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iOS Programming: The Big Nerd Ranch Guide by Joe Conway, Aaron Hillegass

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Strong and Weak References

So far, we’ve said that anytime a pointer variable stores the address of an object, that object has an owner and will stay alive. This is known as a strong reference. However, a variable can optionally not take ownership of an object it points to. A variable that does not take ownership of an object is known as a weak reference.

A weak reference is useful for an unusual situation called a retain cycle. A retain cycle occurs when two or more objects have strong references to each other. This is bad news. When two objects own each other, they will never be destroyed by ARC. Even if every other object in the application releases ownership of these objects, these objects (and any objects that they own) will continue to ...

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