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 alloc::{Global, Alloc, Layout, box_free, oom};
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: Box::into_raw_non_null(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 // Destroy the data at this time, even though we may not free the box
516 // allocation itself (there may still be weak pointers lying around).
517 ptr::drop_in_place(&mut self.ptr.as_mut().data);
519 if self.inner().weak.fetch_sub(1, Release) == 1 {
520 atomic::fence(Acquire);
521 Global.dealloc(self.ptr.as_opaque(), Layout::for_value(self.ptr.as_ref()))
526 #[stable(feature = "ptr_eq", since = "1.17.0")]
527 /// Returns true if the two `Arc`s point to the same value (not
528 /// just values that compare as equal).
533 /// use std::sync::Arc;
535 /// let five = Arc::new(5);
536 /// let same_five = Arc::clone(&five);
537 /// let other_five = Arc::new(5);
539 /// assert!(Arc::ptr_eq(&five, &same_five));
540 /// assert!(!Arc::ptr_eq(&five, &other_five));
542 pub fn ptr_eq(this: &Self, other: &Self) -> bool {
543 this.ptr.as_ptr() == other.ptr.as_ptr()
547 impl<T: ?Sized> Arc<T> {
548 // Allocates an `ArcInner<T>` with sufficient space for an unsized value
549 unsafe fn allocate_for_ptr(ptr: *const T) -> *mut ArcInner<T> {
550 // Create a fake ArcInner to find allocation size and alignment
551 let fake_ptr = ptr as *mut ArcInner<T>;
553 let layout = Layout::for_value(&*fake_ptr);
555 let mem = Global.alloc(layout)
556 .unwrap_or_else(|_| oom());
558 // Initialize the real ArcInner
559 let inner = set_data_ptr(ptr as *mut T, mem.as_ptr() as *mut u8) as *mut ArcInner<T>;
561 ptr::write(&mut (*inner).strong, atomic::AtomicUsize::new(1));
562 ptr::write(&mut (*inner).weak, atomic::AtomicUsize::new(1));
567 fn from_box(v: Box<T>) -> Arc<T> {
569 let box_unique = Box::into_unique(v);
570 let bptr = box_unique.as_ptr();
572 let value_size = size_of_val(&*bptr);
573 let ptr = Self::allocate_for_ptr(bptr);
575 // Copy value as bytes
576 ptr::copy_nonoverlapping(
577 bptr as *const T as *const u8,
578 &mut (*ptr).data as *mut _ as *mut u8,
581 // Free the allocation without dropping its contents
582 box_free(box_unique);
584 Arc { ptr: NonNull::new_unchecked(ptr), phantom: PhantomData }
589 // Sets the data pointer of a `?Sized` raw pointer.
591 // For a slice/trait object, this sets the `data` field and leaves the rest
592 // unchanged. For a sized raw pointer, this simply sets the pointer.
593 unsafe fn set_data_ptr<T: ?Sized, U>(mut ptr: *mut T, data: *mut U) -> *mut T {
594 ptr::write(&mut ptr as *mut _ as *mut *mut u8, data as *mut u8);
599 // Copy elements from slice into newly allocated Arc<[T]>
601 // Unsafe because the caller must either take ownership or bind `T: Copy`
602 unsafe fn copy_from_slice(v: &[T]) -> Arc<[T]> {
603 let v_ptr = v as *const [T];
604 let ptr = Self::allocate_for_ptr(v_ptr);
606 ptr::copy_nonoverlapping(
608 &mut (*ptr).data as *mut [T] as *mut T,
611 Arc { ptr: NonNull::new_unchecked(ptr), phantom: PhantomData }
615 // Specialization trait used for From<&[T]>
616 trait ArcFromSlice<T> {
617 fn from_slice(slice: &[T]) -> Self;
620 impl<T: Clone> ArcFromSlice<T> for Arc<[T]> {
622 default fn from_slice(v: &[T]) -> Self {
623 // Panic guard while cloning T elements.
624 // In the event of a panic, elements that have been written
625 // into the new ArcInner will be dropped, then the memory freed.
633 impl<T> Drop for Guard<T> {
635 use core::slice::from_raw_parts_mut;
638 let slice = from_raw_parts_mut(self.elems, self.n_elems);
639 ptr::drop_in_place(slice);
641 Global.dealloc(self.mem.as_opaque(), self.layout.clone());
647 let v_ptr = v as *const [T];
648 let ptr = Self::allocate_for_ptr(v_ptr);
650 let mem = ptr as *mut _ as *mut u8;
651 let layout = Layout::for_value(&*ptr);
653 // Pointer to first element
654 let elems = &mut (*ptr).data as *mut [T] as *mut T;
656 let mut guard = Guard{
657 mem: NonNull::new_unchecked(mem),
663 for (i, item) in v.iter().enumerate() {
664 ptr::write(elems.offset(i as isize), item.clone());
668 // All clear. Forget the guard so it doesn't free the new ArcInner.
