typename _Pointer=_Tp*, typename _Reference=_Tp&>
struct iterator
{
typedef _Category iterator_category; // 有很多不同种类的iterator，
// 比如下面的reverse_iterator，random_access_iterator等等
typedef _Tp value_type;
typedef _Distance difference_type; // 计算两个iterator之间的相对值
typedef _Pointer pointer;
typedef _Reference reference;
}
// Traits只是将typedef转向到_Iterator上；然后定义了2个偏特化，为了在模板实例化时找到更好的匹配。
template <typename _Iterator>
struct iterator_traits
{
typedef typename _Iterator::iterator_category iterator_category;
typedef typename _Iterator::value_type value_type;
typedef typename _Iterator::difference_type difference_type;
typedef typename _Iterator::pointer pointer;
typedef typename _Iterator::reference reference;
};
template <typename _Tp>
struct iterator_traits<_Tp*>
{
typedef random_access_iterator_tag iterator_category; // 注意这个地方，iterator的类型不是primary template
// 中的iterator::iterator_category。
typedef _Tp value_type;
typedef ptrdiff_t difference_type;
typedef _Tp* pointer;
typedef _Tp& reference;
};
template <typename _Tp>
struct iterator_traits<const _Tp*>
{
typedef random_access_iterator_tag iterator_category;
typedef _Tp value_type;
typedef ptrdiff_t difference_type;
typedef const _Tp* pointer;
typedef const _Tp& reference;
};
template <typename _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>
{
protected:
_Iterator current;
public:
// 实际上，接下来的typedef，我认为是没有有必要的，因为它跟其继承自iterator里的typedef是一样的。
// 但是STL作为一个具有工业强度的类库，难道会包含无用的代码？
// 无解
typedef _Iterator iterator_type;
typedef typename iterator_traits<_Iterator>::difference_type difference_type;
typedef typename iterator_traits<_Iterator>::reference reference;
typedef typename iterator_triats<_Iterator>::pointer pointer;
public:
… // other implementations.
};
Storm - Be Nerdy 