Allow to add metadata to json nodes

* by default no metadata is allowed and the library behaves as it already did
* if a user explicitly adds metadata (last template parameter) each node has a data member of this type and access functions for this member
* add test for this feature

include/nlohmann/detail/json_metadata.hpp

@@ -0,0 +1,30 @@
+#pragma once
+
+namespace nlohmann
+{
+namespace detail
+{
+template<class MetaDataType>
+class json_metadata
+{
+  public:
+    using metadata_t = MetaDataType;
+    metadata_t& metadata()
+    {
+        return m_metadata;
+    }
+    const metadata_t& metadata() const
+    {
+        return m_metadata;
+    }
+  private:
+    metadata_t m_metadata;
+};
+
+template<>
+class json_metadata<void>
+{
+    //no metadata
+};
+}  // namespace detail
+}  // namespace nlohmann

include/nlohmann/detail/macro_scope.hpp

@@ -131,12 +131,13 @@
              class NumberUnsignedType, class NumberFloatType,              \
              template<typename> class AllocatorType,                       \
              template<typename, typename = void> class JSONSerializer,     \
-             class BinaryType>
+             class BinaryType,                                             \
+             class MetaDataType>
 
 #define NLOHMANN_BASIC_JSON_TPL                                            \
     basic_json<ObjectType, ArrayType, StringType, BooleanType,             \
     NumberIntegerType, NumberUnsignedType, NumberFloatType,                \
-    AllocatorType, JSONSerializer, BinaryType>
+    AllocatorType, JSONSerializer, BinaryType, MetaDataType>
 
 // Macros to simplify conversion from/to types
 

include/nlohmann/json.hpp

@@ -63,6 +63,7 @@ SOFTWARE.
 #include <nlohmann/detail/iterators/iteration_proxy.hpp>
 #include <nlohmann/detail/iterators/json_reverse_iterator.hpp>
 #include <nlohmann/detail/iterators/primitive_iterator.hpp>
+#include <nlohmann/detail/json_metadata.hpp>
 #include <nlohmann/detail/json_pointer.hpp>
 #include <nlohmann/detail/json_ref.hpp>
 #include <nlohmann/detail/macro_scope.hpp>
@@ -174,6 +175,7 @@ Format](https://tools.ietf.org/html/rfc8259)
 */
 NLOHMANN_BASIC_JSON_TPL_DECLARATION
 class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-special-member-functions)
+    : public ::nlohmann::detail::json_metadata<MetaDataType>
 {
   private:
     template<detail::value_t> friend struct detail::external_constructor;
@@ -196,6 +198,7 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
 
     /// workaround type for MSVC
     using basic_json_t = NLOHMANN_BASIC_JSON_TPL;
+    using json_metadata_t = ::nlohmann::detail::json_metadata<MetaDataType>;
 
   JSON_PRIVATE_UNLESS_TESTED:
     // convenience aliases for types residing in namespace detail;
@@ -2188,7 +2191,8 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
     @since version 1.0.0
     */
     basic_json(const basic_json& other)
-        : m_type(other.m_type)
+        : json_metadata_t(other),
+          m_type(other.m_type)
     {
         // check of passed value is valid
         other.assert_invariant();
@@ -2280,7 +2284,8 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
     @since version 1.0.0
     */
     basic_json(basic_json&& other) noexcept
-        : m_type(std::move(other.m_type)),
+        : json_metadata_t(std::move(other)),
+          m_type(std::move(other.m_type)),
           m_value(std::move(other.m_value))
     {
         // check that passed value is valid
@@ -2321,12 +2326,15 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
         std::is_nothrow_move_constructible<value_t>::value&&
         std::is_nothrow_move_assignable<value_t>::value&&
         std::is_nothrow_move_constructible<json_value>::value&&
-        std::is_nothrow_move_assignable<json_value>::value
+        std::is_nothrow_move_assignable<json_value>::value&&
+        std::is_nothrow_move_assignable<json_metadata_t>::value
     )
     {
         // check that passed value is valid
         other.assert_invariant();
 
