From: Matt Mongeau <lists@...>
Date: 2013-02-22T04:23:38+09:00
Subject: Re: THE CLASS/OBJECT CHICKEN-AND-EGG PARADOX

void
Init_class_hierarchy(void)
{
    id_attached = rb_intern("__attached__");

    rb_cBasicObject = boot_defclass("BasicObject", 0);
    rb_cObject = boot_defclass("Object", rb_cBasicObject);
    rb_cModule = boot_defclass("Module", rb_cObject);
    rb_cClass =  boot_defclass("Class",  rb_cModule);

    rb_const_set(rb_cObject, rb_intern("BasicObject"), rb_cBasicObject);
    RBASIC(rb_cClass)->klass
  = RBASIC(rb_cModule)->klass
  = RBASIC(rb_cObject)->klass
  = RBASIC(rb_cBasicObject)->klass
  = rb_cClass;
}
This is exactly what is happening. You can see it builds all of these 
first AND then sets the klass AFTER they are created.

But to break it down further...

>> "every object has an internal record"

This is RBasic:
295  struct RObject {
 296      struct RBasic basic;
 297      struct st_table *iv_tbl;
 298  };
290  struct RBasic {
 291      unsigned long flags;
 292      VALUE klass;
 293  };
>> " of what class it’s an instance of,"

See how RObject has an RBasic, see how RBasic has a klass. When 
rb_cObject is first created klass is a null value, later it sets that 
value:

>> "and the internal record inside the object Class"

We're talking about RBasic again here

>> "points back to Class."

 RBASIC(rb_cClass)->klass
  = RBASIC(rb_cModule)->klass
  = RBASIC(rb_cObject)->klass
  = RBASIC(rb_cBasicObject)->klass
  = rb_cClass;
That is done there.

If you want more details about how these things work check out the 
following:

http://rhg.rubyforge.org/chapter02.html
http://rhg.rubyforge.org/chapter04.html

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