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<T>` from a raw pointer.
555 /// The raw pointer must have been previously returned by a call to
556 /// [`Arc<U>::into_raw`][into_raw] where `U` must have the same size and
557 /// alignment as `T`. This is trivially true if `U` is `T`.
558 /// Note that if `U` is not `T` but has the same size and alignment, this is
559 /// basically like transmuting references of different types. See
560 /// [`mem::transmute`][transmute] for more information on what
561 /// restrictions apply in this case.
563 /// The user of `from_raw` has to make sure a specific value of `T` is only
566 /// This function is unsafe because improper use may lead to memory unsafety,
567 /// even if the returned `Arc<T>` is never accessed.
569 /// [into_raw]: struct.Arc.html#method.into_raw
570 /// [transmute]: ../../std/mem/fn.transmute.html
575 /// use std::sync::Arc;
577 /// let x = Arc::new("hello".to_owned());
578 /// let x_ptr = Arc::into_raw(x);
581 /// // Convert back to an `Arc` to prevent leak.
582 /// let x = Arc::from_raw(x_ptr);
583 /// assert_eq!(&*x, "hello");
585 /// // Further calls to `Arc::from_raw(x_ptr)` would be memory-unsafe.
588 /// // The memory was freed when `x` went out of scope above, so `x_ptr` is now dangling!
590 #[stable(feature = "rc_raw", since = "1.17.0")]
591 pub unsafe fn from_raw(ptr: *const T) -> Self {
592 let offset = data_offset(ptr);
594 // Reverse the offset to find the original ArcInner.
595 let fake_ptr = ptr as *mut ArcInner<T>;
596 let arc_ptr = set_data_ptr(fake_ptr, (ptr as *mut u8).offset(-offset));
598 Self::from_ptr(arc_ptr)
601 /// Consumes the `Arc`, returning the wrapped pointer as `NonNull<T>`.
606 /// #![feature(rc_into_raw_non_null)]
608 /// use std::sync::Arc;
610 /// let x = Arc::new("hello".to_owned());
611 /// let ptr = Arc::into_raw_non_null(x);
612 /// let deref = unsafe { ptr.as_ref() };
613 /// assert_eq!(deref, "hello");
615 #[unstable(feature = "rc_into_raw_non_null", issue = "47336")]
617 pub fn into_raw_non_null(this: Self) -> NonNull<T> {
618 // safe because Arc guarantees its pointer is non-null
619 unsafe { NonNull::new_unchecked(Arc::into_raw(this) as *mut _) }
622 /// Creates a new [`Weak`][weak] pointer to this allocation.
624 /// [weak]: struct.Weak.html
629 /// use std::sync::Arc;
631 /// let five = Arc::new(5);
633 /// let weak_five = Arc::downgrade(&five);
635 #[stable(feature = "arc_weak", since = "1.4.0")]
636 pub fn downgrade(this: &Self) -> Weak<T> {
637 // This Relaxed is OK because we're checking the value in the CAS
639 let mut cur = this.inner().weak.load(Relaxed);
642 // check if the weak counter is currently "locked"; if so, spin.
643 if cur == usize::MAX {
644 cur = this.inner().weak.load(Relaxed);
648 // NOTE: this code currently ignores the possibility of overflow
649 // into usize::MAX; in general both Rc and Arc need to be adjusted
650 // to deal with overflow.
652 // Unlike with Clone(), we need this to be an Acquire read to
653 // synchronize with the write coming from `is_unique`, so that the
654 // events prior to that write happen before this read.
655 match this.inner().weak.compare_exchange_weak(cur, cur + 1, Acquire, Relaxed) {
657 // Make sure we do not create a dangling Weak
658 debug_assert!(!is_dangling(this.ptr));
659 return Weak { ptr: this.ptr };
661 Err(old) => cur = old,
666 /// Gets the number of [`Weak`][weak] pointers to this allocation.
668 /// [weak]: struct.Weak.html
672 /// This method by itself is safe, but using it correctly requires extra care.
673 /// Another thread can change the weak count at any time,
674 /// including potentially between calling this method and acting on the result.
679 /// use std::sync::Arc;
681 /// let five = Arc::new(5);
682 /// let _weak_five = Arc::downgrade(&five);
684 /// // This assertion is deterministic because we haven't shared
685 /// // the `Arc` or `Weak` between threads.
686 /// assert_eq!(1, Arc::weak_count(&five));
689 #[stable(feature = "arc_counts", since = "1.15.0")]
690 pub fn weak_count(this: &Self) -> usize {
691 let cnt = this.inner().weak.load(SeqCst);
692 // If the weak count is currently locked, the value of the
693 // count was 0 just before taking the lock.
694 if cnt == usize::MAX { 0 } else { cnt - 1 }
697 /// Gets the number of strong (`Arc`) pointers to this allocation.
701 /// This method by itself is safe, but using it correctly requires extra care.
702 /// Another thread can change the strong count at any time,
703 /// including potentially between calling this method and acting on the result.
708 /// use std::sync::Arc;
710 /// let five = Arc::new(5);
711 /// let _also_five = Arc::clone(&five);
713 /// // This assertion is deterministic because we haven't shared
714 /// // the `Arc` between threads.
715 /// assert_eq!(2, Arc::strong_count(&five));
718 #[stable(feature = "arc_counts", since = "1.15.0")]
719 pub fn strong_count(this: &Self) -> usize {
720 this.inner().strong.load(SeqCst)
724 fn inner(&self) -> &ArcInner<T> {
725 // This unsafety is ok because while this arc is alive we're guaranteed
726 // that the inner pointer is valid. Furthermore, we know that the
727 // `ArcInner` structure itself is `Sync` because the inner data is
728 // `Sync` as well, so we're ok loaning out an immutable pointer to these
730 unsafe { self.ptr.as_ref() }
733 // Non-inlined part of `drop`.
735 unsafe fn drop_slow(&mut self) {
736 // Destroy the data at this time, even though we may not free the box
737 // allocation itself (there may still be weak pointers lying around).
738 ptr::drop_in_place(&mut self.ptr.as_mut().data);
740 if self.inner().weak.fetch_sub(1, Release) == 1 {
741 atomic::fence(Acquire);
742 Global.dealloc(self.ptr.cast(), Layout::for_value(self.ptr.as_ref()))
747 #[stable(feature = "ptr_eq", since = "1.17.0")]
748 /// Returns `true` if the two `Arc`s point to the same allocation
749 /// (in a vein similar to [`ptr::eq`]).
