Re: Userdata as keys

[Asko Kauppi <askok@...>]

The only way I know for this, is making sure you only create _one_  
userdata for each value combination (you should use a lookup in the  
creation function for that). Otherwise, if you end up having multiple  
userdata they aren't usable as table keys. Period. No way I know for  
changing that (think strings, and the way they are immutable; these  
are very profound Lua assumptions that in the end boil down to  
runtime efficiency).

One thing you could do, since coordinates are numbers, is tbl[1000*x 
+y] ?  Maybe?

-asko


Niklas Frykholm kirjoitti 16.10.2006 kello 19.27:

> 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