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 //! Thread-safe reference-counting pointers.
15 //! See the [`Arc<T>`][arc] documentation for more details.
17 //! [arc]: struct.Arc.html
19 use core::sync::atomic;
20 use core::sync::atomic::Ordering::{Acquire, Relaxed, Release, SeqCst};
23 use core::cmp::Ordering;
24 use core::intrinsics::abort;
25 use core::mem::{self, align_of_val, size_of_val, uninitialized};
27 use core::ops::CoerceUnsized;
28 use core::ptr::{self, Shared};
29 use core::marker::Unsize;
30 use core::hash::{Hash, Hasher};
31 use core::{isize, usize};
32 use core::convert::From;
34 use heap::{Heap, Alloc, Layout, box_free};
39 /// A soft limit on the amount of references that may be made to an `Arc`.
41 /// Going above this limit will abort your program (although not
42 /// necessarily) at _exactly_ `MAX_REFCOUNT + 1` references.
43 const MAX_REFCOUNT: usize = (isize::MAX) as usize;
45 /// A thread-safe reference-counting pointer. 'Arc' stands for 'Atomically
46 /// Reference Counted'.
48 /// The type `Arc<T>` provides shared ownership of a value of type `T`,
49 /// allocated in the heap. Invoking [`clone`][clone] on `Arc` produces
50 /// a new pointer to the same value in the heap. When the last `Arc`
51 /// pointer to a given value is destroyed, the pointed-to value is
54 /// Shared references in Rust disallow mutation by default, and `Arc` is no
55 /// exception. If you need to mutate through an `Arc`, use [`Mutex`][mutex],
56 /// [`RwLock`][rwlock], or one of the [`Atomic`][atomic] types.
60 /// Unlike [`Rc<T>`], `Arc<T>` uses atomic operations for its reference
61 /// counting This means that it is thread-safe. The disadvantage is that
62 /// atomic operations are more expensive than ordinary memory accesses. If you
63 /// are not sharing reference-counted values between threads, consider using
64 /// [`Rc<T>`] for lower overhead. [`Rc<T>`] is a safe default, because the
65 /// compiler will catch any attempt to send an [`Rc<T>`] between threads.
66 /// However, a library might choose `Arc<T>` in order to give library consumers
69 /// `Arc<T>` will implement [`Send`] and [`Sync`] as long as the `T` implements
70 /// [`Send`] and [`Sync`]. Why can't you put a non-thread-safe type `T` in an
71 /// `Arc<T>` to make it thread-safe? This may be a bit counter-intuitive at
72 /// first: after all, isn't the point of `Arc<T>` thread safety? The key is
73 /// this: `Arc<T>` makes it thread safe to have multiple ownership of the same
74 /// data, but it doesn't add thread safety to its data. Consider
75 /// `Arc<`[`RefCell<T>`]`>`. [`RefCell<T>`] isn't [`Sync`], and if `Arc<T>` was always
76 /// [`Send`], `Arc<`[`RefCell<T>`]`>` would be as well. But then we'd have a problem:
77 /// [`RefCell<T>`] is not thread safe; it keeps track of the borrowing count using
78 /// non-atomic operations.
80 /// In the end, this means that you may need to pair `Arc<T>` with some sort of
81 /// [`std::sync`] type, usually [`Mutex<T>`][mutex].
83 /// ## Breaking cycles with `Weak`
85 /// The [`downgrade`][downgrade] method can be used to create a non-owning
86 /// [`Weak`][weak] pointer. A [`Weak`][weak] pointer can be [`upgrade`][upgrade]d
87 /// to an `Arc`, but this will return [`None`] if the value has already been
90 /// A cycle between `Arc` pointers will never be deallocated. For this reason,
91 /// [`Weak`][weak] is used to break cycles. For example, a tree could have
92 /// strong `Arc` pointers from parent nodes to children, and [`Weak`][weak]
93 /// pointers from children back to their parents.
95 /// # Cloning references
97 /// Creating a new reference from an existing reference counted pointer is done using the
98 /// `Clone` trait implemented for [`Arc<T>`][arc] and [`Weak<T>`][weak].
101 /// use std::sync::Arc;
102 /// let foo = Arc::new(vec![1.0, 2.0, 3.0]);
103 /// // The two syntaxes below are equivalent.
104 /// let a = foo.clone();
105 /// let b = Arc::clone(&foo);
106 /// // a and b both point to the same memory location as foo.
109 /// The [`Arc::clone(&from)`] syntax is the most idiomatic because it conveys more explicitly
110 /// the meaning of the code. In the example above, this syntax makes it easier to see that
111 /// this code is creating a new reference rather than copying the whole content of foo.
113 /// ## `Deref` behavior
115 /// `Arc<T>` automatically dereferences to `T` (via the [`Deref`][deref] trait),
116 /// so you can call `T`'s methods on a value of type `Arc<T>`. To avoid name
117 /// clashes with `T`'s methods, the methods of `Arc<T>` itself are [associated
118 /// functions][assoc], called using function-like syntax:
121 /// use std::sync::Arc;
122 /// let my_arc = Arc::new(());
124 /// Arc::downgrade(&my_arc);
127 /// [`Weak<T>`][weak] does not auto-dereference to `T`, because the value may have
128 /// already been destroyed.
130 /// [arc]: struct.Arc.html
131 /// [weak]: struct.Weak.html
132 /// [`Rc<T>`]: ../../std/rc/struct.Rc.html
133 /// [clone]: ../../std/clone/trait.Clone.html#tymethod.clone
134 /// [mutex]: ../../std/sync/struct.Mutex.html
135 /// [rwlock]: ../../std/sync/struct.RwLock.html
136 /// [atomic]: ../../std/sync/atomic/index.html
137 /// [`Send`]: ../../std/marker/trait.Send.html
138 /// [`Sync`]: ../../std/marker/trait.Sync.html
139 /// [deref]: ../../std/ops/trait.Deref.html
140 /// [downgrade]: struct.Arc.html#method.downgrade
141 /// [upgrade]: struct.Weak.html#method.upgrade
142 /// [`None`]: ../../std/option/enum.Option.html#variant.None
143 /// [assoc]: ../../book/first-edition/method-syntax.html#associated-functions
144 /// [`RefCell<T>`]: ../../std/cell/struct.RefCell.html
145 /// [`std::sync`]: ../../std/sync/index.html
146 /// [`Arc::clone(&from)`]: #method.clone
150 /// Sharing some immutable data between threads:
152 // Note that we **do not** run these tests here. The windows builders get super
153 // unhappy if a thread outlives the main thread and then exits at the same time
154 // (something deadlocks) so we just avoid this entirely by not running these
157 /// use std::sync::Arc;
160 /// let five = Arc::new(5);
163 /// let five = Arc::clone(&five);
165 /// thread::spawn(move || {
166 /// println!("{:?}", five);
171 /// Sharing a mutable [`AtomicUsize`]:
173 /// [`AtomicUsize`]: ../../std/sync/atomic/struct.AtomicUsize.html
176 /// use std::sync::Arc;
177 /// use std::sync::atomic::{AtomicUsize, Ordering};
180 /// let val = Arc::new(AtomicUsize::new(5));
183 /// let val = Arc::clone(&val);
185 /// thread::spawn(move || {
186 /// let v = val.fetch_add(1, Ordering::SeqCst);
187 /// println!("{:?}", v);
192 /// See the [`rc` documentation][rc_examples] for more examples of reference
193 /// counting in general.
