1 #![stable(feature = "rust1", since = "1.0.0")]
3 //! Thread-safe reference-counting pointers.
5 //! See the [`Arc<T>`][Arc] documentation for more details.
9 use core::cmp::Ordering;
10 use core::convert::{From, TryFrom};
12 use core::hash::{Hash, Hasher};
14 use core::intrinsics::abort;
16 use core::marker::{PhantomData, Unpin, Unsize};
17 use core::mem::{self, align_of_val_raw, size_of_val};
18 use core::ops::{CoerceUnsized, Deref, DispatchFromDyn, Receiver};
20 use core::ptr::{self, NonNull};
21 use core::slice::from_raw_parts_mut;
22 use core::sync::atomic;
23 use core::sync::atomic::Ordering::{Acquire, Relaxed, Release, SeqCst};
26 box_free, handle_alloc_error, AllocError, Allocator, Global, Layout, WriteCloneIntoRaw,
28 use crate::borrow::{Cow, ToOwned};
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 #[cfg(not(sanitize = "thread"))]
44 macro_rules! acquire {
46 atomic::fence(Acquire)
50 // ThreadSanitizer does not support memory fences. To avoid false positive
51 // reports in Arc / Weak implementation use atomic loads for synchronization
53 #[cfg(sanitize = "thread")]
54 macro_rules! acquire {
60 /// A thread-safe reference-counting pointer. 'Arc' stands for 'Atomically
61 /// Reference Counted'.
63 /// The type `Arc<T>` provides shared ownership of a value of type `T`,
64 /// allocated in the heap. Invoking [`clone`][clone] on `Arc` produces
65 /// a new `Arc` instance, which points to the same allocation on the heap as the
66 /// source `Arc`, while increasing a reference count. When the last `Arc`
67 /// pointer to a given allocation is destroyed, the value stored in that allocation (often
68 /// referred to as "inner value") is also dropped.
70 /// Shared references in Rust disallow mutation by default, and `Arc` is no
71 /// exception: you cannot generally obtain a mutable reference to something
72 /// inside an `Arc`. If you need to mutate through an `Arc`, use
73 /// [`Mutex`][mutex], [`RwLock`][rwlock], or one of the [`Atomic`][atomic]
78 /// Unlike [`Rc<T>`], `Arc<T>` uses atomic operations for its reference
79 /// counting. This means that it is thread-safe. The disadvantage is that
80 /// atomic operations are more expensive than ordinary memory accesses. If you
81 /// are not sharing reference-counted allocations between threads, consider using
82 /// [`Rc<T>`] for lower overhead. [`Rc<T>`] is a safe default, because the
83 /// compiler will catch any attempt to send an [`Rc<T>`] between threads.
84 /// However, a library might choose `Arc<T>` in order to give library consumers
87 /// `Arc<T>` will implement [`Send`] and [`Sync`] as long as the `T` implements
88 /// [`Send`] and [`Sync`]. Why can't you put a non-thread-safe type `T` in an
89 /// `Arc<T>` to make it thread-safe? This may be a bit counter-intuitive at
90 /// first: after all, isn't the point of `Arc<T>` thread safety? The key is
91 /// this: `Arc<T>` makes it thread safe to have multiple ownership of the same
92 /// data, but it doesn't add thread safety to its data. Consider
93 /// `Arc<`[`RefCell<T>`]`>`. [`RefCell<T>`] isn't [`Sync`], and if `Arc<T>` was always
94 /// [`Send`], `Arc<`[`RefCell<T>`]`>` would be as well. But then we'd have a problem:
95 /// [`RefCell<T>`] is not thread safe; it keeps track of the borrowing count using
96 /// non-atomic operations.
98 /// In the end, this means that you may need to pair `Arc<T>` with some sort of
99 /// [`std::sync`] type, usually [`Mutex<T>`][mutex].
101 /// ## Breaking cycles with `Weak`
103 /// The [`downgrade`][downgrade] method can be used to create a non-owning
104 /// [`Weak`] pointer. A [`Weak`] pointer can be [`upgrade`][upgrade]d
105 /// to an `Arc`, but this will return [`None`] if the value stored in the allocation has
106 /// already been dropped. In other words, `Weak` pointers do not keep the value
107 /// inside the allocation alive; however, they *do* keep the allocation
108 /// (the backing store for the value) alive.
110 /// A cycle between `Arc` pointers will never be deallocated. For this reason,
111 /// [`Weak`] is used to break cycles. For example, a tree could have
112 /// strong `Arc` pointers from parent nodes to children, and [`Weak`]
113 /// pointers from children back to their parents.
115 /// # Cloning references
117 /// Creating a new reference from an existing reference-counted pointer is done using the
118 /// `Clone` trait implemented for [`Arc<T>`][Arc] and [`Weak<T>`][Weak].
121 /// use std::sync::Arc;
122 /// let foo = Arc::new(vec![1.0, 2.0, 3.0]);
123 /// // The two syntaxes below are equivalent.
124 /// let a = foo.clone();
125 /// let b = Arc::clone(&foo);
126 /// // a, b, and foo are all Arcs that point to the same memory location
129 /// ## `Deref` behavior
131 /// `Arc<T>` automatically dereferences to `T` (via the [`Deref`][deref] trait),
132 /// so you can call `T`'s methods on a value of type `Arc<T>`. To avoid name
133 /// clashes with `T`'s methods, the methods of `Arc<T>` itself are associated
134 /// functions, called using [fully qualified syntax]:
137 /// use std::sync::Arc;
139 /// let my_arc = Arc::new(());
140 /// Arc::downgrade(&my_arc);
143 /// `Arc<T>`'s implementations of traits like `Clone` may also be called using
144 /// fully qualified syntax. Some people prefer to use fully qualified syntax,
145 /// while others prefer using method-call syntax.
148 /// use std::sync::Arc;
150 /// let arc = Arc::new(());
151 /// // Method-call syntax
152 /// let arc2 = arc.clone();
153 /// // Fully qualified syntax
154 /// let arc3 = Arc::clone(&arc);
157 /// [`Weak<T>`][Weak] does not auto-dereference to `T`, because the inner value may have
158 /// already been dropped.
160 /// [`Rc<T>`]: crate::rc::Rc
161 /// [clone]: Clone::clone
162 /// [mutex]: ../../std/sync/struct.Mutex.html
163 /// [rwlock]: ../../std/sync/struct.RwLock.html
164 /// [atomic]: core::sync::atomic
165 /// [`Send`]: core::marker::Send
166 /// [`Sync`]: core::marker::Sync
167 /// [deref]: core::ops::Deref
168 /// [downgrade]: Arc::downgrade
169 /// [upgrade]: Weak::upgrade
170 /// [`RefCell<T>`]: core::cell::RefCell
171 /// [`std::sync`]: ../../std/sync/index.html
172 /// [`Arc::clone(&from)`]: Arc::clone
173 /// [fully qualified syntax]: https://doc.rust-lang.org/book/ch19-03-advanced-traits.html#fully-qualified-syntax-for-disambiguation-calling-methods-with-the-same-name
177 /// Sharing some immutable data between threads:
179 // Note that we **do not** run these tests here. The windows builders get super
180 // unhappy if a thread outlives the main thread and then exits at the same time
181 // (something deadlocks) so we just avoid this entirely by not running these
184 /// use std::sync::Arc;
187 /// let five = Arc::new(5);
190 /// let five = Arc::clone(&five);
192 /// thread::spawn(move || {
193 /// println!("{:?}", five);
198 /// Sharing a mutable [`AtomicUsize`]:
200 /// [`AtomicUsize`]: core::sync::atomic::AtomicUsize
203 /// use std::sync::Arc;
204 /// use std::sync::atomic::{AtomicUsize, Ordering};
207 /// let val = Arc::new(AtomicUsize::new(5));
210 /// let val = Arc::clone(&val);
212 /// thread::spawn(move || {
213 /// let v = val.fetch_add(1, Ordering::SeqCst);
214 /// println!("{:?}", v);
219 /// See the [`rc` documentation][rc_examples] for more examples of reference
220 /// counting in general.
222 /// [rc_examples]: crate::rc#examples
223 #[cfg_attr(not(test), rustc_diagnostic_item = "Arc")]
224 #[stable(feature = "rust1", since = "1.0.0")]
225 pub struct Arc<T: ?Sized> {
226 ptr: NonNull<ArcInner<T>>,
227 phantom: PhantomData<ArcInner<T>>,
230 #[stable(feature = "rust1", since = "1.0.0")]
231 unsafe impl<T: ?Sized + Sync + Send> Send for Arc<T> {}
232 #[stable(feature = "rust1", since = "1.0.0")]
233 unsafe impl<T: ?Sized + Sync + Send> Sync for Arc<T> {}
235 #[unstable(feature = "coerce_unsized", issue = "27732")]
236 impl<T: ?Sized + Unsize<U>, U: ?Sized> CoerceUnsized<Arc<U>> for Arc<T> {}
238 #[unstable(feature = "dispatch_from_dyn", issue = "none")]
239 impl<T: ?Sized + Unsize<U>, U: ?Sized> DispatchFromDyn<Arc<U>> for Arc<T> {}
241 impl<T: ?Sized> Arc<T> {
242 fn from_inner(ptr: NonNull<ArcInner<T>>) -> Self {
243 Self { ptr, phantom: PhantomData }
246 unsafe fn from_ptr(ptr: *mut ArcInner<T>) -> Self {
247 unsafe { Self::from_inner(NonNull::new_unchecked(ptr)) }
251 /// `Weak` is a version of [`Arc`] that holds a non-owning reference to the
252 /// managed allocation. The allocation is accessed by calling [`upgrade`] on the `Weak`
253 /// pointer, which returns an [`Option`]`<`[`Arc`]`<T>>`.
255 /// Since a `Weak` reference does not count towards ownership, it will not
256 /// prevent the value stored in the allocation from being dropped, and `Weak` itself makes no
257 /// guarantees about the value still being present. Thus it may return [`None`]
258 /// when [`upgrade`]d. Note however that a `Weak` reference *does* prevent the allocation
259 /// itself (the backing store) from being deallocated.
261 /// A `Weak` pointer is useful for keeping a temporary reference to the allocation
262 /// managed by [`Arc`] without preventing its inner value from being dropped. It is also used to
263 /// prevent circular references between [`Arc`] pointers, since mutual owning references
264 /// would never allow either [`Arc`] to be dropped. For example, a tree could
265 /// have strong [`Arc`] pointers from parent nodes to children, and `Weak`
266 /// pointers from children back to their parents.
268 /// The typical way to obtain a `Weak` pointer is to call [`Arc::downgrade`].
