Re: COM Events and IDispatch - DWebBrowserEvents2 and HTMLWindowEvents2

From:
"Alexander Nickolov" <agnickolov@mvps.org>
Newsgroups:
microsoft.public.vc.atl
Date:
Mon, 8 Jan 2007 17:55:15 -0800
Message-ID:
<#Oas1E5MHHA.1008@TK2MSFTNGP06.phx.gbl>
No, you are not avoiding the reference counting problem -
you are running headfirst into it. IDispEvent[Simple]Impl
is a roundabout way into a circular reference count.
Implementing your sink on the ATL COM object is a sure
way of getting it.

--
=====================================
Alexander Nickolov
Microsoft MVP [VC], MCSD
email: agnickolov@mvps.org
MVP VC FAQ: http://vcfaq.mvps.org
=====================================

"Lady_A" <alexandra.lev@gmail.com> wrote in message
news:1168302933.548209.160360@42g2000cwt.googlegroups.com...

Thanks a lot!
I do not want to change the existing working code to use IDispEventImpl
instead.
Therefore I am adding another class that will derive only from one
IDispEventImpl and use this to hook into HTMLWindowEvents2, avoiding
the reference counting problem with deriving from 2 IDispEventImpl.

Alexander Nickolov wrote:

But beware the shallow split identity performed by
IDsipEvent[Simple]Impl - it only implements QI but
delegates AddRef/Release to the containing object
thus you have shared reference count. It's highly
recommended to not derive an ATL COM object
class from IDispEvent[Simple]Impl in order to
avoid circular reference counts locking an object
in memory indefinitely.

--
=====================================
Alexander Nickolov
Microsoft MVP [VC], MCSD
email: agnickolov@mvps.org
MVP VC FAQ: http://vcfaq.mvps.org
=====================================

"Igor Tandetnik" <itandetnik@mvps.org> wrote in message
news:OHPTdZTMHHA.2028@TK2MSFTNGP03.phx.gbl...

"Lady_A" <alexandra.lev@gmail.com> wrote in message
news:1168042166.981694.72520@v33g2000cwv.googlegroups.com

I have an IE plugin that is hooked into DWebBrowserEvents2 in the
following way :
[
...
 dual,
...
]
interface IMyInterface : IDispatch {
part of DWebBrowserEvents2 methods with their dispids
};


Why do it this way? Why not use IDispEvent[Simple]Impl, as the nature
intended? See ATLEventHandling sample.

It works fine and now I need to add hooking into
HTMLWindowEvents2::onunload
First of all I am not clear on how can my class can expose in the
COM_MAP an interface DWebBrowserEvents that it does not implement
according to the declaration (it has all the funcs, but there is no ":
public DWebBrowserEvents2")


DWebBrowserEvents2, being a dispinterface, is basically just a synonym
for
IDispatch, which you do implement.

Second of all, how do I add the HTMLWindowEvents2 in this case ?


In general, you don't. Implement a separate class for each separate
event
interface you want to sink events from. Or, do as I suggested and use
IDispEventImpl; your class can derive from IDispEventImpl more than
once,
each such derivation supporting one sink interface. Internally,
IDispEventImpl plays a clever trick called "split COM identity", which
allows one C++ class to pretend to be several COM objects at once.
--
With best wishes,
   Igor Tandetnik

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