Re: handling QueryInterface

From:

"Igor Tandetnik" <itandetnik@mvps.org>

Newsgroups:

microsoft.public.vc.atl

Date:

Wed, 29 Nov 2006 10:55:54 -0500

Message-ID:

<Onhcb78EHHA.1356@TK2MSFTNGP06.phx.gbl>

Jason S <jmsachs@gmail.com> wrote:

What are some ways of overriding the default ATL QueryInterface?

COM_INTERFACE_ENTRY_* macros. E.g. your interface map could look like
this:

BEGIN_COM_MAP(CMyClass)
COM_INTERFACE_ENTRY(IUnknown) // first entry must be simple
COM_INTERFACE_ENTRY_FUNC_BLIND(0, MyQueryInterface)
END_COM_MAP()

then almost all the work of QueryInterface is delegated to
MyQueryInterface.

My understanding of QueryInterface is that a given object has to
provide consistent transitivity, e.g. if you can ever successfully
QueryInterface from any of interfaces in set S = (I1, I2, I3, etc.) to
another, you have to always be able to get from any one of them to any
of the others, & can't decide to return E_NOINTERFACE conditionally
based on something.

I am, however, thinking of implementing an object that offers a
different set S of interfaces depending on how the object was
initialized, which appears to be legal -- my source being "Essential
COM" chapter 4 particularly page 164.

Yes, it is legal.
--
With best wishes,
Igor Tandetnik

With sufficient thrust, pigs fly just fine. However, this is not
necessarily a good idea. It is hard to be sure where they are going to
land, and it could be dangerous sitting under them as they fly
overhead. -- RFC 1925