Property getters and setters

Question

With this simple class I am getting the compiler warning      Attempting to modify access x within its own setter getter   and when I use it like this   var p  point   Point   p x   12   I get an EXC BAD ACCESS  How can I do this without explicit backing ivars   class Point        var x  Int           set               x   newValue   2   Error                   get               return x   2   Error

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Update  Swift 4  In the below class setter and getter is applied to variable sideLength  class Triangle        var sideLength  Double   0 0      init sideLength  Double  name  String      initializer method         self sideLength   sideLength         super init name  name          numberOfSides   3            var perimeter  Double           get      getter             return 3 0   sideLength                   set newValue      setter             sideLength   newValue   4 0                  Creating object  var triangle   Triangle sideLength  3 9  name   a triangle     Getter  print triangle perimeter     invoking getter   Setter  triangle perimeter   9 9    invoking setter

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You are recursively defining x with x  As if someone asks you how old are you  And you answer  I am twice my age   Which is meaningless     You must say I am twice John s age or any other variable but yourself   computed variables are always dependent on another variable     The rule of the thumb is never access the property itself from within the getter ie get  Because that would trigger another get which would trigger another       Don t even print it  Because printing also requires to  get  the value before it can print it   struct Person      var name  String          get              print name     DON T do this                 return  as                    set                     let p1   Person     As that would give the following warning       Attempting to access  name  from within it s own getter    The error looks vague like this     As an alternative you might want to use didSet  With didSet you ll get a hold to the value that is was set before and just got set to  For more see this answer

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Setters and getters in Swift apply to computed properties variables   These properties variables are not actually stored in memory  but rather computed based on the value of stored properties variables   See Apple s Swift documentation on the subject  Swift Variable Declarations

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Try using this    var x Int   var xTimesTwo Int       get           return x   2           set           x   newValue   2           This is basically Jack Wu s answer  but the difference is that in Jack Wu s answer his x variable is var x  Int  in mine  my x variable is like this  var x  Int   so all I did was make it an optional type

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To elaborate on GoZoner s answer   Your real issue here is that you are recursively calling your getter   var x Int               set                       x   newValue   2    This isn t a problem                   get               return x   2    Here is your real issue  you are recursively calling                              your x property s getter                   Like the code comment suggests above  you are infinitely calling the x property s getter  which will continue to execute until you get a EXC BAD ACCESS code  you can see the spinner in the bottom right corner of your Xcode s playground environment    Consider the example from the Swift documentation   struct Point       var x   0 0  y   0 0   struct Size       var width   0 0  height   0 0   struct AlternativeRect       var origin   Point       var size   Size       var center  Point           get               let centerX   origin x    size width   2              let centerY   origin y    size height   2              return Point x  centerX  y  centerY                    set               origin x   newValue x -  size width   2              origin y   newValue y -  size height   2                      Notice how the center computed property never modifies or returns itself in the variable s declaration

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You can customize the set value using property observer  To do this use  didSet  instead of  set    class Point    var x  Int       didSet           x   x   2               As for getter      class Point    var doubleX  Int       get           return x   2

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In order to override setter and getter for swift variables use the below given code   var temX   Int   var x  Int        set newX          temX   newX            get          return temX           We need to keep the value of variable in a temporary variable  since trying to access the same variable whose getter setter is being overridden will result in infinite loops   We can invoke the setter simply like this   x   10   Getter will be invoked on firing below given line of code   var newVar   x

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Setters and Getters apply to computed properties  such properties do not have storage in the instance - the value from the getter is meant to be computed from other instance properties   In your case  there is no x to be assigned   Explicitly   How can I do this without explicit backing ivars    You can t - you ll need something to backup the computed property   Try this   class Point     private var  x  Int   0                 x - gt  backingX   var x  Int       set    x   2   newValue       get   return  x   2           Specifically  in the Swift REPL    15 gt  var pt   Point   pt  Point        x   0    16 gt  pt x   10  17 gt  pt  R3  Point        x   20    18 gt  pt x  R4  Int   10

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I don t know if it is good practice but you can do something like this  class test ancestor       var prop  Int   0     class test  test ancestor       override var prop  Int           get               return super prop    reaching ancestor prop                   set               super prop   newValue   2                    var test instance   test   test instance prop   10  print test instance prop     20  Read more

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Update for Swift 5 1  As of Swift 5 1 you can now get your variable without using get keyword  For example   var helloWorld  String    Hello World

User · Answer

Setters getters in Swift are quite different than ObjC  The property becomes a computed property which means it does not have a backing variable such as  x as it would in ObjC    In the solution code below you can see the xTimesTwo does not store anything  but simply computes the result from x   See Official docs on computed properties   The functionality you want might also be Property Observers   What you need is   var x  Int  var xTimesTwo  Int       set          x   newValue   2           get           return x   2           You can modify other properties within the setter getters  which is what they are meant for

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Here is a theoretical answer  That can be found here      A   get set   property cannot be a constant stored property  It should be a computed property and both get and set should be implemented

[swift] Property getters and setters

Update: Swift 4

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