195 /// [rc_examples]: ../../std/rc/index.html#examples
196 #[stable(feature = "rust1", since = "1.0.0")]
197 pub struct Arc<T: ?Sized> {
198 ptr: Shared<ArcInner<T>>,
201 #[stable(feature = "rust1", since = "1.0.0")]
202 unsafe impl<T: ?Sized + Sync + Send> Send for Arc<T> {}
203 #[stable(feature = "rust1", since = "1.0.0")]
204 unsafe impl<T: ?Sized + Sync + Send> Sync for Arc<T> {}
206 #[unstable(feature = "coerce_unsized", issue = "27732")]
207 impl<T: ?Sized + Unsize<U>, U: ?Sized> CoerceUnsized<Arc<U>> for Arc<T> {}
209 /// `Weak` is a version of [`Arc`] that holds a non-owning reference to the
210 /// managed value. The value is accessed by calling [`upgrade`] on the `Weak`
211 /// pointer, which returns an [`Option`]`<`[`Arc`]`<T>>`.
213 /// Since a `Weak` reference does not count towards ownership, it will not
214 /// prevent the inner value from being dropped, and `Weak` itself makes no
215 /// guarantees about the value still being present and may return [`None`]
216 /// when [`upgrade`]d.
218 /// A `Weak` pointer is useful for keeping a temporary reference to the value
219 /// within [`Arc`] without extending its lifetime. It is also used to prevent
220 /// circular references between [`Arc`] pointers, since mutual owning references
221 /// would never allow either [`Arc`] to be dropped. For example, a tree could
222 /// have strong [`Arc`] pointers from parent nodes to children, and `Weak`
223 /// pointers from children back to their parents.
225 /// The typical way to obtain a `Weak` pointer is to call [`Arc::downgrade`].
227 /// [`Arc`]: struct.Arc.html
228 /// [`Arc::downgrade`]: struct.Arc.html#method.downgrade
229 /// [`upgrade`]: struct.Weak.html#method.upgrade
230 /// [`Option`]: ../../std/option/enum.Option.html
231 /// [`None`]: ../../std/option/enum.Option.html#variant.None
232 #[stable(feature = "arc_weak", since = "1.4.0")]
233 pub struct Weak<T: ?Sized> {
234 ptr: Shared<ArcInner<T>>,
237 #[stable(feature = "arc_weak", since = "1.4.0")]
238 unsafe impl<T: ?Sized + Sync + Send> Send for Weak<T> {}
239 #[stable(feature = "arc_weak", since = "1.4.0")]
240 unsafe impl<T: ?Sized + Sync + Send> Sync for Weak<T> {}
242 #[unstable(feature = "coerce_unsized", issue = "27732")]
243 impl<T: ?Sized + Unsize<U>, U: ?Sized> CoerceUnsized<Weak<U>> for Weak<T> {}
245 #[stable(feature = "arc_weak", since = "1.4.0")]
246 impl<T: ?Sized + fmt::Debug> fmt::Debug for Weak<T> {
247 fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
252 struct ArcInner<T: ?Sized> {
253 strong: atomic::AtomicUsize,
255 // the value usize::MAX acts as a sentinel for temporarily "locking" the
256 // ability to upgrade weak pointers or downgrade strong ones; this is used
257 // to avoid races in `make_mut` and `get_mut`.
258 weak: atomic::AtomicUsize,
263 unsafe impl<T: ?Sized + Sync + Send> Send for ArcInner<T> {}
264 unsafe impl<T: ?Sized + Sync + Send> Sync for ArcInner<T> {}
267 /// Constructs a new `Arc<T>`.
272 /// use std::sync::Arc;
274 /// let five = Arc::new(5);
277 #[stable(feature = "rust1", since = "1.0.0")]
278 pub fn new(data: T) -> Arc<T> {
279 // Start the weak pointer count as 1 which is the weak pointer that's
280 // held by all the strong pointers (kinda), see std/rc.rs for more info
281 let x: Box<_> = box ArcInner {
282 strong: atomic::AtomicUsize::new(1),
283 weak: atomic::AtomicUsize::new(1),
286 Arc { ptr: Shared::from(Box::into_unique(x)) }
289 /// Returns the contained value, if the `Arc` has exactly one strong reference.
291 /// Otherwise, an [`Err`][result] is returned with the same `Arc` that was
294 /// This will succeed even if there are outstanding weak references.
296 /// [result]: ../../std/result/enum.Result.html
301 /// use std::sync::Arc;
303 /// let x = Arc::new(3);
304 /// assert_eq!(Arc::try_unwrap(x), Ok(3));
306 /// let x = Arc::new(4);
307 /// let _y = Arc::clone(&x);
308 /// assert_eq!(*Arc::try_unwrap(x).unwrap_err(), 4);
311 #[stable(feature = "arc_unique", since = "1.4.0")]
312 pub fn try_unwrap(this: Self) -> Result<T, Self> {
313 // See `drop` for why all these atomics are like this
314 if this.inner().strong.compare_exchange(1, 0, Release, Relaxed).is_err() {
318 atomic::fence(Acquire);
321 let elem = ptr::read(&this.ptr.as_ref().data);
323 // Make a weak pointer to clean up the implicit strong-weak reference
324 let _weak = Weak { ptr: this.ptr };
332 impl<T: ?Sized> Arc<T> {
333 /// Consumes the `Arc`, returning the wrapped pointer.
335 /// To avoid a memory leak the pointer must be converted back to an `Arc` using
336 /// [`Arc::from_raw`][from_raw].
338 /// [from_raw]: struct.Arc.html#method.from_raw
343 /// use std::sync::Arc;
345 /// let x = Arc::new(10);
346 /// let x_ptr = Arc::into_raw(x);
347 /// assert_eq!(unsafe { *x_ptr }, 10);
349 #[stable(feature = "rc_raw", since = "1.17.0")]
350 pub fn into_raw(this: Self) -> *const T {
351 let ptr: *const T = &*this;
356 /// Constructs an `Arc` from a raw pointer.
358 /// The raw pointer must have been previously returned by a call to a
359 /// [`Arc::into_raw`][into_raw].
361 /// This function is unsafe because improper use may lead to memory problems. For example, a
362 /// double-free may occur if the function is called twice on the same raw pointer.
364 /// [into_raw]: struct.Arc.html#method.into_raw
369 /// use std::sync::Arc;
371 /// let x = Arc::new(10);
372 /// let x_ptr = Arc::into_raw(x);
375 /// // Convert back to an `Arc` to prevent leak.
376 /// let x = Arc::from_raw(x_ptr);
377 /// assert_eq!(*x, 10);
379 /// // Further calls to `Arc::from_raw(x_ptr)` would be memory unsafe.
382 /// // The memory was freed when `x` went out of scope above, so `x_ptr` is now dangling!
