1 #![stable(feature = "rust1", since = "1.0.0")]
3 //! Thread-safe reference-counting pointers.
5 //! See the [`Arc<T>`][arc] documentation for more details.
7 //! [arc]: struct.Arc.html
10 use core::array::LengthAtMost32;
12 use core::cmp::Ordering;
13 use core::convert::{From, TryFrom};
15 use core::hash::{Hash, Hasher};
16 use core::intrinsics::abort;
18 use core::marker::{PhantomData, Unpin, Unsize};
19 use core::mem::{self, align_of, align_of_val, size_of_val};
20 use core::ops::{CoerceUnsized, Deref, DispatchFromDyn, Receiver};
22 use core::ptr::{self, NonNull};
23 use core::slice::{self, from_raw_parts_mut};
24 use core::sync::atomic;
25 use core::sync::atomic::Ordering::{Acquire, Relaxed, Release, SeqCst};
26 use core::{isize, usize};
28 use crate::alloc::{box_free, handle_alloc_error, Alloc, Global, Layout};
29 use crate::boxed::Box;
30 use crate::rc::is_dangling;
31 use crate::string::String;
37 /// A soft limit on the amount of references that may be made to an `Arc`.
39 /// Going above this limit will abort your program (although not
40 /// necessarily) at _exactly_ `MAX_REFCOUNT + 1` references.
41 const MAX_REFCOUNT: usize = (isize::MAX) as usize;
43 /// A thread-safe reference-counting pointer. 'Arc' stands for 'Atomically
44 /// Reference Counted'.
46 /// The type `Arc<T>` provides shared ownership of a value of type `T`,
47 /// allocated in the heap. Invoking [`clone`][clone] on `Arc` produces
48 /// a new `Arc` instance, which points to the same allocation on the heap as the
49 /// source `Arc`, while increasing a reference count. When the last `Arc`
50 /// pointer to a given allocation is destroyed, the value stored in that allocation (often
51 /// referred to as "inner value") is also dropped.
53 /// Shared references in Rust disallow mutation by default, and `Arc` is no
54 /// exception: you cannot generally obtain a mutable reference to something
55 /// inside an `Arc`. If you need to mutate through an `Arc`, use
56 /// [`Mutex`][mutex], [`RwLock`][rwlock], or one of the [`Atomic`][atomic]
61 /// Unlike [`Rc<T>`], `Arc<T>` uses atomic operations for its reference
62 /// counting. This means that it is thread-safe. The disadvantage is that
63 /// atomic operations are more expensive than ordinary memory accesses. If you
64 /// are not sharing reference-counted allocations between threads, consider using
65 /// [`Rc<T>`] for lower overhead. [`Rc<T>`] is a safe default, because the
66 /// compiler will catch any attempt to send an [`Rc<T>`] between threads.
67 /// However, a library might choose `Arc<T>` in order to give library consumers
70 /// `Arc<T>` will implement [`Send`] and [`Sync`] as long as the `T` implements
71 /// [`Send`] and [`Sync`]. Why can't you put a non-thread-safe type `T` in an
72 /// `Arc<T>` to make it thread-safe? This may be a bit counter-intuitive at
73 /// first: after all, isn't the point of `Arc<T>` thread safety? The key is
74 /// this: `Arc<T>` makes it thread safe to have multiple ownership of the same
75 /// data, but it doesn't add thread safety to its data. Consider
76 /// `Arc<`[`RefCell<T>`]`>`. [`RefCell<T>`] isn't [`Sync`], and if `Arc<T>` was always
77 /// [`Send`], `Arc<`[`RefCell<T>`]`>` would be as well. But then we'd have a problem:
78 /// [`RefCell<T>`] is not thread safe; it keeps track of the borrowing count using
79 /// non-atomic operations.
81 /// In the end, this means that you may need to pair `Arc<T>` with some sort of
82 /// [`std::sync`] type, usually [`Mutex<T>`][mutex].
84 /// ## Breaking cycles with `Weak`
86 /// The [`downgrade`][downgrade] method can be used to create a non-owning
87 /// [`Weak`][weak] pointer. A [`Weak`][weak] pointer can be [`upgrade`][upgrade]d
88 /// to an `Arc`, but this will return [`None`] if the value stored in the allocation has
89 /// already been dropped. In other words, `Weak` pointers do not keep the value
90 /// inside the allocation alive; however, they *do* keep the allocation
91 /// (the backing store for the value) alive.
93 /// A cycle between `Arc` pointers will never be deallocated. For this reason,
94 /// [`Weak`][weak] is used to break cycles. For example, a tree could have
95 /// strong `Arc` pointers from parent nodes to children, and [`Weak`][weak]
96 /// pointers from children back to their parents.
98 /// # Cloning references
100 /// Creating a new reference from an existing reference counted pointer is done using the
101 /// `Clone` trait implemented for [`Arc<T>`][arc] and [`Weak<T>`][weak].
104 /// use std::sync::Arc;
105 /// let foo = Arc::new(vec![1.0, 2.0, 3.0]);
106 /// // The two syntaxes below are equivalent.
107 /// let a = foo.clone();
108 /// let b = Arc::clone(&foo);
109 /// // a, b, and foo are all Arcs that point to the same memory location
112 /// ## `Deref` behavior
114 /// `Arc<T>` automatically dereferences to `T` (via the [`Deref`][deref] trait),
115 /// so you can call `T`'s methods on a value of type `Arc<T>`. To avoid name
116 /// clashes with `T`'s methods, the methods of `Arc<T>` itself are associated
117 /// functions, called using function-like syntax:
120 /// use std::sync::Arc;
121 /// let my_arc = Arc::new(());
123 /// Arc::downgrade(&my_arc);
126 /// [`Weak<T>`][weak] does not auto-dereference to `T`, because the inner value may have
127 /// already been dropped.
129 /// [arc]: struct.Arc.html
130 /// [weak]: struct.Weak.html
131 /// [`Rc<T>`]: ../../std/rc/struct.Rc.html
132 /// [clone]: ../../std/clone/trait.Clone.html#tymethod.clone
133 /// [mutex]: ../../std/sync/struct.Mutex.html
134 /// [rwlock]: ../../std/sync/struct.RwLock.html
135 /// [atomic]: ../../std/sync/atomic/index.html
136 /// [`Send`]: ../../std/marker/trait.Send.html
137 /// [`Sync`]: ../../std/marker/trait.Sync.html
138 /// [deref]: ../../std/ops/trait.Deref.html
139 /// [downgrade]: struct.Arc.html#method.downgrade
140 /// [upgrade]: struct.Weak.html#method.upgrade
141 /// [`None`]: ../../std/option/enum.Option.html#variant.None
142 /// [`RefCell<T>`]: ../../std/cell/struct.RefCell.html
143 /// [`std::sync`]: ../../std/sync/index.html
144 /// [`Arc::clone(&from)`]: #method.clone
148 /// Sharing some immutable data between threads:
150 // Note that we **do not** run these tests here. The windows builders get super
151 // unhappy if a thread outlives the main thread and then exits at the same time
152 // (something deadlocks) so we just avoid this entirely by not running these
155 /// use std::sync::Arc;
158 /// let five = Arc::new(5);
161 /// let five = Arc::clone(&five);
163 /// thread::spawn(move || {
164 /// println!("{:?}", five);
169 /// Sharing a mutable [`AtomicUsize`]:
171 /// [`AtomicUsize`]: ../../std/sync/atomic/struct.AtomicUsize.html
174 /// use std::sync::Arc;
175 /// use std::sync::atomic::{AtomicUsize, Ordering};
178 /// let val = Arc::new(AtomicUsize::new(5));
181 /// let val = Arc::clone(&val);
183 /// thread::spawn(move || {
184 /// let v = val.fetch_add(1, Ordering::SeqCst);
185 /// println!("{:?}", v);
190 /// See the [`rc` documentation][rc_examples] for more examples of reference
191 /// counting in general.
193 /// [rc_examples]: ../../std/rc/index.html#examples
194 #[cfg_attr(not(test), lang = "arc")]
195 #[stable(feature = "rust1", since = "1.0.0")]
196 pub struct Arc<T: ?Sized> {
197 ptr: NonNull<ArcInner<T>>,
198 phantom: PhantomData<ArcInner<T>>,
201 #[stable(feature = "rust1", since = "1.0.0")]
202 unsafe impl<T: ?Sized + Sync + Send> Send for Arc<T> {}
203 #[stable(feature = "rust1", since = "1.0.0")]
204 unsafe impl<T: ?Sized + Sync + Send> Sync for Arc<T> {}
206 #[unstable(feature = "coerce_unsized", issue = "27732")]
207 impl<T: ?Sized + Unsize<U>, U: ?Sized> CoerceUnsized<Arc<U>> for Arc<T> {}
209 #[unstable(feature = "dispatch_from_dyn", issue = "none")]
210 impl<T: ?Sized + Unsize<U>, U: ?Sized> DispatchFromDyn<Arc<U>> for Arc<T> {}
212 impl<T: ?Sized> Arc<T> {
213 fn from_inner(ptr: NonNull<ArcInner<T>>) -> Self {
214 Self { ptr, phantom: PhantomData }
217 unsafe fn from_ptr(ptr: *mut ArcInner<T>) -> Self {
218 Self::from_inner(NonNull::new_unchecked(ptr))
222 /// `Weak` is a version of [`Arc`] that holds a non-owning reference to the
223 /// managed allocation. The allocation is accessed by calling [`upgrade`] on the `Weak`
224 /// pointer, which returns an [`Option`]`<`[`Arc`]`<T>>`.
