::index_sequence<Index...>, std::tuple<Property...> property, Other &&... other) {
+        unroll<Step>(choice<3>, std::move(std::get<Index>(property))..., std::forward<Other>(other)...);
+    }
+
+    template<std::size_t Step = 0, typename... Property, typename... Other>
+    void unroll(choice_t<3>, std::tuple<Property...> property, Other &&... other) {
+        unpack<Step>(std::index_sequence_for<Property...>{}, std::move(property), std::forward<Other>(other)...);
+    }
+
+    template<std::size_t Step = 0, typename... Property, typename... Other>
+    void unroll(choice_t<2>, std::pair<Property...> property, Other &&... other) {
+        assign<Step>(std::move(property.first), std::move(property.second));
+        unroll<Step+1>(choice<3>, std::forward<Other>(other)...);
+    }
+
+    template<std::size_t Step = 0, typename Property, typename... Other>
+    std::enable_if_t<!std::is_invocable_v<Property>>
+    unroll(choice_t<1>, Property &&property, Other &&... other) {
+        assign<Step>(std::forward<Property>(property));
+        unroll<Step+1>(choice<3>, std::forward<Other>(other)...);
+    }
+
+    template<std::size_t Step = 0, typename Func, typename... Other>
+    void unroll(choice_t<0>, Func &&invocable, Other &&... other) {
+        unroll<Step>(choice<3>, std::forward<Func>(invocable)(), std::forward<Other>(other)...);
+    }
+
+    template<std::size_t>
+    void unroll(choice_t<0>) {}
+
+    template<std::size_t = 0, typename Key>
+    void assign(Key &&key, meta_any value = {}) {
+        static meta_any property[2u]{};
+
+        static internal::meta_prop_node node{
+            nullptr,
+            property[0u],
+            property[1u]
+        };
+
+        entt::meta_any instance{std::forward<Key>(key)};
+        ENTT_ASSERT(!internal::find_if_not(&instance, *curr, &node), "Duplicate key");
+        property[0u] = std::move(instance);
+        property[1u] = std::move(value);
+
+        if(!internal::find_if(&node, *curr)) {
+            node.next = *curr;
+            *curr = &node;
+        }
+    }
+
+public:
+    /**
+     * @brief Constructs an extended factory from a given node.
+     * @param target The underlying node to which to assign the properties.
+     */
+    meta_factory(internal::meta_prop_node **target) ENTT_NOEXCEPT
+        : curr{target}
+    {}
+
+    /**
+     * @brief Assigns a property to the last meta object created.
+     *
+     * Both the key and the value (if any) must be at least copy constructible.
+     *
+     * @tparam PropertyOrKey Type of the property or property key.
+     * @tparam Value Optional type of the property value.
+     * @param property_or_key Property or property key.
+     * @param value Optional property value.
+     * @return A meta factory for the parent type.
+     */
+    template<typename PropertyOrKey, typename... Value>
+    auto prop(PropertyOrKey &&property_or_key, Value &&... value) && {
+        if constexpr(sizeof...(Value) == 0) {
+            unroll(choice<3>, std::forward<PropertyOrKey>(property_or_key));
+        } else {
+            assign(std::forward<PropertyOrKey>(property_or_key), std::forward<Value>(value)...);
+        }
+
+        return meta_factory<Type, Spec..., PropertyOrKey, Value...>{curr};
+    }
+
+    /**
+     * @brief Assigns properties to the last meta object created.
+     *
+     * Both the keys and the values (if any) must be at least copy
+     * constructible.
+     *
+     * @tparam Property Types of the properties.
+     * @param property Properties to assign to the last meta object created.
+     * @return A meta factory for the parent type.
+     */
+    template <typename... Property>
+    auto props(Property... property) && {
+        unroll(choice<3>, std::forward<Property>(property)...);
+        return meta_factory<Type, Spec..., Property...>{curr};
+    }
+
+private:
+    internal::meta_prop_node **curr;
+};
+
+
+/**
+ * @brief Basic meta factory to be used for reflection purposes.
+ * @tparam Type Reflected type for which the factory was created.
+ */
+template<typename Type>
+struct meta_factory<Type> {
+    /**
+     * @brief Makes a meta type _searchable_.
+     * @param id Optional unique identifier.
+     * @return An extended meta factory for the given type.
+     */
+    auto type(const id_type id = type_hash<Type>::value()) {
+        auto * const node = internal::meta_info<Type>::resolve();
+
+        ENTT_ASSERT(!internal::find_if_not(id, *internal::meta_context::global(), node), "Duplicate identifier");
+        node->id = id;
+
+        if(!internal::find_if(node, *internal::meta_context::global())) {
+            node->next = *internal::meta_context::global();
+            *internal::meta_context::global() = node;
+        }
+
+        return meta_factory<Type, Type>{&node->prop};
+    }
+
+    /**
+     * @brief Assigns a meta base to a meta type.
+     *
+     * A reflected base class must be a real base class of the reflected type.
+     *
+     * @tparam Base Type of the base class to assign to the meta type.
+     * @return A meta factory for the parent type.
+     */
+    template<typename Base>
+    auto base() ENTT_NOEXCEPT {
+        static_assert(std::is_base_of_v<Base, Type>, "Invalid base type");
+        auto * const type = internal::meta_info<Type>::resolve();
+
+        static internal::meta_base_node node{
+            type,
+            nullptr,
+            &internal::meta_info<Base>::resolve,
+            [](const void *instance) ENTT_NOEXCEPT -> const void * {
+                return static_cast<const Base *>(static_cast<const Type *>(instance));
+            }
+        };
+
+        if(!internal::find_if(&node, type->base)) {
+            node.next = type->base;
+            type->base = &node;
+        }
+
+        return meta_factory<Type>{};
+    }
+
+    /**
+     * @brief Assigns a meta conversion function to a meta type.
+     *
+     * Conversion functions can be either free functions or member
+     * functions.<br/>
+     * In case of free functions, they must accept a const reference to an
+     * instance of the parent type as an argument. In case of member functions,
+     * they should have no arguments at all.
+     *
+     * @tparam Candidate The actual function to use for the conversion.
+     * @return A meta factory for the parent type.
+     */
+    template<auto Candidate>
+    std::enable_if_t<std::is_member_function_pointer_v<decltype(Candidate)>, meta_factory<Type>> conv() ENTT_NOEXCEPT {
+        using conv_type = std::invoke_result_t<decltype(Candidate), Type &>;
+        auto * const type = internal::meta_info<Type>::resolve();
+
+        static internal::meta_conv_node node{
+            type,
+            nullptr,
+            &internal::meta_info<conv_type>::resolve,
+            [](const void *instance) -> meta_any {
+                return (static_cast<const Type *>(instance)->*Candidate)();
+            }
+        };
+
+        if(!internal::find_if(&node, type->conv)) {
+            node.next = type->conv;
+            type->conv = &node;
+        }
+
+        return meta_factory<Type>{};
+    }
+
+    /*! @copydoc conv */
+    template<auto Candidate>
+    std::enable_if_t<!std::is_member_function_pointer_v<decltype(Candidate)>, meta_factory<Type>> conv() ENTT_NOEXCEPT {
+        using conv_type = std::invoke_result_t<decltype(Candidate), Type &>;
+        auto * const type = internal::meta_info<Type>::resolve();
+
+        static internal::meta_conv_node node{
+            type,
+            nullptr,
+            &internal::meta_info<conv_type>::resolve,
+            [](const void *instance) -> meta_any {
+                return Candidate(*static_cast<const Type *>(instance));
+            }
+        };
+
+        if(!internal::find_if(&node, type->conv)) {
+            node.next = type->conv;
+            type->conv = &node;
+        }
+
+        return meta_factory<Type>{};
+    }
+
+    /**
+     * @brief Assigns a meta conversion function to a meta type.
+     *
+     * The given type must be such that an instance of the reflected type can be
+     * converted to it.
+     *
+     * @tparam To Type of the conversion function to assign to the meta type.
+     * @return A meta factory for the parent type.
+     */
+    template<typename To>
+    auto conv() ENTT_NOEXCEPT {
+        static_assert(std::is_convertible_v<Type, To>, "Could not convert to the required type");
+        auto * const type = internal::meta_info<Type>::resolve();
+
+        static internal::meta_conv_node node{
+            type,
+            nullptr,
+            &internal::meta_info<To>::resolve,
+            [](const void *instance) -> meta_any {
+                return static_cast<To>(*static_cast<const Type *>(instance));
+            }
+        };
+
+        if(!internal::find_if(&node, type->conv)) {
+            node.next = type->conv;
+            type->conv = &node;
+        }
+
+        return meta_factory<Type>{};
+    }
+
+    /**
+     * @brief Assigns a meta constructor to a meta type.
+     *
+     * Both member functions and free function can be assigned to meta types in
+     * the role of constructors. All that is required is that they return an
+     * instance of the underlying type.<br/>
+     * From a client's point of view, nothing changes if a constructor of a meta
+     * type is a built-in one or not.
+     *
+     * @tparam Candidate The actual function to use as a constructor.
+     * @tparam Policy Optional policy (no policy set by default).
+     * @return An extended meta factory for the parent type.
+     */
+    template<auto Candidate, typename Policy = as_is_t>
+    auto ctor() ENTT_NOEXCEPT {
+        using descriptor = meta_function_helper_t<Type, decltype(Candidate)>;
+        static_assert(std::is_same_v<std::decay_t<typename descriptor::return_type>, Type>, "The function doesn't return an object of the required type");
+        auto * const type = internal::meta_info<Type>::resolve();
+
+        static internal::meta_ctor_node node{
+            type,
+            nullptr,
+            nullptr,
+            descriptor::args_type::size,
+            [](const typename internal::meta_ctor_node::size_type index) ENTT_NOEXCEPT {
+                return meta_arg(typename descriptor::args_type{}, index);
+            },
+            [](meta_any * const args) {
+                return meta_invoke<Type, Candidate, Policy>({}, args, std::make_index_sequence<descriptor::args_type::size>{});
+            }
+        };
+
+        if(!internal::find_if(&node, type->ctor)) {
+            node.next = type->ctor;
+            type->ctor = &node;
+        }
+
+        return meta_factory<Type, std::integral_constant<decltype(Candidate), Candidate>>{&node.prop};
+    }
+
+    /**
+     * @brief Assigns a meta constructor to a meta type.
+     *
+     * A meta constructor is uniquely identified by the types of its arguments
+     * and is such that there exists an actual constructor of the underlying
+     * type that can be invoked with parameters whose types are those given.
+     *
+     * @tparam Args Types of arguments to use to construct an instance.
+     * @return An extended meta factory for the parent type.
+     */
+    template<typename... Args>
+    auto ctor() ENTT_NOEXCEPT {
+        using descriptor = meta_function_helper_t<Type, Type(*)(Args...)>;
+        auto * const type = internal::meta_info<Type>::resolve();
+
+        static internal::meta_ctor_node node{
+            type,
+            nullptr,
+            nullptr,
+            descriptor::args_type::size,
+            [](const typename internal::meta_ctor_node::size_type index) ENTT_NOEXCEPT {
+                return meta_arg(typename descriptor::args_type{}, index);
+            },
+            [](meta_any * const args) {
+                return meta_construct<Type, Args...>(args, std::make_index_sequence<descriptor::args_type::size>{});
+            }
+        };
+
+        if(!internal::find_if(&node, type->ctor)) {
+            node.next = type->ctor;
+            type->ctor = &node;
+        }
+
+        return meta_factory<Type, Type(Args...)>{&node.prop};
+    }
+
+    /**
+     * @brief Assigns a meta destructor to a meta type.
+     *
+     * Free functions can be assigned to meta types in the role of destructors.
+     * The signature of the function should identical to the following:
+     *
+     * @code{.cpp}
+     * void(Type &);
+     * @endcode
+     *
+     * The purpose is to give users the ability to free up resources that
+     * require special treatment before an object is actually destroyed.
+     *
+     * @tparam Func The actual function to use as a destructor.
+     * @return A meta factory for the parent type.
+     */
+    template<auto Func>
+    auto dtor() ENTT_NOEXCEPT {
+        static_assert(std::is_invocable_v<decltype(Func), Type &>, "The function doesn't accept an object of the type provided");
+        auto * const type = internal::meta_info<Type>::resolve();
+
+        type->dtor = [](void *instance) {
+            Func(*static_cast<Type *>(instance));
+        };
+
+        return meta_factory<Type>{};
+    }
+
+    /**
+     * @brief Assigns a meta data to a meta type.
+     *
+     * Both data members and static and global variables, as well as constants
+     * of any kind, can be assigned to a meta type.<br/>
+     * From a client's point of view, all the variables associated with the
+     * reflected object will appear as if they were part of the type itself.
+     *
+     * @tparam Data The actual variable to attach to the meta type.
+     * @tparam Policy Optional policy (no policy set by default).
+     * @param id Unique identifier.
+     * @return An extended meta factory for the parent type.
+     */
+    template<auto Data, typename Policy = as_is_t>
+    auto data(const id_type id) ENTT_NOEXCEPT {
+        if constexpr(std::is_member_object_pointer_v<decltype(Data)>) {
+            return data<Data, Data, Policy>(id);
+        } else {
+            using data_type = std::remove_pointer_t<decltype(Data)>;
+            auto * const type = internal::meta_info<Type>::resolve();
+
+            static internal::meta_data_node node{
+                {},
+                type,
+                nullptr,
+                nullptr,
+                std::is_same_v<Type, data_type> || std::is_const_v<data_type>,
+                true,
+                &internal::meta_info<data_type>::resolve,
+                &meta_setter<Type, Data>,
+                &meta_getter<Type, Data, Policy>
+            };
+
+            ENTT_ASSERT(!internal::find_if_not(id, type->data, &node), "Duplicate identifier");
+            node.id = id;
+
+            if(!internal::find_if(&node, type->data)) {
+                node.next = type->data;
+                type->data = &node;
+            }
+
+            return meta_factory<Type, std::integral_constant<decltype(Data), Data>>{&node.prop};
+        }
+    }
+
+    /**
+     * @brief Assigns a meta data to a meta type by means of its setter and
+     * getter.
+     *
+     * Setters and getters can be either free functions, member functions or a
+     * mix of them.<br/>
+     * In case of free functions, setters and getters must accept a reference to
+     * an instance of the parent type as their first argument. A setter has then
+     * an extra argument of a type convertible to that of the parameter to
+     * set.<br/>
+     * In case of member functions, getters have no arguments at all, while
+     * setters has an argument of a type convertible to that of the parameter to
+     * set.
+     *
+     * @tparam Setter The actual function to use as a setter.
+     * @tparam Getter The actual function to use as a getter.
+     * @tparam Policy Optional policy (no policy set by default).
+     * @param id Unique identifier.
+     * @return An extended meta factory for the parent type.
+     */
+    template<auto Setter, auto Getter, typename Policy = as_is_t>
+    auto data(const id_type id) ENTT_NOEXCEPT {
+        using underlying_type = std::remove_reference_t<std::invoke_result_t<decltype(Getter), Type &>>;
+        auto * const type = internal::meta_info<Type>::resolve();
+
+        static internal::meta_data_node node{
+            {},
+            type,
+            nullptr,
+            nullptr,
+            std::is_same_v<decltype(Setter), std::nullptr_t> || (std::is_member_object_pointer_v<decltype(Setter)> && std::is_const_v<underlying_type>),
+            false,
+            &internal::meta_info<underlying_type>::resolve,
+            &meta_setter<Type, Setter>,
+            &meta_getter<Type, Getter, Policy>
+        };
+
+        ENTT_ASSERT(!internal::find_if_not(id, type->data, &node), "Duplicate identifier");
+        node.id = id;
+
+        if(!internal::find_if(&node, type->data)) {
+            node.next = type->data;
+            type->data = &node;
+        }
+
+        return meta_factory<Type, std::integral_constant<decltype(Setter), Setter>, std::integral_constant<decltype(Getter), Getter>>{&node.prop};
+    }
+
+    /**
+     * @brief Assigns a meta funcion to a meta type.
+     *
+     * Both member functions and free functions can be assigned to a meta
+     * type.<br/>
+     * From a client's point of view, all the functions associated with the
+     * reflected object will appear as if they were part of the type itself.
+     *
+     * @tparam Candidate The actual function to attach to the meta type.
+     * @tparam Policy Optional policy (no policy set by default).
+     * @param id Unique identifier.
+     * @return An extended meta factory for the parent type.
+     */
+    template<auto Candidate, typename Policy = as_is_t>
+    auto func(const id_type id) ENTT_NOEXCEPT {
+        using descriptor = meta_function_helper_t<Type, decltype(Candidate)>;
+        auto * const type = internal::meta_info<Type>::resolve();
+
+        static internal::meta_func_node node{
+            {},
+            type,
+            nullptr,
+            nullptr,
+            descriptor::args_type::size,
+            descriptor::is_const,
+            descriptor::is_static,
+            &internal::meta_info<std::conditional_t<std::is_same_v<Policy, as_void_t>, void, typename descriptor::return_type>>::resolve,
+            [](const typename internal::meta_func_node::size_type index) ENTT_NOEXCEPT {
+                return meta_arg(typename descriptor::args_type{}, index);
+            },
+            [](meta_handle instance, meta_any *args) {
+                return meta_invoke<Type, Candidate, Policy>(std::move(instance), args, std::make_index_sequence<descriptor::args_type::size>{});
+            }
+        };
+
+        for(auto *it = &type->func; *it; it = &(*it)->next) {
+            if(*it == &node) {
+                *it = node.next;
+                break;
+            }
+        }
+
+        internal::meta_func_node **it = &type->func;
+        for(; *it && (*it)->id != id; it = &(*it)->next);
+        for(; *it && (*it)->id == id && (*it)->arity < node.arity; it = &(*it)->next);
+
+        node.id = id;
+        node.next = *it;
+        *it = &node;
+
+        return meta_factory<Type, std::integral_constant<decltype(Candidate), Candidate>>{&node.prop};
+    }
+};
+
+
+/**
+ * @brief Utility function to use for reflection.
+ *
+ * This is the point from which everything starts.<br/>
+ * By invoking this function with a type that is not yet reflected, a meta type
+ * is created to which it will be possible to attach meta objects through a
+ * dedicated factory.
+ *
+ * @tparam Type Type to reflect.
+ * @return A meta factory for the given type.
+ */
+template<typename Type>
+[[nodiscard]] auto meta() ENTT_NOEXCEPT {
+    auto * const node = internal::meta_info<Type>::resolve();
+    // extended meta factory to allow assigning properties to opaque meta types
+    return meta_factory<Type, Type>{&node->prop};
+}
+
+
+}
+
+
+#endif
+
+// #include "meta/meta.hpp"
+#ifndef ENTT_META_META_HPP
+#define ENTT_META_META_HPP
+
+
+#include <cstddef>
+#include <functional>
+#include <iterator>
+#include <memory>
+#include <type_traits>
+#include <utility>
+// #include "../config/config.h"
+
+// #include "../core/any.hpp"
+
+// #include "../core/fwd.hpp"
+
+// #include "../core/utility.hpp"
+
+// #include "../core/type_info.hpp"
+
+// #include "../core/type_traits.hpp"
+
+// #include "adl_pointer.hpp"
+
+// #include "ctx.hpp"
+
+// #include "node.hpp"
+
+// #include "range.hpp"
+
+// #include "type_traits.hpp"
+
+
+
+namespace entt {
+
+
+class meta_any;
+class meta_type;
+
+
+/*! @brief Proxy object for sequence containers. */
+class meta_sequence_container {
+    template<typename>
+    struct meta_sequence_container_proxy;
+
+    class meta_iterator;
+
+public:
+    /*! @brief Unsigned integer type. */
+    using size_type = std::size_t;
+    /*! @brief Meta iterator type. */
+    using iterator = meta_iterator;
+
+    /*! @brief Default constructor. */
+    meta_sequence_container() ENTT_NOEXCEPT = default;
+
+    /**
+     * @brief Construct a proxy object for sequence containers.
+     * @tparam Type Type of container to wrap.
+     * @param instance The container to wrap.
+     */
+    template<typename Type>
+    meta_sequence_container(std::in_place_type_t<Type>, any instance) ENTT_NOEXCEPT
+        : value_type_fn{&meta_sequence_container_proxy<Type>::value_type},
+          size_fn{&meta_sequence_container_proxy<Type>::size},
+          resize_fn{&meta_sequence_container_proxy<Type>::resize},
+          clear_fn{&meta_sequence_container_proxy<Type>::clear},
+          begin_fn{&meta_sequence_container_proxy<Type>::begin},
+          end_fn{&meta_sequence_container_proxy<Type>::end},
+          insert_fn{&meta_sequence_container_proxy<Type>::insert},
+          erase_fn{&meta_sequence_container_proxy<Type>::erase},
+          get_fn{&meta_sequence_container_proxy<Type>::get},
+          storage{std::move(instance)}
+    {}
+
+    [[nodiscard]] inline meta_type value_type() const ENTT_NOEXCEPT;
+    [[nodiscard]] inline size_type size() const ENTT_NOEXCEPT;
+    inline bool resize(size_type);
+    inline bool clear();
+    [[nodiscard]] inline iterator begin();
+    [[nodiscard]] inline iterator end();
+    inline std::pair<iterator, bool> insert(iterator, meta_any);
+    inline std::pair<iterator, bool> erase(iterator);
+    [[nodiscard]] inline meta_any operator[](size_type);
+    [[nodiscard]] inline explicit operator bool() const ENTT_NOEXCEPT;
+
+private:
+    meta_type(* value_type_fn)() ENTT_NOEXCEPT = nullptr;
+    size_type(* size_fn)(const any &) ENTT_NOEXCEPT = nullptr;
+    bool(* resize_fn)(any &, size_type) = nullptr;
+    bool(* clear_fn)(any &) = nullptr;
+    iterator(* begin_fn)(any &) = nullptr;
+    iterator(* end_fn)(any &) = nullptr;
+    std::pair<iterator, bool>(* insert_fn)(any &, iterator, meta_any &) = nullptr;
+    std::pair<iterator, bool>(* erase_fn)(any &, iterator) = nullptr;
+    meta_any(* get_fn)(any &, size_type) = nullptr;
+    any storage{};
+};
+
+
+/*! @brief Proxy object for associative containers. */
+class meta_associative_container {
+    template<typename>
+    struct meta_associative_container_proxy;
+
+    class meta_iterator;
+
+public:
+    /*! @brief Unsigned integer type. */
+    using size_type = std::size_t;
+    /*! @brief Meta iterator type. */
+    using iterator = meta_iterator;
+
+    /*! @brief Default constructor. */
+    meta_associative_container() ENTT_NOEXCEPT = default;
+
+    /**
+     * @brief Construct a proxy object for associative containers.
+     * @tparam Type Type of container to wrap.
+     * @param instance The container to wrap.
+     */
+    template<typename Type>
+    meta_associative_container(std::in_place_type_t<Type>, any instance) ENTT_NOEXCEPT
+        : key_only_container{is_key_only_meta_associative_container_v<Type>},
+          key_type_fn{&meta_associative_container_proxy<Type>::key_type},
+          mapped_type_fn{&meta_associative_container_proxy<Type>::mapped_type},
+          value_type_fn{&meta_associative_container_proxy<Type>::value_type},
+          size_fn{&meta_associative_container_proxy<Type>::size},
+          clear_fn{&meta_associative_container_proxy<Type>::clear},
+          begin_fn{&meta_associative_container_proxy<Type>::begin},
+          end_fn{&meta_associative_container_proxy<Type>::end},
+          insert_fn{&meta_associative_container_proxy<Type>::insert},
+          erase_fn{&meta_associative_container_proxy<Type>::erase},
+          find_fn{&meta_associative_container_proxy<Type>::find},
+          storage{std::move(instance)}
+    {}
+
+    [[nodiscard]] inline bool key_only() const ENTT_NOEXCEPT;
+    [[nodiscard]] inline meta_type key_type() const ENTT_NOEXCEPT;
+    [[nodiscard]] inline meta_type mapped_type() const ENTT_NOEXCEPT;
+    [[nodiscard]] inline meta_type value_type() const ENTT_NOEXCEPT;
+    [[nodiscard]] inline size_type size() const ENTT_NOEXCEPT;
+    inline bool clear();
+    [[nodiscard]] inline iterator begin();
+    [[nodiscard]] inline iterator end();
+    inline bool insert(meta_any, meta_any);
+    inline bool erase(meta_any);
+    [[nodiscard]] inline iterator find(meta_any);
+    [[nodiscard]] inline explicit operator bool() const ENTT_NOEXCEPT;
+
+private:
+    bool key_only_container{};
+    meta_type(* key_type_fn)() ENTT_NOEXCEPT = nullptr;
+    meta_type(* mapped_type_fn)() ENTT_NOEXCEPT = nullptr;
+    meta_type(* value_type_fn)() ENTT_NOEXCEPT = nullptr;
+    size_type(* size_fn)(const any &) ENTT_NOEXCEPT = nullptr;
+    bool(* clear_fn)(any &) = nullptr;
+    iterator(* begin_fn)(any &) = nullptr;
+    iterator(* end_fn)(any &) = nullptr;
+    bool(* insert_fn)(any &, meta_any &, meta_any &) = nullptr;
+    bool(* erase_fn)(any &, meta_any &) = nullptr;
+    iterator(* find_fn)(any &, meta_any &) = nullptr;
+    any storage{};
+};
+
+
+/*! @brief Opaque wrapper for values of any type. */
+class meta_any {
+    enum class operation { DTOR, DEREF, SEQ, ASSOC };
+
+    using vtable_type = void(const operation, const any &, void *);
+
+    template<typename Type>
+    static void basic_vtable([[maybe_unused]] const operation op, [[maybe_unused]] const any &from, [[maybe_unused]] void *to) {
+        static_assert(std::is_same_v<std::remove_reference_t<std::remove_const_t<Type>>, Type>, "Invalid type");
+
+        if constexpr(!std::is_void_v<Type>) {
+            switch(op) {
+            case operation::DTOR:
+                if(auto *curr = static_cast<internal::meta_type_node *>(to); curr->dtor && from.owner()) {
+                    curr->dtor(const_cast<any &>(from).data());
+                }
+                break;
+            case operation::DEREF:
+                if constexpr(is_meta_pointer_like_v<Type>) {
+                    using element_type = std::remove_const_t<typename std::pointer_traits<Type>::element_type>;
+
+                    if constexpr(std::is_function_v<element_type>) {
+                        *static_cast<meta_any *>(to) = any_cast<Type>(from);
+                    } else if constexpr(!std::is_same_v<std::remove_const_t<typename std::pointer_traits<Type>::element_type>, void>) {
+                        using in_place_type = decltype(adl_meta_pointer_like<Type>::dereference(any_cast<const Type &>(from)));
+                        static_cast<meta_any *>(to)->emplace<in_place_type>(adl_meta_pointer_like<Type>::dereference(any_cast<const Type &>(from)));
+                    }
+                }
+                break;
+            case operation::SEQ:
+                if constexpr(is_complete_v<meta_sequence_container_traits<Type>>) {
+                    *static_cast<meta_sequence_container *>(to) = { std::in_place_type<Type>, std::move(const_cast<any &>(from)) };
+                }
+                break;
+            case operation::ASSOC:
+                if constexpr(is_complete_v<meta_associative_container_traits<Type>>) {
+                    *static_cast<meta_associative_container *>(to) = { std::in_place_type<Type>, std::move(const_cast<any &>(from)) };
+                }
+                break;
+            }
+        }
+    }
+
+    meta_any(const meta_any &other, any ref) ENTT_NOEXCEPT
+        : storage{std::move(ref)},
+          node{storage ? other.node : nullptr},
+          vtable{storage ? other.vtable : &basic_vtable<void>}
+    {}
+
+public:
+    /*! @brief Default constructor. */
+    meta_any() ENTT_NOEXCEPT
+        : storage{},
+          node{},
+          vtable{&basic_vtable<void>}
+    {}
+
+    /**
+     * @brief Constructs a wrapper by directly initializing the new object.
+     * @tparam Type Type of object to use to initialize the wrapper.
+     * @tparam Args Types of arguments to use to construct the new instance.
+     * @param args Parameters to use to construct the instance.
+     */
+    template<typename Type, typename... Args>
+    explicit meta_any(std::in_place_type_t<Type>, Args &&... args)
+        : storage{std::in_place_type<Type>, std::forward<Args>(args)...},
+          node{internal::meta_info<Type>::resolve()},
+          vtable{&basic_vtable<std::remove_const_t<std::remove_reference_t<Type>>>}
+    {}
+
+    /**
+     * @brief Constructs a wrapper that holds an unmanaged object.
+     * @tparam Type Type of object to use to initialize the wrapper.
+     * @param value An instance of an object to use to initialize the wrapper.
+     */
+    template<typename Type>
+    meta_any(std::reference_wrapper<Type> value)
+        : meta_any{}
+    {
+        // invokes deprecated assignment operator (and avoids issues with vs2017)
+        *this = value;
+    }
+
+    /**
+     * @brief Constructs a wrapper from a given value.
+     * @tparam Type Type of object to use to initialize the wrapper.
+     * @param value An instance of an object to use to initialize the wrapper.
+     */
+    template<typename Type, typename = std::enable_if_t<!std::is_same_v<std::decay_t<Type>, meta_any>>>
+    meta_any(Type &&value)
+        : storage{std::forward<Type>(value)},
+          node{internal::meta_info<std::decay_t<Type>>::resolve()},
+          vtable{&basic_vtable<std::decay_t<Type>>}
+    {}
+
+    /**
+     * @brief Copy constructor.
+     * @param other The instance to copy from.
+     */
+    meta_any(const meta_any &other) = default;
+
+    /**
+     * @brief Move constructor.
+     * @param other The instance to move from.
+     */
+    meta_any(meta_any &&other) ENTT_NOEXCEPT
+        : storage{std::move(other.storage)},
+          node{std::exchange(other.node, nullptr)},
+          vtable{std::exchange(other.vtable, &basic_vtable<void>)}
+    {}
+
+    /*! @brief Frees the internal storage, whatever it means. */
+    ~meta_any() {
+        vtable(operation::DTOR, storage, node);
+    }
+
+    /**
+     * @brief Copy assignment operator.
+     * @param other The instance to copy from.
+     * @return This meta any object.
+     */
+    meta_any & operator=(const meta_any &other) {
+        std::exchange(vtable, other.vtable)(operation::DTOR, storage, node);
+        storage = other.storage;
+        node = other.node;
+        return *this;
+    }
+
+    /**
+     * @brief Move assignment operator.
+     * @param other The instance to move from.
+     * @return This meta any object.
+     */
+    meta_any & operator=(meta_any &&other) ENTT_NOEXCEPT {
+        std::exchange(vtable, std::exchange(other.vtable, &basic_vtable<void>))(operation::DTOR, storage, node);
+        storage = std::move(other.storage);
+        node = std::exchange(other.node, nullptr);
+        return *this;
+    }
+
+    /**
+     * @brief Value assignment operator.
+     * @tparam Type Type of object to use to initialize the wrapper.
+     * @param value An instance of an object to use to initialize the wrapper.
+     * @return This meta any object.
+     */
+    template<typename Type>
+    [[deprecated("Use std::in_place_type<T &>, entt::make_meta<T &>, emplace<Type &> or forward_as_meta instead")]]
+    meta_any & operator=(std::reference_wrapper<Type> value) {
+        emplace<Type &>(value.get());
+        return *this;
+    }
+
+    /**
+     * @brief Value assignment operator.
+     * @tparam Type Type of object to use to initialize the wrapper.
+     * @param value An instance of an object to use to initialize the wrapper.
+     * @return This meta any object.
+     */
+    template<typename Type>
+    std::enable_if_t<!std::is_same_v<std::decay_t<Type>, meta_any>, meta_any &>
+    operator=(Type &&value) {
+        emplace<std::decay_t<Type>>(std::forward<Type>(value));
+        return *this;
+    }
+
+    /**
+     * @brief Returns the type of the underlying object.
+     * @return The type of the underlying object, if any.
+     */
+    [[nodiscard]] inline meta_type type() const ENTT_NOEXCEPT;
+
+    /**
+     * @brief Returns an opaque pointer to the contained instance.
+     * @return An opaque pointer the contained instance, if any.
+     */
+    [[nodiscard]] const void * data() const ENTT_NOEXCEPT {
+        return storage.data();
+    }
+
+    /*! @copydoc data */
+    [[nodiscard]] void * data() ENTT_NOEXCEPT {
+        return storage.data();
+    }
+
+    /**
+     * @brief Invokes the underlying function, if possible.
+     *
+     * @sa meta_func::invoke
+     *
+     * @tparam Args Types of arguments to use to invoke the function.
+     * @param id Unique identifier.
+     * @param args Parameters to use to invoke the function.
+     * @return A wrapper containing the returned value, if any.
+     */
+    template<typename... Args>
+    meta_any invoke(const id_type id, Args &&... args) const;
+
+    /*! @copydoc invoke */
+    template<typename... Args>
+    meta_any invoke(const id_type id, Args &&... args);
+
+    /**
+     * @brief Sets the value of a given variable.
+     *
+     * The type of the value must be such that a cast or conversion to the type
+     * of the variable is possible. Otherwise, invoking the setter does nothing.
+     *
+     * @tparam Type Type of value to assign.
+     * @param id Unique identifier.
+     * @param value Parameter to use to set the underlying variable.
+     * @return True in case of success, false otherwise.
+     */
+    template<typename Type>
+    bool set(const id_type id, Type &&value);
+
+    /**
+     * @brief Gets the value of a given variable.
+     * @param id Unique identifier.
+     * @return A wrapper containing the value of the underlying variable.
+     */
+    [[nodiscard]] meta_any get(const id_type id) const;
+
+    /*! @copydoc get */
+    [[nodiscard]] meta_any get(const id_type id);
+
+    /**
+     * @brief Tries to cast an instance to a given type.
+     * @tparam Type Type to which to cast the instance.
+     * @return A (possibly null) pointer to the contained instance.
+     */
+    template<typename Type>
+    [[nodiscard]] const Type * try_cast() const {
+        if(node) {
+            if(const auto info = type_id<Type>(); node->info == info) {
+                return any_cast<Type>(&storage);
+            } else if(const auto *base = internal::meta_visit<&internal::meta_type_node::base>([info](const auto *curr) { return curr->type()->info == info; }, node); base) {
+                return static_cast<const Type *>(base->cast(storage.data()));
+            }
+        }
+
+        return nullptr;
+    }
+
+    /*! @copydoc try_cast */
+    template<typename Type>
+    [[nodiscard]] Type * try_cast() {
+        if(node) {
+            if(const auto info = type_id<Type>(); node->info == info) {
+                return any_cast<Type>(&storage);
+            } else if(const auto *base = internal::meta_visit<&internal::meta_type_node::base>([info](const auto *curr) { return curr->type()->info == info; }, node); base) {
+                return static_cast<Type *>(const_cast<constness_as_t<void, Type> *>(base->cast(static_cast<constness_as_t<any, Type> &>(storage).data())));
+            }
+        }
+
+        return nullptr;
+    }
+
+    /**
+     * @brief Tries to cast an instance to a given type.
+     *
+     * The type of the instance must be such that the cast is possible.
+     *
+     * @warning
+     * Attempting to perform an invalid cast results in undefined behavior.
+     *
+     * @tparam Type Type to which to cast the instance.
+     * @return A reference to the contained instance.
+     */
+    template<typename Type>
+    [[nodiscard]] Type cast() const {
+        auto * const instance = try_cast<std::remove_reference_t<Type>>();
+        ENTT_ASSERT(instance, "Invalid instance");
+        return static_cast<Type>(*instance);
+    }
+
+    /*! @copydoc cast */
+    template<typename Type>
+    [[nodiscard]] Type cast() {
+        // forces const on non-reference types to make them work also with wrappers for const references
+        auto * const instance = try_cast<std::remove_reference_t<const Type>>();
+        ENTT_ASSERT(instance, "Invalid instance");
+        return static_cast<Type>(*instance);
+    }
+
+    /**
+     * @brief Converts an object in such a way that a given cast becomes viable.
+     * @tparam Type Type to which the cast is requested.
+     * @return A valid meta any object if there exists a viable conversion, an
+     * invalid one otherwise.
+     */
+    template<typename Type>
+    [[nodiscard]] meta_any allow_cast() const {
+        if(try_cast<std::remove_reference_t<Type>>() != nullptr) {
+            return as_ref();
+        } else if(node) {
+            if(const auto * const conv = internal::meta_visit<&internal::meta_type_node::conv>([info = type_id<Type>()](const auto *curr) { return curr->type()->info == info; }, node); conv) {
+                return conv->conv(storage.data());
+            }
+        }
+
+        return {};
+    }
+
+    /**
+     * @brief Converts an object in such a way that a given cast becomes viable.
+     * @tparam Type Type to which the cast is requested.
+     * @return True if there exists a viable conversion, false otherwise.
+     */
+    template<typename Type>
+    bool allow_cast() {
+        // forces const on non-reference types to make them work also with wrappers for const references
+        if(try_cast<std::remove_reference_t<const Type>>() != nullptr) {
+            return true;
+        } else if(node) {
+            if(const auto * const conv = internal::meta_visit<&internal::meta_type_node::conv>([info = type_id<Type>()](const auto *curr) { return curr->type()->info == info; }, node); conv) {
+                *this = conv->conv(std::as_const(storage).data());
+                return true;
+            }
+        }
+
+        return false;
+    }
+
+    /**
+     * @brief Replaces the contained object by creating a new instance directly.
+     * @tparam Type Type of object to use to initialize the wrapper.
+     * @tparam Args Types of arguments to use to construct the new instance.
+     * @param args Parameters to use to construct the instance.
+     */
+    template<typename Type, typename... Args>
+    void emplace(Args &&... args) {
+        std::exchange(vtable, &basic_vtable<std::remove_const_t<std::remove_reference_t<Type>>>)(operation::DTOR, storage, node);
+        storage.emplace<Type>(std::forward<Args>(args)...);
+        node = internal::meta_info<Type>::resolve();
+    }
+
+    /*! @brief Destroys contained object */
+    void reset() {
+        std::exchange(vtable, &basic_vtable<void>)(operation::DTOR, storage, node);
+        storage.reset();
+        node = nullptr;
+    }
+
+    /**
+     * @brief Returns a sequence container proxy.
+     * @return A sequence container proxy for the underlying object.
+     */
+    [[nodiscard]] meta_sequence_container as_sequence_container() ENTT_NOEXCEPT {
+        meta_sequence_container proxy;
+        vtable(operation::SEQ, storage.as_ref(), &proxy);
+        return proxy;
+    }
+
+    /*! @copydoc as_sequence_container */
+    [[nodiscard]] meta_sequence_container as_sequence_container() const ENTT_NOEXCEPT {
+        meta_sequence_container proxy;
+        vtable(operation::SEQ, storage.as_ref(), &proxy);
+        return proxy;
+    }
+
+    /**
+     * @brief Returns an associative container proxy.
+     * @return An associative container proxy for the underlying object.
+     */
+    [[nodiscard]] meta_associative_container as_associative_container() ENTT_NOEXCEPT {
+        meta_associative_container proxy;
+        vtable(operation::ASSOC, storage.as_ref(), &proxy);
+        return proxy;
+    }
+
+    /*! @copydoc as_associative_container */
+    [[nodiscard]] meta_associative_container as_associative_container() const ENTT_NOEXCEPT {
+        meta_associative_container proxy;
+        vtable(operation::ASSOC, storage.as_ref(), &proxy);
+        return proxy;
+    }
+
+    /**
+     * @brief Indirection operator for dereferencing opaque objects.
+     * @return A wrapper that shares a reference to an unmanaged object if the
+     * wrapped element is dereferenceable, an invalid meta any otherwise.
+     */
+    [[nodiscard]] meta_any operator*() const ENTT_NOEXCEPT {
+        meta_any ret{};
+        vtable(operation::DEREF, storage, &ret);
+        return ret;
+    }
+
+    /**
+     * @brief Returns false if a wrapper is invalid, true otherwise.
+     * @return False if the wrapper is invalid, true otherwise.
+     */
+    [[nodiscard]] explicit operator bool() const ENTT_NOEXCEPT {
+        return !(node == nullptr);
+    }
+
+    /**
+     * @brief Checks if two wrappers differ in their content.
+     * @param other Wrapper with which to compare.
+     * @return False if the two objects differ in their content, true otherwise.
+     */
+    [[nodiscard]] bool operator==(const meta_any &other) const {
+        return (!node && !other.node) || (node && other.node && node->info == other.node->info && storage == other.storage);
+    }
+
+    /**
+     * @brief Aliasing constructor.
+     * @return A wrapper that shares a reference to an unmanaged object.
+     */
+    [[nodiscard]] meta_any as_ref() ENTT_NOEXCEPT {
+        return meta_any{*this, storage.as_ref()};
+    }
+
+    /*! @copydoc as_ref */
+    [[nodiscard]] meta_any as_ref() const ENTT_NOEXCEPT {
+        return meta_any{*this, storage.as_ref()};
+    }
+
+private:
+    any storage;
+    internal::meta_type_node *node;
+    vtable_type *vtable;
+};
+
+
+/**
+ * @brief Checks if two wrappers differ in their content.
+ * @param lhs A wrapper, either empty or not.
+ * @param rhs A wrapper, either empty or not.
+ * @return True if the two wrappers differ in their content, false otherwise.
+ */
+[[nodiscard]] inline bool operator!=(const meta_any &lhs, const meta_any &rhs) ENTT_NOEXCEPT {
+    return !(lhs == rhs);
+}
+
+
+/**
+ * @brief Constructs a wrapper from a given type, passing it all arguments.
+ * @tparam Type Type of object to use to initialize the wrapper.
+ * @tparam Args Types of arguments to use to construct the new instance.
+ * @param args Parameters to use to construct the instance.
+ * @return A properly initialized wrapper for an object of the given type.
+ */
+template<typename Type, typename... Args>
+meta_any make_meta(Args &&... args) {
+    return meta_any{std::in_place_type<Type>, std::forward<Args>(args)...};
+}
+
+
+/**
+ * @brief Forwards its argument and avoids copies for lvalue references.
+ * @tparam Type Type of argument to use to construct the new instance.
+ * @param value Parameter to use to construct the instance.
+ * @return A properly initialized and not necessarily owning wrapper.
+ */
+template<typename Type>
+meta_any forward_as_meta(Type &&value) {
+    return meta_any{std::in_place_type<std::conditional_t<std::is_rvalue_reference_v<Type>, std::decay_t<Type>, Type>>, std::forward<Type>(value)};
+}
+
+
+/**
+ * @brief Opaque pointers to instances of any type.
+ *
+ * A handle doesn't perform copies and isn't responsible for the contained
+ * object. It doesn't prolong the lifetime of the pointed instance.<br/>
+ * Handles are used to generate references to actual objects when needed.
+ */
+struct meta_handle {
+    /*! @brief Default constructor. */
+    meta_handle() = default;
+
+
+    /*! @brief Default copy constructor, deleted on purpose. */
+    meta_handle(const meta_handle &) = delete;
+
+    /*! @brief Default move constructor. */
+    meta_handle(meta_handle &&) = default;
+
+    /**
+     * @brief Default copy assignment operator, deleted on purpose.
+     * @return This meta handle.
+     */
+    meta_handle & operator=(const meta_handle &) = delete;
+
+    /**
+     * @brief Default move assignment operator.
+     * @return This meta handle.
+     */
+    meta_handle & operator=(meta_handle &&) = default;
+
+    /**
+     * @brief Creates a handle that points to an unmanaged object.
+     * @tparam Type Type of object to use to initialize the handle.
+     * @param value An instance of an object to use to initialize the handle.
+     */
+    template<typename Type, typename = std::enable_if_t<!std::is_same_v<std::decay_t<Type>, meta_handle>>>
+    meta_handle(Type &value) ENTT_NOEXCEPT
+        : meta_handle{}
+    {
+        if constexpr(std::is_same_v<std::decay_t<Type>, meta_any>) {
+            any = value.as_ref();
+        } else {
+            any.emplace<Type &>(value);
+        }
+    }
+
+    /**
+     * @brief Returns false if a handle is invalid, true otherwise.
+     * @return False if the handle is invalid, true otherwise.
+     */
+    [[nodiscard]] explicit operator bool() const ENTT_NOEXCEPT {
+        return static_cast<bool>(any);
+    }
+
+    /**
+     * @brief Access operator for accessing the contained opaque object.
+     * @return A wrapper that shares a reference to an unmanaged object.
+     */
+    [[nodiscard]] meta_any * operator->() {
+        return &any;
+    }
+
+    /*! @copydoc operator-> */
+    [[nodiscard]] const meta_any * operator->() const {
+        return &any;
+    }
+
+private:
+    meta_any any;
+};
+
+
+/*! @brief Opaque wrapper for properties of any type. */
+struct meta_prop {
+    /*! @brief Node type. */
+    using node_type = internal::meta_prop_node;
+
+    /**
+     * @brief Constructs an instance from a given node.
+     * @param curr The underlying node with which to construct the instance.
+     */
+    meta_prop(const node_type *curr = nullptr) ENTT_NOEXCEPT
+        : node{curr}
+    {}
+
+    /**
+     * @brief Returns the stored key as a const reference.
+     * @return A wrapper containing the key stored with the property.
+     */
+    [[nodiscard]] meta_any key() const {
+        return node->id.as_ref();
+    }
+
+    /**
+     * @brief Returns the stored value by copy.
+     * @return A wrapper containing the value stored with the property.
+     */
+    [[nodiscard]] meta_any value() const {
+        return node->value;
+    }
+
+    /**
+     * @brief Returns true if an object is valid, false otherwise.
+     * @return True if the object is valid, false otherwise.
+     */
+    [[nodiscard]] explicit operator bool() const ENTT_NOEXCEPT {
+        return !(node == nullptr);
+    }
+
+private:
+    const node_type *node;
+};
+
+
+/*! @brief Opaque wrapper for constructors. */
+struct meta_ctor {
+    /*! @brief Node type. */
+    using node_type = internal::meta_ctor_node;
+    /*! @brief Unsigned integer type. */
+    using size_type = typename node_type::size_type;
+
+    /*! @copydoc meta_prop::meta_prop */
+    meta_ctor(const node_type *curr = nullptr) ENTT_NOEXCEPT
+        : node{curr}
+    {}
+
+    /**
+     * @brief Returns the type to which an object belongs.
+     * @return The type to which the object belongs.
+     */
+    [[nodiscard]] inline meta_type parent() const ENTT_NOEXCEPT;
+
+    /**
+     * @brief Returns the number of arguments accepted by a constructor.
+     * @return The number of arguments accepted by the constructor.
+     */
+    [[nodiscard]] size_type arity() const ENTT_NOEXCEPT {
+        return node->arity;
+    }
+
+    /**
+     * @brief Returns the type of the i-th argument of a constructor.
+     * @param index Index of the argument of which to return the type.
+     * @return The type of the i-th argument of a constructor.
+     */
+    [[nodiscard]] meta_type arg(size_type index) const ENTT_NOEXCEPT;
+
+    /**
+     * @brief Creates an instance of the underlying type, if possible.
+     *
+     * Parameters must be such that a cast or conversion to the required types
+     * is possible. Otherwise, an empty and thus invalid wrapper is returned.
+     *
+     * @param args Parameters to use to construct the instance.
+     * @param sz Number of parameters to use to construct the instance.
+     * @return A wrapper containing the new instance, if any.
+     */
+    [[nodiscard]] meta_any invoke(meta_any * const args, const size_type sz) const {
+        return sz == arity() ? node->invoke(args) : meta_any{};
+    }
+
+    /**
+     * @copybrief invoke
+     *
+     * @sa invoke
+     *
+     * @tparam Args Types of arguments to use to construct the instance.
+     * @param args Parameters to use to construct the instance.
+     * @return A wrapper containing the new instance, if any.
+     */
+    template<typename... Args>
+    [[nodiscard]] meta_any invoke([[maybe_unused]] Args &&... args) const {
+        meta_any arguments[sizeof...(Args) + 1u]{std::forward<Args>(args)...};
+        return invoke(arguments, sizeof...(Args));
+    }
+
+    /**
+     * @brief Returns a range to use to visit all properties.
+     * @return An iterable range to use to visit all properties.
+     */
+    [[nodiscard]] meta_range<meta_prop> prop() const ENTT_NOEXCEPT {
+        return node->prop;
+    }
+
+    /**
+     * @brief Returns the property associated with a given key.
+     * @param key The key to use to search for a property.
+     * @return The property associated with the given key, if any.
+     */
+    [[nodiscard]] meta_prop prop(meta_any key) const {
+        return internal::meta_visit<&node_type::prop>([&key](const auto *curr) { return curr->id == key; }, node);
+    }
+
+    /**
+     * @brief Returns true if an object is valid, false otherwise.
+     * @return True if the object is valid, false otherwise.
+     */
+    [[nodiscard]] explicit operator bool() const ENTT_NOEXCEPT {
+        return !(node == nullptr);
+    }
+
+private:
+    const node_type *node;
+};
+
+
+/*! @brief Opaque wrapper for data members. */
+struct meta_data {
+    /*! @brief Node type. */
+    using node_type = internal::meta_data_node;
+
+    /*! @copydoc meta_prop::meta_prop */
+    meta_data(const node_type *curr = nullptr) ENTT_NOEXCEPT
+        : node{curr}
+    {}
+
+    /*! @copydoc meta_type::id */
+    [[nodiscard]] id_type id() const ENTT_NOEXCEPT {
+        return node->id;
+    }
+
+    /*! @copydoc meta_ctor::parent */
+    [[nodiscard]] inline meta_type parent() const ENTT_NOEXCEPT;
+
+    /**
+     * @brief Indicates whether a data member is constant or not.
+     * @return True if the data member is constant, false otherwise.
+     */
+    [[nodiscard]] bool is_const() const ENTT_NOEXCEPT {
+        return node->is_const;
+    }
+
+    /**
+     * @brief Indicates whether a data member is static or not.
+     * @return True if the data member is static, false otherwise.
+     */
+    [[nodiscard]] bool is_static() const ENTT_NOEXCEPT {
+        return node->is_static;
+    }
+
+    /*! @copydoc meta_any::type */
+    [[nodiscard]] inline meta_type type() const ENTT_NOEXCEPT;
+
+    /**
+     * @brief Sets the value of a given variable.
+     *
+     * It must be possible to cast the instance to the parent type of the data
+     * member. Otherwise, invoking the setter results in an undefined
+     * behavior.<br/>
+     * The type of the value must be such that a cast or conversion to the type
+     * of the variable is possible. Otherwise, invoking the setter does nothing.
+     *
+     * @tparam Type Type of value to assign.
+     * @param instance An opaque instance of the underlying type.
+     * @param value Parameter to use to set the underlying variable.
+     * @return True in case of success, false otherwise.
+     */
+    template<typename Type>
+    bool set(meta_handle instance, Type &&value) const {
+        return node->set && node->set(std::move(instance), std::forward<Type>(value));
+    }
+
+    /**
+     * @brief Gets the value of a given variable.
+     *
+     * It must be possible to cast the instance to the parent type of the data
+     * member. Otherwise, invoking the getter results in an undefined behavior.
+     *
+     * @param instance An opaque instance of the underlying type.
+     * @return A wrapper containing the value of the underlying variable.
+     */
+    [[nodiscard]] meta_any get(meta_handle instance) const {
+        return node->get(std::move(instance));
+    }
+
+    /*! @copydoc meta_ctor::prop */
+    [[nodiscard]] meta_range<meta_prop> prop() const ENTT_NOEXCEPT {
+        return node->prop;
+    }
+
+    /**
+     * @brief Returns the property associated with a given key.
+     * @param key The key to use to search for a property.
+     * @return The property associated with the given key, if any.
+     */
+    [[nodiscard]] meta_prop prop(meta_any key) const {
+        return internal::meta_visit<&node_type::prop>([&key](const auto *curr) { return curr->id == key; }, node);
+    }
+
+    /**
+     * @brief Returns true if an object is valid, false otherwise.
+     * @return True if the object is valid, false otherwise.
+     */
+    [[nodiscard]] explicit operator bool() const ENTT_NOEXCEPT {
+        return !(node == nullptr);
+    }
+
+private:
+    const node_type *node;
+};
+
+
+/*! @brief Opaque wrapper for member functions. */
+struct meta_func {
+    /*! @brief Node type. */
+    using node_type = internal::meta_func_node;
+    /*! @brief Unsigned integer type. */
+    using size_type = typename node_type::size_type;
+
+    /*! @copydoc meta_prop::meta_prop */
+    meta_func(const node_type *curr = nullptr) ENTT_NOEXCEPT
+        : node{curr}
+    {}
+
+    /*! @copydoc meta_type::id */
+    [[nodiscard]] id_type id() const ENTT_NOEXCEPT {
+        return node->id;
+    }
+
+    /*! @copydoc meta_ctor::parent */
+    [[nodiscard]] inline meta_type parent() const ENTT_NOEXCEPT;
+
+    /**
+     * @brief Returns the number of arguments accepted by a member function.
+     * @return The number of arguments accepted by the member function.
+     */
+    [[nodiscard]] size_type arity() const ENTT_NOEXCEPT {
+        return node->arity;
+    }
+
+    /**
+     * @brief Indicates whether a member function is constant or not.
+     * @return True if the member function is constant, false otherwise.
+     */
+    [[nodiscard]] bool is_const() const ENTT_NOEXCEPT {
+        return node->is_const;
+    }
+
+    /**
+     * @brief Indicates whether a member function is static or not.
+     * @return True if the member function is static, false otherwise.
+     */
+    [[nodiscard]] bool is_static() const ENTT_NOEXCEPT {
+        return node->is_static;
+    }
+
+    /**
+     * @brief Returns the return type of a member function.
+     * @return The return type of the member function.
+     */
+    [[nodiscard]] inline meta_type ret() const ENTT_NOEXCEPT;
+
+    /**
+     * @brief Returns the type of the i-th argument of a member function.
+     * @param index Index of the argument of which to return the type.
+     * @return The type of the i-th argument of a member function.
+     */
+    [[nodiscard]] inline meta_type arg(size_type index) const ENTT_NOEXCEPT;
+
+    /**
+     * @brief Invokes the underlying function, if possible.
+     *
+     * To invoke a member function, the parameters must be such that a cast or
+     * conversion to the required types is possible. Otherwise, an empty and
+     * thus invalid wrapper is returned.<br/>
+     * It must be possible to cast the instance to the parent type of the member
+     * function. Otherwise, invoking the underlying function results in an
+     * undefined behavior.
+     *
+     * @param instance An opaque instance of the underlying type.
+     * @param args Parameters to use to invoke the function.
+     * @param sz Number of parameters to use to invoke the function.
+     * @return A wrapper containing the returned value, if any.
+     */
+    meta_any invoke(meta_handle instance, meta_any * const args, const size_type sz) const {
+        return sz == arity() ? node->invoke(std::move(instance), args) : meta_any{};
+    }
+
+    /**
+     * @copybrief invoke
+     *
+     * @sa invoke
+     *
+     * @tparam Args Types of arguments to use to invoke the function.
+     * @param instance An opaque instance of the underlying type.
+     * @param args Parameters to use to invoke the function.
+     * @return A wrapper containing the new instance, if any.
+     */
+    template<typename... Args>
+    meta_any invoke(meta_handle instance, Args &&... args) const {
+        meta_any arguments[sizeof...(Args) + 1u]{std::forward<Args>(args)...};
+        return invoke(std::move(instance), arguments, sizeof...(Args));
+    }
+
+    /*! @copydoc meta_ctor::prop */
+    [[nodiscard]] meta_range<meta_prop> prop() const ENTT_NOEXCEPT {
+        return node->prop;
+    }
+
+    /**
+     * @brief Returns the property associated with a given key.
+     * @param key The key to use to search for a property.
+     * @return The property associated with the given key, if any.
+     */
+    [[nodiscard]] meta_prop prop(meta_any key) const {
+        return internal::meta_visit<&node_type::prop>([&key](const auto *curr) { return curr->id == key; }, node);
+    }
+
+    /**
+     * @brief Returns true if an object is valid, false otherwise.
+     * @return True if the object is valid, false otherwise.
+     */
+    [[nodiscard]] explicit operator bool() const ENTT_NOEXCEPT {
+        return !(node == nullptr);
+    }
+
+private:
+    const node_type *node;
+};
+
+
+/*! @brief Opaque wrapper for types. */
+class meta_type {
+    static bool can_cast_or_convert(const internal::meta_type_node *type, const type_info info) ENTT_NOEXCEPT {
+        if(type->info == info) {
+            return true;
+        }
+
+        for(const auto *curr = type->conv; curr; curr = curr->next) {
+            if(curr->type()->info == info) {
+                return true;
+            }
+        }
+
+        for(const auto *curr = type->base; curr; curr = curr->next) {
+            if(auto *target = curr->type(); can_cast_or_convert(target, info)) {
+                return true;
+            }
+        }
+
+        return false;
+    }
+
+    template<typename... Args, auto... Index>
+    [[nodiscard]] static const internal::meta_ctor_node * ctor(const internal::meta_ctor_node *curr, std::index_sequence<Index...>) {
+        for(; curr; curr = curr->next) {
+            if(curr->arity == sizeof...(Args) && (can_cast_or_convert(internal::meta_info<Args>::resolve(), curr->arg(Index).info()) && ...)) {
+                return curr;
+            }
+        }
+
+        return nullptr;
+    }
+
+    template<auto... Member, typename Node>
+    void unregister_all(Node **curr) {
+        while(*curr) {
+            (unregister_all(&((*curr)->*Member)), ...);
