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: you cannot generally obtain a mutable reference to something
56 /// inside an `Arc`. If you need to mutate through an `Arc`, use
57 /// [`Mutex`][mutex], [`RwLock`][rwlock], or one of the [`Atomic`][atomic]
62 /// Unlike [`Rc<T>`], `Arc<T>` uses atomic operations for its reference
63 /// counting This means that it is thread-safe. The disadvantage is that
64 /// atomic operations are more expensive than ordinary memory accesses. If you
65 /// are not sharing reference-counted values between threads, consider using
66 /// [`Rc<T>`] for lower overhead. [`Rc<T>`] is a safe default, because the
67 /// compiler will catch any attempt to send an [`Rc<T>`] between threads.
68 /// However, a library might choose `Arc<T>` in order to give library consumers
71 /// `Arc<T>` will implement [`Send`] and [`Sync`] as long as the `T` implements
72 /// [`Send`] and [`Sync`]. Why can't you put a non-thread-safe type `T` in an
73 /// `Arc<T>` to make it thread-safe? This may be a bit counter-intuitive at
74 /// first: after all, isn't the point of `Arc<T>` thread safety? The key is
75 /// this: `Arc<T>` makes it thread safe to have multiple ownership of the same
76 /// data, but it doesn't add thread safety to its data. Consider
77 /// `Arc<`[`RefCell<T>`]`>`. [`RefCell<T>`] isn't [`Sync`], and if `Arc<T>` was always
78 /// [`Send`], `Arc<`[`RefCell<T>`]`>` would be as well. But then we'd have a problem:
79 /// [`RefCell<T>`] is not thread safe; it keeps track of the borrowing count using
80 /// non-atomic operations.
82 /// In the end, this means that you may need to pair `Arc<T>` with some sort of
83 /// [`std::sync`] type, usually [`Mutex<T>`][mutex].
85 /// ## Breaking cycles with `Weak`
87 /// The [`downgrade`][downgrade] method can be used to create a non-owning
88 /// [`Weak`][weak] pointer. A [`Weak`][weak] pointer can be [`upgrade`][upgrade]d
89 /// to an `Arc`, but this will return [`None`] if the value has already been
92 /// A cycle between `Arc` pointers will never be deallocated. For this reason,
93 /// [`Weak`][weak] is used to break cycles. For example, a tree could have
94 /// strong `Arc` pointers from parent nodes to children, and [`Weak`][weak]
95 /// pointers from children back to their parents.
97 /// # Cloning references
99 /// Creating a new reference from an existing reference counted pointer is done using the
100 /// `Clone` trait implemented for [`Arc<T>`][arc] and [`Weak<T>`][weak].
103 /// use std::sync::Arc;
104 /// let foo = Arc::new(vec![1.0, 2.0, 3.0]);
105 /// // The two syntaxes below are equivalent.
106 /// let a = foo.clone();
107 /// let b = Arc::clone(&foo);
108 /// // a and b both point to the same memory location as foo.
111 /// The [`Arc::clone(&from)`] syntax is the most idiomatic because it conveys more explicitly
112 /// the meaning of the code. In the example above, this syntax makes it easier to see that
113 /// this code is creating a new reference rather than copying the whole content of foo.
115 /// ## `Deref` behavior
117 /// `Arc<T>` automatically dereferences to `T` (via the [`Deref`][deref] trait),
118 /// so you can call `T`'s methods on a value of type `Arc<T>`. To avoid name
119 /// clashes with `T`'s methods, the methods of `Arc<T>` itself are [associated
120 /// functions][assoc], called using function-like syntax:
123 /// use std::sync::Arc;
124 /// let my_arc = Arc::new(());
126 /// Arc::downgrade(&my_arc);
129 /// [`Weak<T>`][weak] does not auto-dereference to `T`, because the value may have
130 /// already been destroyed.
132 /// [arc]: struct.Arc.html
133 /// [weak]: struct.Weak.html
134 /// [`Rc<T>`]: ../../std/rc/struct.Rc.html
135 /// [clone]: ../../std/clone/trait.Clone.html#tymethod.clone
136 /// [mutex]: ../../std/sync/struct.Mutex.html
137 /// [rwlock]: ../../std/sync/struct.RwLock.html
138 /// [atomic]: ../../std/sync/atomic/index.html
139 /// [`Send`]: ../../std/marker/trait.Send.html
140 /// [`Sync`]: ../../std/marker/trait.Sync.html
141 /// [deref]: ../../std/ops/trait.Deref.html
142 /// [downgrade]: struct.Arc.html#method.downgrade
143 /// [upgrade]: struct.Weak.html#method.upgrade
144 /// [`None`]: ../../std/option/enum.Option.html#variant.None
145 /// [assoc]: ../../book/first-edition/method-syntax.html#associated-functions
146 /// [`RefCell<T>`]: ../../std/cell/struct.RefCell.html
147 /// [`std::sync`]: ../../std/sync/index.html
148 /// [`Arc::clone(&from)`]: #method.clone
152 /// Sharing some immutable data between threads:
154 // Note that we **do not** run these tests here. The windows builders get super
155 // unhappy if a thread outlives the main thread and then exits at the same time
156 // (something deadlocks) so we just avoid this entirely by not running these
159 /// use std::sync::Arc;
162 /// let five = Arc::new(5);
165 /// let five = Arc::clone(&five);
167 /// thread::spawn(move || {
168 /// println!("{:?}", five);
173 /// Sharing a mutable [`AtomicUsize`]:
175 /// [`AtomicUsize`]: ../../std/sync/atomic/struct.AtomicUsize.html
178 /// use std::sync::Arc;
179 /// use std::sync::atomic::{AtomicUsize, Ordering};
182 /// let val = Arc::new(AtomicUsize::new(5));
185 /// let val = Arc::clone(&val);
187 /// thread::spawn(move || {
188 /// let v = val.fetch_add(1, Ordering::SeqCst);
189 /// println!("{:?}", v);
194 /// See the [`rc` documentation][rc_examples] for more examples of reference
195 /// counting in general.
197 /// [rc_examples]: ../../std/rc/index.html#examples
198 #[stable(feature = "rust1", since = "1.0.0")]
199 pub struct Arc<T: ?Sized> {
200 ptr: Shared<ArcInner<T>>,
203 #[stable(feature = "rust1", since = "1.0.0")]
204 unsafe impl<T: ?Sized + Sync + Send> Send for Arc<T> {}
205 #[stable(feature = "rust1", since = "1.0.0")]
206 unsafe impl<T: ?Sized + Sync + Send> Sync for Arc<T> {}
208 #[unstable(feature = "coerce_unsized", issue = "27732")]
209 impl<T: ?Sized + Unsize<U>, U: ?Sized> CoerceUnsized<Arc<U>> for Arc<T> {}
211 /// `Weak` is a version of [`Arc`] that holds a non-owning reference to the
212 /// managed value. The value is accessed by calling [`upgrade`] on the `Weak`
213 /// pointer, which returns an [`Option`]`<`[`Arc`]`<T>>`.
