Bidirectional Iterator

A Bidirectional Iterator is a Forward Iterator that provides a way to obtain an iterator to the previous element in a sequence.

a bidirectional iterator i is decrementable if there is a "previous" iterator, that is, if prior<i>::type expression is well-defined; iterators pointing to the first element of the sequence are not decrementable.

In addition to the requirements defined in Forward Iterator, the following requirements must be met.

Expression	Type	Complexity
next<i>::type	Bidirectional Iterator	Amortized constant time
prior<i>::type	Bidirectional Iterator	Amortized constant time
i::category	Integral Constant, convertible to bidirectional_iterator_tag	Constant time

typedef[prior](./prior.html)<i>::type j;

For any bidirectional iterators i and j the following invariants always hold:

If i is incrementable, then prior< next<i>::type >::type is a null operation; similarly, if i is decrementable, next< prior<i>::type >::type is a null operation.