alloc/sync.rs
1#![stable(feature = "rust1", since = "1.0.0")]
2
3//! Thread-safe reference-counting pointers.
4//!
5//! See the [`Arc<T>`][Arc] documentation for more details.
6//!
7//! **Note**: This module is only available on platforms that support atomic
8//! loads and stores of pointers. This may be detected at compile time using
9//! `#[cfg(target_has_atomic = "ptr")]`.
10
11use core::any::Any;
12use core::cell::CloneFromCell;
13#[cfg(not(no_global_oom_handling))]
14use core::clone::CloneToUninit;
15use core::clone::UseCloned;
16use core::cmp::Ordering;
17use core::hash::{Hash, Hasher};
18use core::intrinsics::abort;
19#[cfg(not(no_global_oom_handling))]
20use core::iter;
21use core::marker::{PhantomData, Unsize};
22use core::mem::{self, ManuallyDrop, align_of_val_raw};
23use core::num::NonZeroUsize;
24use core::ops::{CoerceUnsized, Deref, DerefMut, DerefPure, DispatchFromDyn, LegacyReceiver};
25use core::panic::{RefUnwindSafe, UnwindSafe};
26use core::pin::{Pin, PinCoerceUnsized};
27use core::ptr::{self, NonNull};
28#[cfg(not(no_global_oom_handling))]
29use core::slice::from_raw_parts_mut;
30use core::sync::atomic::Ordering::{Acquire, Relaxed, Release};
31use core::sync::atomic::{self, Atomic};
32use core::{borrow, fmt, hint};
33
34#[cfg(not(no_global_oom_handling))]
35use crate::alloc::handle_alloc_error;
36use crate::alloc::{AllocError, Allocator, Global, Layout};
37use crate::borrow::{Cow, ToOwned};
38use crate::boxed::Box;
39use crate::rc::is_dangling;
40#[cfg(not(no_global_oom_handling))]
41use crate::string::String;
42#[cfg(not(no_global_oom_handling))]
43use crate::vec::Vec;
44
45/// A soft limit on the amount of references that may be made to an `Arc`.
46///
47/// Going above this limit will abort your program (although not
48/// necessarily) at _exactly_ `MAX_REFCOUNT + 1` references.
49/// Trying to go above it might call a `panic` (if not actually going above it).
50///
51/// This is a global invariant, and also applies when using a compare-exchange loop.
52///
53/// See comment in `Arc::clone`.
54const MAX_REFCOUNT: usize = (isize::MAX) as usize;
55
56/// The error in case either counter reaches above `MAX_REFCOUNT`, and we can `panic` safely.
57const INTERNAL_OVERFLOW_ERROR: &str = "Arc counter overflow";
58
59#[cfg(not(sanitize = "thread"))]
60macro_rules! acquire {
61 ($x:expr) => {
62 atomic::fence(Acquire)
63 };
64}
65
66// ThreadSanitizer does not support memory fences. To avoid false positive
67// reports in Arc / Weak implementation use atomic loads for synchronization
68// instead.
69#[cfg(sanitize = "thread")]
70macro_rules! acquire {
71 ($x:expr) => {
72 $x.load(Acquire)
73 };
74}
75
76/// A thread-safe reference-counting pointer. 'Arc' stands for 'Atomically
77/// Reference Counted'.
78///
79/// The type `Arc<T>` provides shared ownership of a value of type `T`,
80/// allocated in the heap. Invoking [`clone`][clone] on `Arc` produces
81/// a new `Arc` instance, which points to the same allocation on the heap as the
82/// source `Arc`, while increasing a reference count. When the last `Arc`
83/// pointer to a given allocation is destroyed, the value stored in that allocation (often
84/// referred to as "inner value") is also dropped.
85///
86/// Shared references in Rust disallow mutation by default, and `Arc` is no
87/// exception: you cannot generally obtain a mutable reference to something
88/// inside an `Arc`. If you do need to mutate through an `Arc`, you have several options:
89///
90/// 1. Use interior mutability with synchronization primitives like [`Mutex`][mutex],
91/// [`RwLock`][rwlock], or one of the [`Atomic`][atomic] types.
92///
93/// 2. Use clone-on-write semantics with [`Arc::make_mut`] which provides efficient mutation
94/// without requiring interior mutability. This approach clones the data only when
95/// needed (when there are multiple references) and can be more efficient when mutations
96/// are infrequent.
97///
98/// 3. Use [`Arc::get_mut`] when you know your `Arc` is not shared (has a reference count of 1),
99/// which provides direct mutable access to the inner value without any cloning.
100///
101/// ```
102/// use std::sync::Arc;
103///
104/// let mut data = Arc::new(vec![1, 2, 3]);
105///
106/// // This will clone the vector only if there are other references to it
107/// Arc::make_mut(&mut data).push(4);
108///
109/// assert_eq!(*data, vec![1, 2, 3, 4]);
110/// ```
111///
112/// **Note**: This type is only available on platforms that support atomic
113/// loads and stores of pointers, which includes all platforms that support
114/// the `std` crate but not all those which only support [`alloc`](crate).
115/// This may be detected at compile time using `#[cfg(target_has_atomic = "ptr")]`.
116///
117/// ## Thread Safety
118///
119/// Unlike [`Rc<T>`], `Arc<T>` uses atomic operations for its reference
120/// counting. This means that it is thread-safe. The disadvantage is that
121/// atomic operations are more expensive than ordinary memory accesses. If you
122/// are not sharing reference-counted allocations between threads, consider using
123/// [`Rc<T>`] for lower overhead. [`Rc<T>`] is a safe default, because the
124/// compiler will catch any attempt to send an [`Rc<T>`] between threads.
125/// However, a library might choose `Arc<T>` in order to give library consumers
126/// more flexibility.
127///
128/// `Arc<T>` will implement [`Send`] and [`Sync`] as long as the `T` implements
129/// [`Send`] and [`Sync`]. Why can't you put a non-thread-safe type `T` in an
130/// `Arc<T>` to make it thread-safe? This may be a bit counter-intuitive at
131/// first: after all, isn't the point of `Arc<T>` thread safety? The key is
132/// this: `Arc<T>` makes it thread safe to have multiple ownership of the same
133/// data, but it doesn't add thread safety to its data. Consider
134/// <code>Arc<[RefCell\<T>]></code>. [`RefCell<T>`] isn't [`Sync`], and if `Arc<T>` was always
135/// [`Send`], <code>Arc<[RefCell\<T>]></code> would be as well. But then we'd have a problem:
136/// [`RefCell<T>`] is not thread safe; it keeps track of the borrowing count using
137/// non-atomic operations.
138///
139/// In the end, this means that you may need to pair `Arc<T>` with some sort of
140/// [`std::sync`] type, usually [`Mutex<T>`][mutex].
141///
142/// ## Breaking cycles with `Weak`
143///
144/// The [`downgrade`][downgrade] method can be used to create a non-owning
145/// [`Weak`] pointer. A [`Weak`] pointer can be [`upgrade`][upgrade]d
146/// to an `Arc`, but this will return [`None`] if the value stored in the allocation has
147/// already been dropped. In other words, `Weak` pointers do not keep the value
148/// inside the allocation alive; however, they *do* keep the allocation
149/// (the backing store for the value) alive.
150///
151/// A cycle between `Arc` pointers will never be deallocated. For this reason,
152/// [`Weak`] is used to break cycles. For example, a tree could have
153/// strong `Arc` pointers from parent nodes to children, and [`Weak`]
154/// pointers from children back to their parents.
155///
156/// # Cloning references
157///
158/// Creating a new reference from an existing reference-counted pointer is done using the
159/// `Clone` trait implemented for [`Arc<T>`][Arc] and [`Weak<T>`][Weak].
160///
161/// ```
162/// use std::sync::Arc;
163/// let foo = Arc::new(vec![1.0, 2.0, 3.0]);
164/// // The two syntaxes below are equivalent.
165/// let a = foo.clone();
166/// let b = Arc::clone(&foo);
167/// // a, b, and foo are all Arcs that point to the same memory location
168/// ```
169///
170/// ## `Deref` behavior
171///
172/// `Arc<T>` automatically dereferences to `T` (via the [`Deref`] trait),
173/// so you can call `T`'s methods on a value of type `Arc<T>`. To avoid name
174/// clashes with `T`'s methods, the methods of `Arc<T>` itself are associated
175/// functions, called using [fully qualified syntax]:
176///
177/// ```
178/// use std::sync::Arc;
179///
180/// let my_arc = Arc::new(());
181/// let my_weak = Arc::downgrade(&my_arc);
182/// ```
183///
184/// `Arc<T>`'s implementations of traits like `Clone` may also be called using
185/// fully qualified syntax. Some people prefer to use fully qualified syntax,
186/// while others prefer using method-call syntax.
187///
188/// ```
189/// use std::sync::Arc;
190///
191/// let arc = Arc::new(());
192/// // Method-call syntax
193/// let arc2 = arc.clone();
194/// // Fully qualified syntax
195/// let arc3 = Arc::clone(&arc);
196/// ```
197///
198/// [`Weak<T>`][Weak] does not auto-dereference to `T`, because the inner value may have
199/// already been dropped.
200///
201/// [`Rc<T>`]: crate::rc::Rc
202/// [clone]: Clone::clone
203/// [mutex]: ../../std/sync/struct.Mutex.html
204/// [rwlock]: ../../std/sync/struct.RwLock.html
205/// [atomic]: core::sync::atomic
206/// [downgrade]: Arc::downgrade
207/// [upgrade]: Weak::upgrade
208/// [RefCell\<T>]: core::cell::RefCell
209/// [`RefCell<T>`]: core::cell::RefCell
210/// [`std::sync`]: ../../std/sync/index.html
211/// [`Arc::clone(&from)`]: Arc::clone
212/// [fully qualified syntax]: https://doc.rust-lang.org/book/ch19-03-advanced-traits.html#fully-qualified-syntax-for-disambiguation-calling-methods-with-the-same-name
213///
214/// # Examples
215///
216/// Sharing some immutable data between threads:
217///
218/// ```
219/// use std::sync::Arc;
220/// use std::thread;
221///
222/// let five = Arc::new(5);
223///
224/// for _ in 0..10 {
225/// let five = Arc::clone(&five);
226///
227/// thread::spawn(move || {
228/// println!("{five:?}");
229/// });
230/// }
231/// ```
232///
233/// Sharing a mutable [`AtomicUsize`]:
234///
235/// [`AtomicUsize`]: core::sync::atomic::AtomicUsize "sync::atomic::AtomicUsize"
236///
237/// ```
238/// use std::sync::Arc;
239/// use std::sync::atomic::{AtomicUsize, Ordering};
240/// use std::thread;
241///
242/// let val = Arc::new(AtomicUsize::new(5));
243///
244/// for _ in 0..10 {
245/// let val = Arc::clone(&val);
246///
247/// thread::spawn(move || {
248/// let v = val.fetch_add(1, Ordering::Relaxed);
249/// println!("{v:?}");
250/// });
251/// }
252/// ```
253///
254/// See the [`rc` documentation][rc_examples] for more examples of reference
255/// counting in general.
256///
257/// [rc_examples]: crate::rc#examples
258#[doc(search_unbox)]
259#[rustc_diagnostic_item = "Arc"]
260#[stable(feature = "rust1", since = "1.0.0")]
261#[rustc_insignificant_dtor]
262pub struct Arc<
263 T: ?Sized,
264 #[unstable(feature = "allocator_api", issue = "32838")] A: Allocator = Global,
265> {
266 ptr: NonNull<ArcInner<T>>,
267 phantom: PhantomData<ArcInner<T>>,
268 alloc: A,
269}
270
271#[stable(feature = "rust1", since = "1.0.0")]
272unsafe impl<T: ?Sized + Sync + Send, A: Allocator + Send> Send for Arc<T, A> {}
273#[stable(feature = "rust1", since = "1.0.0")]
274unsafe impl<T: ?Sized + Sync + Send, A: Allocator + Sync> Sync for Arc<T, A> {}
275
276#[stable(feature = "catch_unwind", since = "1.9.0")]
277impl<T: RefUnwindSafe + ?Sized, A: Allocator + UnwindSafe> UnwindSafe for Arc<T, A> {}
278
279#[unstable(feature = "coerce_unsized", issue = "18598")]
280impl<T: ?Sized + Unsize<U>, U: ?Sized, A: Allocator> CoerceUnsized<Arc<U, A>> for Arc<T, A> {}
281
282#[unstable(feature = "dispatch_from_dyn", issue = "none")]
283impl<T: ?Sized + Unsize<U>, U: ?Sized> DispatchFromDyn<Arc<U>> for Arc<T> {}
284
285// SAFETY: `Arc::clone` doesn't access any `Cell`s which could contain the `Arc` being cloned.
286#[unstable(feature = "cell_get_cloned", issue = "145329")]
287unsafe impl<T: ?Sized> CloneFromCell for Arc<T> {}
288
289impl<T: ?Sized> Arc<T> {
290 unsafe fn from_inner(ptr: NonNull<ArcInner<T>>) -> Self {
291 unsafe { Self::from_inner_in(ptr, Global) }
292 }
293
294 unsafe fn from_ptr(ptr: *mut ArcInner<T>) -> Self {
295 unsafe { Self::from_ptr_in(ptr, Global) }
296 }
297}
298
299impl<T: ?Sized, A: Allocator> Arc<T, A> {
300 #[inline]
301 fn into_inner_with_allocator(this: Self) -> (NonNull<ArcInner<T>>, A) {
302 let this = mem::ManuallyDrop::new(this);
303 (this.ptr, unsafe { ptr::read(&this.alloc) })
304 }
305
306 #[inline]
307 unsafe fn from_inner_in(ptr: NonNull<ArcInner<T>>, alloc: A) -> Self {
308 Self { ptr, phantom: PhantomData, alloc }
309 }
310
311 #[inline]
312 unsafe fn from_ptr_in(ptr: *mut ArcInner<T>, alloc: A) -> Self {
313 unsafe { Self::from_inner_in(NonNull::new_unchecked(ptr), alloc) }
314 }
315}
316
317/// `Weak` is a version of [`Arc`] that holds a non-owning reference to the
318/// managed allocation.
319///
320/// The allocation is accessed by calling [`upgrade`] on the `Weak`
321/// pointer, which returns an <code>[Option]<[Arc]\<T>></code>.
322///
323/// Since a `Weak` reference does not count towards ownership, it will not
324/// prevent the value stored in the allocation from being dropped, and `Weak` itself makes no
325/// guarantees about the value still being present. Thus it may return [`None`]
326/// when [`upgrade`]d. Note however that a `Weak` reference *does* prevent the allocation
327/// itself (the backing store) from being deallocated.
328///
329/// A `Weak` pointer is useful for keeping a temporary reference to the allocation
330/// managed by [`Arc`] without preventing its inner value from being dropped. It is also used to
331/// prevent circular references between [`Arc`] pointers, since mutual owning references
332/// would never allow either [`Arc`] to be dropped. For example, a tree could
333/// have strong [`Arc`] pointers from parent nodes to children, and `Weak`
334/// pointers from children back to their parents.
335///
336/// The typical way to obtain a `Weak` pointer is to call [`Arc::downgrade`].
337///
338/// [`upgrade`]: Weak::upgrade
339#[stable(feature = "arc_weak", since = "1.4.0")]
340#[rustc_diagnostic_item = "ArcWeak"]
341pub struct Weak<
342 T: ?Sized,
343 #[unstable(feature = "allocator_api", issue = "32838")] A: Allocator = Global,
344> {
345 // This is a `NonNull` to allow optimizing the size of this type in enums,
346 // but it is not necessarily a valid pointer.
347 // `Weak::new` sets this to `usize::MAX` so that it doesn’t need
348 // to allocate space on the heap. That's not a value a real pointer
349 // will ever have because ArcInner has alignment at least 2.
350 ptr: NonNull<ArcInner<T>>,
351 alloc: A,
352}
353
354#[stable(feature = "arc_weak", since = "1.4.0")]
355unsafe impl<T: ?Sized + Sync + Send, A: Allocator + Send> Send for Weak<T, A> {}
356#[stable(feature = "arc_weak", since = "1.4.0")]
357unsafe impl<T: ?Sized + Sync + Send, A: Allocator + Sync> Sync for Weak<T, A> {}
358
359#[unstable(feature = "coerce_unsized", issue = "18598")]
360impl<T: ?Sized + Unsize<U>, U: ?Sized, A: Allocator> CoerceUnsized<Weak<U, A>> for Weak<T, A> {}
361#[unstable(feature = "dispatch_from_dyn", issue = "none")]
362impl<T: ?Sized + Unsize<U>, U: ?Sized> DispatchFromDyn<Weak<U>> for Weak<T> {}
363
364// SAFETY: `Weak::clone` doesn't access any `Cell`s which could contain the `Weak` being cloned.
365#[unstable(feature = "cell_get_cloned", issue = "145329")]
366unsafe impl<T: ?Sized> CloneFromCell for Weak<T> {}
367
368#[stable(feature = "arc_weak", since = "1.4.0")]
369impl<T: ?Sized, A: Allocator> fmt::Debug for Weak<T, A> {
370 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
371 write!(f, "(Weak)")
372 }
373}
374
375// This is repr(C) to future-proof against possible field-reordering, which
376// would interfere with otherwise safe [into|from]_raw() of transmutable
377// inner types.
378// Unlike RcInner, repr(align(2)) is not strictly required because atomic types
379// have the alignment same as its size, but we use it for consistency and clarity.
380#[repr(C, align(2))]
381struct ArcInner<T: ?Sized> {
382 strong: Atomic<usize>,
383
384 // the value usize::MAX acts as a sentinel for temporarily "locking" the
385 // ability to upgrade weak pointers or downgrade strong ones; this is used
386 // to avoid races in `make_mut` and `get_mut`.
387 weak: Atomic<usize>,
388
389 data: T,
390}
391
392/// Calculate layout for `ArcInner<T>` using the inner value's layout
393fn arcinner_layout_for_value_layout(layout: Layout) -> Layout {
394 // Calculate layout using the given value layout.
395 // Previously, layout was calculated on the expression
396 // `&*(ptr as *const ArcInner<T>)`, but this created a misaligned
397 // reference (see #54908).
398 Layout::new::<ArcInner<()>>().extend(layout).unwrap().0.pad_to_align()
399}
400
401unsafe impl<T: ?Sized + Sync + Send> Send for ArcInner<T> {}
402unsafe impl<T: ?Sized + Sync + Send> Sync for ArcInner<T> {}
403
404impl<T> Arc<T> {
405 /// Constructs a new `Arc<T>`.
406 ///
407 /// # Examples
408 ///
409 /// ```
410 /// use std::sync::Arc;
411 ///
412 /// let five = Arc::new(5);
413 /// ```
414 #[cfg(not(no_global_oom_handling))]
415 #[inline]
416 #[stable(feature = "rust1", since = "1.0.0")]
417 pub fn new(data: T) -> Arc<T> {
418 // Start the weak pointer count as 1 which is the weak pointer that's
419 // held by all the strong pointers (kinda), see std/rc.rs for more info
420 let x: Box<_> = Box::new(ArcInner {
421 strong: atomic::AtomicUsize::new(1),
422 weak: atomic::AtomicUsize::new(1),
423 data,
424 });
425 unsafe { Self::from_inner(Box::leak(x).into()) }
426 }
427
428 /// Constructs a new `Arc<T>` while giving you a `Weak<T>` to the allocation,
429 /// to allow you to construct a `T` which holds a weak pointer to itself.
430 ///
431 /// Generally, a structure circularly referencing itself, either directly or
432 /// indirectly, should not hold a strong reference to itself to prevent a memory leak.
433 /// Using this function, you get access to the weak pointer during the
434 /// initialization of `T`, before the `Arc<T>` is created, such that you can
435 /// clone and store it inside the `T`.
436 ///
437 /// `new_cyclic` first allocates the managed allocation for the `Arc<T>`,
438 /// then calls your closure, giving it a `Weak<T>` to this allocation,
439 /// and only afterwards completes the construction of the `Arc<T>` by placing
440 /// the `T` returned from your closure into the allocation.
441 ///
442 /// Since the new `Arc<T>` is not fully-constructed until `Arc<T>::new_cyclic`
443 /// returns, calling [`upgrade`] on the weak reference inside your closure will
444 /// fail and result in a `None` value.
445 ///
446 /// # Panics
447 ///
448 /// If `data_fn` panics, the panic is propagated to the caller, and the
449 /// temporary [`Weak<T>`] is dropped normally.
450 ///
451 /// # Example
452 ///
453 /// ```
454 /// # #![allow(dead_code)]
455 /// use std::sync::{Arc, Weak};
456 ///
457 /// struct Gadget {
458 /// me: Weak<Gadget>,
459 /// }
460 ///
461 /// impl Gadget {
462 /// /// Constructs a reference counted Gadget.
463 /// fn new() -> Arc<Self> {
464 /// // `me` is a `Weak<Gadget>` pointing at the new allocation of the
465 /// // `Arc` we're constructing.
466 /// Arc::new_cyclic(|me| {
467 /// // Create the actual struct here.
468 /// Gadget { me: me.clone() }
469 /// })
470 /// }
471 ///
472 /// /// Returns a reference counted pointer to Self.
473 /// fn me(&self) -> Arc<Self> {
474 /// self.me.upgrade().unwrap()
475 /// }
476 /// }
477 /// ```
478 /// [`upgrade`]: Weak::upgrade
479 #[cfg(not(no_global_oom_handling))]
480 #[inline]
481 #[stable(feature = "arc_new_cyclic", since = "1.60.0")]
482 pub fn new_cyclic<F>(data_fn: F) -> Arc<T>
483 where
484 F: FnOnce(&Weak<T>) -> T,
485 {
486 Self::new_cyclic_in(data_fn, Global)
487 }
488
489 /// Constructs a new `Arc` with uninitialized contents.
