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
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
409 //! [impl-Sum]: Option#impl-Sum%3COption%3CU%3E%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::panicking::{panic, panic_str};
510 ops::{self, ControlFlow, Deref, DerefMut},
513 /// The `Option` type. See [the module level documentation](self) for more.
514 #[derive(Copy, PartialEq, PartialOrd, Eq, Ord, Debug, Hash)]
515 #[rustc_diagnostic_item = "Option"]
516 #[stable(feature = "rust1", since = "1.0.0")]
520 #[stable(feature = "rust1", since = "1.0.0")]
522 /// Some value of type `T`.
524 #[stable(feature = "rust1", since = "1.0.0")]
525 Some(#[stable(feature = "rust1", since = "1.0.0")] T),
528 /////////////////////////////////////////////////////////////////////////////
529 // Type implementation
530 /////////////////////////////////////////////////////////////////////////////
533 /////////////////////////////////////////////////////////////////////////
534 // Querying the contained values
535 /////////////////////////////////////////////////////////////////////////
537 /// Returns `true` if the option is a [`Some`] value.
542 /// let x: Option<u32> = Some(2);
543 /// assert_eq!(x.is_some(), true);
545 /// let x: Option<u32> = None;
546 /// assert_eq!(x.is_some(), false);
548 #[must_use = "if you intended to assert that this has a value, consider `.unwrap()` instead"]
550 #[stable(feature = "rust1", since = "1.0.0")]
551 #[rustc_const_stable(feature = "const_option", since = "1.48.0")]
552 pub const fn is_some(&self) -> bool {
553 matches!(*self, Some(_))
556 /// Returns `true` if the option is a [`Some`] and the value inside of it matches a predicate.
561 /// #![feature(is_some_with)]
563 /// let x: Option<u32> = Some(2);
564 /// assert_eq!(x.is_some_and(|&x| x > 1), true);
566 /// let x: Option<u32> = Some(0);
567 /// assert_eq!(x.is_some_and(|&x| x > 1), false);
569 /// let x: Option<u32> = None;
570 /// assert_eq!(x.is_some_and(|&x| x > 1), false);
574 #[unstable(feature = "is_some_with", issue = "93050")]
575 pub fn is_some_and(&self, f: impl FnOnce(&T) -> bool) -> bool {
576 matches!(self, Some(x) if f(x))
579 /// Returns `true` if the option is a [`None`] value.
584 /// let x: Option<u32> = Some(2);
585 /// assert_eq!(x.is_none(), false);
587 /// let x: Option<u32> = None;
588 /// assert_eq!(x.is_none(), true);
590 #[must_use = "if you intended to assert that this doesn't have a value, consider \
591 `.and_then(|_| panic!(\"`Option` had a value when expected `None`\"))` instead"]
593 #[stable(feature = "rust1", since = "1.0.0")]
594 #[rustc_const_stable(feature = "const_option", since = "1.48.0")]
595 pub const fn is_none(&self) -> bool {
599 /////////////////////////////////////////////////////////////////////////
600 // Adapter for working with references
601 /////////////////////////////////////////////////////////////////////////
603 /// Converts from `&Option<T>` to `Option<&T>`.
607 /// Converts an <code>Option<[String]></code> into an <code>Option<[usize]></code>, preserving
608 /// the original. The [`map`] method takes the `self` argument by value, consuming the original,
609 /// so this technique uses `as_ref` to first take an `Option` to a reference
610 /// to the value inside the original.
612 /// [`map`]: Option::map
613 /// [String]: ../../std/string/struct.String.html "String"
616 /// let text: Option<String> = Some("Hello, world!".to_string());
617 /// // First, cast `Option<String>` to `Option<&String>` with `as_ref`,
618 /// // then consume *that* with `map`, leaving `text` on the stack.
619 /// let text_length: Option<usize> = text.as_ref().map(|s| s.len());
620 /// println!("still can print text: {text:?}");
623 #[rustc_const_stable(feature = "const_option", since = "1.48.0")]
624 #[stable(feature = "rust1", since = "1.0.0")]
625 pub const fn as_ref(&self) -> Option<&T> {
627 Some(ref x) => Some(x),
632 /// Converts from `&mut Option<T>` to `Option<&mut T>`.
637 /// let mut x = Some(2);
638 /// match x.as_mut() {
639 /// Some(v) => *v = 42,
642 /// assert_eq!(x, Some(42));
645 #[stable(feature = "rust1", since = "1.0.0")]
646 #[rustc_const_unstable(feature = "const_option", issue = "67441")]
647 pub const fn as_mut(&mut self) -> Option<&mut T> {
649 Some(ref mut x) => Some(x),
654 /// Converts from <code>[Pin]<[&]Option\<T>></code> to <code>Option<[Pin]<[&]T>></code>.
656 /// [&]: reference "shared reference"
659 #[stable(feature = "pin", since = "1.33.0")]
660 #[rustc_const_unstable(feature = "const_option_ext", issue = "91930")]
661 pub const fn as_pin_ref(self: Pin<&Self>) -> Option<Pin<&T>> {
662 match Pin::get_ref(self).as_ref() {
663 // SAFETY: `x` is guaranteed to be pinned because it comes from `self`
665 Some(x) => unsafe { Some(Pin::new_unchecked(x)) },
670 /// Converts from <code>[Pin]<[&mut] Option\<T>></code> to <code>Option<[Pin]<[&mut] T>></code>.
672 /// [&mut]: reference "mutable reference"
675 #[stable(feature = "pin", since = "1.33.0")]
676 #[rustc_const_unstable(feature = "const_option_ext", issue = "91930")]
677 pub const fn as_pin_mut(self: Pin<&mut Self>) -> Option<Pin<&mut T>> {
678 // SAFETY: `get_unchecked_mut` is never used to move the `Option` inside `self`.
679 // `x` is guaranteed to be pinned because it comes from `self` which is pinned.
681 match Pin::get_unchecked_mut(self).as_mut() {
682 Some(x) => Some(Pin::new_unchecked(x)),
688 /////////////////////////////////////////////////////////////////////////
689 // Getting to contained values
690 /////////////////////////////////////////////////////////////////////////
692 /// Returns the contained [`Some`] value, consuming the `self` value.
696 /// Panics if the value is a [`None`] with a custom panic message provided by
702 /// let x = Some("value");
703 /// assert_eq!(x.expect("fruits are healthy"), "value");
707 /// let x: Option<&str> = None;
708 /// x.expect("fruits are healthy"); // panics with `fruits are healthy`
712 #[stable(feature = "rust1", since = "1.0.0")]
713 #[rustc_const_unstable(feature = "const_option", issue = "67441")]
714 pub const fn expect(self, msg: &str) -> T {
717 None => expect_failed(msg),
721 /// Returns the contained [`Some`] value, consuming the `self` value.
