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operator-(std::counted_iterator<I>, std::default_sentinel_t) - cppreference.com

From cppreference.com

friend constexpr std::iter_difference_t<I> operator-(
    const counted_iterator& x, std::default_sentinel_t );
(1) (since C++20) 
friend constexpr std::iter_difference_t<I> operator-(
    std::default_sentinel_t, const counted_iterator& y );
(2) (since C++20)

1) Returns the negative distance to the end.

2) Returns the positive distance to the end.

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.

Parameters

x, y - iterator adaptors to compute the difference of

Return value

1) -x.count()

2) y.count()

Example

#include <initializer_list>
#include <iterator>

int main()
{
    constexpr static auto v = {1, 2, 3, 4};
    constexpr std::counted_iterator<std::initializer_list<int>::iterator>
        it{v.begin(), 3};
    constexpr auto d1 = it - std::default_sentinel;
    static_assert(d1 == -3); // (1)
    constexpr auto d2 = std::default_sentinel - it;
    static_assert(d2 == +3); // (2)
}

See also