490 ///
491 /// # Examples
492 ///
493 /// ```
494 /// use std::sync::Arc;
495 ///
496 /// let mut five = Arc::<u32>::new_uninit();
497 ///
498 /// // Deferred initialization:
499 /// Arc::get_mut(&mut five).unwrap().write(5);
500 ///
501 /// let five = unsafe { five.assume_init() };
502 ///
503 /// assert_eq!(*five, 5)
504 /// ```
505 #[cfg(not(no_global_oom_handling))]
506 #[inline]
507 #[stable(feature = "new_uninit", since = "1.82.0")]
508 #[must_use]
509 pub fn new_uninit() -> Arc<mem::MaybeUninit<T>> {
510 unsafe {
511 Arc::from_ptr(Arc::allocate_for_layout(
512 Layout::new::<T>(),
513 |layout| Global.allocate(layout),
514 <*mut u8>::cast,
515 ))
516 }
517 }
518
519 /// Constructs a new `Arc` with uninitialized contents, with the memory
520 /// being filled with `0` bytes.
521 ///
522 /// See [`MaybeUninit::zeroed`][zeroed] for examples of correct and incorrect usage
523 /// of this method.
524 ///
525 /// # Examples
526 ///
527 /// ```
528 /// use std::sync::Arc;
529 ///
530 /// let zero = Arc::<u32>::new_zeroed();
531 /// let zero = unsafe { zero.assume_init() };
532 ///
533 /// assert_eq!(*zero, 0)
534 /// ```
535 ///
536 /// [zeroed]: mem::MaybeUninit::zeroed
537 #[cfg(not(no_global_oom_handling))]
538 #[inline]
539 #[stable(feature = "new_zeroed_alloc", since = "CURRENT_RUSTC_VERSION")]
540 #[must_use]
541 pub fn new_zeroed() -> Arc<mem::MaybeUninit<T>> {
542 unsafe {
543 Arc::from_ptr(Arc::allocate_for_layout(
544 Layout::new::<T>(),
545 |layout| Global.allocate_zeroed(layout),
546 <*mut u8>::cast,
547 ))
548 }
549 }
550
551 /// Constructs a new `Pin<Arc<T>>`. If `T` does not implement `Unpin`, then
552 /// `data` will be pinned in memory and unable to be moved.
553 #[cfg(not(no_global_oom_handling))]
554 #[stable(feature = "pin", since = "1.33.0")]
555 #[must_use]
556 pub fn pin(data: T) -> Pin<Arc<T>> {
557 unsafe { Pin::new_unchecked(Arc::new(data)) }
558 }
559
560 /// Constructs a new `Pin<Arc<T>>`, return an error if allocation fails.
561 #[unstable(feature = "allocator_api", issue = "32838")]
562 #[inline]
563 pub fn try_pin(data: T) -> Result<Pin<Arc<T>>, AllocError> {
564 unsafe { Ok(Pin::new_unchecked(Arc::try_new(data)?)) }
565 }
566
567 /// Constructs a new `Arc<T>`, returning an error if allocation fails.
568 ///
569 /// # Examples
570 ///
571 /// ```
572 /// #![feature(allocator_api)]
573 /// use std::sync::Arc;
574 ///
575 /// let five = Arc::try_new(5)?;
576 /// # Ok::<(), std::alloc::AllocError>(())
577 /// ```
578 #[unstable(feature = "allocator_api", issue = "32838")]
579 #[inline]
580 pub fn try_new(data: T) -> Result<Arc<T>, AllocError> {
581 // Start the weak pointer count as 1 which is the weak pointer that's
582 // held by all the strong pointers (kinda), see std/rc.rs for more info
583 let x: Box<_> = Box::try_new(ArcInner {
584 strong: atomic::AtomicUsize::new(1),
585 weak: atomic::AtomicUsize::new(1),
586 data,
587 })?;
588 unsafe { Ok(Self::from_inner(Box::leak(x).into())) }
589 }
590
591 /// Constructs a new `Arc` with uninitialized contents, returning an error
592 /// if allocation fails.
593 ///
594 /// # Examples
595 ///
596 /// ```
597 /// #![feature(allocator_api)]
598 ///
599 /// use std::sync::Arc;
600 ///
601 /// let mut five = Arc::<u32>::try_new_uninit()?;
602 ///
603 /// // Deferred initialization:
604 /// Arc::get_mut(&mut five).unwrap().write(5);
605 ///
606 /// let five = unsafe { five.assume_init() };
607 ///
608 /// assert_eq!(*five, 5);
609 /// # Ok::<(), std::alloc::AllocError>(())
610 /// ```
611 #[unstable(feature = "allocator_api", issue = "32838")]
612 pub fn try_new_uninit() -> Result<Arc<mem::MaybeUninit<T>>, AllocError> {
613 unsafe {
614 Ok(Arc::from_ptr(Arc::try_allocate_for_layout(
615 Layout::new::<T>(),
616 |layout| Global.allocate(layout),
617 <*mut u8>::cast,
618 )?))
619 }
620 }
621
622 /// Constructs a new `Arc` with uninitialized contents, with the memory
623 /// being filled with `0` bytes, returning an error if allocation fails.
624 ///
625 /// See [`MaybeUninit::zeroed`][zeroed] for examples of correct and incorrect usage
626 /// of this method.
627 ///
628 /// # Examples
629 ///
630 /// ```
631 /// #![feature( allocator_api)]
632 ///
633 /// use std::sync::Arc;
634 ///
635 /// let zero = Arc::<u32>::try_new_zeroed()?;
636 /// let zero = unsafe { zero.assume_init() };
637 ///
638 /// assert_eq!(*zero, 0);
639 /// # Ok::<(), std::alloc::AllocError>(())
640 /// ```
641 ///
642 /// [zeroed]: mem::MaybeUninit::zeroed
643 #[unstable(feature = "allocator_api", issue = "32838")]
644 pub fn try_new_zeroed() -> Result<Arc<mem::MaybeUninit<T>>, AllocError> {
645 unsafe {
646 Ok(Arc::from_ptr(Arc::try_allocate_for_layout(
647 Layout::new::<T>(),
648 |layout| Global.allocate_zeroed(layout),
649 <*mut u8>::cast,
650 )?))
651 }
652 }
653}
654
655impl<T, A: Allocator> Arc<T, A> {
656 /// Constructs a new `Arc<T>` in the provided allocator.
657 ///
658 /// # Examples
659 ///
660 /// ```
661 /// #![feature(allocator_api)]
662 ///
663 /// use std::sync::Arc;
664 /// use std::alloc::System;
665 ///
666 /// let five = Arc::new_in(5, System);
667 /// ```
668 #[inline]
669 #[cfg(not(no_global_oom_handling))]
670 #[unstable(feature = "allocator_api", issue = "32838")]
671 pub fn new_in(data: T, alloc: A) -> Arc<T, A> {
672 // Start the weak pointer count as 1 which is the weak pointer that's
673 // held by all the strong pointers (kinda), see std/rc.rs for more info
674 let x = Box::new_in(
675 ArcInner {
676 strong: atomic::AtomicUsize::new(1),
677 weak: atomic::AtomicUsize::new(1),
678 data,
679 },
680 alloc,
681 );
682 let (ptr, alloc) = Box::into_unique(x);
683 unsafe { Self::from_inner_in(ptr.into(), alloc) }
684 }
685
686 /// Constructs a new `Arc` with uninitialized contents in the provided allocator.
687 ///
688 /// # Examples
689 ///
690 /// ```
691 /// #![feature(get_mut_unchecked)]
692 /// #![feature(allocator_api)]
693 ///
694 /// use std::sync::Arc;
695 /// use std::alloc::System;
696 ///
697 /// let mut five = Arc::<u32, _>::new_uninit_in(System);
698 ///
699 /// let five = unsafe {
700 /// // Deferred initialization:
701 /// Arc::get_mut_unchecked(&mut five).as_mut_ptr().write(5);
702 ///
703 /// five.assume_init()
704 /// };
705 ///
706 /// assert_eq!(*five, 5)
707 /// ```
708 #[cfg(not(no_global_oom_handling))]
709 #[unstable(feature = "allocator_api", issue = "32838")]
710 #[inline]
711 pub fn new_uninit_in(alloc: A) -> Arc<mem::MaybeUninit<T>, A> {
712 unsafe {
713 Arc::from_ptr_in(
714 Arc::allocate_for_layout(
715 Layout::new::<T>(),
716 |layout| alloc.allocate(layout),
717 <*mut u8>::cast,
718 ),
719 alloc,
720 )
721 }
722 }
723
724 /// Constructs a new `Arc` with uninitialized contents, with the memory
725 /// being filled with `0` bytes, in the provided allocator.
726 ///
727 /// See [`MaybeUninit::zeroed`][zeroed] for examples of correct and incorrect usage
728 /// of this method.
729 ///
730 /// # Examples
731 ///
732 /// ```
733 /// #![feature(allocator_api)]
734 ///
735 /// use std::sync::Arc;
736 /// use std::alloc::System;
737 ///
738 /// let zero = Arc::<u32, _>::new_zeroed_in(System);
739 /// let zero = unsafe { zero.assume_init() };
740 ///
741 /// assert_eq!(*zero, 0)
742 /// ```
743 ///
744 /// [zeroed]: mem::MaybeUninit::zeroed
745 #[cfg(not(no_global_oom_handling))]
746 #[unstable(feature = "allocator_api", issue = "32838")]
747 #[inline]
748 pub fn new_zeroed_in(alloc: A) -> Arc<mem::MaybeUninit<T>, A> {
749 unsafe {
750 Arc::from_ptr_in(
751 Arc::allocate_for_layout(
752 Layout::new::<T>(),
753 |layout| alloc.allocate_zeroed(layout),
754 <*mut u8>::cast,
755 ),
756 alloc,
757 )
758 }
759 }
760
761 /// Constructs a new `Arc<T, A>` in the given allocator while giving you a `Weak<T, A>` to the allocation,
762 /// to allow you to construct a `T` which holds a weak pointer to itself.
763 ///
764 /// Generally, a structure circularly referencing itself, either directly or
765 /// indirectly, should not hold a strong reference to itself to prevent a memory leak.
766 /// Using this function, you get access to the weak pointer during the
767 /// initialization of `T`, before the `Arc<T, A>` is created, such that you can
768 /// clone and store it inside the `T`.
769 ///
770 /// `new_cyclic_in` first allocates the managed allocation for the `Arc<T, A>`,
771 /// then calls your closure, giving it a `Weak<T, A>` to this allocation,
772 /// and only afterwards completes the construction of the `Arc<T, A>` by placing
773 /// the `T` returned from your closure into the allocation.
774 ///
775 /// Since the new `Arc<T, A>` is not fully-constructed until `Arc<T, A>::new_cyclic_in`
776 /// returns, calling [`upgrade`] on the weak reference inside your closure will
777 /// fail and result in a `None` value.
778 ///
779 /// # Panics
780 ///
781 /// If `data_fn` panics, the panic is propagated to the caller, and the
782 /// temporary [`Weak<T>`] is dropped normally.
783 ///
784 /// # Example
785 ///
786 /// See [`new_cyclic`]
787 ///
788 /// [`new_cyclic`]: Arc::new_cyclic
789 /// [`upgrade`]: Weak::upgrade
790 #[cfg(not(no_global_oom_handling))]
791 #[inline]
792 #[unstable(feature = "allocator_api", issue = "32838")]
793 pub fn new_cyclic_in<F>(data_fn: F, alloc: A) -> Arc<T, A>
794 where
795 F: FnOnce(&Weak<T, A>) -> T,
796 {
797 // Construct the inner in the "uninitialized" state with a single
798 // weak reference.
799 let (uninit_raw_ptr, alloc) = Box::into_raw_with_allocator(Box::new_in(
800 ArcInner {
801 strong: atomic::AtomicUsize::new(0),
802 weak: atomic::AtomicUsize::new(1),
803 data: mem::MaybeUninit::<T>::uninit(),
804 },
805 alloc,
806 ));
807 let uninit_ptr: NonNull<_> = (unsafe { &mut *uninit_raw_ptr }).into();
808 let init_ptr: NonNull<ArcInner<T>> = uninit_ptr.cast();
809
810 let weak = Weak { ptr: init_ptr, alloc };
811
812 // It's important we don't give up ownership of the weak pointer, or
813 // else the memory might be freed by the time `data_fn` returns. If
814 // we really wanted to pass ownership, we could create an additional
815 // weak pointer for ourselves, but this would result in additional
816 // updates to the weak reference count which might not be necessary
817 // otherwise.
818 let data = data_fn(&weak);
819
820 // Now we can properly initialize the inner value and turn our weak
821 // reference into a strong reference.
822 let strong = unsafe {
823 let inner = init_ptr.as_ptr();
824 ptr::write(&raw mut (*inner).data, data);
825
826 // The above write to the data field must be visible to any threads which
827 // observe a non-zero strong count. Therefore we need at least "Release" ordering
828 // in order to synchronize with the `compare_exchange_weak` in `Weak::upgrade`.
829 //
830 // "Acquire" ordering is not required. When considering the possible behaviors
831 // of `data_fn` we only need to look at what it could do with a reference to a
832 // non-upgradeable `Weak`:
833 // - It can *clone* the `Weak`, increasing the weak reference count.
834 // - It can drop those clones, decreasing the weak reference count (but never to zero).
835 //
836 // These side effects do not impact us in any way, and no other side effects are
837 // possible with safe code alone.
838 let prev_value = (*inner).strong.fetch_add(1, Release);
839 debug_assert_eq!(prev_value, 0, "No prior strong references should exist");
840
841 // Strong references should collectively own a shared weak reference,
842 // so don't run the destructor for our old weak reference.
843 // Calling into_raw_with_allocator has the double effect of giving us back the allocator,
844 // and forgetting the weak reference.
845 let alloc = weak.into_raw_with_allocator().1;
846
847 Arc::from_inner_in(init_ptr, alloc)
848 };
849
850 strong
851 }
852
853 /// Constructs a new `Pin<Arc<T, A>>` in the provided allocator. If `T` does not implement `Unpin`,
854 /// then `data` will be pinned in memory and unable to be moved.
855 #[cfg(not(no_global_oom_handling))]
856 #[unstable(feature = "allocator_api", issue = "32838")]
857 #[inline]
858 pub fn pin_in(data: T, alloc: A) -> Pin<Arc<T, A>>
859 where
860 A: 'static,
861 {
862 unsafe { Pin::new_unchecked(Arc::new_in(data, alloc)) }
863 }
864
865 /// Constructs a new `Pin<Arc<T, A>>` in the provided allocator, return an error if allocation
866 /// fails.
867 #[inline]
868 #[unstable(feature = "allocator_api", issue = "32838")]
869 pub fn try_pin_in(data: T, alloc: A) -> Result<Pin<Arc<T, A>>, AllocError>
870 where
871 A: 'static,
872 {
873 unsafe { Ok(Pin::new_unchecked(Arc::try_new_in(data, alloc)?)) }
874 }
875
876 /// Constructs a new `Arc<T, A>` in the provided allocator, returning an error if allocation fails.
877 ///
878 /// # Examples
879 ///
880 /// ```
881 /// #![feature(allocator_api)]
882 ///
883 /// use std::sync::Arc;
884 /// use std::alloc::System;
885 ///
886 /// let five = Arc::try_new_in(5, System)?;
887 /// # Ok::<(), std::alloc::AllocError>(())
888 /// ```
889 #[unstable(feature = "allocator_api", issue = "32838")]
890 #[inline]
891 pub fn try_new_in(data: T, alloc: A) -> Result<Arc<T, A>, AllocError> {
892 // Start the weak pointer count as 1 which is the weak pointer that's
893 // held by all the strong pointers (kinda), see std/rc.rs for more info
894 let x = Box::try_new_in(
895 ArcInner {
896 strong: atomic::AtomicUsize::new(1),
897 weak: atomic::AtomicUsize::new(1),
898 data,
899 },
900 alloc,
901 )?;
902 let (ptr, alloc) = Box::into_unique(x);
903 Ok(unsafe { Self::from_inner_in(ptr.into(), alloc) })
904 }
905
906 /// Constructs a new `Arc` with uninitialized contents, in the provided allocator, returning an
907 /// error if allocation fails.
908 ///
909 /// # Examples
910 ///
911 /// ```
912 /// #![feature(allocator_api)]
913 /// #![feature(get_mut_unchecked)]
914 ///
915 /// use std::sync::Arc;
916 /// use std::alloc::System;
917 ///
918 /// let mut five = Arc::<u32, _>::try_new_uninit_in(System)?;
919 ///
920 /// let five = unsafe {
921 /// // Deferred initialization:
922 /// Arc::get_mut_unchecked(&mut five).as_mut_ptr().write(5);
923 ///
924 /// five.assume_init()
925 /// };
926 ///
927 /// assert_eq!(*five, 5);
928 /// # Ok::<(), std::alloc::AllocError>(())
929 /// ```
930 #[unstable(feature = "allocator_api", issue = "32838")]
931 #[inline]
932 pub fn try_new_uninit_in(alloc: A) -> Result<Arc<mem::MaybeUninit<T>, A>, AllocError> {
933 unsafe {
934 Ok(Arc::from_ptr_in(
935 Arc::try_allocate_for_layout(
936 Layout::new::<T>(),
937 |layout| alloc.allocate(layout),
938 <*mut u8>::cast,
939 )?,
940 alloc,
941 ))
942 }
943 }
944
945 /// Constructs a new `Arc` with uninitialized contents, with the memory
946 /// being filled with `0` bytes, in the provided allocator, returning an error if allocation
947 /// fails.
948 ///
949 /// See [`MaybeUninit::zeroed`][zeroed] for examples of correct and incorrect usage
950 /// of this method.
951 ///
952 /// # Examples
953 ///
954 /// ```
955 /// #![feature(allocator_api)]
956 ///
957 /// use std::sync::Arc;
958 /// use std::alloc::System;
959 ///
960 /// let zero = Arc::<u32, _>::try_new_zeroed_in(System)?;
961 /// let zero = unsafe { zero.assume_init() };
962 ///
963 /// assert_eq!(*zero, 0);
964 /// # Ok::<(), std::alloc::AllocError>(())
965 /// ```
966 ///
967 /// [zeroed]: mem::MaybeUninit::zeroed
968 #[unstable(feature = "allocator_api", issue = "32838")]
969 #[inline]
970 pub fn try_new_zeroed_in(alloc: A) -> Result<Arc<mem::MaybeUninit<T>, A>, AllocError> {
971 unsafe {
972 Ok(Arc::from_ptr_in(
973 Arc::try_allocate_for_layout(
974 Layout::new::<T>(),
975 |layout| alloc.allocate_zeroed(layout),
976 <*mut u8>::cast,
977 )?,
978 alloc,
979 ))
980 }
981 }
982 /// Returns the inner value, if the `Arc` has exactly one strong reference.
983 ///
984 /// Otherwise, an [`Err`] is returned with the same `Arc` that was
985 /// passed in.
986 ///
987 /// This will succeed even if there are outstanding weak references.
988 ///
989 /// It is strongly recommended to use [`Arc::into_inner`] instead if you don't
990 /// keep the `Arc` in the [`Err`] case.
991 /// Immediately dropping the [`Err`]-value, as the expression
992 /// `Arc::try_unwrap(this).ok()` does, can cause the strong count to
993 /// drop to zero and the inner value of the `Arc` to be dropped.
994 /// For instance, if two threads execute such an expression in parallel,
995 /// there is a race condition without the possibility of unsafety:
996 /// The threads could first both check whether they own the last instance
997 /// in `Arc::try_unwrap`, determine that they both do not, and then both
998 /// discard and drop their instance in the call to [`ok`][`Result::ok`].
999 /// In this scenario, the value inside the `Arc` is safely destroyed
1000 /// by exactly one of the threads, but neither thread will ever be able
1001 /// to use the value.
1002 ///
1003 /// # Examples
1004 ///
1005 /// ```
1006 /// use std::sync::Arc;
1007 ///
1008 /// let x = Arc::new(3);
1009 /// assert_eq!(Arc::try_unwrap(x), Ok(3));
1010 ///
1011 /// let x = Arc::new(4);
1012 /// let _y = Arc::clone(&x);
1013 /// assert_eq!(*Arc::try_unwrap(x).unwrap_err(), 4);
1014 /// ```
1015 #[inline]
1016 #[stable(feature = "arc_unique", since = "1.4.0")]
1017 pub fn try_unwrap(this: Self) -> Result<T, Self> {
1018 if this.inner().strong.compare_exchange(1, 0, Relaxed, Relaxed).is_err() {
1019 return Err(this);
1020 }
1021
1022 acquire!(this.inner().strong);
1023
1024 let this = ManuallyDrop::new(this);
1025 let elem: T = unsafe { ptr::read(&this.ptr.as_ref().data) };
1026 let alloc: A = unsafe { ptr::read(&this.alloc) }; // copy the allocator
1027
1028 // Make a weak pointer to clean up the implicit strong-weak reference
1029 let _weak = Weak { ptr: this.ptr, alloc };
1030
1031 Ok(elem)
1032 }
1033
1034 /// Returns the inner value, if the `Arc` has exactly one strong reference.
1035 ///
1036 /// Otherwise, [`None`] is returned and the `Arc` is dropped.
1037 ///
1038 /// This will succeed even if there are outstanding weak references.
1039 ///
1040 /// If `Arc::into_inner` is called on every clone of this `Arc`,
1041 /// it is guaranteed that exactly one of the calls returns the inner value.
1042 /// This means in particular that the inner value is not dropped.
1043 ///
1044 /// [`Arc::try_unwrap`] is conceptually similar to `Arc::into_inner`, but it
1045 /// is meant for different use-cases. If used as a direct replacement
1046 /// for `Arc::into_inner` anyway, such as with the expression
1047 /// <code>[Arc::try_unwrap]\(this).[ok][Result::ok]()</code>, then it does
1048 /// **not** give the same guarantee as described in the previous paragraph.
