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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

And, given

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#Concept.

 (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

(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

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