1.0.0[−][src]Enum std::option::Option
The Option type. See the module level documentation for more.
Variants
No value
Some value T
Methods
impl<T> Option<T>[src]
#[must_use = "if you intended to assert that this has a value, consider `.unwrap()` instead"]
pub fn is_some(&self) -> bool[src]
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);Run
#[must_use = "if you intended to assert that this doesn\'t have a value, consider `.and_then(|| panic!(\"`Option` had a value when expected `None`\"))` instead"]
pub fn is_none(&self) -> bool[src]
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);Run
#[must_use]
pub fn contains<U>(&self, x: &U) -> bool where
U: PartialEq<T>, [src]
U: PartialEq<T>,
Returns true if the option is a Some value containing the given value.
Examples
#![feature(option_result_contains)] let x: Option<u32> = Some(2); assert_eq!(x.contains(&2), true); let x: Option<u32> = Some(3); assert_eq!(x.contains(&2), false); let x: Option<u32> = None; assert_eq!(x.contains(&2), false);Run
pub fn as_ref(&self) -> Option<&T>[src]
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);Run
pub fn as_mut(&mut self) -> Option<&mut T>[src]
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));Run
pub fn as_pin_ref(self: Pin<&Option<T>>) -> Option<Pin<&T>>1.33.0[src]
pub fn as_pin_mut(self: Pin<&mut Option<T>>) -> Option<Pin<&mut T>>1.33.0[src]
pub fn expect(self, msg: &str) -> T[src]
Returns the contained Some value, consuming the self value.
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("fruits are healthy"), "value");Run
let x: Option<&str> = None; x.expect("fruits are healthy"); // panics with `fruits are healthy`Run
pub fn unwrap(self) -> T[src]
Returns the contained Some value, consuming the self value.
Because this function may panic, its use is generally discouraged.
Instead, prefer to use pattern matching and handle the None
case explicitly, or call unwrap_or, unwrap_or_else, or
unwrap_or_default.
Panics
Panics if the self value equals None.
Examples
let x = Some("air"); assert_eq!(x.unwrap(), "air");Run
let x: Option<&str> = None; assert_eq!(x.unwrap(), "air"); // failsRun
pub fn unwrap_or(self, default: T) -> T[src]
Returns the contained Some value or a provided 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");Run
pub fn unwrap_or_else<F>(self, f: F) -> T where
F: FnOnce() -> T, [src]
F: FnOnce() -> T,
Returns the contained Some 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);Run
pub fn map<U, F>(self, f: F) -> Option<U> where
F: FnOnce(T) -> U, [src]
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));Run
pub fn map_or<U, F>(self, default: U, f: F) -> U where
F: FnOnce(T) -> U, [src]
F: FnOnce(T) -> U,
Applies a function to the contained value (if any), or returns the provided default (if not).
Arguments passed to map_or are eagerly evaluated; if you are passing
the result of a function call, it is recommended to use map_or_else,
which is lazily evaluated.
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);Run
pub fn map_or_else<U, D, F>(self, default: D, f: F) -> U where
D: FnOnce() -> U,
F: FnOnce(T) -> U, [src]
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);Run
pub fn ok_or<E>(self, err: E) -> Result<T, E>[src]
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));Run
pub fn ok_or_else<E, F>(self, err: F) -> Result<T, E> where
F: FnOnce() -> E, [src]
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));Run
pub fn iter(&self) -> Iter<T>[src]
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);Run
pub fn iter_mut(&mut self) -> IterMut<T>[src]
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);Run
pub fn and<U>(self, optb: Option<U>) -> Option<U>[src]
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);Run
pub fn and_then<U, F>(self, f: F) -> Option<U> where
F: FnOnce(T) -> Option<U>, [src]
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);Run
pub fn filter<P>(self, predicate: P) -> Option<T> where
P: FnOnce(&T) -> bool, 1.27.0[src]
P: FnOnce(&T) -> bool,
Returns None if the option is None, otherwise calls predicate
with the wrapped value and returns:
Some(t)ifpredicatereturnstrue(wheretis the wrapped value), andNoneifpredicatereturnsfalse.
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));Run
pub fn or(self, optb: Option<T>) -> Option<T>[src]
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);Run
pub fn or_else<F>(self, f: F) -> Option<T> where
F: FnOnce() -> Option<T>, [src]
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);Run
pub fn xor(self, optb: Option<T>) -> Option<T>1.37.0[src]
Returns Some if exactly one of self, optb is Some, otherwise returns None.
