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};
23 use core::ptr::{self, NonNull};
24 #[cfg(not(no_global_oom_handling))]
25 use core::slice::from_raw_parts_mut;
26 use core::sync::atomic;
27 use core::sync::atomic::Ordering::{Acquire, Relaxed, Release, SeqCst};
29 #[cfg(not(no_global_oom_handling))]
30 use crate::alloc::handle_alloc_error;
31 #[cfg(not(no_global_oom_handling))]
32 use crate::alloc::{box_free, WriteCloneIntoRaw};
33 use crate::alloc::{AllocError, Allocator, Global, Layout};
34 use crate::borrow::{Cow, ToOwned};
35 use crate::boxed::Box;
36 use crate::rc::is_dangling;
37 #[cfg(not(no_global_oom_handling))]
38 use crate::string::String;
39 #[cfg(not(no_global_oom_handling))]
45 /// A soft limit on the amount of references that may be made to an `Arc`.
47 /// Going above this limit will abort your program (although not
48 /// necessarily) at _exactly_ `MAX_REFCOUNT + 1` references.
49 const MAX_REFCOUNT: usize = (isize::MAX) as usize;
51 #[cfg(not(sanitize = "thread"))]
52 macro_rules! acquire {
54 atomic::fence(Acquire)
58 // ThreadSanitizer does not support memory fences. To avoid false positive
59 // reports in Arc / Weak implementation use atomic loads for synchronization
61 #[cfg(sanitize = "thread")]
62 macro_rules! acquire {
68 /// A thread-safe reference-counting pointer. 'Arc' stands for 'Atomically
69 /// Reference Counted'.
71 /// The type `Arc<T>` provides shared ownership of a value of type `T`,
72 /// allocated in the heap. Invoking [`clone`][clone] on `Arc` produces
73 /// a new `Arc` instance, which points to the same allocation on the heap as the
74 /// source `Arc`, while increasing a reference count. When the last `Arc`
75 /// pointer to a given allocation is destroyed, the value stored in that allocation (often
76 /// referred to as "inner value") is also dropped.
78 /// Shared references in Rust disallow mutation by default, and `Arc` is no
79 /// exception: you cannot generally obtain a mutable reference to something
80 /// inside an `Arc`. If you need to mutate through an `Arc`, use
81 /// [`Mutex`][mutex], [`RwLock`][rwlock], or one of the [`Atomic`][atomic]
86 /// Unlike [`Rc<T>`], `Arc<T>` uses atomic operations for its reference
87 /// counting. This means that it is thread-safe. The disadvantage is that
88 /// atomic operations are more expensive than ordinary memory accesses. If you
89 /// are not sharing reference-counted allocations between threads, consider using
90 /// [`Rc<T>`] for lower overhead. [`Rc<T>`] is a safe default, because the
91 /// compiler will catch any attempt to send an [`Rc<T>`] between threads.
92 /// However, a library might choose `Arc<T>` in order to give library consumers
95 /// `Arc<T>` will implement [`Send`] and [`Sync`] as long as the `T` implements
96 /// [`Send`] and [`Sync`]. Why can't you put a non-thread-safe type `T` in an
97 /// `Arc<T>` to make it thread-safe? This may be a bit counter-intuitive at
98 /// first: after all, isn't the point of `Arc<T>` thread safety? The key is
99 /// this: `Arc<T>` makes it thread safe to have multiple ownership of the same
100 /// data, but it doesn't add thread safety to its data. Consider
101 /// `Arc<`[`RefCell<T>`]`>`. [`RefCell<T>`] isn't [`Sync`], and if `Arc<T>` was always
102 /// [`Send`], `Arc<`[`RefCell<T>`]`>` would be as well. But then we'd have a problem:
103 /// [`RefCell<T>`] is not thread safe; it keeps track of the borrowing count using
104 /// non-atomic operations.
106 /// In the end, this means that you may need to pair `Arc<T>` with some sort of
107 /// [`std::sync`] type, usually [`Mutex<T>`][mutex].
109 /// ## Breaking cycles with `Weak`
111 /// The [`downgrade`][downgrade] method can be used to create a non-owning
112 /// [`Weak`] pointer. A [`Weak`] pointer can be [`upgrade`][upgrade]d
113 /// to an `Arc`, but this will return [`None`] if the value stored in the allocation has
114 /// already been dropped. In other words, `Weak` pointers do not keep the value
115 /// inside the allocation alive; however, they *do* keep the allocation
116 /// (the backing store for the value) alive.
118 /// A cycle between `Arc` pointers will never be deallocated. For this reason,
119 /// [`Weak`] is used to break cycles. For example, a tree could have
120 /// strong `Arc` pointers from parent nodes to children, and [`Weak`]
121 /// pointers from children back to their parents.
123 /// # Cloning references
125 /// Creating a new reference from an existing reference-counted pointer is done using the
126 /// `Clone` trait implemented for [`Arc<T>`][Arc] and [`Weak<T>`][Weak].
129 /// use std::sync::Arc;
130 /// let foo = Arc::new(vec![1.0, 2.0, 3.0]);
131 /// // The two syntaxes below are equivalent.
132 /// let a = foo.clone();
133 /// let b = Arc::clone(&foo);
134 /// // a, b, and foo are all Arcs that point to the same memory location
137 /// ## `Deref` behavior
139 /// `Arc<T>` automatically dereferences to `T` (via the [`Deref`][deref] trait),
140 /// so you can call `T`'s methods on a value of type `Arc<T>`. To avoid name
141 /// clashes with `T`'s methods, the methods of `Arc<T>` itself are associated
142 /// functions, called using [fully qualified syntax]:
145 /// use std::sync::Arc;
147 /// let my_arc = Arc::new(());
148 /// Arc::downgrade(&my_arc);
151 /// `Arc<T>`'s implementations of traits like `Clone` may also be called using
152 /// fully qualified syntax. Some people prefer to use fully qualified syntax,
153 /// while others prefer using method-call syntax.
156 /// use std::sync::Arc;
158 /// let arc = Arc::new(());
159 /// // Method-call syntax
160 /// let arc2 = arc.clone();
161 /// // Fully qualified syntax
162 /// let arc3 = Arc::clone(&arc);
165 /// [`Weak<T>`][Weak] does not auto-dereference to `T`, because the inner value may have
166 /// already been dropped.
168 /// [`Rc<T>`]: crate::rc::Rc
169 /// [clone]: Clone::clone
170 /// [mutex]: ../../std/sync/struct.Mutex.html
171 /// [rwlock]: ../../std/sync/struct.RwLock.html
172 /// [atomic]: core::sync::atomic
173 /// [`Send`]: core::marker::Send
174 /// [`Sync`]: core::marker::Sync
175 /// [deref]: core::ops::Deref
176 /// [downgrade]: Arc::downgrade
177 /// [upgrade]: Weak::upgrade
178 /// [`RefCell<T>`]: core::cell::RefCell
179 /// [`std::sync`]: ../../std/sync/index.html
180 /// [`Arc::clone(&from)`]: Arc::clone
181 /// [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
185 /// Sharing some immutable data between threads:
187 // Note that we **do not** run these tests here. The windows builders get super
188 // unhappy if a thread outlives the main thread and then exits at the same time
189 // (something deadlocks) so we just avoid this entirely by not running these
192 /// use std::sync::Arc;
195 /// let five = Arc::new(5);
198 /// let five = Arc::clone(&five);
200 /// thread::spawn(move || {
201 /// println!("{:?}", five);
206 /// Sharing a mutable [`AtomicUsize`]:
208 /// [`AtomicUsize`]: core::sync::atomic::AtomicUsize
211 /// use std::sync::Arc;
212 /// use std::sync::atomic::{AtomicUsize, Ordering};
215 /// let val = Arc::new(AtomicUsize::new(5));
218 /// let val = Arc::clone(&val);
220 /// thread::spawn(move || {
221 /// let v = val.fetch_add(1, Ordering::SeqCst);
222 /// println!("{:?}", v);
227 /// See the [`rc` documentation][rc_examples] for more examples of reference
228 /// counting in general.
230 /// [rc_examples]: crate::rc#examples
231 #[cfg_attr(not(test), rustc_diagnostic_item = "Arc")]
232 #[stable(feature = "rust1", since = "1.0.0")]
233 pub struct Arc<T: ?Sized> {
234 ptr: NonNull<ArcInner<T>>,
235 phantom: PhantomData<ArcInner<T>>,
238 #[stable(feature = "rust1", since = "1.0.0")]
239 unsafe impl<T: ?Sized + Sync + Send> Send for Arc<T> {}
240 #[stable(feature = "rust1", since = "1.0.0")]
241 unsafe impl<T: ?Sized + Sync + Send> Sync for Arc<T> {}
243 #[unstable(feature = "coerce_unsized", issue = "27732")]
244 impl<T: ?Sized + Unsize<U>, U: ?Sized> CoerceUnsized<Arc<U>> for Arc<T> {}
246 #[unstable(feature = "dispatch_from_dyn", issue = "none")]
247 impl<T: ?Sized + Unsize<U>, U: ?Sized> DispatchFromDyn<Arc<U>> for Arc<T> {}
249 impl<T: ?Sized> Arc<T> {
250 fn from_inner(ptr: NonNull<ArcInner<T>>) -> Self {
251 Self { ptr, phantom: PhantomData }
254 unsafe fn from_ptr(ptr: *mut ArcInner<T>) -> Self {
255 unsafe { Self::from_inner(NonNull::new_unchecked(ptr)) }
259 /// `Weak` is a version of [`Arc`] that holds a non-owning reference to the
260 /// managed allocation. The allocation is accessed by calling [`upgrade`] on the `Weak`
261 /// pointer, which returns an [`Option`]`<`[`Arc`]`<T>>`.
263 /// Since a `Weak` reference does not count towards ownership, it will not
264 /// prevent the value stored in the allocation from being dropped, and `Weak` itself makes no
265 /// guarantees about the value still being present. Thus it may return [`None`]
266 /// when [`upgrade`]d. Note however that a `Weak` reference *does* prevent the allocation
267 /// itself (the backing store) from being deallocated.
269 /// A `Weak` pointer is useful for keeping a temporary reference to the allocation
270 /// managed by [`Arc`] without preventing its inner value from being dropped. It is also used to
271 /// prevent circular references between [`Arc`] pointers, since mutual owning references
272 /// would never allow either [`Arc`] to be dropped. For example, a tree could
273 /// have strong [`Arc`] pointers from parent nodes to children, and `Weak`
274 /// pointers from children back to their parents.
276 /// The typical way to obtain a `Weak` pointer is to call [`Arc::downgrade`].
278 /// [`upgrade`]: Weak::upgrade
279 #[stable(feature = "arc_weak", since = "1.4.0")]
280 pub struct Weak<T: ?Sized> {
281 // This is a `NonNull` to allow optimizing the size of this type in enums,
282 // but it is not necessarily a valid pointer.
