IUnknown::AddRef

The IUnknown::AddRef method increments the reference count for a the calling interface on an object. It should be called for every new copy of a pointer to an interface on a given object.

ULONG AddRef(void);
Return Value
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.

Comments
Objects use a reference counting mechanism to ensure that the lifetime of the object includes the lifetime of references to it. You use IUnknown::AddRef to stabilize a copy of an interface pointer. It can also be called when the life of a cloned pointer must extend beyond the lifetime of the original pointer. The cloned pointer must be released by calling IUnknown::Release.
Objects must be able to maintain (2 (31) )-1 outstanding pointer references. Therefore, the internal reference counter that IUnknown::AddRef maintains must be a 32-bit unsigned integer.

Notes to Callers
Call this function for every new copy of an interface pointer that you make. For example, if you are passing a copy of a pointer back from a function, you must call IUnknown::AddRef on that pointer. You must also call IUnknown::AddRef on a pointer before passing it as an in-out parameter to a function; the function will call IUnknown::Release before copying the out-value on top of it.