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

[Sam Roberts <vieuxtech@...>]

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.

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 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