Boost C++ Libraries: Ticket #5279: [Foreach] Compile-time const rvalue detection fails with gcc 4.6

attachment set

mimomorin@… — Tue, 08 Mar 2011 12:22:43 GMT

attachment → foreach.patch

Disables compile-time const rvalue detection in gcc 4.6 (and newer versions)

status, milestone changed

Eric Niebler — Tue, 08 Mar 2011 12:56:45 GMT

status new → assigned
milestone To Be Determined → Boost 1.47.0

I wonder if there is a different hack for gcc 4.6 that lets us detect const-qualified rvalues at compile time. gcc 4.6 supports rvalue references, right? It seems any compiler that supports rvalue references can detect rvalues at compile time. A general fix would be VERY GOOD.

mimomorin@… — Tue, 08 Mar 2011 17:05:29 GMT

Yeah, with the C++0x features (-std=c++0x), we can easily detect rvalueness. Personally, I use the following code for rvalue detection

#ifndef BOOST_NO_DECLTYPE
#define BOOST_IS_LVALUE(xxx) boost::is_lvalue_reference<decltype( (xxx) )>::value
#define BOOST_IS_RVALUE(xxx) (!BOOST_IS_LVALUE(xxx))
#endif

Is it better to develop a method that only needs rvalue reference support (i.e. without using the decltype feature)?

mimomorin@… — Wed, 09 Mar 2011 12:57:45 GMT

I will attach a patch which only needs rvalue reference support. I ran the tests locally using gcc 4.6 (w/wo -std=c++0x), and all the tests passed.

attachment set

mimomorin@… — Wed, 09 Mar 2011 13:02:32 GMT

attachment → foreach.2.patch

A patch against trunk. Disables compile-time const rvalue detection in gcc 4.6 (and newer versions) and enables it when a compiler supports rvalue references.

Eric Niebler — Thu, 10 Mar 2011 01:56:36 GMT

Thanks for the patch. I hope to get to this soon.

Eric Niebler — Sun, 13 Mar 2011 14:02:19 GMT

Hrm, for me this fails with gcc 4.5.0 with -std=gnu++0x. Can you confirm?

Eric Niebler — Sun, 13 Mar 2011 14:24:29 GMT

Disregard. There seems to be something wrong with my setup that makes it impossible for me to test with gcc in 0x mode. I'll commit this (modulo some minor changes) and hope for the best.

mimomorin@… — Thu, 14 Apr 2011 23:38:32 GMT

Thanks, Eric! The regression test results seem OK.

I've also tried to support rvalue reference binding of temporary collections in Boost.Foreach (auto or decltype C++0x feature is needed to extend the lifetime of temporaries). A new ticket and patches for this coming soon.

status changed; resolution set

Eric Niebler — Fri, 15 Apr 2011 03:22:55 GMT

status assigned → closed
resolution → fixed

merged to release. thanks for your help with this.

pmachata@… — Thu, 21 Jul 2011 21:51:31 GMT

Hi there. So for GCC 4.6, foreach will use BOOST_FOREACH_RUN_TIME_CONST_RVALUE_DETECTION, which implies that BOOST_FOREACH cannot be used to iterate over noncopyable collections, as noted in the file. Is there a plan to fix this? I'm currently hitting this case when building Wesnoth. Thanks, Petr Machata.