384 #[stable(feature = "rc_raw", since = "1.17.0")]
385 pub unsafe fn from_raw(ptr: *const T) -> Self {
386 // Align the unsized value to the end of the ArcInner.
387 // Because it is ?Sized, it will always be the last field in memory.
388 let align = align_of_val(&*ptr);
389 let layout = Layout::new::<ArcInner<()>>();
390 let offset = (layout.size() + layout.padding_needed_for(align)) as isize;
392 // Reverse the offset to find the original ArcInner.
393 let fake_ptr = ptr as *mut ArcInner<T>;
394 let arc_ptr = set_data_ptr(fake_ptr, (ptr as *mut u8).offset(-offset));
397 ptr: Shared::new_unchecked(arc_ptr),
401 /// Creates a new [`Weak`][weak] pointer to this value.
403 /// [weak]: struct.Weak.html
408 /// use std::sync::Arc;
410 /// let five = Arc::new(5);
412 /// let weak_five = Arc::downgrade(&five);
414 #[stable(feature = "arc_weak", since = "1.4.0")]
415 pub fn downgrade(this: &Self) -> Weak<T> {
416 // This Relaxed is OK because we're checking the value in the CAS
418 let mut cur = this.inner().weak.load(Relaxed);
421 // check if the weak counter is currently "locked"; if so, spin.
422 if cur == usize::MAX {
423 cur = this.inner().weak.load(Relaxed);
427 // NOTE: this code currently ignores the possibility of overflow
428 // into usize::MAX; in general both Rc and Arc need to be adjusted
429 // to deal with overflow.
431 // Unlike with Clone(), we need this to be an Acquire read to
432 // synchronize with the write coming from `is_unique`, so that the
433 // events prior to that write happen before this read.
434 match this.inner().weak.compare_exchange_weak(cur, cur + 1, Acquire, Relaxed) {
435 Ok(_) => return Weak { ptr: this.ptr },
436 Err(old) => cur = old,
441 /// Gets the number of [`Weak`][weak] pointers to this value.
443 /// [weak]: struct.Weak.html
447 /// This method by itself is safe, but using it correctly requires extra care.
448 /// Another thread can change the weak count at any time,
449 /// including potentially between calling this method and acting on the result.
454 /// use std::sync::Arc;
456 /// let five = Arc::new(5);
457 /// let _weak_five = Arc::downgrade(&five);
459 /// // This assertion is deterministic because we haven't shared
460 /// // the `Arc` or `Weak` between threads.
461 /// assert_eq!(1, Arc::weak_count(&five));
464 #[stable(feature = "arc_counts", since = "1.15.0")]
465 pub fn weak_count(this: &Self) -> usize {
466 let cnt = this.inner().weak.load(SeqCst);
467 // If the weak count is currently locked, the value of the
468 // count was 0 just before taking the lock.
469 if cnt == usize::MAX { 0 } else { cnt - 1 }
472 /// Gets the number of strong (`Arc`) pointers to this value.
476 /// This method by itself is safe, but using it correctly requires extra care.
477 /// Another thread can change the strong count at any time,
478 /// including potentially between calling this method and acting on the result.
483 /// use std::sync::Arc;
485 /// let five = Arc::new(5);
486 /// let _also_five = Arc::clone(&five);
488 /// // This assertion is deterministic because we haven't shared
489 /// // the `Arc` between threads.
490 /// assert_eq!(2, Arc::strong_count(&five));
493 #[stable(feature = "arc_counts", since = "1.15.0")]
494 pub fn strong_count(this: &Self) -> usize {
495 this.inner().strong.load(SeqCst)
499 fn inner(&self) -> &ArcInner<T> {
500 // This unsafety is ok because while this arc is alive we're guaranteed
501 // that the inner pointer is valid. Furthermore, we know that the
502 // `ArcInner` structure itself is `Sync` because the inner data is
503 // `Sync` as well, so we're ok loaning out an immutable pointer to these
505 unsafe { self.ptr.as_ref() }
508 // Non-inlined part of `drop`.
510 unsafe fn drop_slow(&mut self) {
511 let ptr = self.ptr.as_ptr();
513 // Destroy the data at this time, even though we may not free the box
514 // allocation itself (there may still be weak pointers lying around).
515 ptr::drop_in_place(&mut self.ptr.as_mut().data);
517 if self.inner().weak.fetch_sub(1, Release) == 1 {
518 atomic::fence(Acquire);
519 Heap.dealloc(ptr as *mut u8, Layout::for_value(&*ptr))
524 #[stable(feature = "ptr_eq", since = "1.17.0")]
525 /// Returns true if the two `Arc`s point to the same value (not
526 /// just values that compare as equal).
531 /// use std::sync::Arc;
533 /// let five = Arc::new(5);
534 /// let same_five = Arc::clone(&five);
535 /// let other_five = Arc::new(5);
537 /// assert!(Arc::ptr_eq(&five, &same_five));
538 /// assert!(!Arc::ptr_eq(&five, &other_five));
540 pub fn ptr_eq(this: &Self, other: &Self) -> bool {
541 this.ptr.as_ptr() == other.ptr.as_ptr()
545 impl<T: ?Sized> Arc<T> {
546 // Allocates an `ArcInner<T>` with sufficient space for an unsized value
547 unsafe fn allocate_for_ptr(ptr: *const T) -> *mut ArcInner<T> {
548 // Create a fake ArcInner to find allocation size and alignment
549 let fake_ptr = ptr as *mut ArcInner<T>;
551 let layout = Layout::for_value(&*fake_ptr);
553 let mem = Heap.alloc(layout)
554 .unwrap_or_else(|e| Heap.oom(e));
556 // Initialize the real ArcInner
557 let inner = set_data_ptr(ptr as *mut T, mem) as *mut ArcInner<T>;
559 ptr::write(&mut (*inner).strong, atomic::AtomicUsize::new(1));
560 ptr::write(&mut (*inner).weak, atomic::AtomicUsize::new(1));
565 fn from_box(v: Box<T>) -> Arc<T> {
567 let bptr = Box::into_raw(v);
569 let value_size = size_of_val(&*bptr);
570 let ptr = Self::allocate_for_ptr(bptr);
572 // Copy value as bytes
573 ptr::copy_nonoverlapping(
574 bptr as *const T as *const u8,
575 &mut (*ptr).data as *mut _ as *mut u8,
578 // Free the allocation without dropping its contents
581 Arc { ptr: Shared::new_unchecked(ptr) }
586 // Sets the data pointer of a `?Sized` raw pointer.
588 // For a slice/trait object, this sets the `data` field and leaves the rest
589 // unchanged. For a sized raw pointer, this simply sets the pointer.
590 unsafe fn set_data_ptr<T: ?Sized, U>(mut ptr: *mut T, data: *mut U) -> *mut T {
591 ptr::write(&mut ptr as *mut _ as *mut *mut u8, data as *mut u8);
596 // Copy elements from slice into newly allocated Arc<[T]>
598 // Unsafe because the caller must either take ownership or bind `T: Copy`
599 unsafe fn copy_from_slice(v: &[T]) -> Arc<[T]> {
600 let v_ptr = v as *const [T];
601 let ptr = Self::allocate_for_ptr(v_ptr);
603 ptr::copy_nonoverlapping(
605 &mut (*ptr).data as *mut [T] as *mut T,
608 Arc { ptr: Shared::new_unchecked(ptr) }
612 // Specialization trait used for From<&[T]>
613 trait ArcFromSlice<T> {
614 fn from_slice(slice: &[T]) -> Self;
617 impl<T: Clone> ArcFromSlice<T> for Arc<[T]> {
619 default fn from_slice(v: &[T]) -> Self {
620 // Panic guard while cloning T elements.
