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, NonNull};
29 use core::marker::{Unsize, PhantomData};
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: NonNull<ArcInner<T>>,
201 phantom: PhantomData<T>,
204 #[stable(feature = "rust1", since = "1.0.0")]
205 unsafe impl<T: ?Sized + Sync + Send> Send for Arc<T> {}
206 #[stable(feature = "rust1", since = "1.0.0")]
207 unsafe impl<T: ?Sized + Sync + Send> Sync for Arc<T> {}
209 #[unstable(feature = "coerce_unsized", issue = "27732")]
210 impl<T: ?Sized + Unsize<U>, U: ?Sized> CoerceUnsized<Arc<U>> for Arc<T> {}
212 /// `Weak` is a version of [`Arc`] that holds a non-owning reference to the
213 /// managed value. The value is accessed by calling [`upgrade`] on the `Weak`
214 /// pointer, which returns an [`Option`]`<`[`Arc`]`<T>>`.
216 /// Since a `Weak` reference does not count towards ownership, it will not
217 /// prevent the inner value from being dropped, and `Weak` itself makes no
218 /// guarantees about the value still being present and may return [`None`]
219 /// when [`upgrade`]d.
221 /// A `Weak` pointer is useful for keeping a temporary reference to the value
222 /// within [`Arc`] without extending its lifetime. It is also used to prevent
223 /// circular references between [`Arc`] pointers, since mutual owning references
224 /// would never allow either [`Arc`] to be dropped. For example, a tree could
225 /// have strong [`Arc`] pointers from parent nodes to children, and `Weak`
226 /// pointers from children back to their parents.
228 /// The typical way to obtain a `Weak` pointer is to call [`Arc::downgrade`].
230 /// [`Arc`]: struct.Arc.html
231 /// [`Arc::downgrade`]: struct.Arc.html#method.downgrade
232 /// [`upgrade`]: struct.Weak.html#method.upgrade
233 /// [`Option`]: ../../std/option/enum.Option.html
234 /// [`None`]: ../../std/option/enum.Option.html#variant.None
235 #[stable(feature = "arc_weak", since = "1.4.0")]
236 pub struct Weak<T: ?Sized> {
237 ptr: NonNull<ArcInner<T>>,
240 #[stable(feature = "arc_weak", since = "1.4.0")]
241 unsafe impl<T: ?Sized + Sync + Send> Send for Weak<T> {}
242 #[stable(feature = "arc_weak", since = "1.4.0")]
243 unsafe impl<T: ?Sized + Sync + Send> Sync for Weak<T> {}
245 #[unstable(feature = "coerce_unsized", issue = "27732")]
246 impl<T: ?Sized + Unsize<U>, U: ?Sized> CoerceUnsized<Weak<U>> for Weak<T> {}
248 #[stable(feature = "arc_weak", since = "1.4.0")]
249 impl<T: ?Sized + fmt::Debug> fmt::Debug for Weak<T> {
250 fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
255 struct ArcInner<T: ?Sized> {
256 strong: atomic::AtomicUsize,
258 // the value usize::MAX acts as a sentinel for temporarily "locking" the
259 // ability to upgrade weak pointers or downgrade strong ones; this is used
260 // to avoid races in `make_mut` and `get_mut`.
261 weak: atomic::AtomicUsize,
266 unsafe impl<T: ?Sized + Sync + Send> Send for ArcInner<T> {}
267 unsafe impl<T: ?Sized + Sync + Send> Sync for ArcInner<T> {}
270 /// Constructs a new `Arc<T>`.
275 /// use std::sync::Arc;
277 /// let five = Arc::new(5);
280 #[stable(feature = "rust1", since = "1.0.0")]
281 pub fn new(data: T) -> Arc<T> {
282 // Start the weak pointer count as 1 which is the weak pointer that's
283 // held by all the strong pointers (kinda), see std/rc.rs for more info
284 let x: Box<_> = box ArcInner {
285 strong: atomic::AtomicUsize::new(1),
286 weak: atomic::AtomicUsize::new(1),
289 Arc { ptr: NonNull::from(Box::into_unique(x)), phantom: PhantomData }
292 /// Returns the contained value, if the `Arc` has exactly one strong reference.
294 /// Otherwise, an [`Err`][result] is returned with the same `Arc` that was
297 /// This will succeed even if there are outstanding weak references.
299 /// [result]: ../../std/result/enum.Result.html
304 /// use std::sync::Arc;
306 /// let x = Arc::new(3);
307 /// assert_eq!(Arc::try_unwrap(x), Ok(3));
309 /// let x = Arc::new(4);
310 /// let _y = Arc::clone(&x);
311 /// assert_eq!(*Arc::try_unwrap(x).unwrap_err(), 4);
314 #[stable(feature = "arc_unique", since = "1.4.0")]
315 pub fn try_unwrap(this: Self) -> Result<T, Self> {
316 // See `drop` for why all these atomics are like this
317 if this.inner().strong.compare_exchange(1, 0, Release, Relaxed).is_err() {
321 atomic::fence(Acquire);
324 let elem = ptr::read(&this.ptr.as_ref().data);
326 // Make a weak pointer to clean up the implicit strong-weak reference
327 let _weak = Weak { ptr: this.ptr };
335 impl<T: ?Sized> Arc<T> {
336 /// Consumes the `Arc`, returning the wrapped pointer.
338 /// To avoid a memory leak the pointer must be converted back to an `Arc` using
339 /// [`Arc::from_raw`][from_raw].
341 /// [from_raw]: struct.Arc.html#method.from_raw
346 /// use std::sync::Arc;
348 /// let x = Arc::new(10);
349 /// let x_ptr = Arc::into_raw(x);
350 /// assert_eq!(unsafe { *x_ptr }, 10);
352 #[stable(feature = "rc_raw", since = "1.17.0")]
353 pub fn into_raw(this: Self) -> *const T {
354 let ptr: *const T = &*this;
359 /// Constructs an `Arc` from a raw pointer.
361 /// The raw pointer must have been previously returned by a call to a
362 /// [`Arc::into_raw`][into_raw].
364 /// This function is unsafe because improper use may lead to memory problems. For example, a
365 /// double-free may occur if the function is called twice on the same raw pointer.
