| use crate::cell::{Cell, OnceCell}; |
| use crate::fmt; |
| use crate::ops::Deref; |
| |
| /// A value which is initialized on the first access. |
| /// |
| /// For a thread-safe version of this struct, see [`std::sync::LazyLock`]. |
| /// |
| /// [`std::sync::LazyLock`]: ../../std/sync/struct.LazyLock.html |
| /// |
| /// # Examples |
| /// |
| /// ``` |
| /// #![feature(once_cell)] |
| /// |
| /// use std::cell::LazyCell; |
| /// |
| /// let lazy: LazyCell<i32> = LazyCell::new(|| { |
| /// println!("initializing"); |
| /// 92 |
| /// }); |
| /// println!("ready"); |
| /// println!("{}", *lazy); |
| /// println!("{}", *lazy); |
| /// |
| /// // Prints: |
| /// // ready |
| /// // initializing |
| /// // 92 |
| /// // 92 |
| /// ``` |
| #[unstable(feature = "once_cell", issue = "74465")] |
| pub struct LazyCell<T, F = fn() -> T> { |
| cell: OnceCell<T>, |
| init: Cell<Option<F>>, |
| } |
| |
| impl<T, F> LazyCell<T, F> { |
| /// Creates a new lazy value with the given initializing function. |
| /// |
| /// # Examples |
| /// |
| /// ``` |
| /// #![feature(once_cell)] |
| /// |
| /// use std::cell::LazyCell; |
| /// |
| /// let hello = "Hello, World!".to_string(); |
| /// |
| /// let lazy = LazyCell::new(|| hello.to_uppercase()); |
| /// |
| /// assert_eq!(&*lazy, "HELLO, WORLD!"); |
| /// ``` |
| #[unstable(feature = "once_cell", issue = "74465")] |
| pub const fn new(init: F) -> LazyCell<T, F> { |
| LazyCell { cell: OnceCell::new(), init: Cell::new(Some(init)) } |
| } |
| } |
| |
| impl<T, F: FnOnce() -> T> LazyCell<T, F> { |
| /// Forces the evaluation of this lazy value and returns a reference to |
| /// the result. |
| /// |
| /// This is equivalent to the `Deref` impl, but is explicit. |
| /// |
| /// # Examples |
| /// |
| /// ``` |
| /// #![feature(once_cell)] |
| /// |
| /// use std::cell::LazyCell; |
| /// |
| /// let lazy = LazyCell::new(|| 92); |
| /// |
| /// assert_eq!(LazyCell::force(&lazy), &92); |
| /// assert_eq!(&*lazy, &92); |
| /// ``` |
| #[unstable(feature = "once_cell", issue = "74465")] |
| pub fn force(this: &LazyCell<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 LazyCell<T, F> { |
| type Target = T; |
| fn deref(&self) -> &T { |
| LazyCell::force(self) |
| } |
| } |
| |
| #[unstable(feature = "once_cell", issue = "74465")] |
| impl<T: Default> Default for LazyCell<T> { |
| /// Creates a new lazy value using `Default` as the initializing function. |
| fn default() -> LazyCell<T> { |
| LazyCell::new(T::default) |
| } |
| } |
| |
| #[unstable(feature = "once_cell", issue = "74465")] |
| impl<T: fmt::Debug, F> fmt::Debug for LazyCell<T, F> { |
| fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result { |
| f.debug_struct("Lazy").field("cell", &self.cell).field("init", &"..").finish() |
| } |
| } |