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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

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 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.

See also

(C++23)
a wrapper that contains either an expected or error value
(class template)
(C++17)
a wrapper that may or may not hold an object
(class template)
(C++17)
a type-safe discriminated union
(class template)
(C++17)
objects that hold instances of any CopyConstructible type
(class)

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