1 // Copyright 2012-2014 The Rust Project Developers. See the COPYRIGHT
2 // file at the top-level directory of this distribution and at
3 // http://rust-lang.org/COPYRIGHT.
5 // Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
6 // http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
7 // <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
8 // option. This file may not be copied, modified, or distributed
9 // except according to those terms.
11 #![stable(feature = "rust1", since = "1.0.0")]
13 //! Threadsafe reference-counted boxes (the `Arc<T>` type).
15 //! The `Arc<T>` type provides shared ownership of an immutable value.
16 //! Destruction is deterministic, and will occur as soon as the last owner is
17 //! gone. It is marked as `Send` because it uses atomic reference counting.
19 //! If you do not need thread-safety, and just need shared ownership, consider
20 //! the [`Rc<T>` type](../rc/struct.Rc.html). It is the same as `Arc<T>`, but
21 //! does not use atomics, making it both thread-unsafe as well as significantly
22 //! faster when updating the reference count.
24 //! The `downgrade` method can be used to create a non-owning `Weak<T>` pointer
25 //! to the box. A `Weak<T>` pointer can be upgraded to an `Arc<T>` pointer, but
26 //! will return `None` if the value has already been dropped.
28 //! For example, a tree with parent pointers can be represented by putting the
29 //! nodes behind strong `Arc<T>` pointers, and then storing the parent pointers
30 //! as `Weak<T>` pointers.
34 //! Sharing some immutable data between threads:
37 //! use std::sync::Arc;
40 //! let five = Arc::new(5);
43 //! let five = five.clone();
45 //! thread::spawn(move || {
46 //! println!("{:?}", five);
51 //! Sharing mutable data safely between threads with a `Mutex`:
54 //! use std::sync::{Arc, Mutex};
57 //! let five = Arc::new(Mutex::new(5));
60 //! let five = five.clone();
62 //! thread::spawn(move || {
63 //! let mut number = five.lock().unwrap();
67 //! println!("{}", *number); // prints 6
74 use core::sync::atomic;
75 use core::sync::atomic::Ordering::{Relaxed, Release, Acquire, SeqCst};
78 use core::cmp::Ordering;
79 use core::mem::{align_of_val, size_of_val};
80 use core::intrinsics::abort;
82 use core::mem::uninitialized;
84 use core::ops::CoerceUnsized;
85 use core::ptr::{self, Shared};
86 use core::marker::Unsize;
87 use core::hash::{Hash, Hasher};
88 use core::{usize, isize};
89 use core::convert::From;
92 const MAX_REFCOUNT: usize = (isize::MAX) as usize;
94 /// An atomically reference counted wrapper for shared state.
98 /// In this example, a large vector is shared between several threads.
99 /// With simple pipes, without `Arc`, a copy would have to be made for each
102 /// When you clone an `Arc<T>`, it will create another pointer to the data and
103 /// increase the reference counter.
106 /// use std::sync::Arc;
110 /// let numbers: Vec<_> = (0..100u32).collect();
111 /// let shared_numbers = Arc::new(numbers);
114 /// let child_numbers = shared_numbers.clone();
116 /// thread::spawn(move || {
117 /// let local_numbers = &child_numbers[..];
119 /// // Work with the local numbers
124 #[unsafe_no_drop_flag]
125 #[stable(feature = "rust1", since = "1.0.0")]
126 pub struct Arc<T: ?Sized> {
127 // FIXME #12808: strange name to try to avoid interfering with
128 // field accesses of the contained type via Deref
129 _ptr: Shared<ArcInner<T>>,
132 #[stable(feature = "rust1", since = "1.0.0")]
133 unsafe impl<T: ?Sized + Sync + Send> Send for Arc<T> {}
134 #[stable(feature = "rust1", since = "1.0.0")]
135 unsafe impl<T: ?Sized + Sync + Send> Sync for Arc<T> {}
137 #[unstable(feature = "coerce_unsized", issue = "27732")]
138 impl<T: ?Sized + Unsize<U>, U: ?Sized> CoerceUnsized<Arc<U>> for Arc<T> {}
140 /// A weak pointer to an `Arc`.
142 /// Weak pointers will not keep the data inside of the `Arc` alive, and can be
143 /// used to break cycles between `Arc` pointers.
