How can Class be of the Class class and not have Class instance methods?

Question

I was studying how the Ruby interpreter is implemented, and one question occurred that didn't get an answer yet for me. That's the one in the title: since Class (r_cClass) has super set to itself (ignoring metaclasses, since actually super is the metaclass of r_cClass), if I send one method to the Class object, this will be looked in the method table of Class' class. But Class' class is Class, so shouldn't I end up looking the instance methods of Class? But that's not the case since in the documentation Class class methods and Class instance methods are separated. In the search_method in eval.c of Ruby, I didn't find any special check for the Class class. Can anyone shed some light on this?

Answer 1

Your beliefs about the way it should work seem right, but I'm not sure why you think it doesn't work that way.

In Ruby 1.8.7:

irb> a = Class.new.methods - Object.new.methods
=> [... 36 element array ...]
irb> b = Class.methods - Object.new.methods
=> [... 37 element array ...]
irb> b - a
=> ["nesting"]

A normal class instance (Class.new) has 36 instance methods. If I look at Class itself, which is also a normal class instance, it has the same 36 instance methods, plus 1 additional class method (nesting), which exists only because it is inherited from its superclass Module.

Note that adding an instance method to Class automatically adds it as a class method as well, but adding a class to Class's metaclass will not.

irb> class Class ; def everywhere ; true ; end ; end
irb> class << Class ; def only_singleton ; true ; end ; end
irb> Class.everywhere
=> true
irb> Class.new.everywhere
=> true
irb> Class.only_singleton
=> true
irb> Class.new.only_singleton
NoMethodError: undefined method 'only_in_singleton' for #<Class:0x4800ac8>