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 unsafe impl<T: ?Sized + Sync + Send> Send for ArcInner<T> {}
337 unsafe impl<T: ?Sized + Sync + Send> Sync for ArcInner<T> {}
340 /// Constructs a new `Arc<T>`.
345 /// use std::sync::Arc;
347 /// let five = Arc::new(5);
349 #[cfg(not(no_global_oom_handling))]
351 #[stable(feature = "rust1", since = "1.0.0")]
352 pub fn new(data: T) -> Arc<T> {
353 // Start the weak pointer count as 1 which is the weak pointer that's
354 // held by all the strong pointers (kinda), see std/rc.rs for more info
355 let x: Box<_> = Box::new(ArcInner {
356 strong: atomic::AtomicUsize::new(1),
357 weak: atomic::AtomicUsize::new(1),
360 unsafe { Self::from_inner(Box::leak(x).into()) }
363 /// Constructs a new `Arc<T>` while giving you a `Weak<T>` to the allocation,
364 /// to allow you to construct a `T` which holds a weak pointer to itself.
366 /// Generally, a structure circularly referencing itself, either directly or
367 /// indirectly, should not hold a strong reference to itself to prevent a memory leak.
368 /// Using this function, you get access to the weak pointer during the
369 /// initialization of `T`, before the `Arc<T>` is created, such that you can
370 /// clone and store it inside the `T`.
372 /// `new_cyclic` first allocates the managed allocation for the `Arc<T>`,
373 /// then calls your closure, giving it a `Weak<T>` to this allocation,
374 /// and only afterwards completes the construction of the `Arc<T>` by placing
375 /// the `T` returned from your closure into the allocation.
377 /// Since the new `Arc<T>` is not fully-constructed until `Arc<T>::new_cyclic`
378 /// returns, calling [`upgrade`] on the weak reference inside your closure will
379 /// fail and result in a `None` value.
383 /// If `data_fn` panics, the panic is propagated to the caller, and the
384 /// temporary [`Weak<T>`] is dropped normally.
389 /// # #![allow(dead_code)]
390 /// use std::sync::{Arc, Weak};
393 /// me: Weak<Gadget>,
397 /// /// Construct a reference counted Gadget.
398 /// fn new() -> Arc<Self> {
399 /// // `me` is a `Weak<Gadget>` pointing at the new allocation of the
400 /// // `Arc` we're constructing.
401 /// Arc::new_cyclic(|me| {
402 /// // Create the actual struct here.
403 /// Gadget { me: me.clone() }
407 /// /// Return a reference counted pointer to Self.
408 /// fn me(&self) -> Arc<Self> {
409 /// self.me.upgrade().unwrap()
413 /// [`upgrade`]: Weak::upgrade
414 #[cfg(not(no_global_oom_handling))]
416 #[stable(feature = "arc_new_cyclic", since = "1.60.0")]
417 pub fn new_cyclic<F>(data_fn: F) -> Arc<T>
419 F: FnOnce(&Weak<T>) -> T,
421 // Construct the inner in the "uninitialized" state with a single
423 let uninit_ptr: NonNull<_> = Box::leak(Box::new(ArcInner {
424 strong: atomic::AtomicUsize::new(0),
425 weak: atomic::AtomicUsize::new(1),
426 data: mem::MaybeUninit::<T>::uninit(),
429 let init_ptr: NonNull<ArcInner<T>> = uninit_ptr.cast();
431 let weak = Weak { ptr: init_ptr };
433 // It's important we don't give up ownership of the weak pointer, or
434 // else the memory might be freed by the time `data_fn` returns. If
435 // we really wanted to pass ownership, we could create an additional
436 // weak pointer for ourselves, but this would result in additional
437 // updates to the weak reference count which might not be necessary
439 let data = data_fn(&weak);
441 // Now we can properly initialize the inner value and turn our weak
442 // reference into a strong reference.
443 let strong = unsafe {
444 let inner = init_ptr.as_ptr();
445 ptr::write(ptr::addr_of_mut!((*inner).data), data);
447 // The above write to the data field must be visible to any threads which
448 // observe a non-zero strong count. Therefore we need at least "Release" ordering
449 // in order to synchronize with the `compare_exchange_weak` in `Weak::upgrade`.
451 // "Acquire" ordering is not required. When considering the possible behaviours
452 // of `data_fn` we only need to look at what it could do with a reference to a
453 // non-upgradeable `Weak`:
454 // - It can *clone* the `Weak`, increasing the weak reference count.
455 // - It can drop those clones, decreasing the weak reference count (but never to zero).
457 // These side effects do not impact us in any way, and no other side effects are
458 // possible with safe code alone.
459 let prev_value = (*inner).strong.fetch_add(1, Release);
460 debug_assert_eq!(prev_value, 0, "No prior strong references should exist");
462 Arc::from_inner(init_ptr)
465 // Strong references should collectively own a shared weak reference,
466 // so don't run the destructor for our old weak reference.
471 /// Constructs a new `Arc` with uninitialized contents.
476 /// #![feature(new_uninit)]
477 /// #![feature(get_mut_unchecked)]
479 /// use std::sync::Arc;
481 /// let mut five = Arc::<u32>::new_uninit();
483 /// // Deferred initialization:
484 /// Arc::get_mut(&mut five).unwrap().write(5);
486 /// let five = unsafe { five.assume_init() };
488 /// assert_eq!(*five, 5)
490 #[cfg(not(no_global_oom_handling))]
491 #[unstable(feature = "new_uninit", issue = "63291")]
493 pub fn new_uninit() -> Arc<mem::MaybeUninit<T>> {
495 Arc::from_ptr(Arc::allocate_for_layout(
497 |layout| Global.allocate(layout),
498 |mem| mem as *mut ArcInner<mem::MaybeUninit<T>>,
503 /// Constructs a new `Arc` with uninitialized contents, with the memory
504 /// being filled with `0` bytes.
506 /// See [`MaybeUninit::zeroed`][zeroed] for examples of correct and incorrect usage
512 /// #![feature(new_uninit)]
514 /// use std::sync::Arc;
516 /// let zero = Arc::<u32>::new_zeroed();
517 /// let zero = unsafe { zero.assume_init() };
519 /// assert_eq!(*zero, 0)
522 /// [zeroed]: mem::MaybeUninit::zeroed
523 #[cfg(not(no_global_oom_handling))]
524 #[unstable(feature = "new_uninit", issue = "63291")]
526 pub fn new_zeroed() -> Arc<mem::MaybeUninit<T>> {
528 Arc::from_ptr(Arc::allocate_for_layout(
530 |layout| Global.allocate_zeroed(layout),
531 |mem| mem as *mut ArcInner<mem::MaybeUninit<T>>,
536 /// Constructs a new `Pin<Arc<T>>`. If `T` does not implement `Unpin`, then
537 /// `data` will be pinned in memory and unable to be moved.
538 #[cfg(not(no_global_oom_handling))]
539 #[stable(feature = "pin", since = "1.33.0")]
541 pub fn pin(data: T) -> Pin<Arc<T>> {
542 unsafe { Pin::new_unchecked(Arc::new(data)) }
545 /// Constructs a new `Pin<Arc<T>>`, return an error if allocation fails.
546 #[unstable(feature = "allocator_api", issue = "32838")]
548 pub fn try_pin(data: T) -> Result<Pin<Arc<T>>, AllocError> {
549 unsafe { Ok(Pin::new_unchecked(Arc::try_new(data)?)) }
552 /// Constructs a new `Arc<T>`, returning an error if allocation fails.
557 /// #![feature(allocator_api)]
558 /// use std::sync::Arc;
560 /// let five = Arc::try_new(5)?;
561 /// # Ok::<(), std::alloc::AllocError>(())
563 #[unstable(feature = "allocator_api", issue = "32838")]
565 pub fn try_new(data: T) -> Result<Arc<T>, AllocError> {
566 // Start the weak pointer count as 1 which is the weak pointer that's
567 // held by all the strong pointers (kinda), see std/rc.rs for more info
568 let x: Box<_> = Box::try_new(ArcInner {
569 strong: atomic::AtomicUsize::new(1),
570 weak: atomic::AtomicUsize::new(1),
573 unsafe { Ok(Self::from_inner(Box::leak(x).into())) }
576 /// Constructs a new `Arc` with uninitialized contents, returning an error
577 /// if allocation fails.
582 /// #![feature(new_uninit, allocator_api)]
583 /// #![feature(get_mut_unchecked)]
585 /// use std::sync::Arc;
587 /// let mut five = Arc::<u32>::try_new_uninit()?;
589 /// // Deferred initialization:
590 /// Arc::get_mut(&mut five).unwrap().write(5);
592 /// let five = unsafe { five.assume_init() };
594 /// assert_eq!(*five, 5);
595 /// # Ok::<(), std::alloc::AllocError>(())
597 #[unstable(feature = "allocator_api", issue = "32838")]
598 // #[unstable(feature = "new_uninit", issue = "63291")]
599 pub fn try_new_uninit() -> Result<Arc<mem::MaybeUninit<T>>, AllocError> {
601 Ok(Arc::from_ptr(Arc::try_allocate_for_layout(
603 |layout| Global.allocate(layout),
604 |mem| mem as *mut ArcInner<mem::MaybeUninit<T>>,
609 /// Constructs a new `Arc` with uninitialized contents, with the memory
610 /// being filled with `0` bytes, returning an error if allocation fails.
612 /// See [`MaybeUninit::zeroed`][zeroed] for examples of correct and incorrect usage
618 /// #![feature(new_uninit, allocator_api)]
620 /// use std::sync::Arc;
622 /// let zero = Arc::<u32>::try_new_zeroed()?;
623 /// let zero = unsafe { zero.assume_init() };
625 /// assert_eq!(*zero, 0);
626 /// # Ok::<(), std::alloc::AllocError>(())
629 /// [zeroed]: mem::MaybeUninit::zeroed
630 #[unstable(feature = "allocator_api", issue = "32838")]
631 // #[unstable(feature = "new_uninit", issue = "63291")]
632 pub fn try_new_zeroed() -> Result<Arc<mem::MaybeUninit<T>>, AllocError> {
634 Ok(Arc::from_ptr(Arc::try_allocate_for_layout(
636 |layout| Global.allocate_zeroed(layout),
637 |mem| mem as *mut ArcInner<mem::MaybeUninit<T>>,
641 /// Returns the inner value, if the `Arc` has exactly one strong reference.