671 Arc { ptr: NonNull::new_unchecked(ptr), phantom: PhantomData }
676 impl<T: Copy> ArcFromSlice<T> for Arc<[T]> {
678 fn from_slice(v: &[T]) -> Self {
679 unsafe { Arc::copy_from_slice(v) }
683 #[stable(feature = "rust1", since = "1.0.0")]
684 impl<T: ?Sized> Clone for Arc<T> {
685 /// Makes a clone of the `Arc` pointer.
687 /// This creates another pointer to the same inner value, increasing the
688 /// strong reference count.
693 /// use std::sync::Arc;
695 /// let five = Arc::new(5);
697 /// Arc::clone(&five);
700 fn clone(&self) -> Arc<T> {
701 // Using a relaxed ordering is alright here, as knowledge of the
702 // original reference prevents other threads from erroneously deleting
705 // As explained in the [Boost documentation][1], Increasing the
706 // reference counter can always be done with memory_order_relaxed: New
707 // references to an object can only be formed from an existing
708 // reference, and passing an existing reference from one thread to
709 // another must already provide any required synchronization.
711 // [1]: (www.boost.org/doc/libs/1_55_0/doc/html/atomic/usage_examples.html)
712 let old_size = self.inner().strong.fetch_add(1, Relaxed);
714 // However we need to guard against massive refcounts in case someone
715 // is `mem::forget`ing Arcs. If we don't do this the count can overflow
716 // and users will use-after free. We racily saturate to `isize::MAX` on
717 // the assumption that there aren't ~2 billion threads incrementing
718 // the reference count at once. This branch will never be taken in
719 // any realistic program.
721 // We abort because such a program is incredibly degenerate, and we
722 // don't care to support it.
723 if old_size > MAX_REFCOUNT {
729 Arc { ptr: self.ptr, phantom: PhantomData }
733 #[stable(feature = "rust1", since = "1.0.0")]
734 impl<T: ?Sized> Deref for Arc<T> {
738 fn deref(&self) -> &T {
743 impl<T: Clone> Arc<T> {
744 /// Makes a mutable reference into the given `Arc`.
746 /// If there are other `Arc` or [`Weak`][weak] pointers to the same value,
747 /// then `make_mut` will invoke [`clone`][clone] on the inner value to
748 /// ensure unique ownership. This is also referred to as clone-on-write.
750 /// See also [`get_mut`][get_mut], which will fail rather than cloning.
752 /// [weak]: struct.Weak.html
753 /// [clone]: ../../std/clone/trait.Clone.html#tymethod.clone
754 /// [get_mut]: struct.Arc.html#method.get_mut
759 /// use std::sync::Arc;
761 /// let mut data = Arc::new(5);
763 /// *Arc::make_mut(&mut data) += 1; // Won't clone anything
764 /// let mut other_data = Arc::clone(&data); // Won't clone inner data
765 /// *Arc::make_mut(&mut data) += 1; // Clones inner data
766 /// *Arc::make_mut(&mut data) += 1; // Won't clone anything
767 /// *Arc::make_mut(&mut other_data) *= 2; // Won't clone anything
769 /// // Now `data` and `other_data` point to different values.
770 /// assert_eq!(*data, 8);
771 /// assert_eq!(*other_data, 12);
774 #[stable(feature = "arc_unique", since = "1.4.0")]
775 pub fn make_mut(this: &mut Self) -> &mut T {
776 // Note that we hold both a strong reference and a weak reference.
777 // Thus, releasing our strong reference only will not, by itself, cause
778 // the memory to be deallocated.
780 // Use Acquire to ensure that we see any writes to `weak` that happen
781 // before release writes (i.e., decrements) to `strong`. Since we hold a
782 // weak count, there's no chance the ArcInner itself could be
784 if this.inner().strong.compare_exchange(1, 0, Acquire, Relaxed).is_err() {
785 // Another strong pointer exists; clone
786 *this = Arc::new((**this).clone());
787 } else if this.inner().weak.load(Relaxed) != 1 {
788 // Relaxed suffices in the above because this is fundamentally an
789 // optimization: we are always racing with weak pointers being
790 // dropped. Worst case, we end up allocated a new Arc unnecessarily.
792 // We removed the last strong ref, but there are additional weak
793 // refs remaining. We'll move the contents to a new Arc, and
794 // invalidate the other weak refs.
796 // Note that it is not possible for the read of `weak` to yield
797 // usize::MAX (i.e., locked), since the weak count can only be
798 // locked by a thread with a strong reference.
800 // Materialize our own implicit weak pointer, so that it can clean
801 // up the ArcInner as needed.
802 let weak = Weak { ptr: this.ptr };
804 // mark the data itself as already deallocated
806 // there is no data race in the implicit write caused by `read`
807 // here (due to zeroing) because data is no longer accessed by
808 // other threads (due to there being no more strong refs at this
810 let mut swap = Arc::new(ptr::read(&weak.ptr.as_ref().data));
811 mem::swap(this, &mut swap);
815 // We were the sole reference of either kind; bump back up the
817 this.inner().strong.store(1, Release);
820 // As with `get_mut()`, the unsafety is ok because our reference was
821 // either unique to begin with, or became one upon cloning the contents.
