Enum serialization and X-macros

[chrysante]

The documentation gives this example of how to serialize an enum with the library.

NLOHMANN_JSON_SERIALIZE_ENUM( TaskState, {
    {TS_INVALID, nullptr},
    {TS_STOPPED, "stopped"},
    {TS_RUNNING, "running"},
    {TS_COMPLETED, "completed"},
})

Unfortunately this macro does not work with X-macros, because you cannot #include within a macro argument list. It would be great to have three additional macros that can be used like this:

NLOHMANN_JSON_SERIALIZE_ENUM_BEGIN(TaskState)
NLOHMANN_JSON_SERIALIZE_ENUM_ELEM(TS_INVALID, nullptr)
NLOHMANN_JSON_SERIALIZE_ENUM_ELEM(TS_STOPPED, "stopped")
NLOHMANN_JSON_SERIALIZE_ENUM_ELEM(TS_RUNNING, "running")
NLOHMANN_JSON_SERIALIZE_ENUM_ELEM(TS_COMPLETED, "completed")
NLOHMANN_JSON_SERIALIZE_ENUM_END()

The intent is that I can then write this code:

NLOHMANN_JSON_SERIALIZE_ENUM_BEGIN(TaskState)
// Use preprocessor to convert enum cases to strings
#define TASKSTATE_DEF(State) NLOHMANN_JSON_SERIALIZE_ENUM_ELEM(State, #State) 
#include "taskstatedef.h"
NLOHMANN_JSON_SERIALIZE_ENUM_END()

This does not work with the existing macro because you cannot #include within a macro argument list.

I looked at the implementation of NLOHMANN_JSON_SERIALIZE_ENUM and it does not seem trivial to add the proposed macros, because the argument list __VA_ARGS__ is used twice, once in to_json() and once in from_json(). However in both functions it is used identically to create a static array that maps the enum values to JSON objects.

Perhaps one way one implement the macros could be to add a class template enum_serializer to the nlohmann namespace that stores the static mapping array. The macros then specialize this template and the from_json and to_json functions could be generic templates that SFINAE out if enum_serializer<enum-type> is not defined. This also would mean that the static array would only be stored once.

What do you think of this idea? If it seems worthwhile I could get around to implement it the next days.

[gregmarr]

What do you mean by "X-macros"?

[chrysante]

I guess X-macro is not entirely correct. Basically I mean the same as here except that the list is defined in a separate header file without include guards that can be included multiple times. LLVM for example does this all over the place to have one single source of thruth for all repetitive declarations.

[gregmarr]

I don't see any difference between writing the first and writing the second, unless you're saying that you'd allow other things between the BEGIN and END. Are you trying to make something that would generate this for you? If so, why would you be able to generate the second but not the first?

[chrysante]

Sorry, I might have not made that clear enough. I want to include a header file between NLOHMANN_JSON_SERIALIZE_ENUM_BEGIN and NLOHMANN_JSON_SERIALIZE_ENUM_END. That header file uses a macro I defined right before to define the enum mapping. But with the existing NLOHMANN_JSON_SERIALIZE_ENUM macro this does not work because you cannot include a header file in the argument list of a macro 'invocation'.

I updated the OP to clarify this.

[gregmarr]

Try this:

#define TASKSTATE_DEF(State) { State, #State },
std::pair<TASKSTATE, std::string> taskstate_def[] = {
#include "taskstatedef.h"
};

NLOHMANN_JSON_SERIALIZE_ENUM(TASKSTATE, taskstate_def);

[chrysante]

Thanks you, that's a great idea!