Path: utzoo!attcan!uunet!husc6!ukma!gatech!cumulus!hsu
From: hsu@cumulus (Yung-Kao Hsu)
Newsgroups: comp.lang.c++
Subject: Re: Can we hide the private part of a class ?
Keywords: header dependency, efficiency.
Message-ID: <1369@cumulus>
Date: 3 Oct 88 14:01:59 GMT
References: <1368@cumulus> <10681@ulysses.homer.nj.att.com>
Reply-To: hsu@cumulus.UUCP (Yung-Kao Hsu)
Organization: Georgia Institute of Technology
Lines: 34

In article <10681@ulysses.homer.nj.att.com> ekrell@hector.UUCP (Eduardo Krell) writes:
>In article <1368@cumulus> hsu@cumulus.UUCP (Yung-Kao Hsu) writes:
>
>>The header dependency simply forces re-compilation of all program
>>units if some private declarations were changed (which shouldn't cause any
>>problem to any other classes).
>
>You can't really say that. What if something like sizeof() was used?
>Adding or removing private members from a class may change its size.
>    
>Eduardo Krell                   AT&T Bell Laboratories, Murray Hill, NJ
>
>UUCP: {att,decvax,ucbvax}!ulysses!ekrell  Internet: ekrell@ulysses.att.com

The problem you mentioned above really only hit half of the target. If the
sizeof() was used, the data type involved really is not private as far as
I can see. If you consider the example I mentioned in the posting,
i.e, an object that handles a table of values. The changes of table size
should not affect those object who use it, since they are only interested
in storing and retrieve data elements. But the changes of the table element
size should, since the data element type is part of interface specification.

But, C++ simply does not support any mechansim like ada and modula-2 that
help in distincting these situations. Thus, in most cases, you have to take
care of what should be recompiled and what needs not. I feel this is a very
serious problem if we want to use C++ in a large project or consider it as
an object-oriented language.

---------
-- 
YUNG-KAO HSU
School Of Infomation & Computer Science, Georgia Tech Atlanta GA30332
Internet:  hsu@cumulus.gatech.edu	 CSNet:  hsu%cumulus@gatech	
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