class b2Body {
private:
	b2Body() {}
	friend class b2World;
};

class Body : public b2Body {
private:
	Body() {}
	friend class World;
};

class b2World {
public:
	b2Body *CreateBody() {
		return new b2Body;
	}
};

class World : public b2World {
public:
	Body *CreateBody() {
		return new Body;
	}
};

int main() {
	World w;
	Body *b = w.CreateBody();
	return 0;
}

t.cpp: In constructor 'Body::Body()':
Line 3: error: 'b2Body::b2Body()' is private
compilation terminated due to -Wfatal-errors.