24 Iterators library [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 [tab:iterator.forward.requirements] 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 ]

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.