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 [`LazyCell`], and can be used in statics.
11 /// [`LazyCell`]: crate::cell::LazyCell
16 /// #![feature(once_cell)]
18 /// use std::collections::HashMap;
20 /// use std::sync::LazyLock;
22 /// static HASHMAP: LazyLock<HashMap<i32, String>> = LazyLock::new(|| {
23 /// println!("initializing");
24 /// let mut m = HashMap::new();
25 /// m.insert(13, "Spica".to_string());
26 /// m.insert(74, "Hoyten".to_string());
31 /// println!("ready");
32 /// std::thread::spawn(|| {
33 /// println!("{:?}", HASHMAP.get(&13));
34 /// }).join().unwrap();
35 /// println!("{:?}", HASHMAP.get(&74));
44 #[unstable(feature = "once_cell", issue = "74465")]
45 pub struct LazyLock<T, F = fn() -> T> {
47 init: Cell<Option<F>>,
49 impl<T, F: FnOnce() -> T> LazyLock<T, F> {
50 /// Creates a new lazy value with the given initializing
52 #[unstable(feature = "once_cell", issue = "74465")]
53 pub const fn new(f: F) -> LazyLock<T, F> {
54 LazyLock { cell: OnceLock::new(), init: Cell::new(Some(f)) }
57 /// Forces the evaluation of this lazy value and
58 /// returns a reference to result. This is equivalent
59 /// to the `Deref` impl, but is explicit.
64 /// #![feature(once_cell)]
66 /// use std::sync::LazyLock;
68 /// let lazy = LazyLock::new(|| 92);
70 /// assert_eq!(LazyLock::force(&lazy), &92);
71 /// assert_eq!(&*lazy, &92);
73 #[unstable(feature = "once_cell", issue = "74465")]
74 pub fn force(this: &LazyLock<T, F>) -> &T {
75 this.cell.get_or_init(|| match this.init.take() {
77 None => panic!("Lazy instance has previously been poisoned"),
82 #[unstable(feature = "once_cell", issue = "74465")]
83 impl<T, F: FnOnce() -> T> Deref for LazyLock<T, F> {
85 fn deref(&self) -> &T {
90 #[unstable(feature = "once_cell", issue = "74465")]
91 impl<T: Default> Default for LazyLock<T> {
92 /// Creates a new lazy value using `Default` as the initializing function.
93 fn default() -> LazyLock<T> {
94 LazyLock::new(T::default)
98 #[unstable(feature = "once_cell", issue = "74465")]
99 impl<T: fmt::Debug, F> fmt::Debug for LazyLock<T, F> {
100 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
101 f.debug_struct("Lazy").field("cell", &self.cell).finish_non_exhaustive()
105 // We never create a `&F` from a `&LazyLock<T, F>` so it is fine
106 // to not impl `Sync` for `F`
107 // we do create a `&mut Option<F>` in `force`, but this is
108 // properly synchronized, so it only happens once
109 // so it also does not contribute to this impl.
110 #[unstable(feature = "once_cell", issue = "74465")]
111 unsafe impl<T, F: Send> Sync for LazyLock<T, F> where OnceLock<T>: Sync {}
112 // auto-derived `Send` impl is OK.
114 #[unstable(feature = "once_cell", issue = "74465")]
115 impl<T, F: UnwindSafe> RefUnwindSafe for LazyLock<T, F> where OnceLock<T>: RefUnwindSafe {}
116 #[unstable(feature = "once_cell", issue = "74465")]
117 impl<T, F: UnwindSafe> UnwindSafe for LazyLock<T, F> where OnceLock<T>: UnwindSafe {}