operator==,!=,<,<=,>,>=,<=> (std::reverse_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::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
Retrieved from " https://en.cppreference.com/mwiki/index.php?title=cpp/iterator/reverse_iterator/operator_cmp&oldid=177504 "