描述： 定义表示形状的抽象类及相应的派生类，并实现相关操作符重载。



（1）定义表示形状的抽象类Shape：



添加公有成员函数double Area()，用于计算形状面积；定义为纯虚函数；



添加公有成员函数void Show()，用于显示形状信息，定义为纯虚函数；



定义虚的析构函数；



重载比较操作符：==、>和<，用于比较两个形状面积的大小关系，返回值类型为bool，可以定义为成员函数或友元函数。

-Descr iption: Definition represent the shape abstract class and derived class, and relevant operator overloading. (1) defines the shape abstract class Shape: add a public member function double Area (), is used to calculate the shape of the area　defined as a pure virtual function　Add public member function void Show () is used to display the shape information, defined as pure virtual function　defined virtual destructor　overloaded comparison operators: ==>　and < for the relationship of the size of the area of ​ ​ two shapes, the return value type bool can be defined as a member function or a friend function.