Click here to Skip to main content
Rate this: bad
good
Please Sign up or sign in to vote.
See more: C++
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:
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 8:50am
Edited 11-Jan-13 9:24am
v5

1 solution

Rate this: bad
good
Please Sign up or sign in to vote.

Solution 1

The crux if the issue is that CComPtr 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 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 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 for safety.
 
Hope this helps.

 
(update)
The COM overview is here Component Object Model[^]
Rules for managing reference counts are here: Reference Counts[^]
  Permalink  
v2

This content, along with any associated source code and files, is licensed under The Code Project Open License (CPOL)

  Print Answers RSS
0 OriginalGriff 7,903
1 Sergey Alexandrovich Kryukov 7,127
2 DamithSL 5,604
3 Manas Bhardwaj 4,986
4 Maciej Los 4,820


Advertise | Privacy | Mobile
Web01 | 2.8.1411023.1 | Last Updated 11 Jan 2013
Copyright © CodeProject, 1999-2014
All Rights Reserved. Terms of Service
Layout: fixed | fluid

CodeProject, 503-250 Ferrand Drive Toronto Ontario, M3C 3G8 Canada +1 416-849-8900 x 100