Swift Optionals

Unlike its predecessors C and Objective-C, Swift has the concept of Optional types. By default, variables in Swift cannot be set to nil, unless they are defined as an Optional type. This is represented syntactically by appending a ? to the end of a type. For example, if you have an assignment, var s: String = “”, you would change String to String?. This will let you set s to nil: s = nil.

Unwrapping

However, to use a variable with an Optional type, it must be unwrapped. There are several ways of doing this: with an if-let statement, guard-let statement, or using a ?. operator. The ?. operator, is similar to the . operator except it checks if the variable is nil before unwrapping it and continuing with the . operation.

Since str1 is not nil, the lowercased() method is called on the unwrapped value of str1, while str2 is nil and the method cannot be called, so the expression results in nil.

There is another, slightly more risky way of unwrapping an Optional and that is the ! operator instead of the ?, which unwraps an Optional without checking if it is nil. This tells the compiler that you “know” that the variable you are about forcibly unwrap is in fact not nil. For example, using the code above, str1!.lowercased() would work just the same as str1?.lowercased(). However, if the variable is nil, then a fatal error is thrown, which would be the case if str2?.lowercased() was changed to str2!.lowercased().

Although ?. is the only operator described here, you can also use ? before subscripting ([...]) and calling.

Implicitly Unwrapped Optionals

A Optional type can also be defined as implicitly-unwrapped, which is represented by appending an ! to the end of a type, for example String!. This means that a variable of Optional type will be forcibly unwrapped without the need for the ? or ! operator. Since the variable can still be nil, this can cause fatal errors to occur if not used properly.

If-let and guard-let

Perhaps one of the most useful statements in Swift is an if-let and its sibling guard-let. As described above, an if-let allows you to unwrap an optional type, if the value is not nil, and use it in the body of the if-let statement.

The if-let above takes str, checks if it is nil, and if it isn’t assigns unwrappedStr to the value of str and executes the code inside the curly braces allowing access to unwrappedStr, otherwise it will execute the else statement. Notice, that once outside of the if-let block, unwrappedStr is no longer available.

Now, the guard-let does almost the opposite of an it-let:

In the example above, the guard-let takes the variable str of Optional type, checks if it is nil, and if it isn’t the value of str is assigned to unwrappedStr, otherwise the code inside the block is executed. Note the return statement inside of the guard-let: you must either have a return or break (or something else that would cause the code to terminate such as a throw), since str is nil, it can’t be unwrapped and unwrappedStr cannot be used.

If you have any questions about Optional types or anything Swift, or if you like what you’ve read and want to know more Swifty things, please leave a comment here or tweet or PM me on Twitter. Happy Swifting!