std::is_scoped_enum

template< class T >
struct is_scoped_enum;

Checks whether T is a scoped enumeration type. Provides the member constant value which is equal to true, if T is a scoped enumeration type. Otherwise, value is equal to false.

The behavior of a program that adds specializations for is_scoped_enum or is_scoped_enum_v is undefined.

Template parameters

Helper variable template

template< class T >
inline constexpr bool is_scoped_enum_v = is_scoped_enum<T>::value;

Notes

As currently the standard allows the name of an enumeration type appears in its enum-base, for example, enum class E : std::enable_if_t<std::is_scoped_enum_v<E>, int> {}; is allowed, std::is_scoped_enum cannot be implemented without extensions, because scoped and unscoped enumeration types are indistinguishable in expressions when they are incomplete.

However, all known compilers consider an enumeration type is still undeclared in its enum-base. As a result, a scoped enumeration type can never be incomplete, and std::is_scoped_enum can be implemented like below. This is CWG issue 2516.

Feature testing macro: __cpp_lib_is_scoped_enum.

Possible implementation

This implementation assumes that scoped enumeration types are always complete.

namespace detail {
namespace { // avoid ODR-violation
template<class T>
auto test_sizable(int) -> decltype(sizeof(T), std::true_type{});
template<class>
auto test_sizable(...) -> std::false_type;
 
template<class T>
auto test_nonconvertible_to_int(int)
    -> decltype(static_cast<std::false_type (*)(int)>(nullptr)(std::declval<T>()));
template<class>
auto test_nonconvertible_to_int(...) -> std::true_type;
 
template<class T>
constexpr bool is_scoped_enum_impl = std::conjunction_v<
    decltype(test_sizable<T>(0)),
    decltype(test_nonconvertible_to_int<T>(0))
>;
}
} // namespace detail
 
template<class>
struct is_scoped_enum : std::false_type {};
 
template<class E>
    requires std::is_enum_v<E>
struct is_scoped_enum<E> : std::bool_constant<detail::is_scoped_enum_impl<E>> {};

Example

#include <iostream>
#include <type_traits>
 
class A {};
 
enum E {};
 
enum struct Es { oz };
 
enum class Ec : int {};
 
int main() 
{
    std::cout << std::boolalpha;
    std::cout << std::is_scoped_enum_v<A> << '\n';
    std::cout << std::is_scoped_enum_v<E> << '\n';
    std::cout << std::is_scoped_enum_v<Es> << '\n';
    std::cout << std::is_scoped_enum_v<Ec> << '\n';
    std::cout << std::is_scoped_enum_v<int> << '\n';
}

false
false
true
true
false

See also