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std::underlying_type

Defined in header `<type_traits>`
template< class T > struct underlying_type;		(since C++11)

If T is a complete enumeration (enum) type, provides a member typedef type that names the underlying type of T.

Otherwise, the behavior is undefined.	(until C++20)
Otherwise, if `T` is not an enumeration type, there is no member `type`. Otherwise (`T` is an incomplete enumeration type), the program is ill-formed.	(since C++20)

The behavior of a program that adds specializations for underlying_type is undefined.

Member types

Name	Definition
`type`	the underlying type of `T`

Helper types

template< class T >
using underlying_type_t = typename underlying_type<T>::type;

(since C++14)

Notes

Each enumeration type has an underlying type, which can be.

Specified explicitly (both scoped and unscoped enumerations);
Omitted, in which case it is int for scoped enumerations or an implementation-defined integral type capable of representing all values of the enum (for unscoped enumerations).

Example

#include <iostream>
#include <type_traits>
 
enum e1 {};
enum class e2 {};
enum class e3 : unsigned {};
enum class e4 : int {};
 
int main()
{
    constexpr bool e1_t = std::is_same_v<std::underlying_type_t<e1>, int>;
    constexpr bool e2_t = std::is_same_v<std::underlying_type_t<e2>, int>;
    constexpr bool e3_t = std::is_same_v<std::underlying_type_t<e3>, int>;
    constexpr bool e4_t = std::is_same_v<std::underlying_type_t<e4>, int>;
 
    std::cout
        << "underlying type for 'e1' is " << (e1_t ? "int" : "non-int") << '\n'
        << "underlying type for 'e2' is " << (e2_t ? "int" : "non-int") << '\n'
        << "underlying type for 'e3' is " << (e3_t ? "int" : "non-int") << '\n'
        << "underlying type for 'e4' is " << (e4_t ? "int" : "non-int") << '\n';
}

Possible output:

underlying type for 'e1' is non-int
underlying type for 'e2' is int
underlying type for 'e3' is non-int
underlying type for 'e4' is int

Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR	Applied to	Behavior as published	Correct behavior
LWG 2396	C++11	incomplete enumeration types were allowed	complete enumeration type required