编程声明一个SHAPE基类，再派生出RECTANGLE和CIRCLE类，二者都有GETAREA函数计算对象的面积。使用RECTANGLE类创建意个派生类SQUARE-Programming a SHAPE statement base class, RECTANGLE and CIRCLE send another type of birth, the two have GETAREA function of the size of the object. RECTANGLE use category is intended to create a derived class SQUARE