27 Iterators library [iterators]

27.2 Iterator requirements [iterator.requirements]

27.2.6 Bidirectional iterators [bidirectional.iterators]

A class or pointer type X satisfies the requirements of a bidirectional iterator if, in addition to satisfying the requirements for forward iterators, the following expressions are valid as shown in Table 98.

Table 98 — Bidirectional iterator requirements (in addition to forward iterator)
ExpressionReturn typeOperationalAssertion/note
--r X& Requires: there exists s such that r == ++s.
Postconditions: r is dereferenceable.
--(++r) == r.
--r == --s implies r == s.
&r == &--r.
r-- convertible to const X& { X tmp = r;
return tmp; }
*r-- reference

[Note: Bidirectional iterators allow algorithms to move iterators backward as well as forward. end note]