Re: Enumerating class objects

[Rici Lake <lua@...>]

On 28-Oct-05, at 4:24 AM, Zulfiqar Malik wrote:

> Last time I posted it on the list, someone told me that this was not
> possible since LUA is not a type safe language and you cannot enumerate
> structures and classes registered from C++ as such, but obviously LUA 
> does
> it somehow. E.g. an expression a = object:var1 + object:var2 works and 
> it
> properly calculates the offset of var1 and var2 w.r.t. the base 
> object, and
> its hard for me to believe that someone from the application side 
> can't do
> it!

object:var1 is not really an "offset" (and actually that doesn't work, 
it
would need to either be object.var1 or object:var1()) -- but in any 
case it
will end up being a lookup, possibly recursive.

There are many times when you can "look things up" without being able 
to enumerate them. The activities are quite different. In Lua, the 
looking a key up in a table can cause an arbitrary function to execute 
(a metamethod) and the key will be deemed to exist if and only if the 
function returns a non-nil value. Since Lua is turing-complete, being 
able to enumerate the possible keys in such a table is equivalent to 
the halting problem which is well known to be unsolvable.