Swift optionals and nil

Jun 7, 2021

Learn how optionals and nil work in Swift, how to declare an optional with a question mark, and how to safely unwrap its value using an if let statement.

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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:

Swift code showing Int("37") returns Optional<Int> type when converting string to integer

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:

Swift code showing Int("test") returns nil and crashes when force unwrapped with exclamation mark

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

Swift code showing optional variable assigned nil and comparison returning true

Swift compiler error showing nil cannot be assigned to non-optional Int type

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

var value: Int? = 2

if let age = value {
    print(age)
}

Tagged: Swift · All topics

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