json/docs/mkdocs/docs/features/enum_conversion.md

Specializing enum conversion

By default, enum values are serialized to JSON as integers. In some cases, this could result in undesired behavior. If the integer values of any enum values are changed after data using those enum values has been serialized to JSON, then deserializing that JSON would result in a different enum value being restored, or the value not being found at all.

It is possible to more precisely specify how a given enum is mapped to and from JSON as shown below:

// example enum type declaration
enum TaskState {
    TS_STOPPED,
    TS_RUNNING,
    TS_COMPLETED,
    TS_INVALID=-1,
};

// map TaskState values to JSON as strings
NLOHMANN_JSON_SERIALIZE_ENUM( TaskState, {
    {TS_INVALID, nullptr},
    {TS_STOPPED, "stopped"},
    {TS_RUNNING, "running"},
    {TS_COMPLETED, "completed"},
})

The NLOHMANN_JSON_SERIALIZE_ENUM() macro declares a set of to_json() / from_json() functions for type TaskState while avoiding repetition and boilerplate serialization code.

Usage

Serialization converts an enum value to its mapped string, deserialization does the reverse, and an unrecognized JSON value deserializes to the first pair in the map:

// enum to JSON as string
json j = TS_STOPPED;
assert(j == "stopped");

// json string to enum
json j3 = "running";
assert(j3.get<TaskState>() == TS_RUNNING);

// undefined json value to enum (where the first map entry above is the default)
json jPi = 3.14;
assert(jPi.get<TaskState>() == TS_INVALID );

??? example "Example: serializing/deserializing enums, including a second enum type"

```cpp
--8<-- "examples/nlohmann_json_serialize_enum.cpp"
```

Output:

```json
--8<-- "examples/nlohmann_json_serialize_enum.output"
```

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 before including the library to store them as objects, with the keys converted by the enum's to_json() function:

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 above,

• NLOHMANN_JSON_SERIALIZE_ENUM() MUST be declared in your enum type's namespace (which can be the global namespace), or the library will not be able to locate it, and it will default to integer serialization.
• It MUST be available (e.g., proper headers must be included) everywhere you use the conversions.

Other Important points:

• When using get<ENUM_TYPE>(), undefined JSON values will default to the first pair specified in your map. Select this default pair carefully. If you desire an exception in this circumstance use NLOHMANN_JSON_SERIALIZE_ENUM_STRICT() which behaves identically except for throwing an out_of_range.410 exception on unrecognized values, both when serializing an enum value not listed in the map and when deserializing a JSON value that matches none of the map's entries.
• If an enum or JSON value is specified more than once in your map, the first matching occurrence from the top of the map will be returned when converting to or from JSON.
• To disable the default serialization of enumerators as integers and force a compiler error instead, see JSON_DISABLE_ENUM_SERIALIZATION.

??? example "Example: NLOHMANN_JSON_SERIALIZE_ENUM_STRICT throwing on unrecognized values"

```cpp
--8<-- "examples/nlohmann_json_serialize_enum_strict_err.cpp"
```

Output:

```json
--8<-- "examples/nlohmann_json_serialize_enum_strict_err.output"
```