Fix quadratic parent updates for ordered_json with JSON_DIAGNOSTICS

set_parent() re-parented the whole container for ordered_json on every
call (push_back, emplace_back, at(), operator[] on existing keys),
making array building quadratic. at() and array append now set only the
one child's parent; object insertion re-parents only if the vector
capacity changed. Also drop the redundant set_parents() in the copy
constructor. Adds a parent-pointer invariant test.

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

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

@@ -68,14 +68,6 @@ basic_json(basic_json&& other) noexcept;
      `std::unordered_map`, `std::multimap`, and `std::unordered_multimap` with a `key_type` compatible to `string_t`
      and a `value_type` from which a `basic_json` value can be constructed.
     - **strings**: `string_t`, string literals, and all compatible string containers can be used.
-
-    !!! note "Rvalue containers are moved from"
-
-        If `val` is an rvalue of a compatible array-like or object-like container other than `array_t`/`object_t`
-        (e.g., `json j = std::move(vec);` or `json j(std::vector<std::string>{...});`), the elements are moved into
-        the JSON value instead of copied. Afterwards, the elements of the source container are in a valid but
-        unspecified state (**moved-from**). Keys of associative containers are `const` and therefore still copied.
-        Lvalues and `const` rvalues are copied, and the source is left unchanged.
     - **numbers**: [`number_integer_t`](number_integer_t.md), [`number_unsigned_t`](number_unsigned_t.md),
      [`number_float_t`](number_float_t.md), and all convertible number types such as `int`, `size_t`, `int64_t`, `float`
      or `double` can be used.
@@ -244,9 +236,7 @@ basic_json(basic_json&& other) noexcept;
 2. No-throw guarantee: this constructor never throws exceptions.
 3. Depends on the called constructor. For types directly supported by the library (i.e., all types for which no
    `to_json()` function was provided), a strong guarantee holds: if an exception is thrown, there are no changes to any
-   JSON value. **Exception:** when `val` is an rvalue array-like or object-like container, its elements are moved one
-   by one; if an exception is thrown in the middle of the conversion (e.g., an allocation failure), the JSON value is
-   unchanged, but the source container may be left partially moved-from.
+   JSON value.
 4. Depends on the called constructor. For types directly supported by the library (i.e., all types for which no
    `to_json()` function was provided), a strong guarantee holds: if an exception is thrown, there are no changes to any
    JSON value.

docs/mkdocs/docs/home/releases.md

@@ -9,15 +9,6 @@ published on the [GitHub releases page](https://github.com/nlohmann/json/release
     This documentation is built from the `develop` branch and may describe changes that are not part of a release
     yet. Their version numbers are followed by an <span class="unreleased-version">unreleased</span> badge.
 
-## Unreleased
-
-- Converting an rvalue container to JSON (`json j = std::move(vec);`, `json j = std::map<std::string, std::string>{...};`)
-  now **moves** the elements instead of copying them (e.g., `std::vector<std::string>` with 100,000 strings: 4.7 ms to
-  2.3 ms). Two consequences: the source container is left in a valid but unspecified (moved-from) state, and if an
-  exception is thrown during the conversion, the source may be partially moved-from, so the strong exception guarantee
-  no longer holds for the source of an rvalue conversion (the JSON value itself is unaffected). Lvalues and `const`
-  objects are still copied. See [`basic_json`](../api/basic_json/basic_json.md).
-
 ## v3.12.0 (2025-04-11)
 
 Fixes bugs found in 3.11.3 and adds several features. All changes are backward-compatible.

include/nlohmann/detail/conversions/to_json.hpp

@@ -167,17 +167,6 @@ struct external_constructor<value_t::array>
         j.replace_value(value_t::array, value);
     }
 
-    /// an rvalue container: move its elements
-    template<typename BasicJsonType, typename CompatibleArrayType>
-    static void construct_moving(BasicJsonType& j, CompatibleArrayType& arr)
-    {
-        using std::begin;
-        using std::end;
-
-        const typename BasicJsonType::json_value value(j.template create<typename BasicJsonType::array_t>(std::make_move_iterator(begin(arr)), std::make_move_iterator(end(arr))));
-        j.replace_value(value_t::array, value);
-    }
-
     template<typename BasicJsonType>
     static void construct(BasicJsonType& j, const std::vector<bool>& arr)
     {
@@ -228,17 +217,6 @@ struct external_constructor<value_t::object>
         j.replace_value(value_t::object, value);
     }
 