723 /// Because this function may panic, its use is generally discouraged.
724 /// Instead, prefer to use pattern matching and handle the [`None`]
725 /// case explicitly, or call [`unwrap_or`], [`unwrap_or_else`], or
726 /// [`unwrap_or_default`].
728 /// [`unwrap_or`]: Option::unwrap_or
729 /// [`unwrap_or_else`]: Option::unwrap_or_else
730 /// [`unwrap_or_default`]: Option::unwrap_or_default
734 /// Panics if the self value equals [`None`].
739 /// let x = Some("air");
740 /// assert_eq!(x.unwrap(), "air");
744 /// let x: Option<&str> = None;
745 /// assert_eq!(x.unwrap(), "air"); // fails
749 #[stable(feature = "rust1", since = "1.0.0")]
750 #[rustc_const_unstable(feature = "const_option", issue = "67441")]
751 pub const fn unwrap(self) -> T {
754 None => panic("called `Option::unwrap()` on a `None` value"),
758 /// Returns the contained [`Some`] value or a provided default.
760 /// Arguments passed to `unwrap_or` are eagerly evaluated; if you are passing
761 /// the result of a function call, it is recommended to use [`unwrap_or_else`],
762 /// which is lazily evaluated.
764 /// [`unwrap_or_else`]: Option::unwrap_or_else
769 /// assert_eq!(Some("car").unwrap_or("bike"), "car");
770 /// assert_eq!(None.unwrap_or("bike"), "bike");
773 #[stable(feature = "rust1", since = "1.0.0")]
774 #[rustc_const_unstable(feature = "const_option_ext", issue = "91930")]
775 pub const fn unwrap_or(self, default: T) -> T
785 /// Returns the contained [`Some`] value or computes it from a closure.
791 /// assert_eq!(Some(4).unwrap_or_else(|| 2 * k), 4);
792 /// assert_eq!(None.unwrap_or_else(|| 2 * k), 20);
795 #[stable(feature = "rust1", since = "1.0.0")]
796 #[rustc_const_unstable(feature = "const_option_ext", issue = "91930")]
797 pub const fn unwrap_or_else<F>(self, f: F) -> T
799 F: ~const FnOnce() -> T,
808 /// Returns the contained [`Some`] value or a default.
810 /// Consumes the `self` argument then, if [`Some`], returns the contained
811 /// value, otherwise if [`None`], returns the [default value] for that
816 /// Converts a string to an integer, turning poorly-formed strings
817 /// into 0 (the default value for integers). [`parse`] converts
818 /// a string to any other type that implements [`FromStr`], returning
819 /// [`None`] on error.
822 /// let good_year_from_input = "1909";
823 /// let bad_year_from_input = "190blarg";
824 /// let good_year = good_year_from_input.parse().ok().unwrap_or_default();
825 /// let bad_year = bad_year_from_input.parse().ok().unwrap_or_default();
827 /// assert_eq!(1909, good_year);
828 /// assert_eq!(0, bad_year);
831 /// [default value]: Default::default
832 /// [`parse`]: str::parse
833 /// [`FromStr`]: crate::str::FromStr
835 #[stable(feature = "rust1", since = "1.0.0")]
836 #[rustc_const_unstable(feature = "const_option_ext", issue = "91930")]
837 pub const fn unwrap_or_default(self) -> T
843 None => Default::default(),
847 /// Returns the contained [`Some`] value, consuming the `self` value,
848 /// without checking that the value is not [`None`].
852 /// Calling this method on [`None`] is *[undefined behavior]*.
854 /// [undefined behavior]: https://doc.rust-lang.org/reference/behavior-considered-undefined.html
859 /// let x = Some("air");
860 /// assert_eq!(unsafe { x.unwrap_unchecked() }, "air");
864 /// let x: Option<&str> = None;
865 /// assert_eq!(unsafe { x.unwrap_unchecked() }, "air"); // Undefined behavior!
869 #[stable(feature = "option_result_unwrap_unchecked", since = "1.58.0")]
870 #[rustc_const_unstable(feature = "const_option_ext", issue = "91930")]
871 pub const unsafe fn unwrap_unchecked(self) -> T {
872 debug_assert!(self.is_some());
875 // SAFETY: the safety contract must be upheld by the caller.
876 None => unsafe { hint::unreachable_unchecked() },
880 /////////////////////////////////////////////////////////////////////////
881 // Transforming contained values
882 /////////////////////////////////////////////////////////////////////////
884 /// Maps an `Option<T>` to `Option<U>` by applying a function to a contained value.
888 /// Converts an <code>Option<[String]></code> into an <code>Option<[usize]></code>, consuming
891 /// [String]: ../../std/string/struct.String.html "String"
893 /// let maybe_some_string = Some(String::from("Hello, World!"));
894 /// // `Option::map` takes self *by value*, consuming `maybe_some_string`
895 /// let maybe_some_len = maybe_some_string.map(|s| s.len());
897 /// assert_eq!(maybe_some_len, Some(13));
900 #[stable(feature = "rust1", since = "1.0.0")]
901 #[rustc_const_unstable(feature = "const_option_ext", issue = "91930")]
902 pub const fn map<U, F>(self, f: F) -> Option<U>
904 F: ~const FnOnce(T) -> U,
908 Some(x) => Some(f(x)),
913 /// Calls the provided closure with a reference to the contained value (if [`Some`]).
918 /// #![feature(result_option_inspect)]
920 /// let v = vec![1, 2, 3, 4, 5];
922 /// // prints "got: 4"
923 /// let x: Option<&usize> = v.get(3).inspect(|x| println!("got: {x}"));
925 /// // prints nothing
926 /// let x: Option<&usize> = v.get(5).inspect(|x| println!("got: {x}"));
929 #[unstable(feature = "result_option_inspect", issue = "91345")]
930 #[rustc_const_unstable(feature = "const_option_ext", issue = "91930")]
931 pub const fn inspect<F>(self, f: F) -> Self
933 F: ~const FnOnce(&T),
936 if let Some(ref x) = self {
943 /// Returns the provided default result (if none),
944 /// or applies a function to the contained value (if any).
946 /// Arguments passed to `map_or` are eagerly evaluated; if you are passing
947 /// the result of a function call, it is recommended to use [`map_or_else`],
948 /// which is lazily evaluated.
