std::is_scoped_enum

Defined in header `<type_traits>`
template< class T > struct is_scoped_enum;		(since C++23)

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

T

-

a type to check

Helper variable template

template< class T > inline constexpr bool is_scoped_enum_v = is_scoped_enum<T>::value;		(since C++23)

Inherited from std::integral_constant

Member constants

value

[static]

true if T is a scoped enumeration type , false otherwise
(public static member constant)

Member functions

operator bool	converts the object to bool, returns `value` (public member function)
operator() (C++14)	returns `value` (public member function)

Member types

Type	Definition
`value_type`	`bool`
`type`	std::integral_constant<bool, 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-test 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

Run this code

#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';
}

Output:

false
false
true
true
false

Compiler support
Freestanding and hosted
Language
Standard library headers
Named requirements
Feature test macros (C++20)
Language support library
Concepts library (C++20)
Metaprogramming library (C++11)
Diagnostics library
General utilities library
Strings library
Containers library
Iterators library
Ranges library (C++20)
Algorithms library
Numerics library
Localizations library
Input/output library
Filesystem library (C++17)
Regular expressions library (C++11)
Concurrency support library (C++11)
Technical specifications
Symbols index
External libraries

is_integral (C++11)	checks if a type is an integral type (class template)
is_arithmetic (C++11)	checks if a type is an arithmetic type (class template)
is_scalar (C++11)	checks if a type is a scalar type (class template)
is_enum (C++11)	checks if a type is an enumeration type (class template)