Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!tut.cis.ohio-state.edu!ucbvax!hplabs!hp-pcd!hplsla!jima
From: jima@hplsla.HP.COM (Jim Adcock)
Newsgroups: comp.lang.c++
Subject: Re: Virtual destructors - why aren't they all?
Message-ID: <6590296@hplsla.HP.COM>
Date: 11 Oct 89 21:36:28 GMT
References: <10627@csli.Stanford.EDU>
Organization: HP Lake Stevens, WA
Lines: 21

>keith@csli.Stanford.EDU (Keith Nishihara) /  5:55 pm  Oct 10, 1989 /
>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?

Well, one answer is that declaring a function virtual causes both a space
penalty in an object of that class [said penalty constant as long as one
or more virtual functions are declared] and also a time penalty in invoking
that virtual function.  Presumably, if someone gives you a class with a 
non-virtual destructor and you'd rather have a virtual destructor, then 
you can trivially derive a new class that has a virtual destructor.  But
you can't go the other way.  Once a superclass declares a virtual function
subclasses always get to eat the space and/or time penalties.

This behavior is true of all C++ I have seen.  If you want a function to
be virtual, declare it virtual.  This requirement also has the advantage of 
language consistency.