Enum std::option::Option

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

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

Variants

None

No value

Some(T)

Some value T

Methods

impl<T> Option<T>

#[must_use] pub fn is_some(&self) -> bool

Returns true if the option is a Some value.

Examples

let x: Option<u32> = Some(2);
assert_eq!(x.is_some(), true);

let x: Option<u32> = None;
assert_eq!(x.is_some(), false);

#[must_use] pub fn is_none(&self) -> bool

Returns true if the option is a None value.

Examples

let x: Option<u32> = Some(2);
assert_eq!(x.is_none(), false);

let x: Option<u32> = None;
assert_eq!(x.is_none(), true);

pub fn as_ref(&self) -> Option<&T>

Converts from &Option<T> to Option<&T>.

Examples

Converts an Option<String> into an Option<usize>, preserving the original. The map method takes the self argument by value, consuming the original, so this technique uses as_ref to first take an Option to a reference to the value inside the original.

let text: Option<String> = Some("Hello, world!".to_string());
// First, cast `Option<String>` to `Option<&String>` with `as_ref`,
// then consume *that* with `map`, leaving `text` on the stack.
let text_length: Option<usize> = text.as_ref().map(|s| s.len());
println!("still can print text: {:?}", text);

pub fn as_mut(&mut self) -> Option<&mut T>

Converts from &mut Option<T> to Option<&mut T>.

Examples

let mut x = Some(2);
match x.as_mut() {
    Some(v) => *v = 42,
    None => {},
}
assert_eq!(x, Some(42));

pub fn as_pin_ref(self: Pin<&'a Option<T>>) -> Option<Pin<&'a T>>

Converts from Pin<&Option<T>> to Option<Pin<&T>>

pub fn as_pin_mut(self: Pin<&'a mut Option<T>>) -> Option<Pin<&'a mut T>>

Converts from Pin<&mut Option<T>> to Option<Pin<&mut T>>

pub fn expect(self, msg: &str) -> T

Unwraps an option, yielding the content of a Some.

Panics

Panics if the value is a None with a custom panic message provided by msg.

Examples

let x = Some("value");
assert_eq!(x.expect("the world is ending"), "value");

let x: Option<&str> = None;
x.expect("the world is ending"); // panics with `the world is ending`

pub fn unwrap(self) -> T

Moves the value v out of the Option<T> if it is Some(v).

In general, because this function may panic, its use is discouraged. Instead, prefer to use pattern matching and handle the None case explicitly.

Panics

Panics if the self value equals None.

Examples

let x = Some("air");
assert_eq!(x.unwrap(), "air");

let x: Option<&str> = None;
assert_eq!(x.unwrap(), "air"); // fails

pub fn unwrap_or(self, def: T) -> T

Returns the contained value or a default.

Arguments passed to unwrap_or are eagerly evaluated; if you are passing the result of a function call, it is recommended to use unwrap_or_else, which is lazily evaluated.

Examples

assert_eq!(Some("car").unwrap_or("bike"), "car");
assert_eq!(None.unwrap_or("bike"), "bike");

pub fn unwrap_or_else<F>(self, f: F) -> T where
    F: FnOnce() -> T, 

Returns the contained value or computes it from a closure.

Examples

let k = 10;
assert_eq!(Some(4).unwrap_or_else(|| 2 * k), 4);
assert_eq!(None.unwrap_or_else(|| 2 * k), 20);

pub fn map<U, F>(self, f: F) -> Option<U> where
    F: FnOnce(T) -> U, 

Maps an Option<T> to Option<U> by applying a function to a contained value.

Examples

Converts an Option<String> into an Option<usize>, consuming the original:

let maybe_some_string = Some(String::from("Hello, World!"));
// `Option::map` takes self *by value*, consuming `maybe_some_string`
let maybe_some_len = maybe_some_string.map(|s| s.len());

assert_eq!(maybe_some_len, Some(13));

pub fn map_or<U, F>(self, default: U, f: F) -> U where
    F: FnOnce(T) -> U, 

Applies a function to the contained value (if any), or returns the provided default (if not).

