Re: Hi and pushing existing UserData from pointer

[Chris Camacho <c.camacho@...>]

> Be careful. Lightuserdata all share the same metatable. This may not be your intent.


By adding the "cpShape" metatable I can use the "cpShape" methods
it is defiantly returning the correct pointer as the right object reacts
when I use that userdata's methods, how can what I'm returning
to lua be sharing the same metatable if I can call a method that's
unique to cpShape???


> My approach to your problem would be to make a userdata the size of your pointer and then 
> essentially use double indirection on any appropriate pointer you receive.

really? that seems a lot of work (there must be something easier and
more elegant!)

Best
Chris



delbu9c1@erau.edu wrote:
> Be careful. Lightuserdata all share the same metatable. This may not be your intent.
>
> Lightuserdata does not create any kind of proxy. It is a pointer and that's it.
>
> My approach to your problem would be to make a userdata the size of your pointer and then essentially use double indirection on any appropriate pointer you receive.
>
> Matt
> ------Original Message------
> From: Chris Camacho
> Sender: lua-bounces@bazar2.conectiva.com.br
> To: Lua list
> ReplyTo: Lua list
> Subject: Re: Hi and pushing existing UserData from pointer
> Sent: Aug 16, 2009 9:49 AM
>
> Im not too sure what you're getting at, however
> I have found one answer....
>
>    lua_pushlightuserdata(L, a);
>    luaL_getmetatable(L, "cpShape");
>    lua_setmetatable(L, -2);
>
> does actually work! I'm getting userdata of the appropriate
> type and if I add an impulse to the colliding shape the correct
> body moves so the above *must* be passing a userdata reference
> correctly, although what I suspect I'm doing is making a
> dummy user data with the pointer inside ???
>
> (This and some other "types" I'm going to remove from the
> lua GC, in any case as it make more sense to pass back these
> objects back to the library when you want to remove them
> from the simulation)
>
> Can you explain your method? can you provide a simple
> code example that shows your technique working?
>
> Best
> Chris
>
> Tony Finch wrote:
>   
> > On Sun, 16 Aug 2009, Chris Camacho wrote:
> >   
> >     
> > > Is there any way to make a new userdata that is a reference to an
> > > already allocated userdata from just a C pointer?
> > >     
> > >       
> > Create a weak table mapping lightuserdata to the full userdata. You can
> > convert a C pointer directly to a lightuserdata which you can use to index
> > the table. The table must have weak values so that your objects are still
> > collected. Lightuserdata doesn't affect collection of the objects it
> > points to.
> >
> > Tony.
> >   
> >     
>
>
>   


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