950 /// [`map_or_else`]: Option::map_or_else
955 /// let x = Some("foo");
956 /// assert_eq!(x.map_or(42, |v| v.len()), 3);
958 /// let x: Option<&str> = None;
959 /// assert_eq!(x.map_or(42, |v| v.len()), 42);
962 #[stable(feature = "rust1", since = "1.0.0")]
963 #[rustc_const_unstable(feature = "const_option_ext", issue = "91930")]
964 pub const fn map_or<U, F>(self, default: U, f: F) -> U
966 F: ~const FnOnce(T) -> U,
976 /// Computes a default function result (if none), or
977 /// applies a different function to the contained value (if any).
984 /// let x = Some("foo");
985 /// assert_eq!(x.map_or_else(|| 2 * k, |v| v.len()), 3);
987 /// let x: Option<&str> = None;
988 /// assert_eq!(x.map_or_else(|| 2 * k, |v| v.len()), 42);
991 #[stable(feature = "rust1", since = "1.0.0")]
992 #[rustc_const_unstable(feature = "const_option_ext", issue = "91930")]
993 pub const fn map_or_else<U, D, F>(self, default: D, f: F) -> U
995 D: ~const FnOnce() -> U,
997 F: ~const FnOnce(T) -> U,
1006 /// Transforms the `Option<T>` into a [`Result<T, E>`], mapping [`Some(v)`] to
1007 /// [`Ok(v)`] and [`None`] to [`Err(err)`].
1009 /// Arguments passed to `ok_or` are eagerly evaluated; if you are passing the
1010 /// result of a function call, it is recommended to use [`ok_or_else`], which is
1011 /// lazily evaluated.
1014 /// [`Err(err)`]: Err
1015 /// [`Some(v)`]: Some
1016 /// [`ok_or_else`]: Option::ok_or_else
1021 /// let x = Some("foo");
1022 /// assert_eq!(x.ok_or(0), Ok("foo"));
1024 /// let x: Option<&str> = None;
1025 /// assert_eq!(x.ok_or(0), Err(0));
1028 #[stable(feature = "rust1", since = "1.0.0")]
1029 #[rustc_const_unstable(feature = "const_option_ext", issue = "91930")]
1030 pub const fn ok_or<E>(self, err: E) -> Result<T, E>
1040 /// Transforms the `Option<T>` into a [`Result<T, E>`], mapping [`Some(v)`] to
1041 /// [`Ok(v)`] and [`None`] to [`Err(err())`].
1044 /// [`Err(err())`]: Err
1045 /// [`Some(v)`]: Some
1050 /// let x = Some("foo");
1051 /// assert_eq!(x.ok_or_else(|| 0), Ok("foo"));
1053 /// let x: Option<&str> = None;
1054 /// assert_eq!(x.ok_or_else(|| 0), Err(0));
1057 #[stable(feature = "rust1", since = "1.0.0")]
1058 #[rustc_const_unstable(feature = "const_option_ext", issue = "91930")]
1059 pub const fn ok_or_else<E, F>(self, err: F) -> Result<T, E>
1061 F: ~const FnOnce() -> E,
1070 /// Converts from `Option<T>` (or `&Option<T>`) to `Option<&T::Target>`.
1072 /// Leaves the original Option in-place, creating a new one with a reference
1073 /// to the original one, additionally coercing the contents via [`Deref`].
1078 /// let x: Option<String> = Some("hey".to_owned());
1079 /// assert_eq!(x.as_deref(), Some("hey"));
1081 /// let x: Option<String> = None;
1082 /// assert_eq!(x.as_deref(), None);
1084 #[stable(feature = "option_deref", since = "1.40.0")]
1085 #[rustc_const_unstable(feature = "const_option_ext", issue = "91930")]
1086 pub const fn as_deref(&self) -> Option<&T::Target>
1090 match self.as_ref() {
1091 Some(t) => Some(t.deref()),
1096 /// Converts from `Option<T>` (or `&mut Option<T>`) to `Option<&mut T::Target>`.
1098 /// Leaves the original `Option` in-place, creating a new one containing a mutable reference to
1099 /// the inner type's [`Deref::Target`] type.
1104 /// let mut x: Option<String> = Some("hey".to_owned());
1105 /// assert_eq!(x.as_deref_mut().map(|x| {
1106 /// x.make_ascii_uppercase();
1108 /// }), Some("HEY".to_owned().as_mut_str()));
1110 #[stable(feature = "option_deref", since = "1.40.0")]
1111 #[rustc_const_unstable(feature = "const_option_ext", issue = "91930")]
1112 pub const fn as_deref_mut(&mut self) -> Option<&mut T::Target>
1116 match self.as_mut() {
1117 Some(t) => Some(t.deref_mut()),
1122 /////////////////////////////////////////////////////////////////////////
1123 // Iterator constructors
1124 /////////////////////////////////////////////////////////////////////////
1126 /// Returns an iterator over the possibly contained value.
1131 /// let x = Some(4);
1132 /// assert_eq!(x.iter().next(), Some(&4));
1134 /// let x: Option<u32> = None;
1135 /// assert_eq!(x.iter().next(), None);
1138 #[rustc_const_unstable(feature = "const_option", issue = "67441")]
1139 #[stable(feature = "rust1", since = "1.0.0")]
1140 pub const fn iter(&self) -> Iter<'_, T> {
1141 Iter { inner: Item { opt: self.as_ref() } }
1144 /// Returns a mutable iterator over the possibly contained value.
1149 /// let mut x = Some(4);
1150 /// match x.iter_mut().next() {
1151 /// Some(v) => *v = 42,
1154 /// assert_eq!(x, Some(42));
1156 /// let mut x: Option<u32> = None;
1157 /// assert_eq!(x.iter_mut().next(), None);
1160 #[stable(feature = "rust1", since = "1.0.0")]
1161 pub fn iter_mut(&mut self) -> IterMut<'_, T> {
1162 IterMut { inner: Item { opt: self.as_mut() } }
1165 /////////////////////////////////////////////////////////////////////////
1166 // Boolean operations on the values, eager and lazy
1167 /////////////////////////////////////////////////////////////////////////
1169 /// Returns [`None`] if the option is [`None`], otherwise returns `optb`.
