Re: [Boost-users] [boost::thread] using futures

Subject: Re: [Boost-users] [boost::thread] using futures
From: Kazz (sfx810_at_[hidden])
Date: 2011-05-09 16:57:38

Thanks for quick reply.
Now I understand that you can only set value once for a promise. How can we
modify the existing example, that which ever worker thread has finished and
called the  set_value(), the next set_value() call gets ignored. One
solution  (plz correct me here if wrong)  could be to wrap both set_value()
with try catch block. Is there any other solution to get a final result from
multiple or thread pools and once the result is obtained, discarding further
calls to set_value() ?

On Mon, May 9, 2011 at 9:26 PM, Anthony Williams <anthony.ajw_at_[hidden]>wrote:

> Kazz <sfx810_at_[hidden]> writes:
>
> > Hi All. I am trying to use Futures, but not getting the expected result.
> The
> > code below works some times, and other times it throws exception
> >
> > "terminate called after throwing an instance of
> > 'boost::exception_detail::clone_impl
> >
> <boost::exception_detail::error_info_injector<boost::promise_already_satisfied>
> >>'
> >   what():  Promise already satisfied"
>
> That exception is thrown if you call set_value a second time on the same
> promise.
>
> > boost::promise<int> pt;    // holds final calculated result
> >
> > void worker1()
> > {
> >     pt.set_value(1);
> > }
> >
> > void worker2()
> > {
> >     pt.set_value(2);
> > }
>
> Both workers set the same promise. One of them will therefore throw, if
> they both get to the set_value. An uncaught exception in a thread will
> terminate the application.
>
> > int main() {
> >     boost::shared_future<int> ans(pt.get_future());
> >
> >     boost::thread th1(worker1);
> >     boost::thread th2(worker2);
> >
> >     ans.wait();
> >
> >     std::cout<<ans.get()<<std::endl;
> >
> >     std::cout<<"exiting"<<std::endl;
> >
> >     return 0;
> > }
>
> Both threads are detached here, and then the process exits. If
> the second thread to call set_value is still sleeping, so hasn't yet got
> to the set_value call, it will be terminated normally as part of process
> exit. If the second thread gets to the set_value call before the process
> exits it will abort the process due to the unhandled exception.
>
> Anthony
> --
> Author of C++ Concurrency in Action     http://www.stdthread.co.uk/book/
> just::thread C++0x thread library             http://www.stdthread.co.uk
> Just Software Solutions Ltd       http://www.justsoftwaresolutions.co.uk
> 15 Carrallack Mews, St Just, Cornwall, TR19 7UL, UK. Company No. 5478976
>
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