27 Iterators library [iterators]

27.2 Iterator requirements [iterator.requirements]

27.2.6 Bidirectional iterators [bidirectional.iterators]

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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 93.
Table 93 — Bidirectional iterator requirements (in addition to forward iterator)
Expression
Return type
Operational
Assertion/note
semantics
pre-/post-condition
--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;
--r;
return tmp; }
*r--
reference
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[ Note
:
Bidirectional iterators allow algorithms to move iterators backward as well as forward.
— end note
 ]