std::underlying_type

 

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

If T is a complete(since C++17) enumeration type, provides a member typedef type that names the underlying type for T.

Otherwise, the behavior is undefined.

Member types

Name	Definition
type	the underlying type for 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

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)

Example

Run this code

#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';
}

Output:

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