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, <code>[Option]<[Box\<T>]></code>.
52 //! [Box\<T>]: ../../std/boxed/struct.Box.html
54 //! The following example uses [`Option`] to create an optional box of
55 //! [`i32`]. Notice that in order to use the inner [`i32`] value, the
56 //! `check_optional` function first needs to use pattern matching to
57 //! determine whether the box has a value (i.e., it is [`Some(...)`][`Some`]) or
61 //! let optional = None;
62 //! check_optional(optional);
64 //! let optional = Some(Box::new(9000));
65 //! check_optional(optional);
67 //! fn check_optional(optional: Option<Box<i32>>) {
69 //! Some(p) => println!("has value {p}"),
70 //! None => println!("has no value"),
77 //! Rust guarantees to optimize the following types `T` such that
78 //! [`Option<T>`] has the same size as `T`:
83 //! * `fn`, `extern "C" fn`[^extern_fn]
84 //! * [`num::NonZero*`]
85 //! * [`ptr::NonNull<U>`]
86 //! * `#[repr(transparent)]` struct around one of the types in this list.
88 //! [^extern_fn]: this remains true for any other ABI: `extern "abi" fn` (_e.g._, `extern "system" fn`)
90 //! [`Box<U>`]: ../../std/boxed/struct.Box.html
91 //! [`num::NonZero*`]: crate::num
92 //! [`ptr::NonNull<U>`]: crate::ptr::NonNull
94 //! This is called the "null pointer optimization" or NPO.
96 //! It is further guaranteed that, for the cases above, one can
97 //! [`mem::transmute`] from all valid values of `T` to `Option<T>` and
98 //! from `Some::<T>(_)` to `T` (but transmuting `None::<T>` to `T`
99 //! is undefined behaviour).
101 //! # Method overview
103 //! In addition to working with pattern matching, [`Option`] provides a wide
104 //! variety of different methods.
106 //! ## Querying the variant
108 //! The [`is_some`] and [`is_none`] methods return [`true`] if the [`Option`]
109 //! is [`Some`] or [`None`], respectively.
111 //! [`is_none`]: Option::is_none
112 //! [`is_some`]: Option::is_some
114 //! ## Adapters for working with references
116 //! * [`as_ref`] converts from <code>[&][][Option]\<T></code> to <code>[Option]<[&]T></code>
117 //! * [`as_mut`] converts from <code>[&mut] [Option]\<T></code> to <code>[Option]<[&mut] T></code>
118 //! * [`as_deref`] converts from <code>[&][][Option]\<T></code> to
119 //! <code>[Option]<[&]T::[Target]></code>
120 //! * [`as_deref_mut`] converts from <code>[&mut] [Option]\<T></code> to
121 //! <code>[Option]<[&mut] T::[Target]></code>
122 //! * [`as_pin_ref`] converts from <code>[Pin]<[&][][Option]\<T>></code> to
123 //! <code>[Option]<[Pin]<[&]T>></code>
124 //! * [`as_pin_mut`] converts from <code>[Pin]<[&mut] [Option]\<T>></code> to
125 //! <code>[Option]<[Pin]<[&mut] T>></code>
127 //! [&]: reference "shared reference"
128 //! [&mut]: reference "mutable reference"
129 //! [Target]: Deref::Target "ops::Deref::Target"
130 //! [`as_deref`]: Option::as_deref
131 //! [`as_deref_mut`]: Option::as_deref_mut
132 //! [`as_mut`]: Option::as_mut
133 //! [`as_pin_mut`]: Option::as_pin_mut
134 //! [`as_pin_ref`]: Option::as_pin_ref
135 //! [`as_ref`]: Option::as_ref
137 //! ## Extracting the contained value
139 //! These methods extract the contained value in an [`Option<T>`] when it
140 //! is the [`Some`] variant. If the [`Option`] is [`None`]:
142 //! * [`expect`] panics with a provided custom message
143 //! * [`unwrap`] panics with a generic message
144 //! * [`unwrap_or`] returns the provided default value
145 //! * [`unwrap_or_default`] returns the default value of the type `T`
146 //! (which must implement the [`Default`] trait)
147 //! * [`unwrap_or_else`] returns the result of evaluating the provided
150 //! [`expect`]: Option::expect
151 //! [`unwrap`]: Option::unwrap
152 //! [`unwrap_or`]: Option::unwrap_or
153 //! [`unwrap_or_default`]: Option::unwrap_or_default
154 //! [`unwrap_or_else`]: Option::unwrap_or_else
156 //! ## Transforming contained values
158 //! These methods transform [`Option`] to [`Result`]:
160 //! * [`ok_or`] transforms [`Some(v)`] to [`Ok(v)`], and [`None`] to
161 //! [`Err(err)`] using the provided default `err` value
162 //! * [`ok_or_else`] transforms [`Some(v)`] to [`Ok(v)`], and [`None`] to
163 //! a value of [`Err`] using the provided function
164 //! * [`transpose`] transposes an [`Option`] of a [`Result`] into a
165 //! [`Result`] of an [`Option`]
167 //! [`Err(err)`]: Err
169 //! [`Some(v)`]: Some
170 //! [`ok_or`]: Option::ok_or
171 //! [`ok_or_else`]: Option::ok_or_else
172 //! [`transpose`]: Option::transpose
174 //! These methods transform the [`Some`] variant:
176 //! * [`filter`] calls the provided predicate function on the contained
177 //! value `t` if the [`Option`] is [`Some(t)`], and returns [`Some(t)`]
178 //! if the function returns `true`; otherwise, returns [`None`]
179 //! * [`flatten`] removes one level of nesting from an
180 //! [`Option<Option<T>>`]
181 //! * [`map`] transforms [`Option<T>`] to [`Option<U>`] by applying the
182 //! provided function to the contained value of [`Some`] and leaving
183 //! [`None`] values unchanged
185 //! [`Some(t)`]: Some
186 //! [`filter`]: Option::filter
187 //! [`flatten`]: Option::flatten
188 //! [`map`]: Option::map
190 //! These methods transform [`Option<T>`] to a value of a possibly
191 //! different type `U`:
193 //! * [`map_or`] applies the provided function to the contained value of
194 //! [`Some`], or returns the provided default value if the [`Option`] is
196 //! * [`map_or_else`] applies the provided function to the contained value
197 //! of [`Some`], or returns the result of evaluating the provided
198 //! fallback function if the [`Option`] is [`None`]
200 //! [`map_or`]: Option::map_or
201 //! [`map_or_else`]: Option::map_or_else
203 //! These methods combine the [`Some`] variants of two [`Option`] values:
205 //! * [`zip`] returns [`Some((s, o))`] if `self` is [`Some(s)`] and the
206 //! provided [`Option`] value is [`Some(o)`]; otherwise, returns [`None`]
207 //! * [`zip_with`] calls the provided function `f` and returns
208 //! [`Some(f(s, o))`] if `self` is [`Some(s)`] and the provided
209 //! [`Option`] value is [`Some(o)`]; otherwise, returns [`None`]
211 //! [`Some(f(s, o))`]: Some
212 //! [`Some(o)`]: Some
213 //! [`Some(s)`]: Some
214 //! [`Some((s, o))`]: Some
215 //! [`zip`]: Option::zip
216 //! [`zip_with`]: Option::zip_with
218 //! ## Boolean operators
220 //! These methods treat the [`Option`] as a boolean value, where [`Some`]
221 //! acts like [`true`] and [`None`] acts like [`false`]. There are two
222 //! categories of these methods: ones that take an [`Option`] as input, and
223 //! ones that take a function as input (to be lazily evaluated).
225 //! The [`and`], [`or`], and [`xor`] methods take another [`Option`] as
226 //! input, and produce an [`Option`] as output. Only the [`and`] method can
227 //! produce an [`Option<U>`] value having a different inner type `U` than
230 //! | method | self | input | output |
231 //! |---------|-----------|-----------|-----------|
232 //! | [`and`] | `None` | (ignored) | `None` |
233 //! | [`and`] | `Some(x)` | `None` | `None` |
234 //! | [`and`] | `Some(x)` | `Some(y)` | `Some(y)` |
235 //! | [`or`] | `None` | `None` | `None` |
236 //! | [`or`] | `None` | `Some(y)` | `Some(y)` |
237 //! | [`or`] | `Some(x)` | (ignored) | `Some(x)` |
238 //! | [`xor`] | `None` | `None` | `None` |
239 //! | [`xor`] | `None` | `Some(y)` | `Some(y)` |
240 //! | [`xor`] | `Some(x)` | `None` | `Some(x)` |
241 //! | [`xor`] | `Some(x)` | `Some(y)` | `None` |
243 //! [`and`]: Option::and
244 //! [`or`]: Option::or
245 //! [`xor`]: Option::xor
247 //! The [`and_then`] and [`or_else`] methods take a function as input, and
248 //! only evaluate the function when they need to produce a new value. Only
249 //! the [`and_then`] method can produce an [`Option<U>`] value having a
250 //! different inner type `U` than [`Option<T>`].
