AddRef and Release
AddRef() function increments the reference count of the object. Release() function decrements the reference count of the object and when the life time expires or rather the reference count reaches zero, server deallocates the object.
IUnknown and functions
IUnknown interface has three methods or functions. They are QueryInterface, AddRef and Release.
The IUnknown::AddRef method increments the reference count for an interface on an object. It should be called for every new copy of a pointer to an interface on a given object.
Returns an integer from 1 to n, the value of the new reference count. This information is meant to be used for diagnostic/testing purposes only, because, in certain situations, the value may be unstable.
Objects use a reference counting mechanism to ensure that the lifetime of the object includes the lifetime of references to it. Every COM object includes a 32 bit unsigned integer member variable for reference counting. Initial value of this reference count is zero. IUnknown::AddRef is called to stabilize a copy of an interface pointer and increments it by one. The updated value is returned. This is called by IUnknown::QueryInteface in server side and it can also be called when object is cloned as pointer. Reference count can have values from 1 to (2^31)-1. Reference count increment should be atomic in case of objects are handled in multi-threaded apartment.
Decrements the reference count for the calling interface on a object. If the reference count on the object falls to 0, the object is freed from memory.
Return ValueReturns the resulting value of the reference count, which is used for diagnostic/testing purposes only. If you need to know that resources have been freed, use an interface with higher-level semantics.
IUnknown::Release returns the current reference count and decrements the reference count of the object. Reference count decrement should be atomic if client is accessing multi-threaded apartment model. IUnknown::Release should be called by the client at the end when this object is no longer needed in the program. COM server deletes or frees the memory allocated by the object once this is called. Client should not call any interface function after this call.
Implement AddRef & Release
Declare the IDL interface file
Declare the C++ prototype header
Implement AddRef and Release
Call AddRef in QueryInterface
Reference counting all objects
We have used a counter named as g_nComObjsInUse to manage the count of live objects in the COM server. A COM server implements multiple objects. It often implements an IClassFactory class where objects are created with CreateInstance. So all class constructors should increment this global reference counter and all destructor should decrement it. DllCanUnloadNow() is called by the COM/DCOM framework when any client releases the objects and then terminates with CoUninitialize(). This g_nComObjsInUse counter will reach zero when all objects are deleted and this server is no longer needed. DllCanUnloadNow() should return S_OK or zero and this COM DLL will be unloaded from the memory.
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