283 // `Weak::new` sets this to `usize::MAX` so that it doesn’t need
284 // to allocate space on the heap. That's not a value a real pointer
285 // will ever have because RcBox has alignment at least 2.
286 // This is only possible when `T: Sized`; unsized `T` never dangle.
287 ptr: NonNull<ArcInner<T>>,
290 #[stable(feature = "arc_weak", since = "1.4.0")]
291 unsafe impl<T: ?Sized + Sync + Send> Send for Weak<T> {}
292 #[stable(feature = "arc_weak", since = "1.4.0")]
293 unsafe impl<T: ?Sized + Sync + Send> Sync for Weak<T> {}
295 #[unstable(feature = "coerce_unsized", issue = "27732")]
296 impl<T: ?Sized + Unsize<U>, U: ?Sized> CoerceUnsized<Weak<U>> for Weak<T> {}
297 #[unstable(feature = "dispatch_from_dyn", issue = "none")]
298 impl<T: ?Sized + Unsize<U>, U: ?Sized> DispatchFromDyn<Weak<U>> for Weak<T> {}
300 #[stable(feature = "arc_weak", since = "1.4.0")]
301 impl<T: ?Sized + fmt::Debug> fmt::Debug for Weak<T> {
302 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
307 // This is repr(C) to future-proof against possible field-reordering, which
308 // would interfere with otherwise safe [into|from]_raw() of transmutable
311 struct ArcInner<T: ?Sized> {
312 strong: atomic::AtomicUsize,
314 // the value usize::MAX acts as a sentinel for temporarily "locking" the
315 // ability to upgrade weak pointers or downgrade strong ones; this is used
316 // to avoid races in `make_mut` and `get_mut`.
317 weak: atomic::AtomicUsize,
322 unsafe impl<T: ?Sized + Sync + Send> Send for ArcInner<T> {}
323 unsafe impl<T: ?Sized + Sync + Send> Sync for ArcInner<T> {}
326 /// Constructs a new `Arc<T>`.
331 /// use std::sync::Arc;
333 /// let five = Arc::new(5);
336 #[stable(feature = "rust1", since = "1.0.0")]
337 pub fn new(data: T) -> Arc<T> {
338 // Start the weak pointer count as 1 which is the weak pointer that's
339 // held by all the strong pointers (kinda), see std/rc.rs for more info
340 let x: Box<_> = box ArcInner {
341 strong: atomic::AtomicUsize::new(1),
342 weak: atomic::AtomicUsize::new(1),
345 Self::from_inner(Box::leak(x).into())
348 /// Constructs a new `Arc<T>` using a weak reference to itself. Attempting
349 /// to upgrade the weak reference before this function returns will result
350 /// in a `None` value. However, the weak reference may be cloned freely and
351 /// stored for use at a later time.
355 /// #![feature(arc_new_cyclic)]
356 /// #![allow(dead_code)]
358 /// use std::sync::{Arc, Weak};
364 /// let foo = Arc::new_cyclic(|me| Foo {
369 #[unstable(feature = "arc_new_cyclic", issue = "75861")]
370 pub fn new_cyclic(data_fn: impl FnOnce(&Weak<T>) -> T) -> Arc<T> {
371 // Construct the inner in the "uninitialized" state with a single
373 let uninit_ptr: NonNull<_> = Box::leak(box ArcInner {
374 strong: atomic::AtomicUsize::new(0),
375 weak: atomic::AtomicUsize::new(1),
376 data: mem::MaybeUninit::<T>::uninit(),
379 let init_ptr: NonNull<ArcInner<T>> = uninit_ptr.cast();
381 let weak = Weak { ptr: init_ptr };
383 // It's important we don't give up ownership of the weak pointer, or
384 // else the memory might be freed by the time `data_fn` returns. If
385 // we really wanted to pass ownership, we could create an additional
386 // weak pointer for ourselves, but this would result in additional
387 // updates to the weak reference count which might not be necessary
389 let data = data_fn(&weak);
391 // Now we can properly initialize the inner value and turn our weak
392 // reference into a strong reference.
394 let inner = init_ptr.as_ptr();
395 ptr::write(ptr::addr_of_mut!((*inner).data), data);
397 // The above write to the data field must be visible to any threads which
398 // observe a non-zero strong count. Therefore we need at least "Release" ordering
399 // in order to synchronize with the `compare_exchange_weak` in `Weak::upgrade`.
401 // "Acquire" ordering is not required. When considering the possible behaviours
402 // of `data_fn` we only need to look at what it could do with a reference to a
403 // non-upgradeable `Weak`:
404 // - It can *clone* the `Weak`, increasing the weak reference count.
405 // - It can drop those clones, decreasing the weak reference count (but never to zero).
407 // These side effects do not impact us in any way, and no other side effects are
408 // possible with safe code alone.
409 let prev_value = (*inner).strong.fetch_add(1, Release);
410 debug_assert_eq!(prev_value, 0, "No prior strong references should exist");
413 let strong = Arc::from_inner(init_ptr);
415 // Strong references should collectively own a shared weak reference,
416 // so don't run the destructor for our old weak reference.
421 /// Constructs a new `Arc` with uninitialized contents.
426 /// #![feature(new_uninit)]
427 /// #![feature(get_mut_unchecked)]
429 /// use std::sync::Arc;
431 /// let mut five = Arc::<u32>::new_uninit();
433 /// let five = unsafe {
434 /// // Deferred initialization:
435 /// Arc::get_mut_unchecked(&mut five).as_mut_ptr().write(5);
437 /// five.assume_init()
440 /// assert_eq!(*five, 5)
442 #[cfg(not(no_global_oom_handling))]
443 #[unstable(feature = "new_uninit", issue = "63291")]
444 pub fn new_uninit() -> Arc<mem::MaybeUninit<T>> {
446 Arc::from_ptr(Arc::allocate_for_layout(
448 |layout| Global.allocate(layout),
449 |mem| mem as *mut ArcInner<mem::MaybeUninit<T>>,
454 /// Constructs a new `Arc` with uninitialized contents, with the memory
455 /// being filled with `0` bytes.
457 /// See [`MaybeUninit::zeroed`][zeroed] for examples of correct and incorrect usage
463 /// #![feature(new_uninit)]
465 /// use std::sync::Arc;
467 /// let zero = Arc::<u32>::new_zeroed();
468 /// let zero = unsafe { zero.assume_init() };
470 /// assert_eq!(*zero, 0)
473 /// [zeroed]: ../../std/mem/union.MaybeUninit.html#method.zeroed
474 #[cfg(not(no_global_oom_handling))]
475 #[unstable(feature = "new_uninit", issue = "63291")]
476 pub fn new_zeroed() -> Arc<mem::MaybeUninit<T>> {
478 Arc::from_ptr(Arc::allocate_for_layout(
480 |layout| Global.allocate_zeroed(layout),
481 |mem| mem as *mut ArcInner<mem::MaybeUninit<T>>,
486 /// Constructs a new `Pin<Arc<T>>`. If `T` does not implement `Unpin`, then
487 /// `data` will be pinned in memory and unable to be moved.
488 #[stable(feature = "pin", since = "1.33.0")]
489 pub fn pin(data: T) -> Pin<Arc<T>> {
490 unsafe { Pin::new_unchecked(Arc::new(data)) }
493 /// Constructs a new `Arc<T>`, returning an error if allocation fails.
498 /// #![feature(allocator_api)]
499 /// use std::sync::Arc;
501 /// let five = Arc::try_new(5)?;
502 /// # Ok::<(), std::alloc::AllocError>(())
504 #[unstable(feature = "allocator_api", issue = "32838")]
506 pub fn try_new(data: T) -> Result<Arc<T>, AllocError> {
507 // Start the weak pointer count as 1 which is the weak pointer that's
508 // held by all the strong pointers (kinda), see std/rc.rs for more info
509 let x: Box<_> = Box::try_new(ArcInner {
510 strong: atomic::AtomicUsize::new(1),
511 weak: atomic::AtomicUsize::new(1),
514 Ok(Self::from_inner(Box::leak(x).into()))
517 /// Constructs a new `Arc` with uninitialized contents, returning an error
518 /// if allocation fails.
523 /// #![feature(new_uninit, allocator_api)]
524 /// #![feature(get_mut_unchecked)]
526 /// use std::sync::Arc;
528 /// let mut five = Arc::<u32>::try_new_uninit()?;
530 /// let five = unsafe {
531 /// // Deferred initialization:
532 /// Arc::get_mut_unchecked(&mut five).as_mut_ptr().write(5);
534 /// five.assume_init()
537 /// assert_eq!(*five, 5);
538 /// # Ok::<(), std::alloc::AllocError>(())
540 #[unstable(feature = "allocator_api", issue = "32838")]
541 // #[unstable(feature = "new_uninit", issue = "63291")]
542 pub fn try_new_uninit() -> Result<Arc<mem::MaybeUninit<T>>, AllocError> {
544 Ok(Arc::from_ptr(Arc::try_allocate_for_layout(
546 |layout| Global.allocate(layout),
547 |mem| mem as *mut ArcInner<mem::MaybeUninit<T>>,
552 /// Constructs a new `Arc` with uninitialized contents, with the memory
553 /// being filled with `0` bytes, returning an error if allocation fails.
555 /// See [`MaybeUninit::zeroed`][zeroed] for examples of correct and incorrect usage
561 /// #![feature(new_uninit, allocator_api)]
563 /// use std::sync::Arc;
565 /// let zero = Arc::<u32>::try_new_zeroed()?;
566 /// let zero = unsafe { zero.assume_init() };
568 /// assert_eq!(*zero, 0);
569 /// # Ok::<(), std::alloc::AllocError>(())
572 /// [zeroed]: mem::MaybeUninit::zeroed
573 #[unstable(feature = "allocator_api", issue = "32838")]
574 // #[unstable(feature = "new_uninit", issue = "63291")]
575 pub fn try_new_zeroed() -> Result<Arc<mem::MaybeUninit<T>>, AllocError> {
577 Ok(Arc::from_ptr(Arc::try_allocate_for_layout(
579 |layout| Global.allocate_zeroed(layout),
580 |mem| mem as *mut ArcInner<mem::MaybeUninit<T>>,
584 /// Returns the inner value, if the `Arc` has exactly one strong reference.
586 /// Otherwise, an [`Err`] is returned with the same `Arc` that was
589 /// This will succeed even if there are outstanding weak references.
594 /// use std::sync::Arc;
596 /// let x = Arc::new(3);
597 /// assert_eq!(Arc::try_unwrap(x), Ok(3));
599 /// let x = Arc::new(4);
600 /// let _y = Arc::clone(&x);
601 /// assert_eq!(*Arc::try_unwrap(x).unwrap_err(), 4);
604 #[stable(feature = "arc_unique", since = "1.4.0")]
605 pub fn try_unwrap(this: Self) -> Result<T, Self> {
606 if this.inner().strong.compare_exchange(1, 0, Relaxed, Relaxed).is_err() {
610 acquire!(this.inner().strong);
613 let elem = ptr::read(&this.ptr.as_ref().data);
615 // Make a weak pointer to clean up the implicit strong-weak reference
616 let _weak = Weak { ptr: this.ptr };
625 /// Constructs a new atomically reference-counted slice with uninitialized contents.
