3 //! Type [`Option`] represents an optional value: every [`Option`]
4 //! is either [`Some`] and contains a value, or [`None`], and
5 //! does not. [`Option`] types are very common in Rust code, as
6 //! they have a number of uses:
9 //! * Return values for functions that are not defined
10 //! over their entire input range (partial functions)
11 //! * Return value for otherwise reporting simple errors, where [`None`] is
13 //! * Optional struct fields
14 //! * Struct fields that can be loaned or "taken"
15 //! * Optional function arguments
16 //! * Nullable pointers
17 //! * Swapping things out of difficult situations
19 //! [`Option`]s are commonly paired with pattern matching to query the presence
20 //! of a value and take action, always accounting for the [`None`] case.
23 //! fn divide(numerator: f64, denominator: f64) -> Option<f64> {
24 //! if denominator == 0.0 {
27 //! Some(numerator / denominator)
31 //! // The return value of the function is an option
32 //! let result = divide(2.0, 3.0);
34 //! // Pattern match to retrieve the value
36 //! // The division was valid
37 //! Some(x) => println!("Result: {}", x),
38 //! // The division was invalid
39 //! None => println!("Cannot divide by 0"),
44 // FIXME: Show how `Option` is used in practice, with lots of methods
46 //! # Options and pointers ("nullable" pointers)
48 //! Rust's pointer types must always point to a valid location; there are
49 //! no "null" references. Instead, Rust has *optional* pointers, like
50 //! the optional owned box, [`Option`]`<`[`Box<T>`]`>`.
52 //! The following example uses [`Option`] to create an optional box of
53 //! [`i32`]. Notice that in order to use the inner [`i32`] value first, the
54 //! `check_optional` function needs to use pattern matching to
55 //! determine whether the box has a value (i.e., it is [`Some(...)`][`Some`]) or
59 //! let optional = None;
60 //! check_optional(optional);
62 //! let optional = Some(Box::new(9000));
63 //! check_optional(optional);
65 //! fn check_optional(optional: Option<Box<i32>>) {
67 //! Some(p) => println!("has value {}", p),
68 //! None => println!("has no value"),
73 //! This usage of [`Option`] to create safe nullable pointers is so
74 //! common that Rust does special optimizations to make the
75 //! representation of [`Option`]`<`[`Box<T>`]`>` a single pointer. Optional pointers
76 //! in Rust are stored as efficiently as any other pointer type.
80 //! Basic pattern matching on [`Option`]:
83 //! let msg = Some("howdy");
85 //! // Take a reference to the contained string
86 //! if let Some(m) = &msg {
87 //! println!("{}", *m);
90 //! // Remove the contained string, destroying the Option
91 //! let unwrapped_msg = msg.unwrap_or("default message");
94 //! Initialize a result to [`None`] before a loop:
97 //! enum Kingdom { Plant(u32, &'static str), Animal(u32, &'static str) }
99 //! // A list of data to search through.
100 //! let all_the_big_things = [
101 //! Kingdom::Plant(250, "redwood"),
102 //! Kingdom::Plant(230, "noble fir"),
103 //! Kingdom::Plant(229, "sugar pine"),
104 //! Kingdom::Animal(25, "blue whale"),
105 //! Kingdom::Animal(19, "fin whale"),
106 //! Kingdom::Animal(15, "north pacific right whale"),
109 //! // We're going to search for the name of the biggest animal,
110 //! // but to start with we've just got `None`.
111 //! let mut name_of_biggest_animal = None;
112 //! let mut size_of_biggest_animal = 0;
113 //! for big_thing in &all_the_big_things {
114 //! match *big_thing {
115 //! Kingdom::Animal(size, name) if size > size_of_biggest_animal => {
116 //! // Now we've found the name of some big animal
117 //! size_of_biggest_animal = size;
118 //! name_of_biggest_animal = Some(name);
120 //! Kingdom::Animal(..) | Kingdom::Plant(..) => ()
124 //! match name_of_biggest_animal {
125 //! Some(name) => println!("the biggest animal is {}", name),
126 //! None => println!("there are no animals :("),
130 //! [`Box<T>`]: ../../std/boxed/struct.Box.html
132 #![stable(feature = "rust1", since = "1.0.0")]
134 use crate::iter::{FromIterator, FusedIterator, TrustedLen};
137 convert, fmt, hint, mem,
138 ops::{self, Deref, DerefMut},
141 /// The `Option` type. See [the module level documentation](self) for more.
142 #[derive(Copy, PartialEq, PartialOrd, Eq, Ord, Debug, Hash)]
143 #[rustc_diagnostic_item = "option_type"]
144 #[stable(feature = "rust1", since = "1.0.0")]
148 #[stable(feature = "rust1", since = "1.0.0")]
152 #[stable(feature = "rust1", since = "1.0.0")]
153 Some(#[stable(feature = "rust1", since = "1.0.0")] T),
156 /////////////////////////////////////////////////////////////////////////////
157 // Type implementation
158 /////////////////////////////////////////////////////////////////////////////
161 /////////////////////////////////////////////////////////////////////////
162 // Querying the contained values
163 /////////////////////////////////////////////////////////////////////////
165 /// Returns `true` if the option is a [`Some`] value.
170 /// let x: Option<u32> = Some(2);
171 /// assert_eq!(x.is_some(), true);
173 /// let x: Option<u32> = None;
174 /// assert_eq!(x.is_some(), false);
176 #[must_use = "if you intended to assert that this has a value, consider `.unwrap()` instead"]
178 #[rustc_const_unstable(feature = "const_option", issue = "67441")]
179 #[stable(feature = "rust1", since = "1.0.0")]
180 pub const fn is_some(&self) -> bool {
181 matches!(*self, Some(_))
184 /// Returns `true` if the option is a [`None`] value.
