id	summary	reporter	owner	description	type	status	milestone	component	version	severity	resolution	keywords	cc
8674	Futures as local named objects can't be returned with implicit move.	mjklaim@…	viboes	"I could test only with VS 2012 Update 2 so this might be related to a compiler bug, but I can't test the boost code with other compilers/platforms right now.

I have this simple test:


{{{
#include <iostream>

#define USE_STD 0
#define USE_BOOST 1

#define USED_THREAD_API USE_BOOST

#if USED_THREAD_API == USE_BOOST

#	define BOOST_THREAD_VERSION 4
#	include <boost/thread/future.hpp>

	using boost::future;
	using boost::async;

#endif
#if USED_THREAD_API == USE_STD
#	include <future>
	using std::future;
	using std::async;
#endif



future<void> do_something()
{
	auto result = async( []{ std::cout<< ""A\n""; } );
	std::cout << ""B\n"";
	return result; // error here
}

int main()
{
	do_something().wait();
	std::cout << ""Hello, World!"" << std::endl;

}

}}}

Both default Debug and Release modes fail to compile on VS2012 U2:

{{{

1>------ Build started: Project: Test_MoveReturnFuture, Configuration: Debug Win32 ------
1>  main.cpp
1>e:\projects\tests\test_movereturnfuture\test_movereturnfuture\main.cpp(29): error C2248: 'boost::future<R>::future' : cannot access private member declared in class 'boost::future<R>'
1>          with
1>          [
1>              R=void
1>          ]
1>          e:\projects\sdk\boost\boost\include\boost-1_53\boost\thread\future.hpp(1406) : see declaration of 'boost::future<R>::future'
1>          with
1>          [
1>              R=void
1>          ]
========== Build: 0 succeeded, 1 failed, 0 up-to-date, 0 skipped ==========
}}}

Changing this line:

{{{
#define USED_THREAD_API USE_BOOST
}}}

To this:

{{{
#define USED_THREAD_API USE_STD
}}}

Makes it compile and execute fine.

I see that there are move constructors in the definition of future so I don't know at all what's the difference between the two implementations.

Another way to avoid the problem is to explicitly move the future:


{{{
future<void> do_something()
{
	auto result = async( []{ std::cout<< ""A\n""; } );
	std::cout << ""B\n"";
	return std::move(result);
}
}}}


Which is what I have been doing in my code because I was initially thinking it was a compiler bug, but this Q/A made me reconsider: http://stackoverflow.com/questions/4986673/c11-rvalues-and-move-semantics-confusion

I cannot test Boost 1.54 at the moment."	Bugs	closed	Boost 1.54.0	thread	Boost 1.53.0	Problem	fixed