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 //! [`Box<T>`]: ../../std/boxed/struct.Box.html
132 #![stable(feature = "rust1", since = "1.0.0")]
134 use crate::iter::{FromIterator, FusedIterator, TrustedLen};
137 convert, fmt, hint, mem,
138 ops::{self, Deref, DerefMut},
141 /// The `Option` type. See [the module level documentation](self) for more.
142 #[derive(Copy, PartialEq, PartialOrd, Eq, Ord, Debug, Hash)]
143 #[rustc_diagnostic_item = "option_type"]
144 #[stable(feature = "rust1", since = "1.0.0")]
147 #[stable(feature = "rust1", since = "1.0.0")]
150 #[stable(feature = "rust1", since = "1.0.0")]
151 Some(#[stable(feature = "rust1", since = "1.0.0")] T),
154 /////////////////////////////////////////////////////////////////////////////
155 // Type implementation
156 /////////////////////////////////////////////////////////////////////////////
159 /////////////////////////////////////////////////////////////////////////
160 // Querying the contained values
161 /////////////////////////////////////////////////////////////////////////
163 /// Returns `true` if the option is a [`Some`] value.
168 /// let x: Option<u32> = Some(2);
169 /// assert_eq!(x.is_some(), true);
171 /// let x: Option<u32> = None;
172 /// assert_eq!(x.is_some(), false);
174 #[must_use = "if you intended to assert that this has a value, consider `.unwrap()` instead"]
176 #[rustc_const_unstable(feature = "const_option", issue = "67441")]
177 #[stable(feature = "rust1", since = "1.0.0")]
178 pub const fn is_some(&self) -> bool {
179 matches!(*self, Some(_))
182 /// Returns `true` if the option is a [`None`] value.
187 /// let x: Option<u32> = Some(2);
188 /// assert_eq!(x.is_none(), false);
190 /// let x: Option<u32> = None;
191 /// assert_eq!(x.is_none(), true);
193 #[must_use = "if you intended to assert that this doesn't have a value, consider \
194 `.and_then(|| panic!(\"`Option` had a value when expected `None`\"))` instead"]
196 #[rustc_const_unstable(feature = "const_option", issue = "67441")]
197 #[stable(feature = "rust1", since = "1.0.0")]
198 pub const fn is_none(&self) -> bool {
202 /// Returns `true` if the option is a [`Some`] value containing the given value.
207 /// #![feature(option_result_contains)]
209 /// let x: Option<u32> = Some(2);
210 /// assert_eq!(x.contains(&2), true);
212 /// let x: Option<u32> = Some(3);
213 /// assert_eq!(x.contains(&2), false);
215 /// let x: Option<u32> = None;
216 /// assert_eq!(x.contains(&2), false);
220 #[unstable(feature = "option_result_contains", issue = "62358")]
221 pub fn contains<U>(&self, x: &U) -> bool
231 /////////////////////////////////////////////////////////////////////////
232 // Adapter for working with references
233 /////////////////////////////////////////////////////////////////////////
235 /// Converts from `&Option<T>` to `Option<&T>`.
239 /// Converts an `Option<`[`String`]`>` into an `Option<`[`usize`]`>`, preserving the original.
240 /// The [`map`] method takes the `self` argument by value, consuming the original,
241 /// so this technique uses `as_ref` to first take an `Option` to a reference
242 /// to the value inside the original.
244 /// [`map`]: Option::map
245 /// [`String`]: ../../std/string/struct.String.html
248 /// let text: Option<String> = Some("Hello, world!".to_string());
249 /// // First, cast `Option<String>` to `Option<&String>` with `as_ref`,
250 /// // then consume *that* with `map`, leaving `text` on the stack.
251 /// let text_length: Option<usize> = text.as_ref().map(|s| s.len());
252 /// println!("still can print text: {:?}", text);
255 #[rustc_const_unstable(feature = "const_option", issue = "67441")]
256 #[stable(feature = "rust1", since = "1.0.0")]
257 pub const fn as_ref(&self) -> Option<&T> {
259 Some(ref x) => Some(x),
264 /// Converts from `&mut Option<T>` to `Option<&mut T>`.
269 /// let mut x = Some(2);
270 /// match x.as_mut() {
271 /// Some(v) => *v = 42,
274 /// assert_eq!(x, Some(42));
277 #[stable(feature = "rust1", since = "1.0.0")]
278 pub fn as_mut(&mut self) -> Option<&mut T> {
280 Some(ref mut x) => Some(x),
285 /// Converts from [`Pin`]`<&Option<T>>` to `Option<`[`Pin`]`<&T>>`.
287 #[stable(feature = "pin", since = "1.33.0")]
288 pub fn as_pin_ref(self: Pin<&Self>) -> Option<Pin<&T>> {
289 // SAFETY: `x` is guaranteed to be pinned because it comes from `self`
291 unsafe { Pin::get_ref(self).as_ref().map(|x| Pin::new_unchecked(x)) }
294 /// Converts from [`Pin`]`<&mut Option<T>>` to `Option<`[`Pin`]`<&mut T>>`.
296 #[stable(feature = "pin", since = "1.33.0")]
297 pub fn as_pin_mut(self: Pin<&mut Self>) -> Option<Pin<&mut T>> {
298 // SAFETY: `get_unchecked_mut` is never used to move the `Option` inside `self`.
299 // `x` is guaranteed to be pinned because it comes from `self` which is pinned.
