Deserializing maps with enum keys

[onavratil-monetplus]

The library provides cool features to work with enums/enum classes. Lets have

enum class MyEnum{
        INVALID,
        ONE,
        TWO
    };
    NLOHMANN_JSON_SERIALIZE_ENUM(MyEnum, {
        {MyEnum::INVALID, nullptr},
        {MyEnum::ONE, "one"},
        {MyEnum::TWO, "two"}
    })

Now I want to have a map whose keys are of the enum class type. I can do something like....

auto obj = "{\"one\": 123}"_json;
auto myMap = obj.get<std::map<std::string, int>>();
for(const auto &[k, v] : myMap){
    MyEnum key = nlohmann::json(k).get<MyEnum>();
}

However, if i try a easier way

auto myMapEasier =  obj.get<std::map<MyEnum, int>>();

...i get a strange error
[json.exception.type_error.302] type must be array, but is object
I have read through some issues, but mostly, they are concerned about serialization of such a map, not the opposite way.

Is this a desired behavior? Or is there some easier workaround I am missing?

[gregmarr]

I believe that this is because the type std::map<NonString, something> is serialized by the library as if it were std::vector<std::pair<NonString, something>>.

[onavratil-monetplus]

Thank you for the answer, I understand it much better now. Just to be a little more verbose...

The problem lies in the fact that my enum class indeed is not directly convertible to string. It can be converted via json thanks to the NLOHMANN_JSON_SERIALIZE_ENUM macro, but this is not a direct enum to string cast. I did this in
nlohmann::json(k).get<MyEnum>(); (first create json and then convert it to string).

The library indeed makes the difference based on std::is_convertible<KeyType,std::string>::value which does not take any detours. For now, the important fact is that the "roundtrip holds", i.e. a std::map<MyEnum, int>, when serialized by nlohmann::json, will be represented as an array of pairs (2-element arrays), which is exactly the format that is expected and works for deserialization.

The only way out of this would be to have an (implicit or explicit) conversion of MyEnum to std::string, however, state-of-art C++, as far as i know, offers no way to overload cast operators for enum class