+        json_metadata_t::operator=(std::move(other));
+
         using std::swap;
         swap(m_type, other.m_type);
         swap(m_value, other.m_value);

include/nlohmann/json_fwd.hpp

@@ -34,7 +34,8 @@ template<template<typename U, typename V, typename... Args> class ObjectType =
          template<typename U> class AllocatorType = std::allocator,
          template<typename T, typename SFINAE = void> class JSONSerializer =
          adl_serializer,
-         class BinaryType = std::vector<std::uint8_t>>
+         class BinaryType = std::vector<std::uint8_t>,
+         class MetaDataType = void>
 class basic_json;
 
 /*!

single_include/nlohmann/json.hpp

@@ -2343,12 +2343,13 @@ JSON_HEDLEY_DIAGNOSTIC_POP
              class NumberUnsignedType, class NumberFloatType,              \
              template<typename> class AllocatorType,                       \
              template<typename, typename = void> class JSONSerializer,     \
-             class BinaryType>
+             class BinaryType,                                             \
+             class MetaDataType>
 
 #define NLOHMANN_BASIC_JSON_TPL                                            \
     basic_json<ObjectType, ArrayType, StringType, BooleanType,             \
     NumberIntegerType, NumberUnsignedType, NumberFloatType,                \
-    AllocatorType, JSONSerializer, BinaryType>
+    AllocatorType, JSONSerializer, BinaryType, MetaDataType>
 
 // Macros to simplify conversion from/to types
 
@@ -3380,7 +3381,8 @@ template<template<typename U, typename V, typename... Args> class ObjectType =
          template<typename U> class AllocatorType = std::allocator,
          template<typename T, typename SFINAE = void> class JSONSerializer =
          adl_serializer,
-         class BinaryType = std::vector<std::uint8_t>>
+         class BinaryType = std::vector<std::uint8_t>,
+         class MetaDataType = void>
 class basic_json;
 
 /*!
@@ -12354,6 +12356,38 @@ class json_reverse_iterator : public std::reverse_iterator<Base>
 
 // #include <nlohmann/detail/iterators/primitive_iterator.hpp>
 
+// #include <nlohmann/detail/json_metadata.hpp>
+
+
+namespace nlohmann
+{
+namespace detail
+{
+template<class MetaDataType>
+class json_metadata
+{
+  public:
+    using metadata_t = MetaDataType;
+    metadata_t& metadata()
+    {
+        return m_metadata;
+    }
+    const metadata_t& metadata() const
+    {
+        return m_metadata;
+    }
+  private:
+    metadata_t m_metadata;
+};
+
+template<>
+class json_metadata<void>
+{
+    //no metadata
+};
+}  // namespace detail
+}  // namespace nlohmann
+
 // #include <nlohmann/detail/json_pointer.hpp>
 
 
@@ -17580,6 +17614,7 @@ Format](https://tools.ietf.org/html/rfc8259)
 */
 NLOHMANN_BASIC_JSON_TPL_DECLARATION
 class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-special-member-functions)
+    : public ::nlohmann::detail::json_metadata<MetaDataType>
 {
   private:
     template<detail::value_t> friend struct detail::external_constructor;
@@ -17602,6 +17637,7 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
 
     /// workaround type for MSVC
     using basic_json_t = NLOHMANN_BASIC_JSON_TPL;
+    using json_metadata_t = ::nlohmann::detail::json_metadata<MetaDataType>;
 
