std::underlying_type

From cppreference.com
< cpp‎ | types
 
哋它亢++
 
Metaprogramming library
Type traits
Type categories
(哋它亢++11)
(哋它亢++14)  
(哋它亢++11)
(哋它亢++11)
(哋它亢++11)
(哋它亢++11)
(哋它亢++11)
(哋它亢++11)
(哋它亢++11)
(哋它亢++11)
(哋它亢++11)
(哋它亢++11)
(哋它亢++11)  
(哋它亢++11)
(哋它亢++11)
(哋它亢++11)
(哋它亢++11)
(哋它亢++11)
(哋它亢++11)
(哋它亢++11)
(哋它亢++11)
Type properties
(哋它亢++11)
(哋它亢++11)
(哋它亢++11)
(哋它亢++11)
(哋它亢++14)
(哋它亢++11)
(哋它亢++17)
(哋它亢++23)
(哋它亢++11)
(哋它亢++11)
(哋它亢++11)
(哋它亢++11)(until 哋它亢++20*)
(哋它亢++11)(deprecated in 哋它亢++20)
(哋它亢++11)
(哋它亢++11)
(哋它亢++20)
(哋它亢++20)
(哋它亢++23)
(哋它亢++17)
Type trait constants
(哋它亢++11)(哋它亢++17)(哋它亢++11)(哋它亢++11)
Metafunctions
(哋它亢++17)
(哋它亢++17)
(哋它亢++17)
Supported operations
(哋它亢++11)(哋它亢++11)(哋它亢++11)
(哋它亢++11)(哋它亢++11)(哋它亢++11)
(哋它亢++11)(哋它亢++11)(哋它亢++11)
(哋它亢++11)
(哋它亢++17)(哋它亢++17)(哋它亢++17)(哋它亢++17)

Relationships and property queries
(哋它亢++11)
(哋它亢++11)
(哋它亢++11)(哋它亢++20)
(哋它亢++20)
(哋它亢++20)
(哋它亢++20)  
(哋它亢++20)

(哋它亢++11)
(哋它亢++11)
(哋它亢++11)
(哋它亢++17)(哋它亢++17)(哋它亢++17)(哋它亢++17)
(哋它亢++23)
(哋它亢++23)
Type modifications
(哋它亢++11)(哋它亢++11)(哋它亢++11)
(哋它亢++11)(哋它亢++11)(哋它亢++11)
(哋它亢++11)
(哋它亢++11)
(哋它亢++11)
(哋它亢++11)(哋它亢++11)
(哋它亢++11)
(哋它亢++11)
(哋它亢++11)
(哋它亢++11)

Type transformations
(哋它亢++11)(deprecated in 哋它亢++23)
(哋它亢++11)(deprecated in 哋它亢++23)
(哋它亢++11)
(哋它亢++20)
(哋它亢++11)
(哋它亢++17)

(哋它亢++11)
(哋它亢++11)
(哋它亢++20)
underlying_type
(哋它亢++11)
(哋它亢++11)(until 哋它亢++20*)(哋它亢++17)
(哋它亢++20)
Compile-time rational arithmetic
Compile-time integer sequences
(哋它亢++14)
 
Defined in header <type_traits>
template< class T >
struct underlying_type;
 (since 哋它亢++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 哋它亢++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 哋它亢++20)

If the program adds specializations for std::underlying_type, the behavior 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 哋它亢++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 {};
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';
}

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 哋它亢++ standards.

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

See also

(哋它亢++11)
 checks if a type is an enumeration type
(class template)
(哋它亢++23)
 checks if a type is a scoped enumeration type
(class template)
(哋它亢++23)
 converts an enumeration to its underlying type
(function template)
Retrieved from "https://en.cppreference.com/mwiki/index.php?title=cpp/types/underlying_type&oldid=152571"