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        let mut d = f.debug_tuple("Lazy");
49        let d = if let Some(x) = self.cell.get() {
50            d.field(&x)
51        } else {
52            d.field(&format_args!("<uninit>"))
53        };
54        d.finish()
55    }
56}
57
58// We never create a `&F` from a `&Lazy<T, F>` so it is fine
59// to not impl `Sync` for `F`
60// we do create a `&mut Option<F>` in `force`, but this is
61// properly synchronized, so it only happens once
62// so it also does not contribute to this impl.
63unsafe impl<T, F: Send> Sync for Lazy<T, F> where Once<T>: Sync {}
64// auto-derived `Send` impl is OK.
65
66impl<T, F, R> Lazy<T, F, R> {
67    /// Creates a new lazy value with the given initializing
68    /// function.
69    pub const fn new(f: F) -> Self {
70        Self {
71            cell: Once::new(),
72            init: Cell::new(Some(f)),
73        }
74    }
75    /// Retrieves a mutable pointer to the inner data.
76    ///
77    /// This is especially useful when interfacing with low level code or FFI where the caller
78    /// explicitly knows that it has exclusive access to the inner data. Note that reading from
79    /// this pointer is UB until initialized or directly written to.
80    pub fn as_mut_ptr(&self) -> *mut T {
81        self.cell.as_mut_ptr()
82    }
83}
84
85impl<T, F: FnOnce() -> T, R: RelaxStrategy> Lazy<T, F, R> {
86    /// Forces the evaluation of this lazy value and
87    /// returns a reference to result. This is equivalent
88    /// to the `Deref` impl, but is explicit.
89    ///
90    /// # Examples
91    ///
92    /// ```
93    /// use spin::Lazy;
94    ///
95    /// let lazy = Lazy::new(|| 92);
96    ///
97    /// assert_eq!(Lazy::force(&lazy), &92);
98    /// assert_eq!(&*lazy, &92);
99    /// ```
100    pub fn force(this: &Self) -> &T {
101        this.cell.call_once(|| match this.init.take() {
102            Some(f) => f(),
103            None => panic!("Lazy instance has previously been poisoned"),
104        })
105    }
106}
107
108impl<T, F: FnOnce() -> T, R: RelaxStrategy> Deref for Lazy<T, F, R> {
109    type Target = T;
110
111    fn deref(&self) -> &T {
112        Self::force(self)
113    }
114}
115
116impl<T: Default, R> Default for Lazy<T, fn() -> T, R> {
117    /// Creates a new lazy value using `Default` as the initializing function.
118    fn default() -> Self {
119        Self::new(T::default)
120    }
121}
spin/lazy.rs

spin/
lazy.rs
