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
20 use core::sync::atomic;
21 use core::sync::atomic::Ordering::{Acquire, Relaxed, Release, SeqCst};
24 use core::cmp::Ordering;
25 use core::intrinsics::abort;
26 use core::mem::{self, align_of_val, size_of_val};
28 use core::ops::CoerceUnsized;
29 use core::ptr::{self, NonNull};
30 use core::marker::{Unsize, PhantomData};
31 use core::hash::{Hash, Hasher};
32 use core::{isize, usize};
33 use core::convert::From;
35 use alloc::{Global, Alloc, Layout, box_free, handle_alloc_error};
41 /// A soft limit on the amount of references that may be made to an `Arc`.
43 /// Going above this limit will abort your program (although not
44 /// necessarily) at _exactly_ `MAX_REFCOUNT + 1` references.
45 const MAX_REFCOUNT: usize = (isize::MAX) as usize;
47 /// A thread-safe reference-counting pointer. 'Arc' stands for 'Atomically
48 /// Reference Counted'.
50 /// The type `Arc<T>` provides shared ownership of a value of type `T`,
51 /// allocated in the heap. Invoking [`clone`][clone] on `Arc` produces
52 /// a new pointer to the same value in the heap. When the last `Arc`
53 /// pointer to a given value is destroyed, the pointed-to value is
56 /// Shared references in Rust disallow mutation by default, and `Arc` is no
57 /// exception: you cannot generally obtain a mutable reference to something
58 /// inside an `Arc`. If you need to mutate through an `Arc`, use
59 /// [`Mutex`][mutex], [`RwLock`][rwlock], or one of the [`Atomic`][atomic]
64 /// Unlike [`Rc<T>`], `Arc<T>` uses atomic operations for its reference
65 /// counting. This means that it is thread-safe. The disadvantage is that
66 /// atomic operations are more expensive than ordinary memory accesses. If you
67 /// are not sharing reference-counted values between threads, consider using
68 /// [`Rc<T>`] for lower overhead. [`Rc<T>`] is a safe default, because the
69 /// compiler will catch any attempt to send an [`Rc<T>`] between threads.
70 /// However, a library might choose `Arc<T>` in order to give library consumers
73 /// `Arc<T>` will implement [`Send`] and [`Sync`] as long as the `T` implements
74 /// [`Send`] and [`Sync`]. Why can't you put a non-thread-safe type `T` in an
75 /// `Arc<T>` to make it thread-safe? This may be a bit counter-intuitive at
76 /// first: after all, isn't the point of `Arc<T>` thread safety? The key is
77 /// this: `Arc<T>` makes it thread safe to have multiple ownership of the same
78 /// data, but it doesn't add thread safety to its data. Consider
79 /// `Arc<`[`RefCell<T>`]`>`. [`RefCell<T>`] isn't [`Sync`], and if `Arc<T>` was always
80 /// [`Send`], `Arc<`[`RefCell<T>`]`>` would be as well. But then we'd have a problem:
81 /// [`RefCell<T>`] is not thread safe; it keeps track of the borrowing count using
82 /// non-atomic operations.
84 /// In the end, this means that you may need to pair `Arc<T>` with some sort of
85 /// [`std::sync`] type, usually [`Mutex<T>`][mutex].
87 /// ## Breaking cycles with `Weak`
89 /// The [`downgrade`][downgrade] method can be used to create a non-owning
90 /// [`Weak`][weak] pointer. A [`Weak`][weak] pointer can be [`upgrade`][upgrade]d
91 /// to an `Arc`, but this will return [`None`] if the value has already been
94 /// A cycle between `Arc` pointers will never be deallocated. For this reason,
95 /// [`Weak`][weak] is used to break cycles. For example, a tree could have
96 /// strong `Arc` pointers from parent nodes to children, and [`Weak`][weak]
97 /// pointers from children back to their parents.
99 /// # Cloning references
101 /// Creating a new reference from an existing reference counted pointer is done using the
102 /// `Clone` trait implemented for [`Arc<T>`][arc] and [`Weak<T>`][weak].
105 /// use std::sync::Arc;
106 /// let foo = Arc::new(vec![1.0, 2.0, 3.0]);
107 /// // The two syntaxes below are equivalent.
108 /// let a = foo.clone();
109 /// let b = Arc::clone(&foo);
110 /// // a and b both point to the same memory location as foo.
113 /// The [`Arc::clone(&from)`] syntax is the most idiomatic because it conveys more explicitly
114 /// the meaning of the code. In the example above, this syntax makes it easier to see that
115 /// this code is creating a new reference rather than copying the whole content of foo.
117 /// ## `Deref` behavior
119 /// `Arc<T>` automatically dereferences to `T` (via the [`Deref`][deref] trait),
120 /// so you can call `T`'s methods on a value of type `Arc<T>`. To avoid name
121 /// clashes with `T`'s methods, the methods of `Arc<T>` itself are [associated
122 /// functions][assoc], called using function-like syntax:
125 /// use std::sync::Arc;
126 /// let my_arc = Arc::new(());
128 /// Arc::downgrade(&my_arc);
131 /// [`Weak<T>`][weak] does not auto-dereference to `T`, because the value may have
132 /// already been destroyed.
134 /// [arc]: struct.Arc.html
135 /// [weak]: struct.Weak.html
136 /// [`Rc<T>`]: ../../std/rc/struct.Rc.html
137 /// [clone]: ../../std/clone/trait.Clone.html#tymethod.clone
138 /// [mutex]: ../../std/sync/struct.Mutex.html
139 /// [rwlock]: ../../std/sync/struct.RwLock.html
140 /// [atomic]: ../../std/sync/atomic/index.html
141 /// [`Send`]: ../../std/marker/trait.Send.html
142 /// [`Sync`]: ../../std/marker/trait.Sync.html
143 /// [deref]: ../../std/ops/trait.Deref.html
144 /// [downgrade]: struct.Arc.html#method.downgrade
145 /// [upgrade]: struct.Weak.html#method.upgrade
146 /// [`None`]: ../../std/option/enum.Option.html#variant.None
147 /// [assoc]: ../../book/first-edition/method-syntax.html#associated-functions
148 /// [`RefCell<T>`]: ../../std/cell/struct.RefCell.html
149 /// [`std::sync`]: ../../std/sync/index.html
150 /// [`Arc::clone(&from)`]: #method.clone
154 /// Sharing some immutable data between threads:
156 // Note that we **do not** run these tests here. The windows builders get super
157 // unhappy if a thread outlives the main thread and then exits at the same time
158 // (something deadlocks) so we just avoid this entirely by not running these
161 /// use std::sync::Arc;
164 /// let five = Arc::new(5);
167 /// let five = Arc::clone(&five);
169 /// thread::spawn(move || {
170 /// println!("{:?}", five);
175 /// Sharing a mutable [`AtomicUsize`]:
177 /// [`AtomicUsize`]: ../../std/sync/atomic/struct.AtomicUsize.html
180 /// use std::sync::Arc;
181 /// use std::sync::atomic::{AtomicUsize, Ordering};
184 /// let val = Arc::new(AtomicUsize::new(5));
187 /// let val = Arc::clone(&val);
189 /// thread::spawn(move || {
190 /// let v = val.fetch_add(1, Ordering::SeqCst);
191 /// println!("{:?}", v);
196 /// See the [`rc` documentation][rc_examples] for more examples of reference
197 /// counting in general.
