1 #![stable(feature = "rust1", since = "1.0.0")]
3 //! Thread-safe reference-counting pointers.
5 //! See the [`Arc<T>`][Arc] documentation for more details.
7 //! **Note**: This module is only available on platforms that support atomic
8 //! loads and stores of pointers. This may be detected at compile time using
9 //! `#[cfg(target_has_atomic = "ptr")]`.
13 use core::cmp::Ordering;
14 use core::convert::{From, TryFrom};
16 use core::hash::{Hash, Hasher};
18 use core::intrinsics::abort;
19 #[cfg(not(no_global_oom_handling))]
21 use core::marker::{PhantomData, Unpin, Unsize};
22 #[cfg(not(no_global_oom_handling))]
23 use core::mem::size_of_val;
24 use core::mem::{self, align_of_val_raw};
25 use core::ops::{CoerceUnsized, Deref, DispatchFromDyn, Receiver};
26 use core::panic::{RefUnwindSafe, UnwindSafe};
28 use core::ptr::{self, NonNull};
29 #[cfg(not(no_global_oom_handling))]
30 use core::slice::from_raw_parts_mut;
31 use core::sync::atomic;
32 use core::sync::atomic::Ordering::{Acquire, Relaxed, Release};
34 #[cfg(not(no_global_oom_handling))]
35 use crate::alloc::handle_alloc_error;
36 #[cfg(not(no_global_oom_handling))]
37 use crate::alloc::{box_free, WriteCloneIntoRaw};
38 use crate::alloc::{AllocError, Allocator, Global, Layout};
39 use crate::borrow::{Cow, ToOwned};
40 use crate::boxed::Box;
41 use crate::rc::is_dangling;
42 #[cfg(not(no_global_oom_handling))]
43 use crate::string::String;
44 #[cfg(not(no_global_oom_handling))]
50 /// A soft limit on the amount of references that may be made to an `Arc`.
52 /// Going above this limit will abort your program (although not
53 /// necessarily) at _exactly_ `MAX_REFCOUNT + 1` references.
54 const MAX_REFCOUNT: usize = (isize::MAX) as usize;
56 #[cfg(not(sanitize = "thread"))]
57 macro_rules! acquire {
59 atomic::fence(Acquire)
63 // ThreadSanitizer does not support memory fences. To avoid false positive
64 // reports in Arc / Weak implementation use atomic loads for synchronization
66 #[cfg(sanitize = "thread")]
67 macro_rules! acquire {
73 /// A thread-safe reference-counting pointer. 'Arc' stands for 'Atomically
74 /// Reference Counted'.
76 /// The type `Arc<T>` provides shared ownership of a value of type `T`,
77 /// allocated in the heap. Invoking [`clone`][clone] on `Arc` produces
78 /// a new `Arc` instance, which points to the same allocation on the heap as the
79 /// source `Arc`, while increasing a reference count. When the last `Arc`
80 /// pointer to a given allocation is destroyed, the value stored in that allocation (often
81 /// referred to as "inner value") is also dropped.
83 /// Shared references in Rust disallow mutation by default, and `Arc` is no
84 /// exception: you cannot generally obtain a mutable reference to something
85 /// inside an `Arc`. If you need to mutate through an `Arc`, use
86 /// [`Mutex`][mutex], [`RwLock`][rwlock], or one of the [`Atomic`][atomic]
89 /// **Note**: This type is only available on platforms that support atomic
90 /// loads and stores of pointers, which includes all platforms that support
91 /// the `std` crate but not all those which only support [`alloc`](crate).
92 /// This may be detected at compile time using `#[cfg(target_has_atomic = "ptr")]`.
96 /// Unlike [`Rc<T>`], `Arc<T>` uses atomic operations for its reference
97 /// counting. This means that it is thread-safe. The disadvantage is that
98 /// atomic operations are more expensive than ordinary memory accesses. If you
99 /// are not sharing reference-counted allocations between threads, consider using
100 /// [`Rc<T>`] for lower overhead. [`Rc<T>`] is a safe default, because the
101 /// compiler will catch any attempt to send an [`Rc<T>`] between threads.
102 /// However, a library might choose `Arc<T>` in order to give library consumers
103 /// more flexibility.
105 /// `Arc<T>` will implement [`Send`] and [`Sync`] as long as the `T` implements
106 /// [`Send`] and [`Sync`]. Why can't you put a non-thread-safe type `T` in an
107 /// `Arc<T>` to make it thread-safe? This may be a bit counter-intuitive at
108 /// first: after all, isn't the point of `Arc<T>` thread safety? The key is
109 /// this: `Arc<T>` makes it thread safe to have multiple ownership of the same
110 /// data, but it doesn't add thread safety to its data. Consider
111 /// <code>Arc<[RefCell\<T>]></code>. [`RefCell<T>`] isn't [`Sync`], and if `Arc<T>` was always
112 /// [`Send`], <code>Arc<[RefCell\<T>]></code> would be as well. But then we'd have a problem:
113 /// [`RefCell<T>`] is not thread safe; it keeps track of the borrowing count using
114 /// non-atomic operations.
116 /// In the end, this means that you may need to pair `Arc<T>` with some sort of
117 /// [`std::sync`] type, usually [`Mutex<T>`][mutex].
119 /// ## Breaking cycles with `Weak`
121 /// The [`downgrade`][downgrade] method can be used to create a non-owning
122 /// [`Weak`] pointer. A [`Weak`] pointer can be [`upgrade`][upgrade]d
123 /// to an `Arc`, but this will return [`None`] if the value stored in the allocation has
124 /// already been dropped. In other words, `Weak` pointers do not keep the value
125 /// inside the allocation alive; however, they *do* keep the allocation
126 /// (the backing store for the value) alive.
128 /// A cycle between `Arc` pointers will never be deallocated. For this reason,
129 /// [`Weak`] is used to break cycles. For example, a tree could have
130 /// strong `Arc` pointers from parent nodes to children, and [`Weak`]
131 /// pointers from children back to their parents.
133 /// # Cloning references
135 /// Creating a new reference from an existing reference-counted pointer is done using the
136 /// `Clone` trait implemented for [`Arc<T>`][Arc] and [`Weak<T>`][Weak].
139 /// use std::sync::Arc;
140 /// let foo = Arc::new(vec![1.0, 2.0, 3.0]);
141 /// // The two syntaxes below are equivalent.
142 /// let a = foo.clone();
143 /// let b = Arc::clone(&foo);
144 /// // a, b, and foo are all Arcs that point to the same memory location
147 /// ## `Deref` behavior
149 /// `Arc<T>` automatically dereferences to `T` (via the [`Deref`][deref] trait),
150 /// so you can call `T`'s methods on a value of type `Arc<T>`. To avoid name
151 /// clashes with `T`'s methods, the methods of `Arc<T>` itself are associated
152 /// functions, called using [fully qualified syntax]:
155 /// use std::sync::Arc;
157 /// let my_arc = Arc::new(());
158 /// let my_weak = Arc::downgrade(&my_arc);
161 /// `Arc<T>`'s implementations of traits like `Clone` may also be called using
162 /// fully qualified syntax. Some people prefer to use fully qualified syntax,
163 /// while others prefer using method-call syntax.
166 /// use std::sync::Arc;
168 /// let arc = Arc::new(());
169 /// // Method-call syntax
170 /// let arc2 = arc.clone();
171 /// // Fully qualified syntax
172 /// let arc3 = Arc::clone(&arc);
175 /// [`Weak<T>`][Weak] does not auto-dereference to `T`, because the inner value may have
176 /// already been dropped.
178 /// [`Rc<T>`]: crate::rc::Rc
179 /// [clone]: Clone::clone
180 /// [mutex]: ../../std/sync/struct.Mutex.html
181 /// [rwlock]: ../../std/sync/struct.RwLock.html
182 /// [atomic]: core::sync::atomic
183 /// [`Send`]: core::marker::Send
184 /// [`Sync`]: core::marker::Sync
185 /// [deref]: core::ops::Deref
186 /// [downgrade]: Arc::downgrade
187 /// [upgrade]: Weak::upgrade
188 /// [RefCell\<T>]: core::cell::RefCell
189 /// [`RefCell<T>`]: core::cell::RefCell
190 /// [`std::sync`]: ../../std/sync/index.html
191 /// [`Arc::clone(&from)`]: Arc::clone
192 /// [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
196 /// Sharing some immutable data between threads:
198 // Note that we **do not** run these tests here. The windows builders get super
199 // unhappy if a thread outlives the main thread and then exits at the same time
200 // (something deadlocks) so we just avoid this entirely by not running these
203 /// use std::sync::Arc;
206 /// let five = Arc::new(5);
209 /// let five = Arc::clone(&five);
211 /// thread::spawn(move || {
212 /// println!("{five:?}");
217 /// Sharing a mutable [`AtomicUsize`]:
219 /// [`AtomicUsize`]: core::sync::atomic::AtomicUsize "sync::atomic::AtomicUsize"
222 /// use std::sync::Arc;
223 /// use std::sync::atomic::{AtomicUsize, Ordering};
226 /// let val = Arc::new(AtomicUsize::new(5));
229 /// let val = Arc::clone(&val);
231 /// thread::spawn(move || {
232 /// let v = val.fetch_add(1, Ordering::SeqCst);
233 /// println!("{v:?}");
238 /// See the [`rc` documentation][rc_examples] for more examples of reference
239 /// counting in general.
241 /// [rc_examples]: crate::rc#examples
242 #[cfg_attr(not(test), rustc_diagnostic_item = "Arc")]
243 #[stable(feature = "rust1", since = "1.0.0")]
244 pub struct Arc<T: ?Sized> {
245 ptr: NonNull<ArcInner<T>>,
246 phantom: PhantomData<ArcInner<T>>,
249 #[stable(feature = "rust1", since = "1.0.0")]
250 unsafe impl<T: ?Sized + Sync + Send> Send for Arc<T> {}
251 #[stable(feature = "rust1", since = "1.0.0")]
252 unsafe impl<T: ?Sized + Sync + Send> Sync for Arc<T> {}
254 #[stable(feature = "catch_unwind", since = "1.9.0")]
255 impl<T: RefUnwindSafe + ?Sized> UnwindSafe for Arc<T> {}
257 #[unstable(feature = "coerce_unsized", issue = "27732")]
258 impl<T: ?Sized + Unsize<U>, U: ?Sized> CoerceUnsized<Arc<U>> for Arc<T> {}
260 #[unstable(feature = "dispatch_from_dyn", issue = "none")]
261 impl<T: ?Sized + Unsize<U>, U: ?Sized> DispatchFromDyn<Arc<U>> for Arc<T> {}
263 impl<T: ?Sized> Arc<T> {
264 unsafe fn from_inner(ptr: NonNull<ArcInner<T>>) -> Self {
265 Self { ptr, phantom: PhantomData }
268 unsafe fn from_ptr(ptr: *mut ArcInner<T>) -> Self {
269 unsafe { Self::from_inner(NonNull::new_unchecked(ptr)) }
273 /// `Weak` is a version of [`Arc`] that holds a non-owning reference to the
274 /// managed allocation. The allocation is accessed by calling [`upgrade`] on the `Weak`
275 /// pointer, which returns an <code>[Option]<[Arc]\<T>></code>.
277 /// Since a `Weak` reference does not count towards ownership, it will not
278 /// prevent the value stored in the allocation from being dropped, and `Weak` itself makes no
279 /// guarantees about the value still being present. Thus it may return [`None`]
280 /// when [`upgrade`]d. Note however that a `Weak` reference *does* prevent the allocation
281 /// itself (the backing store) from being deallocated.
283 /// A `Weak` pointer is useful for keeping a temporary reference to the allocation
284 /// managed by [`Arc`] without preventing its inner value from being dropped. It is also used to
285 /// prevent circular references between [`Arc`] pointers, since mutual owning references
286 /// would never allow either [`Arc`] to be dropped. For example, a tree could
287 /// have strong [`Arc`] pointers from parent nodes to children, and `Weak`
288 /// pointers from children back to their parents.
290 /// The typical way to obtain a `Weak` pointer is to call [`Arc::downgrade`].
292 /// [`upgrade`]: Weak::upgrade
293 #[stable(feature = "arc_weak", since = "1.4.0")]
294 pub struct Weak<T: ?Sized> {
295 // This is a `NonNull` to allow optimizing the size of this type in enums,
296 // but it is not necessarily a valid pointer.