643 /// Otherwise, an [`Err`] is returned with the same `Arc` that was
646 /// This will succeed even if there are outstanding weak references.
651 /// use std::sync::Arc;
653 /// let x = Arc::new(3);
654 /// assert_eq!(Arc::try_unwrap(x), Ok(3));
656 /// let x = Arc::new(4);
657 /// let _y = Arc::clone(&x);
658 /// assert_eq!(*Arc::try_unwrap(x).unwrap_err(), 4);
661 #[stable(feature = "arc_unique", since = "1.4.0")]
662 pub fn try_unwrap(this: Self) -> Result<T, Self> {
663 if this.inner().strong.compare_exchange(1, 0, Relaxed, Relaxed).is_err() {
667 acquire!(this.inner().strong);
670 let elem = ptr::read(&this.ptr.as_ref().data);
672 // Make a weak pointer to clean up the implicit strong-weak reference
673 let _weak = Weak { ptr: this.ptr };
682 /// Constructs a new atomically reference-counted slice with uninitialized contents.
687 /// #![feature(new_uninit)]
688 /// #![feature(get_mut_unchecked)]
690 /// use std::sync::Arc;
692 /// let mut values = Arc::<[u32]>::new_uninit_slice(3);
694 /// // Deferred initialization:
695 /// let data = Arc::get_mut(&mut values).unwrap();
696 /// data[0].write(1);
697 /// data[1].write(2);
698 /// data[2].write(3);
700 /// let values = unsafe { values.assume_init() };
702 /// assert_eq!(*values, [1, 2, 3])
704 #[cfg(not(no_global_oom_handling))]
705 #[unstable(feature = "new_uninit", issue = "63291")]
707 pub fn new_uninit_slice(len: usize) -> Arc<[mem::MaybeUninit<T>]> {
708 unsafe { Arc::from_ptr(Arc::allocate_for_slice(len)) }
711 /// Constructs a new atomically reference-counted slice with uninitialized contents, with the memory being
712 /// filled with `0` bytes.
714 /// See [`MaybeUninit::zeroed`][zeroed] for examples of correct and
715 /// incorrect usage of this method.
720 /// #![feature(new_uninit)]
722 /// use std::sync::Arc;
724 /// let values = Arc::<[u32]>::new_zeroed_slice(3);
725 /// let values = unsafe { values.assume_init() };
727 /// assert_eq!(*values, [0, 0, 0])
730 /// [zeroed]: mem::MaybeUninit::zeroed
731 #[cfg(not(no_global_oom_handling))]
732 #[unstable(feature = "new_uninit", issue = "63291")]
734 pub fn new_zeroed_slice(len: usize) -> Arc<[mem::MaybeUninit<T>]> {
736 Arc::from_ptr(Arc::allocate_for_layout(
737 Layout::array::<T>(len).unwrap(),
738 |layout| Global.allocate_zeroed(layout),
740 ptr::slice_from_raw_parts_mut(mem as *mut T, len)
741 as *mut ArcInner<[mem::MaybeUninit<T>]>
748 impl<T> Arc<mem::MaybeUninit<T>> {
749 /// Converts to `Arc<T>`.
753 /// As with [`MaybeUninit::assume_init`],
754 /// it is up to the caller to guarantee that the inner value
755 /// really is in an initialized state.
756 /// Calling this when the content is not yet fully initialized
757 /// causes immediate undefined behavior.
759 /// [`MaybeUninit::assume_init`]: mem::MaybeUninit::assume_init
764 /// #![feature(new_uninit)]
765 /// #![feature(get_mut_unchecked)]
767 /// use std::sync::Arc;
769 /// let mut five = Arc::<u32>::new_uninit();
771 /// // Deferred initialization:
772 /// Arc::get_mut(&mut five).unwrap().write(5);
774 /// let five = unsafe { five.assume_init() };
776 /// assert_eq!(*five, 5)
778 #[unstable(feature = "new_uninit", issue = "63291")]
779 #[must_use = "`self` will be dropped if the result is not used"]
781 pub unsafe fn assume_init(self) -> Arc<T> {
782 unsafe { Arc::from_inner(mem::ManuallyDrop::new(self).ptr.cast()) }
786 impl<T> Arc<[mem::MaybeUninit<T>]> {
787 /// Converts to `Arc<[T]>`.
791 /// As with [`MaybeUninit::assume_init`],
792 /// it is up to the caller to guarantee that the inner value
793 /// really is in an initialized state.
794 /// Calling this when the content is not yet fully initialized
795 /// causes immediate undefined behavior.
797 /// [`MaybeUninit::assume_init`]: mem::MaybeUninit::assume_init
802 /// #![feature(new_uninit)]
803 /// #![feature(get_mut_unchecked)]
805 /// use std::sync::Arc;
807 /// let mut values = Arc::<[u32]>::new_uninit_slice(3);
809 /// // Deferred initialization:
810 /// let data = Arc::get_mut(&mut values).unwrap();
811 /// data[0].write(1);
812 /// data[1].write(2);
813 /// data[2].write(3);
815 /// let values = unsafe { values.assume_init() };
817 /// assert_eq!(*values, [1, 2, 3])
819 #[unstable(feature = "new_uninit", issue = "63291")]
820 #[must_use = "`self` will be dropped if the result is not used"]
822 pub unsafe fn assume_init(self) -> Arc<[T]> {
823 unsafe { Arc::from_ptr(mem::ManuallyDrop::new(self).ptr.as_ptr() as _) }
827 impl<T: ?Sized> Arc<T> {
828 /// Consumes the `Arc`, returning the wrapped pointer.
830 /// To avoid a memory leak the pointer must be converted back to an `Arc` using
831 /// [`Arc::from_raw`].
836 /// use std::sync::Arc;
838 /// let x = Arc::new("hello".to_owned());
839 /// let x_ptr = Arc::into_raw(x);
840 /// assert_eq!(unsafe { &*x_ptr }, "hello");
842 #[must_use = "losing the pointer will leak memory"]
843 #[stable(feature = "rc_raw", since = "1.17.0")]
844 pub fn into_raw(this: Self) -> *const T {
845 let ptr = Self::as_ptr(&this);
850 /// Provides a raw pointer to the data.
852 /// The counts are not affected in any way and the `Arc` is not consumed. The pointer is valid for
853 /// as long as there are strong counts in the `Arc`.
858 /// use std::sync::Arc;
860 /// let x = Arc::new("hello".to_owned());
861 /// let y = Arc::clone(&x);
862 /// let x_ptr = Arc::as_ptr(&x);
863 /// assert_eq!(x_ptr, Arc::as_ptr(&y));
864 /// assert_eq!(unsafe { &*x_ptr }, "hello");
867 #[stable(feature = "rc_as_ptr", since = "1.45.0")]
868 pub fn as_ptr(this: &Self) -> *const T {
869 let ptr: *mut ArcInner<T> = NonNull::as_ptr(this.ptr);
871 // SAFETY: This cannot go through Deref::deref or RcBoxPtr::inner because
872 // this is required to retain raw/mut provenance such that e.g. `get_mut` can
873 // write through the pointer after the Rc is recovered through `from_raw`.
874 unsafe { ptr::addr_of_mut!((*ptr).data) }
877 /// Constructs an `Arc<T>` from a raw pointer.
879 /// The raw pointer must have been previously returned by a call to
880 /// [`Arc<U>::into_raw`][into_raw] where `U` must have the same size and
881 /// alignment as `T`. This is trivially true if `U` is `T`.
882 /// Note that if `U` is not `T` but has the same size and alignment, this is
883 /// basically like transmuting references of different types. See
884 /// [`mem::transmute`][transmute] for more information on what
885 /// restrictions apply in this case.
887 /// The user of `from_raw` has to make sure a specific value of `T` is only
890 /// This function is unsafe because improper use may lead to memory unsafety,
891 /// even if the returned `Arc<T>` is never accessed.
893 /// [into_raw]: Arc::into_raw
894 /// [transmute]: core::mem::transmute
899 /// use std::sync::Arc;
901 /// let x = Arc::new("hello".to_owned());
902 /// let x_ptr = Arc::into_raw(x);
905 /// // Convert back to an `Arc` to prevent leak.
906 /// let x = Arc::from_raw(x_ptr);
907 /// assert_eq!(&*x, "hello");
909 /// // Further calls to `Arc::from_raw(x_ptr)` would be memory-unsafe.
912 /// // The memory was freed when `x` went out of scope above, so `x_ptr` is now dangling!
914 #[stable(feature = "rc_raw", since = "1.17.0")]
915 pub unsafe fn from_raw(ptr: *const T) -> Self {
917 let offset = data_offset(ptr);
919 // Reverse the offset to find the original ArcInner.
920 let arc_ptr = ptr.byte_sub(offset) as *mut ArcInner<T>;
922 Self::from_ptr(arc_ptr)
926 /// Creates a new [`Weak`] pointer to this allocation.
931 /// use std::sync::Arc;
933 /// let five = Arc::new(5);
935 /// let weak_five = Arc::downgrade(&five);
937 #[must_use = "this returns a new `Weak` pointer, \
938 without modifying the original `Arc`"]
939 #[stable(feature = "arc_weak", since = "1.4.0")]
940 pub fn downgrade(this: &Self) -> Weak<T> {
941 // This Relaxed is OK because we're checking the value in the CAS
943 let mut cur = this.inner().weak.load(Relaxed);
946 // check if the weak counter is currently "locked"; if so, spin.
947 if cur == usize::MAX {
949 cur = this.inner().weak.load(Relaxed);
953 // NOTE: this code currently ignores the possibility of overflow
954 // into usize::MAX; in general both Rc and Arc need to be adjusted
955 // to deal with overflow.
957 // Unlike with Clone(), we need this to be an Acquire read to
958 // synchronize with the write coming from `is_unique`, so that the
959 // events prior to that write happen before this read.
