1 #![stable(feature = "rust1", since = "1.0.0")]
3 //! Thread-safe reference-counting pointers.
5 //! See the [`Arc<T>`][Arc] documentation for more details.
9 use core::cmp::Ordering;
10 use core::convert::{From, TryFrom};
12 use core::hash::{Hash, Hasher};
14 use core::intrinsics::abort;
15 #[cfg(not(no_global_oom_handling))]
17 use core::marker::{PhantomData, Unpin, Unsize};
18 #[cfg(not(no_global_oom_handling))]
19 use core::mem::size_of_val;
20 use core::mem::{self, align_of_val_raw};
21 use core::ops::{CoerceUnsized, Deref, DispatchFromDyn, Receiver};
22 use core::panic::{RefUnwindSafe, UnwindSafe};
24 use core::ptr::{self, NonNull};
25 #[cfg(not(no_global_oom_handling))]
26 use core::slice::from_raw_parts_mut;
27 use core::sync::atomic;
28 use core::sync::atomic::Ordering::{Acquire, Relaxed, Release, SeqCst};
30 #[cfg(not(no_global_oom_handling))]
31 use crate::alloc::handle_alloc_error;
32 #[cfg(not(no_global_oom_handling))]
33 use crate::alloc::{box_free, WriteCloneIntoRaw};
34 use crate::alloc::{AllocError, Allocator, Global, Layout};
35 use crate::borrow::{Cow, ToOwned};
36 use crate::boxed::Box;
37 use crate::rc::is_dangling;
38 #[cfg(not(no_global_oom_handling))]
39 use crate::string::String;
40 #[cfg(not(no_global_oom_handling))]
46 /// A soft limit on the amount of references that may be made to an `Arc`.
48 /// Going above this limit will abort your program (although not
49 /// necessarily) at _exactly_ `MAX_REFCOUNT + 1` references.
50 const MAX_REFCOUNT: usize = (isize::MAX) as usize;
52 #[cfg(not(sanitize = "thread"))]
53 macro_rules! acquire {
55 atomic::fence(Acquire)
59 // ThreadSanitizer does not support memory fences. To avoid false positive
60 // reports in Arc / Weak implementation use atomic loads for synchronization
62 #[cfg(sanitize = "thread")]
63 macro_rules! acquire {
69 /// A thread-safe reference-counting pointer. 'Arc' stands for 'Atomically
70 /// Reference Counted'.
72 /// The type `Arc<T>` provides shared ownership of a value of type `T`,
73 /// allocated in the heap. Invoking [`clone`][clone] on `Arc` produces
74 /// a new `Arc` instance, which points to the same allocation on the heap as the
75 /// source `Arc`, while increasing a reference count. When the last `Arc`
76 /// pointer to a given allocation is destroyed, the value stored in that allocation (often
77 /// referred to as "inner value") is also dropped.
79 /// Shared references in Rust disallow mutation by default, and `Arc` is no
80 /// exception: you cannot generally obtain a mutable reference to something
81 /// inside an `Arc`. If you need to mutate through an `Arc`, use
82 /// [`Mutex`][mutex], [`RwLock`][rwlock], or one of the [`Atomic`][atomic]
87 /// Unlike [`Rc<T>`], `Arc<T>` uses atomic operations for its reference
88 /// counting. This means that it is thread-safe. The disadvantage is that
89 /// atomic operations are more expensive than ordinary memory accesses. If you
90 /// are not sharing reference-counted allocations between threads, consider using
91 /// [`Rc<T>`] for lower overhead. [`Rc<T>`] is a safe default, because the
92 /// compiler will catch any attempt to send an [`Rc<T>`] between threads.
93 /// However, a library might choose `Arc<T>` in order to give library consumers
96 /// `Arc<T>` will implement [`Send`] and [`Sync`] as long as the `T` implements
97 /// [`Send`] and [`Sync`]. Why can't you put a non-thread-safe type `T` in an
98 /// `Arc<T>` to make it thread-safe? This may be a bit counter-intuitive at
99 /// first: after all, isn't the point of `Arc<T>` thread safety? The key is
100 /// this: `Arc<T>` makes it thread safe to have multiple ownership of the same
101 /// data, but it doesn't add thread safety to its data. Consider
102 /// `Arc<`[`RefCell<T>`]`>`. [`RefCell<T>`] isn't [`Sync`], and if `Arc<T>` was always
103 /// [`Send`], `Arc<`[`RefCell<T>`]`>` would be as well. But then we'd have a problem:
104 /// [`RefCell<T>`] is not thread safe; it keeps track of the borrowing count using
105 /// non-atomic operations.
107 /// In the end, this means that you may need to pair `Arc<T>` with some sort of
108 /// [`std::sync`] type, usually [`Mutex<T>`][mutex].
110 /// ## Breaking cycles with `Weak`
112 /// The [`downgrade`][downgrade] method can be used to create a non-owning
113 /// [`Weak`] pointer. A [`Weak`] pointer can be [`upgrade`][upgrade]d
114 /// to an `Arc`, but this will return [`None`] if the value stored in the allocation has
115 /// already been dropped. In other words, `Weak` pointers do not keep the value
116 /// inside the allocation alive; however, they *do* keep the allocation
117 /// (the backing store for the value) alive.
119 /// A cycle between `Arc` pointers will never be deallocated. For this reason,
120 /// [`Weak`] is used to break cycles. For example, a tree could have
121 /// strong `Arc` pointers from parent nodes to children, and [`Weak`]
122 /// pointers from children back to their parents.
124 /// # Cloning references
126 /// Creating a new reference from an existing reference-counted pointer is done using the
127 /// `Clone` trait implemented for [`Arc<T>`][Arc] and [`Weak<T>`][Weak].
130 /// use std::sync::Arc;
131 /// let foo = Arc::new(vec![1.0, 2.0, 3.0]);
132 /// // The two syntaxes below are equivalent.
133 /// let a = foo.clone();
134 /// let b = Arc::clone(&foo);
135 /// // a, b, and foo are all Arcs that point to the same memory location
138 /// ## `Deref` behavior
140 /// `Arc<T>` automatically dereferences to `T` (via the [`Deref`][deref] trait),
141 /// so you can call `T`'s methods on a value of type `Arc<T>`. To avoid name
142 /// clashes with `T`'s methods, the methods of `Arc<T>` itself are associated
143 /// functions, called using [fully qualified syntax]:
146 /// use std::sync::Arc;
148 /// let my_arc = Arc::new(());
149 /// Arc::downgrade(&my_arc);
152 /// `Arc<T>`'s implementations of traits like `Clone` may also be called using
153 /// fully qualified syntax. Some people prefer to use fully qualified syntax,
154 /// while others prefer using method-call syntax.
157 /// use std::sync::Arc;
159 /// let arc = Arc::new(());
160 /// // Method-call syntax
161 /// let arc2 = arc.clone();
162 /// // Fully qualified syntax
163 /// let arc3 = Arc::clone(&arc);
166 /// [`Weak<T>`][Weak] does not auto-dereference to `T`, because the inner value may have
167 /// already been dropped.
169 /// [`Rc<T>`]: crate::rc::Rc
170 /// [clone]: Clone::clone
171 /// [mutex]: ../../std/sync/struct.Mutex.html
172 /// [rwlock]: ../../std/sync/struct.RwLock.html
173 /// [atomic]: core::sync::atomic
174 /// [`Send`]: core::marker::Send
175 /// [`Sync`]: core::marker::Sync
176 /// [deref]: core::ops::Deref
177 /// [downgrade]: Arc::downgrade
178 /// [upgrade]: Weak::upgrade
179 /// [`RefCell<T>`]: core::cell::RefCell
180 /// [`std::sync`]: ../../std/sync/index.html
181 /// [`Arc::clone(&from)`]: Arc::clone
182 /// [fully qualified syntax]: https://doc.rust-lang.org/book/ch19-03-advanced-traits.html#fully-qualified-syntax-for-disambiguation-calling-methods-with-the-same-name
186 /// Sharing some immutable data between threads:
188 // Note that we **do not** run these tests here. The windows builders get super
189 // unhappy if a thread outlives the main thread and then exits at the same time
190 // (something deadlocks) so we just avoid this entirely by not running these
193 /// use std::sync::Arc;
196 /// let five = Arc::new(5);
199 /// let five = Arc::clone(&five);
201 /// thread::spawn(move || {
202 /// println!("{:?}", five);
207 /// Sharing a mutable [`AtomicUsize`]:
209 /// [`AtomicUsize`]: core::sync::atomic::AtomicUsize
212 /// use std::sync::Arc;
213 /// use std::sync::atomic::{AtomicUsize, Ordering};
216 /// let val = Arc::new(AtomicUsize::new(5));
219 /// let val = Arc::clone(&val);
221 /// thread::spawn(move || {
222 /// let v = val.fetch_add(1, Ordering::SeqCst);
223 /// println!("{:?}", v);
228 /// See the [`rc` documentation][rc_examples] for more examples of reference
229 /// counting in general.
231 /// [rc_examples]: crate::rc#examples
232 #[cfg_attr(not(test), rustc_diagnostic_item = "Arc")]
233 #[stable(feature = "rust1", since = "1.0.0")]
234 pub struct Arc<T: ?Sized> {
235 ptr: NonNull<ArcInner<T>>,
236 phantom: PhantomData<ArcInner<T>>,
239 #[stable(feature = "rust1", since = "1.0.0")]
240 unsafe impl<T: ?Sized + Sync + Send> Send for Arc<T> {}
241 #[stable(feature = "rust1", since = "1.0.0")]
242 unsafe impl<T: ?Sized + Sync + Send> Sync for Arc<T> {}
244 #[stable(feature = "catch_unwind", since = "1.9.0")]
245 impl<T: RefUnwindSafe + ?Sized> UnwindSafe for Arc<T> {}
247 #[unstable(feature = "coerce_unsized", issue = "27732")]
248 impl<T: ?Sized + Unsize<U>, U: ?Sized> CoerceUnsized<Arc<U>> for Arc<T> {}
250 #[unstable(feature = "dispatch_from_dyn", issue = "none")]
251 impl<T: ?Sized + Unsize<U>, U: ?Sized> DispatchFromDyn<Arc<U>> for Arc<T> {}
253 impl<T: ?Sized> Arc<T> {
254 fn from_inner(ptr: NonNull<ArcInner<T>>) -> Self {
255 Self { ptr, phantom: PhantomData }
258 unsafe fn from_ptr(ptr: *mut ArcInner<T>) -> Self {
259 unsafe { Self::from_inner(NonNull::new_unchecked(ptr)) }
263 /// `Weak` is a version of [`Arc`] that holds a non-owning reference to the
264 /// managed allocation. The allocation is accessed by calling [`upgrade`] on the `Weak`
265 /// pointer, which returns an [`Option`]`<`[`Arc`]`<T>>`.
267 /// Since a `Weak` reference does not count towards ownership, it will not
268 /// prevent the value stored in the allocation from being dropped, and `Weak` itself makes no
269 /// guarantees about the value still being present. Thus it may return [`None`]
270 /// when [`upgrade`]d. Note however that a `Weak` reference *does* prevent the allocation
271 /// itself (the backing store) from being deallocated.
