transparent manipulation of C data

[Tim Conkling <tconkling@...>]

In my game, each sprite is associated with a script that can modify its 
member variables on a per-frame basis. Right now I'm simply exposing 
some accessor and mutator functions (like get/setSpeed and 
get/setAnimation) to the Lua environment that the scripts run in, and 
the scripts call those functions directly to manipulate the sprites.

This isn't so bad, but it's kind of ugly:

setSpeed(-getSpeed())
setAnimation("walk right")

It would be nice if users could instead do:

self.speed = self.speed * 2;
self.animation = "walk right"

So I was thinking of doing something like this:

mt = {}

mt.__newIndex(t, k, v)
	if(k == "speed") then
		setSpeed(v);
	elseif(k == "animation") then
		setAnimation(v);
	else
		rawset(t, k, v)
	end
end

mt.__index(t, k)
	if(k == "speed") then
		return getSpeed();
	elseif(k == "animation") then
		return getAnimation();
	endif

	return rawget(t, k)
end

I'm worried, however, that I'll be slowing things down unnecessarily by 
adopting this strategy. For sprites, there will probably be about 15 or 
20 different variables that can modified, which is a lot of 
if-elseif-elseif... checks. Is there a better way to do what I'm trying 
to do? Also, are there any pitfalls to watch out for with this method?

Thanks,
Tim