Garbage collection of userdata

["Paul Chiusano" <pchiusano@...>]

Hi,

This is probably a simple question. I'm working on a simple numeric
vector class, and I'd like to be able to create 'views' of these
vectors which don't require making a copy of the original vector, aka:

a = numvector.new(100) -- initialize a vector w/ 100 elements, zeroed out
b = a:view(4,10)  -- create a view which shares the same memory block as 'a'
b[1] = 42  -- a sees this change: so now a[4]==42

On the C side, I have this:

typedef struct NumVector {
	unsigned int start, stop;
	lua_Number *values;
} NumVector;

So, in creating a new 'view', the newly created object will have the
same values array, but with possibly different values for 'start' and
'stop'.

If I were to just allocate a NumVector with a block, like:

/* unsigned int size = something */
NumVector *a = (NumVector *)lua_newuserdata(L, sizeof(NumVector) +
size*sizeof(lua_Number));

...then 'a' would be subject to garbage collection by Lua. Great!
Except that when a becomes garbage, its values array will be
deallocated, so any views that point to it will now be invalid. I
could allocate the values array separately:

NumVector *a = (NumVector *)lua_newuserdata(L, sizeof(NumVector));
a->values = (lua_Number *)lua_newuserdata(L, size*sizeof(lua_Number));

but I don't think this is correct. I'd imagine Lua could immediately
collect a->values since nothing on the Lua side will point to it. (or
does the mark and sweep follow references on the C side, too?). What
would be the cannonical way to handle this situation?

-Paul