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"),
75 //! Rust guarantees to optimize the following types `T` such that
76 //! [`Option<T>`] has the same size as `T`:
81 //! * `fn`, `extern "C" fn`
82 //! * [`num::NonZero*`]
83 //! * [`ptr::NonNull<U>`]
84 //! * `#[repr(transparent)]` struct around one of the types in this list.
86 //! It is further guaranteed that, for the cases above, one can
87 //! [`mem::transmute`] from all valid values of `T` to `Option<T>` and
88 //! from `Some::<T>(_)` to `T` (but transmuting `None::<T>` to `T`
89 //! is undefined behaviour).
93 //! Basic pattern matching on [`Option`]:
96 //! let msg = Some("howdy");
98 //! // Take a reference to the contained string
99 //! if let Some(m) = &msg {
100 //! println!("{}", *m);
103 //! // Remove the contained string, destroying the Option
104 //! let unwrapped_msg = msg.unwrap_or("default message");
107 //! Initialize a result to [`None`] before a loop:
110 //! enum Kingdom { Plant(u32, &'static str), Animal(u32, &'static str) }
112 //! // A list of data to search through.
113 //! let all_the_big_things = [
114 //! Kingdom::Plant(250, "redwood"),
115 //! Kingdom::Plant(230, "noble fir"),
116 //! Kingdom::Plant(229, "sugar pine"),
117 //! Kingdom::Animal(25, "blue whale"),
118 //! Kingdom::Animal(19, "fin whale"),
119 //! Kingdom::Animal(15, "north pacific right whale"),
122 //! // We're going to search for the name of the biggest animal,
123 //! // but to start with we've just got `None`.
124 //! let mut name_of_biggest_animal = None;
125 //! let mut size_of_biggest_animal = 0;
126 //! for big_thing in &all_the_big_things {
127 //! match *big_thing {
128 //! Kingdom::Animal(size, name) if size > size_of_biggest_animal => {
129 //! // Now we've found the name of some big animal
130 //! size_of_biggest_animal = size;
131 //! name_of_biggest_animal = Some(name);
133 //! Kingdom::Animal(..) | Kingdom::Plant(..) => ()
137 //! match name_of_biggest_animal {
138 //! Some(name) => println!("the biggest animal is {}", name),
139 //! None => println!("there are no animals :("),
143 //! [`Box<T>`]: ../../std/boxed/struct.Box.html
144 //! [`Box<U>`]: ../../std/boxed/struct.Box.html
145 //! [`num::NonZero*`]: crate::num
146 //! [`ptr::NonNull<U>`]: crate::ptr::NonNull
148 #![stable(feature = "rust1", since = "1.0.0")]
150 use crate::iter::{FromIterator, FusedIterator, TrustedLen};
153 convert, fmt, hint, mem,
154 ops::{self, Deref, DerefMut},
157 /// The `Option` type. See [the module level documentation](self) for more.
158 #[derive(Copy, PartialEq, PartialOrd, Eq, Ord, Debug, Hash)]
159 #[rustc_diagnostic_item = "option_type"]
160 #[stable(feature = "rust1", since = "1.0.0")]
164 #[stable(feature = "rust1", since = "1.0.0")]
168 #[stable(feature = "rust1", since = "1.0.0")]
169 Some(#[stable(feature = "rust1", since = "1.0.0")] T),
172 /////////////////////////////////////////////////////////////////////////////
173 // Type implementation
174 /////////////////////////////////////////////////////////////////////////////
177 /////////////////////////////////////////////////////////////////////////
178 // Querying the contained values
179 /////////////////////////////////////////////////////////////////////////
181 /// Returns `true` if the option is a [`Some`] value.
186 /// let x: Option<u32> = Some(2);
187 /// assert_eq!(x.is_some(), true);
189 /// let x: Option<u32> = None;
190 /// assert_eq!(x.is_some(), false);
192 #[must_use = "if you intended to assert that this has a value, consider `.unwrap()` instead"]
194 #[rustc_const_stable(feature = "const_option", since = "1.48.0")]
195 #[stable(feature = "rust1", since = "1.0.0")]
196 pub const fn is_some(&self) -> bool {
197 matches!(*self, Some(_))
200 /// Returns `true` if the option is a [`None`] value.
205 /// let x: Option<u32> = Some(2);
206 /// assert_eq!(x.is_none(), false);
208 /// let x: Option<u32> = None;
209 /// assert_eq!(x.is_none(), true);
211 #[must_use = "if you intended to assert that this doesn't have a value, consider \
212 `.and_then(|| panic!(\"`Option` had a value when expected `None`\"))` instead"]
214 #[rustc_const_stable(feature = "const_option", since = "1.48.0")]
215 #[stable(feature = "rust1", since = "1.0.0")]
216 pub const fn is_none(&self) -> bool {
220 /// Returns `true` if the option is a [`Some`] value containing the given value.
225 /// #![feature(option_result_contains)]
227 /// let x: Option<u32> = Some(2);
228 /// assert_eq!(x.contains(&2), true);
230 /// let x: Option<u32> = Some(3);
231 /// assert_eq!(x.contains(&2), false);
233 /// let x: Option<u32> = None;
234 /// assert_eq!(x.contains(&2), false);
238 #[unstable(feature = "option_result_contains", issue = "62358")]
239 pub fn contains<U>(&self, x: &U) -> bool
249 /////////////////////////////////////////////////////////////////////////
250 // Adapter for working with references
251 /////////////////////////////////////////////////////////////////////////
253 /// Converts from `&Option<T>` to `Option<&T>`.
257 /// Converts an `Option<`[`String`]`>` into an `Option<`[`usize`]`>`, preserving the original.
258 /// The [`map`] method takes the `self` argument by value, consuming the original,
259 /// so this technique uses `as_ref` to first take an `Option` to a reference
260 /// to the value inside the original.
