# Enum std::option::Option

1.0.0 · source ·

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

Expand description

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

## Variants§

§

### None

No value.

§

### Some(T)

Some value of type `T`.

## Implementations§

source§

### impl<T> Option<T>

const: 1.48.0 · source

#### pub const 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
1.70.0 · source

#### pub fn is_some_and(self, f: impl FnOnce(T) -> bool) -> bool

Returns `true` if the option is a `Some` and the value inside of it matches a predicate.

##### Examples

    let x: Option<u32> = Some(2);
    assert_eq!(x.is_some_and(|x| x > 1), true);

    let x: Option<u32> = Some(0);
    assert_eq!(x.is_some_and(|x| x > 1), false);

    let x: Option<u32> = None;
    assert_eq!(x.is_some_and(|x| x > 1), false);

Run
const: 1.48.0 · source

#### pub const 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
const: 1.48.0 · source

#### pub const fn as_ref(&self) -> Option<&T>

Converts from `&Option<T>` to `Option<&T>`.

##### Examples

Calculates the length of an `Option<String>` as an `Option<usize>` without moving the `String`. 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:?}");

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const: unstable · source

#### pub fn as_mut(&mut self) -> Option<&mut T>

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
1.33.0 (const: unstable) · source

#### pub fn as_pin_ref(self: Pin<&Option<T>>) -> Option<Pin<&T>>

Converts from `Pin<&Option<T>>` to `Option<Pin<&T>>`.

1.33.0 (const: unstable) · source

#### pub fn as_pin_mut(self: Pin<&mut Option<T>>) -> Option<Pin<&mut T>>

Converts from `Pin<&mut Option<T>>` to `Option<Pin<&mut T>>`.

1.75.0 · source

#### pub fn as_slice(&self) -> &[T]

Returns a slice of the contained value, if any. If this is `None`, an empty slice is returned. This can be useful to have a single type of iterator over an `Option` or slice.

Note: Should you have an `Option<&T>` and wish to get a slice of `T`, you can unpack it via `opt.map_or(&[], std::slice::from_ref)`.

##### Examples

    assert_eq!(
        [Some(1234).as_slice(), None.as_slice()],
        [&[1234][..], &[][..]],
    );

Run

The inverse of this function is (discounting borrowing) `[_]::first`:

    for i in [Some(1234_u16), None] {
        assert_eq!(i.as_ref(), i.as_slice().first());
    }

Run
1.75.0 · source

#### pub fn as_mut_slice(&mut self) -> &mut [T]

Returns a mutable slice of the contained value, if any. If this is `None`, an empty slice is returned. This can be useful to have a single type of iterator over an `Option` or slice.

Note: Should you have an `Option<&mut T>` instead of a `&mut Option<T>`, which this method takes, you can obtain a mutable slice via `opt.map_or(&mut [], std::slice::from_mut)`.

##### Examples

    assert_eq!(
        [Some(1234).as_mut_slice(), None.as_mut_slice()],
        [&mut [1234][..], &mut [][..]],
    );

Run

The result is a mutable slice of zero or one items that points into our original `Option`:

    let mut x = Some(1234);
    x.as_mut_slice()[0] += 1;
    assert_eq!(x, Some(1235));

Run

The inverse of this method (discounting borrowing) is `[_]::first_mut`:

    assert_eq!(Some(123).as_mut_slice().first_mut(), Some(&mut 123))

Run
const: unstable · source

#### pub fn expect(self, msg: &str) -> T

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");

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ⓘ

    let x: Option<&str> = None;
    x.expect("fruits are healthy"); // panics with `fruits are healthy`

Run

##### Recommended Message Style

We recommend that `expect` messages are used to describe the reason you expect the `Option` should be `Some`.

ⓘ

    let item = slice.get(0)
        .expect("slice should not be empty");

Run

Hint: If you’re having trouble remembering how to phrase expect error messages remember to focus on the word “should” as in “env variable should be set by blah” or “the given binary should be available and executable by the current user”.