1049 /// For more information, see the examples below and read the documentation
1050 /// of [`Arc::try_unwrap`].
1051 ///
1052 /// # Examples
1053 ///
1054 /// Minimal example demonstrating the guarantee that `Arc::into_inner` gives.
1055 /// ```
1056 /// use std::sync::Arc;
1057 ///
1058 /// let x = Arc::new(3);
1059 /// let y = Arc::clone(&x);
1060 ///
1061 /// // Two threads calling `Arc::into_inner` on both clones of an `Arc`:
1062 /// let x_thread = std::thread::spawn(|| Arc::into_inner(x));
1063 /// let y_thread = std::thread::spawn(|| Arc::into_inner(y));
1064 ///
1065 /// let x_inner_value = x_thread.join().unwrap();
1066 /// let y_inner_value = y_thread.join().unwrap();
1067 ///
1068 /// // One of the threads is guaranteed to receive the inner value:
1069 /// assert!(matches!(
1070 /// (x_inner_value, y_inner_value),
1071 /// (None, Some(3)) | (Some(3), None)
1072 /// ));
1073 /// // The result could also be `(None, None)` if the threads called
1074 /// // `Arc::try_unwrap(x).ok()` and `Arc::try_unwrap(y).ok()` instead.
1075 /// ```
1076 ///
1077 /// A more practical example demonstrating the need for `Arc::into_inner`:
1078 /// ```
1079 /// use std::sync::Arc;
1080 ///
1081 /// // Definition of a simple singly linked list using `Arc`:
1082 /// #[derive(Clone)]
1083 /// struct LinkedList<T>(Option<Arc<Node<T>>>);
1084 /// struct Node<T>(T, Option<Arc<Node<T>>>);
1085 ///
1086 /// // Dropping a long `LinkedList<T>` relying on the destructor of `Arc`
1087 /// // can cause a stack overflow. To prevent this, we can provide a
1088 /// // manual `Drop` implementation that does the destruction in a loop:
1089 /// impl<T> Drop for LinkedList<T> {
1090 /// fn drop(&mut self) {
1091 /// let mut link = self.0.take();
1092 /// while let Some(arc_node) = link.take() {
1093 /// if let Some(Node(_value, next)) = Arc::into_inner(arc_node) {
1094 /// link = next;
1095 /// }
1096 /// }
1097 /// }
1098 /// }
1099 ///
1100 /// // Implementation of `new` and `push` omitted
1101 /// impl<T> LinkedList<T> {
1102 /// /* ... */
1103 /// # fn new() -> Self {
1104 /// # LinkedList(None)
1105 /// # }
1106 /// # fn push(&mut self, x: T) {
1107 /// # self.0 = Some(Arc::new(Node(x, self.0.take())));
1108 /// # }
1109 /// }
1110 ///
1111 /// // The following code could have still caused a stack overflow
1112 /// // despite the manual `Drop` impl if that `Drop` impl had used
1113 /// // `Arc::try_unwrap(arc).ok()` instead of `Arc::into_inner(arc)`.
1114 ///
1115 /// // Create a long list and clone it
1116 /// let mut x = LinkedList::new();
1117 /// let size = 100000;
1118 /// # let size = if cfg!(miri) { 100 } else { size };
1119 /// for i in 0..size {
1120 /// x.push(i); // Adds i to the front of x
1121 /// }
1122 /// let y = x.clone();
1123 ///
1124 /// // Drop the clones in parallel
1125 /// let x_thread = std::thread::spawn(|| drop(x));
1126 /// let y_thread = std::thread::spawn(|| drop(y));
1127 /// x_thread.join().unwrap();
1128 /// y_thread.join().unwrap();
1129 /// ```
1130 #[inline]
1131 #[stable(feature = "arc_into_inner", since = "1.70.0")]
1132 pub fn into_inner(this: Self) -> Option<T> {
1133 // Make sure that the ordinary `Drop` implementation isn’t called as well
1134 let mut this = mem::ManuallyDrop::new(this);
1135
1136 // Following the implementation of `drop` and `drop_slow`
1137 if this.inner().strong.fetch_sub(1, Release) != 1 {
1138 return None;
1139 }
1140
1141 acquire!(this.inner().strong);
1142
1143 // SAFETY: This mirrors the line
1144 //
1145 // unsafe { ptr::drop_in_place(Self::get_mut_unchecked(self)) };
1146 //
1147 // in `drop_slow`. Instead of dropping the value behind the pointer,
1148 // it is read and eventually returned; `ptr::read` has the same
1149 // safety conditions as `ptr::drop_in_place`.
1150
1151 let inner = unsafe { ptr::read(Self::get_mut_unchecked(&mut this)) };
1152 let alloc = unsafe { ptr::read(&this.alloc) };
1153
1154 drop(Weak { ptr: this.ptr, alloc });
1155
1156 Some(inner)
1157 }
1158}
1159
1160impl<T> Arc<[T]> {
1161 /// Constructs a new atomically reference-counted slice with uninitialized contents.
1162 ///
1163 /// # Examples
1164 ///
1165 /// ```
1166 /// use std::sync::Arc;
1167 ///
1168 /// let mut values = Arc::<[u32]>::new_uninit_slice(3);
1169 ///
1170 /// // Deferred initialization:
1171 /// let data = Arc::get_mut(&mut values).unwrap();
1172 /// data[0].write(1);
1173 /// data[1].write(2);
1174 /// data[2].write(3);
1175 ///
1176 /// let values = unsafe { values.assume_init() };
1177 ///
1178 /// assert_eq!(*values, [1, 2, 3])
1179 /// ```
1180 #[cfg(not(no_global_oom_handling))]
1181 #[inline]
1182 #[stable(feature = "new_uninit", since = "1.82.0")]
1183 #[must_use]
1184 pub fn new_uninit_slice(len: usize) -> Arc<[mem::MaybeUninit<T>]> {
1185 unsafe { Arc::from_ptr(Arc::allocate_for_slice(len)) }
1186 }
1187
1188 /// Constructs a new atomically reference-counted slice with uninitialized contents, with the memory being
1189 /// filled with `0` bytes.
1190 ///
1191 /// See [`MaybeUninit::zeroed`][zeroed] for examples of correct and
1192 /// incorrect usage of this method.
1193 ///
1194 /// # Examples
1195 ///
1196 /// ```
1197 /// use std::sync::Arc;
1198 ///
1199 /// let values = Arc::<[u32]>::new_zeroed_slice(3);
1200 /// let values = unsafe { values.assume_init() };
1201 ///
1202 /// assert_eq!(*values, [0, 0, 0])
1203 /// ```
1204 ///
1205 /// [zeroed]: mem::MaybeUninit::zeroed
1206 #[cfg(not(no_global_oom_handling))]
1207 #[inline]
1208 #[stable(feature = "new_zeroed_alloc", since = "CURRENT_RUSTC_VERSION")]
1209 #[must_use]
1210 pub fn new_zeroed_slice(len: usize) -> Arc<[mem::MaybeUninit<T>]> {
1211 unsafe {
1212 Arc::from_ptr(Arc::allocate_for_layout(
1213 Layout::array::<T>(len).unwrap(),
1214 |layout| Global.allocate_zeroed(layout),
1215 |mem| {
1216 ptr::slice_from_raw_parts_mut(mem as *mut T, len)
1217 as *mut ArcInner<[mem::MaybeUninit<T>]>
1218 },
1219 ))
1220 }
1221 }
1222
1223 /// Converts the reference-counted slice into a reference-counted array.
1224 ///
1225 /// This operation does not reallocate; the underlying array of the slice is simply reinterpreted as an array type.
1226 ///
1227 /// If `N` is not exactly equal to the length of `self`, then this method returns `None`.
1228 #[unstable(feature = "slice_as_array", issue = "133508")]
1229 #[inline]
1230 #[must_use]
1231 pub fn into_array<const N: usize>(self) -> Option<Arc<[T; N]>> {
1232 if self.len() == N {
1233 let ptr = Self::into_raw(self) as *const [T; N];
1234
1235 // SAFETY: The underlying array of a slice has the exact same layout as an actual array `[T; N]` if `N` is equal to the slice's length.
1236 let me = unsafe { Arc::from_raw(ptr) };
1237 Some(me)
1238 } else {
1239 None
1240 }
1241 }
1242}
1243
1244impl<T, A: Allocator> Arc<[T], A> {
1245 /// Constructs a new atomically reference-counted slice with uninitialized contents in the
1246 /// provided allocator.
1247 ///
1248 /// # Examples
1249 ///
1250 /// ```
1251 /// #![feature(get_mut_unchecked)]
1252 /// #![feature(allocator_api)]
1253 ///
1254 /// use std::sync::Arc;
1255 /// use std::alloc::System;
1256 ///
1257 /// let mut values = Arc::<[u32], _>::new_uninit_slice_in(3, System);
1258 ///
1259 /// let values = unsafe {
1260 /// // Deferred initialization:
1261 /// Arc::get_mut_unchecked(&mut values)[0].as_mut_ptr().write(1);
1262 /// Arc::get_mut_unchecked(&mut values)[1].as_mut_ptr().write(2);
1263 /// Arc::get_mut_unchecked(&mut values)[2].as_mut_ptr().write(3);
1264 ///
1265 /// values.assume_init()
1266 /// };
1267 ///
1268 /// assert_eq!(*values, [1, 2, 3])
1269 /// ```
1270 #[cfg(not(no_global_oom_handling))]
1271 #[unstable(feature = "allocator_api", issue = "32838")]
1272 #[inline]
1273 pub fn new_uninit_slice_in(len: usize, alloc: A) -> Arc<[mem::MaybeUninit<T>], A> {
1274 unsafe { Arc::from_ptr_in(Arc::allocate_for_slice_in(len, &alloc), alloc) }
1275 }
1276
1277 /// Constructs a new atomically reference-counted slice with uninitialized contents, with the memory being
1278 /// filled with `0` bytes, in the provided allocator.
1279 ///
1280 /// See [`MaybeUninit::zeroed`][zeroed] for examples of correct and
1281 /// incorrect usage of this method.
1282 ///
1283 /// # Examples
1284 ///
1285 /// ```
1286 /// #![feature(allocator_api)]
1287 ///
1288 /// use std::sync::Arc;
1289 /// use std::alloc::System;
1290 ///
1291 /// let values = Arc::<[u32], _>::new_zeroed_slice_in(3, System);
1292 /// let values = unsafe { values.assume_init() };
1293 ///
1294 /// assert_eq!(*values, [0, 0, 0])
1295 /// ```
1296 ///
1297 /// [zeroed]: mem::MaybeUninit::zeroed
1298 #[cfg(not(no_global_oom_handling))]
1299 #[unstable(feature = "allocator_api", issue = "32838")]
1300 #[inline]
1301 pub fn new_zeroed_slice_in(len: usize, alloc: A) -> Arc<[mem::MaybeUninit<T>], A> {
1302 unsafe {
1303 Arc::from_ptr_in(
1304 Arc::allocate_for_layout(
1305 Layout::array::<T>(len).unwrap(),
1306 |layout| alloc.allocate_zeroed(layout),
1307 |mem| {
1308 ptr::slice_from_raw_parts_mut(mem.cast::<T>(), len)
1309 as *mut ArcInner<[mem::MaybeUninit<T>]>
1310 },
1311 ),
1312 alloc,
1313 )
1314 }
1315 }
1316}
1317
1318impl<T, A: Allocator> Arc<mem::MaybeUninit<T>, A> {
1319 /// Converts to `Arc<T>`.
1320 ///
1321 /// # Safety
1322 ///
1323 /// As with [`MaybeUninit::assume_init`],
1324 /// it is up to the caller to guarantee that the inner value
1325 /// really is in an initialized state.
1326 /// Calling this when the content is not yet fully initialized
1327 /// causes immediate undefined behavior.
1328 ///
1329 /// [`MaybeUninit::assume_init`]: mem::MaybeUninit::assume_init
1330 ///
1331 /// # Examples
1332 ///
1333 /// ```
1334 /// use std::sync::Arc;
1335 ///
1336 /// let mut five = Arc::<u32>::new_uninit();
1337 ///
1338 /// // Deferred initialization:
1339 /// Arc::get_mut(&mut five).unwrap().write(5);
1340 ///
1341 /// let five = unsafe { five.assume_init() };
1342 ///
1343 /// assert_eq!(*five, 5)
1344 /// ```
1345 #[stable(feature = "new_uninit", since = "1.82.0")]
1346 #[must_use = "`self` will be dropped if the result is not used"]
1347 #[inline]
1348 pub unsafe fn assume_init(self) -> Arc<T, A> {
1349 let (ptr, alloc) = Arc::into_inner_with_allocator(self);
1350 unsafe { Arc::from_inner_in(ptr.cast(), alloc) }
1351 }
1352}
1353
1354impl<T, A: Allocator> Arc<[mem::MaybeUninit<T>], A> {
1355 /// Converts to `Arc<[T]>`.
1356 ///
1357 /// # Safety
1358 ///
1359 /// As with [`MaybeUninit::assume_init`],
1360 /// it is up to the caller to guarantee that the inner value
1361 /// really is in an initialized state.
1362 /// Calling this when the content is not yet fully initialized
1363 /// causes immediate undefined behavior.
1364 ///
1365 /// [`MaybeUninit::assume_init`]: mem::MaybeUninit::assume_init
1366 ///
1367 /// # Examples
1368 ///
1369 /// ```
1370 /// use std::sync::Arc;
1371 ///
1372 /// let mut values = Arc::<[u32]>::new_uninit_slice(3);
1373 ///
1374 /// // Deferred initialization:
1375 /// let data = Arc::get_mut(&mut values).unwrap();
1376 /// data[0].write(1);
1377 /// data[1].write(2);
1378 /// data[2].write(3);
1379 ///
1380 /// let values = unsafe { values.assume_init() };
1381 ///
1382 /// assert_eq!(*values, [1, 2, 3])
1383 /// ```
1384 #[stable(feature = "new_uninit", since = "1.82.0")]
1385 #[must_use = "`self` will be dropped if the result is not used"]
1386 #[inline]
1387 pub unsafe fn assume_init(self) -> Arc<[T], A> {
1388 let (ptr, alloc) = Arc::into_inner_with_allocator(self);
1389 unsafe { Arc::from_ptr_in(ptr.as_ptr() as _, alloc) }
1390 }
1391}
1392
1393impl<T: ?Sized> Arc<T> {
1394 /// Constructs an `Arc<T>` from a raw pointer.
1395 ///
1396 /// The raw pointer must have been previously returned by a call to
1397 /// [`Arc<U>::into_raw`][into_raw] with the following requirements:
1398 ///
1399 /// * If `U` is sized, it must have the same size and alignment as `T`. This
1400 /// is trivially true if `U` is `T`.
1401 /// * If `U` is unsized, its data pointer must have the same size and
1402 /// alignment as `T`. This is trivially true if `Arc<U>` was constructed
1403 /// through `Arc<T>` and then converted to `Arc<U>` through an [unsized
1404 /// coercion].
1405 ///
1406 /// Note that if `U` or `U`'s data pointer is not `T` but has the same size
1407 /// and alignment, this is basically like transmuting references of
1408 /// different types. See [`mem::transmute`][transmute] for more information
1409 /// on what restrictions apply in this case.
1410 ///
1411 /// The raw pointer must point to a block of memory allocated by the global allocator.
1412 ///
1413 /// The user of `from_raw` has to make sure a specific value of `T` is only
1414 /// dropped once.
1415 ///
1416 /// This function is unsafe because improper use may lead to memory unsafety,
1417 /// even if the returned `Arc<T>` is never accessed.
1418 ///
1419 /// [into_raw]: Arc::into_raw
1420 /// [transmute]: core::mem::transmute
1421 /// [unsized coercion]: https://doc.rust-lang.org/reference/type-coercions.html#unsized-coercions
1422 ///
1423 /// # Examples
1424 ///
1425 /// ```
1426 /// use std::sync::Arc;
1427 ///
1428 /// let x = Arc::new("hello".to_owned());
1429 /// let x_ptr = Arc::into_raw(x);
1430 ///
1431 /// unsafe {
1432 /// // Convert back to an `Arc` to prevent leak.
1433 /// let x = Arc::from_raw(x_ptr);
1434 /// assert_eq!(&*x, "hello");
1435 ///
1436 /// // Further calls to `Arc::from_raw(x_ptr)` would be memory-unsafe.
1437 /// }
1438 ///
1439 /// // The memory was freed when `x` went out of scope above, so `x_ptr` is now dangling!
1440 /// ```
1441 ///
1442 /// Convert a slice back into its original array:
1443 ///
1444 /// ```
1445 /// use std::sync::Arc;
1446 ///
1447 /// let x: Arc<[u32]> = Arc::new([1, 2, 3]);
1448 /// let x_ptr: *const [u32] = Arc::into_raw(x);
1449 ///
1450 /// unsafe {
1451 /// let x: Arc<[u32; 3]> = Arc::from_raw(x_ptr.cast::<[u32; 3]>());
1452 /// assert_eq!(&*x, &[1, 2, 3]);
1453 /// }
1454 /// ```
1455 #[inline]
1456 #[stable(feature = "rc_raw", since = "1.17.0")]
1457 pub unsafe fn from_raw(ptr: *const T) -> Self {
1458 unsafe { Arc::from_raw_in(ptr, Global) }
1459 }
1460
1461 /// Consumes the `Arc`, returning the wrapped pointer.
1462 ///
1463 /// To avoid a memory leak the pointer must be converted back to an `Arc` using
1464 /// [`Arc::from_raw`].
1465 ///
1466 /// # Examples
1467 ///
1468 /// ```
1469 /// use std::sync::Arc;
1470 ///
1471 /// let x = Arc::new("hello".to_owned());
1472 /// let x_ptr = Arc::into_raw(x);
1473 /// assert_eq!(unsafe { &*x_ptr }, "hello");
1474 /// # // Prevent leaks for Miri.
1475 /// # drop(unsafe { Arc::from_raw(x_ptr) });
1476 /// ```
1477 #[must_use = "losing the pointer will leak memory"]
1478 #[stable(feature = "rc_raw", since = "1.17.0")]
1479 #[rustc_never_returns_null_ptr]
1480 pub fn into_raw(this: Self) -> *const T {
1481 let this = ManuallyDrop::new(this);
1482 Self::as_ptr(&*this)
1483 }
1484
1485 /// Increments the strong reference count on the `Arc<T>` associated with the
1486 /// provided pointer by one.
1487 ///
1488 /// # Safety
1489 ///
1490 /// The pointer must have been obtained through `Arc::into_raw` and must satisfy the
1491 /// same layout requirements specified in [`Arc::from_raw_in`][from_raw_in].
1492 /// The associated `Arc` instance must be valid (i.e. the strong count must be at
1493 /// least 1) for the duration of this method, and `ptr` must point to a block of memory
1494 /// allocated by the global allocator.
1495 ///
1496 /// [from_raw_in]: Arc::from_raw_in
1497 ///
1498 /// # Examples
1499 ///
1500 /// ```
1501 /// use std::sync::Arc;
1502 ///
1503 /// let five = Arc::new(5);
1504 ///
1505 /// unsafe {
1506 /// let ptr = Arc::into_raw(five);
1507 /// Arc::increment_strong_count(ptr);
1508 ///
1509 /// // This assertion is deterministic because we haven't shared
1510 /// // the `Arc` between threads.
1511 /// let five = Arc::from_raw(ptr);
1512 /// assert_eq!(2, Arc::strong_count(&five));
1513 /// # // Prevent leaks for Miri.
1514 /// # Arc::decrement_strong_count(ptr);
1515 /// }
1516 /// ```
1517 #[inline]
1518 #[stable(feature = "arc_mutate_strong_count", since = "1.51.0")]
1519 pub unsafe fn increment_strong_count(ptr: *const T) {
1520 unsafe { Arc::increment_strong_count_in(ptr, Global) }
1521 }
1522
1523 /// Decrements the strong reference count on the `Arc<T>` associated with the
1524 /// provided pointer by one.
1525 ///
1526 /// # Safety
1527 ///
1528 /// The pointer must have been obtained through `Arc::into_raw` and must satisfy the
1529 /// same layout requirements specified in [`Arc::from_raw_in`][from_raw_in].
1530 /// The associated `Arc` instance must be valid (i.e. the strong count must be at
1531 /// least 1) when invoking this method, and `ptr` must point to a block of memory
1532 /// allocated by the global allocator. This method can be used to release the final
1533 /// `Arc` and backing storage, but **should not** be called after the final `Arc` has been
1534 /// released.
1535 ///
1536 /// [from_raw_in]: Arc::from_raw_in
1537 ///
1538 /// # Examples
1539 ///
1540 /// ```
1541 /// use std::sync::Arc;
1542 ///
1543 /// let five = Arc::new(5);
1544 ///
1545 /// unsafe {
1546 /// let ptr = Arc::into_raw(five);
1547 /// Arc::increment_strong_count(ptr);
1548 ///
1549 /// // Those assertions are deterministic because we haven't shared
1550 /// // the `Arc` between threads.
1551 /// let five = Arc::from_raw(ptr);
1552 /// assert_eq!(2, Arc::strong_count(&five));
1553 /// Arc::decrement_strong_count(ptr);
1554 /// assert_eq!(1, Arc::strong_count(&five));
1555 /// }
1556 /// ```
1557 #[inline]
1558 #[stable(feature = "arc_mutate_strong_count", since = "1.51.0")]
1559 pub unsafe fn decrement_strong_count(ptr: *const T) {
1560 unsafe { Arc::decrement_strong_count_in(ptr, Global) }
1561 }
1562}
1563
1564impl<T: ?Sized, A: Allocator> Arc<T, A> {
1565 /// Returns a reference to the underlying allocator.