754 /// use std::sync::Arc;
756 /// let five = Arc::new(5);
757 /// let same_five = Arc::clone(&five);
758 /// let other_five = Arc::new(5);
760 /// assert!(Arc::ptr_eq(&five, &same_five));
761 /// assert!(!Arc::ptr_eq(&five, &other_five));
764 /// [`ptr::eq`]: ../../std/ptr/fn.eq.html
765 pub fn ptr_eq(this: &Self, other: &Self) -> bool {
766 this.ptr.as_ptr() == other.ptr.as_ptr()
770 impl<T: ?Sized> Arc<T> {
771 /// Allocates an `ArcInner<T>` with sufficient space for
772 /// a possibly-unsized inner value where the value has the layout provided.
774 /// The function `mem_to_arcinner` is called with the data pointer
775 /// and must return back a (potentially fat)-pointer for the `ArcInner<T>`.
776 unsafe fn allocate_for_layout(
777 value_layout: Layout,
778 mem_to_arcinner: impl FnOnce(*mut u8) -> *mut ArcInner<T>,
779 ) -> *mut ArcInner<T> {
780 // Calculate layout using the given value layout.
781 // Previously, layout was calculated on the expression
782 // `&*(ptr as *const ArcInner<T>)`, but this created a misaligned
783 // reference (see #54908).
784 let layout = Layout::new::<ArcInner<()>>().extend(value_layout).unwrap().0.pad_to_align();
786 let mem = Global.alloc(layout).unwrap_or_else(|_| handle_alloc_error(layout));
788 // Initialize the ArcInner
789 let inner = mem_to_arcinner(mem.as_ptr());
790 debug_assert_eq!(Layout::for_value(&*inner), layout);
792 ptr::write(&mut (*inner).strong, atomic::AtomicUsize::new(1));
793 ptr::write(&mut (*inner).weak, atomic::AtomicUsize::new(1));
798 /// Allocates an `ArcInner<T>` with sufficient space for an unsized inner value.
799 unsafe fn allocate_for_ptr(ptr: *const T) -> *mut ArcInner<T> {
800 // Allocate for the `ArcInner<T>` using the given value.
801 Self::allocate_for_layout(Layout::for_value(&*ptr), |mem| {
802 set_data_ptr(ptr as *mut T, mem) as *mut ArcInner<T>
806 fn from_box(v: Box<T>) -> Arc<T> {
808 let box_unique = Box::into_unique(v);
809 let bptr = box_unique.as_ptr();
811 let value_size = size_of_val(&*bptr);
812 let ptr = Self::allocate_for_ptr(bptr);
814 // Copy value as bytes
815 ptr::copy_nonoverlapping(
816 bptr as *const T as *const u8,
817 &mut (*ptr).data as *mut _ as *mut u8,
821 // Free the allocation without dropping its contents
822 box_free(box_unique);
830 /// Allocates an `ArcInner<[T]>` with the given length.
831 unsafe fn allocate_for_slice(len: usize) -> *mut ArcInner<[T]> {
832 Self::allocate_for_layout(Layout::array::<T>(len).unwrap(), |mem| {
833 ptr::slice_from_raw_parts_mut(mem as *mut T, len) as *mut ArcInner<[T]>
838 /// Sets the data pointer of a `?Sized` raw pointer.
840 /// For a slice/trait object, this sets the `data` field and leaves the rest
841 /// unchanged. For a sized raw pointer, this simply sets the pointer.
842 unsafe fn set_data_ptr<T: ?Sized, U>(mut ptr: *mut T, data: *mut U) -> *mut T {
843 ptr::write(&mut ptr as *mut _ as *mut *mut u8, data as *mut u8);
848 /// Copy elements from slice into newly allocated Arc<[T]>
850 /// Unsafe because the caller must either take ownership or bind `T: Copy`.
851 unsafe fn copy_from_slice(v: &[T]) -> Arc<[T]> {
852 let ptr = Self::allocate_for_slice(v.len());
854 ptr::copy_nonoverlapping(v.as_ptr(), &mut (*ptr).data as *mut [T] as *mut T, v.len());
859 /// Constructs an `Arc<[T]>` from an iterator known to be of a certain size.
861 /// Behavior is undefined should the size be wrong.
862 unsafe fn from_iter_exact(iter: impl iter::Iterator<Item = T>, len: usize) -> Arc<[T]> {
863 // Panic guard while cloning T elements.
864 // In the event of a panic, elements that have been written
865 // into the new ArcInner will be dropped, then the memory freed.
873 impl<T> Drop for Guard<T> {
876 let slice = from_raw_parts_mut(self.elems, self.n_elems);
877 ptr::drop_in_place(slice);
879 Global.dealloc(self.mem.cast(), self.layout);
884 let ptr = Self::allocate_for_slice(len);
886 let mem = ptr as *mut _ as *mut u8;
887 let layout = Layout::for_value(&*ptr);
889 // Pointer to first element
890 let elems = &mut (*ptr).data as *mut [T] as *mut T;
892 let mut guard = Guard { mem: NonNull::new_unchecked(mem), elems, layout, n_elems: 0 };
894 for (i, item) in iter.enumerate() {
895 ptr::write(elems.add(i), item);
899 // All clear. Forget the guard so it doesn't free the new ArcInner.
906 /// Specialization trait used for `From<&[T]>`.
907 trait ArcFromSlice<T> {
908 fn from_slice(slice: &[T]) -> Self;
911 impl<T: Clone> ArcFromSlice<T> for Arc<[T]> {
913 default fn from_slice(v: &[T]) -> Self {
914 unsafe { Self::from_iter_exact(v.iter().cloned(), v.len()) }
918 impl<T: Copy> ArcFromSlice<T> for Arc<[T]> {
920 fn from_slice(v: &[T]) -> Self {
921 unsafe { Arc::copy_from_slice(v) }
925 #[stable(feature = "rust1", since = "1.0.0")]
926 impl<T: ?Sized> Clone for Arc<T> {
927 /// Makes a clone of the `Arc` pointer.
929 /// This creates another pointer to the same allocation, increasing the
930 /// strong reference count.
935 /// use std::sync::Arc;
937 /// let five = Arc::new(5);
939 /// let _ = Arc::clone(&five);
942 fn clone(&self) -> Arc<T> {
943 // Using a relaxed ordering is alright here, as knowledge of the
944 // original reference prevents other threads from erroneously deleting
947 // As explained in the [Boost documentation][1], Increasing the
948 // reference counter can always be done with memory_order_relaxed: New
949 // references to an object can only be formed from an existing
950 // reference, and passing an existing reference from one thread to
951 // another must already provide any required synchronization.
