Re: [boost] Executor associated to the future continuations (was [thread] boost::future::then)

Le 10/10/15 15:26, Vicente J. Botet Escriba a écrit :

Le 10/10/15 07:57, Vladimir Prus a écrit :

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On 09-Oct-15 10:49 PM, Vicente J. Botet Escriba wrote:

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Le 09/10/15 15:44, Vladimir Prus a écrit :

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On 09-Oct-15 4:02 PM, Mikael Olenfalk wrote:

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On Fri, Oct 9, 2015 at 9:43 AM, Vladimir Prus wrote:

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In particular, suppose I have this:

// Thread 1

boost::promise<int> pi; boost::future<int> fi = pi.get_future();

// pass pi to thread 2 somehow

fi.then(....);

// Thread 2

pi.set_value(100);

And I want continuation to be executed in thread 1, assuming that thread 1 runs Qt event loop, allowing me to use QTimer::singleShot, or queued signal, or similar mechanism. How would I go about making boost::future do it? Has anybody done so?

... I might have misunderstood the question but I would assume a solution would be to build boost.thread with executors enabled[1]. And then wrap the QT event loop in an executor interface[2] and use the boost::future<T>::then(Executor&, ...) overload.

Mikael,

thanks for the response. I suppose it would work - although if I need to pass this executor any time I call 'then', it become rather awkward rather quickly. Well this is the current interface :) It would be nicer if promise could have an executor, and pass it to future and then to futures returned by 'then', so that I only need to to specify custom behaviour when creating a promise - that can be easily wrapped in a function.

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If your future is created with auto f = async(ex, fct)

then the call to

f.then(fct2);

would use the same executor ex.

Vicente,

thanks, that's good to know. What if I don't want to use async, but rather create promise/future directly? Can I specified executor when creating future? Not now. The executors interface is experimental, so we can consider adding a way to associate a future to an executor. The sources of a future are promise, packaged task, async and make_ready_future/make_exceptional_future.

I've not though too much on it but I believe that it is better to associate the Executor interface to the source instead of changing it directly on the future even if this multiplies the interfaces. This is because I would need to associate the Executor at creation time.

I'm not for providing a set_executor function, as this would imply that the shared state must always be aware of a possible executor and so the Executor should be type-erased which is more costly than needed.

We could add a set_executor function, but this function will work only if the shared state is allocated lazily, which I(m not sure it is mandated by the standard.

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Suppose I'm in UI thread, and I ask backend thread to do something. I would like for the work to be done in backend thread, and then for all continuations to be executed in UI thread. Or, more generically, in whatever thread has initialized the operation. I understand your use case, but what is wrong wrapping the back-end task with the executor that initialized it and use this executor every time you need it.

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So, if I implement a "run in UI thread" executor, I don't want async(ex, fct) to execute fct in UI thread.

async(ex, fct) will be executed on the thread(s) associated to ex, not the current thread. What am I missing.

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So, the function must be executed in different thread from all the continuations, and it would seem I'd need to something set executor on promise for that to work?

I will see how adding an Executor parameter to promise, packaged_task constructors and make_ready_future/make_exceptional_future could be implemented if this will solve your use case.

I've create https://svn.boost.org/trac/boost/ticket/11717 to track this feature request Vicente