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From: mittle@blinn.watson.ibm.com (Josh Mittleman)
Newsgroups: comp.lang.c++
Subject: Re: protected member functions
Message-ID: <1991Mar15.165538.17532@watson.ibm.com>
Date: 15 Mar 91 16:55:38 GMT
References: <1991Mar14.215717.28199@asc.slb.com>
Sender: @watson.ibm.com
Reply-To: mittle@ibm.com
Organization: IBM T. J. Watson Research
Lines: 34

ARM, p.253, reads "A friend or member function of a derived class can
access a protected nonstatic member of base of its base classes only
through a pointer to, reference to, or object of the derived class (or any
class derived from that class)."  A discussion of this restriction can be
found on pp.254-5.

A::SetState() is a protected member of A and, by virtue of the public
derivation, B::SetState() is a protected member of B.  But they aren't the
same thing, although they have the same implementation.  Member functions
of B can access B::SetState(), but they have no access to the protected
parts of an A object; only members and friends of A can access those.

Given your original code, suppose there were some other class C : public A,
in which SetState() were overloaded:

class C : public A
{
  protected: void SetState();
};

Now, we do this:

  C objectC;
  B objectB(&C);  // C is a A, so the C* is automatically converted to A*.
  B.Foo();        // wrong: invokes C->A::SetState() instead of
                  // C->C::SetState(). 

Of course, this error could be avoided by making SetState() virtual, but it
still is an incorrect result.  
 

===========================================================================
Josh Mittleman (mittle@ibm.com or joshua@paul.rutgers.edu)
J2-C28 T.J. Watson Research Center, PO Box 704, Yorktown Heights, NY  10598