| //! 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" references. Instead, Rust has *optional* pointers, like |
| //! the optional owned box, <code>[Option]<[Box\<T>]></code>. |
| //! |
| //! [Box\<T>]: ../../std/boxed/struct.Box.html |
| //! |
| //! The following example uses [`Option`] to create an optional box of |
| //! [`i32`]. Notice that in order to use the inner [`i32`] value, the |
| //! `check_optional` function first 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(p) => println!("has value {p}"), |
| //! None => println!("has no value"), |
| //! } |
| //! } |
| //! ``` |
| //! |
| //! # The question mark operator, `?` |
| //! |
| //! Similar to the [`Result`] type, when writing code that calls many functions that return the |
| //! [`Option`] type, handling `Some`/`None` can be tedious. The question mark |
| //! operator, [`?`], hides some of the boilerplate of propagating values |
| //! up the call stack. |
| //! |
| //! It replaces this: |
| //! |
| //! ``` |
| //! # #![allow(dead_code)] |
| //! fn add_last_numbers(stack: &mut Vec<i32>) -> Option<i32> { |
| //! let a = stack.pop(); |
| //! let b = stack.pop(); |
| //! |
| //! match (a, b) { |
| //! (Some(x), Some(y)) => Some(x + y), |
| //! _ => None, |
| //! } |
| //! } |
| //! |
| //! ``` |
| //! |
| //! With this: |
| //! |
| //! ``` |
| //! # #![allow(dead_code)] |
| //! fn add_last_numbers(stack: &mut Vec<i32>) -> Option<i32> { |
| //! Some(stack.pop()? + stack.pop()?) |
| //! } |
| //! ``` |
| //! |
| //! *It's much nicer!* |
| //! |
| //! Ending the expression with [`?`] will result in the [`Some`]'s unwrapped value, unless the |
| //! result is [`None`], in which case [`None`] is returned early from the enclosing function. |
| //! |
| //! [`?`] can be used in functions that return [`Option`] because of the |
| //! early return of [`None`] that it provides. |
| //! |
| //! [`?`]: crate::ops::Try |
| //! [`Some`]: Some |
| //! [`None`]: None |
| //! |
| //! # Representation |
| //! |
| //! Rust guarantees to optimize the following types `T` such that |
| //! [`Option<T>`] has the same size as `T`: |
| //! |
| //! * [`Box<U>`] |
| //! * `&U` |
| //! * `&mut U` |
| //! * `fn`, `extern "C" fn`[^extern_fn] |
| //! * [`num::NonZero*`] |
| //! * [`ptr::NonNull<U>`] |
| //! * `#[repr(transparent)]` struct around one of the types in this list. |
| //! |
| //! [^extern_fn]: this remains true for any other ABI: `extern "abi" fn` (_e.g._, `extern "system" fn`) |
| //! |
| //! [`Box<U>`]: ../../std/boxed/struct.Box.html |
| //! [`num::NonZero*`]: crate::num |
| //! [`ptr::NonNull<U>`]: crate::ptr::NonNull |
| //! |
| //! This is called the "null pointer optimization" or NPO. |
| //! |
| //! It is further guaranteed that, for the cases above, one can |
| //! [`mem::transmute`] from all valid values of `T` to `Option<T>` and |
| //! from `Some::<T>(_)` to `T` (but transmuting `None::<T>` to `T` |
| //! is undefined behaviour). |
| //! |
| //! # Method overview |
| //! |
| //! In addition to working with pattern matching, [`Option`] provides a wide |
| //! variety of different methods. |
| //! |
| //! ## Querying the variant |
| //! |
| //! The [`is_some`] and [`is_none`] methods return [`true`] if the [`Option`] |
| //! is [`Some`] or [`None`], respectively. |
| //! |
| //! [`is_none`]: Option::is_none |
| //! [`is_some`]: Option::is_some |
| //! |
| //! ## Adapters for working with references |
| //! |
| //! * [`as_ref`] converts from <code>[&][][Option]\<T></code> to <code>[Option]<[&]T></code> |
| //! * [`as_mut`] converts from <code>[&mut] [Option]\<T></code> to <code>[Option]<[&mut] T></code> |
| //! * [`as_deref`] converts from <code>[&][][Option]\<T></code> to |
| //! <code>[Option]<[&]T::[Target]></code> |
| //! * [`as_deref_mut`] converts from <code>[&mut] [Option]\<T></code> to |
| //! <code>[Option]<[&mut] T::[Target]></code> |
| //! * [`as_pin_ref`] converts from <code>[Pin]<[&][][Option]\<T>></code> to |
| //! <code>[Option]<[Pin]<[&]T>></code> |
| //! * [`as_pin_mut`] converts from <code>[Pin]<[&mut] [Option]\<T>></code> to |
| //! <code>[Option]<[Pin]<[&mut] T>></code> |
| //! |
| //! [&]: reference "shared reference" |
| //! [&mut]: reference "mutable reference" |
| //! [Target]: Deref::Target "ops::Deref::Target" |
| //! [`as_deref`]: Option::as_deref |
| //! [`as_deref_mut`]: Option::as_deref_mut |
| //! [`as_mut`]: Option::as_mut |
| //! [`as_pin_mut`]: Option::as_pin_mut |
| //! [`as_pin_ref`]: Option::as_pin_ref |
| //! [`as_ref`]: Option::as_ref |
| //! |
| //! ## Extracting the contained value |
| //! |
| //! These methods extract the contained value in an [`Option<T>`] when it |
| //! is the [`Some`] variant. If the [`Option`] is [`None`]: |
| //! |
| //! * [`expect`] panics with a provided custom message |
| //! * [`unwrap`] panics with a generic message |
| //! * [`unwrap_or`] returns the provided default value |
| //! * [`unwrap_or_default`] returns the default value of the type `T` |
| //! (which must implement the [`Default`] trait) |
| //! * [`unwrap_or_else`] returns the result of evaluating the provided |
| //! function |
| //! |
| //! [`expect`]: Option::expect |
| //! [`unwrap`]: Option::unwrap |
| //! [`unwrap_or`]: Option::unwrap_or |
| //! [`unwrap_or_default`]: Option::unwrap_or_default |
| //! [`unwrap_or_else`]: Option::unwrap_or_else |
| //! |
| //! ## Transforming contained values |
| //! |
| //! These methods transform [`Option`] to [`Result`]: |
| //! |
| //! * [`ok_or`] transforms [`Some(v)`] to [`Ok(v)`], and [`None`] to |
| //! [`Err(err)`] using the provided default `err` value |
| //! * [`ok_or_else`] transforms [`Some(v)`] to [`Ok(v)`], and [`None`] to |
| //! a value of [`Err`] using the provided function |
| //! * [`transpose`] transposes an [`Option`] of a [`Result`] into a |
| //! [`Result`] of an [`Option`] |
| //! |
| //! [`Err(err)`]: Err |
| //! [`Ok(v)`]: Ok |
| //! [`Some(v)`]: Some |
| //! [`ok_or`]: Option::ok_or |
| //! [`ok_or_else`]: Option::ok_or_else |
| //! [`transpose`]: Option::transpose |
| //! |
| //! These methods transform the [`Some`] variant: |
| //! |
| //! * [`filter`] calls the provided predicate function on the contained |
| //! value `t` if the [`Option`] is [`Some(t)`], and returns [`Some(t)`] |
| //! if the function returns `true`; otherwise, returns [`None`] |
| //! * [`flatten`] removes one level of nesting from an |
| //! [`Option<Option<T>>`] |
| //! * [`map`] transforms [`Option<T>`] to [`Option<U>`] by applying the |
| //! provided function to the contained value of [`Some`] and leaving |
| //! [`None`] values unchanged |
| //! |
| //! [`Some(t)`]: Some |
| //! [`filter`]: Option::filter |
| //! [`flatten`]: Option::flatten |
| //! [`map`]: Option::map |
| //! |
| //! These methods transform [`Option<T>`] to a value of a possibly |
| //! different type `U`: |
| //! |
| //! * [`map_or`] applies the provided function to the contained value of |
| //! [`Some`], or returns the provided default value if the [`Option`] is |
| //! [`None`] |
| //! * [`map_or_else`] applies the provided function to the contained value |
| //! of [`Some`], or returns the result of evaluating the provided |
| //! fallback function if the [`Option`] is [`None`] |
| //! |
| //! [`map_or`]: Option::map_or |
| //! [`map_or_else`]: Option::map_or_else |
| //! |
| //! These methods combine the [`Some`] variants of two [`Option`] values: |
| //! |
| //! * [`zip`] returns [`Some((s, o))`] if `self` is [`Some(s)`] and the |
