Apple's weak @Property example

gistfile1.m

// The following declaration is similar to "@property(assign) MyClass *myObject;"
// except that if the MyClass instance is deallocated,
// the property value is set to nil instead of remaining as a dangling pointer.
@property(weak) MyClass *myObject;

Here's simple NSObject category that can be used to debug it http://www.componentix.com/blog/28/debug-usage-of-objectivec-weak-properties-with-kvo