273 /// A `Weak` pointer is useful for keeping a temporary reference to the allocation
274 /// managed by [`Arc`] without preventing its inner value from being dropped. It is also used to
275 /// prevent circular references between [`Arc`] pointers, since mutual owning references
276 /// would never allow either [`Arc`] to be dropped. For example, a tree could
277 /// have strong [`Arc`] pointers from parent nodes to children, and `Weak`
278 /// pointers from children back to their parents.
280 /// The typical way to obtain a `Weak` pointer is to call [`Arc::downgrade`].
282 /// [`upgrade`]: Weak::upgrade
283 #[stable(feature = "arc_weak", since = "1.4.0")]
284 pub struct Weak<T: ?Sized> {
285 // This is a `NonNull` to allow optimizing the size of this type in enums,
286 // but it is not necessarily a valid pointer.
287 // `Weak::new` sets this to `usize::MAX` so that it doesn’t need
288 // to allocate space on the heap. That's not a value a real pointer
289 // will ever have because RcBox has alignment at least 2.
290 // This is only possible when `T: Sized`; unsized `T` never dangle.
291 ptr: NonNull<ArcInner<T>>,
294 #[stable(feature = "arc_weak", since = "1.4.0")]
295 unsafe impl<T: ?Sized + Sync + Send> Send for Weak<T> {}
296 #[stable(feature = "arc_weak", since = "1.4.0")]
297 unsafe impl<T: ?Sized + Sync + Send> Sync for Weak<T> {}
299 #[unstable(feature = "coerce_unsized", issue = "27732")]
300 impl<T: ?Sized + Unsize<U>, U: ?Sized> CoerceUnsized<Weak<U>> for Weak<T> {}
301 #[unstable(feature = "dispatch_from_dyn", issue = "none")]
302 impl<T: ?Sized + Unsize<U>, U: ?Sized> DispatchFromDyn<Weak<U>> for Weak<T> {}
304 #[stable(feature = "arc_weak", since = "1.4.0")]
305 impl<T: ?Sized + fmt::Debug> fmt::Debug for Weak<T> {
306 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
311 // This is repr(C) to future-proof against possible field-reordering, which
312 // would interfere with otherwise safe [into|from]_raw() of transmutable
315 struct ArcInner<T: ?Sized> {
316 strong: atomic::AtomicUsize,
318 // the value usize::MAX acts as a sentinel for temporarily "locking" the
319 // ability to upgrade weak pointers or downgrade strong ones; this is used
320 // to avoid races in `make_mut` and `get_mut`.
321 weak: atomic::AtomicUsize,
326 unsafe impl<T: ?Sized + Sync + Send> Send for ArcInner<T> {}
327 unsafe impl<T: ?Sized + Sync + Send> Sync for ArcInner<T> {}
330 /// Constructs a new `Arc<T>`.
335 /// use std::sync::Arc;
337 /// let five = Arc::new(5);
339 #[cfg(not(no_global_oom_handling))]
341 #[stable(feature = "rust1", since = "1.0.0")]
342 pub fn new(data: T) -> Arc<T> {
343 // Start the weak pointer count as 1 which is the weak pointer that's
344 // held by all the strong pointers (kinda), see std/rc.rs for more info
345 let x: Box<_> = box ArcInner {
346 strong: atomic::AtomicUsize::new(1),
347 weak: atomic::AtomicUsize::new(1),
350 Self::from_inner(Box::leak(x).into())
353 /// Constructs a new `Arc<T>` using a weak reference to itself. Attempting
354 /// to upgrade the weak reference before this function returns will result
355 /// in a `None` value. However, the weak reference may be cloned freely and
356 /// stored for use at a later time.
360 /// #![feature(arc_new_cyclic)]
361 /// #![allow(dead_code)]
363 /// use std::sync::{Arc, Weak};
369 /// let foo = Arc::new_cyclic(|me| Foo {
373 #[cfg(not(no_global_oom_handling))]
375 #[unstable(feature = "arc_new_cyclic", issue = "75861")]
376 pub fn new_cyclic(data_fn: impl FnOnce(&Weak<T>) -> T) -> Arc<T> {
377 // Construct the inner in the "uninitialized" state with a single
379 let uninit_ptr: NonNull<_> = Box::leak(box ArcInner {
380 strong: atomic::AtomicUsize::new(0),
381 weak: atomic::AtomicUsize::new(1),
382 data: mem::MaybeUninit::<T>::uninit(),
385 let init_ptr: NonNull<ArcInner<T>> = uninit_ptr.cast();
387 let weak = Weak { ptr: init_ptr };
389 // It's important we don't give up ownership of the weak pointer, or
390 // else the memory might be freed by the time `data_fn` returns. If
391 // we really wanted to pass ownership, we could create an additional
392 // weak pointer for ourselves, but this would result in additional
393 // updates to the weak reference count which might not be necessary
395 let data = data_fn(&weak);
397 // Now we can properly initialize the inner value and turn our weak
398 // reference into a strong reference.
400 let inner = init_ptr.as_ptr();
401 ptr::write(ptr::addr_of_mut!((*inner).data), data);
403 // The above write to the data field must be visible to any threads which
404 // observe a non-zero strong count. Therefore we need at least "Release" ordering
405 // in order to synchronize with the `compare_exchange_weak` in `Weak::upgrade`.
407 // "Acquire" ordering is not required. When considering the possible behaviours
408 // of `data_fn` we only need to look at what it could do with a reference to a
409 // non-upgradeable `Weak`:
410 // - It can *clone* the `Weak`, increasing the weak reference count.
411 // - It can drop those clones, decreasing the weak reference count (but never to zero).
413 // These side effects do not impact us in any way, and no other side effects are
414 // possible with safe code alone.
415 let prev_value = (*inner).strong.fetch_add(1, Release);
416 debug_assert_eq!(prev_value, 0, "No prior strong references should exist");
419 let strong = Arc::from_inner(init_ptr);
421 // Strong references should collectively own a shared weak reference,
422 // so don't run the destructor for our old weak reference.
427 /// Constructs a new `Arc` with uninitialized contents.
432 /// #![feature(new_uninit)]
433 /// #![feature(get_mut_unchecked)]
435 /// use std::sync::Arc;
437 /// let mut five = Arc::<u32>::new_uninit();
439 /// let five = unsafe {
440 /// // Deferred initialization:
441 /// Arc::get_mut_unchecked(&mut five).as_mut_ptr().write(5);
443 /// five.assume_init()
446 /// assert_eq!(*five, 5)
448 #[cfg(not(no_global_oom_handling))]
449 #[unstable(feature = "new_uninit", issue = "63291")]
450 pub fn new_uninit() -> Arc<mem::MaybeUninit<T>> {
452 Arc::from_ptr(Arc::allocate_for_layout(
454 |layout| Global.allocate(layout),
455 |mem| mem as *mut ArcInner<mem::MaybeUninit<T>>,
460 /// Constructs a new `Arc` with uninitialized contents, with the memory
461 /// being filled with `0` bytes.
463 /// See [`MaybeUninit::zeroed`][zeroed] for examples of correct and incorrect usage
469 /// #![feature(new_uninit)]
471 /// use std::sync::Arc;
473 /// let zero = Arc::<u32>::new_zeroed();
474 /// let zero = unsafe { zero.assume_init() };
476 /// assert_eq!(*zero, 0)
479 /// [zeroed]: ../../std/mem/union.MaybeUninit.html#method.zeroed
480 #[cfg(not(no_global_oom_handling))]
481 #[unstable(feature = "new_uninit", issue = "63291")]
482 pub fn new_zeroed() -> Arc<mem::MaybeUninit<T>> {
484 Arc::from_ptr(Arc::allocate_for_layout(
486 |layout| Global.allocate_zeroed(layout),
487 |mem| mem as *mut ArcInner<mem::MaybeUninit<T>>,
492 /// Constructs a new `Pin<Arc<T>>`. If `T` does not implement `Unpin`, then
493 /// `data` will be pinned in memory and unable to be moved.
494 #[cfg(not(no_global_oom_handling))]
495 #[stable(feature = "pin", since = "1.33.0")]
496 pub fn pin(data: T) -> Pin<Arc<T>> {
497 unsafe { Pin::new_unchecked(Arc::new(data)) }
500 /// Constructs a new `Pin<Arc<T>>`, return an error if allocation fails.
501 #[unstable(feature = "allocator_api", issue = "32838")]
503 pub fn try_pin(data: T) -> Result<Pin<Arc<T>>, AllocError> {
504 unsafe { Ok(Pin::new_unchecked(Arc::try_new(data)?)) }
507 /// Constructs a new `Arc<T>`, returning an error if allocation fails.
512 /// #![feature(allocator_api)]
513 /// use std::sync::Arc;
515 /// let five = Arc::try_new(5)?;
516 /// # Ok::<(), std::alloc::AllocError>(())
518 #[unstable(feature = "allocator_api", issue = "32838")]
520 pub fn try_new(data: T) -> Result<Arc<T>, AllocError> {
521 // Start the weak pointer count as 1 which is the weak pointer that's
522 // held by all the strong pointers (kinda), see std/rc.rs for more info
523 let x: Box<_> = Box::try_new(ArcInner {
524 strong: atomic::AtomicUsize::new(1),
525 weak: atomic::AtomicUsize::new(1),
528 Ok(Self::from_inner(Box::leak(x).into()))
531 /// Constructs a new `Arc` with uninitialized contents, returning an error
532 /// if allocation fails.
537 /// #![feature(new_uninit, allocator_api)]
538 /// #![feature(get_mut_unchecked)]
540 /// use std::sync::Arc;
542 /// let mut five = Arc::<u32>::try_new_uninit()?;
544 /// let five = unsafe {
545 /// // Deferred initialization:
546 /// Arc::get_mut_unchecked(&mut five).as_mut_ptr().write(5);
548 /// five.assume_init()
551 /// assert_eq!(*five, 5);
552 /// # Ok::<(), std::alloc::AllocError>(())
554 #[unstable(feature = "allocator_api", issue = "32838")]
555 // #[unstable(feature = "new_uninit", issue = "63291")]
556 pub fn try_new_uninit() -> Result<Arc<mem::MaybeUninit<T>>, AllocError> {
558 Ok(Arc::from_ptr(Arc::try_allocate_for_layout(
560 |layout| Global.allocate(layout),
561 |mem| mem as *mut ArcInner<mem::MaybeUninit<T>>,
566 /// Constructs a new `Arc` with uninitialized contents, with the memory
567 /// being filled with `0` bytes, returning an error if allocation fails.
569 /// See [`MaybeUninit::zeroed`][zeroed] for examples of correct and incorrect usage
575 /// #![feature(new_uninit, allocator_api)]
577 /// use std::sync::Arc;
579 /// let zero = Arc::<u32>::try_new_zeroed()?;
580 /// let zero = unsafe { zero.assume_init() };
582 /// assert_eq!(*zero, 0);
583 /// # Ok::<(), std::alloc::AllocError>(())
586 /// [zeroed]: mem::MaybeUninit::zeroed
587 #[unstable(feature = "allocator_api", issue = "32838")]
588 // #[unstable(feature = "new_uninit", issue = "63291")]
589 pub fn try_new_zeroed() -> Result<Arc<mem::MaybeUninit<T>>, AllocError> {
591 Ok(Arc::from_ptr(Arc::try_allocate_for_layout(
593 |layout| Global.allocate_zeroed(layout),
594 |mem| mem as *mut ArcInner<mem::MaybeUninit<T>>,
598 /// Returns the inner value, if the `Arc` has exactly one strong reference.
