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reverse_iterator<RandomAccessIterator, T, Reference, Distance>

Categories: iterators, adaptors Component type: type

Description

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

Example

template <class T>
void forw(const vector<T>& V)
{
   vector<T>::iterator first = V.begin();
   vector<T>::iterator last = V.end();
   while (first != last) 
      cout << *first++ << endl;
}      

template <class T>
void rev(const vector<T>& V)
{
   typedef reverse_iterator<vector<T>::iterator,
                            T,
                            vector<T>::reference_type,
                            vector<T>::difference_type> 
           reverse_iterator; [2]
   reverse_iterator rfirst(V.end());
   reverse_iterator rlast(V.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]

Definition

Defined in the standard header iterator, and in the nonstandard backward-compatibility header iterator.h.

Template parameters

Parameter Description Default
RandomAccessIterator The base iterator class. Incrementing an object of class reverse_iterator<Iterator> corresponds to decrementing an object of class Iterator.  
T The reverse iterator's value type. This should always be the same as the base iterator's value type.  
Reference The reverse iterator's reference type. This should always be the same as the base iterator's reference type. T&
Distance The reverse iterator's distance type. This should always be the same as the base iterator's distance type. ptrdiff_t

Model of

Random Access Iterator

Type requirements

The base iterator type (that is, the template parameter RandomAccessIterator) must be a Random Access Iterator. The reverse_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.

Public base classes

None.

Members

Member Where defined Description
self reverse_iterator See below
reverse_iterator() Trivial Iterator The default constructor
reverse_iterator(const reverse_iterator& x) Trivial Iterator The copy constructor
reverse_iterator& operator=(const reverse_iterator& x) Trivial Iterator The assignment operator
reverse_iterator(RandomAccessIterator x) reverse_iterator See below.
RandomAccessIterator base() reverse_iterator See below.
Reference operator*() const Trivial Iterator The dereference operator
reverse_iterator& operator++() Forward Iterator Preincrement
reverse_iterator operator++(int) Forward Iterator Postincrement
reverse_iterator& operator--() Bidirectional Iterator Predecrement
reverse_iterator operator--(int) Bidirectional Iterator Postdecrement
reverse_iterator operator+(Distance) Random Access Iterator Iterator addition
reverse_iterator& operator+=(Distance) Random Access Iterator Iterator addition
reverse_iterator operator-(Distance) Random Access Iterator Iterator subtraction
reverse_iterator& operator-=(Distance) Random Access Iterator Iterator subtraction
Reference operator[](Distance) Random Access Iterator Random access to an element.
reverse_iterator operator+(Distance, reverse_iterator) Random Access Iterator Iterator addition. This is a global function, not a member function.
Distance operator-(const reverse_iterator&, const reverse_iterator&) Random Access Iterator Finds the distance between two iterators. This is a global function, not a member function.
bool operator==(const reverse_iterator&, const reverse_iterator&) Trivial Iterator Compares two iterators for equality. This is a global function, not a member function.
bool operator<(const reverse_iterator&, const reverse_iterator&) Random Access Iterator Determines whether the first argument precedes the second. This is a global function, not a member function.
random_access_iterator_tag iterator_category(const reverse_iterator&) Iterator tags Returns the iterator's category. This is a global function, not a member function.
T* value_type(const reverse_iterator&) Iterator tags Returns the iterator's value type. This is a global function, not a member function.
Distance* distance_type(const reverse_iterator&) Iterator tags Returns the iterator's distance type. This is a global function, not a member function.

New members

These members are not defined in the Random Access Iterator requirements, but are specific to reverse_iterator.
Member Description
self A typedef for reverse_iterator<RandomAccessIterator, T, Reference, Distance>.
RandomAccessIterator base() Returns the current value of the reverse_iterator's base iterator. If ri is a reverse iterator and i is any iterator, the two fundamental identities of reverse iterators can be written as reverse_iterator(i).base() == i and &*ri == &*(ri.base() - 1).
reverse_iterator(RandomAccessIterator i) Constructs a reverse_iterator whose base iterator is i.

Notes

[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_iterator. Vector is a Reversible Container, so it provides a typedef for the appropriate instantiation of reverse_iterator. The usual way of declaring these variables is much simpler:

    vector<T>::reverse_iterator rfirst = rbegin();
    vector<T>::reverse_iterator rlast = rend();

[3] Note the implications of this remark. The variable rfirst is initialized as reverse_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_iterator(i)) == &*(i - 1). This code sample shows why this identity is important: if [f, l) is a valid range, then it allows [reverse_iterator(l), reverse_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.

See also

Reversible Container, reverse_bidirectional_iterator, Random Access Iterator, iterator tags, Iterator Overview


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