1 #![stable(feature = "rust1", since = "1.0.0")]
3 //! Thread-safe reference-counting pointers.
5 //! See the [`Arc<T>`][Arc] documentation for more details.
9 use core::cmp::Ordering;
10 use core::convert::{From, TryFrom};
12 use core::hash::{Hash, Hasher};
14 use core::intrinsics::abort;
15 #[cfg(not(no_global_oom_handling))]
17 use core::marker::{PhantomData, Unpin, Unsize};
18 #[cfg(not(no_global_oom_handling))]
19 use core::mem::size_of_val;
20 use core::mem::{self, align_of_val_raw};
21 use core::ops::{CoerceUnsized, Deref, DispatchFromDyn, Receiver};
22 use core::panic::{RefUnwindSafe, UnwindSafe};
24 use core::ptr::{self, NonNull};
25 #[cfg(not(no_global_oom_handling))]
26 use core::slice::from_raw_parts_mut;
27 use core::sync::atomic;
28 use core::sync::atomic::Ordering::{Acquire, Relaxed, Release, SeqCst};
30 #[cfg(not(no_global_oom_handling))]
31 use crate::alloc::handle_alloc_error;
32 #[cfg(not(no_global_oom_handling))]
33 use crate::alloc::{box_free, WriteCloneIntoRaw};
34 use crate::alloc::{AllocError, Allocator, Global, Layout};
35 use crate::borrow::{Cow, ToOwned};
36 use crate::boxed::Box;
37 use crate::rc::is_dangling;
38 #[cfg(not(no_global_oom_handling))]
39 use crate::string::String;
40 #[cfg(not(no_global_oom_handling))]
46 /// A soft limit on the amount of references that may be made to an `Arc`.
48 /// Going above this limit will abort your program (although not
49 /// necessarily) at _exactly_ `MAX_REFCOUNT + 1` references.
50 const MAX_REFCOUNT: usize = (isize::MAX) as usize;
52 #[cfg(not(sanitize = "thread"))]
53 macro_rules! acquire {
55 atomic::fence(Acquire)
59 // ThreadSanitizer does not support memory fences. To avoid false positive
60 // reports in Arc / Weak implementation use atomic loads for synchronization
62 #[cfg(sanitize = "thread")]
63 macro_rules! acquire {
69 /// A thread-safe reference-counting pointer. 'Arc' stands for 'Atomically
70 /// Reference Counted'.
72 /// The type `Arc<T>` provides shared ownership of a value of type `T`,
73 /// allocated in the heap. Invoking [`clone`][clone] on `Arc` produces
74 /// a new `Arc` instance, which points to the same allocation on the heap as the
75 /// source `Arc`, while increasing a reference count. When the last `Arc`
76 /// pointer to a given allocation is destroyed, the value stored in that allocation (often
77 /// referred to as "inner value") is also dropped.
79 /// Shared references in Rust disallow mutation by default, and `Arc` is no
80 /// exception: you cannot generally obtain a mutable reference to something
81 /// inside an `Arc`. If you need to mutate through an `Arc`, use
82 /// [`Mutex`][mutex], [`RwLock`][rwlock], or one of the [`Atomic`][atomic]
87 /// Unlike [`Rc<T>`], `Arc<T>` uses atomic operations for its reference
88 /// counting. This means that it is thread-safe. The disadvantage is that
89 /// atomic operations are more expensive than ordinary memory accesses. If you
90 /// are not sharing reference-counted allocations between threads, consider using
91 /// [`Rc<T>`] for lower overhead. [`Rc<T>`] is a safe default, because the
92 /// compiler will catch any attempt to send an [`Rc<T>`] between threads.
93 /// However, a library might choose `Arc<T>` in order to give library consumers
96 /// `Arc<T>` will implement [`Send`] and [`Sync`] as long as the `T` implements
97 /// [`Send`] and [`Sync`]. Why can't you put a non-thread-safe type `T` in an
98 /// `Arc<T>` to make it thread-safe? This may be a bit counter-intuitive at
99 /// first: after all, isn't the point of `Arc<T>` thread safety? The key is
100 /// this: `Arc<T>` makes it thread safe to have multiple ownership of the same
101 /// data, but it doesn't add thread safety to its data. Consider
102 /// <code>Arc<[RefCell\<T>]></code>. [`RefCell<T>`] isn't [`Sync`], and if `Arc<T>` was always
103 /// [`Send`], <code>Arc<[RefCell\<T>]></code> would be as well. But then we'd have a problem:
104 /// [`RefCell<T>`] is not thread safe; it keeps track of the borrowing count using
105 /// non-atomic operations.
107 /// In the end, this means that you may need to pair `Arc<T>` with some sort of
108 /// [`std::sync`] type, usually [`Mutex<T>`][mutex].
110 /// ## Breaking cycles with `Weak`
112 /// The [`downgrade`][downgrade] method can be used to create a non-owning
113 /// [`Weak`] pointer. A [`Weak`] pointer can be [`upgrade`][upgrade]d
114 /// to an `Arc`, but this will return [`None`] if the value stored in the allocation has
115 /// already been dropped. In other words, `Weak` pointers do not keep the value
116 /// inside the allocation alive; however, they *do* keep the allocation
117 /// (the backing store for the value) alive.
119 /// A cycle between `Arc` pointers will never be deallocated. For this reason,
120 /// [`Weak`] is used to break cycles. For example, a tree could have
121 /// strong `Arc` pointers from parent nodes to children, and [`Weak`]
122 /// pointers from children back to their parents.
124 /// # Cloning references
126 /// Creating a new reference from an existing reference-counted pointer is done using the
127 /// `Clone` trait implemented for [`Arc<T>`][Arc] and [`Weak<T>`][Weak].
130 /// use std::sync::Arc;
131 /// let foo = Arc::new(vec![1.0, 2.0, 3.0]);
132 /// // The two syntaxes below are equivalent.
133 /// let a = foo.clone();
134 /// let b = Arc::clone(&foo);
135 /// // a, b, and foo are all Arcs that point to the same memory location
138 /// ## `Deref` behavior
140 /// `Arc<T>` automatically dereferences to `T` (via the [`Deref`][deref] trait),
141 /// so you can call `T`'s methods on a value of type `Arc<T>`. To avoid name
142 /// clashes with `T`'s methods, the methods of `Arc<T>` itself are associated
143 /// functions, called using [fully qualified syntax]:
146 /// use std::sync::Arc;
148 /// let my_arc = Arc::new(());
149 /// Arc::downgrade(&my_arc);
152 /// `Arc<T>`'s implementations of traits like `Clone` may also be called using
153 /// fully qualified syntax. Some people prefer to use fully qualified syntax,
154 /// while others prefer using method-call syntax.
157 /// use std::sync::Arc;
159 /// let arc = Arc::new(());
160 /// // Method-call syntax
161 /// let arc2 = arc.clone();
162 /// // Fully qualified syntax
163 /// let arc3 = Arc::clone(&arc);
166 /// [`Weak<T>`][Weak] does not auto-dereference to `T`, because the inner value may have
167 /// already been dropped.
169 /// [`Rc<T>`]: crate::rc::Rc
170 /// [clone]: Clone::clone
171 /// [mutex]: ../../std/sync/struct.Mutex.html
172 /// [rwlock]: ../../std/sync/struct.RwLock.html
173 /// [atomic]: core::sync::atomic
174 /// [`Send`]: core::marker::Send
175 /// [`Sync`]: core::marker::Sync
176 /// [deref]: core::ops::Deref
177 /// [downgrade]: Arc::downgrade
178 /// [upgrade]: Weak::upgrade
179 /// [RefCell\<T>]: core::cell::RefCell
180 /// [`RefCell<T>`]: core::cell::RefCell
181 /// [`std::sync`]: ../../std/sync/index.html
182 /// [`Arc::clone(&from)`]: Arc::clone
183 /// [fully qualified syntax]: https://doc.rust-lang.org/book/ch19-03-advanced-traits.html#fully-qualified-syntax-for-disambiguation-calling-methods-with-the-same-name
187 /// Sharing some immutable data between threads:
189 // Note that we **do not** run these tests here. The windows builders get super
190 // unhappy if a thread outlives the main thread and then exits at the same time
191 // (something deadlocks) so we just avoid this entirely by not running these
194 /// use std::sync::Arc;
197 /// let five = Arc::new(5);
200 /// let five = Arc::clone(&five);
202 /// thread::spawn(move || {
203 /// println!("{:?}", five);
208 /// Sharing a mutable [`AtomicUsize`]:
210 /// [`AtomicUsize`]: core::sync::atomic::AtomicUsize "sync::atomic::AtomicUsize"
213 /// use std::sync::Arc;
214 /// use std::sync::atomic::{AtomicUsize, Ordering};
217 /// let val = Arc::new(AtomicUsize::new(5));
220 /// let val = Arc::clone(&val);
222 /// thread::spawn(move || {
223 /// let v = val.fetch_add(1, Ordering::SeqCst);
224 /// println!("{:?}", v);
229 /// See the [`rc` documentation][rc_examples] for more examples of reference
230 /// counting in general.
232 /// [rc_examples]: crate::rc#examples
233 #[cfg_attr(not(test), rustc_diagnostic_item = "Arc")]
234 #[stable(feature = "rust1", since = "1.0.0")]
235 pub struct Arc<T: ?Sized> {
236 ptr: NonNull<ArcInner<T>>,
237 phantom: PhantomData<ArcInner<T>>,
240 #[stable(feature = "rust1", since = "1.0.0")]
241 unsafe impl<T: ?Sized + Sync + Send> Send for Arc<T> {}
242 #[stable(feature = "rust1", since = "1.0.0")]
243 unsafe impl<T: ?Sized + Sync + Send> Sync for Arc<T> {}
245 #[stable(feature = "catch_unwind", since = "1.9.0")]
246 impl<T: RefUnwindSafe + ?Sized> UnwindSafe for Arc<T> {}
248 #[unstable(feature = "coerce_unsized", issue = "27732")]
249 impl<T: ?Sized + Unsize<U>, U: ?Sized> CoerceUnsized<Arc<U>> for Arc<T> {}
251 #[unstable(feature = "dispatch_from_dyn", issue = "none")]
252 impl<T: ?Sized + Unsize<U>, U: ?Sized> DispatchFromDyn<Arc<U>> for Arc<T> {}
254 impl<T: ?Sized> Arc<T> {
255 fn from_inner(ptr: NonNull<ArcInner<T>>) -> Self {
256 Self { ptr, phantom: PhantomData }
259 unsafe fn from_ptr(ptr: *mut ArcInner<T>) -> Self {
260 unsafe { Self::from_inner(NonNull::new_unchecked(ptr)) }
264 /// `Weak` is a version of [`Arc`] that holds a non-owning reference to the
265 /// managed allocation. The allocation is accessed by calling [`upgrade`] on the `Weak`
266 /// pointer, which returns an <code>[Option]<[Arc]\<T>></code>.
268 /// Since a `Weak` reference does not count towards ownership, it will not
269 /// prevent the value stored in the allocation from being dropped, and `Weak` itself makes no
270 /// guarantees about the value still being present. Thus it may return [`None`]
271 /// when [`upgrade`]d. Note however that a `Weak` reference *does* prevent the allocation
272 /// itself (the backing store) from being deallocated.