226 /// Since a `Weak` reference does not count towards ownership, it will not
227 /// prevent the value stored in the allocation from being dropped, and `Weak` itself makes no
228 /// guarantees about the value still being present. Thus it may return [`None`]
229 /// when [`upgrade`]d. Note however that a `Weak` reference *does* prevent the allocation
230 /// itself (the backing store) from being deallocated.
232 /// A `Weak` pointer is useful for keeping a temporary reference to the allocation
233 /// managed by [`Arc`] without preventing its inner value from being dropped. It is also used to
234 /// prevent circular references between [`Arc`] pointers, since mutual owning references
235 /// would never allow either [`Arc`] to be dropped. For example, a tree could
236 /// have strong [`Arc`] pointers from parent nodes to children, and `Weak`
237 /// pointers from children back to their parents.
239 /// The typical way to obtain a `Weak` pointer is to call [`Arc::downgrade`].
241 /// [`Arc`]: struct.Arc.html
242 /// [`Arc::downgrade`]: struct.Arc.html#method.downgrade
243 /// [`upgrade`]: struct.Weak.html#method.upgrade
244 /// [`Option`]: ../../std/option/enum.Option.html
245 /// [`None`]: ../../std/option/enum.Option.html#variant.None
246 #[stable(feature = "arc_weak", since = "1.4.0")]
247 pub struct Weak<T: ?Sized> {
248 // This is a `NonNull` to allow optimizing the size of this type in enums,
249 // but it is not necessarily a valid pointer.
250 // `Weak::new` sets this to `usize::MAX` so that it doesn’t need
251 // to allocate space on the heap. That's not a value a real pointer
252 // will ever have because RcBox has alignment at least 2.
253 ptr: NonNull<ArcInner<T>>,
256 #[stable(feature = "arc_weak", since = "1.4.0")]
257 unsafe impl<T: ?Sized + Sync + Send> Send for Weak<T> {}
258 #[stable(feature = "arc_weak", since = "1.4.0")]
259 unsafe impl<T: ?Sized + Sync + Send> Sync for Weak<T> {}
261 #[unstable(feature = "coerce_unsized", issue = "27732")]
262 impl<T: ?Sized + Unsize<U>, U: ?Sized> CoerceUnsized<Weak<U>> for Weak<T> {}
263 #[unstable(feature = "dispatch_from_dyn", issue = "none")]
264 impl<T: ?Sized + Unsize<U>, U: ?Sized> DispatchFromDyn<Weak<U>> for Weak<T> {}
266 #[stable(feature = "arc_weak", since = "1.4.0")]
267 impl<T: ?Sized + fmt::Debug> fmt::Debug for Weak<T> {
268 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
273 struct ArcInner<T: ?Sized> {
274 strong: atomic::AtomicUsize,
276 // the value usize::MAX acts as a sentinel for temporarily "locking" the
277 // ability to upgrade weak pointers or downgrade strong ones; this is used
278 // to avoid races in `make_mut` and `get_mut`.
279 weak: atomic::AtomicUsize,
284 unsafe impl<T: ?Sized + Sync + Send> Send for ArcInner<T> {}
285 unsafe impl<T: ?Sized + Sync + Send> Sync for ArcInner<T> {}
288 /// Constructs a new `Arc<T>`.
293 /// use std::sync::Arc;
295 /// let five = Arc::new(5);
298 #[stable(feature = "rust1", since = "1.0.0")]
299 pub fn new(data: T) -> Arc<T> {
300 // Start the weak pointer count as 1 which is the weak pointer that's
301 // held by all the strong pointers (kinda), see std/rc.rs for more info
302 let x: Box<_> = box ArcInner {
303 strong: atomic::AtomicUsize::new(1),
304 weak: atomic::AtomicUsize::new(1),
307 Self::from_inner(Box::into_raw_non_null(x))
310 /// Constructs a new `Arc` with uninitialized contents.
315 /// #![feature(new_uninit)]
316 /// #![feature(get_mut_unchecked)]
318 /// use std::sync::Arc;
320 /// let mut five = Arc::<u32>::new_uninit();
322 /// let five = unsafe {
323 /// // Deferred initialization:
324 /// Arc::get_mut_unchecked(&mut five).as_mut_ptr().write(5);
326 /// five.assume_init()
329 /// assert_eq!(*five, 5)
331 #[unstable(feature = "new_uninit", issue = "63291")]
332 pub fn new_uninit() -> Arc<mem::MaybeUninit<T>> {
334 Arc::from_ptr(Arc::allocate_for_layout(Layout::new::<T>(), |mem| {
335 mem as *mut ArcInner<mem::MaybeUninit<T>>
340 /// Constructs a new `Arc` with uninitialized contents, with the memory
341 /// being filled with `0` bytes.
343 /// See [`MaybeUninit::zeroed`][zeroed] for examples of correct and incorrect usage
349 /// #![feature(new_uninit)]
351 /// use std::sync::Arc;
353 /// let zero = Arc::<u32>::new_zeroed();
354 /// let zero = unsafe { zero.assume_init() };
356 /// assert_eq!(*zero, 0)
359 /// [zeroed]: ../../std/mem/union.MaybeUninit.html#method.zeroed
360 #[unstable(feature = "new_uninit", issue = "63291")]
361 pub fn new_zeroed() -> Arc<mem::MaybeUninit<T>> {
363 let mut uninit = Self::new_uninit();
364 ptr::write_bytes::<T>(Arc::get_mut_unchecked(&mut uninit).as_mut_ptr(), 0, 1);
369 /// Constructs a new `Pin<Arc<T>>`. If `T` does not implement `Unpin`, then
370 /// `data` will be pinned in memory and unable to be moved.
371 #[stable(feature = "pin", since = "1.33.0")]
372 pub fn pin(data: T) -> Pin<Arc<T>> {
373 unsafe { Pin::new_unchecked(Arc::new(data)) }
376 /// Returns the inner value, if the `Arc` has exactly one strong reference.
378 /// Otherwise, an [`Err`][result] is returned with the same `Arc` that was
381 /// This will succeed even if there are outstanding weak references.
383 /// [result]: ../../std/result/enum.Result.html
388 /// use std::sync::Arc;
390 /// let x = Arc::new(3);
391 /// assert_eq!(Arc::try_unwrap(x), Ok(3));
393 /// let x = Arc::new(4);
394 /// let _y = Arc::clone(&x);
395 /// assert_eq!(*Arc::try_unwrap(x).unwrap_err(), 4);
398 #[stable(feature = "arc_unique", since = "1.4.0")]
399 pub fn try_unwrap(this: Self) -> Result<T, Self> {
400 // See `drop` for why all these atomics are like this
401 if this.inner().strong.compare_exchange(1, 0, Release, Relaxed).is_err() {
405 atomic::fence(Acquire);
408 let elem = ptr::read(&this.ptr.as_ref().data);
410 // Make a weak pointer to clean up the implicit strong-weak reference
411 let _weak = Weak { ptr: this.ptr };
420 /// Constructs a new reference-counted slice with uninitialized contents.
425 /// #![feature(new_uninit)]
426 /// #![feature(get_mut_unchecked)]
428 /// use std::sync::Arc;
430 /// let mut values = Arc::<[u32]>::new_uninit_slice(3);
432 /// let values = unsafe {
433 /// // Deferred initialization:
434 /// Arc::get_mut_unchecked(&mut values)[0].as_mut_ptr().write(1);
435 /// Arc::get_mut_unchecked(&mut values)[1].as_mut_ptr().write(2);
436 /// Arc::get_mut_unchecked(&mut values)[2].as_mut_ptr().write(3);
438 /// values.assume_init()
441 /// assert_eq!(*values, [1, 2, 3])
443 #[unstable(feature = "new_uninit", issue = "63291")]
444 pub fn new_uninit_slice(len: usize) -> Arc<[mem::MaybeUninit<T>]> {
445 unsafe { Arc::from_ptr(Arc::allocate_for_slice(len)) }
449 impl<T> Arc<mem::MaybeUninit<T>> {
450 /// Converts to `Arc<T>`.
454 /// As with [`MaybeUninit::assume_init`],
455 /// it is up to the caller to guarantee that the inner value
456 /// really is in an initialized state.
457 /// Calling this when the content is not yet fully initialized
458 /// causes immediate undefined behavior.
