Re: Conceptual problem with gettable tag method

[Diego Nehab <diego@...>]

Hi Falko,

I once had a similar decision problem while implementing an image data
type in Lua. I believe the simpler solution to your problem is to stick
to usertag carrying pointers to your C structure. That is, don't push
lua tables from C to Lua. Instead, stick to C pointers to your C
structures, passed to lua as usertag objects. You should define the
gettable and settable methods for the new vector tag. Your getglobal
method would push a usertag object into the c2lua stack, carrying the
pointer to the appropriate C structure.

Here are the new actions for the function calls:

localvec = newVector(10, 10, 10)
-- calls lua_pushusertag to push a pointer to a C structure containing
the 
-- vector (10, 10, 10), as an usertag object with the vector tag, into
the c2lua stack

localvec = GlobalVec
-- calls lua_pushusertag to push the pointer to the C structure
corresponding to the C 
-- global variable "GlobalVec", as an usertag object with the vector
tag, into the c2lua stack

localnum = GlobalVec.x
-- lua will call getglobal for "GlobalVec" and your method will do the
same as it did in the
-- line above. Then, lua will call the gettable tagmethod for the vector
tag, passing
-- your usertag vector object. your tagmethod should then push the
number value of
-- "x" into the c2lua stack

GlobalVec.x = localnum
-- lua will call getglobal for "GlobalVec" and your method will do the
same as it did in the
-- previous examples. Then, lua will call the settable tagmethod for the
vector tag, passing
-- your usertag vector object and the value of localnum. your tagmethod
should then update
-- the field corresponding to "x" in the C structure

You might want to implement the gc tag method for your vectors, to avoid
memory leaks.

Hope it helps,
regards,
Diego.