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 #[rustc_diagnostic_item = "option_type"]
155 #[stable(feature = "rust1", since = "1.0.0")]
158 #[stable(feature = "rust1", since = "1.0.0")]
161 #[stable(feature = "rust1", since = "1.0.0")]
162 Some(#[stable(feature = "rust1", since = "1.0.0")] T),
165 /////////////////////////////////////////////////////////////////////////////
166 // Type implementation
167 /////////////////////////////////////////////////////////////////////////////
170 /////////////////////////////////////////////////////////////////////////
171 // Querying the contained values
172 /////////////////////////////////////////////////////////////////////////
174 /// Returns `true` if the option is a [`Some`] value.
179 /// let x: Option<u32> = Some(2);
180 /// assert_eq!(x.is_some(), true);
182 /// let x: Option<u32> = None;
183 /// assert_eq!(x.is_some(), false);
186 /// [`Some`]: #variant.Some
187 #[must_use = "if you intended to assert that this has a value, consider `.unwrap()` instead"]
189 #[stable(feature = "rust1", since = "1.0.0")]
190 pub fn is_some(&self) -> bool {
191 matches!(*self, Some(_))
194 /// Returns `true` if the option is a [`None`] value.
199 /// let x: Option<u32> = Some(2);
200 /// assert_eq!(x.is_none(), false);
202 /// let x: Option<u32> = None;
203 /// assert_eq!(x.is_none(), true);
206 /// [`None`]: #variant.None
207 #[must_use = "if you intended to assert that this doesn't have a value, consider \
208 `.and_then(|| panic!(\"`Option` had a value when expected `None`\"))` instead"]
210 #[stable(feature = "rust1", since = "1.0.0")]
211 pub fn is_none(&self) -> bool {
215 /// Returns `true` if the option is a [`Some`] value containing the given value.
220 /// #![feature(option_result_contains)]
222 /// let x: Option<u32> = Some(2);
223 /// assert_eq!(x.contains(&2), true);
225 /// let x: Option<u32> = Some(3);
226 /// assert_eq!(x.contains(&2), false);
228 /// let x: Option<u32> = None;
229 /// assert_eq!(x.contains(&2), false);
233 #[unstable(feature = "option_result_contains", issue = "62358")]
234 pub fn contains<U>(&self, x: &U) -> bool
244 /////////////////////////////////////////////////////////////////////////
245 // Adapter for working with references
246 /////////////////////////////////////////////////////////////////////////
248 /// Converts from `&Option<T>` to `Option<&T>`.
252 /// Converts an `Option<`[`String`]`>` into an `Option<`[`usize`]`>`, preserving the original.
253 /// The [`map`] method takes the `self` argument by value, consuming the original,
254 /// so this technique uses `as_ref` to first take an `Option` to a reference
255 /// to the value inside the original.
257 /// [`map`]: enum.Option.html#method.map
258 /// [`String`]: ../../std/string/struct.String.html
259 /// [`usize`]: ../../std/primitive.usize.html
262 /// let text: Option<String> = Some("Hello, world!".to_string());
263 /// // First, cast `Option<String>` to `Option<&String>` with `as_ref`,
264 /// // then consume *that* with `map`, leaving `text` on the stack.
265 /// let text_length: Option<usize> = text.as_ref().map(|s| s.len());
266 /// println!("still can print text: {:?}", text);
269 #[stable(feature = "rust1", since = "1.0.0")]
270 pub fn as_ref(&self) -> Option<&T> {
272 Some(ref x) => Some(x),
277 /// Converts from `&mut Option<T>` to `Option<&mut T>`.
282 /// let mut x = Some(2);
283 /// match x.as_mut() {
284 /// Some(v) => *v = 42,
287 /// assert_eq!(x, Some(42));
290 #[stable(feature = "rust1", since = "1.0.0")]
291 pub fn as_mut(&mut self) -> Option<&mut T> {
293 Some(ref mut x) => Some(x),
298 /// Converts from [`Pin`]`<&Option<T>>` to `Option<`[`Pin`]`<&T>>`.
300 /// [`Pin`]: ../pin/struct.Pin.html
302 #[stable(feature = "pin", since = "1.33.0")]
303 pub fn as_pin_ref(self: Pin<&Self>) -> Option<Pin<&T>> {
304 unsafe { Pin::get_ref(self).as_ref().map(|x| Pin::new_unchecked(x)) }
307 /// Converts from [`Pin`]`<&mut Option<T>>` to `Option<`[`Pin`]`<&mut T>>`.
309 /// [`Pin`]: ../pin/struct.Pin.html
311 #[stable(feature = "pin", since = "1.33.0")]
312 pub fn as_pin_mut(self: Pin<&mut Self>) -> Option<Pin<&mut T>> {
313 unsafe { Pin::get_unchecked_mut(self).as_mut().map(|x| Pin::new_unchecked(x)) }
316 /////////////////////////////////////////////////////////////////////////
317 // Getting to contained values
318 /////////////////////////////////////////////////////////////////////////
320 /// Unwraps an option, yielding the content of a [`Some`].
