4 use crate::panic::{RefUnwindSafe, UnwindSafe};
5 use crate::sync::OnceLock;
7 /// A value which is initialized on the first access.
9 /// This type is a thread-safe `Lazy`, and can be used in statics.
14 /// #![feature(once_cell)]
16 /// use std::collections::HashMap;
18 /// use std::sync::LazyLock;
20 /// static HASHMAP: LazyLock<HashMap<i32, String>> = LazyLock::new(|| {
21 /// println!("initializing");
22 /// let mut m = HashMap::new();
23 /// m.insert(13, "Spica".to_string());
24 /// m.insert(74, "Hoyten".to_string());
29 /// println!("ready");
30 /// std::thread::spawn(|| {
31 /// println!("{:?}", HASHMAP.get(&13));
32 /// }).join().unwrap();
33 /// println!("{:?}", HASHMAP.get(&74));
42 #[unstable(feature = "once_cell", issue = "74465")]
43 pub struct LazyLock<T, F = fn() -> T> {
45 init: Cell<Option<F>>,
48 impl<T, F> LazyLock<T, F> {
49 /// Creates a new lazy value with the given initializing
51 #[unstable(feature = "once_cell", issue = "74465")]
52 pub const fn new(f: F) -> LazyLock<T, F> {
53 LazyLock { cell: OnceLock::new(), init: Cell::new(Some(f)) }
57 impl<T, F: FnOnce() -> T> LazyLock<T, F> {
58 /// Forces the evaluation of this lazy value and
59 /// returns a reference to result. This is equivalent
60 /// to the `Deref` impl, but is explicit.
65 /// #![feature(once_cell)]
67 /// use std::sync::LazyLock;
69 /// let lazy = LazyLock::new(|| 92);
71 /// assert_eq!(LazyLock::force(&lazy), &92);
72 /// assert_eq!(&*lazy, &92);
74 #[unstable(feature = "once_cell", issue = "74465")]
75 pub fn force(this: &LazyLock<T, F>) -> &T {
76 this.cell.get_or_init(|| match this.init.take() {
78 None => panic!("Lazy instance has previously been poisoned"),
83 #[unstable(feature = "once_cell", issue = "74465")]
84 impl<T, F: FnOnce() -> T> Deref for LazyLock<T, F> {
86 fn deref(&self) -> &T {
91 #[unstable(feature = "once_cell", issue = "74465")]
92 impl<T: Default> Default for LazyLock<T> {
93 /// Creates a new lazy value using `Default` as the initializing function.
94 fn default() -> LazyLock<T> {
95 LazyLock::new(T::default)
99 #[unstable(feature = "once_cell", issue = "74465")]
100 impl<T: fmt::Debug, F> fmt::Debug for LazyLock<T, F> {
101 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
102 f.debug_struct("Lazy").field("cell", &self.cell).finish_non_exhaustive()
106 // We never create a `&F` from a `&LazyLock<T, F>` so it is fine
107 // to not impl `Sync` for `F`
108 // we do create a `&mut Option<F>` in `force`, but this is
109 // properly synchronized, so it only happens once
110 // so it also does not contribute to this impl.
111 #[unstable(feature = "once_cell", issue = "74465")]
112 unsafe impl<T, F: Send> Sync for LazyLock<T, F> where OnceLock<T>: Sync {}
113 // auto-derived `Send` impl is OK.
115 #[unstable(feature = "once_cell", issue = "74465")]
116 impl<T, F: UnwindSafe> RefUnwindSafe for LazyLock<T, F> where OnceLock<T>: RefUnwindSafe {}
117 #[unstable(feature = "once_cell", issue = "74465")]
118 impl<T, F: UnwindSafe> UnwindSafe for LazyLock<T, F> where OnceLock<T>: UnwindSafe {}