Re: CreateInstance

From:

"Igor Tandetnik" <itandetnik@mvps.org>

Newsgroups:

microsoft.public.vc.language

Date:

Mon, 4 Feb 2008 00:06:19 -0500

Message-ID:

<uxPMwuuZIHA.484@TK2MSFTNGP06.phx.gbl>

"George" <George@discussions.microsoft.com> wrote in message
news:8652DA36-A029-4697-A32E-7A8C9C5BACFF@microsoft.com

In the book "Inside COM", there is an interesting code segment like
this,

[Code]
IUnknown* CreateInstance()
{
   IUnknown* pI = static_cast <IX*> (new CA);
   pI -> AddRef();
   return pI;
}

// IX an interface is derived from IUnkown, IY is an interface
derived from // IUnknown and CA is a class directly derived from both
IX and IY. [/Code]

My question is, why static_cast <IX*> to result in IUnknown*?

static_cast<IX*> results in IX pointer, of course. IX pointer is then
implicitly convertible to IUnknown*, IUnknown being IX's base class.

Why not
static_cast <IUnknown*> to result in IUnknown* to be more
straightforward

Try it, you get'll a compiler error. I'll leave it as an exercise for
the reader to figure out why you get this error. In your test, don't
forget that CA is supposed to derive from two interfaces, IX and IY.
--
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