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From: "Lucretia" <lucretia9@lycos.co.uk>
Newsgroups: comp.lang.ada
Subject: Re: Basic Explaination of OO in Ada
Date: 18 Sep 2006 12:37:16 -0700
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Date: 2006-09-18T12:37:16-07:00
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Hi,

One thing that (I don't think) is really explained clearly enough is
the idea of *class-wide*. For example a class-wide type is one which
consists of every type in that hierarchy, take this hierarchy:

Shape
  + Circle
  + Polygon
     + Square
     + Triangle

If you were to say Shape'Class, this basically means any type in the
Shape hierarchy, and if you were to say Polygon'Class this means
anything in the Polygon hierarchy (but not including Shape).

Another part of the class-wide is the subprogram, a class-wide
subprogram applies to all types in that hierarchy and cannot be
overridden - it can be overloaded though:

function X(Self : Shape'Class) return Float;

This function applies to Shape, Circle, Polygon, Square, Triangle,
whereas:

function Circumference(Self : Circle'Class) return Float;

Applies onto to the hierarchy rooted at Circle (nothing below it - you
can't pass a Shape to that function).

I found this confusing because the literature isn't (IMO) clear.

Luke.