215 /// Since a `Weak` reference does not count towards ownership, it will not
216 /// prevent the inner value from being dropped, and `Weak` itself makes no
217 /// guarantees about the value still being present and may return [`None`]
218 /// when [`upgrade`]d.
220 /// A `Weak` pointer is useful for keeping a temporary reference to the value
221 /// within [`Arc`] without extending its lifetime. It is also used to prevent
222 /// circular references between [`Arc`] pointers, since mutual owning references
223 /// would never allow either [`Arc`] to be dropped. For example, a tree could
224 /// have strong [`Arc`] pointers from parent nodes to children, and `Weak`
225 /// pointers from children back to their parents.
227 /// The typical way to obtain a `Weak` pointer is to call [`Arc::downgrade`].
229 /// [`Arc`]: struct.Arc.html
230 /// [`Arc::downgrade`]: struct.Arc.html#method.downgrade
231 /// [`upgrade`]: struct.Weak.html#method.upgrade
232 /// [`Option`]: ../../std/option/enum.Option.html
233 /// [`None`]: ../../std/option/enum.Option.html#variant.None
234 #[stable(feature = "arc_weak", since = "1.4.0")]
235 pub struct Weak<T: ?Sized> {
236 ptr: Shared<ArcInner<T>>,
239 #[stable(feature = "arc_weak", since = "1.4.0")]
240 unsafe impl<T: ?Sized + Sync + Send> Send for Weak<T> {}
241 #[stable(feature = "arc_weak", since = "1.4.0")]
242 unsafe impl<T: ?Sized + Sync + Send> Sync for Weak<T> {}
244 #[unstable(feature = "coerce_unsized", issue = "27732")]
245 impl<T: ?Sized + Unsize<U>, U: ?Sized> CoerceUnsized<Weak<U>> for Weak<T> {}
247 #[stable(feature = "arc_weak", since = "1.4.0")]
248 impl<T: ?Sized + fmt::Debug> fmt::Debug for Weak<T> {
249 fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
254 struct ArcInner<T: ?Sized> {
255 strong: atomic::AtomicUsize,
257 // the value usize::MAX acts as a sentinel for temporarily "locking" the
258 // ability to upgrade weak pointers or downgrade strong ones; this is used
259 // to avoid races in `make_mut` and `get_mut`.
260 weak: atomic::AtomicUsize,
265 unsafe impl<T: ?Sized + Sync + Send> Send for ArcInner<T> {}
266 unsafe impl<T: ?Sized + Sync + Send> Sync for ArcInner<T> {}
269 /// Constructs a new `Arc<T>`.
274 /// use std::sync::Arc;
276 /// let five = Arc::new(5);
279 #[stable(feature = "rust1", since = "1.0.0")]
280 pub fn new(data: T) -> Arc<T> {
281 // Start the weak pointer count as 1 which is the weak pointer that's
282 // held by all the strong pointers (kinda), see std/rc.rs for more info
283 let x: Box<_> = box ArcInner {
284 strong: atomic::AtomicUsize::new(1),
285 weak: atomic::AtomicUsize::new(1),
288 Arc { ptr: Shared::from(Box::into_unique(x)) }
291 /// Returns the contained value, if the `Arc` has exactly one strong reference.
293 /// Otherwise, an [`Err`][result] is returned with the same `Arc` that was
296 /// This will succeed even if there are outstanding weak references.
298 /// [result]: ../../std/result/enum.Result.html
303 /// use std::sync::Arc;
305 /// let x = Arc::new(3);
306 /// assert_eq!(Arc::try_unwrap(x), Ok(3));
308 /// let x = Arc::new(4);
309 /// let _y = Arc::clone(&x);
310 /// assert_eq!(*Arc::try_unwrap(x).unwrap_err(), 4);
313 #[stable(feature = "arc_unique", since = "1.4.0")]
314 pub fn try_unwrap(this: Self) -> Result<T, Self> {
315 // See `drop` for why all these atomics are like this
316 if this.inner().strong.compare_exchange(1, 0, Release, Relaxed).is_err() {
320 atomic::fence(Acquire);
323 let elem = ptr::read(&this.ptr.as_ref().data);
325 // Make a weak pointer to clean up the implicit strong-weak reference
326 let _weak = Weak { ptr: this.ptr };
334 impl<T: ?Sized> Arc<T> {
335 /// Consumes the `Arc`, returning the wrapped pointer.
337 /// To avoid a memory leak the pointer must be converted back to an `Arc` using
338 /// [`Arc::from_raw`][from_raw].
340 /// [from_raw]: struct.Arc.html#method.from_raw
345 /// use std::sync::Arc;
347 /// let x = Arc::new(10);
348 /// let x_ptr = Arc::into_raw(x);
349 /// assert_eq!(unsafe { *x_ptr }, 10);
351 #[stable(feature = "rc_raw", since = "1.17.0")]
352 pub fn into_raw(this: Self) -> *const T {
353 let ptr: *const T = &*this;
358 /// Constructs an `Arc` from a raw pointer.
360 /// The raw pointer must have been previously returned by a call to a
361 /// [`Arc::into_raw`][into_raw].
363 /// This function is unsafe because improper use may lead to memory problems. For example, a
364 /// double-free may occur if the function is called twice on the same raw pointer.
366 /// [into_raw]: struct.Arc.html#method.into_raw
371 /// use std::sync::Arc;
373 /// let x = Arc::new(10);
374 /// let x_ptr = Arc::into_raw(x);
377 /// // Convert back to an `Arc` to prevent leak.
378 /// let x = Arc::from_raw(x_ptr);
379 /// assert_eq!(*x, 10);
381 /// // Further calls to `Arc::from_raw(x_ptr)` would be memory unsafe.
384 /// // The memory was freed when `x` went out of scope above, so `x_ptr` is now dangling!
386 #[stable(feature = "rc_raw", since = "1.17.0")]
387 pub unsafe fn from_raw(ptr: *const T) -> Self {
388 // Align the unsized value to the end of the ArcInner.
389 // Because it is ?Sized, it will always be the last field in memory.
390 let align = align_of_val(&*ptr);
391 let layout = Layout::new::<ArcInner<()>>();
392 let offset = (layout.size() + layout.padding_needed_for(align)) as isize;
394 // Reverse the offset to find the original ArcInner.
395 let fake_ptr = ptr as *mut ArcInner<T>;
396 let arc_ptr = set_data_ptr(fake_ptr, (ptr as *mut u8).offset(-offset));
399 ptr: Shared::new_unchecked(arc_ptr),
403 /// Creates a new [`Weak`][weak] pointer to this value.
405 /// [weak]: struct.Weak.html
410 /// use std::sync::Arc;
412 /// let five = Arc::new(5);
414 /// let weak_five = Arc::downgrade(&five);
416 #[stable(feature = "arc_weak", since = "1.4.0")]
417 pub fn downgrade(this: &Self) -> Weak<T> {
418 // This Relaxed is OK because we're checking the value in the CAS
420 let mut cur = this.inner().weak.load(Relaxed);
423 // check if the weak counter is currently "locked"; if so, spin.
