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
11 use core::cmp::Ordering;
12 use core::convert::{From, TryFrom};
14 use core::hash::{Hash, Hasher};
15 use core::intrinsics::abort;
17 use core::marker::{PhantomData, Unpin, Unsize};
18 use core::mem::{self, align_of_val, size_of_val};
19 use core::ops::{CoerceUnsized, Deref, DispatchFromDyn, Receiver};
21 use core::ptr::{self, NonNull};
22 use core::slice::from_raw_parts_mut;
23 use core::sync::atomic;
24 use core::sync::atomic::Ordering::{Acquire, Relaxed, Release, SeqCst};
26 use crate::alloc::{box_free, handle_alloc_error, AllocRef, Global, Layout};
27 use crate::borrow::{Cow, ToOwned};
28 use crate::boxed::Box;
29 use crate::rc::is_dangling;
30 use crate::string::String;
36 /// A soft limit on the amount of references that may be made to an `Arc`.
38 /// Going above this limit will abort your program (although not
39 /// necessarily) at _exactly_ `MAX_REFCOUNT + 1` references.
40 const MAX_REFCOUNT: usize = (isize::MAX) as usize;
42 #[cfg(not(sanitize = "thread"))]
43 macro_rules! acquire {
45 atomic::fence(Acquire)
49 // ThreadSanitizer does not support memory fences. To avoid false positive
50 // reports in Arc / Weak implementation use atomic loads for synchronization
52 #[cfg(sanitize = "thread")]
53 macro_rules! acquire {
59 /// A thread-safe reference-counting pointer. 'Arc' stands for 'Atomically
60 /// Reference Counted'.
62 /// The type `Arc<T>` provides shared ownership of a value of type `T`,
63 /// allocated in the heap. Invoking [`clone`][clone] on `Arc` produces
64 /// a new `Arc` instance, which points to the same allocation on the heap as the
65 /// source `Arc`, while increasing a reference count. When the last `Arc`
66 /// pointer to a given allocation is destroyed, the value stored in that allocation (often
67 /// referred to as "inner value") is also dropped.
69 /// Shared references in Rust disallow mutation by default, and `Arc` is no
70 /// exception: you cannot generally obtain a mutable reference to something
71 /// inside an `Arc`. If you need to mutate through an `Arc`, use
72 /// [`Mutex`][mutex], [`RwLock`][rwlock], or one of the [`Atomic`][atomic]
77 /// Unlike [`Rc<T>`], `Arc<T>` uses atomic operations for its reference
78 /// counting. This means that it is thread-safe. The disadvantage is that
79 /// atomic operations are more expensive than ordinary memory accesses. If you
80 /// are not sharing reference-counted allocations between threads, consider using
81 /// [`Rc<T>`] for lower overhead. [`Rc<T>`] is a safe default, because the
82 /// compiler will catch any attempt to send an [`Rc<T>`] between threads.
83 /// However, a library might choose `Arc<T>` in order to give library consumers
86 /// `Arc<T>` will implement [`Send`] and [`Sync`] as long as the `T` implements
87 /// [`Send`] and [`Sync`]. Why can't you put a non-thread-safe type `T` in an
88 /// `Arc<T>` to make it thread-safe? This may be a bit counter-intuitive at
89 /// first: after all, isn't the point of `Arc<T>` thread safety? The key is
90 /// this: `Arc<T>` makes it thread safe to have multiple ownership of the same
91 /// data, but it doesn't add thread safety to its data. Consider
92 /// `Arc<`[`RefCell<T>`]`>`. [`RefCell<T>`] isn't [`Sync`], and if `Arc<T>` was always
93 /// [`Send`], `Arc<`[`RefCell<T>`]`>` would be as well. But then we'd have a problem:
94 /// [`RefCell<T>`] is not thread safe; it keeps track of the borrowing count using
95 /// non-atomic operations.
97 /// In the end, this means that you may need to pair `Arc<T>` with some sort of
98 /// [`std::sync`] type, usually [`Mutex<T>`][mutex].
100 /// ## Breaking cycles with `Weak`
102 /// The [`downgrade`][downgrade] method can be used to create a non-owning
103 /// [`Weak`][weak] pointer. A [`Weak`][weak] pointer can be [`upgrade`][upgrade]d
104 /// to an `Arc`, but this will return [`None`] if the value stored in the allocation has
105 /// already been dropped. In other words, `Weak` pointers do not keep the value
106 /// inside the allocation alive; however, they *do* keep the allocation
107 /// (the backing store for the value) alive.
109 /// A cycle between `Arc` pointers will never be deallocated. For this reason,
110 /// [`Weak`][weak] is used to break cycles. For example, a tree could have
111 /// strong `Arc` pointers from parent nodes to children, and [`Weak`][weak]
112 /// pointers from children back to their parents.
114 /// # Cloning references
116 /// Creating a new reference from an existing reference counted pointer is done using the
117 /// `Clone` trait implemented for [`Arc<T>`][arc] and [`Weak<T>`][weak].
120 /// use std::sync::Arc;
121 /// let foo = Arc::new(vec![1.0, 2.0, 3.0]);
122 /// // The two syntaxes below are equivalent.
123 /// let a = foo.clone();
124 /// let b = Arc::clone(&foo);
125 /// // a, b, and foo are all Arcs that point to the same memory location
128 /// ## `Deref` behavior
130 /// `Arc<T>` automatically dereferences to `T` (via the [`Deref`][deref] trait),
131 /// so you can call `T`'s methods on a value of type `Arc<T>`. To avoid name
132 /// clashes with `T`'s methods, the methods of `Arc<T>` itself are associated
133 /// functions, called using function-like syntax:
136 /// use std::sync::Arc;
137 /// let my_arc = Arc::new(());
139 /// Arc::downgrade(&my_arc);
142 /// [`Weak<T>`][weak] does not auto-dereference to `T`, because the inner value may have
143 /// already been dropped.
145 /// [arc]: struct.Arc.html
146 /// [weak]: struct.Weak.html
147 /// [`Rc<T>`]: ../../std/rc/struct.Rc.html
148 /// [clone]: ../../std/clone/trait.Clone.html#tymethod.clone
149 /// [mutex]: ../../std/sync/struct.Mutex.html
150 /// [rwlock]: ../../std/sync/struct.RwLock.html
151 /// [atomic]: ../../std/sync/atomic/index.html
152 /// [`Send`]: ../../std/marker/trait.Send.html
153 /// [`Sync`]: ../../std/marker/trait.Sync.html
154 /// [deref]: ../../std/ops/trait.Deref.html
155 /// [downgrade]: struct.Arc.html#method.downgrade
156 /// [upgrade]: struct.Weak.html#method.upgrade
157 /// [`None`]: ../../std/option/enum.Option.html#variant.None
158 /// [`RefCell<T>`]: ../../std/cell/struct.RefCell.html
159 /// [`std::sync`]: ../../std/sync/index.html
160 /// [`Arc::clone(&from)`]: #method.clone
164 /// Sharing some immutable data between threads:
166 // Note that we **do not** run these tests here. The windows builders get super
167 // unhappy if a thread outlives the main thread and then exits at the same time
168 // (something deadlocks) so we just avoid this entirely by not running these
171 /// use std::sync::Arc;
174 /// let five = Arc::new(5);
177 /// let five = Arc::clone(&five);
179 /// thread::spawn(move || {
180 /// println!("{:?}", five);
185 /// Sharing a mutable [`AtomicUsize`]:
187 /// [`AtomicUsize`]: ../../std/sync/atomic/struct.AtomicUsize.html
190 /// use std::sync::Arc;
191 /// use std::sync::atomic::{AtomicUsize, Ordering};
194 /// let val = Arc::new(AtomicUsize::new(5));
197 /// let val = Arc::clone(&val);
199 /// thread::spawn(move || {
200 /// let v = val.fetch_add(1, Ordering::SeqCst);
201 /// println!("{:?}", v);
206 /// See the [`rc` documentation][rc_examples] for more examples of reference
207 /// counting in general.
209 /// [rc_examples]: ../../std/rc/index.html#examples
210 #[cfg_attr(not(test), rustc_diagnostic_item = "Arc")]
211 #[stable(feature = "rust1", since = "1.0.0")]
212 pub struct Arc<T: ?Sized> {
213 ptr: NonNull<ArcInner<T>>,
214 phantom: PhantomData<ArcInner<T>>,
217 #[stable(feature = "rust1", since = "1.0.0")]
218 unsafe impl<T: ?Sized + Sync + Send> Send for Arc<T> {}
219 #[stable(feature = "rust1", since = "1.0.0")]
220 unsafe impl<T: ?Sized + Sync + Send> Sync for Arc<T> {}
222 #[unstable(feature = "coerce_unsized", issue = "27732")]
223 impl<T: ?Sized + Unsize<U>, U: ?Sized> CoerceUnsized<Arc<U>> for Arc<T> {}
225 #[unstable(feature = "dispatch_from_dyn", issue = "none")]
226 impl<T: ?Sized + Unsize<U>, U: ?Sized> DispatchFromDyn<Arc<U>> for Arc<T> {}
228 impl<T: ?Sized> Arc<T> {
229 fn from_inner(ptr: NonNull<ArcInner<T>>) -> Self {
230 Self { ptr, phantom: PhantomData }
233 unsafe fn from_ptr(ptr: *mut ArcInner<T>) -> Self {
234 unsafe { Self::from_inner(NonNull::new_unchecked(ptr)) }
238 /// `Weak` is a version of [`Arc`] that holds a non-owning reference to the
239 /// managed allocation. The allocation is accessed by calling [`upgrade`] on the `Weak`
240 /// pointer, which returns an [`Option`]`<`[`Arc`]`<T>>`.
