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From: Simon Wright <simon@pushface.org>
Newsgroups: comp.lang.ada
Subject: Re: A good program that not compile
Date: Wed, 21 May 2014 14:49:30 +0100
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Date: 2014-05-21T14:49:30+01:00
List-Id: <comp.lang.ada>

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).



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
   is (raise Program_Error);

   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;

package body My is

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