IUnknown

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In programming, the IUnknown interface is the fundamental interface in the Component Object Model (COM). The published COM specification mandates that COM objects must minimally implement this interface. Furthermore, every other COM interface must be derived from IUnknown.

1 Methods
2 Miscellaneous
3 Notes
4 See also

[edit] Methods

The IUnknown interface exposes three methods (sometimes called functions): QueryInterface, AddRef, and Release:^[1]

QueryInterface is used to obtain a pointer to another interface, given a GUID that uniquely identifies that interface (commonly known as an interface ID, or IID). If the COM object does not implement that interface, an E_NOINTERFACE 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 ID is defined as a GUID with the value of {00000000-0000-0000-C000-000000000046}.

[edit] Miscellaneous

Components designed under the ActiveX standard must, at minimal, implement the IUnknown interface.^[2]
IUnknown serves as the base for Mac OS X's Core Foundation CFPlugIn framework.