242 /// Since a `Weak` reference does not count towards ownership, it will not
243 /// prevent the value stored in the allocation from being dropped, and `Weak` itself makes no
244 /// guarantees about the value still being present. Thus it may return [`None`]
245 /// when [`upgrade`]d. Note however that a `Weak` reference *does* prevent the allocation
246 /// itself (the backing store) from being deallocated.
248 /// A `Weak` pointer is useful for keeping a temporary reference to the allocation
249 /// managed by [`Arc`] without preventing its inner value from being dropped. It is also used to
250 /// prevent circular references between [`Arc`] pointers, since mutual owning references
251 /// would never allow either [`Arc`] to be dropped. For example, a tree could
252 /// have strong [`Arc`] pointers from parent nodes to children, and `Weak`
253 /// pointers from children back to their parents.
255 /// The typical way to obtain a `Weak` pointer is to call [`Arc::downgrade`].
257 /// [`Arc`]: struct.Arc.html
258 /// [`Arc::downgrade`]: struct.Arc.html#method.downgrade
259 /// [`upgrade`]: struct.Weak.html#method.upgrade
260 /// [`Option`]: ../../std/option/enum.Option.html
261 /// [`None`]: ../../std/option/enum.Option.html#variant.None
262 #[stable(feature = "arc_weak", since = "1.4.0")]
263 pub struct Weak<T: ?Sized> {
264 // This is a `NonNull` to allow optimizing the size of this type in enums,
265 // but it is not necessarily a valid pointer.
266 // `Weak::new` sets this to `usize::MAX` so that it doesn’t need
267 // to allocate space on the heap. That's not a value a real pointer
268 // will ever have because RcBox has alignment at least 2.
269 // This is only possible when `T: Sized`; unsized `T` never dangle.
270 ptr: NonNull<ArcInner<T>>,
273 #[stable(feature = "arc_weak", since = "1.4.0")]
274 unsafe impl<T: ?Sized + Sync + Send> Send for Weak<T> {}
275 #[stable(feature = "arc_weak", since = "1.4.0")]
276 unsafe impl<T: ?Sized + Sync + Send> Sync for Weak<T> {}
278 #[unstable(feature = "coerce_unsized", issue = "27732")]
279 impl<T: ?Sized + Unsize<U>, U: ?Sized> CoerceUnsized<Weak<U>> for Weak<T> {}
280 #[unstable(feature = "dispatch_from_dyn", issue = "none")]
281 impl<T: ?Sized + Unsize<U>, U: ?Sized> DispatchFromDyn<Weak<U>> for Weak<T> {}
283 #[stable(feature = "arc_weak", since = "1.4.0")]
284 impl<T: ?Sized + fmt::Debug> fmt::Debug for Weak<T> {
285 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
290 // This is repr(C) to future-proof against possible field-reordering, which
291 // would interfere with otherwise safe [into|from]_raw() of transmutable
294 struct ArcInner<T: ?Sized> {
295 strong: atomic::AtomicUsize,
297 // the value usize::MAX acts as a sentinel for temporarily "locking" the
298 // ability to upgrade weak pointers or downgrade strong ones; this is used
299 // to avoid races in `make_mut` and `get_mut`.
300 weak: atomic::AtomicUsize,
305 unsafe impl<T: ?Sized + Sync + Send> Send for ArcInner<T> {}
306 unsafe impl<T: ?Sized + Sync + Send> Sync for ArcInner<T> {}
309 /// Constructs a new `Arc<T>`.
314 /// use std::sync::Arc;
316 /// let five = Arc::new(5);
319 #[stable(feature = "rust1", since = "1.0.0")]
320 pub fn new(data: T) -> Arc<T> {
321 // Start the weak pointer count as 1 which is the weak pointer that's
322 // held by all the strong pointers (kinda), see std/rc.rs for more info
323 let x: Box<_> = box ArcInner {
324 strong: atomic::AtomicUsize::new(1),
325 weak: atomic::AtomicUsize::new(1),
328 Self::from_inner(Box::leak(x).into())
331 /// Constructs a new `Arc` with uninitialized contents.
336 /// #![feature(new_uninit)]
337 /// #![feature(get_mut_unchecked)]
339 /// use std::sync::Arc;
341 /// let mut five = Arc::<u32>::new_uninit();
343 /// let five = unsafe {
344 /// // Deferred initialization:
345 /// Arc::get_mut_unchecked(&mut five).as_mut_ptr().write(5);
347 /// five.assume_init()
350 /// assert_eq!(*five, 5)
352 #[unstable(feature = "new_uninit", issue = "63291")]
353 pub fn new_uninit() -> Arc<mem::MaybeUninit<T>> {
355 Arc::from_ptr(Arc::allocate_for_layout(Layout::new::<T>(), |mem| {
356 mem as *mut ArcInner<mem::MaybeUninit<T>>
361 /// Constructs a new `Arc` with uninitialized contents, with the memory
362 /// being filled with `0` bytes.
364 /// See [`MaybeUninit::zeroed`][zeroed] for examples of correct and incorrect usage
370 /// #![feature(new_uninit)]
372 /// use std::sync::Arc;
374 /// let zero = Arc::<u32>::new_zeroed();
375 /// let zero = unsafe { zero.assume_init() };
377 /// assert_eq!(*zero, 0)
380 /// [zeroed]: ../../std/mem/union.MaybeUninit.html#method.zeroed
381 #[unstable(feature = "new_uninit", issue = "63291")]
382 pub fn new_zeroed() -> Arc<mem::MaybeUninit<T>> {
384 let mut uninit = Self::new_uninit();
385 ptr::write_bytes::<T>(Arc::get_mut_unchecked(&mut uninit).as_mut_ptr(), 0, 1);
390 /// Constructs a new `Pin<Arc<T>>`. If `T` does not implement `Unpin`, then
391 /// `data` will be pinned in memory and unable to be moved.
392 #[stable(feature = "pin", since = "1.33.0")]
393 pub fn pin(data: T) -> Pin<Arc<T>> {
394 unsafe { Pin::new_unchecked(Arc::new(data)) }
397 /// Returns the inner value, if the `Arc` has exactly one strong reference.
399 /// Otherwise, an [`Err`][result] is returned with the same `Arc` that was
402 /// This will succeed even if there are outstanding weak references.
404 /// [result]: ../../std/result/enum.Result.html
409 /// use std::sync::Arc;
411 /// let x = Arc::new(3);
412 /// assert_eq!(Arc::try_unwrap(x), Ok(3));
414 /// let x = Arc::new(4);
415 /// let _y = Arc::clone(&x);
416 /// assert_eq!(*Arc::try_unwrap(x).unwrap_err(), 4);
419 #[stable(feature = "arc_unique", since = "1.4.0")]
420 pub fn try_unwrap(this: Self) -> Result<T, Self> {
421 if this.inner().strong.compare_exchange(1, 0, Relaxed, Relaxed).is_err() {
425 acquire!(this.inner().strong);
428 let elem = ptr::read(&this.ptr.as_ref().data);
430 // Make a weak pointer to clean up the implicit strong-weak reference
431 let _weak = Weak { ptr: this.ptr };
440 /// Constructs a new reference-counted slice with uninitialized contents.
445 /// #![feature(new_uninit)]
446 /// #![feature(get_mut_unchecked)]
448 /// use std::sync::Arc;
450 /// let mut values = Arc::<[u32]>::new_uninit_slice(3);
452 /// let values = unsafe {
453 /// // Deferred initialization:
454 /// Arc::get_mut_unchecked(&mut values)[0].as_mut_ptr().write(1);
455 /// Arc::get_mut_unchecked(&mut values)[1].as_mut_ptr().write(2);
456 /// Arc::get_mut_unchecked(&mut values)[2].as_mut_ptr().write(3);
458 /// values.assume_init()
461 /// assert_eq!(*values, [1, 2, 3])
463 #[unstable(feature = "new_uninit", issue = "63291")]
464 pub fn new_uninit_slice(len: usize) -> Arc<[mem::MaybeUninit<T>]> {
465 unsafe { Arc::from_ptr(Arc::allocate_for_slice(len)) }
469 impl<T> Arc<mem::MaybeUninit<T>> {
470 /// Converts to `Arc<T>`.
474 /// As with [`MaybeUninit::assume_init`],
475 /// it is up to the caller to guarantee that the inner value
476 /// really is in an initialized state.
477 /// Calling this when the content is not yet fully initialized
478 /// causes immediate undefined behavior.
480 /// [`MaybeUninit::assume_init`]: ../../std/mem/union.MaybeUninit.html#method.assume_init
485 /// #![feature(new_uninit)]
486 /// #![feature(get_mut_unchecked)]
488 /// use std::sync::Arc;
490 /// let mut five = Arc::<u32>::new_uninit();
492 /// let five = unsafe {
493 /// // Deferred initialization:
494 /// Arc::get_mut_unchecked(&mut five).as_mut_ptr().write(5);
496 /// five.assume_init()
499 /// assert_eq!(*five, 5)
501 #[unstable(feature = "new_uninit", issue = "63291")]
503 pub unsafe fn assume_init(self) -> Arc<T> {
504 Arc::from_inner(mem::ManuallyDrop::new(self).ptr.cast())
508 impl<T> Arc<[mem::MaybeUninit<T>]> {
509 /// Converts to `Arc<[T]>`.
513 /// As with [`MaybeUninit::assume_init`],
514 /// it is up to the caller to guarantee that the inner value
515 /// really is in an initialized state.
516 /// Calling this when the content is not yet fully initialized
517 /// causes immediate undefined behavior.
