3 //! Type [`Option`] represents an optional value: every [`Option`]
4 //! is either [`Some`] and contains a value, or [`None`], and
5 //! does not. [`Option`] types are very common in Rust code, as
6 //! they have a number of uses:
9 //! * Return values for functions that are not defined
10 //! over their entire input range (partial functions)
11 //! * Return value for otherwise reporting simple errors, where [`None`] is
13 //! * Optional struct fields
14 //! * Struct fields that can be loaned or "taken"
15 //! * Optional function arguments
16 //! * Nullable pointers
17 //! * Swapping things out of difficult situations
19 //! [`Option`]s are commonly paired with pattern matching to query the presence
20 //! of a value and take action, always accounting for the [`None`] case.
23 //! fn divide(numerator: f64, denominator: f64) -> Option<f64> {
24 //! if denominator == 0.0 {
27 //! Some(numerator / denominator)
31 //! // The return value of the function is an option
32 //! let result = divide(2.0, 3.0);
34 //! // Pattern match to retrieve the value
36 //! // The division was valid
37 //! Some(x) => println!("Result: {}", x),
38 //! // The division was invalid
39 //! None => println!("Cannot divide by 0"),
44 // FIXME: Show how `Option` is used in practice, with lots of methods
46 //! # Options and pointers ("nullable" pointers)
48 //! Rust's pointer types must always point to a valid location; there are
49 //! no "null" references. Instead, Rust has *optional* pointers, like
50 //! the optional owned box, [`Option`]`<`[`Box<T>`]`>`.
52 //! The following example uses [`Option`] to create an optional box of
53 //! [`i32`]. Notice that in order to use the inner [`i32`] value first, the
54 //! `check_optional` function needs to use pattern matching to
55 //! determine whether the box has a value (i.e., it is [`Some(...)`][`Some`]) or
59 //! let optional = None;
60 //! check_optional(optional);
62 //! let optional = Some(Box::new(9000));
63 //! check_optional(optional);
65 //! fn check_optional(optional: Option<Box<i32>>) {
67 //! Some(p) => println!("has value {}", p),
68 //! None => println!("has no value"),
73 //! This usage of [`Option`] to create safe nullable pointers is so
74 //! common that Rust does special optimizations to make the
75 //! representation of [`Option`]`<`[`Box<T>`]`>` a single pointer. Optional pointers
76 //! in Rust are stored as efficiently as any other pointer type.
80 //! Basic pattern matching on [`Option`]:
83 //! let msg = Some("howdy");
85 //! // Take a reference to the contained string
86 //! if let Some(m) = &msg {
87 //! println!("{}", *m);
90 //! // Remove the contained string, destroying the Option
91 //! let unwrapped_msg = msg.unwrap_or("default message");
94 //! Initialize a result to [`None`] before a loop:
97 //! enum Kingdom { Plant(u32, &'static str), Animal(u32, &'static str) }
99 //! // A list of data to search through.
100 //! let all_the_big_things = [
101 //! Kingdom::Plant(250, "redwood"),
102 //! Kingdom::Plant(230, "noble fir"),
103 //! Kingdom::Plant(229, "sugar pine"),
104 //! Kingdom::Animal(25, "blue whale"),
105 //! Kingdom::Animal(19, "fin whale"),
106 //! Kingdom::Animal(15, "north pacific right whale"),
109 //! // We're going to search for the name of the biggest animal,
110 //! // but to start with we've just got `None`.
111 //! let mut name_of_biggest_animal = None;
112 //! let mut size_of_biggest_animal = 0;
113 //! for big_thing in &all_the_big_things {
114 //! match *big_thing {
115 //! Kingdom::Animal(size, name) if size > size_of_biggest_animal => {
116 //! // Now we've found the name of some big animal
117 //! size_of_biggest_animal = size;
118 //! name_of_biggest_animal = Some(name);
120 //! Kingdom::Animal(..) | Kingdom::Plant(..) => ()
124 //! match name_of_biggest_animal {
125 //! Some(name) => println!("the biggest animal is {}", name),
126 //! None => println!("there are no animals :("),
130 //! [`Option`]: enum.Option.html
131 //! [`Some`]: enum.Option.html#variant.Some
132 //! [`None`]: enum.Option.html#variant.None
133 //! [`Box<T>`]: ../../std/boxed/struct.Box.html
134 //! [`i32`]: ../../std/primitive.i32.html
136 // ignore-tidy-undocumented-unsafe
138 #![stable(feature = "rust1", since = "1.0.0")]
140 use crate::iter::{FromIterator, FusedIterator, TrustedLen};
143 convert, fmt, hint, mem,
144 ops::{self, Deref, DerefMut},
147 // Note that this is not a lang item per se, but it has a hidden dependency on
148 // `Iterator`, which is one. The compiler assumes that the `next` method of
149 // `Iterator` is an enumeration with one type parameter and two variants,
150 // which basically means it must be `Option`.
152 /// The `Option` type. See [the module level documentation](index.html) for more.
153 #[derive(Copy, PartialEq, PartialOrd, Eq, Ord, Debug, Hash)]
154 #[stable(feature = "rust1", since = "1.0.0")]
157 #[stable(feature = "rust1", since = "1.0.0")]
160 #[stable(feature = "rust1", since = "1.0.0")]
161 Some(#[stable(feature = "rust1", since = "1.0.0")] T),
164 /////////////////////////////////////////////////////////////////////////////
165 // Type implementation
166 /////////////////////////////////////////////////////////////////////////////
169 /////////////////////////////////////////////////////////////////////////
170 // Querying the contained values
171 /////////////////////////////////////////////////////////////////////////
173 /// Returns `true` if the option is a [`Some`] value.
