Upgrading to 3.x: to_/from_json with enum class

[endorph-soft]

I'm in the process of updating a project from version 2.1.1 to the version 3.x series.

I have the following two functions defined to handle enum classes:

template <typename JSON, typename Enum>
void to_json(JSON & j, Enum e) {
  j = ToString(e);
}

template <typename JSON, typename Enum>
void from_json(const JSON & j, Enum & e) {
  if (!FromString(e, j)) {
    throw std::invalid_argument("Badness...");
  }
}

There are definitions of ToString and FromString for all the enumerations of interest. I'm not using SFINAE to restrict Enum to enumerations, because it's not currently needed. With version 2.1.1, everything is working fine.

With the update to 3.x, these functions are no longer found by the lookup. If I comment out the new enum to_json and from_json inside the library, the errors disappear, and all is well again.

How can I make my functions work? Do I need a more specific signature? I don't want to define a function for every enumeration, if I can get away with it.

[nlohmann]

Are the functions in the same namespace as the respective Enum?

[endorph-soft]

Yep, although they are in a different header file (which is included). The internal ToString and FromString are in the global namespace, but I don't think that matters.

It all works fine with 2.1.1.

[theodelrieu]

I'll try to take a look in the next days, meanwhile I would suggest partially specialize adl_serializer on enum classes.

I don't remember when we did add support for enums, I think it was in 2.1.0, which would be even stranger that it breaks now

[endorph-soft]

In 2.1.1 there is code for Unscoped Enums, using this:

template<typename T>
using is_unscoped_enum = 
    std::integral_constant<bool, std::is_convertible<T, int>::value and 
    std::is_enum<T>::value>;

I don't think it will pick up enum classes (by design, probably). The newer code picks up both enum and enum class.

I'll see if I can figure out the adl_serialiser thing. Thanks.

[theodelrieu]

I did take a look, and in order to pick up your overload, you have to constrain your template a bit:

template <typename JSON, typename Enum, typename = std::enable_if_t<std::is_enum<Enum>::value>>
void to_json(JSON & j, Enum e) {
  j = ToString(e);
}

No need to fiddle with adl_serializer in this case.

[endorph-soft]

Thanks for that, and apologies for the slow response. It'll be a few days before I can get back to rolling out new library versions, but I'm sure it will work. Thanks.

[nlohmann]

Any news on this?

[endorph-soft]

Sorry for the lack of response. We've been approaching a release, and I've been busy putting out other fires. I'll try and have a look shortly. Thanks again for your help, and for the library. It's very useful.

[nlohmann]

@endorph-soft Any news one this?

[nlohmann]

Can I close this issue?

[alexhadd]

Hi everyone, I was solving a problem very similar to this, and @theodelrieu 's response was very close to being correct. I could not get things to work until I changed the template a little. I went from...

template <typename JSON, typename Enum, typename = std::enable_if_t<std::is_enum<Enum>::value>>
void to_json(JSON & j, Enum e) {
  j = ToString(e);
}

to

typename Enum, typename = std::enable_if_t<std::is_enum<Enum>::value>>
void to_json(nlohmann::json& j, Enum e) {
  j = ToString(e);
}

Before making this change, I was still being told that my type had no from_json defined when using .get<namespace::mytype>(). I'm quite uncertain as to why directly referring to the nlohmann::json object fixed the issue, but it did.

[nlohmann]

Thanks for checking back!

[theodelrieu]

Hmm, this is quite weird, I'd like to know what is the real difference between both...

Anyway, glad you could solve it.