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//! Optional values. //! //! Type [`Option`] represents an optional value: every [`Option`] //! is either [`Some`] and contains a value, or [`None`], and //! does not. [`Option`] types are very common in Rust code, as //! they have a number of uses: //! //! * Initial values //! * Return values for functions that are not defined //! over their entire input range (partial functions) //! * Return value for otherwise reporting simple errors, where [`None`] is //! returned on error //! * Optional struct fields //! * Struct fields that can be loaned or "taken" //! * Optional function arguments //! * Nullable pointers //! * Swapping things out of difficult situations //! //! [`Option`]s are commonly paired with pattern matching to query the presence //! of a value and take action, always accounting for the [`None`] case. //! //! ``` //! fn divide(numerator: f64, denominator: f64) -> Option<f64> { //! if denominator == 0.0 { //! None //! } else { //! Some(numerator / denominator) //! } //! } //! //! // The return value of the function is an option //! let result = divide(2.0, 3.0); //! //! // Pattern match to retrieve the value //! match result { //! // The division was valid //! Some(x) => println!("Result: {}", x), //! // The division was invalid //! None => println!("Cannot divide by 0"), //! } //! ``` //! // // FIXME: Show how `Option` is used in practice, with lots of methods // //! # Options and pointers ("nullable" pointers) //! //! Rust's pointer types must always point to a valid location; there are //! no "null" pointers. Instead, Rust has *optional* pointers, like //! the optional owned box, [`Option`]`<`[`Box<T>`]`>`. //! //! The following example uses [`Option`] to create an optional box of //! [`i32`]. Notice that in order to use the inner [`i32`] value first, the //! `check_optional` function needs to use pattern matching to //! determine whether the box has a value (i.e., it is [`Some(...)`][`Some`]) or //! not ([`None`]). //! //! ``` //! let optional = None; //! check_optional(optional); //! //! let optional = Some(Box::new(9000)); //! check_optional(optional); //! //! fn check_optional(optional: Option<Box<i32>>) { //! match optional { //! Some(ref p) => println!("has value {}", p), //! None => println!("has no value"), //! } //! } //! ``` //! //! This usage of [`Option`] to create safe nullable pointers is so //! common that Rust does special optimizations to make the //! representation of [`Option`]`<`[`Box<T>`]`>` a single pointer. Optional pointers //! in Rust are stored as efficiently as any other pointer type. //! //! # Examples //! //! Basic pattern matching on [`Option`]: //! //! ``` //! let msg = Some("howdy"); //! //! // Take a reference to the contained string //! if let Some(ref m) = msg { //! println!("{}", *m); //! } //! //! // Remove the contained string, destroying the Option //! let unwrapped_msg = msg.unwrap_or("default message"); //! ``` //! //! Initialize a result to [`None`] before a loop: //! //! ``` //! enum Kingdom { Plant(u32, &'static str), Animal(u32, &'static str) } //! //! // A list of data to search through. //! let all_the_big_things = [ //! Kingdom::Plant(250, "redwood"), //! Kingdom::Plant(230, "noble fir"), //! Kingdom::Plant(229, "sugar pine"), //! Kingdom::Animal(25, "blue whale"), //! Kingdom::Animal(19, "fin whale"), //! Kingdom::Animal(15, "north pacific right whale"), //! ]; //! //! // We're going to search for the name of the biggest animal, //! // but to start with we've just got `None`. //! let mut name_of_biggest_animal = None; //! let mut size_of_biggest_animal = 0; //! for big_thing in &all_the_big_things { //! match *big_thing { //! Kingdom::Animal(size, name) if size > size_of_biggest_animal => { //! // Now we've found the name of some big animal //! size_of_biggest_animal = size; //! name_of_biggest_animal = Some(name); //! } //! Kingdom::Animal(..) | Kingdom::Plant(..) => () //! } //! } //! //! match name_of_biggest_animal { //! Some(name) => println!("the biggest animal is {}", name), //! None => println!("there are no animals :("), //! } //! ``` //! //! [`Option`]: enum.Option.html //! [`Some`]: enum.Option.html#variant.Some //! [`None`]: enum.Option.html#variant.None //! [`Box<T>`]: ../../std/boxed/struct.Box.html //! [`i32`]: ../../std/primitive.i32.html #![stable(feature = "rust1", since = "1.0.0")] use crate::iter::{FromIterator, FusedIterator, TrustedLen}; use crate::{convert, fmt, hint, mem, ops::{self, Deref, DerefMut}}; use crate::pin::Pin; // Note that this is not a lang item per se, but it has a hidden dependency on // `Iterator`, which is one. The compiler assumes that the `next` method of // `Iterator` is an enumeration with one type parameter and two variants, // which basically means it must be `Option`. /// The `Option` type. See [the module level documentation](index.html) for more. #[derive(Copy, PartialEq, PartialOrd, Eq, Ord, Debug, Hash)] #[stable(feature = "rust1", since = "1.0.0")] pub enum Option<T> { /// No value #[stable(feature = "rust1", since = "1.0.0")] None, /// Some value `T` #[stable(feature = "rust1", since = "1.0.0")] Some(#[stable(feature = "rust1", since = "1.0.0")] T), } ///////////////////////////////////////////////////////////////////////////// // Type implementation ///////////////////////////////////////////////////////////////////////////// impl<T> Option<T> { ///////////////////////////////////////////////////////////////////////// // Querying the contained values ///////////////////////////////////////////////////////////////////////// /// Returns `true` if the option is a [`Some`] value. /// /// # Examples /// /// ``` /// let x: Option<u32> = Some(2); /// assert_eq!