Userdata as keys

[Niklas Frykholm <niklas@...>]

In my Lua application I have userdata objects with value semantics 
(position coordinates) that I sometimes want to use as table keys. I.e., 
I want table lookup based on equality rather than identity.

Currently, my best idea for doing this is to have a function key() in 
the userdata that returns a string representation of the data and use 
that string as the table key. This has the disadvantage of 1) not being 
very elegant and 2) creating a lot of unnecessary strings.

Is there a better way of doing this?

// Niklas