From: Robert Klemme <bob.news@...>
Date: 2004-03-24T16:54:28+09:00
Subject: Re: How to avoid duplicate object creations?


"Stig Sandbeck Mathisen" <ssm@fnord.no> schrieb im Newsbeitrag
news:90lad274i50.fsf@talisker.nsc.no...
> "Robert Klemme" <bob.news@gmx.net> writes:
>
> > module SomeDocumentation
> >   class Named
> >     attr :name
> >
> >     def initialize(name)
> >       @name = name
> >     end
> >   end
> >
> >   class Host < Named
> >     INSTANCES = Hash.new {|h,name| h[name] = self.new(name) }
> >   end
> >
> >   class Role < Named
> >     INSTANCES = Hash.new {|h,name| h[name] = self.new(name) }
> >   end
> > end
> >
> > SomeDocumentation::Host::INSTANCES["foo"]
> > SomeDocumentation::Role::INSTANCES["foo"]
>
> Brilliant, thank you.  I think I need to study it for a while, to see
> exactly what it does in combination with my code, though. :)
>
> In addition to this, I also got a message outside the newsgroup
> telling me to use a @@instances hash in the classes, which seems to be
> about the same solution.

I would not choose class variables in this case.  First, you want scoping
by class and not through a hierarchy.  If Host and Role somehow derive
from each other then you have to choose a more comples hash key (i.e. pair
of class and name) to ensure that you don't mix up stuff.

The other is a bit more personal: I generally try to prevent the usage of
class variables since their usage can lead to unexpected results as well
as subtle errors.  One thing is that order matters in which a class var is
referenced. Try this:

#!/usr/bin/ruby

def t(cl)
  p [cl, cl.new.test]
end

# ------------------

class Base1
  (@@foo ||= []) << "base1"

  def test; @@foo; end
end

class Derived1 < Base1
  (@@foo ||= []) << "derived1"
end

# ------------------

class Base2
  def test; @@foo; end
end

class Derived2 < Base2
  (@@foo ||= []) << "derived2"
end

class Base2
  (@@foo ||= []) << "base2"
end

# ------------------

class Base3
end

class Derived3 < Base3
  (@@foo ||= []) << "derived3"
  def test; @@foo; end
end

class Base3
  (@@foo ||= []) << "base3"
  def test; @@foo; end
end

# ------------------

t Base1
t Derived1
t Base2
t Derived2
t Base3
t Derived3


Regards

    robert