621 // In the event of a panic, elements that have been written
622 // into the new ArcInner will be dropped, then the memory freed.
630 impl<T> Drop for Guard<T> {
632 use core::slice::from_raw_parts_mut;
635 let slice = from_raw_parts_mut(self.elems, self.n_elems);
636 ptr::drop_in_place(slice);
638 Heap.dealloc(self.mem, self.layout.clone());
644 let v_ptr = v as *const [T];
645 let ptr = Self::allocate_for_ptr(v_ptr);
647 let mem = ptr as *mut _ as *mut u8;
648 let layout = Layout::for_value(&*ptr);
650 // Pointer to first element
651 let elems = &mut (*ptr).data as *mut [T] as *mut T;
653 let mut guard = Guard{
660 for (i, item) in v.iter().enumerate() {
661 ptr::write(elems.offset(i as isize), item.clone());
665 // All clear. Forget the guard so it doesn't free the new ArcInner.
668 Arc { ptr: Shared::new_unchecked(ptr) }
673 impl<T: Copy> ArcFromSlice<T> for Arc<[T]> {
675 fn from_slice(v: &[T]) -> Self {
676 unsafe { Arc::copy_from_slice(v) }
680 #[stable(feature = "rust1", since = "1.0.0")]
681 impl<T: ?Sized> Clone for Arc<T> {
682 /// Makes a clone of the `Arc` pointer.
684 /// This creates another pointer to the same inner value, increasing the
685 /// strong reference count.
690 /// use std::sync::Arc;
692 /// let five = Arc::new(5);
694 /// Arc::clone(&five);
697 fn clone(&self) -> Arc<T> {
698 // Using a relaxed ordering is alright here, as knowledge of the
699 // original reference prevents other threads from erroneously deleting
702 // As explained in the [Boost documentation][1], Increasing the
703 // reference counter can always be done with memory_order_relaxed: New
704 // references to an object can only be formed from an existing
705 // reference, and passing an existing reference from one thread to
706 // another must already provide any required synchronization.
708 // [1]: (www.boost.org/doc/libs/1_55_0/doc/html/atomic/usage_examples.html)
709 let old_size = self.inner().strong.fetch_add(1, Relaxed);
711 // However we need to guard against massive refcounts in case someone
712 // is `mem::forget`ing Arcs. If we don't do this the count can overflow
713 // and users will use-after free. We racily saturate to `isize::MAX` on
714 // the assumption that there aren't ~2 billion threads incrementing
715 // the reference count at once. This branch will never be taken in
716 // any realistic program.
718 // We abort because such a program is incredibly degenerate, and we
719 // don't care to support it.
720 if old_size > MAX_REFCOUNT {
726 Arc { ptr: self.ptr }
730 #[stable(feature = "rust1", since = "1.0.0")]
731 impl<T: ?Sized> Deref for Arc<T> {
735 fn deref(&self) -> &T {
740 impl<T: Clone> Arc<T> {
741 /// Makes a mutable reference into the given `Arc`.
743 /// If there are other `Arc` or [`Weak`][weak] pointers to the same value,
744 /// then `make_mut` will invoke [`clone`][clone] on the inner value to
745 /// ensure unique ownership. This is also referred to as clone-on-write.
747 /// See also [`get_mut`][get_mut], which will fail rather than cloning.
749 /// [weak]: struct.Weak.html
750 /// [clone]: ../../std/clone/trait.Clone.html#tymethod.clone
751 /// [get_mut]: struct.Arc.html#method.get_mut
756 /// use std::sync::Arc;
758 /// let mut data = Arc::new(5);
760 /// *Arc::make_mut(&mut data) += 1; // Won't clone anything
761 /// let mut other_data = Arc::clone(&data); // Won't clone inner data
762 /// *Arc::make_mut(&mut data) += 1; // Clones inner data
763 /// *Arc::make_mut(&mut data) += 1; // Won't clone anything
764 /// *Arc::make_mut(&mut other_data) *= 2; // Won't clone anything
766 /// // Now `data` and `other_data` point to different values.
767 /// assert_eq!(*data, 8);
768 /// assert_eq!(*other_data, 12);
771 #[stable(feature = "arc_unique", since = "1.4.0")]
772 pub fn make_mut(this: &mut Self) -> &mut T {
773 // Note that we hold both a strong reference and a weak reference.
774 // Thus, releasing our strong reference only will not, by itself, cause
775 // the memory to be deallocated.
777 // Use Acquire to ensure that we see any writes to `weak` that happen
778 // before release writes (i.e., decrements) to `strong`. Since we hold a
779 // weak count, there's no chance the ArcInner itself could be
781 if this.inner().strong.compare_exchange(1, 0, Acquire, Relaxed).is_err() {
782 // Another strong pointer exists; clone
783 *this = Arc::new((**this).clone());
784 } else if this.inner().weak.load(Relaxed) != 1 {
785 // Relaxed suffices in the above because this is fundamentally an
786 // optimization: we are always racing with weak pointers being
787 // dropped. Worst case, we end up allocated a new Arc unnecessarily.
789 // We removed the last strong ref, but there are additional weak
790 // refs remaining. We'll move the contents to a new Arc, and
791 // invalidate the other weak refs.
793 // Note that it is not possible for the read of `weak` to yield
794 // usize::MAX (i.e., locked), since the weak count can only be
795 // locked by a thread with a strong reference.
797 // Materialize our own implicit weak pointer, so that it can clean
798 // up the ArcInner as needed.
799 let weak = Weak { ptr: this.ptr };
801 // mark the data itself as already deallocated
803 // there is no data race in the implicit write caused by `read`
804 // here (due to zeroing) because data is no longer accessed by
805 // other threads (due to there being no more strong refs at this
807 let mut swap = Arc::new(ptr::read(&weak.ptr.as_ref().data));
808 mem::swap(this, &mut swap);
812 // We were the sole reference of either kind; bump back up the
814 this.inner().strong.store(1, Release);
817 // As with `get_mut()`, the unsafety is ok because our reference was
818 // either unique to begin with, or became one upon cloning the contents.
820 &mut this.ptr.as_mut().data
825 impl<T: ?Sized> Arc<T> {
826 /// Returns a mutable reference to the inner value, if there are
827 /// no other `Arc` or [`Weak`][weak] pointers to the same value.
829 /// Returns [`None`][option] otherwise, because it is not safe to
830 /// mutate a shared value.
832 /// See also [`make_mut`][make_mut], which will [`clone`][clone]
833 /// the inner value when it's shared.
