| //! Synchronization primitives for lazy evaluation. |
| //! |
| //! Implementation adapted from the `SyncLazy` type of the standard library. See: |
| //! <https://doc.rust-lang.org/std/lazy/struct.SyncLazy.html> |
| |
| use core::{cell::Cell, fmt, ops::Deref}; |
| use crate::{once::Once, RelaxStrategy, Spin}; |
| |
| /// A value which is initialized on the first access. |
| /// |
| /// This type is a thread-safe `Lazy`, and can be used in statics. |
| /// |
| /// # Examples |
| /// |
| /// ``` |
| /// use std::collections::HashMap; |
| /// use spin::Lazy; |
| /// |
| /// static HASHMAP: Lazy<HashMap<i32, String>> = Lazy::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") |
| /// } |
| /// ``` |
| pub struct Lazy<T, F = fn() -> T, R = Spin> { |
| cell: Once<T, R>, |
| init: Cell<Option<F>>, |
| } |
| |
| impl<T: fmt::Debug, F, R> fmt::Debug for Lazy<T, F, R> { |
| fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result { |
| f.debug_struct("Lazy").field("cell", &self.cell).field("init", &"..").finish() |
| } |
| } |
| |
| // We never create a `&F` from a `&Lazy<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. |
| unsafe impl<T, F: Send> Sync for Lazy<T, F> where Once<T>: Sync {} |
| // auto-derived `Send` impl is OK. |
| |
| impl<T, F, R> Lazy<T, F, R> { |
| /// Creates a new lazy value with the given initializing |
| /// function. |
| pub const fn new(f: F) -> Self { |
| Self { cell: Once::new(), init: Cell::new(Some(f)) } |
| } |
| /// Retrieves a mutable pointer to the inner data. |
| /// |
| /// This is especially useful when interfacing with low level code or FFI where the caller |
| /// explicitly knows that it has exclusive access to the inner data. Note that reading from |
| /// this pointer is UB until initialized or directly written to. |
| pub fn as_mut_ptr(&self) -> *mut T { |
| self.cell.as_mut_ptr() |
| } |
| } |
| |
| impl<T, F: FnOnce() -> T, R: RelaxStrategy> Lazy<T, F, R> { |
| /// Forces the evaluation of this lazy value and |
| /// returns a reference to result. This is equivalent |
| /// to the `Deref` impl, but is explicit. |
| /// |
| /// # Examples |
| /// |
| /// ``` |
| /// use spin::Lazy; |
| /// |
| /// let lazy = Lazy::new(|| 92); |
| /// |
| /// assert_eq!(Lazy::force(&lazy), &92); |
| /// assert_eq!(&*lazy, &92); |
| /// ``` |
| pub fn force(this: &Self) -> &T { |
| this.cell.call_once(|| match this.init.take() { |
| Some(f) => f(), |
| None => panic!("Lazy instance has previously been poisoned"), |
| }) |
| } |
| } |
| |
| impl<T, F: FnOnce() -> T, R: RelaxStrategy> Deref for Lazy<T, F, R> { |
| type Target = T; |
| |
| fn deref(&self) -> &T { |
| Self::force(self) |
| } |
| } |
| |
| impl<T: Default, R> Default for Lazy<T, fn() -> T, R> { |
| /// Creates a new lazy value using `Default` as the initializing function. |
| fn default() -> Self { |
| Self::new(T::default) |
| } |
| } |