953 // [1]: (www.boost.org/doc/libs/1_55_0/doc/html/atomic/usage_examples.html)
954 let old_size = self.inner().strong.fetch_add(1, Relaxed);
956 // However we need to guard against massive refcounts in case someone
957 // is `mem::forget`ing Arcs. If we don't do this the count can overflow
958 // and users will use-after free. We racily saturate to `isize::MAX` on
959 // the assumption that there aren't ~2 billion threads incrementing
960 // the reference count at once. This branch will never be taken in
961 // any realistic program.
963 // We abort because such a program is incredibly degenerate, and we
964 // don't care to support it.
965 if old_size > MAX_REFCOUNT {
971 Self::from_inner(self.ptr)
975 #[stable(feature = "rust1", since = "1.0.0")]
976 impl<T: ?Sized> Deref for Arc<T> {
980 fn deref(&self) -> &T {
985 #[unstable(feature = "receiver_trait", issue = "none")]
986 impl<T: ?Sized> Receiver for Arc<T> {}
988 impl<T: Clone> Arc<T> {
989 /// Makes a mutable reference into the given `Arc`.
991 /// If there are other `Arc` or [`Weak`][weak] pointers to the same allocation,
992 /// then `make_mut` will create a new allocation and invoke [`clone`][clone] on the inner value
993 /// to ensure unique ownership. This is also referred to as clone-on-write.
995 /// Note that this differs from the behavior of [`Rc::make_mut`] which disassociates
996 /// any remaining `Weak` pointers.
998 /// See also [`get_mut`][get_mut], which will fail rather than cloning.
1000 /// [weak]: struct.Weak.html
1001 /// [clone]: ../../std/clone/trait.Clone.html#tymethod.clone
1002 /// [get_mut]: struct.Arc.html#method.get_mut
1003 /// [`Rc::make_mut`]: ../rc/struct.Rc.html#method.make_mut
1008 /// use std::sync::Arc;
1010 /// let mut data = Arc::new(5);
1012 /// *Arc::make_mut(&mut data) += 1; // Won't clone anything
1013 /// let mut other_data = Arc::clone(&data); // Won't clone inner data
1014 /// *Arc::make_mut(&mut data) += 1; // Clones inner data
1015 /// *Arc::make_mut(&mut data) += 1; // Won't clone anything
1016 /// *Arc::make_mut(&mut other_data) *= 2; // Won't clone anything
1018 /// // Now `data` and `other_data` point to different allocations.
1019 /// assert_eq!(*data, 8);
1020 /// assert_eq!(*other_data, 12);
1023 #[stable(feature = "arc_unique", since = "1.4.0")]
1024 pub fn make_mut(this: &mut Self) -> &mut T {
1025 // Note that we hold both a strong reference and a weak reference.
1026 // Thus, releasing our strong reference only will not, by itself, cause
1027 // the memory to be deallocated.
1029 // Use Acquire to ensure that we see any writes to `weak` that happen
1030 // before release writes (i.e., decrements) to `strong`. Since we hold a
1031 // weak count, there's no chance the ArcInner itself could be
1033 if this.inner().strong.compare_exchange(1, 0, Acquire, Relaxed).is_err() {
1034 // Another strong pointer exists; clone
1035 *this = Arc::new((**this).clone());
1036 } else if this.inner().weak.load(Relaxed) != 1 {
1037 // Relaxed suffices in the above because this is fundamentally an
1038 // optimization: we are always racing with weak pointers being
1039 // dropped. Worst case, we end up allocated a new Arc unnecessarily.
1041 // We removed the last strong ref, but there are additional weak
1042 // refs remaining. We'll move the contents to a new Arc, and
1043 // invalidate the other weak refs.
1045 // Note that it is not possible for the read of `weak` to yield
1046 // usize::MAX (i.e., locked), since the weak count can only be
1047 // locked by a thread with a strong reference.
1049 // Materialize our own implicit weak pointer, so that it can clean
1050 // up the ArcInner as needed.
1051 let weak = Weak { ptr: this.ptr };
1053 // mark the data itself as already deallocated
1055 // there is no data race in the implicit write caused by `read`
1056 // here (due to zeroing) because data is no longer accessed by
1057 // other threads (due to there being no more strong refs at this
1059 let mut swap = Arc::new(ptr::read(&weak.ptr.as_ref().data));
1060 mem::swap(this, &mut swap);
1064 // We were the sole reference of either kind; bump back up the
1065 // strong ref count.
1066 this.inner().strong.store(1, Release);
1069 // As with `get_mut()`, the unsafety is ok because our reference was
1070 // either unique to begin with, or became one upon cloning the contents.
1071 unsafe { &mut this.ptr.as_mut().data }
1075 impl<T: ?Sized> Arc<T> {
1076 /// Returns a mutable reference into the given `Arc`, if there are
1077 /// no other `Arc` or [`Weak`][weak] pointers to the same allocation.
1079 /// Returns [`None`][option] otherwise, because it is not safe to
1080 /// mutate a shared value.
1082 /// See also [`make_mut`][make_mut], which will [`clone`][clone]
1083 /// the inner value when there are other pointers.
1085 /// [weak]: struct.Weak.html
1086 /// [option]: ../../std/option/enum.Option.html
1087 /// [make_mut]: struct.Arc.html#method.make_mut
1088 /// [clone]: ../../std/clone/trait.Clone.html#tymethod.clone
1093 /// use std::sync::Arc;
1095 /// let mut x = Arc::new(3);
1096 /// *Arc::get_mut(&mut x).unwrap() = 4;
1097 /// assert_eq!(*x, 4);
1099 /// let _y = Arc::clone(&x);
1100 /// assert!(Arc::get_mut(&mut x).is_none());
1103 #[stable(feature = "arc_unique", since = "1.4.0")]
1104 pub fn get_mut(this: &mut Self) -> Option<&mut T> {
1105 if this.is_unique() {
1106 // This unsafety is ok because we're guaranteed that the pointer
1107 // returned is the *only* pointer that will ever be returned to T. Our
1108 // reference count is guaranteed to be 1 at this point, and we required
1109 // the Arc itself to be `mut`, so we're returning the only possible
1110 // reference to the inner data.
1111 unsafe { Some(Arc::get_mut_unchecked(this)) }
1117 /// Returns a mutable reference into the given `Arc`,
1118 /// without any check.
1120 /// See also [`get_mut`], which is safe and does appropriate checks.
1122 /// [`get_mut`]: struct.Arc.html#method.get_mut
1126 /// Any other `Arc` or [`Weak`] pointers to the same allocation must not be dereferenced
1127 /// for the duration of the returned borrow.
1128 /// This is trivially the case if no such pointers exist,
1129 /// for example immediately after `Arc::new`.
