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From: ark@alice.UUCP (Andrew Koenig)
Newsgroups: comp.lang.c++
Subject: Re: Derived can't access protected members of base's
Message-ID: <10593@alice.UUCP>
Date: 18 Mar 90 14:16:35 GMT
References: <56.UUL1.3#5109@pantor.UUCP>
Organization: AT&T Bell Laboratories, Liberty Corner NJ
Lines: 34

In article <56.UUL1.3#5109@pantor.UUCP>, richard@pantor.UUCP (Richard Sargent) writes:

> To our mind, we have a class derivation tree (root class and a number
> of derived branches). We think that any object of a type from that
> tree of classes should be able to access the protected members of
> other objects also of that tree, but no class not in the tree.

1.2 enforced the restriction too, and I don't think the enforcement
changed significantly between 1.2 and 2.0.

One of the main reasons for the restriction is that otherwise there's
very little practical difference between `protected' and `public.'
For example:

	class Foo {
	protected:
		int i;
	};

Now, suppose I want to clobber member `i' of a `foo' object.  I would
like to write this:

	void clobber(Foo& f, int n) { f.i = n; }

but I can't quite do that directly.  What if I do this, though?

	class dummy: public Foo { friend void clobber(Foo&, int); };

Now `dummy' can get at any protected member of Foo, and so can its
friends.  By making `clobber' a friend, I open Foo up to it, and
completely without permission from the author of Foo.
-- 
				--Andrew Koenig
				  ark@europa.att.com