274 /// A `Weak` pointer is useful for keeping a temporary reference to the allocation
275 /// managed by [`Arc`] without preventing its inner value from being dropped. It is also used to
276 /// prevent circular references between [`Arc`] pointers, since mutual owning references
277 /// would never allow either [`Arc`] to be dropped. For example, a tree could
278 /// have strong [`Arc`] pointers from parent nodes to children, and `Weak`
279 /// pointers from children back to their parents.
281 /// The typical way to obtain a `Weak` pointer is to call [`Arc::downgrade`].
283 /// [`upgrade`]: Weak::upgrade
284 #[stable(feature = "arc_weak", since = "1.4.0")]
285 pub struct Weak<T: ?Sized> {
286 // This is a `NonNull` to allow optimizing the size of this type in enums,
287 // but it is not necessarily a valid pointer.
288 // `Weak::new` sets this to `usize::MAX` so that it doesn’t need
289 // to allocate space on the heap. That's not a value a real pointer
290 // will ever have because RcBox has alignment at least 2.
291 // This is only possible when `T: Sized`; unsized `T` never dangle.
292 ptr: NonNull<ArcInner<T>>,
295 #[stable(feature = "arc_weak", since = "1.4.0")]
296 unsafe impl<T: ?Sized + Sync + Send> Send for Weak<T> {}
297 #[stable(feature = "arc_weak", since = "1.4.0")]
298 unsafe impl<T: ?Sized + Sync + Send> Sync for Weak<T> {}
300 #[unstable(feature = "coerce_unsized", issue = "27732")]
301 impl<T: ?Sized + Unsize<U>, U: ?Sized> CoerceUnsized<Weak<U>> for Weak<T> {}
302 #[unstable(feature = "dispatch_from_dyn", issue = "none")]
303 impl<T: ?Sized + Unsize<U>, U: ?Sized> DispatchFromDyn<Weak<U>> for Weak<T> {}
305 #[stable(feature = "arc_weak", since = "1.4.0")]
306 impl<T: ?Sized + fmt::Debug> fmt::Debug for Weak<T> {
307 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
312 // This is repr(C) to future-proof against possible field-reordering, which
313 // would interfere with otherwise safe [into|from]_raw() of transmutable
316 struct ArcInner<T: ?Sized> {
317 strong: atomic::AtomicUsize,
319 // the value usize::MAX acts as a sentinel for temporarily "locking" the
320 // ability to upgrade weak pointers or downgrade strong ones; this is used
321 // to avoid races in `make_mut` and `get_mut`.
322 weak: atomic::AtomicUsize,
327 unsafe impl<T: ?Sized + Sync + Send> Send for ArcInner<T> {}
328 unsafe impl<T: ?Sized + Sync + Send> Sync for ArcInner<T> {}
331 /// Constructs a new `Arc<T>`.
336 /// use std::sync::Arc;
338 /// let five = Arc::new(5);
340 #[cfg(not(no_global_oom_handling))]
342 #[stable(feature = "rust1", since = "1.0.0")]
343 pub fn new(data: T) -> Arc<T> {
344 // Start the weak pointer count as 1 which is the weak pointer that's
345 // held by all the strong pointers (kinda), see std/rc.rs for more info
346 let x: Box<_> = box ArcInner {
347 strong: atomic::AtomicUsize::new(1),
348 weak: atomic::AtomicUsize::new(1),
351 Self::from_inner(Box::leak(x).into())
354 /// Constructs a new `Arc<T>` using a weak reference to itself. Attempting
355 /// to upgrade the weak reference before this function returns will result
356 /// in a `None` value. However, the weak reference may be cloned freely and
357 /// stored for use at a later time.
361 /// #![feature(arc_new_cyclic)]
362 /// #![allow(dead_code)]
364 /// use std::sync::{Arc, Weak};
370 /// let foo = Arc::new_cyclic(|me| Foo {
374 #[cfg(not(no_global_oom_handling))]
376 #[unstable(feature = "arc_new_cyclic", issue = "75861")]
377 pub fn new_cyclic(data_fn: impl FnOnce(&Weak<T>) -> T) -> Arc<T> {
378 // Construct the inner in the "uninitialized" state with a single
380 let uninit_ptr: NonNull<_> = Box::leak(box ArcInner {
381 strong: atomic::AtomicUsize::new(0),
382 weak: atomic::AtomicUsize::new(1),
383 data: mem::MaybeUninit::<T>::uninit(),
386 let init_ptr: NonNull<ArcInner<T>> = uninit_ptr.cast();
388 let weak = Weak { ptr: init_ptr };
390 // It's important we don't give up ownership of the weak pointer, or
391 // else the memory might be freed by the time `data_fn` returns. If
392 // we really wanted to pass ownership, we could create an additional
393 // weak pointer for ourselves, but this would result in additional
394 // updates to the weak reference count which might not be necessary
396 let data = data_fn(&weak);
398 // Now we can properly initialize the inner value and turn our weak
399 // reference into a strong reference.
401 let inner = init_ptr.as_ptr();
402 ptr::write(ptr::addr_of_mut!((*inner).data), data);
404 // The above write to the data field must be visible to any threads which
405 // observe a non-zero strong count. Therefore we need at least "Release" ordering
406 // in order to synchronize with the `compare_exchange_weak` in `Weak::upgrade`.
408 // "Acquire" ordering is not required. When considering the possible behaviours
409 // of `data_fn` we only need to look at what it could do with a reference to a
410 // non-upgradeable `Weak`:
411 // - It can *clone* the `Weak`, increasing the weak reference count.
412 // - It can drop those clones, decreasing the weak reference count (but never to zero).
414 // These side effects do not impact us in any way, and no other side effects are
415 // possible with safe code alone.
416 let prev_value = (*inner).strong.fetch_add(1, Release);
417 debug_assert_eq!(prev_value, 0, "No prior strong references should exist");
420 let strong = Arc::from_inner(init_ptr);
422 // Strong references should collectively own a shared weak reference,
423 // so don't run the destructor for our old weak reference.
428 /// Constructs a new `Arc` with uninitialized contents.
433 /// #![feature(new_uninit)]
434 /// #![feature(get_mut_unchecked)]
436 /// use std::sync::Arc;
438 /// let mut five = Arc::<u32>::new_uninit();
440 /// let five = unsafe {
441 /// // Deferred initialization:
442 /// Arc::get_mut_unchecked(&mut five).as_mut_ptr().write(5);
444 /// five.assume_init()
447 /// assert_eq!(*five, 5)
449 #[cfg(not(no_global_oom_handling))]
450 #[unstable(feature = "new_uninit", issue = "63291")]
451 pub fn new_uninit() -> Arc<mem::MaybeUninit<T>> {
453 Arc::from_ptr(Arc::allocate_for_layout(
455 |layout| Global.allocate(layout),
456 |mem| mem as *mut ArcInner<mem::MaybeUninit<T>>,
461 /// Constructs a new `Arc` with uninitialized contents, with the memory
462 /// being filled with `0` bytes.
464 /// See [`MaybeUninit::zeroed`][zeroed] for examples of correct and incorrect usage
470 /// #![feature(new_uninit)]
472 /// use std::sync::Arc;
474 /// let zero = Arc::<u32>::new_zeroed();
475 /// let zero = unsafe { zero.assume_init() };
477 /// assert_eq!(*zero, 0)
480 /// [zeroed]: mem::MaybeUninit::zeroed
481 #[cfg(not(no_global_oom_handling))]
482 #[unstable(feature = "new_uninit", issue = "63291")]
483 pub fn new_zeroed() -> Arc<mem::MaybeUninit<T>> {
485 Arc::from_ptr(Arc::allocate_for_layout(
487 |layout| Global.allocate_zeroed(layout),
488 |mem| mem as *mut ArcInner<mem::MaybeUninit<T>>,
493 /// Constructs a new `Pin<Arc<T>>`. If `T` does not implement `Unpin`, then
494 /// `data` will be pinned in memory and unable to be moved.
495 #[cfg(not(no_global_oom_handling))]
496 #[stable(feature = "pin", since = "1.33.0")]
497 pub fn pin(data: T) -> Pin<Arc<T>> {
498 unsafe { Pin::new_unchecked(Arc::new(data)) }
501 /// Constructs a new `Pin<Arc<T>>`, return an error if allocation fails.
502 #[unstable(feature = "allocator_api", issue = "32838")]
504 pub fn try_pin(data: T) -> Result<Pin<Arc<T>>, AllocError> {
505 unsafe { Ok(Pin::new_unchecked(Arc::try_new(data)?)) }
508 /// Constructs a new `Arc<T>`, returning an error if allocation fails.
513 /// #![feature(allocator_api)]
514 /// use std::sync::Arc;
516 /// let five = Arc::try_new(5)?;
517 /// # Ok::<(), std::alloc::AllocError>(())
519 #[unstable(feature = "allocator_api", issue = "32838")]
521 pub fn try_new(data: T) -> Result<Arc<T>, AllocError> {
522 // Start the weak pointer count as 1 which is the weak pointer that's
523 // held by all the strong pointers (kinda), see std/rc.rs for more info
524 let x: Box<_> = Box::try_new(ArcInner {
525 strong: atomic::AtomicUsize::new(1),
526 weak: atomic::AtomicUsize::new(1),
529 Ok(Self::from_inner(Box::leak(x).into()))
532 /// Constructs a new `Arc` with uninitialized contents, returning an error
533 /// if allocation fails.
538 /// #![feature(new_uninit, allocator_api)]
539 /// #![feature(get_mut_unchecked)]
541 /// use std::sync::Arc;
543 /// let mut five = Arc::<u32>::try_new_uninit()?;
545 /// let five = unsafe {
546 /// // Deferred initialization:
547 /// Arc::get_mut_unchecked(&mut five).as_mut_ptr().write(5);
549 /// five.assume_init()
552 /// assert_eq!(*five, 5);
553 /// # Ok::<(), std::alloc::AllocError>(())
555 #[unstable(feature = "allocator_api", issue = "32838")]
556 // #[unstable(feature = "new_uninit", issue = "63291")]
557 pub fn try_new_uninit() -> Result<Arc<mem::MaybeUninit<T>>, AllocError> {
559 Ok(Arc::from_ptr(Arc::try_allocate_for_layout(
561 |layout| Global.allocate(layout),
562 |mem| mem as *mut ArcInner<mem::MaybeUninit<T>>,
567 /// Constructs a new `Arc` with uninitialized contents, with the memory
568 /// being filled with `0` bytes, returning an error if allocation fails.
570 /// See [`MaybeUninit::zeroed`][zeroed] for examples of correct and incorrect usage
576 /// #![feature(new_uninit, allocator_api)]
578 /// use std::sync::Arc;
580 /// let zero = Arc::<u32>::try_new_zeroed()?;
581 /// let zero = unsafe { zero.assume_init() };
583 /// assert_eq!(*zero, 0);
584 /// # Ok::<(), std::alloc::AllocError>(())
587 /// [zeroed]: mem::MaybeUninit::zeroed
588 #[unstable(feature = "allocator_api", issue = "32838")]
589 // #[unstable(feature = "new_uninit", issue = "63291")]
590 pub fn try_new_zeroed() -> Result<Arc<mem::MaybeUninit<T>>, AllocError> {
592 Ok(Arc::from_ptr(Arc::try_allocate_for_layout(
594 |layout| Global.allocate_zeroed(layout),
595 |mem| mem as *mut ArcInner<mem::MaybeUninit<T>>,
599 /// Returns the inner value, if the `Arc` has exactly one strong reference.