1134 /// #![feature(get_mut_unchecked)]
1136 /// use std::sync::Arc;
1138 /// let mut x = Arc::new(String::new());
1140 /// Arc::get_mut_unchecked(&mut x).push_str("foo")
1142 /// assert_eq!(*x, "foo");
1145 #[unstable(feature = "get_mut_unchecked", issue = "63292")]
1146 pub unsafe fn get_mut_unchecked(this: &mut Self) -> &mut T {
1147 &mut this.ptr.as_mut().data
1150 /// Determine whether this is the unique reference (including weak refs) to
1151 /// the underlying data.
1153 /// Note that this requires locking the weak ref count.
1154 fn is_unique(&mut self) -> bool {
1155 // lock the weak pointer count if we appear to be the sole weak pointer
1158 // The acquire label here ensures a happens-before relationship with any
1159 // writes to `strong` (in particular in `Weak::upgrade`) prior to decrements
1160 // of the `weak` count (via `Weak::drop`, which uses release). If the upgraded
1161 // weak ref was never dropped, the CAS here will fail so we do not care to synchronize.
1162 if self.inner().weak.compare_exchange(1, usize::MAX, Acquire, Relaxed).is_ok() {
1163 // This needs to be an `Acquire` to synchronize with the decrement of the `strong`
1164 // counter in `drop` -- the only access that happens when any but the last reference
1165 // is being dropped.
1166 let unique = self.inner().strong.load(Acquire) == 1;
1168 // The release write here synchronizes with a read in `downgrade`,
1169 // effectively preventing the above read of `strong` from happening
1171 self.inner().weak.store(1, Release); // release the lock
1179 #[stable(feature = "rust1", since = "1.0.0")]
1180 unsafe impl<#[may_dangle] T: ?Sized> Drop for Arc<T> {
1181 /// Drops the `Arc`.
1183 /// This will decrement the strong reference count. If the strong reference
1184 /// count reaches zero then the only other references (if any) are
1185 /// [`Weak`], so we `drop` the inner value.
1190 /// use std::sync::Arc;
1194 /// impl Drop for Foo {
1195 /// fn drop(&mut self) {
1196 /// println!("dropped!");
1200 /// let foo = Arc::new(Foo);
1201 /// let foo2 = Arc::clone(&foo);
1203 /// drop(foo); // Doesn't print anything
1204 /// drop(foo2); // Prints "dropped!"
1207 /// [`Weak`]: ../../std/sync/struct.Weak.html
1209 fn drop(&mut self) {
1210 // Because `fetch_sub` is already atomic, we do not need to synchronize
1211 // with other threads unless we are going to delete the object. This
1212 // same logic applies to the below `fetch_sub` to the `weak` count.
1213 if self.inner().strong.fetch_sub(1, Release) != 1 {
1217 // This fence is needed to prevent reordering of use of the data and
1218 // deletion of the data. Because it is marked `Release`, the decreasing
1219 // of the reference count synchronizes with this `Acquire` fence. This
1220 // means that use of the data happens before decreasing the reference
1221 // count, which happens before this fence, which happens before the
1222 // deletion of the data.
1224 // As explained in the [Boost documentation][1],
1226 // > It is important to enforce any possible access to the object in one
1227 // > thread (through an existing reference) to *happen before* deleting
1228 // > the object in a different thread. This is achieved by a "release"
1229 // > operation after dropping a reference (any access to the object
1230 // > through this reference must obviously happened before), and an
1231 // > "acquire" operation before deleting the object.
1233 // In particular, while the contents of an Arc are usually immutable, it's
1234 // possible to have interior writes to something like a Mutex<T>. Since a
1235 // Mutex is not acquired when it is deleted, we can't rely on its
1236 // synchronization logic to make writes in thread A visible to a destructor
1237 // running in thread B.
1239 // Also note that the Acquire fence here could probably be replaced with an
1240 // Acquire load, which could improve performance in highly-contended
1241 // situations. See [2].
1243 // [1]: (www.boost.org/doc/libs/1_55_0/doc/html/atomic/usage_examples.html)
1244 // [2]: (https://github.com/rust-lang/rust/pull/41714)
1245 atomic::fence(Acquire);
1253 impl Arc<dyn Any + Send + Sync> {
1255 #[stable(feature = "rc_downcast", since = "1.29.0")]
1256 /// Attempt to downcast the `Arc<dyn Any + Send + Sync>` to a concrete type.
1261 /// use std::any::Any;
1262 /// use std::sync::Arc;
1264 /// fn print_if_string(value: Arc<dyn Any + Send + Sync>) {
1265 /// if let Ok(string) = value.downcast::<String>() {
1266 /// println!("String ({}): {}", string.len(), string);
1270 /// let my_string = "Hello World".to_string();
1271 /// print_if_string(Arc::new(my_string));
1272 /// print_if_string(Arc::new(0i8));
1274 pub fn downcast<T>(self) -> Result<Arc<T>, Self>
1276 T: Any + Send + Sync + 'static,
1278 if (*self).is::<T>() {
1279 let ptr = self.ptr.cast::<ArcInner<T>>();
1281 Ok(Arc::from_inner(ptr))
1289 /// Constructs a new `Weak<T>`, without allocating any memory.
1290 /// Calling [`upgrade`] on the return value always gives [`None`].
1292 /// [`upgrade`]: struct.Weak.html#method.upgrade
1293 /// [`None`]: ../../std/option/enum.Option.html#variant.None
1298 /// use std::sync::Weak;
1300 /// let empty: Weak<i64> = Weak::new();
1301 /// assert!(empty.upgrade().is_none());
1303 #[stable(feature = "downgraded_weak", since = "1.10.0")]
1304 pub fn new() -> Weak<T> {
1305 Weak { ptr: NonNull::new(usize::MAX as *mut ArcInner<T>).expect("MAX is not 0") }
1308 /// Returns a raw pointer to the object `T` pointed to by this `Weak<T>`.
1310 /// The pointer is valid only if there are some strong references. The pointer may be dangling
1311 /// or even [`null`] otherwise.
1316 /// #![feature(weak_into_raw)]
1318 /// use std::sync::Arc;
1321 /// let strong = Arc::new("hello".to_owned());
1322 /// let weak = Arc::downgrade(&strong);
1323 /// // Both point to the same object
1324 /// assert!(ptr::eq(&*strong, weak.as_raw()));
1325 /// // The strong here keeps it alive, so we can still access the object.
1326 /// assert_eq!("hello", unsafe { &*weak.as_raw() });
1329 /// // But not any more. We can do weak.as_raw(), but accessing the pointer would lead to
1330 /// // undefined behaviour.
