Re: [Boost-bugs] [Boost C++ Libraries] #7418: Copyable and explicitly movable classes

Subject: Re: [Boost-bugs] [Boost C++ Libraries] #7418: Copyable and explicitly movable classes
From: Boost C++ Libraries (noreply_at_[hidden])
Date: 2013-06-13 10:04:56

#7418: Copyable and explicitly movable classes
-------------------------------+------------------------
  Reporter:  danieljames       |      Owner:  igaztanaga
      Type:  Patches           |     Status:  new
 Milestone:  To Be Determined  |  Component:  move
   Version:  Boost 1.53.0      |   Severity:  Problem
Resolution:                    |   Keywords:
-------------------------------+------------------------
Changes (by KaiSt <k.stuhlemmer@…>):

 * version:  Boost 1.52.0 => Boost 1.53.0
 * type:  Feature Requests => Patches

Comment:

 Hi, I locally solved the problem in Boost 1.53. May changes are attached
 as "boost-move-1.53.diff". To be sure we are talking about the same
 problem, let me explain, which problem I see. Let's assume the following
 use case:

 {{{
   class Movable
   {
     private:
       BOOST_MOVABLE_BUT_NOT_COPYABLE(Movable);
     public:
       Movable() : mNrMoves(0)
       { }

       Movable(BOOST_RV_REF(Movable) other)
       : mNrMoves(other.mNrMoves + 1)
       {
         other.mNrMoves = 0;
       }

       unsigned mNrMoves;
   };

   struct MovableUser
   {
     Movable m;

     MovableUser(BOOST_RV_REF_WRAPPER(Movable) mi)
     : m(boost::move(mi))
     { }
   };
 }}}

 If I use this implementation, pre-C++11 compiler users might use it with
 bad surprising behaviour:

 {{{
   Movable m;
   MovableUser mu(m); // this silently compile wit pre-C++11 compiler,
 C++11 compiler correctly fails because of missing boost::move()
 }}}

 My fix just added a wrapper class for boost::rv and an accompanying macro
 BOOST_RV_REF_WRAPPER(). If You change Movable(BOOST_RV_REF(Movable) other)
 to
 Movable(BOOST_RV_REF_WRAPPER(Movable) other), then the example above also
 fails with pre-C++11 compiler enforcing the usage of boost::move():

 {{{
   Movable m;
   MovableUser mu(boost::move(m)); // now correct
 }}}

 I hope, that helps. Please consider to add this patch or a similar
 solution to new releases.

-- 
Ticket URL: <https://svn.boost.org/trac/boost/ticket/7418#comment:1>
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