519 /// [`MaybeUninit::assume_init`]: ../../std/mem/union.MaybeUninit.html#method.assume_init
524 /// #![feature(new_uninit)]
525 /// #![feature(get_mut_unchecked)]
527 /// use std::sync::Arc;
529 /// let mut values = Arc::<[u32]>::new_uninit_slice(3);
531 /// let values = unsafe {
532 /// // Deferred initialization:
533 /// Arc::get_mut_unchecked(&mut values)[0].as_mut_ptr().write(1);
534 /// Arc::get_mut_unchecked(&mut values)[1].as_mut_ptr().write(2);
535 /// Arc::get_mut_unchecked(&mut values)[2].as_mut_ptr().write(3);
537 /// values.assume_init()
540 /// assert_eq!(*values, [1, 2, 3])
542 #[unstable(feature = "new_uninit", issue = "63291")]
544 pub unsafe fn assume_init(self) -> Arc<[T]> {
545 unsafe { Arc::from_ptr(mem::ManuallyDrop::new(self).ptr.as_ptr() as _) }
549 impl<T: ?Sized> Arc<T> {
550 /// Consumes the `Arc`, returning the wrapped pointer.
552 /// To avoid a memory leak the pointer must be converted back to an `Arc` using
553 /// [`Arc::from_raw`][from_raw].
555 /// [from_raw]: struct.Arc.html#method.from_raw
560 /// use std::sync::Arc;
562 /// let x = Arc::new("hello".to_owned());
563 /// let x_ptr = Arc::into_raw(x);
564 /// assert_eq!(unsafe { &*x_ptr }, "hello");
566 #[stable(feature = "rc_raw", since = "1.17.0")]
567 pub fn into_raw(this: Self) -> *const T {
568 let ptr = Self::as_ptr(&this);
573 /// Provides a raw pointer to the data.
575 /// The counts are not affected in any way and the `Arc` is not consumed. The pointer is valid for
576 /// as long as there are strong counts in the `Arc`.
581 /// use std::sync::Arc;
583 /// let x = Arc::new("hello".to_owned());
584 /// let y = Arc::clone(&x);
585 /// let x_ptr = Arc::as_ptr(&x);
586 /// assert_eq!(x_ptr, Arc::as_ptr(&y));
587 /// assert_eq!(unsafe { &*x_ptr }, "hello");
589 #[stable(feature = "rc_as_ptr", since = "1.45.0")]
590 pub fn as_ptr(this: &Self) -> *const T {
591 let ptr: *mut ArcInner<T> = NonNull::as_ptr(this.ptr);
593 // SAFETY: This cannot go through Deref::deref or RcBoxPtr::inner because
594 // this is required to retain raw/mut provenance such that e.g. `get_mut` can
595 // write through the pointer after the Rc is recovered through `from_raw`.
596 unsafe { &raw const (*ptr).data }
599 /// Constructs an `Arc<T>` from a raw pointer.
601 /// The raw pointer must have been previously returned by a call to
602 /// [`Arc<U>::into_raw`][into_raw] where `U` must have the same size and
603 /// alignment as `T`. This is trivially true if `U` is `T`.
604 /// Note that if `U` is not `T` but has the same size and alignment, this is
605 /// basically like transmuting references of different types. See
606 /// [`mem::transmute`][transmute] for more information on what
607 /// restrictions apply in this case.
609 /// The user of `from_raw` has to make sure a specific value of `T` is only
612 /// This function is unsafe because improper use may lead to memory unsafety,
613 /// even if the returned `Arc<T>` is never accessed.
615 /// [into_raw]: struct.Arc.html#method.into_raw
616 /// [transmute]: ../../std/mem/fn.transmute.html
621 /// use std::sync::Arc;
623 /// let x = Arc::new("hello".to_owned());
624 /// let x_ptr = Arc::into_raw(x);
627 /// // Convert back to an `Arc` to prevent leak.
628 /// let x = Arc::from_raw(x_ptr);
629 /// assert_eq!(&*x, "hello");
631 /// // Further calls to `Arc::from_raw(x_ptr)` would be memory-unsafe.
634 /// // The memory was freed when `x` went out of scope above, so `x_ptr` is now dangling!
636 #[stable(feature = "rc_raw", since = "1.17.0")]
637 pub unsafe fn from_raw(ptr: *const T) -> Self {
639 let offset = data_offset(ptr);
641 // Reverse the offset to find the original ArcInner.
642 let fake_ptr = ptr as *mut ArcInner<T>;
643 let arc_ptr = set_data_ptr(fake_ptr, (ptr as *mut u8).offset(-offset));
645 Self::from_ptr(arc_ptr)
649 /// Creates a new [`Weak`][weak] pointer to this allocation.
651 /// [weak]: struct.Weak.html
656 /// use std::sync::Arc;
658 /// let five = Arc::new(5);
660 /// let weak_five = Arc::downgrade(&five);
662 #[stable(feature = "arc_weak", since = "1.4.0")]
663 pub fn downgrade(this: &Self) -> Weak<T> {
664 // This Relaxed is OK because we're checking the value in the CAS
666 let mut cur = this.inner().weak.load(Relaxed);
669 // check if the weak counter is currently "locked"; if so, spin.
670 if cur == usize::MAX {
671 cur = this.inner().weak.load(Relaxed);
675 // NOTE: this code currently ignores the possibility of overflow
676 // into usize::MAX; in general both Rc and Arc need to be adjusted
677 // to deal with overflow.
679 // Unlike with Clone(), we need this to be an Acquire read to
680 // synchronize with the write coming from `is_unique`, so that the
681 // events prior to that write happen before this read.
682 match this.inner().weak.compare_exchange_weak(cur, cur + 1, Acquire, Relaxed) {
684 // Make sure we do not create a dangling Weak
685 debug_assert!(!is_dangling(this.ptr));
686 return Weak { ptr: this.ptr };
688 Err(old) => cur = old,
693 /// Gets the number of [`Weak`][weak] pointers to this allocation.
695 /// [weak]: struct.Weak.html
699 /// This method by itself is safe, but using it correctly requires extra care.
700 /// Another thread can change the weak count at any time,
701 /// including potentially between calling this method and acting on the result.
706 /// use std::sync::Arc;
708 /// let five = Arc::new(5);
709 /// let _weak_five = Arc::downgrade(&five);
711 /// // This assertion is deterministic because we haven't shared
712 /// // the `Arc` or `Weak` between threads.
713 /// assert_eq!(1, Arc::weak_count(&five));
716 #[stable(feature = "arc_counts", since = "1.15.0")]
717 pub fn weak_count(this: &Self) -> usize {
718 let cnt = this.inner().weak.load(SeqCst);
719 // If the weak count is currently locked, the value of the
720 // count was 0 just before taking the lock.
721 if cnt == usize::MAX { 0 } else { cnt - 1 }
724 /// Gets the number of strong (`Arc`) pointers to this allocation.
728 /// This method by itself is safe, but using it correctly requires extra care.
729 /// Another thread can change the strong count at any time,
730 /// including potentially between calling this method and acting on the result.
735 /// use std::sync::Arc;
737 /// let five = Arc::new(5);
738 /// let _also_five = Arc::clone(&five);
740 /// // This assertion is deterministic because we haven't shared
741 /// // the `Arc` between threads.
742 /// assert_eq!(2, Arc::strong_count(&five));
745 #[stable(feature = "arc_counts", since = "1.15.0")]
746 pub fn strong_count(this: &Self) -> usize {
747 this.inner().strong.load(SeqCst)
750 /// Increments the strong reference count on the `Arc<T>` associated with the
751 /// provided pointer by one.
755 /// The pointer must have been obtained through `Arc::into_raw`, and the
756 /// associated `Arc` instance must be valid (i.e. the strong count must be at
757 /// least 1) for the duration of this method.
762 /// #![feature(arc_mutate_strong_count)]
764 /// use std::sync::Arc;
766 /// let five = Arc::new(5);
769 /// let ptr = Arc::into_raw(five);
770 /// Arc::incr_strong_count(ptr);
772 /// // This assertion is deterministic because we haven't shared
773 /// // the `Arc` between threads.
774 /// let five = Arc::from_raw(ptr);
775 /// assert_eq!(2, Arc::strong_count(&five));
779 #[unstable(feature = "arc_mutate_strong_count", issue = "71983")]
780 pub unsafe fn incr_strong_count(ptr: *const T) {
781 // Retain Arc, but don't touch refcount by wrapping in ManuallyDrop
782 let arc = unsafe { mem::ManuallyDrop::new(Arc::<T>::from_raw(ptr)) };
783 // Now increase refcount, but don't drop new refcount either
784 let _arc_clone: mem::ManuallyDrop<_> = arc.clone();
787 /// Decrements the strong reference count on the `Arc<T>` associated with the
788 /// provided pointer by one.
792 /// The pointer must have been obtained through `Arc::into_raw`, and the
793 /// associated `Arc` instance must be valid (i.e. the strong count must be at
794 /// least 1) when invoking this method. This method can be used to release the final
795 /// `Arc` and backing storage, but **should not** be called after the final `Arc` has been
801 /// #![feature(arc_mutate_strong_count)]
803 /// use std::sync::Arc;
805 /// let five = Arc::new(5);
808 /// let ptr = Arc::into_raw(five);
809 /// Arc::incr_strong_count(ptr);
811 /// // Those assertions are deterministic because we haven't shared
812 /// // the `Arc` between threads.
