Optional values.

Type [Option] represents an optional value: every [Option] is either Some and contains a value, or None, and does not. [Option] types are very common in Rust code, as they have a number of uses:

• Initial values
• Return values for functions that are not defined over their entire input range (partial functions)
• Return value for otherwise reporting simple errors, where None is returned on error
• Optional struct fields
• Struct fields that can be loaned or "taken"
• Optional function arguments
• Nullable pointers
• Swapping things out of difficult situations

[Option]s are commonly paired with pattern matching to query the presence of a value and take action, always accounting for the None case.

fn divide(numerator: f64, denominator: f64) -> Option<f64> {
    if denominator == 0.0 {
        None
    } else {
        Some(numerator / denominator)
    }
}

// The return value of the function is an option
let result = divide(2.0, 3.0);

// Pattern match to retrieve the value
match result {
    // The division was valid
    Some(x) => println!("Result: {x}"),
    // The division was invalid
    None    => println!("Cannot divide by 0"),
}

Options and pointers ("nullable" pointers)

Rust's pointer types must always point to a valid location; there are no "null" references. Instead, Rust has optional pointers, like the optional owned box, [Option]<Box<T>>.

The following example uses [Option] to create an optional box of i32. Notice that in order to use the inner i32 value, the check_optional function first needs to use pattern matching to determine whether the box has a value (i.e., it is Some(...)) or not (None).

let optional = None;
check_optional(optional);

let optional = Some(Box::new(9000));
check_optional(optional);

fn check_optional(optional: Option<Box<i32>>) {
    match optional {
        Some(p) => println!("has value {p}"),
        None => println!("has no value"),
    }
}

The question mark operator, ?

Similar to the Result type, when writing code that calls many functions that return the [Option] type, handling Some/None can be tedious. The question mark operator, ?, hides some of the boilerplate of propagating values up the call stack.

It replaces this:

# #![allow(dead_code)]
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 this:

# #![allow(dead_code)]
fn add_last_numbers(stack: &mut Vec<i32>) -> Option<i32> {
    Some(stack.pop()? + stack.pop()?)
}

It's much nicer!

Ending the expression with ? will result in the Some's unwrapped value, unless the result is None, in which case None is returned early from the enclosing function.

? can be used in functions that return [Option] because of the early return of None that it provides.

Representation

Rust guarantees to optimize the following types T such that [Option<T>] has the same size, alignment, and function call ABI as T. It is therefore sound, when T is one of these types, to transmute a value t of type T to type Option<T> (producing the value Some(t)) and to transmute a value Some(t) of type Option<T> to type T (producing the value t).

In some of these cases, Rust further guarantees the following:

• transmute::<_, Option<T>>([0u8; size_of::<T>()]) is sound and produces Option::<T>::None
• transmute::<_, [u8; size_of::<T>()]>(Option::<T>::None) is sound and produces [0u8; size_of::<T>()]

These cases are identified by the second column:

Under some conditions the above types T are also null pointer optimized when wrapped in a Result.

This is called the "null pointer optimization" or NPO.

It is further guaranteed that, for the cases above, one can mem::transmute from all valid values of T to Option<T> and from Some::<T>(_) to T (but transmuting None::<T> to T is undefined behavior).

Method overview

In addition to working with pattern matching, [Option] provides a wide variety of different methods.

Querying the variant

The is_some and is_none methods return true if the [Option] is Some or None, respectively.

The is_some_and and is_none_or methods apply the provided function to the contents of the [Option] to produce a boolean value. If this is None then a default result is returned instead without executing the function.

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>>
• as_slice returns a one-element slice of the contained value, if any. If this is None, an empty slice is returned.
• as_mut_slice returns a mutable one-element slice of the contained value, if any. If this is None, an empty slice is returned.