178 /// let x: Option<u32> = Some(2);
179 /// assert_eq!(x.is_some(), true);
181 /// let x: Option<u32> = None;
182 /// assert_eq!(x.is_some(), false);
185 /// [`Some`]: #variant.Some
186 #[must_use = "if you intended to assert that this has a value, consider `.unwrap()` instead"]
188 #[stable(feature = "rust1", since = "1.0.0")]
189 pub fn is_some(&self) -> bool {
196 /// Returns `true` if the option is a [`None`] value.
201 /// let x: Option<u32> = Some(2);
202 /// assert_eq!(x.is_none(), false);
204 /// let x: Option<u32> = None;
205 /// assert_eq!(x.is_none(), true);
208 /// [`None`]: #variant.None
209 #[must_use = "if you intended to assert that this doesn't have a value, consider \
210 `.and_then(|| panic!(\"`Option` had a value when expected `None`\"))` instead"]
212 #[stable(feature = "rust1", since = "1.0.0")]
213 pub fn is_none(&self) -> bool {
217 /// Returns `true` if the option is a [`Some`] value containing the given value.
222 /// #![feature(option_result_contains)]
224 /// let x: Option<u32> = Some(2);
225 /// assert_eq!(x.contains(&2), true);
227 /// let x: Option<u32> = Some(3);
228 /// assert_eq!(x.contains(&2), false);
230 /// let x: Option<u32> = None;
231 /// assert_eq!(x.contains(&2), false);
235 #[unstable(feature = "option_result_contains", issue = "62358")]
236 pub fn contains<U>(&self, x: &U) -> bool
246 /////////////////////////////////////////////////////////////////////////
247 // Adapter for working with references
248 /////////////////////////////////////////////////////////////////////////
250 /// Converts from `&Option<T>` to `Option<&T>`.
254 /// Converts an `Option<`[`String`]`>` into an `Option<`[`usize`]`>`, preserving the original.
255 /// The [`map`] method takes the `self` argument by value, consuming the original,
256 /// so this technique uses `as_ref` to first take an `Option` to a reference
257 /// to the value inside the original.
259 /// [`map`]: enum.Option.html#method.map
260 /// [`String`]: ../../std/string/struct.String.html
261 /// [`usize`]: ../../std/primitive.usize.html
264 /// let text: Option<String> = Some("Hello, world!".to_string());
265 /// // First, cast `Option<String>` to `Option<&String>` with `as_ref`,
266 /// // then consume *that* with `map`, leaving `text` on the stack.
267 /// let text_length: Option<usize> = text.as_ref().map(|s| s.len());
268 /// println!("still can print text: {:?}", text);
271 #[stable(feature = "rust1", since = "1.0.0")]
272 pub fn as_ref(&self) -> Option<&T> {
274 Some(ref x) => Some(x),
279 /// Converts from `&mut Option<T>` to `Option<&mut T>`.
284 /// let mut x = Some(2);
285 /// match x.as_mut() {
286 /// Some(v) => *v = 42,
289 /// assert_eq!(x, Some(42));
292 #[stable(feature = "rust1", since = "1.0.0")]
293 pub fn as_mut(&mut self) -> Option<&mut T> {
295 Some(ref mut x) => Some(x),
300 /// Converts from [`Pin`]`<&Option<T>>` to `Option<`[`Pin`]`<&T>>`.
302 /// [`Pin`]: ../pin/struct.Pin.html
304 #[stable(feature = "pin", since = "1.33.0")]
305 pub fn as_pin_ref(self: Pin<&Self>) -> Option<Pin<&T>> {
306 unsafe { Pin::get_ref(self).as_ref().map(|x| Pin::new_unchecked(x)) }
309 /// Converts from [`Pin`]`<&mut Option<T>>` to `Option<`[`Pin`]`<&mut T>>`.
311 /// [`Pin`]: ../pin/struct.Pin.html
313 #[stable(feature = "pin", since = "1.33.0")]
314 pub fn as_pin_mut(self: Pin<&mut Self>) -> Option<Pin<&mut T>> {
315 unsafe { Pin::get_unchecked_mut(self).as_mut().map(|x| Pin::new_unchecked(x)) }
318 /////////////////////////////////////////////////////////////////////////
319 // Getting to contained values
320 /////////////////////////////////////////////////////////////////////////
322 /// Unwraps an option, yielding the content of a [`Some`].
326 /// Panics if the value is a [`None`] with a custom panic message provided by
329 /// [`Some`]: #variant.Some
330 /// [`None`]: #variant.None
335 /// let x = Some("value");
336 /// assert_eq!(x.expect("the world is ending"), "value");
339 /// ```{.should_panic}
340 /// let x: Option<&str> = None;
341 /// x.expect("the world is ending"); // panics with `the world is ending`
345 #[stable(feature = "rust1", since = "1.0.0")]
346 pub fn expect(self, msg: &str) -> T {
349 None => expect_failed(msg),
353 /// Moves the value `v` out of the `Option<T>` if it is [`Some(v)`].
355 /// In general, because this function may panic, its use is discouraged.
356 /// Instead, prefer to use pattern matching and handle the [`None`]
361 /// Panics if the self value equals [`None`].
363 /// [`Some(v)`]: #variant.Some
364 /// [`None`]: #variant.None
369 /// let x = Some("air");
370 /// assert_eq!(x.unwrap(), "air");
373 /// ```{.should_panic}
374 /// let x: Option<&str> = None;
375 /// assert_eq!(x.unwrap(), "air"); // fails
379 #[stable(feature = "rust1", since = "1.0.0")]
380 pub fn unwrap(self) -> T {
383 None => panic!("called `Option::unwrap()` on a `None` value"),
387 /// Returns the contained value or a default.
389 /// Arguments passed to `unwrap_or` are eagerly evaluated; if you are passing
390 /// the result of a function call, it is recommended to use [`unwrap_or_else`],
391 /// which is lazily evaluated.
