Options

enum Option<T> {
    None,
    Some(T),
}

let some_number = Some(5);
let some_char = Some('e');

let absent_number: Option<i32> = None;

as_mut

• The as_mut  method is a method of optional types , like Option<T> .

• It is used to convert an Option<T>  into an Option<&mut T> . This allows accessing the contained value mutably without moving the value.

let mut opt: Option<String> = Some(String::from("Hello"));

if let Some(value) = opt.as_mut() {
    value.push_str(", world!"); // Modifies the contained value
}

println!("{:?}", opt); // Output: Some("Hello, world!")

Adapters for working with references

• as_ref  converts from &Option<T>  to Option<&T> .

• as_mut  converts from &mut Option<T>  to Option<&mut T> .

• as_deref  converts from &Option<T>  to Option<&T::Target> .

• as_deref_mut  converts from &mut Option<T>  to Option<&mut T::Target> .

• as_pin_ref  converts from Pin<&Option<T>>  to Option<Pin<&T>> .

• as_pin_mut  converts from Pin<&mut Option<T>>  to Option<Pin<&mut T>> .

Using ?

• Without ? :

  fn add_last_numbers(stack: &mut Vec<i32>) -> Option<i32> {
      let a = stack.pop();
      let b = stack.pop();
  
      match (a, b) {
          (Some(x), Some(y)) => Some(x + y),
          _ => None,
      }
  }
  
  

• With ? :

  fn add_last_numbers(stack: &mut Vec<i32>) -> Option<i32> {
      Some(stack.pop()? + stack.pop()?)
  }
  

Extracting the contained value

• expect  panics with a provided custom message.

• unwrap  panics with a generic message.

• unwrap_or  returns the provided default value.

• unwrap_or_default  returns the default value of the type T  (which must implement the Default  trait).

• unwrap_or_else  returns the result of evaluating the provided function.

If let

let config_max = Some(3u8);
if let Some(max) = config_max {
    println!("The maximum is configured to be {max}");
}

Matching

fn plus_one(x: Option<i32>) -> Option<i32> {
    match x {
        None => None,
        Some(i) => Some(i + 1),
    }
}

let five = Some(5);
let six = plus_one(five);
let none = plus_one(None);