252 //! | method | self | function input | function result | output |
253 //! |--------------|-----------|----------------|-----------------|-----------|
254 //! | [`and_then`] | `None` | (not provided) | (not evaluated) | `None` |
255 //! | [`and_then`] | `Some(x)` | `x` | `None` | `None` |
256 //! | [`and_then`] | `Some(x)` | `x` | `Some(y)` | `Some(y)` |
257 //! | [`or_else`] | `None` | (not provided) | `None` | `None` |
258 //! | [`or_else`] | `None` | (not provided) | `Some(y)` | `Some(y)` |
259 //! | [`or_else`] | `Some(x)` | (not provided) | (not evaluated) | `Some(x)` |
261 //! [`and_then`]: Option::and_then
262 //! [`or_else`]: Option::or_else
264 //! This is an example of using methods like [`and_then`] and [`or`] in a
265 //! pipeline of method calls. Early stages of the pipeline pass failure
266 //! values ([`None`]) through unchanged, and continue processing on
267 //! success values ([`Some`]). Toward the end, [`or`] substitutes an error
268 //! message if it receives [`None`].
271 //! # use std::collections::BTreeMap;
272 //! let mut bt = BTreeMap::new();
273 //! bt.insert(20u8, "foo");
274 //! bt.insert(42u8, "bar");
275 //! let res = [0u8, 1, 11, 200, 22]
278 //! // `checked_sub()` returns `None` on error
280 //! // same with `checked_mul()`
281 //! .and_then(|x| x.checked_mul(2))
282 //! // `BTreeMap::get` returns `None` on error
283 //! .and_then(|x| bt.get(&x))
284 //! // Substitute an error message if we have `None` so far
285 //! .or(Some(&"error!"))
287 //! // Won't panic because we unconditionally used `Some` above
290 //! .collect::<Vec<_>>();
291 //! assert_eq!(res, ["error!", "error!", "foo", "error!", "bar"]);
294 //! ## Comparison operators
296 //! If `T` implements [`PartialOrd`] then [`Option<T>`] will derive its
297 //! [`PartialOrd`] implementation. With this order, [`None`] compares as
298 //! less than any [`Some`], and two [`Some`] compare the same way as their
299 //! contained values would in `T`. If `T` also implements
300 //! [`Ord`], then so does [`Option<T>`].
303 //! assert!(None < Some(0));
304 //! assert!(Some(0) < Some(1));
307 //! ## Iterating over `Option`
309 //! An [`Option`] can be iterated over. This can be helpful if you need an
310 //! iterator that is conditionally empty. The iterator will either produce
311 //! a single value (when the [`Option`] is [`Some`]), or produce no values
312 //! (when the [`Option`] is [`None`]). For example, [`into_iter`] acts like
313 //! [`once(v)`] if the [`Option`] is [`Some(v)`], and like [`empty()`] if
314 //! the [`Option`] is [`None`].
316 //! [`Some(v)`]: Some
317 //! [`empty()`]: crate::iter::empty
318 //! [`once(v)`]: crate::iter::once
320 //! Iterators over [`Option<T>`] come in three types:
322 //! * [`into_iter`] consumes the [`Option`] and produces the contained
324 //! * [`iter`] produces an immutable reference of type `&T` to the
326 //! * [`iter_mut`] produces a mutable reference of type `&mut T` to the
329 //! [`into_iter`]: Option::into_iter
330 //! [`iter`]: Option::iter
331 //! [`iter_mut`]: Option::iter_mut
333 //! An iterator over [`Option`] can be useful when chaining iterators, for
334 //! example, to conditionally insert items. (It's not always necessary to
335 //! explicitly call an iterator constructor: many [`Iterator`] methods that
336 //! accept other iterators will also accept iterable types that implement
337 //! [`IntoIterator`], which includes [`Option`].)
340 //! let yep = Some(42);
342 //! // chain() already calls into_iter(), so we don't have to do so
343 //! let nums: Vec<i32> = (0..4).chain(yep).chain(4..8).collect();
344 //! assert_eq!(nums, [0, 1, 2, 3, 42, 4, 5, 6, 7]);
345 //! let nums: Vec<i32> = (0..4).chain(nope).chain(4..8).collect();
346 //! assert_eq!(nums, [0, 1, 2, 3, 4, 5, 6, 7]);
349 //! One reason to chain iterators in this way is that a function returning
350 //! `impl Iterator` must have all possible return values be of the same
351 //! concrete type. Chaining an iterated [`Option`] can help with that.
354 //! fn make_iter(do_insert: bool) -> impl Iterator<Item = i32> {
355 //! // Explicit returns to illustrate return types matching
356 //! match do_insert {
357 //! true => return (0..4).chain(Some(42)).chain(4..8),
358 //! false => return (0..4).chain(None).chain(4..8),
361 //! println!("{:?}", make_iter(true).collect::<Vec<_>>());
362 //! println!("{:?}", make_iter(false).collect::<Vec<_>>());
365 //! If we try to do the same thing, but using [`once()`] and [`empty()`],
366 //! we can't return `impl Iterator` anymore because the concrete types of
367 //! the return values differ.
369 //! [`empty()`]: crate::iter::empty
370 //! [`once()`]: crate::iter::once
372 //! ```compile_fail,E0308
373 //! # use std::iter::{empty, once};
374 //! // This won't compile because all possible returns from the function
375 //! // must have the same concrete type.
376 //! fn make_iter(do_insert: bool) -> impl Iterator<Item = i32> {
377 //! // Explicit returns to illustrate return types not matching
378 //! match do_insert {
379 //! true => return (0..4).chain(once(42)).chain(4..8),
380 //! false => return (0..4).chain(empty()).chain(4..8),
385 //! ## Collecting into `Option`
387 //! [`Option`] implements the [`FromIterator`][impl-FromIterator] trait,
388 //! which allows an iterator over [`Option`] values to be collected into an
389 //! [`Option`] of a collection of each contained value of the original
390 //! [`Option`] values, or [`None`] if any of the elements was [`None`].
392 //! [impl-FromIterator]: Option#impl-FromIterator%3COption%3CA%3E%3E-for-Option%3CV%3E
395 //! let v = [Some(2), Some(4), None, Some(8)];
396 //! let res: Option<Vec<_>> = v.into_iter().collect();
397 //! assert_eq!(res, None);
398 //! let v = [Some(2), Some(4), Some(8)];
399 //! let res: Option<Vec<_>> = v.into_iter().collect();
400 //! assert_eq!(res, Some(vec![2, 4, 8]));
403 //! [`Option`] also implements the [`Product`][impl-Product] and
404 //! [`Sum`][impl-Sum] traits, allowing an iterator over [`Option`] values
405 //! to provide the [`product`][Iterator::product] and
406 //! [`sum`][Iterator::sum] methods.
408 //! [impl-Product]: Option#impl-Product%3COption%3CU%3E%3E-for-Option%3CT%3E
409 //! [impl-Sum]: Option#impl-Sum%3COption%3CU%3E%3E-for-Option%3CT%3E
412 //! let v = [None, Some(1), Some(2), Some(3)];
413 //! let res: Option<i32> = v.into_iter().sum();
414 //! assert_eq!(res, None);
415 //! let v = [Some(1), Some(2), Some(21)];
416 //! let res: Option<i32> = v.into_iter().product();
417 //! assert_eq!(res, Some(42));
420 //! ## Modifying an [`Option`] in-place
422 //! These methods return a mutable reference to the contained value of an
425 //! * [`insert`] inserts a value, dropping any old contents
426 //! * [`get_or_insert`] gets the current value, inserting a provided
427 //! default value if it is [`None`]
428 //! * [`get_or_insert_default`] gets the current value, inserting the
429 //! default value of type `T` (which must implement [`Default`]) if it is
431 //! * [`get_or_insert_with`] gets the current value, inserting a default
432 //! computed by the provided function if it is [`None`]
434 //! [`get_or_insert`]: Option::get_or_insert
435 //! [`get_or_insert_default`]: Option::get_or_insert_default
436 //! [`get_or_insert_with`]: Option::get_or_insert_with
437 //! [`insert`]: Option::insert
439 //! These methods transfer ownership of the contained value of an
442 //! * [`take`] takes ownership of the contained value of an [`Option`], if
443 //! any, replacing the [`Option`] with [`None`]
444 //! * [`replace`] takes ownership of the contained value of an [`Option`],
445 //! if any, replacing the [`Option`] with a [`Some`] containing the
448 //! [`replace`]: Option::replace
449 //! [`take`]: Option::take
453 //! Basic pattern matching on [`Option`]:
456 //! let msg = Some("howdy");
458 //! // Take a reference to the contained string
459 //! if let Some(m) = &msg {
460 //! println!("{}", *m);
463 //! // Remove the contained string, destroying the Option
464 //! let unwrapped_msg = msg.unwrap_or("default message");
467 //! Initialize a result to [`None`] before a loop:
470 //! enum Kingdom { Plant(u32, &'static str), Animal(u32, &'static str) }
472 //! // A list of data to search through.
