reverse_iterator

Library:  Iterators

Does not inherit

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An iterator adaptor that traverses a collection backwards.

#include <iterator>

namespace std {
  template <class Iterator>
  class reverse_iterator;
}

The class template reverse_iterator adapts any random access iterator or bidirectional_iterator to facilitate the traversal of collections in the opposite direction. The fundamental relationship between a reverse iterator and its corresponding iterator i is established by the identity:

        
&*(reverse_iterator(i)) == &*(i - 1);

This mapping is necessary because, while an iterator past the end of a sequence is still valid but not dereferenceable, there might not be a valid iterator before its beginning.

namespace std {

  template <class Iterator>
  class reverse_iterator
    : public iterator
       <typename iterator_traits<iterator>::iterator_category,
       <typename iterator_traits<iterator>::value_type,
       <typename iterator_traits<iterator>::difference_type,
       <typename iterator_traits<iterator>::pointer,
       <typename iterator_traits<iterator>::reference>
  {

    public:
      typedef Iterator iterator_type;

      reverse_iterator();
      explicit reverse_iterator(iterator_type);

      iterator_type base();

      reference operator*();

      reverse_iterator& operator++();
      reverse_iterator  operator++(int);
      reverse_iterator& operator--();
      reverse_iterator  operator--(int);

      reverse_iterator  operator+  (difference_type) const;
      reverse_iterator& operator+= (difference_type);
      reverse_iterator operator-   (difference_type) const;
      reverse_iterator& operator-= (difference_type);

      reference operator[] (difference_type) const;
  };

  // Nonmember Operators

  template <class Iterator> 
  bool operator== (const reverse_iterator<Iterator>&,
                   const reverse_iterator<Iterator>&);

  template <class Iterator> 
  bool operator!= (const reverse_iterator<Iterator>&,
                   const reverse_iterator<Iterator>&);

  template <class Iterator> 
  bool operator< (const reverse_iterator<Iterator>&,
                  const reverse_iterator<Iterator>&);

  template <class Iterator> 
  bool operator> (const reverse_iterator<Iterator>&,
                  const reverse_iterator<Iterator>&);

  template <class Iterator> 
  bool operator<= (const reverse_iterator<Iterator>&,
                   const reverse_iterator<Iterator>&);

  template <class Iterator> 
  bool operator>= (const reverse_iterator<Iterator>&,
                   const reverse_iterator<Iterator>&);

  template <class Iterator> 
  typename reverse_iterator<Iterator>::difference_type
  operator- (const reverse_iterator<Iterator>&,
             const reverse_iterator<Iterator>&);

  template <class Iterator> 
  reverse_iterator<Iterator>
  operator+ (
      typename reverse_iterator<Iterator>::difference_type,
      const reverse_iterator<Iterator>&);
}

//
//  rev_itr.cpp
//

#include <iostream>   // for cout, endl
#include <vector>     // for vector


int main ()
{
    typedef std::vector<int, std::allocator<int> > Vector;

    // Initialize a vector using an array.
    const Vector::value_type arr[] = { 3, 4, 7, 8 };
    const Vector v (arr + 0, arr + sizeof arr / sizeof *arr);

    // Output the original vector.
    std::cout << "Traversing vector with iterator: \n     ";
    for (Vector::const_iterator i = v.begin (); 
         i != v.end (); ++i)
        std::cout << *i << " ";

    // Output the vector backwards.
    std::cout << "\n\nSame vector, same loop, "
              << "reverse_iterator: \n     ";
    for (Vector::const_reverse_iterator j (v.end ());
         j != Vector::const_reverse_iterator (v.begin ());
         ++j)
        std::cout << *j << " ";

    std::cout << std::endl;

    return 0;
}


Program Output:

Traversing vector with iterator: 
     3 4 7 8 

Same vector, same loop, reverse_iterator: 
     8 7 4 3 

All iterator operations are required to take at most amortized constant time.

Iterators

ISO/IEC 14882:1998 -- International Standard for Information Systems -- Programming Language C++, Section 24.4.1.1