Path: utzoo!attcan!uunet!tut.cis.ohio-state.edu!att!drutx!kvt
From: kvt@drutx.ATT.COM (TranKV)
Newsgroups: comp.lang.c++
Subject: Pure Virtual Destructors
Message-ID: <5033@drutx.ATT.COM>
Date: 6 Feb 90 22:34:04 GMT
References: <4800082@m.cs.uiuc.edu> <4924@drutx.ATT.COM> <10382@alice.UUCP> <4982@drutx.ATT.COM> <10422@alice.UUCP>
Reply-To: kvt@drutx.ATT.COM (TranKV)
Organization: AT&T, Denver, CO
Lines: 38

Commenting on PURE VIRTUAL DESTRUCTORS

In article <10422@alice.UUCP> ark@alice.UUCP (Andrew Koenig) writes:
....

>Therefore, it doesn't make sense to have a pure virtual
>destructor, as it would preclude destroying objects not
>only of that class but of any class derived from it.

I might be ignorant here but to counter Andrew's comments:

    1. "preclude destroying objects ... of that class"
    From C++ usage point of view, this is perfectly OK since the base
    class might just be a place holder (i.e. no data members declared in
    class definition; therefore nothing needs to be destroyed.) I 
    understand that in C implementation, every C++ class requires some 
    data; but as a C++ user, should I be required to be aware of its 
    C implementation?

    2. "... not only of that class but of any class derived from it."
    I don't understand this. As I understand C++, a VIRTUAL BASE
    destructor forces the VIRTUAL DERIVED class destructor to be called
    first (when the derived object is pointed at by a base class 
    pointer), and then other destructors in the class hierarchy. If 
    that's so, why a PURE virtual base class destructor would prevent 
    the VIRTUAL DERIVED class destructor to be called? I'd assume 
    everything works as expected except when it's the turn  of the 
    base class destructor when  nothing will be done. But that's OK
    (see my argument above.)

    I'm confused.

Besides, if pure virtual destructors don't really make sense, why Cfront
doesn't flag it as errors but let linker complain about it?

Kim Tran
Bell Labs
kvt@drutx.att.com