1566 ///
1567 /// Note: this is an associated function, which means that you have
1568 /// to call it as `Arc::allocator(&a)` instead of `a.allocator()`. This
1569 /// is so that there is no conflict with a method on the inner type.
1570 #[inline]
1571 #[unstable(feature = "allocator_api", issue = "32838")]
1572 pub fn allocator(this: &Self) -> &A {
1573 &this.alloc
1574 }
1575
1576 /// Consumes the `Arc`, returning the wrapped pointer and allocator.
1577 ///
1578 /// To avoid a memory leak the pointer must be converted back to an `Arc` using
1579 /// [`Arc::from_raw_in`].
1580 ///
1581 /// # Examples
1582 ///
1583 /// ```
1584 /// #![feature(allocator_api)]
1585 /// use std::sync::Arc;
1586 /// use std::alloc::System;
1587 ///
1588 /// let x = Arc::new_in("hello".to_owned(), System);
1589 /// let (ptr, alloc) = Arc::into_raw_with_allocator(x);
1590 /// assert_eq!(unsafe { &*ptr }, "hello");
1591 /// let x = unsafe { Arc::from_raw_in(ptr, alloc) };
1592 /// assert_eq!(&*x, "hello");
1593 /// ```
1594 #[must_use = "losing the pointer will leak memory"]
1595 #[unstable(feature = "allocator_api", issue = "32838")]
1596 pub fn into_raw_with_allocator(this: Self) -> (*const T, A) {
1597 let this = mem::ManuallyDrop::new(this);
1598 let ptr = Self::as_ptr(&this);
1599 // Safety: `this` is ManuallyDrop so the allocator will not be double-dropped
1600 let alloc = unsafe { ptr::read(&this.alloc) };
1601 (ptr, alloc)
1602 }
1603
1604 /// Provides a raw pointer to the data.
1605 ///
1606 /// The counts are not affected in any way and the `Arc` is not consumed. The pointer is valid for
1607 /// as long as there are strong counts in the `Arc`.
1608 ///
1609 /// # Examples
1610 ///
1611 /// ```
1612 /// use std::sync::Arc;
1613 ///
1614 /// let x = Arc::new("hello".to_owned());
1615 /// let y = Arc::clone(&x);
1616 /// let x_ptr = Arc::as_ptr(&x);
1617 /// assert_eq!(x_ptr, Arc::as_ptr(&y));
1618 /// assert_eq!(unsafe { &*x_ptr }, "hello");
1619 /// ```
1620 #[must_use]
1621 #[stable(feature = "rc_as_ptr", since = "1.45.0")]
1622 #[rustc_never_returns_null_ptr]
1623 pub fn as_ptr(this: &Self) -> *const T {
1624 let ptr: *mut ArcInner<T> = NonNull::as_ptr(this.ptr);
1625
1626 // SAFETY: This cannot go through Deref::deref or ArcInnerPtr::inner because
1627 // this is required to retain raw/mut provenance such that e.g. `get_mut` can
1628 // write through the pointer after the Arc is recovered through `from_raw`.
1629 unsafe { &raw mut (*ptr).data }
1630 }
1631
1632 /// Constructs an `Arc<T, A>` from a raw pointer.
1633 ///
1634 /// The raw pointer must have been previously returned by a call to [`Arc<U,
1635 /// A>::into_raw`][into_raw] with the following requirements:
1636 ///
1637 /// * If `U` is sized, it must have the same size and alignment as `T`. This
1638 /// is trivially true if `U` is `T`.
1639 /// * If `U` is unsized, its data pointer must have the same size and
1640 /// alignment as `T`. This is trivially true if `Arc<U>` was constructed
1641 /// through `Arc<T>` and then converted to `Arc<U>` through an [unsized
1642 /// coercion].
1643 ///
1644 /// Note that if `U` or `U`'s data pointer is not `T` but has the same size
1645 /// and alignment, this is basically like transmuting references of
1646 /// different types. See [`mem::transmute`][transmute] for more information
1647 /// on what restrictions apply in this case.
1648 ///
1649 /// The raw pointer must point to a block of memory allocated by `alloc`
1650 ///
1651 /// The user of `from_raw` has to make sure a specific value of `T` is only
1652 /// dropped once.
1653 ///
1654 /// This function is unsafe because improper use may lead to memory unsafety,
1655 /// even if the returned `Arc<T>` is never accessed.
1656 ///
1657 /// [into_raw]: Arc::into_raw
1658 /// [transmute]: core::mem::transmute
1659 /// [unsized coercion]: https://doc.rust-lang.org/reference/type-coercions.html#unsized-coercions
1660 ///
1661 /// # Examples
1662 ///
1663 /// ```
1664 /// #![feature(allocator_api)]
1665 ///
1666 /// use std::sync::Arc;
1667 /// use std::alloc::System;
1668 ///
1669 /// let x = Arc::new_in("hello".to_owned(), System);
1670 /// let (x_ptr, alloc) = Arc::into_raw_with_allocator(x);
1671 ///
1672 /// unsafe {
1673 /// // Convert back to an `Arc` to prevent leak.
1674 /// let x = Arc::from_raw_in(x_ptr, System);
1675 /// assert_eq!(&*x, "hello");
1676 ///
1677 /// // Further calls to `Arc::from_raw(x_ptr)` would be memory-unsafe.
1678 /// }
1679 ///
1680 /// // The memory was freed when `x` went out of scope above, so `x_ptr` is now dangling!
1681 /// ```
1682 ///
1683 /// Convert a slice back into its original array:
1684 ///
1685 /// ```
1686 /// #![feature(allocator_api)]
1687 ///
1688 /// use std::sync::Arc;
1689 /// use std::alloc::System;
1690 ///
1691 /// let x: Arc<[u32], _> = Arc::new_in([1, 2, 3], System);
1692 /// let x_ptr: *const [u32] = Arc::into_raw_with_allocator(x).0;
1693 ///
1694 /// unsafe {
1695 /// let x: Arc<[u32; 3], _> = Arc::from_raw_in(x_ptr.cast::<[u32; 3]>(), System);
1696 /// assert_eq!(&*x, &[1, 2, 3]);
1697 /// }
1698 /// ```
1699 #[inline]
1700 #[unstable(feature = "allocator_api", issue = "32838")]
1701 pub unsafe fn from_raw_in(ptr: *const T, alloc: A) -> Self {
1702 unsafe {
1703 let offset = data_offset(ptr);
1704
1705 // Reverse the offset to find the original ArcInner.
1706 let arc_ptr = ptr.byte_sub(offset) as *mut ArcInner<T>;
1707
1708 Self::from_ptr_in(arc_ptr, alloc)
1709 }
1710 }
1711
1712 /// Creates a new [`Weak`] pointer to this allocation.
1713 ///
1714 /// # Examples
1715 ///
1716 /// ```
1717 /// use std::sync::Arc;
1718 ///
1719 /// let five = Arc::new(5);
1720 ///
1721 /// let weak_five = Arc::downgrade(&five);
1722 /// ```
1723 #[must_use = "this returns a new `Weak` pointer, \
1724 without modifying the original `Arc`"]
1725 #[stable(feature = "arc_weak", since = "1.4.0")]
1726 pub fn downgrade(this: &Self) -> Weak<T, A>
1727 where
1728 A: Clone,
1729 {
1730 // This Relaxed is OK because we're checking the value in the CAS
1731 // below.
1732 let mut cur = this.inner().weak.load(Relaxed);
1733
1734 loop {
1735 // check if the weak counter is currently "locked"; if so, spin.
1736 if cur == usize::MAX {
1737 hint::spin_loop();
1738 cur = this.inner().weak.load(Relaxed);
1739 continue;
1740 }
1741
1742 // We can't allow the refcount to increase much past `MAX_REFCOUNT`.
1743 assert!(cur <= MAX_REFCOUNT, "{}", INTERNAL_OVERFLOW_ERROR);
1744
1745 // NOTE: this code currently ignores the possibility of overflow
1746 // into usize::MAX; in general both Rc and Arc need to be adjusted
1747 // to deal with overflow.
1748
1749 // Unlike with Clone(), we need this to be an Acquire read to
1750 // synchronize with the write coming from `is_unique`, so that the
1751 // events prior to that write happen before this read.
1752 match this.inner().weak.compare_exchange_weak(cur, cur + 1, Acquire, Relaxed) {
1753 Ok(_) => {
1754 // Make sure we do not create a dangling Weak
1755 debug_assert!(!is_dangling(this.ptr.as_ptr()));
1756 return Weak { ptr: this.ptr, alloc: this.alloc.clone() };
1757 }
1758 Err(old) => cur = old,
1759 }
1760 }
1761 }
1762
1763 /// Gets the number of [`Weak`] pointers to this allocation.
1764 ///
1765 /// # Safety
1766 ///
1767 /// This method by itself is safe, but using it correctly requires extra care.
1768 /// Another thread can change the weak count at any time,
1769 /// including potentially between calling this method and acting on the result.
1770 ///
1771 /// # Examples
1772 ///
1773 /// ```
1774 /// use std::sync::Arc;
1775 ///
1776 /// let five = Arc::new(5);
1777 /// let _weak_five = Arc::downgrade(&five);
1778 ///
1779 /// // This assertion is deterministic because we haven't shared
1780 /// // the `Arc` or `Weak` between threads.
1781 /// assert_eq!(1, Arc::weak_count(&five));
1782 /// ```
1783 #[inline]
1784 #[must_use]
1785 #[stable(feature = "arc_counts", since = "1.15.0")]
1786 pub fn weak_count(this: &Self) -> usize {
1787 let cnt = this.inner().weak.load(Relaxed);
1788 // If the weak count is currently locked, the value of the
1789 // count was 0 just before taking the lock.
1790 if cnt == usize::MAX { 0 } else { cnt - 1 }
1791 }
1792
1793 /// Gets the number of strong (`Arc`) pointers to this allocation.
1794 ///
1795 /// # Safety
1796 ///
1797 /// This method by itself is safe, but using it correctly requires extra care.
1798 /// Another thread can change the strong count at any time,
1799 /// including potentially between calling this method and acting on the result.
1800 ///
1801 /// # Examples
1802 ///
1803 /// ```
1804 /// use std::sync::Arc;
1805 ///
1806 /// let five = Arc::new(5);
1807 /// let _also_five = Arc::clone(&five);
1808 ///
1809 /// // This assertion is deterministic because we haven't shared
1810 /// // the `Arc` between threads.
1811 /// assert_eq!(2, Arc::strong_count(&five));
1812 /// ```
1813 #[inline]
1814 #[must_use]
1815 #[stable(feature = "arc_counts", since = "1.15.0")]
1816 pub fn strong_count(this: &Self) -> usize {
1817 this.inner().strong.load(Relaxed)
1818 }
1819
1820 /// Increments the strong reference count on the `Arc<T>` associated with the
1821 /// provided pointer by one.
1822 ///
1823 /// # Safety
1824 ///
1825 /// The pointer must have been obtained through `Arc::into_raw` and must satisfy the
1826 /// same layout requirements specified in [`Arc::from_raw_in`][from_raw_in].
1827 /// The associated `Arc` instance must be valid (i.e. the strong count must be at
1828 /// least 1) for the duration of this method, and `ptr` must point to a block of memory
1829 /// allocated by `alloc`.
1830 ///
1831 /// [from_raw_in]: Arc::from_raw_in
1832 ///
1833 /// # Examples
1834 ///
1835 /// ```
1836 /// #![feature(allocator_api)]
1837 ///
1838 /// use std::sync::Arc;
1839 /// use std::alloc::System;
1840 ///
1841 /// let five = Arc::new_in(5, System);
1842 ///
1843 /// unsafe {
1844 /// let (ptr, _alloc) = Arc::into_raw_with_allocator(five);
1845 /// Arc::increment_strong_count_in(ptr, System);
1846 ///
1847 /// // This assertion is deterministic because we haven't shared
1848 /// // the `Arc` between threads.
1849 /// let five = Arc::from_raw_in(ptr, System);
1850 /// assert_eq!(2, Arc::strong_count(&five));
1851 /// # // Prevent leaks for Miri.
1852 /// # Arc::decrement_strong_count_in(ptr, System);
1853 /// }
1854 /// ```
1855 #[inline]
1856 #[unstable(feature = "allocator_api", issue = "32838")]
1857 pub unsafe fn increment_strong_count_in(ptr: *const T, alloc: A)
1858 where
1859 A: Clone,
1860 {
1861 // Retain Arc, but don't touch refcount by wrapping in ManuallyDrop
1862 let arc = unsafe { mem::ManuallyDrop::new(Arc::from_raw_in(ptr, alloc)) };
1863 // Now increase refcount, but don't drop new refcount either
1864 let _arc_clone: mem::ManuallyDrop<_> = arc.clone();
1865 }
1866
1867 /// Decrements the strong reference count on the `Arc<T>` associated with the
1868 /// provided pointer by one.
1869 ///
1870 /// # Safety
1871 ///
1872 /// The pointer must have been obtained through `Arc::into_raw` and must satisfy the
1873 /// same layout requirements specified in [`Arc::from_raw_in`][from_raw_in].
1874 /// The associated `Arc` instance must be valid (i.e. the strong count must be at
1875 /// least 1) when invoking this method, and `ptr` must point to a block of memory
1876 /// allocated by `alloc`. This method can be used to release the final
1877 /// `Arc` and backing storage, but **should not** be called after the final `Arc` has been
1878 /// released.
1879 ///
1880 /// [from_raw_in]: Arc::from_raw_in
1881 ///
1882 /// # Examples
1883 ///
1884 /// ```
1885 /// #![feature(allocator_api)]
1886 ///
1887 /// use std::sync::Arc;
1888 /// use std::alloc::System;
1889 ///
1890 /// let five = Arc::new_in(5, System);
1891 ///
1892 /// unsafe {
1893 /// let (ptr, _alloc) = Arc::into_raw_with_allocator(five);
1894 /// Arc::increment_strong_count_in(ptr, System);
1895 ///
1896 /// // Those assertions are deterministic because we haven't shared
1897 /// // the `Arc` between threads.
1898 /// let five = Arc::from_raw_in(ptr, System);
1899 /// assert_eq!(2, Arc::strong_count(&five));
1900 /// Arc::decrement_strong_count_in(ptr, System);
1901 /// assert_eq!(1, Arc::strong_count(&five));
1902 /// }
1903 /// ```
1904 #[inline]
1905 #[unstable(feature = "allocator_api", issue = "32838")]
1906 pub unsafe fn decrement_strong_count_in(ptr: *const T, alloc: A) {
1907 unsafe { drop(Arc::from_raw_in(ptr, alloc)) };
1908 }
1909
1910 #[inline]
1911 fn inner(&self) -> &ArcInner<T> {
1912 // This unsafety is ok because while this arc is alive we're guaranteed
1913 // that the inner pointer is valid. Furthermore, we know that the
1914 // `ArcInner` structure itself is `Sync` because the inner data is
1915 // `Sync` as well, so we're ok loaning out an immutable pointer to these
1916 // contents.
1917 unsafe { self.ptr.as_ref() }
1918 }
1919
1920 // Non-inlined part of `drop`.
1921 #[inline(never)]
1922 unsafe fn drop_slow(&mut self) {
1923 // Drop the weak ref collectively held by all strong references when this
1924 // variable goes out of scope. This ensures that the memory is deallocated
1925 // even if the destructor of `T` panics.
1926 // Take a reference to `self.alloc` instead of cloning because 1. it'll last long
1927 // enough, and 2. you should be able to drop `Arc`s with unclonable allocators
1928 let _weak = Weak { ptr: self.ptr, alloc: &self.alloc };
1929
1930 // Destroy the data at this time, even though we must not free the box
1931 // allocation itself (there might still be weak pointers lying around).
1932 // We cannot use `get_mut_unchecked` here, because `self.alloc` is borrowed.
1933 unsafe { ptr::drop_in_place(&mut (*self.ptr.as_ptr()).data) };
1934 }
1935
1936 /// Returns `true` if the two `Arc`s point to the same allocation in a vein similar to
1937 /// [`ptr::eq`]. This function ignores the metadata of `dyn Trait` pointers.
1938 ///
1939 /// # Examples
1940 ///
1941 /// ```
1942 /// use std::sync::Arc;
1943 ///
1944 /// let five = Arc::new(5);
1945 /// let same_five = Arc::clone(&five);
1946 /// let other_five = Arc::new(5);
1947 ///
1948 /// assert!(Arc::ptr_eq(&five, &same_five));
1949 /// assert!(!Arc::ptr_eq(&five, &other_five));
1950 /// ```
1951 ///
1952 /// [`ptr::eq`]: core::ptr::eq "ptr::eq"
1953 #[inline]
1954 #[must_use]
1955 #[stable(feature = "ptr_eq", since = "1.17.0")]
1956 pub fn ptr_eq(this: &Self, other: &Self) -> bool {
1957 ptr::addr_eq(this.ptr.as_ptr(), other.ptr.as_ptr())
1958 }
1959}
1960
1961impl<T: ?Sized> Arc<T> {
1962 /// Allocates an `ArcInner<T>` with sufficient space for
1963 /// a possibly-unsized inner value where the value has the layout provided.
1964 ///
1965 /// The function `mem_to_arcinner` is called with the data pointer
1966 /// and must return back a (potentially fat)-pointer for the `ArcInner<T>`.
1967 #[cfg(not(no_global_oom_handling))]
1968 unsafe fn allocate_for_layout(
1969 value_layout: Layout,
1970 allocate: impl FnOnce(Layout) -> Result<NonNull<[u8]>, AllocError>,
1971 mem_to_arcinner: impl FnOnce(*mut u8) -> *mut ArcInner<T>,
1972 ) -> *mut ArcInner<T> {
1973 let layout = arcinner_layout_for_value_layout(value_layout);
1974
1975 let ptr = allocate(layout).unwrap_or_else(|_| handle_alloc_error(layout));
1976
1977 unsafe { Self::initialize_arcinner(ptr, layout, mem_to_arcinner) }
1978 }
1979
1980 /// Allocates an `ArcInner<T>` with sufficient space for
1981 /// a possibly-unsized inner value where the value has the layout provided,
1982 /// returning an error if allocation fails.
1983 ///
1984 /// The function `mem_to_arcinner` is called with the data pointer
1985 /// and must return back a (potentially fat)-pointer for the `ArcInner<T>`.
1986 unsafe fn try_allocate_for_layout(
1987 value_layout: Layout,
1988 allocate: impl FnOnce(Layout) -> Result<NonNull<[u8]>, AllocError>,
1989 mem_to_arcinner: impl FnOnce(*mut u8) -> *mut ArcInner<T>,
1990 ) -> Result<*mut ArcInner<T>, AllocError> {
1991 let layout = arcinner_layout_for_value_layout(value_layout);
1992
1993 let ptr = allocate(layout)?;
1994
1995 let inner = unsafe { Self::initialize_arcinner(ptr, layout, mem_to_arcinner) };
1996
1997 Ok(inner)
1998 }
1999
2000 unsafe fn initialize_arcinner(
2001 ptr: NonNull<[u8]>,
2002 layout: Layout,
2003 mem_to_arcinner: impl FnOnce(*mut u8) -> *mut ArcInner<T>,
2004 ) -> *mut ArcInner<T> {
2005 let inner = mem_to_arcinner(ptr.as_non_null_ptr().as_ptr());
2006 debug_assert_eq!(unsafe { Layout::for_value_raw(inner) }, layout);
2007
2008 unsafe {
2009 (&raw mut (*inner).strong).write(atomic::AtomicUsize::new(1));
2010 (&raw mut (*inner).weak).write(atomic::AtomicUsize::new(1));
2011 }
2012
2013 inner
2014 }
2015}
2016
2017impl<T: ?Sized, A: Allocator> Arc<T, A> {
2018 /// Allocates an `ArcInner<T>` with sufficient space for an unsized inner value.
2019 #[inline]
2020 #[cfg(not(no_global_oom_handling))]
2021 unsafe fn allocate_for_ptr_in(ptr: *const T, alloc: &A) -> *mut ArcInner<T> {
2022 // Allocate for the `ArcInner<T>` using the given value.
2023 unsafe {
2024 Arc::allocate_for_layout(
2025 Layout::for_value_raw(ptr),
2026 |layout| alloc.allocate(layout),
2027 |mem| mem.with_metadata_of(ptr as *const ArcInner<T>),
2028 )
2029 }
2030 }
2031
2032 #[cfg(not(no_global_oom_handling))]
2033 fn from_box_in(src: Box<T, A>) -> Arc<T, A> {
2034 unsafe {
2035 let value_size = size_of_val(&*src);
2036 let ptr = Self::allocate_for_ptr_in(&*src, Box::allocator(&src));
2037
2038 // Copy value as bytes
2039 ptr::copy_nonoverlapping(
2040 (&raw const *src) as *const u8,
2041 (&raw mut (*ptr).data) as *mut u8,
2042 value_size,
2043 );
2044
2045 // Free the allocation without dropping its contents
2046 let (bptr, alloc) = Box::into_raw_with_allocator(src);
2047 let src = Box::from_raw_in(bptr as *mut mem::ManuallyDrop<T>, alloc.by_ref());
2048 drop(src);
2049
2050 Self::from_ptr_in(ptr, alloc)
2051 }
2052 }
2053}
2054
2055impl<T> Arc<[T]> {
2056 /// Allocates an `ArcInner<[T]>` with the given length.
2057 #[cfg(not(no_global_oom_handling))]
2058 unsafe fn allocate_for_slice(len: usize) -> *mut ArcInner<[T]> {
2059 unsafe {
2060 Self::allocate_for_layout(
2061 Layout::array::<T>(len).unwrap(),
2062 |layout| Global.allocate(layout),
2063 |mem| ptr::slice_from_raw_parts_mut(mem.cast::<T>(), len) as *mut ArcInner<[T]>,
2064 )
2065 }
2066 }
2067
2068 /// Copy elements from slice into newly allocated `Arc<[T]>`
2069 ///
2070 /// Unsafe because the caller must either take ownership or bind `T: Copy`.