460 /// [`MaybeUninit::assume_init`]: ../../std/mem/union.MaybeUninit.html#method.assume_init
465 /// #![feature(new_uninit)]
466 /// #![feature(get_mut_unchecked)]
468 /// use std::sync::Arc;
470 /// let mut five = Arc::<u32>::new_uninit();
472 /// let five = unsafe {
473 /// // Deferred initialization:
474 /// Arc::get_mut_unchecked(&mut five).as_mut_ptr().write(5);
476 /// five.assume_init()
479 /// assert_eq!(*five, 5)
481 #[unstable(feature = "new_uninit", issue = "63291")]
483 pub unsafe fn assume_init(self) -> Arc<T> {
484 Arc::from_inner(mem::ManuallyDrop::new(self).ptr.cast())
488 impl<T> Arc<[mem::MaybeUninit<T>]> {
489 /// Converts to `Arc<[T]>`.
493 /// As with [`MaybeUninit::assume_init`],
494 /// it is up to the caller to guarantee that the inner value
495 /// really is in an initialized state.
496 /// Calling this when the content is not yet fully initialized
497 /// causes immediate undefined behavior.
499 /// [`MaybeUninit::assume_init`]: ../../std/mem/union.MaybeUninit.html#method.assume_init
504 /// #![feature(new_uninit)]
505 /// #![feature(get_mut_unchecked)]
507 /// use std::sync::Arc;
509 /// let mut values = Arc::<[u32]>::new_uninit_slice(3);
511 /// let values = unsafe {
512 /// // Deferred initialization:
513 /// Arc::get_mut_unchecked(&mut values)[0].as_mut_ptr().write(1);
514 /// Arc::get_mut_unchecked(&mut values)[1].as_mut_ptr().write(2);
515 /// Arc::get_mut_unchecked(&mut values)[2].as_mut_ptr().write(3);
517 /// values.assume_init()
520 /// assert_eq!(*values, [1, 2, 3])
522 #[unstable(feature = "new_uninit", issue = "63291")]
524 pub unsafe fn assume_init(self) -> Arc<[T]> {
525 Arc::from_ptr(mem::ManuallyDrop::new(self).ptr.as_ptr() as _)
529 impl<T: ?Sized> Arc<T> {
530 /// Consumes the `Arc`, returning the wrapped pointer.
532 /// To avoid a memory leak the pointer must be converted back to an `Arc` using
533 /// [`Arc::from_raw`][from_raw].
535 /// [from_raw]: struct.Arc.html#method.from_raw
540 /// use std::sync::Arc;
542 /// let x = Arc::new("hello".to_owned());
543 /// let x_ptr = Arc::into_raw(x);
544 /// assert_eq!(unsafe { &*x_ptr }, "hello");
546 #[stable(feature = "rc_raw", since = "1.17.0")]
547 pub fn into_raw(this: Self) -> *const T {
548 let ptr: *const T = &*this;
553 /// Constructs an `Arc` from a raw pointer.
555 /// The raw pointer must have been previously returned by a call to a
556 /// [`Arc::into_raw`][into_raw].
558 /// This function is unsafe because improper use may lead to memory problems. For example, a
559 /// double-free may occur if the function is called twice on the same raw pointer.
561 /// [into_raw]: struct.Arc.html#method.into_raw
566 /// use std::sync::Arc;
568 /// let x = Arc::new("hello".to_owned());
569 /// let x_ptr = Arc::into_raw(x);
572 /// // Convert back to an `Arc` to prevent leak.
573 /// let x = Arc::from_raw(x_ptr);
574 /// assert_eq!(&*x, "hello");
576 /// // Further calls to `Arc::from_raw(x_ptr)` would be memory-unsafe.
579 /// // The memory was freed when `x` went out of scope above, so `x_ptr` is now dangling!
581 #[stable(feature = "rc_raw", since = "1.17.0")]
582 pub unsafe fn from_raw(ptr: *const T) -> Self {
583 let offset = data_offset(ptr);
585 // Reverse the offset to find the original ArcInner.
586 let fake_ptr = ptr as *mut ArcInner<T>;
587 let arc_ptr = set_data_ptr(fake_ptr, (ptr as *mut u8).offset(-offset));
589 Self::from_ptr(arc_ptr)
592 /// Consumes the `Arc`, returning the wrapped pointer as `NonNull<T>`.
597 /// #![feature(rc_into_raw_non_null)]
599 /// use std::sync::Arc;
601 /// let x = Arc::new("hello".to_owned());
602 /// let ptr = Arc::into_raw_non_null(x);
603 /// let deref = unsafe { ptr.as_ref() };
604 /// assert_eq!(deref, "hello");
606 #[unstable(feature = "rc_into_raw_non_null", issue = "47336")]
608 pub fn into_raw_non_null(this: Self) -> NonNull<T> {
609 // safe because Arc guarantees its pointer is non-null
610 unsafe { NonNull::new_unchecked(Arc::into_raw(this) as *mut _) }
613 /// Creates a new [`Weak`][weak] pointer to this allocation.
615 /// [weak]: struct.Weak.html
620 /// use std::sync::Arc;
622 /// let five = Arc::new(5);
624 /// let weak_five = Arc::downgrade(&five);
626 #[stable(feature = "arc_weak", since = "1.4.0")]
627 pub fn downgrade(this: &Self) -> Weak<T> {
628 // This Relaxed is OK because we're checking the value in the CAS
630 let mut cur = this.inner().weak.load(Relaxed);
633 // check if the weak counter is currently "locked"; if so, spin.
634 if cur == usize::MAX {
635 cur = this.inner().weak.load(Relaxed);
639 // NOTE: this code currently ignores the possibility of overflow
640 // into usize::MAX; in general both Rc and Arc need to be adjusted
641 // to deal with overflow.
643 // Unlike with Clone(), we need this to be an Acquire read to
644 // synchronize with the write coming from `is_unique`, so that the
645 // events prior to that write happen before this read.
646 match this.inner().weak.compare_exchange_weak(cur, cur + 1, Acquire, Relaxed) {
648 // Make sure we do not create a dangling Weak
649 debug_assert!(!is_dangling(this.ptr));
650 return Weak { ptr: this.ptr };
652 Err(old) => cur = old,
657 /// Gets the number of [`Weak`][weak] pointers to this allocation.
659 /// [weak]: struct.Weak.html
663 /// This method by itself is safe, but using it correctly requires extra care.
664 /// Another thread can change the weak count at any time,
665 /// including potentially between calling this method and acting on the result.
670 /// use std::sync::Arc;
672 /// let five = Arc::new(5);
673 /// let _weak_five = Arc::downgrade(&five);
675 /// // This assertion is deterministic because we haven't shared
676 /// // the `Arc` or `Weak` between threads.
677 /// assert_eq!(1, Arc::weak_count(&five));
680 #[stable(feature = "arc_counts", since = "1.15.0")]
681 pub fn weak_count(this: &Self) -> usize {
682 let cnt = this.inner().weak.load(SeqCst);
683 // If the weak count is currently locked, the value of the
684 // count was 0 just before taking the lock.
685 if cnt == usize::MAX { 0 } else { cnt - 1 }
688 /// Gets the number of strong (`Arc`) pointers to this allocation.
692 /// This method by itself is safe, but using it correctly requires extra care.
693 /// Another thread can change the strong count at any time,
694 /// including potentially between calling this method and acting on the result.
699 /// use std::sync::Arc;
701 /// let five = Arc::new(5);
702 /// let _also_five = Arc::clone(&five);
704 /// // This assertion is deterministic because we haven't shared
705 /// // the `Arc` between threads.
706 /// assert_eq!(2, Arc::strong_count(&five));
709 #[stable(feature = "arc_counts", since = "1.15.0")]
710 pub fn strong_count(this: &Self) -> usize {
711 this.inner().strong.load(SeqCst)
715 fn inner(&self) -> &ArcInner<T> {
716 // This unsafety is ok because while this arc is alive we're guaranteed
717 // that the inner pointer is valid. Furthermore, we know that the
718 // `ArcInner` structure itself is `Sync` because the inner data is
719 // `Sync` as well, so we're ok loaning out an immutable pointer to these
721 unsafe { self.ptr.as_ref() }
724 // Non-inlined part of `drop`.
726 unsafe fn drop_slow(&mut self) {
727 // Destroy the data at this time, even though we may not free the box
728 // allocation itself (there may still be weak pointers lying around).
729 ptr::drop_in_place(&mut self.ptr.as_mut().data);
731 if self.inner().weak.fetch_sub(1, Release) == 1 {
732 atomic::fence(Acquire);
733 Global.dealloc(self.ptr.cast(), Layout::for_value(self.ptr.as_ref()))
738 #[stable(feature = "ptr_eq", since = "1.17.0")]
739 /// Returns `true` if the two `Arc`s point to the same allocation
740 /// (in a vein similar to [`ptr::eq`]).
745 /// use std::sync::Arc;
747 /// let five = Arc::new(5);
748 /// let same_five = Arc::clone(&five);
749 /// let other_five = Arc::new(5);
751 /// assert!(Arc::ptr_eq(&five, &same_five));
752 /// assert!(!Arc::ptr_eq(&five, &other_five));
755 /// [`ptr::eq`]: ../../std/ptr/fn.eq.html
756 pub fn ptr_eq(this: &Self, other: &Self) -> bool {
757 this.ptr.as_ptr() == other.ptr.as_ptr()
761 impl<T: ?Sized> Arc<T> {
762 /// Allocates an `ArcInner<T>` with sufficient space for
763 /// a possibly-unsized inner value where the value has the layout provided.