144 #[unsafe_no_drop_flag]
145 #[stable(feature = "arc_weak", since = "1.4.0")]
146 pub struct Weak<T: ?Sized> {
147 // FIXME #12808: strange name to try to avoid interfering with
148 // field accesses of the contained type via Deref
149 _ptr: Shared<ArcInner<T>>,
152 #[stable(feature = "rust1", since = "1.0.0")]
153 unsafe impl<T: ?Sized + Sync + Send> Send for Weak<T> {}
154 #[stable(feature = "rust1", since = "1.0.0")]
155 unsafe impl<T: ?Sized + Sync + Send> Sync for Weak<T> {}
157 #[unstable(feature = "coerce_unsized", issue = "27732")]
158 impl<T: ?Sized + Unsize<U>, U: ?Sized> CoerceUnsized<Weak<U>> for Weak<T> {}
160 #[stable(feature = "rust1", since = "1.0.0")]
161 impl<T: ?Sized + fmt::Debug> fmt::Debug for Weak<T> {
162 fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
167 struct ArcInner<T: ?Sized> {
168 strong: atomic::AtomicUsize,
170 // the value usize::MAX acts as a sentinel for temporarily "locking" the
171 // ability to upgrade weak pointers or downgrade strong ones; this is used
172 // to avoid races in `make_mut` and `get_mut`.
173 weak: atomic::AtomicUsize,
178 unsafe impl<T: ?Sized + Sync + Send> Send for ArcInner<T> {}
179 unsafe impl<T: ?Sized + Sync + Send> Sync for ArcInner<T> {}
182 /// Constructs a new `Arc<T>`.
187 /// use std::sync::Arc;
189 /// let five = Arc::new(5);
192 #[stable(feature = "rust1", since = "1.0.0")]
193 pub fn new(data: T) -> Arc<T> {
194 // Start the weak pointer count as 1 which is the weak pointer that's
195 // held by all the strong pointers (kinda), see std/rc.rs for more info
196 let x: Box<_> = box ArcInner {
197 strong: atomic::AtomicUsize::new(1),
198 weak: atomic::AtomicUsize::new(1),
201 Arc { _ptr: unsafe { Shared::new(Box::into_raw(x)) } }
204 /// Unwraps the contained value if the `Arc<T>` has only one strong reference.
205 /// This will succeed even if there are outstanding weak references.
207 /// Otherwise, an `Err` is returned with the same `Arc<T>`.
212 /// use std::sync::Arc;
214 /// let x = Arc::new(3);
215 /// assert_eq!(Arc::try_unwrap(x), Ok(3));
217 /// let x = Arc::new(4);
218 /// let _y = x.clone();
219 /// assert_eq!(Arc::try_unwrap(x), Err(Arc::new(4)));
222 #[stable(feature = "arc_unique", since = "1.4.0")]
223 pub fn try_unwrap(this: Self) -> Result<T, Self> {
224 // See `drop` for why all these atomics are like this
225 if this.inner().strong.compare_and_swap(1, 0, Release) != 1 {
229 atomic::fence(Acquire);
232 let ptr = *this._ptr;
233 let elem = ptr::read(&(*ptr).data);
235 // Make a weak pointer to clean up the implicit strong-weak reference
236 let _weak = Weak { _ptr: this._ptr };
244 impl<T: ?Sized> Arc<T> {
245 /// Downgrades the `Arc<T>` to a `Weak<T>` reference.
250 /// use std::sync::Arc;
252 /// let five = Arc::new(5);
254 /// let weak_five = Arc::downgrade(&five);
256 #[stable(feature = "arc_weak", since = "1.4.0")]
257 pub fn downgrade(this: &Self) -> Weak<T> {
259 // This Relaxed is OK because we're checking the value in the CAS
261 let cur = this.inner().weak.load(Relaxed);
263 // check if the weak counter is currently "locked"; if so, spin.
264 if cur == usize::MAX {
268 // NOTE: this code currently ignores the possibility of overflow
269 // into usize::MAX; in general both Rc and Arc need to be adjusted
270 // to deal with overflow.
272 // Unlike with Clone(), we need this to be an Acquire read to
273 // synchronize with the write coming from `is_unique`, so that the
274 // events prior to that write happen before this read.
275 if this.inner().weak.compare_and_swap(cur, cur + 1, Acquire) == cur {
276 return Weak { _ptr: this._ptr };
281 /// Get the number of weak references to this value.