| //! provided [`Option`] value is [`Some(o)`]; otherwise, returns [`None`] |
| //! * [`zip_with`] calls the provided function `f` and returns |
| //! [`Some(f(s, o))`] if `self` is [`Some(s)`] and the provided |
| //! [`Option`] value is [`Some(o)`]; otherwise, returns [`None`] |
| //! |
| //! [`Some(f(s, o))`]: Some |
| //! [`Some(o)`]: Some |
| //! [`Some(s)`]: Some |
| //! [`Some((s, o))`]: Some |
| //! [`zip`]: Option::zip |
| //! [`zip_with`]: Option::zip_with |
| //! |
| //! ## Boolean operators |
| //! |
| //! These methods treat the [`Option`] as a boolean value, where [`Some`] |
| //! acts like [`true`] and [`None`] acts like [`false`]. There are two |
| //! categories of these methods: ones that take an [`Option`] as input, and |
| //! ones that take a function as input (to be lazily evaluated). |
| //! |
| //! The [`and`], [`or`], and [`xor`] methods take another [`Option`] as |
| //! input, and produce an [`Option`] as output. Only the [`and`] method can |
| //! produce an [`Option<U>`] value having a different inner type `U` than |
| //! [`Option<T>`]. |
| //! |
| //! | method | self | input | output | |
| //! |---------|-----------|-----------|-----------| |
| //! | [`and`] | `None` | (ignored) | `None` | |
| //! | [`and`] | `Some(x)` | `None` | `None` | |
| //! | [`and`] | `Some(x)` | `Some(y)` | `Some(y)` | |
| //! | [`or`] | `None` | `None` | `None` | |
| //! | [`or`] | `None` | `Some(y)` | `Some(y)` | |
| //! | [`or`] | `Some(x)` | (ignored) | `Some(x)` | |
| //! | [`xor`] | `None` | `None` | `None` | |
| //! | [`xor`] | `None` | `Some(y)` | `Some(y)` | |
| //! | [`xor`] | `Some(x)` | `None` | `Some(x)` | |
| //! | [`xor`] | `Some(x)` | `Some(y)` | `None` | |
| //! |
| //! [`and`]: Option::and |
| //! [`or`]: Option::or |
| //! [`xor`]: Option::xor |
| //! |
| //! The [`and_then`] and [`or_else`] methods take a function as input, and |
| //! only evaluate the function when they need to produce a new value. Only |
| //! the [`and_then`] method can produce an [`Option<U>`] value having a |
| //! different inner type `U` than [`Option<T>`]. |
| //! |
| //! | method | self | function input | function result | output | |
| //! |--------------|-----------|----------------|-----------------|-----------| |
| //! | [`and_then`] | `None` | (not provided) | (not evaluated) | `None` | |
| //! | [`and_then`] | `Some(x)` | `x` | `None` | `None` | |
| //! | [`and_then`] | `Some(x)` | `x` | `Some(y)` | `Some(y)` | |
| //! | [`or_else`] | `None` | (not provided) | `None` | `None` | |
| //! | [`or_else`] | `None` | (not provided) | `Some(y)` | `Some(y)` | |
| //! | [`or_else`] | `Some(x)` | (not provided) | (not evaluated) | `Some(x)` | |
| //! |
| //! [`and_then`]: Option::and_then |
| //! [`or_else`]: Option::or_else |
| //! |
| //! This is an example of using methods like [`and_then`] and [`or`] in a |
| //! pipeline of method calls. Early stages of the pipeline pass failure |
| //! values ([`None`]) through unchanged, and continue processing on |
| //! success values ([`Some`]). Toward the end, [`or`] substitutes an error |
| //! message if it receives [`None`]. |
| //! |
| //! ``` |
| //! # use std::collections::BTreeMap; |
| //! let mut bt = BTreeMap::new(); |
| //! bt.insert(20u8, "foo"); |
| //! bt.insert(42u8, "bar"); |
| //! let res = [0u8, 1, 11, 200, 22] |
| //! .into_iter() |
| //! .map(|x| { |
| //! // `checked_sub()` returns `None` on error |
| //! x.checked_sub(1) |
| //! // same with `checked_mul()` |
| //! .and_then(|x| x.checked_mul(2)) |
| //! // `BTreeMap::get` returns `None` on error |
| //! .and_then(|x| bt.get(&x)) |
| //! // Substitute an error message if we have `None` so far |
| //! .or(Some(&"error!")) |
| //! .copied() |
| //! // Won't panic because we unconditionally used `Some` above |
| //! .unwrap() |
| //! }) |
| //! .collect::<Vec<_>>(); |
| //! assert_eq!(res, ["error!", "error!", "foo", "error!", "bar"]); |
| //! ``` |
| //! |
| //! ## Comparison operators |
| //! |
| //! If `T` implements [`PartialOrd`] then [`Option<T>`] will derive its |
| //! [`PartialOrd`] implementation. With this order, [`None`] compares as |
| //! less than any [`Some`], and two [`Some`] compare the same way as their |
| //! contained values would in `T`. If `T` also implements |
| //! [`Ord`], then so does [`Option<T>`]. |
| //! |
| //! ``` |
| //! assert!(None < Some(0)); |
| //! assert!(Some(0) < Some(1)); |
| //! ``` |
| //! |
| //! ## Iterating over `Option` |
| //! |
| //! An [`Option`] can be iterated over. This can be helpful if you need an |
| //! iterator that is conditionally empty. The iterator will either produce |
| //! a single value (when the [`Option`] is [`Some`]), or produce no values |
| //! (when the [`Option`] is [`None`]). For example, [`into_iter`] acts like |
| //! [`once(v)`] if the [`Option`] is [`Some(v)`], and like [`empty()`] if |
| //! the [`Option`] is [`None`]. |
| //! |
| //! [`Some(v)`]: Some |
| //! [`empty()`]: crate::iter::empty |
| //! [`once(v)`]: crate::iter::once |
| //! |
| //! Iterators over [`Option<T>`] come in three types: |
| //! |
| //! * [`into_iter`] consumes the [`Option`] and produces the contained |
| //! value |
| //! * [`iter`] produces an immutable reference of type `&T` to the |
| //! contained value |
| //! * [`iter_mut`] produces a mutable reference of type `&mut T` to the |
| //! contained value |
| //! |
| //! [`into_iter`]: Option::into_iter |
| //! [`iter`]: Option::iter |
| //! [`iter_mut`]: Option::iter_mut |
| //! |
| //! An iterator over [`Option`] can be useful when chaining iterators, for |
| //! example, to conditionally insert items. (It's not always necessary to |
| //! explicitly call an iterator constructor: many [`Iterator`] methods that |
| //! accept other iterators will also accept iterable types that implement |
| //! [`IntoIterator`], which includes [`Option`].) |
| //! |
| //! ``` |
| //! let yep = Some(42); |
| //! let nope = None; |
| //! // chain() already calls into_iter(), so we don't have to do so |
| //! let nums: Vec<i32> = (0..4).chain(yep).chain(4..8).collect(); |
| //! assert_eq!(nums, [0, 1, 2, 3, 42, 4, 5, 6, 7]); |
| //! let nums: Vec<i32> = (0..4).chain(nope).chain(4..8).collect(); |
| //! assert_eq!(nums, [0, 1, 2, 3, 4, 5, 6, 7]); |
| //! ``` |
| //! |
| //! One reason to chain iterators in this way is that a function returning |
| //! `impl Iterator` must have all possible return values be of the same |
| //! concrete type. Chaining an iterated [`Option`] can help with that. |
| //! |
| //! ``` |
| //! fn make_iter(do_insert: bool) -> impl Iterator<Item = i32> { |
| //! // Explicit returns to illustrate return types matching |
| //! match do_insert { |
| //! true => return (0..4).chain(Some(42)).chain(4..8), |
| //! false => return (0..4).chain(None).chain(4..8), |
| //! } |
| //! } |
| //! println!("{:?}", make_iter(true).collect::<Vec<_>>()); |
| //! println!("{:?}", make_iter(false).collect::<Vec<_>>()); |
| //! ``` |
| //! |
| //! If we try to do the same thing, but using [`once()`] and [`empty()`], |
| //! we can't return `impl Iterator` anymore because the concrete types of |
| //! the return values differ. |
| //! |
| //! [`empty()`]: crate::iter::empty |
| //! [`once()`]: crate::iter::once |
| //! |
| //! ```compile_fail,E0308 |
| //! # use std::iter::{empty, once}; |
| //! // This won't compile because all possible returns from the function |
| //! // must have the same concrete type. |
| //! fn make_iter(do_insert: bool) -> impl Iterator<Item = i32> { |
