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;
15 #[cfg(not(no_global_oom_handling))]
17 use core::marker::{PhantomData, Unpin, Unsize};
18 #[cfg(not(no_global_oom_handling))]
19 use core::mem::size_of_val;
20 use core::mem::{self, align_of_val_raw};
21 use core::ops::{CoerceUnsized, Deref, DispatchFromDyn, Receiver};
22 use core::panic::{RefUnwindSafe, UnwindSafe};
24 use core::ptr::{self, NonNull};
25 #[cfg(not(no_global_oom_handling))]
26 use core::slice::from_raw_parts_mut;
27 use core::sync::atomic;
28 use core::sync::atomic::Ordering::{Acquire, Relaxed, Release, SeqCst};
30 #[cfg(not(no_global_oom_handling))]
31 use crate::alloc::handle_alloc_error;
32 #[cfg(not(no_global_oom_handling))]
33 use crate::alloc::{box_free, WriteCloneIntoRaw};
34 use crate::alloc::{AllocError, Allocator, Global, Layout};
35 use crate::borrow::{Cow, ToOwned};
36 use crate::boxed::Box;
37 use crate::rc::is_dangling;
38 #[cfg(not(no_global_oom_handling))]
39 use crate::string::String;
40 #[cfg(not(no_global_oom_handling))]
46 /// A soft limit on the amount of references that may be made to an `Arc`.
48 /// Going above this limit will abort your program (although not
49 /// necessarily) at _exactly_ `MAX_REFCOUNT + 1` references.
50 const MAX_REFCOUNT: usize = (isize::MAX) as usize;
52 #[cfg(not(sanitize = "thread"))]
53 macro_rules! acquire {
55 atomic::fence(Acquire)
59 // ThreadSanitizer does not support memory fences. To avoid false positive
60 // reports in Arc / Weak implementation use atomic loads for synchronization
62 #[cfg(sanitize = "thread")]
63 macro_rules! acquire {
69 /// A thread-safe reference-counting pointer. 'Arc' stands for 'Atomically
70 /// Reference Counted'.
72 /// The type `Arc<T>` provides shared ownership of a value of type `T`,
73 /// allocated in the heap. Invoking [`clone`][clone] on `Arc` produces
74 /// a new `Arc` instance, which points to the same allocation on the heap as the
75 /// source `Arc`, while increasing a reference count. When the last `Arc`
76 /// pointer to a given allocation is destroyed, the value stored in that allocation (often
77 /// referred to as "inner value") is also dropped.
79 /// Shared references in Rust disallow mutation by default, and `Arc` is no
80 /// exception: you cannot generally obtain a mutable reference to something
81 /// inside an `Arc`. If you need to mutate through an `Arc`, use
82 /// [`Mutex`][mutex], [`RwLock`][rwlock], or one of the [`Atomic`][atomic]
87 /// Unlike [`Rc<T>`], `Arc<T>` uses atomic operations for its reference
88 /// counting. This means that it is thread-safe. The disadvantage is that
89 /// atomic operations are more expensive than ordinary memory accesses. If you
90 /// are not sharing reference-counted allocations between threads, consider using
91 /// [`Rc<T>`] for lower overhead. [`Rc<T>`] is a safe default, because the
92 /// compiler will catch any attempt to send an [`Rc<T>`] between threads.
93 /// However, a library might choose `Arc<T>` in order to give library consumers
96 /// `Arc<T>` will implement [`Send`] and [`Sync`] as long as the `T` implements
97 /// [`Send`] and [`Sync`]. Why can't you put a non-thread-safe type `T` in an
98 /// `Arc<T>` to make it thread-safe? This may be a bit counter-intuitive at
99 /// first: after all, isn't the point of `Arc<T>` thread safety? The key is
100 /// this: `Arc<T>` makes it thread safe to have multiple ownership of the same
101 /// data, but it doesn't add thread safety to its data. Consider
102 /// <code>Arc<[RefCell\<T>]></code>. [`RefCell<T>`] isn't [`Sync`], and if `Arc<T>` was always
103 /// [`Send`], <code>Arc<[RefCell\<T>]></code> would be as well. But then we'd have a problem:
104 /// [`RefCell<T>`] is not thread safe; it keeps track of the borrowing count using
105 /// non-atomic operations.
107 /// In the end, this means that you may need to pair `Arc<T>` with some sort of
108 /// [`std::sync`] type, usually [`Mutex<T>`][mutex].
110 /// ## Breaking cycles with `Weak`
112 /// The [`downgrade`][downgrade] method can be used to create a non-owning
113 /// [`Weak`] pointer. A [`Weak`] pointer can be [`upgrade`][upgrade]d
114 /// to an `Arc`, but this will return [`None`] if the value stored in the allocation has
115 /// already been dropped. In other words, `Weak` pointers do not keep the value
116 /// inside the allocation alive; however, they *do* keep the allocation
117 /// (the backing store for the value) alive.
119 /// A cycle between `Arc` pointers will never be deallocated. For this reason,
120 /// [`Weak`] is used to break cycles. For example, a tree could have
121 /// strong `Arc` pointers from parent nodes to children, and [`Weak`]
122 /// pointers from children back to their parents.
124 /// # Cloning references
126 /// Creating a new reference from an existing reference-counted pointer is done using the
127 /// `Clone` trait implemented for [`Arc<T>`][Arc] and [`Weak<T>`][Weak].
130 /// use std::sync::Arc;
131 /// let foo = Arc::new(vec![1.0, 2.0, 3.0]);
132 /// // The two syntaxes below are equivalent.
133 /// let a = foo.clone();
134 /// let b = Arc::clone(&foo);
135 /// // a, b, and foo are all Arcs that point to the same memory location
138 /// ## `Deref` behavior
140 /// `Arc<T>` automatically dereferences to `T` (via the [`Deref`][deref] trait),
141 /// so you can call `T`'s methods on a value of type `Arc<T>`. To avoid name
142 /// clashes with `T`'s methods, the methods of `Arc<T>` itself are associated
143 /// functions, called using [fully qualified syntax]:
146 /// use std::sync::Arc;
148 /// let my_arc = Arc::new(());
149 /// let my_weak = Arc::downgrade(&my_arc);
152 /// `Arc<T>`'s implementations of traits like `Clone` may also be called using
153 /// fully qualified syntax. Some people prefer to use fully qualified syntax,
154 /// while others prefer using method-call syntax.
157 /// use std::sync::Arc;
159 /// let arc = Arc::new(());
160 /// // Method-call syntax
161 /// let arc2 = arc.clone();
162 /// // Fully qualified syntax
163 /// let arc3 = Arc::clone(&arc);
166 /// [`Weak<T>`][Weak] does not auto-dereference to `T`, because the inner value may have
167 /// already been dropped.
169 /// [`Rc<T>`]: crate::rc::Rc
170 /// [clone]: Clone::clone
171 /// [mutex]: ../../std/sync/struct.Mutex.html
172 /// [rwlock]: ../../std/sync/struct.RwLock.html
173 /// [atomic]: core::sync::atomic
174 /// [`Send`]: core::marker::Send
175 /// [`Sync`]: core::marker::Sync
176 /// [deref]: core::ops::Deref
177 /// [downgrade]: Arc::downgrade
178 /// [upgrade]: Weak::upgrade
179 /// [RefCell\<T>]: core::cell::RefCell
180 /// [`RefCell<T>`]: core::cell::RefCell
181 /// [`std::sync`]: ../../std/sync/index.html
182 /// [`Arc::clone(&from)`]: Arc::clone
183 /// [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
187 /// Sharing some immutable data between threads:
189 // Note that we **do not** run these tests here. The windows builders get super
190 // unhappy if a thread outlives the main thread and then exits at the same time
191 // (something deadlocks) so we just avoid this entirely by not running these
194 /// use std::sync::Arc;
197 /// let five = Arc::new(5);
200 /// let five = Arc::clone(&five);
202 /// thread::spawn(move || {
203 /// println!("{:?}", five);
208 /// Sharing a mutable [`AtomicUsize`]:
210 /// [`AtomicUsize`]: core::sync::atomic::AtomicUsize "sync::atomic::AtomicUsize"
213 /// use std::sync::Arc;
214 /// use std::sync::atomic::{AtomicUsize, Ordering};
217 /// let val = Arc::new(AtomicUsize::new(5));
220 /// let val = Arc::clone(&val);
222 /// thread::spawn(move || {
223 /// let v = val.fetch_add(1, Ordering::SeqCst);
224 /// println!("{:?}", v);
229 /// See the [`rc` documentation][rc_examples] for more examples of reference
230 /// counting in general.
232 /// [rc_examples]: crate::rc#examples
233 #[cfg_attr(not(test), rustc_diagnostic_item = "Arc")]
234 #[stable(feature = "rust1", since = "1.0.0")]
235 pub struct Arc<T: ?Sized> {
236 ptr: NonNull<ArcInner<T>>,
237 phantom: PhantomData<ArcInner<T>>,
240 #[stable(feature = "rust1", since = "1.0.0")]
241 unsafe impl<T: ?Sized + Sync + Send> Send for Arc<T> {}
242 #[stable(feature = "rust1", since = "1.0.0")]
243 unsafe impl<T: ?Sized + Sync + Send> Sync for Arc<T> {}
245 #[stable(feature = "catch_unwind", since = "1.9.0")]
246 impl<T: RefUnwindSafe + ?Sized> UnwindSafe for Arc<T> {}
248 #[unstable(feature = "coerce_unsized", issue = "27732")]
249 impl<T: ?Sized + Unsize<U>, U: ?Sized> CoerceUnsized<Arc<U>> for Arc<T> {}
251 #[unstable(feature = "dispatch_from_dyn", issue = "none")]
252 impl<T: ?Sized + Unsize<U>, U: ?Sized> DispatchFromDyn<Arc<U>> for Arc<T> {}
254 impl<T: ?Sized> Arc<T> {
255 fn from_inner(ptr: NonNull<ArcInner<T>>) -> Self {
256 Self { ptr, phantom: PhantomData }
259 unsafe fn from_ptr(ptr: *mut ArcInner<T>) -> Self {
260 unsafe { Self::from_inner(NonNull::new_unchecked(ptr)) }
264 /// `Weak` is a version of [`Arc`] that holds a non-owning reference to the
265 /// managed allocation. The allocation is accessed by calling [`upgrade`] on the `Weak`
266 /// pointer, which returns an <code>[Option]<[Arc]\<T>></code>.
268 /// Since a `Weak` reference does not count towards ownership, it will not
269 /// prevent the value stored in the allocation from being dropped, and `Weak` itself makes no
270 /// guarantees about the value still being present. Thus it may return [`None`]
271 /// when [`upgrade`]d. Note however that a `Weak` reference *does* prevent the allocation
272 /// itself (the backing store) from being deallocated.
274 /// A `Weak` pointer is useful for keeping a temporary reference to the allocation
275 /// managed by [`Arc`] without preventing its inner value from being dropped. It is also used to
276 /// prevent circular references between [`Arc`] pointers, since mutual owning references
277 /// would never allow either [`Arc`] to be dropped. For example, a tree could
278 /// have strong [`Arc`] pointers from parent nodes to children, and `Weak`
279 /// pointers from children back to their parents.
