一个日期类（年,月，日）。通过重载构造函数：实现用三个整数或一个整数初始化日期对象；定义成员函数：实现计算某一日期是当年的第几天；定义另一个成员函数判断当年是否为闰年。-A date class (year, month, day). Overloaded constructor: objects with three integer or an integer initialization date　defined member functions: calculate a date of the first few days of the year　define another member function to determine the year is a leap year.