Option

We have already seen some use of Option<T>. It stores either a value of type T or nothing. For example, String::find returns an Option<usize>.

fn main() {
    let name = "Löwe 老虎 Léopard Gepardi";
    let mut position: Option<usize> = name.find('é');
    println!("find returned {position:?}");
    assert_eq!(position.unwrap(), 14);
    position = name.find('Z');
    println!("find returned {position:?}");
    assert_eq!(position.expect("Character not found"), 0);
}
This slide should take about 10 minutes.

  • Option is widely used, not just in the standard library.

  • unwrap will return the value in an Option, or panic. expect is similar but takes an error message.

    • You can panic on None, but you can't "accidentally" forget to check for None.
    • It's common to unwrap/expect all over the place when hacking something together, but production code typically handles None in a nicer fashion.

  • The "niche optimization" means that Option<T> often has the same size in memory as T, if there is some representation that is not a valid value of T. For example, a reference cannot be NULL, so Option<&T> automatically uses NULL to represent the None variant, and thus can be stored in the same memory as &T.