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operator==,!=,<,<=,>,>=,<=>(std::reverse_iterator) - cppreference.com

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Defined in header <iterator>

template< class Iter1, class Iter2 > bool operator==( const std::reverse_iterator<Iter1>& lhs, const std::reverse_iterator<Iter2>& rhs );

 (1) (constexpr since C++17)

template< class Iter1, class Iter2 > bool operator!=( const std::reverse_iterator<Iter1>& lhs, const std::reverse_iterator<Iter2>& rhs );

 (2) (constexpr since C++17)

template< class Iter1, class Iter2 > bool operator< ( const std::reverse_iterator<Iter1>& lhs, const std::reverse_iterator<Iter2>& rhs );

 (3) (constexpr since C++17)

template< class Iter1, class Iter2 > bool operator<=( const std::reverse_iterator<Iter1>& lhs, const std::reverse_iterator<Iter2>& rhs );

 (4) (constexpr since C++17)

template< class Iter1, class Iter2 > bool operator> ( const std::reverse_iterator<Iter1>& lhs, const std::reverse_iterator<Iter2>& rhs );

 (5) (constexpr since C++17)

template< class Iter1, class Iter2 > bool operator>=( const std::reverse_iterator<Iter1>& lhs, const std::reverse_iterator<Iter2>& rhs );

 (6) (constexpr since C++17)

template< class Iter1, std::three_way_comparable_with<Iter1> Iter2 > constexpr std::compare_three_way_result_t<Iter1, Iter2> operator<=>( const std::reverse_iterator<Iter1>& lhs, const std::reverse_iterator<Iter2>& rhs );

 (7) (since C++20)

Compares the underlying iterators of lhs and rhs.

  • The result of equality comparisons are preserved (i.e. equal underlying iterators imply equal reverse iterators).
  • The result of relational comparisons are reversed (i.e. a greater underlying iterator implies a lesser reverse iterator).

1) This overload participates in overload resolution only if lhs.base() == rhs.base() is well-formed and convertible to bool.

2) This overload participates in overload resolution only if lhs.base() != rhs.base() is well-formed and convertible to bool.

3) This overload participates in overload resolution only if lhs.base() > rhs.base() is well-formed and convertible to bool.

4) This overload participates in overload resolution only if lhs.base() >= rhs.base() is well-formed and convertible to bool.

5) This overload participates in overload resolution only if lhs.base() < rhs.base() is well-formed and convertible to bool.

6) This overload participates in overload resolution only if lhs.base() <= rhs.base() is well-formed and convertible to bool.

 (since C++20)

Parameters

lhs, rhs - iterator adaptors to compare

Return value

1) lhs.base() == rhs.base()

2) lhs.base() != rhs.base()

3) lhs.base() > rhs.base()

4) lhs.base() >= rhs.base()

5) lhs.base() < rhs.base()

6) lhs.base() <= rhs.base()

7) rhs.base() <=> lhs.base()

Notes

operator<=> returns rhs.base() <=> lhs.base() rather than lhs.base() <=> rhs.base() because this is a reverse iterator.

Example

#include <cassert>
#include <iterator>

int main()
{
    int a[]{0, 1, 2, 3};
    //            ↑  └───── x, y
    //            └──────── z
    // “x” and “y” are equal, but “x” is less than “z” (reversely)
    std::reverse_iterator<int*>
        x{std::rend(a) - std::size(a)},
        y{std::rend(a) - std::size(a)},
        z{std::rbegin(a) + 1};

    // two-way comparisons
    assert(  x == y );
    assert(!(x != y));
    assert(!(x <  y));
    assert(  x <= y );
    assert(!(x == z));
    assert(  x != z );
    assert(  x <  z );
    assert(  x <= z );

    // three-way comparisons
    assert(  x <=> y == 0 );
    assert(!(x <=> y <  0));
    assert(!(x <=> y >  0));
    assert(!(x <=> z == 0));
    assert(  x <=> z <  0 );
    assert(!(x <=> z >  0));
}

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 280 C++98 heterogeneous assignment was not allowed allowed