21

Is there a way to ensure a template parameter is an enum-class type?

I know type_traits has std::is_enum, but I don't want it to match regular enums, just enum_classes.

Example of the wanted effect:

enum class EnumClass {};
enum Enum {};
class Class {};

template <typename T>
void Example()
{
    static_assert(/* T is EnumClass */, "`T` must be an enum class");
}

Example<EnumClass>(); // Ok
Example<Enum>(); // Error
Example<Class>(); // Error

I am using C++11, and unfortunately cannot go any higher (though I'd be curious to know the solution anyway, even if it involves newer standards).

Is it possible?

Gilad Naaman
  • 6,390
  • 15
  • 52
  • 82

1 Answers1

22

You can achieve it with:

template<typename T>
using is_class_enum = std::integral_constant<
   bool,
   std::is_enum<T>::value && !std::is_convertible<T, int>::value>;

Here a demo.

If you prefer using SFINAE, the same can be achieved with:

template<typename T, typename _ = void>
struct is_class_enum : std::false_type {
};

template<typename T>
struct is_class_enum <
  T,
  typename std::enable_if<std::is_enum<T>::value &&
                          !std::is_convertible<T, int>::value>::type> :
    public std::true_type {
};
BiagioF
  • 9,368
  • 2
  • 26
  • 50
  • Correct me if I'm wrong, but you're simply depending on the fact that enum classes are not implicitly convertible to integrals? Isn't it possible to define an implicit conversion operator? – Gilad Naaman Oct 01 '16 at 14:24
  • 1
    @GiladNaaman how do you define an implicit operator from an `enum class`? – BiagioF Oct 01 '16 at 14:26
  • Yeah, my bad, you can't actually define operators or functions inside an `enum class`. – Gilad Naaman Oct 01 '16 at 14:29
  • Shouldn't `std::underlying_type_t` be used instead of just an `int` as a `std::is_convertible<>` check type? – Sergey Nikitin Oct 03 '16 at 11:10
  • 1
    @SergeyNikitin There's no point, and doing `underlying_type` right is actually tricky (you cannot instantiate it until you know that `T` is actually an enum type). – T.C. Oct 05 '16 at 09:46