GeezerVison is showing doc.rust-lang.org — Option in std::option - Rust pub enum Option<T> {
None,
Some(T),
}
Expand descriptionVariants§Implementations§
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);
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);
let x: Option<String> = Some("ownership".to_string());
assert_eq!(x.as_ref().is_some_and(|x| x.len() > 1), true);
println!("still alive {:?}", x);
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);
Returns true if the option is a None or the value inside of it matches a predicate.
§Examples
let x: Option<u32> = Some(2);
assert_eq!(x.is_none_or(|x| x > 1), true);
let x: Option<u32> = Some(0);
assert_eq!(x.is_none_or(|x| x > 1), false);
let x: Option<u32> = None;
assert_eq!(x.is_none_or(|x| x > 1), true);
let x: Option<String> = Some("ownership".to_string());
assert_eq!(x.as_ref().is_none_or(|x| x.len() > 1), true);
println!("still alive {:?}", x);
1.0.0 (const: 1.48.0) · SourceConverts 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: 1.83.0) · SourceConverts 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));
1.33.0 (const: 1.84.0) · SourceConverts from Pin<&Option<T>> to Option<Pin<&T>>.
1.33.0 (const: 1.84.0) · Source
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][..], &[][..]],
);
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());
}
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 [][..]],
);
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));
The inverse of this method (discounting borrowing)
is [_]::first_mut:
assert_eq!(Some(123).as_mut_slice().first_mut(), Some(&mut 123))
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");
ⓘlet x: Option<&str> = None;
x.expect("fruits are healthy"); // panics with `fruits are healthy`
§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");
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: 1.83.0) · Sourcepub const fn unwrap(self) -> TReturns the contained Some value, consuming the self value.
Because this function may panic, its use is generally discouraged.
Panics are meant for unrecoverable errors, and
may abort the entire program.
Instead, prefer to use pattern matching and handle the None
case explicitly, or call unwrap_or, unwrap_or_else, or
unwrap_or_default. In functions returning Option, you can use
the ? (try) operator.
§Panics
Panics if the self value equals None.
§Examples
let x = Some("air");
assert_eq!(x.unwrap(), "air");
ⓘlet x: Option<&str> = None;
assert_eq!(x.unwrap(), "air"); // fails
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");
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);
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);
1.58.0 (const: 1.83.0) · SourceReturns 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");
let x: Option<&str> = None;
assert_eq!(unsafe { x.unwrap_unchecked() }, "air"); // Undefined behavior!
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);
Calls a function with a reference to the contained value if Some.
Returns the original option.
§Examples
let list = vec![1, 2, 3];
// prints "got: 2"
let x = list
.get(1)
.inspect(|x| println!("got: {x}"))
.expect("list should be long enough");
// prints nothing
list.get(5).inspect(|x| println!("got: {x}"));
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);
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.
let v: u64 = std::env::args()
.nth(1)
.map_or_else(|| std::fs::read_to_string("/etc/someconfig.conf"), Ok)?
.parse()?;
Maps an Option<T> to a U by applying function f to the contained
value if the option is Some, otherwise if None, returns the
default value for the type U.
§Examples
let x: Option<&str> = Some("hi");
let y: Option<&str> = None;
assert_eq!(x.map_or_default(|x| x.len()), 2);
assert_eq!(y.map_or_default(|y| y.len()), 0);
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));
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));
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);
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()));
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);
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);
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);
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.
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);
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), andNone 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));
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);
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);
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);
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);
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));
Inserts the default value 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_default();
assert_eq!(y, &0);
*y = 7;
}
assert_eq!(x, Some(7));
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));
🔬This is a nightly-only experimental API. (option_get_or_try_insert_with #143648)
If the option is None, calls the closure and inserts its output if successful.
If the closure returns a residual value such as Err or None,
that residual value is returned and nothing is inserted.
If the option is Some, nothing is inserted.
Unless a residual is returned, a mutable reference to the value
of the option will be output.
§Examples
#![feature(option_get_or_try_insert_with)]
let mut o1: Option<u32> = None;
let mut o2: Option<u8> = None;
let number = "12345";
assert_eq!(o1.get_or_try_insert_with(|| number.parse()).copied(), Ok(12345));
assert!(o2.get_or_try_insert_with(|| number.parse()).is_err());
assert_eq!(o1, Some(12345));
assert_eq!(o2, None);
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);
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
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));
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);
Makes a tuple of the value in self and the value in 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);
🔬This is a nightly-only experimental API. (option_zip #70086)
Combines the value in self with the value in another Option, using the 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);
🔬This is a nightly-only experimental API. (option_reduce #144273)
Reduces two options into one, using the provided function if both are Some.
If self is Some(s) and other is Some(o), this method returns Some(f(s, o)).
Otherwise, if only one of self and other is Some, that one is returned.
If both self and other are None, None is returned.
§Examples
#![feature(option_reduce)]
let s12 = Some(12);
let s17 = Some(17);
let n = None;
let f = |a, b| a + b;
assert_eq!(s12.reduce(s17, f), Some(29));
assert_eq!(s12.reduce(n, f), Some(12));
assert_eq!(n.reduce(s17, f), Some(17));
assert_eq!(n.reduce(n, f), None);
🔬This is a nightly-only experimental API. (option_into_flat_iter #148441)
Transforms an optional iterator into an iterator.
If self is None, the resulting iterator is empty.
Otherwise, an iterator is made from the Some value and returned.
