Re: [Boost-users] Why does boost thread copy a functor?

Subject: Re: [Boost-users] Why does boost thread copy a functor?
From: Chris Cleeland (chris.cleeland_at_[hidden])
Date: 2012-01-15 20:36:10

Lars,

Thanks for the explanation and suggestion! It makes sense.

In my case, the main thread is the long-lived thread, while the boost
thread is not. Moreover, there is state in the Task that is intended to be
shared between both threads.

I wonder if there's a way to make the copying more explicit? Had it not
been for the unexpected behavior of the file descriptor being closed, the
copying notion would have been even more subtle--and my misunderstanding
even harder to find.

On Sunday, January 15, 2012, Lars Viklund <zao_at_[hidden]> wrote:
> On Sun, Jan 15, 2012 at 06:25:52PM -0600, Chris Cleeland wrote:
>> The spawned thread gets an instance of Task, but it's not the same
instance
>> as from the main.
>>
>> What is the rationale?
>
> Functors in general should be expected to be copied, so it might not be
> an explicit decision.
>
> It is, however, a good decision. The thread of execution generally
> outlives the scope where the thread is started. If the thread used a
> reference to the functor, then pretty much _all_ cases would force the
> user to store away function objects elsewhere and destroy them only when
> the thread has terminated completely.
>
> It would also prevent the use of temporaries as thread callables.
>
> If you want to get reference semantics, use a reference_wrapper.
>
> boost::thread t(boost::ref(task)); will have the reference semantics you
> crave.
>
> --
> Lars Viklund | zao_at_[hidden]
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-- 
Chris Cleeland