class A;

class Lock {
	friend class A; //have friendship here, so that A can access private Lock() {} in initializing
			//but in class B, Lock() is not accessable
private:
	Lock() {};
};

class A: virtual public Lock { //virtual public is used, so that A can not be inherited
			       //virtual public is used, so base class (class Lock)'s initialization would be depolyed
			       //if class Lock { private:Lock() {}}; is commented, this would be ok
public:
	A() {};
	A(int n) {};
};

class B: public A { //Point is that Lock作为虚基类；在继承体系中B是最远派生类，初始化时候要负责调用Lock的初始化函数，但是Lock::Lock()是private，所以无法被调用
public:
	B() {};
};