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

I just took another look at this... On Wed, Apr 4, 2012 at 9:26 PM, John M. Dlugosz wrote:

I'm trying to use a packaged_task, and get an error on creation.

template 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::task_object(F)' : member function already defined or declared with [ R=size_t, F=std::tr1::_Bind,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::task_object' with [ R=size_t, F=std::tr1::_Bind,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