反向迭代器

以list为例,我们完全可以再添加一个__list_reverse_iterator结构体,只需要修改++和–的逻辑

template <class T, class Ref, class Ptr>
struct __list_reverse_iterator
{...self& operator++(){_node = _node->_prev;return *this;}self operator++(int){self tmp(*this);_node = _node->_prev;return tmp;}self& operator--(){_node = _node->_next;return *this;}self operator--(int){self tmp(*this);_node = _node->_next;return tmp;}...
};
// 链表
template<class T>
class list
{
public:...typedef __list_reverse_iterator<T, T&, T*> reverse_iterator;reverse_iterator rbegin() { return _head->_prev; }reverse_iterator rend() { return _head; }...
}

但是这样代码重复度太高了!而且自定义类型的迭代器才可以这样用,所以SGI版本的STL使用了迭代器适配模式
创立一个reverse_iterator.h,弱化了源码的萃取版本

template<class Iterator, class Ref, class Ptr>
class reverse_iterator
{Iterator current;
public:typedef reverse_iterator<Iterator, Ref, Ptr> self;reverse_iterator(Iterator it) : current(it) {}Ref operator*(){// 这样做的原因是因为rbegin()最开始在越位位置Iterator tmp = current;--tmp;return *tmp;}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;}bool operator!=(const self& x){return current != x.current;}
};// 链表
template<class T>
class list
{
public:...typedef reverse_iterator<iterator, T&, T*> reverse_iterator;typedef reverse_iterator<iterator, const T&, const T*> const_reverse_iterator;reverse_iterator rbegin() { return reverse_iterator(end()); }reverse_iterator rend() { return reverse_iterator(begin()); }const_reverse_iterator rbegin() const { return const_reverse_iterator(end()); }const_reverse_iterator rend() const { return const_reverse_iterator(begin()); }...
}