300 unsafe { Pin::get_unchecked_mut(self).as_mut().map(|x| Pin::new_unchecked(x)) }
303 /////////////////////////////////////////////////////////////////////////
304 // Getting to contained values
305 /////////////////////////////////////////////////////////////////////////
307 /// Returns the contained [`Some`] value, consuming the `self` value.
311 /// Panics if the value is a [`None`] with a custom panic message provided by
317 /// let x = Some("value");
318 /// assert_eq!(x.expect("fruits are healthy"), "value");
321 /// ```{.should_panic}
322 /// let x: Option<&str> = None;
323 /// x.expect("fruits are healthy"); // panics with `fruits are healthy`
327 #[stable(feature = "rust1", since = "1.0.0")]
328 pub fn expect(self, msg: &str) -> T {
331 None => expect_failed(msg),
335 /// Returns the contained [`Some`] value, consuming the `self` value.
337 /// Because this function may panic, its use is generally discouraged.
338 /// Instead, prefer to use pattern matching and handle the [`None`]
339 /// case explicitly, or call [`unwrap_or`], [`unwrap_or_else`], or
340 /// [`unwrap_or_default`].
342 /// [`unwrap_or`]: Option::unwrap_or
343 /// [`unwrap_or_else`]: Option::unwrap_or_else
344 /// [`unwrap_or_default`]: Option::unwrap_or_default
348 /// Panics if the self value equals [`None`].
353 /// let x = Some("air");
354 /// assert_eq!(x.unwrap(), "air");
357 /// ```{.should_panic}
358 /// let x: Option<&str> = None;
359 /// assert_eq!(x.unwrap(), "air"); // fails
363 #[stable(feature = "rust1", since = "1.0.0")]
364 #[rustc_const_unstable(feature = "const_option", issue = "67441")]
365 pub const fn unwrap(self) -> T {
368 None => panic!("called `Option::unwrap()` on a `None` value"),
372 /// Returns the contained [`Some`] value or a provided default.
374 /// Arguments passed to `unwrap_or` are eagerly evaluated; if you are passing
375 /// the result of a function call, it is recommended to use [`unwrap_or_else`],
376 /// which is lazily evaluated.
378 /// [`unwrap_or_else`]: Option::unwrap_or_else
383 /// assert_eq!(Some("car").unwrap_or("bike"), "car");
384 /// assert_eq!(None.unwrap_or("bike"), "bike");
387 #[stable(feature = "rust1", since = "1.0.0")]
388 pub fn unwrap_or(self, default: T) -> T {
395 /// Returns the contained [`Some`] value or computes it from a closure.
401 /// assert_eq!(Some(4).unwrap_or_else(|| 2 * k), 4);
402 /// assert_eq!(None.unwrap_or_else(|| 2 * k), 20);
405 #[stable(feature = "rust1", since = "1.0.0")]
406 pub fn unwrap_or_else<F: FnOnce() -> T>(self, f: F) -> T {
413 /////////////////////////////////////////////////////////////////////////
414 // Transforming contained values
415 /////////////////////////////////////////////////////////////////////////
417 /// Maps an `Option<T>` to `Option<U>` by applying a function to a contained value.
421 /// Converts an `Option<`[`String`]`>` into an `Option<`[`usize`]`>`, consuming the original:
423 /// [`String`]: ../../std/string/struct.String.html
425 /// let maybe_some_string = Some(String::from("Hello, World!"));
426 /// // `Option::map` takes self *by value*, consuming `maybe_some_string`
427 /// let maybe_some_len = maybe_some_string.map(|s| s.len());
429 /// assert_eq!(maybe_some_len, Some(13));
432 #[stable(feature = "rust1", since = "1.0.0")]
433 pub fn map<U, F: FnOnce(T) -> U>(self, f: F) -> Option<U> {
435 Some(x) => Some(f(x)),
440 /// Applies a function to the contained value (if any),
441 /// or returns the provided default (if not).
443 /// Arguments passed to `map_or` are eagerly evaluated; if you are passing
444 /// the result of a function call, it is recommended to use [`map_or_else`],
445 /// which is lazily evaluated.
447 /// [`map_or_else`]: Option::map_or_else
452 /// let x = Some("foo");
453 /// assert_eq!(x.map_or(42, |v| v.len()), 3);
455 /// let x: Option<&str> = None;
456 /// assert_eq!(x.map_or(42, |v| v.len()), 42);
459 #[stable(feature = "rust1", since = "1.0.0")]
460 pub fn map_or<U, F: FnOnce(T) -> U>(self, default: U, f: F) -> U {
467 /// Applies a function to the contained value (if any),
468 /// or computes a default (if not).
475 /// let x = Some("foo");
476 /// assert_eq!(x.map_or_else(|| 2 * k, |v| v.len()), 3);
478 /// let x: Option<&str> = None;
479 /// assert_eq!(x.map_or_else(|| 2 * k, |v| v.len()), 42);
482 #[stable(feature = "rust1", since = "1.0.0")]
483 pub fn map_or_else<U, D: FnOnce() -> U, F: FnOnce(T) -> U>(self, default: D, f: F) -> U {
490 /// Transforms the `Option<T>` into a [`Result<T, E>`], mapping [`Some(v)`] to
491 /// [`Ok(v)`] and [`None`] to [`Err(err)`].
493 /// Arguments passed to `ok_or` are eagerly evaluated; if you are passing the
494 /// result of a function call, it is recommended to use [`ok_or_else`], which is
495 /// lazily evaluated.
