How are static variables stored in memory in Swift?

Question

How are static variables in Swift stored?

1. What happens if I never call the func useStaticVar() ? Are these variables initialised or not?

2. What happens if I do call useStaticVar() and then never access them again? Ar

   struct Something {
       static var myVariable = 0
       static let myConstant = 3
       static var myString: String?
   
       static func useStaticVar() {
           myVariable = myConstant
           myString = String(myVariable)
       }
   }
   

Answer 1

Check this: Type Properties

NOTE

Unlike stored instance properties, you must always give stored type properties a default value. This is because the type itself does not have an initializer that can assign a value to a stored type property at initialization time.

Stored type properties are lazily initialized on their first access. They are guaranteed to be initialized only once, even when accessed by multiple threads simultaneously, and they do not need to be marked with the lazy modifier.

you must always give stored type properties a default value.

Your code lacks a default value for myString, in such cases, Swift give it a default nil.

Stored type properties are lazily initialized on their first access.

You can test and see how stored type properties are initilized with some code like this:

func ZeroGenerator() -> Int {
    print(#function)
    return 0
}

func ThreeGenerator() -> Int {
    print(#function)
    return 3
}

func NilGeneartor() -> String? {
    print(#function)
    return nil
}

struct Something {
    static var myVariable = ZeroGenerator()
    static let myConstant = ThreeGenerator()
    static var myString: String? = NilGeneartor()

    static func useStaticVar() {
        print(#function)
        myVariable = myConstant
        myString = String(myVariable)
    }
}

Something.useStaticVar()

Output:

useStaticVar()
ZeroGenerator()
ThreeGenerator()
NilGeneartor()

Compiler may make some optimizations for constant default values, but semantically no difference.