A
istream &operator<<(ostream & input,类名&对象名){...return input;}
B
istream &operator<<(ostream & output,类名&对象名){... return input;}
C
ostream &operator<<(ostream & intput,类名&对象名){... return output;}
D
ostream &operator<<(ostream & output,类名&对象名){...return output;}