Understanding Optional and Exclamation Mark in Swift

Swift is very strongly typed. All variables have a type. There is a type in Swift called optional. Optional can have only two values:

1. One value is not set, such as never been set or someone has set it to be not set state, i.e. unset it, to the value nil.

2. The other value is something, it’s set to something.

For example:

var something = display.text

What type is “something”? We didn’t specify the type of this variable. Swift can infer the type from the context. It makes “something” to be the same type of display.text. Let's say “display.text” is a type of string, then “something” is an optional string, which is optional that can be a string.

How to get a string from an optional? You can unwrap the optional, meaning you look in there and get the associated value, with an exclamation mark.

var something = display.text!

Now the type of “something” is a string, not an optional anymore. Note that if display.text value is nil, then your program is gonna crash!

Originally published at victorleungtw.com on November 13, 2015.