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operator==(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. 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

#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

operator==(std::common_iterator)

Parameters

Return value

Example

Defect reports

See also