Exchange the CustomBaseClass subobject in basic_json::swap()

basic_json::swap() (and the friend swap() and the pre-C++20 std::swap
overload that forward to it) only exchanged m_data.m_type/m_data.m_value,
leaving each value's json_base_class_t subobject in place. This is
inconsistent with the copy and move constructors and copy assignment,
which all carry the base class along with the value, so after
a.swap(b) any metadata stored in a CustomBaseClass ended up attached to
the wrong value. Algorithms that mix swap() with moves, such as
std::sort, scrambled the metadata across the whole container.

Fix the member swap() to also exchange the json_base_class_t subobject
and extend the noexcept specifications of swap() and the friend swap()
accordingly.

Fixes #5653.

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

docs/mkdocs/docs/api/basic_json/swap.md

@@ -6,7 +6,9 @@ void swap(reference other) noexcept (
     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_constructible<json_base_class_t>::value &&
+    std::is_nothrow_move_assignable<json_base_class_t>::value
 );
 
 // (2)
@@ -14,7 +16,9 @@ friend void swap(reference left, reference right) noexcept (
     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_constructible<json_base_class_t>::value &&
+    std::is_nothrow_move_assignable<json_base_class_t>::value
 );
 
 // (3)
@@ -37,11 +41,15 @@ void swap(typename binary_t::container_type& other);
    individual elements. All iterators and references remain valid. The past-the-end iterator is invalidated. If macro
    [`JSON_DIAGNOSTIC_POSITIONS`](../macros/json_diagnostic_positions.md) is defined to `#!cpp 1`, the
    [`start_pos()`](start_pos.md)/[`end_pos()`](end_pos.md) diagnostic positions are exchanged along with the value.
+   The [`json_base_class_t`](json_base_class_t.md) subobject is exchanged along with the value as well, the same way it
+   is copied or moved by the copy/move constructors and assignment operators.
 2. Exchanges the contents of the JSON value from `left` with those of `right`. Does not invoke any move, copy, or swap
    operations on individual elements. All iterators and references remain valid. The past-the-end iterator is
    invalidated. Implemented as a friend function callable via ADL. If macro
    [`JSON_DIAGNOSTIC_POSITIONS`](../macros/json_diagnostic_positions.md) is defined to `#!cpp 1`, the
    [`start_pos()`](start_pos.md)/[`end_pos()`](end_pos.md) diagnostic positions are exchanged along with the value.
+   The [`json_base_class_t`](json_base_class_t.md) subobject is exchanged along with the value as well, the same way it
+   is copied or moved by the copy/move constructors and assignment operators.
 3. Exchanges the contents of a JSON array with those of `other`. Does not invoke any move, copy, or swap operations on
    individual elements. All iterators and references remain valid. The past-the-end iterator is invalidated. 
 4. Exchanges the contents of a JSON object with those of `other`. Does not invoke any move, copy, or swap operations on
@@ -164,8 +172,8 @@ Constant.
 
 ## Version history
 
-1. Since version 1.0.0.
+1. Since version 1.0.0. Exchanges the `json_base_class_t` subobject along with the value since version 3.13.0.
-2. Since version 1.0.0.
+2. Since version 1.0.0. Exchanges the `json_base_class_t` subobject along with the value since version 3.13.0.
 3. Since version 1.0.0.
 4. Since version 1.0.0.
 5. Since version 1.0.0.

include/nlohmann/json.hpp

@@ -4335,12 +4335,21 @@ 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&& // NOLINT(cppcoreguidelines-noexcept-swap,performance-noexcept-swap)
-        std::is_nothrow_move_assignable<json_value>::value
+        std::is_nothrow_move_assignable<json_value>::value&&
+        std::is_nothrow_move_constructible<json_base_class_t>::value&&
+        std::is_nothrow_move_assignable<json_base_class_t>::value
     )
     {
         std::swap(m_data.m_type, other.m_data.m_type);
         std::swap(m_data.m_value, other.m_data.m_value);
 