473 //! let all_the_big_things = [
474 //! Kingdom::Plant(250, "redwood"),
475 //! Kingdom::Plant(230, "noble fir"),
476 //! Kingdom::Plant(229, "sugar pine"),
477 //! Kingdom::Animal(25, "blue whale"),
478 //! Kingdom::Animal(19, "fin whale"),
479 //! Kingdom::Animal(15, "north pacific right whale"),
482 //! // We're going to search for the name of the biggest animal,
483 //! // but to start with we've just got `None`.
484 //! let mut name_of_biggest_animal = None;
485 //! let mut size_of_biggest_animal = 0;
486 //! for big_thing in &all_the_big_things {
487 //! match *big_thing {
488 //! Kingdom::Animal(size, name) if size > size_of_biggest_animal => {
489 //! // Now we've found the name of some big animal
490 //! size_of_biggest_animal = size;
491 //! name_of_biggest_animal = Some(name);
493 //! Kingdom::Animal(..) | Kingdom::Plant(..) => ()
497 //! match name_of_biggest_animal {
498 //! Some(name) => println!("the biggest animal is {name}"),
499 //! None => println!("there are no animals :("),
503 #![stable(feature = "rust1", since = "1.0.0")]
505 use crate::iter::{self, FromIterator, FusedIterator, TrustedLen};
506 use crate::marker::Destruct;
507 use crate::panicking::{panic, panic_str};
511 ops::{self, ControlFlow, Deref, DerefMut},
514 /// The `Option` type. See [the module level documentation](self) for more.
515 #[derive(Copy, PartialEq, PartialOrd, Eq, Ord, Debug, Hash)]
516 #[rustc_diagnostic_item = "Option"]
517 #[stable(feature = "rust1", since = "1.0.0")]
521 #[stable(feature = "rust1", since = "1.0.0")]
523 /// Some value of type `T`.
525 #[stable(feature = "rust1", since = "1.0.0")]
526 Some(#[stable(feature = "rust1", since = "1.0.0")] T),
529 /////////////////////////////////////////////////////////////////////////////
530 // Type implementation
531 /////////////////////////////////////////////////////////////////////////////
534 /////////////////////////////////////////////////////////////////////////
535 // Querying the contained values
536 /////////////////////////////////////////////////////////////////////////
538 /// Returns `true` if the option is a [`Some`] value.
543 /// let x: Option<u32> = Some(2);
544 /// assert_eq!(x.is_some(), true);
546 /// let x: Option<u32> = None;
547 /// assert_eq!(x.is_some(), false);
549 #[must_use = "if you intended to assert that this has a value, consider `.unwrap()` instead"]
551 #[stable(feature = "rust1", since = "1.0.0")]
552 #[rustc_const_stable(feature = "const_option_basics", since = "1.48.0")]
553 pub const fn is_some(&self) -> bool {
554 matches!(*self, Some(_))
557 /// Returns `true` if the option is a [`Some`] and the value inside of it matches a predicate.
562 /// #![feature(is_some_with)]
564 /// let x: Option<u32> = Some(2);
565 /// assert_eq!(x.is_some_and(|&x| x > 1), true);
567 /// let x: Option<u32> = Some(0);
568 /// assert_eq!(x.is_some_and(|&x| x > 1), false);
570 /// let x: Option<u32> = None;
571 /// assert_eq!(x.is_some_and(|&x| x > 1), false);
575 #[unstable(feature = "is_some_with", issue = "93050")]
576 pub fn is_some_and(&self, f: impl FnOnce(&T) -> bool) -> bool {
577 matches!(self, Some(x) if f(x))
580 /// Returns `true` if the option is a [`None`] value.
585 /// let x: Option<u32> = Some(2);
586 /// assert_eq!(x.is_none(), false);
588 /// let x: Option<u32> = None;
589 /// assert_eq!(x.is_none(), true);
591 #[must_use = "if you intended to assert that this doesn't have a value, consider \
592 `.and_then(|_| panic!(\"`Option` had a value when expected `None`\"))` instead"]
594 #[stable(feature = "rust1", since = "1.0.0")]
595 #[rustc_const_stable(feature = "const_option_basics", since = "1.48.0")]
596 pub const fn is_none(&self) -> bool {
600 /////////////////////////////////////////////////////////////////////////
601 // Adapter for working with references
602 /////////////////////////////////////////////////////////////////////////
604 /// Converts from `&Option<T>` to `Option<&T>`.
608 /// Converts an <code>Option<[String]></code> into an <code>Option<[usize]></code>, preserving
609 /// the original. The [`map`] method takes the `self` argument by value, consuming the original,
610 /// so this technique uses `as_ref` to first take an `Option` to a reference
611 /// to the value inside the original.
613 /// [`map`]: Option::map
614 /// [String]: ../../std/string/struct.String.html "String"
617 /// let text: Option<String> = Some("Hello, world!".to_string());
618 /// // First, cast `Option<String>` to `Option<&String>` with `as_ref`,
619 /// // then consume *that* with `map`, leaving `text` on the stack.
620 /// let text_length: Option<usize> = text.as_ref().map(|s| s.len());
621 /// println!("still can print text: {text:?}");
624 #[rustc_const_stable(feature = "const_option_basics", since = "1.48.0")]
625 #[stable(feature = "rust1", since = "1.0.0")]
626 pub const fn as_ref(&self) -> Option<&T> {
628 Some(ref x) => Some(x),
633 /// Converts from `&mut Option<T>` to `Option<&mut T>`.
638 /// let mut x = Some(2);
639 /// match x.as_mut() {
640 /// Some(v) => *v = 42,
643 /// assert_eq!(x, Some(42));
646 #[stable(feature = "rust1", since = "1.0.0")]
647 #[rustc_const_unstable(feature = "const_option", issue = "67441")]
648 pub const fn as_mut(&mut self) -> Option<&mut T> {
650 Some(ref mut x) => Some(x),
655 /// Converts from <code>[Pin]<[&]Option\<T>></code> to <code>Option<[Pin]<[&]T>></code>.
657 /// [&]: reference "shared reference"
660 #[stable(feature = "pin", since = "1.33.0")]
661 #[rustc_const_unstable(feature = "const_option_ext", issue = "91930")]
662 pub const fn as_pin_ref(self: Pin<&Self>) -> Option<Pin<&T>> {
663 match Pin::get_ref(self).as_ref() {
664 // SAFETY: `x` is guaranteed to be pinned because it comes from `self`
666 Some(x) => unsafe { Some(Pin::new_unchecked(x)) },
671 /// Converts from <code>[Pin]<[&mut] Option\<T>></code> to <code>Option<[Pin]<[&mut] T>></code>.
673 /// [&mut]: reference "mutable reference"
676 #[stable(feature = "pin", since = "1.33.0")]
677 #[rustc_const_unstable(feature = "const_option_ext", issue = "91930")]
678 pub const fn as_pin_mut(self: Pin<&mut Self>) -> Option<Pin<&mut T>> {
679 // SAFETY: `get_unchecked_mut` is never used to move the `Option` inside `self`.
680 // `x` is guaranteed to be pinned because it comes from `self` which is pinned.
682 match Pin::get_unchecked_mut(self).as_mut() {
683 Some(x) => Some(Pin::new_unchecked(x)),
689 /////////////////////////////////////////////////////////////////////////
690 // Getting to contained values
691 /////////////////////////////////////////////////////////////////////////
693 /// Returns the contained [`Some`] value, consuming the `self` value.
697 /// Panics if the value is a [`None`] with a custom panic message provided by
703 /// let x = Some("value");
704 /// assert_eq!(x.expect("fruits are healthy"), "value");
708 /// let x: Option<&str> = None;
709 /// x.expect("fruits are healthy"); // panics with `fruits are healthy`
712 /// # Recommended Message Style
714 /// We recommend that `expect` messages are used to describe the reason you
715 /// _expect_ the `Option` should be `Some`.
718 /// # let slice: &[u8] = &[];
719 /// let item = slice.get(0)
720 /// .expect("slice should not be empty");
723 /// **Hint**: If you're having trouble remembering how to phrase expect
724 /// error messages remember to focus on the word "should" as in "env
725 /// variable should be set by blah" or "the given binary should be available
726 /// and executable by the current user".
728 /// For more detail on expect message styles and the reasoning behind our
729 /// recommendation please refer to the section on ["Common Message
730 /// Styles"](../../std/error/index.html#common-message-styles) in the [`std::error`](../../std/error/index.html) module docs.
733 #[stable(feature = "rust1", since = "1.0.0")]
734 #[rustc_const_unstable(feature = "const_option", issue = "67441")]
735 pub const fn expect(self, msg: &str) -> T {
738 None => expect_failed(msg),
742 /// Returns the contained [`Some`] value, consuming the `self` value.
744 /// Because this function may panic, its use is generally discouraged.
745 /// Instead, prefer to use pattern matching and handle the [`None`]
746 /// case explicitly, or call [`unwrap_or`], [`unwrap_or_else`], or
747 /// [`unwrap_or_default`].
