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 //! [arc]: struct.Arc.html
10 use core::array::LengthAtMost32;
11 use core::sync::atomic;
12 use core::sync::atomic::Ordering::{Acquire, Relaxed, Release, SeqCst};
15 use core::cmp::{self, Ordering};
17 use core::intrinsics::abort;
18 use core::mem::{self, align_of, align_of_val, size_of_val};
19 use core::ops::{Deref, Receiver, CoerceUnsized, DispatchFromDyn};
21 use core::ptr::{self, NonNull};
22 use core::marker::{Unpin, Unsize, PhantomData};
23 use core::hash::{Hash, Hasher};
24 use core::{isize, usize};
25 use core::convert::{From, TryFrom};
26 use core::slice::{self, from_raw_parts_mut};
28 use crate::alloc::{Global, Alloc, Layout, box_free, handle_alloc_error};
29 use crate::boxed::Box;
30 use crate::rc::is_dangling;
31 use crate::string::String;
37 /// A soft limit on the amount of references that may be made to an `Arc`.
39 /// Going above this limit will abort your program (although not
40 /// necessarily) at _exactly_ `MAX_REFCOUNT + 1` references.
41 const MAX_REFCOUNT: usize = (isize::MAX) as usize;
43 /// A thread-safe reference-counting pointer. 'Arc' stands for 'Atomically
44 /// Reference Counted'.
46 /// The type `Arc<T>` provides shared ownership of a value of type `T`,
47 /// allocated in the heap. Invoking [`clone`][clone] on `Arc` produces
48 /// a new `Arc` instance, which points to the same value on the heap as the
49 /// source `Arc`, while increasing a reference count. When the last `Arc`
50 /// pointer to a given value is destroyed, the pointed-to value is also
53 /// Shared references in Rust disallow mutation by default, and `Arc` is no
54 /// exception: you cannot generally obtain a mutable reference to something
55 /// inside an `Arc`. If you need to mutate through an `Arc`, use
56 /// [`Mutex`][mutex], [`RwLock`][rwlock], or one of the [`Atomic`][atomic]
61 /// Unlike [`Rc<T>`], `Arc<T>` uses atomic operations for its reference
62 /// counting. This means that it is thread-safe. The disadvantage is that
63 /// atomic operations are more expensive than ordinary memory accesses. If you
64 /// are not sharing reference-counted values between threads, consider using
65 /// [`Rc<T>`] for lower overhead. [`Rc<T>`] is a safe default, because the
66 /// compiler will catch any attempt to send an [`Rc<T>`] between threads.
67 /// However, a library might choose `Arc<T>` in order to give library consumers
70 /// `Arc<T>` will implement [`Send`] and [`Sync`] as long as the `T` implements
71 /// [`Send`] and [`Sync`]. Why can't you put a non-thread-safe type `T` in an
72 /// `Arc<T>` to make it thread-safe? This may be a bit counter-intuitive at
73 /// first: after all, isn't the point of `Arc<T>` thread safety? The key is
74 /// this: `Arc<T>` makes it thread safe to have multiple ownership of the same
75 /// data, but it doesn't add thread safety to its data. Consider
76 /// `Arc<`[`RefCell<T>`]`>`. [`RefCell<T>`] isn't [`Sync`], and if `Arc<T>` was always
77 /// [`Send`], `Arc<`[`RefCell<T>`]`>` would be as well. But then we'd have a problem:
78 /// [`RefCell<T>`] is not thread safe; it keeps track of the borrowing count using
79 /// non-atomic operations.
81 /// In the end, this means that you may need to pair `Arc<T>` with some sort of
82 /// [`std::sync`] type, usually [`Mutex<T>`][mutex].
84 /// ## Breaking cycles with `Weak`
86 /// The [`downgrade`][downgrade] method can be used to create a non-owning
87 /// [`Weak`][weak] pointer. A [`Weak`][weak] pointer can be [`upgrade`][upgrade]d
88 /// to an `Arc`, but this will return [`None`] if the value has already been
91 /// A cycle between `Arc` pointers will never be deallocated. For this reason,
92 /// [`Weak`][weak] is used to break cycles. For example, a tree could have
93 /// strong `Arc` pointers from parent nodes to children, and [`Weak`][weak]
94 /// pointers from children back to their parents.
96 /// # Cloning references
98 /// Creating a new reference from an existing reference counted pointer is done using the
99 /// `Clone` trait implemented for [`Arc<T>`][arc] and [`Weak<T>`][weak].
102 /// use std::sync::Arc;
103 /// let foo = Arc::new(vec![1.0, 2.0, 3.0]);
104 /// // The two syntaxes below are equivalent.
105 /// let a = foo.clone();
106 /// let b = Arc::clone(&foo);
107 /// // a, b, and foo are all Arcs that point to the same memory location
110 /// ## `Deref` behavior
112 /// `Arc<T>` automatically dereferences to `T` (via the [`Deref`][deref] trait),
113 /// so you can call `T`'s methods on a value of type `Arc<T>`. To avoid name
114 /// clashes with `T`'s methods, the methods of `Arc<T>` itself are associated
115 /// functions, called using function-like syntax:
118 /// use std::sync::Arc;
119 /// let my_arc = Arc::new(());
121 /// Arc::downgrade(&my_arc);
124 /// [`Weak<T>`][weak] does not auto-dereference to `T`, because the value may have
125 /// already been destroyed.
127 /// [arc]: struct.Arc.html
128 /// [weak]: struct.Weak.html
129 /// [`Rc<T>`]: ../../std/rc/struct.Rc.html
130 /// [clone]: ../../std/clone/trait.Clone.html#tymethod.clone
131 /// [mutex]: ../../std/sync/struct.Mutex.html
132 /// [rwlock]: ../../std/sync/struct.RwLock.html
133 /// [atomic]: ../../std/sync/atomic/index.html
134 /// [`Send`]: ../../std/marker/trait.Send.html
135 /// [`Sync`]: ../../std/marker/trait.Sync.html
136 /// [deref]: ../../std/ops/trait.Deref.html
137 /// [downgrade]: struct.Arc.html#method.downgrade
138 /// [upgrade]: struct.Weak.html#method.upgrade
139 /// [`None`]: ../../std/option/enum.Option.html#variant.None
140 /// [`RefCell<T>`]: ../../std/cell/struct.RefCell.html
141 /// [`std::sync`]: ../../std/sync/index.html
142 /// [`Arc::clone(&from)`]: #method.clone
146 /// Sharing some immutable data between threads:
148 // Note that we **do not** run these tests here. The windows builders get super
149 // unhappy if a thread outlives the main thread and then exits at the same time
150 // (something deadlocks) so we just avoid this entirely by not running these
153 /// use std::sync::Arc;
156 /// let five = Arc::new(5);
159 /// let five = Arc::clone(&five);
161 /// thread::spawn(move || {
162 /// println!("{:?}", five);
167 /// Sharing a mutable [`AtomicUsize`]:
169 /// [`AtomicUsize`]: ../../std/sync/atomic/struct.AtomicUsize.html
172 /// use std::sync::Arc;
173 /// use std::sync::atomic::{AtomicUsize, Ordering};
176 /// let val = Arc::new(AtomicUsize::new(5));
179 /// let val = Arc::clone(&val);
181 /// thread::spawn(move || {
182 /// let v = val.fetch_add(1, Ordering::SeqCst);
183 /// println!("{:?}", v);
188 /// See the [`rc` documentation][rc_examples] for more examples of reference
189 /// counting in general.
191 /// [rc_examples]: ../../std/rc/index.html#examples
192 #[cfg_attr(not(test), lang = "arc")]
193 #[stable(feature = "rust1", since = "1.0.0")]
194 pub struct Arc<T: ?Sized> {
195 ptr: NonNull<ArcInner<T>>,
196 phantom: PhantomData<T>,
199 #[stable(feature = "rust1", since = "1.0.0")]
200 unsafe impl<T: ?Sized + Sync + Send> Send for Arc<T> {}
201 #[stable(feature = "rust1", since = "1.0.0")]
202 unsafe impl<T: ?Sized + Sync + Send> Sync for Arc<T> {}
204 #[unstable(feature = "coerce_unsized", issue = "27732")]
205 impl<T: ?Sized + Unsize<U>, U: ?Sized> CoerceUnsized<Arc<U>> for Arc<T> {}
207 #[unstable(feature = "dispatch_from_dyn", issue = "0")]
208 impl<T: ?Sized + Unsize<U>, U: ?Sized> DispatchFromDyn<Arc<U>> for Arc<T> {}
210 impl<T: ?Sized> Arc<T> {
211 fn from_inner(ptr: NonNull<ArcInner<T>>) -> Self {
214 phantom: PhantomData,
218 unsafe fn from_ptr(ptr: *mut ArcInner<T>) -> Self {
219 Self::from_inner(NonNull::new_unchecked(ptr))
223 /// `Weak` is a version of [`Arc`] that holds a non-owning reference to the
224 /// managed value. The value is accessed by calling [`upgrade`] on the `Weak`
225 /// pointer, which returns an [`Option`]`<`[`Arc`]`<T>>`.
