Path: utzoo!attcan!utgpu!jarvis.csri.toronto.edu!mailrus!wuarchive!psuvax1!rutgers!att!dptg!ulysses!andante!alice!shopiro
From: shopiro@alice.UUCP (Jonathan Shopiro)
Newsgroups: comp.lang.c++
Subject: Re: Virtual destructors - why aren't they all?
Summary: Some classes don't have any virtual functions
Message-ID: <10021@alice.UUCP>
Date: 14 Oct 89 20:57:05 GMT
References: <10627@csli.Stanford.EDU>
Organization: AT&T Bell Laboratories, Murray Hill NJ
Lines: 19

In article <10627@csli.Stanford.EDU>, keith@csli.Stanford.EDU (Keith Nishihara) writes:
- I was surprised to discover that calling delete on a derived class
- object pointed to by a pointer to the base class failed to call
- the derived class destructor. (cfront 1.2) (See example below.)
- I cannot think of any case where this would be the desirable behaviour.
- Fortunately, it appears that declaring the destructor virtual causes
- the appropriate behaviour to happen.  Why are not all desctructors
- virtual?  Is this true in other C++ systems?

It's true at least in C++ 2.0.  The reason why is that some classes
have no virtual functions, and adding a virtual destructor would cost
one word per object of the class.  Some classes are not intended to be
inherited from, so they don't need any virtual functions.  I think it's
generally true that if a class has any virtual functions, it should
have a virtual destructor, but it's up to you to do it.
-- 
		Jonathan Shopiro
		AT&T Bell Laboratories, Warren, NJ  07060-0908
		research!shopiro   (201) 580-4229