424 if cur == usize::MAX {
425 cur = this.inner().weak.load(Relaxed);
429 // NOTE: this code currently ignores the possibility of overflow
430 // into usize::MAX; in general both Rc and Arc need to be adjusted
431 // to deal with overflow.
433 // Unlike with Clone(), we need this to be an Acquire read to
434 // synchronize with the write coming from `is_unique`, so that the
435 // events prior to that write happen before this read.
436 match this.inner().weak.compare_exchange_weak(cur, cur + 1, Acquire, Relaxed) {
437 Ok(_) => return Weak { ptr: this.ptr },
438 Err(old) => cur = old,
443 /// Gets the number of [`Weak`][weak] pointers to this value.
445 /// [weak]: struct.Weak.html
449 /// This method by itself is safe, but using it correctly requires extra care.
450 /// Another thread can change the weak count at any time,
451 /// including potentially between calling this method and acting on the result.
456 /// use std::sync::Arc;
458 /// let five = Arc::new(5);
459 /// let _weak_five = Arc::downgrade(&five);
461 /// // This assertion is deterministic because we haven't shared
462 /// // the `Arc` or `Weak` between threads.
463 /// assert_eq!(1, Arc::weak_count(&five));
466 #[stable(feature = "arc_counts", since = "1.15.0")]
467 pub fn weak_count(this: &Self) -> usize {
468 let cnt = this.inner().weak.load(SeqCst);
469 // If the weak count is currently locked, the value of the
470 // count was 0 just before taking the lock.
471 if cnt == usize::MAX { 0 } else { cnt - 1 }
474 /// Gets the number of strong (`Arc`) pointers to this value.
478 /// This method by itself is safe, but using it correctly requires extra care.
479 /// Another thread can change the strong count at any time,
480 /// including potentially between calling this method and acting on the result.
485 /// use std::sync::Arc;
487 /// let five = Arc::new(5);
488 /// let _also_five = Arc::clone(&five);
490 /// // This assertion is deterministic because we haven't shared
491 /// // the `Arc` between threads.
492 /// assert_eq!(2, Arc::strong_count(&five));
495 #[stable(feature = "arc_counts", since = "1.15.0")]
496 pub fn strong_count(this: &Self) -> usize {
497 this.inner().strong.load(SeqCst)
501 fn inner(&self) -> &ArcInner<T> {
502 // This unsafety is ok because while this arc is alive we're guaranteed
503 // that the inner pointer is valid. Furthermore, we know that the
504 // `ArcInner` structure itself is `Sync` because the inner data is
505 // `Sync` as well, so we're ok loaning out an immutable pointer to these
507 unsafe { self.ptr.as_ref() }
510 // Non-inlined part of `drop`.
512 unsafe fn drop_slow(&mut self) {
513 let ptr = self.ptr.as_ptr();
515 // Destroy the data at this time, even though we may not free the box
516 // allocation itself (there may still be weak pointers lying around).
517 ptr::drop_in_place(&mut self.ptr.as_mut().data);
519 if self.inner().weak.fetch_sub(1, Release) == 1 {
520 atomic::fence(Acquire);
521 Heap.dealloc(ptr as *mut u8, Layout::for_value(&*ptr))
526 #[stable(feature = "ptr_eq", since = "1.17.0")]
527 /// Returns true if the two `Arc`s point to the same value (not
528 /// just values that compare as equal).
533 /// use std::sync::Arc;
535 /// let five = Arc::new(5);
536 /// let same_five = Arc::clone(&five);
537 /// let other_five = Arc::new(5);
539 /// assert!(Arc::ptr_eq(&five, &same_five));
540 /// assert!(!Arc::ptr_eq(&five, &other_five));
542 pub fn ptr_eq(this: &Self, other: &Self) -> bool {
543 this.ptr.as_ptr() == other.ptr.as_ptr()
547 impl<T: ?Sized> Arc<T> {
548 // Allocates an `ArcInner<T>` with sufficient space for an unsized value
549 unsafe fn allocate_for_ptr(ptr: *const T) -> *mut ArcInner<T> {
550 // Create a fake ArcInner to find allocation size and alignment
551 let fake_ptr = ptr as *mut ArcInner<T>;
553 let layout = Layout::for_value(&*fake_ptr);
555 let mem = Heap.alloc(layout)
556 .unwrap_or_else(|e| Heap.oom(e));
558 // Initialize the real ArcInner
559 let inner = set_data_ptr(ptr as *mut T, mem) as *mut ArcInner<T>;
561 ptr::write(&mut (*inner).strong, atomic::AtomicUsize::new(1));
562 ptr::write(&mut (*inner).weak, atomic::AtomicUsize::new(1));
567 fn from_box(v: Box<T>) -> Arc<T> {
569 let bptr = Box::into_raw(v);
571 let value_size = size_of_val(&*bptr);
572 let ptr = Self::allocate_for_ptr(bptr);
574 // Copy value as bytes
575 ptr::copy_nonoverlapping(
576 bptr as *const T as *const u8,
577 &mut (*ptr).data as *mut _ as *mut u8,
580 // Free the allocation without dropping its contents
583 Arc { ptr: Shared::new_unchecked(ptr) }
588 // Sets the data pointer of a `?Sized` raw pointer.
590 // For a slice/trait object, this sets the `data` field and leaves the rest
591 // unchanged. For a sized raw pointer, this simply sets the pointer.
592 unsafe fn set_data_ptr<T: ?Sized, U>(mut ptr: *mut T, data: *mut U) -> *mut T {
593 ptr::write(&mut ptr as *mut _ as *mut *mut u8, data as *mut u8);
598 // Copy elements from slice into newly allocated Arc<[T]>
600 // Unsafe because the caller must either take ownership or bind `T: Copy`
601 unsafe fn copy_from_slice(v: &[T]) -> Arc<[T]> {
602 let v_ptr = v as *const [T];
603 let ptr = Self::allocate_for_ptr(v_ptr);
605 ptr::copy_nonoverlapping(
607 &mut (*ptr).data as *mut [T] as *mut T,
610 Arc { ptr: Shared::new_unchecked(ptr) }
614 // Specialization trait used for From<&[T]>
615 trait ArcFromSlice<T> {
616 fn from_slice(slice: &[T]) -> Self;
619 impl<T: Clone> ArcFromSlice<T> for Arc<[T]> {
621 default fn from_slice(v: &[T]) -> Self {
622 // Panic guard while cloning T elements.
623 // In the event of a panic, elements that have been written
624 // into the new ArcInner will be dropped, then the memory freed.
