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

Question

I'm already aware of how private(set) works. But the below code is give compile-time error,

class Person {
    private(set) let name: String //Error.
    private(set) let age: Int //Error.

    init(name: String, age: Int){
        self.name = name
        self.age = age
    }
}

Error:

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

Since name and age are not read-only properties, it shouldn't give such an error.

If I use let instead of var, it is working fine. Just trying to know why?

Answer 1

private(set) let is a contradiction in terms. Any setter requires a variable.

Please be aware that assigning a default value to a property is not setting the property in terms of initialization. For this reason the didSet property observer is not called after assigning a default value to a property.

In Swift there is only one case to use private(set): If the class contains code which modifies the variable

class Foo {
    let name : String
    private(set) var expiryDate : Date

    init(name: String, expiryDate: Date){
        self.name = name
        self.expiryDate = expiryDate
    }

    func extendExpiryDate(to newDate : Date) {
       expiryDate = newDate
    }
}

If the property is only initialized during init it's a constant – as correctly mentioned in the other answers – so declare it as let constant. Unlike other programming languages Swift provides explicit constants with the benefit of more security, less memory usage and better performance.