600 /// Otherwise, an [`Err`] is returned with the same `Arc` that was
603 /// This will succeed even if there are outstanding weak references.
608 /// use std::sync::Arc;
610 /// let x = Arc::new(3);
611 /// assert_eq!(Arc::try_unwrap(x), Ok(3));
613 /// let x = Arc::new(4);
614 /// let _y = Arc::clone(&x);
615 /// assert_eq!(*Arc::try_unwrap(x).unwrap_err(), 4);
618 #[stable(feature = "arc_unique", since = "1.4.0")]
619 pub fn try_unwrap(this: Self) -> Result<T, Self> {
620 if this.inner().strong.compare_exchange(1, 0, Relaxed, Relaxed).is_err() {
624 acquire!(this.inner().strong);
627 let elem = ptr::read(&this.ptr.as_ref().data);
629 // Make a weak pointer to clean up the implicit strong-weak reference
630 let _weak = Weak { ptr: this.ptr };
639 /// Constructs a new atomically reference-counted slice with uninitialized contents.
644 /// #![feature(new_uninit)]
645 /// #![feature(get_mut_unchecked)]
647 /// use std::sync::Arc;
649 /// let mut values = Arc::<[u32]>::new_uninit_slice(3);
651 /// let values = unsafe {
652 /// // Deferred initialization:
653 /// Arc::get_mut_unchecked(&mut values)[0].as_mut_ptr().write(1);
654 /// Arc::get_mut_unchecked(&mut values)[1].as_mut_ptr().write(2);
655 /// Arc::get_mut_unchecked(&mut values)[2].as_mut_ptr().write(3);
657 /// values.assume_init()
660 /// assert_eq!(*values, [1, 2, 3])
662 #[cfg(not(no_global_oom_handling))]
663 #[unstable(feature = "new_uninit", issue = "63291")]
664 pub fn new_uninit_slice(len: usize) -> Arc<[mem::MaybeUninit<T>]> {
665 unsafe { Arc::from_ptr(Arc::allocate_for_slice(len)) }
668 /// Constructs a new atomically reference-counted slice with uninitialized contents, with the memory being
669 /// filled with `0` bytes.
671 /// See [`MaybeUninit::zeroed`][zeroed] for examples of correct and
672 /// incorrect usage of this method.
677 /// #![feature(new_uninit)]
679 /// use std::sync::Arc;
681 /// let values = Arc::<[u32]>::new_zeroed_slice(3);
682 /// let values = unsafe { values.assume_init() };
684 /// assert_eq!(*values, [0, 0, 0])
687 /// [zeroed]: ../../std/mem/union.MaybeUninit.html#method.zeroed
688 #[cfg(not(no_global_oom_handling))]
689 #[unstable(feature = "new_uninit", issue = "63291")]
690 pub fn new_zeroed_slice(len: usize) -> Arc<[mem::MaybeUninit<T>]> {
692 Arc::from_ptr(Arc::allocate_for_layout(
693 Layout::array::<T>(len).unwrap(),
694 |layout| Global.allocate_zeroed(layout),
696 ptr::slice_from_raw_parts_mut(mem as *mut T, len)
697 as *mut ArcInner<[mem::MaybeUninit<T>]>
704 impl<T> Arc<mem::MaybeUninit<T>> {
705 /// Converts to `Arc<T>`.
709 /// As with [`MaybeUninit::assume_init`],
710 /// it is up to the caller to guarantee that the inner value
711 /// really is in an initialized state.
712 /// Calling this when the content is not yet fully initialized
713 /// causes immediate undefined behavior.
715 /// [`MaybeUninit::assume_init`]: ../../std/mem/union.MaybeUninit.html#method.assume_init
720 /// #![feature(new_uninit)]
721 /// #![feature(get_mut_unchecked)]
723 /// use std::sync::Arc;
725 /// let mut five = Arc::<u32>::new_uninit();
727 /// let five = unsafe {
728 /// // Deferred initialization:
729 /// Arc::get_mut_unchecked(&mut five).as_mut_ptr().write(5);
731 /// five.assume_init()
734 /// assert_eq!(*five, 5)
736 #[unstable(feature = "new_uninit", issue = "63291")]
738 pub unsafe fn assume_init(self) -> Arc<T> {
739 Arc::from_inner(mem::ManuallyDrop::new(self).ptr.cast())
743 impl<T> Arc<[mem::MaybeUninit<T>]> {
744 /// Converts to `Arc<[T]>`.
748 /// As with [`MaybeUninit::assume_init`],
749 /// it is up to the caller to guarantee that the inner value
750 /// really is in an initialized state.
751 /// Calling this when the content is not yet fully initialized
752 /// causes immediate undefined behavior.
754 /// [`MaybeUninit::assume_init`]: ../../std/mem/union.MaybeUninit.html#method.assume_init
759 /// #![feature(new_uninit)]
760 /// #![feature(get_mut_unchecked)]
762 /// use std::sync::Arc;
764 /// let mut values = Arc::<[u32]>::new_uninit_slice(3);
766 /// let values = unsafe {
767 /// // Deferred initialization:
768 /// Arc::get_mut_unchecked(&mut values)[0].as_mut_ptr().write(1);
769 /// Arc::get_mut_unchecked(&mut values)[1].as_mut_ptr().write(2);
770 /// Arc::get_mut_unchecked(&mut values)[2].as_mut_ptr().write(3);
772 /// values.assume_init()
775 /// assert_eq!(*values, [1, 2, 3])
777 #[unstable(feature = "new_uninit", issue = "63291")]
779 pub unsafe fn assume_init(self) -> Arc<[T]> {
780 unsafe { Arc::from_ptr(mem::ManuallyDrop::new(self).ptr.as_ptr() as _) }
784 impl<T: ?Sized> Arc<T> {
785 /// Consumes the `Arc`, returning the wrapped pointer.
787 /// To avoid a memory leak the pointer must be converted back to an `Arc` using
788 /// [`Arc::from_raw`].
793 /// use std::sync::Arc;
795 /// let x = Arc::new("hello".to_owned());
796 /// let x_ptr = Arc::into_raw(x);
797 /// assert_eq!(unsafe { &*x_ptr }, "hello");
799 #[stable(feature = "rc_raw", since = "1.17.0")]
800 pub fn into_raw(this: Self) -> *const T {
801 let ptr = Self::as_ptr(&this);
806 /// Provides a raw pointer to the data.
808 /// The counts are not affected in any way and the `Arc` is not consumed. The pointer is valid for
809 /// as long as there are strong counts in the `Arc`.
814 /// use std::sync::Arc;
816 /// let x = Arc::new("hello".to_owned());
817 /// let y = Arc::clone(&x);
818 /// let x_ptr = Arc::as_ptr(&x);
819 /// assert_eq!(x_ptr, Arc::as_ptr(&y));
820 /// assert_eq!(unsafe { &*x_ptr }, "hello");
822 #[stable(feature = "rc_as_ptr", since = "1.45.0")]
823 pub fn as_ptr(this: &Self) -> *const T {
824 let ptr: *mut ArcInner<T> = NonNull::as_ptr(this.ptr);
826 // SAFETY: This cannot go through Deref::deref or RcBoxPtr::inner because
827 // this is required to retain raw/mut provenance such that e.g. `get_mut` can
828 // write through the pointer after the Rc is recovered through `from_raw`.
829 unsafe { ptr::addr_of_mut!((*ptr).data) }
832 /// Constructs an `Arc<T>` from a raw pointer.
834 /// The raw pointer must have been previously returned by a call to
835 /// [`Arc<U>::into_raw`][into_raw] where `U` must have the same size and
836 /// alignment as `T`. This is trivially true if `U` is `T`.
837 /// Note that if `U` is not `T` but has the same size and alignment, this is
838 /// basically like transmuting references of different types. See
839 /// [`mem::transmute`][transmute] for more information on what
840 /// restrictions apply in this case.
842 /// The user of `from_raw` has to make sure a specific value of `T` is only
845 /// This function is unsafe because improper use may lead to memory unsafety,
846 /// even if the returned `Arc<T>` is never accessed.
848 /// [into_raw]: Arc::into_raw
849 /// [transmute]: core::mem::transmute
854 /// use std::sync::Arc;
856 /// let x = Arc::new("hello".to_owned());
857 /// let x_ptr = Arc::into_raw(x);
860 /// // Convert back to an `Arc` to prevent leak.
861 /// let x = Arc::from_raw(x_ptr);
862 /// assert_eq!(&*x, "hello");
864 /// // Further calls to `Arc::from_raw(x_ptr)` would be memory-unsafe.
867 /// // The memory was freed when `x` went out of scope above, so `x_ptr` is now dangling!
869 #[stable(feature = "rc_raw", since = "1.17.0")]
870 pub unsafe fn from_raw(ptr: *const T) -> Self {
872 let offset = data_offset(ptr);
874 // Reverse the offset to find the original ArcInner.
875 let arc_ptr = (ptr as *mut ArcInner<T>).set_ptr_value((ptr as *mut u8).offset(-offset));
877 Self::from_ptr(arc_ptr)
881 /// Creates a new [`Weak`] pointer to this allocation.
886 /// use std::sync::Arc;
888 /// let five = Arc::new(5);
890 /// let weak_five = Arc::downgrade(&five);
892 #[stable(feature = "arc_weak", since = "1.4.0")]
893 pub fn downgrade(this: &Self) -> Weak<T> {
894 // This Relaxed is OK because we're checking the value in the CAS
896 let mut cur = this.inner().weak.load(Relaxed);
899 // check if the weak counter is currently "locked"; if so, spin.
900 if cur == usize::MAX {
902 cur = this.inner().weak.load(Relaxed);
906 // NOTE: this code currently ignores the possibility of overflow
907 // into usize::MAX; in general both Rc and Arc need to be adjusted
908 // to deal with overflow.
910 // Unlike with Clone(), we need this to be an Acquire read to
911 // synchronize with the write coming from `is_unique`, so that the
912 // events prior to that write happen before this read.
913 match this.inner().weak.compare_exchange_weak(cur, cur + 1, Acquire, Relaxed) {
915 // Make sure we do not create a dangling Weak
916 debug_assert!(!is_dangling(this.ptr.as_ptr()));
917 return Weak { ptr: this.ptr };
919 Err(old) => cur = old,
924 /// Gets the number of [`Weak`] pointers to this allocation.
928 /// This method by itself is safe, but using it correctly requires extra care.
929 /// Another thread can change the weak count at any time,
930 /// including potentially between calling this method and acting on the result.
935 /// use std::sync::Arc;
937 /// let five = Arc::new(5);
938 /// let _weak_five = Arc::downgrade(&five);
940 /// // This assertion is deterministic because we haven't shared
941 /// // the `Arc` or `Weak` between threads.
