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From: danmcleran@hotmail.com
Newsgroups: comp.lang.ada
Subject: Re: Possible Ada deficiency?
Date: 1 Jan 2005 15:28:10 -0800
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Date: 2005-01-01T15:28:10-08:00
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Here's a different way to illustrate this problem. If I want to extend
a type, I can do this with a non-child package. But, if I make the
package containing the extended type a child package, I then have
access to all of the private area as well. So, one can extend a type
with or without access to the private area, depending on whether one
makes the packages containing the derived type a child package or not.
In my example, notice that the derived type in the child package can
change the private area, whereas the derived type in the non-child
package cannot.

Consider these files (Main.adb is the test program):

--Some_Package.ads

package Some_Package is
type Secret_Type is tagged private;

procedure Print_Secret_Value (Value : in out Secret_Type'Class);
private
type Secret_Type is tagged record
Secret_Value : Integer := 0;
end record;
end Some_Package;

--Some_Package.adb

with Ada.Text_IO;

package body Some_Package is
procedure Print_Secret_Value (Value : in out Secret_Type'Class) is
begin
Ada.Text_IO.Put_Line (Item => "Secret value = " &
Integer'Image(Value.Secret_Value));
end Print_Secret_Value;
end Some_Package;

--Some_Package.Child_Package.ads

package Some_Package.Child_Package is
type Some_Child_Type is new Secret_Type with null record;

procedure Badness (
Child     : in out Some_Child_Type;
New_Value : in Integer);


end Some_Package.Child_Package;

--Some_Package.Child_Package.adb

package body Some_Package.Child_Package is
procedure Badness (
Child     : in out Some_Child_Type;
New_Value : in Integer) is
begin
Child.Secret_Value := New_Value;
end Badness;
end Some_Package.Child_Package;

--Non_Child_Package.ads

with Some_Package;

package Non_Child_Package is
type Derived_Type is new Some_Package.Secret_Type with null record;

procedure Badness (
Derived   : in out Derived_Type;
New_Value : in Integer);

end Non_Child_Package;

--Non_Child_Package.adb

package body Non_Child_Package is
procedure Badness (
Derived   : in out Derived_Type;
New_Value : in Integer) is
begin
--Derived.Secret_Value := New_Value;--won't compile
null;
end Badness;
end Non_Child_Package;

--Main.adb

with Some_Package;
with Some_Package.Child_Package;
with Non_Child_Package;

procedure Main is
Child_Value : Some_Package.Child_Package.Some_Child_Type;
Non_Child   : Non_Child_Package.Derived_Type;
begin
Some_Package.Print_Secret_Value (Value => Child_Value);
Some_Package.Print_Secret_Value (Value => Non_Child);
Some_Package.Child_Package.Badness (Child => Child_Value, New_Value
=> 100);
Non_Child_Package.Badness (Derived => Non_Child, New_Value => 100);
Some_Package.Print_Secret_Value (Value => Child_Value);
Some_Package.Print_Secret_Value (Value => Non_Child);
end Main;