IUnknown

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The published COM specification mandates that COM objects must minimally implement the IUnknown interface. This interface comprises three functions - QueryInterface, AddRef and Release.

QueryInterface is used to obtain a pointer to another interface, given a GUID that uniquely identifies that interface (commonly known as an IID). If the COM object does not implement that interface, an error is returned instead.
AddRef is used by clients to indicate that a COM object is being referenced. This is necessary to ensure that a COM object is not disposed prematurely
Release is used by clients to indicate that they have finished using the COM object. An unreferenced COM object may be safely disposed.

interface IUnknown
{
  virtual HRESULT QueryInterface(REFIID riid, void **ppvObject) = 0;
  virtual ULONG AddRef(void) = 0; 
  virtual ULONG Release(void) = 0; 
};

The IUnknown interface is defined as a GUID with the value of {00000000-0000-0000-C0000-00000000046}.