Newby question: How to change __index userdata method in C?

[Sebastian Wolff <sw@...>]

Hi list,

I am pretty new to Lua and do have only experience wrapping C libraries 
using SWIG and luabind. Both are great tools. Now I have a problem 
where I have to write custom binding code and where I do not have any 
idea how to solve it:

I want to write __index and __newindex methods which accept multiple 
data types as a key, eg. I like to use

 object[0] = number; object[{1,2}] = number; etc.

The thing is: the bindings of "object" have been created using one of 
the mentioned wrappers which do not support such an extended way of 
keys. What I want to do is to change the __index and __newindex methods 
after the actual registration of my class. I already found out how this 
could be done in Lua:

function myown_index_method(table,key)
 if (type(key) == "number") then
   -- do something  elseif (... other types, eg. tables ...)
   ...
 else
   return __oldindex(table,key)
 end
end

local metaarray = getmetatable(MyClass())
__oldindex = metaarray.__index
metaarray.__index = myown_index_method(table,key)

(analogously for __newindex)

The problem is: I would like to implement this on C level for 
performance. Now I know how to write the new __index and __newindex 
methods in C.

But what I do not know is:
(1) How to call the original __index/__newindex methods from their new 
C replacements? Particularly I do not want to create global Lua 
variables such as '__oldindex' in the example. Maybe a C function 
pointer?
(2) How to set the __index/__newindex to the new methods? I think I 
could register them manually in their own scope and then set them using 
a Lua script such as the above. But then, again, I would waste the 
namespace.

Does anyone have a simple idea how to solve this?

Thanks !