# operator==,!=,<,<=,>,>=,<=>(std::reverse_iterator)

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< cpp‎ | iterator‎ | reverse iterator

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

Run this code

    #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

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