From: dblack@...
Date: 2007-08-17T09:06:37+09:00
Subject: Re: Does singleton variables have any meaning ?

Hi --

On Fri, 17 Aug 2007, Arno J. wrote:

>>>  class << self  #or class << A
>>>
>>>    @singleton_variable # <- Can this be used/called elsewhere ?
>>
>> Like all instance variables, it belongs to whatever object is 'self'
>> at the point where the instance variable appears, and multiple
>> references to the same instance variable in different contexts where
>> self is the same, will be references to the same instance variable.
>
> But here self is #<Class:A>. Here it really the same context/self as for
> class instance variables (A).

self is #<Class:A> (the singleton class of A), so @singleton_variable
belongs to #<Class:A>.  It has no connection to A.

>
>> I don't find the term "singleton variable" very helpful. All the
>> variables in your example are just instance variables. Calling them
>> "class instance variables" is sometimes useful, since there are
>
>
> So, let's take an another example. After every "puts self" I put the
> result.
>
> class A
>  puts "Inside class"
>  puts self # A -> This is the self that our instance variable would be
> linked to, making a "class instance variable"
>
>  def meth
>    puts "Inside method"
>    puts self # #<A:0x2ebd140>
>  end
>
>  def self.methc
>    puts "Inside class method"
>    puts self # A
>  end
>
>  class << self
>    puts "Inside singleton"
>    puts self # #<Class:A> -> and this is the self that our instance
> variable would be linked to...
>
>    def meths
>      puts "Inside singleton method"
>      puts self # A
>    end
>  end
> end
>
> a = A.new
> #<A:0x2ebd140>
>
> With a being #<A:0x2ebd140>, the results are :
> - A
> - #<A:0x2ebd140>
> - #<Class:A>
>
> So, when defining what's usually called a "class instance variable", the
> self is "A". For what I call a "singleton variable", but that you called
> a "class instance variable", the self is "#<Class:A>".
> To me, there's a difference, but I don't know what that "#<Class:A>" is.

#<Class:A> is the singleton class (sometimes called the "metaclass")
of A. It's a separate object; it has its own instance variables, and
doesn't share them with A.

As for the terminology; here:

   class << A
     @var
   end

@var is an instance variable, and self is a class. So we can call it
an instance variable or a class instance variable. I don't think it's
a good idea to introduce yet another term (singleton variable) for it;
it just gets too confusing.

> It's the same thing with a self.method.
> Can someone tell me how to use those @my_variable and self.method in the
> following class :
>
> class A
>  class << self
>    @my_variable
>    def self.my_method
>    end
>  end
>  # This is where I'd like to use it
> end

You can't use it there, because you're in a new 'self' context (A
rather than #<Class:A>). Therefore the instance variables belong to a
different object.

You could get indirect access to it if the object that owns it creates
wrapper methods for it. But if you put @my_variable where your "This"
comment is, it will be a completely different @my_variable.


David

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