Path: utzoo!utgpu!news-server.csri.toronto.edu!rpi!zaphod.mps.ohio-state.edu!usc!elroy.jpl.nasa.gov!ames!sgi!shinobu!odin!sgi.com!linton
From: linton@sgi.com (Mark Linton)
Newsgroups: comp.lang.c++
Subject: Re: Abstract Base Class Enforcement
Keywords: class, abstract, c++
Message-ID: <1991Mar25.200522.3451@odin.corp.sgi.com>
Date: 25 Mar 91 20:05:22 GMT
References: <358@intertel.UUCP>
Sender: news@odin.corp.sgi.com (Net News)
Reply-To: linton@sgi.com (Mark Linton)
Organization: sgi
Lines: 27

In article <358@intertel.UUCP>, 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.)  Given that you  want to
|> disallow instances of  the abstract base  class from  being created, which  of
|> these alternatives is best?
|> 
|> 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).

Use option 2, making all constructors protected (define a protected no-parameter
constructor if the class otherwise has no constructors).  Adding an unnecessary
pure virtual would confuse people.  Pure virtual means subclasses must provide
an implementation, which is not what you intend.  Similarly, a pure virtual destructor,
if that works, is not what you want (you needn't force subclasses to define a destructor).

Unfortunately, section 10.3 of the ARM confuses people with the statement

    A class is abstract if it has at least one pure virtual function.

This statement is correct, but many people interpret the "if" to mean "if and only if",
which is not correct.  All classes with pure virtual functions are abstract, but not
all abstract classes have a pure virtual function.