1331 /// // assert_eq!("hello", unsafe { &*weak.as_raw() });
1334 /// [`null`]: ../../std/ptr/fn.null.html
1335 #[unstable(feature = "weak_into_raw", issue = "60728")]
1336 pub fn as_raw(&self) -> *const T {
1337 match self.inner() {
1338 None => ptr::null(),
1340 let offset = data_offset_sized::<T>();
1341 let ptr = inner as *const ArcInner<T>;
1342 // Note: while the pointer we create may already point to dropped value, the
1343 // allocation still lives (it must hold the weak point as long as we are alive).
1344 // Therefore, the offset is OK to do, it won't get out of the allocation.
1345 let ptr = unsafe { (ptr as *const u8).offset(offset) };
1351 /// Consumes the `Weak<T>` and turns it into a raw pointer.
1353 /// This converts the weak pointer into a raw pointer, preserving the original weak count. It
1354 /// can be turned back into the `Weak<T>` with [`from_raw`].
1356 /// The same restrictions of accessing the target of the pointer as with
1357 /// [`as_raw`] apply.
1362 /// #![feature(weak_into_raw)]
1364 /// use std::sync::{Arc, Weak};
1366 /// let strong = Arc::new("hello".to_owned());
1367 /// let weak = Arc::downgrade(&strong);
1368 /// let raw = weak.into_raw();
1370 /// assert_eq!(1, Arc::weak_count(&strong));
1371 /// assert_eq!("hello", unsafe { &*raw });
1373 /// drop(unsafe { Weak::from_raw(raw) });
1374 /// assert_eq!(0, Arc::weak_count(&strong));
1377 /// [`from_raw`]: struct.Weak.html#method.from_raw
1378 /// [`as_raw`]: struct.Weak.html#method.as_raw
1379 #[unstable(feature = "weak_into_raw", issue = "60728")]
1380 pub fn into_raw(self) -> *const T {
1381 let result = self.as_raw();
1386 /// Converts a raw pointer previously created by [`into_raw`] back into
1389 /// This can be used to safely get a strong reference (by calling [`upgrade`]
1390 /// later) or to deallocate the weak count by dropping the `Weak<T>`.
1392 /// It takes ownership of one weak count (with the exception of pointers created by [`new`],
1393 /// as these don't have any corresponding weak count).
1397 /// The pointer must have originated from the [`into_raw`] (or [`as_raw'], provided there was
1398 /// a corresponding [`forget`] on the `Weak<T>`) and must still own its potential weak reference
1401 /// It is allowed for the strong count to be 0 at the time of calling this, but the weak count
1402 /// must be non-zero or the pointer must have originated from a dangling `Weak<T>` (one created
1408 /// #![feature(weak_into_raw)]
1410 /// use std::sync::{Arc, Weak};
1412 /// let strong = Arc::new("hello".to_owned());
1414 /// let raw_1 = Arc::downgrade(&strong).into_raw();
1415 /// let raw_2 = Arc::downgrade(&strong).into_raw();
1417 /// assert_eq!(2, Arc::weak_count(&strong));
1419 /// assert_eq!("hello", &*unsafe { Weak::from_raw(raw_1) }.upgrade().unwrap());
1420 /// assert_eq!(1, Arc::weak_count(&strong));
1424 /// // Decrement the last weak count.
1425 /// assert!(unsafe { Weak::from_raw(raw_2) }.upgrade().is_none());
1428 /// [`as_raw`]: struct.Weak.html#method.as_raw
1429 /// [`new`]: struct.Weak.html#method.new
1430 /// [`into_raw`]: struct.Weak.html#method.into_raw
1431 /// [`upgrade`]: struct.Weak.html#method.upgrade
1432 /// [`Weak`]: struct.Weak.html
1433 /// [`Arc`]: struct.Arc.html
1434 /// [`forget`]: ../../std/mem/fn.forget.html
1435 #[unstable(feature = "weak_into_raw", issue = "60728")]
1436 pub unsafe fn from_raw(ptr: *const T) -> Self {
1440 // See Arc::from_raw for details
1441 let offset = data_offset(ptr);
1442 let fake_ptr = ptr as *mut ArcInner<T>;
1443 let ptr = set_data_ptr(fake_ptr, (ptr as *mut u8).offset(-offset));
1444 Weak { ptr: NonNull::new(ptr).expect("Invalid pointer passed to from_raw") }
1449 impl<T: ?Sized> Weak<T> {
1450 /// Attempts to upgrade the `Weak` pointer to an [`Arc`], delaying
1451 /// dropping of the inner value if successful.
1453 /// Returns [`None`] if the inner value has since been dropped.
1455 /// [`Arc`]: struct.Arc.html
1456 /// [`None`]: ../../std/option/enum.Option.html#variant.None
1461 /// use std::sync::Arc;
1463 /// let five = Arc::new(5);
1465 /// let weak_five = Arc::downgrade(&five);
1467 /// let strong_five: Option<Arc<_>> = weak_five.upgrade();
1468 /// assert!(strong_five.is_some());
1470 /// // Destroy all strong pointers.
1471 /// drop(strong_five);
1474 /// assert!(weak_five.upgrade().is_none());
1476 #[stable(feature = "arc_weak", since = "1.4.0")]
1477 pub fn upgrade(&self) -> Option<Arc<T>> {
1478 // We use a CAS loop to increment the strong count instead of a
1479 // fetch_add because once the count hits 0 it must never be above 0.
1480 let inner = self.inner()?;
1482 // Relaxed load because any write of 0 that we can observe
1483 // leaves the field in a permanently zero state (so a
1484 // "stale" read of 0 is fine), and any other value is
1485 // confirmed via the CAS below.
1486 let mut n = inner.strong.load(Relaxed);
1493 // See comments in `Arc::clone` for why we do this (for `mem::forget`).
1494 if n > MAX_REFCOUNT {
1500 // Relaxed is valid for the same reason it is on Arc's Clone impl
1501 match inner.strong.compare_exchange_weak(n, n + 1, Relaxed, Relaxed) {
1502 Ok(_) => return Some(Arc::from_inner(self.ptr)), // null checked above
1503 Err(old) => n = old,
1508 /// Gets the number of strong (`Arc`) pointers pointing to this allocation.
1510 /// If `self` was created using [`Weak::new`], this will return 0.
1512 /// [`Weak::new`]: #method.new
1513 #[stable(feature = "weak_counts", since = "1.41.0")]
1514 pub fn strong_count(&self) -> usize {
1515 if let Some(inner) = self.inner() { inner.strong.load(SeqCst) } else { 0 }
1518 /// Gets an approximation of the number of `Weak` pointers pointing to this
1521 /// If `self` was created using [`Weak::new`], or if there are no remaining
1522 /// strong pointers, this will return 0.