270 /// [`upgrade`]: Weak::upgrade
271 #[stable(feature = "arc_weak", since = "1.4.0")]
272 pub struct Weak<T: ?Sized> {
273 // This is a `NonNull` to allow optimizing the size of this type in enums,
274 // but it is not necessarily a valid pointer.
275 // `Weak::new` sets this to `usize::MAX` so that it doesn’t need
276 // to allocate space on the heap. That's not a value a real pointer
277 // will ever have because RcBox has alignment at least 2.
278 // This is only possible when `T: Sized`; unsized `T` never dangle.
279 ptr: NonNull<ArcInner<T>>,
282 #[stable(feature = "arc_weak", since = "1.4.0")]
283 unsafe impl<T: ?Sized + Sync + Send> Send for Weak<T> {}
284 #[stable(feature = "arc_weak", since = "1.4.0")]
285 unsafe impl<T: ?Sized + Sync + Send> Sync for Weak<T> {}
287 #[unstable(feature = "coerce_unsized", issue = "27732")]
288 impl<T: ?Sized + Unsize<U>, U: ?Sized> CoerceUnsized<Weak<U>> for Weak<T> {}
289 #[unstable(feature = "dispatch_from_dyn", issue = "none")]
290 impl<T: ?Sized + Unsize<U>, U: ?Sized> DispatchFromDyn<Weak<U>> for Weak<T> {}
292 #[stable(feature = "arc_weak", since = "1.4.0")]
293 impl<T: ?Sized + fmt::Debug> fmt::Debug for Weak<T> {
294 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
299 // This is repr(C) to future-proof against possible field-reordering, which
300 // would interfere with otherwise safe [into|from]_raw() of transmutable
303 struct ArcInner<T: ?Sized> {
304 strong: atomic::AtomicUsize,
306 // the value usize::MAX acts as a sentinel for temporarily "locking" the
307 // ability to upgrade weak pointers or downgrade strong ones; this is used
308 // to avoid races in `make_mut` and `get_mut`.
309 weak: atomic::AtomicUsize,
314 unsafe impl<T: ?Sized + Sync + Send> Send for ArcInner<T> {}
315 unsafe impl<T: ?Sized + Sync + Send> Sync for ArcInner<T> {}
318 /// Constructs a new `Arc<T>`.
323 /// use std::sync::Arc;
325 /// let five = Arc::new(5);
328 #[stable(feature = "rust1", since = "1.0.0")]
329 pub fn new(data: T) -> Arc<T> {
330 // Start the weak pointer count as 1 which is the weak pointer that's
331 // held by all the strong pointers (kinda), see std/rc.rs for more info
332 let x: Box<_> = box ArcInner {
333 strong: atomic::AtomicUsize::new(1),
334 weak: atomic::AtomicUsize::new(1),
337 Self::from_inner(Box::leak(x).into())
340 /// Constructs a new `Arc<T>` using a weak reference to itself. Attempting
341 /// to upgrade the weak reference before this function returns will result
342 /// in a `None` value. However, the weak reference may be cloned freely and
343 /// stored for use at a later time.
347 /// #![feature(arc_new_cyclic)]
348 /// #![allow(dead_code)]
350 /// use std::sync::{Arc, Weak};
356 /// let foo = Arc::new_cyclic(|me| Foo {
361 #[unstable(feature = "arc_new_cyclic", issue = "75861")]
362 pub fn new_cyclic(data_fn: impl FnOnce(&Weak<T>) -> T) -> Arc<T> {
363 // Construct the inner in the "uninitialized" state with a single
365 let uninit_ptr: NonNull<_> = Box::leak(box ArcInner {
366 strong: atomic::AtomicUsize::new(0),
367 weak: atomic::AtomicUsize::new(1),
368 data: mem::MaybeUninit::<T>::uninit(),
371 let init_ptr: NonNull<ArcInner<T>> = uninit_ptr.cast();
373 let weak = Weak { ptr: init_ptr };
375 // It's important we don't give up ownership of the weak pointer, or
376 // else the memory might be freed by the time `data_fn` returns. If
377 // we really wanted to pass ownership, we could create an additional
378 // weak pointer for ourselves, but this would result in additional
379 // updates to the weak reference count which might not be necessary
381 let data = data_fn(&weak);
383 // Now we can properly initialize the inner value and turn our weak
384 // reference into a strong reference.
386 let inner = init_ptr.as_ptr();
387 ptr::write(ptr::addr_of_mut!((*inner).data), data);
389 // The above write to the data field must be visible to any threads which
390 // observe a non-zero strong count. Therefore we need at least "Release" ordering
391 // in order to synchronize with the `compare_exchange_weak` in `Weak::upgrade`.
393 // "Acquire" ordering is not required. When considering the possible behaviours
394 // of `data_fn` we only need to look at what it could do with a reference to a
395 // non-upgradeable `Weak`:
396 // - It can *clone* the `Weak`, increasing the weak reference count.
397 // - It can drop those clones, decreasing the weak reference count (but never to zero).
399 // These side effects do not impact us in any way, and no other side effects are
400 // possible with safe code alone.
401 let prev_value = (*inner).strong.fetch_add(1, Release);
402 debug_assert_eq!(prev_value, 0, "No prior strong references should exist");
405 let strong = Arc::from_inner(init_ptr);
407 // Strong references should collectively own a shared weak reference,
408 // so don't run the destructor for our old weak reference.
413 /// Constructs a new `Arc` with uninitialized contents.
418 /// #![feature(new_uninit)]
419 /// #![feature(get_mut_unchecked)]
421 /// use std::sync::Arc;
423 /// let mut five = Arc::<u32>::new_uninit();
425 /// let five = unsafe {
426 /// // Deferred initialization:
427 /// Arc::get_mut_unchecked(&mut five).as_mut_ptr().write(5);
429 /// five.assume_init()
432 /// assert_eq!(*five, 5)
434 #[unstable(feature = "new_uninit", issue = "63291")]
435 pub fn new_uninit() -> Arc<mem::MaybeUninit<T>> {
437 Arc::from_ptr(Arc::allocate_for_layout(
439 |layout| Global.allocate(layout),
440 |mem| mem as *mut ArcInner<mem::MaybeUninit<T>>,
445 /// Constructs a new `Arc` with uninitialized contents, with the memory
446 /// being filled with `0` bytes.
448 /// See [`MaybeUninit::zeroed`][zeroed] for examples of correct and incorrect usage
454 /// #![feature(new_uninit)]
456 /// use std::sync::Arc;
458 /// let zero = Arc::<u32>::new_zeroed();
459 /// let zero = unsafe { zero.assume_init() };
461 /// assert_eq!(*zero, 0)
464 /// [zeroed]: ../../std/mem/union.MaybeUninit.html#method.zeroed
465 #[unstable(feature = "new_uninit", issue = "63291")]
466 pub fn new_zeroed() -> Arc<mem::MaybeUninit<T>> {
468 Arc::from_ptr(Arc::allocate_for_layout(
470 |layout| Global.allocate_zeroed(layout),
471 |mem| mem as *mut ArcInner<mem::MaybeUninit<T>>,
476 /// Constructs a new `Pin<Arc<T>>`. If `T` does not implement `Unpin`, then
477 /// `data` will be pinned in memory and unable to be moved.
478 #[stable(feature = "pin", since = "1.33.0")]
479 pub fn pin(data: T) -> Pin<Arc<T>> {
480 unsafe { Pin::new_unchecked(Arc::new(data)) }
483 /// Constructs a new `Arc<T>`, returning an error if allocation fails.
488 /// #![feature(allocator_api)]
489 /// use std::sync::Arc;
491 /// let five = Arc::try_new(5)?;
492 /// # Ok::<(), std::alloc::AllocError>(())
494 #[unstable(feature = "allocator_api", issue = "32838")]
496 pub fn try_new(data: T) -> Result<Arc<T>, AllocError> {
497 // Start the weak pointer count as 1 which is the weak pointer that's
498 // held by all the strong pointers (kinda), see std/rc.rs for more info
499 let x: Box<_> = Box::try_new(ArcInner {
500 strong: atomic::AtomicUsize::new(1),
501 weak: atomic::AtomicUsize::new(1),
504 Ok(Self::from_inner(Box::leak(x).into()))
507 /// Constructs a new `Arc` with uninitialized contents, returning an error
508 /// if allocation fails.
513 /// #![feature(new_uninit, allocator_api)]
514 /// #![feature(get_mut_unchecked)]
516 /// use std::sync::Arc;
518 /// let mut five = Arc::<u32>::try_new_uninit()?;
520 /// let five = unsafe {
521 /// // Deferred initialization:
522 /// Arc::get_mut_unchecked(&mut five).as_mut_ptr().write(5);
524 /// five.assume_init()
527 /// assert_eq!(*five, 5);
528 /// # Ok::<(), std::alloc::AllocError>(())
530 #[unstable(feature = "allocator_api", issue = "32838")]
531 // #[unstable(feature = "new_uninit", issue = "63291")]
532 pub fn try_new_uninit() -> Result<Arc<mem::MaybeUninit<T>>, AllocError> {
534 Ok(Arc::from_ptr(Arc::try_allocate_for_layout(
536 |layout| Global.allocate(layout),
537 |mem| mem as *mut ArcInner<mem::MaybeUninit<T>>,
542 /// Constructs a new `Arc` with uninitialized contents, with the memory
543 /// being filled with `0` bytes, returning an error if allocation fails.
545 /// See [`MaybeUninit::zeroed`][zeroed] for examples of correct and incorrect usage
551 /// #![feature(new_uninit, allocator_api)]
553 /// use std::sync::Arc;
555 /// let zero = Arc::<u32>::try_new_zeroed()?;
556 /// let zero = unsafe { zero.assume_init() };
558 /// assert_eq!(*zero, 0);
559 /// # Ok::<(), std::alloc::AllocError>(())
562 /// [zeroed]: mem::MaybeUninit::zeroed
563 #[unstable(feature = "allocator_api", issue = "32838")]
564 // #[unstable(feature = "new_uninit", issue = "63291")]
565 pub fn try_new_zeroed() -> Result<Arc<mem::MaybeUninit<T>>, AllocError> {
567 Ok(Arc::from_ptr(Arc::try_allocate_for_layout(
569 |layout| Global.allocate_zeroed(layout),
570 |mem| mem as *mut ArcInner<mem::MaybeUninit<T>>,
574 /// Returns the inner value, if the `Arc` has exactly one strong reference.
576 /// Otherwise, an [`Err`] is returned with the same `Arc` that was
579 /// This will succeed even if there are outstanding weak references.
