3 //! Type [`Option`] represents an optional value: every [`Option`]
4 //! is either [`Some`] and contains a value, or [`None`], and
5 //! does not. [`Option`] types are very common in Rust code, as
6 //! they have a number of uses:
9 //! * Return values for functions that are not defined
10 //! over their entire input range (partial functions)
11 //! * Return value for otherwise reporting simple errors, where [`None`] is
13 //! * Optional struct fields
14 //! * Struct fields that can be loaned or "taken"
15 //! * Optional function arguments
16 //! * Nullable pointers
17 //! * Swapping things out of difficult situations
19 //! [`Option`]s are commonly paired with pattern matching to query the presence
20 //! of a value and take action, always accounting for the [`None`] case.
23 //! fn divide(numerator: f64, denominator: f64) -> Option<f64> {
24 //! if denominator == 0.0 {
27 //! Some(numerator / denominator)
31 //! // The return value of the function is an option
32 //! let result = divide(2.0, 3.0);
34 //! // Pattern match to retrieve the value
36 //! // The division was valid
37 //! Some(x) => println!("Result: {}", x),
38 //! // The division was invalid
39 //! None => println!("Cannot divide by 0"),
44 // FIXME: Show how `Option` is used in practice, with lots of methods
46 //! # Options and pointers ("nullable" pointers)
48 //! Rust's pointer types must always point to a valid location; there are
49 //! no "null" references. Instead, Rust has *optional* pointers, like
50 //! the optional owned box, [`Option`]`<`[`Box<T>`]`>`.
52 //! The following example uses [`Option`] to create an optional box of
53 //! [`i32`]. Notice that in order to use the inner [`i32`] value first, the
54 //! `check_optional` function needs to use pattern matching to
55 //! determine whether the box has a value (i.e., it is [`Some(...)`][`Some`]) or
59 //! let optional = None;
60 //! check_optional(optional);
62 //! let optional = Some(Box::new(9000));
63 //! check_optional(optional);
65 //! fn check_optional(optional: Option<Box<i32>>) {
67 //! Some(p) => println!("has value {}", p),
68 //! None => println!("has no value"),
73 //! This usage of [`Option`] to create safe nullable pointers is so
74 //! common that Rust does special optimizations to make the
75 //! representation of [`Option`]`<`[`Box<T>`]`>` a single pointer. Optional pointers
76 //! in Rust are stored as efficiently as any other pointer type.
80 //! Basic pattern matching on [`Option`]:
83 //! let msg = Some("howdy");
85 //! // Take a reference to the contained string
86 //! if let Some(m) = &msg {
87 //! println!("{}", *m);
90 //! // Remove the contained string, destroying the Option
91 //! let unwrapped_msg = msg.unwrap_or("default message");
94 //! Initialize a result to [`None`] before a loop:
97 //! enum Kingdom { Plant(u32, &'static str), Animal(u32, &'static str) }
99 //! // A list of data to search through.
100 //! let all_the_big_things = [
101 //! Kingdom::Plant(250, "redwood"),
102 //! Kingdom::Plant(230, "noble fir"),
103 //! Kingdom::Plant(229, "sugar pine"),
104 //! Kingdom::Animal(25, "blue whale"),
105 //! Kingdom::Animal(19, "fin whale"),
106 //! Kingdom::Animal(15, "north pacific right whale"),
109 //! // We're going to search for the name of the biggest animal,
110 //! // but to start with we've just got `None`.
111 //! let mut name_of_biggest_animal = None;
112 //! let mut size_of_biggest_animal = 0;
113 //! for big_thing in &all_the_big_things {
114 //! match *big_thing {
115 //! Kingdom::Animal(size, name) if size > size_of_biggest_animal => {
116 //! // Now we've found the name of some big animal
117 //! size_of_biggest_animal = size;
118 //! name_of_biggest_animal = Some(name);
120 //! Kingdom::Animal(..) | Kingdom::Plant(..) => ()
124 //! match name_of_biggest_animal {
125 //! Some(name) => println!("the biggest animal is {}", name),
126 //! None => println!("there are no animals :("),
130 //! [`Option`]: enum.Option.html
131 //! [`Some`]: enum.Option.html#variant.Some
132 //! [`None`]: enum.Option.html#variant.None
133 //! [`Box<T>`]: ../../std/boxed/struct.Box.html
134 //! [`i32`]: ../../std/primitive.i32.html
136 // ignore-tidy-undocumented-unsafe
138 #![stable(feature = "rust1", since = "1.0.0")]
140 use crate::iter::{FromIterator, FusedIterator, TrustedLen};
143 convert, fmt, hint, mem,
144 ops::{self, Deref, DerefMut},
147 // Note that this is not a lang item per se, but it has a hidden dependency on
148 // `Iterator`, which is one. The compiler assumes that the `next` method of
149 // `Iterator` is an enumeration with one type parameter and two variants,
150 // which basically means it must be `Option`.
152 /// The `Option` type. See [the module level documentation](index.html) for more.
153 #[derive(Copy, PartialEq, PartialOrd, Eq, Ord, Debug, Hash)]
154 #[stable(feature = "rust1", since = "1.0.0")]
157 #[stable(feature = "rust1", since = "1.0.0")]
160 #[stable(feature = "rust1", since = "1.0.0")]
161 Some(#[stable(feature = "rust1", since = "1.0.0")] T),
164 /////////////////////////////////////////////////////////////////////////////
165 // Type implementation
166 /////////////////////////////////////////////////////////////////////////////
169 /////////////////////////////////////////////////////////////////////////
170 // Querying the contained values
171 /////////////////////////////////////////////////////////////////////////
173 /// Returns `true` if the option is a [`Some`] value.