324 /// Panics if the value is a [`None`] with a custom panic message provided by
327 /// [`Some`]: #variant.Some
328 /// [`None`]: #variant.None
333 /// let x = Some("value");
334 /// assert_eq!(x.expect("fruits are healthy"), "value");
337 /// ```{.should_panic}
338 /// let x: Option<&str> = None;
339 /// x.expect("fruits are healthy"); // panics with `fruits are healthy`
343 #[stable(feature = "rust1", since = "1.0.0")]
344 pub fn expect(self, msg: &str) -> T {
347 None => expect_failed(msg),
351 /// Moves the value `v` out of the `Option<T>` if it is [`Some(v)`].
353 /// In general, because this function may panic, its use is discouraged.
354 /// Instead, prefer to use pattern matching and handle the [`None`]
359 /// Panics if the self value equals [`None`].
361 /// [`Some(v)`]: #variant.Some
362 /// [`None`]: #variant.None
367 /// let x = Some("air");
368 /// assert_eq!(x.unwrap(), "air");
371 /// ```{.should_panic}
372 /// let x: Option<&str> = None;
373 /// assert_eq!(x.unwrap(), "air"); // fails
377 #[stable(feature = "rust1", since = "1.0.0")]
378 pub fn unwrap(self) -> T {
381 None => panic!("called `Option::unwrap()` on a `None` value"),
385 /// Returns the contained value or a default.
387 /// Arguments passed to `unwrap_or` are eagerly evaluated; if you are passing
388 /// the result of a function call, it is recommended to use [`unwrap_or_else`],
389 /// which is lazily evaluated.
391 /// [`unwrap_or_else`]: #method.unwrap_or_else
396 /// assert_eq!(Some("car").unwrap_or("bike"), "car");
397 /// assert_eq!(None.unwrap_or("bike"), "bike");
400 #[stable(feature = "rust1", since = "1.0.0")]
401 pub fn unwrap_or(self, default: T) -> T {
408 /// Returns the contained value or computes it from a closure.
414 /// assert_eq!(Some(4).unwrap_or_else(|| 2 * k), 4);
415 /// assert_eq!(None.unwrap_or_else(|| 2 * k), 20);
418 #[stable(feature = "rust1", since = "1.0.0")]
419 pub fn unwrap_or_else<F: FnOnce() -> T>(self, f: F) -> T {
426 /////////////////////////////////////////////////////////////////////////
427 // Transforming contained values
428 /////////////////////////////////////////////////////////////////////////
430 /// Maps an `Option<T>` to `Option<U>` by applying a function to a contained value.
434 /// Converts an `Option<`[`String`]`>` into an `Option<`[`usize`]`>`, consuming the original:
436 /// [`String`]: ../../std/string/struct.String.html
437 /// [`usize`]: ../../std/primitive.usize.html
440 /// let maybe_some_string = Some(String::from("Hello, World!"));
441 /// // `Option::map` takes self *by value*, consuming `maybe_some_string`
442 /// let maybe_some_len = maybe_some_string.map(|s| s.len());
444 /// assert_eq!(maybe_some_len, Some(13));
447 #[stable(feature = "rust1", since = "1.0.0")]
448 pub fn map<U, F: FnOnce(T) -> U>(self, f: F) -> Option<U> {
450 Some(x) => Some(f(x)),
455 /// Applies a function to the contained value (if any),
456 /// or returns the provided default (if not).
458 /// Arguments passed to `map_or` are eagerly evaluated; if you are passing
459 /// the result of a function call, it is recommended to use [`map_or_else`],
460 /// which is lazily evaluated.
462 /// [`map_or_else`]: #method.map_or_else
467 /// let x = Some("foo");
468 /// assert_eq!(x.map_or(42, |v| v.len()), 3);
470 /// let x: Option<&str> = None;
471 /// assert_eq!(x.map_or(42, |v| v.len()), 42);
474 #[stable(feature = "rust1", since = "1.0.0")]
475 pub fn map_or<U, F: FnOnce(T) -> U>(self, default: U, f: F) -> U {
482 /// Applies a function to the contained value (if any),
483 /// or computes a default (if not).
490 /// let x = Some("foo");
491 /// assert_eq!(x.map_or_else(|| 2 * k, |v| v.len()), 3);
493 /// let x: Option<&str> = None;
494 /// assert_eq!(x.map_or_else(|| 2 * k, |v| v.len()), 42);
497 #[stable(feature = "rust1", since = "1.0.0")]
498 pub fn map_or_else<U, D: FnOnce() -> U, F: FnOnce(T) -> U>(self, default: D, f: F) -> U {
505 /// Transforms the `Option<T>` into a [`Result<T, E>`], mapping [`Some(v)`] to
506 /// [`Ok(v)`] and [`None`] to [`Err(err)`].
508 /// Arguments passed to `ok_or` are eagerly evaluated; if you are passing the
509 /// result of a function call, it is recommended to use [`ok_or_else`], which is
510 /// lazily evaluated.
512 /// [`Result<T, E>`]: ../../std/result/enum.Result.html
513 /// [`Ok(v)`]: ../../std/result/enum.Result.html#variant.Ok
514 /// [`Err(err)`]: ../../std/result/enum.Result.html#variant.Err
515 /// [`None`]: #variant.None
516 /// [`Some(v)`]: #variant.Some
517 /// [`ok_or_else`]: #method.ok_or_else
522 /// let x = Some("foo");
523 /// assert_eq!(x.ok_or(0), Ok("foo"));
525 /// let x: Option<&str> = None;
526 /// assert_eq!(x.ok_or(0), Err(0));
529 #[stable(feature = "rust1", since = "1.0.0")]
530 pub fn ok_or<E>(self, err: E) -> Result<T, E> {
537 /// Transforms the `Option<T>` into a [`Result<T, E>`], mapping [`Some(v)`] to
538 /// [`Ok(v)`] and [`None`] to [`Err(err())`].