393 /// [`unwrap_or_else`]: #method.unwrap_or_else
398 /// assert_eq!(Some("car").unwrap_or("bike"), "car");
399 /// assert_eq!(None.unwrap_or("bike"), "bike");
402 #[stable(feature = "rust1", since = "1.0.0")]
403 pub fn unwrap_or(self, default: T) -> T {
410 /// Returns the contained value or computes it from a closure.
416 /// assert_eq!(Some(4).unwrap_or_else(|| 2 * k), 4);
417 /// assert_eq!(None.unwrap_or_else(|| 2 * k), 20);
420 #[stable(feature = "rust1", since = "1.0.0")]
421 pub fn unwrap_or_else<F: FnOnce() -> T>(self, f: F) -> T {
428 /////////////////////////////////////////////////////////////////////////
429 // Transforming contained values
430 /////////////////////////////////////////////////////////////////////////
432 /// Maps an `Option<T>` to `Option<U>` by applying a function to a contained value.
436 /// Converts an `Option<`[`String`]`>` into an `Option<`[`usize`]`>`, consuming the original:
438 /// [`String`]: ../../std/string/struct.String.html
439 /// [`usize`]: ../../std/primitive.usize.html
442 /// let maybe_some_string = Some(String::from("Hello, World!"));
443 /// // `Option::map` takes self *by value*, consuming `maybe_some_string`
444 /// let maybe_some_len = maybe_some_string.map(|s| s.len());
446 /// assert_eq!(maybe_some_len, Some(13));
449 #[stable(feature = "rust1", since = "1.0.0")]
450 pub fn map<U, F: FnOnce(T) -> U>(self, f: F) -> Option<U> {
452 Some(x) => Some(f(x)),
457 /// Applies a function to the contained value (if any),
458 /// or returns the provided default (if not).
463 /// let x = Some("foo");
464 /// assert_eq!(x.map_or(42, |v| v.len()), 3);
466 /// let x: Option<&str> = None;
467 /// assert_eq!(x.map_or(42, |v| v.len()), 42);
470 #[stable(feature = "rust1", since = "1.0.0")]
471 pub fn map_or<U, F: FnOnce(T) -> U>(self, default: U, f: F) -> U {
478 /// Applies a function to the contained value (if any),
479 /// or computes a default (if not).
486 /// let x = Some("foo");
487 /// assert_eq!(x.map_or_else(|| 2 * k, |v| v.len()), 3);
489 /// let x: Option<&str> = None;
490 /// assert_eq!(x.map_or_else(|| 2 * k, |v| v.len()), 42);
493 #[stable(feature = "rust1", since = "1.0.0")]
494 pub fn map_or_else<U, D: FnOnce() -> U, F: FnOnce(T) -> U>(self, default: D, f: F) -> U {
501 /// Transforms the `Option<T>` into a [`Result<T, E>`], mapping [`Some(v)`] to
502 /// [`Ok(v)`] and [`None`] to [`Err(err)`].
504 /// Arguments passed to `ok_or` are eagerly evaluated; if you are passing the
505 /// result of a function call, it is recommended to use [`ok_or_else`], which is
506 /// lazily evaluated.
508 /// [`Result<T, E>`]: ../../std/result/enum.Result.html
509 /// [`Ok(v)`]: ../../std/result/enum.Result.html#variant.Ok
510 /// [`Err(err)`]: ../../std/result/enum.Result.html#variant.Err
511 /// [`None`]: #variant.None
512 /// [`Some(v)`]: #variant.Some
513 /// [`ok_or_else`]: #method.ok_or_else
518 /// let x = Some("foo");
519 /// assert_eq!(x.ok_or(0), Ok("foo"));
521 /// let x: Option<&str> = None;
522 /// assert_eq!(x.ok_or(0), Err(0));
525 #[stable(feature = "rust1", since = "1.0.0")]
526 pub fn ok_or<E>(self, err: E) -> Result<T, E> {
533 /// Transforms the `Option<T>` into a [`Result<T, E>`], mapping [`Some(v)`] to
534 /// [`Ok(v)`] and [`None`] to [`Err(err())`].
536 /// [`Result<T, E>`]: ../../std/result/enum.Result.html
537 /// [`Ok(v)`]: ../../std/result/enum.Result.html#variant.Ok
538 /// [`Err(err())`]: ../../std/result/enum.Result.html#variant.Err
539 /// [`None`]: #variant.None
540 /// [`Some(v)`]: #variant.Some
545 /// let x = Some("foo");
546 /// assert_eq!(x.ok_or_else(|| 0), Ok("foo"));
548 /// let x: Option<&str> = None;
549 /// assert_eq!(x.ok_or_else(|| 0), Err(0));
552 #[stable(feature = "rust1", since = "1.0.0")]
553 pub fn ok_or_else<E, F: FnOnce() -> E>(self, err: F) -> Result<T, E> {
560 /////////////////////////////////////////////////////////////////////////
561 // Iterator constructors
562 /////////////////////////////////////////////////////////////////////////
564 /// Returns an iterator over the possibly contained value.
570 /// assert_eq!(x.iter().next(), Some(&4));
572 /// let x: Option<u32> = None;
573 /// assert_eq!(x.iter().next(), None);
576 #[stable(feature = "rust1", since = "1.0.0")]
577 pub fn iter(&self) -> Iter<'_, T> {
578 Iter { inner: Item { opt: self.as_ref() } }
581 /// Returns a mutable iterator over the possibly contained value.
586 /// let mut x = Some(4);
587 /// match x.iter_mut().next() {
588 /// Some(v) => *v = 42,
591 /// assert_eq!(x, Some(42));
593 /// let mut x: Option<u32> = None;
594 /// assert_eq!(x.iter_mut().next(), None);
597 #[stable(feature = "rust1", since = "1.0.0")]
598 pub fn iter_mut(&mut self) -> IterMut<'_, T> {
599 IterMut { inner: Item { opt: self.as_mut() } }
602 /////////////////////////////////////////////////////////////////////////
603 // Boolean operations on the values, eager and lazy
604 /////////////////////////////////////////////////////////////////////////
606 /// Returns [`None`] if the option is [`None`], otherwise returns `optb`.
