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, AllocInit, 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 /// Consumes the `Arc`, returning the wrapped pointer as `NonNull<T>`.
654 /// #![feature(rc_into_raw_non_null)]
655 /// #![allow(deprecated)]
657 /// use std::sync::Arc;
659 /// let x = Arc::new("hello".to_owned());
660 /// let ptr = Arc::into_raw_non_null(x);
661 /// let deref = unsafe { ptr.as_ref() };
662 /// assert_eq!(deref, "hello");
664 #[unstable(feature = "rc_into_raw_non_null", issue = "47336")]
665 #[rustc_deprecated(since = "1.44.0", reason = "use `Arc::into_raw` instead")]
667 pub fn into_raw_non_null(this: Self) -> NonNull<T> {
668 // safe because Arc guarantees its pointer is non-null
669 unsafe { NonNull::new_unchecked(Arc::into_raw(this) as *mut _) }
672 /// Creates a new [`Weak`][weak] pointer to this allocation.
674 /// [weak]: struct.Weak.html
679 /// use std::sync::Arc;
681 /// let five = Arc::new(5);
683 /// let weak_five = Arc::downgrade(&five);
685 #[stable(feature = "arc_weak", since = "1.4.0")]
686 pub fn downgrade(this: &Self) -> Weak<T> {
687 // This Relaxed is OK because we're checking the value in the CAS
689 let mut cur = this.inner().weak.load(Relaxed);
692 // check if the weak counter is currently "locked"; if so, spin.
693 if cur == usize::MAX {
694 cur = this.inner().weak.load(Relaxed);
698 // NOTE: this code currently ignores the possibility of overflow
699 // into usize::MAX; in general both Rc and Arc need to be adjusted
700 // to deal with overflow.
702 // Unlike with Clone(), we need this to be an Acquire read to
703 // synchronize with the write coming from `is_unique`, so that the
704 // events prior to that write happen before this read.
705 match this.inner().weak.compare_exchange_weak(cur, cur + 1, Acquire, Relaxed) {
707 // Make sure we do not create a dangling Weak
708 debug_assert!(!is_dangling(this.ptr));
709 return Weak { ptr: this.ptr };
711 Err(old) => cur = old,
716 /// Gets the number of [`Weak`][weak] pointers to this allocation.
718 /// [weak]: struct.Weak.html
722 /// This method by itself is safe, but using it correctly requires extra care.
723 /// Another thread can change the weak count at any time,
724 /// including potentially between calling this method and acting on the result.
729 /// use std::sync::Arc;
731 /// let five = Arc::new(5);
732 /// let _weak_five = Arc::downgrade(&five);
734 /// // This assertion is deterministic because we haven't shared
735 /// // the `Arc` or `Weak` between threads.
736 /// assert_eq!(1, Arc::weak_count(&five));
739 #[stable(feature = "arc_counts", since = "1.15.0")]
740 pub fn weak_count(this: &Self) -> usize {
741 let cnt = this.inner().weak.load(SeqCst);
742 // If the weak count is currently locked, the value of the
743 // count was 0 just before taking the lock.
744 if cnt == usize::MAX { 0 } else { cnt - 1 }
747 /// Gets the number of strong (`Arc`) pointers to this allocation.
751 /// This method by itself is safe, but using it correctly requires extra care.
752 /// Another thread can change the strong count at any time,
753 /// including potentially between calling this method and acting on the result.
758 /// use std::sync::Arc;
760 /// let five = Arc::new(5);
761 /// let _also_five = Arc::clone(&five);
763 /// // This assertion is deterministic because we haven't shared
764 /// // the `Arc` between threads.
765 /// assert_eq!(2, Arc::strong_count(&five));
768 #[stable(feature = "arc_counts", since = "1.15.0")]
769 pub fn strong_count(this: &Self) -> usize {
770 this.inner().strong.load(SeqCst)
773 /// Increments the strong reference count on the `Arc<T>` associated with the
774 /// provided pointer by one.
778 /// The pointer must have been obtained through `Arc::into_raw`, and the
779 /// associated `Arc` instance must be valid (i.e. the strong count must be at
780 /// least 1) for the duration of this method.
785 /// #![feature(arc_mutate_strong_count)]
787 /// use std::sync::Arc;
789 /// let five = Arc::new(5);
792 /// let ptr = Arc::into_raw(five);
793 /// Arc::incr_strong_count(ptr);
795 /// // This assertion is deterministic because we haven't shared
796 /// // the `Arc` between threads.
797 /// let five = Arc::from_raw(ptr);
798 /// assert_eq!(2, Arc::strong_count(&five));
802 #[unstable(feature = "arc_mutate_strong_count", issue = "71983")]
803 pub unsafe fn incr_strong_count(ptr: *const T) {
804 // Retain Arc, but don't touch refcount by wrapping in ManuallyDrop
805 let arc = unsafe { mem::ManuallyDrop::new(Arc::<T>::from_raw(ptr)) };
806 // Now increase refcount, but don't drop new refcount either
807 let _arc_clone: mem::ManuallyDrop<_> = arc.clone();
810 /// Decrements the strong reference count on the `Arc<T>` associated with the
811 /// provided pointer by one.
815 /// The pointer must have been obtained through `Arc::into_raw`, and the
816 /// associated `Arc` instance must be valid (i.e. the strong count must be at
817 /// least 1) when invoking this method. This method can be used to release the final
818 /// `Arc` and backing storage, but **should not** be called after the final `Arc` has been
824 /// #![feature(arc_mutate_strong_count)]
826 /// use std::sync::Arc;
828 /// let five = Arc::new(5);
831 /// let ptr = Arc::into_raw(five);
832 /// Arc::incr_strong_count(ptr);
834 /// // Those assertions are deterministic because we haven't shared
835 /// // the `Arc` between threads.
836 /// let five = Arc::from_raw(ptr);
837 /// assert_eq!(2, Arc::strong_count(&five));
838 /// Arc::decr_strong_count(ptr);
839 /// assert_eq!(1, Arc::strong_count(&five));
843 #[unstable(feature = "arc_mutate_strong_count", issue = "71983")]
844 pub unsafe fn decr_strong_count(ptr: *const T) {
845 unsafe { mem::drop(Arc::from_raw(ptr)) };
849 fn inner(&self) -> &ArcInner<T> {
850 // This unsafety is ok because while this arc is alive we're guaranteed
851 // that the inner pointer is valid. Furthermore, we know that the
852 // `ArcInner` structure itself is `Sync` because the inner data is
853 // `Sync` as well, so we're ok loaning out an immutable pointer to these
855 unsafe { self.ptr.as_ref() }
858 // Non-inlined part of `drop`.
860 unsafe fn drop_slow(&mut self) {
861 // Destroy the data at this time, even though we may not free the box
862 // allocation itself (there may still be weak pointers lying around).
