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};
27 use crate::alloc::{box_free, handle_alloc_error, AllocInit, AllocRef, Global, Layout};
28 use crate::boxed::Box;
29 use crate::rc::is_dangling;
30 use crate::string::String;
36 /// A soft limit on the amount of references that may be made to an `Arc`.
38 /// Going above this limit will abort your program (although not
39 /// necessarily) at _exactly_ `MAX_REFCOUNT + 1` references.
40 const MAX_REFCOUNT: usize = (isize::MAX) as usize;
42 #[cfg(not(sanitize = "thread"))]
43 macro_rules! acquire {
45 atomic::fence(Acquire)
49 // ThreadSanitizer does not support memory fences. To avoid false positive
50 // reports in Arc / Weak implementation use atomic loads for synchronization
52 #[cfg(sanitize = "thread")]
53 macro_rules! acquire {
59 /// A thread-safe reference-counting pointer. 'Arc' stands for 'Atomically
60 /// Reference Counted'.
62 /// The type `Arc<T>` provides shared ownership of a value of type `T`,
63 /// allocated in the heap. Invoking [`clone`][clone] on `Arc` produces
64 /// a new `Arc` instance, which points to the same allocation on the heap as the
65 /// source `Arc`, while increasing a reference count. When the last `Arc`
66 /// pointer to a given allocation is destroyed, the value stored in that allocation (often
67 /// referred to as "inner value") is also dropped.
69 /// Shared references in Rust disallow mutation by default, and `Arc` is no
70 /// exception: you cannot generally obtain a mutable reference to something
71 /// inside an `Arc`. If you need to mutate through an `Arc`, use
72 /// [`Mutex`][mutex], [`RwLock`][rwlock], or one of the [`Atomic`][atomic]
77 /// Unlike [`Rc<T>`], `Arc<T>` uses atomic operations for its reference
78 /// counting. This means that it is thread-safe. The disadvantage is that
79 /// atomic operations are more expensive than ordinary memory accesses. If you
80 /// are not sharing reference-counted allocations between threads, consider using
81 /// [`Rc<T>`] for lower overhead. [`Rc<T>`] is a safe default, because the
82 /// compiler will catch any attempt to send an [`Rc<T>`] between threads.
83 /// However, a library might choose `Arc<T>` in order to give library consumers
86 /// `Arc<T>` will implement [`Send`] and [`Sync`] as long as the `T` implements
87 /// [`Send`] and [`Sync`]. Why can't you put a non-thread-safe type `T` in an
88 /// `Arc<T>` to make it thread-safe? This may be a bit counter-intuitive at
89 /// first: after all, isn't the point of `Arc<T>` thread safety? The key is
90 /// this: `Arc<T>` makes it thread safe to have multiple ownership of the same
91 /// data, but it doesn't add thread safety to its data. Consider
92 /// `Arc<`[`RefCell<T>`]`>`. [`RefCell<T>`] isn't [`Sync`], and if `Arc<T>` was always
93 /// [`Send`], `Arc<`[`RefCell<T>`]`>` would be as well. But then we'd have a problem:
94 /// [`RefCell<T>`] is not thread safe; it keeps track of the borrowing count using
95 /// non-atomic operations.
97 /// In the end, this means that you may need to pair `Arc<T>` with some sort of
98 /// [`std::sync`] type, usually [`Mutex<T>`][mutex].
100 /// ## Breaking cycles with `Weak`
102 /// The [`downgrade`][downgrade] method can be used to create a non-owning
103 /// [`Weak`][weak] pointer. A [`Weak`][weak] pointer can be [`upgrade`][upgrade]d
104 /// to an `Arc`, but this will return [`None`] if the value stored in the allocation has
105 /// already been dropped. In other words, `Weak` pointers do not keep the value
106 /// inside the allocation alive; however, they *do* keep the allocation
107 /// (the backing store for the value) alive.
109 /// A cycle between `Arc` pointers will never be deallocated. For this reason,
110 /// [`Weak`][weak] is used to break cycles. For example, a tree could have
111 /// strong `Arc` pointers from parent nodes to children, and [`Weak`][weak]
112 /// pointers from children back to their parents.
114 /// # Cloning references
116 /// Creating a new reference from an existing reference counted pointer is done using the
117 /// `Clone` trait implemented for [`Arc<T>`][arc] and [`Weak<T>`][weak].
120 /// use std::sync::Arc;
121 /// let foo = Arc::new(vec![1.0, 2.0, 3.0]);
122 /// // The two syntaxes below are equivalent.
123 /// let a = foo.clone();
124 /// let b = Arc::clone(&foo);
125 /// // a, b, and foo are all Arcs that point to the same memory location
128 /// ## `Deref` behavior
130 /// `Arc<T>` automatically dereferences to `T` (via the [`Deref`][deref] trait),
131 /// so you can call `T`'s methods on a value of type `Arc<T>`. To avoid name
132 /// clashes with `T`'s methods, the methods of `Arc<T>` itself are associated
133 /// functions, called using function-like syntax:
136 /// use std::sync::Arc;
137 /// let my_arc = Arc::new(());
139 /// Arc::downgrade(&my_arc);
142 /// [`Weak<T>`][weak] does not auto-dereference to `T`, because the inner value may have
143 /// already been dropped.
145 /// [arc]: struct.Arc.html
146 /// [weak]: struct.Weak.html
147 /// [`Rc<T>`]: ../../std/rc/struct.Rc.html
148 /// [clone]: ../../std/clone/trait.Clone.html#tymethod.clone
149 /// [mutex]: ../../std/sync/struct.Mutex.html
150 /// [rwlock]: ../../std/sync/struct.RwLock.html
151 /// [atomic]: ../../std/sync/atomic/index.html
152 /// [`Send`]: ../../std/marker/trait.Send.html
153 /// [`Sync`]: ../../std/marker/trait.Sync.html
154 /// [deref]: ../../std/ops/trait.Deref.html
155 /// [downgrade]: struct.Arc.html#method.downgrade
156 /// [upgrade]: struct.Weak.html#method.upgrade
157 /// [`None`]: ../../std/option/enum.Option.html#variant.None
158 /// [`RefCell<T>`]: ../../std/cell/struct.RefCell.html
159 /// [`std::sync`]: ../../std/sync/index.html
160 /// [`Arc::clone(&from)`]: #method.clone
164 /// Sharing some immutable data between threads:
166 // Note that we **do not** run these tests here. The windows builders get super
167 // unhappy if a thread outlives the main thread and then exits at the same time
168 // (something deadlocks) so we just avoid this entirely by not running these
171 /// use std::sync::Arc;
174 /// let five = Arc::new(5);
177 /// let five = Arc::clone(&five);
179 /// thread::spawn(move || {
180 /// println!("{:?}", five);
185 /// Sharing a mutable [`AtomicUsize`]:
187 /// [`AtomicUsize`]: ../../std/sync/atomic/struct.AtomicUsize.html
190 /// use std::sync::Arc;
191 /// use std::sync::atomic::{AtomicUsize, Ordering};
194 /// let val = Arc::new(AtomicUsize::new(5));
197 /// let val = Arc::clone(&val);
199 /// thread::spawn(move || {
200 /// let v = val.fetch_add(1, Ordering::SeqCst);
201 /// println!("{:?}", v);
206 /// See the [`rc` documentation][rc_examples] for more examples of reference
207 /// counting in general.
209 /// [rc_examples]: ../../std/rc/index.html#examples
210 #[cfg_attr(all(bootstrap, not(test)), lang = "arc")]
211 #[cfg_attr(not(test), rustc_diagnostic_item = "Arc")]
212 #[stable(feature = "rust1", since = "1.0.0")]
213 pub struct Arc<T: ?Sized> {
214 ptr: NonNull<ArcInner<T>>,
215 phantom: PhantomData<ArcInner<T>>,
218 #[stable(feature = "rust1", since = "1.0.0")]
219 unsafe impl<T: ?Sized + Sync + Send> Send for Arc<T> {}
220 #[stable(feature = "rust1", since = "1.0.0")]
221 unsafe impl<T: ?Sized + Sync + Send> Sync for Arc<T> {}
223 #[unstable(feature = "coerce_unsized", issue = "27732")]
224 impl<T: ?Sized + Unsize<U>, U: ?Sized> CoerceUnsized<Arc<U>> for Arc<T> {}
226 #[unstable(feature = "dispatch_from_dyn", issue = "none")]
227 impl<T: ?Sized + Unsize<U>, U: ?Sized> DispatchFromDyn<Arc<U>> for Arc<T> {}
229 impl<T: ?Sized> Arc<T> {
230 fn from_inner(ptr: NonNull<ArcInner<T>>) -> Self {
231 Self { ptr, phantom: PhantomData }
234 unsafe fn from_ptr(ptr: *mut ArcInner<T>) -> Self {
235 Self::from_inner(NonNull::new_unchecked(ptr))
239 /// `Weak` is a version of [`Arc`] that holds a non-owning reference to the
240 /// managed allocation. The allocation is accessed by calling [`upgrade`] on the `Weak`
241 /// pointer, which returns an [`Option`]`<`[`Arc`]`<T>>`.
