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

[Rich E]

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?

Thank you kindly,
Rich


1: http://www.boost.org/doc/libs/1_53_0/doc/html/lockfree.html
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