From: Gavin Sinclair <gsinclair@...>
Date: 2003-02-12T01:27:19+09:00
Subject: Re: inheriting from base classes

On Wednesday, February 12, 2003, 1:26:50 AM, dblack wrote:

> [On ClassRoster < Array]
>
> But then there's the question: if a ClassRoster is essentially an
> ordered list of Student objects, and that list is going to be added
> to, sorted, subtracted from, mapped over, etc. etc.... what is the
> disadvantage to having it subclass Array (rather than sort of shadow
> Array via instance variables and rewriting of methods that Array
> already has)?  I'm willing to believe there are disadvantages, but
> I'm not succeeding in picturing an actual scenario where it would
> play out badly.

If I were writing code that needed to perform all those operations on
class rosters, I would be inclined to do this:

  class_roster = []

Then you can apply all the Array methods you like! :)

But you want some ClassRoster-specific methods.  I'd be looking at
hiding the complex manipulations of the class roster from the outside
world.

  class ClassRoster
    def initialise
      @roster = []
    end

    def foo
      @roster.manipulate
    end

    ...
  end


That's what you should be aiming for in OO code: users don't have to
do any low-level stuff on your objects.  You implement just the
methods they need.  They shouldn't have to know that Array is used to
implement it.

Of course, you need a way of building up a roster.  That might be by
parsing a text file, or you may need to resort to:

  class ClassRoster
    def <<(o)
      @roster = []
    end
  end

Rather than say "gosh, what a pain doing such a boring method", look
for ways to eliminate the need for it.  Or, if a few such methods are
indeed perfectly justifiable, then use one of Ruby's forwarding
mechanisms, or try to do more than just delegate (error checking, for
instance).

At the end of the day, the fewer methods per class, the better.
Superfluous methods cloud the intention of the class.  (String et al
are excluded from this rule for obvious reasons.)

Gavin