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Showing content from https://en.cppreference.com/w/cpp/language/../named_req/BidirectionalIterator.html below:

C++ named requirements: LegacyBidirectionalIterator - cppreference.com

A LegacyBidirectionalIterator is a LegacyForwardIterator that can be moved in both directions (i.e. incremented and decremented).

If a LegacyBidirectionalIterator it originates from a Container, then it's value_type is the same as the container's, so dereferencing (*it) obtains the container's value_type.

[edit] Requirements

The type It satisfies LegacyBidirectionalIterator if

The type It satisfies LegacyForwardIterator

And, given

a and b, lvalues of type It
reference, the type denoted by std::iterator_traits<It>::reference

The following expressions must be valid and have their specified effects:

Expression Return Equivalent expression Notes --a It& Preconditions:

a is decrementable (there exists such b that a == ++b)

Postconditions:

a is dereferenceable
--(++a) == a
If --a == --b then a == b
a and --a designate the same iterator object

a-- convertible to const It& It temp = a; --a; return temp; *a-- reference

A mutable LegacyBidirectionalIterator is a LegacyBidirectionalIterator that additionally satisfies the LegacyOutputIterator requirements.

[edit] Notes

The begin iterator is not decrementable and the behavior is undefined if --container.begin() is evaluated.

A bidirectional iterator does not have to be dereferenceable to be decrementable (in particular, the end iterator is not dereferenceable but is decrementable).

Concept

For the definition of std::iterator_traits, the following exposition-only concept is defined.

where the exposition-only concept __LegacyForwardIterator is described in LegacyForwardIterator.

(since C++20) [edit] Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR Applied to Behavior as published Correct behavior LWG 299
(N3066) C++98 the return type of *a-- was
required to be convertible to T changed the return
type to reference^[1] LWG 383 C++98 b was required to be dereferenceable after --a a is required to be dereferenceable instead LWG 1212
(N3066) C++98 the return type of *a-- did not match the return
type of *a++ required by LegacyForwardIterator changed the return
type to reference

â This issue was initially resolved by N2758 (iterator concepts), which was dropped later from the C++ standard.

[edit] See also

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