367 /// [into_raw]: struct.Arc.html#method.into_raw
372 /// use std::sync::Arc;
374 /// let x = Arc::new(10);
375 /// let x_ptr = Arc::into_raw(x);
378 /// // Convert back to an `Arc` to prevent leak.
379 /// let x = Arc::from_raw(x_ptr);
380 /// assert_eq!(*x, 10);
382 /// // Further calls to `Arc::from_raw(x_ptr)` would be memory unsafe.
385 /// // The memory was freed when `x` went out of scope above, so `x_ptr` is now dangling!
387 #[stable(feature = "rc_raw", since = "1.17.0")]
388 pub unsafe fn from_raw(ptr: *const T) -> Self {
389 // Align the unsized value to the end of the ArcInner.
390 // Because it is ?Sized, it will always be the last field in memory.
391 let align = align_of_val(&*ptr);
392 let layout = Layout::new::<ArcInner<()>>();
393 let offset = (layout.size() + layout.padding_needed_for(align)) as isize;
395 // Reverse the offset to find the original ArcInner.
396 let fake_ptr = ptr as *mut ArcInner<T>;
397 let arc_ptr = set_data_ptr(fake_ptr, (ptr as *mut u8).offset(-offset));
400 ptr: NonNull::new_unchecked(arc_ptr),
401 phantom: PhantomData,
405 /// Creates a new [`Weak`][weak] pointer to this value.
407 /// [weak]: struct.Weak.html
412 /// use std::sync::Arc;
414 /// let five = Arc::new(5);
416 /// let weak_five = Arc::downgrade(&five);
418 #[stable(feature = "arc_weak", since = "1.4.0")]
419 pub fn downgrade(this: &Self) -> Weak<T> {
420 // This Relaxed is OK because we're checking the value in the CAS
422 let mut cur = this.inner().weak.load(Relaxed);
425 // check if the weak counter is currently "locked"; if so, spin.
426 if cur == usize::MAX {
427 cur = this.inner().weak.load(Relaxed);
431 // NOTE: this code currently ignores the possibility of overflow
432 // into usize::MAX; in general both Rc and Arc need to be adjusted
433 // to deal with overflow.
435 // Unlike with Clone(), we need this to be an Acquire read to
436 // synchronize with the write coming from `is_unique`, so that the
437 // events prior to that write happen before this read.
438 match this.inner().weak.compare_exchange_weak(cur, cur + 1, Acquire, Relaxed) {
439 Ok(_) => return Weak { ptr: this.ptr },
440 Err(old) => cur = old,
445 /// Gets the number of [`Weak`][weak] pointers to this value.
447 /// [weak]: struct.Weak.html
451 /// This method by itself is safe, but using it correctly requires extra care.
452 /// Another thread can change the weak count at any time,
453 /// including potentially between calling this method and acting on the result.
458 /// use std::sync::Arc;
460 /// let five = Arc::new(5);
461 /// let _weak_five = Arc::downgrade(&five);
463 /// // This assertion is deterministic because we haven't shared
464 /// // the `Arc` or `Weak` between threads.
465 /// assert_eq!(1, Arc::weak_count(&five));
468 #[stable(feature = "arc_counts", since = "1.15.0")]
469 pub fn weak_count(this: &Self) -> usize {
470 let cnt = this.inner().weak.load(SeqCst);
471 // If the weak count is currently locked, the value of the
472 // count was 0 just before taking the lock.
473 if cnt == usize::MAX { 0 } else { cnt - 1 }
476 /// Gets the number of strong (`Arc`) pointers to this value.
480 /// This method by itself is safe, but using it correctly requires extra care.
481 /// Another thread can change the strong count at any time,
482 /// including potentially between calling this method and acting on the result.
487 /// use std::sync::Arc;
489 /// let five = Arc::new(5);
490 /// let _also_five = Arc::clone(&five);
492 /// // This assertion is deterministic because we haven't shared
493 /// // the `Arc` between threads.
494 /// assert_eq!(2, Arc::strong_count(&five));
497 #[stable(feature = "arc_counts", since = "1.15.0")]
498 pub fn strong_count(this: &Self) -> usize {
499 this.inner().strong.load(SeqCst)
503 fn inner(&self) -> &ArcInner<T> {
504 // This unsafety is ok because while this arc is alive we're guaranteed
505 // that the inner pointer is valid. Furthermore, we know that the
506 // `ArcInner` structure itself is `Sync` because the inner data is
507 // `Sync` as well, so we're ok loaning out an immutable pointer to these
509 unsafe { self.ptr.as_ref() }
512 // Non-inlined part of `drop`.
514 unsafe fn drop_slow(&mut self) {
515 let ptr = self.ptr.as_ptr();
517 // Destroy the data at this time, even though we may not free the box
518 // allocation itself (there may still be weak pointers lying around).
519 ptr::drop_in_place(&mut self.ptr.as_mut().data);
521 if self.inner().weak.fetch_sub(1, Release) == 1 {
522 atomic::fence(Acquire);
523 Heap.dealloc(ptr as *mut u8, Layout::for_value(&*ptr))
528 #[stable(feature = "ptr_eq", since = "1.17.0")]
529 /// Returns true if the two `Arc`s point to the same value (not
530 /// just values that compare as equal).
535 /// use std::sync::Arc;
537 /// let five = Arc::new(5);
538 /// let same_five = Arc::clone(&five);
539 /// let other_five = Arc::new(5);
541 /// assert!(Arc::ptr_eq(&five, &same_five));
542 /// assert!(!Arc::ptr_eq(&five, &other_five));
544 pub fn ptr_eq(this: &Self, other: &Self) -> bool {
545 this.ptr.as_ptr() == other.ptr.as_ptr()
549 impl<T: ?Sized> Arc<T> {
550 // Allocates an `ArcInner<T>` with sufficient space for an unsized value
551 unsafe fn allocate_for_ptr(ptr: *const T) -> *mut ArcInner<T> {
552 // Create a fake ArcInner to find allocation size and alignment
553 let fake_ptr = ptr as *mut ArcInner<T>;
555 let layout = Layout::for_value(&*fake_ptr);
557 let mem = Heap.alloc(layout)
558 .unwrap_or_else(|e| Heap.oom(e));
560 // Initialize the real ArcInner
561 let inner = set_data_ptr(ptr as *mut T, mem) as *mut ArcInner<T>;
563 ptr::write(&mut (*inner).strong, atomic::AtomicUsize::new(1));
564 ptr::write(&mut (*inner).weak, atomic::AtomicUsize::new(1));
569 fn from_box(v: Box<T>) -> Arc<T> {
571 let bptr = Box::into_raw(v);
573 let value_size = size_of_val(&*bptr);
574 let ptr = Self::allocate_for_ptr(bptr);
576 // Copy value as bytes
577 ptr::copy_nonoverlapping(
578 bptr as *const T as *const u8,
579 &mut (*ptr).data as *mut _ as *mut u8,
582 // Free the allocation without dropping its contents
585 Arc { ptr: NonNull::new_unchecked(ptr), phantom: PhantomData }
590 // Sets the data pointer of a `?Sized` raw pointer.
