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From: jbuck@galileo.berkeley.edu (Joe Buck)
Newsgroups: comp.lang.c++
Subject: Re: Overloaded operator new VS. inheritance
Message-ID: <10351@pasteur.Berkeley.EDU>
Date: 21 Jan 91 03:13:54 GMT
References: <1991Jan10.195541.6003@usenet@scion.CS.ORST.EDU> <1991Jan10.224933.7518@Neon.Stanford.EDU> <1991Jan17.200447.6794@sco.COM>
Sender: news@pasteur.Berkeley.EDU
Reply-To: jbuck@galileo.berkeley.edu (Joe Buck)
Lines: 32

In article <1991Jan17.200447.6794@sco.COM>, tom@sco.COM (Tom Kelly) writes:
> In the example below, the size passed to the destructor is always
> only the size of the base class, even when the pointer points
> to a derived class.
 
> Note that adding a virtual destructor to the base class fixes
> the problem.

Ah, but that is correct behavior on the part of the compiler.  Without
the virtual destructor, the compiler has absolutely no information
that the pointer is really to a derived class (what if there are no
virtual functions defined in the base class?  Then the code has no
way, short of ESP, to tell that the pointer is really to a derived
class).

In short: ALWAYS make the destructor virtual if you plan to derive
from the class, and if at any point in your code you have

	Base* b;
	...
	delete b;

but b may point to an object of a different class (shorthand version
of this rule: always make the baseclass destructor virtual unless
you will never derive from that class).

I expect that you'll see identical behavior on any C++ compiler.


--
Joe Buck
jbuck@galileo.berkeley.edu	 {uunet,ucbvax}!galileo.berkeley.edu!jbuck