iterator_traints原理

algorithm.h中的distance函數原型如下：

    template <class Iterator>
    typename iterator_traints<Iterator>::difference_type distance(Iterator first,Iterator last){
        typedef  typename iterator_traints<Iterator>::iterator_category category;
        return __distance(first,last,category());//category is a struct
    }

靠iterator_traints而來的category來分別5種迭代器。

template <class T>
struct iterator_traints{
    typedef typename T::iterator_category iterator_category;
    typedef typename T::size_type size_type;
    typedef typename T::difference_type difference_type;

};

iterator_traints其實是一個模板類，可以得到傳入其中的類型的category,所以，迭代器中必須定義iterator_category，size_type，difference_type這三個東西。
iterator_category取值爲5個類中的一個

struct input_iterator_tag{};
struct output_iterator_tag{};
struct forward_iterator_tag:public input_iterator_tag{};
struct bidirectional_iterator_tag:public forward_iterator_tag{};
struct random_access_iterator_tag:public bidirectional_iterator_tag{};

所以，list的迭代器如下:

class __list_iterator{

    public:
        typedef  bidirectional_iterator_tag iterator_category;
        typedef size_t size_type;
        typedef ptrdiff_t  difference_type;
        __list_iterator(__list_node<T>* x):node(x){}
        __list_iterator<T>& operator++(){//前置++
            node=node->next;
            return *this;
        }
        __list_iterator<T>& operator++(int){
            __list_iterator<T>tmp=*this;
            ++*this;
            return tmp;
        }
        __list_iterator<T>& operator--(){//前置++
            node=node->pre;
            return *this;
        }
        __list_iterator<T>& operator--(int){
            __list_iterator<T>tmp=*this;
            --*this;
            return tmp;
        }
        T& operator*(){return node->data;}
        bool operator==(const __list_iterator &x)const{return node==x.node;}
        bool operator!=(const __list_iterator &x)const{return node!=x.node;}
        __list_node<T>*node;
    };

有上面可知，list的迭代器iterator_category爲bidirectional_iterator_tag，傳入distance之後，iterator_traints得到的category爲bidirectional_iterator_tag,根據重載規則，會調用到

   template <class InputIterator>
    typename iterator_traints<InputIterator>::difference_type  __distance(InputIterator first,InputIterator last,input_iterator_tag){
        typename iterator_traints<InputIterator>::difference_type n=0;//here typename is not must need,because here difference must be a type
        std::cout<<"inputIterator"<<std::endl;
        while(first!=last){
            ++first;++n;
        }
        return n;
    }

爲什麼不是調用到__distance(InputIterator first,InputIterator last,bidirectional_iterator_tag),因爲bidirectional_iterator_tag繼承input_iterator，
由於有些迭代器是指針，而不是類，無法通過iterator_traints得到category，所以，可以偏特化實現指針形的

template <class T>
struct iterator_traints<T*>{//偏特化，生成指針版本
    typedef random_access_iterator_tag iterator_category;
    typedef size_t  size_type;
    typedef ptrdiff_t  difference_type;
};