#include <iostream>
using namespace std;
 
struct bb {
	virtual void a() {cout << "bb-a" << endl; }
	virtual void b() {cout << "bb-b" << endl; }
};
 
struct d1 : public bb {
	virtual void a() {cout << "d1-a" << endl; }
private:
	virtual void b() {cout << "d1-b" << endl; }
};
 
struct d2 : public bb {
	virtual void b() {cout << "d2-b" << endl; }
private:
	virtual void a() {cout << "d2-a" << endl; }
};
 
int main() {
	d1 x1;
	x1.a();
	// x1.b(); // <<== This will not compile
	d2 x2;
	// x2.a(); // <<== This will not compile
	x2.b();
	bb &bx1(x1);
	bx1.a();
	bx1.b();
	bb &bx2(x2);
	bx2.a();
	bx2.b();
	return 0;
}

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