声明一个Cat类，拥有静态成员catnum，记录Cat的个体数目；声明静态成员函数GetHowMany()，存取catnum。设计程序测试这个类，体会静态数据成员和静态成员函数的用法。(Declare a Cat class that has the static member catnum, the number of individuals that log Cat; declare the static member function GetHowMany (), and access catnum. The design program tests this class to understand the usage of static data members and static member functions.)