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Reference Counting

COM itself does not automatically try to remove an object from memory when it thinks the object is no longer being used. Instead, the object programmer must remove the unused object. The programmer determines whether an object can be removed based on a reference count.

COM uses the IUnknown methods, AddRef and Release, to manage the reference count of interfaces on an object. The general rules for calling these methods are:

  • Whenever a client receives an interface pointer, AddRef must be called on the interface.

  • Whenever the client has finished using the interface pointer, it must call Release.

In a simple implementation, each AddRef call increments and each Release call decrements a counter variable inside the object. When the count returns to zero, the interface no longer has any users and is free to remove itself from memory.

Reference counting can also be implemented so that each reference to the object (not to an individual interface) is counted. In this case, each AddRef and Release call delegates to a central implementation on the object, and Release frees the entire object when its reference count reaches zero.

Note Note

When a CComObject-derived object is constructed using the new operator, the reference count is 0. Therefore, a call to AddRef must be made after successfully creating the CComObject-derived object.

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