Re: [boost] [thread] using Boost.Thread with move-only arguments

Subject: Re: [boost] [thread] using Boost.Thread with move-only arguments
From: Vicente J. Botet Escriba (vicente.botet_at_[hidden])
Date: 2013-04-04 02:19:18

Le 04/04/13 04:40, Fernando Pelliccioni a écrit :
> Hi Vicente,
>
> I have an issue with Boost.Thread contructor using move-only type arguments.
>
> The "Thread Constructor with arguments" section of the documentation says
> ...
>
> template <class F,class A1,class A2,...>
> thread(F f,A1 a1,A2 a2,...);
>
> Preconditions:
> F and each An must by copyable or movable.
>
> http://www.boost.org/doc/libs/1_53_0/doc/html/thread/thread_management.html#thread.thread_management.thread.multiple_argument_constructor
>
> "An" could be copyable or movable, but the following example fails to
> compile...
>
> error: use of deleted function 'A::A(const A&)'
>
Sorry,  if the documentation is not clear.  I should add a link to the 
Move emulation section 
(http://www.boost.org/doc/libs/1_53_0/doc/html/thread/emulations.html#thread.emulations.move) 
for each function having rvalue references if the documentation is not 
clear. Please could you create a Ticket for that?

Which Boost.Thread version are you using? That is, BOOST_THREAD_VERSION 
is 2, 3 or 4?
Could you try by defining BOOST_THREAD_USES_MOVE, BOOST_THREAD_VERSION 3 
or BOOST_THREAD_VERSION 4?
>
>
>
> I think we should modify ...
>
>      template <class F,class A1>
>      thread(F f,A1 a1,typename
> disable_if<boost::thread_detail::is_convertible<F&,thread_attributes >,
> dummy* >::type=0):
>          thread_info(make_thread_info(boost::bind(boost::type<void>(),f,a1)))
>      {
>          start_thread();
>      }
>
>
> ... by ...
>
>      template <class F,class A1>
>      thread(F&& f,A1&& a1,typename
> disable_if<boost::thread_detail::is_convertible<F&,thread_attributes >,
> dummy* >::type=0):
>
> thread_info(make_thread_info(boost::bind(boost::type<void>(),f,boost::forward<A1>(a1))))
>      {
>          start_thread();
>      }
>
> ( In the all family of related ctors in "boost/thread/detail/thread.hpp"  )
>
> But, I don't know if boost::bind is forwarding its arguments
No. The problem is that boost::bind doesn't supports move semantics 
(Boost.Move).
> I think we should modify Boost.Bind to be similar to std::bind.
> Is there any reason why Bind has not changed so far? Have you found any
> limitations?
IIRC, I requested the feature long-long time ago.
>
> If Bind can not be changed, I think Boost.Thread should use some other tool
> similar to Bind to make forwarding parameters.
> GCC Libstdc++ use a Bind-lite tool for std::thread.
I have taken in account some cases, but not everything works yet. E.g 
the prototype that you need is

         template <class F>
         explicit thread(BOOST_THREAD_RV_REF(F) f
         , typename disable_if<is_same<typename decay<F>::type, thread>, 
dummy* >::type=0
         );

I suspect that you need to follow the instructions given in 
http://www.boost.org/doc/libs/1_53_0/doc/html/thread/emulations.html#thread.emulations.move.portable 
to make your class A movable :(
> I can work on this if you want.
> What do you think?
>
I'm all for a boost::bind or boost::bind-lite supporting boost semantics 
(using Boost.Move). If you can work on this a propose a patch to boost 
bind I'm sure all the Boost community will thank you.
There are a lot of basic Boost libraries that need an update to support 
move semantics (as Tuple, Fusion, SmartPtr, Exception?) so that other 
libraries can built on them. Any effort on this sens will be much 
appreciated.

Please, let me know if you have some trouble after defining one of the 
following

#define BOOST_THREAD_USES_MOVE
#define BOOST_THREAD_VERSION 3
#define BOOST_THREAD_VERSION 4

Best,
Vicente