262 /// [`map`]: Option::map
263 /// [`String`]: ../../std/string/struct.String.html
266 /// let text: Option<String> = Some("Hello, world!".to_string());
267 /// // First, cast `Option<String>` to `Option<&String>` with `as_ref`,
268 /// // then consume *that* with `map`, leaving `text` on the stack.
269 /// let text_length: Option<usize> = text.as_ref().map(|s| s.len());
270 /// println!("still can print text: {:?}", text);
273 #[rustc_const_stable(feature = "const_option", since = "1.48.0")]
274 #[stable(feature = "rust1", since = "1.0.0")]
275 pub const fn as_ref(&self) -> Option<&T> {
277 Some(ref x) => Some(x),
282 /// Converts from `&mut Option<T>` to `Option<&mut T>`.
287 /// let mut x = Some(2);
288 /// match x.as_mut() {
289 /// Some(v) => *v = 42,
292 /// assert_eq!(x, Some(42));
295 #[stable(feature = "rust1", since = "1.0.0")]
296 pub fn as_mut(&mut self) -> Option<&mut T> {
298 Some(ref mut x) => Some(x),
303 /// Converts from [`Pin`]`<&Option<T>>` to `Option<`[`Pin`]`<&T>>`.
305 #[stable(feature = "pin", since = "1.33.0")]
306 pub fn as_pin_ref(self: Pin<&Self>) -> Option<Pin<&T>> {
307 // SAFETY: `x` is guaranteed to be pinned because it comes from `self`
309 unsafe { Pin::get_ref(self).as_ref().map(|x| Pin::new_unchecked(x)) }
312 /// Converts from [`Pin`]`<&mut Option<T>>` to `Option<`[`Pin`]`<&mut T>>`.
314 #[stable(feature = "pin", since = "1.33.0")]
315 pub fn as_pin_mut(self: Pin<&mut Self>) -> Option<Pin<&mut T>> {
316 // SAFETY: `get_unchecked_mut` is never used to move the `Option` inside `self`.
317 // `x` is guaranteed to be pinned because it comes from `self` which is pinned.
318 unsafe { Pin::get_unchecked_mut(self).as_mut().map(|x| Pin::new_unchecked(x)) }
321 /////////////////////////////////////////////////////////////////////////
322 // Getting to contained values
323 /////////////////////////////////////////////////////////////////////////
325 /// Returns the contained [`Some`] value, consuming the `self` value.
329 /// Panics if the value is a [`None`] with a custom panic message provided by
335 /// let x = Some("value");
336 /// assert_eq!(x.expect("fruits are healthy"), "value");
339 /// ```{.should_panic}
340 /// let x: Option<&str> = None;
341 /// x.expect("fruits are healthy"); // panics with `fruits are healthy`
345 #[stable(feature = "rust1", since = "1.0.0")]
346 pub fn expect(self, msg: &str) -> T {
349 None => expect_failed(msg),
353 /// Returns the contained [`Some`] value, consuming the `self` value.
355 /// Because this function may panic, its use is generally discouraged.
356 /// Instead, prefer to use pattern matching and handle the [`None`]
357 /// case explicitly, or call [`unwrap_or`], [`unwrap_or_else`], or
358 /// [`unwrap_or_default`].
360 /// [`unwrap_or`]: Option::unwrap_or
361 /// [`unwrap_or_else`]: Option::unwrap_or_else
362 /// [`unwrap_or_default`]: Option::unwrap_or_default
366 /// Panics if the self value equals [`None`].
371 /// let x = Some("air");
372 /// assert_eq!(x.unwrap(), "air");
375 /// ```{.should_panic}
376 /// let x: Option<&str> = None;
377 /// assert_eq!(x.unwrap(), "air"); // fails
381 #[stable(feature = "rust1", since = "1.0.0")]
382 #[rustc_const_unstable(feature = "const_option", issue = "67441")]
383 pub const fn unwrap(self) -> T {
386 None => panic!("called `Option::unwrap()` on a `None` value"),
390 /// Returns the contained [`Some`] value or a provided default.
392 /// Arguments passed to `unwrap_or` are eagerly evaluated; if you are passing
393 /// the result of a function call, it is recommended to use [`unwrap_or_else`],
394 /// which is lazily evaluated.
396 /// [`unwrap_or_else`]: Option::unwrap_or_else
401 /// assert_eq!(Some("car").unwrap_or("bike"), "car");
402 /// assert_eq!(None.unwrap_or("bike"), "bike");
405 #[stable(feature = "rust1", since = "1.0.0")]
406 pub fn unwrap_or(self, default: T) -> T {
413 /// Returns the contained [`Some`] value or computes it from a closure.
419 /// assert_eq!(Some(4).unwrap_or_else(|| 2 * k), 4);
420 /// assert_eq!(None.unwrap_or_else(|| 2 * k), 20);
423 #[stable(feature = "rust1", since = "1.0.0")]
424 pub fn unwrap_or_else<F: FnOnce() -> T>(self, f: F) -> T {
431 /////////////////////////////////////////////////////////////////////////
432 // Transforming contained values
433 /////////////////////////////////////////////////////////////////////////
435 /// Maps an `Option<T>` to `Option<U>` by applying a function to a contained value.
439 /// Converts an `Option<`[`String`]`>` into an `Option<`[`usize`]`>`, consuming the original:
441 /// [`String`]: ../../std/string/struct.String.html
443 /// let maybe_some_string = Some(String::from("Hello, World!"));
444 /// // `Option::map` takes self *by value*, consuming `maybe_some_string`
445 /// let maybe_some_len = maybe_some_string.map(|s| s.len());
447 /// assert_eq!(maybe_some_len, Some(13));
450 #[stable(feature = "rust1", since = "1.0.0")]
451 pub fn map<U, F: FnOnce(T) -> U>(self, f: F) -> Option<U> {
453 Some(x) => Some(f(x)),
458 /// Applies a function to the contained value (if any),
459 /// or returns the provided default (if not).