592 // For a slice/trait object, this sets the `data` field and leaves the rest
593 // unchanged. For a sized raw pointer, this simply sets the pointer.
594 unsafe fn set_data_ptr<T: ?Sized, U>(mut ptr: *mut T, data: *mut U) -> *mut T {
595 ptr::write(&mut ptr as *mut _ as *mut *mut u8, data as *mut u8);
600 // Copy elements from slice into newly allocated Arc<[T]>
602 // Unsafe because the caller must either take ownership or bind `T: Copy`
603 unsafe fn copy_from_slice(v: &[T]) -> Arc<[T]> {
604 let v_ptr = v as *const [T];
605 let ptr = Self::allocate_for_ptr(v_ptr);
607 ptr::copy_nonoverlapping(
609 &mut (*ptr).data as *mut [T] as *mut T,
612 Arc { ptr: NonNull::new_unchecked(ptr), phantom: PhantomData }
616 // Specialization trait used for From<&[T]>
617 trait ArcFromSlice<T> {
618 fn from_slice(slice: &[T]) -> Self;
621 impl<T: Clone> ArcFromSlice<T> for Arc<[T]> {
623 default fn from_slice(v: &[T]) -> Self {
624 // Panic guard while cloning T elements.
625 // In the event of a panic, elements that have been written
626 // into the new ArcInner will be dropped, then the memory freed.
634 impl<T> Drop for Guard<T> {
636 use core::slice::from_raw_parts_mut;
639 let slice = from_raw_parts_mut(self.elems, self.n_elems);
640 ptr::drop_in_place(slice);
642 Heap.dealloc(self.mem, self.layout.clone());
648 let v_ptr = v as *const [T];
649 let ptr = Self::allocate_for_ptr(v_ptr);
651 let mem = ptr as *mut _ as *mut u8;
652 let layout = Layout::for_value(&*ptr);
654 // Pointer to first element
655 let elems = &mut (*ptr).data as *mut [T] as *mut T;
657 let mut guard = Guard{
664 for (i, item) in v.iter().enumerate() {
665 ptr::write(elems.offset(i as isize), item.clone());
669 // All clear. Forget the guard so it doesn't free the new ArcInner.
672 Arc { ptr: NonNull::new_unchecked(ptr), phantom: PhantomData }
677 impl<T: Copy> ArcFromSlice<T> for Arc<[T]> {
679 fn from_slice(v: &[T]) -> Self {
680 unsafe { Arc::copy_from_slice(v) }
684 #[stable(feature = "rust1", since = "1.0.0")]
685 impl<T: ?Sized> Clone for Arc<T> {
686 /// Makes a clone of the `Arc` pointer.
688 /// This creates another pointer to the same inner value, increasing the
689 /// strong reference count.
694 /// use std::sync::Arc;
696 /// let five = Arc::new(5);
698 /// Arc::clone(&five);
701 fn clone(&self) -> Arc<T> {
702 // Using a relaxed ordering is alright here, as knowledge of the
703 // original reference prevents other threads from erroneously deleting
706 // As explained in the [Boost documentation][1], Increasing the
707 // reference counter can always be done with memory_order_relaxed: New
708 // references to an object can only be formed from an existing
709 // reference, and passing an existing reference from one thread to
710 // another must already provide any required synchronization.
712 // [1]: (www.boost.org/doc/libs/1_55_0/doc/html/atomic/usage_examples.html)
713 let old_size = self.inner().strong.fetch_add(1, Relaxed);
715 // However we need to guard against massive refcounts in case someone
716 // is `mem::forget`ing Arcs. If we don't do this the count can overflow
717 // and users will use-after free. We racily saturate to `isize::MAX` on
718 // the assumption that there aren't ~2 billion threads incrementing
719 // the reference count at once. This branch will never be taken in
720 // any realistic program.
722 // We abort because such a program is incredibly degenerate, and we
723 // don't care to support it.
724 if old_size > MAX_REFCOUNT {
730 Arc { ptr: self.ptr, phantom: PhantomData }
734 #[stable(feature = "rust1", since = "1.0.0")]
735 impl<T: ?Sized> Deref for Arc<T> {
739 fn deref(&self) -> &T {
744 impl<T: Clone> Arc<T> {
745 /// Makes a mutable reference into the given `Arc`.
747 /// If there are other `Arc` or [`Weak`][weak] pointers to the same value,
748 /// then `make_mut` will invoke [`clone`][clone] on the inner value to
749 /// ensure unique ownership. This is also referred to as clone-on-write.
751 /// See also [`get_mut`][get_mut], which will fail rather than cloning.
753 /// [weak]: struct.Weak.html
754 /// [clone]: ../../std/clone/trait.Clone.html#tymethod.clone
755 /// [get_mut]: struct.Arc.html#method.get_mut
760 /// use std::sync::Arc;
762 /// let mut data = Arc::new(5);
764 /// *Arc::make_mut(&mut data) += 1; // Won't clone anything
765 /// let mut other_data = Arc::clone(&data); // Won't clone inner data
766 /// *Arc::make_mut(&mut data) += 1; // Clones inner data
767 /// *Arc::make_mut(&mut data) += 1; // Won't clone anything
768 /// *Arc::make_mut(&mut other_data) *= 2; // Won't clone anything
770 /// // Now `data` and `other_data` point to different values.
771 /// assert_eq!(*data, 8);
772 /// assert_eq!(*other_data, 12);
775 #[stable(feature = "arc_unique", since = "1.4.0")]
776 pub fn make_mut(this: &mut Self) -> &mut T {
777 // Note that we hold both a strong reference and a weak reference.