1174 /// let x = Some(2);
1175 /// let y: Option<&str> = None;
1176 /// assert_eq!(x.and(y), None);
1178 /// let x: Option<u32> = None;
1179 /// let y = Some("foo");
1180 /// assert_eq!(x.and(y), None);
1182 /// let x = Some(2);
1183 /// let y = Some("foo");
1184 /// assert_eq!(x.and(y), Some("foo"));
1186 /// let x: Option<u32> = None;
1187 /// let y: Option<&str> = None;
1188 /// assert_eq!(x.and(y), None);
1191 #[stable(feature = "rust1", since = "1.0.0")]
1192 #[rustc_const_unstable(feature = "const_option_ext", issue = "91930")]
1193 pub const fn and<U>(self, optb: Option<U>) -> Option<U>
1204 /// Returns [`None`] if the option is [`None`], otherwise calls `f` with the
1205 /// wrapped value and returns the result.
1207 /// Some languages call this operation flatmap.
1212 /// fn sq_then_to_string(x: u32) -> Option<String> {
1213 /// x.checked_mul(x).map(|sq| sq.to_string())
1216 /// assert_eq!(Some(2).and_then(sq_then_to_string), Some(4.to_string()));
1217 /// assert_eq!(Some(1_000_000).and_then(sq_then_to_string), None); // overflowed!
1218 /// assert_eq!(None.and_then(sq_then_to_string), None);
1221 /// Often used to chain fallible operations that may return [`None`].
1224 /// let arr_2d = [["A0", "A1"], ["B0", "B1"]];
1226 /// let item_0_1 = arr_2d.get(0).and_then(|row| row.get(1));
1227 /// assert_eq!(item_0_1, Some(&"A1"));
1229 /// let item_2_0 = arr_2d.get(2).and_then(|row| row.get(0));
1230 /// assert_eq!(item_2_0, None);
1233 #[stable(feature = "rust1", since = "1.0.0")]
1234 #[rustc_const_unstable(feature = "const_option_ext", issue = "91930")]
1235 pub const fn and_then<U, F>(self, f: F) -> Option<U>
1237 F: ~const FnOnce(T) -> Option<U>,
1246 /// Returns [`None`] if the option is [`None`], otherwise calls `predicate`
1247 /// with the wrapped value and returns:
1249 /// - [`Some(t)`] if `predicate` returns `true` (where `t` is the wrapped
1251 /// - [`None`] if `predicate` returns `false`.
1253 /// This function works similar to [`Iterator::filter()`]. You can imagine
1254 /// the `Option<T>` being an iterator over one or zero elements. `filter()`
1255 /// lets you decide which elements to keep.
1260 /// fn is_even(n: &i32) -> bool {
1264 /// assert_eq!(None.filter(is_even), None);
1265 /// assert_eq!(Some(3).filter(is_even), None);
1266 /// assert_eq!(Some(4).filter(is_even), Some(4));
1269 /// [`Some(t)`]: Some
1271 #[stable(feature = "option_filter", since = "1.27.0")]
1272 #[rustc_const_unstable(feature = "const_option_ext", issue = "91930")]
1273 pub const fn filter<P>(self, predicate: P) -> Self
1276 P: ~const FnOnce(&T) -> bool,
1279 if let Some(x) = self {
1287 /// Returns the option if it contains a value, otherwise returns `optb`.
1289 /// Arguments passed to `or` are eagerly evaluated; if you are passing the
1290 /// result of a function call, it is recommended to use [`or_else`], which is
1291 /// lazily evaluated.
1293 /// [`or_else`]: Option::or_else
1298 /// let x = Some(2);
1300 /// assert_eq!(x.or(y), Some(2));
1303 /// let y = Some(100);
1304 /// assert_eq!(x.or(y), Some(100));
1306 /// let x = Some(2);
1307 /// let y = Some(100);
1308 /// assert_eq!(x.or(y), Some(2));
1310 /// let x: Option<u32> = None;
1312 /// assert_eq!(x.or(y), None);
1315 #[stable(feature = "rust1", since = "1.0.0")]
1316 #[rustc_const_unstable(feature = "const_option_ext", issue = "91930")]
1317 pub const fn or(self, optb: Option<T>) -> Option<T>
1327 /// Returns the option if it contains a value, otherwise calls `f` and
1328 /// returns the result.
1333 /// fn nobody() -> Option<&'static str> { None }
1334 /// fn vikings() -> Option<&'static str> { Some("vikings") }
1336 /// assert_eq!(Some("barbarians").or_else(vikings), Some("barbarians"));
1337 /// assert_eq!(None.or_else(vikings), Some("vikings"));
1338 /// assert_eq!(None.or_else(nobody), None);
1341 #[stable(feature = "rust1", since = "1.0.0")]
1342 #[rustc_const_unstable(feature = "const_option_ext", issue = "91930")]
1343 pub const fn or_else<F>(self, f: F) -> Option<T>
1345 F: ~const FnOnce() -> Option<T>,
1354 /// Returns [`Some`] if exactly one of `self`, `optb` is [`Some`], otherwise returns [`None`].
1359 /// let x = Some(2);
1360 /// let y: Option<u32> = None;
1361 /// assert_eq!(x.xor(y), Some(2));
1363 /// let x: Option<u32> = None;
1364 /// let y = Some(2);
1365 /// assert_eq!(x.xor(y), Some(2));
1367 /// let x = Some(2);
1368 /// let y = Some(2);
1369 /// assert_eq!(x.xor(y), None);
1371 /// let x: Option<u32> = None;
1372 /// let y: Option<u32> = None;
1373 /// assert_eq!(x.xor(y), None);
1376 #[stable(feature = "option_xor", since = "1.37.0")]
1377 #[rustc_const_unstable(feature = "const_option_ext", issue = "91930")]
1378 pub const fn xor(self, optb: Option<T>) -> Option<T>
1382 match (self, optb) {
1383 (Some(a), None) => Some(a),
1384 (None, Some(b)) => Some(b),
1389 /////////////////////////////////////////////////////////////////////////
1390 // Entry-like operations to insert a value and return a reference
1391 /////////////////////////////////////////////////////////////////////////
1393 /// Inserts `value` into the option, then returns a mutable reference to it.
1395 /// If the option already contains a value, the old value is dropped.
1397 /// See also [`Option::get_or_insert`], which doesn't update the value if
1398 /// the option already contains [`Some`].