-    /// an rvalue container: move its values
-    template<typename BasicJsonType, typename CompatibleObjectType>
-    static void construct_moving(BasicJsonType& j, CompatibleObjectType& obj)
-    {
-        using std::begin;
-        using std::end;
-
-        const typename BasicJsonType::json_value value(j.template create<typename BasicJsonType::object_t>(std::make_move_iterator(begin(obj)), std::make_move_iterator(end(obj))));
-        j.replace_value(value_t::object, value);
-    }
-
     template < typename BasicJsonType, typename CompatibleObjectType,
                enable_if_t < !std::is_same<CompatibleObjectType, typename BasicJsonType::object_t>::value, int > = 0 >
     static void construct(BasicJsonType& j, const CompatibleObjectType& obj)
@@ -376,45 +354,6 @@ inline void to_json(BasicJsonType& j, const CompatibleArrayType& arr)
     external_constructor<value_t::array>::construct(j, arr);
 }
 
-#if defined(__cpp_lib_char8_t)
-// std::basic_string<char8_t> is converted to a string by a dedicated to_json overload
-template<typename T>
-struct is_u8string : std::false_type {};
-
-template<typename Tr, typename Allocator>
-struct is_u8string<std::basic_string<char8_t, Tr, Allocator>> : std::true_type {};
-#endif
-
-// rvalue containers: their elements are moved instead of copied
-template < typename BasicJsonType, typename CompatibleArrayType,
-           enable_if_t < !std::is_lvalue_reference<CompatibleArrayType>::value
-                         && !std::is_const<CompatibleArrayType>::value
-                         && !std::is_same<CompatibleArrayType, typename BasicJsonType::array_t>::value
-                         && !std::is_same<CompatibleArrayType, std::vector<bool>>::value
-                         && !is_basic_json<CompatibleArrayType>::value
-                         && is_compatible_array_type<BasicJsonType, CompatibleArrayType>::value
-                         && !is_compatible_object_type<BasicJsonType, CompatibleArrayType>::value
-                         && !is_compatible_string_type<BasicJsonType, CompatibleArrayType>::value
-                         && !std::is_same<typename BasicJsonType::binary_t, CompatibleArrayType>::value
-                         && !is_compatible_binary_type<BasicJsonType, CompatibleArrayType>::value
-#if JSON_HAS_FILESYSTEM || JSON_HAS_EXPERIMENTAL_FILESYSTEM
-    && !std::is_same<CompatibleArrayType, std_fs::path>::value // has its own to_json
-#endif
-#if defined(__cpp_lib_char8_t)
-    && !is_u8string<CompatibleArrayType>::value // has its own to_json
-#endif
-#if JSON_HAS_RANGE_VIEW_CONVERSION
-    && !is_compatible_range_view<CompatibleArrayType>::value
-#endif
-#if JSON_USE_OBJECTS_FOR_ENUM_KEYED_MAPS
-    && !is_enum_keyed_map<CompatibleArrayType>::value
-#endif
-                         , int > = 0 >
-inline void to_json(BasicJsonType& j, CompatibleArrayType && arr)
-{
-    external_constructor<value_t::array>::construct_moving(j, arr);
-}
-
 #if JSON_HAS_RANGE_VIEW_CONVERSION
 template < typename BasicJsonType, typename T,
            enable_if_t < is_compatible_range_view<std::remove_cvref_t<T>>::value
@@ -488,21 +427,6 @@ inline void to_json(BasicJsonType& j, const EnumKeyedMap& map)
 }
 #endif
 
-template < typename BasicJsonType, typename CompatibleObjectType,
-           enable_if_t < !std::is_lvalue_reference<CompatibleObjectType>::value
-                         && !std::is_const<CompatibleObjectType>::value
-                         && !std::is_same<CompatibleObjectType, typename BasicJsonType::object_t>::value
-                         && is_compatible_object_type<BasicJsonType, CompatibleObjectType>::value
-                         && !is_basic_json<CompatibleObjectType>::value
-#if JSON_USE_OBJECTS_FOR_ENUM_KEYED_MAPS
-    && !is_enum_keyed_map<CompatibleObjectType>::value
-#endif
-                         , int > = 0 >
-inline void to_json(BasicJsonType& j, CompatibleObjectType && obj)
-{
-    external_constructor<value_t::object>::construct_moving(j, obj);
-}
-
 template<typename BasicJsonType>
 inline void to_json(BasicJsonType& j, typename BasicJsonType::object_t&& obj)
 {