823 &mut this.ptr.as_mut().data
828 impl<T: ?Sized> Arc<T> {
829 /// Returns a mutable reference to the inner value, if there are
830 /// no other `Arc` or [`Weak`][weak] pointers to the same value.
832 /// Returns [`None`][option] otherwise, because it is not safe to
833 /// mutate a shared value.
835 /// See also [`make_mut`][make_mut], which will [`clone`][clone]
836 /// the inner value when it's shared.
838 /// [weak]: struct.Weak.html
839 /// [option]: ../../std/option/enum.Option.html
840 /// [make_mut]: struct.Arc.html#method.make_mut
841 /// [clone]: ../../std/clone/trait.Clone.html#tymethod.clone
846 /// use std::sync::Arc;
848 /// let mut x = Arc::new(3);
849 /// *Arc::get_mut(&mut x).unwrap() = 4;
850 /// assert_eq!(*x, 4);
852 /// let _y = Arc::clone(&x);
853 /// assert!(Arc::get_mut(&mut x).is_none());
856 #[stable(feature = "arc_unique", since = "1.4.0")]
857 pub fn get_mut(this: &mut Self) -> Option<&mut T> {
858 if this.is_unique() {
859 // This unsafety is ok because we're guaranteed that the pointer
860 // returned is the *only* pointer that will ever be returned to T. Our
861 // reference count is guaranteed to be 1 at this point, and we required
862 // the Arc itself to be `mut`, so we're returning the only possible
863 // reference to the inner data.
865 Some(&mut this.ptr.as_mut().data)
872 /// Determine whether this is the unique reference (including weak refs) to
873 /// the underlying data.
875 /// Note that this requires locking the weak ref count.
876 fn is_unique(&mut self) -> bool {
877 // lock the weak pointer count if we appear to be the sole weak pointer
880 // The acquire label here ensures a happens-before relationship with any
881 // writes to `strong` prior to decrements of the `weak` count (via drop,
882 // which uses Release).
883 if self.inner().weak.compare_exchange(1, usize::MAX, Acquire, Relaxed).is_ok() {
884 // Due to the previous acquire read, this will observe any writes to
885 // `strong` that were due to upgrading weak pointers; only strong
886 // clones remain, which require that the strong count is > 1 anyway.
887 let unique = self.inner().strong.load(Relaxed) == 1;
889 // The release write here synchronizes with a read in `downgrade`,
890 // effectively preventing the above read of `strong` from happening
892 self.inner().weak.store(1, Release); // release the lock
900 #[stable(feature = "rust1", since = "1.0.0")]
901 unsafe impl<#[may_dangle] T: ?Sized> Drop for Arc<T> {
904 /// This will decrement the strong reference count. If the strong reference
905 /// count reaches zero then the only other references (if any) are
906 /// [`Weak`][weak], so we `drop` the inner value.
908 /// [weak]: struct.Weak.html
913 /// use std::sync::Arc;
917 /// impl Drop for Foo {
918 /// fn drop(&mut self) {
919 /// println!("dropped!");
923 /// let foo = Arc::new(Foo);
924 /// let foo2 = Arc::clone(&foo);
926 /// drop(foo); // Doesn't print anything
927 /// drop(foo2); // Prints "dropped!"
931 // Because `fetch_sub` is already atomic, we do not need to synchronize
932 // with other threads unless we are going to delete the object. This
933 // same logic applies to the below `fetch_sub` to the `weak` count.
934 if self.inner().strong.fetch_sub(1, Release) != 1 {
938 // This fence is needed to prevent reordering of use of the data and
939 // deletion of the data. Because it is marked `Release`, the decreasing
940 // of the reference count synchronizes with this `Acquire` fence. This
941 // means that use of the data happens before decreasing the reference
942 // count, which happens before this fence, which happens before the
943 // deletion of the data.
945 // As explained in the [Boost documentation][1],
947 // > It is important to enforce any possible access to the object in one
948 // > thread (through an existing reference) to *happen before* deleting
949 // > the object in a different thread. This is achieved by a "release"
950 // > operation after dropping a reference (any access to the object
951 // > through this reference must obviously happened before), and an
952 // > "acquire" operation before deleting the object.
954 // In particular, while the contents of an Arc are usually immutable, it's
955 // possible to have interior writes to something like a Mutex<T>. Since a
956 // Mutex is not acquired when it is deleted, we can't rely on its
957 // synchronization logic to make writes in thread A visible to a destructor
958 // running in thread B.
960 // Also note that the Acquire fence here could probably be replaced with an
961 // Acquire load, which could improve performance in highly-contended
962 // situations. See [2].
