returning userdata

[Enno Rehling <enno@...>]

Let's say I've got a C struct like this:

typedef struct data {
  struct data * next;
  int x;
} data;

I want to write a "next" handler in teh "index" metatable that will return a 
userdata object. However, I've figured out from the manual and 
experimentation that I can't just do a lua_pushlightuserdata(L, d->next), 
because light userdata doesn't have a metatable, and if I write 
print(a.next.x), lua complains that I'm using an index function of a.next, 
which is light userdata. How do I make my "next" handler return non-light 
userdata? I don't want to create a new userdata object, because what I want 
to return is the actual object that's behind the ->next in C.

One way I thought of to solve that wasn't nice: I could to wrap all my 
objects in ynother layer of data, and make userdata objects that contain 
data* pointers (which I could then easily copy). I'd have something like

typedef struct lua_data {
  data * cdata;
} lua_data;

and would write something like:
  ((lua_data*)lua_newuserdata(L, sizeof(lua_data)))->cdata = a->cdata->next;

but really, that's pretty ugly, and a lot of 4-byte mallocs.

Enno.