include/nlohmann/json.hpp

@@ -1123,7 +1123,60 @@ public:
 #else
         static_cast<void>(old_capacity);
 #endif
-        return set_parent(j);
+        set_parent_plain(j);
+        return j;
+    }
+
+    /// @brief capacity of the stored object if it is vector-backed (ordered_map), else 0
+#if JSON_DIAGNOSTICS
+    template < typename O = object_t, detail::enable_if_t < detail::has_capacity<O>::value, int > = 0 >
+    std::size_t object_capacity() const noexcept
+    {
+        return m_data.m_value.object->capacity();
+    }
+
+    template < typename O = object_t, detail::enable_if_t < !detail::has_capacity<O>::value, int > = 0 >
+    std::size_t object_capacity() const noexcept
+    {
+        return 0;
+    }
+#else
+    static constexpr std::size_t object_capacity() noexcept
+    {
+        return 0;
+    }
+#endif
+
+    /// @brief set parent of an element that was just looked up or inserted into an object
+    reference set_parent_after_object_emplace(reference j, std::size_t old_capacity)
+    {
+#if JSON_DIAGNOSTICS
+        if (JSON_HEDLEY_UNLIKELY(object_capacity() != old_capacity))
+        {
+            set_parents();
+            return j;
+        }
+        j.m_parent = this;
+#else
+        static_cast<void>(old_capacity);
+#endif
+        return j;
+    }
+
+    void set_parent_plain(reference j) noexcept
+    {
+#if JSON_DIAGNOSTICS
+        j.m_parent = this;
+#else
+        static_cast<void>(j);
+#endif
+    }
+
+    /// @brief set the parent of an element accessed via at(); never moves elements
+    reference set_parent_at(reference j) noexcept
+    {
+        set_parent_plain(j);
+        return j;
     }
 
     reference set_parent(reference j)
@@ -2557,7 +2610,6 @@ public:
             copy_leaf_value(other, *this);
         }
 
-        set_parents();
         assert_invariant();
     }
 
@@ -3445,7 +3497,7 @@ public:
     /// @sa https://json.nlohmann.me/api/basic_json/at/
     reference at(size_type idx)
     {
-        return set_parent(array_at(*this, idx));
+        return set_parent_at(array_at(*this, idx));
     }
 
     /// @brief access specified array element with bounds checking
@@ -3459,7 +3511,7 @@ public:
     /// @sa https://json.nlohmann.me/api/basic_json/at/
     reference at(const typename object_t::key_type& key)
     {
-        return set_parent(object_at(*this, key));
+        return set_parent_at(object_at(*this, key));
     }
 
     /// @brief access specified object element with bounds checking
@@ -3468,7 +3520,7 @@ public:
                  detail::is_usable_as_basic_json_key_type<basic_json_t, KeyType>::value, int> = 0>
     reference at(KeyType && key)
     {
-        return set_parent(object_at(*this, std::forward<KeyType>(key)));
+        return set_parent_at(object_at(*this, std::forward<KeyType>(key)));
     }
 
     /// @brief access specified object element with bounds checking
@@ -3566,8 +3618,9 @@ public:
         // operator[] only works for objects
         if (JSON_HEDLEY_LIKELY(is_object()))
         {
+            const auto old_capacity = object_capacity();
             auto result = m_data.m_value.object->emplace(std::move(key), nullptr);
-            return set_parent(result.first->second);
+            return set_parent_after_object_emplace(result.first->second, old_capacity);
         }
 
         throw_subscript_wrong_type("string");
@@ -3617,8 +3670,9 @@ public:
         // operator[] only works for objects
         if (JSON_HEDLEY_LIKELY(is_object()))
         {
+            const auto old_capacity = object_capacity();
             auto result = m_data.m_value.object->emplace(lookup_key(std::forward<KeyType>(key)), nullptr);
-            return set_parent(result.first->second);
+            return set_parent_after_object_emplace(result.first->second, old_capacity);
         }
 
         throw_subscript_wrong_type("string");
@@ -4585,8 +4639,9 @@ public:
         }
 
         // add the element to the object
+        const auto old_capacity = object_capacity();
         auto res = m_data.m_value.object->insert(val);
-        set_parent(res.first->second);
+        set_parent_after_object_emplace(res.first->second, old_capacity);
     }
 
     /// @brief add an object to an object
@@ -4662,8 +4717,9 @@ public:
         }
 
         // add the element to the array (perfect forwarding)
+        const auto old_capacity = object_capacity();
         auto res = m_data.m_value.object->emplace(std::forward<Args>(args)...);
-        set_parent(res.first->second);
+        set_parent_after_object_emplace(res.first->second, old_capacity);
 
         // create a result iterator and set iterator to the result of emplace
         auto it = begin();

single_include/nlohmann/json.hpp

@@ -7114,17 +7114,6 @@ struct external_constructor<value_t::array>
         j.replace_value(value_t::array, value);
     }
 