283 #[unstable(feature = "arc_counts", reason = "not clearly useful, and racy",
285 pub fn weak_count(this: &Self) -> usize {
286 this.inner().weak.load(SeqCst) - 1
289 /// Get the number of strong references to this value.
291 #[unstable(feature = "arc_counts", reason = "not clearly useful, and racy",
293 pub fn strong_count(this: &Self) -> usize {
294 this.inner().strong.load(SeqCst)
298 fn inner(&self) -> &ArcInner<T> {
299 // This unsafety is ok because while this arc is alive we're guaranteed
300 // that the inner pointer is valid. Furthermore, we know that the
301 // `ArcInner` structure itself is `Sync` because the inner data is
302 // `Sync` as well, so we're ok loaning out an immutable pointer to these
304 unsafe { &**self._ptr }
307 // Non-inlined part of `drop`.
309 unsafe fn drop_slow(&mut self) {
310 let ptr = *self._ptr;
312 // Destroy the data at this time, even though we may not free the box
313 // allocation itself (there may still be weak pointers lying around).
314 ptr::drop_in_place(&mut (*ptr).data);
316 if self.inner().weak.fetch_sub(1, Release) == 1 {
317 atomic::fence(Acquire);
318 deallocate(ptr as *mut u8, size_of_val(&*ptr), align_of_val(&*ptr))
323 #[stable(feature = "rust1", since = "1.0.0")]
324 impl<T: ?Sized> Clone for Arc<T> {
325 /// Makes a clone of the `Arc<T>`.
327 /// This increases the strong reference count.
332 /// use std::sync::Arc;
334 /// let five = Arc::new(5);
339 fn clone(&self) -> Arc<T> {
340 // Using a relaxed ordering is alright here, as knowledge of the
341 // original reference prevents other threads from erroneously deleting
344 // As explained in the [Boost documentation][1], Increasing the
345 // reference counter can always be done with memory_order_relaxed: New
346 // references to an object can only be formed from an existing
347 // reference, and passing an existing reference from one thread to
348 // another must already provide any required synchronization.
350 // [1]: (www.boost.org/doc/libs/1_55_0/doc/html/atomic/usage_examples.html)
351 let old_size = self.inner().strong.fetch_add(1, Relaxed);
353 // However we need to guard against massive refcounts in case someone
354 // is `mem::forget`ing Arcs. If we don't do this the count can overflow
355 // and users will use-after free. We racily saturate to `isize::MAX` on
356 // the assumption that there aren't ~2 billion threads incrementing
357 // the reference count at once. This branch will never be taken in
358 // any realistic program.
360 // We abort because such a program is incredibly degenerate, and we
361 // don't care to support it.
362 if old_size > MAX_REFCOUNT {
368 Arc { _ptr: self._ptr }
372 #[stable(feature = "rust1", since = "1.0.0")]
373 impl<T: ?Sized> Deref for Arc<T> {
377 fn deref(&self) -> &T {
382 impl<T: Clone> Arc<T> {
383 /// Make a mutable reference into the given `Arc<T>` by cloning the inner
384 /// data if the `Arc<T>` doesn't have one strong reference and no weak
387 /// This is also referred to as a copy-on-write.
392 /// use std::sync::Arc;
394 /// let mut data = Arc::new(5);
396 /// *Arc::make_mut(&mut data) += 1; // Won't clone anything
397 /// let mut other_data = data.clone(); // Won't clone inner data
398 /// *Arc::make_mut(&mut data) += 1; // Clones inner data
399 /// *Arc::make_mut(&mut data) += 1; // Won't clone anything
400 /// *Arc::make_mut(&mut other_data) *= 2; // Won't clone anything
402 /// // Note: data and other_data now point to different numbers
403 /// assert_eq!(*data, 8);
404 /// assert_eq!(*other_data, 12);
408 #[stable(feature = "arc_unique", since = "1.4.0")]
409 pub fn make_mut(this: &mut Self) -> &mut T {
410 // Note that we hold both a strong reference and a weak reference.
411 // Thus, releasing our strong reference only will not, by itself, cause
412 // the memory to be deallocated.
414 // Use Acquire to ensure that we see any writes to `weak` that happen
415 // before release writes (i.e., decrements) to `strong`. Since we hold a
416 // weak count, there's no chance the ArcInner itself could be
418 if this.inner().strong.compare_and_swap(1, 0, Acquire) != 1 {
419 // Another strong pointer exists; clone
420 *this = Arc::new((**this).clone());
421 } else if this.inner().weak.load(Relaxed) != 1 {
422 // Relaxed suffices in the above because this is fundamentally an
423 // optimization: we are always racing with weak pointers being
424 // dropped. Worst case, we end up allocated a new Arc unnecessarily.