960 match this.inner().weak.compare_exchange_weak(cur, cur + 1, Acquire, Relaxed) {
962 // Make sure we do not create a dangling Weak
963 debug_assert!(!is_dangling(this.ptr.as_ptr()));
964 return Weak { ptr: this.ptr };
966 Err(old) => cur = old,
971 /// Gets the number of [`Weak`] pointers to this allocation.
975 /// This method by itself is safe, but using it correctly requires extra care.
976 /// Another thread can change the weak count at any time,
977 /// including potentially between calling this method and acting on the result.
982 /// use std::sync::Arc;
984 /// let five = Arc::new(5);
985 /// let _weak_five = Arc::downgrade(&five);
987 /// // This assertion is deterministic because we haven't shared
988 /// // the `Arc` or `Weak` between threads.
989 /// assert_eq!(1, Arc::weak_count(&five));
993 #[stable(feature = "arc_counts", since = "1.15.0")]
994 pub fn weak_count(this: &Self) -> usize {
995 let cnt = this.inner().weak.load(Acquire);
996 // If the weak count is currently locked, the value of the
997 // count was 0 just before taking the lock.
998 if cnt == usize::MAX { 0 } else { cnt - 1 }
1001 /// Gets the number of strong (`Arc`) pointers to this allocation.
1005 /// This method by itself is safe, but using it correctly requires extra care.
1006 /// Another thread can change the strong count at any time,
1007 /// including potentially between calling this method and acting on the result.
1012 /// use std::sync::Arc;
1014 /// let five = Arc::new(5);
1015 /// let _also_five = Arc::clone(&five);
1017 /// // This assertion is deterministic because we haven't shared
1018 /// // the `Arc` between threads.
1019 /// assert_eq!(2, Arc::strong_count(&five));
1023 #[stable(feature = "arc_counts", since = "1.15.0")]
1024 pub fn strong_count(this: &Self) -> usize {
1025 this.inner().strong.load(Acquire)
1028 /// Increments the strong reference count on the `Arc<T>` associated with the
1029 /// provided pointer by one.
1033 /// The pointer must have been obtained through `Arc::into_raw`, and the
1034 /// associated `Arc` instance must be valid (i.e. the strong count must be at
1035 /// least 1) for the duration of this method.
1040 /// use std::sync::Arc;
1042 /// let five = Arc::new(5);
1045 /// let ptr = Arc::into_raw(five);
1046 /// Arc::increment_strong_count(ptr);
1048 /// // This assertion is deterministic because we haven't shared
1049 /// // the `Arc` between threads.
1050 /// let five = Arc::from_raw(ptr);
1051 /// assert_eq!(2, Arc::strong_count(&five));
1055 #[stable(feature = "arc_mutate_strong_count", since = "1.51.0")]
1056 pub unsafe fn increment_strong_count(ptr: *const T) {
1057 // Retain Arc, but don't touch refcount by wrapping in ManuallyDrop
1058 let arc = unsafe { mem::ManuallyDrop::new(Arc::<T>::from_raw(ptr)) };
1059 // Now increase refcount, but don't drop new refcount either
1060 let _arc_clone: mem::ManuallyDrop<_> = arc.clone();
1063 /// Decrements the strong reference count on the `Arc<T>` associated with the
1064 /// provided pointer by one.
1068 /// The pointer must have been obtained through `Arc::into_raw`, and the
1069 /// associated `Arc` instance must be valid (i.e. the strong count must be at
1070 /// least 1) when invoking this method. This method can be used to release the final
1071 /// `Arc` and backing storage, but **should not** be called after the final `Arc` has been
1077 /// use std::sync::Arc;
1079 /// let five = Arc::new(5);
1082 /// let ptr = Arc::into_raw(five);
1083 /// Arc::increment_strong_count(ptr);
1085 /// // Those assertions are deterministic because we haven't shared
1086 /// // the `Arc` between threads.
1087 /// let five = Arc::from_raw(ptr);
1088 /// assert_eq!(2, Arc::strong_count(&five));
1089 /// Arc::decrement_strong_count(ptr);
1090 /// assert_eq!(1, Arc::strong_count(&five));
1094 #[stable(feature = "arc_mutate_strong_count", since = "1.51.0")]
1095 pub unsafe fn decrement_strong_count(ptr: *const T) {
1096 unsafe { mem::drop(Arc::from_raw(ptr)) };
1100 fn inner(&self) -> &ArcInner<T> {
1101 // This unsafety is ok because while this arc is alive we're guaranteed
1102 // that the inner pointer is valid. Furthermore, we know that the
1103 // `ArcInner` structure itself is `Sync` because the inner data is
1104 // `Sync` as well, so we're ok loaning out an immutable pointer to these
1106 unsafe { self.ptr.as_ref() }
1109 // Non-inlined part of `drop`.
1111 unsafe fn drop_slow(&mut self) {
1112 // Destroy the data at this time, even though we must not free the box
1113 // allocation itself (there might still be weak pointers lying around).
1114 unsafe { ptr::drop_in_place(Self::get_mut_unchecked(self)) };
1116 // Drop the weak ref collectively held by all strong references
1117 drop(Weak { ptr: self.ptr });
1120 /// Returns `true` if the two `Arc`s point to the same allocation
1121 /// (in a vein similar to [`ptr::eq`]).
1126 /// use std::sync::Arc;
1128 /// let five = Arc::new(5);
1129 /// let same_five = Arc::clone(&five);
1130 /// let other_five = Arc::new(5);
1132 /// assert!(Arc::ptr_eq(&five, &same_five));
1133 /// assert!(!Arc::ptr_eq(&five, &other_five));
1136 /// [`ptr::eq`]: core::ptr::eq "ptr::eq"
1139 #[stable(feature = "ptr_eq", since = "1.17.0")]
1140 pub fn ptr_eq(this: &Self, other: &Self) -> bool {
1141 this.ptr.as_ptr() == other.ptr.as_ptr()
1145 impl<T: ?Sized> Arc<T> {
1146 /// Allocates an `ArcInner<T>` with sufficient space for
1147 /// a possibly-unsized inner value where the value has the layout provided.
1149 /// The function `mem_to_arcinner` is called with the data pointer
1150 /// and must return back a (potentially fat)-pointer for the `ArcInner<T>`.
1151 #[cfg(not(no_global_oom_handling))]
1152 unsafe fn allocate_for_layout(
1153 value_layout: Layout,
1154 allocate: impl FnOnce(Layout) -> Result<NonNull<[u8]>, AllocError>,
1155 mem_to_arcinner: impl FnOnce(*mut u8) -> *mut ArcInner<T>,
1156 ) -> *mut ArcInner<T> {
1157 // Calculate layout using the given value layout.
1158 // Previously, layout was calculated on the expression
1159 // `&*(ptr as *const ArcInner<T>)`, but this created a misaligned
1160 // reference (see #54908).
1161 let layout = Layout::new::<ArcInner<()>>().extend(value_layout).unwrap().0.pad_to_align();
1163 Arc::try_allocate_for_layout(value_layout, allocate, mem_to_arcinner)
1164 .unwrap_or_else(|_| handle_alloc_error(layout))
1168 /// Allocates an `ArcInner<T>` with sufficient space for
1169 /// a possibly-unsized inner value where the value has the layout provided,
1170 /// returning an error if allocation fails.
1172 /// The function `mem_to_arcinner` is called with the data pointer
1173 /// and must return back a (potentially fat)-pointer for the `ArcInner<T>`.
1174 unsafe fn try_allocate_for_layout(
1175 value_layout: Layout,
1176 allocate: impl FnOnce(Layout) -> Result<NonNull<[u8]>, AllocError>,
1177 mem_to_arcinner: impl FnOnce(*mut u8) -> *mut ArcInner<T>,
1178 ) -> Result<*mut ArcInner<T>, AllocError> {
1179 // Calculate layout using the given value layout.
1180 // Previously, layout was calculated on the expression
1181 // `&*(ptr as *const ArcInner<T>)`, but this created a misaligned
1182 // reference (see #54908).
1183 let layout = Layout::new::<ArcInner<()>>().extend(value_layout).unwrap().0.pad_to_align();
1185 let ptr = allocate(layout)?;
1187 // Initialize the ArcInner
1188 let inner = mem_to_arcinner(ptr.as_non_null_ptr().as_ptr());
1189 debug_assert_eq!(unsafe { Layout::for_value(&*inner) }, layout);
1192 ptr::write(&mut (*inner).strong, atomic::AtomicUsize::new(1));
1193 ptr::write(&mut (*inner).weak, atomic::AtomicUsize::new(1));
1199 /// Allocates an `ArcInner<T>` with sufficient space for an unsized inner value.
1200 #[cfg(not(no_global_oom_handling))]
1201 unsafe fn allocate_for_ptr(ptr: *const T) -> *mut ArcInner<T> {
1202 // Allocate for the `ArcInner<T>` using the given value.
1204 Self::allocate_for_layout(
1205 Layout::for_value(&*ptr),
1206 |layout| Global.allocate(layout),
1207 |mem| mem.with_metadata_of(ptr as *const ArcInner<T>),
1212 #[cfg(not(no_global_oom_handling))]
1213 fn from_box(v: Box<T>) -> Arc<T> {
1215 let (box_unique, alloc) = Box::into_unique(v);
1216 let bptr = box_unique.as_ptr();
1218 let value_size = size_of_val(&*bptr);
1219 let ptr = Self::allocate_for_ptr(bptr);
1221 // Copy value as bytes
1222 ptr::copy_nonoverlapping(
1223 bptr as *const T as *const u8,
1224 &mut (*ptr).data as *mut _ as *mut u8,
1228 // Free the allocation without dropping its contents
1229 box_free(box_unique, alloc);
1237 /// Allocates an `ArcInner<[T]>` with the given length.
1238 #[cfg(not(no_global_oom_handling))]
1239 unsafe fn allocate_for_slice(len: usize) -> *mut ArcInner<[T]> {
1241 Self::allocate_for_layout(
1242 Layout::array::<T>(len).unwrap(),
1243 |layout| Global.allocate(layout),
1244 |mem| ptr::slice_from_raw_parts_mut(mem as *mut T, len) as *mut ArcInner<[T]>,
1249 /// Copy elements from slice into newly allocated Arc<\[T\]>
1251 /// Unsafe because the caller must either take ownership or bind `T: Copy`.