199 /// [rc_examples]: ../../std/rc/index.html#examples
200 #[stable(feature = "rust1", since = "1.0.0")]
201 pub struct Arc<T: ?Sized> {
202 ptr: NonNull<ArcInner<T>>,
203 phantom: PhantomData<T>,
206 #[stable(feature = "rust1", since = "1.0.0")]
207 unsafe impl<T: ?Sized + Sync + Send> Send for Arc<T> {}
208 #[stable(feature = "rust1", since = "1.0.0")]
209 unsafe impl<T: ?Sized + Sync + Send> Sync for Arc<T> {}
211 #[unstable(feature = "coerce_unsized", issue = "27732")]
212 impl<T: ?Sized + Unsize<U>, U: ?Sized> CoerceUnsized<Arc<U>> for Arc<T> {}
214 /// `Weak` is a version of [`Arc`] that holds a non-owning reference to the
215 /// managed value. The value is accessed by calling [`upgrade`] on the `Weak`
216 /// pointer, which returns an [`Option`]`<`[`Arc`]`<T>>`.
218 /// Since a `Weak` reference does not count towards ownership, it will not
219 /// prevent the inner value from being dropped, and `Weak` itself makes no
220 /// guarantees about the value still being present and may return [`None`]
221 /// when [`upgrade`]d.
223 /// A `Weak` pointer is useful for keeping a temporary reference to the value
224 /// within [`Arc`] without extending its lifetime. It is also used to prevent
225 /// circular references between [`Arc`] pointers, since mutual owning references
226 /// would never allow either [`Arc`] to be dropped. For example, a tree could
227 /// have strong [`Arc`] pointers from parent nodes to children, and `Weak`
228 /// pointers from children back to their parents.
230 /// The typical way to obtain a `Weak` pointer is to call [`Arc::downgrade`].
232 /// [`Arc`]: struct.Arc.html
233 /// [`Arc::downgrade`]: struct.Arc.html#method.downgrade
234 /// [`upgrade`]: struct.Weak.html#method.upgrade
235 /// [`Option`]: ../../std/option/enum.Option.html
236 /// [`None`]: ../../std/option/enum.Option.html#variant.None
237 #[stable(feature = "arc_weak", since = "1.4.0")]
238 pub struct Weak<T: ?Sized> {
239 // This is a `NonNull` to allow optimizing the size of this type in enums,
240 // but it is not necessarily a valid pointer.
241 // `Weak::new` sets this to a dangling pointer so that it doesn’t need
242 // to allocate space on the heap.
243 ptr: NonNull<ArcInner<T>>,
246 #[stable(feature = "arc_weak", since = "1.4.0")]
247 unsafe impl<T: ?Sized + Sync + Send> Send for Weak<T> {}
248 #[stable(feature = "arc_weak", since = "1.4.0")]
249 unsafe impl<T: ?Sized + Sync + Send> Sync for Weak<T> {}
251 #[unstable(feature = "coerce_unsized", issue = "27732")]
252 impl<T: ?Sized + Unsize<U>, U: ?Sized> CoerceUnsized<Weak<U>> for Weak<T> {}
254 #[stable(feature = "arc_weak", since = "1.4.0")]
255 impl<T: ?Sized + fmt::Debug> fmt::Debug for Weak<T> {
256 fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
261 struct ArcInner<T: ?Sized> {
262 strong: atomic::AtomicUsize,
264 // the value usize::MAX acts as a sentinel for temporarily "locking" the
265 // ability to upgrade weak pointers or downgrade strong ones; this is used
266 // to avoid races in `make_mut` and `get_mut`.
267 weak: atomic::AtomicUsize,
272 unsafe impl<T: ?Sized + Sync + Send> Send for ArcInner<T> {}
273 unsafe impl<T: ?Sized + Sync + Send> Sync for ArcInner<T> {}
276 /// Constructs a new `Arc<T>`.
281 /// use std::sync::Arc;
283 /// let five = Arc::new(5);
286 #[stable(feature = "rust1", since = "1.0.0")]
287 pub fn new(data: T) -> Arc<T> {
288 // Start the weak pointer count as 1 which is the weak pointer that's
289 // held by all the strong pointers (kinda), see std/rc.rs for more info
290 let x: Box<_> = box ArcInner {
291 strong: atomic::AtomicUsize::new(1),
292 weak: atomic::AtomicUsize::new(1),
295 Arc { ptr: Box::into_raw_non_null(x), phantom: PhantomData }
298 /// Returns the contained value, if the `Arc` has exactly one strong reference.
300 /// Otherwise, an [`Err`][result] is returned with the same `Arc` that was
303 /// This will succeed even if there are outstanding weak references.
305 /// [result]: ../../std/result/enum.Result.html
310 /// use std::sync::Arc;
312 /// let x = Arc::new(3);
313 /// assert_eq!(Arc::try_unwrap(x), Ok(3));
315 /// let x = Arc::new(4);
316 /// let _y = Arc::clone(&x);
317 /// assert_eq!(*Arc::try_unwrap(x).unwrap_err(), 4);
320 #[stable(feature = "arc_unique", since = "1.4.0")]
321 pub fn try_unwrap(this: Self) -> Result<T, Self> {
322 // See `drop` for why all these atomics are like this
323 if this.inner().strong.compare_exchange(1, 0, Release, Relaxed).is_err() {
327 atomic::fence(Acquire);
330 let elem = ptr::read(&this.ptr.as_ref().data);
332 // Make a weak pointer to clean up the implicit strong-weak reference
333 let _weak = Weak { ptr: this.ptr };
341 impl<T: ?Sized> Arc<T> {
342 /// Consumes the `Arc`, returning the wrapped pointer.
344 /// To avoid a memory leak the pointer must be converted back to an `Arc` using
345 /// [`Arc::from_raw`][from_raw].
347 /// [from_raw]: struct.Arc.html#method.from_raw
352 /// use std::sync::Arc;
354 /// let x = Arc::new(10);
355 /// let x_ptr = Arc::into_raw(x);
356 /// assert_eq!(unsafe { *x_ptr }, 10);
358 #[stable(feature = "rc_raw", since = "1.17.0")]
359 pub fn into_raw(this: Self) -> *const T {
360 let ptr: *const T = &*this;
365 /// Constructs an `Arc` from a raw pointer.
367 /// The raw pointer must have been previously returned by a call to a
368 /// [`Arc::into_raw`][into_raw].
370 /// This function is unsafe because improper use may lead to memory problems. For example, a
371 /// double-free may occur if the function is called twice on the same raw pointer.
373 /// [into_raw]: struct.Arc.html#method.into_raw
378 /// use std::sync::Arc;
380 /// let x = Arc::new(10);
381 /// let x_ptr = Arc::into_raw(x);
384 /// // Convert back to an `Arc` to prevent leak.
385 /// let x = Arc::from_raw(x_ptr);
386 /// assert_eq!(*x, 10);
388 /// // Further calls to `Arc::from_raw(x_ptr)` would be memory unsafe.
391 /// // The memory was freed when `x` went out of scope above, so `x_ptr` is now dangling!
393 #[stable(feature = "rc_raw", since = "1.17.0")]
394 pub unsafe fn from_raw(ptr: *const T) -> Self {
395 // Align the unsized value to the end of the ArcInner.
396 // Because it is ?Sized, it will always be the last field in memory.
397 let align = align_of_val(&*ptr);
398 let layout = Layout::new::<ArcInner<()>>();
399 let offset = (layout.size() + layout.padding_needed_for(align)) as isize;
401 // Reverse the offset to find the original ArcInner.
