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>`.
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
147 /// Sharing some immutable data between threads:
149 // Note that we **do not** run these tests here. The windows builders get super
150 // unhappy if a thread outlives the main thread and then exits at the same time
151 // (something deadlocks) so we just avoid this entirely by not running these
154 /// use std::sync::Arc;
157 /// let five = Arc::new(5);
160 /// let five = Arc::clone(&five);
162 /// thread::spawn(move || {
163 /// println!("{:?}", five);
168 /// Sharing a mutable [`AtomicUsize`]:
170 /// [`AtomicUsize`]: ../../std/sync/atomic/struct.AtomicUsize.html
173 /// use std::sync::Arc;
174 /// use std::sync::atomic::{AtomicUsize, Ordering};
177 /// let val = Arc::new(AtomicUsize::new(5));
180 /// let val = Arc::clone(&val);
182 /// thread::spawn(move || {
183 /// let v = val.fetch_add(1, Ordering::SeqCst);
184 /// println!("{:?}", v);
189 /// See the [`rc` documentation][rc_examples] for more examples of reference
190 /// counting in general.
192 /// [rc_examples]: ../../std/rc/index.html#examples
193 #[stable(feature = "rust1", since = "1.0.0")]
194 pub struct Arc<T: ?Sized> {
195 ptr: Shared<ArcInner<T>>,
198 #[stable(feature = "rust1", since = "1.0.0")]
199 unsafe impl<T: ?Sized + Sync + Send> Send for Arc<T> {}
200 #[stable(feature = "rust1", since = "1.0.0")]
201 unsafe impl<T: ?Sized + Sync + Send> Sync for Arc<T> {}
203 #[unstable(feature = "coerce_unsized", issue = "27732")]
204 impl<T: ?Sized + Unsize<U>, U: ?Sized> CoerceUnsized<Arc<U>> for Arc<T> {}
206 /// `Weak` is a version of [`Arc`] that holds a non-owning reference to the
207 /// managed value. The value is accessed by calling [`upgrade`] on the `Weak`
208 /// pointer, which returns an [`Option`]`<`[`Arc`]`<T>>`.
210 /// Since a `Weak` reference does not count towards ownership, it will not
211 /// prevent the inner value from being dropped, and `Weak` itself makes no
212 /// guarantees about the value still being present and may return [`None`]
213 /// when [`upgrade`]d.
215 /// A `Weak` pointer is useful for keeping a temporary reference to the value
216 /// within [`Arc`] without extending its lifetime. It is also used to prevent
217 /// circular references between [`Arc`] pointers, since mutual owning references
218 /// would never allow either [`Arc`] to be dropped. For example, a tree could
219 /// have strong [`Arc`] pointers from parent nodes to children, and `Weak`
220 /// pointers from children back to their parents.
222 /// The typical way to obtain a `Weak` pointer is to call [`Arc::downgrade`].
224 /// [`Arc`]: struct.Arc.html
225 /// [`Arc::downgrade`]: struct.Arc.html#method.downgrade
226 /// [`upgrade`]: struct.Weak.html#method.upgrade
227 /// [`Option`]: ../../std/option/enum.Option.html
228 /// [`None`]: ../../std/option/enum.Option.html#variant.None
229 #[stable(feature = "arc_weak", since = "1.4.0")]
230 pub struct Weak<T: ?Sized> {
231 ptr: Shared<ArcInner<T>>,
234 #[stable(feature = "arc_weak", since = "1.4.0")]
235 unsafe impl<T: ?Sized + Sync + Send> Send for Weak<T> {}
236 #[stable(feature = "arc_weak", since = "1.4.0")]
237 unsafe impl<T: ?Sized + Sync + Send> Sync for Weak<T> {}
239 #[unstable(feature = "coerce_unsized", issue = "27732")]
240 impl<T: ?Sized + Unsize<U>, U: ?Sized> CoerceUnsized<Weak<U>> for Weak<T> {}
242 #[stable(feature = "arc_weak", since = "1.4.0")]
243 impl<T: ?Sized + fmt::Debug> fmt::Debug for Weak<T> {
244 fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
249 struct ArcInner<T: ?Sized> {
250 strong: atomic::AtomicUsize,
252 // the value usize::MAX acts as a sentinel for temporarily "locking" the
253 // ability to upgrade weak pointers or downgrade strong ones; this is used
254 // to avoid races in `make_mut` and `get_mut`.
255 weak: atomic::AtomicUsize,
260 unsafe impl<T: ?Sized + Sync + Send> Send for ArcInner<T> {}
261 unsafe impl<T: ?Sized + Sync + Send> Sync for ArcInner<T> {}
264 /// Constructs a new `Arc<T>`.
269 /// use std::sync::Arc;
271 /// let five = Arc::new(5);
274 #[stable(feature = "rust1", since = "1.0.0")]
275 pub fn new(data: T) -> Arc<T> {
276 // Start the weak pointer count as 1 which is the weak pointer that's
277 // held by all the strong pointers (kinda), see std/rc.rs for more info
278 let x: Box<_> = box ArcInner {
279 strong: atomic::AtomicUsize::new(1),
280 weak: atomic::AtomicUsize::new(1),
283 Arc { ptr: Shared::from(Box::into_unique(x)) }
286 /// Returns the contained value, if the `Arc` has exactly one strong reference.
288 /// Otherwise, an [`Err`][result] is returned with the same `Arc` that was
291 /// This will succeed even if there are outstanding weak references.
293 /// [result]: ../../std/result/enum.Result.html
298 /// use std::sync::Arc;
300 /// let x = Arc::new(3);
301 /// assert_eq!(Arc::try_unwrap(x), Ok(3));
303 /// let x = Arc::new(4);
304 /// let _y = Arc::clone(&x);
305 /// assert_eq!(*Arc::try_unwrap(x).unwrap_err(), 4);
308 #[stable(feature = "arc_unique", since = "1.4.0")]
309 pub fn try_unwrap(this: Self) -> Result<T, Self> {
310 // See `drop` for why all these atomics are like this
311 if this.inner().strong.compare_exchange(1, 0, Release, Relaxed).is_err() {
315 atomic::fence(Acquire);
318 let elem = ptr::read(&this.ptr.as_ref().data);
320 // Make a weak pointer to clean up the implicit strong-weak reference
321 let _weak = Weak { ptr: this.ptr };
329 impl<T: ?Sized> Arc<T> {
330 /// Consumes the `Arc`, returning the wrapped pointer.
