json/api/macros/nlohmann_json_serialize_enum.md

# NLOHMANN_JSON_SERIALIZE_ENUM

```cpp
#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:

```cpp
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"
    #include <iostream>
    #include <nlohmann/json.hpp>
    
    using json = nlohmann::json;
    
    namespace ns
    {
    enum TaskState
    {
        TS_STOPPED,
        TS_RUNNING,
        TS_COMPLETED,
        TS_INVALID = -1
    };
    
    NLOHMANN_JSON_SERIALIZE_ENUM(TaskState,
    {
        { TS_INVALID, nullptr },
        { TS_STOPPED, "stopped" },
        { TS_RUNNING, "running" },
        { TS_COMPLETED, "completed" }
    })
    
    enum class Color
    {
        red, green, blue, unknown
    };
    
    NLOHMANN_JSON_SERIALIZE_ENUM(Color,
    {
        { Color::unknown, "unknown" }, { Color::red, "red" },
        { Color::green, "green" }, { Color::blue, "blue" }
    })
    } // namespace ns
    
    int main()
    {
        // serialization
        json j_stopped = ns::TS_STOPPED;
        json j_red = ns::Color::red;
        std::cout << "ns::TS_STOPPED -> " << j_stopped
                  << ", ns::Color::red -> " << j_red << std::endl;
    
        // deserialization
        json j_running = "running";
        json j_blue = "blue";
        auto running = j_running.get<ns::TaskState>();
        auto blue = j_blue.get<ns::Color>();
        std::cout << j_running << " -> " << running
                  << ", " << j_blue << " -> " << static_cast<int>(blue) << std::endl;
    
        // deserializing undefined JSON value to enum
        // (where the first map entry above is the default)
        json j_pi = 3.14;
        auto invalid = j_pi.get<ns::TaskState>();
        auto unknown = j_pi.get<ns::Color>();
        std::cout << j_pi << " -> " << invalid << ", "
                  << j_pi << " -> " << static_cast<int>(unknown) << std::endl;
    }
    
    ```

    Output:
    
    ```json
    ns::TS_STOPPED -> "stopped", ns::Color::red -> "red"
    "running" -> 1, "blue" -> 2
    3.14 -> -1, 3.14 -> 3
    
    ```

??? 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"
    #include <iostream>
    #include <nlohmann/json.hpp>
    
    using json = nlohmann::json;
    
    namespace ns
    {
    enum class Color
    {
        red, green, blue, unknown
    };
    
    NLOHMANN_JSON_SERIALIZE_ENUM(Color,
    {
        { Color::unknown, "unknown" }, { Color::red, "red" },
        { Color::green, "green" }, { Color::blue, "blue" },
        { Color::red, "rot" } // a second conversion for Color::red
    })
    }
    
    int main()
    {
        // serialization
        json j_red = ns::Color::red;
        std::cout << static_cast<int>(ns::Color::red) << " -> " << j_red << std::endl;
    
        // deserialization
        json j_rot = "rot";
        auto rot = j_rot.get<ns::Color>();
        auto red = j_red.get<ns::Color>();
        std::cout << j_rot << " -> " << static_cast<int>(rot) << std::endl;
        std::cout << j_red << " -> " << static_cast<int>(red) << std::endl;
    }
    
    ```

    Output:
    
    ```json
    0 -> "red"
    "rot" -> 0
    "red" -> 0
    
    ```

## See also

- [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

Added in version 3.4.0.