Examples

let x = Some("foo");
assert_eq!(x.map_or(42, |v| v.len()), 3);

let x: Option<&str> = None;
assert_eq!(x.map_or(42, |v| v.len()), 42);

pub fn map_or_else<U, D, F>(self, default: D, f: F) -> U where
    D: FnOnce() -> U,
    F: FnOnce(T) -> U, 

Applies a function to the contained value (if any), or computes a default (if not).

Examples

let k = 21;

let x = Some("foo");
assert_eq!(x.map_or_else(|| 2 * k, |v| v.len()), 3);

let x: Option<&str> = None;
assert_eq!(x.map_or_else(|| 2 * k, |v| v.len()), 42);

pub fn ok_or<E>(self, err: E) -> Result<T, E>

Transforms the Option<T> into a Result<T, E>, mapping Some(v) to Ok(v) and None to Err(err).

Arguments passed to ok_or are eagerly evaluated; if you are passing the result of a function call, it is recommended to use ok_or_else, which is lazily evaluated.

Examples

let x = Some("foo");
assert_eq!(x.ok_or(0), Ok("foo"));

let x: Option<&str> = None;
assert_eq!(x.ok_or(0), Err(0));

pub fn ok_or_else<E, F>(self, err: F) -> Result<T, E> where
    F: FnOnce() -> E, 

Transforms the Option<T> into a Result<T, E>, mapping Some(v) to Ok(v) and None to Err(err()).

Examples

let x = Some("foo");
assert_eq!(x.ok_or_else(|| 0), Ok("foo"));

let x: Option<&str> = None;
assert_eq!(x.ok_or_else(|| 0), Err(0));

Important traits for Iter<'a, A>

impl<'a, A> Iterator for Iter<'a, A> type Item = &'a A;

pub fn iter(&self) -> Iter<T>

Returns an iterator over the possibly contained value.

Examples

let x = Some(4);
assert_eq!(x.iter().next(), Some(&4));

let x: Option<u32> = None;
assert_eq!(x.iter().next(), None);

Important traits for IterMut<'a, A>

impl<'a, A> Iterator for IterMut<'a, A> type Item = &'a mut A;

pub fn iter_mut(&mut self) -> IterMut<T>

Returns a mutable iterator over the possibly contained value.

Examples

let mut x = Some(4);
match x.iter_mut().next() {
    Some(v) => *v = 42,
    None => {},
}
assert_eq!(x, Some(42));

let mut x: Option<u32> = None;
assert_eq!(x.iter_mut().next(), None);

pub fn and<U>(self, optb: Option<U>) -> Option<U>

Returns None if the option is None, otherwise returns optb.

Examples

let x = Some(2);
let y: Option<&str> = None;
assert_eq!(x.and(y), None);

let x: Option<u32> = None;
let y = Some("foo");
assert_eq!(x.and(y), None);

let x = Some(2);
let y = Some("foo");
assert_eq!(x.and(y), Some("foo"));

let x: Option<u32> = None;
let y: Option<&str> = None;
assert_eq!(x.and(y), None);

pub fn and_then<U, F>(self, f: F) -> Option<U> where
    F: FnOnce(T) -> Option<U>, 

Returns None if the option is None, otherwise calls f with the wrapped value and returns the result.

Some languages call this operation flatmap.

Examples

fn sq(x: u32) -> Option<u32> { Some(x * x) }
fn nope(_: u32) -> Option<u32> { None }

assert_eq!(Some(2).and_then(sq).and_then(sq), Some(16));
assert_eq!(Some(2).and_then(sq).and_then(nope), None);
assert_eq!(Some(2).and_then(nope).and_then(sq), None);
assert_eq!(None.and_then(sq).and_then(sq), None);

pub fn filter<P>(self, predicate: P) -> Option<T> where
    P: FnOnce(&T) -> bool, 

Returns None if the option is None, otherwise calls predicate with the wrapped value and returns:

• Some(t) if predicate returns true (where t is the wrapped value), and
• None if predicate returns false.

This function works similar to Iterator::filter(). You can imagine the Option<T> being an iterator over one or zero elements. filter() lets you decide which elements to keep.

