operator==(std::common_iterator)

 

C++

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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)

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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)

 

template <class I2, std::sentinel_for<I> S2>   requires std::sentinel_for<S, I2> friend constexpr bool operator==( const common_iterator& x,                                   const std::common_iterator<I2, S2>& y );	(1)	(since C++20)
template <class I2, std::sentinel_for<I> S2>   requires std::sentinel_for<S, I2> && std::equality_comparable_with<I, I2> friend constexpr bool operator==( const common_iterator& x,                                   const std::common_iterator<I2, S2>& y );	(2)	(since C++20)

Compares the iterators and/or sentinels held by underlying std::variant member objects var. Two incomparable iterators or two sentinels are considered equal.

The behavior is undefined if either x or y is in an invalid state, i.e. x.var.valueless_by_exception() || y.var.valueless_by_exception() is equal to true.

Let i be x.var.index() and j be y.var.index().

1) if i == j (i.e. both x and y hold iterators or both hold sentinels), returns true, otherwise returns std::get<i>(x.var) == std::get<j>(y.var);

2) if i == 1 && j == 1 (i.e. both x and y hold sentinels), returns true, otherwise returns std::get<i>(x.var) == std::get<j>(y.var).

The != operator is synthesized from operator==.

These function templates are not visible to ordinary unqualified or qualified lookup, and can only be found by argument-dependent lookup when std::common_iterator<I> is an associated class of the arguments.

Parameters

x, y

-

iterator adaptors to compare

Return value

true if underlying iterators and/or sentinels are equal.

Example

Run this code

#include <algorithm>
#include <iostream>
#include <iterator>
#include <initializer_list>
 
int main()
{
    int a[] {0, 1, 2, 3};
 
    using CI = std::common_iterator<
                   std::counted_iterator<int*>,
                   std::default_sentinel_t
                   >;
 
    CI i1{ std::counted_iterator{ a + 0, 2 } };
    CI i2{ std::counted_iterator{ a + 1, 2 } };
    CI i3{ std::counted_iterator{ a + 0, 3 } };
    CI i4{ std::counted_iterator{ a + 0, 0 } };
    CI s1{ std::default_sentinel };
    CI s2{ std::default_sentinel };
 
    std::cout << std::boolalpha
              << (i1 == i2) << ' '   // true
              << (i1 == i3) << ' '   // false
              << (i2 == i3) << ' '   // false
              << (s1 == s2) << ' '   // true
              << (i1 == s1) << ' '   // false
              << (i4 == s1) << '\n'; // true
}

Output:

true false false true false true

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

See also

operator- (C++20)

computes the distance between two iterator adaptors (function template)