584 /// use std::sync::Arc;
586 /// let x = Arc::new(3);
587 /// assert_eq!(Arc::try_unwrap(x), Ok(3));
589 /// let x = Arc::new(4);
590 /// let _y = Arc::clone(&x);
591 /// assert_eq!(*Arc::try_unwrap(x).unwrap_err(), 4);
594 #[stable(feature = "arc_unique", since = "1.4.0")]
595 pub fn try_unwrap(this: Self) -> Result<T, Self> {
596 if this.inner().strong.compare_exchange(1, 0, Relaxed, Relaxed).is_err() {
600 acquire!(this.inner().strong);
603 let elem = ptr::read(&this.ptr.as_ref().data);
605 // Make a weak pointer to clean up the implicit strong-weak reference
606 let _weak = Weak { ptr: this.ptr };
615 /// Constructs a new atomically reference-counted slice with uninitialized contents.
620 /// #![feature(new_uninit)]
621 /// #![feature(get_mut_unchecked)]
623 /// use std::sync::Arc;
625 /// let mut values = Arc::<[u32]>::new_uninit_slice(3);
627 /// let values = unsafe {
628 /// // Deferred initialization:
629 /// Arc::get_mut_unchecked(&mut values)[0].as_mut_ptr().write(1);
630 /// Arc::get_mut_unchecked(&mut values)[1].as_mut_ptr().write(2);
631 /// Arc::get_mut_unchecked(&mut values)[2].as_mut_ptr().write(3);
633 /// values.assume_init()
636 /// assert_eq!(*values, [1, 2, 3])
638 #[unstable(feature = "new_uninit", issue = "63291")]
639 pub fn new_uninit_slice(len: usize) -> Arc<[mem::MaybeUninit<T>]> {
640 unsafe { Arc::from_ptr(Arc::allocate_for_slice(len)) }
643 /// Constructs a new atomically reference-counted slice with uninitialized contents, with the memory being
644 /// filled with `0` bytes.
646 /// See [`MaybeUninit::zeroed`][zeroed] for examples of correct and
647 /// incorrect usage of this method.
652 /// #![feature(new_uninit)]
654 /// use std::sync::Arc;
656 /// let values = Arc::<[u32]>::new_zeroed_slice(3);
657 /// let values = unsafe { values.assume_init() };
659 /// assert_eq!(*values, [0, 0, 0])
662 /// [zeroed]: ../../std/mem/union.MaybeUninit.html#method.zeroed
663 #[unstable(feature = "new_uninit", issue = "63291")]
664 pub fn new_zeroed_slice(len: usize) -> Arc<[mem::MaybeUninit<T>]> {
666 Arc::from_ptr(Arc::allocate_for_layout(
667 Layout::array::<T>(len).unwrap(),
668 |layout| Global.allocate_zeroed(layout),
670 ptr::slice_from_raw_parts_mut(mem as *mut T, len)
671 as *mut ArcInner<[mem::MaybeUninit<T>]>
678 impl<T> Arc<mem::MaybeUninit<T>> {
679 /// Converts to `Arc<T>`.
683 /// As with [`MaybeUninit::assume_init`],
684 /// it is up to the caller to guarantee that the inner value
685 /// really is in an initialized state.
686 /// Calling this when the content is not yet fully initialized
687 /// causes immediate undefined behavior.
689 /// [`MaybeUninit::assume_init`]: ../../std/mem/union.MaybeUninit.html#method.assume_init
694 /// #![feature(new_uninit)]
695 /// #![feature(get_mut_unchecked)]
697 /// use std::sync::Arc;
699 /// let mut five = Arc::<u32>::new_uninit();
701 /// let five = unsafe {
702 /// // Deferred initialization:
703 /// Arc::get_mut_unchecked(&mut five).as_mut_ptr().write(5);
705 /// five.assume_init()
708 /// assert_eq!(*five, 5)
710 #[unstable(feature = "new_uninit", issue = "63291")]
712 pub unsafe fn assume_init(self) -> Arc<T> {
713 Arc::from_inner(mem::ManuallyDrop::new(self).ptr.cast())
717 impl<T> Arc<[mem::MaybeUninit<T>]> {
718 /// Converts to `Arc<[T]>`.
722 /// As with [`MaybeUninit::assume_init`],
723 /// it is up to the caller to guarantee that the inner value
724 /// really is in an initialized state.
725 /// Calling this when the content is not yet fully initialized
726 /// causes immediate undefined behavior.
728 /// [`MaybeUninit::assume_init`]: ../../std/mem/union.MaybeUninit.html#method.assume_init
733 /// #![feature(new_uninit)]
734 /// #![feature(get_mut_unchecked)]
736 /// use std::sync::Arc;
738 /// let mut values = Arc::<[u32]>::new_uninit_slice(3);
740 /// let values = unsafe {
741 /// // Deferred initialization:
742 /// Arc::get_mut_unchecked(&mut values)[0].as_mut_ptr().write(1);
743 /// Arc::get_mut_unchecked(&mut values)[1].as_mut_ptr().write(2);
744 /// Arc::get_mut_unchecked(&mut values)[2].as_mut_ptr().write(3);
746 /// values.assume_init()
749 /// assert_eq!(*values, [1, 2, 3])
751 #[unstable(feature = "new_uninit", issue = "63291")]
753 pub unsafe fn assume_init(self) -> Arc<[T]> {
754 unsafe { Arc::from_ptr(mem::ManuallyDrop::new(self).ptr.as_ptr() as _) }
758 impl<T: ?Sized> Arc<T> {
759 /// Consumes the `Arc`, returning the wrapped pointer.
761 /// To avoid a memory leak the pointer must be converted back to an `Arc` using
762 /// [`Arc::from_raw`].
767 /// use std::sync::Arc;
769 /// let x = Arc::new("hello".to_owned());
770 /// let x_ptr = Arc::into_raw(x);
771 /// assert_eq!(unsafe { &*x_ptr }, "hello");
773 #[stable(feature = "rc_raw", since = "1.17.0")]
774 pub fn into_raw(this: Self) -> *const T {
775 let ptr = Self::as_ptr(&this);
780 /// Provides a raw pointer to the data.
782 /// The counts are not affected in any way and the `Arc` is not consumed. The pointer is valid for
783 /// as long as there are strong counts in the `Arc`.
788 /// use std::sync::Arc;
790 /// let x = Arc::new("hello".to_owned());
791 /// let y = Arc::clone(&x);
792 /// let x_ptr = Arc::as_ptr(&x);
793 /// assert_eq!(x_ptr, Arc::as_ptr(&y));
794 /// assert_eq!(unsafe { &*x_ptr }, "hello");
796 #[stable(feature = "rc_as_ptr", since = "1.45.0")]
797 pub fn as_ptr(this: &Self) -> *const T {
798 let ptr: *mut ArcInner<T> = NonNull::as_ptr(this.ptr);
800 // SAFETY: This cannot go through Deref::deref or RcBoxPtr::inner because
801 // this is required to retain raw/mut provenance such that e.g. `get_mut` can
802 // write through the pointer after the Rc is recovered through `from_raw`.
803 unsafe { ptr::addr_of_mut!((*ptr).data) }
806 /// Constructs an `Arc<T>` from a raw pointer.
808 /// The raw pointer must have been previously returned by a call to
809 /// [`Arc<U>::into_raw`][into_raw] where `U` must have the same size and
810 /// alignment as `T`. This is trivially true if `U` is `T`.
811 /// Note that if `U` is not `T` but has the same size and alignment, this is
812 /// basically like transmuting references of different types. See
813 /// [`mem::transmute`][transmute] for more information on what
814 /// restrictions apply in this case.
816 /// The user of `from_raw` has to make sure a specific value of `T` is only
819 /// This function is unsafe because improper use may lead to memory unsafety,
820 /// even if the returned `Arc<T>` is never accessed.
822 /// [into_raw]: Arc::into_raw
823 /// [transmute]: core::mem::transmute
828 /// use std::sync::Arc;
830 /// let x = Arc::new("hello".to_owned());
831 /// let x_ptr = Arc::into_raw(x);
834 /// // Convert back to an `Arc` to prevent leak.
835 /// let x = Arc::from_raw(x_ptr);
836 /// assert_eq!(&*x, "hello");
838 /// // Further calls to `Arc::from_raw(x_ptr)` would be memory-unsafe.
841 /// // The memory was freed when `x` went out of scope above, so `x_ptr` is now dangling!
843 #[stable(feature = "rc_raw", since = "1.17.0")]
844 pub unsafe fn from_raw(ptr: *const T) -> Self {
846 let offset = data_offset(ptr);
848 // Reverse the offset to find the original ArcInner.
849 let arc_ptr = (ptr as *mut ArcInner<T>).set_ptr_value((ptr as *mut u8).offset(-offset));
851 Self::from_ptr(arc_ptr)
855 /// Creates a new [`Weak`] pointer to this allocation.
860 /// use std::sync::Arc;
862 /// let five = Arc::new(5);
864 /// let weak_five = Arc::downgrade(&five);
866 #[stable(feature = "arc_weak", since = "1.4.0")]
867 pub fn downgrade(this: &Self) -> Weak<T> {
868 // This Relaxed is OK because we're checking the value in the CAS
870 let mut cur = this.inner().weak.load(Relaxed);
873 // check if the weak counter is currently "locked"; if so, spin.
874 if cur == usize::MAX {
876 cur = this.inner().weak.load(Relaxed);
880 // NOTE: this code currently ignores the possibility of overflow
881 // into usize::MAX; in general both Rc and Arc need to be adjusted
882 // to deal with overflow.
884 // Unlike with Clone(), we need this to be an Acquire read to
885 // synchronize with the write coming from `is_unique`, so that the
886 // events prior to that write happen before this read.
887 match this.inner().weak.compare_exchange_weak(cur, cur + 1, Acquire, Relaxed) {
889 // Make sure we do not create a dangling Weak
890 debug_assert!(!is_dangling(this.ptr.as_ptr()));
891 return Weak { ptr: this.ptr };
893 Err(old) => cur = old,
898 /// Gets the number of [`Weak`] pointers to this allocation.
902 /// This method by itself is safe, but using it correctly requires extra care.
903 /// Another thread can change the weak count at any time,
904 /// including potentially between calling this method and acting on the result.
909 /// use std::sync::Arc;
911 /// let five = Arc::new(5);
912 /// let _weak_five = Arc::downgrade(&five);
914 /// // This assertion is deterministic because we haven't shared
915 /// // the `Arc` or `Weak` between threads.
916 /// assert_eq!(1, Arc::weak_count(&five));
919 #[stable(feature = "arc_counts", since = "1.15.0")]
920 pub fn weak_count(this: &Self) -> usize {
921 let cnt = this.inner().weak.load(SeqCst);
922 // If the weak count is currently locked, the value of the
923 // count was 0 just before taking the lock.