540 /// [`Result<T, E>`]: ../../std/result/enum.Result.html
541 /// [`Ok(v)`]: ../../std/result/enum.Result.html#variant.Ok
542 /// [`Err(err())`]: ../../std/result/enum.Result.html#variant.Err
543 /// [`None`]: #variant.None
544 /// [`Some(v)`]: #variant.Some
549 /// let x = Some("foo");
550 /// assert_eq!(x.ok_or_else(|| 0), Ok("foo"));
552 /// let x: Option<&str> = None;
553 /// assert_eq!(x.ok_or_else(|| 0), Err(0));
556 #[stable(feature = "rust1", since = "1.0.0")]
557 pub fn ok_or_else<E, F: FnOnce() -> E>(self, err: F) -> Result<T, E> {
564 /////////////////////////////////////////////////////////////////////////
565 // Iterator constructors
566 /////////////////////////////////////////////////////////////////////////
568 /// Returns an iterator over the possibly contained value.
574 /// assert_eq!(x.iter().next(), Some(&4));
576 /// let x: Option<u32> = None;
577 /// assert_eq!(x.iter().next(), None);
580 #[stable(feature = "rust1", since = "1.0.0")]
581 pub fn iter(&self) -> Iter<'_, T> {
582 Iter { inner: Item { opt: self.as_ref() } }
585 /// Returns a mutable iterator over the possibly contained value.
590 /// let mut x = Some(4);
591 /// match x.iter_mut().next() {
592 /// Some(v) => *v = 42,
595 /// assert_eq!(x, Some(42));
597 /// let mut x: Option<u32> = None;
598 /// assert_eq!(x.iter_mut().next(), None);
601 #[stable(feature = "rust1", since = "1.0.0")]
602 pub fn iter_mut(&mut self) -> IterMut<'_, T> {
603 IterMut { inner: Item { opt: self.as_mut() } }
606 /////////////////////////////////////////////////////////////////////////
607 // Boolean operations on the values, eager and lazy
608 /////////////////////////////////////////////////////////////////////////
610 /// Returns [`None`] if the option is [`None`], otherwise returns `optb`.
612 /// [`None`]: #variant.None
618 /// let y: Option<&str> = None;
619 /// assert_eq!(x.and(y), None);
621 /// let x: Option<u32> = None;
622 /// let y = Some("foo");
623 /// assert_eq!(x.and(y), None);
626 /// let y = Some("foo");
627 /// assert_eq!(x.and(y), Some("foo"));
629 /// let x: Option<u32> = None;
630 /// let y: Option<&str> = None;
631 /// assert_eq!(x.and(y), None);
634 #[stable(feature = "rust1", since = "1.0.0")]
635 pub fn and<U>(self, optb: Option<U>) -> Option<U> {
642 /// Returns [`None`] if the option is [`None`], otherwise calls `f` with the
643 /// wrapped value and returns the result.
645 /// Some languages call this operation flatmap.
647 /// [`None`]: #variant.None
652 /// fn sq(x: u32) -> Option<u32> { Some(x * x) }
653 /// fn nope(_: u32) -> Option<u32> { None }
655 /// assert_eq!(Some(2).and_then(sq).and_then(sq), Some(16));
656 /// assert_eq!(Some(2).and_then(sq).and_then(nope), None);
657 /// assert_eq!(Some(2).and_then(nope).and_then(sq), None);
658 /// assert_eq!(None.and_then(sq).and_then(sq), None);
661 #[stable(feature = "rust1", since = "1.0.0")]
662 pub fn and_then<U, F: FnOnce(T) -> Option<U>>(self, f: F) -> Option<U> {
669 /// Returns [`None`] if the option is [`None`], otherwise calls `predicate`
670 /// with the wrapped value and returns:
672 /// - [`Some(t)`] if `predicate` returns `true` (where `t` is the wrapped
674 /// - [`None`] if `predicate` returns `false`.
676 /// This function works similar to [`Iterator::filter()`]. You can imagine
677 /// the `Option<T>` being an iterator over one or zero elements. `filter()`
678 /// lets you decide which elements to keep.
683 /// fn is_even(n: &i32) -> bool {
687 /// assert_eq!(None.filter(is_even), None);
688 /// assert_eq!(Some(3).filter(is_even), None);
689 /// assert_eq!(Some(4).filter(is_even), Some(4));
692 /// [`None`]: #variant.None
693 /// [`Some(t)`]: #variant.Some
694 /// [`Iterator::filter()`]: ../../std/iter/trait.Iterator.html#method.filter
696 #[stable(feature = "option_filter", since = "1.27.0")]
697 pub fn filter<P: FnOnce(&T) -> bool>(self, predicate: P) -> Self {
698 if let Some(x) = self {
706 /// Returns the option if it contains a value, otherwise returns `optb`.