332 /// To avoid a memory leak the pointer must be converted back to an `Arc` using
333 /// [`Arc::from_raw`][from_raw].
335 /// [from_raw]: struct.Arc.html#method.from_raw
340 /// use std::sync::Arc;
342 /// let x = Arc::new(10);
343 /// let x_ptr = Arc::into_raw(x);
344 /// assert_eq!(unsafe { *x_ptr }, 10);
346 #[stable(feature = "rc_raw", since = "1.17.0")]
347 pub fn into_raw(this: Self) -> *const T {
348 let ptr: *const T = &*this;
353 /// Constructs an `Arc` from a raw pointer.
355 /// The raw pointer must have been previously returned by a call to a
356 /// [`Arc::into_raw`][into_raw].
358 /// This function is unsafe because improper use may lead to memory problems. For example, a
359 /// double-free may occur if the function is called twice on the same raw pointer.
361 /// [into_raw]: struct.Arc.html#method.into_raw
366 /// use std::sync::Arc;
368 /// let x = Arc::new(10);
369 /// let x_ptr = Arc::into_raw(x);
372 /// // Convert back to an `Arc` to prevent leak.
373 /// let x = Arc::from_raw(x_ptr);
374 /// assert_eq!(*x, 10);
376 /// // Further calls to `Arc::from_raw(x_ptr)` would be memory unsafe.
379 /// // The memory was freed when `x` went out of scope above, so `x_ptr` is now dangling!
381 #[stable(feature = "rc_raw", since = "1.17.0")]
382 pub unsafe fn from_raw(ptr: *const T) -> Self {
383 // Align the unsized value to the end of the ArcInner.
384 // Because it is ?Sized, it will always be the last field in memory.
385 let align = align_of_val(&*ptr);
386 let layout = Layout::new::<ArcInner<()>>();
387 let offset = (layout.size() + layout.padding_needed_for(align)) as isize;
389 // Reverse the offset to find the original ArcInner.
390 let fake_ptr = ptr as *mut ArcInner<T>;
391 let arc_ptr = set_data_ptr(fake_ptr, (ptr as *mut u8).offset(-offset));
394 ptr: Shared::new_unchecked(arc_ptr),
398 /// Creates a new [`Weak`][weak] pointer to this value.
400 /// [weak]: struct.Weak.html
405 /// use std::sync::Arc;
407 /// let five = Arc::new(5);
409 /// let weak_five = Arc::downgrade(&five);
411 #[stable(feature = "arc_weak", since = "1.4.0")]
412 pub fn downgrade(this: &Self) -> Weak<T> {
413 // This Relaxed is OK because we're checking the value in the CAS
415 let mut cur = this.inner().weak.load(Relaxed);
418 // check if the weak counter is currently "locked"; if so, spin.
419 if cur == usize::MAX {
420 cur = this.inner().weak.load(Relaxed);
424 // NOTE: this code currently ignores the possibility of overflow
425 // into usize::MAX; in general both Rc and Arc need to be adjusted
426 // to deal with overflow.
428 // Unlike with Clone(), we need this to be an Acquire read to
429 // synchronize with the write coming from `is_unique`, so that the
430 // events prior to that write happen before this read.
431 match this.inner().weak.compare_exchange_weak(cur, cur + 1, Acquire, Relaxed) {
432 Ok(_) => return Weak { ptr: this.ptr },
433 Err(old) => cur = old,
438 /// Gets the number of [`Weak`][weak] pointers to this value.
440 /// [weak]: struct.Weak.html
444 /// This method by itself is safe, but using it correctly requires extra care.
445 /// Another thread can change the weak count at any time,
446 /// including potentially between calling this method and acting on the result.
451 /// use std::sync::Arc;
453 /// let five = Arc::new(5);
454 /// let _weak_five = Arc::downgrade(&five);
456 /// // This assertion is deterministic because we haven't shared
457 /// // the `Arc` or `Weak` between threads.
458 /// assert_eq!(1, Arc::weak_count(&five));
461 #[stable(feature = "arc_counts", since = "1.15.0")]
462 pub fn weak_count(this: &Self) -> usize {
463 let cnt = this.inner().weak.load(SeqCst);
464 // If the weak count is currently locked, the value of the
465 // count was 0 just before taking the lock.
466 if cnt == usize::MAX { 0 } else { cnt - 1 }
469 /// Gets the number of strong (`Arc`) pointers to this value.
473 /// This method by itself is safe, but using it correctly requires extra care.
474 /// Another thread can change the strong count at any time,
475 /// including potentially between calling this method and acting on the result.
480 /// use std::sync::Arc;
482 /// let five = Arc::new(5);
483 /// let _also_five = Arc::clone(&five);
485 /// // This assertion is deterministic because we haven't shared
486 /// // the `Arc` between threads.
487 /// assert_eq!(2, Arc::strong_count(&five));
490 #[stable(feature = "arc_counts", since = "1.15.0")]
491 pub fn strong_count(this: &Self) -> usize {
492 this.inner().strong.load(SeqCst)
496 fn inner(&self) -> &ArcInner<T> {
497 // This unsafety is ok because while this arc is alive we're guaranteed
498 // that the inner pointer is valid. Furthermore, we know that the
499 // `ArcInner` structure itself is `Sync` because the inner data is
500 // `Sync` as well, so we're ok loaning out an immutable pointer to these
502 unsafe { self.ptr.as_ref() }
505 // Non-inlined part of `drop`.
507 unsafe fn drop_slow(&mut self) {
508 let ptr = self.ptr.as_ptr();
510 // Destroy the data at this time, even though we may not free the box
511 // allocation itself (there may still be weak pointers lying around).
512 ptr::drop_in_place(&mut self.ptr.as_mut().data);
514 if self.inner().weak.fetch_sub(1, Release) == 1 {
515 atomic::fence(Acquire);
516 Heap.dealloc(ptr as *mut u8, Layout::for_value(&*ptr))
521 #[stable(feature = "ptr_eq", since = "1.17.0")]
522 /// Returns true if the two `Arc`s point to the same value (not
523 /// just values that compare as equal).
