From cppreference.com

template< class Iterator >

typename reverse_iterator<Iterator>::difference_type
    operator-( const reverse_iterator<Iterator>& lhs,

               const reverse_iterator<Iterator>& rhs );
(until C++11)
template< class Iterator1, class Iterator2 >

auto operator-( const reverse_iterator<Iterator1>& lhs,

                const reverse_iterator<Iterator2>& rhs ) -> decltype(rhs.base() - lhs.base());
(until C++11)

Returns the distance between two iterator adaptors.

[edit] Parameters

lhs, rhs - iterator adaptors to compute the difference of

[edit] Return value

rhs.base() - lhs.base()

[edit] Example