632 impl<T> Drop for Guard<T> {
634 use core::slice::from_raw_parts_mut;
637 let slice = from_raw_parts_mut(self.elems, self.n_elems);
638 ptr::drop_in_place(slice);
640 Heap.dealloc(self.mem, self.layout.clone());
646 let v_ptr = v as *const [T];
647 let ptr = Self::allocate_for_ptr(v_ptr);
649 let mem = ptr as *mut _ as *mut u8;
650 let layout = Layout::for_value(&*ptr);
652 // Pointer to first element
653 let elems = &mut (*ptr).data as *mut [T] as *mut T;
655 let mut guard = Guard{
662 for (i, item) in v.iter().enumerate() {
663 ptr::write(elems.offset(i as isize), item.clone());
667 // All clear. Forget the guard so it doesn't free the new ArcInner.
670 Arc { ptr: Shared::new_unchecked(ptr) }
675 impl<T: Copy> ArcFromSlice<T> for Arc<[T]> {
677 fn from_slice(v: &[T]) -> Self {
678 unsafe { Arc::copy_from_slice(v) }
682 #[stable(feature = "rust1", since = "1.0.0")]
683 impl<T: ?Sized> Clone for Arc<T> {
684 /// Makes a clone of the `Arc` pointer.
686 /// This creates another pointer to the same inner value, increasing the
687 /// strong reference count.
692 /// use std::sync::Arc;
694 /// let five = Arc::new(5);
696 /// Arc::clone(&five);
699 fn clone(&self) -> Arc<T> {
700 // Using a relaxed ordering is alright here, as knowledge of the
701 // original reference prevents other threads from erroneously deleting
704 // As explained in the [Boost documentation][1], Increasing the
705 // reference counter can always be done with memory_order_relaxed: New
706 // references to an object can only be formed from an existing
707 // reference, and passing an existing reference from one thread to
708 // another must already provide any required synchronization.
710 // [1]: (www.boost.org/doc/libs/1_55_0/doc/html/atomic/usage_examples.html)
711 let old_size = self.inner().strong.fetch_add(1, Relaxed);
713 // However we need to guard against massive refcounts in case someone
714 // is `mem::forget`ing Arcs. If we don't do this the count can overflow
715 // and users will use-after free. We racily saturate to `isize::MAX` on
716 // the assumption that there aren't ~2 billion threads incrementing
717 // the reference count at once. This branch will never be taken in
718 // any realistic program.
720 // We abort because such a program is incredibly degenerate, and we
721 // don't care to support it.
722 if old_size > MAX_REFCOUNT {
728 Arc { ptr: self.ptr }
732 #[stable(feature = "rust1", since = "1.0.0")]
733 impl<T: ?Sized> Deref for Arc<T> {
737 fn deref(&self) -> &T {
742 impl<T: Clone> Arc<T> {
743 /// Makes a mutable reference into the given `Arc`.
745 /// If there are other `Arc` or [`Weak`][weak] pointers to the same value,
746 /// then `make_mut` will invoke [`clone`][clone] on the inner value to
747 /// ensure unique ownership. This is also referred to as clone-on-write.
749 /// See also [`get_mut`][get_mut], which will fail rather than cloning.
751 /// [weak]: struct.Weak.html
752 /// [clone]: ../../std/clone/trait.Clone.html#tymethod.clone
753 /// [get_mut]: struct.Arc.html#method.get_mut
758 /// use std::sync::Arc;
760 /// let mut data = Arc::new(5);
762 /// *Arc::make_mut(&mut data) += 1; // Won't clone anything
763 /// let mut other_data = Arc::clone(&data); // Won't clone inner data
764 /// *Arc::make_mut(&mut data) += 1; // Clones inner data
765 /// *Arc::make_mut(&mut data) += 1; // Won't clone anything
766 /// *Arc::make_mut(&mut other_data) *= 2; // Won't clone anything
768 /// // Now `data` and `other_data` point to different values.
769 /// assert_eq!(*data, 8);
770 /// assert_eq!(*other_data, 12);
773 #[stable(feature = "arc_unique", since = "1.4.0")]
774 pub fn make_mut(this: &mut Self) -> &mut T {
775 // Note that we hold both a strong reference and a weak reference.
776 // Thus, releasing our strong reference only will not, by itself, cause
777 // the memory to be deallocated.
779 // Use Acquire to ensure that we see any writes to `weak` that happen
780 // before release writes (i.e., decrements) to `strong`. Since we hold a
781 // weak count, there's no chance the ArcInner itself could be
783 if this.inner().strong.compare_exchange(1, 0, Acquire, Relaxed).is_err() {
784 // Another strong pointer exists; clone
785 *this = Arc::new((**this).clone());
786 } else if this.inner().weak.load(Relaxed) != 1 {
787 // Relaxed suffices in the above because this is fundamentally an
788 // optimization: we are always racing with weak pointers being
789 // dropped. Worst case, we end up allocated a new Arc unnecessarily.
791 // We removed the last strong ref, but there are additional weak
792 // refs remaining. We'll move the contents to a new Arc, and
793 // invalidate the other weak refs.
795 // Note that it is not possible for the read of `weak` to yield
796 // usize::MAX (i.e., locked), since the weak count can only be
797 // locked by a thread with a strong reference.
799 // Materialize our own implicit weak pointer, so that it can clean
800 // up the ArcInner as needed.
801 let weak = Weak { ptr: this.ptr };
803 // mark the data itself as already deallocated
805 // there is no data race in the implicit write caused by `read`
806 // here (due to zeroing) because data is no longer accessed by
807 // other threads (due to there being no more strong refs at this
809 let mut swap = Arc::new(ptr::read(&weak.ptr.as_ref().data));
810 mem::swap(this, &mut swap);
814 // We were the sole reference of either kind; bump back up the
816 this.inner().strong.store(1, Release);
819 // As with `get_mut()`, the unsafety is ok because our reference was
820 // either unique to begin with, or became one upon cloning the contents.
822 &mut this.ptr.as_mut().data
827 impl<T: ?Sized> Arc<T> {
828 /// Returns a mutable reference to the inner value, if there are
829 /// no other `Arc` or [`Weak`][weak] pointers to the same value.
831 /// Returns [`None`][option] otherwise, because it is not safe to
832 /// mutate a shared value.
834 /// See also [`make_mut`][make_mut], which will [`clone`][clone]
835 /// the inner value when it's shared.
837 /// [weak]: struct.Weak.html
838 /// [option]: ../../std/option/enum.Option.html
839 /// [make_mut]: struct.Arc.html#method.make_mut
840 /// [clone]: ../../std/clone/trait.Clone.html#tymethod.clone
845 /// use std::sync::Arc;
847 /// let mut x = Arc::new(3);
848 /// *Arc::get_mut(&mut x).unwrap() = 4;
849 /// assert_eq!(*x, 4);
851 /// let _y = Arc::clone(&x);
852 /// assert!(Arc::get_mut(&mut x).is_none());
855 #[stable(feature = "arc_unique", since = "1.4.0")]
856 pub fn get_mut(this: &mut Self) -> Option<&mut T> {
857 if this.is_unique() {
858 // This unsafety is ok because we're guaranteed that the pointer
859 // returned is the *only* pointer that will ever be returned to T. Our
860 // reference count is guaranteed to be 1 at this point, and we required
861 // the Arc itself to be `mut`, so we're returning the only possible
862 // reference to the inner data.
