Proper way to handle a fail to init

Question

I am looking for a proper way to handle a invalid argument during a initialization.

I am unsure how to do it using Swift as the init has't a return type. How can I tell whoever is trying to initialize this class that you are doing something wrong?

init (timeInterval: Int) {
    if timeInterval > 0
        self.timeInterval = timeInterval
    else
        //???? (return false?)
}

Thank you!

Answer 1

Use a failable initializer. Such an initializer looks very similar to a regular designated initializer, but has a '?' character right after init and is allowed to return nil. A failable initializer creates an optional value.

struct Animal {
    let species: String
    init?(species: String) {
        if species.isEmpty { return nil }
        self.species = species
    }
}

See Apple's documentation on failable initializers for more detail.

Answer 2

In swift, you can't really abort a task half way through execution. There are no exceptions in swift and in general the philosophy is that aborting a task is dangerous and leads to bugs, so it just should't be done.

So, you verify a value like this:

assert(timeInterval > 0)

Which will terminate the program if an invalid value is provided.

You should also change timeInterval to be a UInt so that there will be a compiler error if anybody tries to give a < 0 value or an integer value that could be < 0.

It's probably not the answer you're looking for. But the goal is to check for bad parameters as early as possible, and that means doing it before you create any objects with those parameters. Ideally the check should be done at compile time but that doesn't always work.