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iOS Swift Closures Functions as First Class Citizens Capturing Variables

Matt Frerichs
Matt Frerichs
4,015 Points
Posted by Matt Frerichs
Matt Frerichs
4,015 Points

What is a Void return type?

I am having trouble understanding exactly what the void "()" return type does. I don't understand why you can't just use a return type of Int since the function parameter is of type Int.

2 Answers

Martin Wildfeuer
PLUS
Martin Wildfeuer
Courses Plus Student 11,071 Points
Martin Wildfeuer
Martin Wildfeuer
Courses Plus Student 11,071 Points

If you do not want to return anything from your function, you use either -> Void, -> () or nothing at all. Moreover, the return type has nothing to do with the parameters at all, you could easily return a String representation of a math operation that requires two Ints.

// We just want to print to the console,
// so we don't need to return anything
func sayHello() {
   println("Hello")
}

// You would call this function
// without assigning it to a variable/constant
sayHello();

// That would trigger a warning, as
// the function returns nothing
let test = sayHello()

// Oh, and that would trigger an error, as we
// don't expect the function to return anything...
func sayHello() {
   return "Hello"
}

// ...whereas this also triggers an error. If we specify
// a return type, we have to return something
func sayHello()  -> String {
}
Why would we need a function that returns nothing?

That is a very common use case. For example, if you had a custom view that you wanted to hide, you would implement that method in the class so you can call it from outside.

class CustomAlert : UIViewController {
   ...
   func show() {
     // Code to show this view
   }

   func hide() {
     // Code to hide this view
   }
}

let customAlert = CustomAlert()
customAlert.show()

See? There is no return value needed at all!

Matt Frerichs
Matt Frerichs
4,015 Points
Matt Frerichs
Matt Frerichs
4,015 Points

This helps clear things up for me. Thank you for the help!

Greg Kaleka
Greg Kaleka
39,021 Points
Greg Kaleka
Greg Kaleka
39,021 Points

Hi Matt,

The concept Pasan Premaratne is going over is pretty complicated, but to answer your question specifically, a function that returns void is essentially one that doesn't return anything.

This function has a return type string, so I can pass its returned value to a constant:

func sayHello(name: String) -> String {
    return "Hello, \(name)"
}

let greeting = sayHello("Matt")  // I can do this because sayHello spits out a String

This function has a return type of void, so it doesn't spit anything out that I can use:

func logError(name: String) -> () {  // the arrow and parens are entirely optional
   println(\(name) did something wrong!)  // sorry, couldn't resist :)
}

Does that make sense?

In Pasan's example, that inner function is just printing something to the console, rather than returning a value. Functions with void as a return type are pretty common - they are handy for doing "work" but not necessarily returning something you would use elsewhere. The function viewDidLoad is probably the best example of a function with a return type of void.