528 /// use std::sync::Arc;
530 /// let five = Arc::new(5);
531 /// let same_five = Arc::clone(&five);
532 /// let other_five = Arc::new(5);
534 /// assert!(Arc::ptr_eq(&five, &same_five));
535 /// assert!(!Arc::ptr_eq(&five, &other_five));
537 pub fn ptr_eq(this: &Self, other: &Self) -> bool {
538 this.ptr.as_ptr() == other.ptr.as_ptr()
542 impl<T: ?Sized> Arc<T> {
543 // Allocates an `ArcInner<T>` with sufficient space for an unsized value
544 unsafe fn allocate_for_ptr(ptr: *const T) -> *mut ArcInner<T> {
545 // Create a fake ArcInner to find allocation size and alignment
546 let fake_ptr = ptr as *mut ArcInner<T>;
548 let layout = Layout::for_value(&*fake_ptr);
550 let mem = Heap.alloc(layout)
551 .unwrap_or_else(|e| Heap.oom(e));
553 // Initialize the real ArcInner
554 let inner = set_data_ptr(ptr as *mut T, mem) as *mut ArcInner<T>;
556 ptr::write(&mut (*inner).strong, atomic::AtomicUsize::new(1));
557 ptr::write(&mut (*inner).weak, atomic::AtomicUsize::new(1));
562 fn from_box(v: Box<T>) -> Arc<T> {
564 let bptr = Box::into_raw(v);
566 let value_size = size_of_val(&*bptr);
567 let ptr = Self::allocate_for_ptr(bptr);
569 // Copy value as bytes
570 ptr::copy_nonoverlapping(
571 bptr as *const T as *const u8,
572 &mut (*ptr).data as *mut _ as *mut u8,
575 // Free the allocation without dropping its contents
578 Arc { ptr: Shared::new_unchecked(ptr) }
583 // Sets the data pointer of a `?Sized` raw pointer.
585 // For a slice/trait object, this sets the `data` field and leaves the rest
586 // unchanged. For a sized raw pointer, this simply sets the pointer.
587 unsafe fn set_data_ptr<T: ?Sized, U>(mut ptr: *mut T, data: *mut U) -> *mut T {
588 ptr::write(&mut ptr as *mut _ as *mut *mut u8, data as *mut u8);
593 // Copy elements from slice into newly allocated Arc<[T]>
595 // Unsafe because the caller must either take ownership or bind `T: Copy`
596 unsafe fn copy_from_slice(v: &[T]) -> Arc<[T]> {
597 let v_ptr = v as *const [T];
598 let ptr = Self::allocate_for_ptr(v_ptr);
600 ptr::copy_nonoverlapping(
602 &mut (*ptr).data as *mut [T] as *mut T,
605 Arc { ptr: Shared::new_unchecked(ptr) }
609 // Specialization trait used for From<&[T]>
610 trait ArcFromSlice<T> {
611 fn from_slice(slice: &[T]) -> Self;
614 impl<T: Clone> ArcFromSlice<T> for Arc<[T]> {
616 default fn from_slice(v: &[T]) -> Self {
617 // Panic guard while cloning T elements.
618 // In the event of a panic, elements that have been written
619 // into the new ArcInner will be dropped, then the memory freed.
627 impl<T> Drop for Guard<T> {
629 use core::slice::from_raw_parts_mut;
632 let slice = from_raw_parts_mut(self.elems, self.n_elems);
633 ptr::drop_in_place(slice);
635 Heap.dealloc(self.mem, self.layout.clone());
641 let v_ptr = v as *const [T];
642 let ptr = Self::allocate_for_ptr(v_ptr);
644 let mem = ptr as *mut _ as *mut u8;
645 let layout = Layout::for_value(&*ptr);
647 // Pointer to first element
648 let elems = &mut (*ptr).data as *mut [T] as *mut T;
650 let mut guard = Guard{
657 for (i, item) in v.iter().enumerate() {
658 ptr::write(elems.offset(i as isize), item.clone());
662 // All clear. Forget the guard so it doesn't free the new ArcInner.
665 Arc { ptr: Shared::new_unchecked(ptr) }
670 impl<T: Copy> ArcFromSlice<T> for Arc<[T]> {
672 fn from_slice(v: &[T]) -> Self {
673 unsafe { Arc::copy_from_slice(v) }
677 #[stable(feature = "rust1", since = "1.0.0")]
678 impl<T: ?Sized> Clone for Arc<T> {
679 /// Makes a clone of the `Arc` pointer.
681 /// This creates another pointer to the same inner value, increasing the
682 /// strong reference count.
687 /// use std::sync::Arc;
689 /// let five = Arc::new(5);
691 /// Arc::clone(&five);
694 fn clone(&self) -> Arc<T> {
695 // Using a relaxed ordering is alright here, as knowledge of the
696 // original reference prevents other threads from erroneously deleting
699 // As explained in the [Boost documentation][1], Increasing the
700 // reference counter can always be done with memory_order_relaxed: New
701 // references to an object can only be formed from an existing
702 // reference, and passing an existing reference from one thread to
703 // another must already provide any required synchronization.
705 // [1]: (www.boost.org/doc/libs/1_55_0/doc/html/atomic/usage_examples.html)
706 let old_size = self.inner().strong.fetch_add(1, Relaxed);
708 // However we need to guard against massive refcounts in case someone
709 // is `mem::forget`ing Arcs. If we don't do this the count can overflow
710 // and users will use-after free. We racily saturate to `isize::MAX` on
711 // the assumption that there aren't ~2 billion threads incrementing
712 // the reference count at once. This branch will never be taken in
713 // any realistic program.
715 // We abort because such a program is incredibly degenerate, and we
716 // don't care to support it.
717 if old_size > MAX_REFCOUNT {
723 Arc { ptr: self.ptr }
727 #[stable(feature = "rust1", since = "1.0.0")]
728 impl<T: ?Sized> Deref for Arc<T> {
732 fn deref(&self) -> &T {
737 impl<T: Clone> Arc<T> {
738 /// Makes a mutable reference into the given `Arc`.
740 /// If there are other `Arc` or [`Weak`][weak] pointers to the same value,
741 /// then `make_mut` will invoke [`clone`][clone] on the inner value to
742 /// ensure unique ownership. This is also referred to as clone-on-write.
744 /// See also [`get_mut`][get_mut], which will fail rather than cloning.
