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From: Richard D Riehle <laoXhai@ix.netcom.com>
Subject: Re: Ada OO Mechanism
Date: 1999/05/28
Message-ID: <7imjg9$83r@sjx-ixn6.ix.netcom.com>#1/1
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Newsgroups: comp.lang.ada
Date: 1999-05-28T10:24:25-07:00
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In article <86lne9qh28.fsf@ppp-159-70.villette.club-internet.fr>,
	Laurent Guerby <guerby@acm.org> wrote:

>Hyman Rosen <hymie@prolifics.com> writes:
>> [...]
>> From reading the Rationale, I thought that the assignment to z would
>> result in a dispatching call to Get, using the tag of z that was set
>> at its initialization. What am I doing wrong here? How do controlling
>> return types work?
>
>A variable, once given an initial value, never change its type in
>Ada. So the assignment to a class wide variable can only be done in a
>declarative part.
>
>--LG

Declarative parts can appear in subprograms or declare blocks. Therefore,
one can take advantage of the ability to declare classwide variables in
some interesting ways.  Given a tagged type,

          type Machine is tagged ...

with derviations,

          type Big_Machine is new Machine with ...
          type Solenoid is new Machine with ...
          type Tractor is new Big_Machine with ...

one could dynamically dispatch to some operation overridden for each kind 
of machine such as,

          procedure Turn_On(M : in out Machine);

If we also provide a classwide Get operation that retrieves objects
of type Machine'Class from some kind of container,

          function Get (C : in Container) return Machine'Class;

then we can have a loop that looks like this,

         loop
            declare
               M : Machine'Class := Get(container-name);
                  -- constrains M with classwide Get function
            begin
               Turn_On(M);
            end;
         end loop;

where the classwide variable will be constrained by the Get operation
each time through the loop.  This technique can be used in subprograms,
tasks, or declare blocks.  I have an example I use in some of my classes
that demonstrates several variations on this for Stream_IO files. For
Stream_IO you would want to use Machine'Class'Input(filename). It
works quite nicely and is simple to implement and understand.

Richard Riehle
richard@adaworks.com
http://www.adaworks.com