定义一个学生类student，学生类中有3个私有数据成员：num(学号)、name(姓名)、age(年龄)；2个公有成员函数：display、paixu。使用paixu函数按照姓名的第一个字母从小到大排序，display负责显示学生的基本信息。 -Define a student class student, the student class has three private data members: num (student number), name (name), age (the age of)　two public member functions: display, paixu. Use paixu function according to the first letter of the name of small to large, display student is responsible for displaying basic information.