924 if cnt == usize::MAX { 0 } else { cnt - 1 }
927 /// Gets the number of strong (`Arc`) pointers to this allocation.
931 /// This method by itself is safe, but using it correctly requires extra care.
932 /// Another thread can change the strong count at any time,
933 /// including potentially between calling this method and acting on the result.
938 /// use std::sync::Arc;
940 /// let five = Arc::new(5);
941 /// let _also_five = Arc::clone(&five);
943 /// // This assertion is deterministic because we haven't shared
944 /// // the `Arc` between threads.
945 /// assert_eq!(2, Arc::strong_count(&five));
948 #[stable(feature = "arc_counts", since = "1.15.0")]
949 pub fn strong_count(this: &Self) -> usize {
950 this.inner().strong.load(SeqCst)
953 /// Increments the strong reference count on the `Arc<T>` associated with the
954 /// provided pointer by one.
958 /// The pointer must have been obtained through `Arc::into_raw`, and the
959 /// associated `Arc` instance must be valid (i.e. the strong count must be at
960 /// least 1) for the duration of this method.
965 /// use std::sync::Arc;
967 /// let five = Arc::new(5);
970 /// let ptr = Arc::into_raw(five);
971 /// Arc::increment_strong_count(ptr);
973 /// // This assertion is deterministic because we haven't shared
974 /// // the `Arc` between threads.
975 /// let five = Arc::from_raw(ptr);
976 /// assert_eq!(2, Arc::strong_count(&five));
980 #[stable(feature = "arc_mutate_strong_count", since = "1.51.0")]
981 pub unsafe fn increment_strong_count(ptr: *const T) {
982 // Retain Arc, but don't touch refcount by wrapping in ManuallyDrop
983 let arc = unsafe { mem::ManuallyDrop::new(Arc::<T>::from_raw(ptr)) };
984 // Now increase refcount, but don't drop new refcount either
985 let _arc_clone: mem::ManuallyDrop<_> = arc.clone();
988 /// Decrements the strong reference count on the `Arc<T>` associated with the
989 /// provided pointer by one.
993 /// The pointer must have been obtained through `Arc::into_raw`, and the
994 /// associated `Arc` instance must be valid (i.e. the strong count must be at
995 /// least 1) when invoking this method. This method can be used to release the final
996 /// `Arc` and backing storage, but **should not** be called after the final `Arc` has been
1002 /// use std::sync::Arc;
1004 /// let five = Arc::new(5);
1007 /// let ptr = Arc::into_raw(five);
1008 /// Arc::increment_strong_count(ptr);
1010 /// // Those assertions are deterministic because we haven't shared
1011 /// // the `Arc` between threads.
1012 /// let five = Arc::from_raw(ptr);
1013 /// assert_eq!(2, Arc::strong_count(&five));
1014 /// Arc::decrement_strong_count(ptr);
1015 /// assert_eq!(1, Arc::strong_count(&five));
1019 #[stable(feature = "arc_mutate_strong_count", since = "1.51.0")]
1020 pub unsafe fn decrement_strong_count(ptr: *const T) {
1021 unsafe { mem::drop(Arc::from_raw(ptr)) };
1025 fn inner(&self) -> &ArcInner<T> {
1026 // This unsafety is ok because while this arc is alive we're guaranteed
1027 // that the inner pointer is valid. Furthermore, we know that the
1028 // `ArcInner` structure itself is `Sync` because the inner data is
1029 // `Sync` as well, so we're ok loaning out an immutable pointer to these
1031 unsafe { self.ptr.as_ref() }
1034 // Non-inlined part of `drop`.
1036 unsafe fn drop_slow(&mut self) {
1037 // Destroy the data at this time, even though we may not free the box
1038 // allocation itself (there may still be weak pointers lying around).
1039 unsafe { ptr::drop_in_place(Self::get_mut_unchecked(self)) };
1041 // Drop the weak ref collectively held by all strong references
1042 drop(Weak { ptr: self.ptr });
1046 #[stable(feature = "ptr_eq", since = "1.17.0")]
1047 /// Returns `true` if the two `Arc`s point to the same allocation
1048 /// (in a vein similar to [`ptr::eq`]).
1053 /// use std::sync::Arc;
1055 /// let five = Arc::new(5);
1056 /// let same_five = Arc::clone(&five);
1057 /// let other_five = Arc::new(5);
1059 /// assert!(Arc::ptr_eq(&five, &same_five));
1060 /// assert!(!Arc::ptr_eq(&five, &other_five));
1063 /// [`ptr::eq`]: core::ptr::eq
1064 pub fn ptr_eq(this: &Self, other: &Self) -> bool {
1065 this.ptr.as_ptr() == other.ptr.as_ptr()
1069 impl<T: ?Sized> Arc<T> {
1070 /// Allocates an `ArcInner<T>` with sufficient space for
1071 /// a possibly-unsized inner value where the value has the layout provided.
1073 /// The function `mem_to_arcinner` is called with the data pointer
1074 /// and must return back a (potentially fat)-pointer for the `ArcInner<T>`.
1075 unsafe fn allocate_for_layout(
1076 value_layout: Layout,
1077 allocate: impl FnOnce(Layout) -> Result<NonNull<[u8]>, AllocError>,
1078 mem_to_arcinner: impl FnOnce(*mut u8) -> *mut ArcInner<T>,
1079 ) -> *mut ArcInner<T> {
1080 // Calculate layout using the given value layout.
1081 // Previously, layout was calculated on the expression
1082 // `&*(ptr as *const ArcInner<T>)`, but this created a misaligned
1083 // reference (see #54908).
1084 let layout = Layout::new::<ArcInner<()>>().extend(value_layout).unwrap().0.pad_to_align();
1086 Arc::try_allocate_for_layout(value_layout, allocate, mem_to_arcinner)
1087 .unwrap_or_else(|_| handle_alloc_error(layout))
1091 /// Allocates an `ArcInner<T>` with sufficient space for
1092 /// a possibly-unsized inner value where the value has the layout provided,
1093 /// returning an error if allocation fails.
1095 /// The function `mem_to_arcinner` is called with the data pointer
1096 /// and must return back a (potentially fat)-pointer for the `ArcInner<T>`.
1097 unsafe fn try_allocate_for_layout(
1098 value_layout: Layout,
1099 allocate: impl FnOnce(Layout) -> Result<NonNull<[u8]>, AllocError>,
1100 mem_to_arcinner: impl FnOnce(*mut u8) -> *mut ArcInner<T>,
1101 ) -> Result<*mut ArcInner<T>, AllocError> {
1102 // Calculate layout using the given value layout.
1103 // Previously, layout was calculated on the expression
1104 // `&*(ptr as *const ArcInner<T>)`, but this created a misaligned
1105 // reference (see #54908).
1106 let layout = Layout::new::<ArcInner<()>>().extend(value_layout).unwrap().0.pad_to_align();
1108 let ptr = allocate(layout)?;
1110 // Initialize the ArcInner
1111 let inner = mem_to_arcinner(ptr.as_non_null_ptr().as_ptr());
1112 debug_assert_eq!(unsafe { Layout::for_value(&*inner) }, layout);
1115 ptr::write(&mut (*inner).strong, atomic::AtomicUsize::new(1));
1116 ptr::write(&mut (*inner).weak, atomic::AtomicUsize::new(1));
1122 /// Allocates an `ArcInner<T>` with sufficient space for an unsized inner value.
1123 unsafe fn allocate_for_ptr(ptr: *const T) -> *mut ArcInner<T> {
1124 // Allocate for the `ArcInner<T>` using the given value.
1126 Self::allocate_for_layout(
1127 Layout::for_value(&*ptr),
1128 |layout| Global.allocate(layout),
1129 |mem| (ptr as *mut ArcInner<T>).set_ptr_value(mem) as *mut ArcInner<T>,
1134 fn from_box(v: Box<T>) -> Arc<T> {
1136 let (box_unique, alloc) = Box::into_unique(v);
1137 let bptr = box_unique.as_ptr();
1139 let value_size = size_of_val(&*bptr);
1140 let ptr = Self::allocate_for_ptr(bptr);
1142 // Copy value as bytes
1143 ptr::copy_nonoverlapping(
1144 bptr as *const T as *const u8,
1145 &mut (*ptr).data as *mut _ as *mut u8,
1149 // Free the allocation without dropping its contents
1150 box_free(box_unique, alloc);
1158 /// Allocates an `ArcInner<[T]>` with the given length.
1159 unsafe fn allocate_for_slice(len: usize) -> *mut ArcInner<[T]> {
1161 Self::allocate_for_layout(
1162 Layout::array::<T>(len).unwrap(),
1163 |layout| Global.allocate(layout),
1164 |mem| ptr::slice_from_raw_parts_mut(mem as *mut T, len) as *mut ArcInner<[T]>,
1169 /// Copy elements from slice into newly allocated Arc<\[T\]>
1171 /// Unsafe because the caller must either take ownership or bind `T: Copy`.
1172 unsafe fn copy_from_slice(v: &[T]) -> Arc<[T]> {
1174 let ptr = Self::allocate_for_slice(v.len());
1176 ptr::copy_nonoverlapping(v.as_ptr(), &mut (*ptr).data as *mut [T] as *mut T, v.len());
1182 /// Constructs an `Arc<[T]>` from an iterator known to be of a certain size.
1184 /// Behavior is undefined should the size be wrong.
1185 unsafe fn from_iter_exact(iter: impl iter::Iterator<Item = T>, len: usize) -> Arc<[T]> {
1186 // Panic guard while cloning T elements.
1187 // In the event of a panic, elements that have been written
1188 // into the new ArcInner will be dropped, then the memory freed.
1196 impl<T> Drop for Guard<T> {
1197 fn drop(&mut self) {
1199 let slice = from_raw_parts_mut(self.elems, self.n_elems);
1200 ptr::drop_in_place(slice);
1202 Global.deallocate(self.mem, self.layout);
1208 let ptr = Self::allocate_for_slice(len);
1210 let mem = ptr as *mut _ as *mut u8;
1211 let layout = Layout::for_value(&*ptr);
1213 // Pointer to first element
1214 let elems = &mut (*ptr).data as *mut [T] as *mut T;
1216 let mut guard = Guard { mem: NonNull::new_unchecked(mem), elems, layout, n_elems: 0 };
1218 for (i, item) in iter.enumerate() {
1219 ptr::write(elems.add(i), item);
1223 // All clear. Forget the guard so it doesn't free the new ArcInner.
1231 /// Specialization trait used for `From<&[T]>`.