746 /// [weak]: struct.Weak.html
747 /// [clone]: ../../std/clone/trait.Clone.html#tymethod.clone
748 /// [get_mut]: struct.Arc.html#method.get_mut
753 /// use std::sync::Arc;
755 /// let mut data = Arc::new(5);
757 /// *Arc::make_mut(&mut data) += 1; // Won't clone anything
758 /// let mut other_data = Arc::clone(&data); // Won't clone inner data
759 /// *Arc::make_mut(&mut data) += 1; // Clones inner data
760 /// *Arc::make_mut(&mut data) += 1; // Won't clone anything
761 /// *Arc::make_mut(&mut other_data) *= 2; // Won't clone anything
763 /// // Now `data` and `other_data` point to different values.
764 /// assert_eq!(*data, 8);
765 /// assert_eq!(*other_data, 12);
768 #[stable(feature = "arc_unique", since = "1.4.0")]
769 pub fn make_mut(this: &mut Self) -> &mut T {
770 // Note that we hold both a strong reference and a weak reference.
771 // Thus, releasing our strong reference only will not, by itself, cause
772 // the memory to be deallocated.
774 // Use Acquire to ensure that we see any writes to `weak` that happen
775 // before release writes (i.e., decrements) to `strong`. Since we hold a
776 // weak count, there's no chance the ArcInner itself could be
778 if this.inner().strong.compare_exchange(1, 0, Acquire, Relaxed).is_err() {
779 // Another strong pointer exists; clone
780 *this = Arc::new((**this).clone());
781 } else if this.inner().weak.load(Relaxed) != 1 {
782 // Relaxed suffices in the above because this is fundamentally an
783 // optimization: we are always racing with weak pointers being
784 // dropped. Worst case, we end up allocated a new Arc unnecessarily.
786 // We removed the last strong ref, but there are additional weak
787 // refs remaining. We'll move the contents to a new Arc, and
788 // invalidate the other weak refs.
790 // Note that it is not possible for the read of `weak` to yield
791 // usize::MAX (i.e., locked), since the weak count can only be
792 // locked by a thread with a strong reference.
794 // Materialize our own implicit weak pointer, so that it can clean
795 // up the ArcInner as needed.
796 let weak = Weak { ptr: this.ptr };
798 // mark the data itself as already deallocated
800 // there is no data race in the implicit write caused by `read`
801 // here (due to zeroing) because data is no longer accessed by
802 // other threads (due to there being no more strong refs at this
804 let mut swap = Arc::new(ptr::read(&weak.ptr.as_ref().data));
805 mem::swap(this, &mut swap);
809 // We were the sole reference of either kind; bump back up the
811 this.inner().strong.store(1, Release);
814 // As with `get_mut()`, the unsafety is ok because our reference was
815 // either unique to begin with, or became one upon cloning the contents.
817 &mut this.ptr.as_mut().data
822 impl<T: ?Sized> Arc<T> {
823 /// Returns a mutable reference to the inner value, if there are
824 /// no other `Arc` or [`Weak`][weak] pointers to the same value.
826 /// Returns [`None`][option] otherwise, because it is not safe to
827 /// mutate a shared value.
829 /// See also [`make_mut`][make_mut], which will [`clone`][clone]
830 /// the inner value when it's shared.
832 /// [weak]: struct.Weak.html
833 /// [option]: ../../std/option/enum.Option.html
834 /// [make_mut]: struct.Arc.html#method.make_mut
835 /// [clone]: ../../std/clone/trait.Clone.html#tymethod.clone
840 /// use std::sync::Arc;
842 /// let mut x = Arc::new(3);
843 /// *Arc::get_mut(&mut x).unwrap() = 4;
844 /// assert_eq!(*x, 4);
846 /// let _y = Arc::clone(&x);
847 /// assert!(Arc::get_mut(&mut x).is_none());
850 #[stable(feature = "arc_unique", since = "1.4.0")]
851 pub fn get_mut(this: &mut Self) -> Option<&mut T> {
852 if this.is_unique() {
853 // This unsafety is ok because we're guaranteed that the pointer
854 // returned is the *only* pointer that will ever be returned to T. Our
855 // reference count is guaranteed to be 1 at this point, and we required
856 // the Arc itself to be `mut`, so we're returning the only possible
857 // reference to the inner data.
859 Some(&mut this.ptr.as_mut().data)
866 /// Determine whether this is the unique reference (including weak refs) to
867 /// the underlying data.
869 /// Note that this requires locking the weak ref count.
870 fn is_unique(&mut self) -> bool {
871 // lock the weak pointer count if we appear to be the sole weak pointer
874 // The acquire label here ensures a happens-before relationship with any
875 // writes to `strong` prior to decrements of the `weak` count (via drop,
876 // which uses Release).
877 if self.inner().weak.compare_exchange(1, usize::MAX, Acquire, Relaxed).is_ok() {
878 // Due to the previous acquire read, this will observe any writes to
879 // `strong` that were due to upgrading weak pointers; only strong
880 // clones remain, which require that the strong count is > 1 anyway.
881 let unique = self.inner().strong.load(Relaxed) == 1;
883 // The release write here synchronizes with a read in `downgrade`,
884 // effectively preventing the above read of `strong` from happening
886 self.inner().weak.store(1, Release); // release the lock
894 #[stable(feature = "rust1", since = "1.0.0")]
895 unsafe impl<#[may_dangle] T: ?Sized> Drop for Arc<T> {
898 /// This will decrement the strong reference count. If the strong reference
899 /// count reaches zero then the only other references (if any) are
900 /// [`Weak`][weak], so we `drop` the inner value.
902 /// [weak]: struct.Weak.html
907 /// use std::sync::Arc;
911 /// impl Drop for Foo {
912 /// fn drop(&mut self) {
913 /// println!("dropped!");
917 /// let foo = Arc::new(Foo);
918 /// let foo2 = Arc::clone(&foo);
920 /// drop(foo); // Doesn't print anything
921 /// drop(foo2); // Prints "dropped!"
925 // Because `fetch_sub` is already atomic, we do not need to synchronize
926 // with other threads unless we are going to delete the object. This
927 // same logic applies to the below `fetch_sub` to the `weak` count.
928 if self.inner().strong.fetch_sub(1, Release) != 1 {
932 // This fence is needed to prevent reordering of use of the data and
933 // deletion of the data. Because it is marked `Release`, the decreasing
934 // of the reference count synchronizes with this `Acquire` fence. This
935 // means that use of the data happens before decreasing the reference
936 // count, which happens before this fence, which happens before the
937 // deletion of the data.
