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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

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< cpp‎ | utility
 
 
Utilities library
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(C++17)

 
Defined in header <utility>
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 >
inline 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 >
inline 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:

(C++17)
objects that hold instances of any CopyConstructible type
(class) [edit]
(C++23)
a wrapper that contains either an expected or error value
(class template) [edit]
wraps callable object of any type with specified function call signature
(class template) [edit]
(C++17)
a wrapper that may or may not hold an object
(class template) [edit]
(C++17)
a type-safe discriminated union
(class template) [edit]

[edit] See also

indicates that elements of a range are sorted and unique
(tag)[edit]
indicates that elements of a range are sorted (uniqueness is not required)
(tag)[edit]