Difference Between weak & unowned in respect to ARC memory management in Swift 3.1

Question

I am working on Swift and I have little bit confusion on usage of unowned and weak keywords for memory management in Swift.

Can any one help me to understand where to use unowned ?

Thanks in advance!

Answer 1

The most important difference is that unowned variables are very dangerous in Swift.

weak is an optional type, unowned is NOT.

This means that you can create code that uses weak as part of the logic. (think of asynchronous actions that take place only if a view controller is still being shown on the screen, when the code is ran, if the reference is "weak" it will fail silently without causing any other issues if the code is written correctly)

On the other hand, when a variable is unowned you are basically saying that this variable will always refer to something. And unlike weak, if you call it and there's nothing it will crash.

You generally never wanna use unowned. (I haven't come across any case where I had to). On the other hand, "weak self" variables are quite common when writing blocks.

You can see a good explanation on this question:

Shall we always use [unowned self] inside closure in Swift

EDIT: Actually, check the first and second most voted answers. They provide a clear explanation on WHEN to use unowned and how it works. The second even has pictures!

Answer 2

As per apple docs

There are some pretty good examples given on the developer website, check this link

As far as blocks are concerned as per Apple Docs:

Define a capture in a closure as an unowned reference when the closure and the instance it captures will always refer to each other, and will always be deallocated at the same time.

Conversely, define a capture as a weak reference when the captured reference may become nil at some point in the future. Weak references are always of an optional type, and automatically become nil when the instance they reference is deallocated. This enables you to check for their existence within the closure’s body.

To conclude Use a weak reference whenever it is valid for that reference to become nil at some point during its lifetime. Conversely, use an unowned reference when you know that the reference will never be nil once it has been set during initialization.”