'operator <<' is ambiguous

'operator <<' is ambiguous

class Time{
public:
  int hour, minute, second;
public:
  void set(int h, int m, int s){ hour=h, minute=m, second=s; }
  friend Time& operator++(Time& a);
  friend Time operator++(Time& a, int);
  friend ostream& operator<<(ostream& o, const Time& t);
};

自定义类中重载操作符后，调用时，编译提示错误：'operator <<' is ambiguous

解决方法：
VC6.0 需要先声明类和重载操作符
class Time;
ostream& operator<<(ostream& o, const Time& t);
VC 6.0 在SP3之前, 对iostream与friend有Bug, 需要patch----SP5。可能这样的错误与编译器有关吧。