#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 de-serialized 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.
type
(in) : name of the enum to serialize/deserialize
conversion
(in) : a pair of an enumerator and a JSON serialization; arbitrary pairs can can be given as comma-separated list
The macros add two friend functions to the class 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);
!!! 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.
??? example “Example 1: 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 2: 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" ```
Added in version 3.4.0.