§Examples
#![feature(option_into_flat_iter)]
let o1 = Some([1, 2]);
let o2 = None::<&[usize]>;
assert_eq!(o1.into_flat_iter().collect::<Vec<_>>(), [1, 2]);
assert_eq!(o2.into_flat_iter().collect::<Vec<_>>(), Vec::<&usize>::new());
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));
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));
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));
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));
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));
1.33.0 (const: 1.83.0) · SourceTransposes an Option of a Result into a Result of an Option.
Some(Ok(_)) is mapped to Ok(Some(_)),
Some(Err(_)) is mapped to Err(_),
and None will be mapped to Ok(None).
§Examples
#[derive(Debug, Eq, PartialEq)]
struct SomeErr;
let x: Option<Result<i32, SomeErr>> = Some(Ok(5));
let y: Result<Option<i32>, SomeErr> = Ok(Some(5));
assert_eq!(x.transpose(), y);
1.40.0 (const: 1.83.0) · SourceConverts 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:
let x: Option<Option<Option<u32>>> = Some(Some(Some(6)));
assert_eq!(Some(Some(6)), x.flatten());
assert_eq!(Some(6), x.flatten().flatten());
🔬This is a nightly-only experimental API. (option_reference_flattening #149221)
Converts from Option<&Option<T>> to Option<&T>.
§Examples
Basic usage:
#![feature(option_reference_flattening)]
let x: Option<&Option<u32>> = Some(&Some(6));
assert_eq!(Some(&6), x.flatten_ref());
let x: Option<&Option<u32>> = Some(&None);
assert_eq!(None, x.flatten_ref());
let x: Option<&Option<u32>> = None;
assert_eq!(None, x.flatten_ref());
🔬This is a nightly-only experimental API. (option_reference_flattening #149221)
Converts from Option<&mut Option<T>> to &Option<T>.
§Examples
Basic usage:
#![feature(option_reference_flattening)]
let y = &mut Some(6);
let x: Option<&mut Option<u32>> = Some(y);
assert_eq!(Some(&6), x.flatten_ref());
let y: &mut Option<u32> = &mut None;
let x: Option<&mut Option<u32>> = Some(y);
assert_eq!(None, x.flatten_ref());
let x: Option<&mut Option<u32>> = None;
assert_eq!(None, x.flatten_ref());
🔬This is a nightly-only experimental API. (option_reference_flattening #149221)
Converts from Option<&mut Option<T>> to Option<&mut T>.
§Examples
Basic usage:
#![feature(option_reference_flattening)]
let y: &mut Option<u32> = &mut Some(6);
let x: Option<&mut Option<u32>> = Some(y);
assert_eq!(Some(&mut 6), x.flatten_mut());
let y: &mut Option<u32> = &mut None;
let x: Option<&mut Option<u32>> = Some(y);
assert_eq!(None, x.flatten_mut());
let x: Option<&mut Option<u32>> = None;
assert_eq!(None, x.flatten_mut());
Trait Implementations§
Performs copy-assignment from source. Read more
Formats the value using the given formatter. Read more
Returns None.
§Examples
let opt: Option<u32> = Option::default();
assert!(opt.is_none());
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));
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!")));
Moves val into a new Some.
§Examples
let o: Option<u8> = Option::from(67);
assert_eq!(Some(67), o);
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.
🔬This is a nightly-only experimental API. (try_trait_v2 #84277) Constructs the type from a compatible Residual type. Read more
🔬This is a nightly-only experimental API. (try_trait_v2 #84277) Constructs the type from a compatible Residual type. Read more
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());
The type of the elements being iterated over.
Which kind of iterator are we turning this into?
The type of the elements being iterated over.
Which kind of iterator are we turning this into?
The type of the elements being iterated over.
Which kind of iterator are we turning this into?
Compares and returns the maximum of two values. Read more
Compares and returns the minimum of two values. Read more
Restrict a value to a certain interval. Read more
This method returns an ordering between self and other values if one exists. Read more
Tests less than (for self and other) and is used by the < operator. Read more
Tests less than or equal to (for self and other) and is used by the
<= operator. Read more
Tests greater than (for self and other) and is used by the >
operator. Read more
Tests greater than or equal to (for self and other) and is used by
the >= operator. Read more
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);
🔬This is a nightly-only experimental API. (try_trait_v2_residual #91285) The “return” type of this meta-function.
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);
🔬This is a nightly-only experimental API. (try_trait_v2 #84277) The type of the value produced by ? when not short-circuiting.
🔬This is a nightly-only experimental API. (try_trait_v2 #84277)
🔬This is a nightly-only experimental API. (try_trait_v2 #84277) Constructs the type from its Output type. Read more
🔬This is a nightly-only experimental API. (try_trait_v2 #84277) Auto Trait Implementations§Blanket Implementations§
Immutably borrows from an owned value. Read more
Mutably borrows from an owned value. Read more
🔬This is a nightly-only experimental API. (clone_to_uninit #126799) Performs copy-assignment from self to dest. Read more
Converts to this type from the input type.
Returns the argument unchanged.
Calls U::from(self).
That is, this conversion is whatever the implementation of
From<T> for U chooses to do.
The resulting type after obtaining ownership.
Creates owned data from borrowed data, usually by cloning. Read more
Uses borrowed data to replace owned data, usually by cloning. Read more
The type returned in the event of a conversion error.
Performs the conversion.
The type returned in the event of a conversion error.
Performs the conversion. |