749 /// [`unwrap_or`]: Option::unwrap_or
750 /// [`unwrap_or_else`]: Option::unwrap_or_else
751 /// [`unwrap_or_default`]: Option::unwrap_or_default
755 /// Panics if the self value equals [`None`].
760 /// let x = Some("air");
761 /// assert_eq!(x.unwrap(), "air");
765 /// let x: Option<&str> = None;
766 /// assert_eq!(x.unwrap(), "air"); // fails
770 #[stable(feature = "rust1", since = "1.0.0")]
771 #[rustc_const_unstable(feature = "const_option", issue = "67441")]
772 pub const fn unwrap(self) -> T {
775 None => panic("called `Option::unwrap()` on a `None` value"),
779 /// Returns the contained [`Some`] value or a provided default.
781 /// Arguments passed to `unwrap_or` are eagerly evaluated; if you are passing
782 /// the result of a function call, it is recommended to use [`unwrap_or_else`],
783 /// which is lazily evaluated.
785 /// [`unwrap_or_else`]: Option::unwrap_or_else
790 /// assert_eq!(Some("car").unwrap_or("bike"), "car");
791 /// assert_eq!(None.unwrap_or("bike"), "bike");
794 #[stable(feature = "rust1", since = "1.0.0")]
795 #[rustc_const_unstable(feature = "const_option_ext", issue = "91930")]
796 pub const fn unwrap_or(self, default: T) -> T
806 /// Returns the contained [`Some`] value or computes it from a closure.
812 /// assert_eq!(Some(4).unwrap_or_else(|| 2 * k), 4);
813 /// assert_eq!(None.unwrap_or_else(|| 2 * k), 20);
816 #[stable(feature = "rust1", since = "1.0.0")]
817 #[rustc_const_unstable(feature = "const_option_ext", issue = "91930")]
818 pub const fn unwrap_or_else<F>(self, f: F) -> T
820 F: ~const FnOnce() -> T,
829 /// Returns the contained [`Some`] value or a default.
831 /// Consumes the `self` argument then, if [`Some`], returns the contained
832 /// value, otherwise if [`None`], returns the [default value] for that
838 /// let x: Option<u32> = None;
839 /// let y: Option<u32> = Some(12);
841 /// assert_eq!(x.unwrap_or_default(), 0);
842 /// assert_eq!(y.unwrap_or_default(), 12);
845 /// [default value]: Default::default
846 /// [`parse`]: str::parse
847 /// [`FromStr`]: crate::str::FromStr
849 #[stable(feature = "rust1", since = "1.0.0")]
850 #[rustc_const_unstable(feature = "const_option_ext", issue = "91930")]
851 pub const fn unwrap_or_default(self) -> T
857 None => Default::default(),
861 /// Returns the contained [`Some`] value, consuming the `self` value,
862 /// without checking that the value is not [`None`].
866 /// Calling this method on [`None`] is *[undefined behavior]*.
868 /// [undefined behavior]: https://doc.rust-lang.org/reference/behavior-considered-undefined.html
873 /// let x = Some("air");
874 /// assert_eq!(unsafe { x.unwrap_unchecked() }, "air");
878 /// let x: Option<&str> = None;
879 /// assert_eq!(unsafe { x.unwrap_unchecked() }, "air"); // Undefined behavior!
883 #[stable(feature = "option_result_unwrap_unchecked", since = "1.58.0")]
884 #[rustc_const_unstable(feature = "const_option_ext", issue = "91930")]
885 pub const unsafe fn unwrap_unchecked(self) -> T {
886 debug_assert!(self.is_some());
889 // SAFETY: the safety contract must be upheld by the caller.
890 None => unsafe { hint::unreachable_unchecked() },
894 /////////////////////////////////////////////////////////////////////////
895 // Transforming contained values
896 /////////////////////////////////////////////////////////////////////////
898 /// Maps an `Option<T>` to `Option<U>` by applying a function to a contained value.
902 /// Converts an <code>Option<[String]></code> into an <code>Option<[usize]></code>, consuming
905 /// [String]: ../../std/string/struct.String.html "String"
907 /// let maybe_some_string = Some(String::from("Hello, World!"));
908 /// // `Option::map` takes self *by value*, consuming `maybe_some_string`
909 /// let maybe_some_len = maybe_some_string.map(|s| s.len());
911 /// assert_eq!(maybe_some_len, Some(13));
914 #[stable(feature = "rust1", since = "1.0.0")]
915 #[rustc_const_unstable(feature = "const_option_ext", issue = "91930")]
916 pub const fn map<U, F>(self, f: F) -> Option<U>
918 F: ~const FnOnce(T) -> U,
922 Some(x) => Some(f(x)),
927 /// Calls the provided closure with a reference to the contained value (if [`Some`]).
932 /// #![feature(result_option_inspect)]
934 /// let v = vec![1, 2, 3, 4, 5];
936 /// // prints "got: 4"
937 /// let x: Option<&usize> = v.get(3).inspect(|x| println!("got: {x}"));
939 /// // prints nothing
940 /// let x: Option<&usize> = v.get(5).inspect(|x| println!("got: {x}"));
943 #[unstable(feature = "result_option_inspect", issue = "91345")]
944 #[rustc_const_unstable(feature = "const_option_ext", issue = "91930")]
945 pub const fn inspect<F>(self, f: F) -> Self
947 F: ~const FnOnce(&T),
950 if let Some(ref x) = self {
957 /// Returns the provided default result (if none),
958 /// or applies a function to the contained value (if any).
960 /// Arguments passed to `map_or` are eagerly evaluated; if you are passing
961 /// the result of a function call, it is recommended to use [`map_or_else`],
962 /// which is lazily evaluated.
964 /// [`map_or_else`]: Option::map_or_else
969 /// let x = Some("foo");
970 /// assert_eq!(x.map_or(42, |v| v.len()), 3);
972 /// let x: Option<&str> = None;
973 /// assert_eq!(x.map_or(42, |v| v.len()), 42);
976 #[stable(feature = "rust1", since = "1.0.0")]
977 #[rustc_const_unstable(feature = "const_option_ext", issue = "91930")]
978 pub const fn map_or<U, F>(self, default: U, f: F) -> U
980 F: ~const FnOnce(T) -> U,
990 /// Computes a default function result (if none), or
991 /// applies a different function to the contained value (if any).
998 /// let x = Some("foo");
999 /// assert_eq!(x.map_or_else(|| 2 * k, |v| v.len()), 3);
1001 /// let x: Option<&str> = None;
1002 /// assert_eq!(x.map_or_else(|| 2 * k, |v| v.len()), 42);
1005 #[stable(feature = "rust1", since = "1.0.0")]
1006 #[rustc_const_unstable(feature = "const_option_ext", issue = "91930")]
1007 pub const fn map_or_else<U, D, F>(self, default: D, f: F) -> U
1009 D: ~const FnOnce() -> U,
1011 F: ~const FnOnce(T) -> U,
1020 /// Transforms the `Option<T>` into a [`Result<T, E>`], mapping [`Some(v)`] to
1021 /// [`Ok(v)`] and [`None`] to [`Err(err)`].
1023 /// Arguments passed to `ok_or` are eagerly evaluated; if you are passing the
1024 /// result of a function call, it is recommended to use [`ok_or_else`], which is
1025 /// lazily evaluated.
1028 /// [`Err(err)`]: Err
1029 /// [`Some(v)`]: Some
1030 /// [`ok_or_else`]: Option::ok_or_else
1035 /// let x = Some("foo");
1036 /// assert_eq!(x.ok_or(0), Ok("foo"));
1038 /// let x: Option<&str> = None;
1039 /// assert_eq!(x.ok_or(0), Err(0));
1042 #[stable(feature = "rust1", since = "1.0.0")]
1043 #[rustc_const_unstable(feature = "const_option_ext", issue = "91930")]
1044 pub const fn ok_or<E>(self, err: E) -> Result<T, E>
1054 /// Transforms the `Option<T>` into a [`Result<T, E>`], mapping [`Some(v)`] to
1055 /// [`Ok(v)`] and [`None`] to [`Err(err())`].
1058 /// [`Err(err())`]: Err
1059 /// [`Some(v)`]: Some
1064 /// let x = Some("foo");
1065 /// assert_eq!(x.ok_or_else(|| 0), Ok("foo"));
1067 /// let x: Option<&str> = None;
1068 /// assert_eq!(x.ok_or_else(|| 0), Err(0));
1071 #[stable(feature = "rust1", since = "1.0.0")]
1072 #[rustc_const_unstable(feature = "const_option_ext", issue = "91930")]
1073 pub const fn ok_or_else<E, F>(self, err: F) -> Result<T, E>
1075 F: ~const FnOnce() -> E,
1084 /// Converts from `Option<T>` (or `&Option<T>`) to `Option<&T::Target>`.
1086 /// Leaves the original Option in-place, creating a new one with a reference
1087 /// to the original one, additionally coercing the contents via [`Deref`].