778 // Thus, releasing our strong reference only will not, by itself, cause
779 // the memory to be deallocated.
781 // Use Acquire to ensure that we see any writes to `weak` that happen
782 // before release writes (i.e., decrements) to `strong`. Since we hold a
783 // weak count, there's no chance the ArcInner itself could be
785 if this.inner().strong.compare_exchange(1, 0, Acquire, Relaxed).is_err() {
786 // Another strong pointer exists; clone
787 *this = Arc::new((**this).clone());
788 } else if this.inner().weak.load(Relaxed) != 1 {
789 // Relaxed suffices in the above because this is fundamentally an
790 // optimization: we are always racing with weak pointers being
791 // dropped. Worst case, we end up allocated a new Arc unnecessarily.
793 // We removed the last strong ref, but there are additional weak
794 // refs remaining. We'll move the contents to a new Arc, and
795 // invalidate the other weak refs.
797 // Note that it is not possible for the read of `weak` to yield
798 // usize::MAX (i.e., locked), since the weak count can only be
799 // locked by a thread with a strong reference.
801 // Materialize our own implicit weak pointer, so that it can clean
802 // up the ArcInner as needed.
803 let weak = Weak { ptr: this.ptr };
805 // mark the data itself as already deallocated
807 // there is no data race in the implicit write caused by `read`
808 // here (due to zeroing) because data is no longer accessed by
809 // other threads (due to there being no more strong refs at this
811 let mut swap = Arc::new(ptr::read(&weak.ptr.as_ref().data));
812 mem::swap(this, &mut swap);
816 // We were the sole reference of either kind; bump back up the
818 this.inner().strong.store(1, Release);
821 // As with `get_mut()`, the unsafety is ok because our reference was
822 // either unique to begin with, or became one upon cloning the contents.
824 &mut this.ptr.as_mut().data
829 impl<T: ?Sized> Arc<T> {
830 /// Returns a mutable reference to the inner value, if there are
831 /// no other `Arc` or [`Weak`][weak] pointers to the same value.
833 /// Returns [`None`][option] otherwise, because it is not safe to
834 /// mutate a shared value.
836 /// See also [`make_mut`][make_mut], which will [`clone`][clone]
837 /// the inner value when it's shared.
839 /// [weak]: struct.Weak.html
840 /// [option]: ../../std/option/enum.Option.html
841 /// [make_mut]: struct.Arc.html#method.make_mut
842 /// [clone]: ../../std/clone/trait.Clone.html#tymethod.clone
847 /// use std::sync::Arc;
849 /// let mut x = Arc::new(3);
850 /// *Arc::get_mut(&mut x).unwrap() = 4;
851 /// assert_eq!(*x, 4);
853 /// let _y = Arc::clone(&x);
854 /// assert!(Arc::get_mut(&mut x).is_none());
857 #[stable(feature = "arc_unique", since = "1.4.0")]
858 pub fn get_mut(this: &mut Self) -> Option<&mut T> {
859 if this.is_unique() {
860 // This unsafety is ok because we're guaranteed that the pointer
861 // returned is the *only* pointer that will ever be returned to T. Our
862 // reference count is guaranteed to be 1 at this point, and we required
863 // the Arc itself to be `mut`, so we're returning the only possible
864 // reference to the inner data.
866 Some(&mut this.ptr.as_mut().data)
873 /// Determine whether this is the unique reference (including weak refs) to
874 /// the underlying data.
876 /// Note that this requires locking the weak ref count.
877 fn is_unique(&mut self) -> bool {
878 // lock the weak pointer count if we appear to be the sole weak pointer
881 // The acquire label here ensures a happens-before relationship with any
882 // writes to `strong` prior to decrements of the `weak` count (via drop,
883 // which uses Release).
884 if self.inner().weak.compare_exchange(1, usize::MAX, Acquire, Relaxed).is_ok() {
885 // Due to the previous acquire read, this will observe any writes to
886 // `strong` that were due to upgrading weak pointers; only strong
887 // clones remain, which require that the strong count is > 1 anyway.
888 let unique = self.inner().strong.load(Relaxed) == 1;
890 // The release write here synchronizes with a read in `downgrade`,
891 // effectively preventing the above read of `strong` from happening
893 self.inner().weak.store(1, Release); // release the lock
901 #[stable(feature = "rust1", since = "1.0.0")]
902 unsafe impl<#[may_dangle] T: ?Sized> Drop for Arc<T> {
905 /// This will decrement the strong reference count. If the strong reference
906 /// count reaches zero then the only other references (if any) are
907 /// [`Weak`][weak], so we `drop` the inner value.
909 /// [weak]: struct.Weak.html
914 /// use std::sync::Arc;
918 /// impl Drop for Foo {
919 /// fn drop(&mut self) {
920 /// println!("dropped!");
924 /// let foo = Arc::new(Foo);
925 /// let foo2 = Arc::clone(&foo);
927 /// drop(foo); // Doesn't print anything
928 /// drop(foo2); // Prints "dropped!"
932 // Because `fetch_sub` is already atomic, we do not need to synchronize
933 // with other threads unless we are going to delete the object. This
934 // same logic applies to the below `fetch_sub` to the `weak` count.
935 if self.inner().strong.fetch_sub(1, Release) != 1 {
939 // This fence is needed to prevent reordering of use of the data and
940 // deletion of the data. Because it is marked `Release`, the decreasing
941 // of the reference count synchronizes with this `Acquire` fence. This
942 // means that use of the data happens before decreasing the reference
943 // count, which happens before this fence, which happens before the
944 // deletion of the data.
946 // As explained in the [Boost documentation][1],
948 // > It is important to enforce any possible access to the object in one
949 // > thread (through an existing reference) to *happen before* deleting
950 // > the object in a different thread. This is achieved by a "release"
951 // > operation after dropping a reference (any access to the object
952 // > through this reference must obviously happened before), and an
953 // > "acquire" operation before deleting the object.
955 // In particular, while the contents of an Arc are usually immutable, it's
956 // possible to have interior writes to something like a Mutex<T>. Since a
957 // Mutex is not acquired when it is deleted, we can't rely on its
958 // synchronization logic to make writes in thread A visible to a destructor
959 // running in thread B.