863 unsafe { ptr::drop_in_place(Self::get_mut_unchecked(self)) };
865 // Drop the weak ref collectively held by all strong references
866 drop(Weak { ptr: self.ptr });
870 #[stable(feature = "ptr_eq", since = "1.17.0")]
871 /// Returns `true` if the two `Arc`s point to the same allocation
872 /// (in a vein similar to [`ptr::eq`]).
877 /// use std::sync::Arc;
879 /// let five = Arc::new(5);
880 /// let same_five = Arc::clone(&five);
881 /// let other_five = Arc::new(5);
883 /// assert!(Arc::ptr_eq(&five, &same_five));
884 /// assert!(!Arc::ptr_eq(&five, &other_five));
887 /// [`ptr::eq`]: ../../std/ptr/fn.eq.html
888 pub fn ptr_eq(this: &Self, other: &Self) -> bool {
889 this.ptr.as_ptr() == other.ptr.as_ptr()
893 impl<T: ?Sized> Arc<T> {
894 /// Allocates an `ArcInner<T>` with sufficient space for
895 /// a possibly-unsized inner value where the value has the layout provided.
897 /// The function `mem_to_arcinner` is called with the data pointer
898 /// and must return back a (potentially fat)-pointer for the `ArcInner<T>`.
899 unsafe fn allocate_for_layout(
900 value_layout: Layout,
901 mem_to_arcinner: impl FnOnce(*mut u8) -> *mut ArcInner<T>,
902 ) -> *mut ArcInner<T> {
903 // Calculate layout using the given value layout.
904 // Previously, layout was calculated on the expression
905 // `&*(ptr as *const ArcInner<T>)`, but this created a misaligned
906 // reference (see #54908).
907 let layout = Layout::new::<ArcInner<()>>().extend(value_layout).unwrap().0.pad_to_align();
910 .alloc(layout, AllocInit::Uninitialized)
911 .unwrap_or_else(|_| handle_alloc_error(layout));
913 // Initialize the ArcInner
914 let inner = mem_to_arcinner(mem.ptr.as_ptr());
915 debug_assert_eq!(unsafe { Layout::for_value(&*inner) }, layout);
918 ptr::write(&mut (*inner).strong, atomic::AtomicUsize::new(1));
919 ptr::write(&mut (*inner).weak, atomic::AtomicUsize::new(1));
925 /// Allocates an `ArcInner<T>` with sufficient space for an unsized inner value.
926 unsafe fn allocate_for_ptr(ptr: *const T) -> *mut ArcInner<T> {
927 // Allocate for the `ArcInner<T>` using the given value.
929 Self::allocate_for_layout(Layout::for_value(&*ptr), |mem| {
930 set_data_ptr(ptr as *mut T, mem) as *mut ArcInner<T>
935 fn from_box(v: Box<T>) -> Arc<T> {
937 let box_unique = Box::into_unique(v);
938 let bptr = box_unique.as_ptr();
940 let value_size = size_of_val(&*bptr);
941 let ptr = Self::allocate_for_ptr(bptr);
943 // Copy value as bytes
944 ptr::copy_nonoverlapping(
945 bptr as *const T as *const u8,
946 &mut (*ptr).data as *mut _ as *mut u8,
950 // Free the allocation without dropping its contents
951 box_free(box_unique);
959 /// Allocates an `ArcInner<[T]>` with the given length.
960 unsafe fn allocate_for_slice(len: usize) -> *mut ArcInner<[T]> {
962 Self::allocate_for_layout(Layout::array::<T>(len).unwrap(), |mem| {
963 ptr::slice_from_raw_parts_mut(mem as *mut T, len) as *mut ArcInner<[T]>
969 /// Sets the data pointer of a `?Sized` raw pointer.
971 /// For a slice/trait object, this sets the `data` field and leaves the rest
972 /// unchanged. For a sized raw pointer, this simply sets the pointer.
973 unsafe fn set_data_ptr<T: ?Sized, U>(mut ptr: *mut T, data: *mut U) -> *mut T {
975 ptr::write(&mut ptr as *mut _ as *mut *mut u8, data as *mut u8);
981 /// Copy elements from slice into newly allocated Arc<\[T\]>
983 /// Unsafe because the caller must either take ownership or bind `T: Copy`.
984 unsafe fn copy_from_slice(v: &[T]) -> Arc<[T]> {
986 let ptr = Self::allocate_for_slice(v.len());
988 ptr::copy_nonoverlapping(v.as_ptr(), &mut (*ptr).data as *mut [T] as *mut T, v.len());
994 /// Constructs an `Arc<[T]>` from an iterator known to be of a certain size.
996 /// Behavior is undefined should the size be wrong.
997 unsafe fn from_iter_exact(iter: impl iter::Iterator<Item = T>, len: usize) -> Arc<[T]> {
998 // Panic guard while cloning T elements.
999 // In the event of a panic, elements that have been written
1000 // into the new ArcInner will be dropped, then the memory freed.
1008 impl<T> Drop for Guard<T> {
1009 fn drop(&mut self) {
1011 let slice = from_raw_parts_mut(self.elems, self.n_elems);
1012 ptr::drop_in_place(slice);
1014 Global.dealloc(self.mem.cast(), self.layout);
1020 let ptr = Self::allocate_for_slice(len);
1022 let mem = ptr as *mut _ as *mut u8;
1023 let layout = Layout::for_value(&*ptr);
1025 // Pointer to first element
1026 let elems = &mut (*ptr).data as *mut [T] as *mut T;
1028 let mut guard = Guard { mem: NonNull::new_unchecked(mem), elems, layout, n_elems: 0 };
1030 for (i, item) in iter.enumerate() {
1031 ptr::write(elems.add(i), item);
1035 // All clear. Forget the guard so it doesn't free the new ArcInner.
1043 /// Specialization trait used for `From<&[T]>`.
1044 trait ArcFromSlice<T> {
1045 fn from_slice(slice: &[T]) -> Self;
1048 impl<T: Clone> ArcFromSlice<T> for Arc<[T]> {
1050 default fn from_slice(v: &[T]) -> Self {
1051 unsafe { Self::from_iter_exact(v.iter().cloned(), v.len()) }
1055 impl<T: Copy> ArcFromSlice<T> for Arc<[T]> {
1057 fn from_slice(v: &[T]) -> Self {
1058 unsafe { Arc::copy_from_slice(v) }
1062 #[stable(feature = "rust1", since = "1.0.0")]
1063 impl<T: ?Sized> Clone for Arc<T> {
1064 /// Makes a clone of the `Arc` pointer.
1066 /// This creates another pointer to the same allocation, increasing the
1067 /// strong reference count.