964 // [1]: (www.boost.org/doc/libs/1_55_0/doc/html/atomic/usage_examples.html)
965 // [2]: (https://github.com/rust-lang/rust/pull/41714)
966 atomic::fence(Acquire);
975 /// Constructs a new `Weak<T>`, allocating memory for `T` without initializing
976 /// it. Calling [`upgrade`] on the return value always gives [`None`].
978 /// [`upgrade`]: struct.Weak.html#method.upgrade
979 /// [`None`]: ../../std/option/enum.Option.html#variant.None
984 /// use std::sync::Weak;
986 /// let empty: Weak<i64> = Weak::new();
987 /// assert!(empty.upgrade().is_none());
989 #[stable(feature = "downgraded_weak", since = "1.10.0")]
990 pub fn new() -> Weak<T> {
993 ptr: Box::into_raw_non_null(box ArcInner {
994 strong: atomic::AtomicUsize::new(0),
995 weak: atomic::AtomicUsize::new(1),
996 data: uninitialized(),
1003 impl<T: ?Sized> Weak<T> {
1004 /// Attempts to upgrade the `Weak` pointer to an [`Arc`], extending
1005 /// the lifetime of the value if successful.
1007 /// Returns [`None`] if the value has since been dropped.
1009 /// [`Arc`]: struct.Arc.html
1010 /// [`None`]: ../../std/option/enum.Option.html#variant.None
1015 /// use std::sync::Arc;
1017 /// let five = Arc::new(5);
1019 /// let weak_five = Arc::downgrade(&five);
1021 /// let strong_five: Option<Arc<_>> = weak_five.upgrade();
1022 /// assert!(strong_five.is_some());
1024 /// // Destroy all strong pointers.
1025 /// drop(strong_five);
1028 /// assert!(weak_five.upgrade().is_none());
1030 #[stable(feature = "arc_weak", since = "1.4.0")]
1031 pub fn upgrade(&self) -> Option<Arc<T>> {
1032 // We use a CAS loop to increment the strong count instead of a
1033 // fetch_add because once the count hits 0 it must never be above 0.
1034 let inner = self.inner();
1036 // Relaxed load because any write of 0 that we can observe
1037 // leaves the field in a permanently zero state (so a
1038 // "stale" read of 0 is fine), and any other value is
1039 // confirmed via the CAS below.
1040 let mut n = inner.strong.load(Relaxed);
1047 // See comments in `Arc::clone` for why we do this (for `mem::forget`).
1048 if n > MAX_REFCOUNT {
1054 // Relaxed is valid for the same reason it is on Arc's Clone impl
1055 match inner.strong.compare_exchange_weak(n, n + 1, Relaxed, Relaxed) {
1056 Ok(_) => return Some(Arc { ptr: self.ptr, phantom: PhantomData }),
1057 Err(old) => n = old,
1063 fn inner(&self) -> &ArcInner<T> {
1064 // See comments above for why this is "safe"
1065 unsafe { self.ptr.as_ref() }
1069 #[stable(feature = "arc_weak", since = "1.4.0")]
1070 impl<T: ?Sized> Clone for Weak<T> {
1071 /// Makes a clone of the `Weak` pointer that points to the same value.
1076 /// use std::sync::{Arc, Weak};
1078 /// let weak_five = Arc::downgrade(&Arc::new(5));
1080 /// Weak::clone(&weak_five);
1083 fn clone(&self) -> Weak<T> {
1084 // See comments in Arc::clone() for why this is relaxed. This can use a
1085 // fetch_add (ignoring the lock) because the weak count is only locked
1086 // where are *no other* weak pointers in existence. (So we can't be
1087 // running this code in that case).
1088 let old_size = self.inner().weak.fetch_add(1, Relaxed);
1090 // See comments in Arc::clone() for why we do this (for mem::forget).
1091 if old_size > MAX_REFCOUNT {
1097 return Weak { ptr: self.ptr };
1101 #[stable(feature = "downgraded_weak", since = "1.10.0")]
1102 impl<T> Default for Weak<T> {
1103 /// Constructs a new `Weak<T>`, allocating memory for `T` without initializing
1104 /// it. Calling [`upgrade`] on the return value always gives [`None`].
1106 /// [`upgrade`]: struct.Weak.html#method.upgrade
1107 /// [`None`]: ../../std/option/enum.Option.html#variant.None
1112 /// use std::sync::Weak;
1114 /// let empty: Weak<i64> = Default::default();
1115 /// assert!(empty.upgrade().is_none());
1117 fn default() -> Weak<T> {
1122 #[stable(feature = "arc_weak", since = "1.4.0")]
1123 impl<T: ?Sized> Drop for Weak<T> {
1124 /// Drops the `Weak` pointer.
1129 /// use std::sync::{Arc, Weak};
1133 /// impl Drop for Foo {
1134 /// fn drop(&mut self) {
1135 /// println!("dropped!");
1139 /// let foo = Arc::new(Foo);
1140 /// let weak_foo = Arc::downgrade(&foo);
1141 /// let other_weak_foo = Weak::clone(&weak_foo);
1143 /// drop(weak_foo); // Doesn't print anything
1144 /// drop(foo); // Prints "dropped!"