601 /// Otherwise, an [`Err`] is returned with the same `Arc` that was
604 /// This will succeed even if there are outstanding weak references.
609 /// use std::sync::Arc;
611 /// let x = Arc::new(3);
612 /// assert_eq!(Arc::try_unwrap(x), Ok(3));
614 /// let x = Arc::new(4);
615 /// let _y = Arc::clone(&x);
616 /// assert_eq!(*Arc::try_unwrap(x).unwrap_err(), 4);
619 #[stable(feature = "arc_unique", since = "1.4.0")]
620 pub fn try_unwrap(this: Self) -> Result<T, Self> {
621 if this.inner().strong.compare_exchange(1, 0, Relaxed, Relaxed).is_err() {
625 acquire!(this.inner().strong);
628 let elem = ptr::read(&this.ptr.as_ref().data);
630 // Make a weak pointer to clean up the implicit strong-weak reference
631 let _weak = Weak { ptr: this.ptr };
640 /// Constructs a new atomically reference-counted slice with uninitialized contents.
645 /// #![feature(new_uninit)]
646 /// #![feature(get_mut_unchecked)]
648 /// use std::sync::Arc;
650 /// let mut values = Arc::<[u32]>::new_uninit_slice(3);
652 /// let values = unsafe {
653 /// // Deferred initialization:
654 /// Arc::get_mut_unchecked(&mut values)[0].as_mut_ptr().write(1);
655 /// Arc::get_mut_unchecked(&mut values)[1].as_mut_ptr().write(2);
656 /// Arc::get_mut_unchecked(&mut values)[2].as_mut_ptr().write(3);
658 /// values.assume_init()
661 /// assert_eq!(*values, [1, 2, 3])
663 #[cfg(not(no_global_oom_handling))]
664 #[unstable(feature = "new_uninit", issue = "63291")]
665 pub fn new_uninit_slice(len: usize) -> Arc<[mem::MaybeUninit<T>]> {
666 unsafe { Arc::from_ptr(Arc::allocate_for_slice(len)) }
669 /// Constructs a new atomically reference-counted slice with uninitialized contents, with the memory being
670 /// filled with `0` bytes.
672 /// See [`MaybeUninit::zeroed`][zeroed] for examples of correct and
673 /// incorrect usage of this method.
678 /// #![feature(new_uninit)]
680 /// use std::sync::Arc;
682 /// let values = Arc::<[u32]>::new_zeroed_slice(3);
683 /// let values = unsafe { values.assume_init() };
685 /// assert_eq!(*values, [0, 0, 0])
688 /// [zeroed]: mem::MaybeUninit::zeroed
689 #[cfg(not(no_global_oom_handling))]
690 #[unstable(feature = "new_uninit", issue = "63291")]
691 pub fn new_zeroed_slice(len: usize) -> Arc<[mem::MaybeUninit<T>]> {
693 Arc::from_ptr(Arc::allocate_for_layout(
694 Layout::array::<T>(len).unwrap(),
695 |layout| Global.allocate_zeroed(layout),
697 ptr::slice_from_raw_parts_mut(mem as *mut T, len)
698 as *mut ArcInner<[mem::MaybeUninit<T>]>
705 impl<T> Arc<mem::MaybeUninit<T>> {
706 /// Converts to `Arc<T>`.
710 /// As with [`MaybeUninit::assume_init`],
711 /// it is up to the caller to guarantee that the inner value
712 /// really is in an initialized state.
713 /// Calling this when the content is not yet fully initialized
714 /// causes immediate undefined behavior.
716 /// [`MaybeUninit::assume_init`]: mem::MaybeUninit::assume_init
721 /// #![feature(new_uninit)]
722 /// #![feature(get_mut_unchecked)]
724 /// use std::sync::Arc;
726 /// let mut five = Arc::<u32>::new_uninit();
728 /// let five = unsafe {
729 /// // Deferred initialization:
730 /// Arc::get_mut_unchecked(&mut five).as_mut_ptr().write(5);
732 /// five.assume_init()
735 /// assert_eq!(*five, 5)
737 #[unstable(feature = "new_uninit", issue = "63291")]
739 pub unsafe fn assume_init(self) -> Arc<T> {
740 Arc::from_inner(mem::ManuallyDrop::new(self).ptr.cast())
744 impl<T> Arc<[mem::MaybeUninit<T>]> {
745 /// Converts to `Arc<[T]>`.
749 /// As with [`MaybeUninit::assume_init`],
750 /// it is up to the caller to guarantee that the inner value
751 /// really is in an initialized state.
752 /// Calling this when the content is not yet fully initialized
753 /// causes immediate undefined behavior.
755 /// [`MaybeUninit::assume_init`]: mem::MaybeUninit::assume_init
760 /// #![feature(new_uninit)]
761 /// #![feature(get_mut_unchecked)]
763 /// use std::sync::Arc;
765 /// let mut values = Arc::<[u32]>::new_uninit_slice(3);
767 /// let values = unsafe {
768 /// // Deferred initialization:
769 /// Arc::get_mut_unchecked(&mut values)[0].as_mut_ptr().write(1);
770 /// Arc::get_mut_unchecked(&mut values)[1].as_mut_ptr().write(2);
771 /// Arc::get_mut_unchecked(&mut values)[2].as_mut_ptr().write(3);
773 /// values.assume_init()
776 /// assert_eq!(*values, [1, 2, 3])
778 #[unstable(feature = "new_uninit", issue = "63291")]
780 pub unsafe fn assume_init(self) -> Arc<[T]> {
781 unsafe { Arc::from_ptr(mem::ManuallyDrop::new(self).ptr.as_ptr() as _) }
785 impl<T: ?Sized> Arc<T> {
786 /// Consumes the `Arc`, returning the wrapped pointer.
788 /// To avoid a memory leak the pointer must be converted back to an `Arc` using
789 /// [`Arc::from_raw`].
794 /// use std::sync::Arc;
796 /// let x = Arc::new("hello".to_owned());
797 /// let x_ptr = Arc::into_raw(x);
798 /// assert_eq!(unsafe { &*x_ptr }, "hello");
800 #[stable(feature = "rc_raw", since = "1.17.0")]
801 pub fn into_raw(this: Self) -> *const T {
802 let ptr = Self::as_ptr(&this);
807 /// Provides a raw pointer to the data.
809 /// The counts are not affected in any way and the `Arc` is not consumed. The pointer is valid for
810 /// as long as there are strong counts in the `Arc`.
815 /// use std::sync::Arc;
817 /// let x = Arc::new("hello".to_owned());
818 /// let y = Arc::clone(&x);
819 /// let x_ptr = Arc::as_ptr(&x);
820 /// assert_eq!(x_ptr, Arc::as_ptr(&y));
821 /// assert_eq!(unsafe { &*x_ptr }, "hello");
823 #[stable(feature = "rc_as_ptr", since = "1.45.0")]
824 pub fn as_ptr(this: &Self) -> *const T {
825 let ptr: *mut ArcInner<T> = NonNull::as_ptr(this.ptr);
827 // SAFETY: This cannot go through Deref::deref or RcBoxPtr::inner because
828 // this is required to retain raw/mut provenance such that e.g. `get_mut` can
829 // write through the pointer after the Rc is recovered through `from_raw`.
830 unsafe { ptr::addr_of_mut!((*ptr).data) }
833 /// Constructs an `Arc<T>` from a raw pointer.
835 /// The raw pointer must have been previously returned by a call to
836 /// [`Arc<U>::into_raw`][into_raw] where `U` must have the same size and
837 /// alignment as `T`. This is trivially true if `U` is `T`.
838 /// Note that if `U` is not `T` but has the same size and alignment, this is
839 /// basically like transmuting references of different types. See
840 /// [`mem::transmute`][transmute] for more information on what
841 /// restrictions apply in this case.
843 /// The user of `from_raw` has to make sure a specific value of `T` is only
846 /// This function is unsafe because improper use may lead to memory unsafety,
847 /// even if the returned `Arc<T>` is never accessed.
849 /// [into_raw]: Arc::into_raw
850 /// [transmute]: core::mem::transmute
855 /// use std::sync::Arc;
857 /// let x = Arc::new("hello".to_owned());
858 /// let x_ptr = Arc::into_raw(x);
861 /// // Convert back to an `Arc` to prevent leak.
862 /// let x = Arc::from_raw(x_ptr);
863 /// assert_eq!(&*x, "hello");
865 /// // Further calls to `Arc::from_raw(x_ptr)` would be memory-unsafe.
868 /// // The memory was freed when `x` went out of scope above, so `x_ptr` is now dangling!
870 #[stable(feature = "rc_raw", since = "1.17.0")]
871 pub unsafe fn from_raw(ptr: *const T) -> Self {
873 let offset = data_offset(ptr);
875 // Reverse the offset to find the original ArcInner.
876 let arc_ptr = (ptr as *mut ArcInner<T>).set_ptr_value((ptr as *mut u8).offset(-offset));
878 Self::from_ptr(arc_ptr)
882 /// Creates a new [`Weak`] pointer to this allocation.
887 /// use std::sync::Arc;
889 /// let five = Arc::new(5);
891 /// let weak_five = Arc::downgrade(&five);
893 #[stable(feature = "arc_weak", since = "1.4.0")]
894 pub fn downgrade(this: &Self) -> Weak<T> {
895 // This Relaxed is OK because we're checking the value in the CAS
897 let mut cur = this.inner().weak.load(Relaxed);
900 // check if the weak counter is currently "locked"; if so, spin.
901 if cur == usize::MAX {
903 cur = this.inner().weak.load(Relaxed);
907 // NOTE: this code currently ignores the possibility of overflow
908 // into usize::MAX; in general both Rc and Arc need to be adjusted
909 // to deal with overflow.
911 // Unlike with Clone(), we need this to be an Acquire read to
912 // synchronize with the write coming from `is_unique`, so that the
913 // events prior to that write happen before this read.
914 match this.inner().weak.compare_exchange_weak(cur, cur + 1, Acquire, Relaxed) {
916 // Make sure we do not create a dangling Weak
917 debug_assert!(!is_dangling(this.ptr.as_ptr()));
918 return Weak { ptr: this.ptr };
920 Err(old) => cur = old,
925 /// Gets the number of [`Weak`] pointers to this allocation.
929 /// This method by itself is safe, but using it correctly requires extra care.
930 /// Another thread can change the weak count at any time,
931 /// including potentially between calling this method and acting on the result.
936 /// use std::sync::Arc;
938 /// let five = Arc::new(5);
939 /// let _weak_five = Arc::downgrade(&five);
941 /// // This assertion is deterministic because we haven't shared
942 /// // the `Arc` or `Weak` between threads.
