example code to show c++ enum class to NLOHMANN json conversion

enum_class_json_conversion.cpp

/*
NLOHMANN_JSON_SERIALIZE_ENUM expanded to inline functions, so this macro should be placed in header where enum class is declared

also this macro must be declared within enum type's namespace
```
// include loguru header here, declear enum NamedVerbosity
// this code injection happen in your project header file, no need to modify loguru.hpp
namespace loguru
{
    // this macro must be called in the enum declaring namespace
    NLOHMANN_JSON_SERIALIZE_ENUM(NamedVerbosity, {
                                                     {Verbosity_OFF, "OFF"},
                                                     {Verbosity_FATAL, "FATAL"},     /// program will terminate
                                                     {Verbosity_ERROR, "ERROR"},     /// red text print in console
                                                     {Verbosity_WARNING, "WARNING"}, /// yellow text in console
                                                     {Verbosity_INFO, "INFO"},  /// default 0 for loguru, 20 for python
                                                     {Verbosity_1, "PROGRESS"}, /// 1 for loguru, write into log file
                                                     {Verbosity_2, "DEBUG"},    /// 2 for loguru, write into file
                                                 });
} // namespace loguru
```
*/

#include "json.hpp"
#include <assert.h>
#include <iostream>

// copy of the NLOHMANN_JSON_SERIALIZE_ENUM here for illustration
#define JSON_SERIALIZE_ENUM(ENUM_TYPE, ...)                                           \
    template<typename BasicJsonType>                                                           \
    inline void to_json(BasicJsonType& j, const ENUM_TYPE& e)                                  \
    {                                                                                          \
        static_assert(std::is_enum<ENUM_TYPE>::value, #ENUM_TYPE " must be an enum!");         \
        static const std::pair<ENUM_TYPE, BasicJsonType> m[] = __VA_ARGS__;                    \
        auto it = std::find_if(std::begin(m), std::end(m),                                     \
                               [e](const std::pair<ENUM_TYPE, BasicJsonType>& ej_pair) -> bool \
        {                                                                                      \
            return ej_pair.first == e;                                                         \
        });                                                                                    \
        j = ((it != std::end(m)) ? it : std::begin(m))->second;                                \
    }                                                                                          \
    template<typename BasicJsonType>                                                           \
    inline void from_json(const BasicJsonType& j, ENUM_TYPE& e)                                \
    {                                                                                          \
        static_assert(std::is_enum<ENUM_TYPE>::value, #ENUM_TYPE " must be an enum!");         \
        static const std::pair<ENUM_TYPE, BasicJsonType> m[] = __VA_ARGS__;                    \
        auto it = std::find_if(std::begin(m), std::end(m),                                     \
                               [j](const std::pair<ENUM_TYPE, BasicJsonType>& ej_pair) -> bool \
        {                                                                                      \
            return ej_pair.second == j;                                                        \
        });                                                                                    \
        e = ((it != std::end(m)) ? it : std::begin(m))->first;                                 \
    }


enum  class DevicePreference
{
    CPU, ///  item count is small, but processing each item take times
    GPU, /// large items but small job
};

// use the official macro
NLOHMANN_JSON_SERIALIZE_ENUM(DevicePreference, {
                                                        {DevicePreference::CPU, "CPU"},
                                                        {DevicePreference::GPU, "GPU"},
                                                    })

/*  this is the expansion of macro, for debugging, review purpose
template <typename BasicJsonType> inline void to_json(BasicJsonType& j, const DevicePreference& e)
{
    static_assert(std::is_enum<DevicePreference>::value, "DevicePreference"
                                                              " must be an enum!");
    static const std::pair<DevicePreference, BasicJsonType> m[] = {
        {DevicePreference::CPU, "CPU"},
        {DevicePreference::GPU, "GPU"},
    };
    auto it = std::find_if(
        std::begin(m), std::end(m),
        [e](const std::pair<DevicePreference, BasicJsonType>& ej_pair) -> bool { return ej_pair.first == e; });
    j = ((it != std::end(m)) ? it : std::begin(m))->second;
}

template <typename BasicJsonType> inline void from_json(const BasicJsonType& j, DevicePreference& e)
{
    static_assert(std::is_enum<DevicePreference>::value, "DevicePreference"
                                                              " must be an enum!");
    static const std::pair<DevicePreference, BasicJsonType> m[] = {
        {DevicePreference::CPU, "CPU"},
        {DevicePreference::GPU, "GPU"},
    };
    auto it = std::find_if(
        std::begin(m), std::end(m),
        [j](const std::pair<DevicePreference, BasicJsonType>& ej_pair) -> bool { return ej_pair.second == j; });
    e = ((it != std::end(m)) ? it : std::begin(m))->first;
}
*/

int main()
{

// Usage:
// enum -> json string
nlohmann::json j = DevicePreference::CPU;
assert(j == "CPU");
std::cout << j << std::endl;

// it is fine to scope enum value by enum type
DevicePreference  ts = DevicePreference::GPU;
// json string-> enum
nlohmann::json j3 = "GPU";
assert( j3.get<DevicePreference>() == DevicePreference::GPU);

//std::cout << j3.get<DevicePreference>();    // << operator is not supported

// unmapped json -> enum (default value is first pair)
nlohmann::json jPi = "unknown";
assert( jPi.get<DevicePreference>() == DevicePreference::CPU );

return 0;

}