operator==(std::common_iterator)

From cppreference.com
C++
Iterator library
Iterator concepts
Iterator primitives
Algorithm concepts and utilities
Indirect callable concepts
Common algorithm requirements
(C++20)
(C++20)
(C++20)
Utilities
(C++20)
Iterator adaptors
Range access
(C++11)(C++14)
(C++14)(C++14)
(C++11)(C++14)
(C++14)(C++14)
(C++17)(C++20)
(C++17)
(C++17)
std::common_iterator
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

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

Parameters

x, y - iterator adaptors to compare

Return value

true if underlying iterators and/or sentinels are equal.

Example

Run this code
#include <cassert>
#include <iterator>
 
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};
 
    assert((i1 == i2) == true);
    assert((i1 == i3) == false);
    assert((i2 == i3) == false);
    assert((s1 == s2) == true);
    assert((i1 == s1) == false);
    assert((i4 == s1) == 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

(C++20)
 computes the distance between two iterator adaptors
(function template)
Retrieved from "https://en.cppreference.com/mwiki/index.php?title=cpp/iterator/common_iterator/operator_cmp&oldid=159362