+            *curr = std::exchange((*curr)->next, nullptr);
+        }
+    }
+
+public:
+    /*! @brief Node type. */
+    using node_type = internal::meta_type_node;
+    /*! @brief Node type. */
+    using base_node_type = internal::meta_base_node;
+    /*! @brief Unsigned integer type. */
+    using size_type = typename node_type::size_type;
+
+    /*! @copydoc meta_prop::meta_prop */
+    meta_type(node_type *curr = nullptr) ENTT_NOEXCEPT
+        : node{curr}
+    {}
+
+    /**
+     * @brief Constructs an instance from a given base node.
+     * @param curr The base node with which to construct the instance.
+     */
+    meta_type(base_node_type *curr) ENTT_NOEXCEPT
+        : node{curr ? curr->type() : nullptr}
+    {}
+
+    /**
+     * @brief Returns the type info object of the underlying type.
+     * @return The type info object of the underlying type.
+     */
+    [[nodiscard]] type_info info() const ENTT_NOEXCEPT {
+        return node->info;
+    }
+
+    /**
+     * @brief Returns the identifier assigned to a type.
+     * @return The identifier assigned to the type.
+     */
+    [[nodiscard]] id_type id() const ENTT_NOEXCEPT {
+        return node->id;
+    }
+
+    /**
+     * @brief Returns the size of the underlying type if known.
+     * @return The size of the underlying type if known, 0 otherwise.
+     */
+    [[nodiscard]] size_type size_of() const ENTT_NOEXCEPT {
+        return node->size_of;
+    }
+
+    /**
+     * @brief Checks whether a type refers to void or not.
+     * @return True if the underlying type is void, false otherwise.
+     */
+    [[nodiscard]] bool is_void() const ENTT_NOEXCEPT {
+        return node->is_void;
+    }
+
+    /**
+     * @brief Checks whether a type refers to an integral type or not.
+     * @return True if the underlying type is an integral type, false otherwise.
+     */
+    [[nodiscard]] bool is_integral() const ENTT_NOEXCEPT {
+        return node->is_integral;
+    }
+
+    /**
+     * @brief Checks whether a type refers to a floating-point type or not.
+     * @return True if the underlying type is a floating-point type, false
+     * otherwise.
+     */
+    [[nodiscard]] bool is_floating_point() const ENTT_NOEXCEPT {
+        return node->is_floating_point;
+    }
+
+    /**
+     * @brief Checks whether a type refers to an array type or not.
+     * @return True if the underlying type is an array type, false otherwise.
+     */
+    [[nodiscard]] bool is_array() const ENTT_NOEXCEPT {
+        return node->is_array;
+    }
+
+    /**
+     * @brief Checks whether a type refers to an enum or not.
+     * @return True if the underlying type is an enum, false otherwise.
+     */
+    [[nodiscard]] bool is_enum() const ENTT_NOEXCEPT {
+        return node->is_enum;
+    }
+
+    /**
+     * @brief Checks whether a type refers to an union or not.
+     * @return True if the underlying type is an union, false otherwise.
+     */
+    [[nodiscard]] bool is_union() const ENTT_NOEXCEPT {
+        return node->is_union;
+    }
+
+    /**
+     * @brief Checks whether a type refers to a class or not.
+     * @return True if the underlying type is a class, false otherwise.
+     */
+    [[nodiscard]] bool is_class() const ENTT_NOEXCEPT {
+        return node->is_class;
+    }
+
+    /**
+     * @brief Checks whether a type refers to a pointer or not.
+     * @return True if the underlying type is a pointer, false otherwise.
+     */
+    [[nodiscard]] bool is_pointer() const ENTT_NOEXCEPT {
+        return node->is_pointer;
+    }
+
+    /**
+     * @brief Checks whether a type refers to a function pointer or not.
+     * @return True if the underlying type is a function pointer, false
+     * otherwise.
+     */
+    [[nodiscard]] bool is_function_pointer() const ENTT_NOEXCEPT {
+        return node->is_function_pointer;
+    }
+
+    /**
+     * @brief Checks whether a type refers to a pointer to data member or not.
+     * @return True if the underlying type is a pointer to data member, false
+     * otherwise.
+     */
+    [[nodiscard]] bool is_member_object_pointer() const ENTT_NOEXCEPT {
+        return node->is_member_object_pointer;
+    }
+
+    /**
+     * @brief Checks whether a type refers to a pointer to member function or
+     * not.
+     * @return True if the underlying type is a pointer to member function,
+     * false otherwise.
+     */
+    [[nodiscard]] bool is_member_function_pointer() const ENTT_NOEXCEPT {
+        return node->is_member_function_pointer;
+    }
+
+    /**
+     * @brief Checks whether a type is a pointer-like type or not.
+     * @return True if the underlying type is a pointer-like one, false
+     * otherwise.
+     */
+    [[nodiscard]] bool is_pointer_like() const ENTT_NOEXCEPT {
+        return node->is_pointer_like;
+    }
+
+    /**
+     * @brief Checks whether a type refers to a sequence container or not.
+     * @return True if the type is a sequence container, false otherwise.
+     */
+    [[nodiscard]] bool is_sequence_container() const ENTT_NOEXCEPT {
+        return node->is_sequence_container;
+    }
+
+    /**
+     * @brief Checks whether a type refers to an associative container or not.
+     * @return True if the type is an associative container, false otherwise.
+     */
+    [[nodiscard]] bool is_associative_container() const ENTT_NOEXCEPT {
+        return node->is_associative_container;
+    }
+
+    /**
+     * @brief Checks whether a type refers to a recognized class template
+     * specialization or not.
+     * @return True if the type is a recognized class template specialization,
+     * false otherwise.
+     */
+    [[nodiscard]] bool is_template_specialization() const ENTT_NOEXCEPT {
+        return node->template_info.is_template_specialization;
+    }
+
+    /**
+     * @brief Returns the number of template arguments, if any.
+     * @return The number of template arguments, if any.
+     */
+    [[nodiscard]] size_type template_arity() const ENTT_NOEXCEPT {
+        return node->template_info.arity;
+    }
+
+    /**
+     * @brief Returns a tag for the class template of the underlying type.
+     *
+     * @sa meta_class_template_tag
+     *
+     * @return The tag for the class template of the underlying type.
+     */
+    [[nodiscard]] inline meta_type template_type() const ENTT_NOEXCEPT {
+        return is_template_specialization() ?  node->template_info.type() : meta_type{};
+    }
+
+    /**
+     * @brief Returns the type of the i-th template argument of a type.
+     * @param index Index of the template argument of which to return the type.
+     * @return The type of the i-th template argument of a type.
+     */
+    [[nodiscard]] inline meta_type template_arg(size_type index) const ENTT_NOEXCEPT {
+        return index < template_arity() ? node->template_info.arg(index) : meta_type{};
+    }
+
+    /**
+     * @brief Provides the number of dimensions of an array type.
+     * @return The number of dimensions in case of array types, 0 otherwise.
+     */
+    [[nodiscard]] size_type rank() const ENTT_NOEXCEPT {
+        return node->rank;
+    }
+
+    /**
+     * @brief The number of elements along the given dimension of an array type.
+     * @param dim The dimension of which to return the number of elements.
+     * @return The number of elements along the given dimension in case of array
+     * types, 0 otherwise.
+     */
+    [[nodiscard]] size_type extent(size_type dim = {}) const ENTT_NOEXCEPT {
+        return node->extent(dim);
+    }
+
+    /**
+     * @brief Provides the type for which the pointer is defined.
+     * @return The type for which the pointer is defined or this type if it
+     * doesn't refer to a pointer type.
+     */
+    [[nodiscard]] meta_type remove_pointer() const ENTT_NOEXCEPT {
+        return node->remove_pointer();
+    }
+
+    /**
+     * @brief Provides the type for which the array is defined.
+     * @return The type for which the array is defined or this type if it
+     * doesn't refer to an array type.
+     */
+    [[nodiscard]] meta_type remove_extent() const ENTT_NOEXCEPT {
+        return node->remove_extent();
+    }
+
+    /**
+     * @brief Returns a range to use to visit top-level base meta types.
+     * @return An iterable range to use to visit top-level base meta types.
+     */
+    [[nodiscard]] meta_range<meta_type, internal::meta_base_node> base() const ENTT_NOEXCEPT {
+        return node->base;
+    }
+
+    /**
+     * @brief Returns the base meta type associated with a given identifier.
+     * @param id Unique identifier.
+     * @return The base meta type associated with the given identifier, if any.
+     */
+    [[nodiscard]] meta_type base(const id_type id) const {
+        return internal::meta_visit<&node_type::base>([id](const auto *curr) { return curr->type()->id == id; }, node);
+    }
+
+    /**
+     * @brief Returns a range to use to visit top-level constructors.
+     * @return An iterable range to use to visit top-level constructors.
+     */
+    [[nodiscard]] meta_range<meta_ctor> ctor() const ENTT_NOEXCEPT {
+        return node->ctor;
+    }
+
+    /**
+     * @brief Returns a constructor for a given list of types of arguments.
+     * @tparam Args Constructor arguments.
+     * @return The requested constructor, if any.
+     */
+    template<typename... Args>
+    [[nodiscard]] meta_ctor ctor() const {
+        return ctor<Args...>(node->ctor, std::make_index_sequence<sizeof...(Args)>{});
+    }
+
+    /**
+     * @brief Returns a range to use to visit top-level data.
+     * @return An iterable range to use to visit top-level data.
+     */
+    [[nodiscard]] meta_range<meta_data> data() const ENTT_NOEXCEPT {
+        return node->data;
+    }
+
+    /**
+     * @brief Returns the data associated with a given identifier.
+     *
+     * The data of the base classes will also be visited, if any.
+     *
+     * @param id Unique identifier.
+     * @return The data associated with the given identifier, if any.
+     */
+    [[nodiscard]] meta_data data(const id_type id) const {
+        return internal::meta_visit<&node_type::data>([id](const auto *curr) { return curr->id == id; }, node);
+    }
+
+    /**
+     * @brief Returns a range to use to visit top-level functions.
+     * @return An iterable range to use to visit top-level functions.
+     */
+    [[nodiscard]] meta_range<meta_func> func() const ENTT_NOEXCEPT {
+        return node->func;
+    }
+
+    /**
+     * @brief Returns the function associated with a given identifier.
+     *
+     * The functions of the base classes will also be visited, if any.<br/>
+     * In the case of overloaded functions, the first one with the required
+     * identifier will be returned.
+     *
+     * @param id Unique identifier.
+     * @return The function associated with the given identifier, if any.
+     */
+    [[nodiscard]] meta_func func(const id_type id) const {
+        return internal::meta_visit<&node_type::func>([id](const auto *curr) { return curr->id == id; }, node);
+    }
+
+    /**
+     * @brief Creates an instance of the underlying type, if possible.
+     *
+     * Parameters must be such that a cast or conversion to the required types
+     * is possible. Otherwise, an empty and thus invalid wrapper is returned.
+     *
+     * @param args Parameters to use to construct the instance.
+     * @param sz Number of parameters to use to construct the instance.
+     * @return A wrapper containing the new instance, if any.
+     */
+    [[nodiscard]] meta_any construct(meta_any * const args, const size_type sz) const {
+        meta_any ret{};
+        internal::meta_visit<&node_type::ctor>([args, sz, &ret](const auto *curr) { return (curr->arity == sz) && (ret = curr->invoke(args)); }, node);
+        return ret;
+    }
+
+    /**
+     * @copybrief construct
+     *
+     * @sa construct
+     *
+     * @tparam Args Types of arguments to use to construct the instance.
+     * @param args Parameters to use to construct the instance.
+     * @return A wrapper containing the new instance, if any.
+     */
+    template<typename... Args>
+    [[nodiscard]] meta_any construct(Args &&... args) const {
+        meta_any arguments[sizeof...(Args) + 1u]{std::forward<Args>(args)...};
+        return construct(arguments, sizeof...(Args));
+    }
+
+    /**
+     * @brief Invokes a function given an identifier, if possible.
+     *
+     * It must be possible to cast the instance to the parent type of the member
+     * function. Otherwise, invoking the underlying function results in an
+     * undefined behavior.
+     *
+     * @sa meta_func::invoke
+     *
+     * @param id Unique identifier.
+     * @param instance An opaque instance of the underlying type.
+     * @param args Parameters to use to invoke the function.
+     * @param sz Number of parameters to use to invoke the function.
+     * @return A wrapper containing the returned value, if any.
+     */
+    meta_any invoke(const id_type id, meta_handle instance, meta_any * const args, const size_type sz) const {
+        const internal::meta_func_node* candidate{};
+        size_type extent{sz + 1u};
+        bool ambiguous{};
+
+        for(auto *it = internal::meta_visit<&node_type::func>([id, sz](const auto *curr) { return curr->id == id && curr->arity == sz; }, node); it && it->id == id && it->arity == sz; it = it->next) {
+            size_type direct{};
+            size_type ext{};
+
+            for(size_type next{}; next < sz && next == (direct + ext); ++next) {
+                const auto type = args[next].type();
+                const auto req = it->arg(next).info();
+                type.info() == req ? ++direct : (ext += can_cast_or_convert(type.node, req));
+            }
+
+            if((direct + ext) == sz) {
+                if(ext < extent) {
+                    candidate = it;
+                    extent = ext;
+                    ambiguous = false;
+                } else if(ext == extent) {
+                    ambiguous = true;
+                }
+            }
+        }
+
+        return (candidate && !ambiguous) ? candidate->invoke(std::move(instance), args) : meta_any{};
+    }
+
+    /**
+     * @copybrief invoke
+     *
+     * @sa invoke
+     *
+     * @param id Unique identifier.
+     * @tparam Args Types of arguments to use to invoke the function.
+     * @param instance An opaque instance of the underlying type.
+     * @param args Parameters to use to invoke the function.
+     * @return A wrapper containing the new instance, if any.
+     */
+    template<typename... Args>
+    meta_any invoke(const id_type id, meta_handle instance, Args &&... args) const {
+        meta_any arguments[sizeof...(Args) + 1u]{std::forward<Args>(args)...};
+        return invoke(id, std::move(instance), arguments, sizeof...(Args));
+    }
+
+    /**
+     * @brief Sets the value of a given variable.
+     *
+     * It must be possible to cast the instance to the parent type of the data
+     * member. Otherwise, invoking the setter results in an undefined
+     * behavior.<br/>
+     * The type of the value must be such that a cast or conversion to the type
+     * of the variable is possible. Otherwise, invoking the setter does nothing.
+     *
+     * @tparam Type Type of value to assign.
+     * @param id Unique identifier.
+     * @param instance An opaque instance of the underlying type.
+     * @param value Parameter to use to set the underlying variable.
+     * @return True in case of success, false otherwise.
+     */
+    template<typename Type>
+    bool set(const id_type id, meta_handle instance, Type &&value) const {
+        const auto candidate = data(id);
+        return candidate && candidate.set(std::move(instance), std::forward<Type>(value));
+    }
+
+    /**
+     * @brief Gets the value of a given variable.
+     *
+     * It must be possible to cast the instance to the parent type of the data
+     * member. Otherwise, invoking the getter results in an undefined behavior.
+     *
+     * @param id Unique identifier.
+     * @param instance An opaque instance of the underlying type.
+     * @return A wrapper containing the value of the underlying variable.
+     */
+    [[nodiscard]] meta_any get(const id_type id, meta_handle instance) const {
+        const auto candidate = data(id);
+        return candidate ? candidate.get(std::move(instance)) : meta_any{};
+    }
+
+    /**
+     * @brief Returns a range to use to visit top-level properties.
+     * @return An iterable range to use to visit top-level properties.
+     */
+    [[nodiscard]] meta_range<meta_prop> prop() const ENTT_NOEXCEPT {
+        return node->prop;
+    }
+
+    /**
+     * @brief Returns the property associated with a given key.
+     *
+     * Properties of the base classes will also be visited, if any.
+     *
+     * @param key The key to use to search for a property.
+     * @return The property associated with the given key, if any.
+     */
+    [[nodiscard]] meta_prop prop(meta_any key) const {
+        return internal::meta_visit<&node_type::prop>([&key](const auto *curr) { return curr->id == key; }, node);
+    }
+
+    /**
+     * @brief Returns true if an object is valid, false otherwise.
+     * @return True if the object is valid, false otherwise.
+     */
+    [[nodiscard]] explicit operator bool() const ENTT_NOEXCEPT {
+        return !(node == nullptr);
+    }
+
+    /**
+     * @brief Checks if two objects refer to the same type.
+     * @param other The object with which to compare.
+     * @return True if the objects refer to the same type, false otherwise.
+     */
+    [[nodiscard]] bool operator==(const meta_type &other) const ENTT_NOEXCEPT {
+        return (!node && !other.node) || (node && other.node && node->info == other.node->info);
+    }
+
+    /**
+     * @brief Resets a type and all its parts.
+     *
+     * This function resets a type and all its data members, member functions
+     * and properties, as well as its constructors, destructors and conversion
+     * functions if any.<br/>
+     * Base classes aren't reset but the link between the two types is removed.
+     *
+     * The type is also removed from the list of searchable types.
+     */
+    void reset() ENTT_NOEXCEPT {
+        for(auto** it = internal::meta_context::global(); *it; it = &(*it)->next) {
+            if(*it == node) {
+                *it = (*it)->next;
+                break;
+            }
+        }
+
+        unregister_all(&node->prop);
+        unregister_all(&node->base);
+        unregister_all(&node->conv);
+        unregister_all<&internal::meta_ctor_node::prop>(&node->ctor);
+        unregister_all<&internal::meta_data_node::prop>(&node->data);
+        unregister_all<&internal::meta_func_node::prop>(&node->func);
+
+        node->id = {};
+        node->ctor = node->def_ctor;
+        node->dtor = nullptr;
+    }
+
+private:
+    node_type *node;
+};
+
+
+/**
+ * @brief Checks if two objects refer to the same type.
+ * @param lhs An object, either valid or not.
+ * @param rhs An object, either valid or not.
+ * @return False if the objects refer to the same node, true otherwise.
+ */
+[[nodiscard]] inline bool operator!=(const meta_type &lhs, const meta_type &rhs) ENTT_NOEXCEPT {
+    return !(lhs == rhs);
+}
+
+
+[[nodiscard]] inline meta_type meta_any::type() const ENTT_NOEXCEPT {
+    return node;
+}
+
+
+template<typename... Args>
+meta_any meta_any::invoke(const id_type id, Args &&... args) const {
+    return type().invoke(id, *this, std::forward<Args>(args)...);
+}
+
+
+template<typename... Args>
+meta_any meta_any::invoke(const id_type id, Args &&... args) {
+    return type().invoke(id, *this, std::forward<Args>(args)...);
+}
+
+
+template<typename Type>
+bool meta_any::set(const id_type id, Type &&value) {
+    return type().set(id, *this, std::forward<Type>(value));
+}
+
+
+[[nodiscard]] inline meta_any meta_any::get(const id_type id) const {
+    return type().get(id, *this);
+}
+
+
+[[nodiscard]] inline meta_any meta_any::get(const id_type id) {
+    return type().get(id, *this);
+}
+
+
+[[nodiscard]] inline meta_type meta_ctor::parent() const ENTT_NOEXCEPT {
+    return node->parent;
+}
+
+
+[[nodiscard]] inline meta_type meta_ctor::arg(size_type index) const ENTT_NOEXCEPT {
+    return index < arity() ? node->arg(index) : meta_type{};
+}
+
+
+[[nodiscard]] inline meta_type meta_data::parent() const ENTT_NOEXCEPT {
+    return node->parent;
+}
+
+
+[[nodiscard]] inline meta_type meta_data::type() const ENTT_NOEXCEPT {
+    return node->type();
+}
+
+
+[[nodiscard]] inline meta_type meta_func::parent() const ENTT_NOEXCEPT {
+    return node->parent;
+}
+
+
+[[nodiscard]] inline meta_type meta_func::ret() const ENTT_NOEXCEPT {
+    return node->ret();
+}
+
+
+[[nodiscard]] inline meta_type meta_func::arg(size_type index) const ENTT_NOEXCEPT {
+    return index < arity() ? node->arg(index) : meta_type{};
+}
+
+
+/*! @brief Opaque iterator for sequence containers. */
+class meta_sequence_container::meta_iterator {
+    /*! @brief A sequence container can access the underlying iterator. */
+    friend class meta_sequence_container;
+
+    enum class operation { INCR, DEREF };
+
+    using vtable_type = void(const operation, const any &, void *);
+
+    template<typename It>
+    static void basic_vtable(const operation op, const any &from, void *to) {
+        switch(op) {
+        case operation::INCR:
+            ++any_cast<It &>(const_cast<any &>(from));
+            break;
+        case operation::DEREF:
+            static_cast<meta_any *>(to)->emplace<typename std::iterator_traits<It>::reference>(*any_cast<const It &>(from));
+            break;
+        }
+    }
+
+public:
+    /*! @brief Signed integer type. */
+    using difference_type = std::ptrdiff_t;
+    /*! @brief Type of elements returned by the iterator. */
+    using value_type = meta_any;
+    /*! @brief Pointer type, `void` on purpose. */
+    using pointer = void;
+    /*! @brief Reference type, it is **not** an actual reference. */
+    using reference = value_type;
+    /*! @brief Iterator category. */
+    using iterator_category = std::input_iterator_tag;
+
+    /*! @brief Default constructor. */
+    meta_iterator() ENTT_NOEXCEPT = default;
+
+    /**
+     * @brief Constructs a meta iterator from a given iterator.
+     * @tparam It Type of actual iterator with which to build the meta iterator.
+     * @param iter The actual iterator with which to build the meta iterator.
+     */
+    template<typename It>
+    meta_iterator(It iter)
+        : vtable{&basic_vtable<It>},
+          handle{std::move(iter)}
+    {}
+
+    /*! @brief Pre-increment operator. @return This iterator. */
+    meta_iterator & operator++() ENTT_NOEXCEPT {
+        return vtable(operation::INCR, handle, nullptr), *this;
+    }
+
+    /*! @brief Post-increment operator. @return This iterator. */
+    meta_iterator operator++(int) ENTT_NOEXCEPT {
+        meta_iterator orig = *this;
+        return ++(*this), orig;
+    }
+
+    /**
+     * @brief Checks if two iterators refer to the same element.
+     * @param other The iterator with which to compare.
+     * @return True if the iterators refer to the same element, false otherwise.
+     */
+    [[nodiscard]] bool operator==(const meta_iterator &other) const ENTT_NOEXCEPT {
+        return handle == other.handle;
+    }
+
+    /**
+     * @brief Checks if two iterators refer to the same element.
+     * @param other The iterator with which to compare.
+     * @return False if the iterators refer to the same element, true otherwise.
+     */
+    [[nodiscard]] bool operator!=(const meta_iterator &other) const ENTT_NOEXCEPT {
+        return !(*this == other);
+    }
+
+    /**
+     * @brief Indirection operator.
+     * @return The element to which the iterator points.
+     */
+    [[nodiscard]] reference operator*() const {
+        meta_any other;
+        vtable(operation::DEREF, handle, &other);
+        return other;
+    }
+
+    /**
+     * @brief Returns false if an iterator is invalid, true otherwise.
+     * @return False if the iterator is invalid, true otherwise.
+     */
+    [[nodiscard]] explicit operator bool() const ENTT_NOEXCEPT {
+        return static_cast<bool>(handle);
+    }
+
+private:
+    vtable_type *vtable{};
+    any handle{};
+};
+
+
+template<typename Type>
+struct meta_sequence_container::meta_sequence_container_proxy {
+    using traits_type = meta_sequence_container_traits<Type>;
+
+    [[nodiscard]] static meta_type value_type() ENTT_NOEXCEPT {
+        return internal::meta_info<typename Type::value_type>::resolve();
+    }
+
+    [[nodiscard]] static size_type size(const any &container) ENTT_NOEXCEPT {
+        return traits_type::size(any_cast<const Type &>(container));
+    }
+
+    [[nodiscard]] static bool resize(any &container, size_type sz) {
+        auto * const cont = any_cast<Type>(&container);
+        return cont && traits_type::resize(*cont, sz);
+    }
+
+    [[nodiscard]] static bool clear(any &container) {
+        auto * const cont = any_cast<Type>(&container);
+        return cont && traits_type::clear(*cont);
+    }
+
+    [[nodiscard]] static iterator begin(any &container) {
+        if(auto * const cont = any_cast<Type>(&container); cont) {
+            return iterator{traits_type::begin(*cont)};
+        }
+
+        return iterator{traits_type::cbegin(any_cast<const Type &>(container))};
+    }
+
+    [[nodiscard]] static iterator end(any &container) {
+        if(auto * const cont = any_cast<Type>(&container); cont) {
+            return iterator{traits_type::end(*cont)};
+        }
+
+        return iterator{traits_type::cend(any_cast<const Type &>(container))};
+    }
+
+    [[nodiscard]] static std::pair<iterator, bool> insert(any &container, iterator it, meta_any &value) {
+        if(auto * const cont = any_cast<Type>(&container); cont) {
+            // this abomination is necessary because only on macos value_type and const_reference are different types for std::vector<bool>
+            if(value.allow_cast<typename Type::const_reference>() || value.allow_cast<typename Type::value_type>()) {
+                const auto *element = value.try_cast<std::remove_reference_t<typename Type::const_reference>>();
+                auto ret = traits_type::insert(*cont, any_cast<const typename Type::iterator &>(it.handle), element ? *element : value.cast<typename Type::value_type>());
+                return { iterator{std::move(ret.first)}, ret.second };
+            }
+        }
+
+        return {};
+    }
+
+    [[nodiscard]] static std::pair<iterator, bool> erase(any &container, iterator it) {
+        if(auto * const cont = any_cast<Type>(&container); cont) {
+            auto ret = traits_type::erase(*cont, any_cast<const typename Type::iterator &>(it.handle));
+            return { iterator{std::move(ret.first)}, ret.second };
+        }
+
+        return {};
+    }
+
+    [[nodiscard]] static meta_any get(any &container, size_type pos) {
+        if(auto * const cont = any_cast<Type>(&container); cont) {
+            return meta_any{std::in_place_type<typename Type::reference>, traits_type::get(*cont, pos)};
+        }
+
+        return meta_any{std::in_place_type<typename Type::const_reference>, traits_type::cget(any_cast<const Type &>(container), pos)};
+    }
+};
+
+
+/**
+ * @brief Returns the meta value type of a container.
+ * @return The meta value type of the container.
+ */
+[[nodiscard]] inline meta_type meta_sequence_container::value_type() const ENTT_NOEXCEPT {
+    return value_type_fn();
+}
+
+
+/**
+ * @brief Returns the size of a container.
+ * @return The size of the container.
+ */
+[[nodiscard]] inline meta_sequence_container::size_type meta_sequence_container::size() const ENTT_NOEXCEPT {
+    return size_fn(storage);
+}
+
+
+/**
+ * @brief Resizes a container to contain a given number of elements.
+ * @param sz The new size of the container.
+ * @return True in case of success, false otherwise.
+ */
+inline bool meta_sequence_container::resize(size_type sz) {
+    return resize_fn(storage, sz);
+}
+
+
+/**
+ * @brief Clears the content of a container.
+ * @return True in case of success, false otherwise.
+ */
+inline bool meta_sequence_container::clear() {
+    return clear_fn(storage);
+}
+
+
+/**
+ * @brief Returns an iterator to the first element of a container.
+ * @return An iterator to the first element of the container.
+ */
+[[nodiscard]] inline meta_sequence_container::iterator meta_sequence_container::begin() {
+    return begin_fn(storage);
+}
+
+
+/**
+ * @brief Returns an iterator that is past the last element of a container.
+ * @return An iterator that is past the last element of the container.
+ */
+[[nodiscard]] inline meta_sequence_container::iterator meta_sequence_container::end() {
+    return end_fn(storage);
+}
+
+
+/**
+ * @brief Inserts an element at a specified location of a container.
+ * @param it Iterator before which the element will be inserted.
+ * @param value Element value to insert.
+ * @return A pair consisting of an iterator to the inserted element (in case of
+ * success) and a bool denoting whether the insertion took place.
+ */
+inline std::pair<meta_sequence_container::iterator, bool> meta_sequence_container::insert(iterator it, meta_any value) {
+    return insert_fn(storage, it, value);
+}
+
+
+/**
+ * @brief Removes a given element from a container.
+ * @param it Iterator to the element to remove.
+ * @return A pair consisting of an iterator following the last removed element
+ * (in case of success) and a bool denoting whether the insertion took place.
+ */
+inline std::pair<meta_sequence_container::iterator, bool> meta_sequence_container::erase(iterator it) {
+    return erase_fn(storage, it);
+}
+
+
+/**
+ * @brief Returns a reference to the element at a given location of a container
+ * (no bounds checking is performed).
+ * @param pos The position of the element to return.
+ * @return A reference to the requested element properly wrapped.
+ */
+[[nodiscard]] inline meta_any meta_sequence_container::operator[](size_type pos) {
+    return get_fn(storage, pos);
+}
+
+
+/**
+ * @brief Returns false if a proxy is invalid, true otherwise.
+ * @return False if the proxy is invalid, true otherwise.
+ */
+[[nodiscard]] inline meta_sequence_container::operator bool() const ENTT_NOEXCEPT {
+    return static_cast<bool>(storage);
+}
+
+
+/*! @brief Opaque iterator for associative containers. */
+class meta_associative_container::meta_iterator {
+    enum operation { INCR, DEREF };
+
+    using vtable_type = void(const operation, const any &, void *);
+
+    template<bool KeyOnly, typename It>
+    static void basic_vtable(const operation op, const any &from, void *to) {
+        switch(op) {
+        case operation::INCR:
+            ++any_cast<It &>(const_cast<any &>(from));
+            break;
+        case operation::DEREF:
+            const auto &it = any_cast<const It &>(from);
+            if constexpr(KeyOnly) {
+                static_cast<std::pair<meta_any, meta_any> *>(to)->first.emplace<decltype(*it)>(*it);
+            } else {
+                static_cast<std::pair<meta_any, meta_any> *>(to)->first.emplace<decltype((it->first))>(it->first);
+                static_cast<std::pair<meta_any, meta_any> *>(to)->second.emplace<decltype((it->second))>(it->second);
+            }
+            break;
+        }
+    }
+
+public:
+    /*! @brief Signed integer type. */
+    using difference_type = std::ptrdiff_t;
+    /*! @brief Type of elements returned by the iterator. */
+    using value_type = std::pair<meta_any, meta_any>;
+    /*! @brief Pointer type, `void` on purpose. */
+    using pointer = void;
+    /*! @brief Reference type, it is **not** an actual reference. */
+    using reference = value_type;
+    /*! @brief Iterator category. */
+    using iterator_category = std::input_iterator_tag;
+
+    /*! @brief Default constructor. */
+    meta_iterator() ENTT_NOEXCEPT = default;
+
+    /**
+     * @brief Constructs an meta iterator from a given iterator.
+     * @tparam KeyOnly True if the container is also key-only, false otherwise.
+     * @tparam It Type of actual iterator with which to build the meta iterator.
+     * @param iter The actual iterator with which to build the meta iterator.
+     */
+    template<bool KeyOnly, typename It>
+    meta_iterator(std::integral_constant<bool, KeyOnly>, It iter)
+        : vtable{&basic_vtable<KeyOnly, It>},
+          handle{std::move(iter)}
+    {}
+
+    /*! @brief Pre-increment operator. @return This iterator. */
+    meta_iterator & operator++() ENTT_NOEXCEPT {
+        return vtable(operation::INCR, handle, nullptr), *this;
+    }
+
+    /*! @brief Post-increment operator. @return This iterator. */
+    meta_iterator operator++(int) ENTT_NOEXCEPT {
+        meta_iterator orig = *this;
+        return ++(*this), orig;
+    }
+
+    /**
+     * @brief Checks if two iterators refer to the same element.
+     * @param other The iterator with which to compare.
+     * @return True if the iterators refer to the same element, false otherwise.
+     */
+    [[nodiscard]] bool operator==(const meta_iterator &other) const ENTT_NOEXCEPT {
+        return handle == other.handle;
+    }
+
+    /**
+     * @brief Checks if two iterators refer to the same element.
+     * @param other The iterator with which to compare.
+     * @return False if the iterators refer to the same element, true otherwise.
+     */
+    [[nodiscard]] bool operator!=(const meta_iterator &other) const ENTT_NOEXCEPT {
+        return !(*this == other);
+    }
+
+    /**
+     * @brief Indirection operator.
+     * @return The element to which the iterator points.
+     */
+    [[nodiscard]] reference operator*() const {
+        reference other;
+        vtable(operation::DEREF, handle, &other);
+        return other;
+    }
+
+    /**
+     * @brief Returns false if an iterator is invalid, true otherwise.
+     * @return False if the iterator is invalid, true otherwise.
+     */
+    [[nodiscard]] explicit operator bool() const ENTT_NOEXCEPT {
+        return static_cast<bool>(handle);
+    }
+
+private:
+    vtable_type *vtable{};
+    any handle{};
+};
+
+
+template<typename Type>
+struct meta_associative_container::meta_associative_container_proxy {
+    using traits_type = meta_associative_container_traits<Type>;
+
+    [[nodiscard]] static meta_type key_type() ENTT_NOEXCEPT {
+        return internal::meta_info<typename Type::key_type>::resolve();
+    }
+
+    [[nodiscard]] static meta_type mapped_type() ENTT_NOEXCEPT {
+        if constexpr(is_key_only_meta_associative_container_v<Type>) {
+            return meta_type{};
+        } else {
+            return internal::meta_info<typename Type::mapped_type>::resolve();
+        }
+    }
+
+    [[nodiscard]] static meta_type value_type() ENTT_NOEXCEPT {
+        return internal::meta_info<typename Type::value_type>::resolve();
+    }
+
+    [[nodiscard]] static size_type size(const any &container) ENTT_NOEXCEPT {
+        return traits_type::size(any_cast<const Type &>(container));
+    }
+
+    [[nodiscard]] static bool clear(any &container) {
+        auto * const cont = any_cast<Type>(&container);
+        return cont && traits_type::clear(*cont);
+    }
+
+    [[nodiscard]] static iterator begin(any &container) {
+        if(auto * const cont = any_cast<Type>(&container); cont) {
+            return iterator{is_key_only_meta_associative_container<Type>{}, traits_type::begin(*cont)};
+        }
+
+        return iterator{is_key_only_meta_associative_container<Type>{}, traits_type::cbegin(any_cast<const Type &>(container))};
+    }
+
+    [[nodiscard]] static iterator end(any &container) {
+        if(auto * const cont = any_cast<Type>(&container); cont) {
+            return iterator{is_key_only_meta_associative_container<Type>{}, traits_type::end(*cont)};
+        }
+
+        return iterator{is_key_only_meta_associative_container<Type>{}, traits_type::cend(any_cast<const Type &>(container))};
+    }
+
+    [[nodiscard]] static bool insert(any &container, meta_any &key, meta_any &value) {
+        if(auto * const cont = any_cast<Type>(&container); cont && key.allow_cast<const typename Type::key_type &>()) {
+            if constexpr(is_key_only_meta_associative_container_v<Type>) {
+                return traits_type::insert(*cont, key.cast<const typename Type::key_type &>());
+            } else {
+                return value.allow_cast<const typename Type::mapped_type &>()
+                    && traits_type::insert(*cont, key.cast<const typename Type::key_type &>(), value.cast<const typename Type::mapped_type &>());
+            }
+        }
+
+        return false;
+    }
+
+    [[nodiscard]] static bool erase(any &container, meta_any &key) {
+        if(auto * const cont = any_cast<Type>(&container); cont && key.allow_cast<const typename Type::key_type &>()) {
+            return traits_type::erase(*cont, key.cast<const typename Type::key_type &>());
+        }
+
+        return false;
+    }
+
+    [[nodiscard]] static iterator find(any &container, meta_any &key) {
+        if(key.allow_cast<const typename Type::key_type &>()) {
+            if(auto * const cont = any_cast<Type>(&container); cont) {
+                return iterator{is_key_only_meta_associative_container<Type>{}, traits_type::find(*cont, key.cast<const typename Type::key_type &>())};
+            }
+
+            return iterator{is_key_only_meta_associative_container<Type>{}, traits_type::cfind(any_cast<const Type &>(container), key.cast<const typename Type::key_type &>())};
+        }
+
+        return {};
+    }
+};
+
+
+/**
+ * @brief Returns true if a container is also key-only, false otherwise.
+ * @return True if the associative container is also key-only, false otherwise.
+ */
+[[nodiscard]] inline bool meta_associative_container::key_only() const ENTT_NOEXCEPT {
+    return key_only_container;
+}
+
+
+/**
+ * @brief Returns the meta key type of a container.
+ * @return The meta key type of the a container.
+ */
+[[nodiscard]] inline meta_type meta_associative_container::key_type() const ENTT_NOEXCEPT {
+    return key_type_fn();
+}
+
+
+/**
+ * @brief Returns the meta mapped type of a container.
+ * @return The meta mapped type of the a container.
+ */
+[[nodiscard]] inline meta_type meta_associative_container::mapped_type() const ENTT_NOEXCEPT {
+    return mapped_type_fn();
+}
+
+
+/*! @copydoc meta_sequence_container::value_type */
+[[nodiscard]] inline meta_type meta_associative_container::value_type() const ENTT_NOEXCEPT {
+    return value_type_fn();
+}
+
+
+/*! @copydoc meta_sequence_container::size */
+[[nodiscard]] inline meta_associative_container::size_type meta_associative_container::size() const ENTT_NOEXCEPT {
+    return size_fn(storage);
+}
+
+
+/*! @copydoc meta_sequence_container::clear */
+inline bool meta_associative_container::clear() {
+    return clear_fn(storage);
+}
+
+
+/*! @copydoc meta_sequence_container::begin */
+[[nodiscard]] inline meta_associative_container::iterator meta_associative_container::begin() {
+    return begin_fn(storage);
+}
+
+
+/*! @copydoc meta_sequence_container::end */
+[[nodiscard]] inline meta_associative_container::iterator meta_associative_container::end() {
+    return end_fn(storage);
+}
+
+
+/**
+ * @brief Inserts an element (a key/value pair) into a container.
+ * @param key The key of the element to insert.
+ * @param value The value of the element to insert.
+ * @return A bool denoting whether the insertion took place.
+ */
+inline bool meta_associative_container::insert(meta_any key, meta_any value = {}) {
+    return insert_fn(storage, key, value);
+}
+
+
+/**
+ * @brief Removes the specified element from a container.
+ * @param key The key of the element to remove.
+ * @return A bool denoting whether the removal took place.
+ */
+inline bool meta_associative_container::erase(meta_any key) {
+    return erase_fn(storage, key);
+}
+
+
+/**
+ * @brief Returns an iterator to the element with a given key, if any.
+ * @param key The key of the element to search.
+ * @return An iterator to the element with the given key, if any.
+ */
+[[nodiscard]] inline meta_associative_container::iterator meta_associative_container::find(meta_any key) {
+    return find_fn(storage, key);
+}
+
+
+/**
+ * @brief Returns false if a proxy is invalid, true otherwise.
+ * @return False if the proxy is invalid, true otherwise.
+ */
+[[nodiscard]] inline meta_associative_container::operator bool() const ENTT_NOEXCEPT {
+    return static_cast<bool>(storage);
+}
+
+
+}
+
+
+#endif
+
+// #include "meta/node.hpp"
+#ifndef ENTT_META_NODE_HPP
+#define ENTT_META_NODE_HPP
+
+
+#include <cstddef>
+#include <type_traits>
+#include <utility>
+// #include "../config/config.h"
+
+// #include "../core/attribute.h"
+
+// #include "../core/fwd.hpp"
+
+// #include "../core/type_info.hpp"
+
+// #include "../core/type_traits.hpp"
+
+// #include "type_traits.hpp"
+
+
+
+namespace entt {
+
+
+class meta_any;
+class meta_type;
+struct meta_handle;
+
+
+/**
+ * @cond TURN_OFF_DOXYGEN
+ * Internal details not to be documented.
+ */
+
+
+namespace internal {
+
+
+struct meta_type_node;
+
+
+struct meta_prop_node {
+    meta_prop_node * next;
+    const meta_any &id;
+    meta_any &value;
+};
+
+
+struct meta_base_node {
+    meta_type_node * const parent;
+    meta_base_node * next;
+    meta_type_node *(* const type)() ENTT_NOEXCEPT;
+    const void *(* const cast)(const void *) ENTT_NOEXCEPT;
+};
+
+
+struct meta_conv_node {
+    meta_type_node * const parent;
+    meta_conv_node * next;
+    meta_type_node *(* const type)() ENTT_NOEXCEPT;
+    meta_any(* const conv)(const void *);
+};
+
+
+struct meta_ctor_node {
+    using size_type = std::size_t;
+    meta_type_node * const parent;
+    meta_ctor_node * next;
+    meta_prop_node * prop;
+    const size_type arity;
+    meta_type(* const arg)(const size_type) ENTT_NOEXCEPT;
+    meta_any(* const invoke)(meta_any * const);
+};
+
+
+struct meta_data_node {
+    id_type id;
+    meta_type_node * const parent;
+    meta_data_node * next;
+    meta_prop_node * prop;
+    const bool is_const;
+    const bool is_static;
+    meta_type_node *(* const type)() ENTT_NOEXCEPT;
+    bool(* const set)(meta_handle, meta_any);
+    meta_any(* const get)(meta_handle);
+};
+
+
+struct meta_func_node {
+    using size_type = std::size_t;
+    id_type id;
+    meta_type_node * const parent;
+    meta_func_node * next;
+    meta_prop_node * prop;
+    const size_type arity;
+    const bool is_const;
+    const bool is_static;
+    meta_type_node *(* const ret)() ENTT_NOEXCEPT;
+    meta_type(* const arg)(const size_type) ENTT_NOEXCEPT;
+    meta_any(* const invoke)(meta_handle, meta_any *);
+};
+
+
+struct meta_template_info {
+    using size_type = std::size_t;
+    const bool is_template_specialization;
+    const size_type arity;
+    meta_type_node *(* const type)() ENTT_NOEXCEPT;
+    meta_type_node *(* const arg)(const size_type) ENTT_NOEXCEPT;
+};
+
+
+struct meta_type_node {
+    using size_type = std::size_t;
+    const type_info info;
+    id_type id;
+    meta_type_node * next;
+    meta_prop_node * prop;
+    const size_type size_of;
+    const bool is_void;
+    const bool is_integral;
+    const bool is_floating_point;
+    const bool is_array;
+    const bool is_enum;
+    const bool is_union;
+    const bool is_class;
+    const bool is_pointer;
+    const bool is_function_pointer;
+    const bool is_member_object_pointer;
+    const bool is_member_function_pointer;
+    const bool is_pointer_like;
+    const bool is_sequence_container;
+    const bool is_associative_container;
+    const meta_template_info template_info;
+    const size_type rank;
+    size_type(* const extent)(const size_type) ENTT_NOEXCEPT ;
+    meta_type_node *(* const remove_pointer)() ENTT_NOEXCEPT;
+    meta_type_node *(* const remove_extent)() ENTT_NOEXCEPT;
+    meta_ctor_node * const def_ctor;
+    meta_ctor_node *ctor{nullptr};
+    meta_base_node *base{nullptr};
+    meta_conv_node *conv{nullptr};
+    meta_data_node *data{nullptr};
+    meta_func_node *func{nullptr};
+    void(* dtor)(void *){nullptr};
+};
+
+
+template<auto Member, typename Op, typename Node>
+auto meta_visit(const Op &op, const Node *node)
+-> std::decay_t<decltype(node->*Member)> {
+    for(auto *curr = node->*Member; curr; curr = curr->next) {
+        if(op(curr)) {
+            return curr;
+        }
+    }
+
+    if constexpr(std::is_same_v<Node, meta_type_node>) {
+        for(auto *curr = node->base; curr; curr = curr->next) {
+            if(auto *ret = meta_visit<Member>(op, curr->type()); ret) {
+                return ret;
+            }
+        }
+    }
+
+    return nullptr;
+}
+
+
+template<typename... Args>
+meta_type_node * meta_arg_node(type_list<Args...>, const std::size_t index) ENTT_NOEXCEPT;
+
+
+template<typename Type>
+class ENTT_API meta_node {
+    static_assert(std::is_same_v<Type, std::remove_cv_t<std::remove_reference_t<Type>>>, "Invalid type");
+
+    template<std::size_t... Index>
+    [[nodiscard]] static auto extent(const meta_type_node::size_type dim, std::index_sequence<Index...>) ENTT_NOEXCEPT {
+        meta_type_node::size_type ext{};
+        ((ext = (dim == Index ? std::extent_v<Type, Index> : ext)), ...);
+        return ext;
+    }
+
+    [[nodiscard]] static meta_ctor_node * meta_default_constructor([[maybe_unused]] meta_type_node *type) ENTT_NOEXCEPT {
+        if constexpr(std::is_default_constructible_v<Type>) {
+            static meta_ctor_node node{
+                type,
+                nullptr,
+                nullptr,
+                0u,
+                nullptr,
+                [](meta_any * const) { return meta_any{std::in_place_type<Type>}; }
+            };
+
+            return &node;
+        } else {
+            return nullptr;
+        }
+    }
+
+    [[nodiscard]] static meta_template_info meta_template_descriptor() ENTT_NOEXCEPT {
+        if constexpr(is_complete_v<meta_template_traits<Type>>) {
+            return {
+                true,
+                meta_template_traits<Type>::args_type::size,
+                &meta_node<typename meta_template_traits<Type>::class_type>::resolve,
+                [](const std::size_t index) ENTT_NOEXCEPT {
+                    return meta_arg_node(typename meta_template_traits<Type>::args_type{}, index);
+                }
+            };
+        } else {
+            return { false, 0u, nullptr, nullptr };
+        }
+    }
+
+public:
+    [[nodiscard]] static meta_type_node * resolve() ENTT_NOEXCEPT {
+        static meta_type_node node{
+            type_id<Type>(),
+            {},
+            nullptr,
+            nullptr,
+            size_of_v<Type>,
+            std::is_void_v<Type>,
+            std::is_integral_v<Type>,
+            std::is_floating_point_v<Type>,
+            std::is_array_v<Type>,
+            std::is_enum_v<Type>,
+            std::is_union_v<Type>,
+            std::is_class_v<Type>,
+            std::is_pointer_v<Type>,
+            std::is_pointer_v<Type> && std::is_function_v<std::remove_pointer_t<Type>>,
+            std::is_member_object_pointer_v<Type>,
+            std::is_member_function_pointer_v<Type>,
+            is_meta_pointer_like_v<Type>,
+            is_complete_v<meta_sequence_container_traits<Type>>,
+            is_complete_v<meta_associative_container_traits<Type>>,
+            meta_template_descriptor(),
+            std::rank_v<Type>,
+            [](meta_type_node::size_type dim) ENTT_NOEXCEPT { return extent(dim, std::make_index_sequence<std::rank_v<Type>>{}); },
+            &meta_node<std::remove_cv_t<std::remove_reference_t<std::remove_pointer_t<Type>>>>::resolve,
+            &meta_node<std::remove_cv_t<std::remove_reference_t<std::remove_extent_t<Type>>>>::resolve,
+            meta_default_constructor(&node),
+            meta_default_constructor(&node)
+        };
+
+        return &node;
+    }
+};
+
+
+template<typename Type>
+struct meta_info: meta_node<std::remove_cv_t<std::remove_reference_t<Type>>> {};
+
+
+template<typename... Args>
+meta_type_node * meta_arg_node(type_list<Args...>, const std::size_t index) ENTT_NOEXCEPT {
+    meta_type_node *args[sizeof...(Args) + 1u]{nullptr, internal::meta_info<Args>::resolve()...};
+    return args[index + 1u];
+}
+
+
+}
+
+
+/**
+ * Internal details not to be documented.
+ * @endcond
+ */
+
+
+}
+
+
+#endif
+
+// #include "meta/pointer.hpp"
+#ifndef ENTT_META_POINTER_HPP
+#define ENTT_META_POINTER_HPP
+
+
+#include <memory>
+#include <type_traits>
+// #include "type_traits.hpp"
+
+
+
+namespace entt {
+
+
+/**
+ * @brief Makes plain pointers pointer-like types for the meta system.
+ * @tparam Type Element type.
+ */
+template<typename Type>
+struct is_meta_pointer_like<Type *>
+        : std::true_type
+{};
+
+
+/**
+ * @brief Partial specialization used to reject pointers to arrays.
+ * @tparam Type Type of elements of the array.
+ * @tparam N Number of elements of the array.
+ */
+template<typename Type, std::size_t N>
+struct is_meta_pointer_like<Type(*)[N]>
+    : std::false_type
+{};
+
+
+/**
+ * @brief Makes `std::shared_ptr`s of any type pointer-like types for the meta
+ * system.
+ * @tparam Type Element type.
+ */
+template<typename Type>
+struct is_meta_pointer_like<std::shared_ptr<Type>>
+        : std::true_type
+{};
+
+
+/**
+ * @brief Makes `std::unique_ptr`s of any type pointer-like types for the meta
+ * system.
+ * @tparam Type Element type.
+ * @tparam Args Other arguments.
+ */
+template<typename Type, typename... Args>
+struct is_meta_pointer_like<std::unique_ptr<Type, Args...>>
+        : std::true_type
+{};
+
+
+}
+
+
+#endif
+
+// #include "meta/policy.hpp"
+#ifndef ENTT_META_POLICY_HPP
+#define ENTT_META_POLICY_HPP
+
+
+namespace entt {
+
+
+/*! @brief Empty class type used to request the _as ref_ policy. */
+struct as_ref_t {};
+
+
+/*! @brief Empty class type used to request the _as cref_ policy. */
+struct as_cref_t {};
+
+
+/*! @brief Empty class type used to request the _as-is_ policy. */
+struct as_is_t {};
+
+
+/*! @brief Empty class type used to request the _as void_ policy. */
+struct as_void_t {};
+
+
+}
+
+
+#endif
+
+// #include "meta/range.hpp"
+#ifndef ENTT_META_RANGE_HPP
+#define ENTT_META_RANGE_HPP
+
+
+#include <cstddef>
+#include <iterator>
+
+
+namespace entt {
+
+
+/**
+ * @brief Iterable range to use to iterate all types of meta objects.
+ * @tparam Type Type of meta objects returned.
+ * @tparam Node Type of meta nodes iterated.
+ */
+template<typename Type, typename Node = typename Type::node_type>
+class meta_range {
+    struct range_iterator {
+        using difference_type = std::ptrdiff_t;
+        using value_type = Type;
+        using pointer = void;
+        using reference = value_type;
+        using iterator_category = std::input_iterator_tag;
+        using node_type = Node;
+
+        range_iterator() ENTT_NOEXCEPT = default;
+
+        range_iterator(node_type *head) ENTT_NOEXCEPT
+            : it{head}
+        {}
+
+        range_iterator & operator++() ENTT_NOEXCEPT {
+            return (it = it->next), *this;
+        }
+
+        range_iterator operator++(int) ENTT_NOEXCEPT {
+            range_iterator orig = *this;
+            return ++(*this), orig;
+        }
+
+        [[nodiscard]] reference operator*() const ENTT_NOEXCEPT {
+            return it;
+        }
+
+        [[nodiscard]] bool operator==(const range_iterator &other) const ENTT_NOEXCEPT {
+            return other.it == it;
+        }
+
+        [[nodiscard]] bool operator!=(const range_iterator &other) const ENTT_NOEXCEPT {
+            return !(*this == other);
+        }
+
+    private:
+        node_type *it{};
+    };
+
+public:
+    /*! @brief Node type. */
+    using node_type = Node;
+    /*! @brief Input iterator type. */
+    using iterator = range_iterator;
+
+    /*! @brief Default constructor. */
+    meta_range() ENTT_NOEXCEPT = default;
+
+    /**
+     * @brief Constructs a meta range from a given node.
+     * @param head The underlying node with which to construct the range.
+     */
+    meta_range(node_type *head)
+        : node{head}
+    {}
+
+    /**
+     * @brief Returns an iterator to the beginning.
+     * @return An iterator to the first meta object of the range.
+     */
+    [[nodiscard]] iterator begin() const ENTT_NOEXCEPT {
+        return iterator{node};
+    }
+
+    /**
+     * @brief Returns an iterator to the end.
+     * @return An iterator to the element following the last meta object of the
+     * range.
+     */
+    [[nodiscard]] iterator end() const ENTT_NOEXCEPT {
+        return iterator{};
+    }
+
+private:
+    node_type *node{nullptr};
+};
+
+
+}
+
+
+#endif
+
+// #include "meta/resolve.hpp"
+#ifndef ENTT_META_RESOLVE_HPP
+#define ENTT_META_RESOLVE_HPP
+
+
+#include <algorithm>
+// #include "../core/type_info.hpp"
+
+// #include "ctx.hpp"
+
+// #include "meta.hpp"
+
+// #include "node.hpp"
+
+// #include "range.hpp"
+
+
+
+namespace entt {
+
+
+/**
+ * @brief Returns the meta type associated with a given type.
+ * @tparam Type Type to use to search for a meta type.
+ * @return The meta type associated with the given type, if any.
+ */
+template<typename Type>
+[[nodiscard]] meta_type resolve() ENTT_NOEXCEPT {
+    return internal::meta_info<Type>::resolve();
+}
+
+
+/**
+ * @brief Returns a range to use to visit all meta types.
+ * @return An iterable range to use to visit all meta types.
+ */
+[[nodiscard]] inline meta_range<meta_type> resolve() {
+    return *internal::meta_context::global();
+}
+
+
+/**
+ * @brief Returns the meta type associated with a given identifier, if any.
+ * @param id Unique identifier.
+ * @return The meta type associated with the given identifier, if any.
+ */
+[[nodiscard]] inline meta_type resolve(const id_type id) ENTT_NOEXCEPT {
+    for(auto *curr = *internal::meta_context::global(); curr; curr = curr->next) {
+        if(curr->id == id) {
+            return curr;
+        }
+    }
+
+    return {};
+}
+
+
+/**
+ * @brief Returns the meta type associated with a given type info object, if
+ * any.
+ * @param info The type info object of the requested type.
+ * @return The meta type associated with the given type info object, if any.
+ */
+[[nodiscard]] inline meta_type resolve(const type_info info) ENTT_NOEXCEPT {
+    for(auto *curr = *internal::meta_context::global(); curr; curr = curr->next) {
+        if(curr->info == info) {
+            return curr;
+        }
+    }
+
+    return {};
+}
+
+
+}
+
+
+#endif
+
+// #include "meta/template.hpp"
+#ifndef ENTT_META_TEMPLATE_HPP
+#define ENTT_META_TEMPLATE_HPP
+
+
+// #include "../core/type_traits.hpp"
+
+
+
+namespace entt {
+
+
+/*! @brief Utility class to disambiguate class templates. */
+template<template<typename...> typename>
+struct meta_class_template_tag {};
+
+
+/**
+ * @brief General purpose traits class for generating meta template information.
+ * @tparam Clazz Type of class template.
+ * @tparam Args Types of template arguments.
+ */
+template<template<typename...> typename Clazz, typename... Args>
+struct meta_template_traits<Clazz<Args...>> {
+    /*! @brief Wrapped class template. */
+    using class_type = meta_class_template_tag<Clazz>;
+    /*! @brief List of template arguments. */
+    using args_type = type_list<Args...>;
+};
+
+
+}
+
+
+#endif
+
+// #include "meta/type_traits.hpp"
+#ifndef ENTT_META_TYPE_TRAITS_HPP
+#define ENTT_META_TYPE_TRAITS_HPP
+
+
+#include <type_traits>
+
+
+namespace entt {
+
+
+/**
+ * @brief Traits class template to be specialized to enable support for meta
+ * template information.
+ */
+template<typename>
+struct meta_template_traits;
+
+
+/**
+ * @brief Traits class template to be specialized to enable support for meta
+ * sequence containers.
+ */
+template<typename>
+struct meta_sequence_container_traits;
+
+
+/**
+ * @brief Traits class template to be specialized to enable support for meta
+ * associative containers.
+ */
+template<typename>
+struct meta_associative_container_traits;
+
+
+/**
+ * @brief Provides the member constant `value` to true if a meta associative
+ * container claims to wrap a key-only type, false otherwise.
+ * @tparam Type Potentially key-only meta associative container type.
+ */
+template<typename, typename = void>
+struct is_key_only_meta_associative_container: std::true_type {};
+
+
+/*! @copydoc is_key_only_meta_associative_container */
+template<typename Type>
+struct is_key_only_meta_associative_container<Type, std::void_t<typename meta_associative_container_traits<Type>::type::mapped_type>>
+    : std::false_type
+{};
+
+
+/**
+ * @brief Helper variable template.
+ * @tparam Type Potentially key-only meta associative container type.
+ */
+template<typename Type>
+inline constexpr auto is_key_only_meta_associative_container_v = is_key_only_meta_associative_container<Type>::value;
+
+
+/**
+ * @brief Provides the member constant `value` to true if a given type is a
+ * pointer-like type from the point of view of the meta system, false otherwise.
+ * @tparam Type Potentially pointer-like type.
+ */
+template<typename>
+struct is_meta_pointer_like: std::false_type {};
+
+
+/**
+ * @brief Partial specialization to ensure that const pointer-like types are
+ * also accepted.
+ * @tparam Type Potentially pointer-like type.
+ */
+template<typename Type>
+struct is_meta_pointer_like<const Type>: is_meta_pointer_like<Type> {};
+
+
+/**
+ * @brief Helper variable template.
+ * @tparam Type Potentially pointer-like type.
+ */
+template<typename Type>
+inline constexpr auto is_meta_pointer_like_v = is_meta_pointer_like<Type>::value;
+
+
+}
+
+
+#endif
+
+// #include "meta/utility.hpp"
+#ifndef ENTT_META_UTILITY_HPP
+#define ENTT_META_UTILITY_HPP
+
+
+#include <cstddef>
+#include <functional>
+#include <type_traits>
+#include <utility>
+// #include "../config/config.h"
+
+// #include "../core/type_traits.hpp"
+
+// #include "meta.hpp"
+
+// #include "node.hpp"
+
+// #include "policy.hpp"
+
+
+
+namespace entt {
+
+
+/*! @brief Primary template isn't defined on purpose. */
+template<typename, typename>
+struct meta_function_descriptor;
+
+
+/**
+ * @brief Meta function descriptor.
+ * @tparam Type Reflected type to which the meta function is associated.
+ * @tparam Ret Function return type.
+ * @tparam Class Actual owner of the member function.
+ * @tparam Args Function arguments.
+ */
+template<typename Type, typename Ret, typename Class, typename... Args>
+struct meta_function_descriptor<Type, Ret(Class:: *)(Args...) const> {
+    /*! @brief Meta function return type. */
+    using return_type = Ret;
+    /*! @brief Meta function arguments. */
+    using args_type = std::conditional_t<std::is_same_v<Type, Class>, type_list<Args...>, type_list<const Class &, Args...>>;
+
+    /*! @brief True if the meta function is const, false otherwise. */
+    static constexpr auto is_const = true;
+    /*! @brief True if the meta function is static, false otherwise. */
+    static constexpr auto is_static = !std::is_same_v<Type, Class>;
+};
+
+
+/**
+ * @brief Meta function descriptor.
