Question

Why isn’t first passed as a reference and const as well?

template <typename Iterator>
    int distance(Iterator first, const Iterator & last) {
    int count;
    for ( ; first != last; first++)
        count++;
    return count;
}

Answer 1

It cannot be const because it is incremented inside the function, and it is not passed by reference because it probably makes no sense to do so for the caller.

Furthermore, if it were non-const reference, it would not be possible to use a temporary. For example, you wouldn’t be able to do tis:

std::vector<int> v{ 1, 2, 3, 4 };
auto distance = std::distance(v.begin(), v.end());