From: nobu.nokada@...
Date: 2001-12-28T11:38:00+09:00
Subject: [ruby-talk:29506] Re: GC and values in extensions

Hi,

At Fri, 28 Dec 2001 05:34:13 +0900,
Johann Hibschman <johann@physics.berkeley.edu> wrote:
> 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.

When I wrote such codes once, I used a static Array to hold all
VALUEs referred by C++ objects.  You can use a Hash instead in
recent 1.7.

class RBObject
{
    VALUE obj;
    static VALUE initialize(VALUE obj = 0);
  public:
    RBObject(VALUE v) : obj(initialize(v)) {}
    virtual ~Object();
};

#ifdef HAVE_RB_HASH_DELETE
# define TABLE_NEW() rb_hash_new()
# define TABLE_ADD(obj) rb_hash_aset(objects, (obj), Qfalse)
# define TABLE_DEL(obj) rb_hash_delete(objects, (obj))
#else
# define TABLE_NEW() rb_ary_new()
# define TABLE_ADD(obj) rb_ary_push(objects, (obj))
# define TABLE_DEL(obj) rb_ary_delete(objects, (obj))
#endif
static VALUE objects = (ruby_init(), rb_gc_register_address(&objects), TABLE_NEW());

VALUE
RBObject::initialize(VALUE obj)
{
    if (!rb_special_const_p(obj))
	TABLE_ADD(obj);
    return obj;
}

RBObject::~Object()
{
    if (!rb_special_const_p(obj))
	TABLE_DEL(obj);
    obj = 0;
}

I put complete sample code at
http://member.nifty.ne.jp/nokada/archive/libruby++-0.2.0.tar.bz2


> 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.
> }

In this case, since the caller of foo() can hold r_obj, you
don't need to hold reference within foo().


Nobu Nakada