Xref: utzoo comp.std.c++:495 comp.lang.c++:10830
Path: utzoo!mnetor!tmsoft!torsqnt!news-server.csri.toronto.edu!cs.utexas.edu!uunet!microsoft!jimad
From: jimad@microsoft.UUCP (Jim ADCOCK)
Newsgroups: comp.std.c++,comp.lang.c++
Subject: Re: Accesibility change in virtual functions
Message-ID: <59918@microsoft.UUCP>
Date: 18 Dec 90 19:29:31 GMT
References: <1990Dec7.205909.14021@relay.wpd.sgi.com>
Reply-To: jimad@microsoft.UUCP (Jim ADCOCK)
Organization: Microsoft Corp., Redmond WA
Lines: 100

In article <1990Dec7.205909.14021@relay.wpd.sgi.com> hitz@csi.UOttawa.CA writes:
|I am posting this for Martin Hitz, who does not have posting access to the net.
|Please send e-mail replies to Martin at hitz@csi.UOttawa.CA.
|
|Note: I checked that this is the neahiour of cfront 2.o also, and
|indeed is explicitly stated in the ARM (p.255, section 11.6).  Martin
|would like to raise the issue for discussion, hence I am posting to
|comp.lang.c++ and comp.std.c++.
|I find it awkward that it is possible (at least with g++ and Zortech 2.0)
|to implicitely call a private member function via the virtual mechanism as
|in the following example:
|
|	class B {
|	 public:
|		virtual void f() { cout << "B\n"; }
|	};
|
|	class D: B {
|		void f() { cout << "D\n"; }
|	};
|
|	main()
|	{
|		D  d;
|		B& b = d;	
|		b.f();		// prints "D"
|	}
|
|My opinion is that this shouldn't be possible, i.e. the compiler
|should complain about the private version of f() in the derived 
|class D.
|
|Any comments?

First, I think D needs to derive from a public B otherwise your compiler
ought to prohibit the implied coercion of a D ref to a B ref.  If you don't
mean for B to be a public base, then this discussion is not an issue --
your compiler ought to prohibit it.  Is this a point of confusion?

Next, for comparison, I've added a function g() below for which I've tried to 
adjust access control.  (Your C++ compiler should prohibit this attempt)

For reference, what section 11.6 says is : "The access rules (sec11) for a 
virtual function are determined by its declaration and are not affected by the 
rules for a function that later overrides it."

So compilers indeed ought to accept this program [given D inherits publicly]
as you suggest.  The question then, is should this rule be changed, and if so, 
how?

Note that some people might actually consider this definition a "feature."
B might require some low level methods that it doesn't generally make
sense to expose in D, but which are methods that a class D needs to implement
in order for B to work correctly.  An example pops to mind:  Let f() return
a class's metaclass representation.  At the primitive level we desire to 
allow people access to runtime type information, but at a higher programming
level we want to prohibit such access -- lest programmers fall into the
bad habit of programming based on exact type.  At the primitive level 
however, low level functionality such as file I/O needs access to exact
type information, so it makes sense to allow access at the base class
level.  But D needs to override f() in order to return D's correct metaclass
information.  Voila -- the access to virtual functions rules aren't a bug,
their a feature! 1/2 :-)

Pragmatically, one might note that class B may be compiled long before
D, and therefor the only way to prohibit access to virtual D::f() is
to prohibit conversion of a D ref to A B ref.  One might ask then if the
assignment of a D ref to B ref shouldn't require an explicit conversion?

Still, I think when D inherits from a public B, it is stating that a D
can be used as a B.  This is only possible if all B's public members remain 
accessible.  If you don't want a D to be usable as a B, inherit B privately,
and use access control to promote those methods of B to public access
that you wish D to support.  Compilers then will correctly prohibit 
conversion of a D to a B.

I recommend no change.

// comparison of attempt to change access control verses overloaded virtual
// function follows [shouldn't compile]:

	class B {
	 public:
		virtual void f() { }
		virtual void g() { }
	};

	class D: public B {
	 private:
		void f() { }
		B::g;
	};

	main()
	{
		D  d;
		B& b = d;	
		b.f();
		b.g();
	}