figury dziedziczenie virtual void

#include <iostream>
#include <string>
using namespace std;
class figura
{
    public:
        virtual double pole()
        {

        };
};
    class kwadrat:public figura
    {
          private:
          int a;
          public:
                 kwadrat()
                 {
                 a=0;
                 };

                 double pole(int a)
                 {
                        int kwadrat=a*a;
                        return kwadrat;
                        };
                        };



        class prostokat:public figura
    {
          public:
                 int a;
                 int b;
                 double pole(int a, int b)
                 {
                        int prostokat=a*b;
                        return prostokat;
                        };
                        };


        class kolo:public figura
    {
          public:
                 int r;
                 double pole(int r)
                 {
                        double kolo=3.14*r*r;
                        return kolo;
                        };};



            class romb:public figura
    {
          public:
                 int e;int f;
                 double pole(int e, int f)
                 {
                        int romb=e*f;
                        return romb;
                        };};

int main(int argc, char *argv[])
{
    int a,b,r,e,f;
    kwadrat kwa1;
    prostokat pr1;
    kolo ko1;
    romb ro1;
    cout << "Kwadrat\n";
    cin >> a;

    cout << kwa1.pole(a);
    cout<<endl;
    cout << "Prostokat\n";
    cin >> a;
    cin >> b;
    cout << pr1.pole(a,b);
    cout << "\n";
    cout << "Kolo, podaj promien\n";
    cin >> r;
    cout << ko1.pole(r);
    cout << "\n";
    cout << "romb\n";
    cin >> e;
    cout << "\n";
    cin >> f;

    cout << ro1.pole(e,f);
     cout << "\n";
    system("PAUSE");
    return EXIT_SUCCESS;
}