Examples
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);Run
pub fn get_or_insert(&mut self, v: T) -> &mut T1.20.0[src]
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));Run
pub fn get_or_insert_with<F>(&mut self, f: F) -> &mut T where
F: FnOnce() -> T, 1.20.0[src]
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));Run
pub fn take(&mut self) -> Option<T>[src]
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);Run
pub fn replace(&mut self, value: T) -> Option<T>1.31.0[src]
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);Run
impl<'_, T> Option<&'_ T> where
T: Copy, [src]
T: Copy,
impl<'_, T> Option<&'_ mut T> where
T: Copy, [src]
T: Copy,
impl<'_, T> Option<&'_ T> where
T: Clone, [src]
T: Clone,
impl<'_, T> Option<&'_ mut T> where
T: Clone, [src]
T: Clone,
impl<T> Option<T> where
T: Debug, [src]
T: Debug,
pub fn expect_none(self, msg: &str)[src]
🔬 This is a nightly-only experimental API. (option_expect_none #62633)
newly added
Consumes self while expecting None and returning nothing.
Panics
Panics if the value is a Some, with a panic message including the
passed message, and the content of the Some.
Examples
#![feature(option_expect_none)] use std::collections::HashMap; let mut squares = HashMap::new(); for i in -10..=10 { // This will not panic, since all keys are unique. squares.insert(i, i * i).expect_none("duplicate key"); }Run
#![feature(option_expect_none)] use std::collections::HashMap; let mut sqrts = HashMap::new(); for i in -10..=10 { // This will panic, since both negative and positive `i` will // insert the same `i * i` key, returning the old `Some(i)`. sqrts.insert(i * i, i).expect_none("duplicate key"); }Run
pub fn unwrap_none(self)[src]
🔬 This is a nightly-only experimental API. (option_unwrap_none #62633)
newly added
Consumes self while expecting None and returning nothing.
Panics
Panics if the value is a Some, with a custom panic message provided
by the Some's value.
Examples
#![feature(option_unwrap_none)] use std::collections::HashMap; let mut squares = HashMap::new(); for i in -10..=10 { // This will not panic, since all keys are unique. squares.insert(i, i * i).unwrap_none(); }Run
#![feature(option_unwrap_none)] use std::collections::HashMap; let mut sqrts = HashMap::new(); for i in -10..=10 { // This will panic, since both negative and positive `i` will // insert the same `i * i` key, returning the old `Some(i)`. sqrts.insert(i * i, i).unwrap_none(); }Run
impl<T> Option<T> where
T: Default, [src]
T: Default,
pub fn unwrap_or_default(self) -> T[src]
Returns the contained Some 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);Run
impl<T> Option<T> where
T: Deref, [src]
T: Deref,
pub fn as_deref(&self) -> Option<&<T as Deref>::Target>1.40.0[src]
Converts from Option<T> (or &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.
Examples
let x: Option<String> = Some("hey".to_owned()); assert_eq!(x.as_deref(), Some("hey")); let x: Option<String> = None; assert_eq!(x.as_deref(), None);Run
impl<T> Option<T> where
T: DerefMut, [src]
T: DerefMut,
pub fn as_deref_mut(&mut self) -> Option<&mut <T as Deref>::Target>1.40.0[src]
Converts from Option<T> (or &mut Option<T>) to Option<&mut T::Target>.
Leaves the original Option in-place, creating a new one containing a mutable reference to
the inner type's Deref::Target type.
Examples
let mut x: Option<String> = Some("hey".to_owned()); assert_eq!(x.as_deref_mut().map(|x| { x.make_ascii_uppercase(); x }), Some("HEY".to_owned().as_mut_str()));Run
impl<T, E> Option<Result<T, E>>[src]
pub fn transpose(self) -> Result<Option<T>, E>1.33.0[src]
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());Run
impl<T> Option<Option<T>>[src]
pub fn flatten(self) -> Option<T>1.40.0[src]
Converts from Option<Option<T>> to Option<T>
Examples
Basic usage:
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());Run
Flattening once only removes one level of nesting:
let x: Option<Option<Option<u32>>> = Some(Some(Some(6))); assert_eq!(Some(Some(6)), x.flatten()); assert_eq!(Some(6), x.flatten().flatten());Run
Trait Implementations
impl<T> Clone for Option<T> where
T: Clone, [src]
T: Clone,
impl<T> Copy for Option<T> where
T: Copy, [src]
T: Copy,
impl<T> Debug for Option<T> where
T: Debug, [src]
T: Debug,
impl<T> Default for Option<T>[src]
impl<T> Eq for Option<T> where
T: Eq, [src]
T: Eq,
impl<'a, T> From<&'a Option<T>> for Option<&'a T>1.30.0[src]
impl<'a, T> From<&'a mut Option<T>> for Option<&'a mut T>1.30.0[src]
impl<T> From<T> for Option<T>1.12.0[src]
impl<A, V> FromIterator<Option<A>> for Option<V> where
V: FromIterator<A>, [src]
V: FromIterator<A>,
fn from_iter<I>(iter: I) -> Option<V> where
I: IntoIterator<Item = Option<A>>, [src]
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]));Run
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);Run
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);Run
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> Hash for Option<T> where
T: Hash, [src]
T: Hash,
fn hash<__H>(&self, state: &mut __H) where
__H: Hasher, [src]
__H: Hasher,
fn hash_slice<H>(data: &[Self], state: &mut H) where
H: Hasher, 1.3.0[src]
H: Hasher,
impl<'a, T> IntoIterator for &'a mut Option<T>1.4.0[src]
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?
