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)


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).


#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 >;
    << "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

See also

checks if a type is an enumeration type
(class template)
checks if a type is a scoped enumeration type
(class template)
converts an enumeration to its underlying type
(function template)

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