Re: IUnknown

From:

"Giovanni Dicanio" <giovanni.dicanio@invalid.com>

Newsgroups:

microsoft.public.vc.language

Date:

Sat, 2 Feb 2008 12:18:01 +0100

Message-ID:

<e73jz1YZIHA.5612@TK2MSFTNGP06.phx.gbl>

"George" <George@discussions.microsoft.com> ha scritto nel messaggio
news:10443EBF-25C1-4DED-A5D1-4226951A4F2F@microsoft.com...

you can see QueryInterface has two forms
of implementation, one is pure virtual function, the other is not. From
traditional COM book, QueryInterface should be a pure virtual function.
Correct me if I am wrong.

George: all COM *interfaces* are a set of pure virtual functions.
Then, when you implement a COM interface, you inherit from that interface,
and you write (or reuse) a body for the interface pure virtual functions,
which become class methods with a body, in C++.

Any ideas?

       IUnknown
       {
       public:
           BEGIN_INTERFACE
           virtual HRESULT STDMETHODCALLTYPE QueryInterface(
               /* [in] */ REFIID riid,
               /* [iid_is][out] */ void __RPC_FAR *__RPC_FAR *ppvObject) =
0;

This is a pure virtual function, in fact it has the form "virtual ....
f(...) = 0 ".

"virtual" --> virtual function (or better virtual method)
"= 0" --> pure

           template<class Q>
   HRESULT STDMETHODCALLTYPE QueryInterface(Q** pp)
   {
       return QueryInterface(__uuidof(Q), (void **)pp);
   }

This seems a C++ template "helper" *implementation* of QueryInterface, that
allows the caller to use an interface (Q), instead of the interaface ID.
In fact, this implementation gets the Q interface ID using __uuidof.
The "real" QueryInterface specification is the first you read.
This second one is just an helper template implementation for QueryInterface
virtual method, to allow programmers to write something like this

IMyInterface * pMyInterface;

...QueryInterface( &pMyInterface )

instead of the more "verbose":

.... QueryInterface( IID_IMyInterface, reinterpret_cast<void **>(
&pMyInterface ) );

HTH,
Giovanni