297 // `Weak::new` sets this to `usize::MAX` so that it doesn’t need
298 // to allocate space on the heap. That's not a value a real pointer
299 // will ever have because RcBox has alignment at least 2.
300 // This is only possible when `T: Sized`; unsized `T` never dangle.
301 ptr: NonNull<ArcInner<T>>,
304 #[stable(feature = "arc_weak", since = "1.4.0")]
305 unsafe impl<T: ?Sized + Sync + Send> Send for Weak<T> {}
306 #[stable(feature = "arc_weak", since = "1.4.0")]
307 unsafe impl<T: ?Sized + Sync + Send> Sync for Weak<T> {}
309 #[unstable(feature = "coerce_unsized", issue = "27732")]
310 impl<T: ?Sized + Unsize<U>, U: ?Sized> CoerceUnsized<Weak<U>> for Weak<T> {}
311 #[unstable(feature = "dispatch_from_dyn", issue = "none")]
312 impl<T: ?Sized + Unsize<U>, U: ?Sized> DispatchFromDyn<Weak<U>> for Weak<T> {}
314 #[stable(feature = "arc_weak", since = "1.4.0")]
315 impl<T: ?Sized + fmt::Debug> fmt::Debug for Weak<T> {
316 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
321 // This is repr(C) to future-proof against possible field-reordering, which
322 // would interfere with otherwise safe [into|from]_raw() of transmutable
325 struct ArcInner<T: ?Sized> {
326 strong: atomic::AtomicUsize,
328 // the value usize::MAX acts as a sentinel for temporarily "locking" the
329 // ability to upgrade weak pointers or downgrade strong ones; this is used
330 // to avoid races in `make_mut` and `get_mut`.
331 weak: atomic::AtomicUsize,
336 /// Calculate layout for `ArcInner<T>` using the inner value's layout
337 fn arcinner_layout_for_value_layout(layout: Layout) -> Layout {
338 // Calculate layout using the given value layout.
339 // Previously, layout was calculated on the expression
340 // `&*(ptr as *const ArcInner<T>)`, but this created a misaligned
341 // reference (see #54908).
342 Layout::new::<ArcInner<()>>().extend(layout).unwrap().0.pad_to_align()
345 unsafe impl<T: ?Sized + Sync + Send> Send for ArcInner<T> {}
346 unsafe impl<T: ?Sized + Sync + Send> Sync for ArcInner<T> {}
349 /// Constructs a new `Arc<T>`.
354 /// use std::sync::Arc;
356 /// let five = Arc::new(5);
358 #[cfg(not(no_global_oom_handling))]
360 #[stable(feature = "rust1", since = "1.0.0")]
361 pub fn new(data: T) -> Arc<T> {
362 // Start the weak pointer count as 1 which is the weak pointer that's
363 // held by all the strong pointers (kinda), see std/rc.rs for more info
364 let x: Box<_> = Box::new(ArcInner {
365 strong: atomic::AtomicUsize::new(1),
366 weak: atomic::AtomicUsize::new(1),
369 unsafe { Self::from_inner(Box::leak(x).into()) }
372 /// Constructs a new `Arc<T>` while giving you a `Weak<T>` to the allocation,
373 /// to allow you to construct a `T` which holds a weak pointer to itself.
375 /// Generally, a structure circularly referencing itself, either directly or
376 /// indirectly, should not hold a strong reference to itself to prevent a memory leak.
377 /// Using this function, you get access to the weak pointer during the
378 /// initialization of `T`, before the `Arc<T>` is created, such that you can
379 /// clone and store it inside the `T`.
381 /// `new_cyclic` first allocates the managed allocation for the `Arc<T>`,
382 /// then calls your closure, giving it a `Weak<T>` to this allocation,
383 /// and only afterwards completes the construction of the `Arc<T>` by placing
384 /// the `T` returned from your closure into the allocation.
386 /// Since the new `Arc<T>` is not fully-constructed until `Arc<T>::new_cyclic`
387 /// returns, calling [`upgrade`] on the weak reference inside your closure will
388 /// fail and result in a `None` value.
392 /// If `data_fn` panics, the panic is propagated to the caller, and the
393 /// temporary [`Weak<T>`] is dropped normally.
398 /// # #![allow(dead_code)]
399 /// use std::sync::{Arc, Weak};
402 /// me: Weak<Gadget>,
406 /// /// Construct a reference counted Gadget.
407 /// fn new() -> Arc<Self> {
408 /// // `me` is a `Weak<Gadget>` pointing at the new allocation of the
409 /// // `Arc` we're constructing.
410 /// Arc::new_cyclic(|me| {
411 /// // Create the actual struct here.
412 /// Gadget { me: me.clone() }
416 /// /// Return a reference counted pointer to Self.
417 /// fn me(&self) -> Arc<Self> {
418 /// self.me.upgrade().unwrap()
422 /// [`upgrade`]: Weak::upgrade
423 #[cfg(not(no_global_oom_handling))]
425 #[stable(feature = "arc_new_cyclic", since = "1.60.0")]
426 pub fn new_cyclic<F>(data_fn: F) -> Arc<T>
428 F: FnOnce(&Weak<T>) -> T,
430 // Construct the inner in the "uninitialized" state with a single
432 let uninit_ptr: NonNull<_> = Box::leak(Box::new(ArcInner {
433 strong: atomic::AtomicUsize::new(0),
434 weak: atomic::AtomicUsize::new(1),
435 data: mem::MaybeUninit::<T>::uninit(),
438 let init_ptr: NonNull<ArcInner<T>> = uninit_ptr.cast();
440 let weak = Weak { ptr: init_ptr };
442 // It's important we don't give up ownership of the weak pointer, or
443 // else the memory might be freed by the time `data_fn` returns. If
444 // we really wanted to pass ownership, we could create an additional
445 // weak pointer for ourselves, but this would result in additional
446 // updates to the weak reference count which might not be necessary
448 let data = data_fn(&weak);
450 // Now we can properly initialize the inner value and turn our weak
451 // reference into a strong reference.
452 let strong = unsafe {
453 let inner = init_ptr.as_ptr();
454 ptr::write(ptr::addr_of_mut!((*inner).data), data);
456 // The above write to the data field must be visible to any threads which
457 // observe a non-zero strong count. Therefore we need at least "Release" ordering
458 // in order to synchronize with the `compare_exchange_weak` in `Weak::upgrade`.
460 // "Acquire" ordering is not required. When considering the possible behaviours
461 // of `data_fn` we only need to look at what it could do with a reference to a
462 // non-upgradeable `Weak`:
463 // - It can *clone* the `Weak`, increasing the weak reference count.
464 // - It can drop those clones, decreasing the weak reference count (but never to zero).
466 // These side effects do not impact us in any way, and no other side effects are
467 // possible with safe code alone.
468 let prev_value = (*inner).strong.fetch_add(1, Release);
469 debug_assert_eq!(prev_value, 0, "No prior strong references should exist");
471 Arc::from_inner(init_ptr)
474 // Strong references should collectively own a shared weak reference,
475 // so don't run the destructor for our old weak reference.
480 /// Constructs a new `Arc` with uninitialized contents.
485 /// #![feature(new_uninit)]
486 /// #![feature(get_mut_unchecked)]
488 /// use std::sync::Arc;
490 /// let mut five = Arc::<u32>::new_uninit();
492 /// // Deferred initialization:
493 /// Arc::get_mut(&mut five).unwrap().write(5);
495 /// let five = unsafe { five.assume_init() };
497 /// assert_eq!(*five, 5)
499 #[cfg(not(no_global_oom_handling))]
500 #[unstable(feature = "new_uninit", issue = "63291")]
502 pub fn new_uninit() -> Arc<mem::MaybeUninit<T>> {
504 Arc::from_ptr(Arc::allocate_for_layout(
506 |layout| Global.allocate(layout),
507 |mem| mem as *mut ArcInner<mem::MaybeUninit<T>>,
512 /// Constructs a new `Arc` with uninitialized contents, with the memory
513 /// being filled with `0` bytes.
515 /// See [`MaybeUninit::zeroed`][zeroed] for examples of correct and incorrect usage
521 /// #![feature(new_uninit)]
523 /// use std::sync::Arc;
525 /// let zero = Arc::<u32>::new_zeroed();
526 /// let zero = unsafe { zero.assume_init() };
528 /// assert_eq!(*zero, 0)
531 /// [zeroed]: mem::MaybeUninit::zeroed
532 #[cfg(not(no_global_oom_handling))]
533 #[unstable(feature = "new_uninit", issue = "63291")]
535 pub fn new_zeroed() -> Arc<mem::MaybeUninit<T>> {
537 Arc::from_ptr(Arc::allocate_for_layout(
539 |layout| Global.allocate_zeroed(layout),
540 |mem| mem as *mut ArcInner<mem::MaybeUninit<T>>,
545 /// Constructs a new `Pin<Arc<T>>`. If `T` does not implement `Unpin`, then
546 /// `data` will be pinned in memory and unable to be moved.
547 #[cfg(not(no_global_oom_handling))]
548 #[stable(feature = "pin", since = "1.33.0")]
550 pub fn pin(data: T) -> Pin<Arc<T>> {
551 unsafe { Pin::new_unchecked(Arc::new(data)) }
554 /// Constructs a new `Pin<Arc<T>>`, return an error if allocation fails.
555 #[unstable(feature = "allocator_api", issue = "32838")]
557 pub fn try_pin(data: T) -> Result<Pin<Arc<T>>, AllocError> {
558 unsafe { Ok(Pin::new_unchecked(Arc::try_new(data)?)) }
561 /// Constructs a new `Arc<T>`, returning an error if allocation fails.
566 /// #![feature(allocator_api)]
567 /// use std::sync::Arc;
569 /// let five = Arc::try_new(5)?;
570 /// # Ok::<(), std::alloc::AllocError>(())
572 #[unstable(feature = "allocator_api", issue = "32838")]
574 pub fn try_new(data: T) -> Result<Arc<T>, AllocError> {
575 // Start the weak pointer count as 1 which is the weak pointer that's
576 // held by all the strong pointers (kinda), see std/rc.rs for more info
577 let x: Box<_> = Box::try_new(ArcInner {
578 strong: atomic::AtomicUsize::new(1),
579 weak: atomic::AtomicUsize::new(1),
582 unsafe { Ok(Self::from_inner(Box::leak(x).into())) }
585 /// Constructs a new `Arc` with uninitialized contents, returning an error
586 /// if allocation fails.
591 /// #![feature(new_uninit, allocator_api)]
592 /// #![feature(get_mut_unchecked)]
594 /// use std::sync::Arc;
596 /// let mut five = Arc::<u32>::try_new_uninit()?;
598 /// // Deferred initialization:
599 /// Arc::get_mut(&mut five).unwrap().write(5);
601 /// let five = unsafe { five.assume_init() };
603 /// assert_eq!(*five, 5);
604 /// # Ok::<(), std::alloc::AllocError>(())
606 #[unstable(feature = "allocator_api", issue = "32838")]
607 // #[unstable(feature = "new_uninit", issue = "63291")]
608 pub fn try_new_uninit() -> Result<Arc<mem::MaybeUninit<T>>, AllocError> {
610 Ok(Arc::from_ptr(Arc::try_allocate_for_layout(
612 |layout| Global.allocate(layout),
613 |mem| mem as *mut ArcInner<mem::MaybeUninit<T>>,
618 /// Constructs a new `Arc` with uninitialized contents, with the memory
619 /// being filled with `0` bytes, returning an error if allocation fails.
621 /// See [`MaybeUninit::zeroed`][zeroed] for examples of correct and incorrect usage
627 /// #![feature(new_uninit, allocator_api)]
629 /// use std::sync::Arc;
631 /// let zero = Arc::<u32>::try_new_zeroed()?;
632 /// let zero = unsafe { zero.assume_init() };
634 /// assert_eq!(*zero, 0);
635 /// # Ok::<(), std::alloc::AllocError>(())
638 /// [zeroed]: mem::MaybeUninit::zeroed
639 #[unstable(feature = "allocator_api", issue = "32838")]
640 // #[unstable(feature = "new_uninit", issue = "63291")]
641 pub fn try_new_zeroed() -> Result<Arc<mem::MaybeUninit<T>>, AllocError> {
643 Ok(Arc::from_ptr(Arc::try_allocate_for_layout(
645 |layout| Global.allocate_zeroed(layout),
646 |mem| mem as *mut ArcInner<mem::MaybeUninit<T>>,
650 /// Returns the inner value, if the `Arc` has exactly one strong reference.
