How to declare but not initialize a property in a Swift class

Question

class car {
    var oneWheel : Wheel
    func run(inputWheel:Wheel){
        oneWheel = inputWheel
        ....
    }
}

I don't want to implement init() and I don't want to initialize the wheel.

Answer 1

Create an implicitly unwrapped optional - this will act like a normal variable but it does not need to be initialised - its initial value is nil. Just ensure the value is set before it is used otherwise you will get a fatal error for unwrapping nil.

class car {
  var oneWheel: Wheel!

  func run(inputWheel: Wheel) {
    wheel = inputWheel
  }
}

Answer 2

Like so...

class car {
  var oneWheel : Wheel?
  // with this version, in order to pass oneWheel into this function as an argument, you will need to unwrap the optional value first. (see below example)
  func run(inputWheel:Wheel){
    oneWheel = inputWheel
    ....
  }
}

or if you would like the function to take an optional type Wheel as an argument do

class car {
   var oneWheel : Wheel?
   func run(inputWheel:Wheel?){
    //use conditional binding to safely unwrap the optional
    if let wheel = inputWheel {
    oneWheel = wheel
    }
    ....
  }
}

By using conditional binding, instead of an implicitly unwrapped optional, you avoid the potential of crashing due to... unexpectedly found nil while unwrapping an optional value, which occurs when the compiler finds nil where a non-nil value was expected.