-    /// an rvalue container: move its elements
-    template<typename BasicJsonType, typename CompatibleArrayType>
-    static void construct_moving(BasicJsonType& j, CompatibleArrayType& arr)
-    {
-        using std::begin;
-        using std::end;
-
-        const typename BasicJsonType::json_value value(j.template create<typename BasicJsonType::array_t>(std::make_move_iterator(begin(arr)), std::make_move_iterator(end(arr))));
-        j.replace_value(value_t::array, value);
-    }
-
     template<typename BasicJsonType>
     static void construct(BasicJsonType& j, const std::vector<bool>& arr)
     {
@@ -7175,17 +7164,6 @@ struct external_constructor<value_t::object>
         j.replace_value(value_t::object, value);
     }
 
-    /// an rvalue container: move its values
-    template<typename BasicJsonType, typename CompatibleObjectType>
-    static void construct_moving(BasicJsonType& j, CompatibleObjectType& obj)
-    {
-        using std::begin;
-        using std::end;
-
-        const typename BasicJsonType::json_value value(j.template create<typename BasicJsonType::object_t>(std::make_move_iterator(begin(obj)), std::make_move_iterator(end(obj))));
-        j.replace_value(value_t::object, value);
-    }
-
     template < typename BasicJsonType, typename CompatibleObjectType,
                enable_if_t < !std::is_same<CompatibleObjectType, typename BasicJsonType::object_t>::value, int > = 0 >
     static void construct(BasicJsonType& j, const CompatibleObjectType& obj)
@@ -7323,45 +7301,6 @@ inline void to_json(BasicJsonType& j, const CompatibleArrayType& arr)
     external_constructor<value_t::array>::construct(j, arr);
 }
 
-#if defined(__cpp_lib_char8_t)
-// std::basic_string<char8_t> is converted to a string by a dedicated to_json overload
-template<typename T>
-struct is_u8string : std::false_type {};
-
-template<typename Tr, typename Allocator>
-struct is_u8string<std::basic_string<char8_t, Tr, Allocator>> : std::true_type {};
-#endif
-
-// rvalue containers: their elements are moved instead of copied
-template < typename BasicJsonType, typename CompatibleArrayType,
-           enable_if_t < !std::is_lvalue_reference<CompatibleArrayType>::value
-                         && !std::is_const<CompatibleArrayType>::value
-                         && !std::is_same<CompatibleArrayType, typename BasicJsonType::array_t>::value
-                         && !std::is_same<CompatibleArrayType, std::vector<bool>>::value
-                         && !is_basic_json<CompatibleArrayType>::value
-                         && is_compatible_array_type<BasicJsonType, CompatibleArrayType>::value
-                         && !is_compatible_object_type<BasicJsonType, CompatibleArrayType>::value
-                         && !is_compatible_string_type<BasicJsonType, CompatibleArrayType>::value
-                         && !std::is_same<typename BasicJsonType::binary_t, CompatibleArrayType>::value
-                         && !is_compatible_binary_type<BasicJsonType, CompatibleArrayType>::value
-#if JSON_HAS_FILESYSTEM || JSON_HAS_EXPERIMENTAL_FILESYSTEM
-    && !std::is_same<CompatibleArrayType, std_fs::path>::value // has its own to_json
-#endif
-#if defined(__cpp_lib_char8_t)
-    && !is_u8string<CompatibleArrayType>::value // has its own to_json
-#endif
-#if JSON_HAS_RANGE_VIEW_CONVERSION
-    && !is_compatible_range_view<CompatibleArrayType>::value
-#endif
-#if JSON_USE_OBJECTS_FOR_ENUM_KEYED_MAPS
-    && !is_enum_keyed_map<CompatibleArrayType>::value
-#endif
-                         , int > = 0 >
-inline void to_json(BasicJsonType& j, CompatibleArrayType && arr)
-{
-    external_constructor<value_t::array>::construct_moving(j, arr);
-}
-
 #if JSON_HAS_RANGE_VIEW_CONVERSION
 template < typename BasicJsonType, typename T,
            enable_if_t < is_compatible_range_view<std::remove_cvref_t<T>>::value
@@ -7435,21 +7374,6 @@ inline void to_json(BasicJsonType& j, const EnumKeyedMap& map)
 }
 #endif
 
