From: "Mauricio Fernández" <batsman.geo@...>
Date: 2004-09-14T20:32:01+09:00
Subject: Re: Singleton method on object via define_method?

On Tue, Sep 14, 2004 at 07:39:57PM +0900, Robert Klemme wrote:
> > There's at least one case where you can know if the singleton class has
> > been created or not (singletons of singletons).
> 
> How come?  What's special about singletons here?

>> o = Object.new
=> #<Object:0x401fdd20>
>> class << o; class << self; def foo; "foo" end end end
=> nil
>> class << o; class << self; foo end end
=> "foo"
>> class << o; class << self; class << self; end end end
=> nil
>> class << o; class << self; foo end end
NameError: undefined local variable or method `foo' for #<Class:#<Class:#<Object:0x401fdd20>>>
        from (irb):5


> > And you can always know
> > if you're using evil.rb, of course :-)
> 
> How exactly would you want to do it?  I don't see how eval helps here.

eval is evil, but I really meant evil.rb :-)

>> require 'evil'
=> true
>> o = Object.new
=> #<Object:0x4032e35c>
>> o.internal.klass.to_i
=> 1075690784
>> class << o; end
=> nil
>> o.internal.klass.to_i
=> 1076982108
>> class << o; def foo; end end
=> nil
>> ObjectSpace._id2ref(o.internal.klass.to_i/2).instance_methods.include? "foo"
=> true
>> klass = ObjectSpace._id2ref(o.internal.klass.to_i/2)
=> #<Class:#<Object:0x4032e35c>>
>> class << o; self end
=> #<Class:#<Object:0x4032e35c>>

hence

>>  def has_singleton?(obj); obj.class.id * 2 != obj.internal.klass.to_i end
=> nil
>> o = ""
=> ""
>> has_singleton? o
=> false
>> class << o; end
=> nil
>> has_singleton? o
=> true


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