402 let fake_ptr = ptr as *mut ArcInner<T>;
403 let arc_ptr = set_data_ptr(fake_ptr, (ptr as *mut u8).offset(-offset));
406 ptr: NonNull::new_unchecked(arc_ptr),
407 phantom: PhantomData,
411 /// Creates a new [`Weak`][weak] pointer to this value.
413 /// [weak]: struct.Weak.html
418 /// use std::sync::Arc;
420 /// let five = Arc::new(5);
422 /// let weak_five = Arc::downgrade(&five);
424 #[stable(feature = "arc_weak", since = "1.4.0")]
425 pub fn downgrade(this: &Self) -> Weak<T> {
426 // This Relaxed is OK because we're checking the value in the CAS
428 let mut cur = this.inner().weak.load(Relaxed);
431 // check if the weak counter is currently "locked"; if so, spin.
432 if cur == usize::MAX {
433 cur = this.inner().weak.load(Relaxed);
437 // NOTE: this code currently ignores the possibility of overflow
438 // into usize::MAX; in general both Rc and Arc need to be adjusted
439 // to deal with overflow.
441 // Unlike with Clone(), we need this to be an Acquire read to
442 // synchronize with the write coming from `is_unique`, so that the
443 // events prior to that write happen before this read.
444 match this.inner().weak.compare_exchange_weak(cur, cur + 1, Acquire, Relaxed) {
445 Ok(_) => return Weak { ptr: this.ptr },
446 Err(old) => cur = old,
451 /// Gets the number of [`Weak`][weak] pointers to this value.
453 /// [weak]: struct.Weak.html
457 /// This method by itself is safe, but using it correctly requires extra care.
458 /// Another thread can change the weak count at any time,
459 /// including potentially between calling this method and acting on the result.
464 /// use std::sync::Arc;
466 /// let five = Arc::new(5);
467 /// let _weak_five = Arc::downgrade(&five);
469 /// // This assertion is deterministic because we haven't shared
470 /// // the `Arc` or `Weak` between threads.
471 /// assert_eq!(1, Arc::weak_count(&five));
474 #[stable(feature = "arc_counts", since = "1.15.0")]
475 pub fn weak_count(this: &Self) -> usize {
476 let cnt = this.inner().weak.load(SeqCst);
477 // If the weak count is currently locked, the value of the
478 // count was 0 just before taking the lock.
479 if cnt == usize::MAX { 0 } else { cnt - 1 }
482 /// Gets the number of strong (`Arc`) pointers to this value.
486 /// This method by itself is safe, but using it correctly requires extra care.
487 /// Another thread can change the strong count at any time,
488 /// including potentially between calling this method and acting on the result.
493 /// use std::sync::Arc;
495 /// let five = Arc::new(5);
496 /// let _also_five = Arc::clone(&five);
498 /// // This assertion is deterministic because we haven't shared
499 /// // the `Arc` between threads.
500 /// assert_eq!(2, Arc::strong_count(&five));
503 #[stable(feature = "arc_counts", since = "1.15.0")]
504 pub fn strong_count(this: &Self) -> usize {
505 this.inner().strong.load(SeqCst)
509 fn inner(&self) -> &ArcInner<T> {
510 // This unsafety is ok because while this arc is alive we're guaranteed
511 // that the inner pointer is valid. Furthermore, we know that the
512 // `ArcInner` structure itself is `Sync` because the inner data is
513 // `Sync` as well, so we're ok loaning out an immutable pointer to these
515 unsafe { self.ptr.as_ref() }
518 // Non-inlined part of `drop`.
520 unsafe fn drop_slow(&mut self) {
521 // Destroy the data at this time, even though we may not free the box
522 // allocation itself (there may still be weak pointers lying around).
523 ptr::drop_in_place(&mut self.ptr.as_mut().data);
525 if self.inner().weak.fetch_sub(1, Release) == 1 {
526 atomic::fence(Acquire);
527 Global.dealloc(self.ptr.cast(), Layout::for_value(self.ptr.as_ref()))
532 #[stable(feature = "ptr_eq", since = "1.17.0")]
533 /// Returns true if the two `Arc`s point to the same value (not
534 /// just values that compare as equal).
539 /// use std::sync::Arc;
541 /// let five = Arc::new(5);
542 /// let same_five = Arc::clone(&five);
543 /// let other_five = Arc::new(5);
545 /// assert!(Arc::ptr_eq(&five, &same_five));
546 /// assert!(!Arc::ptr_eq(&five, &other_five));
548 pub fn ptr_eq(this: &Self, other: &Self) -> bool {
549 this.ptr.as_ptr() == other.ptr.as_ptr()
553 impl<T: ?Sized> Arc<T> {
554 // Allocates an `ArcInner<T>` with sufficient space for an unsized value
555 unsafe fn allocate_for_ptr(ptr: *const T) -> *mut ArcInner<T> {
556 // Create a fake ArcInner to find allocation size and alignment
557 let fake_ptr = ptr as *mut ArcInner<T>;
559 let layout = Layout::for_value(&*fake_ptr);
561 let mem = Global.alloc(layout)
562 .unwrap_or_else(|_| handle_alloc_error(layout));
564 // Initialize the real ArcInner
565 let inner = set_data_ptr(ptr as *mut T, mem.as_ptr() as *mut u8) as *mut ArcInner<T>;
567 ptr::write(&mut (*inner).strong, atomic::AtomicUsize::new(1));
568 ptr::write(&mut (*inner).weak, atomic::AtomicUsize::new(1));
573 fn from_box(v: Box<T>) -> Arc<T> {
575 let box_unique = Box::into_unique(v);
576 let bptr = box_unique.as_ptr();
578 let value_size = size_of_val(&*bptr);
579 let ptr = Self::allocate_for_ptr(bptr);
581 // Copy value as bytes
582 ptr::copy_nonoverlapping(
583 bptr as *const T as *const u8,
584 &mut (*ptr).data as *mut _ as *mut u8,
587 // Free the allocation without dropping its contents
588 box_free(box_unique);
590 Arc { ptr: NonNull::new_unchecked(ptr), phantom: PhantomData }
595 // Sets the data pointer of a `?Sized` raw pointer.
597 // For a slice/trait object, this sets the `data` field and leaves the rest
598 // unchanged. For a sized raw pointer, this simply sets the pointer.
599 unsafe fn set_data_ptr<T: ?Sized, U>(mut ptr: *mut T, data: *mut U) -> *mut T {
600 ptr::write(&mut ptr as *mut _ as *mut *mut u8, data as *mut u8);
605 // Copy elements from slice into newly allocated Arc<[T]>
607 // Unsafe because the caller must either take ownership or bind `T: Copy`
608 unsafe fn copy_from_slice(v: &[T]) -> Arc<[T]> {
609 let v_ptr = v as *const [T];
610 let ptr = Self::allocate_for_ptr(v_ptr);
612 ptr::copy_nonoverlapping(
614 &mut (*ptr).data as *mut [T] as *mut T,
617 Arc { ptr: NonNull::new_unchecked(ptr), phantom: PhantomData }
621 // Specialization trait used for From<&[T]>
622 trait ArcFromSlice<T> {
623 fn from_slice(slice: &[T]) -> Self;
626 impl<T: Clone> ArcFromSlice<T> for Arc<[T]> {
628 default fn from_slice(v: &[T]) -> Self {
629 // Panic guard while cloning T elements.
630 // In the event of a panic, elements that have been written
631 // into the new ArcInner will be dropped, then the memory freed.