813 /// let five = Arc::from_raw(ptr);
814 /// assert_eq!(2, Arc::strong_count(&five));
815 /// Arc::decr_strong_count(ptr);
816 /// assert_eq!(1, Arc::strong_count(&five));
820 #[unstable(feature = "arc_mutate_strong_count", issue = "71983")]
821 pub unsafe fn decr_strong_count(ptr: *const T) {
822 unsafe { mem::drop(Arc::from_raw(ptr)) };
826 fn inner(&self) -> &ArcInner<T> {
827 // This unsafety is ok because while this arc is alive we're guaranteed
828 // that the inner pointer is valid. Furthermore, we know that the
829 // `ArcInner` structure itself is `Sync` because the inner data is
830 // `Sync` as well, so we're ok loaning out an immutable pointer to these
832 unsafe { self.ptr.as_ref() }
835 // Non-inlined part of `drop`.
837 unsafe fn drop_slow(&mut self) {
838 // Destroy the data at this time, even though we may not free the box
839 // allocation itself (there may still be weak pointers lying around).
840 unsafe { ptr::drop_in_place(Self::get_mut_unchecked(self)) };
842 // Drop the weak ref collectively held by all strong references
843 drop(Weak { ptr: self.ptr });
847 #[stable(feature = "ptr_eq", since = "1.17.0")]
848 /// Returns `true` if the two `Arc`s point to the same allocation
849 /// (in a vein similar to [`ptr::eq`]).
854 /// use std::sync::Arc;
856 /// let five = Arc::new(5);
857 /// let same_five = Arc::clone(&five);
858 /// let other_five = Arc::new(5);
860 /// assert!(Arc::ptr_eq(&five, &same_five));
861 /// assert!(!Arc::ptr_eq(&five, &other_five));
864 /// [`ptr::eq`]: ../../std/ptr/fn.eq.html
865 pub fn ptr_eq(this: &Self, other: &Self) -> bool {
866 this.ptr.as_ptr() == other.ptr.as_ptr()
870 impl<T: ?Sized> Arc<T> {
871 /// Allocates an `ArcInner<T>` with sufficient space for
872 /// a possibly-unsized inner value where the value has the layout provided.
874 /// The function `mem_to_arcinner` is called with the data pointer
875 /// and must return back a (potentially fat)-pointer for the `ArcInner<T>`.
876 unsafe fn allocate_for_layout(
877 value_layout: Layout,
878 mem_to_arcinner: impl FnOnce(*mut u8) -> *mut ArcInner<T>,
879 ) -> *mut ArcInner<T> {
880 // Calculate layout using the given value layout.
881 // Previously, layout was calculated on the expression
882 // `&*(ptr as *const ArcInner<T>)`, but this created a misaligned
883 // reference (see #54908).
884 let layout = Layout::new::<ArcInner<()>>().extend(value_layout).unwrap().0.pad_to_align();
886 let ptr = Global.alloc(layout).unwrap_or_else(|_| handle_alloc_error(layout));
888 // Initialize the ArcInner
889 let inner = mem_to_arcinner(ptr.as_non_null_ptr().as_ptr());
890 debug_assert_eq!(unsafe { Layout::for_value(&*inner) }, layout);
893 ptr::write(&mut (*inner).strong, atomic::AtomicUsize::new(1));
894 ptr::write(&mut (*inner).weak, atomic::AtomicUsize::new(1));
900 /// Allocates an `ArcInner<T>` with sufficient space for an unsized inner value.
901 unsafe fn allocate_for_ptr(ptr: *const T) -> *mut ArcInner<T> {
902 // Allocate for the `ArcInner<T>` using the given value.
904 Self::allocate_for_layout(Layout::for_value(&*ptr), |mem| {
905 set_data_ptr(ptr as *mut T, mem) as *mut ArcInner<T>
910 fn from_box(v: Box<T>) -> Arc<T> {
912 let box_unique = Box::into_unique(v);
913 let bptr = box_unique.as_ptr();
915 let value_size = size_of_val(&*bptr);
916 let ptr = Self::allocate_for_ptr(bptr);
918 // Copy value as bytes
919 ptr::copy_nonoverlapping(
920 bptr as *const T as *const u8,
921 &mut (*ptr).data as *mut _ as *mut u8,
925 // Free the allocation without dropping its contents
926 box_free(box_unique);
934 /// Allocates an `ArcInner<[T]>` with the given length.
935 unsafe fn allocate_for_slice(len: usize) -> *mut ArcInner<[T]> {
937 Self::allocate_for_layout(Layout::array::<T>(len).unwrap(), |mem| {
938 ptr::slice_from_raw_parts_mut(mem as *mut T, len) as *mut ArcInner<[T]>
944 /// Sets the data pointer of a `?Sized` raw pointer.
946 /// For a slice/trait object, this sets the `data` field and leaves the rest
947 /// unchanged. For a sized raw pointer, this simply sets the pointer.
948 unsafe fn set_data_ptr<T: ?Sized, U>(mut ptr: *mut T, data: *mut U) -> *mut T {
950 ptr::write(&mut ptr as *mut _ as *mut *mut u8, data as *mut u8);
956 /// Copy elements from slice into newly allocated Arc<\[T\]>
958 /// Unsafe because the caller must either take ownership or bind `T: Copy`.
959 unsafe fn copy_from_slice(v: &[T]) -> Arc<[T]> {
961 let ptr = Self::allocate_for_slice(v.len());
963 ptr::copy_nonoverlapping(v.as_ptr(), &mut (*ptr).data as *mut [T] as *mut T, v.len());
969 /// Constructs an `Arc<[T]>` from an iterator known to be of a certain size.
971 /// Behavior is undefined should the size be wrong.
972 unsafe fn from_iter_exact(iter: impl iter::Iterator<Item = T>, len: usize) -> Arc<[T]> {
973 // Panic guard while cloning T elements.
974 // In the event of a panic, elements that have been written
975 // into the new ArcInner will be dropped, then the memory freed.
983 impl<T> Drop for Guard<T> {
986 let slice = from_raw_parts_mut(self.elems, self.n_elems);
987 ptr::drop_in_place(slice);
989 Global.dealloc(self.mem, self.layout);
995 let ptr = Self::allocate_for_slice(len);
997 let mem = ptr as *mut _ as *mut u8;
998 let layout = Layout::for_value(&*ptr);
1000 // Pointer to first element
1001 let elems = &mut (*ptr).data as *mut [T] as *mut T;
1003 let mut guard = Guard { mem: NonNull::new_unchecked(mem), elems, layout, n_elems: 0 };
1005 for (i, item) in iter.enumerate() {
1006 ptr::write(elems.add(i), item);
1010 // All clear. Forget the guard so it doesn't free the new ArcInner.
1018 /// Specialization trait used for `From<&[T]>`.
1019 trait ArcFromSlice<T> {
1020 fn from_slice(slice: &[T]) -> Self;
1023 impl<T: Clone> ArcFromSlice<T> for Arc<[T]> {
1025 default fn from_slice(v: &[T]) -> Self {
1026 unsafe { Self::from_iter_exact(v.iter().cloned(), v.len()) }
1030 impl<T: Copy> ArcFromSlice<T> for Arc<[T]> {
1032 fn from_slice(v: &[T]) -> Self {
1033 unsafe { Arc::copy_from_slice(v) }
1037 #[stable(feature = "rust1", since = "1.0.0")]
1038 impl<T: ?Sized> Clone for Arc<T> {
1039 /// Makes a clone of the `Arc` pointer.
1041 /// This creates another pointer to the same allocation, increasing the
1042 /// strong reference count.
1047 /// use std::sync::Arc;
1049 /// let five = Arc::new(5);
1051 /// let _ = Arc::clone(&five);
1054 fn clone(&self) -> Arc<T> {
1055 // Using a relaxed ordering is alright here, as knowledge of the
1056 // original reference prevents other threads from erroneously deleting
1059 // As explained in the [Boost documentation][1], Increasing the
1060 // reference counter can always be done with memory_order_relaxed: New
1061 // references to an object can only be formed from an existing
1062 // reference, and passing an existing reference from one thread to
1063 // another must already provide any required synchronization.
1065 // [1]: (www.boost.org/doc/libs/1_55_0/doc/html/atomic/usage_examples.html)
1066 let old_size = self.inner().strong.fetch_add(1, Relaxed);
1068 // However we need to guard against massive refcounts in case someone
1069 // is `mem::forget`ing Arcs. If we don't do this the count can overflow
1070 // and users will use-after free. We racily saturate to `isize::MAX` on
1071 // the assumption that there aren't ~2 billion threads incrementing
1072 // the reference count at once. This branch will never be taken in
1073 // any realistic program.
1075 // We abort because such a program is incredibly degenerate, and we
1076 // don't care to support it.
1077 if old_size > MAX_REFCOUNT {
1081 Self::from_inner(self.ptr)
1085 #[stable(feature = "rust1", since = "1.0.0")]
1086 impl<T: ?Sized> Deref for Arc<T> {
1090 fn deref(&self) -> &T {
1095 #[unstable(feature = "receiver_trait", issue = "none")]
1096 impl<T: ?Sized> Receiver for Arc<T> {}
1098 impl<T: Clone> Arc<T> {
1099 /// Makes a mutable reference into the given `Arc`.
1101 /// If there are other `Arc` or [`Weak`][weak] pointers to the same allocation,
1102 /// then `make_mut` will create a new allocation and invoke [`clone`][clone] on the inner value
1103 /// to ensure unique ownership. This is also referred to as clone-on-write.
1105 /// Note that this differs from the behavior of [`Rc::make_mut`] which disassociates
1106 /// any remaining `Weak` pointers.
1108 /// See also [`get_mut`][get_mut], which will fail rather than cloning.
