From: Chris Roos <chrisjroos@...>
Date: 2006-09-11T23:58:31+09:00
Subject: Re: Constant lookup starting in superclass, not derived class

Do you have any suggestions of alternative implementation?

I'd rather not use the self.class.const_get method as it clutters the
code somewhat.

I wonder if I'm trying to refactor two seemingly identical methods
that are in fact different.

class Main1
  def Foo; end
  def foo
    Foo.new
  end
end

class Main2
  def Foo; end
  def foo
    Foo.new
  end
end

At this point, I created a superclass to hold the common foo method
definition.  However, if I had the fully qualified namespace for Foo
(Main1::Foo and Main2::Foo) in each method they would be different and
I wouldn't have moved them.  Maybe that is part of my problem.

I understand the point about the base class depending on child classes
not being desirable.  What about a module relying on constants defined
elsewhere (does the same apply)?

module Base
  def foo
    p Foo.new
  end
end

class Foo
  include Base
  class Foo
  end
end

Thanks again,

Chris

On 9/11/06, Robert Klemme <shortcutter@googlemail.com> wrote:
> Chris Roos wrote:
> > I'm no doubt missing something obvious but I found this a little
> > confusing.  I would have expected the constant lookup to start in Bar
> > and therefore succeed.  Instead, it appears to start in Base where Foo
> > is not defined.
> >
> > class Base
> >  def foo
> >    p Foo
> >  end
> > end
> >
> > class Bar < Base
> >  class Foo
> >  end
> > end
> >
> > Bar.new.foo
> > # ~> -:3:in `foo': uninitialized constant Base::Foo (NameError)
> > # ~>     from -:12
> >
> > Can anyone explain what is happening please?
>
> Foo is not in the scope of Base so it cannot be looked up the way you
> did it.  See Matz's explanation how to access it nevertheless.
> Generally this form of dependency is not desirable, because the base
> class should be self contained and not depend on any sub classes.  In
> Java you could make Base abstract and add an abstract method that will
> return a Class instance - but in Ruby there are no abstract classes.
>
> Kind regards
>
>         robert
>
>