943 /// assert_eq!(1, Arc::weak_count(&five));
946 #[stable(feature = "arc_counts", since = "1.15.0")]
947 pub fn weak_count(this: &Self) -> usize {
948 let cnt = this.inner().weak.load(SeqCst);
949 // If the weak count is currently locked, the value of the
950 // count was 0 just before taking the lock.
951 if cnt == usize::MAX { 0 } else { cnt - 1 }
954 /// Gets the number of strong (`Arc`) pointers to this allocation.
958 /// This method by itself is safe, but using it correctly requires extra care.
959 /// Another thread can change the strong count at any time,
960 /// including potentially between calling this method and acting on the result.
965 /// use std::sync::Arc;
967 /// let five = Arc::new(5);
968 /// let _also_five = Arc::clone(&five);
970 /// // This assertion is deterministic because we haven't shared
971 /// // the `Arc` between threads.
972 /// assert_eq!(2, Arc::strong_count(&five));
975 #[stable(feature = "arc_counts", since = "1.15.0")]
976 pub fn strong_count(this: &Self) -> usize {
977 this.inner().strong.load(SeqCst)
980 /// Increments the strong reference count on the `Arc<T>` associated with the
981 /// provided pointer by one.
985 /// The pointer must have been obtained through `Arc::into_raw`, and the
986 /// associated `Arc` instance must be valid (i.e. the strong count must be at
987 /// least 1) for the duration of this method.
992 /// use std::sync::Arc;
994 /// let five = Arc::new(5);
997 /// let ptr = Arc::into_raw(five);
998 /// Arc::increment_strong_count(ptr);
1000 /// // This assertion is deterministic because we haven't shared
1001 /// // the `Arc` between threads.
1002 /// let five = Arc::from_raw(ptr);
1003 /// assert_eq!(2, Arc::strong_count(&five));
1007 #[stable(feature = "arc_mutate_strong_count", since = "1.51.0")]
1008 pub unsafe fn increment_strong_count(ptr: *const T) {
1009 // Retain Arc, but don't touch refcount by wrapping in ManuallyDrop
1010 let arc = unsafe { mem::ManuallyDrop::new(Arc::<T>::from_raw(ptr)) };
1011 // Now increase refcount, but don't drop new refcount either
1012 let _arc_clone: mem::ManuallyDrop<_> = arc.clone();
1015 /// Decrements the strong reference count on the `Arc<T>` associated with the
1016 /// provided pointer by one.
1020 /// The pointer must have been obtained through `Arc::into_raw`, and the
1021 /// associated `Arc` instance must be valid (i.e. the strong count must be at
1022 /// least 1) when invoking this method. This method can be used to release the final
1023 /// `Arc` and backing storage, but **should not** be called after the final `Arc` has been
1029 /// use std::sync::Arc;
1031 /// let five = Arc::new(5);
1034 /// let ptr = Arc::into_raw(five);
1035 /// Arc::increment_strong_count(ptr);
1037 /// // Those assertions are deterministic because we haven't shared
1038 /// // the `Arc` between threads.
1039 /// let five = Arc::from_raw(ptr);
1040 /// assert_eq!(2, Arc::strong_count(&five));
1041 /// Arc::decrement_strong_count(ptr);
1042 /// assert_eq!(1, Arc::strong_count(&five));
1046 #[stable(feature = "arc_mutate_strong_count", since = "1.51.0")]
1047 pub unsafe fn decrement_strong_count(ptr: *const T) {
1048 unsafe { mem::drop(Arc::from_raw(ptr)) };
1052 fn inner(&self) -> &ArcInner<T> {
1053 // This unsafety is ok because while this arc is alive we're guaranteed
1054 // that the inner pointer is valid. Furthermore, we know that the
1055 // `ArcInner` structure itself is `Sync` because the inner data is
1056 // `Sync` as well, so we're ok loaning out an immutable pointer to these
1058 unsafe { self.ptr.as_ref() }
1061 // Non-inlined part of `drop`.
1063 unsafe fn drop_slow(&mut self) {
1064 // Destroy the data at this time, even though we must not free the box
1065 // allocation itself (there might still be weak pointers lying around).
1066 unsafe { ptr::drop_in_place(Self::get_mut_unchecked(self)) };
1068 // Drop the weak ref collectively held by all strong references
1069 drop(Weak { ptr: self.ptr });
1073 #[stable(feature = "ptr_eq", since = "1.17.0")]
1074 /// Returns `true` if the two `Arc`s point to the same allocation
1075 /// (in a vein similar to [`ptr::eq`]).
1080 /// use std::sync::Arc;
1082 /// let five = Arc::new(5);
1083 /// let same_five = Arc::clone(&five);
1084 /// let other_five = Arc::new(5);
1086 /// assert!(Arc::ptr_eq(&five, &same_five));
1087 /// assert!(!Arc::ptr_eq(&five, &other_five));
1090 /// [`ptr::eq`]: core::ptr::eq "ptr::eq"
1091 pub fn ptr_eq(this: &Self, other: &Self) -> bool {
1092 this.ptr.as_ptr() == other.ptr.as_ptr()
1096 impl<T: ?Sized> Arc<T> {
1097 /// Allocates an `ArcInner<T>` with sufficient space for
1098 /// a possibly-unsized inner value where the value has the layout provided.
1100 /// The function `mem_to_arcinner` is called with the data pointer
1101 /// and must return back a (potentially fat)-pointer for the `ArcInner<T>`.
1102 #[cfg(not(no_global_oom_handling))]
1103 unsafe fn allocate_for_layout(
1104 value_layout: Layout,
1105 allocate: impl FnOnce(Layout) -> Result<NonNull<[u8]>, AllocError>,
1106 mem_to_arcinner: impl FnOnce(*mut u8) -> *mut ArcInner<T>,
1107 ) -> *mut ArcInner<T> {
1108 // Calculate layout using the given value layout.
1109 // Previously, layout was calculated on the expression
1110 // `&*(ptr as *const ArcInner<T>)`, but this created a misaligned
1111 // reference (see #54908).
1112 let layout = Layout::new::<ArcInner<()>>().extend(value_layout).unwrap().0.pad_to_align();
1114 Arc::try_allocate_for_layout(value_layout, allocate, mem_to_arcinner)
1115 .unwrap_or_else(|_| handle_alloc_error(layout))
1119 /// Allocates an `ArcInner<T>` with sufficient space for
1120 /// a possibly-unsized inner value where the value has the layout provided,
1121 /// returning an error if allocation fails.
1123 /// The function `mem_to_arcinner` is called with the data pointer
1124 /// and must return back a (potentially fat)-pointer for the `ArcInner<T>`.
1125 unsafe fn try_allocate_for_layout(
1126 value_layout: Layout,
1127 allocate: impl FnOnce(Layout) -> Result<NonNull<[u8]>, AllocError>,
1128 mem_to_arcinner: impl FnOnce(*mut u8) -> *mut ArcInner<T>,
1129 ) -> Result<*mut ArcInner<T>, AllocError> {
1130 // Calculate layout using the given value layout.
1131 // Previously, layout was calculated on the expression
1132 // `&*(ptr as *const ArcInner<T>)`, but this created a misaligned
1133 // reference (see #54908).
1134 let layout = Layout::new::<ArcInner<()>>().extend(value_layout).unwrap().0.pad_to_align();
1136 let ptr = allocate(layout)?;
1138 // Initialize the ArcInner
1139 let inner = mem_to_arcinner(ptr.as_non_null_ptr().as_ptr());
1140 debug_assert_eq!(unsafe { Layout::for_value(&*inner) }, layout);
1143 ptr::write(&mut (*inner).strong, atomic::AtomicUsize::new(1));
1144 ptr::write(&mut (*inner).weak, atomic::AtomicUsize::new(1));
1150 /// Allocates an `ArcInner<T>` with sufficient space for an unsized inner value.
1151 #[cfg(not(no_global_oom_handling))]
1152 unsafe fn allocate_for_ptr(ptr: *const T) -> *mut ArcInner<T> {
1153 // Allocate for the `ArcInner<T>` using the given value.
1155 Self::allocate_for_layout(
1156 Layout::for_value(&*ptr),
1157 |layout| Global.allocate(layout),
1158 |mem| (ptr as *mut ArcInner<T>).set_ptr_value(mem) as *mut ArcInner<T>,
1163 #[cfg(not(no_global_oom_handling))]
1164 fn from_box(v: Box<T>) -> Arc<T> {
1166 let (box_unique, alloc) = Box::into_unique(v);
1167 let bptr = box_unique.as_ptr();
1169 let value_size = size_of_val(&*bptr);
1170 let ptr = Self::allocate_for_ptr(bptr);
1172 // Copy value as bytes
1173 ptr::copy_nonoverlapping(
1174 bptr as *const T as *const u8,
1175 &mut (*ptr).data as *mut _ as *mut u8,
1179 // Free the allocation without dropping its contents
1180 box_free(box_unique, alloc);
1188 /// Allocates an `ArcInner<[T]>` with the given length.
1189 #[cfg(not(no_global_oom_handling))]
1190 unsafe fn allocate_for_slice(len: usize) -> *mut ArcInner<[T]> {
1192 Self::allocate_for_layout(
1193 Layout::array::<T>(len).unwrap(),
1194 |layout| Global.allocate(layout),
1195 |mem| ptr::slice_from_raw_parts_mut(mem as *mut T, len) as *mut ArcInner<[T]>,
1200 /// Copy elements from slice into newly allocated Arc<\[T\]>
1202 /// Unsafe because the caller must either take ownership or bind `T: Copy`.
1203 #[cfg(not(no_global_oom_handling))]
1204 unsafe fn copy_from_slice(v: &[T]) -> Arc<[T]> {
1206 let ptr = Self::allocate_for_slice(v.len());
1208 ptr::copy_nonoverlapping(v.as_ptr(), &mut (*ptr).data as *mut [T] as *mut T, v.len());
1214 /// Constructs an `Arc<[T]>` from an iterator known to be of a certain size.
1216 /// Behavior is undefined should the size be wrong.
1217 #[cfg(not(no_global_oom_handling))]
1218 unsafe fn from_iter_exact(iter: impl iter::Iterator<Item = T>, len: usize) -> Arc<[T]> {
1219 // Panic guard while cloning T elements.
1220 // In the event of a panic, elements that have been written
1221 // into the new ArcInner will be dropped, then the memory freed.
1229 impl<T> Drop for Guard<T> {
1230 fn drop(&mut self) {
1232 let slice = from_raw_parts_mut(self.elems, self.n_elems);
1233 ptr::drop_in_place(slice);
1235 Global.deallocate(self.mem, self.layout);
1241 let ptr = Self::allocate_for_slice(len);
1243 let mem = ptr as *mut _ as *mut u8;
1244 let layout = Layout::for_value(&*ptr);
1246 // Pointer to first element
1247 let elems = &mut (*ptr).data as *mut [T] as *mut T;
1249 let mut guard = Guard { mem: NonNull::new_unchecked(mem), elems, layout, n_elems: 0 };
1251 for (i, item) in iter.enumerate() {
1252 ptr::write(elems.add(i), item);
1256 // All clear. Forget the guard so it doesn't free the new ArcInner.
1264 /// Specialization trait used for `From<&[T]>`.
