What is the difference between static func and class func in Swift

Question

I can see these definitions in the Swift library   extension Bool   BooleanLiteralConvertible       static func convertFromBooleanLiteral value  Bool  - gt  Bool    protocol BooleanLiteralConvertible       typealias BooleanLiteralType     class func convertFromBooleanLiteral value  BooleanLiteralType  - gt  Self     What s the difference between a member function defined as static func and another one defined as class func  Is it simply that static is for static functions of structs and enums  and class for classes and protocols  Are there any other differences that one should know about  What is the rationale for having this distinction in the syntax itself

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From Swift2 0  Apple says    Always prefix type property requirements with the static keyword when you define them in a protocol  This rule pertains even though type property requirements can be prefixed with the class or static keyword when implemented by a class

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According to the Swift 2 2 Book published by apple      You indicate type methods by writing the static keyword before the method   s func keyword  Classes may also use the class keyword to allow subclasses to override the superclass   s implementation of that method

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Is it simply that static is for static functions of structs and enums  and class for classes and protocols    That s the main difference  Some other differences are that class functions are dynamically dispatched and can be overridden by subclasses   Protocols use the class keyword  but it doesn t exclude structs from implementing the protocol  they just use static instead  Class was chosen for protocols so there wouldn t have to be a third keyword to represent static or class   From Chris Lattner on this topic      We considered unifying the syntax  e g  using  type  as the keyword   but that doesn t actually simply things   The keywords  class  and  static  are good for familiarity and are quite descriptive  once you understand how   methods work   and open the door for potentially adding truly static methods to classes   The primary weirdness of this model is that protocols have to pick a keyword  and we chose  class    but on balance it is the right tradeoff    And here s a snippet that shows some of the override behavior of class functions   class MyClass       class func myFunc             println  myClass            class MyOtherClass  MyClass       override class func myFunc             println  myOtherClass            var x  MyClass   MyOtherClass   x dynamicType myFunc     myOtherClass x   MyClass   x dynamicType myFunc     myClass

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I did some experiments in playground and got some conclusions   TL DR   As you can see  in the case of class  the use of class func or static func is just a question of habit   Playground example with explanation   class Dog       final func identity   - gt  String           return  Once a woofer  forever a woofer              class func talk   - gt  String           return  Woof woof              static func eat   - gt  String           return  Miam miam             func sleep   - gt  String           return  Zzz           class Bulldog  Dog          Can not override a final function       override final func identity   - gt  String             return  I m once a dog but now I m a cat                  Can not override a  class func   but redeclare is ok     func talk   - gt  String           return  I m a bulldog  and I don t woof                 Same as  class func      func eat   - gt  String           return  I m a bulldog  and I don t eat                 Normal function can be overridden     override func sleep   - gt  String           return  I m a bulldog  and I don t sleep            let dog   Dog   let bullDog   Bulldog       FINAL FUNC   print Dog identity       compile error print dog identity       print  Once a woofer  forever a woofer     print Bulldog identity       compile error print bullDog identity       print  Once a woofer  forever a woofer         gt   final func  is just a  normal  one but prevented to be overridden nor redeclared by subclasses       CLASS FUNC print Dog talk       print  Woof woof    called directly from class   print dog talk       compile error cause  class func  is meant to be called directly from class  not an instance  print Bulldog talk       print  Woof woof   cause it s called from Bulldog class  not bullDog instance  print bullDog talk       print  I m a bulldog  and I don t woof   cause talk   is redeclared and it s called from bullDig instance       gt   class func  is like a  static  one  must be called directly from class or subclassed  can be redeclared but NOT meant to be overridden      STATIC FUNC print Dog eat       print  Miam miam    print dog eat       compile error cause  static func  is type method print Bulldog eat       print  Miam miam  print bullDog eat       print  I m a bulldog  and I don t eat       NORMAL FUNC   print Dog sleep       compile error print dog sleep       print  Zzz    print Bulldog sleep       compile error print bullDog sleep       print  I m a bulldog  and I don t sleep

