std::in_place,std::in_place_type,std::in_place_index,std::in_place_t,(3)

NAME
std::in_place,std::in_place_type,std::in_place_index,std::in_place_t, - std::in_place,std::in_place_type,std::in_place_index,std::in_place_t,

Synopsis
   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 > (3) (since C++17)
   struct in_place_type_t { explicit in_place_type_t() = default; };
   template< class T > (4) (since C++17)
   inline constexpr std::in_place_type_t<T> in_place_type {};
   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 > (6) (since C++17)
   inline constexpr std::in_place_index_t<I> in_place_index {};
   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:
   expected a wrapper that contains either an expected or error value
   (C++23) (class template)
   optional a wrapper that may or may not hold an object
   (C++17) (class template)
   variant a type-safe discriminated union
   (C++17) (class template)
   any objects that hold instances of any CopyConstructible type
   (C++17) (class)

See also
   sorted_unique indicates that elements of a range are sorted and unique
   sorted_unique_t (tag)
   (C++23)
   sorted_equivalent indicates that elements of a range are sorted (uniqueness is not
   sorted_equivalent_t required)
   (C++23) (tag)