From: Matthew Heaney <matthew_heaney@acm.org>
Subject: Re: Can't export object of private type
Date: 1999/03/02
Date: 1999-03-02T00:00:00+00:00 [thread overview]
Message-ID: <m3678k6v1x.fsf@mheaney.ni.net> (raw)
In-Reply-To: 36dba095.13738466@news.pacbell.net
tmoran@bix.com (Tom Moran) writes:
> >If there is a derived type, it's within the package hierarchy:
>
> >package P is
>
> > type T (<>) is abstract tagged limited private;
> But what about
> with P;
> package Q is
> type Son_Of_T is new P.T with ...
No, there is no such thing. Outside the package hierarchy, there'd be
no way to create an instance of Son_Of_T.
> Or does the original question *only* refer to
> >package P.C is
Yes, this is the case. Derivations only occur in children of P.
> > type NT is new T with private;
> In which case, why even have T in the public part of P? It could be
> in the private part, where it's visible to P.C, and not visible to
> 'outsiders'. If you restrict T to being extended only in children of
> P, then it can be private, and the original question disappears.
No. You're forgetting that clients are still manipulating a singleton
instance of the derived type. This is a public object, of a public
type, with public operations. That NT derives from T is a public
feature of the type, not an implementation detail.
Of course, you could do it the way you suggest. This is sort of an
extreme example, where you have a family of types, and each type in the
family has a singleton instance.
I don't know why Don wants to do this (I've certainly never had such a
need), but I wanted to show that it is possible, and that in fact the
implementation is relatively simple.
It will take some time for programmers to grok the concept of limited
and indefinite types, and what it's buying you. I have a few examples
of this idiom in the ACM patterns archive.
<http://www.acm.org/archives/patterns.html>
I'm working on another pattern now that was inspired by this thread. An
abstract factory, implemented using smart pointers, is used to
initialize a singleton instance of a limited and indefinite type. Whew!
I'll post an announcement on CLA once it has been submitted (probably
tomorrow or the day after.)
next prev parent reply other threads:[~1999-03-02 0:00 UTC|newest]
Thread overview: 128+ messages / expand[flat|nested] mbox.gz Atom feed top
[not found] <F7JoCB.JxB@syd.csa.com.au>
1999-02-24 0:00 ` Can't export object of private type Don Harrison
1999-02-24 0:00 ` Tom Moran
1999-02-24 0:00 ` Samuel Mize
1999-02-24 0:00 ` Tucker Taft
1999-02-25 0:00 ` Don Harrison
1999-02-25 0:00 ` fraser
1999-02-26 0:00 ` Don Harrison
1999-02-26 0:00 ` fraser
1999-03-01 0:00 ` Don Harrison
1999-03-01 0:00 ` Matthew Heaney
[not found] ` <7b6nqe$75m$1@remarq.com>
1999-02-26 0:00 ` fraser
1999-02-27 0:00 ` Nick Roberts
1999-02-26 0:00 ` Samuel Mize
1999-03-01 0:00 ` Don Harrison
1999-03-01 0:00 ` Matthew Heaney
1999-03-02 0:00 ` fraser
1999-03-03 0:00 ` Don Harrison
1999-02-28 0:00 ` Matthew Heaney
1999-02-28 0:00 ` Matthew Heaney
1999-02-25 0:00 ` Samuel Mize
1999-02-26 0:00 ` Don Harrison
1999-02-27 0:00 ` Nick Roberts
1999-03-01 0:00 ` Don Harrison
1999-03-01 0:00 ` Matthew Heaney
1999-03-01 0:00 ` Nick Roberts
1999-03-01 0:00 ` Nick Roberts
1999-03-01 0:00 ` Don Harrison
1999-03-02 0:00 ` Matthew Heaney