1232 trait ArcFromSlice<T> {
1233 fn from_slice(slice: &[T]) -> Self;
1236 impl<T: Clone> ArcFromSlice<T> for Arc<[T]> {
1238 default fn from_slice(v: &[T]) -> Self {
1239 unsafe { Self::from_iter_exact(v.iter().cloned(), v.len()) }
1243 impl<T: Copy> ArcFromSlice<T> for Arc<[T]> {
1245 fn from_slice(v: &[T]) -> Self {
1246 unsafe { Arc::copy_from_slice(v) }
1250 #[stable(feature = "rust1", since = "1.0.0")]
1251 impl<T: ?Sized> Clone for Arc<T> {
1252 /// Makes a clone of the `Arc` pointer.
1254 /// This creates another pointer to the same allocation, increasing the
1255 /// strong reference count.
1260 /// use std::sync::Arc;
1262 /// let five = Arc::new(5);
1264 /// let _ = Arc::clone(&five);
1267 fn clone(&self) -> Arc<T> {
1268 // Using a relaxed ordering is alright here, as knowledge of the
1269 // original reference prevents other threads from erroneously deleting
1272 // As explained in the [Boost documentation][1], Increasing the
1273 // reference counter can always be done with memory_order_relaxed: New
1274 // references to an object can only be formed from an existing
1275 // reference, and passing an existing reference from one thread to
1276 // another must already provide any required synchronization.
1278 // [1]: (www.boost.org/doc/libs/1_55_0/doc/html/atomic/usage_examples.html)
1279 let old_size = self.inner().strong.fetch_add(1, Relaxed);
1281 // However we need to guard against massive refcounts in case someone
1282 // is `mem::forget`ing Arcs. If we don't do this the count can overflow
1283 // and users will use-after free. We racily saturate to `isize::MAX` on
1284 // the assumption that there aren't ~2 billion threads incrementing
1285 // the reference count at once. This branch will never be taken in
1286 // any realistic program.
1288 // We abort because such a program is incredibly degenerate, and we
1289 // don't care to support it.
1290 if old_size > MAX_REFCOUNT {
1294 Self::from_inner(self.ptr)
1298 #[stable(feature = "rust1", since = "1.0.0")]
1299 impl<T: ?Sized> Deref for Arc<T> {
1303 fn deref(&self) -> &T {
1308 #[unstable(feature = "receiver_trait", issue = "none")]
1309 impl<T: ?Sized> Receiver for Arc<T> {}
1311 impl<T: Clone> Arc<T> {
1312 /// Makes a mutable reference into the given `Arc`.
1314 /// If there are other `Arc` or [`Weak`] pointers to the same allocation,
1315 /// then `make_mut` will create a new allocation and invoke [`clone`][clone] on the inner value
1316 /// to ensure unique ownership. This is also referred to as clone-on-write.
1318 /// Note that this differs from the behavior of [`Rc::make_mut`] which disassociates
1319 /// any remaining `Weak` pointers.
1321 /// See also [`get_mut`][get_mut], which will fail rather than cloning.
1323 /// [clone]: Clone::clone
1324 /// [get_mut]: Arc::get_mut
1325 /// [`Rc::make_mut`]: super::rc::Rc::make_mut
1330 /// use std::sync::Arc;
1332 /// let mut data = Arc::new(5);
1334 /// *Arc::make_mut(&mut data) += 1; // Won't clone anything
1335 /// let mut other_data = Arc::clone(&data); // Won't clone inner data
1336 /// *Arc::make_mut(&mut data) += 1; // Clones inner data
1337 /// *Arc::make_mut(&mut data) += 1; // Won't clone anything
1338 /// *Arc::make_mut(&mut other_data) *= 2; // Won't clone anything
1340 /// // Now `data` and `other_data` point to different allocations.
1341 /// assert_eq!(*data, 8);
1342 /// assert_eq!(*other_data, 12);
1345 #[stable(feature = "arc_unique", since = "1.4.0")]
1346 pub fn make_mut(this: &mut Self) -> &mut T {
1347 // Note that we hold both a strong reference and a weak reference.
1348 // Thus, releasing our strong reference only will not, by itself, cause
1349 // the memory to be deallocated.
1351 // Use Acquire to ensure that we see any writes to `weak` that happen
1352 // before release writes (i.e., decrements) to `strong`. Since we hold a
1353 // weak count, there's no chance the ArcInner itself could be
1355 if this.inner().strong.compare_exchange(1, 0, Acquire, Relaxed).is_err() {
1356 // Another strong pointer exists, so we must clone.
1357 // Pre-allocate memory to allow writing the cloned value directly.
1358 let mut arc = Self::new_uninit();
1360 let data = Arc::get_mut_unchecked(&mut arc);
1361 (**this).write_clone_into_raw(data.as_mut_ptr());
1362 *this = arc.assume_init();
1364 } else if this.inner().weak.load(Relaxed) != 1 {
1365 // Relaxed suffices in the above because this is fundamentally an
1366 // optimization: we are always racing with weak pointers being
1367 // dropped. Worst case, we end up allocated a new Arc unnecessarily.
1369 // We removed the last strong ref, but there are additional weak
1370 // refs remaining. We'll move the contents to a new Arc, and
1371 // invalidate the other weak refs.
1373 // Note that it is not possible for the read of `weak` to yield
1374 // usize::MAX (i.e., locked), since the weak count can only be
1375 // locked by a thread with a strong reference.
1377 // Materialize our own implicit weak pointer, so that it can clean
1378 // up the ArcInner as needed.
1379 let _weak = Weak { ptr: this.ptr };
1381 // Can just steal the data, all that's left is Weaks
1382 let mut arc = Self::new_uninit();
1384 let data = Arc::get_mut_unchecked(&mut arc);
1385 data.as_mut_ptr().copy_from_nonoverlapping(&**this, 1);
1386 ptr::write(this, arc.assume_init());
1389 // We were the sole reference of either kind; bump back up the
1390 // strong ref count.
1391 this.inner().strong.store(1, Release);
1394 // As with `get_mut()`, the unsafety is ok because our reference was
1395 // either unique to begin with, or became one upon cloning the contents.
1396 unsafe { Self::get_mut_unchecked(this) }
1400 impl<T: ?Sized> Arc<T> {
1401 /// Returns a mutable reference into the given `Arc`, if there are
1402 /// no other `Arc` or [`Weak`] pointers to the same allocation.
1404 /// Returns [`None`] otherwise, because it is not safe to
1405 /// mutate a shared value.
1407 /// See also [`make_mut`][make_mut], which will [`clone`][clone]
1408 /// the inner value when there are other pointers.
1410 /// [make_mut]: Arc::make_mut
1411 /// [clone]: Clone::clone
1416 /// use std::sync::Arc;
1418 /// let mut x = Arc::new(3);
1419 /// *Arc::get_mut(&mut x).unwrap() = 4;
1420 /// assert_eq!(*x, 4);
1422 /// let _y = Arc::clone(&x);
1423 /// assert!(Arc::get_mut(&mut x).is_none());
1426 #[stable(feature = "arc_unique", since = "1.4.0")]
1427 pub fn get_mut(this: &mut Self) -> Option<&mut T> {
1428 if this.is_unique() {
1429 // This unsafety is ok because we're guaranteed that the pointer
1430 // returned is the *only* pointer that will ever be returned to T. Our
1431 // reference count is guaranteed to be 1 at this point, and we required
1432 // the Arc itself to be `mut`, so we're returning the only possible
1433 // reference to the inner data.
1434 unsafe { Some(Arc::get_mut_unchecked(this)) }
1440 /// Returns a mutable reference into the given `Arc`,
1441 /// without any check.
1443 /// See also [`get_mut`], which is safe and does appropriate checks.
1445 /// [`get_mut`]: Arc::get_mut
1449 /// Any other `Arc` or [`Weak`] pointers to the same allocation must not be dereferenced
1450 /// for the duration of the returned borrow.
1451 /// This is trivially the case if no such pointers exist,
1452 /// for example immediately after `Arc::new`.
1457 /// #![feature(get_mut_unchecked)]
1459 /// use std::sync::Arc;
1461 /// let mut x = Arc::new(String::new());
1463 /// Arc::get_mut_unchecked(&mut x).push_str("foo")
1465 /// assert_eq!(*x, "foo");
1468 #[unstable(feature = "get_mut_unchecked", issue = "63292")]
1469 pub unsafe fn get_mut_unchecked(this: &mut Self) -> &mut T {
1470 // We are careful to *not* create a reference covering the "count" fields, as
1471 // this would alias with concurrent access to the reference counts (e.g. by `Weak`).
1472 unsafe { &mut (*this.ptr.as_ptr()).data }
1475 /// Determine whether this is the unique reference (including weak refs) to
1476 /// the underlying data.
1478 /// Note that this requires locking the weak ref count.
1479 fn is_unique(&mut self) -> bool {
1480 // lock the weak pointer count if we appear to be the sole weak pointer
1483 // The acquire label here ensures a happens-before relationship with any
1484 // writes to `strong` (in particular in `Weak::upgrade`) prior to decrements
1485 // of the `weak` count (via `Weak::drop`, which uses release). If the upgraded
1486 // weak ref was never dropped, the CAS here will fail so we do not care to synchronize.
1487 if self.inner().weak.compare_exchange(1, usize::MAX, Acquire, Relaxed).is_ok() {
1488 // This needs to be an `Acquire` to synchronize with the decrement of the `strong`
1489 // counter in `drop` -- the only access that happens when any but the last reference
1490 // is being dropped.
1491 let unique = self.inner().strong.load(Acquire) == 1;
1493 // The release write here synchronizes with a read in `downgrade`,
1494 // effectively preventing the above read of `strong` from happening
1496 self.inner().weak.store(1, Release); // release the lock
1504 #[stable(feature = "rust1", since = "1.0.0")]
1505 unsafe impl<#[may_dangle] T: ?Sized> Drop for Arc<T> {
1506 /// Drops the `Arc`.
1508 /// This will decrement the strong reference count. If the strong reference
1509 /// count reaches zero then the only other references (if any) are
1510 /// [`Weak`], so we `drop` the inner value.
1515 /// use std::sync::Arc;
1519 /// impl Drop for Foo {
1520 /// fn drop(&mut self) {
1521 /// println!("dropped!");
1525 /// let foo = Arc::new(Foo);
1526 /// let foo2 = Arc::clone(&foo);
1528 /// drop(foo); // Doesn't print anything
1529 /// drop(foo2); // Prints "dropped!"
1532 fn drop(&mut self) {
1533 // Because `fetch_sub` is already atomic, we do not need to synchronize
1534 // with other threads unless we are going to delete the object. This
1535 // same logic applies to the below `fetch_sub` to the `weak` count.