1072 /// use std::sync::Arc;
1074 /// let five = Arc::new(5);
1076 /// let _ = Arc::clone(&five);
1079 fn clone(&self) -> Arc<T> {
1080 // Using a relaxed ordering is alright here, as knowledge of the
1081 // original reference prevents other threads from erroneously deleting
1084 // As explained in the [Boost documentation][1], Increasing the
1085 // reference counter can always be done with memory_order_relaxed: New
1086 // references to an object can only be formed from an existing
1087 // reference, and passing an existing reference from one thread to
1088 // another must already provide any required synchronization.
1090 // [1]: (www.boost.org/doc/libs/1_55_0/doc/html/atomic/usage_examples.html)
1091 let old_size = self.inner().strong.fetch_add(1, Relaxed);
1093 // However we need to guard against massive refcounts in case someone
1094 // is `mem::forget`ing Arcs. If we don't do this the count can overflow
1095 // and users will use-after free. We racily saturate to `isize::MAX` on
1096 // the assumption that there aren't ~2 billion threads incrementing
1097 // the reference count at once. This branch will never be taken in
1098 // any realistic program.
1100 // We abort because such a program is incredibly degenerate, and we
1101 // don't care to support it.
1102 if old_size > MAX_REFCOUNT {
1106 Self::from_inner(self.ptr)
1110 #[stable(feature = "rust1", since = "1.0.0")]
1111 impl<T: ?Sized> Deref for Arc<T> {
1115 fn deref(&self) -> &T {
1120 #[unstable(feature = "receiver_trait", issue = "none")]
1121 impl<T: ?Sized> Receiver for Arc<T> {}
1123 impl<T: Clone> Arc<T> {
1124 /// Makes a mutable reference into the given `Arc`.
1126 /// If there are other `Arc` or [`Weak`][weak] pointers to the same allocation,
1127 /// then `make_mut` will create a new allocation and invoke [`clone`][clone] on the inner value
1128 /// to ensure unique ownership. This is also referred to as clone-on-write.
1130 /// Note that this differs from the behavior of [`Rc::make_mut`] which disassociates
1131 /// any remaining `Weak` pointers.
1133 /// See also [`get_mut`][get_mut], which will fail rather than cloning.
1135 /// [weak]: struct.Weak.html
1136 /// [clone]: ../../std/clone/trait.Clone.html#tymethod.clone
1137 /// [get_mut]: struct.Arc.html#method.get_mut
1138 /// [`Rc::make_mut`]: ../rc/struct.Rc.html#method.make_mut
1143 /// use std::sync::Arc;
1145 /// let mut data = Arc::new(5);
1147 /// *Arc::make_mut(&mut data) += 1; // Won't clone anything
1148 /// let mut other_data = Arc::clone(&data); // Won't clone inner data
1149 /// *Arc::make_mut(&mut data) += 1; // Clones inner data
1150 /// *Arc::make_mut(&mut data) += 1; // Won't clone anything
1151 /// *Arc::make_mut(&mut other_data) *= 2; // Won't clone anything
1153 /// // Now `data` and `other_data` point to different allocations.
1154 /// assert_eq!(*data, 8);
1155 /// assert_eq!(*other_data, 12);
1158 #[stable(feature = "arc_unique", since = "1.4.0")]
1159 pub fn make_mut(this: &mut Self) -> &mut T {
1160 // Note that we hold both a strong reference and a weak reference.
1161 // Thus, releasing our strong reference only will not, by itself, cause
1162 // the memory to be deallocated.
1164 // Use Acquire to ensure that we see any writes to `weak` that happen
1165 // before release writes (i.e., decrements) to `strong`. Since we hold a
1166 // weak count, there's no chance the ArcInner itself could be
1168 if this.inner().strong.compare_exchange(1, 0, Acquire, Relaxed).is_err() {
1169 // Another strong pointer exists; clone
1170 *this = Arc::new((**this).clone());
1171 } else if this.inner().weak.load(Relaxed) != 1 {
1172 // Relaxed suffices in the above because this is fundamentally an
1173 // optimization: we are always racing with weak pointers being
1174 // dropped. Worst case, we end up allocated a new Arc unnecessarily.
1176 // We removed the last strong ref, but there are additional weak
1177 // refs remaining. We'll move the contents to a new Arc, and
1178 // invalidate the other weak refs.
1180 // Note that it is not possible for the read of `weak` to yield
1181 // usize::MAX (i.e., locked), since the weak count can only be
1182 // locked by a thread with a strong reference.
1184 // Materialize our own implicit weak pointer, so that it can clean
1185 // up the ArcInner as needed.
1186 let weak = Weak { ptr: this.ptr };
1188 // mark the data itself as already deallocated
1190 // there is no data race in the implicit write caused by `read`
1191 // here (due to zeroing) because data is no longer accessed by
1192 // other threads (due to there being no more strong refs at this
1194 let mut swap = Arc::new(ptr::read(&weak.ptr.as_ref().data));
1195 mem::swap(this, &mut swap);
1199 // We were the sole reference of either kind; bump back up the
1200 // strong ref count.
1201 this.inner().strong.store(1, Release);
1204 // As with `get_mut()`, the unsafety is ok because our reference was
1205 // either unique to begin with, or became one upon cloning the contents.
1206 unsafe { Self::get_mut_unchecked(this) }
1210 impl<T: ?Sized> Arc<T> {
1211 /// Returns a mutable reference into the given `Arc`, if there are
1212 /// no other `Arc` or [`Weak`][weak] pointers to the same allocation.
1214 /// Returns [`None`][option] otherwise, because it is not safe to
1215 /// mutate a shared value.
1217 /// See also [`make_mut`][make_mut], which will [`clone`][clone]
1218 /// the inner value when there are other pointers.
1220 /// [weak]: struct.Weak.html
1221 /// [option]: ../../std/option/enum.Option.html
1222 /// [make_mut]: struct.Arc.html#method.make_mut
1223 /// [clone]: ../../std/clone/trait.Clone.html#tymethod.clone
1228 /// use std::sync::Arc;
1230 /// let mut x = Arc::new(3);
1231 /// *Arc::get_mut(&mut x).unwrap() = 4;
1232 /// assert_eq!(*x, 4);
1234 /// let _y = Arc::clone(&x);
1235 /// assert!(Arc::get_mut(&mut x).is_none());
1238 #[stable(feature = "arc_unique", since = "1.4.0")]
1239 pub fn get_mut(this: &mut Self) -> Option<&mut T> {
1240 if this.is_unique() {
1241 // This unsafety is ok because we're guaranteed that the pointer
1242 // returned is the *only* pointer that will ever be returned to T. Our
1243 // reference count is guaranteed to be 1 at this point, and we required
1244 // the Arc itself to be `mut`, so we're returning the only possible
1245 // reference to the inner data.
1246 unsafe { Some(Arc::get_mut_unchecked(this)) }
1252 /// Returns a mutable reference into the given `Arc`,
1253 /// without any check.