942 /// assert_eq!(1, Arc::weak_count(&five));
945 #[stable(feature = "arc_counts", since = "1.15.0")]
946 pub fn weak_count(this: &Self) -> usize {
947 let cnt = this.inner().weak.load(SeqCst);
948 // If the weak count is currently locked, the value of the
949 // count was 0 just before taking the lock.
950 if cnt == usize::MAX { 0 } else { cnt - 1 }
953 /// Gets the number of strong (`Arc`) pointers to this allocation.
957 /// This method by itself is safe, but using it correctly requires extra care.
958 /// Another thread can change the strong count at any time,
959 /// including potentially between calling this method and acting on the result.
964 /// use std::sync::Arc;
966 /// let five = Arc::new(5);
967 /// let _also_five = Arc::clone(&five);
969 /// // This assertion is deterministic because we haven't shared
970 /// // the `Arc` between threads.
971 /// assert_eq!(2, Arc::strong_count(&five));
974 #[stable(feature = "arc_counts", since = "1.15.0")]
975 pub fn strong_count(this: &Self) -> usize {
976 this.inner().strong.load(SeqCst)
979 /// Increments the strong reference count on the `Arc<T>` associated with the
980 /// provided pointer by one.
984 /// The pointer must have been obtained through `Arc::into_raw`, and the
985 /// associated `Arc` instance must be valid (i.e. the strong count must be at
986 /// least 1) for the duration of this method.
991 /// use std::sync::Arc;
993 /// let five = Arc::new(5);
996 /// let ptr = Arc::into_raw(five);
997 /// Arc::increment_strong_count(ptr);
999 /// // This assertion is deterministic because we haven't shared
1000 /// // the `Arc` between threads.
1001 /// let five = Arc::from_raw(ptr);
1002 /// assert_eq!(2, Arc::strong_count(&five));
1006 #[stable(feature = "arc_mutate_strong_count", since = "1.51.0")]
1007 pub unsafe fn increment_strong_count(ptr: *const T) {
1008 // Retain Arc, but don't touch refcount by wrapping in ManuallyDrop
1009 let arc = unsafe { mem::ManuallyDrop::new(Arc::<T>::from_raw(ptr)) };
1010 // Now increase refcount, but don't drop new refcount either
1011 let _arc_clone: mem::ManuallyDrop<_> = arc.clone();
1014 /// Decrements the strong reference count on the `Arc<T>` associated with the
1015 /// provided pointer by one.
1019 /// The pointer must have been obtained through `Arc::into_raw`, and the
1020 /// associated `Arc` instance must be valid (i.e. the strong count must be at
1021 /// least 1) when invoking this method. This method can be used to release the final
1022 /// `Arc` and backing storage, but **should not** be called after the final `Arc` has been
1028 /// use std::sync::Arc;
1030 /// let five = Arc::new(5);
1033 /// let ptr = Arc::into_raw(five);
1034 /// Arc::increment_strong_count(ptr);
1036 /// // Those assertions are deterministic because we haven't shared
1037 /// // the `Arc` between threads.
1038 /// let five = Arc::from_raw(ptr);
1039 /// assert_eq!(2, Arc::strong_count(&five));
1040 /// Arc::decrement_strong_count(ptr);
1041 /// assert_eq!(1, Arc::strong_count(&five));
1045 #[stable(feature = "arc_mutate_strong_count", since = "1.51.0")]
1046 pub unsafe fn decrement_strong_count(ptr: *const T) {
1047 unsafe { mem::drop(Arc::from_raw(ptr)) };
1051 fn inner(&self) -> &ArcInner<T> {
1052 // This unsafety is ok because while this arc is alive we're guaranteed
1053 // that the inner pointer is valid. Furthermore, we know that the
1054 // `ArcInner` structure itself is `Sync` because the inner data is
1055 // `Sync` as well, so we're ok loaning out an immutable pointer to these
1057 unsafe { self.ptr.as_ref() }
1060 // Non-inlined part of `drop`.
1062 unsafe fn drop_slow(&mut self) {
1063 // Destroy the data at this time, even though we must not free the box
1064 // allocation itself (there might still be weak pointers lying around).
1065 unsafe { ptr::drop_in_place(Self::get_mut_unchecked(self)) };
1067 // Drop the weak ref collectively held by all strong references
1068 drop(Weak { ptr: self.ptr });
1072 #[stable(feature = "ptr_eq", since = "1.17.0")]
1073 /// Returns `true` if the two `Arc`s point to the same allocation
1074 /// (in a vein similar to [`ptr::eq`]).
1079 /// use std::sync::Arc;
1081 /// let five = Arc::new(5);
1082 /// let same_five = Arc::clone(&five);
1083 /// let other_five = Arc::new(5);
1085 /// assert!(Arc::ptr_eq(&five, &same_five));
1086 /// assert!(!Arc::ptr_eq(&five, &other_five));
1089 /// [`ptr::eq`]: core::ptr::eq
1090 pub fn ptr_eq(this: &Self, other: &Self) -> bool {
1091 this.ptr.as_ptr() == other.ptr.as_ptr()
1095 impl<T: ?Sized> Arc<T> {
1096 /// Allocates an `ArcInner<T>` with sufficient space for
1097 /// a possibly-unsized inner value where the value has the layout provided.
1099 /// The function `mem_to_arcinner` is called with the data pointer
1100 /// and must return back a (potentially fat)-pointer for the `ArcInner<T>`.
1101 #[cfg(not(no_global_oom_handling))]
1102 unsafe fn allocate_for_layout(
1103 value_layout: Layout,
1104 allocate: impl FnOnce(Layout) -> Result<NonNull<[u8]>, AllocError>,
1105 mem_to_arcinner: impl FnOnce(*mut u8) -> *mut ArcInner<T>,
1106 ) -> *mut ArcInner<T> {
1107 // Calculate layout using the given value layout.
1108 // Previously, layout was calculated on the expression
1109 // `&*(ptr as *const ArcInner<T>)`, but this created a misaligned
1110 // reference (see #54908).
1111 let layout = Layout::new::<ArcInner<()>>().extend(value_layout).unwrap().0.pad_to_align();
1113 Arc::try_allocate_for_layout(value_layout, allocate, mem_to_arcinner)
1114 .unwrap_or_else(|_| handle_alloc_error(layout))
1118 /// Allocates an `ArcInner<T>` with sufficient space for
1119 /// a possibly-unsized inner value where the value has the layout provided,
1120 /// returning an error if allocation fails.
1122 /// The function `mem_to_arcinner` is called with the data pointer
1123 /// and must return back a (potentially fat)-pointer for the `ArcInner<T>`.
1124 unsafe fn try_allocate_for_layout(
1125 value_layout: Layout,
1126 allocate: impl FnOnce(Layout) -> Result<NonNull<[u8]>, AllocError>,
1127 mem_to_arcinner: impl FnOnce(*mut u8) -> *mut ArcInner<T>,
1128 ) -> Result<*mut ArcInner<T>, AllocError> {
1129 // Calculate layout using the given value layout.
1130 // Previously, layout was calculated on the expression
1131 // `&*(ptr as *const ArcInner<T>)`, but this created a misaligned
1132 // reference (see #54908).
1133 let layout = Layout::new::<ArcInner<()>>().extend(value_layout).unwrap().0.pad_to_align();
1135 let ptr = allocate(layout)?;
1137 // Initialize the ArcInner
1138 let inner = mem_to_arcinner(ptr.as_non_null_ptr().as_ptr());
1139 debug_assert_eq!(unsafe { Layout::for_value(&*inner) }, layout);
1142 ptr::write(&mut (*inner).strong, atomic::AtomicUsize::new(1));
1143 ptr::write(&mut (*inner).weak, atomic::AtomicUsize::new(1));
1149 /// Allocates an `ArcInner<T>` with sufficient space for an unsized inner value.
1150 #[cfg(not(no_global_oom_handling))]
1151 unsafe fn allocate_for_ptr(ptr: *const T) -> *mut ArcInner<T> {
1152 // Allocate for the `ArcInner<T>` using the given value.
1154 Self::allocate_for_layout(
1155 Layout::for_value(&*ptr),
1156 |layout| Global.allocate(layout),
1157 |mem| (ptr as *mut ArcInner<T>).set_ptr_value(mem) as *mut ArcInner<T>,
1162 #[cfg(not(no_global_oom_handling))]
1163 fn from_box(v: Box<T>) -> Arc<T> {
1165 let (box_unique, alloc) = Box::into_unique(v);
1166 let bptr = box_unique.as_ptr();
1168 let value_size = size_of_val(&*bptr);
1169 let ptr = Self::allocate_for_ptr(bptr);
1171 // Copy value as bytes
1172 ptr::copy_nonoverlapping(
1173 bptr as *const T as *const u8,
1174 &mut (*ptr).data as *mut _ as *mut u8,
1178 // Free the allocation without dropping its contents
1179 box_free(box_unique, alloc);
1187 /// Allocates an `ArcInner<[T]>` with the given length.
1188 #[cfg(not(no_global_oom_handling))]
1189 unsafe fn allocate_for_slice(len: usize) -> *mut ArcInner<[T]> {
1191 Self::allocate_for_layout(
1192 Layout::array::<T>(len).unwrap(),
1193 |layout| Global.allocate(layout),
1194 |mem| ptr::slice_from_raw_parts_mut(mem as *mut T, len) as *mut ArcInner<[T]>,
1199 /// Copy elements from slice into newly allocated Arc<\[T\]>
1201 /// Unsafe because the caller must either take ownership or bind `T: Copy`.
1202 #[cfg(not(no_global_oom_handling))]
1203 unsafe fn copy_from_slice(v: &[T]) -> Arc<[T]> {
1205 let ptr = Self::allocate_for_slice(v.len());
1207 ptr::copy_nonoverlapping(v.as_ptr(), &mut (*ptr).data as *mut [T] as *mut T, v.len());
1213 /// Constructs an `Arc<[T]>` from an iterator known to be of a certain size.
1215 /// Behavior is undefined should the size be wrong.
1216 #[cfg(not(no_global_oom_handling))]
1217 unsafe fn from_iter_exact(iter: impl iter::Iterator<Item = T>, len: usize) -> Arc<[T]> {
1218 // Panic guard while cloning T elements.
1219 // In the event of a panic, elements that have been written
1220 // into the new ArcInner will be dropped, then the memory freed.
1228 impl<T> Drop for Guard<T> {
1229 fn drop(&mut self) {
1231 let slice = from_raw_parts_mut(self.elems, self.n_elems);
1232 ptr::drop_in_place(slice);
1234 Global.deallocate(self.mem, self.layout);
1240 let ptr = Self::allocate_for_slice(len);
1242 let mem = ptr as *mut _ as *mut u8;
1243 let layout = Layout::for_value(&*ptr);
1245 // Pointer to first element
1246 let elems = &mut (*ptr).data as *mut [T] as *mut T;
1248 let mut guard = Guard { mem: NonNull::new_unchecked(mem), elems, layout, n_elems: 0 };
1250 for (i, item) in iter.enumerate() {
1251 ptr::write(elems.add(i), item);
1255 // All clear. Forget the guard so it doesn't free the new ArcInner.
