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extern crate parking_lot; use std::cell::UnsafeCell; use self::parking_lot::{Once, ONCE_INIT, OnceState}; /// A thread-safe cell which can be written to only once. /// /// Unlike `::std::sync::Mutex`, a `OnceCell` provides simple `&` /// references to the contents. /// /// # Example /// ``` /// use once_cell::sync::OnceCell; /// /// static CELL: OnceCell<String> = OnceCell::INIT; /// assert!(CELL.get().is_none()); /// /// ::std::thread::spawn(|| { /// let value: &String = CELL.get_or_init(|| { /// "Hello, World!".to_string() /// }); /// assert_eq!(value, "Hello, World!"); /// }).join().unwrap(); /// /// let value: Option<&String> = CELL.get(); /// assert!(value.is_some()); /// assert_eq!(value.unwrap().as_str(), "Hello, World!"); /// ``` #[derive(Debug)] pub struct OnceCell<T> { once: Once, value: UnsafeCell<Option<T>>, } impl<T> OnceCell<T> { /// An empty cell, for initialization in a `const` context. pub const INIT: OnceCell<T> = OnceCell { once: ONCE_INIT, value: UnsafeCell::new(None), }; /// Creates a new empty cell. pub fn new() -> OnceCell<T> { OnceCell { once: ONCE_INIT, value: UnsafeCell::new(None), } } /// Gets the reference to the underlying value. Returns `None` /// if the cell is empty. pub fn get(&self) -> Option<&T> { if self.once.state() == OnceState::Done { let value: &Option<T> = unsafe { &*self.value.get() }; value.as_ref() } else { None } } /// Sets the contents of this cell to `value`. Returns /// `Ok(())` if the cell was empty and `Err(value)` if it was /// full. /// /// # Example /// ``` /// use once_cell::sync::OnceCell; /// /// static CELL: OnceCell<i32> = OnceCell::INIT; /// /// fn main() { /// assert!(CELL.get().is_none()); /// /// ::std::thread::spawn(|| { /// assert_eq!(CELL.set(92), Ok(())); /// }).join().unwrap(); /// /// assert_eq!(CELL.set(62), Err(62)); /// assert_eq!(CELL.get(), Some(&92)); /// } /// ``` pub fn set(&self, value: T) -> Result<(), T> { let mut value = Some(value); self.once.call_once(|| { let slot: &mut Option<T> = unsafe { &mut *self.value.get() }; *slot = value.take(); }); match value { None => Ok(()), Some(value) => Err(value) } } /// Gets the contents of the cell, initializing it with `f` /// if the cell was empty. May threads may call `get_or_init` /// concurrently with different initializing functions, but /// it is guaranteed that only one function will be executed. /// /// # Example /// ``` /// use once_cell::sync::OnceCell; /// /// let cell = OnceCell::new(); /// let value = cell.get_or_init(|| 92); /// assert_eq!(value, &92); /// let value = cell.get_or_init(|| unreachable!()); /// assert_eq!(value, &92); /// ``` pub fn get_or_init<F: FnOnce() -> T>(&self, f: F) -> &T { self.once.call_once(|| { let value = f(); let slot: &mut Option<T> = unsafe { &mut *self.value.get() }; *slot = Some(value); }); self.get().unwrap() } } // Why do we need `T: Send`? // Thread A creates a `OnceCell` and shares it with // scoped thread B, which fills the cell, which is // then destroyed by A. That is, destructor observes // a sent value. unsafe impl<T: Sync + Send> Sync for OnceCell<T> {} unsafe impl<T: Send> Send for OnceCell<T> {}