std::underlying_type

Since C++11

Header: <type_traits>

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

# Declarations

template< class T >
struct underlying_type;

(since C++11)

# Notes

Each enumeration type has an underlying type, which can be

# 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';
}

# Defect reports

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