Boost C++ Libraries: Ticket #10125: call_once: compile errors in variadic version with rvalue reference emulation

owner, status changed

viboes — Mon, 16 Jun 2014 18:52:32 GMT

owner changed from Anthony Williams to viboes
status new → assigned

Please, could you show an example an the compile error?

raad@… — Mon, 16 Jun 2014 21:17:03 GMT

Please see the attached simple example for MSVC12. The error message is:

boost_call_once_test\boost_call_once_test\boost_call_once_test.cpp(19): error C2665: 'boost::call_once' : none of the 2 overloads could convert all the argument types
1>          boost_call_once_test\boost_call_once_test\boost\thread\win32\once.hpp(232): could be 'void boost::call_once(boost::once_flag &,void (__cdecl *)(void))'
1>          boost_call_once_test\boost_call_once_test\boost\thread\once.hpp(35): or       'void boost::call_once<boost::once_flag>(Function,boost::once_flag &)'
1>          with
1>          [
1>              Function=boost::once_flag
1>          ]
1>          while trying to match the argument list '(boost::once_flag, boost::_bi::bind_t<void,boost::_mfi::mf0<void,test_class>,boost::_bi::list1<boost::_bi::value<T>>>)'
1>          with
1>          [
1>              T=test_class
1>          ]

I don't know why the error message doesn't even mention

void call_once<Function>(once_flag& flag, boost::rv<Function> &f)

, IntelliSense suggests that overload too.

clang gives similar errors.

attachment set

raad@… — Mon, 16 Jun 2014 21:18:06 GMT

attachment → boost_call_once_test.zip

Poject showing the error for MSVC12

viboes — Sun, 29 Jun 2014 21:38:26 GMT

I have reached to make it work with

#define BOOST_THREAD_VERSION 4
//#define BOOST_NO_CXX11_RVALUE_REFERENCES
#define BOOST_THREAD_PROVIDES_ONCE_CXX11

I don't remember why BOOST_THREAD_PROVIDES_ONCE_CXX11 is not defined when BOOST_THREAD_VERSION is defined.

Please could you check if this work for you. I will comeback when I found why I have not defined it.

viboes — Sun, 29 Jun 2014 21:45:57 GMT

I have found this comment

// For C++11 call_once interface the compiler MUST support constexpr.
// Otherwise once_flag would be initialized during dynamic initialization stage, which is not thread-safe.
#if defined(BOOST_THREAD_PROVIDES_ONCE_CXX11)
#if defined(BOOST_NO_CXX11_CONSTEXPR)
#undef BOOST_THREAD_PROVIDES_ONCE_CXX11
#endif
#endif

And

#if BOOST_THREAD_VERSION>=3
// ONCE_CXX11
// fixme BOOST_THREAD_PROVIDES_ONCE_CXX11 doesn't works when thread.cpp is compiled BOOST_THREAD_VERSION 3
#if ! defined BOOST_THREAD_DONT_PROVIDE_ONCE_CXX11 \
 && ! defined BOOST_THREAD_PROVIDES_ONCE_CXX11
#define BOOST_THREAD_DONT_PROVIDE_ONCE_CXX11
#endif

I will comeback with more insight.

raad@… — Mon, 30 Jun 2014 07:16:32 GMT

Defining BOOST_THREAD_PROVIDES_ONCE_CXX11 would work for me as I don't actually need this for MSVC, only for the latest clang. I only use MSVC for testing as I don't have access to clang regularly. But unfortunately I still have to define BOOST_NO_CXX11_RVALUE_REFERENCES as the standard library I have to use with clang is not move-aware and does not provide std::move and std::forward :-(

viboes — Tue, 01 Jul 2014 21:53:00 GMT

Boost doesn't supports this combination. I think that it would be better to remove this checker and don't use C++11 with this library.

raad@… — Wed, 02 Jul 2014 05:32:21 GMT

I see, thank you. I have to change to libc++ as soon as possible. But everything in Boost works except for this single compile error when variadic templates are switched on.

viboes — Wed, 02 Jul 2014 05:51:41 GMT

I have assumed that if the compiler has variadic templates it has move semantic. In order to support the configuration you want I would need to make the distinction. Do you think it is worth the effort?

raad@… — Wed, 02 Jul 2014 09:29:52 GMT

Well, there is a workaround (also disabling variadic templates when disabling rvalue references), so it's not critical.

But as the rest of Boost (including the rest of Boost.Thread) works in this configuration with variadic templates and without rvalue references and it seems that I'm not the only one using the latest clang with an older standard library, I think it's worth it if it's not too much work.

Maybe the simplest solution would be to disable the variadic version of call_once if BOOST_NO_CXX11_RVALUE_REFERENCES is defined? The attached patch works for me with all compiler and standard library combinations.

attachment set

raad@… — Wed, 02 Jul 2014 09:33:05 GMT

attachment → call_once_variadic.patch

viboes — Thu, 03 Jul 2014 06:10:20 GMT

Ok, I will do it.

milestone changed

viboes — Fri, 04 Jul 2014 05:54:23 GMT

milestone To Be Determined → Boost 1.56.0

raad@… — Mon, 07 Jul 2014 10:34:31 GMT

I just saw that this is now in master. Thank you very much!

status changed; resolution set

viboes — Sat, 19 Jul 2014 16:39:53 GMT

status assigned → closed
resolution → fixed