Path: utzoo!utgpu!news-server.csri.toronto.edu!rpi!zaphod.mps.ohio-state.edu!swrinde!elroy.jpl.nasa.gov!decwrl!world!wmm
From: wmm@world.std.com (William M Miller)
Newsgroups: comp.lang.c++
Subject: Re: Abstract Base Class Enforcement
Keywords: class, abstract, c++
Message-ID: <1991Mar24.204943.9998@world.std.com>
Date: 24 Mar 91 20:49:43 GMT
References: <358@intertel.UUCP>
Organization: The World Public Access UNIX, Brookline, MA
Lines: 34

wohlenbe@intertel.UUCP (Greg Wohlenberg) writes:
>      I've  got  a question.   Say you have  a base  class that  you want to be
> abstract.  But, for one reason or another, it  isn't convenient to make any of
> the  class's virtual functions  pure.  (This would happen  if all your virtual
> functions actually "do something" in the base class.)

If you mean that it's all right for a derived class to override none of the
base class's virtual functions, then you do need one of the additional
methods you described or the one I'll mention below.  However, if you mean
that you need a definition in the base class to provide some common
processing that can be used by derived class overriding functions, there's
nothing to stop you from providing a definition for a pure virtual function;
it will never be invoked by an ordinary member function call, but an
explicitly-qualified call (e.g., "Base::pure_virt1()") will invoke the base
class definition.

> 1. Create a extra do-nothing function in the base class that is pure virtual.
>
> 2. Make the base class's no-parameter constructor "protected".  This  makes it
> so only  derived classes  are able to create instances  of the base class (and
> therefore enforces the abstract characteristic of the base class).

Actually, you'd need to make *all* the constructors either protected or
private.

>      Or, is there another alternative that I am not aware of?

How about making the destructor pure?  (I assume it's already virtual; the
rule of thumb is to make the destructor virtual if there are any virtual
functions.)  You still have to provide a definition of it or the program
won't link or run, of course.

-- William M. Miller, Glockenspiel, Ltd.
   wmm@world.std.com