C++ named requirements: LegacyBidirectionalIterator

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


The type It satisfies LegacyBidirectionalIterator if.

And, given.

  • a and b, iterators 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)


  • 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.


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).


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<I&>;
    {  i-- } -> const I&;
    { *i-- } -> std::Same<std::iter_reference_t<I>>;

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

(since C++20)

See also

specifies that a ForwardIterator is a bidirectional iterator, supporting movement backwards

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