+ * @tparam Type Reflected type to which the meta function is associated.
+ * @tparam Ret Function return type.
+ * @tparam Class Actual owner of the member function.
+ * @tparam Args Function arguments.
+ */
+template<typename Type, typename Ret, typename Class, typename... Args>
+struct meta_function_descriptor<Type, Ret(Class:: *)(Args...)> {
+    /*! @brief Meta function return type. */
+    using return_type = Ret;
+    /*! @brief Meta function arguments. */
+    using args_type = std::conditional_t<std::is_same_v<Type, Class>, type_list<Args...>, type_list<Class &, Args...>>;
+
+    /*! @brief True if the meta function is const, false otherwise. */
+    static constexpr auto is_const = false;
+    /*! @brief True if the meta function is static, false otherwise. */
+    static constexpr auto is_static = !std::is_same_v<Type, Class>;
+};
+
+
+/**
+ * @brief Meta function descriptor.
+ * @tparam Type Reflected type to which the meta function is associated.
+ * @tparam Ret Function return type.
+ * @tparam Args Function arguments.
+ */
+template<typename Type, typename Ret, typename... Args>
+struct meta_function_descriptor<Type, Ret(*)(Args...)> {
+    /*! @brief Meta function return type. */
+    using return_type = Ret;
+    /*! @brief Meta function arguments. */
+    using args_type = type_list<Args...>;
+
+    /*! @brief True if the meta function is const, false otherwise. */
+    static constexpr auto is_const = false;
+    /*! @brief True if the meta function is static, false otherwise. */
+    static constexpr auto is_static = true;
+};
+
+
+/**
+ * @brief Meta function helper.
+ *
+ * Converts a function type to be associated with a reflected type into its meta
+ * function descriptor.
+ *
+ * @tparam Type Reflected type to which the meta function is associated.
+ * @tparam Candidate The actual function to associate with the reflected type.
+ */
+template<typename Type, typename Candidate>
+class meta_function_helper {
+    template<typename Ret, typename... Args, typename Class>
+    static constexpr meta_function_descriptor<Type, Ret(Class:: *)(Args...) const> get_rid_of_noexcept(Ret(Class:: *)(Args...) const);
+
+    template<typename Ret, typename... Args, typename Class>
+    static constexpr meta_function_descriptor<Type, Ret(Class:: *)(Args...)> get_rid_of_noexcept(Ret(Class:: *)(Args...));
+
+    template<typename Ret, typename... Args>
+    static constexpr meta_function_descriptor<Type, Ret(*)(Args...)> get_rid_of_noexcept(Ret(*)(Args...));
+
+public:
+    /*! @brief The meta function descriptor of the given function. */
+    using type = decltype(get_rid_of_noexcept(std::declval<Candidate>()));
+};
+
+
+/**
+ * @brief Helper type.
+ * @tparam Type Reflected type to which the meta function is associated.
+ * @tparam Candidate The actual function to associate with the reflected type.
+ */
+template<typename Type, typename Candidate>
+using meta_function_helper_t = typename meta_function_helper<Type, Candidate>::type;
+
+
+/**
+ * @brief Returns the meta type of the i-th element of a list of arguments.
+ * @tparam Args Actual types of arguments.
+ * @return The meta type of the i-th element of the list of arguments.
+ */
+template<typename... Args>
+[[nodiscard]] static meta_type meta_arg(type_list<Args...>, const std::size_t index) ENTT_NOEXCEPT {
+    return internal::meta_arg_node(type_list<Args...>{}, index);
+}
+
+
+/**
+ * @brief Constructs an instance given a list of erased parameters, if possible.
+ * @tparam Type Actual type of the instance to construct.
+ * @tparam Args Types of arguments expected.
+ * @tparam Index Indexes to use to extract erased arguments from their list.
+ * @param args Parameters to use to construct the instance.
+ * @return A meta any containing the new instance, if any.
+ */
+template<typename Type, typename... Args, std::size_t... Index>
+[[nodiscard]] meta_any meta_construct(meta_any * const args, std::index_sequence<Index...>) {
+    if(((args+Index)->allow_cast<Args>() && ...)) {
+        return Type{(args+Index)->cast<Args>()...};
+    }
+
+    return {};
+}
+
+
+/**
+ * @brief Sets the value of a given variable.
+ * @tparam Type Reflected type to which the variable is associated.
+ * @tparam Data The actual variable to set.
+ * @param instance An opaque instance of the underlying type, if required.
+ * @param value Parameter to use to set the variable.
+ * @return True in case of success, false otherwise.
+ */
+template<typename Type, auto Data>
+[[nodiscard]] bool meta_setter([[maybe_unused]] meta_handle instance, [[maybe_unused]] meta_any value) {
+    if constexpr(!std::is_same_v<decltype(Data), Type> && !std::is_same_v<decltype(Data), std::nullptr_t>) {
+        if constexpr(std::is_function_v<std::remove_reference_t<std::remove_pointer_t<decltype(Data)>>>) {
+            using data_type = type_list_element_t<1u, typename meta_function_helper_t<Type, decltype(Data)>::args_type>;
+
+            if(auto * const clazz = instance->try_cast<Type>(); clazz && value.allow_cast<data_type>()) {
+                Data(*clazz, value.cast<data_type>());
+                return true;
+            }
+        } else if constexpr(std::is_member_function_pointer_v<decltype(Data)>) {
+            using data_type = type_list_element_t<0u, typename meta_function_helper_t<Type, decltype(Data)>::args_type>;
+
+            if(auto * const clazz = instance->try_cast<Type>(); clazz && value.allow_cast<data_type>()) {
+                (clazz->*Data)(value.cast<data_type>());
+                return true;
+            }
+        } else if constexpr(std::is_member_object_pointer_v<decltype(Data)>) {
+            using data_type = std::remove_reference_t<decltype(std::declval<Type>().*Data)>;
+
+            if constexpr(!std::is_array_v<data_type> && !std::is_const_v<data_type>) {
+                if(auto * const clazz = instance->try_cast<Type>(); clazz && value.allow_cast<data_type>()) {
+                    clazz->*Data = value.cast<data_type>();
+                    return true;
+                }
+            }
+        } else {
+            using data_type = std::remove_reference_t<decltype(*Data)>;
+
+            if constexpr(!std::is_array_v<data_type> && !std::is_const_v<data_type>) {
+                if(value.allow_cast<data_type>()) {
+                    *Data = value.cast<data_type>();
+                    return true;
+                }
+            }
+        }
+    }
+
+    return false;
+}
+
+
+/**
+ * @brief Wraps a value depending on the given policy.
+ * @tparam Policy Optional policy (no policy set by default).
+ * @tparam Type Type of value to wrap.
+ * @param value Value to wrap.
+ * @return A meta any containing the returned value.
+ */
+template<typename Policy = as_is_t, typename Type>
+meta_any meta_dispatch(Type &&value) {
+    if constexpr(std::is_same_v<Policy, as_void_t>) {
+        return meta_any{std::in_place_type<void>, std::forward<Type>(value)};
+    } else if constexpr(std::is_same_v<Policy, as_ref_t>) {
+        return meta_any{std::in_place_type<Type>, std::forward<Type>(value)};
+    } else if constexpr(std::is_same_v<Policy, as_cref_t>) {
+        static_assert(std::is_lvalue_reference_v<Type>, "Invalid type");
+        return meta_any{std::in_place_type<const std::remove_reference_t<Type> &>, std::as_const(value)};
+    } else {
+        static_assert(std::is_same_v<Policy, as_is_t>, "Policy not supported");
+        return meta_any{std::forward<Type>(value)};
+    }
+}
+
+
+/**
+ * @brief Gets the value of a given variable.
+ * @tparam Type Reflected type to which the variable is associated.
+ * @tparam Data The actual variable to get.
+ * @tparam Policy Optional policy (no policy set by default).
+ * @param instance An opaque instance of the underlying type, if required.
+ * @return A meta any containing the value of the underlying variable.
+ */
+template<typename Type, auto Data, typename Policy = as_is_t>
+[[nodiscard]] meta_any meta_getter([[maybe_unused]] meta_handle instance) {
+    if constexpr(std::is_function_v<std::remove_reference_t<std::remove_pointer_t<decltype(Data)>>>) {
+        auto * const clazz = instance->try_cast<std::conditional_t<std::is_invocable_v<decltype(Data), const Type &>, const Type, Type>>();
+        return clazz ? meta_dispatch<Policy>(Data(*clazz)) : meta_any{};
+    } else if constexpr(std::is_member_function_pointer_v<decltype(Data)>) {
+        auto * const clazz = instance->try_cast<std::conditional_t<std::is_invocable_v<decltype(Data), const Type &>, const Type, Type>>();
+        return clazz ? meta_dispatch<Policy>((clazz->*Data)()) : meta_any{};
+    } else if constexpr(std::is_member_object_pointer_v<decltype(Data)>) {
+        if constexpr(!std::is_array_v<std::remove_cv_t<std::remove_reference_t<decltype(std::declval<Type>().*Data)>>>) {
+            if(auto * clazz = instance->try_cast<Type>(); clazz) {
+                return meta_dispatch<Policy>(clazz->*Data);
+            } else if(auto * fallback = instance->try_cast<const Type>(); fallback) {
+                return meta_dispatch<Policy>(fallback->*Data);
+            }
+        }
+
+        return meta_any{};
+    } else if constexpr(std::is_pointer_v<decltype(Data)>) {
+        if constexpr(std::is_array_v<std::remove_pointer_t<decltype(Data)>>) {
+            return meta_any{};
+        } else {
+            return meta_dispatch<Policy>(*Data);
+        }
+    } else {
+        return meta_dispatch<Policy>(Data);
+    }
+}
+
+
+/**
+ * @brief Invokes a function given a list of erased parameters, if possible.
+ * @tparam Type Reflected type to which the function is associated.
+ * @tparam Candidate The actual function to invoke.
+ * @tparam Policy Optional policy (no policy set by default).
+ * @tparam Index Indexes to use to extract erased arguments from their list.
+ * @param instance An opaque instance of the underlying type, if required.
+ * @param args Parameters to use to invoke the function.
+ * @return A meta any containing the returned value, if any.
+ */
+template<typename Type, auto Candidate, typename Policy = as_is_t, std::size_t... Index>
+[[nodiscard]] std::enable_if_t<!std::is_invocable_v<decltype(Candidate)>, meta_any> meta_invoke([[maybe_unused]] meta_handle instance, meta_any *args, std::index_sequence<Index...>) {
+    using descriptor = meta_function_helper_t<Type, decltype(Candidate)>;
+
+    const auto invoke = [](auto &&maybe_clazz, auto &&... other) {
+        if constexpr(std::is_member_function_pointer_v<decltype(Candidate)>) {
+            if constexpr(std::is_void_v<typename descriptor::return_type>) {
+                (std::forward<decltype(maybe_clazz)>(maybe_clazz).*Candidate)(std::forward<decltype(other)>(other)...);
+                return meta_any{std::in_place_type<void>};
+            } else {
+                return meta_dispatch<Policy>((std::forward<decltype(maybe_clazz)>(maybe_clazz).*Candidate)(std::forward<decltype(other)>(other)...));
+            }
+        } else {
+            if constexpr(std::is_void_v<typename descriptor::return_type>) {
+                Candidate(std::forward<decltype(maybe_clazz)>(maybe_clazz), std::forward<decltype(other)>(other)...);
+                return meta_any{std::in_place_type<void>};
+            } else {
+                return meta_dispatch<Policy>(Candidate(std::forward<decltype(maybe_clazz)>(maybe_clazz), std::forward<decltype(other)>(other)...));
+            }
+        }
+    };
+
+    if constexpr(std::is_invocable_v<decltype(Candidate), const Type &, type_list_element_t<Index, typename descriptor::args_type>...>) {
+        if(const auto * const clazz = instance->try_cast<const Type>(); clazz && ((args+Index)->allow_cast<type_list_element_t<Index, typename descriptor::args_type>>() && ...)) {
+            return invoke(*clazz, (args+Index)->cast<type_list_element_t<Index, typename descriptor::args_type>>()...);
+        }
+    } else if constexpr(std::is_invocable_v<decltype(Candidate), Type &, type_list_element_t<Index, typename descriptor::args_type>...>) {
+        if(auto * const clazz = instance->try_cast<Type>(); clazz && ((args+Index)->allow_cast<type_list_element_t<Index, typename descriptor::args_type>>() && ...)) {
+            return invoke(*clazz, (args+Index)->cast<type_list_element_t<Index, typename descriptor::args_type>>()...);
+        }
+    } else {
+        if(((args+Index)->allow_cast<type_list_element_t<Index, typename descriptor::args_type>>() && ...)) {
+            return invoke((args+Index)->cast<type_list_element_t<Index, typename descriptor::args_type>>()...);
+        }
+    }
+
+    return meta_any{};
+}
+
+
+/**
+ * @brief Invokes a function given a list of erased parameters, if possible.
+ * @tparam Type Reflected type to which the function is associated.
+ * @tparam Candidate The actual function to invoke.
+ * @tparam Policy Optional policy (no policy set by default).
+ * @tparam Index Indexes to use to extract erased arguments from their list.
+ * @return A meta any containing the returned value, if any.
+ */
+template<typename Type, auto Candidate, typename Policy = as_is_t, std::size_t... Index>
+[[nodiscard]] std::enable_if_t<std::is_invocable_v<decltype(Candidate)>, meta_any> meta_invoke(meta_handle, meta_any *, std::index_sequence<Index...>) {
+    if constexpr(std::is_void_v<decltype(Candidate())>) {
+        Candidate();
+        return meta_any{std::in_place_type<void>};
+    } else {
+        return meta_dispatch<Policy>(Candidate());
+    }
+}
+
+
+}
+
+
+#endif
+
+// #include "platform/android-ndk-r17.hpp"
+#ifndef ENTT_PLATFORM_ANDROID_NDK_R17_HPP
+#define ENTT_PLATFORM_ANDROID_NDK_R17_HPP
+
+
+/**
+ * @cond TURN_OFF_DOXYGEN
+ * Internal details not to be documented.
+ */
+
+
+#ifdef __ANDROID__
+#include <android/ndk-version.h>
+#if __NDK_MAJOR__ == 17
+
+
+#include <functional>
+#include <type_traits>
+#include <utility>
+
+
+namespace std {
+
+
+namespace internal {
+
+
+template<typename Func, typename... Args>
+constexpr auto is_invocable(int) -> decltype(std::invoke(std::declval<Func>(), std::declval<Args>()...), std::true_type{});
+
+
+template<typename, typename...>
+constexpr std::false_type is_invocable(...);
+
+
+template<typename Ret, typename Func, typename... Args>
+constexpr auto is_invocable_r(int)
+-> std::enable_if_t<decltype(std::is_convertible_v<decltype(std::invoke(std::declval<Func>(), std::declval<Args>()...)), Ret>, std::true_type>;
+
+
+template<typename, typename, typename...>
+constexpr std::false_type is_invocable_r(...);
+
+
+}
+
+
+template<typename Func, typename... Args>
+struct is_invocable: decltype(internal::is_invocable<Func, Args...>(0)) {};
+
+
+template<typename Func, typename... Argsv>
+inline constexpr bool is_invocable_v = std::is_invocable<Func, Args...>::value;
+
+
+template<typename Ret, typename Func, typename... Args>
+struct is_invocable_r: decltype(internal::is_invocable_r<Ret, Func, Args...>(0)) {};
+
+
+template<typename Ret, typename Func, typename... Args>
+inline constexpr bool is_invocable_r_v = std::is_invocable_r<Ret, Func, Args...>::value;
+
+
+template<typename Func, typename...Args>
+struct invoke_result {
+    using type = decltype(std::invoke(std::declval<Func>(), std::declval<Args>()...));
+};
+
+
+template<typename Func, typename... Args>
+using invoke_result_t = typename std::invoke_result<Func, Args...>::type;
+
+
+}
+
+
+#endif
+#endif
+
+
+/**
+ * Internal details not to be documented.
+ * @endcond
+ */
+
+
+#endif
+
+// #include "poly/poly.hpp"
+#ifndef ENTT_POLY_POLY_HPP
+#define ENTT_POLY_POLY_HPP
+
+
+#include <cstddef>
+#include <functional>
+#include <tuple>
+#include <type_traits>
+#include <utility>
+// #include "../config/config.h"
+#ifndef ENTT_CONFIG_CONFIG_H
+#define ENTT_CONFIG_CONFIG_H
+
+
+#if defined(__cpp_exceptions) && !defined(ENTT_NOEXCEPTION)
+#   define ENTT_NOEXCEPT noexcept
+#   define ENTT_THROW throw
+#   define ENTT_TRY try
+#   define ENTT_CATCH catch(...)
+#else
+#   define ENTT_NOEXCEPT
+#   define ENTT_THROW
+#   define ENTT_TRY if(true)
+#   define ENTT_CATCH if(false)
+#endif
+
+
+#if defined(__cpp_lib_launder) && __cpp_lib_launder >= 201606L
+#   include <new>
+#   define ENTT_LAUNDER(expr) std::launder(expr)
+#else
+#   define ENTT_LAUNDER(expr) expr
+#endif
+
+
+#ifndef ENTT_USE_ATOMIC
+#   define ENTT_MAYBE_ATOMIC(Type) Type
+#else
+#   include <atomic>
+#   define ENTT_MAYBE_ATOMIC(Type) std::atomic<Type>
+#endif
+
+
+#ifndef ENTT_ID_TYPE
+#   include <cstdint>
+#   define ENTT_ID_TYPE std::uint32_t
+#endif
+
+
+#ifdef ENTT_SPARSE_PAGE
+	static_assert(ENTT_SPARSE_PAGE && ((ENTT_SPARSE_PAGE & (ENTT_SPARSE_PAGE - 1)) == 0), "ENTT_SPARSE_PAGE must be a power of two");
+#else
+#   define ENTT_SPARSE_PAGE 4096
+#endif
+
+
+#ifdef ENTT_PACKED_PAGE
+static_assert(ENTT_PACKED_PAGE && ((ENTT_PACKED_PAGE & (ENTT_PACKED_PAGE - 1)) == 0), "ENTT_PACKED_PAGE must be a power of two");
+#else
+#   define ENTT_PACKED_PAGE 1024
+#endif
+
+
+#ifdef ENTT_DISABLE_ASSERT
+#   undef ENTT_ASSERT
+#   define ENTT_ASSERT(...) (void(0))
+#elif !defined ENTT_ASSERT
+#   include <cassert>
+#   define ENTT_ASSERT(condition, ...) assert(condition)
+#endif
+
+
+#ifdef ENTT_NO_ETO
+#   include <type_traits>
+#   define ENTT_IGNORE_IF_EMPTY std::false_type
+#else
+#   include <type_traits>
+#   define ENTT_IGNORE_IF_EMPTY std::true_type
+#endif
+
+
+#ifndef ENTT_STANDARD_CPP
+#    if defined __clang__ || defined __GNUC__
+#       define ENTT_PRETTY_FUNCTION __PRETTY_FUNCTION__
+#       define ENTT_PRETTY_FUNCTION_PREFIX '='
+#       define ENTT_PRETTY_FUNCTION_SUFFIX ']'
+#    elif defined _MSC_VER
+#       define ENTT_PRETTY_FUNCTION __FUNCSIG__
+#       define ENTT_PRETTY_FUNCTION_PREFIX '<'
+#       define ENTT_PRETTY_FUNCTION_SUFFIX '>'
+#   endif
+#endif
+
+
+#endif
+
+// #include "../core/any.hpp"
+#ifndef ENTT_CORE_ANY_HPP
+#define ENTT_CORE_ANY_HPP
+
+
+#include <cstddef>
+#include <functional>
+#include <memory>
+#include <type_traits>
+#include <utility>
+// #include "../config/config.h"
+#ifndef ENTT_CONFIG_CONFIG_H
+#define ENTT_CONFIG_CONFIG_H
+
+
+#if defined(__cpp_exceptions) && !defined(ENTT_NOEXCEPTION)
+#   define ENTT_NOEXCEPT noexcept
+#   define ENTT_THROW throw
+#   define ENTT_TRY try
+#   define ENTT_CATCH catch(...)
+#else
+#   define ENTT_NOEXCEPT
+#   define ENTT_THROW
+#   define ENTT_TRY if(true)
+#   define ENTT_CATCH if(false)
+#endif
+
+
+#if defined(__cpp_lib_launder) && __cpp_lib_launder >= 201606L
+#   include <new>
+#   define ENTT_LAUNDER(expr) std::launder(expr)
+#else
+#   define ENTT_LAUNDER(expr) expr
+#endif
+
+
+#ifndef ENTT_USE_ATOMIC
+#   define ENTT_MAYBE_ATOMIC(Type) Type
+#else
+#   include <atomic>
+#   define ENTT_MAYBE_ATOMIC(Type) std::atomic<Type>
+#endif
+
+
+#ifndef ENTT_ID_TYPE
+#   include <cstdint>
+#   define ENTT_ID_TYPE std::uint32_t
+#endif
+
+
+#ifdef ENTT_SPARSE_PAGE
+	static_assert(ENTT_SPARSE_PAGE && ((ENTT_SPARSE_PAGE & (ENTT_SPARSE_PAGE - 1)) == 0), "ENTT_SPARSE_PAGE must be a power of two");
+#else
+#   define ENTT_SPARSE_PAGE 4096
+#endif
+
+
+#ifdef ENTT_PACKED_PAGE
+static_assert(ENTT_PACKED_PAGE && ((ENTT_PACKED_PAGE & (ENTT_PACKED_PAGE - 1)) == 0), "ENTT_PACKED_PAGE must be a power of two");
+#else
+#   define ENTT_PACKED_PAGE 1024
+#endif
+
+
+#ifdef ENTT_DISABLE_ASSERT
+#   undef ENTT_ASSERT
+#   define ENTT_ASSERT(...) (void(0))
+#elif !defined ENTT_ASSERT
+#   include <cassert>
+#   define ENTT_ASSERT(condition, ...) assert(condition)
+#endif
+
+
+#ifdef ENTT_NO_ETO
+#   include <type_traits>
+#   define ENTT_IGNORE_IF_EMPTY std::false_type
+#else
+#   include <type_traits>
+#   define ENTT_IGNORE_IF_EMPTY std::true_type
+#endif
+
+
+#ifndef ENTT_STANDARD_CPP
+#    if defined __clang__ || defined __GNUC__
+#       define ENTT_PRETTY_FUNCTION __PRETTY_FUNCTION__
+#       define ENTT_PRETTY_FUNCTION_PREFIX '='
+#       define ENTT_PRETTY_FUNCTION_SUFFIX ']'
+#    elif defined _MSC_VER
+#       define ENTT_PRETTY_FUNCTION __FUNCSIG__
+#       define ENTT_PRETTY_FUNCTION_PREFIX '<'
+#       define ENTT_PRETTY_FUNCTION_SUFFIX '>'
+#   endif
+#endif
+
+
+#endif
+
+// #include "../core/utility.hpp"
+#ifndef ENTT_CORE_UTILITY_HPP
+#define ENTT_CORE_UTILITY_HPP
+
+
+#include <utility>
+// #include "../config/config.h"
+#ifndef ENTT_CONFIG_CONFIG_H
+#define ENTT_CONFIG_CONFIG_H
+
+
+#if defined(__cpp_exceptions) && !defined(ENTT_NOEXCEPTION)
+#   define ENTT_NOEXCEPT noexcept
+#   define ENTT_THROW throw
+#   define ENTT_TRY try
+#   define ENTT_CATCH catch(...)
+#else
+#   define ENTT_NOEXCEPT
+#   define ENTT_THROW
+#   define ENTT_TRY if(true)
+#   define ENTT_CATCH if(false)
+#endif
+
+
+#if defined(__cpp_lib_launder) && __cpp_lib_launder >= 201606L
+#   include <new>
+#   define ENTT_LAUNDER(expr) std::launder(expr)
+#else
+#   define ENTT_LAUNDER(expr) expr
+#endif
+
+
+#ifndef ENTT_USE_ATOMIC
+#   define ENTT_MAYBE_ATOMIC(Type) Type
+#else
+#   include <atomic>
+#   define ENTT_MAYBE_ATOMIC(Type) std::atomic<Type>
+#endif
+
+
+#ifndef ENTT_ID_TYPE
+#   include <cstdint>
+#   define ENTT_ID_TYPE std::uint32_t
+#endif
+
+
+#ifdef ENTT_SPARSE_PAGE
+	static_assert(ENTT_SPARSE_PAGE && ((ENTT_SPARSE_PAGE & (ENTT_SPARSE_PAGE - 1)) == 0), "ENTT_SPARSE_PAGE must be a power of two");
+#else
+#   define ENTT_SPARSE_PAGE 4096
+#endif
+
+
+#ifdef ENTT_PACKED_PAGE
+static_assert(ENTT_PACKED_PAGE && ((ENTT_PACKED_PAGE & (ENTT_PACKED_PAGE - 1)) == 0), "ENTT_PACKED_PAGE must be a power of two");
+#else
+#   define ENTT_PACKED_PAGE 1024
+#endif
+
+
+#ifdef ENTT_DISABLE_ASSERT
+#   undef ENTT_ASSERT
+#   define ENTT_ASSERT(...) (void(0))
+#elif !defined ENTT_ASSERT
+#   include <cassert>
+#   define ENTT_ASSERT(condition, ...) assert(condition)
+#endif
+
+
+#ifdef ENTT_NO_ETO
+#   include <type_traits>
+#   define ENTT_IGNORE_IF_EMPTY std::false_type
+#else
+#   include <type_traits>
+#   define ENTT_IGNORE_IF_EMPTY std::true_type
+#endif
+
+
+#ifndef ENTT_STANDARD_CPP
+#    if defined __clang__ || defined __GNUC__
+#       define ENTT_PRETTY_FUNCTION __PRETTY_FUNCTION__
+#       define ENTT_PRETTY_FUNCTION_PREFIX '='
+#       define ENTT_PRETTY_FUNCTION_SUFFIX ']'
+#    elif defined _MSC_VER
+#       define ENTT_PRETTY_FUNCTION __FUNCSIG__
+#       define ENTT_PRETTY_FUNCTION_PREFIX '<'
+#       define ENTT_PRETTY_FUNCTION_SUFFIX '>'
+#   endif
+#endif
+
+
+#endif
+
+
+
+namespace entt {
+
+
+/*! @brief Identity function object (waiting for C++20). */
+struct identity {
+    /**
+     * @brief Returns its argument unchanged.
+     * @tparam Type Type of the argument.
+     * @param value The actual argument.
+     * @return The submitted value as-is.
+     */
+    template<class Type>
+    [[nodiscard]] constexpr Type && operator()(Type &&value) const ENTT_NOEXCEPT {
+        return std::forward<Type>(value);
+    }
+};
+
+
+/**
+ * @brief Constant utility to disambiguate overloaded members of a class.
+ * @tparam Type Type of the desired overload.
+ * @tparam Class Type of class to which the member belongs.
+ * @param member A valid pointer to a member.
+ * @return Pointer to the member.
+ */
+template<typename Type, typename Class>
+[[nodiscard]] constexpr auto overload(Type Class:: *member) ENTT_NOEXCEPT { return member; }
+
+
+/**
+ * @brief Constant utility to disambiguate overloaded functions.
+ * @tparam Func Function type of the desired overload.
+ * @param func A valid pointer to a function.
+ * @return Pointer to the function.
+ */
+template<typename Func>
+[[nodiscard]] constexpr auto overload(Func *func) ENTT_NOEXCEPT { return func; }
+
+
+/**
+ * @brief Helper type for visitors.
+ * @tparam Func Types of function objects.
+ */
+template<class... Func>
+struct overloaded: Func... {
+    using Func::operator()...;
+};
+
+
+/**
+ * @brief Deduction guide.
+ * @tparam Func Types of function objects.
+ */
+template<class... Func>
+overloaded(Func...)
+-> overloaded<Func...>;
+
+
+/**
+ * @brief Basic implementation of a y-combinator.
+ * @tparam Func Type of a potentially recursive function.
+ */
+template<class Func>
+struct y_combinator {
+    /**
+     * @brief Constructs a y-combinator from a given function.
+     * @param recursive A potentially recursive function.
+     */
+    y_combinator(Func recursive):
+        func{std::move(recursive)}
+    {}
+
+    /**
+     * @brief Invokes a y-combinator and therefore its underlying function.
+     * @tparam Args Types of arguments to use to invoke the underlying function.
+     * @param args Parameters to use to invoke the underlying function.
+     * @return Return value of the underlying function, if any.
+     */
+    template <class... Args>
+    decltype(auto) operator()(Args &&... args) const {
+        return func(*this, std::forward<Args>(args)...);
+    }
+
+    /*! @copydoc operator()() */
+    template <class... Args>
+    decltype(auto) operator()(Args &&... args) {
+        return func(*this, std::forward<Args>(args)...);
+    }
+
+private:
+    Func func;
+};
+
+
+}
+
+
+#endif
+
+// #include "fwd.hpp"
+#ifndef ENTT_CORE_FWD_HPP
+#define ENTT_CORE_FWD_HPP
+
+
+#include <type_traits>
+// #include "../config/config.h"
+
+
+
+namespace entt {
+
+
+template<std::size_t Len = sizeof(double[2]), std::size_t = alignof(typename std::aligned_storage_t<Len + !Len>)>
+class basic_any;
+
+
+/*! @brief Alias declaration for type identifiers. */
+using id_type = ENTT_ID_TYPE;
+
+
+/*! @brief Alias declaration for the most common use case. */
+using any = basic_any<>;
+
+
+}
+
+
+#endif
+
+// #include "type_info.hpp"
+#ifndef ENTT_CORE_TYPE_INFO_HPP
+#define ENTT_CORE_TYPE_INFO_HPP
+
+
+#include <string_view>
+#include <type_traits>
+// #include "../config/config.h"
+
+// #include "../core/attribute.h"
+#ifndef ENTT_CORE_ATTRIBUTE_H
+#define ENTT_CORE_ATTRIBUTE_H
+
+
+#ifndef ENTT_EXPORT
+#   if defined _WIN32 || defined __CYGWIN__ || defined _MSC_VER
+#       define ENTT_EXPORT __declspec(dllexport)
+#       define ENTT_IMPORT __declspec(dllimport)
+#       define ENTT_HIDDEN
+#   elif defined __GNUC__ && __GNUC__ >= 4
+#       define ENTT_EXPORT __attribute__((visibility("default")))
+#       define ENTT_IMPORT __attribute__((visibility("default")))
+#       define ENTT_HIDDEN __attribute__((visibility("hidden")))
+#   else /* Unsupported compiler */
+#       define ENTT_EXPORT
+#       define ENTT_IMPORT
+#       define ENTT_HIDDEN
+#   endif
+#endif
+
+
+#ifndef ENTT_API
+#   if defined ENTT_API_EXPORT
+#       define ENTT_API ENTT_EXPORT
+#   elif defined ENTT_API_IMPORT
+#       define ENTT_API ENTT_IMPORT
+#   else /* No API */
+#       define ENTT_API
+#   endif
+#endif
+
+
+#endif
+
+// #include "hashed_string.hpp"
+#ifndef ENTT_CORE_HASHED_STRING_HPP
+#define ENTT_CORE_HASHED_STRING_HPP
+
+
+#include <cstddef>
+#include <cstdint>
+// #include "../config/config.h"
+
+// #include "fwd.hpp"
+
+
+
+namespace entt {
+
+
+/**
+ * @cond TURN_OFF_DOXYGEN
+ * Internal details not to be documented.
+ */
+
+
+namespace internal {
+
+
+template<typename>
+struct fnv1a_traits;
+
+
+template<>
+struct fnv1a_traits<std::uint32_t> {
+    using type = std::uint32_t;
+    static constexpr std::uint32_t offset = 2166136261;
+    static constexpr std::uint32_t prime = 16777619;
+};
+
+
+template<>
+struct fnv1a_traits<std::uint64_t> {
+    using type = std::uint64_t;
+    static constexpr std::uint64_t offset = 14695981039346656037ull;
+    static constexpr std::uint64_t prime = 1099511628211ull;
+};
+
+
+}
+
+
+/**
+ * Internal details not to be documented.
+ * @endcond
+ */
+
+
+/**
+ * @brief Zero overhead unique identifier.
+ *
+ * A hashed string is a compile-time tool that allows users to use
+ * human-readable identifers in the codebase while using their numeric
+ * counterparts at runtime.<br/>
+ * Because of that, a hashed string can also be used in constant expressions if
+ * required.
+ *
+ * @tparam Char Character type.
+ */
+template<typename Char>
+class basic_hashed_string {
+    using traits_type = internal::fnv1a_traits<id_type>;
+
+    struct const_wrapper {
+        // non-explicit constructor on purpose
+        constexpr const_wrapper(const Char *curr) ENTT_NOEXCEPT: str{curr} {}
+        const Char *str;
+    };
+
+    // Fowler–Noll–Vo hash function v. 1a - the good
+    [[nodiscard]] static constexpr id_type helper(const Char *curr) ENTT_NOEXCEPT {
+        auto value = traits_type::offset;
+
+        while(*curr != 0) {
+            value = (value ^ static_cast<traits_type::type>(*(curr++))) * traits_type::prime;
+        }
+
+        return value;
+    }
+
+public:
+    /*! @brief Character type. */
+    using value_type = Char;
+    /*! @brief Unsigned integer type. */
+    using hash_type = id_type;
+
+    /**
+     * @brief Returns directly the numeric representation of a string view.
+     * @param str Human-readable identifer.
+     * @param size Length of the string to hash.
+     * @return The numeric representation of the string.
+     */
+    [[nodiscard]] static constexpr hash_type value(const value_type *str, std::size_t size) ENTT_NOEXCEPT {
+        id_type partial{traits_type::offset};
+        while(size--) { partial = (partial^(str++)[0])*traits_type::prime; }
+        return partial;
+    }
+
+    /**
+     * @brief Returns directly the numeric representation of a string.
+     *
+     * Forcing template resolution avoids implicit conversions. An
+     * human-readable identifier can be anything but a plain, old bunch of
+     * characters.<br/>
+     * Example of use:
+     * @code{.cpp}
+     * const auto value = basic_hashed_string<char>::to_value("my.png");
+     * @endcode
+     *
+     * @tparam N Number of characters of the identifier.
+     * @param str Human-readable identifer.
+     * @return The numeric representation of the string.
+     */
+    template<std::size_t N>
+    [[nodiscard]] static constexpr hash_type value(const value_type (&str)[N]) ENTT_NOEXCEPT {
+        return helper(str);
+    }
+
+    /**
+     * @brief Returns directly the numeric representation of a string.
+     * @param wrapper Helps achieving the purpose by relying on overloading.
+     * @return The numeric representation of the string.
+     */
+    [[nodiscard]] static hash_type value(const_wrapper wrapper) ENTT_NOEXCEPT {
+        return helper(wrapper.str);
+    }
+
+    /*! @brief Constructs an empty hashed string. */
+    constexpr basic_hashed_string() ENTT_NOEXCEPT
+        : str{nullptr}, hash{}
+    {}
+
+    /**
+     * @brief Constructs a hashed string from an array of const characters.
+     *
+     * Forcing template resolution avoids implicit conversions. An
+     * human-readable identifier can be anything but a plain, old bunch of
+     * characters.<br/>
+     * Example of use:
+     * @code{.cpp}
+     * basic_hashed_string<char> hs{"my.png"};
+     * @endcode
+     *
+     * @tparam N Number of characters of the identifier.
+     * @param curr Human-readable identifer.
+     */
+    template<std::size_t N>
+    constexpr basic_hashed_string(const value_type (&curr)[N]) ENTT_NOEXCEPT
+        : str{curr}, hash{helper(curr)}
+    {}
+
+    /**
+     * @brief Explicit constructor on purpose to avoid constructing a hashed
+     * string directly from a `const value_type *`.
+     * @param wrapper Helps achieving the purpose by relying on overloading.
+     */
+    explicit constexpr basic_hashed_string(const_wrapper wrapper) ENTT_NOEXCEPT
+        : str{wrapper.str}, hash{helper(wrapper.str)}
+    {}
+
+    /**
+     * @brief Returns the human-readable representation of a hashed string.
+     * @return The string used to initialize the instance.
+     */
+    [[nodiscard]] constexpr const value_type * data() const ENTT_NOEXCEPT {
+        return str;
+    }
+
+    /**
+     * @brief Returns the numeric representation of a hashed string.
+     * @return The numeric representation of the instance.
+     */
+    [[nodiscard]] constexpr hash_type value() const ENTT_NOEXCEPT {
+        return hash;
+    }
+
+    /*! @copydoc data */
+    [[nodiscard]] constexpr operator const value_type *() const ENTT_NOEXCEPT { return data(); }
+
+    /**
+     * @brief Returns the numeric representation of a hashed string.
+     * @return The numeric representation of the instance.
+     */
+    [[nodiscard]] constexpr operator hash_type() const ENTT_NOEXCEPT { return value(); }
+
+    /**
+     * @brief Compares two hashed strings.
+     * @param other Hashed string with which to compare.
+     * @return True if the two hashed strings are identical, false otherwise.
+     */
+    [[nodiscard]] constexpr bool operator==(const basic_hashed_string &other) const ENTT_NOEXCEPT {
+        return hash == other.hash;
+    }
+
+private:
+    const value_type *str;
+    hash_type hash;
+};
+
+
+/**
+ * @brief Deduction guide.
+ *
+ * It allows to deduce the character type of the hashed string directly from a
+ * human-readable identifer provided to the constructor.
+ *
+ * @tparam Char Character type.
+ * @tparam N Number of characters of the identifier.
+ * @param str Human-readable identifer.
+ */
+template<typename Char, std::size_t N>
+basic_hashed_string(const Char (&str)[N])
+-> basic_hashed_string<Char>;
+
+
+/**
+ * @brief Compares two hashed strings.
+ * @tparam Char Character type.
+ * @param lhs A valid hashed string.
+ * @param rhs A valid hashed string.
+ * @return True if the two hashed strings are identical, false otherwise.
+ */
+template<typename Char>
+[[nodiscard]] constexpr bool operator!=(const basic_hashed_string<Char> &lhs, const basic_hashed_string<Char> &rhs) ENTT_NOEXCEPT {
+    return !(lhs == rhs);
+}
+
+
+/*! @brief Aliases for common character types. */
+using hashed_string = basic_hashed_string<char>;
+
+
+/*! @brief Aliases for common character types. */
+using hashed_wstring = basic_hashed_string<wchar_t>;
+
+
+inline namespace literals {
+
+
+/**
+ * @brief User defined literal for hashed strings.
+ * @param str The literal without its suffix.
+ * @return A properly initialized hashed string.
+ */
+[[nodiscard]] constexpr entt::hashed_string operator"" _hs(const char *str, std::size_t) ENTT_NOEXCEPT {
+    return entt::hashed_string{str};
+}
+
+
+/**
+ * @brief User defined literal for hashed wstrings.
+ * @param str The literal without its suffix.
+ * @return A properly initialized hashed wstring.
+ */
+[[nodiscard]] constexpr entt::hashed_wstring operator"" _hws(const wchar_t *str, std::size_t) ENTT_NOEXCEPT {
+    return entt::hashed_wstring{str};
+}
+
+
+}
+
+
+}
+
+
+#endif
+
+// #include "fwd.hpp"
+
+
+
+namespace entt {
+
+
+/**
+ * @cond TURN_OFF_DOXYGEN
+ * Internal details not to be documented.
+ */
+
+
+namespace internal {
+
+
+struct ENTT_API type_seq final {
+    [[nodiscard]] static id_type next() ENTT_NOEXCEPT {
+        static ENTT_MAYBE_ATOMIC(id_type) value{};
+        return value++;
+    }
+};
+
+
+template<typename Type>
+[[nodiscard]] constexpr auto stripped_type_name() ENTT_NOEXCEPT {
+#if defined ENTT_PRETTY_FUNCTION
+    std::string_view pretty_function{ENTT_PRETTY_FUNCTION};
+    auto first = pretty_function.find_first_not_of(' ', pretty_function.find_first_of(ENTT_PRETTY_FUNCTION_PREFIX)+1);
+    auto value = pretty_function.substr(first, pretty_function.find_last_of(ENTT_PRETTY_FUNCTION_SUFFIX) - first);
+    return value;
+#else
+    return std::string_view{""};
+#endif
+}
+
+
+template<typename Type, auto = stripped_type_name<Type>().find_first_of('.')>
+[[nodiscard]] static constexpr std::string_view type_name(int) ENTT_NOEXCEPT {
+    constexpr auto value = stripped_type_name<Type>();
+    return value;
+}
+
+
+template<typename Type>
+[[nodiscard]] static std::string_view type_name(char) ENTT_NOEXCEPT {
+    static const auto value = stripped_type_name<Type>();
+    return value;
+}
+
+
+template<typename Type, auto = stripped_type_name<Type>().find_first_of('.')>
+[[nodiscard]] static constexpr id_type type_hash(int) ENTT_NOEXCEPT {
+    constexpr auto stripped = stripped_type_name<Type>();
+    constexpr auto value = hashed_string::value(stripped.data(), stripped.size());
+    return value;
+}
+
+
+template<typename Type>
+[[nodiscard]] static id_type type_hash(char) ENTT_NOEXCEPT {
+    static const auto value = [](const auto stripped) {
+        return hashed_string::value(stripped.data(), stripped.size());
+    }(stripped_type_name<Type>());
+    return value;
+}
+
+
+}
+
+
+/**
+ * Internal details not to be documented.
+ * @endcond
+ */
+
+
+/**
+ * @brief Type sequential identifier.
+ * @tparam Type Type for which to generate a sequential identifier.
+ */
+template<typename Type, typename = void>
+struct ENTT_API type_seq final {
+    /**
+     * @brief Returns the sequential identifier of a given type.
+     * @return The sequential identifier of a given type.
+     */
+    [[nodiscard]] static id_type value() ENTT_NOEXCEPT {
+        static const id_type value = internal::type_seq::next();
+        return value;
+    }
+
+    /*! @copydoc value */
+    [[nodiscard]] constexpr operator id_type() const ENTT_NOEXCEPT { return value(); }
+};
+
+
+/**
+ * @brief Type hash.
+ * @tparam Type Type for which to generate a hash value.
+ */
+template<typename Type, typename = void>
+struct type_hash final {
+    /**
+     * @brief Returns the numeric representation of a given type.
+     * @return The numeric representation of the given type.
+     */
+#if defined ENTT_PRETTY_FUNCTION
+    [[nodiscard]] static constexpr id_type value() ENTT_NOEXCEPT {
+        return internal::type_hash<Type>(0);
+#else
+    [[nodiscard]] static constexpr id_type value() ENTT_NOEXCEPT {
+        return type_seq<Type>::value();
+#endif
+    }
+
+    /*! @copydoc value */
+    [[nodiscard]] constexpr operator id_type() const ENTT_NOEXCEPT { return value(); }
+};
+
+
+/**
+ * @brief Type name.
+ * @tparam Type Type for which to generate a name.
+ */
+template<typename Type, typename = void>
+struct type_name final {
+    /**
+     * @brief Returns the name of a given type.
+     * @return The name of the given type.
+     */
+    [[nodiscard]] static constexpr std::string_view value() ENTT_NOEXCEPT {
+        return internal::type_name<Type>(0);
+    }
+
+    /*! @copydoc value */
+    [[nodiscard]] constexpr operator std::string_view() const ENTT_NOEXCEPT { return value(); }
+};
+
+
+/*! @brief Implementation specific information about a type. */
+class type_info final {
+    template<typename>
+    friend type_info type_id() ENTT_NOEXCEPT;
+
+    type_info(id_type seq_v, id_type hash_v, std::string_view name_v) ENTT_NOEXCEPT
+        : seq_value{seq_v},
+          hash_value{hash_v},
+          name_value{name_v}
+    {}
+
+public:
+    /*! @brief Default constructor. */
+    type_info() ENTT_NOEXCEPT
+        : type_info({}, {}, {})
+    {}
+
+    /*! @brief Default copy constructor. */
+    type_info(const type_info &) ENTT_NOEXCEPT = default;
+    /*! @brief Default move constructor. */
+    type_info(type_info &&) ENTT_NOEXCEPT = default;
+
+    /**
+     * @brief Default copy assignment operator.
+     * @return This type info object.
+     */
+    type_info & operator=(const type_info &) ENTT_NOEXCEPT = default;
+
+    /**
+     * @brief Default move assignment operator.
+     * @return This type info object.
+     */
+    type_info & operator=(type_info &&) ENTT_NOEXCEPT = default;
+
+    /**
+     * @brief Checks if a type info object is properly initialized.
+     * @return True if the object is properly initialized, false otherwise.
+     */
+    [[nodiscard]] explicit operator bool() const ENTT_NOEXCEPT {
+        return name_value.data() != nullptr;
+    }
+
+    /**
+     * @brief Type sequential identifier.
+     * @return Type sequential identifier.
+     */
+    [[nodiscard]] id_type seq() const ENTT_NOEXCEPT {
+        return seq_value;
+    }
+
+    /**
+     * @brief Type hash.
+     * @return Type hash.
+     */
+    [[nodiscard]] id_type hash() const ENTT_NOEXCEPT {
+        return hash_value;
+    }
+
+    /**
+     * @brief Type name.
+     * @return Type name.
+     */
+    [[nodiscard]] std::string_view name() const ENTT_NOEXCEPT {
+        return name_value;
+    }
+
+    /**
+     * @brief Compares the contents of two type info objects.
+     * @param other Object with which to compare.
+     * @return False if the two contents differ, true otherwise.
+     */
+    [[nodiscard]] bool operator==(const type_info &other) const ENTT_NOEXCEPT {
+        return hash_value == other.hash_value;
+    }
+
+private:
+    id_type seq_value;
+    id_type hash_value;
+    std::string_view name_value;
+};
+
+
+/**
+ * @brief Compares the contents of two type info objects.
+ * @param lhs A type info object.
+ * @param rhs A type info object.
+ * @return True if the two contents differ, false otherwise.
+ */
+[[nodiscard]] inline bool operator!=(const type_info &lhs, const type_info &rhs) ENTT_NOEXCEPT {
+    return !(lhs == rhs);
+}
+
+
+/**
+ * @brief Returns the type info object for a given type.
+ * @tparam Type Type for which to generate a type info object.
+ * @return The type info object for the given type.
+ */
+template<typename Type>
+[[nodiscard]] type_info type_id() ENTT_NOEXCEPT {
+    return type_info{
+        type_seq<std::remove_cv_t<std::remove_reference_t<Type>>>::value(),
+        type_hash<std::remove_cv_t<std::remove_reference_t<Type>>>::value(),
+        type_name<std::remove_cv_t<std::remove_reference_t<Type>>>::value()
+    };
+}
+
+
+}
+
+
+#endif
+
+// #include "type_traits.hpp"
+#ifndef ENTT_CORE_TYPE_TRAITS_HPP
+#define ENTT_CORE_TYPE_TRAITS_HPP
+
+
+#include <cstddef>
+#include <iterator>
+#include <type_traits>
+#include <utility>
+// #include "../config/config.h"
+
+// #include "fwd.hpp"
+
+
+
+namespace entt {
+
+
+/**
+ * @brief Utility class to disambiguate overloaded functions.
+ * @tparam N Number of choices available.
+ */
+template<std::size_t N>
+struct choice_t
+    // Unfortunately, doxygen cannot parse such a construct.
+    /*! @cond TURN_OFF_DOXYGEN */
+    : choice_t<N-1>
+    /*! @endcond */
+{};
+
+
+/*! @copybrief choice_t */
+template<>
+struct choice_t<0> {};
+
+
+/**
+ * @brief Variable template for the choice trick.
+ * @tparam N Number of choices available.
+ */
+template<std::size_t N>
+inline constexpr choice_t<N> choice{};
+
+
+/**
+ * @brief Identity type trait.
+ *
+ * Useful to establish non-deduced contexts in template argument deduction
+ * (waiting for C++20) or to provide types through function arguments.
+ *
+ * @tparam Type A type.
+ */
+template<typename Type>
+struct type_identity {
+    /*! @brief Identity type. */
+    using type = Type;
+};
+
+
+/**
+ * @brief Helper type.
+ * @tparam Type A type.
+ */
+template<typename Type>
+using type_identity_t = typename type_identity<Type>::type;
+
+
+/**
+ * @brief A type-only `sizeof` wrapper that returns 0 where `sizeof` complains.
+ * @tparam Type The type of which to return the size.
+ * @tparam The size of the type if `sizeof` accepts it, 0 otherwise.
+ */
+template<typename Type, typename = void>
+struct size_of: std::integral_constant<std::size_t, 0u> {};
+
+
+/*! @copydoc size_of */
+template<typename Type>
+struct size_of<Type, std::void_t<decltype(sizeof(Type))>>
+    : std::integral_constant<std::size_t, sizeof(Type)>
+{};
+
+
+/**
+ * @brief Helper variable template.
+ * @tparam Type The type of which to return the size.
+ */
+template<class Type>
+inline constexpr std::size_t size_of_v = size_of<Type>::value;
+
+
+/**
+ * @brief Using declaration to be used to _repeat_ the same type a number of
+ * times equal to the size of a given parameter pack.
+ * @tparam Type A type to repeat.
+ */
+template<typename Type, typename>
+using unpack_as_t = Type;
+
+
+/**
+ * @brief Helper variable template to be used to _repeat_ the same value a
+ * number of times equal to the size of a given parameter pack.
+ * @tparam Value A value to repeat.
+ */
+template<auto Value, typename>
+inline constexpr auto unpack_as_v = Value;
+
+
+/**
+ * @brief Wraps a static constant.
+ * @tparam Value A static constant.
+ */
+template<auto Value>
+using integral_constant = std::integral_constant<decltype(Value), Value>;
+
+
+/**
+ * @brief Alias template to facilitate the creation of named values.
+ * @tparam Value A constant value at least convertible to `id_type`.
+ */
+template<id_type Value>
+using tag = integral_constant<Value>;
+
+
+/**
+ * @brief A class to use to push around lists of types, nothing more.
+ * @tparam Type Types provided by the type list.
+ */
+template<typename... Type>
+struct type_list {
+    /*! @brief Type list type. */
+    using type = type_list;
+    /*! @brief Compile-time number of elements in the type list. */
+    static constexpr auto size = sizeof...(Type);
+};
+
+
+/*! @brief Primary template isn't defined on purpose. */
+template<std::size_t, typename>
+struct type_list_element;
+
+
+/**
+ * @brief Provides compile-time indexed access to the types of a type list.
+ * @tparam Index Index of the type to return.
+ * @tparam Type First type provided by the type list.
+ * @tparam Other Other types provided by the type list.
+ */
+template<std::size_t Index, typename Type, typename... Other>
+struct type_list_element<Index, type_list<Type, Other...>>
+    : type_list_element<Index - 1u, type_list<Other...>>
+{};
+
+
+/**
+ * @brief Provides compile-time indexed access to the types of a type list.
+ * @tparam Type First type provided by the type list.
+ * @tparam Other Other types provided by the type list.
+ */
+template<typename Type, typename... Other>
+struct type_list_element<0u, type_list<Type, Other...>> {
+    /*! @brief Searched type. */
+    using type = Type;
+};
+
+
+/**
+ * @brief Helper type.
+ * @tparam Index Index of the type to return.
+ * @tparam List Type list to search into.
+ */
+template<std::size_t Index, typename List>
+using type_list_element_t = typename type_list_element<Index, List>::type;
+
+
+/**
+ * @brief Concatenates multiple type lists.
+ * @tparam Type Types provided by the first type list.
+ * @tparam Other Types provided by the second type list.
+ * @return A type list composed by the types of both the type lists.
+ */
+template<typename... Type, typename... Other>
+constexpr type_list<Type..., Other...> operator+(type_list<Type...>, type_list<Other...>) { return {}; }
+
+
+/*! @brief Primary template isn't defined on purpose. */
+template<typename...>
+struct type_list_cat;
+
+
+/*! @brief Concatenates multiple type lists. */
+template<>
+struct type_list_cat<> {
+    /*! @brief A type list composed by the types of all the type lists. */
+    using type = type_list<>;
+};
+
+
+/**
+ * @brief Concatenates multiple type lists.
+ * @tparam Type Types provided by the first type list.
+ * @tparam Other Types provided by the second type list.
+ * @tparam List Other type lists, if any.
+ */
+template<typename... Type, typename... Other, typename... List>
+struct type_list_cat<type_list<Type...>, type_list<Other...>, List...> {
+    /*! @brief A type list composed by the types of all the type lists. */
+    using type = typename type_list_cat<type_list<Type..., Other...>, List...>::type;
+};
+
+
+/**
+ * @brief Concatenates multiple type lists.
+ * @tparam Type Types provided by the type list.
+ */
+template<typename... Type>
+struct type_list_cat<type_list<Type...>> {
+    /*! @brief A type list composed by the types of all the type lists. */
+    using type = type_list<Type...>;
+};
+
+
+/**
+ * @brief Helper type.
+ * @tparam List Type lists to concatenate.
+ */
+template<typename... List>
+using type_list_cat_t = typename type_list_cat<List...>::type;
+
+
+/*! @brief Primary template isn't defined on purpose. */
+template<typename>
+struct type_list_unique;
+
+
+/**
+ * @brief Removes duplicates types from a type list.
+ * @tparam Type One of the types provided by the given type list.
+ * @tparam Other The other types provided by the given type list.
+ */
+template<typename Type, typename... Other>
+struct type_list_unique<type_list<Type, Other...>> {
+    /*! @brief A type list without duplicate types. */
+    using type = std::conditional_t<
+        std::disjunction_v<std::is_same<Type, Other>...>,
+        typename type_list_unique<type_list<Other...>>::type,
+        type_list_cat_t<type_list<Type>, typename type_list_unique<type_list<Other...>>::type>
+    >;
+};
+
+
+/*! @brief Removes duplicates types from a type list. */
+template<>
+struct type_list_unique<type_list<>> {
+    /*! @brief A type list without duplicate types. */
+    using type = type_list<>;
+};
+
+
+/**
+ * @brief Helper type.
+ * @tparam Type A type list.
+ */
+template<typename Type>
+using type_list_unique_t = typename type_list_unique<Type>::type;
+
+
+/**
+ * @brief Provides the member constant `value` to true if a type list contains a
+ * given type, false otherwise.
+ * @tparam List Type list.
+ * @tparam Type Type to look for.
+ */
+template<typename List, typename Type>
+struct type_list_contains;
+
+
+/**
+ * @copybrief type_list_contains
+ * @tparam Type Types provided by the type list.
+ * @tparam Other Type to look for.
+ */
+template<typename... Type, typename Other>
+struct type_list_contains<type_list<Type...>, Other>: std::disjunction<std::is_same<Type, Other>...> {};
+
+
+/**
+ * @brief Helper variable template.
+ * @tparam List Type list.
+ * @tparam Type Type to look for.
+ */
+template<class List, typename Type>
+inline constexpr bool type_list_contains_v = type_list_contains<List, Type>::value;
+
+
+/*! @brief Primary template isn't defined on purpose. */
+template<typename...>
+struct type_list_diff;
+
+
+/**
+ * @brief Computes the difference between two type lists.
+ * @tparam Type Types provided by the first type list.
+ * @tparam Other Types provided by the second type list.
+ */
+template<typename... Type, typename... Other>
+struct type_list_diff<type_list<Type...>, type_list<Other...>> {
+    /*! @brief A type list that is the difference between the two type lists. */
+    using type = type_list_cat_t<std::conditional_t<type_list_contains_v<type_list<Other...>, Type>, type_list<>, type_list<Type>>...>;
+};
+
+
+/**
+ * @brief Helper type.
+ * @tparam List Type lists between which to compute the difference.
+ */
+template<typename... List>
+using type_list_diff_t = typename type_list_diff<List...>::type;
+
+
+/**
+ * @brief A class to use to push around lists of constant values, nothing more.
+ * @tparam Value Values provided by the value list.
+ */
+template<auto... Value>
+struct value_list {
+    /*! @brief Value list type. */
+    using type = value_list;
+    /*! @brief Compile-time number of elements in the value list. */
+    static constexpr auto size = sizeof...(Value);
+};
+
+
+/*! @brief Primary template isn't defined on purpose. */
+template<std::size_t, typename>
+struct value_list_element;
+
+
+/**
+ * @brief Provides compile-time indexed access to the values of a value list.
+ * @tparam Index Index of the value to return.
+ * @tparam Value First value provided by the value list.
+ * @tparam Other Other values provided by the value list.
+ */
+template<std::size_t Index, auto Value, auto... Other>
+struct value_list_element<Index, value_list<Value, Other...>>
+    : value_list_element<Index - 1u, value_list<Other...>>
+{};
+
+
+/**
+ * @brief Provides compile-time indexed access to the types of a type list.
+ * @tparam Value First value provided by the value list.
+ * @tparam Other Other values provided by the value list.
+ */
+template<auto Value, auto... Other>
+struct value_list_element<0u, value_list<Value, Other...>> {
+    /*! @brief Searched value. */
+    static constexpr auto value = Value;
+};
+
+
+/**
+ * @brief Helper type.
+ * @tparam Index Index of the value to return.
+ * @tparam List Value list to search into.
+ */
+template<std::size_t Index, typename List>
+inline constexpr auto value_list_element_v = value_list_element<Index, List>::value;
+
+
+/**
+ * @brief Concatenates multiple value lists.
+ * @tparam Value Values provided by the first value list.
+ * @tparam Other Values provided by the second value list.
+ * @return A value list composed by the values of both the value lists.
+ */
+template<auto... Value, auto... Other>
+constexpr value_list<Value..., Other...> operator+(value_list<Value...>, value_list<Other...>) { return {}; }
+
+
+/*! @brief Primary template isn't defined on purpose. */
+template<typename...>
+struct value_list_cat;
+
+
+/*! @brief Concatenates multiple value lists. */
+template<>
+struct value_list_cat<> {
+    /*! @brief A value list composed by the values of all the value lists. */
+    using type = value_list<>;
+};
+
+
+/**
+ * @brief Concatenates multiple value lists.
+ * @tparam Value Values provided by the first value list.
+ * @tparam Other Values provided by the second value list.
+ * @tparam List Other value lists, if any.
+ */
+template<auto... Value, auto... Other, typename... List>
+struct value_list_cat<value_list<Value...>, value_list<Other...>, List...> {
+    /*! @brief A value list composed by the values of all the value lists. */
+    using type = typename value_list_cat<value_list<Value..., Other...>, List...>::type;
+};
+
+
+/**
+ * @brief Concatenates multiple value lists.
+ * @tparam Value Values provided by the value list.
+ */
+template<auto... Value>
+struct value_list_cat<value_list<Value...>> {
+    /*! @brief A value list composed by the values of all the value lists. */
+    using type = value_list<Value...>;
+};
+
+
+/**
+ * @brief Helper type.
+ * @tparam List Value lists to concatenate.
+ */
+template<typename... List>
+using value_list_cat_t = typename value_list_cat<List...>::type;
+
+
+/*! @brief Same as std::is_invocable, but with tuples. */
+template<typename, typename>
+struct is_applicable: std::false_type {};
+
+
+/**
+ * @copybrief is_applicable
+ * @tparam Func A valid function type.
+ * @tparam Tuple Tuple-like type.
+ * @tparam Args The list of arguments to use to probe the function type.
+ */
+template<typename Func, template<typename...> class Tuple, typename... Args>
+struct is_applicable<Func, Tuple<Args...>>: std::is_invocable<Func, Args...> {};
+
+
+/**
+ * @copybrief is_applicable
+ * @tparam Func A valid function type.
+ * @tparam Tuple Tuple-like type.
+ * @tparam Args The list of arguments to use to probe the function type.
+ */
+template<typename Func, template<typename...> class Tuple, typename... Args>
+struct is_applicable<Func, const Tuple<Args...>>: std::is_invocable<Func, Args...> {};
+
+
+/**
+ * @brief Helper variable template.
+ * @tparam Func A valid function type.
+ * @tparam Args The list of arguments to use to probe the function type.
+ */
+template<typename Func, typename Args>
+inline constexpr bool is_applicable_v = is_applicable<Func, Args>::value;
+
+
+/*! @brief Same as std::is_invocable_r, but with tuples for arguments. */
+template<typename, typename, typename>
+struct is_applicable_r: std::false_type {};
+
+
+/**
+ * @copybrief is_applicable_r
+ * @tparam Ret The type to which the return type of the function should be
+ * convertible.
+ * @tparam Func A valid function type.
+ * @tparam Args The list of arguments to use to probe the function type.
+ */
+template<typename Ret, typename Func, typename... Args>
+struct is_applicable_r<Ret, Func, std::tuple<Args...>>: std::is_invocable_r<Ret, Func, Args...> {};
+
+
+/**
+ * @brief Helper variable template.
+ * @tparam Ret The type to which the return type of the function should be
+ * convertible.
+ * @tparam Func A valid function type.
+ * @tparam Args The list of arguments to use to probe the function type.
+ */
+template<typename Ret, typename Func, typename Args>
+inline constexpr bool is_applicable_r_v = is_applicable_r<Ret, Func, Args>::value;
+
+
+/**
+ * @brief Provides the member constant `value` to true if a given type is
+ * complete, false otherwise.
+ * @tparam Type The type to test.
+ */
+template<typename Type, typename = void>
+struct is_complete: std::false_type {};
+
+
+/*! @copydoc is_complete */
+template<typename Type>
+struct is_complete<Type, std::void_t<decltype(sizeof(Type))>>: std::true_type {};
+
+
+/**
+ * @brief Helper variable template.
+ * @tparam Type The type to test.
+ */
+template<typename Type>
+inline constexpr bool is_complete_v = is_complete<Type>::value;
+
+
+/**
+ * @brief Provides the member constant `value` to true if a given type is an
+ * iterator, false otherwise.
+ * @tparam Type The type to test.
+ */
+template<typename Type, typename = void>
+struct is_iterator: std::false_type {};
+
+
+/*! @copydoc is_iterator */
+template<typename Type>
+struct is_iterator<Type, std::void_t<typename std::iterator_traits<Type>::iterator_category>>
+    : std::true_type
+{};
+
+
+/**
+ * @brief Helper variable template.
+ * @tparam Type The type to test.
+ */
+template<typename Type>
+inline constexpr bool is_iterator_v = is_iterator<Type>::value;
+
+
+/**
+ * @brief Provides the member constant `value` to true if a given type is of the
+ * required iterator type, false otherwise.
+ * @tparam Type The type to test.
+ * @tparam It Required iterator type.
+ */
+template<typename Type, typename It, typename = void>
+struct is_iterator_type: std::false_type {};
+
+
+/*! @copydoc is_iterator_type */
+template<typename Type, typename It>
+struct is_iterator_type<Type, It, std::enable_if_t<is_iterator_v<Type> && std::is_same_v<Type, It>>>
+    : std::true_type
+{};
+
+
+/*! @copydoc is_iterator_type */
+template<typename Type, typename It>
+struct is_iterator_type<Type, It, std::enable_if_t<!std::is_same_v<Type, It>, std::void_t<typename It::iterator_type>>>
+    : is_iterator_type<Type, typename It::iterator_type>
+{};
+
+
+/**
+ * @brief Helper variable template.
+ * @tparam Type The type to test.
+ * @tparam It Required iterator type.
+ */
+template<typename Type, typename It>
+inline constexpr bool is_iterator_type_v = is_iterator_type<Type, It>::value;
+
+
+/**
+ * @cond TURN_OFF_DOXYGEN
+ * Internal details not to be documented.
+ */
+
+
+namespace internal {
+
+
+template<typename>
+[[nodiscard]] constexpr bool is_equality_comparable(...) { return false; }
+
+
+template<typename Type>
+[[nodiscard]] constexpr auto is_equality_comparable(choice_t<0>)
+-> decltype(std::declval<Type>() == std::declval<Type>()) { return true; }
+
+
+template<typename Type>
+[[nodiscard]] constexpr auto is_equality_comparable(choice_t<1>)
+-> decltype(std::declval<typename Type::value_type>(), std::declval<Type>() == std::declval<Type>()) {
+    if constexpr(is_iterator_v<Type>) {
+        return true;
+    } else if constexpr(std::is_same_v<typename Type::value_type, Type>) {
+        return is_equality_comparable<Type>(choice<0>);
+    } else {
+        return is_equality_comparable<typename Type::value_type>(choice<2>);
+    }
+}
+
+
+template<typename Type>
+[[nodiscard]] constexpr auto is_equality_comparable(choice_t<2>)
+-> decltype(std::declval<typename Type::mapped_type>(), std::declval<Type>() == std::declval<Type>()) {
+    return is_equality_comparable<typename Type::key_type>(choice<2>) && is_equality_comparable<typename Type::mapped_type>(choice<2>);
+}
+
+
+}
+
+
+/**
+ * Internal details not to be documented.
+ * @endcond
+ */
+
+
+/**
+ * @brief Provides the member constant `value` to true if a given type is
+ * equality comparable, false otherwise.
+ * @tparam Type The type to test.
+ */