1092 /// let x: Option<String> = Some("hey".to_owned());
1093 /// assert_eq!(x.as_deref(), Some("hey"));
1095 /// let x: Option<String> = None;
1096 /// assert_eq!(x.as_deref(), None);
1098 #[stable(feature = "option_deref", since = "1.40.0")]
1099 #[rustc_const_unstable(feature = "const_option_ext", issue = "91930")]
1100 pub const fn as_deref(&self) -> Option<&T::Target>
1104 match self.as_ref() {
1105 Some(t) => Some(t.deref()),
1110 /// Converts from `Option<T>` (or `&mut Option<T>`) to `Option<&mut T::Target>`.
1112 /// Leaves the original `Option` in-place, creating a new one containing a mutable reference to
1113 /// the inner type's [`Deref::Target`] type.
1118 /// let mut x: Option<String> = Some("hey".to_owned());
1119 /// assert_eq!(x.as_deref_mut().map(|x| {
1120 /// x.make_ascii_uppercase();
1122 /// }), Some("HEY".to_owned().as_mut_str()));
1124 #[stable(feature = "option_deref", since = "1.40.0")]
1125 #[rustc_const_unstable(feature = "const_option_ext", issue = "91930")]
1126 pub const fn as_deref_mut(&mut self) -> Option<&mut T::Target>
1130 match self.as_mut() {
1131 Some(t) => Some(t.deref_mut()),
1136 /////////////////////////////////////////////////////////////////////////
1137 // Iterator constructors
1138 /////////////////////////////////////////////////////////////////////////
1140 /// Returns an iterator over the possibly contained value.
1145 /// let x = Some(4);
1146 /// assert_eq!(x.iter().next(), Some(&4));
1148 /// let x: Option<u32> = None;
1149 /// assert_eq!(x.iter().next(), None);
1152 #[rustc_const_unstable(feature = "const_option", issue = "67441")]
1153 #[stable(feature = "rust1", since = "1.0.0")]
1154 pub const fn iter(&self) -> Iter<'_, T> {
1155 Iter { inner: Item { opt: self.as_ref() } }
1158 /// Returns a mutable iterator over the possibly contained value.
1163 /// let mut x = Some(4);
1164 /// match x.iter_mut().next() {
1165 /// Some(v) => *v = 42,
1168 /// assert_eq!(x, Some(42));
1170 /// let mut x: Option<u32> = None;
1171 /// assert_eq!(x.iter_mut().next(), None);
1174 #[stable(feature = "rust1", since = "1.0.0")]
1175 pub fn iter_mut(&mut self) -> IterMut<'_, T> {
1176 IterMut { inner: Item { opt: self.as_mut() } }
1179 /////////////////////////////////////////////////////////////////////////
1180 // Boolean operations on the values, eager and lazy
1181 /////////////////////////////////////////////////////////////////////////
1183 /// Returns [`None`] if the option is [`None`], otherwise returns `optb`.
1185 /// Arguments passed to `and` are eagerly evaluated; if you are passing the
1186 /// result of a function call, it is recommended to use [`and_then`], which is
1187 /// lazily evaluated.
1189 /// [`and_then`]: Option::and_then
1194 /// let x = Some(2);
1195 /// let y: Option<&str> = None;
1196 /// assert_eq!(x.and(y), None);
1198 /// let x: Option<u32> = None;
1199 /// let y = Some("foo");
1200 /// assert_eq!(x.and(y), None);
1202 /// let x = Some(2);
1203 /// let y = Some("foo");
1204 /// assert_eq!(x.and(y), Some("foo"));
1206 /// let x: Option<u32> = None;
1207 /// let y: Option<&str> = None;
1208 /// assert_eq!(x.and(y), None);
1211 #[stable(feature = "rust1", since = "1.0.0")]
1212 #[rustc_const_unstable(feature = "const_option_ext", issue = "91930")]
1213 pub const fn and<U>(self, optb: Option<U>) -> Option<U>
1224 /// Returns [`None`] if the option is [`None`], otherwise calls `f` with the
1225 /// wrapped value and returns the result.
1227 /// Some languages call this operation flatmap.
1232 /// fn sq_then_to_string(x: u32) -> Option<String> {
1233 /// x.checked_mul(x).map(|sq| sq.to_string())
1236 /// assert_eq!(Some(2).and_then(sq_then_to_string), Some(4.to_string()));
1237 /// assert_eq!(Some(1_000_000).and_then(sq_then_to_string), None); // overflowed!
1238 /// assert_eq!(None.and_then(sq_then_to_string), None);
1241 /// Often used to chain fallible operations that may return [`None`].
1244 /// let arr_2d = [["A0", "A1"], ["B0", "B1"]];
1246 /// let item_0_1 = arr_2d.get(0).and_then(|row| row.get(1));
1247 /// assert_eq!(item_0_1, Some(&"A1"));
1249 /// let item_2_0 = arr_2d.get(2).and_then(|row| row.get(0));
1250 /// assert_eq!(item_2_0, None);
1253 #[stable(feature = "rust1", since = "1.0.0")]
1254 #[rustc_const_unstable(feature = "const_option_ext", issue = "91930")]
1255 pub const fn and_then<U, F>(self, f: F) -> Option<U>
1257 F: ~const FnOnce(T) -> Option<U>,
1266 /// Returns [`None`] if the option is [`None`], otherwise calls `predicate`
1267 /// with the wrapped value and returns:
1269 /// - [`Some(t)`] if `predicate` returns `true` (where `t` is the wrapped
1271 /// - [`None`] if `predicate` returns `false`.
1273 /// This function works similar to [`Iterator::filter()`]. You can imagine
1274 /// the `Option<T>` being an iterator over one or zero elements. `filter()`
1275 /// lets you decide which elements to keep.
1280 /// fn is_even(n: &i32) -> bool {
1284 /// assert_eq!(None.filter(is_even), None);
1285 /// assert_eq!(Some(3).filter(is_even), None);
1286 /// assert_eq!(Some(4).filter(is_even), Some(4));
1289 /// [`Some(t)`]: Some
1291 #[stable(feature = "option_filter", since = "1.27.0")]
1292 #[rustc_const_unstable(feature = "const_option_ext", issue = "91930")]
1293 pub const fn filter<P>(self, predicate: P) -> Self
1296 P: ~const FnOnce(&T) -> bool,
1299 if let Some(x) = self {
1307 /// Returns the option if it contains a value, otherwise returns `optb`.
1309 /// Arguments passed to `or` are eagerly evaluated; if you are passing the
1310 /// result of a function call, it is recommended to use [`or_else`], which is
1311 /// lazily evaluated.
1313 /// [`or_else`]: Option::or_else
1318 /// let x = Some(2);
1320 /// assert_eq!(x.or(y), Some(2));
1323 /// let y = Some(100);
1324 /// assert_eq!(x.or(y), Some(100));
1326 /// let x = Some(2);
1327 /// let y = Some(100);
1328 /// assert_eq!(x.or(y), Some(2));
1330 /// let x: Option<u32> = None;
1332 /// assert_eq!(x.or(y), None);
1335 #[stable(feature = "rust1", since = "1.0.0")]
1336 #[rustc_const_unstable(feature = "const_option_ext", issue = "91930")]
1337 pub const fn or(self, optb: Option<T>) -> Option<T>
1347 /// Returns the option if it contains a value, otherwise calls `f` and
1348 /// returns the result.
1353 /// fn nobody() -> Option<&'static str> { None }
1354 /// fn vikings() -> Option<&'static str> { Some("vikings") }
1356 /// assert_eq!(Some("barbarians").or_else(vikings), Some("barbarians"));
1357 /// assert_eq!(None.or_else(vikings), Some("vikings"));
1358 /// assert_eq!(None.or_else(nobody), None);
1361 #[stable(feature = "rust1", since = "1.0.0")]
1362 #[rustc_const_unstable(feature = "const_option_ext", issue = "91930")]
1363 pub const fn or_else<F>(self, f: F) -> Option<T>
1365 F: ~const FnOnce() -> Option<T>,
1374 /// Returns [`Some`] if exactly one of `self`, `optb` is [`Some`], otherwise returns [`None`].
1379 /// let x = Some(2);
1380 /// let y: Option<u32> = None;
1381 /// assert_eq!(x.xor(y), Some(2));
1383 /// let x: Option<u32> = None;
1384 /// let y = Some(2);
1385 /// assert_eq!(x.xor(y), Some(2));
1387 /// let x = Some(2);
1388 /// let y = Some(2);
1389 /// assert_eq!(x.xor(y), None);
1391 /// let x: Option<u32> = None;
1392 /// let y: Option<u32> = None;
1393 /// assert_eq!(x.xor(y), None);
1396 #[stable(feature = "option_xor", since = "1.37.0")]
1397 #[rustc_const_unstable(feature = "const_option_ext", issue = "91930")]
1398 pub const fn xor(self, optb: Option<T>) -> Option<T>
1402 match (self, optb) {
1403 (Some(a), None) => Some(a),
1404 (None, Some(b)) => Some(b),
1409 /////////////////////////////////////////////////////////////////////////
1410 // Entry-like operations to insert a value and return a reference
1411 /////////////////////////////////////////////////////////////////////////
1413 /// Inserts `value` into the option, then returns a mutable reference to it.
