Error in Swift class Property not initialized at super init call

Question

I have two classes  Shape and Square  class Shape       var numberOfSides   0     var name  String     init name String            self name   name           func simpleDescription   - gt  String           return  A shape with   numberOfSides  sides            class Square  Shape       var sideLength  Double      init sideLength Double  name String            super init name name     Error here         self sideLength   sideLength         numberOfSides   4           func area    - gt  Double           return sideLength   sideLength           With the implementation above I get the error   property  self sideLength  not initialized at super init call     super init name name    Why do I have to set self sideLength before calling super init

User · Answer

Swift has a very clear  specific sequence of operations that are done in initializers  Let s start with some basic examples and work our way up to a general case   Let s take an object A  We ll define it as follows   class A       var x  Int     init x  Int            self x   x           Notice that A does not have a superclass  so it cannot call a super init   function as it does not exist   OK  so now let s subclass A with a new class named B   class B  A       var y  Int     init x  Int  y  Int            self y   y         super init x  x            This is a departure from Objective-C where  super init  would typically be called first before anything else  Not so in Swift  You are responsible for ensuring that your instance variables are in a consistent state before you do anything else  including calling methods  which includes your superclass  initializer

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The  super init    should be called after you initialize all your instance variables   In Apple s  Intermediate Swift  video  you can find it in Apple Developer video resource page https   developer apple com videos wwdc 2014    at about 28 40  it is explicit said that all initializers in super class must be called AFTER you initialize your instance variables   In Objective-C  it was the reverse  In Swift  since all properties need to be initialized before it s used  we need to initialize properties first  This is meant to prevent a call to overrided function from super class s  init    method  without initializing properties first   So the implementation of  Square  should be   class Square  Shape       var sideLength  Double      init sideLength Double  name String            self sideLength   sideLength         numberOfSides   4         super init name name     Correct position for  super init              func area    - gt  Double           return sideLength   sideLength

User · Answer

Add nil to the end of the declaration        Must be nil or swift complains var someProtocol SomeProtocol    nil     Init the view override init frame  CGRect      super init frame  frame              This worked for my case  but may not work for yours

User · Answer

From the docs     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      Why do we need a safety check like this   To answer this lets go though the initialization process in swift      Two-Phase Initialization      Class initialization in Swift is a two-phase process  In the first   phase  each stored property is assigned an initial value by the class   that introduced it  Once the initial state for every stored property   has been determined  the second phase begins  and each class is given   the opportunity to customize its stored properties further before the   new instance is considered ready for use       The use of a two-phase initialization process makes initialization   safe  while still giving complete flexibility to each class in a class   hierarchy  Two-phase initialization prevents property values from   being accessed before they are initialized  and prevents property   values from being set to a different value by another initializer   unexpectedly    So  to make sure the two step initialization process is done as defined above  there are four safety checks  one of them is      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    Now  the two phase initialization never talks about order  but this safety check  introduces super init to be ordered  after the initialization of all the properties   Safety check 1 might seem irrelevant as   Two-phase initialization prevents property values from being accessed before they are initialized can be satisfied  without this safety check 1   Like in this sample  class Shape       var name  String     var sides   Int     init sides Int  named  String            self sides   sides         self name   named          class Triangle  Shape       var hypotenuse  Int     init hypotenuse Int            super init sides  3  named   Triangle            self hypotenuse   hypotenuse           Triangle init has initialized  every property before being used  So Safety check 1 seems irrelevant    But then there could be another scenario  a little bit complex    class Shape       var name  String     var sides   Int     init sides Int  named  String            self sides   sides         self name   named         printShapeDescription             func printShapeDescription             print  Shape Name    self name            print  Sides    self sides             class Triangle  Shape       var hypotenuse  Int     init hypotenuse Int            self hypotenuse   hypotenuse         super init sides  3  named   Triangle              override func printShapeDescription             super printShapeDescription           print  Hypotenuse    self hypotenuse             let triangle   Triangle hypotenuse  12    Output    Shape Name  Triangle Sides  3 Hypotenuse  12   Here if we had called the super init before setting the hypotenuse  the super init call would then have called the printShapeDescription   and since that has been overridden it would first fallback to Triangle class implementation of printShapeDescription    The printShapeDescription   of Triangle class access the hypotenuse a non optional property that still has not been initialised  And this is not allowed as Two-phase initialization prevents property values from being accessed before they are initialized  So make sure the Two phase initialization is done as defined  there needs to be a specific order of calling super init  and that is  after initializing all the properties introduced by self class  thus we need a Safety check 1

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Quote from The Swift Programming Language  which answers your question         Swift   s compiler performs four helpful safety-checks to make sure   that two-phase initialization is completed without error          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          Excerpt From  Apple Inc     The Swift Programming Language     iBooks    https   itunes apple com us book swift-programming-language id881256329 mt 11

User · Answer

Edward    You can modify the code in your example like this   var playerShip PlayerShip  var deltaPoint   CGPointZero  init size  CGSize        super init size  size      playerLayerNode addChild playerShip              This is using  an implicitly unwrapped optional   In documentation we can read       As with optionals  if you don   t provide an initial value when you   declare an implicitly unwrapped optional variable or property  it   s   value automatically defaults to nil

User · Answer

You are just initing in the wrong order        class Shape2           var numberOfSides   0         var name  String         init name String                self name   name                   func simpleDescription   - gt  String               return  A shape with   numberOfSides  sides                        class Square2  Shape2           var sideLength  Double          init sideLength Double  name String                 self sideLength   sideLength             super init name name     It should be behind  self sideLength   sideLength              numberOfSides   4                   func area    - gt  Double               return sideLength   sideLength

User · Answer

swift enforces you to initialise every member var before it is ever might ever be used  Since it can t be sure what happens when it is supers turn  it errors out  better safe than sorry

User · Answer

Sorry for ugly formatting  Just put a question character after declaration and everything will be ok  A question tells the compiler that the value is optional   class Square  Shape       var sideLength  Double        lt     like this         init sideLength Double  name String            super init name name     Error here         self sideLength   sideLength         numberOfSides   4           func area    - gt  Double           return sideLength   sideLength           Edit1   There is a better way to skip this error  According to jmaschad s comment there is no reason to use optional in your case cause optionals are not comfortable in use and You always have to check if optional is not nil before accessing it  So all you have to do is to initialize member after declaration   class Square  Shape       var sideLength  Double Double           init sideLength Double  name String            super init name name          self sideLength   sideLength         numberOfSides   4           func area    - gt  Double           return sideLength   sideLength           Edit2   After two minuses got on this answer I found even better way  If you want class member to be initialized in your constructor you must assign initial value to it inside contructor and before super init   call  Like this   class Square  Shape       var sideLength  Double        init sideLength Double  name String            self sideLength   sideLength       lt   before super init call           super init name name          numberOfSides   4           func area    - gt  Double           return sideLength   sideLength           Good luck in learning Swift

User · Answer

Swift will not allow you to initialise super class with out initialising the properties  reverse of Obj C   So you have to initialise all properties before calling  super init     Please go  to http   blog scottlogic com 2014 11 20 swift-initialisation html  It gives a nice explanation to your problem

[properties] Error in Swift class: Property not initialized at super.init call

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