1526 /// Due to implementation details, the returned value can be off by 1 in
1527 /// either direction when other threads are manipulating any `Arc`s or
1528 /// `Weak`s pointing to the same allocation.
1530 /// [`Weak::new`]: #method.new
1531 #[stable(feature = "weak_counts", since = "1.41.0")]
1532 pub fn weak_count(&self) -> usize {
1535 let weak = inner.weak.load(SeqCst);
1536 let strong = inner.strong.load(SeqCst);
1540 // Since we observed that there was at least one strong pointer
1541 // after reading the weak count, we know that the implicit weak
1542 // reference (present whenever any strong references are alive)
1543 // was still around when we observed the weak count, and can
1544 // therefore safely subtract it.
1551 /// Returns `None` when the pointer is dangling and there is no allocated `ArcInner`,
1552 /// (i.e., when this `Weak` was created by `Weak::new`).
1554 fn inner(&self) -> Option<&ArcInner<T>> {
1555 if is_dangling(self.ptr) { None } else { Some(unsafe { self.ptr.as_ref() }) }
1558 /// Returns `true` if the two `Weak`s point to the same allocation (similar to
1559 /// [`ptr::eq`]), or if both don't point to any allocation
1560 /// (because they were created with `Weak::new()`).
1564 /// Since this compares pointers it means that `Weak::new()` will equal each
1565 /// other, even though they don't point to any allocation.
1570 /// use std::sync::Arc;
1572 /// let first_rc = Arc::new(5);
1573 /// let first = Arc::downgrade(&first_rc);
1574 /// let second = Arc::downgrade(&first_rc);
1576 /// assert!(first.ptr_eq(&second));
1578 /// let third_rc = Arc::new(5);
1579 /// let third = Arc::downgrade(&third_rc);
1581 /// assert!(!first.ptr_eq(&third));
1584 /// Comparing `Weak::new`.
1587 /// use std::sync::{Arc, Weak};
1589 /// let first = Weak::new();
1590 /// let second = Weak::new();
1591 /// assert!(first.ptr_eq(&second));
1593 /// let third_rc = Arc::new(());
1594 /// let third = Arc::downgrade(&third_rc);
1595 /// assert!(!first.ptr_eq(&third));
1598 /// [`ptr::eq`]: ../../std/ptr/fn.eq.html
1600 #[stable(feature = "weak_ptr_eq", since = "1.39.0")]
1601 pub fn ptr_eq(&self, other: &Self) -> bool {
1602 self.ptr.as_ptr() == other.ptr.as_ptr()
1606 #[stable(feature = "arc_weak", since = "1.4.0")]
1607 impl<T: ?Sized> Clone for Weak<T> {
1608 /// Makes a clone of the `Weak` pointer that points to the same allocation.
1613 /// use std::sync::{Arc, Weak};
1615 /// let weak_five = Arc::downgrade(&Arc::new(5));
1617 /// let _ = Weak::clone(&weak_five);
1620 fn clone(&self) -> Weak<T> {
1621 let inner = if let Some(inner) = self.inner() {
1624 return Weak { ptr: self.ptr };
1626 // See comments in Arc::clone() for why this is relaxed. This can use a
1627 // fetch_add (ignoring the lock) because the weak count is only locked
1628 // where are *no other* weak pointers in existence. (So we can't be
1629 // running this code in that case).
1630 let old_size = inner.weak.fetch_add(1, Relaxed);
1632 // See comments in Arc::clone() for why we do this (for mem::forget).
1633 if old_size > MAX_REFCOUNT {
1639 Weak { ptr: self.ptr }
1643 #[stable(feature = "downgraded_weak", since = "1.10.0")]
1644 impl<T> Default for Weak<T> {
1645 /// Constructs a new `Weak<T>`, without allocating memory.
1646 /// Calling [`upgrade`] on the return value always
1649 /// [`None`]: ../../std/option/enum.Option.html#variant.None
1650 /// [`upgrade`]: ../../std/sync/struct.Weak.html#method.upgrade
1655 /// use std::sync::Weak;
1657 /// let empty: Weak<i64> = Default::default();
1658 /// assert!(empty.upgrade().is_none());
1660 fn default() -> Weak<T> {
1665 #[stable(feature = "arc_weak", since = "1.4.0")]
1666 impl<T: ?Sized> Drop for Weak<T> {
1667 /// Drops the `Weak` pointer.
1672 /// use std::sync::{Arc, Weak};
1676 /// impl Drop for Foo {
1677 /// fn drop(&mut self) {
1678 /// println!("dropped!");
1682 /// let foo = Arc::new(Foo);
1683 /// let weak_foo = Arc::downgrade(&foo);
1684 /// let other_weak_foo = Weak::clone(&weak_foo);
1686 /// drop(weak_foo); // Doesn't print anything
1687 /// drop(foo); // Prints "dropped!"
1689 /// assert!(other_weak_foo.upgrade().is_none());
1691 fn drop(&mut self) {
1692 // If we find out that we were the last weak pointer, then its time to
1693 // deallocate the data entirely. See the discussion in Arc::drop() about
1694 // the memory orderings
1696 // It's not necessary to check for the locked state here, because the
1697 // weak count can only be locked if there was precisely one weak ref,
1698 // meaning that drop could only subsequently run ON that remaining weak
1699 // ref, which can only happen after the lock is released.
1700 let inner = if let Some(inner) = self.inner() { inner } else { return };
1702 if inner.weak.fetch_sub(1, Release) == 1 {
1703 atomic::fence(Acquire);
1704 unsafe { Global.dealloc(self.ptr.cast(), Layout::for_value(self.ptr.as_ref())) }
1709 #[stable(feature = "rust1", since = "1.0.0")]
1710 trait ArcEqIdent<T: ?Sized + PartialEq> {
1711 fn eq(&self, other: &Arc<T>) -> bool;
1712 fn ne(&self, other: &Arc<T>) -> bool;
1715 #[stable(feature = "rust1", since = "1.0.0")]
1716 impl<T: ?Sized + PartialEq> ArcEqIdent<T> for Arc<T> {
1718 default fn eq(&self, other: &Arc<T>) -> bool {
1722 default fn ne(&self, other: &Arc<T>) -> bool {
1727 /// We're doing this specialization here, and not as a more general optimization on `&T`, because it
1728 /// would otherwise add a cost to all equality checks on refs. We assume that `Arc`s are used to
1729 /// store large values, that are slow to clone, but also heavy to check for equality, causing this
1730 /// cost to pay off more easily. It's also more likely to have two `Arc` clones, that point to
1731 /// the same value, than two `&T`s.