608 /// [`None`]: #variant.None
614 /// let y: Option<&str> = None;
615 /// assert_eq!(x.and(y), None);
617 /// let x: Option<u32> = None;
618 /// let y = Some("foo");
619 /// assert_eq!(x.and(y), None);
622 /// let y = Some("foo");
623 /// assert_eq!(x.and(y), Some("foo"));
625 /// let x: Option<u32> = None;
626 /// let y: Option<&str> = None;
627 /// assert_eq!(x.and(y), None);
630 #[stable(feature = "rust1", since = "1.0.0")]
631 pub fn and<U>(self, optb: Option<U>) -> Option<U> {
638 /// Returns [`None`] if the option is [`None`], otherwise calls `f` with the
639 /// wrapped value and returns the result.
641 /// Some languages call this operation flatmap.
643 /// [`None`]: #variant.None
648 /// fn sq(x: u32) -> Option<u32> { Some(x * x) }
649 /// fn nope(_: u32) -> Option<u32> { None }
651 /// assert_eq!(Some(2).and_then(sq).and_then(sq), Some(16));
652 /// assert_eq!(Some(2).and_then(sq).and_then(nope), None);
653 /// assert_eq!(Some(2).and_then(nope).and_then(sq), None);
654 /// assert_eq!(None.and_then(sq).and_then(sq), None);
657 #[stable(feature = "rust1", since = "1.0.0")]
658 pub fn and_then<U, F: FnOnce(T) -> Option<U>>(self, f: F) -> Option<U> {
665 /// Returns [`None`] if the option is [`None`], otherwise calls `predicate`
666 /// with the wrapped value and returns:
668 /// - [`Some(t)`] if `predicate` returns `true` (where `t` is the wrapped
670 /// - [`None`] if `predicate` returns `false`.
672 /// This function works similar to [`Iterator::filter()`]. You can imagine
673 /// the `Option<T>` being an iterator over one or zero elements. `filter()`
674 /// lets you decide which elements to keep.
679 /// fn is_even(n: &i32) -> bool {
683 /// assert_eq!(None.filter(is_even), None);
684 /// assert_eq!(Some(3).filter(is_even), None);
685 /// assert_eq!(Some(4).filter(is_even), Some(4));
688 /// [`None`]: #variant.None
689 /// [`Some(t)`]: #variant.Some
690 /// [`Iterator::filter()`]: ../../std/iter/trait.Iterator.html#method.filter
692 #[stable(feature = "option_filter", since = "1.27.0")]
693 pub fn filter<P: FnOnce(&T) -> bool>(self, predicate: P) -> Self {
694 if let Some(x) = self {
702 /// Returns the option if it contains a value, otherwise returns `optb`.
704 /// Arguments passed to `or` are eagerly evaluated; if you are passing the
705 /// result of a function call, it is recommended to use [`or_else`], which is
706 /// lazily evaluated.
708 /// [`or_else`]: #method.or_else
715 /// assert_eq!(x.or(y), Some(2));
718 /// let y = Some(100);
719 /// assert_eq!(x.or(y), Some(100));
722 /// let y = Some(100);
723 /// assert_eq!(x.or(y), Some(2));
725 /// let x: Option<u32> = None;
727 /// assert_eq!(x.or(y), None);
730 #[stable(feature = "rust1", since = "1.0.0")]
731 pub fn or(self, optb: Option<T>) -> Option<T> {
738 /// Returns the option if it contains a value, otherwise calls `f` and
739 /// returns the result.
744 /// fn nobody() -> Option<&'static str> { None }
745 /// fn vikings() -> Option<&'static str> { Some("vikings") }
747 /// assert_eq!(Some("barbarians").or_else(vikings), Some("barbarians"));
748 /// assert_eq!(None.or_else(vikings), Some("vikings"));
749 /// assert_eq!(None.or_else(nobody), None);
752 #[stable(feature = "rust1", since = "1.0.0")]
753 pub fn or_else<F: FnOnce() -> Option<T>>(self, f: F) -> Option<T> {
760 /// Returns [`Some`] if exactly one of `self`, `optb` is [`Some`], otherwise returns [`None`].
762 /// [`Some`]: #variant.Some
763 /// [`None`]: #variant.None
769 /// let y: Option<u32> = None;
770 /// assert_eq!(x.xor(y), Some(2));
772 /// let x: Option<u32> = None;
774 /// assert_eq!(x.xor(y), Some(2));
778 /// assert_eq!(x.xor(y), None);
780 /// let x: Option<u32> = None;
781 /// let y: Option<u32> = None;
782 /// assert_eq!(x.xor(y), None);
785 #[stable(feature = "option_xor", since = "1.37.0")]
786 pub fn xor(self, optb: Option<T>) -> Option<T> {
788 (Some(a), None) => Some(a),
789 (None, Some(b)) => Some(b),
794 /////////////////////////////////////////////////////////////////////////
795 // Entry-like operations to insert if None and return a reference
796 /////////////////////////////////////////////////////////////////////////
798 /// Inserts `v` into the option if it is [`None`], then
799 /// returns a mutable reference to the contained value.
801 /// [`None`]: #variant.None
806 /// let mut x = None;
809 /// let y: &mut u32 = x.get_or_insert(5);
810 /// assert_eq!(y, &5);
815 /// assert_eq!(x, Some(7));
818 #[stable(feature = "option_entry", since = "1.20.0")]
819 pub fn get_or_insert(&mut self, v: T) -> &mut T {
820 self.get_or_insert_with(|| v)
823 /// Inserts a value computed from `f` into the option if it is [`None`], then
824 /// returns a mutable reference to the contained value.