630 /// #![feature(new_uninit)]
631 /// #![feature(get_mut_unchecked)]
633 /// use std::sync::Arc;
635 /// let mut values = Arc::<[u32]>::new_uninit_slice(3);
637 /// let values = unsafe {
638 /// // Deferred initialization:
639 /// Arc::get_mut_unchecked(&mut values)[0].as_mut_ptr().write(1);
640 /// Arc::get_mut_unchecked(&mut values)[1].as_mut_ptr().write(2);
641 /// Arc::get_mut_unchecked(&mut values)[2].as_mut_ptr().write(3);
643 /// values.assume_init()
646 /// assert_eq!(*values, [1, 2, 3])
648 #[cfg(not(no_global_oom_handling))]
649 #[unstable(feature = "new_uninit", issue = "63291")]
650 pub fn new_uninit_slice(len: usize) -> Arc<[mem::MaybeUninit<T>]> {
651 unsafe { Arc::from_ptr(Arc::allocate_for_slice(len)) }
654 /// Constructs a new atomically reference-counted slice with uninitialized contents, with the memory being
655 /// filled with `0` bytes.
657 /// See [`MaybeUninit::zeroed`][zeroed] for examples of correct and
658 /// incorrect usage of this method.
663 /// #![feature(new_uninit)]
665 /// use std::sync::Arc;
667 /// let values = Arc::<[u32]>::new_zeroed_slice(3);
668 /// let values = unsafe { values.assume_init() };
670 /// assert_eq!(*values, [0, 0, 0])
673 /// [zeroed]: ../../std/mem/union.MaybeUninit.html#method.zeroed
674 #[cfg(not(no_global_oom_handling))]
675 #[unstable(feature = "new_uninit", issue = "63291")]
676 pub fn new_zeroed_slice(len: usize) -> Arc<[mem::MaybeUninit<T>]> {
678 Arc::from_ptr(Arc::allocate_for_layout(
679 Layout::array::<T>(len).unwrap(),
680 |layout| Global.allocate_zeroed(layout),
682 ptr::slice_from_raw_parts_mut(mem as *mut T, len)
683 as *mut ArcInner<[mem::MaybeUninit<T>]>
690 impl<T> Arc<mem::MaybeUninit<T>> {
691 /// Converts to `Arc<T>`.
695 /// As with [`MaybeUninit::assume_init`],
696 /// it is up to the caller to guarantee that the inner value
697 /// really is in an initialized state.
698 /// Calling this when the content is not yet fully initialized
699 /// causes immediate undefined behavior.
701 /// [`MaybeUninit::assume_init`]: ../../std/mem/union.MaybeUninit.html#method.assume_init
706 /// #![feature(new_uninit)]
707 /// #![feature(get_mut_unchecked)]
709 /// use std::sync::Arc;
711 /// let mut five = Arc::<u32>::new_uninit();
713 /// let five = unsafe {
714 /// // Deferred initialization:
715 /// Arc::get_mut_unchecked(&mut five).as_mut_ptr().write(5);
717 /// five.assume_init()
720 /// assert_eq!(*five, 5)
722 #[unstable(feature = "new_uninit", issue = "63291")]
724 pub unsafe fn assume_init(self) -> Arc<T> {
725 Arc::from_inner(mem::ManuallyDrop::new(self).ptr.cast())
729 impl<T> Arc<[mem::MaybeUninit<T>]> {
730 /// Converts to `Arc<[T]>`.
734 /// As with [`MaybeUninit::assume_init`],
735 /// it is up to the caller to guarantee that the inner value
736 /// really is in an initialized state.
737 /// Calling this when the content is not yet fully initialized
738 /// causes immediate undefined behavior.
740 /// [`MaybeUninit::assume_init`]: ../../std/mem/union.MaybeUninit.html#method.assume_init
745 /// #![feature(new_uninit)]
746 /// #![feature(get_mut_unchecked)]
748 /// use std::sync::Arc;
750 /// let mut values = Arc::<[u32]>::new_uninit_slice(3);
752 /// let values = unsafe {
753 /// // Deferred initialization:
754 /// Arc::get_mut_unchecked(&mut values)[0].as_mut_ptr().write(1);
755 /// Arc::get_mut_unchecked(&mut values)[1].as_mut_ptr().write(2);
756 /// Arc::get_mut_unchecked(&mut values)[2].as_mut_ptr().write(3);
758 /// values.assume_init()
761 /// assert_eq!(*values, [1, 2, 3])
763 #[unstable(feature = "new_uninit", issue = "63291")]
765 pub unsafe fn assume_init(self) -> Arc<[T]> {
766 unsafe { Arc::from_ptr(mem::ManuallyDrop::new(self).ptr.as_ptr() as _) }
770 impl<T: ?Sized> Arc<T> {
771 /// Consumes the `Arc`, returning the wrapped pointer.
773 /// To avoid a memory leak the pointer must be converted back to an `Arc` using
774 /// [`Arc::from_raw`].
779 /// use std::sync::Arc;
781 /// let x = Arc::new("hello".to_owned());
782 /// let x_ptr = Arc::into_raw(x);
783 /// assert_eq!(unsafe { &*x_ptr }, "hello");
785 #[stable(feature = "rc_raw", since = "1.17.0")]
786 pub fn into_raw(this: Self) -> *const T {
787 let ptr = Self::as_ptr(&this);
792 /// Provides a raw pointer to the data.
794 /// The counts are not affected in any way and the `Arc` is not consumed. The pointer is valid for
795 /// as long as there are strong counts in the `Arc`.
800 /// use std::sync::Arc;
802 /// let x = Arc::new("hello".to_owned());
803 /// let y = Arc::clone(&x);
804 /// let x_ptr = Arc::as_ptr(&x);
805 /// assert_eq!(x_ptr, Arc::as_ptr(&y));
806 /// assert_eq!(unsafe { &*x_ptr }, "hello");
808 #[stable(feature = "rc_as_ptr", since = "1.45.0")]
809 pub fn as_ptr(this: &Self) -> *const T {
810 let ptr: *mut ArcInner<T> = NonNull::as_ptr(this.ptr);
812 // SAFETY: This cannot go through Deref::deref or RcBoxPtr::inner because
813 // this is required to retain raw/mut provenance such that e.g. `get_mut` can
814 // write through the pointer after the Rc is recovered through `from_raw`.
815 unsafe { ptr::addr_of_mut!((*ptr).data) }
818 /// Constructs an `Arc<T>` from a raw pointer.
820 /// The raw pointer must have been previously returned by a call to
821 /// [`Arc<U>::into_raw`][into_raw] where `U` must have the same size and
822 /// alignment as `T`. This is trivially true if `U` is `T`.
823 /// Note that if `U` is not `T` but has the same size and alignment, this is
824 /// basically like transmuting references of different types. See
825 /// [`mem::transmute`][transmute] for more information on what
826 /// restrictions apply in this case.
828 /// The user of `from_raw` has to make sure a specific value of `T` is only
831 /// This function is unsafe because improper use may lead to memory unsafety,
832 /// even if the returned `Arc<T>` is never accessed.
834 /// [into_raw]: Arc::into_raw
835 /// [transmute]: core::mem::transmute
840 /// use std::sync::Arc;
842 /// let x = Arc::new("hello".to_owned());
843 /// let x_ptr = Arc::into_raw(x);
846 /// // Convert back to an `Arc` to prevent leak.
847 /// let x = Arc::from_raw(x_ptr);
848 /// assert_eq!(&*x, "hello");
850 /// // Further calls to `Arc::from_raw(x_ptr)` would be memory-unsafe.
853 /// // The memory was freed when `x` went out of scope above, so `x_ptr` is now dangling!
855 #[stable(feature = "rc_raw", since = "1.17.0")]
856 pub unsafe fn from_raw(ptr: *const T) -> Self {
858 let offset = data_offset(ptr);
860 // Reverse the offset to find the original ArcInner.
861 let arc_ptr = (ptr as *mut ArcInner<T>).set_ptr_value((ptr as *mut u8).offset(-offset));
863 Self::from_ptr(arc_ptr)
867 /// Creates a new [`Weak`] pointer to this allocation.
872 /// use std::sync::Arc;
874 /// let five = Arc::new(5);
876 /// let weak_five = Arc::downgrade(&five);
878 #[stable(feature = "arc_weak", since = "1.4.0")]
879 pub fn downgrade(this: &Self) -> Weak<T> {
880 // This Relaxed is OK because we're checking the value in the CAS
882 let mut cur = this.inner().weak.load(Relaxed);
885 // check if the weak counter is currently "locked"; if so, spin.
886 if cur == usize::MAX {
888 cur = this.inner().weak.load(Relaxed);
892 // NOTE: this code currently ignores the possibility of overflow
893 // into usize::MAX; in general both Rc and Arc need to be adjusted
894 // to deal with overflow.
896 // Unlike with Clone(), we need this to be an Acquire read to
897 // synchronize with the write coming from `is_unique`, so that the
898 // events prior to that write happen before this read.
899 match this.inner().weak.compare_exchange_weak(cur, cur + 1, Acquire, Relaxed) {
901 // Make sure we do not create a dangling Weak
902 debug_assert!(!is_dangling(this.ptr.as_ptr()));
903 return Weak { ptr: this.ptr };
905 Err(old) => cur = old,
910 /// Gets the number of [`Weak`] pointers to this allocation.
914 /// This method by itself is safe, but using it correctly requires extra care.
915 /// Another thread can change the weak count at any time,
916 /// including potentially between calling this method and acting on the result.
921 /// use std::sync::Arc;
923 /// let five = Arc::new(5);
924 /// let _weak_five = Arc::downgrade(&five);
926 /// // This assertion is deterministic because we haven't shared
927 /// // the `Arc` or `Weak` between threads.
928 /// assert_eq!(1, Arc::weak_count(&five));
931 #[stable(feature = "arc_counts", since = "1.15.0")]
932 pub fn weak_count(this: &Self) -> usize {
933 let cnt = this.inner().weak.load(SeqCst);
934 // If the weak count is currently locked, the value of the
935 // count was 0 just before taking the lock.
936 if cnt == usize::MAX { 0 } else { cnt - 1 }
939 /// Gets the number of strong (`Arc`) pointers to this allocation.
943 /// This method by itself is safe, but using it correctly requires extra care.
944 /// Another thread can change the strong count at any time,
945 /// including potentially between calling this method and acting on the result.
950 /// use std::sync::Arc;
952 /// let five = Arc::new(5);
953 /// let _also_five = Arc::clone(&five);
955 /// // This assertion is deterministic because we haven't shared
956 /// // the `Arc` between threads.