864 Some(&mut this.ptr.as_mut().data)
871 /// Determine whether this is the unique reference (including weak refs) to
872 /// the underlying data.
874 /// Note that this requires locking the weak ref count.
875 fn is_unique(&mut self) -> bool {
876 // lock the weak pointer count if we appear to be the sole weak pointer
879 // The acquire label here ensures a happens-before relationship with any
880 // writes to `strong` prior to decrements of the `weak` count (via drop,
881 // which uses Release).
882 if self.inner().weak.compare_exchange(1, usize::MAX, Acquire, Relaxed).is_ok() {
883 // Due to the previous acquire read, this will observe any writes to
884 // `strong` that were due to upgrading weak pointers; only strong
885 // clones remain, which require that the strong count is > 1 anyway.
886 let unique = self.inner().strong.load(Relaxed) == 1;
888 // The release write here synchronizes with a read in `downgrade`,
889 // effectively preventing the above read of `strong` from happening
891 self.inner().weak.store(1, Release); // release the lock
899 #[stable(feature = "rust1", since = "1.0.0")]
900 unsafe impl<#[may_dangle] T: ?Sized> Drop for Arc<T> {
903 /// This will decrement the strong reference count. If the strong reference
904 /// count reaches zero then the only other references (if any) are
905 /// [`Weak`][weak], so we `drop` the inner value.
907 /// [weak]: struct.Weak.html
912 /// use std::sync::Arc;
916 /// impl Drop for Foo {
917 /// fn drop(&mut self) {
918 /// println!("dropped!");
922 /// let foo = Arc::new(Foo);
923 /// let foo2 = Arc::clone(&foo);
925 /// drop(foo); // Doesn't print anything
926 /// drop(foo2); // Prints "dropped!"
930 // Because `fetch_sub` is already atomic, we do not need to synchronize
931 // with other threads unless we are going to delete the object. This
932 // same logic applies to the below `fetch_sub` to the `weak` count.
933 if self.inner().strong.fetch_sub(1, Release) != 1 {
937 // This fence is needed to prevent reordering of use of the data and
938 // deletion of the data. Because it is marked `Release`, the decreasing
939 // of the reference count synchronizes with this `Acquire` fence. This
940 // means that use of the data happens before decreasing the reference
941 // count, which happens before this fence, which happens before the
942 // deletion of the data.
944 // As explained in the [Boost documentation][1],
946 // > It is important to enforce any possible access to the object in one
947 // > thread (through an existing reference) to *happen before* deleting
948 // > the object in a different thread. This is achieved by a "release"
949 // > operation after dropping a reference (any access to the object
950 // > through this reference must obviously happened before), and an
951 // > "acquire" operation before deleting the object.
953 // In particular, while the contents of an Arc are usually immutable, it's
954 // possible to have interior writes to something like a Mutex<T>. Since a
955 // Mutex is not acquired when it is deleted, we can't rely on its
956 // synchronization logic to make writes in thread A visible to a destructor
957 // running in thread B.
959 // Also note that the Acquire fence here could probably be replaced with an
960 // Acquire load, which could improve performance in highly-contended
961 // situations. See [2].
963 // [1]: (www.boost.org/doc/libs/1_55_0/doc/html/atomic/usage_examples.html)
964 // [2]: (https://github.com/rust-lang/rust/pull/41714)
965 atomic::fence(Acquire);
974 /// Constructs a new `Weak<T>`, allocating memory for `T` without initializing
975 /// it. Calling [`upgrade`] on the return value always gives [`None`].
977 /// [`upgrade`]: struct.Weak.html#method.upgrade
978 /// [`None`]: ../../std/option/enum.Option.html#variant.None
983 /// use std::sync::Weak;
985 /// let empty: Weak<i64> = Weak::new();
986 /// assert!(empty.upgrade().is_none());
988 #[stable(feature = "downgraded_weak", since = "1.10.0")]
989 pub fn new() -> Weak<T> {
992 ptr: Shared::from(Box::into_unique(box ArcInner {
993 strong: atomic::AtomicUsize::new(0),
994 weak: atomic::AtomicUsize::new(1),
995 data: uninitialized(),
1002 impl<T: ?Sized> Weak<T> {
1003 /// Attempts to upgrade the `Weak` pointer to an [`Arc`], extending
1004 /// the lifetime of the value if successful.
1006 /// Returns [`None`] if the value has since been dropped.
1008 /// [`Arc`]: struct.Arc.html
1009 /// [`None`]: ../../std/option/enum.Option.html#variant.None
1014 /// use std::sync::Arc;
1016 /// let five = Arc::new(5);
1018 /// let weak_five = Arc::downgrade(&five);
1020 /// let strong_five: Option<Arc<_>> = weak_five.upgrade();
1021 /// assert!(strong_five.is_some());
1023 /// // Destroy all strong pointers.
1024 /// drop(strong_five);
1027 /// assert!(weak_five.upgrade().is_none());
1029 #[stable(feature = "arc_weak", since = "1.4.0")]
1030 pub fn upgrade(&self) -> Option<Arc<T>> {
1031 // We use a CAS loop to increment the strong count instead of a
1032 // fetch_add because once the count hits 0 it must never be above 0.
1033 let inner = self.inner();
1035 // Relaxed load because any write of 0 that we can observe
1036 // leaves the field in a permanently zero state (so a
1037 // "stale" read of 0 is fine), and any other value is
1038 // confirmed via the CAS below.
1039 let mut n = inner.strong.load(Relaxed);
1046 // See comments in `Arc::clone` for why we do this (for `mem::forget`).
1047 if n > MAX_REFCOUNT {
1053 // Relaxed is valid for the same reason it is on Arc's Clone impl
1054 match inner.strong.compare_exchange_weak(n, n + 1, Relaxed, Relaxed) {
1055 Ok(_) => return Some(Arc { ptr: self.ptr }),
1056 Err(old) => n = old,
1062 fn inner(&self) -> &ArcInner<T> {
1063 // See comments above for why this is "safe"
1064 unsafe { self.ptr.as_ref() }
1068 #[stable(feature = "arc_weak", since = "1.4.0")]
1069 impl<T: ?Sized> Clone for Weak<T> {
1070 /// Makes a clone of the `Weak` pointer that points to the same value.
1075 /// use std::sync::{Arc, Weak};
1077 /// let weak_five = Arc::downgrade(&Arc::new(5));
1079 /// Weak::clone(&weak_five);
1082 fn clone(&self) -> Weak<T> {
1083 // See comments in Arc::clone() for why this is relaxed. This can use a
1084 // fetch_add (ignoring the lock) because the weak count is only locked
1085 // where are *no other* weak pointers in existence. (So we can't be
1086 // running this code in that case).
1087 let old_size = self.inner().weak.fetch_add(1, Relaxed);
1089 // See comments in Arc::clone() for why we do this (for mem::forget).
