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From: shaunpatterson@gmail.com
Newsgroups: comp.lang.ada
Subject: Re: Pointers explained?
Date: Mon, 30 Jul 2007 10:56:29 -0700
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Date: 2007-07-30T10:56:29-07:00
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------- OKAY sorry... I tried to tab in and accidentally activated the
POST button ----

Hi

   I'm still relatively new to Ada -- coming from a strong C++
background.  Some of this may seem stupid - and feel free to point it
out to me.  I am using Ada 95 - GNAT Pro 3.16a1


I have a message factory I've been screwing around with (converting
it
from Ada to C++).


I basically have an abstract message type:


type Message is abstract tagged null record;
type Message_Class is access all Message'Class;


then all other messages are derived from this.


Now from my factory create method, I'm returning a Message_Class.
This message_class
is stored in another record to be handled again later:


type CallbackEvent is
record
     msg : Message_Class;
     ...
end record;


Now my basic problem -- or not really a problem -- is that to create
the Message_Class I have to use
"new" and allocate and deallocate memory.


Now - my solution was to re-write my message factory to return a type
Message'Class.  However, I went
to change my callback structure to:


type CallbackEvent is
record
    msg : Message'Class
end record;


and I get the message "unconstrained subtype in component declaration"

So, I'm assuming the compiler just doesn't know how to big this
message is.
that's why I believe i should use a pointer (the Message_Class type)
However,
I cannot find a way of converting between a Message'Class to a
Message_Class;

It appears that the only way to get a Message_Class is by some where
in the
code using a "new"

Is this assumption correct?

thanks!

--
Shaun