765 /// The function `mem_to_arcinner` is called with the data pointer
766 /// and must return back a (potentially fat)-pointer for the `ArcInner<T>`.
767 unsafe fn allocate_for_layout(
768 value_layout: Layout,
769 mem_to_arcinner: impl FnOnce(*mut u8) -> *mut ArcInner<T>,
770 ) -> *mut ArcInner<T> {
771 // Calculate layout using the given value layout.
772 // Previously, layout was calculated on the expression
773 // `&*(ptr as *const ArcInner<T>)`, but this created a misaligned
774 // reference (see #54908).
775 let layout = Layout::new::<ArcInner<()>>().extend(value_layout).unwrap().0.pad_to_align();
777 let mem = Global.alloc(layout).unwrap_or_else(|_| handle_alloc_error(layout));
779 // Initialize the ArcInner
780 let inner = mem_to_arcinner(mem.as_ptr());
781 debug_assert_eq!(Layout::for_value(&*inner), layout);
783 ptr::write(&mut (*inner).strong, atomic::AtomicUsize::new(1));
784 ptr::write(&mut (*inner).weak, atomic::AtomicUsize::new(1));
789 /// Allocates an `ArcInner<T>` with sufficient space for an unsized inner value.
790 unsafe fn allocate_for_ptr(ptr: *const T) -> *mut ArcInner<T> {
791 // Allocate for the `ArcInner<T>` using the given value.
792 Self::allocate_for_layout(Layout::for_value(&*ptr), |mem| {
793 set_data_ptr(ptr as *mut T, mem) as *mut ArcInner<T>
797 fn from_box(v: Box<T>) -> Arc<T> {
799 let box_unique = Box::into_unique(v);
800 let bptr = box_unique.as_ptr();
802 let value_size = size_of_val(&*bptr);
803 let ptr = Self::allocate_for_ptr(bptr);
805 // Copy value as bytes
806 ptr::copy_nonoverlapping(
807 bptr as *const T as *const u8,
808 &mut (*ptr).data as *mut _ as *mut u8,
812 // Free the allocation without dropping its contents
813 box_free(box_unique);
821 /// Allocates an `ArcInner<[T]>` with the given length.
822 unsafe fn allocate_for_slice(len: usize) -> *mut ArcInner<[T]> {
823 Self::allocate_for_layout(Layout::array::<T>(len).unwrap(), |mem| {
824 ptr::slice_from_raw_parts_mut(mem as *mut T, len) as *mut ArcInner<[T]>
829 /// Sets the data pointer of a `?Sized` raw pointer.
831 /// For a slice/trait object, this sets the `data` field and leaves the rest
832 /// unchanged. For a sized raw pointer, this simply sets the pointer.
833 unsafe fn set_data_ptr<T: ?Sized, U>(mut ptr: *mut T, data: *mut U) -> *mut T {
834 ptr::write(&mut ptr as *mut _ as *mut *mut u8, data as *mut u8);
839 /// Copy elements from slice into newly allocated Arc<[T]>
841 /// Unsafe because the caller must either take ownership or bind `T: Copy`.
842 unsafe fn copy_from_slice(v: &[T]) -> Arc<[T]> {
843 let ptr = Self::allocate_for_slice(v.len());
845 ptr::copy_nonoverlapping(v.as_ptr(), &mut (*ptr).data as *mut [T] as *mut T, v.len());
850 /// Constructs an `Arc<[T]>` from an iterator known to be of a certain size.
852 /// Behavior is undefined should the size be wrong.
853 unsafe fn from_iter_exact(iter: impl iter::Iterator<Item = T>, len: usize) -> Arc<[T]> {
854 // Panic guard while cloning T elements.
855 // In the event of a panic, elements that have been written
856 // into the new ArcInner will be dropped, then the memory freed.
864 impl<T> Drop for Guard<T> {
867 let slice = from_raw_parts_mut(self.elems, self.n_elems);
868 ptr::drop_in_place(slice);
870 Global.dealloc(self.mem.cast(), self.layout);
875 let ptr = Self::allocate_for_slice(len);
877 let mem = ptr as *mut _ as *mut u8;
878 let layout = Layout::for_value(&*ptr);
880 // Pointer to first element
881 let elems = &mut (*ptr).data as *mut [T] as *mut T;
883 let mut guard = Guard { mem: NonNull::new_unchecked(mem), elems, layout, n_elems: 0 };
885 for (i, item) in iter.enumerate() {
886 ptr::write(elems.add(i), item);
890 // All clear. Forget the guard so it doesn't free the new ArcInner.
897 /// Specialization trait used for `From<&[T]>`.
898 trait ArcFromSlice<T> {
899 fn from_slice(slice: &[T]) -> Self;
902 impl<T: Clone> ArcFromSlice<T> for Arc<[T]> {
904 default fn from_slice(v: &[T]) -> Self {
905 unsafe { Self::from_iter_exact(v.iter().cloned(), v.len()) }
909 impl<T: Copy> ArcFromSlice<T> for Arc<[T]> {
911 fn from_slice(v: &[T]) -> Self {
912 unsafe { Arc::copy_from_slice(v) }
916 #[stable(feature = "rust1", since = "1.0.0")]
917 impl<T: ?Sized> Clone for Arc<T> {
918 /// Makes a clone of the `Arc` pointer.
920 /// This creates another pointer to the same allocation, increasing the
921 /// strong reference count.
926 /// use std::sync::Arc;
928 /// let five = Arc::new(5);
930 /// let _ = Arc::clone(&five);
933 fn clone(&self) -> Arc<T> {
934 // Using a relaxed ordering is alright here, as knowledge of the
935 // original reference prevents other threads from erroneously deleting
938 // As explained in the [Boost documentation][1], Increasing the
939 // reference counter can always be done with memory_order_relaxed: New
940 // references to an object can only be formed from an existing
941 // reference, and passing an existing reference from one thread to
942 // another must already provide any required synchronization.
944 // [1]: (www.boost.org/doc/libs/1_55_0/doc/html/atomic/usage_examples.html)
945 let old_size = self.inner().strong.fetch_add(1, Relaxed);
947 // However we need to guard against massive refcounts in case someone
948 // is `mem::forget`ing Arcs. If we don't do this the count can overflow
949 // and users will use-after free. We racily saturate to `isize::MAX` on
950 // the assumption that there aren't ~2 billion threads incrementing
951 // the reference count at once. This branch will never be taken in
952 // any realistic program.
954 // We abort because such a program is incredibly degenerate, and we
955 // don't care to support it.
956 if old_size > MAX_REFCOUNT {
962 Self::from_inner(self.ptr)
966 #[stable(feature = "rust1", since = "1.0.0")]
967 impl<T: ?Sized> Deref for Arc<T> {
971 fn deref(&self) -> &T {
976 #[unstable(feature = "receiver_trait", issue = "none")]
977 impl<T: ?Sized> Receiver for Arc<T> {}
979 impl<T: Clone> Arc<T> {
980 /// Makes a mutable reference into the given `Arc`.
982 /// If there are other `Arc` or [`Weak`][weak] pointers to the same allocation,
983 /// then `make_mut` will create a new allocation and invoke [`clone`][clone] on the inner value
984 /// to ensure unique ownership. This is also referred to as clone-on-write.
986 /// Note that this differs from the behavior of [`Rc::make_mut`] which disassociates
987 /// any remaining `Weak` pointers.
989 /// See also [`get_mut`][get_mut], which will fail rather than cloning.
991 /// [weak]: struct.Weak.html
992 /// [clone]: ../../std/clone/trait.Clone.html#tymethod.clone
993 /// [get_mut]: struct.Arc.html#method.get_mut
994 /// [`Rc::make_mut`]: ../rc/struct.Rc.html#method.make_mut
999 /// use std::sync::Arc;
1001 /// let mut data = Arc::new(5);
1003 /// *Arc::make_mut(&mut data) += 1; // Won't clone anything
1004 /// let mut other_data = Arc::clone(&data); // Won't clone inner data
1005 /// *Arc::make_mut(&mut data) += 1; // Clones inner data
1006 /// *Arc::make_mut(&mut data) += 1; // Won't clone anything
1007 /// *Arc::make_mut(&mut other_data) *= 2; // Won't clone anything
1009 /// // Now `data` and `other_data` point to different allocations.
1010 /// assert_eq!(*data, 8);
1011 /// assert_eq!(*other_data, 12);
1014 #[stable(feature = "arc_unique", since = "1.4.0")]
1015 pub fn make_mut(this: &mut Self) -> &mut T {
1016 // Note that we hold both a strong reference and a weak reference.
1017 // Thus, releasing our strong reference only will not, by itself, cause
1018 // the memory to be deallocated.
