Merge d1f0f595f3 into ea528bbfda

include/nlohmann/detail/meta/cpp_future.hpp

@@ -138,7 +138,7 @@ using index_sequence_for = make_index_sequence<sizeof...(Ts)>;
 
 // dispatch utility (taken from ranges-v3)
 template<unsigned N> struct priority_tag : priority_tag < N - 1 > {};
-template<> struct priority_tag<0> {};
+template<> struct priority_tag<-1> {};
 
 // taken from ranges-v3
 template<typename T>

include/nlohmann/detail/meta/type_traits.hpp

@@ -145,6 +145,26 @@ struct has_to_json < BasicJsonType, T, enable_if_t < !is_basic_json<T>::value >>
         T>::value;
 };
 
+template<typename Json, typename T>
+struct has_to_json_member {
+private:
+    template<typename U>
+    static auto check(int) -> decltype(std::declval<U>().to_json(std::declval<Json&>()), std::true_type());
+    template<typename U>
+    static std::false_type check(...);
+public:
+    static constexpr bool value = std::is_same_v<decltype(check<T>(0)),std::true_type>;
+};
+template<typename Json, typename T>
+struct has_from_json_member {
+private:
+    template<typename U>
+    static auto check(int) -> decltype(std::declval<U>().from_json(std::declval<const Json&>()), std::true_type());
+    template<typename U>
+    static std::false_type check(...);
+public:
+    static constexpr bool value = std::is_same_v<decltype(check<T>(0)),std::true_type>;
+};
 
 ///////////////////
 // is_ functions //
@@ -274,6 +294,7 @@ struct is_constructible_object_type_impl <
           typename ConstructibleObjectType::mapped_type >::value)) ||
         (has_from_json<BasicJsonType,
          typename ConstructibleObjectType::mapped_type>::value ||
+         has_from_json_member<BasicJsonType, typename ConstructibleObjectType::mapped_type>::value ||
          has_non_default_from_json <
          BasicJsonType,
          typename ConstructibleObjectType::mapped_type >::value);
@@ -379,6 +400,7 @@ detected_t<value_type_t, ConstructibleArrayType >>::value >>
          typename BasicJsonType::array_t::value_type>::value ||
          has_from_json<BasicJsonType,
          typename ConstructibleArrayType::value_type>::value ||
+         has_from_json_member<BasicJsonType, typename ConstructibleArrayType::value_type>::value ||
          has_non_default_from_json <
          BasicJsonType, typename ConstructibleArrayType::value_type >::value);
 };

include/nlohmann/json.hpp

@@ -1655,6 +1655,13 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
         assert_invariant();
     }
 
+    template<typename T, detail::enable_if_t<detail::has_to_json_member<basic_json, T>::value, int> = 0>
+    basic_json(const T& val) {
+        val.to_json(*this);
+        set_parents();
+        assert_invariant();
+    }
+
     /*!
     @brief create a container (array or object) from an initializer list
 
@@ -3114,6 +3121,13 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
         return JSONSerializer<ValueType>::from_json(*this);
     }
 
+    template<typename ValueType, detail::enable_if_t<detail::has_from_json_member<basic_json_t, ValueType>::value, int> = 0>
+    ValueType get_impl(detail::priority_tag<-1>)const{
+        ValueType v{};
+        v.from_json(*this);
+        return v;
+        }
+
     /*!
     @brief get special-case overload
 
@@ -3296,6 +3310,15 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
         return v;
     }
 
+    template<typename ValueType, detail::enable_if_t<
+        !detail::is_basic_json<ValueType>::value &&
+        detail::has_from_json_member<basic_json_t, ValueType>::value, int> = 0>
+    ValueType& get_to(ValueType& v) const
+    {
+        v.from_json(*this);
+        return v;
+    }
+
     // specialization to allow to call get_to with a basic_json value
     // see https://github.com/nlohmann/json/issues/2175
     template<typename ValueType,