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

/*
DERIVED must be publicly derived from class base
and must have a member function named implementation.
Class DERIVED can have only at most one class base among its ancestors.
interface can be called only when the base object is part of a DERIVED object.
*/

template <class DERIVED>
class base {
public:
	void interface() const {
		static_cast<const DERIVED *>(this)->implementation();
	}
};

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

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

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

	derived2 d2;
	d2.interface();

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

	return EXIT_SUCCESS;
}