1 #![stable(feature = "rust1", since = "1.0.0")]
3 //! Thread-safe reference-counting pointers.
5 //! See the [`Arc<T>`][Arc] documentation for more details.
9 use core::cmp::Ordering;
10 use core::convert::{From, TryFrom};
12 use core::hash::{Hash, Hasher};
14 use core::intrinsics::abort;
15 #[cfg(not(no_global_oom_handling))]
17 use core::marker::{PhantomData, Unpin, Unsize};
18 #[cfg(not(no_global_oom_handling))]
19 use core::mem::size_of_val;
20 use core::mem::{self, align_of_val_raw};
21 use core::ops::{CoerceUnsized, Deref, DispatchFromDyn, Receiver};
22 use core::panic::{RefUnwindSafe, UnwindSafe};
24 use core::ptr::{self, NonNull};
25 #[cfg(not(no_global_oom_handling))]
26 use core::slice::from_raw_parts_mut;
27 use core::sync::atomic;
28 use core::sync::atomic::Ordering::{Acquire, Relaxed, Release, SeqCst};
30 #[cfg(not(no_global_oom_handling))]
31 use crate::alloc::handle_alloc_error;
32 #[cfg(not(no_global_oom_handling))]
33 use crate::alloc::{box_free, WriteCloneIntoRaw};
34 use crate::alloc::{AllocError, Allocator, Global, Layout};
35 use crate::borrow::{Cow, ToOwned};
36 use crate::boxed::Box;
37 use crate::rc::is_dangling;
38 #[cfg(not(no_global_oom_handling))]
39 use crate::string::String;
40 #[cfg(not(no_global_oom_handling))]
46 /// A soft limit on the amount of references that may be made to an `Arc`.
48 /// Going above this limit will abort your program (although not
49 /// necessarily) at _exactly_ `MAX_REFCOUNT + 1` references.
50 const MAX_REFCOUNT: usize = (isize::MAX) as usize;
52 #[cfg(not(sanitize = "thread"))]
53 macro_rules! acquire {
55 atomic::fence(Acquire)
59 // ThreadSanitizer does not support memory fences. To avoid false positive
60 // reports in Arc / Weak implementation use atomic loads for synchronization
62 #[cfg(sanitize = "thread")]
63 macro_rules! acquire {
69 /// A thread-safe reference-counting pointer. 'Arc' stands for 'Atomically
70 /// Reference Counted'.
72 /// The type `Arc<T>` provides shared ownership of a value of type `T`,
73 /// allocated in the heap. Invoking [`clone`][clone] on `Arc` produces
74 /// a new `Arc` instance, which points to the same allocation on the heap as the
75 /// source `Arc`, while increasing a reference count. When the last `Arc`
76 /// pointer to a given allocation is destroyed, the value stored in that allocation (often
77 /// referred to as "inner value") is also dropped.
79 /// Shared references in Rust disallow mutation by default, and `Arc` is no
80 /// exception: you cannot generally obtain a mutable reference to something
81 /// inside an `Arc`. If you need to mutate through an `Arc`, use
82 /// [`Mutex`][mutex], [`RwLock`][rwlock], or one of the [`Atomic`][atomic]
87 /// Unlike [`Rc<T>`], `Arc<T>` uses atomic operations for its reference
88 /// counting. This means that it is thread-safe. The disadvantage is that
89 /// atomic operations are more expensive than ordinary memory accesses. If you
90 /// are not sharing reference-counted allocations between threads, consider using
91 /// [`Rc<T>`] for lower overhead. [`Rc<T>`] is a safe default, because the
92 /// compiler will catch any attempt to send an [`Rc<T>`] between threads.
93 /// However, a library might choose `Arc<T>` in order to give library consumers
96 /// `Arc<T>` will implement [`Send`] and [`Sync`] as long as the `T` implements
97 /// [`Send`] and [`Sync`]. Why can't you put a non-thread-safe type `T` in an
98 /// `Arc<T>` to make it thread-safe? This may be a bit counter-intuitive at
99 /// first: after all, isn't the point of `Arc<T>` thread safety? The key is
100 /// this: `Arc<T>` makes it thread safe to have multiple ownership of the same
101 /// data, but it doesn't add thread safety to its data. Consider
102 /// <code>Arc<[RefCell\<T>]></code>. [`RefCell<T>`] isn't [`Sync`], and if `Arc<T>` was always
103 /// [`Send`], <code>Arc<[RefCell\<T>]></code> would be as well. But then we'd have a problem:
104 /// [`RefCell<T>`] is not thread safe; it keeps track of the borrowing count using
105 /// non-atomic operations.
107 /// In the end, this means that you may need to pair `Arc<T>` with some sort of
108 /// [`std::sync`] type, usually [`Mutex<T>`][mutex].
110 /// ## Breaking cycles with `Weak`
112 /// The [`downgrade`][downgrade] method can be used to create a non-owning
113 /// [`Weak`] pointer. A [`Weak`] pointer can be [`upgrade`][upgrade]d
114 /// to an `Arc`, but this will return [`None`] if the value stored in the allocation has
115 /// already been dropped. In other words, `Weak` pointers do not keep the value
116 /// inside the allocation alive; however, they *do* keep the allocation
117 /// (the backing store for the value) alive.
119 /// A cycle between `Arc` pointers will never be deallocated. For this reason,
120 /// [`Weak`] is used to break cycles. For example, a tree could have
121 /// strong `Arc` pointers from parent nodes to children, and [`Weak`]
122 /// pointers from children back to their parents.
124 /// # Cloning references
126 /// Creating a new reference from an existing reference-counted pointer is done using the
127 /// `Clone` trait implemented for [`Arc<T>`][Arc] and [`Weak<T>`][Weak].
130 /// use std::sync::Arc;
131 /// let foo = Arc::new(vec![1.0, 2.0, 3.0]);
132 /// // The two syntaxes below are equivalent.
133 /// let a = foo.clone();
134 /// let b = Arc::clone(&foo);
135 /// // a, b, and foo are all Arcs that point to the same memory location
138 /// ## `Deref` behavior
140 /// `Arc<T>` automatically dereferences to `T` (via the [`Deref`][deref] trait),
141 /// so you can call `T`'s methods on a value of type `Arc<T>`. To avoid name
142 /// clashes with `T`'s methods, the methods of `Arc<T>` itself are associated
143 /// functions, called using [fully qualified syntax]:
146 /// use std::sync::Arc;
148 /// let my_arc = Arc::new(());
149 /// Arc::downgrade(&my_arc);
152 /// `Arc<T>`'s implementations of traits like `Clone` may also be called using
153 /// fully qualified syntax. Some people prefer to use fully qualified syntax,
154 /// while others prefer using method-call syntax.
157 /// use std::sync::Arc;
159 /// let arc = Arc::new(());
160 /// // Method-call syntax
161 /// let arc2 = arc.clone();
162 /// // Fully qualified syntax
163 /// let arc3 = Arc::clone(&arc);
166 /// [`Weak<T>`][Weak] does not auto-dereference to `T`, because the inner value may have
167 /// already been dropped.
169 /// [`Rc<T>`]: crate::rc::Rc
170 /// [clone]: Clone::clone
171 /// [mutex]: ../../std/sync/struct.Mutex.html
172 /// [rwlock]: ../../std/sync/struct.RwLock.html
173 /// [atomic]: core::sync::atomic
174 /// [`Send`]: core::marker::Send
175 /// [`Sync`]: core::marker::Sync
176 /// [deref]: core::ops::Deref
177 /// [downgrade]: Arc::downgrade
178 /// [upgrade]: Weak::upgrade
179 /// [RefCell\<T>]: core::cell::RefCell
180 /// [`RefCell<T>`]: core::cell::RefCell
181 /// [`std::sync`]: ../../std/sync/index.html
182 /// [`Arc::clone(&from)`]: Arc::clone
183 /// [fully qualified syntax]: https://doc.rust-lang.org/book/ch19-03-advanced-traits.html#fully-qualified-syntax-for-disambiguation-calling-methods-with-the-same-name
187 /// Sharing some immutable data between threads:
189 // Note that we **do not** run these tests here. The windows builders get super
190 // unhappy if a thread outlives the main thread and then exits at the same time
191 // (something deadlocks) so we just avoid this entirely by not running these
194 /// use std::sync::Arc;
197 /// let five = Arc::new(5);
200 /// let five = Arc::clone(&five);
202 /// thread::spawn(move || {
203 /// println!("{:?}", five);
208 /// Sharing a mutable [`AtomicUsize`]:
210 /// [`AtomicUsize`]: core::sync::atomic::AtomicUsize "sync::atomic::AtomicUsize"
213 /// use std::sync::Arc;
214 /// use std::sync::atomic::{AtomicUsize, Ordering};
217 /// let val = Arc::new(AtomicUsize::new(5));
220 /// let val = Arc::clone(&val);
222 /// thread::spawn(move || {
223 /// let v = val.fetch_add(1, Ordering::SeqCst);
224 /// println!("{:?}", v);
229 /// See the [`rc` documentation][rc_examples] for more examples of reference
230 /// counting in general.
232 /// [rc_examples]: crate::rc#examples
233 #[cfg_attr(not(test), rustc_diagnostic_item = "Arc")]
234 #[stable(feature = "rust1", since = "1.0.0")]
235 pub struct Arc<T: ?Sized> {
236 ptr: NonNull<ArcInner<T>>,
237 phantom: PhantomData<ArcInner<T>>,
240 #[stable(feature = "rust1", since = "1.0.0")]
241 unsafe impl<T: ?Sized + Sync + Send> Send for Arc<T> {}
242 #[stable(feature = "rust1", since = "1.0.0")]
243 unsafe impl<T: ?Sized + Sync + Send> Sync for Arc<T> {}
245 #[stable(feature = "catch_unwind", since = "1.9.0")]
246 impl<T: RefUnwindSafe + ?Sized> UnwindSafe for Arc<T> {}
248 #[unstable(feature = "coerce_unsized", issue = "27732")]
249 impl<T: ?Sized + Unsize<U>, U: ?Sized> CoerceUnsized<Arc<U>> for Arc<T> {}
251 #[unstable(feature = "dispatch_from_dyn", issue = "none")]
252 impl<T: ?Sized + Unsize<U>, U: ?Sized> DispatchFromDyn<Arc<U>> for Arc<T> {}
254 impl<T: ?Sized> Arc<T> {
255 fn from_inner(ptr: NonNull<ArcInner<T>>) -> Self {
256 Self { ptr, phantom: PhantomData }
259 unsafe fn from_ptr(ptr: *mut ArcInner<T>) -> Self {
260 unsafe { Self::from_inner(NonNull::new_unchecked(ptr)) }
264 /// `Weak` is a version of [`Arc`] that holds a non-owning reference to the
265 /// managed allocation. The allocation is accessed by calling [`upgrade`] on the `Weak`
266 /// pointer, which returns an <code>[Option]<[Arc]\<T>></code>.
268 /// Since a `Weak` reference does not count towards ownership, it will not
269 /// prevent the value stored in the allocation from being dropped, and `Weak` itself makes no
270 /// guarantees about the value still being present. Thus it may return [`None`]
271 /// when [`upgrade`]d. Note however that a `Weak` reference *does* prevent the allocation
272 /// itself (the backing store) from being deallocated.