652 /// Otherwise, an [`Err`] is returned with the same `Arc` that was
655 /// This will succeed even if there are outstanding weak references.
660 /// use std::sync::Arc;
662 /// let x = Arc::new(3);
663 /// assert_eq!(Arc::try_unwrap(x), Ok(3));
665 /// let x = Arc::new(4);
666 /// let _y = Arc::clone(&x);
667 /// assert_eq!(*Arc::try_unwrap(x).unwrap_err(), 4);
670 #[stable(feature = "arc_unique", since = "1.4.0")]
671 pub fn try_unwrap(this: Self) -> Result<T, Self> {
672 if this.inner().strong.compare_exchange(1, 0, Relaxed, Relaxed).is_err() {
676 acquire!(this.inner().strong);
679 let elem = ptr::read(&this.ptr.as_ref().data);
681 // Make a weak pointer to clean up the implicit strong-weak reference
682 let _weak = Weak { ptr: this.ptr };
691 /// Constructs a new atomically reference-counted slice with uninitialized contents.
696 /// #![feature(new_uninit)]
697 /// #![feature(get_mut_unchecked)]
699 /// use std::sync::Arc;
701 /// let mut values = Arc::<[u32]>::new_uninit_slice(3);
703 /// // Deferred initialization:
704 /// let data = Arc::get_mut(&mut values).unwrap();
705 /// data[0].write(1);
706 /// data[1].write(2);
707 /// data[2].write(3);
709 /// let values = unsafe { values.assume_init() };
711 /// assert_eq!(*values, [1, 2, 3])
713 #[cfg(not(no_global_oom_handling))]
714 #[unstable(feature = "new_uninit", issue = "63291")]
716 pub fn new_uninit_slice(len: usize) -> Arc<[mem::MaybeUninit<T>]> {
717 unsafe { Arc::from_ptr(Arc::allocate_for_slice(len)) }
720 /// Constructs a new atomically reference-counted slice with uninitialized contents, with the memory being
721 /// filled with `0` bytes.
723 /// See [`MaybeUninit::zeroed`][zeroed] for examples of correct and
724 /// incorrect usage of this method.
729 /// #![feature(new_uninit)]
731 /// use std::sync::Arc;
733 /// let values = Arc::<[u32]>::new_zeroed_slice(3);
734 /// let values = unsafe { values.assume_init() };
736 /// assert_eq!(*values, [0, 0, 0])
739 /// [zeroed]: mem::MaybeUninit::zeroed
740 #[cfg(not(no_global_oom_handling))]
741 #[unstable(feature = "new_uninit", issue = "63291")]
743 pub fn new_zeroed_slice(len: usize) -> Arc<[mem::MaybeUninit<T>]> {
745 Arc::from_ptr(Arc::allocate_for_layout(
746 Layout::array::<T>(len).unwrap(),
747 |layout| Global.allocate_zeroed(layout),
749 ptr::slice_from_raw_parts_mut(mem as *mut T, len)
750 as *mut ArcInner<[mem::MaybeUninit<T>]>
757 impl<T> Arc<mem::MaybeUninit<T>> {
758 /// Converts to `Arc<T>`.
762 /// As with [`MaybeUninit::assume_init`],
763 /// it is up to the caller to guarantee that the inner value
764 /// really is in an initialized state.
765 /// Calling this when the content is not yet fully initialized
766 /// causes immediate undefined behavior.
768 /// [`MaybeUninit::assume_init`]: mem::MaybeUninit::assume_init
773 /// #![feature(new_uninit)]
774 /// #![feature(get_mut_unchecked)]
776 /// use std::sync::Arc;
778 /// let mut five = Arc::<u32>::new_uninit();
780 /// // Deferred initialization:
781 /// Arc::get_mut(&mut five).unwrap().write(5);
783 /// let five = unsafe { five.assume_init() };
785 /// assert_eq!(*five, 5)
787 #[unstable(feature = "new_uninit", issue = "63291")]
788 #[must_use = "`self` will be dropped if the result is not used"]
790 pub unsafe fn assume_init(self) -> Arc<T> {
791 unsafe { Arc::from_inner(mem::ManuallyDrop::new(self).ptr.cast()) }
795 impl<T> Arc<[mem::MaybeUninit<T>]> {
796 /// Converts to `Arc<[T]>`.
800 /// As with [`MaybeUninit::assume_init`],
801 /// it is up to the caller to guarantee that the inner value
802 /// really is in an initialized state.
803 /// Calling this when the content is not yet fully initialized
804 /// causes immediate undefined behavior.
806 /// [`MaybeUninit::assume_init`]: mem::MaybeUninit::assume_init
811 /// #![feature(new_uninit)]
812 /// #![feature(get_mut_unchecked)]
814 /// use std::sync::Arc;
816 /// let mut values = Arc::<[u32]>::new_uninit_slice(3);
818 /// // Deferred initialization:
819 /// let data = Arc::get_mut(&mut values).unwrap();
820 /// data[0].write(1);
821 /// data[1].write(2);
822 /// data[2].write(3);
824 /// let values = unsafe { values.assume_init() };
826 /// assert_eq!(*values, [1, 2, 3])
828 #[unstable(feature = "new_uninit", issue = "63291")]
829 #[must_use = "`self` will be dropped if the result is not used"]
831 pub unsafe fn assume_init(self) -> Arc<[T]> {
832 unsafe { Arc::from_ptr(mem::ManuallyDrop::new(self).ptr.as_ptr() as _) }
836 impl<T: ?Sized> Arc<T> {
837 /// Consumes the `Arc`, returning the wrapped pointer.
839 /// To avoid a memory leak the pointer must be converted back to an `Arc` using
840 /// [`Arc::from_raw`].
845 /// use std::sync::Arc;
847 /// let x = Arc::new("hello".to_owned());
848 /// let x_ptr = Arc::into_raw(x);
849 /// assert_eq!(unsafe { &*x_ptr }, "hello");
851 #[must_use = "losing the pointer will leak memory"]
852 #[stable(feature = "rc_raw", since = "1.17.0")]
853 pub fn into_raw(this: Self) -> *const T {
854 let ptr = Self::as_ptr(&this);
859 /// Provides a raw pointer to the data.
861 /// The counts are not affected in any way and the `Arc` is not consumed. The pointer is valid for
862 /// as long as there are strong counts in the `Arc`.
867 /// use std::sync::Arc;
869 /// let x = Arc::new("hello".to_owned());
870 /// let y = Arc::clone(&x);
871 /// let x_ptr = Arc::as_ptr(&x);
872 /// assert_eq!(x_ptr, Arc::as_ptr(&y));
873 /// assert_eq!(unsafe { &*x_ptr }, "hello");
876 #[stable(feature = "rc_as_ptr", since = "1.45.0")]
877 pub fn as_ptr(this: &Self) -> *const T {
878 let ptr: *mut ArcInner<T> = NonNull::as_ptr(this.ptr);
880 // SAFETY: This cannot go through Deref::deref or RcBoxPtr::inner because
881 // this is required to retain raw/mut provenance such that e.g. `get_mut` can
882 // write through the pointer after the Rc is recovered through `from_raw`.
883 unsafe { ptr::addr_of_mut!((*ptr).data) }
886 /// Constructs an `Arc<T>` from a raw pointer.
888 /// The raw pointer must have been previously returned by a call to
889 /// [`Arc<U>::into_raw`][into_raw] where `U` must have the same size and
890 /// alignment as `T`. This is trivially true if `U` is `T`.
891 /// Note that if `U` is not `T` but has the same size and alignment, this is
892 /// basically like transmuting references of different types. See
893 /// [`mem::transmute`][transmute] for more information on what
894 /// restrictions apply in this case.
896 /// The user of `from_raw` has to make sure a specific value of `T` is only
899 /// This function is unsafe because improper use may lead to memory unsafety,
900 /// even if the returned `Arc<T>` is never accessed.
902 /// [into_raw]: Arc::into_raw
903 /// [transmute]: core::mem::transmute
908 /// use std::sync::Arc;
910 /// let x = Arc::new("hello".to_owned());
911 /// let x_ptr = Arc::into_raw(x);
914 /// // Convert back to an `Arc` to prevent leak.
915 /// let x = Arc::from_raw(x_ptr);
916 /// assert_eq!(&*x, "hello");
918 /// // Further calls to `Arc::from_raw(x_ptr)` would be memory-unsafe.
921 /// // The memory was freed when `x` went out of scope above, so `x_ptr` is now dangling!
923 #[stable(feature = "rc_raw", since = "1.17.0")]
924 pub unsafe fn from_raw(ptr: *const T) -> Self {
926 let offset = data_offset(ptr);
928 // Reverse the offset to find the original ArcInner.
929 let arc_ptr = ptr.byte_sub(offset) as *mut ArcInner<T>;
931 Self::from_ptr(arc_ptr)
935 /// Creates a new [`Weak`] pointer to this allocation.
940 /// use std::sync::Arc;
942 /// let five = Arc::new(5);
944 /// let weak_five = Arc::downgrade(&five);
946 #[must_use = "this returns a new `Weak` pointer, \
947 without modifying the original `Arc`"]
948 #[stable(feature = "arc_weak", since = "1.4.0")]
949 pub fn downgrade(this: &Self) -> Weak<T> {
950 // This Relaxed is OK because we're checking the value in the CAS
952 let mut cur = this.inner().weak.load(Relaxed);
955 // check if the weak counter is currently "locked"; if so, spin.
956 if cur == usize::MAX {
958 cur = this.inner().weak.load(Relaxed);
962 // NOTE: this code currently ignores the possibility of overflow
963 // into usize::MAX; in general both Rc and Arc need to be adjusted
964 // to deal with overflow.
966 // Unlike with Clone(), we need this to be an Acquire read to
967 // synchronize with the write coming from `is_unique`, so that the
968 // events prior to that write happen before this read.
969 match this.inner().weak.compare_exchange_weak(cur, cur + 1, Acquire, Relaxed) {
971 // Make sure we do not create a dangling Weak
972 debug_assert!(!is_dangling(this.ptr.as_ptr()));
973 return Weak { ptr: this.ptr };
975 Err(old) => cur = old,
980 /// Gets the number of [`Weak`] pointers to this allocation.
984 /// This method by itself is safe, but using it correctly requires extra care.
985 /// Another thread can change the weak count at any time,
986 /// including potentially between calling this method and acting on the result.
991 /// use std::sync::Arc;
993 /// let five = Arc::new(5);
994 /// let _weak_five = Arc::downgrade(&five);
996 /// // This assertion is deterministic because we haven't shared
997 /// // the `Arc` or `Weak` between threads.
998 /// assert_eq!(1, Arc::weak_count(&five));
1002 #[stable(feature = "arc_counts", since = "1.15.0")]
1003 pub fn weak_count(this: &Self) -> usize {
1004 let cnt = this.inner().weak.load(Acquire);
1005 // If the weak count is currently locked, the value of the
1006 // count was 0 just before taking the lock.
1007 if cnt == usize::MAX { 0 } else { cnt - 1 }
1010 /// Gets the number of strong (`Arc`) pointers to this allocation.
1014 /// This method by itself is safe, but using it correctly requires extra care.
1015 /// Another thread can change the strong count at any time,
1016 /// including potentially between calling this method and acting on the result.
1021 /// use std::sync::Arc;
1023 /// let five = Arc::new(5);
1024 /// let _also_five = Arc::clone(&five);
1026 /// // This assertion is deterministic because we haven't shared
1027 /// // the `Arc` between threads.
1028 /// assert_eq!(2, Arc::strong_count(&five));
1032 #[stable(feature = "arc_counts", since = "1.15.0")]
1033 pub fn strong_count(this: &Self) -> usize {
1034 this.inner().strong.load(Acquire)
1037 /// Increments the strong reference count on the `Arc<T>` associated with the
1038 /// provided pointer by one.
1042 /// The pointer must have been obtained through `Arc::into_raw`, and the
1043 /// associated `Arc` instance must be valid (i.e. the strong count must be at
1044 /// least 1) for the duration of this method.
