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?
Keywords: virtual destructor
Message-ID: <1991Jun4.134034.21404@world.std.com>
Date: 4 Jun 91 13:40:34 GMT
References: <183@swampthing.sbi.com> <1991Jun1.192610.18321@borland.com> <185@swampthing.sbi.com>
Organization: Glockenspiel, Ltd.
Lines: 24

wfl@swampthing.sbi.com (W. Linke CPE) writes:
> A pure virtual member function *MAY* be defined for the abstract class in which
> it is declared, but it *MUST* be overridden in some class derived from the
> abstract class in order to create class objects,
>
> but a pure virtual destructor *MUST* be defined for the abstract class in which
> it is declared, while it *MAY* be "overridden" in classes derived from the
> abstract class.
>
> "Pure virtual" in the above really means two different (overlapping) things.

Actually, that's not quite correct.  "Pure virtual" means the same in both
cases; it's just that the special characteristics of destructors cause the
compiler to do certain things for you that have to be done manually for
ordinary member functions.  Specifically, a pure virtual member function
"*MUST* be defined" if it is ever invoked; constructors are guaranteed to be
invoked.  Similarly, a derived class whose base has a destructor is provided
a destructor by the compiler if none is explicitly coded, thus implicitly
overriding a virtual destructor in the base class.

Pure virtual works exactly the same in both cases; there are no special
rules for pure virtual destructors, just interactions between rules.

-- William M.