2071 #[cfg(not(no_global_oom_handling))]
2072 unsafe fn copy_from_slice(v: &[T]) -> Arc<[T]> {
2073 unsafe {
2074 let ptr = Self::allocate_for_slice(v.len());
2075
2076 ptr::copy_nonoverlapping(v.as_ptr(), (&raw mut (*ptr).data) as *mut T, v.len());
2077
2078 Self::from_ptr(ptr)
2079 }
2080 }
2081
2082 /// Constructs an `Arc<[T]>` from an iterator known to be of a certain size.
2083 ///
2084 /// Behavior is undefined should the size be wrong.
2085 #[cfg(not(no_global_oom_handling))]
2086 unsafe fn from_iter_exact(iter: impl Iterator<Item = T>, len: usize) -> Arc<[T]> {
2087 // Panic guard while cloning T elements.
2088 // In the event of a panic, elements that have been written
2089 // into the new ArcInner will be dropped, then the memory freed.
2090 struct Guard<T> {
2091 mem: NonNull<u8>,
2092 elems: *mut T,
2093 layout: Layout,
2094 n_elems: usize,
2095 }
2096
2097 impl<T> Drop for Guard<T> {
2098 fn drop(&mut self) {
2099 unsafe {
2100 let slice = from_raw_parts_mut(self.elems, self.n_elems);
2101 ptr::drop_in_place(slice);
2102
2103 Global.deallocate(self.mem, self.layout);
2104 }
2105 }
2106 }
2107
2108 unsafe {
2109 let ptr = Self::allocate_for_slice(len);
2110
2111 let mem = ptr as *mut _ as *mut u8;
2112 let layout = Layout::for_value_raw(ptr);
2113
2114 // Pointer to first element
2115 let elems = (&raw mut (*ptr).data) as *mut T;
2116
2117 let mut guard = Guard { mem: NonNull::new_unchecked(mem), elems, layout, n_elems: 0 };
2118
2119 for (i, item) in iter.enumerate() {
2120 ptr::write(elems.add(i), item);
2121 guard.n_elems += 1;
2122 }
2123
2124 // All clear. Forget the guard so it doesn't free the new ArcInner.
2125 mem::forget(guard);
2126
2127 Self::from_ptr(ptr)
2128 }
2129 }
2130}
2131
2132impl<T, A: Allocator> Arc<[T], A> {
2133 /// Allocates an `ArcInner<[T]>` with the given length.
2134 #[inline]
2135 #[cfg(not(no_global_oom_handling))]
2136 unsafe fn allocate_for_slice_in(len: usize, alloc: &A) -> *mut ArcInner<[T]> {
2137 unsafe {
2138 Arc::allocate_for_layout(
2139 Layout::array::<T>(len).unwrap(),
2140 |layout| alloc.allocate(layout),
2141 |mem| ptr::slice_from_raw_parts_mut(mem.cast::<T>(), len) as *mut ArcInner<[T]>,
2142 )
2143 }
2144 }
2145}
2146
2147/// Specialization trait used for `From<&[T]>`.
2148#[cfg(not(no_global_oom_handling))]
2149trait ArcFromSlice<T> {
2150 fn from_slice(slice: &[T]) -> Self;
2151}
2152
2153#[cfg(not(no_global_oom_handling))]
2154impl<T: Clone> ArcFromSlice<T> for Arc<[T]> {
2155 #[inline]
2156 default fn from_slice(v: &[T]) -> Self {
2157 unsafe { Self::from_iter_exact(v.iter().cloned(), v.len()) }
2158 }
2159}
2160
2161#[cfg(not(no_global_oom_handling))]
2162impl<T: Copy> ArcFromSlice<T> for Arc<[T]> {
2163 #[inline]
2164 fn from_slice(v: &[T]) -> Self {
2165 unsafe { Arc::copy_from_slice(v) }
2166 }
2167}
2168
2169#[stable(feature = "rust1", since = "1.0.0")]
2170impl<T: ?Sized, A: Allocator + Clone> Clone for Arc<T, A> {
2171 /// Makes a clone of the `Arc` pointer.
2172 ///
2173 /// This creates another pointer to the same allocation, increasing the
2174 /// strong reference count.
2175 ///
2176 /// # Examples
2177 ///
2178 /// ```
2179 /// use std::sync::Arc;
2180 ///
2181 /// let five = Arc::new(5);
2182 ///
2183 /// let _ = Arc::clone(&five);
2184 /// ```
2185 #[inline]
2186 fn clone(&self) -> Arc<T, A> {
2187 // Using a relaxed ordering is alright here, as knowledge of the
2188 // original reference prevents other threads from erroneously deleting
2189 // the object.
2190 //
2191 // As explained in the [Boost documentation][1], Increasing the
2192 // reference counter can always be done with memory_order_relaxed: New
2193 // references to an object can only be formed from an existing
2194 // reference, and passing an existing reference from one thread to
2195 // another must already provide any required synchronization.
2196 //
2197 // [1]: (www.boost.org/doc/libs/1_55_0/doc/html/atomic/usage_examples.html)
2198 let old_size = self.inner().strong.fetch_add(1, Relaxed);
2199
2200 // However we need to guard against massive refcounts in case someone is `mem::forget`ing
2201 // Arcs. If we don't do this the count can overflow and users will use-after free. This
2202 // branch will never be taken in any realistic program. We abort because such a program is
2203 // incredibly degenerate, and we don't care to support it.
2204 //
2205 // This check is not 100% water-proof: we error when the refcount grows beyond `isize::MAX`.
2206 // But we do that check *after* having done the increment, so there is a chance here that
2207 // the worst already happened and we actually do overflow the `usize` counter. However, that
2208 // requires the counter to grow from `isize::MAX` to `usize::MAX` between the increment
2209 // above and the `abort` below, which seems exceedingly unlikely.
2210 //
2211 // This is a global invariant, and also applies when using a compare-exchange loop to increment
2212 // counters in other methods.
2213 // Otherwise, the counter could be brought to an almost-overflow using a compare-exchange loop,
2214 // and then overflow using a few `fetch_add`s.
2215 if old_size > MAX_REFCOUNT {
2216 abort();
2217 }
2218
2219 unsafe { Self::from_inner_in(self.ptr, self.alloc.clone()) }
2220 }
2221}
2222
2223#[unstable(feature = "ergonomic_clones", issue = "132290")]
2224impl<T: ?Sized, A: Allocator + Clone> UseCloned for Arc<T, A> {}
2225
2226#[stable(feature = "rust1", since = "1.0.0")]
2227impl<T: ?Sized, A: Allocator> Deref for Arc<T, A> {
2228 type Target = T;
2229
2230 #[inline]
2231 fn deref(&self) -> &T {
2232 &self.inner().data
2233 }
2234}
2235
2236#[unstable(feature = "pin_coerce_unsized_trait", issue = "123430")]
2237unsafe impl<T: ?Sized, A: Allocator> PinCoerceUnsized for Arc<T, A> {}
2238
2239#[unstable(feature = "pin_coerce_unsized_trait", issue = "123430")]
2240unsafe impl<T: ?Sized, A: Allocator> PinCoerceUnsized for Weak<T, A> {}
2241
2242#[unstable(feature = "deref_pure_trait", issue = "87121")]
2243unsafe impl<T: ?Sized, A: Allocator> DerefPure for Arc<T, A> {}
2244
2245#[unstable(feature = "legacy_receiver_trait", issue = "none")]
2246impl<T: ?Sized> LegacyReceiver for Arc<T> {}
2247
2248#[cfg(not(no_global_oom_handling))]
2249impl<T: ?Sized + CloneToUninit, A: Allocator + Clone> Arc<T, A> {
2250 /// Makes a mutable reference into the given `Arc`.
2251 ///
2252 /// If there are other `Arc` pointers to the same allocation, then `make_mut` will
2253 /// [`clone`] the inner value to a new allocation to ensure unique ownership. This is also
2254 /// referred to as clone-on-write.
2255 ///
2256 /// However, if there are no other `Arc` pointers to this allocation, but some [`Weak`]
2257 /// pointers, then the [`Weak`] pointers will be dissociated and the inner value will not
2258 /// be cloned.
2259 ///
2260 /// See also [`get_mut`], which will fail rather than cloning the inner value
2261 /// or dissociating [`Weak`] pointers.
2262 ///
2263 /// [`clone`]: Clone::clone
2264 /// [`get_mut`]: Arc::get_mut
2265 ///
2266 /// # Examples
2267 ///
2268 /// ```
2269 /// use std::sync::Arc;
2270 ///
2271 /// let mut data = Arc::new(5);
2272 ///
2273 /// *Arc::make_mut(&mut data) += 1; // Won't clone anything
2274 /// let mut other_data = Arc::clone(&data); // Won't clone inner data
2275 /// *Arc::make_mut(&mut data) += 1; // Clones inner data
2276 /// *Arc::make_mut(&mut data) += 1; // Won't clone anything
2277 /// *Arc::make_mut(&mut other_data) *= 2; // Won't clone anything
2278 ///
2279 /// // Now `data` and `other_data` point to different allocations.
2280 /// assert_eq!(*data, 8);
2281 /// assert_eq!(*other_data, 12);
2282 /// ```
2283 ///
2284 /// [`Weak`] pointers will be dissociated:
2285 ///
2286 /// ```
2287 /// use std::sync::Arc;
2288 ///
2289 /// let mut data = Arc::new(75);
2290 /// let weak = Arc::downgrade(&data);
2291 ///
2292 /// assert!(75 == *data);
2293 /// assert!(75 == *weak.upgrade().unwrap());
2294 ///
2295 /// *Arc::make_mut(&mut data) += 1;
2296 ///
2297 /// assert!(76 == *data);
2298 /// assert!(weak.upgrade().is_none());
2299 /// ```
2300 #[inline]
2301 #[stable(feature = "arc_unique", since = "1.4.0")]
2302 pub fn make_mut(this: &mut Self) -> &mut T {
2303 let size_of_val = size_of_val::<T>(&**this);
2304
2305 // Note that we hold both a strong reference and a weak reference.
2306 // Thus, releasing our strong reference only will not, by itself, cause
2307 // the memory to be deallocated.
2308 //
2309 // Use Acquire to ensure that we see any writes to `weak` that happen
2310 // before release writes (i.e., decrements) to `strong`. Since we hold a
2311 // weak count, there's no chance the ArcInner itself could be
2312 // deallocated.
2313 if this.inner().strong.compare_exchange(1, 0, Acquire, Relaxed).is_err() {
2314 // Another strong pointer exists, so we must clone.
2315
2316 let this_data_ref: &T = &**this;
2317 // `in_progress` drops the allocation if we panic before finishing initializing it.
2318 let mut in_progress: UniqueArcUninit<T, A> =
2319 UniqueArcUninit::new(this_data_ref, this.alloc.clone());
2320
2321 let initialized_clone = unsafe {
2322 // Clone. If the clone panics, `in_progress` will be dropped and clean up.
2323 this_data_ref.clone_to_uninit(in_progress.data_ptr().cast());
2324 // Cast type of pointer, now that it is initialized.
2325 in_progress.into_arc()
2326 };
2327 *this = initialized_clone;
2328 } else if this.inner().weak.load(Relaxed) != 1 {
2329 // Relaxed suffices in the above because this is fundamentally an
2330 // optimization: we are always racing with weak pointers being
2331 // dropped. Worst case, we end up allocated a new Arc unnecessarily.
2332
2333 // We removed the last strong ref, but there are additional weak
2334 // refs remaining. We'll move the contents to a new Arc, and
2335 // invalidate the other weak refs.
2336
2337 // Note that it is not possible for the read of `weak` to yield
2338 // usize::MAX (i.e., locked), since the weak count can only be
2339 // locked by a thread with a strong reference.
2340
2341 // Materialize our own implicit weak pointer, so that it can clean
2342 // up the ArcInner as needed.
2343 let _weak = Weak { ptr: this.ptr, alloc: this.alloc.clone() };
2344
2345 // Can just steal the data, all that's left is Weaks
2346 //
2347 // We don't need panic-protection like the above branch does, but we might as well
2348 // use the same mechanism.
2349 let mut in_progress: UniqueArcUninit<T, A> =
2350 UniqueArcUninit::new(&**this, this.alloc.clone());
2351 unsafe {
2352 // Initialize `in_progress` with move of **this.
2353 // We have to express this in terms of bytes because `T: ?Sized`; there is no
2354 // operation that just copies a value based on its `size_of_val()`.
2355 ptr::copy_nonoverlapping(
2356 ptr::from_ref(&**this).cast::<u8>(),
2357 in_progress.data_ptr().cast::<u8>(),
2358 size_of_val,
2359 );
2360
2361 ptr::write(this, in_progress.into_arc());
2362 }
2363 } else {
2364 // We were the sole reference of either kind; bump back up the
2365 // strong ref count.
2366 this.inner().strong.store(1, Release);
2367 }
2368
2369 // As with `get_mut()`, the unsafety is ok because our reference was
2370 // either unique to begin with, or became one upon cloning the contents.
2371 unsafe { Self::get_mut_unchecked(this) }
2372 }
2373}
2374
2375impl<T: Clone, A: Allocator> Arc<T, A> {
2376 /// If we have the only reference to `T` then unwrap it. Otherwise, clone `T` and return the
2377 /// clone.
2378 ///
2379 /// Assuming `arc_t` is of type `Arc<T>`, this function is functionally equivalent to
2380 /// `(*arc_t).clone()`, but will avoid cloning the inner value where possible.
2381 ///
2382 /// # Examples
2383 ///
2384 /// ```
2385 /// # use std::{ptr, sync::Arc};
2386 /// let inner = String::from("test");
2387 /// let ptr = inner.as_ptr();
2388 ///
2389 /// let arc = Arc::new(inner);
2390 /// let inner = Arc::unwrap_or_clone(arc);
2391 /// // The inner value was not cloned
2392 /// assert!(ptr::eq(ptr, inner.as_ptr()));
2393 ///
2394 /// let arc = Arc::new(inner);
2395 /// let arc2 = arc.clone();
2396 /// let inner = Arc::unwrap_or_clone(arc);
2397 /// // Because there were 2 references, we had to clone the inner value.
2398 /// assert!(!ptr::eq(ptr, inner.as_ptr()));
2399 /// // `arc2` is the last reference, so when we unwrap it we get back
2400 /// // the original `String`.
2401 /// let inner = Arc::unwrap_or_clone(arc2);
2402 /// assert!(ptr::eq(ptr, inner.as_ptr()));
2403 /// ```
2404 #[inline]
2405 #[stable(feature = "arc_unwrap_or_clone", since = "1.76.0")]
2406 pub fn unwrap_or_clone(this: Self) -> T {
2407 Arc::try_unwrap(this).unwrap_or_else(|arc| (*arc).clone())
2408 }
2409}
2410
2411impl<T: ?Sized, A: Allocator> Arc<T, A> {
2412 /// Returns a mutable reference into the given `Arc`, if there are
2413 /// no other `Arc` or [`Weak`] pointers to the same allocation.
2414 ///
2415 /// Returns [`None`] otherwise, because it is not safe to
2416 /// mutate a shared value.
2417 ///
2418 /// See also [`make_mut`][make_mut], which will [`clone`][clone]
2419 /// the inner value when there are other `Arc` pointers.
2420 ///
2421 /// [make_mut]: Arc::make_mut
2422 /// [clone]: Clone::clone
2423 ///
2424 /// # Examples
2425 ///
2426 /// ```
2427 /// use std::sync::Arc;
2428 ///
2429 /// let mut x = Arc::new(3);
2430 /// *Arc::get_mut(&mut x).unwrap() = 4;
2431 /// assert_eq!(*x, 4);
2432 ///
2433 /// let _y = Arc::clone(&x);
2434 /// assert!(Arc::get_mut(&mut x).is_none());
2435 /// ```
2436 #[inline]
2437 #[stable(feature = "arc_unique", since = "1.4.0")]
2438 pub fn get_mut(this: &mut Self) -> Option<&mut T> {
2439 if Self::is_unique(this) {
2440 // This unsafety is ok because we're guaranteed that the pointer
2441 // returned is the *only* pointer that will ever be returned to T. Our
2442 // reference count is guaranteed to be 1 at this point, and we required
2443 // the Arc itself to be `mut`, so we're returning the only possible
2444 // reference to the inner data.
2445 unsafe { Some(Arc::get_mut_unchecked(this)) }
2446 } else {
2447 None
2448 }
2449 }
2450
2451 /// Returns a mutable reference into the given `Arc`,
2452 /// without any check.
2453 ///
2454 /// See also [`get_mut`], which is safe and does appropriate checks.
2455 ///
2456 /// [`get_mut`]: Arc::get_mut
2457 ///
2458 /// # Safety
2459 ///
2460 /// If any other `Arc` or [`Weak`] pointers to the same allocation exist, then
2461 /// they must not be dereferenced or have active borrows for the duration
2462 /// of the returned borrow, and their inner type must be exactly the same as the
2463 /// inner type of this Arc (including lifetimes). This is trivially the case if no
2464 /// such pointers exist, for example immediately after `Arc::new`.
2465 ///
2466 /// # Examples
2467 ///
2468 /// ```
2469 /// #![feature(get_mut_unchecked)]
2470 ///
2471 /// use std::sync::Arc;
2472 ///
2473 /// let mut x = Arc::new(String::new());
2474 /// unsafe {
2475 /// Arc::get_mut_unchecked(&mut x).push_str("foo")
2476 /// }
2477 /// assert_eq!(*x, "foo");
2478 /// ```
2479 /// Other `Arc` pointers to the same allocation must be to the same type.
2480 /// ```no_run
2481 /// #![feature(get_mut_unchecked)]
2482 ///
2483 /// use std::sync::Arc;
2484 ///
2485 /// let x: Arc<str> = Arc::from("Hello, world!");
2486 /// let mut y: Arc<[u8]> = x.clone().into();
2487 /// unsafe {
2488 /// // this is Undefined Behavior, because x's inner type is str, not [u8]
2489 /// Arc::get_mut_unchecked(&mut y).fill(0xff); // 0xff is invalid in UTF-8
2490 /// }
2491 /// println!("{}", &*x); // Invalid UTF-8 in a str
2492 /// ```
2493 /// Other `Arc` pointers to the same allocation must be to the exact same type, including lifetimes.
2494 /// ```no_run
2495 /// #![feature(get_mut_unchecked)]
2496 ///
2497 /// use std::sync::Arc;
2498 ///
2499 /// let x: Arc<&str> = Arc::new("Hello, world!");
2500 /// {
2501 /// let s = String::from("Oh, no!");
2502 /// let mut y: Arc<&str> = x.clone();
2503 /// unsafe {
2504 /// // this is Undefined Behavior, because x's inner type
2505 /// // is &'long str, not &'short str
2506 /// *Arc::get_mut_unchecked(&mut y) = &s;
2507 /// }
2508 /// }
2509 /// println!("{}", &*x); // Use-after-free
2510 /// ```
2511 #[inline]
2512 #[unstable(feature = "get_mut_unchecked", issue = "63292")]
2513 pub unsafe fn get_mut_unchecked(this: &mut Self) -> &mut T {
2514 // We are careful to *not* create a reference covering the "count" fields, as
2515 // this would alias with concurrent access to the reference counts (e.g. by `Weak`).
2516 unsafe { &mut (*this.ptr.as_ptr()).data }
2517 }
2518
2519 /// Determine whether this is the unique reference to the underlying data.
2520 ///
2521 /// Returns `true` if there are no other `Arc` or [`Weak`] pointers to the same allocation;
2522 /// returns `false` otherwise.
2523 ///
2524 /// If this function returns `true`, then is guaranteed to be safe to call [`get_mut_unchecked`]
2525 /// on this `Arc`, so long as no clones occur in between.
2526 ///
2527 /// # Examples
2528 ///
2529 /// ```
2530 /// #![feature(arc_is_unique)]
2531 ///
2532 /// use std::sync::Arc;
2533 ///
2534 /// let x = Arc::new(3);
2535 /// assert!(Arc::is_unique(&x));
2536 ///
2537 /// let y = Arc::clone(&x);
2538 /// assert!(!Arc::is_unique(&x));
2539 /// drop(y);
2540 ///
2541 /// // Weak references also count, because they could be upgraded at any time.
2542 /// let z = Arc::downgrade(&x);
2543 /// assert!(!Arc::is_unique(&x));
2544 /// ```
2545 ///
2546 /// # Pointer invalidation
2547 ///
2548 /// This function will always return the same value as `Arc::get_mut(arc).is_some()`. However,
2549 /// unlike that operation it does not produce any mutable references to the underlying data,
2550 /// meaning no pointers to the data inside the `Arc` are invalidated by the call. Thus, the
2551 /// following code is valid, even though it would be UB if it used `Arc::get_mut`:
2552 ///
2553 /// ```
2554 /// #![feature(arc_is_unique)]
2555 ///
2556 /// use std::sync::Arc;
2557 ///
2558 /// let arc = Arc::new(5);
2559 /// let pointer: *const i32 = &*arc;
2560 /// assert!(Arc::is_unique(&arc));
2561 /// assert_eq!(unsafe { *pointer }, 5);
2562 /// ```
2563 ///
2564 /// # Atomic orderings
2565 ///
2566 /// Concurrent drops to other `Arc` pointers to the same allocation will synchronize with this
2567 /// call - that is, this call performs an `Acquire` operation on the underlying strong and weak
2568 /// ref counts. This ensures that calling `get_mut_unchecked` is safe.
2569 ///
2570 /// Note that this operation requires locking the weak ref count, so concurrent calls to
2571 /// `downgrade` may spin-loop for a short period of time.
