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std::projected

Defined in header <iterator>
(1) 
template< std::indirectly_readable I,
          std::indirectly_regular_unary_invocable<I> Proj >
struct projected {
    using value_type = std::remove_cvref_t<std::indirect_result_t<Proj&, I>>;
    std::indirect_result_t<Proj&, I> operator*() const; // not defined
};
 (since C++20)
(until C++26) 
template< std::indirectly_readable I,
          std::indirectly_regular_unary_invocable<I> Proj >
using projected = /*projected-impl*/<I, Proj>::/*__type*/; // see below
 (since C++26) 
template< std::weakly_incrementable I, class Proj >
struct incrementable_traits<std::projected<I, Proj>> {
    using difference_type = std::iter_difference_t<I>;
};
 (2) (since C++20)
(until C++26) 
template< class I, class Proj >
struct /*projected-impl*/ { // exposition only
    struct /*__type*/ { // exposition only
        using value_type = std::remove_cvref_t<std::indirect_result_t<Proj&, I>>;
        using difference_type = std::iter_difference_t<I>; // conditionally present

        std::indirect_result_t<Proj&, I> operator*() const; // not defined
    };
};
 (3) (since C++26)
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

#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
}

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