Re: [asio-users] structured programming with asio

[Stephan M. <sm...@gm...>]

Am Donnerstag, 29. M=E4rz 2007 16:01:02 schrieb Christopher Kohlhoff:
> Can you illustrate this with a bit of pseudocode? I think it may be
> possible to do something similar using multiple io_service objects, but
> I'd like to see how you think the deadline_timer solution would work
> before I can say for sure.

Hi Chris,

I managed to get it to work as I described. I'm not so good in Pseudocode b=
ut=20
I'll try to simplify on my current working example of an async DNS query=20
inside an object:

class DNS: public boost::enable_shared_from_this<DNS> {
public:
   typedef boost::shared_ptr<DNS> pointer;
   static pointer create(asio::io_service &iosrv) {
      return pointer(new DNS(iosrv));
   }
   asio::deadline_timer &getTimer(void) {return _timer;};

   void  do_query(const std::string &ip) throw () {
       asio::ip::tcp::resolver::query query(querystring, "");
       _resolver.async_resolve(query,
            boost::bind(&DNS::handleQuery, shared_from_this(),
            asio::placeholders::error, asio::placeholders::iterator));
   }
   void  handleQuery(const asio::error& err, asio::ip::tcp::resolver::itera=
tor=20
it) {
        // store the result to be accessed from the object later
        _timer.expires_from_now(boost::posix_time::seconds(0));
   }

private:      =20
   DNS(asio::io_service &iosrv) : _resolver(iosrv), _timer(iosrv) {};

   asio::ip::tcp::resolver  _resolver;
   asio::deadline_timer     _timer;
}


class UsingClient {
public:
    UsingClient : _dns(DNS::create(_iosrv)) {};
   =20
    someCallbackStartingTheQuery() {
      _dns->do_query(domain);        // tell the _dns to start the query
      // when finished, this timer will be our trigger
      _dns->getTimer().async_wait(
          boost::bind(&UsingClient::someOtherCallbackFinishing,
             shared_from_this(), asio::placeholders::error));
    }
    someOtherCallbackFinishing() {
         // retrieve the result from _dns object and continue
    }

private:
    boost::shared_ptr<DNS>  _dns;
}


Looking a bit complicated, but the key thing is, it works.=20
With -D_GLIBCXX_DEBUG switched off only though, due to the problem I report=
ed=20
yesterday.

Anyway, it did not work when UsingClient instantiates the DNS object direct=
ly=20
rather than using the shared ptr for unknown reason. I always got a=20
bad_weak_ptr exception when doing the async_wait on the timer. Apparently t=
he=20
object is being moved or destroyed on the way or something. Couldn't figure=
=20
it out, so I'm using the apparently unnecessary shared_ptr to keep at least=
 a=20
reference count of one to the DNS object.

My problem is: All that looks kinda weird and complicated for a seemingly=20
simple thing so I thought there might be a more simple and obvious approach=
=20
I'm missing here.
I hope I could clearify this a bit.

Greetings...

Stephan