826 /// [`None`]: #variant.None
831 /// let mut x = None;
834 /// let y: &mut u32 = x.get_or_insert_with(|| 5);
835 /// assert_eq!(y, &5);
840 /// assert_eq!(x, Some(7));
843 #[stable(feature = "option_entry", since = "1.20.0")]
844 pub fn get_or_insert_with<F: FnOnce() -> T>(&mut self, f: F) -> &mut T {
845 if let None = *self {
850 Some(ref mut v) => v,
851 None => unsafe { hint::unreachable_unchecked() },
855 /////////////////////////////////////////////////////////////////////////
857 /////////////////////////////////////////////////////////////////////////
859 /// Takes the value out of the option, leaving a [`None`] in its place.
861 /// [`None`]: #variant.None
866 /// let mut x = Some(2);
867 /// let y = x.take();
868 /// assert_eq!(x, None);
869 /// assert_eq!(y, Some(2));
871 /// let mut x: Option<u32> = None;
872 /// let y = x.take();
873 /// assert_eq!(x, None);
874 /// assert_eq!(y, None);
877 #[stable(feature = "rust1", since = "1.0.0")]
878 pub fn take(&mut self) -> Option<T> {
882 /// Replaces the actual value in the option by the value given in parameter,
883 /// returning the old value if present,
884 /// leaving a [`Some`] in its place without deinitializing either one.
886 /// [`Some`]: #variant.Some
891 /// let mut x = Some(2);
892 /// let old = x.replace(5);
893 /// assert_eq!(x, Some(5));
894 /// assert_eq!(old, Some(2));
896 /// let mut x = None;
897 /// let old = x.replace(3);
898 /// assert_eq!(x, Some(3));
899 /// assert_eq!(old, None);
902 #[stable(feature = "option_replace", since = "1.31.0")]
903 pub fn replace(&mut self, value: T) -> Option<T> {
904 mem::replace(self, Some(value))
908 impl<T: Copy> Option<&T> {
909 /// Maps an `Option<&T>` to an `Option<T>` by copying the contents of the
916 /// let opt_x = Some(&x);
917 /// assert_eq!(opt_x, Some(&12));
918 /// let copied = opt_x.copied();
919 /// assert_eq!(copied, Some(12));
921 #[stable(feature = "copied", since = "1.35.0")]
922 pub fn copied(self) -> Option<T> {
927 impl<T: Copy> Option<&mut T> {
928 /// Maps an `Option<&mut T>` to an `Option<T>` by copying the contents of the
935 /// let opt_x = Some(&mut x);
936 /// assert_eq!(opt_x, Some(&mut 12));
937 /// let copied = opt_x.copied();
938 /// assert_eq!(copied, Some(12));
940 #[stable(feature = "copied", since = "1.35.0")]
941 pub fn copied(self) -> Option<T> {
946 impl<T: Clone> Option<&T> {
947 /// Maps an `Option<&T>` to an `Option<T>` by cloning the contents of the
954 /// let opt_x = Some(&x);
955 /// assert_eq!(opt_x, Some(&12));
956 /// let cloned = opt_x.cloned();
957 /// assert_eq!(cloned, Some(12));
959 #[stable(feature = "rust1", since = "1.0.0")]
960 pub fn cloned(self) -> Option<T> {
961 self.map(|t| t.clone())
965 impl<T: Clone> Option<&mut T> {
966 /// Maps an `Option<&mut T>` to an `Option<T>` by cloning the contents of the
973 /// let opt_x = Some(&mut x);
974 /// assert_eq!(opt_x, Some(&mut 12));
975 /// let cloned = opt_x.cloned();
976 /// assert_eq!(cloned, Some(12));
978 #[stable(since = "1.26.0", feature = "option_ref_mut_cloned")]
979 pub fn cloned(self) -> Option<T> {
980 self.map(|t| t.clone())
984 impl<T: fmt::Debug> Option<T> {
985 /// Unwraps an option, expecting [`None`] and returning nothing.
989 /// Panics if the value is a [`Some`], with a panic message including the
990 /// passed message, and the content of the [`Some`].
992 /// [`Some`]: #variant.Some
993 /// [`None`]: #variant.None
998 /// #![feature(option_expect_none)]
1000 /// use std::collections::HashMap;
1001 /// let mut squares = HashMap::new();
1002 /// for i in -10..=10 {
1003 /// // This will not panic, since all keys are unique.
1004 /// squares.insert(i, i * i).expect_none("duplicate key");
1008 /// ```{.should_panic}
1009 /// #![feature(option_expect_none)]
1011 /// use std::collections::HashMap;
1012 /// let mut sqrts = HashMap::new();
1013 /// for i in -10..=10 {
1014 /// // This will panic, since both negative and positive `i` will
1015 /// // insert the same `i * i` key, returning the old `Some(i)`.
1016 /// sqrts.insert(i * i, i).expect_none("duplicate key");
1021 #[unstable(feature = "option_expect_none", reason = "newly added", issue = "62633")]
1022 pub fn expect_none(self, msg: &str) {
1023 if let Some(val) = self {
1024 expect_none_failed(msg, &val);
1028 /// Unwraps an option, expecting [`None`] and returning nothing.
1032 /// Panics if the value is a [`Some`], with a custom panic message provided
1033 /// by the [`Some`]'s value.
1035 /// [`Some(v)`]: #variant.Some
1036 /// [`None`]: #variant.None
1041 /// #![feature(option_unwrap_none)]
1043 /// use std::collections::HashMap;
1044 /// let mut squares = HashMap::new();
1045 /// for i in -10..=10 {
1046 /// // This will not panic, since all keys are unique.
1047 /// squares.insert(i, i * i).unwrap_none();
1051 /// ```{.should_panic}
1052 /// #![feature(option_unwrap_none)]
1054 /// use std::collections::HashMap;
1055 /// let mut sqrts = HashMap::new();
1056 /// for i in -10..=10 {
1057 /// // This will panic, since both negative and positive `i` will
1058 /// // insert the same `i * i` key, returning the old `Some(i)`.
