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From: =?utf-8?Q?Yannick_Duch=C3=AAne_=28Hibou57?=
 =?utf-8?Q?=29?=
 <yannick_duchene@yahoo.fr>
Newsgroups: comp.lang.ada
Subject: Overring function and its returned type
Date: Fri, 09 Nov 2012 08:13:23 +0100
Organization: Ada @ Home
Message-ID: <op.wnhvol1wule2fv@cardamome>
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Date: 2012-11-09T08:13:23+01:00
List-Id: <comp.lang.ada>

I feel a bit disappointed with a construct I never attempted before, and=
  =

which seems to not be valid according to GNAT, which I believe is not  =

wrong, as I can't recall to have read it's allowed. First a snippet, the=
n  =

some comments.


     procedure Ex
     is

        -- =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=
=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D

        package P1 is

           type R is interface;
           type T is interface;

           not overriding
           function F
             (E : T)
              return R'Class
              is abstract;
        end;

        -- =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=
=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D

        package P2 is

           type R is interface and P1.R;
           type T is interface and P1.T;

           overriding
           function F
             (E : T)
              return R'Class -- Not overriding :-(
              is abstract;
        end;

        -- =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=
=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D

        package P3 is

           type R is new P1.R with null record;
           type T is new P1.T with null record;

           overriding
           function F
             (E : T)
              return P1.R'Class;
        end;

        -- ----------------------------------------

        package body P3 is

           overriding
           function F
             (E : T)
              return P1.R'Class
           is
              X : R;
           begin
              return X; -- This is OK
           end;
        end;

        -- =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=
=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D

     begin
        null;
     end Ex;


In `P2`, `R` inherits from `P1.R`, so belongs to `P1.R'Class`, and is a =
 =

value returned type for an concrete function `F`, as `P3` shows. But GNA=
T  =

reject it, complaining `P2.F` is not overriding (by the way, the example=
  =

also shows how much it is safer to always add an =E2=80=9C[not] overridi=
ng=E2=80=9D  =

qualification to dispatching subprograms).

As `P2.R'Class` is compatible with `P1.R'Class`, why not allow this for =
an  =

overriding function? This case makes me think of the one fixed by Ada  =

2012, about the returned class wide type of a function and the extended =
 =

returned statement, explained in [Extended return  =

statements](http://www.ada-auth.org/standards/12rat/html/Rat12-4-6.html)=
.

If that's not allowed with good reasons, then what are these?

A work around could be to use overloading instead of overriding, but tha=
t  =

may cause troubles when there is no other arguments to the function, in =
 =

which case this will probably looks ambiguous in some contexts. This can=
  =

also cause issues when there are predicates using `F` in `P1`. I could  =

give a tiny example of such a case if someone need one to see why it may=
  =

be an issue.

If that could be OK to allow overriding functions to change their return=
  =

type to a compatible type when this type is class wide, then is there a =
 =

chance to add it as a last minute change for Ada 2012?


-- =

=E2=80=9CSyntactic sugar causes cancer of the semi-colons.=E2=80=9D [1]
=E2=80=9CStructured Programming supports the law of the excluded muddle.=
=E2=80=9D [1]
[1]: Epigrams on Programming =E2=80=94 Alan J. =E2=80=94 P. Yale Univers=
ity