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From: Simon Wright <simon@pushface.org>
Newsgroups: comp.lang.ada
Subject: Re: Confused about class-wide types
Date: Sun, 20 Mar 2016 12:46:10 +0000
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Xref: news.eternal-september.org comp.lang.ada:29829
Date: 2016-03-20T12:46:10+00:00
List-Id: <comp.lang.ada>

Mart van de Wege <mvdwege@gmail.com> writes:

> with Ada.Containers.Vectors;
> with Ada.Strings.Unbounded; use Ada.Strings.Unbounded;
> package Types_Test is
>    type Event is abstract tagged null record;
>    type Event_Ptr is access Event'Class;
>    type Message_Event is new Event with private;
>    type Message_Event_Ptr is access Message_Event'Class;

You don't need this; Event_Ptr covers Message_Event'Class.

>    procedure Message (M : in out Message_Event_Ptr;
> 		      Mess : in String);

M should be of type Message_Event.

> Test program:
>
> with Types_Test; use Types_Test;
> procedure Test_Type
> is
>    M : Message_Event_Ptr;

This should be of type Event_Ptr.

>    Log : Events.Vector;
> begin
>    M := new Message_Event;
>    Message(M,"Test Message");

Now it gets tricky.

   Message(Message_Event(M.all),"Test Message");

works, because M.All is of type Event'Class and we can do a view
conversion to Message_Event (that's what it was created as in the first
place; we'd get CE if it wasn't).

I think this is difficult because you've introduced operations for the
child (Message_Event) of which the parent (Event) has no analogue at
all. What is the common operation that all Events must support?
Maybe something like

   procedure Handle (E : Event) is abstract;

Your procedure Message is a factory, I think; it might be better as

   function Message (Mess : in String) return Message_Event;