1403 /// let mut opt = None;
1404 /// let val = opt.insert(1);
1405 /// assert_eq!(*val, 1);
1406 /// assert_eq!(opt.unwrap(), 1);
1407 /// let val = opt.insert(2);
1408 /// assert_eq!(*val, 2);
1410 /// assert_eq!(opt.unwrap(), 3);
1412 #[must_use = "if you intended to set a value, consider assignment instead"]
1414 #[stable(feature = "option_insert", since = "1.53.0")]
1415 #[rustc_const_unstable(feature = "const_option_ext", issue = "91930")]
1416 pub const fn insert(&mut self, value: T) -> &mut T
1420 *self = Some(value);
1422 // SAFETY: the code above just filled the option
1423 unsafe { self.as_mut().unwrap_unchecked() }
1426 /// Inserts `value` into the option if it is [`None`], then
1427 /// returns a mutable reference to the contained value.
1429 /// See also [`Option::insert`], which updates the value even if
1430 /// the option already contains [`Some`].
1435 /// let mut x = None;
1438 /// let y: &mut u32 = x.get_or_insert(5);
1439 /// assert_eq!(y, &5);
1444 /// assert_eq!(x, Some(7));
1447 #[stable(feature = "option_entry", since = "1.20.0")]
1448 #[rustc_const_unstable(feature = "const_option_ext", issue = "91930")]
1449 pub const fn get_or_insert(&mut self, value: T) -> &mut T
1453 if let None = *self {
1454 *self = Some(value);
1457 // SAFETY: a `None` variant for `self` would have been replaced by a `Some`
1458 // variant in the code above.
1459 unsafe { self.as_mut().unwrap_unchecked() }
1462 /// Inserts the default value into the option if it is [`None`], then
1463 /// returns a mutable reference to the contained value.
1468 /// #![feature(option_get_or_insert_default)]
1470 /// let mut x = None;
1473 /// let y: &mut u32 = x.get_or_insert_default();
1474 /// assert_eq!(y, &0);
1479 /// assert_eq!(x, Some(7));
1482 #[unstable(feature = "option_get_or_insert_default", issue = "82901")]
1483 #[rustc_const_unstable(feature = "const_option_ext", issue = "91930")]
1484 pub const fn get_or_insert_default(&mut self) -> &mut T
1488 #[rustc_allow_const_fn_unstable(const_fn_trait_bound)]
1489 const fn default<T: ~const Default>() -> T {
1493 self.get_or_insert_with(default)
1496 /// Inserts a value computed from `f` into the option if it is [`None`],
1497 /// then returns a mutable reference to the contained value.
1502 /// let mut x = None;
1505 /// let y: &mut u32 = x.get_or_insert_with(|| 5);
1506 /// assert_eq!(y, &5);
1511 /// assert_eq!(x, Some(7));
1514 #[stable(feature = "option_entry", since = "1.20.0")]
1515 #[rustc_const_unstable(feature = "const_option_ext", issue = "91930")]
1516 pub const fn get_or_insert_with<F>(&mut self, f: F) -> &mut T
1518 F: ~const FnOnce() -> T,
1521 if let None = *self {
1522 // the compiler isn't smart enough to know that we are not dropping a `T`
1523 // here and wants us to ensure `T` can be dropped at compile time.
1524 mem::forget(mem::replace(self, Some(f())))
1527 // SAFETY: a `None` variant for `self` would have been replaced by a `Some`
1528 // variant in the code above.
1529 unsafe { self.as_mut().unwrap_unchecked() }
1532 /////////////////////////////////////////////////////////////////////////
1534 /////////////////////////////////////////////////////////////////////////
1536 /// Takes the value out of the option, leaving a [`None`] in its place.
1541 /// let mut x = Some(2);
1542 /// let y = x.take();
1543 /// assert_eq!(x, None);
1544 /// assert_eq!(y, Some(2));
1546 /// let mut x: Option<u32> = None;
1547 /// let y = x.take();
1548 /// assert_eq!(x, None);
1549 /// assert_eq!(y, None);
1552 #[stable(feature = "rust1", since = "1.0.0")]
1553 #[rustc_const_unstable(feature = "const_option", issue = "67441")]
1554 pub const fn take(&mut self) -> Option<T> {
1555 // FIXME replace `mem::replace` by `mem::take` when the latter is const ready
1556 mem::replace(self, None)
1559 /// Replaces the actual value in the option by the value given in parameter,
1560 /// returning the old value if present,
1561 /// leaving a [`Some`] in its place without deinitializing either one.
1566 /// let mut x = Some(2);
1567 /// let old = x.replace(5);
1568 /// assert_eq!(x, Some(5));
1569 /// assert_eq!(old, Some(2));
1571 /// let mut x = None;
1572 /// let old = x.replace(3);
1573 /// assert_eq!(x, Some(3));
1574 /// assert_eq!(old, None);
1577 #[rustc_const_unstable(feature = "const_option", issue = "67441")]
1578 #[stable(feature = "option_replace", since = "1.31.0")]
1579 pub const fn replace(&mut self, value: T) -> Option<T> {
1580 mem::replace(self, Some(value))
1583 /// Returns `true` if the option is a [`Some`] value containing the given value.
1588 /// #![feature(option_result_contains)]
1590 /// let x: Option<u32> = Some(2);
1591 /// assert_eq!(x.contains(&2), true);
1593 /// let x: Option<u32> = Some(3);
1594 /// assert_eq!(x.contains(&2), false);
1596 /// let x: Option<u32> = None;
1597 /// assert_eq!(x.contains(&2), false);
1601 #[unstable(feature = "option_result_contains", issue = "62358")]
1602 #[rustc_const_unstable(feature = "const_option_ext", issue = "91930")]
1603 pub const fn contains<U>(&self, x: &U) -> bool
1605 U: ~const PartialEq<T>,
1613 /// Zips `self` with another `Option`.
1615 /// If `self` is `Some(s)` and `other` is `Some(o)`, this method returns `Some((s, o))`.
1616 /// Otherwise, `None` is returned.
1621 /// let x = Some(1);
1622 /// let y = Some("hi");
1623 /// let z = None::<u8>;
1625 /// assert_eq!(x.zip(y), Some((1, "hi")));
1626 /// assert_eq!(x.zip(z), None);
1628 #[stable(feature = "option_zip_option", since = "1.46.0")]
1629 #[rustc_const_unstable(feature = "const_option_ext", issue = "91930")]
1630 pub const fn zip<U>(self, other: Option<U>) -> Option<(T, U)>
1635 match (self, other) {
1636 (Some(a), Some(b)) => Some((a, b)),
1641 /// Zips `self` and another `Option` with function `f`.
1643 /// If `self` is `Some(s)` and `other` is `Some(o)`, this method returns `Some(f(s, o))`.
1644 /// Otherwise, `None` is returned.
