Re: [Boost-users] problems with Boost.Thread on VS10

Subject: Re: [Boost-users] problems with Boost.Thread on VS10
From: Jeffrey Lee Hellrung, Jr. (jeffrey.hellrung_at_[hidden])
Date: 2012-04-07 10:42:24

I just took another look at this...

On Wed, Apr 4, 2012 at 9:26 PM, John M. Dlugosz <mpbecey7gu_at_[hidden]>wrote:

> I'm trying to use a packaged_task, and get an error on creation.
>
>        template<typename R,typename F>
>        struct task_object:
>            task_base<R>
>        {
>            F f;
>            task_object(F const& f_):   // << refers to look here
>                f(f_)
>            {}
> #ifndef BOOST_NO_RVALUE_REFERENCES
>            task_object(F&& f_):         // <<<< Error here
>                f(f_)
>            {}
>
>
> E:\boost_1_49\boost/thread/**future.hpp(1251): error C2535:
> 'boost::detail::task_object<R,**F>::task_object(F)' : member function
> already defined or declared
>          with
>          [
>              R=size_t,
>              F=std::tr1::_Bind<size_t,size_**
> t,std::tr1::_Bind1<std::tr1::_**Callable_fun<size_t (__cdecl *const
> )(std::string &),false>,const char *>> &
>

I think this is your problem: F is a (lvalue) reference. So F const & ==
F&& == F due to reference collapsing rules.

         ]
>          E:\boost_1_49\boost/thread/**future.hpp(1247) : see declaration
> of 'boost::detail::task_object<R,**F>::task_object'
>          with
>          [
>              R=size_t,
>              F=std::tr1::_Bind<size_t,size_**
> t,std::tr1::_Bind1<std::tr1::_**Callable_fun<size_t (__cdecl *const
> )(std::string &),false>,const char *>> &
>          ]
>
>
> So I'm thinking that VS10 (Microsoft Visual Studio 2010) has problems
> overloading the constructor with (T const&) and (T&&) ?  But shouldn't
> someone have noticed that long ago?
>

I see two ways out here. One option is to specialize task_object for
reference F's. The other is to use metafunctions to choose the right
parameters for the "F const &" and "F&&" constructors, respectively, e.g.,
use

typename add_reference< typename add_const<F>::type >::type

in place of "F const &" (or call_traits<F>::param_type; if you weren't
looking to support C++03 you could also just use "F const &" as you are
now); and use

struct disabler { };
typename mpl::if_c< is_reference<F>::value, disabler, F&& >::type

in place of "F&&".

HTH,

- Jeff