Forum: Ruby Class Variables for subclases...

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9ea8229b9b6503cde77171ffcfe2eedc?d=identicon&s=25 Sonny Chee (soroe)
on 2007-03-27 04:37
Hey Guys,

Check out the following:

class A
  @@anobject = 'green'
  def fun2
    @@anobject
  end
end

class B < A
  @@anobject = 'blue'
  def fun2
    @@anobject
  end
end

puts B.new.fun2    # blue
puts A.new.fun2    # blue

So it looks like the subclass B is not defining its own class variable
but reusing A's class variable @@anobject. Is there a different way to
specify class variables to overcome this issue?

Sonny.
0ec4920185b657a03edf01fff96b4e9b?d=identicon&s=25 Yukihiro Matsumoto (Guest)
on 2007-03-27 04:44
(Received via mailing list)
Hi,

In message "Re: Class Variables for subclases..."
    on Tue, 27 Mar 2007 11:37:38 +0900, Sonny Chee
<sonny.chee@gmail.com> writes:
|Check out the following:
|
|class A
|  @@anobject = 'green'
|  def fun2
|    @@anobject
|  end
|end
|
|class B < A
|  @@anobject = 'blue'
|  def fun2
|    @@anobject
|  end
|end
|
|puts B.new.fun2    # blue
|puts A.new.fun2    # blue
|
|So it looks like the subclass B is not defining its own class variable
|but reusing A's class variable @@anobject. Is there a different way to
|specify class variables to overcome this issue?

You can use constants-as-class-shared technique.

class A
  Anobject = ['green']
  def fun2
    Anobject[0]
  end
end

class B < A
  Anobject = ['blue']
  def fun2
    Anobject[0]
  end
end

puts B.new.fun2    # blue
puts A.new.fun2    # blue
9ea8229b9b6503cde77171ffcfe2eedc?d=identicon&s=25 Sonny Chee (soroe)
on 2007-03-27 05:25
Yukihiro Matsumoto wrote:

> You can use constants-as-class-shared technique.

Thanks, Matz.  For my edification, is there a reason why class variables
behave in this way when subclassed?  Is this a feature of the language
or the Ruby Interpretter?

Sonny.
47b1910084592eb77a032bc7d8d1a84e?d=identicon&s=25 Joel VanderWerf (Guest)
on 2007-03-27 05:34
(Received via mailing list)
Sonny Chee wrote:
>
> So it looks like the subclass B is not defining its own class variable
> but reusing A's class variable @@anobject. Is there a different way to
> specify class variables to overcome this issue?
>
> Sonny.
>

You may find it more suitable to use instance variables in your class
objects, and write accessors so you can get at them from instances of
your classes.

For example:

class A
   @anobject = 'green'
   def self.anobject
     @anobject
   end
   def fun2
     self.class.anobject
   end
end

class B < A
   @anobject = 'blue'
end

puts B.new.fun2    # blue
puts A.new.fun2    # green


Google (or search the list) for 'ruby class instance variable'.

I think the 'traits' gem does this in a systematic way, though of course
it's worth playing around to understand the concept first.
9ea8229b9b6503cde77171ffcfe2eedc?d=identicon&s=25 Sonny Chee (soroe)
on 2007-03-27 05:38
Thanks for the pointers Joel.

Sonny.
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