Static vs class functions/variables in Swift classes?

Question

The following code compiles in Swift 1.2:

class myClass {
    static func myMethod1() {
    }
    class func myMethod2() {
    }
    static var myVar1 = \"\"         


        

Answer 1

class vs static

class is used inside Reference Type(class):

• computed property
• method
• can be overridden by subclass

static is used inside Reference Type and Value Type(class, enum):

• computed property and stored property
• method
• cannot be changed by subclass

protocol MyProtocol {
//    class var protocolClassVariable : Int { get }//ERROR: Class properties are only allowed within classes
    static var protocolStaticVariable : Int { get }
    
//    class func protocolClassFunc()//ERROR: Class methods are only allowed within classes
    static func protocolStaticFunc()
}

struct ValueTypeStruct: MyProtocol {
    //MyProtocol implementation begin
    static var protocolStaticVariable: Int = 1
    
    static func protocolStaticFunc() {
        
    }
    //MyProtocol implementation end
    
//    class var classVariable = "classVariable"//ERROR: Class properties are only allowed within classes
    static var staticVariable = "staticVariable"

//    class func classFunc() {} //ERROR: Class methods are only allowed within classes
    static func staticFunc() {}
}

class ReferenceTypeClass: MyProtocol {
    //MyProtocol implementation begin
    static var protocolStaticVariable: Int = 2
    
    static func protocolStaticFunc() {
        
    }
    //MyProtocol implementation end
    
    var variable = "variable"

//    class var classStoredPropertyVariable = "classVariable"//ERROR: Class stored properties not supported in classes

    class var classComputedPropertyVariable: Int {
        get {
            return 1
        }
    }

    static var staticStoredPropertyVariable = "staticVariable"

    static var staticComputedPropertyVariable: Int {
        get {
            return 1
        }
    }

    class func classFunc() {}
    static func staticFunc() {}
}

final class FinalSubReferenceTypeClass: ReferenceTypeClass {
    override class var classComputedPropertyVariable: Int {
        get {
            return 2
        }
    }
    override class func classFunc() {}
}

//class SubFinalSubReferenceTypeClass: FinalSubReferenceTypeClass {}// ERROR: Inheritance from a final class

[Reference vs Value Type]

Answer 2

Testing in Swift 4 shows performance difference in simulator. I made a class with "class func" and struct with "static func" and ran them in test.

static func is:

• 20% faster without compiler optimization
• 38% faster when optimization -whole-module-optimization is enabled.

However, running the same code on iPhone 7 under iOS 10.3 shows exactly the same performance.

Here is sample project in Swift 4 for Xcode 9 if you like to test yourself https://github.com/protyagov/StructVsClassPerformance

Answer 3

Regarding to OOP, the answer is too simple:

The subclasses can override class methods, but cannot override static methods.

In addition to your post, if you want to declare a class variable (like you did class var myVar2 = ""), you should do it as follow:

class var myVar2: String {
    return "whatever you want"
}