1059 /// sqrts.insert(i * i, i).unwrap_none();
1064 #[unstable(feature = "option_unwrap_none", reason = "newly added", issue = "62633")]
1065 pub fn unwrap_none(self) {
1066 if let Some(val) = self {
1067 expect_none_failed("called `Option::unwrap_none()` on a `Some` value", &val);
1072 impl<T: Default> Option<T> {
1073 /// Returns the contained value or a default
1075 /// Consumes the `self` argument then, if [`Some`], returns the contained
1076 /// value, otherwise if [`None`], returns the [default value] for that
1081 /// Converts a string to an integer, turning poorly-formed strings
1082 /// into 0 (the default value for integers). [`parse`] converts
1083 /// a string to any other type that implements [`FromStr`], returning
1084 /// [`None`] on error.
1087 /// let good_year_from_input = "1909";
1088 /// let bad_year_from_input = "190blarg";
1089 /// let good_year = good_year_from_input.parse().ok().unwrap_or_default();
1090 /// let bad_year = bad_year_from_input.parse().ok().unwrap_or_default();
1092 /// assert_eq!(1909, good_year);
1093 /// assert_eq!(0, bad_year);
1096 /// [`Some`]: #variant.Some
1097 /// [`None`]: #variant.None
1098 /// [default value]: ../default/trait.Default.html#tymethod.default
1099 /// [`parse`]: ../../std/primitive.str.html#method.parse
1100 /// [`FromStr`]: ../../std/str/trait.FromStr.html
1102 #[stable(feature = "rust1", since = "1.0.0")]
1103 pub fn unwrap_or_default(self) -> T {
1106 None => Default::default(),
1111 impl<T: Deref> Option<T> {
1112 /// Converts from `Option<T>` (or `&Option<T>`) to `Option<&T::Target>`.
1114 /// Leaves the original Option in-place, creating a new one with a reference
1115 /// to the original one, additionally coercing the contents via [`Deref`].
1117 /// [`Deref`]: ../../std/ops/trait.Deref.html
1122 /// let x: Option<String> = Some("hey".to_owned());
1123 /// assert_eq!(x.as_deref(), Some("hey"));
1125 /// let x: Option<String> = None;
1126 /// assert_eq!(x.as_deref(), None);
1128 #[stable(feature = "option_deref", since = "1.40.0")]
1129 pub fn as_deref(&self) -> Option<&T::Target> {
1130 self.as_ref().map(|t| t.deref())
1134 impl<T: DerefMut> Option<T> {
1135 /// Converts from `Option<T>` (or `&mut Option<T>`) to `Option<&mut T::Target>`.
1137 /// Leaves the original `Option` in-place, creating a new one containing a mutable reference to
1138 /// the inner type's `Deref::Target` type.
1143 /// let mut x: Option<String> = Some("hey".to_owned());
1144 /// assert_eq!(x.as_deref_mut().map(|x| {
1145 /// x.make_ascii_uppercase();
1147 /// }), Some("HEY".to_owned().as_mut_str()));
1149 #[stable(feature = "option_deref", since = "1.40.0")]
1150 pub fn as_deref_mut(&mut self) -> Option<&mut T::Target> {
1151 self.as_mut().map(|t| t.deref_mut())
1155 impl<T, E> Option<Result<T, E>> {
1156 /// Transposes an `Option` of a [`Result`] into a [`Result`] of an `Option`.
1158 /// [`None`] will be mapped to [`Ok`]`(`[`None`]`)`.
1159 /// [`Some`]`(`[`Ok`]`(_))` and [`Some`]`(`[`Err`]`(_))` will be mapped to
1160 /// [`Ok`]`(`[`Some`]`(_))` and [`Err`]`(_)`.
1162 /// [`None`]: #variant.None
1163 /// [`Ok`]: ../../std/result/enum.Result.html#variant.Ok
1164 /// [`Some`]: #variant.Some
1165 /// [`Err`]: ../../std/result/enum.Result.html#variant.Err
1170 /// #[derive(Debug, Eq, PartialEq)]
1173 /// let x: Result<Option<i32>, SomeErr> = Ok(Some(5));
1174 /// let y: Option<Result<i32, SomeErr>> = Some(Ok(5));
1175 /// assert_eq!(x, y.transpose());
1178 #[stable(feature = "transpose_result", since = "1.33.0")]
1179 pub fn transpose(self) -> Result<Option<T>, E> {
1181 Some(Ok(x)) => Ok(Some(x)),
1182 Some(Err(e)) => Err(e),
1188 // This is a separate function to reduce the code size of .expect() itself.
1192 fn expect_failed(msg: &str) -> ! {
1196 // This is a separate function to reduce the code size of .expect_none() itself.
1200 fn expect_none_failed(msg: &str, value: &dyn fmt::Debug) -> ! {
1201 panic!("{}: {:?}", msg, value)
1204 /////////////////////////////////////////////////////////////////////////////
1205 // Trait implementations
1206 /////////////////////////////////////////////////////////////////////////////
1208 #[stable(feature = "rust1", since = "1.0.0")]
1209 impl<T: Clone> Clone for Option<T> {
1211 fn clone(&self) -> Self {
1213 Some(x) => Some(x.clone()),
1219 fn clone_from(&mut self, source: &Self) {
1220 match (self, source) {
1221 (Some(to), Some(from)) => to.clone_from(from),
1222 (to, from) => *to = from.clone(),
1227 #[stable(feature = "rust1", since = "1.0.0")]
1228 impl<T> Default for Option<T> {
1229 /// Returns [`None`][Option::None].
1234 /// let opt: Option<u32> = Option::default();
1235 /// assert!(opt.is_none());
1238 fn default() -> Option<T> {
1243 #[stable(feature = "rust1", since = "1.0.0")]
1244 impl<T> IntoIterator for Option<T> {
1246 type IntoIter = IntoIter<T>;
1248 /// Returns a consuming iterator over the possibly contained value.