1146 /// assert!(other_weak_foo.upgrade().is_none());
1148 fn drop(&mut self) {
1149 // If we find out that we were the last weak pointer, then its time to
1150 // deallocate the data entirely. See the discussion in Arc::drop() about
1151 // the memory orderings
1153 // It's not necessary to check for the locked state here, because the
1154 // weak count can only be locked if there was precisely one weak ref,
1155 // meaning that drop could only subsequently run ON that remaining weak
1156 // ref, which can only happen after the lock is released.
1157 if self.inner().weak.fetch_sub(1, Release) == 1 {
1158 atomic::fence(Acquire);
1160 Global.dealloc(self.ptr.as_opaque(), Layout::for_value(self.ptr.as_ref()))
1166 #[stable(feature = "rust1", since = "1.0.0")]
1167 impl<T: ?Sized + PartialEq> PartialEq for Arc<T> {
1168 /// Equality for two `Arc`s.
1170 /// Two `Arc`s are equal if their inner values are equal.
1175 /// use std::sync::Arc;
1177 /// let five = Arc::new(5);
1179 /// assert!(five == Arc::new(5));
1181 fn eq(&self, other: &Arc<T>) -> bool {
1182 *(*self) == *(*other)
1185 /// Inequality for two `Arc`s.
1187 /// Two `Arc`s are unequal if their inner values are unequal.
1192 /// use std::sync::Arc;
1194 /// let five = Arc::new(5);
1196 /// assert!(five != Arc::new(6));
1198 fn ne(&self, other: &Arc<T>) -> bool {
1199 *(*self) != *(*other)
1202 #[stable(feature = "rust1", since = "1.0.0")]
1203 impl<T: ?Sized + PartialOrd> PartialOrd for Arc<T> {
1204 /// Partial comparison for two `Arc`s.
1206 /// The two are compared by calling `partial_cmp()` on their inner values.
1211 /// use std::sync::Arc;
1212 /// use std::cmp::Ordering;
1214 /// let five = Arc::new(5);
1216 /// assert_eq!(Some(Ordering::Less), five.partial_cmp(&Arc::new(6)));
1218 fn partial_cmp(&self, other: &Arc<T>) -> Option<Ordering> {
1219 (**self).partial_cmp(&**other)
1222 /// Less-than comparison for two `Arc`s.
1224 /// The two are compared by calling `<` on their inner values.
1229 /// use std::sync::Arc;
1231 /// let five = Arc::new(5);
1233 /// assert!(five < Arc::new(6));
1235 fn lt(&self, other: &Arc<T>) -> bool {
1236 *(*self) < *(*other)
1239 /// 'Less than or equal to' comparison for two `Arc`s.
1241 /// The two are compared by calling `<=` on their inner values.
1246 /// use std::sync::Arc;
1248 /// let five = Arc::new(5);
1250 /// assert!(five <= Arc::new(5));
1252 fn le(&self, other: &Arc<T>) -> bool {
1253 *(*self) <= *(*other)
1256 /// Greater-than comparison for two `Arc`s.
1258 /// The two are compared by calling `>` on their inner values.
1263 /// use std::sync::Arc;
1265 /// let five = Arc::new(5);
1267 /// assert!(five > Arc::new(4));
1269 fn gt(&self, other: &Arc<T>) -> bool {
1270 *(*self) > *(*other)
1273 /// 'Greater than or equal to' comparison for two `Arc`s.
1275 /// The two are compared by calling `>=` on their inner values.
1280 /// use std::sync::Arc;
1282 /// let five = Arc::new(5);
1284 /// assert!(five >= Arc::new(5));
1286 fn ge(&self, other: &Arc<T>) -> bool {
1287 *(*self) >= *(*other)
1290 #[stable(feature = "rust1", since = "1.0.0")]
1291 impl<T: ?Sized + Ord> Ord for Arc<T> {
1292 /// Comparison for two `Arc`s.
1294 /// The two are compared by calling `cmp()` on their inner values.
1299 /// use std::sync::Arc;
1300 /// use std::cmp::Ordering;
1302 /// let five = Arc::new(5);
1304 /// assert_eq!(Ordering::Less, five.cmp(&Arc::new(6)));
1306 fn cmp(&self, other: &Arc<T>) -> Ordering {
1307 (**self).cmp(&**other)
1310 #[stable(feature = "rust1", since = "1.0.0")]
1311 impl<T: ?Sized + Eq> Eq for Arc<T> {}
1313 #[stable(feature = "rust1", since = "1.0.0")]
1314 impl<T: ?Sized + fmt::Display> fmt::Display for Arc<T> {
1315 fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
1316 fmt::Display::fmt(&**self, f)
1320 #[stable(feature = "rust1", since = "1.0.0")]
1321 impl<T: ?Sized + fmt::Debug> fmt::Debug for Arc<T> {
1322 fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
1323 fmt::Debug::fmt(&**self, f)
1327 #[stable(feature = "rust1", since = "1.0.0")]
1328 impl<T: ?Sized> fmt::Pointer for Arc<T> {
1329 fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
1330 fmt::Pointer::fmt(&(&**self as *const T), f)
1334 #[stable(feature = "rust1", since = "1.0.0")]
1335 impl<T: Default> Default for Arc<T> {
1336 /// Creates a new `Arc<T>`, with the `Default` value for `T`.