For more detail on expect message styles and the reasoning behind our recommendation please refer to the section on “Common Message Styles” in the `std::error` module docs.

const: unstable · source

#### pub fn unwrap(self) -> T

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"); // fails

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source

#### pub fn unwrap_or(self, default: T) -> T

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
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#### pub fn unwrap_or_else<F>(self, f: F) -> Twhere 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);

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source

#### pub fn unwrap_or_default(self) -> Twhere T: Default,

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

    let x: Option<u32> = None;
    let y: Option<u32> = Some(12);

    assert_eq!(x.unwrap_or_default(), 0);
    assert_eq!(y.unwrap_or_default(), 12);

Run
1.58.0 (const: unstable) · source

#### pub unsafe fn unwrap_unchecked(self) -> T

Returns the contained `Some` value, consuming the `self` value, without checking that the value is not `None`.

##### Safety

Calling this method on `None` is undefined behavior.

##### Examples

    let x = Some("air");
    assert_eq!(unsafe { x.unwrap_unchecked() }, "air");

Run

    let x: Option<&str> = None;
    assert_eq!(unsafe { x.unwrap_unchecked() }, "air"); // Undefined behavior!

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source

#### pub 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 (if `Some`) or returns `None` (if `None`).

##### Examples

Calculates the length of an `Option<String>` as 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));

    let x: Option<&str> = None;
    assert_eq!(x.map(|s| s.len()), None);

Run
source

#### pub fn inspect<F>(self, f: F) -> Option<T>where F: FnOnce(&T),

🔬This is a nightly-only experimental API. (`result_option_inspect` #91345)

Calls the provided closure with a reference to the contained value (if `Some`).

##### Examples

    #![feature(result_option_inspect)]

    let v = vec![1, 2, 3, 4, 5];

    // prints "got: 4"
    let x: Option<&usize> = v.get(3).inspect(|x| println!("got: {x}"));

    // prints nothing
    let x: Option<&usize> = v.get(5).inspect(|x| println!("got: {x}"));

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source

#### pub fn map_or<U, F>(self, default: U, f: F) -> Uwhere F: FnOnce(T) -> U,

Returns the provided default result (if none), or applies a function to the contained value (if any).

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
source

#### pub fn map_or_else<U, D, F>(self, default: D, f: F) -> Uwhere D: FnOnce() -> U, F: FnOnce(T) -> U,

Computes a default function result (if none), or applies a different function to the contained value (if any).

##### Basic 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

##### Handling a Result-based fallback

A somewhat common occurrence when dealing with optional values in combination with `Result<T, E>` is the case where one wants to invoke a fallible fallback if the option is not present. This example parses a command line argument (if present), or the contents of a file to an integer. However, unlike accessing the command line argument, reading the file is fallible, so it must be wrapped with `Ok`.

    let v: u64 = std::env::args()
       .nth(1)
       .map_or_else(|| std::fs::read_to_string("/etc/someconfig.conf"), Ok)?
       .parse()?;

Run
source

#### pub 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)`.

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));

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source

#### pub 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));

Run
1.40.0 · source

#### pub fn as_deref(&self) -> Option<&<T as Deref>::Target>where T: Deref,

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
1.40.0 · source

#### pub fn as_deref_mut(&mut self) -> Option<&mut <T as Deref>::Target>where T: DerefMut,

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
const: unstable · source

#### pub 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);

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source

#### pub 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);

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source

#### pub fn and<U>(self, optb: Option<U>) -> Option<U>

Returns `None` if the option is `None`, otherwise returns `optb`.

Arguments passed to `and` are eagerly evaluated; if you are passing the result of a function call, it is recommended to use `and_then`, which is lazily evaluated.

##### 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);

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#### pub 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_then_to_string(x: u32) -> Option<String> {
        x.checked_mul(x).map(|sq| sq.to_string())
    }

    assert_eq!(Some(2).and_then(sq_then_to_string), Some(4.to_string()));
    assert_eq!(Some(1_000_000).and_then(sq_then_to_string), None); // overflowed!
    assert_eq!(None.and_then(sq_then_to_string), None);

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Often used to chain fallible operations that may return `None`.

    let arr_2d = [["A0", "A1"], ["B0", "B1"]];

    let item_0_1 = arr_2d.get(0).and_then(|row| row.get(1));
    assert_eq!(item_0_1, Some(&"A1"));

    let item_2_0 = arr_2d.get(2).and_then(|row| row.get(0));
    assert_eq!(item_2_0, None);

Run
1.27.0 · source

#### pub fn filter<P>(self, predicate: P) -> Option<T>where P: FnOnce(&T) -> bool,

Returns `None` if the option is `None`, otherwise calls `predicate` with the wrapped value and returns:

- `Some(t)` if `predicate` returns `true` (where `t` is the wrapped value), and
- `None` if `predicate` returns `false`.

