27 Iterators library [iterators]

27.2 Iterator requirements [iterator.requirements]

27.2.5 Forward iterators [forward.iterators]

A class or pointer type X satisfies the requirements of a forward iterator if

The domain of == for forward iterators is that of iterators over the same underlying sequence. However, value-initialized iterators may be compared and shall compare equal to other value-initialized iterators of the same type. [Note: Value-initialized iterators behave as if they refer past the end of the same empty sequence. end note]

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

[Note: The requirement that a == b implies ++a == ++b (which is not true for input and output iterators) and the removal of the restrictions on the number of the assignments through a mutable iterator (which applies to output iterators) allows the use of multi-pass one-directional algorithms with forward iterators. end note]

Table 97 — Forward iterator requirements (in addition to input iterator)
ExpressionReturn typeOperationalAssertion/note
r++ convertible to const X& { X tmp = r;
return tmp; }
*r++ reference

If a and b are equal, then either a and b are both dereferenceable or else neither is dereferenceable.

If a and b are both dereferenceable, then a == b if and only if *a and *b are bound to the same object.