Question

I need a way for a single variable to represent two kinds of objects derived from the same base class.

It’s kinda hard to describe but I’ll try the best:

Say the base class:

class Rectangle
{
   float w;
   float h;
   const float area() {return w*h;}
};

And the two derived classes:

class Poker : Rectangle
{
    int style;  // Diamond, Club, ....
    int point;  // A~10, J, Q, K
};

class BusinessCard : Rectangle
{
    string name;
    string address;
    string phone;
};

Now is it possible to declare an object, which could be either a poker or a business-card?

‘cuz the usage below is illegal:

Rectangle* rec;
rec = new Poker();
delete rec;
rec = new BusinessCard();

Polymorphism might be a way but since it’s only good for changing base-class’ member attributes, I need this object to be able to represent exactly either of the derived objects.

EDIT:

Thanks for the all the answers. The public inheritance , the virtual destructor and even the boost::variant typedef are all fantastic hints.

Answer 1

You can do that. The problem is the inheritance modifier for classes is private. Most of the time, private inheritance is not what you want to use. Instead, declare it explicitly as public:

class Rectangle
{
   float w;
   float h;
   const float area() {return w*h; }; // you missed a semicolon here, btw
   virtual ~Rectangle() { } // to make `delete` work correctly
};

class Poker : public Rectangle // note the public keyword
{
    int style;  // Diamond, Club, ....
    int point;  // A~10, J, Q, K
};

class BusinessCard : public Rectangle 
{
    string name;
    string address;
    string phone;
};

Then your code snippet should work.