1341 /// use std::sync::Arc;
1343 /// let x: Arc<i32> = Default::default();
1344 /// assert_eq!(*x, 0);
1346 fn default() -> Arc<T> {
1347 Arc::new(Default::default())
1351 #[stable(feature = "rust1", since = "1.0.0")]
1352 impl<T: ?Sized + Hash> Hash for Arc<T> {
1353 fn hash<H: Hasher>(&self, state: &mut H) {
1354 (**self).hash(state)
1358 #[stable(feature = "from_for_ptrs", since = "1.6.0")]
1359 impl<T> From<T> for Arc<T> {
1360 fn from(t: T) -> Self {
1365 #[stable(feature = "shared_from_slice", since = "1.21.0")]
1366 impl<'a, T: Clone> From<&'a [T]> for Arc<[T]> {
1368 fn from(v: &[T]) -> Arc<[T]> {
1369 <Self as ArcFromSlice<T>>::from_slice(v)
1373 #[stable(feature = "shared_from_slice", since = "1.21.0")]
1374 impl<'a> From<&'a str> for Arc<str> {
1376 fn from(v: &str) -> Arc<str> {
1377 let arc = Arc::<[u8]>::from(v.as_bytes());
1378 unsafe { Arc::from_raw(Arc::into_raw(arc) as *const str) }
1382 #[stable(feature = "shared_from_slice", since = "1.21.0")]
1383 impl From<String> for Arc<str> {
1385 fn from(v: String) -> Arc<str> {
1390 #[stable(feature = "shared_from_slice", since = "1.21.0")]
1391 impl<T: ?Sized> From<Box<T>> for Arc<T> {
1393 fn from(v: Box<T>) -> Arc<T> {
1398 #[stable(feature = "shared_from_slice", since = "1.21.0")]
1399 impl<T> From<Vec<T>> for Arc<[T]> {
1401 fn from(mut v: Vec<T>) -> Arc<[T]> {
1403 let arc = Arc::copy_from_slice(&v);
1405 // Allow the Vec to free its memory, but not destroy its contents
1415 use std::boxed::Box;
1416 use std::clone::Clone;
1417 use std::sync::mpsc::channel;
1420 use std::option::Option;
1421 use std::option::Option::{None, Some};
1422 use std::sync::atomic;
1423 use std::sync::atomic::Ordering::{Acquire, SeqCst};
1425 use std::sync::Mutex;
1426 use std::convert::From;
1428 use super::{Arc, Weak};
1431 struct Canary(*mut atomic::AtomicUsize);
1433 impl Drop for Canary {
1434 fn drop(&mut self) {
1438 (*c).fetch_add(1, SeqCst);
1446 #[cfg_attr(target_os = "emscripten", ignore)]
1447 fn manually_share_arc() {
1448 let v = vec![1, 2, 3, 4, 5, 6, 7, 8, 9, 10];
1449 let arc_v = Arc::new(v);
1451 let (tx, rx) = channel();
1453 let _t = thread::spawn(move || {
1454 let arc_v: Arc<Vec<i32>> = rx.recv().unwrap();
1455 assert_eq!((*arc_v)[3], 4);
1458 tx.send(arc_v.clone()).unwrap();
1460 assert_eq!((*arc_v)[2], 3);
1461 assert_eq!((*arc_v)[4], 5);
1465 fn test_arc_get_mut() {
1466 let mut x = Arc::new(3);
1467 *Arc::get_mut(&mut x).unwrap() = 4;
1470 assert!(Arc::get_mut(&mut x).is_none());
1472 assert!(Arc::get_mut(&mut x).is_some());
1473 let _w = Arc::downgrade(&x);
1474 assert!(Arc::get_mut(&mut x).is_none());
1479 let x = Arc::new(3);
1480 assert_eq!(Arc::try_unwrap(x), Ok(3));
1481 let x = Arc::new(4);
1483 assert_eq!(Arc::try_unwrap(x), Err(Arc::new(4)));
1484 let x = Arc::new(5);
1485 let _w = Arc::downgrade(&x);
1486 assert_eq!(Arc::try_unwrap(x), Ok(5));
1490 fn into_from_raw() {
1491 let x = Arc::new(box "hello");
1494 let x_ptr = Arc::into_raw(x);
1497 assert_eq!(**x_ptr, "hello");
1499 let x = Arc::from_raw(x_ptr);
1500 assert_eq!(**x, "hello");
1502 assert_eq!(Arc::try_unwrap(x).map(|x| *x), Ok("hello"));
1507 fn test_into_from_raw_unsized() {
1508 use std::fmt::Display;
1509 use std::string::ToString;
1511 let arc: Arc<str> = Arc::from("foo");
1513 let ptr = Arc::into_raw(arc.clone());
1514 let arc2 = unsafe { Arc::from_raw(ptr) };
1516 assert_eq!(unsafe { &*ptr }, "foo");