708 /// Arguments passed to `or` are eagerly evaluated; if you are passing the
709 /// result of a function call, it is recommended to use [`or_else`], which is
710 /// lazily evaluated.
712 /// [`or_else`]: #method.or_else
719 /// assert_eq!(x.or(y), Some(2));
722 /// let y = Some(100);
723 /// assert_eq!(x.or(y), Some(100));
726 /// let y = Some(100);
727 /// assert_eq!(x.or(y), Some(2));
729 /// let x: Option<u32> = None;
731 /// assert_eq!(x.or(y), None);
734 #[stable(feature = "rust1", since = "1.0.0")]
735 pub fn or(self, optb: Option<T>) -> Option<T> {
742 /// Returns the option if it contains a value, otherwise calls `f` and
743 /// returns the result.
748 /// fn nobody() -> Option<&'static str> { None }
749 /// fn vikings() -> Option<&'static str> { Some("vikings") }
751 /// assert_eq!(Some("barbarians").or_else(vikings), Some("barbarians"));
752 /// assert_eq!(None.or_else(vikings), Some("vikings"));
753 /// assert_eq!(None.or_else(nobody), None);
756 #[stable(feature = "rust1", since = "1.0.0")]
757 pub fn or_else<F: FnOnce() -> Option<T>>(self, f: F) -> Option<T> {
764 /// Returns [`Some`] if exactly one of `self`, `optb` is [`Some`], otherwise returns [`None`].
766 /// [`Some`]: #variant.Some
767 /// [`None`]: #variant.None
773 /// let y: Option<u32> = None;
774 /// assert_eq!(x.xor(y), Some(2));
776 /// let x: Option<u32> = None;
778 /// assert_eq!(x.xor(y), Some(2));
782 /// assert_eq!(x.xor(y), None);
784 /// let x: Option<u32> = None;
785 /// let y: Option<u32> = None;
786 /// assert_eq!(x.xor(y), None);
789 #[stable(feature = "option_xor", since = "1.37.0")]
790 pub fn xor(self, optb: Option<T>) -> Option<T> {
792 (Some(a), None) => Some(a),
793 (None, Some(b)) => Some(b),
798 /////////////////////////////////////////////////////////////////////////
799 // Entry-like operations to insert if None and return a reference
800 /////////////////////////////////////////////////////////////////////////
802 /// Inserts `v` into the option if it is [`None`], then
803 /// returns a mutable reference to the contained value.
805 /// [`None`]: #variant.None
810 /// let mut x = None;
813 /// let y: &mut u32 = x.get_or_insert(5);
814 /// assert_eq!(y, &5);
819 /// assert_eq!(x, Some(7));
822 #[stable(feature = "option_entry", since = "1.20.0")]
823 pub fn get_or_insert(&mut self, v: T) -> &mut T {
824 self.get_or_insert_with(|| v)
827 /// Inserts a value computed from `f` into the option if it is [`None`], then
828 /// returns a mutable reference to the contained value.
830 /// [`None`]: #variant.None
835 /// let mut x = None;
838 /// let y: &mut u32 = x.get_or_insert_with(|| 5);
839 /// assert_eq!(y, &5);
844 /// assert_eq!(x, Some(7));
847 #[stable(feature = "option_entry", since = "1.20.0")]
848 pub fn get_or_insert_with<F: FnOnce() -> T>(&mut self, f: F) -> &mut T {
849 if let None = *self {
854 Some(ref mut v) => v,
855 None => unsafe { hint::unreachable_unchecked() },
859 /////////////////////////////////////////////////////////////////////////
861 /////////////////////////////////////////////////////////////////////////
863 /// Takes the value out of the option, leaving a [`None`] in its place.
865 /// [`None`]: #variant.None
870 /// let mut x = Some(2);
871 /// let y = x.take();
872 /// assert_eq!(x, None);
873 /// assert_eq!(y, Some(2));
875 /// let mut x: Option<u32> = None;
876 /// let y = x.take();
877 /// assert_eq!(x, None);
878 /// assert_eq!(y, None);
881 #[stable(feature = "rust1", since = "1.0.0")]
882 pub fn take(&mut self) -> Option<T> {
886 /// Replaces the actual value in the option by the value given in parameter,
887 /// returning the old value if present,
888 /// leaving a [`Some`] in its place without deinitializing either one.
