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 | (since C++20) |

The behavior of a program that adds specializations for `underlying_type`

is undefined.

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.

- Specified explicitly (both scoped and unscoped enumerations);
- 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>; 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

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 |

(C++11) | checks if a type is an enumeration type (class template) |

(C++23) | checks if a type is a scoped enumeration type (class template) |

(C++23) | converts an enumeration to its underlying type (function template) |

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