639 impl<T> Drop for Guard<T> {
641 use core::slice::from_raw_parts_mut;
644 let slice = from_raw_parts_mut(self.elems, self.n_elems);
645 ptr::drop_in_place(slice);
647 Global.dealloc(self.mem.cast(), self.layout.clone());
653 let v_ptr = v as *const [T];
654 let ptr = Self::allocate_for_ptr(v_ptr);
656 let mem = ptr as *mut _ as *mut u8;
657 let layout = Layout::for_value(&*ptr);
659 // Pointer to first element
660 let elems = &mut (*ptr).data as *mut [T] as *mut T;
662 let mut guard = Guard{
663 mem: NonNull::new_unchecked(mem),
669 for (i, item) in v.iter().enumerate() {
670 ptr::write(elems.offset(i as isize), item.clone());
674 // All clear. Forget the guard so it doesn't free the new ArcInner.
677 Arc { ptr: NonNull::new_unchecked(ptr), phantom: PhantomData }
682 impl<T: Copy> ArcFromSlice<T> for Arc<[T]> {
684 fn from_slice(v: &[T]) -> Self {
685 unsafe { Arc::copy_from_slice(v) }
689 #[stable(feature = "rust1", since = "1.0.0")]
690 impl<T: ?Sized> Clone for Arc<T> {
691 /// Makes a clone of the `Arc` pointer.
693 /// This creates another pointer to the same inner value, increasing the
694 /// strong reference count.
699 /// use std::sync::Arc;
701 /// let five = Arc::new(5);
703 /// Arc::clone(&five);
706 fn clone(&self) -> Arc<T> {
707 // Using a relaxed ordering is alright here, as knowledge of the
708 // original reference prevents other threads from erroneously deleting
711 // As explained in the [Boost documentation][1], Increasing the
712 // reference counter can always be done with memory_order_relaxed: New
713 // references to an object can only be formed from an existing
714 // reference, and passing an existing reference from one thread to
715 // another must already provide any required synchronization.
717 // [1]: (www.boost.org/doc/libs/1_55_0/doc/html/atomic/usage_examples.html)
718 let old_size = self.inner().strong.fetch_add(1, Relaxed);
720 // However we need to guard against massive refcounts in case someone
721 // is `mem::forget`ing Arcs. If we don't do this the count can overflow
722 // and users will use-after free. We racily saturate to `isize::MAX` on
723 // the assumption that there aren't ~2 billion threads incrementing
724 // the reference count at once. This branch will never be taken in
725 // any realistic program.
727 // We abort because such a program is incredibly degenerate, and we
728 // don't care to support it.
729 if old_size > MAX_REFCOUNT {
735 Arc { ptr: self.ptr, phantom: PhantomData }
739 #[stable(feature = "rust1", since = "1.0.0")]
740 impl<T: ?Sized> Deref for Arc<T> {
744 fn deref(&self) -> &T {
749 impl<T: Clone> Arc<T> {
750 /// Makes a mutable reference into the given `Arc`.
752 /// If there are other `Arc` or [`Weak`][weak] pointers to the same value,
753 /// then `make_mut` will invoke [`clone`][clone] on the inner value to
754 /// ensure unique ownership. This is also referred to as clone-on-write.
756 /// See also [`get_mut`][get_mut], which will fail rather than cloning.
758 /// [weak]: struct.Weak.html
759 /// [clone]: ../../std/clone/trait.Clone.html#tymethod.clone
760 /// [get_mut]: struct.Arc.html#method.get_mut
765 /// use std::sync::Arc;
767 /// let mut data = Arc::new(5);
769 /// *Arc::make_mut(&mut data) += 1; // Won't clone anything
770 /// let mut other_data = Arc::clone(&data); // Won't clone inner data
771 /// *Arc::make_mut(&mut data) += 1; // Clones inner data
772 /// *Arc::make_mut(&mut data) += 1; // Won't clone anything
773 /// *Arc::make_mut(&mut other_data) *= 2; // Won't clone anything
775 /// // Now `data` and `other_data` point to different values.
776 /// assert_eq!(*data, 8);
777 /// assert_eq!(*other_data, 12);
780 #[stable(feature = "arc_unique", since = "1.4.0")]
781 pub fn make_mut(this: &mut Self) -> &mut T {
782 // Note that we hold both a strong reference and a weak reference.
783 // Thus, releasing our strong reference only will not, by itself, cause
784 // the memory to be deallocated.
786 // Use Acquire to ensure that we see any writes to `weak` that happen
787 // before release writes (i.e., decrements) to `strong`. Since we hold a
788 // weak count, there's no chance the ArcInner itself could be
790 if this.inner().strong.compare_exchange(1, 0, Acquire, Relaxed).is_err() {
791 // Another strong pointer exists; clone
792 *this = Arc::new((**this).clone());
793 } else if this.inner().weak.load(Relaxed) != 1 {
794 // Relaxed suffices in the above because this is fundamentally an
795 // optimization: we are always racing with weak pointers being
796 // dropped. Worst case, we end up allocated a new Arc unnecessarily.
798 // We removed the last strong ref, but there are additional weak
799 // refs remaining. We'll move the contents to a new Arc, and
800 // invalidate the other weak refs.
802 // Note that it is not possible for the read of `weak` to yield
803 // usize::MAX (i.e., locked), since the weak count can only be
804 // locked by a thread with a strong reference.
806 // Materialize our own implicit weak pointer, so that it can clean
807 // up the ArcInner as needed.
808 let weak = Weak { ptr: this.ptr };
810 // mark the data itself as already deallocated
812 // there is no data race in the implicit write caused by `read`
813 // here (due to zeroing) because data is no longer accessed by
814 // other threads (due to there being no more strong refs at this
816 let mut swap = Arc::new(ptr::read(&weak.ptr.as_ref().data));
817 mem::swap(this, &mut swap);
821 // We were the sole reference of either kind; bump back up the
823 this.inner().strong.store(1, Release);
826 // As with `get_mut()`, the unsafety is ok because our reference was
827 // either unique to begin with, or became one upon cloning the contents.
829 &mut this.ptr.as_mut().data
834 impl<T: ?Sized> Arc<T> {
835 /// Returns a mutable reference to the inner value, if there are
836 /// no other `Arc` or [`Weak`][weak] pointers to the same value.
838 /// Returns [`None`][option] otherwise, because it is not safe to
839 /// mutate a shared value.
841 /// See also [`make_mut`][make_mut], which will [`clone`][clone]
842 /// the inner value when it's shared.
844 /// [weak]: struct.Weak.html
845 /// [option]: ../../std/option/enum.Option.html
846 /// [make_mut]: struct.Arc.html#method.make_mut
847 /// [clone]: ../../std/clone/trait.Clone.html#tymethod.clone
852 /// use std::sync::Arc;
854 /// let mut x = Arc::new(3);
855 /// *Arc::get_mut(&mut x).unwrap() = 4;
856 /// assert_eq!(*x, 4);
858 /// let _y = Arc::clone(&x);
859 /// assert!(Arc::get_mut(&mut x).is_none());
862 #[stable(feature = "arc_unique", since = "1.4.0")]
863 pub fn get_mut(this: &mut Self) -> Option<&mut T> {
864 if this.is_unique() {
865 // This unsafety is ok because we're guaranteed that the pointer
866 // returned is the *only* pointer that will ever be returned to T. Our
867 // reference count is guaranteed to be 1 at this point, and we required
868 // the Arc itself to be `mut`, so we're returning the only possible
869 // reference to the inner data.