(x.is_some(), true); /// /// let x: Option<u32> = None; /// assert_eq!(x.is_some(), false); /// ``` /// /// [`Some`]: #variant.Some #[must_use = "if you intended to assert that this has a value, consider `.unwrap()` instead"] #[inline] #[stable(feature = "rust1", since = "1.0.0")] pub fn is_some(&self) -> bool { match *self { Some(_) => true, None => false, } } /// Returns `true` if the option is a [`None`] value. /// /// # Examples /// /// ``` /// let x: Option<u32> = Some(2); /// assert_eq!(x.is_none(), false); /// /// let x: Option<u32> = None; /// assert_eq!(x.is_none(), true); /// ``` /// /// [`None`]: #variant.None #[must_use = "if you intended to assert that this doesn't have a value, consider \ `.and_then(|| panic!(\"`Option` had a value when expected `None`\"))` instead"] #[inline] #[stable(feature = "rust1", since = "1.0.0")] pub fn is_none(&self) -> bool { !self.is_some() } /// Returns `true` if the option is a [`Some`] value containing the given value. /// /// # Examples /// /// ``` /// #![feature(option_result_contains)] /// /// let x: Option<u32> = Some(2); /// assert_eq!(x.contains(&2), true); /// /// let x: Option<u32> = Some(3); /// assert_eq!(x.contains(&2), false); /// /// let x: Option<u32> = None; /// assert_eq!(x.contains(&2), false); /// ``` #[must_use] #[inline] #[unstable(feature = "option_result_contains", issue = "62358")] pub fn contains<U>(&self, x: &U) -> bool where U: PartialEq<T> { match self { Some(y) => x == y, None => false, } } ///////////////////////////////////////////////////////////////////////// // Adapter for working with references ///////////////////////////////////////////////////////////////////////// /// Converts from `&Option<T>` to `Option<&T>`. /// /// # Examples /// /// Converts an `Option<`[`String`]`>` into an `Option<`[`usize`]`>`, preserving the original. /// The [`map`] method takes the `self` argument by value, consuming the original, /// so this technique uses `as_ref` to first take an `Option` to a reference /// to the value inside the original. /// /// [`map`]: enum.Option.html#method.map /// [`String`]: ../../std/string/struct.String.html /// [`usize`]: ../../std/primitive.usize.html /// /// ``` /// let text: Option<String> = Some("Hello, world!".to_string()); /// // First, cast `Option<String>` to `Option<&String>` with `as_ref`, /// // then consume *that* with `map`, leaving `text` on the stack. /// let text_length: Option<usize> = text.as_ref().map(|s| s.len()); /// println!("still can print text: {:?}", text); /// ``` #[inline] #[stable(feature = "rust1", since = "1.0.0")] pub fn as_ref(&self) -> Option<&T> { match *self { Some(ref x) => Some(x), None => None, } } /// Converts from `&mut Option<T>` to `Option<&mut T>`. /// /// # Examples /// /// ``` /// let mut x = Some(2); /// match x.as_mut() { /// Some(v) => *v = 42, /// None => {}, /// } /// assert_eq!(x, Some(42)); /// ``` #[inline] #[stable(feature = "rust1", since = "1.0.0")] pub fn as_mut(&mut self) -> Option<&mut T> { match *self { Some(ref mut x) => Some(x), None => None, } } /// Converts from [`Pin`]`<&Option<T>>` to `Option<`[`Pin`]`<&T>>`. /// /// [`Pin`]: ../pin/struct.Pin.html #[inline] #[stable(feature = "pin", since = "1.33.0")] pub fn as_pin_ref(self: Pin<&Self>) -> Option<Pin<&T>> { unsafe { Pin::get_ref(self).as_ref().map(|x| Pin::new_unchecked(x)) } } /// Converts from [`Pin`]`<&mut Option<T>>` to `Option<`[`Pin`]`<&mut T>>`. /// /// [`Pin`]: ../pin/struct.Pin.html #[inline] #[stable(feature = "pin", since = "1.33.0")] pub fn as_pin_mut(self: Pin<&mut Self>) -> Option<Pin<&mut T>> { unsafe { Pin::get_unchecked_mut(self).as_mut().map(|x| Pin::new_unchecked(x)) } } ///////////////////////////////////////////////////////////////////////// // Getting to contained values ///////////////////////////////////////////////////////////////////////// /// Unwraps an option, yielding the content of a [`Some`]. /// /// # Panics /// /// Panics if the value is a [`None`] with a custom panic message provided by /// `msg`. /// /// [`Some`]: #variant.Some /// [`None`]: #variant.None /// /// # Examples /// /// ``` /// let x = Some("value"); /// assert_eq!(x.expect("the world is ending"), "value"); /// ``` /// /// ```{.should_panic} /// let x: Option<&str> = None; /// x.expect("the world is ending"); // panics with `the world is ending` /// ``` #[inline] #[stable(feature = "rust1", since = "1.0.0")] pub fn expect(self, msg: &str) -> T { match self { Some(val) => val, None => expect_failed(msg), } } /// Moves the value `v` out of the `Option<T>` if it is [`Some(v)`]. /// /// In general, because this function may panic, its use is discouraged. /// Instead, prefer to use pattern matching and handle the [`None`] /// case explicitly. /// /// # Panics /// /// Panics if the self value equals [`None`]. /// /// [`Some(v)`]: #variant.Some /// [`None`]: #variant.None /// /// # Examples /// /// ``` /// let x = Some("air"); /// assert_eq!