Program code demonstrating use of abstract class in C++

#include<iostream>using namespace std;class abstractinterface {   public:   virtual void numbers()=0;   void input();   int a, b; };void abstractinterface::input()  {    cout<< "Enter the numbersn";    cin>>a>>b;  }class add : public abstractinterface {  public:  void numbers()   {     int sum;     sum=a+b;     cout<<"sum is "<<sum<<"n";   } };class sub : public abstractinterface {   public:   void numbers()    {     int diff;     diff=a-b;     cout<<"diff is "<<diff<<"n";    } };int main() {  add obj1;  obj1.input();  obj1.numbers();  sub obj2;  obj2.input();  obj2.numbers();return 0; }