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)