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Niels LohmannandMuhammad Amir bin Mohamad Ghazaly
8c1f60a45e
* Store maps with enum keys as objects (opt-in) Maps with enum keys, such as std::map<E, T>, are stored as arrays of [key, value] pairs, because enums are not convertible to the string type of object keys - even if NLOHMANN_JSON_SERIALIZE_ENUM maps them to strings (#4378). The new JSON_USE_OBJECTS_FOR_ENUM_KEYED_MAPS macro stores them as objects instead, converting each key with the enum's to_json. It applies to any map-like type with enum keys (std::map with any comparator, std::unordered_map, ...). A key that does not convert to a string throws type_error.302, and two keys converting to the same string throw the new type_error.318, rather than losing an entry. The macro changes the output of inline functions, so it is part of the ABI tag (_ekmo). Reading needs no macro: std::map and std::unordered_map with enum keys are now also read from objects, converting each key with the enum's from_json. That input was rejected before, and arrays of pairs are still read, so data written either way can be read. This supersedes #4531, which first proposed storing these maps as objects. Co-authored-by: Muhammad Amir bin Mohamad Ghazaly <amirghaz@umich.edu> Signed-off-by: Niels Lohmann <mail@nlohmann.me> * Keep multimaps with enum keys as arrays of pairs 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> * Move the default enum-keyed map tests out of unit-conversions.cpp The Windows clang 20.1.8 job (MinGW, Debug) failed to link test-conversions_cpp17 with "relocation truncated to fit: IMAGE_REL_AMD64_REL32 against .rdata": the object file of unit-conversions.cpp was already close to the limit, and the new "maps with enum keys" test case pushed it over. windows.yml asks to keep these objects small by splitting test files. Move the test case unchanged into unit-enum_keyed_maps_default.cpp, with the three enums it needs. It still honors a -D flag for JSON_USE_OBJECTS_FOR_ENUM_KEYED_MAPS, as before. unit-conversions.cpp is back to its state on develop. Signed-off-by: Niels Lohmann <mail@nlohmann.me> * Build the enum-keyed map test object instead of parsing it ci_test_diagnostic_positions failed in unit-enum_keyed_maps_default.cpp: with JSON_DIAGNOSTIC_POSITIONS, a parsed value adds its byte range to the exception message ("(bytes 0-7) type must be array, but is object"), so the exact-message checks did not match. Build the object in memory, like unit-custom-array-type.cpp does. Signed-off-by: Niels Lohmann <mail@nlohmann.me> --------- Signed-off-by: Niels Lohmann <mail@nlohmann.me> Co-authored-by: Muhammad Amir bin Mohamad Ghazaly <amirghaz@umich.edu>
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NLOHMANN_JSON_SERIALIZE_ENUM
#define NLOHMANN_JSON_SERIALIZE_ENUM(type, conversion...)
By default, enum values are serialized to JSON as integers. In some cases, this could result in undesired behavior. If an enum is modified or re-ordered after data has been serialized to JSON, the later deserialized JSON data may be undefined or a different enum value than was originally intended.
The NLOHMANN_JSON_SERIALIZE_ENUM allows to define a user-defined serialization for every enumerator.
Parameters
type(in)- name of the enum to serialize/deserialize
conversion(in)- a pair of an enumerator and a JSON serialization; arbitrary pairs can be given as a comma-separated list
Default definition
The macro adds two functions to the namespace which take care of the serialization and deserialization:
template<typename BasicJsonType>
inline void to_json(BasicJsonType& j, const type& e);
template<typename BasicJsonType>
inline void from_json(const BasicJsonType& j, type& e);
Notes
!!! info "Prerequisites"
The macro must be used inside the namespace of the enum.
!!! important "Important notes"
- When using [`get<ENUM_TYPE>()`](../basic_json/get.md), undefined JSON values will default to the first specified
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
??? example "Example: Basic usage"
The example shows how `NLOHMANN_JSON_SERIALIZE_ENUM` can be used to serialize/deserialize both classical enums and
C++11 enum classes:
```cpp hl_lines="16 17 18 19 20 21 22 29 30 31 32 33"
--8<-- "examples/nlohmann_json_serialize_enum.cpp"
```
Output:
```json
--8<-- "examples/nlohmann_json_serialize_enum.output"
```
??? example "Example: Multiple conversions for one enumerator"
The example shows how to use multiple conversions for a single enumerator. In the example, `Color::red` will always
be *serialized* to `"red"`, because the first occurring conversion. The second conversion, however, offers an
alternative *deserialization* from `"rot"` to `Color::red`.
```cpp hl_lines="17"
--8<-- "examples/nlohmann_json_serialize_enum_2.cpp"
```
Output:
```json
--8<-- "examples/nlohmann_json_serialize_enum_2.output"
```
See also
- Specializing enum conversion
NLOHMANN_JSON_SERIALIZE_ENUM_STRICTJSON_DISABLE_ENUM_SERIALIZATIONJSON_USE_OBJECTS_FOR_ENUM_KEYED_MAPS
Version history
Added in version 3.4.0.