From: Lyle Johnson <ljohnson@...>
Date: 2001-02-23T07:10:05+09:00
Subject: [ruby-talk:11354] Any way to control order in which objects are GC'd?

I fear that I already know the answer, but I'll go ahead and ask. But first,
the setup.

The problem is that for the Ruby extension module I'm working on, I'm
wrapping C++ objects that can have references (pointers) to other C++
objects. To avoid a problem of dangling pointers with circular references,
one would typically use this kind of pattern in the C++ class destructor:

    // Destructor for SomeClass
    SomeClass::~SomeClass() {
        // clean up all outstanding pointers to me, because I'm about
        // to be destroyed!
    }

Ruby, of course, doesn't have the concept of C++ destructors. When the GC
detects that an object is no longer in use, it will get "collected". So the
problem I'm running into is that when object "Foo" gets garbage collected
(and the underlying C++ object "CFoo" is destroyed), there are still other
C++ objects out there that  have references to "CFoo". As soon as they try
to access the recently deceased object, bad things happen.

So the question is, is there any way to control the order in which objects
get garbage-collected? For the current problem I'm debugging, all of this
action is taking place when the Ruby interpreter shuts down. For those
familiar with the source code, I'm looking at the function
"rb_gc_call_finalizer_at_exit()", which is defined in gc.c. The second loop
in this function loops over all the data objects and GC's them in an
arbitrary order.

Any suggestions are welcome...