Add support for deserialization of STL containers of non-default constructable types (fixes #2574).

include/nlohmann/adl_serializer.hpp

@@ -1,14 +1,17 @@
 #pragma once
 
+#include <type_traits>
 #include <utility>
 
 #include <nlohmann/detail/conversions/from_json.hpp>
 #include <nlohmann/detail/conversions/to_json.hpp>
+#include <nlohmann/detail/meta/identity_tag.hpp>
+#include <nlohmann/detail/meta/type_traits.hpp>
 
 namespace nlohmann
 {
 
-template<typename, typename>
+template<typename ValueType, typename>
 struct adl_serializer
 {
     /*!
@@ -17,17 +20,39 @@ struct adl_serializer
     This function is usually called by the `get()` function of the
     @ref basic_json class (either explicit or via conversion operators).
 
+    @note This function is chosen for default-constructible value types.
+
     @param[in] j        JSON value to read from
     @param[in,out] val  value to write to
     */
-    template<typename BasicJsonType, typename ValueType>
-    static auto from_json(BasicJsonType&& j, ValueType& val) noexcept(
+    template<typename BasicJsonType, typename TargetType = ValueType>
+    static auto from_json(BasicJsonType && j, TargetType& val) noexcept(
         noexcept(::nlohmann::from_json(std::forward<BasicJsonType>(j), val)))
     -> decltype(::nlohmann::from_json(std::forward<BasicJsonType>(j), val), void())
     {
         ::nlohmann::from_json(std::forward<BasicJsonType>(j), val);
     }
 
+    /*!
+    @brief convert a JSON value to any value type
+
+    This function is usually called by the `get()` function of the
+    @ref basic_json class (either explicit or via conversion operators).
+
+    @note This function is chosen for value types which are not default-constructible.
+
+    @param[in] j  JSON value to read from
+
+    @return copy of the JSON value, converted to @a ValueType
+    */
+    template<typename BasicJsonType, typename TargetType = ValueType>
+    static auto from_json(BasicJsonType && j) noexcept(
+    noexcept(::nlohmann::from_json(std::forward<BasicJsonType>(j), detail::identity_tag<TargetType> {})))
+    -> decltype(::nlohmann::from_json(std::forward<BasicJsonType>(j), detail::identity_tag<TargetType> {}))
+    {
+        return ::nlohmann::from_json(std::forward<BasicJsonType>(j), detail::identity_tag<TargetType> {});
+    }
+
     /*!
     @brief convert any value type to a JSON value
 
@@ -37,13 +62,12 @@ struct adl_serializer
     @param[in,out] j  JSON value to write to
     @param[in] val    value to read from
     */
-    template<typename BasicJsonType, typename ValueType>
-    static auto to_json(BasicJsonType& j, ValueType&& val) noexcept(
-        noexcept(::nlohmann::to_json(j, std::forward<ValueType>(val))))
-    -> decltype(::nlohmann::to_json(j, std::forward<ValueType>(val)), void())
+    template<typename BasicJsonType, typename TargetType = ValueType>
+    static auto to_json(BasicJsonType& j, TargetType && val) noexcept(
+        noexcept(::nlohmann::to_json(j, std::forward<TargetType>(val))))
+    -> decltype(::nlohmann::to_json(j, std::forward<TargetType>(val)), void())
     {
-        ::nlohmann::to_json(j, std::forward<ValueType>(val));
+        ::nlohmann::to_json(j, std::forward<TargetType>(val));
     }
 };
-
 }  // namespace nlohmann

include/nlohmann/detail/conversions/from_json.hpp

@@ -15,6 +15,7 @@
 #include <nlohmann/detail/exceptions.hpp>
 #include <nlohmann/detail/macro_scope.hpp>
 #include <nlohmann/detail/meta/cpp_future.hpp>
+#include <nlohmann/detail/meta/identity_tag.hpp>
 #include <nlohmann/detail/meta/type_traits.hpp>
 #include <nlohmann/detail/value_t.hpp>
 
@@ -187,7 +188,10 @@ auto from_json_array_impl(const BasicJsonType& j, std::array<T, N>& arr,
     }
 }
 
-template<typename BasicJsonType, typename ConstructibleArrayType>
+template<typename BasicJsonType, typename ConstructibleArrayType,
+         enable_if_t<
+             std::is_assignable<ConstructibleArrayType&, ConstructibleArrayType>::value,
+             int> = 0>
 auto from_json_array_impl(const BasicJsonType& j, ConstructibleArrayType& arr, priority_tag<1> /*unused*/)
 -> decltype(
     arr.reserve(std::declval<typename ConstructibleArrayType::size_type>()),
@@ -208,7 +212,10 @@ auto from_json_array_impl(const BasicJsonType& j, ConstructibleArrayType& arr, p
     arr = std::move(ret);
 }
 