281 /// The typical way to obtain a `Weak` pointer is to call [`Arc::downgrade`].
283 /// [`upgrade`]: Weak::upgrade
284 #[stable(feature = "arc_weak", since = "1.4.0")]
285 pub struct Weak<T: ?Sized> {
286 // This is a `NonNull` to allow optimizing the size of this type in enums,
287 // but it is not necessarily a valid pointer.
288 // `Weak::new` sets this to `usize::MAX` so that it doesn’t need
289 // to allocate space on the heap. That's not a value a real pointer
290 // will ever have because RcBox has alignment at least 2.
291 // This is only possible when `T: Sized`; unsized `T` never dangle.
292 ptr: NonNull<ArcInner<T>>,
295 #[stable(feature = "arc_weak", since = "1.4.0")]
296 unsafe impl<T: ?Sized + Sync + Send> Send for Weak<T> {}
297 #[stable(feature = "arc_weak", since = "1.4.0")]
298 unsafe impl<T: ?Sized + Sync + Send> Sync for Weak<T> {}
300 #[unstable(feature = "coerce_unsized", issue = "27732")]
301 impl<T: ?Sized + Unsize<U>, U: ?Sized> CoerceUnsized<Weak<U>> for Weak<T> {}
302 #[unstable(feature = "dispatch_from_dyn", issue = "none")]
303 impl<T: ?Sized + Unsize<U>, U: ?Sized> DispatchFromDyn<Weak<U>> for Weak<T> {}
305 #[stable(feature = "arc_weak", since = "1.4.0")]
306 impl<T: ?Sized + fmt::Debug> fmt::Debug for Weak<T> {
307 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
312 // This is repr(C) to future-proof against possible field-reordering, which
313 // would interfere with otherwise safe [into|from]_raw() of transmutable
316 struct ArcInner<T: ?Sized> {
317 strong: atomic::AtomicUsize,
319 // the value usize::MAX acts as a sentinel for temporarily "locking" the
320 // ability to upgrade weak pointers or downgrade strong ones; this is used
321 // to avoid races in `make_mut` and `get_mut`.
322 weak: atomic::AtomicUsize,
327 unsafe impl<T: ?Sized + Sync + Send> Send for ArcInner<T> {}
328 unsafe impl<T: ?Sized + Sync + Send> Sync for ArcInner<T> {}
331 /// Constructs a new `Arc<T>`.
336 /// use std::sync::Arc;
338 /// let five = Arc::new(5);
340 #[cfg(not(no_global_oom_handling))]
342 #[stable(feature = "rust1", since = "1.0.0")]
343 pub fn new(data: T) -> Arc<T> {
344 // Start the weak pointer count as 1 which is the weak pointer that's
345 // held by all the strong pointers (kinda), see std/rc.rs for more info
346 let x: Box<_> = box ArcInner {
347 strong: atomic::AtomicUsize::new(1),
348 weak: atomic::AtomicUsize::new(1),
351 Self::from_inner(Box::leak(x).into())
354 /// Constructs a new `Arc<T>` using a weak reference to itself. Attempting
355 /// to upgrade the weak reference before this function returns will result
356 /// in a `None` value. However, the weak reference may be cloned freely and
357 /// stored for use at a later time.
361 /// #![feature(arc_new_cyclic)]
362 /// #![allow(dead_code)]
364 /// use std::sync::{Arc, Weak};
370 /// let foo = Arc::new_cyclic(|me| Foo {
374 #[cfg(not(no_global_oom_handling))]
376 #[unstable(feature = "arc_new_cyclic", issue = "75861")]
377 pub fn new_cyclic(data_fn: impl FnOnce(&Weak<T>) -> T) -> Arc<T> {
378 // Construct the inner in the "uninitialized" state with a single
380 let uninit_ptr: NonNull<_> = Box::leak(box ArcInner {
381 strong: atomic::AtomicUsize::new(0),
382 weak: atomic::AtomicUsize::new(1),
383 data: mem::MaybeUninit::<T>::uninit(),
386 let init_ptr: NonNull<ArcInner<T>> = uninit_ptr.cast();
388 let weak = Weak { ptr: init_ptr };
390 // It's important we don't give up ownership of the weak pointer, or
391 // else the memory might be freed by the time `data_fn` returns. If
392 // we really wanted to pass ownership, we could create an additional
393 // weak pointer for ourselves, but this would result in additional
394 // updates to the weak reference count which might not be necessary
396 let data = data_fn(&weak);
398 // Now we can properly initialize the inner value and turn our weak
399 // reference into a strong reference.
401 let inner = init_ptr.as_ptr();
402 ptr::write(ptr::addr_of_mut!((*inner).data), data);
404 // The above write to the data field must be visible to any threads which
405 // observe a non-zero strong count. Therefore we need at least "Release" ordering
406 // in order to synchronize with the `compare_exchange_weak` in `Weak::upgrade`.
408 // "Acquire" ordering is not required. When considering the possible behaviours
409 // of `data_fn` we only need to look at what it could do with a reference to a
410 // non-upgradeable `Weak`:
411 // - It can *clone* the `Weak`, increasing the weak reference count.
412 // - It can drop those clones, decreasing the weak reference count (but never to zero).
414 // These side effects do not impact us in any way, and no other side effects are
415 // possible with safe code alone.
416 let prev_value = (*inner).strong.fetch_add(1, Release);
417 debug_assert_eq!(prev_value, 0, "No prior strong references should exist");
420 let strong = Arc::from_inner(init_ptr);
422 // Strong references should collectively own a shared weak reference,
423 // so don't run the destructor for our old weak reference.
428 /// Constructs a new `Arc` with uninitialized contents.
433 /// #![feature(new_uninit)]
434 /// #![feature(get_mut_unchecked)]
436 /// use std::sync::Arc;
438 /// let mut five = Arc::<u32>::new_uninit();
440 /// let five = unsafe {
441 /// // Deferred initialization:
442 /// Arc::get_mut_unchecked(&mut five).as_mut_ptr().write(5);
444 /// five.assume_init()
447 /// assert_eq!(*five, 5)
449 #[cfg(not(no_global_oom_handling))]
450 #[unstable(feature = "new_uninit", issue = "63291")]
452 pub fn new_uninit() -> Arc<mem::MaybeUninit<T>> {
454 Arc::from_ptr(Arc::allocate_for_layout(
456 |layout| Global.allocate(layout),
457 |mem| mem as *mut ArcInner<mem::MaybeUninit<T>>,
462 /// Constructs a new `Arc` with uninitialized contents, with the memory
463 /// being filled with `0` bytes.
465 /// See [`MaybeUninit::zeroed`][zeroed] for examples of correct and incorrect usage
471 /// #![feature(new_uninit)]
473 /// use std::sync::Arc;
475 /// let zero = Arc::<u32>::new_zeroed();
476 /// let zero = unsafe { zero.assume_init() };
478 /// assert_eq!(*zero, 0)
481 /// [zeroed]: mem::MaybeUninit::zeroed
482 #[cfg(not(no_global_oom_handling))]
483 #[unstable(feature = "new_uninit", issue = "63291")]
485 pub fn new_zeroed() -> Arc<mem::MaybeUninit<T>> {
487 Arc::from_ptr(Arc::allocate_for_layout(
489 |layout| Global.allocate_zeroed(layout),
490 |mem| mem as *mut ArcInner<mem::MaybeUninit<T>>,
495 /// Constructs a new `Pin<Arc<T>>`. If `T` does not implement `Unpin`, then
496 /// `data` will be pinned in memory and unable to be moved.
497 #[cfg(not(no_global_oom_handling))]
498 #[stable(feature = "pin", since = "1.33.0")]
500 pub fn pin(data: T) -> Pin<Arc<T>> {
501 unsafe { Pin::new_unchecked(Arc::new(data)) }
504 /// Constructs a new `Pin<Arc<T>>`, return an error if allocation fails.
505 #[unstable(feature = "allocator_api", issue = "32838")]
507 pub fn try_pin(data: T) -> Result<Pin<Arc<T>>, AllocError> {
508 unsafe { Ok(Pin::new_unchecked(Arc::try_new(data)?)) }
511 /// Constructs a new `Arc<T>`, returning an error if allocation fails.
516 /// #![feature(allocator_api)]
517 /// use std::sync::Arc;
519 /// let five = Arc::try_new(5)?;
520 /// # Ok::<(), std::alloc::AllocError>(())
522 #[unstable(feature = "allocator_api", issue = "32838")]
524 pub fn try_new(data: T) -> Result<Arc<T>, AllocError> {
525 // Start the weak pointer count as 1 which is the weak pointer that's
526 // held by all the strong pointers (kinda), see std/rc.rs for more info
527 let x: Box<_> = Box::try_new(ArcInner {
528 strong: atomic::AtomicUsize::new(1),
529 weak: atomic::AtomicUsize::new(1),
532 Ok(Self::from_inner(Box::leak(x).into()))
535 /// Constructs a new `Arc` with uninitialized contents, returning an error
536 /// if allocation fails.
541 /// #![feature(new_uninit, allocator_api)]
542 /// #![feature(get_mut_unchecked)]
544 /// use std::sync::Arc;
546 /// let mut five = Arc::<u32>::try_new_uninit()?;
548 /// let five = unsafe {
549 /// // Deferred initialization:
550 /// Arc::get_mut_unchecked(&mut five).as_mut_ptr().write(5);
552 /// five.assume_init()
555 /// assert_eq!(*five, 5);
556 /// # Ok::<(), std::alloc::AllocError>(())
558 #[unstable(feature = "allocator_api", issue = "32838")]
559 // #[unstable(feature = "new_uninit", issue = "63291")]
560 pub fn try_new_uninit() -> Result<Arc<mem::MaybeUninit<T>>, AllocError> {
562 Ok(Arc::from_ptr(Arc::try_allocate_for_layout(
564 |layout| Global.allocate(layout),
565 |mem| mem as *mut ArcInner<mem::MaybeUninit<T>>,
570 /// Constructs a new `Arc` with uninitialized contents, with the memory
571 /// being filled with `0` bytes, returning an error if allocation fails.
573 /// See [`MaybeUninit::zeroed`][zeroed] for examples of correct and incorrect usage
579 /// #![feature(new_uninit, allocator_api)]
581 /// use std::sync::Arc;
583 /// let zero = Arc::<u32>::try_new_zeroed()?;
584 /// let zero = unsafe { zero.assume_init() };
586 /// assert_eq!(*zero, 0);
587 /// # Ok::<(), std::alloc::AllocError>(())
590 /// [zeroed]: mem::MaybeUninit::zeroed
591 #[unstable(feature = "allocator_api", issue = "32838")]
592 // #[unstable(feature = "new_uninit", issue = "63291")]
593 pub fn try_new_zeroed() -> Result<Arc<mem::MaybeUninit<T>>, AllocError> {
595 Ok(Arc::from_ptr(Arc::try_allocate_for_layout(
597 |layout| Global.allocate_zeroed(layout),
598 |mem| mem as *mut ArcInner<mem::MaybeUninit<T>>,
602 /// Returns the inner value, if the `Arc` has exactly one strong reference.
