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From: Adam Beneschan <adam@irvine.com>
Newsgroups: comp.lang.ada
Subject: Re: Pointers explained?
Date: Mon, 30 Jul 2007 15:20:56 -0700
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Date: 2007-07-30T15:20:56-07:00
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On Jul 30, 12:36 pm, I wrote while half-asleep:

> A couple things I might point out: First, if you have a factory
> function [say Create_Message] that returns Message'Class (i.e. *not* a
> pointer), you can declare variables of type Message'Class as long as
> you use the function (or some other function call, maybe) as an
> initializer:
>
>     Var1 : Message'Class := Create_Message (...);
>
> When you do this, Create_Message can return any type derived from
> Message.  At that point, the program will determine how much space is
> needed for Var1 and allocate it.  Var1's type can't be changed after
> that, though; if Create_Message returns a Message_3, Var1's type will
> be fixed at Message_3, and you can't, say, assign
>
>     Var1 := Y;
>
> unless Y also happens to have type Message_3.  (If it's the wrong
> type, a Constraint_Error exception will be raised.)  This applies to
> record components also

No, it doesn't.  My bad.  Record components have to have definite
subtypes.  Sorry---I should have tried it first to make sure.

                     -- Adam