From: Robert Klemme <shortcutter@...>
Date: 2007-12-13T20:00:52+09:00
Subject: Re: Inheritance and meta-classes

> > 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.

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"]

Note also that #respond_to? is a weak method to determine whether an
instance will respond to a method, because of #method_missing which I
believe is used a lot by Rails.  Also, it is not called on the class
but on the object.

You can use #instance_method(s) to check whether a class implements a
certain instance method but as I said, you might not be lucky.

Kind regards

robert