604 /// Otherwise, an [`Err`] is returned with the same `Arc` that was
607 /// This will succeed even if there are outstanding weak references.
612 /// use std::sync::Arc;
614 /// let x = Arc::new(3);
615 /// assert_eq!(Arc::try_unwrap(x), Ok(3));
617 /// let x = Arc::new(4);
618 /// let _y = Arc::clone(&x);
619 /// assert_eq!(*Arc::try_unwrap(x).unwrap_err(), 4);
622 #[stable(feature = "arc_unique", since = "1.4.0")]
623 pub fn try_unwrap(this: Self) -> Result<T, Self> {
624 if this.inner().strong.compare_exchange(1, 0, Relaxed, Relaxed).is_err() {
628 acquire!(this.inner().strong);
631 let elem = ptr::read(&this.ptr.as_ref().data);
633 // Make a weak pointer to clean up the implicit strong-weak reference
634 let _weak = Weak { ptr: this.ptr };
643 /// Constructs a new atomically reference-counted slice with uninitialized contents.
648 /// #![feature(new_uninit)]
649 /// #![feature(get_mut_unchecked)]
651 /// use std::sync::Arc;
653 /// let mut values = Arc::<[u32]>::new_uninit_slice(3);
655 /// let values = unsafe {
656 /// // Deferred initialization:
657 /// Arc::get_mut_unchecked(&mut values)[0].as_mut_ptr().write(1);
658 /// Arc::get_mut_unchecked(&mut values)[1].as_mut_ptr().write(2);
659 /// Arc::get_mut_unchecked(&mut values)[2].as_mut_ptr().write(3);
661 /// values.assume_init()
664 /// assert_eq!(*values, [1, 2, 3])
666 #[cfg(not(no_global_oom_handling))]
667 #[unstable(feature = "new_uninit", issue = "63291")]
669 pub fn new_uninit_slice(len: usize) -> Arc<[mem::MaybeUninit<T>]> {
670 unsafe { Arc::from_ptr(Arc::allocate_for_slice(len)) }
673 /// Constructs a new atomically reference-counted slice with uninitialized contents, with the memory being
674 /// filled with `0` bytes.
676 /// See [`MaybeUninit::zeroed`][zeroed] for examples of correct and
677 /// incorrect usage of this method.
682 /// #![feature(new_uninit)]
684 /// use std::sync::Arc;
686 /// let values = Arc::<[u32]>::new_zeroed_slice(3);
687 /// let values = unsafe { values.assume_init() };
689 /// assert_eq!(*values, [0, 0, 0])
692 /// [zeroed]: mem::MaybeUninit::zeroed
693 #[cfg(not(no_global_oom_handling))]
694 #[unstable(feature = "new_uninit", issue = "63291")]
696 pub fn new_zeroed_slice(len: usize) -> Arc<[mem::MaybeUninit<T>]> {
698 Arc::from_ptr(Arc::allocate_for_layout(
699 Layout::array::<T>(len).unwrap(),
700 |layout| Global.allocate_zeroed(layout),
702 ptr::slice_from_raw_parts_mut(mem as *mut T, len)
703 as *mut ArcInner<[mem::MaybeUninit<T>]>
710 impl<T> Arc<mem::MaybeUninit<T>> {
711 /// Converts to `Arc<T>`.
715 /// As with [`MaybeUninit::assume_init`],
716 /// it is up to the caller to guarantee that the inner value
717 /// really is in an initialized state.
718 /// Calling this when the content is not yet fully initialized
719 /// causes immediate undefined behavior.
721 /// [`MaybeUninit::assume_init`]: mem::MaybeUninit::assume_init
726 /// #![feature(new_uninit)]
727 /// #![feature(get_mut_unchecked)]
729 /// use std::sync::Arc;
731 /// let mut five = Arc::<u32>::new_uninit();
733 /// let five = unsafe {
734 /// // Deferred initialization:
735 /// Arc::get_mut_unchecked(&mut five).as_mut_ptr().write(5);
737 /// five.assume_init()
740 /// assert_eq!(*five, 5)
742 #[unstable(feature = "new_uninit", issue = "63291")]
743 #[must_use = "`self` will be dropped if the result is not used"]
745 pub unsafe fn assume_init(self) -> Arc<T> {
746 Arc::from_inner(mem::ManuallyDrop::new(self).ptr.cast())
750 impl<T> Arc<[mem::MaybeUninit<T>]> {
751 /// Converts to `Arc<[T]>`.
755 /// As with [`MaybeUninit::assume_init`],
756 /// it is up to the caller to guarantee that the inner value
757 /// really is in an initialized state.
758 /// Calling this when the content is not yet fully initialized
759 /// causes immediate undefined behavior.
761 /// [`MaybeUninit::assume_init`]: mem::MaybeUninit::assume_init
766 /// #![feature(new_uninit)]
767 /// #![feature(get_mut_unchecked)]
769 /// use std::sync::Arc;
771 /// let mut values = Arc::<[u32]>::new_uninit_slice(3);
773 /// let values = unsafe {
774 /// // Deferred initialization:
775 /// Arc::get_mut_unchecked(&mut values)[0].as_mut_ptr().write(1);
776 /// Arc::get_mut_unchecked(&mut values)[1].as_mut_ptr().write(2);
777 /// Arc::get_mut_unchecked(&mut values)[2].as_mut_ptr().write(3);
779 /// values.assume_init()
782 /// assert_eq!(*values, [1, 2, 3])
784 #[unstable(feature = "new_uninit", issue = "63291")]
785 #[must_use = "`self` will be dropped if the result is not used"]
787 pub unsafe fn assume_init(self) -> Arc<[T]> {
788 unsafe { Arc::from_ptr(mem::ManuallyDrop::new(self).ptr.as_ptr() as _) }
792 impl<T: ?Sized> Arc<T> {
793 /// Consumes the `Arc`, returning the wrapped pointer.
795 /// To avoid a memory leak the pointer must be converted back to an `Arc` using
796 /// [`Arc::from_raw`].
801 /// use std::sync::Arc;
803 /// let x = Arc::new("hello".to_owned());
804 /// let x_ptr = Arc::into_raw(x);
805 /// assert_eq!(unsafe { &*x_ptr }, "hello");
807 #[stable(feature = "rc_raw", since = "1.17.0")]
808 pub fn into_raw(this: Self) -> *const T {
809 let ptr = Self::as_ptr(&this);
814 /// Provides a raw pointer to the data.
816 /// The counts are not affected in any way and the `Arc` is not consumed. The pointer is valid for
817 /// as long as there are strong counts in the `Arc`.
822 /// use std::sync::Arc;
824 /// let x = Arc::new("hello".to_owned());
825 /// let y = Arc::clone(&x);
826 /// let x_ptr = Arc::as_ptr(&x);
827 /// assert_eq!(x_ptr, Arc::as_ptr(&y));
828 /// assert_eq!(unsafe { &*x_ptr }, "hello");
831 #[stable(feature = "rc_as_ptr", since = "1.45.0")]
832 pub fn as_ptr(this: &Self) -> *const T {
833 let ptr: *mut ArcInner<T> = NonNull::as_ptr(this.ptr);
835 // SAFETY: This cannot go through Deref::deref or RcBoxPtr::inner because
836 // this is required to retain raw/mut provenance such that e.g. `get_mut` can
837 // write through the pointer after the Rc is recovered through `from_raw`.
838 unsafe { ptr::addr_of_mut!((*ptr).data) }
841 /// Constructs an `Arc<T>` from a raw pointer.
843 /// The raw pointer must have been previously returned by a call to
844 /// [`Arc<U>::into_raw`][into_raw] where `U` must have the same size and
845 /// alignment as `T`. This is trivially true if `U` is `T`.
846 /// Note that if `U` is not `T` but has the same size and alignment, this is
847 /// basically like transmuting references of different types. See
848 /// [`mem::transmute`][transmute] for more information on what
849 /// restrictions apply in this case.
851 /// The user of `from_raw` has to make sure a specific value of `T` is only
854 /// This function is unsafe because improper use may lead to memory unsafety,
855 /// even if the returned `Arc<T>` is never accessed.
857 /// [into_raw]: Arc::into_raw
858 /// [transmute]: core::mem::transmute
863 /// use std::sync::Arc;
865 /// let x = Arc::new("hello".to_owned());
866 /// let x_ptr = Arc::into_raw(x);
869 /// // Convert back to an `Arc` to prevent leak.
870 /// let x = Arc::from_raw(x_ptr);
871 /// assert_eq!(&*x, "hello");
873 /// // Further calls to `Arc::from_raw(x_ptr)` would be memory-unsafe.
876 /// // The memory was freed when `x` went out of scope above, so `x_ptr` is now dangling!
878 #[stable(feature = "rc_raw", since = "1.17.0")]
879 pub unsafe fn from_raw(ptr: *const T) -> Self {
881 let offset = data_offset(ptr);
883 // Reverse the offset to find the original ArcInner.
884 let arc_ptr = (ptr as *mut ArcInner<T>).set_ptr_value((ptr as *mut u8).offset(-offset));
886 Self::from_ptr(arc_ptr)
890 /// Creates a new [`Weak`] pointer to this allocation.
895 /// use std::sync::Arc;
897 /// let five = Arc::new(5);
899 /// let weak_five = Arc::downgrade(&five);
901 #[must_use = "this returns a new `Weak` pointer, \
902 without modifying the original `Arc`"]
903 #[stable(feature = "arc_weak", since = "1.4.0")]
904 pub fn downgrade(this: &Self) -> Weak<T> {
905 // This Relaxed is OK because we're checking the value in the CAS
907 let mut cur = this.inner().weak.load(Relaxed);
910 // check if the weak counter is currently "locked"; if so, spin.
911 if cur == usize::MAX {
913 cur = this.inner().weak.load(Relaxed);
917 // NOTE: this code currently ignores the possibility of overflow
918 // into usize::MAX; in general both Rc and Arc need to be adjusted
919 // to deal with overflow.
921 // Unlike with Clone(), we need this to be an Acquire read to
922 // synchronize with the write coming from `is_unique`, so that the
923 // events prior to that write happen before this read.
924 match this.inner().weak.compare_exchange_weak(cur, cur + 1, Acquire, Relaxed) {
926 // Make sure we do not create a dangling Weak
927 debug_assert!(!is_dangling(this.ptr.as_ptr()));
928 return Weak { ptr: this.ptr };
930 Err(old) => cur = old,
935 /// Gets the number of [`Weak`] pointers to this allocation.
939 /// This method by itself is safe, but using it correctly requires extra care.
940 /// Another thread can change the weak count at any time,
941 /// including potentially between calling this method and acting on the result.
946 /// use std::sync::Arc;
948 /// let five = Arc::new(5);
949 /// let _weak_five = Arc::downgrade(&five);
951 /// // This assertion is deterministic because we haven't shared
952 /// // the `Arc` or `Weak` between threads.
953 /// assert_eq!(1, Arc::weak_count(&five));
956 #[stable(feature = "arc_counts", since = "1.15.0")]
957 pub fn weak_count(this: &Self) -> usize {
958 let cnt = this.inner().weak.load(SeqCst);
959 // If the weak count is currently locked, the value of the
960 // count was 0 just before taking the lock.