274 /// A `Weak` pointer is useful for keeping a temporary reference to the allocation
275 /// managed by [`Arc`] without preventing its inner value from being dropped. It is also used to
276 /// prevent circular references between [`Arc`] pointers, since mutual owning references
277 /// would never allow either [`Arc`] to be dropped. For example, a tree could
278 /// have strong [`Arc`] pointers from parent nodes to children, and `Weak`
279 /// pointers from children back to their parents.
281 /// The typical way to obtain a `Weak` pointer is to call [`Arc::downgrade`].
283 /// [`upgrade`]: Weak::upgrade
284 #[stable(feature = "arc_weak", since = "1.4.0")]
285 pub struct Weak<T: ?Sized> {
286 // This is a `NonNull` to allow optimizing the size of this type in enums,
287 // but it is not necessarily a valid pointer.
288 // `Weak::new` sets this to `usize::MAX` so that it doesn’t need
289 // to allocate space on the heap. That's not a value a real pointer
290 // will ever have because RcBox has alignment at least 2.
291 // This is only possible when `T: Sized`; unsized `T` never dangle.
292 ptr: NonNull<ArcInner<T>>,
295 #[stable(feature = "arc_weak", since = "1.4.0")]
296 unsafe impl<T: ?Sized + Sync + Send> Send for Weak<T> {}
297 #[stable(feature = "arc_weak", since = "1.4.0")]
298 unsafe impl<T: ?Sized + Sync + Send> Sync for Weak<T> {}
300 #[unstable(feature = "coerce_unsized", issue = "27732")]
301 impl<T: ?Sized + Unsize<U>, U: ?Sized> CoerceUnsized<Weak<U>> for Weak<T> {}
302 #[unstable(feature = "dispatch_from_dyn", issue = "none")]
303 impl<T: ?Sized + Unsize<U>, U: ?Sized> DispatchFromDyn<Weak<U>> for Weak<T> {}
305 #[stable(feature = "arc_weak", since = "1.4.0")]
306 impl<T: ?Sized + fmt::Debug> fmt::Debug for Weak<T> {
307 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
312 // This is repr(C) to future-proof against possible field-reordering, which
313 // would interfere with otherwise safe [into|from]_raw() of transmutable
316 struct ArcInner<T: ?Sized> {
317 strong: atomic::AtomicUsize,
319 // the value usize::MAX acts as a sentinel for temporarily "locking" the
320 // ability to upgrade weak pointers or downgrade strong ones; this is used
321 // to avoid races in `make_mut` and `get_mut`.
322 weak: atomic::AtomicUsize,
327 unsafe impl<T: ?Sized + Sync + Send> Send for ArcInner<T> {}
328 unsafe impl<T: ?Sized + Sync + Send> Sync for ArcInner<T> {}
331 /// Constructs a new `Arc<T>`.
336 /// use std::sync::Arc;
338 /// let five = Arc::new(5);
340 #[cfg(not(no_global_oom_handling))]
342 #[stable(feature = "rust1", since = "1.0.0")]
343 pub fn new(data: T) -> Arc<T> {
344 // Start the weak pointer count as 1 which is the weak pointer that's
345 // held by all the strong pointers (kinda), see std/rc.rs for more info
346 let x: Box<_> = box ArcInner {
347 strong: atomic::AtomicUsize::new(1),
348 weak: atomic::AtomicUsize::new(1),
351 Self::from_inner(Box::leak(x).into())
354 /// Constructs a new `Arc<T>` using a weak reference to itself. Attempting
355 /// to upgrade the weak reference before this function returns will result
356 /// in a `None` value. However, the weak reference may be cloned freely and
357 /// stored for use at a later time.
361 /// #![feature(arc_new_cyclic)]
362 /// #![allow(dead_code)]
364 /// use std::sync::{Arc, Weak};
370 /// let foo = Arc::new_cyclic(|me| Foo {
374 #[cfg(not(no_global_oom_handling))]
376 #[unstable(feature = "arc_new_cyclic", issue = "75861")]
377 pub fn new_cyclic(data_fn: impl FnOnce(&Weak<T>) -> T) -> Arc<T> {
378 // Construct the inner in the "uninitialized" state with a single
380 let uninit_ptr: NonNull<_> = Box::leak(box ArcInner {
381 strong: atomic::AtomicUsize::new(0),
382 weak: atomic::AtomicUsize::new(1),
383 data: mem::MaybeUninit::<T>::uninit(),
386 let init_ptr: NonNull<ArcInner<T>> = uninit_ptr.cast();
388 let weak = Weak { ptr: init_ptr };
390 // It's important we don't give up ownership of the weak pointer, or
391 // else the memory might be freed by the time `data_fn` returns. If
392 // we really wanted to pass ownership, we could create an additional
393 // weak pointer for ourselves, but this would result in additional
394 // updates to the weak reference count which might not be necessary
396 let data = data_fn(&weak);
398 // Now we can properly initialize the inner value and turn our weak
399 // reference into a strong reference.
401 let inner = init_ptr.as_ptr();
402 ptr::write(ptr::addr_of_mut!((*inner).data), data);
404 // The above write to the data field must be visible to any threads which
405 // observe a non-zero strong count. Therefore we need at least "Release" ordering
406 // in order to synchronize with the `compare_exchange_weak` in `Weak::upgrade`.
408 // "Acquire" ordering is not required. When considering the possible behaviours
409 // of `data_fn` we only need to look at what it could do with a reference to a
410 // non-upgradeable `Weak`:
411 // - It can *clone* the `Weak`, increasing the weak reference count.
412 // - It can drop those clones, decreasing the weak reference count (but never to zero).
414 // These side effects do not impact us in any way, and no other side effects are
415 // possible with safe code alone.
416 let prev_value = (*inner).strong.fetch_add(1, Release);
417 debug_assert_eq!(prev_value, 0, "No prior strong references should exist");
420 let strong = Arc::from_inner(init_ptr);
422 // Strong references should collectively own a shared weak reference,
423 // so don't run the destructor for our old weak reference.
428 /// Constructs a new `Arc` with uninitialized contents.
433 /// #![feature(new_uninit)]
434 /// #![feature(get_mut_unchecked)]
436 /// use std::sync::Arc;
438 /// let mut five = Arc::<u32>::new_uninit();
440 /// let five = unsafe {
441 /// // Deferred initialization:
442 /// Arc::get_mut_unchecked(&mut five).as_mut_ptr().write(5);
444 /// five.assume_init()
447 /// assert_eq!(*five, 5)
449 #[cfg(not(no_global_oom_handling))]
450 #[unstable(feature = "new_uninit", issue = "63291")]
451 pub fn new_uninit() -> Arc<mem::MaybeUninit<T>> {
453 Arc::from_ptr(Arc::allocate_for_layout(
455 |layout| Global.allocate(layout),
456 |mem| mem as *mut ArcInner<mem::MaybeUninit<T>>,
461 /// Constructs a new `Arc` with uninitialized contents, with the memory
462 /// being filled with `0` bytes.
464 /// See [`MaybeUninit::zeroed`][zeroed] for examples of correct and incorrect usage
470 /// #![feature(new_uninit)]
472 /// use std::sync::Arc;
474 /// let zero = Arc::<u32>::new_zeroed();
475 /// let zero = unsafe { zero.assume_init() };
477 /// assert_eq!(*zero, 0)
480 /// [zeroed]: mem::MaybeUninit::zeroed
481 #[cfg(not(no_global_oom_handling))]
482 #[unstable(feature = "new_uninit", issue = "63291")]
483 pub fn new_zeroed() -> Arc<mem::MaybeUninit<T>> {
485 Arc::from_ptr(Arc::allocate_for_layout(
487 |layout| Global.allocate_zeroed(layout),
488 |mem| mem as *mut ArcInner<mem::MaybeUninit<T>>,
493 /// Constructs a new `Pin<Arc<T>>`. If `T` does not implement `Unpin`, then
494 /// `data` will be pinned in memory and unable to be moved.
495 #[cfg(not(no_global_oom_handling))]
496 #[stable(feature = "pin", since = "1.33.0")]
497 pub fn pin(data: T) -> Pin<Arc<T>> {
498 unsafe { Pin::new_unchecked(Arc::new(data)) }
501 /// Constructs a new `Pin<Arc<T>>`, return an error if allocation fails.
502 #[unstable(feature = "allocator_api", issue = "32838")]
504 pub fn try_pin(data: T) -> Result<Pin<Arc<T>>, AllocError> {
505 unsafe { Ok(Pin::new_unchecked(Arc::try_new(data)?)) }
508 /// Constructs a new `Arc<T>`, returning an error if allocation fails.
513 /// #![feature(allocator_api)]
514 /// use std::sync::Arc;
516 /// let five = Arc::try_new(5)?;
517 /// # Ok::<(), std::alloc::AllocError>(())
519 #[unstable(feature = "allocator_api", issue = "32838")]
521 pub fn try_new(data: T) -> Result<Arc<T>, AllocError> {
522 // Start the weak pointer count as 1 which is the weak pointer that's
523 // held by all the strong pointers (kinda), see std/rc.rs for more info
524 let x: Box<_> = Box::try_new(ArcInner {
525 strong: atomic::AtomicUsize::new(1),
526 weak: atomic::AtomicUsize::new(1),
529 Ok(Self::from_inner(Box::leak(x).into()))
532 /// Constructs a new `Arc` with uninitialized contents, returning an error
533 /// if allocation fails.
538 /// #![feature(new_uninit, allocator_api)]
539 /// #![feature(get_mut_unchecked)]
541 /// use std::sync::Arc;
543 /// let mut five = Arc::<u32>::try_new_uninit()?;
545 /// let five = unsafe {
546 /// // Deferred initialization:
547 /// Arc::get_mut_unchecked(&mut five).as_mut_ptr().write(5);
549 /// five.assume_init()
552 /// assert_eq!(*five, 5);
553 /// # Ok::<(), std::alloc::AllocError>(())
555 #[unstable(feature = "allocator_api", issue = "32838")]
556 // #[unstable(feature = "new_uninit", issue = "63291")]
557 pub fn try_new_uninit() -> Result<Arc<mem::MaybeUninit<T>>, AllocError> {
559 Ok(Arc::from_ptr(Arc::try_allocate_for_layout(
561 |layout| Global.allocate(layout),
562 |mem| mem as *mut ArcInner<mem::MaybeUninit<T>>,
567 /// Constructs a new `Arc` with uninitialized contents, with the memory
568 /// being filled with `0` bytes, returning an error if allocation fails.
570 /// See [`MaybeUninit::zeroed`][zeroed] for examples of correct and incorrect usage
576 /// #![feature(new_uninit, allocator_api)]
578 /// use std::sync::Arc;
580 /// let zero = Arc::<u32>::try_new_zeroed()?;
581 /// let zero = unsafe { zero.assume_init() };
583 /// assert_eq!(*zero, 0);
584 /// # Ok::<(), std::alloc::AllocError>(())
587 /// [zeroed]: mem::MaybeUninit::zeroed
588 #[unstable(feature = "allocator_api", issue = "32838")]
589 // #[unstable(feature = "new_uninit", issue = "63291")]
590 pub fn try_new_zeroed() -> Result<Arc<mem::MaybeUninit<T>>, AllocError> {
592 Ok(Arc::from_ptr(Arc::try_allocate_for_layout(
594 |layout| Global.allocate_zeroed(layout),
595 |mem| mem as *mut ArcInner<mem::MaybeUninit<T>>,
599 /// Returns the inner value, if the `Arc` has exactly one strong reference.
