Calling a member function from a base class

Question

I was reading about delegation and I wanted to able to call with a base class a any function pass as parameter depending on the event, I did it this way and it works but I am not sure if it is the proper way and if it is portable like this.

class base {
 public: 
     typedef void (base::*methodPTR)();
     methodPTR pfn;

    void setMethod(methodPTR fn)
    {
        pfn = fn;
    }
    void run(){
        if(pfn) (this->*pfn)();
    }
};

class a : public base {
 public:
     void fn()
     {
         cout<<"from class a!"<<endl;
     }
};

class b : public base
{
    a ob;
 public:
     void init()
     {
         ob.setMethod(static_cast<base::methodPTR>(&b::fn));
     }

     void fn()
     {
         cout << "from class b!" << endl;
     }

     void test()
     {
         ob.run();
     }
};

int _tmain(int argc, _TCHAR* argv[])
{
    b x;
    x.init();
    x.test();

    return 0;
}

Answer 1

You are calling the member function fn of class b with an instance of class a which will cause undefineded behaviour if you access data memebers of your class.

Replace your classes a and b with this to see the magic :

class a : public base {
    int j;
 public:
    a()
    {
        j = 42;
    }

     void fn()
     {
         cout<<"from class a!"<<endl;
     }
};

class b : public base
{
    int i;
    a ob;
 public:
     void init()
     {
         i = 5;
         ob.setMethod(static_cast<base::methodPTR>(&b::fn));
     }

     void fn()
     {
         cout << "from class b!" << endl;
         cout << "i = " << i << endl;
     }

     void test()
     {
         ob.run();
     }
};