   JSON_PRIVATE_UNLESS_TESTED:
     // convenience aliases for types residing in namespace detail;
@@ -19594,7 +19630,8 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
     @since version 1.0.0
     */
     basic_json(const basic_json& other)
-        : m_type(other.m_type)
+        : json_metadata_t(other),
+          m_type(other.m_type)
     {
         // check of passed value is valid
         other.assert_invariant();
@@ -19686,7 +19723,8 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
     @since version 1.0.0
     */
     basic_json(basic_json&& other) noexcept
-        : m_type(std::move(other.m_type)),
+        : json_metadata_t(std::move(other)),
+          m_type(std::move(other.m_type)),
           m_value(std::move(other.m_value))
     {
         // check that passed value is valid
@@ -19727,12 +19765,15 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
         std::is_nothrow_move_constructible<value_t>::value&&
         std::is_nothrow_move_assignable<value_t>::value&&
         std::is_nothrow_move_constructible<json_value>::value&&
-        std::is_nothrow_move_assignable<json_value>::value
+        std::is_nothrow_move_assignable<json_value>::value&&
+        std::is_nothrow_move_assignable<json_metadata_t>::value
     )
     {
         // check that passed value is valid
         other.assert_invariant();
 
+        json_metadata_t::operator=(std::move(other));
+
         using std::swap;
         swap(m_type, other.m_type);
         swap(m_value, other.m_value);

