Re: smart pointers, slicing, and double deletion

Hmmmm, I must run my test program through valgrind.  And apply that
patch you sent... anyhow,

I think the problem I was hitting is that the smart pointer that the
converter in shared_ptr_convertor.hpp was different to the one that you
got a copy of if you called a C++ function from Lua that took a smart
pointer as a parameter.

I'm not sure which one is a copy of the smart pointer that is actually
holding the instance in the internals of luabind, but I suspect its the
one passed to functions, since besides the slicing, my program was
showing expected ref counts in the pointers.  Perhaps not.

Nigel

On Wed, 2009-11-25 at 10:25 -0500, Evan Wies wrote:
> I am seeing some similar problems.  Valgrind is not happy when my objects are destroyed -- I think
> the lua_State is getting mucked with after it has closed.  In your case, it is probably the thread's
> state that is being altered post-mortem.
> 
> One shouldn't have to jump through hoops to do this.  I'm exploring and will let you know if I come
> up with anything.
> 
> -Evan
> 
> 
> Nigel Atkinson wrote:
> > Smart pointers, slicing, and double deletion.
> > 
> > Sound's like a right mess doesn't it.  I finaly cracked it, and since
> > I'm sure someone else will come across the same problem I had, I thought
> > I'd share.
> > 
> > What I was trying to do:
> > I have a GUI widget (Ogre3d if your wondering), that holds a list of its
> > children widgets.  This widget class is the base for several Lua widget
> > classes.  I'm using smart pointers to avoid adoption issues.
> > 
> > Originally using shared pointers, I found that when only C++ had a
> > pointer to the instance, it would get 'sliced' if it was a Lua class.
> > 
> > Bring in shared_ptr_converter.hpp, and that fixes the slicing.  However
> > because of part of the way it works, it causes double deletion albeit
> > seemingly only at the shut down of a Lua thread. (Its not safe across
> > Lua threads either, but that's easily fixed.)
> > 
> > So to fix the double counting, I started thinking about making my own
> > shared pointer - oh man, do I have to? - then realised that
> > boost::intrusive_ptr would do most of the job.  Also rather than store
> > the counts in the widget class, I just put them in to a map.  Handy for
> > debugging.  I still might move that data to the widget class, however
> > this would work if you could not change the class.
> > 
> > So that's the ref counting working now, on to the slicing problem.  I
> > added code to my intrusive_ptr_add_ref and intrusive_ptr_release
> > functions, to create and remove, a Lua ref i.e. lua_ref(), lua_unref()
> > at the appropriate time.  The trick was getting the pointer to the class
> > representation on to the Lua stack.  After a bit of following the code
> > line by line as my test program made class instances, I found the bit of
> > luabind code I needed.
> > 
> > Attached is my test program, and a Lua script to drive it.  I'd be
> > interested in any comments any of you have about it!
> > 
> > Nigel
> > 
> > 
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> > 
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