Re: [Boost-bugs] [Boost C++ Libraries] #5475: [Foreach] rvalue reference binding of temporary ranges

Subject: Re: [Boost-bugs] [Boost C++ Libraries] #5475: [Foreach] rvalue reference binding of temporary ranges
From: Boost C++ Libraries (noreply_at_[hidden])
Date: 2011-10-17 15:17:34

#5475: [Foreach] rvalue reference binding of temporary ranges
--------------------------------------+-------------------------------------
  Reporter:  mimomorin@…              |       Owner:  eric_niebler  
      Type:  Patches                  |      Status:  closed        
 Milestone:  To Be Determined         |   Component:  foreach       
   Version:  Boost Development Trunk  |    Severity:  Problem       
Resolution:  fixed                    |    Keywords:  foreach, C++0x
--------------------------------------+-------------------------------------

Comment (by Michel Morin <mimomorin@…>):

 On gcc-4.3 (C++11), I found a regression that was not caught by the test.
 Due to auto type deduction bugs (`decltype`) in template functions,
 the following code does not compile on gcc-4.3 (C++11) with Boost trunk

 {{{
 #include <iostream>
 #include <vector>
 #include <boost/foreach.hpp>

 template<typename T>
 typename boost::mpl::if_<boost::is_rvalue_reference<T&&>, T const,
 T>::type
 add_const_if_rvalue(T&& t)
 {
     return t;
 }

 template <typename T>
 void f()
 {
     BOOST_FOREACH(int const& i, add_const_if_rvalue(std::vector<int>(3,
 0))) {
         std::cout << i << std::endl;
     }
 }

 int main (int argc, char* argv[])
 {
     f<void>();
     return 0;
 }
 }}}

 This code compiles fine on gcc-4.3 (C++11) with Boost.1.47.
 It also compiles fine on gcc-4.5, 4.6, 4.7 (C++11) with Boost trunk.

 Do we need to revert to the C++03 BOOST_FOREACH on gcc-4.3 too?
 Patch is pretty simple.

 {{{
 #!diff
 Index: boost/foreach.hpp
 ===================================================================
 --- boost/foreach.hpp   (revision 75012)
 +++ boost/foreach.hpp   (working copy)
 @@ -34,7 +34,7 @@
  // With these C++0x features, temporary collections can be bound to
  // rvalue references and their lifetime is extended. No copy/move is
 needed.
  #if !defined(BOOST_NO_DECLTYPE) && !defined(BOOST_NO_RVALUE_REFERENCES)
 \
 - && !(BOOST_WORKAROUND(__GNUC__, == 4) && (__GNUC_MINOR__ == 4) &&
 !defined(BOOST_INTEL) &&      \
 + && !(BOOST_WORKAROUND(__GNUC__, == 4) && (__GNUC_MINOR__ <= 4) &&
 !defined(BOOST_INTEL) &&      \
 !defined(BOOST_CLANG))
  # define BOOST_FOREACH_USE_RVALUE_REFERENCE_BINDING
  // Some compilers let us detect even const-qualified rvalues at compile-
 time
 }}}

-- 
Ticket URL: <https://svn.boost.org/trac/boost/ticket/5475#comment:29>
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