178 /// let x: Option<u32> = Some(2);
179 /// assert_eq!(x.is_some(), true);
181 /// let x: Option<u32> = None;
182 /// assert_eq!(x.is_some(), false);
185 /// [`Some`]: #variant.Some
186 #[must_use = "if you intended to assert that this has a value, consider `.unwrap()` instead"]
188 #[stable(feature = "rust1", since = "1.0.0")]
189 pub fn is_some(&self) -> bool {
190 matches!(*self, Some(_))
193 /// Returns `true` if the option is a [`None`] value.
198 /// let x: Option<u32> = Some(2);
199 /// assert_eq!(x.is_none(), false);
201 /// let x: Option<u32> = None;
202 /// assert_eq!(x.is_none(), true);
205 /// [`None`]: #variant.None
206 #[must_use = "if you intended to assert that this doesn't have a value, consider \
207 `.and_then(|| panic!(\"`Option` had a value when expected `None`\"))` instead"]
209 #[stable(feature = "rust1", since = "1.0.0")]
210 pub fn is_none(&self) -> bool {
214 /// Returns `true` if the option is a [`Some`] value containing the given value.
219 /// #![feature(option_result_contains)]
221 /// let x: Option<u32> = Some(2);
222 /// assert_eq!(x.contains(&2), true);
224 /// let x: Option<u32> = Some(3);
225 /// assert_eq!(x.contains(&2), false);
227 /// let x: Option<u32> = None;
228 /// assert_eq!(x.contains(&2), false);
232 #[unstable(feature = "option_result_contains", issue = "62358")]
233 pub fn contains<U>(&self, x: &U) -> bool
243 /////////////////////////////////////////////////////////////////////////
244 // Adapter for working with references
245 /////////////////////////////////////////////////////////////////////////
247 /// Converts from `&Option<T>` to `Option<&T>`.
251 /// Converts an `Option<`[`String`]`>` into an `Option<`[`usize`]`>`, preserving the original.
252 /// The [`map`] method takes the `self` argument by value, consuming the original,
253 /// so this technique uses `as_ref` to first take an `Option` to a reference
254 /// to the value inside the original.
256 /// [`map`]: enum.Option.html#method.map
257 /// [`String`]: ../../std/string/struct.String.html
258 /// [`usize`]: ../../std/primitive.usize.html
261 /// let text: Option<String> = Some("Hello, world!".to_string());
262 /// // First, cast `Option<String>` to `Option<&String>` with `as_ref`,
263 /// // then consume *that* with `map`, leaving `text` on the stack.
264 /// let text_length: Option<usize> = text.as_ref().map(|s| s.len());
265 /// println!("still can print text: {:?}", text);
268 #[stable(feature = "rust1", since = "1.0.0")]
269 pub fn as_ref(&self) -> Option<&T> {
271 Some(ref x) => Some(x),
276 /// Converts from `&mut Option<T>` to `Option<&mut T>`.
281 /// let mut x = Some(2);
282 /// match x.as_mut() {
283 /// Some(v) => *v = 42,
286 /// assert_eq!(x, Some(42));
289 #[stable(feature = "rust1", since = "1.0.0")]
290 pub fn as_mut(&mut self) -> Option<&mut T> {
292 Some(ref mut x) => Some(x),
297 /// Converts from [`Pin`]`<&Option<T>>` to `Option<`[`Pin`]`<&T>>`.
299 /// [`Pin`]: ../pin/struct.Pin.html
301 #[stable(feature = "pin", since = "1.33.0")]
302 pub fn as_pin_ref(self: Pin<&Self>) -> Option<Pin<&T>> {
303 unsafe { Pin::get_ref(self).as_ref().map(|x| Pin::new_unchecked(x)) }
306 /// Converts from [`Pin`]`<&mut Option<T>>` to `Option<`[`Pin`]`<&mut T>>`.
308 /// [`Pin`]: ../pin/struct.Pin.html
310 #[stable(feature = "pin", since = "1.33.0")]
311 pub fn as_pin_mut(self: Pin<&mut Self>) -> Option<Pin<&mut T>> {
312 unsafe { Pin::get_unchecked_mut(self).as_mut().map(|x| Pin::new_unchecked(x)) }
315 /////////////////////////////////////////////////////////////////////////
316 // Getting to contained values
317 /////////////////////////////////////////////////////////////////////////
319 /// Unwraps an option, yielding the content of a [`Some`].
323 /// Panics if the value is a [`None`] with a custom panic message provided by
326 /// [`Some`]: #variant.Some
327 /// [`None`]: #variant.None
332 /// let x = Some("value");
333 /// assert_eq!(x.expect("the world is ending"), "value");
336 /// ```{.should_panic}
337 /// let x: Option<&str> = None;
338 /// x.expect("the world is ending"); // panics with `the world is ending`
342 #[stable(feature = "rust1", since = "1.0.0")]
343 pub fn expect(self, msg: &str) -> T {
346 None => expect_failed(msg),
350 /// Moves the value `v` out of the `Option<T>` if it is [`Some(v)`].
352 /// In general, because this function may panic, its use is discouraged.
353 /// Instead, prefer to use pattern matching and handle the [`None`]
358 /// Panics if the self value equals [`None`].
360 /// [`Some(v)`]: #variant.Some
361 /// [`None`]: #variant.None
366 /// let x = Some("air");
367 /// assert_eq!(x.unwrap(), "air");
370 /// ```{.should_panic}
371 /// let x: Option<&str> = None;
372 /// assert_eq!(x.unwrap(), "air"); // fails
376 #[stable(feature = "rust1", since = "1.0.0")]
377 pub fn unwrap(self) -> T {
380 None => panic!("called `Option::unwrap()` on a `None` value"),
384 /// Returns the contained value or a default.
386 /// Arguments passed to `unwrap_or` are eagerly evaluated; if you are passing
387 /// the result of a function call, it is recommended to use [`unwrap_or_else`],
388 /// which is lazily evaluated.