890 /// [`Some`]: #variant.Some
895 /// let mut x = Some(2);
896 /// let old = x.replace(5);
897 /// assert_eq!(x, Some(5));
898 /// assert_eq!(old, Some(2));
900 /// let mut x = None;
901 /// let old = x.replace(3);
902 /// assert_eq!(x, Some(3));
903 /// assert_eq!(old, None);
906 #[stable(feature = "option_replace", since = "1.31.0")]
907 pub fn replace(&mut self, value: T) -> Option<T> {
908 mem::replace(self, Some(value))
912 impl<T: Copy> Option<&T> {
913 /// Maps an `Option<&T>` to an `Option<T>` by copying the contents of the
920 /// let opt_x = Some(&x);
921 /// assert_eq!(opt_x, Some(&12));
922 /// let copied = opt_x.copied();
923 /// assert_eq!(copied, Some(12));
925 #[stable(feature = "copied", since = "1.35.0")]
926 pub fn copied(self) -> Option<T> {
931 impl<T: Copy> Option<&mut T> {
932 /// Maps an `Option<&mut T>` to an `Option<T>` by copying the contents of the
939 /// let opt_x = Some(&mut x);
940 /// assert_eq!(opt_x, Some(&mut 12));
941 /// let copied = opt_x.copied();
942 /// assert_eq!(copied, Some(12));
944 #[stable(feature = "copied", since = "1.35.0")]
945 pub fn copied(self) -> Option<T> {
950 impl<T: Clone> Option<&T> {
951 /// Maps an `Option<&T>` to an `Option<T>` by cloning the contents of the
958 /// let opt_x = Some(&x);
959 /// assert_eq!(opt_x, Some(&12));
960 /// let cloned = opt_x.cloned();
961 /// assert_eq!(cloned, Some(12));
963 #[stable(feature = "rust1", since = "1.0.0")]
964 pub fn cloned(self) -> Option<T> {
965 self.map(|t| t.clone())
969 impl<T: Clone> Option<&mut T> {
970 /// Maps an `Option<&mut T>` to an `Option<T>` by cloning the contents of the
977 /// let opt_x = Some(&mut x);
978 /// assert_eq!(opt_x, Some(&mut 12));
979 /// let cloned = opt_x.cloned();
980 /// assert_eq!(cloned, Some(12));
982 #[stable(since = "1.26.0", feature = "option_ref_mut_cloned")]
983 pub fn cloned(self) -> Option<T> {
984 self.map(|t| t.clone())
988 impl<T: fmt::Debug> Option<T> {
989 /// Unwraps an option, expecting [`None`] and returning nothing.
993 /// Panics if the value is a [`Some`], with a panic message including the
994 /// passed message, and the content of the [`Some`].
996 /// [`Some`]: #variant.Some
997 /// [`None`]: #variant.None
1002 /// #![feature(option_expect_none)]
1004 /// use std::collections::HashMap;
1005 /// let mut squares = HashMap::new();
1006 /// for i in -10..=10 {
1007 /// // This will not panic, since all keys are unique.
1008 /// squares.insert(i, i * i).expect_none("duplicate key");
1012 /// ```{.should_panic}
1013 /// #![feature(option_expect_none)]
1015 /// use std::collections::HashMap;
1016 /// let mut sqrts = HashMap::new();
1017 /// for i in -10..=10 {
1018 /// // This will panic, since both negative and positive `i` will
1019 /// // insert the same `i * i` key, returning the old `Some(i)`.
1020 /// sqrts.insert(i * i, i).expect_none("duplicate key");
1025 #[unstable(feature = "option_expect_none", reason = "newly added", issue = "62633")]
1026 pub fn expect_none(self, msg: &str) {
1027 if let Some(val) = self {
1028 expect_none_failed(msg, &val);
1032 /// Unwraps an option, expecting [`None`] and returning nothing.
1036 /// Panics if the value is a [`Some`], with a custom panic message provided
1037 /// by the [`Some`]'s value.
1039 /// [`Some(v)`]: #variant.Some
1040 /// [`None`]: #variant.None
1045 /// #![feature(option_unwrap_none)]
1047 /// use std::collections::HashMap;
1048 /// let mut squares = HashMap::new();
1049 /// for i in -10..=10 {
1050 /// // This will not panic, since all keys are unique.
1051 /// squares.insert(i, i * i).unwrap_none();
1055 /// ```{.should_panic}
1056 /// #![feature(option_unwrap_none)]
1058 /// use std::collections::HashMap;
1059 /// let mut sqrts = HashMap::new();
1060 /// for i in -10..=10 {
1061 /// // This will panic, since both negative and positive `i` will
1062 /// // insert the same `i * i` key, returning the old `Some(i)`.
1063 /// sqrts.insert(i * i, i).unwrap_none();
1068 #[unstable(feature = "option_unwrap_none", reason = "newly added", issue = "62633")]
1069 pub fn unwrap_none(self) {
1070 if let Some(val) = self {
1071 expect_none_failed("called `Option::unwrap_none()` on a `Some` value", &val);
1076 impl<T: Default> Option<T> {
1077 /// Returns the contained value or a default
1079 /// Consumes the `self` argument then, if [`Some`], returns the contained
1080 /// value, otherwise if [`None`], returns the [default value] for that
1085 /// Converts a string to an integer, turning poorly-formed strings
1086 /// into 0 (the default value for integers). [`parse`] converts
1087 /// a string to any other type that implements [`FromStr`], returning
1088 /// [`None`] on error.
1091 /// let good_year_from_input = "1909";
1092 /// let bad_year_from_input = "190blarg";
1093 /// let good_year = good_year_from_input.parse().ok().unwrap_or_default();
1094 /// let bad_year = bad_year_from_input.parse().ok().unwrap_or_default();
1096 /// assert_eq!(1909, good_year);
1097 /// assert_eq!(0, bad_year);
1100 /// [`Some`]: #variant.Some
1101 /// [`None`]: #variant.None
1102 /// [default value]: ../default/trait.Default.html#tymethod.default
1103 /// [`parse`]: ../../std/primitive.str.html#method.parse
1104 /// [`FromStr`]: ../../std/str/trait.FromStr.html
1106 #[stable(feature = "rust1", since = "1.0.0")]
1107 pub fn unwrap_or_default(self) -> T {
1110 None => Default::default(),
1115 impl<T: Deref> Option<T> {
1116 /// Converts from `Option<T>` (or `&Option<T>`) to `Option<&T::Target>`.
