From mboxrd@z Thu Jan  1 00:00:00 1970
X-Spam-Checker-Version: SpamAssassin 3.4.4 (2020-01-24) on polar.synack.me
X-Spam-Level: 
X-Spam-Status: No, score=0.2 required=5.0 tests=BAYES_00,INVALID_MSGID,
	REPLYTO_WITHOUT_TO_CC autolearn=no autolearn_force=no version=3.4.4
X-Google-Language: ENGLISH,ASCII-7-bit
X-Google-Thread: 103376,db28129c6779d52b,start
X-Google-Attributes: gid103376,public
From: Stephen Leake <Stephen.Leake@gsfc.nasa.gov>
Subject: Re: dispatching query
Date: 1998/02/05
Message-ID: <34DA1D7C.575@gsfc.nasa.gov>#1/1
X-Deja-AN: 322406900
Content-Transfer-Encoding: 7bit
References: <34CFD22D.1D20CEA9@dsto.defence.gov.au>
Mime-Version: 1.0
Reply-To: Stephen.Leake@gsfc.nasa.gov
Content-Type: text/plain; charset=us-ascii
Organization: NASA Goddard Space Flight Center -- Greenbelt, Maryland USA
Newsgroups: comp.lang.ada
Date: 1998-02-05T00:00:00+00:00
List-Id: <comp.lang.ada>


Mark Rutten wrote:
> 
> Is it only possible to dispatch on subprograms which can be inherited
> (i.e. specified in the same declarative block)?

This is correct; see LRM 3.9.2. A dispatching operation is a primitive
operation, which is declared in the same block as the tagged type.

> 
> e.g.
> 
> package messages is
> 
>   type message is abstract tagged null record;
>   type message_ptr is access all message'Class;
> 
>   procedure copy(msg: access message) is abstract;
> 
> end messages;
> 
> Then (in some other package) I declare a type (or several types) which
> extend message. What I would like to do is to define a set of procedures
> in a third package (separate to the declarations of the types) which can

why do you want the operations in a third package?

If you want to hide the operations from some parts of the application,
you can declare the operations in the private part of the package spec.

> be dispatched upon
> 
> e.g.
> 
>   procedure handle(msg: access boolean_message);
>   procedure handle(msg: access integer_message);
> 
> where boolean_message and integer_message have been inherited from
> message and are declared in the second package. Then I would like to
> call
> 
>   handle(msg);
> 
> (msg of type message_ptr) and have this dispatch on the above two
> procedures.
> 
> I guess what I'm really trying to do is to replace what might be a case
> statement over the different inherited types with this set of
> procedures. I've tried the above with gnat3.09 on Solaris, but it
> complains at having too many choices when trying to resolve the
> procedure call, rather than recognising a dispatchable operation. Is
> there a way around this?

As you guessed, you must declare dispatching operations with the types.

-- 
- Stephe