Microsoft Media Foundation and COM

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Hello All.

I am trying to use Microsoft Media Foundation to implement a customized media player that would have some functionality that I have not found in other players. I am a newbie when it comes to MF and COM, and am using the book Developing Microsoft Media Foundation Applications from Microsoft Press as a guide.

I understand that MF interacts with COM. The code examples in the book use the CComPtr class to handle reference counting, etc. For example:

C++

CComPtr<IMFTopology> pTopology = NULL;

That being said, not every initialization of a MF object in the book uses this approach. For example, IMFMediaSession. So my question is this: under what circumstances would CComPtr (or AddRef/Release) need to be used in my code? How do I know which interfaces would require this? Strictly speaking, is reference counting mandatory when utilizing COM from my classes? The book does not cover this question to my satisfaction, and I have not been able to find a sufficient answer via MSDN either.

Hopefully my question is clear. Best regards.

Posted 11-Jan-13 7:50am

Michael Buckman

Updated 11-Jan-13 8:24am

v5

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Rob Grainger · Accepted Answer · 2013-01-11T10:21:00

The crux if the issue is that CComPtr<t> manages this to a large extent, maintaining its own reference and calling AddRef() when created and Release() when destroyed (using the RAII idiom (RAII - Wikipedia[^]). Whenever you hold a pointer to a COM object, generally wrap it in a CComPtr<t> and C++ will correctly managed AddRef/Release for you.

Occasionally, you can use a pointer to the interface directly. For example, if a function you write receives an IMFMediaSession* as a parameter, and the only way you use the parameter is to directly call methods (using -> ), then you don't need to use AddRef/Release (be sure to check for null though).

The main consideration is lifetime of the reference. Always, for example, use CComPtr<t> for:

1. Members of a class or struct, where the sub-object will get destructed (hence Released) when the object is destroyed.
2. In a method, when you invoke another function that may possibly copy the reference (if you're not sure - use CComPtr<t> for safety.

Hope this helps.

(update)
The COM overview is here Component Object Model[^]
Rules for managing reference counts are here: Reference Counts[^]