1252 #[cfg(not(no_global_oom_handling))]
1253 unsafe fn copy_from_slice(v: &[T]) -> Arc<[T]> {
1255 let ptr = Self::allocate_for_slice(v.len());
1257 ptr::copy_nonoverlapping(v.as_ptr(), &mut (*ptr).data as *mut [T] as *mut T, v.len());
1263 /// Constructs an `Arc<[T]>` from an iterator known to be of a certain size.
1265 /// Behavior is undefined should the size be wrong.
1266 #[cfg(not(no_global_oom_handling))]
1267 unsafe fn from_iter_exact(iter: impl iter::Iterator<Item = T>, len: usize) -> Arc<[T]> {
1268 // Panic guard while cloning T elements.
1269 // In the event of a panic, elements that have been written
1270 // into the new ArcInner will be dropped, then the memory freed.
1278 impl<T> Drop for Guard<T> {
1279 fn drop(&mut self) {
1281 let slice = from_raw_parts_mut(self.elems, self.n_elems);
1282 ptr::drop_in_place(slice);
1284 Global.deallocate(self.mem, self.layout);
1290 let ptr = Self::allocate_for_slice(len);
1292 let mem = ptr as *mut _ as *mut u8;
1293 let layout = Layout::for_value(&*ptr);
1295 // Pointer to first element
1296 let elems = &mut (*ptr).data as *mut [T] as *mut T;
1298 let mut guard = Guard { mem: NonNull::new_unchecked(mem), elems, layout, n_elems: 0 };
1300 for (i, item) in iter.enumerate() {
1301 ptr::write(elems.add(i), item);
1305 // All clear. Forget the guard so it doesn't free the new ArcInner.
1313 /// Specialization trait used for `From<&[T]>`.
1314 #[cfg(not(no_global_oom_handling))]
1315 trait ArcFromSlice<T> {
1316 fn from_slice(slice: &[T]) -> Self;
1319 #[cfg(not(no_global_oom_handling))]
1320 impl<T: Clone> ArcFromSlice<T> for Arc<[T]> {
1322 default fn from_slice(v: &[T]) -> Self {
1323 unsafe { Self::from_iter_exact(v.iter().cloned(), v.len()) }
1327 #[cfg(not(no_global_oom_handling))]
1328 impl<T: Copy> ArcFromSlice<T> for Arc<[T]> {
1330 fn from_slice(v: &[T]) -> Self {
1331 unsafe { Arc::copy_from_slice(v) }
1335 #[stable(feature = "rust1", since = "1.0.0")]
1336 impl<T: ?Sized> Clone for Arc<T> {
1337 /// Makes a clone of the `Arc` pointer.
1339 /// This creates another pointer to the same allocation, increasing the
1340 /// strong reference count.
1345 /// use std::sync::Arc;
1347 /// let five = Arc::new(5);
1349 /// let _ = Arc::clone(&five);
1352 fn clone(&self) -> Arc<T> {
1353 // Using a relaxed ordering is alright here, as knowledge of the
1354 // original reference prevents other threads from erroneously deleting
1357 // As explained in the [Boost documentation][1], Increasing the
1358 // reference counter can always be done with memory_order_relaxed: New
1359 // references to an object can only be formed from an existing
1360 // reference, and passing an existing reference from one thread to
1361 // another must already provide any required synchronization.
1363 // [1]: (www.boost.org/doc/libs/1_55_0/doc/html/atomic/usage_examples.html)
1364 let old_size = self.inner().strong.fetch_add(1, Relaxed);
1366 // However we need to guard against massive refcounts in case someone is `mem::forget`ing
1367 // Arcs. If we don't do this the count can overflow and users will use-after free. This
1368 // branch will never be taken in any realistic program. We abort because such a program is
1369 // incredibly degenerate, and we don't care to support it.
1371 // This check is not 100% water-proof: we error when the refcount grows beyond `isize::MAX`.
1372 // But we do that check *after* having done the increment, so there is a chance here that
1373 // the worst already happened and we actually do overflow the `usize` counter. However, that
1374 // requires the counter to grow from `isize::MAX` to `usize::MAX` between the increment
1375 // above and the `abort` below, which seems exceedingly unlikely.
1376 if old_size > MAX_REFCOUNT {
1380 unsafe { Self::from_inner(self.ptr) }
1384 #[stable(feature = "rust1", since = "1.0.0")]
1385 impl<T: ?Sized> Deref for Arc<T> {
1389 fn deref(&self) -> &T {
1394 #[unstable(feature = "receiver_trait", issue = "none")]
1395 impl<T: ?Sized> Receiver for Arc<T> {}
1397 impl<T: Clone> Arc<T> {
1398 /// Makes a mutable reference into the given `Arc`.
1400 /// If there are other `Arc` pointers to the same allocation, then `make_mut` will
1401 /// [`clone`] the inner value to a new allocation to ensure unique ownership. This is also
1402 /// referred to as clone-on-write.
1404 /// However, if there are no other `Arc` pointers to this allocation, but some [`Weak`]
1405 /// pointers, then the [`Weak`] pointers will be dissociated and the inner value will not
1408 /// See also [`get_mut`], which will fail rather than cloning the inner value
1409 /// or dissociating [`Weak`] pointers.
1411 /// [`clone`]: Clone::clone
1412 /// [`get_mut`]: Arc::get_mut
1417 /// use std::sync::Arc;
1419 /// let mut data = Arc::new(5);
1421 /// *Arc::make_mut(&mut data) += 1; // Won't clone anything
1422 /// let mut other_data = Arc::clone(&data); // Won't clone inner data
1423 /// *Arc::make_mut(&mut data) += 1; // Clones inner data
1424 /// *Arc::make_mut(&mut data) += 1; // Won't clone anything
1425 /// *Arc::make_mut(&mut other_data) *= 2; // Won't clone anything
1427 /// // Now `data` and `other_data` point to different allocations.
1428 /// assert_eq!(*data, 8);
1429 /// assert_eq!(*other_data, 12);
1432 /// [`Weak`] pointers will be dissociated:
1435 /// use std::sync::Arc;
1437 /// let mut data = Arc::new(75);
1438 /// let weak = Arc::downgrade(&data);
1440 /// assert!(75 == *data);
1441 /// assert!(75 == *weak.upgrade().unwrap());
1443 /// *Arc::make_mut(&mut data) += 1;
1445 /// assert!(76 == *data);
1446 /// assert!(weak.upgrade().is_none());
1448 #[cfg(not(no_global_oom_handling))]
1450 #[stable(feature = "arc_unique", since = "1.4.0")]
1451 pub fn make_mut(this: &mut Self) -> &mut T {
1452 // Note that we hold both a strong reference and a weak reference.
1453 // Thus, releasing our strong reference only will not, by itself, cause
1454 // the memory to be deallocated.
1456 // Use Acquire to ensure that we see any writes to `weak` that happen
1457 // before release writes (i.e., decrements) to `strong`. Since we hold a
1458 // weak count, there's no chance the ArcInner itself could be
1460 if this.inner().strong.compare_exchange(1, 0, Acquire, Relaxed).is_err() {
1461 // Another strong pointer exists, so we must clone.
1462 // Pre-allocate memory to allow writing the cloned value directly.
1463 let mut arc = Self::new_uninit();
1465 let data = Arc::get_mut_unchecked(&mut arc);
1466 (**this).write_clone_into_raw(data.as_mut_ptr());
1467 *this = arc.assume_init();
1469 } else if this.inner().weak.load(Relaxed) != 1 {
1470 // Relaxed suffices in the above because this is fundamentally an
1471 // optimization: we are always racing with weak pointers being
1472 // dropped. Worst case, we end up allocated a new Arc unnecessarily.
1474 // We removed the last strong ref, but there are additional weak
1475 // refs remaining. We'll move the contents to a new Arc, and
1476 // invalidate the other weak refs.
1478 // Note that it is not possible for the read of `weak` to yield
1479 // usize::MAX (i.e., locked), since the weak count can only be
1480 // locked by a thread with a strong reference.
1482 // Materialize our own implicit weak pointer, so that it can clean
1483 // up the ArcInner as needed.
1484 let _weak = Weak { ptr: this.ptr };
1486 // Can just steal the data, all that's left is Weaks
1487 let mut arc = Self::new_uninit();
1489 let data = Arc::get_mut_unchecked(&mut arc);
1490 data.as_mut_ptr().copy_from_nonoverlapping(&**this, 1);
1491 ptr::write(this, arc.assume_init());
1494 // We were the sole reference of either kind; bump back up the
1495 // strong ref count.
1496 this.inner().strong.store(1, Release);
1499 // As with `get_mut()`, the unsafety is ok because our reference was
1500 // either unique to begin with, or became one upon cloning the contents.
1501 unsafe { Self::get_mut_unchecked(this) }
1504 /// If we have the only reference to `T` then unwrap it. Otherwise, clone `T` and return the
1507 /// Assuming `arc_t` is of type `Arc<T>`, this function is functionally equivalent to
1508 /// `(*arc_t).clone()`, but will avoid cloning the inner value where possible.
1513 /// #![feature(arc_unwrap_or_clone)]
1514 /// # use std::{ptr, sync::Arc};
1515 /// let inner = String::from("test");
1516 /// let ptr = inner.as_ptr();
1518 /// let arc = Arc::new(inner);
1519 /// let inner = Arc::unwrap_or_clone(arc);
1520 /// // The inner value was not cloned
1521 /// assert!(ptr::eq(ptr, inner.as_ptr()));
1523 /// let arc = Arc::new(inner);
1524 /// let arc2 = arc.clone();
1525 /// let inner = Arc::unwrap_or_clone(arc);
1526 /// // Because there were 2 references, we had to clone the inner value.
1527 /// assert!(!ptr::eq(ptr, inner.as_ptr()));
1528 /// // `arc2` is the last reference, so when we unwrap it we get back
1529 /// // the original `String`.