1265 #[cfg(not(no_global_oom_handling))]
1266 trait ArcFromSlice<T> {
1267 fn from_slice(slice: &[T]) -> Self;
1270 #[cfg(not(no_global_oom_handling))]
1271 impl<T: Clone> ArcFromSlice<T> for Arc<[T]> {
1273 default fn from_slice(v: &[T]) -> Self {
1274 unsafe { Self::from_iter_exact(v.iter().cloned(), v.len()) }
1278 #[cfg(not(no_global_oom_handling))]
1279 impl<T: Copy> ArcFromSlice<T> for Arc<[T]> {
1281 fn from_slice(v: &[T]) -> Self {
1282 unsafe { Arc::copy_from_slice(v) }
1286 #[stable(feature = "rust1", since = "1.0.0")]
1287 impl<T: ?Sized> Clone for Arc<T> {
1288 /// Makes a clone of the `Arc` pointer.
1290 /// This creates another pointer to the same allocation, increasing the
1291 /// strong reference count.
1296 /// use std::sync::Arc;
1298 /// let five = Arc::new(5);
1300 /// let _ = Arc::clone(&five);
1303 fn clone(&self) -> Arc<T> {
1304 // Using a relaxed ordering is alright here, as knowledge of the
1305 // original reference prevents other threads from erroneously deleting
1308 // As explained in the [Boost documentation][1], Increasing the
1309 // reference counter can always be done with memory_order_relaxed: New
1310 // references to an object can only be formed from an existing
1311 // reference, and passing an existing reference from one thread to
1312 // another must already provide any required synchronization.
1314 // [1]: (www.boost.org/doc/libs/1_55_0/doc/html/atomic/usage_examples.html)
1315 let old_size = self.inner().strong.fetch_add(1, Relaxed);
1317 // However we need to guard against massive refcounts in case someone
1318 // is `mem::forget`ing Arcs. If we don't do this the count can overflow
1319 // and users will use-after free. We racily saturate to `isize::MAX` on
1320 // the assumption that there aren't ~2 billion threads incrementing
1321 // the reference count at once. This branch will never be taken in
1322 // any realistic program.
1324 // We abort because such a program is incredibly degenerate, and we
1325 // don't care to support it.
1326 if old_size > MAX_REFCOUNT {
1330 Self::from_inner(self.ptr)
1334 #[stable(feature = "rust1", since = "1.0.0")]
1335 impl<T: ?Sized> Deref for Arc<T> {
1339 fn deref(&self) -> &T {
1344 #[unstable(feature = "receiver_trait", issue = "none")]
1345 impl<T: ?Sized> Receiver for Arc<T> {}
1347 impl<T: Clone> Arc<T> {
1348 /// Makes a mutable reference into the given `Arc`.
1350 /// If there are other `Arc` pointers to the same allocation, then `make_mut` will
1351 /// [`clone`] the inner value to a new allocation to ensure unique ownership. This is also
1352 /// referred to as clone-on-write.
1354 /// However, if there are no other `Arc` pointers to this allocation, but some [`Weak`]
1355 /// pointers, then the [`Weak`] pointers will be disassociated and the inner value will not
1358 /// See also [`get_mut`], which will fail rather than cloning the inner value
1359 /// or diassociating [`Weak`] pointers.
1361 /// [`clone`]: Clone::clone
1362 /// [`get_mut`]: Arc::get_mut
1367 /// use std::sync::Arc;
1369 /// let mut data = Arc::new(5);
1371 /// *Arc::make_mut(&mut data) += 1; // Won't clone anything
1372 /// let mut other_data = Arc::clone(&data); // Won't clone inner data
1373 /// *Arc::make_mut(&mut data) += 1; // Clones inner data
1374 /// *Arc::make_mut(&mut data) += 1; // Won't clone anything
1375 /// *Arc::make_mut(&mut other_data) *= 2; // Won't clone anything
1377 /// // Now `data` and `other_data` point to different allocations.
1378 /// assert_eq!(*data, 8);
1379 /// assert_eq!(*other_data, 12);
1382 /// [`Weak`] pointers will be disassociated:
1385 /// use std::sync::Arc;
1387 /// let mut data = Arc::new(75);
1388 /// let weak = Arc::downgrade(&data);
1390 /// assert!(75 == *data);
1391 /// assert!(75 == *weak.upgrade().unwrap());
1393 /// *Arc::make_mut(&mut data) += 1;
1395 /// assert!(76 == *data);
1396 /// assert!(weak.upgrade().is_none());
1398 #[cfg(not(no_global_oom_handling))]
1400 #[stable(feature = "arc_unique", since = "1.4.0")]
1401 pub fn make_mut(this: &mut Self) -> &mut T {
1402 // Note that we hold both a strong reference and a weak reference.
1403 // Thus, releasing our strong reference only will not, by itself, cause
1404 // the memory to be deallocated.
1406 // Use Acquire to ensure that we see any writes to `weak` that happen
1407 // before release writes (i.e., decrements) to `strong`. Since we hold a
1408 // weak count, there's no chance the ArcInner itself could be
1410 if this.inner().strong.compare_exchange(1, 0, Acquire, Relaxed).is_err() {
1411 // Another strong pointer exists, so we must clone.
1412 // Pre-allocate memory to allow writing the cloned value directly.
1413 let mut arc = Self::new_uninit();
1415 let data = Arc::get_mut_unchecked(&mut arc);
1416 (**this).write_clone_into_raw(data.as_mut_ptr());
1417 *this = arc.assume_init();
1419 } else if this.inner().weak.load(Relaxed) != 1 {
1420 // Relaxed suffices in the above because this is fundamentally an
1421 // optimization: we are always racing with weak pointers being
1422 // dropped. Worst case, we end up allocated a new Arc unnecessarily.
1424 // We removed the last strong ref, but there are additional weak
1425 // refs remaining. We'll move the contents to a new Arc, and
1426 // invalidate the other weak refs.
1428 // Note that it is not possible for the read of `weak` to yield
1429 // usize::MAX (i.e., locked), since the weak count can only be
1430 // locked by a thread with a strong reference.
1432 // Materialize our own implicit weak pointer, so that it can clean
1433 // up the ArcInner as needed.
1434 let _weak = Weak { ptr: this.ptr };
1436 // Can just steal the data, all that's left is Weaks
1437 let mut arc = Self::new_uninit();
1439 let data = Arc::get_mut_unchecked(&mut arc);
1440 data.as_mut_ptr().copy_from_nonoverlapping(&**this, 1);
1441 ptr::write(this, arc.assume_init());
1444 // We were the sole reference of either kind; bump back up the
1445 // strong ref count.
1446 this.inner().strong.store(1, Release);
1449 // As with `get_mut()`, the unsafety is ok because our reference was
1450 // either unique to begin with, or became one upon cloning the contents.
1451 unsafe { Self::get_mut_unchecked(this) }
1455 impl<T: ?Sized> Arc<T> {
1456 /// Returns a mutable reference into the given `Arc`, if there are
1457 /// no other `Arc` or [`Weak`] pointers to the same allocation.
1459 /// Returns [`None`] otherwise, because it is not safe to
1460 /// mutate a shared value.
1462 /// See also [`make_mut`][make_mut], which will [`clone`][clone]
1463 /// the inner value when there are other `Arc` pointers.
1465 /// [make_mut]: Arc::make_mut
1466 /// [clone]: Clone::clone
1471 /// use std::sync::Arc;
1473 /// let mut x = Arc::new(3);
1474 /// *Arc::get_mut(&mut x).unwrap() = 4;
1475 /// assert_eq!(*x, 4);
1477 /// let _y = Arc::clone(&x);
1478 /// assert!(Arc::get_mut(&mut x).is_none());
1481 #[stable(feature = "arc_unique", since = "1.4.0")]
1482 pub fn get_mut(this: &mut Self) -> Option<&mut T> {
1483 if this.is_unique() {
1484 // This unsafety is ok because we're guaranteed that the pointer
1485 // returned is the *only* pointer that will ever be returned to T. Our
1486 // reference count is guaranteed to be 1 at this point, and we required
1487 // the Arc itself to be `mut`, so we're returning the only possible
1488 // reference to the inner data.
1489 unsafe { Some(Arc::get_mut_unchecked(this)) }
1495 /// Returns a mutable reference into the given `Arc`,
1496 /// without any check.
1498 /// See also [`get_mut`], which is safe and does appropriate checks.
1500 /// [`get_mut`]: Arc::get_mut
1504 /// Any other `Arc` or [`Weak`] pointers to the same allocation must not be dereferenced
1505 /// for the duration of the returned borrow.
1506 /// This is trivially the case if no such pointers exist,
1507 /// for example immediately after `Arc::new`.
1512 /// #![feature(get_mut_unchecked)]
1514 /// use std::sync::Arc;
1516 /// let mut x = Arc::new(String::new());
1518 /// Arc::get_mut_unchecked(&mut x).push_str("foo")
1520 /// assert_eq!(*x, "foo");
1523 #[unstable(feature = "get_mut_unchecked", issue = "63292")]
1524 pub unsafe fn get_mut_unchecked(this: &mut Self) -> &mut T {
1525 // We are careful to *not* create a reference covering the "count" fields, as
1526 // this would alias with concurrent access to the reference counts (e.g. by `Weak`).
1527 unsafe { &mut (*this.ptr.as_ptr()).data }
1530 /// Determine whether this is the unique reference (including weak refs) to
1531 /// the underlying data.
1533 /// Note that this requires locking the weak ref count.
1534 fn is_unique(&mut self) -> bool {
1535 // lock the weak pointer count if we appear to be the sole weak pointer
1538 // The acquire label here ensures a happens-before relationship with any
1539 // writes to `strong` (in particular in `Weak::upgrade`) prior to decrements
1540 // of the `weak` count (via `Weak::drop`, which uses release). If the upgraded
1541 // weak ref was never dropped, the CAS here will fail so we do not care to synchronize.
1542 if self.inner().weak.compare_exchange(1, usize::MAX, Acquire, Relaxed).is_ok() {
1543 // This needs to be an `Acquire` to synchronize with the decrement of the `strong`
1544 // counter in `drop` -- the only access that happens when any but the last reference
1545 // is being dropped.
1546 let unique = self.inner().strong.load(Acquire) == 1;
1548 // The release write here synchronizes with a read in `downgrade`,
1549 // effectively preventing the above read of `strong` from happening
1551 self.inner().weak.store(1, Release); // release the lock
1559 #[stable(feature = "rust1", since = "1.0.0")]
1560 unsafe impl<#[may_dangle] T: ?Sized> Drop for Arc<T> {
1561 /// Drops the `Arc`.
1563 /// This will decrement the strong reference count. If the strong reference
1564 /// count reaches zero then the only other references (if any) are
1565 /// [`Weak`], so we `drop` the inner value.
1570 /// use std::sync::Arc;
1574 /// impl Drop for Foo {
1575 /// fn drop(&mut self) {
1576 /// println!("dropped!");
1580 /// let foo = Arc::new(Foo);
1581 /// let foo2 = Arc::clone(&foo);
1583 /// drop(foo); // Doesn't print anything
1584 /// drop(foo2); // Prints "dropped!"
