From: Scott Strattner <strattner@...>
Date: 2008-10-29T02:57:35+09:00
Subject: Proper way to define a subclass within parent file?

I am starting to build a script to manipulate various network device
information, including a list of Vlans from a switch. To that end, I
created a Vlan class:

class Vlan
  @@max = 399
  @@min = 1
  @@protected = [1,2,5]
  @@namelen = 32
  def initialize (id,desc)
    self.set_name(desc) if (self.set_id(id))
  end
  def set_id (id)
    return nil unless (id.kind_of? Integer)
    return nil if (@@protected.index(id) || id > @@max || id < @@min)
    @id = id
  end
  def set_name (desc)
    desc.gsub!(/\s+/,"_")
      desc = desc[0..@@namelen-1] if (desc.length > @@namelen)
      @description = desc
  end
  def to_s
    return "#{@id}\t#{@description}"
  end
  def valid?
    return true if (@id.kind_of? Integer)
    return false
  end
  # snip various other methods
end

This works as expected; to test it out I added some debugging code to
loop through some values and verify it only created valid instances:

v = Array.new
20.times { |a|
  temp = Vlan.new(a,"this is vlan id #{a}")
  v.insert(-1,temp) if (temp.valid?)
}
v.length.times { |a| puts "#{a} #{v[a].to_s}" }

I then added a simple subclass, within the same file. This special kind
of Vlan acts like the others, except it uses a different range of valid
id values:

class HMC_Vlan < Vlan
  @@min = 701
  @@max = 799
  @@protected = [700,705]
end

I then added to the debug code:

v2 = Array.new
700.upto(720) { |a|
  temp2 = HMC_Vlan.new(a,"hmcvlan.#{a}")
  v2.insert(-1,temp2) if (temp2.valid?)
}
v2.length.times { |a| puts "#{a} #{v2[a].to_s}" }

When I ran this file, it created HMC_Vlans as expected, but no longer
created any valid Vlans ("v" array was empty). It turns out the class
variables @@max, min, protected in HMC_Vlan are overwriting the class
variables in Vlan - even though no instance of HMC_Vlan is created until
after my debug code creates the "v" array.
It was my understanding that a Class declaration was treated as a
conditional block - *if* someone creates this Class, *then* run through
the contents. But here it appears the class declaration is processed
before any instance of it is made. Even so, shouldn't a class variable
be specific to that class, and not be shared with children? Is there
another kind of inheritance I can use to maintain this separation in
class variables? Is there a specific way to define a subclass within the
same file as the parent? Why does the class HMC_Vlan definition get
executed before any instances are created?

I am running this under Windows (ActiveRuby 1.8.6)
-- 
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