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From: stt@spock.camb.inmet.com (Tucker Taft)
Subject: Re: Deriving from non-null abstract type
Date: 1995/03/29
Message-ID: <D67M37.J6y@inmet.camb.inmet.com>#1/1
X-Deja-AN: 100540692
sender: news@inmet.camb.inmet.com
references: <3lbh2q$8ov@israel-info.datasrv.co.il>
organization: Intermetrics, Inc.
newsgroups: comp.lang.ada
Date: 1995-03-29T00:00:00+00:00
List-Id: <comp.lang.ada>

Moti Ben-Ari (benari@zeus.datasrv.co.il) wrote:
: 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. 

There is no need to use an extension aggregate at all.  You
can use "regular" record aggregates if you are giving all of the
components of a record extension, including those inherited from the 
parent type.  Hence, the following is the appropriate way to initialize
all of the components of a record extension:

    D : Derived_Type := 
           (General_Info => General_Value, Special_Info => S);

or positionally:

    D : Derived_Type (General_Value, S);

: ... 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 kind of aggregate is to be used when you want to 
default-initialize the inherited components.  If you want
to initialize them, then as indicated above, just use
a regular record aggregate.

: 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?

Yes -- use a regular record aggregate.

: If not, why aren't abstract types restricted to null records?

See above.

: Moti

: Moti Ben-Ari
: benari@datasrv.co.il

-Tucker Taft   stt@inmet.com
Intermetrics, Inc.