Re: CreateInstance
Thanks Igor,
1.
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.
--
I think the reason is it is ambiguity since both IX and IY are both derived
from IUnknown and there will be ambiguity issue. This is a short answer.
The long description should be, IX and IY are both derived from IUnknown and
they both may have their overridden implementation of
QueryInterface/AddRef/Release. Some of the implementations may even be
conflicting in IX and IY. CA is derived from both IX and IY, and the
ambiguity issue is compiler does not know whether the converting result
should be function table in IX or in IY. Is that correct?
2.
I am still confused that since IX and IY are interface. If following strict
grammar, they should not contain implementation and only declaration. Since
they both source from IUnknown, so the only implementation should be in class
CA, so there should not be any ambiguity issue?
regards,
George
"Igor Tandetnik" wrote:
"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