Is there no way to upcast into an abstract class and not modify it each time a class is derived from it?

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

This is defeating the purpose of inheritance and abstract interfaces. bye and sayonara both do the same thing (saying goodbye), only in different languages. This means you should have an abstract say_goodbye method that gets overridden for subclasses. I suppose this is a simplified example, so maybe you could describe your actual scenario so we can provide more specific help.

Edit If you want to create a copy of the derived class through an abstract interface, check out this question. If you want to explicitly access the different attributes of your subclasses, you should be asking your self if subclassing es even appropriate here, since your classes don't seem to have much in common.