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));

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source

#### pub fn or(self, optb: Option<T>) -> Option<T>

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);

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source

#### pub 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);

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1.37.0 · source

#### pub fn xor(self, optb: Option<T>) -> Option<T>

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);

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1.53.0 · source

#### pub fn insert(&mut self, value: T) -> &mut T

Inserts `value` into the option, then returns a mutable reference to it.

If the option already contains a value, the old value is dropped.

See also `Option::get_or_insert`, which doesn’t update the value if the option already contains `Some`.

##### Example

    let mut opt = None;
    let val = opt.insert(1);
    assert_eq!(*val, 1);
    assert_eq!(opt.unwrap(), 1);
    let val = opt.insert(2);
    assert_eq!(*val, 2);
    *val = 3;
    assert_eq!(opt.unwrap(), 3);

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1.20.0 · source

#### pub fn get_or_insert(&mut self, value: T) -> &mut T

Inserts `value` into the option if it is `None`, then returns a mutable reference to the contained value.

See also `Option::insert`, which updates the value even if the option already contains `Some`.

##### 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));

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#### pub fn get_or_insert_default(&mut self) -> &mut T where T: Default,

🔬This is a nightly-only experimental API. (`option_get_or_insert_default` #82901)

Inserts the default value into the option if it is `None`, then returns a mutable reference to the contained value.

##### Examples

    #![feature(option_get_or_insert_default)]

    let mut x = None;

    {
        let y: &mut u32 = x.get_or_insert_default();
        assert_eq!(y, &0);

        *y = 7;
    }

    assert_eq!(x, Some(7));

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1.20.0 · source

#### pub fn get_or_insert_with<F>(&mut self, f: F) -> &mut T where 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));

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const: unstable · source

#### pub 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);
    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);

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#### pub fn take_if<P>(&mut self, predicate: P) -> Option<T>where P: FnOnce(&mut T) -> bool,

🔬This is a nightly-only experimental API. (`option_take_if` #98934)

Takes the value out of the option, but only if the predicate evaluates to `true` on a mutable reference to the value.

In other words, replaces `self` with `None` if the predicate returns `true`. This method operates similar to `Option::take` but conditional.

##### Examples

    #![feature(option_take_if)]

    let mut x = Some(42);

    let prev = x.take_if(|v| if *v == 42 {
        *v += 1;
        false
    } else {
        false
    });
    assert_eq!(x, Some(43));
    assert_eq!(prev, None);

    let prev = x.take_if(|v| *v == 43);
    assert_eq!(x, None);
    assert_eq!(prev, Some(43));

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1.31.0 (const: unstable) · source

#### pub fn replace(&mut self, value: T) -> Option<T>

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);

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1.46.0 · source

#### pub fn zip<U>(self, other: Option<U>) -> Option<(T, U)>

Zips `self` with another `Option`.

If `self` is `Some(s)` and `other` is `Some(o)`, this method returns `Some((s, o))`. Otherwise, `None` is returned.

##### Examples

    let x = Some(1);
    let y = Some("hi");
    let z = None::<u8>;

    assert_eq!(x.zip(y), Some((1, "hi")));
    assert_eq!(x.zip(z), None);

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#### pub fn zip_with<U, F, R>(self, other: Option<U>, f: F) -> Option<R>where F: FnOnce(T, U) -> R,

🔬This is a nightly-only experimental API. (`option_zip` #70086)

Zips `self` and another `Option` with function `f`.

If `self` is `Some(s)` and `other` is `Some(o)`, this method returns `Some(f(s, o))`. Otherwise, `None` is returned.

##### Examples

    #![feature(option_zip)]

    #[derive(Debug, PartialEq)]
    struct Point {
        x: f64,
        y: f64,
    }

    impl Point {
        fn new(x: f64, y: f64) -> Self {
            Self { x, y }
        }
    }

    let x = Some(17.5);
    let y = Some(42.7);

    assert_eq!(x.zip_with(y, Point::new), Some(Point { x: 17.5, y: 42.7 }));
    assert_eq!(x.zip_with(None, Point::new), None);

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### impl<T, U> Option<(T, U)>

1.66.0 · source

#### pub fn unzip(self) -> (Option<T>, Option<U>)

Unzips an option containing a tuple of two options.