| //! // Explicit returns to illustrate return types not matching |
| //! match do_insert { |
| //! true => return (0..4).chain(once(42)).chain(4..8), |
| //! false => return (0..4).chain(empty()).chain(4..8), |
| //! } |
| //! } |
| //! ``` |
| //! |
| //! ## Collecting into `Option` |
| //! |
| //! [`Option`] implements the [`FromIterator`][impl-FromIterator] trait, |
| //! which allows an iterator over [`Option`] values to be collected into an |
| //! [`Option`] of a collection of each contained value of the original |
| //! [`Option`] values, or [`None`] if any of the elements was [`None`]. |
| //! |
| //! [impl-FromIterator]: Option#impl-FromIterator%3COption%3CA%3E%3E-for-Option%3CV%3E |
| //! |
| //! ``` |
| //! let v = [Some(2), Some(4), None, Some(8)]; |
| //! let res: Option<Vec<_>> = v.into_iter().collect(); |
| //! assert_eq!(res, None); |
| //! let v = [Some(2), Some(4), Some(8)]; |
| //! let res: Option<Vec<_>> = v.into_iter().collect(); |
| //! assert_eq!(res, Some(vec![2, 4, 8])); |
| //! ``` |
| //! |
| //! [`Option`] also implements the [`Product`][impl-Product] and |
| //! [`Sum`][impl-Sum] traits, allowing an iterator over [`Option`] values |
| //! to provide the [`product`][Iterator::product] and |
| //! [`sum`][Iterator::sum] methods. |
| //! |
| //! [impl-Product]: Option#impl-Product%3COption%3CU%3E%3E-for-Option%3CT%3E |
| //! [impl-Sum]: Option#impl-Sum%3COption%3CU%3E%3E-for-Option%3CT%3E |
| //! |
| //! ``` |
| //! let v = [None, Some(1), Some(2), Some(3)]; |
| //! let res: Option<i32> = v.into_iter().sum(); |
| //! assert_eq!(res, None); |
| //! let v = [Some(1), Some(2), Some(21)]; |
| //! let res: Option<i32> = v.into_iter().product(); |
| //! assert_eq!(res, Some(42)); |
| //! ``` |
| //! |
| //! ## Modifying an [`Option`] in-place |
| //! |
| //! These methods return a mutable reference to the contained value of an |
| //! [`Option<T>`]: |
| //! |
| //! * [`insert`] inserts a value, dropping any old contents |
| //! * [`get_or_insert`] gets the current value, inserting a provided |
| //! default value if it is [`None`] |
| //! * [`get_or_insert_default`] gets the current value, inserting the |
| //! default value of type `T` (which must implement [`Default`]) if it is |
| //! [`None`] |
| //! * [`get_or_insert_with`] gets the current value, inserting a default |
| //! computed by the provided function if it is [`None`] |
| //! |
| //! [`get_or_insert`]: Option::get_or_insert |
| //! [`get_or_insert_default`]: Option::get_or_insert_default |
| //! [`get_or_insert_with`]: Option::get_or_insert_with |
| //! [`insert`]: Option::insert |
| //! |
| //! These methods transfer ownership of the contained value of an |
| //! [`Option`]: |
| //! |
| //! * [`take`] takes ownership of the contained value of an [`Option`], if |
| //! any, replacing the [`Option`] with [`None`] |
| //! * [`replace`] takes ownership of the contained value of an [`Option`], |
| //! if any, replacing the [`Option`] with a [`Some`] containing the |
| //! provided value |
| //! |
| //! [`replace`]: Option::replace |
| //! [`take`]: Option::take |
| //! |
| //! # Examples |
| //! |
| //! Basic pattern matching on [`Option`]: |
| //! |
| //! ``` |
| //! let msg = Some("howdy"); |
| //! |
| //! // Take a reference to the contained string |
| //! if let Some(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 :("), |
| //! } |
| //! ``` |
| |
| #![stable(feature = "rust1", since = "1.0.0")] |
| |
| use crate::iter::{self, FromIterator, FusedIterator, TrustedLen}; |
| use crate::marker::Destruct; |
| use crate::panicking::{panic, panic_str}; |
| use crate::pin::Pin; |
| use crate::{ |
| cmp, convert, hint, mem, |
| ops::{self, ControlFlow, Deref, DerefMut}, |
| slice, |
| }; |
| |
| /// The `Option` type. See [the module level documentation](self) for more. |
| #[derive(Copy, PartialOrd, Eq, Ord, Debug, Hash)] |
| #[rustc_diagnostic_item = "Option"] |
| #[stable(feature = "rust1", since = "1.0.0")] |
| pub enum Option<T> { |
| /// No value. |
| #[lang = "None"] |
| #[stable(feature = "rust1", since = "1.0.0")] |
| None, |
| /// Some value of type `T`. |
| #[lang = "Some"] |
| #[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); |
| /// ``` |
| #[must_use = "if you intended to assert that this has a value, consider `.unwrap()` instead"] |
| #[inline] |
| #[stable(feature = "rust1", since = "1.0.0")] |
| #[rustc_const_stable(feature = "const_option_basics", since = "1.48.0")] |
| pub const fn is_some(&self) -> bool { |
| matches!(*self, Some(_)) |
| } |
| |
| /// Returns `true` if the option is a [`Some`] and the value inside of it matches a predicate. |
| /// |
| /// # Examples |
| /// |
| /// ``` |
| /// #![feature(is_some_and)] |
| /// |
| /// let x: Option<u32> = Some(2); |
| /// assert_eq!(x.is_some_and(|x| x > 1), true); |
| /// |
| /// let x: Option<u32> = Some(0); |
| /// assert_eq!(x.is_some_and(|x| x > 1), false); |
| /// |
| /// let x: Option<u32> = None; |
| /// assert_eq!(x.is_some_and(|x| x > 1), false); |
| /// ``` |
| #[must_use] |
| #[inline] |
| #[unstable(feature = "is_some_and", issue = "93050")] |
| pub fn is_some_and(self, f: impl FnOnce(T) -> bool) -> bool { |
| match self { |
| None => false, |
| Some(x) => f(x), |
| } |
| } |
| |
| /// 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); |
| /// ``` |
| #[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")] |
| #[rustc_const_stable(feature = "const_option_basics", since = "1.48.0")] |
| pub const fn is_none(&self) -> bool { |
| !self.is_some() |
| } |
| |
| ///////////////////////////////////////////////////////////////////////// |
| // Adapter for working with references |
| ///////////////////////////////////////////////////////////////////////// |
| |
| /// Converts from `&Option<T>` to `Option<&T>`. |
| /// |
| /// # Examples |
| /// |
| /// Calculates the length of an <code>Option<[String]></code> as an <code>Option<[usize]></code> |
| /// without moving the [`String`]. 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`]: Option::map |
| /// [String]: ../../std/string/struct.String.html "String" |
| /// [`String`]: ../../std/string/struct.String.html "String" |
| /// |
| /// ``` |
| /// 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] |
| #[rustc_const_stable(feature = "const_option_basics", since = "1.48.0")] |
| #[stable(feature = "rust1", since = "1.0.0")] |
| pub const 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")] |
| #[rustc_const_unstable(feature = "const_option", issue = "67441")] |
| pub const fn as_mut(&mut self) -> Option<&mut T> { |
| match *self { |
| Some(ref mut x) => Some(x), |
| None => None, |
| } |
| } |
| |
| /// Converts from <code>[Pin]<[&]Option\<T>></code> to <code>Option<[Pin]<[&]T>></code>. |
| /// |
| /// [&]: reference "shared reference" |
| #[inline] |
| #[must_use] |
| #[stable(feature = "pin", since = "1.33.0")] |
| #[rustc_const_unstable(feature = "const_option_ext", issue = "91930")] |
| pub const fn as_pin_ref(self: Pin<&Self>) -> Option<Pin<&T>> { |
| match Pin::get_ref(self).as_ref() { |
| // SAFETY: `x` is guaranteed to be pinned because it comes from `self` |
| // which is pinned. |
| Some(x) => unsafe { Some(Pin::new_unchecked(x)) }, |
| None => None, |
| } |
| } |
| |
| /// Converts from <code>[Pin]<[&mut] Option\<T>></code> to <code>Option<[Pin]<[&mut] T>></code>. |
| /// |
| /// [&mut]: reference "mutable reference" |
| #[inline] |
| #[must_use] |
| #[stable(feature = "pin", since = "1.33.0")] |