390 /// [`unwrap_or_else`]: #method.unwrap_or_else
395 /// assert_eq!(Some("car").unwrap_or("bike"), "car");
396 /// assert_eq!(None.unwrap_or("bike"), "bike");
399 #[stable(feature = "rust1", since = "1.0.0")]
400 pub fn unwrap_or(self, default: T) -> T {
407 /// Returns the contained value or computes it from a closure.
413 /// assert_eq!(Some(4).unwrap_or_else(|| 2 * k), 4);
414 /// assert_eq!(None.unwrap_or_else(|| 2 * k), 20);
417 #[stable(feature = "rust1", since = "1.0.0")]
418 pub fn unwrap_or_else<F: FnOnce() -> T>(self, f: F) -> T {
425 /////////////////////////////////////////////////////////////////////////
426 // Transforming contained values
427 /////////////////////////////////////////////////////////////////////////
429 /// Maps an `Option<T>` to `Option<U>` by applying a function to a contained value.
433 /// Converts an `Option<`[`String`]`>` into an `Option<`[`usize`]`>`, consuming the original:
435 /// [`String`]: ../../std/string/struct.String.html
436 /// [`usize`]: ../../std/primitive.usize.html
439 /// let maybe_some_string = Some(String::from("Hello, World!"));
440 /// // `Option::map` takes self *by value*, consuming `maybe_some_string`
441 /// let maybe_some_len = maybe_some_string.map(|s| s.len());
443 /// assert_eq!(maybe_some_len, Some(13));
446 #[stable(feature = "rust1", since = "1.0.0")]
447 pub fn map<U, F: FnOnce(T) -> U>(self, f: F) -> Option<U> {
449 Some(x) => Some(f(x)),
454 /// Applies a function to the contained value (if any),
455 /// or returns the provided default (if not).
460 /// let x = Some("foo");
461 /// assert_eq!(x.map_or(42, |v| v.len()), 3);
463 /// let x: Option<&str> = None;
464 /// assert_eq!(x.map_or(42, |v| v.len()), 42);
467 #[stable(feature = "rust1", since = "1.0.0")]
468 pub fn map_or<U, F: FnOnce(T) -> U>(self, default: U, f: F) -> U {
475 /// Applies a function to the contained value (if any),
476 /// or computes a default (if not).
483 /// let x = Some("foo");
484 /// assert_eq!(x.map_or_else(|| 2 * k, |v| v.len()), 3);
486 /// let x: Option<&str> = None;
487 /// assert_eq!(x.map_or_else(|| 2 * k, |v| v.len()), 42);
490 #[stable(feature = "rust1", since = "1.0.0")]
491 pub fn map_or_else<U, D: FnOnce() -> U, F: FnOnce(T) -> U>(self, default: D, f: F) -> U {
498 /// Transforms the `Option<T>` into a [`Result<T, E>`], mapping [`Some(v)`] to
499 /// [`Ok(v)`] and [`None`] to [`Err(err)`].
501 /// Arguments passed to `ok_or` are eagerly evaluated; if you are passing the
502 /// result of a function call, it is recommended to use [`ok_or_else`], which is
503 /// lazily evaluated.
505 /// [`Result<T, E>`]: ../../std/result/enum.Result.html
506 /// [`Ok(v)`]: ../../std/result/enum.Result.html#variant.Ok
507 /// [`Err(err)`]: ../../std/result/enum.Result.html#variant.Err
508 /// [`None`]: #variant.None
509 /// [`Some(v)`]: #variant.Some
510 /// [`ok_or_else`]: #method.ok_or_else
515 /// let x = Some("foo");
516 /// assert_eq!(x.ok_or(0), Ok("foo"));
518 /// let x: Option<&str> = None;
519 /// assert_eq!(x.ok_or(0), Err(0));
522 #[stable(feature = "rust1", since = "1.0.0")]
523 pub fn ok_or<E>(self, err: E) -> Result<T, E> {
530 /// Transforms the `Option<T>` into a [`Result<T, E>`], mapping [`Some(v)`] to
531 /// [`Ok(v)`] and [`None`] to [`Err(err())`].
533 /// [`Result<T, E>`]: ../../std/result/enum.Result.html
534 /// [`Ok(v)`]: ../../std/result/enum.Result.html#variant.Ok
535 /// [`Err(err())`]: ../../std/result/enum.Result.html#variant.Err
536 /// [`None`]: #variant.None
537 /// [`Some(v)`]: #variant.Some
542 /// let x = Some("foo");
543 /// assert_eq!(x.ok_or_else(|| 0), Ok("foo"));
545 /// let x: Option<&str> = None;
546 /// assert_eq!(x.ok_or_else(|| 0), Err(0));
549 #[stable(feature = "rust1", since = "1.0.0")]
550 pub fn ok_or_else<E, F: FnOnce() -> E>(self, err: F) -> Result<T, E> {
557 /////////////////////////////////////////////////////////////////////////
558 // Iterator constructors
559 /////////////////////////////////////////////////////////////////////////
561 /// Returns an iterator over the possibly contained value.
567 /// assert_eq!(x.iter().next(), Some(&4));
569 /// let x: Option<u32> = None;
570 /// assert_eq!(x.iter().next(), None);
573 #[stable(feature = "rust1", since = "1.0.0")]
574 pub fn iter(&self) -> Iter<'_, T> {
575 Iter { inner: Item { opt: self.as_ref() } }
578 /// Returns a mutable iterator over the possibly contained value.
583 /// let mut x = Some(4);
584 /// match x.iter_mut().next() {
585 /// Some(v) => *v = 42,
588 /// assert_eq!(x, Some(42));
590 /// let mut x: Option<u32> = None;
591 /// assert_eq!(x.iter_mut().next(), None);
594 #[stable(feature = "rust1", since = "1.0.0")]
595 pub fn iter_mut(&mut self) -> IterMut<'_, T> {
596 IterMut { inner: Item { opt: self.as_mut() } }
599 /////////////////////////////////////////////////////////////////////////
600 // Boolean operations on the values, eager and lazy
601 /////////////////////////////////////////////////////////////////////////
603 /// Returns [`None`] if the option is [`None`], otherwise returns `optb`.