243 /// Since a `Weak` reference does not count towards ownership, it will not
244 /// prevent the value stored in the allocation from being dropped, and `Weak` itself makes no
245 /// guarantees about the value still being present. Thus it may return [`None`]
246 /// when [`upgrade`]d. Note however that a `Weak` reference *does* prevent the allocation
247 /// itself (the backing store) from being deallocated.
249 /// A `Weak` pointer is useful for keeping a temporary reference to the allocation
250 /// managed by [`Arc`] without preventing its inner value from being dropped. It is also used to
251 /// prevent circular references between [`Arc`] pointers, since mutual owning references
252 /// would never allow either [`Arc`] to be dropped. For example, a tree could
253 /// have strong [`Arc`] pointers from parent nodes to children, and `Weak`
254 /// pointers from children back to their parents.
256 /// The typical way to obtain a `Weak` pointer is to call [`Arc::downgrade`].
258 /// [`Arc`]: struct.Arc.html
259 /// [`Arc::downgrade`]: struct.Arc.html#method.downgrade
260 /// [`upgrade`]: struct.Weak.html#method.upgrade
261 /// [`Option`]: ../../std/option/enum.Option.html
262 /// [`None`]: ../../std/option/enum.Option.html#variant.None
263 #[stable(feature = "arc_weak", since = "1.4.0")]
264 pub struct Weak<T: ?Sized> {
265 // This is a `NonNull` to allow optimizing the size of this type in enums,
266 // but it is not necessarily a valid pointer.
267 // `Weak::new` sets this to `usize::MAX` so that it doesn’t need
268 // to allocate space on the heap. That's not a value a real pointer
269 // will ever have because RcBox has alignment at least 2.
270 ptr: NonNull<ArcInner<T>>,
273 #[stable(feature = "arc_weak", since = "1.4.0")]
274 unsafe impl<T: ?Sized + Sync + Send> Send for Weak<T> {}
275 #[stable(feature = "arc_weak", since = "1.4.0")]
276 unsafe impl<T: ?Sized + Sync + Send> Sync for Weak<T> {}
278 #[unstable(feature = "coerce_unsized", issue = "27732")]
279 impl<T: ?Sized + Unsize<U>, U: ?Sized> CoerceUnsized<Weak<U>> for Weak<T> {}
280 #[unstable(feature = "dispatch_from_dyn", issue = "none")]
281 impl<T: ?Sized + Unsize<U>, U: ?Sized> DispatchFromDyn<Weak<U>> for Weak<T> {}
283 #[stable(feature = "arc_weak", since = "1.4.0")]
284 impl<T: ?Sized + fmt::Debug> fmt::Debug for Weak<T> {
285 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
290 // This is repr(C) to future-proof against possible field-reordering, which
291 // would interfere with otherwise safe [into|from]_raw() of transmutable
294 struct ArcInner<T: ?Sized> {
295 strong: atomic::AtomicUsize,
297 // the value usize::MAX acts as a sentinel for temporarily "locking" the
298 // ability to upgrade weak pointers or downgrade strong ones; this is used
299 // to avoid races in `make_mut` and `get_mut`.
300 weak: atomic::AtomicUsize,
305 unsafe impl<T: ?Sized + Sync + Send> Send for ArcInner<T> {}
306 unsafe impl<T: ?Sized + Sync + Send> Sync for ArcInner<T> {}
309 /// Constructs a new `Arc<T>`.
314 /// use std::sync::Arc;
316 /// let five = Arc::new(5);
319 #[stable(feature = "rust1", since = "1.0.0")]
320 pub fn new(data: T) -> Arc<T> {
321 // Start the weak pointer count as 1 which is the weak pointer that's
322 // held by all the strong pointers (kinda), see std/rc.rs for more info
323 let x: Box<_> = box ArcInner {
324 strong: atomic::AtomicUsize::new(1),
325 weak: atomic::AtomicUsize::new(1),
328 Self::from_inner(Box::into_raw_non_null(x))
331 /// Constructs a new `Arc` with uninitialized contents.
336 /// #![feature(new_uninit)]
337 /// #![feature(get_mut_unchecked)]
339 /// use std::sync::Arc;
341 /// let mut five = Arc::<u32>::new_uninit();
343 /// let five = unsafe {
344 /// // Deferred initialization:
345 /// Arc::get_mut_unchecked(&mut five).as_mut_ptr().write(5);
347 /// five.assume_init()
350 /// assert_eq!(*five, 5)
352 #[unstable(feature = "new_uninit", issue = "63291")]
353 pub fn new_uninit() -> Arc<mem::MaybeUninit<T>> {
355 Arc::from_ptr(Arc::allocate_for_layout(Layout::new::<T>(), |mem| {
356 mem as *mut ArcInner<mem::MaybeUninit<T>>
361 /// Constructs a new `Arc` with uninitialized contents, with the memory
362 /// being filled with `0` bytes.
364 /// See [`MaybeUninit::zeroed`][zeroed] for examples of correct and incorrect usage
370 /// #![feature(new_uninit)]
372 /// use std::sync::Arc;
374 /// let zero = Arc::<u32>::new_zeroed();
375 /// let zero = unsafe { zero.assume_init() };
377 /// assert_eq!(*zero, 0)
380 /// [zeroed]: ../../std/mem/union.MaybeUninit.html#method.zeroed
381 #[unstable(feature = "new_uninit", issue = "63291")]
382 pub fn new_zeroed() -> Arc<mem::MaybeUninit<T>> {
384 let mut uninit = Self::new_uninit();
385 ptr::write_bytes::<T>(Arc::get_mut_unchecked(&mut uninit).as_mut_ptr(), 0, 1);
390 /// Constructs a new `Pin<Arc<T>>`. If `T` does not implement `Unpin`, then
391 /// `data` will be pinned in memory and unable to be moved.
392 #[stable(feature = "pin", since = "1.33.0")]
393 pub fn pin(data: T) -> Pin<Arc<T>> {
394 unsafe { Pin::new_unchecked(Arc::new(data)) }
397 /// Returns the inner value, if the `Arc` has exactly one strong reference.
399 /// Otherwise, an [`Err`][result] is returned with the same `Arc` that was
402 /// This will succeed even if there are outstanding weak references.
404 /// [result]: ../../std/result/enum.Result.html
409 /// use std::sync::Arc;
411 /// let x = Arc::new(3);
412 /// assert_eq!(Arc::try_unwrap(x), Ok(3));
414 /// let x = Arc::new(4);
415 /// let _y = Arc::clone(&x);
416 /// assert_eq!(*Arc::try_unwrap(x).unwrap_err(), 4);
419 #[stable(feature = "arc_unique", since = "1.4.0")]
420 pub fn try_unwrap(this: Self) -> Result<T, Self> {
421 // See `drop` for why all these atomics are like this
422 if this.inner().strong.compare_exchange(1, 0, Release, Relaxed).is_err() {
426 acquire!(this.inner().strong);
429 let elem = ptr::read(&this.ptr.as_ref().data);
431 // Make a weak pointer to clean up the implicit strong-weak reference
432 let _weak = Weak { ptr: this.ptr };
441 /// Constructs a new reference-counted slice with uninitialized contents.
446 /// #![feature(new_uninit)]
447 /// #![feature(get_mut_unchecked)]
449 /// use std::sync::Arc;
451 /// let mut values = Arc::<[u32]>::new_uninit_slice(3);
453 /// let values = unsafe {
454 /// // Deferred initialization:
455 /// Arc::get_mut_unchecked(&mut values)[0].as_mut_ptr().write(1);
456 /// Arc::get_mut_unchecked(&mut values)[1].as_mut_ptr().write(2);
457 /// Arc::get_mut_unchecked(&mut values)[2].as_mut_ptr().write(3);
459 /// values.assume_init()
462 /// assert_eq!(*values, [1, 2, 3])
464 #[unstable(feature = "new_uninit", issue = "63291")]
465 pub fn new_uninit_slice(len: usize) -> Arc<[mem::MaybeUninit<T>]> {
466 unsafe { Arc::from_ptr(Arc::allocate_for_slice(len)) }
470 impl<T> Arc<mem::MaybeUninit<T>> {
471 /// Converts to `Arc<T>`.
475 /// As with [`MaybeUninit::assume_init`],
476 /// it is up to the caller to guarantee that the inner value
477 /// really is in an initialized state.
478 /// Calling this when the content is not yet fully initialized
479 /// causes immediate undefined behavior.
481 /// [`MaybeUninit::assume_init`]: ../../std/mem/union.MaybeUninit.html#method.assume_init
486 /// #![feature(new_uninit)]
487 /// #![feature(get_mut_unchecked)]
489 /// use std::sync::Arc;
491 /// let mut five = Arc::<u32>::new_uninit();
493 /// let five = unsafe {
494 /// // Deferred initialization:
495 /// Arc::get_mut_unchecked(&mut five).as_mut_ptr().write(5);
497 /// five.assume_init()
500 /// assert_eq!(*five, 5)
502 #[unstable(feature = "new_uninit", issue = "63291")]
504 pub unsafe fn assume_init(self) -> Arc<T> {
505 Arc::from_inner(mem::ManuallyDrop::new(self).ptr.cast())
509 impl<T> Arc<[mem::MaybeUninit<T>]> {
510 /// Converts to `Arc<[T]>`.