test/src/unit-metadata.cpp

@@ -0,0 +1,185 @@
+/*
+    __ _____ _____ _____
+ __|  |   __|     |   | |  JSON for Modern C++ (test suite)
+|  |  |__   |  |  | | | |  version 3.10.2
+|_____|_____|_____|_|___|  https://github.com/nlohmann/json
+
+Licensed under the MIT License <http://opensource.org/licenses/MIT>.
+SPDX-License-Identifier: MIT
+Copyright (c) 2013-2019 Niels Lohmann <http://nlohmann.me>.
+
+Permission is hereby  granted, free of charge, to any  person obtaining a copy
+of this software and associated  documentation files (the "Software"), to deal
+in the Software  without restriction, including without  limitation the rights
+to  use, copy,  modify, merge,  publish, distribute,  sublicense, and/or  sell
+copies  of  the Software,  and  to  permit persons  to  whom  the Software  is
+furnished to do so, subject to the following conditions:
+
+The above copyright notice and this permission notice shall be included in all
+copies or substantial portions of the Software.
+
+THE SOFTWARE  IS PROVIDED "AS  IS", WITHOUT WARRANTY  OF ANY KIND,  EXPRESS OR
+IMPLIED,  INCLUDING BUT  NOT  LIMITED TO  THE  WARRANTIES OF  MERCHANTABILITY,
+FITNESS FOR  A PARTICULAR PURPOSE AND  NONINFRINGEMENT. IN NO EVENT  SHALL THE
+AUTHORS  OR COPYRIGHT  HOLDERS  BE  LIABLE FOR  ANY  CLAIM,  DAMAGES OR  OTHER
+LIABILITY, WHETHER IN AN ACTION OF  CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+OUT OF OR IN CONNECTION WITH THE SOFTWARE  OR THE USE OR OTHER DEALINGS IN THE
+SOFTWARE.
+*/
+
+#include "doctest_compatibility.h"
+
+#include <nlohmann/json.hpp>
+
+template<class T>
+using json_with_metadata =
+    nlohmann::basic_json <
+    std::map,
+    std::vector,
+    std::string,
+    bool,
+    std::int64_t,
+    std::uint64_t,
+    double,
+    std::allocator,
+    nlohmann::adl_serializer,
+    std::vector<std::uint8_t>,
+    T
+    >;
+
+TEST_CASE("JSON Node Metadata")
+{
+    SECTION("type int")
+    {
+        using json = json_with_metadata<int>;
+        json null;
+        auto obj   = json::object();
+        auto array = json::array();
+
+        null.metadata()  = 1;
+        obj.metadata()   = 2;
+        array.metadata() = 3;
+        auto copy = array;
+
+        CHECK(null.metadata()  == 1);
+        CHECK(obj.metadata()   == 2);
+        CHECK(array.metadata() == 3);
+        CHECK(copy.metadata()  == 3);
+    }
+    SECTION("type vector<int>")
+    {
+        using json = json_with_metadata<std::vector<int>>;
+        json value;
+        value.metadata().emplace_back(1);
+        auto copy = value;
+        value.metadata().emplace_back(2);
+
+        CHECK(copy.metadata().size()  == 1);
+        CHECK(copy.metadata().at(0)   == 1);
+        CHECK(value.metadata().size() == 2);
+        CHECK(value.metadata().at(0)  == 1);
+        CHECK(value.metadata().at(1)  == 2);
+    }
+    SECTION("copy ctor")
+    {
+        using json = json_with_metadata<std::vector<int>>;
+        json value;
+        value.metadata().emplace_back(1);
+        value.metadata().emplace_back(2);
+
+        json copy = value;
+
+        CHECK(copy.metadata().size()  == 2);
+        CHECK(copy.metadata().at(0)   == 1);
+        CHECK(copy.metadata().at(1)   == 2);
+        CHECK(value.metadata().size() == 2);
+        CHECK(value.metadata().at(0)  == 1);
+        CHECK(value.metadata().at(1)  == 2);
+
+        value.metadata().clear();
+        CHECK(copy.metadata().size()  == 2);
+        CHECK(value.metadata().size() == 0);
+    }
+    SECTION("move ctor")
+    {
+        using json = json_with_metadata<std::vector<int>>;
+        json value;
+        value.metadata().emplace_back(1);
+        value.metadata().emplace_back(2);
+
+        const json moved = std::move(value);
+
+        CHECK(moved.metadata().size()  == 2);
+        CHECK(moved.metadata().at(0)   == 1);
+        CHECK(moved.metadata().at(1)   == 2);
+        CHECK(value.metadata().size()  == 0);
+    }
+    SECTION("move assign")
+    {
+        using json = json_with_metadata<std::vector<int>>;
+        json value;
+        value.metadata().emplace_back(1);
+        value.metadata().emplace_back(2);
+
+        json moved;
+        moved = std::move(value);
+
+        CHECK(moved.metadata().size()  == 2);
+        CHECK(moved.metadata().at(0)   == 1);
+        CHECK(moved.metadata().at(1)   == 2);
+        CHECK(value.metadata().size()  == 0);
+    }
+    SECTION("copy assign")
+    {
+        using json = json_with_metadata<std::vector<int>>;
+        json value;
+        value.metadata().emplace_back(1);
+        value.metadata().emplace_back(2);
+
+        json copy;
+        copy = value;
+
+        CHECK(copy.metadata().size()  == 2);
+        CHECK(copy.metadata().at(0)   == 1);
+        CHECK(copy.metadata().at(1)   == 2);
+        CHECK(value.metadata().size() == 2);
+        CHECK(value.metadata().at(0)  == 1);
+        CHECK(value.metadata().at(1)  == 2);
+
+        value.metadata().clear();
+        CHECK(copy.metadata().size()  == 2);
+        CHECK(value.metadata().size() == 0);
+    }
+    SECTION("type unique_ptr<int>")
+    {
+        using json = json_with_metadata<std::unique_ptr<int>>;
+        json value;
+        value.metadata().reset(new int);
+        (*value.metadata()) = 42;
+        auto moved = std::move(value);
+
+        CHECK(value.metadata() == nullptr);
+        CHECK(moved.metadata() != nullptr);
+        CHECK(*moved.metadata() == 42);
+    }
+    SECTION("type vector<int> in json array")
+    {
+        using json = json_with_metadata<std::vector<int>>;
+        json value;
+        value.metadata().emplace_back(1);
+        value.metadata().emplace_back(2);
+
+        json array(10, value);
+
+        CHECK(value.metadata().size() == 2);
+        CHECK(value.metadata().at(0)  == 1);
+        CHECK(value.metadata().at(1)  == 2);
+
+        for (const auto& val : array)
+        {
+            CHECK(val.metadata().size() == 2);
+            CHECK(val.metadata().at(0)  == 1);
+            CHECK(val.metadata().at(1)  == 2);
+        }
+    }
+}