Re: Can a child access private types of its parent?

[Martin <martin.dowie@btopenworld.com>]

On Jul 21, 12:42 pm, vlc <just.another.spam.acco...@googlemail.com>
wrote:
> Hi *,
>
> in the following code extract, I try to access a private variable I
> (line 09) in the Parent's type P from Child (in function Get, line
> 23):
>
> 01  with Ada.Integer_Text_Io; use Ada.Integer_Text_Io;
> 02
> 03  procedure Main is
> 04
> 05  package Parent is
> 06        type P is tagged private;
> 07     private
> 08        type P is tagged record
> 09           I : integer := 5;
> 10        end record;
> 11     end Parent;
> 12
> 13     package Child is
> 14        type C is new Parent.P with private;
> 15        function Get (This : C) return integer;
> 16     private
> 17        type C is new Parent.P with null record;
> 18     end Child;
> 19
> 20     package body Child is
> 21        function Get (This : C) return integer is
> 22        begin
> 23           return This.I;
> 24        end Get;
> 25     end Child;
> 26
> 27     Var : Child.C;
> 28
> 29  begin
> 30
> 31     Put (Var.Get);
> 32
> 33  end Main;
>
> But the compiler gives me a
> main.adb:23:21: no selector "I" for type "C" defined at line 17
>
> The code compiles if I make the type P public, i.e. if I comment the
> lines 06 and 07, the code compiles as the variable I gets inherited to
> Child. But then everybody can not only read but also write Var.I, e.g.
>
> 29  begin
> 30
> 31     Var.I := 2;
> 32     Put (Var.Get);
> 33
> 34  end Main;
>
> Is there a way to inherit private types from a Parent to its child?
>
> Thank a lot in advance for any help!

You need to make the Parent - Child packages (at library level).

e.g.
-- File: parent.ads
package Parent is
   -- As you have
end Parent;

-- File: parent-child.ads
package Parent.Child is
   -- Rest as you have, or you can miss the "Parent." bit, e.g.
   -- type C is new P with private;
   ...
end Parent.Child;

-- File: parent-child.adb
package body Parent.Child is
   -- Rest as you have
end Parent.Child;

-- File: main.adb
with Ada.Integer_Text_Io; use Ada.Integer_Text_Io;
with Parent.Child;

procedure Main is
   Var : Parent.Child.C;
begin
   Put (Var.Get);
end Main;

Cheers
-- Martin