1587 fn drop(&mut self) {
1588 // Because `fetch_sub` is already atomic, we do not need to synchronize
1589 // with other threads unless we are going to delete the object. This
1590 // same logic applies to the below `fetch_sub` to the `weak` count.
1591 if self.inner().strong.fetch_sub(1, Release) != 1 {
1595 // This fence is needed to prevent reordering of use of the data and
1596 // deletion of the data. Because it is marked `Release`, the decreasing
1597 // of the reference count synchronizes with this `Acquire` fence. This
1598 // means that use of the data happens before decreasing the reference
1599 // count, which happens before this fence, which happens before the
1600 // deletion of the data.
1602 // As explained in the [Boost documentation][1],
1604 // > It is important to enforce any possible access to the object in one
1605 // > thread (through an existing reference) to *happen before* deleting
1606 // > the object in a different thread. This is achieved by a "release"
1607 // > operation after dropping a reference (any access to the object
1608 // > through this reference must obviously happened before), and an
1609 // > "acquire" operation before deleting the object.
1611 // In particular, while the contents of an Arc are usually immutable, it's
1612 // possible to have interior writes to something like a Mutex<T>. Since a
1613 // Mutex is not acquired when it is deleted, we can't rely on its
1614 // synchronization logic to make writes in thread A visible to a destructor
1615 // running in thread B.
1617 // Also note that the Acquire fence here could probably be replaced with an
1618 // Acquire load, which could improve performance in highly-contended
1619 // situations. See [2].
1621 // [1]: (www.boost.org/doc/libs/1_55_0/doc/html/atomic/usage_examples.html)
1622 // [2]: (https://github.com/rust-lang/rust/pull/41714)
1623 acquire!(self.inner().strong);
1631 impl Arc<dyn Any + Send + Sync> {
1633 #[stable(feature = "rc_downcast", since = "1.29.0")]
1634 /// Attempt to downcast the `Arc<dyn Any + Send + Sync>` to a concrete type.
1639 /// use std::any::Any;
1640 /// use std::sync::Arc;
1642 /// fn print_if_string(value: Arc<dyn Any + Send + Sync>) {
1643 /// if let Ok(string) = value.downcast::<String>() {
1644 /// println!("String ({}): {}", string.len(), string);
1648 /// let my_string = "Hello World".to_string();
1649 /// print_if_string(Arc::new(my_string));
1650 /// print_if_string(Arc::new(0i8));
1652 pub fn downcast<T>(self) -> Result<Arc<T>, Self>
1654 T: Any + Send + Sync + 'static,
1656 if (*self).is::<T>() {
1657 let ptr = self.ptr.cast::<ArcInner<T>>();
1659 Ok(Arc::from_inner(ptr))
1667 /// Constructs a new `Weak<T>`, without allocating any memory.
1668 /// Calling [`upgrade`] on the return value always gives [`None`].
1670 /// [`upgrade`]: Weak::upgrade
1675 /// use std::sync::Weak;
1677 /// let empty: Weak<i64> = Weak::new();
1678 /// assert!(empty.upgrade().is_none());
1680 #[stable(feature = "downgraded_weak", since = "1.10.0")]
1681 pub fn new() -> Weak<T> {
1682 Weak { ptr: NonNull::new(usize::MAX as *mut ArcInner<T>).expect("MAX is not 0") }
1686 /// Helper type to allow accessing the reference counts without
1687 /// making any assertions about the data field.
1688 struct WeakInner<'a> {
1689 weak: &'a atomic::AtomicUsize,
1690 strong: &'a atomic::AtomicUsize,
1693 impl<T: ?Sized> Weak<T> {
1694 /// Returns a raw pointer to the object `T` pointed to by this `Weak<T>`.
1696 /// The pointer is valid only if there are some strong references. The pointer may be dangling,
1697 /// unaligned or even [`null`] otherwise.
1702 /// use std::sync::Arc;
1705 /// let strong = Arc::new("hello".to_owned());
1706 /// let weak = Arc::downgrade(&strong);
1707 /// // Both point to the same object
1708 /// assert!(ptr::eq(&*strong, weak.as_ptr()));
1709 /// // The strong here keeps it alive, so we can still access the object.
1710 /// assert_eq!("hello", unsafe { &*weak.as_ptr() });
1713 /// // But not any more. We can do weak.as_ptr(), but accessing the pointer would lead to
1714 /// // undefined behaviour.
1715 /// // assert_eq!("hello", unsafe { &*weak.as_ptr() });
1718 /// [`null`]: core::ptr::null "ptr::null"
1719 #[stable(feature = "weak_into_raw", since = "1.45.0")]
1720 pub fn as_ptr(&self) -> *const T {
1721 let ptr: *mut ArcInner<T> = NonNull::as_ptr(self.ptr);
1723 if is_dangling(ptr) {
1724 // If the pointer is dangling, we return the sentinel directly. This cannot be
1725 // a valid payload address, as the payload is at least as aligned as ArcInner (usize).
1728 // SAFETY: if is_dangling returns false, then the pointer is dereferencable.
1729 // The payload may be dropped at this point, and we have to maintain provenance,
1730 // so use raw pointer manipulation.
1731 unsafe { ptr::addr_of_mut!((*ptr).data) }
1735 /// Consumes the `Weak<T>` and turns it into a raw pointer.
1737 /// This converts the weak pointer into a raw pointer, while still preserving the ownership of
1738 /// one weak reference (the weak count is not modified by this operation). It can be turned
1739 /// back into the `Weak<T>` with [`from_raw`].
1741 /// The same restrictions of accessing the target of the pointer as with
1742 /// [`as_ptr`] apply.
1747 /// use std::sync::{Arc, Weak};
1749 /// let strong = Arc::new("hello".to_owned());
1750 /// let weak = Arc::downgrade(&strong);
1751 /// let raw = weak.into_raw();
1753 /// assert_eq!(1, Arc::weak_count(&strong));
1754 /// assert_eq!("hello", unsafe { &*raw });
1756 /// drop(unsafe { Weak::from_raw(raw) });
1757 /// assert_eq!(0, Arc::weak_count(&strong));
1760 /// [`from_raw`]: Weak::from_raw
1761 /// [`as_ptr`]: Weak::as_ptr
1762 #[stable(feature = "weak_into_raw", since = "1.45.0")]
1763 pub fn into_raw(self) -> *const T {
1764 let result = self.as_ptr();
1769 /// Converts a raw pointer previously created by [`into_raw`] back into `Weak<T>`.
1771 /// This can be used to safely get a strong reference (by calling [`upgrade`]
1772 /// later) or to deallocate the weak count by dropping the `Weak<T>`.
1774 /// It takes ownership of one weak reference (with the exception of pointers created by [`new`],
1775 /// as these don't own anything; the method still works on them).
1779 /// The pointer must have originated from the [`into_raw`] and must still own its potential
1782 /// It is allowed for the strong count to be 0 at the time of calling this. Nevertheless, this
1783 /// takes ownership of one weak reference currently represented as a raw pointer (the weak
1784 /// count is not modified by this operation) and therefore it must be paired with a previous
1785 /// call to [`into_raw`].
1789 /// use std::sync::{Arc, Weak};
1791 /// let strong = Arc::new("hello".to_owned());
1793 /// let raw_1 = Arc::downgrade(&strong).into_raw();
1794 /// let raw_2 = Arc::downgrade(&strong).into_raw();
1796 /// assert_eq!(2, Arc::weak_count(&strong));
1798 /// assert_eq!("hello", &*unsafe { Weak::from_raw(raw_1) }.upgrade().unwrap());
1799 /// assert_eq!(1, Arc::weak_count(&strong));
1803 /// // Decrement the last weak count.
1804 /// assert!(unsafe { Weak::from_raw(raw_2) }.upgrade().is_none());
1807 /// [`new`]: Weak::new
1808 /// [`into_raw`]: Weak::into_raw
1809 /// [`upgrade`]: Weak::upgrade
1810 #[stable(feature = "weak_into_raw", since = "1.45.0")]
1811 pub unsafe fn from_raw(ptr: *const T) -> Self {
1812 // See Weak::as_ptr for context on how the input pointer is derived.
1814 let ptr = if is_dangling(ptr as *mut T) {
1815 // This is a dangling Weak.
1816 ptr as *mut ArcInner<T>
1818 // Otherwise, we're guaranteed the pointer came from a nondangling Weak.
1819 // SAFETY: data_offset is safe to call, as ptr references a real (potentially dropped) T.
1820 let offset = unsafe { data_offset(ptr) };
1821 // Thus, we reverse the offset to get the whole RcBox.
1822 // SAFETY: the pointer originated from a Weak, so this offset is safe.
1823 unsafe { (ptr as *mut ArcInner<T>).set_ptr_value((ptr as *mut u8).offset(-offset)) }
1826 // SAFETY: we now have recovered the original Weak pointer, so can create the Weak.
1827 Weak { ptr: unsafe { NonNull::new_unchecked(ptr) } }
1831 impl<T: ?Sized> Weak<T> {
1832 /// Attempts to upgrade the `Weak` pointer to an [`Arc`], delaying
1833 /// dropping of the inner value if successful.
1835 /// Returns [`None`] if the inner value has since been dropped.
1840 /// use std::sync::Arc;
1842 /// let five = Arc::new(5);
1844 /// let weak_five = Arc::downgrade(&five);
1846 /// let strong_five: Option<Arc<_>> = weak_five.upgrade();
1847 /// assert!(strong_five.is_some());
1849 /// // Destroy all strong pointers.
1850 /// drop(strong_five);
1853 /// assert!(weak_five.upgrade().is_none());
1855 #[stable(feature = "arc_weak", since = "1.4.0")]
1856 pub fn upgrade(&self) -> Option<Arc<T>> {
1857 // We use a CAS loop to increment the strong count instead of a
1858 // fetch_add as this function should never take the reference count
1859 // from zero to one.
1860 let inner = self.inner()?;
1862 // Relaxed load because any write of 0 that we can observe
1863 // leaves the field in a permanently zero state (so a
1864 // "stale" read of 0 is fine), and any other value is
1865 // confirmed via the CAS below.
1866 let mut n = inner.strong.load(Relaxed);
1873 // See comments in `Arc::clone` for why we do this (for `mem::forget`).
1874 if n > MAX_REFCOUNT {
1878 // Relaxed is fine for the failure case because we don't have any expectations about the new state.
1879 // Acquire is necessary for the success case to synchronise with `Arc::new_cyclic`, when the inner
1880 // value can be initialized after `Weak` references have already been created. In that case, we
1881 // expect to observe the fully initialized value.
1882 match inner.strong.compare_exchange_weak(n, n + 1, Acquire, Relaxed) {
1883 Ok(_) => return Some(Arc::from_inner(self.ptr)), // null checked above
1884 Err(old) => n = old,
1889 /// Gets the number of strong (`Arc`) pointers pointing to this allocation.
1891 /// If `self` was created using [`Weak::new`], this will return 0.
1892 #[stable(feature = "weak_counts", since = "1.41.0")]
1893 pub fn strong_count(&self) -> usize {
1894 if let Some(inner) = self.inner() { inner.strong.load(SeqCst) } else { 0 }
1897 /// Gets an approximation of the number of `Weak` pointers pointing to this
1900 /// If `self` was created using [`Weak::new`], or if there are no remaining
1901 /// strong pointers, this will return 0.
