Re: Nested userdata

[Asko Kauppi <askok@...>]

Maybe you should reconsider your data structures. Do you really want  
to do the "nesting" on the C++ side, or could you do it better in Lua?

I would see such problems arising when adding scriptability to  
existing C++ API's. There one cannot freely change the balance as one  
would when designing a system from scratch. If this is not the case,  
please give reasons as to why you must embed A within B (I presume  
there's more fields, too, otherwise they would be essentially equal?).

-asko


Jonathan kirjoitti 16.2.2009 kello 20:10:

> I would like to access the following C++ types from lua:
>
> struct A { int x, y; };
> struct B { A a; }
>
> and then in lua do:
>
> b = B:new()
> var = b.a.x
>
> So the best way I can explain this is that I want to have "nested"  
> userdata. Here's what I have so far:
>
> static int B_new (lua_State *L)
> {
>  B *b = (B*)lua_newuserdata(L, sizeof(B));
>  luaL_getmetatable(L, "B");
>  lua_setmetatable(L, -2);
>  return 1;
> }
>
> static int A_index (lua_State *L)
> {
>    // push 'x' or 'y' depending on key
> }
>
> static int B_index (lua_State *L)
> {
>    // what do I push if the key is 'a'? I don't want to allocate new  
> userdata, but I also don't want light user data because I need the  
> metatable.
>
>    if (lua_tostring(L, 2) == "a")
>       // .. ? ..
> }