From: dblack@...
Date: 2006-03-02T23:07:08+09:00
Subject: Re: Class method in singleton_methods?

Hi --

On Thu, 2 Mar 2006, Minkoo Seo wrote:

> Hi group.
>
> I've found that I have to query class methods like:
>
> irb(main):001:0> class Foo
> irb(main):002:1>   def Foo.bar
> irb(main):003:2>   end
> irb(main):004:1> end
> => nil
> irb(main):005:0> Foo.singleton_methods
> => ["bar"]
> irb(main):006:0>
>
> It is interesting that a class method is actually a singleton method. I
> know that there are tons of posting regarding metaclasses/objects.
> Unfortunately, there are simply too many to read and understand all of
> them. So, please forgive my naive question.
>
> Here's the thing. I guess Foo is actually defined like:
>
> irb(main):001:0> Foo = Class.new
> => Foo
> irb(main):002:0> class << Foo
> irb(main):003:1>   def Foo.bar
> irb(main):004:2>   end
> irb(main):005:1> end
> => nil
> irb(main):006:0> Foo.singleton_methods
> => ["bar"]
> irb(main):007:0>
>
> Am I correct?

A class method is indeed a singleton method of a Class object.  The
term "class method" is really just a convenient label for this case,
because it occurs quite frequently.

The "def obj.meth" and "class << obj; def meth" techniques differ as
to the visibility of constants:

   X = 1
   class C
     X = 2
     def self.a
       puts X
     end
   end

   class << C
     def b
       puts X
     end
   end

   C.a               # 2  (C::X)
   C.b               # 1  (top-level X)

But in the vast majority of cases you can use them pretty much
interchangeably.


David

-- 
David A. Black (dblack@wobblini.net)
Ruby Power and Light (http://www.rubypowerandlight.com)

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