#include <iostream>
#include <cstdlib>
using namespace std;

class base {
public:
	virtual ~base() {}
	virtual void implementation() const = 0;
	void interface() const {implementation();}
};

class derived1: public base {
public:
	void implementation() const {cout << "derived1\n";}
};

class derived2: public base {
public:
	void implementation() const {cout << "derived2\n";}
};

int main()
{
	derived1 d1;
	d1.interface();

	derived2 d2;
	d2.interface();

	cout << "sizeof (base) == " << sizeof (base) << "\n"
		<< "sizeof (derived1) == " << sizeof (derived1) << "\n"
		<< "sizeof (derived2) == " << sizeof (derived2) << "\n";

	return EXIT_SUCCESS;
}