1255 /// See also [`get_mut`], which is safe and does appropriate checks.
1257 /// [`get_mut`]: struct.Arc.html#method.get_mut
1261 /// Any other `Arc` or [`Weak`] pointers to the same allocation must not be dereferenced
1262 /// for the duration of the returned borrow.
1263 /// This is trivially the case if no such pointers exist,
1264 /// for example immediately after `Arc::new`.
1269 /// #![feature(get_mut_unchecked)]
1271 /// use std::sync::Arc;
1273 /// let mut x = Arc::new(String::new());
1275 /// Arc::get_mut_unchecked(&mut x).push_str("foo")
1277 /// assert_eq!(*x, "foo");
1280 #[unstable(feature = "get_mut_unchecked", issue = "63292")]
1281 pub unsafe fn get_mut_unchecked(this: &mut Self) -> &mut T {
1282 // We are careful to *not* create a reference covering the "count" fields, as
1283 // this would alias with concurrent access to the reference counts (e.g. by `Weak`).
1284 unsafe { &mut (*this.ptr.as_ptr()).data }
1287 /// Determine whether this is the unique reference (including weak refs) to
1288 /// the underlying data.
1290 /// Note that this requires locking the weak ref count.
1291 fn is_unique(&mut self) -> bool {
1292 // lock the weak pointer count if we appear to be the sole weak pointer
1295 // The acquire label here ensures a happens-before relationship with any
1296 // writes to `strong` (in particular in `Weak::upgrade`) prior to decrements
1297 // of the `weak` count (via `Weak::drop`, which uses release). If the upgraded
1298 // weak ref was never dropped, the CAS here will fail so we do not care to synchronize.
1299 if self.inner().weak.compare_exchange(1, usize::MAX, Acquire, Relaxed).is_ok() {
1300 // This needs to be an `Acquire` to synchronize with the decrement of the `strong`
1301 // counter in `drop` -- the only access that happens when any but the last reference
1302 // is being dropped.
1303 let unique = self.inner().strong.load(Acquire) == 1;
1305 // The release write here synchronizes with a read in `downgrade`,
1306 // effectively preventing the above read of `strong` from happening
1308 self.inner().weak.store(1, Release); // release the lock
1316 #[stable(feature = "rust1", since = "1.0.0")]
1317 unsafe impl<#[may_dangle] T: ?Sized> Drop for Arc<T> {
1318 /// Drops the `Arc`.
1320 /// This will decrement the strong reference count. If the strong reference
1321 /// count reaches zero then the only other references (if any) are
1322 /// [`Weak`], so we `drop` the inner value.
1327 /// use std::sync::Arc;
1331 /// impl Drop for Foo {
1332 /// fn drop(&mut self) {
1333 /// println!("dropped!");
1337 /// let foo = Arc::new(Foo);
1338 /// let foo2 = Arc::clone(&foo);
1340 /// drop(foo); // Doesn't print anything
1341 /// drop(foo2); // Prints "dropped!"
1344 /// [`Weak`]: ../../std/sync/struct.Weak.html
1346 fn drop(&mut self) {
1347 // Because `fetch_sub` is already atomic, we do not need to synchronize
1348 // with other threads unless we are going to delete the object. This
1349 // same logic applies to the below `fetch_sub` to the `weak` count.
1350 if self.inner().strong.fetch_sub(1, Release) != 1 {
1354 // This fence is needed to prevent reordering of use of the data and
1355 // deletion of the data. Because it is marked `Release`, the decreasing
1356 // of the reference count synchronizes with this `Acquire` fence. This
1357 // means that use of the data happens before decreasing the reference
1358 // count, which happens before this fence, which happens before the
1359 // deletion of the data.
1361 // As explained in the [Boost documentation][1],
1363 // > It is important to enforce any possible access to the object in one
1364 // > thread (through an existing reference) to *happen before* deleting
1365 // > the object in a different thread. This is achieved by a "release"
1366 // > operation after dropping a reference (any access to the object
1367 // > through this reference must obviously happened before), and an
1368 // > "acquire" operation before deleting the object.
1370 // In particular, while the contents of an Arc are usually immutable, it's
1371 // possible to have interior writes to something like a Mutex<T>. Since a
1372 // Mutex is not acquired when it is deleted, we can't rely on its
1373 // synchronization logic to make writes in thread A visible to a destructor
1374 // running in thread B.
1376 // Also note that the Acquire fence here could probably be replaced with an
1377 // Acquire load, which could improve performance in highly-contended
1378 // situations. See [2].
1380 // [1]: (www.boost.org/doc/libs/1_55_0/doc/html/atomic/usage_examples.html)
1381 // [2]: (https://github.com/rust-lang/rust/pull/41714)
1382 acquire!(self.inner().strong);
1390 impl Arc<dyn Any + Send + Sync> {
1392 #[stable(feature = "rc_downcast", since = "1.29.0")]
1393 /// Attempt to downcast the `Arc<dyn Any + Send + Sync>` to a concrete type.
1398 /// use std::any::Any;
1399 /// use std::sync::Arc;
1401 /// fn print_if_string(value: Arc<dyn Any + Send + Sync>) {
1402 /// if let Ok(string) = value.downcast::<String>() {
1403 /// println!("String ({}): {}", string.len(), string);
1407 /// let my_string = "Hello World".to_string();
1408 /// print_if_string(Arc::new(my_string));
1409 /// print_if_string(Arc::new(0i8));
1411 pub fn downcast<T>(self) -> Result<Arc<T>, Self>
1413 T: Any + Send + Sync + 'static,
1415 if (*self).is::<T>() {
1416 let ptr = self.ptr.cast::<ArcInner<T>>();
1418 Ok(Arc::from_inner(ptr))
1426 /// Constructs a new `Weak<T>`, without allocating any memory.
1427 /// Calling [`upgrade`] on the return value always gives [`None`].
1429 /// [`upgrade`]: struct.Weak.html#method.upgrade
1430 /// [`None`]: ../../std/option/enum.Option.html#variant.None
1435 /// use std::sync::Weak;
1437 /// let empty: Weak<i64> = Weak::new();
1438 /// assert!(empty.upgrade().is_none());
1440 #[stable(feature = "downgraded_weak", since = "1.10.0")]
1441 pub fn new() -> Weak<T> {
1442 Weak { ptr: NonNull::new(usize::MAX as *mut ArcInner<T>).expect("MAX is not 0") }
1445 /// Returns a raw pointer to the object `T` pointed to by this `Weak<T>`.
1447 /// The pointer is valid only if there are some strong references. The pointer may be dangling,
1448 /// unaligned or even [`null`] otherwise.
