std::in_place, std::in_place_t, std::in_place_type_t, std::in_place_index_t

 

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optional

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Defined in header <utility>
std::in_place_tag in_place( /* unspecified */ );	(1)	(since C++17)
template < class T > std::in_place_tag in_place( /* unspecified */<T> );	(2)	(since C++17)
template < std::size_t I > std::in_place_tag in_place( /* unspecified */<I> );	(3)	(since C++17)
using in_place_t = std::in_place_tag (&)( /* unspecified */ );		(since C++17)
template < class T > using in_place_type_t = std::in_place_tag (&)( /* unspecified */<T> );		(since C++17)
template < std::size_t I > using in_place_index_t = std::in_place_tag (&)( /* unspecified */<I> );		(since C++17)

The std::in_place function and function templates are special disambiguation tags that can be passed to the constructors of 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.

They are implemented as a set of overloaded function/function templates with a special return type, to allow users to write all of in_place, in_place<T> (with a type), and in_place<I> (with an integral index). The corresponding alias/alias templates in_place_t, in_place_type_t and in_place_index_t can be used in the constructor's parameter list to match the intended tag.

Actually calling any of the in_place functions results in undefined behavior.

See also

optional (since C++17)	a wrapper that may or may not hold an object (class template)
variant (since C++17)	a type-safe discriminated union (class template)
any (since C++17)	Objects that hold instances of any CopyConstructible type. (class)
in_place_tag (since C++17)	Special tag type used by the in-place construction tags (class)