1415 /// If the option already contains a value, the old value is dropped.
1417 /// See also [`Option::get_or_insert`], which doesn't update the value if
1418 /// the option already contains [`Some`].
1423 /// let mut opt = None;
1424 /// let val = opt.insert(1);
1425 /// assert_eq!(*val, 1);
1426 /// assert_eq!(opt.unwrap(), 1);
1427 /// let val = opt.insert(2);
1428 /// assert_eq!(*val, 2);
1430 /// assert_eq!(opt.unwrap(), 3);
1432 #[must_use = "if you intended to set a value, consider assignment instead"]
1434 #[stable(feature = "option_insert", since = "1.53.0")]
1435 #[rustc_const_unstable(feature = "const_option_ext", issue = "91930")]
1436 pub const fn insert(&mut self, value: T) -> &mut T
1440 *self = Some(value);
1442 // SAFETY: the code above just filled the option
1443 unsafe { self.as_mut().unwrap_unchecked() }
1446 /// Inserts `value` into the option if it is [`None`], then
1447 /// returns a mutable reference to the contained value.
1449 /// See also [`Option::insert`], which updates the value even if
1450 /// the option already contains [`Some`].
1455 /// let mut x = None;
1458 /// let y: &mut u32 = x.get_or_insert(5);
1459 /// assert_eq!(y, &5);
1464 /// assert_eq!(x, Some(7));
1467 #[stable(feature = "option_entry", since = "1.20.0")]
1468 #[rustc_const_unstable(feature = "const_option_ext", issue = "91930")]
1469 pub const fn get_or_insert(&mut self, value: T) -> &mut T
1473 if let None = *self {
1474 *self = Some(value);
1477 // SAFETY: a `None` variant for `self` would have been replaced by a `Some`
1478 // variant in the code above.
1479 unsafe { self.as_mut().unwrap_unchecked() }
1482 /// Inserts the default value into the option if it is [`None`], then
1483 /// returns a mutable reference to the contained value.
1488 /// #![feature(option_get_or_insert_default)]
1490 /// let mut x = None;
1493 /// let y: &mut u32 = x.get_or_insert_default();
1494 /// assert_eq!(y, &0);
1499 /// assert_eq!(x, Some(7));
1502 #[unstable(feature = "option_get_or_insert_default", issue = "82901")]
1503 #[rustc_const_unstable(feature = "const_option_ext", issue = "91930")]
1504 pub const fn get_or_insert_default(&mut self) -> &mut T
1508 const fn default<T: ~const Default>() -> T {
1512 self.get_or_insert_with(default)
1515 /// Inserts a value computed from `f` into the option if it is [`None`],
1516 /// then returns a mutable reference to the contained value.
1521 /// let mut x = None;
1524 /// let y: &mut u32 = x.get_or_insert_with(|| 5);
1525 /// assert_eq!(y, &5);
1530 /// assert_eq!(x, Some(7));
1533 #[stable(feature = "option_entry", since = "1.20.0")]
1534 #[rustc_const_unstable(feature = "const_option_ext", issue = "91930")]
1535 pub const fn get_or_insert_with<F>(&mut self, f: F) -> &mut T
1537 F: ~const FnOnce() -> T,
1540 if let None = *self {
1541 // the compiler isn't smart enough to know that we are not dropping a `T`
1542 // here and wants us to ensure `T` can be dropped at compile time.
1543 mem::forget(mem::replace(self, Some(f())))
1546 // SAFETY: a `None` variant for `self` would have been replaced by a `Some`
1547 // variant in the code above.
1548 unsafe { self.as_mut().unwrap_unchecked() }
1551 /////////////////////////////////////////////////////////////////////////
1553 /////////////////////////////////////////////////////////////////////////
1555 /// Takes the value out of the option, leaving a [`None`] in its place.
1560 /// let mut x = Some(2);
1561 /// let y = x.take();
1562 /// assert_eq!(x, None);
1563 /// assert_eq!(y, Some(2));
1565 /// let mut x: Option<u32> = None;
1566 /// let y = x.take();
1567 /// assert_eq!(x, None);
1568 /// assert_eq!(y, None);
1571 #[stable(feature = "rust1", since = "1.0.0")]
1572 #[rustc_const_unstable(feature = "const_option", issue = "67441")]
1573 pub const fn take(&mut self) -> Option<T> {
1574 // FIXME replace `mem::replace` by `mem::take` when the latter is const ready
1575 mem::replace(self, None)
1578 /// Replaces the actual value in the option by the value given in parameter,
1579 /// returning the old value if present,
1580 /// leaving a [`Some`] in its place without deinitializing either one.
1585 /// let mut x = Some(2);
1586 /// let old = x.replace(5);
1587 /// assert_eq!(x, Some(5));
1588 /// assert_eq!(old, Some(2));
1590 /// let mut x = None;
1591 /// let old = x.replace(3);
1592 /// assert_eq!(x, Some(3));
1593 /// assert_eq!(old, None);
1596 #[rustc_const_unstable(feature = "const_option", issue = "67441")]
1597 #[stable(feature = "option_replace", since = "1.31.0")]
1598 pub const fn replace(&mut self, value: T) -> Option<T> {
1599 mem::replace(self, Some(value))
1602 /// Returns `true` if the option is a [`Some`] value containing the given value.
1607 /// #![feature(option_result_contains)]
1609 /// let x: Option<u32> = Some(2);
1610 /// assert_eq!(x.contains(&2), true);
1612 /// let x: Option<u32> = Some(3);
1613 /// assert_eq!(x.contains(&2), false);
1615 /// let x: Option<u32> = None;
1616 /// assert_eq!(x.contains(&2), false);
1620 #[unstable(feature = "option_result_contains", issue = "62358")]
1621 #[rustc_const_unstable(feature = "const_option_ext", issue = "91930")]
1622 pub const fn contains<U>(&self, x: &U) -> bool
1624 U: ~const PartialEq<T>,
1632 /// Zips `self` with another `Option`.
1634 /// If `self` is `Some(s)` and `other` is `Some(o)`, this method returns `Some((s, o))`.
1635 /// Otherwise, `None` is returned.
1640 /// let x = Some(1);
1641 /// let y = Some("hi");
1642 /// let z = None::<u8>;
1644 /// assert_eq!(x.zip(y), Some((1, "hi")));
1645 /// assert_eq!(x.zip(z), None);
1647 #[stable(feature = "option_zip_option", since = "1.46.0")]
1648 #[rustc_const_unstable(feature = "const_option_ext", issue = "91930")]
1649 pub const fn zip<U>(self, other: Option<U>) -> Option<(T, U)>
1654 match (self, other) {
1655 (Some(a), Some(b)) => Some((a, b)),
1660 /// Zips `self` and another `Option` with function `f`.
1662 /// If `self` is `Some(s)` and `other` is `Some(o)`, this method returns `Some(f(s, o))`.
1663 /// Otherwise, `None` is returned.
1668 /// #![feature(option_zip)]
1670 /// #[derive(Debug, PartialEq)]
1677 /// fn new(x: f64, y: f64) -> Self {
1682 /// let x = Some(17.5);
1683 /// let y = Some(42.7);
1685 /// assert_eq!(x.zip_with(y, Point::new), Some(Point { x: 17.5, y: 42.7 }));
1686 /// assert_eq!(x.zip_with(None, Point::new), None);
1688 #[unstable(feature = "option_zip", issue = "70086")]
1689 #[rustc_const_unstable(feature = "const_option_ext", issue = "91930")]
1690 pub const fn zip_with<U, F, R>(self, other: Option<U>, f: F) -> Option<R>
1692 F: ~const FnOnce(T, U) -> R,
1697 match (self, other) {
1698 (Some(a), Some(b)) => Some(f(a, b)),
1704 impl<T, U> Option<(T, U)> {
1705 /// Unzips an option containing a tuple of two options.
1707 /// If `self` is `Some((a, b))` this method returns `(Some(a), Some(b))`.
1708 /// Otherwise, `(None, None)` is returned.