1110 /// [weak]: struct.Weak.html
1111 /// [clone]: ../../std/clone/trait.Clone.html#tymethod.clone
1112 /// [get_mut]: struct.Arc.html#method.get_mut
1113 /// [`Rc::make_mut`]: ../rc/struct.Rc.html#method.make_mut
1118 /// use std::sync::Arc;
1120 /// let mut data = Arc::new(5);
1122 /// *Arc::make_mut(&mut data) += 1; // Won't clone anything
1123 /// let mut other_data = Arc::clone(&data); // Won't clone inner data
1124 /// *Arc::make_mut(&mut data) += 1; // Clones inner data
1125 /// *Arc::make_mut(&mut data) += 1; // Won't clone anything
1126 /// *Arc::make_mut(&mut other_data) *= 2; // Won't clone anything
1128 /// // Now `data` and `other_data` point to different allocations.
1129 /// assert_eq!(*data, 8);
1130 /// assert_eq!(*other_data, 12);
1133 #[stable(feature = "arc_unique", since = "1.4.0")]
1134 pub fn make_mut(this: &mut Self) -> &mut T {
1135 // Note that we hold both a strong reference and a weak reference.
1136 // Thus, releasing our strong reference only will not, by itself, cause
1137 // the memory to be deallocated.
1139 // Use Acquire to ensure that we see any writes to `weak` that happen
1140 // before release writes (i.e., decrements) to `strong`. Since we hold a
1141 // weak count, there's no chance the ArcInner itself could be
1143 if this.inner().strong.compare_exchange(1, 0, Acquire, Relaxed).is_err() {
1144 // Another strong pointer exists; clone
1145 *this = Arc::new((**this).clone());
1146 } else if this.inner().weak.load(Relaxed) != 1 {
1147 // Relaxed suffices in the above because this is fundamentally an
1148 // optimization: we are always racing with weak pointers being
1149 // dropped. Worst case, we end up allocated a new Arc unnecessarily.
1151 // We removed the last strong ref, but there are additional weak
1152 // refs remaining. We'll move the contents to a new Arc, and
1153 // invalidate the other weak refs.
1155 // Note that it is not possible for the read of `weak` to yield
1156 // usize::MAX (i.e., locked), since the weak count can only be
1157 // locked by a thread with a strong reference.
1159 // Materialize our own implicit weak pointer, so that it can clean
1160 // up the ArcInner as needed.
1161 let weak = Weak { ptr: this.ptr };
1163 // mark the data itself as already deallocated
1165 // there is no data race in the implicit write caused by `read`
1166 // here (due to zeroing) because data is no longer accessed by
1167 // other threads (due to there being no more strong refs at this
1169 let mut swap = Arc::new(ptr::read(&weak.ptr.as_ref().data));
1170 mem::swap(this, &mut swap);
1174 // We were the sole reference of either kind; bump back up the
1175 // strong ref count.
1176 this.inner().strong.store(1, Release);
1179 // As with `get_mut()`, the unsafety is ok because our reference was
1180 // either unique to begin with, or became one upon cloning the contents.
1181 unsafe { Self::get_mut_unchecked(this) }
1185 impl<T: ?Sized> Arc<T> {
1186 /// Returns a mutable reference into the given `Arc`, if there are
1187 /// no other `Arc` or [`Weak`][weak] pointers to the same allocation.
1189 /// Returns [`None`][option] otherwise, because it is not safe to
1190 /// mutate a shared value.
1192 /// See also [`make_mut`][make_mut], which will [`clone`][clone]
1193 /// the inner value when there are other pointers.
1195 /// [weak]: struct.Weak.html
1196 /// [option]: ../../std/option/enum.Option.html
1197 /// [make_mut]: struct.Arc.html#method.make_mut
1198 /// [clone]: ../../std/clone/trait.Clone.html#tymethod.clone
1203 /// use std::sync::Arc;
1205 /// let mut x = Arc::new(3);
1206 /// *Arc::get_mut(&mut x).unwrap() = 4;
1207 /// assert_eq!(*x, 4);
1209 /// let _y = Arc::clone(&x);
1210 /// assert!(Arc::get_mut(&mut x).is_none());
1213 #[stable(feature = "arc_unique", since = "1.4.0")]
1214 pub fn get_mut(this: &mut Self) -> Option<&mut T> {
1215 if this.is_unique() {
1216 // This unsafety is ok because we're guaranteed that the pointer
1217 // returned is the *only* pointer that will ever be returned to T. Our
1218 // reference count is guaranteed to be 1 at this point, and we required
1219 // the Arc itself to be `mut`, so we're returning the only possible
1220 // reference to the inner data.
1221 unsafe { Some(Arc::get_mut_unchecked(this)) }
1227 /// Returns a mutable reference into the given `Arc`,
1228 /// without any check.
1230 /// See also [`get_mut`], which is safe and does appropriate checks.
1232 /// [`get_mut`]: struct.Arc.html#method.get_mut
1236 /// Any other `Arc` or [`Weak`] pointers to the same allocation must not be dereferenced
1237 /// for the duration of the returned borrow.
1238 /// This is trivially the case if no such pointers exist,
1239 /// for example immediately after `Arc::new`.
1244 /// #![feature(get_mut_unchecked)]
1246 /// use std::sync::Arc;
1248 /// let mut x = Arc::new(String::new());
1250 /// Arc::get_mut_unchecked(&mut x).push_str("foo")
1252 /// assert_eq!(*x, "foo");
1255 #[unstable(feature = "get_mut_unchecked", issue = "63292")]
1256 pub unsafe fn get_mut_unchecked(this: &mut Self) -> &mut T {
1257 // We are careful to *not* create a reference covering the "count" fields, as
1258 // this would alias with concurrent access to the reference counts (e.g. by `Weak`).
1259 unsafe { &mut (*this.ptr.as_ptr()).data }
1262 /// Determine whether this is the unique reference (including weak refs) to
1263 /// the underlying data.
1265 /// Note that this requires locking the weak ref count.
1266 fn is_unique(&mut self) -> bool {
1267 // lock the weak pointer count if we appear to be the sole weak pointer
1270 // The acquire label here ensures a happens-before relationship with any
1271 // writes to `strong` (in particular in `Weak::upgrade`) prior to decrements
1272 // of the `weak` count (via `Weak::drop`, which uses release). If the upgraded
1273 // weak ref was never dropped, the CAS here will fail so we do not care to synchronize.
1274 if self.inner().weak.compare_exchange(1, usize::MAX, Acquire, Relaxed).is_ok() {
1275 // This needs to be an `Acquire` to synchronize with the decrement of the `strong`
1276 // counter in `drop` -- the only access that happens when any but the last reference
1277 // is being dropped.
1278 let unique = self.inner().strong.load(Acquire) == 1;
1280 // The release write here synchronizes with a read in `downgrade`,
1281 // effectively preventing the above read of `strong` from happening
1283 self.inner().weak.store(1, Release); // release the lock
1291 #[stable(feature = "rust1", since = "1.0.0")]
1292 unsafe impl<#[may_dangle] T: ?Sized> Drop for Arc<T> {
1293 /// Drops the `Arc`.
1295 /// This will decrement the strong reference count. If the strong reference
1296 /// count reaches zero then the only other references (if any) are
1297 /// [`Weak`], so we `drop` the inner value.
1302 /// use std::sync::Arc;
1306 /// impl Drop for Foo {
1307 /// fn drop(&mut self) {
1308 /// println!("dropped!");
1312 /// let foo = Arc::new(Foo);
1313 /// let foo2 = Arc::clone(&foo);
1315 /// drop(foo); // Doesn't print anything
1316 /// drop(foo2); // Prints "dropped!"
1319 /// [`Weak`]: ../../std/sync/struct.Weak.html
1321 fn drop(&mut self) {
1322 // Because `fetch_sub` is already atomic, we do not need to synchronize
1323 // with other threads unless we are going to delete the object. This
1324 // same logic applies to the below `fetch_sub` to the `weak` count.
1325 if self.inner().strong.fetch_sub(1, Release) != 1 {
1329 // This fence is needed to prevent reordering of use of the data and
1330 // deletion of the data. Because it is marked `Release`, the decreasing
1331 // of the reference count synchronizes with this `Acquire` fence. This
1332 // means that use of the data happens before decreasing the reference
1333 // count, which happens before this fence, which happens before the
1334 // deletion of the data.
1336 // As explained in the [Boost documentation][1],
1338 // > It is important to enforce any possible access to the object in one
1339 // > thread (through an existing reference) to *happen before* deleting
1340 // > the object in a different thread. This is achieved by a "release"
1341 // > operation after dropping a reference (any access to the object
1342 // > through this reference must obviously happened before), and an
1343 // > "acquire" operation before deleting the object.
1345 // In particular, while the contents of an Arc are usually immutable, it's
1346 // possible to have interior writes to something like a Mutex<T>. Since a
1347 // Mutex is not acquired when it is deleted, we can't rely on its
1348 // synchronization logic to make writes in thread A visible to a destructor
1349 // running in thread B.
1351 // Also note that the Acquire fence here could probably be replaced with an
1352 // Acquire load, which could improve performance in highly-contended
1353 // situations. See [2].
1355 // [1]: (www.boost.org/doc/libs/1_55_0/doc/html/atomic/usage_examples.html)
1356 // [2]: (https://github.com/rust-lang/rust/pull/41714)
1357 acquire!(self.inner().strong);
1365 impl Arc<dyn Any + Send + Sync> {
1367 #[stable(feature = "rc_downcast", since = "1.29.0")]
1368 /// Attempt to downcast the `Arc<dyn Any + Send + Sync>` to a concrete type.