1049 /// use std::sync::Arc;
1051 /// let five = Arc::new(5);
1054 /// let ptr = Arc::into_raw(five);
1055 /// Arc::increment_strong_count(ptr);
1057 /// // This assertion is deterministic because we haven't shared
1058 /// // the `Arc` between threads.
1059 /// let five = Arc::from_raw(ptr);
1060 /// assert_eq!(2, Arc::strong_count(&five));
1064 #[stable(feature = "arc_mutate_strong_count", since = "1.51.0")]
1065 pub unsafe fn increment_strong_count(ptr: *const T) {
1066 // Retain Arc, but don't touch refcount by wrapping in ManuallyDrop
1067 let arc = unsafe { mem::ManuallyDrop::new(Arc::<T>::from_raw(ptr)) };
1068 // Now increase refcount, but don't drop new refcount either
1069 let _arc_clone: mem::ManuallyDrop<_> = arc.clone();
1072 /// Decrements the strong reference count on the `Arc<T>` associated with the
1073 /// provided pointer by one.
1077 /// The pointer must have been obtained through `Arc::into_raw`, and the
1078 /// associated `Arc` instance must be valid (i.e. the strong count must be at
1079 /// least 1) when invoking this method. This method can be used to release the final
1080 /// `Arc` and backing storage, but **should not** be called after the final `Arc` has been
1086 /// use std::sync::Arc;
1088 /// let five = Arc::new(5);
1091 /// let ptr = Arc::into_raw(five);
1092 /// Arc::increment_strong_count(ptr);
1094 /// // Those assertions are deterministic because we haven't shared
1095 /// // the `Arc` between threads.
1096 /// let five = Arc::from_raw(ptr);
1097 /// assert_eq!(2, Arc::strong_count(&five));
1098 /// Arc::decrement_strong_count(ptr);
1099 /// assert_eq!(1, Arc::strong_count(&five));
1103 #[stable(feature = "arc_mutate_strong_count", since = "1.51.0")]
1104 pub unsafe fn decrement_strong_count(ptr: *const T) {
1105 unsafe { mem::drop(Arc::from_raw(ptr)) };
1109 fn inner(&self) -> &ArcInner<T> {
1110 // This unsafety is ok because while this arc is alive we're guaranteed
1111 // that the inner pointer is valid. Furthermore, we know that the
1112 // `ArcInner` structure itself is `Sync` because the inner data is
1113 // `Sync` as well, so we're ok loaning out an immutable pointer to these
1115 unsafe { self.ptr.as_ref() }
1118 // Non-inlined part of `drop`.
1120 unsafe fn drop_slow(&mut self) {
1121 // Destroy the data at this time, even though we must not free the box
1122 // allocation itself (there might still be weak pointers lying around).
1123 unsafe { ptr::drop_in_place(Self::get_mut_unchecked(self)) };
1125 // Drop the weak ref collectively held by all strong references
1126 drop(Weak { ptr: self.ptr });
1129 /// Returns `true` if the two `Arc`s point to the same allocation in a vein similar to
1130 /// [`ptr::eq`]. See [that function][`ptr::eq`] for caveats when comparing `dyn Trait` pointers.
1135 /// use std::sync::Arc;
1137 /// let five = Arc::new(5);
1138 /// let same_five = Arc::clone(&five);
1139 /// let other_five = Arc::new(5);
1141 /// assert!(Arc::ptr_eq(&five, &same_five));
1142 /// assert!(!Arc::ptr_eq(&five, &other_five));
1145 /// [`ptr::eq`]: core::ptr::eq "ptr::eq"
1148 #[stable(feature = "ptr_eq", since = "1.17.0")]
1149 pub fn ptr_eq(this: &Self, other: &Self) -> bool {
1150 this.ptr.as_ptr() == other.ptr.as_ptr()
1154 impl<T: ?Sized> Arc<T> {
1155 /// Allocates an `ArcInner<T>` with sufficient space for
1156 /// a possibly-unsized inner value where the value has the layout provided.
1158 /// The function `mem_to_arcinner` is called with the data pointer
1159 /// and must return back a (potentially fat)-pointer for the `ArcInner<T>`.
1160 #[cfg(not(no_global_oom_handling))]
1161 unsafe fn allocate_for_layout(
1162 value_layout: Layout,
1163 allocate: impl FnOnce(Layout) -> Result<NonNull<[u8]>, AllocError>,
1164 mem_to_arcinner: impl FnOnce(*mut u8) -> *mut ArcInner<T>,
1165 ) -> *mut ArcInner<T> {
1166 let layout = arcinner_layout_for_value_layout(value_layout);
1168 Arc::try_allocate_for_layout(value_layout, allocate, mem_to_arcinner)
1169 .unwrap_or_else(|_| handle_alloc_error(layout))
1173 /// Allocates an `ArcInner<T>` with sufficient space for
1174 /// a possibly-unsized inner value where the value has the layout provided,
1175 /// returning an error if allocation fails.
1177 /// The function `mem_to_arcinner` is called with the data pointer
1178 /// and must return back a (potentially fat)-pointer for the `ArcInner<T>`.
1179 unsafe fn try_allocate_for_layout(
1180 value_layout: Layout,
1181 allocate: impl FnOnce(Layout) -> Result<NonNull<[u8]>, AllocError>,
1182 mem_to_arcinner: impl FnOnce(*mut u8) -> *mut ArcInner<T>,
1183 ) -> Result<*mut ArcInner<T>, AllocError> {
1184 let layout = arcinner_layout_for_value_layout(value_layout);
1186 let ptr = allocate(layout)?;
1188 // Initialize the ArcInner
1189 let inner = mem_to_arcinner(ptr.as_non_null_ptr().as_ptr());
1190 debug_assert_eq!(unsafe { Layout::for_value(&*inner) }, layout);
1193 ptr::write(&mut (*inner).strong, atomic::AtomicUsize::new(1));
1194 ptr::write(&mut (*inner).weak, atomic::AtomicUsize::new(1));
1200 /// Allocates an `ArcInner<T>` with sufficient space for an unsized inner value.
1201 #[cfg(not(no_global_oom_handling))]
1202 unsafe fn allocate_for_ptr(ptr: *const T) -> *mut ArcInner<T> {
1203 // Allocate for the `ArcInner<T>` using the given value.
1205 Self::allocate_for_layout(
1206 Layout::for_value(&*ptr),
1207 |layout| Global.allocate(layout),
1208 |mem| mem.with_metadata_of(ptr as *const ArcInner<T>),
1213 #[cfg(not(no_global_oom_handling))]
1214 fn from_box(v: Box<T>) -> Arc<T> {
1216 let (box_unique, alloc) = Box::into_unique(v);
1217 let bptr = box_unique.as_ptr();
1219 let value_size = size_of_val(&*bptr);
1220 let ptr = Self::allocate_for_ptr(bptr);
1222 // Copy value as bytes
1223 ptr::copy_nonoverlapping(
1224 bptr as *const T as *const u8,
1225 &mut (*ptr).data as *mut _ as *mut u8,
1229 // Free the allocation without dropping its contents
1230 box_free(box_unique, alloc);
1238 /// Allocates an `ArcInner<[T]>` with the given length.
1239 #[cfg(not(no_global_oom_handling))]
1240 unsafe fn allocate_for_slice(len: usize) -> *mut ArcInner<[T]> {
1242 Self::allocate_for_layout(
1243 Layout::array::<T>(len).unwrap(),
1244 |layout| Global.allocate(layout),
1245 |mem| ptr::slice_from_raw_parts_mut(mem as *mut T, len) as *mut ArcInner<[T]>,
1250 /// Copy elements from slice into newly allocated `Arc<[T]>`
1252 /// Unsafe because the caller must either take ownership or bind `T: Copy`.
1253 #[cfg(not(no_global_oom_handling))]
1254 unsafe fn copy_from_slice(v: &[T]) -> Arc<[T]> {
1256 let ptr = Self::allocate_for_slice(v.len());
1258 ptr::copy_nonoverlapping(v.as_ptr(), &mut (*ptr).data as *mut [T] as *mut T, v.len());
1264 /// Create an `Arc<[T]>` by reusing the underlying memory
1265 /// of a `Vec<T>`. This will return the vector if the existing allocation
1266 /// is not large enough.
1267 #[cfg(not(no_global_oom_handling))]
1268 fn try_from_vec_in_place(mut v: Vec<T>) -> Result<Arc<[T]>, Vec<T>> {
1269 let layout_elements = Layout::array::<T>(v.len()).unwrap();
1270 let layout_allocation = Layout::array::<T>(v.capacity()).unwrap();
1271 let layout_arcinner = arcinner_layout_for_value_layout(layout_elements);
1272 let mut ptr = NonNull::new(v.as_mut_ptr()).expect("`Vec<T>` stores `NonNull<T>`");
1273 if layout_arcinner.size() > layout_allocation.size()
1274 || layout_arcinner.align() > layout_allocation.align()
1276 // Can't fit - calling `grow` would involve `realloc`
1277 // (which copies the elements), followed by copying again.
1280 if layout_arcinner.size() < layout_allocation.size()
1281 || layout_arcinner.align() < layout_allocation.align()
1283 // We need to shrink the allocation so that it fits
1284 // https://doc.rust-lang.org/nightly/std/alloc/trait.Allocator.html#memory-fitting
1286 // - Vec allocates by requesting `Layout::array::<T>(capacity)`, so this capacity matches
1287 // - `layout_arcinner` is smaller
1288 // If this fails, the ownership has not been transferred
1289 if let Ok(p) = unsafe { Global.shrink(ptr.cast(), layout_allocation, layout_arcinner) }
1296 // Make sure the vec's memory isn't deallocated now
1297 let v = mem::ManuallyDrop::new(v);
1298 let ptr: *mut ArcInner<[T]> = ptr::slice_from_raw_parts_mut(ptr.as_ptr(), v.len()) as _;
1300 ptr::copy(ptr.cast::<T>(), &mut (*ptr).data as *mut [T] as *mut T, v.len());
1301 ptr::write(&mut (*ptr).strong, atomic::AtomicUsize::new(1));
1302 ptr::write(&mut (*ptr).weak, atomic::AtomicUsize::new(1));
1303 Ok(Self::from_ptr(ptr))
1307 /// Constructs an `Arc<[T]>` from an iterator known to be of a certain size.
1309 /// Behavior is undefined should the size be wrong.
1310 #[cfg(not(no_global_oom_handling))]
1311 unsafe fn from_iter_exact(iter: impl iter::Iterator<Item = T>, len: usize) -> Arc<[T]> {
1312 // Panic guard while cloning T elements.
1313 // In the event of a panic, elements that have been written
1314 // into the new ArcInner will be dropped, then the memory freed.
1322 impl<T> Drop for Guard<T> {
1323 fn drop(&mut self) {
1325 let slice = from_raw_parts_mut(self.elems, self.n_elems);
1326 ptr::drop_in_place(slice);
1328 Global.deallocate(self.mem, self.layout);
1334 let ptr = Self::allocate_for_slice(len);
1336 let mem = ptr as *mut _ as *mut u8;
1337 let layout = Layout::for_value(&*ptr);
1339 // Pointer to first element
1340 let elems = &mut (*ptr).data as *mut [T] as *mut T;
1342 let mut guard = Guard { mem: NonNull::new_unchecked(mem), elems, layout, n_elems: 0 };
1344 for (i, item) in iter.enumerate() {
1345 ptr::write(elems.add(i), item);
1349 // All clear. Forget the guard so it doesn't free the new ArcInner.
1357 /// Specialization trait used for `From<&[T]>`.
1358 #[cfg(not(no_global_oom_handling))]
1359 trait ArcFromSlice<T> {
1360 fn from_slice(slice: &[T]) -> Self;
1363 #[cfg(not(no_global_oom_handling))]
1364 impl<T: Clone> ArcFromSlice<T> for Arc<[T]> {
1366 default fn from_slice(v: &[T]) -> Self {
1367 unsafe { Self::from_iter_exact(v.iter().cloned(), v.len()) }
1371 #[cfg(not(no_global_oom_handling))]
1372 impl<T: Copy> ArcFromSlice<T> for Arc<[T]> {
1374 fn from_slice(v: &[T]) -> Self {
1375 unsafe { Arc::copy_from_slice(v) }
1379 #[stable(feature = "rust1", since = "1.0.0")]
1380 impl<T: ?Sized> Clone for Arc<T> {
1381 /// Makes a clone of the `Arc` pointer.
