Enum core::option::Option 1.0.0[−][src]
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
Returns true if the option is a Some value containing the given value.
Examples
#![feature(option_result_contains)]
let x: Option<u32> = Some(2);
assert_eq!(x.contains(&2), true);
let x: Option<u32> = Some(3);
assert_eq!(x.contains(&2), false);
let x: Option<u32> = None;
assert_eq!(x.contains(&2), false);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 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);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`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");let x: Option<&str> = None;
assert_eq!(x.unwrap(), "air"); // failsReturns 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, 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.
Examples
Converts an Option<String> into an Option<usize>, consuming
the original:
let maybe_some_string = Some(String::from("Hello, World!"));
// `Option::map` takes self *by value*, consuming `maybe_some_string`
let maybe_some_len = maybe_some_string.map(|s| s.len());
assert_eq!(maybe_some_len, Some(13));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));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);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));Returns None if the option is None, otherwise returns optb.
Examples
let x = Some(2);
let y: Option<&str> = None;
assert_eq!(x.and(y), None);
let x: Option<u32> = None;
let y = Some("foo");
assert_eq!(x.and(y), None);
let x = Some(2);
let y = Some("foo");
assert_eq!(x.and(y), Some("foo"));
let x: Option<u32> = None;
let y: Option<&str> = None;
assert_eq!(x.and(y), None);Returns None if the option is None, otherwise calls f with the
wrapped value and returns the result.
Some languages call this operation flatmap.
Examples
fn sq(x: u32) -> Option<u32> { Some(x * x) }
fn nope(_: u32) -> Option<u32> { None }
assert_eq!(Some(2).and_then(sq).and_then(sq), Some(16));
assert_eq!(Some(2).and_then(sq).and_then(nope), None);
assert_eq!(Some(2).and_then(nope).and_then(sq), None);
assert_eq!(None.and_then(sq).and_then(sq), None);Returns None if the option is None, otherwise calls predicate
with the wrapped value and returns:
Some(t)ifpredicatereturnstrue(wheretis the wrapped value), andNoneifpredicatereturnsfalse.
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));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);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);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
#![feature(unzip_option)]
let x = Some((1, "hi"));
let y = None::<(u8, u32)>;
assert_eq!(x.unzip(), (Some(1), Some("hi")));
assert_eq!(y.unzip(), (None, None));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
Converts a string to an integer, turning poorly-formed strings
into 0 (the default value for integers). parse converts
a string to any other type that implements FromStr, returning
None on error.
let good_year_from_input = "1909";
let bad_year_from_input = "190blarg";
let good_year = good_year_from_input.parse().ok().unwrap_or_default();
let bad_year = bad_year_from_input.parse().ok().unwrap_or_default();
assert_eq!(1909, good_year);
assert_eq!(0, bad_year);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()));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());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:
let x: Option<Option<Option<u32>>> = Some(Some(Some(6)));
assert_eq!(Some(Some(6)), x.flatten());
assert_eq!(Some(6), x.flatten().flatten());Trait Implementations
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));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.
type Item = T
type Item = T
The type of the elements being iterated over.
This method returns an ordering between self and other values if one exists. Read more
This method tests less than (for self and other) and is used by the < operator. Read more
This method tests less than or equal to (for self and other) and is used by the <=
operator. Read more
This method tests greater than (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 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));type Residual = Option<Infallible>
type Residual = Option<Infallible>
The type of the value passed to FromResidual::from_residual
as part of ? when short-circuiting. Read more
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