Optional vs implicitly unwrapped optional: what’s the difference?

Swift version: 5.10

Paul Hudson @twostraws May 28th 2019

When you’re just learning Swift, the difference between an optional (String?), an implicitly unwrapped optional (String!), and a regular type (String) can seem awfully confusing. Here’s a quick summary that should explain the difference:

When you use String you’re saying this will always have a string inside, and can never have nothing inside. It might be an empty string (""), but even an empty string is still a string.

When you use String? you’re saying this might have a string inside, but it might have nothing at all inside – not even an empty string. Swift won’t let you use these without unwrapping them, which is usually done using if let.

When you use String! you’re saying this might have a string inside, but it might have nothing at all inside – not even an empty string. However, Swift lets you use these as if they were a String, as if they always do have a value, but if you try to use a nil value by accident your code will crash. This effectively lets you say “I know this might be nil, but I’m so sure it has a value that I’m willing for my program to crash if I’m wrong.”

So: String is definitely a string, String? might be nil or might be a string, and String! might be nil but when you use it you’re absolutely sure it has a string.