1020 // Use Acquire to ensure that we see any writes to `weak` that happen
1021 // before release writes (i.e., decrements) to `strong`. Since we hold a
1022 // weak count, there's no chance the ArcInner itself could be
1024 if this.inner().strong.compare_exchange(1, 0, Acquire, Relaxed).is_err() {
1025 // Another strong pointer exists; clone
1026 *this = Arc::new((**this).clone());
1027 } else if this.inner().weak.load(Relaxed) != 1 {
1028 // Relaxed suffices in the above because this is fundamentally an
1029 // optimization: we are always racing with weak pointers being
1030 // dropped. Worst case, we end up allocated a new Arc unnecessarily.
1032 // We removed the last strong ref, but there are additional weak
1033 // refs remaining. We'll move the contents to a new Arc, and
1034 // invalidate the other weak refs.
1036 // Note that it is not possible for the read of `weak` to yield
1037 // usize::MAX (i.e., locked), since the weak count can only be
1038 // locked by a thread with a strong reference.
1040 // Materialize our own implicit weak pointer, so that it can clean
1041 // up the ArcInner as needed.
1042 let weak = Weak { ptr: this.ptr };
1044 // mark the data itself as already deallocated
1046 // there is no data race in the implicit write caused by `read`
1047 // here (due to zeroing) because data is no longer accessed by
1048 // other threads (due to there being no more strong refs at this
1050 let mut swap = Arc::new(ptr::read(&weak.ptr.as_ref().data));
1051 mem::swap(this, &mut swap);
1055 // We were the sole reference of either kind; bump back up the
1056 // strong ref count.
1057 this.inner().strong.store(1, Release);
1060 // As with `get_mut()`, the unsafety is ok because our reference was
1061 // either unique to begin with, or became one upon cloning the contents.
1062 unsafe { &mut this.ptr.as_mut().data }
1066 impl<T: ?Sized> Arc<T> {
1067 /// Returns a mutable reference into the given `Arc`, if there are
1068 /// no other `Arc` or [`Weak`][weak] pointers to the same allocation.
1070 /// Returns [`None`][option] otherwise, because it is not safe to
1071 /// mutate a shared value.
1073 /// See also [`make_mut`][make_mut], which will [`clone`][clone]
1074 /// the inner value when there are other pointers.
1076 /// [weak]: struct.Weak.html
1077 /// [option]: ../../std/option/enum.Option.html
1078 /// [make_mut]: struct.Arc.html#method.make_mut
1079 /// [clone]: ../../std/clone/trait.Clone.html#tymethod.clone
1084 /// use std::sync::Arc;
1086 /// let mut x = Arc::new(3);
1087 /// *Arc::get_mut(&mut x).unwrap() = 4;
1088 /// assert_eq!(*x, 4);
1090 /// let _y = Arc::clone(&x);
1091 /// assert!(Arc::get_mut(&mut x).is_none());
1094 #[stable(feature = "arc_unique", since = "1.4.0")]
1095 pub fn get_mut(this: &mut Self) -> Option<&mut T> {
1096 if this.is_unique() {
1097 // This unsafety is ok because we're guaranteed that the pointer
1098 // returned is the *only* pointer that will ever be returned to T. Our
1099 // reference count is guaranteed to be 1 at this point, and we required
1100 // the Arc itself to be `mut`, so we're returning the only possible
1101 // reference to the inner data.
1102 unsafe { Some(Arc::get_mut_unchecked(this)) }
1108 /// Returns a mutable reference into the given `Arc`,
1109 /// without any check.
1111 /// See also [`get_mut`], which is safe and does appropriate checks.
1113 /// [`get_mut`]: struct.Arc.html#method.get_mut
1117 /// Any other `Arc` or [`Weak`] pointers to the same allocation must not be dereferenced
1118 /// for the duration of the returned borrow.
1119 /// This is trivially the case if no such pointers exist,
1120 /// for example immediately after `Arc::new`.
1125 /// #![feature(get_mut_unchecked)]
1127 /// use std::sync::Arc;
1129 /// let mut x = Arc::new(String::new());
1131 /// Arc::get_mut_unchecked(&mut x).push_str("foo")
1133 /// assert_eq!(*x, "foo");
1136 #[unstable(feature = "get_mut_unchecked", issue = "63292")]
1137 pub unsafe fn get_mut_unchecked(this: &mut Self) -> &mut T {
1138 &mut this.ptr.as_mut().data
1141 /// Determine whether this is the unique reference (including weak refs) to
1142 /// the underlying data.
1144 /// Note that this requires locking the weak ref count.
1145 fn is_unique(&mut self) -> bool {
1146 // lock the weak pointer count if we appear to be the sole weak pointer
1149 // The acquire label here ensures a happens-before relationship with any
1150 // writes to `strong` (in particular in `Weak::upgrade`) prior to decrements
1151 // of the `weak` count (via `Weak::drop`, which uses release). If the upgraded
1152 // weak ref was never dropped, the CAS here will fail so we do not care to synchronize.
1153 if self.inner().weak.compare_exchange(1, usize::MAX, Acquire, Relaxed).is_ok() {
1154 // This needs to be an `Acquire` to synchronize with the decrement of the `strong`
1155 // counter in `drop` -- the only access that happens when any but the last reference
1156 // is being dropped.
1157 let unique = self.inner().strong.load(Acquire) == 1;
1159 // The release write here synchronizes with a read in `downgrade`,
1160 // effectively preventing the above read of `strong` from happening
1162 self.inner().weak.store(1, Release); // release the lock
1170 #[stable(feature = "rust1", since = "1.0.0")]
1171 unsafe impl<#[may_dangle] T: ?Sized> Drop for Arc<T> {
1172 /// Drops the `Arc`.
1174 /// This will decrement the strong reference count. If the strong reference
1175 /// count reaches zero then the only other references (if any) are
1176 /// [`Weak`], so we `drop` the inner value.
1181 /// use std::sync::Arc;
1185 /// impl Drop for Foo {
1186 /// fn drop(&mut self) {
1187 /// println!("dropped!");
1191 /// let foo = Arc::new(Foo);
1192 /// let foo2 = Arc::clone(&foo);
1194 /// drop(foo); // Doesn't print anything
1195 /// drop(foo2); // Prints "dropped!"
1198 /// [`Weak`]: ../../std/sync/struct.Weak.html
1200 fn drop(&mut self) {
1201 // Because `fetch_sub` is already atomic, we do not need to synchronize
1202 // with other threads unless we are going to delete the object. This
1203 // same logic applies to the below `fetch_sub` to the `weak` count.
1204 if self.inner().strong.fetch_sub(1, Release) != 1 {
1208 // This fence is needed to prevent reordering of use of the data and
1209 // deletion of the data. Because it is marked `Release`, the decreasing
1210 // of the reference count synchronizes with this `Acquire` fence. This
1211 // means that use of the data happens before decreasing the reference
1212 // count, which happens before this fence, which happens before the
1213 // deletion of the data.
1215 // As explained in the [Boost documentation][1],
1217 // > It is important to enforce any possible access to the object in one
1218 // > thread (through an existing reference) to *happen before* deleting
1219 // > the object in a different thread. This is achieved by a "release"
1220 // > operation after dropping a reference (any access to the object
1221 // > through this reference must obviously happened before), and an
1222 // > "acquire" operation before deleting the object.
1224 // In particular, while the contents of an Arc are usually immutable, it's
1225 // possible to have interior writes to something like a Mutex<T>. Since a
1226 // Mutex is not acquired when it is deleted, we can't rely on its
1227 // synchronization logic to make writes in thread A visible to a destructor
1228 // running in thread B.
1230 // Also note that the Acquire fence here could probably be replaced with an
1231 // Acquire load, which could improve performance in highly-contended
1232 // situations. See [2].
1234 // [1]: (www.boost.org/doc/libs/1_55_0/doc/html/atomic/usage_examples.html)
1235 // [2]: (https://github.com/rust-lang/rust/pull/41714)
1236 atomic::fence(Acquire);
1244 impl Arc<dyn Any + Send + Sync> {
1246 #[stable(feature = "rc_downcast", since = "1.29.0")]
1247 /// Attempt to downcast the `Arc<dyn Any + Send + Sync>` to a concrete type.
1252 /// use std::any::Any;
1253 /// use std::sync::Arc;
1255 /// fn print_if_string(value: Arc<dyn Any + Send + Sync>) {
1256 /// if let Ok(string) = value.downcast::<String>() {
1257 /// println!("String ({}): {}", string.len(), string);
1261 /// let my_string = "Hello World".to_string();
1262 /// print_if_string(Arc::new(my_string));
1263 /// print_if_string(Arc::new(0i8));
1265 pub fn downcast<T>(self) -> Result<Arc<T>, Self>
1267 T: Any + Send + Sync + 'static,
1269 if (*self).is::<T>() {
1270 let ptr = self.ptr.cast::<ArcInner<T>>();
1272 Ok(Arc::from_inner(ptr))
1280 /// Constructs a new `Weak<T>`, without allocating any memory.
1281 /// Calling [`upgrade`] on the return value always gives [`None`].
