From: Rick DeNatale <rick.denatale@...>
Date: 2007-12-13T23:33:39+09:00
Subject: Re: Inheritance and meta-classes

On 12/13/07, Robert Klemme <shortcutter@googlemail.com> wrote:
> > > How do I find where a function is defined in the inheritance tree?
> > > Because of metaclass inheritance being cyclical I don't know where
> > > to stop looking.
>
> Where exactly do you see cycles?  I am not aware of any.

shadowfax:~/ssanta rick$ irb
irb(main):001:0> def meta
irb(main):002:1>   class << self
irb(main):003:2>      self
irb(main):004:2>   end
irb(main):005:1> end
=> nil
irb(main):006:0> meta
=> #<Class:#<Object:0x349a4>>
irb(main):007:0> Array.meta
=> #<Class:Array>
irb(main):008:0> Array.meta.superclass
=> #<Class:Class>
irb(main):009:0> Array.meta.superclass.superclass
=> #<Class:Class>

> 2007/12/13, Adam Salter <adam.q.salter@gmail.com>:
> > I still don't get how you can query and see where in the chain a
> > method got added to an object...
> >
> > I've now found that you can do:
> > obj.class.ancestors # => Array
> >
> > But that still doesn't include metaclasses...
>
> Correct.
>
> > How does Ruby know??
>
> The meta class is there but it's hidden from #ancestors as you found
> out.  Inheritance wise it sits between an object and its class.
>
> irb(main):001:0> c1 = Class.new
> => #<Class:0x7ff9f70c>
> irb(main):002:0> def c1.x; "X" end
> => nil
> irb(main):003:0> c1.x
> => "X"
> irb(main):004:0> c2 = Class.new c1
> => #<Class:0x7ff8f4b0>
> irb(main):005:0> c2.x
> => "X"
> irb(main):006:0> c2.ancestors
> => [#<Class:0x7ff8f4b0>, #<Class:0x7ff9f70c>, Object, Kernel]
> irb(main):007:0> c1.class.instance_methods.grep /x/
> => ["extend"]
> irb(main):008:0> class <<c2;self end.instance_methods.grep /x/
> => ["x", "extend"]

I don't see any metaclasses here.  c1 and c2 are simply anonymous classes.

A metaclass doesn't sit between an instance and its class. A singleton
class of an instance would, but that's not a metaclass (although the
way the two terms get conflated I can see where that's a common
misconception).  A metaclass is a singleton 'class' which holds the
behavior of a class.

Here's a simplified diagram
http://myskitch.com/rubyredrick/skitched-20071213-093049/

It's simplified since it doesn't include the inclusion of Kernel by Object.

-- 
Rick DeNatale

My blog on Ruby
http://talklikeaduck.denhaven2.com/