If `self` is `Some((a, b))` this method returns `(Some(a), Some(b))`. Otherwise, `(None, None)` is returned.

##### Examples

    let x = Some((1, "hi"));
    let y = None::<(u8, u32)>;

    assert_eq!(x.unzip(), (Some(1), Some("hi")));
    assert_eq!(y.unzip(), (None, None));

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### impl<T> Option<&T>

1.35.0 (const: unstable) · source

#### pub fn copied(self) -> Option<T>where T: Copy,

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

##### Examples

    let x = 12;
    let opt_x = Some(&x);
    assert_eq!(opt_x, Some(&12));
    let copied = opt_x.copied();
    assert_eq!(copied, Some(12));

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#### pub fn cloned(self) -> Option<T>where T: Clone,

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

##### Examples

    let x = 12;
    let opt_x = Some(&x);
    assert_eq!(opt_x, Some(&12));
    let cloned = opt_x.cloned();
    assert_eq!(cloned, Some(12));

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### impl<T> Option<&mut T>

1.35.0 (const: unstable) · source

#### pub fn copied(self) -> Option<T>where T: Copy,

Maps an `Option<&mut T>` to an `Option<T>` by copying the contents of the option.

##### Examples

    let mut x = 12;
    let opt_x = Some(&mut x);
    assert_eq!(opt_x, Some(&mut 12));
    let copied = opt_x.copied();
    assert_eq!(copied, Some(12));

Run
1.26.0 · source

#### pub fn cloned(self) -> Option<T>where T: Clone,

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

##### Examples

    let mut x = 12;
    let opt_x = Some(&mut x);
    assert_eq!(opt_x, Some(&mut 12));
    let cloned = opt_x.cloned();
    assert_eq!(cloned, Some(12));

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### impl<T, E> Option<Result<T, E>>

1.33.0 (const: unstable) · source

#### pub fn transpose(self) -> Result<Option<T>, E>

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());

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### impl<T> Option<Option<T>>

1.40.0 (const: unstable) · source

#### pub fn flatten(self) -> Option<T>

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());

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Flattening only removes one level of nesting at a time:

    let x: Option<Option<Option<u32>>> = Some(Some(Some(6)));
    assert_eq!(Some(Some(6)), x.flatten());
    assert_eq!(Some(6), x.flatten().flatten());

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## Trait Implementations§

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### impl<T> Clone for Option<T>where T: Clone,

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#### fn clone(&self) -> Option<T>

Returns a copy of the value. Read more
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#### fn clone_from(&mut self, source: &Option<T>)

Performs copy-assignment from `source`. Read more
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### impl<T> Debug for Option<T>where T: Debug,

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#### fn fmt(&self, f: &mut Formatter<'_>) -> Result<(), Error>

Formats the value using the given formatter. Read more
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### impl<T> Default for Option<T>

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#### fn default() -> Option<T>

Returns `None`.

##### Examples

    let opt: Option<u32> = Option::default();
    assert!(opt.is_none());

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1.30.0 · source§

### impl<'a, T> From<&'a Option<T>> for Option<&'a T>

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#### fn from(o: &'a Option<T>) -> Option<&'a T>

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 `from` to first take an `Option` to a reference to the value inside the original.

    let s: Option<String> = Some(String::from("Hello, Rustaceans!"));
    let o: Option<usize> = Option::from(&s).map(|ss: &String| ss.len());

    println!("Can still print s: {s:?}");

    assert_eq!(o, Some(18));

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1.30.0 · source§

### impl<'a, T> From<&'a mut Option<T>> for Option<&'a mut T>

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#### fn from(o: &'a mut Option<T>) -> Option<&'a mut T>

Converts from `&mut Option<T>` to `Option<&mut T>`

##### Examples

    let mut s = Some(String::from("Hello"));
    let o: Option<&mut String> = Option::from(&mut s);

    match o {
        Some(t) => *t = String::from("Hello, Rustaceans!"),
        None => (),
    }

    assert_eq!(s, Some(String::from("Hello, Rustaceans!")));

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1.12.0 · source§

### impl<T> From<T> for Option<T>

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#### fn from(val: T) -> Option<T>

Moves `val` into a new `Some`.