-template < typename BasicJsonType, typename CompatibleObjectType,
-           enable_if_t < !std::is_lvalue_reference<CompatibleObjectType>::value
-                         && !std::is_const<CompatibleObjectType>::value
-                         && !std::is_same<CompatibleObjectType, typename BasicJsonType::object_t>::value
-                         && is_compatible_object_type<BasicJsonType, CompatibleObjectType>::value
-                         && !is_basic_json<CompatibleObjectType>::value
-#if JSON_USE_OBJECTS_FOR_ENUM_KEYED_MAPS
-    && !is_enum_keyed_map<CompatibleObjectType>::value
-#endif
-                         , int > = 0 >
-inline void to_json(BasicJsonType& j, CompatibleObjectType && obj)
-{
-    external_constructor<value_t::object>::construct_moving(j, obj);
-}
-
 template<typename BasicJsonType>
 inline void to_json(BasicJsonType& j, typename BasicJsonType::object_t&& obj)
 {
@@ -30547,7 +30471,60 @@ public:
 #else
         static_cast<void>(old_capacity);
 #endif
-        return set_parent(j);
+        set_parent_plain(j);
+        return j;
+    }
+
+    /// @brief capacity of the stored object if it is vector-backed (ordered_map), else 0
+#if JSON_DIAGNOSTICS
+    template < typename O = object_t, detail::enable_if_t < detail::has_capacity<O>::value, int > = 0 >
+    std::size_t object_capacity() const noexcept
+    {
+        return m_data.m_value.object->capacity();
+    }
+
+    template < typename O = object_t, detail::enable_if_t < !detail::has_capacity<O>::value, int > = 0 >
+    std::size_t object_capacity() const noexcept
+    {
+        return 0;
+    }
+#else
+    static constexpr std::size_t object_capacity() noexcept
+    {
+        return 0;
+    }
+#endif
+
+    /// @brief set parent of an element that was just looked up or inserted into an object
+    reference set_parent_after_object_emplace(reference j, std::size_t old_capacity)
+    {
+#if JSON_DIAGNOSTICS
+        if (JSON_HEDLEY_UNLIKELY(object_capacity() != old_capacity))
+        {
+            set_parents();
+            return j;
+        }
+        j.m_parent = this;
+#else
+        static_cast<void>(old_capacity);
+#endif
+        return j;
+    }
+
+    void set_parent_plain(reference j) noexcept
+    {
+#if JSON_DIAGNOSTICS
+        j.m_parent = this;
+#else
+        static_cast<void>(j);
+#endif
+    }
+
+    /// @brief set the parent of an element accessed via at(); never moves elements
+    reference set_parent_at(reference j) noexcept
+    {
+        set_parent_plain(j);
+        return j;
     }
 
     reference set_parent(reference j)
@@ -31981,7 +31958,6 @@ public:
             copy_leaf_value(other, *this);
         }
 
-        set_parents();
         assert_invariant();
     }
 
@@ -32869,7 +32845,7 @@ public:
     /// @sa https://json.nlohmann.me/api/basic_json/at/
     reference at(size_type idx)
     {
-        return set_parent(array_at(*this, idx));
+        return set_parent_at(array_at(*this, idx));
     }
 
     /// @brief access specified array element with bounds checking
@@ -32883,7 +32859,7 @@ public:
     /// @sa https://json.nlohmann.me/api/basic_json/at/
     reference at(const typename object_t::key_type& key)
     {
-        return set_parent(object_at(*this, key));
+        return set_parent_at(object_at(*this, key));
     }
 
     /// @brief access specified object element with bounds checking
@@ -32892,7 +32868,7 @@ public:
                  detail::is_usable_as_basic_json_key_type<basic_json_t, KeyType>::value, int> = 0>
     reference at(KeyType && key)
     {
-        return set_parent(object_at(*this, std::forward<KeyType>(key)));
+        return set_parent_at(object_at(*this, std::forward<KeyType>(key)));
     }
 
     /// @brief access specified object element with bounds checking
@@ -32990,8 +32966,9 @@ public:
         // operator[] only works for objects
         if (JSON_HEDLEY_LIKELY(is_object()))
         {
+            const auto old_capacity = object_capacity();
             auto result = m_data.m_value.object->emplace(std::move(key), nullptr);
-            return set_parent(result.first->second);
+            return set_parent_after_object_emplace(result.first->second, old_capacity);
         }
 
         throw_subscript_wrong_type("string");
@@ -33041,8 +33018,9 @@ public:
         // operator[] only works for objects
         if (JSON_HEDLEY_LIKELY(is_object()))
         {
+            const auto old_capacity = object_capacity();
             auto result = m_data.m_value.object->emplace(lookup_key(std::forward<KeyType>(key)), nullptr);
-            return set_parent(result.first->second);
+            return set_parent_after_object_emplace(result.first->second, old_capacity);
         }
 
         throw_subscript_wrong_type("string");
@@ -34009,8 +33987,9 @@ public:
         }
 
         // add the element to the object
+        const auto old_capacity = object_capacity();
         auto res = m_data.m_value.object->insert(val);
-        set_parent(res.first->second);
+        set_parent_after_object_emplace(res.first->second, old_capacity);
     }
 
