# std::bidirectional_iterator

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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).

### 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)

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