1283 /// [`upgrade`]: struct.Weak.html#method.upgrade
1284 /// [`None`]: ../../std/option/enum.Option.html#variant.None
1289 /// use std::sync::Weak;
1291 /// let empty: Weak<i64> = Weak::new();
1292 /// assert!(empty.upgrade().is_none());
1294 #[stable(feature = "downgraded_weak", since = "1.10.0")]
1295 pub fn new() -> Weak<T> {
1296 Weak { ptr: NonNull::new(usize::MAX as *mut ArcInner<T>).expect("MAX is not 0") }
1299 /// Returns a raw pointer to the object `T` pointed to by this `Weak<T>`.
1301 /// The pointer is valid only if there are some strong references. The pointer may be dangling
1302 /// or even [`null`] otherwise.
1307 /// #![feature(weak_into_raw)]
1309 /// use std::sync::Arc;
1312 /// let strong = Arc::new("hello".to_owned());
1313 /// let weak = Arc::downgrade(&strong);
1314 /// // Both point to the same object
1315 /// assert!(ptr::eq(&*strong, weak.as_raw()));
1316 /// // The strong here keeps it alive, so we can still access the object.
1317 /// assert_eq!("hello", unsafe { &*weak.as_raw() });
1320 /// // But not any more. We can do weak.as_raw(), but accessing the pointer would lead to
1321 /// // undefined behaviour.
1322 /// // assert_eq!("hello", unsafe { &*weak.as_raw() });
1325 /// [`null`]: ../../std/ptr/fn.null.html
1326 #[unstable(feature = "weak_into_raw", issue = "60728")]
1327 pub fn as_raw(&self) -> *const T {
1328 match self.inner() {
1329 None => ptr::null(),
1331 let offset = data_offset_sized::<T>();
1332 let ptr = inner as *const ArcInner<T>;
1333 // Note: while the pointer we create may already point to dropped value, the
1334 // allocation still lives (it must hold the weak point as long as we are alive).
1335 // Therefore, the offset is OK to do, it won't get out of the allocation.
1336 let ptr = unsafe { (ptr as *const u8).offset(offset) };
1342 /// Consumes the `Weak<T>` and turns it into a raw pointer.
1344 /// This converts the weak pointer into a raw pointer, preserving the original weak count. It
1345 /// can be turned back into the `Weak<T>` with [`from_raw`].
1347 /// The same restrictions of accessing the target of the pointer as with
1348 /// [`as_raw`] apply.
1353 /// #![feature(weak_into_raw)]
1355 /// use std::sync::{Arc, Weak};
1357 /// let strong = Arc::new("hello".to_owned());
1358 /// let weak = Arc::downgrade(&strong);
1359 /// let raw = weak.into_raw();
1361 /// assert_eq!(1, Arc::weak_count(&strong));
1362 /// assert_eq!("hello", unsafe { &*raw });
1364 /// drop(unsafe { Weak::from_raw(raw) });
1365 /// assert_eq!(0, Arc::weak_count(&strong));
1368 /// [`from_raw`]: struct.Weak.html#method.from_raw
1369 /// [`as_raw`]: struct.Weak.html#method.as_raw
1370 #[unstable(feature = "weak_into_raw", issue = "60728")]
1371 pub fn into_raw(self) -> *const T {
1372 let result = self.as_raw();
1377 /// Converts a raw pointer previously created by [`into_raw`] back into
1380 /// This can be used to safely get a strong reference (by calling [`upgrade`]
1381 /// later) or to deallocate the weak count by dropping the `Weak<T>`.
1383 /// It takes ownership of one weak count (with the exception of pointers created by [`new`],
1384 /// as these don't have any corresponding weak count).
1388 /// The pointer must have originated from the [`into_raw`] (or [`as_raw'], provided there was
1389 /// a corresponding [`forget`] on the `Weak<T>`) and must still own its potential weak reference
1392 /// It is allowed for the strong count to be 0 at the time of calling this, but the weak count
1393 /// must be non-zero or the pointer must have originated from a dangling `Weak<T>` (one created
1399 /// #![feature(weak_into_raw)]
1401 /// use std::sync::{Arc, Weak};
1403 /// let strong = Arc::new("hello".to_owned());
1405 /// let raw_1 = Arc::downgrade(&strong).into_raw();
1406 /// let raw_2 = Arc::downgrade(&strong).into_raw();
1408 /// assert_eq!(2, Arc::weak_count(&strong));
1410 /// assert_eq!("hello", &*unsafe { Weak::from_raw(raw_1) }.upgrade().unwrap());
1411 /// assert_eq!(1, Arc::weak_count(&strong));
1415 /// // Decrement the last weak count.
1416 /// assert!(unsafe { Weak::from_raw(raw_2) }.upgrade().is_none());
1419 /// [`as_raw`]: struct.Weak.html#method.as_raw
1420 /// [`new`]: struct.Weak.html#method.new
1421 /// [`into_raw`]: struct.Weak.html#method.into_raw
1422 /// [`upgrade`]: struct.Weak.html#method.upgrade
1423 /// [`Weak`]: struct.Weak.html
1424 /// [`Arc`]: struct.Arc.html
1425 /// [`forget`]: ../../std/mem/fn.forget.html
1426 #[unstable(feature = "weak_into_raw", issue = "60728")]
1427 pub unsafe fn from_raw(ptr: *const T) -> Self {
1431 // See Arc::from_raw for details
1432 let offset = data_offset(ptr);
1433 let fake_ptr = ptr as *mut ArcInner<T>;
1434 let ptr = set_data_ptr(fake_ptr, (ptr as *mut u8).offset(-offset));
1435 Weak { ptr: NonNull::new(ptr).expect("Invalid pointer passed to from_raw") }
1440 impl<T: ?Sized> Weak<T> {
1441 /// Attempts to upgrade the `Weak` pointer to an [`Arc`], delaying
1442 /// dropping of the inner value if successful.
1444 /// Returns [`None`] if the inner value has since been dropped.
1446 /// [`Arc`]: struct.Arc.html
1447 /// [`None`]: ../../std/option/enum.Option.html#variant.None
1452 /// use std::sync::Arc;
1454 /// let five = Arc::new(5);
1456 /// let weak_five = Arc::downgrade(&five);
1458 /// let strong_five: Option<Arc<_>> = weak_five.upgrade();
1459 /// assert!(strong_five.is_some());
1461 /// // Destroy all strong pointers.
1462 /// drop(strong_five);
1465 /// assert!(weak_five.upgrade().is_none());
1467 #[stable(feature = "arc_weak", since = "1.4.0")]
1468 pub fn upgrade(&self) -> Option<Arc<T>> {
1469 // We use a CAS loop to increment the strong count instead of a
1470 // fetch_add because once the count hits 0 it must never be above 0.
1471 let inner = self.inner()?;
1473 // Relaxed load because any write of 0 that we can observe
1474 // leaves the field in a permanently zero state (so a
1475 // "stale" read of 0 is fine), and any other value is
1476 // confirmed via the CAS below.
1477 let mut n = inner.strong.load(Relaxed);
1484 // See comments in `Arc::clone` for why we do this (for `mem::forget`).
1485 if n > MAX_REFCOUNT {
1491 // Relaxed is valid for the same reason it is on Arc's Clone impl
1492 match inner.strong.compare_exchange_weak(n, n + 1, Relaxed, Relaxed) {
1493 Ok(_) => return Some(Arc::from_inner(self.ptr)), // null checked above
1494 Err(old) => n = old,
1499 /// Gets the number of strong (`Arc`) pointers pointing to this allocation.
1501 /// If `self` was created using [`Weak::new`], this will return 0.
1503 /// [`Weak::new`]: #method.new
1504 #[stable(feature = "weak_counts", since = "1.41.0")]
1505 pub fn strong_count(&self) -> usize {
1506 if let Some(inner) = self.inner() { inner.strong.load(SeqCst) } else { 0 }
1509 /// Gets an approximation of the number of `Weak` pointers pointing to this
1512 /// If `self` was created using [`Weak::new`], or if there are no remaining
1513 /// strong pointers, this will return 0.
1517 /// Due to implementation details, the returned value can be off by 1 in
1518 /// either direction when other threads are manipulating any `Arc`s or
1519 /// `Weak`s pointing to the same allocation.
1521 /// [`Weak::new`]: #method.new
1522 #[stable(feature = "weak_counts", since = "1.41.0")]
1523 pub fn weak_count(&self) -> usize {
1526 let weak = inner.weak.load(SeqCst);
1527 let strong = inner.strong.load(SeqCst);
1531 // Since we observed that there was at least one strong pointer
1532 // after reading the weak count, we know that the implicit weak
1533 // reference (present whenever any strong references are alive)
1534 // was still around when we observed the weak count, and can
1535 // therefore safely subtract it.
1542 /// Returns `None` when the pointer is dangling and there is no allocated `ArcInner`,
1543 /// (i.e., when this `Weak` was created by `Weak::new`).
1545 fn inner(&self) -> Option<&ArcInner<T>> {
1546 if is_dangling(self.ptr) { None } else { Some(unsafe { self.ptr.as_ref() }) }
1549 /// Returns `true` if the two `Weak`s point to the same allocation (similar to
1550 /// [`ptr::eq`]), or if both don't point to any allocation
1551 /// (because they were created with `Weak::new()`).
