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What is an Optional in Swift

What is the Problem

In C, it is possible to create a variable without giving it a value. This would look something like this:

int number

If you were to try to use the value before assigning it a value, you would get undefined behavior (that is very bad).

In contrast Swift, for safety reasons, requires all variables and constants to always hold a value. This prevents that scenario where the value of a variable can be unknown. However, there are still cases in programming where one wants to represent the absence of a value. A great example of this is when performing a search. One would want to be able to return something from the search that indicates that no value was found.

How is an Optional Defined

To solve this problem, Swift created the type Optional that can either hold no value (None) or hold some value (Some). In fact, because Swift allows enums to have associated values, an optional is defined as an enum:

// slightly simplified
enum Optional {
    case none
    case some(Wrapped)
}

You declare an optional version of a type by adding a ? after the type name (String?).

Unwrapping an Optional

Before you use the value from an Optional you must first "unwrap" it, which basically means that you assert that it does indeed hold a value. We consider an optional value "wrapped" because the real value is actually held inside the enumeration.

Optional Binding

You can unwrap an optional in both a "safe" and "unsafe" way. The safe way is to use Optional Binding:

let possibleString: String? = "Hello"
if let actualString = possibleString {
    // actualString is a normal (non-optional) String value
    // equal to the value stored in possibleString
    print(actualString)
}
else {
    // possibleString did not hold a value, handle that
    // situation
}

Forced Unwrapping

Sometimes you know for sure that a variable holds an actual value and you can assert that with Forced Unwrapping by using an exclamation point (!):

let possibleString: String? = "Hello"
print(possibleString!)

If possibleString were None (did not hold a value), the whole program would crash with a runtime error and therefore, forced unwrapping is considered "unsafe".

Implicitly Unwrapped Optional

An Implicitly Unwrapped Optional is an optional that doesn't need to be unwrapped because it is done implicitly. These types of optionals are declared with an ! instead of a ?:

let possibleString: String! 
print(possibleString)

Notice, I did not use an ! to print out the value of possibleString. Just like forced unwrapping, accessing an implicitly unwrapped optional that is nil will cause the entire program to crash with a runtime error.

For more information on when Implicitly Unwrapped Optionals are appropriate, I recommend my other post Uses For Implicitly Unwrapped Optionals.