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

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 [`template 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.

Examples

??? 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"
```

See also

• Specializing enum conversion
• JSON_DISABLE_ENUM_SERIALIZATION

Version history

Added in version 3.4.0.