Extracting the contained value

These methods extract the contained value in an [Option<T>] when it is the Some variant. If the [Option] is None:

• 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
• unwrap_unchecked produces undefined behavior

Transforming contained values

These methods transform [Option] to [Result]:

• ok_or transforms Some(v) to Ok(v), and None to Err(err) using the provided default err value
• ok_or_else transforms Some(v) to Ok(v), and None to a value of Err using the provided function
• transpose transposes an [Option] of a Result into a Result of an [Option]

These methods transform the Some variant:

• filter calls the provided predicate function on the contained value t if the [Option] is Some(t), and returns Some(t) if the function returns true; otherwise, returns None
• flatten removes one level of nesting from an [Option<Option<T>>]
• inspect method takes ownership of the [Option] and applies the provided function to the contained value by reference if Some
• map transforms [Option<T>] to [Option<U>] by applying the provided function to the contained value of Some and leaving None values unchanged

These methods transform [Option<T>] to a value of a possibly different type U:

• map_or applies the provided function to the contained value of Some, or returns the provided default value if the [Option] is None
• map_or_else applies the provided function to the contained value of Some, or returns the result of evaluating the provided fallback function if the [Option] is None

These methods combine the Some variants of two [Option] values:

• zip returns Some((s, o)) if self is Some(s) and the provided [Option] value is Some(o); otherwise, returns None
• zip_with calls the provided function f and returns Some(f(s, o)) if self is Some(s) and the provided [Option] value is Some(o); otherwise, returns None

Boolean operators

These methods treat the [Option] as a boolean value, where Some acts like true and None acts like false. There are two categories of these methods: ones that take an [Option] as input, and ones that take a function as input (to be lazily evaluated).

The and, or, and xor methods take another [Option] as input, and produce an [Option] as output. Only the and method can produce an [Option<U>] value having a different inner type U than [Option<T>].

The and_then and or_else methods take a function as input, and only evaluate the function when they need to produce a new value. Only the and_then method can produce an [Option<U>] value having a different inner type U than [Option<T>].

This is an example of using methods like and_then and or in a pipeline of method calls. Early stages of the pipeline pass failure values (None) through unchanged, and continue processing on success values (Some). Toward the end, or substitutes an error message if it receives None.

# use std::collections::BTreeMap;
let mut bt = BTreeMap::new();
bt.insert(20u8, "foo");
bt.insert(42u8, "bar");
let res = [0u8, 1, 11, 200, 22]
    .into_iter()
    .map(|x| {
        // `checked_sub()` returns `None` on error
        x.checked_sub(1)
            // same with `checked_mul()`
            .and_then(|x| x.checked_mul(2))
            // `BTreeMap::get` returns `None` on error
            .and_then(|x| bt.get(&x))
            // Substitute an error message if we have `None` so far
            .or(Some(&"error!"))
            .copied()
            // Won't panic because we unconditionally used `Some` above
            .unwrap()
    })
    .collect::<Vec<_>>();
assert_eq!(res, ["error!", "error!", "foo", "error!", "bar"]);

Comparison operators

If T implements PartialOrd then [Option<T>] will derive its PartialOrd implementation. With this order, None compares as less than any Some, and two Some compare the same way as their contained values would in T. If T also implements Ord, then so does [Option<T>].

assert!(None < Some(0));
assert!(Some(0) < Some(1));

Iterating over Option

An [Option] can be iterated over. This can be helpful if you need an iterator that is conditionally empty. The iterator will either produce a single value (when the [Option] is Some), or produce no values (when the [Option] is None). For example, into_iter acts like once(v) if the [Option] is Some(v), and like empty() if the [Option] is None.

Iterators over [Option<T>] come in three types:

• into_iter consumes the [Option] and produces the contained value
• iter produces an immutable reference of type &T to the contained value
• iter_mut produces a mutable reference of type &mut T to the contained value

An iterator over [Option] can be useful when chaining iterators, for example, to conditionally insert items. (It's not always necessary to explicitly call an iterator constructor: many Iterator methods that accept other iterators will also accept iterable types that implement IntoIterator, which includes [Option].)

let yep = Some(42);
let nope = None;
// chain() already calls into_iter(), so we don't have to do so
let nums: Vec<i32> = (0..4).chain(yep).chain(4..8).collect();
assert_eq!(nums, [0, 1, 2, 3, 42, 4, 5, 6, 7]);
let nums: Vec<i32> = (0..4).chain(nope).chain(4..8).collect();
assert_eq!(nums, [0, 1, 2, 3, 4, 5, 6, 7]);