1530 /// let inner = Arc::unwrap_or_clone(arc2);
1531 /// assert!(ptr::eq(ptr, inner.as_ptr()));
1534 #[unstable(feature = "arc_unwrap_or_clone", issue = "93610")]
1535 pub fn unwrap_or_clone(this: Self) -> T {
1536 Arc::try_unwrap(this).unwrap_or_else(|arc| (*arc).clone())
1540 impl<T: ?Sized> Arc<T> {
1541 /// Returns a mutable reference into the given `Arc`, if there are
1542 /// no other `Arc` or [`Weak`] pointers to the same allocation.
1544 /// Returns [`None`] otherwise, because it is not safe to
1545 /// mutate a shared value.
1547 /// See also [`make_mut`][make_mut], which will [`clone`][clone]
1548 /// the inner value when there are other `Arc` pointers.
1550 /// [make_mut]: Arc::make_mut
1551 /// [clone]: Clone::clone
1556 /// use std::sync::Arc;
1558 /// let mut x = Arc::new(3);
1559 /// *Arc::get_mut(&mut x).unwrap() = 4;
1560 /// assert_eq!(*x, 4);
1562 /// let _y = Arc::clone(&x);
1563 /// assert!(Arc::get_mut(&mut x).is_none());
1566 #[stable(feature = "arc_unique", since = "1.4.0")]
1567 pub fn get_mut(this: &mut Self) -> Option<&mut T> {
1568 if this.is_unique() {
1569 // This unsafety is ok because we're guaranteed that the pointer
1570 // returned is the *only* pointer that will ever be returned to T. Our
1571 // reference count is guaranteed to be 1 at this point, and we required
1572 // the Arc itself to be `mut`, so we're returning the only possible
1573 // reference to the inner data.
1574 unsafe { Some(Arc::get_mut_unchecked(this)) }
1580 /// Returns a mutable reference into the given `Arc`,
1581 /// without any check.
1583 /// See also [`get_mut`], which is safe and does appropriate checks.
1585 /// [`get_mut`]: Arc::get_mut
1589 /// Any other `Arc` or [`Weak`] pointers to the same allocation must not be dereferenced
1590 /// for the duration of the returned borrow.
1591 /// This is trivially the case if no such pointers exist,
1592 /// for example immediately after `Arc::new`.
1597 /// #![feature(get_mut_unchecked)]
1599 /// use std::sync::Arc;
1601 /// let mut x = Arc::new(String::new());
1603 /// Arc::get_mut_unchecked(&mut x).push_str("foo")
1605 /// assert_eq!(*x, "foo");
1608 #[unstable(feature = "get_mut_unchecked", issue = "63292")]
1609 pub unsafe fn get_mut_unchecked(this: &mut Self) -> &mut T {
1610 // We are careful to *not* create a reference covering the "count" fields, as
1611 // this would alias with concurrent access to the reference counts (e.g. by `Weak`).
1612 unsafe { &mut (*this.ptr.as_ptr()).data }
1615 /// Determine whether this is the unique reference (including weak refs) to
1616 /// the underlying data.
1618 /// Note that this requires locking the weak ref count.
1619 fn is_unique(&mut self) -> bool {
1620 // lock the weak pointer count if we appear to be the sole weak pointer
1623 // The acquire label here ensures a happens-before relationship with any
1624 // writes to `strong` (in particular in `Weak::upgrade`) prior to decrements
1625 // of the `weak` count (via `Weak::drop`, which uses release). If the upgraded
1626 // weak ref was never dropped, the CAS here will fail so we do not care to synchronize.
1627 if self.inner().weak.compare_exchange(1, usize::MAX, Acquire, Relaxed).is_ok() {
1628 // This needs to be an `Acquire` to synchronize with the decrement of the `strong`
1629 // counter in `drop` -- the only access that happens when any but the last reference
1630 // is being dropped.
1631 let unique = self.inner().strong.load(Acquire) == 1;
1633 // The release write here synchronizes with a read in `downgrade`,
1634 // effectively preventing the above read of `strong` from happening
1636 self.inner().weak.store(1, Release); // release the lock
1644 #[stable(feature = "rust1", since = "1.0.0")]
1645 unsafe impl<#[may_dangle] T: ?Sized> Drop for Arc<T> {
1646 /// Drops the `Arc`.
1648 /// This will decrement the strong reference count. If the strong reference
1649 /// count reaches zero then the only other references (if any) are
1650 /// [`Weak`], so we `drop` the inner value.
1655 /// use std::sync::Arc;
1659 /// impl Drop for Foo {
1660 /// fn drop(&mut self) {
1661 /// println!("dropped!");
1665 /// let foo = Arc::new(Foo);
1666 /// let foo2 = Arc::clone(&foo);
1668 /// drop(foo); // Doesn't print anything
1669 /// drop(foo2); // Prints "dropped!"
1672 fn drop(&mut self) {
1673 // Because `fetch_sub` is already atomic, we do not need to synchronize
1674 // with other threads unless we are going to delete the object. This
1675 // same logic applies to the below `fetch_sub` to the `weak` count.
1676 if self.inner().strong.fetch_sub(1, Release) != 1 {
1680 // This fence is needed to prevent reordering of use of the data and
1681 // deletion of the data. Because it is marked `Release`, the decreasing
1682 // of the reference count synchronizes with this `Acquire` fence. This
1683 // means that use of the data happens before decreasing the reference
1684 // count, which happens before this fence, which happens before the
1685 // deletion of the data.
1687 // As explained in the [Boost documentation][1],
1689 // > It is important to enforce any possible access to the object in one
1690 // > thread (through an existing reference) to *happen before* deleting
1691 // > the object in a different thread. This is achieved by a "release"
1692 // > operation after dropping a reference (any access to the object
1693 // > through this reference must obviously happened before), and an
1694 // > "acquire" operation before deleting the object.
1696 // In particular, while the contents of an Arc are usually immutable, it's
1697 // possible to have interior writes to something like a Mutex<T>. Since a
1698 // Mutex is not acquired when it is deleted, we can't rely on its
1699 // synchronization logic to make writes in thread A visible to a destructor
1700 // running in thread B.
1702 // Also note that the Acquire fence here could probably be replaced with an
1703 // Acquire load, which could improve performance in highly-contended
1704 // situations. See [2].
1706 // [1]: (www.boost.org/doc/libs/1_55_0/doc/html/atomic/usage_examples.html)
1707 // [2]: (https://github.com/rust-lang/rust/pull/41714)
1708 acquire!(self.inner().strong);
1716 impl Arc<dyn Any + Send + Sync> {
1717 /// Attempt to downcast the `Arc<dyn Any + Send + Sync>` to a concrete type.
1722 /// use std::any::Any;
1723 /// use std::sync::Arc;
1725 /// fn print_if_string(value: Arc<dyn Any + Send + Sync>) {
1726 /// if let Ok(string) = value.downcast::<String>() {
1727 /// println!("String ({}): {}", string.len(), string);
1731 /// let my_string = "Hello World".to_string();
1732 /// print_if_string(Arc::new(my_string));
1733 /// print_if_string(Arc::new(0i8));
1736 #[stable(feature = "rc_downcast", since = "1.29.0")]
1737 pub fn downcast<T>(self) -> Result<Arc<T>, Self>
1739 T: Any + Send + Sync,
1741 if (*self).is::<T>() {
1743 let ptr = self.ptr.cast::<ArcInner<T>>();
1745 Ok(Arc::from_inner(ptr))
1752 /// Downcasts the `Arc<dyn Any + Send + Sync>` to a concrete type.
1754 /// For a safe alternative see [`downcast`].
1759 /// #![feature(downcast_unchecked)]
1761 /// use std::any::Any;
1762 /// use std::sync::Arc;
1764 /// let x: Arc<dyn Any + Send + Sync> = Arc::new(1_usize);
1767 /// assert_eq!(*x.downcast_unchecked::<usize>(), 1);
1773 /// The contained value must be of type `T`. Calling this method
1774 /// with the incorrect type is *undefined behavior*.
1777 /// [`downcast`]: Self::downcast
1779 #[unstable(feature = "downcast_unchecked", issue = "90850")]
1780 pub unsafe fn downcast_unchecked<T>(self) -> Arc<T>
1782 T: Any + Send + Sync,
1785 let ptr = self.ptr.cast::<ArcInner<T>>();
1787 Arc::from_inner(ptr)
1793 /// Constructs a new `Weak<T>`, without allocating any memory.
1794 /// Calling [`upgrade`] on the return value always gives [`None`].
1796 /// [`upgrade`]: Weak::upgrade
1801 /// use std::sync::Weak;
1803 /// let empty: Weak<i64> = Weak::new();
1804 /// assert!(empty.upgrade().is_none());
1806 #[stable(feature = "downgraded_weak", since = "1.10.0")]
1807 #[rustc_const_unstable(feature = "const_weak_new", issue = "95091", reason = "recently added")]
1809 pub const fn new() -> Weak<T> {
1810 Weak { ptr: unsafe { NonNull::new_unchecked(ptr::invalid_mut::<ArcInner<T>>(usize::MAX)) } }
1814 /// Helper type to allow accessing the reference counts without
1815 /// making any assertions about the data field.
1816 struct WeakInner<'a> {
1817 weak: &'a atomic::AtomicUsize,
1818 strong: &'a atomic::AtomicUsize,
1821 impl<T: ?Sized> Weak<T> {
1822 /// Returns a raw pointer to the object `T` pointed to by this `Weak<T>`.
1824 /// The pointer is valid only if there are some strong references. The pointer may be dangling,
1825 /// unaligned or even [`null`] otherwise.
1830 /// use std::sync::Arc;
1833 /// let strong = Arc::new("hello".to_owned());
1834 /// let weak = Arc::downgrade(&strong);
1835 /// // Both point to the same object
1836 /// assert!(ptr::eq(&*strong, weak.as_ptr()));
1837 /// // The strong here keeps it alive, so we can still access the object.
1838 /// assert_eq!("hello", unsafe { &*weak.as_ptr() });
1841 /// // But not any more. We can do weak.as_ptr(), but accessing the pointer would lead to
1842 /// // undefined behaviour.