514 /// As with [`MaybeUninit::assume_init`],
515 /// it is up to the caller to guarantee that the inner value
516 /// really is in an initialized state.
517 /// Calling this when the content is not yet fully initialized
518 /// causes immediate undefined behavior.
520 /// [`MaybeUninit::assume_init`]: ../../std/mem/union.MaybeUninit.html#method.assume_init
525 /// #![feature(new_uninit)]
526 /// #![feature(get_mut_unchecked)]
528 /// use std::sync::Arc;
530 /// let mut values = Arc::<[u32]>::new_uninit_slice(3);
532 /// let values = unsafe {
533 /// // Deferred initialization:
534 /// Arc::get_mut_unchecked(&mut values)[0].as_mut_ptr().write(1);
535 /// Arc::get_mut_unchecked(&mut values)[1].as_mut_ptr().write(2);
536 /// Arc::get_mut_unchecked(&mut values)[2].as_mut_ptr().write(3);
538 /// values.assume_init()
541 /// assert_eq!(*values, [1, 2, 3])
543 #[unstable(feature = "new_uninit", issue = "63291")]
545 pub unsafe fn assume_init(self) -> Arc<[T]> {
546 Arc::from_ptr(mem::ManuallyDrop::new(self).ptr.as_ptr() as _)
550 impl<T: ?Sized> Arc<T> {
551 /// Consumes the `Arc`, returning the wrapped pointer.
553 /// To avoid a memory leak the pointer must be converted back to an `Arc` using
554 /// [`Arc::from_raw`][from_raw].
556 /// [from_raw]: struct.Arc.html#method.from_raw
561 /// use std::sync::Arc;
563 /// let x = Arc::new("hello".to_owned());
564 /// let x_ptr = Arc::into_raw(x);
565 /// assert_eq!(unsafe { &*x_ptr }, "hello");
567 #[stable(feature = "rc_raw", since = "1.17.0")]
568 pub fn into_raw(this: Self) -> *const T {
569 let ptr = Self::as_ptr(&this);
574 /// Provides a raw pointer to the data.
576 /// The counts are not affected in way and the `Arc` is not consumed. The pointer is valid for
577 /// as long as there are strong counts in the `Arc`.
582 /// #![feature(weak_into_raw)]
584 /// use std::sync::Arc;
586 /// let x = Arc::new("hello".to_owned());
587 /// let y = Arc::clone(&x);
588 /// let x_ptr = Arc::as_ptr(&x);
589 /// assert_eq!(x_ptr, Arc::as_ptr(&y));
590 /// assert_eq!(unsafe { &*x_ptr }, "hello");
592 #[unstable(feature = "weak_into_raw", issue = "60728")]
593 pub fn as_ptr(this: &Self) -> *const T {
594 let ptr: *mut ArcInner<T> = NonNull::as_ptr(this.ptr);
595 let fake_ptr = ptr as *mut T;
597 // SAFETY: This cannot go through Deref::deref.
598 // Instead, we manually offset the pointer rather than manifesting a reference.
599 // This is so that the returned pointer retains the same provenance as our pointer.
600 // This is required so that e.g. `get_mut` can write through the pointer
601 // after the Arc is recovered through `from_raw`.
603 let offset = data_offset(&(*ptr).data);
604 set_data_ptr(fake_ptr, (ptr as *mut u8).offset(offset))
608 /// Constructs an `Arc<T>` from a raw pointer.
610 /// The raw pointer must have been previously returned by a call to
611 /// [`Arc<U>::into_raw`][into_raw] where `U` must have the same size and
612 /// alignment as `T`. This is trivially true if `U` is `T`.
613 /// Note that if `U` is not `T` but has the same size and alignment, this is
614 /// basically like transmuting references of different types. See
615 /// [`mem::transmute`][transmute] for more information on what
616 /// restrictions apply in this case.
618 /// The user of `from_raw` has to make sure a specific value of `T` is only
621 /// This function is unsafe because improper use may lead to memory unsafety,
622 /// even if the returned `Arc<T>` is never accessed.
624 /// [into_raw]: struct.Arc.html#method.into_raw
625 /// [transmute]: ../../std/mem/fn.transmute.html
630 /// use std::sync::Arc;
632 /// let x = Arc::new("hello".to_owned());
633 /// let x_ptr = Arc::into_raw(x);
636 /// // Convert back to an `Arc` to prevent leak.
637 /// let x = Arc::from_raw(x_ptr);
638 /// assert_eq!(&*x, "hello");
640 /// // Further calls to `Arc::from_raw(x_ptr)` would be memory-unsafe.
643 /// // The memory was freed when `x` went out of scope above, so `x_ptr` is now dangling!
645 #[stable(feature = "rc_raw", since = "1.17.0")]
646 pub unsafe fn from_raw(ptr: *const T) -> Self {
647 let offset = data_offset(ptr);
649 // Reverse the offset to find the original ArcInner.
650 let fake_ptr = ptr as *mut ArcInner<T>;
651 let arc_ptr = set_data_ptr(fake_ptr, (ptr as *mut u8).offset(-offset));
653 Self::from_ptr(arc_ptr)
656 /// Consumes the `Arc`, returning the wrapped pointer as `NonNull<T>`.
661 /// #![feature(rc_into_raw_non_null)]
663 /// use std::sync::Arc;
665 /// let x = Arc::new("hello".to_owned());
666 /// let ptr = Arc::into_raw_non_null(x);
667 /// let deref = unsafe { ptr.as_ref() };
668 /// assert_eq!(deref, "hello");
670 #[unstable(feature = "rc_into_raw_non_null", issue = "47336")]
672 pub fn into_raw_non_null(this: Self) -> NonNull<T> {
673 // safe because Arc guarantees its pointer is non-null
674 unsafe { NonNull::new_unchecked(Arc::into_raw(this) as *mut _) }
677 /// Creates a new [`Weak`][weak] pointer to this allocation.
679 /// [weak]: struct.Weak.html
684 /// use std::sync::Arc;
686 /// let five = Arc::new(5);
688 /// let weak_five = Arc::downgrade(&five);
690 #[stable(feature = "arc_weak", since = "1.4.0")]
691 pub fn downgrade(this: &Self) -> Weak<T> {
692 // This Relaxed is OK because we're checking the value in the CAS
694 let mut cur = this.inner().weak.load(Relaxed);
697 // check if the weak counter is currently "locked"; if so, spin.
698 if cur == usize::MAX {
699 cur = this.inner().weak.load(Relaxed);
703 // NOTE: this code currently ignores the possibility of overflow
704 // into usize::MAX; in general both Rc and Arc need to be adjusted
705 // to deal with overflow.
707 // Unlike with Clone(), we need this to be an Acquire read to
708 // synchronize with the write coming from `is_unique`, so that the
709 // events prior to that write happen before this read.
710 match this.inner().weak.compare_exchange_weak(cur, cur + 1, Acquire, Relaxed) {
712 // Make sure we do not create a dangling Weak
713 debug_assert!(!is_dangling(this.ptr));
714 return Weak { ptr: this.ptr };
716 Err(old) => cur = old,
721 /// Gets the number of [`Weak`][weak] pointers to this allocation.
723 /// [weak]: struct.Weak.html
727 /// This method by itself is safe, but using it correctly requires extra care.
728 /// Another thread can change the weak count at any time,
729 /// including potentially between calling this method and acting on the result.
734 /// use std::sync::Arc;
736 /// let five = Arc::new(5);
737 /// let _weak_five = Arc::downgrade(&five);
739 /// // This assertion is deterministic because we haven't shared
740 /// // the `Arc` or `Weak` between threads.
741 /// assert_eq!(1, Arc::weak_count(&five));
744 #[stable(feature = "arc_counts", since = "1.15.0")]
745 pub fn weak_count(this: &Self) -> usize {
746 let cnt = this.inner().weak.load(SeqCst);
747 // If the weak count is currently locked, the value of the
748 // count was 0 just before taking the lock.
749 if cnt == usize::MAX { 0 } else { cnt - 1 }
752 /// Gets the number of strong (`Arc`) pointers to this allocation.
756 /// This method by itself is safe, but using it correctly requires extra care.
757 /// Another thread can change the strong count at any time,
758 /// including potentially between calling this method and acting on the result.
763 /// use std::sync::Arc;
765 /// let five = Arc::new(5);
766 /// let _also_five = Arc::clone(&five);
768 /// // This assertion is deterministic because we haven't shared
769 /// // the `Arc` between threads.
770 /// assert_eq!(2, Arc::strong_count(&five));
773 #[stable(feature = "arc_counts", since = "1.15.0")]
774 pub fn strong_count(this: &Self) -> usize {
775 this.inner().strong.load(SeqCst)
779 fn inner(&self) -> &ArcInner<T> {
780 // This unsafety is ok because while this arc is alive we're guaranteed
781 // that the inner pointer is valid. Furthermore, we know that the
782 // `ArcInner` structure itself is `Sync` because the inner data is
783 // `Sync` as well, so we're ok loaning out an immutable pointer to these
785 unsafe { self.ptr.as_ref() }
788 // Non-inlined part of `drop`.
790 unsafe fn drop_slow(&mut self) {
791 // Destroy the data at this time, even though we may not free the box
792 // allocation itself (there may still be weak pointers lying around).