1263 /// Specialization trait used for `From<&[T]>`.
1264 #[cfg(not(no_global_oom_handling))]
1265 trait ArcFromSlice<T> {
1266 fn from_slice(slice: &[T]) -> Self;
1269 #[cfg(not(no_global_oom_handling))]
1270 impl<T: Clone> ArcFromSlice<T> for Arc<[T]> {
1272 default fn from_slice(v: &[T]) -> Self {
1273 unsafe { Self::from_iter_exact(v.iter().cloned(), v.len()) }
1277 #[cfg(not(no_global_oom_handling))]
1278 impl<T: Copy> ArcFromSlice<T> for Arc<[T]> {
1280 fn from_slice(v: &[T]) -> Self {
1281 unsafe { Arc::copy_from_slice(v) }
1285 #[stable(feature = "rust1", since = "1.0.0")]
1286 impl<T: ?Sized> Clone for Arc<T> {
1287 /// Makes a clone of the `Arc` pointer.
1289 /// This creates another pointer to the same allocation, increasing the
1290 /// strong reference count.
1295 /// use std::sync::Arc;
1297 /// let five = Arc::new(5);
1299 /// let _ = Arc::clone(&five);
1302 fn clone(&self) -> Arc<T> {
1303 // Using a relaxed ordering is alright here, as knowledge of the
1304 // original reference prevents other threads from erroneously deleting
1307 // As explained in the [Boost documentation][1], Increasing the
1308 // reference counter can always be done with memory_order_relaxed: New
1309 // references to an object can only be formed from an existing
1310 // reference, and passing an existing reference from one thread to
1311 // another must already provide any required synchronization.
1313 // [1]: (www.boost.org/doc/libs/1_55_0/doc/html/atomic/usage_examples.html)
1314 let old_size = self.inner().strong.fetch_add(1, Relaxed);
1316 // However we need to guard against massive refcounts in case someone
1317 // is `mem::forget`ing Arcs. If we don't do this the count can overflow
1318 // and users will use-after free. We racily saturate to `isize::MAX` on
1319 // the assumption that there aren't ~2 billion threads incrementing
1320 // the reference count at once. This branch will never be taken in
1321 // any realistic program.
1323 // We abort because such a program is incredibly degenerate, and we
1324 // don't care to support it.
1325 if old_size > MAX_REFCOUNT {
1329 Self::from_inner(self.ptr)
1333 #[stable(feature = "rust1", since = "1.0.0")]
1334 impl<T: ?Sized> Deref for Arc<T> {
1338 fn deref(&self) -> &T {
1343 #[unstable(feature = "receiver_trait", issue = "none")]
1344 impl<T: ?Sized> Receiver for Arc<T> {}
1346 impl<T: Clone> Arc<T> {
1347 /// Makes a mutable reference into the given `Arc`.
1349 /// If there are other `Arc` or [`Weak`] pointers to the same allocation,
1350 /// then `make_mut` will create a new allocation and invoke [`clone`][clone] on the inner value
1351 /// to ensure unique ownership. This is also referred to as clone-on-write.
1353 /// Note that this differs from the behavior of [`Rc::make_mut`] which disassociates
1354 /// any remaining `Weak` pointers.
1356 /// See also [`get_mut`][get_mut], which will fail rather than cloning.
1358 /// [clone]: Clone::clone
1359 /// [get_mut]: Arc::get_mut
1360 /// [`Rc::make_mut`]: super::rc::Rc::make_mut
1365 /// use std::sync::Arc;
1367 /// let mut data = Arc::new(5);
1369 /// *Arc::make_mut(&mut data) += 1; // Won't clone anything
1370 /// let mut other_data = Arc::clone(&data); // Won't clone inner data
1371 /// *Arc::make_mut(&mut data) += 1; // Clones inner data
1372 /// *Arc::make_mut(&mut data) += 1; // Won't clone anything
1373 /// *Arc::make_mut(&mut other_data) *= 2; // Won't clone anything
1375 /// // Now `data` and `other_data` point to different allocations.
1376 /// assert_eq!(*data, 8);
1377 /// assert_eq!(*other_data, 12);
1379 #[cfg(not(no_global_oom_handling))]
1381 #[stable(feature = "arc_unique", since = "1.4.0")]
1382 pub fn make_mut(this: &mut Self) -> &mut T {
1383 // Note that we hold both a strong reference and a weak reference.
1384 // Thus, releasing our strong reference only will not, by itself, cause
1385 // the memory to be deallocated.
1387 // Use Acquire to ensure that we see any writes to `weak` that happen
1388 // before release writes (i.e., decrements) to `strong`. Since we hold a
1389 // weak count, there's no chance the ArcInner itself could be
1391 if this.inner().strong.compare_exchange(1, 0, Acquire, Relaxed).is_err() {
1392 // Another strong pointer exists, so we must clone.
1393 // Pre-allocate memory to allow writing the cloned value directly.
1394 let mut arc = Self::new_uninit();
1396 let data = Arc::get_mut_unchecked(&mut arc);
1397 (**this).write_clone_into_raw(data.as_mut_ptr());
1398 *this = arc.assume_init();
1400 } else if this.inner().weak.load(Relaxed) != 1 {
1401 // Relaxed suffices in the above because this is fundamentally an
1402 // optimization: we are always racing with weak pointers being
1403 // dropped. Worst case, we end up allocated a new Arc unnecessarily.
1405 // We removed the last strong ref, but there are additional weak
1406 // refs remaining. We'll move the contents to a new Arc, and
1407 // invalidate the other weak refs.
1409 // Note that it is not possible for the read of `weak` to yield
1410 // usize::MAX (i.e., locked), since the weak count can only be
1411 // locked by a thread with a strong reference.
1413 // Materialize our own implicit weak pointer, so that it can clean
1414 // up the ArcInner as needed.
1415 let _weak = Weak { ptr: this.ptr };
1417 // Can just steal the data, all that's left is Weaks
1418 let mut arc = Self::new_uninit();
1420 let data = Arc::get_mut_unchecked(&mut arc);
1421 data.as_mut_ptr().copy_from_nonoverlapping(&**this, 1);
1422 ptr::write(this, arc.assume_init());
1425 // We were the sole reference of either kind; bump back up the
1426 // strong ref count.
1427 this.inner().strong.store(1, Release);
1430 // As with `get_mut()`, the unsafety is ok because our reference was
1431 // either unique to begin with, or became one upon cloning the contents.
1432 unsafe { Self::get_mut_unchecked(this) }
1436 impl<T: ?Sized> Arc<T> {
1437 /// Returns a mutable reference into the given `Arc`, if there are
1438 /// no other `Arc` or [`Weak`] pointers to the same allocation.
1440 /// Returns [`None`] otherwise, because it is not safe to
1441 /// mutate a shared value.
1443 /// See also [`make_mut`][make_mut], which will [`clone`][clone]
1444 /// the inner value when there are other pointers.
1446 /// [make_mut]: Arc::make_mut
1447 /// [clone]: Clone::clone
1452 /// use std::sync::Arc;
1454 /// let mut x = Arc::new(3);
1455 /// *Arc::get_mut(&mut x).unwrap() = 4;
1456 /// assert_eq!(*x, 4);
1458 /// let _y = Arc::clone(&x);
1459 /// assert!(Arc::get_mut(&mut x).is_none());
1462 #[stable(feature = "arc_unique", since = "1.4.0")]
1463 pub fn get_mut(this: &mut Self) -> Option<&mut T> {
1464 if this.is_unique() {
1465 // This unsafety is ok because we're guaranteed that the pointer
1466 // returned is the *only* pointer that will ever be returned to T. Our
1467 // reference count is guaranteed to be 1 at this point, and we required
1468 // the Arc itself to be `mut`, so we're returning the only possible
1469 // reference to the inner data.
1470 unsafe { Some(Arc::get_mut_unchecked(this)) }
1476 /// Returns a mutable reference into the given `Arc`,
1477 /// without any check.
1479 /// See also [`get_mut`], which is safe and does appropriate checks.
1481 /// [`get_mut`]: Arc::get_mut
1485 /// Any other `Arc` or [`Weak`] pointers to the same allocation must not be dereferenced
1486 /// for the duration of the returned borrow.
1487 /// This is trivially the case if no such pointers exist,
1488 /// for example immediately after `Arc::new`.
1493 /// #![feature(get_mut_unchecked)]
1495 /// use std::sync::Arc;
1497 /// let mut x = Arc::new(String::new());
1499 /// Arc::get_mut_unchecked(&mut x).push_str("foo")
1501 /// assert_eq!(*x, "foo");
1504 #[unstable(feature = "get_mut_unchecked", issue = "63292")]
1505 pub unsafe fn get_mut_unchecked(this: &mut Self) -> &mut T {
1506 // We are careful to *not* create a reference covering the "count" fields, as
1507 // this would alias with concurrent access to the reference counts (e.g. by `Weak`).
1508 unsafe { &mut (*this.ptr.as_ptr()).data }
1511 /// Determine whether this is the unique reference (including weak refs) to
1512 /// the underlying data.
1514 /// Note that this requires locking the weak ref count.
1515 fn is_unique(&mut self) -> bool {
1516 // lock the weak pointer count if we appear to be the sole weak pointer
1519 // The acquire label here ensures a happens-before relationship with any
1520 // writes to `strong` (in particular in `Weak::upgrade`) prior to decrements
1521 // of the `weak` count (via `Weak::drop`, which uses release). If the upgraded
1522 // weak ref was never dropped, the CAS here will fail so we do not care to synchronize.
1523 if self.inner().weak.compare_exchange(1, usize::MAX, Acquire, Relaxed).is_ok() {
1524 // This needs to be an `Acquire` to synchronize with the decrement of the `strong`
1525 // counter in `drop` -- the only access that happens when any but the last reference
1526 // is being dropped.
1527 let unique = self.inner().strong.load(Acquire) == 1;
1529 // The release write here synchronizes with a read in `downgrade`,
1530 // effectively preventing the above read of `strong` from happening
1532 self.inner().weak.store(1, Release); // release the lock
1540 #[stable(feature = "rust1", since = "1.0.0")]
1541 unsafe impl<#[may_dangle] T: ?Sized> Drop for Arc<T> {
1542 /// Drops the `Arc`.
1544 /// This will decrement the strong reference count. If the strong reference
1545 /// count reaches zero then the only other references (if any) are
1546 /// [`Weak`], so we `drop` the inner value.
1551 /// use std::sync::Arc;
1555 /// impl Drop for Foo {
1556 /// fn drop(&mut self) {
1557 /// println!("dropped!");
1561 /// let foo = Arc::new(Foo);
1562 /// let foo2 = Arc::clone(&foo);
1564 /// drop(foo); // Doesn't print anything
1565 /// drop(foo2); // Prints "dropped!"
1568 fn drop(&mut self) {
1569 // Because `fetch_sub` is already atomic, we do not need to synchronize
1570 // with other threads unless we are going to delete the object. This
1571 // same logic applies to the below `fetch_sub` to the `weak` count.
