std::underlying_type

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

If T is a complete enumeration 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)

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

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.

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

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

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