793 ptr::drop_in_place(&mut self.ptr.as_mut().data);
795 if self.inner().weak.fetch_sub(1, Release) == 1 {
796 acquire!(self.inner().weak);
797 Global.dealloc(self.ptr.cast(), Layout::for_value(self.ptr.as_ref()))
802 #[stable(feature = "ptr_eq", since = "1.17.0")]
803 /// Returns `true` if the two `Arc`s point to the same allocation
804 /// (in a vein similar to [`ptr::eq`]).
809 /// use std::sync::Arc;
811 /// let five = Arc::new(5);
812 /// let same_five = Arc::clone(&five);
813 /// let other_five = Arc::new(5);
815 /// assert!(Arc::ptr_eq(&five, &same_five));
816 /// assert!(!Arc::ptr_eq(&five, &other_five));
819 /// [`ptr::eq`]: ../../std/ptr/fn.eq.html
820 pub fn ptr_eq(this: &Self, other: &Self) -> bool {
821 this.ptr.as_ptr() == other.ptr.as_ptr()
825 impl<T: ?Sized> Arc<T> {
826 /// Allocates an `ArcInner<T>` with sufficient space for
827 /// a possibly-unsized inner value where the value has the layout provided.
829 /// The function `mem_to_arcinner` is called with the data pointer
830 /// and must return back a (potentially fat)-pointer for the `ArcInner<T>`.
831 unsafe fn allocate_for_layout(
832 value_layout: Layout,
833 mem_to_arcinner: impl FnOnce(*mut u8) -> *mut ArcInner<T>,
834 ) -> *mut ArcInner<T> {
835 // Calculate layout using the given value layout.
836 // Previously, layout was calculated on the expression
837 // `&*(ptr as *const ArcInner<T>)`, but this created a misaligned
838 // reference (see #54908).
839 let layout = Layout::new::<ArcInner<()>>().extend(value_layout).unwrap().0.pad_to_align();
842 .alloc(layout, AllocInit::Uninitialized)
843 .unwrap_or_else(|_| handle_alloc_error(layout));
845 // Initialize the ArcInner
846 let inner = mem_to_arcinner(mem.ptr.as_ptr());
847 debug_assert_eq!(Layout::for_value(&*inner), layout);
849 ptr::write(&mut (*inner).strong, atomic::AtomicUsize::new(1));
850 ptr::write(&mut (*inner).weak, atomic::AtomicUsize::new(1));
855 /// Allocates an `ArcInner<T>` with sufficient space for an unsized inner value.
856 unsafe fn allocate_for_ptr(ptr: *const T) -> *mut ArcInner<T> {
857 // Allocate for the `ArcInner<T>` using the given value.
858 Self::allocate_for_layout(Layout::for_value(&*ptr), |mem| {
859 set_data_ptr(ptr as *mut T, mem) as *mut ArcInner<T>
863 fn from_box(v: Box<T>) -> Arc<T> {
865 let box_unique = Box::into_unique(v);
866 let bptr = box_unique.as_ptr();
868 let value_size = size_of_val(&*bptr);
869 let ptr = Self::allocate_for_ptr(bptr);
871 // Copy value as bytes
872 ptr::copy_nonoverlapping(
873 bptr as *const T as *const u8,
874 &mut (*ptr).data as *mut _ as *mut u8,
878 // Free the allocation without dropping its contents
879 box_free(box_unique);
887 /// Allocates an `ArcInner<[T]>` with the given length.
888 unsafe fn allocate_for_slice(len: usize) -> *mut ArcInner<[T]> {
889 Self::allocate_for_layout(Layout::array::<T>(len).unwrap(), |mem| {
890 ptr::slice_from_raw_parts_mut(mem as *mut T, len) as *mut ArcInner<[T]>
895 /// Sets the data pointer of a `?Sized` raw pointer.
897 /// For a slice/trait object, this sets the `data` field and leaves the rest
898 /// unchanged. For a sized raw pointer, this simply sets the pointer.
899 unsafe fn set_data_ptr<T: ?Sized, U>(mut ptr: *mut T, data: *mut U) -> *mut T {
900 ptr::write(&mut ptr as *mut _ as *mut *mut u8, data as *mut u8);
905 /// Copy elements from slice into newly allocated Arc<\[T\]>
907 /// Unsafe because the caller must either take ownership or bind `T: Copy`.
908 unsafe fn copy_from_slice(v: &[T]) -> Arc<[T]> {
909 let ptr = Self::allocate_for_slice(v.len());
911 ptr::copy_nonoverlapping(v.as_ptr(), &mut (*ptr).data as *mut [T] as *mut T, v.len());
916 /// Constructs an `Arc<[T]>` from an iterator known to be of a certain size.
918 /// Behavior is undefined should the size be wrong.
919 unsafe fn from_iter_exact(iter: impl iter::Iterator<Item = T>, len: usize) -> Arc<[T]> {
920 // Panic guard while cloning T elements.
921 // In the event of a panic, elements that have been written
922 // into the new ArcInner will be dropped, then the memory freed.
930 impl<T> Drop for Guard<T> {
933 let slice = from_raw_parts_mut(self.elems, self.n_elems);
934 ptr::drop_in_place(slice);
936 Global.dealloc(self.mem.cast(), self.layout);
941 let ptr = Self::allocate_for_slice(len);
943 let mem = ptr as *mut _ as *mut u8;
944 let layout = Layout::for_value(&*ptr);
946 // Pointer to first element
947 let elems = &mut (*ptr).data as *mut [T] as *mut T;
949 let mut guard = Guard { mem: NonNull::new_unchecked(mem), elems, layout, n_elems: 0 };
951 for (i, item) in iter.enumerate() {
952 ptr::write(elems.add(i), item);
956 // All clear. Forget the guard so it doesn't free the new ArcInner.
963 /// Specialization trait used for `From<&[T]>`.
964 trait ArcFromSlice<T> {
965 fn from_slice(slice: &[T]) -> Self;
968 impl<T: Clone> ArcFromSlice<T> for Arc<[T]> {
970 default fn from_slice(v: &[T]) -> Self {
971 unsafe { Self::from_iter_exact(v.iter().cloned(), v.len()) }
975 impl<T: Copy> ArcFromSlice<T> for Arc<[T]> {
977 fn from_slice(v: &[T]) -> Self {
978 unsafe { Arc::copy_from_slice(v) }
982 #[stable(feature = "rust1", since = "1.0.0")]
983 impl<T: ?Sized> Clone for Arc<T> {
984 /// Makes a clone of the `Arc` pointer.
986 /// This creates another pointer to the same allocation, increasing the
987 /// strong reference count.
992 /// use std::sync::Arc;
994 /// let five = Arc::new(5);
996 /// let _ = Arc::clone(&five);
999 fn clone(&self) -> Arc<T> {
1000 // Using a relaxed ordering is alright here, as knowledge of the
1001 // original reference prevents other threads from erroneously deleting
1004 // As explained in the [Boost documentation][1], Increasing the
1005 // reference counter can always be done with memory_order_relaxed: New
1006 // references to an object can only be formed from an existing
1007 // reference, and passing an existing reference from one thread to
1008 // another must already provide any required synchronization.
1010 // [1]: (www.boost.org/doc/libs/1_55_0/doc/html/atomic/usage_examples.html)
1011 let old_size = self.inner().strong.fetch_add(1, Relaxed);
1013 // However we need to guard against massive refcounts in case someone
1014 // is `mem::forget`ing Arcs. If we don't do this the count can overflow
1015 // and users will use-after free. We racily saturate to `isize::MAX` on
1016 // the assumption that there aren't ~2 billion threads incrementing
1017 // the reference count at once. This branch will never be taken in
1018 // any realistic program.
1020 // We abort because such a program is incredibly degenerate, and we
1021 // don't care to support it.
1022 if old_size > MAX_REFCOUNT {
1028 Self::from_inner(self.ptr)
1032 #[stable(feature = "rust1", since = "1.0.0")]
1033 impl<T: ?Sized> Deref for Arc<T> {
1037 fn deref(&self) -> &T {
1042 #[unstable(feature = "receiver_trait", issue = "none")]
1043 impl<T: ?Sized> Receiver for Arc<T> {}
1045 impl<T: Clone> Arc<T> {
1046 /// Makes a mutable reference into the given `Arc`.
1048 /// If there are other `Arc` or [`Weak`][weak] pointers to the same allocation,
1049 /// then `make_mut` will create a new allocation and invoke [`clone`][clone] on the inner value
1050 /// to ensure unique ownership. This is also referred to as clone-on-write.
1052 /// Note that this differs from the behavior of [`Rc::make_mut`] which disassociates
1053 /// any remaining `Weak` pointers.
1055 /// See also [`get_mut`][get_mut], which will fail rather than cloning.
1057 /// [weak]: struct.Weak.html
1058 /// [clone]: ../../std/clone/trait.Clone.html#tymethod.clone
1059 /// [get_mut]: struct.Arc.html#method.get_mut
1060 /// [`Rc::make_mut`]: ../rc/struct.Rc.html#method.make_mut
1065 /// use std::sync::Arc;
1067 /// let mut data = Arc::new(5);
1069 /// *Arc::make_mut(&mut data) += 1; // Won't clone anything
1070 /// let mut other_data = Arc::clone(&data); // Won't clone inner data
1071 /// *Arc::make_mut(&mut data) += 1; // Clones inner data
1072 /// *Arc::make_mut(&mut data) += 1; // Won't clone anything
1073 /// *Arc::make_mut(&mut other_data) *= 2; // Won't clone anything
1075 /// // Now `data` and `other_data` point to different allocations.
