Allow custom getter/setter for enum classes?

[AmaiKinono]

I tried to bind the value of a select element to an enum class. To do this, I defined custom getters and setters:

enum class BookType {
  one_way,
  return_flight,
};

// ctor is an Rml::DataModelConstructor
ctor.RegisterScalar<BookType>([](const BookType& booktype, Rml::Variant& variant) {
  if (booktype == BookType::one_way) {
    variant = "one_way";
  } else if (booktype == BookType::return_flight) {
    variant = "return";
  } else {
    Rml::Log::Message(Rml::Log::LT_WARNING, "Invalid booktype.");
  }
 },
  [](BookType& booktype, const Rml::Variant& variant) {
    Rml::String str = variant.Get<Rml::String>();
    if (str == "one_way") {
      booktype = BookType::one_way;
    } else if (str == "return") {
      booktype = BookType::return_flight;
    } else {
      Rml::Log::Message(Rml::Log::LT_WARNING, "Invalid value :'%s'.", str.c_str());
    }
  });

I got this compile error:

<path-to-RmlUi>/Include/RmlUi/Core/DataModelHandle.h:146:51: error: static assertion failed: Cannot register scalar data type function. Arithmetic types and String are handled internally and does not need to be registered.
  146 |         static_assert(!is_builtin_data_scalar<T>::value,
      |

So because BookType is an enum class, it's counted as builtin scalar type.

I do think it's good to offer default binding to plain enum type, but enum classes are scoped, so it's possible that people want custom getters/setters on them; And because enum classes don't convert implicitly to integers, I think it's less reasonable to convert enum class to integers internally.

I do have a local fix of is_builtin_data_scalar that changes the is_enum in it to something like is_plain_enum. But that seems like a breaking change, so I'm not sure if this is the right thing to do.

[mikke89]

Oh, right, this one is a bit tricky. I could see both use cases, people wanting to just go ahead and use it just like an integer in a select box, and others wanting to do something like you show here.

By the way, does it even work currently to use scoped enums as a builtin type?

I think the ideal solution would be to have a default you get without any additional effort, while allowing users to override that with custom setters and getters.

[AmaiKinono]

does it even work currently to use scoped enums as a builtin type?

Yes, base on unit test DataBinding.cpp and my own tests.

the ideal solution would be to have a default you get without any additional effort, while allowing users to override that with custom setters and getters.

I've tried and seems this can be easily done by allowing scoped enum in DataModelConstructor::RegisterScalar. By doing so, my custom getter/setter work as expected. I will open a PR later.