C++ named requirements: LegacyBidirectionalIterator

A LegacyBidirectionalIterator is a LegacyForwardIterator

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

Notes

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

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[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

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

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