461 /// Arguments passed to `map_or` are eagerly evaluated; if you are passing
462 /// the result of a function call, it is recommended to use [`map_or_else`],
463 /// which is lazily evaluated.
465 /// [`map_or_else`]: Option::map_or_else
470 /// let x = Some("foo");
471 /// assert_eq!(x.map_or(42, |v| v.len()), 3);
473 /// let x: Option<&str> = None;
474 /// assert_eq!(x.map_or(42, |v| v.len()), 42);
477 #[stable(feature = "rust1", since = "1.0.0")]
478 pub fn map_or<U, F: FnOnce(T) -> U>(self, default: U, f: F) -> U {
485 /// Applies a function to the contained value (if any),
486 /// or computes a default (if not).
493 /// let x = Some("foo");
494 /// assert_eq!(x.map_or_else(|| 2 * k, |v| v.len()), 3);
496 /// let x: Option<&str> = None;
497 /// assert_eq!(x.map_or_else(|| 2 * k, |v| v.len()), 42);
500 #[stable(feature = "rust1", since = "1.0.0")]
501 pub fn map_or_else<U, D: FnOnce() -> U, F: FnOnce(T) -> U>(self, default: D, f: F) -> U {
508 /// Transforms the `Option<T>` into a [`Result<T, E>`], mapping [`Some(v)`] to
509 /// [`Ok(v)`] and [`None`] to [`Err(err)`].
511 /// Arguments passed to `ok_or` are eagerly evaluated; if you are passing the
512 /// result of a function call, it is recommended to use [`ok_or_else`], which is
513 /// lazily evaluated.
516 /// [`Err(err)`]: Err
517 /// [`Some(v)`]: Some
518 /// [`ok_or_else`]: Option::ok_or_else
523 /// let x = Some("foo");
524 /// assert_eq!(x.ok_or(0), Ok("foo"));
526 /// let x: Option<&str> = None;
527 /// assert_eq!(x.ok_or(0), Err(0));
530 #[stable(feature = "rust1", since = "1.0.0")]
531 pub fn ok_or<E>(self, err: E) -> Result<T, E> {
538 /// Transforms the `Option<T>` into a [`Result<T, E>`], mapping [`Some(v)`] to
539 /// [`Ok(v)`] and [`None`] to [`Err(err())`].
542 /// [`Err(err())`]: Err
543 /// [`Some(v)`]: Some
548 /// let x = Some("foo");
549 /// assert_eq!(x.ok_or_else(|| 0), Ok("foo"));
551 /// let x: Option<&str> = None;
552 /// assert_eq!(x.ok_or_else(|| 0), Err(0));
555 #[stable(feature = "rust1", since = "1.0.0")]
556 pub fn ok_or_else<E, F: FnOnce() -> E>(self, err: F) -> Result<T, E> {
563 /// Inserts `value` into the option then returns a mutable reference to it.
565 /// If the option already contains a value, the old value is dropped.
570 /// #![feature(option_insert)]
572 /// let mut opt = None;
573 /// let val = opt.insert(1);
574 /// assert_eq!(*val, 1);
575 /// assert_eq!(opt.unwrap(), 1);
576 /// let val = opt.insert(2);
577 /// assert_eq!(*val, 2);
579 /// assert_eq!(opt.unwrap(), 3);
582 #[unstable(feature = "option_insert", reason = "newly added", issue = "78271")]
583 pub fn insert(&mut self, value: T) -> &mut T {
588 // SAFETY: the code above just filled the option
589 None => unsafe { hint::unreachable_unchecked() },
593 /////////////////////////////////////////////////////////////////////////
594 // Iterator constructors
595 /////////////////////////////////////////////////////////////////////////
597 /// Returns an iterator over the possibly contained value.
603 /// assert_eq!(x.iter().next(), Some(&4));
605 /// let x: Option<u32> = None;
606 /// assert_eq!(x.iter().next(), None);
609 #[rustc_const_unstable(feature = "const_option", issue = "67441")]
610 #[stable(feature = "rust1", since = "1.0.0")]
611 pub const fn iter(&self) -> Iter<'_, T> {
612 Iter { inner: Item { opt: self.as_ref() } }
615 /// Returns a mutable iterator over the possibly contained value.
620 /// let mut x = Some(4);
621 /// match x.iter_mut().next() {
622 /// Some(v) => *v = 42,
625 /// assert_eq!(x, Some(42));
627 /// let mut x: Option<u32> = None;
628 /// assert_eq!(x.iter_mut().next(), None);
631 #[stable(feature = "rust1", since = "1.0.0")]
632 pub fn iter_mut(&mut self) -> IterMut<'_, T> {
633 IterMut { inner: Item { opt: self.as_mut() } }
636 /////////////////////////////////////////////////////////////////////////
637 // Boolean operations on the values, eager and lazy
638 /////////////////////////////////////////////////////////////////////////
640 /// Returns [`None`] if the option is [`None`], otherwise returns `optb`.
646 /// let y: Option<&str> = None;
647 /// assert_eq!(x.and(y), None);
649 /// let x: Option<u32> = None;
650 /// let y = Some("foo");
651 /// assert_eq!(x.and(y), None);
654 /// let y = Some("foo");
655 /// assert_eq!(x.and(y), Some("foo"));
657 /// let x: Option<u32> = None;
658 /// let y: Option<&str> = None;
659 /// assert_eq!(x.and(y), None);
662 #[stable(feature = "rust1", since = "1.0.0")]
663 pub fn and<U>(self, optb: Option<U>) -> Option<U> {
670 /// Returns [`None`] if the option is [`None`], otherwise calls `f` with the
671 /// wrapped value and returns the result.
673 /// Some languages call this operation flatmap.
