traits编程---萃取容器中迭代器的类型等

　　可以直接利用STL中定义好的traits_iterator来萃取

/*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()
        {
            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=);
    fun(a);

    vector<);
    func(vec.begin(),vec.end());
    ;
}

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<,'a' );
   list< );
   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  

*\  

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