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//! A little crate providing a type for creating a [`Arc`][`std::sync::Arc`] on the stack. use std::mem::MaybeUninit; use std::sync::{ atomic::{AtomicUsize, Ordering::*}, Arc, }; use std::ptr::NonNull; /// A type with the same layout as the allocated value of [`Arc`] that it's initialized /// once you convert a mutable reference to it. /// /// This type consist of the strong,weak count of the [`Arc`] and a [`MaybeUninit`] holding the /// data.The strong it's set to zero until you cast to the first [`Arc`] with [`to_arc`] or /// [`to_arc_with`] thus upgrading the count. /// /// # Examples /// /// The first use-case of this type is lazy-init an mutable static across threads and drop it when /// all ceases to use it. /// /// ``` /// use stackarc::ArcInner; /// use std::thread::{sleep, spawn}; /// use std::time::Duration; /// /// static mut A: ArcInner<String> = ArcInner::uninit(); /// /// fn main() { /// let x = spawn(|| unsafe { /// let _a = A.to_arc_with(|| format!("foobar")); /// sleep(Duration::from_secs(2)); /// }); /// /// let y = spawn(|| unsafe { /// let _a = A.to_arc_with(|| format!("barfoo")); /// sleep(Duration::from_secs(2)); /// }); /// /// // wait one second to wait for the threads to initialize the value /// // which in turn wait another to maintain their reference alive /// // and don't drop the count until create another /// sleep(Duration::from_secs(1)); /// let z = unsafe { A.to_arc_with(|| format!("baz")) }; /// /// assert_ne!(*z, "baz"); /// /// drop(z); /// x.join().unwrap(); /// y.join().unwrap(); /// let z = unsafe { A.to_arc_with(|| format!("foo")) }; /// /// assert_eq!(*z, "foo"); /// } /// ``` /// /// [`to_arc`]: #method.to_arc /// [`to_arc_with`]: #method.to_arc_with #[repr(C)] pub struct ArcInner<T> { strong: AtomicUsize, weak: AtomicUsize, data: MaybeUninit<T> } impl<T> ArcInner<T> { /// Declares a new `ArcInner` in an uninit state,use [`to_arc_with`] or [`to_arc`] to initialize it. /// /// [`to_arc`]: #method.to_arc /// [`to_arc_with`]: #method.to_arc_with #[inline] pub const fn uninit() -> Self { Self { strong: AtomicUsize::new(0), weak: AtomicUsize::new(1), data: MaybeUninit::uninit(), } } /// Returns an [`Arc`] after upgrading the counts and initialize the value calling `default` if /// the strong is zero. /// /// A good example is at the [type docs](#top). // maybe rename to `as_arc_with` as we are converting between references but with a little overhead when it's uninitialized #[inline] pub fn to_arc_with<F: FnOnce() -> T>(&mut self, default: F) -> Arc<T> { unsafe { if self.strong.fetch_add(1, SeqCst) == 0 { // we know that there's no other references that can be dropped // once we reach here so a relaxed order it's fine self.weak.store(2, Relaxed); self.as_mut_ptr().write(default()); } else { self.weak.fetch_add(1, SeqCst); } (&NonNull::from(self) as *const NonNull<Self> as *const Arc<T>).read() } } /// Returns an [`Arc`] after upgrading the counts and initialize the value with `x` if the /// strong is zero. /// /// A good example is at the [type docs](#top). #[inline] pub fn to_arc(&mut self, x: T) -> Arc<T> { self.to_arc_with(|| x) } /// Returns a raw mutable pointer to the inner data. #[inline] pub fn as_mut_ptr(&mut self) -> *mut T { self.data.as_mut_ptr() } /// Extracts a raw pointer to the inner data. #[inline] pub fn as_ptr(&self) -> *const T { self.data.as_ptr() } } #[cfg(test)] mod test { use super::*; #[test] fn a() { let mut a = ArcInner::uninit(); { let b = a.to_arc(1); assert_eq!(1, *b); } let c = a.to_arc(0); drop(a); assert_eq!(0, *c); } }