[Swift] Why 'weak self' Is Not Needed (or Possible) in Structs

Key Takeaway

• weak self is not needed (and not possible) for structs.

Experiment

Let's define a class with a simple closure as follows:

struct Calculator {
    let numA: Double
    let numB: Double
    let closure: (Double, Double) -> Void
    
    init(numA: Double, numB: Double, closure: @escaping (Double, Double) -> Void) {
        self.numA = numA
        self.numB = numB
        self.closure = closure
    }
    
    func calc() {
        closure(numA, numB)
    }
}

Now, let's try to write code where the closure uses a taxRate property held within the struct.

struct SampleStruct {
    private let taxRate = 1.1
    let numA: Double
    let numB: Double
    
    init(numA: Double, numB: Double) {
        self.numA = numA
        self.numB = numB
    }
    
    // ⭕️ This is possible
    func getCalculator() -> Calculator {
        Calculator(numA: numA, numB: numB) { numA, numB in
            print("\((numA + numB) * self.taxRate)")
        }
    }
}

Since self is used in the closure processing, if you try to add [weak self] out of habit, the compiler will complain.

struct SampleStruct {
    private let taxRate = 1.1
    let numA: Double
    let numB: Double
    
    init(numA: Double, numB: Double) {
        self.numA = numA
        self.numB = numB
    }
    
    // ❌ This is not possible
    func getCalculator() -> Calculator {
        Calculator(numA: numA, numB: numB) { [weak self] numA, numB in // 'weak' may only be applied to class and class-bound protocol types, not 'SampleStruct'
            guard let self = self else {
                print("self is nil")
                return
            }
            print("\((numA + numB) * self.taxRate)")
        }
    }
}

Let's look at the error message.

Error message:

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

It seems that 'weak' cannot be used with structs.

Since structs are value types and not reference types like classes, this is actually expected behavior.

That's all.