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This example will clear every aspect   import UIKit  class Parent       final func finalFunc   - gt  String      Final Function  cannot be redeclared          return  Parent Final Function              static func staticFunc   - gt  String      Static Function  can be redeclared          return  Parent Static Function              func staticFunc   - gt  String      Above function redeclared as Normal function          return  Parent Static Function  redeclared with same name but as non-static normal  function              class func classFunc   - gt  String      Class Function  can be redeclared          return  Parent Class Function              func classFunc   - gt  String      Above function redeclared as Normal function          return  Parent Class Function  redeclared with same name but as non-class normal  function              func normalFunc   - gt  String      Normal function  obviously cannot be redeclared          return  Parent Normal Function            class Child Parent           Final functions cannot be overridden       override func staticFunc   - gt  String      This override form is of the redeclared version i e   func staticFunc    so just like any other function of normal type  it can be overridden          return  Child Static Function redeclared and overridden  can simply be called Child Normal Function              override class func classFunc   - gt  String      Class function  can be overidden          return  Child Class Function              override func classFunc   - gt  String      This override form is of the redeclared version i e   func classFunc    so just like any other function of normal type  it can be overridden          return  Child Class Function  redeclared and overridden  can simply be called Child Normal Function              override func normalFunc   - gt  String      Normal function  can be overridden          return  Child Normal Function            let parent   Parent   let child   Child       Final print  1      parent finalFunc         1  Can be called by object  print  2      child finalFunc          2  Can be called by object  parent final  function will be called     Parent finalFunc                    Cannot be called by class name directly     Child finalFunc                     Cannot be called by class name directly      Static print  3      parent staticFunc        3  Cannot be called by object  this is redeclared version  i e  a normal function   print  4      child staticFunc         4  Cannot be called by object  this is override form redeclared version  normal function   print  5      Parent staticFunc        5  Can be called by class name directly  print  6      Child staticFunc         6  Can be called by class name direcly  parent static  function will be called      Class print  7      parent classFunc         7  Cannot be called by object  this is redeclared version  i e  a normal function   print  8      child classFunc          8  Cannot be called by object  this is override form redeclared version  normal function   print  9      Parent classFunc         9  Can be called by class name directly  print  10      Child classFunc         10  Can be called by class name direcly  child class  function will be called      Normal print  11      parent normalFunc        11  Can be called by object  print  12      child normalFunc         12  Can be called by object  child normal  function will be called     Parent normalFunc                    Cannot be called by class name directly     Child normalFunc                     Cannot be called by class name directly       Notes                                                                                 Types------Redeclare------Override------Call by object------Call by Class    Final----------0--------------0---------------1------------------0-------    Static---------1--------------0---------------0------------------1-------    Class----------1--------------1---------------0------------------1-------    Normal---------0--------------1---------------1------------------0-------   ---------------------------------------------------------------------------   Final vs Normal function  Both are same but normal methods can be overridden   Static vs Class function  Both are same but class methods can be overridden        Output

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To declare a type variable property  mark the declaration with the static declaration modifier  Classes may mark type computed properties with the class declaration modifier instead to allow subclasses to override the superclass   s implementation  Type properties are discussed in Type Properties   NOTE In a class declaration  the keyword static has the same effect as marking the declaration with both the class and final declaration modifiers    Source  The Swift Programming Language - Type Variable Properties

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To be clearer  I make an example here   class ClassA     class func func1   - gt  String       return  func1         static func func2   - gt  String       return  func2            same as above   final class func func2   - gt  String       return  func2               static func is same as final class func  Because it is final  we can not override it in subclass as below   class ClassB   ClassA     override class func func1   - gt  String       return  func1 in ClassB            ERROR  Class method overrides a  final  class method   override static func func2   - gt  String       return  func2 in ClassB

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This is called type methods  and are called with dot syntax  like instance methods  However  you call type methods on the type  not on an instance of that type  Here   s how you call a type method on a class called SomeClass

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Both the static and class keywords allow us to attach methods to a class rather than to instances of a class  For example  you might create a Student class with properties such as name and age  then create a static method numberOfStudents that is owned by the Student class itself rather than individual instances  Where static and class differ is how they support inheritance  When you make a static method it becomes owned by the class and can t be changed by subclasses  whereas when you use class it may be overridden if needed  Here is an Example code    class Vehicle       static func getCurrentSpeed   - gt  Int           return 0            class func getCurrentNumberOfPassengers   - gt  Int           return 0                class Bicycle  Vehicle         This is not allowed       Compiler error   quot Cannot override static method quot        static override func getCurrentSpeed   - gt  Int               return 15                class override func getCurrentNumberOfPassengers   - gt  Int           return 1

[class] What is the difference between static func and class func in Swift?

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