1999-03-03 0:00 ` Don Harrison
1999-03-03 0:00 ` Samuel Mize
1999-03-04 0:00 ` Don Harrison
1999-03-07 0:00 ` Ehud Lamm
1999-03-03 0:00 ` Robert A Duff
1999-03-04 0:00 ` Don Harrison
1999-03-04 0:00 ` Robert A Duff
1999-03-01 0:00 ` Don Harrison
1999-03-02 0:00 ` Matthew Heaney
1999-02-28 0:00 ` Matthew Heaney
1999-03-01 0:00 ` Nick Roberts
1999-03-01 0:00 ` Matthew Heaney
1999-03-01 0:00 ` Matthew Heaney
1999-03-02 0:00 ` Nick Roberts
1999-03-01 0:00 ` Samuel Mize
1999-02-25 0:00 ` robert_dewar
1999-02-26 0:00 ` Don Harrison
1999-02-26 0:00 ` robert_dewar
1999-02-26 0:00 ` bourguet
1999-02-26 0:00 ` Samuel T. Harris
1999-02-27 0:00 ` Simon Wright
1999-02-27 0:00 ` Jean-Pierre Rosen
1999-02-28 0:00 ` dewar
1999-03-01 0:00 ` bourguet
1999-02-26 0:00 ` dennison
1999-03-01 0:00 ` Stephen Leake
1999-03-01 0:00 ` Don Harrison
1999-03-01 0:00 ` robert_dewar
1999-03-03 0:00 ` Don Harrison
1999-03-03 0:00 ` robert_dewar
1999-02-27 0:00 ` Brian Rogoff
1999-03-01 0:00 ` robert_dewar
1999-02-28 0:00 ` Matthew Heaney
1999-03-01 0:00 ` Tom Moran
1999-03-02 0:00 ` Matthew Heaney
1999-03-02 0:00 ` Tom Moran
1999-03-02 0:00 ` Matthew Heaney
1999-03-02 0:00 ` Tom Moran
1999-03-02 0:00 ` Matthew Heaney [this message]
1999-03-02 0:00 ` nabbasi
1999-03-02 0:00 ` Matthew Heaney
1999-03-03 0:00 ` Don Harrison
1999-03-03 0:00 ` Single Extension; Polymorphic Arrays Nick Roberts
1999-03-03 0:00 ` Nick Roberts
1999-03-08 0:00 ` Matthew Heaney
1999-03-08 0:00 ` Nick Roberts
1999-03-08 0:00 ` Tucker Taft
[not found] ` <m3ogm40wav.fsf@mheaney.ni.net>
1999-03-08 0:00 ` Tucker Taft
1999-03-08 0:00 ` dennison
1999-03-09 0:00 ` Nick Roberts
1999-03-08 0:00 ` Nick Roberts
1999-03-03 0:00 ` Can't export object of private type Don Harrison
1999-03-03 0:00 ` Don Harrison
1999-03-03 0:00 ` Nick Roberts
1999-03-04 0:00 ` Don Harrison
1999-03-04 0:00 ` Nick Roberts
1999-03-08 0:00 ` Matthew Heaney
1999-03-09 0:00 ` Don Harrison
1999-03-09 0:00 ` Matthew Heaney
1999-03-09 0:00 ` Nick Roberts
1999-03-10 0:00 ` Don Harrison
1999-03-10 0:00 ` Matthew Heaney
1999-03-04 0:00 ` Nick Roberts
1999-03-04 0:00 ` robert_dewar
1999-03-05 0:00 ` Nick Roberts
1999-03-05 0:00 ` Robert A Duff
1999-03-05 0:00 ` Abstract Subprograms of Untagged Types Nick Roberts
1999-03-05 0:00 ` robert_dewar
1999-03-05 0:00 ` Tucker Taft
1999-03-05 0:00 ` Nick Roberts
1999-03-06 0:00 ` robert_dewar
1999-03-04 0:00 ` Can't export object of private type fraser
1999-03-09 0:00 ` Don Harrison
1999-03-08 0:00 ` Matthew Heaney
1999-03-08 0:00 ` Nick Roberts
1999-03-08 0:00 ` Matthew Heaney
1999-03-10 0:00 ` Don Harrison
1999-03-10 0:00 ` Matthew Heaney
1999-03-10 0:00 ` dennison
1999-03-10 0:00 ` robert_dewar
1999-03-10 0:00 ` dennison
1999-03-10 0:00 ` robert_dewar
1999-03-10 0:00 ` dennison
1999-03-11 0:00 ` robert_dewar
1999-03-11 0:00 ` Don Harrison
1999-03-12 0:00 ` robert_dewar
1999-03-11 0:00 ` dennison
1999-03-11 0:00 ` bill
1999-03-11 0:00 ` Scott Ingram
1999-03-11 0:00 ` Larry Kilgallen
1999-03-11 0:00 ` dennison
1999-03-12 0:00 ` dewar
1999-03-11 0:00 ` Don Harrison
1999-03-10 0:00 ` Robert A Duff
1999-03-10 0:00 ` dennison
1999-03-11 0:00 ` dennison
1999-03-10 0:00 ` robert_dewar
1999-03-03 0:00 ` Don Harrison
1999-02-28 0:00 ` Matthew Heaney
1999-02-28 0:00 ` Matthew Heaney
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