23 Iterators library [iterators]

23.4 Iterator primitives [iterator.primitives]

23.4.2 Iterator operations [iterator.operations]

Since only random access iterators provide + and - operators, the library provides two function templates advance and distance.
These function templates use + and - for random access iterators (and are, therefore, constant time for them); for input, forward and bidirectional iterators they use ++ to provide linear time implementations.
template<class InputIterator, class Distance> constexpr void advance(InputIterator& i, Distance n);
Expects: n is negative only for bidirectional iterators.
Effects: Increments i by n if n is non-negative, and decrements i by -n otherwise.
template<class InputIterator> constexpr typename iterator_traits<InputIterator>::difference_type distance(InputIterator first, InputIterator last);
Expects: last is reachable from first, or InputIterator meets the Cpp17RandomAccessIterator requirements and first is reachable from last.
Effects: If InputIterator meets the Cpp17RandomAccessIterator requirements, returns (last - first); otherwise, returns the number of increments needed to get from first to last.
template<class InputIterator> constexpr InputIterator next(InputIterator x, typename iterator_traits<InputIterator>::difference_type n = 1);
Effects: Equivalent to: advance(x, n); return x;
template<class BidirectionalIterator> constexpr BidirectionalIterator prev(BidirectionalIterator x, typename iterator_traits<BidirectionalIterator>::difference_type n = 1);
Effects: Equivalent to: advance(x, -n); return x;