Subj : Re: Reserving another slot for private data
To   : Sterling Bates
From : Brendan Eich
Date : Thu Apr 22 2004 02:55 pm

Sterling Bates wrote:
> I need to store within each object a reference back to the Delphi 
> object.  I can currently do so with standard JS objects, but I need the 
> same for strings and doubles, which occupy their standard private slot. 
>  So I think I would use this slot like so:
> 
> 1. Implement JSClass.reserveSlots to return 1 (since I only need one slot.)


This won't work for strings and doubles, which are not objects, so have 
no slots (reserved, private, or otherwise).

As an aside, you can reserve a constant number n of slots by including 
JSCLASS_HAS_RESERVED_SLOTS(n) in the class's flags initializer.  The new 
reserveSlots hook is optional and only when the number of slots varies 
per object (but does not change once an object has been constructed).


> 2. After the object is created, use JS_SetReservedSlot to assign the 
> slot value.
> 3. When I need to get the Delphi object reference, call JS_GetReservedSlot.


Yes, those are the APIs to use to access reserved slots (I thought you 
were going to list alternatives in 2 and 3; I'll do that below).


> The problem is that conversion to & from a jsval could drop the 
> left-most bit in the pointer, which would be problematic. 


How so?  The jsval is a tagged pointer to the object, string, or double. 
  The reserved slots you want are in each object's slots array.  A 
string is a primitive type; if you wrap it with a String object and hack 
the engine so that String objects have reserved slots, then you can 
indeed associate a pointer with the object -- but not with the primitive 
type value it "boxes".

Note that boxing is not transparent.  You can't use a String object in 
all ways as if it were a string datum.  Ditto for numbers (doubles).


> Is that 
> actually an issue, and how could I avoid this?


I think you're confused by jsval tagging -- ignore it ;-).

The problem you've posed has come up before for objects, and other than 
reserved slots, a perhaps better solution is to maintain your own 
hashtable mapping GC-things to whatever data you like.  You can use 
JS_SetGCCallback and JS_IsAboutToBeFinalized from it to tell when to 
remove table entries.

Why do you need the back-reference per string or double?  Are *data* of 
those primitive types really always associated 1:1 with Delphi *objects*?

/be

.