std::projected

 

C++

Compiler support

Freestanding and hosted

Language

Standard library

Standard library headers

Named requirements

Feature test macros (C++20)

Language support library

Concepts library (C++20)

Diagnostics library

Memory management library

Metaprogramming library (C++11)

General utilities library

Containers library

Iterators library

Ranges library (C++20)

Algorithms library

Strings library

Text processing library

Numerics library

Date and time library

Input/output library

Filesystem library (C++17)

Concurrency support library (C++11)

Execution support library (C++26)

Technical specifications

Symbols index

External libraries

 

Iterator library

Iterator concepts
indirectly_readable (C++20) indirectly_writable (C++20) weakly_incrementable (C++20) incrementable (C++20) is-integer-like is-signed-integer-like (C++20)(C++20)	sentinel_for (C++20) sized_sentinel_for (C++20) input_iterator (C++20) output_iterator (C++20) input_or_output_iterator (C++20)	forward_iterator (C++20) bidirectional_iterator (C++20) random_access_iterator (C++20) contiguous_iterator (C++20)
Iterator primitives
input_iterator_tagoutput_iterator_tagforward_iterator_tagbidirectional_iterator_tagrandom_access_iterator_tagcontiguous_iterator_tag (C++20)	iter_value_titer_difference_titer_reference_titer_const_reference_titer_rvalue_reference_titer_common_reference_t (C++20)(C++20)(C++20)(C++23)(C++20)(C++20)	iterator (deprecated in C++17) iterator_traits incrementable_traits (C++20) indirectly_readable_traits (C++20)
Algorithm concepts and utilities
Indirect callable concepts
indirectly_unary_invocableindirectly_regular_unary_invocable (C++20)(C++20)	indirect_unary_predicate (C++20) indirect_binary_predicate (C++20)	indirect_equivalence_relation (C++20) indirect_strict_weak_order (C++20)
Common algorithm requirements
indirectly_movable (C++20) indirectly_movable_storable (C++20) indirectly_copyable (C++20)	indirectly_copyable_storable (C++20)   indirectly_swappable (C++20) indirectly_comparable (C++20)	permutable (C++20) mergeable (C++20) sortable (C++20)
Utilities
indirect_result_t (C++20)	projected (C++20)	projected_value_t (C++26)
Iterator adaptors
reverse_iterator make_reverse_iterator (C++14) move_iterator (C++11) make_move_iterator (C++11) default_sentinel_tdefault_sentinel (C++20)(C++20) unreachable_sentinel_tunreachable_sentinel (C++20)(C++20)	front_insert_iterator back_insert_iterator inserter insert_iterator front_inserter back_inserter move_sentinel (C++20)	common_iterator (C++20) counted_iterator (C++20) basic_const_iterator (C++23) const_iterator (C++23) const_sentinel (C++23) make_const_iterator (C++23) make_const_sentinel (C++23)
Stream iterators istream_iterator    ostream_iterator istreambuf_iterator ostreambuf_iterator	Iterator customization points ranges::iter_move (C++20) ranges::iter_swap (C++20)
Iterator operations advance distance prev (C++11)   next (C++11) ranges::advance (C++20) ranges::distance (C++20) ranges::prev (C++20) ranges::next (C++20)	Range access begincbegin (C++11)(C++14) rbegincrbegin (C++14)(C++14)   endcend (C++11)(C++14) rendcrend (C++14)(C++14)   sizessize (C++17)(C++20) empty (C++17) data (C++17)

 

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 (3)			(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*/ {     struct /*__type*/     {         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) (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>.

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 (C++26)

computes the value type of an indirectly_readable type by projection (alias template)