Examples

fn is_even(n: &i32) -> bool {
    n % 2 == 0
}

assert_eq!(None.filter(is_even), None);
assert_eq!(Some(3).filter(is_even), None);
assert_eq!(Some(4).filter(is_even), Some(4));

pub fn or(self, optb: Option<T>) -> Option<T>

Returns the option if it contains a value, otherwise returns optb.

Arguments passed to or are eagerly evaluated; if you are passing the result of a function call, it is recommended to use or_else, which is lazily evaluated.

Examples

let x = Some(2);
let y = None;
assert_eq!(x.or(y), Some(2));

let x = None;
let y = Some(100);
assert_eq!(x.or(y), Some(100));

let x = Some(2);
let y = Some(100);
assert_eq!(x.or(y), Some(2));

let x: Option<u32> = None;
let y = None;
assert_eq!(x.or(y), None);

pub fn or_else<F>(self, f: F) -> Option<T> where
    F: FnOnce() -> Option<T>, 

Returns the option if it contains a value, otherwise calls f and returns the result.

Examples

fn nobody() -> Option<&'static str> { None }
fn vikings() -> Option<&'static str> { Some("vikings") }

assert_eq!(Some("barbarians").or_else(vikings), Some("barbarians"));
assert_eq!(None.or_else(vikings), Some("vikings"));
assert_eq!(None.or_else(nobody), None);

pub fn xor(self, optb: Option<T>) -> Option<T>

🔬 This is a nightly-only experimental API. (option_xor #50512)

Returns Some if exactly one of self, optb is Some, otherwise returns None.

Examples

#![feature(option_xor)]

let x = Some(2);
let y: Option<u32> = None;
assert_eq!(x.xor(y), Some(2));

let x: Option<u32> = None;
let y = Some(2);
assert_eq!(x.xor(y), Some(2));

let x = Some(2);
let y = Some(2);
assert_eq!(x.xor(y), None);

let x: Option<u32> = None;
let y: Option<u32> = None;
assert_eq!(x.xor(y), None);

Important traits for &'_ mut F

impl<'_, F> Future for &'_ mut F where
    F: Unpin + Future + ?Sized,  type Output = <F as Future>::Output;impl<'_, I> Iterator for &'_ mut I where
    I: Iterator + ?Sized,  type Item = <I as Iterator>::Item;impl<'_, R: Read + ?Sized> Read for &'_ mut Rimpl<'_, W: Write + ?Sized> Write for &'_ mut W

pub fn get_or_insert(&mut self, v: T) -> &mut T

Inserts v into the option if it is None, then returns a mutable reference to the contained value.

Examples

let mut x = None;

{
    let y: &mut u32 = x.get_or_insert(5);
    assert_eq!(y, &5);

    *y = 7;
}

assert_eq!(x, Some(7));

Important traits for &'_ mut F

impl<'_, F> Future for &'_ mut F where
    F: Unpin + Future + ?Sized,  type Output = <F as Future>::Output;impl<'_, I> Iterator for &'_ mut I where
    I: Iterator + ?Sized,  type Item = <I as Iterator>::Item;impl<'_, R: Read + ?Sized> Read for &'_ mut Rimpl<'_, W: Write + ?Sized> Write for &'_ mut W

pub fn get_or_insert_with<F>(&mut self, f: F) -> &mut T where
    F: FnOnce() -> T, 

Inserts a value computed from f into the option if it is None, then returns a mutable reference to the contained value.

Examples

let mut x = None;

{
    let y: &mut u32 = x.get_or_insert_with(|| 5);
    assert_eq!(y, &5);

    *y = 7;
}

assert_eq!(x, Some(7));

pub fn take(&mut self) -> Option<T>

Takes the value out of the option, leaving a None in its place.

Examples

let mut x = Some(2);
let y = x.take();
assert_eq!(x, None);
assert_eq!(y, Some(2));

let mut x: Option<u32> = None;
let y = x.take();
assert_eq!(x, None);
assert_eq!(y, None);

pub fn replace(&mut self, value: T) -> Option<T>

Replaces the actual value in the option by the value given in parameter, returning the old value if present, leaving a Some in its place without deinitializing either one.