678 /// fn sq(x: u32) -> Option<u32> { Some(x * x) }
679 /// fn nope(_: u32) -> Option<u32> { None }
681 /// assert_eq!(Some(2).and_then(sq).and_then(sq), Some(16));
682 /// assert_eq!(Some(2).and_then(sq).and_then(nope), None);
683 /// assert_eq!(Some(2).and_then(nope).and_then(sq), None);
684 /// assert_eq!(None.and_then(sq).and_then(sq), None);
687 #[stable(feature = "rust1", since = "1.0.0")]
688 pub fn and_then<U, F: FnOnce(T) -> Option<U>>(self, f: F) -> Option<U> {
695 /// Returns [`None`] if the option is [`None`], otherwise calls `predicate`
696 /// with the wrapped value and returns:
698 /// - [`Some(t)`] if `predicate` returns `true` (where `t` is the wrapped
700 /// - [`None`] if `predicate` returns `false`.
702 /// This function works similar to [`Iterator::filter()`]. You can imagine
703 /// the `Option<T>` being an iterator over one or zero elements. `filter()`
704 /// lets you decide which elements to keep.
709 /// fn is_even(n: &i32) -> bool {
713 /// assert_eq!(None.filter(is_even), None);
714 /// assert_eq!(Some(3).filter(is_even), None);
715 /// assert_eq!(Some(4).filter(is_even), Some(4));
718 /// [`Some(t)`]: Some
720 #[stable(feature = "option_filter", since = "1.27.0")]
721 pub fn filter<P: FnOnce(&T) -> bool>(self, predicate: P) -> Self {
722 if let Some(x) = self {
730 /// Returns the option if it contains a value, otherwise returns `optb`.
732 /// Arguments passed to `or` are eagerly evaluated; if you are passing the
733 /// result of a function call, it is recommended to use [`or_else`], which is
734 /// lazily evaluated.
736 /// [`or_else`]: Option::or_else
743 /// assert_eq!(x.or(y), Some(2));
746 /// let y = Some(100);
747 /// assert_eq!(x.or(y), Some(100));
750 /// let y = Some(100);
751 /// assert_eq!(x.or(y), Some(2));
753 /// let x: Option<u32> = None;
755 /// assert_eq!(x.or(y), None);
758 #[stable(feature = "rust1", since = "1.0.0")]
759 pub fn or(self, optb: Option<T>) -> Option<T> {
766 /// Returns the option if it contains a value, otherwise calls `f` and
767 /// returns the result.
772 /// fn nobody() -> Option<&'static str> { None }
773 /// fn vikings() -> Option<&'static str> { Some("vikings") }
775 /// assert_eq!(Some("barbarians").or_else(vikings), Some("barbarians"));
776 /// assert_eq!(None.or_else(vikings), Some("vikings"));
777 /// assert_eq!(None.or_else(nobody), None);
780 #[stable(feature = "rust1", since = "1.0.0")]
781 pub fn or_else<F: FnOnce() -> Option<T>>(self, f: F) -> Option<T> {
788 /// Returns [`Some`] if exactly one of `self`, `optb` is [`Some`], otherwise returns [`None`].
794 /// let y: Option<u32> = None;
795 /// assert_eq!(x.xor(y), Some(2));
797 /// let x: Option<u32> = None;
799 /// assert_eq!(x.xor(y), Some(2));
803 /// assert_eq!(x.xor(y), None);
805 /// let x: Option<u32> = None;
806 /// let y: Option<u32> = None;
807 /// assert_eq!(x.xor(y), None);
810 #[stable(feature = "option_xor", since = "1.37.0")]
811 pub fn xor(self, optb: Option<T>) -> Option<T> {
813 (Some(a), None) => Some(a),
814 (None, Some(b)) => Some(b),
819 /////////////////////////////////////////////////////////////////////////
820 // Entry-like operations to insert if None and return a reference
821 /////////////////////////////////////////////////////////////////////////
823 /// Inserts `value` into the option if it is [`None`], then
824 /// returns a mutable reference to the contained value.
829 /// let mut x = None;
832 /// let y: &mut u32 = x.get_or_insert(5);
833 /// assert_eq!(y, &5);
838 /// assert_eq!(x, Some(7));
841 #[stable(feature = "option_entry", since = "1.20.0")]
842 pub fn get_or_insert(&mut self, value: T) -> &mut T {
843 self.get_or_insert_with(|| value)
846 /// Inserts a value computed from `f` into the option if it is [`None`],
847 /// then returns a mutable reference to the contained value.
852 /// let mut x = None;
855 /// let y: &mut u32 = x.get_or_insert_with(|| 5);
856 /// assert_eq!(y, &5);
861 /// assert_eq!(x, Some(7));
864 #[stable(feature = "option_entry", since = "1.20.0")]
865 pub fn get_or_insert_with<F: FnOnce() -> T>(&mut self, f: F) -> &mut T {
866 if let None = *self {
872 // SAFETY: a `None` variant for `self` would have been replaced by a `Some`
873 // variant in the code above.
874 None => unsafe { hint::unreachable_unchecked() },
878 /////////////////////////////////////////////////////////////////////////
880 /////////////////////////////////////////////////////////////////////////
882 /// Takes the value out of the option, leaving a [`None`] in its place.
887 /// let mut x = Some(2);
888 /// let y = x.take();
889 /// assert_eq!(x, None);
890 /// assert_eq!(y, Some(2));
892 /// let mut x: Option<u32> = None;
893 /// let y = x.take();
894 /// assert_eq!(x, None);
895 /// assert_eq!(y, None);
898 #[stable(feature = "rust1", since = "1.0.0")]
899 pub fn take(&mut self) -> Option<T> {
903 /// Replaces the actual value in the option by the value given in parameter,
904 /// returning the old value if present,
905 /// leaving a [`Some`] in its place without deinitializing either one.
