Re: Can't export object of private type

[Matthew Heaney <matthew_heaney@acm.org>]

tmoran@bix.com (Tom Moran) writes:

> >No.  If you make the type limited and indefinite, no one outside the
> >package can create an instance.
> But given such a type T, if I'm outside and make
>   type Son_Of_T is new T with ...
> how can I create an object of type Son_Of_T, and if such an object
> cannot be created, of what use is Son_Of_T?

The question is wrong.  There's no such thing as "being outside" and
declaring a derived type.

If there is a derived type, it's within the package hierarchy:

package P is

   type T (<>) is abstract tagged limited private;

   procedure Op (O : access T) is abstract;

private

   type T is abstract tagged limited null record;

end P;


package P.C is

   type NT is new T with private;

   procedure Op (O : access NT);


   type NT_Access is access all NT;

   function Singleton return NT_Access;


private

   type NT is new T with null record;

end;


package body P.C is

   O : aliased NT;


   procedure Op (O : access NT) is
   begin
      null;
   end;

   function Singleton return NT_Access is
   begin
      return O'Access;
   end;

end;


When I try to declare an instance of P.C.NT:

with P.C;
procedure Test_P is

  O : P.C.NT;

begin

  null;

end;


I get the following error:

gnatf -x5 /home/matt/test_p.adb
test_p.adb:4:10: unconstrained subtype not allowed (need initialization)

But within the package hierarchy, I'm free to declare instances of the
type (see the body of P.C), because I have privileged access to the
representation of the type (which of course, is _not_ indefinite).

We can get rid of the compiler diagnostic, by using the singleton instance:

with P.C; use P.C;
procedure Test_P is
begin

  Op (Singleton);

end;


We have used the features the language provides us to design a type
hierarchy such that, the client must use the singleton instance(s).  He
has no other choice in the matter, which is the intended effect.