Examples

let mut x = Some(2);
let old = x.replace(5);
assert_eq!(x, Some(5));
assert_eq!(old, Some(2));

let mut x = None;
let old = x.replace(3);
assert_eq!(x, Some(3));
assert_eq!(old, None);

impl<'_, T> Option<&'_ T> where
    T: Copy, 

pub fn copied(self) -> Option<T>

Maps an Option<&T> to an Option<T> by copying the contents of the option.

Examples

let x = 12;
let opt_x = Some(&x);
assert_eq!(opt_x, Some(&12));
let copied = opt_x.copied();
assert_eq!(copied, Some(12));

impl<'_, T> Option<&'_ mut T> where
    T: Copy, 

pub fn copied(self) -> Option<T>

Maps an Option<&mut T> to an Option<T> by copying the contents of the option.

Examples

let mut x = 12;
let opt_x = Some(&mut x);
assert_eq!(opt_x, Some(&mut 12));
let copied = opt_x.copied();
assert_eq!(copied, Some(12));

impl<'_, T> Option<&'_ T> where
    T: Clone, 

pub fn cloned(self) -> Option<T>

Maps an Option<&T> to an Option<T> by cloning the contents of the option.

Examples

let x = 12;
let opt_x = Some(&x);
assert_eq!(opt_x, Some(&12));
let cloned = opt_x.cloned();
assert_eq!(cloned, Some(12));

impl<'_, T> Option<&'_ mut T> where
    T: Clone, 

pub fn cloned(self) -> Option<T>

Maps an Option<&mut T> to an Option<T> by cloning the contents of the option.

Examples

let mut x = 12;
let opt_x = Some(&mut x);
assert_eq!(opt_x, Some(&mut 12));
let cloned = opt_x.cloned();
assert_eq!(cloned, Some(12));

impl<T> Option<T> where
    T: Default, 

pub fn unwrap_or_default(self) -> T

Returns the contained value or a default

Consumes the self argument then, if Some, returns the contained value, otherwise if None, returns the default value for that type.

Examples

Converts a string to an integer, turning poorly-formed strings into 0 (the default value for integers). parse converts a string to any other type that implements FromStr, returning None on error.

let good_year_from_input = "1909";
let bad_year_from_input = "190blarg";
let good_year = good_year_from_input.parse().ok().unwrap_or_default();
let bad_year = bad_year_from_input.parse().ok().unwrap_or_default();

assert_eq!(1909, good_year);
assert_eq!(0, bad_year);

impl<T> Option<T> where
    T: Deref, 

pub fn deref(&self) -> Option<&<T as Deref>::Target>

🔬 This is a nightly-only experimental API. (inner_deref #50264)

newly added

Converts from &Option<T> to Option<&T::Target>.

Leaves the original Option in-place, creating a new one with a reference to the original one, additionally coercing the contents via Deref.

impl<T, E> Option<Result<T, E>>

pub fn transpose(self) -> Result<Option<T>, E>

Transposes an Option of a Result into a Result of an Option.

None will be mapped to Ok(None). Some(Ok(_)) and Some(Err(_)) will be mapped to Ok(Some(_)) and Err(_).

Examples

#[derive(Debug, Eq, PartialEq)]
struct SomeErr;

let x: Result<Option<i32>, SomeErr> = Ok(Some(5));
let y: Option<Result<i32, SomeErr>> = Some(Ok(5));
assert_eq!(x, y.transpose());

impl<T> Option<Option<T>>

pub fn flatten(self) -> Option<T>

🔬 This is a nightly-only experimental API. (option_flattening #60258)

Converts from Option<Option<T>> to Option<T>

Examples

Basic usage:

#![feature(option_flattening)]
let x: Option<Option<u32>> = Some(Some(6));
assert_eq!(Some(6), x.flatten());

let x: Option<Option<u32>> = Some(None);
assert_eq!(None, x.flatten());

let x: Option<Option<u32>> = None;
assert_eq!(None, x.flatten());