605 /// [`None`]: #variant.None
611 /// let y: Option<&str> = None;
612 /// assert_eq!(x.and(y), None);
614 /// let x: Option<u32> = None;
615 /// let y = Some("foo");
616 /// assert_eq!(x.and(y), None);
619 /// let y = Some("foo");
620 /// assert_eq!(x.and(y), Some("foo"));
622 /// let x: Option<u32> = None;
623 /// let y: Option<&str> = None;
624 /// assert_eq!(x.and(y), None);
627 #[stable(feature = "rust1", since = "1.0.0")]
628 pub fn and<U>(self, optb: Option<U>) -> Option<U> {
635 /// Returns [`None`] if the option is [`None`], otherwise calls `f` with the
636 /// wrapped value and returns the result.
638 /// Some languages call this operation flatmap.
640 /// [`None`]: #variant.None
645 /// fn sq(x: u32) -> Option<u32> { Some(x * x) }
646 /// fn nope(_: u32) -> Option<u32> { None }
648 /// assert_eq!(Some(2).and_then(sq).and_then(sq), Some(16));
649 /// assert_eq!(Some(2).and_then(sq).and_then(nope), None);
650 /// assert_eq!(Some(2).and_then(nope).and_then(sq), None);
651 /// assert_eq!(None.and_then(sq).and_then(sq), None);
654 #[stable(feature = "rust1", since = "1.0.0")]
655 pub fn and_then<U, F: FnOnce(T) -> Option<U>>(self, f: F) -> Option<U> {
662 /// Returns [`None`] if the option is [`None`], otherwise calls `predicate`
663 /// with the wrapped value and returns:
665 /// - [`Some(t)`] if `predicate` returns `true` (where `t` is the wrapped
667 /// - [`None`] if `predicate` returns `false`.
669 /// This function works similar to [`Iterator::filter()`]. You can imagine
670 /// the `Option<T>` being an iterator over one or zero elements. `filter()`
671 /// lets you decide which elements to keep.
676 /// fn is_even(n: &i32) -> bool {
680 /// assert_eq!(None.filter(is_even), None);
681 /// assert_eq!(Some(3).filter(is_even), None);
682 /// assert_eq!(Some(4).filter(is_even), Some(4));
685 /// [`None`]: #variant.None
686 /// [`Some(t)`]: #variant.Some
687 /// [`Iterator::filter()`]: ../../std/iter/trait.Iterator.html#method.filter
689 #[stable(feature = "option_filter", since = "1.27.0")]
690 pub fn filter<P: FnOnce(&T) -> bool>(self, predicate: P) -> Self {
691 if let Some(x) = self {
699 /// Returns the option if it contains a value, otherwise returns `optb`.
701 /// Arguments passed to `or` are eagerly evaluated; if you are passing the
702 /// result of a function call, it is recommended to use [`or_else`], which is
703 /// lazily evaluated.
705 /// [`or_else`]: #method.or_else
712 /// assert_eq!(x.or(y), Some(2));
715 /// let y = Some(100);
716 /// assert_eq!(x.or(y), Some(100));
719 /// let y = Some(100);
720 /// assert_eq!(x.or(y), Some(2));
722 /// let x: Option<u32> = None;
724 /// assert_eq!(x.or(y), None);
727 #[stable(feature = "rust1", since = "1.0.0")]
728 pub fn or(self, optb: Option<T>) -> Option<T> {
735 /// Returns the option if it contains a value, otherwise calls `f` and
736 /// returns the result.
741 /// fn nobody() -> Option<&'static str> { None }
742 /// fn vikings() -> Option<&'static str> { Some("vikings") }
744 /// assert_eq!(Some("barbarians").or_else(vikings), Some("barbarians"));
745 /// assert_eq!(None.or_else(vikings), Some("vikings"));
746 /// assert_eq!(None.or_else(nobody), None);
749 #[stable(feature = "rust1", since = "1.0.0")]
750 pub fn or_else<F: FnOnce() -> Option<T>>(self, f: F) -> Option<T> {
757 /// Returns [`Some`] if exactly one of `self`, `optb` is [`Some`], otherwise returns [`None`].
759 /// [`Some`]: #variant.Some
760 /// [`None`]: #variant.None
766 /// let y: Option<u32> = None;
767 /// assert_eq!(x.xor(y), Some(2));
769 /// let x: Option<u32> = None;
771 /// assert_eq!(x.xor(y), Some(2));
775 /// assert_eq!(x.xor(y), None);
777 /// let x: Option<u32> = None;
778 /// let y: Option<u32> = None;
779 /// assert_eq!(x.xor(y), None);
782 #[stable(feature = "option_xor", since = "1.37.0")]
783 pub fn xor(self, optb: Option<T>) -> Option<T> {
785 (Some(a), None) => Some(a),
786 (None, Some(b)) => Some(b),
791 /////////////////////////////////////////////////////////////////////////
792 // Entry-like operations to insert if None and return a reference
793 /////////////////////////////////////////////////////////////////////////
795 /// Inserts `v` into the option if it is [`None`], then
796 /// returns a mutable reference to the contained value.
798 /// [`None`]: #variant.None
803 /// let mut x = None;
806 /// let y: &mut u32 = x.get_or_insert(5);
807 /// assert_eq!(y, &5);
812 /// assert_eq!(x, Some(7));
815 #[stable(feature = "option_entry", since = "1.20.0")]
816 pub fn get_or_insert(&mut self, v: T) -> &mut T {
817 self.get_or_insert_with(|| v)
820 /// Inserts a value computed from `f` into the option if it is [`None`], then
821 /// returns a mutable reference to the contained value.
