visibility vs. accessibility of protected symbols
timon.gehr at gmx.ch
Fri Feb 17 11:02:53 PST 2012
On Friday, 17 February 2012 at 18:35:38 UTC, Jonathan M Davis
> On Friday, February 17, 2012 09:03:32 Steven Schveighoffer
>> On Mon, 13 Feb 2012 21:05:35 -0500, Jonathan M Davis
>> <jmdavisProg at gmx.com>
>> > On Tuesday, February 14, 2012 02:43:29 Timon Gehr wrote:
>> >> On 02/14/2012 02:16 AM, Jonathan M Davis wrote:
>> >> Well, not being able override what cannot be accesses is a
>> >> quite basic
>> >> requirement of security. Private members cannot be
>> >> overriden in a
>> >> different module.
>> > Have you ever read up on NVI? That's how it's supposed to
>> > work. The
>> > whole idea
>> > is to have derived classes override functions without being
>> > able to use
>> > them.
>> > It makes it so that the public function is non-virtual and
>> > contains
>> > whatever
>> > stuff you want to guarantee is called before or after the
>> > private
>> > function,
>> > which is then virtual.
>> > http://www.gotw.ca/publications/mill18.htm
>> This does nothing to protect the implementation. You are
>> yourself if you think you can stop a derived class from
>> calling it's own
>> implementation! In this case, protected is good enough, and I
>> like how
>> private is always non-virtual. As Timon says, it's good for
>> security and
>> module encapsulation.
> I know. That's part of why I'm arguing that protected is enough
> and that private shouldn't be virtual. But pretty much the
> whole point of doing NVI with private is the theory that you
> want to guarantee that the derived classes can't call the
> function. The fact that you can't actually guarantee that just
> makes the arguments for making private virtual for NVI that
> much weaker.
> But Timon's arguments make it sound like he doesn't understand
> what NVI is, which was the point of my post.
> - Jonathan M Davis
I understand what NVI is. It is just that private methods
overridable from a different module don't have many merits,
regardless of how NVI is apparently sometimes implemented in C++.
The fact that C++ has overridable private methods is quite likely
just a side-effect of wanting to give some kind of semantics to a
method declared as both private and virtual. I could argue that
it sounds like you don't understand what NVI is because you
assume C++'s private virtual semantics are required for the
implementation of NVI. ;)
As far as information hiding design goes, we certainly don't want
to borrow heavily from C++. Its system is unsound.
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