Subj : Custom C++ objects with references and garbage collecting in spidermonkey
To   : netscape.public.mozilla.jseng
From : Mads Bondo Dydensborg
Date : Fri Mar 18 2005 10:34 am

Hi all

I have been searching for an answer to this question, and I sure hope 
someone can help me out. Say I have the following classes


class A {
  ...
}

class B {
....
   public:
   class A & getA(int i);
}

and that both are exposed to the JS engine, using the private data to 
store a pointer to the object.

Now, using C++ code, these classes have the problem that the following 
code is wrong:

B b = new B();
A & a = b.getA(0);
delete b;
use(a);  // Error, a is no longer valid.

Now, these classes may be broken - I did not design them and can not 
change them on a source level - but avoiding these situation are pretty 
easy in C++, because of the way the classes are used (B is a sort of a 
container for A, and it does make sense).


However, say I have java code like this, where the classes have been 
exposed using the private data fields:

var B = new B();
var a = b.getA(0); // From this point on, b is never referenced

/// code, here
/// The JS decides to GC b
print( a );   // UPS! a contains a pointer to an object embedded in b, 

               // that is now gone!


How do I solve this problem?

Basically I want to say to the GC: "Do not clean B before a have been 
cleaned".

I have thought about Rooting b in a, but you can have many a's coming 
out from b at the same time:

var B = new B();
var a = b.getA(0);
var anothera = b.getA(1);

I don't think there is an easy way to us GC_AddRoot to that? Or rather, 
if I call GC_RemoveRoot, when a is finalized, I have the problem that 
anothera needs b to live on.

I want to say to the GC: "Do not clean B before a and another a have 
been cleaned"

 From reading the docs, it seems to me, that one perhaps should make a 
and another a properties of b? So, I should set a and anothera as 
properties of b, whenever b.getA(int) returns an a?

Does that sound right?

Any help will be greatly appriciated. A pointer to some docs about how 
to use the SpiderMonkey GC "right" would be also be appreciated.

Thanks in advance,

Mads

.