std::common_iterator<I,S>::operator*,->

 

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)

Metaprogramming library (C++11)

Diagnostics library

General utilities library

Strings library

Containers library

Iterators library

Ranges library (C++20)

Algorithms library

Numerics library

Localizations library

Input/output library

Filesystem library (C++17)

Regular expressions library (C++11)

Concurrency support library (C++11)

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) input_or_output_iterator (C++20) sentinel_for (C++20) sized_sentinel_for (C++20)	input_iterator (C++20) output_iterator (C++20) forward_iterator (C++20) bidirectional_iterator (C++20) random_access_iterator (C++20) contiguous_iterator (C++20)
Iterator primitives
iterator_traits input_iterator_tagoutput_iterator_tagforward_iterator_tagbidirectional_iterator_tagrandom_access_iterator_tagcontiguous_iterator_tag (C++20) iterator (deprecated in C++17)	incrementable_traits (C++20) indirectly_readable_traits (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)
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)
Iterator adaptors
reverse_iterator move_iterator (C++11) 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) back_insert_iterator front_insert_iterator insert_iterator	make_reverse_iterator (C++14) make_move_iterator (C++11) default_sentinel_tdefault_sentinel (C++20)(C++20) unreachable_sentinel_tunreachable_sentinel (C++20)(C++20) make_const_iterator (C++23) make_const_sentinel (C++23) back_inserter front_inserter inserter
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) endcend (C++11)(C++14) sizessize (C++17)(C++20)	rbegincrbegin (C++14)(C++14) rendcrend (C++14)(C++14) empty (C++17) data (C++17)

 

std::common_iterator

Member functions

common_iterator::common_iterator (C++20)

common_iterator::operator= (C++20)

common_iterator::operator*common_iterator::operator-> (C++20)(C++20)

common_iterator::operator++common_iterator::operator++(int) (C++20)(C++20)

Non-member functions

operator== (C++20)

operator- (C++20)

iter_move (C++20)

iter_swap (C++20)

Helper classes

incrementable_traits (C++20)

iterator_traits (C++20)

 

constexpr decltype(auto) operator*();	(1)	(since C++20)
constexpr decltype(auto) operator*() const     requires /*dereferenceable*/<const I>;	(2)	(since C++20)
constexpr auto operator->() const     requires /* see description */;	(3)	(since C++20)
Helper types
class /*proxy*/ {     std::iter_value_t<I> keep_;     constexpr proxy(std::iter_reference_t<I>&& x)       : keep_(std::move(x)) {} public:     constexpr const std::iter_value_t<I>* operator->() const noexcept {         return std::addressof(keep_);     } };	(4)	(exposition only*)

Returns pointer or reference to the current element, or a proxy holding it.

The behavior is undefined if the underlying std::variant member object var does not hold an object of type I, i.e. std::holds_alternative<I>(var) is equal to false.

Let it denote the iterator of type I held by var, that is std::get<I>(var).

1-2) Returns the result of dereferencing it.

3) Returns a pointer or underlying iterator to the current element, or a proxy holding it:

• Equivalent to return it;, if I is a pointer type or if the expression it.operator->() is well-formed
• Otherwise, equivalent to auto&& tmp = *it; return std::addressof(tmp);, if std::iter_reference_t<I> is a reference type,
• Otherwise, equivalent to return proxy(*it);, where proxy is an exposition only class (4).

The expression in the requires-clause is equivalent to
std::indirectly_readable<const I> && (

    requires(const I& i) { i.operator->(); } ||
    std::is_reference_v<std::iter_reference_t<I>> ||
    std::constructible_from<std::iter_value_t<I>, std::iter_reference_t<I>>

).

Parameters

(none)

Return value

1-2) Reference to the current element, or prvalue temporary. Equivalent to *it.

3) Pointer or iterator to the current element or proxy holding it as described above.

Example

Run this code

#include <algorithm>
#include <complex>
#include <initializer_list>
#include <iostream>
#include <iterator>
 
using std::complex_literals::operator""i;
 
int main()
{
    const auto il = {1i, 3.14 + 2i, 3i, 4i, 5i};
 
    using CI = std::common_iterator<
        std::counted_iterator<decltype(il)::iterator>,
        std::default_sentinel_t>;
 
    CI ci{std::counted_iterator{std::next(begin(il), 1), std::ssize(il) - 1}};
 
    std::cout << *ci << ' ' << ci->real() << '\n';
}

Output:

(3.14,2) 3.14

Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR	Applied to	Behavior as published	Correct behavior
LWG 3574	C++20	variant was fully constexpr (P2231R1) but common_iterator was not	also made constexpr
LWG 3595	C++20	functions of the proxy type lacked constexpr and noexcept	added
LWG 3672	C++20	operator-> might return by reference in usual cases	always returns by value

See also

(constructor) (C++20)

constructs a new iterator adaptor (public member function)