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Newsgroups: comp.lang.ada
Date: Tue, 11 Sep 2018 18:49:42 -0700 (PDT)
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Subject: Re: Error: "non-local pointer cannot point to a local object"
From: Brad Moore <bmoore.ada@gmail.com>
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Xref: reader02.eternal-september.org comp.lang.ada:54328
Date: 2018-09-11T18:49:42-07:00
List-Id: <comp.lang.ada>

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 hav=
ing 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 Int=
eger. In this context, Working_Record depends on Master_Record. There can b=
e many Working_Record instances for a single Master_Record. The Copy functi=
on is intended to enforce this rule by acting as an object factory, requiri=
ng 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-friendl=
y record, where Master_Record is not intended to be used directly, except a=
s a place to anchor the variable scope. The resulting code will be written =
as a library object containing two records just like I have here, as well a=
s 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. Tha=
nks in advance!

One solution would be to use access discriminants.


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;

with P; use P;

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;

regards,

Brad