Enum std::option::Option
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[src]
pub enum Option<T> {
None,
Some(T),
}The Option type. See the module level documentation for more.
Variants
None | No value |
Some | Some value |
Methods
impl<T> Option<T>
fn is_some(&self) -> bool
Returns true if the option is a Some value
Examples
fn main() { let x: Option<u32> = Some(2); assert_eq!(x.is_some(), true); let x: Option<u32> = None; assert_eq!(x.is_some(), false); }let x: Option<u32> = Some(2); assert_eq!(x.is_some(), true); let x: Option<u32> = None; assert_eq!(x.is_some(), false);
fn is_none(&self) -> bool
Returns true if the option is a None value
Examples
fn main() { let x: Option<u32> = Some(2); assert_eq!(x.is_none(), false); let x: Option<u32> = None; assert_eq!(x.is_none(), true); }let x: Option<u32> = Some(2); assert_eq!(x.is_none(), false); let x: Option<u32> = None; assert_eq!(x.is_none(), true);
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);
fn as_mut(&mut self) -> Option<&mut T>
Converts from Option<T> to Option<&mut T>
Examples
fn main() { let mut x = Some(2); match x.as_mut() { Some(v) => *v = 42, None => {}, } assert_eq!(x, Some(42)); }let mut x = Some(2); match x.as_mut() { Some(v) => *v = 42, None => {}, } assert_eq!(x, Some(42));
fn as_mut_slice(&mut self) -> &mut [T]
Deprecated since 1.4.0
: niche API, unclear of usefulness
Converts from Option<T> to &mut [T] (without copying)
Examples
#![feature(as_slice)] fn main() { #![allow(deprecated)] let mut x = Some("Diamonds"); { let v = x.as_mut_slice(); assert!(v == ["Diamonds"]); v[0] = "Dirt"; assert!(v == ["Dirt"]); } assert_eq!(x, Some("Dirt")); }#![feature(as_slice)] let mut x = Some("Diamonds"); { let v = x.as_mut_slice(); assert!(v == ["Diamonds"]); v[0] = "Dirt"; assert!(v == ["Dirt"]); } assert_eq!(x, Some("Dirt"));
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
fn main() { let x = Some("value"); assert_eq!(x.expect("the world is ending"), "value"); }let x = Some("value"); assert_eq!(x.expect("the world is ending"), "value");fn main() { let x: Option<&str> = None; x.expect("the world is ending"); // panics with `the world is ending` }
let x: Option<&str> = None; x.expect("the world is ending"); // panics with `the world is ending`
fn unwrap(self) -> T
Moves the value v out of the Option<T> if it is Some(v).
Panics
Panics if the self value equals None.
Safety note
In general, because this function may panic, its use is discouraged.
Instead, prefer to use pattern matching and handle the None
case explicitly.
Examples
fn main() { let x = Some("air"); assert_eq!(x.unwrap(), "air"); }let x = Some("air"); assert_eq!(x.unwrap(), "air");fn main() { let x: Option<&str> = None; assert_eq!(x.unwrap(), "air"); // fails }
let x: Option<&str> = None; assert_eq!(x.unwrap(), "air"); // fails
fn unwrap_or(self, def: T) -> T
Returns the contained value or a default.
Examples
fn main() { assert_eq!(Some("car").unwrap_or("bike"), "car"); assert_eq!(None.unwrap_or("bike"), "bike"); }assert_eq!(Some("car").unwrap_or("bike"), "car"); assert_eq!(None.unwrap_or("bike"), "bike");
fn unwrap_or_else<F>(self, f: F) -> T where F: FnOnce() -> T
Returns the contained value or computes it from a closure.
Examples
fn main() { let k = 10; assert_eq!(Some(4).unwrap_or_else(|| 2 * k), 4); assert_eq!(None.unwrap_or_else(|| 2 * k), 20); }let k = 10; assert_eq!(Some(4).unwrap_or_else(|| 2 * k), 4); assert_eq!(None.unwrap_or_else(|| 2 * k), 20);
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
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));
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 a default (if not).
Examples
fn main() { 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); }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);
fn map_or_else<U, D, F>(self, default: D, f: F) -> U where F: FnOnce(T) -> U, D: FnOnce() -> U
Applies a function to the contained value (if any),
or computes a default (if not).
Examples
fn main() { 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); }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);
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
fn main() { 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)); }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));
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
fn main() { 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)); }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));
fn iter(&self) -> Iter<T>
Returns an iterator over the possibly contained value.
Examples
fn main() { let x = Some(4); assert_eq!(x.iter().next(), Some(&4)); let x: Option<u32> = None; assert_eq!(x.iter().next(), None); }let x = Some(4); assert_eq!(x.iter().next(), Some(&4)); let x: Option<u32> = None; assert_eq!(x.iter().next(), None);
fn iter_mut(&mut self) -> IterMut<T>
Returns a mutable iterator over the possibly contained value.
Examples
fn main() { 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); }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);
fn and<U>(self, optb: Option<U>) -> Option<U>
Returns None if the option is None, otherwise returns optb.
Examples
fn main() { 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); }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);
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 main() { 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); }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);
fn or(self, optb: Option<T>) -> Option<T>
Returns the option if it contains a value, otherwise returns optb.
Examples
fn main() { 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); }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);
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 main() { 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); }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);
fn take(&mut self) -> Option<T>
Takes the value out of the option, leaving a None in its place.
Examples
fn main() { let mut x = Some(2); x.take(); assert_eq!(x, None); let mut x: Option<u32> = None; x.take(); assert_eq!(x, None); }let mut x = Some(2); x.take(); assert_eq!(x, None); let mut x: Option<u32> = None; x.take(); assert_eq!(x, None);
fn as_slice(&self) -> &[T]
Deprecated since 1.4.0
: niche API, unclear of usefulness
Converts from Option<T> to &[T] (without copying)
impl<'a, T> Option<&'a T> where T: Clone
fn cloned(self) -> Option<T>
Maps an Option<&T> to an Option<T> by cloning the contents of the
option.
impl<T> Option<T> where T: Default
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);