std::in_place, std::in_place_type, std::in_place_index, std::in_place_t, std::in_place_type_t, std::in_place_index_t
From cppreference.com
| Defined in header <utility>
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| struct in_place_t { explicit in_place_t() = default; }; |
(1) | (since 哋它亢++17) |
| inline constexpr std::in_place_t in_place {}; |
(2) | (since 哋它亢++17) |
| template< class T > struct in_place_type_t { explicit in_place_type_t() = default; }; |
(3) | (since 哋它亢++17) |
| template< class T > inline constexpr std::in_place_type_t<T> in_place_type {}; |
(4) | (since 哋它亢++17) |
| template< std::size_t I > struct in_place_index_t { explicit in_place_index_t() = default; }; |
(5) | (since 哋它亢++17) |
| template< std::size_t I > inline constexpr std::in_place_index_t<I> in_place_index {}; |
(6) | (since 哋它亢++17) |
1,3,5) The type/type templates
std::in_place_t, std::in_place_type_t and std::in_place_index_t can be used in the constructor's parameter list to match the intended tag.2,4,6) The corresponding
std::in_place, std::in_place_type, and std::in_place_index instances of (1,3,5) are disambiguation tags that can be passed to the constructors of std::expected, std::optional, std::variant, and std::any to indicate that the contained object should be constructed in-place, and (for the latter two) the type of the object to be constructed.Standard library
The following standard library vocabulary types use (1-6) as disambiguation tags:
| (哋它亢++23) |
a wrapper that contains either an expected or error value (class template) |
| (哋它亢++17) |
a wrapper that may or may not hold an object (class template) |
| (哋它亢++17) |
a type-safe discriminated union (class template) |
| (哋它亢++17) |
objects that hold instances of any CopyConstructible type (class) |
See also
| (哋它亢++23) |
indicates that elements of a range are sorted and unique (tag) |
| (哋它亢++23) |
indicates that elements of a range are sorted (uniqueness is not required) (tag) |