961 if cnt == usize::MAX { 0 } else { cnt - 1 }
964 /// Gets the number of strong (`Arc`) pointers to this allocation.
968 /// This method by itself is safe, but using it correctly requires extra care.
969 /// Another thread can change the strong count at any time,
970 /// including potentially between calling this method and acting on the result.
975 /// use std::sync::Arc;
977 /// let five = Arc::new(5);
978 /// let _also_five = Arc::clone(&five);
980 /// // This assertion is deterministic because we haven't shared
981 /// // the `Arc` between threads.
982 /// assert_eq!(2, Arc::strong_count(&five));
985 #[stable(feature = "arc_counts", since = "1.15.0")]
986 pub fn strong_count(this: &Self) -> usize {
987 this.inner().strong.load(SeqCst)
990 /// Increments the strong reference count on the `Arc<T>` associated with the
991 /// provided pointer by one.
995 /// The pointer must have been obtained through `Arc::into_raw`, and the
996 /// associated `Arc` instance must be valid (i.e. the strong count must be at
997 /// least 1) for the duration of this method.
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 /// // This assertion is 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));
1017 #[stable(feature = "arc_mutate_strong_count", since = "1.51.0")]
1018 pub unsafe fn increment_strong_count(ptr: *const T) {
1019 // Retain Arc, but don't touch refcount by wrapping in ManuallyDrop
1020 let arc = unsafe { mem::ManuallyDrop::new(Arc::<T>::from_raw(ptr)) };
1021 // Now increase refcount, but don't drop new refcount either
1022 let _arc_clone: mem::ManuallyDrop<_> = arc.clone();
1025 /// Decrements the strong reference count on the `Arc<T>` associated with the
1026 /// provided pointer by one.
1030 /// The pointer must have been obtained through `Arc::into_raw`, and the
1031 /// associated `Arc` instance must be valid (i.e. the strong count must be at
1032 /// least 1) when invoking this method. This method can be used to release the final
1033 /// `Arc` and backing storage, but **should not** be called after the final `Arc` has been
1039 /// use std::sync::Arc;
1041 /// let five = Arc::new(5);
1044 /// let ptr = Arc::into_raw(five);
1045 /// Arc::increment_strong_count(ptr);
1047 /// // Those assertions are deterministic because we haven't shared
1048 /// // the `Arc` between threads.
1049 /// let five = Arc::from_raw(ptr);
1050 /// assert_eq!(2, Arc::strong_count(&five));
1051 /// Arc::decrement_strong_count(ptr);
1052 /// assert_eq!(1, Arc::strong_count(&five));
1056 #[stable(feature = "arc_mutate_strong_count", since = "1.51.0")]
1057 pub unsafe fn decrement_strong_count(ptr: *const T) {
1058 unsafe { mem::drop(Arc::from_raw(ptr)) };
1062 fn inner(&self) -> &ArcInner<T> {
1063 // This unsafety is ok because while this arc is alive we're guaranteed
1064 // that the inner pointer is valid. Furthermore, we know that the
1065 // `ArcInner` structure itself is `Sync` because the inner data is
1066 // `Sync` as well, so we're ok loaning out an immutable pointer to these
1068 unsafe { self.ptr.as_ref() }
1071 // Non-inlined part of `drop`.
1073 unsafe fn drop_slow(&mut self) {
1074 // Destroy the data at this time, even though we must not free the box
1075 // allocation itself (there might still be weak pointers lying around).
1076 unsafe { ptr::drop_in_place(Self::get_mut_unchecked(self)) };
1078 // Drop the weak ref collectively held by all strong references
1079 drop(Weak { ptr: self.ptr });
1083 #[stable(feature = "ptr_eq", since = "1.17.0")]
1084 /// Returns `true` if the two `Arc`s point to the same allocation
1085 /// (in a vein similar to [`ptr::eq`]).
1090 /// use std::sync::Arc;
1092 /// let five = Arc::new(5);
1093 /// let same_five = Arc::clone(&five);
1094 /// let other_five = Arc::new(5);
1096 /// assert!(Arc::ptr_eq(&five, &same_five));
1097 /// assert!(!Arc::ptr_eq(&five, &other_five));
1100 /// [`ptr::eq`]: core::ptr::eq "ptr::eq"
1101 pub fn ptr_eq(this: &Self, other: &Self) -> bool {
1102 this.ptr.as_ptr() == other.ptr.as_ptr()
1106 impl<T: ?Sized> Arc<T> {
1107 /// Allocates an `ArcInner<T>` with sufficient space for
1108 /// a possibly-unsized inner value where the value has the layout provided.
1110 /// The function `mem_to_arcinner` is called with the data pointer
1111 /// and must return back a (potentially fat)-pointer for the `ArcInner<T>`.
1112 #[cfg(not(no_global_oom_handling))]
1113 unsafe fn allocate_for_layout(
1114 value_layout: Layout,
1115 allocate: impl FnOnce(Layout) -> Result<NonNull<[u8]>, AllocError>,
1116 mem_to_arcinner: impl FnOnce(*mut u8) -> *mut ArcInner<T>,
1117 ) -> *mut ArcInner<T> {
1118 // Calculate layout using the given value layout.
1119 // Previously, layout was calculated on the expression
1120 // `&*(ptr as *const ArcInner<T>)`, but this created a misaligned
1121 // reference (see #54908).
1122 let layout = Layout::new::<ArcInner<()>>().extend(value_layout).unwrap().0.pad_to_align();
1124 Arc::try_allocate_for_layout(value_layout, allocate, mem_to_arcinner)
1125 .unwrap_or_else(|_| handle_alloc_error(layout))
1129 /// Allocates an `ArcInner<T>` with sufficient space for
1130 /// a possibly-unsized inner value where the value has the layout provided,
1131 /// returning an error if allocation fails.
1133 /// The function `mem_to_arcinner` is called with the data pointer
1134 /// and must return back a (potentially fat)-pointer for the `ArcInner<T>`.
1135 unsafe fn try_allocate_for_layout(
1136 value_layout: Layout,
1137 allocate: impl FnOnce(Layout) -> Result<NonNull<[u8]>, AllocError>,
1138 mem_to_arcinner: impl FnOnce(*mut u8) -> *mut ArcInner<T>,
1139 ) -> Result<*mut ArcInner<T>, AllocError> {
1140 // Calculate layout using the given value layout.
1141 // Previously, layout was calculated on the expression
1142 // `&*(ptr as *const ArcInner<T>)`, but this created a misaligned
1143 // reference (see #54908).
1144 let layout = Layout::new::<ArcInner<()>>().extend(value_layout).unwrap().0.pad_to_align();
1146 let ptr = allocate(layout)?;
1148 // Initialize the ArcInner
1149 let inner = mem_to_arcinner(ptr.as_non_null_ptr().as_ptr());
1150 debug_assert_eq!(unsafe { Layout::for_value(&*inner) }, layout);
1153 ptr::write(&mut (*inner).strong, atomic::AtomicUsize::new(1));
1154 ptr::write(&mut (*inner).weak, atomic::AtomicUsize::new(1));
1160 /// Allocates an `ArcInner<T>` with sufficient space for an unsized inner value.
1161 #[cfg(not(no_global_oom_handling))]
1162 unsafe fn allocate_for_ptr(ptr: *const T) -> *mut ArcInner<T> {
1163 // Allocate for the `ArcInner<T>` using the given value.
1165 Self::allocate_for_layout(
1166 Layout::for_value(&*ptr),
1167 |layout| Global.allocate(layout),
1168 |mem| (ptr as *mut ArcInner<T>).set_ptr_value(mem) as *mut ArcInner<T>,
1173 #[cfg(not(no_global_oom_handling))]
1174 fn from_box(v: Box<T>) -> Arc<T> {
1176 let (box_unique, alloc) = Box::into_unique(v);
1177 let bptr = box_unique.as_ptr();
1179 let value_size = size_of_val(&*bptr);
1180 let ptr = Self::allocate_for_ptr(bptr);
1182 // Copy value as bytes
1183 ptr::copy_nonoverlapping(
1184 bptr as *const T as *const u8,
1185 &mut (*ptr).data as *mut _ as *mut u8,
1189 // Free the allocation without dropping its contents
1190 box_free(box_unique, alloc);
1198 /// Allocates an `ArcInner<[T]>` with the given length.
1199 #[cfg(not(no_global_oom_handling))]
1200 unsafe fn allocate_for_slice(len: usize) -> *mut ArcInner<[T]> {
1202 Self::allocate_for_layout(
1203 Layout::array::<T>(len).unwrap(),
1204 |layout| Global.allocate(layout),
1205 |mem| ptr::slice_from_raw_parts_mut(mem as *mut T, len) as *mut ArcInner<[T]>,
1210 /// Copy elements from slice into newly allocated Arc<\[T\]>
1212 /// Unsafe because the caller must either take ownership or bind `T: Copy`.
1213 #[cfg(not(no_global_oom_handling))]
1214 unsafe fn copy_from_slice(v: &[T]) -> Arc<[T]> {
1216 let ptr = Self::allocate_for_slice(v.len());
1218 ptr::copy_nonoverlapping(v.as_ptr(), &mut (*ptr).data as *mut [T] as *mut T, v.len());
1224 /// Constructs an `Arc<[T]>` from an iterator known to be of a certain size.
1226 /// Behavior is undefined should the size be wrong.
1227 #[cfg(not(no_global_oom_handling))]
1228 unsafe fn from_iter_exact(iter: impl iter::Iterator<Item = T>, len: usize) -> Arc<[T]> {
1229 // Panic guard while cloning T elements.
1230 // In the event of a panic, elements that have been written
1231 // into the new ArcInner will be dropped, then the memory freed.
1239 impl<T> Drop for Guard<T> {
1240 fn drop(&mut self) {
1242 let slice = from_raw_parts_mut(self.elems, self.n_elems);
1243 ptr::drop_in_place(slice);
1245 Global.deallocate(self.mem, self.layout);
1251 let ptr = Self::allocate_for_slice(len);
1253 let mem = ptr as *mut _ as *mut u8;
1254 let layout = Layout::for_value(&*ptr);
1256 // Pointer to first element
1257 let elems = &mut (*ptr).data as *mut [T] as *mut T;
1259 let mut guard = Guard { mem: NonNull::new_unchecked(mem), elems, layout, n_elems: 0 };
1261 for (i, item) in iter.enumerate() {
1262 ptr::write(elems.add(i), item);
1266 // All clear. Forget the guard so it doesn't free the new ArcInner.
1274 /// Specialization trait used for `From<&[T]>`.
1275 #[cfg(not(no_global_oom_handling))]
1276 trait ArcFromSlice<T> {
1277 fn from_slice(slice: &[T]) -> Self;
1280 #[cfg(not(no_global_oom_handling))]
1281 impl<T: Clone> ArcFromSlice<T> for Arc<[T]> {
1283 default fn from_slice(v: &[T]) -> Self {
1284 unsafe { Self::from_iter_exact(v.iter().cloned(), v.len()) }
1288 #[cfg(not(no_global_oom_handling))]
1289 impl<T: Copy> ArcFromSlice<T> for Arc<[T]> {
1291 fn from_slice(v: &[T]) -> Self {
1292 unsafe { Arc::copy_from_slice(v) }
1296 #[stable(feature = "rust1", since = "1.0.0")]
1297 impl<T: ?Sized> Clone for Arc<T> {
1298 /// Makes a clone of the `Arc` pointer.
