Delegation, Objekt in Objekt

[HarryMason]

Wie behebt man diesen Fehler ???

#include <iostream>
#include <string>
#include <sstream>
using namespace std;

class Date {
private :
   int _day, _month, _year;
   DateFormat *df;
public :
   Date(int day,int month,int year) {
      _day = day;
      _month = month;
      _year = year;
   }
   string toString() {
      return df->format(this);
   }
   int getDay() { return _day; }
   int getMonth() { return _month; }
   int getYear() { return _year; }     
};

class DateFormat {
public:
   virtual string format(Date *d) {
     ostringstream os;
     os << d->getDay() << ".";
     os << d->getMonth() << ".";
     os << d->getYear();
     return os.str();
   }
};

int main() {
  Date d1(22,3,2000);
  cout << d1.toString();
  getchar();  
  return 0;  
}

Hiernochmal bei codepad : http://codepad.org/O1J9Vt0L

[out]

Das ist Murx. DateFormat will Date benutzen und vice versa. Mach das Design nochmals neu.