Enum core::option::Option1.0.0 [] [src]

pub enum Option<T> {
    None,
    Some(T),
}
1.0.0

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

Variants

None

No value

Some

Some value T

Methods

impl<T> Option<T>
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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.

fn main() { 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); }
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 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: FnOnce() -> T>(self, f: F) -> 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: 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:

fn main() { 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)); }
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: 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

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: FnOnce() -> U, F: FnOnce(T) -> U>(self, default: D, f: F) -> 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: 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

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: 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 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: 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 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);

impl<'a, T: Clone> Option<&'a T>
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fn cloned(self) -> Option<T>

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

impl<T: Default> Option<T>
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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.

fn main() { 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); }
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);

Trait Implementations

impl<T> Default for Option<T>
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fn default() -> Option<T>

Returns the "default value" for a type. Read more

impl<T> IntoIterator for Option<T>
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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>

Returns a consuming iterator over the possibly contained value.

Examples

fn main() { 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()); }
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());

impl<'a, T> IntoIterator for &'a Option<T>
1.4.0
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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
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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<A, V: FromIterator<A>> FromIterator<Option<A>> for Option<V>
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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:

fn main() { 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))); }
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)));

Derived Implementations

impl<T: Hash> Hash for Option<T>
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fn hash<__HT: Hasher>(&self, __arg_0: &mut __HT)

Feeds this value into the state given, updating the hasher as necessary.

fn hash_slice<H: Hasher>(data: &[Self], state: &mut H) where Self: Sized
1.3.0

Feeds a slice of this type into the state provided.

impl<T: Debug> Debug for Option<T>
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fn fmt(&self, __arg_0: &mut Formatter) -> Result

Formats the value using the given formatter.

impl<T: Ord> Ord for Option<T>
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fn cmp(&self, __arg_0: &Option<T>) -> Ordering

This method returns an Ordering between self and other. Read more

impl<T: Eq> Eq for Option<T>
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impl<T: PartialOrd> PartialOrd for Option<T>
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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: PartialEq> PartialEq for Option<T>
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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: Copy> Copy for Option<T>
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impl<T: Clone> Clone for Option<T>
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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