operator-(std::reverse_iterator) - cppreference.com

From cppreference.com

Defined in header `<iterator>`
template< class Iter1, class Iter2 > typename reverse_iterator<Iter>::difference_type operator-( const reverse_iterator<Iter1>& lhs, const reverse_iterator<Iter2>& rhs );		(until C++11)
template< class Iter1, class Iter2 > auto operator-( const reverse_iterator<Iter1>& lhs, const reverse_iterator<Iter2>& rhs ) -> decltype(rhs.base() - lhs.base());		(since C++11) (constexpr since C++17)

Returns the distance between two iterator adaptors.

Parameters

lhs, rhs

iterator adaptors to compute the difference of

Return value

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 <iostream>
#include <iterator>
#include <list>
#include <vector>

int main()
{
    std::vector vec{0, 1, 2, 3};
    std::reverse_iterator<std::vector<int>::iterator>
        vec_ri1{std::reverse_iterator{vec.rbegin()}},
        vec_ri2{std::reverse_iterator{vec.rend()}};
    std::cout << (vec_ri2 - vec_ri1) << ' '; // 4
    std::cout << (vec_ri1 - vec_ri2) << '\n'; // -4
    
    std::list lst{5, 6, 7, 8};
    std::reverse_iterator<std::list<int>::iterator>
        lst_ri1{std::reverse_iterator{lst.rbegin()}},
        lst_ri2{std::reverse_iterator{lst.rend()}};
//  auto n = (lst_ri1 - lst_ri2); // Error: the underlying iterators do not
                                  //        model random access iterator
}

Output:

Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.