【转】traits技术及模板偏特化

#include <iostream>
using namespace std;

struct __xtrue_type { }; // define two mark-type
struct __xfalse_type { };

class CComplexObject // a demo class
{
public:
     virtual void clone() { cout << "in clone" << endl; }
};

class CDerivedComplexObject : public CComplexObject // a demo derived class
{
public:
     virtual void clone() { cout << "in derived clone" << endl; }
};

// A general edtion of Traits
template <typename T>
struct Traits
{
     typedef __xfalse_type has_clone_method; // trait 1: has clone method or not? All types defaultly has no clone method.
};

// Specialized edtion for ComplexObject
template <>
struct Traits<CComplexObject>
{
     typedef __xtrue_type has_clone_method;
};

template <typename T>
class XContainer
{
     template <typename flag>
         class Impl
     {
     };
     template <>
         class Impl <__xtrue_type>
     {
     public:
         void clone(T* pObj)
         {
             pObj->clone();
         }
     };
     template <>
         class Impl <__xfalse_type>
     {
     public:
         void clone(T* pObj)
         {
         }
     };
public:
     void clone(T* pObj)
     {
         Impl<Traits<T>::has_clone_method>().clone(pObj);
     }
};

void main()
{
     int* p1 = 0;
     CComplexObject c2;
     CComplexObject* p2 = &c2;
     CDerivedComplexObject c3;
     CComplexObject* p3 = &c3; // you must point to a derived object by a base-class pointer,
                             //it's a little problem

     XContainer<int> n1;
     XContainer<CComplexObject> n2;
     XContainer<CComplexObject> n3;

     n1.clone(p1);
     n2.clone(p2);
     n3.clone(p3);
}

摘自：http://www.cppblog.com/woaidongmao/archive/2008/11/09/66387.html