Class Variables for subclases

soroe · March 27, 2007, 4:37am

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.

soroe · March 27, 2007, 4:44am

Hi,

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

soroe · March 27, 2007, 5:25am

Yukihiro M. 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.

soroe · March 27, 2007, 5:34am

Sonny C. 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.

soroe · March 27, 2007, 5:38am

Thanks for the pointers Joel.

Sonny.