1300 /// This creates another pointer to the same allocation, increasing the
1301 /// strong reference count.
1306 /// use std::sync::Arc;
1308 /// let five = Arc::new(5);
1310 /// let _ = Arc::clone(&five);
1313 fn clone(&self) -> Arc<T> {
1314 // Using a relaxed ordering is alright here, as knowledge of the
1315 // original reference prevents other threads from erroneously deleting
1318 // As explained in the [Boost documentation][1], Increasing the
1319 // reference counter can always be done with memory_order_relaxed: New
1320 // references to an object can only be formed from an existing
1321 // reference, and passing an existing reference from one thread to
1322 // another must already provide any required synchronization.
1324 // [1]: (www.boost.org/doc/libs/1_55_0/doc/html/atomic/usage_examples.html)
1325 let old_size = self.inner().strong.fetch_add(1, Relaxed);
1327 // However we need to guard against massive refcounts in case someone
1328 // is `mem::forget`ing Arcs. If we don't do this the count can overflow
1329 // and users will use-after free. We racily saturate to `isize::MAX` on
1330 // the assumption that there aren't ~2 billion threads incrementing
1331 // the reference count at once. This branch will never be taken in
1332 // any realistic program.
1334 // We abort because such a program is incredibly degenerate, and we
1335 // don't care to support it.
1336 if old_size > MAX_REFCOUNT {
1340 Self::from_inner(self.ptr)
1344 #[stable(feature = "rust1", since = "1.0.0")]
1345 impl<T: ?Sized> Deref for Arc<T> {
1349 fn deref(&self) -> &T {
1354 #[unstable(feature = "receiver_trait", issue = "none")]
1355 impl<T: ?Sized> Receiver for Arc<T> {}
1357 impl<T: Clone> Arc<T> {
1358 /// Makes a mutable reference into the given `Arc`.
1360 /// If there are other `Arc` pointers to the same allocation, then `make_mut` will
1361 /// [`clone`] the inner value to a new allocation to ensure unique ownership. This is also
1362 /// referred to as clone-on-write.
1364 /// However, if there are no other `Arc` pointers to this allocation, but some [`Weak`]
1365 /// pointers, then the [`Weak`] pointers will be disassociated and the inner value will not
1368 /// See also [`get_mut`], which will fail rather than cloning the inner value
1369 /// or diassociating [`Weak`] pointers.
1371 /// [`clone`]: Clone::clone
1372 /// [`get_mut`]: Arc::get_mut
1377 /// use std::sync::Arc;
1379 /// let mut data = Arc::new(5);
1381 /// *Arc::make_mut(&mut data) += 1; // Won't clone anything
1382 /// let mut other_data = Arc::clone(&data); // Won't clone inner data
1383 /// *Arc::make_mut(&mut data) += 1; // Clones inner data
1384 /// *Arc::make_mut(&mut data) += 1; // Won't clone anything
1385 /// *Arc::make_mut(&mut other_data) *= 2; // Won't clone anything
1387 /// // Now `data` and `other_data` point to different allocations.
1388 /// assert_eq!(*data, 8);
1389 /// assert_eq!(*other_data, 12);
1392 /// [`Weak`] pointers will be disassociated:
1395 /// use std::sync::Arc;
1397 /// let mut data = Arc::new(75);
1398 /// let weak = Arc::downgrade(&data);
1400 /// assert!(75 == *data);
1401 /// assert!(75 == *weak.upgrade().unwrap());
1403 /// *Arc::make_mut(&mut data) += 1;
1405 /// assert!(76 == *data);
1406 /// assert!(weak.upgrade().is_none());
1408 #[cfg(not(no_global_oom_handling))]
1410 #[stable(feature = "arc_unique", since = "1.4.0")]
1411 pub fn make_mut(this: &mut Self) -> &mut T {
1412 // Note that we hold both a strong reference and a weak reference.
1413 // Thus, releasing our strong reference only will not, by itself, cause
1414 // the memory to be deallocated.
1416 // Use Acquire to ensure that we see any writes to `weak` that happen
1417 // before release writes (i.e., decrements) to `strong`. Since we hold a
1418 // weak count, there's no chance the ArcInner itself could be
1420 if this.inner().strong.compare_exchange(1, 0, Acquire, Relaxed).is_err() {
1421 // Another strong pointer exists, so we must clone.
1422 // Pre-allocate memory to allow writing the cloned value directly.
1423 let mut arc = Self::new_uninit();
1425 let data = Arc::get_mut_unchecked(&mut arc);
1426 (**this).write_clone_into_raw(data.as_mut_ptr());
1427 *this = arc.assume_init();
1429 } else if this.inner().weak.load(Relaxed) != 1 {
1430 // Relaxed suffices in the above because this is fundamentally an
1431 // optimization: we are always racing with weak pointers being
1432 // dropped. Worst case, we end up allocated a new Arc unnecessarily.
1434 // We removed the last strong ref, but there are additional weak
1435 // refs remaining. We'll move the contents to a new Arc, and
1436 // invalidate the other weak refs.
1438 // Note that it is not possible for the read of `weak` to yield
1439 // usize::MAX (i.e., locked), since the weak count can only be
1440 // locked by a thread with a strong reference.
1442 // Materialize our own implicit weak pointer, so that it can clean
1443 // up the ArcInner as needed.
1444 let _weak = Weak { ptr: this.ptr };
1446 // Can just steal the data, all that's left is Weaks
1447 let mut arc = Self::new_uninit();
1449 let data = Arc::get_mut_unchecked(&mut arc);
1450 data.as_mut_ptr().copy_from_nonoverlapping(&**this, 1);
1451 ptr::write(this, arc.assume_init());
1454 // We were the sole reference of either kind; bump back up the
1455 // strong ref count.
1456 this.inner().strong.store(1, Release);
1459 // As with `get_mut()`, the unsafety is ok because our reference was
1460 // either unique to begin with, or became one upon cloning the contents.
1461 unsafe { Self::get_mut_unchecked(this) }
1465 impl<T: ?Sized> Arc<T> {
1466 /// Returns a mutable reference into the given `Arc`, if there are
1467 /// no other `Arc` or [`Weak`] pointers to the same allocation.
1469 /// Returns [`None`] otherwise, because it is not safe to
1470 /// mutate a shared value.
1472 /// See also [`make_mut`][make_mut], which will [`clone`][clone]
1473 /// the inner value when there are other `Arc` pointers.
1475 /// [make_mut]: Arc::make_mut
1476 /// [clone]: Clone::clone
1481 /// use std::sync::Arc;
1483 /// let mut x = Arc::new(3);
1484 /// *Arc::get_mut(&mut x).unwrap() = 4;
1485 /// assert_eq!(*x, 4);
1487 /// let _y = Arc::clone(&x);
1488 /// assert!(Arc::get_mut(&mut x).is_none());
1491 #[stable(feature = "arc_unique", since = "1.4.0")]
1492 pub fn get_mut(this: &mut Self) -> Option<&mut T> {
1493 if this.is_unique() {
1494 // This unsafety is ok because we're guaranteed that the pointer
1495 // returned is the *only* pointer that will ever be returned to T. Our
1496 // reference count is guaranteed to be 1 at this point, and we required
1497 // the Arc itself to be `mut`, so we're returning the only possible
1498 // reference to the inner data.
1499 unsafe { Some(Arc::get_mut_unchecked(this)) }
1505 /// Returns a mutable reference into the given `Arc`,
1506 /// without any check.
1508 /// See also [`get_mut`], which is safe and does appropriate checks.
1510 /// [`get_mut`]: Arc::get_mut
1514 /// Any other `Arc` or [`Weak`] pointers to the same allocation must not be dereferenced
1515 /// for the duration of the returned borrow.
1516 /// This is trivially the case if no such pointers exist,
1517 /// for example immediately after `Arc::new`.
1522 /// #![feature(get_mut_unchecked)]
1524 /// use std::sync::Arc;
1526 /// let mut x = Arc::new(String::new());
1528 /// Arc::get_mut_unchecked(&mut x).push_str("foo")
1530 /// assert_eq!(*x, "foo");
1533 #[unstable(feature = "get_mut_unchecked", issue = "63292")]
1534 pub unsafe fn get_mut_unchecked(this: &mut Self) -> &mut T {
1535 // We are careful to *not* create a reference covering the "count" fields, as
1536 // this would alias with concurrent access to the reference counts (e.g. by `Weak`).
1537 unsafe { &mut (*this.ptr.as_ptr()).data }
1540 /// Determine whether this is the unique reference (including weak refs) to
1541 /// the underlying data.
1543 /// Note that this requires locking the weak ref count.
1544 fn is_unique(&mut self) -> bool {
1545 // lock the weak pointer count if we appear to be the sole weak pointer
1548 // The acquire label here ensures a happens-before relationship with any
1549 // writes to `strong` (in particular in `Weak::upgrade`) prior to decrements
1550 // of the `weak` count (via `Weak::drop`, which uses release). If the upgraded
1551 // weak ref was never dropped, the CAS here will fail so we do not care to synchronize.
1552 if self.inner().weak.compare_exchange(1, usize::MAX, Acquire, Relaxed).is_ok() {
1553 // This needs to be an `Acquire` to synchronize with the decrement of the `strong`
1554 // counter in `drop` -- the only access that happens when any but the last reference
1555 // is being dropped.
1556 let unique = self.inner().strong.load(Acquire) == 1;
1558 // The release write here synchronizes with a read in `downgrade`,
1559 // effectively preventing the above read of `strong` from happening
1561 self.inner().weak.store(1, Release); // release the lock
1569 #[stable(feature = "rust1", since = "1.0.0")]
1570 unsafe impl<#[may_dangle] T: ?Sized> Drop for Arc<T> {
1571 /// Drops the `Arc`.
1573 /// This will decrement the strong reference count. If the strong reference
1574 /// count reaches zero then the only other references (if any) are
1575 /// [`Weak`], so we `drop` the inner value.
1580 /// use std::sync::Arc;
1584 /// impl Drop for Foo {
1585 /// fn drop(&mut self) {
1586 /// println!("dropped!");
1590 /// let foo = Arc::new(Foo);
1591 /// let foo2 = Arc::clone(&foo);
1593 /// drop(foo); // Doesn't print anything
1594 /// drop(foo2); // Prints "dropped!"
1597 fn drop(&mut self) {
1598 // Because `fetch_sub` is already atomic, we do not need to synchronize
1599 // with other threads unless we are going to delete the object. This
1600 // same logic applies to the below `fetch_sub` to the `weak` count.
1601 if self.inner().strong.fetch_sub(1, Release) != 1 {
1605 // This fence is needed to prevent reordering of use of the data and
1606 // deletion of the data. Because it is marked `Release`, the decreasing
1607 // of the reference count synchronizes with this `Acquire` fence. This
1608 // means that use of the data happens before decreasing the reference
1609 // count, which happens before this fence, which happens before the
1610 // deletion of the data.