961 // Also note that the Acquire fence here could probably be replaced with an
962 // Acquire load, which could improve performance in highly-contended
963 // situations. See [2].
965 // [1]: (www.boost.org/doc/libs/1_55_0/doc/html/atomic/usage_examples.html)
966 // [2]: (https://github.com/rust-lang/rust/pull/41714)
967 atomic::fence(Acquire);
976 /// Constructs a new `Weak<T>`, allocating memory for `T` without initializing
977 /// it. Calling [`upgrade`] on the return value always gives [`None`].
979 /// [`upgrade`]: struct.Weak.html#method.upgrade
980 /// [`None`]: ../../std/option/enum.Option.html#variant.None
985 /// use std::sync::Weak;
987 /// let empty: Weak<i64> = Weak::new();
988 /// assert!(empty.upgrade().is_none());
990 #[stable(feature = "downgraded_weak", since = "1.10.0")]
991 pub fn new() -> Weak<T> {
994 ptr: NonNull::from(Box::into_unique(box ArcInner {
995 strong: atomic::AtomicUsize::new(0),
996 weak: atomic::AtomicUsize::new(1),
997 data: uninitialized(),
1004 impl<T: ?Sized> Weak<T> {
1005 /// Attempts to upgrade the `Weak` pointer to an [`Arc`], extending
1006 /// the lifetime of the value if successful.
1008 /// Returns [`None`] if the value has since been dropped.
1010 /// [`Arc`]: struct.Arc.html
1011 /// [`None`]: ../../std/option/enum.Option.html#variant.None
1016 /// use std::sync::Arc;
1018 /// let five = Arc::new(5);
1020 /// let weak_five = Arc::downgrade(&five);
1022 /// let strong_five: Option<Arc<_>> = weak_five.upgrade();
1023 /// assert!(strong_five.is_some());
1025 /// // Destroy all strong pointers.
1026 /// drop(strong_five);
1029 /// assert!(weak_five.upgrade().is_none());
1031 #[stable(feature = "arc_weak", since = "1.4.0")]
1032 pub fn upgrade(&self) -> Option<Arc<T>> {
1033 // We use a CAS loop to increment the strong count instead of a
1034 // fetch_add because once the count hits 0 it must never be above 0.
1035 let inner = self.inner();
1037 // Relaxed load because any write of 0 that we can observe
1038 // leaves the field in a permanently zero state (so a
1039 // "stale" read of 0 is fine), and any other value is
1040 // confirmed via the CAS below.
1041 let mut n = inner.strong.load(Relaxed);
1048 // See comments in `Arc::clone` for why we do this (for `mem::forget`).
1049 if n > MAX_REFCOUNT {
1055 // Relaxed is valid for the same reason it is on Arc's Clone impl
1056 match inner.strong.compare_exchange_weak(n, n + 1, Relaxed, Relaxed) {
1057 Ok(_) => return Some(Arc { ptr: self.ptr, phantom: PhantomData }),
1058 Err(old) => n = old,
1064 fn inner(&self) -> &ArcInner<T> {
1065 // See comments above for why this is "safe"
1066 unsafe { self.ptr.as_ref() }
1070 #[stable(feature = "arc_weak", since = "1.4.0")]
1071 impl<T: ?Sized> Clone for Weak<T> {
1072 /// Makes a clone of the `Weak` pointer that points to the same value.
1077 /// use std::sync::{Arc, Weak};
1079 /// let weak_five = Arc::downgrade(&Arc::new(5));
1081 /// Weak::clone(&weak_five);
1084 fn clone(&self) -> Weak<T> {
1085 // See comments in Arc::clone() for why this is relaxed. This can use a
1086 // fetch_add (ignoring the lock) because the weak count is only locked
1087 // where are *no other* weak pointers in existence. (So we can't be
1088 // running this code in that case).
1089 let old_size = self.inner().weak.fetch_add(1, Relaxed);
1091 // See comments in Arc::clone() for why we do this (for mem::forget).
1092 if old_size > MAX_REFCOUNT {
1098 return Weak { ptr: self.ptr };
1102 #[stable(feature = "downgraded_weak", since = "1.10.0")]
1103 impl<T> Default for Weak<T> {
1104 /// Constructs a new `Weak<T>`, allocating memory for `T` without initializing
1105 /// it. Calling [`upgrade`] on the return value always gives [`None`].
1107 /// [`upgrade`]: struct.Weak.html#method.upgrade
1108 /// [`None`]: ../../std/option/enum.Option.html#variant.None
1113 /// use std::sync::Weak;
1115 /// let empty: Weak<i64> = Default::default();
1116 /// assert!(empty.upgrade().is_none());
1118 fn default() -> Weak<T> {
1123 #[stable(feature = "arc_weak", since = "1.4.0")]
1124 impl<T: ?Sized> Drop for Weak<T> {
1125 /// Drops the `Weak` pointer.
1130 /// use std::sync::{Arc, Weak};
1134 /// impl Drop for Foo {
1135 /// fn drop(&mut self) {
1136 /// println!("dropped!");
1140 /// let foo = Arc::new(Foo);
1141 /// let weak_foo = Arc::downgrade(&foo);
1142 /// let other_weak_foo = Weak::clone(&weak_foo);
1144 /// drop(weak_foo); // Doesn't print anything
1145 /// drop(foo); // Prints "dropped!"
1147 /// assert!(other_weak_foo.upgrade().is_none());
1149 fn drop(&mut self) {
1150 let ptr = self.ptr.as_ptr();
1152 // If we find out that we were the last weak pointer, then its time to
1153 // deallocate the data entirely. See the discussion in Arc::drop() about
1154 // the memory orderings
1156 // It's not necessary to check for the locked state here, because the
1157 // weak count can only be locked if there was precisely one weak ref,
1158 // meaning that drop could only subsequently run ON that remaining weak
1159 // ref, which can only happen after the lock is released.
1160 if self.inner().weak.fetch_sub(1, Release) == 1 {
1161 atomic::fence(Acquire);
1163 Heap.dealloc(ptr as *mut u8, Layout::for_value(&*ptr))
1169 #[stable(feature = "rust1", since = "1.0.0")]
1170 impl<T: ?Sized + PartialEq> PartialEq for Arc<T> {
1171 /// Equality for two `Arc`s.