1118 /// Leaves the original Option in-place, creating a new one with a reference
1119 /// to the original one, additionally coercing the contents via [`Deref`].
1121 /// [`Deref`]: ../../std/ops/trait.Deref.html
1126 /// let x: Option<String> = Some("hey".to_owned());
1127 /// assert_eq!(x.as_deref(), Some("hey"));
1129 /// let x: Option<String> = None;
1130 /// assert_eq!(x.as_deref(), None);
1132 #[stable(feature = "option_deref", since = "1.40.0")]
1133 pub fn as_deref(&self) -> Option<&T::Target> {
1134 self.as_ref().map(|t| t.deref())
1138 impl<T: DerefMut> Option<T> {
1139 /// Converts from `Option<T>` (or `&mut Option<T>`) to `Option<&mut T::Target>`.
1141 /// Leaves the original `Option` in-place, creating a new one containing a mutable reference to
1142 /// the inner type's `Deref::Target` type.
1147 /// let mut x: Option<String> = Some("hey".to_owned());
1148 /// assert_eq!(x.as_deref_mut().map(|x| {
1149 /// x.make_ascii_uppercase();
1151 /// }), Some("HEY".to_owned().as_mut_str()));
1153 #[stable(feature = "option_deref", since = "1.40.0")]
1154 pub fn as_deref_mut(&mut self) -> Option<&mut T::Target> {
1155 self.as_mut().map(|t| t.deref_mut())
1159 impl<T, E> Option<Result<T, E>> {
1160 /// Transposes an `Option` of a [`Result`] into a [`Result`] of an `Option`.
1162 /// [`None`] will be mapped to [`Ok`]`(`[`None`]`)`.
1163 /// [`Some`]`(`[`Ok`]`(_))` and [`Some`]`(`[`Err`]`(_))` will be mapped to
1164 /// [`Ok`]`(`[`Some`]`(_))` and [`Err`]`(_)`.
1166 /// [`None`]: #variant.None
1167 /// [`Ok`]: ../../std/result/enum.Result.html#variant.Ok
1168 /// [`Some`]: #variant.Some
1169 /// [`Err`]: ../../std/result/enum.Result.html#variant.Err
1174 /// #[derive(Debug, Eq, PartialEq)]
1177 /// let x: Result<Option<i32>, SomeErr> = Ok(Some(5));
1178 /// let y: Option<Result<i32, SomeErr>> = Some(Ok(5));
1179 /// assert_eq!(x, y.transpose());
1182 #[stable(feature = "transpose_result", since = "1.33.0")]
1183 pub fn transpose(self) -> Result<Option<T>, E> {
1185 Some(Ok(x)) => Ok(Some(x)),
1186 Some(Err(e)) => Err(e),
1192 // This is a separate function to reduce the code size of .expect() itself.
1196 fn expect_failed(msg: &str) -> ! {
1200 // This is a separate function to reduce the code size of .expect_none() itself.
1204 fn expect_none_failed(msg: &str, value: &dyn fmt::Debug) -> ! {
1205 panic!("{}: {:?}", msg, value)
1208 /////////////////////////////////////////////////////////////////////////////
1209 // Trait implementations
1210 /////////////////////////////////////////////////////////////////////////////
1212 #[stable(feature = "rust1", since = "1.0.0")]
1213 impl<T: Clone> Clone for Option<T> {
1215 fn clone(&self) -> Self {
1217 Some(x) => Some(x.clone()),
1223 fn clone_from(&mut self, source: &Self) {
1224 match (self, source) {
1225 (Some(to), Some(from)) => to.clone_from(from),
1226 (to, from) => *to = from.clone(),
1231 #[stable(feature = "rust1", since = "1.0.0")]
1232 impl<T> Default for Option<T> {
1233 /// Returns [`None`][Option::None].
1238 /// let opt: Option<u32> = Option::default();
1239 /// assert!(opt.is_none());
1242 fn default() -> Option<T> {
1247 #[stable(feature = "rust1", since = "1.0.0")]
1248 impl<T> IntoIterator for Option<T> {
1250 type IntoIter = IntoIter<T>;
1252 /// Returns a consuming iterator over the possibly contained value.