1843 /// // assert_eq!("hello", unsafe { &*weak.as_ptr() });
1846 /// [`null`]: core::ptr::null "ptr::null"
1848 #[stable(feature = "weak_into_raw", since = "1.45.0")]
1849 pub fn as_ptr(&self) -> *const T {
1850 let ptr: *mut ArcInner<T> = NonNull::as_ptr(self.ptr);
1852 if is_dangling(ptr) {
1853 // If the pointer is dangling, we return the sentinel directly. This cannot be
1854 // a valid payload address, as the payload is at least as aligned as ArcInner (usize).
1857 // SAFETY: if is_dangling returns false, then the pointer is dereferenceable.
1858 // The payload may be dropped at this point, and we have to maintain provenance,
1859 // so use raw pointer manipulation.
1860 unsafe { ptr::addr_of_mut!((*ptr).data) }
1864 /// Consumes the `Weak<T>` and turns it into a raw pointer.
1866 /// This converts the weak pointer into a raw pointer, while still preserving the ownership of
1867 /// one weak reference (the weak count is not modified by this operation). It can be turned
1868 /// back into the `Weak<T>` with [`from_raw`].
1870 /// The same restrictions of accessing the target of the pointer as with
1871 /// [`as_ptr`] apply.
1876 /// use std::sync::{Arc, Weak};
1878 /// let strong = Arc::new("hello".to_owned());
1879 /// let weak = Arc::downgrade(&strong);
1880 /// let raw = weak.into_raw();
1882 /// assert_eq!(1, Arc::weak_count(&strong));
1883 /// assert_eq!("hello", unsafe { &*raw });
1885 /// drop(unsafe { Weak::from_raw(raw) });
1886 /// assert_eq!(0, Arc::weak_count(&strong));
1889 /// [`from_raw`]: Weak::from_raw
1890 /// [`as_ptr`]: Weak::as_ptr
1891 #[must_use = "`self` will be dropped if the result is not used"]
1892 #[stable(feature = "weak_into_raw", since = "1.45.0")]
1893 pub fn into_raw(self) -> *const T {
1894 let result = self.as_ptr();
1899 /// Converts a raw pointer previously created by [`into_raw`] back into `Weak<T>`.
1901 /// This can be used to safely get a strong reference (by calling [`upgrade`]
1902 /// later) or to deallocate the weak count by dropping the `Weak<T>`.
1904 /// It takes ownership of one weak reference (with the exception of pointers created by [`new`],
1905 /// as these don't own anything; the method still works on them).
1909 /// The pointer must have originated from the [`into_raw`] and must still own its potential
1912 /// It is allowed for the strong count to be 0 at the time of calling this. Nevertheless, this
1913 /// takes ownership of one weak reference currently represented as a raw pointer (the weak
1914 /// count is not modified by this operation) and therefore it must be paired with a previous
1915 /// call to [`into_raw`].
1919 /// use std::sync::{Arc, Weak};
1921 /// let strong = Arc::new("hello".to_owned());
1923 /// let raw_1 = Arc::downgrade(&strong).into_raw();
1924 /// let raw_2 = Arc::downgrade(&strong).into_raw();
1926 /// assert_eq!(2, Arc::weak_count(&strong));
1928 /// assert_eq!("hello", &*unsafe { Weak::from_raw(raw_1) }.upgrade().unwrap());
1929 /// assert_eq!(1, Arc::weak_count(&strong));
1933 /// // Decrement the last weak count.
1934 /// assert!(unsafe { Weak::from_raw(raw_2) }.upgrade().is_none());
1937 /// [`new`]: Weak::new
1938 /// [`into_raw`]: Weak::into_raw
1939 /// [`upgrade`]: Weak::upgrade
1940 #[stable(feature = "weak_into_raw", since = "1.45.0")]
1941 pub unsafe fn from_raw(ptr: *const T) -> Self {
1942 // See Weak::as_ptr for context on how the input pointer is derived.
1944 let ptr = if is_dangling(ptr as *mut T) {
1945 // This is a dangling Weak.
1946 ptr as *mut ArcInner<T>
1948 // Otherwise, we're guaranteed the pointer came from a nondangling Weak.
1949 // SAFETY: data_offset is safe to call, as ptr references a real (potentially dropped) T.
1950 let offset = unsafe { data_offset(ptr) };
1951 // Thus, we reverse the offset to get the whole RcBox.
1952 // SAFETY: the pointer originated from a Weak, so this offset is safe.
1953 unsafe { ptr.byte_sub(offset) as *mut ArcInner<T> }
1956 // SAFETY: we now have recovered the original Weak pointer, so can create the Weak.
1957 Weak { ptr: unsafe { NonNull::new_unchecked(ptr) } }
1961 impl<T: ?Sized> Weak<T> {
1962 /// Attempts to upgrade the `Weak` pointer to an [`Arc`], delaying
1963 /// dropping of the inner value if successful.
1965 /// Returns [`None`] if the inner value has since been dropped.
1970 /// use std::sync::Arc;
1972 /// let five = Arc::new(5);
1974 /// let weak_five = Arc::downgrade(&five);
1976 /// let strong_five: Option<Arc<_>> = weak_five.upgrade();
1977 /// assert!(strong_five.is_some());
1979 /// // Destroy all strong pointers.
1980 /// drop(strong_five);
1983 /// assert!(weak_five.upgrade().is_none());
1985 #[must_use = "this returns a new `Arc`, \
1986 without modifying the original weak pointer"]
1987 #[stable(feature = "arc_weak", since = "1.4.0")]
1988 pub fn upgrade(&self) -> Option<Arc<T>> {
1989 // We use a CAS loop to increment the strong count instead of a
1990 // fetch_add as this function should never take the reference count
1991 // from zero to one.
1994 // Relaxed is fine for the failure case because we don't have any expectations about the new state.
1995 // Acquire is necessary for the success case to synchronise with `Arc::new_cyclic`, when the inner
1996 // value can be initialized after `Weak` references have already been created. In that case, we
1997 // expect to observe the fully initialized value.
1998 .fetch_update(Acquire, Relaxed, |n| {
1999 // Any write of 0 we can observe leaves the field in permanently zero state.
2003 // See comments in `Arc::clone` for why we do this (for `mem::forget`).
2004 if n > MAX_REFCOUNT {
2010 // null checked above
2011 .map(|_| unsafe { Arc::from_inner(self.ptr) })
2014 /// Gets the number of strong (`Arc`) pointers pointing to this allocation.
2016 /// If `self` was created using [`Weak::new`], this will return 0.
2018 #[stable(feature = "weak_counts", since = "1.41.0")]
2019 pub fn strong_count(&self) -> usize {
2020 if let Some(inner) = self.inner() { inner.strong.load(Acquire) } else { 0 }
2023 /// Gets an approximation of the number of `Weak` pointers pointing to this
2026 /// If `self` was created using [`Weak::new`], or if there are no remaining
2027 /// strong pointers, this will return 0.
2031 /// Due to implementation details, the returned value can be off by 1 in
2032 /// either direction when other threads are manipulating any `Arc`s or
2033 /// `Weak`s pointing to the same allocation.
2035 #[stable(feature = "weak_counts", since = "1.41.0")]
2036 pub fn weak_count(&self) -> usize {
2039 let weak = inner.weak.load(Acquire);
2040 let strong = inner.strong.load(Acquire);
2044 // Since we observed that there was at least one strong pointer
2045 // after reading the weak count, we know that the implicit weak
2046 // reference (present whenever any strong references are alive)
2047 // was still around when we observed the weak count, and can
2048 // therefore safely subtract it.
2055 /// Returns `None` when the pointer is dangling and there is no allocated `ArcInner`,
2056 /// (i.e., when this `Weak` was created by `Weak::new`).
2058 fn inner(&self) -> Option<WeakInner<'_>> {
2059 if is_dangling(self.ptr.as_ptr()) {
2062 // We are careful to *not* create a reference covering the "data" field, as
2063 // the field may be mutated concurrently (for example, if the last `Arc`
2064 // is dropped, the data field will be dropped in-place).
2066 let ptr = self.ptr.as_ptr();
2067 WeakInner { strong: &(*ptr).strong, weak: &(*ptr).weak }
2072 /// Returns `true` if the two `Weak`s point to the same allocation (similar to
2073 /// [`ptr::eq`]), or if both don't point to any allocation
2074 /// (because they were created with `Weak::new()`).
2078 /// Since this compares pointers it means that `Weak::new()` will equal each
2079 /// other, even though they don't point to any allocation.
2084 /// use std::sync::Arc;
2086 /// let first_rc = Arc::new(5);
2087 /// let first = Arc::downgrade(&first_rc);
2088 /// let second = Arc::downgrade(&first_rc);
2090 /// assert!(first.ptr_eq(&second));
2092 /// let third_rc = Arc::new(5);
2093 /// let third = Arc::downgrade(&third_rc);
2095 /// assert!(!first.ptr_eq(&third));
2098 /// Comparing `Weak::new`.
2101 /// use std::sync::{Arc, Weak};
2103 /// let first = Weak::new();
2104 /// let second = Weak::new();
2105 /// assert!(first.ptr_eq(&second));
2107 /// let third_rc = Arc::new(());
2108 /// let third = Arc::downgrade(&third_rc);
2109 /// assert!(!first.ptr_eq(&third));
2112 /// [`ptr::eq`]: core::ptr::eq "ptr::eq"
2115 #[stable(feature = "weak_ptr_eq", since = "1.39.0")]
2116 pub fn ptr_eq(&self, other: &Self) -> bool {
2117 self.ptr.as_ptr() == other.ptr.as_ptr()
2121 #[stable(feature = "arc_weak", since = "1.4.0")]
2122 impl<T: ?Sized> Clone for Weak<T> {
2123 /// Makes a clone of the `Weak` pointer that points to the same allocation.
2128 /// use std::sync::{Arc, Weak};
2130 /// let weak_five = Arc::downgrade(&Arc::new(5));
2132 /// let _ = Weak::clone(&weak_five);
2135 fn clone(&self) -> Weak<T> {
2136 let inner = if let Some(inner) = self.inner() {
2139 return Weak { ptr: self.ptr };
2141 // See comments in Arc::clone() for why this is relaxed. This can use a
2142 // fetch_add (ignoring the lock) because the weak count is only locked
2143 // where are *no other* weak pointers in existence. (So we can't be
2144 // running this code in that case).