497 /// [`Result<T, E>`]: Result
499 /// [`Err(err)`]: Err
500 /// [`Some(v)`]: Some
501 /// [`ok_or_else`]: Option::ok_or_else
506 /// let x = Some("foo");
507 /// assert_eq!(x.ok_or(0), Ok("foo"));
509 /// let x: Option<&str> = None;
510 /// assert_eq!(x.ok_or(0), Err(0));
513 #[stable(feature = "rust1", since = "1.0.0")]
514 pub fn ok_or<E>(self, err: E) -> Result<T, E> {
521 /// Transforms the `Option<T>` into a [`Result<T, E>`], mapping [`Some(v)`] to
522 /// [`Ok(v)`] and [`None`] to [`Err(err())`].
524 /// [`Result<T, E>`]: Result
526 /// [`Err(err())`]: Err
527 /// [`Some(v)`]: Some
532 /// let x = Some("foo");
533 /// assert_eq!(x.ok_or_else(|| 0), Ok("foo"));
535 /// let x: Option<&str> = None;
536 /// assert_eq!(x.ok_or_else(|| 0), Err(0));
539 #[stable(feature = "rust1", since = "1.0.0")]
540 pub fn ok_or_else<E, F: FnOnce() -> E>(self, err: F) -> Result<T, E> {
547 /////////////////////////////////////////////////////////////////////////
548 // Iterator constructors
549 /////////////////////////////////////////////////////////////////////////
551 /// Returns an iterator over the possibly contained value.
557 /// assert_eq!(x.iter().next(), Some(&4));
559 /// let x: Option<u32> = None;
560 /// assert_eq!(x.iter().next(), None);
563 #[rustc_const_unstable(feature = "const_option", issue = "67441")]
564 #[stable(feature = "rust1", since = "1.0.0")]
565 pub const fn iter(&self) -> Iter<'_, T> {
566 Iter { inner: Item { opt: self.as_ref() } }
569 /// Returns a mutable iterator over the possibly contained value.
574 /// let mut x = Some(4);
575 /// match x.iter_mut().next() {
576 /// Some(v) => *v = 42,
579 /// assert_eq!(x, Some(42));
581 /// let mut x: Option<u32> = None;
582 /// assert_eq!(x.iter_mut().next(), None);
585 #[stable(feature = "rust1", since = "1.0.0")]
586 pub fn iter_mut(&mut self) -> IterMut<'_, T> {
587 IterMut { inner: Item { opt: self.as_mut() } }
590 /////////////////////////////////////////////////////////////////////////
591 // Boolean operations on the values, eager and lazy
592 /////////////////////////////////////////////////////////////////////////
594 /// Returns [`None`] if the option is [`None`], otherwise returns `optb`.
600 /// let y: Option<&str> = None;
601 /// assert_eq!(x.and(y), None);
603 /// let x: Option<u32> = None;
604 /// let y = Some("foo");
605 /// assert_eq!(x.and(y), None);
608 /// let y = Some("foo");
609 /// assert_eq!(x.and(y), Some("foo"));
611 /// let x: Option<u32> = None;
612 /// let y: Option<&str> = None;
613 /// assert_eq!(x.and(y), None);
616 #[stable(feature = "rust1", since = "1.0.0")]
617 pub fn and<U>(self, optb: Option<U>) -> Option<U> {
624 /// Returns [`None`] if the option is [`None`], otherwise calls `f` with the
625 /// wrapped value and returns the result.
627 /// Some languages call this operation flatmap.
632 /// fn sq(x: u32) -> Option<u32> { Some(x * x) }
633 /// fn nope(_: u32) -> Option<u32> { None }
635 /// assert_eq!(Some(2).and_then(sq).and_then(sq), Some(16));
636 /// assert_eq!(Some(2).and_then(sq).and_then(nope), None);
637 /// assert_eq!(Some(2).and_then(nope).and_then(sq), None);
638 /// assert_eq!(None.and_then(sq).and_then(sq), None);
641 #[stable(feature = "rust1", since = "1.0.0")]
642 pub fn and_then<U, F: FnOnce(T) -> Option<U>>(self, f: F) -> Option<U> {
649 /// Returns [`None`] if the option is [`None`], otherwise calls `predicate`
650 /// with the wrapped value and returns:
652 /// - [`Some(t)`] if `predicate` returns `true` (where `t` is the wrapped
654 /// - [`None`] if `predicate` returns `false`.
656 /// This function works similar to [`Iterator::filter()`]. You can imagine
657 /// the `Option<T>` being an iterator over one or zero elements. `filter()`
658 /// lets you decide which elements to keep.
663 /// fn is_even(n: &i32) -> bool {
667 /// assert_eq!(None.filter(is_even), None);
668 /// assert_eq!(Some(3).filter(is_even), None);
669 /// assert_eq!(Some(4).filter(is_even), Some(4));
673 #[stable(feature = "option_filter", since = "1.27.0")]
674 pub fn filter<P: FnOnce(&T) -> bool>(self, predicate: P) -> Self {
675 if let Some(x) = self {
683 /// Returns the option if it contains a value, otherwise returns `optb`.
685 /// Arguments passed to `or` are eagerly evaluated; if you are passing the
686 /// result of a function call, it is recommended to use [`or_else`], which is
687 /// lazily evaluated.
689 /// [`or_else`]: Option::or_else
696 /// assert_eq!(x.or(y), Some(2));
699 /// let y = Some(100);
700 /// assert_eq!(x.or(y), Some(100));
703 /// let y = Some(100);
704 /// assert_eq!(x.or(y), Some(2));
706 /// let x: Option<u32> = None;
708 /// assert_eq!(x.or(y), None);
711 #[stable(feature = "rust1", since = "1.0.0")]
712 pub fn or(self, optb: Option<T>) -> Option<T> {
719 /// Returns the option if it contains a value, otherwise calls `f` and
720 /// returns the result.
