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: 2011-10-16 07:09:39

#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@…>):

 The current implementation of C++11 BOOST_FOREACH is based on
 **`decltype`**.
 However, gcc-4.4 has a bug that prevents us from using `decltype` in
 `BOOST_FOREACH`.
 (The bug has been fixed in gcc-4.5 and newer versions.)

 The reason I used `decltype` rather than `auto` is that
 * `auto &&` is buggy in gcc-4.4.*, 4.5.*, 4.6.0 and so we should not use
 it. (This bug is already fixed in gcc-4.6.1 and 4.7.)
 * `decltype` is more widely available than `auto`.
 * binding rvalue ranges to **const** references can be easily implemented
 with `decltype` than with `auto`.

 Short-term fixed would be:
 A. Revert to C++03 BOOST_FOREACH on gcc-4.4
 B. Disable **const** reference biding of rvalue ranges on gcc-4.4. That
 is, rvalue ranges are bound to **const or non-const** rvalue ranges using
 `auto &&`. In spite of the above bug, `auto &&` can be safely used in
 gcc-4.4, since lvalues are allowed to be bound to rvalue references in
 gcc-4.4.
 A and B are extremely easy to do without causing any error.

 Long-term fixed would be
 C. Implement "**const** reference binding of rvalue ranges" with `auto &&`
 on gcc-4.4. I already implemented it in this
 [[http://lists.boost.org/Archives/boost/2011/04/180591.php|post]].

 Comments? I'm ready to make patches.

-- 
Ticket URL: <https://svn.boost.org/trac/boost/ticket/5475#comment:23>
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