add strict enum de/serialization macro

[hnampally]

#3992

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• If applicable, an #3992 is referenced.
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[coveralls]

coverage: 99.186%. remained the same
when pulling c6d9ea0 on hnampally:issue-3992
into f06604f on nlohmann:develop.

[nlohmann]

Just a thought: shouldn't a strict macro also take care of this?

[hnampally]

Thank you for catching that! Yes, that makes sense. Could you clarify which exception should be thrown in this case?

[nlohmann]

Is this needed? The other examples work with exceptions without this.

[hnampally]

Yes, for some reason, I encountered a compilation issue without this. I noticed that JSON_THROW is already defined in json.hpp, but the issue persists. Could you let me know if I'm overlooking something?

[hnampally]

It looks like it is being undefined in macro_unscope.hpp at the end of json.hpp. Please find the relevant line at nlohmann/json.hpp.

[hnampally]

I noticed that other examples, such as nlohmann_json_serialize_enum_2.cpp, do not encounter this compilation issue because they use macro functions like NLOHMANN_JSON_SERIALIZE_ENUM, which do not throw exceptions. In macro_scope.hpp, NLOHMANN_JSON_SERIALIZE_ENUM_STRICT is the first macro to trigger the JSON_THROW exception. Therefore, I believe I need to redefine it in this example code.

[nlohmann]

No, this definitely needs to be fixed - a user must not worry about any macros for exceptions. In this case, the exception from NLOHMANN_JSON_SERIALIZE_ENUM_STRICT should be thrown via a plain throw. The JSON_THROW is only used for exceptions that can occur inside of the library whereas it makes no sense for a user to use a different throw mechanism for the the exception in NLOHMANN_JSON_SERIALIZE_ENUM_STRICT.

[gregmarr]

On the other hand, if you don't use JSON_THROW, then NLOHMANN_JSON_SERIALIZE_ENUM_STRICT is not usable with exceptions disabled. An alternative solution to having the macro escape the library could be to have an internal function like this just for this particular usage of throwing from inside one of the macros:

    template<typename T> 
    void json_throw_from_serialize_macro(T&& exception) 
    {
        JSON_THROW std::forward<T>(exception);
    }

[hnampally]

@nlohmann Yes, I completely agree. I will give Greg's proposed solution a try.
@gregmarr thanks for your code snippet.

[nlohmann]

No, this definitely needs to be fixed - a user must not worry about any macros for exceptions. In this case, the exception from NLOHMANN_JSON_SERIALIZE_ENUM_STRICT should be thrown via a plain throw. The JSON_THROW is only used for exceptions that can occur inside of the library whereas it makes no sense for a user to use a different throw mechanism for the the exception in NLOHMANN_JSON_SERIALIZE_ENUM_STRICT.