Inheriting from C++ Classes

[Nick <nick@...>]

I'm making a game engine, which is using Lua for scripting, using Lunar 
to export C++ classes to Lua.

I'd like to be able to add unique methods to individual instances in 
Lua, they don't need to be accessible to C++.

As an example, I have a Sprite class that is used for a lot of the 
objects in the game, I'd like the instances of these objects to define 
their own Update function, but this isn't currently possible with the 
set up that I seem to have (Update is NOT a function in the C++ object):

PlayerSprite = Sprite()
PlayerSprite:SetImage("Player.png")

PlayerSprite.Update = function (self)
--Update the player's sprite e.g., animation, collision detection or 
whatever
end

EnemySprite = Sprite()
EnemySprite:SetImage("Enemy.png")

EnemySprite.Update = function (self)
--Update the enemy sprite
end

Update Functions can be called fine even though they don't exist in C++ 
however calls to PlayerSprite:Update call the same function as 
EnemySprite:Update, seems like its being added to the 'class' not the 
instance.

I've used Ignacio Burgueño's modifications to enable property style 
access, and uncommented this part which could be the problem based on 
this comment in spanish (in Lunar.h):

			// Esto es opcional. Si lo dejo, le puedo agregar funciones
			// a un objeto trabajando sobre una instancia, sino tengo que agregarlas
			// a la clase
			if(lua_type(L, 3)  == LUA_TFUNCTION) {
				lua_pop(L, 1);	// stack: userdata, clave, valor
				lua_rawset(L, lua_upvalueindex(2));
			}
			else {
				lua_pop(L, 1);	// stack: userdata, clave, valor
				lua_rawset(L, lua_upvalueindex(2));
			}

Instead of a lua_rawset(L, lua_upvalueindex(2)) I tried setting to my 
object  on the stack (lua_rawset(L,-3)) but that just crashes the 
program, I assume because its userdata not a table.

Is there any way of doing this along these lines (much preferred) or 
will I need to add some sort of function registration system in the C++ 
code.

Cheers,
Nick

--
*Nick Koirala*

<http://www.littlemonkey.co.nz>