601 /// Otherwise, an [`Err`] is returned with the same `Arc` that was
604 /// This will succeed even if there are outstanding weak references.
609 /// use std::sync::Arc;
611 /// let x = Arc::new(3);
612 /// assert_eq!(Arc::try_unwrap(x), Ok(3));
614 /// let x = Arc::new(4);
615 /// let _y = Arc::clone(&x);
616 /// assert_eq!(*Arc::try_unwrap(x).unwrap_err(), 4);
619 #[stable(feature = "arc_unique", since = "1.4.0")]
620 pub fn try_unwrap(this: Self) -> Result<T, Self> {
621 if this.inner().strong.compare_exchange(1, 0, Relaxed, Relaxed).is_err() {
625 acquire!(this.inner().strong);
628 let elem = ptr::read(&this.ptr.as_ref().data);
630 // Make a weak pointer to clean up the implicit strong-weak reference
631 let _weak = Weak { ptr: this.ptr };
640 /// Constructs a new atomically reference-counted slice with uninitialized contents.
645 /// #![feature(new_uninit)]
646 /// #![feature(get_mut_unchecked)]
648 /// use std::sync::Arc;
650 /// let mut values = Arc::<[u32]>::new_uninit_slice(3);
652 /// let values = unsafe {
653 /// // Deferred initialization:
654 /// Arc::get_mut_unchecked(&mut values)[0].as_mut_ptr().write(1);
655 /// Arc::get_mut_unchecked(&mut values)[1].as_mut_ptr().write(2);
656 /// Arc::get_mut_unchecked(&mut values)[2].as_mut_ptr().write(3);
658 /// values.assume_init()
661 /// assert_eq!(*values, [1, 2, 3])
663 #[cfg(not(no_global_oom_handling))]
664 #[unstable(feature = "new_uninit", issue = "63291")]
665 pub fn new_uninit_slice(len: usize) -> Arc<[mem::MaybeUninit<T>]> {
666 unsafe { Arc::from_ptr(Arc::allocate_for_slice(len)) }
669 /// Constructs a new atomically reference-counted slice with uninitialized contents, with the memory being
670 /// filled with `0` bytes.
672 /// See [`MaybeUninit::zeroed`][zeroed] for examples of correct and
673 /// incorrect usage of this method.
678 /// #![feature(new_uninit)]
680 /// use std::sync::Arc;
682 /// let values = Arc::<[u32]>::new_zeroed_slice(3);
683 /// let values = unsafe { values.assume_init() };
685 /// assert_eq!(*values, [0, 0, 0])
688 /// [zeroed]: mem::MaybeUninit::zeroed
689 #[cfg(not(no_global_oom_handling))]
690 #[unstable(feature = "new_uninit", issue = "63291")]
691 pub fn new_zeroed_slice(len: usize) -> Arc<[mem::MaybeUninit<T>]> {
693 Arc::from_ptr(Arc::allocate_for_layout(
694 Layout::array::<T>(len).unwrap(),
695 |layout| Global.allocate_zeroed(layout),
697 ptr::slice_from_raw_parts_mut(mem as *mut T, len)
698 as *mut ArcInner<[mem::MaybeUninit<T>]>
705 impl<T> Arc<mem::MaybeUninit<T>> {
706 /// Converts to `Arc<T>`.
710 /// As with [`MaybeUninit::assume_init`],
711 /// it is up to the caller to guarantee that the inner value
712 /// really is in an initialized state.
713 /// Calling this when the content is not yet fully initialized
714 /// causes immediate undefined behavior.
716 /// [`MaybeUninit::assume_init`]: mem::MaybeUninit::assume_init
721 /// #![feature(new_uninit)]
722 /// #![feature(get_mut_unchecked)]
724 /// use std::sync::Arc;
726 /// let mut five = Arc::<u32>::new_uninit();
728 /// let five = unsafe {
729 /// // Deferred initialization:
730 /// Arc::get_mut_unchecked(&mut five).as_mut_ptr().write(5);
732 /// five.assume_init()
735 /// assert_eq!(*five, 5)
737 #[unstable(feature = "new_uninit", issue = "63291")]
738 #[must_use = "`self` will be dropped if the result is not used"]
740 pub unsafe fn assume_init(self) -> Arc<T> {
741 Arc::from_inner(mem::ManuallyDrop::new(self).ptr.cast())
745 impl<T> Arc<[mem::MaybeUninit<T>]> {
746 /// Converts to `Arc<[T]>`.
750 /// As with [`MaybeUninit::assume_init`],
751 /// it is up to the caller to guarantee that the inner value
752 /// really is in an initialized state.
753 /// Calling this when the content is not yet fully initialized
754 /// causes immediate undefined behavior.
756 /// [`MaybeUninit::assume_init`]: mem::MaybeUninit::assume_init
761 /// #![feature(new_uninit)]
762 /// #![feature(get_mut_unchecked)]
764 /// use std::sync::Arc;
766 /// let mut values = Arc::<[u32]>::new_uninit_slice(3);
768 /// let values = unsafe {
769 /// // Deferred initialization:
770 /// Arc::get_mut_unchecked(&mut values)[0].as_mut_ptr().write(1);
771 /// Arc::get_mut_unchecked(&mut values)[1].as_mut_ptr().write(2);
772 /// Arc::get_mut_unchecked(&mut values)[2].as_mut_ptr().write(3);
774 /// values.assume_init()
777 /// assert_eq!(*values, [1, 2, 3])
779 #[unstable(feature = "new_uninit", issue = "63291")]
780 #[must_use = "`self` will be dropped if the result is not used"]
782 pub unsafe fn assume_init(self) -> Arc<[T]> {
783 unsafe { Arc::from_ptr(mem::ManuallyDrop::new(self).ptr.as_ptr() as _) }
787 impl<T: ?Sized> Arc<T> {
788 /// Consumes the `Arc`, returning the wrapped pointer.
790 /// To avoid a memory leak the pointer must be converted back to an `Arc` using
791 /// [`Arc::from_raw`].
796 /// use std::sync::Arc;
798 /// let x = Arc::new("hello".to_owned());
799 /// let x_ptr = Arc::into_raw(x);
800 /// assert_eq!(unsafe { &*x_ptr }, "hello");
802 #[stable(feature = "rc_raw", since = "1.17.0")]
803 pub fn into_raw(this: Self) -> *const T {
804 let ptr = Self::as_ptr(&this);
809 /// Provides a raw pointer to the data.
811 /// The counts are not affected in any way and the `Arc` is not consumed. The pointer is valid for
812 /// as long as there are strong counts in the `Arc`.
817 /// use std::sync::Arc;
819 /// let x = Arc::new("hello".to_owned());
820 /// let y = Arc::clone(&x);
821 /// let x_ptr = Arc::as_ptr(&x);
822 /// assert_eq!(x_ptr, Arc::as_ptr(&y));
823 /// assert_eq!(unsafe { &*x_ptr }, "hello");
825 #[stable(feature = "rc_as_ptr", since = "1.45.0")]
826 pub fn as_ptr(this: &Self) -> *const T {
827 let ptr: *mut ArcInner<T> = NonNull::as_ptr(this.ptr);
829 // SAFETY: This cannot go through Deref::deref or RcBoxPtr::inner because
830 // this is required to retain raw/mut provenance such that e.g. `get_mut` can
831 // write through the pointer after the Rc is recovered through `from_raw`.
832 unsafe { ptr::addr_of_mut!((*ptr).data) }
835 /// Constructs an `Arc<T>` from a raw pointer.
837 /// The raw pointer must have been previously returned by a call to
838 /// [`Arc<U>::into_raw`][into_raw] where `U` must have the same size and
839 /// alignment as `T`. This is trivially true if `U` is `T`.
840 /// Note that if `U` is not `T` but has the same size and alignment, this is
841 /// basically like transmuting references of different types. See
842 /// [`mem::transmute`][transmute] for more information on what
843 /// restrictions apply in this case.
845 /// The user of `from_raw` has to make sure a specific value of `T` is only
848 /// This function is unsafe because improper use may lead to memory unsafety,
849 /// even if the returned `Arc<T>` is never accessed.
851 /// [into_raw]: Arc::into_raw
852 /// [transmute]: core::mem::transmute
857 /// use std::sync::Arc;
859 /// let x = Arc::new("hello".to_owned());
860 /// let x_ptr = Arc::into_raw(x);
863 /// // Convert back to an `Arc` to prevent leak.
864 /// let x = Arc::from_raw(x_ptr);
865 /// assert_eq!(&*x, "hello");
867 /// // Further calls to `Arc::from_raw(x_ptr)` would be memory-unsafe.
870 /// // The memory was freed when `x` went out of scope above, so `x_ptr` is now dangling!
872 #[stable(feature = "rc_raw", since = "1.17.0")]
873 pub unsafe fn from_raw(ptr: *const T) -> Self {
875 let offset = data_offset(ptr);
877 // Reverse the offset to find the original ArcInner.
878 let arc_ptr = (ptr as *mut ArcInner<T>).set_ptr_value((ptr as *mut u8).offset(-offset));
880 Self::from_ptr(arc_ptr)
884 /// Creates a new [`Weak`] pointer to this allocation.
889 /// use std::sync::Arc;
891 /// let five = Arc::new(5);
893 /// let weak_five = Arc::downgrade(&five);
895 #[stable(feature = "arc_weak", since = "1.4.0")]
896 pub fn downgrade(this: &Self) -> Weak<T> {
897 // This Relaxed is OK because we're checking the value in the CAS
899 let mut cur = this.inner().weak.load(Relaxed);
902 // check if the weak counter is currently "locked"; if so, spin.
903 if cur == usize::MAX {
905 cur = this.inner().weak.load(Relaxed);
909 // NOTE: this code currently ignores the possibility of overflow
910 // into usize::MAX; in general both Rc and Arc need to be adjusted
911 // to deal with overflow.
913 // Unlike with Clone(), we need this to be an Acquire read to
914 // synchronize with the write coming from `is_unique`, so that the
915 // events prior to that write happen before this read.
916 match this.inner().weak.compare_exchange_weak(cur, cur + 1, Acquire, Relaxed) {
918 // Make sure we do not create a dangling Weak
919 debug_assert!(!is_dangling(this.ptr.as_ptr()));
920 return Weak { ptr: this.ptr };
922 Err(old) => cur = old,
927 /// Gets the number of [`Weak`] pointers to this allocation.
931 /// This method by itself is safe, but using it correctly requires extra care.
932 /// Another thread can change the weak count at any time,
933 /// including potentially between calling this method and acting on the result.
938 /// use std::sync::Arc;
940 /// let five = Arc::new(5);
941 /// let _weak_five = Arc::downgrade(&five);
943 /// // This assertion is deterministic because we haven't shared
944 /// // the `Arc` or `Weak` between threads.