823 /// [`None`]: #variant.None
828 /// let mut x = None;
831 /// let y: &mut u32 = x.get_or_insert_with(|| 5);
832 /// assert_eq!(y, &5);
837 /// assert_eq!(x, Some(7));
840 #[stable(feature = "option_entry", since = "1.20.0")]
841 pub fn get_or_insert_with<F: FnOnce() -> T>(&mut self, f: F) -> &mut T {
842 if let None = *self {
847 Some(ref mut v) => v,
848 None => unsafe { hint::unreachable_unchecked() },
852 /////////////////////////////////////////////////////////////////////////
854 /////////////////////////////////////////////////////////////////////////
856 /// Takes the value out of the option, leaving a [`None`] in its place.
858 /// [`None`]: #variant.None
863 /// let mut x = Some(2);
864 /// let y = x.take();
865 /// assert_eq!(x, None);
866 /// assert_eq!(y, Some(2));
868 /// let mut x: Option<u32> = None;
869 /// let y = x.take();
870 /// assert_eq!(x, None);
871 /// assert_eq!(y, None);
874 #[stable(feature = "rust1", since = "1.0.0")]
875 pub fn take(&mut self) -> Option<T> {
879 /// Replaces the actual value in the option by the value given in parameter,
880 /// returning the old value if present,
881 /// leaving a [`Some`] in its place without deinitializing either one.
883 /// [`Some`]: #variant.Some
888 /// let mut x = Some(2);
889 /// let old = x.replace(5);
890 /// assert_eq!(x, Some(5));
891 /// assert_eq!(old, Some(2));
893 /// let mut x = None;
894 /// let old = x.replace(3);
895 /// assert_eq!(x, Some(3));
896 /// assert_eq!(old, None);
899 #[stable(feature = "option_replace", since = "1.31.0")]
900 pub fn replace(&mut self, value: T) -> Option<T> {
901 mem::replace(self, Some(value))
905 impl<T: Copy> Option<&T> {
906 /// Maps an `Option<&T>` to an `Option<T>` by copying the contents of the
913 /// let opt_x = Some(&x);
914 /// assert_eq!(opt_x, Some(&12));
915 /// let copied = opt_x.copied();
916 /// assert_eq!(copied, Some(12));
918 #[stable(feature = "copied", since = "1.35.0")]
919 pub fn copied(self) -> Option<T> {
924 impl<T: Copy> Option<&mut T> {
925 /// Maps an `Option<&mut T>` to an `Option<T>` by copying the contents of the
932 /// let opt_x = Some(&mut x);
933 /// assert_eq!(opt_x, Some(&mut 12));
934 /// let copied = opt_x.copied();
935 /// assert_eq!(copied, Some(12));
937 #[stable(feature = "copied", since = "1.35.0")]
938 pub fn copied(self) -> Option<T> {
943 impl<T: Clone> Option<&T> {
944 /// Maps an `Option<&T>` to an `Option<T>` by cloning the contents of the
951 /// let opt_x = Some(&x);
952 /// assert_eq!(opt_x, Some(&12));
953 /// let cloned = opt_x.cloned();
954 /// assert_eq!(cloned, Some(12));
956 #[stable(feature = "rust1", since = "1.0.0")]
957 pub fn cloned(self) -> Option<T> {
958 self.map(|t| t.clone())
962 impl<T: Clone> Option<&mut T> {
963 /// Maps an `Option<&mut T>` to an `Option<T>` by cloning the contents of the
970 /// let opt_x = Some(&mut x);
971 /// assert_eq!(opt_x, Some(&mut 12));
972 /// let cloned = opt_x.cloned();
973 /// assert_eq!(cloned, Some(12));
975 #[stable(since = "1.26.0", feature = "option_ref_mut_cloned")]
976 pub fn cloned(self) -> Option<T> {
977 self.map(|t| t.clone())
981 impl<T: fmt::Debug> Option<T> {
982 /// Unwraps an option, expecting [`None`] and returning nothing.
986 /// Panics if the value is a [`Some`], with a panic message including the
987 /// passed message, and the content of the [`Some`].
989 /// [`Some`]: #variant.Some
990 /// [`None`]: #variant.None
995 /// #![feature(option_expect_none)]
997 /// use std::collections::HashMap;
998 /// let mut squares = HashMap::new();
999 /// for i in -10..=10 {
1000 /// // This will not panic, since all keys are unique.
1001 /// squares.insert(i, i * i).expect_none("duplicate key");
1005 /// ```{.should_panic}
1006 /// #![feature(option_expect_none)]
1008 /// use std::collections::HashMap;
1009 /// let mut sqrts = HashMap::new();
1010 /// for i in -10..=10 {
1011 /// // This will panic, since both negative and positive `i` will
1012 /// // insert the same `i * i` key, returning the old `Some(i)`.
1013 /// sqrts.insert(i * i, i).expect_none("duplicate key");
1018 #[unstable(feature = "option_expect_none", reason = "newly added", issue = "62633")]
1019 pub fn expect_none(self, msg: &str) {
1020 if let Some(val) = self {
1021 expect_none_failed(msg, &val);
1025 /// Unwraps an option, expecting [`None`] and returning nothing.
1029 /// Panics if the value is a [`Some`], with a custom panic message provided
1030 /// by the [`Some`]'s value.
1032 /// [`Some(v)`]: #variant.Some
1033 /// [`None`]: #variant.None
1038 /// #![feature(option_unwrap_none)]
1040 /// use std::collections::HashMap;
1041 /// let mut squares = HashMap::new();
1042 /// for i in -10..=10 {
1043 /// // This will not panic, since all keys are unique.
1044 /// squares.insert(i, i * i).unwrap_none();
1048 /// ```{.should_panic}
1049 /// #![feature(option_unwrap_none)]
1051 /// use std::collections::HashMap;
1052 /// let mut sqrts = HashMap::new();
1053 /// for i in -10..=10 {
1054 /// // This will panic, since both negative and positive `i` will
1055 /// // insert the same `i * i` key, returning the old `Some(i)`.