(x.unwrap(), "air"); /// ``` /// /// ```{.should_panic} /// let x: Option<&str> = None; /// assert_eq!(x.unwrap(), "air"); // fails /// ``` #[inline] #[stable(feature = "rust1", since = "1.0.0")] pub fn unwrap(self) -> T { match self { Some(val) => val, None => panic!("called `Option::unwrap()` on a `None` value"), } } /// Returns the contained value or a default. /// /// Arguments passed to `unwrap_or` are eagerly evaluated; if you are passing /// the result of a function call, it is recommended to use [`unwrap_or_else`], /// which is lazily evaluated. /// /// [`unwrap_or_else`]: #method.unwrap_or_else /// /// # Examples /// /// ``` /// assert_eq!(Some("car").unwrap_or("bike"), "car"); /// assert_eq!(None.unwrap_or("bike"), "bike"); /// ``` #[inline] #[stable(feature = "rust1", since = "1.0.0")] pub fn unwrap_or(self, def: T) -> T { match self { Some(x) => x, None => def, } } /// Returns the contained value or computes it from a closure. /// /// # Examples /// /// ``` /// let k = 10; /// assert_eq!(Some(4).unwrap_or_else(|| 2 * k), 4); /// assert_eq!(None.unwrap_or_else(|| 2 * k), 20); /// ``` #[inline] #[stable(feature = "rust1", since = "1.0.0")] pub fn unwrap_or_else<F: FnOnce() -> T>(self, f: F) -> T { match self { Some(x) => x, None => f(), } } ///////////////////////////////////////////////////////////////////////// // Transforming contained values ///////////////////////////////////////////////////////////////////////// /// Maps an `Option<T>` to `Option<U>` by applying a function to a contained value. /// /// # Examples /// /// Converts an `Option<`[`String`]`>` into an `Option<`[`usize`]`>`, consuming the original: /// /// [`String`]: ../../std/string/struct.String.html /// [`usize`]: ../../std/primitive.usize.html /// /// ``` /// let maybe_some_string = Some(String::from("Hello, World!")); /// // `Option::map` takes self *by value*, consuming `maybe_some_string` /// let maybe_some_len = maybe_some_string.map(|s| s.len()); /// /// assert_eq!(maybe_some_len, Some(13)); /// ``` #[inline] #[stable(feature = "rust1", since = "1.0.0")] pub fn map<U, F: FnOnce(T) -> U>(self, f: F) -> Option<U> { match self { Some(x) => Some(f(x)), None => None, } } /// Applies a function to the contained value (if any), /// or returns the provided default (if not). /// /// # Examples /// /// ``` /// let x = Some("foo"); /// assert_eq!(x.map_or(42, |v| v.len()), 3); /// /// let x: Option<&str> = None; /// assert_eq!(x.map_or(42, |v| v.len()), 42); /// ``` #[inline] #[stable(feature = "rust1", since = "1.0.0")] pub fn map_or<U, F: FnOnce(T) -> U>(self, default: U, f: F) -> U { match self { Some(t) => f(t), None => default, } } /// Applies a function to the contained value (if any), /// or computes a default (if not). /// /// # Examples /// /// ``` /// let k = 21; /// /// let x = Some("foo"); /// assert_eq!(x.map_or_else(|| 2 * k, |v| v.len()), 3); /// /// let x: Option<&str> = None; /// assert_eq!(x.map_or_else(|| 2 * k, |v| v.len()), 42); /// ``` #[inline] #[stable(feature = "rust1", since = "1.0.0")] pub fn map_or_else<U, D: FnOnce() -> U, F: FnOnce(T) -> U>(self, default: D, f: F) -> U { match self { Some(t) => f(t), None => default(), } } /// Transforms the `Option<T>` into a [`Result<T, E>`], mapping [`Some(v)`] to /// [`Ok(v)`] and [`None`] to [`Err(err)`]. /// /// Arguments passed to `ok_or` are eagerly evaluated; if you are passing the /// result of a function call, it is recommended to use [`ok_or_else`], which is /// lazily evaluated. /// /// [`Result<T, E>`]: ../../std/result/enum.Result.html /// [`Ok(v)`]: ../../std/result/enum.Result.html#variant.Ok /// [`Err(err)`]: ../../std/result/enum.Result.html#variant.Err /// [`None`]: #variant.None /// [`Some(v)`]: #variant.Some /// [`ok_or_else`]: #method.ok_or_else /// /// # Examples /// /// ``` /// let x = Some("foo"); /// assert_eq!(x.ok_or(0), Ok("foo")); /// /// let x: Option<&str> = None; /// assert_eq!(x.ok_or(0), Err(0)); /// ``` #[inline] #[stable(feature = "rust1", since = "1.0.0")] pub fn ok_or<E>(self, err: E) -> Result<T, E> { match self { Some(v) => Ok(v), None => Err(err), } } /// Transforms the `Option<T>` into a [`Result<T, E>`], mapping [`Some(v)`] to /// [`Ok(v)`] and [`None`] to [`Err(err())`]. /// /// [`Result<T, E>`]: ../../std/result/enum.Result.html /// [`Ok(v)`]: ../../std/result/enum.Result.html#variant.Ok /// [`Err(err())`]: ../../std/result/enum.Result.html#variant.Err /// [`None`]: #variant.None /// [`Some(v)`]: #variant.Some /// /// # Examples /// /// ``` /// let x = Some("foo"); /// assert_eq!(x.ok_or_else(|| 0), Ok("foo")); /// /// let x: Option<&str> = None; /// assert_eq!(x.ok_or_else(|| 0), Err(0)); /// ``` #[inline] #[stable(feature = "rust1", since = "1.0.0")] pub fn ok_or_else<E, F: FnOnce() -> E>(self, err: F) -> Result<T, E> { match self { Some(v) => Ok(v), None => Err(err()), } } ///////////////////////////////////////////////////////////////////////// // Iterator constructors ///////////////////////////////////////////////////////////////////////// /// Returns an iterator over the possibly contained value. /// /// # Examples /// /// ``` /// let x = Some(4); /// assert_eq!(x.iter().next(), Some(&4)); /// /// let x: Option<u32> = None; /// assert_eq!