Re: [Boost-bugs] [Boost C++ Libraries] #5146: Assertion fails in visitation_impl.hpp with nested variants

Subject: Re: [Boost-bugs] [Boost C++ Libraries] #5146: Assertion fails in visitation_impl.hpp with nested variants
From: Boost C++ Libraries (noreply_at_[hidden])
Date: 2011-02-01 19:33:55

#5146: Assertion fails in visitation_impl.hpp with nested variants
------------------------------------------------------+---------------------
  Reporter:  Florian Goujeon <flo.goujeon@…>          |       Owner:  ebf                            
      Type:  Bugs                                     |      Status:  new                            
 Milestone:  To Be Determined                         |   Component:  variant                        
   Version:  Boost 1.45.0                             |    Severity:  Problem                        
Resolution:                                           |    Keywords:  variant assert assertion nested
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Comment (by Florian Goujeon <flo.goujeon@…>):

 OK, I got it.

 Since ptr_to_ptr_to_tv1 kinda contains direct_ptr_to_ptr_to_tv1, variant
 has to destruct direct_ptr_to_ptr_to_tv1 AFTER
 direct_ptr_to_ptr_to_tv1.pointed_test_variant() was copied into
 ptr_to_ptr_to_tv1.

 Currently, I guess variant clears its content (i.e. calls the destructor
 of its contained object) then sets it with a copy of the new object. This
 is the most natural behavior, but it causes problems when the copy
 constructor of the new object needs data that are stored in the previously
 contained object.

 I don't know the code of Boost.Variant, so I won't be able to write a
 patch.
 But basically, the changes I made in my implementation can be summarized
 like this:

 BEFORE:
 {{{
 template<typename Clear, typename Set>
 void
 clear_and_set(const Set& value)
 {
         (*reinterpret_cast<Clear*>(buffer_)).~Clear(); //clear
         new(buffer_) Set(value); //set
 }
 }}}

 AFTER:
 {{{
 template<typename Clear, typename Set>
 void
 clear_and_set(const Set& value)
 {
         char old_buffer[Size];
         for(unsigned int i = 0; i < Size; ++i)
         {
                 old_buffer[i] = buffer_[i];
         }

         new(buffer_) Set(value); //set

         (*reinterpret_cast<Clear*>(old_buffer)).~Clear(); //clear
 }
 }}}

-- 
Ticket URL: <https://svn.boost.org/trac/boost/ticket/5146#comment:1>
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