1649 /// #![feature(option_zip)]
1651 /// #[derive(Debug, PartialEq)]
1658 /// fn new(x: f64, y: f64) -> Self {
1663 /// let x = Some(17.5);
1664 /// let y = Some(42.7);
1666 /// assert_eq!(x.zip_with(y, Point::new), Some(Point { x: 17.5, y: 42.7 }));
1667 /// assert_eq!(x.zip_with(None, Point::new), None);
1669 #[unstable(feature = "option_zip", issue = "70086")]
1670 #[rustc_const_unstable(feature = "const_option_ext", issue = "91930")]
1671 pub const fn zip_with<U, F, R>(self, other: Option<U>, f: F) -> Option<R>
1673 F: ~const FnOnce(T, U) -> R,
1678 match (self, other) {
1679 (Some(a), Some(b)) => Some(f(a, b)),
1685 impl<T, U> Option<(T, U)> {
1686 /// Unzips an option containing a tuple of two options.
1688 /// If `self` is `Some((a, b))` this method returns `(Some(a), Some(b))`.
1689 /// Otherwise, `(None, None)` is returned.
1694 /// #![feature(unzip_option)]
1696 /// let x = Some((1, "hi"));
1697 /// let y = None::<(u8, u32)>;
1699 /// assert_eq!(x.unzip(), (Some(1), Some("hi")));
1700 /// assert_eq!(y.unzip(), (None, None));
1703 #[unstable(feature = "unzip_option", issue = "87800", reason = "recently added")]
1704 pub const fn unzip(self) -> (Option<T>, Option<U>) {
1706 Some((a, b)) => (Some(a), Some(b)),
1707 None => (None, None),
1712 impl<T> Option<&T> {
1713 /// Maps an `Option<&T>` to an `Option<T>` by copying the contents of the
1720 /// let opt_x = Some(&x);
1721 /// assert_eq!(opt_x, Some(&12));
1722 /// let copied = opt_x.copied();
1723 /// assert_eq!(copied, Some(12));
1725 #[must_use = "`self` will be dropped if the result is not used"]
1726 #[stable(feature = "copied", since = "1.35.0")]
1727 #[rustc_const_unstable(feature = "const_option", issue = "67441")]
1728 pub const fn copied(self) -> Option<T>
1732 // FIXME: this implementation, which sidesteps using `Option::map` since it's not const
1733 // ready yet, should be reverted when possible to avoid code repetition
1735 Some(&v) => Some(v),
1740 /// Maps an `Option<&T>` to an `Option<T>` by cloning the contents of the
1747 /// let opt_x = Some(&x);
1748 /// assert_eq!(opt_x, Some(&12));
1749 /// let cloned = opt_x.cloned();
1750 /// assert_eq!(cloned, Some(12));
1752 #[must_use = "`self` will be dropped if the result is not used"]
1753 #[stable(feature = "rust1", since = "1.0.0")]
1754 #[rustc_const_unstable(feature = "const_option_cloned", issue = "91582")]
1755 pub const fn cloned(self) -> Option<T>
1760 Some(t) => Some(t.clone()),
1766 impl<T> Option<&mut T> {
1767 /// Maps an `Option<&mut T>` to an `Option<T>` by copying the contents of the
1774 /// let opt_x = Some(&mut x);
1775 /// assert_eq!(opt_x, Some(&mut 12));
1776 /// let copied = opt_x.copied();
1777 /// assert_eq!(copied, Some(12));
1779 #[must_use = "`self` will be dropped if the result is not used"]
1780 #[stable(feature = "copied", since = "1.35.0")]
1781 #[rustc_const_unstable(feature = "const_option_ext", issue = "91930")]
1782 pub const fn copied(self) -> Option<T>
1787 Some(&mut t) => Some(t),
1792 /// Maps an `Option<&mut T>` to an `Option<T>` by cloning the contents of the
1799 /// let opt_x = Some(&mut x);
1800 /// assert_eq!(opt_x, Some(&mut 12));
1801 /// let cloned = opt_x.cloned();
1802 /// assert_eq!(cloned, Some(12));
1804 #[must_use = "`self` will be dropped if the result is not used"]
1805 #[stable(since = "1.26.0", feature = "option_ref_mut_cloned")]
1806 #[rustc_const_unstable(feature = "const_option_cloned", issue = "91582")]
1807 pub const fn cloned(self) -> Option<T>
1812 Some(t) => Some(t.clone()),
1818 impl<T, E> Option<Result<T, E>> {
1819 /// Transposes an `Option` of a [`Result`] into a [`Result`] of an `Option`.
1821 /// [`None`] will be mapped to <code>[Ok]\([None])</code>.
1822 /// <code>[Some]\([Ok]\(\_))</code> and <code>[Some]\([Err]\(\_))</code> will be mapped to
1823 /// <code>[Ok]\([Some]\(\_))</code> and <code>[Err]\(\_)</code>.
1828 /// #[derive(Debug, Eq, PartialEq)]
1831 /// let x: Result<Option<i32>, SomeErr> = Ok(Some(5));
1832 /// let y: Option<Result<i32, SomeErr>> = Some(Ok(5));
1833 /// assert_eq!(x, y.transpose());
1836 #[stable(feature = "transpose_result", since = "1.33.0")]
1837 #[rustc_const_unstable(feature = "const_option", issue = "67441")]
1838 pub const fn transpose(self) -> Result<Option<T>, E> {
1840 Some(Ok(x)) => Ok(Some(x)),
1841 Some(Err(e)) => Err(e),
1847 // This is a separate function to reduce the code size of .expect() itself.
1848 #[cfg_attr(not(feature = "panic_immediate_abort"), inline(never))]
1849 #[cfg_attr(feature = "panic_immediate_abort", inline)]
1852 #[rustc_const_unstable(feature = "const_option", issue = "67441")]
1853 const fn expect_failed(msg: &str) -> ! {
1857 /////////////////////////////////////////////////////////////////////////////
1858 // Trait implementations
1859 /////////////////////////////////////////////////////////////////////////////
1861 #[stable(feature = "rust1", since = "1.0.0")]
1862 #[rustc_const_unstable(feature = "const_clone", issue = "91805")]
1863 impl<T> const Clone for Option<T>
1865 T: ~const Clone + ~const Drop,
1868 fn clone(&self) -> Self {
1870 Some(x) => Some(x.clone()),
1876 fn clone_from(&mut self, source: &Self) {
1877 match (self, source) {
1878 (Some(to), Some(from)) => to.clone_from(from),
1879 (to, from) => *to = from.clone(),
1884 #[stable(feature = "rust1", since = "1.0.0")]
1885 #[rustc_const_unstable(feature = "const_default_impls", issue = "87864")]
1886 impl<T> const Default for Option<T> {
1887 /// Returns [`None`][Option::None].