910 /// let mut x = Some(2);
911 /// let old = x.replace(5);
912 /// assert_eq!(x, Some(5));
913 /// assert_eq!(old, Some(2));
915 /// let mut x = None;
916 /// let old = x.replace(3);
917 /// assert_eq!(x, Some(3));
918 /// assert_eq!(old, None);
921 #[stable(feature = "option_replace", since = "1.31.0")]
922 pub fn replace(&mut self, value: T) -> Option<T> {
923 mem::replace(self, Some(value))
926 /// Zips `self` with another `Option`.
928 /// If `self` is `Some(s)` and `other` is `Some(o)`, this method returns `Some((s, o))`.
929 /// Otherwise, `None` is returned.
935 /// let y = Some("hi");
936 /// let z = None::<u8>;
938 /// assert_eq!(x.zip(y), Some((1, "hi")));
939 /// assert_eq!(x.zip(z), None);
941 #[stable(feature = "option_zip_option", since = "1.46.0")]
942 pub fn zip<U>(self, other: Option<U>) -> Option<(T, U)> {
943 match (self, other) {
944 (Some(a), Some(b)) => Some((a, b)),
949 /// Zips `self` and another `Option` with function `f`.
951 /// If `self` is `Some(s)` and `other` is `Some(o)`, this method returns `Some(f(s, o))`.
952 /// Otherwise, `None` is returned.
957 /// #![feature(option_zip)]
959 /// #[derive(Debug, PartialEq)]
966 /// fn new(x: f64, y: f64) -> Self {
971 /// let x = Some(17.5);
972 /// let y = Some(42.7);
974 /// assert_eq!(x.zip_with(y, Point::new), Some(Point { x: 17.5, y: 42.7 }));
975 /// assert_eq!(x.zip_with(None, Point::new), None);
977 #[unstable(feature = "option_zip", issue = "70086")]
978 pub fn zip_with<U, F, R>(self, other: Option<U>, f: F) -> Option<R>
980 F: FnOnce(T, U) -> R,
982 Some(f(self?, other?))
986 impl<T: Copy> Option<&T> {
987 /// Maps an `Option<&T>` to an `Option<T>` by copying the contents of the
994 /// let opt_x = Some(&x);
995 /// assert_eq!(opt_x, Some(&12));
996 /// let copied = opt_x.copied();
997 /// assert_eq!(copied, Some(12));
999 #[stable(feature = "copied", since = "1.35.0")]
1000 pub fn copied(self) -> Option<T> {
1005 impl<T: Copy> Option<&mut T> {
1006 /// Maps an `Option<&mut T>` to an `Option<T>` by copying the contents of the
1013 /// let opt_x = Some(&mut x);
1014 /// assert_eq!(opt_x, Some(&mut 12));
1015 /// let copied = opt_x.copied();
1016 /// assert_eq!(copied, Some(12));
1018 #[stable(feature = "copied", since = "1.35.0")]
1019 pub fn copied(self) -> Option<T> {
1020 self.map(|&mut t| t)
1024 impl<T: Clone> Option<&T> {
1025 /// Maps an `Option<&T>` to an `Option<T>` by cloning the contents of the
1032 /// let opt_x = Some(&x);
1033 /// assert_eq!(opt_x, Some(&12));
1034 /// let cloned = opt_x.cloned();
1035 /// assert_eq!(cloned, Some(12));
1037 #[stable(feature = "rust1", since = "1.0.0")]
1038 pub fn cloned(self) -> Option<T> {
1039 self.map(|t| t.clone())
1043 impl<T: Clone> Option<&mut T> {
1044 /// Maps an `Option<&mut T>` to an `Option<T>` by cloning the contents of the
1051 /// let opt_x = Some(&mut x);
1052 /// assert_eq!(opt_x, Some(&mut 12));
1053 /// let cloned = opt_x.cloned();
1054 /// assert_eq!(cloned, Some(12));
1056 #[stable(since = "1.26.0", feature = "option_ref_mut_cloned")]
1057 pub fn cloned(self) -> Option<T> {
1058 self.map(|t| t.clone())
1062 impl<T: fmt::Debug> Option<T> {
1063 /// Consumes `self` while expecting [`None`] and returning nothing.
1067 /// Panics if the value is a [`Some`], with a panic message including the
1068 /// passed message, and the content of the [`Some`].
1073 /// #![feature(option_expect_none)]
1075 /// use std::collections::HashMap;
1076 /// let mut squares = HashMap::new();
1077 /// for i in -10..=10 {
1078 /// // This will not panic, since all keys are unique.
1079 /// squares.insert(i, i * i).expect_none("duplicate key");
1083 /// ```{.should_panic}
1084 /// #![feature(option_expect_none)]
1086 /// use std::collections::HashMap;
1087 /// let mut sqrts = HashMap::new();
1088 /// for i in -10..=10 {
1089 /// // This will panic, since both negative and positive `i` will
1090 /// // insert the same `i * i` key, returning the old `Some(i)`.
1091 /// sqrts.insert(i * i, i).expect_none("duplicate key");
1096 #[unstable(feature = "option_expect_none", reason = "newly added", issue = "62633")]
1097 pub fn expect_none(self, msg: &str) {
1098 if let Some(val) = self {
1099 expect_none_failed(msg, &val);
1103 /// Consumes `self` while expecting [`None`] and returning nothing.
1107 /// Panics if the value is a [`Some`], with a custom panic message provided
1108 /// by the [`Some`]'s value.
1110 /// [`Some(v)`]: Some
1115 /// #![feature(option_unwrap_none)]
1117 /// use std::collections::HashMap;
1118 /// let mut squares = HashMap::new();
1119 /// for i in -10..=10 {
1120 /// // This will not panic, since all keys are unique.
1121 /// squares.insert(i, i * i).unwrap_none();
1125 /// ```{.should_panic}
1126 /// #![feature(option_unwrap_none)]
1128 /// use std::collections::HashMap;
1129 /// let mut sqrts = HashMap::new();
1130 /// for i in -10..=10 {
1131 /// // This will panic, since both negative and positive `i` will
1132 /// // insert the same `i * i` key, returning the old `Some(i)`.
