重载ostream &operator<<(ostream &out, const Calculate &c) ;中, 第一个参数应该为const么?
const ostream &operator<<(const ostream &out, const Calculate &c){ out <<c.first <<" " <<c.second ; return out ;}....int main(){ Calculate c ; cout <<c <<endl ; // vs08这里会报错, 说"无法从“const std::ostream”转换为“std::ostream &”" return 0 ;}