1905 /// Due to implementation details, the returned value can be off by 1 in
1906 /// either direction when other threads are manipulating any `Arc`s or
1907 /// `Weak`s pointing to the same allocation.
1908 #[stable(feature = "weak_counts", since = "1.41.0")]
1909 pub fn weak_count(&self) -> usize {
1912 let weak = inner.weak.load(SeqCst);
1913 let strong = inner.strong.load(SeqCst);
1917 // Since we observed that there was at least one strong pointer
1918 // after reading the weak count, we know that the implicit weak
1919 // reference (present whenever any strong references are alive)
1920 // was still around when we observed the weak count, and can
1921 // therefore safely subtract it.
1928 /// Returns `None` when the pointer is dangling and there is no allocated `ArcInner`,
1929 /// (i.e., when this `Weak` was created by `Weak::new`).
1931 fn inner(&self) -> Option<WeakInner<'_>> {
1932 if is_dangling(self.ptr.as_ptr()) {
1935 // We are careful to *not* create a reference covering the "data" field, as
1936 // the field may be mutated concurrently (for example, if the last `Arc`
1937 // is dropped, the data field will be dropped in-place).
1939 let ptr = self.ptr.as_ptr();
1940 WeakInner { strong: &(*ptr).strong, weak: &(*ptr).weak }
1945 /// Returns `true` if the two `Weak`s point to the same allocation (similar to
1946 /// [`ptr::eq`]), or if both don't point to any allocation
1947 /// (because they were created with `Weak::new()`).
1951 /// Since this compares pointers it means that `Weak::new()` will equal each
1952 /// other, even though they don't point to any allocation.
1957 /// use std::sync::Arc;
1959 /// let first_rc = Arc::new(5);
1960 /// let first = Arc::downgrade(&first_rc);
1961 /// let second = Arc::downgrade(&first_rc);
1963 /// assert!(first.ptr_eq(&second));
1965 /// let third_rc = Arc::new(5);
1966 /// let third = Arc::downgrade(&third_rc);
1968 /// assert!(!first.ptr_eq(&third));
1971 /// Comparing `Weak::new`.
1974 /// use std::sync::{Arc, Weak};
1976 /// let first = Weak::new();
1977 /// let second = Weak::new();
1978 /// assert!(first.ptr_eq(&second));
1980 /// let third_rc = Arc::new(());
1981 /// let third = Arc::downgrade(&third_rc);
1982 /// assert!(!first.ptr_eq(&third));
1985 /// [`ptr::eq`]: core::ptr::eq "ptr::eq"
1987 #[stable(feature = "weak_ptr_eq", since = "1.39.0")]
1988 pub fn ptr_eq(&self, other: &Self) -> bool {
1989 self.ptr.as_ptr() == other.ptr.as_ptr()
1993 #[stable(feature = "arc_weak", since = "1.4.0")]
1994 impl<T: ?Sized> Clone for Weak<T> {
1995 /// Makes a clone of the `Weak` pointer that points to the same allocation.
2000 /// use std::sync::{Arc, Weak};
2002 /// let weak_five = Arc::downgrade(&Arc::new(5));
2004 /// let _ = Weak::clone(&weak_five);
2007 fn clone(&self) -> Weak<T> {
2008 let inner = if let Some(inner) = self.inner() {
2011 return Weak { ptr: self.ptr };
2013 // See comments in Arc::clone() for why this is relaxed. This can use a
2014 // fetch_add (ignoring the lock) because the weak count is only locked
2015 // where are *no other* weak pointers in existence. (So we can't be
2016 // running this code in that case).
2017 let old_size = inner.weak.fetch_add(1, Relaxed);
2019 // See comments in Arc::clone() for why we do this (for mem::forget).
2020 if old_size > MAX_REFCOUNT {
2024 Weak { ptr: self.ptr }
2028 #[stable(feature = "downgraded_weak", since = "1.10.0")]
2029 impl<T> Default for Weak<T> {
2030 /// Constructs a new `Weak<T>`, without allocating memory.
2031 /// Calling [`upgrade`] on the return value always
2034 /// [`upgrade`]: Weak::upgrade
2039 /// use std::sync::Weak;
2041 /// let empty: Weak<i64> = Default::default();
2042 /// assert!(empty.upgrade().is_none());
2044 fn default() -> Weak<T> {
2049 #[stable(feature = "arc_weak", since = "1.4.0")]
2050 unsafe impl<#[may_dangle] T: ?Sized> Drop for Weak<T> {
2051 /// Drops the `Weak` pointer.
2056 /// use std::sync::{Arc, Weak};
2060 /// impl Drop for Foo {
2061 /// fn drop(&mut self) {
2062 /// println!("dropped!");
2066 /// let foo = Arc::new(Foo);
2067 /// let weak_foo = Arc::downgrade(&foo);
2068 /// let other_weak_foo = Weak::clone(&weak_foo);
2070 /// drop(weak_foo); // Doesn't print anything
2071 /// drop(foo); // Prints "dropped!"
2073 /// assert!(other_weak_foo.upgrade().is_none());
2075 fn drop(&mut self) {
2076 // If we find out that we were the last weak pointer, then its time to
2077 // deallocate the data entirely. See the discussion in Arc::drop() about
2078 // the memory orderings
2080 // It's not necessary to check for the locked state here, because the
2081 // weak count can only be locked if there was precisely one weak ref,
2082 // meaning that drop could only subsequently run ON that remaining weak
2083 // ref, which can only happen after the lock is released.
2084 let inner = if let Some(inner) = self.inner() { inner } else { return };
2086 if inner.weak.fetch_sub(1, Release) == 1 {
2087 acquire!(inner.weak);
2088 unsafe { Global.deallocate(self.ptr.cast(), Layout::for_value_raw(self.ptr.as_ptr())) }
2093 #[stable(feature = "rust1", since = "1.0.0")]
2094 trait ArcEqIdent<T: ?Sized + PartialEq> {
2095 fn eq(&self, other: &Arc<T>) -> bool;
2096 fn ne(&self, other: &Arc<T>) -> bool;
2099 #[stable(feature = "rust1", since = "1.0.0")]
2100 impl<T: ?Sized + PartialEq> ArcEqIdent<T> for Arc<T> {
2102 default fn eq(&self, other: &Arc<T>) -> bool {
2106 default fn ne(&self, other: &Arc<T>) -> bool {
2111 /// We're doing this specialization here, and not as a more general optimization on `&T`, because it
2112 /// would otherwise add a cost to all equality checks on refs. We assume that `Arc`s are used to
2113 /// store large values, that are slow to clone, but also heavy to check for equality, causing this
2114 /// cost to pay off more easily. It's also more likely to have two `Arc` clones, that point to
2115 /// the same value, than two `&T`s.
2117 /// We can only do this when `T: Eq` as a `PartialEq` might be deliberately irreflexive.
2118 #[stable(feature = "rust1", since = "1.0.0")]
2119 impl<T: ?Sized + crate::rc::MarkerEq> ArcEqIdent<T> for Arc<T> {
2121 fn eq(&self, other: &Arc<T>) -> bool {
2122 Arc::ptr_eq(self, other) || **self == **other
2126 fn ne(&self, other: &Arc<T>) -> bool {
2127 !Arc::ptr_eq(self, other) && **self != **other
2131 #[stable(feature = "rust1", since = "1.0.0")]
2132 impl<T: ?Sized + PartialEq> PartialEq for Arc<T> {
2133 /// Equality for two `Arc`s.
2135 /// Two `Arc`s are equal if their inner values are equal, even if they are
2136 /// stored in different allocation.
2138 /// If `T` also implements `Eq` (implying reflexivity of equality),
2139 /// two `Arc`s that point to the same allocation are always equal.
2144 /// use std::sync::Arc;
2146 /// let five = Arc::new(5);
2148 /// assert!(five == Arc::new(5));
2151 fn eq(&self, other: &Arc<T>) -> bool {
2152 ArcEqIdent::eq(self, other)
2155 /// Inequality for two `Arc`s.
2157 /// Two `Arc`s are unequal if their inner values are unequal.
2159 /// If `T` also implements `Eq` (implying reflexivity of equality),
2160 /// two `Arc`s that point to the same value are never unequal.
2165 /// use std::sync::Arc;
2167 /// let five = Arc::new(5);
2169 /// assert!(five != Arc::new(6));
2172 fn ne(&self, other: &Arc<T>) -> bool {
2173 ArcEqIdent::ne(self, other)
2177 #[stable(feature = "rust1", since = "1.0.0")]
2178 impl<T: ?Sized + PartialOrd> PartialOrd for Arc<T> {
2179 /// Partial comparison for two `Arc`s.
2181 /// The two are compared by calling `partial_cmp()` on their inner values.
2186 /// use std::sync::Arc;
2187 /// use std::cmp::Ordering;
2189 /// let five = Arc::new(5);
2191 /// assert_eq!(Some(Ordering::Less), five.partial_cmp(&Arc::new(6)));
2193 fn partial_cmp(&self, other: &Arc<T>) -> Option<Ordering> {
2194 (**self).partial_cmp(&**other)
2197 /// Less-than comparison for two `Arc`s.
2199 /// The two are compared by calling `<` on their inner values.
2204 /// use std::sync::Arc;
2206 /// let five = Arc::new(5);
2208 /// assert!(five < Arc::new(6));
2210 fn lt(&self, other: &Arc<T>) -> bool {
2211 *(*self) < *(*other)
2214 /// 'Less than or equal to' comparison for two `Arc`s.
2216 /// The two are compared by calling `<=` on their inner values.
2221 /// use std::sync::Arc;
2223 /// let five = Arc::new(5);
2225 /// assert!(five <= Arc::new(5));
2227 fn le(&self, other: &Arc<T>) -> bool {
2228 *(*self) <= *(*other)
2231 /// Greater-than comparison for two `Arc`s.
2233 /// The two are compared by calling `>` on their inner values.
2238 /// use std::sync::Arc;
2240 /// let five = Arc::new(5);
2242 /// assert!(five > Arc::new(4));
2244 fn gt(&self, other: &Arc<T>) -> bool {
2245 *(*self) > *(*other)
2248 /// 'Greater than or equal to' comparison for two `Arc`s.
2250 /// The two are compared by calling `>=` on their inner values.
2255 /// use std::sync::Arc;
2257 /// let five = Arc::new(5);
2259 /// assert!(five >= Arc::new(5));
2261 fn ge(&self, other: &Arc<T>) -> bool {
2262 *(*self) >= *(*other)
2265 #[stable(feature = "rust1", since = "1.0.0")]
2266 impl<T: ?Sized + Ord> Ord for Arc<T> {
2267 /// Comparison for two `Arc`s.
2269 /// The two are compared by calling `cmp()` on their inner values.
