Re: patching userdata metatable.... ouch!

[Ando Sonenblick <ando@...>]

Edgar,

Thanks for your input:

> This should do it (untested):
>
>   function SetInheritance(obj, parent)
>     setmetatable(getmetatable(obj) or obj, { __index = parent })
>   end
>
> But you have a conceptual problem!  You change not only the
> behaviour of 'obj' but that of all objects that use the same
> metatable!  Beside, the use of that function is _very_ limited.
> A real class-like system needs more work[1].


But I was intentionally trying just to modify obj's inheritance and not 
the behavior of all objects that use the same metatable that obj 
originally uses.

Conceptually I have a userdata object whose behavior is 
implemented/defined by its metatable:

obj
   omt

I'm basically looking to tack on another one:

obj
   omt  (if obj usage is handled here, do so and be done)
      nmt   (else, use this metatable...)

I want this to be on an object by object basis.  So if I have two 
objects both defined by the same metatable:

foo
   omt

bar
   omt

and I modify foo with another mt

foo
   omt
     nmt

bar is unaffected and remains

bar
   omt

There's definitely a way to accomplish this - in lua you can do almost 
anything.  It's finding an efficient and preferably self-contained 
method of doing so...

ando

On Dec 16, 2003, at 9:00 PM, Edgar Toernig wrote:

> Ando,
>
> You wrote:
> > Goal: I now want to have it inherent from another object: x, where is
> > is a table or a userdata
> >
> > x = { Bar = function() print("bar") end }
> >
> > So that after calling SetInhereitance(obj, x) I can then calll: 
> > obj:Bar()...
> >
> > So I want to patch in a new metatable that will, for any __index call 
> > (to
> > start with) it will first call the original metatable and if 
> > something is
> > found, return that. Otherwise it'll index x to see if it has anything,
> > returning the result.
> >
> > But now matter what I try I cant get this to work. I think I have a 
> > solution
> > were obj a table but not for user data. For illustration purposes, 
> > this is
> > the direction I've been going:
> >
> > SetInheritance = function(obj, parent)
> > local nmt
> >
> > obj.omt = getmetatable(sf)
> > nmt = obj.omt
> > nmt.__index = function(object, key)
> > local result = rawget(object, "omt")[key]
> > if result then return result end
> > return parent[key]
> > end
> > application:SetObjectMetatable(sf, nmt)
> > end
>
>
>
> Ciao, ET.
>
> PS: Could you fix your mailer to send plain-text non-flowed
> messages?  Thanks.
>
> [1] You could start with the example I sent you.  Add a
> 'baseclass' field in each class table and let 'classmeta'
> handle the inheritance instead of raising an error.
-----------------------
Ando Sonenblick
SpriTec Software
www.spritec.com