1133 /// sqrts.insert(i * i, i).unwrap_none();
1138 #[unstable(feature = "option_unwrap_none", reason = "newly added", issue = "62633")]
1139 pub fn unwrap_none(self) {
1140 if let Some(val) = self {
1141 expect_none_failed("called `Option::unwrap_none()` on a `Some` value", &val);
1146 impl<T: Default> Option<T> {
1147 /// Returns the contained [`Some`] value or a default
1149 /// Consumes the `self` argument then, if [`Some`], returns the contained
1150 /// value, otherwise if [`None`], returns the [default value] for that
1155 /// Converts a string to an integer, turning poorly-formed strings
1156 /// into 0 (the default value for integers). [`parse`] converts
1157 /// a string to any other type that implements [`FromStr`], returning
1158 /// [`None`] on error.
1161 /// let good_year_from_input = "1909";
1162 /// let bad_year_from_input = "190blarg";
1163 /// let good_year = good_year_from_input.parse().ok().unwrap_or_default();
1164 /// let bad_year = bad_year_from_input.parse().ok().unwrap_or_default();
1166 /// assert_eq!(1909, good_year);
1167 /// assert_eq!(0, bad_year);
1170 /// [default value]: Default::default
1171 /// [`parse`]: str::parse
1172 /// [`FromStr`]: crate::str::FromStr
1174 #[stable(feature = "rust1", since = "1.0.0")]
1175 pub fn unwrap_or_default(self) -> T {
1178 None => Default::default(),
1183 impl<T: Deref> Option<T> {
1184 /// Converts from `Option<T>` (or `&Option<T>`) to `Option<&T::Target>`.
1186 /// Leaves the original Option in-place, creating a new one with a reference
1187 /// to the original one, additionally coercing the contents via [`Deref`].
1192 /// let x: Option<String> = Some("hey".to_owned());
1193 /// assert_eq!(x.as_deref(), Some("hey"));
1195 /// let x: Option<String> = None;
1196 /// assert_eq!(x.as_deref(), None);
1198 #[stable(feature = "option_deref", since = "1.40.0")]
1199 pub fn as_deref(&self) -> Option<&T::Target> {
1200 self.as_ref().map(|t| t.deref())
1204 impl<T: DerefMut> Option<T> {
1205 /// Converts from `Option<T>` (or `&mut Option<T>`) to `Option<&mut T::Target>`.
1207 /// Leaves the original `Option` in-place, creating a new one containing a mutable reference to
1208 /// the inner type's `Deref::Target` type.
1213 /// let mut x: Option<String> = Some("hey".to_owned());
1214 /// assert_eq!(x.as_deref_mut().map(|x| {
1215 /// x.make_ascii_uppercase();
1217 /// }), Some("HEY".to_owned().as_mut_str()));
1219 #[stable(feature = "option_deref", since = "1.40.0")]
1220 pub fn as_deref_mut(&mut self) -> Option<&mut T::Target> {
1221 self.as_mut().map(|t| t.deref_mut())
1225 impl<T, E> Option<Result<T, E>> {
1226 /// Transposes an `Option` of a [`Result`] into a [`Result`] of an `Option`.
1228 /// [`None`] will be mapped to [`Ok`]`(`[`None`]`)`.
1229 /// [`Some`]`(`[`Ok`]`(_))` and [`Some`]`(`[`Err`]`(_))` will be mapped to
1230 /// [`Ok`]`(`[`Some`]`(_))` and [`Err`]`(_)`.
1235 /// #[derive(Debug, Eq, PartialEq)]
1238 /// let x: Result<Option<i32>, SomeErr> = Ok(Some(5));
1239 /// let y: Option<Result<i32, SomeErr>> = Some(Ok(5));
1240 /// assert_eq!(x, y.transpose());
1243 #[stable(feature = "transpose_result", since = "1.33.0")]
1244 pub fn transpose(self) -> Result<Option<T>, E> {
1246 Some(Ok(x)) => Ok(Some(x)),
1247 Some(Err(e)) => Err(e),
1253 // This is a separate function to reduce the code size of .expect() itself.
1257 fn expect_failed(msg: &str) -> ! {
1261 // This is a separate function to reduce the code size of .expect_none() itself.
1265 fn expect_none_failed(msg: &str, value: &dyn fmt::Debug) -> ! {
1266 panic!("{}: {:?}", msg, value)
1269 /////////////////////////////////////////////////////////////////////////////
1270 // Trait implementations
1271 /////////////////////////////////////////////////////////////////////////////
1273 #[stable(feature = "rust1", since = "1.0.0")]
1274 impl<T: Clone> Clone for Option<T> {
1276 fn clone(&self) -> Self {
1278 Some(x) => Some(x.clone()),
1284 fn clone_from(&mut self, source: &Self) {
1285 match (self, source) {
1286 (Some(to), Some(from)) => to.clone_from(from),
1287 (to, from) => *to = from.clone(),
1292 #[stable(feature = "rust1", since = "1.0.0")]
1293 impl<T> Default for Option<T> {
1294 /// Returns [`None`][Option::None].
1299 /// let opt: Option<u32> = Option::default();
1300 /// assert!(opt.is_none());
1303 fn default() -> Option<T> {
1308 #[stable(feature = "rust1", since = "1.0.0")]
1309 impl<T> IntoIterator for Option<T> {
1311 type IntoIter = IntoIter<T>;
1313 /// Returns a consuming iterator over the possibly contained value.