957 /// assert_eq!(2, Arc::strong_count(&five));
960 #[stable(feature = "arc_counts", since = "1.15.0")]
961 pub fn strong_count(this: &Self) -> usize {
962 this.inner().strong.load(SeqCst)
965 /// Increments the strong reference count on the `Arc<T>` associated with the
966 /// provided pointer by one.
970 /// The pointer must have been obtained through `Arc::into_raw`, and the
971 /// associated `Arc` instance must be valid (i.e. the strong count must be at
972 /// least 1) for the duration of this method.
977 /// use std::sync::Arc;
979 /// let five = Arc::new(5);
982 /// let ptr = Arc::into_raw(five);
983 /// Arc::increment_strong_count(ptr);
985 /// // This assertion is deterministic because we haven't shared
986 /// // the `Arc` between threads.
987 /// let five = Arc::from_raw(ptr);
988 /// assert_eq!(2, Arc::strong_count(&five));
992 #[stable(feature = "arc_mutate_strong_count", since = "1.51.0")]
993 pub unsafe fn increment_strong_count(ptr: *const T) {
994 // Retain Arc, but don't touch refcount by wrapping in ManuallyDrop
995 let arc = unsafe { mem::ManuallyDrop::new(Arc::<T>::from_raw(ptr)) };
996 // Now increase refcount, but don't drop new refcount either
997 let _arc_clone: mem::ManuallyDrop<_> = arc.clone();
1000 /// Decrements the strong reference count on the `Arc<T>` associated with the
1001 /// provided pointer by one.
1005 /// The pointer must have been obtained through `Arc::into_raw`, and the
1006 /// associated `Arc` instance must be valid (i.e. the strong count must be at
1007 /// least 1) when invoking this method. This method can be used to release the final
1008 /// `Arc` and backing storage, but **should not** be called after the final `Arc` has been
1014 /// use std::sync::Arc;
1016 /// let five = Arc::new(5);
1019 /// let ptr = Arc::into_raw(five);
1020 /// Arc::increment_strong_count(ptr);
1022 /// // Those assertions are deterministic because we haven't shared
1023 /// // the `Arc` between threads.
1024 /// let five = Arc::from_raw(ptr);
1025 /// assert_eq!(2, Arc::strong_count(&five));
1026 /// Arc::decrement_strong_count(ptr);
1027 /// assert_eq!(1, Arc::strong_count(&five));
1031 #[stable(feature = "arc_mutate_strong_count", since = "1.51.0")]
1032 pub unsafe fn decrement_strong_count(ptr: *const T) {
1033 unsafe { mem::drop(Arc::from_raw(ptr)) };
1037 fn inner(&self) -> &ArcInner<T> {
1038 // This unsafety is ok because while this arc is alive we're guaranteed
1039 // that the inner pointer is valid. Furthermore, we know that the
1040 // `ArcInner` structure itself is `Sync` because the inner data is
1041 // `Sync` as well, so we're ok loaning out an immutable pointer to these
1043 unsafe { self.ptr.as_ref() }
1046 // Non-inlined part of `drop`.
1048 unsafe fn drop_slow(&mut self) {
1049 // Destroy the data at this time, even though we may not free the box
1050 // allocation itself (there may still be weak pointers lying around).
1051 unsafe { ptr::drop_in_place(Self::get_mut_unchecked(self)) };
1053 // Drop the weak ref collectively held by all strong references
1054 drop(Weak { ptr: self.ptr });
1058 #[stable(feature = "ptr_eq", since = "1.17.0")]
1059 /// Returns `true` if the two `Arc`s point to the same allocation
1060 /// (in a vein similar to [`ptr::eq`]).
1065 /// use std::sync::Arc;
1067 /// let five = Arc::new(5);
1068 /// let same_five = Arc::clone(&five);
1069 /// let other_five = Arc::new(5);
1071 /// assert!(Arc::ptr_eq(&five, &same_five));
1072 /// assert!(!Arc::ptr_eq(&five, &other_five));
1075 /// [`ptr::eq`]: core::ptr::eq
1076 pub fn ptr_eq(this: &Self, other: &Self) -> bool {
1077 this.ptr.as_ptr() == other.ptr.as_ptr()
1081 impl<T: ?Sized> Arc<T> {
1082 /// Allocates an `ArcInner<T>` with sufficient space for
1083 /// a possibly-unsized inner value where the value has the layout provided.
1085 /// The function `mem_to_arcinner` is called with the data pointer
1086 /// and must return back a (potentially fat)-pointer for the `ArcInner<T>`.
1087 #[cfg(not(no_global_oom_handling))]
1088 unsafe fn allocate_for_layout(
1089 value_layout: Layout,
1090 allocate: impl FnOnce(Layout) -> Result<NonNull<[u8]>, AllocError>,
1091 mem_to_arcinner: impl FnOnce(*mut u8) -> *mut ArcInner<T>,
1092 ) -> *mut ArcInner<T> {
1093 // Calculate layout using the given value layout.
1094 // Previously, layout was calculated on the expression
1095 // `&*(ptr as *const ArcInner<T>)`, but this created a misaligned
1096 // reference (see #54908).
1097 let layout = Layout::new::<ArcInner<()>>().extend(value_layout).unwrap().0.pad_to_align();
1099 Arc::try_allocate_for_layout(value_layout, allocate, mem_to_arcinner)
1100 .unwrap_or_else(|_| handle_alloc_error(layout))
1104 /// Allocates an `ArcInner<T>` with sufficient space for
1105 /// a possibly-unsized inner value where the value has the layout provided,
1106 /// returning an error if allocation fails.
1108 /// The function `mem_to_arcinner` is called with the data pointer
1109 /// and must return back a (potentially fat)-pointer for the `ArcInner<T>`.
1110 unsafe fn try_allocate_for_layout(
1111 value_layout: Layout,
1112 allocate: impl FnOnce(Layout) -> Result<NonNull<[u8]>, AllocError>,
1113 mem_to_arcinner: impl FnOnce(*mut u8) -> *mut ArcInner<T>,
1114 ) -> Result<*mut ArcInner<T>, AllocError> {
1115 // Calculate layout using the given value layout.
1116 // Previously, layout was calculated on the expression
1117 // `&*(ptr as *const ArcInner<T>)`, but this created a misaligned
1118 // reference (see #54908).
1119 let layout = Layout::new::<ArcInner<()>>().extend(value_layout).unwrap().0.pad_to_align();
1121 let ptr = allocate(layout)?;
1123 // Initialize the ArcInner
1124 let inner = mem_to_arcinner(ptr.as_non_null_ptr().as_ptr());
1125 debug_assert_eq!(unsafe { Layout::for_value(&*inner) }, layout);
1128 ptr::write(&mut (*inner).strong, atomic::AtomicUsize::new(1));
1129 ptr::write(&mut (*inner).weak, atomic::AtomicUsize::new(1));
1135 /// Allocates an `ArcInner<T>` with sufficient space for an unsized inner value.
1136 #[cfg(not(no_global_oom_handling))]
1137 unsafe fn allocate_for_ptr(ptr: *const T) -> *mut ArcInner<T> {
1138 // Allocate for the `ArcInner<T>` using the given value.
1140 Self::allocate_for_layout(
1141 Layout::for_value(&*ptr),
1142 |layout| Global.allocate(layout),
1143 |mem| (ptr as *mut ArcInner<T>).set_ptr_value(mem) as *mut ArcInner<T>,
1148 #[cfg(not(no_global_oom_handling))]
1149 fn from_box(v: Box<T>) -> Arc<T> {
1151 let (box_unique, alloc) = Box::into_unique(v);
1152 let bptr = box_unique.as_ptr();
1154 let value_size = size_of_val(&*bptr);
1155 let ptr = Self::allocate_for_ptr(bptr);
1157 // Copy value as bytes
1158 ptr::copy_nonoverlapping(
1159 bptr as *const T as *const u8,
1160 &mut (*ptr).data as *mut _ as *mut u8,
1164 // Free the allocation without dropping its contents
1165 box_free(box_unique, alloc);
1173 /// Allocates an `ArcInner<[T]>` with the given length.
1174 #[cfg(not(no_global_oom_handling))]
1175 unsafe fn allocate_for_slice(len: usize) -> *mut ArcInner<[T]> {
1177 Self::allocate_for_layout(
1178 Layout::array::<T>(len).unwrap(),
1179 |layout| Global.allocate(layout),
1180 |mem| ptr::slice_from_raw_parts_mut(mem as *mut T, len) as *mut ArcInner<[T]>,
1185 /// Copy elements from slice into newly allocated Arc<\[T\]>
1187 /// Unsafe because the caller must either take ownership or bind `T: Copy`.
1188 #[cfg(not(no_global_oom_handling))]
1189 unsafe fn copy_from_slice(v: &[T]) -> Arc<[T]> {
1191 let ptr = Self::allocate_for_slice(v.len());
1193 ptr::copy_nonoverlapping(v.as_ptr(), &mut (*ptr).data as *mut [T] as *mut T, v.len());
1199 /// Constructs an `Arc<[T]>` from an iterator known to be of a certain size.
1201 /// Behavior is undefined should the size be wrong.
1202 #[cfg(not(no_global_oom_handling))]
1203 unsafe fn from_iter_exact(iter: impl iter::Iterator<Item = T>, len: usize) -> Arc<[T]> {
1204 // Panic guard while cloning T elements.
1205 // In the event of a panic, elements that have been written
1206 // into the new ArcInner will be dropped, then the memory freed.
1214 impl<T> Drop for Guard<T> {
1215 fn drop(&mut self) {
1217 let slice = from_raw_parts_mut(self.elems, self.n_elems);
1218 ptr::drop_in_place(slice);
1220 Global.deallocate(self.mem, self.layout);
1226 let ptr = Self::allocate_for_slice(len);
1228 let mem = ptr as *mut _ as *mut u8;
1229 let layout = Layout::for_value(&*ptr);
1231 // Pointer to first element
1232 let elems = &mut (*ptr).data as *mut [T] as *mut T;
1234 let mut guard = Guard { mem: NonNull::new_unchecked(mem), elems, layout, n_elems: 0 };
1236 for (i, item) in iter.enumerate() {
1237 ptr::write(elems.add(i), item);
1241 // All clear. Forget the guard so it doesn't free the new ArcInner.
1249 /// Specialization trait used for `From<&[T]>`.
1250 #[cfg(not(no_global_oom_handling))]
1251 trait ArcFromSlice<T> {
1252 fn from_slice(slice: &[T]) -> Self;
1255 #[cfg(not(no_global_oom_handling))]
1256 impl<T: Clone> ArcFromSlice<T> for Arc<[T]> {
1258 default fn from_slice(v: &[T]) -> Self {
1259 unsafe { Self::from_iter_exact(v.iter().cloned(), v.len()) }
1263 #[cfg(not(no_global_oom_handling))]
1264 impl<T: Copy> ArcFromSlice<T> for Arc<[T]> {
1266 fn from_slice(v: &[T]) -> Self {
1267 unsafe { Arc::copy_from_slice(v) }
1271 #[stable(feature = "rust1", since = "1.0.0")]
1272 impl<T: ?Sized> Clone for Arc<T> {
1273 /// Makes a clone of the `Arc` pointer.
