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 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.

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 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);

Notes¶

Prerequisites

The macro must be used inside the namespace of the enum.

Important notes

When using template get<ENUM_TYPE>(), 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 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:

#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.template get<ns::TaskState>();
    auto blue = j_blue.template 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.template get<ns::TaskState>();
    auto unknown = j_pi.template get<ns::Color>();
    std::cout << j_pi << " -> " << invalid << ", "
              << j_pi << " -> " << static_cast<int>(unknown) << std::endl;
}

Output:

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

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.

#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.template get<ns::Color>();
    auto red = j_red.template get<ns::Color>();
    std::cout << j_rot << " -> " << static_cast<int>(rot) << std::endl;
    std::cout << j_red << " -> " << static_cast<int>(red) << std::endl;
}

Output:

0 -> "red"
"rot" -> 0
"red" -> 0

Version history¶

Added in version 3.4.0.

Last update: June 11, 2023