| #[rustc_const_unstable(feature = "const_option_ext", issue = "91930")] |
| pub const fn as_pin_mut(self: Pin<&mut Self>) -> Option<Pin<&mut T>> { |
| // SAFETY: `get_unchecked_mut` is never used to move the `Option` inside `self`. |
| // `x` is guaranteed to be pinned because it comes from `self` which is pinned. |
| unsafe { |
| match Pin::get_unchecked_mut(self).as_mut() { |
| Some(x) => Some(Pin::new_unchecked(x)), |
| None => None, |
| } |
| } |
| } |
| |
| const fn get_some_offset() -> isize { |
| if mem::size_of::<Option<T>>() == mem::size_of::<T>() { |
| // niche optimization means the `T` is always stored at the same position as the Option. |
| 0 |
| } else { |
| assert!(mem::size_of::<Option<T>>() == mem::size_of::<Option<mem::MaybeUninit<T>>>()); |
| let some_uninit = Some(mem::MaybeUninit::<T>::uninit()); |
| // SAFETY: This gets the byte offset of the `Some(_)` value following the fact that |
| // niche optimization is not active, and thus Option<T> and Option<MaybeUninit<t>> share |
| // the same layout. |
| unsafe { |
| (some_uninit.as_ref().unwrap() as *const mem::MaybeUninit<T>) |
| .byte_offset_from(&some_uninit as *const Option<mem::MaybeUninit<T>>) |
| } |
| } |
| } |
| |
| /// Returns a slice of the contained value, if any. If this is `None`, an |
| /// empty slice is returned. This can be useful to have a single type of |
| /// iterator over an `Option` or slice. |
| /// |
| /// Note: Should you have an `Option<&T>` and wish to get a slice of `T`, |
| /// you can unpack it via `opt.map_or(&[], std::slice::from_ref)`. |
| /// |
| /// # Examples |
| /// |
| /// ```rust |
| /// #![feature(option_as_slice)] |
| /// |
| /// assert_eq!( |
| /// [Some(1234).as_slice(), None.as_slice()], |
| /// [&[1234][..], &[][..]], |
| /// ); |
| /// ``` |
| /// |
| /// The inverse of this function is (discounting |
| /// borrowing) [`[_]::first`](slice::first): |
| /// |
| /// ```rust |
| /// #![feature(option_as_slice)] |
| /// |
| /// for i in [Some(1234_u16), None] { |
| /// assert_eq!(i.as_ref(), i.as_slice().first()); |
| /// } |
| /// ``` |
| #[inline] |
| #[must_use] |
| #[unstable(feature = "option_as_slice", issue = "108545")] |
| pub fn as_slice(&self) -> &[T] { |
| // SAFETY: This is sound as long as `get_some_offset` returns the |
| // correct offset. Though in the `None` case, the slice may be located |
| // at a pointer pointing into padding, the fact that the slice is |
| // empty, and the padding is at a properly aligned position for a |
| // value of that type makes it sound. |
| unsafe { |
| slice::from_raw_parts( |
| (self as *const Option<T>).wrapping_byte_offset(Self::get_some_offset()) |
| as *const T, |
| self.is_some() as usize, |
| ) |
| } |
| } |
| |
| /// Returns a mutable slice of the contained value, if any. If this is |
| /// `None`, an empty slice is returned. This can be useful to have a |
| /// single type of iterator over an `Option` or slice. |
| /// |
| /// Note: Should you have an `Option<&mut T>` instead of a |
| /// `&mut Option<T>`, which this method takes, you can obtain a mutable |
| /// slice via `opt.map_or(&mut [], std::slice::from_mut)`. |
| /// |
| /// # Examples |
| /// |
| /// ```rust |
| /// #![feature(option_as_slice)] |
| /// |
| /// assert_eq!( |
| /// [Some(1234).as_mut_slice(), None.as_mut_slice()], |
| /// [&mut [1234][..], &mut [][..]], |
| /// ); |
| /// ``` |
| /// |
| /// The result is a mutable slice of zero or one items that points into |
| /// our original `Option`: |
| /// |
| /// ```rust |
| /// #![feature(option_as_slice)] |
| /// |
| /// let mut x = Some(1234); |
| /// x.as_mut_slice()[0] += 1; |
| /// assert_eq!(x, Some(1235)); |
| /// ``` |
| /// |
| /// The inverse of this method (discounting borrowing) |
| /// is [`[_]::first_mut`](slice::first_mut): |
| /// |
| /// ```rust |
| /// #![feature(option_as_slice)] |
| /// |
| /// assert_eq!(Some(123).as_mut_slice().first_mut(), Some(&mut 123)) |
| /// ``` |
| #[inline] |
| #[must_use] |
| #[unstable(feature = "option_as_slice", issue = "108545")] |
| pub fn as_mut_slice(&mut self) -> &mut [T] { |
| // SAFETY: This is sound as long as `get_some_offset` returns the |
| // correct offset. Though in the `None` case, the slice may be located |
| // at a pointer pointing into padding, the fact that the slice is |
| // empty, and the padding is at a properly aligned position for a |
| // value of that type makes it sound. |
| unsafe { |
| slice::from_raw_parts_mut( |
| (self as *mut Option<T>).wrapping_byte_offset(Self::get_some_offset()) as *mut T, |
| self.is_some() as usize, |
| ) |
| } |
| } |
| |
| ///////////////////////////////////////////////////////////////////////// |
| // Getting to contained values |
| ///////////////////////////////////////////////////////////////////////// |
| |
| /// Returns the contained [`Some`] value, consuming the `self` value. |
| /// |
| /// # Panics |
| /// |
| /// Panics if the value is a [`None`] with a custom panic message provided by |
| /// `msg`. |
| /// |
| /// # Examples |
| /// |
| /// ``` |
| /// let x = Some("value"); |
| /// assert_eq!(x.expect("fruits are healthy"), "value"); |
| /// ``` |
| /// |
| /// ```should_panic |
| /// let x: Option<&str> = None; |
| /// x.expect("fruits are healthy"); // panics with `fruits are healthy` |
| /// ``` |
| /// |
| /// # Recommended Message Style |
| /// |
| /// We recommend that `expect` messages are used to describe the reason you |
| /// _expect_ the `Option` should be `Some`. |
| /// |
| /// ```should_panic |
| /// # let slice: &[u8] = &[]; |
| /// let item = slice.get(0) |
| /// .expect("slice should not be empty"); |
| /// ``` |
| /// |
| /// **Hint**: If you're having trouble remembering how to phrase expect |
| /// error messages remember to focus on the word "should" as in "env |
| /// variable should be set by blah" or "the given binary should be available |
| /// and executable by the current user". |
| /// |
| /// For more detail on expect message styles and the reasoning behind our |
| /// recommendation please refer to the section on ["Common Message |
| /// Styles"](../../std/error/index.html#common-message-styles) in the [`std::error`](../../std/error/index.html) module docs. |
| #[inline] |
| #[track_caller] |
| #[stable(feature = "rust1", since = "1.0.0")] |
| #[rustc_const_unstable(feature = "const_option", issue = "67441")] |
| pub const fn expect(self, msg: &str) -> T { |
| match self { |
| Some(val) => val, |
| None => expect_failed(msg), |
| } |
| } |
| |
| /// Returns the contained [`Some`] value, consuming the `self` value. |
| /// |
| /// Because this function may panic, its use is generally discouraged. |
| /// Instead, prefer to use pattern matching and handle the [`None`] |
| /// case explicitly, or call [`unwrap_or`], [`unwrap_or_else`], or |
| /// [`unwrap_or_default`]. |
| /// |
| /// [`unwrap_or`]: Option::unwrap_or |
| /// [`unwrap_or_else`]: Option::unwrap_or_else |
| /// [`unwrap_or_default`]: Option::unwrap_or_default |
| /// |
| /// # Panics |
| /// |
| /// Panics if the self value equals [`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] |
| #[track_caller] |
| #[stable(feature = "rust1", since = "1.0.0")] |
| #[rustc_const_unstable(feature = "const_option", issue = "67441")] |
| pub const fn unwrap(self) -> T { |
| match self { |
| Some(val) => val, |
| None => panic("called `Option::unwrap()` on a `None` value"), |
| } |
| } |
| |
| /// Returns the contained [`Some`] value or a provided 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`]: Option::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")] |
