Path: utzoo!utgpu!news-server.csri.toronto.edu!rpi!usc!cs.utexas.edu!uunet!world!wmm
From: wmm@world.std.com (William M Miller)
Newsgroups: comp.lang.c++
Subject: Re: Pure virtual destructors are illegal?
Message-ID: <1991Jun4.134117.21491@world.std.com>
Date: 4 Jun 91 13:41:17 GMT
References: <183@swampthing.sbi.com> <1410006@hpsciz.sc.hp.com>
Organization: Glockenspiel, Ltd.
Lines: 34

masa@hpsciz.sc.hp.com (Masayoshi Habu) writes:
> Please correct me if I am wrong. A pure virtual function usually has no
> implementation because the intention is to catch a run-time error if this
> function is never overridden and called somehow. 

No, that's not quite right.  The basic idea of a pure virtual function is to
prevent its class from being instantiated, i.e., to make it abstract.  Since
derived classes that do not override a pure virtual function inherit the
base class's pure virtual, it is impossible to create an object for which a
pure virtual function is not overridden.  The compiler guarantees that at
compile time, not at runtime.

A pure virtual function may indeed be defined, if it is to be called (as a
destructor will be).  Also, an implementation is not required to generate a
runtime error if an undefined pure virtual function is invoked; E&S says
only that such an occurrence results in undefined behavior (although an
implementation that did not produce an error message would be less useful
than one that did).

>                                                             So my opinion
> is that the compiler should reject a pure virtual destrcutor.

Although I've never needed one, the rationale I've heard for pure virtual
destructors was that someone wanted to make a class abstract but that all
the ordinary member functions needed to be defined, i.e., not pure.  Making
the destructor pure virtual is a better choice than defining a dummy pure
virtual that is unneeded except for making the class abstract.  In any case,
I see no need to make destructors a special case, and the core language
working group of X3J16, the ANSI C++ Standard Committee, has explicitly
affirmed that pure virtual destructors are intended to be included in the
language.

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