1076 /// assert_eq!(*data, 8);
1077 /// assert_eq!(*other_data, 12);
1080 #[stable(feature = "arc_unique", since = "1.4.0")]
1081 pub fn make_mut(this: &mut Self) -> &mut T {
1082 // Note that we hold both a strong reference and a weak reference.
1083 // Thus, releasing our strong reference only will not, by itself, cause
1084 // the memory to be deallocated.
1086 // Use Acquire to ensure that we see any writes to `weak` that happen
1087 // before release writes (i.e., decrements) to `strong`. Since we hold a
1088 // weak count, there's no chance the ArcInner itself could be
1090 if this.inner().strong.compare_exchange(1, 0, Acquire, Relaxed).is_err() {
1091 // Another strong pointer exists; clone
1092 *this = Arc::new((**this).clone());
1093 } else if this.inner().weak.load(Relaxed) != 1 {
1094 // Relaxed suffices in the above because this is fundamentally an
1095 // optimization: we are always racing with weak pointers being
1096 // dropped. Worst case, we end up allocated a new Arc unnecessarily.
1098 // We removed the last strong ref, but there are additional weak
1099 // refs remaining. We'll move the contents to a new Arc, and
1100 // invalidate the other weak refs.
1102 // Note that it is not possible for the read of `weak` to yield
1103 // usize::MAX (i.e., locked), since the weak count can only be
1104 // locked by a thread with a strong reference.
1106 // Materialize our own implicit weak pointer, so that it can clean
1107 // up the ArcInner as needed.
1108 let weak = Weak { ptr: this.ptr };
1110 // mark the data itself as already deallocated
1112 // there is no data race in the implicit write caused by `read`
1113 // here (due to zeroing) because data is no longer accessed by
1114 // other threads (due to there being no more strong refs at this
1116 let mut swap = Arc::new(ptr::read(&weak.ptr.as_ref().data));
1117 mem::swap(this, &mut swap);
1121 // We were the sole reference of either kind; bump back up the
1122 // strong ref count.
1123 this.inner().strong.store(1, Release);
1126 // As with `get_mut()`, the unsafety is ok because our reference was
1127 // either unique to begin with, or became one upon cloning the contents.
1128 unsafe { &mut this.ptr.as_mut().data }
1132 impl<T: ?Sized> Arc<T> {
1133 /// Returns a mutable reference into the given `Arc`, if there are
1134 /// no other `Arc` or [`Weak`][weak] pointers to the same allocation.
1136 /// Returns [`None`][option] otherwise, because it is not safe to
1137 /// mutate a shared value.
1139 /// See also [`make_mut`][make_mut], which will [`clone`][clone]
1140 /// the inner value when there are other pointers.
1142 /// [weak]: struct.Weak.html
1143 /// [option]: ../../std/option/enum.Option.html
1144 /// [make_mut]: struct.Arc.html#method.make_mut
1145 /// [clone]: ../../std/clone/trait.Clone.html#tymethod.clone
1150 /// use std::sync::Arc;
1152 /// let mut x = Arc::new(3);
1153 /// *Arc::get_mut(&mut x).unwrap() = 4;
1154 /// assert_eq!(*x, 4);
1156 /// let _y = Arc::clone(&x);
1157 /// assert!(Arc::get_mut(&mut x).is_none());
1160 #[stable(feature = "arc_unique", since = "1.4.0")]
1161 pub fn get_mut(this: &mut Self) -> Option<&mut T> {
1162 if this.is_unique() {
1163 // This unsafety is ok because we're guaranteed that the pointer
1164 // returned is the *only* pointer that will ever be returned to T. Our
1165 // reference count is guaranteed to be 1 at this point, and we required
1166 // the Arc itself to be `mut`, so we're returning the only possible
1167 // reference to the inner data.
1168 unsafe { Some(Arc::get_mut_unchecked(this)) }
1174 /// Returns a mutable reference into the given `Arc`,
1175 /// without any check.
1177 /// See also [`get_mut`], which is safe and does appropriate checks.
1179 /// [`get_mut`]: struct.Arc.html#method.get_mut
1183 /// Any other `Arc` or [`Weak`] pointers to the same allocation must not be dereferenced
1184 /// for the duration of the returned borrow.
1185 /// This is trivially the case if no such pointers exist,
1186 /// for example immediately after `Arc::new`.
1191 /// #![feature(get_mut_unchecked)]
1193 /// use std::sync::Arc;
1195 /// let mut x = Arc::new(String::new());
1197 /// Arc::get_mut_unchecked(&mut x).push_str("foo")
1199 /// assert_eq!(*x, "foo");
1202 #[unstable(feature = "get_mut_unchecked", issue = "63292")]
1203 pub unsafe fn get_mut_unchecked(this: &mut Self) -> &mut T {
1204 &mut this.ptr.as_mut().data
1207 /// Determine whether this is the unique reference (including weak refs) to
1208 /// the underlying data.
1210 /// Note that this requires locking the weak ref count.
1211 fn is_unique(&mut self) -> bool {
1212 // lock the weak pointer count if we appear to be the sole weak pointer
1215 // The acquire label here ensures a happens-before relationship with any
1216 // writes to `strong` (in particular in `Weak::upgrade`) prior to decrements
1217 // of the `weak` count (via `Weak::drop`, which uses release). If the upgraded
1218 // weak ref was never dropped, the CAS here will fail so we do not care to synchronize.
1219 if self.inner().weak.compare_exchange(1, usize::MAX, Acquire, Relaxed).is_ok() {
1220 // This needs to be an `Acquire` to synchronize with the decrement of the `strong`
1221 // counter in `drop` -- the only access that happens when any but the last reference
1222 // is being dropped.
1223 let unique = self.inner().strong.load(Acquire) == 1;
1225 // The release write here synchronizes with a read in `downgrade`,
1226 // effectively preventing the above read of `strong` from happening
1228 self.inner().weak.store(1, Release); // release the lock
1236 #[stable(feature = "rust1", since = "1.0.0")]
1237 unsafe impl<#[may_dangle] T: ?Sized> Drop for Arc<T> {
1238 /// Drops the `Arc`.
1240 /// This will decrement the strong reference count. If the strong reference
1241 /// count reaches zero then the only other references (if any) are
1242 /// [`Weak`], so we `drop` the inner value.
1247 /// use std::sync::Arc;
1251 /// impl Drop for Foo {
1252 /// fn drop(&mut self) {
1253 /// println!("dropped!");
1257 /// let foo = Arc::new(Foo);
1258 /// let foo2 = Arc::clone(&foo);
1260 /// drop(foo); // Doesn't print anything
1261 /// drop(foo2); // Prints "dropped!"
1264 /// [`Weak`]: ../../std/sync/struct.Weak.html
1266 fn drop(&mut self) {
1267 // Because `fetch_sub` is already atomic, we do not need to synchronize
1268 // with other threads unless we are going to delete the object. This
1269 // same logic applies to the below `fetch_sub` to the `weak` count.
1270 if self.inner().strong.fetch_sub(1, Release) != 1 {
1274 // This fence is needed to prevent reordering of use of the data and
1275 // deletion of the data. Because it is marked `Release`, the decreasing
1276 // of the reference count synchronizes with this `Acquire` fence. This
1277 // means that use of the data happens before decreasing the reference
1278 // count, which happens before this fence, which happens before the
1279 // deletion of the data.
1281 // As explained in the [Boost documentation][1],
1283 // > It is important to enforce any possible access to the object in one
1284 // > thread (through an existing reference) to *happen before* deleting
1285 // > the object in a different thread. This is achieved by a "release"
1286 // > operation after dropping a reference (any access to the object
1287 // > through this reference must obviously happened before), and an
1288 // > "acquire" operation before deleting the object.
1290 // In particular, while the contents of an Arc are usually immutable, it's
1291 // possible to have interior writes to something like a Mutex<T>. Since a
1292 // Mutex is not acquired when it is deleted, we can't rely on its
1293 // synchronization logic to make writes in thread A visible to a destructor
1294 // running in thread B.
1296 // Also note that the Acquire fence here could probably be replaced with an
1297 // Acquire load, which could improve performance in highly-contended
1298 // situations. See [2].
1300 // [1]: (www.boost.org/doc/libs/1_55_0/doc/html/atomic/usage_examples.html)
1301 // [2]: (https://github.com/rust-lang/rust/pull/41714)
1302 acquire!(self.inner().strong);
1310 impl Arc<dyn Any + Send + Sync> {
1312 #[stable(feature = "rc_downcast", since = "1.29.0")]
1313 /// Attempt to downcast the `Arc<dyn Any + Send + Sync>` to a concrete type.
