1 //! Lazy values and one-time initialization of static data.
3 use crate::cell::{Cell, UnsafeCell};
8 /// A cell which can be written to only once.
10 /// Unlike `RefCell`, a `OnceCell` only provides shared `&T` references to its value.
11 /// Unlike `Cell`, a `OnceCell` doesn't require copying or replacing the value to access it.
16 /// #![feature(once_cell)]
18 /// use std::lazy::OnceCell;
20 /// let cell = OnceCell::new();
21 /// assert!(cell.get().is_none());
23 /// let value: &String = cell.get_or_init(|| {
24 /// "Hello, World!".to_string()
26 /// assert_eq!(value, "Hello, World!");
27 /// assert!(cell.get().is_some());
29 #[unstable(feature = "once_cell", issue = "74465")]
30 pub struct OnceCell<T> {
31 // Invariant: written to at most once.
32 inner: UnsafeCell<Option<T>>,
35 #[unstable(feature = "once_cell", issue = "74465")]
36 impl<T> Default for OnceCell<T> {
37 fn default() -> Self {
42 #[unstable(feature = "once_cell", issue = "74465")]
43 impl<T: fmt::Debug> fmt::Debug for OnceCell<T> {
44 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
46 Some(v) => f.debug_tuple("OnceCell").field(v).finish(),
47 None => f.write_str("OnceCell(Uninit)"),
52 #[unstable(feature = "once_cell", issue = "74465")]
53 impl<T: Clone> Clone for OnceCell<T> {
54 fn clone(&self) -> OnceCell<T> {
55 let res = OnceCell::new();
56 if let Some(value) = self.get() {
57 match res.set(value.clone()) {
59 Err(_) => unreachable!(),
66 #[unstable(feature = "once_cell", issue = "74465")]
67 impl<T: PartialEq> PartialEq for OnceCell<T> {
68 fn eq(&self, other: &Self) -> bool {
69 self.get() == other.get()
73 #[unstable(feature = "once_cell", issue = "74465")]
74 impl<T: Eq> Eq for OnceCell<T> {}
76 #[unstable(feature = "once_cell", issue = "74465")]
77 impl<T> const From<T> for OnceCell<T> {
78 fn from(value: T) -> Self {
79 OnceCell { inner: UnsafeCell::new(Some(value)) }
84 /// Creates a new empty cell.
85 #[unstable(feature = "once_cell", issue = "74465")]
87 pub const fn new() -> OnceCell<T> {
88 OnceCell { inner: UnsafeCell::new(None) }
91 /// Gets the reference to the underlying value.
93 /// Returns `None` if the cell is empty.
94 #[unstable(feature = "once_cell", issue = "74465")]
95 pub fn get(&self) -> Option<&T> {
96 // SAFETY: Safe due to `inner`'s invariant
97 unsafe { &*self.inner.get() }.as_ref()
100 /// Gets the mutable reference to the underlying value.
102 /// Returns `None` if the cell is empty.
103 #[unstable(feature = "once_cell", issue = "74465")]
104 pub fn get_mut(&mut self) -> Option<&mut T> {
105 self.inner.get_mut().as_mut()
108 /// Sets the contents of the cell to `value`.
112 /// This method returns `Ok(())` if the cell was empty and `Err(value)` if
118 /// #![feature(once_cell)]
120 /// use std::lazy::OnceCell;
122 /// let cell = OnceCell::new();
123 /// assert!(cell.get().is_none());
125 /// assert_eq!(cell.set(92), Ok(()));
126 /// assert_eq!(cell.set(62), Err(62));
128 /// assert!(cell.get().is_some());
130 #[unstable(feature = "once_cell", issue = "74465")]
131 pub fn set(&self, value: T) -> Result<(), T> {
132 // SAFETY: Safe because we cannot have overlapping mutable borrows
133 let slot = unsafe { &*self.inner.get() };
138 // SAFETY: This is the only place where we set the slot, no races
139 // due to reentrancy/concurrency are possible, and we've
140 // checked that slot is currently `None`, so this write
141 // maintains the `inner`'s invariant.
142 let slot = unsafe { &mut *self.inner.get() };
147 /// Gets the contents of the cell, initializing it with `f`
148 /// if the cell was empty.
152 /// If `f` panics, the panic is propagated to the caller, and the cell
153 /// remains uninitialized.
155 /// It is an error to reentrantly initialize the cell from `f`. Doing
156 /// so results in a panic.
161 /// #![feature(once_cell)]
163 /// use std::lazy::OnceCell;
165 /// let cell = OnceCell::new();
166 /// let value = cell.get_or_init(|| 92);
167 /// assert_eq!(value, &92);
168 /// let value = cell.get_or_init(|| unreachable!());
169 /// assert_eq!(value, &92);
171 #[unstable(feature = "once_cell", issue = "74465")]
172 pub fn get_or_init<F>(&self, f: F) -> &T
176 match self.get_or_try_init(|| Ok::<T, !>(f())) {
181 /// Gets the contents of the cell, initializing it with `f` if
182 /// the cell was empty. If the cell was empty and `f` failed, an
183 /// error is returned.
187 /// If `f` panics, the panic is propagated to the caller, and the cell
188 /// remains uninitialized.
190 /// It is an error to reentrantly initialize the cell from `f`. Doing
191 /// so results in a panic.
