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

(1.1)
X satisfies the requirements of an input iterator ([input.iterators]),
(1.2)
X satisfies the DefaultConstructible requirements ([utility.arg.requirements]),
(1.3)
if X is a mutable iterator, reference is a reference to T; if X is a constant iterator, reference is a reference to const T,
(1.4)
the expressions in Table 92 are valid and have the indicated semantics, and
(1.5)
objects of type X offer the multi-pass guarantee, described below.

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:

(3.1)
a == b implies ++a == ++b and
(3.2)
X is a pointer type or the expression (void)++X(a), *a is equivalent to the expression *a.

[ 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 92 — Forward iterator requirements (in addition to input iterator)

Expression	Return type	Operational	Assertion/note
		semantics	pre-/post-condition
r++	convertible to const X&	{ X tmp = r; ++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.