189 /// let x: Option<u32> = Some(2);
190 /// assert_eq!(x.is_none(), false);
192 /// let x: Option<u32> = None;
193 /// assert_eq!(x.is_none(), true);
195 #[must_use = "if you intended to assert that this doesn't have a value, consider \
196 `.and_then(|| panic!(\"`Option` had a value when expected `None`\"))` instead"]
198 #[rustc_const_unstable(feature = "const_option", issue = "67441")]
199 #[stable(feature = "rust1", since = "1.0.0")]
200 pub const fn is_none(&self) -> bool {
204 /// Returns `true` if the option is a [`Some`] value containing the given value.
209 /// #![feature(option_result_contains)]
211 /// let x: Option<u32> = Some(2);
212 /// assert_eq!(x.contains(&2), true);
214 /// let x: Option<u32> = Some(3);
215 /// assert_eq!(x.contains(&2), false);
217 /// let x: Option<u32> = None;
218 /// assert_eq!(x.contains(&2), false);
222 #[unstable(feature = "option_result_contains", issue = "62358")]
223 pub fn contains<U>(&self, x: &U) -> bool
233 /////////////////////////////////////////////////////////////////////////
234 // Adapter for working with references
235 /////////////////////////////////////////////////////////////////////////
237 /// Converts from `&Option<T>` to `Option<&T>`.
241 /// Converts an `Option<`[`String`]`>` into an `Option<`[`usize`]`>`, preserving the original.
242 /// The [`map`] method takes the `self` argument by value, consuming the original,
243 /// so this technique uses `as_ref` to first take an `Option` to a reference
244 /// to the value inside the original.
246 /// [`map`]: Option::map
247 /// [`String`]: ../../std/string/struct.String.html
250 /// let text: Option<String> = Some("Hello, world!".to_string());
251 /// // First, cast `Option<String>` to `Option<&String>` with `as_ref`,
252 /// // then consume *that* with `map`, leaving `text` on the stack.
253 /// let text_length: Option<usize> = text.as_ref().map(|s| s.len());
254 /// println!("still can print text: {:?}", text);
257 #[rustc_const_unstable(feature = "const_option", issue = "67441")]
258 #[stable(feature = "rust1", since = "1.0.0")]
259 pub const fn as_ref(&self) -> Option<&T> {
261 Some(ref x) => Some(x),
266 /// Converts from `&mut Option<T>` to `Option<&mut T>`.
271 /// let mut x = Some(2);
272 /// match x.as_mut() {
273 /// Some(v) => *v = 42,
276 /// assert_eq!(x, Some(42));
279 #[stable(feature = "rust1", since = "1.0.0")]
280 pub fn as_mut(&mut self) -> Option<&mut T> {
282 Some(ref mut x) => Some(x),
287 /// Converts from [`Pin`]`<&Option<T>>` to `Option<`[`Pin`]`<&T>>`.
289 #[stable(feature = "pin", since = "1.33.0")]
290 pub fn as_pin_ref(self: Pin<&Self>) -> Option<Pin<&T>> {
291 // SAFETY: `x` is guaranteed to be pinned because it comes from `self`
293 unsafe { Pin::get_ref(self).as_ref().map(|x| Pin::new_unchecked(x)) }
296 /// Converts from [`Pin`]`<&mut Option<T>>` to `Option<`[`Pin`]`<&mut T>>`.
298 #[stable(feature = "pin", since = "1.33.0")]
299 pub fn as_pin_mut(self: Pin<&mut Self>) -> Option<Pin<&mut T>> {
300 // SAFETY: `get_unchecked_mut` is never used to move the `Option` inside `self`.
301 // `x` is guaranteed to be pinned because it comes from `self` which is pinned.
302 unsafe { Pin::get_unchecked_mut(self).as_mut().map(|x| Pin::new_unchecked(x)) }
305 /////////////////////////////////////////////////////////////////////////
306 // Getting to contained values
307 /////////////////////////////////////////////////////////////////////////
309 /// Returns the contained [`Some`] value, consuming the `self` value.
313 /// Panics if the value is a [`None`] with a custom panic message provided by
319 /// let x = Some("value");
320 /// assert_eq!(x.expect("fruits are healthy"), "value");
323 /// ```{.should_panic}
324 /// let x: Option<&str> = None;
325 /// x.expect("fruits are healthy"); // panics with `fruits are healthy`
329 #[stable(feature = "rust1", since = "1.0.0")]
330 pub fn expect(self, msg: &str) -> T {
333 None => expect_failed(msg),
337 /// Returns the contained [`Some`] value, consuming the `self` value.
339 /// Because this function may panic, its use is generally discouraged.
340 /// Instead, prefer to use pattern matching and handle the [`None`]
341 /// case explicitly, or call [`unwrap_or`], [`unwrap_or_else`], or
342 /// [`unwrap_or_default`].
344 /// [`unwrap_or`]: Option::unwrap_or
345 /// [`unwrap_or_else`]: Option::unwrap_or_else
346 /// [`unwrap_or_default`]: Option::unwrap_or_default
350 /// Panics if the self value equals [`None`].
355 /// let x = Some("air");
356 /// assert_eq!(x.unwrap(), "air");
359 /// ```{.should_panic}
360 /// let x: Option<&str> = None;
361 /// assert_eq!(x.unwrap(), "air"); // fails
365 #[stable(feature = "rust1", since = "1.0.0")]
366 #[rustc_const_unstable(feature = "const_option", issue = "67441")]
367 pub const fn unwrap(self) -> T {
370 None => panic!("called `Option::unwrap()` on a `None` value"),
374 /// Returns the contained [`Some`] value or a provided default.
376 /// Arguments passed to `unwrap_or` are eagerly evaluated; if you are passing
377 /// the result of a function call, it is recommended to use [`unwrap_or_else`],
378 /// which is lazily evaluated.
380 /// [`unwrap_or_else`]: Option::unwrap_or_else
385 /// assert_eq!(Some("car").unwrap_or("bike"), "car");
386 /// assert_eq!(None.unwrap_or("bike"), "bike");
389 #[stable(feature = "rust1", since = "1.0.0")]
390 pub fn unwrap_or(self, default: T) -> T {
397 /// Returns the contained [`Some`] value or computes it from a closure.
