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 C++20)
       using value_type = (until C++26)
   std::remove_cvref_t<std::indirect_result_t<Proj&, I>>;
       std::indirect_result_t<Proj&, I> operator*() const; //
   not defined
   };
   template< std::indirectly_readable I,
             std::indirectly_regular_unary_invocable<I> Proj > (since C++26)
   using projected = /*projected-impl*/<I, Proj>::/*__type*/; //
   see (3)
   template< std::weakly_incrementable I, class Proj >
   struct incrementable_traits<std::projected<I, Proj>> (1) (since C++20)
   { (2) (until C++26)
       using difference_type = std::iter_difference_t<I>;
   };
   template< class I, class Proj >
   struct /*projected-impl*/
   {
       struct /*__type*/
       {
           using value_type = (since C++26)
   std::remove_cvref_t<std::indirect_result_t<Proj&, I>>; (3) (exposition
           using difference_type = std::iter_difference_t<I>; // only*)
   conditionally present
           std::indirect_result_t<Proj&, I> operator*() const;
   // not defined
       };
   };
   1)
   Class
   (until C++26)
   Alias
   (since C++26) 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.
   3) An indirect layer used for avoiding unexpected argument-dependent lookup. The
   member type difference_type exists only if I models weakly_incrementable.
   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

Notes
   The indirect layer prevents I and Proj to be associated classes of projected. When
   an associated class of I or Proj is an incomplete class type, the indirect layer
   avoids the unnecessary attempt to inspect the definition of that type that results
   in hard error.

Example
// Run this code
 #include <algorithm>
 #include <cassert>
 #include <functional>
 #include <iterator>
 template<class T>
 struct Holder
 {
     T t;
 };
 struct Incomplete;
 using P = Holder<Incomplete>*;
 static_assert(std::equality_comparable<P>); // OK
 static_assert(std::indirectly_comparable<P*, P*, std::equal_to<>>); // Error before C++26
 static_assert(std::sortable<P*>); // Error before C++26
 int main()
 {
     P a[10] = {}; // ten null pointers
     assert(std::count(a, a + 10, nullptr) == 10); // OK
     assert(std::ranges::count(a, a + 10, nullptr) == 10); // Error before C++26
 }

See also
   projected_value_t computes the value type of an indirectly_readable type by
   (C++26) projection
                     (alias template)