| #[rustc_const_unstable(feature = "const_option_ext", issue = "91930")] |
| pub const fn unwrap_or(self, default: T) -> T |
| where |
| T: ~const Destruct, |
| { |
| match self { |
| Some(x) => x, |
| None => default, |
| } |
| } |
| |
| /// Returns the contained [`Some`] 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")] |
| #[rustc_const_unstable(feature = "const_option_ext", issue = "91930")] |
| pub const fn unwrap_or_else<F>(self, f: F) -> T |
| where |
| F: ~const FnOnce() -> T, |
| F: ~const Destruct, |
| { |
| match self { |
| Some(x) => x, |
| None => f(), |
| } |
| } |
| |
| /// Returns the contained [`Some`] 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 |
| /// |
| /// ``` |
| /// let x: Option<u32> = None; |
| /// let y: Option<u32> = Some(12); |
| /// |
| /// assert_eq!(x.unwrap_or_default(), 0); |
| /// assert_eq!(y.unwrap_or_default(), 12); |
| /// ``` |
| /// |
| /// [default value]: Default::default |
| /// [`parse`]: str::parse |
| /// [`FromStr`]: crate::str::FromStr |
| #[inline] |
| #[stable(feature = "rust1", since = "1.0.0")] |
| #[rustc_const_unstable(feature = "const_option_ext", issue = "91930")] |
| pub const fn unwrap_or_default(self) -> T |
| where |
| T: ~const Default, |
| { |
| match self { |
| Some(x) => x, |
| None => Default::default(), |
| } |
| } |
| |
| /// Returns the contained [`Some`] value, consuming the `self` value, |
| /// without checking that the value is not [`None`]. |
| /// |
| /// # Safety |
| /// |
| /// Calling this method on [`None`] is *[undefined behavior]*. |
| /// |
| /// [undefined behavior]: https://doc.rust-lang.org/reference/behavior-considered-undefined.html |
| /// |
| /// # Examples |
| /// |
| /// ``` |
| /// let x = Some("air"); |
| /// assert_eq!(unsafe { x.unwrap_unchecked() }, "air"); |
| /// ``` |
| /// |
| /// ```no_run |
| /// let x: Option<&str> = None; |
| /// assert_eq!(unsafe { x.unwrap_unchecked() }, "air"); // Undefined behavior! |
| /// ``` |
| #[inline] |
| #[track_caller] |
| #[stable(feature = "option_result_unwrap_unchecked", since = "1.58.0")] |
| #[rustc_const_unstable(feature = "const_option_ext", issue = "91930")] |
| pub const unsafe fn unwrap_unchecked(self) -> T { |
| debug_assert!(self.is_some()); |
| match self { |
| Some(val) => val, |
| // SAFETY: the safety contract must be upheld by the caller. |
| None => unsafe { hint::unreachable_unchecked() }, |
| } |
| } |
| |
| ///////////////////////////////////////////////////////////////////////// |
| // Transforming contained values |
| ///////////////////////////////////////////////////////////////////////// |
| |
| /// Maps an `Option<T>` to `Option<U>` by applying a function to a contained value (if `Some`) or returns `None` (if `None`). |
| /// |
| /// # Examples |
| /// |
| /// Calculates the length of an <code>Option<[String]></code> as an |
| /// <code>Option<[usize]></code>, consuming the original: |
| /// |
| /// [String]: ../../std/string/struct.String.html "String" |
| /// ``` |
| /// 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)); |
| /// |
| /// let x: Option<&str> = None; |
| /// assert_eq!(x.map(|s| s.len()), None); |
| /// ``` |
| #[inline] |
| #[stable(feature = "rust1", since = "1.0.0")] |
| #[rustc_const_unstable(feature = "const_option_ext", issue = "91930")] |
| pub const fn map<U, F>(self, f: F) -> Option<U> |
| where |
| F: ~const FnOnce(T) -> U, |
| F: ~const Destruct, |
| { |
| match self { |
| Some(x) => Some(f(x)), |
| None => None, |
| } |
| } |
| |
| /// Calls the provided closure with a reference to the contained value (if [`Some`]). |
| /// |
| /// # Examples |
| /// |
| /// ``` |
| /// #![feature(result_option_inspect)] |
| /// |
| /// let v = vec![1, 2, 3, 4, 5]; |
| /// |
| /// // prints "got: 4" |
| /// let x: Option<&usize> = v.get(3).inspect(|x| println!("got: {x}")); |
| /// |
| /// // prints nothing |
| /// let x: Option<&usize> = v.get(5).inspect(|x| println!("got: {x}")); |
| /// ``` |
| #[inline] |
| #[unstable(feature = "result_option_inspect", issue = "91345")] |
| #[rustc_const_unstable(feature = "const_option_ext", issue = "91930")] |
| pub const fn inspect<F>(self, f: F) -> Self |
| where |
| F: ~const FnOnce(&T), |
| F: ~const Destruct, |
| { |
| if let Some(ref x) = self { |
| f(x); |
| } |
| |
| self |
| } |
| |
| /// Returns the provided default result (if none), |
| /// or applies a function to the contained value (if any). |
| /// |
| /// Arguments passed to `map_or` are eagerly evaluated; if you are passing |
| /// the result of a function call, it is recommended to use [`map_or_else`], |
| /// which is lazily evaluated. |
| /// |
| /// [`map_or_else`]: Option::map_or_else |
| /// |
| /// # 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")] |
| #[rustc_const_unstable(feature = "const_option_ext", issue = "91930")] |
| pub const fn map_or<U, F>(self, default: U, f: F) -> U |
| where |
| F: ~const FnOnce(T) -> U, |
| F: ~const Destruct, |
| U: ~const Destruct, |
| { |
| match self { |
| Some(t) => f(t), |
| None => default, |
| } |
| } |
| |
| /// Computes a default function result (if none), or |
| /// applies a different function to the contained value (if any). |
| /// |
| /// # 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")] |
| #[rustc_const_unstable(feature = "const_option_ext", issue = "91930")] |
| pub const fn map_or_else<U, D, F>(self, default: D, f: F) -> U |
| where |
| D: ~const FnOnce() -> U, |
| D: ~const Destruct, |
| F: ~const FnOnce(T) -> U, |
| F: ~const Destruct, |
| { |
| 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. |
| /// |
| /// [`Ok(v)`]: Ok |
| /// [`Err(err)`]: Err |
| /// [`Some(v)`]: Some |
| /// [`ok_or_else`]: Option::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")] |
| #[rustc_const_unstable(feature = "const_option_ext", issue = "91930")] |
| pub const fn ok_or<E>(self, err: E) -> Result<T, E> |
| where |
| E: ~const Destruct, |
| { |
| 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())`]. |
| /// |
| /// [`Ok(v)`]: Ok |
| /// [`Err(err())`]: Err |
| /// [`Some(v)`]: 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")] |
| #[rustc_const_unstable(feature = "const_option_ext", issue = "91930")] |
| pub const fn ok_or_else<E, F>(self, err: F) -> Result<T, E> |
| where |
| F: ~const FnOnce() -> E, |
| F: ~const Destruct, |
| { |
| match self { |
| Some(v) => Ok(v), |
| None => Err(err()), |
| } |
| } |
| |
| /// 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`]. |
| /// |
| /// # Examples |
| /// |
| /// ``` |
| /// 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); |
| /// ``` |
| #[stable(feature = "option_deref", since = "1.40.0")] |
| #[rustc_const_unstable(feature = "const_option_ext", issue = "91930")] |
| pub const fn as_deref(&self) -> Option<&T::Target> |
| where |
| T: ~const Deref, |
| { |
| match self.as_ref() { |
| Some(t) => Some(t.deref()), |
| None => None, |
| } |
| } |
| |
| /// 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 |
| /// |
| /// ``` |
| /// 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())); |
| /// ``` |
| #[stable(feature = "option_deref", since = "1.40.0")] |
| #[rustc_const_unstable(feature = "const_option_ext", issue = "91930")] |
| pub const fn as_deref_mut(&mut self) -> Option<&mut T::Target> |
| where |
| T: ~const DerefMut, |
| { |
| match self.as_mut() { |
| Some(t) => Some(t.deref_mut()), |
| None => None, |
| } |
| } |
