Question

I need to access the derived class member variable through Base class variable.

Class A{

};

Class B:public A {
  int data;  
};

now I need to do something like this

A *pb = new B()
pb->data = 10;

but the problem is I cannot access the Derived member class wihtout it.

and Yeah I know how to make it work with virtual functions.

Thanks, I really appreciate your help.

Answer 1

Short answer: You cannot. Because your compiler does not know what pb is. it could be of type A. However, you an use dynamic_cast, which returns a B pointer or NULL if that is not possible.

A *pa = new B();
B *pb = dynamic_cast<B*>(pa);
if (pb) {
    pb->data = 10;
}
else {
    ...
}

Anyhow, if you need to do that it probably means that you should revise your design as upcasting is not a good idea. Sometimes though, you just cannot avoid it. E.g. when using external libraries and such.