403 /// assert_eq!(Some(4).unwrap_or_else(|| 2 * k), 4);
404 /// assert_eq!(None.unwrap_or_else(|| 2 * k), 20);
407 #[stable(feature = "rust1", since = "1.0.0")]
408 pub fn unwrap_or_else<F: FnOnce() -> T>(self, f: F) -> T {
415 /////////////////////////////////////////////////////////////////////////
416 // Transforming contained values
417 /////////////////////////////////////////////////////////////////////////
419 /// Maps an `Option<T>` to `Option<U>` by applying a function to a contained value.
423 /// Converts an `Option<`[`String`]`>` into an `Option<`[`usize`]`>`, consuming the original:
425 /// [`String`]: ../../std/string/struct.String.html
427 /// let maybe_some_string = Some(String::from("Hello, World!"));
428 /// // `Option::map` takes self *by value*, consuming `maybe_some_string`
429 /// let maybe_some_len = maybe_some_string.map(|s| s.len());
431 /// assert_eq!(maybe_some_len, Some(13));
434 #[stable(feature = "rust1", since = "1.0.0")]
435 pub fn map<U, F: FnOnce(T) -> U>(self, f: F) -> Option<U> {
437 Some(x) => Some(f(x)),
442 /// Applies a function to the contained value (if any),
443 /// or returns the provided default (if not).
445 /// Arguments passed to `map_or` are eagerly evaluated; if you are passing
446 /// the result of a function call, it is recommended to use [`map_or_else`],
447 /// which is lazily evaluated.
449 /// [`map_or_else`]: Option::map_or_else
454 /// let x = Some("foo");
455 /// assert_eq!(x.map_or(42, |v| v.len()), 3);
457 /// let x: Option<&str> = None;
458 /// assert_eq!(x.map_or(42, |v| v.len()), 42);
461 #[stable(feature = "rust1", since = "1.0.0")]
462 pub fn map_or<U, F: FnOnce(T) -> U>(self, default: U, f: F) -> U {
469 /// Applies a function to the contained value (if any),
470 /// or computes a default (if not).
477 /// let x = Some("foo");
478 /// assert_eq!(x.map_or_else(|| 2 * k, |v| v.len()), 3);
480 /// let x: Option<&str> = None;
481 /// assert_eq!(x.map_or_else(|| 2 * k, |v| v.len()), 42);
484 #[stable(feature = "rust1", since = "1.0.0")]
485 pub fn map_or_else<U, D: FnOnce() -> U, F: FnOnce(T) -> U>(self, default: D, f: F) -> U {
492 /// Transforms the `Option<T>` into a [`Result<T, E>`], mapping [`Some(v)`] to
493 /// [`Ok(v)`] and [`None`] to [`Err(err)`].
495 /// Arguments passed to `ok_or` are eagerly evaluated; if you are passing the
496 /// result of a function call, it is recommended to use [`ok_or_else`], which is
497 /// lazily evaluated.
499 /// [`Result<T, E>`]: Result
501 /// [`Err(err)`]: Err
502 /// [`Some(v)`]: Some
503 /// [`ok_or_else`]: Option::ok_or_else
508 /// let x = Some("foo");
509 /// assert_eq!(x.ok_or(0), Ok("foo"));
511 /// let x: Option<&str> = None;
512 /// assert_eq!(x.ok_or(0), Err(0));
515 #[stable(feature = "rust1", since = "1.0.0")]
516 pub fn ok_or<E>(self, err: E) -> Result<T, E> {
523 /// Transforms the `Option<T>` into a [`Result<T, E>`], mapping [`Some(v)`] to
524 /// [`Ok(v)`] and [`None`] to [`Err(err())`].
526 /// [`Result<T, E>`]: Result
528 /// [`Err(err())`]: Err
529 /// [`Some(v)`]: Some
534 /// let x = Some("foo");
535 /// assert_eq!(x.ok_or_else(|| 0), Ok("foo"));
537 /// let x: Option<&str> = None;
538 /// assert_eq!(x.ok_or_else(|| 0), Err(0));
541 #[stable(feature = "rust1", since = "1.0.0")]
542 pub fn ok_or_else<E, F: FnOnce() -> E>(self, err: F) -> Result<T, E> {
549 /////////////////////////////////////////////////////////////////////////
550 // Iterator constructors
551 /////////////////////////////////////////////////////////////////////////
553 /// Returns an iterator over the possibly contained value.
559 /// assert_eq!(x.iter().next(), Some(&4));
561 /// let x: Option<u32> = None;
562 /// assert_eq!(x.iter().next(), None);
565 #[rustc_const_unstable(feature = "const_option", issue = "67441")]
566 #[stable(feature = "rust1", since = "1.0.0")]
567 pub const fn iter(&self) -> Iter<'_, T> {
568 Iter { inner: Item { opt: self.as_ref() } }
571 /// Returns a mutable iterator over the possibly contained value.
576 /// let mut x = Some(4);
577 /// match x.iter_mut().next() {
578 /// Some(v) => *v = 42,
581 /// assert_eq!(x, Some(42));
583 /// let mut x: Option<u32> = None;
584 /// assert_eq!(x.iter_mut().next(), None);
587 #[stable(feature = "rust1", since = "1.0.0")]
588 pub fn iter_mut(&mut self) -> IterMut<'_, T> {
589 IterMut { inner: Item { opt: self.as_mut() } }
592 /////////////////////////////////////////////////////////////////////////
593 // Boolean operations on the values, eager and lazy
594 /////////////////////////////////////////////////////////////////////////
596 /// Returns [`None`] if the option is [`None`], otherwise returns `optb`.
602 /// let y: Option<&str> = None;
603 /// assert_eq!(x.and(y), None);
605 /// let x: Option<u32> = None;
606 /// let y = Some("foo");
607 /// assert_eq!(x.and(y), None);
610 /// let y = Some("foo");
611 /// assert_eq!(x.and(y), Some("foo"));
613 /// let x: Option<u32> = None;
614 /// let y: Option<&str> = None;
615 /// assert_eq!(x.and(y), None);
618 #[stable(feature = "rust1", since = "1.0.0")]
619 pub fn and<U>(self, optb: Option<U>) -> Option<U> {
626 /// Returns [`None`] if the option is [`None`], otherwise calls `f` with the
627 /// wrapped value and returns the result.