1090 if old_size > MAX_REFCOUNT {
1096 return Weak { ptr: self.ptr };
1100 #[stable(feature = "downgraded_weak", since = "1.10.0")]
1101 impl<T> Default for Weak<T> {
1102 /// Constructs a new `Weak<T>`, allocating memory for `T` without initializing
1103 /// it. Calling [`upgrade`] on the return value always gives [`None`].
1105 /// [`upgrade`]: struct.Weak.html#method.upgrade
1106 /// [`None`]: ../../std/option/enum.Option.html#variant.None
1111 /// use std::sync::Weak;
1113 /// let empty: Weak<i64> = Default::default();
1114 /// assert!(empty.upgrade().is_none());
1116 fn default() -> Weak<T> {
1121 #[stable(feature = "arc_weak", since = "1.4.0")]
1122 impl<T: ?Sized> Drop for Weak<T> {
1123 /// Drops the `Weak` pointer.
1128 /// use std::sync::{Arc, Weak};
1132 /// impl Drop for Foo {
1133 /// fn drop(&mut self) {
1134 /// println!("dropped!");
1138 /// let foo = Arc::new(Foo);
1139 /// let weak_foo = Arc::downgrade(&foo);
1140 /// let other_weak_foo = Weak::clone(&weak_foo);
1142 /// drop(weak_foo); // Doesn't print anything
1143 /// drop(foo); // Prints "dropped!"
1145 /// assert!(other_weak_foo.upgrade().is_none());
1147 fn drop(&mut self) {
1148 let ptr = self.ptr.as_ptr();
1150 // If we find out that we were the last weak pointer, then its time to
1151 // deallocate the data entirely. See the discussion in Arc::drop() about
1152 // the memory orderings
1154 // It's not necessary to check for the locked state here, because the
1155 // weak count can only be locked if there was precisely one weak ref,
1156 // meaning that drop could only subsequently run ON that remaining weak
1157 // ref, which can only happen after the lock is released.
1158 if self.inner().weak.fetch_sub(1, Release) == 1 {
1159 atomic::fence(Acquire);
1161 Heap.dealloc(ptr as *mut u8, Layout::for_value(&*ptr))
1167 #[stable(feature = "rust1", since = "1.0.0")]
1168 impl<T: ?Sized + PartialEq> PartialEq for Arc<T> {
1169 /// Equality for two `Arc`s.
1171 /// Two `Arc`s are equal if their inner values are equal.
1176 /// use std::sync::Arc;
1178 /// let five = Arc::new(5);
1180 /// assert!(five == Arc::new(5));
1182 fn eq(&self, other: &Arc<T>) -> bool {
1183 *(*self) == *(*other)
1186 /// Inequality for two `Arc`s.
1188 /// Two `Arc`s are unequal if their inner values are unequal.
1193 /// use std::sync::Arc;
1195 /// let five = Arc::new(5);
1197 /// assert!(five != Arc::new(6));
1199 fn ne(&self, other: &Arc<T>) -> bool {
1200 *(*self) != *(*other)
1203 #[stable(feature = "rust1", since = "1.0.0")]
1204 impl<T: ?Sized + PartialOrd> PartialOrd for Arc<T> {
1205 /// Partial comparison for two `Arc`s.
1207 /// The two are compared by calling `partial_cmp()` on their inner values.
1212 /// use std::sync::Arc;
1213 /// use std::cmp::Ordering;
1215 /// let five = Arc::new(5);
1217 /// assert_eq!(Some(Ordering::Less), five.partial_cmp(&Arc::new(6)));
1219 fn partial_cmp(&self, other: &Arc<T>) -> Option<Ordering> {
1220 (**self).partial_cmp(&**other)
1223 /// Less-than comparison for two `Arc`s.
1225 /// The two are compared by calling `<` on their inner values.
1230 /// use std::sync::Arc;
1232 /// let five = Arc::new(5);
1234 /// assert!(five < Arc::new(6));
1236 fn lt(&self, other: &Arc<T>) -> bool {
1237 *(*self) < *(*other)
1240 /// 'Less than or equal to' comparison for two `Arc`s.
1242 /// The two are compared by calling `<=` on their inner values.
1247 /// use std::sync::Arc;
1249 /// let five = Arc::new(5);
1251 /// assert!(five <= Arc::new(5));
1253 fn le(&self, other: &Arc<T>) -> bool {
1254 *(*self) <= *(*other)
1257 /// Greater-than comparison for two `Arc`s.
1259 /// The two are compared by calling `>` on their inner values.
1264 /// use std::sync::Arc;
1266 /// let five = Arc::new(5);
1268 /// assert!(five > Arc::new(4));
1270 fn gt(&self, other: &Arc<T>) -> bool {
1271 *(*self) > *(*other)
1274 /// 'Greater than or equal to' comparison for two `Arc`s.
1276 /// The two are compared by calling `>=` on their inner values.
1281 /// use std::sync::Arc;
1283 /// let five = Arc::new(5);
1285 /// assert!(five >= Arc::new(5));
1287 fn ge(&self, other: &Arc<T>) -> bool {
1288 *(*self) >= *(*other)
1291 #[stable(feature = "rust1", since = "1.0.0")]
1292 impl<T: ?Sized + Ord> Ord for Arc<T> {
1293 /// Comparison for two `Arc`s.
1295 /// The two are compared by calling `cmp()` on their inner values.
1300 /// use std::sync::Arc;
1301 /// use std::cmp::Ordering;
1303 /// let five = Arc::new(5);
1305 /// assert_eq!(Ordering::Less, five.cmp(&Arc::new(6)));
1307 fn cmp(&self, other: &Arc<T>) -> Ordering {
1308 (**self).cmp(&**other)
1311 #[stable(feature = "rust1", since = "1.0.0")]
1312 impl<T: ?Sized + Eq> Eq for Arc<T> {}
1314 #[stable(feature = "rust1", since = "1.0.0")]
1315 impl<T: ?Sized + fmt::Display> fmt::Display for Arc<T> {
1316 fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
1317 fmt::Display::fmt(&**self, f)
1321 #[stable(feature = "rust1", since = "1.0.0")]
1322 impl<T: ?Sized + fmt::Debug> fmt::Debug for Arc<T> {
1323 fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
1324 fmt::Debug::fmt(&**self, f)
1328 #[stable(feature = "rust1", since = "1.0.0")]
1329 impl<T: ?Sized> fmt::Pointer for Arc<T> {
1330 fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
1331 fmt::Pointer::fmt(&self.ptr, f)
1335 #[stable(feature = "rust1", since = "1.0.0")]
1336 impl<T: Default> Default for Arc<T> {
1337 /// Creates a new `Arc<T>`, with the `Default` value for `T`.
