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 |
(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