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;
}

Related examples in the same category

1.	'Interface' for subclass
2.	Demonstrate virtual functons: list interface
3.	Virtual functions: interface class

Use virtual function to define interface. : Interface « Class « C++

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