1453 /// use std::sync::Arc;
1456 /// let strong = Arc::new("hello".to_owned());
1457 /// let weak = Arc::downgrade(&strong);
1458 /// // Both point to the same object
1459 /// assert!(ptr::eq(&*strong, weak.as_ptr()));
1460 /// // The strong here keeps it alive, so we can still access the object.
1461 /// assert_eq!("hello", unsafe { &*weak.as_ptr() });
1464 /// // But not any more. We can do weak.as_ptr(), but accessing the pointer would lead to
1465 /// // undefined behaviour.
1466 /// // assert_eq!("hello", unsafe { &*weak.as_ptr() });
1469 /// [`null`]: ../../std/ptr/fn.null.html
1470 #[stable(feature = "weak_into_raw", since = "1.45.0")]
1471 pub fn as_ptr(&self) -> *const T {
1472 let ptr: *mut ArcInner<T> = NonNull::as_ptr(self.ptr);
1474 // SAFETY: we must offset the pointer manually, and said pointer may be
1475 // a dangling weak (usize::MAX) if T is sized. data_offset is safe to call,
1476 // because we know that a pointer to unsized T was derived from a real
1477 // unsized T, as dangling weaks are only created for sized T. wrapping_offset
1478 // is used so that we can use the same code path for the non-dangling
1479 // unsized case and the potentially dangling sized case.
1481 let offset = data_offset(ptr as *mut T);
1482 set_data_ptr(ptr as *mut T, (ptr as *mut u8).wrapping_offset(offset))
1486 /// Consumes the `Weak<T>` and turns it into a raw pointer.
1488 /// This converts the weak pointer into a raw pointer, preserving the original weak count. It
1489 /// can be turned back into the `Weak<T>` with [`from_raw`].
1491 /// The same restrictions of accessing the target of the pointer as with
1492 /// [`as_ptr`] apply.
1497 /// use std::sync::{Arc, Weak};
1499 /// let strong = Arc::new("hello".to_owned());
1500 /// let weak = Arc::downgrade(&strong);
1501 /// let raw = weak.into_raw();
1503 /// assert_eq!(1, Arc::weak_count(&strong));
1504 /// assert_eq!("hello", unsafe { &*raw });
1506 /// drop(unsafe { Weak::from_raw(raw) });
1507 /// assert_eq!(0, Arc::weak_count(&strong));
1510 /// [`from_raw`]: struct.Weak.html#method.from_raw
1511 /// [`as_ptr`]: struct.Weak.html#method.as_ptr
1512 #[stable(feature = "weak_into_raw", since = "1.45.0")]
1513 pub fn into_raw(self) -> *const T {
1514 let result = self.as_ptr();
1519 /// Converts a raw pointer previously created by [`into_raw`] back into
1522 /// This can be used to safely get a strong reference (by calling [`upgrade`]
1523 /// later) or to deallocate the weak count by dropping the `Weak<T>`.
1525 /// It takes ownership of one weak count (with the exception of pointers created by [`new`],
1526 /// as these don't have any corresponding weak count).
1530 /// The pointer must have originated from the [`into_raw`] and must still own its potential
1531 /// weak reference count.
1533 /// It is allowed for the strong count to be 0 at the time of calling this, but the weak count
1534 /// must be non-zero or the pointer must have originated from a dangling `Weak<T>` (one created
1540 /// use std::sync::{Arc, Weak};
1542 /// let strong = Arc::new("hello".to_owned());
1544 /// let raw_1 = Arc::downgrade(&strong).into_raw();
1545 /// let raw_2 = Arc::downgrade(&strong).into_raw();
1547 /// assert_eq!(2, Arc::weak_count(&strong));
1549 /// assert_eq!("hello", &*unsafe { Weak::from_raw(raw_1) }.upgrade().unwrap());
1550 /// assert_eq!(1, Arc::weak_count(&strong));
1554 /// // Decrement the last weak count.
1555 /// assert!(unsafe { Weak::from_raw(raw_2) }.upgrade().is_none());
1558 /// [`new`]: struct.Weak.html#method.new
1559 /// [`into_raw`]: struct.Weak.html#method.into_raw
1560 /// [`upgrade`]: struct.Weak.html#method.upgrade
1561 /// [`Weak`]: struct.Weak.html
1562 /// [`Arc`]: struct.Arc.html
1563 /// [`forget`]: ../../std/mem/fn.forget.html
1564 #[stable(feature = "weak_into_raw", since = "1.45.0")]
1565 pub unsafe fn from_raw(ptr: *const T) -> Self {
1569 // See Arc::from_raw for details
1571 let offset = data_offset(ptr);
1572 let fake_ptr = ptr as *mut ArcInner<T>;
1573 let ptr = set_data_ptr(fake_ptr, (ptr as *mut u8).offset(-offset));
1574 Weak { ptr: NonNull::new(ptr).expect("Invalid pointer passed to from_raw") }
1580 /// Helper type to allow accessing the reference counts without
1581 /// making any assertions about the data field.
1582 struct WeakInner<'a> {
1583 weak: &'a atomic::AtomicUsize,
1584 strong: &'a atomic::AtomicUsize,
1587 impl<T: ?Sized> Weak<T> {
1588 /// Attempts to upgrade the `Weak` pointer to an [`Arc`], delaying
1589 /// dropping of the inner value if successful.
1591 /// Returns [`None`] if the inner value has since been dropped.
1593 /// [`Arc`]: struct.Arc.html
1594 /// [`None`]: ../../std/option/enum.Option.html#variant.None
1599 /// use std::sync::Arc;
1601 /// let five = Arc::new(5);
1603 /// let weak_five = Arc::downgrade(&five);
1605 /// let strong_five: Option<Arc<_>> = weak_five.upgrade();
1606 /// assert!(strong_five.is_some());
1608 /// // Destroy all strong pointers.
1609 /// drop(strong_five);
1612 /// assert!(weak_five.upgrade().is_none());
1614 #[stable(feature = "arc_weak", since = "1.4.0")]
1615 pub fn upgrade(&self) -> Option<Arc<T>> {
1616 // We use a CAS loop to increment the strong count instead of a
1617 // fetch_add because once the count hits 0 it must never be above 0.
1618 let inner = self.inner()?;
1620 // Relaxed load because any write of 0 that we can observe
1621 // leaves the field in a permanently zero state (so a
1622 // "stale" read of 0 is fine), and any other value is
1623 // confirmed via the CAS below.
1624 let mut n = inner.strong.load(Relaxed);
1631 // See comments in `Arc::clone` for why we do this (for `mem::forget`).
1632 if n > MAX_REFCOUNT {
1636 // Relaxed is valid for the same reason it is on Arc's Clone impl
1637 match inner.strong.compare_exchange_weak(n, n + 1, Relaxed, Relaxed) {
1638 Ok(_) => return Some(Arc::from_inner(self.ptr)), // null checked above
1639 Err(old) => n = old,
1644 /// Gets the number of strong (`Arc`) pointers pointing to this allocation.