| |
| ///////////////////////////////////////////////////////////////////////// |
| // 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] |
| #[rustc_const_unstable(feature = "const_option", issue = "67441")] |
| #[stable(feature = "rust1", since = "1.0.0")] |
| pub const 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`. |
| /// |
| /// Arguments passed to `and` are eagerly evaluated; if you are passing the |
| /// result of a function call, it is recommended to use [`and_then`], which is |
| /// lazily evaluated. |
| /// |
| /// [`and_then`]: Option::and_then |
| /// |
| /// # 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")] |
| #[rustc_const_unstable(feature = "const_option_ext", issue = "91930")] |
| pub const fn and<U>(self, optb: Option<U>) -> Option<U> |
| where |
| T: ~const Destruct, |
| U: ~const Destruct, |
| { |
| 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. |
| /// |
| /// # Examples |
| /// |
| /// ``` |
| /// fn sq_then_to_string(x: u32) -> Option<String> { |
| /// x.checked_mul(x).map(|sq| sq.to_string()) |
| /// } |
| /// |
| /// assert_eq!(Some(2).and_then(sq_then_to_string), Some(4.to_string())); |
| /// assert_eq!(Some(1_000_000).and_then(sq_then_to_string), None); // overflowed! |
| /// assert_eq!(None.and_then(sq_then_to_string), None); |
| /// ``` |
| /// |
| /// Often used to chain fallible operations that may return [`None`]. |
| /// |
| /// ``` |
| /// let arr_2d = [["A0", "A1"], ["B0", "B1"]]; |
| /// |
| /// let item_0_1 = arr_2d.get(0).and_then(|row| row.get(1)); |
| /// assert_eq!(item_0_1, Some(&"A1")); |
| /// |
| /// let item_2_0 = arr_2d.get(2).and_then(|row| row.get(0)); |
| /// assert_eq!(item_2_0, None); |
| /// ``` |
| #[inline] |
| #[stable(feature = "rust1", since = "1.0.0")] |
| #[rustc_const_unstable(feature = "const_option_ext", issue = "91930")] |
| pub const fn and_then<U, F>(self, f: F) -> Option<U> |
| where |
| F: ~const FnOnce(T) -> Option<U>, |
| F: ~const Destruct, |
| { |
| 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)); |
| /// ``` |
| /// |
| /// [`Some(t)`]: Some |
| #[inline] |
| #[stable(feature = "option_filter", since = "1.27.0")] |
| #[rustc_const_unstable(feature = "const_option_ext", issue = "91930")] |
| pub const fn filter<P>(self, predicate: P) -> Self |
| where |
| T: ~const Destruct, |
| P: ~const FnOnce(&T) -> bool, |
| P: ~const Destruct, |
| { |
| 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`]: Option::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")] |
| #[rustc_const_unstable(feature = "const_option_ext", issue = "91930")] |
| pub const fn or(self, optb: Option<T>) -> Option<T> |
| where |
| T: ~const Destruct, |
| { |
| match self { |
| Some(x) => Some(x), |
| 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")] |
| #[rustc_const_unstable(feature = "const_option_ext", issue = "91930")] |
| pub const fn or_else<F>(self, f: F) -> Option<T> |
| where |
| F: ~const FnOnce() -> Option<T>, |
| F: ~const Destruct, |
| { |
| match self { |
| Some(x) => Some(x), |
| None => f(), |
| } |
| } |
| |
| /// Returns [`Some`] if exactly one of `self`, `optb` is [`Some`], otherwise returns [`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")] |
| #[rustc_const_unstable(feature = "const_option_ext", issue = "91930")] |
| pub const fn xor(self, optb: Option<T>) -> Option<T> |
| where |
| T: ~const Destruct, |
| { |
| match (self, optb) { |
| (Some(a), None) => Some(a), |
| (None, Some(b)) => Some(b), |
| _ => None, |
| } |
| } |
| |
| ///////////////////////////////////////////////////////////////////////// |
| // Entry-like operations to insert a value and return a reference |
| ///////////////////////////////////////////////////////////////////////// |
| |
| /// Inserts `value` into the option, then returns a mutable reference to it. |
| /// |
| /// If the option already contains a value, the old value is dropped. |
| /// |
| /// See also [`Option::get_or_insert`], which doesn't update the value if |
| /// the option already contains [`Some`]. |
| /// |
| /// # Example |
| /// |
| /// ``` |
| /// let mut opt = None; |
| /// let val = opt.insert(1); |
| /// assert_eq!(*val, 1); |
| /// assert_eq!(opt.unwrap(), 1); |
| /// let val = opt.insert(2); |
| /// assert_eq!(*val, 2); |
| /// *val = 3; |
| /// assert_eq!(opt.unwrap(), 3); |
| /// ``` |
| #[must_use = "if you intended to set a value, consider assignment instead"] |
| #[inline] |
| #[stable(feature = "option_insert", since = "1.53.0")] |
| #[rustc_const_unstable(feature = "const_option_ext", issue = "91930")] |
| pub const fn insert(&mut self, value: T) -> &mut T |
| where |
| T: ~const Destruct, |
| { |
| *self = Some(value); |
| |
| // SAFETY: the code above just filled the option |
| unsafe { self.as_mut().unwrap_unchecked() } |
| } |
| |
| /// Inserts `value` into the option if it is [`None`], then |
| /// returns a mutable reference to the contained value. |
| /// |
| /// See also [`Option::insert`], which updates the value even if |
| /// the option already contains [`Some`]. |
| /// |
| /// # 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")] |
| #[rustc_const_unstable(feature = "const_option_ext", issue = "91930")] |
| pub const fn get_or_insert(&mut self, value: T) -> &mut T |
| where |
| T: ~const Destruct, |
| { |
| if let None = *self { |
| *self = Some(value); |
| } |
| |
| // SAFETY: a `None` variant for `self` would have been replaced by a `Some` |
| // variant in the code above. |
| unsafe { self.as_mut().unwrap_unchecked() } |
| } |
| |
| /// Inserts the default value into the option if it is [`None`], then |
| /// returns a mutable reference to the contained value. |
| /// |
| /// # Examples |
| /// |
| /// ``` |
| /// #![feature(option_get_or_insert_default)] |
| /// |
| /// let mut x = None; |
| /// |
| /// { |
| /// let y: &mut u32 = x.get_or_insert_default(); |
| /// assert_eq!(y, &0); |
| /// |
| /// *y = 7; |
| /// } |
| /// |
| /// assert_eq!(x, Some(7)); |
| /// ``` |
| #[inline] |
| #[unstable(feature = "option_get_or_insert_default", issue = "82901")] |
| #[rustc_const_unstable(feature = "const_option_ext", issue = "91930")] |
| pub const fn get_or_insert_default(&mut self) -> &mut T |
| where |
| T: ~const Default, |
| { |
| const fn default<T: ~const Default>() -> T { |
| T::default() |
| } |
| |
| self.get_or_insert_with(default) |
| } |
| |
| /// Inserts a value computed from `f` into the option if it is [`None`], |
| /// then returns a mutable reference to the contained value. |
| /// |
| /// # 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")] |
| #[rustc_const_unstable(feature = "const_option_ext", issue = "91930")] |
| pub const fn get_or_insert_with<F>(&mut self, f: F) -> &mut T |
| where |
| F: ~const FnOnce() -> T, |
| F: ~const Destruct, |
| { |
| if let None = *self { |
| // the compiler isn't smart enough to know that we are not dropping a `T` |
| // here and wants us to ensure `T` can be dropped at compile time. |
| mem::forget(mem::replace(self, Some(f()))) |
| } |
| |
| // SAFETY: a `None` variant for `self` would have been replaced by a `Some` |
| // variant in the code above. |
| unsafe { self.as_mut().unwrap_unchecked() } |
| } |
| |
| ///////////////////////////////////////////////////////////////////////// |
| // Misc |
| ///////////////////////////////////////////////////////////////////////// |
| |
| /// Takes the value out of the option, leaving a [`None`] in its place. |
| /// |
| /// # 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")] |
