std::projected

Iterator library

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

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*()

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

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
}

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

std::projected

Template parameters

Notes

Example

See also

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