Fix swap(array_t&)/swap(object_t&) to update parent pointers under JSON_DIAGNOSTICS

Both overloads swapped the underlying container storage but never called
set_parents(), leaving elements moved into *this with stale m_parent
pointers (typically nullptr from the free-standing array_t/object_t).
This produced wrong JSON Pointer paths in diagnostic messages and could
trip assert_invariant() on subsequent copies. Mirrors the fix already
applied in swap(reference other).

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

docs/mkdocs/docs/examples/get__ValueType_const.output

@@ -4,8 +4,8 @@
 Hello, world!
 1 2 3 4 5 
 
+string: "Hello, world!"
 number: {"floating-point":17.23,"integer":42}
 null: null
-string: "Hello, world!"
 boolean: true
 array: [1,2,3,4,5]

docs/mkdocs/docs/examples/get_to.output

@@ -4,8 +4,8 @@
 Hello, world!
 1 2 3 4 5 
 
+string: "Hello, world!"
 number: {"floating-point":17.23,"integer":42}
 null: null
-string: "Hello, world!"
 boolean: true
 array: [1,2,3,4,5]

docs/mkdocs/docs/examples/operator__ValueType.output

@@ -4,9 +4,9 @@
 Hello, world!
 1 2 3 4 5 
 
+string: "Hello, world!"
 number: {"floating-point":17.23,"integer":42}
 null: null
-string: "Hello, world!"
 boolean: true
 array: [1,2,3,4,5]
 [json.exception.type_error.302] type must be boolean, but is string

include/nlohmann/detail/conversions/from_json.hpp

@@ -398,34 +398,6 @@ inline void from_json(const BasicJsonType& j, CompatibleArrayType& bin)
     }
 }
 
-template<typename BasicJsonType, typename ConstructibleObjectType>
-auto from_json_object_impl(const BasicJsonType& j, ConstructibleObjectType& obj, priority_tag<1> /*unused*/)
--> decltype(
-    obj.reserve(std::declval<typename ConstructibleObjectType::size_type>()),
-    void())
-{
-    ConstructibleObjectType ret;
-    const auto* inner_object = j.template get_ptr<const typename BasicJsonType::object_t*>();
-    ret.reserve(inner_object->size());
-    for (const auto& p : *inner_object)
-    {
-        ret.emplace(p.first, p.second.template get<typename ConstructibleObjectType::mapped_type>());
-    }
-    obj = std::move(ret);
-}
-
-template<typename BasicJsonType, typename ConstructibleObjectType>
-inline void from_json_object_impl(const BasicJsonType& j, ConstructibleObjectType& obj, priority_tag<0> /*unused*/)
-{
-    ConstructibleObjectType ret;
-    const auto* inner_object = j.template get_ptr<const typename BasicJsonType::object_t*>();
-    for (const auto& p : *inner_object)
-    {
-        ret.emplace(p.first, p.second.template get<typename ConstructibleObjectType::mapped_type>());
-    }
-    obj = std::move(ret);
-}
-
 template<typename BasicJsonType, typename ConstructibleObjectType,
          enable_if_t<is_constructible_object_type<BasicJsonType, ConstructibleObjectType>::value, int> = 0>
 inline void from_json(const BasicJsonType& j, ConstructibleObjectType& obj)
@@ -435,7 +407,13 @@ inline void from_json(const BasicJsonType& j, ConstructibleObjectType& obj)
         JSON_THROW(type_error::create(302, concat("type must be object, but is ", j.type_name()), &j));
     }
 
-    from_json_object_impl(j, obj, priority_tag<1> {});
+    ConstructibleObjectType ret;
+    const auto* inner_object = j.template get_ptr<const typename BasicJsonType::object_t*>();
+    for (const auto& p : *inner_object)
+    {
+        ret.emplace(p.first, p.second.template get<typename ConstructibleObjectType::mapped_type>());
+    }
+    obj = std::move(ret);
 }
 
 // overload for arithmetic types, not chosen for basic_json template arguments

include/nlohmann/json.hpp

@@ -3573,6 +3573,7 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
         {
             using std::swap;
             swap(*(m_data.m_value.array), other);
+            set_parents();
         }
         else
         {
@@ -3589,6 +3590,7 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
         {
             using std::swap;
             swap(*(m_data.m_value.object), other);
+            set_parents();
         }
         else
         {

single_include/nlohmann/json.hpp

@@ -5693,34 +5693,6 @@ inline void from_json(const BasicJsonType& j, CompatibleArrayType& bin)
     }
 }
 
