[cfe-dev] libc++: help diagnosing the following std::atomic compile error

[Howard Hinnant]

On May 20, 2013, at 3:29 PM, Rich E <reakinator at gmail.com> wrote:

> Hello all,
> 
> I am trying to help along the recently added Boost.Lockfree (1) library utilize libc++'s std::atomic, as it is currently using Boost.Atomic, and an emulated, locking implementation at that.  <atomic> is currently used for this library for modern gcc and msvc compilers, but not yet clang / libc++.
> 
> 
> (note: to enable <atomic> for boost::lockfree, first apply this patch: https://svn.boost.org/trac/boost/attachment/ticket/8593/lockfree_atomic_libcpp.patch )
> 
> The problem can be seen in the compile errors for this simple program:
> 
> #include "boost/lockfree/queue.hpp"
> 
> int main(int argc, const char * argv[])
> {
>   boost::lockfree::queue<int> q( 1 );
>   return 0;
> }
> 
> The diagnostic is:
> 
> In file included from /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/../lib/c++/v1/memory:606:
> /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/../lib/c++/v1/atomic:623:58: error: no viable conversion from 'boost::lockfree::detail::tagged_ptr<boost::lockfree::queue<int, boost::parameter::void_, boost::parameter::void_, boost::parameter::void_>::node>' to '_Atomic(boost::lockfree::detail::tagged_ptr<boost::lockfree::queue<int, boost::parameter::void_, boost::parameter::void_, boost::parameter::void_>::node>)'
>     _LIBCPP_CONSTEXPR __atomic_base(_Tp __d) _NOEXCEPT : __a_(__d) {}
>                                                          ^    ~~~
> /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/../lib/c++/v1/atomic:727:51: note: in instantiation of member function 'std::__1::__atomic_base<boost::lockfree::detail::tagged_ptr<boost::lockfree::queue<int, boost::parameter::void_, boost::parameter::void_, boost::parameter::void_>::node>, false>::__atomic_base' requested here
>     _LIBCPP_CONSTEXPR atomic(_Tp __d) _NOEXCEPT : __base(__d) {}
>                                                   ^
> ../../../../../cinder-dev/boost/boost/lockfree/queue.hpp:192:9: note: in instantiation of member function 'std::__1::atomic<boost::lockfree::detail::tagged_ptr<boost::lockfree::queue<int, boost::parameter::void_, boost::parameter::void_, boost::parameter::void_>::node> >::atomic' requested here
>         head_(tagged_node_handle(0, 0)),
>         ^
> /Users/richeakin/code/cinder/audio-rewrite/audio2/test/BasicTest/xcode/../src/BasicTestApp.cpp:75:30: note: in instantiation of member function 'boost::lockfree::queue<int, boost::parameter::void_, boost::parameter::void_, boost::parameter::void_>::queue' requested here
>         boost::lockfree::queue<int> q( 1 );
>                                     ^
> ../../../../../cinder-dev/boost/boost/lockfree/detail/tagged_ptr_dcas.hpp:112:5: note: candidate function
>     operator bool(void) const
>     ^
> 
> 
> My understanding is (please correct me if wrong) that _Atomic(_Tp) is a directive to the compiler, which is replaced with atomic the true atomic instructions for each given architecture. As such, it doesn't know how to replace boost::lockfree::detail::tagged_ptr<...> with size_t, or whatever other atomic value lockfree expects. But, this is where my understanding of this sort of template metaprogramming reaches its end, I just can't tell why this trips up with libc++ but not with vc11.
> 
> Can anyone see where the translation falls short, or have suggestions on how to proceeed?

This looks like a clang bug to me.  _Atomic seems to not be set up to deal with non-integral types.

Here is a simplified version of Rich's test:

#include <atomic>

struct A
{
    A(int) {}
};

int
main()
{
    std::atomic<A> q(A(1));
}

Howard