Merge branch 'develop' into issue-4378-enum-map-keys

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

docs/mkdocs/docs/api/macros/index.md

@@ -53,6 +53,8 @@ header. See also the [macro overview page](../../features/macros.md).
 - [**JSON_BRACE_INIT_COPY_SEMANTICS**](json_brace_init_copy_semantics.md) - opt in to copy/move semantics for single-element brace initialization
 - [**JSON_DISABLE_ENUM_SERIALIZATION**](json_disable_enum_serialization.md) - switch off default serialization/deserialization functions for enums
 - [**JSON_USE_IMPLICIT_CONVERSIONS**](json_use_implicit_conversions.md) - control implicit conversions
+- [**JSON_USE_OBJECTS_FOR_ENUM_KEYED_MAPS**](json_use_objects_for_enum_keyed_maps.md) - opt in to storing maps with enum
+  keys as objects
 
 ## Comparison behavior
 

docs/mkdocs/docs/api/macros/json_use_objects_for_enum_keyed_maps.md

@@ -0,0 +1,139 @@
+# JSON_USE_OBJECTS_FOR_ENUM_KEYED_MAPS
+
+```cpp
+#define JSON_USE_OBJECTS_FOR_ENUM_KEYED_MAPS /* value */
+```
+
+When defined to `1`, maps whose keys are enums (such as `std::map<E, T>` or `std::unordered_map<E, T>`) are stored as
+JSON objects, using the enum's own conversion for the keys. By default, they are stored as arrays of `[key, value]`
+pairs.
+
+## Default definition
+
+The default value is `0` (disabled — existing behavior is preserved).
+
+```cpp
+#define JSON_USE_OBJECTS_FOR_ENUM_KEYED_MAPS 0
+```
+
+## Notes
+
+!!! note "Background"
+
+    JSON object keys are strings, so a map is only stored as an object if its keys can be converted to a string type.
+    Enums are not, even if [`NLOHMANN_JSON_SERIALIZE_ENUM`](nlohmann_json_serialize_enum.md) maps them to strings, so a
+    map with enum keys becomes an array of `[key, value]` pairs:
+
+    ```json
+    [["stopped", "aa"], ["completed", "bb"]]
+    ```
+
+    With this macro, the same map becomes an object
+    (see [#4378](https://github.com/nlohmann/json/issues/4378)):
+
+    ```json
+    {"completed": "bb", "stopped": "aa"}
+    ```
+
+!!! note "Maps with non-unique keys"
+
+    Maps that allow duplicate keys, such as `std::multimap<E, T>` or `std::unordered_multimap<E, T>`, are not affected
+    by the macro and are still stored as arrays of `[key, value]` pairs, as an object cannot hold duplicate keys.
+
+!!! note "Reading"
+
+    Reading is not affected by the macro: a map with enum keys can always be read from both an array of pairs and an
+    object. For the latter, each key is converted to the enum with its `from_json` function, e.g., the one defined by
+    [`NLOHMANN_JSON_SERIALIZE_ENUM`](nlohmann_json_serialize_enum.md). Data written without the macro can therefore
+    still be read after enabling it.
+
+!!! warning "Keys must serialize to distinct strings"
+
+    Each key is converted with the enum's `to_json` function. If a key is not converted to a string (for instance, an
+    enum without [`NLOHMANN_JSON_SERIALIZE_ENUM`](nlohmann_json_serialize_enum.md), which is stored as an integer, or an
+    enumerator mapped to `nullptr`), [`type_error.302`](../../home/exceptions.md#jsonexceptiontype_error302) is thrown.
+    If two keys are converted to the same string (for instance, because
+    [`NLOHMANN_JSON_SERIALIZE_ENUM`](nlohmann_json_serialize_enum.md) maps an unlisted enumerator to the first entry),
+    [`type_error.318`](../../home/exceptions.md#jsonexceptiontype_error318) is thrown. In both cases, the target value
+    is not changed.
+
+!!! warning "Opt-in only"
+
+    This macro must be defined **before** including `<nlohmann/json.hpp>`. Defining it after the include has no effect.
+
+!!! note "ABI compatibility"
+
+    The value of this macro is encoded in the [namespace](../../features/namespace.md) (tag `_ekmo`), resulting in
+    distinct symbol names. Translation units compiled with and without it can therefore be linked into the same program
+    without One Definition Rule (ODR) violations, but they cannot exchange instances of library types.
+
+## Examples
+
+??? example "Default behavior (macro not defined)"
+
+    Without the macro, a map with enum keys is stored as an array of pairs:
+
+    ```cpp
+    #include <map>
+    #include <nlohmann/json.hpp>
+
+    using json = nlohmann::json;
+
+    enum TaskState { TS_STOPPED, TS_RUNNING, TS_COMPLETED };
+
+    NLOHMANN_JSON_SERIALIZE_ENUM(TaskState, {
+        {TS_STOPPED, "stopped"},
+        {TS_RUNNING, "running"},
+        {TS_COMPLETED, "completed"},
+    })
+
+    int main()
+    {
+        std::map<TaskState, std::string> m = {{TS_STOPPED, "aa"}, {TS_COMPLETED, "bb"}};
+
+        json j = m;
+        // j is [["stopped","aa"],["completed","bb"]]
+    }
+    ```
+
+??? example "Objects for enum-keyed maps (macro defined to 1)"
+
+    With the macro, the same map is stored as an object:
+
+    ```cpp
+    #define JSON_USE_OBJECTS_FOR_ENUM_KEYED_MAPS 1
+    #include <map>
+    #include <nlohmann/json.hpp>
+
+    using json = nlohmann::json;
+
+    enum TaskState { TS_STOPPED, TS_RUNNING, TS_COMPLETED };
+
+    NLOHMANN_JSON_SERIALIZE_ENUM(TaskState, {
+        {TS_STOPPED, "stopped"},
+        {TS_RUNNING, "running"},
+        {TS_COMPLETED, "completed"},
+    })
+
+    int main()
+    {
+        std::map<TaskState, std::string> m = {{TS_STOPPED, "aa"}, {TS_COMPLETED, "bb"}};
+
+        json j = m;
+        // j is {"completed":"bb","stopped":"aa"}
+
+        auto m2 = j.get<std::map<TaskState, std::string>>();
+        // m2 == m
+    }
+    ```
+
+## See also
+
+- [Specializing enum conversion](../../features/enum_conversion.md)
+- [**NLOHMANN_JSON_SERIALIZE_ENUM**](nlohmann_json_serialize_enum.md) - serialize/deserialize an enum
+- [**NLOHMANN_JSON_SERIALIZE_ENUM_STRICT**](nlohmann_json_serialize_enum_strict.md) - serialize/deserialize an enum with
+  exceptions
+
+## Version history
+
+- Added in version 3.13.0.

docs/mkdocs/docs/api/macros/nlohmann_json_serialize_enum.md

@@ -41,6 +41,9 @@ inline void from_json(const BasicJsonType& j, type& e);
       conversion. Select this default pair carefully. See example 1 below.
     - If an enum or JSON value is specified in multiple conversions, the first matching conversion from the top of the
       list will be returned when converting to or from JSON. See example 2 below.
+    - Maps with enum keys (e.g., `std::map<ENUM_TYPE, T>`) are stored as arrays of `[key, value]` pairs by default.
+      Define [`JSON_USE_OBJECTS_FOR_ENUM_KEYED_MAPS`](json_use_objects_for_enum_keyed_maps.md) to store them as objects
+      with the converted keys. Such maps can be read from both forms.
 
 ## Examples
 
@@ -80,6 +83,7 @@ inline void from_json(const BasicJsonType& j, type& e);
 - [Specializing enum conversion](../../features/enum_conversion.md)
 - [`NLOHMANN_JSON_SERIALIZE_ENUM_STRICT`](./nlohmann_json_serialize_enum_strict.md)
 - [`JSON_DISABLE_ENUM_SERIALIZATION`](json_disable_enum_serialization.md)
+- [`JSON_USE_OBJECTS_FOR_ENUM_KEYED_MAPS`](json_use_objects_for_enum_keyed_maps.md)
 
 ## Version history
 

docs/mkdocs/docs/api/macros/nlohmann_json_serialize_enum_strict.md

@@ -44,6 +44,9 @@ inline void from_json(const BasicJsonType& j, type& e);
       `"enum value out of range for <type>"`.
     - If an enum or JSON value is specified in multiple conversions, the first matching conversion from the top of the
       list will be returned when converting to or from JSON. See example 2 below.
+    - Maps with enum keys (e.g., `std::map<ENUM_TYPE, T>`) are stored as arrays of `[key, value]` pairs by default.
+      Define [`JSON_USE_OBJECTS_FOR_ENUM_KEYED_MAPS`](json_use_objects_for_enum_keyed_maps.md) to store them as objects
+      with the converted keys. Such maps can be read from both forms.
 
 ## Examples
 
@@ -99,6 +102,7 @@ inline void from_json(const BasicJsonType& j, type& e);
 - [Specializing enum conversion](../../features/enum_conversion.md)
 - [`NLOHMANN_JSON_SERIALIZE_ENUM`](./nlohmann_json_serialize_enum.md)
 - [`JSON_DISABLE_ENUM_SERIALIZATION`](json_disable_enum_serialization.md)
+- [`JSON_USE_OBJECTS_FOR_ENUM_KEYED_MAPS`](json_use_objects_for_enum_keyed_maps.md)
 