725 /// fn nobody() -> Option<&'static str> { None }
726 /// fn vikings() -> Option<&'static str> { Some("vikings") }
728 /// assert_eq!(Some("barbarians").or_else(vikings), Some("barbarians"));
729 /// assert_eq!(None.or_else(vikings), Some("vikings"));
730 /// assert_eq!(None.or_else(nobody), None);
733 #[stable(feature = "rust1", since = "1.0.0")]
734 pub fn or_else<F: FnOnce() -> Option<T>>(self, f: F) -> Option<T> {
741 /// Returns [`Some`] if exactly one of `self`, `optb` is [`Some`], otherwise returns [`None`].
747 /// let y: Option<u32> = None;
748 /// assert_eq!(x.xor(y), Some(2));
750 /// let x: Option<u32> = None;
752 /// assert_eq!(x.xor(y), Some(2));
756 /// assert_eq!(x.xor(y), None);
758 /// let x: Option<u32> = None;
759 /// let y: Option<u32> = None;
760 /// assert_eq!(x.xor(y), None);
763 #[stable(feature = "option_xor", since = "1.37.0")]
764 pub fn xor(self, optb: Option<T>) -> Option<T> {
766 (Some(a), None) => Some(a),
767 (None, Some(b)) => Some(b),
772 /////////////////////////////////////////////////////////////////////////
773 // Entry-like operations to insert if None and return a reference
774 /////////////////////////////////////////////////////////////////////////
776 /// Inserts `v` into the option if it is [`None`], then
777 /// returns a mutable reference to the contained value.
782 /// let mut x = None;
785 /// let y: &mut u32 = x.get_or_insert(5);
786 /// assert_eq!(y, &5);
791 /// assert_eq!(x, Some(7));
794 #[stable(feature = "option_entry", since = "1.20.0")]
795 pub fn get_or_insert(&mut self, v: T) -> &mut T {
796 self.get_or_insert_with(|| v)
799 /// Inserts a value computed from `f` into the option if it is [`None`], then
800 /// returns a mutable reference to the contained value.
805 /// let mut x = None;
808 /// let y: &mut u32 = x.get_or_insert_with(|| 5);
809 /// assert_eq!(y, &5);
814 /// assert_eq!(x, Some(7));
817 #[stable(feature = "option_entry", since = "1.20.0")]
818 pub fn get_or_insert_with<F: FnOnce() -> T>(&mut self, f: F) -> &mut T {
819 if let None = *self {
824 Some(ref mut v) => v,
825 // SAFETY: a `None` variant for `self` would have been replaced by a `Some`
826 // variant in the code above.
827 None => unsafe { hint::unreachable_unchecked() },
831 /////////////////////////////////////////////////////////////////////////
833 /////////////////////////////////////////////////////////////////////////
835 /// Takes the value out of the option, leaving a [`None`] in its place.
840 /// let mut x = Some(2);
841 /// let y = x.take();
842 /// assert_eq!(x, None);
843 /// assert_eq!(y, Some(2));
845 /// let mut x: Option<u32> = None;
846 /// let y = x.take();
847 /// assert_eq!(x, None);
848 /// assert_eq!(y, None);
851 #[stable(feature = "rust1", since = "1.0.0")]
852 pub fn take(&mut self) -> Option<T> {
856 /// Replaces the actual value in the option by the value given in parameter,
857 /// returning the old value if present,
858 /// leaving a [`Some`] in its place without deinitializing either one.
863 /// let mut x = Some(2);
864 /// let old = x.replace(5);
865 /// assert_eq!(x, Some(5));
866 /// assert_eq!(old, Some(2));
868 /// let mut x = None;
869 /// let old = x.replace(3);
870 /// assert_eq!(x, Some(3));
871 /// assert_eq!(old, None);
874 #[stable(feature = "option_replace", since = "1.31.0")]
875 pub fn replace(&mut self, value: T) -> Option<T> {
876 mem::replace(self, Some(value))
879 /// Zips `self` with another `Option`.
881 /// If `self` is `Some(s)` and `other` is `Some(o)`, this method returns `Some((s, o))`.
882 /// Otherwise, `None` is returned.
888 /// let y = Some("hi");
889 /// let z = None::<u8>;
891 /// assert_eq!(x.zip(y), Some((1, "hi")));
892 /// assert_eq!(x.zip(z), None);
894 #[stable(feature = "option_zip_option", since = "1.46.0")]
895 pub fn zip<U>(self, other: Option<U>) -> Option<(T, U)> {
896 match (self, other) {
897 (Some(a), Some(b)) => Some((a, b)),
902 /// Zips `self` and another `Option` with function `f`.
904 /// If `self` is `Some(s)` and `other` is `Some(o)`, this method returns `Some(f(s, o))`.
905 /// Otherwise, `None` is returned.