(x.iter().next(), None); /// ``` #[inline] #[stable(feature = "rust1", since = "1.0.0")] pub fn iter(&self) -> Iter<'_, T> { Iter { inner: Item { opt: self.as_ref() } } } /// Returns a mutable iterator over the possibly contained value. /// /// # Examples /// /// ``` /// let mut x = Some(4); /// match x.iter_mut().next() { /// Some(v) => *v = 42, /// None => {}, /// } /// assert_eq!(x, Some(42)); /// /// let mut x: Option<u32> = None; /// assert_eq!(x.iter_mut().next(), None); /// ``` #[inline] #[stable(feature = "rust1", since = "1.0.0")] pub fn iter_mut(&mut self) -> IterMut<'_, T> { IterMut { inner: Item { opt: self.as_mut() } } } ///////////////////////////////////////////////////////////////////////// // Boolean operations on the values, eager and lazy ///////////////////////////////////////////////////////////////////////// /// Returns [`None`] if the option is [`None`], otherwise returns `optb`. /// /// [`None`]: #variant.None /// /// # Examples /// /// ``` /// let x = Some(2); /// let y: Option<&str> = None; /// assert_eq!(x.and(y), None); /// /// let x: Option<u32> = None; /// let y = Some("foo"); /// assert_eq!(x.and(y), None); /// /// let x = Some(2); /// let y = Some("foo"); /// assert_eq!(x.and(y), Some("foo")); /// /// let x: Option<u32> = None; /// let y: Option<&str> = None; /// assert_eq!(x.and(y), None); /// ``` #[inline] #[stable(feature = "rust1", since = "1.0.0")] pub fn and<U>(self, optb: Option<U>) -> Option<U> { match self { Some(_) => optb, None => None, } } /// Returns [`None`] if the option is [`None`], otherwise calls `f` with the /// wrapped value and returns the result. /// /// Some languages call this operation flatmap. /// /// [`None`]: #variant.None /// /// # Examples /// /// ``` /// fn sq(x: u32) -> Option<u32> { Some(x * x) } /// fn nope(_: u32) -> Option<u32> { None } /// /// assert_eq!(Some(2).and_then(sq).and_then(sq), Some(16)); /// assert_eq!(Some(2).and_then(sq).and_then(nope), None); /// assert_eq!(Some(2).and_then(nope).and_then(sq), None); /// assert_eq!(None.and_then(sq).and_then(sq), None); /// ``` #[inline] #[stable(feature = "rust1", since = "1.0.0")] pub fn and_then<U, F: FnOnce(T) -> Option<U>>(self, f: F) -> Option<U> { match self { Some(x) => f(x), None => None, } } /// Returns [`None`] if the option is [`None`], otherwise calls `predicate` /// with the wrapped value and returns: /// /// - [`Some(t)`] if `predicate` returns `true` (where `t` is the wrapped /// value), and /// - [`None`] if `predicate` returns `false`. /// /// This function works similar to [`Iterator::filter()`]. You can imagine /// the `Option<T>` being an iterator over one or zero elements. `filter()` /// lets you decide which elements to keep. /// /// # Examples /// /// ```rust /// fn is_even(n: &i32) -> bool { /// n % 2 == 0 /// } /// /// assert_eq!(None.filter(is_even), None); /// assert_eq!(Some(3).filter(is_even), None); /// assert_eq!(Some(4).filter(is_even), Some(4)); /// ``` /// /// [`None`]: #variant.None /// [`Some(t)`]: #variant.Some /// [`Iterator::filter()`]: ../../std/iter/trait.Iterator.html#method.filter #[inline] #[stable(feature = "option_filter", since = "1.27.0")] pub fn filter<P: FnOnce(&T) -> bool>(self, predicate: P) -> Self { if let Some(x) = self { if predicate(&x) { return Some(x) } } None } /// Returns the option if it contains a value, otherwise returns `optb`. /// /// Arguments passed to `or` are eagerly evaluated; if you are passing the /// result of a function call, it is recommended to use [`or_else`], which is /// lazily evaluated. /// /// [`or_else`]: #method.or_else /// /// # Examples /// /// ``` /// let x = Some(2); /// let y = None; /// assert_eq!(x.or(y), Some(2)); /// /// let x = None; /// let y = Some(100); /// assert_eq!(x.or(y), Some(100)); /// /// let x = Some(2); /// let y = Some(100); /// assert_eq!(x.or(y), Some(2)); /// /// let x: Option<u32> = None; /// let y = None; /// assert_eq!(x.or(y), None); /// ``` #[inline] #[stable(feature = "rust1", since = "1.0.0")] pub fn or(self, optb: Option<T>) -> Option<T> { match self { Some(_) => self, None => optb, } } /// Returns the option if it contains a value, otherwise calls `f` and /// returns the result. /// /// # Examples /// /// ``` /// fn nobody() -> Option<&'static str> { None } /// fn vikings() -> Option<&'static str> { Some("vikings") } /// /// assert_eq!(Some("barbarians").or_else(vikings), Some("barbarians")); /// assert_eq!(None.or_else(vikings), Some("vikings")); /// assert_eq!(None.or_else(nobody), None); /// ``` #[inline] #[stable(feature = "rust1", since = "1.0.0")] pub fn or_else<F: FnOnce() -> Option<T>>(self, f: F) -> Option<T> { match self { Some(_) => self, None => f(), } } /// Returns [`Some`] if exactly one of `self`, `optb` is [`Some`], otherwise returns [`None`]. /// /// [`Some`]: #variant.Some /// [`None`]: #variant.None /// /// # Examples /// /// ``` /// let x = Some(2); /// let y: Option<u32> = None; /// assert_eq!(x.xor(y), Some(2)); /// /// let x: Option<u32> = None; /// let y = Some(2); /// assert_eq!(x.xor(y), Some(2)); /// /// let x = Some(2); /// let y = Some(2); /// assert_eq!(x.xor(y), None); /// /// let x: Option<u32> = None; /// let y: Option<u32> = None; /// assert_eq!