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=-1.3 required=5.0 tests=BAYES_00,INVALID_MSGID
	autolearn=no autolearn_force=no version=3.4.4
X-Google-Language: ENGLISH,ASCII-7-bit
X-Google-Thread: 103376,3e9f0e99a5a40441,start
X-Google-Attributes: gid103376,public
From: benari@zeus.datasrv.co.il (Moti Ben-Ari)
Subject: Deriving from non-null abstract type
Date: 1995/03/29
Message-ID: <3lbh2q$8ov@israel-info.datasrv.co.il>#1/1
X-Deja-AN: 100540680
organization: DataServe LTD. (An Internet Access Provider), Israel.
newsgroups: comp.lang.ada
Date: 1995-03-29T00:00:00+00:00
List-Id: <comp.lang.ada>

I would like to derive from a _non-null_ abstract tagged type:

  type Abstract_Type is abstract tagged
    record
      General_Info: Some_Type;
    end record;

  type Derived_Type is new Abstract_Type with
    record
      Special_Info: Some_Other_Type;
    end record;

Of course, you can't create a value of the abstract type,
but it turns out that it is impossible to create a value
of the derived type:

    D: Derived_Type := (G with S);

because G would have to be an aggregate of an abstract type 
which is illegal. The only thing you can do is:

  D: Derived_Type := (Abstract_Type with S);

which means that the component General_Info has to
be initialized in a separate statement.
This seems to be against the philosophy of Ada which
provides aggregates so that you won't forget to initialize
a component.

Is there some way of creating a value that I don't see?
If not, why aren't abstract types restricted to null records?

Moti

Moti Ben-Ari
benari@datasrv.co.il