定义两个类，techer类和student类，将student类对象转换为techer类对象，假设学生毕业后留校，其相关 数据仍然有用，如编号num，姓名name，性别sex。转换后输出相关数据-Define two classes, techer class and student class, the student class object is converted to techer class object, assuming that the students stayed af