1318 /// let x = Some("string");
1319 /// let v: Vec<&str> = x.into_iter().collect();
1320 /// assert_eq!(v, ["string"]);
1323 /// let v: Vec<&str> = x.into_iter().collect();
1324 /// assert!(v.is_empty());
1327 fn into_iter(self) -> IntoIter<T> {
1328 IntoIter { inner: Item { opt: self } }
1332 #[stable(since = "1.4.0", feature = "option_iter")]
1333 impl<'a, T> IntoIterator for &'a Option<T> {
1335 type IntoIter = Iter<'a, T>;
1337 fn into_iter(self) -> Iter<'a, T> {
1342 #[stable(since = "1.4.0", feature = "option_iter")]
1343 impl<'a, T> IntoIterator for &'a mut Option<T> {
1344 type Item = &'a mut T;
1345 type IntoIter = IterMut<'a, T>;
1347 fn into_iter(self) -> IterMut<'a, T> {
1352 #[stable(since = "1.12.0", feature = "option_from")]
1353 impl<T> From<T> for Option<T> {
1354 /// Copies `val` into a new `Some`.
1359 /// let o: Option<u8> = Option::from(67);
1361 /// assert_eq!(Some(67), o);
1363 fn from(val: T) -> Option<T> {
1368 #[stable(feature = "option_ref_from_ref_option", since = "1.30.0")]
1369 impl<'a, T> From<&'a Option<T>> for Option<&'a T> {
1370 /// Converts from `&Option<T>` to `Option<&T>`.
1374 /// Converts an `Option<`[`String`]`>` into an `Option<`[`usize`]`>`, preserving the original.
1375 /// The [`map`] method takes the `self` argument by value, consuming the original,
1376 /// so this technique uses `as_ref` to first take an `Option` to a reference
1377 /// to the value inside the original.
1379 /// [`map`]: Option::map
1380 /// [`String`]: ../../std/string/struct.String.html
1383 /// let s: Option<String> = Some(String::from("Hello, Rustaceans!"));
1384 /// let o: Option<usize> = Option::from(&s).map(|ss: &String| ss.len());
1386 /// println!("Can still print s: {:?}", s);
1388 /// assert_eq!(o, Some(18));
1390 fn from(o: &'a Option<T>) -> Option<&'a T> {
1395 #[stable(feature = "option_ref_from_ref_option", since = "1.30.0")]
1396 impl<'a, T> From<&'a mut Option<T>> for Option<&'a mut T> {
1397 /// Converts from `&mut Option<T>` to `Option<&mut T>`
1402 /// let mut s = Some(String::from("Hello"));
1403 /// let o: Option<&mut String> = Option::from(&mut s);
1406 /// Some(t) => *t = String::from("Hello, Rustaceans!"),
1410 /// assert_eq!(s, Some(String::from("Hello, Rustaceans!")));
1412 fn from(o: &'a mut Option<T>) -> Option<&'a mut T> {
1417 /////////////////////////////////////////////////////////////////////////////
1418 // The Option Iterators
1419 /////////////////////////////////////////////////////////////////////////////
1421 #[derive(Clone, Debug)]
1426 impl<A> Iterator for Item<A> {
1430 fn next(&mut self) -> Option<A> {
1435 fn size_hint(&self) -> (usize, Option<usize>) {
1437 Some(_) => (1, Some(1)),
1438 None => (0, Some(0)),
1443 impl<A> DoubleEndedIterator for Item<A> {
1445 fn next_back(&mut self) -> Option<A> {
1450 impl<A> ExactSizeIterator for Item<A> {}
1451 impl<A> FusedIterator for Item<A> {}
1452 unsafe impl<A> TrustedLen for Item<A> {}
1454 /// An iterator over a reference to the [`Some`] variant of an [`Option`].
1456 /// The iterator yields one value if the [`Option`] is a [`Some`], otherwise none.
1458 /// This `struct` is created by the [`Option::iter`] function.
1459 #[stable(feature = "rust1", since = "1.0.0")]
1461 pub struct Iter<'a, A: 'a> {
1465 #[stable(feature = "rust1", since = "1.0.0")]
1466 impl<'a, A> Iterator for Iter<'a, A> {
1470 fn next(&mut self) -> Option<&'a A> {
1474 fn size_hint(&self) -> (usize, Option<usize>) {
1475 self.inner.size_hint()
1479 #[stable(feature = "rust1", since = "1.0.0")]
1480 impl<'a, A> DoubleEndedIterator for Iter<'a, A> {
1482 fn next_back(&mut self) -> Option<&'a A> {
1483 self.inner.next_back()
1487 #[stable(feature = "rust1", since = "1.0.0")]
1488 impl<A> ExactSizeIterator for Iter<'_, A> {}
1490 #[stable(feature = "fused", since = "1.26.0")]
1491 impl<A> FusedIterator for Iter<'_, A> {}
1493 #[unstable(feature = "trusted_len", issue = "37572")]
1494 unsafe impl<A> TrustedLen for Iter<'_, A> {}
1496 #[stable(feature = "rust1", since = "1.0.0")]
1497 impl<A> Clone for Iter<'_, A> {
1499 fn clone(&self) -> Self {
1500 Iter { inner: self.inner.clone() }
1504 /// An iterator over a mutable reference to the [`Some`] variant of an [`Option`].
1506 /// The iterator yields one value if the [`Option`] is a [`Some`], otherwise none.
1508 /// This `struct` is created by the [`Option::iter_mut`] function.
1509 #[stable(feature = "rust1", since = "1.0.0")]
1511 pub struct IterMut<'a, A: 'a> {
1512 inner: Item<&'a mut A>,
1515 #[stable(feature = "rust1", since = "1.0.0")]
1516 impl<'a, A> Iterator for IterMut<'a, A> {
1517 type Item = &'a mut A;
1520 fn next(&mut self) -> Option<&'a mut A> {
1524 fn size_hint(&self) -> (usize, Option<usize>) {
1525 self.inner.size_hint()
1529 #[stable(feature = "rust1", since = "1.0.0")]
1530 impl<'a, A> DoubleEndedIterator for IterMut<'a, A> {
1532 fn next_back(&mut self) -> Option<&'a mut A> {
1533 self.inner.next_back()
1537 #[stable(feature = "rust1", since = "1.0.0")]
1538 impl<A> ExactSizeIterator for IterMut<'_, A> {}
1540 #[stable(feature = "fused", since = "1.26.0")]
1541 impl<A> FusedIterator for IterMut<'_, A> {}
1542 #[unstable(feature = "trusted_len", issue = "37572")]
1543 unsafe impl<A> TrustedLen for IterMut<'_, A> {}
1545 /// An iterator over the value in [`Some`] variant of an [`Option`].