1713 /// #![feature(unzip_option)]
1715 /// let x = Some((1, "hi"));
1716 /// let y = None::<(u8, u32)>;
1718 /// assert_eq!(x.unzip(), (Some(1), Some("hi")));
1719 /// assert_eq!(y.unzip(), (None, None));
1722 #[unstable(feature = "unzip_option", issue = "87800", reason = "recently added")]
1723 pub const fn unzip(self) -> (Option<T>, Option<U>) {
1725 Some((a, b)) => (Some(a), Some(b)),
1726 None => (None, None),
1731 impl<T> Option<&T> {
1732 /// Maps an `Option<&T>` to an `Option<T>` by copying the contents of the
1739 /// let opt_x = Some(&x);
1740 /// assert_eq!(opt_x, Some(&12));
1741 /// let copied = opt_x.copied();
1742 /// assert_eq!(copied, Some(12));
1744 #[must_use = "`self` will be dropped if the result is not used"]
1745 #[stable(feature = "copied", since = "1.35.0")]
1746 #[rustc_const_unstable(feature = "const_option", issue = "67441")]
1747 pub const fn copied(self) -> Option<T>
1751 // FIXME: this implementation, which sidesteps using `Option::map` since it's not const
1752 // ready yet, should be reverted when possible to avoid code repetition
1754 Some(&v) => Some(v),
1759 /// Maps an `Option<&T>` to an `Option<T>` by cloning the contents of the
1766 /// let opt_x = Some(&x);
1767 /// assert_eq!(opt_x, Some(&12));
1768 /// let cloned = opt_x.cloned();
1769 /// assert_eq!(cloned, Some(12));
1771 #[must_use = "`self` will be dropped if the result is not used"]
1772 #[stable(feature = "rust1", since = "1.0.0")]
1773 #[rustc_const_unstable(feature = "const_option_cloned", issue = "91582")]
1774 pub const fn cloned(self) -> Option<T>
1779 Some(t) => Some(t.clone()),
1785 impl<T> Option<&mut T> {
1786 /// Maps an `Option<&mut T>` to an `Option<T>` by copying the contents of the
1793 /// let opt_x = Some(&mut x);
1794 /// assert_eq!(opt_x, Some(&mut 12));
1795 /// let copied = opt_x.copied();
1796 /// assert_eq!(copied, Some(12));
1798 #[must_use = "`self` will be dropped if the result is not used"]
1799 #[stable(feature = "copied", since = "1.35.0")]
1800 #[rustc_const_unstable(feature = "const_option_ext", issue = "91930")]
1801 pub const fn copied(self) -> Option<T>
1806 Some(&mut t) => Some(t),
1811 /// Maps an `Option<&mut T>` to an `Option<T>` by cloning the contents of the
1818 /// let opt_x = Some(&mut x);
1819 /// assert_eq!(opt_x, Some(&mut 12));
1820 /// let cloned = opt_x.cloned();
1821 /// assert_eq!(cloned, Some(12));
1823 #[must_use = "`self` will be dropped if the result is not used"]
1824 #[stable(since = "1.26.0", feature = "option_ref_mut_cloned")]
1825 #[rustc_const_unstable(feature = "const_option_cloned", issue = "91582")]
1826 pub const fn cloned(self) -> Option<T>
1831 Some(t) => Some(t.clone()),
1837 impl<T, E> Option<Result<T, E>> {
1838 /// Transposes an `Option` of a [`Result`] into a [`Result`] of an `Option`.
1840 /// [`None`] will be mapped to <code>[Ok]\([None])</code>.
1841 /// <code>[Some]\([Ok]\(\_))</code> and <code>[Some]\([Err]\(\_))</code> will be mapped to
1842 /// <code>[Ok]\([Some]\(\_))</code> and <code>[Err]\(\_)</code>.
1847 /// #[derive(Debug, Eq, PartialEq)]
1850 /// let x: Result<Option<i32>, SomeErr> = Ok(Some(5));
1851 /// let y: Option<Result<i32, SomeErr>> = Some(Ok(5));
1852 /// assert_eq!(x, y.transpose());
1855 #[stable(feature = "transpose_result", since = "1.33.0")]
1856 #[rustc_const_unstable(feature = "const_option", issue = "67441")]
1857 pub const fn transpose(self) -> Result<Option<T>, E> {
1859 Some(Ok(x)) => Ok(Some(x)),
1860 Some(Err(e)) => Err(e),
1866 // This is a separate function to reduce the code size of .expect() itself.
1867 #[cfg_attr(not(feature = "panic_immediate_abort"), inline(never))]
1868 #[cfg_attr(feature = "panic_immediate_abort", inline)]
1871 #[rustc_const_unstable(feature = "const_option", issue = "67441")]
1872 const fn expect_failed(msg: &str) -> ! {
1876 /////////////////////////////////////////////////////////////////////////////
1877 // Trait implementations
1878 /////////////////////////////////////////////////////////////////////////////
1880 #[stable(feature = "rust1", since = "1.0.0")]
1881 #[rustc_const_unstable(feature = "const_clone", issue = "91805")]
1882 impl<T> const Clone for Option<T>
1884 T: ~const Clone + ~const Destruct,
1887 fn clone(&self) -> Self {
1889 Some(x) => Some(x.clone()),
1895 fn clone_from(&mut self, source: &Self) {
1896 match (self, source) {
1897 (Some(to), Some(from)) => to.clone_from(from),
1898 (to, from) => *to = from.clone(),
1903 #[stable(feature = "rust1", since = "1.0.0")]
1904 #[rustc_const_unstable(feature = "const_default_impls", issue = "87864")]
1905 impl<T> const Default for Option<T> {
1906 /// Returns [`None`][Option::None].
1911 /// let opt: Option<u32> = Option::default();
1912 /// assert!(opt.is_none());
1915 fn default() -> Option<T> {
1920 #[stable(feature = "rust1", since = "1.0.0")]
1921 impl<T> IntoIterator for Option<T> {
1923 type IntoIter = IntoIter<T>;
1925 /// Returns a consuming iterator over the possibly contained value.
1930 /// let x = Some("string");
1931 /// let v: Vec<&str> = x.into_iter().collect();
1932 /// assert_eq!(v, ["string"]);
1935 /// let v: Vec<&str> = x.into_iter().collect();
1936 /// assert!(v.is_empty());
1939 fn into_iter(self) -> IntoIter<T> {
1940 IntoIter { inner: Item { opt: self } }
1944 #[stable(since = "1.4.0", feature = "option_iter")]
1945 impl<'a, T> IntoIterator for &'a Option<T> {
1947 type IntoIter = Iter<'a, T>;
1949 fn into_iter(self) -> Iter<'a, T> {
1954 #[stable(since = "1.4.0", feature = "option_iter")]
1955 impl<'a, T> IntoIterator for &'a mut Option<T> {
1956 type Item = &'a mut T;
1957 type IntoIter = IterMut<'a, T>;
1959 fn into_iter(self) -> IterMut<'a, T> {
1964 #[stable(since = "1.12.0", feature = "option_from")]
1965 #[rustc_const_unstable(feature = "const_convert", issue = "88674")]
1966 impl<T> const From<T> for Option<T> {
1967 /// Moves `val` into a new [`Some`].
1972 /// let o: Option<u8> = Option::from(67);
1974 /// assert_eq!(Some(67), o);
1976 fn from(val: T) -> Option<T> {
1981 #[stable(feature = "option_ref_from_ref_option", since = "1.30.0")]
1982 #[rustc_const_unstable(feature = "const_convert", issue = "88674")]
1983 impl<'a, T> const From<&'a Option<T>> for Option<&'a T> {
1984 /// Converts from `&Option<T>` to `Option<&T>`.
1988 /// Converts an <code>[Option]<[String]></code> into an <code>[Option]<[usize]></code>, preserving
1989 /// the original. The [`map`] method takes the `self` argument by value, consuming the original,
1990 /// so this technique uses `from` to first take an [`Option`] to a reference
1991 /// to the value inside the original.
1993 /// [`map`]: Option::map
1994 /// [String]: ../../std/string/struct.String.html "String"
1997 /// let s: Option<String> = Some(String::from("Hello, Rustaceans!"));
1998 /// let o: Option<usize> = Option::from(&s).map(|ss: &String| ss.len());
2000 /// println!("Can still print s: {s:?}");
2002 /// assert_eq!(o, Some(18));
2004 fn from(o: &'a Option<T>) -> Option<&'a T> {
2009 #[stable(feature = "option_ref_from_ref_option", since = "1.30.0")]
2010 #[rustc_const_unstable(feature = "const_convert", issue = "88674")]
2011 impl<'a, T> const From<&'a mut Option<T>> for Option<&'a mut T> {
2012 /// Converts from `&mut Option<T>` to `Option<&mut T>`
2017 /// let mut s = Some(String::from("Hello"));
2018 /// let o: Option<&mut String> = Option::from(&mut s);
2021 /// Some(t) => *t = String::from("Hello, Rustaceans!"),
2025 /// assert_eq!(s, Some(String::from("Hello, Rustaceans!")));
2027 fn from(o: &'a mut Option<T>) -> Option<&'a mut T> {
2032 /////////////////////////////////////////////////////////////////////////////
2033 // The Option Iterators
2034 /////////////////////////////////////////////////////////////////////////////
2036 #[derive(Clone, Debug)]
2041 impl<A> Iterator for Item<A> {
2045 fn next(&mut self) -> Option<A> {
2050 fn size_hint(&self) -> (usize, Option<usize>) {
2052 Some(_) => (1, Some(1)),
2053 None => (0, Some(0)),
2058 impl<A> DoubleEndedIterator for Item<A> {
2060 fn next_back(&mut self) -> Option<A> {
2065 impl<A> ExactSizeIterator for Item<A> {}
2066 impl<A> FusedIterator for Item<A> {}
2067 unsafe impl<A> TrustedLen for Item<A> {}
2069 /// An iterator over a reference to the [`Some`] variant of an [`Option`].