     /// @brief add an object to an object
@@ -34086,8 +34065,9 @@ public:
         }
 
         // add the element to the array (perfect forwarding)
+        const auto old_capacity = object_capacity();
         auto res = m_data.m_value.object->emplace(std::forward<Args>(args)...);
-        set_parent(res.first->second);
+        set_parent_after_object_emplace(res.first->second, old_capacity);
 
         // create a result iterator and set iterator to the result of emplace
         auto it = begin();

tests/src/unit-diagnostics.cpp

@@ -13,9 +13,11 @@
 #endif
 
 #define JSON_DIAGNOSTICS 1
+#define JSON_TESTS_PRIVATE
 
 #include <nlohmann/json.hpp>
 using nlohmann::json;
+using nlohmann::ordered_json;
 
 #include <map>
 #include <unordered_map>
@@ -631,3 +633,99 @@ TEST_CASE("Better diagnostics past the descent bound of update(), merge_patch()
         CHECK_THROWS_WITH_AS(p->at("y").at(0), expected_y.c_str(), json::type_error);
     }
 }
+
+namespace
+{
+// check that every child's parent pointer points to the containing value
+template<typename BasicJsonType>
+bool parents_consistent(const BasicJsonType& j)
+{
+    bool ok = true;
+    if (j.is_array() || j.is_object())
+    {
+        for (const auto& child : j)
+        {
+            ok = ok && child.m_parent == &j && parents_consistent(child);
+        }
+    }
+    return ok;
+}
+} // namespace
+
+TEST_CASE_TEMPLATE("Parent pointers stay consistent", J, json, ordered_json)
+{
+    SECTION("parse, copy, move, assign, swap")
+    {
+        J j = J::parse(R"({"a":[1,{"b":[2,3]},[4]],"c":{"d":{"e":[]}}})");
+        CHECK(parents_consistent(j));
+
+        J copy(j);
+        CHECK(parents_consistent(copy));
+
+        J moved(std::move(copy));
+        CHECK(parents_consistent(moved));
+
+        J assigned;
+        assigned = j;
+        CHECK(parents_consistent(assigned));
+
+        J other = {1, 2, {{"k", J::array({1, 2})}}};
+        other.swap(assigned);
+        CHECK(parents_consistent(other));
+        CHECK(parents_consistent(assigned));
+    }
+
+    SECTION("push_back growth")
+    {
+        J arr = J::array();
+        for (int i = 0; i < 500; ++i)
+        {
+            arr.push_back(J::array({i}));
+        }
+        CHECK(parents_consistent(arr));
+
+        J obj = J::object();
+        for (int i = 0; i < 500; ++i)
+        {
+            obj.push_back({std::to_string(i), J::array({i})});
+        }
+        CHECK(parents_consistent(obj));
+    }
+
+    SECTION("emplace_back and emplace growth")
+    {
+        J arr = J::array();
+        J obj = J::object();
+        for (int i = 0; i < 500; ++i)
+        {
+            arr.emplace_back(J::array({i}));
+            obj.emplace(std::to_string(i), J::array({i}));
+        }
+        CHECK(parents_consistent(arr));
+        CHECK(parents_consistent(obj));
+    }
+
+    SECTION("operator[] growth and at()")
+    {
+        J obj = J::object();
+        for (int i = 0; i < 500; ++i)
+        {
+            obj[std::to_string(i)] = J::array({i});
+            CHECK(obj[std::to_string(i / 2)].m_parent == &obj);
+        }
+        CHECK(parents_consistent(obj));
+
+        J arr = J::array();
+        for (int i = 0; i < 500; ++i)
+        {
+            arr[static_cast<size_t>(i)] = J::object({{"x", i}});
+        }
+        for (size_t i = 0; i < 500; ++i)
+        {
+            CHECK(arr.at(i).m_parent == &arr);
+        }
+        CHECK(obj.at("3").m_parent == &obj);
+        CHECK(parents_consistent(arr));
+        CHECK(parents_consistent(obj));
+    }
+}