2274 /// use std::sync::Arc;
2275 /// use std::cmp::Ordering;
2277 /// let five = Arc::new(5);
2279 /// assert_eq!(Ordering::Less, five.cmp(&Arc::new(6)));
2281 fn cmp(&self, other: &Arc<T>) -> Ordering {
2282 (**self).cmp(&**other)
2285 #[stable(feature = "rust1", since = "1.0.0")]
2286 impl<T: ?Sized + Eq> Eq for Arc<T> {}
2288 #[stable(feature = "rust1", since = "1.0.0")]
2289 impl<T: ?Sized + fmt::Display> fmt::Display for Arc<T> {
2290 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
2291 fmt::Display::fmt(&**self, f)
2295 #[stable(feature = "rust1", since = "1.0.0")]
2296 impl<T: ?Sized + fmt::Debug> fmt::Debug for Arc<T> {
2297 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
2298 fmt::Debug::fmt(&**self, f)
2302 #[stable(feature = "rust1", since = "1.0.0")]
2303 impl<T: ?Sized> fmt::Pointer for Arc<T> {
2304 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
2305 fmt::Pointer::fmt(&(&**self as *const T), f)
2309 #[cfg(not(no_global_oom_handling))]
2310 #[stable(feature = "rust1", since = "1.0.0")]
2311 impl<T: Default> Default for Arc<T> {
2312 /// Creates a new `Arc<T>`, with the `Default` value for `T`.
2317 /// use std::sync::Arc;
2319 /// let x: Arc<i32> = Default::default();
2320 /// assert_eq!(*x, 0);
2322 fn default() -> Arc<T> {
2323 Arc::new(Default::default())
2327 #[stable(feature = "rust1", since = "1.0.0")]
2328 impl<T: ?Sized + Hash> Hash for Arc<T> {
2329 fn hash<H: Hasher>(&self, state: &mut H) {
2330 (**self).hash(state)
2334 #[cfg(not(no_global_oom_handling))]
2335 #[stable(feature = "from_for_ptrs", since = "1.6.0")]
2336 impl<T> From<T> for Arc<T> {
2337 /// Converts a `T` into an `Arc<T>`
2339 /// The conversion moves the value into a
2340 /// newly allocated `Arc`. It is equivalent to
2341 /// calling `Arc::new(t)`.
2345 /// # use std::sync::Arc;
2347 /// let arc = Arc::new(5);
2349 /// assert_eq!(Arc::from(x), arc);
2351 fn from(t: T) -> Self {
2356 #[cfg(not(no_global_oom_handling))]
2357 #[stable(feature = "shared_from_slice", since = "1.21.0")]
2358 impl<T: Clone> From<&[T]> for Arc<[T]> {
2359 /// Allocate a reference-counted slice and fill it by cloning `v`'s items.
2364 /// # use std::sync::Arc;
2365 /// let original: &[i32] = &[1, 2, 3];
2366 /// let shared: Arc<[i32]> = Arc::from(original);
2367 /// assert_eq!(&[1, 2, 3], &shared[..]);
2370 fn from(v: &[T]) -> Arc<[T]> {
2371 <Self as ArcFromSlice<T>>::from_slice(v)
2375 #[cfg(not(no_global_oom_handling))]
2376 #[stable(feature = "shared_from_slice", since = "1.21.0")]
2377 impl From<&str> for Arc<str> {
2378 /// Allocate a reference-counted `str` and copy `v` into it.
2383 /// # use std::sync::Arc;
2384 /// let shared: Arc<str> = Arc::from("eggplant");
2385 /// assert_eq!("eggplant", &shared[..]);
2388 fn from(v: &str) -> Arc<str> {
2389 let arc = Arc::<[u8]>::from(v.as_bytes());
2390 unsafe { Arc::from_raw(Arc::into_raw(arc) as *const str) }
2394 #[cfg(not(no_global_oom_handling))]
2395 #[stable(feature = "shared_from_slice", since = "1.21.0")]
2396 impl From<String> for Arc<str> {
2397 /// Allocate a reference-counted `str` and copy `v` into it.
2402 /// # use std::sync::Arc;
2403 /// let unique: String = "eggplant".to_owned();
2404 /// let shared: Arc<str> = Arc::from(unique);
2405 /// assert_eq!("eggplant", &shared[..]);
2408 fn from(v: String) -> Arc<str> {
2413 #[cfg(not(no_global_oom_handling))]
2414 #[stable(feature = "shared_from_slice", since = "1.21.0")]
2415 impl<T: ?Sized> From<Box<T>> for Arc<T> {
2416 /// Move a boxed object to a new, reference-counted allocation.
2421 /// # use std::sync::Arc;
2422 /// let unique: Box<str> = Box::from("eggplant");
2423 /// let shared: Arc<str> = Arc::from(unique);
2424 /// assert_eq!("eggplant", &shared[..]);
2427 fn from(v: Box<T>) -> Arc<T> {
2432 #[cfg(not(no_global_oom_handling))]
2433 #[stable(feature = "shared_from_slice", since = "1.21.0")]
2434 impl<T> From<Vec<T>> for Arc<[T]> {
2435 /// Allocate a reference-counted slice and move `v`'s items into it.
2440 /// # use std::sync::Arc;
2441 /// let unique: Vec<i32> = vec![1, 2, 3];
2442 /// let shared: Arc<[i32]> = Arc::from(unique);
2443 /// assert_eq!(&[1, 2, 3], &shared[..]);
2446 fn from(mut v: Vec<T>) -> Arc<[T]> {
2448 let arc = Arc::copy_from_slice(&v);
2450 // Allow the Vec to free its memory, but not destroy its contents
2458 #[stable(feature = "shared_from_cow", since = "1.45.0")]
2459 impl<'a, B> From<Cow<'a, B>> for Arc<B>
2461 B: ToOwned + ?Sized,
2462 Arc<B>: From<&'a B> + From<B::Owned>,
2464 /// Create an atomically reference-counted pointer from
2465 /// a clone-on-write pointer by copying its content.
2470 /// # use std::sync::Arc;
2471 /// # use std::borrow::Cow;
2472 /// let cow: Cow<str> = Cow::Borrowed("eggplant");
2473 /// let shared: Arc<str> = Arc::from(cow);
2474 /// assert_eq!("eggplant", &shared[..]);
2477 fn from(cow: Cow<'a, B>) -> Arc<B> {
2479 Cow::Borrowed(s) => Arc::from(s),
2480 Cow::Owned(s) => Arc::from(s),
2485 #[stable(feature = "boxed_slice_try_from", since = "1.43.0")]
2486 impl<T, const N: usize> TryFrom<Arc<[T]>> for Arc<[T; N]> {
2487 type Error = Arc<[T]>;
2489 fn try_from(boxed_slice: Arc<[T]>) -> Result<Self, Self::Error> {
2490 if boxed_slice.len() == N {
2491 Ok(unsafe { Arc::from_raw(Arc::into_raw(boxed_slice) as *mut [T; N]) })
2498 #[cfg(not(no_global_oom_handling))]
2499 #[stable(feature = "shared_from_iter", since = "1.37.0")]
2500 impl<T> iter::FromIterator<T> for Arc<[T]> {
2501 /// Takes each element in the `Iterator` and collects it into an `Arc<[T]>`.
2503 /// # Performance characteristics
2505 /// ## The general case
2507 /// In the general case, collecting into `Arc<[T]>` is done by first
2508 /// collecting into a `Vec<T>`. That is, when writing the following:
2511 /// # use std::sync::Arc;
2512 /// let evens: Arc<[u8]> = (0..10).filter(|&x| x % 2 == 0).collect();
2513 /// # assert_eq!(&*evens, &[0, 2, 4, 6, 8]);
2516 /// this behaves as if we wrote:
2519 /// # use std::sync::Arc;
2520 /// let evens: Arc<[u8]> = (0..10).filter(|&x| x % 2 == 0)
2521 /// .collect::<Vec<_>>() // The first set of allocations happens here.
2522 /// .into(); // A second allocation for `Arc<[T]>` happens here.
2523 /// # assert_eq!(&*evens, &[0, 2, 4, 6, 8]);
2526 /// This will allocate as many times as needed for constructing the `Vec<T>`
2527 /// and then it will allocate once for turning the `Vec<T>` into the `Arc<[T]>`.
2529 /// ## Iterators of known length
2531 /// When your `Iterator` implements `TrustedLen` and is of an exact size,
2532 /// a single allocation will be made for the `Arc<[T]>`. For example:
2535 /// # use std::sync::Arc;
2536 /// let evens: Arc<[u8]> = (0..10).collect(); // Just a single allocation happens here.
2537 /// # assert_eq!(&*evens, &*(0..10).collect::<Vec<_>>());
2539 fn from_iter<I: iter::IntoIterator<Item = T>>(iter: I) -> Self {
2540 ToArcSlice::to_arc_slice(iter.into_iter())
2544 /// Specialization trait used for collecting into `Arc<[T]>`.
2545 trait ToArcSlice<T>: Iterator<Item = T> + Sized {
2546 fn to_arc_slice(self) -> Arc<[T]>;
2549 #[cfg(not(no_global_oom_handling))]
2550 impl<T, I: Iterator<Item = T>> ToArcSlice<T> for I {
2551 default fn to_arc_slice(self) -> Arc<[T]> {
2552 self.collect::<Vec<T>>().into()
2556 #[cfg(not(no_global_oom_handling))]
2557 impl<T, I: iter::TrustedLen<Item = T>> ToArcSlice<T> for I {
2558 fn to_arc_slice(self) -> Arc<[T]> {
2559 // This is the case for a `TrustedLen` iterator.
2560 let (low, high) = self.size_hint();
2561 if let Some(high) = high {
2565 "TrustedLen iterator's size hint is not exact: {:?}",
2570 // SAFETY: We need to ensure that the iterator has an exact length and we have.
2571 Arc::from_iter_exact(self, low)
2574 // TrustedLen contract guarantees that `upper_bound == `None` implies an iterator
2575 // length exceeding `usize::MAX`.
2576 // The default implementation would collect into a vec which would panic.
2577 // Thus we panic here immediately without invoking `Vec` code.
2578 panic!("capacity overflow");
2583 #[stable(feature = "rust1", since = "1.0.0")]
2584 impl<T: ?Sized> borrow::Borrow<T> for Arc<T> {
2585 fn borrow(&self) -> &T {
2590 #[stable(since = "1.5.0", feature = "smart_ptr_as_ref")]
2591 impl<T: ?Sized> AsRef<T> for Arc<T> {
2592 fn as_ref(&self) -> &T {
2597 #[stable(feature = "pin", since = "1.33.0")]
2598 impl<T: ?Sized> Unpin for Arc<T> {}
2600 /// Get the offset within an `ArcInner` for the payload behind a pointer.
2604 /// The pointer must point to (and have valid metadata for) a previously
2605 /// valid instance of T, but the T is allowed to be dropped.
2606 unsafe fn data_offset<T: ?Sized>(ptr: *const T) -> isize {
2607 // Align the unsized value to the end of the ArcInner.
2608 // Because RcBox is repr(C), it will always be the last field in memory.
2609 // SAFETY: since the only unsized types possible are slices, trait objects,
2610 // and extern types, the input safety requirement is currently enough to
2611 // satisfy the requirements of align_of_val_raw; this is an implementation
2612 // detail of the language that must not be relied upon outside of std.
2613 unsafe { data_offset_align(align_of_val_raw(ptr)) }
2617 fn data_offset_align(align: usize) -> isize {
2618 let layout = Layout::new::<ArcInner<()>>();
2619 (layout.size() + layout.padding_needed_for(align)) as isize