From mboxrd@z Thu Jan  1 00:00:00 1970
X-Spam-Checker-Version: SpamAssassin 3.4.4 (2020-01-24) on polar.synack.me
X-Spam-Level: 
X-Spam-Status: No, score=-1.9 required=5.0 tests=BAYES_00 autolearn=ham
	autolearn_force=no version=3.4.4
X-Google-Thread: 103376,342dcd67e9ca73ee
X-Google-Attributes: gid103376,public
X-Google-Language: ENGLISH,ASCII-7-bit
Path: 
 g2news1.google.com!news3.google.com!newsfeed2.dallas1.level3.net!news.level3.com!newsfeed-00.mathworks.com!nntp.TheWorld.com!not-for-mail
From: Robert A Duff <bobduff@shell01.TheWorld.com>
Newsgroups: comp.lang.ada
Subject: Re: tagged record child: override constructor?
Date: 13 Sep 2005 14:55:47 -0400
Organization: The World Public Access UNIX, Brookline, MA
Message-ID: <wccd5nc23ws.fsf@shell01.TheWorld.com>
References: <1126591134.797303.318920@z14g2000cwz.googlegroups.com>
 <ReDVe.2$Fv6.0@dfw-service2.ext.ray.com>
NNTP-Posting-Host: shell01.theworld.com
Mime-Version: 1.0
Content-Type: text/plain; charset=us-ascii
X-Trace: pcls4.std.com 1126637748 31760 192.74.137.71 (13 Sep 2005 18:55:48
 GMT)
X-Complaints-To: abuse@TheWorld.com
NNTP-Posting-Date: Tue, 13 Sep 2005 18:55:48 +0000 (UTC)
User-Agent: Gnus/5.09 (Gnus v5.9.0) Emacs/21.2
Xref: g2news1.google.com comp.lang.ada:4623
Date: 2005-09-13T14:55:47-04:00
List-Id: <comp.lang.ada>

Jeffrey Carter <spam@spam.com> writes:

> sean.gilbertson@gmail.com wrote:
> > I have a tagged record that is declared private, along with a
> > constructor function which returns an instance.  I do this to enforce
> > the assignment of several required fields in the record, so if you know
> > how to do this another way, please let me know!
> 
> It depends on what you mean by "enforce the assignment of several
> required fields". It is impossible to force the client to call your
> function, though you can arrange things so that the type is useless to
> the client if the client doesn't first call your function.
[... lots of good possibilities snipped...]

There's another way to "enforce...":

    package P is
        type T(<>) is private;
        function Make_T(...) return T;
    private
        type T is record...
    end P;

    use P;

Now clients can't create uninitialized objects of type T:

    X: T; -- illegal
    type T_Ptr is access T;
    Y: T_Ptr := new T; -- illegal

    type R is
        record
            Component: T; -- illegal
        end record;

    type A is array(...) of T; -- illegal

The client is forced to call Make_T:

    X: T := Make_T(...); -- OK
    Y: T_Ptr := new T'(Make_T(...)); -- OK

etc.

- Bob