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operator==(std::common_iterator)

From cppreference.com
 
 
Iterator library
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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.

Contents

[edit] Parameters

x, y - iterator adaptors to compare

[edit] Return value

true if underlying iterators and/or sentinels are equal.

[edit] Example

#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);
}

[edit] 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

[edit] See also

(C++20)
computes the distance between two iterator adaptors
(function template) [edit]