 ## Version history
 

docs/mkdocs/docs/features/enum_conversion.md

@@ -43,6 +43,23 @@ json jPi = 3.14;
 assert(jPi.get<TaskState>() == TS_INVALID );
 ```
 
+## Maps with enum keys
+
+By default, maps with enum keys, such as `std::map<TaskState, std::string>`, are stored as arrays of `[key, value]`
+pairs, because JSON object keys must be strings. Define
+[`JSON_USE_OBJECTS_FOR_ENUM_KEYED_MAPS`](../api/macros/json_use_objects_for_enum_keyed_maps.md) before including the
+library to store them as objects, with the keys converted by the enum's `to_json()` function:
+
+```cpp
+std::map<TaskState, std::string> m = {{TS_STOPPED, "aa"}, {TS_COMPLETED, "bb"}};
+
+json j = m;
+// default:                                   [["stopped","aa"],["completed","bb"]]
+// with JSON_USE_OBJECTS_FOR_ENUM_KEYED_MAPS: {"completed":"bb","stopped":"aa"}
+```
+
+Either form can be read back, with or without the macro.
+
 ## Notes
 
 Just as in [Arbitrary Type Conversions](arbitrary_types.md) above,

docs/mkdocs/docs/features/macros.md

@@ -167,6 +167,13 @@ behavior is deprecated and switched off (`0`) by default.
 
 See [full documentation of `JSON_USE_LEGACY_DISCARDED_VALUE_COMPARISON`](../api/macros/json_use_legacy_discarded_value_comparison.md).
 
+## `JSON_USE_OBJECTS_FOR_ENUM_KEYED_MAPS`
+
+When defined to `1`, maps with enum keys (e.g., `std::map<E, T>`) are stored as objects, using the enum's conversion for
+the keys, instead of arrays of `[key, value]` pairs. It is switched off (`0`) by default.
+
+See [full documentation of `JSON_USE_OBJECTS_FOR_ENUM_KEYED_MAPS`](../api/macros/json_use_objects_for_enum_keyed_maps.md).
+
 ## `JSON_USE_SIMDUTF`
 
 When defined, UTF-8 validation of JSON strings read from contiguous byte input is delegated to the

docs/mkdocs/docs/features/namespace.md

@@ -20,6 +20,8 @@ The complete default namespace name is derived as follows:
       `_bics`.
     - [`JSON_PRECISE_STREAM_POSITION`](../api/macros/json_precise_stream_position.md) defined non-zero appends `_psp`.
     - [`JSON_STRICT_NUL_HANDLING`](../api/macros/json_strict_nul_handling.md) defined non-zero appends `_snul`.
+    - [`JSON_USE_OBJECTS_FOR_ENUM_KEYED_MAPS`](../api/macros/json_use_objects_for_enum_keyed_maps.md) defined non-zero
+      appends `_ekmo`.
 - The inline namespace ends with the suffix `_v` followed by the 3 components of the version number separated by
   underscores. To omit the version component, see [Disabling the version component](#disabling-the-version-component)
   below.

docs/mkdocs/docs/home/exceptions.md

@@ -596,6 +596,9 @@ During implicit or explicit value conversion, the JSON type must be compatible w
     [json.exception.type_error.302] type must be string, but is object
     ```
 
+This exception is also thrown with [`JSON_USE_OBJECTS_FOR_ENUM_KEYED_MAPS`](../api/macros/json_use_objects_for_enum_keyed_maps.md)
+if a key of a map with enum keys is not converted to a string, for instance, because the enum is stored as an integer.
+
 ### json.exception.type_error.303
 
 To retrieve a reference to a value stored in a `basic_json` object with `get_ref`, the type of the reference must match the value type. For instance, for a JSON array, the `ReferenceType` must be `array_t &`.
@@ -782,6 +785,19 @@ The dynamic type of the object cannot be represented in the requested serializat
 
     Encapsulate the JSON value in an object. That is, instead of serializing `#!json true`, serialize `#!json {"value": true}`
 
+### json.exception.type_error.318
+
+With [`JSON_USE_OBJECTS_FOR_ENUM_KEYED_MAPS`](../api/macros/json_use_objects_for_enum_keyed_maps.md), a map with enum
+keys is stored as an object. This exception is thrown if two of its keys are converted to the same string, so one of the
+entries would be lost. This happens, for instance, if [`NLOHMANN_JSON_SERIALIZE_ENUM`](../api/macros/nlohmann_json_serialize_enum.md)
+does not list an enumerator and it is therefore converted like the first listed one.
+
+!!! failure "Example message"
+
+    ```
+    [json.exception.type_error.318] duplicate object key 'red'
+    ```
+
 ## Out of range
 
 This exception is thrown in case a library function is called on an input parameter that exceeds the expected range, for instance, in the case of array indices or nonexisting object keys.

docs/mkdocs/mkdocs.yml

@@ -303,6 +303,7 @@ nav:
           - 'JSON_USE_GLOBAL_UDLS': api/macros/json_use_global_udls.md
           - 'JSON_USE_IMPLICIT_CONVERSIONS': api/macros/json_use_implicit_conversions.md
           - 'JSON_USE_LEGACY_DISCARDED_VALUE_COMPARISON': api/macros/json_use_legacy_discarded_value_comparison.md
+          - 'JSON_USE_OBJECTS_FOR_ENUM_KEYED_MAPS': api/macros/json_use_objects_for_enum_keyed_maps.md
           - 'JSON_USE_SIMDUTF': api/macros/json_use_simdutf.md
           - 'NLOHMANN_DEFINE_DERIVED_TYPE_INTRUSIVE, NLOHMANN_DEFINE_DERIVED_TYPE_INTRUSIVE_WITH_DEFAULT, NLOHMANN_DEFINE_DERIVED_TYPE_INTRUSIVE_ONLY_SERIALIZE, NLOHMANN_DEFINE_DERIVED_TYPE_NON_INTRUSIVE, NLOHMANN_DEFINE_DERIVED_TYPE_NON_INTRUSIVE_WITH_DEFAULT, NLOHMANN_DEFINE_DERIVED_TYPE_NON_INTRUSIVE_ONLY_SERIALIZE': api/macros/nlohmann_define_derived_type.md
           - 'NLOHMANN_DEFINE_TYPE_INTRUSIVE, NLOHMANN_DEFINE_TYPE_INTRUSIVE_WITH_DEFAULT, NLOHMANN_DEFINE_TYPE_INTRUSIVE_ONLY_SERIALIZE': api/macros/nlohmann_define_type_intrusive.md

include/nlohmann/detail/abi_macros.hpp

@@ -46,6 +46,10 @@
     #define JSON_STRICT_NUL_HANDLING 0
 #endif
 
+#ifndef JSON_USE_OBJECTS_FOR_ENUM_KEYED_MAPS
+    #define JSON_USE_OBJECTS_FOR_ENUM_KEYED_MAPS 0
+#endif
+
 #if JSON_DIAGNOSTICS
     #define NLOHMANN_JSON_ABI_TAG_DIAGNOSTICS _diag
 #else
@@ -82,14 +86,20 @@
     #define NLOHMANN_JSON_ABI_TAG_STRICT_NUL_HANDLING
 #endif
 
+#if JSON_USE_OBJECTS_FOR_ENUM_KEYED_MAPS
+    #define NLOHMANN_JSON_ABI_TAG_OBJECTS_FOR_ENUM_KEYED_MAPS _ekmo
+#else
+    #define NLOHMANN_JSON_ABI_TAG_OBJECTS_FOR_ENUM_KEYED_MAPS
+#endif
+
 #ifndef NLOHMANN_JSON_NAMESPACE_NO_VERSION
     #define NLOHMANN_JSON_NAMESPACE_NO_VERSION 0
 #endif
 
 // Construct the namespace ABI tags component
-#define NLOHMANN_JSON_ABI_TAGS_CONCAT_EX(a, b, c, d, e, f) json_abi ## a ## b ## c ## d ## e ## f
-#define NLOHMANN_JSON_ABI_TAGS_CONCAT(a, b, c, d, e, f) \
-    NLOHMANN_JSON_ABI_TAGS_CONCAT_EX(a, b, c, d, e, f)
+#define NLOHMANN_JSON_ABI_TAGS_CONCAT_EX(a, b, c, d, e, f, g) json_abi ## a ## b ## c ## d ## e ## f ## g
+#define NLOHMANN_JSON_ABI_TAGS_CONCAT(a, b, c, d, e, f, g) \
+    NLOHMANN_JSON_ABI_TAGS_CONCAT_EX(a, b, c, d, e, f, g)
 
 #define NLOHMANN_JSON_ABI_TAGS                                       \
     NLOHMANN_JSON_ABI_TAGS_CONCAT(                                   \
@@ -98,7 +108,8 @@
             NLOHMANN_JSON_ABI_TAG_DIAGNOSTIC_POSITIONS,              \
             NLOHMANN_JSON_ABI_TAG_BRACE_INIT_COPY_SEMANTICS,         \
             NLOHMANN_JSON_ABI_TAG_PRECISE_STREAM_POSITION,           \
-            NLOHMANN_JSON_ABI_TAG_STRICT_NUL_HANDLING)
+            NLOHMANN_JSON_ABI_TAG_STRICT_NUL_HANDLING,               \
+            NLOHMANN_JSON_ABI_TAG_OBJECTS_FOR_ENUM_KEYED_MAPS)
 
 // Construct the namespace version component
 #define NLOHMANN_JSON_NAMESPACE_VERSION_CONCAT_EX(major, minor, patch) \

include/nlohmann/detail/conversions/from_json.hpp

@@ -550,11 +550,40 @@ auto from_json(BasicJsonType&& j, TupleRelated&& t)
     return from_json_tuple_impl(std::forward<BasicJsonType>(j), std::forward<TupleRelated>(t), priority_tag<3> {});
 }
 
+// read a map with enum keys from an object, using the enum's own from_json for
+// the keys (e.g., from NLOHMANN_JSON_SERIALIZE_ENUM); this is the form written
+// with JSON_USE_OBJECTS_FOR_ENUM_KEYED_MAPS
+template<typename BasicJsonType, typename Map>
+inline bool from_json_enum_keyed_object(const BasicJsonType& j, Map& m, std::true_type /*key is enum*/)
+{
+    if (!j.is_object())
+    {
+        return false;
+    }
+    m.clear();
+    for (const auto& p : *j.template get_ptr<const typename BasicJsonType::object_t*>())
+    {
+        m.emplace(BasicJsonType(p.first).template get<typename Map::key_type>(), p.second.template get<typename Map::mapped_type>());
+    }
+    return true;
+}
+
+template<typename BasicJsonType, typename Map>
+inline bool from_json_enum_keyed_object(const BasicJsonType& /*j*/, Map& /*m*/, std::false_type /*key is enum*/)
+{
+    return false;
+}
+
 template < typename BasicJsonType, typename Key, typename Value, typename Compare, typename Allocator,
            typename = enable_if_t < !std::is_constructible <
                                         typename BasicJsonType::string_t, Key >::value >>
 inline void from_json(const BasicJsonType& j, std::map<Key, Value, Compare, Allocator>& m)
 {
+    // NOLINTNEXTLINE(modernize-type-traits) we use C++11
+    if (from_json_enum_keyed_object(j, m, std::is_enum<Key> {}))
+    {
+        return;
+    }
     if (JSON_HEDLEY_UNLIKELY(!j.is_array()))
     {
         JSON_THROW(type_error::create(302, concat("type must be array, but is ", j.type_name()), &j));
@@ -575,6 +604,11 @@ template < typename BasicJsonType, typename Key, typename Value, typename Hash,
                                         typename BasicJsonType::string_t, Key >::value >>
 inline void from_json(const BasicJsonType& j, std::unordered_map<Key, Value, Hash, KeyEqual, Allocator>& m)
 {
+    // NOLINTNEXTLINE(modernize-type-traits) we use C++11
+    if (from_json_enum_keyed_object(j, m, std::is_enum<Key> {}))
+    {
+        return;
+    }
     if (JSON_HEDLEY_UNLIKELY(!j.is_array()))
     {
         JSON_THROW(type_error::create(302, concat("type must be array, but is ", j.type_name()), &j));

include/nlohmann/detail/conversions/to_json.hpp

@@ -23,6 +23,7 @@
 #include <valarray> // valarray
 #include <vector> // vector
 
+#include <nlohmann/detail/exceptions.hpp>
 #include <nlohmann/detail/iterators/iteration_proxy.hpp>
