Re: smart pointers, slicing, and double deletion

Yes that would be one way to go, for less grief.  However I'm creating
more specialised widgets in Lua.  The C++ side of things has no
knowledge of them except that they are a 'type' of widget.  

I guess its not too un-maintainable.  The hard part was figuring out
what and how to do it.  As long as there are comments explaining the
whys and how, someone else or me in a few months should be able to
update it.  The biggest danger in the code is the use of
detail::wrap_access::ref which may change in a future release of
luabind.

Nigel

On Wed, 2009-11-25 at 11:44 -0500, Taesoo Kwon wrote:
> I also would like your approach if it is possible and easy to
> maintain.
> But how about making C++ side control all the memory
> allocation/deallocation?
> 
> For example, in lua part, only widget names are stored, and every time
> a pointer is needed, you use an interface function to check and
> retrieve if such widget exists.
> 
> Taesoo
> 
> On Wed, Nov 25, 2009 at 8:23 AM, Nigel Atkinson
> <[email protected]> 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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