1383 /// This creates another pointer to the same allocation, increasing the
1384 /// strong reference count.
1389 /// use std::sync::Arc;
1391 /// let five = Arc::new(5);
1393 /// let _ = Arc::clone(&five);
1396 fn clone(&self) -> Arc<T> {
1397 // Using a relaxed ordering is alright here, as knowledge of the
1398 // original reference prevents other threads from erroneously deleting
1401 // As explained in the [Boost documentation][1], Increasing the
1402 // reference counter can always be done with memory_order_relaxed: New
1403 // references to an object can only be formed from an existing
1404 // reference, and passing an existing reference from one thread to
1405 // another must already provide any required synchronization.
1407 // [1]: (www.boost.org/doc/libs/1_55_0/doc/html/atomic/usage_examples.html)
1408 let old_size = self.inner().strong.fetch_add(1, Relaxed);
1410 // However we need to guard against massive refcounts in case someone is `mem::forget`ing
1411 // Arcs. If we don't do this the count can overflow and users will use-after free. This
1412 // branch will never be taken in any realistic program. We abort because such a program is
1413 // incredibly degenerate, and we don't care to support it.
1415 // This check is not 100% water-proof: we error when the refcount grows beyond `isize::MAX`.
1416 // But we do that check *after* having done the increment, so there is a chance here that
1417 // the worst already happened and we actually do overflow the `usize` counter. However, that
1418 // requires the counter to grow from `isize::MAX` to `usize::MAX` between the increment
1419 // above and the `abort` below, which seems exceedingly unlikely.
1420 if old_size > MAX_REFCOUNT {
1424 unsafe { Self::from_inner(self.ptr) }
1428 #[stable(feature = "rust1", since = "1.0.0")]
1429 impl<T: ?Sized> Deref for Arc<T> {
1433 fn deref(&self) -> &T {
1438 #[unstable(feature = "receiver_trait", issue = "none")]
1439 impl<T: ?Sized> Receiver for Arc<T> {}
1441 impl<T: Clone> Arc<T> {
1442 /// Makes a mutable reference into the given `Arc`.
1444 /// If there are other `Arc` pointers to the same allocation, then `make_mut` will
1445 /// [`clone`] the inner value to a new allocation to ensure unique ownership. This is also
1446 /// referred to as clone-on-write.
1448 /// However, if there are no other `Arc` pointers to this allocation, but some [`Weak`]
1449 /// pointers, then the [`Weak`] pointers will be dissociated and the inner value will not
1452 /// See also [`get_mut`], which will fail rather than cloning the inner value
1453 /// or dissociating [`Weak`] pointers.
1455 /// [`clone`]: Clone::clone
1456 /// [`get_mut`]: Arc::get_mut
1461 /// use std::sync::Arc;
1463 /// let mut data = Arc::new(5);
1465 /// *Arc::make_mut(&mut data) += 1; // Won't clone anything
1466 /// let mut other_data = Arc::clone(&data); // Won't clone inner data
1467 /// *Arc::make_mut(&mut data) += 1; // Clones inner data
1468 /// *Arc::make_mut(&mut data) += 1; // Won't clone anything
1469 /// *Arc::make_mut(&mut other_data) *= 2; // Won't clone anything
1471 /// // Now `data` and `other_data` point to different allocations.
1472 /// assert_eq!(*data, 8);
1473 /// assert_eq!(*other_data, 12);
1476 /// [`Weak`] pointers will be dissociated:
1479 /// use std::sync::Arc;
1481 /// let mut data = Arc::new(75);
1482 /// let weak = Arc::downgrade(&data);
1484 /// assert!(75 == *data);
1485 /// assert!(75 == *weak.upgrade().unwrap());
1487 /// *Arc::make_mut(&mut data) += 1;
1489 /// assert!(76 == *data);
1490 /// assert!(weak.upgrade().is_none());
1492 #[cfg(not(no_global_oom_handling))]
1494 #[stable(feature = "arc_unique", since = "1.4.0")]
1495 pub fn make_mut(this: &mut Self) -> &mut T {
1496 // Note that we hold both a strong reference and a weak reference.
1497 // Thus, releasing our strong reference only will not, by itself, cause
1498 // the memory to be deallocated.
1500 // Use Acquire to ensure that we see any writes to `weak` that happen
1501 // before release writes (i.e., decrements) to `strong`. Since we hold a
1502 // weak count, there's no chance the ArcInner itself could be
1504 if this.inner().strong.compare_exchange(1, 0, Acquire, Relaxed).is_err() {
1505 // Another strong pointer exists, so we must clone.
1506 // Pre-allocate memory to allow writing the cloned value directly.
1507 let mut arc = Self::new_uninit();
1509 let data = Arc::get_mut_unchecked(&mut arc);
1510 (**this).write_clone_into_raw(data.as_mut_ptr());
1511 *this = arc.assume_init();
1513 } else if this.inner().weak.load(Relaxed) != 1 {
1514 // Relaxed suffices in the above because this is fundamentally an
1515 // optimization: we are always racing with weak pointers being
1516 // dropped. Worst case, we end up allocated a new Arc unnecessarily.
1518 // We removed the last strong ref, but there are additional weak
1519 // refs remaining. We'll move the contents to a new Arc, and
1520 // invalidate the other weak refs.
1522 // Note that it is not possible for the read of `weak` to yield
1523 // usize::MAX (i.e., locked), since the weak count can only be
1524 // locked by a thread with a strong reference.
1526 // Materialize our own implicit weak pointer, so that it can clean
1527 // up the ArcInner as needed.
1528 let _weak = Weak { ptr: this.ptr };
1530 // Can just steal the data, all that's left is Weaks
1531 let mut arc = Self::new_uninit();
1533 let data = Arc::get_mut_unchecked(&mut arc);
1534 data.as_mut_ptr().copy_from_nonoverlapping(&**this, 1);
1535 ptr::write(this, arc.assume_init());
1538 // We were the sole reference of either kind; bump back up the
1539 // strong ref count.
1540 this.inner().strong.store(1, Release);
1543 // As with `get_mut()`, the unsafety is ok because our reference was
1544 // either unique to begin with, or became one upon cloning the contents.
1545 unsafe { Self::get_mut_unchecked(this) }
1548 /// If we have the only reference to `T` then unwrap it. Otherwise, clone `T` and return the
1551 /// Assuming `arc_t` is of type `Arc<T>`, this function is functionally equivalent to
1552 /// `(*arc_t).clone()`, but will avoid cloning the inner value where possible.
1557 /// #![feature(arc_unwrap_or_clone)]
1558 /// # use std::{ptr, sync::Arc};
1559 /// let inner = String::from("test");
1560 /// let ptr = inner.as_ptr();
1562 /// let arc = Arc::new(inner);
1563 /// let inner = Arc::unwrap_or_clone(arc);
1564 /// // The inner value was not cloned
1565 /// assert!(ptr::eq(ptr, inner.as_ptr()));
1567 /// let arc = Arc::new(inner);
1568 /// let arc2 = arc.clone();
1569 /// let inner = Arc::unwrap_or_clone(arc);
1570 /// // Because there were 2 references, we had to clone the inner value.
1571 /// assert!(!ptr::eq(ptr, inner.as_ptr()));
1572 /// // `arc2` is the last reference, so when we unwrap it we get back
1573 /// // the original `String`.
1574 /// let inner = Arc::unwrap_or_clone(arc2);
1575 /// assert!(ptr::eq(ptr, inner.as_ptr()));
1578 #[unstable(feature = "arc_unwrap_or_clone", issue = "93610")]
1579 pub fn unwrap_or_clone(this: Self) -> T {
1580 Arc::try_unwrap(this).unwrap_or_else(|arc| (*arc).clone())
1584 impl<T: ?Sized> Arc<T> {
1585 /// Returns a mutable reference into the given `Arc`, if there are
1586 /// no other `Arc` or [`Weak`] pointers to the same allocation.
1588 /// Returns [`None`] otherwise, because it is not safe to
1589 /// mutate a shared value.
1591 /// See also [`make_mut`][make_mut], which will [`clone`][clone]
1592 /// the inner value when there are other `Arc` pointers.
1594 /// [make_mut]: Arc::make_mut
1595 /// [clone]: Clone::clone
1600 /// use std::sync::Arc;
1602 /// let mut x = Arc::new(3);
1603 /// *Arc::get_mut(&mut x).unwrap() = 4;
1604 /// assert_eq!(*x, 4);
1606 /// let _y = Arc::clone(&x);
1607 /// assert!(Arc::get_mut(&mut x).is_none());
1610 #[stable(feature = "arc_unique", since = "1.4.0")]
1611 pub fn get_mut(this: &mut Self) -> Option<&mut T> {
1612 if this.is_unique() {
1613 // This unsafety is ok because we're guaranteed that the pointer
1614 // returned is the *only* pointer that will ever be returned to T. Our
1615 // reference count is guaranteed to be 1 at this point, and we required
1616 // the Arc itself to be `mut`, so we're returning the only possible
1617 // reference to the inner data.
1618 unsafe { Some(Arc::get_mut_unchecked(this)) }
1624 /// Returns a mutable reference into the given `Arc`,
1625 /// without any check.
1627 /// See also [`get_mut`], which is safe and does appropriate checks.
1629 /// [`get_mut`]: Arc::get_mut
1633 /// If any other `Arc` or [`Weak`] pointers to the same allocation exist, then
1634 /// they must be must not be dereferenced or have active borrows for the duration
1635 /// of the returned borrow, and their inner type must be exactly the same as the
1636 /// inner type of this Rc (including lifetimes). This is trivially the case if no
1637 /// such pointers exist, for example immediately after `Arc::new`.
1642 /// #![feature(get_mut_unchecked)]
1644 /// use std::sync::Arc;
1646 /// let mut x = Arc::new(String::new());
1648 /// Arc::get_mut_unchecked(&mut x).push_str("foo")
1650 /// assert_eq!(*x, "foo");
1652 /// Other `Arc` pointers to the same allocation must be to the same type.
1654 /// #![feature(get_mut_unchecked)]
1656 /// use std::sync::Arc;
1658 /// let x: Arc<str> = Arc::from("Hello, world!");
1659 /// let mut y: Arc<[u8]> = x.clone().into();
1661 /// // this is Undefined Behavior, because x's inner type is str, not [u8]
1662 /// Arc::get_mut_unchecked(&mut y).fill(0xff); // 0xff is invalid in UTF-8
1664 /// println!("{}", &*x); // Invliad UTF-8 in a str
1666 /// Other `Arc` pointers to the same allocation must be to the exact same type, including lifetimes.
1668 /// #![feature(get_mut_unchecked)]
1670 /// use std::sync::Arc;
1672 /// let x: Arc<&str> = Arc::new("Hello, world!");
1674 /// let s = String::from("Oh, no!");
1675 /// let mut y: Arc<&str> = x.clone().into();
1677 /// // this is Undefined Behavior, because x's inner type
1678 /// // is &'long str, not &'short str
1679 /// *Arc::get_mut_unchecked(&mut y) = &s;
1682 /// println!("{}", &*x); // Use-after-free
1685 #[unstable(feature = "get_mut_unchecked", issue = "63292")]
1686 pub unsafe fn get_mut_unchecked(this: &mut Self) -> &mut T {
1687 // We are careful to *not* create a reference covering the "count" fields, as
1688 // this would alias with concurrent access to the reference counts (e.g. by `Weak`).
1689 unsafe { &mut (*this.ptr.as_ptr()).data }
1692 /// Determine whether this is the unique reference (including weak refs) to
1693 /// the underlying data.
1695 /// Note that this requires locking the weak ref count.
1696 fn is_unique(&mut self) -> bool {
1697 // lock the weak pointer count if we appear to be the sole weak pointer
1700 // The acquire label here ensures a happens-before relationship with any
1701 // writes to `strong` (in particular in `Weak::upgrade`) prior to decrements
1702 // of the `weak` count (via `Weak::drop`, which uses release). If the upgraded
1703 // weak ref was never dropped, the CAS here will fail so we do not care to synchronize.
1704 if self.inner().weak.compare_exchange(1, usize::MAX, Acquire, Relaxed).is_ok() {
1705 // This needs to be an `Acquire` to synchronize with the decrement of the `strong`
1706 // counter in `drop` -- the only access that happens when any but the last reference
1707 // is being dropped.
