Module std::option

1.0.0 · source ·

Expand description

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

Options 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"),
}

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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"),
    }
}

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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:

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,
    }
}

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With this:

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

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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 as T:

Box
&U
&mut U
fn, extern "C" fn¹
num::NonZero*
ptr::NonNull
#[repr(transparent)] struct around one of the types in this list.

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 behaviour).

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.

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>>

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

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>>
map transforms Option<T> to Option 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 value having a different inner type U than Option<T>.

method	self	input	output
`and`	`None`	(ignored)	`None`
`and`	`Some(x)`	`None`	`None`
`and`	`Some(x)`	`Some(y)`	`Some(y)`
`or`	`None`	`None`	`None`
`or`	`None`	`Some(y)`	`Some(y)`
`or`	`Some(x)`	(ignored)	`Some(x)`
`xor`	`None`	`None`	`None`
`xor`	`None`	`Some(y)`	`Some(y)`
`xor`	`Some(x)`	`None`	`Some(x)`
`xor`	`Some(x)`	`Some(y)`	`None`

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 value having a different inner type U than Option<T>.

method	self	function input	function result	output
`and_then`	`None`	(not provided)	(not evaluated)	`None`
`and_then`	`Some(x)`	`x`	`None`	`None`
`and_then`	`Some(x)`	`x`	`Some(y)`	`Some(y)`
`or_else`	`None`	(not provided)	`None`	`None`
`or_else`	`None`	(not provided)	`Some(y)`	`Some(y)`
`or_else`	`Some(x)`	(not provided)	(not evaluated)	`Some(x)`

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.

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"]);

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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));

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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]);

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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<_>>());

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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.

// 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),
    }
}

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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]));

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Option also implements the Product and Sum traits, allowing an iterator over Option values to provide the product and 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));

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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");

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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 :("),
}

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this remains true for any other ABI: extern "abi" fn (e.g., extern "system" fn) ↩

Structs

IntoIter
An iterator over the value in Some variant of an Option.
Iter
An iterator over a reference to the Some variant of an Option.
IterMut
An iterator over a mutable reference to the Some variant of an Option.

Enums

Option
The Option type. See the module level documentation for more.

Traits

SpecOptionPartialEqExperimental
This specialization trait is a workaround for LLVM not currently (2023-01) being able to optimize this itself, even though Alive confirms that it would be legal to do so: https://github.com/llvm/llvm-project/issues/52622