1517 assert_eq!(arc, arc2);
1519 let arc: Arc<Display> = Arc::new(123);
1521 let ptr = Arc::into_raw(arc.clone());
1522 let arc2 = unsafe { Arc::from_raw(ptr) };
1524 assert_eq!(unsafe { &*ptr }.to_string(), "123");
1525 assert_eq!(arc2.to_string(), "123");
1529 fn test_cowarc_clone_make_mut() {
1530 let mut cow0 = Arc::new(75);
1531 let mut cow1 = cow0.clone();
1532 let mut cow2 = cow1.clone();
1534 assert!(75 == *Arc::make_mut(&mut cow0));
1535 assert!(75 == *Arc::make_mut(&mut cow1));
1536 assert!(75 == *Arc::make_mut(&mut cow2));
1538 *Arc::make_mut(&mut cow0) += 1;
1539 *Arc::make_mut(&mut cow1) += 2;
1540 *Arc::make_mut(&mut cow2) += 3;
1542 assert!(76 == *cow0);
1543 assert!(77 == *cow1);
1544 assert!(78 == *cow2);
1546 // none should point to the same backing memory
1547 assert!(*cow0 != *cow1);
1548 assert!(*cow0 != *cow2);
1549 assert!(*cow1 != *cow2);
1553 fn test_cowarc_clone_unique2() {
1554 let mut cow0 = Arc::new(75);
1555 let cow1 = cow0.clone();
1556 let cow2 = cow1.clone();
1558 assert!(75 == *cow0);
1559 assert!(75 == *cow1);
1560 assert!(75 == *cow2);
1562 *Arc::make_mut(&mut cow0) += 1;
1563 assert!(76 == *cow0);
1564 assert!(75 == *cow1);
1565 assert!(75 == *cow2);
1567 // cow1 and cow2 should share the same contents
1568 // cow0 should have a unique reference
1569 assert!(*cow0 != *cow1);
1570 assert!(*cow0 != *cow2);
1571 assert!(*cow1 == *cow2);
1575 fn test_cowarc_clone_weak() {
1576 let mut cow0 = Arc::new(75);
1577 let cow1_weak = Arc::downgrade(&cow0);
1579 assert!(75 == *cow0);
1580 assert!(75 == *cow1_weak.upgrade().unwrap());
1582 *Arc::make_mut(&mut cow0) += 1;
1584 assert!(76 == *cow0);
1585 assert!(cow1_weak.upgrade().is_none());
1590 let x = Arc::new(5);
1591 let y = Arc::downgrade(&x);
1592 assert!(y.upgrade().is_some());
1597 let x = Arc::new(5);
1598 let y = Arc::downgrade(&x);
1600 assert!(y.upgrade().is_none());
1604 fn weak_self_cyclic() {
1606 x: Mutex<Option<Weak<Cycle>>>,
1609 let a = Arc::new(Cycle { x: Mutex::new(None) });
1610 let b = Arc::downgrade(&a.clone());
1611 *a.x.lock().unwrap() = Some(b);
1613 // hopefully we don't double-free (or leak)...
1618 let mut canary = atomic::AtomicUsize::new(0);
1619 let x = Arc::new(Canary(&mut canary as *mut atomic::AtomicUsize));
1621 assert!(canary.load(Acquire) == 1);
1625 fn drop_arc_weak() {
1626 let mut canary = atomic::AtomicUsize::new(0);
1627 let arc = Arc::new(Canary(&mut canary as *mut atomic::AtomicUsize));
1628 let arc_weak = Arc::downgrade(&arc);
1629 assert!(canary.load(Acquire) == 0);
1631 assert!(canary.load(Acquire) == 1);
1636 fn test_strong_count() {
1637 let a = Arc::new(0);
1638 assert!(Arc::strong_count(&a) == 1);
1639 let w = Arc::downgrade(&a);
1640 assert!(Arc::strong_count(&a) == 1);
1641 let b = w.upgrade().expect("");
1642 assert!(Arc::strong_count(&b) == 2);
1643 assert!(Arc::strong_count(&a) == 2);
1646 assert!(Arc::strong_count(&b) == 1);
1648 assert!(Arc::strong_count(&b) == 2);
1649 assert!(Arc::strong_count(&c) == 2);
1653 fn test_weak_count() {
1654 let a = Arc::new(0);
1655 assert!(Arc::strong_count(&a) == 1);
1656 assert!(Arc::weak_count(&a) == 0);
1657 let w = Arc::downgrade(&a);
1658 assert!(Arc::strong_count(&a) == 1);
1659 assert!(Arc::weak_count(&a) == 1);
1661 assert!(Arc::weak_count(&a) == 2);
1664 assert!(Arc::strong_count(&a) == 1);
1665 assert!(Arc::weak_count(&a) == 0);
