What is the difference between static func and cla

Question

I can see these definitions in the Swift library:

extension Bool : BooleanLiteralConvertible {
    static func convertFromBooleanLiteral(value: Bool) -> Bool
}

protocol BooleanLiteralConvertible {
    typealias BooleanLiteralType
    class func convertFromBooleanLiteral(value: BooleanLiteralType) -> 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?

Answer 1

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.”

Answer 2

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() -> String {
        return "Once a woofer, forever a woofer!"
    }

    class func talk() -> String {
        return "Woof woof!"
    }

    static func eat() -> String {
        return "Miam miam"
    }

    func sleep() -> String {
        return "Zzz"
    }
}

class Bulldog: Dog {
    // Can not override a final function
//    override final func identity() -> 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() -> String {
        return "I'm a bulldog, and I don't woof."
    }

    // Same as "class func"
    func eat() -> String {
        return "I'm a bulldog, and I don't eat."
    }

    // Normal function can be overridden
    override func sleep() -> 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!"

// => "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

// => "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."

Answer 3

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:

Answer 4

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

Answer 5

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

Answer 6

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:"