重载运算符operator>>

class ABC{

int num;

string str;

public:

//重载运算符operator>>

};

ABC abc;

ifstream in("a.txt");

in>>abc;

*********** 该代码段如何写？ **************

声明：
friend istream& operator >> ( istream& is, ABC& d );

定义：
istream& operator >> ( istream& is, const ABC& d )
{
return is >> d.num >> d.str;
}