939 // As explained in the [Boost documentation][1],
941 // > It is important to enforce any possible access to the object in one
942 // > thread (through an existing reference) to *happen before* deleting
943 // > the object in a different thread. This is achieved by a "release"
944 // > operation after dropping a reference (any access to the object
945 // > through this reference must obviously happened before), and an
946 // > "acquire" operation before deleting the object.
948 // In particular, while the contents of an Arc are usually immutable, it's
949 // possible to have interior writes to something like a Mutex<T>. Since a
950 // Mutex is not acquired when it is deleted, we can't rely on its
951 // synchronization logic to make writes in thread A visible to a destructor
952 // running in thread B.
954 // Also note that the Acquire fence here could probably be replaced with an
955 // Acquire load, which could improve performance in highly-contended
956 // situations. See [2].
958 // [1]: (www.boost.org/doc/libs/1_55_0/doc/html/atomic/usage_examples.html)
959 // [2]: (https://github.com/rust-lang/rust/pull/41714)
960 atomic::fence(Acquire);
969 /// Constructs a new `Weak<T>`, allocating memory for `T` without initializing
970 /// it. Calling [`upgrade`] on the return value always gives [`None`].
972 /// [`upgrade`]: struct.Weak.html#method.upgrade
973 /// [`None`]: ../../std/option/enum.Option.html#variant.None
978 /// use std::sync::Weak;
980 /// let empty: Weak<i64> = Weak::new();
981 /// assert!(empty.upgrade().is_none());
983 #[stable(feature = "downgraded_weak", since = "1.10.0")]
984 pub fn new() -> Weak<T> {
987 ptr: Shared::from(Box::into_unique(box ArcInner {
988 strong: atomic::AtomicUsize::new(0),
989 weak: atomic::AtomicUsize::new(1),
990 data: uninitialized(),
997 impl<T: ?Sized> Weak<T> {
998 /// Attempts to upgrade the `Weak` pointer to an [`Arc`], extending
999 /// the lifetime of the value if successful.
1001 /// Returns [`None`] if the value has since been dropped.
1003 /// [`Arc`]: struct.Arc.html
1004 /// [`None`]: ../../std/option/enum.Option.html#variant.None
1009 /// use std::sync::Arc;
1011 /// let five = Arc::new(5);
1013 /// let weak_five = Arc::downgrade(&five);
1015 /// let strong_five: Option<Arc<_>> = weak_five.upgrade();
1016 /// assert!(strong_five.is_some());
1018 /// // Destroy all strong pointers.
1019 /// drop(strong_five);
1022 /// assert!(weak_five.upgrade().is_none());
1024 #[stable(feature = "arc_weak", since = "1.4.0")]
1025 pub fn upgrade(&self) -> Option<Arc<T>> {
1026 // We use a CAS loop to increment the strong count instead of a
1027 // fetch_add because once the count hits 0 it must never be above 0.
1028 let inner = self.inner();
1030 // Relaxed load because any write of 0 that we can observe
1031 // leaves the field in a permanently zero state (so a
1032 // "stale" read of 0 is fine), and any other value is
1033 // confirmed via the CAS below.
1034 let mut n = inner.strong.load(Relaxed);
1041 // See comments in `Arc::clone` for why we do this (for `mem::forget`).
1042 if n > MAX_REFCOUNT {
1048 // Relaxed is valid for the same reason it is on Arc's Clone impl
1049 match inner.strong.compare_exchange_weak(n, n + 1, Relaxed, Relaxed) {
1050 Ok(_) => return Some(Arc { ptr: self.ptr }),
1051 Err(old) => n = old,
1057 fn inner(&self) -> &ArcInner<T> {
1058 // See comments above for why this is "safe"
1059 unsafe { self.ptr.as_ref() }
1063 #[stable(feature = "arc_weak", since = "1.4.0")]
1064 impl<T: ?Sized> Clone for Weak<T> {
1065 /// Makes a clone of the `Weak` pointer that points to the same value.
1070 /// use std::sync::{Arc, Weak};
1072 /// let weak_five = Arc::downgrade(&Arc::new(5));
1074 /// Weak::clone(&weak_five);
1077 fn clone(&self) -> Weak<T> {
1078 // See comments in Arc::clone() for why this is relaxed. This can use a
1079 // fetch_add (ignoring the lock) because the weak count is only locked
1080 // where are *no other* weak pointers in existence. (So we can't be
1081 // running this code in that case).
1082 let old_size = self.inner().weak.fetch_add(1, Relaxed);
1084 // See comments in Arc::clone() for why we do this (for mem::forget).
1085 if old_size > MAX_REFCOUNT {
1091 return Weak { ptr: self.ptr };
1095 #[stable(feature = "downgraded_weak", since = "1.10.0")]
1096 impl<T> Default for Weak<T> {
1097 /// Constructs a new `Weak<T>`, allocating memory for `T` without initializing
1098 /// it. Calling [`upgrade`] on the return value always gives [`None`].
1100 /// [`upgrade`]: struct.Weak.html#method.upgrade
1101 /// [`None`]: ../../std/option/enum.Option.html#variant.None
1106 /// use std::sync::Weak;
1108 /// let empty: Weak<i64> = Default::default();
1109 /// assert!(empty.upgrade().is_none());
1111 fn default() -> Weak<T> {
1116 #[stable(feature = "arc_weak", since = "1.4.0")]
1117 impl<T: ?Sized> Drop for Weak<T> {
1118 /// Drops the `Weak` pointer.
1123 /// use std::sync::{Arc, Weak};
1127 /// impl Drop for Foo {
1128 /// fn drop(&mut self) {
1129 /// println!("dropped!");
1133 /// let foo = Arc::new(Foo);
1134 /// let weak_foo = Arc::downgrade(&foo);
1135 /// let other_weak_foo = Weak::clone(&weak_foo);
1137 /// drop(weak_foo); // Doesn't print anything
1138 /// drop(foo); // Prints "dropped!"
1140 /// assert!(other_weak_foo.upgrade().is_none());
1142 fn drop(&mut self) {
1143 let ptr = self.ptr.as_ptr();
1145 // If we find out that we were the last weak pointer, then its time to
1146 // deallocate the data entirely. See the discussion in Arc::drop() about
1147 // the memory orderings
1149 // It's not necessary to check for the locked state here, because the
1150 // weak count can only be locked if there was precisely one weak ref,
1151 // meaning that drop could only subsequently run ON that remaining weak
1152 // ref, which can only happen after the lock is released.
