std::bidirectional_iterator

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>

• If ITER_TRAITS<I>::iterator_concept is valid and names a type, /*ITER_CONCEPT*/<I>
• Otherwise, if ITER_TRAITS<I>::iterator_category is valid and names a type, /*ITER_CONCEPT*/<I>
• 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>

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)
• 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

Notes

Unlike the LegacyBidirectionalIterator requirements, the bidirectional_iterator

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