spin/
lazy.rs

1//! Synchronization primitives for lazy evaluation.
2//!
3//! Implementation adapted from the `SyncLazy` type of the standard library. See:
4//! <https://doc.rust-lang.org/std/lazy/struct.SyncLazy.html>
5
6use crate::{once::Once, RelaxStrategy, Spin};
7use core::{cell::Cell, fmt, ops::Deref};
8
9/// A value which is initialized on the first access.
10///
11/// This type is a thread-safe `Lazy`, and can be used in statics.
12///
13/// # Examples
14///
15/// ```
16/// use std::collections::HashMap;
17/// use spin::Lazy;
18///
19/// static HASHMAP: Lazy<HashMap<i32, String>> = Lazy::new(|| {
20///     println!("initializing");
21///     let mut m = HashMap::new();
22///     m.insert(13, "Spica".to_string());
23///     m.insert(74, "Hoyten".to_string());
24///     m
25/// });
26///
27/// fn main() {
28///     println!("ready");
29///     std::thread::spawn(|| {
30///         println!("{:?}", HASHMAP.get(&13));
31///     }).join().unwrap();
32///     println!("{:?}", HASHMAP.get(&74));
33///
34///     // Prints:
35///     //   ready
36///     //   initializing
37///     //   Some("Spica")
38///     //   Some("Hoyten")
39/// }
40/// ```
41pub struct Lazy<T, F = fn() -> T, R = Spin> {
42    cell: Once<T, R>,
43    init: Cell<Option<F>>,
44}
45
46impl<T: fmt::Debug, F, R> fmt::Debug for Lazy<T, F, R> {
47    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
48        f.debug_struct("Lazy")
49            .field("cell", &self.cell)
50            .field("init", &"..")
51            .finish()
52    }
53}
54
55// We never create a `&F` from a `&Lazy<T, F>` so it is fine
56// to not impl `Sync` for `F`
57// we do create a `&mut Option<F>` in `force`, but this is
58// properly synchronized, so it only happens once
59// so it also does not contribute to this impl.
60unsafe impl<T, F: Send> Sync for Lazy<T, F> where Once<T>: Sync {}
61// auto-derived `Send` impl is OK.
62
63impl<T, F, R> Lazy<T, F, R> {
64    /// Creates a new lazy value with the given initializing
65    /// function.
66    pub const fn new(f: F) -> Self {
67        Self {
68            cell: Once::new(),
69            init: Cell::new(Some(f)),
70        }
71    }
72    /// Retrieves a mutable pointer to the inner data.
73    ///
74    /// This is especially useful when interfacing with low level code or FFI where the caller
75    /// explicitly knows that it has exclusive access to the inner data. Note that reading from
76    /// this pointer is UB until initialized or directly written to.
77    pub fn as_mut_ptr(&self) -> *mut T {
78        self.cell.as_mut_ptr()
79    }
80}
81
82impl<T, F: FnOnce() -> T, R: RelaxStrategy> Lazy<T, F, R> {
83    /// Forces the evaluation of this lazy value and
84    /// returns a reference to result. This is equivalent
85    /// to the `Deref` impl, but is explicit.
86    ///
87    /// # Examples
88    ///
89    /// ```
90    /// use spin::Lazy;
91    ///
92    /// let lazy = Lazy::new(|| 92);
93    ///
94    /// assert_eq!(Lazy::force(&lazy), &92);
95    /// assert_eq!(&*lazy, &92);
96    /// ```
97    pub fn force(this: &Self) -> &T {
98        this.cell.call_once(|| match this.init.take() {
99            Some(f) => f(),
100            None => panic!("Lazy instance has previously been poisoned"),
101        })
102    }
103}
104
105impl<T, F: FnOnce() -> T, R: RelaxStrategy> Deref for Lazy<T, F, R> {
106    type Target = T;
107
108    fn deref(&self) -> &T {
109        Self::force(self)
110    }
111}
112
113impl<T: Default, R> Default for Lazy<T, fn() -> T, R> {
114    /// Creates a new lazy value using `Default` as the initializing function.
115    fn default() -> Self {
116        Self::new(T::default)
117    }
118}
spin/lazy.rs

spin/
lazy.rs
