std::projected(3)

NAME
std::projected - std::projected

Synopsis
   Defined in header <iterator>
   template< std::indirectly_readable I,
   std::indirectly_regular_unary_invocable<I> Proj >
   struct projected { (since
   using value_type = std::remove_cvref_t<std::indirect_result_t<Proj&, (1) C++20)
   I>>;
   std::indirect_result_t<Proj&, I> operator*() const; // not defined
   };
   template< std::weakly_incrementable I, class Proj >
   struct incrementable_traits<std::projected<I, Proj>> { (2) (since
   using difference_type = std::iter_difference_t<I>; C++20)
   };
   1) Class template projected combines an indirectly_readable type I and a callable
   object type Proj into a new indirectly_readable type whose reference type is the
   result of applying Proj to the std::iter_reference_t<I>.
   2) This specialization of std::incrementable_traits makes std::projected<I, Proj> a
   weakly_incrementable type when I is also a weakly_incrementable type.
   projected is used only to constrain algorithms that accept callable objects and
   projections, and hence its operator*() is not defined.

Template parameters
   I - an indirectly readable type
   Proj - projection applied to a dereferenced I