1153 if self.inner().weak.fetch_sub(1, Release) == 1 {
1154 atomic::fence(Acquire);
1156 Heap.dealloc(ptr as *mut u8, Layout::for_value(&*ptr))
1162 #[stable(feature = "rust1", since = "1.0.0")]
1163 impl<T: ?Sized + PartialEq> PartialEq for Arc<T> {
1164 /// Equality for two `Arc`s.
1166 /// Two `Arc`s are equal if their inner values are equal.
1171 /// use std::sync::Arc;
1173 /// let five = Arc::new(5);
1175 /// assert!(five == Arc::new(5));
1177 fn eq(&self, other: &Arc<T>) -> bool {
1178 *(*self) == *(*other)
1181 /// Inequality for two `Arc`s.
1183 /// Two `Arc`s are unequal if their inner values are unequal.
1188 /// use std::sync::Arc;
1190 /// let five = Arc::new(5);
1192 /// assert!(five != Arc::new(6));
1194 fn ne(&self, other: &Arc<T>) -> bool {
1195 *(*self) != *(*other)
1198 #[stable(feature = "rust1", since = "1.0.0")]
1199 impl<T: ?Sized + PartialOrd> PartialOrd for Arc<T> {
1200 /// Partial comparison for two `Arc`s.
1202 /// The two are compared by calling `partial_cmp()` on their inner values.
1207 /// use std::sync::Arc;
1208 /// use std::cmp::Ordering;
1210 /// let five = Arc::new(5);
1212 /// assert_eq!(Some(Ordering::Less), five.partial_cmp(&Arc::new(6)));
1214 fn partial_cmp(&self, other: &Arc<T>) -> Option<Ordering> {
1215 (**self).partial_cmp(&**other)
1218 /// Less-than comparison for two `Arc`s.
1220 /// The two are compared by calling `<` on their inner values.
1225 /// use std::sync::Arc;
1227 /// let five = Arc::new(5);
1229 /// assert!(five < Arc::new(6));
1231 fn lt(&self, other: &Arc<T>) -> bool {
1232 *(*self) < *(*other)
1235 /// 'Less than or equal to' comparison for two `Arc`s.
1237 /// The two are compared by calling `<=` on their inner values.
1242 /// use std::sync::Arc;
1244 /// let five = Arc::new(5);
1246 /// assert!(five <= Arc::new(5));
1248 fn le(&self, other: &Arc<T>) -> bool {
1249 *(*self) <= *(*other)
1252 /// Greater-than comparison for two `Arc`s.
1254 /// The two are compared by calling `>` on their inner values.
1259 /// use std::sync::Arc;
1261 /// let five = Arc::new(5);
1263 /// assert!(five > Arc::new(4));
1265 fn gt(&self, other: &Arc<T>) -> bool {
1266 *(*self) > *(*other)
1269 /// 'Greater than or equal to' comparison for two `Arc`s.
1271 /// The two are compared by calling `>=` on their inner values.
1276 /// use std::sync::Arc;
1278 /// let five = Arc::new(5);
1280 /// assert!(five >= Arc::new(5));
1282 fn ge(&self, other: &Arc<T>) -> bool {
1283 *(*self) >= *(*other)
1286 #[stable(feature = "rust1", since = "1.0.0")]
1287 impl<T: ?Sized + Ord> Ord for Arc<T> {
1288 /// Comparison for two `Arc`s.
1290 /// The two are compared by calling `cmp()` on their inner values.
1295 /// use std::sync::Arc;
1296 /// use std::cmp::Ordering;
1298 /// let five = Arc::new(5);
1300 /// assert_eq!(Ordering::Less, five.cmp(&Arc::new(6)));
1302 fn cmp(&self, other: &Arc<T>) -> Ordering {
1303 (**self).cmp(&**other)
1306 #[stable(feature = "rust1", since = "1.0.0")]
1307 impl<T: ?Sized + Eq> Eq for Arc<T> {}
1309 #[stable(feature = "rust1", since = "1.0.0")]
1310 impl<T: ?Sized + fmt::Display> fmt::Display for Arc<T> {
1311 fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
1312 fmt::Display::fmt(&**self, f)
1316 #[stable(feature = "rust1", since = "1.0.0")]
1317 impl<T: ?Sized + fmt::Debug> fmt::Debug for Arc<T> {
1318 fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
1319 fmt::Debug::fmt(&**self, f)
1323 #[stable(feature = "rust1", since = "1.0.0")]
1324 impl<T: ?Sized> fmt::Pointer for Arc<T> {
1325 fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
1326 fmt::Pointer::fmt(&self.ptr, f)
1330 #[stable(feature = "rust1", since = "1.0.0")]
1331 impl<T: Default> Default for Arc<T> {
1332 /// Creates a new `Arc<T>`, with the `Default` value for `T`.
