iterator adapter reverse_iterator

1.迭代器适配器reverse_iterator

template <class _Iterator>
class reverse_iterator
{
protected:
  _Iterator current;  //对应之正向迭代器

public:

  //逆向迭代器的5种associated types都和其相对应

  typedef typename iterator_traits<_Iterator>::iterator_category
          iterator_category;
  typedef typename iterator_traits<_Iterator>::value_type
          value_type;
  typedef typename iterator_traits<_Iterator>::difference_type
          difference_type;
  typedef typename iterator_traits<_Iterator>::pointer
          pointer;
  typedef typename iterator_traits<_Iterator>::reference
          reference;

  typedef _Iterator iterator_type;  //代表正向迭代器
  typedef reverse_iterator<_Iterator> _Self;  //代表逆向迭代器

public:
  reverse_iterator() {}
  explicit reverse_iterator(iterator_type __x) : current(__x) {}

  reverse_iterator(const _Self& __x) : current(__x.current) {}
#ifdef __STL_MEMBER_TEMPLATES
  template <class _Iter>
  reverse_iterator(const reverse_iterator<_Iter>& __x)
    : current(__x.base()) {}
#endif /* __STL_MEMBER_TEMPLATES */

  iterator_type base() const { return current; }  //取出对应的正向迭代器

  //对逆向迭代器取值，就是将对应之正向迭代器退一位取值

  reference operator*() const {
    _Iterator __tmp = current;
    return *--__tmp;
  }
#ifndef __SGI_STL_NO_ARROW_OPERATOR
  pointer operator->() const { return &(operator*()); }

#endif /* __SGI_STL_NO_ARROW_OPERATOR */

  //前进变后退，后退变成前进

  _Self& operator++() {
    --current;
    return *this;
  }
  _Self operator++(int) {
    _Self __tmp = *this;
    --current;
    return __tmp;
  }
  _Self& operator--() {
    ++current;
    return *this;
  }
  _Self operator--(int) {
    _Self __tmp = *this;
    ++current;
    return __tmp;
  }

  _Self operator+(difference_type __n) const {
    return _Self(current - __n);
  }
  _Self& operator+=(difference_type __n) {
    current -= __n;
    return *this;
  }
  _Self operator-(difference_type __n) const {
    return _Self(current + __n);
  }
  _Self& operator-=(difference_type __n) {
    current += __n;
    return *this;
  }
  reference operator[](difference_type __n) const { return *(*this + __n); }  
};

2.rbegin，rend

  reverse_iterator rbegin() { return reverse_iterator(end()); }
  reverse_iterator rend() { return reverse_iterator(begin()); }