stl_iterator.h 一

一. back_insert_iterator

功能:用于在后部插入元素, 迭代器类型: output_iterator_tag.  实现：

template <class _Container>
class back_insert_iterator {
protected:
  _Container* container;
public:
  typedef _Container          container_type;
  typedef output_iterator_tag iterator_category;
  typedef void                value_type;
  typedef void                difference_type;
  typedef void                pointer;
  typedef void                reference;

  explicit back_insert_iterator(_Container& __x) : container(&__x) {}
  back_insert_iterator<_Container>&
  operator=(const typename _Container::value_type& __value) {
    container->push_back(__value);
    return *this;
  }
  back_insert_iterator<_Container>& operator*() { return *this; }
  back_insert_iterator<_Container>& operator++() { return *this; }
  back_insert_iterator<_Container>& operator++(int) { return *this; }
};

接收一个容器作为构造的参数。 重载operator=实现插入。重载其他几个迭代器的必须操作符， 但是不做任何处理， 用以满足迭代器要求。另有一个全局模板函数来生成此迭代器, 如下：

template <class _Container>
inline back_insert_iterator<_Container> back_inserter(_Container& __x) {
  return back_insert_iterator<_Container>(__x);
}

二. front_insert_iterator

除了是从前面而不是后面插入， 同back_insert_iterator基本差不多。生成函数：

template <class _Container>
inline front_insert_iterator<_Container> front_inserter(_Container& __x) {
  return front_insert_iterator<_Container>(__x);
}

三. insert_iterator

除了构造需要多传一个插入位置的参数外，和back_insert_iterator基本差不多。还有就是operator=有点差别, 因为每次插入之后， 插入位置就失效了， 所以需要修改插入位置：

  operator=(const typename _Container::value_type& __value) {
    iter = container->insert(iter, __value);
    ++iter;
    return *this;
  }

生成函数:

template <class _Container, class _Iterator>
inline
insert_iterator<_Container> inserter(_Container& __x, _Iterator __i)
{
  typedef typename _Container::iterator __iter;
  return insert_iterator<_Container>(__x, __iter(__i));
}

四. reverse_bidirectional_iterator

功能:  实现反向迭代。 迭代器类型: bidirectional_iterator_tag, 四个模板参数template <class _BidirectionalIterator, class _Tp, class _Reference,
          class _Distance>

其中为什么只传入引用类型， 却不需要传入指针类型。 我的理解是， 类型为_Tp, 那么指针类型就一定是_Tp*, 但是， 引用类型却不一定， 可能是某个其他的自定义类型，重载了operator*. 可以作为_Tp的引用类型传入， 只要对此类型解引用， 仍然可以得到值。从这个意义上讲， iterator也应当属于应用类型吧。实现：

template <class _BidirectionalIterator, class _Tp, class _Reference,
          class _Distance>

class reverse_bidirectional_iterator {
  typedef reverse_bidirectional_iterator<_BidirectionalIterator, _Tp,
                                         _Reference, _Distance>  _Self;
protected:
  _BidirectionalIterator current;
public:
  typedef bidirectional_iterator_tag iterator_category;
  typedef _Tp                        value_type;
  typedef _Distance                  difference_type;
  typedef _Tp*                       pointer;
  typedef _Reference                 reference;

  reverse_bidirectional_iterator() {}
  explicit reverse_bidirectional_iterator(_BidirectionalIterator __x)
    : current(__x) {}
  _BidirectionalIterator base() const { return current; }
  _Reference operator*() const {
    _BidirectionalIterator __tmp = current;
    return *--__tmp;
  }

 pointer operator->() const { return &(operator*()); }

  _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;
  }

因为current指向的位置并不是当前元素的位置，而是当前元素的下一个元素， 所以， operator*中的实现， 返回的是*--__tmp. 由于是双向迭代器， 所以不仅实现了++, 还实现了--. 然后是一个迭代器的判等。operator->的含义是调用operator*得到当前所指向的值，然后对值取引用， 得到指向那个指的引用。

template <class _BiIter, class _Tp, class _Ref, class _Distance>
inline bool operator==(
    const reverse_bidirectional_iterator<_BiIter, _Tp, _Ref, _Distance>& __x,
    const reverse_bidirectional_iterator<_BiIter, _Tp, _Ref, _Distance>& __y)
{
  return __x.base() == __y.base();
}

五. reverse_iterator

是一个以迭代器为模板参数的模版类， 实现了反向的迭代功能。 operator操作同reverse_bidirectional_iterator的实现. reverse_iterator至少是一个双向迭代器，也有可能是一个随机迭代器。 所以还实现了+=, -=, -, +, []等operator, 并且实现了更多的比较操作。 reverse_iterator是最新的实现， reverse_bidirectional_iterator和另一个随机反向迭代器是原来的实现， 为不能模版特化的版本实现的。

实现：

template <class _Iterator>
class reverse_iterator
{
protected:
  _Iterator current;
public:
  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;
  typedef reverse_iterator<_Iterator> _Self;

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; }
  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 */

  _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); } 
};
 
template <class _Iterator>
inline bool operator==(const reverse_iterator<_Iterator>& __x,
                       const reverse_iterator<_Iterator>& __y) {
  return __x.base() == __y.base();
}

template <class _Iterator>
inline bool operator<(const reverse_iterator<_Iterator>& __x,
                      const reverse_iterator<_Iterator>& __y) {
  return __y.base() < __x.base();
}

#ifdef __STL_FUNCTION_TMPL_PARTIAL_ORDER

template <class _Iterator>
inline bool operator!=(const reverse_iterator<_Iterator>& __x,
                       const reverse_iterator<_Iterator>& __y) {
  return !(__x == __y);
}

template <class _Iterator>
inline bool operator>(const reverse_iterator<_Iterator>& __x,
                      const reverse_iterator<_Iterator>& __y) {
  return __y < __x;
}

template <class _Iterator>
inline bool operator<=(const reverse_iterator<_Iterator>& __x,
                       const reverse_iterator<_Iterator>& __y) {
  return !(__y < __x);
}

template <class _Iterator>
inline bool operator>=(const reverse_iterator<_Iterator>& __x,
                      const reverse_iterator<_Iterator>& __y) {
  return !(__x < __y);
}

#endif /* __STL_FUNCTION_TMPL_PARTIAL_ORDER */

template <class _Iterator>
inline typename reverse_iterator<_Iterator>::difference_type
operator-(const reverse_iterator<_Iterator>& __x,
          const reverse_iterator<_Iterator>& __y) {
  return __y.base() - __x.base();
}

template <class _Iterator>
inline reverse_iterator<_Iterator>
operator+(typename reverse_iterator<_Iterator>::difference_type __n,
          const reverse_iterator<_Iterator>& __x) {
  return reverse_iterator<_Iterator>(__x.base() - __n);
}