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From: Ken Garlington <garlingtonke@lmtas.lmco.com>
Subject: Re: Ada95 OOP Questions
Date: 1996/08/14
Message-ID: <321206C2.1F26@lmtas.lmco.com>#1/1
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references: <4tf3l4$4hu@masala.cc.uh.edu>
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organization: Lockheed Martin Tactical Aircraft Systems
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newsgroups: comp.lang.ada
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Date: 1996-08-14T00:00:00+00:00
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Jon S Anthony wrote:
> 
> In article <dahgq78i0z.fsf@explorer2.clark.net> ecn@explorer2.clark.net (Eric C. Newton) writes:
> 
> > It's a pretty hard sell to say (pardon the syntax, 8-):
> >
> >       struct Person;
> >       Eat(Person);
> >       Walk(Person);
> >       Talk(Person);
> >
> > "That's a person.  It has the features 'Eat, Walk, Talk'."
> >
> > Vs.
> >       class Person
> >       {
> >               Eat();
> >               Walk();
> >               Talk();
> >       };
> >
> 
> I see no reason why _passive_ entities (the so called objects for
> either of the above) should look like they are acting or being
> commanded to act.  If you want that then use an activity which in Ada
> would be modeled by a task:

Here's a more convoluted way to get the syntax in the "right" order.
Granted, it exposes state variables that should be hidden, but by God
the syntax is right!

package Person is

  type State is private;
  Initial_State : constant State;

  type Activity is access procedure ( Using : in out State );
  procedure Eating ( Using : in out State );
  procedure Walking ( Using : in out State );
  procedure Talking ( Using : in out State );

  type Object is tagged
    record
      Eat : Activity := Eating'Access;
      Walk : Activity := Walking'Access;
      Talk : Activity := Talking'Access;
      Current_State : State := Initial_State;
    end record;

private

  type State is
    record
      Stomach_Level : Natural;
      X_Coordinate  : Natural;
      Y_Coordinate  : Natural;
      Volume        : Natural;
    end record;

  Initial_State : constant State := (0,0,0,0);

end Person;

with Person;
procedure Person_Test is

  Joe : Person.Object;

begin

  Joe.Eat(Using => Joe.Current_State);
  Joe.Walk(Using => Joe.Current_State);
  Joe.Talk(Using => Joe.Current_State);

end Person_Test;

:)

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