2145 let old_size = inner.weak.fetch_add(1, Relaxed);
2147 // See comments in Arc::clone() for why we do this (for mem::forget).
2148 if old_size > MAX_REFCOUNT {
2152 Weak { ptr: self.ptr }
2156 #[stable(feature = "downgraded_weak", since = "1.10.0")]
2157 impl<T> Default for Weak<T> {
2158 /// Constructs a new `Weak<T>`, without allocating memory.
2159 /// Calling [`upgrade`] on the return value always
2162 /// [`upgrade`]: Weak::upgrade
2167 /// use std::sync::Weak;
2169 /// let empty: Weak<i64> = Default::default();
2170 /// assert!(empty.upgrade().is_none());
2172 fn default() -> Weak<T> {
2177 #[stable(feature = "arc_weak", since = "1.4.0")]
2178 unsafe impl<#[may_dangle] T: ?Sized> Drop for Weak<T> {
2179 /// Drops the `Weak` pointer.
2184 /// use std::sync::{Arc, Weak};
2188 /// impl Drop for Foo {
2189 /// fn drop(&mut self) {
2190 /// println!("dropped!");
2194 /// let foo = Arc::new(Foo);
2195 /// let weak_foo = Arc::downgrade(&foo);
2196 /// let other_weak_foo = Weak::clone(&weak_foo);
2198 /// drop(weak_foo); // Doesn't print anything
2199 /// drop(foo); // Prints "dropped!"
2201 /// assert!(other_weak_foo.upgrade().is_none());
2203 fn drop(&mut self) {
2204 // If we find out that we were the last weak pointer, then its time to
2205 // deallocate the data entirely. See the discussion in Arc::drop() about
2206 // the memory orderings
2208 // It's not necessary to check for the locked state here, because the
2209 // weak count can only be locked if there was precisely one weak ref,
2210 // meaning that drop could only subsequently run ON that remaining weak
2211 // ref, which can only happen after the lock is released.
2212 let inner = if let Some(inner) = self.inner() { inner } else { return };
2214 if inner.weak.fetch_sub(1, Release) == 1 {
2215 acquire!(inner.weak);
2216 unsafe { Global.deallocate(self.ptr.cast(), Layout::for_value_raw(self.ptr.as_ptr())) }
2221 #[stable(feature = "rust1", since = "1.0.0")]
2222 trait ArcEqIdent<T: ?Sized + PartialEq> {
2223 fn eq(&self, other: &Arc<T>) -> bool;
2224 fn ne(&self, other: &Arc<T>) -> bool;
2227 #[stable(feature = "rust1", since = "1.0.0")]
2228 impl<T: ?Sized + PartialEq> ArcEqIdent<T> for Arc<T> {
2230 default fn eq(&self, other: &Arc<T>) -> bool {
2234 default fn ne(&self, other: &Arc<T>) -> bool {
2239 /// We're doing this specialization here, and not as a more general optimization on `&T`, because it
2240 /// would otherwise add a cost to all equality checks on refs. We assume that `Arc`s are used to
2241 /// store large values, that are slow to clone, but also heavy to check for equality, causing this
2242 /// cost to pay off more easily. It's also more likely to have two `Arc` clones, that point to
2243 /// the same value, than two `&T`s.
2245 /// We can only do this when `T: Eq` as a `PartialEq` might be deliberately irreflexive.
2246 #[stable(feature = "rust1", since = "1.0.0")]
2247 impl<T: ?Sized + crate::rc::MarkerEq> ArcEqIdent<T> for Arc<T> {
2249 fn eq(&self, other: &Arc<T>) -> bool {
2250 Arc::ptr_eq(self, other) || **self == **other
2254 fn ne(&self, other: &Arc<T>) -> bool {
2255 !Arc::ptr_eq(self, other) && **self != **other
2259 #[stable(feature = "rust1", since = "1.0.0")]
2260 impl<T: ?Sized + PartialEq> PartialEq for Arc<T> {
2261 /// Equality for two `Arc`s.
2263 /// Two `Arc`s are equal if their inner values are equal, even if they are
2264 /// stored in different allocation.
2266 /// If `T` also implements `Eq` (implying reflexivity of equality),
2267 /// two `Arc`s that point to the same allocation are always equal.
2272 /// use std::sync::Arc;
2274 /// let five = Arc::new(5);
2276 /// assert!(five == Arc::new(5));
2279 fn eq(&self, other: &Arc<T>) -> bool {
2280 ArcEqIdent::eq(self, other)
2283 /// Inequality for two `Arc`s.
2285 /// Two `Arc`s are unequal if their inner values are unequal.
2287 /// If `T` also implements `Eq` (implying reflexivity of equality),
2288 /// two `Arc`s that point to the same value are never unequal.
2293 /// use std::sync::Arc;
2295 /// let five = Arc::new(5);
2297 /// assert!(five != Arc::new(6));
2300 fn ne(&self, other: &Arc<T>) -> bool {
2301 ArcEqIdent::ne(self, other)
2305 #[stable(feature = "rust1", since = "1.0.0")]
2306 impl<T: ?Sized + PartialOrd> PartialOrd for Arc<T> {
2307 /// Partial comparison for two `Arc`s.
2309 /// The two are compared by calling `partial_cmp()` on their inner values.
2314 /// use std::sync::Arc;
2315 /// use std::cmp::Ordering;
2317 /// let five = Arc::new(5);
2319 /// assert_eq!(Some(Ordering::Less), five.partial_cmp(&Arc::new(6)));
2321 fn partial_cmp(&self, other: &Arc<T>) -> Option<Ordering> {
2322 (**self).partial_cmp(&**other)
2325 /// Less-than comparison for two `Arc`s.
2327 /// The two are compared by calling `<` on their inner values.
2332 /// use std::sync::Arc;
2334 /// let five = Arc::new(5);
2336 /// assert!(five < Arc::new(6));
2338 fn lt(&self, other: &Arc<T>) -> bool {
2339 *(*self) < *(*other)
2342 /// 'Less than or equal to' comparison for two `Arc`s.
2344 /// The two are compared by calling `<=` on their inner values.
2349 /// use std::sync::Arc;
2351 /// let five = Arc::new(5);
2353 /// assert!(five <= Arc::new(5));
2355 fn le(&self, other: &Arc<T>) -> bool {
2356 *(*self) <= *(*other)
2359 /// Greater-than comparison for two `Arc`s.
2361 /// The two are compared by calling `>` on their inner values.
2366 /// use std::sync::Arc;
2368 /// let five = Arc::new(5);
2370 /// assert!(five > Arc::new(4));
2372 fn gt(&self, other: &Arc<T>) -> bool {
2373 *(*self) > *(*other)
2376 /// 'Greater than or equal to' comparison for two `Arc`s.
2378 /// The two are compared by calling `>=` on their inner values.
2383 /// use std::sync::Arc;
2385 /// let five = Arc::new(5);
2387 /// assert!(five >= Arc::new(5));
2389 fn ge(&self, other: &Arc<T>) -> bool {
2390 *(*self) >= *(*other)
2393 #[stable(feature = "rust1", since = "1.0.0")]
2394 impl<T: ?Sized + Ord> Ord for Arc<T> {
2395 /// Comparison for two `Arc`s.
2397 /// The two are compared by calling `cmp()` on their inner values.
2402 /// use std::sync::Arc;
2403 /// use std::cmp::Ordering;
2405 /// let five = Arc::new(5);
2407 /// assert_eq!(Ordering::Less, five.cmp(&Arc::new(6)));
2409 fn cmp(&self, other: &Arc<T>) -> Ordering {
2410 (**self).cmp(&**other)
2413 #[stable(feature = "rust1", since = "1.0.0")]
2414 impl<T: ?Sized + Eq> Eq for Arc<T> {}
2416 #[stable(feature = "rust1", since = "1.0.0")]
2417 impl<T: ?Sized + fmt::Display> fmt::Display for Arc<T> {
2418 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
2419 fmt::Display::fmt(&**self, f)
2423 #[stable(feature = "rust1", since = "1.0.0")]
2424 impl<T: ?Sized + fmt::Debug> fmt::Debug for Arc<T> {
2425 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
2426 fmt::Debug::fmt(&**self, f)
2430 #[stable(feature = "rust1", since = "1.0.0")]
2431 impl<T: ?Sized> fmt::Pointer for Arc<T> {
2432 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
2433 fmt::Pointer::fmt(&(&**self as *const T), f)
2437 #[cfg(not(no_global_oom_handling))]
2438 #[stable(feature = "rust1", since = "1.0.0")]
2439 impl<T: Default> Default for Arc<T> {
2440 /// Creates a new `Arc<T>`, with the `Default` value for `T`.
2445 /// use std::sync::Arc;
2447 /// let x: Arc<i32> = Default::default();
2448 /// assert_eq!(*x, 0);
2450 fn default() -> Arc<T> {
2451 Arc::new(Default::default())
2455 #[stable(feature = "rust1", since = "1.0.0")]
2456 impl<T: ?Sized + Hash> Hash for Arc<T> {
2457 fn hash<H: Hasher>(&self, state: &mut H) {
2458 (**self).hash(state)
2462 #[cfg(not(no_global_oom_handling))]
2463 #[stable(feature = "from_for_ptrs", since = "1.6.0")]
2464 impl<T> From<T> for Arc<T> {
2465 /// Converts a `T` into an `Arc<T>`
2467 /// The conversion moves the value into a
2468 /// newly allocated `Arc`. It is equivalent to
2469 /// calling `Arc::new(t)`.
2473 /// # use std::sync::Arc;
2475 /// let arc = Arc::new(5);
2477 /// assert_eq!(Arc::from(x), arc);
2479 fn from(t: T) -> Self {
2484 #[cfg(not(no_global_oom_handling))]
2485 #[stable(feature = "shared_from_slice", since = "1.21.0")]
2486 impl<T: Clone> From<&[T]> for Arc<[T]> {
2487 /// Allocate a reference-counted slice and fill it by cloning `v`'s items.