| #[rustc_const_unstable(feature = "const_option", issue = "67441")] |
| pub const fn take(&mut self) -> Option<T> { |
| // FIXME replace `mem::replace` by `mem::take` when the latter is const ready |
| mem::replace(self, None) |
| } |
| |
| /// 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. |
| /// |
| /// # 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] |
| #[rustc_const_unstable(feature = "const_option", issue = "67441")] |
| #[stable(feature = "option_replace", since = "1.31.0")] |
| pub const fn replace(&mut self, value: T) -> Option<T> { |
| mem::replace(self, Some(value)) |
| } |
| |
| /// 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")] |
| #[rustc_const_unstable(feature = "const_option_ext", issue = "91930")] |
| pub const fn contains<U>(&self, x: &U) -> bool |
| where |
| U: ~const PartialEq<T>, |
| { |
| match self { |
| Some(y) => x.eq(y), |
| None => false, |
| } |
| } |
| |
| /// Zips `self` with another `Option`. |
| /// |
| /// If `self` is `Some(s)` and `other` is `Some(o)`, this method returns `Some((s, o))`. |
| /// Otherwise, `None` is returned. |
| /// |
| /// # Examples |
| /// |
| /// ``` |
| /// let x = Some(1); |
| /// let y = Some("hi"); |
| /// let z = None::<u8>; |
| /// |
| /// assert_eq!(x.zip(y), Some((1, "hi"))); |
| /// assert_eq!(x.zip(z), None); |
| /// ``` |
| #[stable(feature = "option_zip_option", since = "1.46.0")] |
| #[rustc_const_unstable(feature = "const_option_ext", issue = "91930")] |
| pub const fn zip<U>(self, other: Option<U>) -> Option<(T, U)> |
| where |
| T: ~const Destruct, |
| U: ~const Destruct, |
| { |
| match (self, other) { |
| (Some(a), Some(b)) => Some((a, b)), |
| _ => None, |
| } |
| } |
| |
| /// Zips `self` and another `Option` with function `f`. |
| /// |
| /// If `self` is `Some(s)` and `other` is `Some(o)`, this method returns `Some(f(s, o))`. |
| /// Otherwise, `None` is returned. |
| /// |
| /// # Examples |
| /// |
| /// ``` |
| /// #![feature(option_zip)] |
| /// |
| /// #[derive(Debug, PartialEq)] |
| /// struct Point { |
| /// x: f64, |
| /// y: f64, |
| /// } |
| /// |
| /// impl Point { |
| /// fn new(x: f64, y: f64) -> Self { |
| /// Self { x, y } |
| /// } |
| /// } |
| /// |
| /// let x = Some(17.5); |
| /// let y = Some(42.7); |
| /// |
| /// assert_eq!(x.zip_with(y, Point::new), Some(Point { x: 17.5, y: 42.7 })); |
| /// assert_eq!(x.zip_with(None, Point::new), None); |
| /// ``` |
| #[unstable(feature = "option_zip", issue = "70086")] |
| #[rustc_const_unstable(feature = "const_option_ext", issue = "91930")] |
| pub const fn zip_with<U, F, R>(self, other: Option<U>, f: F) -> Option<R> |
| where |
| F: ~const FnOnce(T, U) -> R, |
| F: ~const Destruct, |
| T: ~const Destruct, |
| U: ~const Destruct, |
| { |
| match (self, other) { |
| (Some(a), Some(b)) => Some(f(a, b)), |
| _ => None, |
| } |
| } |
| } |
| |
| impl<T, U> Option<(T, U)> { |
| /// Unzips an option containing a tuple of two options. |
| /// |
| /// If `self` is `Some((a, b))` this method returns `(Some(a), Some(b))`. |
| /// Otherwise, `(None, None)` is returned. |
| /// |
| /// # Examples |
| /// |
| /// ``` |
| /// let x = Some((1, "hi")); |
| /// let y = None::<(u8, u32)>; |
| /// |
| /// assert_eq!(x.unzip(), (Some(1), Some("hi"))); |
| /// assert_eq!(y.unzip(), (None, None)); |
| /// ``` |
| #[inline] |
| #[stable(feature = "unzip_option", since = "1.66.0")] |
| #[rustc_const_unstable(feature = "const_option", issue = "67441")] |
| pub const fn unzip(self) -> (Option<T>, Option<U>) |
| where |
| T: ~const Destruct, |
| U: ~const Destruct, |
| { |
| match self { |
| Some((a, b)) => (Some(a), Some(b)), |
| None => (None, None), |
| } |
| } |
| } |
| |
| impl<T> 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)); |
| /// ``` |
| #[must_use = "`self` will be dropped if the result is not used"] |
| #[stable(feature = "copied", since = "1.35.0")] |
| #[rustc_const_unstable(feature = "const_option", issue = "67441")] |
| pub const fn copied(self) -> Option<T> |
| where |
| T: Copy, |
| { |
| // FIXME: this implementation, which sidesteps using `Option::map` since it's not const |
| // ready yet, should be reverted when possible to avoid code repetition |
| match self { |
| Some(&v) => Some(v), |
| None => None, |
| } |
| } |
| |
| /// 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)); |
| /// ``` |
| #[must_use = "`self` will be dropped if the result is not used"] |
| #[stable(feature = "rust1", since = "1.0.0")] |
| #[rustc_const_unstable(feature = "const_option_cloned", issue = "91582")] |
| pub const fn cloned(self) -> Option<T> |
| where |
| T: ~const Clone, |
| { |
| match self { |
| Some(t) => Some(t.clone()), |
| None => None, |
| } |
| } |
| } |
| |
| impl<T> 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)); |
| /// ``` |
| #[must_use = "`self` will be dropped if the result is not used"] |
| #[stable(feature = "copied", since = "1.35.0")] |
| #[rustc_const_unstable(feature = "const_option_ext", issue = "91930")] |
| pub const fn copied(self) -> Option<T> |
| where |
| T: Copy, |
| { |
| match self { |
| Some(&mut t) => Some(t), |
| None => None, |
| } |
| } |
| |
| /// 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)); |
| /// ``` |
| #[must_use = "`self` will be dropped if the result is not used"] |
| #[stable(since = "1.26.0", feature = "option_ref_mut_cloned")] |
| #[rustc_const_unstable(feature = "const_option_cloned", issue = "91582")] |
| pub const fn cloned(self) -> Option<T> |
| where |
| T: ~const Clone, |
| { |
| match self { |
| Some(t) => Some(t.clone()), |
| None => None, |
| } |
| } |
| } |
| |
| impl<T, E> Option<Result<T, E>> { |
| /// Transposes an `Option` of a [`Result`] into a [`Result`] of an `Option`. |
| /// |
| /// [`None`] will be mapped to <code>[Ok]\([None])</code>. |
| /// <code>[Some]\([Ok]\(\_))</code> and <code>[Some]\([Err]\(\_))</code> will be mapped to |
| /// <code>[Ok]\([Some]\(\_))</code> and <code>[Err]\(\_)</code>. |
| /// |
| /// # 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")] |
| #[rustc_const_unstable(feature = "const_option", issue = "67441")] |
| pub const 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. |
| #[cfg_attr(not(feature = "panic_immediate_abort"), inline(never))] |
| #[cfg_attr(feature = "panic_immediate_abort", inline)] |
| #[cold] |
| #[track_caller] |
| #[rustc_const_unstable(feature = "const_option", issue = "67441")] |
| const fn expect_failed(msg: &str) -> ! { |
| panic_str(msg) |
| } |
| |
| ///////////////////////////////////////////////////////////////////////////// |
| // Trait implementations |
| ///////////////////////////////////////////////////////////////////////////// |
| |
| #[stable(feature = "rust1", since = "1.0.0")] |
| #[rustc_const_unstable(feature = "const_clone", issue = "91805")] |
| impl<T> const Clone for Option<T> |
| where |
| T: ~const Clone + ~const Destruct, |
| { |
| #[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")] |
| #[rustc_const_unstable(feature = "const_default_impls", issue = "87864")] |
| impl<T> const 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")] |
| #[rustc_const_unstable(feature = "const_convert", issue = "88674")] |
| impl<T> const From<T> for Option<T> { |
| /// Moves `val` into a new [`Some`]. |
| /// |
| /// # Examples |
| /// |
| /// ``` |
| /// let o: Option<u8> = Option::from(67); |
| /// |
| /// assert_eq!(Some(67), o); |
| /// ``` |
| fn from(val: T) -> Option<T> { |
| Some(val) |
| } |
| } |
| |