2572 ///
2573 /// [`get_mut_unchecked`]: Self::get_mut_unchecked
2574 #[inline]
2575 #[unstable(feature = "arc_is_unique", issue = "138938")]
2576 pub fn is_unique(this: &Self) -> bool {
2577 // lock the weak pointer count if we appear to be the sole weak pointer
2578 // holder.
2579 //
2580 // The acquire label here ensures a happens-before relationship with any
2581 // writes to `strong` (in particular in `Weak::upgrade`) prior to decrements
2582 // of the `weak` count (via `Weak::drop`, which uses release). If the upgraded
2583 // weak ref was never dropped, the CAS here will fail so we do not care to synchronize.
2584 if this.inner().weak.compare_exchange(1, usize::MAX, Acquire, Relaxed).is_ok() {
2585 // This needs to be an `Acquire` to synchronize with the decrement of the `strong`
2586 // counter in `drop` -- the only access that happens when any but the last reference
2587 // is being dropped.
2588 let unique = this.inner().strong.load(Acquire) == 1;
2589
2590 // The release write here synchronizes with a read in `downgrade`,
2591 // effectively preventing the above read of `strong` from happening
2592 // after the write.
2593 this.inner().weak.store(1, Release); // release the lock
2594 unique
2595 } else {
2596 false
2597 }
2598 }
2599}
2600
2601#[stable(feature = "rust1", since = "1.0.0")]
2602unsafe impl<#[may_dangle] T: ?Sized, A: Allocator> Drop for Arc<T, A> {
2603 /// Drops the `Arc`.
2604 ///
2605 /// This will decrement the strong reference count. If the strong reference
2606 /// count reaches zero then the only other references (if any) are
2607 /// [`Weak`], so we `drop` the inner value.
2608 ///
2609 /// # Examples
2610 ///
2611 /// ```
2612 /// use std::sync::Arc;
2613 ///
2614 /// struct Foo;
2615 ///
2616 /// impl Drop for Foo {
2617 /// fn drop(&mut self) {
2618 /// println!("dropped!");
2619 /// }
2620 /// }
2621 ///
2622 /// let foo = Arc::new(Foo);
2623 /// let foo2 = Arc::clone(&foo);
2624 ///
2625 /// drop(foo); // Doesn't print anything
2626 /// drop(foo2); // Prints "dropped!"
2627 /// ```
2628 #[inline]
2629 fn drop(&mut self) {
2630 // Because `fetch_sub` is already atomic, we do not need to synchronize
2631 // with other threads unless we are going to delete the object. This
2632 // same logic applies to the below `fetch_sub` to the `weak` count.
2633 if self.inner().strong.fetch_sub(1, Release) != 1 {
2634 return;
2635 }
2636
2637 // This fence is needed to prevent reordering of use of the data and
2638 // deletion of the data. Because it is marked `Release`, the decreasing
2639 // of the reference count synchronizes with this `Acquire` fence. This
2640 // means that use of the data happens before decreasing the reference
2641 // count, which happens before this fence, which happens before the
2642 // deletion of the data.
2643 //
2644 // As explained in the [Boost documentation][1],
2645 //
2646 // > It is important to enforce any possible access to the object in one
2647 // > thread (through an existing reference) to *happen before* deleting
2648 // > the object in a different thread. This is achieved by a "release"
2649 // > operation after dropping a reference (any access to the object
2650 // > through this reference must obviously happened before), and an
2651 // > "acquire" operation before deleting the object.
2652 //
2653 // In particular, while the contents of an Arc are usually immutable, it's
2654 // possible to have interior writes to something like a Mutex<T>. Since a
2655 // Mutex is not acquired when it is deleted, we can't rely on its
2656 // synchronization logic to make writes in thread A visible to a destructor
2657 // running in thread B.
2658 //
2659 // Also note that the Acquire fence here could probably be replaced with an
2660 // Acquire load, which could improve performance in highly-contended
2661 // situations. See [2].
2662 //
2663 // [1]: (www.boost.org/doc/libs/1_55_0/doc/html/atomic/usage_examples.html)
2664 // [2]: (https://github.com/rust-lang/rust/pull/41714)
2665 acquire!(self.inner().strong);
2666
2667 // Make sure we aren't trying to "drop" the shared static for empty slices
2668 // used by Default::default.
2669 debug_assert!(
2670 !ptr::addr_eq(self.ptr.as_ptr(), &STATIC_INNER_SLICE.inner),
2671 "Arcs backed by a static should never reach a strong count of 0. \
2672 Likely decrement_strong_count or from_raw were called too many times.",
2673 );
2674
2675 unsafe {
2676 self.drop_slow();
2677 }
2678 }
2679}
2680
2681impl<A: Allocator> Arc<dyn Any + Send + Sync, A> {
2682 /// Attempts to downcast the `Arc<dyn Any + Send + Sync>` to a concrete type.
2683 ///
2684 /// # Examples
2685 ///
2686 /// ```
2687 /// use std::any::Any;
2688 /// use std::sync::Arc;
2689 ///
2690 /// fn print_if_string(value: Arc<dyn Any + Send + Sync>) {
2691 /// if let Ok(string) = value.downcast::<String>() {
2692 /// println!("String ({}): {}", string.len(), string);
2693 /// }
2694 /// }
2695 ///
2696 /// let my_string = "Hello World".to_string();
2697 /// print_if_string(Arc::new(my_string));
2698 /// print_if_string(Arc::new(0i8));
2699 /// ```
2700 #[inline]
2701 #[stable(feature = "rc_downcast", since = "1.29.0")]
2702 pub fn downcast<T>(self) -> Result<Arc<T, A>, Self>
2703 where
2704 T: Any + Send + Sync,
2705 {
2706 if (*self).is::<T>() {
2707 unsafe {
2708 let (ptr, alloc) = Arc::into_inner_with_allocator(self);
2709 Ok(Arc::from_inner_in(ptr.cast(), alloc))
2710 }
2711 } else {
2712 Err(self)
2713 }
2714 }
2715
2716 /// Downcasts the `Arc<dyn Any + Send + Sync>` to a concrete type.
2717 ///
2718 /// For a safe alternative see [`downcast`].
2719 ///
2720 /// # Examples
2721 ///
2722 /// ```
2723 /// #![feature(downcast_unchecked)]
2724 ///
2725 /// use std::any::Any;
2726 /// use std::sync::Arc;
2727 ///
2728 /// let x: Arc<dyn Any + Send + Sync> = Arc::new(1_usize);
2729 ///
2730 /// unsafe {
2731 /// assert_eq!(*x.downcast_unchecked::<usize>(), 1);
2732 /// }
2733 /// ```
2734 ///
2735 /// # Safety
2736 ///
2737 /// The contained value must be of type `T`. Calling this method
2738 /// with the incorrect type is *undefined behavior*.
2739 ///
2740 ///
2741 /// [`downcast`]: Self::downcast
2742 #[inline]
2743 #[unstable(feature = "downcast_unchecked", issue = "90850")]
2744 pub unsafe fn downcast_unchecked<T>(self) -> Arc<T, A>
2745 where
2746 T: Any + Send + Sync,
2747 {
2748 unsafe {
2749 let (ptr, alloc) = Arc::into_inner_with_allocator(self);
2750 Arc::from_inner_in(ptr.cast(), alloc)
2751 }
2752 }
2753}
2754
2755impl<T> Weak<T> {
2756 /// Constructs a new `Weak<T>`, without allocating any memory.
2757 /// Calling [`upgrade`] on the return value always gives [`None`].
2758 ///
2759 /// [`upgrade`]: Weak::upgrade
2760 ///
2761 /// # Examples
2762 ///
2763 /// ```
2764 /// use std::sync::Weak;
2765 ///
2766 /// let empty: Weak<i64> = Weak::new();
2767 /// assert!(empty.upgrade().is_none());
2768 /// ```
2769 #[inline]
2770 #[stable(feature = "downgraded_weak", since = "1.10.0")]
2771 #[rustc_const_stable(feature = "const_weak_new", since = "1.73.0")]
2772 #[must_use]
2773 pub const fn new() -> Weak<T> {
2774 Weak { ptr: NonNull::without_provenance(NonZeroUsize::MAX), alloc: Global }
2775 }
2776}
2777
2778impl<T, A: Allocator> Weak<T, A> {
2779 /// Constructs a new `Weak<T, A>`, without allocating any memory, technically in the provided
2780 /// allocator.
2781 /// Calling [`upgrade`] on the return value always gives [`None`].
2782 ///
2783 /// [`upgrade`]: Weak::upgrade
2784 ///
2785 /// # Examples
2786 ///
2787 /// ```
2788 /// #![feature(allocator_api)]
2789 ///
2790 /// use std::sync::Weak;
2791 /// use std::alloc::System;
2792 ///
2793 /// let empty: Weak<i64, _> = Weak::new_in(System);
2794 /// assert!(empty.upgrade().is_none());
2795 /// ```
2796 #[inline]
2797 #[unstable(feature = "allocator_api", issue = "32838")]
2798 pub fn new_in(alloc: A) -> Weak<T, A> {
2799 Weak { ptr: NonNull::without_provenance(NonZeroUsize::MAX), alloc }
2800 }
2801}
2802
2803/// Helper type to allow accessing the reference counts without
2804/// making any assertions about the data field.
2805struct WeakInner<'a> {
2806 weak: &'a Atomic<usize>,
2807 strong: &'a Atomic<usize>,
2808}
2809
2810impl<T: ?Sized> Weak<T> {
2811 /// Converts a raw pointer previously created by [`into_raw`] back into `Weak<T>`.
2812 ///
2813 /// This can be used to safely get a strong reference (by calling [`upgrade`]
2814 /// later) or to deallocate the weak count by dropping the `Weak<T>`.
2815 ///
2816 /// It takes ownership of one weak reference (with the exception of pointers created by [`new`],
2817 /// as these don't own anything; the method still works on them).
2818 ///
2819 /// # Safety
2820 ///
2821 /// The pointer must have originated from the [`into_raw`] and must still own its potential
2822 /// weak reference, and must point to a block of memory allocated by global allocator.
2823 ///
2824 /// It is allowed for the strong count to be 0 at the time of calling this. Nevertheless, this
2825 /// takes ownership of one weak reference currently represented as a raw pointer (the weak
2826 /// count is not modified by this operation) and therefore it must be paired with a previous
2827 /// call to [`into_raw`].
2828 /// # Examples
2829 ///
2830 /// ```
2831 /// use std::sync::{Arc, Weak};
2832 ///
2833 /// let strong = Arc::new("hello".to_owned());
2834 ///
2835 /// let raw_1 = Arc::downgrade(&strong).into_raw();
2836 /// let raw_2 = Arc::downgrade(&strong).into_raw();
2837 ///
2838 /// assert_eq!(2, Arc::weak_count(&strong));
2839 ///
2840 /// assert_eq!("hello", &*unsafe { Weak::from_raw(raw_1) }.upgrade().unwrap());
2841 /// assert_eq!(1, Arc::weak_count(&strong));
2842 ///
2843 /// drop(strong);
2844 ///
2845 /// // Decrement the last weak count.
2846 /// assert!(unsafe { Weak::from_raw(raw_2) }.upgrade().is_none());
2847 /// ```
2848 ///
2849 /// [`new`]: Weak::new
2850 /// [`into_raw`]: Weak::into_raw
2851 /// [`upgrade`]: Weak::upgrade
2852 #[inline]
2853 #[stable(feature = "weak_into_raw", since = "1.45.0")]
2854 pub unsafe fn from_raw(ptr: *const T) -> Self {
2855 unsafe { Weak::from_raw_in(ptr, Global) }
2856 }
2857
2858 /// Consumes the `Weak<T>` and turns it into a raw pointer.
2859 ///
2860 /// This converts the weak pointer into a raw pointer, while still preserving the ownership of
2861 /// one weak reference (the weak count is not modified by this operation). It can be turned
2862 /// back into the `Weak<T>` with [`from_raw`].
2863 ///
2864 /// The same restrictions of accessing the target of the pointer as with
2865 /// [`as_ptr`] apply.
2866 ///
2867 /// # Examples
2868 ///
2869 /// ```
2870 /// use std::sync::{Arc, Weak};
2871 ///
2872 /// let strong = Arc::new("hello".to_owned());
2873 /// let weak = Arc::downgrade(&strong);
2874 /// let raw = weak.into_raw();
2875 ///
2876 /// assert_eq!(1, Arc::weak_count(&strong));
2877 /// assert_eq!("hello", unsafe { &*raw });
2878 ///
2879 /// drop(unsafe { Weak::from_raw(raw) });
2880 /// assert_eq!(0, Arc::weak_count(&strong));
2881 /// ```
2882 ///
2883 /// [`from_raw`]: Weak::from_raw
2884 /// [`as_ptr`]: Weak::as_ptr
2885 #[must_use = "losing the pointer will leak memory"]
2886 #[stable(feature = "weak_into_raw", since = "1.45.0")]
2887 pub fn into_raw(self) -> *const T {
2888 ManuallyDrop::new(self).as_ptr()
2889 }
2890}
2891
2892impl<T: ?Sized, A: Allocator> Weak<T, A> {
2893 /// Returns a reference to the underlying allocator.
2894 #[inline]
2895 #[unstable(feature = "allocator_api", issue = "32838")]
2896 pub fn allocator(&self) -> &A {
2897 &self.alloc
2898 }
2899
2900 /// Returns a raw pointer to the object `T` pointed to by this `Weak<T>`.
2901 ///
2902 /// The pointer is valid only if there are some strong references. The pointer may be dangling,
2903 /// unaligned or even [`null`] otherwise.
2904 ///
2905 /// # Examples
2906 ///
2907 /// ```
2908 /// use std::sync::Arc;
2909 /// use std::ptr;
2910 ///
2911 /// let strong = Arc::new("hello".to_owned());
2912 /// let weak = Arc::downgrade(&strong);
2913 /// // Both point to the same object
2914 /// assert!(ptr::eq(&*strong, weak.as_ptr()));
2915 /// // The strong here keeps it alive, so we can still access the object.
2916 /// assert_eq!("hello", unsafe { &*weak.as_ptr() });
2917 ///
2918 /// drop(strong);
2919 /// // But not any more. We can do weak.as_ptr(), but accessing the pointer would lead to
2920 /// // undefined behavior.
2921 /// // assert_eq!("hello", unsafe { &*weak.as_ptr() });
2922 /// ```
2923 ///
2924 /// [`null`]: core::ptr::null "ptr::null"
2925 #[must_use]
2926 #[stable(feature = "weak_into_raw", since = "1.45.0")]
2927 pub fn as_ptr(&self) -> *const T {
2928 let ptr: *mut ArcInner<T> = NonNull::as_ptr(self.ptr);
2929
2930 if is_dangling(ptr) {
2931 // If the pointer is dangling, we return the sentinel directly. This cannot be
2932 // a valid payload address, as the payload is at least as aligned as ArcInner (usize).
2933 ptr as *const T
2934 } else {
2935 // SAFETY: if is_dangling returns false, then the pointer is dereferenceable.
2936 // The payload may be dropped at this point, and we have to maintain provenance,
2937 // so use raw pointer manipulation.
2938 unsafe { &raw mut (*ptr).data }
2939 }
2940 }
2941
2942 /// Consumes the `Weak<T>`, returning the wrapped pointer and allocator.
2943 ///
2944 /// This converts the weak pointer into a raw pointer, while still preserving the ownership of
2945 /// one weak reference (the weak count is not modified by this operation). It can be turned
2946 /// back into the `Weak<T>` with [`from_raw_in`].
2947 ///
2948 /// The same restrictions of accessing the target of the pointer as with
2949 /// [`as_ptr`] apply.
2950 ///
2951 /// # Examples
2952 ///
2953 /// ```
2954 /// #![feature(allocator_api)]
2955 /// use std::sync::{Arc, Weak};
2956 /// use std::alloc::System;
2957 ///
2958 /// let strong = Arc::new_in("hello".to_owned(), System);
2959 /// let weak = Arc::downgrade(&strong);
2960 /// let (raw, alloc) = weak.into_raw_with_allocator();
2961 ///
2962 /// assert_eq!(1, Arc::weak_count(&strong));
2963 /// assert_eq!("hello", unsafe { &*raw });
2964 ///
2965 /// drop(unsafe { Weak::from_raw_in(raw, alloc) });
2966 /// assert_eq!(0, Arc::weak_count(&strong));
2967 /// ```
2968 ///
2969 /// [`from_raw_in`]: Weak::from_raw_in
2970 /// [`as_ptr`]: Weak::as_ptr
2971 #[must_use = "losing the pointer will leak memory"]
2972 #[unstable(feature = "allocator_api", issue = "32838")]
2973 pub fn into_raw_with_allocator(self) -> (*const T, A) {
2974 let this = mem::ManuallyDrop::new(self);
2975 let result = this.as_ptr();
2976 // Safety: `this` is ManuallyDrop so the allocator will not be double-dropped
2977 let alloc = unsafe { ptr::read(&this.alloc) };
2978 (result, alloc)
2979 }
2980
2981 /// Converts a raw pointer previously created by [`into_raw`] back into `Weak<T>` in the provided
2982 /// allocator.
2983 ///
2984 /// This can be used to safely get a strong reference (by calling [`upgrade`]
2985 /// later) or to deallocate the weak count by dropping the `Weak<T>`.
2986 ///
2987 /// It takes ownership of one weak reference (with the exception of pointers created by [`new`],
2988 /// as these don't own anything; the method still works on them).
2989 ///
2990 /// # Safety
2991 ///
2992 /// The pointer must have originated from the [`into_raw`] and must still own its potential
2993 /// weak reference, and must point to a block of memory allocated by `alloc`.
2994 ///
2995 /// It is allowed for the strong count to be 0 at the time of calling this. Nevertheless, this
2996 /// takes ownership of one weak reference currently represented as a raw pointer (the weak
2997 /// count is not modified by this operation) and therefore it must be paired with a previous
2998 /// call to [`into_raw`].
2999 /// # Examples
3000 ///
3001 /// ```
3002 /// use std::sync::{Arc, Weak};
3003 ///
3004 /// let strong = Arc::new("hello".to_owned());
3005 ///
3006 /// let raw_1 = Arc::downgrade(&strong).into_raw();
3007 /// let raw_2 = Arc::downgrade(&strong).into_raw();
3008 ///
3009 /// assert_eq!(2, Arc::weak_count(&strong));
3010 ///
3011 /// assert_eq!("hello", &*unsafe { Weak::from_raw(raw_1) }.upgrade().unwrap());
3012 /// assert_eq!(1, Arc::weak_count(&strong));
3013 ///
3014 /// drop(strong);
3015 ///
3016 /// // Decrement the last weak count.
3017 /// assert!(unsafe { Weak::from_raw(raw_2) }.upgrade().is_none());
3018 /// ```
3019 ///
3020 /// [`new`]: Weak::new
3021 /// [`into_raw`]: Weak::into_raw
3022 /// [`upgrade`]: Weak::upgrade
3023 #[inline]
3024 #[unstable(feature = "allocator_api", issue = "32838")]
3025 pub unsafe fn from_raw_in(ptr: *const T, alloc: A) -> Self {
3026 // See Weak::as_ptr for context on how the input pointer is derived.
3027
3028 let ptr = if is_dangling(ptr) {
3029 // This is a dangling Weak.
3030 ptr as *mut ArcInner<T>
3031 } else {
3032 // Otherwise, we're guaranteed the pointer came from a nondangling Weak.
3033 // SAFETY: data_offset is safe to call, as ptr references a real (potentially dropped) T.
3034 let offset = unsafe { data_offset(ptr) };
3035 // Thus, we reverse the offset to get the whole ArcInner.
3036 // SAFETY: the pointer originated from a Weak, so this offset is safe.
3037 unsafe { ptr.byte_sub(offset) as *mut ArcInner<T> }
3038 };
3039
3040 // SAFETY: we now have recovered the original Weak pointer, so can create the Weak.
3041 Weak { ptr: unsafe { NonNull::new_unchecked(ptr) }, alloc }
3042 }
3043}
3044
3045impl<T: ?Sized, A: Allocator> Weak<T, A> {
3046 /// Attempts to upgrade the `Weak` pointer to an [`Arc`], delaying
3047 /// dropping of the inner value if successful.
3048 ///
3049 /// Returns [`None`] if the inner value has since been dropped.
3050 ///
3051 /// # Examples
3052 ///
3053 /// ```
3054 /// use std::sync::Arc;
3055 ///
3056 /// let five = Arc::new(5);
3057 ///
3058 /// let weak_five = Arc::downgrade(&five);
3059 ///
3060 /// let strong_five: Option<Arc<_>> = weak_five.upgrade();
3061 /// assert!(strong_five.is_some());
3062 ///
3063 /// // Destroy all strong pointers.
3064 /// drop(strong_five);
3065 /// drop(five);
3066 ///
3067 /// assert!(weak_five.upgrade().is_none());
3068 /// ```
3069 #[must_use = "this returns a new `Arc`, \
3070 without modifying the original weak pointer"]
3071 #[stable(feature = "arc_weak", since = "1.4.0")]
3072 pub fn upgrade(&self) -> Option<Arc<T, A>>
3073 where
3074 A: Clone,
3075 {
3076 #[inline]
3077 fn checked_increment(n: usize) -> Option<usize> {
3078 // Any write of 0 we can observe leaves the field in permanently zero state.
3079 if n == 0 {
3080 return None;
3081 }
3082 // See comments in `Arc::clone` for why we do this (for `mem::forget`).
3083 assert!(n <= MAX_REFCOUNT, "{}", INTERNAL_OVERFLOW_ERROR);
3084 Some(n + 1)
3085 }
3086
3087 // We use a CAS loop to increment the strong count instead of a
3088 // fetch_add as this function should never take the reference count
3089 // from zero to one.
