traits编程技法利用了“内嵌型别”的编程技巧与编译器的template参数推导功能。
下面主要看看利用traits编程技法实现的迭代器萃取机制。
5种迭代器类型定义：
struct input_iterator_tag {};
struct output_iterator_tag {};
struct forward_iterator_tag : public input_iterator_tag {};
struct bidirectional_iterarot_tag : public forward_iterator_tag {};
struct random_access_iterator_tag : public bidirectional_iterarot_tag {};
std::iterator的定义
template <class Category, class T, class Distance = ptrdiff_t,
class Pointer = T*, class Reference = T&>
struct iterator {
typedef Category iterator_category;
typedef T value_type;
typedef Distance difference_type;
typedef Pointer pointer;
typedef Reference reference;
};
如果想和STL协同工作，自行定义的迭代器必须定义iterator_category、value_type、difference_type、pointer、reference这五大内嵌型别。为了方便，STL定义了std::iterator这个基类，自定义的迭代器只要继承这个基类
就可以了。但要注意的是，并不是STL中的所有迭代器都继承了这个基类，比如_deque_iterator。
"榨汁机"traits
template <class Iterator>
struct iterator_traits {
typedef typename Iterator::iterator_category iterator_category;
typedef typename Itetator::value_type value_type;
typedef typename Iterator::difference_type difference_type;
typedef typename Iterator::pointer pointer;
typedef typename Iterator::reference reference;
};
由于原生指针也是一种迭代器，但是原生指针没有内嵌型别，因此，iterator_traits需要为原生指针提供特化版本。
template <class T>
struct iterator_traits<T*> {
typedef random_access_iterator_tag iterator_category;
typedef T value_type;
typedef ptrdiff_t difference_type;
typedef T* pointer;
typedef T& reference;
}; template <class T>
struct iterator_traits<const T*> {
typedef random_access_iterator_tag iterator_category;
typedef T value_type;
typedef ptrdiff_t difference_type;
typedef const T* pointer;
typedef const T& reference;
};