Re: Error: "non-local pointer cannot point to a local object"

[NiGHTS <nights@unku.us>]

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 having difficulty solving.
> > 
> > I have two distinct records, in this example called "Master_Record" and "Working_Record". One contains an Integer, the other a "pointer" to that Integer. In this context, Working_Record depends on Master_Record. There can be many Working_Record instances for a single Master_Record. The Copy function is intended to enforce this rule by acting as an object factory, requiring 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.
> > 
> > Ultimately the purpose of this is for Working_Record to be a copy-friendly 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 written 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. 
> > 
> > ----------------------------------------
> > type Master_Record is record
> >         
> >         Root  : aliased Integer;
> >         -- Here is where other general variables are kept
> >                 
> > end record;
> >                 
> > type Working_Record is record
> >         
> >         Root_Pointer    : access Integer;
> >         -- Here is where specific instance variables are kept
> >                 
> > end record;
> >                 
> > function Copy (
> >                 
> >         Source : in out Master_Record
> >                         
> > ) return Working_Record is
> > begin
> >                 
> >         return (Root_Pointer => Source.Root'Access); -- << Error here >>
> >                 
> > end Copy;
> >                 
> > MR : Master_Record;
> > WR : Working_Record := Copy (MR);  
> > ----------------------------------------
> > 
> > Any ideas on how to achieve this goal? I'm sure its something simple. Thanks in advance!
> 
> One solution would be to use access discriminants.
> 
> 
> eg.
> package P is
>                 
>    type Master_Record is record 
>         
>       Root  : aliased Integer; 
>       -- Here is where other general variables are kept 
>                 
>    end record; 
>    
>    type Working_Record (Root : not null access Integer) is record 
>       null;
>       -- Here is where specific instance variables are kept 
>    end record; 
>    
> end P;
> 
> with P; use P;
> 
> procedure Main is
>    MR : aliased Master_Record;
>    WR1 : Working_Record (Root => MR.Root'Access);
>    WR2 : Working_Record (Root => MR.Root'Access);
> begin   
>    WR1.Root.all := 35;
>    WR2.Root.all := 36;
> end Main;
> 
> regards,
> 
> Brad

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