再谈traits编程

正在学习STL源码，看到了traits编程技术，于是上网查了查，发现不用每次都定义类

比如：

/*特性萃取器*/
template <class unknown_class>
struct unknown_class_traits {
    typedef typename unknown_class::return_type return_type;
};

/*特性萃取器 —— 针对原生指针*/
template <class T>
struct unknown_class_traits<T*> {
    typedef T return_type;
};

/*特性萃取其 —— 针对指向常数*/
template <class T>
struct unknown_class_traits<const T*> {
    typedef const T return_type;
};

c++中已经有iterator_traits类，可以直接用，代码如下

#include <iostream>  
#include <typeinfo>
#include <iterator>
#include <vector>

using namespace std;  
 
class A
{
    public:
        A(int i=10)
        {
            k=i;    
        }
    private:
        int k;
};

template<class it>//pointer and value_type and reference
void fun(it)
{
    typedef typename iterator_traits<it>::pointer pointer;
       cout<<"pointer："<<typeid(pointer).name()<<endl;
        
    typedef typename iterator_traits<it>::value_type value_type;
    cout<<"value_type："<<typeid(value_type).name()<<endl;
    
    typedef typename iterator_traits<it>::reference reference;
    cout<<"reference："<<typeid(reference).name()<<endl;
}

template<class it>//iterator_category and difference_type
void func(it t1,it t2)
{
    typedef typename iterator_traits<it>::iterator_category  iterator_category; 
    cout<<"iterator_category："<<typeid(iterator_category).name()<<endl;
    
    typedef typename iterator_traits<it>::difference_type difference_type;
    cout<<"difference_type："<<typeid(difference_type).name()<<endl;
}

int main( )   
{  
    A *a=new A(6);
    fun(a);
    
    vector<int> vec(10);
    func(vec.begin(),vec.end());
    return 0;
}

msdn上的代码：

// iterator_traits.cpp  
// compile with: /EHsc  
#include <iostream>  
#include <iterator>  
#include <vector>  
#include <list>  
  
using namespace std;  
  
template< class it >  
void  
function( it i1, it i2 )  
{  
   iterator_traits<it>::iterator_category cat;  
   cout << typeid( cat ).name( ) << endl;  
   while ( i1 != i2 )  
   {  
      iterator_traits<it>::value_type x;  
      x = *i1;  
      cout << x << " ";  
      i1++;  
   };     
   cout << endl;  
};  
  
int main( )   
{  
   vector<char> vc( 10,'a' );  
   list<int> li( 10 );  
   function( vc.begin( ), vc.end( ) );  
   function( li.begin( ), li.end( ) );  
}  
\* Output:   
struct std::random_access_iterator_tag  
a a a a a a a a a a   
struct std::bidirectional_iterator_tag  
0 0 0 0 0 0 0 0 0 0   
*\

msdn链接：https://msdn.microsoft.com/zh-cn/library/zdxb97eh.aspx

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