W1035 Not a Value
There's often a need to represent the absence of a value. For example, consider a database tracking students' first name, middle name, and last name. Not all students have a middle name. So what value should we store in a variable representing a student's middle name if this particular student doesn't have one?
The name of this special value, representing non-existence, varies from language to language. In Swift, it's called nil.
Nil represents the absence of value of the specified type. We indicate that we intend to accept such nil values by specifying the type explicitly and suffixing the type name with a question mark. For example, in the aforementioned case, rather than specify the type as:
let middleName : String
let middleName : String?
This enables us to specify either a character or the special place-holder for no value, nil:
let middleName : String? = "A"
let middleName : String? = nil
Types which may be nil are termed optionals, because the constant or variable might have a value of the specified type; if not, it will be nil.
While nil is very helpful in these cases, using the value in an expression becomes slightly more complex, because for each such value, we need to take into account the possibility that it may be nil. Let's consider another example:
1let a : Double = 0.4 2let b : Double? = nil 3let c = a + b
Line 3 will result in an error: value of optional type 'Double?' must be unwrapped to a value of type 'Double' Why? Because we assumed that b was not nil. Since it wouldn't make any sense to add nil to a Double, we need to ensure that b is, in fact, not nil before attempting to use it as an addend. We'll learn later various ways to check whether or not an optional contains nil, but, if we are certain that a value is not nil, we can indicate this with an exclamation point at the end of the name of the optional:
1let a : Double = 0.4 2let b : Double? = 0.2 3let c = a + b!
Note that unwrapping refers to removing the "box" around the optional to reveal its contents, which should contain a value of the underlying type. If we are force-unwrapping, by using the exclamation point, our program will terminate with an error if the optional contained nil.
- Optional (Swift Documentation)