871 Some(&mut this.ptr.as_mut().data)
878 /// Determine whether this is the unique reference (including weak refs) to
879 /// the underlying data.
881 /// Note that this requires locking the weak ref count.
882 fn is_unique(&mut self) -> bool {
883 // lock the weak pointer count if we appear to be the sole weak pointer
886 // The acquire label here ensures a happens-before relationship with any
887 // writes to `strong` (in particular in `Weak::upgrade`) prior to decrements
888 // of the `weak` count (via `Weak::drop`, which uses release). If the upgraded
889 // weak ref was never dropped, the CAS here will fail so we do not care to synchronize.
890 if self.inner().weak.compare_exchange(1, usize::MAX, Acquire, Relaxed).is_ok() {
891 // This needs to be an `Acquire` to synchronize with the decrement of the `strong`
892 // counter in `drop` -- the only access that happens when any but the last reference
894 let unique = self.inner().strong.load(Acquire) == 1;
896 // The release write here synchronizes with a read in `downgrade`,
897 // effectively preventing the above read of `strong` from happening
899 self.inner().weak.store(1, Release); // release the lock
907 #[stable(feature = "rust1", since = "1.0.0")]
908 unsafe impl<#[may_dangle] T: ?Sized> Drop for Arc<T> {
911 /// This will decrement the strong reference count. If the strong reference
912 /// count reaches zero then the only other references (if any) are
913 /// [`Weak`][weak], so we `drop` the inner value.
915 /// [weak]: struct.Weak.html
920 /// use std::sync::Arc;
924 /// impl Drop for Foo {
925 /// fn drop(&mut self) {
926 /// println!("dropped!");
930 /// let foo = Arc::new(Foo);
931 /// let foo2 = Arc::clone(&foo);
933 /// drop(foo); // Doesn't print anything
934 /// drop(foo2); // Prints "dropped!"
938 // Because `fetch_sub` is already atomic, we do not need to synchronize
939 // with other threads unless we are going to delete the object. This
940 // same logic applies to the below `fetch_sub` to the `weak` count.
941 if self.inner().strong.fetch_sub(1, Release) != 1 {
945 // This fence is needed to prevent reordering of use of the data and
946 // deletion of the data. Because it is marked `Release`, the decreasing
947 // of the reference count synchronizes with this `Acquire` fence. This
948 // means that use of the data happens before decreasing the reference
949 // count, which happens before this fence, which happens before the
950 // deletion of the data.
952 // As explained in the [Boost documentation][1],
954 // > It is important to enforce any possible access to the object in one
955 // > thread (through an existing reference) to *happen before* deleting
956 // > the object in a different thread. This is achieved by a "release"
957 // > operation after dropping a reference (any access to the object
958 // > through this reference must obviously happened before), and an
959 // > "acquire" operation before deleting the object.
961 // In particular, while the contents of an Arc are usually immutable, it's
962 // possible to have interior writes to something like a Mutex<T>. Since a
963 // Mutex is not acquired when it is deleted, we can't rely on its
964 // synchronization logic to make writes in thread A visible to a destructor
965 // running in thread B.
967 // Also note that the Acquire fence here could probably be replaced with an
968 // Acquire load, which could improve performance in highly-contended
969 // situations. See [2].
971 // [1]: (www.boost.org/doc/libs/1_55_0/doc/html/atomic/usage_examples.html)
972 // [2]: (https://github.com/rust-lang/rust/pull/41714)
973 atomic::fence(Acquire);
981 impl Arc<dyn Any + Send + Sync> {
983 #[stable(feature = "rc_downcast", since = "1.29.0")]
984 /// Attempt to downcast the `Arc<dyn Any + Send + Sync>` to a concrete type.
989 /// use std::any::Any;
990 /// use std::sync::Arc;
992 /// fn print_if_string(value: Arc<dyn Any + Send + Sync>) {
993 /// if let Ok(string) = value.downcast::<String>() {
994 /// println!("String ({}): {}", string.len(), string);
999 /// let my_string = "Hello World".to_string();
1000 /// print_if_string(Arc::new(my_string));
1001 /// print_if_string(Arc::new(0i8));
1004 pub fn downcast<T>(self) -> Result<Arc<T>, Self>
1006 T: Any + Send + Sync + 'static,
1008 if (*self).is::<T>() {
1009 let ptr = self.ptr.cast::<ArcInner<T>>();
1011 Ok(Arc { ptr, phantom: PhantomData })
1019 /// Constructs a new `Weak<T>`, without allocating any memory.
1020 /// Calling [`upgrade`] on the return value always gives [`None`].
1022 /// [`upgrade`]: struct.Weak.html#method.upgrade
1023 /// [`None`]: ../../std/option/enum.Option.html#variant.None
1028 /// use std::sync::Weak;
1030 /// let empty: Weak<i64> = Weak::new();
1031 /// assert!(empty.upgrade().is_none());
1033 #[stable(feature = "downgraded_weak", since = "1.10.0")]
1034 pub fn new() -> Weak<T> {
1036 ptr: NonNull::dangling(),
1041 impl<T: ?Sized> Weak<T> {
1042 /// Attempts to upgrade the `Weak` pointer to an [`Arc`], extending
1043 /// the lifetime of the value if successful.
1045 /// Returns [`None`] if the value has since been dropped.
1047 /// [`Arc`]: struct.Arc.html
1048 /// [`None`]: ../../std/option/enum.Option.html#variant.None
1053 /// use std::sync::Arc;
1055 /// let five = Arc::new(5);
1057 /// let weak_five = Arc::downgrade(&five);
1059 /// let strong_five: Option<Arc<_>> = weak_five.upgrade();
1060 /// assert!(strong_five.is_some());
1062 /// // Destroy all strong pointers.
1063 /// drop(strong_five);
1066 /// assert!(weak_five.upgrade().is_none());
1068 #[stable(feature = "arc_weak", since = "1.4.0")]
1069 pub fn upgrade(&self) -> Option<Arc<T>> {
1070 // We use a CAS loop to increment the strong count instead of a
1071 // fetch_add because once the count hits 0 it must never be above 0.
1072 let inner = self.inner()?;
1074 // Relaxed load because any write of 0 that we can observe
1075 // leaves the field in a permanently zero state (so a
1076 // "stale" read of 0 is fine), and any other value is
1077 // confirmed via the CAS below.
1078 let mut n = inner.strong.load(Relaxed);
1085 // See comments in `Arc::clone` for why we do this (for `mem::forget`).
1086 if n > MAX_REFCOUNT {
1092 // Relaxed is valid for the same reason it is on Arc's Clone impl
1093 match inner.strong.compare_exchange_weak(n, n + 1, Relaxed, Relaxed) {
1094 Ok(_) => return Some(Arc {
1095 // null checked above
1097 phantom: PhantomData,
1099 Err(old) => n = old,
1104 /// Return `None` when the pointer is dangling and there is no allocated `ArcInner`,
1105 /// i.e. this `Weak` was created by `Weak::new`
1107 fn inner(&self) -> Option<&ArcInner<T>> {
1108 if is_dangling(self.ptr) {
1111 Some(unsafe { self.ptr.as_ref() })
1116 #[stable(feature = "arc_weak", since = "1.4.0")]
1117 impl<T: ?Sized> Clone for Weak<T> {
1118 /// Makes a clone of the `Weak` pointer that points to the same value.
