Subj : Re: Custom C++ objects with references and garbage collecting in
To   : netscape.public.mozilla.jseng
From : Shanti Rao
Date : Fri Mar 18 2005 09:16 pm

Hi Mads,

I solved this problem by using smart pointers. The code is at 
http://www.jsdb.org/jsdb.h and it works something like this:

You use these macros to to replace JS_GetPrivate() and JS_SetPrivate()

#define GETPRIVATE(x,obj) (((JSPointer<x>*)JS_GetPrivate(cx,obj))->P)
#define GETPOINTER (((JSPointerBase*)JS_GetPrivate(cx,obj)))

#define SETPRIVATE(obj,x,t,ad,parent) \
  JS_SetPrivate(cx,obj,new JSPointer<x>(parent,t,ad))

#define DELPRIVATE(x) \
JSPointer<x> * t = \
    (JSPointer<x>*)JS_GetPrivate(cx,obj);\
  if (t) delete t; t=0; JS_SetPrivate(cx,obj,NULL)

#define CLOSEPRIVATE(x) \
JSPointer<x> * t = \
    (JSPointer<x>*)JS_GetPrivate(cx,obj);\
  if (t) t->Close()

A finalize function looks like this

void Table_JSFinalize(JSContext *cx, JSObject *obj)
{
  DELPRIVATE(DataTable);
}

When you want to create a JSObject representation of a child class in 
B::GetA(), you call a function like this:

JSObject*
New_Object(JSContext *cx, TParameterList* t,bool autodelete,JSPointerBase* Parent)
{
  JSObject* obj;
  obj = JS_NewObject(cx, ...,..., NULL));
  JS_DefineFunctions(cx,obj,functions);
  JS_DefineProperties(cx,obj,properties);
  SETPRIVATE(obj,TParameterList,t,autodelete,Parent);
  return obj;
}


If 'autodelete' is false, this pointer system will ensure that if B is garbage 
collected, then the actual C++ object representing B will not be deleted. 
Instead, its child A objects keep track of it, and the B object gets cleaned 
up AFTER the last A object is GC'd. Presto!

Shanti


> 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 use(a);  // Error, a is no longer valid.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
>

.