Path: utzoo!attcan!uunet!husc6!bbn!bbn.com!lpringle
From: lpringle@bbn.com (Lewis Pringle)
Newsgroups: comp.lang.c++
Subject: Re: Friend specifier considered harmful
Message-ID: <29433@bbn.COM>
Date: 8 Sep 88 02:28:37 GMT
References: <61@cybaswan.UUCP> <8160@alice.UUCP>
Sender: news@bbn.COM
Reply-To: lpringle@BBN.COM (Lewis Pringle)
Organization: Bolt Beranek and Newman Inc., Cambridge MA
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In article <8160@alice.UUCP> ark@alice.UUCP (Andrew Koenig) writes:
>
>For example, a linear algebra package might have Vector and Matrix
>classes.  Although some people might claim that a vector is
>just a 1xN matrix, others might prefer an Nx1 matrix, and
>therefore I suggest that vectors and matrices are really
>different beasts and neither is a species of the other.
>
>Now, how do you write a function to multiply a Matrix by a
>Vector?  It apparently needs to know about the internal
>representations of both classes.  Moreover, symmetry suggests
>that it should be a member of both or neither -- which
>perforce means neither.  Friends are the only way to make
>this possible.


I am not sure that your example well illustrates your point, since accessing
the size, and elements of both vectors and matrices would be public methods of
those classes.  I think thats really all you need to implement multiplication,
even if you are efficiency hacking (in which case inline functions as accessors
to the matrices would be a good choice).

I am not sure if I like the use of friends, but I can share a place where I
used them, and could not think of a better way.

I was implementing an object-oriented portable windowing system, and I defined
high level window object (class).  It had pointers to implementation specific
structures (e.g. the OS Window pointer) which needed to be referenced by
another class (the Controls).  I could have made a public accessor for this
machine-dependent information, but I wanted the public interface to be entirely
portable, and the use of friends was the only way I could find to acheive that.

It seems to me that the trouble is that C++ provides a restrictive form of
data hiding (not that I know of a better way!): its too "all or nothing".
The use of friends is not exacly what you want.  Perhaps if in
specifying a friend,you didn't give blanket access to the given class,
but instead to just an explicitly stated set of members?

					Lewis.
"OS/2: half an operating system for half a computer."

In Real Life:		Lewis Gordon Pringle Jr.
Electronic Mail:	lpringle@labs-n.bbn.com
Phone:			(617) 873-4433