1318 /// use std::any::Any;
1319 /// use std::sync::Arc;
1321 /// fn print_if_string(value: Arc<dyn Any + Send + Sync>) {
1322 /// if let Ok(string) = value.downcast::<String>() {
1323 /// println!("String ({}): {}", string.len(), string);
1327 /// let my_string = "Hello World".to_string();
1328 /// print_if_string(Arc::new(my_string));
1329 /// print_if_string(Arc::new(0i8));
1331 pub fn downcast<T>(self) -> Result<Arc<T>, Self>
1333 T: Any + Send + Sync + 'static,
1335 if (*self).is::<T>() {
1336 let ptr = self.ptr.cast::<ArcInner<T>>();
1338 Ok(Arc::from_inner(ptr))
1346 /// Constructs a new `Weak<T>`, without allocating any memory.
1347 /// Calling [`upgrade`] on the return value always gives [`None`].
1349 /// [`upgrade`]: struct.Weak.html#method.upgrade
1350 /// [`None`]: ../../std/option/enum.Option.html#variant.None
1355 /// use std::sync::Weak;
1357 /// let empty: Weak<i64> = Weak::new();
1358 /// assert!(empty.upgrade().is_none());
1360 #[stable(feature = "downgraded_weak", since = "1.10.0")]
1361 pub fn new() -> Weak<T> {
1362 Weak { ptr: NonNull::new(usize::MAX as *mut ArcInner<T>).expect("MAX is not 0") }
1365 /// Returns a raw pointer to the object `T` pointed to by this `Weak<T>`.
1367 /// The pointer is valid only if there are some strong references. The pointer may be dangling,
1368 /// unaligned or even [`null`] otherwise.
1373 /// #![feature(weak_into_raw)]
1375 /// use std::sync::Arc;
1378 /// let strong = Arc::new("hello".to_owned());
1379 /// let weak = Arc::downgrade(&strong);
1380 /// // Both point to the same object
1381 /// assert!(ptr::eq(&*strong, weak.as_ptr()));
1382 /// // The strong here keeps it alive, so we can still access the object.
1383 /// assert_eq!("hello", unsafe { &*weak.as_ptr() });
1386 /// // But not any more. We can do weak.as_ptr(), but accessing the pointer would lead to
1387 /// // undefined behaviour.
1388 /// // assert_eq!("hello", unsafe { &*weak.as_ptr() });
1391 /// [`null`]: ../../std/ptr/fn.null.html
1392 #[unstable(feature = "weak_into_raw", issue = "60728")]
1393 pub fn as_ptr(&self) -> *const T {
1394 let offset = data_offset_sized::<T>();
1395 let ptr = self.ptr.cast::<u8>().as_ptr().wrapping_offset(offset);
1399 /// Consumes the `Weak<T>` and turns it into a raw pointer.
1401 /// This converts the weak pointer into a raw pointer, preserving the original weak count. It
1402 /// can be turned back into the `Weak<T>` with [`from_raw`].
1404 /// The same restrictions of accessing the target of the pointer as with
1405 /// [`as_ptr`] apply.
1410 /// #![feature(weak_into_raw)]
1412 /// use std::sync::{Arc, Weak};
1414 /// let strong = Arc::new("hello".to_owned());
1415 /// let weak = Arc::downgrade(&strong);
1416 /// let raw = weak.into_raw();
1418 /// assert_eq!(1, Arc::weak_count(&strong));
1419 /// assert_eq!("hello", unsafe { &*raw });
1421 /// drop(unsafe { Weak::from_raw(raw) });
1422 /// assert_eq!(0, Arc::weak_count(&strong));
1425 /// [`from_raw`]: struct.Weak.html#method.from_raw
1426 /// [`as_ptr`]: struct.Weak.html#method.as_ptr
1427 #[unstable(feature = "weak_into_raw", issue = "60728")]
1428 pub fn into_raw(self) -> *const T {
1429 let result = self.as_ptr();
1434 /// Converts a raw pointer previously created by [`into_raw`] back into
1437 /// This can be used to safely get a strong reference (by calling [`upgrade`]
1438 /// later) or to deallocate the weak count by dropping the `Weak<T>`.
1440 /// It takes ownership of one weak count (with the exception of pointers created by [`new`],
1441 /// as these don't have any corresponding weak count).
1445 /// The pointer must have originated from the [`into_raw`] and must still own its potential
1446 /// weak reference count.
1448 /// It is allowed for the strong count to be 0 at the time of calling this, but the weak count
1449 /// must be non-zero or the pointer must have originated from a dangling `Weak<T>` (one created
1455 /// #![feature(weak_into_raw)]
1457 /// use std::sync::{Arc, Weak};
1459 /// let strong = Arc::new("hello".to_owned());
1461 /// let raw_1 = Arc::downgrade(&strong).into_raw();
1462 /// let raw_2 = Arc::downgrade(&strong).into_raw();
1464 /// assert_eq!(2, Arc::weak_count(&strong));
1466 /// assert_eq!("hello", &*unsafe { Weak::from_raw(raw_1) }.upgrade().unwrap());
1467 /// assert_eq!(1, Arc::weak_count(&strong));
1471 /// // Decrement the last weak count.
1472 /// assert!(unsafe { Weak::from_raw(raw_2) }.upgrade().is_none());
1475 /// [`new`]: struct.Weak.html#method.new
1476 /// [`into_raw`]: struct.Weak.html#method.into_raw
1477 /// [`upgrade`]: struct.Weak.html#method.upgrade
1478 /// [`Weak`]: struct.Weak.html
1479 /// [`Arc`]: struct.Arc.html
1480 /// [`forget`]: ../../std/mem/fn.forget.html
1481 #[unstable(feature = "weak_into_raw", issue = "60728")]
1482 pub unsafe fn from_raw(ptr: *const T) -> Self {
1486 // See Arc::from_raw for details
1487 let offset = data_offset(ptr);
1488 let fake_ptr = ptr as *mut ArcInner<T>;
1489 let ptr = set_data_ptr(fake_ptr, (ptr as *mut u8).offset(-offset));
1490 Weak { ptr: NonNull::new(ptr).expect("Invalid pointer passed to from_raw") }
1495 impl<T: ?Sized> Weak<T> {
1496 /// Attempts to upgrade the `Weak` pointer to an [`Arc`], delaying
1497 /// dropping of the inner value if successful.
1499 /// Returns [`None`] if the inner value has since been dropped.
1501 /// [`Arc`]: struct.Arc.html
1502 /// [`None`]: ../../std/option/enum.Option.html#variant.None
1507 /// use std::sync::Arc;
1509 /// let five = Arc::new(5);
1511 /// let weak_five = Arc::downgrade(&five);
1513 /// let strong_five: Option<Arc<_>> = weak_five.upgrade();
1514 /// assert!(strong_five.is_some());
1516 /// // Destroy all strong pointers.
1517 /// drop(strong_five);
1520 /// assert!(weak_five.upgrade().is_none());
1522 #[stable(feature = "arc_weak", since = "1.4.0")]
1523 pub fn upgrade(&self) -> Option<Arc<T>> {
1524 // We use a CAS loop to increment the strong count instead of a
1525 // fetch_add because once the count hits 0 it must never be above 0.
1526 let inner = self.inner()?;
1528 // Relaxed load because any write of 0 that we can observe
1529 // leaves the field in a permanently zero state (so a
1530 // "stale" read of 0 is fine), and any other value is
1531 // confirmed via the CAS below.
1532 let mut n = inner.strong.load(Relaxed);
1539 // See comments in `Arc::clone` for why we do this (for `mem::forget`).
1540 if n > MAX_REFCOUNT {
1546 // Relaxed is valid for the same reason it is on Arc's Clone impl
1547 match inner.strong.compare_exchange_weak(n, n + 1, Relaxed, Relaxed) {
1548 Ok(_) => return Some(Arc::from_inner(self.ptr)), // null checked above
1549 Err(old) => n = old,
1554 /// Gets the number of strong (`Arc`) pointers pointing to this allocation.
1556 /// If `self` was created using [`Weak::new`], this will return 0.
1558 /// [`Weak::new`]: #method.new
1559 #[stable(feature = "weak_counts", since = "1.41.0")]
1560 pub fn strong_count(&self) -> usize {
1561 if let Some(inner) = self.inner() { inner.strong.load(SeqCst) } else { 0 }
1564 /// Gets an approximation of the number of `Weak` pointers pointing to this
1567 /// If `self` was created using [`Weak::new`], or if there are no remaining
1568 /// strong pointers, this will return 0.
1572 /// Due to implementation details, the returned value can be off by 1 in
1573 /// either direction when other threads are manipulating any `Arc`s or
1574 /// `Weak`s pointing to the same allocation.
1576 /// [`Weak::new`]: #method.new
1577 #[stable(feature = "weak_counts", since = "1.41.0")]
1578 pub fn weak_count(&self) -> usize {
1581 let weak = inner.weak.load(SeqCst);
1582 let strong = inner.strong.load(SeqCst);
1586 // Since we observed that there was at least one strong pointer
1587 // after reading the weak count, we know that the implicit weak
1588 // reference (present whenever any strong references are alive)
1589 // was still around when we observed the weak count, and can
1590 // therefore safely subtract it.
1597 /// Returns `None` when the pointer is dangling and there is no allocated `ArcInner`,
1598 /// (i.e., when this `Weak` was created by `Weak::new`).