945 /// assert_eq!(1, Arc::weak_count(&five));
948 #[stable(feature = "arc_counts", since = "1.15.0")]
949 pub fn weak_count(this: &Self) -> usize {
950 let cnt = this.inner().weak.load(SeqCst);
951 // If the weak count is currently locked, the value of the
952 // count was 0 just before taking the lock.
953 if cnt == usize::MAX { 0 } else { cnt - 1 }
956 /// Gets the number of strong (`Arc`) pointers to this allocation.
960 /// This method by itself is safe, but using it correctly requires extra care.
961 /// Another thread can change the strong count at any time,
962 /// including potentially between calling this method and acting on the result.
967 /// use std::sync::Arc;
969 /// let five = Arc::new(5);
970 /// let _also_five = Arc::clone(&five);
972 /// // This assertion is deterministic because we haven't shared
973 /// // the `Arc` between threads.
974 /// assert_eq!(2, Arc::strong_count(&five));
977 #[stable(feature = "arc_counts", since = "1.15.0")]
978 pub fn strong_count(this: &Self) -> usize {
979 this.inner().strong.load(SeqCst)
982 /// Increments the strong reference count on the `Arc<T>` associated with the
983 /// provided pointer by one.
987 /// The pointer must have been obtained through `Arc::into_raw`, and the
988 /// associated `Arc` instance must be valid (i.e. the strong count must be at
989 /// least 1) for the duration of this method.
994 /// use std::sync::Arc;
996 /// let five = Arc::new(5);
999 /// let ptr = Arc::into_raw(five);
1000 /// Arc::increment_strong_count(ptr);
1002 /// // This assertion is deterministic because we haven't shared
1003 /// // the `Arc` between threads.
1004 /// let five = Arc::from_raw(ptr);
1005 /// assert_eq!(2, Arc::strong_count(&five));
1009 #[stable(feature = "arc_mutate_strong_count", since = "1.51.0")]
1010 pub unsafe fn increment_strong_count(ptr: *const T) {
1011 // Retain Arc, but don't touch refcount by wrapping in ManuallyDrop
1012 let arc = unsafe { mem::ManuallyDrop::new(Arc::<T>::from_raw(ptr)) };
1013 // Now increase refcount, but don't drop new refcount either
1014 let _arc_clone: mem::ManuallyDrop<_> = arc.clone();
1017 /// Decrements the strong reference count on the `Arc<T>` associated with the
1018 /// provided pointer by one.
1022 /// The pointer must have been obtained through `Arc::into_raw`, and the
1023 /// associated `Arc` instance must be valid (i.e. the strong count must be at
1024 /// least 1) when invoking this method. This method can be used to release the final
1025 /// `Arc` and backing storage, but **should not** be called after the final `Arc` has been
1031 /// use std::sync::Arc;
1033 /// let five = Arc::new(5);
1036 /// let ptr = Arc::into_raw(five);
1037 /// Arc::increment_strong_count(ptr);
1039 /// // Those assertions are deterministic because we haven't shared
1040 /// // the `Arc` between threads.
1041 /// let five = Arc::from_raw(ptr);
1042 /// assert_eq!(2, Arc::strong_count(&five));
1043 /// Arc::decrement_strong_count(ptr);
1044 /// assert_eq!(1, Arc::strong_count(&five));
1048 #[stable(feature = "arc_mutate_strong_count", since = "1.51.0")]
1049 pub unsafe fn decrement_strong_count(ptr: *const T) {
1050 unsafe { mem::drop(Arc::from_raw(ptr)) };
1054 fn inner(&self) -> &ArcInner<T> {
1055 // This unsafety is ok because while this arc is alive we're guaranteed
1056 // that the inner pointer is valid. Furthermore, we know that the
1057 // `ArcInner` structure itself is `Sync` because the inner data is
1058 // `Sync` as well, so we're ok loaning out an immutable pointer to these
1060 unsafe { self.ptr.as_ref() }
1063 // Non-inlined part of `drop`.
1065 unsafe fn drop_slow(&mut self) {
1066 // Destroy the data at this time, even though we must not free the box
1067 // allocation itself (there might still be weak pointers lying around).
1068 unsafe { ptr::drop_in_place(Self::get_mut_unchecked(self)) };
1070 // Drop the weak ref collectively held by all strong references
1071 drop(Weak { ptr: self.ptr });
1075 #[stable(feature = "ptr_eq", since = "1.17.0")]
1076 /// Returns `true` if the two `Arc`s point to the same allocation
1077 /// (in a vein similar to [`ptr::eq`]).
1082 /// use std::sync::Arc;
1084 /// let five = Arc::new(5);
1085 /// let same_five = Arc::clone(&five);
1086 /// let other_five = Arc::new(5);
1088 /// assert!(Arc::ptr_eq(&five, &same_five));
1089 /// assert!(!Arc::ptr_eq(&five, &other_five));
1092 /// [`ptr::eq`]: core::ptr::eq "ptr::eq"
1093 pub fn ptr_eq(this: &Self, other: &Self) -> bool {
1094 this.ptr.as_ptr() == other.ptr.as_ptr()
1098 impl<T: ?Sized> Arc<T> {
1099 /// Allocates an `ArcInner<T>` with sufficient space for
1100 /// a possibly-unsized inner value where the value has the layout provided.
1102 /// The function `mem_to_arcinner` is called with the data pointer
1103 /// and must return back a (potentially fat)-pointer for the `ArcInner<T>`.
1104 #[cfg(not(no_global_oom_handling))]
1105 unsafe fn allocate_for_layout(
1106 value_layout: Layout,
1107 allocate: impl FnOnce(Layout) -> Result<NonNull<[u8]>, AllocError>,
1108 mem_to_arcinner: impl FnOnce(*mut u8) -> *mut ArcInner<T>,
1109 ) -> *mut ArcInner<T> {
1110 // Calculate layout using the given value layout.
1111 // Previously, layout was calculated on the expression
1112 // `&*(ptr as *const ArcInner<T>)`, but this created a misaligned
1113 // reference (see #54908).
1114 let layout = Layout::new::<ArcInner<()>>().extend(value_layout).unwrap().0.pad_to_align();
1116 Arc::try_allocate_for_layout(value_layout, allocate, mem_to_arcinner)
1117 .unwrap_or_else(|_| handle_alloc_error(layout))
1121 /// Allocates an `ArcInner<T>` with sufficient space for
1122 /// a possibly-unsized inner value where the value has the layout provided,
1123 /// returning an error if allocation fails.
1125 /// The function `mem_to_arcinner` is called with the data pointer
1126 /// and must return back a (potentially fat)-pointer for the `ArcInner<T>`.
1127 unsafe fn try_allocate_for_layout(
1128 value_layout: Layout,
1129 allocate: impl FnOnce(Layout) -> Result<NonNull<[u8]>, AllocError>,
1130 mem_to_arcinner: impl FnOnce(*mut u8) -> *mut ArcInner<T>,
1131 ) -> Result<*mut ArcInner<T>, AllocError> {
1132 // Calculate layout using the given value layout.
1133 // Previously, layout was calculated on the expression
1134 // `&*(ptr as *const ArcInner<T>)`, but this created a misaligned
1135 // reference (see #54908).
1136 let layout = Layout::new::<ArcInner<()>>().extend(value_layout).unwrap().0.pad_to_align();
1138 let ptr = allocate(layout)?;
1140 // Initialize the ArcInner
1141 let inner = mem_to_arcinner(ptr.as_non_null_ptr().as_ptr());
1142 debug_assert_eq!(unsafe { Layout::for_value(&*inner) }, layout);
1145 ptr::write(&mut (*inner).strong, atomic::AtomicUsize::new(1));
1146 ptr::write(&mut (*inner).weak, atomic::AtomicUsize::new(1));
1152 /// Allocates an `ArcInner<T>` with sufficient space for an unsized inner value.
1153 #[cfg(not(no_global_oom_handling))]
1154 unsafe fn allocate_for_ptr(ptr: *const T) -> *mut ArcInner<T> {
1155 // Allocate for the `ArcInner<T>` using the given value.
1157 Self::allocate_for_layout(
1158 Layout::for_value(&*ptr),
1159 |layout| Global.allocate(layout),
1160 |mem| (ptr as *mut ArcInner<T>).set_ptr_value(mem) as *mut ArcInner<T>,
1165 #[cfg(not(no_global_oom_handling))]
1166 fn from_box(v: Box<T>) -> Arc<T> {
1168 let (box_unique, alloc) = Box::into_unique(v);
1169 let bptr = box_unique.as_ptr();
1171 let value_size = size_of_val(&*bptr);
1172 let ptr = Self::allocate_for_ptr(bptr);
1174 // Copy value as bytes
1175 ptr::copy_nonoverlapping(
1176 bptr as *const T as *const u8,
1177 &mut (*ptr).data as *mut _ as *mut u8,
1181 // Free the allocation without dropping its contents
1182 box_free(box_unique, alloc);
1190 /// Allocates an `ArcInner<[T]>` with the given length.
1191 #[cfg(not(no_global_oom_handling))]
1192 unsafe fn allocate_for_slice(len: usize) -> *mut ArcInner<[T]> {
1194 Self::allocate_for_layout(
1195 Layout::array::<T>(len).unwrap(),
1196 |layout| Global.allocate(layout),
1197 |mem| ptr::slice_from_raw_parts_mut(mem as *mut T, len) as *mut ArcInner<[T]>,
1202 /// Copy elements from slice into newly allocated Arc<\[T\]>
1204 /// Unsafe because the caller must either take ownership or bind `T: Copy`.
1205 #[cfg(not(no_global_oom_handling))]
1206 unsafe fn copy_from_slice(v: &[T]) -> Arc<[T]> {
1208 let ptr = Self::allocate_for_slice(v.len());
1210 ptr::copy_nonoverlapping(v.as_ptr(), &mut (*ptr).data as *mut [T] as *mut T, v.len());
1216 /// Constructs an `Arc<[T]>` from an iterator known to be of a certain size.
1218 /// Behavior is undefined should the size be wrong.
1219 #[cfg(not(no_global_oom_handling))]
1220 unsafe fn from_iter_exact(iter: impl iter::Iterator<Item = T>, len: usize) -> Arc<[T]> {
1221 // Panic guard while cloning T elements.
1222 // In the event of a panic, elements that have been written
1223 // into the new ArcInner will be dropped, then the memory freed.
1231 impl<T> Drop for Guard<T> {
1232 fn drop(&mut self) {
1234 let slice = from_raw_parts_mut(self.elems, self.n_elems);
1235 ptr::drop_in_place(slice);
1237 Global.deallocate(self.mem, self.layout);
1243 let ptr = Self::allocate_for_slice(len);
1245 let mem = ptr as *mut _ as *mut u8;
1246 let layout = Layout::for_value(&*ptr);
1248 // Pointer to first element
1249 let elems = &mut (*ptr).data as *mut [T] as *mut T;
1251 let mut guard = Guard { mem: NonNull::new_unchecked(mem), elems, layout, n_elems: 0 };
1253 for (i, item) in iter.enumerate() {
1254 ptr::write(elems.add(i), item);
1258 // All clear. Forget the guard so it doesn't free the new ArcInner.
1266 /// Specialization trait used for `From<&[T]>`.