629 /// Some languages call this operation flatmap.
634 /// fn sq(x: u32) -> Option<u32> { Some(x * x) }
635 /// fn nope(_: u32) -> Option<u32> { None }
637 /// assert_eq!(Some(2).and_then(sq).and_then(sq), Some(16));
638 /// assert_eq!(Some(2).and_then(sq).and_then(nope), None);
639 /// assert_eq!(Some(2).and_then(nope).and_then(sq), None);
640 /// assert_eq!(None.and_then(sq).and_then(sq), None);
643 #[stable(feature = "rust1", since = "1.0.0")]
644 pub fn and_then<U, F: FnOnce(T) -> Option<U>>(self, f: F) -> Option<U> {
651 /// Returns [`None`] if the option is [`None`], otherwise calls `predicate`
652 /// with the wrapped value and returns:
654 /// - [`Some(t)`] if `predicate` returns `true` (where `t` is the wrapped
656 /// - [`None`] if `predicate` returns `false`.
658 /// This function works similar to [`Iterator::filter()`]. You can imagine
659 /// the `Option<T>` being an iterator over one or zero elements. `filter()`
660 /// lets you decide which elements to keep.
665 /// fn is_even(n: &i32) -> bool {
669 /// assert_eq!(None.filter(is_even), None);
670 /// assert_eq!(Some(3).filter(is_even), None);
671 /// assert_eq!(Some(4).filter(is_even), Some(4));
675 #[stable(feature = "option_filter", since = "1.27.0")]
676 pub fn filter<P: FnOnce(&T) -> bool>(self, predicate: P) -> Self {
677 if let Some(x) = self {
685 /// Returns the option if it contains a value, otherwise returns `optb`.
687 /// Arguments passed to `or` are eagerly evaluated; if you are passing the
688 /// result of a function call, it is recommended to use [`or_else`], which is
689 /// lazily evaluated.
691 /// [`or_else`]: Option::or_else
698 /// assert_eq!(x.or(y), Some(2));
701 /// let y = Some(100);
702 /// assert_eq!(x.or(y), Some(100));
705 /// let y = Some(100);
706 /// assert_eq!(x.or(y), Some(2));
708 /// let x: Option<u32> = None;
710 /// assert_eq!(x.or(y), None);
713 #[stable(feature = "rust1", since = "1.0.0")]
714 pub fn or(self, optb: Option<T>) -> Option<T> {
721 /// Returns the option if it contains a value, otherwise calls `f` and
722 /// returns the result.
727 /// fn nobody() -> Option<&'static str> { None }
728 /// fn vikings() -> Option<&'static str> { Some("vikings") }
730 /// assert_eq!(Some("barbarians").or_else(vikings), Some("barbarians"));
731 /// assert_eq!(None.or_else(vikings), Some("vikings"));
732 /// assert_eq!(None.or_else(nobody), None);
735 #[stable(feature = "rust1", since = "1.0.0")]
736 pub fn or_else<F: FnOnce() -> Option<T>>(self, f: F) -> Option<T> {
743 /// Returns [`Some`] if exactly one of `self`, `optb` is [`Some`], otherwise returns [`None`].
749 /// let y: Option<u32> = None;
750 /// assert_eq!(x.xor(y), Some(2));
752 /// let x: Option<u32> = None;
754 /// assert_eq!(x.xor(y), Some(2));
758 /// assert_eq!(x.xor(y), None);
760 /// let x: Option<u32> = None;
761 /// let y: Option<u32> = None;
762 /// assert_eq!(x.xor(y), None);
765 #[stable(feature = "option_xor", since = "1.37.0")]
766 pub fn xor(self, optb: Option<T>) -> Option<T> {
768 (Some(a), None) => Some(a),
769 (None, Some(b)) => Some(b),
774 /////////////////////////////////////////////////////////////////////////
775 // Entry-like operations to insert if None and return a reference
776 /////////////////////////////////////////////////////////////////////////
778 /// Inserts `v` into the option if it is [`None`], then
779 /// returns a mutable reference to the contained value.
784 /// let mut x = None;
787 /// let y: &mut u32 = x.get_or_insert(5);
788 /// assert_eq!(y, &5);
793 /// assert_eq!(x, Some(7));
796 #[stable(feature = "option_entry", since = "1.20.0")]
797 pub fn get_or_insert(&mut self, v: T) -> &mut T {
798 self.get_or_insert_with(|| v)
801 /// Inserts a value computed from `f` into the option if it is [`None`], then
802 /// returns a mutable reference to the contained value.
807 /// let mut x = None;
810 /// let y: &mut u32 = x.get_or_insert_with(|| 5);
811 /// assert_eq!(y, &5);
816 /// assert_eq!(x, Some(7));
819 #[stable(feature = "option_entry", since = "1.20.0")]
820 pub fn get_or_insert_with<F: FnOnce() -> T>(&mut self, f: F) -> &mut T {
821 if let None = *self {
826 Some(ref mut v) => v,
827 // SAFETY: a `None` variant for `self` would have been replaced by a `Some`
828 // variant in the code above.
829 None => unsafe { hint::unreachable_unchecked() },
833 /////////////////////////////////////////////////////////////////////////
835 /////////////////////////////////////////////////////////////////////////
837 /// Takes the value out of the option, leaving a [`None`] in its place.
842 /// let mut x = Some(2);
843 /// let y = x.take();
844 /// assert_eq!(x, None);
845 /// assert_eq!(y, Some(2));
847 /// let mut x: Option<u32> = None;
848 /// let y = x.take();
849 /// assert_eq!(x, None);
850 /// assert_eq!(y, None);
853 #[stable(feature = "rust1", since = "1.0.0")]
854 pub fn take(&mut self) -> Option<T> {
858 /// Replaces the actual value in the option by the value given in parameter,
859 /// returning the old value if present,
860 /// leaving a [`Some`] in its place without deinitializing either one.
