_id2ref

/*
 *  call-seq:
 *     ObjectSpace._id2ref(object_id) -> an_object
 *
 *  Converts an object id to a reference to the object. May not be
 *  called on an object id passed as a parameter to a finalizer.
 *
 *     s = "I am a string"                    #=> "I am a string"
 *     r = ObjectSpace._id2ref(s.object_id)   #=> "I am a string"
 *     r == s                                 #=> true
 *
 */

static VALUE
id2ref(obj, objid)
    VALUE obj, objid;
{
    unsigned long ptr, p0;
    int type;

    rb_secure(4);
    p0 = ptr = NUM2ULONG(objid);
    if (ptr == Qtrue) return Qtrue;
    if (ptr == Qfalse) return Qfalse;
    if (ptr == Qnil) return Qnil;
    if (FIXNUM_P(ptr)) return (VALUE)ptr;
    ptr = objid ^ FIXNUM_FLAG;  /* unset FIXNUM_FLAG */

    if ((ptr % sizeof(RVALUE)) == (4 << 2)) {
        ID symid = ptr / sizeof(RVALUE);
        if (rb_id2name(symid) == 0)
            rb_raise(rb_eRangeError, "%p is not symbol id value", p0);
        return ID2SYM(symid);
    }

    if (!is_pointer_to_heap((void *)ptr)||
        (type = BUILTIN_TYPE(ptr)) > T_SYMBOL || type == T_ICLASS) {
        rb_raise(rb_eRangeError, "0x%lx is not id value", p0);
    }
    if (BUILTIN_TYPE(ptr) == 0 || RBASIC(ptr)->klass == 0) {
        rb_raise(rb_eRangeError, "0x%lx is recycled object", p0);
    }
    return (VALUE)ptr;
}