1253 /// let x = Some("string");
1254 /// let v: Vec<&str> = x.into_iter().collect();
1255 /// assert_eq!(v, ["string"]);
1258 /// let v: Vec<&str> = x.into_iter().collect();
1259 /// assert!(v.is_empty());
1262 fn into_iter(self) -> IntoIter<T> {
1263 IntoIter { inner: Item { opt: self } }
1267 #[stable(since = "1.4.0", feature = "option_iter")]
1268 impl<'a, T> IntoIterator for &'a Option<T> {
1270 type IntoIter = Iter<'a, T>;
1272 fn into_iter(self) -> Iter<'a, T> {
1277 #[stable(since = "1.4.0", feature = "option_iter")]
1278 impl<'a, T> IntoIterator for &'a mut Option<T> {
1279 type Item = &'a mut T;
1280 type IntoIter = IterMut<'a, T>;
1282 fn into_iter(self) -> IterMut<'a, T> {
1287 #[stable(since = "1.12.0", feature = "option_from")]
1288 impl<T> From<T> for Option<T> {
1289 fn from(val: T) -> Option<T> {
1294 #[stable(feature = "option_ref_from_ref_option", since = "1.30.0")]
1295 impl<'a, T> From<&'a Option<T>> for Option<&'a T> {
1296 fn from(o: &'a Option<T>) -> Option<&'a T> {
1301 #[stable(feature = "option_ref_from_ref_option", since = "1.30.0")]
1302 impl<'a, T> From<&'a mut Option<T>> for Option<&'a mut T> {
1303 fn from(o: &'a mut Option<T>) -> Option<&'a mut T> {
1308 /////////////////////////////////////////////////////////////////////////////
1309 // The Option Iterators
1310 /////////////////////////////////////////////////////////////////////////////
1312 #[derive(Clone, Debug)]
1317 impl<A> Iterator for Item<A> {
1321 fn next(&mut self) -> Option<A> {
1326 fn size_hint(&self) -> (usize, Option<usize>) {
1328 Some(_) => (1, Some(1)),
1329 None => (0, Some(0)),
1334 impl<A> DoubleEndedIterator for Item<A> {
1336 fn next_back(&mut self) -> Option<A> {
1341 impl<A> ExactSizeIterator for Item<A> {}
1342 impl<A> FusedIterator for Item<A> {}
1343 unsafe impl<A> TrustedLen for Item<A> {}
1345 /// An iterator over a reference to the [`Some`] variant of an [`Option`].
1347 /// The iterator yields one value if the [`Option`] is a [`Some`], otherwise none.
1349 /// This `struct` is created by the [`Option::iter`] function.
1351 /// [`Option`]: enum.Option.html
1352 /// [`Some`]: enum.Option.html#variant.Some
1353 /// [`Option::iter`]: enum.Option.html#method.iter
1354 #[stable(feature = "rust1", since = "1.0.0")]
1356 pub struct Iter<'a, A: 'a> {
1360 #[stable(feature = "rust1", since = "1.0.0")]
1361 impl<'a, A> Iterator for Iter<'a, A> {
1365 fn next(&mut self) -> Option<&'a A> {
1369 fn size_hint(&self) -> (usize, Option<usize>) {
1370 self.inner.size_hint()
1374 #[stable(feature = "rust1", since = "1.0.0")]
1375 impl<'a, A> DoubleEndedIterator for Iter<'a, A> {
1377 fn next_back(&mut self) -> Option<&'a A> {
1378 self.inner.next_back()
1382 #[stable(feature = "rust1", since = "1.0.0")]
1383 impl<A> ExactSizeIterator for Iter<'_, A> {}
1385 #[stable(feature = "fused", since = "1.26.0")]
1386 impl<A> FusedIterator for Iter<'_, A> {}
1388 #[unstable(feature = "trusted_len", issue = "37572")]
1389 unsafe impl<A> TrustedLen for Iter<'_, A> {}
1391 #[stable(feature = "rust1", since = "1.0.0")]
1392 impl<A> Clone for Iter<'_, A> {
1394 fn clone(&self) -> Self {
1395 Iter { inner: self.inner.clone() }
1399 /// An iterator over a mutable reference to the [`Some`] variant of an [`Option`].
1401 /// The iterator yields one value if the [`Option`] is a [`Some`], otherwise none.
1403 /// This `struct` is created by the [`Option::iter_mut`] function.
1405 /// [`Option`]: enum.Option.html
1406 /// [`Some`]: enum.Option.html#variant.Some
1407 /// [`Option::iter_mut`]: enum.Option.html#method.iter_mut
1408 #[stable(feature = "rust1", since = "1.0.0")]
1410 pub struct IterMut<'a, A: 'a> {
1411 inner: Item<&'a mut A>,
1414 #[stable(feature = "rust1", since = "1.0.0")]
1415 impl<'a, A> Iterator for IterMut<'a, A> {
1416 type Item = &'a mut A;
1419 fn next(&mut self) -> Option<&'a mut A> {
1423 fn size_hint(&self) -> (usize, Option<usize>) {
1424 self.inner.size_hint()
1428 #[stable(feature = "rust1", since = "1.0.0")]
1429 impl<'a, A> DoubleEndedIterator for IterMut<'a, A> {
1431 fn next_back(&mut self) -> Option<&'a mut A> {
1432 self.inner.next_back()
1436 #[stable(feature = "rust1", since = "1.0.0")]
1437 impl<A> ExactSizeIterator for IterMut<'_, A> {}
1439 #[stable(feature = "fused", since = "1.26.0")]
1440 impl<A> FusedIterator for IterMut<'_, A> {}
1441 #[unstable(feature = "trusted_len", issue = "37572")]
1442 unsafe impl<A> TrustedLen for IterMut<'_, A> {}
1444 /// An iterator over the value in [`Some`] variant of an [`Option`].