-template<typename BasicJsonType, typename ConstructibleArrayType>
+template<typename BasicJsonType, typename ConstructibleArrayType,
+         enable_if_t<
+             std::is_assignable<ConstructibleArrayType&, ConstructibleArrayType>::value,
+             int> = 0>
 void from_json_array_impl(const BasicJsonType& j, ConstructibleArrayType& arr,
                           priority_tag<0> /*unused*/)
 {
@@ -247,6 +254,25 @@ void())
     from_json_array_impl(j, arr, priority_tag<3> {});
 }
 
+template < typename BasicJsonType, typename T, std::size_t... Idx >
+std::array<T, sizeof...(Idx)> from_json_inplace_array_impl(BasicJsonType&& j,
+        identity_tag<std::array<T, sizeof...(Idx)>> /*unused*/, index_sequence<Idx...> /*unused*/)
+{
+    return { { std::forward<BasicJsonType>(j).at(Idx).template get<T>()... } };
+}
+
+template < typename BasicJsonType, typename T, std::size_t N >
+auto from_json(BasicJsonType&& j, identity_tag<std::array<T, N>> tag)
+-> decltype(from_json_inplace_array_impl(std::forward<BasicJsonType>(j), tag, make_index_sequence<N> {}))
+{
+    if (JSON_HEDLEY_UNLIKELY(!j.is_array()))
+    {
+        JSON_THROW(type_error::create(302, "type must be array, but is " + std::string(j.type_name()), j));
+    }
+
+    return from_json_inplace_array_impl(std::forward<BasicJsonType>(j), tag, make_index_sequence<N> {});
+}
+
 template<typename BasicJsonType>
 void from_json(const BasicJsonType& j, typename BasicJsonType::binary_t& bin)
 {
@@ -322,22 +348,47 @@ void from_json(const BasicJsonType& j, ArithmeticType& val)
     }
 }
 
+template<typename BasicJsonType, typename... Args, std::size_t... Idx>
+std::tuple<Args...> from_json_tuple_impl_base(BasicJsonType&& j, index_sequence<Idx...> /*unused*/)
+{
+    return std::make_tuple(std::forward<BasicJsonType>(j).at(Idx).template get<Args>()...);
+}
+
+template < typename BasicJsonType, class A1, class A2 >
+std::pair<A1, A2> from_json_tuple_impl(BasicJsonType&& j, identity_tag<std::pair<A1, A2>> /*unused*/, priority_tag<0> /*unused*/)
+{
+    return {std::forward<BasicJsonType>(j).at(0).template get<A1>(),
+            std::forward<BasicJsonType>(j).at(1).template get<A2>()};
+}
+
 template<typename BasicJsonType, typename A1, typename A2>
-void from_json(const BasicJsonType& j, std::pair<A1, A2>& p)
+void from_json_tuple_impl(BasicJsonType&& j, std::pair<A1, A2>& p, priority_tag<1> /*unused*/)
 {
-    p = {j.at(0).template get<A1>(), j.at(1).template get<A2>()};
+    p = from_json_tuple_impl(std::forward<BasicJsonType>(j), identity_tag<std::pair<A1, A2>> {}, priority_tag<0> {});
 }
 
-template<typename BasicJsonType, typename Tuple, std::size_t... Idx>
-void from_json_tuple_impl(const BasicJsonType& j, Tuple& t, index_sequence<Idx...> /*unused*/)
+template<typename BasicJsonType, typename... Args>
+std::tuple<Args...> from_json_tuple_impl(BasicJsonType&& j, identity_tag<std::tuple<Args...>> /*unused*/, priority_tag<2> /*unused*/)
 {
-    t = std::make_tuple(j.at(Idx).template get<typename std::tuple_element<Idx, Tuple>::type>()...);
+    return from_json_tuple_impl_base<BasicJsonType, Args...>(std::forward<BasicJsonType>(j), index_sequence_for<Args...> {});
 }
 
