Re: C function mapping strategy

["Thomas Lefort" <thomas.lefort@...>]

> I'm working on integrating some C code with Lua. The C code is
> dynamically bound, so I'm able to do things like the following:
>
> void *obj = obj_new("name_of_obj");
> obj_method(obj, "draw");
> obj_method(obj, "scale", 2, [0.5, 0.5]);
> ...
>
> The above C code will allocate an object, then call the object's draw
> method, and then set the object's scale parameter.  What I'd like to
> be able to do with Lua is the following
>
> obj = Obj.new("name_of_obj");
> obj:draw()
> obj.scale = {0.5, 0.5}

You can do it with metatables and closures.

Consider the following function, to be used as the __index field of
your userdata metatable.

local function __index(u, k)
 return function(u, ...) return obj_method(u, k, ...) end
end

With this setup:

0) obj:draw() stands for (obj.draw)(obj)

1) obj.draw triggers the metamethod: __index(obj, "draw")

2) __index returns a closure where k is bound to "draw":

 function(u, ...) return obj_method(u, "draw", ...) end

3) the closure is called with obj: obj_method(obj, "draw")

Once you get the knack of both metatables and closures you can mold
about any object system you like.

For properties, you will need a thin reflection layer to distinguish
them from methods, and to marshal arguments as C arrays like your
example suggests. But the basic mechanisms are the same.

I hope this helps.