std::bidirectional_iterator

The concept bidirectional_iterator refines forward_iterator by adding the ability to move an iterator backward.

[edit] Iterator concept determination

Definition of this concept is specified via an exposition-only alias template /*ITER_CONCEPT*/.

In order to determine /*ITER_CONCEPT*/<I>, let ITER_TRAITS<I> denote I if the specialization std::iterator_traits <I> is generated from the primary template, or std::iterator_traits <I> otherwise:

• If ITER_TRAITS<I>::iterator_concept is valid and names a type, /*ITER_CONCEPT*/<I> denotes the type.
• Otherwise, if ITER_TRAITS<I>::iterator_category is valid and names a type, /*ITER_CONCEPT*/<I> denotes the type.
• Otherwise, if std::iterator_traits <I> is generated from the primary template, /*ITER_CONCEPT*/<I> denotes std::random_access_iterator_tag .
  (That is, std::derived_from </*ITER_CONCEPT*/<I>, std::bidirectional_iterator_tag > is assumed to be true.)
• Otherwise, /*ITER_CONCEPT*/<I> does not denote a type and results in a substitution failure.

[edit] Semantic requirements

A bidirectional iterator r is said to be decrementable if and only if there exists some s such that ++s == r.

std::bidirectional_iterator<I> is modeled only if all the concepts it subsumes are modeled, and given two objects a and b of type I:

• If a is decrementable, a is in the domain of the expressions --a and a--.
• Pre-decrement yields an lvalue that refers to the operand: std::addressof (--a) == std::addressof (a).
• Post-decrement yields the previous value of the operand: if bool(a == b), then bool(a-- == b).
• Post-decrement and pre-decrement perform the same modification on its operand: If bool(a == b), then after evaluating both a-- and --b, bool(a == b) still holds.
• Increment and decrement are inverses of each other:

• If a is incrementable and bool(a == b), then bool(--(++a) == b).
• If a is decrementable and bool(a == b), then bool(++(--a) == b).

[edit] Equality preservation

Expressions declared in requires expressions of the standard library concepts are required to be equality-preserving (except where stated otherwise).

[edit] Notes

Unlike the LegacyBidirectionalIterator requirements, the bidirectional_iterator concept does not require dereference to return an lvalue.

[edit] Example

A minimum bidirectional iterator.

#include <cstddef>
#include <iterator>
 
class SimpleBidiIterator
{
public:
 using difference_type = std::ptrdiff_t ;
 using value_type = int;
 
 SimpleBidiIterator();
 SimpleBidiIterator(const SimpleBidiIterator&);
 
 SimpleBidiIterator& operator=(const SimpleBidiIterator&);
 
 int operator*() const;
 
 SimpleBidiIterator& operator++();
 
 SimpleBidiIterator operator++(int)
 {
 auto tmp = *this;
 ++*this;
 return tmp;
 }
 
 SimpleBidiIterator& operator--();
 
 SimpleBidiIterator operator--(int)
 {
 auto tmp = *this;
 --*this;
 return tmp;
 }
 
 bool operator==(const SimpleBidiIterator&) const;
};
 
static_assert(std::bidirectional_iterator<SimpleBidiIterator>);

[edit] See also

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