227 /// Since a `Weak` reference does not count towards ownership, it will not
228 /// prevent the inner value from being dropped, and `Weak` itself makes no
229 /// guarantees about the value still being present and may return [`None`]
230 /// when [`upgrade`]d.
232 /// A `Weak` pointer is useful for keeping a temporary reference to the value
233 /// within [`Arc`] without extending its lifetime. It is also used to prevent
234 /// circular references between [`Arc`] pointers, since mutual owning references
235 /// would never allow either [`Arc`] to be dropped. For example, a tree could
236 /// have strong [`Arc`] pointers from parent nodes to children, and `Weak`
237 /// pointers from children back to their parents.
239 /// The typical way to obtain a `Weak` pointer is to call [`Arc::downgrade`].
241 /// [`Arc`]: struct.Arc.html
242 /// [`Arc::downgrade`]: struct.Arc.html#method.downgrade
243 /// [`upgrade`]: struct.Weak.html#method.upgrade
244 /// [`Option`]: ../../std/option/enum.Option.html
245 /// [`None`]: ../../std/option/enum.Option.html#variant.None
246 #[stable(feature = "arc_weak", since = "1.4.0")]
247 pub struct Weak<T: ?Sized> {
248 // This is a `NonNull` to allow optimizing the size of this type in enums,
249 // but it is not necessarily a valid pointer.
250 // `Weak::new` sets this to `usize::MAX` so that it doesn’t need
251 // to allocate space on the heap. That's not a value a real pointer
252 // will ever have because RcBox has alignment at least 2.
253 ptr: NonNull<ArcInner<T>>,
256 #[stable(feature = "arc_weak", since = "1.4.0")]
257 unsafe impl<T: ?Sized + Sync + Send> Send for Weak<T> {}
258 #[stable(feature = "arc_weak", since = "1.4.0")]
259 unsafe impl<T: ?Sized + Sync + Send> Sync for Weak<T> {}
261 #[unstable(feature = "coerce_unsized", issue = "27732")]
262 impl<T: ?Sized + Unsize<U>, U: ?Sized> CoerceUnsized<Weak<U>> for Weak<T> {}
263 #[unstable(feature = "dispatch_from_dyn", issue = "0")]
264 impl<T: ?Sized + Unsize<U>, U: ?Sized> DispatchFromDyn<Weak<U>> for Weak<T> {}
266 #[stable(feature = "arc_weak", since = "1.4.0")]
267 impl<T: ?Sized + fmt::Debug> fmt::Debug for Weak<T> {
268 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
273 struct ArcInner<T: ?Sized> {
274 strong: atomic::AtomicUsize,
276 // the value usize::MAX acts as a sentinel for temporarily "locking" the
277 // ability to upgrade weak pointers or downgrade strong ones; this is used
278 // to avoid races in `make_mut` and `get_mut`.
279 weak: atomic::AtomicUsize,
284 unsafe impl<T: ?Sized + Sync + Send> Send for ArcInner<T> {}
285 unsafe impl<T: ?Sized + Sync + Send> Sync for ArcInner<T> {}
288 /// Constructs a new `Arc<T>`.
293 /// use std::sync::Arc;
295 /// let five = Arc::new(5);
298 #[stable(feature = "rust1", since = "1.0.0")]
299 pub fn new(data: T) -> Arc<T> {
300 // Start the weak pointer count as 1 which is the weak pointer that's
301 // held by all the strong pointers (kinda), see std/rc.rs for more info
302 let x: Box<_> = box ArcInner {
303 strong: atomic::AtomicUsize::new(1),
304 weak: atomic::AtomicUsize::new(1),
307 Self::from_inner(Box::into_raw_non_null(x))
310 /// Constructs a new `Arc` with uninitialized contents.
315 /// #![feature(new_uninit)]
316 /// #![feature(get_mut_unchecked)]
318 /// use std::sync::Arc;
320 /// let mut five = Arc::<u32>::new_uninit();
322 /// let five = unsafe {
323 /// // Deferred initialization:
324 /// Arc::get_mut_unchecked(&mut five).as_mut_ptr().write(5);
326 /// five.assume_init()
329 /// assert_eq!(*five, 5)
331 #[unstable(feature = "new_uninit", issue = "63291")]
332 pub fn new_uninit() -> Arc<mem::MaybeUninit<T>> {
334 Arc::from_ptr(Arc::allocate_for_layout(
336 |mem| mem as *mut ArcInner<mem::MaybeUninit<T>>,
341 /// Constructs a new `Pin<Arc<T>>`. If `T` does not implement `Unpin`, then
342 /// `data` will be pinned in memory and unable to be moved.
343 #[stable(feature = "pin", since = "1.33.0")]
344 pub fn pin(data: T) -> Pin<Arc<T>> {
345 unsafe { Pin::new_unchecked(Arc::new(data)) }
348 /// Returns the contained value, if the `Arc` has exactly one strong reference.
350 /// Otherwise, an [`Err`][result] is returned with the same `Arc` that was
353 /// This will succeed even if there are outstanding weak references.
355 /// [result]: ../../std/result/enum.Result.html
360 /// use std::sync::Arc;
362 /// let x = Arc::new(3);
363 /// assert_eq!(Arc::try_unwrap(x), Ok(3));
365 /// let x = Arc::new(4);
366 /// let _y = Arc::clone(&x);
367 /// assert_eq!(*Arc::try_unwrap(x).unwrap_err(), 4);
370 #[stable(feature = "arc_unique", since = "1.4.0")]
371 pub fn try_unwrap(this: Self) -> Result<T, Self> {
372 // See `drop` for why all these atomics are like this
373 if this.inner().strong.compare_exchange(1, 0, Release, Relaxed).is_err() {
377 atomic::fence(Acquire);
380 let elem = ptr::read(&this.ptr.as_ref().data);
382 // Make a weak pointer to clean up the implicit strong-weak reference
383 let _weak = Weak { ptr: this.ptr };
392 /// Constructs a new reference-counted slice with uninitialized contents.
397 /// #![feature(new_uninit)]
398 /// #![feature(get_mut_unchecked)]
400 /// use std::sync::Arc;
402 /// let mut values = Arc::<[u32]>::new_uninit_slice(3);
404 /// let values = unsafe {
405 /// // Deferred initialization:
406 /// Arc::get_mut_unchecked(&mut values)[0].as_mut_ptr().write(1);
407 /// Arc::get_mut_unchecked(&mut values)[1].as_mut_ptr().write(2);
408 /// Arc::get_mut_unchecked(&mut values)[2].as_mut_ptr().write(3);
410 /// values.assume_init()
413 /// assert_eq!(*values, [1, 2, 3])
415 #[unstable(feature = "new_uninit", issue = "63291")]
416 pub fn new_uninit_slice(len: usize) -> Arc<[mem::MaybeUninit<T>]> {
418 Arc::from_ptr(Arc::allocate_for_slice(len))
423 impl<T> Arc<mem::MaybeUninit<T>> {
424 /// Converts to `Arc<T>`.
428 /// As with [`MaybeUninit::assume_init`],
429 /// it is up to the caller to guarantee that the value
430 /// really is in an initialized state.
431 /// Calling this when the content is not yet fully initialized
432 /// causes immediate undefined behavior.
434 /// [`MaybeUninit::assume_init`]: ../../std/mem/union.MaybeUninit.html#method.assume_init
439 /// #![feature(new_uninit)]
440 /// #![feature(get_mut_unchecked)]
442 /// use std::sync::Arc;
444 /// let mut five = Arc::<u32>::new_uninit();
446 /// let five = unsafe {
447 /// // Deferred initialization:
448 /// Arc::get_mut_unchecked(&mut five).as_mut_ptr().write(5);
450 /// five.assume_init()
453 /// assert_eq!(*five, 5)
455 #[unstable(feature = "new_uninit", issue = "63291")]
457 pub unsafe fn assume_init(self) -> Arc<T> {
458 Arc::from_inner(mem::ManuallyDrop::new(self).ptr.cast())
462 impl<T> Arc<[mem::MaybeUninit<T>]> {
463 /// Converts to `Arc<[T]>`.
467 /// As with [`MaybeUninit::assume_init`],
468 /// it is up to the caller to guarantee that the value
469 /// really is in an initialized state.
470 /// Calling this when the content is not yet fully initialized
471 /// causes immediate undefined behavior.