2492 /// # use std::sync::Arc;
2493 /// let original: &[i32] = &[1, 2, 3];
2494 /// let shared: Arc<[i32]> = Arc::from(original);
2495 /// assert_eq!(&[1, 2, 3], &shared[..]);
2498 fn from(v: &[T]) -> Arc<[T]> {
2499 <Self as ArcFromSlice<T>>::from_slice(v)
2503 #[cfg(not(no_global_oom_handling))]
2504 #[stable(feature = "shared_from_slice", since = "1.21.0")]
2505 impl From<&str> for Arc<str> {
2506 /// Allocate a reference-counted `str` and copy `v` into it.
2511 /// # use std::sync::Arc;
2512 /// let shared: Arc<str> = Arc::from("eggplant");
2513 /// assert_eq!("eggplant", &shared[..]);
2516 fn from(v: &str) -> Arc<str> {
2517 let arc = Arc::<[u8]>::from(v.as_bytes());
2518 unsafe { Arc::from_raw(Arc::into_raw(arc) as *const str) }
2522 #[cfg(not(no_global_oom_handling))]
2523 #[stable(feature = "shared_from_slice", since = "1.21.0")]
2524 impl From<String> for Arc<str> {
2525 /// Allocate a reference-counted `str` and copy `v` into it.
2530 /// # use std::sync::Arc;
2531 /// let unique: String = "eggplant".to_owned();
2532 /// let shared: Arc<str> = Arc::from(unique);
2533 /// assert_eq!("eggplant", &shared[..]);
2536 fn from(v: String) -> Arc<str> {
2541 #[cfg(not(no_global_oom_handling))]
2542 #[stable(feature = "shared_from_slice", since = "1.21.0")]
2543 impl<T: ?Sized> From<Box<T>> for Arc<T> {
2544 /// Move a boxed object to a new, reference-counted allocation.
2549 /// # use std::sync::Arc;
2550 /// let unique: Box<str> = Box::from("eggplant");
2551 /// let shared: Arc<str> = Arc::from(unique);
2552 /// assert_eq!("eggplant", &shared[..]);
2555 fn from(v: Box<T>) -> Arc<T> {
2560 #[cfg(not(no_global_oom_handling))]
2561 #[stable(feature = "shared_from_slice", since = "1.21.0")]
2562 impl<T> From<Vec<T>> for Arc<[T]> {
2563 /// Allocate a reference-counted slice and move `v`'s items into it.
2568 /// # use std::sync::Arc;
2569 /// let unique: Vec<i32> = vec![1, 2, 3];
2570 /// let shared: Arc<[i32]> = Arc::from(unique);
2571 /// assert_eq!(&[1, 2, 3], &shared[..]);
2574 fn from(mut v: Vec<T>) -> Arc<[T]> {
2576 let arc = Arc::copy_from_slice(&v);
2578 // Allow the Vec to free its memory, but not destroy its contents
2586 #[stable(feature = "shared_from_cow", since = "1.45.0")]
2587 impl<'a, B> From<Cow<'a, B>> for Arc<B>
2589 B: ToOwned + ?Sized,
2590 Arc<B>: From<&'a B> + From<B::Owned>,
2592 /// Create an atomically reference-counted pointer from
2593 /// a clone-on-write pointer by copying its content.
2598 /// # use std::sync::Arc;
2599 /// # use std::borrow::Cow;
2600 /// let cow: Cow<str> = Cow::Borrowed("eggplant");
2601 /// let shared: Arc<str> = Arc::from(cow);
2602 /// assert_eq!("eggplant", &shared[..]);
2605 fn from(cow: Cow<'a, B>) -> Arc<B> {
2607 Cow::Borrowed(s) => Arc::from(s),
2608 Cow::Owned(s) => Arc::from(s),
2613 #[stable(feature = "shared_from_str", since = "1.62.0")]
2614 impl From<Arc<str>> for Arc<[u8]> {
2615 /// Converts an atomically reference-counted string slice into a byte slice.
2620 /// # use std::sync::Arc;
2621 /// let string: Arc<str> = Arc::from("eggplant");
2622 /// let bytes: Arc<[u8]> = Arc::from(string);
2623 /// assert_eq!("eggplant".as_bytes(), bytes.as_ref());
2626 fn from(rc: Arc<str>) -> Self {
2627 // SAFETY: `str` has the same layout as `[u8]`.
2628 unsafe { Arc::from_raw(Arc::into_raw(rc) as *const [u8]) }
2632 #[stable(feature = "boxed_slice_try_from", since = "1.43.0")]
2633 impl<T, const N: usize> TryFrom<Arc<[T]>> for Arc<[T; N]> {
2634 type Error = Arc<[T]>;
2636 fn try_from(boxed_slice: Arc<[T]>) -> Result<Self, Self::Error> {
2637 if boxed_slice.len() == N {
2638 Ok(unsafe { Arc::from_raw(Arc::into_raw(boxed_slice) as *mut [T; N]) })
2645 #[cfg(not(no_global_oom_handling))]
2646 #[stable(feature = "shared_from_iter", since = "1.37.0")]
2647 impl<T> iter::FromIterator<T> for Arc<[T]> {
2648 /// Takes each element in the `Iterator` and collects it into an `Arc<[T]>`.
2650 /// # Performance characteristics
2652 /// ## The general case
2654 /// In the general case, collecting into `Arc<[T]>` is done by first
2655 /// collecting into a `Vec<T>`. That is, when writing the following:
2658 /// # use std::sync::Arc;
2659 /// let evens: Arc<[u8]> = (0..10).filter(|&x| x % 2 == 0).collect();
2660 /// # assert_eq!(&*evens, &[0, 2, 4, 6, 8]);
2663 /// this behaves as if we wrote:
2666 /// # use std::sync::Arc;
2667 /// let evens: Arc<[u8]> = (0..10).filter(|&x| x % 2 == 0)
2668 /// .collect::<Vec<_>>() // The first set of allocations happens here.
2669 /// .into(); // A second allocation for `Arc<[T]>` happens here.
2670 /// # assert_eq!(&*evens, &[0, 2, 4, 6, 8]);
2673 /// This will allocate as many times as needed for constructing the `Vec<T>`
2674 /// and then it will allocate once for turning the `Vec<T>` into the `Arc<[T]>`.
2676 /// ## Iterators of known length
2678 /// When your `Iterator` implements `TrustedLen` and is of an exact size,
2679 /// a single allocation will be made for the `Arc<[T]>`. For example:
2682 /// # use std::sync::Arc;
2683 /// let evens: Arc<[u8]> = (0..10).collect(); // Just a single allocation happens here.
2684 /// # assert_eq!(&*evens, &*(0..10).collect::<Vec<_>>());
2686 fn from_iter<I: iter::IntoIterator<Item = T>>(iter: I) -> Self {
2687 ToArcSlice::to_arc_slice(iter.into_iter())
2691 /// Specialization trait used for collecting into `Arc<[T]>`.
2692 trait ToArcSlice<T>: Iterator<Item = T> + Sized {
2693 fn to_arc_slice(self) -> Arc<[T]>;
2696 #[cfg(not(no_global_oom_handling))]
2697 impl<T, I: Iterator<Item = T>> ToArcSlice<T> for I {
2698 default fn to_arc_slice(self) -> Arc<[T]> {
2699 self.collect::<Vec<T>>().into()
2703 #[cfg(not(no_global_oom_handling))]
2704 impl<T, I: iter::TrustedLen<Item = T>> ToArcSlice<T> for I {
2705 fn to_arc_slice(self) -> Arc<[T]> {
2706 // This is the case for a `TrustedLen` iterator.
2707 let (low, high) = self.size_hint();
2708 if let Some(high) = high {
2712 "TrustedLen iterator's size hint is not exact: {:?}",
2717 // SAFETY: We need to ensure that the iterator has an exact length and we have.
2718 Arc::from_iter_exact(self, low)
2721 // TrustedLen contract guarantees that `upper_bound == `None` implies an iterator
2722 // length exceeding `usize::MAX`.
2723 // The default implementation would collect into a vec which would panic.
2724 // Thus we panic here immediately without invoking `Vec` code.
2725 panic!("capacity overflow");
2730 #[stable(feature = "rust1", since = "1.0.0")]
2731 impl<T: ?Sized> borrow::Borrow<T> for Arc<T> {
2732 fn borrow(&self) -> &T {
2737 #[stable(since = "1.5.0", feature = "smart_ptr_as_ref")]
2738 impl<T: ?Sized> AsRef<T> for Arc<T> {
2739 fn as_ref(&self) -> &T {
2744 #[stable(feature = "pin", since = "1.33.0")]
2745 impl<T: ?Sized> Unpin for Arc<T> {}
2747 /// Get the offset within an `ArcInner` for the payload behind a pointer.
2751 /// The pointer must point to (and have valid metadata for) a previously
2752 /// valid instance of T, but the T is allowed to be dropped.
2753 unsafe fn data_offset<T: ?Sized>(ptr: *const T) -> usize {
2754 // Align the unsized value to the end of the ArcInner.
2755 // Because RcBox is repr(C), it will always be the last field in memory.
2756 // SAFETY: since the only unsized types possible are slices, trait objects,
2757 // and extern types, the input safety requirement is currently enough to
2758 // satisfy the requirements of align_of_val_raw; this is an implementation
2759 // detail of the language that must not be relied upon outside of std.
2760 unsafe { data_offset_align(align_of_val_raw(ptr)) }
2764 fn data_offset_align(align: usize) -> usize {
2765 let layout = Layout::new::<ArcInner<()>>();
2766 layout.size() + layout.padding_needed_for(align)
2769 #[stable(feature = "arc_error", since = "1.52.0")]
2770 impl<T: core::error::Error + ?Sized> core::error::Error for Arc<T> {
2771 #[allow(deprecated, deprecated_in_future)]
2772 fn description(&self) -> &str {
2773 core::error::Error::description(&**self)
2776 #[allow(deprecated)]
2777 fn cause(&self) -> Option<&dyn core::error::Error> {
2778 core::error::Error::cause(&**self)
2781 fn source(&self) -> Option<&(dyn core::error::Error + 'static)> {
2782 core::error::Error::source(&**self)
2785 fn provide<'a>(&'a self, req: &mut core::any::Demand<'a>) {
2786 core::error::Error::provide(&**self, req);