1708 let unique = self.inner().strong.load(Acquire) == 1;
1710 // The release write here synchronizes with a read in `downgrade`,
1711 // effectively preventing the above read of `strong` from happening
1713 self.inner().weak.store(1, Release); // release the lock
1721 #[stable(feature = "rust1", since = "1.0.0")]
1722 unsafe impl<#[may_dangle] T: ?Sized> Drop for Arc<T> {
1723 /// Drops the `Arc`.
1725 /// This will decrement the strong reference count. If the strong reference
1726 /// count reaches zero then the only other references (if any) are
1727 /// [`Weak`], so we `drop` the inner value.
1732 /// use std::sync::Arc;
1736 /// impl Drop for Foo {
1737 /// fn drop(&mut self) {
1738 /// println!("dropped!");
1742 /// let foo = Arc::new(Foo);
1743 /// let foo2 = Arc::clone(&foo);
1745 /// drop(foo); // Doesn't print anything
1746 /// drop(foo2); // Prints "dropped!"
1749 fn drop(&mut self) {
1750 // Because `fetch_sub` is already atomic, we do not need to synchronize
1751 // with other threads unless we are going to delete the object. This
1752 // same logic applies to the below `fetch_sub` to the `weak` count.
1753 if self.inner().strong.fetch_sub(1, Release) != 1 {
1757 // This fence is needed to prevent reordering of use of the data and
1758 // deletion of the data. Because it is marked `Release`, the decreasing
1759 // of the reference count synchronizes with this `Acquire` fence. This
1760 // means that use of the data happens before decreasing the reference
1761 // count, which happens before this fence, which happens before the
1762 // deletion of the data.
1764 // As explained in the [Boost documentation][1],
1766 // > It is important to enforce any possible access to the object in one
1767 // > thread (through an existing reference) to *happen before* deleting
1768 // > the object in a different thread. This is achieved by a "release"
1769 // > operation after dropping a reference (any access to the object
1770 // > through this reference must obviously happened before), and an
1771 // > "acquire" operation before deleting the object.
1773 // In particular, while the contents of an Arc are usually immutable, it's
1774 // possible to have interior writes to something like a Mutex<T>. Since a
1775 // Mutex is not acquired when it is deleted, we can't rely on its
1776 // synchronization logic to make writes in thread A visible to a destructor
1777 // running in thread B.
1779 // Also note that the Acquire fence here could probably be replaced with an
1780 // Acquire load, which could improve performance in highly-contended
1781 // situations. See [2].
1783 // [1]: (www.boost.org/doc/libs/1_55_0/doc/html/atomic/usage_examples.html)
1784 // [2]: (https://github.com/rust-lang/rust/pull/41714)
1785 acquire!(self.inner().strong);
1793 impl Arc<dyn Any + Send + Sync> {
1794 /// Attempt to downcast the `Arc<dyn Any + Send + Sync>` to a concrete type.
1799 /// use std::any::Any;
1800 /// use std::sync::Arc;
1802 /// fn print_if_string(value: Arc<dyn Any + Send + Sync>) {
1803 /// if let Ok(string) = value.downcast::<String>() {
1804 /// println!("String ({}): {}", string.len(), string);
1808 /// let my_string = "Hello World".to_string();
1809 /// print_if_string(Arc::new(my_string));
1810 /// print_if_string(Arc::new(0i8));
1813 #[stable(feature = "rc_downcast", since = "1.29.0")]
1814 pub fn downcast<T>(self) -> Result<Arc<T>, Self>
1816 T: Any + Send + Sync,
1818 if (*self).is::<T>() {
1820 let ptr = self.ptr.cast::<ArcInner<T>>();
1822 Ok(Arc::from_inner(ptr))
1829 /// Downcasts the `Arc<dyn Any + Send + Sync>` to a concrete type.
1831 /// For a safe alternative see [`downcast`].
1836 /// #![feature(downcast_unchecked)]
1838 /// use std::any::Any;
1839 /// use std::sync::Arc;
1841 /// let x: Arc<dyn Any + Send + Sync> = Arc::new(1_usize);
1844 /// assert_eq!(*x.downcast_unchecked::<usize>(), 1);
1850 /// The contained value must be of type `T`. Calling this method
1851 /// with the incorrect type is *undefined behavior*.
1854 /// [`downcast`]: Self::downcast
1856 #[unstable(feature = "downcast_unchecked", issue = "90850")]
1857 pub unsafe fn downcast_unchecked<T>(self) -> Arc<T>
1859 T: Any + Send + Sync,
1862 let ptr = self.ptr.cast::<ArcInner<T>>();
1864 Arc::from_inner(ptr)
1870 /// Constructs a new `Weak<T>`, without allocating any memory.
1871 /// Calling [`upgrade`] on the return value always gives [`None`].
1873 /// [`upgrade`]: Weak::upgrade
1878 /// use std::sync::Weak;
1880 /// let empty: Weak<i64> = Weak::new();
1881 /// assert!(empty.upgrade().is_none());
1883 #[stable(feature = "downgraded_weak", since = "1.10.0")]
1884 #[rustc_const_unstable(feature = "const_weak_new", issue = "95091", reason = "recently added")]
1886 pub const fn new() -> Weak<T> {
1887 Weak { ptr: unsafe { NonNull::new_unchecked(ptr::invalid_mut::<ArcInner<T>>(usize::MAX)) } }
1891 /// Helper type to allow accessing the reference counts without
1892 /// making any assertions about the data field.
1893 struct WeakInner<'a> {
1894 weak: &'a atomic::AtomicUsize,
1895 strong: &'a atomic::AtomicUsize,
1898 impl<T: ?Sized> Weak<T> {
1899 /// Returns a raw pointer to the object `T` pointed to by this `Weak<T>`.
1901 /// The pointer is valid only if there are some strong references. The pointer may be dangling,
1902 /// unaligned or even [`null`] otherwise.
1907 /// use std::sync::Arc;
1910 /// let strong = Arc::new("hello".to_owned());
1911 /// let weak = Arc::downgrade(&strong);
1912 /// // Both point to the same object
1913 /// assert!(ptr::eq(&*strong, weak.as_ptr()));
1914 /// // The strong here keeps it alive, so we can still access the object.
1915 /// assert_eq!("hello", unsafe { &*weak.as_ptr() });
1918 /// // But not any more. We can do weak.as_ptr(), but accessing the pointer would lead to
1919 /// // undefined behaviour.
1920 /// // assert_eq!("hello", unsafe { &*weak.as_ptr() });
1923 /// [`null`]: core::ptr::null "ptr::null"
1925 #[stable(feature = "weak_into_raw", since = "1.45.0")]
1926 pub fn as_ptr(&self) -> *const T {
1927 let ptr: *mut ArcInner<T> = NonNull::as_ptr(self.ptr);
1929 if is_dangling(ptr) {
1930 // If the pointer is dangling, we return the sentinel directly. This cannot be
1931 // a valid payload address, as the payload is at least as aligned as ArcInner (usize).
1934 // SAFETY: if is_dangling returns false, then the pointer is dereferenceable.
1935 // The payload may be dropped at this point, and we have to maintain provenance,
1936 // so use raw pointer manipulation.
1937 unsafe { ptr::addr_of_mut!((*ptr).data) }
1941 /// Consumes the `Weak<T>` and turns it into a raw pointer.
1943 /// This converts the weak pointer into a raw pointer, while still preserving the ownership of
1944 /// one weak reference (the weak count is not modified by this operation). It can be turned
1945 /// back into the `Weak<T>` with [`from_raw`].
1947 /// The same restrictions of accessing the target of the pointer as with
1948 /// [`as_ptr`] apply.
1953 /// use std::sync::{Arc, Weak};
1955 /// let strong = Arc::new("hello".to_owned());
1956 /// let weak = Arc::downgrade(&strong);
1957 /// let raw = weak.into_raw();
1959 /// assert_eq!(1, Arc::weak_count(&strong));
1960 /// assert_eq!("hello", unsafe { &*raw });
1962 /// drop(unsafe { Weak::from_raw(raw) });
1963 /// assert_eq!(0, Arc::weak_count(&strong));
1966 /// [`from_raw`]: Weak::from_raw
1967 /// [`as_ptr`]: Weak::as_ptr
1968 #[must_use = "`self` will be dropped if the result is not used"]
1969 #[stable(feature = "weak_into_raw", since = "1.45.0")]
1970 pub fn into_raw(self) -> *const T {
1971 let result = self.as_ptr();
1976 /// Converts a raw pointer previously created by [`into_raw`] back into `Weak<T>`.
1978 /// This can be used to safely get a strong reference (by calling [`upgrade`]
1979 /// later) or to deallocate the weak count by dropping the `Weak<T>`.
1981 /// It takes ownership of one weak reference (with the exception of pointers created by [`new`],
1982 /// as these don't own anything; the method still works on them).
1986 /// The pointer must have originated from the [`into_raw`] and must still own its potential
1989 /// It is allowed for the strong count to be 0 at the time of calling this. Nevertheless, this
1990 /// takes ownership of one weak reference currently represented as a raw pointer (the weak
1991 /// count is not modified by this operation) and therefore it must be paired with a previous
1992 /// call to [`into_raw`].
1996 /// use std::sync::{Arc, Weak};
1998 /// let strong = Arc::new("hello".to_owned());
2000 /// let raw_1 = Arc::downgrade(&strong).into_raw();
2001 /// let raw_2 = Arc::downgrade(&strong).into_raw();
2003 /// assert_eq!(2, Arc::weak_count(&strong));
2005 /// assert_eq!("hello", &*unsafe { Weak::from_raw(raw_1) }.upgrade().unwrap());
2006 /// assert_eq!(1, Arc::weak_count(&strong));
2010 /// // Decrement the last weak count.
2011 /// assert!(unsafe { Weak::from_raw(raw_2) }.upgrade().is_none());
2014 /// [`new`]: Weak::new
2015 /// [`into_raw`]: Weak::into_raw
2016 /// [`upgrade`]: Weak::upgrade
2017 #[stable(feature = "weak_into_raw", since = "1.45.0")]
2018 pub unsafe fn from_raw(ptr: *const T) -> Self {
2019 // See Weak::as_ptr for context on how the input pointer is derived.
2021 let ptr = if is_dangling(ptr as *mut T) {
2022 // This is a dangling Weak.
2023 ptr as *mut ArcInner<T>
2025 // Otherwise, we're guaranteed the pointer came from a nondangling Weak.
2026 // SAFETY: data_offset is safe to call, as ptr references a real (potentially dropped) T.
2027 let offset = unsafe { data_offset(ptr) };
2028 // Thus, we reverse the offset to get the whole RcBox.
2029 // SAFETY: the pointer originated from a Weak, so this offset is safe.
2030 unsafe { ptr.byte_sub(offset) as *mut ArcInner<T> }
2033 // SAFETY: we now have recovered the original Weak pointer, so can create the Weak.
2034 Weak { ptr: unsafe { NonNull::new_unchecked(ptr) } }
2038 impl<T: ?Sized> Weak<T> {
2039 /// Attempts to upgrade the `Weak` pointer to an [`Arc`], delaying
2040 /// dropping of the inner value if successful.
2042 /// Returns [`None`] if the inner value has since been dropped.
2047 /// use std::sync::Arc;
2049 /// let five = Arc::new(5);
2051 /// let weak_five = Arc::downgrade(&five);
2053 /// let strong_five: Option<Arc<_>> = weak_five.upgrade();
2054 /// assert!(strong_five.is_some());
2056 /// // Destroy all strong pointers.
2057 /// drop(strong_five);
2060 /// assert!(weak_five.upgrade().is_none());
2062 #[must_use = "this returns a new `Arc`, \
2063 without modifying the original weak pointer"]
2064 #[stable(feature = "arc_weak", since = "1.4.0")]
2065 pub fn upgrade(&self) -> Option<Arc<T>> {
2066 // We use a CAS loop to increment the strong count instead of a
2067 // fetch_add as this function should never take the reference count
2068 // from zero to one.
2071 // Relaxed is fine for the failure case because we don't have any expectations about the new state.
2072 // Acquire is necessary for the success case to synchronise with `Arc::new_cyclic`, when the inner
2073 // value can be initialized after `Weak` references have already been created. In that case, we
2074 // expect to observe the fully initialized value.