473 /// [`MaybeUninit::assume_init`]: ../../std/mem/union.MaybeUninit.html#method.assume_init
478 /// #![feature(new_uninit)]
479 /// #![feature(get_mut_unchecked)]
481 /// use std::sync::Arc;
483 /// let mut values = Arc::<[u32]>::new_uninit_slice(3);
485 /// let values = unsafe {
486 /// // Deferred initialization:
487 /// Arc::get_mut_unchecked(&mut values)[0].as_mut_ptr().write(1);
488 /// Arc::get_mut_unchecked(&mut values)[1].as_mut_ptr().write(2);
489 /// Arc::get_mut_unchecked(&mut values)[2].as_mut_ptr().write(3);
491 /// values.assume_init()
494 /// assert_eq!(*values, [1, 2, 3])
496 #[unstable(feature = "new_uninit", issue = "63291")]
498 pub unsafe fn assume_init(self) -> Arc<[T]> {
499 Arc::from_ptr(mem::ManuallyDrop::new(self).ptr.as_ptr() as _)
503 impl<T: ?Sized> Arc<T> {
504 /// Consumes the `Arc`, returning the wrapped pointer.
506 /// To avoid a memory leak the pointer must be converted back to an `Arc` using
507 /// [`Arc::from_raw`][from_raw].
509 /// [from_raw]: struct.Arc.html#method.from_raw
514 /// use std::sync::Arc;
516 /// let x = Arc::new("hello".to_owned());
517 /// let x_ptr = Arc::into_raw(x);
518 /// assert_eq!(unsafe { &*x_ptr }, "hello");
520 #[stable(feature = "rc_raw", since = "1.17.0")]
521 pub fn into_raw(this: Self) -> *const T {
522 let ptr: *const T = &*this;
527 /// Constructs an `Arc` from a raw pointer.
529 /// The raw pointer must have been previously returned by a call to a
530 /// [`Arc::into_raw`][into_raw].
532 /// This function is unsafe because improper use may lead to memory problems. For example, a
533 /// double-free may occur if the function is called twice on the same raw pointer.
535 /// [into_raw]: struct.Arc.html#method.into_raw
540 /// use std::sync::Arc;
542 /// let x = Arc::new("hello".to_owned());
543 /// let x_ptr = Arc::into_raw(x);
546 /// // Convert back to an `Arc` to prevent leak.
547 /// let x = Arc::from_raw(x_ptr);
548 /// assert_eq!(&*x, "hello");
550 /// // Further calls to `Arc::from_raw(x_ptr)` would be memory-unsafe.
553 /// // The memory was freed when `x` went out of scope above, so `x_ptr` is now dangling!
555 #[stable(feature = "rc_raw", since = "1.17.0")]
556 pub unsafe fn from_raw(ptr: *const T) -> Self {
557 let offset = data_offset(ptr);
559 // Reverse the offset to find the original ArcInner.
560 let fake_ptr = ptr as *mut ArcInner<T>;
561 let arc_ptr = set_data_ptr(fake_ptr, (ptr as *mut u8).offset(-offset));
563 Self::from_ptr(arc_ptr)
566 /// Consumes the `Arc`, returning the wrapped pointer as `NonNull<T>`.
571 /// #![feature(rc_into_raw_non_null)]
573 /// use std::sync::Arc;
575 /// let x = Arc::new("hello".to_owned());
576 /// let ptr = Arc::into_raw_non_null(x);
577 /// let deref = unsafe { ptr.as_ref() };
578 /// assert_eq!(deref, "hello");
580 #[unstable(feature = "rc_into_raw_non_null", issue = "47336")]
582 pub fn into_raw_non_null(this: Self) -> NonNull<T> {
583 // safe because Arc guarantees its pointer is non-null
584 unsafe { NonNull::new_unchecked(Arc::into_raw(this) as *mut _) }
587 /// Creates a new [`Weak`][weak] pointer to this value.
589 /// [weak]: struct.Weak.html
594 /// use std::sync::Arc;
596 /// let five = Arc::new(5);
598 /// let weak_five = Arc::downgrade(&five);
600 #[stable(feature = "arc_weak", since = "1.4.0")]
601 pub fn downgrade(this: &Self) -> Weak<T> {
602 // This Relaxed is OK because we're checking the value in the CAS
604 let mut cur = this.inner().weak.load(Relaxed);
607 // check if the weak counter is currently "locked"; if so, spin.
608 if cur == usize::MAX {
609 cur = this.inner().weak.load(Relaxed);
613 // NOTE: this code currently ignores the possibility of overflow
614 // into usize::MAX; in general both Rc and Arc need to be adjusted
615 // to deal with overflow.
617 // Unlike with Clone(), we need this to be an Acquire read to
618 // synchronize with the write coming from `is_unique`, so that the
619 // events prior to that write happen before this read.
620 match this.inner().weak.compare_exchange_weak(cur, cur + 1, Acquire, Relaxed) {
622 // Make sure we do not create a dangling Weak
623 debug_assert!(!is_dangling(this.ptr));
624 return Weak { ptr: this.ptr };
626 Err(old) => cur = old,
631 /// Gets the number of [`Weak`][weak] pointers to this value.
633 /// [weak]: struct.Weak.html
637 /// This method by itself is safe, but using it correctly requires extra care.
638 /// Another thread can change the weak count at any time,
639 /// including potentially between calling this method and acting on the result.
644 /// use std::sync::Arc;
646 /// let five = Arc::new(5);
647 /// let _weak_five = Arc::downgrade(&five);
649 /// // This assertion is deterministic because we haven't shared
650 /// // the `Arc` or `Weak` between threads.
651 /// assert_eq!(1, Arc::weak_count(&five));
654 #[stable(feature = "arc_counts", since = "1.15.0")]
655 pub fn weak_count(this: &Self) -> usize {
656 let cnt = this.inner().weak.load(SeqCst);
657 // If the weak count is currently locked, the value of the
658 // count was 0 just before taking the lock.
659 if cnt == usize::MAX { 0 } else { cnt - 1 }
662 /// Gets the number of strong (`Arc`) pointers to this value.
666 /// This method by itself is safe, but using it correctly requires extra care.
667 /// Another thread can change the strong count at any time,
668 /// including potentially between calling this method and acting on the result.
673 /// use std::sync::Arc;
675 /// let five = Arc::new(5);
676 /// let _also_five = Arc::clone(&five);
678 /// // This assertion is deterministic because we haven't shared
679 /// // the `Arc` between threads.
680 /// assert_eq!(2, Arc::strong_count(&five));
683 #[stable(feature = "arc_counts", since = "1.15.0")]
684 pub fn strong_count(this: &Self) -> usize {
685 this.inner().strong.load(SeqCst)
689 fn inner(&self) -> &ArcInner<T> {
690 // This unsafety is ok because while this arc is alive we're guaranteed
691 // that the inner pointer is valid. Furthermore, we know that the
692 // `ArcInner` structure itself is `Sync` because the inner data is
693 // `Sync` as well, so we're ok loaning out an immutable pointer to these
695 unsafe { self.ptr.as_ref() }
698 // Non-inlined part of `drop`.
700 unsafe fn drop_slow(&mut self) {
701 // Destroy the data at this time, even though we may not free the box
702 // allocation itself (there may still be weak pointers lying around).
703 ptr::drop_in_place(&mut self.ptr.as_mut().data);
705 if self.inner().weak.fetch_sub(1, Release) == 1 {
706 atomic::fence(Acquire);
707 Global.dealloc(self.ptr.cast(), Layout::for_value(self.ptr.as_ref()))
712 #[stable(feature = "ptr_eq", since = "1.17.0")]
713 /// Returns `true` if the two `Arc`s point to the same value (not
714 /// just values that compare as equal).
719 /// use std::sync::Arc;
721 /// let five = Arc::new(5);
722 /// let same_five = Arc::clone(&five);
723 /// let other_five = Arc::new(5);
725 /// assert!(Arc::ptr_eq(&five, &same_five));
726 /// assert!(!Arc::ptr_eq(&five, &other_five));
728 pub fn ptr_eq(this: &Self, other: &Self) -> bool {
729 this.ptr.as_ptr() == other.ptr.as_ptr()
733 impl<T: ?Sized> Arc<T> {
734 /// Allocates an `ArcInner<T>` with sufficient space for
735 /// a possibly-unsized value where the value has the layout provided.
737 /// The function `mem_to_arcinner` is called with the data pointer
738 /// and must return back a (potentially fat)-pointer for the `ArcInner<T>`.
739 unsafe fn allocate_for_layout(
740 value_layout: Layout,
741 mem_to_arcinner: impl FnOnce(*mut u8) -> *mut ArcInner<T>
742 ) -> *mut ArcInner<T> {
743 // Calculate layout using the given value layout.
744 // Previously, layout was calculated on the expression
745 // `&*(ptr as *const ArcInner<T>)`, but this created a misaligned
746 // reference (see #54908).