1342 /// use std::sync::Arc;
1344 /// let x: Arc<i32> = Default::default();
1345 /// assert_eq!(*x, 0);
1347 fn default() -> Arc<T> {
1348 Arc::new(Default::default())
1352 #[stable(feature = "rust1", since = "1.0.0")]
1353 impl<T: ?Sized + Hash> Hash for Arc<T> {
1354 fn hash<H: Hasher>(&self, state: &mut H) {
1355 (**self).hash(state)
1359 #[stable(feature = "from_for_ptrs", since = "1.6.0")]
1360 impl<T> From<T> for Arc<T> {
1361 fn from(t: T) -> Self {
1366 #[stable(feature = "shared_from_slice", since = "1.21.0")]
1367 impl<'a, T: Clone> From<&'a [T]> for Arc<[T]> {
1369 fn from(v: &[T]) -> Arc<[T]> {
1370 <Self as ArcFromSlice<T>>::from_slice(v)
1374 #[stable(feature = "shared_from_slice", since = "1.21.0")]
1375 impl<'a> From<&'a str> for Arc<str> {
1377 fn from(v: &str) -> Arc<str> {
1378 unsafe { mem::transmute(<Arc<[u8]>>::from(v.as_bytes())) }
1382 #[stable(feature = "shared_from_slice", since = "1.21.0")]
1383 impl From<String> for Arc<str> {
1385 fn from(v: String) -> Arc<str> {
1390 #[stable(feature = "shared_from_slice", since = "1.21.0")]
1391 impl<T: ?Sized> From<Box<T>> for Arc<T> {
1393 fn from(v: Box<T>) -> Arc<T> {
1398 #[stable(feature = "shared_from_slice", since = "1.21.0")]
1399 impl<T> From<Vec<T>> for Arc<[T]> {
1401 fn from(mut v: Vec<T>) -> Arc<[T]> {
1403 let arc = Arc::copy_from_slice(&v);
1405 // Allow the Vec to free its memory, but not destroy its contents
1415 use std::boxed::Box;
1416 use std::clone::Clone;
1417 use std::sync::mpsc::channel;
1420 use std::option::Option;
1421 use std::option::Option::{None, Some};
1422 use std::sync::atomic;
1423 use std::sync::atomic::Ordering::{Acquire, SeqCst};
1425 use std::sync::Mutex;
1426 use std::convert::From;
1428 use super::{Arc, Weak};
1431 struct Canary(*mut atomic::AtomicUsize);
1433 impl Drop for Canary {
1434 fn drop(&mut self) {
1438 (*c).fetch_add(1, SeqCst);
1446 #[cfg_attr(target_os = "emscripten", ignore)]
1447 fn manually_share_arc() {
1448 let v = vec![1, 2, 3, 4, 5, 6, 7, 8, 9, 10];
1449 let arc_v = Arc::new(v);
1451 let (tx, rx) = channel();
1453 let _t = thread::spawn(move || {
1454 let arc_v: Arc<Vec<i32>> = rx.recv().unwrap();
1455 assert_eq!((*arc_v)[3], 4);
1458 tx.send(arc_v.clone()).unwrap();
1460 assert_eq!((*arc_v)[2], 3);
1461 assert_eq!((*arc_v)[4], 5);
1465 fn test_arc_get_mut() {
1466 let mut x = Arc::new(3);
1467 *Arc::get_mut(&mut x).unwrap() = 4;
1470 assert!(Arc::get_mut(&mut x).is_none());
1472 assert!(Arc::get_mut(&mut x).is_some());
1473 let _w = Arc::downgrade(&x);
1474 assert!(Arc::get_mut(&mut x).is_none());
1479 let x = Arc::new(3);
1480 assert_eq!(Arc::try_unwrap(x), Ok(3));
1481 let x = Arc::new(4);
1483 assert_eq!(Arc::try_unwrap(x), Err(Arc::new(4)));
1484 let x = Arc::new(5);
1485 let _w = Arc::downgrade(&x);
1486 assert_eq!(Arc::try_unwrap(x), Ok(5));
1490 fn into_from_raw() {
1491 let x = Arc::new(box "hello");
1494 let x_ptr = Arc::into_raw(x);
1497 assert_eq!(**x_ptr, "hello");
1499 let x = Arc::from_raw(x_ptr);
1500 assert_eq!(**x, "hello");
1502 assert_eq!(Arc::try_unwrap(x).map(|x| *x), Ok("hello"));
1507 fn test_into_from_raw_unsized() {
1508 use std::fmt::Display;
1509 use std::string::ToString;
1511 let arc: Arc<str> = Arc::from("foo");
1513 let ptr = Arc::into_raw(arc.clone());
1514 let arc2 = unsafe { Arc::from_raw(ptr) };
1516 assert_eq!(unsafe { &*ptr }, "foo");
1517 assert_eq!(arc, arc2);
1519 let arc: Arc<Display> = Arc::new(123);
1521 let ptr = Arc::into_raw(arc.clone());
1522 let arc2 = unsafe { Arc::from_raw(ptr) };
1524 assert_eq!(unsafe { &*ptr }.to_string(), "123");
1525 assert_eq!(arc2.to_string(), "123");
1529 fn test_cowarc_clone_make_mut() {
1530 let mut cow0 = Arc::new(75);
1531 let mut cow1 = cow0.clone();
1532 let mut cow2 = cow1.clone();
1534 assert!(75 == *Arc::make_mut(&mut cow0));
1535 assert!(75 == *Arc::make_mut(&mut cow1));
1536 assert!(75 == *Arc::make_mut(&mut cow2));
1538 *Arc::make_mut(&mut cow0) += 1;
1539 *Arc::make_mut(&mut cow1) += 2;
1540 *Arc::make_mut(&mut cow2) += 3;
1542 assert!(76 == *cow0);
1543 assert!(77 == *cow1);
1544 assert!(78 == *cow2);
1546 // none should point to the same backing memory
1547 assert!(*cow0 != *cow1);
1548 assert!(*cow0 != *cow2);
1549 assert!(*cow1 != *cow2);
1553 fn test_cowarc_clone_unique2() {
1554 let mut cow0 = Arc::new(75);
1555 let cow1 = cow0.clone();
1556 let cow2 = cow1.clone();
1558 assert!(75 == *cow0);
1559 assert!(75 == *cow1);
1560 assert!(75 == *cow2);
1562 *Arc::make_mut(&mut cow0) += 1;
1563 assert!(76 == *cow0);
1564 assert!(75 == *cow1);
1565 assert!(75 == *cow2);
1567 // cow1 and cow2 should share the same contents
1568 // cow0 should have a unique reference
1569 assert!(*cow0 != *cow1);
1570 assert!(*cow0 != *cow2);
1571 assert!(*cow1 == *cow2);
1575 fn test_cowarc_clone_weak() {
1576 let mut cow0 = Arc::new(75);
1577 let cow1_weak = Arc::downgrade(&cow0);
1579 assert!(75 == *cow0);
1580 assert!(75 == *cow1_weak.upgrade().unwrap());
1582 *Arc::make_mut(&mut cow0) += 1;
1584 assert!(76 == *cow0);
1585 assert!(cow1_weak.upgrade().is_none());
1590 let x = Arc::new(5);
1591 let y = Arc::downgrade(&x);
1592 assert!(y.upgrade().is_some());
1597 let x = Arc::new(5);
1598 let y = Arc::downgrade(&x);
1600 assert!(y.upgrade().is_none());
1604 fn weak_self_cyclic() {
1606 x: Mutex<Option<Weak<Cycle>>>,
1609 let a = Arc::new(Cycle { x: Mutex::new(None) });
1610 let b = Arc::downgrade(&a.clone());
1611 *a.x.lock().unwrap() = Some(b);
1613 // hopefully we don't double-free (or leak)...