+template<typename Type, typename = void>
+struct is_equality_comparable: std::bool_constant<internal::is_equality_comparable<Type>(choice<2>)> {};
+
+
+/**
+ * @brief Helper variable template.
+ * @tparam Type The type to test.
+ */
+template<class Type>
+inline constexpr bool is_equality_comparable_v = is_equality_comparable<Type>::value;
+
+
+/**
+ * @brief Transcribes the constness of a type to another type.
+ * @tparam To The type to which to transcribe the constness.
+ * @tparam From The type from which to transcribe the constness.
+ */
+template<typename To, typename From>
+struct constness_as {
+    /*! @brief The type resulting from the transcription of the constness. */
+    using type = std::remove_const_t<To>;
+};
+
+
+/*! @copydoc constness_as */
+template<typename To, typename From>
+struct constness_as<To, const From> {
+    /*! @brief The type resulting from the transcription of the constness. */
+    using type = std::add_const_t<To>;
+};
+
+
+/**
+ * @brief Alias template to facilitate the transcription of the constness.
+ * @tparam To The type to which to transcribe the constness.
+ * @tparam From The type from which to transcribe the constness.
+ */
+template<typename To, typename From>
+using constness_as_t = typename constness_as<To, From>::type;
+
+
+/**
+ * @brief Extracts the class of a non-static member object or function.
+ * @tparam Member A pointer to a non-static member object or function.
+ */
+template<typename Member>
+class member_class {
+    static_assert(std::is_member_pointer_v<Member>, "Invalid pointer type to non-static member object or function");
+
+    template<typename Class, typename Ret, typename... Args>
+    static Class * clazz(Ret(Class:: *)(Args...));
+
+    template<typename Class, typename Ret, typename... Args>
+    static Class * clazz(Ret(Class:: *)(Args...) const);
+
+    template<typename Class, typename Type>
+    static Class * clazz(Type Class:: *);
+
+public:
+    /*! @brief The class of the given non-static member object or function. */
+    using type = std::remove_pointer_t<decltype(clazz(std::declval<Member>()))>;
+};
+
+
+/**
+ * @brief Helper type.
+ * @tparam Member A pointer to a non-static member object or function.
+ */
+template<typename Member>
+using member_class_t = typename member_class<Member>::type;
+
+
+}
+
+
+#endif
+
+
+
+namespace entt {
+
+
+/**
+ * @brief A SBO friendly, type-safe container for single values of any type.
+ * @tparam Len Size of the storage reserved for the small buffer optimization.
+ * @tparam Align Optional alignment requirement.
+ */
+template<std::size_t Len, std::size_t Align>
+class basic_any {
+    enum class operation: std::uint8_t { COPY, MOVE, DTOR, COMP, ADDR, CADDR, TYPE };
+    enum class policy: std::uint8_t { OWNER, REF, CREF };
+
+    using storage_type = std::aligned_storage_t<Len + !Len, Align>;
+    using vtable_type = const void *(const operation, const basic_any &, void *);
+
+    template<typename Type>
+    static constexpr bool in_situ = Len && alignof(Type) <= alignof(storage_type) && sizeof(Type) <= sizeof(storage_type) && std::is_nothrow_move_constructible_v<Type>;
+
+    template<typename Type>
+    [[nodiscard]] static constexpr policy type_to_policy() {
+        if constexpr(std::is_lvalue_reference_v<Type>) {
+            if constexpr(std::is_const_v<std::remove_reference_t<Type>>) {
+                return policy::CREF;
+            } else {
+                return policy::REF;
+            }
+        } else {
+            return policy::OWNER;
+        }
+    }
+
+    template<typename Type>
+    [[nodiscard]] static bool compare(const void *lhs, const void *rhs) {
+        if constexpr(!std::is_function_v<Type> && is_equality_comparable_v<Type>) {
+            return *static_cast<const Type *>(lhs) == *static_cast<const Type *>(rhs);
+        } else {
+            return lhs == rhs;
+        }
+    }
+
+    template<typename Type>
+    static const void * basic_vtable([[maybe_unused]] const operation op, [[maybe_unused]] const basic_any &from, [[maybe_unused]] void *to) {
+        static_assert(std::is_same_v<std::remove_reference_t<std::remove_const_t<Type>>, Type>, "Invalid type");
+
+        if constexpr(!std::is_void_v<Type>) {
+            const Type *instance = (in_situ<Type> && from.mode == policy::OWNER)
+                ? ENTT_LAUNDER(reinterpret_cast<const Type *>(&from.storage))
+                : static_cast<const Type *>(from.instance);
+
+            switch(op) {
+            case operation::COPY:
+                if constexpr(std::is_copy_constructible_v<Type>) {
+                    static_cast<basic_any *>(to)->emplace<Type>(*instance);
+                }
+                break;
+            case operation::MOVE:
+                if constexpr(in_situ<Type>) {
+                    if(from.mode == policy::OWNER) {
+                        return new (&static_cast<basic_any *>(to)->storage) Type{std::move(*const_cast<Type *>(instance))};
+                    }
+                }
+
+                return (static_cast<basic_any *>(to)->instance = std::exchange(const_cast<basic_any &>(from).instance, nullptr));
+            case operation::DTOR:
+                if(from.mode == policy::OWNER) {
+                    if constexpr(in_situ<Type>) {
+                        instance->~Type();
+                    } else if constexpr(std::is_array_v<Type>) {
+                        delete[] instance;
+                    } else {
+                        delete instance;
+                    }
+                }
+                break;
+            case operation::COMP:
+                return compare<Type>(instance, (*static_cast<const basic_any **>(to))->data()) ? to : nullptr;
+            case operation::ADDR:
+                if(from.mode == policy::CREF) {
+                    return nullptr;
+                }
+                [[fallthrough]];
+            case operation::CADDR:
+                return instance;
+            case operation::TYPE:
+                *static_cast<type_info *>(to) = type_id<Type>();
+                break;
+            }
+        }
+
+        return nullptr;
+    }
+
+    template<typename Type, typename... Args>
+    void initialize([[maybe_unused]] Args &&... args) {
+        if constexpr(!std::is_void_v<Type>) {
+            if constexpr(std::is_lvalue_reference_v<Type>) {
+                static_assert(sizeof...(Args) == 1u && (std::is_lvalue_reference_v<Args> && ...), "Invalid arguments");
+                instance = (std::addressof(args), ...);
+            } else if constexpr(in_situ<Type>) {
+                if constexpr(sizeof...(Args) != 0u && std::is_aggregate_v<Type>) {
+                    new (&storage) Type{std::forward<Args>(args)...};
+                } else {
+                    new (&storage) Type(std::forward<Args>(args)...);
+                }
+            } else {
+                if constexpr(sizeof...(Args) != 0u && std::is_aggregate_v<Type>) {
+                    instance = new Type{std::forward<Args>(args)...};
+                } else {
+                    instance = new Type(std::forward<Args>(args)...);
+                }
+            }
+        }
+    }
+
+    basic_any(const basic_any &other, const policy pol) ENTT_NOEXCEPT
+        : instance{other.data()},
+          vtable{other.vtable},
+          mode{pol}
+    {}
+
+public:
+    /*! @brief Size of the internal storage. */
+    static constexpr auto length = Len;
+    /*! @brief Alignment requirement. */
+    static constexpr auto alignment = Align;
+
+    /*! @brief Default constructor. */
+    basic_any() ENTT_NOEXCEPT
+        : instance{},
+          vtable{&basic_vtable<void>},
+          mode{policy::OWNER}
+    {}
+
+    /**
+     * @brief Constructs a wrapper by directly initializing the new object.
+     * @tparam Type Type of object to use to initialize the wrapper.
+     * @tparam Args Types of arguments to use to construct the new instance.
+     * @param args Parameters to use to construct the instance.
+     */
+    template<typename Type, typename... Args>
+    explicit basic_any(std::in_place_type_t<Type>, Args &&... args)
+        : instance{},
+          vtable{&basic_vtable<std::remove_const_t<std::remove_reference_t<Type>>>},
+          mode{type_to_policy<Type>()}
+    {
+        initialize<Type>(std::forward<Args>(args)...);
+    }
+
+    /**
+     * @brief Constructs a wrapper that holds an unmanaged object.
+     * @tparam Type Type of object to use to initialize the wrapper.
+     * @param value An instance of an object to use to initialize the wrapper.
+     */
+    template<typename Type>
+    basic_any(std::reference_wrapper<Type> value) ENTT_NOEXCEPT
+        : basic_any{}
+    {
+        // invokes deprecated assignment operator (and avoids issues with vs2017)
+        *this = value;
+    }
+
+    /**
+     * @brief Constructs a wrapper from a given value.
+     * @tparam Type Type of object to use to initialize the wrapper.
+     * @param value An instance of an object to use to initialize the wrapper.
+     */
+    template<typename Type, typename = std::enable_if_t<!std::is_same_v<std::decay_t<Type>, basic_any>>>
+    basic_any(Type &&value)
+        : instance{},
+          vtable{&basic_vtable<std::decay_t<Type>>},
+          mode{policy::OWNER}
+    {
+        initialize<std::decay_t<Type>>(std::forward<Type>(value));
+    }
+
+    /**
+     * @brief Copy constructor.
+     * @param other The instance to copy from.
+     */
+    basic_any(const basic_any &other)
+        : instance{},
+          vtable{&basic_vtable<void>},
+          mode{policy::OWNER}
+    {
+        other.vtable(operation::COPY, other, this);
+    }
+
+    /**
+     * @brief Move constructor.
+     * @param other The instance to move from.
+     */
+    basic_any(basic_any &&other) ENTT_NOEXCEPT
+        : instance{},
+          vtable{other.vtable},
+          mode{other.mode}
+    {
+        vtable(operation::MOVE, other, this);
+    }
+
+    /*! @brief Frees the internal storage, whatever it means. */
+    ~basic_any() {
+        vtable(operation::DTOR, *this, nullptr);
+    }
+
+    /**
+     * @brief Copy assignment operator.
+     * @param other The instance to copy from.
+     * @return This any object.
+     */
+    basic_any & operator=(const basic_any &other) {
+        reset();
+        other.vtable(operation::COPY, other, this);
+        return *this;
+    }
+
+    /**
+     * @brief Move assignment operator.
+     * @param other The instance to move from.
+     * @return This any object.
+     */
+    basic_any & operator=(basic_any &&other) ENTT_NOEXCEPT {
+        std::exchange(vtable, other.vtable)(operation::DTOR, *this, nullptr);
+        other.vtable(operation::MOVE, other, this);
+        mode = other.mode;
+        return *this;
+    }
+
+    /**
+     * @brief Value assignment operator.
+     * @tparam Type Type of object to use to initialize the wrapper.
+     * @param value An instance of an object to use to initialize the wrapper.
+     * @return This any object.
+     */
+    template<typename Type>
+    [[deprecated("Use std::in_place_type<T &>, entt::make_any<T &>, emplace<Type &> or forward_as_any instead")]]
+    basic_any & operator=(std::reference_wrapper<Type> value) ENTT_NOEXCEPT {
+        emplace<Type &>(value.get());
+        return *this;
+    }
+
+    /**
+     * @brief Value assignment operator.
+     * @tparam Type Type of object to use to initialize the wrapper.
+     * @param value An instance of an object to use to initialize the wrapper.
+     * @return This any object.
+     */
+    template<typename Type>
+    std::enable_if_t<!std::is_same_v<std::decay_t<Type>, basic_any>, basic_any &>
+    operator=(Type &&value) {
+        emplace<std::decay_t<Type>>(std::forward<Type>(value));
+        return *this;
+    }
+
+    /**
+     * @brief Returns the type of the contained object.
+     * @return The type of the contained object, if any.
+     */
+    [[nodiscard]] type_info type() const ENTT_NOEXCEPT {
+        type_info info{};
+        vtable(operation::TYPE, *this, &info);
+        return info;
+    }
+
+    /**
+     * @brief Returns an opaque pointer to the contained instance.
+     * @return An opaque pointer the contained instance, if any.
+     */
+    [[nodiscard]] const void * data() const ENTT_NOEXCEPT {
+        return vtable(operation::CADDR, *this, nullptr);
+    }
+
+    /*! @copydoc data */
+    [[nodiscard]] void * data() ENTT_NOEXCEPT {
+        return const_cast<void *>(vtable(operation::ADDR, *this, nullptr));
+    }
+
+    /**
+     * @brief Replaces the contained object by creating a new instance directly.
+     * @tparam Type Type of object to use to initialize the wrapper.
+     * @tparam Args Types of arguments to use to construct the new instance.
+     * @param args Parameters to use to construct the instance.
+     */
+    template<typename Type, typename... Args>
+    void emplace(Args &&... args) {
+        std::exchange(vtable, &basic_vtable<std::remove_const_t<std::remove_reference_t<Type>>>)(operation::DTOR, *this, nullptr);
+        mode = type_to_policy<Type>();
+        initialize<Type>(std::forward<Args>(args)...);
+    }
+
+    /*! @brief Destroys contained object */
+    void reset() {
+        std::exchange(vtable, &basic_vtable<void>)(operation::DTOR, *this, nullptr);
+        mode = policy::OWNER;
+    }
+
+    /**
+     * @brief Returns false if a wrapper is empty, true otherwise.
+     * @return False if the wrapper is empty, true otherwise.
+     */
+    [[nodiscard]] explicit operator bool() const ENTT_NOEXCEPT {
+        return !(vtable(operation::CADDR, *this, nullptr) == nullptr);
+    }
+
+    /**
+     * @brief Checks if two wrappers differ in their content.
+     * @param other Wrapper with which to compare.
+     * @return False if the two objects differ in their content, true otherwise.
+     */
+    bool operator==(const basic_any &other) const ENTT_NOEXCEPT {
+        const basic_any *trampoline = &other;
+        return type() == other.type() && (vtable(operation::COMP, *this, &trampoline) || !other.data());
+    }
+
+    /**
+     * @brief Aliasing constructor.
+     * @return A wrapper that shares a reference to an unmanaged object.
+     */
+    [[nodiscard]] basic_any as_ref() ENTT_NOEXCEPT {
+        return basic_any{*this, (mode == policy::CREF ? policy::CREF : policy::REF)};
+    }
+
+    /*! @copydoc as_ref */
+    [[nodiscard]] basic_any as_ref() const ENTT_NOEXCEPT {
+        return basic_any{*this, policy::CREF};
+    }
+
+    /**
+     * @brief Returns true if a wrapper owns its object, false otherwise.
+     * @return True if the wrapper owns its object, false otherwise.
+     */
+    [[nodiscard]] bool owner() const ENTT_NOEXCEPT {
+        return (mode == policy::OWNER);
+    }
+
+private:
+    union { const void *instance; storage_type storage; };
+    vtable_type *vtable;
+    policy mode;
+};
+
+
+/**
+ * @brief Checks if two wrappers differ in their content.
+ * @tparam Len Size of the storage reserved for the small buffer optimization.
+ * @tparam Align Alignment requirement.
+ * @param lhs A wrapper, either empty or not.
+ * @param rhs A wrapper, either empty or not.
+ * @return True if the two wrappers differ in their content, false otherwise.
+ */
+template<std::size_t Len, std::size_t Align>
+[[nodiscard]] inline bool operator!=(const basic_any<Len, Align> &lhs, const basic_any<Len, Align> &rhs) ENTT_NOEXCEPT {
+    return !(lhs == rhs);
+}
+
+
+/**
+ * @brief Performs type-safe access to the contained object.
+ * @tparam Type Type to which conversion is required.
+ * @tparam Len Size of the storage reserved for the small buffer optimization.
+ * @tparam Align Alignment requirement.
+ * @param data Target any object.
+ * @return The element converted to the requested type.
+ */
+template<typename Type, std::size_t Len, std::size_t Align>
+Type any_cast(const basic_any<Len, Align> &data) ENTT_NOEXCEPT {
+    const auto * const instance = any_cast<std::remove_reference_t<Type>>(&data);
+    ENTT_ASSERT(instance, "Invalid instance");
+    return static_cast<Type>(*instance);
+}
+
+
+/*! @copydoc any_cast */
+template<typename Type, std::size_t Len, std::size_t Align>
+Type any_cast(basic_any<Len, Align> &data) ENTT_NOEXCEPT {
+    // forces const on non-reference types to make them work also with wrappers for const references
+    auto * const instance = any_cast<std::remove_reference_t<const Type>>(&data);
+    ENTT_ASSERT(instance, "Invalid instance");
+    return static_cast<Type>(*instance);
+}
+
+
+/*! @copydoc any_cast */
+template<typename Type, std::size_t Len, std::size_t Align>
+Type any_cast(basic_any<Len, Align> &&data) ENTT_NOEXCEPT {
+    // forces const on non-reference types to make them work also with wrappers for const references
+    auto * const instance = any_cast<std::remove_reference_t<const Type>>(&data);
+    ENTT_ASSERT(instance, "Invalid instance");
+    return static_cast<Type>(std::move(*instance));
+}
+
+
+/*! @copydoc any_cast */
+template<typename Type, std::size_t Len, std::size_t Align>
+const Type * any_cast(const basic_any<Len, Align> *data) ENTT_NOEXCEPT {
+    return (data->type() == type_id<Type>() ? static_cast<const Type *>(data->data()) : nullptr);
+}
+
+
+/*! @copydoc any_cast */
+template<typename Type, std::size_t Len, std::size_t Align>
+Type * any_cast(basic_any<Len, Align> *data) ENTT_NOEXCEPT {
+    // last attempt to make wrappers for const references return their values
+    return (data->type() == type_id<Type>() ? static_cast<Type *>(static_cast<constness_as_t<basic_any<Len, Align>, Type> *>(data)->data()) : nullptr);
+}
+
+
+/**
+ * @brief Constructs a wrapper from a given type, passing it all arguments.
+ * @tparam Type Type of object to use to initialize the wrapper.
+ * @tparam Len Size of the storage reserved for the small buffer optimization.
+ * @tparam Align Optional alignment requirement.
+ * @tparam Args Types of arguments to use to construct the new instance.
+ * @param args Parameters to use to construct the instance.
+ * @return A properly initialized wrapper for an object of the given type.
+ */
+template<typename Type, std::size_t Len = basic_any<>::length, std::size_t Align = basic_any<Len>::alignment, typename... Args>
+basic_any<Len, Align> make_any(Args &&... args) {
+    return basic_any<Len, Align>{std::in_place_type<Type>, std::forward<Args>(args)...};
+}
+
+
+/**
+ * @brief Forwards its argument and avoids copies for lvalue references.
+ * @tparam Len Size of the storage reserved for the small buffer optimization.
+ * @tparam Align Optional alignment requirement.
+ * @tparam Type Type of argument to use to construct the new instance.
+ * @param value Parameter to use to construct the instance.
+ * @return A properly initialized and not necessarily owning wrapper.
+ */
+template<std::size_t Len = basic_any<>::length, std::size_t Align = basic_any<Len>::alignment, typename Type>
+basic_any<Len, Align> forward_as_any(Type &&value) {
+    return basic_any<Len, Align>{std::in_place_type<std::conditional_t<std::is_rvalue_reference_v<Type>, std::decay_t<Type>, Type>>, std::forward<Type>(value)};
+}
+
+
+}
+
+
+#endif
+
+// #include "../core/type_info.hpp"
+#ifndef ENTT_CORE_TYPE_INFO_HPP
+#define ENTT_CORE_TYPE_INFO_HPP
+
+
+#include <string_view>
+#include <type_traits>
+// #include "../config/config.h"
+
+// #include "../core/attribute.h"
+
+// #include "hashed_string.hpp"
+
+// #include "fwd.hpp"
+
+
+
+namespace entt {
+
+
+/**
+ * @cond TURN_OFF_DOXYGEN
+ * Internal details not to be documented.
+ */
+
+
+namespace internal {
+
+
+struct ENTT_API type_seq final {
+    [[nodiscard]] static id_type next() ENTT_NOEXCEPT {
+        static ENTT_MAYBE_ATOMIC(id_type) value{};
+        return value++;
+    }
+};
+
+
+template<typename Type>
+[[nodiscard]] constexpr auto stripped_type_name() ENTT_NOEXCEPT {
+#if defined ENTT_PRETTY_FUNCTION
+    std::string_view pretty_function{ENTT_PRETTY_FUNCTION};
+    auto first = pretty_function.find_first_not_of(' ', pretty_function.find_first_of(ENTT_PRETTY_FUNCTION_PREFIX)+1);
+    auto value = pretty_function.substr(first, pretty_function.find_last_of(ENTT_PRETTY_FUNCTION_SUFFIX) - first);
+    return value;
+#else
+    return std::string_view{""};
+#endif
+}
+
+
+template<typename Type, auto = stripped_type_name<Type>().find_first_of('.')>
+[[nodiscard]] static constexpr std::string_view type_name(int) ENTT_NOEXCEPT {
+    constexpr auto value = stripped_type_name<Type>();
+    return value;
+}
+
+
+template<typename Type>
+[[nodiscard]] static std::string_view type_name(char) ENTT_NOEXCEPT {
+    static const auto value = stripped_type_name<Type>();
+    return value;
+}
+
+
+template<typename Type, auto = stripped_type_name<Type>().find_first_of('.')>
+[[nodiscard]] static constexpr id_type type_hash(int) ENTT_NOEXCEPT {
+    constexpr auto stripped = stripped_type_name<Type>();
+    constexpr auto value = hashed_string::value(stripped.data(), stripped.size());
+    return value;
+}
+
+
+template<typename Type>
+[[nodiscard]] static id_type type_hash(char) ENTT_NOEXCEPT {
+    static const auto value = [](const auto stripped) {
+        return hashed_string::value(stripped.data(), stripped.size());
+    }(stripped_type_name<Type>());
+    return value;
+}
+
+
+}
+
+
+/**
+ * Internal details not to be documented.
+ * @endcond
+ */
+
+
+/**
+ * @brief Type sequential identifier.
+ * @tparam Type Type for which to generate a sequential identifier.
+ */
+template<typename Type, typename = void>
+struct ENTT_API type_seq final {
+    /**
+     * @brief Returns the sequential identifier of a given type.
+     * @return The sequential identifier of a given type.
+     */
+    [[nodiscard]] static id_type value() ENTT_NOEXCEPT {
+        static const id_type value = internal::type_seq::next();
+        return value;
+    }
+
+    /*! @copydoc value */
+    [[nodiscard]] constexpr operator id_type() const ENTT_NOEXCEPT { return value(); }
+};
+
+
+/**
+ * @brief Type hash.
+ * @tparam Type Type for which to generate a hash value.
+ */
+template<typename Type, typename = void>
+struct type_hash final {
+    /**
+     * @brief Returns the numeric representation of a given type.
+     * @return The numeric representation of the given type.
+     */
+#if defined ENTT_PRETTY_FUNCTION
+    [[nodiscard]] static constexpr id_type value() ENTT_NOEXCEPT {
+        return internal::type_hash<Type>(0);
+#else
+    [[nodiscard]] static constexpr id_type value() ENTT_NOEXCEPT {
+        return type_seq<Type>::value();
+#endif
+    }
+
+    /*! @copydoc value */
+    [[nodiscard]] constexpr operator id_type() const ENTT_NOEXCEPT { return value(); }
+};
+
+
+/**
+ * @brief Type name.
+ * @tparam Type Type for which to generate a name.
+ */
+template<typename Type, typename = void>
+struct type_name final {
+    /**
+     * @brief Returns the name of a given type.
+     * @return The name of the given type.
+     */
+    [[nodiscard]] static constexpr std::string_view value() ENTT_NOEXCEPT {
+        return internal::type_name<Type>(0);
+    }
+
+    /*! @copydoc value */
+    [[nodiscard]] constexpr operator std::string_view() const ENTT_NOEXCEPT { return value(); }
+};
+
+
+/*! @brief Implementation specific information about a type. */
+class type_info final {
+    template<typename>
+    friend type_info type_id() ENTT_NOEXCEPT;
+
+    type_info(id_type seq_v, id_type hash_v, std::string_view name_v) ENTT_NOEXCEPT
+        : seq_value{seq_v},
+          hash_value{hash_v},
+          name_value{name_v}
+    {}
+
+public:
+    /*! @brief Default constructor. */
+    type_info() ENTT_NOEXCEPT
+        : type_info({}, {}, {})
+    {}
+
+    /*! @brief Default copy constructor. */
+    type_info(const type_info &) ENTT_NOEXCEPT = default;
+    /*! @brief Default move constructor. */
+    type_info(type_info &&) ENTT_NOEXCEPT = default;
+
+    /**
+     * @brief Default copy assignment operator.
+     * @return This type info object.
+     */
+    type_info & operator=(const type_info &) ENTT_NOEXCEPT = default;
+
+    /**
+     * @brief Default move assignment operator.
+     * @return This type info object.
+     */
+    type_info & operator=(type_info &&) ENTT_NOEXCEPT = default;
+
+    /**
+     * @brief Checks if a type info object is properly initialized.
+     * @return True if the object is properly initialized, false otherwise.
+     */
+    [[nodiscard]] explicit operator bool() const ENTT_NOEXCEPT {
+        return name_value.data() != nullptr;
+    }
+
+    /**
+     * @brief Type sequential identifier.
+     * @return Type sequential identifier.
+     */
+    [[nodiscard]] id_type seq() const ENTT_NOEXCEPT {
+        return seq_value;
+    }
+
+    /**
+     * @brief Type hash.
+     * @return Type hash.
+     */
+    [[nodiscard]] id_type hash() const ENTT_NOEXCEPT {
+        return hash_value;
+    }
+
+    /**
+     * @brief Type name.
+     * @return Type name.
+     */
+    [[nodiscard]] std::string_view name() const ENTT_NOEXCEPT {
+        return name_value;
+    }
+
+    /**
+     * @brief Compares the contents of two type info objects.
+     * @param other Object with which to compare.
+     * @return False if the two contents differ, true otherwise.
+     */
+    [[nodiscard]] bool operator==(const type_info &other) const ENTT_NOEXCEPT {
+        return hash_value == other.hash_value;
+    }
+
+private:
+    id_type seq_value;
+    id_type hash_value;
+    std::string_view name_value;
+};
+
+
+/**
+ * @brief Compares the contents of two type info objects.
+ * @param lhs A type info object.
+ * @param rhs A type info object.
+ * @return True if the two contents differ, false otherwise.
+ */
+[[nodiscard]] inline bool operator!=(const type_info &lhs, const type_info &rhs) ENTT_NOEXCEPT {
+    return !(lhs == rhs);
+}
+
+
+/**
+ * @brief Returns the type info object for a given type.
+ * @tparam Type Type for which to generate a type info object.
+ * @return The type info object for the given type.
+ */
+template<typename Type>
+[[nodiscard]] type_info type_id() ENTT_NOEXCEPT {
+    return type_info{
+        type_seq<std::remove_cv_t<std::remove_reference_t<Type>>>::value(),
+        type_hash<std::remove_cv_t<std::remove_reference_t<Type>>>::value(),
+        type_name<std::remove_cv_t<std::remove_reference_t<Type>>>::value()
+    };
+}
+
+
+}
+
+
+#endif
+
+// #include "../core/type_traits.hpp"
+#ifndef ENTT_CORE_TYPE_TRAITS_HPP
+#define ENTT_CORE_TYPE_TRAITS_HPP
+
+
+#include <cstddef>
+#include <iterator>
+#include <type_traits>
+#include <utility>
+// #include "../config/config.h"
+
+// #include "fwd.hpp"
+
+
+
+namespace entt {
+
+
+/**
+ * @brief Utility class to disambiguate overloaded functions.
+ * @tparam N Number of choices available.
+ */
+template<std::size_t N>
+struct choice_t
+    // Unfortunately, doxygen cannot parse such a construct.
+    /*! @cond TURN_OFF_DOXYGEN */
+    : choice_t<N-1>
+    /*! @endcond */
+{};
+
+
+/*! @copybrief choice_t */
+template<>
+struct choice_t<0> {};
+
+
+/**
+ * @brief Variable template for the choice trick.
+ * @tparam N Number of choices available.
+ */
+template<std::size_t N>
+inline constexpr choice_t<N> choice{};
+
+
+/**
+ * @brief Identity type trait.
+ *
+ * Useful to establish non-deduced contexts in template argument deduction
+ * (waiting for C++20) or to provide types through function arguments.
+ *
+ * @tparam Type A type.
+ */
+template<typename Type>
+struct type_identity {
+    /*! @brief Identity type. */
+    using type = Type;
+};
+
+
+/**
+ * @brief Helper type.
+ * @tparam Type A type.
+ */
+template<typename Type>
+using type_identity_t = typename type_identity<Type>::type;
+
+
+/**
+ * @brief A type-only `sizeof` wrapper that returns 0 where `sizeof` complains.
+ * @tparam Type The type of which to return the size.
+ * @tparam The size of the type if `sizeof` accepts it, 0 otherwise.
+ */
+template<typename Type, typename = void>
+struct size_of: std::integral_constant<std::size_t, 0u> {};
+
+
+/*! @copydoc size_of */
+template<typename Type>
+struct size_of<Type, std::void_t<decltype(sizeof(Type))>>
+    : std::integral_constant<std::size_t, sizeof(Type)>
+{};
+
+
+/**
+ * @brief Helper variable template.
+ * @tparam Type The type of which to return the size.
+ */
+template<class Type>
+inline constexpr std::size_t size_of_v = size_of<Type>::value;
+
+
+/**
+ * @brief Using declaration to be used to _repeat_ the same type a number of
+ * times equal to the size of a given parameter pack.
+ * @tparam Type A type to repeat.
+ */
+template<typename Type, typename>
+using unpack_as_t = Type;
+
+
+/**
+ * @brief Helper variable template to be used to _repeat_ the same value a
+ * number of times equal to the size of a given parameter pack.
+ * @tparam Value A value to repeat.
+ */
+template<auto Value, typename>
+inline constexpr auto unpack_as_v = Value;
+
+
+/**
+ * @brief Wraps a static constant.
+ * @tparam Value A static constant.
+ */
+template<auto Value>
+using integral_constant = std::integral_constant<decltype(Value), Value>;
+
+
+/**
+ * @brief Alias template to facilitate the creation of named values.
+ * @tparam Value A constant value at least convertible to `id_type`.
+ */
+template<id_type Value>
+using tag = integral_constant<Value>;
+
+
+/**
+ * @brief A class to use to push around lists of types, nothing more.
+ * @tparam Type Types provided by the type list.
+ */
+template<typename... Type>
+struct type_list {
+    /*! @brief Type list type. */
+    using type = type_list;
+    /*! @brief Compile-time number of elements in the type list. */
+    static constexpr auto size = sizeof...(Type);
+};
+
+
+/*! @brief Primary template isn't defined on purpose. */
+template<std::size_t, typename>
+struct type_list_element;
+
+
+/**
+ * @brief Provides compile-time indexed access to the types of a type list.
+ * @tparam Index Index of the type to return.
+ * @tparam Type First type provided by the type list.
+ * @tparam Other Other types provided by the type list.
+ */
+template<std::size_t Index, typename Type, typename... Other>
+struct type_list_element<Index, type_list<Type, Other...>>
+    : type_list_element<Index - 1u, type_list<Other...>>
+{};
+
+
+/**
+ * @brief Provides compile-time indexed access to the types of a type list.
+ * @tparam Type First type provided by the type list.
+ * @tparam Other Other types provided by the type list.
+ */
+template<typename Type, typename... Other>
+struct type_list_element<0u, type_list<Type, Other...>> {
+    /*! @brief Searched type. */
+    using type = Type;
+};
+
+
+/**
+ * @brief Helper type.
+ * @tparam Index Index of the type to return.
+ * @tparam List Type list to search into.
+ */
+template<std::size_t Index, typename List>
+using type_list_element_t = typename type_list_element<Index, List>::type;
+
+
+/**
+ * @brief Concatenates multiple type lists.
+ * @tparam Type Types provided by the first type list.
+ * @tparam Other Types provided by the second type list.
+ * @return A type list composed by the types of both the type lists.
+ */
+template<typename... Type, typename... Other>
+constexpr type_list<Type..., Other...> operator+(type_list<Type...>, type_list<Other...>) { return {}; }
+
+
+/*! @brief Primary template isn't defined on purpose. */
+template<typename...>
+struct type_list_cat;
+
+
+/*! @brief Concatenates multiple type lists. */
+template<>
+struct type_list_cat<> {
+    /*! @brief A type list composed by the types of all the type lists. */
+    using type = type_list<>;
+};
+
+
+/**
+ * @brief Concatenates multiple type lists.
+ * @tparam Type Types provided by the first type list.
+ * @tparam Other Types provided by the second type list.
+ * @tparam List Other type lists, if any.
+ */
+template<typename... Type, typename... Other, typename... List>
+struct type_list_cat<type_list<Type...>, type_list<Other...>, List...> {
+    /*! @brief A type list composed by the types of all the type lists. */
+    using type = typename type_list_cat<type_list<Type..., Other...>, List...>::type;
+};
+
+
+/**
+ * @brief Concatenates multiple type lists.
+ * @tparam Type Types provided by the type list.
+ */
+template<typename... Type>
+struct type_list_cat<type_list<Type...>> {
+    /*! @brief A type list composed by the types of all the type lists. */
+    using type = type_list<Type...>;
+};
+
+
+/**
+ * @brief Helper type.
+ * @tparam List Type lists to concatenate.
+ */
+template<typename... List>
+using type_list_cat_t = typename type_list_cat<List...>::type;
+
+
+/*! @brief Primary template isn't defined on purpose. */
+template<typename>
+struct type_list_unique;
+
+
+/**
+ * @brief Removes duplicates types from a type list.
+ * @tparam Type One of the types provided by the given type list.
+ * @tparam Other The other types provided by the given type list.
+ */
+template<typename Type, typename... Other>
+struct type_list_unique<type_list<Type, Other...>> {
+    /*! @brief A type list without duplicate types. */
+    using type = std::conditional_t<
+        std::disjunction_v<std::is_same<Type, Other>...>,
+        typename type_list_unique<type_list<Other...>>::type,
+        type_list_cat_t<type_list<Type>, typename type_list_unique<type_list<Other...>>::type>
+    >;
+};
+
+
+/*! @brief Removes duplicates types from a type list. */
+template<>
+struct type_list_unique<type_list<>> {
+    /*! @brief A type list without duplicate types. */
+    using type = type_list<>;
+};
+
+
+/**
+ * @brief Helper type.
+ * @tparam Type A type list.
+ */
+template<typename Type>
+using type_list_unique_t = typename type_list_unique<Type>::type;
+
+
+/**
+ * @brief Provides the member constant `value` to true if a type list contains a
+ * given type, false otherwise.
+ * @tparam List Type list.
+ * @tparam Type Type to look for.
+ */
+template<typename List, typename Type>
+struct type_list_contains;
+
+
+/**
+ * @copybrief type_list_contains
+ * @tparam Type Types provided by the type list.
+ * @tparam Other Type to look for.
+ */
+template<typename... Type, typename Other>
+struct type_list_contains<type_list<Type...>, Other>: std::disjunction<std::is_same<Type, Other>...> {};
+
+
+/**
+ * @brief Helper variable template.
+ * @tparam List Type list.
+ * @tparam Type Type to look for.
+ */
+template<class List, typename Type>
+inline constexpr bool type_list_contains_v = type_list_contains<List, Type>::value;
+
+
+/*! @brief Primary template isn't defined on purpose. */
+template<typename...>
+struct type_list_diff;
+
+
+/**
+ * @brief Computes the difference between two type lists.
+ * @tparam Type Types provided by the first type list.
+ * @tparam Other Types provided by the second type list.
+ */
+template<typename... Type, typename... Other>
+struct type_list_diff<type_list<Type...>, type_list<Other...>> {
+    /*! @brief A type list that is the difference between the two type lists. */
+    using type = type_list_cat_t<std::conditional_t<type_list_contains_v<type_list<Other...>, Type>, type_list<>, type_list<Type>>...>;
+};
+
+
+/**
+ * @brief Helper type.
+ * @tparam List Type lists between which to compute the difference.
+ */
+template<typename... List>
+using type_list_diff_t = typename type_list_diff<List...>::type;
+
+
+/**
+ * @brief A class to use to push around lists of constant values, nothing more.
+ * @tparam Value Values provided by the value list.
+ */
+template<auto... Value>
+struct value_list {
+    /*! @brief Value list type. */
+    using type = value_list;
+    /*! @brief Compile-time number of elements in the value list. */
+    static constexpr auto size = sizeof...(Value);
+};
+
+
+/*! @brief Primary template isn't defined on purpose. */
+template<std::size_t, typename>
+struct value_list_element;
+
+
+/**
+ * @brief Provides compile-time indexed access to the values of a value list.
+ * @tparam Index Index of the value to return.
+ * @tparam Value First value provided by the value list.
+ * @tparam Other Other values provided by the value list.
+ */
+template<std::size_t Index, auto Value, auto... Other>
+struct value_list_element<Index, value_list<Value, Other...>>
+    : value_list_element<Index - 1u, value_list<Other...>>
+{};
+
+
+/**
+ * @brief Provides compile-time indexed access to the types of a type list.
+ * @tparam Value First value provided by the value list.
+ * @tparam Other Other values provided by the value list.
+ */
+template<auto Value, auto... Other>
+struct value_list_element<0u, value_list<Value, Other...>> {
+    /*! @brief Searched value. */
+    static constexpr auto value = Value;
+};
+
+
+/**
+ * @brief Helper type.
+ * @tparam Index Index of the value to return.
+ * @tparam List Value list to search into.
+ */
+template<std::size_t Index, typename List>
+inline constexpr auto value_list_element_v = value_list_element<Index, List>::value;
+
+
+/**
+ * @brief Concatenates multiple value lists.
+ * @tparam Value Values provided by the first value list.
+ * @tparam Other Values provided by the second value list.
+ * @return A value list composed by the values of both the value lists.
+ */
+template<auto... Value, auto... Other>
+constexpr value_list<Value..., Other...> operator+(value_list<Value...>, value_list<Other...>) { return {}; }
+
+
+/*! @brief Primary template isn't defined on purpose. */
+template<typename...>
+struct value_list_cat;
+
+
+/*! @brief Concatenates multiple value lists. */
+template<>
+struct value_list_cat<> {
+    /*! @brief A value list composed by the values of all the value lists. */
+    using type = value_list<>;
+};
+
+
+/**
+ * @brief Concatenates multiple value lists.
+ * @tparam Value Values provided by the first value list.
+ * @tparam Other Values provided by the second value list.
+ * @tparam List Other value lists, if any.
+ */
+template<auto... Value, auto... Other, typename... List>
+struct value_list_cat<value_list<Value...>, value_list<Other...>, List...> {
+    /*! @brief A value list composed by the values of all the value lists. */
+    using type = typename value_list_cat<value_list<Value..., Other...>, List...>::type;
+};
+
+
+/**
+ * @brief Concatenates multiple value lists.
+ * @tparam Value Values provided by the value list.
+ */
+template<auto... Value>
+struct value_list_cat<value_list<Value...>> {
+    /*! @brief A value list composed by the values of all the value lists. */
+    using type = value_list<Value...>;
+};
+
+
+/**
+ * @brief Helper type.
+ * @tparam List Value lists to concatenate.
+ */
+template<typename... List>
+using value_list_cat_t = typename value_list_cat<List...>::type;
+
+
+/*! @brief Same as std::is_invocable, but with tuples. */
+template<typename, typename>
+struct is_applicable: std::false_type {};
+
+
+/**
+ * @copybrief is_applicable
+ * @tparam Func A valid function type.
+ * @tparam Tuple Tuple-like type.
+ * @tparam Args The list of arguments to use to probe the function type.
+ */
+template<typename Func, template<typename...> class Tuple, typename... Args>
+struct is_applicable<Func, Tuple<Args...>>: std::is_invocable<Func, Args...> {};
+
+
+/**
+ * @copybrief is_applicable
+ * @tparam Func A valid function type.
+ * @tparam Tuple Tuple-like type.
+ * @tparam Args The list of arguments to use to probe the function type.
+ */
+template<typename Func, template<typename...> class Tuple, typename... Args>
+struct is_applicable<Func, const Tuple<Args...>>: std::is_invocable<Func, Args...> {};
+
+
+/**
+ * @brief Helper variable template.
+ * @tparam Func A valid function type.
+ * @tparam Args The list of arguments to use to probe the function type.
+ */
+template<typename Func, typename Args>
+inline constexpr bool is_applicable_v = is_applicable<Func, Args>::value;
+
+
+/*! @brief Same as std::is_invocable_r, but with tuples for arguments. */
+template<typename, typename, typename>
+struct is_applicable_r: std::false_type {};
+
+
+/**
+ * @copybrief is_applicable_r
+ * @tparam Ret The type to which the return type of the function should be
+ * convertible.
+ * @tparam Func A valid function type.
+ * @tparam Args The list of arguments to use to probe the function type.
+ */
+template<typename Ret, typename Func, typename... Args>
+struct is_applicable_r<Ret, Func, std::tuple<Args...>>: std::is_invocable_r<Ret, Func, Args...> {};
+
+
+/**
+ * @brief Helper variable template.
+ * @tparam Ret The type to which the return type of the function should be
+ * convertible.
+ * @tparam Func A valid function type.
+ * @tparam Args The list of arguments to use to probe the function type.
+ */
+template<typename Ret, typename Func, typename Args>
+inline constexpr bool is_applicable_r_v = is_applicable_r<Ret, Func, Args>::value;
+
+
+/**
+ * @brief Provides the member constant `value` to true if a given type is
+ * complete, false otherwise.
+ * @tparam Type The type to test.
+ */
+template<typename Type, typename = void>
+struct is_complete: std::false_type {};
+
+
+/*! @copydoc is_complete */
+template<typename Type>
+struct is_complete<Type, std::void_t<decltype(sizeof(Type))>>: std::true_type {};
+
+
+/**
+ * @brief Helper variable template.
+ * @tparam Type The type to test.
+ */
+template<typename Type>
+inline constexpr bool is_complete_v = is_complete<Type>::value;
+
+
+/**
+ * @brief Provides the member constant `value` to true if a given type is an
+ * iterator, false otherwise.
+ * @tparam Type The type to test.
+ */
+template<typename Type, typename = void>
+struct is_iterator: std::false_type {};
+
+
+/*! @copydoc is_iterator */
+template<typename Type>
+struct is_iterator<Type, std::void_t<typename std::iterator_traits<Type>::iterator_category>>
+    : std::true_type
+{};
+
+
+/**
+ * @brief Helper variable template.
+ * @tparam Type The type to test.
+ */
+template<typename Type>
+inline constexpr bool is_iterator_v = is_iterator<Type>::value;
+
+
+/**
+ * @brief Provides the member constant `value` to true if a given type is of the
+ * required iterator type, false otherwise.
+ * @tparam Type The type to test.
+ * @tparam It Required iterator type.
+ */
+template<typename Type, typename It, typename = void>
+struct is_iterator_type: std::false_type {};
+
+
+/*! @copydoc is_iterator_type */
+template<typename Type, typename It>
+struct is_iterator_type<Type, It, std::enable_if_t<is_iterator_v<Type> && std::is_same_v<Type, It>>>
+    : std::true_type
+{};
+
+
+/*! @copydoc is_iterator_type */
+template<typename Type, typename It>
+struct is_iterator_type<Type, It, std::enable_if_t<!std::is_same_v<Type, It>, std::void_t<typename It::iterator_type>>>
+    : is_iterator_type<Type, typename It::iterator_type>
+{};
+
+
+/**
+ * @brief Helper variable template.
+ * @tparam Type The type to test.
+ * @tparam It Required iterator type.
+ */
+template<typename Type, typename It>
+inline constexpr bool is_iterator_type_v = is_iterator_type<Type, It>::value;
+
+
+/**
+ * @cond TURN_OFF_DOXYGEN
+ * Internal details not to be documented.
+ */
+
+
+namespace internal {
+
+
+template<typename>
+[[nodiscard]] constexpr bool is_equality_comparable(...) { return false; }
+
+
+template<typename Type>
+[[nodiscard]] constexpr auto is_equality_comparable(choice_t<0>)
+-> decltype(std::declval<Type>() == std::declval<Type>()) { return true; }
+
+
+template<typename Type>
+[[nodiscard]] constexpr auto is_equality_comparable(choice_t<1>)
+-> decltype(std::declval<typename Type::value_type>(), std::declval<Type>() == std::declval<Type>()) {
+    if constexpr(is_iterator_v<Type>) {
+        return true;
+    } else if constexpr(std::is_same_v<typename Type::value_type, Type>) {
+        return is_equality_comparable<Type>(choice<0>);
+    } else {
+        return is_equality_comparable<typename Type::value_type>(choice<2>);
+    }
+}
+
+
+template<typename Type>
+[[nodiscard]] constexpr auto is_equality_comparable(choice_t<2>)
+-> decltype(std::declval<typename Type::mapped_type>(), std::declval<Type>() == std::declval<Type>()) {
+    return is_equality_comparable<typename Type::key_type>(choice<2>) && is_equality_comparable<typename Type::mapped_type>(choice<2>);
+}
+
+
+}
+
+
+/**
+ * Internal details not to be documented.
+ * @endcond
+ */
+
+
+/**
+ * @brief Provides the member constant `value` to true if a given type is
+ * equality comparable, false otherwise.
+ * @tparam Type The type to test.
+ */
+template<typename Type, typename = void>
+struct is_equality_comparable: std::bool_constant<internal::is_equality_comparable<Type>(choice<2>)> {};
+
+
+/**
+ * @brief Helper variable template.
+ * @tparam Type The type to test.
+ */
+template<class Type>
+inline constexpr bool is_equality_comparable_v = is_equality_comparable<Type>::value;
+
+
+/**
+ * @brief Transcribes the constness of a type to another type.
+ * @tparam To The type to which to transcribe the constness.
+ * @tparam From The type from which to transcribe the constness.
+ */
+template<typename To, typename From>
+struct constness_as {
+    /*! @brief The type resulting from the transcription of the constness. */
+    using type = std::remove_const_t<To>;
+};
+
+
+/*! @copydoc constness_as */
+template<typename To, typename From>
+struct constness_as<To, const From> {
+    /*! @brief The type resulting from the transcription of the constness. */
+    using type = std::add_const_t<To>;
+};
+
+
+/**
+ * @brief Alias template to facilitate the transcription of the constness.
+ * @tparam To The type to which to transcribe the constness.
+ * @tparam From The type from which to transcribe the constness.
+ */
+template<typename To, typename From>
+using constness_as_t = typename constness_as<To, From>::type;
+
+
+/**
+ * @brief Extracts the class of a non-static member object or function.
+ * @tparam Member A pointer to a non-static member object or function.
+ */
+template<typename Member>
+class member_class {
+    static_assert(std::is_member_pointer_v<Member>, "Invalid pointer type to non-static member object or function");
+
+    template<typename Class, typename Ret, typename... Args>
+    static Class * clazz(Ret(Class:: *)(Args...));
+
+    template<typename Class, typename Ret, typename... Args>
+    static Class * clazz(Ret(Class:: *)(Args...) const);
+
+    template<typename Class, typename Type>
+    static Class * clazz(Type Class:: *);
+
+public:
+    /*! @brief The class of the given non-static member object or function. */
+    using type = std::remove_pointer_t<decltype(clazz(std::declval<Member>()))>;
+};
+
+
+/**
+ * @brief Helper type.
+ * @tparam Member A pointer to a non-static member object or function.
+ */
+template<typename Member>
+using member_class_t = typename member_class<Member>::type;
+
+
+}
+
+
+#endif
+
+// #include "fwd.hpp"
+#ifndef ENTT_POLY_FWD_HPP
+#define ENTT_POLY_FWD_HPP
+
+
+#include <type_traits>
+
+
+namespace entt {
+
+
+template<typename, std::size_t Len, std::size_t = alignof(typename std::aligned_storage_t<Len + !Len>)>
+class basic_poly;
+
+
+/**
+ * @brief Alias declaration for the most common use case.
+ * @tparam Concept Concept descriptor.
+ */
+template<typename Concept>
+using poly = basic_poly<Concept, sizeof(double[2])>;
+
+
+}
+
+
+#endif
+
+
+
+namespace entt {
+
+
+/*! @brief Inspector class used to infer the type of the virtual table. */
+struct poly_inspector {
+    /**
+     * @brief Generic conversion operator (definition only).
+     * @tparam Type Type to which conversion is requested.
+     */
+    template <class Type>
+    operator Type &&() const;
+
+    /**
+     * @brief Dummy invocation function (definition only).
+     * @tparam Member Index of the function to invoke.
+     * @tparam Args Types of arguments to pass to the function.
+     * @param args The arguments to pass to the function.
+     * @return A poly inspector convertible to any type.
+     */
+    template<auto Member, typename... Args>
+    poly_inspector invoke(Args &&... args) const;
+
+    /*! @copydoc invoke */
+    template<auto Member, typename... Args>
+    poly_inspector invoke(Args &&... args);
+};
+
+
+/**
+ * @brief Static virtual table factory.
+ * @tparam Concept Concept descriptor.
+ * @tparam Len Size of the storage reserved for the small buffer optimization.
+ * @tparam Align Alignment requirement.
+ */
+template<typename Concept, std::size_t Len, std::size_t Align>
+class poly_vtable {
+    using inspector = typename Concept::template type<poly_inspector>;
+
+    template<typename Ret, typename... Args>
+    static auto vtable_entry(Ret(*)(inspector &, Args...)) -> Ret(*)(basic_any<Len, Align> &, Args...);
+
+    template<typename Ret, typename... Args>
+    static auto vtable_entry(Ret(*)(const inspector &, Args...)) -> Ret(*)(const basic_any<Len, Align> &, Args...);
+
+    template<typename Ret, typename... Args>
+    static auto vtable_entry(Ret(*)(Args...)) -> Ret(*)(const basic_any<Len, Align> &, Args...);
+
+    template<typename Ret, typename... Args>
+    static auto vtable_entry(Ret(inspector:: *)(Args...)) -> Ret(*)(basic_any<Len, Align> &, Args...);
+
+    template<typename Ret, typename... Args>
+    static auto vtable_entry(Ret(inspector:: *)(Args...) const) -> Ret(*)(const basic_any<Len, Align> &, Args...);
+
+    template<auto... Candidate>
+    static auto make_vtable(value_list<Candidate...>)
+    -> decltype(std::make_tuple(vtable_entry(Candidate)...));
+
+    template<typename... Func>
+    [[nodiscard]] static constexpr auto make_vtable(type_list<Func...>)  {
+        if constexpr(sizeof...(Func) == 0) {
+            return decltype(make_vtable(typename Concept::template impl<inspector>{})){};
+        } else if constexpr((std::is_function_v<Func> && ...)) {
+            return decltype(std::make_tuple(vtable_entry(std::declval<Func inspector:: *>())...)){};
+        }
+    }
+
+    template<typename Type, auto Candidate, typename Ret, typename Any, typename... Args>
+    static void fill_vtable_entry(Ret(* &entry)(Any &, Args...)) {
+        if constexpr(std::is_invocable_r_v<Ret, decltype(Candidate), Args...>) {
+            entry = +[](Any &, Args... args) -> Ret {
+                return std::invoke(Candidate, std::forward<Args>(args)...);
+            };
+        } else {
+            entry = +[](Any &instance, Args... args) -> Ret {
+                return static_cast<Ret>(std::invoke(Candidate, any_cast<constness_as_t<Type, Any> &>(instance), std::forward<Args>(args)...));
+            };
+        }
+    }
+
+    template<typename Type, auto... Index>
+    [[nodiscard]] static auto fill_vtable(std::index_sequence<Index...>) {
+        type impl{};
+        (fill_vtable_entry<Type, value_list_element_v<Index, typename Concept::template impl<Type>>>(std::get<Index>(impl)), ...);
+        return impl;
+    }
+
+public:
+    /*! @brief Virtual table type. */
+    using type = decltype(make_vtable(Concept{}));
+
+    /**
+     * @brief Returns a static virtual table for a specific concept and type.
+     * @tparam Type The type for which to generate the virtual table.
+     * @return A static virtual table for the given concept and type.
+     */
+    template<typename Type>
+    [[nodiscard]] static const auto * instance() {
+        static_assert(std::is_same_v<Type, std::decay_t<Type>>, "Type differs from its decayed form");
+        static const auto vtable = fill_vtable<Type>(std::make_index_sequence<Concept::template impl<Type>::size>{});
+        return &vtable;
+    }
+};
+
+
+/**
+ * @brief Poly base class used to inject functionalities into concepts.
+ * @tparam Poly The outermost poly class.
+ */
+template<typename Poly>
+struct poly_base {
+    /**
+     * @brief Invokes a function from the static virtual table.
+     * @tparam Member Index of the function to invoke.
+     * @tparam Args Types of arguments to pass to the function.
+     * @param self A reference to the poly object that made the call.
+     * @param args The arguments to pass to the function.
+     * @return The return value of the invoked function, if any.
+     */
+    template<auto Member, typename... Args>
+    [[nodiscard]] decltype(auto) invoke(const poly_base &self, Args &&... args) const {
+        const auto &poly = static_cast<const Poly &>(self);
+        return std::get<Member>(*poly.vtable)(poly.storage, std::forward<Args>(args)...);
+    }
+
+    /*! @copydoc invoke */
+    template<auto Member, typename... Args>
+    [[nodiscard]] decltype(auto) invoke(poly_base &self, Args &&... args) {
+        auto &poly = static_cast<Poly &>(self);
+        return std::get<Member>(*poly.vtable)(poly.storage, std::forward<Args>(args)...);
+    }
+};
+
+
+/**
+ * @brief Shortcut for calling `poly_base<Type>::invoke`.
+ * @tparam Member Index of the function to invoke.
+ * @tparam Poly A fully defined poly object.
+ * @tparam Args Types of arguments to pass to the function.
+ * @param self A reference to the poly object that made the call.
+ * @param args The arguments to pass to the function.
+ * @return The return value of the invoked function, if any.
+ */
+template<auto Member, typename Poly, typename... Args>
+decltype(auto) poly_call(Poly &&self, Args &&... args) {
+    return std::forward<Poly>(self).template invoke<Member>(self, std::forward<Args>(args)...);
+}
+
+
+/**
+ * @brief Static polymorphism made simple and within everyone's reach.
+ *
+ * Static polymorphism is a very powerful tool in C++, albeit sometimes
+ * cumbersome to obtain.<br/>
+ * This class aims to make it simple and easy to use.
+ *
+ * @note
+ * Both deduced and defined static virtual tables are supported.<br/>
+ * Moreover, the `poly` class template also works with unmanaged objects.
+ *
+ * @tparam Concept Concept descriptor.
+ * @tparam Len Size of the storage reserved for the small buffer optimization.
+ * @tparam Align Optional alignment requirement.
+ */
+template<typename Concept, std::size_t Len, std::size_t Align>
+class basic_poly: private Concept::template type<poly_base<basic_poly<Concept, Len, Align>>> {
+    /*! @brief A poly base is allowed to snoop into a poly object. */
+    friend struct poly_base<basic_poly>;
+
+    using vtable_type = typename poly_vtable<Concept, Len, Align>::type;
+
+public:
+    /*! @brief Concept type. */
+    using concept_type = typename Concept::template type<poly_base<basic_poly>>;
+
+    /*! @brief Default constructor. */
+    basic_poly() ENTT_NOEXCEPT
+        : storage{},
+          vtable{}
+    {}
+
+    /**
+     * @brief Constructs a poly by directly initializing the new object.
+     * @tparam Type Type of object to use to initialize the poly.
+     * @tparam Args Types of arguments to use to construct the new instance.
+     * @param args Parameters to use to construct the instance.
+     */
+    template<typename Type, typename... Args>
+    explicit basic_poly(std::in_place_type_t<Type>, Args &&... args)
+        : storage{std::in_place_type<Type>, std::forward<Args>(args)...},
+          vtable{poly_vtable<Concept, Len, Align>::template instance<std::remove_const_t<std::remove_reference_t<Type>>>()}
+    {}
+
+    /**
+     * @brief Constructs a poly from a given value.
+     * @tparam Type Type of object to use to initialize the poly.
+     * @param value An instance of an object to use to initialize the poly.
+     */
+    template<typename Type, typename = std::enable_if_t<!std::is_same_v<std::remove_cv_t<std::remove_reference_t<Type>>, basic_poly>>>
+    basic_poly(Type &&value) ENTT_NOEXCEPT
+        : basic_poly{std::in_place_type<std::remove_cv_t<std::remove_reference_t<Type>>>, std::forward<Type>(value)}
+    {}
+
+    /**
+     * @brief Copy constructor.
+     * @param other The instance to copy from.
+     */
+    basic_poly(const basic_poly &other) = default;
+
+    /**
+     * @brief Move constructor.
+     * @param other The instance to move from.
+     */
+    basic_poly(basic_poly &&other) ENTT_NOEXCEPT
+        : basic_poly{}
+    {
+        swap(*this, other);
+    }
+
+    /**
+     * @brief Assignment operator.
+     * @param other The instance to assign from.
+     * @return This poly object.
+     */
+    basic_poly & operator=(basic_poly other) {
+        swap(other, *this);
+        return *this;
+    }
+
+    /**
+     * @brief Returns the type of the contained object.
+     * @return The type of the contained object, if any.
+     */
+    [[nodiscard]] type_info type() const ENTT_NOEXCEPT {
+        return storage.type();
+    }
+
+    /**
+     * @brief Returns an opaque pointer to the contained instance.
+     * @return An opaque pointer the contained instance, if any.
+     */
+    [[nodiscard]] const void * data() const ENTT_NOEXCEPT {
+        return storage.data();
+    }
+
+    /*! @copydoc data */
+    [[nodiscard]] void * data() ENTT_NOEXCEPT {
+        return storage.data();
+    }
+
+    /**
+     * @brief Replaces the contained object by creating a new instance directly.
+     * @tparam Type Type of object to use to initialize the poly.
+     * @tparam Args Types of arguments to use to construct the new instance.
+     * @param args Parameters to use to construct the instance.
+     */
+    template<typename Type, typename... Args>
+    void emplace(Args &&... args) {
+        *this = basic_poly{std::in_place_type<Type>, std::forward<Args>(args)...};
+    }
+
+    /*! @brief Destroys contained object */
+    void reset() {
+        *this = basic_poly{};
+    }
+
+    /**
+     * @brief Returns false if a poly is empty, true otherwise.
+     * @return False if the poly is empty, true otherwise.
+     */
+    [[nodiscard]] explicit operator bool() const ENTT_NOEXCEPT {
+        return !(vtable == nullptr);
+    }
+
+    /**
+     * @brief Returns a pointer to the underlying concept.
+     * @return A pointer to the underlying concept.
+     */
+    [[nodiscard]] concept_type * operator->() ENTT_NOEXCEPT {
+        return this;
+    }
+
+    /*! @copydoc operator-> */
+    [[nodiscard]] const concept_type * operator->() const ENTT_NOEXCEPT {
+        return this;
+    }
+
+    /**
+     * @brief Swaps two poly objects.
+     * @param lhs A valid poly object.
+     * @param rhs A valid poly object.
+     */
+    friend void swap(basic_poly &lhs, basic_poly &rhs) {
+        using std::swap;
+        swap(lhs.storage, rhs.storage);
+        swap(lhs.vtable, rhs.vtable);
+    }
+
+    /**
+     * @brief Aliasing constructor.
+     * @return A poly that shares a reference to an unmanaged object.
+     */
+    [[nodiscard]] basic_poly as_ref() ENTT_NOEXCEPT {
+        basic_poly ref = std::as_const(*this).as_ref();
+        ref.storage = storage.as_ref();
+        return ref;
+    }
+
+    /*! @copydoc as_ref */
+    [[nodiscard]] basic_poly as_ref() const ENTT_NOEXCEPT {
+        basic_poly ref{};
+        ref.storage = storage.as_ref();
+        ref.vtable = vtable;
+        return ref;
+    }
+
+private:
+    basic_any<Len, Align> storage;
+    const vtable_type *vtable;
+};
+
+
+}
+
+
+#endif
+
+// #include "process/process.hpp"
+#ifndef ENTT_PROCESS_PROCESS_HPP
+#define ENTT_PROCESS_PROCESS_HPP
+
+
+#include <utility>
+#include <type_traits>
+// #include "../config/config.h"
+#ifndef ENTT_CONFIG_CONFIG_H
+#define ENTT_CONFIG_CONFIG_H
+
+
+#if defined(__cpp_exceptions) && !defined(ENTT_NOEXCEPTION)
+#   define ENTT_NOEXCEPT noexcept
+#   define ENTT_THROW throw
+#   define ENTT_TRY try
+#   define ENTT_CATCH catch(...)
+#else