865 /// let mut x = Some(2);
866 /// let old = x.replace(5);
867 /// assert_eq!(x, Some(5));
868 /// assert_eq!(old, Some(2));
870 /// let mut x = None;
871 /// let old = x.replace(3);
872 /// assert_eq!(x, Some(3));
873 /// assert_eq!(old, None);
876 #[stable(feature = "option_replace", since = "1.31.0")]
877 pub fn replace(&mut self, value: T) -> Option<T> {
878 mem::replace(self, Some(value))
881 /// Zips `self` with another `Option`.
883 /// If `self` is `Some(s)` and `other` is `Some(o)`, this method returns `Some((s, o))`.
884 /// Otherwise, `None` is returned.
890 /// let y = Some("hi");
891 /// let z = None::<u8>;
893 /// assert_eq!(x.zip(y), Some((1, "hi")));
894 /// assert_eq!(x.zip(z), None);
896 #[stable(feature = "option_zip_option", since = "1.46.0")]
897 pub fn zip<U>(self, other: Option<U>) -> Option<(T, U)> {
898 match (self, other) {
899 (Some(a), Some(b)) => Some((a, b)),
904 /// Zips `self` and another `Option` with function `f`.
906 /// If `self` is `Some(s)` and `other` is `Some(o)`, this method returns `Some(f(s, o))`.
907 /// Otherwise, `None` is returned.
912 /// #![feature(option_zip)]
914 /// #[derive(Debug, PartialEq)]
921 /// fn new(x: f64, y: f64) -> Self {
926 /// let x = Some(17.5);
927 /// let y = Some(42.7);
929 /// assert_eq!(x.zip_with(y, Point::new), Some(Point { x: 17.5, y: 42.7 }));
930 /// assert_eq!(x.zip_with(None, Point::new), None);
932 #[unstable(feature = "option_zip", issue = "70086")]
933 pub fn zip_with<U, F, R>(self, other: Option<U>, f: F) -> Option<R>
935 F: FnOnce(T, U) -> R,
937 Some(f(self?, other?))
941 impl<T: Copy> Option<&T> {
942 /// Maps an `Option<&T>` to an `Option<T>` by copying the contents of the
949 /// let opt_x = Some(&x);
950 /// assert_eq!(opt_x, Some(&12));
951 /// let copied = opt_x.copied();
952 /// assert_eq!(copied, Some(12));
954 #[stable(feature = "copied", since = "1.35.0")]
955 pub fn copied(self) -> Option<T> {
960 impl<T: Copy> Option<&mut T> {
961 /// Maps an `Option<&mut T>` to an `Option<T>` by copying the contents of the
968 /// let opt_x = Some(&mut x);
969 /// assert_eq!(opt_x, Some(&mut 12));
970 /// let copied = opt_x.copied();
971 /// assert_eq!(copied, Some(12));
973 #[stable(feature = "copied", since = "1.35.0")]
974 pub fn copied(self) -> Option<T> {
979 impl<T: Clone> Option<&T> {
980 /// Maps an `Option<&T>` to an `Option<T>` by cloning the contents of the
987 /// let opt_x = Some(&x);
988 /// assert_eq!(opt_x, Some(&12));
989 /// let cloned = opt_x.cloned();
990 /// assert_eq!(cloned, Some(12));
992 #[stable(feature = "rust1", since = "1.0.0")]
993 pub fn cloned(self) -> Option<T> {
994 self.map(|t| t.clone())
998 impl<T: Clone> Option<&mut T> {
999 /// Maps an `Option<&mut T>` to an `Option<T>` by cloning the contents of the
1006 /// let opt_x = Some(&mut x);
1007 /// assert_eq!(opt_x, Some(&mut 12));
1008 /// let cloned = opt_x.cloned();
1009 /// assert_eq!(cloned, Some(12));
1011 #[stable(since = "1.26.0", feature = "option_ref_mut_cloned")]
1012 pub fn cloned(self) -> Option<T> {
1013 self.map(|t| t.clone())
1017 impl<T: fmt::Debug> Option<T> {
1018 /// Consumes `self` while expecting [`None`] and returning nothing.
1022 /// Panics if the value is a [`Some`], with a panic message including the
1023 /// passed message, and the content of the [`Some`].
1028 /// #![feature(option_expect_none)]
1030 /// use std::collections::HashMap;
1031 /// let mut squares = HashMap::new();
1032 /// for i in -10..=10 {
1033 /// // This will not panic, since all keys are unique.
1034 /// squares.insert(i, i * i).expect_none("duplicate key");
1038 /// ```{.should_panic}
1039 /// #![feature(option_expect_none)]
1041 /// use std::collections::HashMap;
1042 /// let mut sqrts = HashMap::new();
1043 /// for i in -10..=10 {
1044 /// // This will panic, since both negative and positive `i` will
1045 /// // insert the same `i * i` key, returning the old `Some(i)`.
1046 /// sqrts.insert(i * i, i).expect_none("duplicate key");
1051 #[unstable(feature = "option_expect_none", reason = "newly added", issue = "62633")]
1052 pub fn expect_none(self, msg: &str) {
1053 if let Some(val) = self {
1054 expect_none_failed(msg, &val);
1058 /// Consumes `self` while expecting [`None`] and returning nothing.
1062 /// Panics if the value is a [`Some`], with a custom panic message provided
1063 /// by the [`Some`]'s value.
1065 /// [`Some(v)`]: Some
1070 /// #![feature(option_unwrap_none)]
1072 /// use std::collections::HashMap;
1073 /// let mut squares = HashMap::new();
1074 /// for i in -10..=10 {
1075 /// // This will not panic, since all keys are unique.