1446 /// The iterator yields one value if the [`Option`] is a [`Some`], otherwise none.
1448 /// This `struct` is created by the [`Option::into_iter`] function.
1450 /// [`Option`]: enum.Option.html
1451 /// [`Some`]: enum.Option.html#variant.Some
1452 /// [`Option::into_iter`]: enum.Option.html#method.into_iter
1453 #[derive(Clone, Debug)]
1454 #[stable(feature = "rust1", since = "1.0.0")]
1455 pub struct IntoIter<A> {
1459 #[stable(feature = "rust1", since = "1.0.0")]
1460 impl<A> Iterator for IntoIter<A> {
1464 fn next(&mut self) -> Option<A> {
1468 fn size_hint(&self) -> (usize, Option<usize>) {
1469 self.inner.size_hint()
1473 #[stable(feature = "rust1", since = "1.0.0")]
1474 impl<A> DoubleEndedIterator for IntoIter<A> {
1476 fn next_back(&mut self) -> Option<A> {
1477 self.inner.next_back()
1481 #[stable(feature = "rust1", since = "1.0.0")]
1482 impl<A> ExactSizeIterator for IntoIter<A> {}
1484 #[stable(feature = "fused", since = "1.26.0")]
1485 impl<A> FusedIterator for IntoIter<A> {}
1487 #[unstable(feature = "trusted_len", issue = "37572")]
1488 unsafe impl<A> TrustedLen for IntoIter<A> {}
1490 /////////////////////////////////////////////////////////////////////////////
1492 /////////////////////////////////////////////////////////////////////////////
1494 #[stable(feature = "rust1", since = "1.0.0")]
1495 impl<A, V: FromIterator<A>> FromIterator<Option<A>> for Option<V> {
1496 /// Takes each element in the [`Iterator`]: if it is [`None`][Option::None],
1497 /// no further elements are taken, and the [`None`][Option::None] is
1498 /// returned. Should no [`None`][Option::None] occur, a container with the
1499 /// values of each [`Option`] is returned.
1503 /// Here is an example which increments every integer in a vector.
1504 /// We use the checked variant of `add` that returns `None` when the
1505 /// calculation would result in an overflow.
1508 /// let items = vec![0_u16, 1, 2];
1510 /// let res: Option<Vec<u16>> = items
1512 /// .map(|x| x.checked_add(1))
1515 /// assert_eq!(res, Some(vec![1, 2, 3]));
1518 /// As you can see, this will return the expected, valid items.
1520 /// Here is another example that tries to subtract one from another list
1521 /// of integers, this time checking for underflow:
1524 /// let items = vec![2_u16, 1, 0];
1526 /// let res: Option<Vec<u16>> = items
1528 /// .map(|x| x.checked_sub(1))
1531 /// assert_eq!(res, None);
1534 /// Since the last element is zero, it would underflow. Thus, the resulting
1535 /// value is `None`.
1537 /// Here is a variation on the previous example, showing that no
1538 /// further elements are taken from `iter` after the first `None`.
1541 /// let items = vec![3_u16, 2, 1, 10];
1543 /// let mut shared = 0;
1545 /// let res: Option<Vec<u16>> = items
1547 /// .map(|x| { shared += x; x.checked_sub(2) })
1550 /// assert_eq!(res, None);
1551 /// assert_eq!(shared, 6);
1554 /// Since the third element caused an underflow, no further elements were taken,
1555 /// so the final value of `shared` is 6 (= `3 + 2 + 1`), not 16.
1557 /// [`Iterator`]: ../iter/trait.Iterator.html
1559 fn from_iter<I: IntoIterator<Item = Option<A>>>(iter: I) -> Option<V> {
1560 // FIXME(#11084): This could be replaced with Iterator::scan when this
1561 // performance bug is closed.
1563 iter.into_iter().map(|x| x.ok_or(())).collect::<Result<_, _>>().ok()
1567 /// The error type that results from applying the try operator (`?`) to a `None` value. If you wish
1568 /// to allow `x?` (where `x` is an `Option<T>`) to be converted into your error type, you can
1569 /// implement `impl From<NoneError>` for `YourErrorType`. In that case, `x?` within a function that
1570 /// returns `Result<_, YourErrorType>` will translate a `None` value into an `Err` result.
1571 #[unstable(feature = "try_trait", issue = "42327")]
1572 #[derive(Clone, Copy, PartialEq, PartialOrd, Eq, Ord, Debug, Hash)]
1573 pub struct NoneError;
1575 #[unstable(feature = "try_trait", issue = "42327")]
1576 impl<T> ops::Try for Option<T> {
1578 type Error = NoneError;
1581 fn into_result(self) -> Result<T, NoneError> {
1582 self.ok_or(NoneError)
1586 fn from_ok(v: T) -> Self {
1591 fn from_error(_: NoneError) -> Self {
1596 impl<T> Option<Option<T>> {
1597 /// Converts from `Option<Option<T>>` to `Option<T>`
1602 /// let x: Option<Option<u32>> = Some(Some(6));
1603 /// assert_eq!(Some(6), x.flatten());
1605 /// let x: Option<Option<u32>> = Some(None);
1606 /// assert_eq!(None, x.flatten());
1608 /// let x: Option<Option<u32>> = None;
1609 /// assert_eq!(None, x.flatten());
1611 /// Flattening once only removes one level of nesting:
1613 /// let x: Option<Option<Option<u32>>> = Some(Some(Some(6)));
1614 /// assert_eq!(Some(Some(6)), x.flatten());
1615 /// assert_eq!(Some(6), x.flatten().flatten());
1618 #[stable(feature = "option_flattening", since = "1.40.0")]
1619 pub fn flatten(self) -> Option<T> {
1620 self.and_then(convert::identity)