(x.xor(y), None); /// ``` #[inline] #[stable(feature = "option_xor", since = "1.37.0")] pub fn xor(self, optb: Option<T>) -> Option<T> { match (self, optb) { (Some(a), None) => Some(a), (None, Some(b)) => Some(b), _ => None, } } ///////////////////////////////////////////////////////////////////////// // Entry-like operations to insert if None and return a reference ///////////////////////////////////////////////////////////////////////// /// Inserts `v` into the option if it is [`None`], then /// returns a mutable reference to the contained value. /// /// [`None`]: #variant.None /// /// # Examples /// /// ``` /// let mut x = None; /// /// { /// let y: &mut u32 = x.get_or_insert(5); /// assert_eq!(y, &5); /// /// *y = 7; /// } /// /// assert_eq!(x, Some(7)); /// ``` #[inline] #[stable(feature = "option_entry", since = "1.20.0")] pub fn get_or_insert(&mut self, v: T) -> &mut T { self.get_or_insert_with(|| v) } /// Inserts a value computed from `f` into the option if it is [`None`], then /// returns a mutable reference to the contained value. /// /// [`None`]: #variant.None /// /// # Examples /// /// ``` /// let mut x = None; /// /// { /// let y: &mut u32 = x.get_or_insert_with(|| 5); /// assert_eq!(y, &5); /// /// *y = 7; /// } /// /// assert_eq!(x, Some(7)); /// ``` #[inline] #[stable(feature = "option_entry", since = "1.20.0")] pub fn get_or_insert_with<F: FnOnce() -> T>(&mut self, f: F) -> &mut T { match *self { None => *self = Some(f()), _ => (), } match *self { Some(ref mut v) => v, None => unsafe { hint::unreachable_unchecked() }, } } ///////////////////////////////////////////////////////////////////////// // Misc ///////////////////////////////////////////////////////////////////////// /// Takes the value out of the option, leaving a [`None`] in its place. /// /// [`None`]: #variant.None /// /// # Examples /// /// ``` /// let mut x = Some(2); /// let y = x.take(); /// assert_eq!(x, None); /// assert_eq!(y, Some(2)); /// /// let mut x: Option<u32> = None; /// let y = x.take(); /// assert_eq!(x, None); /// assert_eq!(y, None); /// ``` #[inline] #[stable(feature = "rust1", since = "1.0.0")] pub fn take(&mut self) -> Option<T> { mem::take(self) } /// Replaces the actual value in the option by the value given in parameter, /// returning the old value if present, /// leaving a [`Some`] in its place without deinitializing either one. /// /// [`Some`]: #variant.Some /// /// # Examples /// /// ``` /// let mut x = Some(2); /// let old = x.replace(5); /// assert_eq!(x, Some(5)); /// assert_eq!(old, Some(2)); /// /// let mut x = None; /// let old = x.replace(3); /// assert_eq!(x, Some(3)); /// assert_eq!(old, None); /// ``` #[inline] #[stable(feature = "option_replace", since = "1.31.0")] pub fn replace(&mut self, value: T) -> Option<T> { mem::replace(self, Some(value)) } } impl<T: Copy> Option<&T> { /// Maps an `Option<&T>` to an `Option<T>` by copying the contents of the /// option. /// /// # Examples /// /// ``` /// let x = 12; /// let opt_x = Some(&x); /// assert_eq!(opt_x, Some(&12)); /// let copied = opt_x.copied(); /// assert_eq!(copied, Some(12)); /// ``` #[stable(feature = "copied", since = "1.35.0")] pub fn copied(self) -> Option<T> { self.map(|&t| t) } } impl<T: Copy> Option<&mut T> { /// Maps an `Option<&mut T>` to an `Option<T>` by copying the contents of the /// option. /// /// # Examples /// /// ``` /// let mut x = 12; /// let opt_x = Some(&mut x); /// assert_eq!(opt_x, Some(&mut 12)); /// let copied = opt_x.copied(); /// assert_eq!(copied, Some(12)); /// ``` #[stable(feature = "copied", since = "1.35.0")] pub fn copied(self) -> Option<T> { self.map(|&mut t| t) } } impl<T: Clone> Option<&T> { /// Maps an `Option<&T>` to an `Option<T>` by cloning the contents of the /// option. /// /// # Examples /// /// ``` /// let x = 12; /// let opt_x = Some(&x); /// assert_eq!(opt_x, Some(&12)); /// let cloned = opt_x.cloned(); /// assert_eq!(cloned, Some(12)); /// ``` #[stable(feature = "rust1", since = "1.0.0")] pub fn cloned(self) -> Option<T> { self.map(|t| t.clone()) } } impl<T: Clone> Option<&mut T> { /// Maps an `Option<&mut T>` to an `Option<T>` by cloning the contents of the /// option. /// /// # Examples /// /// ``` /// let mut x = 12; /// let opt_x = Some(&mut x); /// assert_eq!(opt_x, Some(&mut 12)); /// let cloned = opt_x.cloned(); /// assert_eq!(cloned, Some(12)); /// ``` #[stable(since = "1.26.0", feature = "option_ref_mut_cloned")] pub fn cloned(self) -> Option<T> { self.map(|t| t.clone()) } } impl<T: fmt::Debug> Option<T> { /// Unwraps an option, expecting [`None`] and returning nothing. /// /// # Panics /// /// Panics if the value is a [`Some`], with a panic message including the /// passed message, and the content of the [`Some`]. /// /// [`Some`]: #variant.Some /// [`None`]: #variant.None /// /// # Examples /// /// ``` /// #![feature(option_expect_none)] /// /// use std::collections::HashMap; /// let mut squares = HashMap::new(); /// for i in -10..=10 { /// // This will not panic, since all keys are unique. /// squares.insert(i, i * i).expect_none("duplicate key"); /// } /// ``` /// /// ```{.should_panic} /// #![feature(option_expect_none)] /// /// use std::collections::HashMap; /// let mut sqrts = HashMap::new(); /// for i in -10..