tests/src/unit-to_json-rvalue.cpp

@@ -1,257 +0,0 @@
-//     __ _____ _____ _____
-//  __|  |   __|     |   | |  JSON for Modern C++ (supporting code)
-// |  |  |__   |  |  | | | |  version 3.12.0
-// |_____|_____|_____|_|___|  https://github.com/nlohmann/json
-//
-// SPDX-FileCopyrightText: 2013-2026 Niels Lohmann <https://nlohmann.me>
-// SPDX-License-Identifier: MIT
-
-#include "doctest_compatibility.h"
-
-#include <nlohmann/json.hpp>
-using nlohmann::json;
-
-#include <array>
-#include <deque>
-#include <forward_list>
-#include <list>
-#include <map>
-#include <set>
-#include <string>
-#include <unordered_map>
-#include <utility>
-#include <valarray>
-#include <vector>
-
-#if (defined(__cplusplus) && __cplusplus >= 201703L) || (defined(_HAS_CXX17) && _HAS_CXX17 == 1)
-    #if __has_include(<filesystem>)
-        #include <filesystem>
-        #define TO_JSON_RVALUE_HAS_FS 1
-    #endif
-#endif
-
-namespace
-{
-// element type that counts how it is converted to JSON: a conversion from an
-// lvalue copies, a conversion from an rvalue moves
-struct tracked
-{
-    std::string payload;
-    bool moved_from = false;
-
-    explicit tracked(std::string p) : payload(std::move(p)) {}
-    tracked(const tracked&) = default;
-    tracked& operator=(const tracked&) = default;
-    tracked(tracked&& other) noexcept : payload(std::move(other.payload)), moved_from(other.moved_from)
-    {
-        other.moved_from = true;
-    }
-    tracked& operator=(tracked&& other) noexcept
-    {
-        payload = std::move(other.payload);
-        moved_from = other.moved_from;
-        other.moved_from = true;
-        return *this;
-    }
-    ~tracked() = default;
-};
-
-int tracked_copies = 0; // NOLINT(cppcoreguidelines-avoid-non-const-global-variables,misc-use-internal-linkage)
-int tracked_moves = 0;  // NOLINT(cppcoreguidelines-avoid-non-const-global-variables,misc-use-internal-linkage)
-
-void to_json(json& j, const tracked& t)
-{
-    ++tracked_copies;
-    j = t.payload;
-}
-
-void to_json(json& j, tracked&& t)
-{
-    ++tracked_moves;
-    j = std::move(t.payload);
-    t.moved_from = true;
-}
-
-// user type with its own rvalue-aware serializer
-struct udt_container
-{
-    std::vector<int> values;
-};
-} // namespace
-
-namespace nlohmann
-{
-template<>
-struct adl_serializer<udt_container>
-{
-    static void to_json(json& j, const udt_container& c)
-    {
-        j = c.values;
-        j.push_back("copied");
-    }
-    static void to_json(json& j, udt_container&& c)
-    {
-        j = std::move(c.values);
-        j.push_back("moved");
-    }
-};
-} // namespace nlohmann
-
-TEST_CASE("to_json from rvalue containers")
-{
-    const std::string long_a(64, 'a');
-    const std::string long_b(64, 'b');
-
-    SECTION("vector<std::string>: elements are moved")
-    {
-        std::vector<std::string> v{long_a, long_b};
-        const json j = std::move(v);
-        CHECK(j == json::array({long_a, long_b}));
-        for (const auto& s : v) // NOLINT(bugprone-use-after-move,hicpp-invalid-access-moved)
-        {
-            CHECK(s.empty()); // long strings leave the source element empty when moved
-        }
-    }
-
-    SECTION("vector<std::string>: lvalues are copied and left untouched")
-    {
-        const std::vector<std::string> v{long_a, long_b};
-        const json j = v;
-        CHECK(j == json::array({long_a, long_b}));
-        CHECK(v[0] == long_a);
-        CHECK(v[1] == long_b);
-
-        std::vector<std::string> v2{long_a, long_b};
-        const json j2 = v2;
-        CHECK(j2 == j);
-        CHECK(v2[0] == long_a);
-        CHECK(v2[1] == long_b);
-    }
-
-    SECTION("tracked elements: rvalue moves, lvalue copies")
-    {
-        tracked_copies = 0;
-        tracked_moves = 0;
-
-        std::vector<tracked> v;
-        v.emplace_back("x");
-        v.emplace_back("y");
-
-        const json copy = v;
-        CHECK(copy == json::array({"x", "y"}));
-        CHECK(tracked_copies == 2);
-        CHECK(tracked_moves == 0);
-        CHECK_FALSE(v[0].moved_from);
-
-        const json moved = std::move(v);