+#   define ENTT_NOEXCEPT
+#   define ENTT_THROW
+#   define ENTT_TRY if(true)
+#   define ENTT_CATCH if(false)
+#endif
+
+
+#if defined(__cpp_lib_launder) && __cpp_lib_launder >= 201606L
+#   include <new>
+#   define ENTT_LAUNDER(expr) std::launder(expr)
+#else
+#   define ENTT_LAUNDER(expr) expr
+#endif
+
+
+#ifndef ENTT_USE_ATOMIC
+#   define ENTT_MAYBE_ATOMIC(Type) Type
+#else
+#   include <atomic>
+#   define ENTT_MAYBE_ATOMIC(Type) std::atomic<Type>
+#endif
+
+
+#ifndef ENTT_ID_TYPE
+#   include <cstdint>
+#   define ENTT_ID_TYPE std::uint32_t
+#endif
+
+
+#ifdef ENTT_SPARSE_PAGE
+	static_assert(ENTT_SPARSE_PAGE && ((ENTT_SPARSE_PAGE & (ENTT_SPARSE_PAGE - 1)) == 0), "ENTT_SPARSE_PAGE must be a power of two");
+#else
+#   define ENTT_SPARSE_PAGE 4096
+#endif
+
+
+#ifdef ENTT_PACKED_PAGE
+static_assert(ENTT_PACKED_PAGE && ((ENTT_PACKED_PAGE & (ENTT_PACKED_PAGE - 1)) == 0), "ENTT_PACKED_PAGE must be a power of two");
+#else
+#   define ENTT_PACKED_PAGE 1024
+#endif
+
+
+#ifdef ENTT_DISABLE_ASSERT
+#   undef ENTT_ASSERT
+#   define ENTT_ASSERT(...) (void(0))
+#elif !defined ENTT_ASSERT
+#   include <cassert>
+#   define ENTT_ASSERT(condition, ...) assert(condition)
+#endif
+
+
+#ifdef ENTT_NO_ETO
+#   include <type_traits>
+#   define ENTT_IGNORE_IF_EMPTY std::false_type
+#else
+#   include <type_traits>
+#   define ENTT_IGNORE_IF_EMPTY std::true_type
+#endif
+
+
+#ifndef ENTT_STANDARD_CPP
+#    if defined __clang__ || defined __GNUC__
+#       define ENTT_PRETTY_FUNCTION __PRETTY_FUNCTION__
+#       define ENTT_PRETTY_FUNCTION_PREFIX '='
+#       define ENTT_PRETTY_FUNCTION_SUFFIX ']'
+#    elif defined _MSC_VER
+#       define ENTT_PRETTY_FUNCTION __FUNCSIG__
+#       define ENTT_PRETTY_FUNCTION_PREFIX '<'
+#       define ENTT_PRETTY_FUNCTION_SUFFIX '>'
+#   endif
+#endif
+
+
+#endif
+
+
+
+namespace entt {
+
+
+/**
+ * @brief Base class for processes.
+ *
+ * This class stays true to the CRTP idiom. Derived classes must specify what's
+ * the intended type for elapsed times.<br/>
+ * A process should expose publicly the following member functions whether
+ * required:
+ *
+ * * @code{.cpp}
+ *   void update(Delta, void *);
+ *   @endcode
+ *
+ *   It's invoked once per tick until a process is explicitly aborted or it
+ *   terminates either with or without errors. Even though it's not mandatory to
+ *   declare this member function, as a rule of thumb each process should at
+ *   least define it to work properly. The `void *` parameter is an opaque
+ *   pointer to user data (if any) forwarded directly to the process during an
+ *   update.
+ *
+ * * @code{.cpp}
+ *   void init();
+ *   @endcode
+ *
+ *   It's invoked when the process joins the running queue of a scheduler. This
+ *   happens as soon as it's attached to the scheduler if the process is a top
+ *   level one, otherwise when it replaces its parent if the process is a
+ *   continuation.
+ *
+ * * @code{.cpp}
+ *   void succeeded();
+ *   @endcode
+ *
+ *   It's invoked in case of success, immediately after an update and during the
+ *   same tick.
+ *
+ * * @code{.cpp}
+ *   void failed();
+ *   @endcode
+ *
+ *   It's invoked in case of errors, immediately after an update and during the
+ *   same tick.
+ *
+ * * @code{.cpp}
+ *   void aborted();
+ *   @endcode
+ *
+ *   It's invoked only if a process is explicitly aborted. There is no guarantee
+ *   that it executes in the same tick, this depends solely on whether the
+ *   process is aborted immediately or not.
+ *
+ * Derived classes can change the internal state of a process by invoking the
+ * `succeed` and `fail` protected member functions and even pause or unpause the
+ * process itself.
+ *
+ * @sa scheduler
+ *
+ * @tparam Derived Actual type of process that extends the class template.
+ * @tparam Delta Type to use to provide elapsed time.
+ */
+template<typename Derived, typename Delta>
+class process {
+    enum class state: unsigned int {
+        UNINITIALIZED = 0,
+        RUNNING,
+        PAUSED,
+        SUCCEEDED,
+        FAILED,
+        ABORTED,
+        FINISHED,
+        REJECTED
+    };
+
+    template<typename Target = Derived>
+    auto next(std::integral_constant<state, state::UNINITIALIZED>)
+    -> decltype(std::declval<Target>().init(), void()) {
+        static_cast<Target *>(this)->init();
+    }
+
+    template<typename Target = Derived>
+    auto next(std::integral_constant<state, state::RUNNING>, Delta delta, void *data)
+    -> decltype(std::declval<Target>().update(delta, data), void()) {
+        static_cast<Target *>(this)->update(delta, data);
+    }
+
+    template<typename Target = Derived>
+    auto next(std::integral_constant<state, state::SUCCEEDED>)
+    -> decltype(std::declval<Target>().succeeded(), void()) {
+        static_cast<Target *>(this)->succeeded();
+    }
+
+    template<typename Target = Derived>
+    auto next(std::integral_constant<state, state::FAILED>)
+    -> decltype(std::declval<Target>().failed(), void()) {
+        static_cast<Target *>(this)->failed();
+    }
+
+    template<typename Target = Derived>
+    auto next(std::integral_constant<state, state::ABORTED>)
+    -> decltype(std::declval<Target>().aborted(), void()) {
+        static_cast<Target *>(this)->aborted();
+    }
+
+    void next(...) const ENTT_NOEXCEPT {}
+
+protected:
+    /**
+     * @brief Terminates a process with success if it's still alive.
+     *
+     * The function is idempotent and it does nothing if the process isn't
+     * alive.
+     */
+    void succeed() ENTT_NOEXCEPT {
+        if(alive()) {
+            current = state::SUCCEEDED;
+        }
+    }
+
+    /**
+     * @brief Terminates a process with errors if it's still alive.
+     *
+     * The function is idempotent and it does nothing if the process isn't
+     * alive.
+     */
+    void fail() ENTT_NOEXCEPT {
+        if(alive()) {
+            current = state::FAILED;
+        }
+    }
+
+    /**
+     * @brief Stops a process if it's in a running state.
+     *
+     * The function is idempotent and it does nothing if the process isn't
+     * running.
+     */
+    void pause() ENTT_NOEXCEPT {
+        if(current == state::RUNNING) {
+            current = state::PAUSED;
+        }
+    }
+
+    /**
+     * @brief Restarts a process if it's paused.
+     *
+     * The function is idempotent and it does nothing if the process isn't
+     * paused.
+     */
+    void unpause() ENTT_NOEXCEPT {
+        if(current  == state::PAUSED) {
+            current  = state::RUNNING;
+        }
+    }
+
+public:
+    /*! @brief Type used to provide elapsed time. */
+    using delta_type = Delta;
+
+    /*! @brief Default destructor. */
+    virtual ~process() {
+        static_assert(std::is_base_of_v<process, Derived>, "Incorrect use of the class template");
+    }
+
+    /**
+     * @brief Aborts a process if it's still alive.
+     *
+     * The function is idempotent and it does nothing if the process isn't
+     * alive.
+     *
+     * @param immediately Requests an immediate operation.
+     */
+    void abort(const bool immediately = false) {
+        if(alive()) {
+            current = state::ABORTED;
+
+            if(immediately) {
+                tick({});
+            }
+        }
+    }
+
+    /**
+     * @brief Returns true if a process is either running or paused.
+     * @return True if the process is still alive, false otherwise.
+     */
+    [[nodiscard]] bool alive() const ENTT_NOEXCEPT {
+        return current == state::RUNNING || current == state::PAUSED;
+    }
+
+    /**
+     * @brief Returns true if a process is already terminated.
+     * @return True if the process is terminated, false otherwise.
+     */
+    [[nodiscard]] bool finished() const ENTT_NOEXCEPT {
+        return current == state::FINISHED;
+    }
+
+    /**
+     * @brief Returns true if a process is currently paused.
+     * @return True if the process is paused, false otherwise.
+     */
+    [[nodiscard]] bool paused() const ENTT_NOEXCEPT {
+        return current == state::PAUSED;
+    }
+
+    /**
+     * @brief Returns true if a process terminated with errors.
+     * @return True if the process terminated with errors, false otherwise.
+     */
+    [[nodiscard]] bool rejected() const ENTT_NOEXCEPT {
+        return current == state::REJECTED;
+    }
+
+    /**
+     * @brief Updates a process and its internal state if required.
+     * @param delta Elapsed time.
+     * @param data Optional data.
+     */
+    void tick(const Delta delta, void *data = nullptr) {
+        switch (current) {
+        case state::UNINITIALIZED:
+            next(std::integral_constant<state, state::UNINITIALIZED>{});
+            current = state::RUNNING;
+            break;
+        case state::RUNNING:
+            next(std::integral_constant<state, state::RUNNING>{}, delta, data);
+            break;
+        default:
+            // suppress warnings
+            break;
+        }
+
+        // if it's dead, it must be notified and removed immediately
+        switch(current) {
+        case state::SUCCEEDED:
+            next(std::integral_constant<state, state::SUCCEEDED>{});
+            current = state::FINISHED;
+            break;
+        case state::FAILED:
+            next(std::integral_constant<state, state::FAILED>{});
+            current = state::REJECTED;
+            break;
+        case state::ABORTED:
+            next(std::integral_constant<state, state::ABORTED>{});
+            current = state::REJECTED;
+            break;
+        default:
+            // suppress warnings
+            break;
+        }
+    }
+
+private:
+    state current{state::UNINITIALIZED};
+};
+
+
+/**
+ * @brief Adaptor for lambdas and functors to turn them into processes.
+ *
+ * Lambdas and functors can't be used directly with a scheduler for they are not
+ * properly defined processes with managed life cycles.<br/>
+ * This class helps in filling the gap and turning lambdas and functors into
+ * full featured processes usable by a scheduler.
+ *
+ * The signature of the function call operator should be equivalent to the
+ * following:
  *
  * @code{.cpp}
- * std::integral_constant<ENTT_ID_TYPE, entt::basic_hashed_string{#type}>
+ * void(Delta delta, void *data, auto succeed, auto fail);
+ * @endcode
+ *
+ * Where:
+ *
+ * * `delta` is the elapsed time.
+ * * `data` is an opaque pointer to user data if any, `nullptr` otherwise.
+ * * `succeed` is a function to call when a process terminates with success.
+ * * `fail` is a function to call when a process terminates with errors.
+ *
+ * The signature of the function call operator of both `succeed` and `fail`
+ * is equivalent to the following:
+ *
+ * @code{.cpp}
+ * void();
+ * @endcode
+ *
+ * Usually users shouldn't worry about creating adaptors. A scheduler will
+ * create them internally each and avery time a lambda or a functor is used as
+ * a process.
+ *
+ * @sa process
+ * @sa scheduler
+ *
+ * @tparam Func Actual type of process.
+ * @tparam Delta Type to use to provide elapsed time.
+ */
+template<typename Func, typename Delta>
+struct process_adaptor: process<process_adaptor<Func, Delta>, Delta>, private Func {
+    /**
+     * @brief Constructs a process adaptor from a lambda or a functor.
+     * @tparam Args Types of arguments to use to initialize the actual process.
+     * @param args Parameters to use to initialize the actual process.
+     */
+    template<typename... Args>
+    process_adaptor(Args &&... args)
+        : Func{std::forward<Args>(args)...}
+    {}
+
+    /**
+     * @brief Updates a process and its internal state if required.
+     * @param delta Elapsed time.
+     * @param data Optional data.
+     */
+    void update(const Delta delta, void *data) {
+        Func::operator()(delta, data, [this]() { this->succeed(); }, [this]() { this->fail(); });
+    }
+};
+
+
+}
+
+
+#endif
+
+// #include "process/scheduler.hpp"
+#ifndef ENTT_PROCESS_SCHEDULER_HPP
+#define ENTT_PROCESS_SCHEDULER_HPP
+
+
+#include <vector>
+#include <memory>
+#include <utility>
+#include <algorithm>
+#include <type_traits>
+// #include "../config/config.h"
+
+// #include "process.hpp"
+#ifndef ENTT_PROCESS_PROCESS_HPP
+#define ENTT_PROCESS_PROCESS_HPP
+
+
+#include <utility>
+#include <type_traits>
+// #include "../config/config.h"
+
+
+
+namespace entt {
+
+
+/**
+ * @brief Base class for processes.
+ *
+ * This class stays true to the CRTP idiom. Derived classes must specify what's
+ * the intended type for elapsed times.<br/>
+ * A process should expose publicly the following member functions whether
+ * required:
+ *
+ * * @code{.cpp}
+ *   void update(Delta, void *);
+ *   @endcode
+ *
+ *   It's invoked once per tick until a process is explicitly aborted or it
+ *   terminates either with or without errors. Even though it's not mandatory to
+ *   declare this member function, as a rule of thumb each process should at
+ *   least define it to work properly. The `void *` parameter is an opaque
+ *   pointer to user data (if any) forwarded directly to the process during an
+ *   update.
+ *
+ * * @code{.cpp}
+ *   void init();
+ *   @endcode
+ *
+ *   It's invoked when the process joins the running queue of a scheduler. This
+ *   happens as soon as it's attached to the scheduler if the process is a top
+ *   level one, otherwise when it replaces its parent if the process is a
+ *   continuation.
+ *
+ * * @code{.cpp}
+ *   void succeeded();
+ *   @endcode
+ *
+ *   It's invoked in case of success, immediately after an update and during the
+ *   same tick.
+ *
+ * * @code{.cpp}
+ *   void failed();
+ *   @endcode
+ *
+ *   It's invoked in case of errors, immediately after an update and during the
+ *   same tick.
+ *
+ * * @code{.cpp}
+ *   void aborted();
+ *   @endcode
+ *
+ *   It's invoked only if a process is explicitly aborted. There is no guarantee
+ *   that it executes in the same tick, this depends solely on whether the
+ *   process is aborted immediately or not.
+ *
+ * Derived classes can change the internal state of a process by invoking the
+ * `succeed` and `fail` protected member functions and even pause or unpause the
+ * process itself.
+ *
+ * @sa scheduler
+ *
+ * @tparam Derived Actual type of process that extends the class template.
+ * @tparam Delta Type to use to provide elapsed time.
+ */
+template<typename Derived, typename Delta>
+class process {
+    enum class state: unsigned int {
+        UNINITIALIZED = 0,
+        RUNNING,
+        PAUSED,
+        SUCCEEDED,
+        FAILED,
+        ABORTED,
+        FINISHED,
+        REJECTED
+    };
+
+    template<typename Target = Derived>
+    auto next(std::integral_constant<state, state::UNINITIALIZED>)
+    -> decltype(std::declval<Target>().init(), void()) {
+        static_cast<Target *>(this)->init();
+    }
+
+    template<typename Target = Derived>
+    auto next(std::integral_constant<state, state::RUNNING>, Delta delta, void *data)
+    -> decltype(std::declval<Target>().update(delta, data), void()) {
+        static_cast<Target *>(this)->update(delta, data);
+    }
+
+    template<typename Target = Derived>
+    auto next(std::integral_constant<state, state::SUCCEEDED>)
+    -> decltype(std::declval<Target>().succeeded(), void()) {
+        static_cast<Target *>(this)->succeeded();
+    }
+
+    template<typename Target = Derived>
+    auto next(std::integral_constant<state, state::FAILED>)
+    -> decltype(std::declval<Target>().failed(), void()) {
+        static_cast<Target *>(this)->failed();
+    }
+
+    template<typename Target = Derived>
+    auto next(std::integral_constant<state, state::ABORTED>)
+    -> decltype(std::declval<Target>().aborted(), void()) {
+        static_cast<Target *>(this)->aborted();
+    }
+
+    void next(...) const ENTT_NOEXCEPT {}
+
+protected:
+    /**
+     * @brief Terminates a process with success if it's still alive.
+     *
+     * The function is idempotent and it does nothing if the process isn't
+     * alive.
+     */
+    void succeed() ENTT_NOEXCEPT {
+        if(alive()) {
+            current = state::SUCCEEDED;
+        }
+    }
+
+    /**
+     * @brief Terminates a process with errors if it's still alive.
+     *
+     * The function is idempotent and it does nothing if the process isn't
+     * alive.
+     */
+    void fail() ENTT_NOEXCEPT {
+        if(alive()) {
+            current = state::FAILED;
+        }
+    }
+
+    /**
+     * @brief Stops a process if it's in a running state.
+     *
+     * The function is idempotent and it does nothing if the process isn't
+     * running.
+     */
+    void pause() ENTT_NOEXCEPT {
+        if(current == state::RUNNING) {
+            current = state::PAUSED;
+        }
+    }
+
+    /**
+     * @brief Restarts a process if it's paused.
+     *
+     * The function is idempotent and it does nothing if the process isn't
+     * paused.
+     */
+    void unpause() ENTT_NOEXCEPT {
+        if(current  == state::PAUSED) {
+            current  = state::RUNNING;
+        }
+    }
+
+public:
+    /*! @brief Type used to provide elapsed time. */
+    using delta_type = Delta;
+
+    /*! @brief Default destructor. */
+    virtual ~process() {
+        static_assert(std::is_base_of_v<process, Derived>, "Incorrect use of the class template");
+    }
+
+    /**
+     * @brief Aborts a process if it's still alive.
+     *
+     * The function is idempotent and it does nothing if the process isn't
+     * alive.
+     *
+     * @param immediately Requests an immediate operation.
+     */
+    void abort(const bool immediately = false) {
+        if(alive()) {
+            current = state::ABORTED;
+
+            if(immediately) {
+                tick({});
+            }
+        }
+    }
+
+    /**
+     * @brief Returns true if a process is either running or paused.
+     * @return True if the process is still alive, false otherwise.
+     */
+    [[nodiscard]] bool alive() const ENTT_NOEXCEPT {
+        return current == state::RUNNING || current == state::PAUSED;
+    }
+
+    /**
+     * @brief Returns true if a process is already terminated.
+     * @return True if the process is terminated, false otherwise.
+     */
+    [[nodiscard]] bool finished() const ENTT_NOEXCEPT {
+        return current == state::FINISHED;
+    }
+
+    /**
+     * @brief Returns true if a process is currently paused.
+     * @return True if the process is paused, false otherwise.
+     */
+    [[nodiscard]] bool paused() const ENTT_NOEXCEPT {
+        return current == state::PAUSED;
+    }
+
+    /**
+     * @brief Returns true if a process terminated with errors.
+     * @return True if the process terminated with errors, false otherwise.
+     */
+    [[nodiscard]] bool rejected() const ENTT_NOEXCEPT {
+        return current == state::REJECTED;
+    }
+
+    /**
+     * @brief Updates a process and its internal state if required.
+     * @param delta Elapsed time.
+     * @param data Optional data.
+     */
+    void tick(const Delta delta, void *data = nullptr) {
+        switch (current) {
+        case state::UNINITIALIZED:
+            next(std::integral_constant<state, state::UNINITIALIZED>{});
+            current = state::RUNNING;
+            break;
+        case state::RUNNING:
+            next(std::integral_constant<state, state::RUNNING>{}, delta, data);
+            break;
+        default:
+            // suppress warnings
+            break;
+        }
+
+        // if it's dead, it must be notified and removed immediately
+        switch(current) {
+        case state::SUCCEEDED:
+            next(std::integral_constant<state, state::SUCCEEDED>{});
+            current = state::FINISHED;
+            break;
+        case state::FAILED:
+            next(std::integral_constant<state, state::FAILED>{});
+            current = state::REJECTED;
+            break;
+        case state::ABORTED:
+            next(std::integral_constant<state, state::ABORTED>{});
+            current = state::REJECTED;
+            break;
+        default:
+            // suppress warnings
+            break;
+        }
+    }
+
+private:
+    state current{state::UNINITIALIZED};
+};
+
+
+/**
+ * @brief Adaptor for lambdas and functors to turn them into processes.
+ *
+ * Lambdas and functors can't be used directly with a scheduler for they are not
+ * properly defined processes with managed life cycles.<br/>
+ * This class helps in filling the gap and turning lambdas and functors into
+ * full featured processes usable by a scheduler.
+ *
+ * The signature of the function call operator should be equivalent to the
+ * following:
+ *
+ * @code{.cpp}
+ * void(Delta delta, void *data, auto succeed, auto fail);
+ * @endcode
+ *
+ * Where:
+ *
+ * * `delta` is the elapsed time.
+ * * `data` is an opaque pointer to user data if any, `nullptr` otherwise.
+ * * `succeed` is a function to call when a process terminates with success.
+ * * `fail` is a function to call when a process terminates with errors.
+ *
+ * The signature of the function call operator of both `succeed` and `fail`
+ * is equivalent to the following:
+ *
+ * @code{.cpp}
+ * void();
+ * @endcode
+ *
+ * Usually users shouldn't worry about creating adaptors. A scheduler will
+ * create them internally each and avery time a lambda or a functor is used as
+ * a process.
+ *
+ * @sa process
+ * @sa scheduler
+ *
+ * @tparam Func Actual type of process.
+ * @tparam Delta Type to use to provide elapsed time.
+ */
+template<typename Func, typename Delta>
+struct process_adaptor: process<process_adaptor<Func, Delta>, Delta>, private Func {
+    /**
+     * @brief Constructs a process adaptor from a lambda or a functor.
+     * @tparam Args Types of arguments to use to initialize the actual process.
+     * @param args Parameters to use to initialize the actual process.
+     */
+    template<typename... Args>
+    process_adaptor(Args &&... args)
+        : Func{std::forward<Args>(args)...}
+    {}
+
+    /**
+     * @brief Updates a process and its internal state if required.
+     * @param delta Elapsed time.
+     * @param data Optional data.
+     */
+    void update(const Delta delta, void *data) {
+        Func::operator()(delta, data, [this]() { this->succeed(); }, [this]() { this->fail(); });
+    }
+};
+
+
+}
+
+
+#endif
+
+
+
+namespace entt {
+
+
+/**
+ * @brief Cooperative scheduler for processes.
+ *
+ * A cooperative scheduler runs processes and helps managing their life cycles.
+ *
+ * Each process is invoked once per tick. If a process terminates, it's
+ * removed automatically from the scheduler and it's never invoked again.<br/>
+ * A process can also have a child. In this case, the process is replaced with
+ * its child when it terminates if it returns with success. In case of errors,
+ * both the process and its child are discarded.
+ *
+ * Example of use (pseudocode):
+ *
+ * @code{.cpp}
+ * scheduler.attach([](auto delta, void *, auto succeed, auto fail) {
+ *     // code
+ * }).then<my_process>(arguments...);
+ * @endcode
+ *
+ * In order to invoke all scheduled processes, call the `update` member function
+ * passing it the elapsed time to forward to the tasks.
+ *
+ * @sa process
+ *
+ * @tparam Delta Type to use to provide elapsed time.
+ */
+template<typename Delta>
+class scheduler {
+    struct process_handler {
+        using instance_type = std::unique_ptr<void, void(*)(void *)>;
+        using update_fn_type = bool(process_handler &, Delta, void *);
+        using abort_fn_type = void(process_handler &, bool);
+        using next_type = std::unique_ptr<process_handler>;
+
+        instance_type instance;
+        update_fn_type *update;
+        abort_fn_type *abort;
+        next_type next;
+    };
+
+    struct continuation {
+        continuation(process_handler *ref)
+            : handler{ref}
+        {}
+
+        template<typename Proc, typename... Args>
+        continuation then(Args &&... args) {
+            static_assert(std::is_base_of_v<process<Proc, Delta>, Proc>, "Invalid process type");
+            auto proc = typename process_handler::instance_type{new Proc{std::forward<Args>(args)...}, &scheduler::deleter<Proc>};
+            handler->next.reset(new process_handler{std::move(proc), &scheduler::update<Proc>, &scheduler::abort<Proc>, nullptr});
+            handler = handler->next.get();
+            return *this;
+        }
+
+        template<typename Func>
+        continuation then(Func &&func) {
+            return then<process_adaptor<std::decay_t<Func>, Delta>>(std::forward<Func>(func));
+        }
+
+    private:
+        process_handler *handler;
+    };
+
+    template<typename Proc>
+    [[nodiscard]] static bool update(process_handler &handler, const Delta delta, void *data) {
+        auto *process = static_cast<Proc *>(handler.instance.get());
+        process->tick(delta, data);
+
+        if(process->rejected()) {
+            return true;
+        } else if(process->finished()) {
+            if(handler.next) {
+                handler = std::move(*handler.next);
+                // forces the process to exit the uninitialized state
+                return handler.update(handler, {}, nullptr);
+            }
+
+            return true;
+        }
+
+        return false;
+    }
+
+    template<typename Proc>
+    static void abort(process_handler &handler, const bool immediately) {
+        static_cast<Proc *>(handler.instance.get())->abort(immediately);
+    }
+
+    template<typename Proc>
+    static void deleter(void *proc) {
+        delete static_cast<Proc *>(proc);
+    }
+
+public:
+    /*! @brief Unsigned integer type. */
+    using size_type = std::size_t;
+
+    /*! @brief Default constructor. */
+    scheduler() = default;
+
+    /*! @brief Default move constructor. */
+    scheduler(scheduler &&) = default;
+
+    /*! @brief Default move assignment operator. @return This scheduler. */
+    scheduler & operator=(scheduler &&) = default;
+
+    /**
+     * @brief Number of processes currently scheduled.
+     * @return Number of processes currently scheduled.
+     */
+    [[nodiscard]] size_type size() const ENTT_NOEXCEPT {
+        return handlers.size();
+    }
+
+    /**
+     * @brief Returns true if at least a process is currently scheduled.
+     * @return True if there are scheduled processes, false otherwise.
+     */
+    [[nodiscard]] bool empty() const ENTT_NOEXCEPT {
+        return handlers.empty();
+    }
+
+    /**
+     * @brief Discards all scheduled processes.
+     *
+     * Processes aren't aborted. They are discarded along with their children
+     * and never executed again.
+     */
+    void clear() {
+        handlers.clear();
+    }
+
+    /**
+     * @brief Schedules a process for the next tick.
+     *
+     * Returned value is an opaque object that can be used to attach a child to
+     * the given process. The child is automatically scheduled when the process
+     * terminates and only if the process returns with success.
+     *
+     * Example of use (pseudocode):
+     *
+     * @code{.cpp}
+     * // schedules a task in the form of a process class
+     * scheduler.attach<my_process>(arguments...)
+     * // appends a child in the form of a lambda function
+     * .then([](auto delta, void *, auto succeed, auto fail) {
+     *     // code
+     * })
+     * // appends a child in the form of another process class
+     * .then<my_other_process>();
+     * @endcode
+     *
+     * @tparam Proc Type of process to schedule.
+     * @tparam Args Types of arguments to use to initialize the process.
+     * @param args Parameters to use to initialize the process.
+     * @return An opaque object to use to concatenate processes.
+     */
+    template<typename Proc, typename... Args>
+    auto attach(Args &&... args) {
+        static_assert(std::is_base_of_v<process<Proc, Delta>, Proc>, "Invalid process type");
+        auto proc = typename process_handler::instance_type{new Proc{std::forward<Args>(args)...}, &scheduler::deleter<Proc>};
+        process_handler handler{std::move(proc), &scheduler::update<Proc>, &scheduler::abort<Proc>, nullptr};
+        // forces the process to exit the uninitialized state
+        handler.update(handler, {}, nullptr);
+        return continuation{&handlers.emplace_back(std::move(handler))};
+    }
+
+    /**
+     * @brief Schedules a process for the next tick.
+     *
+     * A process can be either a lambda or a functor. The scheduler wraps both
+     * of them in a process adaptor internally.<br/>
+     * The signature of the function call operator should be equivalent to the
+     * following:
+     *
+     * @code{.cpp}
+     * void(Delta delta, void *data, auto succeed, auto fail);
+     * @endcode
+     *
+     * Where:
+     *
+     * * `delta` is the elapsed time.
+     * * `data` is an opaque pointer to user data if any, `nullptr` otherwise.
+     * * `succeed` is a function to call when a process terminates with success.
+     * * `fail` is a function to call when a process terminates with errors.
+     *
+     * The signature of the function call operator of both `succeed` and `fail`
+     * is equivalent to the following:
+     *
+     * @code{.cpp}
+     * void();
+     * @endcode
+     *
+     * Returned value is an opaque object that can be used to attach a child to
+     * the given process. The child is automatically scheduled when the process
+     * terminates and only if the process returns with success.
+     *
+     * Example of use (pseudocode):
+     *
+     * @code{.cpp}
+     * // schedules a task in the form of a lambda function
+     * scheduler.attach([](auto delta, void *, auto succeed, auto fail) {
+     *     // code
+     * })
+     * // appends a child in the form of another lambda function
+     * .then([](auto delta, void *, auto succeed, auto fail) {
+     *     // code
+     * })
+     * // appends a child in the form of a process class
+     * .then<my_process>(arguments...);
+     * @endcode
+     *
+     * @sa process_adaptor
+     *
+     * @tparam Func Type of process to schedule.
+     * @param func Either a lambda or a functor to use as a process.
+     * @return An opaque object to use to concatenate processes.
+     */
+    template<typename Func>
+    auto attach(Func &&func) {
+        using Proc = process_adaptor<std::decay_t<Func>, Delta>;
+        return attach<Proc>(std::forward<Func>(func));
+    }
+
+    /**
+     * @brief Updates all scheduled processes.
+     *
+     * All scheduled processes are executed in no specific order.<br/>
+     * If a process terminates with success, it's replaced with its child, if
+     * any. Otherwise, if a process terminates with an error, it's removed along
+     * with its child.
+     *
+     * @param delta Elapsed time.
+     * @param data Optional data.
+     */
+    void update(const Delta delta, void *data = nullptr) {
+        auto sz = handlers.size();
+
+        for(auto pos = handlers.size(); pos; --pos) {
+            auto &handler = handlers[pos-1];
+
+            if(const auto dead = handler.update(handler, delta, data); dead) {
+                std::swap(handler, handlers[--sz]);
+            }
+        }
+
+        handlers.erase(handlers.begin() + sz, handlers.end());
+    }
+
+    /**
+     * @brief Aborts all scheduled processes.
+     *
+     * Unless an immediate operation is requested, the abort is scheduled for
+     * the next tick. Processes won't be executed anymore in any case.<br/>
+     * Once a process is fully aborted and thus finished, it's discarded along
+     * with its child, if any.
+     *
+     * @param immediately Requests an immediate operation.
+     */
+    void abort(const bool immediately = false) {
+        decltype(handlers) exec;
+        exec.swap(handlers);
+
+        for(auto &&handler: exec) {
+            handler.abort(handler, immediately);
+        }
+
+        std::move(handlers.begin(), handlers.end(), std::back_inserter(exec));
+        handlers.swap(exec);
+    }
+
+private:
+    std::vector<process_handler> handlers{};
+};
+
+
+}
+
+
+#endif
+
+// #include "resource/cache.hpp"
+#ifndef ENTT_RESOURCE_CACHE_HPP
+#define ENTT_RESOURCE_CACHE_HPP
+
+
+#include <type_traits>
+#include <unordered_map>
+#include <utility>
+// #include "../config/config.h"
+#ifndef ENTT_CONFIG_CONFIG_H
+#define ENTT_CONFIG_CONFIG_H
+
+
+#if defined(__cpp_exceptions) && !defined(ENTT_NOEXCEPTION)
+#   define ENTT_NOEXCEPT noexcept
+#   define ENTT_THROW throw
+#   define ENTT_TRY try
+#   define ENTT_CATCH catch(...)
+#else
+#   define ENTT_NOEXCEPT
+#   define ENTT_THROW
+#   define ENTT_TRY if(true)
+#   define ENTT_CATCH if(false)
+#endif
+
+
+#if defined(__cpp_lib_launder) && __cpp_lib_launder >= 201606L
+#   include <new>
+#   define ENTT_LAUNDER(expr) std::launder(expr)
+#else
+#   define ENTT_LAUNDER(expr) expr
+#endif
+
+
+#ifndef ENTT_USE_ATOMIC
+#   define ENTT_MAYBE_ATOMIC(Type) Type
+#else
+#   include <atomic>
+#   define ENTT_MAYBE_ATOMIC(Type) std::atomic<Type>
+#endif
+
+
+#ifndef ENTT_ID_TYPE
+#   include <cstdint>
+#   define ENTT_ID_TYPE std::uint32_t
+#endif
+
+
+#ifdef ENTT_SPARSE_PAGE
+	static_assert(ENTT_SPARSE_PAGE && ((ENTT_SPARSE_PAGE & (ENTT_SPARSE_PAGE - 1)) == 0), "ENTT_SPARSE_PAGE must be a power of two");
+#else
+#   define ENTT_SPARSE_PAGE 4096
+#endif
+
+
+#ifdef ENTT_PACKED_PAGE
+static_assert(ENTT_PACKED_PAGE && ((ENTT_PACKED_PAGE & (ENTT_PACKED_PAGE - 1)) == 0), "ENTT_PACKED_PAGE must be a power of two");
+#else
+#   define ENTT_PACKED_PAGE 1024
+#endif
+
+
+#ifdef ENTT_DISABLE_ASSERT
+#   undef ENTT_ASSERT
+#   define ENTT_ASSERT(...) (void(0))
+#elif !defined ENTT_ASSERT
+#   include <cassert>
+#   define ENTT_ASSERT(condition, ...) assert(condition)
+#endif
+
+
+#ifdef ENTT_NO_ETO
+#   include <type_traits>
+#   define ENTT_IGNORE_IF_EMPTY std::false_type
+#else
+#   include <type_traits>
+#   define ENTT_IGNORE_IF_EMPTY std::true_type
+#endif
+
+
+#ifndef ENTT_STANDARD_CPP
+#    if defined __clang__ || defined __GNUC__
+#       define ENTT_PRETTY_FUNCTION __PRETTY_FUNCTION__
+#       define ENTT_PRETTY_FUNCTION_PREFIX '='
+#       define ENTT_PRETTY_FUNCTION_SUFFIX ']'
+#    elif defined _MSC_VER
+#       define ENTT_PRETTY_FUNCTION __FUNCSIG__
+#       define ENTT_PRETTY_FUNCTION_PREFIX '<'
+#       define ENTT_PRETTY_FUNCTION_SUFFIX '>'
+#   endif
+#endif
+
+
+#endif
+
+// #include "../core/fwd.hpp"
+#ifndef ENTT_CORE_FWD_HPP
+#define ENTT_CORE_FWD_HPP
+
+
+#include <type_traits>
+// #include "../config/config.h"
+#ifndef ENTT_CONFIG_CONFIG_H
+#define ENTT_CONFIG_CONFIG_H
+
+
+#if defined(__cpp_exceptions) && !defined(ENTT_NOEXCEPTION)
+#   define ENTT_NOEXCEPT noexcept
+#   define ENTT_THROW throw
+#   define ENTT_TRY try
+#   define ENTT_CATCH catch(...)
+#else
+#   define ENTT_NOEXCEPT
+#   define ENTT_THROW
+#   define ENTT_TRY if(true)
+#   define ENTT_CATCH if(false)
+#endif
+
+
+#if defined(__cpp_lib_launder) && __cpp_lib_launder >= 201606L
+#   include <new>
+#   define ENTT_LAUNDER(expr) std::launder(expr)
+#else
+#   define ENTT_LAUNDER(expr) expr
+#endif
+
+
+#ifndef ENTT_USE_ATOMIC
+#   define ENTT_MAYBE_ATOMIC(Type) Type
+#else
+#   include <atomic>
+#   define ENTT_MAYBE_ATOMIC(Type) std::atomic<Type>
+#endif
+
+
+#ifndef ENTT_ID_TYPE
+#   include <cstdint>
+#   define ENTT_ID_TYPE std::uint32_t
+#endif
+
+
+#ifdef ENTT_SPARSE_PAGE
+	static_assert(ENTT_SPARSE_PAGE && ((ENTT_SPARSE_PAGE & (ENTT_SPARSE_PAGE - 1)) == 0), "ENTT_SPARSE_PAGE must be a power of two");
+#else
+#   define ENTT_SPARSE_PAGE 4096
+#endif
+
+
+#ifdef ENTT_PACKED_PAGE
+static_assert(ENTT_PACKED_PAGE && ((ENTT_PACKED_PAGE & (ENTT_PACKED_PAGE - 1)) == 0), "ENTT_PACKED_PAGE must be a power of two");
+#else
+#   define ENTT_PACKED_PAGE 1024
+#endif
+
+
+#ifdef ENTT_DISABLE_ASSERT
+#   undef ENTT_ASSERT
+#   define ENTT_ASSERT(...) (void(0))
+#elif !defined ENTT_ASSERT
+#   include <cassert>
+#   define ENTT_ASSERT(condition, ...) assert(condition)
+#endif
+
+
+#ifdef ENTT_NO_ETO
+#   include <type_traits>
+#   define ENTT_IGNORE_IF_EMPTY std::false_type
+#else
+#   include <type_traits>
+#   define ENTT_IGNORE_IF_EMPTY std::true_type
+#endif
+
+
+#ifndef ENTT_STANDARD_CPP
+#    if defined __clang__ || defined __GNUC__
+#       define ENTT_PRETTY_FUNCTION __PRETTY_FUNCTION__
+#       define ENTT_PRETTY_FUNCTION_PREFIX '='
+#       define ENTT_PRETTY_FUNCTION_SUFFIX ']'
+#    elif defined _MSC_VER
+#       define ENTT_PRETTY_FUNCTION __FUNCSIG__
+#       define ENTT_PRETTY_FUNCTION_PREFIX '<'
+#       define ENTT_PRETTY_FUNCTION_SUFFIX '>'
+#   endif
+#endif
+
+
+#endif
+
+
+
+namespace entt {
+
+
+template<std::size_t Len = sizeof(double[2]), std::size_t = alignof(typename std::aligned_storage_t<Len + !Len>)>
+class basic_any;
+
+
+/*! @brief Alias declaration for type identifiers. */
+using id_type = ENTT_ID_TYPE;
+
+
+/*! @brief Alias declaration for the most common use case. */
+using any = basic_any<>;
+
+
+}
+
+
+#endif
+
+// #include "handle.hpp"
+#ifndef ENTT_RESOURCE_HANDLE_HPP
+#define ENTT_RESOURCE_HANDLE_HPP
+
+
+#include <memory>
+#include <type_traits>
+#include <utility>
+// #include "../config/config.h"
+
+// #include "fwd.hpp"
+#ifndef ENTT_RESOURCE_FWD_HPP
+#define ENTT_RESOURCE_FWD_HPP
+
+
+namespace entt {
+
+
+template<typename>
+struct resource_cache;
+
+
+template<typename>
+class resource_handle;
+
+
+template<typename, typename>
+class resource_loader;
+
+
+}
+
+
+#endif
+
+
+
+namespace entt {
+
+
+/**
+ * @brief Shared resource handle.
+ *
+ * A shared resource handle is a small class that wraps a resource and keeps it
+ * alive even if it's deleted from the cache. It can be either copied or
+ * moved. A handle shares a reference to the same resource with all the other
+ * handles constructed for the same identifier.<br/>
+ * As a rule of thumb, resources should never be copied nor moved. Handles are
+ * the way to go to keep references to them.
+ *
+ * @tparam Resource Type of resource managed by a handle.
+ */
+template<typename Resource>
+class resource_handle {
+    /*! @brief Resource handles are friends with each other. */
+    template<typename>
+    friend class resource_handle;
+
+public:
+    /*! @brief Default constructor. */
+    resource_handle() ENTT_NOEXCEPT = default;
+
+    /**
+     * @brief Creates a handle from a shared pointer, namely a resource.
+     * @param res A pointer to a properly initialized resource.
+     */
+    resource_handle(std::shared_ptr<Resource> res) ENTT_NOEXCEPT
+        : resource{std::move(res)}
+    {}
+
+    /**
+     * @brief Copy constructor.
+     * @param other The instance to copy from.
+     */
+    resource_handle(const resource_handle<Resource> &other) ENTT_NOEXCEPT = default;
+
+    /**
+     * @brief Move constructor.
+     * @param other The instance to move from.
+     */
+    resource_handle(resource_handle<Resource> &&other) ENTT_NOEXCEPT = default;
+
+    /**
+     * @brief Copy constructs a handle which shares ownership of the resource.
+     * @tparam Other Type of resource managed by the received handle.
+     * @param other The handle to copy from.
+     */
+    template<typename Other, typename = std::enable_if_t<!std::is_same_v<Other, Resource> && std::is_base_of_v<Resource, Other>>>
+    resource_handle(const resource_handle<Other> &other) ENTT_NOEXCEPT
+        : resource{other.resource}
+    {}
+
+    /**
+     * @brief Move constructs a handle which takes ownership of the resource.
+     * @tparam Other Type of resource managed by the received handle.
+     * @param other The handle to move from.
+     */
+    template<typename Other, typename = std::enable_if_t<!std::is_same_v<Other, Resource> && std::is_base_of_v<Resource, Other>>>
+    resource_handle(resource_handle<Other> &&other) ENTT_NOEXCEPT
+        : resource{std::move(other.resource)}
+    {}
+
+    /**
+     * @brief Copy assignment operator.
+     * @param other The instance to copy from.
+     * @return This resource handle.
+     */
+    resource_handle & operator=(const resource_handle<Resource> &other) ENTT_NOEXCEPT = default;
+
+    /**
+     * @brief Move assignment operator.
+     * @param other The instance to move from.
+     * @return This resource handle.
+     */
+    resource_handle & operator=(resource_handle<Resource> &&other) ENTT_NOEXCEPT = default;
+
+    /**
+     * @brief Copy assignment operator from foreign handle.
+     * @tparam Other Type of resource managed by the received handle.
+     * @param other The handle to copy from.
+     * @return This resource handle.
+     */
+    template<typename Other>
+    std::enable_if_t<!std::is_same_v<Other, Resource> && std::is_base_of_v<Resource, Other>, resource_handle &>
+    operator=(const resource_handle<Other> &other) ENTT_NOEXCEPT {
+        resource = other.resource;
+        return *this;
+    }
+
+    /**
+     * @brief Move assignment operator from foreign handle.
+     * @tparam Other Type of resource managed by the received handle.
+     * @param other The handle to move from.
+     * @return This resource handle.
+     */
+    template<typename Other>
+    std::enable_if_t<!std::is_same_v<Other, Resource> && std::is_base_of_v<Resource, Other>, resource_handle &>
+    operator=(resource_handle<Other> &&other) ENTT_NOEXCEPT {
+        resource = std::move(other.resource);
+        return *this;
+    }
+
+    /**
+     * @brief Gets a reference to the managed resource.
+     *
+     * @warning
+     * The behavior is undefined if the handle doesn't contain a resource.
+     *
+     * @return A reference to the managed resource.
+     */
+    [[nodiscard]] const Resource & get() const ENTT_NOEXCEPT {
+        ENTT_ASSERT(static_cast<bool>(resource), "Invalid resource");
+        return *resource;
+    }
+
+    /*! @copydoc get */
+    [[nodiscard]] Resource & get() ENTT_NOEXCEPT {
+        return const_cast<Resource &>(std::as_const(*this).get());
+    }
+
+    /*! @copydoc get */
+    [[nodiscard]] operator const Resource & () const ENTT_NOEXCEPT {
+        return get();
+    }
+
+    /*! @copydoc get */
+    [[nodiscard]] operator Resource & () ENTT_NOEXCEPT {
+        return get();
+    }
+
+    /*! @copydoc get */
+    [[nodiscard]] const Resource & operator *() const ENTT_NOEXCEPT {
+        return get();
+    }
+
+    /*! @copydoc get */
+    [[nodiscard]] Resource & operator *() ENTT_NOEXCEPT {
+        return get();
+    }
+
+    /**
+     * @brief Gets a pointer to the managed resource.
+     *
+     * @warning
+     * The behavior is undefined if the handle doesn't contain a resource.
+     *
+     * @return A pointer to the managed resource or `nullptr` if the handle
+     * contains no resource at all.
+     */
+    [[nodiscard]] const Resource * operator->() const ENTT_NOEXCEPT {
+        return resource.get();
+    }
+
+    /*! @copydoc operator-> */
+    [[nodiscard]] Resource * operator->() ENTT_NOEXCEPT {
+        return const_cast<Resource *>(std::as_const(*this).operator->());
+    }
+
+    /**
+     * @brief Returns true if a handle contains a resource, false otherwise.
+     * @return True if the handle contains a resource, false otherwise.
+     */
+    [[nodiscard]] explicit operator bool() const ENTT_NOEXCEPT {
+        return static_cast<bool>(resource);
+    }
+
+private:
+    std::shared_ptr<Resource> resource;
+};
+
+
+}
+
+
+#endif
+
+// #include "loader.hpp"
+#ifndef ENTT_RESOURCE_LOADER_HPP
+#define ENTT_RESOURCE_LOADER_HPP
+
+
+// #include "fwd.hpp"
+
+// #include "handle.hpp"
+
+
+
+namespace entt {
+
+
+/**
+ * @brief Base class for resource loaders.
+ *
+ * Resource loaders must inherit from this class and stay true to the CRTP
+ * idiom. Moreover, a resource loader must expose a public, const member
+ * function named `load` that accepts a variable number of arguments and returns
+ * a handle to the resource just created.<br/>
+ * As an example:
+ *
+ * @code{.cpp}
+ * struct my_resource {};
+ *
+ * struct my_loader: entt::resource_loader<my_loader, my_resource> {
+ *     resource_handle<my_resource> load(int value) const {
+ *         // use the integer value somehow
+ *         return std::make_shared<my_resource>();
+ *     }
+ * };
+ * @endcode
+ *
+ * In general, resource loaders should not have a state or retain data of any
+ * type. They should let the cache manage their resources instead.
+ *
+ * @note
+ * Base class and CRTP idiom aren't strictly required with the current
+ * implementation. One could argue that a cache can easily work with loaders of
+ * any type. However, future changes won't be breaking ones by forcing the use
+ * of a base class today and that's why the model is already in its place.
+ *
+ * @tparam Loader Type of the derived class.
+ * @tparam Resource Type of resource for which to use the loader.
+ */
+template<typename Loader, typename Resource>
+class resource_loader {
+    /*! @brief Resource loaders are friends of their caches. */
+    template<typename Other>
+    friend struct resource_cache;
+
+    /**
+     * @brief Loads the resource and returns it.
+     * @tparam Args Types of arguments for the loader.
+     * @param args Arguments for the loader.
+     * @return The resource just loaded or an empty pointer in case of errors.
+     */
+    template<typename... Args>
+    [[nodiscard]] resource_handle<Resource> get(Args &&... args) const {
+        return static_cast<const Loader *>(this)->load(std::forward<Args>(args)...);
+    }
+};
+
+
+}
+
+
+#endif
+
+// #include "fwd.hpp"
+
+
+
+namespace entt {
+
+
+/**
+ * @brief Simple cache for resources of a given type.
+ *
+ * Minimal implementation of a cache for resources of a given type. It doesn't
+ * offer much functionalities but it's suitable for small or medium sized
+ * applications and can be freely inherited to add targeted functionalities for
+ * large sized applications.
+ *
+ * @tparam Resource Type of resources managed by a cache.
+ */
+template<typename Resource>
+struct resource_cache {
+    /*! @brief Unsigned integer type. */
+    using size_type = std::size_t;
+    /*! @brief Type of resources managed by a cache. */
+    using resource_type = Resource;
+
+    /*! @brief Default constructor. */
+    resource_cache() = default;
+
+    /*! @brief Default move constructor. */
+    resource_cache(resource_cache &&) = default;
+
+    /*! @brief Default move assignment operator. @return This cache. */
+    resource_cache & operator=(resource_cache &&) = default;
+
+    /**
+     * @brief Number of resources managed by a cache.
+     * @return Number of resources currently stored.
+     */
+    [[nodiscard]] size_type size() const ENTT_NOEXCEPT {
+        return resources.size();
+    }
+
+    /**
+     * @brief Returns true if a cache contains no resources, false otherwise.
+     * @return True if the cache contains no resources, false otherwise.
+     */
+    [[nodiscard]] bool empty() const ENTT_NOEXCEPT {
+        return resources.empty();
+    }
+
+    /**
+     * @brief Clears a cache and discards all its resources.
+     *
+     * Handles are not invalidated and the memory used by a resource isn't
+     * freed as long as at least a handle keeps the resource itself alive.
+     */
+    void clear() ENTT_NOEXCEPT {
+        resources.clear();
+    }
+
+    /**
+     * @brief Loads the resource that corresponds to a given identifier.
+     *
+     * In case an identifier isn't already present in the cache, it loads its
+     * resource and stores it aside for future uses. Arguments are forwarded
+     * directly to the loader in order to construct properly the requested
+     * resource.
+     *
+     * @note
+     * If the identifier is already present in the cache, this function does
+     * nothing and the arguments are simply discarded.
+     *
+     * @warning
+     * If the resource cannot be loaded correctly, the returned handle will be
+     * invalid and any use of it will result in undefined behavior.
+     *
+     * @tparam Loader Type of loader to use to load the resource if required.
+     * @tparam Args Types of arguments to use to load the resource if required.
+     * @param id Unique resource identifier.
+     * @param args Arguments to use to load the resource if required.
+     * @return A handle for the given resource.
+     */
+    template<typename Loader, typename... Args>
+    resource_handle<Resource> load(const id_type id, Args &&... args) {
+        if(auto it = resources.find(id); it == resources.cend()) {
+            if(auto handle = temp<Loader>(std::forward<Args>(args)...); handle) {
+                return (resources[id] = std::move(handle));
+            }
+        } else {
+            return it->second;
+        }
+
+        return {};
+    }
+
+    /**
+     * @brief Reloads a resource or loads it for the first time if not present.
+     *
+     * Equivalent to the following snippet (pseudocode):
+     *
+     * @code{.cpp}
+     * cache.discard(id);
+     * cache.load(id, args...);
+     * @endcode
+     *
+     * Arguments are forwarded directly to the loader in order to construct
+     * properly the requested resource.
+     *
+     * @warning
+     * If the resource cannot be loaded correctly, the returned handle will be
+     * invalid and any use of it will result in undefined behavior.
+     *
+     * @tparam Loader Type of loader to use to load the resource.
+     * @tparam Args Types of arguments to use to load the resource.
+     * @param id Unique resource identifier.
+     * @param args Arguments to use to load the resource.
+     * @return A handle for the given resource.
+     */
+    template<typename Loader, typename... Args>
+    resource_handle<Resource> reload(const id_type id, Args &&... args) {
+        return (discard(id), load<Loader>(id, std::forward<Args>(args)...));
+    }
+
+    /**
+     * @brief Creates a temporary handle for a resource.
+     *
+     * Arguments are forwarded directly to the loader in order to construct
+     * properly the requested resource. The handle isn't stored aside and the
+     * cache isn't in charge of the lifetime of the resource itself.
+     *
+     * @tparam Loader Type of loader to use to load the resource.
+     * @tparam Args Types of arguments to use to load the resource.
+     * @param args Arguments to use to load the resource.
+     * @return A handle for the given resource.
+     */
+    template<typename Loader, typename... Args>
+    [[nodiscard]] resource_handle<Resource> temp(Args &&... args) const {
+        return Loader{}.get(std::forward<Args>(args)...);
+    }
+
+    /**
+     * @brief Creates a handle for a given resource identifier.
+     *
+     * A resource handle can be in a either valid or invalid state. In other
+     * terms, a resource handle is properly initialized with a resource if the
+     * cache contains the resource itself. Otherwise the returned handle is
+     * uninitialized and accessing it results in undefined behavior.
+     *
+     * @sa resource_handle
+     *
+     * @param id Unique resource identifier.
+     * @return A handle for the given resource.
+     */
+    [[nodiscard]] resource_handle<Resource> handle(const id_type id) const {
+        if(auto it = resources.find(id); it != resources.cend()) {
+            return it->second;
+        }
+
+        return {};
+    }
+
+    /**
+     * @brief Checks if a cache contains a given identifier.
+     * @param id Unique resource identifier.
+     * @return True if the cache contains the resource, false otherwise.
+     */
+    [[nodiscard]] bool contains(const id_type id) const {
+        return (resources.find(id) != resources.cend());
+    }
+
+    /**
+     * @brief Discards the resource that corresponds to a given identifier.
+     *
+     * Handles are not invalidated and the memory used by the resource isn't
+     * freed as long as at least a handle keeps the resource itself alive.
+     *
+     * @param id Unique resource identifier.
+     */
+    void discard(const id_type id) {
+        if(auto it = resources.find(id); it != resources.end()) {
+            resources.erase(it);
+        }
+    }
+
+    /**
+     * @brief Iterates all resources.
+     *
+     * The function object is invoked for each element. It is provided with
+     * either the resource identifier, the resource handle or both of them.<br/>
+     * The signature of the function must be equivalent to one of the following
+     * forms:
+     *
+     * @code{.cpp}
+     * void(const entt::id_type);
+     * void(entt::resource_handle<Resource>);
+     * void(const entt::id_type, entt::resource_handle<Resource>);
+     * @endcode
+     *
+     * @tparam Func Type of the function object to invoke.
+     * @param func A valid function object.
+     */
+    template <typename Func>
+    void each(Func func) const {
+        auto begin = resources.begin();
+        auto end = resources.end();
+
+        while(begin != end) {
+            auto curr = begin++;
+
+            if constexpr(std::is_invocable_v<Func, id_type>) {
+                func(curr->first);
+            } else if constexpr(std::is_invocable_v<Func, resource_handle<Resource>>) {
+                func(curr->second);
+            } else {
+                func(curr->first, curr->second);
+            }
+        }
+    }
+
+private:
+    std::unordered_map<id_type, resource_handle<Resource>> resources;
+};
+
+
+}
+
+
+#endif
+
+// #include "resource/handle.hpp"
+#ifndef ENTT_RESOURCE_HANDLE_HPP
+#define ENTT_RESOURCE_HANDLE_HPP
+
+
+#include <memory>
+#include <type_traits>
+#include <utility>
+// #include "../config/config.h"
+
+// #include "fwd.hpp"
+
+
+
+namespace entt {
+
+
+/**
+ * @brief Shared resource handle.
+ *
+ * A shared resource handle is a small class that wraps a resource and keeps it
+ * alive even if it's deleted from the cache. It can be either copied or
+ * moved. A handle shares a reference to the same resource with all the other
+ * handles constructed for the same identifier.<br/>
+ * As a rule of thumb, resources should never be copied nor moved. Handles are
+ * the way to go to keep references to them.
+ *
+ * @tparam Resource Type of resource managed by a handle.
+ */
+template<typename Resource>
+class resource_handle {
+    /*! @brief Resource handles are friends with each other. */
+    template<typename>
+    friend class resource_handle;
+
+public:
+    /*! @brief Default constructor. */
+    resource_handle() ENTT_NOEXCEPT = default;
+
+    /**
+     * @brief Creates a handle from a shared pointer, namely a resource.
+     * @param res A pointer to a properly initialized resource.
+     */
+    resource_handle(std::shared_ptr<Resource> res) ENTT_NOEXCEPT
+        : resource{std::move(res)}
+    {}
+
+    /**
+     * @brief Copy constructor.
+     * @param other The instance to copy from.
+     */
+    resource_handle(const resource_handle<Resource> &other) ENTT_NOEXCEPT = default;
+
+    /**
+     * @brief Move constructor.
+     * @param other The instance to move from.
+     */
+    resource_handle(resource_handle<Resource> &&other) ENTT_NOEXCEPT = default;
+
+    /**
+     * @brief Copy constructs a handle which shares ownership of the resource.
+     * @tparam Other Type of resource managed by the received handle.
+     * @param other The handle to copy from.
+     */
+    template<typename Other, typename = std::enable_if_t<!std::is_same_v<Other, Resource> && std::is_base_of_v<Resource, Other>>>
+    resource_handle(const resource_handle<Other> &other) ENTT_NOEXCEPT
+        : resource{other.resource}
+    {}
+
+    /**
+     * @brief Move constructs a handle which takes ownership of the resource.
+     * @tparam Other Type of resource managed by the received handle.
+     * @param other The handle to move from.
+     */
+    template<typename Other, typename = std::enable_if_t<!std::is_same_v<Other, Resource> && std::is_base_of_v<Resource, Other>>>
+    resource_handle(resource_handle<Other> &&other) ENTT_NOEXCEPT
+        : resource{std::move(other.resource)}
+    {}
+
+    /**
+     * @brief Copy assignment operator.
+     * @param other The instance to copy from.
+     * @return This resource handle.
+     */
+    resource_handle & operator=(const resource_handle<Resource> &other) ENTT_NOEXCEPT = default;
+
+    /**
+     * @brief Move assignment operator.
+     * @param other The instance to move from.
+     * @return This resource handle.
+     */
+    resource_handle & operator=(resource_handle<Resource> &&other) ENTT_NOEXCEPT = default;
+
+    /**
+     * @brief Copy assignment operator from foreign handle.
+     * @tparam Other Type of resource managed by the received handle.
+     * @param other The handle to copy from.
+     * @return This resource handle.
+     */
+    template<typename Other>
+    std::enable_if_t<!std::is_same_v<Other, Resource> && std::is_base_of_v<Resource, Other>, resource_handle &>
+    operator=(const resource_handle<Other> &other) ENTT_NOEXCEPT {
+        resource = other.resource;
+        return *this;
+    }
+
+    /**
+     * @brief Move assignment operator from foreign handle.
+     * @tparam Other Type of resource managed by the received handle.
+     * @param other The handle to move from.
+     * @return This resource handle.
+     */
+    template<typename Other>
+    std::enable_if_t<!std::is_same_v<Other, Resource> && std::is_base_of_v<Resource, Other>, resource_handle &>
+    operator=(resource_handle<Other> &&other) ENTT_NOEXCEPT {
+        resource = std::move(other.resource);
+        return *this;
+    }
+
+    /**
+     * @brief Gets a reference to the managed resource.
+     *
+     * @warning
+     * The behavior is undefined if the handle doesn't contain a resource.
+     *
+     * @return A reference to the managed resource.
+     */
+    [[nodiscard]] const Resource & get() const ENTT_NOEXCEPT {
+        ENTT_ASSERT(static_cast<bool>(resource), "Invalid resource");
+        return *resource;
+    }
+
+    /*! @copydoc get */
+    [[nodiscard]] Resource & get() ENTT_NOEXCEPT {
+        return const_cast<Resource &>(std::as_const(*this).get());
+    }
+
+    /*! @copydoc get */
+    [[nodiscard]] operator const Resource & () const ENTT_NOEXCEPT {
+        return get();
+    }
+
+    /*! @copydoc get */
+    [[nodiscard]] operator Resource & () ENTT_NOEXCEPT {
+        return get();
+    }
+
+    /*! @copydoc get */
+    [[nodiscard]] const Resource & operator *() const ENTT_NOEXCEPT {
+        return get();
+    }
+
+    /*! @copydoc get */
+    [[nodiscard]] Resource & operator *() ENTT_NOEXCEPT {
+        return get();
+    }
+
+    /**
+     * @brief Gets a pointer to the managed resource.
+     *
+     * @warning
+     * The behavior is undefined if the handle doesn't contain a resource.
+     *
+     * @return A pointer to the managed resource or `nullptr` if the handle
+     * contains no resource at all.
+     */
+    [[nodiscard]] const Resource * operator->() const ENTT_NOEXCEPT {
+        return resource.get();
+    }
+
+    /*! @copydoc operator-> */
+    [[nodiscard]] Resource * operator->() ENTT_NOEXCEPT {
+        return const_cast<Resource *>(std::as_const(*this).operator->());
+    }
+
+    /**
+     * @brief Returns true if a handle contains a resource, false otherwise.
+     * @return True if the handle contains a resource, false otherwise.
+     */
