Subj : Re: Confused about enumerating
To   : Brian Genisio <BrianGenisio@yahoo.com>
From : Brendan Eich <brendan@meer.net>
Date : Fri Feb 20 2004 10:53 pm

Brian Genisio wrote:

> I am creating an enumeration JSID based off of the Ids of the objects 
> in the collection.  For a collection with two elements, I expect the 
> following to be printed, using my alert method:
>
> ALERT: length
> ALERT: value1ID
> ALERT: value2ID
>
> Instead, I see the following:
>
> ALERT: value1ID
> ALERT: value2ID
>
> If I change my embedded code to give "Length", instead of "length", I 
> get the following:
>
> ALERT: Length
> ALERT: value1ID
> ALERT: value2ID
>
> If I keep the value to "length", but get rid of "length" as a defined 
> property of the object, it comes out the way I expect it.


This is for the reason I said earlier: the code for for..in loops in 
jsinterp.c will suppress duplicate property id enumeration in the 
following sort of case:

var p = {q: 0, r: 1}, o = {__proto__: p, q: 2, r: 3};
for (var i in o) print(i, o[i]);

This script should print

q 2
r 3

The jsinterp.c code will find q twice, once when enumerating o, and 
later when enumerating its prototype object p.  In the latter case, it 
will suppress the enumeration of 'q' as the value of i because it looks 
up 'q' in o, the original object, and finds it first in o, which is not 
the same object as the one currently being enumerated, namely p.

In your case, you haven't actually defined a property 'length' in the 
original object, so the lookup finds 'length' in the prototype, notices 
that that object is not the same as the one currently being enumerated, 
and suppresses the "duplicate".

The moral is: don't lie in your enumerate implementation: return only 
ids that you will show as being in the object in question, via resolve 
if lazy (if reflecting properties only on demand), or getProperty if 
eager (if the property of that id in the class's object is being defined 
up front).

In short, it seems your resolve hook is not consistent with your 
enumerate hook.  You are enumerating 'length', but you are not resolving 
'length'.  That's always a bug.

Second bug: you shouldn't allocate and free (via new and delete) a word 
of storage for an unsigned long.  Just store an int-tagged jsval 
directly in *statep, using JSVAL_TO_INT and INT_TO_JSVAL.  I doubt 
you'll overflow 2^30 - 1.  What you are doing now with JSVAL_TO_PRIVATE 
and PRIVATE_TO_JSVAL is wrong.  See the definitions of those macros in 
jsapi.h -- they are for decoding and encoding a pointer aligned on a (0 
mod 2) or looser boundary as a jsval.  PRIVATE_TO_JSVAL will turn an 
even unsigned long value into the next higher odd number, and 
JSVAL_TO_PRIVATE will turn an odd number into its lower even neighbor.

/be

.