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From: Robert Dewar <robert_dewar@my-deja.com>
Subject: Re: friend classes in ada95
Date: 2000/04/20
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Date: 2000-04-20T00:00:00+00:00
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In article <m3hfcxjt22.fsf@ns58.infomatch.bc.ca>,
  Ray Blaak <blaak@infomatch.com> wrote:

> OO programming being essentially about ADTs covers the most
> important and worthwhile point. What makes OO programming
> different from ADTs, in least in my opinion, is language
> support for inheritance and polymorphism.

nope! that has nothing to do with object oriented programming.
inheritance and polymorphism are concepts that perfectly well
fit into the general world of ADT's.

Example. Suppose we define an abstract algebra as a tagged
type. The operations are purely value oriented (e.g. operation
+ on two elements computes a third element).



The notion of extending the abstract algebra to a concrete
field makes perfect sense, and the notion of dynamic dispatching
to allow routines to deal with different algebras also makes
sense.

What does not make sense is to think of this as object
oriented programming, since the two fundamental components
of objects:

  1. state, so that objects can be changed

  2. methods with message passing semantics so that the
     operations are viewed as passing a message to the
     distinguished object (the prefix in languages that
     use this syntax)

are completely missing.

When Simula-67 was introduced, ADT's were of course very well
known and understood (this was after all a decade after Algol-60
was introduced!) Simula-67 had something original and
interesting to add to the programmer's knowledge base.

If you think that OO is simply about ADT's then you are back
in the 60's and have missed something :-)

On the other hand, if you think that type extension, information
hiding, and dynamic dispatching are ONLY about object oriented
programming, then you have also missed something!


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