1618 let mut canary = atomic::AtomicUsize::new(0);
1619 let x = Arc::new(Canary(&mut canary as *mut atomic::AtomicUsize));
1621 assert!(canary.load(Acquire) == 1);
1625 fn drop_arc_weak() {
1626 let mut canary = atomic::AtomicUsize::new(0);
1627 let arc = Arc::new(Canary(&mut canary as *mut atomic::AtomicUsize));
1628 let arc_weak = Arc::downgrade(&arc);
1629 assert!(canary.load(Acquire) == 0);
1631 assert!(canary.load(Acquire) == 1);
1636 fn test_strong_count() {
1637 let a = Arc::new(0);
1638 assert!(Arc::strong_count(&a) == 1);
1639 let w = Arc::downgrade(&a);
1640 assert!(Arc::strong_count(&a) == 1);
1641 let b = w.upgrade().expect("");
1642 assert!(Arc::strong_count(&b) == 2);
1643 assert!(Arc::strong_count(&a) == 2);
1646 assert!(Arc::strong_count(&b) == 1);
1648 assert!(Arc::strong_count(&b) == 2);
1649 assert!(Arc::strong_count(&c) == 2);
1653 fn test_weak_count() {
1654 let a = Arc::new(0);
1655 assert!(Arc::strong_count(&a) == 1);
1656 assert!(Arc::weak_count(&a) == 0);
1657 let w = Arc::downgrade(&a);
1658 assert!(Arc::strong_count(&a) == 1);
1659 assert!(Arc::weak_count(&a) == 1);
1661 assert!(Arc::weak_count(&a) == 2);
1664 assert!(Arc::strong_count(&a) == 1);
1665 assert!(Arc::weak_count(&a) == 0);
1667 assert!(Arc::strong_count(&a) == 2);
1668 assert!(Arc::weak_count(&a) == 0);
1669 let d = Arc::downgrade(&c);
1670 assert!(Arc::weak_count(&c) == 1);
1671 assert!(Arc::strong_count(&c) == 2);
1680 let a = Arc::new(5);
1681 assert_eq!(format!("{:?}", a), "5");
1684 // Make sure deriving works with Arc<T>
1685 #[derive(Eq, Ord, PartialEq, PartialOrd, Clone, Debug, Default)]
1692 let x: Arc<[i32]> = Arc::new([1, 2, 3]);
1693 assert_eq!(format!("{:?}", x), "[1, 2, 3]");
1694 let y = Arc::downgrade(&x.clone());
1696 assert!(y.upgrade().is_none());
1700 fn test_from_owned() {
1702 let foo_arc = Arc::from(foo);
1703 assert!(123 == *foo_arc);
1707 fn test_new_weak() {
1708 let foo: Weak<usize> = Weak::new();
1709 assert!(foo.upgrade().is_none());
1714 let five = Arc::new(5);
1715 let same_five = five.clone();
1716 let other_five = Arc::new(5);
1718 assert!(Arc::ptr_eq(&five, &same_five));
1719 assert!(!Arc::ptr_eq(&five, &other_five));
1723 #[cfg_attr(target_os = "emscripten", ignore)]
1724 fn test_weak_count_locked() {
1725 let mut a = Arc::new(atomic::AtomicBool::new(false));
1727 let t = thread::spawn(move || {
1728 for _i in 0..1000000 {
1729 Arc::get_mut(&mut a);
1731 a.store(true, SeqCst);
1734 while !a2.load(SeqCst) {
1735 let n = Arc::weak_count(&a2);
1736 assert!(n < 2, "bad weak count: {}", n);
1742 fn test_from_str() {
1743 let r: Arc<str> = Arc::from("foo");
1745 assert_eq!(&r[..], "foo");
1749 fn test_copy_from_slice() {
1750 let s: &[u32] = &[1, 2, 3];
1751 let r: Arc<[u32]> = Arc::from(s);
1753 assert_eq!(&r[..], [1, 2, 3]);
1757 fn test_clone_from_slice() {
1758 #[derive(Clone, Debug, Eq, PartialEq)]
1761 let s: &[X] = &[X(1), X(2), X(3)];
1762 let r: Arc<[X]> = Arc::from(s);
1764 assert_eq!(&r[..], s);
1769 fn test_clone_from_slice_panic() {
1770 use std::string::{String, ToString};
1772 struct Fail(u32, String);
1774 impl Clone for Fail {
1775 fn clone(&self) -> Fail {
1779 Fail(self.0, self.1.clone())
1784 Fail(0, "foo".to_string()),
1785 Fail(1, "bar".to_string()),
1786 Fail(2, "baz".to_string()),
1789 // Should panic, but not cause memory corruption
1790 let _r: Arc<[Fail]> = Arc::from(s);
1794 fn test_from_box() {
1795 let b: Box<u32> = box 123;
1796 let r: Arc<u32> = Arc::from(b);
1798 assert_eq!(*r, 123);
1802 fn test_from_box_str() {
1803 use std::string::String;
1805 let s = String::from("foo").into_boxed_str();
1806 let r: Arc<str> = Arc::from(s);
1808 assert_eq!(&r[..], "foo");
1812 fn test_from_box_slice() {
1813 let s = vec![1, 2, 3].into_boxed_slice();
1814 let r: Arc<[u32]> = Arc::from(s);
1816 assert_eq!(&r[..], [1, 2, 3]);
1820 fn test_from_box_trait() {
1821 use std::fmt::Display;
1822 use std::string::ToString;
1824 let b: Box<Display> = box 123;
1825 let r: Arc<Display> = Arc::from(b);
1827 assert_eq!(r.to_string(), "123");
1831 fn test_from_box_trait_zero_sized() {
1832 use std::fmt::Debug;
1834 let b: Box<Debug> = box ();
1835 let r: Arc<Debug> = Arc::from(b);
1837 assert_eq!(format!("{:?}", r), "()");
1841 fn test_from_vec() {
1842 let v = vec![1, 2, 3];
1843 let r: Arc<[u32]> = Arc::from(v);
1845 assert_eq!(&r[..], [1, 2, 3]);
1849 #[stable(feature = "rust1", since = "1.0.0")]
1850 impl<T: ?Sized> borrow::Borrow<T> for Arc<T> {
1851 fn borrow(&self) -> &T {
1856 #[stable(since = "1.5.0", feature = "smart_ptr_as_ref")]
1857 impl<T: ?Sized> AsRef<T> for Arc<T> {
1858 fn as_ref(&self) -> &T {