1892 /// let opt: Option<u32> = Option::default();
1893 /// assert!(opt.is_none());
1896 fn default() -> Option<T> {
1901 #[stable(feature = "rust1", since = "1.0.0")]
1902 impl<T> IntoIterator for Option<T> {
1904 type IntoIter = IntoIter<T>;
1906 /// Returns a consuming iterator over the possibly contained value.
1911 /// let x = Some("string");
1912 /// let v: Vec<&str> = x.into_iter().collect();
1913 /// assert_eq!(v, ["string"]);
1916 /// let v: Vec<&str> = x.into_iter().collect();
1917 /// assert!(v.is_empty());
1920 fn into_iter(self) -> IntoIter<T> {
1921 IntoIter { inner: Item { opt: self } }
1925 #[stable(since = "1.4.0", feature = "option_iter")]
1926 impl<'a, T> IntoIterator for &'a Option<T> {
1928 type IntoIter = Iter<'a, T>;
1930 fn into_iter(self) -> Iter<'a, T> {
1935 #[stable(since = "1.4.0", feature = "option_iter")]
1936 impl<'a, T> IntoIterator for &'a mut Option<T> {
1937 type Item = &'a mut T;
1938 type IntoIter = IterMut<'a, T>;
1940 fn into_iter(self) -> IterMut<'a, T> {
1945 #[stable(since = "1.12.0", feature = "option_from")]
1946 #[rustc_const_unstable(feature = "const_convert", issue = "88674")]
1947 impl<T> const From<T> for Option<T> {
1948 /// Moves `val` into a new [`Some`].
1953 /// let o: Option<u8> = Option::from(67);
1955 /// assert_eq!(Some(67), o);
1957 fn from(val: T) -> Option<T> {
1962 #[stable(feature = "option_ref_from_ref_option", since = "1.30.0")]
1963 #[rustc_const_unstable(feature = "const_convert", issue = "88674")]
1964 impl<'a, T> const From<&'a Option<T>> for Option<&'a T> {
1965 /// Converts from `&Option<T>` to `Option<&T>`.
1969 /// Converts an <code>[Option]<[String]></code> into an <code>[Option]<[usize]></code>, preserving
1970 /// the original. The [`map`] method takes the `self` argument by value, consuming the original,
1971 /// so this technique uses `from` to first take an [`Option`] to a reference
1972 /// to the value inside the original.
1974 /// [`map`]: Option::map
1975 /// [String]: ../../std/string/struct.String.html "String"
1978 /// let s: Option<String> = Some(String::from("Hello, Rustaceans!"));
1979 /// let o: Option<usize> = Option::from(&s).map(|ss: &String| ss.len());
1981 /// println!("Can still print s: {s:?}");
1983 /// assert_eq!(o, Some(18));
1985 fn from(o: &'a Option<T>) -> Option<&'a T> {
1990 #[stable(feature = "option_ref_from_ref_option", since = "1.30.0")]
1991 #[rustc_const_unstable(feature = "const_convert", issue = "88674")]
1992 impl<'a, T> const From<&'a mut Option<T>> for Option<&'a mut T> {
1993 /// Converts from `&mut Option<T>` to `Option<&mut T>`
1998 /// let mut s = Some(String::from("Hello"));
1999 /// let o: Option<&mut String> = Option::from(&mut s);
2002 /// Some(t) => *t = String::from("Hello, Rustaceans!"),
2006 /// assert_eq!(s, Some(String::from("Hello, Rustaceans!")));
2008 fn from(o: &'a mut Option<T>) -> Option<&'a mut T> {
2013 /////////////////////////////////////////////////////////////////////////////
2014 // The Option Iterators
2015 /////////////////////////////////////////////////////////////////////////////
2017 #[derive(Clone, Debug)]
2022 impl<A> Iterator for Item<A> {
2026 fn next(&mut self) -> Option<A> {
2031 fn size_hint(&self) -> (usize, Option<usize>) {
2033 Some(_) => (1, Some(1)),
2034 None => (0, Some(0)),
2039 impl<A> DoubleEndedIterator for Item<A> {
2041 fn next_back(&mut self) -> Option<A> {
2046 impl<A> ExactSizeIterator for Item<A> {}
2047 impl<A> FusedIterator for Item<A> {}
2048 unsafe impl<A> TrustedLen for Item<A> {}
2050 /// An iterator over a reference to the [`Some`] variant of an [`Option`].
2052 /// The iterator yields one value if the [`Option`] is a [`Some`], otherwise none.
2054 /// This `struct` is created by the [`Option::iter`] function.
2055 #[stable(feature = "rust1", since = "1.0.0")]
2057 pub struct Iter<'a, A: 'a> {
2061 #[stable(feature = "rust1", since = "1.0.0")]
2062 impl<'a, A> Iterator for Iter<'a, A> {
2066 fn next(&mut self) -> Option<&'a A> {
2070 fn size_hint(&self) -> (usize, Option<usize>) {
2071 self.inner.size_hint()
2075 #[stable(feature = "rust1", since = "1.0.0")]
2076 impl<'a, A> DoubleEndedIterator for Iter<'a, A> {
2078 fn next_back(&mut self) -> Option<&'a A> {
2079 self.inner.next_back()
2083 #[stable(feature = "rust1", since = "1.0.0")]
2084 impl<A> ExactSizeIterator for Iter<'_, A> {}
2086 #[stable(feature = "fused", since = "1.26.0")]
2087 impl<A> FusedIterator for Iter<'_, A> {}
2089 #[unstable(feature = "trusted_len", issue = "37572")]
2090 unsafe impl<A> TrustedLen for Iter<'_, A> {}
2092 #[stable(feature = "rust1", since = "1.0.0")]
2093 impl<A> Clone for Iter<'_, A> {
2095 fn clone(&self) -> Self {
2096 Iter { inner: self.inner.clone() }
2100 /// An iterator over a mutable reference to the [`Some`] variant of an [`Option`].
2102 /// The iterator yields one value if the [`Option`] is a [`Some`], otherwise none.
2104 /// This `struct` is created by the [`Option::iter_mut`] function.
