Re: [Boost-users] boost::thread::attributes reference overload?

Subject: Re: [Boost-users] boost::thread::attributes reference overload?
From: Vicente J. Botet Escriba (vicente.botet_at_[hidden])
Date: 2013-06-13 02:33:25

Le 13/06/13 07:16, Anthony Foiani a e'crit :
> Greetings.
>
> I have a class that wraps a boost::thread object, and I would like to
> specify certain thread attributes at construction time.
>
> My first attempt looked basically like this:
>
>      struct my_thread
>      {
>
>          static boost::thread::attributes getThreadAttr()
>          {
>              boost::thread::attributes attrs;
>              attrs.set_stack_size( 256 * 1024 );
>              return attrs;
>          }
>
>          my_thread( Runnable func )
>              : thread_( getThreadAttr(), func )
>          {}
>
>          boost::thread thread_;
>      };
>
> But that fails:
>
>      g++ -std=c++0x -o thread-attr thread-attr.cpp \
>          -lboost_thread-mt -lboost_system -pthread
>      ...
>      /usr/include/boost/thread/detail/thread.hpp:191:9: note: \
>        template argument deduction/substitution failed:
>          thread-attr.cpp:48:42: note:   cannot convert ¡my_thread::getThreadAttr()()¢ (type \
>            ¡boost::thread::attributes {aka boost::thread_attributes}¢) to type
>            ¡boost::thread::attributes& {aka boost::thread_attributes&}¢
Which Boost version are you using. the thread constructor is expecting a 
constant reference.
         thread(attributes const& attrs, BOOST_THREAD_RV_REF(F) f):
I would analyze later on why your example doesn't works.

> Replacing 'getThreadAttr' with a method that returns a non-const
> reference to internal static variables allows the program to compile
> and run successfully:
>
>      static boost::thread::attributes & getThreadAttr()
>      {
>          static boost::thread::attributes attrs;
>          static bool init = false;
>
>          if ( ! init )
>          {
>              attrs.set_stack_size( 256 * 1024 );
>              init = true;
>          }
>
>          return attrs;
>      }
>
> The full program, with Linux compile lines, can be found at the bottom
> of this message and also at:
>
>      http://foiani.home.dyndns.org/~tony/boost/thread-attr.cpp
>
> I did find a closely-related question on StackOverflow:
>
>      http://stackoverflow.com/questions/13868619
>
> Where Vicente replied (http://stackoverflow.com/a/13894968/784478):
>
>      The needed overload
>
>          template <class F, class Arg, class ...Args>
>          thread(attributes const& attrs, F&& f, Arg&& arg, Args&&... args)
>
>      is defined only if the compiler supports variadic templates and
>      rvalue references. This could explain your problem.
>
>      I guess the documentation could be improved to signal this
>      clearly. Please could you create a Trac ticket to follow this
>      issue?
The issue you mention needed variadic templates

|  boost::thread causeTrouble(attrs,foo,42);|

> However, the compiler I'm using (g++ 4.7.2) has supported both of
> these features for years.  According to this page:
>
>      http://gcc.gnu.org/projects/cxx0x.html
>
> g++ has supported both of these features since 4.3.
This is a little bit more tricky

// VARIADIC_THREAD
#if ! defined BOOST_THREAD_PROVIDES_VARIADIC_THREAD \
  && ! defined BOOST_THREAD_DONT_PROVIDE_VARIADIC_THREAD

#if ! defined(BOOST_NO_SFINAE_EXPR) && \
     ! defined(BOOST_NO_CXX11_VARIADIC_TEMPLATES) && \
     ! defined(BOOST_NO_CXX11_DECLTYPE) && \
*    ! defined(BOOST_NO_CXX11_DECLTYPE_N3276) && \*
     ! defined(BOOST_NO_CXX11_AUTO) && \
     ! defined(BOOST_NO_CXX11_RVALUE_REFERENCES) && \
     ! defined(BOOST_NO_CXX11_HDR_TUPLE)

#define BOOST_THREAD_PROVIDES_VARIADIC_THREAD
#endif
#endif
> Josef_K (on freenode IRC #boost channel) was kind enough to try the
> sample code in their own environment, and they encountered the same
> errors on both g++ 4.8.1 and clang 3.4.
>
> The most likely explanation is that I'm doing something stupid.  If
> anyone has the time to hit me with the clue-by-four, I would
> appreciate it.
>
> There is the possibility that Boost.Thread v4 fixes this (I'm on
> 1_51_0, which is Boost.Thread v3.0.1).  I'd rather not try to
> transition libraries at this point, but if it has been fixed,
> confirmation would be welcome.
>
> For now, I'll most likely implement the "return reference to internal
> static" method in my project.  It's a bit racy, but since I do all my
> thread creation through this one routine, I should be able to manage
> it.
An alternative could be

         my_thread( Runnable func )
             : thread_(  )
         {
             boost::thread::attributes attrs;
             attrs.set_stack_size( 256 * 1024 );
             thread_ = thread(attrs, func);
        }

         boost::thread thread_;
     };

> Anyway.  I'm left feeling that there is a gap somewhere; whether it's
> in my understanding, in the older version of Boost.Threads, in
> compiler support / quirks, or elsewhere, I don't know.
I agree that the documentation can be improved to signal all the 
non-supported features due to lack of compiler features (or that I have 
not yet found an C++98 portable version :()
>
> Regardless, thanks for the excellent library, and happy hacking!
>
>
You are welcome.

Vicente