What comes first in Ruby's object model?

Question

I've been reading Metaprogramming Ruby and the object model is like the chicken or egg dilemma.

In Ruby 1.8, the Object class is an instance of Class. Module's superclass is Object and is an instance of Class. Class' superclass is Module, and it is an instance of Class (self-referential). Say class SomeClass; end is defined somewhere; SomeClass is an instance of Class, however its superclass is Object. Why does an instance of Class have Object as the superclass instead of nil?

Also, if Object is to exist, then Class has to exist, but then Module has to exist, but for Module to exist Object has to exist. How are these classes created?

Answer 1

Here is a complete Ruby Class Diagram (for Ruby 1.8): http://banisterfiend.wordpress.com/2008/11/25/a-complete-ruby-class-diagram/

To help you understand the strange seemingly impossible self-reflexive nature of the class diagram it is useful to know that Class pointers and Super class pointers can be assigned at any time in the C API. That is, you can create an object (in the C API) and after-the-fact decide what the Class and Super class pointers point to.

Also, to understand the order of definition, look at Init_Object() in object.c (in Ruby 1.9)

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);

metaclass = rb_make_metaclass(rb_cBasicObject, rb_cClass);
metaclass = rb_make_metaclass(rb_cObject, metaclass);
metaclass = rb_make_metaclass(rb_cModule, metaclass);
metaclass = rb_make_metaclass(rb_cClass, metaclass);
boot_defmetametaclass(rb_cModule, metaclass);
boot_defmetametaclass(rb_cObject, metaclass);
boot_defmetametaclass(rb_cBasicObject, metaclass);

Where rb_cBasicObject is BasicObject in Ruby, rb_cObject is Object in Ruby, and so on.

Answer 2

• http://ruby-doc.org/core/classes/Class.html <-- This shows the self referential diagram.
• http://ruby-doc.org/core/classes/Object.html

The easiest thing to consider is that everything in ruby is an Object. Everything else is a modification of this concept. So the class Class adds class like functionality, by modifying Object. As a result every Object is a class, etc.

The superclass of Object is nil. (Reference)

The superclass of Object is BasicObject (in 1.9) and the superclass of BasicObject is nil.

Answer 3

You can find some more details in the Ruby Hacking Guide: http://rhg.rubyforge.org/

Especially Chapter 04: Bootstrap