1275 /// This creates another pointer to the same allocation, increasing the
1276 /// strong reference count.
1281 /// use std::sync::Arc;
1283 /// let five = Arc::new(5);
1285 /// let _ = Arc::clone(&five);
1288 fn clone(&self) -> Arc<T> {
1289 // Using a relaxed ordering is alright here, as knowledge of the
1290 // original reference prevents other threads from erroneously deleting
1293 // As explained in the [Boost documentation][1], Increasing the
1294 // reference counter can always be done with memory_order_relaxed: New
1295 // references to an object can only be formed from an existing
1296 // reference, and passing an existing reference from one thread to
1297 // another must already provide any required synchronization.
1299 // [1]: (www.boost.org/doc/libs/1_55_0/doc/html/atomic/usage_examples.html)
1300 let old_size = self.inner().strong.fetch_add(1, Relaxed);
1302 // However we need to guard against massive refcounts in case someone
1303 // is `mem::forget`ing Arcs. If we don't do this the count can overflow
1304 // and users will use-after free. We racily saturate to `isize::MAX` on
1305 // the assumption that there aren't ~2 billion threads incrementing
1306 // the reference count at once. This branch will never be taken in
1307 // any realistic program.
1309 // We abort because such a program is incredibly degenerate, and we
1310 // don't care to support it.
1311 if old_size > MAX_REFCOUNT {
1315 Self::from_inner(self.ptr)
1319 #[stable(feature = "rust1", since = "1.0.0")]
1320 impl<T: ?Sized> Deref for Arc<T> {
1324 fn deref(&self) -> &T {
1329 #[unstable(feature = "receiver_trait", issue = "none")]
1330 impl<T: ?Sized> Receiver for Arc<T> {}
1332 impl<T: Clone> Arc<T> {
1333 /// Makes a mutable reference into the given `Arc`.
1335 /// If there are other `Arc` or [`Weak`] pointers to the same allocation,
1336 /// then `make_mut` will create a new allocation and invoke [`clone`][clone] on the inner value
1337 /// to ensure unique ownership. This is also referred to as clone-on-write.
1339 /// Note that this differs from the behavior of [`Rc::make_mut`] which disassociates
1340 /// any remaining `Weak` pointers.
1342 /// See also [`get_mut`][get_mut], which will fail rather than cloning.
1344 /// [clone]: Clone::clone
1345 /// [get_mut]: Arc::get_mut
1346 /// [`Rc::make_mut`]: super::rc::Rc::make_mut
1351 /// use std::sync::Arc;
1353 /// let mut data = Arc::new(5);
1355 /// *Arc::make_mut(&mut data) += 1; // Won't clone anything
1356 /// let mut other_data = Arc::clone(&data); // Won't clone inner data
1357 /// *Arc::make_mut(&mut data) += 1; // Clones inner data
1358 /// *Arc::make_mut(&mut data) += 1; // Won't clone anything
1359 /// *Arc::make_mut(&mut other_data) *= 2; // Won't clone anything
1361 /// // Now `data` and `other_data` point to different allocations.
1362 /// assert_eq!(*data, 8);
1363 /// assert_eq!(*other_data, 12);
1365 #[cfg(not(no_global_oom_handling))]
1367 #[stable(feature = "arc_unique", since = "1.4.0")]
1368 pub fn make_mut(this: &mut Self) -> &mut T {
1369 // Note that we hold both a strong reference and a weak reference.
1370 // Thus, releasing our strong reference only will not, by itself, cause
1371 // the memory to be deallocated.
1373 // Use Acquire to ensure that we see any writes to `weak` that happen
1374 // before release writes (i.e., decrements) to `strong`. Since we hold a
1375 // weak count, there's no chance the ArcInner itself could be
1377 if this.inner().strong.compare_exchange(1, 0, Acquire, Relaxed).is_err() {
1378 // Another strong pointer exists, so we must clone.
1379 // Pre-allocate memory to allow writing the cloned value directly.
1380 let mut arc = Self::new_uninit();
1382 let data = Arc::get_mut_unchecked(&mut arc);
1383 (**this).write_clone_into_raw(data.as_mut_ptr());
1384 *this = arc.assume_init();
1386 } else if this.inner().weak.load(Relaxed) != 1 {
1387 // Relaxed suffices in the above because this is fundamentally an
1388 // optimization: we are always racing with weak pointers being
1389 // dropped. Worst case, we end up allocated a new Arc unnecessarily.
1391 // We removed the last strong ref, but there are additional weak
1392 // refs remaining. We'll move the contents to a new Arc, and
1393 // invalidate the other weak refs.
1395 // Note that it is not possible for the read of `weak` to yield
1396 // usize::MAX (i.e., locked), since the weak count can only be
1397 // locked by a thread with a strong reference.
1399 // Materialize our own implicit weak pointer, so that it can clean
1400 // up the ArcInner as needed.
1401 let _weak = Weak { ptr: this.ptr };
1403 // Can just steal the data, all that's left is Weaks
1404 let mut arc = Self::new_uninit();
1406 let data = Arc::get_mut_unchecked(&mut arc);
1407 data.as_mut_ptr().copy_from_nonoverlapping(&**this, 1);
1408 ptr::write(this, arc.assume_init());
1411 // We were the sole reference of either kind; bump back up the
1412 // strong ref count.
1413 this.inner().strong.store(1, Release);
1416 // As with `get_mut()`, the unsafety is ok because our reference was
1417 // either unique to begin with, or became one upon cloning the contents.
1418 unsafe { Self::get_mut_unchecked(this) }
1422 impl<T: ?Sized> Arc<T> {
1423 /// Returns a mutable reference into the given `Arc`, if there are
1424 /// no other `Arc` or [`Weak`] pointers to the same allocation.
1426 /// Returns [`None`] otherwise, because it is not safe to
1427 /// mutate a shared value.
1429 /// See also [`make_mut`][make_mut], which will [`clone`][clone]
1430 /// the inner value when there are other pointers.
1432 /// [make_mut]: Arc::make_mut
1433 /// [clone]: Clone::clone
1438 /// use std::sync::Arc;
1440 /// let mut x = Arc::new(3);
1441 /// *Arc::get_mut(&mut x).unwrap() = 4;
1442 /// assert_eq!(*x, 4);
1444 /// let _y = Arc::clone(&x);
1445 /// assert!(Arc::get_mut(&mut x).is_none());
1448 #[stable(feature = "arc_unique", since = "1.4.0")]
1449 pub fn get_mut(this: &mut Self) -> Option<&mut T> {
1450 if this.is_unique() {
1451 // This unsafety is ok because we're guaranteed that the pointer
1452 // returned is the *only* pointer that will ever be returned to T. Our
1453 // reference count is guaranteed to be 1 at this point, and we required
1454 // the Arc itself to be `mut`, so we're returning the only possible
1455 // reference to the inner data.
1456 unsafe { Some(Arc::get_mut_unchecked(this)) }
1462 /// Returns a mutable reference into the given `Arc`,
1463 /// without any check.
1465 /// See also [`get_mut`], which is safe and does appropriate checks.
1467 /// [`get_mut`]: Arc::get_mut
1471 /// Any other `Arc` or [`Weak`] pointers to the same allocation must not be dereferenced
1472 /// for the duration of the returned borrow.
1473 /// This is trivially the case if no such pointers exist,
1474 /// for example immediately after `Arc::new`.
1479 /// #![feature(get_mut_unchecked)]
1481 /// use std::sync::Arc;
1483 /// let mut x = Arc::new(String::new());
1485 /// Arc::get_mut_unchecked(&mut x).push_str("foo")
1487 /// assert_eq!(*x, "foo");
1490 #[unstable(feature = "get_mut_unchecked", issue = "63292")]
1491 pub unsafe fn get_mut_unchecked(this: &mut Self) -> &mut T {
1492 // We are careful to *not* create a reference covering the "count" fields, as
1493 // this would alias with concurrent access to the reference counts (e.g. by `Weak`).
1494 unsafe { &mut (*this.ptr.as_ptr()).data }
1497 /// Determine whether this is the unique reference (including weak refs) to
1498 /// the underlying data.
1500 /// Note that this requires locking the weak ref count.
1501 fn is_unique(&mut self) -> bool {
1502 // lock the weak pointer count if we appear to be the sole weak pointer
1505 // The acquire label here ensures a happens-before relationship with any
1506 // writes to `strong` (in particular in `Weak::upgrade`) prior to decrements
1507 // of the `weak` count (via `Weak::drop`, which uses release). If the upgraded
1508 // weak ref was never dropped, the CAS here will fail so we do not care to synchronize.
1509 if self.inner().weak.compare_exchange(1, usize::MAX, Acquire, Relaxed).is_ok() {
1510 // This needs to be an `Acquire` to synchronize with the decrement of the `strong`
1511 // counter in `drop` -- the only access that happens when any but the last reference
1512 // is being dropped.
1513 let unique = self.inner().strong.load(Acquire) == 1;
1515 // The release write here synchronizes with a read in `downgrade`,
1516 // effectively preventing the above read of `strong` from happening
1518 self.inner().weak.store(1, Release); // release the lock
1526 #[stable(feature = "rust1", since = "1.0.0")]
1527 unsafe impl<#[may_dangle] T: ?Sized> Drop for Arc<T> {
1528 /// Drops the `Arc`.
1530 /// This will decrement the strong reference count. If the strong reference
1531 /// count reaches zero then the only other references (if any) are
1532 /// [`Weak`], so we `drop` the inner value.
1537 /// use std::sync::Arc;
1541 /// impl Drop for Foo {
1542 /// fn drop(&mut self) {
1543 /// println!("dropped!");
1547 /// let foo = Arc::new(Foo);
1548 /// let foo2 = Arc::clone(&foo);
1550 /// drop(foo); // Doesn't print anything
1551 /// drop(foo2); // Prints "dropped!"
1554 fn drop(&mut self) {
1555 // Because `fetch_sub` is already atomic, we do not need to synchronize
1556 // with other threads unless we are going to delete the object. This
1557 // same logic applies to the below `fetch_sub` to the `weak` count.
1558 if self.inner().strong.fetch_sub(1, Release) != 1 {
1562 // This fence is needed to prevent reordering of use of the data and
1563 // deletion of the data. Because it is marked `Release`, the decreasing
1564 // of the reference count synchronizes with this `Acquire` fence. This
1565 // means that use of the data happens before decreasing the reference
1566 // count, which happens before this fence, which happens before the
1567 // deletion of the data.
1569 // As explained in the [Boost documentation][1],
1571 // > It is important to enforce any possible access to the object in one
1572 // > thread (through an existing reference) to *happen before* deleting
1573 // > the object in a different thread. This is achieved by a "release"
1574 // > operation after dropping a reference (any access to the object
1575 // > through this reference must obviously happened before), and an
1576 // > "acquire" operation before deleting the object.