1536 if self.inner().strong.fetch_sub(1, Release) != 1 {
1540 // This fence is needed to prevent reordering of use of the data and
1541 // deletion of the data. Because it is marked `Release`, the decreasing
1542 // of the reference count synchronizes with this `Acquire` fence. This
1543 // means that use of the data happens before decreasing the reference
1544 // count, which happens before this fence, which happens before the
1545 // deletion of the data.
1547 // As explained in the [Boost documentation][1],
1549 // > It is important to enforce any possible access to the object in one
1550 // > thread (through an existing reference) to *happen before* deleting
1551 // > the object in a different thread. This is achieved by a "release"
1552 // > operation after dropping a reference (any access to the object
1553 // > through this reference must obviously happened before), and an
1554 // > "acquire" operation before deleting the object.
1556 // In particular, while the contents of an Arc are usually immutable, it's
1557 // possible to have interior writes to something like a Mutex<T>. Since a
1558 // Mutex is not acquired when it is deleted, we can't rely on its
1559 // synchronization logic to make writes in thread A visible to a destructor
1560 // running in thread B.
1562 // Also note that the Acquire fence here could probably be replaced with an
1563 // Acquire load, which could improve performance in highly-contended
1564 // situations. See [2].
1566 // [1]: (www.boost.org/doc/libs/1_55_0/doc/html/atomic/usage_examples.html)
1567 // [2]: (https://github.com/rust-lang/rust/pull/41714)
1568 acquire!(self.inner().strong);
1576 impl Arc<dyn Any + Send + Sync> {
1578 #[stable(feature = "rc_downcast", since = "1.29.0")]
1579 /// Attempt to downcast the `Arc<dyn Any + Send + Sync>` to a concrete type.
1584 /// use std::any::Any;
1585 /// use std::sync::Arc;
1587 /// fn print_if_string(value: Arc<dyn Any + Send + Sync>) {
1588 /// if let Ok(string) = value.downcast::<String>() {
1589 /// println!("String ({}): {}", string.len(), string);
1593 /// let my_string = "Hello World".to_string();
1594 /// print_if_string(Arc::new(my_string));
1595 /// print_if_string(Arc::new(0i8));
1597 pub fn downcast<T>(self) -> Result<Arc<T>, Self>
1599 T: Any + Send + Sync + 'static,
1601 if (*self).is::<T>() {
1602 let ptr = self.ptr.cast::<ArcInner<T>>();
1604 Ok(Arc::from_inner(ptr))
1612 /// Constructs a new `Weak<T>`, without allocating any memory.
1613 /// Calling [`upgrade`] on the return value always gives [`None`].
1615 /// [`upgrade`]: Weak::upgrade
1620 /// use std::sync::Weak;
1622 /// let empty: Weak<i64> = Weak::new();
1623 /// assert!(empty.upgrade().is_none());
1625 #[stable(feature = "downgraded_weak", since = "1.10.0")]
1626 pub fn new() -> Weak<T> {
1627 Weak { ptr: NonNull::new(usize::MAX as *mut ArcInner<T>).expect("MAX is not 0") }
1631 /// Helper type to allow accessing the reference counts without
1632 /// making any assertions about the data field.
1633 struct WeakInner<'a> {
1634 weak: &'a atomic::AtomicUsize,
1635 strong: &'a atomic::AtomicUsize,
1638 impl<T: ?Sized> Weak<T> {
1639 /// Returns a raw pointer to the object `T` pointed to by this `Weak<T>`.
1641 /// The pointer is valid only if there are some strong references. The pointer may be dangling,
1642 /// unaligned or even [`null`] otherwise.
1647 /// use std::sync::Arc;
1650 /// let strong = Arc::new("hello".to_owned());
1651 /// let weak = Arc::downgrade(&strong);
1652 /// // Both point to the same object
1653 /// assert!(ptr::eq(&*strong, weak.as_ptr()));
1654 /// // The strong here keeps it alive, so we can still access the object.
1655 /// assert_eq!("hello", unsafe { &*weak.as_ptr() });
1658 /// // But not any more. We can do weak.as_ptr(), but accessing the pointer would lead to
1659 /// // undefined behaviour.
1660 /// // assert_eq!("hello", unsafe { &*weak.as_ptr() });
1663 /// [`null`]: core::ptr::null
1664 #[stable(feature = "weak_into_raw", since = "1.45.0")]
1665 pub fn as_ptr(&self) -> *const T {
1666 let ptr: *mut ArcInner<T> = NonNull::as_ptr(self.ptr);
1668 if is_dangling(ptr) {
1669 // If the pointer is dangling, we return the sentinel directly. This cannot be
1670 // a valid payload address, as the payload is at least as aligned as ArcInner (usize).
1673 // SAFETY: if is_dangling returns false, then the pointer is dereferencable.
1674 // The payload may be dropped at this point, and we have to maintain provenance,
1675 // so use raw pointer manipulation.
1676 unsafe { ptr::addr_of_mut!((*ptr).data) }
1680 /// Consumes the `Weak<T>` and turns it into a raw pointer.
1682 /// This converts the weak pointer into a raw pointer, while still preserving the ownership of
1683 /// one weak reference (the weak count is not modified by this operation). It can be turned
1684 /// back into the `Weak<T>` with [`from_raw`].
1686 /// The same restrictions of accessing the target of the pointer as with
1687 /// [`as_ptr`] apply.
1692 /// use std::sync::{Arc, Weak};
1694 /// let strong = Arc::new("hello".to_owned());
1695 /// let weak = Arc::downgrade(&strong);
1696 /// let raw = weak.into_raw();
1698 /// assert_eq!(1, Arc::weak_count(&strong));
1699 /// assert_eq!("hello", unsafe { &*raw });
1701 /// drop(unsafe { Weak::from_raw(raw) });
1702 /// assert_eq!(0, Arc::weak_count(&strong));
1705 /// [`from_raw`]: Weak::from_raw
1706 /// [`as_ptr`]: Weak::as_ptr
1707 #[stable(feature = "weak_into_raw", since = "1.45.0")]
1708 pub fn into_raw(self) -> *const T {
1709 let result = self.as_ptr();
1714 /// Converts a raw pointer previously created by [`into_raw`] back into `Weak<T>`.
1716 /// This can be used to safely get a strong reference (by calling [`upgrade`]
1717 /// later) or to deallocate the weak count by dropping the `Weak<T>`.
1719 /// It takes ownership of one weak reference (with the exception of pointers created by [`new`],
1720 /// as these don't own anything; the method still works on them).
1724 /// The pointer must have originated from the [`into_raw`] and must still own its potential
1727 /// It is allowed for the strong count to be 0 at the time of calling this. Nevertheless, this
1728 /// takes ownership of one weak reference currently represented as a raw pointer (the weak
1729 /// count is not modified by this operation) and therefore it must be paired with a previous
1730 /// call to [`into_raw`].
1734 /// use std::sync::{Arc, Weak};
1736 /// let strong = Arc::new("hello".to_owned());
1738 /// let raw_1 = Arc::downgrade(&strong).into_raw();
1739 /// let raw_2 = Arc::downgrade(&strong).into_raw();
1741 /// assert_eq!(2, Arc::weak_count(&strong));
1743 /// assert_eq!("hello", &*unsafe { Weak::from_raw(raw_1) }.upgrade().unwrap());
1744 /// assert_eq!(1, Arc::weak_count(&strong));
1748 /// // Decrement the last weak count.
1749 /// assert!(unsafe { Weak::from_raw(raw_2) }.upgrade().is_none());
1752 /// [`new`]: Weak::new
1753 /// [`into_raw`]: Weak::into_raw
1754 /// [`upgrade`]: Weak::upgrade
1755 /// [`forget`]: std::mem::forget
1756 #[stable(feature = "weak_into_raw", since = "1.45.0")]
1757 pub unsafe fn from_raw(ptr: *const T) -> Self {
1758 // See Weak::as_ptr for context on how the input pointer is derived.
1760 let ptr = if is_dangling(ptr as *mut T) {
1761 // This is a dangling Weak.
1762 ptr as *mut ArcInner<T>
1764 // Otherwise, we're guaranteed the pointer came from a nondangling Weak.
1765 // SAFETY: data_offset is safe to call, as ptr references a real (potentially dropped) T.
1766 let offset = unsafe { data_offset(ptr) };
1767 // Thus, we reverse the offset to get the whole RcBox.
1768 // SAFETY: the pointer originated from a Weak, so this offset is safe.
1769 unsafe { (ptr as *mut ArcInner<T>).set_ptr_value((ptr as *mut u8).offset(-offset)) }
1772 // SAFETY: we now have recovered the original Weak pointer, so can create the Weak.
1773 Weak { ptr: unsafe { NonNull::new_unchecked(ptr) } }
1777 impl<T: ?Sized> Weak<T> {
1778 /// Attempts to upgrade the `Weak` pointer to an [`Arc`], delaying
1779 /// dropping of the inner value if successful.
1781 /// Returns [`None`] if the inner value has since been dropped.
1786 /// use std::sync::Arc;
1788 /// let five = Arc::new(5);
1790 /// let weak_five = Arc::downgrade(&five);
1792 /// let strong_five: Option<Arc<_>> = weak_five.upgrade();
1793 /// assert!(strong_five.is_some());
1795 /// // Destroy all strong pointers.
1796 /// drop(strong_five);
1799 /// assert!(weak_five.upgrade().is_none());
1801 #[stable(feature = "arc_weak", since = "1.4.0")]
1802 pub fn upgrade(&self) -> Option<Arc<T>> {
1803 // We use a CAS loop to increment the strong count instead of a
1804 // fetch_add as this function should never take the reference count
1805 // from zero to one.
1806 let inner = self.inner()?;
1808 // Relaxed load because any write of 0 that we can observe
1809 // leaves the field in a permanently zero state (so a
1810 // "stale" read of 0 is fine), and any other value is
1811 // confirmed via the CAS below.
1812 let mut n = inner.strong.load(Relaxed);
1819 // See comments in `Arc::clone` for why we do this (for `mem::forget`).
1820 if n > MAX_REFCOUNT {
1824 // Relaxed is fine for the failure case because we don't have any expectations about the new state.
1825 // Acquire is necessary for the success case to synchronise with `Arc::new_cyclic`, when the inner
1826 // value can be initialized after `Weak` references have already been created. In that case, we
1827 // expect to observe the fully initialized value.
1828 match inner.strong.compare_exchange_weak(n, n + 1, Acquire, Relaxed) {
1829 Ok(_) => return Some(Arc::from_inner(self.ptr)), // null checked above
1830 Err(old) => n = old,
1835 /// Gets the number of strong (`Arc`) pointers pointing to this allocation.