747 let layout = Layout::new::<ArcInner<()>>()
748 .extend(value_layout).unwrap().0
749 .pad_to_align().unwrap();
751 let mem = Global.alloc(layout)
752 .unwrap_or_else(|_| handle_alloc_error(layout));
754 // Initialize the ArcInner
755 let inner = mem_to_arcinner(mem.as_ptr());
756 debug_assert_eq!(Layout::for_value(&*inner), layout);
758 ptr::write(&mut (*inner).strong, atomic::AtomicUsize::new(1));
759 ptr::write(&mut (*inner).weak, atomic::AtomicUsize::new(1));
764 /// Allocates an `ArcInner<T>` with sufficient space for an unsized value.
765 unsafe fn allocate_for_ptr(ptr: *const T) -> *mut ArcInner<T> {
766 // Allocate for the `ArcInner<T>` using the given value.
767 Self::allocate_for_layout(
768 Layout::for_value(&*ptr),
769 |mem| set_data_ptr(ptr as *mut T, mem) as *mut ArcInner<T>,
773 fn from_box(v: Box<T>) -> Arc<T> {
775 let box_unique = Box::into_unique(v);
776 let bptr = box_unique.as_ptr();
778 let value_size = size_of_val(&*bptr);
779 let ptr = Self::allocate_for_ptr(bptr);
781 // Copy value as bytes
782 ptr::copy_nonoverlapping(
783 bptr as *const T as *const u8,
784 &mut (*ptr).data as *mut _ as *mut u8,
787 // Free the allocation without dropping its contents
788 box_free(box_unique);
796 /// Allocates an `ArcInner<[T]>` with the given length.
797 unsafe fn allocate_for_slice(len: usize) -> *mut ArcInner<[T]> {
798 Self::allocate_for_layout(
799 Layout::array::<T>(len).unwrap(),
800 |mem| ptr::slice_from_raw_parts_mut(mem as *mut T, len) as *mut ArcInner<[T]>,
805 /// Sets the data pointer of a `?Sized` raw pointer.
807 /// For a slice/trait object, this sets the `data` field and leaves the rest
808 /// unchanged. For a sized raw pointer, this simply sets the pointer.
809 unsafe fn set_data_ptr<T: ?Sized, U>(mut ptr: *mut T, data: *mut U) -> *mut T {
810 ptr::write(&mut ptr as *mut _ as *mut *mut u8, data as *mut u8);
815 /// Copy elements from slice into newly allocated Arc<[T]>
817 /// Unsafe because the caller must either take ownership or bind `T: Copy`.
818 unsafe fn copy_from_slice(v: &[T]) -> Arc<[T]> {
819 let ptr = Self::allocate_for_slice(v.len());
821 ptr::copy_nonoverlapping(
823 &mut (*ptr).data as *mut [T] as *mut T,
829 /// Constructs an `Arc<[T]>` from an iterator known to be of a certain size.
831 /// Behavior is undefined should the size be wrong.
832 unsafe fn from_iter_exact(iter: impl iter::Iterator<Item = T>, len: usize) -> Arc<[T]> {
833 // Panic guard while cloning T elements.
834 // In the event of a panic, elements that have been written
835 // into the new ArcInner will be dropped, then the memory freed.
843 impl<T> Drop for Guard<T> {
846 let slice = from_raw_parts_mut(self.elems, self.n_elems);
847 ptr::drop_in_place(slice);
849 Global.dealloc(self.mem.cast(), self.layout);
854 let ptr = Self::allocate_for_slice(len);
856 let mem = ptr as *mut _ as *mut u8;
857 let layout = Layout::for_value(&*ptr);
859 // Pointer to first element
860 let elems = &mut (*ptr).data as *mut [T] as *mut T;
862 let mut guard = Guard {
863 mem: NonNull::new_unchecked(mem),
869 for (i, item) in iter.enumerate() {
870 ptr::write(elems.add(i), item);
874 // All clear. Forget the guard so it doesn't free the new ArcInner.
881 /// Specialization trait used for `From<&[T]>`.
882 trait ArcFromSlice<T> {
883 fn from_slice(slice: &[T]) -> Self;
886 impl<T: Clone> ArcFromSlice<T> for Arc<[T]> {
888 default fn from_slice(v: &[T]) -> Self {
890 Self::from_iter_exact(v.iter().cloned(), v.len())
895 impl<T: Copy> ArcFromSlice<T> for Arc<[T]> {
897 fn from_slice(v: &[T]) -> Self {
898 unsafe { Arc::copy_from_slice(v) }
902 #[stable(feature = "rust1", since = "1.0.0")]
903 impl<T: ?Sized> Clone for Arc<T> {
904 /// Makes a clone of the `Arc` pointer.
906 /// This creates another pointer to the same inner value, increasing the
907 /// strong reference count.
912 /// use std::sync::Arc;
914 /// let five = Arc::new(5);
916 /// let _ = Arc::clone(&five);
919 fn clone(&self) -> Arc<T> {
920 // Using a relaxed ordering is alright here, as knowledge of the
921 // original reference prevents other threads from erroneously deleting
924 // As explained in the [Boost documentation][1], Increasing the
925 // reference counter can always be done with memory_order_relaxed: New
926 // references to an object can only be formed from an existing
927 // reference, and passing an existing reference from one thread to
928 // another must already provide any required synchronization.
930 // [1]: (www.boost.org/doc/libs/1_55_0/doc/html/atomic/usage_examples.html)
931 let old_size = self.inner().strong.fetch_add(1, Relaxed);
933 // However we need to guard against massive refcounts in case someone
934 // is `mem::forget`ing Arcs. If we don't do this the count can overflow
935 // and users will use-after free. We racily saturate to `isize::MAX` on
936 // the assumption that there aren't ~2 billion threads incrementing
937 // the reference count at once. This branch will never be taken in
938 // any realistic program.
940 // We abort because such a program is incredibly degenerate, and we
941 // don't care to support it.
942 if old_size > MAX_REFCOUNT {
948 Self::from_inner(self.ptr)
952 #[stable(feature = "rust1", since = "1.0.0")]
953 impl<T: ?Sized> Deref for Arc<T> {
957 fn deref(&self) -> &T {
962 #[unstable(feature = "receiver_trait", issue = "0")]
963 impl<T: ?Sized> Receiver for Arc<T> {}
965 impl<T: Clone> Arc<T> {
966 /// Makes a mutable reference into the given `Arc`.
968 /// If there are other `Arc` or [`Weak`][weak] pointers to the same value,
969 /// then `make_mut` will invoke [`clone`][clone] on the inner value to
970 /// ensure unique ownership. This is also referred to as clone-on-write.
972 /// See also [`get_mut`][get_mut], which will fail rather than cloning.
974 /// [weak]: struct.Weak.html
975 /// [clone]: ../../std/clone/trait.Clone.html#tymethod.clone
976 /// [get_mut]: struct.Arc.html#method.get_mut
981 /// use std::sync::Arc;
983 /// let mut data = Arc::new(5);
985 /// *Arc::make_mut(&mut data) += 1; // Won't clone anything
986 /// let mut other_data = Arc::clone(&data); // Won't clone inner data
987 /// *Arc::make_mut(&mut data) += 1; // Clones inner data
988 /// *Arc::make_mut(&mut data) += 1; // Won't clone anything
989 /// *Arc::make_mut(&mut other_data) *= 2; // Won't clone anything
991 /// // Now `data` and `other_data` point to different values.
992 /// assert_eq!(*data, 8);
993 /// assert_eq!(*other_data, 12);
996 #[stable(feature = "arc_unique", since = "1.4.0")]
997 pub fn make_mut(this: &mut Self) -> &mut T {
998 // Note that we hold both a strong reference and a weak reference.
999 // Thus, releasing our strong reference only will not, by itself, cause
1000 // the memory to be deallocated.
1002 // Use Acquire to ensure that we see any writes to `weak` that happen
1003 // before release writes (i.e., decrements) to `strong`. Since we hold a
1004 // weak count, there's no chance the ArcInner itself could be
1006 if this.inner().strong.compare_exchange(1, 0, Acquire, Relaxed).is_err() {
1007 // Another strong pointer exists; clone
1008 *this = Arc::new((**this).clone());
1009 } else if this.inner().weak.load(Relaxed) != 1 {
1010 // Relaxed suffices in the above because this is fundamentally an
1011 // optimization: we are always racing with weak pointers being
1012 // dropped. Worst case, we end up allocated a new Arc unnecessarily.
1014 // We removed the last strong ref, but there are additional weak
1015 // refs remaining. We'll move the contents to a new Arc, and
1016 // invalidate the other weak refs.
1018 // Note that it is not possible for the read of `weak` to yield
1019 // usize::MAX (i.e., locked), since the weak count can only be
1020 // locked by a thread with a strong reference.
1022 // Materialize our own implicit weak pointer, so that it can clean
1023 // up the ArcInner as needed.
