From: Rick DeNatale <rick.denatale@...>
Date: 2009-05-08T01:21:41+09:00
Subject: Re: Superclass of eigenclass

On Thu, May 7, 2009 at 11:50 AM, Rick DeNatale <rick.denatale@gmail.com> wrote:
> On Thu, May 7, 2009 at 8:35 AM, Danny O cuiv <danny.ocuiv@gmail.com> wrote:
>> On page 261 of The Ruby Programming Language, they state:
>>
>> "Class objects are special: they have superclasses. The eigenclasses of
>> class objects are also special: they have superclasses, too. The
>> eigenclass of an ordinary object stands alone and has no superclass."
>
> I'm pretty sure that this is a typo.

Just for the record, I just submitted the following erratum report to
the O'Reilly site for the book:

Page 261 2nd Paragraph:
The paragraph says:

"Class objects are special: they have superclasses. The eigenclasses of
class objects are also special: they have superclasses, too. The
eigenclass of an ordinary object stands alone and has no superclass."

Both the eigenclass of a class object and the eigenclass of an
ordinary object have a superclass.  If not then sending a message to
an ordinary object would end up invoking method_missing.

I think that the paragraph should read:

"Class objects are special: they can have subclasses. The eigenclasses of
class objects are also special: they can have subclasses, too. The
eigenclass of an ordinary object can have neither a subclass, nor any
other instance except that single ordinary object.  If an ordinary
object with an eigenclass is duplicated with the Object#dup method,
the duplicate will not acquire any of it's singleton methods. If it is
cloned with the Object#clone method, the result will get an eigenclass
which is a duplicate of the original objects eigenclass, so that each
eigenclass has but a single instance."

The following ruby code (which behaves identically on Ruby 1.8.6,
1.8.7 and 1.9.1) illustrates the above:

module Kernel
  def eigenclass
    class << self;self;end
  end
end

class TestClass

  def bar
    :bar
  end

  def self.foo
    :foo
  end
end

t = TestClass.new
def t.gorp
  :gorp
end

p t_eigenclass = t.eigenclass # => #<Class:#<TestClass:0x232d0>>
p t_eigenclass.superclass # => #<Class:TestClass>

p t.bar # => :bar
p t.gorp # => :gorp

p t_dup = t.dup
p t_clone = t.clone
p t_clone.respond_to?(:gorp) # => true
p t_clone.eigenclass == t_eigenclass # => false
p t_dup.respond_to?(:gorp) # => false

p t.class.foo # => :foo
p t.eigenclass.foo # => :foo


-- 
Rick DeNatale

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