Calling an optional function in Swift

Question

I'm trying to work out a way to call methods dynamically by number. Here's a simplified version of what I'm doing.

class C {

    func a() {}
    func b() {}

    let f = [0: a, 1: b]

    func call(n: Int) {
        f[n]?()
    }
}

let c = C()

c.call(0)

When I run this in a playground I get

Playground execution failed: error: <REPL>:10:13: error: could not find an overload for 'subscript' that accepts the supplied arguments
    f[n]?()
    ~~~~^~~

Yet if I run

func a() {}
func b() {}

let f = [0: a, 1: b]

f[0]?()

directly with no containing class it works as expected. What's going on?

Answer 1

This is really interesting! I noticed that your first bit of code ran fine if I moved the function definitions outside of the class but kept everything else the same. From the error message I was able to conclude that when the functions are declared within the class they need to be called with an instance of the class. When you just call a() inside the class the compiler is automatically interpreting that as self.a(). However, when the function is stored in a variable (f[0], f[1], etc.) it needs to be passed an instance of class C (self) first. This works:

class C {

    func a() {println("in a")}
    func b() {println("in b")}

    let f = [0: a, 1: b]

    func call(n: Int) {
        a() // works because it's auto-translated to self.a()
        f[n]?(self)() // works because self is passed in manually
    }
}

let c = C()

c.call(0)