写一个不能被继承的类（友元的不可继承性）

class A
{
public:
	friend T;
private:
	A() {}
	~A() {}
};

class B:virtual public A<B>
{
public:
	B() {}
	~B() {}
};

class C :public B
{
public:
	C() {}  // 报错，A<T>::A(T = B)不可访问
	~C() {}
};