 #include <nlohmann/detail/meta/cpp_future.hpp>
 #include <nlohmann/detail/meta/std_fs.hpp>
@@ -381,6 +382,9 @@ template < typename BasicJsonType, typename CompatibleArrayType,
                          !is_basic_json<CompatibleArrayType>::value
 #if JSON_HAS_RANGES && !defined(__MINGW32__)
     && !is_compatible_range_view<CompatibleArrayType>::value
+#endif
+#if JSON_USE_OBJECTS_FOR_ENUM_KEYED_MAPS
+    && !is_enum_keyed_map<CompatibleArrayType>::value
 #endif
                          ,
                          int > = 0 >
@@ -435,6 +439,33 @@ inline void to_json(BasicJsonType& j, const CompatibleObjectType& obj)
     external_constructor<value_t::object>::construct(j, obj);
 }
 
+#if JSON_USE_OBJECTS_FOR_ENUM_KEYED_MAPS
+// store a map with enum keys as an object, using the enum's own to_json for the
+// keys (e.g., from NLOHMANN_JSON_SERIALIZE_ENUM); without the macro, such maps
+// are stored as arrays of [key, value] pairs
+template < typename BasicJsonType, typename EnumKeyedMap,
+           enable_if_t < is_enum_keyed_map<EnumKeyedMap>::value&& !is_basic_json<EnumKeyedMap>::value, int > = 0 >
+inline void to_json(BasicJsonType& j, const EnumKeyedMap& map)
+{
+    typename BasicJsonType::object_t obj;
+    for (const auto& p : map)
+    {
+        BasicJsonType key = p.first;
+        if (JSON_HEDLEY_UNLIKELY(!key.is_string()))
+        {
+            JSON_THROW(type_error::create(302, concat("type must be string, but is ", key.type_name()), &key));
+        }
+
+        auto& key_string = *key.template get_ptr<typename BasicJsonType::string_t*>();
+        if (JSON_HEDLEY_UNLIKELY(!obj.emplace(key_string, BasicJsonType(p.second)).second))
+        {
+            JSON_THROW(type_error::create(318, concat("duplicate object key '", key_string, "'"), &key));
+        }
+    }
+    external_constructor<value_t::object>::construct(j, std::move(obj));
+}
+#endif
+
 template<typename BasicJsonType>
 inline void to_json(BasicJsonType& j, typename BasicJsonType::object_t&& obj)
 {

include/nlohmann/detail/input/binary_reader.hpp

@@ -3344,8 +3344,8 @@ class binary_reader
         return enter_object(detail::unknown_size());
     }
 
-    // Note, UBJSON has no binary type of its own; BJData, which shares this
-    // reader, decodes optimized 'B' arrays as binary in get_ubjson_array().
+    // Note, no reader for UBJSON binary types is implemented because they do
+    // not exist
 
     bool get_ubjson_high_precision_number()
     {
@@ -4052,7 +4052,7 @@ class binary_reader
 #endif
     }
 
-    /*!
+    /*
     @brief read a number from the input
 
     @tparam NumberType the type of the number
@@ -4062,10 +4062,10 @@ class binary_reader
     @return whether conversion completed
 
     @note This function needs to respect the system's endianness, because
-          bytes in CBOR, MessagePack, UBJSON, and BON8 are stored in network
-          order (big endian) and therefore need reordering on little endian
-          systems. On the other hand, BSON and BJData use little endian and
-          should reorder on big endian systems.
+          bytes in CBOR, MessagePack, and UBJSON are stored in network order
+          (big endian) and therefore need reordering on little endian systems.
+          On the other hand, BSON and BJData use little endian and should reorder
+          on big endian systems.
     */
     template<typename NumberType, bool InputIsLittleEndian = false>
     bool get_number(const input_format_t format, NumberType& result)

include/nlohmann/detail/input/input_adapters.hpp

@@ -8,12 +8,12 @@
 
 #pragma once
 
-#include <algorithm> // min
 #include <array> // array
 #include <cstddef> // size_t
-#include <cstdint> // uint32_t
 #include <cstring> // strlen
 #include <iterator> // begin, end, iterator_traits, random_access_iterator_tag, distance, next
+#include <memory> // shared_ptr, make_shared, addressof
+#include <numeric> // accumulate
 #include <streambuf> // streambuf
 #include <string> // string, char_traits
 #include <type_traits> // enable_if, is_base_of, is_pointer, is_integral, remove_pointer
@@ -28,7 +28,6 @@
 #include <nlohmann/detail/iterators/iterator_traits.hpp>
 #include <nlohmann/detail/macro_scope.hpp>
 #include <nlohmann/detail/meta/type_traits.hpp>
-#include <nlohmann/detail/string_utils.hpp>
 
 NLOHMANN_JSON_NAMESPACE_BEGIN
 namespace detail
@@ -83,9 +82,8 @@ class file_input_adapter
 };
 
 /*!
-Input adapter for a (caching) istream. Does not skip a UTF Byte Order Mark
-itself; that is done by the lexer's skip_bom(). Does not support changing
-the underlying std::streambuf
+Input adapter for a (caching) istream. Ignores a UFT Byte Order Mark at
+beginning of input. Does not support changing the underlying std::streambuf
 in mid-input. Maintains underlying std::istream and std::streambuf to support
 subsequent use of standard std::istream operations to process any input
 characters following those used in parsing the JSON input.  Clears the
@@ -450,14 +448,32 @@ struct wide_string_input_helper<BaseInputAdapter, 4>
             // get the current character
             const auto wc = input.get_character();
 
-            if (wc <= 0x10FFFF)
+            // UTF-32 to UTF-8 encoding
+            if (wc < 0x80)
             {
-                // UTF-32 to UTF-8 encoding
-                utf8_bytes_filled = 0;
-                encode_utf8(static_cast<std::uint32_t>(wc), [&utf8_bytes, &utf8_bytes_filled](std::uint32_t byte)
-                {
-                    utf8_bytes[utf8_bytes_filled++] = static_cast<std::char_traits<char>::int_type>(byte);
-                });
+                utf8_bytes[0] = static_cast<std::char_traits<char>::int_type>(wc);
+                utf8_bytes_filled = 1;
+            }
+            else if (wc <= 0x7FF)
+            {
+                utf8_bytes[0] = static_cast<std::char_traits<char>::int_type>(0xC0u | ((static_cast<unsigned int>(wc) >> 6u) & 0x1Fu));
+                utf8_bytes[1] = static_cast<std::char_traits<char>::int_type>(0x80u | (static_cast<unsigned int>(wc) & 0x3Fu));
+                utf8_bytes_filled = 2;
+            }
+            else if (wc <= 0xFFFF)
+            {
+                utf8_bytes[0] = static_cast<std::char_traits<char>::int_type>(0xE0u | ((static_cast<unsigned int>(wc) >> 12u) & 0x0Fu));
+                utf8_bytes[1] = static_cast<std::char_traits<char>::int_type>(0x80u | ((static_cast<unsigned int>(wc) >> 6u) & 0x3Fu));
+                utf8_bytes[2] = static_cast<std::char_traits<char>::int_type>(0x80u | (static_cast<unsigned int>(wc) & 0x3Fu));
+                utf8_bytes_filled = 3;
+            }
+            else if (wc <= 0x10FFFF)
+            {
+                utf8_bytes[0] = static_cast<std::char_traits<char>::int_type>(0xF0u | ((static_cast<unsigned int>(wc) >> 18u) & 0x07u));
+                utf8_bytes[1] = static_cast<std::char_traits<char>::int_type>(0x80u | ((static_cast<unsigned int>(wc) >> 12u) & 0x3Fu));
+                utf8_bytes[2] = static_cast<std::char_traits<char>::int_type>(0x80u | ((static_cast<unsigned int>(wc) >> 6u) & 0x3Fu));
+                utf8_bytes[3] = static_cast<std::char_traits<char>::int_type>(0x80u | (static_cast<unsigned int>(wc) & 0x3Fu));
+                utf8_bytes_filled = 4;
             }
             else
             {
@@ -494,15 +510,24 @@ struct wide_string_input_helper<BaseInputAdapter, 2>
             // get the current character
             const auto wc = input.get_character();
 
-            if (0xD800 > wc || wc >= 0xE000)
+            // UTF-16 to UTF-8 encoding
+            if (wc < 0x80)
             {
-                // a UTF-16 code unit outside the surrogate range is a valid
-                // code point (at most U+FFFF) on its own
-                utf8_bytes_filled = 0;
-                encode_utf8(static_cast<std::uint32_t>(wc), [&utf8_bytes, &utf8_bytes_filled](std::uint32_t byte)
-                {
-                    utf8_bytes[utf8_bytes_filled++] = static_cast<std::char_traits<char>::int_type>(byte);
-                });
+                utf8_bytes[0] = static_cast<std::char_traits<char>::int_type>(wc);
+                utf8_bytes_filled = 1;
+            }
+            else if (wc <= 0x7FF)
+            {
+                utf8_bytes[0] = static_cast<std::char_traits<char>::int_type>(0xC0u | ((static_cast<unsigned int>(wc) >> 6u)));
+                utf8_bytes[1] = static_cast<std::char_traits<char>::int_type>(0x80u | (static_cast<unsigned int>(wc) & 0x3Fu));
+                utf8_bytes_filled = 2;
+            }
+            else if (0xD800 > wc || wc >= 0xE000)
+            {
+                utf8_bytes[0] = static_cast<std::char_traits<char>::int_type>(0xE0u | ((static_cast<unsigned int>(wc) >> 12u)));
+                utf8_bytes[1] = static_cast<std::char_traits<char>::int_type>(0x80u | ((static_cast<unsigned int>(wc) >> 6u) & 0x3Fu));
+                utf8_bytes[2] = static_cast<std::char_traits<char>::int_type>(0x80u | (static_cast<unsigned int>(wc) & 0x3Fu));
+                utf8_bytes_filled = 3;
             }
             else
             {
@@ -520,11 +545,11 @@ struct wide_string_input_helper<BaseInputAdapter, 2>
                     if (0xDC00 <= wc2 && wc2 <= 0xDFFF)
                     {
                         const auto charcode = 0x10000u + (((static_cast<unsigned int>(wc) & 0x3FFu) << 10u) | (wc2 & 0x3FFu));
-                        utf8_bytes_filled = 0;
-                        encode_utf8(charcode, [&utf8_bytes, &utf8_bytes_filled](std::uint32_t byte)
-                        {
-                            utf8_bytes[utf8_bytes_filled++] = static_cast<std::char_traits<char>::int_type>(byte);
-                        });
+                        utf8_bytes[0] = static_cast<std::char_traits<char>::int_type>(0xF0u | (charcode >> 18u));
+                        utf8_bytes[1] = static_cast<std::char_traits<char>::int_type>(0x80u | ((charcode >> 12u) & 0x3Fu));