1572 if self.inner().strong.fetch_sub(1, Release) != 1 {
1576 // This fence is needed to prevent reordering of use of the data and
1577 // deletion of the data. Because it is marked `Release`, the decreasing
1578 // of the reference count synchronizes with this `Acquire` fence. This
1579 // means that use of the data happens before decreasing the reference
1580 // count, which happens before this fence, which happens before the
1581 // deletion of the data.
1583 // As explained in the [Boost documentation][1],
1585 // > It is important to enforce any possible access to the object in one
1586 // > thread (through an existing reference) to *happen before* deleting
1587 // > the object in a different thread. This is achieved by a "release"
1588 // > operation after dropping a reference (any access to the object
1589 // > through this reference must obviously happened before), and an
1590 // > "acquire" operation before deleting the object.
1592 // In particular, while the contents of an Arc are usually immutable, it's
1593 // possible to have interior writes to something like a Mutex<T>. Since a
1594 // Mutex is not acquired when it is deleted, we can't rely on its
1595 // synchronization logic to make writes in thread A visible to a destructor
1596 // running in thread B.
1598 // Also note that the Acquire fence here could probably be replaced with an
1599 // Acquire load, which could improve performance in highly-contended
1600 // situations. See [2].
1602 // [1]: (www.boost.org/doc/libs/1_55_0/doc/html/atomic/usage_examples.html)
1603 // [2]: (https://github.com/rust-lang/rust/pull/41714)
1604 acquire!(self.inner().strong);
1612 impl Arc<dyn Any + Send + Sync> {
1614 #[stable(feature = "rc_downcast", since = "1.29.0")]
1615 /// Attempt to downcast the `Arc<dyn Any + Send + Sync>` to a concrete type.
1620 /// use std::any::Any;
1621 /// use std::sync::Arc;
1623 /// fn print_if_string(value: Arc<dyn Any + Send + Sync>) {
1624 /// if let Ok(string) = value.downcast::<String>() {
1625 /// println!("String ({}): {}", string.len(), string);
1629 /// let my_string = "Hello World".to_string();
1630 /// print_if_string(Arc::new(my_string));
1631 /// print_if_string(Arc::new(0i8));
1633 pub fn downcast<T>(self) -> Result<Arc<T>, Self>
1635 T: Any + Send + Sync + 'static,
1637 if (*self).is::<T>() {
1638 let ptr = self.ptr.cast::<ArcInner<T>>();
1640 Ok(Arc::from_inner(ptr))
1648 /// Constructs a new `Weak<T>`, without allocating any memory.
1649 /// Calling [`upgrade`] on the return value always gives [`None`].
1651 /// [`upgrade`]: Weak::upgrade
1656 /// use std::sync::Weak;
1658 /// let empty: Weak<i64> = Weak::new();
1659 /// assert!(empty.upgrade().is_none());
1661 #[stable(feature = "downgraded_weak", since = "1.10.0")]
1662 pub fn new() -> Weak<T> {
1663 Weak { ptr: NonNull::new(usize::MAX as *mut ArcInner<T>).expect("MAX is not 0") }
1667 /// Helper type to allow accessing the reference counts without
1668 /// making any assertions about the data field.
1669 struct WeakInner<'a> {
1670 weak: &'a atomic::AtomicUsize,
1671 strong: &'a atomic::AtomicUsize,
1674 impl<T: ?Sized> Weak<T> {
1675 /// Returns a raw pointer to the object `T` pointed to by this `Weak<T>`.
1677 /// The pointer is valid only if there are some strong references. The pointer may be dangling,
1678 /// unaligned or even [`null`] otherwise.
1683 /// use std::sync::Arc;
1686 /// let strong = Arc::new("hello".to_owned());
1687 /// let weak = Arc::downgrade(&strong);
1688 /// // Both point to the same object
1689 /// assert!(ptr::eq(&*strong, weak.as_ptr()));
1690 /// // The strong here keeps it alive, so we can still access the object.
1691 /// assert_eq!("hello", unsafe { &*weak.as_ptr() });
1694 /// // But not any more. We can do weak.as_ptr(), but accessing the pointer would lead to
1695 /// // undefined behaviour.
1696 /// // assert_eq!("hello", unsafe { &*weak.as_ptr() });
1699 /// [`null`]: core::ptr::null
1700 #[stable(feature = "weak_into_raw", since = "1.45.0")]
1701 pub fn as_ptr(&self) -> *const T {
1702 let ptr: *mut ArcInner<T> = NonNull::as_ptr(self.ptr);
1704 if is_dangling(ptr) {
1705 // If the pointer is dangling, we return the sentinel directly. This cannot be
1706 // a valid payload address, as the payload is at least as aligned as ArcInner (usize).
1709 // SAFETY: if is_dangling returns false, then the pointer is dereferencable.
1710 // The payload may be dropped at this point, and we have to maintain provenance,
1711 // so use raw pointer manipulation.
1712 unsafe { ptr::addr_of_mut!((*ptr).data) }
1716 /// Consumes the `Weak<T>` and turns it into a raw pointer.
1718 /// This converts the weak pointer into a raw pointer, while still preserving the ownership of
1719 /// one weak reference (the weak count is not modified by this operation). It can be turned
1720 /// back into the `Weak<T>` with [`from_raw`].
1722 /// The same restrictions of accessing the target of the pointer as with
1723 /// [`as_ptr`] apply.
1728 /// use std::sync::{Arc, Weak};
1730 /// let strong = Arc::new("hello".to_owned());
1731 /// let weak = Arc::downgrade(&strong);
1732 /// let raw = weak.into_raw();
1734 /// assert_eq!(1, Arc::weak_count(&strong));
1735 /// assert_eq!("hello", unsafe { &*raw });
1737 /// drop(unsafe { Weak::from_raw(raw) });
1738 /// assert_eq!(0, Arc::weak_count(&strong));
1741 /// [`from_raw`]: Weak::from_raw
1742 /// [`as_ptr`]: Weak::as_ptr
1743 #[stable(feature = "weak_into_raw", since = "1.45.0")]
1744 pub fn into_raw(self) -> *const T {
1745 let result = self.as_ptr();
1750 /// Converts a raw pointer previously created by [`into_raw`] back into `Weak<T>`.
1752 /// This can be used to safely get a strong reference (by calling [`upgrade`]
1753 /// later) or to deallocate the weak count by dropping the `Weak<T>`.
1755 /// It takes ownership of one weak reference (with the exception of pointers created by [`new`],
1756 /// as these don't own anything; the method still works on them).
1760 /// The pointer must have originated from the [`into_raw`] and must still own its potential
1763 /// It is allowed for the strong count to be 0 at the time of calling this. Nevertheless, this
1764 /// takes ownership of one weak reference currently represented as a raw pointer (the weak
1765 /// count is not modified by this operation) and therefore it must be paired with a previous
1766 /// call to [`into_raw`].
1770 /// use std::sync::{Arc, Weak};
1772 /// let strong = Arc::new("hello".to_owned());
1774 /// let raw_1 = Arc::downgrade(&strong).into_raw();
1775 /// let raw_2 = Arc::downgrade(&strong).into_raw();
1777 /// assert_eq!(2, Arc::weak_count(&strong));
1779 /// assert_eq!("hello", &*unsafe { Weak::from_raw(raw_1) }.upgrade().unwrap());
1780 /// assert_eq!(1, Arc::weak_count(&strong));
1784 /// // Decrement the last weak count.
1785 /// assert!(unsafe { Weak::from_raw(raw_2) }.upgrade().is_none());
1788 /// [`new`]: Weak::new
1789 /// [`into_raw`]: Weak::into_raw
1790 /// [`upgrade`]: Weak::upgrade
1791 /// [`forget`]: std::mem::forget
1792 #[stable(feature = "weak_into_raw", since = "1.45.0")]
1793 pub unsafe fn from_raw(ptr: *const T) -> Self {
1794 // See Weak::as_ptr for context on how the input pointer is derived.
1796 let ptr = if is_dangling(ptr as *mut T) {
1797 // This is a dangling Weak.
1798 ptr as *mut ArcInner<T>
1800 // Otherwise, we're guaranteed the pointer came from a nondangling Weak.
1801 // SAFETY: data_offset is safe to call, as ptr references a real (potentially dropped) T.
1802 let offset = unsafe { data_offset(ptr) };
1803 // Thus, we reverse the offset to get the whole RcBox.
1804 // SAFETY: the pointer originated from a Weak, so this offset is safe.
1805 unsafe { (ptr as *mut ArcInner<T>).set_ptr_value((ptr as *mut u8).offset(-offset)) }
1808 // SAFETY: we now have recovered the original Weak pointer, so can create the Weak.
1809 Weak { ptr: unsafe { NonNull::new_unchecked(ptr) } }
1813 impl<T: ?Sized> Weak<T> {
1814 /// Attempts to upgrade the `Weak` pointer to an [`Arc`], delaying
1815 /// dropping of the inner value if successful.
1817 /// Returns [`None`] if the inner value has since been dropped.
1822 /// use std::sync::Arc;
1824 /// let five = Arc::new(5);
1826 /// let weak_five = Arc::downgrade(&five);
1828 /// let strong_five: Option<Arc<_>> = weak_five.upgrade();
1829 /// assert!(strong_five.is_some());
1831 /// // Destroy all strong pointers.
1832 /// drop(strong_five);
1835 /// assert!(weak_five.upgrade().is_none());
1837 #[stable(feature = "arc_weak", since = "1.4.0")]
1838 pub fn upgrade(&self) -> Option<Arc<T>> {
1839 // We use a CAS loop to increment the strong count instead of a
1840 // fetch_add as this function should never take the reference count
1841 // from zero to one.
1842 let inner = self.inner()?;
1844 // Relaxed load because any write of 0 that we can observe
1845 // leaves the field in a permanently zero state (so a
1846 // "stale" read of 0 is fine), and any other value is
1847 // confirmed via the CAS below.
1848 let mut n = inner.strong.load(Relaxed);
1855 // See comments in `Arc::clone` for why we do this (for `mem::forget`).
1856 if n > MAX_REFCOUNT {
1860 // Relaxed is fine for the failure case because we don't have any expectations about the new state.
1861 // Acquire is necessary for the success case to synchronise with `Arc::new_cyclic`, when the inner
1862 // value can be initialized after `Weak` references have already been created. In that case, we
1863 // expect to observe the fully initialized value.
1864 match inner.strong.compare_exchange_weak(n, n + 1, Acquire, Relaxed) {
1865 Ok(_) => return Some(Arc::from_inner(self.ptr)), // null checked above
1866 Err(old) => n = old,
1871 /// Gets the number of strong (`Arc`) pointers pointing to this allocation.
1873 /// If `self` was created using [`Weak::new`], this will return 0.
1874 #[stable(feature = "weak_counts", since = "1.41.0")]
1875 pub fn strong_count(&self) -> usize {
1876 if let Some(inner) = self.inner() { inner.strong.load(SeqCst) } else { 0 }
1879 /// Gets an approximation of the number of `Weak` pointers pointing to this
1882 /// If `self` was created using [`Weak::new`], or if there are no remaining
1883 /// strong pointers, this will return 0.
