Re: pushing full userdata

[Peter Shook <pshook@...>]

Roger D. Vargas wrote:
> I was playing with the example about accessing C structures from lua in the 
> wiki and i noticed that objects are created from inside lua scripts. i was 
> wondering if there is a way to create the structure in C and then push it to 
> the lua state as a full userdata.

Here is the simple approach:

  Foo *fp = (Foo*)lua_newuserdata(L, sizeof(Foo));
  fp->x = ...;
  fp->y = ...;
  fp->whatever = something_other_than_this;
  luaL_getmetatable(L, FOO);
  lua_setmetatable(L, -2);
  lua_setglobal(L, "myfoo");

Now when you pass the value of the 'myfoo' variable to some function, it 
will need to cast the userdata back to a pointer to Foo:

  Foo *fp = (Foo*)luaL_checkudata(L, index, FOO);
  if (fp == NULL) luaL_typerror(L, index, FOO);

I usually wrap these up in 'push' and 'check' functions and think of 
them analogous to lua_pushnumber and luaL_checknumber.

But I get the feeling you know this already and there is more to your 
question.  If your question is "given some pointer, can I bless it so 
that it becomes a full userdata without calling lua_newuserdata", then 
the answer is "no".  Just put your pointer inside the userdata in this case.

It can get a little more tricky if there are resources allocated, *and* 
you need to push the userdata more than once, *and* you want the __gc 
event to do the clean up, in which case you want to push a unique 
userdata so that the __gc event only deallocates the resource once.
See http://lua-users.org/lists/lua-l/2001-09/msg00121.html

Also if you manually deallocate the resouce, you'll need to indicate 
this somehow so that the __gc event doesn't deallocate the resource 
again.  See lua-5.0/src/lib/liolib.c

- Peter