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From: "Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de>
Subject: Re: Another problem with "interface"
Newsgroups: comp.lang.ada
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References: <mJednUhUY_4tqgTUnZ2dneKdnZydnZ2d@posted.plusnet>
Date: Mon, 16 Feb 2009 14:29:50 +0100
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Date: 2009-02-16T14:29:50+01:00
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On Mon, 16 Feb 2009 09:53:59 +0000, Robert_Matthews wrote:

> In using interface types with GNAT I have encountered another problem.
> Consider the following package:
> 
> package Test is
> 
>    type A_Type is limited interface;
> 
>    procedure P (A : in out A_Type; D : Integer) is abstract;
> 
>    protected type New_A_Type is new A_Type
>    with
>       procedure P (D : Integer);
>       --  other subprograms...
>    private
>       F : Integer;
>    end New_A_Type;
> 
>    function Set_A return New_A_Type;
> 
> end Test;
> 
> GNAT gives an error for the function Set_A: 
> "operation can be dispatching in only one type",
> which leaves me mystified. Am I making another
> dumb mistake? Please help!
> 
> The version of GNAT is GNAT GPL 2008 (20080521).

This looks like a compiler bug to me. The compiler thinks that Set_A is a
protected function of New_A_Type and thus has the hidden argument
New_A_Type and the result New_A_Type, so it complains. But protected type
is not tagged so Set_A cannot be dispatching.

However you can trick the compiler by ensuring that Set_A declaration were
beyond the freezing point of New_A_Type (whatever that might mean for a
protected type). For example:

   type A_Type is limited interface;

   procedure P (A : in out A_Type; D : Integer) is abstract;

   protected type New_A_Type is new A_Type with
      procedure P (D : Integer);
      --  other subprograms...
   private
      F : Integer;
   end New_A_Type;

   package Foo is  -- Package brackets around it
      function Set_A return New_A_Type;
   end Foo;

A more logical way to do it would be:

   function Set_A return New_A_Type'Class;

Alas, this does not work, because there seem to be no way to create
New_A_Type'Class, since New_A_Type is protected.

However you could use A_Type'Class instead:

   function Set_A return A_Type'Class;

P.S. Returning New_A_Type from a function is tricky because it is limited,
yet does not have aggregates. You can use the return statement to work this
around:

   function Set_A return New_A_Type is
   begin
      return Result : New_A_Type do
         null;
      end return;
   end Set_A;

-- 
Regards,
Dmitry A. Kazakov
http://www.dmitry-kazakov.de