910 /// #![feature(option_zip)]
912 /// #[derive(Debug, PartialEq)]
919 /// fn new(x: f64, y: f64) -> Self {
924 /// let x = Some(17.5);
925 /// let y = Some(42.7);
927 /// assert_eq!(x.zip_with(y, Point::new), Some(Point { x: 17.5, y: 42.7 }));
928 /// assert_eq!(x.zip_with(None, Point::new), None);
930 #[unstable(feature = "option_zip", issue = "70086")]
931 pub fn zip_with<U, F, R>(self, other: Option<U>, f: F) -> Option<R>
933 F: FnOnce(T, U) -> R,
935 Some(f(self?, other?))
939 impl<T: Copy> Option<&T> {
940 /// Maps an `Option<&T>` to an `Option<T>` by copying the contents of the
947 /// let opt_x = Some(&x);
948 /// assert_eq!(opt_x, Some(&12));
949 /// let copied = opt_x.copied();
950 /// assert_eq!(copied, Some(12));
952 #[stable(feature = "copied", since = "1.35.0")]
953 pub fn copied(self) -> Option<T> {
958 impl<T: Copy> Option<&mut T> {
959 /// Maps an `Option<&mut T>` to an `Option<T>` by copying the contents of the
966 /// let opt_x = Some(&mut x);
967 /// assert_eq!(opt_x, Some(&mut 12));
968 /// let copied = opt_x.copied();
969 /// assert_eq!(copied, Some(12));
971 #[stable(feature = "copied", since = "1.35.0")]
972 pub fn copied(self) -> Option<T> {
977 impl<T: Clone> Option<&T> {
978 /// Maps an `Option<&T>` to an `Option<T>` by cloning the contents of the
985 /// let opt_x = Some(&x);
986 /// assert_eq!(opt_x, Some(&12));
987 /// let cloned = opt_x.cloned();
988 /// assert_eq!(cloned, Some(12));
990 #[stable(feature = "rust1", since = "1.0.0")]
991 pub fn cloned(self) -> Option<T> {
992 self.map(|t| t.clone())
996 impl<T: Clone> Option<&mut T> {
997 /// Maps an `Option<&mut T>` to an `Option<T>` by cloning the contents of the
1004 /// let opt_x = Some(&mut x);
1005 /// assert_eq!(opt_x, Some(&mut 12));
1006 /// let cloned = opt_x.cloned();
1007 /// assert_eq!(cloned, Some(12));
1009 #[stable(since = "1.26.0", feature = "option_ref_mut_cloned")]
1010 pub fn cloned(self) -> Option<T> {
1011 self.map(|t| t.clone())
1015 impl<T: fmt::Debug> Option<T> {
1016 /// Consumes `self` while expecting [`None`] and returning nothing.
1020 /// Panics if the value is a [`Some`], with a panic message including the
1021 /// passed message, and the content of the [`Some`].
1026 /// #![feature(option_expect_none)]
1028 /// use std::collections::HashMap;
1029 /// let mut squares = HashMap::new();
1030 /// for i in -10..=10 {
1031 /// // This will not panic, since all keys are unique.
1032 /// squares.insert(i, i * i).expect_none("duplicate key");
1036 /// ```{.should_panic}
1037 /// #![feature(option_expect_none)]
1039 /// use std::collections::HashMap;
1040 /// let mut sqrts = HashMap::new();
1041 /// for i in -10..=10 {
1042 /// // This will panic, since both negative and positive `i` will
1043 /// // insert the same `i * i` key, returning the old `Some(i)`.
1044 /// sqrts.insert(i * i, i).expect_none("duplicate key");
1049 #[unstable(feature = "option_expect_none", reason = "newly added", issue = "62633")]
1050 pub fn expect_none(self, msg: &str) {
1051 if let Some(val) = self {
1052 expect_none_failed(msg, &val);
1056 /// Consumes `self` while expecting [`None`] and returning nothing.
1060 /// Panics if the value is a [`Some`], with a custom panic message provided
1061 /// by the [`Some`]'s value.
1063 /// [`Some(v)`]: Some
1068 /// #![feature(option_unwrap_none)]
1070 /// use std::collections::HashMap;
1071 /// let mut squares = HashMap::new();
1072 /// for i in -10..=10 {
1073 /// // This will not panic, since all keys are unique.
1074 /// squares.insert(i, i * i).unwrap_none();
1078 /// ```{.should_panic}
1079 /// #![feature(option_unwrap_none)]
1081 /// use std::collections::HashMap;
1082 /// let mut sqrts = HashMap::new();
1083 /// for i in -10..=10 {
1084 /// // This will panic, since both negative and positive `i` will
1085 /// // insert the same `i * i` key, returning the old `Some(i)`.
1086 /// sqrts.insert(i * i, i).unwrap_none();
1091 #[unstable(feature = "option_unwrap_none", reason = "newly added", issue = "62633")]
1092 pub fn unwrap_none(self) {
1093 if let Some(val) = self {
1094 expect_none_failed("called `Option::unwrap_none()` on a `Some` value", &val);
1099 impl<T: Default> Option<T> {
1100 /// Returns the contained [`Some`] value or a default
1102 /// Consumes the `self` argument then, if [`Some`], returns the contained
1103 /// value, otherwise if [`None`], returns the [default value] for that
1108 /// Converts a string to an integer, turning poorly-formed strings
1109 /// into 0 (the default value for integers). [`parse`] converts
1110 /// a string to any other type that implements [`FromStr`], returning
1111 /// [`None`] on error.
1114 /// let good_year_from_input = "1909";
1115 /// let bad_year_from_input = "190blarg";
1116 /// let good_year = good_year_from_input.parse().ok().unwrap_or_default();
1117 /// let bad_year = bad_year_from_input.parse().ok().unwrap_or_default();
1119 /// assert_eq!(1909, good_year);
1120 /// assert_eq!(0, bad_year);
1123 /// [default value]: Default::default
1124 /// [`parse`]: str::parse
1125 /// [`FromStr`]: crate::str::FromStr
1127 #[stable(feature = "rust1", since = "1.0.0")]
1128 pub fn unwrap_or_default(self) -> T {
1131 None => Default::default(),
1136 impl<T: Deref> Option<T> {
1137 /// Converts from `Option<T>` (or `&Option<T>`) to `Option<&T::Target>`.