+        // the custom base class travels with the value when it is copied or
+        // moved, so it is exchanged along with it
+        {
+            using std::swap;
+            swap(static_cast<json_base_class_t&>(*this), static_cast<json_base_class_t&>(other));
+        }
+
 #if JSON_DIAGNOSTIC_POSITIONS
         std::swap(start_position, other.start_position);
         std::swap(end_position, other.end_position);
@@ -4357,7 +4366,9 @@ 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&& // NOLINT(cppcoreguidelines-noexcept-swap,performance-noexcept-swap)
-        std::is_nothrow_move_assignable<json_value>::value
+        std::is_nothrow_move_assignable<json_value>::value&&
+        std::is_nothrow_move_constructible<json_base_class_t>::value&&
+        std::is_nothrow_move_assignable<json_base_class_t>::value
     )
     {
         left.swap(right);

single_include/nlohmann/json.hpp

@@ -30416,12 +30416,21 @@ 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&& // NOLINT(cppcoreguidelines-noexcept-swap,performance-noexcept-swap)
-        std::is_nothrow_move_assignable<json_value>::value
+        std::is_nothrow_move_assignable<json_value>::value&&
+        std::is_nothrow_move_constructible<json_base_class_t>::value&&
+        std::is_nothrow_move_assignable<json_base_class_t>::value
     )
     {
         std::swap(m_data.m_type, other.m_data.m_type);
         std::swap(m_data.m_value, other.m_data.m_value);
 
+        // the custom base class travels with the value when it is copied or
+        // moved, so it is exchanged along with it
+        {
+            using std::swap;
+            swap(static_cast<json_base_class_t&>(*this), static_cast<json_base_class_t&>(other));
+        }
+
 #if JSON_DIAGNOSTIC_POSITIONS
         std::swap(start_position, other.start_position);
         std::swap(end_position, other.end_position);
@@ -30438,7 +30447,9 @@ 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&& // NOLINT(cppcoreguidelines-noexcept-swap,performance-noexcept-swap)
-        std::is_nothrow_move_assignable<json_value>::value
+        std::is_nothrow_move_assignable<json_value>::value&&
+        std::is_nothrow_move_constructible<json_base_class_t>::value&&
+        std::is_nothrow_move_assignable<json_base_class_t>::value
     )
     {
         left.swap(right);

tests/src/unit-custom-base-class.cpp

@@ -6,9 +6,11 @@
 // SPDX-FileCopyrightText: 2013-2026 Niels Lohmann <https://nlohmann.me>
 // SPDX-License-Identifier: MIT
 
+#include <algorithm>
 #include <set>
 #include <sstream>
 #include <string>
+#include <vector>
 
 #include "doctest_compatibility.h"
 
@@ -180,6 +182,76 @@ TEST_CASE("JSON Node Metadata")
             CHECK(val.metadata().at(1)  == 2);
         }
     }
+    SECTION("member swap")
+    {
+        using json = json_with_metadata<int>;
+        json a = 1;
+        a.metadata() = 100;
+        json b = 2;
+        b.metadata() = 200;
+
+        a.swap(b);
+
+        CHECK(a.get<int>()  == 2);
+        CHECK(b.get<int>()  == 1);
+        CHECK(a.metadata()  == 200);
+        CHECK(b.metadata()  == 100);
+    }
+    SECTION("nonmember swap")
+    {
+        using json = json_with_metadata<int>;
+        json a = 1;
+        a.metadata() = 100;
+        json b = 2;
+        b.metadata() = 200;
+
+        using std::swap;
+        swap(a, b);
+
+        CHECK(a.get<int>()  == 2);
+        CHECK(b.get<int>()  == 1);
+        CHECK(a.metadata()  == 200);
+        CHECK(b.metadata()  == 100);
+    }
+    SECTION("std::swap")
+    {
+        using json = json_with_metadata<int>;
+        json a = 1;
+        a.metadata() = 100;
+        json b = 2;
+        b.metadata() = 200;
+
+        std::swap(a, b);
+
+        CHECK(a.get<int>()  == 2);
+        CHECK(b.get<int>()  == 1);
+        CHECK(a.metadata()  == 200);
+        CHECK(b.metadata()  == 100);
+    }
+    SECTION("std::sort keeps metadata attached to its value")
+    {
+        // std::sort mixes swap() with moves; each value's metadata must
+        // travel with it, just as it does for copy, move, and assignment
+        using json = json_with_metadata<int>;
+        std::vector<json> values;
+        for (int v :
+                {
+                    5, 3, 9, 1, 7, 2, 8, 4, 6, 0, 15, 13, 19, 11, 17, 12, 18, 14, 16, 10,
+                    25, 23, 29, 21, 27, 22, 28, 24, 26, 20, 35, 33
+                })
+        {
+            json value = v;
+            value.metadata() = v;
+            values.push_back(value);
+        }
+
+        std::sort(values.begin(), values.end());
+
+        for (const auto& value : values)
+        {
+            CHECK(value.metadata() == value.get<int>());
+        }
+    }
 }
 
 // Test extending nlohmann::json by using a custom base class.