1887 /// Due to implementation details, the returned value can be off by 1 in
1888 /// either direction when other threads are manipulating any `Arc`s or
1889 /// `Weak`s pointing to the same allocation.
1890 #[stable(feature = "weak_counts", since = "1.41.0")]
1891 pub fn weak_count(&self) -> usize {
1894 let weak = inner.weak.load(SeqCst);
1895 let strong = inner.strong.load(SeqCst);
1899 // Since we observed that there was at least one strong pointer
1900 // after reading the weak count, we know that the implicit weak
1901 // reference (present whenever any strong references are alive)
1902 // was still around when we observed the weak count, and can
1903 // therefore safely subtract it.
1910 /// Returns `None` when the pointer is dangling and there is no allocated `ArcInner`,
1911 /// (i.e., when this `Weak` was created by `Weak::new`).
1913 fn inner(&self) -> Option<WeakInner<'_>> {
1914 if is_dangling(self.ptr.as_ptr()) {
1917 // We are careful to *not* create a reference covering the "data" field, as
1918 // the field may be mutated concurrently (for example, if the last `Arc`
1919 // is dropped, the data field will be dropped in-place).
1921 let ptr = self.ptr.as_ptr();
1922 WeakInner { strong: &(*ptr).strong, weak: &(*ptr).weak }
1927 /// Returns `true` if the two `Weak`s point to the same allocation (similar to
1928 /// [`ptr::eq`]), or if both don't point to any allocation
1929 /// (because they were created with `Weak::new()`).
1933 /// Since this compares pointers it means that `Weak::new()` will equal each
1934 /// other, even though they don't point to any allocation.
1939 /// use std::sync::Arc;
1941 /// let first_rc = Arc::new(5);
1942 /// let first = Arc::downgrade(&first_rc);
1943 /// let second = Arc::downgrade(&first_rc);
1945 /// assert!(first.ptr_eq(&second));
1947 /// let third_rc = Arc::new(5);
1948 /// let third = Arc::downgrade(&third_rc);
1950 /// assert!(!first.ptr_eq(&third));
1953 /// Comparing `Weak::new`.
1956 /// use std::sync::{Arc, Weak};
1958 /// let first = Weak::new();
1959 /// let second = Weak::new();
1960 /// assert!(first.ptr_eq(&second));
1962 /// let third_rc = Arc::new(());
1963 /// let third = Arc::downgrade(&third_rc);
1964 /// assert!(!first.ptr_eq(&third));
1967 /// [`ptr::eq`]: core::ptr::eq
1969 #[stable(feature = "weak_ptr_eq", since = "1.39.0")]
1970 pub fn ptr_eq(&self, other: &Self) -> bool {
1971 self.ptr.as_ptr() == other.ptr.as_ptr()
1975 #[stable(feature = "arc_weak", since = "1.4.0")]
1976 impl<T: ?Sized> Clone for Weak<T> {
1977 /// Makes a clone of the `Weak` pointer that points to the same allocation.
1982 /// use std::sync::{Arc, Weak};
1984 /// let weak_five = Arc::downgrade(&Arc::new(5));
1986 /// let _ = Weak::clone(&weak_five);
1989 fn clone(&self) -> Weak<T> {
1990 let inner = if let Some(inner) = self.inner() {
1993 return Weak { ptr: self.ptr };
1995 // See comments in Arc::clone() for why this is relaxed. This can use a
1996 // fetch_add (ignoring the lock) because the weak count is only locked
1997 // where are *no other* weak pointers in existence. (So we can't be
1998 // running this code in that case).
1999 let old_size = inner.weak.fetch_add(1, Relaxed);
2001 // See comments in Arc::clone() for why we do this (for mem::forget).
2002 if old_size > MAX_REFCOUNT {
2006 Weak { ptr: self.ptr }
2010 #[stable(feature = "downgraded_weak", since = "1.10.0")]
2011 impl<T> Default for Weak<T> {
2012 /// Constructs a new `Weak<T>`, without allocating memory.
2013 /// Calling [`upgrade`] on the return value always
2016 /// [`upgrade`]: Weak::upgrade
2021 /// use std::sync::Weak;
2023 /// let empty: Weak<i64> = Default::default();
2024 /// assert!(empty.upgrade().is_none());
2026 fn default() -> Weak<T> {
2031 #[stable(feature = "arc_weak", since = "1.4.0")]
2032 unsafe impl<#[may_dangle] T: ?Sized> Drop for Weak<T> {
2033 /// Drops the `Weak` pointer.
2038 /// use std::sync::{Arc, Weak};
2042 /// impl Drop for Foo {
2043 /// fn drop(&mut self) {
2044 /// println!("dropped!");
2048 /// let foo = Arc::new(Foo);
2049 /// let weak_foo = Arc::downgrade(&foo);
2050 /// let other_weak_foo = Weak::clone(&weak_foo);
2052 /// drop(weak_foo); // Doesn't print anything
2053 /// drop(foo); // Prints "dropped!"
2055 /// assert!(other_weak_foo.upgrade().is_none());
2057 fn drop(&mut self) {
2058 // If we find out that we were the last weak pointer, then its time to
2059 // deallocate the data entirely. See the discussion in Arc::drop() about
2060 // the memory orderings
2062 // It's not necessary to check for the locked state here, because the
2063 // weak count can only be locked if there was precisely one weak ref,
2064 // meaning that drop could only subsequently run ON that remaining weak
2065 // ref, which can only happen after the lock is released.
2066 let inner = if let Some(inner) = self.inner() { inner } else { return };
2068 if inner.weak.fetch_sub(1, Release) == 1 {
2069 acquire!(inner.weak);
2070 unsafe { Global.deallocate(self.ptr.cast(), Layout::for_value_raw(self.ptr.as_ptr())) }
2075 #[stable(feature = "rust1", since = "1.0.0")]
2076 trait ArcEqIdent<T: ?Sized + PartialEq> {
2077 fn eq(&self, other: &Arc<T>) -> bool;
2078 fn ne(&self, other: &Arc<T>) -> bool;
2081 #[stable(feature = "rust1", since = "1.0.0")]
2082 impl<T: ?Sized + PartialEq> ArcEqIdent<T> for Arc<T> {
2084 default fn eq(&self, other: &Arc<T>) -> bool {
2088 default fn ne(&self, other: &Arc<T>) -> bool {
2093 /// We're doing this specialization here, and not as a more general optimization on `&T`, because it
2094 /// would otherwise add a cost to all equality checks on refs. We assume that `Arc`s are used to
2095 /// store large values, that are slow to clone, but also heavy to check for equality, causing this
2096 /// cost to pay off more easily. It's also more likely to have two `Arc` clones, that point to
2097 /// the same value, than two `&T`s.
2099 /// We can only do this when `T: Eq` as a `PartialEq` might be deliberately irreflexive.
2100 #[stable(feature = "rust1", since = "1.0.0")]
2101 impl<T: ?Sized + crate::rc::MarkerEq> ArcEqIdent<T> for Arc<T> {
2103 fn eq(&self, other: &Arc<T>) -> bool {
2104 Arc::ptr_eq(self, other) || **self == **other
2108 fn ne(&self, other: &Arc<T>) -> bool {
2109 !Arc::ptr_eq(self, other) && **self != **other
2113 #[stable(feature = "rust1", since = "1.0.0")]
2114 impl<T: ?Sized + PartialEq> PartialEq for Arc<T> {
2115 /// Equality for two `Arc`s.
2117 /// Two `Arc`s are equal if their inner values are equal, even if they are
2118 /// stored in different allocation.
2120 /// If `T` also implements `Eq` (implying reflexivity of equality),
2121 /// two `Arc`s that point to the same allocation are always equal.
2126 /// use std::sync::Arc;
2128 /// let five = Arc::new(5);
2130 /// assert!(five == Arc::new(5));
2133 fn eq(&self, other: &Arc<T>) -> bool {
2134 ArcEqIdent::eq(self, other)
2137 /// Inequality for two `Arc`s.
2139 /// Two `Arc`s are unequal if their inner values are unequal.
2141 /// If `T` also implements `Eq` (implying reflexivity of equality),
2142 /// two `Arc`s that point to the same value are never unequal.
2147 /// use std::sync::Arc;
2149 /// let five = Arc::new(5);
2151 /// assert!(five != Arc::new(6));
2154 fn ne(&self, other: &Arc<T>) -> bool {
2155 ArcEqIdent::ne(self, other)
2159 #[stable(feature = "rust1", since = "1.0.0")]
2160 impl<T: ?Sized + PartialOrd> PartialOrd for Arc<T> {
2161 /// Partial comparison for two `Arc`s.
2163 /// The two are compared by calling `partial_cmp()` on their inner values.
2168 /// use std::sync::Arc;
2169 /// use std::cmp::Ordering;
2171 /// let five = Arc::new(5);
2173 /// assert_eq!(Some(Ordering::Less), five.partial_cmp(&Arc::new(6)));
2175 fn partial_cmp(&self, other: &Arc<T>) -> Option<Ordering> {
2176 (**self).partial_cmp(&**other)
2179 /// Less-than comparison for two `Arc`s.
2181 /// The two are compared by calling `<` on their inner values.
2186 /// use std::sync::Arc;
2188 /// let five = Arc::new(5);
2190 /// assert!(five < Arc::new(6));
2192 fn lt(&self, other: &Arc<T>) -> bool {
2193 *(*self) < *(*other)
2196 /// 'Less than or equal to' comparison for two `Arc`s.
2198 /// The two are compared by calling `<=` on their inner values.
2203 /// use std::sync::Arc;
2205 /// let five = Arc::new(5);
2207 /// assert!(five <= Arc::new(5));
2209 fn le(&self, other: &Arc<T>) -> bool {
2210 *(*self) <= *(*other)
2213 /// Greater-than comparison for two `Arc`s.
2215 /// The two are compared by calling `>` on their inner values.
2220 /// use std::sync::Arc;
2222 /// let five = Arc::new(5);
2224 /// assert!(five > Arc::new(4));
2226 fn gt(&self, other: &Arc<T>) -> bool {
2227 *(*self) > *(*other)
2230 /// 'Greater than or equal to' comparison for two `Arc`s.
2232 /// The two are compared by calling `>=` on their inner values.
2237 /// use std::sync::Arc;
2239 /// let five = Arc::new(5);
2241 /// assert!(five >= Arc::new(5));
2243 fn ge(&self, other: &Arc<T>) -> bool {
2244 *(*self) >= *(*other)
2247 #[stable(feature = "rust1", since = "1.0.0")]
2248 impl<T: ?Sized + Ord> Ord for Arc<T> {
2249 /// Comparison for two `Arc`s.
2251 /// The two are compared by calling `cmp()` on their inner values.
