From: Robert Klemme <shortcutter@...>
Date: 2008-09-24T15:34:52+09:00
Subject: Re: understanding inheritance and class singleton methods

On 24.09.2008 07:55, Ittay Dror wrote:
> Hi,
> 
> I'll start with what I understand:
> class Foo
>    class << self
>       def hi
>          puts 'HI'
>       end
>    end
> end
> 
> Creates an instance of a singleton class that has the method #hi. So 
> Foo.hi calls the method defined in the class of Foo -> the singleton 
> class (which inherits from Class)
> 
> My question:
> class Bar < Foo
> end
> 
> Bar.hi --> 'HI'
> 
> It looks like two things are happening: 1. Bar points to Foo, so 
> instances of Bar lookup methods in Foo also. 2. The instance Bar has a 
> class that inherits from Foo's instance class.
> 
> Is this correct?

Basically yes.  Although singleton classes are a bit tricky - they are 
partly hidden (for example, you do not see them via Module#ancestors).

> Is there somewhere where I can find the exact flow of 
> '<' (and '<<')?

AFAIK you cannot really "see" this:

irb(main):001:0> class Foo; end
=> nil
irb(main):002:0> class Bar < Foo; end
=> nil
irb(main):003:0> $fc = class <<Foo; self end
=> #<Class:Foo>
irb(main):004:0> $bc = class <<Bar; self end
=> #<Class:Bar>
irb(main):005:0> $bc < $fc
=> nil
irb(main):006:0> $bc > $fc
=> false
irb(main):007:0> $bc.ancestors
=> [Class, Module, Object, PP::ObjectMixin, Kernel]
irb(main):008:0> $fc.ancestors
=> [Class, Module, Object, PP::ObjectMixin, Kernel]
irb(main):009:0> Foo < $fc
=> nil
irb(main):011:0> $fc.superclass
=> #<Class:Class>
irb(main):012:0> $bc.superclass
=> #<Class:Class>

I keep remembering that instance methods of a singleton class are 
inherited by singleton classes of subclasses.  This has served me well 
in the past. :-)

Cheers

	robert