1667 assert!(Arc::strong_count(&a) == 2);
1668 assert!(Arc::weak_count(&a) == 0);
1669 let d = Arc::downgrade(&c);
1670 assert!(Arc::weak_count(&c) == 1);
1671 assert!(Arc::strong_count(&c) == 2);
1680 let a = Arc::new(5);
1681 assert_eq!(format!("{:?}", a), "5");
1684 // Make sure deriving works with Arc<T>
1685 #[derive(Eq, Ord, PartialEq, PartialOrd, Clone, Debug, Default)]
1692 let x: Arc<[i32]> = Arc::new([1, 2, 3]);
1693 assert_eq!(format!("{:?}", x), "[1, 2, 3]");
1694 let y = Arc::downgrade(&x.clone());
1696 assert!(y.upgrade().is_none());
1700 fn test_from_owned() {
1702 let foo_arc = Arc::from(foo);
1703 assert!(123 == *foo_arc);
1707 fn test_new_weak() {
1708 let foo: Weak<usize> = Weak::new();
1709 assert!(foo.upgrade().is_none());
1714 let five = Arc::new(5);
1715 let same_five = five.clone();
1716 let other_five = Arc::new(5);
1718 assert!(Arc::ptr_eq(&five, &same_five));
1719 assert!(!Arc::ptr_eq(&five, &other_five));
1723 #[cfg_attr(target_os = "emscripten", ignore)]
1724 fn test_weak_count_locked() {
1725 let mut a = Arc::new(atomic::AtomicBool::new(false));
1727 let t = thread::spawn(move || {
1728 for _i in 0..1000000 {
1729 Arc::get_mut(&mut a);
1731 a.store(true, SeqCst);
1734 while !a2.load(SeqCst) {
1735 let n = Arc::weak_count(&a2);
1736 assert!(n < 2, "bad weak count: {}", n);
1742 fn test_from_str() {
1743 let r: Arc<str> = Arc::from("foo");
1745 assert_eq!(&r[..], "foo");
1749 fn test_copy_from_slice() {
1750 let s: &[u32] = &[1, 2, 3];
1751 let r: Arc<[u32]> = Arc::from(s);
1753 assert_eq!(&r[..], [1, 2, 3]);
1757 fn test_clone_from_slice() {
1758 #[derive(Clone, Debug, Eq, PartialEq)]
1761 let s: &[X] = &[X(1), X(2), X(3)];
1762 let r: Arc<[X]> = Arc::from(s);
1764 assert_eq!(&r[..], s);
1769 fn test_clone_from_slice_panic() {
1770 use std::string::{String, ToString};
1772 struct Fail(u32, String);
1774 impl Clone for Fail {
1775 fn clone(&self) -> Fail {
1779 Fail(self.0, self.1.clone())
1784 Fail(0, "foo".to_string()),
1785 Fail(1, "bar".to_string()),
1786 Fail(2, "baz".to_string()),
1789 // Should panic, but not cause memory corruption
1790 let _r: Arc<[Fail]> = Arc::from(s);
1794 fn test_from_box() {
1795 let b: Box<u32> = box 123;
1796 let r: Arc<u32> = Arc::from(b);
1798 assert_eq!(*r, 123);
1802 fn test_from_box_str() {
1803 use std::string::String;
1805 let s = String::from("foo").into_boxed_str();
1806 let r: Arc<str> = Arc::from(s);
1808 assert_eq!(&r[..], "foo");
1812 fn test_from_box_slice() {
1813 let s = vec![1, 2, 3].into_boxed_slice();
1814 let r: Arc<[u32]> = Arc::from(s);
1816 assert_eq!(&r[..], [1, 2, 3]);
1820 fn test_from_box_trait() {
1821 use std::fmt::Display;
1822 use std::string::ToString;
1824 let b: Box<Display> = box 123;
1825 let r: Arc<Display> = Arc::from(b);
1827 assert_eq!(r.to_string(), "123");
1831 fn test_from_box_trait_zero_sized() {
1832 use std::fmt::Debug;
1834 let b: Box<Debug> = box ();
1835 let r: Arc<Debug> = Arc::from(b);
1837 assert_eq!(format!("{:?}", r), "()");
1841 fn test_from_vec() {
1842 let v = vec![1, 2, 3];
1843 let r: Arc<[u32]> = Arc::from(v);
1845 assert_eq!(&r[..], [1, 2, 3]);
1849 #[stable(feature = "rust1", since = "1.0.0")]
1850 impl<T: ?Sized> borrow::Borrow<T> for Arc<T> {
1851 fn borrow(&self) -> &T {
1856 #[stable(since = "1.5.0", feature = "smart_ptr_as_ref")]
1857 impl<T: ?Sized> AsRef<T> for Arc<T> {
1858 fn as_ref(&self) -> &T {