1076 /// squares.insert(i, i * i).unwrap_none();
1080 /// ```{.should_panic}
1081 /// #![feature(option_unwrap_none)]
1083 /// use std::collections::HashMap;
1084 /// let mut sqrts = HashMap::new();
1085 /// for i in -10..=10 {
1086 /// // This will panic, since both negative and positive `i` will
1087 /// // insert the same `i * i` key, returning the old `Some(i)`.
1088 /// sqrts.insert(i * i, i).unwrap_none();
1093 #[unstable(feature = "option_unwrap_none", reason = "newly added", issue = "62633")]
1094 pub fn unwrap_none(self) {
1095 if let Some(val) = self {
1096 expect_none_failed("called `Option::unwrap_none()` on a `Some` value", &val);
1101 impl<T: Default> Option<T> {
1102 /// Returns the contained [`Some`] value or a default
1104 /// Consumes the `self` argument then, if [`Some`], returns the contained
1105 /// value, otherwise if [`None`], returns the [default value] for that
1110 /// Converts a string to an integer, turning poorly-formed strings
1111 /// into 0 (the default value for integers). [`parse`] converts
1112 /// a string to any other type that implements [`FromStr`], returning
1113 /// [`None`] on error.
1116 /// let good_year_from_input = "1909";
1117 /// let bad_year_from_input = "190blarg";
1118 /// let good_year = good_year_from_input.parse().ok().unwrap_or_default();
1119 /// let bad_year = bad_year_from_input.parse().ok().unwrap_or_default();
1121 /// assert_eq!(1909, good_year);
1122 /// assert_eq!(0, bad_year);
1125 /// [default value]: Default::default
1126 /// [`parse`]: str::parse
1127 /// [`FromStr`]: crate::str::FromStr
1129 #[stable(feature = "rust1", since = "1.0.0")]
1130 pub fn unwrap_or_default(self) -> T {
1133 None => Default::default(),
1138 impl<T: Deref> Option<T> {
1139 /// Converts from `Option<T>` (or `&Option<T>`) to `Option<&T::Target>`.
1141 /// Leaves the original Option in-place, creating a new one with a reference
1142 /// to the original one, additionally coercing the contents via [`Deref`].
1147 /// let x: Option<String> = Some("hey".to_owned());
1148 /// assert_eq!(x.as_deref(), Some("hey"));
1150 /// let x: Option<String> = None;
1151 /// assert_eq!(x.as_deref(), None);
1153 #[stable(feature = "option_deref", since = "1.40.0")]
1154 pub fn as_deref(&self) -> Option<&T::Target> {
1155 self.as_ref().map(|t| t.deref())
1159 impl<T: DerefMut> Option<T> {
1160 /// Converts from `Option<T>` (or `&mut Option<T>`) to `Option<&mut T::Target>`.
1162 /// Leaves the original `Option` in-place, creating a new one containing a mutable reference to
1163 /// the inner type's `Deref::Target` type.
1168 /// let mut x: Option<String> = Some("hey".to_owned());
1169 /// assert_eq!(x.as_deref_mut().map(|x| {
1170 /// x.make_ascii_uppercase();
1172 /// }), Some("HEY".to_owned().as_mut_str()));
1174 #[stable(feature = "option_deref", since = "1.40.0")]
1175 pub fn as_deref_mut(&mut self) -> Option<&mut T::Target> {
1176 self.as_mut().map(|t| t.deref_mut())
1180 impl<T, E> Option<Result<T, E>> {
1181 /// Transposes an `Option` of a [`Result`] into a [`Result`] of an `Option`.
1183 /// [`None`] will be mapped to [`Ok`]`(`[`None`]`)`.
1184 /// [`Some`]`(`[`Ok`]`(_))` and [`Some`]`(`[`Err`]`(_))` will be mapped to
1185 /// [`Ok`]`(`[`Some`]`(_))` and [`Err`]`(_)`.
1190 /// #[derive(Debug, Eq, PartialEq)]
1193 /// let x: Result<Option<i32>, SomeErr> = Ok(Some(5));
1194 /// let y: Option<Result<i32, SomeErr>> = Some(Ok(5));
1195 /// assert_eq!(x, y.transpose());
1198 #[stable(feature = "transpose_result", since = "1.33.0")]
1199 pub fn transpose(self) -> Result<Option<T>, E> {
1201 Some(Ok(x)) => Ok(Some(x)),
1202 Some(Err(e)) => Err(e),
1208 // This is a separate function to reduce the code size of .expect() itself.
1212 fn expect_failed(msg: &str) -> ! {
1216 // This is a separate function to reduce the code size of .expect_none() itself.
1220 fn expect_none_failed(msg: &str, value: &dyn fmt::Debug) -> ! {
1221 panic!("{}: {:?}", msg, value)
1224 /////////////////////////////////////////////////////////////////////////////
1225 // Trait implementations
1226 /////////////////////////////////////////////////////////////////////////////
1228 #[stable(feature = "rust1", since = "1.0.0")]
1229 impl<T: Clone> Clone for Option<T> {
1231 fn clone(&self) -> Self {
1233 Some(x) => Some(x.clone()),
1239 fn clone_from(&mut self, source: &Self) {
1240 match (self, source) {
1241 (Some(to), Some(from)) => to.clone_from(from),
1242 (to, from) => *to = from.clone(),
1247 #[stable(feature = "rust1", since = "1.0.0")]
1248 impl<T> Default for Option<T> {
1249 /// Returns [`None`][Option::None].
1254 /// let opt: Option<u32> = Option::default();
1255 /// assert!(opt.is_none());
1258 fn default() -> Option<T> {
1263 #[stable(feature = "rust1", since = "1.0.0")]
1264 impl<T> IntoIterator for Option<T> {
1266 type IntoIter = IntoIter<T>;
1268 /// Returns a consuming iterator over the possibly contained value.