1139 /// Leaves the original Option in-place, creating a new one with a reference
1140 /// to the original one, additionally coercing the contents via [`Deref`].
1145 /// let x: Option<String> = Some("hey".to_owned());
1146 /// assert_eq!(x.as_deref(), Some("hey"));
1148 /// let x: Option<String> = None;
1149 /// assert_eq!(x.as_deref(), None);
1151 #[stable(feature = "option_deref", since = "1.40.0")]
1152 pub fn as_deref(&self) -> Option<&T::Target> {
1153 self.as_ref().map(|t| t.deref())
1157 impl<T: DerefMut> Option<T> {
1158 /// Converts from `Option<T>` (or `&mut Option<T>`) to `Option<&mut T::Target>`.
1160 /// Leaves the original `Option` in-place, creating a new one containing a mutable reference to
1161 /// the inner type's `Deref::Target` type.
1166 /// let mut x: Option<String> = Some("hey".to_owned());
1167 /// assert_eq!(x.as_deref_mut().map(|x| {
1168 /// x.make_ascii_uppercase();
1170 /// }), Some("HEY".to_owned().as_mut_str()));
1172 #[stable(feature = "option_deref", since = "1.40.0")]
1173 pub fn as_deref_mut(&mut self) -> Option<&mut T::Target> {
1174 self.as_mut().map(|t| t.deref_mut())
1178 impl<T, E> Option<Result<T, E>> {
1179 /// Transposes an `Option` of a [`Result`] into a [`Result`] of an `Option`.
1181 /// [`None`] will be mapped to [`Ok`]`(`[`None`]`)`.
1182 /// [`Some`]`(`[`Ok`]`(_))` and [`Some`]`(`[`Err`]`(_))` will be mapped to
1183 /// [`Ok`]`(`[`Some`]`(_))` and [`Err`]`(_)`.
1188 /// #[derive(Debug, Eq, PartialEq)]
1191 /// let x: Result<Option<i32>, SomeErr> = Ok(Some(5));
1192 /// let y: Option<Result<i32, SomeErr>> = Some(Ok(5));
1193 /// assert_eq!(x, y.transpose());
1196 #[stable(feature = "transpose_result", since = "1.33.0")]
1197 pub fn transpose(self) -> Result<Option<T>, E> {
1199 Some(Ok(x)) => Ok(Some(x)),
1200 Some(Err(e)) => Err(e),
1206 // This is a separate function to reduce the code size of .expect() itself.
1210 fn expect_failed(msg: &str) -> ! {
1214 // This is a separate function to reduce the code size of .expect_none() itself.
1218 fn expect_none_failed(msg: &str, value: &dyn fmt::Debug) -> ! {
1219 panic!("{}: {:?}", msg, value)
1222 /////////////////////////////////////////////////////////////////////////////
1223 // Trait implementations
1224 /////////////////////////////////////////////////////////////////////////////
1226 #[stable(feature = "rust1", since = "1.0.0")]
1227 impl<T: Clone> Clone for Option<T> {
1229 fn clone(&self) -> Self {
1231 Some(x) => Some(x.clone()),
1237 fn clone_from(&mut self, source: &Self) {
1238 match (self, source) {
1239 (Some(to), Some(from)) => to.clone_from(from),
1240 (to, from) => *to = from.clone(),
1245 #[stable(feature = "rust1", since = "1.0.0")]
1246 impl<T> Default for Option<T> {
1247 /// Returns [`None`][Option::None].
1252 /// let opt: Option<u32> = Option::default();
1253 /// assert!(opt.is_none());
1256 fn default() -> Option<T> {
1261 #[stable(feature = "rust1", since = "1.0.0")]
1262 impl<T> IntoIterator for Option<T> {
1264 type IntoIter = IntoIter<T>;
1266 /// Returns a consuming iterator over the possibly contained value.
1271 /// let x = Some("string");
1272 /// let v: Vec<&str> = x.into_iter().collect();
1273 /// assert_eq!(v, ["string"]);
1276 /// let v: Vec<&str> = x.into_iter().collect();
1277 /// assert!(v.is_empty());
1280 fn into_iter(self) -> IntoIter<T> {
1281 IntoIter { inner: Item { opt: self } }
1285 #[stable(since = "1.4.0", feature = "option_iter")]
1286 impl<'a, T> IntoIterator for &'a Option<T> {
1288 type IntoIter = Iter<'a, T>;
1290 fn into_iter(self) -> Iter<'a, T> {
1295 #[stable(since = "1.4.0", feature = "option_iter")]
1296 impl<'a, T> IntoIterator for &'a mut Option<T> {
1297 type Item = &'a mut T;
1298 type IntoIter = IterMut<'a, T>;
1300 fn into_iter(self) -> IterMut<'a, T> {
1305 #[stable(since = "1.12.0", feature = "option_from")]
1306 impl<T> From<T> for Option<T> {
1307 /// Copies `val` into a new `Some`.
1312 /// let o: Option<u8> = Option::from(67);
1314 /// assert_eq!(Some(67), o);
1316 fn from(val: T) -> Option<T> {
1321 #[stable(feature = "option_ref_from_ref_option", since = "1.30.0")]
1322 impl<'a, T> From<&'a Option<T>> for Option<&'a T> {
1323 /// Converts from `&Option<T>` to `Option<&T>`.
