weak vs unowned in Swift. What are the internal differences?

Question

I understand the usage and superficial differences between weak and unowned in Swift:

The simplest examples I've seen is that if there is a Dog and a Bone, the Bone may have a weak reference to the Dog (and vice versa) because the each can exist independent of each other.

On the other hand, in the case of a Human and a Heart, the Heart may have an unowned reference to the human, because as soon as the Human becomes... "dereferenced", the Heart can no longer reasonably be accessed. That and the classic example with the Customer and the CreditCard.

So this is not a duplicate of questions asking about that.

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My question is, what is the point in having two such similar concepts? What are the internal differences that necessitate having two keywords for what seem essentially 99% the same thing? The question is WHY the differences exist, not what the differences are.

Given that we can just set up a variable like this: weak var customer: Customer!, the advantage of unowned variables being non-optional is a moot point.

The only practical advantage I can see of using unowned vs implicitly unwrapping a weak variable via ! is that we can make unowned references constant via let.

... and that maybe the compiler can make more effective optimizations for that reason.

Is that true, or is there something else happening behind the scenes that provides a compelling argument to keeping both keywords (even though the slight distinction is – based on Stack Overflow traffic – evidently confusing to new and experienced developers alike).

I'd be most interested to hear from people who have worked on the Swift compiler (or other compilers).

Answer 1

My question is, what is the point in having two such similar concepts? What are the internal differences that necessitate having two keywords for what seem essentially 99% the same thing?

They are not at all similar. They are as different as they can be.

• weak is a highly complex concept, introduced when ARC was introduced. It performs the near-miraculous task of allowing you to prevent a retain a cycle (by avoiding a strong reference) without risking a crash from a dangling pointer when the referenced object goes out of existence — something that used to happen all the time before ARC was introduced.

• unowned, on the other hand, is non-ARC weak (to be specific, it is the same as non-ARC assign). It is what we used to have to risk, it is what caused so many crashes, before ARC was introduced. It is highly dangerous, because you can get a dangling pointer and a crash if the referenced object goes out of existence.

The reason for the difference is that weak, in order to perform its miracle, involves a lot of extra overhead for the runtime, inserted behind the scenes by the compiler. weak references are memory-managed for you. In particular, the runtime must maintain a scratchpad of all references marked in this way, keeping track of them so that if an object weakly referenced goes out of existence, the runtime can locate that reference and replace it by nil to prevent a dangling pointer.

In Swift, as a consequence, a weak reference is always to an Optional (exactly so that it can be replaced by nil). This is an additional source of overhead, because working with an Optional entails extra work, as it must always be unwrapped in order to get anything done with it.

For this reason, unowned is always to be preferred wherever it is applicable. But never use it unless it is absolutely safe to do so! With unowned, you are throwing away automatic memory management and safety. You are deliberately reverting to the bad old days before ARC.

In my usage, the common case arises in situations where a closure needs a capture list involving self in order to avoid a retain cycle. In such a situation, it is almost always possible to say [unowned self] in the capture list. When we do:

• It is more convenient for the programmer because there is nothing to unwrap. [weak self] would be an Optional in need of unwrapping in order to use it.

• It is more efficient, partly for the same reason (unwrapping always adds an extra level of indirection) and partly because it is one fewer weak reference for the runtime's scratchpad list to keep track of.