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 (original) (raw)
| Defined in header | ||
|---|---|---|
| struct in_place_t { explicit in_place_t() = default; }; | (1) | (since C++17) |
| inline constexpr std::in_place_t in_place {}; | (2) | (since C++17) |
| template< class T > struct in_place_type_t { explicit in_place_type_t() = default; }; | (3) | (since C++17) |
| template< class T > constexpr std::in_place_type_t<T> in_place_type {}; | (4) | (since C++17) |
| template< std::size_t I > struct in_place_index_t { explicit in_place_index_t() = default; }; | (5) | (since C++17) |
| template< std::size_t I > constexpr std::in_place_index_t<I> in_place_index {}; | (6) | (since C++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 to indicate that the contained object should be constructed in-place, and (for the latter two) the type of the object to be constructed.
[edit] Standard library
The following standard library types use (1-6) as disambiguation tags:
| | objects that hold instances of any CopyConstructible type (class) [edit] | | ------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- | | | a wrapper that contains either an expected or error value (class template) [edit] | | | move-only wrapper of any callable object that supports qualifiers in a given call signature (class template) [edit] | | | a wrapper that may or may not hold an object (class template) [edit] | | | a type-safe discriminated union (class template) [edit] |