1612 // As explained in the [Boost documentation][1],
1614 // > It is important to enforce any possible access to the object in one
1615 // > thread (through an existing reference) to *happen before* deleting
1616 // > the object in a different thread. This is achieved by a "release"
1617 // > operation after dropping a reference (any access to the object
1618 // > through this reference must obviously happened before), and an
1619 // > "acquire" operation before deleting the object.
1621 // In particular, while the contents of an Arc are usually immutable, it's
1622 // possible to have interior writes to something like a Mutex<T>. Since a
1623 // Mutex is not acquired when it is deleted, we can't rely on its
1624 // synchronization logic to make writes in thread A visible to a destructor
1625 // running in thread B.
1627 // Also note that the Acquire fence here could probably be replaced with an
1628 // Acquire load, which could improve performance in highly-contended
1629 // situations. See [2].
1631 // [1]: (www.boost.org/doc/libs/1_55_0/doc/html/atomic/usage_examples.html)
1632 // [2]: (https://github.com/rust-lang/rust/pull/41714)
1633 acquire!(self.inner().strong);
1641 impl Arc<dyn Any + Send + Sync> {
1643 #[stable(feature = "rc_downcast", since = "1.29.0")]
1644 /// Attempt to downcast the `Arc<dyn Any + Send + Sync>` to a concrete type.
1649 /// use std::any::Any;
1650 /// use std::sync::Arc;
1652 /// fn print_if_string(value: Arc<dyn Any + Send + Sync>) {
1653 /// if let Ok(string) = value.downcast::<String>() {
1654 /// println!("String ({}): {}", string.len(), string);
1658 /// let my_string = "Hello World".to_string();
1659 /// print_if_string(Arc::new(my_string));
1660 /// print_if_string(Arc::new(0i8));
1662 pub fn downcast<T>(self) -> Result<Arc<T>, Self>
1664 T: Any + Send + Sync + 'static,
1666 if (*self).is::<T>() {
1667 let ptr = self.ptr.cast::<ArcInner<T>>();
1669 Ok(Arc::from_inner(ptr))
1677 /// Constructs a new `Weak<T>`, without allocating any memory.
1678 /// Calling [`upgrade`] on the return value always gives [`None`].
1680 /// [`upgrade`]: Weak::upgrade
1685 /// use std::sync::Weak;
1687 /// let empty: Weak<i64> = Weak::new();
1688 /// assert!(empty.upgrade().is_none());
1690 #[stable(feature = "downgraded_weak", since = "1.10.0")]
1692 pub fn new() -> Weak<T> {
1693 Weak { ptr: NonNull::new(usize::MAX as *mut ArcInner<T>).expect("MAX is not 0") }
1697 /// Helper type to allow accessing the reference counts without
1698 /// making any assertions about the data field.
1699 struct WeakInner<'a> {
1700 weak: &'a atomic::AtomicUsize,
1701 strong: &'a atomic::AtomicUsize,
1704 impl<T: ?Sized> Weak<T> {
1705 /// Returns a raw pointer to the object `T` pointed to by this `Weak<T>`.
1707 /// The pointer is valid only if there are some strong references. The pointer may be dangling,
1708 /// unaligned or even [`null`] otherwise.
1713 /// use std::sync::Arc;
1716 /// let strong = Arc::new("hello".to_owned());
1717 /// let weak = Arc::downgrade(&strong);
1718 /// // Both point to the same object
1719 /// assert!(ptr::eq(&*strong, weak.as_ptr()));
1720 /// // The strong here keeps it alive, so we can still access the object.
1721 /// assert_eq!("hello", unsafe { &*weak.as_ptr() });
1724 /// // But not any more. We can do weak.as_ptr(), but accessing the pointer would lead to
1725 /// // undefined behaviour.
1726 /// // assert_eq!("hello", unsafe { &*weak.as_ptr() });
1729 /// [`null`]: core::ptr::null "ptr::null"
1731 #[stable(feature = "weak_into_raw", since = "1.45.0")]
1732 pub fn as_ptr(&self) -> *const T {
1733 let ptr: *mut ArcInner<T> = NonNull::as_ptr(self.ptr);
1735 if is_dangling(ptr) {
1736 // If the pointer is dangling, we return the sentinel directly. This cannot be
1737 // a valid payload address, as the payload is at least as aligned as ArcInner (usize).
1740 // SAFETY: if is_dangling returns false, then the pointer is dereferencable.
1741 // The payload may be dropped at this point, and we have to maintain provenance,
1742 // so use raw pointer manipulation.
1743 unsafe { ptr::addr_of_mut!((*ptr).data) }
1747 /// Consumes the `Weak<T>` and turns it into a raw pointer.
1749 /// This converts the weak pointer into a raw pointer, while still preserving the ownership of
1750 /// one weak reference (the weak count is not modified by this operation). It can be turned
1751 /// back into the `Weak<T>` with [`from_raw`].
1753 /// The same restrictions of accessing the target of the pointer as with
1754 /// [`as_ptr`] apply.
1759 /// use std::sync::{Arc, Weak};
1761 /// let strong = Arc::new("hello".to_owned());
1762 /// let weak = Arc::downgrade(&strong);
1763 /// let raw = weak.into_raw();
1765 /// assert_eq!(1, Arc::weak_count(&strong));
1766 /// assert_eq!("hello", unsafe { &*raw });
1768 /// drop(unsafe { Weak::from_raw(raw) });
1769 /// assert_eq!(0, Arc::weak_count(&strong));
1772 /// [`from_raw`]: Weak::from_raw
1773 /// [`as_ptr`]: Weak::as_ptr
1774 #[must_use = "`self` will be dropped if the result is not used"]
1775 #[stable(feature = "weak_into_raw", since = "1.45.0")]
1776 pub fn into_raw(self) -> *const T {
1777 let result = self.as_ptr();
1782 /// Converts a raw pointer previously created by [`into_raw`] back into `Weak<T>`.
1784 /// This can be used to safely get a strong reference (by calling [`upgrade`]
1785 /// later) or to deallocate the weak count by dropping the `Weak<T>`.
1787 /// It takes ownership of one weak reference (with the exception of pointers created by [`new`],
1788 /// as these don't own anything; the method still works on them).
1792 /// The pointer must have originated from the [`into_raw`] and must still own its potential
1795 /// It is allowed for the strong count to be 0 at the time of calling this. Nevertheless, this
1796 /// takes ownership of one weak reference currently represented as a raw pointer (the weak
1797 /// count is not modified by this operation) and therefore it must be paired with a previous
1798 /// call to [`into_raw`].
1802 /// use std::sync::{Arc, Weak};
1804 /// let strong = Arc::new("hello".to_owned());
1806 /// let raw_1 = Arc::downgrade(&strong).into_raw();
1807 /// let raw_2 = Arc::downgrade(&strong).into_raw();
1809 /// assert_eq!(2, Arc::weak_count(&strong));
1811 /// assert_eq!("hello", &*unsafe { Weak::from_raw(raw_1) }.upgrade().unwrap());
1812 /// assert_eq!(1, Arc::weak_count(&strong));
1816 /// // Decrement the last weak count.
1817 /// assert!(unsafe { Weak::from_raw(raw_2) }.upgrade().is_none());
1820 /// [`new`]: Weak::new
1821 /// [`into_raw`]: Weak::into_raw
1822 /// [`upgrade`]: Weak::upgrade
1823 #[stable(feature = "weak_into_raw", since = "1.45.0")]
1824 pub unsafe fn from_raw(ptr: *const T) -> Self {
1825 // See Weak::as_ptr for context on how the input pointer is derived.
1827 let ptr = if is_dangling(ptr as *mut T) {
1828 // This is a dangling Weak.
1829 ptr as *mut ArcInner<T>
1831 // Otherwise, we're guaranteed the pointer came from a nondangling Weak.
1832 // SAFETY: data_offset is safe to call, as ptr references a real (potentially dropped) T.
1833 let offset = unsafe { data_offset(ptr) };
1834 // Thus, we reverse the offset to get the whole RcBox.
1835 // SAFETY: the pointer originated from a Weak, so this offset is safe.
1836 unsafe { (ptr as *mut ArcInner<T>).set_ptr_value((ptr as *mut u8).offset(-offset)) }
1839 // SAFETY: we now have recovered the original Weak pointer, so can create the Weak.
1840 Weak { ptr: unsafe { NonNull::new_unchecked(ptr) } }
1844 impl<T: ?Sized> Weak<T> {
1845 /// Attempts to upgrade the `Weak` pointer to an [`Arc`], delaying
1846 /// dropping of the inner value if successful.
1848 /// Returns [`None`] if the inner value has since been dropped.
1853 /// use std::sync::Arc;
1855 /// let five = Arc::new(5);
1857 /// let weak_five = Arc::downgrade(&five);
1859 /// let strong_five: Option<Arc<_>> = weak_five.upgrade();
1860 /// assert!(strong_five.is_some());
1862 /// // Destroy all strong pointers.
1863 /// drop(strong_five);
1866 /// assert!(weak_five.upgrade().is_none());
1868 #[must_use = "this returns a new `Arc`, \
1869 without modifying the original weak pointer"]
1870 #[stable(feature = "arc_weak", since = "1.4.0")]
1871 pub fn upgrade(&self) -> Option<Arc<T>> {
1872 // We use a CAS loop to increment the strong count instead of a
1873 // fetch_add as this function should never take the reference count
1874 // from zero to one.
1875 let inner = self.inner()?;
1877 // Relaxed load because any write of 0 that we can observe
1878 // leaves the field in a permanently zero state (so a
1879 // "stale" read of 0 is fine), and any other value is
1880 // confirmed via the CAS below.
1881 let mut n = inner.strong.load(Relaxed);
1888 // See comments in `Arc::clone` for why we do this (for `mem::forget`).
1889 if n > MAX_REFCOUNT {
1893 // Relaxed is fine for the failure case because we don't have any expectations about the new state.
1894 // Acquire is necessary for the success case to synchronise with `Arc::new_cyclic`, when the inner
1895 // value can be initialized after `Weak` references have already been created. In that case, we
1896 // expect to observe the fully initialized value.
1897 match inner.strong.compare_exchange_weak(n, n + 1, Acquire, Relaxed) {
1898 Ok(_) => return Some(Arc::from_inner(self.ptr)), // null checked above
1899 Err(old) => n = old,
1904 /// Gets the number of strong (`Arc`) pointers pointing to this allocation.
1906 /// If `self` was created using [`Weak::new`], this will return 0.
1907 #[stable(feature = "weak_counts", since = "1.41.0")]
1908 pub fn strong_count(&self) -> usize {
1909 if let Some(inner) = self.inner() { inner.strong.load(SeqCst) } else { 0 }
1912 /// Gets an approximation of the number of `Weak` pointers pointing to this
1915 /// If `self` was created using [`Weak::new`], or if there are no remaining
1916 /// strong pointers, this will return 0.
