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Showing content from https://en.cppreference.com/w/cpp/language/../algorithm/../ranges/../iterator/projected.html below:

std::projected - cppreference.com

(1) (since C++20)
(until C++26) (since C++26) (2) (since C++20)
(until C++26)

Helper templates

(3) (since C++26)
(exposition only*) 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>

For the exposition-only nested class

/*type*/

, the nested 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.

[edit] Template parameters I - an indirectly readable type Proj - projection applied to a dereferenced I [edit] 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.

[edit] 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
}

[edit] See also

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