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
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
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
let mut x = Some(2); match x.as_mut() { Some(v) => *v = 42, None => {}, } assert_eq!(x, Some(42));
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`
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
let x = Some("air"); assert_eq!(x.unwrap(), "air");
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
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
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
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 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);
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));
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));
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);
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);
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);
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);
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);
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);
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); x.take(); assert_eq!(x, None); let mut x: Option<u32> = None; x.take(); assert_eq!(x, None);
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);