Enum core::option::Option 1.0.0
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[src]
pub enum Option<T> {
None,
Some(T),
}The Option type. See the module level documentation for more.
Variants
NoneNo value
Some(T)Some value T
Methods
impl<T> Option<T>[src]
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);Run
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);Run
fn as_ref(&self) -> Option<&T>
Converts from Option<T> to Option<&T>.
Examples
Convert 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 num_as_str: Option<String> = Some("10".to_string()); // First, cast `Option<String>` to `Option<&String>` with `as_ref`, // then consume *that* with `map`, leaving `num_as_str` on the stack. let num_as_int: Option<usize> = num_as_str.as_ref().map(|n| n.len()); println!("still can print num_as_str: {:?}", num_as_str);Run
fn as_mut(&mut self) -> Option<&mut T>
Converts from 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
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");Run
let x: Option<&str> = None; x.expect("the world is ending"); // panics with `the world is ending`Run
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");Run
let x: Option<&str> = None; assert_eq!(x.unwrap(), "air"); // failsRun
fn unwrap_or(self, def: T) -> T
Returns the contained value or a default.
Examples
assert_eq!(Some("car").unwrap_or("bike"), "car"); assert_eq!(None.unwrap_or("bike"), "bike");Run
fn unwrap_or_else<F: FnOnce() -> T>(self, f: F) -> 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);Run
fn map<U, F: FnOnce(T) -> U>(self, f: F) -> Option<U>
Maps an Option<T> to Option<U> by applying a function to a contained value.
Examples
Convert 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
fn map_or<U, F: FnOnce(T) -> U>(self, default: U, f: F) -> U
Applies a function to the contained value (if any),
or returns a 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);Run
fn map_or_else<U, D: FnOnce() -> U, F: FnOnce(T) -> U>(
self,
default: D,
f: F
) -> U
self,
default: D,
f: F
) -> 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
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).
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
fn ok_or_else<E, F: FnOnce() -> E>(self, err: F) -> Result<T, 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
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);Run
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);Run
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);Run
fn and_then<U, F: FnOnce(T) -> Option<U>>(self, f: F) -> 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
fn or(self, optb: Option<T>) -> Option<T>
Returns the option if it contains a value, otherwise returns optb.
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
fn or_else<F: FnOnce() -> Option<T>>(self, f: F) -> 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
fn get_or_insert(&mut self, v: T) -> &mut T1.20.0
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
fn get_or_insert_with<F: FnOnce() -> T>(&mut self, f: F) -> &mut T1.20.0
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
fn take(&mut self) -> Option<T>
impl<'a, T: Clone> Option<&'a T>[src]
impl<T: Default> Option<T>[src]
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
Convert 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
Trait Implementations
impl<T: Clone> Clone for Option<T>[src]
fn clone(&self) -> Option<T>
Returns a copy of the value. Read more
fn clone_from(&mut self, source: &Self)
Performs copy-assignment from source. Read more
impl<T: Copy> Copy for Option<T>[src]
impl<T: PartialEq> PartialEq for Option<T>[src]
fn eq(&self, __arg_0: &Option<T>) -> bool
This method tests for self and other values to be equal, and is used by ==. Read more
fn ne(&self, __arg_0: &Option<T>) -> bool
This method tests for !=.
impl<T: PartialOrd> PartialOrd for Option<T>[src]
fn partial_cmp(&self, __arg_0: &Option<T>) -> Option<Ordering>
This method returns an ordering between self and other values if one exists. Read more
fn lt(&self, __arg_0: &Option<T>) -> bool
This method tests less than (for self and other) and is used by the < operator. Read more
fn le(&self, __arg_0: &Option<T>) -> bool
This method tests less than or equal to (for self and other) and is used by the <= operator. Read more
fn gt(&self, __arg_0: &Option<T>) -> bool
This method tests greater than (for self and other) and is used by the > operator. Read more
fn ge(&self, __arg_0: &Option<T>) -> bool
This method tests greater than or equal to (for self and other) and is used by the >= operator. Read more
impl<T: Eq> Eq for Option<T>[src]
impl<T: Ord> Ord for Option<T>[src]
fn cmp(&self, __arg_0: &Option<T>) -> Ordering
This method returns an Ordering between self and other. Read more
fn max(self, other: Self) -> Self where
Self: Sized,
Self: Sized,
Compares and returns the maximum of two values. Read more
fn min(self, other: Self) -> Self where
Self: Sized,
Self: Sized,
Compares and returns the minimum of two values. Read more
impl<T: Debug> Debug for Option<T>[src]
impl<T: Hash> Hash for Option<T>[src]
fn hash<__HT: Hasher>(&self, __arg_0: &mut __HT)
Feeds this value into the given [Hasher]. Read more
fn hash_slice<H: Hasher>(data: &[Self], state: &mut H) where
Self: Sized, 1.3.0
Self: Sized,
Feeds a slice of this type into the given [Hasher]. Read more
impl<T> Default for Option<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>
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>
Creates an iterator from a value. Read more
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>
Creates an iterator from a value. Read more
impl<T> From<T> for Option<T>1.12.0[src]
impl<A, V: FromIterator<A>> FromIterator<Option<A>> for Option<V>[src]
fn from_iter<I: IntoIterator<Item = Option<A>>>(iter: I) -> Option<V>
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.
Here is an example which increments every integer in a vector, checking for overflow:
use std::u16; let v = vec![1, 2]; let res: Option<Vec<u16>> = v.iter().map(|&x: &u16| if x == u16::MAX { None } else { Some(x + 1) } ).collect(); assert!(res == Some(vec![2, 3]));Run