upcasting : Assigning a pointer of a derived class type to a pointer of its base class type (将派生类类型指针赋值给基类类型指针

downcasting : Assigning a pointer of a base class type to a pointer of its derived class type. (将基类类型指针赋值给派生类类型指针)

注意这里的upcasting/downcasting的探讨范围是指针的赋值：且所谓向上向下方向是从右向坐看方向。

上转可不适用dynamic_cast而隐式转换

eg:GeometricObject *g = new Circle(1);

    Circle *c = new Circle(2);

    g = c; //Correct

下转必须显式执行

c = dynamic_cast(g);／／不可以直接赋值，进行隐式转换，只能dynamic_cast

 注意：基类与派生类的赋值互操作（对象，指针，引用）都遵从一个原则：可以截断，不可无中生有。

casting