1267 #[cfg(not(no_global_oom_handling))]
1268 trait ArcFromSlice<T> {
1269 fn from_slice(slice: &[T]) -> Self;
1272 #[cfg(not(no_global_oom_handling))]
1273 impl<T: Clone> ArcFromSlice<T> for Arc<[T]> {
1275 default fn from_slice(v: &[T]) -> Self {
1276 unsafe { Self::from_iter_exact(v.iter().cloned(), v.len()) }
1280 #[cfg(not(no_global_oom_handling))]
1281 impl<T: Copy> ArcFromSlice<T> for Arc<[T]> {
1283 fn from_slice(v: &[T]) -> Self {
1284 unsafe { Arc::copy_from_slice(v) }
1288 #[stable(feature = "rust1", since = "1.0.0")]
1289 impl<T: ?Sized> Clone for Arc<T> {
1290 /// Makes a clone of the `Arc` pointer.
1292 /// This creates another pointer to the same allocation, increasing the
1293 /// strong reference count.
1298 /// use std::sync::Arc;
1300 /// let five = Arc::new(5);
1302 /// let _ = Arc::clone(&five);
1305 fn clone(&self) -> Arc<T> {
1306 // Using a relaxed ordering is alright here, as knowledge of the
1307 // original reference prevents other threads from erroneously deleting
1310 // As explained in the [Boost documentation][1], Increasing the
1311 // reference counter can always be done with memory_order_relaxed: New
1312 // references to an object can only be formed from an existing
1313 // reference, and passing an existing reference from one thread to
1314 // another must already provide any required synchronization.
1316 // [1]: (www.boost.org/doc/libs/1_55_0/doc/html/atomic/usage_examples.html)
1317 let old_size = self.inner().strong.fetch_add(1, Relaxed);
1319 // However we need to guard against massive refcounts in case someone
1320 // is `mem::forget`ing Arcs. If we don't do this the count can overflow
1321 // and users will use-after free. We racily saturate to `isize::MAX` on
1322 // the assumption that there aren't ~2 billion threads incrementing
1323 // the reference count at once. This branch will never be taken in
1324 // any realistic program.
1326 // We abort because such a program is incredibly degenerate, and we
1327 // don't care to support it.
1328 if old_size > MAX_REFCOUNT {
1332 Self::from_inner(self.ptr)
1336 #[stable(feature = "rust1", since = "1.0.0")]
1337 impl<T: ?Sized> Deref for Arc<T> {
1341 fn deref(&self) -> &T {
1346 #[unstable(feature = "receiver_trait", issue = "none")]
1347 impl<T: ?Sized> Receiver for Arc<T> {}
1349 impl<T: Clone> Arc<T> {
1350 /// Makes a mutable reference into the given `Arc`.
1352 /// If there are other `Arc` pointers to the same allocation, then `make_mut` will
1353 /// [`clone`] the inner value to a new allocation to ensure unique ownership. This is also
1354 /// referred to as clone-on-write.
1356 /// However, if there are no other `Arc` pointers to this allocation, but some [`Weak`]
1357 /// pointers, then the [`Weak`] pointers will be disassociated and the inner value will not
1360 /// See also [`get_mut`], which will fail rather than cloning the inner value
1361 /// or diassociating [`Weak`] pointers.
1363 /// [`clone`]: Clone::clone
1364 /// [`get_mut`]: Arc::get_mut
1369 /// use std::sync::Arc;
1371 /// let mut data = Arc::new(5);
1373 /// *Arc::make_mut(&mut data) += 1; // Won't clone anything
1374 /// let mut other_data = Arc::clone(&data); // Won't clone inner data
1375 /// *Arc::make_mut(&mut data) += 1; // Clones inner data
1376 /// *Arc::make_mut(&mut data) += 1; // Won't clone anything
1377 /// *Arc::make_mut(&mut other_data) *= 2; // Won't clone anything
1379 /// // Now `data` and `other_data` point to different allocations.
1380 /// assert_eq!(*data, 8);
1381 /// assert_eq!(*other_data, 12);
1384 /// [`Weak`] pointers will be disassociated:
1387 /// use std::sync::Arc;
1389 /// let mut data = Arc::new(75);
1390 /// let weak = Arc::downgrade(&data);
1392 /// assert!(75 == *data);
1393 /// assert!(75 == *weak.upgrade().unwrap());
1395 /// *Arc::make_mut(&mut data) += 1;
1397 /// assert!(76 == *data);
1398 /// assert!(weak.upgrade().is_none());
1400 #[cfg(not(no_global_oom_handling))]
1402 #[stable(feature = "arc_unique", since = "1.4.0")]
1403 pub fn make_mut(this: &mut Self) -> &mut T {
1404 // Note that we hold both a strong reference and a weak reference.
1405 // Thus, releasing our strong reference only will not, by itself, cause
1406 // the memory to be deallocated.
1408 // Use Acquire to ensure that we see any writes to `weak` that happen
1409 // before release writes (i.e., decrements) to `strong`. Since we hold a
1410 // weak count, there's no chance the ArcInner itself could be
1412 if this.inner().strong.compare_exchange(1, 0, Acquire, Relaxed).is_err() {
1413 // Another strong pointer exists, so we must clone.
1414 // Pre-allocate memory to allow writing the cloned value directly.
1415 let mut arc = Self::new_uninit();
1417 let data = Arc::get_mut_unchecked(&mut arc);
1418 (**this).write_clone_into_raw(data.as_mut_ptr());
1419 *this = arc.assume_init();
1421 } else if this.inner().weak.load(Relaxed) != 1 {
1422 // Relaxed suffices in the above because this is fundamentally an
1423 // optimization: we are always racing with weak pointers being
1424 // dropped. Worst case, we end up allocated a new Arc unnecessarily.
1426 // We removed the last strong ref, but there are additional weak
1427 // refs remaining. We'll move the contents to a new Arc, and
1428 // invalidate the other weak refs.
1430 // Note that it is not possible for the read of `weak` to yield
1431 // usize::MAX (i.e., locked), since the weak count can only be
1432 // locked by a thread with a strong reference.
1434 // Materialize our own implicit weak pointer, so that it can clean
1435 // up the ArcInner as needed.
1436 let _weak = Weak { ptr: this.ptr };
1438 // Can just steal the data, all that's left is Weaks
1439 let mut arc = Self::new_uninit();
1441 let data = Arc::get_mut_unchecked(&mut arc);
1442 data.as_mut_ptr().copy_from_nonoverlapping(&**this, 1);
1443 ptr::write(this, arc.assume_init());
1446 // We were the sole reference of either kind; bump back up the
1447 // strong ref count.
1448 this.inner().strong.store(1, Release);
1451 // As with `get_mut()`, the unsafety is ok because our reference was
1452 // either unique to begin with, or became one upon cloning the contents.
1453 unsafe { Self::get_mut_unchecked(this) }
1457 impl<T: ?Sized> Arc<T> {
1458 /// Returns a mutable reference into the given `Arc`, if there are
1459 /// no other `Arc` or [`Weak`] pointers to the same allocation.
1461 /// Returns [`None`] otherwise, because it is not safe to
1462 /// mutate a shared value.
1464 /// See also [`make_mut`][make_mut], which will [`clone`][clone]
1465 /// the inner value when there are other `Arc` pointers.
1467 /// [make_mut]: Arc::make_mut
1468 /// [clone]: Clone::clone
1473 /// use std::sync::Arc;
1475 /// let mut x = Arc::new(3);
1476 /// *Arc::get_mut(&mut x).unwrap() = 4;
1477 /// assert_eq!(*x, 4);
1479 /// let _y = Arc::clone(&x);
1480 /// assert!(Arc::get_mut(&mut x).is_none());
1483 #[stable(feature = "arc_unique", since = "1.4.0")]
1484 pub fn get_mut(this: &mut Self) -> Option<&mut T> {
1485 if this.is_unique() {
1486 // This unsafety is ok because we're guaranteed that the pointer
1487 // returned is the *only* pointer that will ever be returned to T. Our
1488 // reference count is guaranteed to be 1 at this point, and we required
1489 // the Arc itself to be `mut`, so we're returning the only possible
1490 // reference to the inner data.
1491 unsafe { Some(Arc::get_mut_unchecked(this)) }
1497 /// Returns a mutable reference into the given `Arc`,
1498 /// without any check.
1500 /// See also [`get_mut`], which is safe and does appropriate checks.
1502 /// [`get_mut`]: Arc::get_mut
1506 /// Any other `Arc` or [`Weak`] pointers to the same allocation must not be dereferenced
1507 /// for the duration of the returned borrow.
1508 /// This is trivially the case if no such pointers exist,
1509 /// for example immediately after `Arc::new`.
1514 /// #![feature(get_mut_unchecked)]
1516 /// use std::sync::Arc;
1518 /// let mut x = Arc::new(String::new());
1520 /// Arc::get_mut_unchecked(&mut x).push_str("foo")
1522 /// assert_eq!(*x, "foo");
1525 #[unstable(feature = "get_mut_unchecked", issue = "63292")]
1526 pub unsafe fn get_mut_unchecked(this: &mut Self) -> &mut T {
1527 // We are careful to *not* create a reference covering the "count" fields, as
1528 // this would alias with concurrent access to the reference counts (e.g. by `Weak`).
1529 unsafe { &mut (*this.ptr.as_ptr()).data }
1532 /// Determine whether this is the unique reference (including weak refs) to
1533 /// the underlying data.
1535 /// Note that this requires locking the weak ref count.
1536 fn is_unique(&mut self) -> bool {
1537 // lock the weak pointer count if we appear to be the sole weak pointer
1540 // The acquire label here ensures a happens-before relationship with any
1541 // writes to `strong` (in particular in `Weak::upgrade`) prior to decrements
1542 // of the `weak` count (via `Weak::drop`, which uses release). If the upgraded
1543 // weak ref was never dropped, the CAS here will fail so we do not care to synchronize.
1544 if self.inner().weak.compare_exchange(1, usize::MAX, Acquire, Relaxed).is_ok() {
1545 // This needs to be an `Acquire` to synchronize with the decrement of the `strong`
1546 // counter in `drop` -- the only access that happens when any but the last reference
1547 // is being dropped.
1548 let unique = self.inner().strong.load(Acquire) == 1;
1550 // The release write here synchronizes with a read in `downgrade`,
1551 // effectively preventing the above read of `strong` from happening
1553 self.inner().weak.store(1, Release); // release the lock
1561 #[stable(feature = "rust1", since = "1.0.0")]
1562 unsafe impl<#[may_dangle] T: ?Sized> Drop for Arc<T> {
1563 /// Drops the `Arc`.
1565 /// This will decrement the strong reference count. If the strong reference
1566 /// count reaches zero then the only other references (if any) are
1567 /// [`Weak`], so we `drop` the inner value.
1572 /// use std::sync::Arc;
1576 /// impl Drop for Foo {
1577 /// fn drop(&mut self) {
1578 /// println!("dropped!");
1582 /// let foo = Arc::new(Foo);
1583 /// let foo2 = Arc::clone(&foo);
1585 /// drop(foo); // Doesn't print anything
1586 /// drop(foo2); // Prints "dropped!"