426 // We removed the last strong ref, but there are additional weak
427 // refs remaining. We'll move the contents to a new Arc, and
428 // invalidate the other weak refs.
430 // Note that it is not possible for the read of `weak` to yield
431 // usize::MAX (i.e., locked), since the weak count can only be
432 // locked by a thread with a strong reference.
434 // Materialize our own implicit weak pointer, so that it can clean
435 // up the ArcInner as needed.
436 let weak = Weak { _ptr: this._ptr };
438 // mark the data itself as already deallocated
440 // there is no data race in the implicit write caused by `read`
441 // here (due to zeroing) because data is no longer accessed by
442 // other threads (due to there being no more strong refs at this
444 let mut swap = Arc::new(ptr::read(&(**weak._ptr).data));
445 mem::swap(this, &mut swap);
449 // We were the sole reference of either kind; bump back up the
451 this.inner().strong.store(1, Release);
454 // As with `get_mut()`, the unsafety is ok because our reference was
455 // either unique to begin with, or became one upon cloning the contents.
457 let inner = &mut **this._ptr;
463 impl<T: ?Sized> Arc<T> {
464 /// Returns a mutable reference to the contained value if the `Arc<T>` has
465 /// one strong reference and no weak references.
470 /// use std::sync::Arc;
472 /// let mut x = Arc::new(3);
473 /// *Arc::get_mut(&mut x).unwrap() = 4;
474 /// assert_eq!(*x, 4);
476 /// let _y = x.clone();
477 /// assert!(Arc::get_mut(&mut x).is_none());
480 #[stable(feature = "arc_unique", since = "1.4.0")]
481 pub fn get_mut(this: &mut Self) -> Option<&mut T> {
482 if this.is_unique() {
483 // This unsafety is ok because we're guaranteed that the pointer
484 // returned is the *only* pointer that will ever be returned to T. Our
485 // reference count is guaranteed to be 1 at this point, and we required
486 // the Arc itself to be `mut`, so we're returning the only possible
487 // reference to the inner data.
489 let inner = &mut **this._ptr;
490 Some(&mut inner.data)
497 /// Determine whether this is the unique reference (including weak refs) to
498 /// the underlying data.
500 /// Note that this requires locking the weak ref count.
501 fn is_unique(&mut self) -> bool {
502 // lock the weak pointer count if we appear to be the sole weak pointer
505 // The acquire label here ensures a happens-before relationship with any
506 // writes to `strong` prior to decrements of the `weak` count (via drop,
507 // which uses Release).
508 if self.inner().weak.compare_and_swap(1, usize::MAX, Acquire) == 1 {
509 // Due to the previous acquire read, this will observe any writes to
510 // `strong` that were due to upgrading weak pointers; only strong
511 // clones remain, which require that the strong count is > 1 anyway.
512 let unique = self.inner().strong.load(Relaxed) == 1;
514 // The release write here synchronizes with a read in `downgrade`,
515 // effectively preventing the above read of `strong` from happening
517 self.inner().weak.store(1, Release); // release the lock
525 #[stable(feature = "rust1", since = "1.0.0")]
526 impl<T: ?Sized> Drop for Arc<T> {
527 /// Drops the `Arc<T>`.
529 /// This will decrement the strong reference count. If the strong reference
530 /// count becomes zero and the only other references are `Weak<T>` ones,
531 /// `drop`s the inner value.
536 /// use std::sync::Arc;
539 /// let five = Arc::new(5);
543 /// drop(five); // explicit drop
546 /// let five = Arc::new(5);
550 /// } // implicit drop
552 #[unsafe_destructor_blind_to_params]
555 // This structure has #[unsafe_no_drop_flag], so this drop glue may run
556 // more than once (but it is guaranteed to be zeroed after the first if
557 // it's run more than once)
558 let ptr = *self._ptr;
559 // if ptr.is_null() { return }
560 if ptr as *mut u8 as usize == 0 || ptr as *mut u8 as usize == mem::POST_DROP_USIZE {
564 // Because `fetch_sub` is already atomic, we do not need to synchronize
565 // with other threads unless we are going to delete the object. This
566 // same logic applies to the below `fetch_sub` to the `weak` count.