1555 /// Since this compares pointers it means that `Weak::new()` will equal each
1556 /// other, even though they don't point to any allocation.
1561 /// use std::sync::Arc;
1563 /// let first_rc = Arc::new(5);
1564 /// let first = Arc::downgrade(&first_rc);
1565 /// let second = Arc::downgrade(&first_rc);
1567 /// assert!(first.ptr_eq(&second));
1569 /// let third_rc = Arc::new(5);
1570 /// let third = Arc::downgrade(&third_rc);
1572 /// assert!(!first.ptr_eq(&third));
1575 /// Comparing `Weak::new`.
1578 /// use std::sync::{Arc, Weak};
1580 /// let first = Weak::new();
1581 /// let second = Weak::new();
1582 /// assert!(first.ptr_eq(&second));
1584 /// let third_rc = Arc::new(());
1585 /// let third = Arc::downgrade(&third_rc);
1586 /// assert!(!first.ptr_eq(&third));
1589 /// [`ptr::eq`]: ../../std/ptr/fn.eq.html
1591 #[stable(feature = "weak_ptr_eq", since = "1.39.0")]
1592 pub fn ptr_eq(&self, other: &Self) -> bool {
1593 self.ptr.as_ptr() == other.ptr.as_ptr()
1597 #[stable(feature = "arc_weak", since = "1.4.0")]
1598 impl<T: ?Sized> Clone for Weak<T> {
1599 /// Makes a clone of the `Weak` pointer that points to the same allocation.
1604 /// use std::sync::{Arc, Weak};
1606 /// let weak_five = Arc::downgrade(&Arc::new(5));
1608 /// let _ = Weak::clone(&weak_five);
1611 fn clone(&self) -> Weak<T> {
1612 let inner = if let Some(inner) = self.inner() {
1615 return Weak { ptr: self.ptr };
1617 // See comments in Arc::clone() for why this is relaxed. This can use a
1618 // fetch_add (ignoring the lock) because the weak count is only locked
1619 // where are *no other* weak pointers in existence. (So we can't be
1620 // running this code in that case).
1621 let old_size = inner.weak.fetch_add(1, Relaxed);
1623 // See comments in Arc::clone() for why we do this (for mem::forget).
1624 if old_size > MAX_REFCOUNT {
1630 Weak { ptr: self.ptr }
1634 #[stable(feature = "downgraded_weak", since = "1.10.0")]
1635 impl<T> Default for Weak<T> {
1636 /// Constructs a new `Weak<T>`, without allocating memory.
1637 /// Calling [`upgrade`] on the return value always
1640 /// [`None`]: ../../std/option/enum.Option.html#variant.None
1641 /// [`upgrade`]: ../../std/sync/struct.Weak.html#method.upgrade
1646 /// use std::sync::Weak;
1648 /// let empty: Weak<i64> = Default::default();
1649 /// assert!(empty.upgrade().is_none());
1651 fn default() -> Weak<T> {
1656 #[stable(feature = "arc_weak", since = "1.4.0")]
1657 impl<T: ?Sized> Drop for Weak<T> {
1658 /// Drops the `Weak` pointer.
1663 /// use std::sync::{Arc, Weak};
1667 /// impl Drop for Foo {
1668 /// fn drop(&mut self) {
1669 /// println!("dropped!");
1673 /// let foo = Arc::new(Foo);
1674 /// let weak_foo = Arc::downgrade(&foo);
1675 /// let other_weak_foo = Weak::clone(&weak_foo);
1677 /// drop(weak_foo); // Doesn't print anything
1678 /// drop(foo); // Prints "dropped!"
1680 /// assert!(other_weak_foo.upgrade().is_none());
1682 fn drop(&mut self) {
1683 // If we find out that we were the last weak pointer, then its time to
1684 // deallocate the data entirely. See the discussion in Arc::drop() about
1685 // the memory orderings
1687 // It's not necessary to check for the locked state here, because the
1688 // weak count can only be locked if there was precisely one weak ref,
1689 // meaning that drop could only subsequently run ON that remaining weak
1690 // ref, which can only happen after the lock is released.
1691 let inner = if let Some(inner) = self.inner() { inner } else { return };
1693 if inner.weak.fetch_sub(1, Release) == 1 {
1694 atomic::fence(Acquire);
1695 unsafe { Global.dealloc(self.ptr.cast(), Layout::for_value(self.ptr.as_ref())) }
1700 #[stable(feature = "rust1", since = "1.0.0")]
1701 trait ArcEqIdent<T: ?Sized + PartialEq> {
1702 fn eq(&self, other: &Arc<T>) -> bool;
1703 fn ne(&self, other: &Arc<T>) -> bool;
1706 #[stable(feature = "rust1", since = "1.0.0")]
1707 impl<T: ?Sized + PartialEq> ArcEqIdent<T> for Arc<T> {
1709 default fn eq(&self, other: &Arc<T>) -> bool {
1713 default fn ne(&self, other: &Arc<T>) -> bool {
1718 /// We're doing this specialization here, and not as a more general optimization on `&T`, because it
1719 /// would otherwise add a cost to all equality checks on refs. We assume that `Arc`s are used to
1720 /// store large values, that are slow to clone, but also heavy to check for equality, causing this
1721 /// cost to pay off more easily. It's also more likely to have two `Arc` clones, that point to
1722 /// the same value, than two `&T`s.
1724 /// We can only do this when `T: Eq` as a `PartialEq` might be deliberately irreflexive.
1725 #[stable(feature = "rust1", since = "1.0.0")]
1726 impl<T: ?Sized + Eq> ArcEqIdent<T> for Arc<T> {
1728 fn eq(&self, other: &Arc<T>) -> bool {
1729 Arc::ptr_eq(self, other) || **self == **other
1733 fn ne(&self, other: &Arc<T>) -> bool {
1734 !Arc::ptr_eq(self, other) && **self != **other
1738 #[stable(feature = "rust1", since = "1.0.0")]
1739 impl<T: ?Sized + PartialEq> PartialEq for Arc<T> {
1740 /// Equality for two `Arc`s.
1742 /// Two `Arc`s are equal if their inner values are equal, even if they are
1743 /// stored in different allocation.
1745 /// If `T` also implements `Eq` (implying reflexivity of equality),
1746 /// two `Arc`s that point to the same allocation are always equal.
1751 /// use std::sync::Arc;
1753 /// let five = Arc::new(5);
1755 /// assert!(five == Arc::new(5));
1758 fn eq(&self, other: &Arc<T>) -> bool {
1759 ArcEqIdent::eq(self, other)
1762 /// Inequality for two `Arc`s.
1764 /// Two `Arc`s are unequal if their inner values are unequal.
1766 /// If `T` also implements `Eq` (implying reflexivity of equality),
1767 /// two `Arc`s that point to the same value are never unequal.
1772 /// use std::sync::Arc;
1774 /// let five = Arc::new(5);
1776 /// assert!(five != Arc::new(6));
1779 fn ne(&self, other: &Arc<T>) -> bool {
1780 ArcEqIdent::ne(self, other)
1784 #[stable(feature = "rust1", since = "1.0.0")]
1785 impl<T: ?Sized + PartialOrd> PartialOrd for Arc<T> {
1786 /// Partial comparison for two `Arc`s.
1788 /// The two are compared by calling `partial_cmp()` on their inner values.
1793 /// use std::sync::Arc;
1794 /// use std::cmp::Ordering;
1796 /// let five = Arc::new(5);
1798 /// assert_eq!(Some(Ordering::Less), five.partial_cmp(&Arc::new(6)));
1800 fn partial_cmp(&self, other: &Arc<T>) -> Option<Ordering> {
1801 (**self).partial_cmp(&**other)
1804 /// Less-than comparison for two `Arc`s.
1806 /// The two are compared by calling `<` on their inner values.
1811 /// use std::sync::Arc;
1813 /// let five = Arc::new(5);
1815 /// assert!(five < Arc::new(6));
1817 fn lt(&self, other: &Arc<T>) -> bool {
1818 *(*self) < *(*other)
1821 /// 'Less than or equal to' comparison for two `Arc`s.
1823 /// The two are compared by calling `<=` on their inner values.
1828 /// use std::sync::Arc;
1830 /// let five = Arc::new(5);
1832 /// assert!(five <= Arc::new(5));
1834 fn le(&self, other: &Arc<T>) -> bool {
1835 *(*self) <= *(*other)
1838 /// Greater-than comparison for two `Arc`s.
1840 /// The two are compared by calling `>` on their inner values.
1845 /// use std::sync::Arc;
1847 /// let five = Arc::new(5);
1849 /// assert!(five > Arc::new(4));
1851 fn gt(&self, other: &Arc<T>) -> bool {
1852 *(*self) > *(*other)
1855 /// 'Greater than or equal to' comparison for two `Arc`s.
1857 /// The two are compared by calling `>=` on their inner values.
1862 /// use std::sync::Arc;
1864 /// let five = Arc::new(5);
1866 /// assert!(five >= Arc::new(5));
1868 fn ge(&self, other: &Arc<T>) -> bool {
1869 *(*self) >= *(*other)
1872 #[stable(feature = "rust1", since = "1.0.0")]
1873 impl<T: ?Sized + Ord> Ord for Arc<T> {
1874 /// Comparison for two `Arc`s.
