advance.cpp

template <typename Iterator, typename IteratorType>
struct __advance {
	static Iterator f(Iterator it, int n) {
		while (n > 0) { ++it; --n; }
		while (n < 0) { --it; ++n; }
		return it;
	}
};

template <typename Iterator>
struct __advance<Iterator, std::random_access_iterator_tag> {
	static Iterator f(Iterator it, int n) {
		return it + n;
	}
};

template <typename Iterator>
Iterator advance(Iterator it, int n) {
	return
			__advance
			<
				Iterator,
				typename std::iterator_traits<Iterator>::iterator_category
			>::f(it, n);
}