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.
  • 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).


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

See also

specifies that an input_iterator is a forward iterator, supporting equality comparison and multi-pass
specifies that a bidirectional_iterator is a random-access iterator, supporting advancement in constant time and subscripting

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