1589 fn drop(&mut self) {
1590 // Because `fetch_sub` is already atomic, we do not need to synchronize
1591 // with other threads unless we are going to delete the object. This
1592 // same logic applies to the below `fetch_sub` to the `weak` count.
1593 if self.inner().strong.fetch_sub(1, Release) != 1 {
1597 // This fence is needed to prevent reordering of use of the data and
1598 // deletion of the data. Because it is marked `Release`, the decreasing
1599 // of the reference count synchronizes with this `Acquire` fence. This
1600 // means that use of the data happens before decreasing the reference
1601 // count, which happens before this fence, which happens before the
1602 // deletion of the data.
1604 // As explained in the [Boost documentation][1],
1606 // > It is important to enforce any possible access to the object in one
1607 // > thread (through an existing reference) to *happen before* deleting
1608 // > the object in a different thread. This is achieved by a "release"
1609 // > operation after dropping a reference (any access to the object
1610 // > through this reference must obviously happened before), and an
1611 // > "acquire" operation before deleting the object.
1613 // In particular, while the contents of an Arc are usually immutable, it's
1614 // possible to have interior writes to something like a Mutex<T>. Since a
1615 // Mutex is not acquired when it is deleted, we can't rely on its
1616 // synchronization logic to make writes in thread A visible to a destructor
1617 // running in thread B.
1619 // Also note that the Acquire fence here could probably be replaced with an
1620 // Acquire load, which could improve performance in highly-contended
1621 // situations. See [2].
1623 // [1]: (www.boost.org/doc/libs/1_55_0/doc/html/atomic/usage_examples.html)
1624 // [2]: (https://github.com/rust-lang/rust/pull/41714)
1625 acquire!(self.inner().strong);
1633 impl Arc<dyn Any + Send + Sync> {
1635 #[stable(feature = "rc_downcast", since = "1.29.0")]
1636 /// Attempt to downcast the `Arc<dyn Any + Send + Sync>` to a concrete type.
1641 /// use std::any::Any;
1642 /// use std::sync::Arc;
1644 /// fn print_if_string(value: Arc<dyn Any + Send + Sync>) {
1645 /// if let Ok(string) = value.downcast::<String>() {
1646 /// println!("String ({}): {}", string.len(), string);
1650 /// let my_string = "Hello World".to_string();
1651 /// print_if_string(Arc::new(my_string));
1652 /// print_if_string(Arc::new(0i8));
1654 pub fn downcast<T>(self) -> Result<Arc<T>, Self>
1656 T: Any + Send + Sync + 'static,
1658 if (*self).is::<T>() {
1659 let ptr = self.ptr.cast::<ArcInner<T>>();
1661 Ok(Arc::from_inner(ptr))
1669 /// Constructs a new `Weak<T>`, without allocating any memory.
1670 /// Calling [`upgrade`] on the return value always gives [`None`].
1672 /// [`upgrade`]: Weak::upgrade
1677 /// use std::sync::Weak;
1679 /// let empty: Weak<i64> = Weak::new();
1680 /// assert!(empty.upgrade().is_none());
1682 #[stable(feature = "downgraded_weak", since = "1.10.0")]
1683 pub fn new() -> Weak<T> {
1684 Weak { ptr: NonNull::new(usize::MAX as *mut ArcInner<T>).expect("MAX is not 0") }
1688 /// Helper type to allow accessing the reference counts without
1689 /// making any assertions about the data field.
1690 struct WeakInner<'a> {
1691 weak: &'a atomic::AtomicUsize,
1692 strong: &'a atomic::AtomicUsize,
1695 impl<T: ?Sized> Weak<T> {
1696 /// Returns a raw pointer to the object `T` pointed to by this `Weak<T>`.
1698 /// The pointer is valid only if there are some strong references. The pointer may be dangling,
1699 /// unaligned or even [`null`] otherwise.
1704 /// use std::sync::Arc;
1707 /// let strong = Arc::new("hello".to_owned());
1708 /// let weak = Arc::downgrade(&strong);
1709 /// // Both point to the same object
1710 /// assert!(ptr::eq(&*strong, weak.as_ptr()));
1711 /// // The strong here keeps it alive, so we can still access the object.
1712 /// assert_eq!("hello", unsafe { &*weak.as_ptr() });
1715 /// // But not any more. We can do weak.as_ptr(), but accessing the pointer would lead to
1716 /// // undefined behaviour.
1717 /// // assert_eq!("hello", unsafe { &*weak.as_ptr() });
1720 /// [`null`]: core::ptr::null "ptr::null"
1721 #[stable(feature = "weak_into_raw", since = "1.45.0")]
1722 pub fn as_ptr(&self) -> *const T {
1723 let ptr: *mut ArcInner<T> = NonNull::as_ptr(self.ptr);
1725 if is_dangling(ptr) {
1726 // If the pointer is dangling, we return the sentinel directly. This cannot be
1727 // a valid payload address, as the payload is at least as aligned as ArcInner (usize).
1730 // SAFETY: if is_dangling returns false, then the pointer is dereferencable.
1731 // The payload may be dropped at this point, and we have to maintain provenance,
1732 // so use raw pointer manipulation.
1733 unsafe { ptr::addr_of_mut!((*ptr).data) }
1737 /// Consumes the `Weak<T>` and turns it into a raw pointer.
1739 /// This converts the weak pointer into a raw pointer, while still preserving the ownership of
1740 /// one weak reference (the weak count is not modified by this operation). It can be turned
1741 /// back into the `Weak<T>` with [`from_raw`].
1743 /// The same restrictions of accessing the target of the pointer as with
1744 /// [`as_ptr`] apply.
1749 /// use std::sync::{Arc, Weak};
1751 /// let strong = Arc::new("hello".to_owned());
1752 /// let weak = Arc::downgrade(&strong);
1753 /// let raw = weak.into_raw();
1755 /// assert_eq!(1, Arc::weak_count(&strong));
1756 /// assert_eq!("hello", unsafe { &*raw });
1758 /// drop(unsafe { Weak::from_raw(raw) });
1759 /// assert_eq!(0, Arc::weak_count(&strong));
1762 /// [`from_raw`]: Weak::from_raw
1763 /// [`as_ptr`]: Weak::as_ptr
1764 #[must_use = "`self` will be dropped if the result is not used"]
1765 #[stable(feature = "weak_into_raw", since = "1.45.0")]
1766 pub fn into_raw(self) -> *const T {
1767 let result = self.as_ptr();
1772 /// Converts a raw pointer previously created by [`into_raw`] back into `Weak<T>`.
1774 /// This can be used to safely get a strong reference (by calling [`upgrade`]
1775 /// later) or to deallocate the weak count by dropping the `Weak<T>`.
1777 /// It takes ownership of one weak reference (with the exception of pointers created by [`new`],
1778 /// as these don't own anything; the method still works on them).
1782 /// The pointer must have originated from the [`into_raw`] and must still own its potential
1785 /// It is allowed for the strong count to be 0 at the time of calling this. Nevertheless, this
1786 /// takes ownership of one weak reference currently represented as a raw pointer (the weak
1787 /// count is not modified by this operation) and therefore it must be paired with a previous
1788 /// call to [`into_raw`].
1792 /// use std::sync::{Arc, Weak};
1794 /// let strong = Arc::new("hello".to_owned());
1796 /// let raw_1 = Arc::downgrade(&strong).into_raw();
1797 /// let raw_2 = Arc::downgrade(&strong).into_raw();
1799 /// assert_eq!(2, Arc::weak_count(&strong));
1801 /// assert_eq!("hello", &*unsafe { Weak::from_raw(raw_1) }.upgrade().unwrap());
1802 /// assert_eq!(1, Arc::weak_count(&strong));
1806 /// // Decrement the last weak count.
1807 /// assert!(unsafe { Weak::from_raw(raw_2) }.upgrade().is_none());
1810 /// [`new`]: Weak::new
1811 /// [`into_raw`]: Weak::into_raw
1812 /// [`upgrade`]: Weak::upgrade
1813 #[stable(feature = "weak_into_raw", since = "1.45.0")]
1814 pub unsafe fn from_raw(ptr: *const T) -> Self {
1815 // See Weak::as_ptr for context on how the input pointer is derived.
1817 let ptr = if is_dangling(ptr as *mut T) {
1818 // This is a dangling Weak.
1819 ptr as *mut ArcInner<T>
1821 // Otherwise, we're guaranteed the pointer came from a nondangling Weak.
1822 // SAFETY: data_offset is safe to call, as ptr references a real (potentially dropped) T.
1823 let offset = unsafe { data_offset(ptr) };
1824 // Thus, we reverse the offset to get the whole RcBox.
1825 // SAFETY: the pointer originated from a Weak, so this offset is safe.
1826 unsafe { (ptr as *mut ArcInner<T>).set_ptr_value((ptr as *mut u8).offset(-offset)) }
1829 // SAFETY: we now have recovered the original Weak pointer, so can create the Weak.
1830 Weak { ptr: unsafe { NonNull::new_unchecked(ptr) } }
1834 impl<T: ?Sized> Weak<T> {
1835 /// Attempts to upgrade the `Weak` pointer to an [`Arc`], delaying
1836 /// dropping of the inner value if successful.
1838 /// Returns [`None`] if the inner value has since been dropped.
1843 /// use std::sync::Arc;
1845 /// let five = Arc::new(5);
1847 /// let weak_five = Arc::downgrade(&five);
1849 /// let strong_five: Option<Arc<_>> = weak_five.upgrade();
1850 /// assert!(strong_five.is_some());
1852 /// // Destroy all strong pointers.
1853 /// drop(strong_five);
1856 /// assert!(weak_five.upgrade().is_none());
1858 #[stable(feature = "arc_weak", since = "1.4.0")]
1859 pub fn upgrade(&self) -> Option<Arc<T>> {
1860 // We use a CAS loop to increment the strong count instead of a
1861 // fetch_add as this function should never take the reference count
1862 // from zero to one.
1863 let inner = self.inner()?;
1865 // Relaxed load because any write of 0 that we can observe
1866 // leaves the field in a permanently zero state (so a
1867 // "stale" read of 0 is fine), and any other value is
1868 // confirmed via the CAS below.
1869 let mut n = inner.strong.load(Relaxed);
1876 // See comments in `Arc::clone` for why we do this (for `mem::forget`).
1877 if n > MAX_REFCOUNT {
1881 // Relaxed is fine for the failure case because we don't have any expectations about the new state.
1882 // Acquire is necessary for the success case to synchronise with `Arc::new_cyclic`, when the inner
1883 // value can be initialized after `Weak` references have already been created. In that case, we
1884 // expect to observe the fully initialized value.
1885 match inner.strong.compare_exchange_weak(n, n + 1, Acquire, Relaxed) {
1886 Ok(_) => return Some(Arc::from_inner(self.ptr)), // null checked above
1887 Err(old) => n = old,
1892 /// Gets the number of strong (`Arc`) pointers pointing to this allocation.
1894 /// If `self` was created using [`Weak::new`], this will return 0.
1895 #[stable(feature = "weak_counts", since = "1.41.0")]
1896 pub fn strong_count(&self) -> usize {
1897 if let Some(inner) = self.inner() { inner.strong.load(SeqCst) } else { 0 }
1900 /// Gets an approximation of the number of `Weak` pointers pointing to this
1903 /// If `self` was created using [`Weak::new`], or if there are no remaining
1904 /// strong pointers, this will return 0.