567 if self.inner().strong.fetch_sub(1, Release) != 1 {
571 // This fence is needed to prevent reordering of use of the data and
572 // deletion of the data. Because it is marked `Release`, the decreasing
573 // of the reference count synchronizes with this `Acquire` fence. This
574 // means that use of the data happens before decreasing the reference
575 // count, which happens before this fence, which happens before the
576 // deletion of the data.
578 // As explained in the [Boost documentation][1],
580 // > It is important to enforce any possible access to the object in one
581 // > thread (through an existing reference) to *happen before* deleting
582 // > the object in a different thread. This is achieved by a "release"
583 // > operation after dropping a reference (any access to the object
584 // > through this reference must obviously happened before), and an
585 // > "acquire" operation before deleting the object.
587 // [1]: (www.boost.org/doc/libs/1_55_0/doc/html/atomic/usage_examples.html)
588 atomic::fence(Acquire);
596 impl<T: ?Sized> Weak<T> {
597 /// Upgrades a weak reference to a strong reference.
599 /// Upgrades the `Weak<T>` reference to an `Arc<T>`, if possible.
601 /// Returns `None` if there were no strong references and the data was
607 /// use std::sync::Arc;
609 /// let five = Arc::new(5);
611 /// let weak_five = Arc::downgrade(&five);
613 /// let strong_five: Option<Arc<_>> = weak_five.upgrade();
615 #[stable(feature = "arc_weak", since = "1.4.0")]
616 pub fn upgrade(&self) -> Option<Arc<T>> {
617 // We use a CAS loop to increment the strong count instead of a
618 // fetch_add because once the count hits 0 it must never be above 0.
619 let inner = self.inner();
621 // Relaxed load because any write of 0 that we can observe
622 // leaves the field in a permanently zero state (so a
623 // "stale" read of 0 is fine), and any other value is
624 // confirmed via the CAS below.
625 let n = inner.strong.load(Relaxed);
630 // See comments in `Arc::clone` for why we do this (for `mem::forget`).
631 if n > MAX_REFCOUNT {
635 // Relaxed is valid for the same reason it is on Arc's Clone impl
636 let old = inner.strong.compare_and_swap(n, n + 1, Relaxed);
638 return Some(Arc { _ptr: self._ptr });
644 fn inner(&self) -> &ArcInner<T> {
645 // See comments above for why this is "safe"
646 unsafe { &**self._ptr }
650 #[stable(feature = "arc_weak", since = "1.4.0")]
651 impl<T: ?Sized> Clone for Weak<T> {
652 /// Makes a clone of the `Weak<T>`.
654 /// This increases the weak reference count.
659 /// use std::sync::Arc;
661 /// let weak_five = Arc::downgrade(&Arc::new(5));
663 /// weak_five.clone();
666 fn clone(&self) -> Weak<T> {
667 // See comments in Arc::clone() for why this is relaxed. This can use a
668 // fetch_add (ignoring the lock) because the weak count is only locked
669 // where are *no other* weak pointers in existence. (So we can't be
670 // running this code in that case).
671 let old_size = self.inner().weak.fetch_add(1, Relaxed);
673 // See comments in Arc::clone() for why we do this (for mem::forget).
674 if old_size > MAX_REFCOUNT {
680 return Weak { _ptr: self._ptr };
684 #[stable(feature = "rust1", since = "1.0.0")]
685 impl<T: ?Sized> Drop for Weak<T> {
686 /// Drops the `Weak<T>`.
688 /// This will decrement the weak reference count.
693 /// use std::sync::Arc;
696 /// let five = Arc::new(5);
697 /// let weak_five = Arc::downgrade(&five);
701 /// drop(weak_five); // explicit drop
704 /// let five = Arc::new(5);
705 /// let weak_five = Arc::downgrade(&five);
709 /// } // implicit drop
712 let ptr = *self._ptr;
714 // see comments above for why this check is here
715 if ptr as *mut u8 as usize == 0 || ptr as *mut u8 as usize == mem::POST_DROP_USIZE {
719 // If we find out that we were the last weak pointer, then its time to
720 // deallocate the data entirely. See the discussion in Arc::drop() about
721 // the memory orderings
723 // It's not necessary to check for the locked state here, because the
724 // weak count can only be locked if there was precisely one weak ref,
725 // meaning that drop could only subsequently run ON that remaining weak
726 // ref, which can only happen after the lock is released.
