Re: [Boost-bugs] [Boost C++ Libraries] #3604: Access violation on diamond inheritance

Subject: Re: [Boost-bugs] [Boost C++ Libraries] #3604: Access violation on diamond inheritance
From: Boost C++ Libraries (noreply_at_[hidden])
Date: 2009-11-29 07:29:18

#3604: Access violation on diamond inheritance
----------------------------------+-----------------------------------------
  Reporter:  kondo@…              |       Owner:  ramey        
      Type:  Bugs                 |      Status:  closed       
 Milestone:  Boost 1.41.0         |   Component:  serialization
   Version:  Boost 1.40.0         |    Severity:  Problem      
Resolution:  invalid              |    Keywords:               
----------------------------------+-----------------------------------------
Changes (by ramey):

  * status:  assigned => closed
  * resolution:  => invalid

Comment:

 Making a small change in your example illustrates the problem here.

 Turns out you can downcast a pointer from a virtual base class.  It
 makes sense when you think about it.  The serialization library does
 this at runtime through an intermediate void * so it doesn't detect
 the error at compile time.  And the MSVC compiler doesn't detect it
 unless it's done with a static cast.  It turns out that the vbc data
 is "attached" to the first subclass.  So the problem only occurs when
 we try to use the second - sub2.

 If you make the following change in your program, and try to compile
 I think the problem will be obvious. (note to self, always use
 static_cast rather than implicit pointer conversions !!!)

 {{{
         // Serialize pTarget via VBase pointer.
         {
         std::ofstream ofs("outtarget.xml");
         boost::archive::xml_oarchive oa(ofs);
         assert(ofs);
         Target* pTarget = new Target;
         VBase * pVBase = static_cast<VBase *>(pTarget);
         Target* pTarget2 = static_cast<Target *>(pVBase);
         oa << boost::serialization::make_nvp("pVBase", pVBase);
     }
     {
         std::ifstream ifs("outtarget.xml");
         assert(ifs);
         boost::archive::xml_iarchive ia(ifs);
         VBase * pVBase;
         ia >> boost::serialization::make_nvp("pVBase", pVBase);
     }
 }}}

 PS I did in fact find some problems with void cast registry - but I don't
 think they're related to this.  After I fixed them, I still had the
 problem.

 Robert Ramey

-- 
Ticket URL: <https://svn.boost.org/trac/boost/ticket/3604#comment:2>
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