1056 /// sqrts.insert(i * i, i).unwrap_none();
1061 #[unstable(feature = "option_unwrap_none", reason = "newly added", issue = "62633")]
1062 pub fn unwrap_none(self) {
1063 if let Some(val) = self {
1064 expect_none_failed("called `Option::unwrap_none()` on a `Some` value", &val);
1069 impl<T: Default> Option<T> {
1070 /// Returns the contained value or a default
1072 /// Consumes the `self` argument then, if [`Some`], returns the contained
1073 /// value, otherwise if [`None`], returns the [default value] for that
1078 /// Converts a string to an integer, turning poorly-formed strings
1079 /// into 0 (the default value for integers). [`parse`] converts
1080 /// a string to any other type that implements [`FromStr`], returning
1081 /// [`None`] on error.
1084 /// let good_year_from_input = "1909";
1085 /// let bad_year_from_input = "190blarg";
1086 /// let good_year = good_year_from_input.parse().ok().unwrap_or_default();
1087 /// let bad_year = bad_year_from_input.parse().ok().unwrap_or_default();
1089 /// assert_eq!(1909, good_year);
1090 /// assert_eq!(0, bad_year);
1093 /// [`Some`]: #variant.Some
1094 /// [`None`]: #variant.None
1095 /// [default value]: ../default/trait.Default.html#tymethod.default
1096 /// [`parse`]: ../../std/primitive.str.html#method.parse
1097 /// [`FromStr`]: ../../std/str/trait.FromStr.html
1099 #[stable(feature = "rust1", since = "1.0.0")]
1100 pub fn unwrap_or_default(self) -> T {
1103 None => Default::default(),
1108 impl<T: Deref> Option<T> {
1109 /// Converts from `Option<T>` (or `&Option<T>`) to `Option<&T::Target>`.
1111 /// Leaves the original Option in-place, creating a new one with a reference
1112 /// to the original one, additionally coercing the contents via [`Deref`].
1114 /// [`Deref`]: ../../std/ops/trait.Deref.html
1119 /// let x: Option<String> = Some("hey".to_owned());
1120 /// assert_eq!(x.as_deref(), Some("hey"));
1122 /// let x: Option<String> = None;
1123 /// assert_eq!(x.as_deref(), None);
1125 #[stable(feature = "option_deref", since = "1.40.0")]
1126 pub fn as_deref(&self) -> Option<&T::Target> {
1127 self.as_ref().map(|t| t.deref())
1131 impl<T: DerefMut> Option<T> {
1132 /// Converts from `Option<T>` (or `&mut Option<T>`) to `Option<&mut T::Target>`.
1134 /// Leaves the original `Option` in-place, creating a new one containing a mutable reference to
1135 /// the inner type's `Deref::Target` type.
1140 /// let mut x: Option<String> = Some("hey".to_owned());
1141 /// assert_eq!(x.as_deref_mut().map(|x| {
1142 /// x.make_ascii_uppercase();
1144 /// }), Some("HEY".to_owned().as_mut_str()));
1146 #[stable(feature = "option_deref", since = "1.40.0")]
1147 pub fn as_deref_mut(&mut self) -> Option<&mut T::Target> {
1148 self.as_mut().map(|t| t.deref_mut())
1152 impl<T, E> Option<Result<T, E>> {
1153 /// Transposes an `Option` of a [`Result`] into a [`Result`] of an `Option`.
1155 /// [`None`] will be mapped to [`Ok`]`(`[`None`]`)`.
1156 /// [`Some`]`(`[`Ok`]`(_))` and [`Some`]`(`[`Err`]`(_))` will be mapped to
1157 /// [`Ok`]`(`[`Some`]`(_))` and [`Err`]`(_)`.
1159 /// [`None`]: #variant.None
1160 /// [`Ok`]: ../../std/result/enum.Result.html#variant.Ok
1161 /// [`Some`]: #variant.Some
1162 /// [`Err`]: ../../std/result/enum.Result.html#variant.Err
1167 /// #[derive(Debug, Eq, PartialEq)]
1170 /// let x: Result<Option<i32>, SomeErr> = Ok(Some(5));
1171 /// let y: Option<Result<i32, SomeErr>> = Some(Ok(5));
1172 /// assert_eq!(x, y.transpose());
1175 #[stable(feature = "transpose_result", since = "1.33.0")]
1176 pub fn transpose(self) -> Result<Option<T>, E> {
1178 Some(Ok(x)) => Ok(Some(x)),
1179 Some(Err(e)) => Err(e),
1185 // This is a separate function to reduce the code size of .expect() itself.
1189 fn expect_failed(msg: &str) -> ! {
1193 // This is a separate function to reduce the code size of .expect_none() itself.
1197 fn expect_none_failed(msg: &str, value: &dyn fmt::Debug) -> ! {
1198 panic!("{}: {:?}", msg, value)
1201 /////////////////////////////////////////////////////////////////////////////
1202 // Trait implementations
1203 /////////////////////////////////////////////////////////////////////////////
1205 #[stable(feature = "rust1", since = "1.0.0")]
1206 impl<T: Clone> Clone for Option<T> {
1208 fn clone(&self) -> Self {
1210 Some(x) => Some(x.clone()),
1216 fn clone_from(&mut self, source: &Self) {
1217 match (self, source) {
1218 (Some(to), Some(from)) => to.clone_from(from),
1219 (to, from) => *to = from.clone(),
1224 #[stable(feature = "rust1", since = "1.0.0")]
1225 impl<T> Default for Option<T> {
1226 /// Returns [`None`][Option::None].
1231 /// let opt: Option<u32> = Option::default();
1232 /// assert!(opt.is_none());
1235 fn default() -> Option<T> {
1240 #[stable(feature = "rust1", since = "1.0.0")]
1241 impl<T> IntoIterator for Option<T> {
1243 type IntoIter = IntoIter<T>;
1245 /// Returns a consuming iterator over the possibly contained value.