##### Examples

    let o: Option<u8> = Option::from(67);

    assert_eq!(Some(67), o);

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### impl<A, V> FromIterator<Option<A>> for Option<V>where V: FromIterator<A>,

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#### fn from_iter<I>(iter: I) -> Option<V>where 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 of type `V` containing 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.

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### impl<T> FromResidual<Yeet<()>> for Option<T>

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#### fn from_residual(_: Yeet<()>) -> Option<T>

🔬This is a nightly-only experimental API. (`try_trait_v2` #84277)
Constructs the type from a compatible `Residual` type. Read more
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### impl<T> FromResidual for Option<T>

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#### fn from_residual(residual: Option<Infallible>) -> Option<T>

🔬This is a nightly-only experimental API. (`try_trait_v2` #84277)
Constructs the type from a compatible `Residual` type. Read more
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### impl<T> Hash for Option<T>where T: Hash,

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#### fn hash<__H>(&self, state: &mut __H)where __H: Hasher,

Feeds this value into the given `Hasher`. Read more
1.3.0 · source§

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

Feeds a slice of this type into the given `Hasher`. Read more
1.4.0 · source§

### impl<'a, T> IntoIterator for &'a Option<T>

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#### type Item = &'a T

The type of the elements being iterated over.
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#### type IntoIter = Iter<'a, T>

Which kind of iterator are we turning this into?
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#### fn into_iter(self) -> Iter<'a, T> ⓘ

Creates an iterator from a value. Read more
1.4.0 · source§

### impl<'a, T> IntoIterator for &'a mut Option<T>

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#### type Item = &'a mut T

The type of the elements being iterated over.
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#### type IntoIter = IterMut<'a, T>

Which kind of iterator are we turning this into?
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#### fn into_iter(self) -> IterMut<'a, T> ⓘ

Creates an iterator from a value. Read more
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### impl<T> IntoIterator for Option<T>

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#### fn into_iter(self) -> IntoIter<T> ⓘ

Returns a consuming iterator over the possibly contained value.

##### Examples

    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());

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#### type Item = T

The type of the elements being iterated over.
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#### type IntoIter = IntoIter<T>

Which kind of iterator are we turning this into?
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### impl<T> Ord for Option<T>where T: Ord,

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#### fn cmp(&self, other: &Option<T>) -> Ordering

This method returns an `Ordering` between `self` and `other`. Read more
1.21.0 · source§

#### fn max(self, other: Self) -> Selfwhere Self: Sized,

Compares and returns the maximum of two values. Read more
1.21.0 · source§

#### fn min(self, other: Self) -> Selfwhere Self: Sized,

Compares and returns the minimum of two values. Read more
1.50.0 · source§

#### fn clamp(self, min: Self, max: Self) -> Selfwhere Self: Sized + PartialOrd,

Restrict a value to a certain interval. Read more
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### impl<T> PartialEq for Option<T>where T: PartialEq,

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#### fn eq(&self, other: &Option<T>) -> bool

This method tests for `self` and `other` values to be equal, and is used by `==`.
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#### fn ne(&self, other: &Rhs) -> bool

This method tests for `!=`. The default implementation is almost always sufficient, and should not be overridden without very good reason.
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### impl<T> PartialOrd for Option<T>where T: PartialOrd,

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#### fn partial_cmp(&self, other: &Option<T>) -> Option<Ordering>

This method returns an ordering between `self` and `other` values if one exists. Read more
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#### fn lt(&self, other: &Rhs) -> bool

This method tests less than (for `self` and `other`) and is used by the `<` operator. Read more
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#### fn le(&self, other: &Rhs) -> bool

This method tests less than or equal to (for `self` and `other`) and is used by the `<=` operator. Read more
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#### fn gt(&self, other: &Rhs) -> bool

This method tests greater than (for `self` and `other`) and is used by the `>` operator. Read more
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#### fn ge(&self, other: &Rhs) -> bool

This method tests greater than or equal to (for `self` and `other`) and is used by the `>=` operator. Read more
1.37.0 · source§

### impl<T, U> Product<Option<U>> for Option<T>where T: Product<U>,

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#### fn product<I>(iter: I) -> Option<T>where 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.