1837 /// If `self` was created using [`Weak::new`], this will return 0.
1838 #[stable(feature = "weak_counts", since = "1.41.0")]
1839 pub fn strong_count(&self) -> usize {
1840 if let Some(inner) = self.inner() { inner.strong.load(SeqCst) } else { 0 }
1843 /// Gets an approximation of the number of `Weak` pointers pointing to this
1846 /// If `self` was created using [`Weak::new`], or if there are no remaining
1847 /// strong pointers, this will return 0.
1851 /// Due to implementation details, the returned value can be off by 1 in
1852 /// either direction when other threads are manipulating any `Arc`s or
1853 /// `Weak`s pointing to the same allocation.
1854 #[stable(feature = "weak_counts", since = "1.41.0")]
1855 pub fn weak_count(&self) -> usize {
1858 let weak = inner.weak.load(SeqCst);
1859 let strong = inner.strong.load(SeqCst);
1863 // Since we observed that there was at least one strong pointer
1864 // after reading the weak count, we know that the implicit weak
1865 // reference (present whenever any strong references are alive)
1866 // was still around when we observed the weak count, and can
1867 // therefore safely subtract it.
1874 /// Returns `None` when the pointer is dangling and there is no allocated `ArcInner`,
1875 /// (i.e., when this `Weak` was created by `Weak::new`).
1877 fn inner(&self) -> Option<WeakInner<'_>> {
1878 if is_dangling(self.ptr.as_ptr()) {
1881 // We are careful to *not* create a reference covering the "data" field, as
1882 // the field may be mutated concurrently (for example, if the last `Arc`
1883 // is dropped, the data field will be dropped in-place).
1885 let ptr = self.ptr.as_ptr();
1886 WeakInner { strong: &(*ptr).strong, weak: &(*ptr).weak }
1891 /// Returns `true` if the two `Weak`s point to the same allocation (similar to
1892 /// [`ptr::eq`]), or if both don't point to any allocation
1893 /// (because they were created with `Weak::new()`).
1897 /// Since this compares pointers it means that `Weak::new()` will equal each
1898 /// other, even though they don't point to any allocation.
1903 /// use std::sync::Arc;
1905 /// let first_rc = Arc::new(5);
1906 /// let first = Arc::downgrade(&first_rc);
1907 /// let second = Arc::downgrade(&first_rc);
1909 /// assert!(first.ptr_eq(&second));
1911 /// let third_rc = Arc::new(5);
1912 /// let third = Arc::downgrade(&third_rc);
1914 /// assert!(!first.ptr_eq(&third));
1917 /// Comparing `Weak::new`.
1920 /// use std::sync::{Arc, Weak};
1922 /// let first = Weak::new();
1923 /// let second = Weak::new();
1924 /// assert!(first.ptr_eq(&second));
1926 /// let third_rc = Arc::new(());
1927 /// let third = Arc::downgrade(&third_rc);
1928 /// assert!(!first.ptr_eq(&third));
1931 /// [`ptr::eq`]: core::ptr::eq
1933 #[stable(feature = "weak_ptr_eq", since = "1.39.0")]
1934 pub fn ptr_eq(&self, other: &Self) -> bool {
1935 self.ptr.as_ptr() == other.ptr.as_ptr()
1939 #[stable(feature = "arc_weak", since = "1.4.0")]
1940 impl<T: ?Sized> Clone for Weak<T> {
1941 /// Makes a clone of the `Weak` pointer that points to the same allocation.
1946 /// use std::sync::{Arc, Weak};
1948 /// let weak_five = Arc::downgrade(&Arc::new(5));
1950 /// let _ = Weak::clone(&weak_five);
1953 fn clone(&self) -> Weak<T> {
1954 let inner = if let Some(inner) = self.inner() {
1957 return Weak { ptr: self.ptr };
1959 // See comments in Arc::clone() for why this is relaxed. This can use a
1960 // fetch_add (ignoring the lock) because the weak count is only locked
1961 // where are *no other* weak pointers in existence. (So we can't be
1962 // running this code in that case).
1963 let old_size = inner.weak.fetch_add(1, Relaxed);
1965 // See comments in Arc::clone() for why we do this (for mem::forget).
1966 if old_size > MAX_REFCOUNT {
1970 Weak { ptr: self.ptr }
1974 #[stable(feature = "downgraded_weak", since = "1.10.0")]
1975 impl<T> Default for Weak<T> {
1976 /// Constructs a new `Weak<T>`, without allocating memory.
1977 /// Calling [`upgrade`] on the return value always
1980 /// [`upgrade`]: Weak::upgrade
1985 /// use std::sync::Weak;
1987 /// let empty: Weak<i64> = Default::default();
1988 /// assert!(empty.upgrade().is_none());
1990 fn default() -> Weak<T> {
1995 #[stable(feature = "arc_weak", since = "1.4.0")]
1996 impl<T: ?Sized> Drop for Weak<T> {
1997 /// Drops the `Weak` pointer.
2002 /// use std::sync::{Arc, Weak};
2006 /// impl Drop for Foo {
2007 /// fn drop(&mut self) {
2008 /// println!("dropped!");
2012 /// let foo = Arc::new(Foo);
2013 /// let weak_foo = Arc::downgrade(&foo);
2014 /// let other_weak_foo = Weak::clone(&weak_foo);
2016 /// drop(weak_foo); // Doesn't print anything
2017 /// drop(foo); // Prints "dropped!"
2019 /// assert!(other_weak_foo.upgrade().is_none());
2021 fn drop(&mut self) {
2022 // If we find out that we were the last weak pointer, then its time to
2023 // deallocate the data entirely. See the discussion in Arc::drop() about
2024 // the memory orderings
2026 // It's not necessary to check for the locked state here, because the
2027 // weak count can only be locked if there was precisely one weak ref,
2028 // meaning that drop could only subsequently run ON that remaining weak
2029 // ref, which can only happen after the lock is released.
2030 let inner = if let Some(inner) = self.inner() { inner } else { return };
2032 if inner.weak.fetch_sub(1, Release) == 1 {
2033 acquire!(inner.weak);
2034 unsafe { Global.deallocate(self.ptr.cast(), Layout::for_value_raw(self.ptr.as_ptr())) }
2039 #[stable(feature = "rust1", since = "1.0.0")]
2040 trait ArcEqIdent<T: ?Sized + PartialEq> {
2041 fn eq(&self, other: &Arc<T>) -> bool;
2042 fn ne(&self, other: &Arc<T>) -> bool;
2045 #[stable(feature = "rust1", since = "1.0.0")]
2046 impl<T: ?Sized + PartialEq> ArcEqIdent<T> for Arc<T> {
2048 default fn eq(&self, other: &Arc<T>) -> bool {
2052 default fn ne(&self, other: &Arc<T>) -> bool {
2057 /// We're doing this specialization here, and not as a more general optimization on `&T`, because it
2058 /// would otherwise add a cost to all equality checks on refs. We assume that `Arc`s are used to
2059 /// store large values, that are slow to clone, but also heavy to check for equality, causing this
2060 /// cost to pay off more easily. It's also more likely to have two `Arc` clones, that point to
2061 /// the same value, than two `&T`s.
2063 /// We can only do this when `T: Eq` as a `PartialEq` might be deliberately irreflexive.
2064 #[stable(feature = "rust1", since = "1.0.0")]
2065 impl<T: ?Sized + crate::rc::MarkerEq> ArcEqIdent<T> for Arc<T> {
2067 fn eq(&self, other: &Arc<T>) -> bool {
2068 Arc::ptr_eq(self, other) || **self == **other
2072 fn ne(&self, other: &Arc<T>) -> bool {
2073 !Arc::ptr_eq(self, other) && **self != **other
2077 #[stable(feature = "rust1", since = "1.0.0")]
2078 impl<T: ?Sized + PartialEq> PartialEq for Arc<T> {
2079 /// Equality for two `Arc`s.
2081 /// Two `Arc`s are equal if their inner values are equal, even if they are
2082 /// stored in different allocation.
2084 /// If `T` also implements `Eq` (implying reflexivity of equality),
2085 /// two `Arc`s that point to the same allocation are always equal.
2090 /// use std::sync::Arc;
2092 /// let five = Arc::new(5);
2094 /// assert!(five == Arc::new(5));
2097 fn eq(&self, other: &Arc<T>) -> bool {
2098 ArcEqIdent::eq(self, other)
2101 /// Inequality for two `Arc`s.
2103 /// Two `Arc`s are unequal if their inner values are unequal.
2105 /// If `T` also implements `Eq` (implying reflexivity of equality),
2106 /// two `Arc`s that point to the same value are never unequal.
2111 /// use std::sync::Arc;
2113 /// let five = Arc::new(5);
2115 /// assert!(five != Arc::new(6));
2118 fn ne(&self, other: &Arc<T>) -> bool {
2119 ArcEqIdent::ne(self, other)
2123 #[stable(feature = "rust1", since = "1.0.0")]
2124 impl<T: ?Sized + PartialOrd> PartialOrd for Arc<T> {
2125 /// Partial comparison for two `Arc`s.
2127 /// The two are compared by calling `partial_cmp()` on their inner values.
2132 /// use std::sync::Arc;
2133 /// use std::cmp::Ordering;
2135 /// let five = Arc::new(5);
2137 /// assert_eq!(Some(Ordering::Less), five.partial_cmp(&Arc::new(6)));
2139 fn partial_cmp(&self, other: &Arc<T>) -> Option<Ordering> {
2140 (**self).partial_cmp(&**other)
2143 /// Less-than comparison for two `Arc`s.
2145 /// The two are compared by calling `<` on their inner values.
2150 /// use std::sync::Arc;
2152 /// let five = Arc::new(5);
2154 /// assert!(five < Arc::new(6));
2156 fn lt(&self, other: &Arc<T>) -> bool {
2157 *(*self) < *(*other)
2160 /// 'Less than or equal to' comparison for two `Arc`s.
2162 /// The two are compared by calling `<=` on their inner values.
2167 /// use std::sync::Arc;
2169 /// let five = Arc::new(5);
2171 /// assert!(five <= Arc::new(5));
2173 fn le(&self, other: &Arc<T>) -> bool {
2174 *(*self) <= *(*other)
2177 /// Greater-than comparison for two `Arc`s.
2179 /// The two are compared by calling `>` on their inner values.
2184 /// use std::sync::Arc;
2186 /// let five = Arc::new(5);
2188 /// assert!(five > Arc::new(4));
2190 fn gt(&self, other: &Arc<T>) -> bool {
2191 *(*self) > *(*other)
2194 /// 'Greater than or equal to' comparison for two `Arc`s.