+                        utf8_bytes[2] = static_cast<std::char_traits<char>::int_type>(0x80u | ((charcode >> 6u) & 0x3Fu));
+                        utf8_bytes[3] = static_cast<std::char_traits<char>::int_type>(0x80u | (charcode & 0x3Fu));
+                        utf8_bytes_filled = 4;
                         valid_pair = true;
                     }
                 }
@@ -837,9 +862,9 @@ auto input_adapter(T (&array)[N]) -> decltype(input_adapter(array, array + N)) /
     return input_adapter(array, array + N);
 }
 
-// This class only handles inputs that construct a contiguous_bytes_input_adapter
-// (e.g. span_input_adapter). It's required so that expressions like {ptr, len}
-// can be implicitly cast to the correct adapter.
+// This class only handles inputs of input_buffer_adapter type.
+// It's required so that expressions like {ptr, len} can be implicitly cast
+// to the correct adapter.
 class span_input_adapter
 {
   public:

include/nlohmann/detail/input/json_sax.hpp

@@ -10,7 +10,6 @@
 
 #include <algorithm> // find_if, min
 #include <cstddef>
-#include <limits> // numeric_limits
 #include <string> // string
 #include <type_traits> // enable_if_t
 #include <utility> // move, pair
@@ -176,88 +175,6 @@ template<typename ArrayType>
 inline void reserve_array(ArrayType& /*arr*/, std::size_t /*len*/, priority_tag<0> /*unused*/)
 {}
 
-#if JSON_DIAGNOSTIC_POSITIONS
-/*!
-@brief set the diagnostic positions of a value the DOM SAX parsers just stored
-
-Shared by json_sax_dom_parser and json_sax_dom_callback_parser. basic_json
-befriends this struct, as the position members are private.
-*/
-struct diagnostic_positions
-{
-    /*!
-    @param[in,out] v  the value that was just parsed
-    @param[in] lexer  the lexer that read it, or nullptr to leave @a v alone
-    */
-    template<typename BasicJsonType, typename LexerType>
-    static void set_from_lexer(BasicJsonType& v, LexerType* lexer)
-    {
-        if (lexer)
-        {
-            // Lexer has read past the current field value, so set the end position to the current position.
-            // The start position will be set below based on the length of the string representation
-            // of the value.
-            v.end_position = lexer->get_position();
-
-            switch (v.type())
-            {
-                case value_t::boolean:
-                {
-                    // 4 and 5 are the string length of "true" and "false"
-                    v.start_position = v.end_position - (v.m_data.m_value.boolean ? 4 : 5);
-                    break;
-                }
-
-                case value_t::null:
-                {
-                    // 4 is the string length of "null"
-                    v.start_position = v.end_position - 4;
-                    break;
-                }
-
-                case value_t::string:
-                {
-                    // escape sequences make the token longer than the value it
-                    // parses to, so the start position cannot be derived from
-                    // the value; use the offset the lexer recorded instead
-                    v.start_position = lexer->get_token_start_position();
-                    break;
-                }
-
-                case value_t::discarded:
-                {
-                    // an object or array the callback of
-                    // json_sax_dom_callback_parser rejected has no position
-                    v.end_position = std::string::npos;
-                    v.start_position = v.end_position;
-                    break;
-                }
-
-                case value_t::binary:
-                case value_t::number_integer:
-                case value_t::number_unsigned:
-                case value_t::number_float:
-                {
-                    v.start_position = v.end_position - lexer->get_string().size();
-                    break;
-                }
-
-                case value_t::object:
-                case value_t::array:
-                {
-                    // object and array are handled in start_object() and start_array() handlers
-                    // skip setting the values here.
-                    break;
-                }
-                default: // LCOV_EXCL_LINE
-                    // Handle all possible types discretely, default handler should never be reached.
-                    JSON_ASSERT(false); // NOLINT(cert-dcl03-c,hicpp-static-assert,misc-static-assert) LCOV_EXCL_LINE
-            }
-        }
-    }
-};
-#endif
-
 /*!
 @brief SAX implementation to create a JSON value from SAX events
 
@@ -459,6 +376,76 @@ class json_sax_dom_parser
 
   private:
 
+#if JSON_DIAGNOSTIC_POSITIONS
+    void handle_diagnostic_positions_for_json_value(BasicJsonType& v)
+    {
+        if (m_lexer_ref)
+        {
+            // Lexer has read past the current field value, so set the end position to the current position.
+            // The start position will be set below based on the length of the string representation
+            // of the value.
+            v.end_position = m_lexer_ref->get_position();
+
+            switch (v.type())
+            {
+                case value_t::boolean:
+                {
+                    // 4 and 5 are the string length of "true" and "false"
+                    v.start_position = v.end_position - (v.m_data.m_value.boolean ? 4 : 5);
+                    break;
+                }
+
+                case value_t::null:
+                {
+                    // 4 is the string length of "null"
+                    v.start_position = v.end_position - 4;
+                    break;
+                }
+
+                case value_t::string:
+                {
+                    // escape sequences make the token longer than the value it
+                    // parses to, so the start position cannot be derived from
+                    // the value; use the offset the lexer recorded instead
+                    v.start_position = m_lexer_ref->get_token_start_position();
+                    break;
+                }
+
+                // As we handle the start and end positions for values created during parsing,
+                // we do not expect the following value type to be called. Regardless, set the positions
+                // in case this is created manually or through a different constructor. Exclude from lcov
+                // since the exact condition of this switch is esoteric.
+                // LCOV_EXCL_START
+                case value_t::discarded:
+                {
+                    v.end_position = std::string::npos;
+                    v.start_position = v.end_position;
+                    break;
+                }
+                // LCOV_EXCL_STOP
+                case value_t::binary:
+                case value_t::number_integer:
+                case value_t::number_unsigned:
+                case value_t::number_float:
+                {
+                    v.start_position = v.end_position - m_lexer_ref->get_string().size();
+                    break;
+                }
+                case value_t::object:
+                case value_t::array:
+                {
+                    // object and array are handled in start_object() and start_array() handlers
+                    // skip setting the values here.
+                    break;
+                }
+                default: // LCOV_EXCL_LINE
+                    // Handle all possible types discretely, default handler should never be reached.
+                    JSON_ASSERT(false); // NOLINT(cert-dcl03-c,hicpp-static-assert,misc-static-assert,-warnings-as-errors) LCOV_EXCL_LINE
+            }
+        }
+    }
+#endif
+
     /*!
     @invariant If the ref stack is empty, then the passed value will be the new
                root.
@@ -474,7 +461,7 @@ class json_sax_dom_parser
             root = BasicJsonType(std::forward<Value>(v));
 
 #if JSON_DIAGNOSTIC_POSITIONS
-            diagnostic_positions::set_from_lexer(root, m_lexer_ref);
+            handle_diagnostic_positions_for_json_value(root);
 #endif
 
             return &root;
@@ -487,7 +474,7 @@ class json_sax_dom_parser
             ref_stack.back()->m_data.m_value.array->emplace_back(std::forward<Value>(v));
 
 #if JSON_DIAGNOSTIC_POSITIONS
-            diagnostic_positions::set_from_lexer(ref_stack.back()->m_data.m_value.array->back(), m_lexer_ref);
+            handle_diagnostic_positions_for_json_value(ref_stack.back()->m_data.m_value.array->back());
 #endif
 
             return &(ref_stack.back()->m_data.m_value.array->back());
@@ -498,7 +485,7 @@ class json_sax_dom_parser
         *object_element = BasicJsonType(std::forward<Value>(v));
 
 #if JSON_DIAGNOSTIC_POSITIONS
-        diagnostic_positions::set_from_lexer(*object_element, m_lexer_ref);
+        handle_diagnostic_positions_for_json_value(*object_element);
 #endif
 
         return object_element;
@@ -675,7 +662,7 @@ class json_sax_dom_callback_parser
 
 #if JSON_DIAGNOSTIC_POSITIONS
                     // Set start/end positions for discarded object.
-                    diagnostic_positions::set_from_lexer(*ref_stack.back(), m_lexer_ref);
+                    handle_diagnostic_positions_for_json_value(*ref_stack.back());
 #endif
                 }
             }
@@ -791,7 +778,7 @@ class json_sax_dom_callback_parser
 
 #if JSON_DIAGNOSTIC_POSITIONS
                     // Set start/end positions for discarded array.
-                    diagnostic_positions::set_from_lexer(*ref_stack.back(), m_lexer_ref);
+                    handle_diagnostic_positions_for_json_value(*ref_stack.back());
 #endif
                 }
             }
@@ -844,6 +831,72 @@ class json_sax_dom_callback_parser
 
   private:
 
+#if JSON_DIAGNOSTIC_POSITIONS
+    void handle_diagnostic_positions_for_json_value(BasicJsonType& v)
