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From: Robert A Duff <bobduff@shell01.TheWorld.com>
Newsgroups: comp.lang.ada
Subject: Re: private types
Date: 18 Mar 2006 07:56:45 -0500
Organization: The World Public Access UNIX, Brookline, MA
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Date: 2006-03-18T07:56:45-05:00
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Justin Gombos <rpbkbq.xax.gld@uluv.kbq> writes:

> On 2006-03-17, Robert A Duff <bobduff@shell01.TheWorld.com> wrote:
> > Justin Gombos <rpbkbq.xax.gld@uluv.kbq> writes:
> >
> > I'm not sure what the right answer is, but surely all the arguments
> > for and against dummy values apply equally to access types.
> 
> I don't agree with that.  Null is a standard abnormal object for
> access types in all languages, and can never be taken for something
> valid.

Null is neither "abnormal" nor "invalid" in Ada.  As for "all
languages", some have a concept of "null" or "nil" or whatever that is
the same in this regard.  Some languages have no such concept.

In Ada, if an object of an access type has no explicit initial value,
you can't easily tell whether that means "null is a meaningful value for
this variable, and that's the default I want" versus "this variable will
be initialized to a meaningful (non-null) value later".

This is exactly analogous to the case with integers -- if they were
default-initialized to zero, you can't easily tell whether zero is
intended as a meaningful initial value, versus later initialization to a
meaningful value.

- Bob