Path: utzoo!attcan!uunet!lll-winken!lll-tis!ames!mailrus!tut.cis.ohio-state.edu!rutgers!bellcore!faline!thumper!ulysses!andante!alice!shopiro
From: shopiro@alice.UUCP (Jonathan Shopiro)
Newsgroups: comp.lang.c++
Subject: Re: Friend specifier considered harmful
Summary: friend status can be granted to individual class members
Message-ID: <8180@alice.UUCP>
Date: 9 Sep 88 15:35:44 GMT
References: <61@cybaswan.UUCP> <8160@alice.UUCP> <29433@bbn.COM>
Organization: AT&T Bell Laboratories, Murray Hill NJ
Lines: 34

In article <29433@bbn.COM>, lpringle@bbn.com (Lewis Pringle) writes:
> ...
> 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?

Good idea!  In fact you can do exactly that, for example

class Foo {
	int	bar();
	int	baz();
};
class Foe {
	friend int Foo::bar();
	int	priv;
};

Then Foo:bar() can access priv in any Foe object, but Foo::baz() cannot.

I should also say that there seems no limit to the gradations of data
hiding that a programmer might want, for example you might want one
private member to be accessible to one external function and another
private member to be accessible only to a different external function.
However, the fundamental purpose of data hiding in C++ is to help the
programmer avoid bugs and it has no impact whatsoever on correct
programs.  Therefore, more elaborate data hiding schemes have two
strikes against themselves at the outset:  they make things more
complex by their very existence, but their justification must be that
they make things simpler.
-- 
		Jonathan Shopiro
		AT&T Bell Laboratories, Murray Hill, NJ  07974
		research!shopiro   (201) 582-4179