 template<typename BasicJsonType, typename... Args>
-void from_json(const BasicJsonType& j, std::tuple<Args...>& t)
+void from_json_tuple_impl(BasicJsonType&& j, std::tuple<Args...>& t, priority_tag<3> /*unused*/)
 {
-    from_json_tuple_impl(j, t, index_sequence_for<Args...> {});
+    t = from_json_tuple_impl_base<BasicJsonType, Args...>(std::forward<BasicJsonType>(j), index_sequence_for<Args...> {});
+}
+
+template<typename BasicJsonType, typename TupleRelated>
+auto from_json(BasicJsonType&& j, TupleRelated&& t)
+-> decltype(from_json_tuple_impl(std::forward<BasicJsonType>(j), std::forward<TupleRelated>(t), priority_tag<3> {}))
+{
+    if (JSON_HEDLEY_UNLIKELY(!j.is_array()))
+    {
+        JSON_THROW(type_error::create(302, "type must be array, but is " + std::string(j.type_name()), j));
+    }
+
+    return from_json_tuple_impl(std::forward<BasicJsonType>(j), std::forward<TupleRelated>(t), priority_tag<3> {});
 }
 
 template < typename BasicJsonType, typename Key, typename Value, typename Compare, typename Allocator,
@@ -383,11 +434,11 @@ void from_json(const BasicJsonType& j, std::unordered_map<Key, Value, Hash, KeyE
 struct from_json_fn
 {
     template<typename BasicJsonType, typename T>
-    auto operator()(const BasicJsonType& j, T& val) const
-    noexcept(noexcept(from_json(j, val)))
-    -> decltype(from_json(j, val), void())
+    auto operator()(const BasicJsonType& j, T&& val) const
+    noexcept(noexcept(from_json(j, std::forward<T>(val))))
+    -> decltype(from_json(j, std::forward<T>(val)))
     {
-        return from_json(j, val);
+        return from_json(j, std::forward<T>(val));
     }
 };
 }  // namespace detail

include/nlohmann/detail/meta/identity_tag.hpp

@@ -0,0 +1,10 @@
+#pragma once
+
+namespace nlohmann
+{
+namespace detail
+{
+// dispatching helper struct
+template <class T> struct identity_tag {};
+}  // namespace detail
+}  // namespace nlohmann

include/nlohmann/detail/meta/type_traits.hpp

@@ -106,8 +106,7 @@ struct is_getable
 };
 
 template<typename BasicJsonType, typename T>
-struct has_from_json < BasicJsonType, T,
-           enable_if_t < !is_basic_json<T>::value >>
+struct has_from_json < BasicJsonType, T, enable_if_t < !is_basic_json<T>::value >>
 {
     using serializer = typename BasicJsonType::template json_serializer<T, void>;
 
@@ -151,6 +150,52 @@ struct has_to_json < BasicJsonType, T, enable_if_t < !is_basic_json<T>::value >>
 // is_ functions //
 ///////////////////
 
+// https://en.cppreference.com/w/cpp/types/conjunction
+template<class...> struct conjunction : std::true_type { };
+template<class B1> struct conjunction<B1> : B1 { };
+template<class B1, class... Bn>
+struct conjunction<B1, Bn...>
+: std::conditional<bool(B1::value), conjunction<Bn...>, B1>::type {};
+
+// Reimplementation of is_constructible and is_default_constructible, due to them being broken for
+// std::pair and std::tuple until LWG 2367 fix (see https://cplusplus.github.io/LWG/lwg-defects.html#2367).
+// This causes compile errors in e.g. clang 3.5 or gcc 4.9.
+template <typename T>
+struct is_default_constructible : std::is_default_constructible<T> {};
+
+template <typename T1, typename T2>
+struct is_default_constructible<std::pair<T1, T2>>
+            : conjunction<is_default_constructible<T1>, is_default_constructible<T2>> {};
+
+template <typename T1, typename T2>
+struct is_default_constructible<const std::pair<T1, T2>>
+            : conjunction<is_default_constructible<T1>, is_default_constructible<T2>> {};
+
+template <typename... Ts>
+struct is_default_constructible<std::tuple<Ts...>>
+            : conjunction<is_default_constructible<Ts>...> {};
+
+template <typename... Ts>
+struct is_default_constructible<const std::tuple<Ts...>>
+            : conjunction<is_default_constructible<Ts>...> {};
+
+
+template <typename T, typename... Args>
+struct is_constructible : std::is_constructible<T, Args...> {};
+
+template <typename T1, typename T2>
+struct is_constructible<std::pair<T1, T2>> : is_default_constructible<std::pair<T1, T2>> {};
+
+template <typename T1, typename T2>
+struct is_constructible<const std::pair<T1, T2>> : is_default_constructible<const std::pair<T1, T2>> {};
+
+template <typename... Ts>
+struct is_constructible<std::tuple<Ts...>> : is_default_constructible<std::tuple<Ts...>> {};
+
+template <typename... Ts>
+struct is_constructible<const std::tuple<Ts...>> : is_default_constructible<const std::tuple<Ts...>> {};
+
+
 template<typename T, typename = void>
 struct is_iterator_traits : std::false_type {};
 
