Default optional parameter in Swift function

When I set firstThing to default nil this will work, without the default value of nil I get a error that there is a missing parameter when calling the function.

By typing Int? I thought it made it optional with a default value of nil, am I right? And if so, why doesn't it work without the = nil?

func test(firstThing: Int? = nil) {
    if firstThing != nil {
        print(firstThing!)
    }
    print("done")
}
test()

Solution 1:

Optionals and default parameters are two different things.

An Optional is a variable that can be nil, that's it.

Default parameters use a default value when you omit that parameter, this default value is specified like this: func test(param: Int = 0)

If you specify a parameter that is an optional, you have to provide it, even if the value you want to pass is nil. If your function looks like this func test(param: Int?), you can't call it like this test(). Even though the parameter is optional, it doesn't have a default value.

You can also combine the two and have a parameter that takes an optional where nil is the default value, like this: func test(param: Int? = nil).

Solution 2:

The default argument allows you to call the function without passing an argument. If you don't pass the argument, then the default argument is supplied. So using your code, this...

test()

...is exactly the same as this:

test(nil)

If you leave out the default argument like this...

func test(firstThing: Int?) {
    if firstThing != nil {
        print(firstThing!)
    }
    print("done")
}

...then you can no longer do this...

test()

If you do, you will get the "missing argument" error that you described. You must pass an argument every time, even if that argument is just nil:

test(nil)   // this works