-        CHECK(moved == copy);
-        CHECK(tracked_copies == 2);
-        CHECK(tracked_moves == 2);
-    }
-
-    SECTION("const rvalue is copied")
-    {
-        tracked_copies = 0;
-        tracked_moves = 0;
-        const std::vector<tracked> v(2, tracked("z"));
-        const json j = std::move(v); // NOLINT(performance-move-const-arg,hicpp-move-const-arg)
-        CHECK(j == json::array({"z", "z"}));
-        CHECK(tracked_copies == 2);
-        CHECK(tracked_moves == 0);
-    }
-
-    SECTION("other sequence containers")
-    {
-        CHECK(json(std::deque<std::string> {long_a, long_b}) == json::array({long_a, long_b}));
-        CHECK(json(std::list<std::string> {long_a, long_b}) == json::array({long_a, long_b}));
-        CHECK(json(std::forward_list<std::string> {long_a, long_b}) == json::array({long_a, long_b}));
-        CHECK(json(std::array<std::string, 2> {{long_a, long_b}}) == json::array({long_a, long_b}));
-        CHECK(json(std::set<std::string> {long_a, long_b}) == json::array({long_a, long_b}));
-        CHECK(json(std::vector<bool> {true, false}) == json::array({true, false}));
-        CHECK(json(std::vector<int> {1, 2, 3}) == json::array({1, 2, 3}));
-        CHECK(json(std::vector<std::vector<std::string>> {{long_a}, {long_b}}) == json::array({json::array({long_a}), json::array({long_b})}));
-        CHECK(json(std::vector<json> {1, "a"}) == json::array({1, "a"}));
-    }
-
-    SECTION("valarray")
-    {
-        std::valarray<int> va(3);
-        va[0] = 1;
-        va[1] = 2;
-        va[2] = 3;
-        CHECK(json(std::move(va)) == json::array({1, 2, 3})); // NOLINT(performance-move-const-arg,hicpp-move-const-arg)
-        CHECK(json(std::valarray<int>(2)) == json::array({0, 0}));
-    }
-
-    SECTION("array_t and json arrays")
-    {
-        json::array_t arr{1, 2};
-        CHECK(json(std::move(arr)) == json::array({1, 2}));
-        json a = json::array({1, 2});
-        const json b = std::move(a);
-        CHECK(b == json::array({1, 2}));
-    }
-
-    SECTION("object-like containers")
-    {
-        std::map<std::string, std::string> m{{"a", long_a}, {"b", long_b}};
-        const json j = std::move(m);
-        CHECK(j == json({{"a", long_a}, {"b", long_b}}));
-        for (const auto& kv : m) // NOLINT(bugprone-use-after-move,hicpp-invalid-access-moved)
-        {
-            CHECK(kv.second.empty()); // values moved, keys (const) copied
-        }
-
-        const std::map<std::string, std::string> cm{{"a", long_a}};
-        const json jc = cm;
-        CHECK(jc["a"] == long_a);
-        CHECK(cm.at("a") == long_a);
-
-        std::unordered_map<std::string, std::string> um{{"a", long_a}};
-        CHECK(json(std::move(um)) == json({{"a", long_a}}));
-
-        std::multimap<std::string, int> mm{{"a", 1}};
-        CHECK(json(std::move(mm)) == json({{"a", 1}}));
-
-        tracked_copies = 0;
-        tracked_moves = 0;
-        std::map<std::string, tracked> mt;
-        mt.emplace("k", tracked("v"));
-        const json jt = std::move(mt);
-        CHECK(jt == json({{"k", "v"}}));
-        CHECK(tracked_moves == 1);
-        CHECK(tracked_copies == 0);
-
-        json::object_t obj{{"a", 1}};
-        CHECK(json(std::move(obj)) == json({{"a", 1}}));
-    }
-
-    SECTION("user type with its own to_json overloads")
-    {
-        udt_container c{{1, 2}};
-        const json lv = c;
-        CHECK(lv == json::array({1, 2, "copied"}));
-        const json rv = std::move(c);
-        CHECK(rv == json::array({1, 2, "moved"}));
-    }
-
-    SECTION("assignment and implicit conversion")
-    {
-        std::vector<std::string> v{long_a};
-        json j;
-        j = std::move(v);
-        CHECK(j == json::array({long_a}));
-        json k = {std::vector<std::string>{long_b}};
-        CHECK(k.size() == 1);
-    }
-
-#ifdef TO_JSON_RVALUE_HAS_FS
-    SECTION("std::filesystem::path rvalue is still a string")
-    {
-        CHECK(json(std::filesystem::path("a/b")) == json("a/b"));
-#ifdef __cpp_lib_char8_t
-        CHECK(json(std::u8string(u8"a/b")) == json("a/b"));
-#endif
-    }
-#endif
-}