1327 /// Converts an `Option<`[`String`]`>` into an `Option<`[`usize`]`>`, preserving the original.
1328 /// The [`map`] method takes the `self` argument by value, consuming the original,
1329 /// so this technique uses `as_ref` to first take an `Option` to a reference
1330 /// to the value inside the original.
1332 /// [`map`]: Option::map
1333 /// [`String`]: ../../std/string/struct.String.html
1336 /// let s: Option<String> = Some(String::from("Hello, Rustaceans!"));
1337 /// let o: Option<usize> = Option::from(&s).map(|ss: &String| ss.len());
1339 /// println!("Can still print s: {:?}", s);
1341 /// assert_eq!(o, Some(18));
1343 fn from(o: &'a Option<T>) -> Option<&'a T> {
1348 #[stable(feature = "option_ref_from_ref_option", since = "1.30.0")]
1349 impl<'a, T> From<&'a mut Option<T>> for Option<&'a mut T> {
1350 /// Converts from `&mut Option<T>` to `Option<&mut T>`
1355 /// let mut s = Some(String::from("Hello"));
1356 /// let o: Option<&mut String> = Option::from(&mut s);
1359 /// Some(t) => *t = String::from("Hello, Rustaceans!"),
1363 /// assert_eq!(s, Some(String::from("Hello, Rustaceans!")));
1365 fn from(o: &'a mut Option<T>) -> Option<&'a mut T> {
1370 /////////////////////////////////////////////////////////////////////////////
1371 // The Option Iterators
1372 /////////////////////////////////////////////////////////////////////////////
1374 #[derive(Clone, Debug)]
1379 impl<A> Iterator for Item<A> {
1383 fn next(&mut self) -> Option<A> {
1388 fn size_hint(&self) -> (usize, Option<usize>) {
1390 Some(_) => (1, Some(1)),
1391 None => (0, Some(0)),
1396 impl<A> DoubleEndedIterator for Item<A> {
1398 fn next_back(&mut self) -> Option<A> {
1403 impl<A> ExactSizeIterator for Item<A> {}
1404 impl<A> FusedIterator for Item<A> {}
1405 unsafe impl<A> TrustedLen for Item<A> {}
1407 /// An iterator over a reference to the [`Some`] variant of an [`Option`].
1409 /// The iterator yields one value if the [`Option`] is a [`Some`], otherwise none.
1411 /// This `struct` is created by the [`Option::iter`] function.
1412 #[stable(feature = "rust1", since = "1.0.0")]
1414 pub struct Iter<'a, A: 'a> {
1418 #[stable(feature = "rust1", since = "1.0.0")]
1419 impl<'a, A> Iterator for Iter<'a, A> {
1423 fn next(&mut self) -> Option<&'a A> {
1427 fn size_hint(&self) -> (usize, Option<usize>) {
1428 self.inner.size_hint()
1432 #[stable(feature = "rust1", since = "1.0.0")]
1433 impl<'a, A> DoubleEndedIterator for Iter<'a, A> {
1435 fn next_back(&mut self) -> Option<&'a A> {
1436 self.inner.next_back()
1440 #[stable(feature = "rust1", since = "1.0.0")]
1441 impl<A> ExactSizeIterator for Iter<'_, A> {}
1443 #[stable(feature = "fused", since = "1.26.0")]
1444 impl<A> FusedIterator for Iter<'_, A> {}
1446 #[unstable(feature = "trusted_len", issue = "37572")]
1447 unsafe impl<A> TrustedLen for Iter<'_, A> {}
1449 #[stable(feature = "rust1", since = "1.0.0")]
1450 impl<A> Clone for Iter<'_, A> {
1452 fn clone(&self) -> Self {
1453 Iter { inner: self.inner.clone() }
1457 /// An iterator over a mutable reference to the [`Some`] variant of an [`Option`].
1459 /// The iterator yields one value if the [`Option`] is a [`Some`], otherwise none.
1461 /// This `struct` is created by the [`Option::iter_mut`] function.
1462 #[stable(feature = "rust1", since = "1.0.0")]
1464 pub struct IterMut<'a, A: 'a> {
1465 inner: Item<&'a mut A>,
1468 #[stable(feature = "rust1", since = "1.0.0")]
1469 impl<'a, A> Iterator for IterMut<'a, A> {
1470 type Item = &'a mut A;
1473 fn next(&mut self) -> Option<&'a mut A> {
1477 fn size_hint(&self) -> (usize, Option<usize>) {
1478 self.inner.size_hint()
1482 #[stable(feature = "rust1", since = "1.0.0")]
1483 impl<'a, A> DoubleEndedIterator for IterMut<'a, A> {
1485 fn next_back(&mut self) -> Option<&'a mut A> {
1486 self.inner.next_back()
1490 #[stable(feature = "rust1", since = "1.0.0")]
1491 impl<A> ExactSizeIterator for IterMut<'_, A> {}
1493 #[stable(feature = "fused", since = "1.26.0")]
1494 impl<A> FusedIterator for IterMut<'_, A> {}
1495 #[unstable(feature = "trusted_len", issue = "37572")]
1496 unsafe impl<A> TrustedLen for IterMut<'_, A> {}
1498 /// An iterator over the value in [`Some`] variant of an [`Option`].
1500 /// The iterator yields one value if the [`Option`] is a [`Some`], otherwise none.
