25 Iterators library [iterators]

25.4 Iterator primitives [iterator.primitives]

25.4.3 Iterator operations [iterator.operations]

1
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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.
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template<class InputIterator, class Distance> constexpr void advance(InputIterator& i, Distance n);
2
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Preconditions: n is negative only for bidirectional iterators.
3
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Effects: Increments i by n if n is non-negative, and decrements i by -n otherwise.
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template<class InputIterator> constexpr typename iterator_traits<InputIterator>::difference_type distance(InputIterator first, InputIterator last);
4
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Preconditions: last is reachable from first, or InputIterator meets the Cpp17RandomAccessIterator requirements and first is reachable from last.
5
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Effects: If InputIterator meets the Cpp17RandomAccessIterator requirements, returns (last - first); otherwise, increments first until last is reached and returns the number of increments.
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template<class InputIterator> constexpr InputIterator next(InputIterator x, typename iterator_traits<InputIterator>::difference_type n = 1);
6
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Effects: Equivalent to: advance(x, n); return x;
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template<class BidirectionalIterator> constexpr BidirectionalIterator prev(BidirectionalIterator x, typename iterator_traits<BidirectionalIterator>::difference_type n = 1);
7
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Effects: Equivalent to: advance(x, -n); return x;