=10 { /// // This will panic, since both negative and positive `i` will /// // insert the same `i * i` key, returning the old `Some(i)`. /// sqrts.insert(i * i, i).expect_none("duplicate key"); /// } /// ``` #[inline] #[unstable(feature = "option_expect_none", reason = "newly added", issue = "62633")] pub fn expect_none(self, msg: &str) { if let Some(val) = self { expect_none_failed(msg, &val); } } /// Unwraps an option, expecting [`None`] and returning nothing. /// /// # Panics /// /// Panics if the value is a [`Some`], with a custom panic message provided /// by the [`Some`]'s value. /// /// [`Some(v)`]: #variant.Some /// [`None`]: #variant.None /// /// # Examples /// /// ``` /// #![feature(option_unwrap_none)] /// /// use std::collections::HashMap; /// let mut squares = HashMap::new(); /// for i in -10..=10 { /// // This will not panic, since all keys are unique. /// squares.insert(i, i * i).unwrap_none(); /// } /// ``` /// /// ```{.should_panic} /// #![feature(option_unwrap_none)] /// /// use std::collections::HashMap; /// let mut sqrts = HashMap::new(); /// for i in -10..=10 { /// // This will panic, since both negative and positive `i` will /// // insert the same `i * i` key, returning the old `Some(i)`. /// sqrts.insert(i * i, i).unwrap_none(); /// } /// ``` #[inline] #[unstable(feature = "option_unwrap_none", reason = "newly added", issue = "62633")] pub fn unwrap_none(self) { if let Some(val) = self { expect_none_failed("called `Option::unwrap_none()` on a `Some` value", &val); } } } impl<T: Default> Option<T> { /// Returns the contained value or a default /// /// Consumes the `self` argument then, if [`Some`], returns the contained /// value, otherwise if [`None`], returns the [default value] for that /// type. /// /// # Examples /// /// Converts a string to an integer, turning poorly-formed strings /// into 0 (the default value for integers). [`parse`] converts /// a string to any other type that implements [`FromStr`], returning /// [`None`] on error. /// /// ``` /// let good_year_from_input = "1909"; /// let bad_year_from_input = "190blarg"; /// let good_year = good_year_from_input.parse().ok().unwrap_or_default(); /// let bad_year = bad_year_from_input.parse().ok().unwrap_or_default(); /// /// assert_eq!(1909, good_year); /// assert_eq!(0, bad_year); /// ``` /// /// [`Some`]: #variant.Some /// [`None`]: #variant.None /// [default value]: ../default/trait.Default.html#tymethod.default /// [`parse`]: ../../std/primitive.str.html#method.parse /// [`FromStr`]: ../../std/str/trait.FromStr.html #[inline] #[stable(feature = "rust1", since = "1.0.0")] pub fn unwrap_or_default(self) -> T { match self { Some(x) => x, None => Default::default(), } } } #[unstable(feature = "inner_deref", reason = "newly added", issue = "50264")] impl<T: Deref> Option<T> { /// Converts from `Option<T>` (or `&Option<T>`) to `Option<&T::Target>`. /// /// Leaves the original Option in-place, creating a new one with a reference /// to the original one, additionally coercing the contents via [`Deref`]. /// /// [`Deref`]: ../../std/ops/trait.Deref.html /// /// # Examples /// /// ``` /// #![feature(inner_deref)] /// /// let x: Option<String> = Some("hey".to_owned()); /// assert_eq!(x.as_deref(), Some("hey")); /// /// let x: Option<String> = None; /// assert_eq!(x.as_deref(), None); /// ``` pub fn as_deref(&self) -> Option<&T::Target> { self.as_ref().map(|t| t.deref()) } } #[unstable(feature = "inner_deref", reason = "newly added", issue = "50264")] impl<T: DerefMut> Option<T> { /// Converts from `Option<T>` (or `&mut Option<T>`) to `Option<&mut T::Target>`. /// /// Leaves the original `Option` in-place, creating a new one containing a mutable reference to /// the inner type's `Deref::Target` type. /// /// # Examples /// /// ``` /// #![feature(inner_deref)] /// /// let mut x: Option<String> = Some("hey".to_owned()); /// assert_eq!(x.as_deref_mut().map(|x| { /// x.make_ascii_uppercase(); /// x /// }), Some("HEY".to_owned().as_mut_str())); /// ``` pub fn as_deref_mut(&mut self) -> Option<&mut T::Target> { self.as_mut().map(|t| t.deref_mut()) } } impl<T, E> Option<Result<T, E>> { /// Transposes an `Option` of a [`Result`] into a [`Result`] of an `Option`. /// /// [`None`] will be mapped to [`Ok`]`(`[`None`]`)`. /// [`Some`]`(`[`Ok`]`(_))` and [`Some`]`(`[`Err`]`(_))` will be mapped to /// [`Ok`]`(`[`Some`]`(_))` and [`Err`]`(_)`. /// /// [`None`]: #variant.None /// [`Ok`]: ../../std/result/enum.Result.html#variant.Ok /// [`Some`]: #variant.Some /// [`Err`]: ../../std/result/enum.Result.html#variant.Err /// /// # Examples /// /// ``` /// #[derive(Debug, Eq, PartialEq)] /// struct SomeErr; /// /// let x: Result<Option<i32>, SomeErr> = Ok(Some(5)); /// let y: Option<Result<i32, SomeErr>> = Some(Ok(5)); /// assert_eq!(x, y.transpose()); /// ``` #[inline] #[stable(feature = "transpose_result", since = "1.33.0")] pub fn transpose(self) -> Result<Option<T>, E> { match self { Some(Ok(x)) => Ok(Some(x)), Some(Err(e)) => Err(e), None => Ok(None), } } } // This is a separate function to reduce the code size of .expect() itself. #[inline(never)] #[cold] fn expect_failed(msg: &str) -> ! { panic!("{}", msg) } // This is a separate function to reduce the code size of .expect_none() itself. #[inline(never)] #[cold] fn expect_none_failed(msg: &str, value: &dyn fmt::Debug) -> ! { panic!("{}: {:?