24 Iterators library [iterators]

24.3 Iterator requirements [iterator.requirements]

24.3.5 C++17 iterator requirements [iterator.cpp17]

24.3.5.6 Bidirectional iterators [bidirectional.iterators]

1
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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 77.
Table 77 — 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
2
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[Note 1: 
Bidirectional iterators allow algorithms to move iterators backward as well as forward.
— end note]