Why does calling super.init() call my default init()

Question

So I have a class with a designated initializer that takes values for every stored property. All my stored properties also have a default values, so I assume this class get a default init.

In my designated init, I call super.init()

The problem is, if I call it at the end of my init, it loads all properties to default values, but if I call it at the start it works as I expect.

The book says:

Safety check 1 A designated initializer must ensure that all of the properties introduced by its class are initialized before it delegates up to a superclass initializer.

As mentioned above, the memory for an object is only considered fully initialized once the initial state of all of its stored properties is known. In order for this rule to be satisfied, a designated initializer must make sure that all its own properties are initialized before it hands off up the chain.

So I am not sure if things have changed or I am doing something wrong?

Code:

class ORQuizViewController: UIViewController {
let imageView: UIImageView = UIImageView(image: nil)
let questionLabel: UILabel = UILabel()
let choicesArray: [BlackWhiteButton] = [BlackWhiteButton]()
let correctAnswer: Int = -1

init(image: UIImage!, question: String, choices: [String], answerIndex: Int) {
    super.init()
    imageView = UIImageView(image:image)
    questionLabel = UILabel()
    questionLabel.text = question
    var tempChoices = [BlackWhiteButton]()
    for choice in choices {
        var choiceLabel = BlackWhiteButton()
        choiceLabel.setTitle(choice, forState: .Normal)
        tempChoices.append(choiceLabel)
    }
    choicesArray = tempChoices
    correctAnswer = answerIndex
}

Are the properties defined by this class or are they inherited from the super class? — Paulw11
They are defined by this class, so based on Apple's doc, I should be initializing before calling super. — Vipul Patil
Is your initialiser marked as convenience or is it the designated initialiser? — Paulw11
Why do you set your values to defaults and have a designated initialiser that sets defaults? — Paulw11

Paulw11 Paulw11 · Accepted Answer · 2014-09-14T23:22:37

The correct solution is to call super.init before you assign the final values, as you have observed. Safety check 1, as you said, requires that all of the properties introduced by the subclass are initialised before delegating up - this is taken care of by your default assignments.

Safety check 1 is applied during phase 1 of the initialisation process.

Phase 2 of the initialisation process then occurs -

Phase 2

Working back down from the top of the chain, each designated initializer in the chain has the option to customize the instance further. Initializers are now able to access self and can modify its properties, call its instance methods, and so on. Finally, any convenience initializers in the chain have the option to customize the instance and to work with self.

So you need to assign the final values during phase 2.

The documentation is a little unclear, but it seems that you should continue to use the typical convention from Objective C of calling the superclass init first.

Why does calling super.init() call my default init()

2 Answers