What does a weak reference in a Side Table structure refer to?

In the Side Table structure in iOS, a weak reference usually points to an object that has additional information added through the associated objects mechanism. The Side Table is an internal mechanism of the Objective-C runtime that stores associated objects for class instances.

A weak reference is needed to avoid retain cycles: if the Side Table holds a strong reference to an object, and the object references back, it will lead to a memory leak. Therefore, a weak reference allows the runtime to correctly release objects when they are no longer needed.