[email protected] wrote:
The way I see it, there’s two reasons: a more metaphysical design
reason and a more practical reason. Let’s start with the metaphysical
In Object-Oriented Design we try to model concepts and entities from
the real world with Objects in our programming language. There are
correspondences between our Objects and our concepts as well as
between the relationships between our Objects and the relationships
between our concepts. In particular: an instance-of relationship
between an Object and a Class as well as a subclass-of relationship
between a Class and another Class correspond to an is-a relationship
between concepts or entities.
So, to add yet another cats and dogs and cars example to the huge
existing pile …
class Mammal; end
class Dog < Mammal; end
lassie = Dog.new
Now Lassie is an instance of Dog, and Dog is a subclass of Mammal. In
the real world this corresponds to “Lassie is a Dog” and “Every Dog is
a Mammal”. If you can get in front of a mirror, look yourself in the
eye and say this with a straight face, then making Lassie an instance
of Dog and Dog a subclass of Mammal is likely correct, otherwise you
should think twice.
Now, can you say with a straight face that “Every Deck of Cards is an
Array”? Maybe not.
So, on to reason number two, the practical reason: Arrays in Ruby are
an unwieldy beast. Traditionally, Collections have been very hard to
model in classical object-oriented programming languages. Is it
Array < OrderedCollectionWithDuplicateEntries <
or Array < OrderedCollectionWithDuplicateEntries < OrderedCollection?
There are many ways to solve this, Multiple Inheritance or Traits for
example, but Ruby has found a rather elegant solution: just squish the
entire Smalltalk Collection Hierarchy in one single Class! And so, a
Ruby Array is not only an Array but also a List (first), a Queue
(shift/unshift), a Stack (push/pop), a Vector, a Bag, a Set (uniq, |,
&), a Tuple and even a Matrix (transpose)!
This power comes with a cost, though: 71 public instance methods on
Array (not counting the methods inherited from Object or mixed in from
Enumerable)! Do you really want to have all of that on your Deck of
Cards? What does “transpose” mean in the context of a SongList?
Ruby makes it very easy to delegate to an Array instead of
inheriting from it, through the use of metaprogramming, Delegator,
Forwardable and friends. It also makes it easy to keep the Array “look
and feel” on your own objects, because a lot of what looks like Array
specific operators are actually just methods (e.g. , =, <<) that
you can just as well implement on your own class. And thirdly, several
useful methods aren’t actually methods of the Array Class but of the
Enumerable Module that you can mix in into your own class.
So, by delegating a couple of methods to an Array and mixing in
Enumerable you get something that looks an awful lot like an Array but
is actually a Domain Object custom-tailored to your specific needs.
An alternate approach if you don’t want a specialized Domain Object
but rather a specialized generic Collection is described here:
The basic idea is: copy the Array Class (remember, Classes are just
Objects, too!), remove the methods that you don’t want, et
voilà,there is a List or a Stack or a Queue which isn’t a subclass of Array
but actually “inherits” its behaviour.