1733 /// We can only do this when `T: Eq` as a `PartialEq` might be deliberately irreflexive.
1734 #[stable(feature = "rust1", since = "1.0.0")]
1735 impl<T: ?Sized + Eq> ArcEqIdent<T> for Arc<T> {
1737 fn eq(&self, other: &Arc<T>) -> bool {
1738 Arc::ptr_eq(self, other) || **self == **other
1742 fn ne(&self, other: &Arc<T>) -> bool {
1743 !Arc::ptr_eq(self, other) && **self != **other
1747 #[stable(feature = "rust1", since = "1.0.0")]
1748 impl<T: ?Sized + PartialEq> PartialEq for Arc<T> {
1749 /// Equality for two `Arc`s.
1751 /// Two `Arc`s are equal if their inner values are equal, even if they are
1752 /// stored in different allocation.
1754 /// If `T` also implements `Eq` (implying reflexivity of equality),
1755 /// two `Arc`s that point to the same allocation are always equal.
1760 /// use std::sync::Arc;
1762 /// let five = Arc::new(5);
1764 /// assert!(five == Arc::new(5));
1767 fn eq(&self, other: &Arc<T>) -> bool {
1768 ArcEqIdent::eq(self, other)
1771 /// Inequality for two `Arc`s.
1773 /// Two `Arc`s are unequal if their inner values are unequal.
1775 /// If `T` also implements `Eq` (implying reflexivity of equality),
1776 /// two `Arc`s that point to the same value are never unequal.
1781 /// use std::sync::Arc;
1783 /// let five = Arc::new(5);
1785 /// assert!(five != Arc::new(6));
1788 fn ne(&self, other: &Arc<T>) -> bool {
1789 ArcEqIdent::ne(self, other)
1793 #[stable(feature = "rust1", since = "1.0.0")]
1794 impl<T: ?Sized + PartialOrd> PartialOrd for Arc<T> {
1795 /// Partial comparison for two `Arc`s.
1797 /// The two are compared by calling `partial_cmp()` on their inner values.
1802 /// use std::sync::Arc;
1803 /// use std::cmp::Ordering;
1805 /// let five = Arc::new(5);
1807 /// assert_eq!(Some(Ordering::Less), five.partial_cmp(&Arc::new(6)));
1809 fn partial_cmp(&self, other: &Arc<T>) -> Option<Ordering> {
1810 (**self).partial_cmp(&**other)
1813 /// Less-than comparison for two `Arc`s.
1815 /// The two are compared by calling `<` on their inner values.
1820 /// use std::sync::Arc;
1822 /// let five = Arc::new(5);
1824 /// assert!(five < Arc::new(6));
1826 fn lt(&self, other: &Arc<T>) -> bool {
1827 *(*self) < *(*other)
1830 /// 'Less than or equal to' comparison for two `Arc`s.
1832 /// The two are compared by calling `<=` on their inner values.
1837 /// use std::sync::Arc;
1839 /// let five = Arc::new(5);
1841 /// assert!(five <= Arc::new(5));
1843 fn le(&self, other: &Arc<T>) -> bool {
1844 *(*self) <= *(*other)
1847 /// Greater-than comparison for two `Arc`s.
1849 /// The two are compared by calling `>` on their inner values.
1854 /// use std::sync::Arc;
1856 /// let five = Arc::new(5);
1858 /// assert!(five > Arc::new(4));
1860 fn gt(&self, other: &Arc<T>) -> bool {
1861 *(*self) > *(*other)
1864 /// 'Greater than or equal to' comparison for two `Arc`s.
1866 /// The two are compared by calling `>=` on their inner values.
1871 /// use std::sync::Arc;
1873 /// let five = Arc::new(5);
1875 /// assert!(five >= Arc::new(5));
1877 fn ge(&self, other: &Arc<T>) -> bool {
1878 *(*self) >= *(*other)
1881 #[stable(feature = "rust1", since = "1.0.0")]
1882 impl<T: ?Sized + Ord> Ord for Arc<T> {
1883 /// Comparison for two `Arc`s.
1885 /// The two are compared by calling `cmp()` on their inner values.
1890 /// use std::sync::Arc;
1891 /// use std::cmp::Ordering;
1893 /// let five = Arc::new(5);
1895 /// assert_eq!(Ordering::Less, five.cmp(&Arc::new(6)));
1897 fn cmp(&self, other: &Arc<T>) -> Ordering {
1898 (**self).cmp(&**other)
1901 #[stable(feature = "rust1", since = "1.0.0")]
1902 impl<T: ?Sized + Eq> Eq for Arc<T> {}
1904 #[stable(feature = "rust1", since = "1.0.0")]
1905 impl<T: ?Sized + fmt::Display> fmt::Display for Arc<T> {
1906 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
1907 fmt::Display::fmt(&**self, f)
1911 #[stable(feature = "rust1", since = "1.0.0")]
1912 impl<T: ?Sized + fmt::Debug> fmt::Debug for Arc<T> {
1913 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
1914 fmt::Debug::fmt(&**self, f)
1918 #[stable(feature = "rust1", since = "1.0.0")]
1919 impl<T: ?Sized> fmt::Pointer for Arc<T> {
1920 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
1921 fmt::Pointer::fmt(&(&**self as *const T), f)
1925 #[stable(feature = "rust1", since = "1.0.0")]
1926 impl<T: Default> Default for Arc<T> {
1927 /// Creates a new `Arc<T>`, with the `Default` value for `T`.
