Why can the keyword "weak" only be applied to class and class-bound protocol types

Question

When I'm declaring variables as weak in Swift, I sometimes get the error message from Xcode:

'weak' may only be applied to class and class-bound protocol types

or

'weak' must not be applied to non-class-bound 'SomeProtocol'; consider adding a protocol conformance that has a class bound

I'm wondering why the keyword weak can only applied to class and class-bound protocol types? What is the reason behind this requirement?

`weak` is only relevant to reference counting and only classes are reference counted — dan, Aug 09 '16 at 02:30

score 137 · Answer 1 · edited Dec 11 '19 at 13:30

137

One common reason for this error is that you have declared you own protocol, but forgot to inherit from AnyObject:

protocol PenguinDelegate: AnyObject {
    func userDidTapThePenguin()
}

class MyViewController: UIViewController {
    weak var delegate: PenguinDelegate?
}

The code above will give you the error if you forget to inherit from AnyObject. The reason being that weak only makes sense for reference types (classes). So you make the compiler less nervous by clearly stating that the PenguinDelegate is intended for classes, and not value types.

edited Dec 11 '19 at 13:30

Michal Šrůtek

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answered Oct 22 '16 at 16:37

Jak

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1

what is the advantage of inheriting from NSObjectProtocol? My own delegates don't do that and I haven't encountered any problem in my uses – Apostolos Apostolidis Apr 25 '17 at 12:07
3

@Apostolos Weak references are only valid in classes. By inheriting from NSObjectProtocol your are guaranteeing to the compiler that the protocol will only be used for classes (and not for enums, etc.). – Vince O'Sullivan May 06 '17 at 06:08
@VinceO'Sullivan What's the advantage of guaranteeing that to the compiler? Isn't is easier to just not inherit from `NSObjectProtocol`? For one you don't have to deal with adding the `weak` modifier. – yesthisisjoe Aug 02 '17 at 22:08
12

Yep, it works. But you don't know why. `weak` is a qualifier for reference types, it makes no sense with value types because value types cannot be weak by definition. A protocol can be adopted by reference types *and* value types. So you should constrain it to be implemented *only* by reference types: `protocol PenguinDelegate: class { }` Here, you are constraining the protocol to be implemented only by `NSObjectProtocol`s, whose are reference types too, that's the reason why it works. – Martin Aug 10 '17 at 10:08
4

Use `protocol PenguinDelegate: class` which does not depend on an Objective-C runtime but still solves the problem. – Feuermurmel Jan 14 '18 at 20:43

score 84 · Accepted Answer · answered Aug 09 '16 at 02:37

weak is a qualifier for reference types (as opposed to value types, such as structs and built-in value types).

Reference types let you have multiple references to the same object. The object gets deallocated when the last strong reference stops referencing it (weak references do not count).

Value types, on the other hand, are assigned by copy. Reference counting does not apply, so weak modifier does not make sense with them.

score 65 · Answer 3 · answered Jun 11 '17 at 12:14

65

protocol PenguinDelegate: class {
    func userDidTapThePenguin()
}

class MyViewController: UIViewController {
    weak var delegate: PenguinDelegate?
}

If you type class after your protocol it works as well and seems more appropriate that for NSObjectProtocol.

answered Jun 11 '17 at 12:14

aaronium112

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I feel like the combination of this and the answer from @dasblinkenlight would fully answer this question. dasblinkenlight explained why the error message is showing and this explains how to achieve what you were likely trying to do as a developer. – stuckj Apr 10 '18 at 17:48

score 13 · Answer 4 · edited Feb 16 '18 at 06:44

Well just in case anyone else thinks that you have everything correct in your code like me, check that you did not mistakenly replaced the : by an =.

