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//! Lazy values and one-time initialization of static data.
use crate::cell::{Cell, UnsafeCell};
use crate::fmt;
use crate::mem;
use crate::ops::Deref;
/// A cell which can be written to only once.
///
/// Unlike `RefCell`, a `OnceCell` only provides shared `&T` references to its value.
/// Unlike `Cell`, a `OnceCell` doesn't require copying or replacing the value to access it.
///
/// # Examples
///
/// ```
/// #![feature(once_cell)]
///
/// use std::lazy::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());
/// ```
#[unstable(feature = "once_cell", issue = "74465")]
pub struct OnceCell<T> {
// Invariant: written to at most once.
inner: UnsafeCell<Option<T>>,
}
#[unstable(feature = "once_cell", issue = "74465")]
impl<T> Default for OnceCell<T> {
fn default() -> Self {
Self::new()
}
}
#[unstable(feature = "once_cell", issue = "74465")]
impl<T: fmt::Debug> fmt::Debug for OnceCell<T> {
fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
match self.get() {
Some(v) => f.debug_tuple("OnceCell").field(v).finish(),
None => f.write_str("OnceCell(Uninit)"),
}
}
}
#[unstable(feature = "once_cell", issue = "74465")]
impl<T: Clone> Clone for OnceCell<T> {
fn clone(&self) -> OnceCell<T> {
let res = OnceCell::new();
if let Some(value) = self.get() {
match res.set(value.clone()) {
Ok(()) => (),
Err(_) => unreachable!(),
}
}
res
}
}
#[unstable(feature = "once_cell", issue = "74465")]
impl<T: PartialEq> PartialEq for OnceCell<T> {
fn eq(&self, other: &Self) -> bool {
self.get() == other.get()
}
}
#[unstable(feature = "once_cell", issue = "74465")]
impl<T: Eq> Eq for OnceCell<T> {}
#[unstable(feature = "once_cell", issue = "74465")]
impl<T> const From<T> for OnceCell<T> {
/// Creates a new `OnceCell<T>` which already contains the given `value`.
fn from(value: T) -> Self {
OnceCell { inner: UnsafeCell::new(Some(value)) }
}
}
impl<T> OnceCell<T> {
/// Creates a new empty cell.
#[unstable(feature = "once_cell", issue = "74465")]
#[must_use]
pub const fn new() -> OnceCell<T> {
OnceCell { inner: UnsafeCell::new(None) }
}
/// Gets the reference to the underlying value.
///
/// Returns `None` if the cell is empty.
#[unstable(feature = "once_cell", issue = "74465")]
pub fn get(&self) -> Option<&T> {
// SAFETY: Safe due to `inner`'s invariant
unsafe { &*self.inner.get() }.as_ref()
}
/// Gets the mutable reference to the underlying value.
///
/// Returns `None` if the cell is empty.
#[unstable(feature = "once_cell", issue = "74465")]
pub fn get_mut(&mut self) -> Option<&mut T> {
self.inner.get_mut().as_mut()
}
/// Sets the contents of the cell to `value`.
///
/// # Errors
///
/// This method returns `Ok(())` if the cell was empty and `Err(value)` if
/// it was full.
///
/// # Examples
///
/// ```
/// #![feature(once_cell)]
///
/// use std::lazy::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());
/// ```
#[unstable(feature = "once_cell", issue = "74465")]
pub fn set(&self, value: T) -> Result<(), T> {
// SAFETY: Safe because we cannot have overlapping mutable borrows
let slot = unsafe { &*self.inner.get() };
if slot.is_some() {
return Err(value);
}
// SAFETY: 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.
let slot = unsafe { &mut *self.inner.get() };
*slot = Some(value);
Ok(())
}
/// Gets the contents of the cell, initializing it with `f`
/// if the cell was empty.
///
/// # Panics
///
/// If `f` panics, the panic is propagated to the caller, and the cell
/// remains uninitialized.
///
/// It is an error to reentrantly initialize the cell from `f`. Doing
/// so results in a panic.
///
/// # Examples
///
/// ```
/// #![feature(once_cell)]
///
/// use std::lazy::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);
/// ```
#[unstable(feature = "once_cell", issue = "74465")]
pub fn get_or_init<F>(&self, f: F) -> &T
where
F: FnOnce() -> T,
{
match self.get_or_try_init(|| Ok::<T, !>(f())) {
Ok(val) => val,
}
}
/// Gets the contents of the cell, initializing it with `f` if
/// the cell was empty. If the cell was empty and `f` failed, an
/// error is returned.
///
/// # Panics
///
/// If `f` panics, the panic is propagated to the caller, and the cell
/// remains uninitialized.
///
/// It is an error to reentrantly initialize the cell from `f`. Doing
/// so results in a panic.