+    {
+        if (m_lexer_ref)
+        {
+            // Lexer has read past the current field value, so set the end position to the current position.
+            // The start position will be set below based on the length of the string representation
+            // of the value.
+            v.end_position = m_lexer_ref->get_position();
+
+            switch (v.type())
+            {
+                case value_t::boolean:
+                {
+                    // 4 and 5 are the string length of "true" and "false"
+                    v.start_position = v.end_position - (v.m_data.m_value.boolean ? 4 : 5);
+                    break;
+                }
+
+                case value_t::null:
+                {
+                    // 4 is the string length of "null"
+                    v.start_position = v.end_position - 4;
+                    break;
+                }
+
+                case value_t::string:
+                {
+                    // escape sequences make the token longer than the value it
+                    // parses to, so the start position cannot be derived from
+                    // the value; use the offset the lexer recorded instead
+                    v.start_position = m_lexer_ref->get_token_start_position();
+                    break;
+                }
+
+                case value_t::discarded:
+                {
+                    v.end_position = std::string::npos;
+                    v.start_position = v.end_position;
+                    break;
+                }
+
+                case value_t::binary:
+                case value_t::number_integer:
+                case value_t::number_unsigned:
+                case value_t::number_float:
+                {
+                    v.start_position = v.end_position - m_lexer_ref->get_string().size();
+                    break;
+                }
+
+                case value_t::object:
+                case value_t::array:
+                {
+                    // object and array are handled in start_object() and start_array() handlers
+                    // skip setting the values here.
+                    break;
+                }
+                default: // LCOV_EXCL_LINE
+                    // Handle all possible types discretely, default handler should never be reached.
+                    JSON_ASSERT(false); // NOLINT(cert-dcl03-c,hicpp-static-assert,misc-static-assert,-warnings-as-errors) LCOV_EXCL_LINE
+            }
+        }
+    }
+#endif
+
     /// if there is a pending duplicate-key stash entry for this exact slot,
     /// remove it from the stash; if restore_value is true, the stashed
     /// previous value is moved back into the slot first (use this when the
@@ -965,7 +1018,7 @@ class json_sax_dom_callback_parser
         auto value = BasicJsonType(std::forward<Value>(v));
 
 #if JSON_DIAGNOSTIC_POSITIONS
-        diagnostic_positions::set_from_lexer(value, m_lexer_ref);
+        handle_diagnostic_positions_for_json_value(value);
 #endif
 
         // check callback

include/nlohmann/detail/input/lexer.hpp

@@ -11,7 +11,6 @@
 #include <array> // array
 #include <clocale> // localeconv
 #include <cstddef> // size_t
-#include <cstdint> // uint32_t
 #include <cstdio> // snprintf
 #include <cstdlib> // strtof, strtod, strtold, strtoll, strtoull
 #include <initializer_list> // initializer_list
@@ -25,7 +24,6 @@
 #include <nlohmann/detail/input/string_scan.hpp>
 #include <nlohmann/detail/macro_scope.hpp>
 #include <nlohmann/detail/meta/type_traits.hpp>
-#include <nlohmann/detail/string_utils.hpp>
 
 NLOHMANN_JSON_NAMESPACE_BEGIN
 namespace detail
@@ -502,10 +500,32 @@ class lexer : public lexer_base<BasicJsonType>
                             JSON_ASSERT(0x00 <= codepoint && codepoint <= 0x10FFFF);
 
                             // translate codepoint into bytes
-                            encode_utf8(static_cast<std::uint32_t>(codepoint), [this](std::uint32_t byte)
+                            if (codepoint < 0x80)
                             {
-                                add(static_cast<char_int_type>(byte));
-                            });
+                                // 1-byte characters: 0xxxxxxx (ASCII)
+                                add(static_cast<char_int_type>(codepoint));
+                            }
+                            else if (codepoint <= 0x7FF)
+                            {
+                                // 2-byte characters: 110xxxxx 10xxxxxx
+                                add(static_cast<char_int_type>(0xC0u | (static_cast<unsigned int>(codepoint) >> 6u)));
+                                add(static_cast<char_int_type>(0x80u | (static_cast<unsigned int>(codepoint) & 0x3Fu)));
+                            }
+                            else if (codepoint <= 0xFFFF)
+                            {
+                                // 3-byte characters: 1110xxxx 10xxxxxx 10xxxxxx
+                                add(static_cast<char_int_type>(0xE0u | (static_cast<unsigned int>(codepoint) >> 12u)));
+                                add(static_cast<char_int_type>(0x80u | ((static_cast<unsigned int>(codepoint) >> 6u) & 0x3Fu)));
+                                add(static_cast<char_int_type>(0x80u | (static_cast<unsigned int>(codepoint) & 0x3Fu)));
+                            }
+                            else
+                            {
+                                // 4-byte characters: 11110xxx 10xxxxxx 10xxxxxx 10xxxxxx
+                                add(static_cast<char_int_type>(0xF0u | (static_cast<unsigned int>(codepoint) >> 18u)));
+                                add(static_cast<char_int_type>(0x80u | ((static_cast<unsigned int>(codepoint) >> 12u) & 0x3Fu)));
+                                add(static_cast<char_int_type>(0x80u | ((static_cast<unsigned int>(codepoint) >> 6u) & 0x3Fu)));
+                                add(static_cast<char_int_type>(0x80u | (static_cast<unsigned int>(codepoint) & 0x3Fu)));
+                            }
 
                             break;
                         }
@@ -1474,30 +1494,45 @@ scan_number_done:
     */
     token_type convert_number(token_type number_type, std::size_t mantissa_end)
     {
-        // accept() only needs to know whether the input is valid, so it sets
-        // discard_number_values (see json.hpp), and an integer token whose
-        // digit count shows that it fits is reported without calling
-        // convert_integer(). A number with up to 18 digits always fits into
-        // both std::uint64_t and std::int64_t (18 nines is about 1e18, below
-        // INT64_MAX, which is about 9.2e18). Longer tokens take the exact path
-        // below, including the fallback to floating point when the value does
-        // not fit.
+        // If the caller does not need the converted value (only whether the
+        // input is syntactically valid; see json_sax_acceptor/accept()), an
+        // unsigned/integer token can be reported without calling
+        // strtoull()/strtoll() at all, *provided* we can already tell from
+        // the digit count alone that the conversion cannot overflow 64 bits.
+        // Such tokens are always finite and are accepted unconditionally by
+        // the parser regardless of their actual value (parser::sax_parse_internal()
+        // never checks finiteness for value_unsigned/value_integer), so the
+        // classification below is all that is needed.
         //
-        // With a narrower number_unsigned_t/number_integer_t (e.g.
-        // std::uint32_t), the exact path would reclassify some of these tokens
-        // as (finite) floats, while this check reports integers. That does not
-        // change the result of accept(): it always parses through
-        // json_sax_acceptor, whose number callbacks discard their argument and
-        // return true, and the parser rejects neither integers nor finite
-        // floats. value_unsigned/value_integer are left unset here, so a caller
-        // that reads the converted value must not set discard_number_values.
+        // A decimal number with up to 18 digits is always representable in
+        // both std::uint64_t and std::int64_t (18 nines is ~1e18, well below
+        // both UINT64_MAX ~1.8e19 and INT64_MAX ~9.2e18), so strtoull()/strtoll()
+        // could not have set errno to ERANGE for it. Numbers with more digits
+        // (rare in practice) fall through to the exact code below, unchanged,
+        // so their handling -- including reclassification to value_float when
+        // the value overflows 64 bits, and rejection when it is not even
+        // finite as a double -- is bit-for-bit identical to before this
+        // optimization.
         //
-        // On contiguous input, scan_number_bulk_contiguous() converts integer
-        // tokens itself and does not pass them to this function, unless
-        // JSON_DIAGNOSTIC_POSITIONS is enabled. This check is therefore only
-        // reached for input without bulk access (e.g. streams), with
-        // JSON_DIAGNOSTIC_POSITIONS, or when scan_number_bulk_contiguous()
-        // falls back to scan_number().
+        // Note this reasons about std::uint64_t/std::int64_t, not about
+        // number_unsigned_t/number_integer_t (BasicJsonType's own, possibly
+        // narrower, template parameters -- e.g. std::uint32_t). That is fine
+        // *only* because discard_number_values is exclusively set by
+        // accept() (see json.hpp), and accept() always parses through the
+        // library's own json_sax_acceptor -- never a user-supplied SAX
+        // consumer -- whose number_unsigned()/number_integer()/number_float()
+        // callbacks unconditionally discard their argument and return true.
+        // So for every caller that can reach this branch, neither the token
