What is "iunknown"

In computer programming, the IUnknown (custom) 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. IUnknown exposes two essential features of all COM objects: object lifetime management through reference counting, and access to various predefined interfaces.

A custom IUnknown interface consists of a pointer to a virtual method table that contains a list of pointers to the functions that implement the functions declared in the interface, in the same order that they are declared in the interface. The in-process invocation overhead is therefore comparable to virtual method calls in C++.