1502 /// This `struct` is created by the [`Option::into_iter`] function.
1504 /// [`Option::into_iter`]: enum.Option.html#method.into_iter
1505 #[derive(Clone, Debug)]
1506 #[stable(feature = "rust1", since = "1.0.0")]
1507 pub struct IntoIter<A> {
1511 #[stable(feature = "rust1", since = "1.0.0")]
1512 impl<A> Iterator for IntoIter<A> {
1516 fn next(&mut self) -> Option<A> {
1520 fn size_hint(&self) -> (usize, Option<usize>) {
1521 self.inner.size_hint()
1525 #[stable(feature = "rust1", since = "1.0.0")]
1526 impl<A> DoubleEndedIterator for IntoIter<A> {
1528 fn next_back(&mut self) -> Option<A> {
1529 self.inner.next_back()
1533 #[stable(feature = "rust1", since = "1.0.0")]
1534 impl<A> ExactSizeIterator for IntoIter<A> {}
1536 #[stable(feature = "fused", since = "1.26.0")]
1537 impl<A> FusedIterator for IntoIter<A> {}
1539 #[unstable(feature = "trusted_len", issue = "37572")]
1540 unsafe impl<A> TrustedLen for IntoIter<A> {}
1542 /////////////////////////////////////////////////////////////////////////////
1544 /////////////////////////////////////////////////////////////////////////////
1546 #[stable(feature = "rust1", since = "1.0.0")]
1547 impl<A, V: FromIterator<A>> FromIterator<Option<A>> for Option<V> {
1548 /// Takes each element in the [`Iterator`]: if it is [`None`][Option::None],
1549 /// no further elements are taken, and the [`None`][Option::None] is
1550 /// returned. Should no [`None`][Option::None] occur, a container with the
1551 /// values of each [`Option`] is returned.
1555 /// Here is an example which increments every integer in a vector.
1556 /// We use the checked variant of `add` that returns `None` when the
1557 /// calculation would result in an overflow.
1560 /// let items = vec![0_u16, 1, 2];
1562 /// let res: Option<Vec<u16>> = items
1564 /// .map(|x| x.checked_add(1))
1567 /// assert_eq!(res, Some(vec![1, 2, 3]));
1570 /// As you can see, this will return the expected, valid items.
1572 /// Here is another example that tries to subtract one from another list
1573 /// of integers, this time checking for underflow:
1576 /// let items = vec![2_u16, 1, 0];
1578 /// let res: Option<Vec<u16>> = items
1580 /// .map(|x| x.checked_sub(1))
1583 /// assert_eq!(res, None);
1586 /// Since the last element is zero, it would underflow. Thus, the resulting
1587 /// value is `None`.
1589 /// Here is a variation on the previous example, showing that no
1590 /// further elements are taken from `iter` after the first `None`.
1593 /// let items = vec![3_u16, 2, 1, 10];
1595 /// let mut shared = 0;
1597 /// let res: Option<Vec<u16>> = items
1599 /// .map(|x| { shared += x; x.checked_sub(2) })
1602 /// assert_eq!(res, None);
1603 /// assert_eq!(shared, 6);
1606 /// Since the third element caused an underflow, no further elements were taken,
1607 /// so the final value of `shared` is 6 (= `3 + 2 + 1`), not 16.
1609 fn from_iter<I: IntoIterator<Item = Option<A>>>(iter: I) -> Option<V> {
1610 // FIXME(#11084): This could be replaced with Iterator::scan when this
1611 // performance bug is closed.
1613 iter.into_iter().map(|x| x.ok_or(())).collect::<Result<_, _>>().ok()
1617 /// The error type that results from applying the try operator (`?`) to a `None` value. If you wish
1618 /// to allow `x?` (where `x` is an `Option<T>`) to be converted into your error type, you can
1619 /// implement `impl From<NoneError>` for `YourErrorType`. In that case, `x?` within a function that
1620 /// returns `Result<_, YourErrorType>` will translate a `None` value into an `Err` result.
1621 #[rustc_diagnostic_item = "none_error"]
1622 #[unstable(feature = "try_trait", issue = "42327")]
1623 #[derive(Clone, Copy, PartialEq, PartialOrd, Eq, Ord, Debug, Hash)]
1624 pub struct NoneError;
1626 #[unstable(feature = "try_trait", issue = "42327")]
1627 impl<T> ops::Try for Option<T> {
1629 type Error = NoneError;
1632 fn into_result(self) -> Result<T, NoneError> {
1633 self.ok_or(NoneError)
1637 fn from_ok(v: T) -> Self {
1642 fn from_error(_: NoneError) -> Self {
1647 impl<T> Option<Option<T>> {
1648 /// Converts from `Option<Option<T>>` to `Option<T>`
1653 /// let x: Option<Option<u32>> = Some(Some(6));
1654 /// assert_eq!(Some(6), x.flatten());
1656 /// let x: Option<Option<u32>> = Some(None);
1657 /// assert_eq!(None, x.flatten());
1659 /// let x: Option<Option<u32>> = None;
1660 /// assert_eq!(None, x.flatten());
1662 /// Flattening once only removes one level of nesting:
1664 /// let x: Option<Option<Option<u32>>> = Some(Some(Some(6)));
1665 /// assert_eq!(Some(Some(6)), x.flatten());
1666 /// assert_eq!(Some(6), x.flatten().flatten());
1669 #[stable(feature = "option_flattening", since = "1.40.0")]
1670 pub fn flatten(self) -> Option<T> {
1671 self.and_then(convert::identity)