+        // classification below nor the eventual (possibly narrowed, and on
+        // this fast path left stale/unset) value_unsigned/value_integer is
+        // ever consulted -- an unsigned/integer token is accepted outright,
+        // and even a >18-digit token that this fast path deliberately falls
+        // through for is, once reclassified to value_float, still finite
+        // (and thus accepted) for any digit count that fits in number_unsigned_t
+        // or number_integer_t regardless of that type's width. If this
+        // function is ever taught to run with discard_number_values true for
+        // a caller that *does* read the converted value, this reasoning (and
+        // the fast path below) would need to be revisited.
         if (discard_number_values)
         {
             constexpr std::size_t safe_digit_count = 18;
@@ -1988,7 +2023,7 @@ scan_number_done:
         return value_float;
     }
 
-    /// return current string value
+    /// return current string value (implicitly resets the token; useful only once)
     string_t& get_string()
     {
         // a number token holds '.' regardless of the locale (#4084)
@@ -2290,11 +2325,11 @@ scan_number_done:
     /// the position of the decimal point in token_buffer
     std::size_t decimal_point_position = std::string::npos;
 
-    /// whether the caller only needs the token types and never looks at the
-    /// converted numeric values; set only by accept(), which parses through
-    /// json_sax_acceptor. When set, convert_number() skips converting integer
-    /// tokens whose digit count guarantees that they fit into 64 bits (see
-    /// there)
+    /// whether the caller (e.g. accept()/json_sax_acceptor) only needs the
+    /// token classification and never looks at the converted numeric value;
+    /// when set, scan_number() may skip strtoull()/strtoll() for
+    /// value_unsigned/value_integer tokens whose digit count guarantees they
+    /// fit into 64 bits (see scan_number())
     const bool discard_number_values = false;
 };
 

include/nlohmann/detail/input/parser.hpp

@@ -54,8 +54,7 @@ using parser_callback_t =
 /*!
 @brief syntax analysis
 
-This class implements an iterative parser that keeps the open containers on
-an explicit stack and reports what it reads as SAX events.
+This class implements a recursive descent parser.
 */
 template<typename BasicJsonType, typename InputAdapterType>
 class parser
@@ -99,9 +98,28 @@ class parser
         if (callback)
         {
             json_sax_dom_callback_parser<BasicJsonType, InputAdapterType> sdp(result, callback, allow_exceptions, &m_lexer);
+            sax_parse_internal(&sdp);
+
+            if (strict)
+            {
+                // in strict mode, input must be completely read
+                if (get_token() != token_type::end_of_input)
+                {
+                    sdp.parse_error(m_lexer.get_position(),
+                                    m_lexer.get_token_string(),
+                                    parse_error::create(101, m_lexer.get_position(),
+                                                        exception_message(token_type::end_of_input, "value"), nullptr));
+                }
+            }
+            else
+            {
+                // the caller keeps using the input: position it right after
+                // the value by leaving the character that terminated it
+                m_lexer.release_lookahead();
+            }
 
             // in case of an error, return a discarded value
-            if (!parse_dom(sdp, strict))
+            if (sdp.is_errored())
             {
                 result = value_t::discarded;
                 return;
@@ -117,9 +135,26 @@ class parser
         else
         {
             json_sax_dom_parser<BasicJsonType, InputAdapterType> sdp(result, allow_exceptions, &m_lexer);
+            sax_parse_internal(&sdp);
+
+            if (strict)
+            {
+                // in strict mode, input must be completely read
+                if (get_token() != token_type::end_of_input)
+                {
+                    sdp.parse_error(m_lexer.get_position(),
+                                    m_lexer.get_token_string(),
+                                    parse_error::create(101, m_lexer.get_position(), exception_message(token_type::end_of_input, "value"), nullptr));
+                }
+            }
+            else
+            {
+                // see above
+                m_lexer.release_lookahead();
+            }
 
             // in case of an error, return a discarded value
-            if (!parse_dom(sdp, strict))
+            if (sdp.is_errored())
             {
                 result = value_t::discarded;
                 return;
@@ -172,46 +207,6 @@ class parser
     }
 
   private:
-    /*!
-    @brief run a DOM SAX parser to completion and position the lexer
-
-    Shared by both branches of @ref parse(): builds no SAX parser itself,
-    but drives an already-constructed @a json_sax_dom_parser or
-    @ref json_sax_dom_callback_parser through @ref sax_parse_internal(),
-    then applies the strict-EOF check (reporting parse_error.101 through
-    @a sdp on failure) or, in non-strict mode, releases the lookahead so
-    the caller can keep reading the input right after the parsed value.
-
-    @param[in,out] sdp     the DOM SAX parser to run
-    @param[in] strict      whether to expect the last token to be EOF
-    @return whether @a sdp did not report an error
-    */
-    template<typename DomSax>
-    bool parse_dom(DomSax& sdp, const bool strict)
-    {
-        sax_parse_internal(&sdp);
-
-        if (strict)
-        {
-            // in strict mode, input must be completely read
-            if (get_token() != token_type::end_of_input)
-            {
-                sdp.parse_error(m_lexer.get_position(),
-                                m_lexer.get_token_string(),
-                                parse_error::create(101, m_lexer.get_position(),
-                                                    exception_message(token_type::end_of_input, "value"), nullptr));
-            }
-        }
-        else
-        {
-            // the caller keeps using the input: position it right after
-            // the value by leaving the character that terminated it
-            m_lexer.release_lookahead();
-        }
-
-        return !sdp.is_errored();
-    }
-
     template<typename SAX>
     JSON_HEDLEY_NON_NULL(2)
     bool sax_parse_internal(SAX* sax)
@@ -444,9 +439,8 @@ class parser
 
                     // We are done with this array. Before we can parse a
                     // new value, we need to evaluate the new state first.
-                    // By setting skip_to_state_evaluation to true, the next
-                    // iteration skips parsing a value and evaluates the
-                    // enclosing state directly.
+                    // By setting skip_to_state_evaluation to false, we
+                    // are effectively jumping to the beginning of this if.
                     JSON_ASSERT(!states.empty());
                     states.pop_back();
                     skip_to_state_evaluation = true;
@@ -506,9 +500,8 @@ class parser
 
                 // We are done with this object. Before we can parse a
                 // new value, we need to evaluate the new state first.
-                // By setting skip_to_state_evaluation to true, the next
-                // iteration skips parsing a value and evaluates the
-                // enclosing state directly.
+                // By setting skip_to_state_evaluation to false, we
+                // are effectively jumping to the beginning of this if.
                 JSON_ASSERT(!states.empty());
                 states.pop_back();
                 skip_to_state_evaluation = true;

include/nlohmann/detail/macro_unscope.hpp

@@ -46,6 +46,7 @@
     #undef JSON_BRACE_INIT_COPY_SEMANTICS
     #undef JSON_PRECISE_STREAM_POSITION
     #undef JSON_STRICT_NUL_HANDLING
+    #undef JSON_USE_OBJECTS_FOR_ENUM_KEYED_MAPS
 #endif
 
 #include <nlohmann/thirdparty/hedley/hedley_undef.hpp>

include/nlohmann/detail/meta/type_traits.hpp

@@ -400,6 +400,30 @@ 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>
+struct is_enum_keyed_map : std::false_type {};
+
+template<typename T>
+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 &&
+                                  !(is_detected<insert_result_t, T>::value && !is_detected<insert_result_second_t, T>::value);
+};
+
 template<typename BasicJsonType, typename ConstructibleObjectType,
          typename = void>
 struct is_constructible_object_type_impl : std::false_type {};

include/nlohmann/detail/string_utils.hpp

@@ -36,61 +36,6 @@ StringType to_string(std::size_t value)
     return result;
 }
 
-///////////////////
-// UTF-8 encoding //
-///////////////////
-
-/*!
-@brief encode a Unicode code point as UTF-8
-
-Used to turn a decoded code point back into bytes: by the wide-string input
-adapters in input_adapters.hpp (one code point per UTF-32 unit, per UTF-16
-unit outside the surrogate range, and per valid UTF-16 surrogate pair), and
-by the lexer's `\uXXXX`/`\uXXXX\uYYYY` handling in lexer.hpp. Passing a
-code point above U+10FFFF, or one in the surrogate range U+D800..U+DFFF, is
-undefined behavior; callers are expected to have rejected those already
-(the wide-string adapters pass malformed units through unencoded instead of
-calling this function, and the lexer rejects unpaired surrogates before
-reaching it).
-
-@tparam Out    a callable invoked with one byte (as std::uint32_t, 0x00..0xFF)
-               at a time, most significant byte first
-@param[in] cp   the code point to encode (at most U+10FFFF)
-@param[in] out  called once for each byte of the UTF-8 encoding of @a cp
-*/
-template<typename Out>