1600 fn inner(&self) -> Option<&ArcInner<T>> {
1601 if is_dangling(self.ptr) { None } else { Some(unsafe { self.ptr.as_ref() }) }
1604 /// Returns `true` if the two `Weak`s point to the same allocation (similar to
1605 /// [`ptr::eq`]), or if both don't point to any allocation
1606 /// (because they were created with `Weak::new()`).
1610 /// Since this compares pointers it means that `Weak::new()` will equal each
1611 /// other, even though they don't point to any allocation.
1616 /// use std::sync::Arc;
1618 /// let first_rc = Arc::new(5);
1619 /// let first = Arc::downgrade(&first_rc);
1620 /// let second = Arc::downgrade(&first_rc);
1622 /// assert!(first.ptr_eq(&second));
1624 /// let third_rc = Arc::new(5);
1625 /// let third = Arc::downgrade(&third_rc);
1627 /// assert!(!first.ptr_eq(&third));
1630 /// Comparing `Weak::new`.
1633 /// use std::sync::{Arc, Weak};
1635 /// let first = Weak::new();
1636 /// let second = Weak::new();
1637 /// assert!(first.ptr_eq(&second));
1639 /// let third_rc = Arc::new(());
1640 /// let third = Arc::downgrade(&third_rc);
1641 /// assert!(!first.ptr_eq(&third));
1644 /// [`ptr::eq`]: ../../std/ptr/fn.eq.html
1646 #[stable(feature = "weak_ptr_eq", since = "1.39.0")]
1647 pub fn ptr_eq(&self, other: &Self) -> bool {
1648 self.ptr.as_ptr() == other.ptr.as_ptr()
1652 #[stable(feature = "arc_weak", since = "1.4.0")]
1653 impl<T: ?Sized> Clone for Weak<T> {
1654 /// Makes a clone of the `Weak` pointer that points to the same allocation.
1659 /// use std::sync::{Arc, Weak};
1661 /// let weak_five = Arc::downgrade(&Arc::new(5));
1663 /// let _ = Weak::clone(&weak_five);
1666 fn clone(&self) -> Weak<T> {
1667 let inner = if let Some(inner) = self.inner() {
1670 return Weak { ptr: self.ptr };
1672 // See comments in Arc::clone() for why this is relaxed. This can use a
1673 // fetch_add (ignoring the lock) because the weak count is only locked
1674 // where are *no other* weak pointers in existence. (So we can't be
1675 // running this code in that case).
1676 let old_size = inner.weak.fetch_add(1, Relaxed);
1678 // See comments in Arc::clone() for why we do this (for mem::forget).
1679 if old_size > MAX_REFCOUNT {
1685 Weak { ptr: self.ptr }
1689 #[stable(feature = "downgraded_weak", since = "1.10.0")]
1690 impl<T> Default for Weak<T> {
1691 /// Constructs a new `Weak<T>`, without allocating memory.
1692 /// Calling [`upgrade`] on the return value always
1695 /// [`None`]: ../../std/option/enum.Option.html#variant.None
1696 /// [`upgrade`]: ../../std/sync/struct.Weak.html#method.upgrade
1701 /// use std::sync::Weak;
1703 /// let empty: Weak<i64> = Default::default();
1704 /// assert!(empty.upgrade().is_none());
1706 fn default() -> Weak<T> {
1711 #[stable(feature = "arc_weak", since = "1.4.0")]
1712 impl<T: ?Sized> Drop for Weak<T> {
1713 /// Drops the `Weak` pointer.
1718 /// use std::sync::{Arc, Weak};
1722 /// impl Drop for Foo {
1723 /// fn drop(&mut self) {
1724 /// println!("dropped!");
1728 /// let foo = Arc::new(Foo);
1729 /// let weak_foo = Arc::downgrade(&foo);
1730 /// let other_weak_foo = Weak::clone(&weak_foo);
1732 /// drop(weak_foo); // Doesn't print anything
1733 /// drop(foo); // Prints "dropped!"
1735 /// assert!(other_weak_foo.upgrade().is_none());
1737 fn drop(&mut self) {
1738 // If we find out that we were the last weak pointer, then its time to
1739 // deallocate the data entirely. See the discussion in Arc::drop() about
1740 // the memory orderings
1742 // It's not necessary to check for the locked state here, because the
1743 // weak count can only be locked if there was precisely one weak ref,
1744 // meaning that drop could only subsequently run ON that remaining weak
1745 // ref, which can only happen after the lock is released.
1746 let inner = if let Some(inner) = self.inner() { inner } else { return };
1748 if inner.weak.fetch_sub(1, Release) == 1 {
1749 acquire!(inner.weak);
1750 unsafe { Global.dealloc(self.ptr.cast(), Layout::for_value(self.ptr.as_ref())) }
1755 #[stable(feature = "rust1", since = "1.0.0")]
1756 trait ArcEqIdent<T: ?Sized + PartialEq> {
1757 fn eq(&self, other: &Arc<T>) -> bool;
1758 fn ne(&self, other: &Arc<T>) -> bool;
1761 #[stable(feature = "rust1", since = "1.0.0")]
1762 impl<T: ?Sized + PartialEq> ArcEqIdent<T> for Arc<T> {
1764 default fn eq(&self, other: &Arc<T>) -> bool {
1768 default fn ne(&self, other: &Arc<T>) -> bool {
1773 /// We're doing this specialization here, and not as a more general optimization on `&T`, because it
1774 /// would otherwise add a cost to all equality checks on refs. We assume that `Arc`s are used to
1775 /// store large values, that are slow to clone, but also heavy to check for equality, causing this
1776 /// cost to pay off more easily. It's also more likely to have two `Arc` clones, that point to
1777 /// the same value, than two `&T`s.
1779 /// We can only do this when `T: Eq` as a `PartialEq` might be deliberately irreflexive.
1780 #[stable(feature = "rust1", since = "1.0.0")]
1781 impl<T: ?Sized + Eq> ArcEqIdent<T> for Arc<T> {
1783 fn eq(&self, other: &Arc<T>) -> bool {
1784 Arc::ptr_eq(self, other) || **self == **other
1788 fn ne(&self, other: &Arc<T>) -> bool {
1789 !Arc::ptr_eq(self, other) && **self != **other
1793 #[stable(feature = "rust1", since = "1.0.0")]
1794 impl<T: ?Sized + PartialEq> PartialEq for Arc<T> {
1795 /// Equality for two `Arc`s.
1797 /// Two `Arc`s are equal if their inner values are equal, even if they are
1798 /// stored in different allocation.
1800 /// If `T` also implements `Eq` (implying reflexivity of equality),
1801 /// two `Arc`s that point to the same allocation are always equal.
1806 /// use std::sync::Arc;
1808 /// let five = Arc::new(5);
1810 /// assert!(five == Arc::new(5));
1813 fn eq(&self, other: &Arc<T>) -> bool {
1814 ArcEqIdent::eq(self, other)
1817 /// Inequality for two `Arc`s.
1819 /// Two `Arc`s are unequal if their inner values are unequal.
1821 /// If `T` also implements `Eq` (implying reflexivity of equality),
1822 /// two `Arc`s that point to the same value are never unequal.
1827 /// use std::sync::Arc;
1829 /// let five = Arc::new(5);
1831 /// assert!(five != Arc::new(6));
1834 fn ne(&self, other: &Arc<T>) -> bool {
1835 ArcEqIdent::ne(self, other)
1839 #[stable(feature = "rust1", since = "1.0.0")]
1840 impl<T: ?Sized + PartialOrd> PartialOrd for Arc<T> {
1841 /// Partial comparison for two `Arc`s.
1843 /// The two are compared by calling `partial_cmp()` on their inner values.
1848 /// use std::sync::Arc;
1849 /// use std::cmp::Ordering;
1851 /// let five = Arc::new(5);
1853 /// assert_eq!(Some(Ordering::Less), five.partial_cmp(&Arc::new(6)));
1855 fn partial_cmp(&self, other: &Arc<T>) -> Option<Ordering> {
1856 (**self).partial_cmp(&**other)
1859 /// Less-than comparison for two `Arc`s.
1861 /// The two are compared by calling `<` on their inner values.
1866 /// use std::sync::Arc;
1868 /// let five = Arc::new(5);
1870 /// assert!(five < Arc::new(6));
1872 fn lt(&self, other: &Arc<T>) -> bool {
1873 *(*self) < *(*other)
1876 /// 'Less than or equal to' comparison for two `Arc`s.
1878 /// The two are compared by calling `<=` on their inner values.
1883 /// use std::sync::Arc;
1885 /// let five = Arc::new(5);
1887 /// assert!(five <= Arc::new(5));
1889 fn le(&self, other: &Arc<T>) -> bool {
1890 *(*self) <= *(*other)
1893 /// Greater-than comparison for two `Arc`s.
1895 /// The two are compared by calling `>` on their inner values.
1900 /// use std::sync::Arc;
1902 /// let five = Arc::new(5);
1904 /// assert!(five > Arc::new(4));
1906 fn gt(&self, other: &Arc<T>) -> bool {
1907 *(*self) > *(*other)
1910 /// 'Greater than or equal to' comparison for two `Arc`s.
1912 /// The two are compared by calling `>=` on their inner values.
