template< std::common_with<I> I2 >
  friend constexpr std::iter_difference_t<I2> operator-(
    const counted_iterator& x, const counted_iterator<I2>& y );
(since C++20)

Computes the distance between two iterator adaptors.

The behavior is undefined if x and y do not point to elements of the same sequence. That is, there must exist some n such that std::next(x.base(), x.count() + n) and std::next(y.base(), y.count() + n) refer to the same element.

This function template is not visible to ordinary unqualified or qualified lookup, and can only be found by argument-dependent lookup when std::counted_iterator<I> is an associated class of the arguments.


x, y - iterator adaptors to compute the difference of

Return value

y.count() - x.count().


Since the length counts down, not up, the order of the arguments of operator- in the underlying expression is reversed, i.e. y is lhs and x is rhs.


#include <initializer_list>
#include <iterator>
int main()
    static constexpr auto v = {1, 2, 3, 4, 5, 6};
    constexpr std::counted_iterator<std::initializer_list<int>::iterator>
        it1 {v.begin(), 5},
        it2 {it1 + 3},
        it3 {v.begin(), 2};
    static_assert(it1 - it2 == -3);
    static_assert(it2 - it1 == +3);
    // static_assert(it1 - it3 == -3); // UB: operands of operator- do not refer to
                                       // elements of the same sequence

See also

advances or decrements the iterator
(public member function)
advances the iterator
(function template)
computes the signed distance to the end
(function template)

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