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From: dan@cs.uq.oz.au (Dan Johnston D.B.)
Newsgroups: comp.lang.ada
Subject: Re: Deriving from non-null abstract type
Date: 30 Mar 1995 01:53:48 GMT
Organization: Computer Science Dept, University of Queensland
Distribution: world
Message-ID: <3ld2vc$1cu@uqcspe.cs.uq.oz.au>
References: <3lbh2q$8ov@israel-info.datasrv.co.il>
Reply-To: dan@cs.uq.oz.au
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Date: 1995-03-30T01:53:48+00:00
List-Id: <comp.lang.ada>

In <3lbh2q$8ov@israel-info.datasrv.co.il> benari@zeus.datasrv.co.il (Moti Ben-Ari) writes:

>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:

You can use an ordinary aggregate
     ( General_Info => Some_Type, Special_Info => Some_Other_Type )

I sympathise because this problem had me hung up for a considerable time too.
You (and I also) obviously expected that an aggregate of an extension type
must be an extension-aggregate, but this is not the case.
The only real confirmation of this that I can find in the manual is the last
example in section 3.9.1 where the Painted_Point aggregate is an example
of an extended type represented as an ordinary aggregate.

Perhaps there should be a note in section 3.9.2 of the manual saying that
aggregates of extended types don't have to be extension-aggregates.
    dan.          dan@cs.uq.oz.au