727 if self.inner().weak.fetch_sub(1, Release) == 1 {
728 atomic::fence(Acquire);
729 unsafe { deallocate(ptr as *mut u8, size_of_val(&*ptr), align_of_val(&*ptr)) }
734 #[stable(feature = "rust1", since = "1.0.0")]
735 impl<T: ?Sized + PartialEq> PartialEq for Arc<T> {
736 /// Equality for two `Arc<T>`s.
738 /// Two `Arc<T>`s are equal if their inner value are equal.
743 /// use std::sync::Arc;
745 /// let five = Arc::new(5);
747 /// five == Arc::new(5);
749 fn eq(&self, other: &Arc<T>) -> bool {
750 *(*self) == *(*other)
753 /// Inequality for two `Arc<T>`s.
755 /// Two `Arc<T>`s are unequal if their inner value are unequal.
760 /// use std::sync::Arc;
762 /// let five = Arc::new(5);
764 /// five != Arc::new(5);
766 fn ne(&self, other: &Arc<T>) -> bool {
767 *(*self) != *(*other)
770 #[stable(feature = "rust1", since = "1.0.0")]
771 impl<T: ?Sized + PartialOrd> PartialOrd for Arc<T> {
772 /// Partial comparison for two `Arc<T>`s.
774 /// The two are compared by calling `partial_cmp()` on their inner values.
779 /// use std::sync::Arc;
781 /// let five = Arc::new(5);
783 /// five.partial_cmp(&Arc::new(5));
785 fn partial_cmp(&self, other: &Arc<T>) -> Option<Ordering> {
786 (**self).partial_cmp(&**other)
789 /// Less-than comparison for two `Arc<T>`s.
791 /// The two are compared by calling `<` on their inner values.
796 /// use std::sync::Arc;
798 /// let five = Arc::new(5);
800 /// five < Arc::new(5);
802 fn lt(&self, other: &Arc<T>) -> bool {
806 /// 'Less-than or equal to' comparison for two `Arc<T>`s.
808 /// The two are compared by calling `<=` on their inner values.
813 /// use std::sync::Arc;
815 /// let five = Arc::new(5);
817 /// five <= Arc::new(5);
819 fn le(&self, other: &Arc<T>) -> bool {
820 *(*self) <= *(*other)
823 /// Greater-than comparison for two `Arc<T>`s.
825 /// The two are compared by calling `>` on their inner values.
830 /// use std::sync::Arc;
832 /// let five = Arc::new(5);
834 /// five > Arc::new(5);
836 fn gt(&self, other: &Arc<T>) -> bool {
840 /// 'Greater-than or equal to' comparison for two `Arc<T>`s.
842 /// The two are compared by calling `>=` on their inner values.
847 /// use std::sync::Arc;
849 /// let five = Arc::new(5);
851 /// five >= Arc::new(5);
853 fn ge(&self, other: &Arc<T>) -> bool {
854 *(*self) >= *(*other)
857 #[stable(feature = "rust1", since = "1.0.0")]
858 impl<T: ?Sized + Ord> Ord for Arc<T> {
859 fn cmp(&self, other: &Arc<T>) -> Ordering {
860 (**self).cmp(&**other)
863 #[stable(feature = "rust1", since = "1.0.0")]
864 impl<T: ?Sized + Eq> Eq for Arc<T> {}
866 #[stable(feature = "rust1", since = "1.0.0")]
867 impl<T: ?Sized + fmt::Display> fmt::Display for Arc<T> {
868 fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
869 fmt::Display::fmt(&**self, f)
873 #[stable(feature = "rust1", since = "1.0.0")]
874 impl<T: ?Sized + fmt::Debug> fmt::Debug for Arc<T> {
875 fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
876 fmt::Debug::fmt(&**self, f)
880 #[stable(feature = "rust1", since = "1.0.0")]
881 impl<T> fmt::Pointer for Arc<T> {
882 fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
883 fmt::Pointer::fmt(&*self._ptr, f)
887 #[stable(feature = "rust1", since = "1.0.0")]
888 impl<T: Default> Default for Arc<T> {
889 fn default() -> Arc<T> {
890 Arc::new(Default::default())
894 #[stable(feature = "rust1", since = "1.0.0")]
895 impl<T: ?Sized + Hash> Hash for Arc<T> {
896 fn hash<H: Hasher>(&self, state: &mut H) {
901 #[stable(feature = "from_for_ptrs", since = "1.6.0")]
902 impl<T> From<T> for Arc<T> {
903 fn from(t: T) -> Self {
909 /// Constructs a new `Weak<T>` without an accompanying instance of T.