1024 let weak = Weak { ptr: this.ptr };
1026 // mark the data itself as already deallocated
1028 // there is no data race in the implicit write caused by `read`
1029 // here (due to zeroing) because data is no longer accessed by
1030 // other threads (due to there being no more strong refs at this
1032 let mut swap = Arc::new(ptr::read(&weak.ptr.as_ref().data));
1033 mem::swap(this, &mut swap);
1037 // We were the sole reference of either kind; bump back up the
1038 // strong ref count.
1039 this.inner().strong.store(1, Release);
1042 // As with `get_mut()`, the unsafety is ok because our reference was
1043 // either unique to begin with, or became one upon cloning the contents.
1045 &mut this.ptr.as_mut().data
1050 impl<T: ?Sized> Arc<T> {
1051 /// Returns a mutable reference to the inner value, if there are
1052 /// no other `Arc` or [`Weak`][weak] pointers to the same value.
1054 /// Returns [`None`][option] otherwise, because it is not safe to
1055 /// mutate a shared value.
1057 /// See also [`make_mut`][make_mut], which will [`clone`][clone]
1058 /// the inner value when it's shared.
1060 /// [weak]: struct.Weak.html
1061 /// [option]: ../../std/option/enum.Option.html
1062 /// [make_mut]: struct.Arc.html#method.make_mut
1063 /// [clone]: ../../std/clone/trait.Clone.html#tymethod.clone
1068 /// use std::sync::Arc;
1070 /// let mut x = Arc::new(3);
1071 /// *Arc::get_mut(&mut x).unwrap() = 4;
1072 /// assert_eq!(*x, 4);
1074 /// let _y = Arc::clone(&x);
1075 /// assert!(Arc::get_mut(&mut x).is_none());
1078 #[stable(feature = "arc_unique", since = "1.4.0")]
1079 pub fn get_mut(this: &mut Self) -> Option<&mut T> {
1080 if this.is_unique() {
1081 // This unsafety is ok because we're guaranteed that the pointer
1082 // returned is the *only* pointer that will ever be returned to T. Our
1083 // reference count is guaranteed to be 1 at this point, and we required
1084 // the Arc itself to be `mut`, so we're returning the only possible
1085 // reference to the inner data.
1087 Some(Arc::get_mut_unchecked(this))
1094 /// Returns a mutable reference to the inner value,
1095 /// without any check.
1097 /// See also [`get_mut`], which is safe and does appropriate checks.
1099 /// [`get_mut`]: struct.Arc.html#method.get_mut
1103 /// Any other `Arc` or [`Weak`] pointers to the same value must not be dereferenced
1104 /// for the duration of the returned borrow.
1105 /// This is trivially the case if no such pointers exist,
1106 /// for example immediately after `Arc::new`.
1111 /// #![feature(get_mut_unchecked)]
1113 /// use std::sync::Arc;
1115 /// let mut x = Arc::new(String::new());
1117 /// Arc::get_mut_unchecked(&mut x).push_str("foo")
1119 /// assert_eq!(*x, "foo");
1122 #[unstable(feature = "get_mut_unchecked", issue = "63292")]
1123 pub unsafe fn get_mut_unchecked(this: &mut Self) -> &mut T {
1124 &mut this.ptr.as_mut().data
1127 /// Determine whether this is the unique reference (including weak refs) to
1128 /// the underlying data.
1130 /// Note that this requires locking the weak ref count.
1131 fn is_unique(&mut self) -> bool {
1132 // lock the weak pointer count if we appear to be the sole weak pointer
1135 // The acquire label here ensures a happens-before relationship with any
1136 // writes to `strong` (in particular in `Weak::upgrade`) prior to decrements
1137 // of the `weak` count (via `Weak::drop`, which uses release). If the upgraded
1138 // weak ref was never dropped, the CAS here will fail so we do not care to synchronize.
1139 if self.inner().weak.compare_exchange(1, usize::MAX, Acquire, Relaxed).is_ok() {
1140 // This needs to be an `Acquire` to synchronize with the decrement of the `strong`
1141 // counter in `drop` -- the only access that happens when any but the last reference
1142 // is being dropped.
1143 let unique = self.inner().strong.load(Acquire) == 1;
1145 // The release write here synchronizes with a read in `downgrade`,
1146 // effectively preventing the above read of `strong` from happening
1148 self.inner().weak.store(1, Release); // release the lock
1156 #[stable(feature = "rust1", since = "1.0.0")]
1157 unsafe impl<#[may_dangle] T: ?Sized> Drop for Arc<T> {
1158 /// Drops the `Arc`.
1160 /// This will decrement the strong reference count. If the strong reference
1161 /// count reaches zero then the only other references (if any) are
1162 /// [`Weak`], so we `drop` the inner value.
1167 /// use std::sync::Arc;
1171 /// impl Drop for Foo {
1172 /// fn drop(&mut self) {
1173 /// println!("dropped!");
1177 /// let foo = Arc::new(Foo);
1178 /// let foo2 = Arc::clone(&foo);
1180 /// drop(foo); // Doesn't print anything
1181 /// drop(foo2); // Prints "dropped!"
1184 /// [`Weak`]: ../../std/sync/struct.Weak.html
1186 fn drop(&mut self) {
1187 // Because `fetch_sub` is already atomic, we do not need to synchronize
1188 // with other threads unless we are going to delete the object. This
1189 // same logic applies to the below `fetch_sub` to the `weak` count.
1190 if self.inner().strong.fetch_sub(1, Release) != 1 {
1194 // This fence is needed to prevent reordering of use of the data and
1195 // deletion of the data. Because it is marked `Release`, the decreasing
1196 // of the reference count synchronizes with this `Acquire` fence. This
1197 // means that use of the data happens before decreasing the reference
1198 // count, which happens before this fence, which happens before the
1199 // deletion of the data.
1201 // As explained in the [Boost documentation][1],
1203 // > It is important to enforce any possible access to the object in one
1204 // > thread (through an existing reference) to *happen before* deleting
1205 // > the object in a different thread. This is achieved by a "release"
1206 // > operation after dropping a reference (any access to the object
1207 // > through this reference must obviously happened before), and an
1208 // > "acquire" operation before deleting the object.
1210 // In particular, while the contents of an Arc are usually immutable, it's
1211 // possible to have interior writes to something like a Mutex<T>. Since a
1212 // Mutex is not acquired when it is deleted, we can't rely on its
1213 // synchronization logic to make writes in thread A visible to a destructor
1214 // running in thread B.
1216 // Also note that the Acquire fence here could probably be replaced with an
1217 // Acquire load, which could improve performance in highly-contended
1218 // situations. See [2].
1220 // [1]: (www.boost.org/doc/libs/1_55_0/doc/html/atomic/usage_examples.html)
1221 // [2]: (https://github.com/rust-lang/rust/pull/41714)
1222 atomic::fence(Acquire);
1230 impl Arc<dyn Any + Send + Sync> {
1232 #[stable(feature = "rc_downcast", since = "1.29.0")]
1233 /// Attempt to downcast the `Arc<dyn Any + Send + Sync>` to a concrete type.
1238 /// use std::any::Any;
1239 /// use std::sync::Arc;
1241 /// fn print_if_string(value: Arc<dyn Any + Send + Sync>) {
1242 /// if let Ok(string) = value.downcast::<String>() {
1243 /// println!("String ({}): {}", string.len(), string);
1247 /// let my_string = "Hello World".to_string();
1248 /// print_if_string(Arc::new(my_string));
1249 /// print_if_string(Arc::new(0i8));
1251 pub fn downcast<T>(self) -> Result<Arc<T>, Self>
1253 T: Any + Send + Sync + 'static,
1255 if (*self).is::<T>() {
1256 let ptr = self.ptr.cast::<ArcInner<T>>();
1258 Ok(Arc::from_inner(ptr))
1266 /// Constructs a new `Weak<T>`, without allocating any memory.
1267 /// Calling [`upgrade`] on the return value always gives [`None`].
1269 /// [`upgrade`]: struct.Weak.html#method.upgrade
1270 /// [`None`]: ../../std/option/enum.Option.html#variant.None
1275 /// use std::sync::Weak;
1277 /// let empty: Weak<i64> = Weak::new();
1278 /// assert!(empty.upgrade().is_none());
1280 #[stable(feature = "downgraded_weak", since = "1.10.0")]
1281 pub fn new() -> Weak<T> {
1283 ptr: NonNull::new(usize::MAX as *mut ArcInner<T>).expect("MAX is not 0"),
1287 /// Returns a raw pointer to the object `T` pointed to by this `Weak<T>`.
1289 /// It is up to the caller to ensure that the object is still alive when accessing it through
1292 /// The pointer may be [`null`] or be dangling in case the object has already been destroyed.
