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

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

----- Original Message ----- 
From: "Sam Roberts" <vieuxtech@gmail.com>
To: "Lua list" <lua@bazar2.conectiva.com.br>
Sent: Friday, August 14, 2009 4:29 PM
Subject: Re: how does GC work with complex expressions using metamethods?


> On Fri, Aug 14, 2009 at 4:06 PM, Michael
> Newberry<mnewberry@mirametrics.com> wrote:
> > 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.
>
> This isn't going to work.

   Maybe I'm not understanding what you are saying here, but what I 
described has worked for years. But it requires the user call 
CData:delete(). So it isn't automatic. Is it "not being automatic" that you 
mean isn't going to work? If so, yes, that is the problem.

> CreateData needs to return a (full) userdata. That userdata will
> probably contain nothing but your c++ pointer, and must have a
> metatable, with a __gc function that will delete that pointer.

   I already tried to do that, but maybe I goofed something. Here was the 
code inside the Cfunction CreateData():

   // ... create the C++ object with pointer pData

   CData** ptr = (CData**)lua_newuserdata( L, sizeof(CData*) );
   *ptr = pData;
   return 1;

Back in the lua new() method, the value of self.Cptr has "something". But 
what is that something? And how do I attach a __gc metamethod to it?

Michael

> I can think of ways of getting around this, but they are unstable
> hacks, and besides, userdata was designed to do exactly what you are
> doing: encapsulate a C/C++ object/pointer, and give you hooks into
> garbage collection so you can manage their lifecycle.
>
> Cheers,
> Sam