Functions and OOP: Optionals and Nil

This tutorial belongs to the Swift series

Optionals are one key feature of Swift.

When you don’t know if a value will be present or absent, you declare the type as an optional.

The optional wraps another value, with its own type. Or maybe not.

We declare an optional adding a question mark after its type, like this:

var value: Int? = 10

Now value is not an Int value. It’s an optional wrapping an Int value.

To find out if the optional wraps a value, you must unwrap it.

We do so using an exclamation mark:

var value: Int? = 10
print(value!) //10

Swift methods often return an optional. For example the Int type initializer accepts a string, and returns an Int optional:

This is because it does not know if the string can be converted to a number.

If the optional does not contain a value, it evaluates as nil, and you cannot unwrap it:

nil is a special value that cannot be assigned to a variable. Only to an optional:

You typically use if statements to unwrap values in your code, like this:

var value: Int? = 2

if let age = value {
    print(age)
}