1578 // In particular, while the contents of an Arc are usually immutable, it's
1579 // possible to have interior writes to something like a Mutex<T>. Since a
1580 // Mutex is not acquired when it is deleted, we can't rely on its
1581 // synchronization logic to make writes in thread A visible to a destructor
1582 // running in thread B.
1584 // Also note that the Acquire fence here could probably be replaced with an
1585 // Acquire load, which could improve performance in highly-contended
1586 // situations. See [2].
1588 // [1]: (www.boost.org/doc/libs/1_55_0/doc/html/atomic/usage_examples.html)
1589 // [2]: (https://github.com/rust-lang/rust/pull/41714)
1590 acquire!(self.inner().strong);
1598 impl Arc<dyn Any + Send + Sync> {
1600 #[stable(feature = "rc_downcast", since = "1.29.0")]
1601 /// Attempt to downcast the `Arc<dyn Any + Send + Sync>` to a concrete type.
1606 /// use std::any::Any;
1607 /// use std::sync::Arc;
1609 /// fn print_if_string(value: Arc<dyn Any + Send + Sync>) {
1610 /// if let Ok(string) = value.downcast::<String>() {
1611 /// println!("String ({}): {}", string.len(), string);
1615 /// let my_string = "Hello World".to_string();
1616 /// print_if_string(Arc::new(my_string));
1617 /// print_if_string(Arc::new(0i8));
1619 pub fn downcast<T>(self) -> Result<Arc<T>, Self>
1621 T: Any + Send + Sync + 'static,
1623 if (*self).is::<T>() {
1624 let ptr = self.ptr.cast::<ArcInner<T>>();
1626 Ok(Arc::from_inner(ptr))
1634 /// Constructs a new `Weak<T>`, without allocating any memory.
1635 /// Calling [`upgrade`] on the return value always gives [`None`].
1637 /// [`upgrade`]: Weak::upgrade
1642 /// use std::sync::Weak;
1644 /// let empty: Weak<i64> = Weak::new();
1645 /// assert!(empty.upgrade().is_none());
1647 #[stable(feature = "downgraded_weak", since = "1.10.0")]
1648 pub fn new() -> Weak<T> {
1649 Weak { ptr: NonNull::new(usize::MAX as *mut ArcInner<T>).expect("MAX is not 0") }
1653 /// Helper type to allow accessing the reference counts without
1654 /// making any assertions about the data field.
1655 struct WeakInner<'a> {
1656 weak: &'a atomic::AtomicUsize,
1657 strong: &'a atomic::AtomicUsize,
1660 impl<T: ?Sized> Weak<T> {
1661 /// Returns a raw pointer to the object `T` pointed to by this `Weak<T>`.
1663 /// The pointer is valid only if there are some strong references. The pointer may be dangling,
1664 /// unaligned or even [`null`] otherwise.
1669 /// use std::sync::Arc;
1672 /// let strong = Arc::new("hello".to_owned());
1673 /// let weak = Arc::downgrade(&strong);
1674 /// // Both point to the same object
1675 /// assert!(ptr::eq(&*strong, weak.as_ptr()));
1676 /// // The strong here keeps it alive, so we can still access the object.
1677 /// assert_eq!("hello", unsafe { &*weak.as_ptr() });
1680 /// // But not any more. We can do weak.as_ptr(), but accessing the pointer would lead to
1681 /// // undefined behaviour.
1682 /// // assert_eq!("hello", unsafe { &*weak.as_ptr() });
1685 /// [`null`]: core::ptr::null
1686 #[stable(feature = "weak_into_raw", since = "1.45.0")]
1687 pub fn as_ptr(&self) -> *const T {
1688 let ptr: *mut ArcInner<T> = NonNull::as_ptr(self.ptr);
1690 if is_dangling(ptr) {
1691 // If the pointer is dangling, we return the sentinel directly. This cannot be
1692 // a valid payload address, as the payload is at least as aligned as ArcInner (usize).
1695 // SAFETY: if is_dangling returns false, then the pointer is dereferencable.
1696 // The payload may be dropped at this point, and we have to maintain provenance,
1697 // so use raw pointer manipulation.
1698 unsafe { ptr::addr_of_mut!((*ptr).data) }
1702 /// Consumes the `Weak<T>` and turns it into a raw pointer.
1704 /// This converts the weak pointer into a raw pointer, while still preserving the ownership of
1705 /// one weak reference (the weak count is not modified by this operation). It can be turned
1706 /// back into the `Weak<T>` with [`from_raw`].
1708 /// The same restrictions of accessing the target of the pointer as with
1709 /// [`as_ptr`] apply.
1714 /// use std::sync::{Arc, Weak};
1716 /// let strong = Arc::new("hello".to_owned());
1717 /// let weak = Arc::downgrade(&strong);
1718 /// let raw = weak.into_raw();
1720 /// assert_eq!(1, Arc::weak_count(&strong));
1721 /// assert_eq!("hello", unsafe { &*raw });
1723 /// drop(unsafe { Weak::from_raw(raw) });
1724 /// assert_eq!(0, Arc::weak_count(&strong));
1727 /// [`from_raw`]: Weak::from_raw
1728 /// [`as_ptr`]: Weak::as_ptr
1729 #[stable(feature = "weak_into_raw", since = "1.45.0")]
1730 pub fn into_raw(self) -> *const T {
1731 let result = self.as_ptr();
1736 /// Converts a raw pointer previously created by [`into_raw`] back into `Weak<T>`.
1738 /// This can be used to safely get a strong reference (by calling [`upgrade`]
1739 /// later) or to deallocate the weak count by dropping the `Weak<T>`.
1741 /// It takes ownership of one weak reference (with the exception of pointers created by [`new`],
1742 /// as these don't own anything; the method still works on them).
1746 /// The pointer must have originated from the [`into_raw`] and must still own its potential
1749 /// It is allowed for the strong count to be 0 at the time of calling this. Nevertheless, this
1750 /// takes ownership of one weak reference currently represented as a raw pointer (the weak
1751 /// count is not modified by this operation) and therefore it must be paired with a previous
1752 /// call to [`into_raw`].
1756 /// use std::sync::{Arc, Weak};
1758 /// let strong = Arc::new("hello".to_owned());
1760 /// let raw_1 = Arc::downgrade(&strong).into_raw();
1761 /// let raw_2 = Arc::downgrade(&strong).into_raw();
1763 /// assert_eq!(2, Arc::weak_count(&strong));
1765 /// assert_eq!("hello", &*unsafe { Weak::from_raw(raw_1) }.upgrade().unwrap());
1766 /// assert_eq!(1, Arc::weak_count(&strong));
1770 /// // Decrement the last weak count.
1771 /// assert!(unsafe { Weak::from_raw(raw_2) }.upgrade().is_none());
1774 /// [`new`]: Weak::new
1775 /// [`into_raw`]: Weak::into_raw
1776 /// [`upgrade`]: Weak::upgrade
1777 /// [`forget`]: std::mem::forget
1778 #[stable(feature = "weak_into_raw", since = "1.45.0")]
1779 pub unsafe fn from_raw(ptr: *const T) -> Self {
1780 // See Weak::as_ptr for context on how the input pointer is derived.
1782 let ptr = if is_dangling(ptr as *mut T) {
1783 // This is a dangling Weak.
1784 ptr as *mut ArcInner<T>
1786 // Otherwise, we're guaranteed the pointer came from a nondangling Weak.
1787 // SAFETY: data_offset is safe to call, as ptr references a real (potentially dropped) T.
1788 let offset = unsafe { data_offset(ptr) };
1789 // Thus, we reverse the offset to get the whole RcBox.
1790 // SAFETY: the pointer originated from a Weak, so this offset is safe.
1791 unsafe { (ptr as *mut ArcInner<T>).set_ptr_value((ptr as *mut u8).offset(-offset)) }
1794 // SAFETY: we now have recovered the original Weak pointer, so can create the Weak.
1795 Weak { ptr: unsafe { NonNull::new_unchecked(ptr) } }
1799 impl<T: ?Sized> Weak<T> {
1800 /// Attempts to upgrade the `Weak` pointer to an [`Arc`], delaying
1801 /// dropping of the inner value if successful.
1803 /// Returns [`None`] if the inner value has since been dropped.
1808 /// use std::sync::Arc;
1810 /// let five = Arc::new(5);
1812 /// let weak_five = Arc::downgrade(&five);
1814 /// let strong_five: Option<Arc<_>> = weak_five.upgrade();
1815 /// assert!(strong_five.is_some());
1817 /// // Destroy all strong pointers.
1818 /// drop(strong_five);
1821 /// assert!(weak_five.upgrade().is_none());
1823 #[stable(feature = "arc_weak", since = "1.4.0")]
1824 pub fn upgrade(&self) -> Option<Arc<T>> {
1825 // We use a CAS loop to increment the strong count instead of a
1826 // fetch_add as this function should never take the reference count
1827 // from zero to one.
1828 let inner = self.inner()?;
1830 // Relaxed load because any write of 0 that we can observe
1831 // leaves the field in a permanently zero state (so a
1832 // "stale" read of 0 is fine), and any other value is
1833 // confirmed via the CAS below.
1834 let mut n = inner.strong.load(Relaxed);
1841 // See comments in `Arc::clone` for why we do this (for `mem::forget`).
1842 if n > MAX_REFCOUNT {
1846 // Relaxed is fine for the failure case because we don't have any expectations about the new state.
1847 // Acquire is necessary for the success case to synchronise with `Arc::new_cyclic`, when the inner
1848 // value can be initialized after `Weak` references have already been created. In that case, we
1849 // expect to observe the fully initialized value.
1850 match inner.strong.compare_exchange_weak(n, n + 1, Acquire, Relaxed) {
1851 Ok(_) => return Some(Arc::from_inner(self.ptr)), // null checked above
1852 Err(old) => n = old,
1857 /// Gets the number of strong (`Arc`) pointers pointing to this allocation.
1859 /// If `self` was created using [`Weak::new`], this will return 0.
1860 #[stable(feature = "weak_counts", since = "1.41.0")]
1861 pub fn strong_count(&self) -> usize {
1862 if let Some(inner) = self.inner() { inner.strong.load(SeqCst) } else { 0 }
1865 /// Gets an approximation of the number of `Weak` pointers pointing to this
1868 /// If `self` was created using [`Weak::new`], or if there are no remaining
1869 /// strong pointers, this will return 0.
1873 /// Due to implementation details, the returned value can be off by 1 in
1874 /// either direction when other threads are manipulating any `Arc`s or
1875 /// `Weak`s pointing to the same allocation.