1250 /// let x = Some("string");
1251 /// let v: Vec<&str> = x.into_iter().collect();
1252 /// assert_eq!(v, ["string"]);
1255 /// let v: Vec<&str> = x.into_iter().collect();
1256 /// assert!(v.is_empty());
1259 fn into_iter(self) -> IntoIter<T> {
1260 IntoIter { inner: Item { opt: self } }
1264 #[stable(since = "1.4.0", feature = "option_iter")]
1265 impl<'a, T> IntoIterator for &'a Option<T> {
1267 type IntoIter = Iter<'a, T>;
1269 fn into_iter(self) -> Iter<'a, T> {
1274 #[stable(since = "1.4.0", feature = "option_iter")]
1275 impl<'a, T> IntoIterator for &'a mut Option<T> {
1276 type Item = &'a mut T;
1277 type IntoIter = IterMut<'a, T>;
1279 fn into_iter(self) -> IterMut<'a, T> {
1284 #[stable(since = "1.12.0", feature = "option_from")]
1285 impl<T> From<T> for Option<T> {
1286 fn from(val: T) -> Option<T> {
1291 #[stable(feature = "option_ref_from_ref_option", since = "1.30.0")]
1292 impl<'a, T> From<&'a Option<T>> for Option<&'a T> {
1293 fn from(o: &'a Option<T>) -> Option<&'a T> {
1298 #[stable(feature = "option_ref_from_ref_option", since = "1.30.0")]
1299 impl<'a, T> From<&'a mut Option<T>> for Option<&'a mut T> {
1300 fn from(o: &'a mut Option<T>) -> Option<&'a mut T> {
1305 /////////////////////////////////////////////////////////////////////////////
1306 // The Option Iterators
1307 /////////////////////////////////////////////////////////////////////////////
1309 #[derive(Clone, Debug)]
1314 impl<A> Iterator for Item<A> {
1318 fn next(&mut self) -> Option<A> {
1323 fn size_hint(&self) -> (usize, Option<usize>) {
1325 Some(_) => (1, Some(1)),
1326 None => (0, Some(0)),
1331 impl<A> DoubleEndedIterator for Item<A> {
1333 fn next_back(&mut self) -> Option<A> {
1338 impl<A> ExactSizeIterator for Item<A> {}
1339 impl<A> FusedIterator for Item<A> {}
1340 unsafe impl<A> TrustedLen for Item<A> {}
1342 /// An iterator over a reference to the [`Some`] variant of an [`Option`].
1344 /// The iterator yields one value if the [`Option`] is a [`Some`], otherwise none.
1346 /// This `struct` is created by the [`Option::iter`] function.
1348 /// [`Option`]: enum.Option.html
1349 /// [`Some`]: enum.Option.html#variant.Some
1350 /// [`Option::iter`]: enum.Option.html#method.iter
1351 #[stable(feature = "rust1", since = "1.0.0")]
1353 pub struct Iter<'a, A: 'a> {
1357 #[stable(feature = "rust1", since = "1.0.0")]
1358 impl<'a, A> Iterator for Iter<'a, A> {
1362 fn next(&mut self) -> Option<&'a A> {
1366 fn size_hint(&self) -> (usize, Option<usize>) {
1367 self.inner.size_hint()
1371 #[stable(feature = "rust1", since = "1.0.0")]
1372 impl<'a, A> DoubleEndedIterator for Iter<'a, A> {
1374 fn next_back(&mut self) -> Option<&'a A> {
1375 self.inner.next_back()
1379 #[stable(feature = "rust1", since = "1.0.0")]
1380 impl<A> ExactSizeIterator for Iter<'_, A> {}
1382 #[stable(feature = "fused", since = "1.26.0")]
1383 impl<A> FusedIterator for Iter<'_, A> {}
1385 #[unstable(feature = "trusted_len", issue = "37572")]
1386 unsafe impl<A> TrustedLen for Iter<'_, A> {}
1388 #[stable(feature = "rust1", since = "1.0.0")]
1389 impl<A> Clone for Iter<'_, A> {
1391 fn clone(&self) -> Self {
1392 Iter { inner: self.inner.clone() }
1396 /// An iterator over a mutable reference to the [`Some`] variant of an [`Option`].
1398 /// The iterator yields one value if the [`Option`] is a [`Some`], otherwise none.
1400 /// This `struct` is created by the [`Option::iter_mut`] function.
1402 /// [`Option`]: enum.Option.html
1403 /// [`Some`]: enum.Option.html#variant.Some
1404 /// [`Option::iter_mut`]: enum.Option.html#method.iter_mut
1405 #[stable(feature = "rust1", since = "1.0.0")]
1407 pub struct IterMut<'a, A: 'a> {
1408 inner: Item<&'a mut A>,
1411 #[stable(feature = "rust1", since = "1.0.0")]
1412 impl<'a, A> Iterator for IterMut<'a, A> {
1413 type Item = &'a mut A;
1416 fn next(&mut self) -> Option<&'a mut A> {
1420 fn size_hint(&self) -> (usize, Option<usize>) {
1421 self.inner.size_hint()
1425 #[stable(feature = "rust1", since = "1.0.0")]
1426 impl<'a, A> DoubleEndedIterator for IterMut<'a, A> {
1428 fn next_back(&mut self) -> Option<&'a mut A> {
1429 self.inner.next_back()
1433 #[stable(feature = "rust1", since = "1.0.0")]
1434 impl<A> ExactSizeIterator for IterMut<'_, A> {}
1436 #[stable(feature = "fused", since = "1.26.0")]
1437 impl<A> FusedIterator for IterMut<'_, A> {}
1438 #[unstable(feature = "trusted_len", issue = "37572")]
1439 unsafe impl<A> TrustedLen for IterMut<'_, A> {}
1441 /// An iterator over the value in [`Some`] variant of an [`Option`].
