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extern crate parking_lot;
#[macro_use]
pub mod unsync {
use std::{
ops::Deref,
cell::UnsafeCell,
};
/// A cell which can be written to only once. Not thread safe.
///
/// Unlike `::std::cell::RefCell`, a `OnceCell` provides simple `&`
/// references to the contents.
///
/// # Example
/// ```
/// use once_cell::unsync::OnceCell;
///
/// let cell = OnceCell::new();
/// assert!(cell.get().is_none());
///
/// let value: &String = cell.get_or_init(|| {
/// "Hello, World!".to_string()
/// });
/// assert_eq!(value, "Hello, World!");
/// assert!(cell.get().is_some());
/// ```
#[derive(Debug, Default)]
pub struct OnceCell<T> {
// Invariant: written to at most once.
inner: UnsafeCell<Option<T>>,
}
impl<T> OnceCell<T> {
/// An empty cell, for initialization in a `const` context.
pub const INIT: OnceCell<T> = OnceCell { inner: UnsafeCell::new(None) };
/// Creates a new empty cell.
pub fn new() -> OnceCell<T> {
OnceCell { inner: UnsafeCell::new(None) }
}
/// Gets the reference to the underlying value. Returns `None`
/// if the cell is empty.
pub fn get(&self) -> Option<&T> {
// Safe due to `inner`'s invariant
unsafe { &*self.inner.get() }.as_ref()
}
/// 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::unsync::OnceCell;
///
/// let cell = OnceCell::new();
/// assert!(cell.get().is_none());
///
/// assert_eq!(cell.set(92), Ok(()));
/// assert_eq!(cell.set(62), Err(62));
///
/// assert!(cell.get().is_some());
/// ```
pub fn set(&self, value: T) -> Result<(), T> {
let slot = unsafe { &mut *self.inner.get() };
if slot.is_some() {
return Err(value);
}
// This is the only place where we set the slot, no races
// due to reentrancy/concurrency are possible, and we've
// checked that slot is currently `None`, so this write
// maintains the `inner`'s invariant.
*slot = Some(value);
Ok(())
}
/// Gets the contents of the cell, initializing it with `f`
/// if cell was empty.
///
/// # Example
/// ```
/// use once_cell::unsync::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 {
enum Void {}
match self.get_or_try_init(|| Ok::<T, Void>(f())) {
Ok(val) => val,
Err(void) => match void {},
}
}
/// Gets the contents of the cell, initializing it with `f` if
/// cell was empty. If the cell was empty and `f` failed, an
/// error is returned.
///
/// # Example
/// ```
/// use once_cell::unsync::OnceCell;
///
/// let cell = OnceCell::new();
/// assert_eq!(cell.get_or_try_init(|| Err(())), Err(()));
/// assert!(cell.get().is_none());
/// let value = cell.get_or_try_init(|| -> Result<i32, ()> {
/// Ok(92)
/// });
/// assert_eq!(value, Ok(&92));
/// assert_eq!(cell.get(), Some(&92))
/// ```
pub fn get_or_try_init<F: FnOnce() -> Result<T, E>, E>(&self, f: F) -> Result<&T, E> {
if let Some(val) = self.get() {
return Ok(val);
}
let val = f()?;
assert!(self.set(val).is_ok(), "reentrant init");
Ok(self.get().unwrap())
}
}
/// A value which is initialized on the first access.
///
/// # Example
/// ```
/// use once_cell::unsync::Lazy;
///
/// let lazy: Lazy<i32> = Lazy::new(|| {
/// println!("initializing");
/// 92
/// });
/// println!("A");
/// println!("{}", *lazy);
/// println!("{}", *lazy);
///
/// // prints
/// // A
/// // initializing
/// // 92
/// // 92
/// ```
#[derive(Debug)]
pub struct Lazy<T, F: Fn() -> T = fn() -> T> {
#[doc(hidden)]
pub __cell: OnceCell<T>,
#[doc(hidden)]
pub __init: F,
}
impl<T, F: Fn() -> T> Lazy<T, F> {
/// Creates a new lazy value with the given initializing
/// function.
pub fn new(init: F) -> Lazy<T, F> {
Lazy {
__cell: OnceCell::INIT,
__init: init,
}
}
}
impl<T, F: Fn() -> T> Deref for Lazy<T, F> {
type Target = T;
fn deref(&self) -> &T {
self.__cell.get_or_init(|| (self.__init)())
}
}
/// Creates a new lazy value initialized by the given closure block.
