Path: utzoo!utgpu!attcan!uunet!lll-winken!lll-tis!ames!ncar!gatech!rutgers!bellcore!faline!thumper!ulysses!andante!alice!ark
From: ark@alice.UUCP (Andrew Koenig)
Newsgroups: comp.lang.c++
Subject: Re: Friend specifier considered harmful
Message-ID: <8160@alice.UUCP>
Date: 2 Sep 88 14:25:34 GMT
References: <61@cybaswan.UUCP>
Organization: AT&T Bell Laboratories, Liberty Corner NJ
Lines: 35

In article <61@cybaswan.UUCP>, eeartym@cybaswan.UUCP (Dr R.Artym eleceng ) writes:

> 	Do YOU have a c++ program that couldn't have been written without
> the friend specifier?

Obviously not -- I can write a universal Turing machine
simulator without the friend specifier.  But the real question is:
Are there problems for which friends are the ``best'' solution?

My answer is YES.  Friends are useful any time you want to have
several interrelated classes that can't be naturally described
by inheritance.

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.

If you are tempted to respond that this just shows that
matrices and vectors should either be the same class
or one should inherit from the other, my response is that
my argument still holds for any two classes that need
to be linked by some means other than inheritance --
you won't convince me that friends are unnecessary until
you have convinced me that no such classes can ever exist.
-- 
				--Andrew Koenig
				  ark@europa.att.com