What is weak (iOS4 = unsafe_unretained ) in Objective C

-It says "keep this as long as someone else points to it strongly"
the same thing as assign, no retain or release
-A "weak" reference is a reference that you do not retain.

-We generally use weak for IBOutlets (UIViewController's Childs).This works because the child object only 

needs to exist as long as the parent object does.

-a weak reference is a reference that does not protect the referenced object from collection by a garbage collector.

-Weak is essentially assign, a unretained property. Except the when the object is deallocated the weak pointer is automatically set to nil

Example :
@property (weak, nonatomic) IBOutlet UIButton *myButton; 
@synthesize myButton;

Explain:
Imagine our object is a dog, and that the dog wants to run away (be deallocated).

Strong pointers are like a leash on the dog. As long as you have the leash attached to the dog, the dog will not run away. If five people attach their leash to one dog, (five strong pointers to one object), then the dog will not run away until all five leashes are detached.

Weak pointers, on the other hand, are like little kids pointing at the dog and saying "Look! A dog!" As long as the dog is still on the leash, the little kids can still see the dog, and they'll still point to it. As soon as all the leashes are detached, though, the dog runs away no matter how many little kids are pointing to it.

As soon as the last strong pointer (leash) no longer points to an object, the object will be deallocated, and all weak pointers will be zeroed out.