835 /// [weak]: struct.Weak.html
836 /// [option]: ../../std/option/enum.Option.html
837 /// [make_mut]: struct.Arc.html#method.make_mut
838 /// [clone]: ../../std/clone/trait.Clone.html#tymethod.clone
843 /// use std::sync::Arc;
845 /// let mut x = Arc::new(3);
846 /// *Arc::get_mut(&mut x).unwrap() = 4;
847 /// assert_eq!(*x, 4);
849 /// let _y = Arc::clone(&x);
850 /// assert!(Arc::get_mut(&mut x).is_none());
853 #[stable(feature = "arc_unique", since = "1.4.0")]
854 pub fn get_mut(this: &mut Self) -> Option<&mut T> {
855 if this.is_unique() {
856 // This unsafety is ok because we're guaranteed that the pointer
857 // returned is the *only* pointer that will ever be returned to T. Our
858 // reference count is guaranteed to be 1 at this point, and we required
859 // the Arc itself to be `mut`, so we're returning the only possible
860 // reference to the inner data.
862 Some(&mut this.ptr.as_mut().data)
869 /// Determine whether this is the unique reference (including weak refs) to
870 /// the underlying data.
872 /// Note that this requires locking the weak ref count.
873 fn is_unique(&mut self) -> bool {
874 // lock the weak pointer count if we appear to be the sole weak pointer
877 // The acquire label here ensures a happens-before relationship with any
878 // writes to `strong` prior to decrements of the `weak` count (via drop,
879 // which uses Release).
880 if self.inner().weak.compare_exchange(1, usize::MAX, Acquire, Relaxed).is_ok() {
881 // Due to the previous acquire read, this will observe any writes to
882 // `strong` that were due to upgrading weak pointers; only strong
883 // clones remain, which require that the strong count is > 1 anyway.
884 let unique = self.inner().strong.load(Relaxed) == 1;
886 // The release write here synchronizes with a read in `downgrade`,
887 // effectively preventing the above read of `strong` from happening
889 self.inner().weak.store(1, Release); // release the lock
897 #[stable(feature = "rust1", since = "1.0.0")]
898 unsafe impl<#[may_dangle] T: ?Sized> Drop for Arc<T> {
901 /// This will decrement the strong reference count. If the strong reference
902 /// count reaches zero then the only other references (if any) are
903 /// [`Weak`][weak], so we `drop` the inner value.
905 /// [weak]: struct.Weak.html
910 /// use std::sync::Arc;
914 /// impl Drop for Foo {
915 /// fn drop(&mut self) {
916 /// println!("dropped!");
920 /// let foo = Arc::new(Foo);
921 /// let foo2 = Arc::clone(&foo);
923 /// drop(foo); // Doesn't print anything
924 /// drop(foo2); // Prints "dropped!"
928 // Because `fetch_sub` is already atomic, we do not need to synchronize
929 // with other threads unless we are going to delete the object. This
930 // same logic applies to the below `fetch_sub` to the `weak` count.
931 if self.inner().strong.fetch_sub(1, Release) != 1 {
935 // This fence is needed to prevent reordering of use of the data and
936 // deletion of the data. Because it is marked `Release`, the decreasing
937 // of the reference count synchronizes with this `Acquire` fence. This
938 // means that use of the data happens before decreasing the reference
939 // count, which happens before this fence, which happens before the
940 // deletion of the data.
942 // As explained in the [Boost documentation][1],
944 // > It is important to enforce any possible access to the object in one
945 // > thread (through an existing reference) to *happen before* deleting
946 // > the object in a different thread. This is achieved by a "release"
947 // > operation after dropping a reference (any access to the object
948 // > through this reference must obviously happened before), and an
949 // > "acquire" operation before deleting the object.
951 // In particular, while the contents of an Arc are usually immutable, it's
952 // possible to have interior writes to something like a Mutex<T>. Since a
953 // Mutex is not acquired when it is deleted, we can't rely on its
954 // synchronization logic to make writes in thread A visible to a destructor
955 // running in thread B.
957 // Also note that the Acquire fence here could probably be replaced with an
958 // Acquire load, which could improve performance in highly-contended
959 // situations. See [2].
961 // [1]: (www.boost.org/doc/libs/1_55_0/doc/html/atomic/usage_examples.html)
962 // [2]: (https://github.com/rust-lang/rust/pull/41714)
963 atomic::fence(Acquire);
972 /// Constructs a new `Weak<T>`, allocating memory for `T` without initializing
973 /// it. Calling [`upgrade`] on the return value always gives [`None`].
975 /// [`upgrade`]: struct.Weak.html#method.upgrade
976 /// [`None`]: ../../std/option/enum.Option.html#variant.None
981 /// use std::sync::Weak;
983 /// let empty: Weak<i64> = Weak::new();
984 /// assert!(empty.upgrade().is_none());
986 #[stable(feature = "downgraded_weak", since = "1.10.0")]
987 pub fn new() -> Weak<T> {
990 ptr: Shared::from(Box::into_unique(box ArcInner {
991 strong: atomic::AtomicUsize::new(0),
992 weak: atomic::AtomicUsize::new(1),
993 data: uninitialized(),
1000 impl<T: ?Sized> Weak<T> {
1001 /// Attempts to upgrade the `Weak` pointer to an [`Arc`], extending
1002 /// the lifetime of the value if successful.
1004 /// Returns [`None`] if the value has since been dropped.
1006 /// [`Arc`]: struct.Arc.html
1007 /// [`None`]: ../../std/option/enum.Option.html#variant.None
1012 /// use std::sync::Arc;
1014 /// let five = Arc::new(5);
1016 /// let weak_five = Arc::downgrade(&five);
1018 /// let strong_five: Option<Arc<_>> = weak_five.upgrade();
1019 /// assert!(strong_five.is_some());
1021 /// // Destroy all strong pointers.
1022 /// drop(strong_five);
1025 /// assert!(weak_five.upgrade().is_none());
1027 #[stable(feature = "arc_weak", since = "1.4.0")]
1028 pub fn upgrade(&self) -> Option<Arc<T>> {
1029 // We use a CAS loop to increment the strong count instead of a
1030 // fetch_add because once the count hits 0 it must never be above 0.
1031 let inner = self.inner();
1033 // Relaxed load because any write of 0 that we can observe
1034 // leaves the field in a permanently zero state (so a
1035 // "stale" read of 0 is fine), and any other value is
1036 // confirmed via the CAS below.
1037 let mut n = inner.strong.load(Relaxed);
1044 // See comments in `Arc::clone` for why we do this (for `mem::forget`).
1045 if n > MAX_REFCOUNT {
1051 // Relaxed is valid for the same reason it is on Arc's Clone impl
1052 match inner.strong.compare_exchange_weak(n, n + 1, Relaxed, Relaxed) {
1053 Ok(_) => return Some(Arc { ptr: self.ptr }),
1054 Err(old) => n = old,
1060 fn inner(&self) -> &ArcInner<T> {
1061 // See comments above for why this is "safe"
1062 unsafe { self.ptr.as_ref() }
1066 #[stable(feature = "arc_weak", since = "1.4.0")]
1067 impl<T: ?Sized> Clone for Weak<T> {
1068 /// Makes a clone of the `Weak` pointer that points to the same value.
