C++ named requirements: LegacyBidirectionalIterator

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 [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