##### Examples

This multiplies each number in a vector of strings, if a string could not be parsed the operation returns `None`:

    let nums = vec!["5", "10", "1", "2"];
    let total: Option<usize> = nums.iter().map(|w| w.parse::<usize>().ok()).product();
    assert_eq!(total, Some(100));
    let nums = vec!["5", "10", "one", "2"];
    let total: Option<usize> = nums.iter().map(|w| w.parse::<usize>().ok()).product();
    assert_eq!(total, None);

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### impl<T> Residual<T> for Option<Infallible>

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#### type TryType = Option<T>

🔬This is a nightly-only experimental API. (`try_trait_v2_residual` #91285)
The “return” type of this meta-function.
1.37.0 · source§

### impl<T, U> Sum<Option<U>> for Option<T>where T: Sum<U>,

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#### fn sum<I>(iter: I) -> Option<T>where 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));
    let words = vec!["have", "a", "good", "day"];
    let total: Option<usize> = words.iter().map(|w| w.find('a')).sum();
    assert_eq!(total, None);

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### impl<T> Try for Option<T>

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#### type Output = T

🔬This is a nightly-only experimental API. (`try_trait_v2` #84277)
The type of the value produced by `?` when not short-circuiting.
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#### type Residual = Option<Infallible>

🔬This is a nightly-only experimental API. (`try_trait_v2` #84277)
The type of the value passed to `FromResidual::from_residual` as part of `?` when short-circuiting. Read more
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#### fn from_output(output: <Option<T> as Try>::Output) -> Option<T>

🔬This is a nightly-only experimental API. (`try_trait_v2` #84277)
Constructs the type from its `Output` type. Read more
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#### fn branch( self ) -> ControlFlow<<Option<T> as Try>::Residual, <Option<T> as Try>::Output>

🔬This is a nightly-only experimental API. (`try_trait_v2` #84277)
Used in `?` to decide whether the operator should produce a value (because this returned `ControlFlow::Continue`) or propagate a value back to the caller (because this returned `ControlFlow::Break`). Read more
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### impl<T> Copy for Option<T>where T: Copy,

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### impl<T> Eq for Option<T>where T: Eq,

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### impl<T> StructuralEq for Option<T>

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### impl<T> StructuralPartialEq for Option<T>

## Auto Trait Implementations§

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### impl<T> RefUnwindSafe for Option<T>where T: RefUnwindSafe,

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### impl<T> Send for Option<T>where T: Send,

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### impl<T> Sync for Option<T>where T: Sync,

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### impl<T> Unpin for Option<T>where T: Unpin,

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### impl<T> UnwindSafe for Option<T>where T: UnwindSafe,

## Blanket Implementations§

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### impl<T> Any for Twhere T: 'static + ?Sized,

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#### fn type_id(&self) -> TypeId

Gets the `TypeId` of `self`. Read more
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### impl<T> Borrow<T> for Twhere T: ?Sized,

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#### fn borrow(&self) -> &T

Immutably borrows from an owned value. Read more
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### impl<T> BorrowMut<T> for Twhere T: ?Sized,

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#### fn borrow_mut(&mut self) -> &mut T

Mutably borrows from an owned value. Read more
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### impl<T> From<!> for T

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#### fn from(t: !) -> T

Converts to this type from the input type.
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### impl<T> From<T> for T

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#### fn from(t: T) -> T

Returns the argument unchanged.

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### impl<T, U> Into<U> for Twhere U: From<T>,

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#### fn into(self) -> U

Calls `U::from(self)`.

That is, this conversion is whatever the implementation of `From<T> for U` chooses to do.

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### impl<T> ToOwned for Twhere T: Clone,

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#### type Owned = T

The resulting type after obtaining ownership.
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#### fn to_owned(&self) -> T

Creates owned data from borrowed data, usually by cloning. Read more
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#### fn clone_into(&self, target: &mut T)

Uses borrowed data to replace owned data, usually by cloning. Read more
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### impl<T, U> TryFrom<U> for Twhere U: Into<T>,

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#### type Error = Infallible

The type returned in the event of a conversion error.
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#### fn try_from(value: U) -> Result<T, <T as TryFrom<U>>::Error>

Performs the conversion.
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### impl<T, U> TryInto<U> for Twhere U: TryFrom<T>,

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#### type Error = <U as TryFrom<T>>::Error

The type returned in the event of a conversion error.
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#### fn try_into(self) -> Result<U, <U as TryFrom<T>>::Error>

Performs the conversion.