@@ -193,9 +238,9 @@ struct is_compatible_object_type_impl <
 
     // macOS's is_constructible does not play well with nonesuch...
     static constexpr bool value =
-        std::is_constructible<typename object_t::key_type,
+        is_constructible<typename object_t::key_type,
         typename CompatibleObjectType::key_type>::value &&
-        std::is_constructible<typename object_t::mapped_type,
+        is_constructible<typename object_t::mapped_type,
         typename CompatibleObjectType::mapped_type>::value;
 };
 
@@ -216,10 +261,10 @@ struct is_constructible_object_type_impl <
     using object_t = typename BasicJsonType::object_t;
 
     static constexpr bool value =
-        (std::is_default_constructible<ConstructibleObjectType>::value &&
+        (is_default_constructible<ConstructibleObjectType>::value &&
          (std::is_move_assignable<ConstructibleObjectType>::value ||
           std::is_copy_assignable<ConstructibleObjectType>::value) &&
-         (std::is_constructible<typename ConstructibleObjectType::key_type,
+         (is_constructible<typename ConstructibleObjectType::key_type,
           typename object_t::key_type>::value &&
           std::is_same <
           typename object_t::mapped_type,
@@ -247,7 +292,7 @@ struct is_compatible_string_type_impl <
     value_type_t, CompatibleStringType>::value >>
 {
     static constexpr auto value =
-        std::is_constructible<typename BasicJsonType::string_t, CompatibleStringType>::value;
+        is_constructible<typename BasicJsonType::string_t, CompatibleStringType>::value;
 };
 
 template<typename BasicJsonType, typename ConstructibleStringType>
@@ -265,7 +310,7 @@ struct is_constructible_string_type_impl <
     value_type_t, ConstructibleStringType>::value >>
 {
     static constexpr auto value =
-        std::is_constructible<ConstructibleStringType,
+        is_constructible<ConstructibleStringType,
         typename BasicJsonType::string_t>::value;
 };
 
@@ -288,7 +333,7 @@ struct is_compatible_array_type_impl <
     iterator_traits<CompatibleArrayType >>::value >>
 {
     static constexpr bool value =
-        std::is_constructible<BasicJsonType,
+        is_constructible<BasicJsonType,
         typename CompatibleArrayType::value_type>::value;
 };
 
@@ -311,7 +356,7 @@ struct is_constructible_array_type_impl <
     BasicJsonType, ConstructibleArrayType,
     enable_if_t < !std::is_same<ConstructibleArrayType,
     typename BasicJsonType::value_type>::value&&
-    std::is_default_constructible<ConstructibleArrayType>::value&&
+    is_default_constructible<ConstructibleArrayType>::value&&
 (std::is_move_assignable<ConstructibleArrayType>::value ||
  std::is_copy_assignable<ConstructibleArrayType>::value)&&
 is_detected<value_type_t, ConstructibleArrayType>::value&&
@@ -355,7 +400,7 @@ struct is_compatible_integer_type_impl <
     using CompatibleLimits = std::numeric_limits<CompatibleNumberIntegerType>;
 
     static constexpr auto value =
-        std::is_constructible<RealIntegerType,
+        is_constructible<RealIntegerType,
         CompatibleNumberIntegerType>::value &&
         CompatibleLimits::is_integer &&
         RealLimits::is_signed == CompatibleLimits::is_signed;
@@ -382,17 +427,10 @@ template<typename BasicJsonType, typename CompatibleType>
 struct is_compatible_type
     : is_compatible_type_impl<BasicJsonType, CompatibleType> {};
 
-// https://en.cppreference.com/w/cpp/types/conjunction
-template<class...> struct conjunction : std::true_type { };
-template<class B1> struct conjunction<B1> : B1 { };
-template<class B1, class... Bn>
-struct conjunction<B1, Bn...>
-: std::conditional<bool(B1::value), conjunction<Bn...>, B1>::type {};
-
 template<typename T1, typename T2>
 struct is_constructible_tuple : std::false_type {};
 
 template<typename T1, typename... Args>
-struct is_constructible_tuple<T1, std::tuple<Args...>> : conjunction<std::is_constructible<T1, Args>...> {};
+struct is_constructible_tuple<T1, std::tuple<Args...>> : conjunction<is_constructible<T1, Args>...> {};
 }  // namespace detail
 }  // namespace nlohmann