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-Language: ENGLISH,ASCII-7-bit
X-Google-Thread: 103376,c944ce2ba25eeb02
X-Google-Attributes: gid103376,public
X-Google-ArrivalTime: 2001-04-19 13:41:04 PST
Path: 
 supernews.google.com!newsfeed.google.com!newsfeed.stanford.edu!canoe.uoregon.edu!logbridge.uoregon.edu!news.huji.ac.il!not-for-mail
From: "Ehud Lamm" <mslamm@mscc.huji.ac.il>
Newsgroups: comp.lang.ada
Subject: Re: Generic Packages
Date: Thu, 19 Apr 2001 23:26:25 +0300
Organization: The Hebrew University of Jerusalem
Message-ID: <9bni0i$ugu$1@news.huji.ac.il>
References: <9bnh7f$5di$1@news.inter.net.il>
NNTP-Posting-Host: di3-92.dialin.huji.ac.il
X-Trace: news.huji.ac.il 987712339 31262 132.64.13.92 (19 Apr 2001 20:32:19
 GMT)
X-Complaints-To: abuse@news.huji.ac.il
NNTP-Posting-Date: Thu, 19 Apr 2001 20:32:19 +0000 (UTC)
X-Priority: 3
X-MSMail-Priority: Normal
X-Newsreader: Microsoft Outlook Express 5.00.2014.211
X-MimeOLE: Produced By Microsoft MimeOLE V5.00.2014.211
Xref: supernews.google.com comp.lang.ada:7017
Date: 2001-04-19T23:26:25+03:00
List-Id: <comp.lang.ada>

First thing you have to realize is that when you want a generic unit to work
ona private or limited private type, the package must somehow be told how to
work with objects of the opaque type.

Basically, you do this by supplying routines as generic parameters. These
routines are the ones that know how to use the type (they usually come from
the package that exports the time). They, in essence, provide an interface
that the generic unit needs in order to work with the type. This is a
restricted form of the itnerface exported by the ADT package.

I usually describe this situation with the following metahpor.There is a
secrect, and this secret is how to handle the abstract data type. You want
to pass this secret to the generic unit. However generic units don't have
ears so you can't just lean over and whisper the secret. What you do is
package the secret inside a routine (a more techincal term would be
"procedural abstraction") and pass the routine to the generic unit.

This subject is explained very well in the Ada83 Ratioanale chapter on
generic units. I highly recommend reading it (you instructor should have a
copy of this chapter, and you can also find it online).

Good luck with the assignment!

Ehud Lamm