1876 /// The two are compared by calling `cmp()` on their inner values.
1881 /// use std::sync::Arc;
1882 /// use std::cmp::Ordering;
1884 /// let five = Arc::new(5);
1886 /// assert_eq!(Ordering::Less, five.cmp(&Arc::new(6)));
1888 fn cmp(&self, other: &Arc<T>) -> Ordering {
1889 (**self).cmp(&**other)
1892 #[stable(feature = "rust1", since = "1.0.0")]
1893 impl<T: ?Sized + Eq> Eq for Arc<T> {}
1895 #[stable(feature = "rust1", since = "1.0.0")]
1896 impl<T: ?Sized + fmt::Display> fmt::Display for Arc<T> {
1897 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
1898 fmt::Display::fmt(&**self, f)
1902 #[stable(feature = "rust1", since = "1.0.0")]
1903 impl<T: ?Sized + fmt::Debug> fmt::Debug for Arc<T> {
1904 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
1905 fmt::Debug::fmt(&**self, f)
1909 #[stable(feature = "rust1", since = "1.0.0")]
1910 impl<T: ?Sized> fmt::Pointer for Arc<T> {
1911 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
1912 fmt::Pointer::fmt(&(&**self as *const T), f)
1916 #[stable(feature = "rust1", since = "1.0.0")]
1917 impl<T: Default> Default for Arc<T> {
1918 /// Creates a new `Arc<T>`, with the `Default` value for `T`.
1923 /// use std::sync::Arc;
1925 /// let x: Arc<i32> = Default::default();
1926 /// assert_eq!(*x, 0);
1928 fn default() -> Arc<T> {
1929 Arc::new(Default::default())
1933 #[stable(feature = "rust1", since = "1.0.0")]
1934 impl<T: ?Sized + Hash> Hash for Arc<T> {
1935 fn hash<H: Hasher>(&self, state: &mut H) {
1936 (**self).hash(state)
1940 #[stable(feature = "from_for_ptrs", since = "1.6.0")]
1941 impl<T> From<T> for Arc<T> {
1942 fn from(t: T) -> Self {
1947 #[stable(feature = "shared_from_slice", since = "1.21.0")]
1948 impl<T: Clone> From<&[T]> for Arc<[T]> {
1950 fn from(v: &[T]) -> Arc<[T]> {
1951 <Self as ArcFromSlice<T>>::from_slice(v)
1955 #[stable(feature = "shared_from_slice", since = "1.21.0")]
1956 impl From<&str> for Arc<str> {
1958 fn from(v: &str) -> Arc<str> {
1959 let arc = Arc::<[u8]>::from(v.as_bytes());
1960 unsafe { Arc::from_raw(Arc::into_raw(arc) as *const str) }
1964 #[stable(feature = "shared_from_slice", since = "1.21.0")]
1965 impl From<String> for Arc<str> {
1967 fn from(v: String) -> Arc<str> {
1972 #[stable(feature = "shared_from_slice", since = "1.21.0")]
1973 impl<T: ?Sized> From<Box<T>> for Arc<T> {
1975 fn from(v: Box<T>) -> Arc<T> {
1980 #[stable(feature = "shared_from_slice", since = "1.21.0")]
1981 impl<T> From<Vec<T>> for Arc<[T]> {
1983 fn from(mut v: Vec<T>) -> Arc<[T]> {
1985 let arc = Arc::copy_from_slice(&v);
1987 // Allow the Vec to free its memory, but not destroy its contents
1995 #[unstable(feature = "boxed_slice_try_from", issue = "none")]
1996 impl<T, const N: usize> TryFrom<Arc<[T]>> for Arc<[T; N]>
1998 [T; N]: LengthAtMost32,
2000 type Error = Arc<[T]>;
2002 fn try_from(boxed_slice: Arc<[T]>) -> Result<Self, Self::Error> {
2003 if boxed_slice.len() == N {
2004 Ok(unsafe { Arc::from_raw(Arc::into_raw(boxed_slice) as *mut [T; N]) })
2011 #[stable(feature = "shared_from_iter", since = "1.37.0")]
2012 impl<T> iter::FromIterator<T> for Arc<[T]> {
2013 /// Takes each element in the `Iterator` and collects it into an `Arc<[T]>`.
2015 /// # Performance characteristics
2017 /// ## The general case
2019 /// In the general case, collecting into `Arc<[T]>` is done by first
2020 /// collecting into a `Vec<T>`. That is, when writing the following:
2023 /// # use std::sync::Arc;
2024 /// let evens: Arc<[u8]> = (0..10).filter(|&x| x % 2 == 0).collect();
2025 /// # assert_eq!(&*evens, &[0, 2, 4, 6, 8]);
2028 /// this behaves as if we wrote:
2031 /// # use std::sync::Arc;
2032 /// let evens: Arc<[u8]> = (0..10).filter(|&x| x % 2 == 0)
2033 /// .collect::<Vec<_>>() // The first set of allocations happens here.
2034 /// .into(); // A second allocation for `Arc<[T]>` happens here.
2035 /// # assert_eq!(&*evens, &[0, 2, 4, 6, 8]);
2038 /// This will allocate as many times as needed for constructing the `Vec<T>`
2039 /// and then it will allocate once for turning the `Vec<T>` into the `Arc<[T]>`.
2041 /// ## Iterators of known length
2043 /// When your `Iterator` implements `TrustedLen` and is of an exact size,
2044 /// a single allocation will be made for the `Arc<[T]>`. For example:
2047 /// # use std::sync::Arc;
2048 /// let evens: Arc<[u8]> = (0..10).collect(); // Just a single allocation happens here.
2049 /// # assert_eq!(&*evens, &*(0..10).collect::<Vec<_>>());
2051 fn from_iter<I: iter::IntoIterator<Item = T>>(iter: I) -> Self {
2052 ArcFromIter::from_iter(iter.into_iter())
2056 /// Specialization trait used for collecting into `Arc<[T]>`.
2057 trait ArcFromIter<T, I> {
2058 fn from_iter(iter: I) -> Self;
2061 impl<T, I: Iterator<Item = T>> ArcFromIter<T, I> for Arc<[T]> {
2062 default fn from_iter(iter: I) -> Self {
2063 iter.collect::<Vec<T>>().into()
2067 impl<T, I: iter::TrustedLen<Item = T>> ArcFromIter<T, I> for Arc<[T]> {
2068 default fn from_iter(iter: I) -> Self {
2069 // This is the case for a `TrustedLen` iterator.
2070 let (low, high) = iter.size_hint();
2071 if let Some(high) = high {
2075 "TrustedLen iterator's size hint is not exact: {:?}",
2080 // SAFETY: We need to ensure that the iterator has an exact length and we have.
2081 Arc::from_iter_exact(iter, low)
2084 // Fall back to normal implementation.
2085 iter.collect::<Vec<T>>().into()
2090 impl<'a, T: 'a + Clone> ArcFromIter<&'a T, slice::Iter<'a, T>> for Arc<[T]> {
2091 fn from_iter(iter: slice::Iter<'a, T>) -> Self {
2092 // Delegate to `impl<T: Clone> From<&[T]> for Arc<[T]>`.
2094 // In the case that `T: Copy`, we get to use `ptr::copy_nonoverlapping`
2095 // which is even more performant.
2097 // In the fall-back case we have `T: Clone`. This is still better
2098 // than the `TrustedLen` implementation as slices have a known length
2099 // and so we get to avoid calling `size_hint` and avoid the branching.
2100 iter.as_slice().into()
2104 #[stable(feature = "rust1", since = "1.0.0")]
2105 impl<T: ?Sized> borrow::Borrow<T> for Arc<T> {
2106 fn borrow(&self) -> &T {
2111 #[stable(since = "1.5.0", feature = "smart_ptr_as_ref")]
2112 impl<T: ?Sized> AsRef<T> for Arc<T> {
2113 fn as_ref(&self) -> &T {
2118 #[stable(feature = "pin", since = "1.33.0")]
2119 impl<T: ?Sized> Unpin for Arc<T> {}
2121 /// Computes the offset of the data field within `ArcInner`.
2122 unsafe fn data_offset<T: ?Sized>(ptr: *const T) -> isize {
2123 // Align the unsized value to the end of the `ArcInner`.
2124 // Because it is `?Sized`, it will always be the last field in memory.
2125 // Note: This is a detail of the current implementation of the compiler,
2126 // and is not a guaranteed language detail. Do not rely on it outside of std.
2127 data_offset_align(align_of_val(&*ptr))
2130 /// Computes the offset of the data field within `ArcInner`.
2132 /// Unlike [`data_offset`], this doesn't need the pointer, but it works only on `T: Sized`.
2133 fn data_offset_sized<T>() -> isize {
2134 data_offset_align(align_of::<T>())
2138 fn data_offset_align(align: usize) -> isize {
2139 let layout = Layout::new::<ArcInner<()>>();
2140 (layout.size() + layout.padding_needed_for(align)) as isize