1443 /// The iterator yields one value if the [`Option`] is a [`Some`], otherwise none.
1445 /// This `struct` is created by the [`Option::into_iter`] function.
1447 /// [`Option`]: enum.Option.html
1448 /// [`Some`]: enum.Option.html#variant.Some
1449 /// [`Option::into_iter`]: enum.Option.html#method.into_iter
1450 #[derive(Clone, Debug)]
1451 #[stable(feature = "rust1", since = "1.0.0")]
1452 pub struct IntoIter<A> {
1456 #[stable(feature = "rust1", since = "1.0.0")]
1457 impl<A> Iterator for IntoIter<A> {
1461 fn next(&mut self) -> Option<A> {
1465 fn size_hint(&self) -> (usize, Option<usize>) {
1466 self.inner.size_hint()
1470 #[stable(feature = "rust1", since = "1.0.0")]
1471 impl<A> DoubleEndedIterator for IntoIter<A> {
1473 fn next_back(&mut self) -> Option<A> {
1474 self.inner.next_back()
1478 #[stable(feature = "rust1", since = "1.0.0")]
1479 impl<A> ExactSizeIterator for IntoIter<A> {}
1481 #[stable(feature = "fused", since = "1.26.0")]
1482 impl<A> FusedIterator for IntoIter<A> {}
1484 #[unstable(feature = "trusted_len", issue = "37572")]
1485 unsafe impl<A> TrustedLen for IntoIter<A> {}
1487 /////////////////////////////////////////////////////////////////////////////
1489 /////////////////////////////////////////////////////////////////////////////
1491 #[stable(feature = "rust1", since = "1.0.0")]
1492 impl<A, V: FromIterator<A>> FromIterator<Option<A>> for Option<V> {
1493 /// Takes each element in the [`Iterator`]: if it is [`None`][Option::None],
1494 /// no further elements are taken, and the [`None`][Option::None] is
1495 /// returned. Should no [`None`][Option::None] occur, a container with the
1496 /// values of each [`Option`] is returned.
1500 /// Here is an example which increments every integer in a vector.
1501 /// We use the checked variant of `add` that returns `None` when the
1502 /// calculation would result in an overflow.
1505 /// let items = vec![0_u16, 1, 2];
1507 /// let res: Option<Vec<u16>> = items
1509 /// .map(|x| x.checked_add(1))
1512 /// assert_eq!(res, Some(vec![1, 2, 3]));
1515 /// As you can see, this will return the expected, valid items.
1517 /// Here is another example that tries to subtract one from another list
1518 /// of integers, this time checking for underflow:
1521 /// let items = vec![2_u16, 1, 0];
1523 /// let res: Option<Vec<u16>> = items
1525 /// .map(|x| x.checked_sub(1))
1528 /// assert_eq!(res, None);
1531 /// Since the last element is zero, it would underflow. Thus, the resulting
1532 /// value is `None`.
1534 /// Here is a variation on the previous example, showing that no
1535 /// further elements are taken from `iter` after the first `None`.
1538 /// let items = vec![3_u16, 2, 1, 10];
1540 /// let mut shared = 0;
1542 /// let res: Option<Vec<u16>> = items
1544 /// .map(|x| { shared += x; x.checked_sub(2) })
1547 /// assert_eq!(res, None);
1548 /// assert_eq!(shared, 6);
1551 /// Since the third element caused an underflow, no further elements were taken,
1552 /// so the final value of `shared` is 6 (= `3 + 2 + 1`), not 16.
1554 /// [`Iterator`]: ../iter/trait.Iterator.html
1556 fn from_iter<I: IntoIterator<Item = Option<A>>>(iter: I) -> Option<V> {
1557 // FIXME(#11084): This could be replaced with Iterator::scan when this
1558 // performance bug is closed.
1560 iter.into_iter().map(|x| x.ok_or(())).collect::<Result<_, _>>().ok()
1564 /// The error type that results from applying the try operator (`?`) to a `None` value. If you wish
1565 /// to allow `x?` (where `x` is an `Option<T>`) to be converted into your error type, you can
1566 /// implement `impl From<NoneError>` for `YourErrorType`. In that case, `x?` within a function that
1567 /// returns `Result<_, YourErrorType>` will translate a `None` value into an `Err` result.
1568 #[unstable(feature = "try_trait", issue = "42327")]
1569 #[derive(Clone, Copy, PartialEq, PartialOrd, Eq, Ord, Debug, Hash)]
1570 pub struct NoneError;
1572 #[unstable(feature = "try_trait", issue = "42327")]
1573 impl<T> ops::Try for Option<T> {
1575 type Error = NoneError;
1578 fn into_result(self) -> Result<T, NoneError> {
1579 self.ok_or(NoneError)
1583 fn from_ok(v: T) -> Self {
1588 fn from_error(_: NoneError) -> Self {
1593 impl<T> Option<Option<T>> {
1594 /// Converts from `Option<Option<T>>` to `Option<T>`
1599 /// let x: Option<Option<u32>> = Some(Some(6));
1600 /// assert_eq!(Some(6), x.flatten());
1602 /// let x: Option<Option<u32>> = Some(None);
1603 /// assert_eq!(None, x.flatten());
1605 /// let x: Option<Option<u32>> = None;
1606 /// assert_eq!(None, x.flatten());
1608 /// Flattening once only removes one level of nesting:
1610 /// let x: Option<Option<Option<u32>>> = Some(Some(Some(6)));
1611 /// assert_eq!(Some(Some(6)), x.flatten());
1612 /// assert_eq!(Some(6), x.flatten().flatten());
1615 #[stable(feature = "option_flattening", since = "1.40.0")]
1616 pub fn flatten(self) -> Option<T> {
1617 self.and_then(convert::identity)