pub enum Option<T> {
None,
Some(T),
}
Expand description
The Option
type. See the module level documentation for more.
Variants§
Implementations§
source§impl<T> Option<T>
impl<T> Option<T>
1.70.0 · sourcepub fn is_some_and(self, f: impl FnOnce(T) -> bool) -> bool
pub fn is_some_and(self, f: impl FnOnce(T) -> bool) -> bool
1.82.0 · sourcepub fn is_none_or(self, f: impl FnOnce(T) -> bool) -> bool
pub fn is_none_or(self, f: impl FnOnce(T) -> bool) -> bool
1.0.0 (const: 1.48.0) · sourcepub const fn as_ref(&self) -> Option<&T>
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:?}");
1.0.0 (const: unstable) · sourcepub fn as_mut(&mut self) -> Option<&mut T>
pub fn as_mut(&mut self) -> Option<&mut T>
Converts from &mut Option<T>
to Option<&mut T>
.
§Examples
1.75.0 (const: unstable) · sourcepub fn as_slice(&self) -> &[T]
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
The inverse of this function is (discounting
borrowing) [_]::first
:
1.75.0 (const: unstable) · sourcepub fn as_mut_slice(&mut self) -> &mut [T]
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
The result is a mutable slice of zero or one items that points into
our original Option
:
The inverse of this method (discounting borrowing)
is [_]::first_mut
:
1.0.0 (const: unstable) · sourcepub fn expect(self, msg: &str) -> T
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
§Recommended Message Style
We recommend that expect
messages are used to describe the reason you
expect the Option
should be Some
.
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.
1.0.0 (const: unstable) · sourcepub fn unwrap(self) -> T
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
1.0.0 · sourcepub fn unwrap_or(self, default: T) -> T
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
1.0.0 · sourcepub fn unwrap_or_else<F>(self, f: F) -> Twhere
F: FnOnce() -> T,
pub fn unwrap_or_else<F>(self, f: F) -> Twhere
F: FnOnce() -> T,
1.0.0 · sourcepub fn unwrap_or_default(self) -> Twhere
T: Default,
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
1.58.0 (const: unstable) · sourcepub unsafe fn unwrap_unchecked(self) -> T
pub unsafe fn unwrap_unchecked(self) -> T
1.0.0 · sourcepub fn map<U, F>(self, f: F) -> Option<U>where
F: FnOnce(T) -> U,
pub fn map<U, F>(self, f: F) -> Option<U>where
F: FnOnce(T) -> U,
1.76.0 · sourcepub fn inspect<F: FnOnce(&T)>(self, f: F) -> Self
pub fn inspect<F: FnOnce(&T)>(self, f: F) -> Self
1.0.0 · sourcepub fn map_or<U, F>(self, default: U, f: F) -> Uwhere
F: FnOnce(T) -> U,
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
1.0.0 · sourcepub fn map_or_else<U, D, F>(self, default: D, f: F) -> U
pub fn map_or_else<U, D, F>(self, default: D, f: F) -> 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);
§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
.
1.0.0 · sourcepub fn ok_or<E>(self, err: E) -> Result<T, E>
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
1.0.0 · sourcepub fn ok_or_else<E, F>(self, err: F) -> Result<T, E>where
F: FnOnce() -> E,
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
1.40.0 · sourcepub fn as_deref(&self) -> Option<&T::Target>where
T: Deref,
pub fn as_deref(&self) -> Option<&T::Target>where
T: Deref,
1.40.0 · sourcepub fn as_deref_mut(&mut self) -> Option<&mut T::Target>where
T: DerefMut,
pub fn as_deref_mut(&mut self) -> Option<&mut T::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
1.0.0 (const: unstable) · sourcepub fn iter(&self) -> Iter<'_, T> ⓘ
pub fn iter(&self) -> Iter<'_, T> ⓘ
Returns an iterator over the possibly contained value.
§Examples
1.0.0 · sourcepub fn iter_mut(&mut self) -> IterMut<'_, T> ⓘ
pub fn iter_mut(&mut self) -> IterMut<'_, T> ⓘ
Returns a mutable iterator over the possibly contained value.
§Examples
1.0.0 · sourcepub fn and<U>(self, optb: Option<U>) -> Option<U>
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);
1.0.0 · sourcepub fn and_then<U, F>(self, f: F) -> Option<U>
pub fn and_then<U, F>(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_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);
Often used to chain fallible operations that may return None
.
1.27.0 · sourcepub fn filter<P>(self, predicate: P) -> Self
pub fn filter<P>(self, predicate: P) -> Self
Returns None
if the option is None
, otherwise calls predicate
with the wrapped value and returns:
Some(t)
ifpredicate
returnstrue
(wheret
is the wrapped value), andNone
ifpredicate
returnsfalse
.
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
1.0.0 · sourcepub fn or(self, optb: Option<T>) -> Option<T>
pub fn or(self, optb: Option<T>) -> Option<T>
1.0.0 · sourcepub fn or_else<F>(self, f: F) -> Option<T>
pub fn or_else<F>(self, f: F) -> Option<T>
Returns the option if it contains a value, otherwise calls f
and
returns the result.
