std::bidirectional_iterator

 

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Stream iterators istream_iterator    ostream_iterator istreambuf_iterator ostreambuf_iterator	Iterator customization points ranges::iter_move (C++20) ranges::iter_swap (C++20)
Iterator operations advance distance prev (C++11)   next (C++11) ranges::advance (C++20) ranges::distance (C++20) ranges::prev (C++20) ranges::next (C++20)	Range access begincbegin (C++11)(C++14) rbegincrbegin (C++14)(C++14)   endcend (C++11)(C++14) rendcrend (C++14)(C++14)   sizessize (C++17)(C++20) empty (C++17) data (C++17)

 

Defined in header <iterator>
template<class I >     concept bidirectional_iterator =         std::forward_iterator<I>&&         std::derived_from</*ITER_CONCEPT*/<I>, std::bidirectional_iterator_tag>&&         requires(I i){             {--i }->std::same_as<I&>;             { i--}->std::same_as<I>;         };		(since C++20)

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

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.

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).

Equality preservation

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

Notes

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

Example

A minimum bidirectional iterator.

#include <cstddef>#include <iterator>   struct SimpleBidiIterator {using difference_type =std::ptrdiff_t;using value_type =int;   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>);

See also

forward_iterator (C++20)	specifies that an input_iterator is a forward iterator, supporting equality comparison and multi-pass (concept)
random_access_iterator (C++20)	specifies that a bidirectional_iterator is a random-access iterator, supporting advancement in constant time and subscripting (concept)