1297 /// #![feature(weak_into_raw)]
1299 /// use std::sync::Arc;
1302 /// let strong = Arc::new("hello".to_owned());
1303 /// let weak = Arc::downgrade(&strong);
1304 /// // Both point to the same object
1305 /// assert!(ptr::eq(&*strong, weak.as_raw()));
1306 /// // The strong here keeps it alive, so we can still access the object.
1307 /// assert_eq!("hello", unsafe { &*weak.as_raw() });
1310 /// // But not any more. We can do weak.as_raw(), but accessing the pointer would lead to
1311 /// // undefined behaviour.
1312 /// // assert_eq!("hello", unsafe { &*weak.as_raw() });
1315 /// [`null`]: ../../std/ptr/fn.null.html
1316 #[unstable(feature = "weak_into_raw", issue = "60728")]
1317 pub fn as_raw(&self) -> *const T {
1318 match self.inner() {
1319 None => ptr::null(),
1321 let offset = data_offset_sized::<T>();
1322 let ptr = inner as *const ArcInner<T>;
1323 // Note: while the pointer we create may already point to dropped value, the
1324 // allocation still lives (it must hold the weak point as long as we are alive).
1325 // Therefore, the offset is OK to do, it won't get out of the allocation.
1326 let ptr = unsafe { (ptr as *const u8).offset(offset) };
1332 /// Consumes the `Weak<T>` and turns it into a raw pointer.
1334 /// This converts the weak pointer into a raw pointer, preserving the original weak count. It
1335 /// can be turned back into the `Weak<T>` with [`from_raw`].
1337 /// The same restrictions of accessing the target of the pointer as with
1338 /// [`as_raw`] apply.
1343 /// #![feature(weak_into_raw)]
1345 /// use std::sync::{Arc, Weak};
1347 /// let strong = Arc::new("hello".to_owned());
1348 /// let weak = Arc::downgrade(&strong);
1349 /// let raw = weak.into_raw();
1351 /// assert_eq!(1, Arc::weak_count(&strong));
1352 /// assert_eq!("hello", unsafe { &*raw });
1354 /// drop(unsafe { Weak::from_raw(raw) });
1355 /// assert_eq!(0, Arc::weak_count(&strong));
1358 /// [`from_raw`]: struct.Weak.html#method.from_raw
1359 /// [`as_raw`]: struct.Weak.html#method.as_raw
1360 #[unstable(feature = "weak_into_raw", issue = "60728")]
1361 pub fn into_raw(self) -> *const T {
1362 let result = self.as_raw();
1367 /// Converts a raw pointer previously created by [`into_raw`] back into
1370 /// This can be used to safely get a strong reference (by calling [`upgrade`]
1371 /// later) or to deallocate the weak count by dropping the `Weak<T>`.
1373 /// It takes ownership of one weak count. In case a [`null`] is passed, a dangling [`Weak`] is
1378 /// The pointer must represent one valid weak count. In other words, it must point to `T` which
1379 /// is or *was* managed by an [`Arc`] and the weak count of that [`Arc`] must not have reached
1380 /// 0. It is allowed for the strong count to be 0.
1385 /// #![feature(weak_into_raw)]
1387 /// use std::sync::{Arc, Weak};
1389 /// let strong = Arc::new("hello".to_owned());
1391 /// let raw_1 = Arc::downgrade(&strong).into_raw();
1392 /// let raw_2 = Arc::downgrade(&strong).into_raw();
1394 /// assert_eq!(2, Arc::weak_count(&strong));
1396 /// assert_eq!("hello", &*unsafe { Weak::from_raw(raw_1) }.upgrade().unwrap());
1397 /// assert_eq!(1, Arc::weak_count(&strong));
1401 /// // Decrement the last weak count.
1402 /// assert!(unsafe { Weak::from_raw(raw_2) }.upgrade().is_none());
1405 /// [`null`]: ../../std/ptr/fn.null.html
1406 /// [`into_raw`]: struct.Weak.html#method.into_raw
1407 /// [`upgrade`]: struct.Weak.html#method.upgrade
1408 /// [`Weak`]: struct.Weak.html
1409 /// [`Arc`]: struct.Arc.html
1410 #[unstable(feature = "weak_into_raw", issue = "60728")]
1411 pub unsafe fn from_raw(ptr: *const T) -> Self {
1415 // See Arc::from_raw for details
1416 let offset = data_offset(ptr);
1417 let fake_ptr = ptr as *mut ArcInner<T>;
1418 let ptr = set_data_ptr(fake_ptr, (ptr as *mut u8).offset(-offset));
1420 ptr: NonNull::new(ptr).expect("Invalid pointer passed to from_raw"),
1426 impl<T: ?Sized> Weak<T> {
1427 /// Attempts to upgrade the `Weak` pointer to an [`Arc`], extending
1428 /// the lifetime of the value if successful.
1430 /// Returns [`None`] if the value has since been dropped.
1432 /// [`Arc`]: struct.Arc.html
1433 /// [`None`]: ../../std/option/enum.Option.html#variant.None
1438 /// use std::sync::Arc;
1440 /// let five = Arc::new(5);
1442 /// let weak_five = Arc::downgrade(&five);
1444 /// let strong_five: Option<Arc<_>> = weak_five.upgrade();
1445 /// assert!(strong_five.is_some());
1447 /// // Destroy all strong pointers.
1448 /// drop(strong_five);
1451 /// assert!(weak_five.upgrade().is_none());
1453 #[stable(feature = "arc_weak", since = "1.4.0")]
1454 pub fn upgrade(&self) -> Option<Arc<T>> {
1455 // We use a CAS loop to increment the strong count instead of a
1456 // fetch_add because once the count hits 0 it must never be above 0.
1457 let inner = self.inner()?;
1459 // Relaxed load because any write of 0 that we can observe
1460 // leaves the field in a permanently zero state (so a
1461 // "stale" read of 0 is fine), and any other value is
1462 // confirmed via the CAS below.
1463 let mut n = inner.strong.load(Relaxed);
1470 // See comments in `Arc::clone` for why we do this (for `mem::forget`).
1471 if n > MAX_REFCOUNT {
1477 // Relaxed is valid for the same reason it is on Arc's Clone impl
1478 match inner.strong.compare_exchange_weak(n, n + 1, Relaxed, Relaxed) {
1479 Ok(_) => return Some(Arc::from_inner(self.ptr)), // null checked above
1480 Err(old) => n = old,
1485 /// Gets the number of strong (`Arc`) pointers pointing to this value.
1487 /// If `self` was created using [`Weak::new`], this will return 0.
1489 /// [`Weak::new`]: #method.new
1490 #[unstable(feature = "weak_counts", issue = "57977")]
1491 pub fn strong_count(&self) -> usize {
1492 if let Some(inner) = self.inner() {
1493 inner.strong.load(SeqCst)
1499 /// Gets an approximation of the number of `Weak` pointers pointing to this
1502 /// If `self` was created using [`Weak::new`], this will return 0. If not,
1503 /// the returned value is at least 1, since `self` still points to the
1508 /// Due to implementation details, the returned value can be off by 1 in
1509 /// either direction when other threads are manipulating any `Arc`s or
1510 /// `Weak`s pointing to the same value.
1512 /// [`Weak::new`]: #method.new
1513 #[unstable(feature = "weak_counts", issue = "57977")]
1514 pub fn weak_count(&self) -> Option<usize> {
1515 // Due to the implicit weak pointer added when any strong pointers are
1516 // around, we cannot implement `weak_count` correctly since it
1517 // necessarily requires accessing the strong count and weak count in an
1518 // unsynchronized fashion. So this version is a bit racy.
1519 self.inner().map(|inner| {
1520 let strong = inner.strong.load(SeqCst);
1521 let weak = inner.weak.load(SeqCst);
1523 // If the last `Arc` has *just* been dropped, it might not yet
1524 // have removed the implicit weak count, so the value we get
1525 // here might be 1 too high.
1528 // As long as there's still at least 1 `Arc` around, subtract
1529 // the implicit weak pointer.
1530 // Note that the last `Arc` might get dropped between the 2
1531 // loads we do above, removing the implicit weak pointer. This
1532 // means that the value might be 1 too low here. In order to not
1533 // return 0 here (which would happen if we're the only weak
1534 // pointer), we guard against that specifically.
1535 cmp::max(1, weak - 1)
1540 /// Returns `None` when the pointer is dangling and there is no allocated `ArcInner`,
1541 /// (i.e., when this `Weak` was created by `Weak::new`).
1543 fn inner(&self) -> Option<&ArcInner<T>> {
1544 if is_dangling(self.ptr) {
1547 Some(unsafe { self.ptr.as_ref() })
1551 /// Returns `true` if the two `Weak`s point to the same value (not just
1552 /// values that compare as equal), or if both don't point to any value
1553 /// (because they were created with `Weak::new()`).
1557 /// Since this compares pointers it means that `Weak::new()` will equal each
1558 /// other, even though they don't point to any value.