1073 /// use std::sync::{Arc, Weak};
1075 /// let weak_five = Arc::downgrade(&Arc::new(5));
1077 /// Weak::clone(&weak_five);
1080 fn clone(&self) -> Weak<T> {
1081 // See comments in Arc::clone() for why this is relaxed. This can use a
1082 // fetch_add (ignoring the lock) because the weak count is only locked
1083 // where are *no other* weak pointers in existence. (So we can't be
1084 // running this code in that case).
1085 let old_size = self.inner().weak.fetch_add(1, Relaxed);
1087 // See comments in Arc::clone() for why we do this (for mem::forget).
1088 if old_size > MAX_REFCOUNT {
1094 return Weak { ptr: self.ptr };
1098 #[stable(feature = "downgraded_weak", since = "1.10.0")]
1099 impl<T> Default for Weak<T> {
1100 /// Constructs a new `Weak<T>`, allocating memory for `T` without initializing
1101 /// it. Calling [`upgrade`] on the return value always gives [`None`].
1103 /// [`upgrade`]: struct.Weak.html#method.upgrade
1104 /// [`None`]: ../../std/option/enum.Option.html#variant.None
1109 /// use std::sync::Weak;
1111 /// let empty: Weak<i64> = Default::default();
1112 /// assert!(empty.upgrade().is_none());
1114 fn default() -> Weak<T> {
1119 #[stable(feature = "arc_weak", since = "1.4.0")]
1120 impl<T: ?Sized> Drop for Weak<T> {
1121 /// Drops the `Weak` pointer.
1126 /// use std::sync::{Arc, Weak};
1130 /// impl Drop for Foo {
1131 /// fn drop(&mut self) {
1132 /// println!("dropped!");
1136 /// let foo = Arc::new(Foo);
1137 /// let weak_foo = Arc::downgrade(&foo);
1138 /// let other_weak_foo = Weak::clone(&weak_foo);
1140 /// drop(weak_foo); // Doesn't print anything
1141 /// drop(foo); // Prints "dropped!"
1143 /// assert!(other_weak_foo.upgrade().is_none());
1145 fn drop(&mut self) {
1146 let ptr = self.ptr.as_ptr();
1148 // If we find out that we were the last weak pointer, then its time to
1149 // deallocate the data entirely. See the discussion in Arc::drop() about
1150 // the memory orderings
1152 // It's not necessary to check for the locked state here, because the
1153 // weak count can only be locked if there was precisely one weak ref,
1154 // meaning that drop could only subsequently run ON that remaining weak
1155 // ref, which can only happen after the lock is released.
1156 if self.inner().weak.fetch_sub(1, Release) == 1 {
1157 atomic::fence(Acquire);
1159 Heap.dealloc(ptr as *mut u8, Layout::for_value(&*ptr))
1165 #[stable(feature = "rust1", since = "1.0.0")]
1166 impl<T: ?Sized + PartialEq> PartialEq for Arc<T> {
1167 /// Equality for two `Arc`s.
1169 /// Two `Arc`s are equal if their inner values are equal.
1174 /// use std::sync::Arc;
1176 /// let five = Arc::new(5);
1178 /// assert!(five == Arc::new(5));
1180 fn eq(&self, other: &Arc<T>) -> bool {
1181 *(*self) == *(*other)
1184 /// Inequality for two `Arc`s.
1186 /// Two `Arc`s are unequal if their inner values are unequal.
1191 /// use std::sync::Arc;
1193 /// let five = Arc::new(5);
1195 /// assert!(five != Arc::new(6));
1197 fn ne(&self, other: &Arc<T>) -> bool {
1198 *(*self) != *(*other)
1201 #[stable(feature = "rust1", since = "1.0.0")]
1202 impl<T: ?Sized + PartialOrd> PartialOrd for Arc<T> {
1203 /// Partial comparison for two `Arc`s.
1205 /// The two are compared by calling `partial_cmp()` on their inner values.
1210 /// use std::sync::Arc;
1211 /// use std::cmp::Ordering;
1213 /// let five = Arc::new(5);
1215 /// assert_eq!(Some(Ordering::Less), five.partial_cmp(&Arc::new(6)));
1217 fn partial_cmp(&self, other: &Arc<T>) -> Option<Ordering> {
1218 (**self).partial_cmp(&**other)
1221 /// Less-than comparison for two `Arc`s.
1223 /// The two are compared by calling `<` on their inner values.
1228 /// use std::sync::Arc;
1230 /// let five = Arc::new(5);
1232 /// assert!(five < Arc::new(6));
1234 fn lt(&self, other: &Arc<T>) -> bool {
1235 *(*self) < *(*other)
1238 /// 'Less than or equal to' comparison for two `Arc`s.
1240 /// The two are compared by calling `<=` on their inner values.
1245 /// use std::sync::Arc;
1247 /// let five = Arc::new(5);
1249 /// assert!(five <= Arc::new(5));
1251 fn le(&self, other: &Arc<T>) -> bool {
1252 *(*self) <= *(*other)
1255 /// Greater-than comparison for two `Arc`s.
1257 /// The two are compared by calling `>` on their inner values.
1262 /// use std::sync::Arc;
1264 /// let five = Arc::new(5);
1266 /// assert!(five > Arc::new(4));
1268 fn gt(&self, other: &Arc<T>) -> bool {
1269 *(*self) > *(*other)
1272 /// 'Greater than or equal to' comparison for two `Arc`s.
1274 /// The two are compared by calling `>=` on their inner values.
1279 /// use std::sync::Arc;
1281 /// let five = Arc::new(5);
1283 /// assert!(five >= Arc::new(5));
1285 fn ge(&self, other: &Arc<T>) -> bool {
1286 *(*self) >= *(*other)
1289 #[stable(feature = "rust1", since = "1.0.0")]
1290 impl<T: ?Sized + Ord> Ord for Arc<T> {
1291 /// Comparison for two `Arc`s.
1293 /// The two are compared by calling `cmp()` on their inner values.
1298 /// use std::sync::Arc;
1299 /// use std::cmp::Ordering;
1301 /// let five = Arc::new(5);
1303 /// assert_eq!(Ordering::Less, five.cmp(&Arc::new(6)));
1305 fn cmp(&self, other: &Arc<T>) -> Ordering {
1306 (**self).cmp(&**other)
1309 #[stable(feature = "rust1", since = "1.0.0")]
1310 impl<T: ?Sized + Eq> Eq for Arc<T> {}
1312 #[stable(feature = "rust1", since = "1.0.0")]
1313 impl<T: ?Sized + fmt::Display> fmt::Display for Arc<T> {
1314 fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
1315 fmt::Display::fmt(&**self, f)
1319 #[stable(feature = "rust1", since = "1.0.0")]
1320 impl<T: ?Sized + fmt::Debug> fmt::Debug for Arc<T> {
1321 fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
1322 fmt::Debug::fmt(&**self, f)
1326 #[stable(feature = "rust1", since = "1.0.0")]
1327 impl<T: ?Sized> fmt::Pointer for Arc<T> {
1328 fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
1329 fmt::Pointer::fmt(&self.ptr, f)
1333 #[stable(feature = "rust1", since = "1.0.0")]
1334 impl<T: Default> Default for Arc<T> {
1335 /// Creates a new `Arc<T>`, with the `Default` value for `T`.
