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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: Re: Generic access type convention and aliasing
Date: Tue, 07 May 2013 08:02:58 +0200
Organization: Ada @ Home
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Date: 2013-05-07T08:02:58+02:00
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Le Tue, 07 May 2013 02:56:34 +0200, Randy Brukardt <randy@rrsoftware.com=
>  =

a =C3=A9crit:
> We eventually realized that access discriminants already had the corre=
ct  =

> semantics so we didn't need a new feature for that.

I don't see how:


     package Storages is

        type Element_Type =E2=80=A6; -- With the real case, a generic pa=
rameter.

        type Element_Access is access Element_Type;

        type Instance_Type
          (Element : Element_Access :=3D null) is
           limited
           private;

        function Create
           return Instance_Type;

     private

        type Instance_Type
          (Element : Element_Access :=3D null) is
           null record; -- With the real case, management data.

        function Create
           return Instance_Type
           is (Instance_Type'(Element =3D> null));

     end Storages;

     S1 : Storages.Instance_Type  :=3D Storages.Create;
     S2 : Storages.Instance_Type  :=3D S1; -- Illegal.
     A  : Storages.Element_Access :=3D S1.Element; -- However still lega=
l.


That makes the `Element` member constant (one of the thing I wanted); th=
at  =

does not prevent from copying it. Implicit deference, even if it avoids =
 =

direct use of the access member (syntactic sugar, not added safety,  =

although a good idea), does not prevent copying it neither, as it requir=
es  =

to expose the access discriminant in the public part. I know no way to  =

prevent copying this member, except wrapping every use of the access  =

member in many functions and procedures or using reader/updater callback=
s,  =

which is something I prefer to avoid; that's why I was looking at this A=
I  =

about limited access types (didn't care enough for its status, sorry, I =
 =

note this is indeed =E2=80=9CNo action=E2=80=9D).

By the way as a side note, even if using a boolean discriminant for the =
 =

storage allocation status is expressive, using null for the access and  =

that access as a discriminant, is more resilient, as an error with it wh=
en  =

null, will be more surely caught at runtime whenever compiled without  =

validity checking, where a not=E2=80=91really existing member, thus retu=
rning  =

probably not=E2=80=91null garbage, may have enough time to break somethi=
ng before  =

being caught.


-- =

=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