Re: smart pointers, slicing, and double deletion

I've found a problem with my code.  The lua ref that is created keeps
the Lua class instance alive, regardless if there is *anything* else
referencing it.

The lua_ref call bumps the smart pointers ref count up by one.

Additional Lua vars pointing at the instance do *not* increase the smart
pointers ref count.

So how to tell that the reference associated with the instance is the
last one, and should be removed? No idea. 

The shared_ptr_converter must have this problem too?

Time for a beer and turn the computer off methinks.

Nigel

On Thu, 2009-11-26 at 02:23 +1300, 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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