3090 //
3091 // Relaxed is fine for the failure case because we don't have any expectations about the new state.
3092 // Acquire is necessary for the success case to synchronise with `Arc::new_cyclic`, when the inner
3093 // value can be initialized after `Weak` references have already been created. In that case, we
3094 // expect to observe the fully initialized value.
3095 if self.inner()?.strong.fetch_update(Acquire, Relaxed, checked_increment).is_ok() {
3096 // SAFETY: pointer is not null, verified in checked_increment
3097 unsafe { Some(Arc::from_inner_in(self.ptr, self.alloc.clone())) }
3098 } else {
3099 None
3100 }
3101 }
3102
3103 /// Gets the number of strong (`Arc`) pointers pointing to this allocation.
3104 ///
3105 /// If `self` was created using [`Weak::new`], this will return 0.
3106 #[must_use]
3107 #[stable(feature = "weak_counts", since = "1.41.0")]
3108 pub fn strong_count(&self) -> usize {
3109 if let Some(inner) = self.inner() { inner.strong.load(Relaxed) } else { 0 }
3110 }
3111
3112 /// Gets an approximation of the number of `Weak` pointers pointing to this
3113 /// allocation.
3114 ///
3115 /// If `self` was created using [`Weak::new`], or if there are no remaining
3116 /// strong pointers, this will return 0.
3117 ///
3118 /// # Accuracy
3119 ///
3120 /// Due to implementation details, the returned value can be off by 1 in
3121 /// either direction when other threads are manipulating any `Arc`s or
3122 /// `Weak`s pointing to the same allocation.
3123 #[must_use]
3124 #[stable(feature = "weak_counts", since = "1.41.0")]
3125 pub fn weak_count(&self) -> usize {
3126 if let Some(inner) = self.inner() {
3127 let weak = inner.weak.load(Acquire);
3128 let strong = inner.strong.load(Relaxed);
3129 if strong == 0 {
3130 0
3131 } else {
3132 // Since we observed that there was at least one strong pointer
3133 // after reading the weak count, we know that the implicit weak
3134 // reference (present whenever any strong references are alive)
3135 // was still around when we observed the weak count, and can
3136 // therefore safely subtract it.
3137 weak - 1
3138 }
3139 } else {
3140 0
3141 }
3142 }
3143
3144 /// Returns `None` when the pointer is dangling and there is no allocated `ArcInner`,
3145 /// (i.e., when this `Weak` was created by `Weak::new`).
3146 #[inline]
3147 fn inner(&self) -> Option<WeakInner<'_>> {
3148 let ptr = self.ptr.as_ptr();
3149 if is_dangling(ptr) {
3150 None
3151 } else {
3152 // We are careful to *not* create a reference covering the "data" field, as
3153 // the field may be mutated concurrently (for example, if the last `Arc`
3154 // is dropped, the data field will be dropped in-place).
3155 Some(unsafe { WeakInner { strong: &(*ptr).strong, weak: &(*ptr).weak } })
3156 }
3157 }
3158
3159 /// Returns `true` if the two `Weak`s point to the same allocation similar to [`ptr::eq`], or if
3160 /// both don't point to any allocation (because they were created with `Weak::new()`). However,
3161 /// this function ignores the metadata of `dyn Trait` pointers.
3162 ///
3163 /// # Notes
3164 ///
3165 /// Since this compares pointers it means that `Weak::new()` will equal each
3166 /// other, even though they don't point to any allocation.
3167 ///
3168 /// # Examples
3169 ///
3170 /// ```
3171 /// use std::sync::Arc;
3172 ///
3173 /// let first_rc = Arc::new(5);
3174 /// let first = Arc::downgrade(&first_rc);
3175 /// let second = Arc::downgrade(&first_rc);
3176 ///
3177 /// assert!(first.ptr_eq(&second));
3178 ///
3179 /// let third_rc = Arc::new(5);
3180 /// let third = Arc::downgrade(&third_rc);
3181 ///
3182 /// assert!(!first.ptr_eq(&third));
3183 /// ```
3184 ///
3185 /// Comparing `Weak::new`.
3186 ///
3187 /// ```
3188 /// use std::sync::{Arc, Weak};
3189 ///
3190 /// let first = Weak::new();
3191 /// let second = Weak::new();
3192 /// assert!(first.ptr_eq(&second));
3193 ///
3194 /// let third_rc = Arc::new(());
3195 /// let third = Arc::downgrade(&third_rc);
3196 /// assert!(!first.ptr_eq(&third));
3197 /// ```
3198 ///
3199 /// [`ptr::eq`]: core::ptr::eq "ptr::eq"
3200 #[inline]
3201 #[must_use]
3202 #[stable(feature = "weak_ptr_eq", since = "1.39.0")]
3203 pub fn ptr_eq(&self, other: &Self) -> bool {
3204 ptr::addr_eq(self.ptr.as_ptr(), other.ptr.as_ptr())
3205 }
3206}
3207
3208#[stable(feature = "arc_weak", since = "1.4.0")]
3209impl<T: ?Sized, A: Allocator + Clone> Clone for Weak<T, A> {
3210 /// Makes a clone of the `Weak` pointer that points to the same allocation.
3211 ///
3212 /// # Examples
3213 ///
3214 /// ```
3215 /// use std::sync::{Arc, Weak};
3216 ///
3217 /// let weak_five = Arc::downgrade(&Arc::new(5));
3218 ///
3219 /// let _ = Weak::clone(&weak_five);
3220 /// ```
3221 #[inline]
3222 fn clone(&self) -> Weak<T, A> {
3223 if let Some(inner) = self.inner() {
3224 // See comments in Arc::clone() for why this is relaxed. This can use a
3225 // fetch_add (ignoring the lock) because the weak count is only locked
3226 // where are *no other* weak pointers in existence. (So we can't be
3227 // running this code in that case).
3228 let old_size = inner.weak.fetch_add(1, Relaxed);
3229
3230 // See comments in Arc::clone() for why we do this (for mem::forget).
3231 if old_size > MAX_REFCOUNT {
3232 abort();
3233 }
3234 }
3235
3236 Weak { ptr: self.ptr, alloc: self.alloc.clone() }
3237 }
3238}
3239
3240#[unstable(feature = "ergonomic_clones", issue = "132290")]
3241impl<T: ?Sized, A: Allocator + Clone> UseCloned for Weak<T, A> {}
3242
3243#[stable(feature = "downgraded_weak", since = "1.10.0")]
3244impl<T> Default for Weak<T> {
3245 /// Constructs a new `Weak<T>`, without allocating memory.
3246 /// Calling [`upgrade`] on the return value always
3247 /// gives [`None`].
3248 ///
3249 /// [`upgrade`]: Weak::upgrade
3250 ///
3251 /// # Examples
3252 ///
3253 /// ```
3254 /// use std::sync::Weak;
3255 ///
3256 /// let empty: Weak<i64> = Default::default();
3257 /// assert!(empty.upgrade().is_none());
3258 /// ```
3259 fn default() -> Weak<T> {
3260 Weak::new()
3261 }
3262}
3263
3264#[stable(feature = "arc_weak", since = "1.4.0")]
3265unsafe impl<#[may_dangle] T: ?Sized, A: Allocator> Drop for Weak<T, A> {
3266 /// Drops the `Weak` pointer.
3267 ///
3268 /// # Examples
3269 ///
3270 /// ```
3271 /// use std::sync::{Arc, Weak};
3272 ///
3273 /// struct Foo;
3274 ///
3275 /// impl Drop for Foo {
3276 /// fn drop(&mut self) {
3277 /// println!("dropped!");
3278 /// }
3279 /// }
3280 ///
3281 /// let foo = Arc::new(Foo);
3282 /// let weak_foo = Arc::downgrade(&foo);
3283 /// let other_weak_foo = Weak::clone(&weak_foo);
3284 ///
3285 /// drop(weak_foo); // Doesn't print anything
3286 /// drop(foo); // Prints "dropped!"
3287 ///
3288 /// assert!(other_weak_foo.upgrade().is_none());
3289 /// ```
3290 fn drop(&mut self) {
3291 // If we find out that we were the last weak pointer, then its time to
3292 // deallocate the data entirely. See the discussion in Arc::drop() about
3293 // the memory orderings
3294 //
3295 // It's not necessary to check for the locked state here, because the
3296 // weak count can only be locked if there was precisely one weak ref,
3297 // meaning that drop could only subsequently run ON that remaining weak
3298 // ref, which can only happen after the lock is released.
3299 let inner = if let Some(inner) = self.inner() { inner } else { return };
3300
3301 if inner.weak.fetch_sub(1, Release) == 1 {
3302 acquire!(inner.weak);
3303
3304 // Make sure we aren't trying to "deallocate" the shared static for empty slices
3305 // used by Default::default.
3306 debug_assert!(
3307 !ptr::addr_eq(self.ptr.as_ptr(), &STATIC_INNER_SLICE.inner),
3308 "Arc/Weaks backed by a static should never be deallocated. \
3309 Likely decrement_strong_count or from_raw were called too many times.",
3310 );
3311
3312 unsafe {
3313 self.alloc.deallocate(self.ptr.cast(), Layout::for_value_raw(self.ptr.as_ptr()))
3314 }
3315 }
3316 }
3317}
3318
3319#[stable(feature = "rust1", since = "1.0.0")]
3320trait ArcEqIdent<T: ?Sized + PartialEq, A: Allocator> {
3321 fn eq(&self, other: &Arc<T, A>) -> bool;
3322 fn ne(&self, other: &Arc<T, A>) -> bool;
3323}
3324
3325#[stable(feature = "rust1", since = "1.0.0")]
3326impl<T: ?Sized + PartialEq, A: Allocator> ArcEqIdent<T, A> for Arc<T, A> {
3327 #[inline]
3328 default fn eq(&self, other: &Arc<T, A>) -> bool {
3329 **self == **other
3330 }
3331 #[inline]
3332 default fn ne(&self, other: &Arc<T, A>) -> bool {
3333 **self != **other
3334 }
3335}
3336
3337/// We're doing this specialization here, and not as a more general optimization on `&T`, because it
3338/// would otherwise add a cost to all equality checks on refs. We assume that `Arc`s are used to
3339/// store large values, that are slow to clone, but also heavy to check for equality, causing this
3340/// cost to pay off more easily. It's also more likely to have two `Arc` clones, that point to
3341/// the same value, than two `&T`s.
3342///
3343/// We can only do this when `T: Eq` as a `PartialEq` might be deliberately irreflexive.
3344#[stable(feature = "rust1", since = "1.0.0")]
3345impl<T: ?Sized + crate::rc::MarkerEq, A: Allocator> ArcEqIdent<T, A> for Arc<T, A> {
3346 #[inline]
3347 fn eq(&self, other: &Arc<T, A>) -> bool {
3348 Arc::ptr_eq(self, other) || **self == **other
3349 }
3350
3351 #[inline]
3352 fn ne(&self, other: &Arc<T, A>) -> bool {
3353 !Arc::ptr_eq(self, other) && **self != **other
3354 }
3355}
3356
3357#[stable(feature = "rust1", since = "1.0.0")]
3358impl<T: ?Sized + PartialEq, A: Allocator> PartialEq for Arc<T, A> {
3359 /// Equality for two `Arc`s.
3360 ///
3361 /// Two `Arc`s are equal if their inner values are equal, even if they are
3362 /// stored in different allocation.
3363 ///
3364 /// If `T` also implements `Eq` (implying reflexivity of equality),
3365 /// two `Arc`s that point to the same allocation are always equal.
3366 ///
3367 /// # Examples
3368 ///
3369 /// ```
3370 /// use std::sync::Arc;
3371 ///
3372 /// let five = Arc::new(5);
3373 ///
3374 /// assert!(five == Arc::new(5));
3375 /// ```
3376 #[inline]
3377 fn eq(&self, other: &Arc<T, A>) -> bool {
3378 ArcEqIdent::eq(self, other)
3379 }
3380
3381 /// Inequality for two `Arc`s.
3382 ///
3383 /// Two `Arc`s are not equal if their inner values are not equal.
3384 ///
3385 /// If `T` also implements `Eq` (implying reflexivity of equality),
3386 /// two `Arc`s that point to the same value are always equal.
3387 ///
3388 /// # Examples
3389 ///
3390 /// ```
3391 /// use std::sync::Arc;
3392 ///
3393 /// let five = Arc::new(5);
3394 ///
3395 /// assert!(five != Arc::new(6));
3396 /// ```
3397 #[inline]
3398 fn ne(&self, other: &Arc<T, A>) -> bool {
3399 ArcEqIdent::ne(self, other)
3400 }
3401}
3402
3403#[stable(feature = "rust1", since = "1.0.0")]
3404impl<T: ?Sized + PartialOrd, A: Allocator> PartialOrd for Arc<T, A> {
3405 /// Partial comparison for two `Arc`s.
3406 ///
3407 /// The two are compared by calling `partial_cmp()` on their inner values.
3408 ///
3409 /// # Examples
3410 ///
3411 /// ```
3412 /// use std::sync::Arc;
3413 /// use std::cmp::Ordering;
3414 ///
3415 /// let five = Arc::new(5);
3416 ///
3417 /// assert_eq!(Some(Ordering::Less), five.partial_cmp(&Arc::new(6)));
3418 /// ```
3419 fn partial_cmp(&self, other: &Arc<T, A>) -> Option<Ordering> {
3420 (**self).partial_cmp(&**other)
3421 }
3422
3423 /// Less-than comparison for two `Arc`s.
3424 ///
3425 /// The two are compared by calling `<` on their inner values.
3426 ///
3427 /// # Examples
3428 ///
3429 /// ```
3430 /// use std::sync::Arc;
3431 ///
3432 /// let five = Arc::new(5);
3433 ///
3434 /// assert!(five < Arc::new(6));
3435 /// ```
3436 fn lt(&self, other: &Arc<T, A>) -> bool {
3437 *(*self) < *(*other)
3438 }
3439
3440 /// 'Less than or equal to' comparison for two `Arc`s.
3441 ///
3442 /// The two are compared by calling `<=` on their inner values.
3443 ///
3444 /// # Examples
3445 ///
3446 /// ```
3447 /// use std::sync::Arc;
3448 ///
3449 /// let five = Arc::new(5);
3450 ///
3451 /// assert!(five <= Arc::new(5));
3452 /// ```
3453 fn le(&self, other: &Arc<T, A>) -> bool {
3454 *(*self) <= *(*other)
3455 }
3456
3457 /// Greater-than comparison for two `Arc`s.
3458 ///
3459 /// The two are compared by calling `>` on their inner values.
3460 ///
3461 /// # Examples
3462 ///
3463 /// ```
3464 /// use std::sync::Arc;
3465 ///
3466 /// let five = Arc::new(5);
3467 ///
3468 /// assert!(five > Arc::new(4));
3469 /// ```
3470 fn gt(&self, other: &Arc<T, A>) -> bool {
3471 *(*self) > *(*other)
3472 }
3473
3474 /// 'Greater than or equal to' comparison for two `Arc`s.
3475 ///
3476 /// The two are compared by calling `>=` on their inner values.
3477 ///
3478 /// # Examples
3479 ///
3480 /// ```
3481 /// use std::sync::Arc;
3482 ///
3483 /// let five = Arc::new(5);
3484 ///
3485 /// assert!(five >= Arc::new(5));
3486 /// ```
3487 fn ge(&self, other: &Arc<T, A>) -> bool {
3488 *(*self) >= *(*other)
3489 }
3490}
3491#[stable(feature = "rust1", since = "1.0.0")]
3492impl<T: ?Sized + Ord, A: Allocator> Ord for Arc<T, A> {
3493 /// Comparison for two `Arc`s.
3494 ///
3495 /// The two are compared by calling `cmp()` on their inner values.
3496 ///
3497 /// # Examples
3498 ///
3499 /// ```
3500 /// use std::sync::Arc;
3501 /// use std::cmp::Ordering;
3502 ///
3503 /// let five = Arc::new(5);
3504 ///
3505 /// assert_eq!(Ordering::Less, five.cmp(&Arc::new(6)));
3506 /// ```
3507 fn cmp(&self, other: &Arc<T, A>) -> Ordering {
3508 (**self).cmp(&**other)
3509 }
3510}
3511#[stable(feature = "rust1", since = "1.0.0")]
3512impl<T: ?Sized + Eq, A: Allocator> Eq for Arc<T, A> {}
3513
3514#[stable(feature = "rust1", since = "1.0.0")]
3515impl<T: ?Sized + fmt::Display, A: Allocator> fmt::Display for Arc<T, A> {
3516 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
3517 fmt::Display::fmt(&**self, f)
3518 }
3519}
3520
3521#[stable(feature = "rust1", since = "1.0.0")]
3522impl<T: ?Sized + fmt::Debug, A: Allocator> fmt::Debug for Arc<T, A> {
3523 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
3524 fmt::Debug::fmt(&**self, f)
3525 }
3526}
3527
3528#[stable(feature = "rust1", since = "1.0.0")]
3529impl<T: ?Sized, A: Allocator> fmt::Pointer for Arc<T, A> {
3530 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
3531 fmt::Pointer::fmt(&(&raw const **self), f)
3532 }
3533}
3534
3535#[cfg(not(no_global_oom_handling))]
3536#[stable(feature = "rust1", since = "1.0.0")]
3537impl<T: Default> Default for Arc<T> {
3538 /// Creates a new `Arc<T>`, with the `Default` value for `T`.
3539 ///
3540 /// # Examples
3541 ///
3542 /// ```
3543 /// use std::sync::Arc;
3544 ///
3545 /// let x: Arc<i32> = Default::default();
3546 /// assert_eq!(*x, 0);
3547 /// ```
3548 fn default() -> Arc<T> {
3549 unsafe {
3550 Self::from_inner(
3551 Box::leak(Box::write(
3552 Box::new_uninit(),
3553 ArcInner {
3554 strong: atomic::AtomicUsize::new(1),
3555 weak: atomic::AtomicUsize::new(1),
3556 data: T::default(),
3557 },
3558 ))
3559 .into(),
3560 )
3561 }
3562 }
3563}
3564
3565/// Struct to hold the static `ArcInner` used for empty `Arc<str/CStr/[T]>` as
3566/// returned by `Default::default`.
3567///
3568/// Layout notes:
3569/// * `repr(align(16))` so we can use it for `[T]` with `align_of::<T>() <= 16`.
3570/// * `repr(C)` so `inner` is at offset 0 (and thus guaranteed to actually be aligned to 16).
3571/// * `[u8; 1]` (to be initialized with 0) so it can be used for `Arc<CStr>`.
3572#[repr(C, align(16))]
3573struct SliceArcInnerForStatic {
3574 inner: ArcInner<[u8; 1]>,
3575}
3576#[cfg(not(no_global_oom_handling))]
3577const MAX_STATIC_INNER_SLICE_ALIGNMENT: usize = 16;
3578
3579static STATIC_INNER_SLICE: SliceArcInnerForStatic = SliceArcInnerForStatic {
3580 inner: ArcInner {
3581 strong: atomic::AtomicUsize::new(1),
3582 weak: atomic::AtomicUsize::new(1),
3583 data: [0],
3584 },
3585};
3586
3587#[cfg(not(no_global_oom_handling))]
3588#[stable(feature = "more_rc_default_impls", since = "1.80.0")]
3589impl Default for Arc<str> {
3590 /// Creates an empty str inside an Arc
3591 ///
3592 /// This may or may not share an allocation with other Arcs.
3593 #[inline]
3594 fn default() -> Self {
3595 let arc: Arc<[u8]> = Default::default();
3596 debug_assert!(core::str::from_utf8(&*arc).is_ok());
3597 let (ptr, alloc) = Arc::into_inner_with_allocator(arc);
3598 unsafe { Arc::from_ptr_in(ptr.as_ptr() as *mut ArcInner<str>, alloc) }
3599 }
3600}
3601
3602#[cfg(not(no_global_oom_handling))]
3603#[stable(feature = "more_rc_default_impls", since = "1.80.0")]
3604impl Default for Arc<core::ffi::CStr> {
3605 /// Creates an empty CStr inside an Arc
3606 ///
3607 /// This may or may not share an allocation with other Arcs.
3608 #[inline]
3609 fn default() -> Self {
3610 use core::ffi::CStr;
3611 let inner: NonNull<ArcInner<[u8]>> = NonNull::from(&STATIC_INNER_SLICE.inner);
3612 let inner: NonNull<ArcInner<CStr>> =
3613 NonNull::new(inner.as_ptr() as *mut ArcInner<CStr>).unwrap();
3614 // `this` semantically is the Arc "owned" by the static, so make sure not to drop it.
3615 let this: mem::ManuallyDrop<Arc<CStr>> =
3616 unsafe { mem::ManuallyDrop::new(Arc::from_inner(inner)) };
3617 (*this).clone()
3618 }
3619}
3620
3621#[cfg(not(no_global_oom_handling))]
3622#[stable(feature = "more_rc_default_impls", since = "1.80.0")]
3623impl<T> Default for Arc<[T]> {
3624 /// Creates an empty `[T]` inside an Arc
3625 ///
3626 /// This may or may not share an allocation with other Arcs.
3627 #[inline]
3628 fn default() -> Self {
3629 if align_of::<T>() <= MAX_STATIC_INNER_SLICE_ALIGNMENT {
3630 // We take a reference to the whole struct instead of the ArcInner<[u8; 1]> inside it so
3631 // we don't shrink the range of bytes the ptr is allowed to access under Stacked Borrows.
3632 // (Miri complains on 32-bit targets with Arc<[Align16]> otherwise.)
3633 // (Note that NonNull::from(&STATIC_INNER_SLICE.inner) is fine under Tree Borrows.)
3634 let inner: NonNull<SliceArcInnerForStatic> = NonNull::from(&STATIC_INNER_SLICE);
3635 let inner: NonNull<ArcInner<[T; 0]>> = inner.cast();
3636 // `this` semantically is the Arc "owned" by the static, so make sure not to drop it.