2071 /// The iterator yields one value if the [`Option`] is a [`Some`], otherwise none.
2073 /// This `struct` is created by the [`Option::iter`] function.
2074 #[stable(feature = "rust1", since = "1.0.0")]
2076 pub struct Iter<'a, A: 'a> {
2080 #[stable(feature = "rust1", since = "1.0.0")]
2081 impl<'a, A> Iterator for Iter<'a, A> {
2085 fn next(&mut self) -> Option<&'a A> {
2089 fn size_hint(&self) -> (usize, Option<usize>) {
2090 self.inner.size_hint()
2094 #[stable(feature = "rust1", since = "1.0.0")]
2095 impl<'a, A> DoubleEndedIterator for Iter<'a, A> {
2097 fn next_back(&mut self) -> Option<&'a A> {
2098 self.inner.next_back()
2102 #[stable(feature = "rust1", since = "1.0.0")]
2103 impl<A> ExactSizeIterator for Iter<'_, A> {}
2105 #[stable(feature = "fused", since = "1.26.0")]
2106 impl<A> FusedIterator for Iter<'_, A> {}
2108 #[unstable(feature = "trusted_len", issue = "37572")]
2109 unsafe impl<A> TrustedLen for Iter<'_, A> {}
2111 #[stable(feature = "rust1", since = "1.0.0")]
2112 impl<A> Clone for Iter<'_, A> {
2114 fn clone(&self) -> Self {
2115 Iter { inner: self.inner.clone() }
2119 /// An iterator over a mutable reference to the [`Some`] variant of an [`Option`].
2121 /// The iterator yields one value if the [`Option`] is a [`Some`], otherwise none.
2123 /// This `struct` is created by the [`Option::iter_mut`] function.
2124 #[stable(feature = "rust1", since = "1.0.0")]
2126 pub struct IterMut<'a, A: 'a> {
2127 inner: Item<&'a mut A>,
2130 #[stable(feature = "rust1", since = "1.0.0")]
2131 impl<'a, A> Iterator for IterMut<'a, A> {
2132 type Item = &'a mut A;
2135 fn next(&mut self) -> Option<&'a mut A> {
2139 fn size_hint(&self) -> (usize, Option<usize>) {
2140 self.inner.size_hint()
2144 #[stable(feature = "rust1", since = "1.0.0")]
2145 impl<'a, A> DoubleEndedIterator for IterMut<'a, A> {
2147 fn next_back(&mut self) -> Option<&'a mut A> {
2148 self.inner.next_back()
2152 #[stable(feature = "rust1", since = "1.0.0")]
2153 impl<A> ExactSizeIterator for IterMut<'_, A> {}
2155 #[stable(feature = "fused", since = "1.26.0")]
2156 impl<A> FusedIterator for IterMut<'_, A> {}
2157 #[unstable(feature = "trusted_len", issue = "37572")]
2158 unsafe impl<A> TrustedLen for IterMut<'_, A> {}
2160 /// An iterator over the value in [`Some`] variant of an [`Option`].
2162 /// The iterator yields one value if the [`Option`] is a [`Some`], otherwise none.
2164 /// This `struct` is created by the [`Option::into_iter`] function.
2165 #[derive(Clone, Debug)]
2166 #[stable(feature = "rust1", since = "1.0.0")]
2167 pub struct IntoIter<A> {
2171 #[stable(feature = "rust1", since = "1.0.0")]
2172 impl<A> Iterator for IntoIter<A> {
2176 fn next(&mut self) -> Option<A> {
2180 fn size_hint(&self) -> (usize, Option<usize>) {
2181 self.inner.size_hint()
2185 #[stable(feature = "rust1", since = "1.0.0")]
2186 impl<A> DoubleEndedIterator for IntoIter<A> {
2188 fn next_back(&mut self) -> Option<A> {
2189 self.inner.next_back()
2193 #[stable(feature = "rust1", since = "1.0.0")]
2194 impl<A> ExactSizeIterator for IntoIter<A> {}
2196 #[stable(feature = "fused", since = "1.26.0")]
2197 impl<A> FusedIterator for IntoIter<A> {}
2199 #[unstable(feature = "trusted_len", issue = "37572")]
2200 unsafe impl<A> TrustedLen for IntoIter<A> {}
2202 /////////////////////////////////////////////////////////////////////////////
2204 /////////////////////////////////////////////////////////////////////////////
2206 #[stable(feature = "rust1", since = "1.0.0")]
2207 impl<A, V: FromIterator<A>> FromIterator<Option<A>> for Option<V> {
2208 /// Takes each element in the [`Iterator`]: if it is [`None`][Option::None],
2209 /// no further elements are taken, and the [`None`][Option::None] is
2210 /// returned. Should no [`None`][Option::None] occur, a container of type
2211 /// `V` containing the values of each [`Option`] is returned.
2215 /// Here is an example which increments every integer in a vector.
2216 /// We use the checked variant of `add` that returns `None` when the
2217 /// calculation would result in an overflow.
2220 /// let items = vec![0_u16, 1, 2];
2222 /// let res: Option<Vec<u16>> = items
2224 /// .map(|x| x.checked_add(1))
2227 /// assert_eq!(res, Some(vec![1, 2, 3]));
2230 /// As you can see, this will return the expected, valid items.
2232 /// Here is another example that tries to subtract one from another list
2233 /// of integers, this time checking for underflow:
2236 /// let items = vec![2_u16, 1, 0];
2238 /// let res: Option<Vec<u16>> = items
2240 /// .map(|x| x.checked_sub(1))
2243 /// assert_eq!(res, None);
2246 /// Since the last element is zero, it would underflow. Thus, the resulting
2247 /// value is `None`.
2249 /// Here is a variation on the previous example, showing that no
2250 /// further elements are taken from `iter` after the first `None`.
2253 /// let items = vec![3_u16, 2, 1, 10];
2255 /// let mut shared = 0;
2257 /// let res: Option<Vec<u16>> = items
2259 /// .map(|x| { shared += x; x.checked_sub(2) })
2262 /// assert_eq!(res, None);
2263 /// assert_eq!(shared, 6);
2266 /// Since the third element caused an underflow, no further elements were taken,
2267 /// so the final value of `shared` is 6 (= `3 + 2 + 1`), not 16.
2269 fn from_iter<I: IntoIterator<Item = Option<A>>>(iter: I) -> Option<V> {
2270 // FIXME(#11084): This could be replaced with Iterator::scan when this
2271 // performance bug is closed.
2273 iter::try_process(iter.into_iter(), |i| i.collect())
2277 #[unstable(feature = "try_trait_v2", issue = "84277")]
2278 #[rustc_const_unstable(feature = "const_convert", issue = "88674")]
2279 impl<T> const ops::Try for Option<T> {
2281 type Residual = Option<convert::Infallible>;
2284 fn from_output(output: Self::Output) -> Self {
2289 fn branch(self) -> ControlFlow<Self::Residual, Self::Output> {
2291 Some(v) => ControlFlow::Continue(v),
2292 None => ControlFlow::Break(None),
2297 #[unstable(feature = "try_trait_v2", issue = "84277")]
2298 #[rustc_const_unstable(feature = "const_convert", issue = "88674")]
2299 impl<T> const ops::FromResidual for Option<T> {
2301 fn from_residual(residual: Option<convert::Infallible>) -> Self {
2308 #[unstable(feature = "try_trait_v2_yeet", issue = "96374")]
2309 impl<T> ops::FromResidual<ops::Yeet<()>> for Option<T> {
2311 fn from_residual(ops::Yeet(()): ops::Yeet<()>) -> Self {
2316 #[unstable(feature = "try_trait_v2_residual", issue = "91285")]
2317 #[rustc_const_unstable(feature = "const_try", issue = "74935")]
2318 impl<T> const ops::Residual<T> for Option<convert::Infallible> {
2319 type TryType = Option<T>;
2322 impl<T> Option<Option<T>> {
2323 /// Converts from `Option<Option<T>>` to `Option<T>`.
2330 /// let x: Option<Option<u32>> = Some(Some(6));
2331 /// assert_eq!(Some(6), x.flatten());
2333 /// let x: Option<Option<u32>> = Some(None);
2334 /// assert_eq!(None, x.flatten());
2336 /// let x: Option<Option<u32>> = None;
2337 /// assert_eq!(None, x.flatten());
2340 /// Flattening only removes one level of nesting at a time:
2343 /// let x: Option<Option<Option<u32>>> = Some(Some(Some(6)));
2344 /// assert_eq!(Some(Some(6)), x.flatten());
2345 /// assert_eq!(Some(6), x.flatten().flatten());
2348 #[stable(feature = "option_flattening", since = "1.40.0")]
2349 #[rustc_const_unstable(feature = "const_option", issue = "67441")]
2350 pub const fn flatten(self) -> Option<T> {
2352 Some(inner) => inner,