2196 /// The two are compared by calling `>=` on their inner values.
2201 /// use std::sync::Arc;
2203 /// let five = Arc::new(5);
2205 /// assert!(five >= Arc::new(5));
2207 fn ge(&self, other: &Arc<T>) -> bool {
2208 *(*self) >= *(*other)
2211 #[stable(feature = "rust1", since = "1.0.0")]
2212 impl<T: ?Sized + Ord> Ord for Arc<T> {
2213 /// Comparison for two `Arc`s.
2215 /// The two are compared by calling `cmp()` on their inner values.
2220 /// use std::sync::Arc;
2221 /// use std::cmp::Ordering;
2223 /// let five = Arc::new(5);
2225 /// assert_eq!(Ordering::Less, five.cmp(&Arc::new(6)));
2227 fn cmp(&self, other: &Arc<T>) -> Ordering {
2228 (**self).cmp(&**other)
2231 #[stable(feature = "rust1", since = "1.0.0")]
2232 impl<T: ?Sized + Eq> Eq for Arc<T> {}
2234 #[stable(feature = "rust1", since = "1.0.0")]
2235 impl<T: ?Sized + fmt::Display> fmt::Display for Arc<T> {
2236 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
2237 fmt::Display::fmt(&**self, f)
2241 #[stable(feature = "rust1", since = "1.0.0")]
2242 impl<T: ?Sized + fmt::Debug> fmt::Debug for Arc<T> {
2243 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
2244 fmt::Debug::fmt(&**self, f)
2248 #[stable(feature = "rust1", since = "1.0.0")]
2249 impl<T: ?Sized> fmt::Pointer for Arc<T> {
2250 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
2251 fmt::Pointer::fmt(&(&**self as *const T), f)
2255 #[stable(feature = "rust1", since = "1.0.0")]
2256 impl<T: Default> Default for Arc<T> {
2257 /// Creates a new `Arc<T>`, with the `Default` value for `T`.
2262 /// use std::sync::Arc;
2264 /// let x: Arc<i32> = Default::default();
2265 /// assert_eq!(*x, 0);
2267 fn default() -> Arc<T> {
2268 Arc::new(Default::default())
2272 #[stable(feature = "rust1", since = "1.0.0")]
2273 impl<T: ?Sized + Hash> Hash for Arc<T> {
2274 fn hash<H: Hasher>(&self, state: &mut H) {
2275 (**self).hash(state)
2279 #[stable(feature = "from_for_ptrs", since = "1.6.0")]
2280 impl<T> From<T> for Arc<T> {
2281 fn from(t: T) -> Self {
2286 #[stable(feature = "shared_from_slice", since = "1.21.0")]
2287 impl<T: Clone> From<&[T]> for Arc<[T]> {
2288 /// Allocate a reference-counted slice and fill it by cloning `v`'s items.
2293 /// # use std::sync::Arc;
2294 /// let original: &[i32] = &[1, 2, 3];
2295 /// let shared: Arc<[i32]> = Arc::from(original);
2296 /// assert_eq!(&[1, 2, 3], &shared[..]);
2299 fn from(v: &[T]) -> Arc<[T]> {
2300 <Self as ArcFromSlice<T>>::from_slice(v)
2304 #[stable(feature = "shared_from_slice", since = "1.21.0")]
2305 impl From<&str> for Arc<str> {
2306 /// Allocate a reference-counted `str` and copy `v` into it.
2311 /// # use std::sync::Arc;
2312 /// let shared: Arc<str> = Arc::from("eggplant");
2313 /// assert_eq!("eggplant", &shared[..]);
2316 fn from(v: &str) -> Arc<str> {
2317 let arc = Arc::<[u8]>::from(v.as_bytes());
2318 unsafe { Arc::from_raw(Arc::into_raw(arc) as *const str) }
2322 #[stable(feature = "shared_from_slice", since = "1.21.0")]
2323 impl From<String> for Arc<str> {
2324 /// Allocate a reference-counted `str` and copy `v` into it.
2329 /// # use std::sync::Arc;
2330 /// let unique: String = "eggplant".to_owned();
2331 /// let shared: Arc<str> = Arc::from(unique);
2332 /// assert_eq!("eggplant", &shared[..]);
2335 fn from(v: String) -> Arc<str> {
2340 #[stable(feature = "shared_from_slice", since = "1.21.0")]
2341 impl<T: ?Sized> From<Box<T>> for Arc<T> {
2342 /// Move a boxed object to a new, reference-counted allocation.
2347 /// # use std::sync::Arc;
2348 /// let unique: Box<str> = Box::from("eggplant");
2349 /// let shared: Arc<str> = Arc::from(unique);
2350 /// assert_eq!("eggplant", &shared[..]);
2353 fn from(v: Box<T>) -> Arc<T> {
2358 #[stable(feature = "shared_from_slice", since = "1.21.0")]
2359 impl<T> From<Vec<T>> for Arc<[T]> {
2360 /// Allocate a reference-counted slice and move `v`'s items into it.
2365 /// # use std::sync::Arc;
2366 /// let unique: Vec<i32> = vec![1, 2, 3];
2367 /// let shared: Arc<[i32]> = Arc::from(unique);
2368 /// assert_eq!(&[1, 2, 3], &shared[..]);
2371 fn from(mut v: Vec<T>) -> Arc<[T]> {
2373 let arc = Arc::copy_from_slice(&v);
2375 // Allow the Vec to free its memory, but not destroy its contents
2383 #[stable(feature = "shared_from_cow", since = "1.45.0")]
2384 impl<'a, B> From<Cow<'a, B>> for Arc<B>
2386 B: ToOwned + ?Sized,
2387 Arc<B>: From<&'a B> + From<B::Owned>,
2390 fn from(cow: Cow<'a, B>) -> Arc<B> {
2392 Cow::Borrowed(s) => Arc::from(s),
2393 Cow::Owned(s) => Arc::from(s),
2398 #[stable(feature = "boxed_slice_try_from", since = "1.43.0")]
2399 impl<T, const N: usize> TryFrom<Arc<[T]>> for Arc<[T; N]> {
2400 type Error = Arc<[T]>;
2402 fn try_from(boxed_slice: Arc<[T]>) -> Result<Self, Self::Error> {
2403 if boxed_slice.len() == N {
2404 Ok(unsafe { Arc::from_raw(Arc::into_raw(boxed_slice) as *mut [T; N]) })
2411 #[stable(feature = "shared_from_iter", since = "1.37.0")]
2412 impl<T> iter::FromIterator<T> for Arc<[T]> {
2413 /// Takes each element in the `Iterator` and collects it into an `Arc<[T]>`.
2415 /// # Performance characteristics
2417 /// ## The general case
2419 /// In the general case, collecting into `Arc<[T]>` is done by first
2420 /// collecting into a `Vec<T>`. That is, when writing the following:
2423 /// # use std::sync::Arc;
2424 /// let evens: Arc<[u8]> = (0..10).filter(|&x| x % 2 == 0).collect();
2425 /// # assert_eq!(&*evens, &[0, 2, 4, 6, 8]);
2428 /// this behaves as if we wrote:
2431 /// # use std::sync::Arc;
2432 /// let evens: Arc<[u8]> = (0..10).filter(|&x| x % 2 == 0)
2433 /// .collect::<Vec<_>>() // The first set of allocations happens here.
2434 /// .into(); // A second allocation for `Arc<[T]>` happens here.
2435 /// # assert_eq!(&*evens, &[0, 2, 4, 6, 8]);
2438 /// This will allocate as many times as needed for constructing the `Vec<T>`
2439 /// and then it will allocate once for turning the `Vec<T>` into the `Arc<[T]>`.
2441 /// ## Iterators of known length
2443 /// When your `Iterator` implements `TrustedLen` and is of an exact size,
2444 /// a single allocation will be made for the `Arc<[T]>`. For example:
2447 /// # use std::sync::Arc;
2448 /// let evens: Arc<[u8]> = (0..10).collect(); // Just a single allocation happens here.
2449 /// # assert_eq!(&*evens, &*(0..10).collect::<Vec<_>>());
2451 fn from_iter<I: iter::IntoIterator<Item = T>>(iter: I) -> Self {
2452 ToArcSlice::to_arc_slice(iter.into_iter())
2456 /// Specialization trait used for collecting into `Arc<[T]>`.
2457 trait ToArcSlice<T>: Iterator<Item = T> + Sized {
2458 fn to_arc_slice(self) -> Arc<[T]>;
2461 impl<T, I: Iterator<Item = T>> ToArcSlice<T> for I {
2462 default fn to_arc_slice(self) -> Arc<[T]> {
2463 self.collect::<Vec<T>>().into()
2467 impl<T, I: iter::TrustedLen<Item = T>> ToArcSlice<T> for I {
2468 fn to_arc_slice(self) -> Arc<[T]> {
2469 // This is the case for a `TrustedLen` iterator.
2470 let (low, high) = self.size_hint();
2471 if let Some(high) = high {
2475 "TrustedLen iterator's size hint is not exact: {:?}",
2480 // SAFETY: We need to ensure that the iterator has an exact length and we have.
2481 Arc::from_iter_exact(self, low)
2484 // Fall back to normal implementation.
2485 self.collect::<Vec<T>>().into()
2490 #[stable(feature = "rust1", since = "1.0.0")]
2491 impl<T: ?Sized> borrow::Borrow<T> for Arc<T> {
2492 fn borrow(&self) -> &T {
2497 #[stable(since = "1.5.0", feature = "smart_ptr_as_ref")]
2498 impl<T: ?Sized> AsRef<T> for Arc<T> {
2499 fn as_ref(&self) -> &T {
2504 #[stable(feature = "pin", since = "1.33.0")]
2505 impl<T: ?Sized> Unpin for Arc<T> {}
2507 /// Get the offset within an `ArcInner` for the payload behind a pointer.
2511 /// The pointer must point to (and have valid metadata for) a previously
2512 /// valid instance of T, but the T is allowed to be dropped.
2513 unsafe fn data_offset<T: ?Sized>(ptr: *const T) -> isize {
2514 // Align the unsized value to the end of the ArcInner.
2515 // Because RcBox is repr(C), it will always be the last field in memory.
2516 // SAFETY: since the only unsized types possible are slices, trait objects,
2517 // and extern types, the input safety requirement is currently enough to
2518 // satisfy the requirements of align_of_val_raw; this is an implementation
2519 // detail of the language that may not be relied upon outside of std.
2520 unsafe { data_offset_align(align_of_val_raw(ptr)) }
2524 fn data_offset_align(align: usize) -> isize {
2525 let layout = Layout::new::<ArcInner<()>>();
2526 (layout.size() + layout.padding_needed_for(align)) as isize