Re: [boost] [thread] boost::future::then

Subject: Re: [boost] [thread] boost::future::then
From: Vicente J. Botet Escriba (vicente.botet_at_[hidden])
Date: 2015-10-09 15:38:57

Le 09/10/15 09:43, Vladimir Prus a écrit :
>
> Hi,
>
> looking through docs for some other futures implementation [1], I see 
> this:
>
>     Why not use boost::future? #
>
>     At the time of writing, 1.53 (the first version with the requisite 
> features) was brand new, not well-tested,
>     and not available to [...] developers.
>     It is still a bit buggy/bleeding-edge
>     They haven't fleshed out the threading model very well yet, e.g. 
> every single then currently spawns a new
>     thread unless you explicitly ask it to work on this thread only, 
> and executor support was nonexistent
>     (and now, is still experimental).
>
> Now, 1.53 is ancient. Can anybody comment on these statements 
> regarding boost::future::thread state in 1.59?
The comment should be almost right, the first implementations had a lot 
of issues.
Even in 1.59 the feature is broken (at least respect to what the 
standard TS Concurrency says) as the future resulting from  ::then() 
blocks. If you admit that the resulting futures blocks the develop 
branch is OK for you (which includes some fixes respect to 1.59).

I have a branch 
(https://github.com/boostorg/thread/tree/feature/non_blocking_futures) 
that don't blocks on any future, but this will break async.
I have another branch that has copyable executors and non-blocking 
futures 
(https://github.com/boostorg/thread/tree/feature/make_executors_copyable).

I expect to have this branch merged for 1.60.

>
> 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?
>
You need the Executor interface.

::then(ex, f);

http://www.boost.org/doc/libs/1_59_0/doc/html/thread/synchronization.html#thread.synchronization.futures.reference.unique_future.then

Vicente