2075 .fetch_update(Acquire, Relaxed, |n| {
2076 // Any write of 0 we can observe leaves the field in permanently zero state.
2080 // See comments in `Arc::clone` for why we do this (for `mem::forget`).
2081 if n > MAX_REFCOUNT {
2087 // null checked above
2088 .map(|_| unsafe { Arc::from_inner(self.ptr) })
2091 /// Gets the number of strong (`Arc`) pointers pointing to this allocation.
2093 /// If `self` was created using [`Weak::new`], this will return 0.
2095 #[stable(feature = "weak_counts", since = "1.41.0")]
2096 pub fn strong_count(&self) -> usize {
2097 if let Some(inner) = self.inner() { inner.strong.load(Acquire) } else { 0 }
2100 /// Gets an approximation of the number of `Weak` pointers pointing to this
2103 /// If `self` was created using [`Weak::new`], or if there are no remaining
2104 /// strong pointers, this will return 0.
2108 /// Due to implementation details, the returned value can be off by 1 in
2109 /// either direction when other threads are manipulating any `Arc`s or
2110 /// `Weak`s pointing to the same allocation.
2112 #[stable(feature = "weak_counts", since = "1.41.0")]
2113 pub fn weak_count(&self) -> usize {
2116 let weak = inner.weak.load(Acquire);
2117 let strong = inner.strong.load(Acquire);
2121 // Since we observed that there was at least one strong pointer
2122 // after reading the weak count, we know that the implicit weak
2123 // reference (present whenever any strong references are alive)
2124 // was still around when we observed the weak count, and can
2125 // therefore safely subtract it.
2132 /// Returns `None` when the pointer is dangling and there is no allocated `ArcInner`,
2133 /// (i.e., when this `Weak` was created by `Weak::new`).
2135 fn inner(&self) -> Option<WeakInner<'_>> {
2136 if is_dangling(self.ptr.as_ptr()) {
2139 // We are careful to *not* create a reference covering the "data" field, as
2140 // the field may be mutated concurrently (for example, if the last `Arc`
2141 // is dropped, the data field will be dropped in-place).
2143 let ptr = self.ptr.as_ptr();
2144 WeakInner { strong: &(*ptr).strong, weak: &(*ptr).weak }
2149 /// Returns `true` if the two `Weak`s point to the same allocation similar to [`ptr::eq`], or if
2150 /// both don't point to any allocation (because they were created with `Weak::new()`). See [that
2151 /// function][`ptr::eq`] for caveats when comparing `dyn Trait` pointers.
2155 /// Since this compares pointers it means that `Weak::new()` will equal each
2156 /// other, even though they don't point to any allocation.
2161 /// use std::sync::Arc;
2163 /// let first_rc = Arc::new(5);
2164 /// let first = Arc::downgrade(&first_rc);
2165 /// let second = Arc::downgrade(&first_rc);
2167 /// assert!(first.ptr_eq(&second));
2169 /// let third_rc = Arc::new(5);
2170 /// let third = Arc::downgrade(&third_rc);
2172 /// assert!(!first.ptr_eq(&third));
2175 /// Comparing `Weak::new`.
2178 /// use std::sync::{Arc, Weak};
2180 /// let first = Weak::new();
2181 /// let second = Weak::new();
2182 /// assert!(first.ptr_eq(&second));
2184 /// let third_rc = Arc::new(());
2185 /// let third = Arc::downgrade(&third_rc);
2186 /// assert!(!first.ptr_eq(&third));
2189 /// [`ptr::eq`]: core::ptr::eq "ptr::eq"
2192 #[stable(feature = "weak_ptr_eq", since = "1.39.0")]
2193 pub fn ptr_eq(&self, other: &Self) -> bool {
2194 self.ptr.as_ptr() == other.ptr.as_ptr()
2198 #[stable(feature = "arc_weak", since = "1.4.0")]
2199 impl<T: ?Sized> Clone for Weak<T> {
2200 /// Makes a clone of the `Weak` pointer that points to the same allocation.
2205 /// use std::sync::{Arc, Weak};
2207 /// let weak_five = Arc::downgrade(&Arc::new(5));
2209 /// let _ = Weak::clone(&weak_five);
2212 fn clone(&self) -> Weak<T> {
2213 let inner = if let Some(inner) = self.inner() {
2216 return Weak { ptr: self.ptr };
2218 // See comments in Arc::clone() for why this is relaxed. This can use a
2219 // fetch_add (ignoring the lock) because the weak count is only locked
2220 // where are *no other* weak pointers in existence. (So we can't be
2221 // running this code in that case).
2222 let old_size = inner.weak.fetch_add(1, Relaxed);
2224 // See comments in Arc::clone() for why we do this (for mem::forget).
2225 if old_size > MAX_REFCOUNT {
2229 Weak { ptr: self.ptr }
2233 #[stable(feature = "downgraded_weak", since = "1.10.0")]
2234 impl<T> Default for Weak<T> {
2235 /// Constructs a new `Weak<T>`, without allocating memory.
2236 /// Calling [`upgrade`] on the return value always
2239 /// [`upgrade`]: Weak::upgrade
2244 /// use std::sync::Weak;
2246 /// let empty: Weak<i64> = Default::default();
2247 /// assert!(empty.upgrade().is_none());
2249 fn default() -> Weak<T> {
2254 #[stable(feature = "arc_weak", since = "1.4.0")]
2255 unsafe impl<#[may_dangle] T: ?Sized> Drop for Weak<T> {
2256 /// Drops the `Weak` pointer.
2261 /// use std::sync::{Arc, Weak};
2265 /// impl Drop for Foo {
2266 /// fn drop(&mut self) {
2267 /// println!("dropped!");
2271 /// let foo = Arc::new(Foo);
2272 /// let weak_foo = Arc::downgrade(&foo);
2273 /// let other_weak_foo = Weak::clone(&weak_foo);
2275 /// drop(weak_foo); // Doesn't print anything
2276 /// drop(foo); // Prints "dropped!"
2278 /// assert!(other_weak_foo.upgrade().is_none());
2280 fn drop(&mut self) {
2281 // If we find out that we were the last weak pointer, then its time to
2282 // deallocate the data entirely. See the discussion in Arc::drop() about
2283 // the memory orderings
2285 // It's not necessary to check for the locked state here, because the
2286 // weak count can only be locked if there was precisely one weak ref,
2287 // meaning that drop could only subsequently run ON that remaining weak
2288 // ref, which can only happen after the lock is released.
2289 let inner = if let Some(inner) = self.inner() { inner } else { return };
2291 if inner.weak.fetch_sub(1, Release) == 1 {
2292 acquire!(inner.weak);
2293 unsafe { Global.deallocate(self.ptr.cast(), Layout::for_value_raw(self.ptr.as_ptr())) }
2298 #[stable(feature = "rust1", since = "1.0.0")]
2299 trait ArcEqIdent<T: ?Sized + PartialEq> {
2300 fn eq(&self, other: &Arc<T>) -> bool;
2301 fn ne(&self, other: &Arc<T>) -> bool;
2304 #[stable(feature = "rust1", since = "1.0.0")]
2305 impl<T: ?Sized + PartialEq> ArcEqIdent<T> for Arc<T> {
2307 default fn eq(&self, other: &Arc<T>) -> bool {
2311 default fn ne(&self, other: &Arc<T>) -> bool {
2316 /// We're doing this specialization here, and not as a more general optimization on `&T`, because it
2317 /// would otherwise add a cost to all equality checks on refs. We assume that `Arc`s are used to
2318 /// store large values, that are slow to clone, but also heavy to check for equality, causing this
2319 /// cost to pay off more easily. It's also more likely to have two `Arc` clones, that point to
2320 /// the same value, than two `&T`s.
2322 /// We can only do this when `T: Eq` as a `PartialEq` might be deliberately irreflexive.
2323 #[stable(feature = "rust1", since = "1.0.0")]
2324 impl<T: ?Sized + crate::rc::MarkerEq> ArcEqIdent<T> for Arc<T> {
2326 fn eq(&self, other: &Arc<T>) -> bool {
2327 Arc::ptr_eq(self, other) || **self == **other
2331 fn ne(&self, other: &Arc<T>) -> bool {
2332 !Arc::ptr_eq(self, other) && **self != **other
2336 #[stable(feature = "rust1", since = "1.0.0")]
2337 impl<T: ?Sized + PartialEq> PartialEq for Arc<T> {
2338 /// Equality for two `Arc`s.
2340 /// Two `Arc`s are equal if their inner values are equal, even if they are
2341 /// stored in different allocation.
2343 /// If `T` also implements `Eq` (implying reflexivity of equality),
2344 /// two `Arc`s that point to the same allocation are always equal.
2349 /// use std::sync::Arc;
2351 /// let five = Arc::new(5);
2353 /// assert!(five == Arc::new(5));
2356 fn eq(&self, other: &Arc<T>) -> bool {
2357 ArcEqIdent::eq(self, other)
2360 /// Inequality for two `Arc`s.
2362 /// Two `Arc`s are unequal if their inner values are unequal.
2364 /// If `T` also implements `Eq` (implying reflexivity of equality),
2365 /// two `Arc`s that point to the same value are never unequal.
2370 /// use std::sync::Arc;
2372 /// let five = Arc::new(5);
2374 /// assert!(five != Arc::new(6));
2377 fn ne(&self, other: &Arc<T>) -> bool {
2378 ArcEqIdent::ne(self, other)
2382 #[stable(feature = "rust1", since = "1.0.0")]
2383 impl<T: ?Sized + PartialOrd> PartialOrd for Arc<T> {
2384 /// Partial comparison for two `Arc`s.
2386 /// The two are compared by calling `partial_cmp()` on their inner values.
2391 /// use std::sync::Arc;
2392 /// use std::cmp::Ordering;
2394 /// let five = Arc::new(5);
2396 /// assert_eq!(Some(Ordering::Less), five.partial_cmp(&Arc::new(6)));
2398 fn partial_cmp(&self, other: &Arc<T>) -> Option<Ordering> {
2399 (**self).partial_cmp(&**other)
2402 /// Less-than comparison for two `Arc`s.
2404 /// The two are compared by calling `<` on their inner values.
2409 /// use std::sync::Arc;
2411 /// let five = Arc::new(5);
2413 /// assert!(five < Arc::new(6));
2415 fn lt(&self, other: &Arc<T>) -> bool {
2416 *(*self) < *(*other)
2419 /// 'Less than or equal to' comparison for two `Arc`s.
2421 /// The two are compared by calling `<=` on their inner values.
2426 /// use std::sync::Arc;
2428 /// let five = Arc::new(5);
2430 /// assert!(five <= Arc::new(5));
2432 fn le(&self, other: &Arc<T>) -> bool {
2433 *(*self) <= *(*other)
2436 /// Greater-than comparison for two `Arc`s.
2438 /// The two are compared by calling `>` on their inner values.
2443 /// use std::sync::Arc;
2445 /// let five = Arc::new(5);
2447 /// assert!(five > Arc::new(4));
2449 fn gt(&self, other: &Arc<T>) -> bool {
2450 *(*self) > *(*other)
2453 /// 'Greater than or equal to' comparison for two `Arc`s.
2455 /// The two are compared by calling `>=` on their inner values.
2460 /// use std::sync::Arc;
2462 /// let five = Arc::new(5);
2464 /// assert!(five >= Arc::new(5));
2466 fn ge(&self, other: &Arc<T>) -> bool {
2467 *(*self) >= *(*other)
2470 #[stable(feature = "rust1", since = "1.0.0")]
2471 impl<T: ?Sized + Ord> Ord for Arc<T> {
2472 /// Comparison for two `Arc`s.
2474 /// The two are compared by calling `cmp()` on their inner values.
2479 /// use std::sync::Arc;
2480 /// use std::cmp::Ordering;
2482 /// let five = Arc::new(5);
2484 /// assert_eq!(Ordering::Less, five.cmp(&Arc::new(6)));
2486 fn cmp(&self, other: &Arc<T>) -> Ordering {
2487 (**self).cmp(&**other)
2490 #[stable(feature = "rust1", since = "1.0.0")]
2491 impl<T: ?Sized + Eq> Eq for Arc<T> {}
2493 #[stable(feature = "rust1", since = "1.0.0")]
2494 impl<T: ?Sized + fmt::Display> fmt::Display for Arc<T> {
2495 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
2496 fmt::Display::fmt(&**self, f)
2500 #[stable(feature = "rust1", since = "1.0.0")]
2501 impl<T: ?Sized + fmt::Debug> fmt::Debug for Arc<T> {
2502 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
2503 fmt::Debug::fmt(&**self, f)
2507 #[stable(feature = "rust1", since = "1.0.0")]
2508 impl<T: ?Sized> fmt::Pointer for Arc<T> {
2509 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
2510 fmt::Pointer::fmt(&(&**self as *const T), f)
2514 #[cfg(not(no_global_oom_handling))]
2515 #[stable(feature = "rust1", since = "1.0.0")]
2516 impl<T: Default> Default for Arc<T> {
2517 /// Creates a new `Arc<T>`, with the `Default` value for `T`.
