Re: Dual interface

"Igor Tandetnik" <>
Tue, 23 Sep 2008 08:14:07 -0400
"George" <> wrote in message

while never be
able to do anything else?

Why can't it do anything else?

I agree with all of your comments except this one above. The
component could do anything it like, but not be utilized by outside
-- since outside could only call AddRef/Release.

It's not a comment - it's a question. Can you answer it? Can you explain
to me why you believe AddRef and Release are limited to updating the
component's reference count, and can't do anything else?

My question is not what the component could do, but how other parties
from outside could use it -- since outside could only use by
interface and methods in such interface.

Correct. So the outside parties would call methods on the only interface
accessible on this component, namely IUnknown. There's a reason that
STGMEDIUM field is called pUnkForRelease.

I think some code is more clear to solve all our confusions. It is
appreciated if you could write some simple code logic which
illustrates in the context of STDMEDIUM, an object which only
implements IUnknown is useful by only allowing other parties to call
AddRef/Release. :-)

void SetUpMedium(STGMEDIUM* medium);
void CleanUpMedium(STGMEDIUM* medium);

class MediumReleaser : public IUnknown {
    ULONG ref;
    STDMEDIUM* medium;

    MediumReleaser(STDMEDIUM* m) : ref(0), medium(m) {}

    ULONG AddRef() { return ++ref; }
    ULONG Release() {
        ULONG r = --ref;
        if (ref == 0) {
            delete this;
        return r;
    // Trivial implementation of QueryInterface
    // omitted for brevity.

void SetUpMedium(STGMEDIUM* medium) {
    // Set up medium fields in some way that requires unusual cleanup.
    // Cleanup implemented in CleanUpMedium (not shown).

    MediumReleaser* p = new MediumReleaser(medium);
    medium->pUnkForRelease = p;

With best wishes,
    Igor Tandetnik

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