| use crate::cell::Cell; |
| use crate::fmt; |
| use crate::ops::Deref; |
| use crate::panic::{RefUnwindSafe, UnwindSafe}; |
| use crate::sync::OnceLock; |
| |
| /// A value which is initialized on the first access. |
| /// |
| /// This type is a thread-safe [`LazyCell`], and can be used in statics. |
| /// |
| /// [`LazyCell`]: crate::cell::LazyCell |
| /// |
| /// # Examples |
| /// |
| /// ``` |
| /// #![feature(once_cell)] |
| /// |
| /// use std::collections::HashMap; |
| /// |
| /// use std::sync::LazyLock; |
| /// |
| /// static HASHMAP: LazyLock<HashMap<i32, String>> = LazyLock::new(|| { |
| /// println!("initializing"); |
| /// let mut m = HashMap::new(); |
| /// m.insert(13, "Spica".to_string()); |
| /// m.insert(74, "Hoyten".to_string()); |
| /// m |
| /// }); |
| /// |
| /// fn main() { |
| /// println!("ready"); |
| /// std::thread::spawn(|| { |
| /// println!("{:?}", HASHMAP.get(&13)); |
| /// }).join().unwrap(); |
| /// println!("{:?}", HASHMAP.get(&74)); |
| /// |
| /// // Prints: |
| /// // ready |
| /// // initializing |
| /// // Some("Spica") |
| /// // Some("Hoyten") |
| /// } |
| /// ``` |
| #[unstable(feature = "once_cell", issue = "74465")] |
| pub struct LazyLock<T, F = fn() -> T> { |
| cell: OnceLock<T>, |
| init: Cell<Option<F>>, |
| } |
| |
| impl<T, F> LazyLock<T, F> { |
| /// Creates a new lazy value with the given initializing |
| /// function. |
| #[unstable(feature = "once_cell", issue = "74465")] |
| pub const fn new(f: F) -> LazyLock<T, F> { |
| LazyLock { cell: OnceLock::new(), init: Cell::new(Some(f)) } |
| } |
| } |
| |
| impl<T, F: FnOnce() -> T> LazyLock<T, F> { |
| /// Forces the evaluation of this lazy value and |
| /// returns a reference to result. This is equivalent |
| /// to the `Deref` impl, but is explicit. |
| /// |
| /// # Examples |
| /// |
| /// ``` |
| /// #![feature(once_cell)] |
| /// |
| /// use std::sync::LazyLock; |
| /// |
| /// let lazy = LazyLock::new(|| 92); |
| /// |
| /// assert_eq!(LazyLock::force(&lazy), &92); |
| /// assert_eq!(&*lazy, &92); |
| /// ``` |
| #[unstable(feature = "once_cell", issue = "74465")] |
| pub fn force(this: &LazyLock<T, F>) -> &T { |
| this.cell.get_or_init(|| match this.init.take() { |
| Some(f) => f(), |
| None => panic!("Lazy instance has previously been poisoned"), |
| }) |
| } |
| } |
| |
| #[unstable(feature = "once_cell", issue = "74465")] |
| impl<T, F: FnOnce() -> T> Deref for LazyLock<T, F> { |
| type Target = T; |
| fn deref(&self) -> &T { |
| LazyLock::force(self) |
| } |
| } |
| |
| #[unstable(feature = "once_cell", issue = "74465")] |
| impl<T: Default> Default for LazyLock<T> { |
| /// Creates a new lazy value using `Default` as the initializing function. |
| fn default() -> LazyLock<T> { |
| LazyLock::new(T::default) |
| } |
| } |
| |
| #[unstable(feature = "once_cell", issue = "74465")] |
| impl<T: fmt::Debug, F> fmt::Debug for LazyLock<T, F> { |
| fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result { |
| f.debug_struct("Lazy").field("cell", &self.cell).finish_non_exhaustive() |
| } |
| } |
| |
| // We never create a `&F` from a `&LazyLock<T, F>` so it is fine |
| // to not impl `Sync` for `F` |
| // we do create a `&mut Option<F>` in `force`, but this is |
| // properly synchronized, so it only happens once |
| // so it also does not contribute to this impl. |
| #[unstable(feature = "once_cell", issue = "74465")] |
| unsafe impl<T, F: Send> Sync for LazyLock<T, F> where OnceLock<T>: Sync {} |
| // auto-derived `Send` impl is OK. |
| |
| #[unstable(feature = "once_cell", issue = "74465")] |
| impl<T, F: UnwindSafe> RefUnwindSafe for LazyLock<T, F> where OnceLock<T>: RefUnwindSafe {} |
| #[unstable(feature = "once_cell", issue = "74465")] |
| impl<T, F: UnwindSafe> UnwindSafe for LazyLock<T, F> where OnceLock<T>: UnwindSafe {} |
| |
| #[cfg(test)] |
| mod tests; |