2256 /// use std::sync::Arc;
2257 /// use std::cmp::Ordering;
2259 /// let five = Arc::new(5);
2261 /// assert_eq!(Ordering::Less, five.cmp(&Arc::new(6)));
2263 fn cmp(&self, other: &Arc<T>) -> Ordering {
2264 (**self).cmp(&**other)
2267 #[stable(feature = "rust1", since = "1.0.0")]
2268 impl<T: ?Sized + Eq> Eq for Arc<T> {}
2270 #[stable(feature = "rust1", since = "1.0.0")]
2271 impl<T: ?Sized + fmt::Display> fmt::Display for Arc<T> {
2272 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
2273 fmt::Display::fmt(&**self, f)
2277 #[stable(feature = "rust1", since = "1.0.0")]
2278 impl<T: ?Sized + fmt::Debug> fmt::Debug for Arc<T> {
2279 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
2280 fmt::Debug::fmt(&**self, f)
2284 #[stable(feature = "rust1", since = "1.0.0")]
2285 impl<T: ?Sized> fmt::Pointer for Arc<T> {
2286 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
2287 fmt::Pointer::fmt(&(&**self as *const T), f)
2291 #[cfg(not(no_global_oom_handling))]
2292 #[stable(feature = "rust1", since = "1.0.0")]
2293 impl<T: Default> Default for Arc<T> {
2294 /// Creates a new `Arc<T>`, with the `Default` value for `T`.
2299 /// use std::sync::Arc;
2301 /// let x: Arc<i32> = Default::default();
2302 /// assert_eq!(*x, 0);
2304 fn default() -> Arc<T> {
2305 Arc::new(Default::default())
2309 #[stable(feature = "rust1", since = "1.0.0")]
2310 impl<T: ?Sized + Hash> Hash for Arc<T> {
2311 fn hash<H: Hasher>(&self, state: &mut H) {
2312 (**self).hash(state)
2316 #[cfg(not(no_global_oom_handling))]
2317 #[stable(feature = "from_for_ptrs", since = "1.6.0")]
2318 impl<T> From<T> for Arc<T> {
2319 /// Converts a `T` into an `Arc<T>`
2321 /// The conversion moves the value into a
2322 /// newly allocated `Arc`. It is equivalent to
2323 /// calling `Arc::new(t)`.
2327 /// # use std::sync::Arc;
2329 /// let arc = Arc::new(5);
2331 /// assert_eq!(Arc::from(x), arc);
2333 fn from(t: T) -> Self {
2338 #[cfg(not(no_global_oom_handling))]
2339 #[stable(feature = "shared_from_slice", since = "1.21.0")]
2340 impl<T: Clone> From<&[T]> for Arc<[T]> {
2341 /// Allocate a reference-counted slice and fill it by cloning `v`'s items.
2346 /// # use std::sync::Arc;
2347 /// let original: &[i32] = &[1, 2, 3];
2348 /// let shared: Arc<[i32]> = Arc::from(original);
2349 /// assert_eq!(&[1, 2, 3], &shared[..]);
2352 fn from(v: &[T]) -> Arc<[T]> {
2353 <Self as ArcFromSlice<T>>::from_slice(v)
2357 #[cfg(not(no_global_oom_handling))]
2358 #[stable(feature = "shared_from_slice", since = "1.21.0")]
2359 impl From<&str> for Arc<str> {
2360 /// Allocate a reference-counted `str` and copy `v` into it.
2365 /// # use std::sync::Arc;
2366 /// let shared: Arc<str> = Arc::from("eggplant");
2367 /// assert_eq!("eggplant", &shared[..]);
2370 fn from(v: &str) -> Arc<str> {
2371 let arc = Arc::<[u8]>::from(v.as_bytes());
2372 unsafe { Arc::from_raw(Arc::into_raw(arc) as *const str) }
2376 #[cfg(not(no_global_oom_handling))]
2377 #[stable(feature = "shared_from_slice", since = "1.21.0")]
2378 impl From<String> for Arc<str> {
2379 /// Allocate a reference-counted `str` and copy `v` into it.
2384 /// # use std::sync::Arc;
2385 /// let unique: String = "eggplant".to_owned();
2386 /// let shared: Arc<str> = Arc::from(unique);
2387 /// assert_eq!("eggplant", &shared[..]);
2390 fn from(v: String) -> Arc<str> {
2395 #[cfg(not(no_global_oom_handling))]
2396 #[stable(feature = "shared_from_slice", since = "1.21.0")]
2397 impl<T: ?Sized> From<Box<T>> for Arc<T> {
2398 /// Move a boxed object to a new, reference-counted allocation.
2403 /// # use std::sync::Arc;
2404 /// let unique: Box<str> = Box::from("eggplant");
2405 /// let shared: Arc<str> = Arc::from(unique);
2406 /// assert_eq!("eggplant", &shared[..]);
2409 fn from(v: Box<T>) -> Arc<T> {
2414 #[cfg(not(no_global_oom_handling))]
2415 #[stable(feature = "shared_from_slice", since = "1.21.0")]
2416 impl<T> From<Vec<T>> for Arc<[T]> {
2417 /// Allocate a reference-counted slice and move `v`'s items into it.
2422 /// # use std::sync::Arc;
2423 /// let unique: Vec<i32> = vec![1, 2, 3];
2424 /// let shared: Arc<[i32]> = Arc::from(unique);
2425 /// assert_eq!(&[1, 2, 3], &shared[..]);
2428 fn from(mut v: Vec<T>) -> Arc<[T]> {
2430 let arc = Arc::copy_from_slice(&v);
2432 // Allow the Vec to free its memory, but not destroy its contents
2440 #[stable(feature = "shared_from_cow", since = "1.45.0")]
2441 impl<'a, B> From<Cow<'a, B>> for Arc<B>
2443 B: ToOwned + ?Sized,
2444 Arc<B>: From<&'a B> + From<B::Owned>,
2446 /// Create an atomically reference-counted pointer from
2447 /// a clone-on-write pointer by copying its content.
2452 /// # use std::sync::Arc;
2453 /// # use std::borrow::Cow;
2454 /// let cow: Cow<str> = Cow::Borrowed("eggplant");
2455 /// let shared: Arc<str> = Arc::from(cow);
2456 /// assert_eq!("eggplant", &shared[..]);
2459 fn from(cow: Cow<'a, B>) -> Arc<B> {
2461 Cow::Borrowed(s) => Arc::from(s),
2462 Cow::Owned(s) => Arc::from(s),
2467 #[stable(feature = "boxed_slice_try_from", since = "1.43.0")]
2468 impl<T, const N: usize> TryFrom<Arc<[T]>> for Arc<[T; N]> {
2469 type Error = Arc<[T]>;
2471 fn try_from(boxed_slice: Arc<[T]>) -> Result<Self, Self::Error> {
2472 if boxed_slice.len() == N {
2473 Ok(unsafe { Arc::from_raw(Arc::into_raw(boxed_slice) as *mut [T; N]) })
2480 #[cfg(not(no_global_oom_handling))]
2481 #[stable(feature = "shared_from_iter", since = "1.37.0")]
2482 impl<T> iter::FromIterator<T> for Arc<[T]> {
2483 /// Takes each element in the `Iterator` and collects it into an `Arc<[T]>`.
2485 /// # Performance characteristics
2487 /// ## The general case
2489 /// In the general case, collecting into `Arc<[T]>` is done by first
2490 /// collecting into a `Vec<T>`. That is, when writing the following:
2493 /// # use std::sync::Arc;
2494 /// let evens: Arc<[u8]> = (0..10).filter(|&x| x % 2 == 0).collect();
2495 /// # assert_eq!(&*evens, &[0, 2, 4, 6, 8]);
2498 /// this behaves as if we wrote:
2501 /// # use std::sync::Arc;
2502 /// let evens: Arc<[u8]> = (0..10).filter(|&x| x % 2 == 0)
2503 /// .collect::<Vec<_>>() // The first set of allocations happens here.
2504 /// .into(); // A second allocation for `Arc<[T]>` happens here.
2505 /// # assert_eq!(&*evens, &[0, 2, 4, 6, 8]);
2508 /// This will allocate as many times as needed for constructing the `Vec<T>`
2509 /// and then it will allocate once for turning the `Vec<T>` into the `Arc<[T]>`.
2511 /// ## Iterators of known length
2513 /// When your `Iterator` implements `TrustedLen` and is of an exact size,
2514 /// a single allocation will be made for the `Arc<[T]>`. For example:
2517 /// # use std::sync::Arc;
2518 /// let evens: Arc<[u8]> = (0..10).collect(); // Just a single allocation happens here.
2519 /// # assert_eq!(&*evens, &*(0..10).collect::<Vec<_>>());
2521 fn from_iter<I: iter::IntoIterator<Item = T>>(iter: I) -> Self {
2522 ToArcSlice::to_arc_slice(iter.into_iter())
2526 /// Specialization trait used for collecting into `Arc<[T]>`.
2527 trait ToArcSlice<T>: Iterator<Item = T> + Sized {
2528 fn to_arc_slice(self) -> Arc<[T]>;
2531 #[cfg(not(no_global_oom_handling))]
2532 impl<T, I: Iterator<Item = T>> ToArcSlice<T> for I {
2533 default fn to_arc_slice(self) -> Arc<[T]> {
2534 self.collect::<Vec<T>>().into()
2538 #[cfg(not(no_global_oom_handling))]
2539 impl<T, I: iter::TrustedLen<Item = T>> ToArcSlice<T> for I {
2540 fn to_arc_slice(self) -> Arc<[T]> {
2541 // This is the case for a `TrustedLen` iterator.
2542 let (low, high) = self.size_hint();
2543 if let Some(high) = high {
2547 "TrustedLen iterator's size hint is not exact: {:?}",
2552 // SAFETY: We need to ensure that the iterator has an exact length and we have.
2553 Arc::from_iter_exact(self, low)
2556 // TrustedLen contract guarantees that `upper_bound == `None` implies an iterator
2557 // length exceeding `usize::MAX`.
2558 // The default implementation would collect into a vec which would panic.
2559 // Thus we panic here immediately without invoking `Vec` code.
2560 panic!("capacity overflow");
2565 #[stable(feature = "rust1", since = "1.0.0")]
2566 impl<T: ?Sized> borrow::Borrow<T> for Arc<T> {
2567 fn borrow(&self) -> &T {
2572 #[stable(since = "1.5.0", feature = "smart_ptr_as_ref")]
2573 impl<T: ?Sized> AsRef<T> for Arc<T> {
2574 fn as_ref(&self) -> &T {
2579 #[stable(feature = "pin", since = "1.33.0")]
2580 impl<T: ?Sized> Unpin for Arc<T> {}
2582 /// Get the offset within an `ArcInner` for the payload behind a pointer.
2586 /// The pointer must point to (and have valid metadata for) a previously
2587 /// valid instance of T, but the T is allowed to be dropped.
2588 unsafe fn data_offset<T: ?Sized>(ptr: *const T) -> isize {
2589 // Align the unsized value to the end of the ArcInner.
2590 // Because RcBox is repr(C), it will always be the last field in memory.
2591 // SAFETY: since the only unsized types possible are slices, trait objects,
2592 // and extern types, the input safety requirement is currently enough to
2593 // satisfy the requirements of align_of_val_raw; this is an implementation
2594 // detail of the language that must not be relied upon outside of std.
2595 unsafe { data_offset_align(align_of_val_raw(ptr)) }
2599 fn data_offset_align(align: usize) -> isize {
2600 let layout = Layout::new::<ArcInner<()>>();
2601 (layout.size() + layout.padding_needed_for(align)) as isize