1373 /// use std::any::Any;
1374 /// use std::sync::Arc;
1376 /// fn print_if_string(value: Arc<dyn Any + Send + Sync>) {
1377 /// if let Ok(string) = value.downcast::<String>() {
1378 /// println!("String ({}): {}", string.len(), string);
1382 /// let my_string = "Hello World".to_string();
1383 /// print_if_string(Arc::new(my_string));
1384 /// print_if_string(Arc::new(0i8));
1386 pub fn downcast<T>(self) -> Result<Arc<T>, Self>
1388 T: Any + Send + Sync + 'static,
1390 if (*self).is::<T>() {
1391 let ptr = self.ptr.cast::<ArcInner<T>>();
1393 Ok(Arc::from_inner(ptr))
1401 /// Constructs a new `Weak<T>`, without allocating any memory.
1402 /// Calling [`upgrade`] on the return value always gives [`None`].
1404 /// [`upgrade`]: struct.Weak.html#method.upgrade
1405 /// [`None`]: ../../std/option/enum.Option.html#variant.None
1410 /// use std::sync::Weak;
1412 /// let empty: Weak<i64> = Weak::new();
1413 /// assert!(empty.upgrade().is_none());
1415 #[stable(feature = "downgraded_weak", since = "1.10.0")]
1416 pub fn new() -> Weak<T> {
1417 Weak { ptr: NonNull::new(usize::MAX as *mut ArcInner<T>).expect("MAX is not 0") }
1420 /// Returns a raw pointer to the object `T` pointed to by this `Weak<T>`.
1422 /// The pointer is valid only if there are some strong references. The pointer may be dangling,
1423 /// unaligned or even [`null`] otherwise.
1428 /// use std::sync::Arc;
1431 /// let strong = Arc::new("hello".to_owned());
1432 /// let weak = Arc::downgrade(&strong);
1433 /// // Both point to the same object
1434 /// assert!(ptr::eq(&*strong, weak.as_ptr()));
1435 /// // The strong here keeps it alive, so we can still access the object.
1436 /// assert_eq!("hello", unsafe { &*weak.as_ptr() });
1439 /// // But not any more. We can do weak.as_ptr(), but accessing the pointer would lead to
1440 /// // undefined behaviour.
1441 /// // assert_eq!("hello", unsafe { &*weak.as_ptr() });
1444 /// [`null`]: ../../std/ptr/fn.null.html
1445 #[stable(feature = "weak_into_raw", since = "1.45.0")]
1446 pub fn as_ptr(&self) -> *const T {
1447 let ptr: *mut ArcInner<T> = NonNull::as_ptr(self.ptr);
1449 // SAFETY: we must offset the pointer manually, and said pointer may be
1450 // a dangling weak (usize::MAX) if T is sized. data_offset is safe to call,
1451 // because we know that a pointer to unsized T was derived from a real
1452 // unsized T, as dangling weaks are only created for sized T. wrapping_offset
1453 // is used so that we can use the same code path for the non-dangling
1454 // unsized case and the potentially dangling sized case.
1456 let offset = data_offset(ptr as *mut T);
1457 set_data_ptr(ptr as *mut T, (ptr as *mut u8).wrapping_offset(offset))
1461 /// Consumes the `Weak<T>` and turns it into a raw pointer.
1463 /// This converts the weak pointer into a raw pointer, while still preserving the ownership of
1464 /// one weak reference (the weak count is not modified by this operation). It can be turned
1465 /// back into the `Weak<T>` with [`from_raw`].
1467 /// The same restrictions of accessing the target of the pointer as with
1468 /// [`as_ptr`] apply.
1473 /// use std::sync::{Arc, Weak};
1475 /// let strong = Arc::new("hello".to_owned());
1476 /// let weak = Arc::downgrade(&strong);
1477 /// let raw = weak.into_raw();
1479 /// assert_eq!(1, Arc::weak_count(&strong));
1480 /// assert_eq!("hello", unsafe { &*raw });
1482 /// drop(unsafe { Weak::from_raw(raw) });
1483 /// assert_eq!(0, Arc::weak_count(&strong));
1486 /// [`from_raw`]: struct.Weak.html#method.from_raw
1487 /// [`as_ptr`]: struct.Weak.html#method.as_ptr
1488 #[stable(feature = "weak_into_raw", since = "1.45.0")]
1489 pub fn into_raw(self) -> *const T {
1490 let result = self.as_ptr();
1495 /// Converts a raw pointer previously created by [`into_raw`] back into `Weak<T>`.
1497 /// This can be used to safely get a strong reference (by calling [`upgrade`]
1498 /// later) or to deallocate the weak count by dropping the `Weak<T>`.
1500 /// It takes ownership of one weak reference (with the exception of pointers created by [`new`],
1501 /// as these don't own anything; the method still works on them).
1505 /// The pointer must have originated from the [`into_raw`] and must still own its potential
1508 /// It is allowed for the strong count to be 0 at the time of calling this. Nevertheless, this
1509 /// takes ownership of one weak reference currently represented as a raw pointer (the weak
1510 /// count is not modified by this operation) and therefore it must be paired with a previous
1511 /// call to [`into_raw`].
1515 /// use std::sync::{Arc, Weak};
1517 /// let strong = Arc::new("hello".to_owned());
1519 /// let raw_1 = Arc::downgrade(&strong).into_raw();
1520 /// let raw_2 = Arc::downgrade(&strong).into_raw();
1522 /// assert_eq!(2, Arc::weak_count(&strong));
1524 /// assert_eq!("hello", &*unsafe { Weak::from_raw(raw_1) }.upgrade().unwrap());
1525 /// assert_eq!(1, Arc::weak_count(&strong));
1529 /// // Decrement the last weak count.
1530 /// assert!(unsafe { Weak::from_raw(raw_2) }.upgrade().is_none());
1533 /// [`new`]: struct.Weak.html#method.new
1534 /// [`into_raw`]: struct.Weak.html#method.into_raw
1535 /// [`upgrade`]: struct.Weak.html#method.upgrade
1536 /// [`Weak`]: struct.Weak.html
1537 /// [`Arc`]: struct.Arc.html
1538 /// [`forget`]: ../../std/mem/fn.forget.html
1539 #[stable(feature = "weak_into_raw", since = "1.45.0")]
1540 pub unsafe fn from_raw(ptr: *const T) -> Self {
1544 // See Arc::from_raw for details
1546 let offset = data_offset(ptr);
1547 let fake_ptr = ptr as *mut ArcInner<T>;
1548 let ptr = set_data_ptr(fake_ptr, (ptr as *mut u8).offset(-offset));
1549 Weak { ptr: NonNull::new(ptr).expect("Invalid pointer passed to from_raw") }
1555 /// Helper type to allow accessing the reference counts without
1556 /// making any assertions about the data field.
1557 struct WeakInner<'a> {
1558 weak: &'a atomic::AtomicUsize,
1559 strong: &'a atomic::AtomicUsize,
1562 impl<T: ?Sized> Weak<T> {
1563 /// Attempts to upgrade the `Weak` pointer to an [`Arc`], delaying
1564 /// dropping of the inner value if successful.
1566 /// Returns [`None`] if the inner value has since been dropped.
1568 /// [`Arc`]: struct.Arc.html
1569 /// [`None`]: ../../std/option/enum.Option.html#variant.None
1574 /// use std::sync::Arc;
1576 /// let five = Arc::new(5);
1578 /// let weak_five = Arc::downgrade(&five);
1580 /// let strong_five: Option<Arc<_>> = weak_five.upgrade();
1581 /// assert!(strong_five.is_some());
1583 /// // Destroy all strong pointers.
1584 /// drop(strong_five);
1587 /// assert!(weak_five.upgrade().is_none());
1589 #[stable(feature = "arc_weak", since = "1.4.0")]
1590 pub fn upgrade(&self) -> Option<Arc<T>> {
1591 // We use a CAS loop to increment the strong count instead of a
1592 // fetch_add because once the count hits 0 it must never be above 0.
1593 let inner = self.inner()?;
1595 // Relaxed load because any write of 0 that we can observe
1596 // leaves the field in a permanently zero state (so a
1597 // "stale" read of 0 is fine), and any other value is
1598 // confirmed via the CAS below.
1599 let mut n = inner.strong.load(Relaxed);
1606 // See comments in `Arc::clone` for why we do this (for `mem::forget`).
1607 if n > MAX_REFCOUNT {
1611 // Relaxed is valid for the same reason it is on Arc's Clone impl
1612 match inner.strong.compare_exchange_weak(n, n + 1, Relaxed, Relaxed) {
1613 Ok(_) => return Some(Arc::from_inner(self.ptr)), // null checked above
1614 Err(old) => n = old,
1619 /// Gets the number of strong (`Arc`) pointers pointing to this allocation.
1621 /// If `self` was created using [`Weak::new`], this will return 0.
1623 /// [`Weak::new`]: #method.new
1624 #[stable(feature = "weak_counts", since = "1.41.0")]
1625 pub fn strong_count(&self) -> usize {
1626 if let Some(inner) = self.inner() { inner.strong.load(SeqCst) } else { 0 }
1629 /// Gets an approximation of the number of `Weak` pointers pointing to this
1632 /// If `self` was created using [`Weak::new`], or if there are no remaining
1633 /// strong pointers, this will return 0.
1637 /// Due to implementation details, the returned value can be off by 1 in
1638 /// either direction when other threads are manipulating any `Arc`s or
1639 /// `Weak`s pointing to the same allocation.
