Date_Time(继承)

#include "iostream"using std::cin;using std::cout;using std::endl;using std::istream;using std::ostream;class Time{protected:  int hour;  int minute;  int second;public:  Time(int h = 0, int m = 0, int s = 0)  :hour(h), minute(m), second(s)  {    int s1 = second / 60;    second = second % 60;    minute = minute + s1;    int m1 = minute / 60;    minute = minute % 60;    hour = hour + m1;    hour = hour % 24;  }  Time(const Time &t)  :hour(t.hour), minute(t.minute), second(t.second){}  friend ostream& operator << (ostream &output,const Time &t);  friend istream& operator >> (istream &input, Time &t);};class Date{protected:  int year;  int month;  int day;public:  Date(int y = 0, int m = 0, int d = 0)  :year(y), month(m), day(d)  {}  friend ostream& operator << (ostream &output, const Date &d);  friend istream& operator >> (istream &input, Date &d);};class DateTime : public Date, public Time{public:  DateTime(int y = 0, int m = 0, int d = 0, int h = 0, int mi = 0, int s = 0)  :Date(y, m, d), Time(h, mi, s)  {}  friend ostream& operator << (ostream &output, const DateTime &d);  friend istream& operator >> (istream &input, DateTime &d);  DateTime operator + (const DateTime &d);  DateTime operator - (const DateTime &d);};ostream& operator << (ostream &output,const Time &t){  output << t.hour <<" h " << t.minute << " m " << t.second << " s";  return output;}istream& operator >> (istream &input, Time &t){  input >> t.hour >> t.minute >> t.second;  if(t.hour < 0) t.hour = 24 - (-t.hour % 24);  if(t.minute < 0) t.minute = 60 - (-t.minute % 60);  if(t.second < 0) t.second = 60 - (-t.second % 60);  int s1 = t.second / 60;  t.second = t.second % 60;  t.minute = t.minute + s1;  int m1 = t.minute / 60;  t.minute = t.minute % 60;  t.hour = t.hour + m1;  t.hour = t.hour % 24;  return input;}ostream& operator << (ostream &output, const Date &d){  output << d.year << " year " << d.month << " month " << d.day << " day";  return output; }istream& operator >> (istream &input, Date &d){  cin >> d.year >> d.month >> d.day;  return input;}ostream& operator << (ostream &output, const DateTime &d){  output << (Date) d << " ";  output << (Time) d;  return output;}istream& operator >> (istream &input, DateTime &d){  input >> d.year >> d.month >> d.day >> d.hour >> d.minute >> d.second;  return input;}DateTime DateTime::operator - (const DateTime &d){  return DateTime(year - d.year, month - d.month, day - d.day,hour - d.hour, minute - d.minute, second - d.second);}int main(int argc, char const *argv[]){  DateTime a(2015, 3, 2, 15, 23, 12);  DateTime b(2015, 5, 6, 19, 27, 30);  cout << "DateTime a = " << a << endl;  cin >> a;  cout << "DateTime a = " << a << endl;  cout << "DateTime b = " << b << endl;  cout << "b - a = " << b - a << endl;  return 0;}