1273 /// let x = Some("string");
1274 /// let v: Vec<&str> = x.into_iter().collect();
1275 /// assert_eq!(v, ["string"]);
1278 /// let v: Vec<&str> = x.into_iter().collect();
1279 /// assert!(v.is_empty());
1282 fn into_iter(self) -> IntoIter<T> {
1283 IntoIter { inner: Item { opt: self } }
1287 #[stable(since = "1.4.0", feature = "option_iter")]
1288 impl<'a, T> IntoIterator for &'a Option<T> {
1290 type IntoIter = Iter<'a, T>;
1292 fn into_iter(self) -> Iter<'a, T> {
1297 #[stable(since = "1.4.0", feature = "option_iter")]
1298 impl<'a, T> IntoIterator for &'a mut Option<T> {
1299 type Item = &'a mut T;
1300 type IntoIter = IterMut<'a, T>;
1302 fn into_iter(self) -> IterMut<'a, T> {
1307 #[stable(since = "1.12.0", feature = "option_from")]
1308 impl<T> From<T> for Option<T> {
1309 /// Copies `val` into a new `Some`.
1314 /// let o: Option<u8> = Option::from(67);
1316 /// assert_eq!(Some(67), o);
1318 fn from(val: T) -> Option<T> {
1323 #[stable(feature = "option_ref_from_ref_option", since = "1.30.0")]
1324 impl<'a, T> From<&'a Option<T>> for Option<&'a T> {
1325 /// Converts from `&Option<T>` to `Option<&T>`.
1329 /// Converts an `Option<`[`String`]`>` into an `Option<`[`usize`]`>`, preserving the original.
1330 /// The [`map`] method takes the `self` argument by value, consuming the original,
1331 /// so this technique uses `as_ref` to first take an `Option` to a reference
1332 /// to the value inside the original.
1334 /// [`map`]: Option::map
1335 /// [`String`]: ../../std/string/struct.String.html
1338 /// let s: Option<String> = Some(String::from("Hello, Rustaceans!"));
1339 /// let o: Option<usize> = Option::from(&s).map(|ss: &String| ss.len());
1341 /// println!("Can still print s: {:?}", s);
1343 /// assert_eq!(o, Some(18));
1345 fn from(o: &'a Option<T>) -> Option<&'a T> {
1350 #[stable(feature = "option_ref_from_ref_option", since = "1.30.0")]
1351 impl<'a, T> From<&'a mut Option<T>> for Option<&'a mut T> {
1352 /// Converts from `&mut Option<T>` to `Option<&mut T>`
1357 /// let mut s = Some(String::from("Hello"));
1358 /// let o: Option<&mut String> = Option::from(&mut s);
1361 /// Some(t) => *t = String::from("Hello, Rustaceans!"),
1365 /// assert_eq!(s, Some(String::from("Hello, Rustaceans!")));
1367 fn from(o: &'a mut Option<T>) -> Option<&'a mut T> {
1372 /////////////////////////////////////////////////////////////////////////////
1373 // The Option Iterators
1374 /////////////////////////////////////////////////////////////////////////////
1376 #[derive(Clone, Debug)]
1381 impl<A> Iterator for Item<A> {
1385 fn next(&mut self) -> Option<A> {
1390 fn size_hint(&self) -> (usize, Option<usize>) {
1392 Some(_) => (1, Some(1)),
1393 None => (0, Some(0)),
1398 impl<A> DoubleEndedIterator for Item<A> {
1400 fn next_back(&mut self) -> Option<A> {
1405 impl<A> ExactSizeIterator for Item<A> {}
1406 impl<A> FusedIterator for Item<A> {}
1407 unsafe impl<A> TrustedLen for Item<A> {}
1409 /// An iterator over a reference to the [`Some`] variant of an [`Option`].
1411 /// The iterator yields one value if the [`Option`] is a [`Some`], otherwise none.
1413 /// This `struct` is created by the [`Option::iter`] function.
1414 #[stable(feature = "rust1", since = "1.0.0")]
1416 pub struct Iter<'a, A: 'a> {
1420 #[stable(feature = "rust1", since = "1.0.0")]
1421 impl<'a, A> Iterator for Iter<'a, A> {
1425 fn next(&mut self) -> Option<&'a A> {
1429 fn size_hint(&self) -> (usize, Option<usize>) {
1430 self.inner.size_hint()
1434 #[stable(feature = "rust1", since = "1.0.0")]
1435 impl<'a, A> DoubleEndedIterator for Iter<'a, A> {
1437 fn next_back(&mut self) -> Option<&'a A> {
1438 self.inner.next_back()
1442 #[stable(feature = "rust1", since = "1.0.0")]
1443 impl<A> ExactSizeIterator for Iter<'_, A> {}
1445 #[stable(feature = "fused", since = "1.26.0")]
1446 impl<A> FusedIterator for Iter<'_, A> {}
1448 #[unstable(feature = "trusted_len", issue = "37572")]
1449 unsafe impl<A> TrustedLen for Iter<'_, A> {}
1451 #[stable(feature = "rust1", since = "1.0.0")]
1452 impl<A> Clone for Iter<'_, A> {
1454 fn clone(&self) -> Self {
1455 Iter { inner: self.inner.clone() }
1459 /// An iterator over a mutable reference to the [`Some`] variant of an [`Option`].
1461 /// The iterator yields one value if the [`Option`] is a [`Some`], otherwise none.
1463 /// This `struct` is created by the [`Option::iter_mut`] function.
1464 #[stable(feature = "rust1", since = "1.0.0")]
1466 pub struct IterMut<'a, A: 'a> {
1467 inner: Item<&'a mut A>,
1470 #[stable(feature = "rust1", since = "1.0.0")]
1471 impl<'a, A> Iterator for IterMut<'a, A> {
1472 type Item = &'a mut A;
1475 fn next(&mut self) -> Option<&'a mut A> {
1479 fn size_hint(&self) -> (usize, Option<usize>) {
1480 self.inner.size_hint()
1484 #[stable(feature = "rust1", since = "1.0.0")]
1485 impl<'a, A> DoubleEndedIterator for IterMut<'a, A> {
1487 fn next_back(&mut self) -> Option<&'a mut A> {
1488 self.inner.next_back()
1492 #[stable(feature = "rust1", since = "1.0.0")]
1493 impl<A> ExactSizeIterator for IterMut<'_, A> {}
1495 #[stable(feature = "fused", since = "1.26.0")]
1496 impl<A> FusedIterator for IterMut<'_, A> {}
1497 #[unstable(feature = "trusted_len", issue = "37572")]
1498 unsafe impl<A> TrustedLen for IterMut<'_, A> {}
1500 /// An iterator over the value in [`Some`] variant of an [`Option`].