Here is what I had. It was also giving me the same error as above:

protocol PenguinDelegate: class {
    func userDidTapThePenguin()
}

class MyViewController: UIViewController {
    weak var delegate = PenguinDelegate?
}

But the correct way is:

protocol PenguinDelegate: class {
    func userDidTapThePenguin()
}

class MyViewController: UIViewController {
    weak var delegate: PenguinDelegate?
}

Do you see the difference? It took me a while to see that I had an equal sign instead of a colon. Also note that I did get other errors for the same line for I had decided my first error seem like the most likely to be the real problem :

-weak may only be applied to class and class-bound protocol types

:-<

score 5 · Answer 5 · answered Oct 03 '17 at 17:10

I find out in one case where you even have class type but still you get this error message.

For example,

class MyVC: UIViewController {
   var myText: UITextView = {
      [weak self]
      let text = UITextView()
      // some codes using self
      return text
   }()
}

Here an UITextView object is returned from an anonymous block as initialization of var myText. I got the same type of error message. To resolve the issue, the var has to be marked as lazy:

class MyVC: UIViewController {
   lasy var myText: UITextView = {
      [weak self]
      let text = UITextView()
      // some codes using self
      return text
   }()
}

Dren · Answer 6 · 2022-02-22T13:53:46.340

3

Just FYI and who is not updated. After swift proposal SE-0156 https://github.com/apple/swift-evolution/blob/master/proposals/0156-subclass-existentials.md was implemented, there is in the Swift docs "Class-Only Protocols section" https://docs.swift.org/swift-book/LanguageGuide/Protocols.html#ID281 now described to use AnyObject instead of class. So, it is possible for : class to be deprecated in future.

edited Feb 22 '22 at 13:53

answered Jun 26 '18 at 10:22

Dren

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score 2 · Answer 7 · answered Aug 09 '16 at 02:34

2

weak is for ARC(Automatic Reference Counting). It means not adding reference count. So it only works for Class. And in Swift, you will get optional value for security.

answered Aug 09 '16 at 02:34

Lumialxk

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score 1 · Answer 8 · edited Jun 20 '20 at 09:12

1

I tried to capture String and Array-typed properties for a closure. I got these errors:

'weak' may only be applied to class and class-bound protocol types, not '[String]'

'weak' may only be applied to class and class-bound protocol types, not 'String'

I played a while in the playground, and it turned out, capturing self is enough for these types.

edited Jun 20 '20 at 09:12

Community

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answered Feb 27 '18 at 18:19

Display Name

4,502
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score 1 · Answer 9 · edited Jun 20 '20 at 09:12

1

I was using objective C class in swift for a scrolView. I created IBOutlet of that scroll view. And while compiling code this error started showing.

So to fix this kind of issue, import that class in your bridging header

import "YourClass.h"

I was using Xcode 9.2 with swift 3.2

edited Jun 20 '20 at 09:12

Community

1
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answered May 07 '18 at 06:38

Pramod More

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score 0 · Answer 10 · answered Sep 25 '19 at 15:29

0

weak only works for reference type, so Xcode would report an error if you are calling from struct (instead of class).

answered Sep 25 '19 at 15:29

superarts.org

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well, if closures are reference types, then why XCode shows an error `'weak' may only be applied to class and class-bound protocol types, not '() -> Void'`? – Kuvonchbek Yakubov Apr 17 '22 at 20:58

score 0 · Answer 11 · answered Oct 23 '19 at 12:01

0

weak is not for value type.
weak comes to the picture only for the class.

"weak" can apply anything which is inherited from class or class-bound protocol types

Class Protocol: protocol ViewControllerDelegate : class { func getInformationk(value: String?) }
NSObjectProtocol:

protocol ViewControllerDelegate : NSObjectProtocol { func getInformation(value: String?) }

answered Oct 23 '19 at 12:01

Divesh singh

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Can you explain a bit more? – Dieter Meemken Oct 23 '19 at 12:23

Why can the keyword "weak" only be applied to class and class-bound protocol types

11 Answers11

import "YourClass.h"

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