+    [[nodiscard]] explicit operator bool() const ENTT_NOEXCEPT {
+        return static_cast<bool>(resource);
+    }
+
+private:
+    std::shared_ptr<Resource> resource;
+};
+
+
+}
+
+
+#endif
+
+// #include "resource/loader.hpp"
+#ifndef ENTT_RESOURCE_LOADER_HPP
+#define ENTT_RESOURCE_LOADER_HPP
+
+
+// #include "fwd.hpp"
+
+// #include "handle.hpp"
+
+
+
+namespace entt {
+
+
+/**
+ * @brief Base class for resource loaders.
+ *
+ * Resource loaders must inherit from this class and stay true to the CRTP
+ * idiom. Moreover, a resource loader must expose a public, const member
+ * function named `load` that accepts a variable number of arguments and returns
+ * a handle to the resource just created.<br/>
+ * As an example:
+ *
+ * @code{.cpp}
+ * struct my_resource {};
+ *
+ * struct my_loader: entt::resource_loader<my_loader, my_resource> {
+ *     resource_handle<my_resource> load(int value) const {
+ *         // use the integer value somehow
+ *         return std::make_shared<my_resource>();
+ *     }
+ * };
  * @endcode
  *
- * In order to support even sligthly older compilers, I prefer to stick to the
- * implementation below.
- *
- * @param type Type to assign a name to.
- */
-#define ENTT_NAMED_TYPE(type)\
-    template<>\
-    struct entt::named_type_traits<type>\
-        : std::integral_constant<ENTT_ID_TYPE, entt::basic_hashed_string<std::remove_cv_t<std::remove_pointer_t<std::decay_t<decltype(#type)>>>>{#type}>\
-    {\
-        static_assert(std::is_same_v<std::decay_t<type>, type>);\
-    };
-
-
-/**
- * @brief Defines a named type (to use for structs).
- * @param clazz Name of the type to define.
- * @param body Body of the type to define.
- */
-#define ENTT_NAMED_STRUCT_ONLY(clazz, body)\
-    struct clazz body;\
-    ENTT_NAMED_TYPE(clazz)
-
-
-/**
- * @brief Defines a named type (to use for structs).
- * @param ns Namespace where to define the named type.
- * @param clazz Name of the type to define.
- * @param body Body of the type to define.
- */
-#define ENTT_NAMED_STRUCT_WITH_NAMESPACE(ns, clazz, body)\
-    namespace ns { struct clazz body; }\
-    ENTT_NAMED_TYPE(ns::clazz)
-
-
-/*! @brief Utility function to simulate macro overloading. */
-#define ENTT_NAMED_STRUCT_OVERLOAD(_1, _2, _3, FUNC, ...) FUNC
-/*! @brief Defines a named type (to use for structs). */
-#define ENTT_NAMED_STRUCT(...) ENTT_EXPAND(ENTT_NAMED_STRUCT_OVERLOAD(__VA_ARGS__, ENTT_NAMED_STRUCT_WITH_NAMESPACE, ENTT_NAMED_STRUCT_ONLY,)(__VA_ARGS__))
-
-
-/**
- * @brief Defines a named type (to use for classes).
- * @param clazz Name of the type to define.
- * @param body Body of the type to define.
- */
-#define ENTT_NAMED_CLASS_ONLY(clazz, body)\
-    class clazz body;\
-    ENTT_NAMED_TYPE(clazz)
-
-
-/**
- * @brief Defines a named type (to use for classes).
- * @param ns Namespace where to define the named type.
- * @param clazz Name of the type to define.
- * @param body Body of the type to define.
- */
-#define ENTT_NAMED_CLASS_WITH_NAMESPACE(ns, clazz, body)\
-    namespace ns { class clazz body; }\
-    ENTT_NAMED_TYPE(ns::clazz)
-
-
-/*! @brief Utility function to simulate macro overloading. */
-#define ENTT_NAMED_CLASS_MACRO(_1, _2, _3, FUNC, ...) FUNC
-/*! @brief Defines a named type (to use for classes). */
-#define ENTT_NAMED_CLASS(...) ENTT_EXPAND(ENTT_NAMED_CLASS_MACRO(__VA_ARGS__, ENTT_NAMED_CLASS_WITH_NAMESPACE, ENTT_NAMED_CLASS_ONLY,)(__VA_ARGS__))
-
-
-#endif // ENTT_CORE_TYPE_TRAITS_HPP
+ * In general, resource loaders should not have a state or retain data of any
+ * type. They should let the cache manage their resources instead.
+ *
+ * @note
+ * Base class and CRTP idiom aren't strictly required with the current
+ * implementation. One could argue that a cache can easily work with loaders of
+ * any type. However, future changes won't be breaking ones by forcing the use
+ * of a base class today and that's why the model is already in its place.
+ *
+ * @tparam Loader Type of the derived class.
+ * @tparam Resource Type of resource for which to use the loader.
+ */
+template<typename Loader, typename Resource>
+class resource_loader {
+    /*! @brief Resource loaders are friends of their caches. */
+    template<typename Other>
+    friend struct resource_cache;
+
+    /**
+     * @brief Loads the resource and returns it.
+     * @tparam Args Types of arguments for the loader.
+     * @param args Arguments for the loader.
+     * @return The resource just loaded or an empty pointer in case of errors.
+     */
+    template<typename... Args>
+    [[nodiscard]] resource_handle<Resource> get(Args &&... args) const {
+        return static_cast<const Loader *>(this)->load(std::forward<Args>(args)...);
+    }
+};
+
+
+}
+
+
+#endif
+
+// #include "signal/delegate.hpp"
+#ifndef ENTT_SIGNAL_DELEGATE_HPP
+#define ENTT_SIGNAL_DELEGATE_HPP
+
+
+#include <tuple>
+#include <cstddef>
+#include <utility>
+#include <functional>
+#include <type_traits>
+// #include "../core/type_traits.hpp"
+#ifndef ENTT_CORE_TYPE_TRAITS_HPP
+#define ENTT_CORE_TYPE_TRAITS_HPP
+
+
+#include <cstddef>
+#include <iterator>
+#include <type_traits>
+#include <utility>
+// #include "../config/config.h"
+#ifndef ENTT_CONFIG_CONFIG_H
+#define ENTT_CONFIG_CONFIG_H
+
+
+#if defined(__cpp_exceptions) && !defined(ENTT_NOEXCEPTION)
+#   define ENTT_NOEXCEPT noexcept
+#   define ENTT_THROW throw
+#   define ENTT_TRY try
+#   define ENTT_CATCH catch(...)
+#else
+#   define ENTT_NOEXCEPT
+#   define ENTT_THROW
+#   define ENTT_TRY if(true)
+#   define ENTT_CATCH if(false)
+#endif
+
+
+#if defined(__cpp_lib_launder) && __cpp_lib_launder >= 201606L
+#   include <new>
+#   define ENTT_LAUNDER(expr) std::launder(expr)
+#else
+#   define ENTT_LAUNDER(expr) expr
+#endif
+
+
+#ifndef ENTT_USE_ATOMIC
+#   define ENTT_MAYBE_ATOMIC(Type) Type
+#else
+#   include <atomic>
+#   define ENTT_MAYBE_ATOMIC(Type) std::atomic<Type>
+#endif
+
+
+#ifndef ENTT_ID_TYPE
+#   include <cstdint>
+#   define ENTT_ID_TYPE std::uint32_t
+#endif
+
+
+#ifdef ENTT_SPARSE_PAGE
+	static_assert(ENTT_SPARSE_PAGE && ((ENTT_SPARSE_PAGE & (ENTT_SPARSE_PAGE - 1)) == 0), "ENTT_SPARSE_PAGE must be a power of two");
+#else
+#   define ENTT_SPARSE_PAGE 4096
+#endif
+
+
+#ifdef ENTT_PACKED_PAGE
+static_assert(ENTT_PACKED_PAGE && ((ENTT_PACKED_PAGE & (ENTT_PACKED_PAGE - 1)) == 0), "ENTT_PACKED_PAGE must be a power of two");
+#else
+#   define ENTT_PACKED_PAGE 1024
+#endif
+
+
+#ifdef ENTT_DISABLE_ASSERT
+#   undef ENTT_ASSERT
+#   define ENTT_ASSERT(...) (void(0))
+#elif !defined ENTT_ASSERT
+#   include <cassert>
+#   define ENTT_ASSERT(condition, ...) assert(condition)
+#endif
+
+
+#ifdef ENTT_NO_ETO
+#   include <type_traits>
+#   define ENTT_IGNORE_IF_EMPTY std::false_type
+#else
+#   include <type_traits>
+#   define ENTT_IGNORE_IF_EMPTY std::true_type
+#endif
+
+
+#ifndef ENTT_STANDARD_CPP
+#    if defined __clang__ || defined __GNUC__
+#       define ENTT_PRETTY_FUNCTION __PRETTY_FUNCTION__
+#       define ENTT_PRETTY_FUNCTION_PREFIX '='
+#       define ENTT_PRETTY_FUNCTION_SUFFIX ']'
+#    elif defined _MSC_VER
+#       define ENTT_PRETTY_FUNCTION __FUNCSIG__
+#       define ENTT_PRETTY_FUNCTION_PREFIX '<'
+#       define ENTT_PRETTY_FUNCTION_SUFFIX '>'
+#   endif
+#endif
+
+
+#endif
+
+// #include "fwd.hpp"
+#ifndef ENTT_CORE_FWD_HPP
+#define ENTT_CORE_FWD_HPP
+
+
+#include <type_traits>
+// #include "../config/config.h"
+
+
+
+namespace entt {
+
+
+template<std::size_t Len = sizeof(double[2]), std::size_t = alignof(typename std::aligned_storage_t<Len + !Len>)>
+class basic_any;
+
+
+/*! @brief Alias declaration for type identifiers. */
+using id_type = ENTT_ID_TYPE;
+
+
+/*! @brief Alias declaration for the most common use case. */
+using any = basic_any<>;
+
+
+}
+
+
+#endif
+
+
+
+namespace entt {
+
+
+/**
+ * @brief Utility class to disambiguate overloaded functions.
+ * @tparam N Number of choices available.
+ */
+template<std::size_t N>
+struct choice_t
+    // Unfortunately, doxygen cannot parse such a construct.
+    /*! @cond TURN_OFF_DOXYGEN */
+    : choice_t<N-1>
+    /*! @endcond */
+{};
+
+
+/*! @copybrief choice_t */
+template<>
+struct choice_t<0> {};
+
+
+/**
+ * @brief Variable template for the choice trick.
+ * @tparam N Number of choices available.
+ */
+template<std::size_t N>
+inline constexpr choice_t<N> choice{};
+
+
+/**
+ * @brief Identity type trait.
+ *
+ * Useful to establish non-deduced contexts in template argument deduction
+ * (waiting for C++20) or to provide types through function arguments.
+ *
+ * @tparam Type A type.
+ */
+template<typename Type>
+struct type_identity {
+    /*! @brief Identity type. */
+    using type = Type;
+};
+
+
+/**
+ * @brief Helper type.
+ * @tparam Type A type.
+ */
+template<typename Type>
+using type_identity_t = typename type_identity<Type>::type;
+
+
+/**
+ * @brief A type-only `sizeof` wrapper that returns 0 where `sizeof` complains.
+ * @tparam Type The type of which to return the size.
+ * @tparam The size of the type if `sizeof` accepts it, 0 otherwise.
+ */
+template<typename Type, typename = void>
+struct size_of: std::integral_constant<std::size_t, 0u> {};
+
+
+/*! @copydoc size_of */
+template<typename Type>
+struct size_of<Type, std::void_t<decltype(sizeof(Type))>>
+    : std::integral_constant<std::size_t, sizeof(Type)>
+{};
+
+
+/**
+ * @brief Helper variable template.
+ * @tparam Type The type of which to return the size.
+ */
+template<class Type>
+inline constexpr std::size_t size_of_v = size_of<Type>::value;
+
+
+/**
+ * @brief Using declaration to be used to _repeat_ the same type a number of
+ * times equal to the size of a given parameter pack.
+ * @tparam Type A type to repeat.
+ */
+template<typename Type, typename>
+using unpack_as_t = Type;
+
+
+/**
+ * @brief Helper variable template to be used to _repeat_ the same value a
+ * number of times equal to the size of a given parameter pack.
+ * @tparam Value A value to repeat.
+ */
+template<auto Value, typename>
+inline constexpr auto unpack_as_v = Value;
+
+
+/**
+ * @brief Wraps a static constant.
+ * @tparam Value A static constant.
+ */
+template<auto Value>
+using integral_constant = std::integral_constant<decltype(Value), Value>;
+
+
+/**
+ * @brief Alias template to facilitate the creation of named values.
+ * @tparam Value A constant value at least convertible to `id_type`.
+ */
+template<id_type Value>
+using tag = integral_constant<Value>;
+
+
+/**
+ * @brief A class to use to push around lists of types, nothing more.
+ * @tparam Type Types provided by the type list.
+ */
+template<typename... Type>
+struct type_list {
+    /*! @brief Type list type. */
+    using type = type_list;
+    /*! @brief Compile-time number of elements in the type list. */
+    static constexpr auto size = sizeof...(Type);
+};
+
+
+/*! @brief Primary template isn't defined on purpose. */
+template<std::size_t, typename>
+struct type_list_element;
+
+
+/**
+ * @brief Provides compile-time indexed access to the types of a type list.
+ * @tparam Index Index of the type to return.
+ * @tparam Type First type provided by the type list.
+ * @tparam Other Other types provided by the type list.
+ */
+template<std::size_t Index, typename Type, typename... Other>
+struct type_list_element<Index, type_list<Type, Other...>>
+    : type_list_element<Index - 1u, type_list<Other...>>
+{};
+
+
+/**
+ * @brief Provides compile-time indexed access to the types of a type list.
+ * @tparam Type First type provided by the type list.
+ * @tparam Other Other types provided by the type list.
+ */
+template<typename Type, typename... Other>
+struct type_list_element<0u, type_list<Type, Other...>> {
+    /*! @brief Searched type. */
+    using type = Type;
+};
+
+
+/**
+ * @brief Helper type.
+ * @tparam Index Index of the type to return.
+ * @tparam List Type list to search into.
+ */
+template<std::size_t Index, typename List>
+using type_list_element_t = typename type_list_element<Index, List>::type;
+
+
+/**
+ * @brief Concatenates multiple type lists.
+ * @tparam Type Types provided by the first type list.
+ * @tparam Other Types provided by the second type list.
+ * @return A type list composed by the types of both the type lists.
+ */
+template<typename... Type, typename... Other>
+constexpr type_list<Type..., Other...> operator+(type_list<Type...>, type_list<Other...>) { return {}; }
+
+
+/*! @brief Primary template isn't defined on purpose. */
+template<typename...>
+struct type_list_cat;
+
+
+/*! @brief Concatenates multiple type lists. */
+template<>
+struct type_list_cat<> {
+    /*! @brief A type list composed by the types of all the type lists. */
+    using type = type_list<>;
+};
+
+
+/**
+ * @brief Concatenates multiple type lists.
+ * @tparam Type Types provided by the first type list.
+ * @tparam Other Types provided by the second type list.
+ * @tparam List Other type lists, if any.
+ */
+template<typename... Type, typename... Other, typename... List>
+struct type_list_cat<type_list<Type...>, type_list<Other...>, List...> {
+    /*! @brief A type list composed by the types of all the type lists. */
+    using type = typename type_list_cat<type_list<Type..., Other...>, List...>::type;
+};
+
+
+/**
+ * @brief Concatenates multiple type lists.
+ * @tparam Type Types provided by the type list.
+ */
+template<typename... Type>
+struct type_list_cat<type_list<Type...>> {
+    /*! @brief A type list composed by the types of all the type lists. */
+    using type = type_list<Type...>;
+};
+
+
+/**
+ * @brief Helper type.
+ * @tparam List Type lists to concatenate.
+ */
+template<typename... List>
+using type_list_cat_t = typename type_list_cat<List...>::type;
+
+
+/*! @brief Primary template isn't defined on purpose. */
+template<typename>
+struct type_list_unique;
+
+
+/**
+ * @brief Removes duplicates types from a type list.
+ * @tparam Type One of the types provided by the given type list.
+ * @tparam Other The other types provided by the given type list.
+ */
+template<typename Type, typename... Other>
+struct type_list_unique<type_list<Type, Other...>> {
+    /*! @brief A type list without duplicate types. */
+    using type = std::conditional_t<
+        std::disjunction_v<std::is_same<Type, Other>...>,
+        typename type_list_unique<type_list<Other...>>::type,
+        type_list_cat_t<type_list<Type>, typename type_list_unique<type_list<Other...>>::type>
+    >;
+};
+
+
+/*! @brief Removes duplicates types from a type list. */
+template<>
+struct type_list_unique<type_list<>> {
+    /*! @brief A type list without duplicate types. */
+    using type = type_list<>;
+};
+
+
+/**
+ * @brief Helper type.
+ * @tparam Type A type list.
+ */
+template<typename Type>
+using type_list_unique_t = typename type_list_unique<Type>::type;
+
+
+/**
+ * @brief Provides the member constant `value` to true if a type list contains a
+ * given type, false otherwise.
+ * @tparam List Type list.
+ * @tparam Type Type to look for.
+ */
+template<typename List, typename Type>
+struct type_list_contains;
+
+
+/**
+ * @copybrief type_list_contains
+ * @tparam Type Types provided by the type list.
+ * @tparam Other Type to look for.
+ */
+template<typename... Type, typename Other>
+struct type_list_contains<type_list<Type...>, Other>: std::disjunction<std::is_same<Type, Other>...> {};
+
+
+/**
+ * @brief Helper variable template.
+ * @tparam List Type list.
+ * @tparam Type Type to look for.
+ */
+template<class List, typename Type>
+inline constexpr bool type_list_contains_v = type_list_contains<List, Type>::value;
+
+
+/*! @brief Primary template isn't defined on purpose. */
+template<typename...>
+struct type_list_diff;
+
+
+/**
+ * @brief Computes the difference between two type lists.
+ * @tparam Type Types provided by the first type list.
+ * @tparam Other Types provided by the second type list.
+ */
+template<typename... Type, typename... Other>
+struct type_list_diff<type_list<Type...>, type_list<Other...>> {
+    /*! @brief A type list that is the difference between the two type lists. */
+    using type = type_list_cat_t<std::conditional_t<type_list_contains_v<type_list<Other...>, Type>, type_list<>, type_list<Type>>...>;
+};
+
+
+/**
+ * @brief Helper type.
+ * @tparam List Type lists between which to compute the difference.
+ */
+template<typename... List>
+using type_list_diff_t = typename type_list_diff<List...>::type;
+
+
+/**
+ * @brief A class to use to push around lists of constant values, nothing more.
+ * @tparam Value Values provided by the value list.
+ */
+template<auto... Value>
+struct value_list {
+    /*! @brief Value list type. */
+    using type = value_list;
+    /*! @brief Compile-time number of elements in the value list. */
+    static constexpr auto size = sizeof...(Value);
+};
+
+
+/*! @brief Primary template isn't defined on purpose. */
+template<std::size_t, typename>
+struct value_list_element;
+
+
+/**
+ * @brief Provides compile-time indexed access to the values of a value list.
+ * @tparam Index Index of the value to return.
+ * @tparam Value First value provided by the value list.
+ * @tparam Other Other values provided by the value list.
+ */
+template<std::size_t Index, auto Value, auto... Other>
+struct value_list_element<Index, value_list<Value, Other...>>
+    : value_list_element<Index - 1u, value_list<Other...>>
+{};
+
+
+/**
+ * @brief Provides compile-time indexed access to the types of a type list.
+ * @tparam Value First value provided by the value list.
+ * @tparam Other Other values provided by the value list.
+ */
+template<auto Value, auto... Other>
+struct value_list_element<0u, value_list<Value, Other...>> {
+    /*! @brief Searched value. */
+    static constexpr auto value = Value;
+};
+
+
+/**
+ * @brief Helper type.
+ * @tparam Index Index of the value to return.
+ * @tparam List Value list to search into.
+ */
+template<std::size_t Index, typename List>
+inline constexpr auto value_list_element_v = value_list_element<Index, List>::value;
+
+
+/**
+ * @brief Concatenates multiple value lists.
+ * @tparam Value Values provided by the first value list.
+ * @tparam Other Values provided by the second value list.
+ * @return A value list composed by the values of both the value lists.
+ */
+template<auto... Value, auto... Other>
+constexpr value_list<Value..., Other...> operator+(value_list<Value...>, value_list<Other...>) { return {}; }
+
+
+/*! @brief Primary template isn't defined on purpose. */
+template<typename...>
+struct value_list_cat;
+
+
+/*! @brief Concatenates multiple value lists. */
+template<>
+struct value_list_cat<> {
+    /*! @brief A value list composed by the values of all the value lists. */
+    using type = value_list<>;
+};
+
+
+/**
+ * @brief Concatenates multiple value lists.
+ * @tparam Value Values provided by the first value list.
+ * @tparam Other Values provided by the second value list.
+ * @tparam List Other value lists, if any.
+ */
+template<auto... Value, auto... Other, typename... List>
+struct value_list_cat<value_list<Value...>, value_list<Other...>, List...> {
+    /*! @brief A value list composed by the values of all the value lists. */
+    using type = typename value_list_cat<value_list<Value..., Other...>, List...>::type;
+};
+
+
+/**
+ * @brief Concatenates multiple value lists.
+ * @tparam Value Values provided by the value list.
+ */
+template<auto... Value>
+struct value_list_cat<value_list<Value...>> {
+    /*! @brief A value list composed by the values of all the value lists. */
+    using type = value_list<Value...>;
+};
+
+
+/**
+ * @brief Helper type.
+ * @tparam List Value lists to concatenate.
+ */
+template<typename... List>
+using value_list_cat_t = typename value_list_cat<List...>::type;
+
+
+/*! @brief Same as std::is_invocable, but with tuples. */
+template<typename, typename>
+struct is_applicable: std::false_type {};
+
+
+/**
+ * @copybrief is_applicable
+ * @tparam Func A valid function type.
+ * @tparam Tuple Tuple-like type.
+ * @tparam Args The list of arguments to use to probe the function type.
+ */
+template<typename Func, template<typename...> class Tuple, typename... Args>
+struct is_applicable<Func, Tuple<Args...>>: std::is_invocable<Func, Args...> {};
+
+
+/**
+ * @copybrief is_applicable
+ * @tparam Func A valid function type.
+ * @tparam Tuple Tuple-like type.
+ * @tparam Args The list of arguments to use to probe the function type.
+ */
+template<typename Func, template<typename...> class Tuple, typename... Args>
+struct is_applicable<Func, const Tuple<Args...>>: std::is_invocable<Func, Args...> {};
+
+
+/**
+ * @brief Helper variable template.
+ * @tparam Func A valid function type.
+ * @tparam Args The list of arguments to use to probe the function type.
+ */
+template<typename Func, typename Args>
+inline constexpr bool is_applicable_v = is_applicable<Func, Args>::value;
+
+
+/*! @brief Same as std::is_invocable_r, but with tuples for arguments. */
+template<typename, typename, typename>
+struct is_applicable_r: std::false_type {};
+
+
+/**
+ * @copybrief is_applicable_r
+ * @tparam Ret The type to which the return type of the function should be
+ * convertible.
+ * @tparam Func A valid function type.
+ * @tparam Args The list of arguments to use to probe the function type.
+ */
+template<typename Ret, typename Func, typename... Args>
+struct is_applicable_r<Ret, Func, std::tuple<Args...>>: std::is_invocable_r<Ret, Func, Args...> {};
+
+
+/**
+ * @brief Helper variable template.
+ * @tparam Ret The type to which the return type of the function should be
+ * convertible.
+ * @tparam Func A valid function type.
+ * @tparam Args The list of arguments to use to probe the function type.
+ */
+template<typename Ret, typename Func, typename Args>
+inline constexpr bool is_applicable_r_v = is_applicable_r<Ret, Func, Args>::value;
+
+
+/**
+ * @brief Provides the member constant `value` to true if a given type is
+ * complete, false otherwise.
+ * @tparam Type The type to test.
+ */
+template<typename Type, typename = void>
+struct is_complete: std::false_type {};
+
+
+/*! @copydoc is_complete */
+template<typename Type>
+struct is_complete<Type, std::void_t<decltype(sizeof(Type))>>: std::true_type {};
+
+
+/**
+ * @brief Helper variable template.
+ * @tparam Type The type to test.
+ */
+template<typename Type>
+inline constexpr bool is_complete_v = is_complete<Type>::value;
+
+
+/**
+ * @brief Provides the member constant `value` to true if a given type is an
+ * iterator, false otherwise.
+ * @tparam Type The type to test.
+ */
+template<typename Type, typename = void>
+struct is_iterator: std::false_type {};
+
+
+/*! @copydoc is_iterator */
+template<typename Type>
+struct is_iterator<Type, std::void_t<typename std::iterator_traits<Type>::iterator_category>>
+    : std::true_type
+{};
+
+
+/**
+ * @brief Helper variable template.
+ * @tparam Type The type to test.
+ */
+template<typename Type>
+inline constexpr bool is_iterator_v = is_iterator<Type>::value;
+
+
+/**
+ * @brief Provides the member constant `value` to true if a given type is of the
+ * required iterator type, false otherwise.
+ * @tparam Type The type to test.
+ * @tparam It Required iterator type.
+ */
+template<typename Type, typename It, typename = void>
+struct is_iterator_type: std::false_type {};
+
+
+/*! @copydoc is_iterator_type */
+template<typename Type, typename It>
+struct is_iterator_type<Type, It, std::enable_if_t<is_iterator_v<Type> && std::is_same_v<Type, It>>>
+    : std::true_type
+{};
+
+
+/*! @copydoc is_iterator_type */
+template<typename Type, typename It>
+struct is_iterator_type<Type, It, std::enable_if_t<!std::is_same_v<Type, It>, std::void_t<typename It::iterator_type>>>
+    : is_iterator_type<Type, typename It::iterator_type>
+{};
+
+
+/**
+ * @brief Helper variable template.
+ * @tparam Type The type to test.
+ * @tparam It Required iterator type.
+ */
+template<typename Type, typename It>
+inline constexpr bool is_iterator_type_v = is_iterator_type<Type, It>::value;
+
+
+/**
+ * @cond TURN_OFF_DOXYGEN
+ * Internal details not to be documented.
+ */
+
+
+namespace internal {
+
+
+template<typename>
+[[nodiscard]] constexpr bool is_equality_comparable(...) { return false; }
+
+
+template<typename Type>
+[[nodiscard]] constexpr auto is_equality_comparable(choice_t<0>)
+-> decltype(std::declval<Type>() == std::declval<Type>()) { return true; }
+
+
+template<typename Type>
+[[nodiscard]] constexpr auto is_equality_comparable(choice_t<1>)
+-> decltype(std::declval<typename Type::value_type>(), std::declval<Type>() == std::declval<Type>()) {
+    if constexpr(is_iterator_v<Type>) {
+        return true;
+    } else if constexpr(std::is_same_v<typename Type::value_type, Type>) {
+        return is_equality_comparable<Type>(choice<0>);
+    } else {
+        return is_equality_comparable<typename Type::value_type>(choice<2>);
+    }
+}
+
+
+template<typename Type>
+[[nodiscard]] constexpr auto is_equality_comparable(choice_t<2>)
+-> decltype(std::declval<typename Type::mapped_type>(), std::declval<Type>() == std::declval<Type>()) {
+    return is_equality_comparable<typename Type::key_type>(choice<2>) && is_equality_comparable<typename Type::mapped_type>(choice<2>);
+}
+
+
+}
+
+
+/**
+ * Internal details not to be documented.
+ * @endcond
+ */
+
+
+/**
+ * @brief Provides the member constant `value` to true if a given type is
+ * equality comparable, false otherwise.
+ * @tparam Type The type to test.
+ */
+template<typename Type, typename = void>
+struct is_equality_comparable: std::bool_constant<internal::is_equality_comparable<Type>(choice<2>)> {};
+
+
+/**
+ * @brief Helper variable template.
+ * @tparam Type The type to test.
+ */
+template<class Type>
+inline constexpr bool is_equality_comparable_v = is_equality_comparable<Type>::value;
+
+
+/**
+ * @brief Transcribes the constness of a type to another type.
+ * @tparam To The type to which to transcribe the constness.
+ * @tparam From The type from which to transcribe the constness.
+ */
+template<typename To, typename From>
+struct constness_as {
+    /*! @brief The type resulting from the transcription of the constness. */
+    using type = std::remove_const_t<To>;
+};
+
+
+/*! @copydoc constness_as */
+template<typename To, typename From>
+struct constness_as<To, const From> {
+    /*! @brief The type resulting from the transcription of the constness. */
+    using type = std::add_const_t<To>;
+};
+
+
+/**
+ * @brief Alias template to facilitate the transcription of the constness.
+ * @tparam To The type to which to transcribe the constness.
+ * @tparam From The type from which to transcribe the constness.
+ */
+template<typename To, typename From>
+using constness_as_t = typename constness_as<To, From>::type;
+
+
+/**
+ * @brief Extracts the class of a non-static member object or function.
+ * @tparam Member A pointer to a non-static member object or function.
+ */
+template<typename Member>
+class member_class {
+    static_assert(std::is_member_pointer_v<Member>, "Invalid pointer type to non-static member object or function");
+
+    template<typename Class, typename Ret, typename... Args>
+    static Class * clazz(Ret(Class:: *)(Args...));
+
+    template<typename Class, typename Ret, typename... Args>
+    static Class * clazz(Ret(Class:: *)(Args...) const);
+
+    template<typename Class, typename Type>
+    static Class * clazz(Type Class:: *);
+
+public:
+    /*! @brief The class of the given non-static member object or function. */
+    using type = std::remove_pointer_t<decltype(clazz(std::declval<Member>()))>;
+};
+
+
+/**
+ * @brief Helper type.
+ * @tparam Member A pointer to a non-static member object or function.
+ */
+template<typename Member>
+using member_class_t = typename member_class<Member>::type;
+
+
+}
+
+
+#endif
+
+// #include "../config/config.h"
+#ifndef ENTT_CONFIG_CONFIG_H
+#define ENTT_CONFIG_CONFIG_H
+
+
+#if defined(__cpp_exceptions) && !defined(ENTT_NOEXCEPTION)
+#   define ENTT_NOEXCEPT noexcept
+#   define ENTT_THROW throw
+#   define ENTT_TRY try
+#   define ENTT_CATCH catch(...)
+#else
+#   define ENTT_NOEXCEPT
+#   define ENTT_THROW
+#   define ENTT_TRY if(true)
+#   define ENTT_CATCH if(false)
+#endif
+
+
+#if defined(__cpp_lib_launder) && __cpp_lib_launder >= 201606L
+#   include <new>
+#   define ENTT_LAUNDER(expr) std::launder(expr)
+#else
+#   define ENTT_LAUNDER(expr) expr
+#endif
+
+
+#ifndef ENTT_USE_ATOMIC
+#   define ENTT_MAYBE_ATOMIC(Type) Type
+#else
+#   include <atomic>
+#   define ENTT_MAYBE_ATOMIC(Type) std::atomic<Type>
+#endif
+
+
+#ifndef ENTT_ID_TYPE
+#   include <cstdint>
+#   define ENTT_ID_TYPE std::uint32_t
+#endif
+
+
+#ifdef ENTT_SPARSE_PAGE
+	static_assert(ENTT_SPARSE_PAGE && ((ENTT_SPARSE_PAGE & (ENTT_SPARSE_PAGE - 1)) == 0), "ENTT_SPARSE_PAGE must be a power of two");
+#else
+#   define ENTT_SPARSE_PAGE 4096
+#endif
+
+
+#ifdef ENTT_PACKED_PAGE
+static_assert(ENTT_PACKED_PAGE && ((ENTT_PACKED_PAGE & (ENTT_PACKED_PAGE - 1)) == 0), "ENTT_PACKED_PAGE must be a power of two");
+#else
+#   define ENTT_PACKED_PAGE 1024
+#endif
+
+
+#ifdef ENTT_DISABLE_ASSERT
+#   undef ENTT_ASSERT
+#   define ENTT_ASSERT(...) (void(0))
+#elif !defined ENTT_ASSERT
+#   include <cassert>
+#   define ENTT_ASSERT(condition, ...) assert(condition)
+#endif
+
+
+#ifdef ENTT_NO_ETO
+#   include <type_traits>
+#   define ENTT_IGNORE_IF_EMPTY std::false_type
+#else
+#   include <type_traits>
+#   define ENTT_IGNORE_IF_EMPTY std::true_type
+#endif
+
+
+#ifndef ENTT_STANDARD_CPP
+#    if defined __clang__ || defined __GNUC__
+#       define ENTT_PRETTY_FUNCTION __PRETTY_FUNCTION__
+#       define ENTT_PRETTY_FUNCTION_PREFIX '='
+#       define ENTT_PRETTY_FUNCTION_SUFFIX ']'
+#    elif defined _MSC_VER
+#       define ENTT_PRETTY_FUNCTION __FUNCSIG__
+#       define ENTT_PRETTY_FUNCTION_PREFIX '<'
+#       define ENTT_PRETTY_FUNCTION_SUFFIX '>'
+#   endif
+#endif
+
+
+#endif
+
+
+
+namespace entt {
+
+
+/**
+ * @cond TURN_OFF_DOXYGEN
+ * Internal details not to be documented.
+ */
+
+
+namespace internal {
+
+
+template<typename Ret, typename... Args>
+auto function_pointer(Ret(*)(Args...)) -> Ret(*)(Args...);
+
+
+template<typename Ret, typename Type, typename... Args, typename Other>
+auto function_pointer(Ret(*)(Type, Args...), Other &&) -> Ret(*)(Args...);
+
+
+template<typename Class, typename Ret, typename... Args, typename... Other>
+auto function_pointer(Ret(Class:: *)(Args...), Other &&...) -> Ret(*)(Args...);
+
+
+template<typename Class, typename Ret, typename... Args, typename... Other>
+auto function_pointer(Ret(Class:: *)(Args...) const, Other &&...) -> Ret(*)(Args...);
+
+
+template<typename Class, typename Type, typename... Other>
+auto function_pointer(Type Class:: *, Other &&...) -> Type(*)();
+
+
+template<typename... Type>
+using function_pointer_t = decltype(internal::function_pointer(std::declval<Type>()...));
+
+
+template<typename... Class, typename Ret, typename... Args>
+[[nodiscard]] constexpr auto index_sequence_for(Ret(*)(Args...)) {
+    return std::index_sequence_for<Class..., Args...>{};
+}
+
+
+}
+
+
+/**
+ * Internal details not to be documented.
+ * @endcond
+ */
+
+
+/*! @brief Used to wrap a function or a member of a specified type. */
+template<auto>
+struct connect_arg_t {};
+
+
+/*! @brief Constant of type connect_arg_t used to disambiguate calls. */
+template<auto Func>
+inline constexpr connect_arg_t<Func> connect_arg{};
+
+
+/**
+ * @brief Basic delegate implementation.
+ *
+ * Primary template isn't defined on purpose. All the specializations give a
+ * compile-time error unless the template parameter is a function type.
+ */
+template<typename>
+class delegate;
+
+
+/**
+ * @brief Utility class to use to send around functions and members.
+ *
+ * Unmanaged delegate for function pointers and members. Users of this class are
+ * in charge of disconnecting instances before deleting them.
+ *
+ * A delegate can be used as a general purpose invoker without memory overhead
+ * for free functions possibly with payloads and bound or unbound members.
+ *
+ * @tparam Ret Return type of a function type.
+ * @tparam Args Types of arguments of a function type.
+ */
+template<typename Ret, typename... Args>
+class delegate<Ret(Args...)> {
+    template<auto Candidate, std::size_t... Index>
+    [[nodiscard]] auto wrap(std::index_sequence<Index...>) ENTT_NOEXCEPT {
+        return [](const void *, Args... args) -> Ret {
+            [[maybe_unused]] const auto arguments = std::forward_as_tuple(std::forward<Args>(args)...);
+            return static_cast<Ret>(std::invoke(Candidate, std::forward<type_list_element_t<Index, type_list<Args...>>>(std::get<Index>(arguments))...));
+        };
+    }
+
+    template<auto Candidate, typename Type, std::size_t... Index>
+    [[nodiscard]] auto wrap(Type &, std::index_sequence<Index...>) ENTT_NOEXCEPT {
+        return [](const void *payload, Args... args) -> Ret {
+            [[maybe_unused]] const auto arguments = std::forward_as_tuple(std::forward<Args>(args)...);
+            Type *curr = static_cast<Type *>(const_cast<constness_as_t<void, Type> *>(payload));
+            return static_cast<Ret>(std::invoke(Candidate, *curr, std::forward<type_list_element_t<Index, type_list<Args...>>>(std::get<Index>(arguments))...));
+        };
+    }
+
+    template<auto Candidate, typename Type, std::size_t... Index>
+    [[nodiscard]] auto wrap(Type *, std::index_sequence<Index...>) ENTT_NOEXCEPT {
+        return [](const void *payload, Args... args) -> Ret {
+            [[maybe_unused]] const auto arguments = std::forward_as_tuple(std::forward<Args>(args)...);
+            Type *curr = static_cast<Type *>(const_cast<constness_as_t<void, Type> *>(payload));
+            return static_cast<Ret>(std::invoke(Candidate, curr, std::forward<type_list_element_t<Index, type_list<Args...>>>(std::get<Index>(arguments))...));
+        };
+    }
+
+public:
+    /*! @brief Function type of the contained target. */
+    using function_type = Ret(const void *, Args...);
+    /*! @brief Function type of the delegate. */
+    using type = Ret(Args...);
+    /*! @brief Return type of the delegate. */
+    using result_type = Ret;
+
+    /*! @brief Default constructor. */
+    delegate() ENTT_NOEXCEPT
+        : fn{nullptr}, data{nullptr}
+    {}
+
+    /**
+     * @brief Constructs a delegate and connects a free function or an unbound
+     * member.
+     * @tparam Candidate Function or member to connect to the delegate.
+     */
+    template<auto Candidate>
+    delegate(connect_arg_t<Candidate>) ENTT_NOEXCEPT {
+        connect<Candidate>();
+    }
+
+    /**
+     * @brief Constructs a delegate and connects a free function with payload or
+     * a bound member.
+     * @tparam Candidate Function or member to connect to the delegate.
+     * @tparam Type Type of class or type of payload.
+     * @param value_or_instance A valid object that fits the purpose.
+     */
+    template<auto Candidate, typename Type>
+    delegate(connect_arg_t<Candidate>, Type &&value_or_instance) ENTT_NOEXCEPT {
+        connect<Candidate>(std::forward<Type>(value_or_instance));
+    }
+
+    /**
+     * @brief Constructs a delegate and connects an user defined function with
+     * optional payload.
+     * @param function Function to connect to the delegate.
+     * @param payload User defined arbitrary data.
+     */
+    delegate(function_type *function, const void *payload = nullptr) ENTT_NOEXCEPT {
+        connect(function, payload);
+    }
+
+    /**
+     * @brief Connects a free function or an unbound member to a delegate.
+     * @tparam Candidate Function or member to connect to the delegate.
+     */
+    template<auto Candidate>
+    void connect() ENTT_NOEXCEPT {
+        data = nullptr;
+
+        if constexpr(std::is_invocable_r_v<Ret, decltype(Candidate), Args...>) {
+            fn = [](const void *, Args... args) -> Ret {
+                return Ret(std::invoke(Candidate, std::forward<Args>(args)...));
+            };
+        } else if constexpr(std::is_member_pointer_v<decltype(Candidate)>) {
+            fn = wrap<Candidate>(internal::index_sequence_for<type_list_element_t<0, type_list<Args...>>>(internal::function_pointer_t<decltype(Candidate)>{}));
+        } else {
+            fn = wrap<Candidate>(internal::index_sequence_for(internal::function_pointer_t<decltype(Candidate)>{}));
+        }
+    }
+
+    /**
+     * @brief Connects a free function with payload or a bound member to a
+     * delegate.
+     *
+     * The delegate isn't responsible for the connected object or the payload.
+     * Users must always guarantee that the lifetime of the instance overcomes
+     * the one of the delegate.<br/>
+     * When used to connect a free function with payload, its signature must be
+     * such that the instance is the first argument before the ones used to
+     * define the delegate itself.
+     *
+     * @tparam Candidate Function or member to connect to the delegate.
+     * @tparam Type Type of class or type of payload.
+     * @param value_or_instance A valid reference that fits the purpose.
+     */
+    template<auto Candidate, typename Type>
+    void connect(Type &value_or_instance) ENTT_NOEXCEPT {
+        data = &value_or_instance;
+
+        if constexpr(std::is_invocable_r_v<Ret, decltype(Candidate), Type &, Args...>) {
+            fn = [](const void *payload, Args... args) -> Ret {
+                Type *curr = static_cast<Type *>(const_cast<constness_as_t<void, Type> *>(payload));
+                return Ret(std::invoke(Candidate, *curr, std::forward<Args>(args)...));
+            };
+        } else {
+            fn = wrap<Candidate>(value_or_instance, internal::index_sequence_for(internal::function_pointer_t<decltype(Candidate), Type>{}));
+        }
+    }
+
+    /**
+     * @brief Connects a free function with payload or a bound member to a
+     * delegate.
+     *
+     * @sa connect(Type &)
+     *
+     * @tparam Candidate Function or member to connect to the delegate.
+     * @tparam Type Type of class or type of payload.
+     * @param value_or_instance A valid pointer that fits the purpose.
+     */
+    template<auto Candidate, typename Type>
+    void connect(Type *value_or_instance) ENTT_NOEXCEPT {
+        data = value_or_instance;
+
+        if constexpr(std::is_invocable_r_v<Ret, decltype(Candidate), Type *, Args...>) {
+            fn = [](const void *payload, Args... args) -> Ret {
+                Type *curr = static_cast<Type *>(const_cast<constness_as_t<void, Type> *>(payload));
+                return Ret(std::invoke(Candidate, curr, std::forward<Args>(args)...));
+            };
+        } else {
+            fn = wrap<Candidate>(value_or_instance, internal::index_sequence_for(internal::function_pointer_t<decltype(Candidate), Type>{}));
+        }
+    }
+
+    /**
+     * @brief Connects an user defined function with optional payload to a
+     * delegate.
+     *
+     * The delegate isn't responsible for the connected object or the payload.
+     * Users must always guarantee that the lifetime of an instance overcomes
+     * the one of the delegate.<br/>
+     * The payload is returned as the first argument to the target function in
+     * all cases.
+     *
+     * @param function Function to connect to the delegate.
+     * @param payload User defined arbitrary data.
+     */
+    void connect(function_type *function, const void *payload = nullptr) ENTT_NOEXCEPT {
+        fn = function;
+        data = payload;
+    }
+
+    /**
+     * @brief Resets a delegate.
+     *
+     * After a reset, a delegate cannot be invoked anymore.
+     */
+    void reset() ENTT_NOEXCEPT {
+        fn = nullptr;
+        data = nullptr;
+    }
+
+    /**
+     * @brief Returns the instance or the payload linked to a delegate, if any.
+     * @return An opaque pointer to the underlying data.
+     */
+    [[nodiscard]] const void * instance() const ENTT_NOEXCEPT {
+        return data;
+    }
+
+    /**
+     * @brief Triggers a delegate.
+     *
+     * The delegate invokes the underlying function and returns the result.
+     *
+     * @warning
+     * Attempting to trigger an invalid delegate results in undefined
+     * behavior.
+     *
+     * @param args Arguments to use to invoke the underlying function.
+     * @return The value returned by the underlying function.
+     */
+    Ret operator()(Args... args) const {
+        ENTT_ASSERT(static_cast<bool>(*this), "Uninitialized delegate");
+        return fn(data, std::forward<Args>(args)...);
+    }
+
+    /**
+     * @brief Checks whether a delegate actually stores a listener.
+     * @return False if the delegate is empty, true otherwise.
+     */
+    [[nodiscard]] explicit operator bool() const ENTT_NOEXCEPT {
+        // no need to test also data
+        return !(fn == nullptr);
+    }
+
+    /**
+     * @brief Compares the contents of two delegates.
+     * @param other Delegate with which to compare.
+     * @return False if the two contents differ, true otherwise.
+     */
+    [[nodiscard]] bool operator==(const delegate<Ret(Args...)> &other) const ENTT_NOEXCEPT {
+        return fn == other.fn && data == other.data;
+    }
+
+private:
+    function_type *fn;
+    const void *data;
+};
+
+
+/**
+ * @brief Compares the contents of two delegates.
+ * @tparam Ret Return type of a function type.
+ * @tparam Args Types of arguments of a function type.
+ * @param lhs A valid delegate object.
+ * @param rhs A valid delegate object.
+ * @return True if the two contents differ, false otherwise.
+ */
+template<typename Ret, typename... Args>
+[[nodiscard]] bool operator!=(const delegate<Ret(Args...)> &lhs, const delegate<Ret(Args...)> &rhs) ENTT_NOEXCEPT {
+    return !(lhs == rhs);
+}
+
+
+/**
+ * @brief Deduction guide.
+ * @tparam Candidate Function or member to connect to the delegate.
+ */
+template<auto Candidate>
+delegate(connect_arg_t<Candidate>)
+-> delegate<std::remove_pointer_t<internal::function_pointer_t<decltype(Candidate)>>>;
+
+
+/**
+ * @brief Deduction guide.
+ * @tparam Candidate Function or member to connect to the delegate.
+ * @tparam Type Type of class or type of payload.
+ */
+template<auto Candidate, typename Type>
+delegate(connect_arg_t<Candidate>, Type &&)
+-> delegate<std::remove_pointer_t<internal::function_pointer_t<decltype(Candidate), Type>>>;
+
+
+/**
+ * @brief Deduction guide.
+ * @tparam Ret Return type of a function type.
+ * @tparam Args Types of arguments of a function type.
+ */
+template<typename Ret, typename... Args>
+delegate(Ret(*)(const void *, Args...), const void * = nullptr)
+-> delegate<Ret(Args...)>;
+
+
+}
+
+
+#endif
+
+// #include "signal/dispatcher.hpp"
+#ifndef ENTT_SIGNAL_DISPATCHER_HPP
+#define ENTT_SIGNAL_DISPATCHER_HPP
+
+
+#include <cstddef>
+#include <memory>
+#include <type_traits>
+#include <utility>
+#include <vector>
+// #include "../config/config.h"
+
+// #include "../core/fwd.hpp"
+#ifndef ENTT_CORE_FWD_HPP
+#define ENTT_CORE_FWD_HPP
+
+
+#include <type_traits>
+// #include "../config/config.h"
+
+
+
+namespace entt {
+
+
+template<std::size_t Len = sizeof(double[2]), std::size_t = alignof(typename std::aligned_storage_t<Len + !Len>)>
+class basic_any;
+
+
+/*! @brief Alias declaration for type identifiers. */
+using id_type = ENTT_ID_TYPE;
+
+
+/*! @brief Alias declaration for the most common use case. */
+using any = basic_any<>;
+
+
+}
+
+
+#endif
+
+// #include "../core/type_info.hpp"
+#ifndef ENTT_CORE_TYPE_INFO_HPP
+#define ENTT_CORE_TYPE_INFO_HPP
+
+
+#include <string_view>
+#include <type_traits>
+// #include "../config/config.h"
+
+// #include "../core/attribute.h"
+#ifndef ENTT_CORE_ATTRIBUTE_H
+#define ENTT_CORE_ATTRIBUTE_H
+
+
+#ifndef ENTT_EXPORT
+#   if defined _WIN32 || defined __CYGWIN__ || defined _MSC_VER
+#       define ENTT_EXPORT __declspec(dllexport)
+#       define ENTT_IMPORT __declspec(dllimport)
+#       define ENTT_HIDDEN
+#   elif defined __GNUC__ && __GNUC__ >= 4
+#       define ENTT_EXPORT __attribute__((visibility("default")))
+#       define ENTT_IMPORT __attribute__((visibility("default")))
+#       define ENTT_HIDDEN __attribute__((visibility("hidden")))
+#   else /* Unsupported compiler */
+#       define ENTT_EXPORT
+#       define ENTT_IMPORT
+#       define ENTT_HIDDEN
+#   endif
+#endif
+
+
+#ifndef ENTT_API
+#   if defined ENTT_API_EXPORT
+#       define ENTT_API ENTT_EXPORT
+#   elif defined ENTT_API_IMPORT
+#       define ENTT_API ENTT_IMPORT
+#   else /* No API */
+#       define ENTT_API
+#   endif
+#endif
+
+
+#endif
+
+// #include "hashed_string.hpp"
+#ifndef ENTT_CORE_HASHED_STRING_HPP
+#define ENTT_CORE_HASHED_STRING_HPP
+
+
+#include <cstddef>
+#include <cstdint>
+// #include "../config/config.h"
+
+// #include "fwd.hpp"
+
+
+
+namespace entt {
+
+
+/**
+ * @cond TURN_OFF_DOXYGEN
+ * Internal details not to be documented.
+ */
+
+
+namespace internal {
+
+
+template<typename>
+struct fnv1a_traits;
+
+
+template<>
+struct fnv1a_traits<std::uint32_t> {
+    using type = std::uint32_t;
+    static constexpr std::uint32_t offset = 2166136261;
+    static constexpr std::uint32_t prime = 16777619;
+};
+
+
+template<>
+struct fnv1a_traits<std::uint64_t> {
+    using type = std::uint64_t;
+    static constexpr std::uint64_t offset = 14695981039346656037ull;
+    static constexpr std::uint64_t prime = 1099511628211ull;
+};
+
+
+}
+
+
+/**
+ * Internal details not to be documented.
+ * @endcond
+ */
+
+
+/**
+ * @brief Zero overhead unique identifier.
+ *
+ * A hashed string is a compile-time tool that allows users to use
+ * human-readable identifers in the codebase while using their numeric
+ * counterparts at runtime.<br/>
+ * Because of that, a hashed string can also be used in constant expressions if
+ * required.
+ *
+ * @tparam Char Character type.
+ */
+template<typename Char>
+class basic_hashed_string {
+    using traits_type = internal::fnv1a_traits<id_type>;
+
+    struct const_wrapper {
+        // non-explicit constructor on purpose
+        constexpr const_wrapper(const Char *curr) ENTT_NOEXCEPT: str{curr} {}
+        const Char *str;
+    };
+
+    // Fowler–Noll–Vo hash function v. 1a - the good
+    [[nodiscard]] static constexpr id_type helper(const Char *curr) ENTT_NOEXCEPT {
+        auto value = traits_type::offset;
+
+        while(*curr != 0) {
+            value = (value ^ static_cast<traits_type::type>(*(curr++))) * traits_type::prime;
+        }
+
+        return value;
+    }
+
+public:
+    /*! @brief Character type. */
+    using value_type = Char;
+    /*! @brief Unsigned integer type. */
+    using hash_type = id_type;
+
+    /**
+     * @brief Returns directly the numeric representation of a string view.
+     * @param str Human-readable identifer.
+     * @param size Length of the string to hash.
+     * @return The numeric representation of the string.
+     */
+    [[nodiscard]] static constexpr hash_type value(const value_type *str, std::size_t size) ENTT_NOEXCEPT {
+        id_type partial{traits_type::offset};
+        while(size--) { partial = (partial^(str++)[0])*traits_type::prime; }
+        return partial;
+    }
+
+    /**
+     * @brief Returns directly the numeric representation of a string.
+     *
+     * Forcing template resolution avoids implicit conversions. An
+     * human-readable identifier can be anything but a plain, old bunch of
+     * characters.<br/>
+     * Example of use:
+     * @code{.cpp}
+     * const auto value = basic_hashed_string<char>::to_value("my.png");
+     * @endcode
+     *
+     * @tparam N Number of characters of the identifier.
+     * @param str Human-readable identifer.
+     * @return The numeric representation of the string.
+     */
+    template<std::size_t N>
+    [[nodiscard]] static constexpr hash_type value(const value_type (&str)[N]) ENTT_NOEXCEPT {
+        return helper(str);
+    }
+
+    /**
+     * @brief Returns directly the numeric representation of a string.
+     * @param wrapper Helps achieving the purpose by relying on overloading.
+     * @return The numeric representation of the string.
+     */
+    [[nodiscard]] static hash_type value(const_wrapper wrapper) ENTT_NOEXCEPT {
+        return helper(wrapper.str);
+    }
+
+    /*! @brief Constructs an empty hashed string. */
+    constexpr basic_hashed_string() ENTT_NOEXCEPT
+        : str{nullptr}, hash{}
+    {}
+
+    /**
+     * @brief Constructs a hashed string from an array of const characters.
+     *
+     * Forcing template resolution avoids implicit conversions. An
+     * human-readable identifier can be anything but a plain, old bunch of
+     * characters.<br/>
+     * Example of use:
+     * @code{.cpp}
+     * basic_hashed_string<char> hs{"my.png"};
+     * @endcode
+     *
+     * @tparam N Number of characters of the identifier.
+     * @param curr Human-readable identifer.
+     */
+    template<std::size_t N>
+    constexpr basic_hashed_string(const value_type (&curr)[N]) ENTT_NOEXCEPT
+        : str{curr}, hash{helper(curr)}
+    {}
+
+    /**
+     * @brief Explicit constructor on purpose to avoid constructing a hashed
+     * string directly from a `const value_type *`.
+     * @param wrapper Helps achieving the purpose by relying on overloading.
+     */
+    explicit constexpr basic_hashed_string(const_wrapper wrapper) ENTT_NOEXCEPT
+        : str{wrapper.str}, hash{helper(wrapper.str)}
+    {}
+
+    /**
+     * @brief Returns the human-readable representation of a hashed string.
+     * @return The string used to initialize the instance.
+     */
+    [[nodiscard]] constexpr const value_type * data() const ENTT_NOEXCEPT {
+        return str;
+    }
+
+    /**
+     * @brief Returns the numeric representation of a hashed string.
+     * @return The numeric representation of the instance.
+     */
+    [[nodiscard]] constexpr hash_type value() const ENTT_NOEXCEPT {
+        return hash;
+    }
+
+    /*! @copydoc data */
+    [[nodiscard]] constexpr operator const value_type *() const ENTT_NOEXCEPT { return data(); }
+
+    /**
+     * @brief Returns the numeric representation of a hashed string.
+     * @return The numeric representation of the instance.
+     */
+    [[nodiscard]] constexpr operator hash_type() const ENTT_NOEXCEPT { return value(); }
+
+    /**
+     * @brief Compares two hashed strings.
+     * @param other Hashed string with which to compare.
+     * @return True if the two hashed strings are identical, false otherwise.
+     */
+    [[nodiscard]] constexpr bool operator==(const basic_hashed_string &other) const ENTT_NOEXCEPT {
+        return hash == other.hash;
+    }
+
+private:
+    const value_type *str;
+    hash_type hash;
+};
+
+
+/**
+ * @brief Deduction guide.
+ *
+ * It allows to deduce the character type of the hashed string directly from a
+ * human-readable identifer provided to the constructor.
+ *
+ * @tparam Char Character type.
+ * @tparam N Number of characters of the identifier.
+ * @param str Human-readable identifer.
+ */
+template<typename Char, std::size_t N>
+basic_hashed_string(const Char (&str)[N])
+-> basic_hashed_string<Char>;
+
+
+/**
+ * @brief Compares two hashed strings.
+ * @tparam Char Character type.
+ * @param lhs A valid hashed string.
+ * @param rhs A valid hashed string.
+ * @return True if the two hashed strings are identical, false otherwise.
+ */
+template<typename Char>
+[[nodiscard]] constexpr bool operator!=(const basic_hashed_string<Char> &lhs, const basic_hashed_string<Char> &rhs) ENTT_NOEXCEPT {
+    return !(lhs == rhs);
+}
+
+
+/*! @brief Aliases for common character types. */
+using hashed_string = basic_hashed_string<char>;
+
+
+/*! @brief Aliases for common character types. */
+using hashed_wstring = basic_hashed_string<wchar_t>;
+
+
+inline namespace literals {
+
+
+/**
+ * @brief User defined literal for hashed strings.
+ * @param str The literal without its suffix.
+ * @return A properly initialized hashed string.
+ */
+[[nodiscard]] constexpr entt::hashed_string operator"" _hs(const char *str, std::size_t) ENTT_NOEXCEPT {
+    return entt::hashed_string{str};
+}
+
+
+/**
+ * @brief User defined literal for hashed wstrings.
+ * @param str The literal without its suffix.
+ * @return A properly initialized hashed wstring.
+ */
+[[nodiscard]] constexpr entt::hashed_wstring operator"" _hws(const wchar_t *str, std::size_t) ENTT_NOEXCEPT {
+    return entt::hashed_wstring{str};
+}
+
+
+}
+
+
+}
+
+
+#endif
+
+// #include "fwd.hpp"
+
+
+
+namespace entt {
+
+
+/**
+ * @cond TURN_OFF_DOXYGEN
+ * Internal details not to be documented.
+ */
+
+
+namespace internal {
+
+
+struct ENTT_API type_seq final {
+    [[nodiscard]] static id_type next() ENTT_NOEXCEPT {
+        static ENTT_MAYBE_ATOMIC(id_type) value{};
+        return value++;
+    }
+};
+
+
+template<typename Type>
+[[nodiscard]] constexpr auto stripped_type_name() ENTT_NOEXCEPT {
+#if defined ENTT_PRETTY_FUNCTION
+    std::string_view pretty_function{ENTT_PRETTY_FUNCTION};
+    auto first = pretty_function.find_first_not_of(' ', pretty_function.find_first_of(ENTT_PRETTY_FUNCTION_PREFIX)+1);
+    auto value = pretty_function.substr(first, pretty_function.find_last_of(ENTT_PRETTY_FUNCTION_SUFFIX) - first);
+    return value;
+#else
+    return std::string_view{""};
+#endif
+}
+
+
+template<typename Type, auto = stripped_type_name<Type>().find_first_of('.')>
+[[nodiscard]] static constexpr std::string_view type_name(int) ENTT_NOEXCEPT {
+    constexpr auto value = stripped_type_name<Type>();
+    return value;
+}
+
+
+template<typename Type>
+[[nodiscard]] static std::string_view type_name(char) ENTT_NOEXCEPT {
+    static const auto value = stripped_type_name<Type>();
+    return value;
+}
+
+
+template<typename Type, auto = stripped_type_name<Type>().find_first_of('.')>
+[[nodiscard]] static constexpr id_type type_hash(int) ENTT_NOEXCEPT {
+    constexpr auto stripped = stripped_type_name<Type>();
+    constexpr auto value = hashed_string::value(stripped.data(), stripped.size());
+    return value;
+}
+
+
+template<typename Type>
+[[nodiscard]] static id_type type_hash(char) ENTT_NOEXCEPT {
+    static const auto value = [](const auto stripped) {
+        return hashed_string::value(stripped.data(), stripped.size());
+    }(stripped_type_name<Type>());
+    return value;
+}
+
+
+}
+
+
+/**
+ * Internal details not to be documented.
+ * @endcond
+ */
+
+
+/**
+ * @brief Type sequential identifier.
+ * @tparam Type Type for which to generate a sequential identifier.
+ */
+template<typename Type, typename = void>
+struct ENTT_API type_seq final {
+    /**
+     * @brief Returns the sequential identifier of a given type.
+     * @return The sequential identifier of a given type.
+     */
+    [[nodiscard]] static id_type value() ENTT_NOEXCEPT {
+        static const id_type value = internal::type_seq::next();
+        return value;
+    }
+
+    /*! @copydoc value */
+    [[nodiscard]] constexpr operator id_type() const ENTT_NOEXCEPT { return value(); }
+};
+
+
+/**
+ * @brief Type hash.
+ * @tparam Type Type for which to generate a hash value.
+ */
+template<typename Type, typename = void>
+struct type_hash final {
+    /**
+     * @brief Returns the numeric representation of a given type.
+     * @return The numeric representation of the given type.
+     */
+#if defined ENTT_PRETTY_FUNCTION
+    [[nodiscard]] static constexpr id_type value() ENTT_NOEXCEPT {
+        return internal::type_hash<Type>(0);
+#else
+    [[nodiscard]] static constexpr id_type value() ENTT_NOEXCEPT {
+        return type_seq<Type>::value();
+#endif
+    }
+
+    /*! @copydoc value */
+    [[nodiscard]] constexpr operator id_type() const ENTT_NOEXCEPT { return value(); }
+};
+
+
+/**
+ * @brief Type name.
+ * @tparam Type Type for which to generate a name.
+ */
+template<typename Type, typename = void>
+struct type_name final {
+    /**
+     * @brief Returns the name of a given type.
+     * @return The name of the given type.
+     */
+    [[nodiscard]] static constexpr std::string_view value() ENTT_NOEXCEPT {
+        return internal::type_name<Type>(0);
+    }
+
+    /*! @copydoc value */
+    [[nodiscard]] constexpr operator std::string_view() const ENTT_NOEXCEPT { return value(); }
+};
+
+
+/*! @brief Implementation specific information about a type. */
+class type_info final {
+    template<typename>
+    friend type_info type_id() ENTT_NOEXCEPT;
+
+    type_info(id_type seq_v, id_type hash_v, std::string_view name_v) ENTT_NOEXCEPT
+        : seq_value{seq_v},
+          hash_value{hash_v},
+          name_value{name_v}
+    {}
+
+public:
+    /*! @brief Default constructor. */
+    type_info() ENTT_NOEXCEPT
+        : type_info({}, {}, {})
+    {}
+
+    /*! @brief Default copy constructor. */
+    type_info(const type_info &) ENTT_NOEXCEPT = default;
+    /*! @brief Default move constructor. */
+    type_info(type_info &&) ENTT_NOEXCEPT = default;
+
+    /**
+     * @brief Default copy assignment operator.
+     * @return This type info object.
+     */
+    type_info & operator=(const type_info &) ENTT_NOEXCEPT = default;
+
+    /**
+     * @brief Default move assignment operator.
+     * @return This type info object.
+     */
+    type_info & operator=(type_info &&) ENTT_NOEXCEPT = default;
+
+    /**
+     * @brief Checks if a type info object is properly initialized.
+     * @return True if the object is properly initialized, false otherwise.
+     */
+    [[nodiscard]] explicit operator bool() const ENTT_NOEXCEPT {
+        return name_value.data() != nullptr;
+    }
+
+    /**
+     * @brief Type sequential identifier.
+     * @return Type sequential identifier.
+     */
+    [[nodiscard]] id_type seq() const ENTT_NOEXCEPT {
+        return seq_value;
+    }
+
+    /**
+     * @brief Type hash.
+     * @return Type hash.
+     */
+    [[nodiscard]] id_type hash() const ENTT_NOEXCEPT {
+        return hash_value;
+    }
+
+    /**
+     * @brief Type name.
+     * @return Type name.
+     */
+    [[nodiscard]] std::string_view name() const ENTT_NOEXCEPT {
+        return name_value;
+    }
+
+    /**
+     * @brief Compares the contents of two type info objects.
+     * @param other Object with which to compare.
+     * @return False if the two contents differ, true otherwise.
+     */
+    [[nodiscard]] bool operator==(const type_info &other) const ENTT_NOEXCEPT {
+        return hash_value == other.hash_value;
+    }
+
+private:
+    id_type seq_value;
+    id_type hash_value;
+    std::string_view name_value;
+};
+
+
+/**
+ * @brief Compares the contents of two type info objects.
+ * @param lhs A type info object.
+ * @param rhs A type info object.
+ * @return True if the two contents differ, false otherwise.
+ */
+[[nodiscard]] inline bool operator!=(const type_info &lhs, const type_info &rhs) ENTT_NOEXCEPT {
+    return !(lhs == rhs);
+}
+
+
+/**
+ * @brief Returns the type info object for a given type.
+ * @tparam Type Type for which to generate a type info object.
+ * @return The type info object for the given type.
+ */
+template<typename Type>
+[[nodiscard]] type_info type_id() ENTT_NOEXCEPT {
+    return type_info{
+        type_seq<std::remove_cv_t<std::remove_reference_t<Type>>>::value(),
+        type_hash<std::remove_cv_t<std::remove_reference_t<Type>>>::value(),
+        type_name<std::remove_cv_t<std::remove_reference_t<Type>>>::value()
+    };
+}
+
+
+}
+
+
+#endif
 
