Re: best way to add field to userdata?

[Mark Hamburg <mark@...>]

In the particular case of width and height, I've found a "dimensions" method that returns both to be useful. Multiple value return is something that gets forgotten about far too often by people coming from other languages. But that's just a special case and would not, for example, handle a property like the alignment on a sprite.

There are various interesting proposals for how to modify Lua to better support this, but we won't go there at this point (though they might be a reasonable discussion for 5.2).

Essentially, you need your __index function to implement the following logic:

	__index = function( t, k )
		local v = methods[ k ]
		if v ~= nil then return v end
		local get = getters[ k ]
		if get ~= nil then
			return get( t, k )
		end
		error( "Bad key: " .. k ) -- Insert better error message here
	end

methods and getters are upvalues for the function.

You could implement this function either in C or in Lua. I'm not sure which would work more efficiently and it may depend on how you implement your getter functions. Using upvalues should make this fairly easy, however, to write in C since you can just access the method and getter tables using pseudo-indices.

Mark