1257 /// let x = Some("string");
1258 /// let v: Vec<&str> = x.into_iter().collect();
1259 /// assert_eq!(v, ["string"]);
1262 /// let v: Vec<&str> = x.into_iter().collect();
1263 /// assert!(v.is_empty());
1266 fn into_iter(self) -> IntoIter<T> {
1267 IntoIter { inner: Item { opt: self } }
1271 #[stable(since = "1.4.0", feature = "option_iter")]
1272 impl<'a, T> IntoIterator for &'a Option<T> {
1274 type IntoIter = Iter<'a, T>;
1276 fn into_iter(self) -> Iter<'a, T> {
1281 #[stable(since = "1.4.0", feature = "option_iter")]
1282 impl<'a, T> IntoIterator for &'a mut Option<T> {
1283 type Item = &'a mut T;
1284 type IntoIter = IterMut<'a, T>;
1286 fn into_iter(self) -> IterMut<'a, T> {
1291 #[stable(since = "1.12.0", feature = "option_from")]
1292 impl<T> From<T> for Option<T> {
1293 fn from(val: T) -> Option<T> {
1298 #[stable(feature = "option_ref_from_ref_option", since = "1.30.0")]
1299 impl<'a, T> From<&'a Option<T>> for Option<&'a T> {
1300 fn from(o: &'a Option<T>) -> Option<&'a T> {
1305 #[stable(feature = "option_ref_from_ref_option", since = "1.30.0")]
1306 impl<'a, T> From<&'a mut Option<T>> for Option<&'a mut T> {
1307 fn from(o: &'a mut Option<T>) -> Option<&'a mut T> {
1312 /////////////////////////////////////////////////////////////////////////////
1313 // The Option Iterators
1314 /////////////////////////////////////////////////////////////////////////////
1316 #[derive(Clone, Debug)]
1321 impl<A> Iterator for Item<A> {
1325 fn next(&mut self) -> Option<A> {
1330 fn size_hint(&self) -> (usize, Option<usize>) {
1332 Some(_) => (1, Some(1)),
1333 None => (0, Some(0)),
1338 impl<A> DoubleEndedIterator for Item<A> {
1340 fn next_back(&mut self) -> Option<A> {
1345 impl<A> ExactSizeIterator for Item<A> {}
1346 impl<A> FusedIterator for Item<A> {}
1347 unsafe impl<A> TrustedLen for Item<A> {}
1349 /// An iterator over a reference to the [`Some`] variant of an [`Option`].
1351 /// The iterator yields one value if the [`Option`] is a [`Some`], otherwise none.
1353 /// This `struct` is created by the [`Option::iter`] function.
1355 /// [`Option`]: enum.Option.html
1356 /// [`Some`]: enum.Option.html#variant.Some
1357 /// [`Option::iter`]: enum.Option.html#method.iter
1358 #[stable(feature = "rust1", since = "1.0.0")]
1360 pub struct Iter<'a, A: 'a> {
1364 #[stable(feature = "rust1", since = "1.0.0")]
1365 impl<'a, A> Iterator for Iter<'a, A> {
1369 fn next(&mut self) -> Option<&'a A> {
1373 fn size_hint(&self) -> (usize, Option<usize>) {
1374 self.inner.size_hint()
1378 #[stable(feature = "rust1", since = "1.0.0")]
1379 impl<'a, A> DoubleEndedIterator for Iter<'a, A> {
1381 fn next_back(&mut self) -> Option<&'a A> {
1382 self.inner.next_back()
1386 #[stable(feature = "rust1", since = "1.0.0")]
1387 impl<A> ExactSizeIterator for Iter<'_, A> {}
1389 #[stable(feature = "fused", since = "1.26.0")]
1390 impl<A> FusedIterator for Iter<'_, A> {}
1392 #[unstable(feature = "trusted_len", issue = "37572")]
1393 unsafe impl<A> TrustedLen for Iter<'_, A> {}
1395 #[stable(feature = "rust1", since = "1.0.0")]
1396 impl<A> Clone for Iter<'_, A> {
1398 fn clone(&self) -> Self {
1399 Iter { inner: self.inner.clone() }
1403 /// An iterator over a mutable reference to the [`Some`] variant of an [`Option`].
1405 /// The iterator yields one value if the [`Option`] is a [`Some`], otherwise none.
1407 /// This `struct` is created by the [`Option::iter_mut`] function.
1409 /// [`Option`]: enum.Option.html
1410 /// [`Some`]: enum.Option.html#variant.Some
1411 /// [`Option::iter_mut`]: enum.Option.html#method.iter_mut
1412 #[stable(feature = "rust1", since = "1.0.0")]
1414 pub struct IterMut<'a, A: 'a> {
1415 inner: Item<&'a mut A>,
1418 #[stable(feature = "rust1", since = "1.0.0")]
1419 impl<'a, A> Iterator for IterMut<'a, A> {
1420 type Item = &'a mut A;
1423 fn next(&mut self) -> Option<&'a mut 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 IterMut<'a, A> {
1435 fn next_back(&mut self) -> Option<&'a mut A> {
1436 self.inner.next_back()
1440 #[stable(feature = "rust1", since = "1.0.0")]
1441 impl<A> ExactSizeIterator for IterMut<'_, A> {}
1443 #[stable(feature = "fused", since = "1.26.0")]
1444 impl<A> FusedIterator for IterMut<'_, A> {}
1445 #[unstable(feature = "trusted_len", issue = "37572")]
1446 unsafe impl<A> TrustedLen for IterMut<'_, A> {}
1448 /// An iterator over the value in [`Some`] variant of an [`Option`].
1450 /// The iterator yields one value if the [`Option`] is a [`Some`], otherwise none.
1452 /// This `struct` is created by the [`Option::into_iter`] function.
