Re: how does GC work with complex expressions using metamethods?

["Michael Newberry" <mnewberry@...>]

----- Original Message ----- 
From: "Javier Guerra" <javier@guerrag.com>
To: "Lua list" <lua@bazar2.conectiva.com.br>
Sent: Friday, August 14, 2009 3:51 PM
Subject: Re: how does GC work with complex expressions using metamethods?


> On Fri, Aug 14, 2009 at 5:37 PM, Michael
> Newberry<mnewberry@mirametrics.com> wrote:
> > I wonder if anyone can help me with the following questions. I am trying 
> > to
> > add metamethods like _add and __mul to a lua class (a table configured 
> > in
> > the appropriate way).
>
> so your objects are not userdatas, right? just tables with metatables?


Right, my classes are tables, declared generally like this:

   CData = { Cptr = nil }

   function CData:new()
     local o = {}
     setmetatable(o,self)
     self.__index = self
     o.Cptr = CreateData()
     return o
   end

When the user uses  A = CData:new()  the registered function "CreateData" 
is called and it creates the C++ object aand returns a pointer to it. Class 
methods use o.Cptr to talk to the C++ object.


> > Each of these arithmetic metamethods must return a new
> > object. For example, suppose B and C are objects and I use
> >
> > A = B + C
> >
> > The result A is a new object, which is fine. I can delete A and grab its
> > memory whenever I want to.
>
> no, you can't _delete_ an object, you just remove any reference to it,
> and the GC will eventually collect it.

Sorry, my terminology wasn't clear. When the user does  A:delete(), the 
delete method calls a CFunction with self.Cptr which allows the C++ object 
to get deleted. Then delete() sets self = nil and I leave the rest to the 
lua GC.

So my question comes down to how to automatically get the Cfunction called 
with the object pointer so the C++ object can be free'd?

Michael