1173 /// Two `Arc`s are equal if their inner values are equal.
1178 /// use std::sync::Arc;
1180 /// let five = Arc::new(5);
1182 /// assert!(five == Arc::new(5));
1184 fn eq(&self, other: &Arc<T>) -> bool {
1185 *(*self) == *(*other)
1188 /// Inequality for two `Arc`s.
1190 /// Two `Arc`s are unequal if their inner values are unequal.
1195 /// use std::sync::Arc;
1197 /// let five = Arc::new(5);
1199 /// assert!(five != Arc::new(6));
1201 fn ne(&self, other: &Arc<T>) -> bool {
1202 *(*self) != *(*other)
1205 #[stable(feature = "rust1", since = "1.0.0")]
1206 impl<T: ?Sized + PartialOrd> PartialOrd for Arc<T> {
1207 /// Partial comparison for two `Arc`s.
1209 /// The two are compared by calling `partial_cmp()` on their inner values.
1214 /// use std::sync::Arc;
1215 /// use std::cmp::Ordering;
1217 /// let five = Arc::new(5);
1219 /// assert_eq!(Some(Ordering::Less), five.partial_cmp(&Arc::new(6)));
1221 fn partial_cmp(&self, other: &Arc<T>) -> Option<Ordering> {
1222 (**self).partial_cmp(&**other)
1225 /// Less-than comparison for two `Arc`s.
1227 /// The two are compared by calling `<` on their inner values.
1232 /// use std::sync::Arc;
1234 /// let five = Arc::new(5);
1236 /// assert!(five < Arc::new(6));
1238 fn lt(&self, other: &Arc<T>) -> bool {
1239 *(*self) < *(*other)
1242 /// 'Less than or equal to' comparison for two `Arc`s.
1244 /// The two are compared by calling `<=` on their inner values.
1249 /// use std::sync::Arc;
1251 /// let five = Arc::new(5);
1253 /// assert!(five <= Arc::new(5));
1255 fn le(&self, other: &Arc<T>) -> bool {
1256 *(*self) <= *(*other)
1259 /// Greater-than comparison for two `Arc`s.
1261 /// The two are compared by calling `>` on their inner values.
1266 /// use std::sync::Arc;
1268 /// let five = Arc::new(5);
1270 /// assert!(five > Arc::new(4));
1272 fn gt(&self, other: &Arc<T>) -> bool {
1273 *(*self) > *(*other)
1276 /// 'Greater than or equal to' comparison for two `Arc`s.
1278 /// The two are compared by calling `>=` on their inner values.
1283 /// use std::sync::Arc;
1285 /// let five = Arc::new(5);
1287 /// assert!(five >= Arc::new(5));
1289 fn ge(&self, other: &Arc<T>) -> bool {
1290 *(*self) >= *(*other)
1293 #[stable(feature = "rust1", since = "1.0.0")]
1294 impl<T: ?Sized + Ord> Ord for Arc<T> {
1295 /// Comparison for two `Arc`s.
1297 /// The two are compared by calling `cmp()` on their inner values.
1302 /// use std::sync::Arc;
1303 /// use std::cmp::Ordering;
1305 /// let five = Arc::new(5);
1307 /// assert_eq!(Ordering::Less, five.cmp(&Arc::new(6)));
1309 fn cmp(&self, other: &Arc<T>) -> Ordering {
1310 (**self).cmp(&**other)
1313 #[stable(feature = "rust1", since = "1.0.0")]
1314 impl<T: ?Sized + Eq> Eq for Arc<T> {}
1316 #[stable(feature = "rust1", since = "1.0.0")]
1317 impl<T: ?Sized + fmt::Display> fmt::Display for Arc<T> {
1318 fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
1319 fmt::Display::fmt(&**self, f)
1323 #[stable(feature = "rust1", since = "1.0.0")]
1324 impl<T: ?Sized + fmt::Debug> fmt::Debug for Arc<T> {
1325 fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
1326 fmt::Debug::fmt(&**self, f)
1330 #[stable(feature = "rust1", since = "1.0.0")]
1331 impl<T: ?Sized> fmt::Pointer for Arc<T> {
1332 fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
1333 fmt::Pointer::fmt(&(&**self as *const T), f)
1337 #[stable(feature = "rust1", since = "1.0.0")]
1338 impl<T: Default> Default for Arc<T> {
1339 /// Creates a new `Arc<T>`, with the `Default` value for `T`.