-template<typename BasicJsonType, typename ConstructibleObjectType>
-auto from_json_object_impl(const BasicJsonType& j, ConstructibleObjectType& obj, priority_tag<1> /*unused*/)
--> decltype(
-    obj.reserve(std::declval<typename ConstructibleObjectType::size_type>()),
-    void())
-{
-    ConstructibleObjectType ret;
-    const auto* inner_object = j.template get_ptr<const typename BasicJsonType::object_t*>();
-    ret.reserve(inner_object->size());
-    for (const auto& p : *inner_object)
-    {
-        ret.emplace(p.first, p.second.template get<typename ConstructibleObjectType::mapped_type>());
-    }
-    obj = std::move(ret);
-}
-
-template<typename BasicJsonType, typename ConstructibleObjectType>
-inline void from_json_object_impl(const BasicJsonType& j, ConstructibleObjectType& obj, priority_tag<0> /*unused*/)
-{
-    ConstructibleObjectType ret;
-    const auto* inner_object = j.template get_ptr<const typename BasicJsonType::object_t*>();
-    for (const auto& p : *inner_object)
-    {
-        ret.emplace(p.first, p.second.template get<typename ConstructibleObjectType::mapped_type>());
-    }
-    obj = std::move(ret);
-}
-
 template<typename BasicJsonType, typename ConstructibleObjectType,
          enable_if_t<is_constructible_object_type<BasicJsonType, ConstructibleObjectType>::value, int> = 0>
 inline void from_json(const BasicJsonType& j, ConstructibleObjectType& obj)
@@ -5730,7 +5702,13 @@ inline void from_json(const BasicJsonType& j, ConstructibleObjectType& obj)
         JSON_THROW(type_error::create(302, concat("type must be object, but is ", j.type_name()), &j));
     }
 
-    from_json_object_impl(j, obj, priority_tag<1> {});
+    ConstructibleObjectType ret;
+    const auto* inner_object = j.template get_ptr<const typename BasicJsonType::object_t*>();
+    for (const auto& p : *inner_object)
+    {
+        ret.emplace(p.first, p.second.template get<typename ConstructibleObjectType::mapped_type>());
+    }
+    obj = std::move(ret);
 }
 
 // overload for arithmetic types, not chosen for basic_json template arguments
@@ -25023,6 +25001,7 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
         {
             using std::swap;
             swap(*(m_data.m_value.array), other);
+            set_parents();
         }
         else
         {
@@ -25039,6 +25018,7 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
         {
             using std::swap;
             swap(*(m_data.m_value.object), other);
+            set_parents();
         }
         else
         {

tests/src/unit-conversions.cpp

@@ -1389,37 +1389,6 @@ TEST_CASE("value conversion")
                 // CHECK(m5["one"] == "eins");
             }
 
-            SECTION("reserve is called on containers that support it (#5406)")
-            {
-                // build a larger object so that a missing/incorrect reserve()
-                // call would be more likely to corrupt or drop elements
-                json j_large;
-                for (int i = 0; i < 100; ++i)
-                {
-                    j_large[std::to_string(i)] = i;
-                }
-
-                SECTION("std::unordered_map (supports reserve)")
-                {
-                    const auto m = j_large.get<std::unordered_map<std::string, int>>();
-                    CHECK(m.size() == 100);
-                    for (int i = 0; i < 100; ++i)
-                    {
-                        CHECK(m.at(std::to_string(i)) == i);
-                    }
-                }
-
-                SECTION("std::map (no reserve, fallback path)")
-                {
-                    const auto m = j_large.get<std::map<std::string, int>>();
-                    CHECK(m.size() == 100);
-                    for (int i = 0; i < 100; ++i)
-                    {
-                        CHECK(m.at(std::to_string(i)) == i);
-                    }
-                }
-            }
-
             SECTION("std::multimap")
             {
                 j1.get<std::multimap<std::string, int>>();

tests/src/unit-diagnostics.cpp

@@ -273,5 +273,36 @@ TEST_CASE("Regression tests for extended diagnostics")
         CHECK(j1["numbers"]["two"] == 2);
         CHECK(j1["string"] == "t");
     }
+
+    SECTION("Regression test - swap(array_t&)/swap(object_t&) must update JSON_DIAGNOSTICS parent pointers")
+    {
+        // swap(array_t&)
+        {
+            json j = json::array();
+            json::array_t arr = {json::array({1})};
+            j.swap(arr);
+
+            // parent pointers of the moved-in elements must point into j, not
+            // into the now-defunct free-standing array_t
+            CHECK_THROWS_WITH_AS(j[0][0].get<std::string>(), "[json.exception.type_error.302] (/0/0) type must be string, but is number", json::type_error);
+
+            // must not trigger assert_invariant() in a debug/assert-enabled build
+            json const k = j;
+            CHECK(k == j);
+        }
+
+        // swap(object_t&)
+        {
+            json o = json::object();
+            json::object_t obj = {{"a", json::array({1})}};
+            o.swap(obj);
+
+            CHECK_THROWS_WITH_AS(o["a"][0].get<std::string>(), "[json.exception.type_error.302] (/a/0) type must be string, but is number", json::type_error);
+
+            // must not trigger assert_invariant() in a debug/assert-enabled build
+            json const p = o;
+            CHECK(p == o);
+        }
+    }
 }