1646 /// If `self` was created using [`Weak::new`], this will return 0.
1648 /// [`Weak::new`]: #method.new
1649 #[stable(feature = "weak_counts", since = "1.41.0")]
1650 pub fn strong_count(&self) -> usize {
1651 if let Some(inner) = self.inner() { inner.strong.load(SeqCst) } else { 0 }
1654 /// Gets an approximation of the number of `Weak` pointers pointing to this
1657 /// If `self` was created using [`Weak::new`], or if there are no remaining
1658 /// strong pointers, this will return 0.
1662 /// Due to implementation details, the returned value can be off by 1 in
1663 /// either direction when other threads are manipulating any `Arc`s or
1664 /// `Weak`s pointing to the same allocation.
1666 /// [`Weak::new`]: #method.new
1667 #[stable(feature = "weak_counts", since = "1.41.0")]
1668 pub fn weak_count(&self) -> usize {
1671 let weak = inner.weak.load(SeqCst);
1672 let strong = inner.strong.load(SeqCst);
1676 // Since we observed that there was at least one strong pointer
1677 // after reading the weak count, we know that the implicit weak
1678 // reference (present whenever any strong references are alive)
1679 // was still around when we observed the weak count, and can
1680 // therefore safely subtract it.
1687 /// Returns `None` when the pointer is dangling and there is no allocated `ArcInner`,
1688 /// (i.e., when this `Weak` was created by `Weak::new`).
1690 fn inner(&self) -> Option<WeakInner<'_>> {
1691 if is_dangling(self.ptr) {
1694 // We are careful to *not* create a reference covering the "data" field, as
1695 // the field may be mutated concurrently (for example, if the last `Arc`
1696 // is dropped, the data field will be dropped in-place).
1698 let ptr = self.ptr.as_ptr();
1699 WeakInner { strong: &(*ptr).strong, weak: &(*ptr).weak }
1704 /// Returns `true` if the two `Weak`s point to the same allocation (similar to
1705 /// [`ptr::eq`]), or if both don't point to any allocation
1706 /// (because they were created with `Weak::new()`).
1710 /// Since this compares pointers it means that `Weak::new()` will equal each
1711 /// other, even though they don't point to any allocation.
1716 /// use std::sync::Arc;
1718 /// let first_rc = Arc::new(5);
1719 /// let first = Arc::downgrade(&first_rc);
1720 /// let second = Arc::downgrade(&first_rc);
1722 /// assert!(first.ptr_eq(&second));
1724 /// let third_rc = Arc::new(5);
1725 /// let third = Arc::downgrade(&third_rc);
1727 /// assert!(!first.ptr_eq(&third));
1730 /// Comparing `Weak::new`.
1733 /// use std::sync::{Arc, Weak};
1735 /// let first = Weak::new();
1736 /// let second = Weak::new();
1737 /// assert!(first.ptr_eq(&second));
1739 /// let third_rc = Arc::new(());
1740 /// let third = Arc::downgrade(&third_rc);
1741 /// assert!(!first.ptr_eq(&third));
1744 /// [`ptr::eq`]: ../../std/ptr/fn.eq.html
1746 #[stable(feature = "weak_ptr_eq", since = "1.39.0")]
1747 pub fn ptr_eq(&self, other: &Self) -> bool {
1748 self.ptr.as_ptr() == other.ptr.as_ptr()
1752 #[stable(feature = "arc_weak", since = "1.4.0")]
1753 impl<T: ?Sized> Clone for Weak<T> {
1754 /// Makes a clone of the `Weak` pointer that points to the same allocation.
1759 /// use std::sync::{Arc, Weak};
1761 /// let weak_five = Arc::downgrade(&Arc::new(5));
1763 /// let _ = Weak::clone(&weak_five);
1766 fn clone(&self) -> Weak<T> {
1767 let inner = if let Some(inner) = self.inner() {
1770 return Weak { ptr: self.ptr };
1772 // See comments in Arc::clone() for why this is relaxed. This can use a
1773 // fetch_add (ignoring the lock) because the weak count is only locked
1774 // where are *no other* weak pointers in existence. (So we can't be
1775 // running this code in that case).
1776 let old_size = inner.weak.fetch_add(1, Relaxed);
1778 // See comments in Arc::clone() for why we do this (for mem::forget).
1779 if old_size > MAX_REFCOUNT {
1783 Weak { ptr: self.ptr }
1787 #[stable(feature = "downgraded_weak", since = "1.10.0")]
1788 impl<T> Default for Weak<T> {
1789 /// Constructs a new `Weak<T>`, without allocating memory.
1790 /// Calling [`upgrade`] on the return value always
1793 /// [`None`]: ../../std/option/enum.Option.html#variant.None
1794 /// [`upgrade`]: ../../std/sync/struct.Weak.html#method.upgrade
1799 /// use std::sync::Weak;
1801 /// let empty: Weak<i64> = Default::default();
1802 /// assert!(empty.upgrade().is_none());
1804 fn default() -> Weak<T> {
1809 #[stable(feature = "arc_weak", since = "1.4.0")]
1810 impl<T: ?Sized> Drop for Weak<T> {
1811 /// Drops the `Weak` pointer.
1816 /// use std::sync::{Arc, Weak};
1820 /// impl Drop for Foo {
1821 /// fn drop(&mut self) {
1822 /// println!("dropped!");
1826 /// let foo = Arc::new(Foo);
1827 /// let weak_foo = Arc::downgrade(&foo);
1828 /// let other_weak_foo = Weak::clone(&weak_foo);
1830 /// drop(weak_foo); // Doesn't print anything
1831 /// drop(foo); // Prints "dropped!"
1833 /// assert!(other_weak_foo.upgrade().is_none());
1835 fn drop(&mut self) {
1836 // If we find out that we were the last weak pointer, then its time to
1837 // deallocate the data entirely. See the discussion in Arc::drop() about
1838 // the memory orderings
1840 // It's not necessary to check for the locked state here, because the
1841 // weak count can only be locked if there was precisely one weak ref,
1842 // meaning that drop could only subsequently run ON that remaining weak
1843 // ref, which can only happen after the lock is released.