1641 /// [`Weak::new`]: #method.new
1642 #[stable(feature = "weak_counts", since = "1.41.0")]
1643 pub fn weak_count(&self) -> usize {
1646 let weak = inner.weak.load(SeqCst);
1647 let strong = inner.strong.load(SeqCst);
1651 // Since we observed that there was at least one strong pointer
1652 // after reading the weak count, we know that the implicit weak
1653 // reference (present whenever any strong references are alive)
1654 // was still around when we observed the weak count, and can
1655 // therefore safely subtract it.
1662 /// Returns `None` when the pointer is dangling and there is no allocated `ArcInner`,
1663 /// (i.e., when this `Weak` was created by `Weak::new`).
1665 fn inner(&self) -> Option<WeakInner<'_>> {
1666 if is_dangling(self.ptr) {
1669 // We are careful to *not* create a reference covering the "data" field, as
1670 // the field may be mutated concurrently (for example, if the last `Arc`
1671 // is dropped, the data field will be dropped in-place).
1673 let ptr = self.ptr.as_ptr();
1674 WeakInner { strong: &(*ptr).strong, weak: &(*ptr).weak }
1679 /// Returns `true` if the two `Weak`s point to the same allocation (similar to
1680 /// [`ptr::eq`]), or if both don't point to any allocation
1681 /// (because they were created with `Weak::new()`).
1685 /// Since this compares pointers it means that `Weak::new()` will equal each
1686 /// other, even though they don't point to any allocation.
1691 /// use std::sync::Arc;
1693 /// let first_rc = Arc::new(5);
1694 /// let first = Arc::downgrade(&first_rc);
1695 /// let second = Arc::downgrade(&first_rc);
1697 /// assert!(first.ptr_eq(&second));
1699 /// let third_rc = Arc::new(5);
1700 /// let third = Arc::downgrade(&third_rc);
1702 /// assert!(!first.ptr_eq(&third));
1705 /// Comparing `Weak::new`.
1708 /// use std::sync::{Arc, Weak};
1710 /// let first = Weak::new();
1711 /// let second = Weak::new();
1712 /// assert!(first.ptr_eq(&second));
1714 /// let third_rc = Arc::new(());
1715 /// let third = Arc::downgrade(&third_rc);
1716 /// assert!(!first.ptr_eq(&third));
1719 /// [`ptr::eq`]: ../../std/ptr/fn.eq.html
1721 #[stable(feature = "weak_ptr_eq", since = "1.39.0")]
1722 pub fn ptr_eq(&self, other: &Self) -> bool {
1723 self.ptr.as_ptr() == other.ptr.as_ptr()
1727 #[stable(feature = "arc_weak", since = "1.4.0")]
1728 impl<T: ?Sized> Clone for Weak<T> {
1729 /// Makes a clone of the `Weak` pointer that points to the same allocation.
1734 /// use std::sync::{Arc, Weak};
1736 /// let weak_five = Arc::downgrade(&Arc::new(5));
1738 /// let _ = Weak::clone(&weak_five);
1741 fn clone(&self) -> Weak<T> {
1742 let inner = if let Some(inner) = self.inner() {
1745 return Weak { ptr: self.ptr };
1747 // See comments in Arc::clone() for why this is relaxed. This can use a
1748 // fetch_add (ignoring the lock) because the weak count is only locked
1749 // where are *no other* weak pointers in existence. (So we can't be
1750 // running this code in that case).
1751 let old_size = inner.weak.fetch_add(1, Relaxed);
1753 // See comments in Arc::clone() for why we do this (for mem::forget).
1754 if old_size > MAX_REFCOUNT {
1758 Weak { ptr: self.ptr }
1762 #[stable(feature = "downgraded_weak", since = "1.10.0")]
1763 impl<T> Default for Weak<T> {
1764 /// Constructs a new `Weak<T>`, without allocating memory.
1765 /// Calling [`upgrade`] on the return value always
1768 /// [`None`]: ../../std/option/enum.Option.html#variant.None
1769 /// [`upgrade`]: ../../std/sync/struct.Weak.html#method.upgrade
1774 /// use std::sync::Weak;
1776 /// let empty: Weak<i64> = Default::default();
1777 /// assert!(empty.upgrade().is_none());
1779 fn default() -> Weak<T> {
1784 #[stable(feature = "arc_weak", since = "1.4.0")]
1785 impl<T: ?Sized> Drop for Weak<T> {
1786 /// Drops the `Weak` pointer.
1791 /// use std::sync::{Arc, Weak};
1795 /// impl Drop for Foo {
1796 /// fn drop(&mut self) {
1797 /// println!("dropped!");
1801 /// let foo = Arc::new(Foo);
1802 /// let weak_foo = Arc::downgrade(&foo);
1803 /// let other_weak_foo = Weak::clone(&weak_foo);
1805 /// drop(weak_foo); // Doesn't print anything
1806 /// drop(foo); // Prints "dropped!"
1808 /// assert!(other_weak_foo.upgrade().is_none());
1810 fn drop(&mut self) {
1811 // If we find out that we were the last weak pointer, then its time to
1812 // deallocate the data entirely. See the discussion in Arc::drop() about
1813 // the memory orderings
1815 // It's not necessary to check for the locked state here, because the
1816 // weak count can only be locked if there was precisely one weak ref,
1817 // meaning that drop could only subsequently run ON that remaining weak
1818 // ref, which can only happen after the lock is released.
1819 let inner = if let Some(inner) = self.inner() { inner } else { return };
1821 if inner.weak.fetch_sub(1, Release) == 1 {
1822 acquire!(inner.weak);
1823 unsafe { Global.dealloc(self.ptr.cast(), Layout::for_value(self.ptr.as_ref())) }
1828 #[stable(feature = "rust1", since = "1.0.0")]
1829 trait ArcEqIdent<T: ?Sized + PartialEq> {
1830 fn eq(&self, other: &Arc<T>) -> bool;
1831 fn ne(&self, other: &Arc<T>) -> bool;
1834 #[stable(feature = "rust1", since = "1.0.0")]
1835 impl<T: ?Sized + PartialEq> ArcEqIdent<T> for Arc<T> {
1837 default fn eq(&self, other: &Arc<T>) -> bool {
1841 default fn ne(&self, other: &Arc<T>) -> bool {
1846 /// We're doing this specialization here, and not as a more general optimization on `&T`, because it
1847 /// would otherwise add a cost to all equality checks on refs. We assume that `Arc`s are used to
1848 /// store large values, that are slow to clone, but also heavy to check for equality, causing this
1849 /// cost to pay off more easily. It's also more likely to have two `Arc` clones, that point to
1850 /// the same value, than two `&T`s.
1852 /// We can only do this when `T: Eq` as a `PartialEq` might be deliberately irreflexive.
1853 #[stable(feature = "rust1", since = "1.0.0")]
1854 impl<T: ?Sized + crate::rc::MarkerEq> ArcEqIdent<T> for Arc<T> {
1856 fn eq(&self, other: &Arc<T>) -> bool {
1857 Arc::ptr_eq(self, other) || **self == **other
1861 fn ne(&self, other: &Arc<T>) -> bool {
1862 !Arc::ptr_eq(self, other) && **self != **other
1866 #[stable(feature = "rust1", since = "1.0.0")]
1867 impl<T: ?Sized + PartialEq> PartialEq for Arc<T> {
1868 /// Equality for two `Arc`s.
1870 /// Two `Arc`s are equal if their inner values are equal, even if they are
1871 /// stored in different allocation.
1873 /// If `T` also implements `Eq` (implying reflexivity of equality),
1874 /// two `Arc`s that point to the same allocation are always equal.
1879 /// use std::sync::Arc;
1881 /// let five = Arc::new(5);
1883 /// assert!(five == Arc::new(5));
1886 fn eq(&self, other: &Arc<T>) -> bool {
1887 ArcEqIdent::eq(self, other)
1890 /// Inequality for two `Arc`s.
1892 /// Two `Arc`s are unequal if their inner values are unequal.
1894 /// If `T` also implements `Eq` (implying reflexivity of equality),
1895 /// two `Arc`s that point to the same value are never unequal.
1900 /// use std::sync::Arc;
1902 /// let five = Arc::new(5);
1904 /// assert!(five != Arc::new(6));
1907 fn ne(&self, other: &Arc<T>) -> bool {
1908 ArcEqIdent::ne(self, other)
1912 #[stable(feature = "rust1", since = "1.0.0")]
1913 impl<T: ?Sized + PartialOrd> PartialOrd for Arc<T> {
1914 /// Partial comparison for two `Arc`s.
1916 /// The two are compared by calling `partial_cmp()` on their inner values.
1921 /// use std::sync::Arc;
1922 /// use std::cmp::Ordering;
1924 /// let five = Arc::new(5);
1926 /// assert_eq!(Some(Ordering::Less), five.partial_cmp(&Arc::new(6)));
1928 fn partial_cmp(&self, other: &Arc<T>) -> Option<Ordering> {
1929 (**self).partial_cmp(&**other)
1932 /// Less-than comparison for two `Arc`s.
1934 /// The two are compared by calling `<` on their inner values.
1939 /// use std::sync::Arc;
1941 /// let five = Arc::new(5);
1943 /// assert!(five < Arc::new(6));
1945 fn lt(&self, other: &Arc<T>) -> bool {
1946 *(*self) < *(*other)
1949 /// 'Less than or equal to' comparison for two `Arc`s.
1951 /// The two are compared by calling `<=` on their inner values.
1956 /// use std::sync::Arc;
1958 /// let five = Arc::new(5);
1960 /// assert!(five <= Arc::new(5));
1962 fn le(&self, other: &Arc<T>) -> bool {
1963 *(*self) <= *(*other)
1966 /// Greater-than comparison for two `Arc`s.
