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 [tab:iterator.bidirectional.requirements].
| Expression | Return type | Operational | Assertion/note |
| semantics | pre-/post-condition | ||
| -- r | X& |
pre: there exists s such that r == ++s. post: 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 |
[ Note: Bidirectional iterators allow algorithms to move iterators backward as well as forward. — end note ]