1844 let inner = if let Some(inner) = self.inner() { inner } else { return };
1846 if inner.weak.fetch_sub(1, Release) == 1 {
1847 acquire!(inner.weak);
1848 unsafe { Global.dealloc(self.ptr.cast(), Layout::for_value(self.ptr.as_ref())) }
1853 #[stable(feature = "rust1", since = "1.0.0")]
1854 trait ArcEqIdent<T: ?Sized + PartialEq> {
1855 fn eq(&self, other: &Arc<T>) -> bool;
1856 fn ne(&self, other: &Arc<T>) -> bool;
1859 #[stable(feature = "rust1", since = "1.0.0")]
1860 impl<T: ?Sized + PartialEq> ArcEqIdent<T> for Arc<T> {
1862 default fn eq(&self, other: &Arc<T>) -> bool {
1866 default fn ne(&self, other: &Arc<T>) -> bool {
1871 /// We're doing this specialization here, and not as a more general optimization on `&T`, because it
1872 /// would otherwise add a cost to all equality checks on refs. We assume that `Arc`s are used to
1873 /// store large values, that are slow to clone, but also heavy to check for equality, causing this
1874 /// cost to pay off more easily. It's also more likely to have two `Arc` clones, that point to
1875 /// the same value, than two `&T`s.
1877 /// We can only do this when `T: Eq` as a `PartialEq` might be deliberately irreflexive.
1878 #[stable(feature = "rust1", since = "1.0.0")]
1879 impl<T: ?Sized + crate::rc::MarkerEq> ArcEqIdent<T> for Arc<T> {
1881 fn eq(&self, other: &Arc<T>) -> bool {
1882 Arc::ptr_eq(self, other) || **self == **other
1886 fn ne(&self, other: &Arc<T>) -> bool {
1887 !Arc::ptr_eq(self, other) && **self != **other
1891 #[stable(feature = "rust1", since = "1.0.0")]
1892 impl<T: ?Sized + PartialEq> PartialEq for Arc<T> {
1893 /// Equality for two `Arc`s.
1895 /// Two `Arc`s are equal if their inner values are equal, even if they are
1896 /// stored in different allocation.
1898 /// If `T` also implements `Eq` (implying reflexivity of equality),
1899 /// two `Arc`s that point to the same allocation are always equal.
1904 /// use std::sync::Arc;
1906 /// let five = Arc::new(5);
1908 /// assert!(five == Arc::new(5));
1911 fn eq(&self, other: &Arc<T>) -> bool {
1912 ArcEqIdent::eq(self, other)
1915 /// Inequality for two `Arc`s.
1917 /// Two `Arc`s are unequal if their inner values are unequal.
1919 /// If `T` also implements `Eq` (implying reflexivity of equality),
1920 /// two `Arc`s that point to the same value are never unequal.
1925 /// use std::sync::Arc;
1927 /// let five = Arc::new(5);
1929 /// assert!(five != Arc::new(6));
1932 fn ne(&self, other: &Arc<T>) -> bool {
1933 ArcEqIdent::ne(self, other)
1937 #[stable(feature = "rust1", since = "1.0.0")]
1938 impl<T: ?Sized + PartialOrd> PartialOrd for Arc<T> {
1939 /// Partial comparison for two `Arc`s.
1941 /// The two are compared by calling `partial_cmp()` on their inner values.
1946 /// use std::sync::Arc;
1947 /// use std::cmp::Ordering;
1949 /// let five = Arc::new(5);
1951 /// assert_eq!(Some(Ordering::Less), five.partial_cmp(&Arc::new(6)));
1953 fn partial_cmp(&self, other: &Arc<T>) -> Option<Ordering> {
1954 (**self).partial_cmp(&**other)
1957 /// Less-than comparison for two `Arc`s.
1959 /// The two are compared by calling `<` on their inner values.
1964 /// use std::sync::Arc;
1966 /// let five = Arc::new(5);
1968 /// assert!(five < Arc::new(6));
1970 fn lt(&self, other: &Arc<T>) -> bool {
1971 *(*self) < *(*other)
1974 /// 'Less than or equal to' comparison for two `Arc`s.
1976 /// The two are compared by calling `<=` on their inner values.
1981 /// use std::sync::Arc;
1983 /// let five = Arc::new(5);
1985 /// assert!(five <= Arc::new(5));
1987 fn le(&self, other: &Arc<T>) -> bool {
1988 *(*self) <= *(*other)
1991 /// Greater-than comparison for two `Arc`s.
1993 /// The two are compared by calling `>` on their inner values.
1998 /// use std::sync::Arc;
2000 /// let five = Arc::new(5);
2002 /// assert!(five > Arc::new(4));
2004 fn gt(&self, other: &Arc<T>) -> bool {
2005 *(*self) > *(*other)
2008 /// 'Greater than or equal to' comparison for two `Arc`s.
2010 /// The two are compared by calling `>=` on their inner values.
2015 /// use std::sync::Arc;
2017 /// let five = Arc::new(5);
2019 /// assert!(five >= Arc::new(5));
2021 fn ge(&self, other: &Arc<T>) -> bool {
2022 *(*self) >= *(*other)
2025 #[stable(feature = "rust1", since = "1.0.0")]
2026 impl<T: ?Sized + Ord> Ord for Arc<T> {
2027 /// Comparison for two `Arc`s.
2029 /// The two are compared by calling `cmp()` on their inner values.
2034 /// use std::sync::Arc;
2035 /// use std::cmp::Ordering;
2037 /// let five = Arc::new(5);
2039 /// assert_eq!(Ordering::Less, five.cmp(&Arc::new(6)));
2041 fn cmp(&self, other: &Arc<T>) -> Ordering {
2042 (**self).cmp(&**other)
2045 #[stable(feature = "rust1", since = "1.0.0")]
2046 impl<T: ?Sized + Eq> Eq for Arc<T> {}
2048 #[stable(feature = "rust1", since = "1.0.0")]
2049 impl<T: ?Sized + fmt::Display> fmt::Display for Arc<T> {
2050 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
2051 fmt::Display::fmt(&**self, f)
2055 #[stable(feature = "rust1", since = "1.0.0")]
2056 impl<T: ?Sized + fmt::Debug> fmt::Debug for Arc<T> {
2057 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
2058 fmt::Debug::fmt(&**self, f)
2062 #[stable(feature = "rust1", since = "1.0.0")]
2063 impl<T: ?Sized> fmt::Pointer for Arc<T> {
2064 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
2065 fmt::Pointer::fmt(&(&**self as *const T), f)
2069 #[stable(feature = "rust1", since = "1.0.0")]
2070 impl<T: Default> Default for Arc<T> {
2071 /// Creates a new `Arc<T>`, with the `Default` value for `T`.
