--- a/cpp/json_util/json_util.hpp	Mon Mar 26 13:26:31 2018 +0200
+++ b/cpp/json_util/json_util.hpp	Mon Mar 26 13:56:05 2018 +0200
@@ -302,15 +302,15 @@
      * \param def the default value if property is undefined
      * \return the value, boost::none or def
      */
-    template <typename Type>
-    inline boost::optional<Type> optional(const std::string& key, Type&& def) const noexcept
+    template <typename Type, typename DefaultValue>
+    inline boost::optional<Type> optional(const std::string& key, DefaultValue&& def) const noexcept
     {
         static_assert(parser_type_traits<Type>::value, "type not supported");
 
         const auto it = json_.find(key);
 
         if (it == json_.end())
-            return def;
+            return boost::optional<Type>(std::forward<DefaultValue>(def));
 
         return parser_type_traits<Type>::get(*it);
     }