27 Iterators library [iterators]

27.4 Iterator primitives [iterator.primitives]

27.4.3 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);
Requires: n shall be negative only for bidirectional and random access iterators.
Effects: Increments (or decrements for negative n) iterator reference i by n.
template<class InputIterator> constexpr typename iterator_traits<InputIterator>::difference_type distance(InputIterator first, InputIterator last);
Effects: If InputIterator meets the requirements of random access iterator, returns (last - first); otherwise, returns the number of increments needed to get from first to last.
Requires: If InputIterator meets the requirements of random access iterator, last shall be reachable from first or first shall be reachable from last; otherwise, last shall be reachable from first.
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;