Question

#include<iostream>
using namespace std;

class Abs
{
        public:
        virtual void hi()=0;
};

class B:public Abs
{
        public:
        void hi() {cout<<"B Hi"<<endl;}
        void bye() {cout<<"B Bye"<<endl;}
};
class C:public Abs
{
        public:
        void hi() {cout<<"C Hi"<<endl;}
        void sayonara() {cout<<"C Sayonara"<<endl;}
};

int main()
{
        Abs *bb=new B;
        bb->bye();
        Abs *cc=new C;
        cc->sayonara();
}//main

The compiler says

test2.cpp: In function ‘int main()’:
test2.cpp:26: error: ‘class Abs’ has no member named ‘bye’
test2.cpp:28: error: ‘class Abs’ has no member named ‘sayonara’

Because of this problem, I’ll have to add functions to the Abs class each time I create a new derived class which inherits from it (Upcasting is compulsory for me to do. The program I’m planning requires it to be so). I don’t want to touch the base class once it’s created.
Doesn’t this problem violate the principle that once you make a base class, you won’t have to modify it ever. Any way to resolve this problem?
p.s: I’ve seen the factory design pattern and the prototype design patterns, but both of them can’t seem to be able to solve it.

Answer 1

Well, i’m not sure to understand exactly what you want (and why you want it that way) but:

int main()
{
        Abs *bb=new B;
        static_cast<B*>(bb)->bye();
        Abs *cc=new C;
        static_cast<C*>(cc)->sayonara();
}//main

Will work.

You just have to be sure that bb is really a B* before you static_cast.

You may also use dynamic_cast which will return a null pointer if bb is not of the correct type.