1908 /// Due to implementation details, the returned value can be off by 1 in
1909 /// either direction when other threads are manipulating any `Arc`s or
1910 /// `Weak`s pointing to the same allocation.
1911 #[stable(feature = "weak_counts", since = "1.41.0")]
1912 pub fn weak_count(&self) -> usize {
1915 let weak = inner.weak.load(SeqCst);
1916 let strong = inner.strong.load(SeqCst);
1920 // Since we observed that there was at least one strong pointer
1921 // after reading the weak count, we know that the implicit weak
1922 // reference (present whenever any strong references are alive)
1923 // was still around when we observed the weak count, and can
1924 // therefore safely subtract it.
1931 /// Returns `None` when the pointer is dangling and there is no allocated `ArcInner`,
1932 /// (i.e., when this `Weak` was created by `Weak::new`).
1934 fn inner(&self) -> Option<WeakInner<'_>> {
1935 if is_dangling(self.ptr.as_ptr()) {
1938 // We are careful to *not* create a reference covering the "data" field, as
1939 // the field may be mutated concurrently (for example, if the last `Arc`
1940 // is dropped, the data field will be dropped in-place).
1942 let ptr = self.ptr.as_ptr();
1943 WeakInner { strong: &(*ptr).strong, weak: &(*ptr).weak }
1948 /// Returns `true` if the two `Weak`s point to the same allocation (similar to
1949 /// [`ptr::eq`]), or if both don't point to any allocation
1950 /// (because they were created with `Weak::new()`).
1954 /// Since this compares pointers it means that `Weak::new()` will equal each
1955 /// other, even though they don't point to any allocation.
1960 /// use std::sync::Arc;
1962 /// let first_rc = Arc::new(5);
1963 /// let first = Arc::downgrade(&first_rc);
1964 /// let second = Arc::downgrade(&first_rc);
1966 /// assert!(first.ptr_eq(&second));
1968 /// let third_rc = Arc::new(5);
1969 /// let third = Arc::downgrade(&third_rc);
1971 /// assert!(!first.ptr_eq(&third));
1974 /// Comparing `Weak::new`.
1977 /// use std::sync::{Arc, Weak};
1979 /// let first = Weak::new();
1980 /// let second = Weak::new();
1981 /// assert!(first.ptr_eq(&second));
1983 /// let third_rc = Arc::new(());
1984 /// let third = Arc::downgrade(&third_rc);
1985 /// assert!(!first.ptr_eq(&third));
1988 /// [`ptr::eq`]: core::ptr::eq "ptr::eq"
1990 #[stable(feature = "weak_ptr_eq", since = "1.39.0")]
1991 pub fn ptr_eq(&self, other: &Self) -> bool {
1992 self.ptr.as_ptr() == other.ptr.as_ptr()
1996 #[stable(feature = "arc_weak", since = "1.4.0")]
1997 impl<T: ?Sized> Clone for Weak<T> {
1998 /// Makes a clone of the `Weak` pointer that points to the same allocation.
2003 /// use std::sync::{Arc, Weak};
2005 /// let weak_five = Arc::downgrade(&Arc::new(5));
2007 /// let _ = Weak::clone(&weak_five);
2010 fn clone(&self) -> Weak<T> {
2011 let inner = if let Some(inner) = self.inner() {
2014 return Weak { ptr: self.ptr };
2016 // See comments in Arc::clone() for why this is relaxed. This can use a
2017 // fetch_add (ignoring the lock) because the weak count is only locked
2018 // where are *no other* weak pointers in existence. (So we can't be
2019 // running this code in that case).
2020 let old_size = inner.weak.fetch_add(1, Relaxed);
2022 // See comments in Arc::clone() for why we do this (for mem::forget).
2023 if old_size > MAX_REFCOUNT {
2027 Weak { ptr: self.ptr }
2031 #[stable(feature = "downgraded_weak", since = "1.10.0")]
2032 impl<T> Default for Weak<T> {
2033 /// Constructs a new `Weak<T>`, without allocating memory.
2034 /// Calling [`upgrade`] on the return value always
2037 /// [`upgrade`]: Weak::upgrade
2042 /// use std::sync::Weak;
2044 /// let empty: Weak<i64> = Default::default();
2045 /// assert!(empty.upgrade().is_none());
2047 fn default() -> Weak<T> {
2052 #[stable(feature = "arc_weak", since = "1.4.0")]
2053 unsafe impl<#[may_dangle] T: ?Sized> Drop for Weak<T> {
2054 /// Drops the `Weak` pointer.
2059 /// use std::sync::{Arc, Weak};
2063 /// impl Drop for Foo {
2064 /// fn drop(&mut self) {
2065 /// println!("dropped!");
2069 /// let foo = Arc::new(Foo);
2070 /// let weak_foo = Arc::downgrade(&foo);
2071 /// let other_weak_foo = Weak::clone(&weak_foo);
2073 /// drop(weak_foo); // Doesn't print anything
2074 /// drop(foo); // Prints "dropped!"
2076 /// assert!(other_weak_foo.upgrade().is_none());
2078 fn drop(&mut self) {
2079 // If we find out that we were the last weak pointer, then its time to
2080 // deallocate the data entirely. See the discussion in Arc::drop() about
2081 // the memory orderings
2083 // It's not necessary to check for the locked state here, because the
2084 // weak count can only be locked if there was precisely one weak ref,
2085 // meaning that drop could only subsequently run ON that remaining weak
2086 // ref, which can only happen after the lock is released.
2087 let inner = if let Some(inner) = self.inner() { inner } else { return };
2089 if inner.weak.fetch_sub(1, Release) == 1 {
2090 acquire!(inner.weak);
2091 unsafe { Global.deallocate(self.ptr.cast(), Layout::for_value_raw(self.ptr.as_ptr())) }
2096 #[stable(feature = "rust1", since = "1.0.0")]
2097 trait ArcEqIdent<T: ?Sized + PartialEq> {
2098 fn eq(&self, other: &Arc<T>) -> bool;
2099 fn ne(&self, other: &Arc<T>) -> bool;
2102 #[stable(feature = "rust1", since = "1.0.0")]
2103 impl<T: ?Sized + PartialEq> ArcEqIdent<T> for Arc<T> {
2105 default fn eq(&self, other: &Arc<T>) -> bool {
2109 default fn ne(&self, other: &Arc<T>) -> bool {
2114 /// We're doing this specialization here, and not as a more general optimization on `&T`, because it
2115 /// would otherwise add a cost to all equality checks on refs. We assume that `Arc`s are used to
2116 /// store large values, that are slow to clone, but also heavy to check for equality, causing this
2117 /// cost to pay off more easily. It's also more likely to have two `Arc` clones, that point to
2118 /// the same value, than two `&T`s.
2120 /// We can only do this when `T: Eq` as a `PartialEq` might be deliberately irreflexive.
2121 #[stable(feature = "rust1", since = "1.0.0")]
2122 impl<T: ?Sized + crate::rc::MarkerEq> ArcEqIdent<T> for Arc<T> {
2124 fn eq(&self, other: &Arc<T>) -> bool {
2125 Arc::ptr_eq(self, other) || **self == **other
2129 fn ne(&self, other: &Arc<T>) -> bool {
2130 !Arc::ptr_eq(self, other) && **self != **other
2134 #[stable(feature = "rust1", since = "1.0.0")]
2135 impl<T: ?Sized + PartialEq> PartialEq for Arc<T> {
2136 /// Equality for two `Arc`s.
2138 /// Two `Arc`s are equal if their inner values are equal, even if they are
2139 /// stored in different allocation.
2141 /// If `T` also implements `Eq` (implying reflexivity of equality),
2142 /// two `Arc`s that point to the same allocation are always equal.
2147 /// use std::sync::Arc;
2149 /// let five = Arc::new(5);
2151 /// assert!(five == Arc::new(5));
2154 fn eq(&self, other: &Arc<T>) -> bool {
2155 ArcEqIdent::eq(self, other)
2158 /// Inequality for two `Arc`s.
2160 /// Two `Arc`s are unequal if their inner values are unequal.
2162 /// If `T` also implements `Eq` (implying reflexivity of equality),
2163 /// two `Arc`s that point to the same value are never unequal.
2168 /// use std::sync::Arc;
2170 /// let five = Arc::new(5);
2172 /// assert!(five != Arc::new(6));
2175 fn ne(&self, other: &Arc<T>) -> bool {
2176 ArcEqIdent::ne(self, other)
2180 #[stable(feature = "rust1", since = "1.0.0")]
2181 impl<T: ?Sized + PartialOrd> PartialOrd for Arc<T> {
2182 /// Partial comparison for two `Arc`s.
2184 /// The two are compared by calling `partial_cmp()` on their inner values.
2189 /// use std::sync::Arc;
2190 /// use std::cmp::Ordering;
2192 /// let five = Arc::new(5);
2194 /// assert_eq!(Some(Ordering::Less), five.partial_cmp(&Arc::new(6)));
2196 fn partial_cmp(&self, other: &Arc<T>) -> Option<Ordering> {
2197 (**self).partial_cmp(&**other)
2200 /// Less-than comparison for two `Arc`s.
2202 /// The two are compared by calling `<` on their inner values.
2207 /// use std::sync::Arc;
2209 /// let five = Arc::new(5);
2211 /// assert!(five < Arc::new(6));
2213 fn lt(&self, other: &Arc<T>) -> bool {
2214 *(*self) < *(*other)
2217 /// 'Less than or equal to' comparison for two `Arc`s.
2219 /// The two are compared by calling `<=` on their inner values.
2224 /// use std::sync::Arc;
2226 /// let five = Arc::new(5);
2228 /// assert!(five <= Arc::new(5));
2230 fn le(&self, other: &Arc<T>) -> bool {
2231 *(*self) <= *(*other)
2234 /// Greater-than comparison for two `Arc`s.
2236 /// The two are compared by calling `>` on their inner values.
2241 /// use std::sync::Arc;
2243 /// let five = Arc::new(5);
2245 /// assert!(five > Arc::new(4));
2247 fn gt(&self, other: &Arc<T>) -> bool {
2248 *(*self) > *(*other)
2251 /// 'Greater than or equal to' comparison for two `Arc`s.
2253 /// The two are compared by calling `>=` on their inner values.
2258 /// use std::sync::Arc;
2260 /// let five = Arc::new(5);
2262 /// assert!(five >= Arc::new(5));
2264 fn ge(&self, other: &Arc<T>) -> bool {
2265 *(*self) >= *(*other)
2268 #[stable(feature = "rust1", since = "1.0.0")]
2269 impl<T: ?Sized + Ord> Ord for Arc<T> {
2270 /// Comparison for two `Arc`s.
2272 /// The two are compared by calling `cmp()` on their inner values.