1123 /// use std::sync::{Arc, Weak};
1125 /// let weak_five = Arc::downgrade(&Arc::new(5));
1127 /// Weak::clone(&weak_five);
1130 fn clone(&self) -> Weak<T> {
1131 let inner = if let Some(inner) = self.inner() {
1134 return Weak { ptr: self.ptr };
1136 // See comments in Arc::clone() for why this is relaxed. This can use a
1137 // fetch_add (ignoring the lock) because the weak count is only locked
1138 // where are *no other* weak pointers in existence. (So we can't be
1139 // running this code in that case).
1140 let old_size = inner.weak.fetch_add(1, Relaxed);
1142 // See comments in Arc::clone() for why we do this (for mem::forget).
1143 if old_size > MAX_REFCOUNT {
1149 return Weak { ptr: self.ptr };
1153 #[stable(feature = "downgraded_weak", since = "1.10.0")]
1154 impl<T> Default for Weak<T> {
1155 /// Constructs a new `Weak<T>`, without allocating memory.
1156 /// Calling [`upgrade`] on the return value always gives [`None`].
1158 /// [`upgrade`]: struct.Weak.html#method.upgrade
1159 /// [`None`]: ../../std/option/enum.Option.html#variant.None
1164 /// use std::sync::Weak;
1166 /// let empty: Weak<i64> = Default::default();
1167 /// assert!(empty.upgrade().is_none());
1169 fn default() -> Weak<T> {
1174 #[stable(feature = "arc_weak", since = "1.4.0")]
1175 impl<T: ?Sized> Drop for Weak<T> {
1176 /// Drops the `Weak` pointer.
1181 /// use std::sync::{Arc, Weak};
1185 /// impl Drop for Foo {
1186 /// fn drop(&mut self) {
1187 /// println!("dropped!");
1191 /// let foo = Arc::new(Foo);
1192 /// let weak_foo = Arc::downgrade(&foo);
1193 /// let other_weak_foo = Weak::clone(&weak_foo);
1195 /// drop(weak_foo); // Doesn't print anything
1196 /// drop(foo); // Prints "dropped!"
1198 /// assert!(other_weak_foo.upgrade().is_none());
1200 fn drop(&mut self) {
1201 // If we find out that we were the last weak pointer, then its time to
1202 // deallocate the data entirely. See the discussion in Arc::drop() about
1203 // the memory orderings
1205 // It's not necessary to check for the locked state here, because the
1206 // weak count can only be locked if there was precisely one weak ref,
1207 // meaning that drop could only subsequently run ON that remaining weak
1208 // ref, which can only happen after the lock is released.
1209 let inner = if let Some(inner) = self.inner() {
1215 if inner.weak.fetch_sub(1, Release) == 1 {
1216 atomic::fence(Acquire);
1218 Global.dealloc(self.ptr.cast(), Layout::for_value(self.ptr.as_ref()))
1224 #[stable(feature = "rust1", since = "1.0.0")]
1225 impl<T: ?Sized + PartialEq> PartialEq for Arc<T> {
1226 /// Equality for two `Arc`s.
1228 /// Two `Arc`s are equal if their inner values are equal.
1233 /// use std::sync::Arc;
1235 /// let five = Arc::new(5);
1237 /// assert!(five == Arc::new(5));
1239 fn eq(&self, other: &Arc<T>) -> bool {
1240 *(*self) == *(*other)
1243 /// Inequality for two `Arc`s.
1245 /// Two `Arc`s are unequal if their inner values are unequal.
1250 /// use std::sync::Arc;
1252 /// let five = Arc::new(5);
1254 /// assert!(five != Arc::new(6));
1256 fn ne(&self, other: &Arc<T>) -> bool {
1257 *(*self) != *(*other)
1260 #[stable(feature = "rust1", since = "1.0.0")]
1261 impl<T: ?Sized + PartialOrd> PartialOrd for Arc<T> {
1262 /// Partial comparison for two `Arc`s.
1264 /// The two are compared by calling `partial_cmp()` on their inner values.
1269 /// use std::sync::Arc;
1270 /// use std::cmp::Ordering;
1272 /// let five = Arc::new(5);
1274 /// assert_eq!(Some(Ordering::Less), five.partial_cmp(&Arc::new(6)));
1276 fn partial_cmp(&self, other: &Arc<T>) -> Option<Ordering> {
1277 (**self).partial_cmp(&**other)
1280 /// Less-than comparison for two `Arc`s.
1282 /// The two are compared by calling `<` on their inner values.
1287 /// use std::sync::Arc;
1289 /// let five = Arc::new(5);
1291 /// assert!(five < Arc::new(6));
1293 fn lt(&self, other: &Arc<T>) -> bool {
1294 *(*self) < *(*other)
1297 /// 'Less than or equal to' comparison for two `Arc`s.
1299 /// The two are compared by calling `<=` on their inner values.
1304 /// use std::sync::Arc;
1306 /// let five = Arc::new(5);
1308 /// assert!(five <= Arc::new(5));
1310 fn le(&self, other: &Arc<T>) -> bool {
1311 *(*self) <= *(*other)
1314 /// Greater-than comparison for two `Arc`s.
1316 /// The two are compared by calling `>` on their inner values.
1321 /// use std::sync::Arc;
1323 /// let five = Arc::new(5);
1325 /// assert!(five > Arc::new(4));
1327 fn gt(&self, other: &Arc<T>) -> bool {
1328 *(*self) > *(*other)
1331 /// 'Greater than or equal to' comparison for two `Arc`s.
1333 /// The two are compared by calling `>=` on their inner values.
1338 /// use std::sync::Arc;
1340 /// let five = Arc::new(5);
1342 /// assert!(five >= Arc::new(5));
1344 fn ge(&self, other: &Arc<T>) -> bool {
1345 *(*self) >= *(*other)
1348 #[stable(feature = "rust1", since = "1.0.0")]
1349 impl<T: ?Sized + Ord> Ord for Arc<T> {
1350 /// Comparison for two `Arc`s.
1352 /// The two are compared by calling `cmp()` on their inner values.
1357 /// use std::sync::Arc;
1358 /// use std::cmp::Ordering;
1360 /// let five = Arc::new(5);
1362 /// assert_eq!(Ordering::Less, five.cmp(&Arc::new(6)));
1364 fn cmp(&self, other: &Arc<T>) -> Ordering {
1365 (**self).cmp(&**other)
1368 #[stable(feature = "rust1", since = "1.0.0")]
1369 impl<T: ?Sized + Eq> Eq for Arc<T> {}
1371 #[stable(feature = "rust1", since = "1.0.0")]
1372 impl<T: ?Sized + fmt::Display> fmt::Display for Arc<T> {
1373 fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
1374 fmt::Display::fmt(&**self, f)
1378 #[stable(feature = "rust1", since = "1.0.0")]
1379 impl<T: ?Sized + fmt::Debug> fmt::Debug for Arc<T> {
1380 fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
1381 fmt::Debug::fmt(&**self, f)
1385 #[stable(feature = "rust1", since = "1.0.0")]
1386 impl<T: ?Sized> fmt::Pointer for Arc<T> {
1387 fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
1388 fmt::Pointer::fmt(&(&**self as *const T), f)
1392 #[stable(feature = "rust1", since = "1.0.0")]
1393 impl<T: Default> Default for Arc<T> {
1394 /// Creates a new `Arc<T>`, with the `Default` value for `T`.