1340 /// use std::sync::Arc;
1342 /// let x: Arc<i32> = Default::default();
1343 /// assert_eq!(*x, 0);
1345 fn default() -> Arc<T> {
1346 Arc::new(Default::default())
1350 #[stable(feature = "rust1", since = "1.0.0")]
1351 impl<T: ?Sized + Hash> Hash for Arc<T> {
1352 fn hash<H: Hasher>(&self, state: &mut H) {
1353 (**self).hash(state)
1357 #[stable(feature = "from_for_ptrs", since = "1.6.0")]
1358 impl<T> From<T> for Arc<T> {
1359 fn from(t: T) -> Self {
1364 #[stable(feature = "shared_from_slice", since = "1.21.0")]
1365 impl<'a, T: Clone> From<&'a [T]> for Arc<[T]> {
1367 fn from(v: &[T]) -> Arc<[T]> {
1368 <Self as ArcFromSlice<T>>::from_slice(v)
1372 #[stable(feature = "shared_from_slice", since = "1.21.0")]
1373 impl<'a> From<&'a str> for Arc<str> {
1375 fn from(v: &str) -> Arc<str> {
1376 unsafe { mem::transmute(<Arc<[u8]>>::from(v.as_bytes())) }
1380 #[stable(feature = "shared_from_slice", since = "1.21.0")]
1381 impl From<String> for Arc<str> {
1383 fn from(v: String) -> Arc<str> {
1388 #[stable(feature = "shared_from_slice", since = "1.21.0")]
1389 impl<T: ?Sized> From<Box<T>> for Arc<T> {
1391 fn from(v: Box<T>) -> Arc<T> {
1396 #[stable(feature = "shared_from_slice", since = "1.21.0")]
1397 impl<T> From<Vec<T>> for Arc<[T]> {
1399 fn from(mut v: Vec<T>) -> Arc<[T]> {
1401 let arc = Arc::copy_from_slice(&v);
1403 // Allow the Vec to free its memory, but not destroy its contents
1413 use std::boxed::Box;
1414 use std::clone::Clone;
1415 use std::sync::mpsc::channel;
1418 use std::option::Option;
1419 use std::option::Option::{None, Some};
1420 use std::sync::atomic;
1421 use std::sync::atomic::Ordering::{Acquire, SeqCst};
1423 use std::sync::Mutex;
1424 use std::convert::From;
1426 use super::{Arc, Weak};
1429 struct Canary(*mut atomic::AtomicUsize);
1431 impl Drop for Canary {
1432 fn drop(&mut self) {
1436 (*c).fetch_add(1, SeqCst);
1444 #[cfg_attr(target_os = "emscripten", ignore)]
1445 fn manually_share_arc() {
1446 let v = vec![1, 2, 3, 4, 5, 6, 7, 8, 9, 10];
1447 let arc_v = Arc::new(v);
1449 let (tx, rx) = channel();
1451 let _t = thread::spawn(move || {
1452 let arc_v: Arc<Vec<i32>> = rx.recv().unwrap();
1453 assert_eq!((*arc_v)[3], 4);
1456 tx.send(arc_v.clone()).unwrap();
1458 assert_eq!((*arc_v)[2], 3);
1459 assert_eq!((*arc_v)[4], 5);
1463 fn test_arc_get_mut() {
1464 let mut x = Arc::new(3);
1465 *Arc::get_mut(&mut x).unwrap() = 4;
1468 assert!(Arc::get_mut(&mut x).is_none());
1470 assert!(Arc::get_mut(&mut x).is_some());
1471 let _w = Arc::downgrade(&x);
1472 assert!(Arc::get_mut(&mut x).is_none());
1477 let x = Arc::new(3);
1478 assert_eq!(Arc::try_unwrap(x), Ok(3));
1479 let x = Arc::new(4);
1481 assert_eq!(Arc::try_unwrap(x), Err(Arc::new(4)));
1482 let x = Arc::new(5);
1483 let _w = Arc::downgrade(&x);
1484 assert_eq!(Arc::try_unwrap(x), Ok(5));
1488 fn into_from_raw() {
1489 let x = Arc::new(box "hello");
1492 let x_ptr = Arc::into_raw(x);
1495 assert_eq!(**x_ptr, "hello");
1497 let x = Arc::from_raw(x_ptr);
1498 assert_eq!(**x, "hello");
1500 assert_eq!(Arc::try_unwrap(x).map(|x| *x), Ok("hello"));
1505 fn test_into_from_raw_unsized() {
1506 use std::fmt::Display;
1507 use std::string::ToString;
1509 let arc: Arc<str> = Arc::from("foo");
1511 let ptr = Arc::into_raw(arc.clone());
1512 let arc2 = unsafe { Arc::from_raw(ptr) };
1514 assert_eq!(unsafe { &*ptr }, "foo");
1515 assert_eq!(arc, arc2);
1517 let arc: Arc<Display> = Arc::new(123);
1519 let ptr = Arc::into_raw(arc.clone());
1520 let arc2 = unsafe { Arc::from_raw(ptr) };
1522 assert_eq!(unsafe { &*ptr }.to_string(), "123");
1523 assert_eq!(arc2.to_string(), "123");
1527 fn test_cowarc_clone_make_mut() {
1528 let mut cow0 = Arc::new(75);
1529 let mut cow1 = cow0.clone();
1530 let mut cow2 = cow1.clone();
1532 assert!(75 == *Arc::make_mut(&mut cow0));
1533 assert!(75 == *Arc::make_mut(&mut cow1));
1534 assert!(75 == *Arc::make_mut(&mut cow2));
1536 *Arc::make_mut(&mut cow0) += 1;
1537 *Arc::make_mut(&mut cow1) += 2;
1538 *Arc::make_mut(&mut cow2) += 3;
1540 assert!(76 == *cow0);
1541 assert!(77 == *cow1);
1542 assert!(78 == *cow2);
1544 // none should point to the same backing memory
1545 assert!(*cow0 != *cow1);
1546 assert!(*cow0 != *cow2);
1547 assert!(*cow1 != *cow2);
1551 fn test_cowarc_clone_unique2() {
1552 let mut cow0 = Arc::new(75);
1553 let cow1 = cow0.clone();
1554 let cow2 = cow1.clone();
1556 assert!(75 == *cow0);
1557 assert!(75 == *cow1);
1558 assert!(75 == *cow2);
1560 *Arc::make_mut(&mut cow0) += 1;
1561 assert!(76 == *cow0);
1562 assert!(75 == *cow1);
1563 assert!(75 == *cow2);
1565 // cow1 and cow2 should share the same contents
1566 // cow0 should have a unique reference
1567 assert!(*cow0 != *cow1);
1568 assert!(*cow0 != *cow2);
1569 assert!(*cow1 == *cow2);
1573 fn test_cowarc_clone_weak() {
1574 let mut cow0 = Arc::new(75);
1575 let cow1_weak = Arc::downgrade(&cow0);
1577 assert!(75 == *cow0);
1578 assert!(75 == *cow1_weak.upgrade().unwrap());
1580 *Arc::make_mut(&mut cow0) += 1;
1582 assert!(76 == *cow0);
1583 assert!(cow1_weak.upgrade().is_none());
1588 let x = Arc::new(5);
1589 let y = Arc::downgrade(&x);
1590 assert!(y.upgrade().is_some());
1595 let x = Arc::new(5);
1596 let y = Arc::downgrade(&x);
1598 assert!(y.upgrade().is_none());
1602 fn weak_self_cyclic() {
1604 x: Mutex<Option<Weak<Cycle>>>,
1607 let a = Arc::new(Cycle { x: Mutex::new(None) });
1608 let b = Arc::downgrade(&a.clone());
1609 *a.x.lock().unwrap() = Some(b);
1611 // hopefully we don't double-free (or leak)...
