定义一个日期类DateType，它的数据成员y, m, d用来表示当前日期的年、月、日。设计该类实现的功能（相应的类成员函数）。-Definition of a date class DateType, its data member y, m, d is used to indicate the current date of the year, month, day. Designed to achieve the function of the class (the correspondi