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

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

Doug,

It is not true that anyone has told me explicitly to return full userdata 
using lua_newuserdata(). Perhaps you think someone has told me that, but it 
isn't so. With all due respect, you've been leaving that particular part for 
me to guess! I had tried returning the userdata pointer from ptr = 
lua_newmetatable(), like this:

   lua_pushlightuserdata( L, pData );
   lua_pushlightuserdata( L, ptr );
   return 2;

But, obviously, that did not work. So, after you just now told me 
*explicitly* that I need to return lua_newuserdata(), rather than the value 
it returns, I simply moved my C++ object pointer up above the call to 
lua_newuserdata() as shown below.

   // C++ object pointer pData exists at this point
   lua_pushlightuserdata( L, pData );    // my object pointer

   // create a userdata and add it to the userdata metatable for CData
   CData** ptr = (CData**)lua_newuserdata( L, sizeof(CData*) );
   *ptr = pData;
   luaL_getmetatable( L, "CData_type" );
   lua_setmetatable( L, -2 );
   return 2;

After that change, the __gc is not called right away and it does get called 
correctly at the end of the script. I'll do some testing and see if this 
change solves the problem. Thanks for mentioning lua_newuserdata() explitly.

BTW, Yes, I know I can get the C++ object pointer from the userdata, so I 
don;t need both. If this code now gives me what I want, then I'll see about 
cleaning that up.

Michael

----- Original Message ----- 
From: "Doug Currie" <doug.currie@gmail.com>
To: "Lua list" <lua@bazar2.conectiva.com.br>
Sent: Sunday, August 16, 2009 12:02 PM
Subject: Re: how does GC work with complex expressions using metamethods?


> On Aug 16, 2009, at 2:54 PM, Michael Newberry wrote:
>
> > No, I returned both lightuserdata and the metatable containing the  GC 
> > method.
>
> You did not return (full) userdata. You returned lightuserdata. We are 
> all telling you, over and over again, this won't work.
>
> > I also tried returning the pointer to the userdata using 
> > lua_pushlightuserdata and that made no difference.
>
> lua_pushlightuserdata does not return (full) userdata.
>
> lua_newuserdata pushes the (full) userdata on the Lua stack. That is  what 
> you need to return from your function.
>
> > Remember I am returning two values, and I am assigning the second  value 
> > to a table element back in the lua code. The two return values  are 
> > stored in a table which I has setup as a class.
>
> That doesn't help since neither is the (full) userdata you need to  store.
>
> e