Path: utzoo!utgpu!jarvis.csri.toronto.edu!cs.utexas.edu!ut-emx!walt.cc.utexas.edu!jkuhn
From: jkuhn@walt.cc.utexas.edu (Jeff Kuhn)
Newsgroups: comp.lang.c++
Subject: Access privilege question
Message-ID: <25397@ut-emx.UUCP>
Date: 2 Mar 90 06:53:31 GMT
Sender: news@ut-emx.UUCP
Reply-To: jkuhn@walt.cc.utexas.edu (Jeff Kuhn)
Organization: The University of Texas at Austin, Austin, Texas
Lines: 46

Here is an interresting problem that I did not have to worry about until
an upgrade from 1.7 to 2.0.

I need to access the protected members of a base class within a method of a
derived class THROUGH a pointer to an object of the base class.

Consider the following bit of code:

class Base {
	//...
protected:
	//...
	int	i;
};

class Derived : public Base {
	//...
public:
	//...
	int getMember (Base *);
};

int Derived::getMember (Base *bp)
{
	return bp->i;	// illegal: access through Base pointer not allowed
}

I believe that this could be done in 1.7, but 2.0 "fixed" the problem.  There
are two solutions that I have come up with.
1) Declare Derived a friend of Base:  This works, but any classes derived from
   Derived must be declared friends also.
2) cast the Base pointer to a Derived pointer and resolve the scope of the
   member, i:
     return ((Derived *)bp)->Base::i;
   This works, but is a hack.  Suppose that instead of i, a virtual function
   was desired.  If bp was actually a pointer to an object of class Derived,
   would the correct virtual function be called? (Base::f() or Derived::f())

I need to be able to do this correctly in order to have class Derived handle a
list of Base classes.  Derived needs to be able to pass messages to objects in
the list, regardless of whether they are of class Base or Derived.

Any comments/suggestions would be appreciated.

Jeff R. Kuhn
jkuhn@ccwf.cc.utexas.edu