< cpp‎ | experimental‎ | ranges
Technical Specification
Filesystem library (filesystem TS)
Library fundamentals (library fundamentals TS)
Library fundamentals 2 (library fundamentals TS v2)
Library fundamentals 3 (library fundamentals TS v3)
Extensions for parallelism (parallelism TS)
Extensions for parallelism 2 (parallelism TS v2)
Extensions for concurrency (concurrency TS)
Extensions for concurrency 2 (concurrency TS v2)
Concepts (concepts TS)
Ranges (ranges TS)
Reflection (reflection TS)
Mathematical special functions (special functions TR)
Experimental Non-TS
Pattern Matching
Linear Algebra
2D Graphics
Iterators library
Iterator concepts
Indirect callable concepts
Common algorithm requirements
Concept utilities
Iterator utilities and operations
Iterator traits
Iterator adaptors
Stream iterators
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 [is) must remain valid while [is) continues to denote a range.

Let a and b be two dereferenceable 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.

[edit] 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.