From: dblack@...
Date: 2006-11-08T11:45:37+09:00
Subject: Re: Can't bind a singleton method to a subclass?

Hi --

On Wed, 8 Nov 2006, David Walker wrote:

> The behavior of this snippet doesn't make sense to me:
>
> class A
> def A.class_method
>   puts "Class method worked in #{self}"
> end
> end
>
> class B < A; end
>
> aklass = (class << A; self; end)
> bklass = (class << B; self; end)
>
> p RUBY_VERSION
>
> A.class_method
> B.class_method
>
> p A.method(:class_method)
> p B.method(:class_method)
> p aklass.instance_method(:class_method)
> p bklass.instance_method(:class_method)
> p A.method(:class_method).unbind
> p B.method(:class_method).unbind
> p A.method(:class_method).unbind.bind(A)
> p B.method(:class_method).unbind.bind(B)
>
> The output (for me) is:
>
> "1.8.4"
> Class method worked in A
> Class method worked in B
> #<Method: A.class_method>
> #<Method: B(A).class_method>
> #<UnboundMethod: #<Class:A>#class_method>
> #<UnboundMethod: #<Class:A>#class_method>
> #<UnboundMethod: #<Class:A>#class_method>
> #<UnboundMethod: #<Class:A>#class_method>
> #<Method: A.class_method>
> /workplace2/test.rb:24:in `bind': singleton method called for a
> different object (TypeError)
> 	from /workplace2/test.rb:24
>
> In my mind, the last call should succeed, instead of raising an error.
> Is it just plain impossible to re-bind the method to yield a #<Method:
> B(A).class_method>?

The fact that B can call A.class_method is due to the special-casing
of singleton classes of Class objects; it's the one case where a given
object can execute singleton methods of another object.  But those
singleton methods still belong, unambiguously, to the first object, A.
The reason B can call them is that A's singleton class serves as the
superclass of B's singleton class -- so class_method lies on the
method look-up path of B.  But class_method is still defined only in
A's singleton class.


David

-- 
                   David A. Black | dblack@wobblini.net
Author of "Ruby for Rails"   [1] | Ruby/Rails training & consultancy [3]
DABlog (DAB's Weblog)        [2] | Co-director, Ruby Central, Inc.   [4]
[1] http://www.manning.com/black | [3] http://www.rubypowerandlight.com
[2] http://dablog.rubypal.com    | [4] http://www.rubycentral.org