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Reverse Iterator

Author: David Abrahams, Jeremy Siek, Thomas Witt
Contact: dave@boost-consulting.com, jsiek@osl.iu.edu, witt@ive.uni-hannover.de
Organization: Boost Consulting, Indiana University Open Systems Lab, University of Hanover Institute for Transport Railway Operation and Construction
Date: 2004-11-01
Copyright: Copyright David Abrahams, Jeremy Siek, and Thomas Witt 2003.
abstract:The reverse iterator adaptor iterates through the adapted iterator range in the opposite direction.

Table of Contents

reverse_iterator synopsis

template <class Iterator>
class reverse_iterator
{
public:
  typedef iterator_traits<Iterator>::value_type value_type;
  typedef iterator_traits<Iterator>::reference reference;
  typedef iterator_traits<Iterator>::pointer pointer;
  typedef iterator_traits<Iterator>::difference_type difference_type;
  typedef /* see below */ iterator_category;

  reverse_iterator() {}
  explicit reverse_iterator(Iterator x) ;

  template<class OtherIterator>
  reverse_iterator(
      reverse_iterator<OtherIterator> const& r
    , typename enable_if_convertible<OtherIterator, Iterator>::type* = 0 // exposition
  );
  Iterator const& base() const;
  reference operator*() const;
  reverse_iterator& operator++();
  reverse_iterator& operator--();
private:
  Iterator m_iterator; // exposition
};

If Iterator models Random Access Traversal Iterator and Readable Lvalue Iterator, then iterator_category is convertible to random_access_iterator_tag. Otherwise, if Iterator models Bidirectional Traversal Iterator and Readable Lvalue Iterator, then iterator_category is convertible to bidirectional_iterator_tag. Otherwise, iterator_category is convertible to input_iterator_tag.

reverse_iterator requirements

Iterator must be a model of Bidirectional Traversal Iterator. The type iterator_traits<Iterator>::reference must be the type of *i, where i is an object of type Iterator.

reverse_iterator models

A specialization of reverse_iterator models the same iterator traversal and iterator access concepts modeled by its Iterator argument. In addition, it may model old iterator concepts specified in the following table:

If I models then reverse_iterator<I> models
Readable Lvalue Iterator, Bidirectional Traversal Iterator Bidirectional Iterator
Writable Lvalue Iterator, Bidirectional Traversal Iterator Mutable Bidirectional Iterator
Readable Lvalue Iterator, Random Access Traversal Iterator Random Access Iterator
Writable Lvalue Iterator, Random Access Traversal Iterator Mutable Random Access Iterator

reverse_iterator<X> is interoperable with reverse_iterator<Y> if and only if X is interoperable with Y.

reverse_iterator operations

In addition to the operations required by the concepts modeled by reverse_iterator, reverse_iterator provides the following operations.

reverse_iterator();

Requires:Iterator must be Default Constructible.
Effects:Constructs an instance of reverse_iterator with m_iterator default constructed.

explicit reverse_iterator(Iterator x);

Effects:Constructs an instance of reverse_iterator with m_iterator copy constructed from x.
template<class OtherIterator>
reverse_iterator(
    reverse_iterator<OtherIterator> const& r
  , typename enable_if_convertible<OtherIterator, Iterator>::type* = 0 // exposition
);
Requires:OtherIterator is implicitly convertible to Iterator.
Effects:Constructs instance of reverse_iterator whose m_iterator subobject is constructed from y.base().

Iterator const& base() const;

Returns:m_iterator

reference operator*() const;

Effects:
Iterator tmp = m_iterator;
return *--tmp;

reverse_iterator& operator++();

Effects:--m_iterator
Returns:*this

reverse_iterator& operator--();

Effects:++m_iterator
Returns:*this
template <class BidirectionalIterator>
reverse_iterator<BidirectionalIterator>n
make_reverse_iterator(BidirectionalIterator x);
Returns:An instance of reverse_iterator<BidirectionalIterator> with a current constructed from x.

Example

The following example prints an array of characters in reverse order using reverse_iterator.

char letters_[] = "hello world!";
const int N = sizeof(letters_)/sizeof(char) - 1;
typedef char* base_iterator;
base_iterator letters(letters_);
std::cout << "original sequence of letters:\t\t\t" << letters_ << std::endl;

boost::reverse_iterator<base_iterator>
  reverse_letters_first(letters + N),
  reverse_letters_last(letters);

std::cout << "sequence in reverse order:\t\t\t";
std::copy(reverse_letters_first, reverse_letters_last,
          std::ostream_iterator<char>(std::cout));
std::cout << std::endl;

std::cout << "sequence in double-reversed (normal) order:\t";
std::copy(boost::make_reverse_iterator(reverse_letters_last),
          boost::make_reverse_iterator(reverse_letters_first),
          std::ostream_iterator<char>(std::cout));
std::cout << std::endl;

The output is:

original sequence of letters:                   hello world!
sequence in reverse order:                      !dlrow olleh
sequence in double-reversed (normal) order:     hello world!

The source code for this example can be found here.