1917 /// use std::sync::Arc;
1919 /// let five = Arc::new(5);
1921 /// assert!(five >= Arc::new(5));
1923 fn ge(&self, other: &Arc<T>) -> bool {
1924 *(*self) >= *(*other)
1927 #[stable(feature = "rust1", since = "1.0.0")]
1928 impl<T: ?Sized + Ord> Ord for Arc<T> {
1929 /// Comparison for two `Arc`s.
1931 /// The two are compared by calling `cmp()` on their inner values.
1936 /// use std::sync::Arc;
1937 /// use std::cmp::Ordering;
1939 /// let five = Arc::new(5);
1941 /// assert_eq!(Ordering::Less, five.cmp(&Arc::new(6)));
1943 fn cmp(&self, other: &Arc<T>) -> Ordering {
1944 (**self).cmp(&**other)
1947 #[stable(feature = "rust1", since = "1.0.0")]
1948 impl<T: ?Sized + Eq> Eq for Arc<T> {}
1950 #[stable(feature = "rust1", since = "1.0.0")]
1951 impl<T: ?Sized + fmt::Display> fmt::Display for Arc<T> {
1952 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
1953 fmt::Display::fmt(&**self, f)
1957 #[stable(feature = "rust1", since = "1.0.0")]
1958 impl<T: ?Sized + fmt::Debug> fmt::Debug for Arc<T> {
1959 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
1960 fmt::Debug::fmt(&**self, f)
1964 #[stable(feature = "rust1", since = "1.0.0")]
1965 impl<T: ?Sized> fmt::Pointer for Arc<T> {
1966 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
1967 fmt::Pointer::fmt(&(&**self as *const T), f)
1971 #[stable(feature = "rust1", since = "1.0.0")]
1972 impl<T: Default> Default for Arc<T> {
1973 /// Creates a new `Arc<T>`, with the `Default` value for `T`.
1978 /// use std::sync::Arc;
1980 /// let x: Arc<i32> = Default::default();
1981 /// assert_eq!(*x, 0);
1983 fn default() -> Arc<T> {
1984 Arc::new(Default::default())
1988 #[stable(feature = "rust1", since = "1.0.0")]
1989 impl<T: ?Sized + Hash> Hash for Arc<T> {
1990 fn hash<H: Hasher>(&self, state: &mut H) {
1991 (**self).hash(state)
1995 #[stable(feature = "from_for_ptrs", since = "1.6.0")]
1996 impl<T> From<T> for Arc<T> {
1997 fn from(t: T) -> Self {
2002 #[stable(feature = "shared_from_slice", since = "1.21.0")]
2003 impl<T: Clone> From<&[T]> for Arc<[T]> {
2005 fn from(v: &[T]) -> Arc<[T]> {
2006 <Self as ArcFromSlice<T>>::from_slice(v)
2010 #[stable(feature = "shared_from_slice", since = "1.21.0")]
2011 impl From<&str> for Arc<str> {
2013 fn from(v: &str) -> Arc<str> {
2014 let arc = Arc::<[u8]>::from(v.as_bytes());
2015 unsafe { Arc::from_raw(Arc::into_raw(arc) as *const str) }
2019 #[stable(feature = "shared_from_slice", since = "1.21.0")]
2020 impl From<String> for Arc<str> {
2022 fn from(v: String) -> Arc<str> {
2027 #[stable(feature = "shared_from_slice", since = "1.21.0")]
2028 impl<T: ?Sized> From<Box<T>> for Arc<T> {
2030 fn from(v: Box<T>) -> Arc<T> {
2035 #[stable(feature = "shared_from_slice", since = "1.21.0")]
2036 impl<T> From<Vec<T>> for Arc<[T]> {
2038 fn from(mut v: Vec<T>) -> Arc<[T]> {
2040 let arc = Arc::copy_from_slice(&v);
2042 // Allow the Vec to free its memory, but not destroy its contents
2050 #[stable(feature = "boxed_slice_try_from", since = "1.43.0")]
2051 impl<T, const N: usize> TryFrom<Arc<[T]>> for Arc<[T; N]>
2053 [T; N]: LengthAtMost32,
2055 type Error = Arc<[T]>;
2057 fn try_from(boxed_slice: Arc<[T]>) -> Result<Self, Self::Error> {
2058 if boxed_slice.len() == N {
2059 Ok(unsafe { Arc::from_raw(Arc::into_raw(boxed_slice) as *mut [T; N]) })
2066 #[stable(feature = "shared_from_iter", since = "1.37.0")]
2067 impl<T> iter::FromIterator<T> for Arc<[T]> {
2068 /// Takes each element in the `Iterator` and collects it into an `Arc<[T]>`.
2070 /// # Performance characteristics
2072 /// ## The general case
2074 /// In the general case, collecting into `Arc<[T]>` is done by first
2075 /// collecting into a `Vec<T>`. That is, when writing the following:
2078 /// # use std::sync::Arc;
2079 /// let evens: Arc<[u8]> = (0..10).filter(|&x| x % 2 == 0).collect();
2080 /// # assert_eq!(&*evens, &[0, 2, 4, 6, 8]);
2083 /// this behaves as if we wrote:
2086 /// # use std::sync::Arc;
2087 /// let evens: Arc<[u8]> = (0..10).filter(|&x| x % 2 == 0)
2088 /// .collect::<Vec<_>>() // The first set of allocations happens here.
2089 /// .into(); // A second allocation for `Arc<[T]>` happens here.
2090 /// # assert_eq!(&*evens, &[0, 2, 4, 6, 8]);
2093 /// This will allocate as many times as needed for constructing the `Vec<T>`
2094 /// and then it will allocate once for turning the `Vec<T>` into the `Arc<[T]>`.
2096 /// ## Iterators of known length
2098 /// When your `Iterator` implements `TrustedLen` and is of an exact size,
2099 /// a single allocation will be made for the `Arc<[T]>`. For example:
2102 /// # use std::sync::Arc;
2103 /// let evens: Arc<[u8]> = (0..10).collect(); // Just a single allocation happens here.
2104 /// # assert_eq!(&*evens, &*(0..10).collect::<Vec<_>>());
2106 fn from_iter<I: iter::IntoIterator<Item = T>>(iter: I) -> Self {
2107 ArcFromIter::from_iter(iter.into_iter())
2111 /// Specialization trait used for collecting into `Arc<[T]>`.
2112 trait ArcFromIter<T, I> {
2113 fn from_iter(iter: I) -> Self;
2116 impl<T, I: Iterator<Item = T>> ArcFromIter<T, I> for Arc<[T]> {
2117 default fn from_iter(iter: I) -> Self {
2118 iter.collect::<Vec<T>>().into()
2122 impl<T, I: iter::TrustedLen<Item = T>> ArcFromIter<T, I> for Arc<[T]> {
2123 default fn from_iter(iter: I) -> Self {
2124 // This is the case for a `TrustedLen` iterator.
2125 let (low, high) = iter.size_hint();
2126 if let Some(high) = high {
2130 "TrustedLen iterator's size hint is not exact: {:?}",
2135 // SAFETY: We need to ensure that the iterator has an exact length and we have.
2136 Arc::from_iter_exact(iter, low)
2139 // Fall back to normal implementation.
2140 iter.collect::<Vec<T>>().into()
2145 impl<'a, T: 'a + Clone> ArcFromIter<&'a T, slice::Iter<'a, T>> for Arc<[T]> {
2146 fn from_iter(iter: slice::Iter<'a, T>) -> Self {
2147 // Delegate to `impl<T: Clone> From<&[T]> for Arc<[T]>`.
2149 // In the case that `T: Copy`, we get to use `ptr::copy_nonoverlapping`
2150 // which is even more performant.
2152 // In the fall-back case we have `T: Clone`. This is still better
2153 // than the `TrustedLen` implementation as slices have a known length
2154 // and so we get to avoid calling `size_hint` and avoid the branching.
2155 iter.as_slice().into()
2159 #[stable(feature = "rust1", since = "1.0.0")]
2160 impl<T: ?Sized> borrow::Borrow<T> for Arc<T> {
2161 fn borrow(&self) -> &T {
2166 #[stable(since = "1.5.0", feature = "smart_ptr_as_ref")]
2167 impl<T: ?Sized> AsRef<T> for Arc<T> {
2168 fn as_ref(&self) -> &T {
2173 #[stable(feature = "pin", since = "1.33.0")]
2174 impl<T: ?Sized> Unpin for Arc<T> {}
2176 /// Computes the offset of the data field within `ArcInner`.
2177 unsafe fn data_offset<T: ?Sized>(ptr: *const T) -> isize {
2178 // Align the unsized value to the end of the `ArcInner`.
2179 // Because it is `?Sized`, it will always be the last field in memory.
2180 // Note: This is a detail of the current implementation of the compiler,
2181 // and is not a guaranteed language detail. Do not rely on it outside of std.
2182 data_offset_align(align_of_val(&*ptr))
2185 /// Computes the offset of the data field within `ArcInner`.
2187 /// Unlike [`data_offset`], this doesn't need the pointer, but it works only on `T: Sized`.
2188 fn data_offset_sized<T>() -> isize {
2189 data_offset_align(align_of::<T>())
2193 fn data_offset_align(align: usize) -> isize {
2194 let layout = Layout::new::<ArcInner<()>>();
2195 (layout.size() + layout.padding_needed_for(align)) as isize