1344 /// use std::sync::Arc;
1346 /// let x: Arc<i32> = Default::default();
1347 /// assert_eq!(*x, 0);
1349 fn default() -> Arc<T> {
1350 Arc::new(Default::default())
1354 #[stable(feature = "rust1", since = "1.0.0")]
1355 impl<T: ?Sized + Hash> Hash for Arc<T> {
1356 fn hash<H: Hasher>(&self, state: &mut H) {
1357 (**self).hash(state)
1361 #[stable(feature = "from_for_ptrs", since = "1.6.0")]
1362 impl<T> From<T> for Arc<T> {
1363 fn from(t: T) -> Self {
1368 #[stable(feature = "shared_from_slice", since = "1.21.0")]
1369 impl<'a, T: Clone> From<&'a [T]> for Arc<[T]> {
1371 fn from(v: &[T]) -> Arc<[T]> {
1372 <Self as ArcFromSlice<T>>::from_slice(v)
1376 #[stable(feature = "shared_from_slice", since = "1.21.0")]
1377 impl<'a> From<&'a str> for Arc<str> {
1379 fn from(v: &str) -> Arc<str> {
1380 let arc = Arc::<[u8]>::from(v.as_bytes());
1381 unsafe { Arc::from_raw(Arc::into_raw(arc) as *const str) }
1385 #[stable(feature = "shared_from_slice", since = "1.21.0")]
1386 impl From<String> for Arc<str> {
1388 fn from(v: String) -> Arc<str> {
1393 #[stable(feature = "shared_from_slice", since = "1.21.0")]
1394 impl<T: ?Sized> From<Box<T>> for Arc<T> {
1396 fn from(v: Box<T>) -> Arc<T> {
1401 #[stable(feature = "shared_from_slice", since = "1.21.0")]
1402 impl<T> From<Vec<T>> for Arc<[T]> {
1404 fn from(mut v: Vec<T>) -> Arc<[T]> {
1406 let arc = Arc::copy_from_slice(&v);
1408 // Allow the Vec to free its memory, but not destroy its contents
1418 use std::boxed::Box;
1419 use std::clone::Clone;
1420 use std::sync::mpsc::channel;
1423 use std::option::Option;
1424 use std::option::Option::{None, Some};
1425 use std::sync::atomic;
1426 use std::sync::atomic::Ordering::{Acquire, SeqCst};
1428 use std::sync::Mutex;
1429 use std::convert::From;
1431 use super::{Arc, Weak};
1434 struct Canary(*mut atomic::AtomicUsize);
1436 impl Drop for Canary {
1437 fn drop(&mut self) {
1441 (*c).fetch_add(1, SeqCst);
1449 #[cfg_attr(target_os = "emscripten", ignore)]
1450 fn manually_share_arc() {
1451 let v = vec![1, 2, 3, 4, 5, 6, 7, 8, 9, 10];
1452 let arc_v = Arc::new(v);
1454 let (tx, rx) = channel();
1456 let _t = thread::spawn(move || {
1457 let arc_v: Arc<Vec<i32>> = rx.recv().unwrap();
1458 assert_eq!((*arc_v)[3], 4);
1461 tx.send(arc_v.clone()).unwrap();
1463 assert_eq!((*arc_v)[2], 3);
1464 assert_eq!((*arc_v)[4], 5);
1468 fn test_arc_get_mut() {
1469 let mut x = Arc::new(3);
1470 *Arc::get_mut(&mut x).unwrap() = 4;
1473 assert!(Arc::get_mut(&mut x).is_none());
1475 assert!(Arc::get_mut(&mut x).is_some());
1476 let _w = Arc::downgrade(&x);
1477 assert!(Arc::get_mut(&mut x).is_none());
1482 let x = Arc::new(3);
1483 assert_eq!(Arc::try_unwrap(x), Ok(3));
1484 let x = Arc::new(4);
1486 assert_eq!(Arc::try_unwrap(x), Err(Arc::new(4)));
1487 let x = Arc::new(5);
1488 let _w = Arc::downgrade(&x);
1489 assert_eq!(Arc::try_unwrap(x), Ok(5));
1493 fn into_from_raw() {
1494 let x = Arc::new(box "hello");
1497 let x_ptr = Arc::into_raw(x);
1500 assert_eq!(**x_ptr, "hello");
1502 let x = Arc::from_raw(x_ptr);
1503 assert_eq!(**x, "hello");
1505 assert_eq!(Arc::try_unwrap(x).map(|x| *x), Ok("hello"));
1510 fn test_into_from_raw_unsized() {
1511 use std::fmt::Display;
1512 use std::string::ToString;
1514 let arc: Arc<str> = Arc::from("foo");
1516 let ptr = Arc::into_raw(arc.clone());
1517 let arc2 = unsafe { Arc::from_raw(ptr) };
1519 assert_eq!(unsafe { &*ptr }, "foo");
1520 assert_eq!(arc, arc2);
1522 let arc: Arc<Display> = Arc::new(123);
1524 let ptr = Arc::into_raw(arc.clone());
1525 let arc2 = unsafe { Arc::from_raw(ptr) };
1527 assert_eq!(unsafe { &*ptr }.to_string(), "123");
1528 assert_eq!(arc2.to_string(), "123");
1532 fn test_cowarc_clone_make_mut() {
1533 let mut cow0 = Arc::new(75);
1534 let mut cow1 = cow0.clone();
1535 let mut cow2 = cow1.clone();
1537 assert!(75 == *Arc::make_mut(&mut cow0));
1538 assert!(75 == *Arc::make_mut(&mut cow1));
1539 assert!(75 == *Arc::make_mut(&mut cow2));
1541 *Arc::make_mut(&mut cow0) += 1;
1542 *Arc::make_mut(&mut cow1) += 2;
1543 *Arc::make_mut(&mut cow2) += 3;
1545 assert!(76 == *cow0);
1546 assert!(77 == *cow1);
1547 assert!(78 == *cow2);
1549 // none should point to the same backing memory
1550 assert!(*cow0 != *cow1);
1551 assert!(*cow0 != *cow2);
1552 assert!(*cow1 != *cow2);
1556 fn test_cowarc_clone_unique2() {
1557 let mut cow0 = Arc::new(75);
1558 let cow1 = cow0.clone();
1559 let cow2 = cow1.clone();
1561 assert!(75 == *cow0);
1562 assert!(75 == *cow1);
1563 assert!(75 == *cow2);
1565 *Arc::make_mut(&mut cow0) += 1;
1566 assert!(76 == *cow0);
1567 assert!(75 == *cow1);
1568 assert!(75 == *cow2);
1570 // cow1 and cow2 should share the same contents
1571 // cow0 should have a unique reference
1572 assert!(*cow0 != *cow1);
1573 assert!(*cow0 != *cow2);
1574 assert!(*cow1 == *cow2);
1578 fn test_cowarc_clone_weak() {
1579 let mut cow0 = Arc::new(75);
1580 let cow1_weak = Arc::downgrade(&cow0);
1582 assert!(75 == *cow0);
1583 assert!(75 == *cow1_weak.upgrade().unwrap());
1585 *Arc::make_mut(&mut cow0) += 1;
1587 assert!(76 == *cow0);
1588 assert!(cow1_weak.upgrade().is_none());
1593 let x = Arc::new(5);
1594 let y = Arc::downgrade(&x);
1595 assert!(y.upgrade().is_some());
1600 let x = Arc::new(5);
1601 let y = Arc::downgrade(&x);
1603 assert!(y.upgrade().is_none());
1607 fn weak_self_cyclic() {
1609 x: Mutex<Option<Weak<Cycle>>>,
1612 let a = Arc::new(Cycle { x: Mutex::new(None) });
1613 let b = Arc::downgrade(&a.clone());
1614 *a.x.lock().unwrap() = Some(b);
1616 // hopefully we don't double-free (or leak)...
