Question

If I have an IUnknown *ptr, do I need to call Release() on every interface I obtain through ptr->QueryInterface(), in addition to calling ptr->Release() when I’m done with ptr?

I used to think that the answer is “Yes”, but this quote from MSDN confused me:

Occasionally you may need to obtain a weak reference to an object (that is, you may wish to obtain a pointer to one of its interfaces without incrementing the reference count), but it is not acceptable to do this by calling QueryInterface followed by Release.

I don’t understand why that’s problematic — if I call ptr->QueryInterface() and then call Release on the resulting pointer, shouldn’t the reference count on the object still be positive? How does that result in an invalid pointer?

Answer 1

The documentation is correct. And you need to follow reference counting rules – that includes calling Release on interfaces obtained from QueryInterface in addition to after you created the object.

To clear up why you can’t do weak pointers with Release – there exists a race condition in calling QueryInterface and then Release immediately after.

• Thread1 creates object – reference count 1
• Thread2 calls QueryInterface for weak reference – reference count 2
• Thread1 releases object – reference count 1
• Thread2 calls Release for weak reference – reference count 0. Object is destroyed.
• Thread2 tries to use object – error.

The warning is there to guard against the above – presumably some programmers think that they can “call ptr->QueryInterface() and then call Release on the resulting pointer” and then use the object…