1502 /// The iterator yields one value if the [`Option`] is a [`Some`], otherwise none.
1504 /// This `struct` is created by the [`Option::into_iter`] function.
1506 /// [`Option::into_iter`]: enum.Option.html#method.into_iter
1507 #[derive(Clone, Debug)]
1508 #[stable(feature = "rust1", since = "1.0.0")]
1509 pub struct IntoIter<A> {
1513 #[stable(feature = "rust1", since = "1.0.0")]
1514 impl<A> Iterator for IntoIter<A> {
1518 fn next(&mut self) -> Option<A> {
1522 fn size_hint(&self) -> (usize, Option<usize>) {
1523 self.inner.size_hint()
1527 #[stable(feature = "rust1", since = "1.0.0")]
1528 impl<A> DoubleEndedIterator for IntoIter<A> {
1530 fn next_back(&mut self) -> Option<A> {
1531 self.inner.next_back()
1535 #[stable(feature = "rust1", since = "1.0.0")]
1536 impl<A> ExactSizeIterator for IntoIter<A> {}
1538 #[stable(feature = "fused", since = "1.26.0")]
1539 impl<A> FusedIterator for IntoIter<A> {}
1541 #[unstable(feature = "trusted_len", issue = "37572")]
1542 unsafe impl<A> TrustedLen for IntoIter<A> {}
1544 /////////////////////////////////////////////////////////////////////////////
1546 /////////////////////////////////////////////////////////////////////////////
1548 #[stable(feature = "rust1", since = "1.0.0")]
1549 impl<A, V: FromIterator<A>> FromIterator<Option<A>> for Option<V> {
1550 /// Takes each element in the [`Iterator`]: if it is [`None`][Option::None],
1551 /// no further elements are taken, and the [`None`][Option::None] is
1552 /// returned. Should no [`None`][Option::None] occur, a container with the
1553 /// values of each [`Option`] is returned.
1557 /// Here is an example which increments every integer in a vector.
1558 /// We use the checked variant of `add` that returns `None` when the
1559 /// calculation would result in an overflow.
1562 /// let items = vec![0_u16, 1, 2];
1564 /// let res: Option<Vec<u16>> = items
1566 /// .map(|x| x.checked_add(1))
1569 /// assert_eq!(res, Some(vec![1, 2, 3]));
1572 /// As you can see, this will return the expected, valid items.
1574 /// Here is another example that tries to subtract one from another list
1575 /// of integers, this time checking for underflow:
1578 /// let items = vec![2_u16, 1, 0];
1580 /// let res: Option<Vec<u16>> = items
1582 /// .map(|x| x.checked_sub(1))
1585 /// assert_eq!(res, None);
1588 /// Since the last element is zero, it would underflow. Thus, the resulting
1589 /// value is `None`.
1591 /// Here is a variation on the previous example, showing that no
1592 /// further elements are taken from `iter` after the first `None`.
1595 /// let items = vec![3_u16, 2, 1, 10];
1597 /// let mut shared = 0;
1599 /// let res: Option<Vec<u16>> = items
1601 /// .map(|x| { shared += x; x.checked_sub(2) })
1604 /// assert_eq!(res, None);
1605 /// assert_eq!(shared, 6);
1608 /// Since the third element caused an underflow, no further elements were taken,
1609 /// so the final value of `shared` is 6 (= `3 + 2 + 1`), not 16.
1611 fn from_iter<I: IntoIterator<Item = Option<A>>>(iter: I) -> Option<V> {
1612 // FIXME(#11084): This could be replaced with Iterator::scan when this
1613 // performance bug is closed.
1615 iter.into_iter().map(|x| x.ok_or(())).collect::<Result<_, _>>().ok()
1619 /// The error type that results from applying the try operator (`?`) to a `None` value. If you wish
1620 /// to allow `x?` (where `x` is an `Option<T>`) to be converted into your error type, you can
1621 /// implement `impl From<NoneError>` for `YourErrorType`. In that case, `x?` within a function that
1622 /// returns `Result<_, YourErrorType>` will translate a `None` value into an `Err` result.
1623 #[rustc_diagnostic_item = "none_error"]
1624 #[unstable(feature = "try_trait", issue = "42327")]
1625 #[derive(Clone, Copy, PartialEq, PartialOrd, Eq, Ord, Debug, Hash)]
1626 pub struct NoneError;
1628 #[unstable(feature = "try_trait", issue = "42327")]
1629 impl<T> ops::Try for Option<T> {
1631 type Error = NoneError;
1634 fn into_result(self) -> Result<T, NoneError> {
1635 self.ok_or(NoneError)
1639 fn from_ok(v: T) -> Self {
1644 fn from_error(_: NoneError) -> Self {
1649 impl<T> Option<Option<T>> {
1650 /// Converts from `Option<Option<T>>` to `Option<T>`
1655 /// let x: Option<Option<u32>> = Some(Some(6));
1656 /// assert_eq!(Some(6), x.flatten());
1658 /// let x: Option<Option<u32>> = Some(None);
1659 /// assert_eq!(None, x.flatten());
1661 /// let x: Option<Option<u32>> = None;
1662 /// assert_eq!(None, x.flatten());
1664 /// Flattening once only removes one level of nesting:
1666 /// let x: Option<Option<Option<u32>>> = Some(Some(Some(6)));
1667 /// assert_eq!(Some(Some(6)), x.flatten());
1668 /// assert_eq!(Some(6), x.flatten().flatten());
1671 #[stable(feature = "option_flattening", since = "1.40.0")]
1672 pub fn flatten(self) -> Option<T> {
1673 self.and_then(convert::identity)