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From: Victor Porton <porton@narod.ru>
Newsgroups: comp.lang.ada
Subject: Re: A good program that not compile
Date: Wed, 21 May 2014 17:10:14 +0300
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Date: 2014-05-21T17:10:14+03:00
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Simon Wright wrote:

> Victor Porton <porton@narod.ru> writes:
> 
>> The following does not compile.
> 
> For very good reasons.
> 
> Obtain_Handle(File_Handle) ought to be a function!
> It should take an in parameter, not in out!
> 
> I take it your point is that you want to be able to declare
> 
>    function Obtain_Handle(Object: Abstract_Handle) return Integer
>    is abstract;
> 
> but I find that (with the latest compilers) this works:
> 
>    function Obtain_Handle(Object: Abstract_Handle) return Integer
>    is (raise Program_Error);
> 
> with the slight disadvantage of postponing failure to run time (which
> might happen anyway with your proposal).

I have rewritten it with your suggestions. Now it does compile, but instead
as of to work as intended (Assigning 123 to the Handle) it raises
Program_Error.

Can we save my program and make it work as intended?

The listing follows:

-- my.ads
with Ada.Finalization;

package My is

   type Abstract_Handle is new Ada.Finalization.Limited_Controlled with
      record
         Handle: Integer;
      end record;

   function Obtain_Handle(Object: Abstract_Handle) return Integer;

   overriding procedure Initialize(Object: in out Abstract_Handle);

   type File_Handle is new Abstract_Handle with null record;

   overriding function Obtain_Handle(Object: File_Handle) return Integer;

end My;


-- my.adb
package body My is

   function Obtain_Handle(Object: Abstract_Handle) return Integer is
   begin
      raise Program_Error;
      return 0;
   end;

   overriding procedure Initialize(Object: in out Abstract_Handle) is
   begin
      Object.Handle := Obtain_Handle(Object);
   end;

   overriding function Obtain_Handle(Object: File_Handle) return Integer is
   begin
      return 123; -- Suppose 123 is a file handle, for an example
   end;

end My;

-- main.adb
with My;
with Ada.Text_IO;

procedure Main is
   X: My.File_Handle;
begin
   Ada.Text_IO.Put_Line(Integer'Image(X.Handle));
end;

-- 
Victor Porton - http://portonvictor.org