Property initializers run before 'self' is available

Question

Seems like I'm having a problem with something that shouldn't be the case... But I would like to ask for some help.

There are some explanations here on the Stack I don't get.

Having two simple classes where one refers to another, as per below:

class User {
  lazy var name: String = ""
  lazy var age: Int = 0

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

class MyOwn {
  let myUser: User = User(name: "John", age: 100)
  var life = myUser.age 
  //Cannot use instance member 'myUser' within property initializer
  //property initializers run before 'self' is available
}

I get the commented compile error. May someone please tell me what should I do to solve the case?

Put the code in a method. You cannot initialize properties which depend on each other on the top level of the class. Or use a computed property. Btw: the `lazy` attributes in the `User` class are useless. — vadian, Apr 21 '17 at 19:54
that's a v surprising limitation in `swift` : any ideas why it is like this? — WestCoastProjects, May 25 '20 at 21:25

Paulo Mattos · Accepted Answer · 2022-01-29T12:24:29.603

60

As correctly pointed out by vadian you should create an init in such scenarios:

class MyOwn {
    let myUser: User
    var life: Int

    init() {
        self.myUser = User(name: "John", age: 100)
        self.life = myUser.age 
    }
}

You can't provide a default value for a stored property that depends on another instance property.

edited Jan 29 '22 at 12:24

answered Apr 21 '17 at 19:59

Paulo Mattos

18,845
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2

this one surely answers to my problems. Thank you! – RafalK Apr 22 '17 at 06:28
2

Why does our lovely IDE not propose this? – brainray Apr 03 '21 at 18:34

score 24 · Answer 2 · answered Apr 21 '17 at 19:54

24

You should declare life like this:

lazy var life:Int = {
    return self.myUser.age
}()

Because you are trying to initialise one property(variable) with another during initialisation process. At this time variables are not available yet.

answered Apr 21 '17 at 19:54

Eugene Laminskiy

594
2
11

Should work fine, but isn't using `lazy` for that an overkill? ;-) – Paulo Mattos Apr 21 '17 at 20:02
3

@PauloMattos The code architecture presented in your answer is more acceptable, at least for me. But I can not say why a person needed the architecture that he is trying to implement. I just showed him how to do it without compilation errors and explained what was his mistake – Eugene Laminskiy Apr 21 '17 at 20:07

score 0 · Answer 3 · answered Feb 27 '23 at 19:46

What worked for me is turning the variable into a computed property

// Previous declaration
var temperatureString = String(format:"%.1f", temperature) // "Cannot use instance member 'temperature' within property initializer; property initializers run before 'self' is available" 

// Computed Property
var temperatureString : String { 
   return String(format:"%.1f", temperature)
}

Property initializers run before 'self' is available

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