2522 /// use std::sync::Arc;
2524 /// let x: Arc<i32> = Default::default();
2525 /// assert_eq!(*x, 0);
2527 fn default() -> Arc<T> {
2528 Arc::new(Default::default())
2532 #[stable(feature = "rust1", since = "1.0.0")]
2533 impl<T: ?Sized + Hash> Hash for Arc<T> {
2534 fn hash<H: Hasher>(&self, state: &mut H) {
2535 (**self).hash(state)
2539 #[cfg(not(no_global_oom_handling))]
2540 #[stable(feature = "from_for_ptrs", since = "1.6.0")]
2541 impl<T> From<T> for Arc<T> {
2542 /// Converts a `T` into an `Arc<T>`
2544 /// The conversion moves the value into a
2545 /// newly allocated `Arc`. It is equivalent to
2546 /// calling `Arc::new(t)`.
2550 /// # use std::sync::Arc;
2552 /// let arc = Arc::new(5);
2554 /// assert_eq!(Arc::from(x), arc);
2556 fn from(t: T) -> Self {
2561 #[cfg(not(no_global_oom_handling))]
2562 #[stable(feature = "shared_from_slice", since = "1.21.0")]
2563 impl<T: Clone> From<&[T]> for Arc<[T]> {
2564 /// Allocate a reference-counted slice and fill it by cloning `v`'s items.
2569 /// # use std::sync::Arc;
2570 /// let original: &[i32] = &[1, 2, 3];
2571 /// let shared: Arc<[i32]> = Arc::from(original);
2572 /// assert_eq!(&[1, 2, 3], &shared[..]);
2575 fn from(v: &[T]) -> Arc<[T]> {
2576 <Self as ArcFromSlice<T>>::from_slice(v)
2580 #[cfg(not(no_global_oom_handling))]
2581 #[stable(feature = "shared_from_slice", since = "1.21.0")]
2582 impl From<&str> for Arc<str> {
2583 /// Allocate a reference-counted `str` and copy `v` into it.
2588 /// # use std::sync::Arc;
2589 /// let shared: Arc<str> = Arc::from("eggplant");
2590 /// assert_eq!("eggplant", &shared[..]);
2593 fn from(v: &str) -> Arc<str> {
2594 let arc = Arc::<[u8]>::from(v.as_bytes());
2595 unsafe { Arc::from_raw(Arc::into_raw(arc) as *const str) }
2599 #[cfg(not(no_global_oom_handling))]
2600 #[stable(feature = "shared_from_slice", since = "1.21.0")]
2601 impl From<String> for Arc<str> {
2602 /// Allocate a reference-counted `str` and copy `v` into it.
2607 /// # use std::sync::Arc;
2608 /// let unique: String = "eggplant".to_owned();
2609 /// let shared: Arc<str> = Arc::from(unique);
2610 /// assert_eq!("eggplant", &shared[..]);
2613 fn from(v: String) -> Arc<str> {
2618 #[cfg(not(no_global_oom_handling))]
2619 #[stable(feature = "shared_from_slice", since = "1.21.0")]
2620 impl<T: ?Sized> From<Box<T>> for Arc<T> {
2621 /// Move a boxed object to a new, reference-counted allocation.
2626 /// # use std::sync::Arc;
2627 /// let unique: Box<str> = Box::from("eggplant");
2628 /// let shared: Arc<str> = Arc::from(unique);
2629 /// assert_eq!("eggplant", &shared[..]);
2632 fn from(v: Box<T>) -> Arc<T> {
2637 #[cfg(not(no_global_oom_handling))]
2638 #[stable(feature = "shared_from_slice", since = "1.21.0")]
2639 impl<T> From<Vec<T>> for Arc<[T]> {
2640 /// Allocate a reference-counted slice and move `v`'s items into it.
2645 /// # use std::sync::Arc;
2646 /// let unique: Vec<i32> = vec![1, 2, 3];
2647 /// let shared: Arc<[i32]> = Arc::from(unique);
2648 /// assert_eq!(&[1, 2, 3], &shared[..]);
2651 fn from(v: Vec<T>) -> Arc<[T]> {
2652 match Arc::try_from_vec_in_place(v) {
2656 let rc = Arc::copy_from_slice(&v);
2657 // Allow the Vec to free its memory, but not destroy its contents
2666 #[stable(feature = "shared_from_cow", since = "1.45.0")]
2667 impl<'a, B> From<Cow<'a, B>> for Arc<B>
2669 B: ToOwned + ?Sized,
2670 Arc<B>: From<&'a B> + From<B::Owned>,
2672 /// Create an atomically reference-counted pointer from
2673 /// a clone-on-write pointer by copying its content.
2678 /// # use std::sync::Arc;
2679 /// # use std::borrow::Cow;
2680 /// let cow: Cow<str> = Cow::Borrowed("eggplant");
2681 /// let shared: Arc<str> = Arc::from(cow);
2682 /// assert_eq!("eggplant", &shared[..]);
2685 fn from(cow: Cow<'a, B>) -> Arc<B> {
2687 Cow::Borrowed(s) => Arc::from(s),
2688 Cow::Owned(s) => Arc::from(s),
2693 #[stable(feature = "shared_from_str", since = "1.62.0")]
2694 impl From<Arc<str>> for Arc<[u8]> {
2695 /// Converts an atomically reference-counted string slice into a byte slice.
2700 /// # use std::sync::Arc;
2701 /// let string: Arc<str> = Arc::from("eggplant");
2702 /// let bytes: Arc<[u8]> = Arc::from(string);
2703 /// assert_eq!("eggplant".as_bytes(), bytes.as_ref());
2706 fn from(rc: Arc<str>) -> Self {
2707 // SAFETY: `str` has the same layout as `[u8]`.
2708 unsafe { Arc::from_raw(Arc::into_raw(rc) as *const [u8]) }
2712 #[stable(feature = "boxed_slice_try_from", since = "1.43.0")]
2713 impl<T, const N: usize> TryFrom<Arc<[T]>> for Arc<[T; N]> {
2714 type Error = Arc<[T]>;
2716 fn try_from(boxed_slice: Arc<[T]>) -> Result<Self, Self::Error> {
2717 if boxed_slice.len() == N {
2718 Ok(unsafe { Arc::from_raw(Arc::into_raw(boxed_slice) as *mut [T; N]) })
2725 #[cfg(not(no_global_oom_handling))]
2726 #[stable(feature = "shared_from_iter", since = "1.37.0")]
2727 impl<T> iter::FromIterator<T> for Arc<[T]> {
2728 /// Takes each element in the `Iterator` and collects it into an `Arc<[T]>`.
2730 /// # Performance characteristics
2732 /// ## The general case
2734 /// In the general case, collecting into `Arc<[T]>` is done by first
2735 /// collecting into a `Vec<T>`. That is, when writing the following:
2738 /// # use std::sync::Arc;
2739 /// let evens: Arc<[u8]> = (0..10).filter(|&x| x % 2 == 0).collect();
2740 /// # assert_eq!(&*evens, &[0, 2, 4, 6, 8]);
2743 /// this behaves as if we wrote:
2746 /// # use std::sync::Arc;
2747 /// let evens: Arc<[u8]> = (0..10).filter(|&x| x % 2 == 0)
2748 /// .collect::<Vec<_>>() // The first set of allocations happens here.
2749 /// .into(); // A second allocation for `Arc<[T]>` happens here.
2750 /// # assert_eq!(&*evens, &[0, 2, 4, 6, 8]);
2753 /// This will allocate as many times as needed for constructing the `Vec<T>`
2754 /// and then it will allocate once for turning the `Vec<T>` into the `Arc<[T]>`.
2756 /// ## Iterators of known length
2758 /// When your `Iterator` implements `TrustedLen` and is of an exact size,
2759 /// a single allocation will be made for the `Arc<[T]>`. For example:
2762 /// # use std::sync::Arc;
2763 /// let evens: Arc<[u8]> = (0..10).collect(); // Just a single allocation happens here.
2764 /// # assert_eq!(&*evens, &*(0..10).collect::<Vec<_>>());
2766 fn from_iter<I: iter::IntoIterator<Item = T>>(iter: I) -> Self {
2767 ToArcSlice::to_arc_slice(iter.into_iter())
2771 /// Specialization trait used for collecting into `Arc<[T]>`.
2772 trait ToArcSlice<T>: Iterator<Item = T> + Sized {
2773 fn to_arc_slice(self) -> Arc<[T]>;
2776 #[cfg(not(no_global_oom_handling))]
2777 impl<T, I: Iterator<Item = T>> ToArcSlice<T> for I {
2778 default fn to_arc_slice(self) -> Arc<[T]> {
2779 self.collect::<Vec<T>>().into()
2783 #[cfg(not(no_global_oom_handling))]
2784 impl<T, I: iter::TrustedLen<Item = T>> ToArcSlice<T> for I {
2785 fn to_arc_slice(self) -> Arc<[T]> {
2786 // This is the case for a `TrustedLen` iterator.
2787 let (low, high) = self.size_hint();
2788 if let Some(high) = high {
2792 "TrustedLen iterator's size hint is not exact: {:?}",
2797 // SAFETY: We need to ensure that the iterator has an exact length and we have.
2798 Arc::from_iter_exact(self, low)
2801 // TrustedLen contract guarantees that `upper_bound == `None` implies an iterator
2802 // length exceeding `usize::MAX`.
2803 // The default implementation would collect into a vec which would panic.
2804 // Thus we panic here immediately without invoking `Vec` code.
2805 panic!("capacity overflow");
2810 #[stable(feature = "rust1", since = "1.0.0")]
2811 impl<T: ?Sized> borrow::Borrow<T> for Arc<T> {
2812 fn borrow(&self) -> &T {
2817 #[stable(since = "1.5.0", feature = "smart_ptr_as_ref")]
2818 impl<T: ?Sized> AsRef<T> for Arc<T> {
2819 fn as_ref(&self) -> &T {
2824 #[stable(feature = "pin", since = "1.33.0")]
2825 impl<T: ?Sized> Unpin for Arc<T> {}
2827 /// Get the offset within an `ArcInner` for the payload behind a pointer.
2831 /// The pointer must point to (and have valid metadata for) a previously
2832 /// valid instance of T, but the T is allowed to be dropped.
2833 unsafe fn data_offset<T: ?Sized>(ptr: *const T) -> usize {
2834 // Align the unsized value to the end of the ArcInner.
2835 // Because RcBox is repr(C), it will always be the last field in memory.
2836 // SAFETY: since the only unsized types possible are slices, trait objects,
2837 // and extern types, the input safety requirement is currently enough to
2838 // satisfy the requirements of align_of_val_raw; this is an implementation
2839 // detail of the language that must not be relied upon outside of std.
2840 unsafe { data_offset_align(align_of_val_raw(ptr)) }
2844 fn data_offset_align(align: usize) -> usize {
2845 let layout = Layout::new::<ArcInner<()>>();
2846 layout.size() + layout.padding_needed_for(align)
2849 #[stable(feature = "arc_error", since = "1.52.0")]
2850 impl<T: core::error::Error + ?Sized> core::error::Error for Arc<T> {
2851 #[allow(deprecated, deprecated_in_future)]
2852 fn description(&self) -> &str {
2853 core::error::Error::description(&**self)
2856 #[allow(deprecated)]
2857 fn cause(&self) -> Option<&dyn core::error::Error> {
2858 core::error::Error::cause(&**self)
2861 fn source(&self) -> Option<&(dyn core::error::Error + 'static)> {
2862 core::error::Error::source(&**self)
2865 fn provide<'a>(&'a self, req: &mut core::any::Demand<'a>) {
2866 core::error::Error::provide(&**self, req);