1547 /// The iterator yields one value if the [`Option`] is a [`Some`], otherwise none.
1549 /// This `struct` is created by the [`Option::into_iter`] function.
1550 #[derive(Clone, Debug)]
1551 #[stable(feature = "rust1", since = "1.0.0")]
1552 pub struct IntoIter<A> {
1556 #[stable(feature = "rust1", since = "1.0.0")]
1557 impl<A> Iterator for IntoIter<A> {
1561 fn next(&mut self) -> Option<A> {
1565 fn size_hint(&self) -> (usize, Option<usize>) {
1566 self.inner.size_hint()
1570 #[stable(feature = "rust1", since = "1.0.0")]
1571 impl<A> DoubleEndedIterator for IntoIter<A> {
1573 fn next_back(&mut self) -> Option<A> {
1574 self.inner.next_back()
1578 #[stable(feature = "rust1", since = "1.0.0")]
1579 impl<A> ExactSizeIterator for IntoIter<A> {}
1581 #[stable(feature = "fused", since = "1.26.0")]
1582 impl<A> FusedIterator for IntoIter<A> {}
1584 #[unstable(feature = "trusted_len", issue = "37572")]
1585 unsafe impl<A> TrustedLen for IntoIter<A> {}
1587 /////////////////////////////////////////////////////////////////////////////
1589 /////////////////////////////////////////////////////////////////////////////
1591 #[stable(feature = "rust1", since = "1.0.0")]
1592 impl<A, V: FromIterator<A>> FromIterator<Option<A>> for Option<V> {
1593 /// Takes each element in the [`Iterator`]: if it is [`None`][Option::None],
1594 /// no further elements are taken, and the [`None`][Option::None] is
1595 /// returned. Should no [`None`][Option::None] occur, a container with the
1596 /// values of each [`Option`] is returned.
1600 /// Here is an example which increments every integer in a vector.
1601 /// We use the checked variant of `add` that returns `None` when the
1602 /// calculation would result in an overflow.
1605 /// let items = vec![0_u16, 1, 2];
1607 /// let res: Option<Vec<u16>> = items
1609 /// .map(|x| x.checked_add(1))
1612 /// assert_eq!(res, Some(vec![1, 2, 3]));
1615 /// As you can see, this will return the expected, valid items.
1617 /// Here is another example that tries to subtract one from another list
1618 /// of integers, this time checking for underflow:
1621 /// let items = vec![2_u16, 1, 0];
1623 /// let res: Option<Vec<u16>> = items
1625 /// .map(|x| x.checked_sub(1))
1628 /// assert_eq!(res, None);
1631 /// Since the last element is zero, it would underflow. Thus, the resulting
1632 /// value is `None`.
1634 /// Here is a variation on the previous example, showing that no
1635 /// further elements are taken from `iter` after the first `None`.
1638 /// let items = vec![3_u16, 2, 1, 10];
1640 /// let mut shared = 0;
1642 /// let res: Option<Vec<u16>> = items
1644 /// .map(|x| { shared += x; x.checked_sub(2) })
1647 /// assert_eq!(res, None);
1648 /// assert_eq!(shared, 6);
1651 /// Since the third element caused an underflow, no further elements were taken,
1652 /// so the final value of `shared` is 6 (= `3 + 2 + 1`), not 16.
1654 fn from_iter<I: IntoIterator<Item = Option<A>>>(iter: I) -> Option<V> {
1655 // FIXME(#11084): This could be replaced with Iterator::scan when this
1656 // performance bug is closed.
1658 iter.into_iter().map(|x| x.ok_or(())).collect::<Result<_, _>>().ok()
1662 /// The error type that results from applying the try operator (`?`) to a `None` value. If you wish
1663 /// to allow `x?` (where `x` is an `Option<T>`) to be converted into your error type, you can
1664 /// implement `impl From<NoneError>` for `YourErrorType`. In that case, `x?` within a function that
1665 /// returns `Result<_, YourErrorType>` will translate a `None` value into an `Err` result.
1666 #[rustc_diagnostic_item = "none_error"]
1667 #[unstable(feature = "try_trait", issue = "42327")]
1668 #[derive(Clone, Copy, PartialEq, PartialOrd, Eq, Ord, Debug, Hash)]
1669 pub struct NoneError;
1671 #[unstable(feature = "try_trait", issue = "42327")]
1672 impl<T> ops::Try for Option<T> {
1674 type Error = NoneError;
1677 fn into_result(self) -> Result<T, NoneError> {
1678 self.ok_or(NoneError)
1682 fn from_ok(v: T) -> Self {
1687 fn from_error(_: NoneError) -> Self {
1692 impl<T> Option<Option<T>> {
1693 /// Converts from `Option<Option<T>>` to `Option<T>`
1700 /// let x: Option<Option<u32>> = Some(Some(6));
1701 /// assert_eq!(Some(6), x.flatten());
1703 /// let x: Option<Option<u32>> = Some(None);
1704 /// assert_eq!(None, x.flatten());
1706 /// let x: Option<Option<u32>> = None;
1707 /// assert_eq!(None, x.flatten());
1710 /// Flattening only removes one level of nesting at a time:
1713 /// let x: Option<Option<Option<u32>>> = Some(Some(Some(6)));
1714 /// assert_eq!(Some(Some(6)), x.flatten());
1715 /// assert_eq!(Some(6), x.flatten().flatten());
1718 #[stable(feature = "option_flattening", since = "1.40.0")]
1719 pub fn flatten(self) -> Option<T> {
1720 self.and_then(convert::identity)