Re: Pure virtual, forced implementation, and base class methods

From:

Richard Damon <news.x.richarddamon@xoxy.net>

Newsgroups:

comp.lang.c++

Date:

Wed, 28 Sep 2011 21:05:53 -0400

Message-ID:

<j60g9f$5r3$1@dont-email.me>

On 9/28/11 6:02 PM, Andy Champ wrote:

.. So what we want to do is expose a class that's got some methods in
that are pure virtual, so you have to override them - but they're
incompletely implemented in the base class. There's some useful stuff in
the base class implementation, just not enough. A diagnostic dump
function is an example.

The exposed class is the tip of a hierarchy. It is _not_ the one with
the implementation. This minimal example shows roughly what we have. C
is the exposed class; D is locally derived from it. In D we want to call
into a base class method; B forces us to implement that method. It looks
as though in the most derived method we have to know which of the
classes in the hierarchy has an implementation in order to call it. Yet
if we leave out the declaration that forces us to implement a derived
method we can call through _without_ knowing which of the classes
implements it.

Is this just the way it is, an MS bug, or is there a better way?

If B's pure virtual function is only to force you to define it below,
then make A's m() an implemented pure virtual, i.e. declared

virtual void m()=0;

but still include in your code

void A:m() {
.....
}

The B layer is unneeded.

When you call C::m() in D::m(), the compiler calls the version of m()
that is statically in scope at that point, which as presented is B::m()
(being marked pure doesn't negate being declared).

A very common use of this is in pure virtual destructors to make a class
abstract, but that destructor still needs to be implemented (ALL classes
have a destructor, but it may be just implicitly defined).