Flattening once only removes one level of nesting:

#![feature(option_flattening)]
let x: Option<Option<Option<u32>>> = Some(Some(Some(6)));
assert_eq!(Some(Some(6)), x.flatten());
assert_eq!(Some(6), x.flatten().flatten());

Trait Implementations

impl<T> From<T> for Option<T>

fn from(val: T) -> Option<T>

Performs the conversion.

impl<'a, T> From<&'a Option<T>> for Option<&'a T>

fn from(o: &'a Option<T>) -> Option<&'a T>

Performs the conversion.

impl<'a, T> From<&'a mut Option<T>> for Option<&'a mut T>

fn from(o: &'a mut Option<T>) -> Option<&'a mut T>

Performs the conversion.

impl<T, U> Product<Option<U>> for Option<T> where
    T: Product<U>, 

fn product<I>(iter: I) -> Option<T> where
    I: Iterator<Item = Option<U>>, 

Takes each element in the Iterator: if it is a None, no further elements are taken, and the None is returned. Should no None occur, the product of all elements is returned.

impl<A, V> FromIterator<Option<A>> for Option<V> where
    V: FromIterator<A>, 

fn from_iter<I>(iter: I) -> Option<V> where
    I: IntoIterator<Item = Option<A>>, 

Takes each element in the Iterator: if it is None, no further elements are taken, and the None is returned. Should no None occur, a container with the values of each Option is returned.

Examples

Here is an example which increments every integer in a vector. We use the checked variant of add that returns None when the calculation would result in an overflow.

let items = vec![0_u16, 1, 2];

let res: Option<Vec<u16>> = items
    .iter()
    .map(|x| x.checked_add(1))
    .collect();

assert_eq!(res, Some(vec![1, 2, 3]));

As you can see, this will return the expected, valid items.

Here is another example that tries to subtract one from another list of integers, this time checking for underflow:

let items = vec![2_u16, 1, 0];

let res: Option<Vec<u16>> = items
    .iter()
    .map(|x| x.checked_sub(1))
    .collect();

assert_eq!(res, None);

Since the last element is zero, it would underflow. Thus, the resulting value is None.

Here is a variation on the previous example, showing that no further elements are taken from iter after the first None.

let items = vec![3_u16, 2, 1, 10];

let mut shared = 0;

let res: Option<Vec<u16>> = items
    .iter()
    .map(|x| { shared += x; x.checked_sub(2) })
    .collect();

assert_eq!(res, None);
assert_eq!(shared, 6);

Since the third element caused an underflow, no further elements were taken, so the final value of shared is 6 (= 3 + 2 + 1), not 16.

impl<T> PartialOrd<Option<T>> for Option<T> where
    T: PartialOrd<T>, 

fn partial_cmp(&self, other: &Option<T>) -> Option<Ordering>

This method returns an ordering between self and other values if one exists.

fn lt(&self, other: &Option<T>) -> bool

This method tests less than (for self and other) and is used by the < operator.

fn le(&self, other: &Option<T>) -> bool

This method tests less than or equal to (for self and other) and is used by the <= operator.

fn gt(&self, other: &Option<T>) -> bool

This method tests greater than (for self and other) and is used by the > operator.

fn ge(&self, other: &Option<T>) -> bool

This method tests greater than or equal to (for self and other) and is used by the >= operator.

impl<T> Hash for Option<T> where
    T: Hash, 

fn hash<__HT>(&self, state: &mut __HT) where
    __HT: Hasher, 

Feeds this value into the given [Hasher].

fn hash_slice<H>(data: &[Self], state: &mut H) where
    H: Hasher, 

Feeds a slice of this type into the given [Hasher].

impl<T> Default for Option<T>

fn default() -> Option<T>

Returns None.

impl<T> Eq for Option<T> where
    T: Eq, 

impl<T, U> Sum<Option<U>> for Option<T> where
    T: Sum<U>, 

fn sum<I>(iter: I) -> Option<T> where
    I: Iterator<Item = Option<U>>, 

Takes each element in the Iterator: if it is a None, no further elements are taken, and the None is returned. Should no None occur, the sum of all elements is returned.

