Re: Dual interface

Tim Roberts <>
Sat, 20 Sep 2008 20:00:49 -0700
George <> wrote:

My confusion is, struct STGMEDIUM holds a pointer of IUnknown, and as you
and others mentioned the pointer to IUnknown is a COM object which only
expose IUnknown interface.

IUnknown is NOT an object. It is an interface. All COM objects happen to
implement it, but IUnknown is not an object. You need to get the
terminology straight -- it's important.

For the COM object which struct STGMEDIUM holds, how could other component
utilizes it? Could you describe a usage scenario please? :-)

After all of the messages in this thread, I don't think you yet understand
how interfaces work.

Let me construct an example. Let's say I define a class like this:

   class SillySample
       : public IUnknown
       , public IBaseFilter
       , public IDispatch
       , public IPin
       , public IDropTarget

This is a silly sample, because no real object would ever implement that
particular set of interfaces.

Now, I can construct an object of that class in several ways, but let's say
I do it in a simple way.

    SillySample * pss = new SillySample;

Now, using the object "pss", I can call any of the methods in any of those
interfaces. ALL of the methods are part of "class SillySample". But, to
use this in a COM-like way, I would use interfaces. So, let's say I pass
that object to a function that expects an IDropTarget.

    void Func1( IDropTarget * pidt );

I can pass my object to that function:

    Func1( pss );

Now, inside of Func1, "pidt" is still the SAME OBJECT. However, using that
pointer, I can only access the methods of IDropTarget. Func1 doesn't have
to know about any of the other methods, although they are still there.

Now, let's say Func1 knows something about the object it was given, and
wants to access its IBaseFilter methods. To do that, it has to get an
IBaseFilter pointer. To do THAT, it has to call QueryInterface. I'm going
to do that in two steps, just to make a point:

    IUnknown * punk = pidt;
    IBaseFilter * pfilt;
    punk->QueryInterface( IBaseFilter, *pfilt );

Now, I have pidt, punk, and pfilt. They all have different methods, but
they are ALL THE SAME OBJECT. They are all the SillySample object that I
created outside of Func1. The parts that use the IBaseFilter interface
don't need to know that the object it has can do a whole bunch of other
things. They don't care. They have the interface that does what they need
to do.

* * *

A STGMEDIUM structure holds information about a chunk of memory. It is
used by several different COM interfaces. The memory it holds could have
been allocated in many different ways. It could have been allocated off
the heap, or it could be a COM object that needs to be destroyed. *IF* it
is a COM object, the ONLY THING that I need in order to destroy it is its
Release() method. I DONT'T CARE what other functionality the object might
have. So, all I need to do is store its IUnknown interface. Other parts
of the program can use other interfaces on this same object. That doesn't
Tim Roberts,
Providenza & Boekelheide, Inc.

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