2076 /// use std::sync::Arc;
2078 /// let x: Arc<i32> = Default::default();
2079 /// assert_eq!(*x, 0);
2081 fn default() -> Arc<T> {
2082 Arc::new(Default::default())
2086 #[stable(feature = "rust1", since = "1.0.0")]
2087 impl<T: ?Sized + Hash> Hash for Arc<T> {
2088 fn hash<H: Hasher>(&self, state: &mut H) {
2089 (**self).hash(state)
2093 #[stable(feature = "from_for_ptrs", since = "1.6.0")]
2094 impl<T> From<T> for Arc<T> {
2095 fn from(t: T) -> Self {
2100 #[stable(feature = "shared_from_slice", since = "1.21.0")]
2101 impl<T: Clone> From<&[T]> for Arc<[T]> {
2103 fn from(v: &[T]) -> Arc<[T]> {
2104 <Self as ArcFromSlice<T>>::from_slice(v)
2108 #[stable(feature = "shared_from_slice", since = "1.21.0")]
2109 impl From<&str> for Arc<str> {
2111 fn from(v: &str) -> Arc<str> {
2112 let arc = Arc::<[u8]>::from(v.as_bytes());
2113 unsafe { Arc::from_raw(Arc::into_raw(arc) as *const str) }
2117 #[stable(feature = "shared_from_slice", since = "1.21.0")]
2118 impl From<String> for Arc<str> {
2120 fn from(v: String) -> Arc<str> {
2125 #[stable(feature = "shared_from_slice", since = "1.21.0")]
2126 impl<T: ?Sized> From<Box<T>> for Arc<T> {
2128 fn from(v: Box<T>) -> Arc<T> {
2133 #[stable(feature = "shared_from_slice", since = "1.21.0")]
2134 impl<T> From<Vec<T>> for Arc<[T]> {
2136 fn from(mut v: Vec<T>) -> Arc<[T]> {
2138 let arc = Arc::copy_from_slice(&v);
2140 // Allow the Vec to free its memory, but not destroy its contents
2148 #[stable(feature = "shared_from_cow", since = "1.45.0")]
2149 impl<'a, B> From<Cow<'a, B>> for Arc<B>
2151 B: ToOwned + ?Sized,
2152 Arc<B>: From<&'a B> + From<B::Owned>,
2155 fn from(cow: Cow<'a, B>) -> Arc<B> {
2157 Cow::Borrowed(s) => Arc::from(s),
2158 Cow::Owned(s) => Arc::from(s),
2163 #[stable(feature = "boxed_slice_try_from", since = "1.43.0")]
2164 impl<T, const N: usize> TryFrom<Arc<[T]>> for Arc<[T; N]> {
2165 type Error = Arc<[T]>;
2167 fn try_from(boxed_slice: Arc<[T]>) -> Result<Self, Self::Error> {
2168 if boxed_slice.len() == N {
2169 Ok(unsafe { Arc::from_raw(Arc::into_raw(boxed_slice) as *mut [T; N]) })
2176 #[stable(feature = "shared_from_iter", since = "1.37.0")]
2177 impl<T> iter::FromIterator<T> for Arc<[T]> {
2178 /// Takes each element in the `Iterator` and collects it into an `Arc<[T]>`.
2180 /// # Performance characteristics
2182 /// ## The general case
2184 /// In the general case, collecting into `Arc<[T]>` is done by first
2185 /// collecting into a `Vec<T>`. That is, when writing the following:
2188 /// # use std::sync::Arc;
2189 /// let evens: Arc<[u8]> = (0..10).filter(|&x| x % 2 == 0).collect();
2190 /// # assert_eq!(&*evens, &[0, 2, 4, 6, 8]);
2193 /// this behaves as if we wrote:
2196 /// # use std::sync::Arc;
2197 /// let evens: Arc<[u8]> = (0..10).filter(|&x| x % 2 == 0)
2198 /// .collect::<Vec<_>>() // The first set of allocations happens here.
2199 /// .into(); // A second allocation for `Arc<[T]>` happens here.
2200 /// # assert_eq!(&*evens, &[0, 2, 4, 6, 8]);
2203 /// This will allocate as many times as needed for constructing the `Vec<T>`
2204 /// and then it will allocate once for turning the `Vec<T>` into the `Arc<[T]>`.
2206 /// ## Iterators of known length
2208 /// When your `Iterator` implements `TrustedLen` and is of an exact size,
2209 /// a single allocation will be made for the `Arc<[T]>`. For example:
2212 /// # use std::sync::Arc;
2213 /// let evens: Arc<[u8]> = (0..10).collect(); // Just a single allocation happens here.
2214 /// # assert_eq!(&*evens, &*(0..10).collect::<Vec<_>>());
2216 fn from_iter<I: iter::IntoIterator<Item = T>>(iter: I) -> Self {
2217 ToArcSlice::to_arc_slice(iter.into_iter())
2221 /// Specialization trait used for collecting into `Arc<[T]>`.
2222 trait ToArcSlice<T>: Iterator<Item = T> + Sized {
2223 fn to_arc_slice(self) -> Arc<[T]>;
2226 impl<T, I: Iterator<Item = T>> ToArcSlice<T> for I {
2227 default fn to_arc_slice(self) -> Arc<[T]> {
2228 self.collect::<Vec<T>>().into()
2232 impl<T, I: iter::TrustedLen<Item = T>> ToArcSlice<T> for I {
2233 fn to_arc_slice(self) -> Arc<[T]> {
2234 // This is the case for a `TrustedLen` iterator.
2235 let (low, high) = self.size_hint();
2236 if let Some(high) = high {
2240 "TrustedLen iterator's size hint is not exact: {:?}",
2245 // SAFETY: We need to ensure that the iterator has an exact length and we have.
2246 Arc::from_iter_exact(self, low)
2249 // Fall back to normal implementation.
2250 self.collect::<Vec<T>>().into()
2255 #[stable(feature = "rust1", since = "1.0.0")]
2256 impl<T: ?Sized> borrow::Borrow<T> for Arc<T> {
2257 fn borrow(&self) -> &T {
2262 #[stable(since = "1.5.0", feature = "smart_ptr_as_ref")]
2263 impl<T: ?Sized> AsRef<T> for Arc<T> {
2264 fn as_ref(&self) -> &T {
2269 #[stable(feature = "pin", since = "1.33.0")]
2270 impl<T: ?Sized> Unpin for Arc<T> {}
2272 /// Get the offset within an `ArcInner` for
2273 /// a payload of type described by a pointer.
2277 /// This has the same safety requirements as `align_of_val_raw`. In effect:
2279 /// - This function is safe for any argument if `T` is sized, and
2280 /// - if `T` is unsized, the pointer must have appropriate pointer metadata
2281 /// aquired from the real instance that you are getting this offset for.
2282 unsafe fn data_offset<T: ?Sized>(ptr: *const T) -> isize {
2283 // Align the unsized value to the end of the `ArcInner`.
2284 // Because it is `?Sized`, it will always be the last field in memory.
2285 // Note: This is a detail of the current implementation of the compiler,
2286 // and is not a guaranteed language detail. Do not rely on it outside of std.
2287 unsafe { data_offset_align(align_of_val(&*ptr)) }
2291 fn data_offset_align(align: usize) -> isize {
2292 let layout = Layout::new::<ArcInner<()>>();
2293 (layout.size() + layout.padding_needed_for(align)) as isize