Subj : Re: GetProperty
To   : netscape.public.mozilla.jseng
From : Brendan Eich <brendan@meer.net>
Date : Tue Feb 10 2004 05:45 pm

Brendan Eich wrote:

>> JSBool ScrSFVec3f::JSGetProperty(JSContext* iContext, JSObject* iObject,
>> jsval id, jsval *vp)
>> {
>>    if (JSVAL_IS_INT (id))
>>    {
>>          SoSFVec3f* lVector = (SoSFVec3f*) JS_GetPrivate(iContext,
>> iObject);
>>
>>            static double lValue;
>>            lValue = lVector->getValue()[JSVAL_TO_INT(id)];
>>            *vp = DOUBLE_TO_JSVAL(&lValue);
>>        }
>>        return JS_TRUE;
>> }
>>
>>
>> Now, my question is if the Javascript engine makes a copy of the 
>> double, or does it come back to the memory location of lValue 
>> whenever it needs to read the value of the double. This is important 
>> for the following javascript 
>
>
> It's important for the above example too -- you're returning the 
> address of a stack temporary that goes out of scope as soon as 
> ScrSFVec3f::JSGetProperty returns!
>
> Be careful with pointers (addresses) in C ;-).


And I should be careful with my skimming of your code!  Sorry, I missed 
the "static" storage class on double lValue (need my eyes examined).

But my final point stands.  You can't take any old jsdouble address and 
tag it as a jsval and store it in a property or other place known to the 
GC.  Only GC-allocated jsdoubles can be so tagged.  Hope this is all 
clear now.

/be

.