Assign value with optional question mark

Question

I'm currently learning Swift from basics. Now I'm on optionals and I'm trying to understand this case:

var text: String? = nil

text? = "some text"

What happens exactly if we assign value with question mark? I don't understand why the value of text is nil. Can you explain me what is the difference between assigning text = "some text" and text? = "some text"?

Answer 1

You're right to be surprised. text? = means "If text is nil don't perform the assignment."

You've stumbled across a highly obscure language feature. See https://ericasadun.com/2017/01/25/pretty-much-every-way-to-assign-optionals/ (As she rightly says, you can count on the fingers of zero hands the number of times you'll ever actually talk like this, because who would ever want to assign a value only just in case the lvalue is already non-nil?)

NOTE I prefer to look at this as a zero-length variant of optional chaining. It is extremely useful to be able to say e.g.

self.navigationController?.hidesBarsOnTap = true

meaning, if self.navigationController is nil, fuhgeddaboudit. Well, your use case is sort of a variant of that, with nothing after the question mark. Most people are unaware that if the last object in the chain is an Optional, the chain can end in a question mark. The expressions in f are all legal:

class C {
    struct T {
        var text : String?
    }
    var t : T?
    func f() {
        self.t?.text = "howdy"
        self.t? = T()
        self.t?.text? = "howdy"
    }
}

But only the first one, self.t?.text =, is common to say in real life.