CODEWITHSUNDEEP
swiftcomputed-properties
manismku
manismku

Reputation: 2180

Swift: What does this error: 'private(set)' modifier cannot be applied to read-only properties mean?

I am bit confused if we can create computed property which is read-only Somethig like:

extension ToMyClass {
    private(set) var isEmpty: Bool {
        return head == nil
    }
}

While trying to create I got following error:

 error: 'private(set)' modifier cannot be applied to read-only properties

Upvotes: 9

Views: 4183

Answers (3)

Natasha
Natasha

Reputation: 6893

Let's say, you have a struct -

struct StudentData {
    let id: String
    private(set) var maxScore: Int 

    var scoreNotUptoMark: Bool {
        return maxScore < 60
    }
    
    init(id: String, score: Int) {
        self.id = id
        self.maxScore = score
    }
    
    mutating func scoreUp(by score: Int) -> Int {
        self.maxScore = maxScore + score
        return maxScore
    }
}

If you add private(set) modifier to id or scoreNotUptoMark we will get an error stating -

private(set)' modifier cannot be applied to read-only properties

Have a look at the code, is there any way, one can set value to id after it is initialized. No, right? A constant can only be initialized when it is declared with value or from the initializer of the struct or class, it belongs to. Hence it is a read-only properly.

Same goes for scoreNotUptoMark. Can you set any value to a computed properties? No, right? So, by behavior, it is a read-only property

Upvotes: -1

mfaani
mfaani

Reputation: 36317

I got the same error but for a totally different reason. My code was as such:

protocol Foo {
    var bar: String { get }
}
class Baz: Foo {
    private (set) let bar: String // Error
    
    init(bar: String) {
        self.bar = bar
    }
}

I just had to change:

private (set) let bar: String

to:

private (set) var bar: String

let makes properties immutable and that was causing issues.

Upvotes: 2

Woof
Woof

Reputation: 1443

You are trying to set a modfier for a computed property, which is always read-only

The code below was taken from: The Swift Programming Language (Swift 4)

struct TrackedString {
    private(set) var numberOfEdits = 0
    var value: String = "" {
        didSet {
            numberOfEdits += 1
        }
    }
}

It should be a stored property

Upvotes: 10

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