STL源码剖析中 traits的使用

#include <iostream>using std::cout;using std::endl;//归根到底就是使用了function template的参数推导机制(argument deduction)//traits函数 生成一个该类型的临时对象，然后返回该对象//而真正的调用函数根据临时对象的类型决定调用哪个函数，当然这些函数是重载的，只是根据类型不同调用不同的重载函数而已////////迭代器的类型，某些函数根据其类型选择迭代方式struct input_iter {};struct forward_iter : public input_iter {};struct random_access_iter : public forward_iter {};template<class Category, class T, class Distance = ptrdiff_t, class Pointer = T*,class Reference = T&>class iter{typedef Category iterator_category;   //迭代器的类型typedef T value_type;                 //迭代器中的值的类型typedef Distance difference_type;     //typedef Pointer pointer;typedef Reference reference;};//根据Iterator萃取其类型template<class Iterator>struct iter_traits{typedef typename Iterator::iterator_category iterator_category;typedef typename Iterator::value_type value_type;typedef typename Iterator::distance_type distance_type;typedef typename Iterator::pointer pointer;typedef typename Iterator::reference reference;};//针对原生指针(naive pointer)的traits的偏特化版template <class T>struct iter_traits<T*>{typedef random_access_iter iterator_category;  //原生指针应该是随机存取的迭代器typedef T value_type;typedef ptrdiff_t distance_type;typedef T* pointer;typedef T& reference;};template <class T>struct iter_traits<const T*>{typedef random_access_iter iterator_category;typedef T value_type;typedef ptrdiff_t distance_type;typedef T* pointer;typedef T& reference;};//根据Iterator获取其迭代器类型template <class Iterator>inline typename iter_traits<Iterator>::iterator_category iter_category(const Iterator&){typedef typename iter_traits<Iterator>::iterator_category category;return category();}//根据Iterator获取其迭代器中值的类型template <class Iterator>inline typename iter_traits<Iterator>::value_type* iter_value_type(const Iterator&){return static_cast<typename iter_traits<Iterator>::value_type*>(0);}class A {};void fun(int *a){cout << "int *a " << endl;}void fun(A *a){cout << "class A" << endl;}int main(){A *pa = new A;int *a = 0;fun(iter_value_type(pa));fun(iter_value_type(a));return 0;}