1932 /// use std::sync::Arc;
1934 /// let x: Arc<i32> = Default::default();
1935 /// assert_eq!(*x, 0);
1937 fn default() -> Arc<T> {
1938 Arc::new(Default::default())
1942 #[stable(feature = "rust1", since = "1.0.0")]
1943 impl<T: ?Sized + Hash> Hash for Arc<T> {
1944 fn hash<H: Hasher>(&self, state: &mut H) {
1945 (**self).hash(state)
1949 #[stable(feature = "from_for_ptrs", since = "1.6.0")]
1950 impl<T> From<T> for Arc<T> {
1951 fn from(t: T) -> Self {
1956 #[stable(feature = "shared_from_slice", since = "1.21.0")]
1957 impl<T: Clone> From<&[T]> for Arc<[T]> {
1959 fn from(v: &[T]) -> Arc<[T]> {
1960 <Self as ArcFromSlice<T>>::from_slice(v)
1964 #[stable(feature = "shared_from_slice", since = "1.21.0")]
1965 impl From<&str> for Arc<str> {
1967 fn from(v: &str) -> Arc<str> {
1968 let arc = Arc::<[u8]>::from(v.as_bytes());
1969 unsafe { Arc::from_raw(Arc::into_raw(arc) as *const str) }
1973 #[stable(feature = "shared_from_slice", since = "1.21.0")]
1974 impl From<String> for Arc<str> {
1976 fn from(v: String) -> Arc<str> {
1981 #[stable(feature = "shared_from_slice", since = "1.21.0")]
1982 impl<T: ?Sized> From<Box<T>> for Arc<T> {
1984 fn from(v: Box<T>) -> Arc<T> {
1989 #[stable(feature = "shared_from_slice", since = "1.21.0")]
1990 impl<T> From<Vec<T>> for Arc<[T]> {
1992 fn from(mut v: Vec<T>) -> Arc<[T]> {
1994 let arc = Arc::copy_from_slice(&v);
1996 // Allow the Vec to free its memory, but not destroy its contents
2004 #[unstable(feature = "boxed_slice_try_from", issue = "none")]
2005 impl<T, const N: usize> TryFrom<Arc<[T]>> for Arc<[T; N]>
2007 [T; N]: LengthAtMost32,
2009 type Error = Arc<[T]>;
2011 fn try_from(boxed_slice: Arc<[T]>) -> Result<Self, Self::Error> {
2012 if boxed_slice.len() == N {
2013 Ok(unsafe { Arc::from_raw(Arc::into_raw(boxed_slice) as *mut [T; N]) })
2020 #[stable(feature = "shared_from_iter", since = "1.37.0")]
2021 impl<T> iter::FromIterator<T> for Arc<[T]> {
2022 /// Takes each element in the `Iterator` and collects it into an `Arc<[T]>`.
2024 /// # Performance characteristics
2026 /// ## The general case
2028 /// In the general case, collecting into `Arc<[T]>` is done by first
2029 /// collecting into a `Vec<T>`. That is, when writing the following:
2032 /// # use std::sync::Arc;
2033 /// let evens: Arc<[u8]> = (0..10).filter(|&x| x % 2 == 0).collect();
2034 /// # assert_eq!(&*evens, &[0, 2, 4, 6, 8]);
2037 /// this behaves as if we wrote:
2040 /// # use std::sync::Arc;
2041 /// let evens: Arc<[u8]> = (0..10).filter(|&x| x % 2 == 0)
2042 /// .collect::<Vec<_>>() // The first set of allocations happens here.
2043 /// .into(); // A second allocation for `Arc<[T]>` happens here.
2044 /// # assert_eq!(&*evens, &[0, 2, 4, 6, 8]);
2047 /// This will allocate as many times as needed for constructing the `Vec<T>`
2048 /// and then it will allocate once for turning the `Vec<T>` into the `Arc<[T]>`.
2050 /// ## Iterators of known length
2052 /// When your `Iterator` implements `TrustedLen` and is of an exact size,
2053 /// a single allocation will be made for the `Arc<[T]>`. For example:
2056 /// # use std::sync::Arc;
2057 /// let evens: Arc<[u8]> = (0..10).collect(); // Just a single allocation happens here.
2058 /// # assert_eq!(&*evens, &*(0..10).collect::<Vec<_>>());
2060 fn from_iter<I: iter::IntoIterator<Item = T>>(iter: I) -> Self {
2061 ArcFromIter::from_iter(iter.into_iter())
2065 /// Specialization trait used for collecting into `Arc<[T]>`.
2066 trait ArcFromIter<T, I> {
2067 fn from_iter(iter: I) -> Self;
2070 impl<T, I: Iterator<Item = T>> ArcFromIter<T, I> for Arc<[T]> {
2071 default fn from_iter(iter: I) -> Self {
2072 iter.collect::<Vec<T>>().into()
2076 impl<T, I: iter::TrustedLen<Item = T>> ArcFromIter<T, I> for Arc<[T]> {
2077 default fn from_iter(iter: I) -> Self {
2078 // This is the case for a `TrustedLen` iterator.
2079 let (low, high) = iter.size_hint();
2080 if let Some(high) = high {
2084 "TrustedLen iterator's size hint is not exact: {:?}",
2089 // SAFETY: We need to ensure that the iterator has an exact length and we have.
2090 Arc::from_iter_exact(iter, low)
2093 // Fall back to normal implementation.
2094 iter.collect::<Vec<T>>().into()
2099 impl<'a, T: 'a + Clone> ArcFromIter<&'a T, slice::Iter<'a, T>> for Arc<[T]> {
2100 fn from_iter(iter: slice::Iter<'a, T>) -> Self {
2101 // Delegate to `impl<T: Clone> From<&[T]> for Arc<[T]>`.
2103 // In the case that `T: Copy`, we get to use `ptr::copy_nonoverlapping`
2104 // which is even more performant.
2106 // In the fall-back case we have `T: Clone`. This is still better
2107 // than the `TrustedLen` implementation as slices have a known length
2108 // and so we get to avoid calling `size_hint` and avoid the branching.
2109 iter.as_slice().into()
2113 #[stable(feature = "rust1", since = "1.0.0")]
2114 impl<T: ?Sized> borrow::Borrow<T> for Arc<T> {
2115 fn borrow(&self) -> &T {
2120 #[stable(since = "1.5.0", feature = "smart_ptr_as_ref")]
2121 impl<T: ?Sized> AsRef<T> for Arc<T> {
2122 fn as_ref(&self) -> &T {
2127 #[stable(feature = "pin", since = "1.33.0")]
2128 impl<T: ?Sized> Unpin for Arc<T> {}
2130 /// Computes the offset of the data field within `ArcInner`.
2131 unsafe fn data_offset<T: ?Sized>(ptr: *const T) -> isize {
2132 // Align the unsized value to the end of the `ArcInner`.
2133 // Because it is `?Sized`, it will always be the last field in memory.
2134 // Note: This is a detail of the current implementation of the compiler,
2135 // and is not a guaranteed language detail. Do not rely on it outside of std.
2136 data_offset_align(align_of_val(&*ptr))
2139 /// Computes the offset of the data field within `ArcInner`.
2141 /// Unlike [`data_offset`], this doesn't need the pointer, but it works only on `T: Sized`.
2142 fn data_offset_sized<T>() -> isize {
2143 data_offset_align(align_of::<T>())
2147 fn data_offset_align(align: usize) -> isize {
2148 let layout = Layout::new::<ArcInner<()>>();
2149 (layout.size() + layout.padding_needed_for(align)) as isize