1337 /// use std::sync::Arc;
1339 /// let x: Arc<i32> = Default::default();
1340 /// assert_eq!(*x, 0);
1342 fn default() -> Arc<T> {
1343 Arc::new(Default::default())
1347 #[stable(feature = "rust1", since = "1.0.0")]
1348 impl<T: ?Sized + Hash> Hash for Arc<T> {
1349 fn hash<H: Hasher>(&self, state: &mut H) {
1350 (**self).hash(state)
1354 #[stable(feature = "from_for_ptrs", since = "1.6.0")]
1355 impl<T> From<T> for Arc<T> {
1356 fn from(t: T) -> Self {
1361 #[stable(feature = "shared_from_slice", since = "1.21.0")]
1362 impl<'a, T: Clone> From<&'a [T]> for Arc<[T]> {
1364 fn from(v: &[T]) -> Arc<[T]> {
1365 <Self as ArcFromSlice<T>>::from_slice(v)
1369 #[stable(feature = "shared_from_slice", since = "1.21.0")]
1370 impl<'a> From<&'a str> for Arc<str> {
1372 fn from(v: &str) -> Arc<str> {
1373 unsafe { mem::transmute(<Arc<[u8]>>::from(v.as_bytes())) }
1377 #[stable(feature = "shared_from_slice", since = "1.21.0")]
1378 impl From<String> for Arc<str> {
1380 fn from(v: String) -> Arc<str> {
1385 #[stable(feature = "shared_from_slice", since = "1.21.0")]
1386 impl<T: ?Sized> From<Box<T>> for Arc<T> {
1388 fn from(v: Box<T>) -> Arc<T> {
1393 #[stable(feature = "shared_from_slice", since = "1.21.0")]
1394 impl<T> From<Vec<T>> for Arc<[T]> {
1396 fn from(mut v: Vec<T>) -> Arc<[T]> {
1398 let arc = Arc::copy_from_slice(&v);
1400 // Allow the Vec to free its memory, but not destroy its contents
1410 use std::boxed::Box;
1411 use std::clone::Clone;
1412 use std::sync::mpsc::channel;
1415 use std::option::Option;
1416 use std::option::Option::{None, Some};
1417 use std::sync::atomic;
1418 use std::sync::atomic::Ordering::{Acquire, SeqCst};
1420 use std::sync::Mutex;
1421 use std::convert::From;
1423 use super::{Arc, Weak};
1426 struct Canary(*mut atomic::AtomicUsize);
1428 impl Drop for Canary {
1429 fn drop(&mut self) {
1433 (*c).fetch_add(1, SeqCst);
1441 #[cfg_attr(target_os = "emscripten", ignore)]
1442 fn manually_share_arc() {
1443 let v = vec![1, 2, 3, 4, 5, 6, 7, 8, 9, 10];
1444 let arc_v = Arc::new(v);
1446 let (tx, rx) = channel();
1448 let _t = thread::spawn(move || {
1449 let arc_v: Arc<Vec<i32>> = rx.recv().unwrap();
1450 assert_eq!((*arc_v)[3], 4);
1453 tx.send(arc_v.clone()).unwrap();
1455 assert_eq!((*arc_v)[2], 3);
1456 assert_eq!((*arc_v)[4], 5);
1460 fn test_arc_get_mut() {
1461 let mut x = Arc::new(3);
1462 *Arc::get_mut(&mut x).unwrap() = 4;
1465 assert!(Arc::get_mut(&mut x).is_none());
1467 assert!(Arc::get_mut(&mut x).is_some());
1468 let _w = Arc::downgrade(&x);
1469 assert!(Arc::get_mut(&mut x).is_none());
1474 let x = Arc::new(3);
1475 assert_eq!(Arc::try_unwrap(x), Ok(3));
1476 let x = Arc::new(4);
1478 assert_eq!(Arc::try_unwrap(x), Err(Arc::new(4)));
1479 let x = Arc::new(5);
1480 let _w = Arc::downgrade(&x);
1481 assert_eq!(Arc::try_unwrap(x), Ok(5));
1485 fn into_from_raw() {
1486 let x = Arc::new(box "hello");
1489 let x_ptr = Arc::into_raw(x);
1492 assert_eq!(**x_ptr, "hello");
1494 let x = Arc::from_raw(x_ptr);
1495 assert_eq!(**x, "hello");
1497 assert_eq!(Arc::try_unwrap(x).map(|x| *x), Ok("hello"));
1502 fn test_into_from_raw_unsized() {
1503 use std::fmt::Display;
1504 use std::string::ToString;
1506 let arc: Arc<str> = Arc::from("foo");
1508 let ptr = Arc::into_raw(arc.clone());
1509 let arc2 = unsafe { Arc::from_raw(ptr) };
1511 assert_eq!(unsafe { &*ptr }, "foo");
1512 assert_eq!(arc, arc2);
1514 let arc: Arc<Display> = Arc::new(123);
1516 let ptr = Arc::into_raw(arc.clone());
1517 let arc2 = unsafe { Arc::from_raw(ptr) };
1519 assert_eq!(unsafe { &*ptr }.to_string(), "123");
1520 assert_eq!(arc2.to_string(), "123");
1524 fn test_cowarc_clone_make_mut() {
1525 let mut cow0 = Arc::new(75);
1526 let mut cow1 = cow0.clone();
1527 let mut cow2 = cow1.clone();
1529 assert!(75 == *Arc::make_mut(&mut cow0));
1530 assert!(75 == *Arc::make_mut(&mut cow1));
1531 assert!(75 == *Arc::make_mut(&mut cow2));
1533 *Arc::make_mut(&mut cow0) += 1;
1534 *Arc::make_mut(&mut cow1) += 2;
1535 *Arc::make_mut(&mut cow2) += 3;
1537 assert!(76 == *cow0);
1538 assert!(77 == *cow1);
1539 assert!(78 == *cow2);
1541 // none should point to the same backing memory
1542 assert!(*cow0 != *cow1);
1543 assert!(*cow0 != *cow2);
1544 assert!(*cow1 != *cow2);
1548 fn test_cowarc_clone_unique2() {
1549 let mut cow0 = Arc::new(75);
1550 let cow1 = cow0.clone();
1551 let cow2 = cow1.clone();
1553 assert!(75 == *cow0);
1554 assert!(75 == *cow1);
1555 assert!(75 == *cow2);
1557 *Arc::make_mut(&mut cow0) += 1;
1558 assert!(76 == *cow0);
1559 assert!(75 == *cow1);
1560 assert!(75 == *cow2);
1562 // cow1 and cow2 should share the same contents
1563 // cow0 should have a unique reference
1564 assert!(*cow0 != *cow1);
1565 assert!(*cow0 != *cow2);
1566 assert!(*cow1 == *cow2);
1570 fn test_cowarc_clone_weak() {
1571 let mut cow0 = Arc::new(75);
1572 let cow1_weak = Arc::downgrade(&cow0);
1574 assert!(75 == *cow0);
1575 assert!(75 == *cow1_weak.upgrade().unwrap());
1577 *Arc::make_mut(&mut cow0) += 1;
1579 assert!(76 == *cow0);
1580 assert!(cow1_weak.upgrade().is_none());
1585 let x = Arc::new(5);
1586 let y = Arc::downgrade(&x);
1587 assert!(y.upgrade().is_some());
1592 let x = Arc::new(5);
1593 let y = Arc::downgrade(&x);
1595 assert!(y.upgrade().is_none());
1599 fn weak_self_cyclic() {
1601 x: Mutex<Option<Weak<Cycle>>>,
1604 let a = Arc::new(Cycle { x: Mutex::new(None) });
1605 let b = Arc::downgrade(&a.clone());
1606 *a.x.lock().unwrap() = Some(b);
1608 // hopefully we don't double-free (or leak)...