1454 /// [`Option`]: enum.Option.html
1455 /// [`Some`]: enum.Option.html#variant.Some
1456 /// [`Option::into_iter`]: enum.Option.html#method.into_iter
1457 #[derive(Clone, Debug)]
1458 #[stable(feature = "rust1", since = "1.0.0")]
1459 pub struct IntoIter<A> {
1463 #[stable(feature = "rust1", since = "1.0.0")]
1464 impl<A> Iterator for IntoIter<A> {
1468 fn next(&mut self) -> Option<A> {
1472 fn size_hint(&self) -> (usize, Option<usize>) {
1473 self.inner.size_hint()
1477 #[stable(feature = "rust1", since = "1.0.0")]
1478 impl<A> DoubleEndedIterator for IntoIter<A> {
1480 fn next_back(&mut self) -> Option<A> {
1481 self.inner.next_back()
1485 #[stable(feature = "rust1", since = "1.0.0")]
1486 impl<A> ExactSizeIterator for IntoIter<A> {}
1488 #[stable(feature = "fused", since = "1.26.0")]
1489 impl<A> FusedIterator for IntoIter<A> {}
1491 #[unstable(feature = "trusted_len", issue = "37572")]
1492 unsafe impl<A> TrustedLen for IntoIter<A> {}
1494 /////////////////////////////////////////////////////////////////////////////
1496 /////////////////////////////////////////////////////////////////////////////
1498 #[stable(feature = "rust1", since = "1.0.0")]
1499 impl<A, V: FromIterator<A>> FromIterator<Option<A>> for Option<V> {
1500 /// Takes each element in the [`Iterator`]: if it is [`None`][Option::None],
1501 /// no further elements are taken, and the [`None`][Option::None] is
1502 /// returned. Should no [`None`][Option::None] occur, a container with the
1503 /// values of each [`Option`] is returned.
1507 /// Here is an example which increments every integer in a vector.
1508 /// We use the checked variant of `add` that returns `None` when the
1509 /// calculation would result in an overflow.
1512 /// let items = vec![0_u16, 1, 2];
1514 /// let res: Option<Vec<u16>> = items
1516 /// .map(|x| x.checked_add(1))
1519 /// assert_eq!(res, Some(vec![1, 2, 3]));
1522 /// As you can see, this will return the expected, valid items.
1524 /// Here is another example that tries to subtract one from another list
1525 /// of integers, this time checking for underflow:
1528 /// let items = vec![2_u16, 1, 0];
1530 /// let res: Option<Vec<u16>> = items
1532 /// .map(|x| x.checked_sub(1))
1535 /// assert_eq!(res, None);
1538 /// Since the last element is zero, it would underflow. Thus, the resulting
1539 /// value is `None`.
1541 /// Here is a variation on the previous example, showing that no
1542 /// further elements are taken from `iter` after the first `None`.
1545 /// let items = vec![3_u16, 2, 1, 10];
1547 /// let mut shared = 0;
1549 /// let res: Option<Vec<u16>> = items
1551 /// .map(|x| { shared += x; x.checked_sub(2) })
1554 /// assert_eq!(res, None);
1555 /// assert_eq!(shared, 6);
1558 /// Since the third element caused an underflow, no further elements were taken,
1559 /// so the final value of `shared` is 6 (= `3 + 2 + 1`), not 16.
1561 /// [`Iterator`]: ../iter/trait.Iterator.html
1563 fn from_iter<I: IntoIterator<Item = Option<A>>>(iter: I) -> Option<V> {
1564 // FIXME(#11084): This could be replaced with Iterator::scan when this
1565 // performance bug is closed.
1567 iter.into_iter().map(|x| x.ok_or(())).collect::<Result<_, _>>().ok()
1571 /// The error type that results from applying the try operator (`?`) to a `None` value. If you wish
1572 /// to allow `x?` (where `x` is an `Option<T>`) to be converted into your error type, you can
1573 /// implement `impl From<NoneError>` for `YourErrorType`. In that case, `x?` within a function that
1574 /// returns `Result<_, YourErrorType>` will translate a `None` value into an `Err` result.
1575 #[unstable(feature = "try_trait", issue = "42327")]
1576 #[derive(Clone, Copy, PartialEq, PartialOrd, Eq, Ord, Debug, Hash)]
1577 pub struct NoneError;
1579 #[unstable(feature = "try_trait", issue = "42327")]
1580 impl<T> ops::Try for Option<T> {
1582 type Error = NoneError;
1585 fn into_result(self) -> Result<T, NoneError> {
1586 self.ok_or(NoneError)
1590 fn from_ok(v: T) -> Self {
1595 fn from_error(_: NoneError) -> Self {
1600 impl<T> Option<Option<T>> {
1601 /// Converts from `Option<Option<T>>` to `Option<T>`
1606 /// let x: Option<Option<u32>> = Some(Some(6));
1607 /// assert_eq!(Some(6), x.flatten());
1609 /// let x: Option<Option<u32>> = Some(None);
1610 /// assert_eq!(None, x.flatten());
1612 /// let x: Option<Option<u32>> = None;
1613 /// assert_eq!(None, x.flatten());
1615 /// Flattening once only removes one level of nesting:
1617 /// let x: Option<Option<Option<u32>>> = Some(Some(Some(6)));
1618 /// assert_eq!(Some(Some(6)), x.flatten());
1619 /// assert_eq!(Some(6), x.flatten().flatten());
1622 #[stable(feature = "option_flattening", since = "1.40.0")]
1623 pub fn flatten(self) -> Option<T> {
1624 self.and_then(convert::identity)