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=-1.9 required=5.0 tests=BAYES_00
	autolearn=unavailable autolearn_force=no version=3.4.4
X-Received: by 2002:ad4:55e8:: with SMTP id bu8mr1934153qvb.61.1579859246796;
        Fri, 24 Jan 2020 01:47:26 -0800 (PST)
X-Received: by 2002:a9d:3d0a:: with SMTP id a10mr2036951otc.327.1579859246512;
 Fri, 24 Jan 2020 01:47:26 -0800 (PST)
Path: 
 eternal-september.org!reader01.eternal-september.org!feeder.eternal-september.org!news.gegeweb.eu!gegeweb.org!usenet-fr.net!proxad.net!feeder1-2.proxad.net!209.85.160.216.MISMATCH!g89no5602778qtd.0!news-out.google.com!o19ni791qtr.1!nntp.google.com!g89no5602772qtd.0!postnews.google.com!google-groups.googlegroups.com!not-for-mail
Newsgroups: comp.lang.ada
Date: Fri, 24 Jan 2020 01:47:26 -0800 (PST)
In-Reply-To: <lyo8uunmk9.fsf@pushface.org>
Complaints-To: groups-abuse@google.com
Injection-Info: google-groups.googlegroups.com; posting-host=136.163.208.2;
 posting-account=HFCrOQoAAABZD_f-UUbYHm3lJDIrh-UX
NNTP-Posting-Host: 136.163.208.2
References: <b820f7d0-1b5e-4e1d-9dd1-3f873e2f13e1@googlegroups.com>
 <e99b708d-d0bd-4ae6-b099-3bcfb9cadc77@googlegroups.com>
 <6dbb0da9-8c68-435e-945d-d0e1eefeda4c@googlegroups.com>
 <e073de39-42bf-4571-aaaf-c7107d9f6962@googlegroups.com>
 <c54a537d-4ac1-417e-9f25-47f450b79d4f@googlegroups.com>
 <lyo8uunmk9.fsf@pushface.org>
User-Agent: G2/1.0
MIME-Version: 1.0
Message-ID: <e8f81d93-a54f-4470-8d45-fecc00f29d95@googlegroups.com>
Subject: Re: Calling a record type's methods (functions or procedure) when
 record is in an array
From: joakimds@kth.se
Injection-Date: Fri, 24 Jan 2020 09:47:26 +0000
Content-Type: text/plain; charset="UTF-8"
Content-Transfer-Encoding: quoted-printable
Xref: reader01.eternal-september.org comp.lang.ada:57931
Date: 2020-01-24T01:47:26-08:00
List-Id: <comp.lang.ada>

Den torsdag 23 januari 2020 kl. 17:51:22 UTC+1 skrev Simon Wright:
> joakimds@kth.se writes:
>=20
> > Other issues?
>=20
> Alignment?
>=20
> =3D=3D=3D=3D=3D=3D=3D=3D=3D
>=20
> Looks to me like something it would be better not to do, even if you can.

Hi Simon,

Not entirely sure how Alignment can interfere but I can realize that using =
controlled types in the Taft type defined in the body may yield unexpected =
results. Right now I am leaning towards better not to do even if one can. H=
owever, here is another implementation that may be less error prone:

package Cars is

   type Any_Holder is abstract tagged limited null record;
  =20
   function Make_Holder return Any_Holder'Class;
  =20
   type Any_Car (<>) is limited private;
   --  This type is defined with unknown discriminant in order to make
   --  sure instances are properly initialized by allocator function.
  =20
   function Allocate_Car (Holder : in out Any_Holder'Class) return Any_Car;
   --  The allocator function.

   type Car_Passenger_Count is range 0 .. 5;
  =20
   function Passenger_Count (Car : Any_Car) return Car_Passenger_Count;

   procedure Set_Passenger_Count
     (Car   : in out Any_Car;
      Value : Car_Passenger_Count);

private

   type Taft_Car;

   type Any_Car
     (Reference : not null access Taft_Car)
   is limited null record;

end Cars;

package body Cars is

   type Taft_Car is record
      Passenger_Count : Car_Passenger_Count;
   end record;

   type Car_Holder is new Any_Holder with record
      Car : aliased Taft_Car;
   end record;
  =20
   function Make_Holder return Any_Holder'Class is
   begin
      return C : Car_Holder do
         C.Car.Passenger_Count :=3D 0;
      end return;
   end Make_Holder;

   function Allocate_Car (Holder : in out Any_Holder'Class) return Any_Car =
is
      H : Car_Holder renames Car_Holder (Holder);
   begin
      return Car : Any_Car (H.Car'Access) do
         null;
      end return;
   end Allocate_Car;

   function Passenger_Count (Car : Any_Car) return Car_Passenger_Count is
   begin
      return Car.Reference.Passenger_Count;
   end Passenger_Count;

   procedure Set_Passenger_Count
     (Car   : in out Any_Car;
      Value : Car_Passenger_Count) is
   begin
      Car.Reference.all.Passenger_Count :=3D Value;
   end Set_Passenger_Count;
  =20
end Cars;

with Ada.Text_IO;

with Cars;
use  Cars;

procedure Main is
   Holder : Cars.Any_Holder'Class :=3D Cars.Make_Holder;
   Car : Cars.Any_Car :=3D Cars.Allocate_Car (Holder);
begin
   Set_Passenger_Count (Car, 3);
   Ada.Text_IO.Put_Line (Passenger_Count (Car)'Image);
end Main;

The issues with the above implementation is:
1. Usage of downward conversion at "H : Car_Holder renames Car_Holder (Hold=
er);". It is a feature of Ada that can lead to inefficient code generation =
and there is a gnatcheck or AdaControl rule to check/forbid such usage.
2. Usage of anonymous access type "(Reference : not null access Taft_Car)".=
 It isn't obvious from the code as it is written that it is correct althoug=
h it is accepted by the GNAT compiler due to the complexity of the anonymou=
s access type rules. Perhaps somebody in the Ada community would like to mo=
tivate using the anonymous access type rules in the Reference Manual why it=
 is correct? :)

Best regards,
Joakim