From mboxrd@z Thu Jan  1 00:00:00 1970
X-Spam-Checker-Version: SpamAssassin 3.4.4 (2020-01-24) on polar.synack.me
X-Spam-Level: 
X-Spam-Status: No, score=-0.9 required=5.0 tests=BAYES_00,FORGED_GMAIL_RCVD,
	FREEMAIL_FROM autolearn=no autolearn_force=no version=3.4.4
Path: 
 eternal-september.org!reader01.eternal-september.org!reader02.eternal-september.org!news.eternal-september.org!mx02.eternal-september.org!feeder.eternal-september.org!news.swapon.de!fu-berlin.de!uni-berlin.de!individual.net!not-for-mail
From: Mart van de Wege <mvdwege@gmail.com>
Newsgroups: comp.lang.ada
Subject: Confused about class-wide types
Date: Sun, 20 Mar 2016 12:15:58 +0100
Message-ID: <86oaa97635.fsf@gaheris.avalon.lan>
Mime-Version: 1.0
Content-Type: text/plain
X-Trace: individual.net 3NLErs21+mwTjLHwCESjNgNKIw8iLrR+W3dzsTh8PzBDkzbDGR
X-Orig-Path: gaheris.avalon.lan!not-for-mail
Cancel-Lock: sha1:409GSK0vOtoHAhWXs551tlzz3WM=
 sha1:RAql8suifUPKIFd7Vg1NsuFynEI=
User-Agent: Gnus/5.13 (Gnus v5.13) Emacs/24.5 (gnu/linux)
Xref: news.eternal-september.org comp.lang.ada:29827
Date: 2016-03-20T12:15:58+01:00
List-Id: <comp.lang.ada>

Hi,

I'm finding myself a little confused about the interaction between
class-wide types.

Here's some example code:

Spec:

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;
   procedure Message (M : in out Message_Event_Ptr;
		      Mess : in String);
   package Events is new Ada.Containers.Vectors ( Index_Type => Natural,
						 Element_Type => Event_Ptr);
private
   type Message_Event is new Event with record
      Message : Unbounded_String;
   end record;
end Types_Test;

Test program:

with Types_Test; use Types_Test;
procedure Test_Type
is
   M : Message_Event_Ptr;
   Log : Events.Vector;
begin
   M := new Message_Event;
   Message(M,"Test Message");
   Log.Append(M);
end Test_Type;

The idea is that Events.Vector should be able to hold different types of
Events. Yet if I compile it this way, GNAT bombs out with:

test_type.adb:9:15: expected type "Event_Ptr" defined at types_test.ads:5
test_type.adb:9:15: found type "Message_Event_Ptr" defined at types_test.ads:7

Obviously, I'm doing something wrong; I'm probably misunderstanding
something, but what?

-- 
"We will need a longer wall when the revolution comes."
    --- AJS, quoting an uncertain source.