1920 /// Due to implementation details, the returned value can be off by 1 in
1921 /// either direction when other threads are manipulating any `Arc`s or
1922 /// `Weak`s pointing to the same allocation.
1923 #[stable(feature = "weak_counts", since = "1.41.0")]
1924 pub fn weak_count(&self) -> usize {
1927 let weak = inner.weak.load(SeqCst);
1928 let strong = inner.strong.load(SeqCst);
1932 // Since we observed that there was at least one strong pointer
1933 // after reading the weak count, we know that the implicit weak
1934 // reference (present whenever any strong references are alive)
1935 // was still around when we observed the weak count, and can
1936 // therefore safely subtract it.
1943 /// Returns `None` when the pointer is dangling and there is no allocated `ArcInner`,
1944 /// (i.e., when this `Weak` was created by `Weak::new`).
1946 fn inner(&self) -> Option<WeakInner<'_>> {
1947 if is_dangling(self.ptr.as_ptr()) {
1950 // We are careful to *not* create a reference covering the "data" field, as
1951 // the field may be mutated concurrently (for example, if the last `Arc`
1952 // is dropped, the data field will be dropped in-place).
1954 let ptr = self.ptr.as_ptr();
1955 WeakInner { strong: &(*ptr).strong, weak: &(*ptr).weak }
1960 /// Returns `true` if the two `Weak`s point to the same allocation (similar to
1961 /// [`ptr::eq`]), or if both don't point to any allocation
1962 /// (because they were created with `Weak::new()`).
1966 /// Since this compares pointers it means that `Weak::new()` will equal each
1967 /// other, even though they don't point to any allocation.
1972 /// use std::sync::Arc;
1974 /// let first_rc = Arc::new(5);
1975 /// let first = Arc::downgrade(&first_rc);
1976 /// let second = Arc::downgrade(&first_rc);
1978 /// assert!(first.ptr_eq(&second));
1980 /// let third_rc = Arc::new(5);
1981 /// let third = Arc::downgrade(&third_rc);
1983 /// assert!(!first.ptr_eq(&third));
1986 /// Comparing `Weak::new`.
1989 /// use std::sync::{Arc, Weak};
1991 /// let first = Weak::new();
1992 /// let second = Weak::new();
1993 /// assert!(first.ptr_eq(&second));
1995 /// let third_rc = Arc::new(());
1996 /// let third = Arc::downgrade(&third_rc);
1997 /// assert!(!first.ptr_eq(&third));
2000 /// [`ptr::eq`]: core::ptr::eq "ptr::eq"
2002 #[stable(feature = "weak_ptr_eq", since = "1.39.0")]
2003 pub fn ptr_eq(&self, other: &Self) -> bool {
2004 self.ptr.as_ptr() == other.ptr.as_ptr()
2008 #[stable(feature = "arc_weak", since = "1.4.0")]
2009 impl<T: ?Sized> Clone for Weak<T> {
2010 /// Makes a clone of the `Weak` pointer that points to the same allocation.
2015 /// use std::sync::{Arc, Weak};
2017 /// let weak_five = Arc::downgrade(&Arc::new(5));
2019 /// let _ = Weak::clone(&weak_five);
2022 fn clone(&self) -> Weak<T> {
2023 let inner = if let Some(inner) = self.inner() {
2026 return Weak { ptr: self.ptr };
2028 // See comments in Arc::clone() for why this is relaxed. This can use a
2029 // fetch_add (ignoring the lock) because the weak count is only locked
2030 // where are *no other* weak pointers in existence. (So we can't be
2031 // running this code in that case).
2032 let old_size = inner.weak.fetch_add(1, Relaxed);
2034 // See comments in Arc::clone() for why we do this (for mem::forget).
2035 if old_size > MAX_REFCOUNT {
2039 Weak { ptr: self.ptr }
2043 #[stable(feature = "downgraded_weak", since = "1.10.0")]
2044 impl<T> Default for Weak<T> {
2045 /// Constructs a new `Weak<T>`, without allocating memory.
2046 /// Calling [`upgrade`] on the return value always
2049 /// [`upgrade`]: Weak::upgrade
2054 /// use std::sync::Weak;
2056 /// let empty: Weak<i64> = Default::default();
2057 /// assert!(empty.upgrade().is_none());
2059 fn default() -> Weak<T> {
2064 #[stable(feature = "arc_weak", since = "1.4.0")]
2065 unsafe impl<#[may_dangle] T: ?Sized> Drop for Weak<T> {
2066 /// Drops the `Weak` pointer.
2071 /// use std::sync::{Arc, Weak};
2075 /// impl Drop for Foo {
2076 /// fn drop(&mut self) {
2077 /// println!("dropped!");
2081 /// let foo = Arc::new(Foo);
2082 /// let weak_foo = Arc::downgrade(&foo);
2083 /// let other_weak_foo = Weak::clone(&weak_foo);
2085 /// drop(weak_foo); // Doesn't print anything
2086 /// drop(foo); // Prints "dropped!"
2088 /// assert!(other_weak_foo.upgrade().is_none());
2090 fn drop(&mut self) {
2091 // If we find out that we were the last weak pointer, then its time to
2092 // deallocate the data entirely. See the discussion in Arc::drop() about
2093 // the memory orderings
2095 // It's not necessary to check for the locked state here, because the
2096 // weak count can only be locked if there was precisely one weak ref,
2097 // meaning that drop could only subsequently run ON that remaining weak
2098 // ref, which can only happen after the lock is released.
2099 let inner = if let Some(inner) = self.inner() { inner } else { return };
2101 if inner.weak.fetch_sub(1, Release) == 1 {
2102 acquire!(inner.weak);
2103 unsafe { Global.deallocate(self.ptr.cast(), Layout::for_value_raw(self.ptr.as_ptr())) }
2108 #[stable(feature = "rust1", since = "1.0.0")]
2109 trait ArcEqIdent<T: ?Sized + PartialEq> {
2110 fn eq(&self, other: &Arc<T>) -> bool;
2111 fn ne(&self, other: &Arc<T>) -> bool;
2114 #[stable(feature = "rust1", since = "1.0.0")]
2115 impl<T: ?Sized + PartialEq> ArcEqIdent<T> for Arc<T> {
2117 default fn eq(&self, other: &Arc<T>) -> bool {
2121 default fn ne(&self, other: &Arc<T>) -> bool {
2126 /// We're doing this specialization here, and not as a more general optimization on `&T`, because it
2127 /// would otherwise add a cost to all equality checks on refs. We assume that `Arc`s are used to
2128 /// store large values, that are slow to clone, but also heavy to check for equality, causing this
2129 /// cost to pay off more easily. It's also more likely to have two `Arc` clones, that point to
2130 /// the same value, than two `&T`s.
2132 /// We can only do this when `T: Eq` as a `PartialEq` might be deliberately irreflexive.
2133 #[stable(feature = "rust1", since = "1.0.0")]
2134 impl<T: ?Sized + crate::rc::MarkerEq> ArcEqIdent<T> for Arc<T> {
2136 fn eq(&self, other: &Arc<T>) -> bool {
2137 Arc::ptr_eq(self, other) || **self == **other
2141 fn ne(&self, other: &Arc<T>) -> bool {
2142 !Arc::ptr_eq(self, other) && **self != **other
2146 #[stable(feature = "rust1", since = "1.0.0")]
2147 impl<T: ?Sized + PartialEq> PartialEq for Arc<T> {
2148 /// Equality for two `Arc`s.
2150 /// Two `Arc`s are equal if their inner values are equal, even if they are
2151 /// stored in different allocation.
2153 /// If `T` also implements `Eq` (implying reflexivity of equality),
2154 /// two `Arc`s that point to the same allocation are always equal.
2159 /// use std::sync::Arc;
2161 /// let five = Arc::new(5);
2163 /// assert!(five == Arc::new(5));
2166 fn eq(&self, other: &Arc<T>) -> bool {
2167 ArcEqIdent::eq(self, other)
2170 /// Inequality for two `Arc`s.
2172 /// Two `Arc`s are unequal if their inner values are unequal.
2174 /// If `T` also implements `Eq` (implying reflexivity of equality),
2175 /// two `Arc`s that point to the same value are never unequal.
2180 /// use std::sync::Arc;
2182 /// let five = Arc::new(5);
2184 /// assert!(five != Arc::new(6));
2187 fn ne(&self, other: &Arc<T>) -> bool {
2188 ArcEqIdent::ne(self, other)
2192 #[stable(feature = "rust1", since = "1.0.0")]
2193 impl<T: ?Sized + PartialOrd> PartialOrd for Arc<T> {
2194 /// Partial comparison for two `Arc`s.
2196 /// The two are compared by calling `partial_cmp()` on their inner values.
2201 /// use std::sync::Arc;
2202 /// use std::cmp::Ordering;
2204 /// let five = Arc::new(5);
2206 /// assert_eq!(Some(Ordering::Less), five.partial_cmp(&Arc::new(6)));
2208 fn partial_cmp(&self, other: &Arc<T>) -> Option<Ordering> {
2209 (**self).partial_cmp(&**other)
2212 /// Less-than comparison for two `Arc`s.
2214 /// The two are compared by calling `<` on their inner values.
2219 /// use std::sync::Arc;
2221 /// let five = Arc::new(5);
2223 /// assert!(five < Arc::new(6));
2225 fn lt(&self, other: &Arc<T>) -> bool {
2226 *(*self) < *(*other)
2229 /// 'Less than or equal to' comparison for two `Arc`s.
2231 /// The two are compared by calling `<=` on their inner values.
2236 /// use std::sync::Arc;
2238 /// let five = Arc::new(5);
2240 /// assert!(five <= Arc::new(5));
2242 fn le(&self, other: &Arc<T>) -> bool {
2243 *(*self) <= *(*other)
2246 /// Greater-than comparison for two `Arc`s.
2248 /// The two are compared by calling `>` on their inner values.
2253 /// use std::sync::Arc;
2255 /// let five = Arc::new(5);
2257 /// assert!(five > Arc::new(4));
2259 fn gt(&self, other: &Arc<T>) -> bool {
2260 *(*self) > *(*other)
2263 /// 'Greater than or equal to' comparison for two `Arc`s.
2265 /// The two are compared by calling `>=` on their inner values.
2270 /// use std::sync::Arc;
2272 /// let five = Arc::new(5);
2274 /// assert!(five >= Arc::new(5));
2276 fn ge(&self, other: &Arc<T>) -> bool {
2277 *(*self) >= *(*other)
2280 #[stable(feature = "rust1", since = "1.0.0")]
2281 impl<T: ?Sized + Ord> Ord for Arc<T> {
2282 /// Comparison for two `Arc`s.
2284 /// The two are compared by calling `cmp()` on their inner values.