§Examples
1.37.0 · sourcepub fn xor(self, optb: Option<T>) -> Option<T>
pub fn xor(self, optb: Option<T>) -> Option<T>
1.53.0 · sourcepub fn insert(&mut self, value: T) -> &mut T
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
1.20.0 · sourcepub fn get_or_insert(&mut self, value: T) -> &mut T
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
sourcepub fn get_or_insert_default(&mut self) -> &mut Twhere
T: Default,
🔬This is a nightly-only experimental API. (option_get_or_insert_default
#82901)
pub fn get_or_insert_default(&mut self) -> &mut Twhere
T: Default,
option_get_or_insert_default
#82901)1.20.0 · sourcepub fn get_or_insert_with<F>(&mut self, f: F) -> &mut Twhere
F: FnOnce() -> T,
pub fn get_or_insert_with<F>(&mut self, f: F) -> &mut Twhere
F: FnOnce() -> T,
1.80.0 · sourcepub fn take_if<P>(&mut self, predicate: P) -> Option<T>
pub fn take_if<P>(&mut self, predicate: P) -> Option<T>
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
1.31.0 (const: unstable) · sourcepub fn replace(&mut self, value: T) -> Option<T>
pub fn replace(&mut self, value: T) -> Option<T>
1.46.0 · sourcepub fn zip<U>(self, other: Option<U>) -> Option<(T, U)>
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
sourcepub 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)
pub fn zip_with<U, F, R>(self, other: Option<U>, f: F) -> Option<R>where
F: FnOnce(T, U) -> R,
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);
source§impl<T, U> Option<(T, U)>
impl<T, U> Option<(T, U)>
source§impl<T> Option<&T>
impl<T> Option<&T>
source§impl<T> Option<&mut T>
impl<T> Option<&mut T>
source§impl<T, E> Option<Result<T, E>>
impl<T, E> Option<Result<T, E>>
source§impl<T> Option<Option<T>>
impl<T> Option<Option<T>>
1.40.0 (const: unstable) · sourcepub fn flatten(self) -> Option<T>
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());
Flattening only removes one level of nesting at a time:
Trait Implementations§
1.30.0 · source§impl<'a, T> From<&'a Option<T>> for Option<&'a T>
impl<'a, T> From<&'a Option<T>> for Option<&'a T>
source§fn from(o: &'a Option<T>) -> Option<&'a T>
fn from(o: &'a Option<T>) -> Option<&'a T>
1.30.0 · source§impl<'a, T> From<&'a mut Option<T>> for Option<&'a mut T>
impl<'a, T> From<&'a mut Option<T>> for Option<&'a mut T>
1.0.0 · source§impl<A, V: FromIterator<A>> FromIterator<Option<A>> for Option<V>
impl<A, V: FromIterator<A>> FromIterator<Option<A>> for Option<V>
source§fn from_iter<I: IntoIterator<Item = Option<A>>>(iter: I) -> Option<V>
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 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]));
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);
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);
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.
source§impl<T> FromResidual<Option<Infallible>> for Option<T>
impl<T> FromResidual<Option<Infallible>> for Option<T>
source§fn from_residual(residual: Option<Infallible>) -> Self
fn from_residual(residual: Option<Infallible>) -> Self
try_trait_v2
#84277)Residual
type. Read more1.4.0 · source§impl<'a, T> IntoIterator for &'a Option<T>
impl<'a, T> IntoIterator for &'a Option<T>
1.4.0 · source§impl<'a, T> IntoIterator for &'a mut Option<T>
impl<'a, T> IntoIterator for &'a mut Option<T>
1.0.0 · source§impl<T> IntoIterator for Option<T>
impl<T> IntoIterator for Option<T>
1.0.0 · source§impl<T: Ord> Ord for Option<T>
impl<T: Ord> Ord for Option<T>
1.0.0 · source§impl<T: PartialOrd> PartialOrd for Option<T>
impl<T: PartialOrd> PartialOrd for Option<T>
1.37.0 · source§impl<T, U> Product<Option<U>> for Option<T>where
T: Product<U>,
impl<T, U> Product<Option<U>> for Option<T>where
T: Product<U>,
source§fn product<I>(iter: I) -> Option<T>
fn product<I>(iter: I) -> Option<T>
source§impl<T> Residual<T> for Option<Infallible>
impl<T> Residual<T> for Option<Infallible>
1.37.0 · source§impl<T, U> Sum<Option<U>> for Option<T>where
T: Sum<U>,
impl<T, U> Sum<Option<U>> for Option<T>where
T: Sum<U>,
source§fn sum<I>(iter: I) -> Option<T>
fn sum<I>(iter: I) -> Option<T>
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
:
source§impl<T> Try for Option<T>
impl<T> Try for Option<T>
source§type Output = T
type Output = T
try_trait_v2
#84277)?
when not short-circuiting.source§type Residual = Option<Infallible>
type Residual = Option<Infallible>
try_trait_v2
#84277)FromResidual::from_residual
as part of ?
when short-circuiting. Read moresource§fn from_output(output: Self::Output) -> Self
fn from_output(output: Self::Output) -> Self
try_trait_v2
#84277)Output
type. Read moresource§fn branch(self) -> ControlFlow<Self::Residual, Self::Output>
fn branch(self) -> ControlFlow<Self::Residual, Self::Output>
try_trait_v2
#84277)?
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