CODEWITHSUNDEEP
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Colton Allen
Colton Allen

Reputation: 3060

How to access class constants and variables

Why can I not perform operations with a variable or constant at the class level? Is this not allowed or is there a keyword I need to use to access them? Is this bad practice?

Controller:

class ViewController: UIViewController {
    let one = 1
    let two = 2
    var sum = one + two
}

Error:

ViewController.Type does not have a member named 'one'

Upvotes: 1

Views: 242

Answers (2)

Arkku
Arkku

Reputation: 42159

Class variables and constants must be static, e.g., static let one = 1.

Here it suffices for the two let constants to be static for them to be usable in initializing both class and instance variables. The following works for me:

class MyClass {
    static let one = 1
    static let two = 2
    static var sum = one + two
    var instanceProduct = one * two
}

MyClass.one
MyClass.sum
MyClass().instanceProduct

Note that in the above example you can do MyClass.sum = 5. If you meant the sum to be constant as well, simply change it to static let sum = one + two.

The requirement is that the constants you use outside of any functions and closures be declared static let. The implication of this is that they are truly constant for the entire class. If you need instance-specific constants, you cannot use them outside of functions or closures (as you've noticed) – as a workaround for a variable sum I would suggest a lazy variable initialized by a closure:

class MyClass {
    let one: Int // value given in `init`
    let two = 2
    lazy var sum: Int = { self.one + self.two }()

    init(one: Int = 1) {
        self.one = one
    }
}
MyClass().sum // 3
MyClass(one: 2).sum // 4

Upvotes: 3

DogCoffee
DogCoffee

Reputation: 19966

Simple fix 

   override func viewDidLoad() {
        super.viewDidLoad()
        var result = one + two
    }

Upvotes: 0

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