///
/// # Examples
///
/// ```
/// #![feature(once_cell)]
///
/// use std::lazy::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))
/// ```
#[unstable(feature = "once_cell", issue = "74465")]
pub fn get_or_try_init<F, E>(&self, f: F) -> Result<&T, E>
where
F: FnOnce() -> Result<T, E>,
{
if let Some(val) = self.get() {
return Ok(val);
}
/// Avoid inlining the initialization closure into the common path that fetches
/// the already initialized value
#[cold]
fn outlined_call<F, T, E>(f: F) -> Result<T, E>
where
F: FnOnce() -> Result<T, E>,
{
f()
}
let val = outlined_call(f)?;
// Note that *some* forms of reentrant initialization might lead to
// UB (see `reentrant_init` test). I believe that just removing this
// `assert`, while keeping `set/get` would be sound, but it seems
// better to panic, rather than to silently use an old value.
assert!(self.set(val).is_ok(), "reentrant init");
Ok(self.get().unwrap())
}
/// Consumes the cell, returning the wrapped value.
///
/// Returns `None` if the cell was empty.
///
/// # Examples
///
/// ```
/// #![feature(once_cell)]
///
/// use std::lazy::OnceCell;
///
/// let cell: OnceCell<String> = OnceCell::new();
/// assert_eq!(cell.into_inner(), None);
///
/// let cell = OnceCell::new();
/// cell.set("hello".to_string()).unwrap();
/// assert_eq!(cell.into_inner(), Some("hello".to_string()));
/// ```
#[unstable(feature = "once_cell", issue = "74465")]
pub fn into_inner(self) -> Option<T> {
// Because `into_inner` takes `self` by value, the compiler statically verifies
// that it is not currently borrowed. So it is safe to move out `Option<T>`.
self.inner.into_inner()
}
/// Takes the value out of this `OnceCell`, moving it back to an uninitialized state.
///
/// Has no effect and returns `None` if the `OnceCell` hasn't been initialized.
///
/// Safety is guaranteed by requiring a mutable reference.
///
/// # Examples
///
/// ```
/// #![feature(once_cell)]
///
/// use std::lazy::OnceCell;
///
/// let mut cell: OnceCell<String> = OnceCell::new();
/// assert_eq!(cell.take(), None);
///
/// let mut cell = OnceCell::new();
/// cell.set("hello".to_string()).unwrap();
/// assert_eq!(cell.take(), Some("hello".to_string()));
/// assert_eq!(cell.get(), None);
/// ```
#[unstable(feature = "once_cell", issue = "74465")]
pub fn take(&mut self) -> Option<T> {
mem::take(self).into_inner()
}
}
/// A value which is initialized on the first access.
///
/// # Examples
///
/// ```
/// #![feature(once_cell)]
///
/// use std::lazy::Lazy;
///
/// let lazy: Lazy<i32> = Lazy::new(|| {
/// println!("initializing");
/// 92
/// });
/// println!("ready");
/// println!("{}", *lazy);
/// println!("{}", *lazy);
///
/// // Prints:
/// // ready
/// // initializing
/// // 92
/// // 92
/// ```
#[unstable(feature = "once_cell", issue = "74465")]
pub struct Lazy<T, F = fn() -> T> {
cell: OnceCell<T>,
init: Cell<Option<F>>,
}
#[unstable(feature = "once_cell", issue = "74465")]
impl<T: fmt::Debug, F> fmt::Debug for Lazy<T, F> {
fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
f.debug_struct("Lazy").field("cell", &self.cell).field("init", &"..").finish()
}
}
impl<T, F> Lazy<T, F> {
/// Creates a new lazy value with the given initializing function.
///
/// # Examples
///
/// ```
/// #![feature(once_cell)]
///
/// # fn main() {
/// use std::lazy::Lazy;
///
/// let hello = "Hello, World!".to_string();
///
/// let lazy = Lazy::new(|| hello.to_uppercase());
///
/// assert_eq!(&*lazy, "HELLO, WORLD!");
/// # }
/// ```
#[unstable(feature = "once_cell", issue = "74465")]
pub const fn new(init: F) -> Lazy<T, F> {
Lazy { cell: OnceCell::new(), init: Cell::new(Some(init)) }
}
}
impl<T, F: FnOnce() -> T> Lazy<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::lazy::Lazy;
///
/// let lazy = Lazy::new(|| 92);
///
/// assert_eq!(Lazy::force(&lazy), &92);
/// assert_eq!(&*lazy, &92);
/// ```
#[unstable(feature = "once_cell", issue = "74465")]
pub fn force(this: &Lazy<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 Lazy<T, F> {
type Target = T;
fn deref(&self) -> &T {
Lazy::force(self)
}
}
#[unstable(feature = "once_cell", issue = "74465")]
impl<T: Default> Default for Lazy<T> {
/// Creates a new lazy value using `Default` as the initializing function.
fn default() -> Lazy<T> {
Lazy::new(T::default)
}
}