From: Ron Jeffries <ronjeffries@...>
Date: 2002-02-26T03:24:18+09:00
Subject: Re: Style Question

On Mon, 25 Feb 2002 16:24:18 GMT, Dave Thomas
<Dave@PragmaticProgrammer.com> wrote:

>Ron Jeffries <ronjeffries@REMOVEacm.org> writes:
>
>> I could build a separate class, SetFactory, and do things over there,
>> but in Ruby I see no reason to do that. I'm comfortable with a bunch of
>> factory methods on Set, such as Set.create, except that for those
>> methods to compile, all the concrete subclasses that Set uses have to be
>> at least declared before they are used. So the top of the code looks
>> like this:
>
>How about having the concrete classes extend set to add their own
>factories in
>
>  # untested
>  class Set
>    class <<self
>       protected :new
>    end
>  end

I don't understand the notation above with class << etc. What's up with
that? How is it different from

class Set
  protected :new
end

>  class ArraySet < Set
>    def Set.fromArray(ary)
>      s = Set.new
>      ary.each {|i| s.add(i) }
>    end
>  end
>
>  class WibbleSet < Set
>    def Set.fromWibble(wib)
>      # ...
>    end
>  end

I think I get this. It's basically the same deal but avoids having to
predeclare the subclasses up at the top, right? Definitely a better way.
>
>
>  as = Set.fromArray [ 1, 2, 3, 4 ]
>  wb = Set.fromWibble( wibble, wobble, whee! )
>
>This schema means you can add new concrete representations without
>ever changing the base class.

I guess you mean without ever editing the base class's file if it has
its own. Certainly defining a method on it is changing the class itself?

Thanks!

Ronald E Jeffries
http://www.XProgramming.com
http://www.objectmentor.com
I'm giving the best advice I have. You get to decide whether it's true for you.