 // #include "sigh.hpp"
 #ifndef ENTT_SIGNAL_SIGH_HPP
 #define ENTT_SIGNAL_SIGH_HPP
 
 
+#include <vector>
+#include <utility>
+#include <iterator>
 #include <algorithm>
-#include <utility>
-#include <vector>
 #include <functional>
 #include <type_traits>
 // #include "../config/config.h"
 
 // #include "delegate.hpp"
+#ifndef ENTT_SIGNAL_DELEGATE_HPP
+#define ENTT_SIGNAL_DELEGATE_HPP
+
+
+#include <tuple>
+#include <cstddef>
+#include <utility>
+#include <functional>
+#include <type_traits>
+// #include "../core/type_traits.hpp"
+
+// #include "../config/config.h"
+
+
+
+namespace entt {
+
+
+/**
+ * @cond TURN_OFF_DOXYGEN
+ * Internal details not to be documented.
+ */
+
+
+namespace internal {
+
+
+template<typename Ret, typename... Args>
+auto function_pointer(Ret(*)(Args...)) -> Ret(*)(Args...);
+
+
+template<typename Ret, typename Type, typename... Args, typename Other>
+auto function_pointer(Ret(*)(Type, Args...), Other &&) -> Ret(*)(Args...);
+
+
+template<typename Class, typename Ret, typename... Args, typename... Other>
+auto function_pointer(Ret(Class:: *)(Args...), Other &&...) -> Ret(*)(Args...);
+
+
+template<typename Class, typename Ret, typename... Args, typename... Other>
+auto function_pointer(Ret(Class:: *)(Args...) const, Other &&...) -> Ret(*)(Args...);
+
+
+template<typename Class, typename Type, typename... Other>
+auto function_pointer(Type Class:: *, Other &&...) -> Type(*)();
+
+
+template<typename... Type>
+using function_pointer_t = decltype(internal::function_pointer(std::declval<Type>()...));
+
+
+template<typename... Class, typename Ret, typename... Args>
+[[nodiscard]] constexpr auto index_sequence_for(Ret(*)(Args...)) {
+    return std::index_sequence_for<Class..., Args...>{};
+}
+
+
+}
+
+
+/**
+ * Internal details not to be documented.
+ * @endcond
+ */
+
+
+/*! @brief Used to wrap a function or a member of a specified type. */
+template<auto>
+struct connect_arg_t {};
+
+
+/*! @brief Constant of type connect_arg_t used to disambiguate calls. */
+template<auto Func>
+inline constexpr connect_arg_t<Func> connect_arg{};
+
+
+/**
+ * @brief Basic delegate implementation.
+ *
+ * Primary template isn't defined on purpose. All the specializations give a
+ * compile-time error unless the template parameter is a function type.
+ */
+template<typename>
+class delegate;
+
+
+/**
+ * @brief Utility class to use to send around functions and members.
+ *
+ * Unmanaged delegate for function pointers and members. Users of this class are
+ * in charge of disconnecting instances before deleting them.
+ *
+ * A delegate can be used as a general purpose invoker without memory overhead
+ * for free functions possibly with payloads and bound or unbound members.
+ *
+ * @tparam Ret Return type of a function type.
+ * @tparam Args Types of arguments of a function type.
+ */
+template<typename Ret, typename... Args>
+class delegate<Ret(Args...)> {
+    template<auto Candidate, std::size_t... Index>
+    [[nodiscard]] auto wrap(std::index_sequence<Index...>) ENTT_NOEXCEPT {
+        return [](const void *, Args... args) -> Ret {
+            [[maybe_unused]] const auto arguments = std::forward_as_tuple(std::forward<Args>(args)...);
+            return static_cast<Ret>(std::invoke(Candidate, std::forward<type_list_element_t<Index, type_list<Args...>>>(std::get<Index>(arguments))...));
+        };
+    }
+
+    template<auto Candidate, typename Type, std::size_t... Index>
+    [[nodiscard]] auto wrap(Type &, std::index_sequence<Index...>) ENTT_NOEXCEPT {
+        return [](const void *payload, Args... args) -> Ret {
+            [[maybe_unused]] const auto arguments = std::forward_as_tuple(std::forward<Args>(args)...);
+            Type *curr = static_cast<Type *>(const_cast<constness_as_t<void, Type> *>(payload));
+            return static_cast<Ret>(std::invoke(Candidate, *curr, std::forward<type_list_element_t<Index, type_list<Args...>>>(std::get<Index>(arguments))...));
+        };
+    }
+
+    template<auto Candidate, typename Type, std::size_t... Index>
+    [[nodiscard]] auto wrap(Type *, std::index_sequence<Index...>) ENTT_NOEXCEPT {
+        return [](const void *payload, Args... args) -> Ret {
+            [[maybe_unused]] const auto arguments = std::forward_as_tuple(std::forward<Args>(args)...);
+            Type *curr = static_cast<Type *>(const_cast<constness_as_t<void, Type> *>(payload));
+            return static_cast<Ret>(std::invoke(Candidate, curr, std::forward<type_list_element_t<Index, type_list<Args...>>>(std::get<Index>(arguments))...));
+        };
+    }
+
+public:
+    /*! @brief Function type of the contained target. */
+    using function_type = Ret(const void *, Args...);
+    /*! @brief Function type of the delegate. */
+    using type = Ret(Args...);
+    /*! @brief Return type of the delegate. */
+    using result_type = Ret;
+
+    /*! @brief Default constructor. */
+    delegate() ENTT_NOEXCEPT
+        : fn{nullptr}, data{nullptr}
+    {}
+
+    /**
+     * @brief Constructs a delegate and connects a free function or an unbound
+     * member.
+     * @tparam Candidate Function or member to connect to the delegate.
+     */
+    template<auto Candidate>
+    delegate(connect_arg_t<Candidate>) ENTT_NOEXCEPT {
+        connect<Candidate>();
+    }
+
+    /**
+     * @brief Constructs a delegate and connects a free function with payload or
+     * a bound member.
+     * @tparam Candidate Function or member to connect to the delegate.
+     * @tparam Type Type of class or type of payload.
+     * @param value_or_instance A valid object that fits the purpose.
+     */
+    template<auto Candidate, typename Type>
+    delegate(connect_arg_t<Candidate>, Type &&value_or_instance) ENTT_NOEXCEPT {
+        connect<Candidate>(std::forward<Type>(value_or_instance));
+    }
+
+    /**
+     * @brief Constructs a delegate and connects an user defined function with
+     * optional payload.
+     * @param function Function to connect to the delegate.
+     * @param payload User defined arbitrary data.
+     */
+    delegate(function_type *function, const void *payload = nullptr) ENTT_NOEXCEPT {
+        connect(function, payload);
+    }
+
+    /**
+     * @brief Connects a free function or an unbound member to a delegate.
+     * @tparam Candidate Function or member to connect to the delegate.
+     */
+    template<auto Candidate>
+    void connect() ENTT_NOEXCEPT {
+        data = nullptr;
+
+        if constexpr(std::is_invocable_r_v<Ret, decltype(Candidate), Args...>) {
+            fn = [](const void *, Args... args) -> Ret {
+                return Ret(std::invoke(Candidate, std::forward<Args>(args)...));
+            };
+        } else if constexpr(std::is_member_pointer_v<decltype(Candidate)>) {
+            fn = wrap<Candidate>(internal::index_sequence_for<type_list_element_t<0, type_list<Args...>>>(internal::function_pointer_t<decltype(Candidate)>{}));
+        } else {
+            fn = wrap<Candidate>(internal::index_sequence_for(internal::function_pointer_t<decltype(Candidate)>{}));
+        }
+    }
+
+    /**
+     * @brief Connects a free function with payload or a bound member to a
+     * delegate.
+     *
+     * The delegate isn't responsible for the connected object or the payload.
+     * Users must always guarantee that the lifetime of the instance overcomes
+     * the one of the delegate.<br/>
+     * When used to connect a free function with payload, its signature must be
+     * such that the instance is the first argument before the ones used to
+     * define the delegate itself.
+     *
+     * @tparam Candidate Function or member to connect to the delegate.
+     * @tparam Type Type of class or type of payload.
+     * @param value_or_instance A valid reference that fits the purpose.
+     */
+    template<auto Candidate, typename Type>
+    void connect(Type &value_or_instance) ENTT_NOEXCEPT {
+        data = &value_or_instance;
+
+        if constexpr(std::is_invocable_r_v<Ret, decltype(Candidate), Type &, Args...>) {
+            fn = [](const void *payload, Args... args) -> Ret {
+                Type *curr = static_cast<Type *>(const_cast<constness_as_t<void, Type> *>(payload));
+                return Ret(std::invoke(Candidate, *curr, std::forward<Args>(args)...));
+            };
+        } else {
+            fn = wrap<Candidate>(value_or_instance, internal::index_sequence_for(internal::function_pointer_t<decltype(Candidate), Type>{}));
+        }
+    }
+
+    /**
+     * @brief Connects a free function with payload or a bound member to a
+     * delegate.
+     *
+     * @sa connect(Type &)
+     *
+     * @tparam Candidate Function or member to connect to the delegate.
+     * @tparam Type Type of class or type of payload.
+     * @param value_or_instance A valid pointer that fits the purpose.
+     */
+    template<auto Candidate, typename Type>
+    void connect(Type *value_or_instance) ENTT_NOEXCEPT {
+        data = value_or_instance;
+
+        if constexpr(std::is_invocable_r_v<Ret, decltype(Candidate), Type *, Args...>) {
+            fn = [](const void *payload, Args... args) -> Ret {
+                Type *curr = static_cast<Type *>(const_cast<constness_as_t<void, Type> *>(payload));
+                return Ret(std::invoke(Candidate, curr, std::forward<Args>(args)...));
+            };
+        } else {
+            fn = wrap<Candidate>(value_or_instance, internal::index_sequence_for(internal::function_pointer_t<decltype(Candidate), Type>{}));
+        }
+    }
+
+    /**
+     * @brief Connects an user defined function with optional payload to a
+     * delegate.
+     *
+     * The delegate isn't responsible for the connected object or the payload.
+     * Users must always guarantee that the lifetime of an instance overcomes
+     * the one of the delegate.<br/>
+     * The payload is returned as the first argument to the target function in
+     * all cases.
+     *
+     * @param function Function to connect to the delegate.
+     * @param payload User defined arbitrary data.
+     */
+    void connect(function_type *function, const void *payload = nullptr) ENTT_NOEXCEPT {
+        fn = function;
+        data = payload;
+    }
+
+    /**
+     * @brief Resets a delegate.
+     *
+     * After a reset, a delegate cannot be invoked anymore.
+     */
+    void reset() ENTT_NOEXCEPT {
+        fn = nullptr;
+        data = nullptr;
+    }
+
+    /**
+     * @brief Returns the instance or the payload linked to a delegate, if any.
+     * @return An opaque pointer to the underlying data.
+     */
+    [[nodiscard]] const void * instance() const ENTT_NOEXCEPT {
+        return data;
+    }
+
+    /**
+     * @brief Triggers a delegate.
+     *
+     * The delegate invokes the underlying function and returns the result.
+     *
+     * @warning
+     * Attempting to trigger an invalid delegate results in undefined
+     * behavior.
+     *
+     * @param args Arguments to use to invoke the underlying function.
+     * @return The value returned by the underlying function.
+     */
+    Ret operator()(Args... args) const {
+        ENTT_ASSERT(static_cast<bool>(*this), "Uninitialized delegate");
+        return fn(data, std::forward<Args>(args)...);
+    }
+
+    /**
+     * @brief Checks whether a delegate actually stores a listener.
+     * @return False if the delegate is empty, true otherwise.
+     */
+    [[nodiscard]] explicit operator bool() const ENTT_NOEXCEPT {
+        // no need to test also data
+        return !(fn == nullptr);
+    }
+
+    /**
+     * @brief Compares the contents of two delegates.
+     * @param other Delegate with which to compare.
+     * @return False if the two contents differ, true otherwise.
+     */
+    [[nodiscard]] bool operator==(const delegate<Ret(Args...)> &other) const ENTT_NOEXCEPT {
+        return fn == other.fn && data == other.data;
+    }
+
+private:
+    function_type *fn;
+    const void *data;
+};
+
+
+/**
+ * @brief Compares the contents of two delegates.
+ * @tparam Ret Return type of a function type.
+ * @tparam Args Types of arguments of a function type.
+ * @param lhs A valid delegate object.
+ * @param rhs A valid delegate object.
+ * @return True if the two contents differ, false otherwise.
+ */
+template<typename Ret, typename... Args>
+[[nodiscard]] bool operator!=(const delegate<Ret(Args...)> &lhs, const delegate<Ret(Args...)> &rhs) ENTT_NOEXCEPT {
+    return !(lhs == rhs);
+}
+
+
+/**
+ * @brief Deduction guide.
+ * @tparam Candidate Function or member to connect to the delegate.
+ */
+template<auto Candidate>
+delegate(connect_arg_t<Candidate>)
+-> delegate<std::remove_pointer_t<internal::function_pointer_t<decltype(Candidate)>>>;
+
+
+/**
+ * @brief Deduction guide.
+ * @tparam Candidate Function or member to connect to the delegate.
+ * @tparam Type Type of class or type of payload.
+ */
+template<auto Candidate, typename Type>
+delegate(connect_arg_t<Candidate>, Type &&)
+-> delegate<std::remove_pointer_t<internal::function_pointer_t<decltype(Candidate), Type>>>;
+
+
+/**
+ * @brief Deduction guide.
+ * @tparam Ret Return type of a function type.
+ * @tparam Args Types of arguments of a function type.
+ */
+template<typename Ret, typename... Args>
+delegate(Ret(*)(const void *, Args...), const void * = nullptr)
+-> delegate<Ret(Args...)>;
+
+
+}
+
+
+#endif
 
