From: 7stud -- <bbxx789_05ss@...>
Date: 2008-01-23T17:13:12+09:00
Subject: Re: singleton definition differences

Nicholas Randal wrote:
> would someone please explain the difference between these definitions
> 
> class A
>   class << self
>     def hello
>       puts 'hello'
>     end
>   end
> end
> 
> class B
>   def B.hello
>     puts 'hello'
>   end
> end
> 
> A.hello
> B.hello

According to p. 36, "Programming Ruby(2nd ed.)", they are different 
syntaxes for accomplishing the same thing.   If you define class A like 
this:

class A
  def sayhi
    puts "hi"
  end
end

a = A.new
a.sayhi


then you have this situation:


a -- instance of class A
|
|
|
V
class A
methods:
---sayhi()
|
|
|
V
class Object
methods:
---send(), etc.


If you define class A like you did:

> class A
>   class << self
>     def hello
>       puts 'hello'
>     end
>   end
> end

then this is what happens:

a -- instance of class A
|
|
|
V
class A
methods:
-- <no methods>
|
|
|
V
class A' -- a superclass ( or "singleton class") for A only containing 
A's class methods
methods:
---hello()
|
|
|
V
class Object
methods
--send(), etc.

In effect, you inserted a new class between class A and class Object. 
The dot notation does the same thing.
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