虚函数实现动态多态性，设计一个类CShape，并在CShape的基础上有继承类圆CCircle、矩形CRect和椭圆CEll。-Virtual functions to achieve dynamic polymorphism, to design a class CShape, and CShape the basis of a derived class round CCircle, rectangular and oval CRect CEll.

用C++实现一个具体事例，形状作为基类，正方形，三角形等等作为派生类。来体现类的多态性，其中涉及多态，继承，虚函数的使用。-using a C++ example to illustrate polymorphism, virtual function and etc. The example has a base class of shape, and many concrete shapes as derived classes.