1621 let mut canary = atomic::AtomicUsize::new(0);
1622 let x = Arc::new(Canary(&mut canary as *mut atomic::AtomicUsize));
1624 assert!(canary.load(Acquire) == 1);
1628 fn drop_arc_weak() {
1629 let mut canary = atomic::AtomicUsize::new(0);
1630 let arc = Arc::new(Canary(&mut canary as *mut atomic::AtomicUsize));
1631 let arc_weak = Arc::downgrade(&arc);
1632 assert!(canary.load(Acquire) == 0);
1634 assert!(canary.load(Acquire) == 1);
1639 fn test_strong_count() {
1640 let a = Arc::new(0);
1641 assert!(Arc::strong_count(&a) == 1);
1642 let w = Arc::downgrade(&a);
1643 assert!(Arc::strong_count(&a) == 1);
1644 let b = w.upgrade().expect("");
1645 assert!(Arc::strong_count(&b) == 2);
1646 assert!(Arc::strong_count(&a) == 2);
1649 assert!(Arc::strong_count(&b) == 1);
1651 assert!(Arc::strong_count(&b) == 2);
1652 assert!(Arc::strong_count(&c) == 2);
1656 fn test_weak_count() {
1657 let a = Arc::new(0);
1658 assert!(Arc::strong_count(&a) == 1);
1659 assert!(Arc::weak_count(&a) == 0);
1660 let w = Arc::downgrade(&a);
1661 assert!(Arc::strong_count(&a) == 1);
1662 assert!(Arc::weak_count(&a) == 1);
1664 assert!(Arc::weak_count(&a) == 2);
1667 assert!(Arc::strong_count(&a) == 1);
1668 assert!(Arc::weak_count(&a) == 0);
1670 assert!(Arc::strong_count(&a) == 2);
1671 assert!(Arc::weak_count(&a) == 0);
1672 let d = Arc::downgrade(&c);
1673 assert!(Arc::weak_count(&c) == 1);
1674 assert!(Arc::strong_count(&c) == 2);
1683 let a = Arc::new(5);
1684 assert_eq!(format!("{:?}", a), "5");
1687 // Make sure deriving works with Arc<T>
1688 #[derive(Eq, Ord, PartialEq, PartialOrd, Clone, Debug, Default)]
1695 let x: Arc<[i32]> = Arc::new([1, 2, 3]);
1696 assert_eq!(format!("{:?}", x), "[1, 2, 3]");
1697 let y = Arc::downgrade(&x.clone());
1699 assert!(y.upgrade().is_none());
1703 fn test_from_owned() {
1705 let foo_arc = Arc::from(foo);
1706 assert!(123 == *foo_arc);
1710 fn test_new_weak() {
1711 let foo: Weak<usize> = Weak::new();
1712 assert!(foo.upgrade().is_none());
1717 let five = Arc::new(5);
1718 let same_five = five.clone();
1719 let other_five = Arc::new(5);
1721 assert!(Arc::ptr_eq(&five, &same_five));
1722 assert!(!Arc::ptr_eq(&five, &other_five));
1726 #[cfg_attr(target_os = "emscripten", ignore)]
1727 fn test_weak_count_locked() {
1728 let mut a = Arc::new(atomic::AtomicBool::new(false));
1730 let t = thread::spawn(move || {
1731 for _i in 0..1000000 {
1732 Arc::get_mut(&mut a);
1734 a.store(true, SeqCst);
1737 while !a2.load(SeqCst) {
1738 let n = Arc::weak_count(&a2);
1739 assert!(n < 2, "bad weak count: {}", n);
1745 fn test_from_str() {
1746 let r: Arc<str> = Arc::from("foo");
1748 assert_eq!(&r[..], "foo");
1752 fn test_copy_from_slice() {
1753 let s: &[u32] = &[1, 2, 3];
1754 let r: Arc<[u32]> = Arc::from(s);
1756 assert_eq!(&r[..], [1, 2, 3]);
1760 fn test_clone_from_slice() {
1761 #[derive(Clone, Debug, Eq, PartialEq)]
1764 let s: &[X] = &[X(1), X(2), X(3)];
1765 let r: Arc<[X]> = Arc::from(s);
1767 assert_eq!(&r[..], s);
1772 fn test_clone_from_slice_panic() {
1773 use std::string::{String, ToString};
1775 struct Fail(u32, String);
1777 impl Clone for Fail {
1778 fn clone(&self) -> Fail {
1782 Fail(self.0, self.1.clone())
1787 Fail(0, "foo".to_string()),
1788 Fail(1, "bar".to_string()),
1789 Fail(2, "baz".to_string()),
1792 // Should panic, but not cause memory corruption
1793 let _r: Arc<[Fail]> = Arc::from(s);
1797 fn test_from_box() {
1798 let b: Box<u32> = box 123;
1799 let r: Arc<u32> = Arc::from(b);
1801 assert_eq!(*r, 123);
1805 fn test_from_box_str() {
1806 use std::string::String;
1808 let s = String::from("foo").into_boxed_str();
1809 let r: Arc<str> = Arc::from(s);
1811 assert_eq!(&r[..], "foo");
1815 fn test_from_box_slice() {
1816 let s = vec![1, 2, 3].into_boxed_slice();
1817 let r: Arc<[u32]> = Arc::from(s);
1819 assert_eq!(&r[..], [1, 2, 3]);
1823 fn test_from_box_trait() {
1824 use std::fmt::Display;
1825 use std::string::ToString;
1827 let b: Box<Display> = box 123;
1828 let r: Arc<Display> = Arc::from(b);
1830 assert_eq!(r.to_string(), "123");
1834 fn test_from_box_trait_zero_sized() {
1835 use std::fmt::Debug;
1837 let b: Box<Debug> = box ();
1838 let r: Arc<Debug> = Arc::from(b);
1840 assert_eq!(format!("{:?}", r), "()");
1844 fn test_from_vec() {
1845 let v = vec![1, 2, 3];
1846 let r: Arc<[u32]> = Arc::from(v);
1848 assert_eq!(&r[..], [1, 2, 3]);
1852 #[stable(feature = "rust1", since = "1.0.0")]
1853 impl<T: ?Sized> borrow::Borrow<T> for Arc<T> {
1854 fn borrow(&self) -> &T {
1859 #[stable(since = "1.5.0", feature = "smart_ptr_as_ref")]
1860 impl<T: ?Sized> AsRef<T> for Arc<T> {
1861 fn as_ref(&self) -> &T {