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Exchange the CustomBaseClass subobject in basic_json::swap()

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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>
This commit is contained in:
Niels Lohmann
2026-09-29 23:38:56 +02:00
parent 633de8e44b
commit 07fee4fd55
4 changed files with 110 additions and 8 deletions
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single_include/nlohmann/json.hpp
+13 -2
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@@ -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);