2289 /// use std::sync::Arc;
2290 /// use std::cmp::Ordering;
2292 /// let five = Arc::new(5);
2294 /// assert_eq!(Ordering::Less, five.cmp(&Arc::new(6)));
2296 fn cmp(&self, other: &Arc<T>) -> Ordering {
2297 (**self).cmp(&**other)
2300 #[stable(feature = "rust1", since = "1.0.0")]
2301 impl<T: ?Sized + Eq> Eq for Arc<T> {}
2303 #[stable(feature = "rust1", since = "1.0.0")]
2304 impl<T: ?Sized + fmt::Display> fmt::Display for Arc<T> {
2305 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
2306 fmt::Display::fmt(&**self, f)
2310 #[stable(feature = "rust1", since = "1.0.0")]
2311 impl<T: ?Sized + fmt::Debug> fmt::Debug for Arc<T> {
2312 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
2313 fmt::Debug::fmt(&**self, f)
2317 #[stable(feature = "rust1", since = "1.0.0")]
2318 impl<T: ?Sized> fmt::Pointer for Arc<T> {
2319 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
2320 fmt::Pointer::fmt(&(&**self as *const T), f)
2324 #[cfg(not(no_global_oom_handling))]
2325 #[stable(feature = "rust1", since = "1.0.0")]
2326 impl<T: Default> Default for Arc<T> {
2327 /// Creates a new `Arc<T>`, with the `Default` value for `T`.
2332 /// use std::sync::Arc;
2334 /// let x: Arc<i32> = Default::default();
2335 /// assert_eq!(*x, 0);
2337 fn default() -> Arc<T> {
2338 Arc::new(Default::default())
2342 #[stable(feature = "rust1", since = "1.0.0")]
2343 impl<T: ?Sized + Hash> Hash for Arc<T> {
2344 fn hash<H: Hasher>(&self, state: &mut H) {
2345 (**self).hash(state)
2349 #[cfg(not(no_global_oom_handling))]
2350 #[stable(feature = "from_for_ptrs", since = "1.6.0")]
2351 impl<T> From<T> for Arc<T> {
2352 /// Converts a `T` into an `Arc<T>`
2354 /// The conversion moves the value into a
2355 /// newly allocated `Arc`. It is equivalent to
2356 /// calling `Arc::new(t)`.
2360 /// # use std::sync::Arc;
2362 /// let arc = Arc::new(5);
2364 /// assert_eq!(Arc::from(x), arc);
2366 fn from(t: T) -> Self {
2371 #[cfg(not(no_global_oom_handling))]
2372 #[stable(feature = "shared_from_slice", since = "1.21.0")]
2373 impl<T: Clone> From<&[T]> for Arc<[T]> {
2374 /// Allocate a reference-counted slice and fill it by cloning `v`'s items.
2379 /// # use std::sync::Arc;
2380 /// let original: &[i32] = &[1, 2, 3];
2381 /// let shared: Arc<[i32]> = Arc::from(original);
2382 /// assert_eq!(&[1, 2, 3], &shared[..]);
2385 fn from(v: &[T]) -> Arc<[T]> {
2386 <Self as ArcFromSlice<T>>::from_slice(v)
2390 #[cfg(not(no_global_oom_handling))]
2391 #[stable(feature = "shared_from_slice", since = "1.21.0")]
2392 impl From<&str> for Arc<str> {
2393 /// Allocate a reference-counted `str` and copy `v` into it.
2398 /// # use std::sync::Arc;
2399 /// let shared: Arc<str> = Arc::from("eggplant");
2400 /// assert_eq!("eggplant", &shared[..]);
2403 fn from(v: &str) -> Arc<str> {
2404 let arc = Arc::<[u8]>::from(v.as_bytes());
2405 unsafe { Arc::from_raw(Arc::into_raw(arc) as *const str) }
2409 #[cfg(not(no_global_oom_handling))]
2410 #[stable(feature = "shared_from_slice", since = "1.21.0")]
2411 impl From<String> for Arc<str> {
2412 /// Allocate a reference-counted `str` and copy `v` into it.
2417 /// # use std::sync::Arc;
2418 /// let unique: String = "eggplant".to_owned();
2419 /// let shared: Arc<str> = Arc::from(unique);
2420 /// assert_eq!("eggplant", &shared[..]);
2423 fn from(v: String) -> Arc<str> {
2428 #[cfg(not(no_global_oom_handling))]
2429 #[stable(feature = "shared_from_slice", since = "1.21.0")]
2430 impl<T: ?Sized> From<Box<T>> for Arc<T> {
2431 /// Move a boxed object to a new, reference-counted allocation.
2436 /// # use std::sync::Arc;
2437 /// let unique: Box<str> = Box::from("eggplant");
2438 /// let shared: Arc<str> = Arc::from(unique);
2439 /// assert_eq!("eggplant", &shared[..]);
2442 fn from(v: Box<T>) -> Arc<T> {
2447 #[cfg(not(no_global_oom_handling))]
2448 #[stable(feature = "shared_from_slice", since = "1.21.0")]
2449 impl<T> From<Vec<T>> for Arc<[T]> {
2450 /// Allocate a reference-counted slice and move `v`'s items into it.
2455 /// # use std::sync::Arc;
2456 /// let unique: Vec<i32> = vec![1, 2, 3];
2457 /// let shared: Arc<[i32]> = Arc::from(unique);
2458 /// assert_eq!(&[1, 2, 3], &shared[..]);
2461 fn from(mut v: Vec<T>) -> Arc<[T]> {
2463 let arc = Arc::copy_from_slice(&v);
2465 // Allow the Vec to free its memory, but not destroy its contents
2473 #[stable(feature = "shared_from_cow", since = "1.45.0")]
2474 impl<'a, B> From<Cow<'a, B>> for Arc<B>
2476 B: ToOwned + ?Sized,
2477 Arc<B>: From<&'a B> + From<B::Owned>,
2479 /// Create an atomically reference-counted pointer from
2480 /// a clone-on-write pointer by copying its content.
2485 /// # use std::sync::Arc;
2486 /// # use std::borrow::Cow;
2487 /// let cow: Cow<str> = Cow::Borrowed("eggplant");
2488 /// let shared: Arc<str> = Arc::from(cow);
2489 /// assert_eq!("eggplant", &shared[..]);
2492 fn from(cow: Cow<'a, B>) -> Arc<B> {
2494 Cow::Borrowed(s) => Arc::from(s),
2495 Cow::Owned(s) => Arc::from(s),
2500 #[stable(feature = "boxed_slice_try_from", since = "1.43.0")]
2501 impl<T, const N: usize> TryFrom<Arc<[T]>> for Arc<[T; N]> {
2502 type Error = Arc<[T]>;
2504 fn try_from(boxed_slice: Arc<[T]>) -> Result<Self, Self::Error> {
2505 if boxed_slice.len() == N {
2506 Ok(unsafe { Arc::from_raw(Arc::into_raw(boxed_slice) as *mut [T; N]) })
2513 #[cfg(not(no_global_oom_handling))]
2514 #[stable(feature = "shared_from_iter", since = "1.37.0")]
2515 impl<T> iter::FromIterator<T> for Arc<[T]> {
2516 /// Takes each element in the `Iterator` and collects it into an `Arc<[T]>`.
2518 /// # Performance characteristics
2520 /// ## The general case
2522 /// In the general case, collecting into `Arc<[T]>` is done by first
2523 /// collecting into a `Vec<T>`. That is, when writing the following:
2526 /// # use std::sync::Arc;
2527 /// let evens: Arc<[u8]> = (0..10).filter(|&x| x % 2 == 0).collect();
2528 /// # assert_eq!(&*evens, &[0, 2, 4, 6, 8]);
2531 /// this behaves as if we wrote:
2534 /// # use std::sync::Arc;
2535 /// let evens: Arc<[u8]> = (0..10).filter(|&x| x % 2 == 0)
2536 /// .collect::<Vec<_>>() // The first set of allocations happens here.
2537 /// .into(); // A second allocation for `Arc<[T]>` happens here.
2538 /// # assert_eq!(&*evens, &[0, 2, 4, 6, 8]);
2541 /// This will allocate as many times as needed for constructing the `Vec<T>`
2542 /// and then it will allocate once for turning the `Vec<T>` into the `Arc<[T]>`.
2544 /// ## Iterators of known length
2546 /// When your `Iterator` implements `TrustedLen` and is of an exact size,
2547 /// a single allocation will be made for the `Arc<[T]>`. For example:
2550 /// # use std::sync::Arc;
2551 /// let evens: Arc<[u8]> = (0..10).collect(); // Just a single allocation happens here.
2552 /// # assert_eq!(&*evens, &*(0..10).collect::<Vec<_>>());
2554 fn from_iter<I: iter::IntoIterator<Item = T>>(iter: I) -> Self {
2555 ToArcSlice::to_arc_slice(iter.into_iter())
2559 /// Specialization trait used for collecting into `Arc<[T]>`.
2560 trait ToArcSlice<T>: Iterator<Item = T> + Sized {
2561 fn to_arc_slice(self) -> Arc<[T]>;
2564 #[cfg(not(no_global_oom_handling))]
2565 impl<T, I: Iterator<Item = T>> ToArcSlice<T> for I {
2566 default fn to_arc_slice(self) -> Arc<[T]> {
2567 self.collect::<Vec<T>>().into()
2571 #[cfg(not(no_global_oom_handling))]
2572 impl<T, I: iter::TrustedLen<Item = T>> ToArcSlice<T> for I {
2573 fn to_arc_slice(self) -> Arc<[T]> {
2574 // This is the case for a `TrustedLen` iterator.
2575 let (low, high) = self.size_hint();
2576 if let Some(high) = high {
2580 "TrustedLen iterator's size hint is not exact: {:?}",
2585 // SAFETY: We need to ensure that the iterator has an exact length and we have.
2586 Arc::from_iter_exact(self, low)
2589 // TrustedLen contract guarantees that `upper_bound == `None` implies an iterator
2590 // length exceeding `usize::MAX`.
2591 // The default implementation would collect into a vec which would panic.
2592 // Thus we panic here immediately without invoking `Vec` code.
2593 panic!("capacity overflow");
2598 #[stable(feature = "rust1", since = "1.0.0")]
2599 impl<T: ?Sized> borrow::Borrow<T> for Arc<T> {
2600 fn borrow(&self) -> &T {
2605 #[stable(since = "1.5.0", feature = "smart_ptr_as_ref")]
2606 impl<T: ?Sized> AsRef<T> for Arc<T> {
2607 fn as_ref(&self) -> &T {
2612 #[stable(feature = "pin", since = "1.33.0")]
2613 impl<T: ?Sized> Unpin for Arc<T> {}
2615 /// Get the offset within an `ArcInner` for the payload behind a pointer.
2619 /// The pointer must point to (and have valid metadata for) a previously
2620 /// valid instance of T, but the T is allowed to be dropped.
2621 unsafe fn data_offset<T: ?Sized>(ptr: *const T) -> isize {
2622 // Align the unsized value to the end of the ArcInner.
2623 // Because RcBox is repr(C), it will always be the last field in memory.
2624 // SAFETY: since the only unsized types possible are slices, trait objects,
2625 // and extern types, the input safety requirement is currently enough to
2626 // satisfy the requirements of align_of_val_raw; this is an implementation
2627 // detail of the language that must not be relied upon outside of std.
2628 unsafe { data_offset_align(align_of_val_raw(ptr)) }
2632 fn data_offset_align(align: usize) -> isize {
2633 let layout = Layout::new::<ArcInner<()>>();
2634 (layout.size() + layout.padding_needed_for(align)) as isize