One reason to chain iterators in this way is that a function returning impl Iterator must have all possible return values be of the same concrete type. Chaining an iterated [Option] can help with that.

fn make_iter(do_insert: bool) -> impl Iterator<Item = i32> {
    // Explicit returns to illustrate return types matching
    match do_insert {
        true => return (0..4).chain(Some(42)).chain(4..8),
        false => return (0..4).chain(None).chain(4..8),
    }
}
println!("{:?}", make_iter(true).collect::<Vec<_>>());
println!("{:?}", make_iter(false).collect::<Vec<_>>());

If we try to do the same thing, but using once() and empty(), we can't return impl Iterator anymore because the concrete types of the return values differ.

# use std::iter::{empty, once};
// This won't compile because all possible returns from the function
// must have the same concrete type.
fn make_iter(do_insert: bool) -> impl Iterator<Item = i32> {
    // Explicit returns to illustrate return types not matching
    match do_insert {
        true => return (0..4).chain(once(42)).chain(4..8),
        false => return (0..4).chain(empty()).chain(4..8),
    }
}

Collecting into Option

[Option] implements the FromIterator trait, which allows an iterator over [Option] values to be collected into an [Option] of a collection of each contained value of the original [Option] values, or None if any of the elements was None.

let v = [Some(2), Some(4), None, Some(8)];
let res: Option<Vec<_>> = v.into_iter().collect();
assert_eq!(res, None);
let v = [Some(2), Some(4), Some(8)];
let res: Option<Vec<_>> = v.into_iter().collect();
assert_eq!(res, Some(vec![2, 4, 8]));

[Option] also implements the Product and Sum traits, allowing an iterator over [Option] values to provide the [product][Iterator::product] and [sum][Iterator::sum] methods.

let v = [None, Some(1), Some(2), Some(3)];
let res: Option<i32> = v.into_iter().sum();
assert_eq!(res, None);
let v = [Some(1), Some(2), Some(21)];
let res: Option<i32> = v.into_iter().product();
assert_eq!(res, Some(42));

Modifying an [Option] in-place

These methods return a mutable reference to the contained value of an [Option<T>]:

• insert inserts a value, dropping any old contents
• get_or_insert gets the current value, inserting a provided default value if it is None
• get_or_insert_default gets the current value, inserting the default value of type T (which must implement Default) if it is None
• get_or_insert_with gets the current value, inserting a default computed by the provided function if it is None

These methods transfer ownership of the contained value of an [Option]:

• take takes ownership of the contained value of an [Option], if any, replacing the [Option] with None
• replace takes ownership of the contained value of an [Option], if any, replacing the [Option] with a Some containing the provided value

Examples

Basic pattern matching on [Option]:

let msg = Some("howdy");

// Take a reference to the contained string
if let Some(m) = &msg {
    println!("{}", *m);
}

// Remove the contained string, destroying the Option
let unwrapped_msg = msg.unwrap_or("default message");

Initialize a result to None before a loop:

enum Kingdom { Plant(u32, &'static str), Animal(u32, &'static str) }

// A list of data to search through.
let all_the_big_things = [
    Kingdom::Plant(250, "redwood"),
    Kingdom::Plant(230, "noble fir"),
    Kingdom::Plant(229, "sugar pine"),
    Kingdom::Animal(25, "blue whale"),
    Kingdom::Animal(19, "fin whale"),
    Kingdom::Animal(15, "north pacific right whale"),
];

// We're going to search for the name of the biggest animal,
// but to start with we've just got `None`.
let mut name_of_biggest_animal = None;
let mut size_of_biggest_animal = 0;
for big_thing in &all_the_big_things {
    match *big_thing {
        Kingdom::Animal(size, name) if size > size_of_biggest_animal => {
            // Now we've found the name of some big animal
            size_of_biggest_animal = size;
            name_of_biggest_animal = Some(name);
        }
        Kingdom::Animal(..) | Kingdom::Plant(..) => ()
    }
}

match name_of_biggest_animal {
    Some(name) => println!("the biggest animal is {name}"),
    None => println!("there are no animals :("),
}