[bidirectional.iterators]

A class or pointer type X meets the requirements of a bidirectional iterator if, in addition to meeting the Cpp17ForwardIterator requirements, the following expressions are valid as shown in Table 82 .

Table 82 — Cpp17BidirectionalIterator requirements (in addition to Cpp17ForwardIterator) [tab:bidirectionaliterator]

🔗 Expression	Return type	Operational	Assertion/note
🔗		semantics	pre-/post-condition
🔗 --r	X&		Preconditions: there exists s such that r == ++s. Postconditions: r is dereferenceable. --(+r) == r. --r == --s implies r == s. addressof(r) == addressof(-r).
🔗 r--	convertible to const X&	{ X tmp = r; --r; return tmp; }
🔗 *r--	reference

[Note 1:

Bidirectional iterators allow algorithms to move iterators backward as well as forward.

— end note]