Subj : Re: JS_NewObject
To   : netscape.public.mozilla.jseng
From : "Jochen Stier" <jstier@cs.uvic.ca>
Date : Mon Mar 29 2004 01:51 pm

Thanks,


Really helpful stuff. I will go and do my homework first, before I bother
you with any more
of my elucid assumptions, LOL.

BTW, you did not understand my question.... I said

>With "internal class" I meant a C++ class for which I have created a
>corresponding JavaScript class using JS_DefineClass. Now I would like to
instantiate an object of the
>"internal class" from within the C++ code !!!!!!!!!!===> and not the
JavaScript <==== !!!!!!!! In the >constructor of the internal class I call
JS_NewObject to obtain a JSObject*. I can now use that JSObject* as >one of
the parameters to a JS_CallFunction and everything works fine. I can access
the methods and
>members of the internal class via JavaScript. What I am confused about is
the GC ? At what point
>does the GC delete the JSObject*, if at all ?

I am NOT creating a new object from within JavaScript. I am creating it
inside the C++ app and then pass it to a JavaScript as a parameter in the
call JS_CallFunction (or whatever the proper name of that function may be).
I am just wondering how I can let the GC know that a JSObjects exists and
when it can remove them.

But never mind, I will figure it out....

Thanks a lot for your help !

Cheers
Jochen


BTW, if not GC then what is reference counting good for ????


"Brendan Eich" <brendan@meer.net> wrote in message
news:40688414.6070702@meer.net...
> Jochen Stier wrote:
> > Hi Brendan,
> >
> >
> > With "internal class" I meant a C++ class for which I have created a
> > corresponding JavaScript
> > class using JS_DefineClass.
>
>
> Do you mean JS_InitClass?
>
>
> > Now I would like to instantiate an object of the "internal class" from
> > within the C++ code and not the JavaScript. In the constructor of the
> > internal class I call
> > JS_NewObject to obtain a JSObject*.
>
>
> Why do you call JS_NewObject instead of using the obj parameter, which
> if your class constructor has been called via operator new (tested by
> JS_IsConstructing(cx) from within the constructor code), the obj param
> will refer to a new object of your class, created for you by the engine?
>
> Only if you want your constructor to create a new object when invoked as
> a function (without operator new), as Array and Function do, should you
> code if (!JS_IsConstructing(cx)) { obj = JS_NewObject(...); ... }.
>
>
> > I can now use that JSObject* as one of the parameters to
> > a JS_CallFunction and everything works fine. I can access the methods
and
> > members of the internal class via JavaScript.
>
>
> Do you return the new object via *rval from your constructor function?
>
>
> > What I am confused about is the GC ? At what point does the GC delete
the
> > JSObject*, if at all ?
>
>
> The GC marks all live GC-things starting from its set of roots, some of
> which are known by the code, some of which you add via JS_AddNamedRoot.
>   Any object, string, or double not reached during the mark phase will
> be collected and finalized during the sweep phase.
>
> If the new object you create in your constructor is returned via *rval,
> then the caller's reference will keep that object live until the caller
> returns or finishes.  But within your constructor, after you call
> JS_NewObject, if you haven't protected the newborn, and if you call
> JS_NewObject again, the newborn root in cx that protects the last new
> object allocated using cx will be overwritten, and the first object may
> be collected.
>
> Please read and understand
> http://www.mozilla.org/js/spidermonkey/gctips.html.
>
>
> > I can certainly delete an object of the "internal class"
>
>
> You seem to be using delete and object here in a C++ sense.  Don't
> confuse those meanings with JS's delete and object -- which are quite
> different.
>
>
> > but there must be some call to
> > the Javascript engine to decrement the reference count to the JS_Object.
>
>
> Why do you assume a JSObject (no _) has a reference count?
>
> Garbage collection does not use a reference count; reference-counting is
> not garbage collection.
>
>
> > OpenInventor has
> > something like somevar->ref() and somevar->unref(). Am I correct to
assume
> > that JS_AddRoot
> > and JS_RemoveRoot are equivalent to the the ref() and unref() calls of
> > OpenInventor ?
>
>
> No.  I really think you ought to study the docs
> (http://www.mozilla.org/js/spidermonkey/) and code
> (http://lxr.mozilla.org/mozilla/source/js/src/) more before assuming
> facts not in evidence and then asking for elucidation in the newsgroup.
>
>
> > What about
> > the JS_DeleteProperty that was mentioned by Sterling in the previous
answer
> > ?
>
>
> What about it?  Please read any JS book, or the ECMA-262 spec
> (http://www.mozilla.org/js/language/E262-3.pdf), and learn what the
> delete operator does in JS.  It is not a memory deallocator.
>
> /be

.