Книга: Standard Template Library Programmer

reverse_bidirectional_iterator

reverse_bidirectional_iterator<BidirectionalIterator, T, Reference, Distance>

Categories: iterators, adaptors

Component type: type

Reverse_bidirectional_iterator is an iterator adaptor that enables backwards traversal of a range. Operator++ applied to an object of class reverse_bidirectional_iterator<BidirectionalIterator> means the same thing as operator-- applied to an object of class BidirectionalIterator. There are two different reverse iterator adaptors: the class reverse_bidirectional_iterator has a template argument that is a Bidirectional Iterator, and the class reverse_iterator has a template argument that is a Random Access Iterator. [1]

template <class T>
void forw(const list <T>& L) {
 list<T>::iterator first = L.begin();
 list<T>::iterator last = L.end();
 while (first != last) cout << *first++ << endl;
}
template <class T>
void rev(const list <T>& L) {
 typedef reverse_bidirectional_iterator<list<T>::iterator, T, list<T>::reference_type, list<T>::difference_type> reverse_iterator; [2]
 reverse_iterator rfirst(L.end());
 reverse_iterator rlast(L.begin());
 while (rfirst != rlast) cout << *rfirst++ << endl;
}

In the function forw, the elements are printed in the order *first, *(first+1) , …, *(last-1). In the function rev, they are printed in the order *(last – 1), *(last - 2), …, *first. [3]

Defined in the standard header iterator, and in the nonstandard backward-compatibility header iterator.h. This class is no longer part of the C++ standard, but it was present in early drafts, and it is retained in this implementation for backward compatibility.

Bidirectional Iterator.

The base iterator type (that is, the template parameter BidirectionalIterator) must be a Bidirectional Iterator. The reverse_bidirectional_iterator's value type, reference type, and distance type (that is, the template parameters T, Reference, and Distance, respectively) must be the same as the base iterator's value type, reference type, and distance type.

None.

These members are not defined in the Bidirectional Iterator requirements, but are specific to reverse_bidirectional_iterator.

[1] There isn't really any good reason to have two separate classes: this separation is purely because of a technical limitation in some of today's C++ compilers. If the two classes were combined into one, then there would be no way to declare the return types of the iterator tag functions iterator_category, distance_type and value_type correctly. The iterator traits class solves this problem: it addresses the same issues as the iterator tag functions, but in a cleaner and more flexible manner. Iterator traits, however, rely on partial specialization, and many C++ compilers do not yet implement partial specialization. Once compilers that support partial specialization become more common, these two different reverse iterator classes will be combined into a single class.

[2] The declarations for rfirst and rlast are written in this clumsy form simply as an illustration of how to declare a reverse_bidirectional_iterator. List is a Reversible Container, so it provides a typedef for the appropriate instantiation of reverse_bidirectional_iterator. The usual way of declaring these variables is much simpler:

list<T>::reverse_bidirectional_iterator rfirst = rbegin();
list<T>::reverse_bidirectional_iterator rlast = rend();

[3] Note the implications of this remark. The variable rfirst is initialized as reverse_bidirectional_iterator<…> rfirst(V.end());. The value obtained when it is dereferenced, however, is *(V.end() – 1). This is a general property: the fundamental identity of reverse iterators is &*(reverse_bidirectional_iterator(i)) == &*(i – 1). This code sample shows why this identity is important: if [f, l) is a valid range, then it allows [reverse_bidirectional_iterator(l), reverse_bidirectional_iterator(f)) to be a valid range as well. Note that the iterator l is not part of the range, but it is required to be dereferenceable or past-the-end. There is no requirement that any such iterator precedes f.

Reversible Container, reverse_iterator, Bidirectional Iterator, iterator tags, Iterator overview

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