911 /// This allocates memory for T, but does not initialize it. Calling
912 /// Weak<T>::upgrade() on the return value always gives None.
917 /// #![feature(downgraded_weak)]
919 /// use std::sync::Weak;
921 /// let empty: Weak<i64> = Weak::new();
923 #[unstable(feature = "downgraded_weak",
924 reason = "recently added",
926 pub fn new() -> Weak<T> {
928 Weak { _ptr: Shared::new(Box::into_raw(box ArcInner {
929 strong: atomic::AtomicUsize::new(0),
930 weak: atomic::AtomicUsize::new(1),
931 data: uninitialized(),
939 use std::clone::Clone;
940 use std::sync::mpsc::channel;
943 use std::option::Option;
944 use std::option::Option::{Some, None};
945 use std::sync::atomic;
946 use std::sync::atomic::Ordering::{Acquire, SeqCst};
949 use super::{Arc, Weak};
950 use std::sync::Mutex;
951 use std::convert::From;
953 struct Canary(*mut atomic::AtomicUsize);
955 impl Drop for Canary {
960 (*c).fetch_add(1, SeqCst);
968 fn manually_share_arc() {
969 let v = vec![1, 2, 3, 4, 5, 6, 7, 8, 9, 10];
970 let arc_v = Arc::new(v);
972 let (tx, rx) = channel();
974 let _t = thread::spawn(move || {
975 let arc_v: Arc<Vec<i32>> = rx.recv().unwrap();
976 assert_eq!((*arc_v)[3], 4);
979 tx.send(arc_v.clone()).unwrap();
981 assert_eq!((*arc_v)[2], 3);
982 assert_eq!((*arc_v)[4], 5);
986 fn test_arc_get_mut() {
987 let mut x = Arc::new(3);
988 *Arc::get_mut(&mut x).unwrap() = 4;
991 assert!(Arc::get_mut(&mut x).is_none());
993 assert!(Arc::get_mut(&mut x).is_some());
994 let _w = Arc::downgrade(&x);
995 assert!(Arc::get_mut(&mut x).is_none());
1000 let x = Arc::new(3);
1001 assert_eq!(Arc::try_unwrap(x), Ok(3));
1002 let x = Arc::new(4);
1004 assert_eq!(Arc::try_unwrap(x), Err(Arc::new(4)));
1005 let x = Arc::new(5);
1006 let _w = Arc::downgrade(&x);
1007 assert_eq!(Arc::try_unwrap(x), Ok(5));
1011 fn test_cowarc_clone_make_mut() {
1012 let mut cow0 = Arc::new(75);
1013 let mut cow1 = cow0.clone();
1014 let mut cow2 = cow1.clone();
1016 assert!(75 == *Arc::make_mut(&mut cow0));
1017 assert!(75 == *Arc::make_mut(&mut cow1));
1018 assert!(75 == *Arc::make_mut(&mut cow2));
1020 *Arc::make_mut(&mut cow0) += 1;
1021 *Arc::make_mut(&mut cow1) += 2;
1022 *Arc::make_mut(&mut cow2) += 3;
1024 assert!(76 == *cow0);
1025 assert!(77 == *cow1);
1026 assert!(78 == *cow2);
1028 // none should point to the same backing memory
1029 assert!(*cow0 != *cow1);
1030 assert!(*cow0 != *cow2);
1031 assert!(*cow1 != *cow2);
1035 fn test_cowarc_clone_unique2() {
1036 let mut cow0 = Arc::new(75);
1037 let cow1 = cow0.clone();
1038 let cow2 = cow1.clone();
1040 assert!(75 == *cow0);
1041 assert!(75 == *cow1);
1042 assert!(75 == *cow2);
1044 *Arc::make_mut(&mut cow0) += 1;
1045 assert!(76 == *cow0);
1046 assert!(75 == *cow1);
1047 assert!(75 == *cow2);
1049 // cow1 and cow2 should share the same contents
1050 // cow0 should have a unique reference
1051 assert!(*cow0 != *cow1);
1052 assert!(*cow0 != *cow2);
1053 assert!(*cow1 == *cow2);
1057 fn test_cowarc_clone_weak() {
1058 let mut cow0 = Arc::new(75);
1059 let cow1_weak = Arc::downgrade(&cow0);
1061 assert!(75 == *cow0);
1062 assert!(75 == *cow1_weak.upgrade().unwrap());
1064 *Arc::make_mut(&mut cow0) += 1;
1066 assert!(76 == *cow0);
1067 assert!(cow1_weak.upgrade().is_none());
1072 let x = Arc::new(5);
1073 let y = Arc::downgrade(&x);
1074 assert!(y.upgrade().is_some());
1079 let x = Arc::new(5);
1080 let y = Arc::downgrade(&x);
1082 assert!(y.upgrade().is_none());
1086 fn weak_self_cyclic() {
1088 x: Mutex<Option<Weak<Cycle>>>,
1091 let a = Arc::new(Cycle { x: Mutex::new(None) });
1092 let b = Arc::downgrade(&a.clone());
1093 *a.x.lock().unwrap() = Some(b);
1095 // hopefully we don't double-free (or leak)...