196 /// #![feature(once_cell)]
198 /// use std::lazy::OnceCell;
200 /// let cell = OnceCell::new();
201 /// assert_eq!(cell.get_or_try_init(|| Err(())), Err(()));
202 /// assert!(cell.get().is_none());
203 /// let value = cell.get_or_try_init(|| -> Result<i32, ()> {
206 /// assert_eq!(value, Ok(&92));
207 /// assert_eq!(cell.get(), Some(&92))
209 #[unstable(feature = "once_cell", issue = "74465")]
210 pub fn get_or_try_init<F, E>(&self, f: F) -> Result<&T, E>
212 F: FnOnce() -> Result<T, E>,
214 if let Some(val) = self.get() {
217 /// Avoid inlining the initialization closure into the common path that fetches
218 /// the already initialized value
220 fn outlined_call<F, T, E>(f: F) -> Result<T, E>
222 F: FnOnce() -> Result<T, E>,
226 let val = outlined_call(f)?;
227 // Note that *some* forms of reentrant initialization might lead to
228 // UB (see `reentrant_init` test). I believe that just removing this
229 // `assert`, while keeping `set/get` would be sound, but it seems
230 // better to panic, rather than to silently use an old value.
231 assert!(self.set(val).is_ok(), "reentrant init");
232 Ok(self.get().unwrap())
235 /// Consumes the cell, returning the wrapped value.
237 /// Returns `None` if the cell was empty.
242 /// #![feature(once_cell)]
244 /// use std::lazy::OnceCell;
246 /// let cell: OnceCell<String> = OnceCell::new();
247 /// assert_eq!(cell.into_inner(), None);
249 /// let cell = OnceCell::new();
250 /// cell.set("hello".to_string()).unwrap();
251 /// assert_eq!(cell.into_inner(), Some("hello".to_string()));
253 #[unstable(feature = "once_cell", issue = "74465")]
254 pub fn into_inner(self) -> Option<T> {
255 // Because `into_inner` takes `self` by value, the compiler statically verifies
256 // that it is not currently borrowed. So it is safe to move out `Option<T>`.
257 self.inner.into_inner()
260 /// Takes the value out of this `OnceCell`, moving it back to an uninitialized state.
262 /// Has no effect and returns `None` if the `OnceCell` hasn't been initialized.
264 /// Safety is guaranteed by requiring a mutable reference.
269 /// #![feature(once_cell)]
271 /// use std::lazy::OnceCell;
273 /// let mut cell: OnceCell<String> = OnceCell::new();
274 /// assert_eq!(cell.take(), None);
276 /// let mut cell = OnceCell::new();
277 /// cell.set("hello".to_string()).unwrap();
278 /// assert_eq!(cell.take(), Some("hello".to_string()));
279 /// assert_eq!(cell.get(), None);
281 #[unstable(feature = "once_cell", issue = "74465")]
282 pub fn take(&mut self) -> Option<T> {
283 mem::take(self).into_inner()
287 /// A value which is initialized on the first access.
292 /// #![feature(once_cell)]
294 /// use std::lazy::Lazy;
296 /// let lazy: Lazy<i32> = Lazy::new(|| {
297 /// println!("initializing");
300 /// println!("ready");
301 /// println!("{}", *lazy);
302 /// println!("{}", *lazy);
310 #[unstable(feature = "once_cell", issue = "74465")]
311 pub struct Lazy<T, F = fn() -> T> {
313 init: Cell<Option<F>>,
316 #[unstable(feature = "once_cell", issue = "74465")]
317 impl<T: fmt::Debug, F> fmt::Debug for Lazy<T, F> {
318 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
319 f.debug_struct("Lazy").field("cell", &self.cell).field("init", &"..").finish()
323 impl<T, F> Lazy<T, F> {
324 /// Creates a new lazy value with the given initializing function.
329 /// #![feature(once_cell)]
332 /// use std::lazy::Lazy;
334 /// let hello = "Hello, World!".to_string();
336 /// let lazy = Lazy::new(|| hello.to_uppercase());
338 /// assert_eq!(&*lazy, "HELLO, WORLD!");
341 #[unstable(feature = "once_cell", issue = "74465")]
342 pub const fn new(init: F) -> Lazy<T, F> {
343 Lazy { cell: OnceCell::new(), init: Cell::new(Some(init)) }
347 impl<T, F: FnOnce() -> T> Lazy<T, F> {
348 /// Forces the evaluation of this lazy value and returns a reference to
351 /// This is equivalent to the `Deref` impl, but is explicit.
356 /// #![feature(once_cell)]
358 /// use std::lazy::Lazy;
360 /// let lazy = Lazy::new(|| 92);
362 /// assert_eq!(Lazy::force(&lazy), &92);
363 /// assert_eq!(&*lazy, &92);
365 #[unstable(feature = "once_cell", issue = "74465")]
366 pub fn force(this: &Lazy<T, F>) -> &T {
367 this.cell.get_or_init(|| match this.init.take() {
369 None => panic!("`Lazy` instance has previously been poisoned"),
374 #[unstable(feature = "once_cell", issue = "74465")]
375 impl<T, F: FnOnce() -> T> Deref for Lazy<T, F> {
377 fn deref(&self) -> &T {
382 #[unstable(feature = "once_cell", issue = "74465")]
383 impl<T: Default> Default for Lazy<T> {
384 /// Creates a new lazy value using `Default` as the initializing function.
385 fn default() -> Lazy<T> {
386 Lazy::new(T::default)