Examples

This sums up the position of the character 'a' in a vector of strings, if a word did not have the character 'a' the operation returns None:

let words = vec!["have", "a", "great", "day"];
let total: Option<usize> = words.iter().map(|w| w.find('a')).sum();
assert_eq!(total, Some(5));

impl<T> Clone for Option<T> where
    T: Clone, 

fn clone(&self) -> Option<T>

Returns a copy of the value.

fn clone_from(&mut self, source: &Self)

Performs copy-assignment from source.

impl<T> Try for Option<T>

type Ok = T

🔬 This is a nightly-only experimental API. (try_trait #42327)

The type of this value when viewed as successful.

type Error = NoneError

🔬 This is a nightly-only experimental API. (try_trait #42327)

The type of this value when viewed as failed.

fn into_result(self) -> Result<T, NoneError>

🔬 This is a nightly-only experimental API. (try_trait #42327)

Applies the "?" operator. A return of Ok(t) means that the execution should continue normally, and the result of ? is the value t. A return of Err(e) means that execution should branch to the innermost enclosing catch, or return from the function.

fn from_ok(v: T) -> Option<T>

🔬 This is a nightly-only experimental API. (try_trait #42327)

Wrap an OK value to construct the composite result. For example, Result::Ok(x) and Result::from_ok(x) are equivalent.

fn from_error(NoneError) -> Option<T>

🔬 This is a nightly-only experimental API. (try_trait #42327)

Wrap an error value to construct the composite result. For example, Result::Err(x) and Result::from_error(x) are equivalent.

impl<T> Debug for Option<T> where
    T: Debug, 

fn fmt(&self, f: &mut Formatter) -> Result<(), Error>

Formats the value using the given formatter.

impl<T> Copy for Option<T> where
    T: Copy, 

impl<T> Ord for Option<T> where
    T: Ord, 

fn cmp(&self, other: &Option<T>) -> Ordering

This method returns an Ordering between self and other.

fn max(self, other: Self) -> Self

Compares and returns the maximum of two values.

fn min(self, other: Self) -> Self

Compares and returns the minimum of two values.

fn clamp(self, min: Self, max: Self) -> Self

🔬 This is a nightly-only experimental API. (clamp #44095)

Restrict a value to a certain interval.

impl<T> PartialEq<Option<T>> for Option<T> where
    T: PartialEq<T>, 

fn eq(&self, other: &Option<T>) -> bool

This method tests for self and other values to be equal, and is used by ==.

fn ne(&self, other: &Option<T>) -> bool

This method tests for !=.

impl<'a, T> IntoIterator for &'a mut Option<T>

type Item = &'a mut T

The type of the elements being iterated over.

type IntoIter = IterMut<'a, T>

Which kind of iterator are we turning this into?

Important traits for IterMut<'a, A>

impl<'a, A> Iterator for IterMut<'a, A> type Item = &'a mut A;

fn into_iter(self) -> IterMut<'a, T>

Creates an iterator from a value.

impl<'a, T> IntoIterator for &'a Option<T>

type Item = &'a T

The type of the elements being iterated over.

type IntoIter = Iter<'a, T>

Which kind of iterator are we turning this into?

Important traits for Iter<'a, A>

impl<'a, A> Iterator for Iter<'a, A> type Item = &'a A;

fn into_iter(self) -> Iter<'a, T>

Creates an iterator from a value.

impl<T> IntoIterator for Option<T>

type Item = T

The type of the elements being iterated over.

type IntoIter = IntoIter<T>

Which kind of iterator are we turning this into?

Important traits for IntoIter<A>

impl<A> Iterator for IntoIter<A> type Item = A;

fn into_iter(self) -> IntoIter<T>

Returns a consuming iterator over the possibly contained value.

Examples

let x = Some("string");
let v: Vec<&str> = x.into_iter().collect();
assert_eq!(v, ["string"]);

let x = None;
let v: Vec<&str> = x.into_iter().collect();
assert!(v.is_empty());