Re: double deletion problem with shared pointers.

On Thu, 2009-11-26 at 11:00 +0100, Daniel Wallin wrote:
> Nigel Atkinson wrote:
> > The attached code shows the double deletion problem I encounted.  I
> > previously thought that the problem only surfaced when using the
> > shared_ptr_converter.  However it looks like it happens either way.
> 
> Right. This fails because the widget_wrapper instance that corresponds
> to your Lua widget instance is held in a global shared_ptr, which is
> destructed after the Lua state is destroyed.
> 
> widget_wrapper will hold a weak back reference to the Lua instance, so
> that it can call back to overridden functions in Lua. When it is
> destructed it will clean up the weak reference, and in this case that
> means accessing a destroyed Lua state.
> 
> shared_ptr_converter has the same problem. It stores a back reference to
> the Lua instance, keeping it alive so that you don't get the slicing you
> encountered, and when it's destructed it cleans that up.
> 
> AFAIK there's really no way around this. You need to ensure that you
> don't destroy the Lua state while you still have references to Lua
> objects around.
> 
If I use intrusive pointers, that is, the ref count is with the object
rather than the individual pointer, it should work.  Pointers made from
a raw pointer have the correct ref count.  However I've hit a problem of
the luaRef that keeps the object from slicing, also keeps the ref count
up by one, and it is indistinguishable from other (none or more)
references to the object.  It like I need some sort of ref counting in
the Lua code too. 

So close!

The other option would be to get a copy of the global shared pointer
that holds the widget_wrapper, and store that.

Of course the easiest way for my current problem would be to just store
the list of widget children in Lua.

Thank you for your help Daniel, and Evan, much appreciated. And everyone
else too.

Nigel


------------------------------------------------------------------------------
Let Crystal Reports handle the reporting - Free Crystal Reports 2008 30-Day 
trial. Simplify your report design, integration and deployment - and focus on 
what you do best, core application coding. Discover what's new with
Crystal Reports now.  http://p.sf.net/sfu/bobj-july