From: Robert Klemme <shortcutter@...>
Date: 2007-12-16T21:24:58+09:00
Subject: Re: Inheritance and meta-classes

On 13.12.2007 15:33, Rick DeNatale wrote:
> 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>

Ah, now I see!  Should've put on my glasses. ;-)

>> 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.

According to the definition given by you the class that's used in 
statement 8 is a meta class.

> 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

Since a metaclass - according to your definition - is a class's 
singleton class it sits between the instance (the class object, e.g. 
Array) and its class (Class) in the same way an ordinary's object's 
singleton class does - which is also shown by the image you referred to 
in your posting.

> 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.

You mean "class<<Array;self;end", do you?

> 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.

Nice colors. ;-)  I reckon Array' is intended to mean the singleton 
class of object Array.  I am not sure I get your point where you wanted 
to correct me.  As far as I can see we are completely in sync.

Generally singleton classes of class instances and of ordinary instances 
behave the same.  The only difference I can see so far is that, since 
classes can inherit from each other, a subclass also shares instance 
methods defined for its superclass (see my irb session).  Btw, something 
similar happens with instances when cloned:

irb(main):001:0> o = Object.new
=> #<Object:0x7ff9e474>
irb(main):002:0> def o.xxx;1;end
=> nil
irb(main):003:0> o.xxx
=> 1
irb(main):004:0> class <<o;self;end.instance_methods.grep /xx/
=> ["xxx"]
irb(main):005:0> o1 = o.clone
=> #<Object:0x7ff80eec>
irb(main):006:0> o1.xxx
=> 1
irb(main):007:0> class <<o1;self;end.instance_methods.grep /xx/
=> ["xxx"]
irb(main):008:0> o2 = o.dup
=> #<Object:0x7ff73cec>
irb(main):009:0> o2.xxx
NoMethodError: undefined method `xxx' for #<Object:0x7ff73cec>
         from (irb):9
         from :0
irb(main):010:0> class <<o2;self;end.instance_methods.grep /xx/
=> []

Kind regards

	robert