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Newsgroups: comp.lang.ada
Date: Wed, 12 Sep 2018 07:41:38 -0700 (PDT)
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Subject: Re: Error: "non-local pointer cannot point to a local object"
From: NiGHTS <nights@unku.us>
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Xref: reader02.eternal-september.org comp.lang.ada:54331
Date: 2018-09-12T07:41:38-07:00
List-Id: <comp.lang.ada>

On Tuesday, September 11, 2018 at 9:49:44 PM UTC-4, Brad Moore wrote:
> On Tuesday, September 11, 2018 at 4:21:00 PM UTC-6, NiGHTS wrote:
> > As the subject line indicates, I am having a compile error which I am h=
aving difficulty solving.
> >=20
> > I have two distinct records, in this example called "Master_Record" and=
 "Working_Record". One contains an Integer, the other a "pointer" to that I=
nteger. In this context, Working_Record depends on Master_Record. There can=
 be many Working_Record instances for a single Master_Record. The Copy func=
tion is intended to enforce this rule by acting as an object factory, requi=
ring an instance of Master_Record to create Working_Record. I know this isn=
't perfect, so any suggestions on a better strategy would be appreciated.
> >=20
> > Ultimately the purpose of this is for Working_Record to be a copy-frien=
dly record, where Master_Record is not intended to be used directly, except=
 as a place to anchor the variable scope. The resulting code will be writte=
n as a library object containing two records just like I have here, as well=
 as a Copy function. The user of this library object is expected to create =
an instance of Master_Record, then copy it to Working_Record.=20
> >=20
> > ----------------------------------------
> > type Master_Record is record
> >        =20
> >         Root  : aliased Integer;
> >         -- Here is where other general variables are kept
> >                =20
> > end record;
> >                =20
> > type Working_Record is record
> >        =20
> >         Root_Pointer    : access Integer;
> >         -- Here is where specific instance variables are kept
> >                =20
> > end record;
> >                =20
> > function Copy (
> >                =20
> >         Source : in out Master_Record
> >                        =20
> > ) return Working_Record is
> > begin
> >                =20
> >         return (Root_Pointer =3D> Source.Root'Access); -- << Error here=
 >>
> >                =20
> > end Copy;
> >                =20
> > MR : Master_Record;
> > WR : Working_Record :=3D Copy (MR); =20
> > ----------------------------------------
> >=20
> > Any ideas on how to achieve this goal? I'm sure its something simple. T=
hanks in advance!
>=20
> One solution would be to use access discriminants.
>=20
>=20
> eg.
> package P is
>                =20
>    type Master_Record is record=20
>        =20
>       Root  : aliased Integer;=20
>       -- Here is where other general variables are kept=20
>                =20
>    end record;=20
>   =20
>    type Working_Record (Root : not null access Integer) is record=20
>       null;
>       -- Here is where specific instance variables are kept=20
>    end record;=20
>   =20
> end P;
>=20
> with P; use P;
>=20
> procedure Main is
>    MR : aliased Master_Record;
>    WR1 : Working_Record (Root =3D> MR.Root'Access);
>    WR2 : Working_Record (Root =3D> MR.Root'Access);
> begin  =20
>    WR1.Root.all :=3D 35;
>    WR2.Root.all :=3D 36;
> end Main;
>=20
> regards,
>=20
> Brad

I like this strategy as it enforces my dependency requirement. Thank you.