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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 16:58:43 +0100
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Date: 2014-05-21T16:58:43+01:00
List-Id: <comp.lang.ada>

Victor Porton <porton@narod.ru> writes:

> 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.

You have written (as before)

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

but I wrote

   overriding procedure Initialize(Object: in out Abstract_Handle) is
   begin
      Object.Handle := Obtain_Handle(Abstract_Handle'Class (Object));
      --                             ^^^^^^^^^^^^^^^^^^^^^^^      ^
      --                        use view conversion to get dispatching
   end;

In your case, the call to Obtain_Handle is *not* dispatching. You need a
classwide object (or access) to get dispatching; as you see, you can get
this by a view conversion.