std::underlying_type

template< class T >
struct underlying_type;

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

Otherwise, the behavior is undefined.

Member types

Helper types

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

Notes

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

1. Specified explicitly (both scoped and unscoped enumerations).

2. 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).

Defect reports

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

Example

#include <iostream>
#include <type_traits>
 
enum e1 {};
enum class e2: int {};
 
int main() {
    bool e1_type = std::is_same<
        unsigned
       ,typename std::underlying_type<e1>::type
    >::value; 
 
    bool e2_type = std::is_same<
        int
       ,typename std::underlying_type<e2>::type
    >::value;
 
    std::cout
    << "underlying type for 'e1' is " << (e1_type?"unsigned":"non-unsigned") << '\n'
    << "underlying type for 'e2' is " << (e2_type?"int":"non-int") << '\n';
}

underlying type for 'e1' is unsigned
underlying type for 'e2' is int