1399 /// use std::sync::Arc;
1401 /// let x: Arc<i32> = Default::default();
1402 /// assert_eq!(*x, 0);
1404 fn default() -> Arc<T> {
1405 Arc::new(Default::default())
1409 #[stable(feature = "rust1", since = "1.0.0")]
1410 impl<T: ?Sized + Hash> Hash for Arc<T> {
1411 fn hash<H: Hasher>(&self, state: &mut H) {
1412 (**self).hash(state)
1416 #[stable(feature = "from_for_ptrs", since = "1.6.0")]
1417 impl<T> From<T> for Arc<T> {
1418 fn from(t: T) -> Self {
1423 #[stable(feature = "shared_from_slice", since = "1.21.0")]
1424 impl<'a, T: Clone> From<&'a [T]> for Arc<[T]> {
1426 fn from(v: &[T]) -> Arc<[T]> {
1427 <Self as ArcFromSlice<T>>::from_slice(v)
1431 #[stable(feature = "shared_from_slice", since = "1.21.0")]
1432 impl<'a> From<&'a str> for Arc<str> {
1434 fn from(v: &str) -> Arc<str> {
1435 let arc = Arc::<[u8]>::from(v.as_bytes());
1436 unsafe { Arc::from_raw(Arc::into_raw(arc) as *const str) }
1440 #[stable(feature = "shared_from_slice", since = "1.21.0")]
1441 impl From<String> for Arc<str> {
1443 fn from(v: String) -> Arc<str> {
1448 #[stable(feature = "shared_from_slice", since = "1.21.0")]
1449 impl<T: ?Sized> From<Box<T>> for Arc<T> {
1451 fn from(v: Box<T>) -> Arc<T> {
1456 #[stable(feature = "shared_from_slice", since = "1.21.0")]
1457 impl<T> From<Vec<T>> for Arc<[T]> {
1459 fn from(mut v: Vec<T>) -> Arc<[T]> {
1461 let arc = Arc::copy_from_slice(&v);
1463 // Allow the Vec to free its memory, but not destroy its contents
1473 use std::boxed::Box;
1474 use std::clone::Clone;
1475 use std::sync::mpsc::channel;
1478 use std::option::Option;
1479 use std::option::Option::{None, Some};
1480 use std::sync::atomic;
1481 use std::sync::atomic::Ordering::{Acquire, SeqCst};
1483 use std::sync::Mutex;
1484 use std::convert::From;
1486 use super::{Arc, Weak};
1489 struct Canary(*mut atomic::AtomicUsize);
1491 impl Drop for Canary {
1492 fn drop(&mut self) {
1496 (*c).fetch_add(1, SeqCst);
1504 #[cfg_attr(target_os = "emscripten", ignore)]
1505 fn manually_share_arc() {
1506 let v = vec![1, 2, 3, 4, 5, 6, 7, 8, 9, 10];
1507 let arc_v = Arc::new(v);
1509 let (tx, rx) = channel();
1511 let _t = thread::spawn(move || {
1512 let arc_v: Arc<Vec<i32>> = rx.recv().unwrap();
1513 assert_eq!((*arc_v)[3], 4);
1516 tx.send(arc_v.clone()).unwrap();
1518 assert_eq!((*arc_v)[2], 3);
1519 assert_eq!((*arc_v)[4], 5);
1523 fn test_arc_get_mut() {
1524 let mut x = Arc::new(3);
1525 *Arc::get_mut(&mut x).unwrap() = 4;
1528 assert!(Arc::get_mut(&mut x).is_none());
1530 assert!(Arc::get_mut(&mut x).is_some());
1531 let _w = Arc::downgrade(&x);
1532 assert!(Arc::get_mut(&mut x).is_none());
1537 let x = Arc::new(3);
1538 assert_eq!(Arc::try_unwrap(x), Ok(3));
1539 let x = Arc::new(4);
1541 assert_eq!(Arc::try_unwrap(x), Err(Arc::new(4)));
1542 let x = Arc::new(5);
1543 let _w = Arc::downgrade(&x);
1544 assert_eq!(Arc::try_unwrap(x), Ok(5));
1548 fn into_from_raw() {
1549 let x = Arc::new(box "hello");
1552 let x_ptr = Arc::into_raw(x);
1555 assert_eq!(**x_ptr, "hello");
1557 let x = Arc::from_raw(x_ptr);
1558 assert_eq!(**x, "hello");
1560 assert_eq!(Arc::try_unwrap(x).map(|x| *x), Ok("hello"));
1565 fn test_into_from_raw_unsized() {
1566 use std::fmt::Display;
1567 use std::string::ToString;
1569 let arc: Arc<str> = Arc::from("foo");
1571 let ptr = Arc::into_raw(arc.clone());
1572 let arc2 = unsafe { Arc::from_raw(ptr) };
1574 assert_eq!(unsafe { &*ptr }, "foo");
1575 assert_eq!(arc, arc2);
1577 let arc: Arc<dyn Display> = Arc::new(123);
1579 let ptr = Arc::into_raw(arc.clone());
1580 let arc2 = unsafe { Arc::from_raw(ptr) };
1582 assert_eq!(unsafe { &*ptr }.to_string(), "123");
1583 assert_eq!(arc2.to_string(), "123");
1587 fn test_cowarc_clone_make_mut() {
1588 let mut cow0 = Arc::new(75);
1589 let mut cow1 = cow0.clone();
1590 let mut cow2 = cow1.clone();
1592 assert!(75 == *Arc::make_mut(&mut cow0));
1593 assert!(75 == *Arc::make_mut(&mut cow1));
1594 assert!(75 == *Arc::make_mut(&mut cow2));
1596 *Arc::make_mut(&mut cow0) += 1;
1597 *Arc::make_mut(&mut cow1) += 2;
1598 *Arc::make_mut(&mut cow2) += 3;
1600 assert!(76 == *cow0);
1601 assert!(77 == *cow1);
1602 assert!(78 == *cow2);
1604 // none should point to the same backing memory
1605 assert!(*cow0 != *cow1);
1606 assert!(*cow0 != *cow2);
1607 assert!(*cow1 != *cow2);
1611 fn test_cowarc_clone_unique2() {
1612 let mut cow0 = Arc::new(75);
1613 let cow1 = cow0.clone();
1614 let cow2 = cow1.clone();
1616 assert!(75 == *cow0);
1617 assert!(75 == *cow1);
1618 assert!(75 == *cow2);
1620 *Arc::make_mut(&mut cow0) += 1;
1621 assert!(76 == *cow0);
1622 assert!(75 == *cow1);
1623 assert!(75 == *cow2);
1625 // cow1 and cow2 should share the same contents
1626 // cow0 should have a unique reference
1627 assert!(*cow0 != *cow1);
1628 assert!(*cow0 != *cow2);
1629 assert!(*cow1 == *cow2);
1633 fn test_cowarc_clone_weak() {
1634 let mut cow0 = Arc::new(75);
1635 let cow1_weak = Arc::downgrade(&cow0);
1637 assert!(75 == *cow0);
1638 assert!(75 == *cow1_weak.upgrade().unwrap());
1640 *Arc::make_mut(&mut cow0) += 1;
1642 assert!(76 == *cow0);
1643 assert!(cow1_weak.upgrade().is_none());
1648 let x = Arc::new(5);
1649 let y = Arc::downgrade(&x);
1650 assert!(y.upgrade().is_some());
1655 let x = Arc::new(5);
1656 let y = Arc::downgrade(&x);
1658 assert!(y.upgrade().is_none());
1662 fn weak_self_cyclic() {
1664 x: Mutex<Option<Weak<Cycle>>>,
1667 let a = Arc::new(Cycle { x: Mutex::new(None) });
1668 let b = Arc::downgrade(&a.clone());
1669 *a.x.lock().unwrap() = Some(b);
1671 // hopefully we don't double-free (or leak)...