 // #include "fwd.hpp"
 #ifndef ENTT_SIGNAL_FWD_HPP
 #define ENTT_SIGNAL_FWD_HPP
 
 
-// #include "../config/config.h"
-
-
-
-namespace entt {
-
-
-/*! @class delegate */
+namespace entt {
+
+
 template<typename>
 class delegate;
 
-/*! @class sink */
+
+class dispatcher;
+
+
+template<typename>
+class emitter;
+
+
+class connection;
+
+
+struct scoped_connection;
+
+
 template<typename>
 class sink;
 
-/*! @class sigh */
+
 template<typename>
 class sigh;
 
@@ -15364,7 +47017,7 @@
 }
 
 
-#endif // ENTT_SIGNAL_FWD_HPP
+#endif
 
 
 
@@ -15417,9 +47070,9 @@
 
 public:
     /*! @brief Unsigned integer type. */
-    using size_type = typename std::vector<delegate<Ret(Args...)>>::size_type;
+    using size_type = std::size_t;
     /*! @brief Sink type. */
-    using sink_type = entt::sink<Ret(Args...)>;
+    using sink_type = sink<Ret(Args...)>;
 
     /**
      * @brief Instance type when it comes to connecting member functions.
@@ -15432,7 +47085,7 @@
      * @brief Number of listeners connected to the signal.
      * @return Number of listeners currently connected.
      */
-    size_type size() const ENTT_NOEXCEPT {
+    [[nodiscard]] size_type size() const ENTT_NOEXCEPT {
         return calls.size();
     }
 
@@ -15440,7 +47093,7 @@
      * @brief Returns false if at least a listener is connected to the signal.
      * @return True if the signal has no listeners connected, false otherwise.
      */
-    bool empty() const ENTT_NOEXCEPT {
+    [[nodiscard]] bool empty() const ENTT_NOEXCEPT {
         return calls.empty();
     }
 
@@ -15452,8 +47105,8 @@
      * @param args Arguments to use to invoke listeners.
      */
     void publish(Args... args) const {
-        for(auto pos = calls.size(); pos; --pos) {
-            calls[pos-1](args...);
+        for(auto &&call: std::as_const(calls)) {
+            call(args...);
         }
     }
 
@@ -15473,22 +47126,20 @@
      */
     template<typename Func>
     void collect(Func func, Args... args) const {
-        bool stop = false;
-
-        for(auto pos = calls.size(); pos && !stop; --pos) {
+        for(auto &&call: calls) {
             if constexpr(std::is_void_v<Ret>) {
                 if constexpr(std::is_invocable_r_v<bool, Func>) {
-                    calls[pos-1](args...);
-                    stop = func();
+                    call(args...);
+                    if(func()) { break; }
                 } else {
-                    calls[pos-1](args...);
+                    call(args...);
                     func();
                 }
             } else {
                 if constexpr(std::is_invocable_r_v<bool, Func, Ret>) {
-                    stop = func(calls[pos-1](args...));
+                    if(func(call(args...))) { break; }
                 } else {
-                    func(calls[pos-1](args...));
+                    func(call(args...));
                 }
             }
         }
@@ -15516,46 +47167,14 @@
     {}
 
 public:
-    /*! Default constructor. */
+    /*! @brief Default constructor. */
     connection() = default;
 
-    /*! @brief Default copy constructor. */
-    connection(const connection &) = default;
-
-    /**
-     * @brief Default move constructor.
-     * @param other The instance to move from.
-     */
-    connection(connection &&other)
-        : connection{}
-    {
-        std::swap(disconnect, other.disconnect);
-        std::swap(signal, other.signal);
-    }
-
-    /*! @brief Default copy assignment operator. @return This connection. */
-    connection & operator=(const connection &) = default;
-
-    /**
-     * @brief Default move assignment operator.
-     * @param other The instance to move from.
-     * @return This connection.
-     */
-    connection & operator=(connection &&other) {
-        if(this != &other) {
-            auto tmp{std::move(other)};
-            disconnect = tmp.disconnect;
-            signal = tmp.signal;
-        }
-
-        return *this;
-    }
-
     /**
      * @brief Checks whether a connection is properly initialized.
      * @return True if the connection is properly initialized, false otherwise.
      */
-    explicit operator bool() const ENTT_NOEXCEPT {
+    [[nodiscard]] explicit operator bool() const ENTT_NOEXCEPT {
         return static_cast<bool>(disconnect);
     }
 
@@ -15582,27 +47201,24 @@
  * A scoped connection automatically breaks the link between the two objects
  * when it goes out of scope.
  */
-struct scoped_connection: private connection {
-    /*! Default constructor. */
+struct scoped_connection {
+    /*! @brief Default constructor. */
     scoped_connection() = default;
 
     /**
      * @brief Constructs a scoped connection from a basic connection.
-     * @param conn A valid connection object.
-     */
-    scoped_connection(const connection &conn)
-        : connection{conn}
+     * @param other A valid connection object.
+     */
+    scoped_connection(const connection &other)
+        : conn{other}
     {}
 
     /*! @brief Default copy constructor, deleted on purpose. */
     scoped_connection(const scoped_connection &) = delete;
 
-    /*! @brief Default move constructor. */
-    scoped_connection(scoped_connection &&) = default;
-
     /*! @brief Automatically breaks the link on destruction. */
     ~scoped_connection() {
-        connection::release();
+        conn.release();
     }
 
     /**
@@ -15612,33 +47228,30 @@
     scoped_connection & operator=(const scoped_connection &) = delete;
 
     /**
-     * @brief Default move assignment operator.
-     * @return This scoped connection.
-     */
-    scoped_connection & operator=(scoped_connection &&) = default;
-
-    /**
-     * @brief Copies a connection.
-     * @param other The connection object to copy.
+     * @brief Acquires a connection.
+     * @param other The connection object to acquire.
      * @return This scoped connection.
      */
-    scoped_connection & operator=(const connection &other) {
-        static_cast<connection &>(*this) = other;
-        return *this;
-    }
-
-    /**
-     * @brief Moves a connection.
-     * @param other The connection object to move.
-     * @return This scoped connection.
-     */
-    scoped_connection & operator=(connection &&other) {
-        static_cast<connection &>(*this) = std::move(other);
-        return *this;
-    }
-
-    using connection::operator bool;
-    using connection::release;
+    scoped_connection & operator=(connection other) {
+        conn = std::move(other);
+        return *this;
+    }
+
+    /**
+     * @brief Checks whether a scoped connection is properly initialized.
+     * @return True if the connection is properly initialized, false otherwise.
+     */
+    [[nodiscard]] explicit operator bool() const ENTT_NOEXCEPT {
+        return static_cast<bool>(conn);
+    }
+
+    /*! @brief Breaks the connection. */
+    void release() {
+        conn.release();
+    }
+
+private:
+    connection conn;
 };
 
 
@@ -15653,21 +47266,26 @@
  * as private data member without exposing the publish functionality to the
  * users of the class.
  *
+ * @warning
+ * Lifetime of a sink must not overcome that of the signal to which it refers.
+ * In any other case, attempting to use a sink results in undefined behavior.
+ *
  * @tparam Ret Return type of a function type.
  * @tparam Args Types of arguments of a function type.
  */
 template<typename Ret, typename... Args>
 class sink<Ret(Args...)> {
     using signal_type = sigh<Ret(Args...)>;
+    using difference_type = typename std::iterator_traits<typename decltype(signal_type::calls)::iterator>::difference_type;
 
     template<auto Candidate, typename Type>
-    static void release(Type &value_or_instance, void *signal) {
+    static void release(Type value_or_instance, void *signal) {
         sink{*static_cast<signal_type *>(signal)}.disconnect<Candidate>(value_or_instance);
     }
 
-    template<auto Function>
+    template<auto Candidate>
     static void release(void *signal) {
-        sink{*static_cast<signal_type *>(signal)}.disconnect<Function>();
+        sink{*static_cast<signal_type *>(signal)}.disconnect<Candidate>();
     }
 
 public:
@@ -15676,100 +47294,207 @@
      * @param ref A valid reference to a signal object.
      */
     sink(sigh<Ret(Args...)> &ref) ENTT_NOEXCEPT
-        : signal{&ref}
+        : offset{},
+          signal{&ref}
     {}
 
     /**
      * @brief Returns false if at least a listener is connected to the sink.
      * @return True if the sink has no listeners connected, false otherwise.
      */
-    bool empty() const ENTT_NOEXCEPT {
+    [[nodiscard]] bool empty() const ENTT_NOEXCEPT {
         return signal->calls.empty();
     }
 
     /**
-     * @brief Connects a free function to a signal.
-     *
-     * The signal handler performs checks to avoid multiple connections for free
-     * functions.
-     *
+     * @brief Returns a sink that connects before a given free function or an
+     * unbound member.
      * @tparam Function A valid free function pointer.
-     * @return A properly initialized connection object.
+     * @return A properly initialized sink object.
      */
     template<auto Function>
+    [[nodiscard]] sink before() {
+        delegate<Ret(Args...)> call{};
+        call.template connect<Function>();
+
+        const auto &calls = signal->calls;
+        const auto it = std::find(calls.cbegin(), calls.cend(), std::move(call));
+
+        sink other{*this};
+        other.offset = std::distance(it, calls.cend());
+        return other;
+    }
+
+    /**
+     * @brief Returns a sink that connects before a free function with payload
+     * or a bound member.
+     * @tparam Candidate Member or free function to look for.
+     * @tparam Type Type of class or type of payload.
+     * @param value_or_instance A valid object that fits the purpose.
+     * @return A properly initialized sink object.
+     */
+    template<auto Candidate, typename Type>
+    [[nodiscard]] sink before(Type &&value_or_instance) {
+        delegate<Ret(Args...)> call{};
+        call.template connect<Candidate>(value_or_instance);
+
+        const auto &calls = signal->calls;
+        const auto it = std::find(calls.cbegin(), calls.cend(), std::move(call));
+
+        sink other{*this};
+        other.offset = std::distance(it, calls.cend());
+        return other;
+    }
+
+    /**
+     * @brief Returns a sink that connects before a given instance or specific
+     * payload.
+     * @tparam Type Type of class or type of payload.
+     * @param value_or_instance A valid object that fits the purpose.
+     * @return A properly initialized sink object.
+     */
+    template<typename Type>
+    [[nodiscard]] sink before(Type &value_or_instance) {
+        return before(&value_or_instance);
+    }
+
+    /**
+     * @brief Returns a sink that connects before a given instance or specific
+     * payload.
+     * @tparam Type Type of class or type of payload.
+     * @param value_or_instance A valid pointer that fits the purpose.
+     * @return A properly initialized sink object.
+     */
+    template<typename Type>
+    [[nodiscard]] sink before(Type *value_or_instance) {
+        sink other{*this};
+
+        if(value_or_instance) {
+            const auto &calls = signal->calls;
+            const auto it = std::find_if(calls.cbegin(), calls.cend(), [value_or_instance](const auto &delegate) {
+                return delegate.instance() == value_or_instance;
+            });
+
+            other.offset = std::distance(it, calls.cend());
+        }
+
+        return other;
+    }
+
+    /**
+     * @brief Returns a sink that connects before anything else.
+     * @return A properly initialized sink object.
+     */
+    [[nodiscard]] sink before() {
+        sink other{*this};
+        other.offset = signal->calls.size();
+        return other;
+    }
+
+    /**
+     * @brief Connects a free function or an unbound member to a signal.
+     *
+     * The signal handler performs checks to avoid multiple connections for the
+     * same function.
+     *
+     * @tparam Candidate Function or member to connect to the signal.
+     * @return A properly initialized connection object.
+     */
+    template<auto Candidate>
     connection connect() {
-        disconnect<Function>();
+        disconnect<Candidate>();
+
+        delegate<Ret(Args...)> call{};
+        call.template connect<Candidate>();
+        signal->calls.insert(signal->calls.end() - offset, std::move(call));
+
         delegate<void(void *)> conn{};
-        conn.template connect<&release<Function>>();
-        signal->calls.emplace_back(delegate<Ret(Args...)>{connect_arg<Function>});
+        conn.template connect<&release<Candidate>>();
         return { std::move(conn), signal };
     }
 
     /**
-     * @brief Connects a member function or a free function with payload to a
+     * @brief Connects a free function with payload or a bound member to a
      * signal.
      *
      * The signal isn't responsible for the connected object or the payload.
      * Users must always guarantee that the lifetime of the instance overcomes
-     * the one  of the delegate. On the other side, the signal handler performs
+     * the one of the signal. On the other side, the signal handler performs
      * checks to avoid multiple connections for the same function.<br/>
      * When used to connect a free function with payload, its signature must be
      * such that the instance is the first argument before the ones used to
-     * define the delegate itself.
-     *
-     * @tparam Candidate Member or free function to connect to the signal.
+     * define the signal itself.
+     *
+     * @tparam Candidate Function or member to connect to the signal.
      * @tparam Type Type of class or type of payload.
-     * @param value_or_instance A valid reference that fits the purpose.
+     * @param value_or_instance A valid object that fits the purpose.
      * @return A properly initialized connection object.
      */
     template<auto Candidate, typename Type>
-    connection connect(Type &value_or_instance) {
+    connection connect(Type &&value_or_instance) {
         disconnect<Candidate>(value_or_instance);
+
+        delegate<Ret(Args...)> call{};
+        call.template connect<Candidate>(value_or_instance);
+        signal->calls.insert(signal->calls.end() - offset, std::move(call));
+
         delegate<void(void *)> conn{};
-        conn.template connect<&sink::release<Candidate, Type>>(value_or_instance);
-        signal->calls.emplace_back(delegate<Ret(Args...)>{connect_arg<Candidate>, value_or_instance});
+        conn.template connect<&release<Candidate, Type>>(value_or_instance);
         return { std::move(conn), signal };
     }
 
     /**
-     * @brief Disconnects a free function from a signal.
-     * @tparam Function A valid free function pointer.
-     */
-    template<auto Function>
+     * @brief Disconnects a free function or an unbound member from a signal.
+     * @tparam Candidate Function or member to disconnect from the signal.
+     */
+    template<auto Candidate>
     void disconnect() {
         auto &calls = signal->calls;
-        delegate<Ret(Args...)> delegate{};
-        delegate.template connect<Function>();
-        calls.erase(std::remove(calls.begin(), calls.end(), delegate), calls.end());
-    }
-
-    /**
-     * @brief Disconnects a member function or a free function with payload from
-     * a signal.
-     * @tparam Candidate Member or free function to disconnect from the signal.
+        delegate<Ret(Args...)> call{};
+        call.template connect<Candidate>();
+        calls.erase(std::remove(calls.begin(), calls.end(), std::move(call)), calls.end());
+    }
+
+    /**
+     * @brief Disconnects a free function with payload or a bound member from a
+     * signal.
+     * @tparam Candidate Function or member to disconnect from the signal.
      * @tparam Type Type of class or type of payload.
-     * @param value_or_instance A valid reference that fits the purpose.
+     * @param value_or_instance A valid object that fits the purpose.
      */
     template<auto Candidate, typename Type>
-    void disconnect(Type &value_or_instance) {
+    void disconnect(Type &&value_or_instance) {
         auto &calls = signal->calls;
-        delegate<Ret(Args...)> delegate{};
-        delegate.template connect<Candidate>(value_or_instance);
-        calls.erase(std::remove(calls.begin(), calls.end(), delegate), calls.end());
-    }
-
-    /**
-     * @brief Disconnects member functions or free functions based on an
-     * instance or specific payload.
+        delegate<Ret(Args...)> call{};
+        call.template connect<Candidate>(value_or_instance);
+        calls.erase(std::remove(calls.begin(), calls.end(), std::move(call)), calls.end());
+    }
+
+    /**
+     * @brief Disconnects free functions with payload or bound members from a
+     * signal.
      * @tparam Type Type of class or type of payload.
-     * @param value_or_instance A valid reference that fits the purpose.
-     */
-    template<typename Type>
-    void disconnect(const Type &value_or_instance) {
-        auto &calls = signal->calls;
-        calls.erase(std::remove_if(calls.begin(), calls.end(), [&value_or_instance](const auto &delegate) {
-            return delegate.instance() == &value_or_instance;
-        }), calls.end());
+     * @param value_or_instance A valid object that fits the purpose.
+     */
+    template<typename Type>
+    void disconnect(Type &value_or_instance) {
+        disconnect(&value_or_instance);
+    }
+
+    /**
+     * @brief Disconnects free functions with payload or bound members from a
+     * signal.
+     * @tparam Type Type of class or type of payload.
+     * @param value_or_instance A valid object that fits the purpose.
+     */
+    template<typename Type>
+    void disconnect(Type *value_or_instance) {
+        if(value_or_instance) {
+            auto &calls = signal->calls;
+            calls.erase(std::remove_if(calls.begin(), calls.end(), [value_or_instance](const auto &delegate) {
+                return delegate.instance() == value_or_instance;
+            }), calls.end());
+        }
     }
 
     /*! @brief Disconnects all the listeners from a signal. */
@@ -15778,6 +47503,7 @@
     }
 
 private:
+    difference_type offset;
     signal_type *signal;
 };
 
@@ -15792,13 +47518,14 @@
  * @tparam Args Types of arguments of a function type.
  */
 template<typename Ret, typename... Args>
-sink(sigh<Ret(Args...)> &) ENTT_NOEXCEPT -> sink<Ret(Args...)>;
-
-
-}
-
-
-#endif // ENTT_SIGNAL_SIGH_HPP
+sink(sigh<Ret(Args...)> &)
+-> sink<Ret(Args...)>;
+
+
+}
+
+
+#endif
 
 
 
@@ -15812,117 +47539,101 @@
  * events to be published all together once per tick.<br/>
  * Listeners are provided in the form of member functions. For each event of
  * type `Event`, listeners are such that they can be invoked with an argument of
- * type `const Event &`, no matter what the return type is.
- *
- * The types of the instances are `Class &`. Users must guarantee that the
- * lifetimes of the objects overcome the one of the dispatcher itself to avoid
- * crashes.
+ * type `Event &`, no matter what the return type is.
+ *
+ * The dispatcher creates instances of the `sigh` class internally. Refer to the
+ * documentation of the latter for more details.
  */
 class dispatcher {
-    using event_family = family<struct internal_dispatcher_event_family>;
-
-    template<typename Class, typename Event>
-    using instance_type = typename sigh<void(const Event &)>::template instance_type<Class>;
-
-    struct base_wrapper {
-        virtual ~base_wrapper() = default;
+    struct basic_pool {
+        virtual ~basic_pool() = default;
         virtual void publish() = 0;
+        virtual void disconnect(void *) = 0;
+        virtual void clear() ENTT_NOEXCEPT = 0;
     };
 
     template<typename Event>
-    struct signal_wrapper: base_wrapper {
-        using signal_type = sigh<void(const Event &)>;
+    struct pool_handler final: basic_pool {
+        static_assert(std::is_same_v<Event, std::decay_t<Event>>, "Invalid event type");
+
+        using signal_type = sigh<void(Event &)>;
         using sink_type = typename signal_type::sink_type;
 
         void publish() override {
-            for(const auto &event: events[current]) {
-                signal.publish(event);
-            }
-
-            events[current++].clear();
-            current %= std::extent<decltype(events)>::value;
-        }
-
-        sink_type sink() ENTT_NOEXCEPT {
+            const auto length = events.size();
+
+            for(std::size_t pos{}; pos < length; ++pos) {
+                signal.publish(events[pos]);
+            }
+
+            events.erase(events.cbegin(), events.cbegin()+length);
+        }
+
+        void disconnect(void *instance) override {
+            sink().disconnect(instance);
+        }
+
+        void clear() ENTT_NOEXCEPT override {
+            events.clear();
+        }
+
+        [[nodiscard]] sink_type sink() ENTT_NOEXCEPT {
             return entt::sink{signal};
         }
 
         template<typename... Args>
         void trigger(Args &&... args) {
-            signal.publish({ std::forward<Args>(args)... });
+            Event instance{std::forward<Args>(args)...};
+            signal.publish(instance);
         }
 
         template<typename... Args>
         void enqueue(Args &&... args) {
-            events[current].emplace_back(std::forward<Args>(args)...);
+            if constexpr(std::is_aggregate_v<Event>) {
+                events.push_back(Event{std::forward<Args>(args)...});
+            } else {
+                events.emplace_back(std::forward<Args>(args)...);
+            }
         }
 
     private:
         signal_type signal{};
-        std::vector<Event> events[2];
-        int current{};
-    };
-
-    struct wrapper_data {
-        std::unique_ptr<base_wrapper> wrapper;
-        ENTT_ID_TYPE runtime_type;
+        std::vector<Event> events;
     };
 
     template<typename Event>
-    static auto type() ENTT_NOEXCEPT {
-        if constexpr(is_named_type_v<Event>) {
-            return named_type_traits<Event>::value;
-        } else {
-            return event_family::type<Event>;
-        }
-    }
-
-    template<typename Event>
-    signal_wrapper<Event> & assure() {
-        const auto wtype = type<Event>();
-        wrapper_data *wdata = nullptr;
-
-        if constexpr(is_named_type_v<Event>) {
-            const auto it = std::find_if(wrappers.begin(), wrappers.end(), [wtype](const auto &candidate) {
-                return candidate.wrapper && candidate.runtime_type == wtype;
-            });
-
-            wdata = (it == wrappers.cend() ? &wrappers.emplace_back() : &(*it));
-        } else {
-            if(!(wtype < wrappers.size())) {
-                wrappers.resize(wtype+1);
-            }
-
-            wdata = &wrappers[wtype];
-
-            if(wdata->wrapper && wdata->runtime_type != wtype) {
-                wrappers.emplace_back();
-                std::swap(wrappers[wtype], wrappers.back());
-                wdata = &wrappers[wtype];
-            }
-        }
-
-        if(!wdata->wrapper) {
-            wdata->wrapper = std::make_unique<signal_wrapper<Event>>();
-            wdata->runtime_type = wtype;
-        }
-
-        return static_cast<signal_wrapper<Event> &>(*wdata->wrapper);
-    }
-
-public:
-    /*! @brief Type of sink for the given event. */
-    template<typename Event>
-    using sink_type = typename signal_wrapper<Event>::sink_type;
+    [[nodiscard]] pool_handler<Event> & assure() {
+        const auto index = type_seq<Event>::value();
+
+        if(!(index < pools.size())) {
+            pools.resize(std::size_t(index)+1u);
+        }
+
+        if(!pools[index]) {
+            pools[index].reset(new pool_handler<Event>{});
+        }
+
+        return static_cast<pool_handler<Event> &>(*pools[index]);
+    }
+
+public:
+    /*! @brief Default constructor. */
+    dispatcher() = default;
+
+    /*! @brief Default move constructor. */
+    dispatcher(dispatcher &&) = default;
+
+    /*! @brief Default move assignment operator. @return This dispatcher. */
+    dispatcher & operator=(dispatcher &&) = default;
 
     /**
      * @brief Returns a sink object for the given event.
      *
      * A sink is an opaque object used to connect listeners to events.
      *
-     * The function type for a listener is:
-     * @code{.cpp}
-     * void(const Event &);
+     * The function type for a listener is _compatible_ with:
+     * @code{.cpp}
+     * void(Event &);
      * @endcode
      *
      * The order of invocation of the listeners isn't guaranteed.
@@ -15933,7 +47644,7 @@
      * @return A temporary sink object.
      */
     template<typename Event>
-    sink_type<Event> sink() ENTT_NOEXCEPT {
+    [[nodiscard]] auto sink() {
         return assure<Event>().sink();
     }
 
@@ -15996,6 +47707,53 @@
     }
 
     /**
+     * @brief Utility function to disconnect everything related to a given value
+     * or instance from a dispatcher.
+     * @tparam Type Type of class or type of payload.
+     * @param value_or_instance A valid object that fits the purpose.
+     */
+    template<typename Type>
+    void disconnect(Type &value_or_instance) {
+        disconnect(&value_or_instance);
+    }
+
+    /**
+     * @brief Utility function to disconnect everything related to a given value
+     * or instance from a dispatcher.
+     * @tparam Type Type of class or type of payload.
+     * @param value_or_instance A valid object that fits the purpose.
+     */
+    template<typename Type>
+    void disconnect(Type *value_or_instance) {
+        for(auto &&cpool: pools) {
+            if(cpool) {
+                cpool->disconnect(value_or_instance);
+            }
+        }
+    }
+
+    /**
+     * @brief Discards all the events queued so far.
+     *
+     * If no types are provided, the dispatcher will clear all the existing
+     * pools.
+     *
+     * @tparam Event Type of events to discard.
+     */
+    template<typename... Event>
+    void clear() {
+        if constexpr(sizeof...(Event) == 0) {
+            for(auto &&cpool: pools) {
+                if(cpool) {
+                    cpool->clear();
+                }
+            }
+        } else {
+            (assure<Event>().clear(), ...);
+        }
+    }
+
+    /**
      * @brief Delivers all the pending events of the given type.
      *
      * This method is blocking and it doesn't return until all the events are
@@ -16017,42 +47775,41 @@
      * to reduce at a minimum the time spent in the bodies of the listeners.
      */
     void update() const {
-        for(auto pos = wrappers.size(); pos; --pos) {
-            auto &wdata = wrappers[pos-1];
-
-            if(wdata.wrapper) {
-                wdata.wrapper->publish();
-            }
-        }
-    }
-
-private:
-    std::vector<wrapper_data> wrappers;
-};
-
-
-}
-
-
-#endif // ENTT_SIGNAL_DISPATCHER_HPP
+        for(auto pos = pools.size(); pos; --pos) {
+            if(auto &&cpool = pools[pos-1]; cpool) {
+                cpool->publish();
+            }
+        }
+    }
+
+private:
+    std::vector<std::unique_ptr<basic_pool>> pools;
+};
+
+
+}
+
+
+#endif
 
 // #include "signal/emitter.hpp"
 #ifndef ENTT_SIGNAL_EMITTER_HPP
 #define ENTT_SIGNAL_EMITTER_HPP
 
 
-#include <type_traits>
+#include <algorithm>
 #include <functional>
-#include <algorithm>
-#include <utility>
+#include <iterator>
+#include <list>
 #include <memory>
+#include <type_traits>
+#include <utility>
 #include <vector>
-#include <list>
-// #include "../config/config.h"
-
-// #include "../core/family.hpp"
-
-// #include "../core/type_traits.hpp"
+// #include "../config/config.h"
+
+// #include "../core/fwd.hpp"
+
+// #include "../core/type_info.hpp"
 
 
 
@@ -16071,33 +47828,33 @@
  * }
  * @endcode
  *
- * Handlers for the type of events are created internally on the fly. It's not
+ * Pools for the type of events are created internally on the fly. It's not
  * required to specify in advance the full list of accepted types.<br/>
  * Moreover, whenever an event is published, an emitter provides the listeners
- * with a reference to itself along with a const reference to the event.
- * Therefore listeners have an handy way to work with it without incurring in
- * the need of capturing a reference to the emitter.
+ * with a reference to itself along with a reference to the event. Therefore
+ * listeners have an handy way to work with it without incurring in the need of
+ * capturing a reference to the emitter.
  *
  * @tparam Derived Actual type of emitter that extends the class template.
  */
 template<typename Derived>
 class emitter {
-    using handler_family = family<struct internal_emitter_handler_family>;
-
-    struct base_handler {
-        virtual ~base_handler() = default;
+    struct basic_pool {
+        virtual ~basic_pool() = default;
         virtual bool empty() const ENTT_NOEXCEPT = 0;
         virtual void clear() ENTT_NOEXCEPT = 0;
     };
 
     template<typename Event>
-    struct event_handler: base_handler {
-        using listener_type = std::function<void(const Event &, Derived &)>;
+    struct pool_handler final: basic_pool {
+        static_assert(std::is_same_v<Event, std::decay_t<Event>>, "Invalid event type");
+
+        using listener_type = std::function<void(Event &, Derived &)>;
         using element_type = std::pair<bool, listener_type>;
         using container_type = std::list<element_type>;
         using connection_type = typename container_type::iterator;
 
-        bool empty() const ENTT_NOEXCEPT override {
+        [[nodiscard]] bool empty() const ENTT_NOEXCEPT override {
             auto pred = [](auto &&element) { return element.first; };
 
             return std::all_of(once_list.cbegin(), once_list.cend(), pred) &&
@@ -16106,9 +47863,13 @@
 
         void clear() ENTT_NOEXCEPT override {
             if(publishing) {
-                auto func = [](auto &&element) { element.first = true; };
-                std::for_each(once_list.begin(), once_list.end(), func);
-                std::for_each(on_list.begin(), on_list.end(), func);
+                for(auto &&element: once_list) {
+                    element.first = true;
+                }
+
+                for(auto &&element: on_list) {
+                    element.first = true;
+                }
             } else {
                 once_list.clear();
                 on_list.clear();
@@ -16123,7 +47884,7 @@
             return on_list.emplace(on_list.cend(), false, std::move(listener));
         }
 
-        void erase(connection_type conn) ENTT_NOEXCEPT {
+        void erase(connection_type conn) {
             conn->first = true;
 
             if(!publishing) {
@@ -16133,18 +47894,19 @@
             }
         }
 
-        void publish(const Event &event, Derived &ref) {
+        void publish(Event &event, Derived &ref) {
             container_type swap_list;
             once_list.swap(swap_list);
 
-            auto func = [&event, &ref](auto &&element) {
-                return element.first ? void() : element.second(event, ref);
-            };
-
             publishing = true;
 
-            std::for_each(on_list.rbegin(), on_list.rend(), func);
-            std::for_each(swap_list.rbegin(), swap_list.rend(), func);
+            for(auto &&element: on_list) {
+                element.first ? void() : element.second(event, ref);
+            }
+
+            for(auto &&element: swap_list) {
+                element.first ? void() : element.second(event, ref);
+            }
 
             publishing = false;
 
@@ -16157,57 +47919,31 @@
         container_type on_list{};
     };
 
-    struct handler_data {
-        std::unique_ptr<base_handler> handler;
-        ENTT_ID_TYPE runtime_type;
-    };
-
     template<typename Event>
-    static auto type() ENTT_NOEXCEPT {
-        if constexpr(is_named_type_v<Event>) {
-            return named_type_traits<Event>::value;
-        } else {
-            return handler_family::type<Event>;
-        }
+    [[nodiscard]] pool_handler<Event> * assure() {
+        const auto index = type_seq<Event>::value();
+
+        if(!(index < pools.size())) {
+            pools.resize(std::size_t(index)+1u);
+        }
+
+        if(!pools[index]) {
+            pools[index].reset(new pool_handler<Event>{});
+        }
+
+        return static_cast<pool_handler<Event> *>(pools[index].get());
     }
 
     template<typename Event>
-    event_handler<Event> * assure() const ENTT_NOEXCEPT {
-        const auto htype = type<Event>();
-        handler_data *hdata = nullptr;
-
-        if constexpr(is_named_type_v<Event>) {
-            const auto it = std::find_if(handlers.begin(), handlers.end(), [htype](const auto &candidate) {
-                return candidate.handler && candidate.runtime_type == htype;
-            });
-
-            hdata = (it == handlers.cend() ? &handlers.emplace_back() : &(*it));
-        } else {
-            if(!(htype < handlers.size())) {
-                handlers.resize(htype+1);
-            }
-
-            hdata = &handlers[htype];
-
-            if(hdata->handler && hdata->runtime_type != htype) {
-                handlers.emplace_back();
-                std::swap(handlers[htype], handlers.back());
-                hdata = &handlers[htype];
-            }
-        }
-
-        if(!hdata->handler) {
-            hdata->handler = std::make_unique<event_handler<Event>>();
-            hdata->runtime_type = htype;
-        }
-
-        return static_cast<event_handler<Event> *>(hdata->handler.get());
+    [[nodiscard]] const pool_handler<Event> * assure() const {
+        const auto index = type_seq<Event>::value();
+        return (!(index < pools.size()) || !pools[index]) ? nullptr : static_cast<const pool_handler<Event> *>(pools[index].get());
     }
 
 public:
     /** @brief Type of listeners accepted for the given event. */
     template<typename Event>
-    using listener = typename event_handler<Event>::listener_type;
+    using listener = typename pool_handler<Event>::listener_type;
 
     /**
      * @brief Generic connection type for events.
@@ -16219,28 +47955,28 @@
      * @tparam Event Type of event for which the connection is created.
      */
     template<typename Event>
-    struct connection: private event_handler<Event>::connection_type {
+    struct connection: private pool_handler<Event>::connection_type {
         /** @brief Event emitters are friend classes of connections. */
         friend class emitter;
 
         /*! @brief Default constructor. */
-        connection() ENTT_NOEXCEPT = default;
+        connection() = default;
 
         /**
          * @brief Creates a connection that wraps its underlying instance.
          * @param conn A connection object to wrap.
          */
-        connection(typename event_handler<Event>::connection_type conn)
-            : event_handler<Event>::connection_type{std::move(conn)}
+        connection(typename pool_handler<Event>::connection_type conn)
+            : pool_handler<Event>::connection_type{std::move(conn)}
         {}
     };
 
     /*! @brief Default constructor. */
-    emitter() ENTT_NOEXCEPT = default;
+    emitter() = default;
 
     /*! @brief Default destructor. */
-    virtual ~emitter() ENTT_NOEXCEPT {
-        static_assert(std::is_base_of_v<emitter<Derived>, Derived>);
+    virtual ~emitter() {
+        static_assert(std::is_base_of_v<emitter<Derived>, Derived>, "Incorrect use of the class template");
     }
 
     /*! @brief Default move constructor. */
@@ -16262,7 +47998,8 @@
      */
     template<typename Event, typename... Args>
     void publish(Args &&... args) {
-        assure<Event>()->publish({ std::forward<Args>(args)... }, *static_cast<Derived *>(this));
+        Event instance{std::forward<Args>(args)...};
+        assure<Event>()->publish(instance, *static_cast<Derived *>(this));
     }
 
     /**
@@ -16274,7 +48011,7 @@
      * to be used later to disconnect the listener if required.
      *
      * The listener is as a callable object that can be moved and the type of
-     * which is `void(const Event &, Derived &)`.
+     * which is _compatible_ with `void(Event &, Derived &)`.
      *
      * @note
      * Whenever an event is emitted, the emitter provides the listener with a
@@ -16299,7 +48036,7 @@
      * to be used later to disconnect the listener if required.
      *
      * The listener is as a callable object that can be moved and the type of
-     * which is `void(const Event &, Derived &)`.
+     * which is _compatible_ with `void(Event &, Derived &)`.
      *
      * @note
      * Whenever an event is emitted, the emitter provides the listener with a
@@ -16325,7 +48062,7 @@
      * @param conn A valid connection.
      */
     template<typename Event>
-    void erase(connection<Event> conn) ENTT_NOEXCEPT {
+    void erase(connection<Event> conn) {
         assure<Event>()->erase(std::move(conn));
     }
 
@@ -16338,7 +48075,7 @@
      * @tparam Event Type of event to reset.
      */
     template<typename Event>
-    void clear() ENTT_NOEXCEPT {
+    void clear() {
         assure<Event>()->clear();
     }
 
@@ -16349,9 +48086,11 @@
      * results in undefined behavior.
      */
     void clear() ENTT_NOEXCEPT {
-        std::for_each(handlers.begin(), handlers.end(), [](auto &&hdata) {
-            return hdata.handler ? hdata.handler->clear() : void();
-        });
+        for(auto &&cpool: pools) {
+            if(cpool) {
+                cpool->clear();
+            }
+        }
     }
 
     /**
@@ -16360,29 +48099,553 @@
      * @return True if there are no listeners registered, false otherwise.
      */
     template<typename Event>
-    bool empty() const ENTT_NOEXCEPT {
-        return assure<Event>()->empty();
+    [[nodiscard]] bool empty() const {
+        const auto *cpool = assure<Event>();
+        return !cpool || cpool->empty();
     }
 
     /**
      * @brief Checks if there are listeners registered with the event emitter.
      * @return True if there are no listeners registered, false otherwise.
      */
-    bool empty() const ENTT_NOEXCEPT {
-        return std::all_of(handlers.cbegin(), handlers.cend(), [](auto &&hdata) {
-            return !hdata.handler || hdata.handler->empty();
+    [[nodiscard]] bool empty() const ENTT_NOEXCEPT {
+        return std::all_of(pools.cbegin(), pools.cend(), [](auto &&cpool) {
+            return !cpool || cpool->empty();
         });
     }
 
 private:
-    mutable std::vector<handler_data> handlers{};
-};
-
-
-}
-
-
-#endif // ENTT_SIGNAL_EMITTER_HPP
+    std::vector<std::unique_ptr<basic_pool>> pools{};
+};
+
+
+}
+
+
+#endif
 
 // #include "signal/sigh.hpp"
-
+#ifndef ENTT_SIGNAL_SIGH_HPP
+#define ENTT_SIGNAL_SIGH_HPP
+
+
+#include <vector>
+#include <utility>
+#include <iterator>
+#include <algorithm>
+#include <functional>
+#include <type_traits>
+// #include "../config/config.h"
+
+// #include "delegate.hpp"
+
+// #include "fwd.hpp"
+
+
+
+namespace entt {
+
+
+/**
+ * @brief Sink class.
+ *
+ * Primary template isn't defined on purpose. All the specializations give a
+ * compile-time error unless the template parameter is a function type.
+ *
+ * @tparam Function A valid function type.
+ */
+template<typename Function>
+class sink;
+
+
+/**
+ * @brief Unmanaged signal handler.
+ *
+ * Primary template isn't defined on purpose. All the specializations give a
+ * compile-time error unless the template parameter is a function type.
+ *
+ * @tparam Function A valid function type.
+ */
+template<typename Function>
+class sigh;
+
+
+/**
+ * @brief Unmanaged signal handler.
+ *
+ * It works directly with references to classes and pointers to member functions
+ * as well as pointers to free functions. Users of this class are in charge of
+ * disconnecting instances before deleting them.
+ *
+ * This class serves mainly two purposes:
+ *
+ * * Creating signals to use later to notify a bunch of listeners.
+ * * Collecting results from a set of functions like in a voting system.
+ *
+ * @tparam Ret Return type of a function type.
+ * @tparam Args Types of arguments of a function type.
+ */
+template<typename Ret, typename... Args>
+class sigh<Ret(Args...)> {
+    /*! @brief A sink is allowed to modify a signal. */
+    friend class sink<Ret(Args...)>;
+
+public:
+    /*! @brief Unsigned integer type. */
+    using size_type = std::size_t;
+    /*! @brief Sink type. */
+    using sink_type = sink<Ret(Args...)>;
+
+    /**
+     * @brief Instance type when it comes to connecting member functions.
+     * @tparam Class Type of class to which the member function belongs.
+     */
+    template<typename Class>
+    using instance_type = Class *;
+
+    /**
+     * @brief Number of listeners connected to the signal.
+     * @return Number of listeners currently connected.
+     */
+    [[nodiscard]] size_type size() const ENTT_NOEXCEPT {
+        return calls.size();
+    }
+
+    /**
+     * @brief Returns false if at least a listener is connected to the signal.
+     * @return True if the signal has no listeners connected, false otherwise.
+     */
+    [[nodiscard]] bool empty() const ENTT_NOEXCEPT {
+        return calls.empty();
+    }
+
+    /**
+     * @brief Triggers a signal.
+     *
+     * All the listeners are notified. Order isn't guaranteed.
+     *
+     * @param args Arguments to use to invoke listeners.
+     */
+    void publish(Args... args) const {
+        for(auto &&call: std::as_const(calls)) {
+            call(args...);
+        }
+    }
+
+    /**
+     * @brief Collects return values from the listeners.
+     *
+     * The collector must expose a call operator with the following properties:
+     *
+     * * The return type is either `void` or such that it's convertible to
+     *   `bool`. In the second case, a true value will stop the iteration.
+     * * The list of parameters is empty if `Ret` is `void`, otherwise it
+     *   contains a single element such that `Ret` is convertible to it.
+     *
+     * @tparam Func Type of collector to use, if any.
+     * @param func A valid function object.
+     * @param args Arguments to use to invoke listeners.
+     */
+    template<typename Func>
+    void collect(Func func, Args... args) const {
+        for(auto &&call: calls) {
+            if constexpr(std::is_void_v<Ret>) {
+                if constexpr(std::is_invocable_r_v<bool, Func>) {
+                    call(args...);
+                    if(func()) { break; }
+                } else {
+                    call(args...);
+                    func();
+                }
+            } else {
+                if constexpr(std::is_invocable_r_v<bool, Func, Ret>) {
+                    if(func(call(args...))) { break; }
+                } else {
+                    func(call(args...));
+                }
+            }
+        }
+    }
+
+private:
+    std::vector<delegate<Ret(Args...)>> calls;
+};
+
+
+/**
+ * @brief Connection class.
+ *
+ * Opaque object the aim of which is to allow users to release an already
+ * estabilished connection without having to keep a reference to the signal or
+ * the sink that generated it.
+ */
+class connection {
+    /*! @brief A sink is allowed to create connection objects. */
+    template<typename>
+    friend class sink;
+
+    connection(delegate<void(void *)> fn, void *ref)
+        : disconnect{fn}, signal{ref}
+    {}
+
+public:
+    /*! @brief Default constructor. */
+    connection() = default;
+
+    /**
+     * @brief Checks whether a connection is properly initialized.
+     * @return True if the connection is properly initialized, false otherwise.
+     */
+    [[nodiscard]] explicit operator bool() const ENTT_NOEXCEPT {
+        return static_cast<bool>(disconnect);
+    }
+
+    /*! @brief Breaks the connection. */
+    void release() {
+        if(disconnect) {
+            disconnect(signal);
+            disconnect.reset();
+        }
+    }
+
+private:
+    delegate<void(void *)> disconnect;
+    void *signal{};
+};
+
+
+/**
+ * @brief Scoped connection class.
+ *
+ * Opaque object the aim of which is to allow users to release an already
+ * estabilished connection without having to keep a reference to the signal or
+ * the sink that generated it.<br/>
+ * A scoped connection automatically breaks the link between the two objects
+ * when it goes out of scope.
+ */
+struct scoped_connection {
+    /*! @brief Default constructor. */
+    scoped_connection() = default;
+
+    /**
+     * @brief Constructs a scoped connection from a basic connection.
+     * @param other A valid connection object.
+     */
+    scoped_connection(const connection &other)
+        : conn{other}
+    {}
+
+    /*! @brief Default copy constructor, deleted on purpose. */
+    scoped_connection(const scoped_connection &) = delete;
+
+    /*! @brief Automatically breaks the link on destruction. */
+    ~scoped_connection() {
+        conn.release();
+    }
+
+    /**
+     * @brief Default copy assignment operator, deleted on purpose.
+     * @return This scoped connection.
+     */
+    scoped_connection & operator=(const scoped_connection &) = delete;
+
+    /**
+     * @brief Acquires a connection.
+     * @param other The connection object to acquire.
+     * @return This scoped connection.
+     */
+    scoped_connection & operator=(connection other) {
+        conn = std::move(other);
+        return *this;
+    }
+
+    /**
+     * @brief Checks whether a scoped connection is properly initialized.
+     * @return True if the connection is properly initialized, false otherwise.
+     */
+    [[nodiscard]] explicit operator bool() const ENTT_NOEXCEPT {
+        return static_cast<bool>(conn);
+    }
+
+    /*! @brief Breaks the connection. */
+    void release() {
+        conn.release();
+    }
+
+private:
+    connection conn;
+};
+
+
+/**
+ * @brief Sink class.
+ *
+ * A sink is used to connect listeners to signals and to disconnect them.<br/>
+ * The function type for a listener is the one of the signal to which it
+ * belongs.
+ *
+ * The clear separation between a signal and a sink permits to store the former
+ * as private data member without exposing the publish functionality to the
+ * users of the class.
+ *
+ * @warning
+ * Lifetime of a sink must not overcome that of the signal to which it refers.
+ * In any other case, attempting to use a sink results in undefined behavior.
+ *
+ * @tparam Ret Return type of a function type.
+ * @tparam Args Types of arguments of a function type.
+ */
+template<typename Ret, typename... Args>
+class sink<Ret(Args...)> {
+    using signal_type = sigh<Ret(Args...)>;
+    using difference_type = typename std::iterator_traits<typename decltype(signal_type::calls)::iterator>::difference_type;
+
+    template<auto Candidate, typename Type>
+    static void release(Type value_or_instance, void *signal) {
+        sink{*static_cast<signal_type *>(signal)}.disconnect<Candidate>(value_or_instance);
+    }
+
+    template<auto Candidate>
+    static void release(void *signal) {
+        sink{*static_cast<signal_type *>(signal)}.disconnect<Candidate>();
+    }
+
+public:
+    /**
+     * @brief Constructs a sink that is allowed to modify a given signal.
+     * @param ref A valid reference to a signal object.
+     */
+    sink(sigh<Ret(Args...)> &ref) ENTT_NOEXCEPT
+        : offset{},
+          signal{&ref}
+    {}
+
+    /**
+     * @brief Returns false if at least a listener is connected to the sink.
+     * @return True if the sink has no listeners connected, false otherwise.
+     */
+    [[nodiscard]] bool empty() const ENTT_NOEXCEPT {
+        return signal->calls.empty();
+    }
+
+    /**
+     * @brief Returns a sink that connects before a given free function or an
+     * unbound member.
+     * @tparam Function A valid free function pointer.
+     * @return A properly initialized sink object.
+     */
+    template<auto Function>
+    [[nodiscard]] sink before() {
+        delegate<Ret(Args...)> call{};
+        call.template connect<Function>();
+
+        const auto &calls = signal->calls;
+        const auto it = std::find(calls.cbegin(), calls.cend(), std::move(call));
+
+        sink other{*this};
+        other.offset = std::distance(it, calls.cend());
+        return other;
+    }
+
+    /**
+     * @brief Returns a sink that connects before a free function with payload
+     * or a bound member.
+     * @tparam Candidate Member or free function to look for.
+     * @tparam Type Type of class or type of payload.
+     * @param value_or_instance A valid object that fits the purpose.
+     * @return A properly initialized sink object.
+     */
+    template<auto Candidate, typename Type>
+    [[nodiscard]] sink before(Type &&value_or_instance) {
+        delegate<Ret(Args...)> call{};
+        call.template connect<Candidate>(value_or_instance);
+
+        const auto &calls = signal->calls;
+        const auto it = std::find(calls.cbegin(), calls.cend(), std::move(call));
+
+        sink other{*this};
+        other.offset = std::distance(it, calls.cend());
+        return other;
+    }
+
+    /**
+     * @brief Returns a sink that connects before a given instance or specific
+     * payload.
+     * @tparam Type Type of class or type of payload.
+     * @param value_or_instance A valid object that fits the purpose.
+     * @return A properly initialized sink object.
+     */
+    template<typename Type>
+    [[nodiscard]] sink before(Type &value_or_instance) {
+        return before(&value_or_instance);
+    }
+
+    /**
+     * @brief Returns a sink that connects before a given instance or specific
+     * payload.
+     * @tparam Type Type of class or type of payload.
+     * @param value_or_instance A valid pointer that fits the purpose.
+     * @return A properly initialized sink object.
+     */
+    template<typename Type>
+    [[nodiscard]] sink before(Type *value_or_instance) {
+        sink other{*this};
+
+        if(value_or_instance) {
+            const auto &calls = signal->calls;
+            const auto it = std::find_if(calls.cbegin(), calls.cend(), [value_or_instance](const auto &delegate) {
+                return delegate.instance() == value_or_instance;
+            });
+
+            other.offset = std::distance(it, calls.cend());
+        }
+
+        return other;
+    }
+
+    /**
+     * @brief Returns a sink that connects before anything else.
+     * @return A properly initialized sink object.
+     */
+    [[nodiscard]] sink before() {
+        sink other{*this};
+        other.offset = signal->calls.size();
+        return other;
+    }
+
+    /**
+     * @brief Connects a free function or an unbound member to a signal.
+     *
+     * The signal handler performs checks to avoid multiple connections for the
+     * same function.
+     *
+     * @tparam Candidate Function or member to connect to the signal.
+     * @return A properly initialized connection object.
+     */
+    template<auto Candidate>
+    connection connect() {
+        disconnect<Candidate>();
+
+        delegate<Ret(Args...)> call{};
+        call.template connect<Candidate>();
+        signal->calls.insert(signal->calls.end() - offset, std::move(call));
+
+        delegate<void(void *)> conn{};
+        conn.template connect<&release<Candidate>>();
+        return { std::move(conn), signal };
+    }
+
+    /**
+     * @brief Connects a free function with payload or a bound member to a
+     * signal.
+     *
+     * The signal isn't responsible for the connected object or the payload.
+     * Users must always guarantee that the lifetime of the instance overcomes
+     * the one of the signal. On the other side, the signal handler performs
+     * checks to avoid multiple connections for the same function.<br/>
+     * When used to connect a free function with payload, its signature must be
+     * such that the instance is the first argument before the ones used to
+     * define the signal itself.
+     *
+     * @tparam Candidate Function or member to connect to the signal.
+     * @tparam Type Type of class or type of payload.
+     * @param value_or_instance A valid object that fits the purpose.
+     * @return A properly initialized connection object.
+     */
+    template<auto Candidate, typename Type>
+    connection connect(Type &&value_or_instance) {
+        disconnect<Candidate>(value_or_instance);
+
+        delegate<Ret(Args...)> call{};
+        call.template connect<Candidate>(value_or_instance);
+        signal->calls.insert(signal->calls.end() - offset, std::move(call));
+
+        delegate<void(void *)> conn{};
+        conn.template connect<&release<Candidate, Type>>(value_or_instance);
+        return { std::move(conn), signal };
+    }
+
+    /**
+     * @brief Disconnects a free function or an unbound member from a signal.
+     * @tparam Candidate Function or member to disconnect from the signal.
+     */
+    template<auto Candidate>
+    void disconnect() {
+        auto &calls = signal->calls;
+        delegate<Ret(Args...)> call{};
+        call.template connect<Candidate>();
+        calls.erase(std::remove(calls.begin(), calls.end(), std::move(call)), calls.end());
+    }
+
+    /**
+     * @brief Disconnects a free function with payload or a bound member from a
+     * signal.
+     * @tparam Candidate Function or member to disconnect from the signal.
+     * @tparam Type Type of class or type of payload.
+     * @param value_or_instance A valid object that fits the purpose.
+     */
+    template<auto Candidate, typename Type>
+    void disconnect(Type &&value_or_instance) {
+        auto &calls = signal->calls;
+        delegate<Ret(Args...)> call{};
+        call.template connect<Candidate>(value_or_instance);
+        calls.erase(std::remove(calls.begin(), calls.end(), std::move(call)), calls.end());
+    }
+
+    /**
+     * @brief Disconnects free functions with payload or bound members from a
+     * signal.
+     * @tparam Type Type of class or type of payload.
+     * @param value_or_instance A valid object that fits the purpose.
+     */
+    template<typename Type>
+    void disconnect(Type &value_or_instance) {
+        disconnect(&value_or_instance);
+    }
+
+    /**
+     * @brief Disconnects free functions with payload or bound members from a
+     * signal.
+     * @tparam Type Type of class or type of payload.
+     * @param value_or_instance A valid object that fits the purpose.
+     */
+    template<typename Type>
+    void disconnect(Type *value_or_instance) {
+        if(value_or_instance) {
+            auto &calls = signal->calls;
+            calls.erase(std::remove_if(calls.begin(), calls.end(), [value_or_instance](const auto &delegate) {
+                return delegate.instance() == value_or_instance;
+            }), calls.end());
+        }
+    }
+
+    /*! @brief Disconnects all the listeners from a signal. */
+    void disconnect() {
+        signal->calls.clear();
+    }
+
+private:
+    difference_type offset;
+    signal_type *signal;
+};
+
+
+/**
+ * @brief Deduction guide.
+ *
+ * It allows to deduce the function type of a sink directly from the signal it
+ * refers to.
+ *
+ * @tparam Ret Return type of a function type.
+ * @tparam Args Types of arguments of a function type.
+ */
+template<typename Ret, typename... Args>
+sink(sigh<Ret(Args...)> &)
+-> sink<Ret(Args...)>;
+
+
+}
+
+
+#endif
+