}", msg, value) } ///////////////////////////////////////////////////////////////////////////// // Trait implementations ///////////////////////////////////////////////////////////////////////////// #[stable(feature = "rust1", since = "1.0.0")] impl<T: Clone> Clone for Option<T> { #[inline] fn clone(&self) -> Self { match self { Some(x) => Some(x.clone()), None => None, } } #[inline] fn clone_from(&mut self, source: &Self) { match (self, source) { (Some(to), Some(from)) => to.clone_from(from), (to, from) => *to = from.clone(), } } } #[stable(feature = "rust1", since = "1.0.0")] impl<T> Default for Option<T> { /// Returns [`None`][Option::None]. /// /// # Examples /// /// ``` /// let opt: Option<u32> = Option::default(); /// assert!(opt.is_none()); /// ``` #[inline] fn default() -> Option<T> { None } } #[stable(feature = "rust1", since = "1.0.0")] impl<T> IntoIterator for Option<T> { type Item = T; type IntoIter = IntoIter<T>; /// Returns a consuming iterator over the possibly contained value. /// /// # Examples /// /// ``` /// let x = Some("string"); /// let v: Vec<&str> = x.into_iter().collect(); /// assert_eq!(v, ["string"]); /// /// let x = None; /// let v: Vec<&str> = x.into_iter().collect(); /// assert!(v.is_empty()); /// ``` #[inline] fn into_iter(self) -> IntoIter<T> { IntoIter { inner: Item { opt: self } } } } #[stable(since = "1.4.0", feature = "option_iter")] impl<'a, T> IntoIterator for &'a Option<T> { type Item = &'a T; type IntoIter = Iter<'a, T>; fn into_iter(self) -> Iter<'a, T> { self.iter() } } #[stable(since = "1.4.0", feature = "option_iter")] impl<'a, T> IntoIterator for &'a mut Option<T> { type Item = &'a mut T; type IntoIter = IterMut<'a, T>; fn into_iter(self) -> IterMut<'a, T> { self.iter_mut() } } #[stable(since = "1.12.0", feature = "option_from")] impl<T> From<T> for Option<T> { fn from(val: T) -> Option<T> { Some(val) } } #[stable(feature = "option_ref_from_ref_option", since = "1.30.0")] impl<'a, T> From<&'a Option<T>> for Option<&'a T> { fn from(o: &'a Option<T>) -> Option<&'a T> { o.as_ref() } } #[stable(feature = "option_ref_from_ref_option", since = "1.30.0")] impl<'a, T> From<&'a mut Option<T>> for Option<&'a mut T> { fn from(o: &'a mut Option<T>) -> Option<&'a mut T> { o.as_mut() } } ///////////////////////////////////////////////////////////////////////////// // The Option Iterators ///////////////////////////////////////////////////////////////////////////// #[derive(Clone, Debug)] struct Item<A> { opt: Option<A> } impl<A> Iterator for Item<A> { type Item = A; #[inline] fn next(&mut self) -> Option<A> { self.opt.take() } #[inline] fn size_hint(&self) -> (usize, Option<usize>) { match self.opt { Some(_) => (1, Some(1)), None => (0, Some(0)), } } } impl<A> DoubleEndedIterator for Item<A> { #[inline] fn next_back(&mut self) -> Option<A> { self.opt.take() } } impl<A> ExactSizeIterator for Item<A> {} impl<A> FusedIterator for Item<A> {} unsafe impl<A> TrustedLen for Item<A> {} /// An iterator over a reference to the [`Some`] variant of an [`Option`]. /// /// The iterator yields one value if the [`Option`] is a [`Some`], otherwise none. /// /// This `struct` is created by the [`Option::iter`] function. /// /// [`Option`]: enum.Option.html /// [`Some`]: enum.Option.html#variant.Some /// [`Option::iter`]: enum.Option.html#method.iter #[stable(feature = "rust1", since = "1.0.0")] #[derive(Debug)] pub struct Iter<'a, A: 'a> { inner: Item<&'a A> } #[stable(feature = "rust1", since = "1.0.0")] impl<'a, A> Iterator for Iter<'a, A> { type Item = &'a A; #[inline] fn next(&mut self) -> Option<&'a A> { self.inner.next() } #[inline] fn size_hint(&self) -> (usize, Option<usize>) { self.inner.size_hint() } } #[stable(feature = "rust1", since = "1.0.0")] impl<'a, A> DoubleEndedIterator for Iter<'a, A> { #[inline] fn next_back(&mut self) -> Option<&'a A> { self.inner.next_back() } } #[stable(feature = "rust1", since = "1.0.0")] impl<A> ExactSizeIterator for Iter<'_, A> {} #[stable(feature = "fused", since = "1.26.0")] impl<A> FusedIterator for Iter<'_, A> {} #[unstable(feature = "trusted_len", issue = "37572")] unsafe impl<A> TrustedLen for Iter<'_, A> {} #[stable(feature = "rust1", since = "1.0.0")] impl<A> Clone for Iter<'_, A> { #[inline] fn clone(&self) -> Self { Iter { inner: self.inner.clone() } } } /// An iterator over a mutable reference to the [`Some`] variant of an [`Option`]. /// /// The iterator yields one value if the [`Option`] is a [`Some`], otherwise none. /// /// This `struct` is created by the [`Option::iter_mut`] function. /// /// [`Option`]: enum.Option.html /// [`Some`]: enum.Option.html#variant.Some /// [`Option::iter_mut`]: enum.Option.html#method.iter_mut #[stable(feature = "rust1", since = "1.0.0")] #[derive(Debug)] pub struct IterMut<'a, A: 'a> { inner: Item<&'a mut A> } #[stable(feature = "rust1", since = "1.0.0")] impl<'a, A> Iterator for IterMut<'a, A> { type Item = &'a mut A; #[inline] fn next(&mut self) -> Option<&'a mut A> { self.inner.next() } #[inline] fn size_hint(&self) -> (usize, Option<usize>) { self.inner.size_hint() } } #[stable(feature = "rust1", since = "1.0.0")] impl<'a, A> DoubleEndedIterator for IterMut<'a, A> { #[inline] fn next_back(&mut self) -> Option<&'a mut A> { self.inner.next_back() } } #[stable(feature = "rust1", since = "1.0.0")] impl<A> ExactSizeIterator for IterMut<'_, A> {} #[stable(feature = "fused", since = "1.26.0")] impl<A> FusedIterator for IterMut<'_, A> {} #[unstable(feature = "trusted_len", issue = "37572")] unsafe impl<A> TrustedLen for IterMut<'_, A> {} /// An iterator over the value in [`Some`] variant of an [`Option`]. /// /// The iterator yields one value if the [`Option`] is a [`Some`], otherwise none. /// /// This `struct` is created by the [`Option::into_iter`] function. /// /// [`Option`]: enum.Option.html /// [`Some`]: enum.Option.html#variant.Some /// [`Option::into_iter`]: enum.Option.html#method.into_iter #[derive(Clone, Debug)] #[stable(feature = "rust1", since = "1.0.0")] pub struct IntoIter<A> { inner: Item<A> } #[stable(feature = "rust1", since = "1.0.0")] impl<A> Iterator for IntoIter<A> { type Item = A; #[inline] fn next(&mut self) -> Option<A> { self.inner.next() } #[inline] fn size_hint(&self) -> (usize, Option<usize>) { self.inner.size_hint() } } #[stable(feature = "rust1", since = "1.0.0")] impl<A> DoubleEndedIterator for IntoIter<A> { #[inline] fn next_back(&mut self) -> Option<A> { self.inner.next_back() } } #[stable(feature = "rust1", since = "1.0.0")] impl<A> ExactSizeIterator for IntoIter<A> {} #[stable(feature = "fused", since = "1.26.0")] impl<A> FusedIterator for IntoIter<A> {} #[unstable(feature = "trusted_len", issue = "37572")] unsafe impl<A> TrustedLen for IntoIter<A> {} ///////////////////////////////////////////////////////////////////////////// // FromIterator ///////////////////////////////////////////////////////////////////////////// #[stable(feature = "rust1", since = "1.0.0")] impl<A, V: FromIterator<A>> FromIterator<Option<A>> for Option<V> { /// Takes each element in the [`Iterator`]: if it is [`None`][Option::None], /// no further elements are taken, and the [`None`][Option::None] is /// returned. Should no [`None`][Option::None] occur, a container with the /// values of each [`Option`] is returned. /// /// # Examples /// /// Here is an example which increments every integer in a vector. /// We use the checked variant of `add` that returns `None` when the /// calculation would result in an overflow. /// /// ``` /// let items = vec![0_u16, 1, 2]; /// /// let res: Option<Vec<u16>> = items /// .iter() /// .map(|x| x.checked_add(1)) /// .collect(); /// /// assert_eq!(res, Some(vec![1, 2, 3])); /// ``` /// /// As you can see, this will return the expected, valid items. /// /// Here is another example that tries to subtract one from another list /// of integers, this time checking for underflow: /// /// ``` /// let items = vec![2_u16, 1, 0]; /// /// let res: Option<Vec<u16>> = items /// .iter() /// .map(|x| x.checked_sub(1)) /// .collect(); /// /// assert_eq!(res, None); /// ``` /// /// Since the last element is zero, it would underflow. Thus, the resulting /// value is `None`. /// /// Here is a variation on the previous example, showing that no /// further elements are taken from `iter` after the first `None`. /// /// ``` /// let items = vec![3_u16, 2, 1, 10]; /// /// let mut shared = 0; /// /// let res: Option<Vec<u16>> = items /// .iter() /// .map(|x| { shared += x; x.checked_sub(2) }) /// .collect(); /// /// assert_eq!(res, None); /// assert_eq!(shared, 6); /// ``` /// /// Since the third element caused an underflow, no further elements were taken, /// so the final value of `shared` is 6 (= `3 + 2 + 1`), not 16. /// /// [`Iterator`]: ../iter/trait.Iterator.html #[inline] fn from_iter<I: IntoIterator<Item=Option<A>>>(iter: I) -> Option<V> { // FIXME(#11084): This could be replaced with Iterator::scan when this // performance bug is closed. iter.into_iter() .map(|x| x.ok_or(())) .collect::<Result<_, _>>() .ok() } } /// The error type that results from applying the try operator (`?`) to a `None` value. If you wish /// to allow `x?` (where `x` is an `Option<T>`) to be converted into your error type, you can /// implement `impl From<NoneError>` for `YourErrorType`. In that case, `x?` within a function that /// returns `Result<_, YourErrorType>` will translate a `None` value into an `Err` result. #[unstable(feature = "try_trait", issue = "42327")] #[derive(Clone, Copy, PartialEq, PartialOrd, Eq, Ord, Debug, Hash)] pub struct NoneError; #[unstable(feature = "try_trait", issue = "42327")] impl<T> ops::Try for Option<T> { type Ok = T; type Error = NoneError; #[inline] fn into_result(self) -> Result<T, NoneError> { self.ok_or(NoneError) } #[inline] fn from_ok(v: T) -> Self { Some(v) } #[inline] fn from_error(_: NoneError) -> Self { None } } impl<T> Option<Option<T>> { /// Converts from `Option<Option<T>>` to `Option<T>` /// /// # Examples /// Basic usage: /// ``` /// #![feature(option_flattening)] /// let x: Option<Option<u32>> = Some(Some(6)); /// assert_eq!(Some(6), x.flatten()); /// /// let x: Option<Option<u32>> = Some(None); /// assert_eq!(None, x.flatten()); /// /// let x: Option<Option<u32>> = None; /// assert_eq!(None, x.flatten()); /// ``` /// Flattening once only removes one level of nesting: /// ``` /// #![feature(option_flattening)] /// let x: Option<Option<Option<u32>>> = Some(Some(Some(6))); /// assert_eq!(Some(Some(6)), x.flatten()); /// assert_eq!(Some(6), x.flatten().flatten()); /// ``` #[inline] #[unstable(feature = "option_flattening", issue = "60258")] pub fn flatten(self) -> Option<T> { self.and_then(convert::identity) } }