何使用派生类指针指向基类，即downcast向下转型

基类指针指向派生类，我们已经很熟了。假如我们想用派生类反过来指向基类，就需要有两个要求：1）马克-to-win：基类指针开始时指向派生类，2）我们还需要清清楚楚的转型一下。

if you want to use derived class pointe

point to base class, there are two requirements:

1) base class pointer is initially the type of the derived class like Animal a2 = new Dog();

2) 马克-to-win：we still need to explicitly cast it like Dog d = (Dog)a2;

更多请见：https://blog.csdn.net/qq_44639795/article/details/103117238