Overview_Optionals

Optionals

One of the main benefits of Swift is type safety. Attempting to access a property with an unexpected nil value is a major cause of bugs. Optionals exist to remove any uncertainty around nil.

Using Optionals well will give you confidence when accessing variables in your code. At compile time, you will always know if the property you're accessing might have a value or must have a value.

Properties can be declared as variables (var) or constants (let). For each of these types of declarations, they can be:

• Optional (variable) by adding ? after the type. This can either be declared with var or let.
• Required (constant) by declaring it as let (but without the ?).
• Optional, but implicitly unwrapped (by adding ! after the type. Use this with var declarations).

For example:

struct MyExampleStruct {
    var firstName : String? // Optional
    let lastName : String // Non-Optional const. This value MUST be set during initialization.
    let apiKey : String = "xQ7tt4$" // Non-Optional const, but initialized here.
    var gender : String! // Implicitly Unwrapped Optional

    init(lastname: String) {
            self.lastName = lastname
    }
    init(firstname: String?, lastname: String, gender: String!) {
            self.firstName = firstname
            self.lastName = lastname
            self.gender = gender
    }
}

Note that the following init will fail, because apiKey is a constant (let) whose value had already been assigned when declared:

   init(firstname: String?, lastname: String, apiKey: String!, gender: String!) {

            self.firstName = firstname
            self.lastName = last name
            self.apiKey = apiKey
            self.gender = gender
    }

var s = MyExampleStruct(lastname: "Fred")
    //{firstName: nil, lastName: "Fred", apiKey: "xQ7tt4$", gender: nil}
var u = MyExampleStruct(firstname: "Howard", lastname: "Duck", gender: "M")
    //{firstName: "Howard", lastName: "Duck", apiKey: "xQ7tt4$", gender: "M"}

Where possible, strive to keep your properties safe, meaning Optional. Valid reasons for non-safety (implicitly unwrapped optionals) are:

• Instances where you are absolutely certain that at run-time, they will never be nil (@IBOutlet is a good example of this).
• Lazy vars (but with a caveat, discussed below).