The forward_iterator concept adds copyability, equality comparison, and the multi-pass guarantee, specified below.

The domain of == for forward iterators is that of iterators over the same underlying sequence.

However, value-initialized iterators of the same type may be compared and shall compare equal to other value-initialized iterators of the same type.

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

Two dereferenceable iterators a and b of type X offer the multi-pass guarantee if

• a == b implies ++a == ++b and
• the expression ((void)[](X x){++x;}(a), *a) is equivalent to the expression *a.

[Note 2:

The requirement that a == b implies ++a == ++b and the removal of the restrictions on the number of assignments through a mutable iterator (which applies to output iterators) allow the use of multi-pass one-directional algorithms with forward iterators.

— end note]