1563 /// use std::sync::Arc;
1565 /// let first_rc = Arc::new(5);
1566 /// let first = Arc::downgrade(&first_rc);
1567 /// let second = Arc::downgrade(&first_rc);
1569 /// assert!(first.ptr_eq(&second));
1571 /// let third_rc = Arc::new(5);
1572 /// let third = Arc::downgrade(&third_rc);
1574 /// assert!(!first.ptr_eq(&third));
1577 /// Comparing `Weak::new`.
1580 /// use std::sync::{Arc, Weak};
1582 /// let first = Weak::new();
1583 /// let second = Weak::new();
1584 /// assert!(first.ptr_eq(&second));
1586 /// let third_rc = Arc::new(());
1587 /// let third = Arc::downgrade(&third_rc);
1588 /// assert!(!first.ptr_eq(&third));
1591 #[stable(feature = "weak_ptr_eq", since = "1.39.0")]
1592 pub fn ptr_eq(&self, other: &Self) -> bool {
1593 self.ptr.as_ptr() == other.ptr.as_ptr()
1597 #[stable(feature = "arc_weak", since = "1.4.0")]
1598 impl<T: ?Sized> Clone for Weak<T> {
1599 /// Makes a clone of the `Weak` pointer that points to the same value.
1604 /// use std::sync::{Arc, Weak};
1606 /// let weak_five = Arc::downgrade(&Arc::new(5));
1608 /// let _ = Weak::clone(&weak_five);
1611 fn clone(&self) -> Weak<T> {
1612 let inner = if let Some(inner) = self.inner() {
1615 return Weak { ptr: self.ptr };
1617 // See comments in Arc::clone() for why this is relaxed. This can use a
1618 // fetch_add (ignoring the lock) because the weak count is only locked
1619 // where are *no other* weak pointers in existence. (So we can't be
1620 // running this code in that case).
1621 let old_size = inner.weak.fetch_add(1, Relaxed);
1623 // See comments in Arc::clone() for why we do this (for mem::forget).
1624 if old_size > MAX_REFCOUNT {
1630 return Weak { ptr: self.ptr };
1634 #[stable(feature = "downgraded_weak", since = "1.10.0")]
1635 impl<T> Default for Weak<T> {
1636 /// Constructs a new `Weak<T>`, without allocating memory.
1637 /// Calling [`upgrade`] on the return value always
1640 /// [`None`]: ../../std/option/enum.Option.html#variant.None
1641 /// [`upgrade`]: ../../std/sync/struct.Weak.html#method.upgrade
1646 /// use std::sync::Weak;
1648 /// let empty: Weak<i64> = Default::default();
1649 /// assert!(empty.upgrade().is_none());
1651 fn default() -> Weak<T> {
1656 #[stable(feature = "arc_weak", since = "1.4.0")]
1657 impl<T: ?Sized> Drop for Weak<T> {
1658 /// Drops the `Weak` pointer.
1663 /// use std::sync::{Arc, Weak};
1667 /// impl Drop for Foo {
1668 /// fn drop(&mut self) {
1669 /// println!("dropped!");
1673 /// let foo = Arc::new(Foo);
1674 /// let weak_foo = Arc::downgrade(&foo);
1675 /// let other_weak_foo = Weak::clone(&weak_foo);
1677 /// drop(weak_foo); // Doesn't print anything
1678 /// drop(foo); // Prints "dropped!"
1680 /// assert!(other_weak_foo.upgrade().is_none());
1682 fn drop(&mut self) {
1683 // If we find out that we were the last weak pointer, then its time to
1684 // deallocate the data entirely. See the discussion in Arc::drop() about
1685 // the memory orderings
1687 // It's not necessary to check for the locked state here, because the
1688 // weak count can only be locked if there was precisely one weak ref,
1689 // meaning that drop could only subsequently run ON that remaining weak
1690 // ref, which can only happen after the lock is released.
1691 let inner = if let Some(inner) = self.inner() {
1697 if inner.weak.fetch_sub(1, Release) == 1 {
1698 atomic::fence(Acquire);
1700 Global.dealloc(self.ptr.cast(), Layout::for_value(self.ptr.as_ref()))
1706 #[stable(feature = "rust1", since = "1.0.0")]
1707 trait ArcEqIdent<T: ?Sized + PartialEq> {
1708 fn eq(&self, other: &Arc<T>) -> bool;
1709 fn ne(&self, other: &Arc<T>) -> bool;
1712 #[stable(feature = "rust1", since = "1.0.0")]
1713 impl<T: ?Sized + PartialEq> ArcEqIdent<T> for Arc<T> {
1715 default fn eq(&self, other: &Arc<T>) -> bool {
1719 default fn ne(&self, other: &Arc<T>) -> bool {
1724 /// We're doing this specialization here, and not as a more general optimization on `&T`, because it
1725 /// would otherwise add a cost to all equality checks on refs. We assume that `Arc`s are used to
1726 /// store large values, that are slow to clone, but also heavy to check for equality, causing this
1727 /// cost to pay off more easily. It's also more likely to have two `Arc` clones, that point to
1728 /// the same value, than two `&T`s.
1729 #[stable(feature = "rust1", since = "1.0.0")]
1730 impl<T: ?Sized + Eq> ArcEqIdent<T> for Arc<T> {
1732 fn eq(&self, other: &Arc<T>) -> bool {
1733 Arc::ptr_eq(self, other) || **self == **other
1737 fn ne(&self, other: &Arc<T>) -> bool {
1738 !Arc::ptr_eq(self, other) && **self != **other
1742 #[stable(feature = "rust1", since = "1.0.0")]
1743 impl<T: ?Sized + PartialEq> PartialEq for Arc<T> {
1744 /// Equality for two `Arc`s.
1746 /// Two `Arc`s are equal if their inner values are equal.
1748 /// If `T` also implements `Eq`, two `Arc`s that point to the same value are
1754 /// use std::sync::Arc;
1756 /// let five = Arc::new(5);
1758 /// assert!(five == Arc::new(5));
1761 fn eq(&self, other: &Arc<T>) -> bool {
1762 ArcEqIdent::eq(self, other)
1765 /// Inequality for two `Arc`s.
1767 /// Two `Arc`s are unequal if their inner values are unequal.
1769 /// If `T` also implements `Eq`, two `Arc`s that point to the same value are
1775 /// use std::sync::Arc;
1777 /// let five = Arc::new(5);
1779 /// assert!(five != Arc::new(6));
1782 fn ne(&self, other: &Arc<T>) -> bool {
1783 ArcEqIdent::ne(self, other)
1787 #[stable(feature = "rust1", since = "1.0.0")]
1788 impl<T: ?Sized + PartialOrd> PartialOrd for Arc<T> {
1789 /// Partial comparison for two `Arc`s.
1791 /// The two are compared by calling `partial_cmp()` on their inner values.
1796 /// use std::sync::Arc;
1797 /// use std::cmp::Ordering;
1799 /// let five = Arc::new(5);
1801 /// assert_eq!(Some(Ordering::Less), five.partial_cmp(&Arc::new(6)));
1803 fn partial_cmp(&self, other: &Arc<T>) -> Option<Ordering> {
1804 (**self).partial_cmp(&**other)
1807 /// Less-than comparison for two `Arc`s.
1809 /// The two are compared by calling `<` on their inner values.
1814 /// use std::sync::Arc;
1816 /// let five = Arc::new(5);
1818 /// assert!(five < Arc::new(6));
1820 fn lt(&self, other: &Arc<T>) -> bool {
1821 *(*self) < *(*other)
1824 /// 'Less than or equal to' comparison for two `Arc`s.
1826 /// The two are compared by calling `<=` on their inner values.
1831 /// use std::sync::Arc;
1833 /// let five = Arc::new(5);
1835 /// assert!(five <= Arc::new(5));
1837 fn le(&self, other: &Arc<T>) -> bool {
1838 *(*self) <= *(*other)
1841 /// Greater-than comparison for two `Arc`s.
1843 /// The two are compared by calling `>` on their inner values.
1848 /// use std::sync::Arc;
1850 /// let five = Arc::new(5);
1852 /// assert!(five > Arc::new(4));
1854 fn gt(&self, other: &Arc<T>) -> bool {
1855 *(*self) > *(*other)
1858 /// 'Greater than or equal to' comparison for two `Arc`s.
1860 /// The two are compared by calling `>=` on their inner values.
1865 /// use std::sync::Arc;
1867 /// let five = Arc::new(5);
1869 /// assert!(five >= Arc::new(5));
1871 fn ge(&self, other: &Arc<T>) -> bool {
1872 *(*self) >= *(*other)
1875 #[stable(feature = "rust1", since = "1.0.0")]
1876 impl<T: ?Sized + Ord> Ord for Arc<T> {
1877 /// Comparison for two `Arc`s.