1676 let mut canary = atomic::AtomicUsize::new(0);
1677 let x = Arc::new(Canary(&mut canary as *mut atomic::AtomicUsize));
1679 assert!(canary.load(Acquire) == 1);
1683 fn drop_arc_weak() {
1684 let mut canary = atomic::AtomicUsize::new(0);
1685 let arc = Arc::new(Canary(&mut canary as *mut atomic::AtomicUsize));
1686 let arc_weak = Arc::downgrade(&arc);
1687 assert!(canary.load(Acquire) == 0);
1689 assert!(canary.load(Acquire) == 1);
1694 fn test_strong_count() {
1695 let a = Arc::new(0);
1696 assert!(Arc::strong_count(&a) == 1);
1697 let w = Arc::downgrade(&a);
1698 assert!(Arc::strong_count(&a) == 1);
1699 let b = w.upgrade().expect("");
1700 assert!(Arc::strong_count(&b) == 2);
1701 assert!(Arc::strong_count(&a) == 2);
1704 assert!(Arc::strong_count(&b) == 1);
1706 assert!(Arc::strong_count(&b) == 2);
1707 assert!(Arc::strong_count(&c) == 2);
1711 fn test_weak_count() {
1712 let a = Arc::new(0);
1713 assert!(Arc::strong_count(&a) == 1);
1714 assert!(Arc::weak_count(&a) == 0);
1715 let w = Arc::downgrade(&a);
1716 assert!(Arc::strong_count(&a) == 1);
1717 assert!(Arc::weak_count(&a) == 1);
1719 assert!(Arc::weak_count(&a) == 2);
1722 assert!(Arc::strong_count(&a) == 1);
1723 assert!(Arc::weak_count(&a) == 0);
1725 assert!(Arc::strong_count(&a) == 2);
1726 assert!(Arc::weak_count(&a) == 0);
1727 let d = Arc::downgrade(&c);
1728 assert!(Arc::weak_count(&c) == 1);
1729 assert!(Arc::strong_count(&c) == 2);
1738 let a = Arc::new(5);
1739 assert_eq!(format!("{:?}", a), "5");
1742 // Make sure deriving works with Arc<T>
1743 #[derive(Eq, Ord, PartialEq, PartialOrd, Clone, Debug, Default)]
1750 let x: Arc<[i32]> = Arc::new([1, 2, 3]);
1751 assert_eq!(format!("{:?}", x), "[1, 2, 3]");
1752 let y = Arc::downgrade(&x.clone());
1754 assert!(y.upgrade().is_none());
1758 fn test_from_owned() {
1760 let foo_arc = Arc::from(foo);
1761 assert!(123 == *foo_arc);
1765 fn test_new_weak() {
1766 let foo: Weak<usize> = Weak::new();
1767 assert!(foo.upgrade().is_none());
1772 let five = Arc::new(5);
1773 let same_five = five.clone();
1774 let other_five = Arc::new(5);
1776 assert!(Arc::ptr_eq(&five, &same_five));
1777 assert!(!Arc::ptr_eq(&five, &other_five));
1781 #[cfg_attr(target_os = "emscripten", ignore)]
1782 fn test_weak_count_locked() {
1783 let mut a = Arc::new(atomic::AtomicBool::new(false));
1785 let t = thread::spawn(move || {
1786 for _i in 0..1000000 {
1787 Arc::get_mut(&mut a);
1789 a.store(true, SeqCst);
1792 while !a2.load(SeqCst) {
1793 let n = Arc::weak_count(&a2);
1794 assert!(n < 2, "bad weak count: {}", n);
1800 fn test_from_str() {
1801 let r: Arc<str> = Arc::from("foo");
1803 assert_eq!(&r[..], "foo");
1807 fn test_copy_from_slice() {
1808 let s: &[u32] = &[1, 2, 3];
1809 let r: Arc<[u32]> = Arc::from(s);
1811 assert_eq!(&r[..], [1, 2, 3]);
1815 fn test_clone_from_slice() {
1816 #[derive(Clone, Debug, Eq, PartialEq)]
1819 let s: &[X] = &[X(1), X(2), X(3)];
1820 let r: Arc<[X]> = Arc::from(s);
1822 assert_eq!(&r[..], s);
1827 fn test_clone_from_slice_panic() {
1828 use std::string::{String, ToString};
1830 struct Fail(u32, String);
1832 impl Clone for Fail {
1833 fn clone(&self) -> Fail {
1837 Fail(self.0, self.1.clone())
1842 Fail(0, "foo".to_string()),
1843 Fail(1, "bar".to_string()),
1844 Fail(2, "baz".to_string()),
1847 // Should panic, but not cause memory corruption
1848 let _r: Arc<[Fail]> = Arc::from(s);
1852 fn test_from_box() {
1853 let b: Box<u32> = box 123;
1854 let r: Arc<u32> = Arc::from(b);
1856 assert_eq!(*r, 123);
1860 fn test_from_box_str() {
1861 use std::string::String;
1863 let s = String::from("foo").into_boxed_str();
1864 let r: Arc<str> = Arc::from(s);
1866 assert_eq!(&r[..], "foo");
1870 fn test_from_box_slice() {
1871 let s = vec![1, 2, 3].into_boxed_slice();
1872 let r: Arc<[u32]> = Arc::from(s);
1874 assert_eq!(&r[..], [1, 2, 3]);
1878 fn test_from_box_trait() {
1879 use std::fmt::Display;
1880 use std::string::ToString;
1882 let b: Box<dyn Display> = box 123;
1883 let r: Arc<dyn Display> = Arc::from(b);
1885 assert_eq!(r.to_string(), "123");
1889 fn test_from_box_trait_zero_sized() {
1890 use std::fmt::Debug;
1892 let b: Box<dyn Debug> = box ();
1893 let r: Arc<dyn Debug> = Arc::from(b);
1895 assert_eq!(format!("{:?}", r), "()");
1899 fn test_from_vec() {
1900 let v = vec![1, 2, 3];
1901 let r: Arc<[u32]> = Arc::from(v);
1903 assert_eq!(&r[..], [1, 2, 3]);
1907 fn test_downcast() {
1910 let r1: Arc<dyn Any + Send + Sync> = Arc::new(i32::max_value());
1911 let r2: Arc<dyn Any + Send + Sync> = Arc::new("abc");
1913 assert!(r1.clone().downcast::<u32>().is_err());
1915 let r1i32 = r1.downcast::<i32>();
1916 assert!(r1i32.is_ok());
1917 assert_eq!(r1i32.unwrap(), Arc::new(i32::max_value()));
1919 assert!(r2.clone().downcast::<i32>().is_err());
1921 let r2str = r2.downcast::<&'static str>();
1922 assert!(r2str.is_ok());
1923 assert_eq!(r2str.unwrap(), Arc::new("abc"));
1927 #[stable(feature = "rust1", since = "1.0.0")]
1928 impl<T: ?Sized> borrow::Borrow<T> for Arc<T> {
1929 fn borrow(&self) -> &T {
1934 #[stable(since = "1.5.0", feature = "smart_ptr_as_ref")]
1935 impl<T: ?Sized> AsRef<T> for Arc<T> {
1936 fn as_ref(&self) -> &T {