Use virtual function to define interface.



#include <iostream>
using namespace std;

class area {
  double dim1, dim2; 
public:
  void setarea(double d1, double d2)
  {
    dim1 = d1;
    dim2 = d2;
  }
  void getdim(double &d1, double &d2)
  {
    d1 = dim1;
    d2 = dim2;
  }
  virtual double getarea()
  { 
    cout << "You must override this function\n";
    return 0.0;
  }
};

class rectangle : public area {
public:
  double getarea() 
  {
    double d1, d2;
    getdim(d1, d2);
    return d1 * d2;
  }
};

class triangle : public area {
public:
  double getarea()
  {
    double d1, d2;
  
    getdim(d1, d2);
    return 0.5 * d1 * d2;
  }
};

int main()
{
  area *p;
  rectangle r;
  triangle t;

  r.setarea(3.3, 4.5);
  t.setarea(4.0, 5.0);

  p = &r;
  cout << "Rectangle has area: " << p->getarea() << '\n';

  p = &t;
  cout << "Triangle has area: " << p->getarea() << '\n';

  return 0;
}

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4.	Virtual functions retain virtual nature when inherited.
5.	Virtual function and public inheritance
6.	Virtual function and three level inheritance
7.	Virtual function for two derived classes
8.	Class pointer and virtual function

Use virtual function to define interface. : Virtual « Function « C++

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