2277 /// use std::sync::Arc;
2278 /// use std::cmp::Ordering;
2280 /// let five = Arc::new(5);
2282 /// assert_eq!(Ordering::Less, five.cmp(&Arc::new(6)));
2284 fn cmp(&self, other: &Arc<T>) -> Ordering {
2285 (**self).cmp(&**other)
2288 #[stable(feature = "rust1", since = "1.0.0")]
2289 impl<T: ?Sized + Eq> Eq for Arc<T> {}
2291 #[stable(feature = "rust1", since = "1.0.0")]
2292 impl<T: ?Sized + fmt::Display> fmt::Display for Arc<T> {
2293 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
2294 fmt::Display::fmt(&**self, f)
2298 #[stable(feature = "rust1", since = "1.0.0")]
2299 impl<T: ?Sized + fmt::Debug> fmt::Debug for Arc<T> {
2300 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
2301 fmt::Debug::fmt(&**self, f)
2305 #[stable(feature = "rust1", since = "1.0.0")]
2306 impl<T: ?Sized> fmt::Pointer for Arc<T> {
2307 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
2308 fmt::Pointer::fmt(&(&**self as *const T), f)
2312 #[cfg(not(no_global_oom_handling))]
2313 #[stable(feature = "rust1", since = "1.0.0")]
2314 impl<T: Default> Default for Arc<T> {
2315 /// Creates a new `Arc<T>`, with the `Default` value for `T`.
2320 /// use std::sync::Arc;
2322 /// let x: Arc<i32> = Default::default();
2323 /// assert_eq!(*x, 0);
2325 fn default() -> Arc<T> {
2326 Arc::new(Default::default())
2330 #[stable(feature = "rust1", since = "1.0.0")]
2331 impl<T: ?Sized + Hash> Hash for Arc<T> {
2332 fn hash<H: Hasher>(&self, state: &mut H) {
2333 (**self).hash(state)
2337 #[cfg(not(no_global_oom_handling))]
2338 #[stable(feature = "from_for_ptrs", since = "1.6.0")]
2339 impl<T> From<T> for Arc<T> {
2340 /// Converts a `T` into an `Arc<T>`
2342 /// The conversion moves the value into a
2343 /// newly allocated `Arc`. It is equivalent to
2344 /// calling `Arc::new(t)`.
2348 /// # use std::sync::Arc;
2350 /// let arc = Arc::new(5);
2352 /// assert_eq!(Arc::from(x), arc);
2354 fn from(t: T) -> Self {
2359 #[cfg(not(no_global_oom_handling))]
2360 #[stable(feature = "shared_from_slice", since = "1.21.0")]
2361 impl<T: Clone> From<&[T]> for Arc<[T]> {
2362 /// Allocate a reference-counted slice and fill it by cloning `v`'s items.
2367 /// # use std::sync::Arc;
2368 /// let original: &[i32] = &[1, 2, 3];
2369 /// let shared: Arc<[i32]> = Arc::from(original);
2370 /// assert_eq!(&[1, 2, 3], &shared[..]);
2373 fn from(v: &[T]) -> Arc<[T]> {
2374 <Self as ArcFromSlice<T>>::from_slice(v)
2378 #[cfg(not(no_global_oom_handling))]
2379 #[stable(feature = "shared_from_slice", since = "1.21.0")]
2380 impl From<&str> for Arc<str> {
2381 /// Allocate a reference-counted `str` and copy `v` into it.
2386 /// # use std::sync::Arc;
2387 /// let shared: Arc<str> = Arc::from("eggplant");
2388 /// assert_eq!("eggplant", &shared[..]);
2391 fn from(v: &str) -> Arc<str> {
2392 let arc = Arc::<[u8]>::from(v.as_bytes());
2393 unsafe { Arc::from_raw(Arc::into_raw(arc) as *const str) }
2397 #[cfg(not(no_global_oom_handling))]
2398 #[stable(feature = "shared_from_slice", since = "1.21.0")]
2399 impl From<String> for Arc<str> {
2400 /// Allocate a reference-counted `str` and copy `v` into it.
2405 /// # use std::sync::Arc;
2406 /// let unique: String = "eggplant".to_owned();
2407 /// let shared: Arc<str> = Arc::from(unique);
2408 /// assert_eq!("eggplant", &shared[..]);
2411 fn from(v: String) -> Arc<str> {
2416 #[cfg(not(no_global_oom_handling))]
2417 #[stable(feature = "shared_from_slice", since = "1.21.0")]
2418 impl<T: ?Sized> From<Box<T>> for Arc<T> {
2419 /// Move a boxed object to a new, reference-counted allocation.
2424 /// # use std::sync::Arc;
2425 /// let unique: Box<str> = Box::from("eggplant");
2426 /// let shared: Arc<str> = Arc::from(unique);
2427 /// assert_eq!("eggplant", &shared[..]);
2430 fn from(v: Box<T>) -> Arc<T> {
2435 #[cfg(not(no_global_oom_handling))]
2436 #[stable(feature = "shared_from_slice", since = "1.21.0")]
2437 impl<T> From<Vec<T>> for Arc<[T]> {
2438 /// Allocate a reference-counted slice and move `v`'s items into it.
2443 /// # use std::sync::Arc;
2444 /// let unique: Vec<i32> = vec![1, 2, 3];
2445 /// let shared: Arc<[i32]> = Arc::from(unique);
2446 /// assert_eq!(&[1, 2, 3], &shared[..]);
2449 fn from(mut v: Vec<T>) -> Arc<[T]> {
2451 let arc = Arc::copy_from_slice(&v);
2453 // Allow the Vec to free its memory, but not destroy its contents
2461 #[stable(feature = "shared_from_cow", since = "1.45.0")]
2462 impl<'a, B> From<Cow<'a, B>> for Arc<B>
2464 B: ToOwned + ?Sized,
2465 Arc<B>: From<&'a B> + From<B::Owned>,
2467 /// Create an atomically reference-counted pointer from
2468 /// a clone-on-write pointer by copying its content.
2473 /// # use std::sync::Arc;
2474 /// # use std::borrow::Cow;
2475 /// let cow: Cow<str> = Cow::Borrowed("eggplant");
2476 /// let shared: Arc<str> = Arc::from(cow);
2477 /// assert_eq!("eggplant", &shared[..]);
2480 fn from(cow: Cow<'a, B>) -> Arc<B> {
2482 Cow::Borrowed(s) => Arc::from(s),
2483 Cow::Owned(s) => Arc::from(s),
2488 #[stable(feature = "boxed_slice_try_from", since = "1.43.0")]
2489 impl<T, const N: usize> TryFrom<Arc<[T]>> for Arc<[T; N]> {
2490 type Error = Arc<[T]>;
2492 fn try_from(boxed_slice: Arc<[T]>) -> Result<Self, Self::Error> {
2493 if boxed_slice.len() == N {
2494 Ok(unsafe { Arc::from_raw(Arc::into_raw(boxed_slice) as *mut [T; N]) })
2501 #[cfg(not(no_global_oom_handling))]
2502 #[stable(feature = "shared_from_iter", since = "1.37.0")]
2503 impl<T> iter::FromIterator<T> for Arc<[T]> {
2504 /// Takes each element in the `Iterator` and collects it into an `Arc<[T]>`.
2506 /// # Performance characteristics
2508 /// ## The general case
2510 /// In the general case, collecting into `Arc<[T]>` is done by first
2511 /// collecting into a `Vec<T>`. That is, when writing the following:
2514 /// # use std::sync::Arc;
2515 /// let evens: Arc<[u8]> = (0..10).filter(|&x| x % 2 == 0).collect();
2516 /// # assert_eq!(&*evens, &[0, 2, 4, 6, 8]);
2519 /// this behaves as if we wrote:
2522 /// # use std::sync::Arc;
2523 /// let evens: Arc<[u8]> = (0..10).filter(|&x| x % 2 == 0)
2524 /// .collect::<Vec<_>>() // The first set of allocations happens here.
2525 /// .into(); // A second allocation for `Arc<[T]>` happens here.
2526 /// # assert_eq!(&*evens, &[0, 2, 4, 6, 8]);
2529 /// This will allocate as many times as needed for constructing the `Vec<T>`
2530 /// and then it will allocate once for turning the `Vec<T>` into the `Arc<[T]>`.
2532 /// ## Iterators of known length
2534 /// When your `Iterator` implements `TrustedLen` and is of an exact size,
2535 /// a single allocation will be made for the `Arc<[T]>`. For example:
2538 /// # use std::sync::Arc;
2539 /// let evens: Arc<[u8]> = (0..10).collect(); // Just a single allocation happens here.
2540 /// # assert_eq!(&*evens, &*(0..10).collect::<Vec<_>>());
2542 fn from_iter<I: iter::IntoIterator<Item = T>>(iter: I) -> Self {
2543 ToArcSlice::to_arc_slice(iter.into_iter())
2547 /// Specialization trait used for collecting into `Arc<[T]>`.
2548 trait ToArcSlice<T>: Iterator<Item = T> + Sized {
2549 fn to_arc_slice(self) -> Arc<[T]>;
2552 #[cfg(not(no_global_oom_handling))]
2553 impl<T, I: Iterator<Item = T>> ToArcSlice<T> for I {
2554 default fn to_arc_slice(self) -> Arc<[T]> {
2555 self.collect::<Vec<T>>().into()
2559 #[cfg(not(no_global_oom_handling))]
2560 impl<T, I: iter::TrustedLen<Item = T>> ToArcSlice<T> for I {
2561 fn to_arc_slice(self) -> Arc<[T]> {
2562 // This is the case for a `TrustedLen` iterator.
2563 let (low, high) = self.size_hint();
2564 if let Some(high) = high {
2568 "TrustedLen iterator's size hint is not exact: {:?}",
2573 // SAFETY: We need to ensure that the iterator has an exact length and we have.
2574 Arc::from_iter_exact(self, low)
2577 // TrustedLen contract guarantees that `upper_bound == `None` implies an iterator
2578 // length exceeding `usize::MAX`.
2579 // The default implementation would collect into a vec which would panic.
2580 // Thus we panic here immediately without invoking `Vec` code.
2581 panic!("capacity overflow");
2586 #[stable(feature = "rust1", since = "1.0.0")]
2587 impl<T: ?Sized> borrow::Borrow<T> for Arc<T> {
2588 fn borrow(&self) -> &T {
2593 #[stable(since = "1.5.0", feature = "smart_ptr_as_ref")]
2594 impl<T: ?Sized> AsRef<T> for Arc<T> {
2595 fn as_ref(&self) -> &T {
2600 #[stable(feature = "pin", since = "1.33.0")]
2601 impl<T: ?Sized> Unpin for Arc<T> {}
2603 /// Get the offset within an `ArcInner` for the payload behind a pointer.
2607 /// The pointer must point to (and have valid metadata for) a previously
2608 /// valid instance of T, but the T is allowed to be dropped.
2609 unsafe fn data_offset<T: ?Sized>(ptr: *const T) -> isize {
2610 // Align the unsized value to the end of the ArcInner.
2611 // Because RcBox is repr(C), it will always be the last field in memory.
2612 // SAFETY: since the only unsized types possible are slices, trait objects,
2613 // and extern types, the input safety requirement is currently enough to
2614 // satisfy the requirements of align_of_val_raw; this is an implementation
2615 // detail of the language that must not be relied upon outside of std.
2616 unsafe { data_offset_align(align_of_val_raw(ptr)) }
2620 fn data_offset_align(align: usize) -> isize {
2621 let layout = Layout::new::<ArcInner<()>>();
2622 (layout.size() + layout.padding_needed_for(align)) as isize