/// This macro works in const contexts.
/// If you need a `move` closure, use `Lazy::new` constructor function.
///
/// # Example
/// ```
/// # #[macro_use] extern crate once_cell;
/// # fn main() {
/// use once_cell::unsync::Lazy;
///
/// let hello = "Hello, World!".to_string();
///
/// let lazy = unsync_lazy! {
/// hello.to_uppercase()
/// };
///
/// assert_eq!(&*lazy, "HELLO, WORLD!");
/// # }
/// ```
#[macro_export]
macro_rules! unsync_lazy {
($($block:tt)*) => {
$crate::unsync::Lazy {
__cell: $crate::unsync::OnceCell::INIT,
__init: || { $($block)* },
}
};
}
}
#[macro_use]
pub mod sync {
use std::{
ptr,
sync::{atomic::{AtomicPtr, Ordering::Relaxed}},
};
use parking_lot::{Once, ONCE_INIT};
#[derive(Debug)]
pub struct OnceCell<T> {
// Invariant 1: `inner` is written to only from within `once.call_once`.
// Corollary 1: inner is written at most once.
// Corollary 2: all reads & writes to inner are fine with `Relaxed` ordering.
// Invariant 2: if not null, ptr came from `Box::into_raw`.
inner: AtomicPtr<T>,
once: Once,
}
// 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> {}
impl<T> OnceCell<T> {
pub const INIT: OnceCell<T> = OnceCell {
inner: AtomicPtr::new(ptr::null_mut()),
once: ONCE_INIT,
};
pub fn new() -> OnceCell<T> {
OnceCell {
inner: AtomicPtr::new(ptr::null_mut()),
once: Once::new(),
}
}
pub fn get(&self) -> Option<&T> {
let ptr = self.inner.load(Relaxed);
// Safe due to Corollary 1
unsafe { ptr.as_ref() }
}
pub fn set(&self, value: T) -> Result<(), T> {
let mut value = Some(value);
self.once.call_once(|| {
let value = value.take().unwrap();
unsafe { self.set_inner(value) }
});
match value {
None => Ok(()),
Some(value) => Err(value)
}
}
/// Guarantees that only one `f` is ever called.
pub fn get_or_init<F: FnOnce() -> T>(&self, f: F) -> &T {
self.once.call_once(|| {
let value = f();
unsafe { self.set_inner(value); }
});
self.get().unwrap()
}
// Invariant: must be called from `self.once`.
unsafe fn set_inner(&self, value: T) {
let ptr = Box::into_raw(Box::new(value));
self.inner.store(ptr, Relaxed);
}
}
impl<T> Drop for OnceCell<T> {
fn drop(&mut self) {
let ptr = self.inner.load(Relaxed);
if !ptr.is_null() {
// Safe due to Corollary 2
drop(unsafe { Box::from_raw(ptr) })
}
}
}
#[derive(Debug)]
pub struct Lazy<T, F: Fn() -> T = fn() -> T> {
#[doc(hidden)]
pub __cell: OnceCell<T>,
#[doc(hidden)]
pub __init: F,
}
impl<T, F: Fn() -> T> Lazy<T, F> {
pub fn new(f: F) -> Lazy<T, F> {
Lazy {
__cell: OnceCell::new(),
__init: f,
}
}
}
impl<T, F: Fn() -> T> ::std::ops::Deref for Lazy<T, F> {
type Target = T;
fn deref(&self) -> &T {
self.__cell.get_or_init(|| (self.__init)())
}
}
#[macro_export]
macro_rules! sync_lazy {
($($block:tt)*) => {
$crate::sync::Lazy {
__cell: $crate::sync::OnceCell::INIT,
__init: || { $($block)* },
}
};
}
}