Subj : Re: Is this gc-safe
To   : netscape.public.mozilla.jseng
From : Brendan Eich
Date : Wed Aug 31 2005 10:29 am

Thomas Sondergaard wrote:
> On my javascript object I want a property called 'geometry' that returns 
> an array with the upper left and lower right corners of a box, ie. the 
> value of the property might be: [ [0,0], [1,1] ].
> 
> This is the body of the getter function for the property. I wonder if 
> this is okay, or do I need to root the objects as I create them to avoid 
> garbage collection?
> 
>     vector<double> ul = getUpperLeftCorner();
>     vector<double> lr = getLowerRightCorner();
>     jsval ulj[2] = {
>       DOUBLE_TO_JSVAL(JS_NewDouble(ctx, ul[0])),
>       DOUBLE_TO_JSVAL(JS_NewDouble(ctx, ul[1])),

If your malloc implementation can return null, you should null-test 
these and propagate failure (meaning out of memory error was reported) 
if either returns null.

You should also use JS_NewNumberValue, for two reasons:
- It optimizes integral jsdoubles that fit in a jsval, normalizing them 
so they compare with other such normalized doubles.
- JS_NewNumberValue returns the tagged jsval for you via an out param, 
and returns false on failure (error thrown as exception, or reported if 
out of memory).

>     };
>     jsval lrj[2] = {
>       DOUBLE_TO_JSVAL(JS_NewDouble(ctx, lr[0])),
>       DOUBLE_TO_JSVAL(JS_NewDouble(ctx, lr[1])),
>     };
>     jsval ret[2] = {
>       OBJECT_TO_JSVAL(JS_NewArrayObject(ctx, 2, ulj)),
>       OBJECT_TO_JSVAL(JS_NewArrayObject(ctx, 2, lrj))
>     };
>     *vp = OBJECT_TO_JSVAL(JS_NewArrayObject(ctx, 2, ret));
>     return JS_TRUE;
> 
> 
> What jsapi function calls can invoke the gc?

The GC's allocator can invoke the collector, so you need to root as you 
go.  Since you are using C++, you might find JSAutoLocalRootScope in the 
latest CVS jsapi.h handy.

/be

.