继承、多继承、虚继承的使用！ 如果要想使一个公共基类在派生类中只产生一个基类子对象，则必须将这个基类设定为虚基类。-inheritance, multiple inheritance, the use of virtual inheritance! If we want to make a public base class in the derived class only have a base class Object son, they would have to set the b