1100 let mut canary = atomic::AtomicUsize::new(0);
1101 let x = Arc::new(Canary(&mut canary as *mut atomic::AtomicUsize));
1103 assert!(canary.load(Acquire) == 1);
1107 fn drop_arc_weak() {
1108 let mut canary = atomic::AtomicUsize::new(0);
1109 let arc = Arc::new(Canary(&mut canary as *mut atomic::AtomicUsize));
1110 let arc_weak = Arc::downgrade(&arc);
1111 assert!(canary.load(Acquire) == 0);
1113 assert!(canary.load(Acquire) == 1);
1118 fn test_strong_count() {
1119 let a = Arc::new(0u32);
1120 assert!(Arc::strong_count(&a) == 1);
1121 let w = Arc::downgrade(&a);
1122 assert!(Arc::strong_count(&a) == 1);
1123 let b = w.upgrade().expect("");
1124 assert!(Arc::strong_count(&b) == 2);
1125 assert!(Arc::strong_count(&a) == 2);
1128 assert!(Arc::strong_count(&b) == 1);
1130 assert!(Arc::strong_count(&b) == 2);
1131 assert!(Arc::strong_count(&c) == 2);
1135 fn test_weak_count() {
1136 let a = Arc::new(0u32);
1137 assert!(Arc::strong_count(&a) == 1);
1138 assert!(Arc::weak_count(&a) == 0);
1139 let w = Arc::downgrade(&a);
1140 assert!(Arc::strong_count(&a) == 1);
1141 assert!(Arc::weak_count(&a) == 1);
1143 assert!(Arc::weak_count(&a) == 2);
1146 assert!(Arc::strong_count(&a) == 1);
1147 assert!(Arc::weak_count(&a) == 0);
1149 assert!(Arc::strong_count(&a) == 2);
1150 assert!(Arc::weak_count(&a) == 0);
1151 let d = Arc::downgrade(&c);
1152 assert!(Arc::weak_count(&c) == 1);
1153 assert!(Arc::strong_count(&c) == 2);
1162 let a = Arc::new(5u32);
1163 assert_eq!(format!("{:?}", a), "5");
1166 // Make sure deriving works with Arc<T>
1167 #[derive(Eq, Ord, PartialEq, PartialOrd, Clone, Debug, Default)]
1174 let x: Arc<[i32]> = Arc::new([1, 2, 3]);
1175 assert_eq!(format!("{:?}", x), "[1, 2, 3]");
1176 let y = Arc::downgrade(&x.clone());
1178 assert!(y.upgrade().is_none());
1182 fn test_from_owned() {
1184 let foo_arc = Arc::from(foo);
1185 assert!(123 == *foo_arc);
1189 fn test_new_weak() {
1190 let foo: Weak<usize> = Weak::new();
1191 assert!(foo.upgrade().is_none());
1195 #[stable(feature = "rust1", since = "1.0.0")]
1196 impl<T: ?Sized> borrow::Borrow<T> for Arc<T> {
1197 fn borrow(&self) -> &T {
1202 #[stable(since = "1.5.0", feature = "smart_ptr_as_ref")]
1203 impl<T: ?Sized> AsRef<T> for Arc<T> {
1204 fn as_ref(&self) -> &T {