# std::experimental::ranges::ForwardIterator

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< cpp‎ | experimental‎ | ranges

Defined in header `<experimental/ranges/iterator>` |  |
template < class I > concept bool ForwardIterator = InputIterator <I> && DerivedFrom <ranges::iterator_category_t<I>, ranges::forward_iterator_tag> && Incrementable <I> && Sentinel <I, I>; |  | (ranges TS)

The concept `ForwardIterator<I>` refines `InputIterator` by adding equality comparison and the multi-pass guarantee.

The domain of `==` for forward iterators is that of iterators over the same underlying sequence, except that value-initialized iterators of the same type may be compared with each other and shall compare equal.

Pointers and references obtained from a forward iterator into a range `[i, s)` must remain valid while `[i, s)` continues to denote a range.

Let `a` and `b` be two deferenceable iterators of type `I`. `ForwardIterator<I>` is satisfied only if:

- a == b implies ++a == ++b; and
- The expression ([](X x){++x;}(a), *a) is equivalent to *a, i.e., incrementing a copy of `a` has no effect on the result of dereferencing `a`.

This is known as the multi-pass guarantee.

### Notes

A forward iterator cannot be "stashing": it cannot return a reference to an object within itself, because such references would be invalidated by the destruction of the forward iterator.

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