-void encode_utf8(std::uint32_t cp, Out&& out)
-{
-    JSON_ASSERT(cp <= 0x10FFFF);
-
-    if (cp < 0x80)
-    {
-        // 1-byte characters: 0xxxxxxx (ASCII)
-        out(cp);
-    }
-    else if (cp <= 0x7FF)
-    {
-        // 2-byte characters: 110xxxxx 10xxxxxx
-        out(0xC0u | (cp >> 6u));
-        out(0x80u | (cp & 0x3Fu));
-    }
-    else if (cp <= 0xFFFF)
-    {
-        // 3-byte characters: 1110xxxx 10xxxxxx 10xxxxxx
-        out(0xE0u | (cp >> 12u));
-        out(0x80u | ((cp >> 6u) & 0x3Fu));
-        out(0x80u | (cp & 0x3Fu));
-    }
-    else
-    {
-        // 4-byte characters: 11110xxx 10xxxxxx 10xxxxxx 10xxxxxx
-        out(0xF0u | (cp >> 18u));
-        out(0x80u | ((cp >> 12u) & 0x3Fu));
-        out(0x80u | ((cp >> 6u) & 0x3Fu));
-        out(0x80u | (cp & 0x3Fu));
-    }
-}
-
 ///////////////////
 // UTF-8 decoding //
 ///////////////////
@@ -106,23 +51,11 @@ This is a single-byte step of a "shift-based" UTF-8 decoder originally
 written by Björn Hoehrmann. See
 http://bjoern.hoehrmann.de/utf-8/decoder/dfa/ for details.
 
-The library checks UTF-8 well-formedness (RFC 3629, section 4) in four
-places, which differ in speed, diagnostics, and how they read the input:
-
-- decode() and @ref is_valid_utf8 below: the serializer (to escape and, in
-  strict mode, reject ill-formed UTF-8 when dumping a string) and the CBOR,
-  MessagePack, BSON, UBJSON and BJData readers (to reject ill-formed UTF-8 in
-  text strings at decode time).
-- the per-lead-byte switch in lexer::scan_string(): JSON text, with a
-  diagnostic for each kind of error.
-- validate_one_utf8() and valid_utf8_prefix() in string_scan.hpp: the lexer's
-  bulk string scan, the bulk path of the BON8 reader, and the BON8 writer.
-  They must accept exactly what the lexer's switch accepts.
-- the byte path of binary_reader::get_bon8_string(): BON8 input without bulk
-  access, and the bytes the bulk path leaves to it.
-
-All four must accept the same set of sequences, so a change to one needs a
-matching change to the others.
+This decoder is the single source of truth for UTF-8 validation in this
+library: it is used both by the serializer (to escape and, in strict mode,
+reject ill-formed UTF-8 when dumping a string) and by the binary readers
+(to reject ill-formed UTF-8 in CBOR/MessagePack/BSON/UBJSON text strings at
+decode time; see @ref is_valid_utf8 below).
 
 @param[in,out] state  the current decoder state
 @param[in,out] codep  codepoint (valid only if resulting state is UTF8_ACCEPT)

include/nlohmann/json.hpp

@@ -149,9 +149,6 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
     friend class ::nlohmann::detail::json_sax_dom_parser;
     template<typename BasicJsonType, typename InputAdapterType>
     friend class ::nlohmann::detail::json_sax_dom_callback_parser;
-#if JSON_DIAGNOSTIC_POSITIONS
-    friend struct ::nlohmann::detail::diagnostic_positions;
-#endif
     friend class ::nlohmann::detail::exception;
 
     /// workaround type for MSVC

single_include/nlohmann/json_fwd.hpp

@@ -64,6 +64,10 @@
     #define JSON_STRICT_NUL_HANDLING 0
 #endif
 
+#ifndef JSON_USE_OBJECTS_FOR_ENUM_KEYED_MAPS
+    #define JSON_USE_OBJECTS_FOR_ENUM_KEYED_MAPS 0
+#endif
+
 #if JSON_DIAGNOSTICS
     #define NLOHMANN_JSON_ABI_TAG_DIAGNOSTICS _diag
 #else
@@ -100,14 +104,20 @@
     #define NLOHMANN_JSON_ABI_TAG_STRICT_NUL_HANDLING
 #endif
 
+#if JSON_USE_OBJECTS_FOR_ENUM_KEYED_MAPS
+    #define NLOHMANN_JSON_ABI_TAG_OBJECTS_FOR_ENUM_KEYED_MAPS _ekmo
+#else
+    #define NLOHMANN_JSON_ABI_TAG_OBJECTS_FOR_ENUM_KEYED_MAPS
+#endif
+
 #ifndef NLOHMANN_JSON_NAMESPACE_NO_VERSION
     #define NLOHMANN_JSON_NAMESPACE_NO_VERSION 0
 #endif
 
 // Construct the namespace ABI tags component
-#define NLOHMANN_JSON_ABI_TAGS_CONCAT_EX(a, b, c, d, e, f) json_abi ## a ## b ## c ## d ## e ## f
-#define NLOHMANN_JSON_ABI_TAGS_CONCAT(a, b, c, d, e, f) \
-    NLOHMANN_JSON_ABI_TAGS_CONCAT_EX(a, b, c, d, e, f)
+#define NLOHMANN_JSON_ABI_TAGS_CONCAT_EX(a, b, c, d, e, f, g) json_abi ## a ## b ## c ## d ## e ## f ## g
+#define NLOHMANN_JSON_ABI_TAGS_CONCAT(a, b, c, d, e, f, g) \
+    NLOHMANN_JSON_ABI_TAGS_CONCAT_EX(a, b, c, d, e, f, g)
 
 #define NLOHMANN_JSON_ABI_TAGS                                       \
     NLOHMANN_JSON_ABI_TAGS_CONCAT(                                   \
@@ -116,7 +126,8 @@
             NLOHMANN_JSON_ABI_TAG_DIAGNOSTIC_POSITIONS,              \
             NLOHMANN_JSON_ABI_TAG_BRACE_INIT_COPY_SEMANTICS,         \
             NLOHMANN_JSON_ABI_TAG_PRECISE_STREAM_POSITION,           \
-            NLOHMANN_JSON_ABI_TAG_STRICT_NUL_HANDLING)
+            NLOHMANN_JSON_ABI_TAG_STRICT_NUL_HANDLING,               \
+            NLOHMANN_JSON_ABI_TAG_OBJECTS_FOR_ENUM_KEYED_MAPS)
 
 // Construct the namespace version component
 #define NLOHMANN_JSON_NAMESPACE_VERSION_CONCAT_EX(major, minor, patch) \

tests/abi/config/default.cpp

@@ -44,6 +44,10 @@ TEST_CASE("default namespace")
         expected += "_snul";
 #endif
 
+#if JSON_USE_OBJECTS_FOR_ENUM_KEYED_MAPS
+        expected += "_ekmo";
+#endif
+
         expected += "_v" STRINGIZE(NLOHMANN_JSON_VERSION_MAJOR);
         expected += "_" STRINGIZE(NLOHMANN_JSON_VERSION_MINOR);
         expected += "_" STRINGIZE(NLOHMANN_JSON_VERSION_PATCH) "::basic_json";

tests/abi/config/noversion.cpp

@@ -45,6 +45,10 @@ TEST_CASE("default namespace without version component")
         expected += "_snul";
 #endif
 
+#if JSON_USE_OBJECTS_FOR_ENUM_KEYED_MAPS
+        expected += "_ekmo";
+#endif
+
         expected += "::basic_json";
 
         // fallback for Clang

tests/src/unit-enum_keyed_maps.cpp

@@ -0,0 +1,246 @@
+//     __ _____ _____ _____
+//  __|  |   __|     |   | |  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"
+
+// skip tests if JSON_DisableEnumSerialization=ON (#4384)
+#if defined(JSON_DISABLE_ENUM_SERIALIZATION) && (JSON_DISABLE_ENUM_SERIALIZATION == 1)
+    #define SKIP_TESTS_FOR_ENUM_SERIALIZATION
+#endif
+
+// This file tests the opt-in JSON_USE_OBJECTS_FOR_ENUM_KEYED_MAPS, so it defines
+// the macro itself rather than relying on a -D flag, and runs in every build.
+// The default behavior is tested in unit-enum_keyed_maps_default.cpp.
+#ifdef JSON_USE_OBJECTS_FOR_ENUM_KEYED_MAPS
+    #undef JSON_USE_OBJECTS_FOR_ENUM_KEYED_MAPS
+#endif
+
+#define JSON_USE_OBJECTS_FOR_ENUM_KEYED_MAPS 1
+
+#include <nlohmann/json.hpp>
+using nlohmann::json;
+using nlohmann::ordered_json;
+
+#include <cstddef>
+#include <functional>
+#include <map>
+#include <string>
+#include <unordered_map>
+#include <utility>
+#include <vector>
+
+#define STRINGIZE_EX(x) #x
+#define STRINGIZE(x) STRINGIZE_EX(x)
+
+// NLOHMANN_JSON_SERIALIZE_ENUM uses a static std::pair
+DOCTEST_CLANG_SUPPRESS_WARNING_PUSH
+DOCTEST_CLANG_SUPPRESS_WARNING("-Wexit-time-destructors")
+
+namespace
+{
+// std::hash is only required for enums since C++14
+struct enum_hash
+{
+    template<typename T>
+    std::size_t operator()(T t) const noexcept
+    {
+        return static_cast<std::size_t>(t);
+    }
+};
+} // namespace
+
+// the example from #4378
+enum TaskState // NOLINT(cert-int09-c,readability-enum-initial-value,cppcoreguidelines-use-enum-class)
+{
+    TS_STOPPED,
+    TS_RUNNING,
+    TS_COMPLETED,
+    TS_INVALID = -1,
+};
+
+// NOLINTNEXTLINE(misc-const-correctness,misc-use-internal-linkage,cppcoreguidelines-avoid-c-arrays,hicpp-avoid-c-arrays,modernize-avoid-c-arrays) - false positive
+NLOHMANN_JSON_SERIALIZE_ENUM(TaskState,
+{
+    {TS_INVALID, nullptr},
+    {TS_STOPPED, "stopped"},
+    {TS_RUNNING, "running"},
+    {TS_COMPLETED, "completed"},
+})
+
+enum class color {red, green, blue}; // blue is not mapped and falls back to "red"
+
+// NOLINTNEXTLINE(misc-const-correctness,misc-use-internal-linkage,cppcoreguidelines-avoid-c-arrays,hicpp-avoid-c-arrays,modernize-avoid-c-arrays) - false positive
+NLOHMANN_JSON_SERIALIZE_ENUM(color,
+{
+    {color::red, "red"},
+    {color::green, "green"},
+})
+
+enum class strict_color {red, green, blue}; // blue is not mapped
+
+// NOLINTNEXTLINE(misc-const-correctness,misc-use-internal-linkage,cppcoreguidelines-avoid-c-arrays,hicpp-avoid-c-arrays,modernize-avoid-c-arrays) - false positive
+NLOHMANN_JSON_SERIALIZE_ENUM_STRICT(strict_color,
+{
+    {strict_color::red, "red"},
+    {strict_color::green, "green"},
+})
+
+enum class digit {zero, one};
+
+// NOLINTNEXTLINE(misc-const-correctness,misc-use-internal-linkage,cppcoreguidelines-avoid-c-arrays,hicpp-avoid-c-arrays,modernize-avoid-c-arrays) - false positive
+NLOHMANN_JSON_SERIALIZE_ENUM(digit,
+{
+    {digit::zero, 0},
+    {digit::one, 1},
+})
+
+#ifndef SKIP_TESTS_FOR_ENUM_SERIALIZATION
+enum class plain {zero, one}; // serialized as integer
+#endif
+
+TEST_CASE("JSON_USE_OBJECTS_FOR_ENUM_KEYED_MAPS")
+{
+    SECTION("the macro is part of the ABI tag")
+    {
+        const std::string ns = STRINGIZE(NLOHMANN_JSON_NAMESPACE);
+        CHECK(ns.find("_ekmo") != std::string::npos);
+    }
+
+    SECTION("std::map (#4378)")
+    {
+        using task_map = std::map<TaskState, std::string>;
+        const task_map m = {{TS_STOPPED, "aa"}, {TS_COMPLETED, "bb"}};
+        const json j = m;
+        CHECK(j == json::parse(R"({"stopped":"aa","completed":"bb"})"));
+        CHECK(j.get<task_map>() == m);
+
+        json j2;
+        j2["x"] = m;
+        CHECK(j2.dump() == R"({"x":{"completed":"bb","stopped":"aa"}})");
+    }
+
+    SECTION("std::map with custom comparator")
+    {
+        using task_map = std::map<TaskState, int, std::greater<TaskState>>;
+        const task_map m = {{TS_STOPPED, 1}, {TS_RUNNING, 2}};
+        const json j = m;
+        CHECK(j == json::parse(R"({"stopped":1,"running":2})"));
+        CHECK(j.get<task_map>() == m);
+    }
+
+    SECTION("std::unordered_map")
+    {
+        using task_map = std::unordered_map<TaskState, int, enum_hash>;