1879 /// The two are compared by calling `cmp()` on their inner values.
1884 /// use std::sync::Arc;
1885 /// use std::cmp::Ordering;
1887 /// let five = Arc::new(5);
1889 /// assert_eq!(Ordering::Less, five.cmp(&Arc::new(6)));
1891 fn cmp(&self, other: &Arc<T>) -> Ordering {
1892 (**self).cmp(&**other)
1895 #[stable(feature = "rust1", since = "1.0.0")]
1896 impl<T: ?Sized + Eq> Eq for Arc<T> {}
1898 #[stable(feature = "rust1", since = "1.0.0")]
1899 impl<T: ?Sized + fmt::Display> fmt::Display for Arc<T> {
1900 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
1901 fmt::Display::fmt(&**self, f)
1905 #[stable(feature = "rust1", since = "1.0.0")]
1906 impl<T: ?Sized + fmt::Debug> fmt::Debug for Arc<T> {
1907 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
1908 fmt::Debug::fmt(&**self, f)
1912 #[stable(feature = "rust1", since = "1.0.0")]
1913 impl<T: ?Sized> fmt::Pointer for Arc<T> {
1914 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
1915 fmt::Pointer::fmt(&(&**self as *const T), f)
1919 #[stable(feature = "rust1", since = "1.0.0")]
1920 impl<T: Default> Default for Arc<T> {
1921 /// Creates a new `Arc<T>`, with the `Default` value for `T`.
1926 /// use std::sync::Arc;
1928 /// let x: Arc<i32> = Default::default();
1929 /// assert_eq!(*x, 0);
1931 fn default() -> Arc<T> {
1932 Arc::new(Default::default())
1936 #[stable(feature = "rust1", since = "1.0.0")]
1937 impl<T: ?Sized + Hash> Hash for Arc<T> {
1938 fn hash<H: Hasher>(&self, state: &mut H) {
1939 (**self).hash(state)
1943 #[stable(feature = "from_for_ptrs", since = "1.6.0")]
1944 impl<T> From<T> for Arc<T> {
1945 fn from(t: T) -> Self {
1950 #[stable(feature = "shared_from_slice", since = "1.21.0")]
1951 impl<T: Clone> From<&[T]> for Arc<[T]> {
1953 fn from(v: &[T]) -> Arc<[T]> {
1954 <Self as ArcFromSlice<T>>::from_slice(v)
1958 #[stable(feature = "shared_from_slice", since = "1.21.0")]
1959 impl From<&str> for Arc<str> {
1961 fn from(v: &str) -> Arc<str> {
1962 let arc = Arc::<[u8]>::from(v.as_bytes());
1963 unsafe { Arc::from_raw(Arc::into_raw(arc) as *const str) }
1967 #[stable(feature = "shared_from_slice", since = "1.21.0")]
1968 impl From<String> for Arc<str> {
1970 fn from(v: String) -> Arc<str> {
1975 #[stable(feature = "shared_from_slice", since = "1.21.0")]
1976 impl<T: ?Sized> From<Box<T>> for Arc<T> {
1978 fn from(v: Box<T>) -> Arc<T> {
1983 #[stable(feature = "shared_from_slice", since = "1.21.0")]
1984 impl<T> From<Vec<T>> for Arc<[T]> {
1986 fn from(mut v: Vec<T>) -> Arc<[T]> {
1988 let arc = Arc::copy_from_slice(&v);
1990 // Allow the Vec to free its memory, but not destroy its contents
1998 #[unstable(feature = "boxed_slice_try_from", issue = "0")]
1999 impl<T, const N: usize> TryFrom<Arc<[T]>> for Arc<[T; N]>
2001 [T; N]: LengthAtMost32,
2003 type Error = Arc<[T]>;
2005 fn try_from(boxed_slice: Arc<[T]>) -> Result<Self, Self::Error> {
2006 if boxed_slice.len() == N {
2007 Ok(unsafe { Arc::from_raw(Arc::into_raw(boxed_slice) as *mut [T; N]) })
2014 #[stable(feature = "shared_from_iter", since = "1.37.0")]
2015 impl<T> iter::FromIterator<T> for Arc<[T]> {
2016 /// Takes each element in the `Iterator` and collects it into an `Arc<[T]>`.
2018 /// # Performance characteristics
2020 /// ## The general case
2022 /// In the general case, collecting into `Arc<[T]>` is done by first
2023 /// collecting into a `Vec<T>`. That is, when writing the following:
2026 /// # use std::sync::Arc;
2027 /// let evens: Arc<[u8]> = (0..10).filter(|&x| x % 2 == 0).collect();
2028 /// # assert_eq!(&*evens, &[0, 2, 4, 6, 8]);
2031 /// this behaves as if we wrote:
2034 /// # use std::sync::Arc;
2035 /// let evens: Arc<[u8]> = (0..10).filter(|&x| x % 2 == 0)
2036 /// .collect::<Vec<_>>() // The first set of allocations happens here.
2037 /// .into(); // A second allocation for `Arc<[T]>` happens here.
2038 /// # assert_eq!(&*evens, &[0, 2, 4, 6, 8]);
2041 /// This will allocate as many times as needed for constructing the `Vec<T>`
2042 /// and then it will allocate once for turning the `Vec<T>` into the `Arc<[T]>`.
2044 /// ## Iterators of known length
2046 /// When your `Iterator` implements `TrustedLen` and is of an exact size,
2047 /// a single allocation will be made for the `Arc<[T]>`. For example:
2050 /// # use std::sync::Arc;
2051 /// let evens: Arc<[u8]> = (0..10).collect(); // Just a single allocation happens here.
2052 /// # assert_eq!(&*evens, &*(0..10).collect::<Vec<_>>());
2054 fn from_iter<I: iter::IntoIterator<Item = T>>(iter: I) -> Self {
2055 ArcFromIter::from_iter(iter.into_iter())
2059 /// Specialization trait used for collecting into `Arc<[T]>`.
2060 trait ArcFromIter<T, I> {
2061 fn from_iter(iter: I) -> Self;
2064 impl<T, I: Iterator<Item = T>> ArcFromIter<T, I> for Arc<[T]> {
2065 default fn from_iter(iter: I) -> Self {
2066 iter.collect::<Vec<T>>().into()
2070 impl<T, I: iter::TrustedLen<Item = T>> ArcFromIter<T, I> for Arc<[T]> {
2071 default fn from_iter(iter: I) -> Self {
2072 // This is the case for a `TrustedLen` iterator.
2073 let (low, high) = iter.size_hint();
2074 if let Some(high) = high {
2077 "TrustedLen iterator's size hint is not exact: {:?}",
2082 // SAFETY: We need to ensure that the iterator has an exact length and we have.
2083 Arc::from_iter_exact(iter, low)
2086 // Fall back to normal implementation.
2087 iter.collect::<Vec<T>>().into()
2092 impl<'a, T: 'a + Clone> ArcFromIter<&'a T, slice::Iter<'a, T>> for Arc<[T]> {
2093 fn from_iter(iter: slice::Iter<'a, T>) -> Self {
2094 // Delegate to `impl<T: Clone> From<&[T]> for Arc<[T]>`.
2096 // In the case that `T: Copy`, we get to use `ptr::copy_nonoverlapping`
2097 // which is even more performant.
2099 // In the fall-back case we have `T: Clone`. This is still better
2100 // than the `TrustedLen` implementation as slices have a known length
2101 // and so we get to avoid calling `size_hint` and avoid the branching.
2102 iter.as_slice().into()
2106 #[stable(feature = "rust1", since = "1.0.0")]
2107 impl<T: ?Sized> borrow::Borrow<T> for Arc<T> {
2108 fn borrow(&self) -> &T {
2113 #[stable(since = "1.5.0", feature = "smart_ptr_as_ref")]
2114 impl<T: ?Sized> AsRef<T> for Arc<T> {
2115 fn as_ref(&self) -> &T {
2120 #[stable(feature = "pin", since = "1.33.0")]
2121 impl<T: ?Sized> Unpin for Arc<T> { }
2123 /// Computes the offset of the data field within `ArcInner`.
2124 unsafe fn data_offset<T: ?Sized>(ptr: *const T) -> isize {
2125 // Align the unsized value to the end of the `ArcInner`.
2126 // Because it is `?Sized`, it will always be the last field in memory.
2127 data_offset_align(align_of_val(&*ptr))
2130 /// Computes the offset of the data field within `ArcInner`.
2132 /// Unlike [`data_offset`], this doesn't need the pointer, but it works only on `T: Sized`.
2133 fn data_offset_sized<T>() -> isize {
2134 data_offset_align(align_of::<T>())
2138 fn data_offset_align(align: usize) -> isize {
2139 let layout = Layout::new::<ArcInner<()>>();
2140 (layout.size() + layout.padding_needed_for(align)) as isize