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From: je@bton.ac.uk (John English)
Subject: Re: dispatching query
Date: 1998/02/04
Message-ID: <6b8c5m$8hb@saturn.brighton.ac.uk>#1/1
X-Deja-AN: 321888665
References: <34CFD22D.1D20CEA9@dsto.defence.gov.au>
Organization: University of Brighton
Newsgroups: comp.lang.ada
Date: 1998-02-04T00:00:00+00:00
List-Id: <comp.lang.ada>


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

You're talking about primitives. Dispatching happens when you have a
class wide type (X'Class).

: 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
: be dispatched upon
: e.g.
:   procedure handle(msg: access boolean_message);
:   procedure handle(msg: access integer_message);

So, make handle a primitive of Message (abstract if necessary) and
then hand it a class-wide pointer (an access Message'Class) to make
it dispatch. That is, add
  procedure Handle (Msg : access Message) is abstract;
to Messages, and then call Handle(This_Message'Access)
(where This_Message is a Message'Class object).

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