1616 let mut canary = atomic::AtomicUsize::new(0);
1617 let x = Arc::new(Canary(&mut canary as *mut atomic::AtomicUsize));
1619 assert!(canary.load(Acquire) == 1);
1623 fn drop_arc_weak() {
1624 let mut canary = atomic::AtomicUsize::new(0);
1625 let arc = Arc::new(Canary(&mut canary as *mut atomic::AtomicUsize));
1626 let arc_weak = Arc::downgrade(&arc);
1627 assert!(canary.load(Acquire) == 0);
1629 assert!(canary.load(Acquire) == 1);
1634 fn test_strong_count() {
1635 let a = Arc::new(0);
1636 assert!(Arc::strong_count(&a) == 1);
1637 let w = Arc::downgrade(&a);
1638 assert!(Arc::strong_count(&a) == 1);
1639 let b = w.upgrade().expect("");
1640 assert!(Arc::strong_count(&b) == 2);
1641 assert!(Arc::strong_count(&a) == 2);
1644 assert!(Arc::strong_count(&b) == 1);
1646 assert!(Arc::strong_count(&b) == 2);
1647 assert!(Arc::strong_count(&c) == 2);
1651 fn test_weak_count() {
1652 let a = Arc::new(0);
1653 assert!(Arc::strong_count(&a) == 1);
1654 assert!(Arc::weak_count(&a) == 0);
1655 let w = Arc::downgrade(&a);
1656 assert!(Arc::strong_count(&a) == 1);
1657 assert!(Arc::weak_count(&a) == 1);
1659 assert!(Arc::weak_count(&a) == 2);
1662 assert!(Arc::strong_count(&a) == 1);
1663 assert!(Arc::weak_count(&a) == 0);
1665 assert!(Arc::strong_count(&a) == 2);
1666 assert!(Arc::weak_count(&a) == 0);
1667 let d = Arc::downgrade(&c);
1668 assert!(Arc::weak_count(&c) == 1);
1669 assert!(Arc::strong_count(&c) == 2);
1678 let a = Arc::new(5);
1679 assert_eq!(format!("{:?}", a), "5");
1682 // Make sure deriving works with Arc<T>
1683 #[derive(Eq, Ord, PartialEq, PartialOrd, Clone, Debug, Default)]
1690 let x: Arc<[i32]> = Arc::new([1, 2, 3]);
1691 assert_eq!(format!("{:?}", x), "[1, 2, 3]");
1692 let y = Arc::downgrade(&x.clone());
1694 assert!(y.upgrade().is_none());
1698 fn test_from_owned() {
1700 let foo_arc = Arc::from(foo);
1701 assert!(123 == *foo_arc);
1705 fn test_new_weak() {
1706 let foo: Weak<usize> = Weak::new();
1707 assert!(foo.upgrade().is_none());
1712 let five = Arc::new(5);
1713 let same_five = five.clone();
1714 let other_five = Arc::new(5);
1716 assert!(Arc::ptr_eq(&five, &same_five));
1717 assert!(!Arc::ptr_eq(&five, &other_five));
1721 #[cfg_attr(target_os = "emscripten", ignore)]
1722 fn test_weak_count_locked() {
1723 let mut a = Arc::new(atomic::AtomicBool::new(false));
1725 let t = thread::spawn(move || {
1726 for _i in 0..1000000 {
1727 Arc::get_mut(&mut a);
1729 a.store(true, SeqCst);
1732 while !a2.load(SeqCst) {
1733 let n = Arc::weak_count(&a2);
1734 assert!(n < 2, "bad weak count: {}", n);
1740 fn test_from_str() {
1741 let r: Arc<str> = Arc::from("foo");
1743 assert_eq!(&r[..], "foo");
1747 fn test_copy_from_slice() {
1748 let s: &[u32] = &[1, 2, 3];
1749 let r: Arc<[u32]> = Arc::from(s);
1751 assert_eq!(&r[..], [1, 2, 3]);
1755 fn test_clone_from_slice() {
1756 #[derive(Clone, Debug, Eq, PartialEq)]
1759 let s: &[X] = &[X(1), X(2), X(3)];
1760 let r: Arc<[X]> = Arc::from(s);
1762 assert_eq!(&r[..], s);
1767 fn test_clone_from_slice_panic() {
1768 use std::string::{String, ToString};
1770 struct Fail(u32, String);
1772 impl Clone for Fail {
1773 fn clone(&self) -> Fail {
1777 Fail(self.0, self.1.clone())
1782 Fail(0, "foo".to_string()),
1783 Fail(1, "bar".to_string()),
1784 Fail(2, "baz".to_string()),
1787 // Should panic, but not cause memory corruption
1788 let _r: Arc<[Fail]> = Arc::from(s);
1792 fn test_from_box() {
1793 let b: Box<u32> = box 123;
1794 let r: Arc<u32> = Arc::from(b);
1796 assert_eq!(*r, 123);
1800 fn test_from_box_str() {
1801 use std::string::String;
1803 let s = String::from("foo").into_boxed_str();
1804 let r: Arc<str> = Arc::from(s);
1806 assert_eq!(&r[..], "foo");
1810 fn test_from_box_slice() {
1811 let s = vec![1, 2, 3].into_boxed_slice();
1812 let r: Arc<[u32]> = Arc::from(s);
1814 assert_eq!(&r[..], [1, 2, 3]);
1818 fn test_from_box_trait() {
1819 use std::fmt::Display;
1820 use std::string::ToString;
1822 let b: Box<Display> = box 123;
1823 let r: Arc<Display> = Arc::from(b);
1825 assert_eq!(r.to_string(), "123");
1829 fn test_from_box_trait_zero_sized() {
1830 use std::fmt::Debug;
1832 let b: Box<Debug> = box ();
1833 let r: Arc<Debug> = Arc::from(b);
1835 assert_eq!(format!("{:?}", r), "()");
1839 fn test_from_vec() {
1840 let v = vec![1, 2, 3];
1841 let r: Arc<[u32]> = Arc::from(v);
1843 assert_eq!(&r[..], [1, 2, 3]);
1847 #[stable(feature = "rust1", since = "1.0.0")]
1848 impl<T: ?Sized> borrow::Borrow<T> for Arc<T> {
1849 fn borrow(&self) -> &T {
1854 #[stable(since = "1.5.0", feature = "smart_ptr_as_ref")]
1855 impl<T: ?Sized> AsRef<T> for Arc<T> {
1856 fn as_ref(&self) -> &T {