1876 #[stable(feature = "weak_counts", since = "1.41.0")]
1877 pub fn weak_count(&self) -> usize {
1880 let weak = inner.weak.load(SeqCst);
1881 let strong = inner.strong.load(SeqCst);
1885 // Since we observed that there was at least one strong pointer
1886 // after reading the weak count, we know that the implicit weak
1887 // reference (present whenever any strong references are alive)
1888 // was still around when we observed the weak count, and can
1889 // therefore safely subtract it.
1896 /// Returns `None` when the pointer is dangling and there is no allocated `ArcInner`,
1897 /// (i.e., when this `Weak` was created by `Weak::new`).
1899 fn inner(&self) -> Option<WeakInner<'_>> {
1900 if is_dangling(self.ptr.as_ptr()) {
1903 // We are careful to *not* create a reference covering the "data" field, as
1904 // the field may be mutated concurrently (for example, if the last `Arc`
1905 // is dropped, the data field will be dropped in-place).
1907 let ptr = self.ptr.as_ptr();
1908 WeakInner { strong: &(*ptr).strong, weak: &(*ptr).weak }
1913 /// Returns `true` if the two `Weak`s point to the same allocation (similar to
1914 /// [`ptr::eq`]), or if both don't point to any allocation
1915 /// (because they were created with `Weak::new()`).
1919 /// Since this compares pointers it means that `Weak::new()` will equal each
1920 /// other, even though they don't point to any allocation.
1925 /// use std::sync::Arc;
1927 /// let first_rc = Arc::new(5);
1928 /// let first = Arc::downgrade(&first_rc);
1929 /// let second = Arc::downgrade(&first_rc);
1931 /// assert!(first.ptr_eq(&second));
1933 /// let third_rc = Arc::new(5);
1934 /// let third = Arc::downgrade(&third_rc);
1936 /// assert!(!first.ptr_eq(&third));
1939 /// Comparing `Weak::new`.
1942 /// use std::sync::{Arc, Weak};
1944 /// let first = Weak::new();
1945 /// let second = Weak::new();
1946 /// assert!(first.ptr_eq(&second));
1948 /// let third_rc = Arc::new(());
1949 /// let third = Arc::downgrade(&third_rc);
1950 /// assert!(!first.ptr_eq(&third));
1953 /// [`ptr::eq`]: core::ptr::eq
1955 #[stable(feature = "weak_ptr_eq", since = "1.39.0")]
1956 pub fn ptr_eq(&self, other: &Self) -> bool {
1957 self.ptr.as_ptr() == other.ptr.as_ptr()
1961 #[stable(feature = "arc_weak", since = "1.4.0")]
1962 impl<T: ?Sized> Clone for Weak<T> {
1963 /// Makes a clone of the `Weak` pointer that points to the same allocation.
1968 /// use std::sync::{Arc, Weak};
1970 /// let weak_five = Arc::downgrade(&Arc::new(5));
1972 /// let _ = Weak::clone(&weak_five);
1975 fn clone(&self) -> Weak<T> {
1976 let inner = if let Some(inner) = self.inner() {
1979 return Weak { ptr: self.ptr };
1981 // See comments in Arc::clone() for why this is relaxed. This can use a
1982 // fetch_add (ignoring the lock) because the weak count is only locked
1983 // where are *no other* weak pointers in existence. (So we can't be
1984 // running this code in that case).
1985 let old_size = inner.weak.fetch_add(1, Relaxed);
1987 // See comments in Arc::clone() for why we do this (for mem::forget).
1988 if old_size > MAX_REFCOUNT {
1992 Weak { ptr: self.ptr }
1996 #[stable(feature = "downgraded_weak", since = "1.10.0")]
1997 impl<T> Default for Weak<T> {
1998 /// Constructs a new `Weak<T>`, without allocating memory.
1999 /// Calling [`upgrade`] on the return value always
2002 /// [`upgrade`]: Weak::upgrade
2007 /// use std::sync::Weak;
2009 /// let empty: Weak<i64> = Default::default();
2010 /// assert!(empty.upgrade().is_none());
2012 fn default() -> Weak<T> {
2017 #[stable(feature = "arc_weak", since = "1.4.0")]
2018 unsafe impl<#[may_dangle] T: ?Sized> Drop for Weak<T> {
2019 /// Drops the `Weak` pointer.
2024 /// use std::sync::{Arc, Weak};
2028 /// impl Drop for Foo {
2029 /// fn drop(&mut self) {
2030 /// println!("dropped!");
2034 /// let foo = Arc::new(Foo);
2035 /// let weak_foo = Arc::downgrade(&foo);
2036 /// let other_weak_foo = Weak::clone(&weak_foo);
2038 /// drop(weak_foo); // Doesn't print anything
2039 /// drop(foo); // Prints "dropped!"
2041 /// assert!(other_weak_foo.upgrade().is_none());
2043 fn drop(&mut self) {
2044 // If we find out that we were the last weak pointer, then its time to
2045 // deallocate the data entirely. See the discussion in Arc::drop() about
2046 // the memory orderings
2048 // It's not necessary to check for the locked state here, because the
2049 // weak count can only be locked if there was precisely one weak ref,
2050 // meaning that drop could only subsequently run ON that remaining weak
2051 // ref, which can only happen after the lock is released.
2052 let inner = if let Some(inner) = self.inner() { inner } else { return };
2054 if inner.weak.fetch_sub(1, Release) == 1 {
2055 acquire!(inner.weak);
2056 unsafe { Global.deallocate(self.ptr.cast(), Layout::for_value_raw(self.ptr.as_ptr())) }
2061 #[stable(feature = "rust1", since = "1.0.0")]
2062 trait ArcEqIdent<T: ?Sized + PartialEq> {
2063 fn eq(&self, other: &Arc<T>) -> bool;
2064 fn ne(&self, other: &Arc<T>) -> bool;
2067 #[stable(feature = "rust1", since = "1.0.0")]
2068 impl<T: ?Sized + PartialEq> ArcEqIdent<T> for Arc<T> {
2070 default fn eq(&self, other: &Arc<T>) -> bool {
2074 default fn ne(&self, other: &Arc<T>) -> bool {
2079 /// We're doing this specialization here, and not as a more general optimization on `&T`, because it
2080 /// would otherwise add a cost to all equality checks on refs. We assume that `Arc`s are used to
2081 /// store large values, that are slow to clone, but also heavy to check for equality, causing this
2082 /// cost to pay off more easily. It's also more likely to have two `Arc` clones, that point to
2083 /// the same value, than two `&T`s.
2085 /// We can only do this when `T: Eq` as a `PartialEq` might be deliberately irreflexive.
2086 #[stable(feature = "rust1", since = "1.0.0")]
2087 impl<T: ?Sized + crate::rc::MarkerEq> ArcEqIdent<T> for Arc<T> {
2089 fn eq(&self, other: &Arc<T>) -> bool {
2090 Arc::ptr_eq(self, other) || **self == **other
2094 fn ne(&self, other: &Arc<T>) -> bool {
2095 !Arc::ptr_eq(self, other) && **self != **other
2099 #[stable(feature = "rust1", since = "1.0.0")]
2100 impl<T: ?Sized + PartialEq> PartialEq for Arc<T> {
2101 /// Equality for two `Arc`s.
2103 /// Two `Arc`s are equal if their inner values are equal, even if they are
2104 /// stored in different allocation.
2106 /// If `T` also implements `Eq` (implying reflexivity of equality),
2107 /// two `Arc`s that point to the same allocation are always equal.
2112 /// use std::sync::Arc;
2114 /// let five = Arc::new(5);
2116 /// assert!(five == Arc::new(5));
2119 fn eq(&self, other: &Arc<T>) -> bool {
2120 ArcEqIdent::eq(self, other)
2123 /// Inequality for two `Arc`s.
2125 /// Two `Arc`s are unequal if their inner values are unequal.
2127 /// If `T` also implements `Eq` (implying reflexivity of equality),
2128 /// two `Arc`s that point to the same value are never unequal.
2133 /// use std::sync::Arc;
2135 /// let five = Arc::new(5);
2137 /// assert!(five != Arc::new(6));
2140 fn ne(&self, other: &Arc<T>) -> bool {
2141 ArcEqIdent::ne(self, other)
2145 #[stable(feature = "rust1", since = "1.0.0")]
2146 impl<T: ?Sized + PartialOrd> PartialOrd for Arc<T> {
2147 /// Partial comparison for two `Arc`s.
2149 /// The two are compared by calling `partial_cmp()` on their inner values.
2154 /// use std::sync::Arc;
2155 /// use std::cmp::Ordering;
2157 /// let five = Arc::new(5);
2159 /// assert_eq!(Some(Ordering::Less), five.partial_cmp(&Arc::new(6)));
2161 fn partial_cmp(&self, other: &Arc<T>) -> Option<Ordering> {
2162 (**self).partial_cmp(&**other)
2165 /// Less-than comparison for two `Arc`s.
2167 /// The two are compared by calling `<` on their inner values.
2172 /// use std::sync::Arc;
2174 /// let five = Arc::new(5);
2176 /// assert!(five < Arc::new(6));
2178 fn lt(&self, other: &Arc<T>) -> bool {
2179 *(*self) < *(*other)
2182 /// 'Less than or equal to' comparison for two `Arc`s.
2184 /// The two are compared by calling `<=` on their inner values.
2189 /// use std::sync::Arc;
2191 /// let five = Arc::new(5);
2193 /// assert!(five <= Arc::new(5));
2195 fn le(&self, other: &Arc<T>) -> bool {
2196 *(*self) <= *(*other)
2199 /// Greater-than comparison for two `Arc`s.
2201 /// The two are compared by calling `>` on their inner values.
2206 /// use std::sync::Arc;
2208 /// let five = Arc::new(5);
2210 /// assert!(five > Arc::new(4));
2212 fn gt(&self, other: &Arc<T>) -> bool {
2213 *(*self) > *(*other)
2216 /// 'Greater than or equal to' comparison for two `Arc`s.
2218 /// The two are compared by calling `>=` on their inner values.
2223 /// use std::sync::Arc;
2225 /// let five = Arc::new(5);
2227 /// assert!(five >= Arc::new(5));
2229 fn ge(&self, other: &Arc<T>) -> bool {
2230 *(*self) >= *(*other)
2233 #[stable(feature = "rust1", since = "1.0.0")]
2234 impl<T: ?Sized + Ord> Ord for Arc<T> {
2235 /// Comparison for two `Arc`s.
2237 /// The two are compared by calling `cmp()` on their inner values.