1968 /// The two are compared by calling `>` on their inner values.
1973 /// use std::sync::Arc;
1975 /// let five = Arc::new(5);
1977 /// assert!(five > Arc::new(4));
1979 fn gt(&self, other: &Arc<T>) -> bool {
1980 *(*self) > *(*other)
1983 /// 'Greater than or equal to' comparison for two `Arc`s.
1985 /// The two are compared by calling `>=` on their inner values.
1990 /// use std::sync::Arc;
1992 /// let five = Arc::new(5);
1994 /// assert!(five >= Arc::new(5));
1996 fn ge(&self, other: &Arc<T>) -> bool {
1997 *(*self) >= *(*other)
2000 #[stable(feature = "rust1", since = "1.0.0")]
2001 impl<T: ?Sized + Ord> Ord for Arc<T> {
2002 /// Comparison for two `Arc`s.
2004 /// The two are compared by calling `cmp()` on their inner values.
2009 /// use std::sync::Arc;
2010 /// use std::cmp::Ordering;
2012 /// let five = Arc::new(5);
2014 /// assert_eq!(Ordering::Less, five.cmp(&Arc::new(6)));
2016 fn cmp(&self, other: &Arc<T>) -> Ordering {
2017 (**self).cmp(&**other)
2020 #[stable(feature = "rust1", since = "1.0.0")]
2021 impl<T: ?Sized + Eq> Eq for Arc<T> {}
2023 #[stable(feature = "rust1", since = "1.0.0")]
2024 impl<T: ?Sized + fmt::Display> fmt::Display for Arc<T> {
2025 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
2026 fmt::Display::fmt(&**self, f)
2030 #[stable(feature = "rust1", since = "1.0.0")]
2031 impl<T: ?Sized + fmt::Debug> fmt::Debug for Arc<T> {
2032 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
2033 fmt::Debug::fmt(&**self, f)
2037 #[stable(feature = "rust1", since = "1.0.0")]
2038 impl<T: ?Sized> fmt::Pointer for Arc<T> {
2039 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
2040 fmt::Pointer::fmt(&(&**self as *const T), f)
2044 #[stable(feature = "rust1", since = "1.0.0")]
2045 impl<T: Default> Default for Arc<T> {
2046 /// Creates a new `Arc<T>`, with the `Default` value for `T`.
2051 /// use std::sync::Arc;
2053 /// let x: Arc<i32> = Default::default();
2054 /// assert_eq!(*x, 0);
2056 fn default() -> Arc<T> {
2057 Arc::new(Default::default())
2061 #[stable(feature = "rust1", since = "1.0.0")]
2062 impl<T: ?Sized + Hash> Hash for Arc<T> {
2063 fn hash<H: Hasher>(&self, state: &mut H) {
2064 (**self).hash(state)
2068 #[stable(feature = "from_for_ptrs", since = "1.6.0")]
2069 impl<T> From<T> for Arc<T> {
2070 fn from(t: T) -> Self {
2075 #[stable(feature = "shared_from_slice", since = "1.21.0")]
2076 impl<T: Clone> From<&[T]> for Arc<[T]> {
2078 fn from(v: &[T]) -> Arc<[T]> {
2079 <Self as ArcFromSlice<T>>::from_slice(v)
2083 #[stable(feature = "shared_from_slice", since = "1.21.0")]
2084 impl From<&str> for Arc<str> {
2086 fn from(v: &str) -> Arc<str> {
2087 let arc = Arc::<[u8]>::from(v.as_bytes());
2088 unsafe { Arc::from_raw(Arc::into_raw(arc) as *const str) }
2092 #[stable(feature = "shared_from_slice", since = "1.21.0")]
2093 impl From<String> for Arc<str> {
2095 fn from(v: String) -> Arc<str> {
2100 #[stable(feature = "shared_from_slice", since = "1.21.0")]
2101 impl<T: ?Sized> From<Box<T>> for Arc<T> {
2103 fn from(v: Box<T>) -> Arc<T> {
2108 #[stable(feature = "shared_from_slice", since = "1.21.0")]
2109 impl<T> From<Vec<T>> for Arc<[T]> {
2111 fn from(mut v: Vec<T>) -> Arc<[T]> {
2113 let arc = Arc::copy_from_slice(&v);
2115 // Allow the Vec to free its memory, but not destroy its contents
2123 #[stable(feature = "shared_from_cow", since = "1.45.0")]
2124 impl<'a, B> From<Cow<'a, B>> for Arc<B>
2126 B: ToOwned + ?Sized,
2127 Arc<B>: From<&'a B> + From<B::Owned>,
2130 fn from(cow: Cow<'a, B>) -> Arc<B> {
2132 Cow::Borrowed(s) => Arc::from(s),
2133 Cow::Owned(s) => Arc::from(s),
2138 #[stable(feature = "boxed_slice_try_from", since = "1.43.0")]
2139 impl<T, const N: usize> TryFrom<Arc<[T]>> for Arc<[T; N]> {
2140 type Error = Arc<[T]>;
2142 fn try_from(boxed_slice: Arc<[T]>) -> Result<Self, Self::Error> {
2143 if boxed_slice.len() == N {
2144 Ok(unsafe { Arc::from_raw(Arc::into_raw(boxed_slice) as *mut [T; N]) })
2151 #[stable(feature = "shared_from_iter", since = "1.37.0")]
2152 impl<T> iter::FromIterator<T> for Arc<[T]> {
2153 /// Takes each element in the `Iterator` and collects it into an `Arc<[T]>`.
2155 /// # Performance characteristics
2157 /// ## The general case
2159 /// In the general case, collecting into `Arc<[T]>` is done by first
2160 /// collecting into a `Vec<T>`. That is, when writing the following:
2163 /// # use std::sync::Arc;
2164 /// let evens: Arc<[u8]> = (0..10).filter(|&x| x % 2 == 0).collect();
2165 /// # assert_eq!(&*evens, &[0, 2, 4, 6, 8]);
2168 /// this behaves as if we wrote:
2171 /// # use std::sync::Arc;
2172 /// let evens: Arc<[u8]> = (0..10).filter(|&x| x % 2 == 0)
2173 /// .collect::<Vec<_>>() // The first set of allocations happens here.
2174 /// .into(); // A second allocation for `Arc<[T]>` happens here.
2175 /// # assert_eq!(&*evens, &[0, 2, 4, 6, 8]);
2178 /// This will allocate as many times as needed for constructing the `Vec<T>`
2179 /// and then it will allocate once for turning the `Vec<T>` into the `Arc<[T]>`.
2181 /// ## Iterators of known length
2183 /// When your `Iterator` implements `TrustedLen` and is of an exact size,
2184 /// a single allocation will be made for the `Arc<[T]>`. For example:
2187 /// # use std::sync::Arc;
2188 /// let evens: Arc<[u8]> = (0..10).collect(); // Just a single allocation happens here.
2189 /// # assert_eq!(&*evens, &*(0..10).collect::<Vec<_>>());
2191 fn from_iter<I: iter::IntoIterator<Item = T>>(iter: I) -> Self {
2192 ToArcSlice::to_arc_slice(iter.into_iter())
2196 /// Specialization trait used for collecting into `Arc<[T]>`.
2197 trait ToArcSlice<T>: Iterator<Item = T> + Sized {
2198 fn to_arc_slice(self) -> Arc<[T]>;
2201 impl<T, I: Iterator<Item = T>> ToArcSlice<T> for I {
2202 default fn to_arc_slice(self) -> Arc<[T]> {
2203 self.collect::<Vec<T>>().into()
2207 impl<T, I: iter::TrustedLen<Item = T>> ToArcSlice<T> for I {
2208 fn to_arc_slice(self) -> Arc<[T]> {
2209 // This is the case for a `TrustedLen` iterator.
2210 let (low, high) = self.size_hint();
2211 if let Some(high) = high {
2215 "TrustedLen iterator's size hint is not exact: {:?}",
2220 // SAFETY: We need to ensure that the iterator has an exact length and we have.
2221 Arc::from_iter_exact(self, low)
2224 // Fall back to normal implementation.
2225 self.collect::<Vec<T>>().into()
2230 #[stable(feature = "rust1", since = "1.0.0")]
2231 impl<T: ?Sized> borrow::Borrow<T> for Arc<T> {
2232 fn borrow(&self) -> &T {
2237 #[stable(since = "1.5.0", feature = "smart_ptr_as_ref")]
2238 impl<T: ?Sized> AsRef<T> for Arc<T> {
2239 fn as_ref(&self) -> &T {
2244 #[stable(feature = "pin", since = "1.33.0")]
2245 impl<T: ?Sized> Unpin for Arc<T> {}
2247 /// Get the offset within an `ArcInner` for
2248 /// a payload of type described by a pointer.
2252 /// This has the same safety requirements as `align_of_val_raw`. In effect:
2254 /// - This function is safe for any argument if `T` is sized, and
2255 /// - if `T` is unsized, the pointer must have appropriate pointer metadata
2256 /// acquired from the real instance that you are getting this offset for.
2257 unsafe fn data_offset<T: ?Sized>(ptr: *const T) -> isize {
2258 // Align the unsized value to the end of the `ArcInner`.
2259 // Because it is `?Sized`, it will always be the last field in memory.
2260 // Note: This is a detail of the current implementation of the compiler,
2261 // and is not a guaranteed language detail. Do not rely on it outside of std.
2262 unsafe { data_offset_align(align_of_val(&*ptr)) }
2266 fn data_offset_align(align: usize) -> isize {
2267 let layout = Layout::new::<ArcInner<()>>();
2268 (layout.size() + layout.padding_needed_for(align)) as isize