Subj : Re: Property Getter & Setter Question
To   : Digital Man
From : Brendan Eich
Date : Sat Nov 20 2004 09:48 am

Digital Man wrote:

> Is there any advantage or disadvantage to setting this attribute (memory
> consumption, performance, etc.)?

Obviously, if there's no slot allocated for the property, then there's a 
memory savings.  Slots are pointer-sized words allocated contiguously 
from the malloc heap, so if you have 100 getter/setter pairs and need no 
slots, then you can save 100 words (400 bytes on most platforms) per object.

> If you have getter/setter functions for a
> property, is there any reason that you *shouldn't* set this attribute?

If the getter and setter want to see the last-got-or-set value in the 
*vp parameter passed in as well as out.  A slot is a strong reference in 
the live thing graph the GC scans, provided the object owning the slot 
is alive.

If you don't use a slot, and you want a reference to be stored for 
return by later gets, you will have to manage not only the storage for 
that reference (perhaps in a private data struct associated with the 
object via JS_SetPrivate), but you'll also have to make sure the 
reference is scanned by the GC (by implementing JSClass.mark).

If you only ever store a translation of the value in a peer object 
space, and need no persistent JS value associated for future gets to 
return, then you need no slot.

> I didn't know about this attribute as it's not documented at:
> http://www.mozilla.org/js/spidermonkey/apidoc/sparse-frameset.html

Patches welcome: mozilla/js/docs/jsref.xml.

/be

.