Re: Unusual usage of IUknown

From:

Scot T Brennecke <ScotB@Spamhater.MVPs.org>

Newsgroups:

microsoft.public.vc.language

Date:

Tue, 01 Sep 2009 01:41:17 -0500

Message-ID:

<#aAM18sKKHA.3708@TK2MSFTNGP02.phx.gbl>

Gingko wrote:

"Ben Voigt [C++ MVP]" <bvoigt@newsgroup.nospam> a ?crit dans le message de
news: 53AC1D3E-48E3-46AD-A1C8-3D8B75E30477@microsoft.com...

/// Minimal reference decrement implementation (with object destruction
when
reaching zero)
ULONG STDMETHODCALLTYPE CUnknownStub::Release()
{
if (--m_cRef == 0) {
delete this;

This line worries me in a base class. Is it guaranteed that this is the
same pointer returned from new, which should also be passed to delete? Or
could "this" represent a sub-object at (potentially) a different address?
It's probably only a problem if this isn't the leftmost base class at
every level, but COM usually does use multiple inheritance. Just a trap
for the unwary.

       return 0; // "this" have been deleted, so don't return
this->m_cRef
   }
   return m_cRef;
}

I thought that using a virtual destructor was the mean to avoid this problem
?

Was I wrong ?

Gilles

A virtual destructor only helps to make sure the most-derived destructor is actually called. If this is a sub-object aggregated
into another object, you've still got big trouble.