virtual void watch_fd(int fd, poll_flag flag) = 0;
virtual bool unwatch_fd(int fd, poll_flag flag) = 0;
virtual bool is_watched(int fd, poll_flag flag) = 0;
virtual void watch_fd(int fd, poll_flag flag) = 0;
virtual bool unwatch_fd(int fd, poll_flag flag) = 0;
virtual bool is_watched(int fd, poll_flag flag) = 0;