From: Johann Hibschman <johann@...>
Date: 2001-12-28T05:34:13+09:00
Subject: [ruby-talk:29483] GC and values in extensions

Hi folks,

When is it safe to leave a VALUE floating around in an extension
module?  I want to have C++ classes with VALUEs as members, but
I don't know how to protect those from the garbage collector.

Is this really a problem, and, if so, what do I have to do about it?
In theory, it seems like I would have to somehow tell Ruby that the
variable "tmp" contains a ruby object.  rb_define_variable will do
that for globals, but not for objects that will go away.

rb_gc_register_address looks promising, but I'm left wondering if
anything bad would happen if I registered a variable which is already
accessible through Ruby, or if I registered the same memory location
twice.

I'm wondering about this, since I'm trying to build a C++ "handle"
for Ruby objects, which should somehow tell Ruby that it's holding
a reference to the the object upon creation, then tell ruby that
it's done with that reference upon destruction, like:

void foo(VALUE r_obj) {
    RBObject cpp_obj(r_obj);     // Hold a reference to r_obj
    <<do stuff with cpp_obj, maybe stashing it away somewhere>>
    return;                      // Destruction of cpp_obj frees ref.
}

(Well, okay, it doesn't "free" the ref at the end, it just tells the
interpreter that it can stop worrying about that address in memory.)

If you can give me any pointers, I'd appreciate it.

--Johann