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Path: utzoo!utgpu!water!watnot!watmath!clyde!rutgers!princeton!allegra!alice!bs
From: bs@alice.UUCP
Newsgroups: comp.lang.c++
Subject: Re: proposed info-hiding mechanism
Message-ID: <6763@alice.uUCp>
Date: Mon, 30-Mar-87 22:22:55 EST
Article-I.D.: alice.6763
Posted: Mon Mar 30 22:22:55 1987
Date-Received: Wed, 1-Apr-87 06:59:00 EST
Organization: AT&T Bell Laboratories, Murray Hill NJ
Lines: 53
Keywords: friend   protected



shapiro@inria.UUCP (Marc Shapiro) writes:

 >   class sosObject {
 > 	long size;
 > 	int fd;
 >    protected:
 > 	write_me_to_disk () {return ::write(fd, (void*)this, size);}
 > 	}
 > 
 >   class A: public sosObject {
 > 	...
 > 	}
 > 
 >   class B: public sosObject {
 > 	foo (A *a) { a -> write_me_to_disk(); }		// legal !
 > 	}
 > 
 > The above example shows that even though B is not derived from A, B has
 > access to A's 'protected' members.

Correct. The unit of protection in C++ is a class; not an individual object.
There are cases where the class is the ideal unit of protection and cases
where the object is. Concievably an ideal language would give the programmer
a choice.

mikem@otc.UUCP (Mike Mobray) writes:

 >    class Base {
 >	protected:
 >	    prot_member();
 >    };
 >
 >    class Derived : public Base {
 >	 // Derived can use prot_member()
 >
 >	 friend somefunc(Derived *d) { d->prot_member(); }
 >
 >			      // somefunc should be able to use prot_member()
 >			      // for an object of type Derived. In release
 >			      // 1.1 this gives an error message complaining
 >			      // that prot_member is protected.
 >   };

Again, cfront is correct. A friend does not have access to protected members
of a base class of the class it is a friend of. A protected member is accessible
by members of a derived class and by friends of its class. In the example
above somefunc() is neither a member of Derived nor a friend of Base.

One might conceiveably use the less restrictive rule that ``a friend can
access anything a member can'', but the implications of such a relaxation
of the rules are not obvious and quite nasty to implement.