1613 let mut canary = atomic::AtomicUsize::new(0);
1614 let x = Arc::new(Canary(&mut canary as *mut atomic::AtomicUsize));
1616 assert!(canary.load(Acquire) == 1);
1620 fn drop_arc_weak() {
1621 let mut canary = atomic::AtomicUsize::new(0);
1622 let arc = Arc::new(Canary(&mut canary as *mut atomic::AtomicUsize));
1623 let arc_weak = Arc::downgrade(&arc);
1624 assert!(canary.load(Acquire) == 0);
1626 assert!(canary.load(Acquire) == 1);
1631 fn test_strong_count() {
1632 let a = Arc::new(0);
1633 assert!(Arc::strong_count(&a) == 1);
1634 let w = Arc::downgrade(&a);
1635 assert!(Arc::strong_count(&a) == 1);
1636 let b = w.upgrade().expect("");
1637 assert!(Arc::strong_count(&b) == 2);
1638 assert!(Arc::strong_count(&a) == 2);
1641 assert!(Arc::strong_count(&b) == 1);
1643 assert!(Arc::strong_count(&b) == 2);
1644 assert!(Arc::strong_count(&c) == 2);
1648 fn test_weak_count() {
1649 let a = Arc::new(0);
1650 assert!(Arc::strong_count(&a) == 1);
1651 assert!(Arc::weak_count(&a) == 0);
1652 let w = Arc::downgrade(&a);
1653 assert!(Arc::strong_count(&a) == 1);
1654 assert!(Arc::weak_count(&a) == 1);
1656 assert!(Arc::weak_count(&a) == 2);
1659 assert!(Arc::strong_count(&a) == 1);
1660 assert!(Arc::weak_count(&a) == 0);
1662 assert!(Arc::strong_count(&a) == 2);
1663 assert!(Arc::weak_count(&a) == 0);
1664 let d = Arc::downgrade(&c);
1665 assert!(Arc::weak_count(&c) == 1);
1666 assert!(Arc::strong_count(&c) == 2);
1675 let a = Arc::new(5);
1676 assert_eq!(format!("{:?}", a), "5");
1679 // Make sure deriving works with Arc<T>
1680 #[derive(Eq, Ord, PartialEq, PartialOrd, Clone, Debug, Default)]
1687 let x: Arc<[i32]> = Arc::new([1, 2, 3]);
1688 assert_eq!(format!("{:?}", x), "[1, 2, 3]");
1689 let y = Arc::downgrade(&x.clone());
1691 assert!(y.upgrade().is_none());
1695 fn test_from_owned() {
1697 let foo_arc = Arc::from(foo);
1698 assert!(123 == *foo_arc);
1702 fn test_new_weak() {
1703 let foo: Weak<usize> = Weak::new();
1704 assert!(foo.upgrade().is_none());
1709 let five = Arc::new(5);
1710 let same_five = five.clone();
1711 let other_five = Arc::new(5);
1713 assert!(Arc::ptr_eq(&five, &same_five));
1714 assert!(!Arc::ptr_eq(&five, &other_five));
1718 #[cfg_attr(target_os = "emscripten", ignore)]
1719 fn test_weak_count_locked() {
1720 let mut a = Arc::new(atomic::AtomicBool::new(false));
1722 let t = thread::spawn(move || {
1723 for _i in 0..1000000 {
1724 Arc::get_mut(&mut a);
1726 a.store(true, SeqCst);
1729 while !a2.load(SeqCst) {
1730 let n = Arc::weak_count(&a2);
1731 assert!(n < 2, "bad weak count: {}", n);
1737 fn test_from_str() {
1738 let r: Arc<str> = Arc::from("foo");
1740 assert_eq!(&r[..], "foo");
1744 fn test_copy_from_slice() {
1745 let s: &[u32] = &[1, 2, 3];
1746 let r: Arc<[u32]> = Arc::from(s);
1748 assert_eq!(&r[..], [1, 2, 3]);
1752 fn test_clone_from_slice() {
1753 #[derive(Clone, Debug, Eq, PartialEq)]
1756 let s: &[X] = &[X(1), X(2), X(3)];
1757 let r: Arc<[X]> = Arc::from(s);
1759 assert_eq!(&r[..], s);
1764 fn test_clone_from_slice_panic() {
1765 use std::string::{String, ToString};
1767 struct Fail(u32, String);
1769 impl Clone for Fail {
1770 fn clone(&self) -> Fail {
1774 Fail(self.0, self.1.clone())
1779 Fail(0, "foo".to_string()),
1780 Fail(1, "bar".to_string()),
1781 Fail(2, "baz".to_string()),
1784 // Should panic, but not cause memory corruption
1785 let _r: Arc<[Fail]> = Arc::from(s);
1789 fn test_from_box() {
1790 let b: Box<u32> = box 123;
1791 let r: Arc<u32> = Arc::from(b);
1793 assert_eq!(*r, 123);
1797 fn test_from_box_str() {
1798 use std::string::String;
1800 let s = String::from("foo").into_boxed_str();
1801 let r: Arc<str> = Arc::from(s);
1803 assert_eq!(&r[..], "foo");
1807 fn test_from_box_slice() {
1808 let s = vec![1, 2, 3].into_boxed_slice();
1809 let r: Arc<[u32]> = Arc::from(s);
1811 assert_eq!(&r[..], [1, 2, 3]);
1815 fn test_from_box_trait() {
1816 use std::fmt::Display;
1817 use std::string::ToString;
1819 let b: Box<Display> = box 123;
1820 let r: Arc<Display> = Arc::from(b);
1822 assert_eq!(r.to_string(), "123");
1826 fn test_from_box_trait_zero_sized() {
1827 use std::fmt::Debug;
1829 let b: Box<Debug> = box ();
1830 let r: Arc<Debug> = Arc::from(b);
1832 assert_eq!(format!("{:?}", r), "()");
1836 fn test_from_vec() {
1837 let v = vec![1, 2, 3];
1838 let r: Arc<[u32]> = Arc::from(v);
1840 assert_eq!(&r[..], [1, 2, 3]);
1844 #[stable(feature = "rust1", since = "1.0.0")]
1845 impl<T: ?Sized> borrow::Borrow<T> for Arc<T> {
1846 fn borrow(&self) -> &T {
1851 #[stable(since = "1.5.0", feature = "smart_ptr_as_ref")]
1852 impl<T: ?Sized> AsRef<T> for Arc<T> {
1853 fn as_ref(&self) -> &T {