2242 /// use std::sync::Arc;
2243 /// use std::cmp::Ordering;
2245 /// let five = Arc::new(5);
2247 /// assert_eq!(Ordering::Less, five.cmp(&Arc::new(6)));
2249 fn cmp(&self, other: &Arc<T>) -> Ordering {
2250 (**self).cmp(&**other)
2253 #[stable(feature = "rust1", since = "1.0.0")]
2254 impl<T: ?Sized + Eq> Eq for Arc<T> {}
2256 #[stable(feature = "rust1", since = "1.0.0")]
2257 impl<T: ?Sized + fmt::Display> fmt::Display for Arc<T> {
2258 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
2259 fmt::Display::fmt(&**self, f)
2263 #[stable(feature = "rust1", since = "1.0.0")]
2264 impl<T: ?Sized + fmt::Debug> fmt::Debug for Arc<T> {
2265 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
2266 fmt::Debug::fmt(&**self, f)
2270 #[stable(feature = "rust1", since = "1.0.0")]
2271 impl<T: ?Sized> fmt::Pointer for Arc<T> {
2272 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
2273 fmt::Pointer::fmt(&(&**self as *const T), f)
2277 #[stable(feature = "rust1", since = "1.0.0")]
2278 impl<T: Default> Default for Arc<T> {
2279 /// Creates a new `Arc<T>`, with the `Default` value for `T`.
2284 /// use std::sync::Arc;
2286 /// let x: Arc<i32> = Default::default();
2287 /// assert_eq!(*x, 0);
2289 fn default() -> Arc<T> {
2290 Arc::new(Default::default())
2294 #[stable(feature = "rust1", since = "1.0.0")]
2295 impl<T: ?Sized + Hash> Hash for Arc<T> {
2296 fn hash<H: Hasher>(&self, state: &mut H) {
2297 (**self).hash(state)
2301 #[stable(feature = "from_for_ptrs", since = "1.6.0")]
2302 impl<T> From<T> for Arc<T> {
2303 /// Converts a `T` into an `Arc<T>`
2305 /// The conversion moves the value into a
2306 /// newly allocated `Arc`. It is equivalent to
2307 /// calling `Arc::new(t)`.
2311 /// # use std::sync::Arc;
2313 /// let arc = Arc::new(5);
2315 /// assert_eq!(Arc::from(x), arc);
2317 fn from(t: T) -> Self {
2322 #[cfg(not(no_global_oom_handling))]
2323 #[stable(feature = "shared_from_slice", since = "1.21.0")]
2324 impl<T: Clone> From<&[T]> for Arc<[T]> {
2325 /// Allocate a reference-counted slice and fill it by cloning `v`'s items.
2330 /// # use std::sync::Arc;
2331 /// let original: &[i32] = &[1, 2, 3];
2332 /// let shared: Arc<[i32]> = Arc::from(original);
2333 /// assert_eq!(&[1, 2, 3], &shared[..]);
2336 fn from(v: &[T]) -> Arc<[T]> {
2337 <Self as ArcFromSlice<T>>::from_slice(v)
2341 #[cfg(not(no_global_oom_handling))]
2342 #[stable(feature = "shared_from_slice", since = "1.21.0")]
2343 impl From<&str> for Arc<str> {
2344 /// Allocate a reference-counted `str` and copy `v` into it.
2349 /// # use std::sync::Arc;
2350 /// let shared: Arc<str> = Arc::from("eggplant");
2351 /// assert_eq!("eggplant", &shared[..]);
2354 fn from(v: &str) -> Arc<str> {
2355 let arc = Arc::<[u8]>::from(v.as_bytes());
2356 unsafe { Arc::from_raw(Arc::into_raw(arc) as *const str) }
2360 #[cfg(not(no_global_oom_handling))]
2361 #[stable(feature = "shared_from_slice", since = "1.21.0")]
2362 impl From<String> for Arc<str> {
2363 /// Allocate a reference-counted `str` and copy `v` into it.
2368 /// # use std::sync::Arc;
2369 /// let unique: String = "eggplant".to_owned();
2370 /// let shared: Arc<str> = Arc::from(unique);
2371 /// assert_eq!("eggplant", &shared[..]);
2374 fn from(v: String) -> Arc<str> {
2379 #[cfg(not(no_global_oom_handling))]
2380 #[stable(feature = "shared_from_slice", since = "1.21.0")]
2381 impl<T: ?Sized> From<Box<T>> for Arc<T> {
2382 /// Move a boxed object to a new, reference-counted allocation.
2387 /// # use std::sync::Arc;
2388 /// let unique: Box<str> = Box::from("eggplant");
2389 /// let shared: Arc<str> = Arc::from(unique);
2390 /// assert_eq!("eggplant", &shared[..]);
2393 fn from(v: Box<T>) -> Arc<T> {
2398 #[cfg(not(no_global_oom_handling))]
2399 #[stable(feature = "shared_from_slice", since = "1.21.0")]
2400 impl<T> From<Vec<T>> for Arc<[T]> {
2401 /// Allocate a reference-counted slice and move `v`'s items into it.
2406 /// # use std::sync::Arc;
2407 /// let unique: Vec<i32> = vec![1, 2, 3];
2408 /// let shared: Arc<[i32]> = Arc::from(unique);
2409 /// assert_eq!(&[1, 2, 3], &shared[..]);
2412 fn from(mut v: Vec<T>) -> Arc<[T]> {
2414 let arc = Arc::copy_from_slice(&v);
2416 // Allow the Vec to free its memory, but not destroy its contents
2424 #[stable(feature = "shared_from_cow", since = "1.45.0")]
2425 impl<'a, B> From<Cow<'a, B>> for Arc<B>
2427 B: ToOwned + ?Sized,
2428 Arc<B>: From<&'a B> + From<B::Owned>,
2430 /// Create an atomically reference-counted pointer from
2431 /// a clone-on-write pointer by copying its content.
2436 /// # use std::sync::Arc;
2437 /// # use std::borrow::Cow;
2438 /// let cow: Cow<str> = Cow::Borrowed("eggplant");
2439 /// let shared: Arc<str> = Arc::from(cow);
2440 /// assert_eq!("eggplant", &shared[..]);
2443 fn from(cow: Cow<'a, B>) -> Arc<B> {
2445 Cow::Borrowed(s) => Arc::from(s),
2446 Cow::Owned(s) => Arc::from(s),
2451 #[stable(feature = "boxed_slice_try_from", since = "1.43.0")]
2452 impl<T, const N: usize> TryFrom<Arc<[T]>> for Arc<[T; N]> {
2453 type Error = Arc<[T]>;
2455 fn try_from(boxed_slice: Arc<[T]>) -> Result<Self, Self::Error> {
2456 if boxed_slice.len() == N {
2457 Ok(unsafe { Arc::from_raw(Arc::into_raw(boxed_slice) as *mut [T; N]) })
2464 #[cfg(not(no_global_oom_handling))]
2465 #[stable(feature = "shared_from_iter", since = "1.37.0")]
2466 impl<T> iter::FromIterator<T> for Arc<[T]> {
2467 /// Takes each element in the `Iterator` and collects it into an `Arc<[T]>`.
2469 /// # Performance characteristics
2471 /// ## The general case
2473 /// In the general case, collecting into `Arc<[T]>` is done by first
2474 /// collecting into a `Vec<T>`. That is, when writing the following:
2477 /// # use std::sync::Arc;
2478 /// let evens: Arc<[u8]> = (0..10).filter(|&x| x % 2 == 0).collect();
2479 /// # assert_eq!(&*evens, &[0, 2, 4, 6, 8]);
2482 /// this behaves as if we wrote:
2485 /// # use std::sync::Arc;
2486 /// let evens: Arc<[u8]> = (0..10).filter(|&x| x % 2 == 0)
2487 /// .collect::<Vec<_>>() // The first set of allocations happens here.
2488 /// .into(); // A second allocation for `Arc<[T]>` happens here.
2489 /// # assert_eq!(&*evens, &[0, 2, 4, 6, 8]);
2492 /// This will allocate as many times as needed for constructing the `Vec<T>`
2493 /// and then it will allocate once for turning the `Vec<T>` into the `Arc<[T]>`.
2495 /// ## Iterators of known length
2497 /// When your `Iterator` implements `TrustedLen` and is of an exact size,
2498 /// a single allocation will be made for the `Arc<[T]>`. For example:
2501 /// # use std::sync::Arc;
2502 /// let evens: Arc<[u8]> = (0..10).collect(); // Just a single allocation happens here.
2503 /// # assert_eq!(&*evens, &*(0..10).collect::<Vec<_>>());
2505 fn from_iter<I: iter::IntoIterator<Item = T>>(iter: I) -> Self {
2506 ToArcSlice::to_arc_slice(iter.into_iter())
2510 /// Specialization trait used for collecting into `Arc<[T]>`.
2511 trait ToArcSlice<T>: Iterator<Item = T> + Sized {
2512 fn to_arc_slice(self) -> Arc<[T]>;
2515 #[cfg(not(no_global_oom_handling))]
2516 impl<T, I: Iterator<Item = T>> ToArcSlice<T> for I {
2517 default fn to_arc_slice(self) -> Arc<[T]> {
2518 self.collect::<Vec<T>>().into()
2522 #[cfg(not(no_global_oom_handling))]
2523 impl<T, I: iter::TrustedLen<Item = T>> ToArcSlice<T> for I {
2524 fn to_arc_slice(self) -> Arc<[T]> {
2525 // This is the case for a `TrustedLen` iterator.
2526 let (low, high) = self.size_hint();
2527 if let Some(high) = high {
2531 "TrustedLen iterator's size hint is not exact: {:?}",
2536 // SAFETY: We need to ensure that the iterator has an exact length and we have.
2537 Arc::from_iter_exact(self, low)
2540 // TrustedLen contract guarantees that `upper_bound == `None` implies an iterator
2541 // length exceeding `usize::MAX`.
2542 // The default implementation would collect into a vec which would panic.
2543 // Thus we panic here immediately without invoking `Vec` code.
2544 panic!("capacity overflow");
2549 #[stable(feature = "rust1", since = "1.0.0")]
2550 impl<T: ?Sized> borrow::Borrow<T> for Arc<T> {
2551 fn borrow(&self) -> &T {
2556 #[stable(since = "1.5.0", feature = "smart_ptr_as_ref")]
2557 impl<T: ?Sized> AsRef<T> for Arc<T> {
2558 fn as_ref(&self) -> &T {
2563 #[stable(feature = "pin", since = "1.33.0")]
2564 impl<T: ?Sized> Unpin for Arc<T> {}
2566 /// Get the offset within an `ArcInner` for the payload behind a pointer.
2570 /// The pointer must point to (and have valid metadata for) a previously
2571 /// valid instance of T, but the T is allowed to be dropped.
2572 unsafe fn data_offset<T: ?Sized>(ptr: *const T) -> isize {
2573 // Align the unsized value to the end of the ArcInner.
2574 // Because RcBox is repr(C), it will always be the last field in memory.
2575 // SAFETY: since the only unsized types possible are slices, trait objects,
2576 // and extern types, the input safety requirement is currently enough to
2577 // satisfy the requirements of align_of_val_raw; this is an implementation
2578 // detail of the language that may not be relied upon outside of std.
2579 unsafe { data_offset_align(align_of_val_raw(ptr)) }
2583 fn data_offset_align(align: usize) -> isize {
2584 let layout = Layout::new::<ArcInner<()>>();
2585 (layout.size() + layout.padding_needed_for(align)) as isize