C++ named requirements: LegacyBidirectionalIterator

 

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C++ named requirements

Basic DefaultConstructible MoveConstructible (C++11) CopyConstructible CopyAssignable MoveAssignable (C++11) Destructible Type properties ScalarType PODType TriviallyCopyable (C++11) TrivialType (C++11) StandardLayoutType (C++11)   ImplicitLifetimeType Library-wide BooleanTestable EqualityComparable LessThanComparable Swappable ValueSwappable (C++11) NullablePointer (C++11) Hash (C++11) Allocator FunctionObject Callable Predicate BinaryPredicate Compare

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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.

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.

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. template<class I> concept __LegacyBidirectionalIterator =     __LegacyForwardIterator<I> && requires(I i)     {         {  --i } -> std::same_as<I&>;         {  i-- } -> std::convertible_to<const I&>;         { *i-- } -> std::same_as<std::iter_reference_t<I>>;     }; where the exposition-only concept __LegacyForwardIterator is described in LegacyForwardIterator.

(since C++20)

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
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

See also

bidirectional_iterator (C++20)	specifies that a forward_iterator is a bidirectional iterator, supporting movement backwards (concept)
Iterator library	provides definitions for iterators, iterator traits, adaptors, and utility functions