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) |

Name | Definition |
---|---|

`type` | the underlying type of `T` |

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).

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 |

#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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