fn into_iter(self) -> IterMut<'a, T>[src]
impl<T> IntoIterator for Option<T>[src]
type Item = T
The type of the elements being iterated over.
type IntoIter = IntoIter<T>
Which kind of iterator are we turning this into?
fn into_iter(self) -> IntoIter<T>[src]
impl<'a, T> IntoIterator for &'a Option<T>1.4.0[src]
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?
fn into_iter(self) -> Iter<'a, T>[src]
impl<T> Ord for Option<T> where
T: Ord, [src]
T: Ord,
fn cmp(&self, other: &Option<T>) -> Ordering[src]
#[must_use]
fn max(self, other: Self) -> Self1.21.0[src]
#[must_use]
fn min(self, other: Self) -> Self1.21.0[src]
#[must_use]
fn clamp(self, min: Self, max: Self) -> Self[src]
impl<T> PartialEq<Option<T>> for Option<T> where
T: PartialEq<T>, [src]
T: PartialEq<T>,
impl<T> PartialOrd<Option<T>> for Option<T> where
T: PartialOrd<T>, [src]
T: PartialOrd<T>,
fn partial_cmp(&self, other: &Option<T>) -> Option<Ordering>[src]
fn lt(&self, other: &Option<T>) -> bool[src]
fn le(&self, other: &Option<T>) -> bool[src]
fn gt(&self, other: &Option<T>) -> bool[src]
fn ge(&self, other: &Option<T>) -> bool[src]
impl<T, U> Product<Option<U>> for Option<T> where
T: Product<U>, 1.37.0[src]
T: Product<U>,
fn product<I>(iter: I) -> Option<T> where
I: Iterator<Item = Option<U>>, [src]
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<T> StructuralEq for Option<T>[src]
impl<T> StructuralPartialEq for Option<T>[src]
impl<T, U> Sum<Option<U>> for Option<T> where
T: Sum<U>, 1.37.0[src]
T: Sum<U>,
fn sum<I>(iter: I) -> Option<T> where
I: Iterator<Item = Option<U>>, [src]
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));Run
impl<T> Try for Option<T>[src]
Auto Trait Implementations
impl<T> RefUnwindSafe for Option<T> where
T: RefUnwindSafe,
T: RefUnwindSafe,
impl<T> Send for Option<T> where
T: Send,
T: Send,
impl<T> Sync for Option<T> where
T: Sync,
T: Sync,
impl<T> Unpin for Option<T> where
T: Unpin,
T: Unpin,
impl<T> UnwindSafe for Option<T> where
T: UnwindSafe,
T: UnwindSafe,
Blanket Implementations
impl<T> Any for T where
T: 'static + ?Sized, [src]
T: 'static + ?Sized,
impl<T> Borrow<T> for T where
T: ?Sized, [src]
T: ?Sized,
impl<T> BorrowMut<T> for T where
T: ?Sized, [src]
T: ?Sized,
fn borrow_mut(&mut self) -> &mut T[src]
impl<T> From<!> for T[src]
impl<T> From<T> for T[src]
impl<T, U> Into<U> for T where
U: From<T>, [src]
U: From<T>,
impl<I> IntoIterator for I where
I: Iterator, [src]
I: Iterator,
type Item = <I as Iterator>::Item
The type of the elements being iterated over.
type IntoIter = I
Which kind of iterator are we turning this into?
fn into_iter(self) -> I[src]
impl<T> ToOwned for T where
T: Clone, [src]
T: Clone,
type Owned = T
The resulting type after obtaining ownership.
fn to_owned(&self) -> T[src]
fn clone_into(&self, target: &mut T)[src]
impl<T, U> TryFrom<U> for T where
U: Into<T>, [src]
U: Into<T>,
type Error = Infallible
The type returned in the event of a conversion error.
fn try_from(value: U) -> Result<T, <T as TryFrom<U>>::Error>[src]
impl<T, U> TryInto<U> for T where
U: TryFrom<T>, [src]
U: TryFrom<T>,