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Keep multimaps with enum keys as arrays of pairs

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With JSON_USE_OBJECTS_FOR_ENUM_KEYED_MAPS, is_enum_keyed_map also matched
std::multimap and std::unordered_multimap. Storing them as objects throws
type_error.318 as soon as a key occurs twice, which is the normal case for
a multimap, so such values could no longer be serialized at all once the
macro was enabled, although they are stored losslessly as arrays of
[key, value] pairs without it.

Exclude maps with non-unique keys from is_enum_keyed_map. They are
detected by insert(value_type) returning an iterator rather than a
pair<iterator, bool>. Map-like types without such an insert() are still
treated as before.

Signed-off-by: Niels Lohmann <mail@nlohmann.me>
This commit is contained in:
Niels Lohmann
2026-09-27 18:24:48 +02:00
parent d914e3a027
commit 99716a5ade
4 changed files with 35 additions and 2 deletions
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single_include/nlohmann/json.hpp
+11 -1
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@@ -4421,6 +4421,12 @@ template<typename BasicJsonType, typename CompatibleObjectType>
struct is_compatible_object_type
: is_compatible_object_type_impl<BasicJsonType, CompatibleObjectType> {};
template<typename T>
using insert_result_t = decltype(std::declval<T&>().insert(std::declval<const value_type_t<T>&>()));
template<typename T>
using insert_result_second_t = decltype(std::declval<T&>().insert(std::declval<const value_type_t<T>&>()).second);
// a map-like type (std::map, std::unordered_map, ...) whose keys are enums; see
// JSON_USE_OBJECTS_FOR_ENUM_KEYED_MAPS
template<typename T, typename = void>
@@ -4431,8 +4437,12 @@ struct is_enum_keyed_map <
T, enable_if_t < is_detected<mapped_type_t, T>::value&&
is_detected<key_type_t, T>::value >>
{
// maps with non-unique keys (std::multimap, std::unordered_multimap, ...)
// are excluded, because an object cannot hold duplicate keys; they are
// detected by insert() returning an iterator instead of a pair<iterator, bool>
// NOLINTNEXTLINE(modernize-type-traits) we use C++11
static constexpr bool value = std::is_enum<typename T::key_type>::value;
static constexpr bool value = std::is_enum<typename T::key_type>::value &&
!(is_detected<insert_result_t, T>::value && !is_detected<insert_result_second_t, T>::value);
};
template<typename BasicJsonType, typename ConstructibleObjectType,