| #[stable(feature = "option_ref_from_ref_option", since = "1.30.0")] |
| #[rustc_const_unstable(feature = "const_convert", issue = "88674")] |
| impl<'a, T> const From<&'a Option<T>> for Option<&'a T> { |
| /// Converts from `&Option<T>` to `Option<&T>`. |
| /// |
| /// # Examples |
| /// |
| /// Converts an <code>[Option]<[String]></code> into an <code>[Option]<[usize]></code>, preserving |
| /// the original. The [`map`] method takes the `self` argument by value, consuming the original, |
| /// so this technique uses `from` to first take an [`Option`] to a reference |
| /// to the value inside the original. |
| /// |
| /// [`map`]: Option::map |
| /// [String]: ../../std/string/struct.String.html "String" |
| /// |
| /// ``` |
| /// let s: Option<String> = Some(String::from("Hello, Rustaceans!")); |
| /// let o: Option<usize> = Option::from(&s).map(|ss: &String| ss.len()); |
| /// |
| /// println!("Can still print s: {s:?}"); |
| /// |
| /// assert_eq!(o, Some(18)); |
| /// ``` |
| fn from(o: &'a Option<T>) -> Option<&'a T> { |
| o.as_ref() |
| } |
| } |
| |
| #[stable(feature = "option_ref_from_ref_option", since = "1.30.0")] |
| #[rustc_const_unstable(feature = "const_convert", issue = "88674")] |
| impl<'a, T> const From<&'a mut Option<T>> for Option<&'a mut T> { |
| /// Converts from `&mut Option<T>` to `Option<&mut T>` |
| /// |
| /// # Examples |
| /// |
| /// ``` |
| /// let mut s = Some(String::from("Hello")); |
| /// let o: Option<&mut String> = Option::from(&mut s); |
| /// |
| /// match o { |
| /// Some(t) => *t = String::from("Hello, Rustaceans!"), |
| /// None => (), |
| /// } |
| /// |
| /// assert_eq!(s, Some(String::from("Hello, Rustaceans!"))); |
| /// ``` |
| fn from(o: &'a mut Option<T>) -> Option<&'a mut T> { |
| o.as_mut() |
| } |
| } |
| |
| #[stable(feature = "rust1", since = "1.0.0")] |
| impl<T> crate::marker::StructuralPartialEq for Option<T> {} |
| #[stable(feature = "rust1", since = "1.0.0")] |
| impl<T: PartialEq> PartialEq for Option<T> { |
| #[inline] |
| fn eq(&self, other: &Self) -> bool { |
| SpecOptionPartialEq::eq(self, other) |
| } |
| } |
| |
| /// This specialization trait is a workaround for LLVM not currently (2023-01) |
| /// being able to optimize this itself, even though Alive confirms that it would |
| /// be legal to do so: <https://github.com/llvm/llvm-project/issues/52622> |
| /// |
| /// Once that's fixed, `Option` should go back to deriving `PartialEq`, as |
| /// it used to do before <https://github.com/rust-lang/rust/pull/103556>. |
| #[unstable(feature = "spec_option_partial_eq", issue = "none", reason = "exposed only for rustc")] |
| #[doc(hidden)] |
| pub trait SpecOptionPartialEq: Sized { |
| fn eq(l: &Option<Self>, other: &Option<Self>) -> bool; |
| } |
| |
| #[unstable(feature = "spec_option_partial_eq", issue = "none", reason = "exposed only for rustc")] |
| impl<T: PartialEq> SpecOptionPartialEq for T { |
| #[inline] |
| default fn eq(l: &Option<T>, r: &Option<T>) -> bool { |
| match (l, r) { |
| (Some(l), Some(r)) => *l == *r, |
| (None, None) => true, |
| _ => false, |
| } |
| } |
| } |
| |
| macro_rules! non_zero_option { |
| ( $( #[$stability: meta] $NZ:ty; )+ ) => { |
| $( |
| #[$stability] |
| impl SpecOptionPartialEq for $NZ { |
| #[inline] |
| fn eq(l: &Option<Self>, r: &Option<Self>) -> bool { |
| l.map(Self::get).unwrap_or(0) == r.map(Self::get).unwrap_or(0) |
| } |
| } |
| )+ |
| }; |
| } |
| |
| non_zero_option! { |
| #[stable(feature = "nonzero", since = "1.28.0")] crate::num::NonZeroU8; |
| #[stable(feature = "nonzero", since = "1.28.0")] crate::num::NonZeroU16; |
| #[stable(feature = "nonzero", since = "1.28.0")] crate::num::NonZeroU32; |
| #[stable(feature = "nonzero", since = "1.28.0")] crate::num::NonZeroU64; |
| #[stable(feature = "nonzero", since = "1.28.0")] crate::num::NonZeroU128; |
| #[stable(feature = "nonzero", since = "1.28.0")] crate::num::NonZeroUsize; |
| #[stable(feature = "signed_nonzero", since = "1.34.0")] crate::num::NonZeroI8; |
| #[stable(feature = "signed_nonzero", since = "1.34.0")] crate::num::NonZeroI16; |
| #[stable(feature = "signed_nonzero", since = "1.34.0")] crate::num::NonZeroI32; |
| #[stable(feature = "signed_nonzero", since = "1.34.0")] crate::num::NonZeroI64; |
| #[stable(feature = "signed_nonzero", since = "1.34.0")] crate::num::NonZeroI128; |
| #[stable(feature = "signed_nonzero", since = "1.34.0")] crate::num::NonZeroIsize; |
| } |
| |
| #[stable(feature = "nonnull", since = "1.25.0")] |
| impl<T> SpecOptionPartialEq for crate::ptr::NonNull<T> { |
| #[inline] |
| fn eq(l: &Option<Self>, r: &Option<Self>) -> bool { |
| l.map(Self::as_ptr).unwrap_or_else(|| crate::ptr::null_mut()) |
| == r.map(Self::as_ptr).unwrap_or_else(|| crate::ptr::null_mut()) |
| } |
| } |
| |
| #[stable(feature = "rust1", since = "1.0.0")] |
| impl SpecOptionPartialEq for cmp::Ordering { |
| #[inline] |
| fn eq(l: &Option<Self>, r: &Option<Self>) -> bool { |
| l.map_or(2, |x| x as i8) == r.map_or(2, |x| x as i8) |
| } |
| } |
| |
| ///////////////////////////////////////////////////////////////////////////// |
| // 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. |
| #[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. |
| #[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. |
| #[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 of type |
| /// `V` containing 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. |
| #[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::try_process(iter.into_iter(), |i| i.collect()) |
| } |
| } |
| |
| #[unstable(feature = "try_trait_v2", issue = "84277")] |
| #[rustc_const_unstable(feature = "const_convert", issue = "88674")] |
| impl<T> const ops::Try for Option<T> { |
| type Output = T; |
| type Residual = Option<convert::Infallible>; |
| |
| #[inline] |
| fn from_output(output: Self::Output) -> Self { |
| Some(output) |
| } |
| |
| #[inline] |
| fn branch(self) -> ControlFlow<Self::Residual, Self::Output> { |
| match self { |
| Some(v) => ControlFlow::Continue(v), |
| None => ControlFlow::Break(None), |
| } |
| } |
| } |
| |
| #[unstable(feature = "try_trait_v2", issue = "84277")] |
| #[rustc_const_unstable(feature = "const_convert", issue = "88674")] |
| impl<T> const ops::FromResidual for Option<T> { |
| #[inline] |
| fn from_residual(residual: Option<convert::Infallible>) -> Self { |
| match residual { |
| None => None, |
| } |
| } |
| } |
| |
| #[unstable(feature = "try_trait_v2_yeet", issue = "96374")] |
| impl<T> ops::FromResidual<ops::Yeet<()>> for Option<T> { |
| #[inline] |
| fn from_residual(ops::Yeet(()): ops::Yeet<()>) -> Self { |
| None |
| } |
| } |
| |
| #[unstable(feature = "try_trait_v2_residual", issue = "91285")] |
| #[rustc_const_unstable(feature = "const_try", issue = "74935")] |
| impl<T> const ops::Residual<T> for Option<convert::Infallible> { |
| type TryType = Option<T>; |
| } |
| |
| impl<T> Option<Option<T>> { |
| /// Converts from `Option<Option<T>>` to `Option<T>`. |
| /// |
| /// # Examples |
| /// |
| /// Basic usage: |
| /// |
| /// ``` |
| /// 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 only removes one level of nesting at a time: |
| /// |
| /// ``` |
| /// 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] |
| #[stable(feature = "option_flattening", since = "1.40.0")] |
| #[rustc_const_unstable(feature = "const_option", issue = "67441")] |
| pub const fn flatten(self) -> Option<T> { |
| match self { |
| Some(inner) => inner, |
| None => None, |
| } |
| } |
| } |