2105 #[stable(feature = "rust1", since = "1.0.0")]
2107 pub struct IterMut<'a, A: 'a> {
2108 inner: Item<&'a mut A>,
2111 #[stable(feature = "rust1", since = "1.0.0")]
2112 impl<'a, A> Iterator for IterMut<'a, A> {
2113 type Item = &'a mut A;
2116 fn next(&mut self) -> Option<&'a mut A> {
2120 fn size_hint(&self) -> (usize, Option<usize>) {
2121 self.inner.size_hint()
2125 #[stable(feature = "rust1", since = "1.0.0")]
2126 impl<'a, A> DoubleEndedIterator for IterMut<'a, A> {
2128 fn next_back(&mut self) -> Option<&'a mut A> {
2129 self.inner.next_back()
2133 #[stable(feature = "rust1", since = "1.0.0")]
2134 impl<A> ExactSizeIterator for IterMut<'_, A> {}
2136 #[stable(feature = "fused", since = "1.26.0")]
2137 impl<A> FusedIterator for IterMut<'_, A> {}
2138 #[unstable(feature = "trusted_len", issue = "37572")]
2139 unsafe impl<A> TrustedLen for IterMut<'_, A> {}
2141 /// An iterator over the value in [`Some`] variant of an [`Option`].
2143 /// The iterator yields one value if the [`Option`] is a [`Some`], otherwise none.
2145 /// This `struct` is created by the [`Option::into_iter`] function.
2146 #[derive(Clone, Debug)]
2147 #[stable(feature = "rust1", since = "1.0.0")]
2148 pub struct IntoIter<A> {
2152 #[stable(feature = "rust1", since = "1.0.0")]
2153 impl<A> Iterator for IntoIter<A> {
2157 fn next(&mut self) -> Option<A> {
2161 fn size_hint(&self) -> (usize, Option<usize>) {
2162 self.inner.size_hint()
2166 #[stable(feature = "rust1", since = "1.0.0")]
2167 impl<A> DoubleEndedIterator for IntoIter<A> {
2169 fn next_back(&mut self) -> Option<A> {
2170 self.inner.next_back()
2174 #[stable(feature = "rust1", since = "1.0.0")]
2175 impl<A> ExactSizeIterator for IntoIter<A> {}
2177 #[stable(feature = "fused", since = "1.26.0")]
2178 impl<A> FusedIterator for IntoIter<A> {}
2180 #[unstable(feature = "trusted_len", issue = "37572")]
2181 unsafe impl<A> TrustedLen for IntoIter<A> {}
2183 /////////////////////////////////////////////////////////////////////////////
2185 /////////////////////////////////////////////////////////////////////////////
2187 #[stable(feature = "rust1", since = "1.0.0")]
2188 impl<A, V: FromIterator<A>> FromIterator<Option<A>> for Option<V> {
2189 /// Takes each element in the [`Iterator`]: if it is [`None`][Option::None],
2190 /// no further elements are taken, and the [`None`][Option::None] is
2191 /// returned. Should no [`None`][Option::None] occur, a container of type
2192 /// `V` containing the values of each [`Option`] is returned.
2196 /// Here is an example which increments every integer in a vector.
2197 /// We use the checked variant of `add` that returns `None` when the
2198 /// calculation would result in an overflow.
2201 /// let items = vec![0_u16, 1, 2];
2203 /// let res: Option<Vec<u16>> = items
2205 /// .map(|x| x.checked_add(1))
2208 /// assert_eq!(res, Some(vec![1, 2, 3]));
2211 /// As you can see, this will return the expected, valid items.
2213 /// Here is another example that tries to subtract one from another list
2214 /// of integers, this time checking for underflow:
2217 /// let items = vec![2_u16, 1, 0];
2219 /// let res: Option<Vec<u16>> = items
2221 /// .map(|x| x.checked_sub(1))
2224 /// assert_eq!(res, None);
2227 /// Since the last element is zero, it would underflow. Thus, the resulting
2228 /// value is `None`.
2230 /// Here is a variation on the previous example, showing that no
2231 /// further elements are taken from `iter` after the first `None`.
2234 /// let items = vec![3_u16, 2, 1, 10];
2236 /// let mut shared = 0;
2238 /// let res: Option<Vec<u16>> = items
2240 /// .map(|x| { shared += x; x.checked_sub(2) })
2243 /// assert_eq!(res, None);
2244 /// assert_eq!(shared, 6);
2247 /// Since the third element caused an underflow, no further elements were taken,
2248 /// so the final value of `shared` is 6 (= `3 + 2 + 1`), not 16.
2250 fn from_iter<I: IntoIterator<Item = Option<A>>>(iter: I) -> Option<V> {
2251 // FIXME(#11084): This could be replaced with Iterator::scan when this
2252 // performance bug is closed.
2254 iter::try_process(iter.into_iter(), |i| i.collect())
2258 #[unstable(feature = "try_trait_v2", issue = "84277")]
2259 #[rustc_const_unstable(feature = "const_convert", issue = "88674")]
2260 impl<T> const ops::Try for Option<T> {
2262 type Residual = Option<convert::Infallible>;
2265 fn from_output(output: Self::Output) -> Self {
2270 fn branch(self) -> ControlFlow<Self::Residual, Self::Output> {
2272 Some(v) => ControlFlow::Continue(v),
2273 None => ControlFlow::Break(None),
2278 #[unstable(feature = "try_trait_v2", issue = "84277")]
2279 #[rustc_const_unstable(feature = "const_convert", issue = "88674")]
2280 impl<T> const ops::FromResidual for Option<T> {
2282 fn from_residual(residual: Option<convert::Infallible>) -> Self {
2289 #[unstable(feature = "try_trait_v2_residual", issue = "91285")]
2290 impl<T> ops::Residual<T> for Option<convert::Infallible> {
2291 type TryType = Option<T>;
2294 impl<T> Option<Option<T>> {
2295 /// Converts from `Option<Option<T>>` to `Option<T>`.
2302 /// let x: Option<Option<u32>> = Some(Some(6));
2303 /// assert_eq!(Some(6), x.flatten());
2305 /// let x: Option<Option<u32>> = Some(None);
2306 /// assert_eq!(None, x.flatten());
2308 /// let x: Option<Option<u32>> = None;
2309 /// assert_eq!(None, x.flatten());
2312 /// Flattening only removes one level of nesting at a time:
2315 /// let x: Option<Option<Option<u32>>> = Some(Some(Some(6)));
2316 /// assert_eq!(Some(Some(6)), x.flatten());
2317 /// assert_eq!(Some(6), x.flatten().flatten());
2320 #[stable(feature = "option_flattening", since = "1.40.0")]
2321 #[rustc_const_unstable(feature = "const_option", issue = "67441")]
2322 pub const fn flatten(self) -> Option<T> {
2324 Some(inner) => inner,