+        const task_map m = {{TS_STOPPED, 1}, {TS_RUNNING, 2}};
+        const json j = m;
+        CHECK(j == json::parse(R"({"stopped":1,"running":2})"));
+        CHECK(j.get<task_map>() == m);
+    }
+
+    SECTION("nested maps")
+    {
+        using nested_map = std::map<color, std::map<TaskState, int>>;
+        const nested_map m = {{color::green, {{TS_RUNNING, 1}}}, {color::red, {}}};
+        const json j = m;
+        CHECK(j == json::parse(R"({"green":{"running":1},"red":{}})"));
+        CHECK(j.get<nested_map>() == m);
+    }
+
+    SECTION("ordered_json keeps the order of the map")
+    {
+        using task_map = std::map<TaskState, int>;
+        const task_map m = {{TS_STOPPED, 1}, {TS_RUNNING, 2}, {TS_COMPLETED, 3}};
+        const ordered_json j = m;
+        CHECK(j.dump() == R"({"stopped":1,"running":2,"completed":3})");
+        CHECK(j.get<task_map>() == m);
+    }
+
+    SECTION("empty map")
+    {
+        const json j = std::map<TaskState, int>();
+        CHECK(j.is_object());
+        CHECK(j.empty());
+    }
+
+    SECTION("NLOHMANN_JSON_SERIALIZE_ENUM_STRICT")
+    {
+        using color_map = std::map<strict_color, int>;
+        const color_map m = {{strict_color::red, 1}, {strict_color::green, 2}};
+        const json j = m;
+        CHECK(j == json::parse(R"({"red":1,"green":2})"));
+        CHECK(j.get<color_map>() == m);
+
+        const color_map unmapped = {{strict_color::blue, 1}};
+        json _;
+        CHECK_THROWS_WITH_AS(_ = unmapped,
+                             "[json.exception.out_of_range.410] enum value out of range for strict_color", json::out_of_range&);
+    }
+
+    SECTION("arrays of [key, value] pairs are still read")
+    {
+        using task_map = std::map<TaskState, int>;
+        const task_map m = {{TS_STOPPED, 1}};
+        CHECK(json::parse(R"([["stopped",1]])").get<task_map>() == m);
+    }
+
+    SECTION("other containers are not affected")
+    {
+        const std::vector<std::pair<TaskState, int>> pairs = {{TS_STOPPED, 1}};
+        const std::map<std::string, TaskState> string_keys = {{"a", TS_STOPPED}};
+        const std::map<int, int> int_keys = {{1, 2}};
+        CHECK(json(pairs) == json::parse(R"([["stopped",1]])"));
+        CHECK(json(string_keys) == json::parse(R"({"a":"stopped"})"));
+        CHECK(json(int_keys) == json::parse("[[1,2]]"));
+    }
+
+    SECTION("maps with non-unique keys are still stored as arrays of pairs")
+    {
+        const std::multimap<TaskState, int> mm = {{TS_STOPPED, 1}, {TS_STOPPED, 2}};
+        const std::unordered_multimap<TaskState, int, enum_hash> umm = {{TS_RUNNING, 3}, {TS_RUNNING, 3}};
+        CHECK(json(mm) == json::parse(R"([["stopped",1],["stopped",2]])"));
+        CHECK(json(umm) == json::parse(R"([["running",3],["running",3]])"));
+    }
+
+    SECTION("keys that do not serialize to strings")
+    {
+        const std::map<TaskState, int> null_key = {{TS_INVALID, 1}};
+        const std::map<digit, int> number_key = {{digit::zero, 1}};
+        json j = "unchanged";
+
+        // mapped to null
+        CHECK_THROWS_WITH_AS(j = null_key,
+                             "[json.exception.type_error.302] type must be string, but is null", json::type_error&);
+
+        // mapped to a number
+        CHECK_THROWS_WITH_AS(j = number_key,
+                             "[json.exception.type_error.302] type must be string, but is number", json::type_error&);
+
+#ifndef SKIP_TESTS_FOR_ENUM_SERIALIZATION
+        // enum without NLOHMANN_JSON_SERIALIZE_ENUM
+        const std::map<plain, int> plain_key = {{plain::zero, 1}};
+        CHECK_THROWS_WITH_AS(j = plain_key,
+                             "[json.exception.type_error.302] type must be string, but is number", json::type_error&);
+#endif
+
+        CHECK(j == "unchanged");
+    }
+
+    SECTION("keys that serialize to the same string")
+    {
+        const std::map<color, int> m = {{color::red, 1}, {color::blue, 2}};
+        json j = "unchanged";
+
+        // color::blue is not mapped and falls back to "red"
+        CHECK_THROWS_WITH_AS(j = m,
+                             "[json.exception.type_error.318] duplicate object key 'red'", json::type_error&);
+
+        CHECK(j == "unchanged");
+    }
+}
+
+DOCTEST_CLANG_SUPPRESS_WARNING_POP

tests/src/unit-enum_keyed_maps_default.cpp

@@ -0,0 +1,144 @@
+//     __ _____ _____ _____
+//  __|  |   __|     |   | |  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"
+
+// This file tests maps with enum keys with the default setting of
+// JSON_USE_OBJECTS_FOR_ENUM_KEYED_MAPS (or whatever a -D flag sets it to).
+// unit-enum_keyed_maps.cpp tests JSON_USE_OBJECTS_FOR_ENUM_KEYED_MAPS=1.
+// These tests are not part of unit-conversions.cpp, because that object file
+// is already too big for the MinGW linker of some compilers.
+
+#include <nlohmann/json.hpp>
+using nlohmann::json;
+
+#include <cstddef>
+#include <functional>
+#include <map>
+#include <string>
+#include <unordered_map>
+
+// NLOHMANN_JSON_SERIALIZE_ENUM uses a static std::pair
+DOCTEST_CLANG_SUPPRESS_WARNING_PUSH
+DOCTEST_CLANG_SUPPRESS_WARNING("-Wexit-time-destructors")
+
+enum class cards {kreuz, pik, herz, karo};
+
+// NOLINTNEXTLINE(misc-use-internal-linkage,misc-const-correctness,cppcoreguidelines-avoid-c-arrays,hicpp-avoid-c-arrays,modernize-avoid-c-arrays) - false positive
+NLOHMANN_JSON_SERIALIZE_ENUM(cards,
+{
+    {cards::kreuz, "kreuz"},
+    {cards::pik, "pik"},
+    {cards::herz, "herz"},
+    {cards::karo, "karo"}
+})
+
+enum TaskState // NOLINT(cert-int09-c,readability-enum-initial-value,cppcoreguidelines-use-enum-class)
+{
+    TS_STOPPED,
+    TS_RUNNING,
+    TS_COMPLETED,
+    TS_INVALID = -1,
+};
+
+// NOLINTNEXTLINE(misc-const-correctness,misc-use-internal-linkage,cppcoreguidelines-avoid-c-arrays,hicpp-avoid-c-arrays,modernize-avoid-c-arrays) - false positive
+NLOHMANN_JSON_SERIALIZE_ENUM(TaskState,
+{
+    {TS_INVALID, nullptr},
+    {TS_STOPPED, "stopped"},
+    {TS_RUNNING, "running"},
+    {TS_COMPLETED, "completed"},
+})
+
+enum class strict_cards {kreuz, pik, herz, karo, andere}; // andere not included in mapping
+
+// NOLINTNEXTLINE(misc-use-internal-linkage,misc-const-correctness,cppcoreguidelines-avoid-c-arrays,hicpp-avoid-c-arrays,modernize-avoid-c-arrays) - false positive
+NLOHMANN_JSON_SERIALIZE_ENUM_STRICT(strict_cards,
+{
+    {strict_cards::kreuz, "kreuz"},
+    {strict_cards::pik, "pik"},
+    {strict_cards::herz, "herz"},
+    {strict_cards::karo, "karo"}
+})
+
+namespace
+{
+// std::hash is only required for enums since C++14
+struct enum_hash
+{
+    template<typename T>
+    std::size_t operator()(T t) const noexcept
+    {
+        return static_cast<std::size_t>(t);
+    }
+};
+} // namespace
+
+// see unit-enum_keyed_maps.cpp for JSON_USE_OBJECTS_FOR_ENUM_KEYED_MAPS=1
+TEST_CASE("maps with enum keys")
+{
+    using task_map = std::map<TaskState, std::string>;
+    using task_umap = std::unordered_map<TaskState, std::string, enum_hash>;
+    using task_gmap = std::map<TaskState, std::string, std::greater<TaskState>>;
+    using nested_map = std::map<cards, std::map<TaskState, int>>;
+    using strict_map = std::map<strict_cards, int>;
+    using int_map = std::map<int, int>;
+    using int_umap = std::unordered_map<int, int>;
+
+    const task_map m = {{TS_STOPPED, "aa"}, {TS_COMPLETED, "bb"}};
+
+#if !JSON_USE_OBJECTS_FOR_ENUM_KEYED_MAPS
+    SECTION("stored as array of pairs")
+    {
+        CHECK(json(m) == json::parse(R"([["stopped","aa"],["completed","bb"]])"));
+        CHECK(json(task_umap {{TS_RUNNING, "cc"}}) == json::parse(R"([["running","cc"]])"));
+    }
+#endif
+
+    SECTION("read from array of pairs")
+    {
+        CHECK(json::parse(R"([["stopped","aa"],["completed","bb"]])").get<task_map>() == m);
+    }
+
+    SECTION("read from object (#4378)")
+    {
+        const json j = json::parse(R"({"stopped":"aa","completed":"bb"})");
+        CHECK(j.get<task_map>() == m);
+        CHECK(j.get<task_umap>() == task_umap(m.begin(), m.end()));
+        CHECK(j.get<task_gmap>() == task_gmap(m.begin(), m.end()));
+        CHECK(json::parse(R"({"kreuz":{"stopped":1}})").get<nested_map>() == nested_map {{cards::kreuz, {{TS_STOPPED, 1}}}});
+        CHECK(nlohmann::ordered_json::parse(R"({"stopped":"aa","completed":"bb"})").get<task_map>() == m);
+
+        // object keys go through the enum's from_json
+        strict_map sm;
+        CHECK_THROWS_WITH_AS(json::parse(R"({"what?":1})").get_to(sm),
+                             "[json.exception.out_of_range.410] enum value out of range for strict_cards: \"what?\"", json::out_of_range&);
+    }
+
+    SECTION("objects are only read for enum keys")
+    {
+        // built rather than parsed, so that the messages do not gain a byte
+        // range with JSON_DIAGNOSTIC_POSITIONS
+        const json j = {{"1", 2}};
+        int_map im;
+        int_umap ium;
+        CHECK_THROWS_WITH_AS(j.get_to(im),
+                             "[json.exception.type_error.302] type must be array, but is object", json::type_error&);
+        CHECK_THROWS_WITH_AS(j.get_to(ium),
+                             "[json.exception.type_error.302] type must be array, but is object", json::type_error&);
+    }
+
+    SECTION("other types are rejected")
+    {
+        task_map tm;
+        CHECK_THROWS_WITH_AS(json("stopped").get_to(tm),
+                             "[json.exception.type_error.302] type must be array, but is string", json::type_error&);
+    }
+}
+
+DOCTEST_CLANG_SUPPRESS_WARNING_POP

tools/generate_natvis/generate_natvis.py

@@ -20,7 +20,7 @@ if __name__ == '__main__':
 
     namespaces = ['nlohmann']
     abi_prefix = 'json_abi'
-    abi_tags = ['_diag', '_ldvcmp', '_dp', '_bics', '_psp', '_snul']
+    abi_tags = ['_diag', '_ldvcmp', '_dp', '_bics', '_psp', '_snul', '_ekmo']
     version = '_v' + args.version.replace('.', '_')
     inline_namespaces = []