3637 let this: mem::ManuallyDrop<Arc<[T; 0]>> =
3638 unsafe { mem::ManuallyDrop::new(Arc::from_inner(inner)) };
3639 return (*this).clone();
3640 }
3641
3642 // If T's alignment is too large for the static, make a new unique allocation.
3643 let arr: [T; 0] = [];
3644 Arc::from(arr)
3645 }
3646}
3647
3648#[cfg(not(no_global_oom_handling))]
3649#[stable(feature = "pin_default_impls", since = "1.91.0")]
3650impl<T> Default for Pin<Arc<T>>
3651where
3652 T: ?Sized,
3653 Arc<T>: Default,
3654{
3655 #[inline]
3656 fn default() -> Self {
3657 unsafe { Pin::new_unchecked(Arc::<T>::default()) }
3658 }
3659}
3660
3661#[stable(feature = "rust1", since = "1.0.0")]
3662impl<T: ?Sized + Hash, A: Allocator> Hash for Arc<T, A> {
3663 fn hash<H: Hasher>(&self, state: &mut H) {
3664 (**self).hash(state)
3665 }
3666}
3667
3668#[cfg(not(no_global_oom_handling))]
3669#[stable(feature = "from_for_ptrs", since = "1.6.0")]
3670impl<T> From<T> for Arc<T> {
3671 /// Converts a `T` into an `Arc<T>`
3672 ///
3673 /// The conversion moves the value into a
3674 /// newly allocated `Arc`. It is equivalent to
3675 /// calling `Arc::new(t)`.
3676 ///
3677 /// # Example
3678 /// ```rust
3679 /// # use std::sync::Arc;
3680 /// let x = 5;
3681 /// let arc = Arc::new(5);
3682 ///
3683 /// assert_eq!(Arc::from(x), arc);
3684 /// ```
3685 fn from(t: T) -> Self {
3686 Arc::new(t)
3687 }
3688}
3689
3690#[cfg(not(no_global_oom_handling))]
3691#[stable(feature = "shared_from_array", since = "1.74.0")]
3692impl<T, const N: usize> From<[T; N]> for Arc<[T]> {
3693 /// Converts a [`[T; N]`](prim@array) into an `Arc<[T]>`.
3694 ///
3695 /// The conversion moves the array into a newly allocated `Arc`.
3696 ///
3697 /// # Example
3698 ///
3699 /// ```
3700 /// # use std::sync::Arc;
3701 /// let original: [i32; 3] = [1, 2, 3];
3702 /// let shared: Arc<[i32]> = Arc::from(original);
3703 /// assert_eq!(&[1, 2, 3], &shared[..]);
3704 /// ```
3705 #[inline]
3706 fn from(v: [T; N]) -> Arc<[T]> {
3707 Arc::<[T; N]>::from(v)
3708 }
3709}
3710
3711#[cfg(not(no_global_oom_handling))]
3712#[stable(feature = "shared_from_slice", since = "1.21.0")]
3713impl<T: Clone> From<&[T]> for Arc<[T]> {
3714 /// Allocates a reference-counted slice and fills it by cloning `v`'s items.
3715 ///
3716 /// # Example
3717 ///
3718 /// ```
3719 /// # use std::sync::Arc;
3720 /// let original: &[i32] = &[1, 2, 3];
3721 /// let shared: Arc<[i32]> = Arc::from(original);
3722 /// assert_eq!(&[1, 2, 3], &shared[..]);
3723 /// ```
3724 #[inline]
3725 fn from(v: &[T]) -> Arc<[T]> {
3726 <Self as ArcFromSlice<T>>::from_slice(v)
3727 }
3728}
3729
3730#[cfg(not(no_global_oom_handling))]
3731#[stable(feature = "shared_from_mut_slice", since = "1.84.0")]
3732impl<T: Clone> From<&mut [T]> for Arc<[T]> {
3733 /// Allocates a reference-counted slice and fills it by cloning `v`'s items.
3734 ///
3735 /// # Example
3736 ///
3737 /// ```
3738 /// # use std::sync::Arc;
3739 /// let mut original = [1, 2, 3];
3740 /// let original: &mut [i32] = &mut original;
3741 /// let shared: Arc<[i32]> = Arc::from(original);
3742 /// assert_eq!(&[1, 2, 3], &shared[..]);
3743 /// ```
3744 #[inline]
3745 fn from(v: &mut [T]) -> Arc<[T]> {
3746 Arc::from(&*v)
3747 }
3748}
3749
3750#[cfg(not(no_global_oom_handling))]
3751#[stable(feature = "shared_from_slice", since = "1.21.0")]
3752impl From<&str> for Arc<str> {
3753 /// Allocates a reference-counted `str` and copies `v` into it.
3754 ///
3755 /// # Example
3756 ///
3757 /// ```
3758 /// # use std::sync::Arc;
3759 /// let shared: Arc<str> = Arc::from("eggplant");
3760 /// assert_eq!("eggplant", &shared[..]);
3761 /// ```
3762 #[inline]
3763 fn from(v: &str) -> Arc<str> {
3764 let arc = Arc::<[u8]>::from(v.as_bytes());
3765 unsafe { Arc::from_raw(Arc::into_raw(arc) as *const str) }
3766 }
3767}
3768
3769#[cfg(not(no_global_oom_handling))]
3770#[stable(feature = "shared_from_mut_slice", since = "1.84.0")]
3771impl From<&mut str> for Arc<str> {
3772 /// Allocates a reference-counted `str` and copies `v` into it.
3773 ///
3774 /// # Example
3775 ///
3776 /// ```
3777 /// # use std::sync::Arc;
3778 /// let mut original = String::from("eggplant");
3779 /// let original: &mut str = &mut original;
3780 /// let shared: Arc<str> = Arc::from(original);
3781 /// assert_eq!("eggplant", &shared[..]);
3782 /// ```
3783 #[inline]
3784 fn from(v: &mut str) -> Arc<str> {
3785 Arc::from(&*v)
3786 }
3787}
3788
3789#[cfg(not(no_global_oom_handling))]
3790#[stable(feature = "shared_from_slice", since = "1.21.0")]
3791impl From<String> for Arc<str> {
3792 /// Allocates a reference-counted `str` and copies `v` into it.
3793 ///
3794 /// # Example
3795 ///
3796 /// ```
3797 /// # use std::sync::Arc;
3798 /// let unique: String = "eggplant".to_owned();
3799 /// let shared: Arc<str> = Arc::from(unique);
3800 /// assert_eq!("eggplant", &shared[..]);
3801 /// ```
3802 #[inline]
3803 fn from(v: String) -> Arc<str> {
3804 Arc::from(&v[..])
3805 }
3806}
3807
3808#[cfg(not(no_global_oom_handling))]
3809#[stable(feature = "shared_from_slice", since = "1.21.0")]
3810impl<T: ?Sized, A: Allocator> From<Box<T, A>> for Arc<T, A> {
3811 /// Move a boxed object to a new, reference-counted allocation.
3812 ///
3813 /// # Example
3814 ///
3815 /// ```
3816 /// # use std::sync::Arc;
3817 /// let unique: Box<str> = Box::from("eggplant");
3818 /// let shared: Arc<str> = Arc::from(unique);
3819 /// assert_eq!("eggplant", &shared[..]);
3820 /// ```
3821 #[inline]
3822 fn from(v: Box<T, A>) -> Arc<T, A> {
3823 Arc::from_box_in(v)
3824 }
3825}
3826
3827#[cfg(not(no_global_oom_handling))]
3828#[stable(feature = "shared_from_slice", since = "1.21.0")]
3829impl<T, A: Allocator + Clone> From<Vec<T, A>> for Arc<[T], A> {
3830 /// Allocates a reference-counted slice and moves `v`'s items into it.
3831 ///
3832 /// # Example
3833 ///
3834 /// ```
3835 /// # use std::sync::Arc;
3836 /// let unique: Vec<i32> = vec![1, 2, 3];
3837 /// let shared: Arc<[i32]> = Arc::from(unique);
3838 /// assert_eq!(&[1, 2, 3], &shared[..]);
3839 /// ```
3840 #[inline]
3841 fn from(v: Vec<T, A>) -> Arc<[T], A> {
3842 unsafe {
3843 let (vec_ptr, len, cap, alloc) = v.into_raw_parts_with_alloc();
3844
3845 let rc_ptr = Self::allocate_for_slice_in(len, &alloc);
3846 ptr::copy_nonoverlapping(vec_ptr, (&raw mut (*rc_ptr).data) as *mut T, len);
3847
3848 // Create a `Vec<T, &A>` with length 0, to deallocate the buffer
3849 // without dropping its contents or the allocator
3850 let _ = Vec::from_raw_parts_in(vec_ptr, 0, cap, &alloc);
3851
3852 Self::from_ptr_in(rc_ptr, alloc)
3853 }
3854 }
3855}
3856
3857#[stable(feature = "shared_from_cow", since = "1.45.0")]
3858impl<'a, B> From<Cow<'a, B>> for Arc<B>
3859where
3860 B: ToOwned + ?Sized,
3861 Arc<B>: From<&'a B> + From<B::Owned>,
3862{
3863 /// Creates an atomically reference-counted pointer from a clone-on-write
3864 /// pointer by copying its content.
3865 ///
3866 /// # Example
3867 ///
3868 /// ```rust
3869 /// # use std::sync::Arc;
3870 /// # use std::borrow::Cow;
3871 /// let cow: Cow<'_, str> = Cow::Borrowed("eggplant");
3872 /// let shared: Arc<str> = Arc::from(cow);
3873 /// assert_eq!("eggplant", &shared[..]);
3874 /// ```
3875 #[inline]
3876 fn from(cow: Cow<'a, B>) -> Arc<B> {
3877 match cow {
3878 Cow::Borrowed(s) => Arc::from(s),
3879 Cow::Owned(s) => Arc::from(s),
3880 }
3881 }
3882}
3883
3884#[stable(feature = "shared_from_str", since = "1.62.0")]
3885impl From<Arc<str>> for Arc<[u8]> {
3886 /// Converts an atomically reference-counted string slice into a byte slice.
3887 ///
3888 /// # Example
3889 ///
3890 /// ```
3891 /// # use std::sync::Arc;
3892 /// let string: Arc<str> = Arc::from("eggplant");
3893 /// let bytes: Arc<[u8]> = Arc::from(string);
3894 /// assert_eq!("eggplant".as_bytes(), bytes.as_ref());
3895 /// ```
3896 #[inline]
3897 fn from(rc: Arc<str>) -> Self {
3898 // SAFETY: `str` has the same layout as `[u8]`.
3899 unsafe { Arc::from_raw(Arc::into_raw(rc) as *const [u8]) }
3900 }
3901}
3902
3903#[stable(feature = "boxed_slice_try_from", since = "1.43.0")]
3904impl<T, A: Allocator, const N: usize> TryFrom<Arc<[T], A>> for Arc<[T; N], A> {
3905 type Error = Arc<[T], A>;
3906
3907 fn try_from(boxed_slice: Arc<[T], A>) -> Result<Self, Self::Error> {
3908 if boxed_slice.len() == N {
3909 let (ptr, alloc) = Arc::into_inner_with_allocator(boxed_slice);
3910 Ok(unsafe { Arc::from_inner_in(ptr.cast(), alloc) })
3911 } else {
3912 Err(boxed_slice)
3913 }
3914 }
3915}
3916
3917#[cfg(not(no_global_oom_handling))]
3918#[stable(feature = "shared_from_iter", since = "1.37.0")]
3919impl<T> FromIterator<T> for Arc<[T]> {
3920 /// Takes each element in the `Iterator` and collects it into an `Arc<[T]>`.
3921 ///
3922 /// # Performance characteristics
3923 ///
3924 /// ## The general case
3925 ///
3926 /// In the general case, collecting into `Arc<[T]>` is done by first
3927 /// collecting into a `Vec<T>`. That is, when writing the following:
3928 ///
3929 /// ```rust
3930 /// # use std::sync::Arc;
3931 /// let evens: Arc<[u8]> = (0..10).filter(|&x| x % 2 == 0).collect();
3932 /// # assert_eq!(&*evens, &[0, 2, 4, 6, 8]);
3933 /// ```
3934 ///
3935 /// this behaves as if we wrote:
3936 ///
3937 /// ```rust
3938 /// # use std::sync::Arc;
3939 /// let evens: Arc<[u8]> = (0..10).filter(|&x| x % 2 == 0)
3940 /// .collect::<Vec<_>>() // The first set of allocations happens here.
3941 /// .into(); // A second allocation for `Arc<[T]>` happens here.
3942 /// # assert_eq!(&*evens, &[0, 2, 4, 6, 8]);
3943 /// ```
3944 ///
3945 /// This will allocate as many times as needed for constructing the `Vec<T>`
3946 /// and then it will allocate once for turning the `Vec<T>` into the `Arc<[T]>`.
3947 ///
3948 /// ## Iterators of known length
3949 ///
3950 /// When your `Iterator` implements `TrustedLen` and is of an exact size,
3951 /// a single allocation will be made for the `Arc<[T]>`. For example:
3952 ///
3953 /// ```rust
3954 /// # use std::sync::Arc;
3955 /// let evens: Arc<[u8]> = (0..10).collect(); // Just a single allocation happens here.
3956 /// # assert_eq!(&*evens, &*(0..10).collect::<Vec<_>>());
3957 /// ```
3958 fn from_iter<I: IntoIterator<Item = T>>(iter: I) -> Self {
3959 ToArcSlice::to_arc_slice(iter.into_iter())
3960 }
3961}
3962
3963#[cfg(not(no_global_oom_handling))]
3964/// Specialization trait used for collecting into `Arc<[T]>`.
3965trait ToArcSlice<T>: Iterator<Item = T> + Sized {
3966 fn to_arc_slice(self) -> Arc<[T]>;
3967}
3968
3969#[cfg(not(no_global_oom_handling))]
3970impl<T, I: Iterator<Item = T>> ToArcSlice<T> for I {
3971 default fn to_arc_slice(self) -> Arc<[T]> {
3972 self.collect::<Vec<T>>().into()
3973 }
3974}
3975
3976#[cfg(not(no_global_oom_handling))]
3977impl<T, I: iter::TrustedLen<Item = T>> ToArcSlice<T> for I {
3978 fn to_arc_slice(self) -> Arc<[T]> {
3979 // This is the case for a `TrustedLen` iterator.
3980 let (low, high) = self.size_hint();
3981 if let Some(high) = high {
3982 debug_assert_eq!(
3983 low,
3984 high,
3985 "TrustedLen iterator's size hint is not exact: {:?}",
3986 (low, high)
3987 );
3988
3989 unsafe {
3990 // SAFETY: We need to ensure that the iterator has an exact length and we have.
3991 Arc::from_iter_exact(self, low)
3992 }
3993 } else {
3994 // TrustedLen contract guarantees that `upper_bound == None` implies an iterator
3995 // length exceeding `usize::MAX`.
3996 // The default implementation would collect into a vec which would panic.
3997 // Thus we panic here immediately without invoking `Vec` code.
3998 panic!("capacity overflow");
3999 }
4000 }
4001}
4002
4003#[stable(feature = "rust1", since = "1.0.0")]
4004impl<T: ?Sized, A: Allocator> borrow::Borrow<T> for Arc<T, A> {
4005 fn borrow(&self) -> &T {
4006 &**self
4007 }
4008}
4009
4010#[stable(since = "1.5.0", feature = "smart_ptr_as_ref")]
4011impl<T: ?Sized, A: Allocator> AsRef<T> for Arc<T, A> {
4012 fn as_ref(&self) -> &T {
4013 &**self
4014 }
4015}
4016
4017#[stable(feature = "pin", since = "1.33.0")]
4018impl<T: ?Sized, A: Allocator> Unpin for Arc<T, A> {}
4019
4020/// Gets the offset within an `ArcInner` for the payload behind a pointer.
4021///
4022/// # Safety
4023///
4024/// The pointer must point to (and have valid metadata for) a previously
4025/// valid instance of T, but the T is allowed to be dropped.
4026unsafe fn data_offset<T: ?Sized>(ptr: *const T) -> usize {
4027 // Align the unsized value to the end of the ArcInner.
4028 // Because ArcInner is repr(C), it will always be the last field in memory.
4029 // SAFETY: since the only unsized types possible are slices, trait objects,
4030 // and extern types, the input safety requirement is currently enough to
4031 // satisfy the requirements of align_of_val_raw; this is an implementation
4032 // detail of the language that must not be relied upon outside of std.
4033 unsafe { data_offset_align(align_of_val_raw(ptr)) }
4034}
4035
4036#[inline]
4037fn data_offset_align(align: usize) -> usize {
4038 let layout = Layout::new::<ArcInner<()>>();
4039 layout.size() + layout.padding_needed_for(align)
4040}
4041
4042/// A unique owning pointer to an [`ArcInner`] **that does not imply the contents are initialized,**
4043/// but will deallocate it (without dropping the value) when dropped.
4044///
4045/// This is a helper for [`Arc::make_mut()`] to ensure correct cleanup on panic.
4046#[cfg(not(no_global_oom_handling))]
4047struct UniqueArcUninit<T: ?Sized, A: Allocator> {
4048 ptr: NonNull<ArcInner<T>>,
4049 layout_for_value: Layout,
4050 alloc: Option<A>,
4051}
4052
4053#[cfg(not(no_global_oom_handling))]
4054impl<T: ?Sized, A: Allocator> UniqueArcUninit<T, A> {
4055 /// Allocates an ArcInner with layout suitable to contain `for_value` or a clone of it.
4056 fn new(for_value: &T, alloc: A) -> UniqueArcUninit<T, A> {
4057 let layout = Layout::for_value(for_value);
4058 let ptr = unsafe {
4059 Arc::allocate_for_layout(
4060 layout,
4061 |layout_for_arcinner| alloc.allocate(layout_for_arcinner),
4062 |mem| mem.with_metadata_of(ptr::from_ref(for_value) as *const ArcInner<T>),
4063 )
4064 };
4065 Self { ptr: NonNull::new(ptr).unwrap(), layout_for_value: layout, alloc: Some(alloc) }
4066 }
4067
4068 /// Returns the pointer to be written into to initialize the [`Arc`].
4069 fn data_ptr(&mut self) -> *mut T {
4070 let offset = data_offset_align(self.layout_for_value.align());
4071 unsafe { self.ptr.as_ptr().byte_add(offset) as *mut T }
4072 }
4073
4074 /// Upgrade this into a normal [`Arc`].
4075 ///
4076 /// # Safety
4077 ///
4078 /// The data must have been initialized (by writing to [`Self::data_ptr()`]).
4079 unsafe fn into_arc(self) -> Arc<T, A> {
4080 let mut this = ManuallyDrop::new(self);
4081 let ptr = this.ptr.as_ptr();
4082 let alloc = this.alloc.take().unwrap();
4083
4084 // SAFETY: The pointer is valid as per `UniqueArcUninit::new`, and the caller is responsible
4085 // for having initialized the data.
4086 unsafe { Arc::from_ptr_in(ptr, alloc) }
4087 }
4088}
4089
4090#[cfg(not(no_global_oom_handling))]
4091impl<T: ?Sized, A: Allocator> Drop for UniqueArcUninit<T, A> {
4092 fn drop(&mut self) {
4093 // SAFETY:
4094 // * new() produced a pointer safe to deallocate.
4095 // * We own the pointer unless into_arc() was called, which forgets us.
4096 unsafe {
4097 self.alloc.take().unwrap().deallocate(
4098 self.ptr.cast(),
4099 arcinner_layout_for_value_layout(self.layout_for_value),
4100 );
4101 }
4102 }
4103}
4104
4105#[stable(feature = "arc_error", since = "1.52.0")]
4106impl<T: core::error::Error + ?Sized> core::error::Error for Arc<T> {
4107 #[allow(deprecated)]
4108 fn cause(&self) -> Option<&dyn core::error::Error> {
4109 core::error::Error::cause(&**self)
4110 }
4111
4112 fn source(&self) -> Option<&(dyn core::error::Error + 'static)> {
4113 core::error::Error::source(&**self)
4114 }
4115
4116 fn provide<'a>(&'a self, req: &mut core::error::Request<'a>) {
4117 core::error::Error::provide(&**self, req);
4118 }
4119}
4120
4121/// A uniquely owned [`Arc`].
4122///
4123/// This represents an `Arc` that is known to be uniquely owned -- that is, have exactly one strong
4124/// reference. Multiple weak pointers can be created, but attempts to upgrade those to strong
4125/// references will fail unless the `UniqueArc` they point to has been converted into a regular `Arc`.
4126///
4127/// Because it is uniquely owned, the contents of a `UniqueArc` can be freely mutated. A common
4128/// use case is to have an object be mutable during its initialization phase but then have it become
4129/// immutable and converted to a normal `Arc`.
4130///
4131/// This can be used as a flexible way to create cyclic data structures, as in the example below.
4132///
4133/// ```
4134/// #![feature(unique_rc_arc)]
4135/// use std::sync::{Arc, Weak, UniqueArc};
4136///
4137/// struct Gadget {
4138/// me: Weak<Gadget>,
4139/// }
4140///
4141/// fn create_gadget() -> Option<Arc<Gadget>> {
4142/// let mut rc = UniqueArc::new(Gadget {
4143/// me: Weak::new(),
4144/// });
4145/// rc.me = UniqueArc::downgrade(&rc);
4146/// Some(UniqueArc::into_arc(rc))
4147/// }
4148///
4149/// create_gadget().unwrap();
4150/// ```
4151///
4152/// An advantage of using `UniqueArc` over [`Arc::new_cyclic`] to build cyclic data structures is that
4153/// [`Arc::new_cyclic`]'s `data_fn` parameter cannot be async or return a [`Result`]. As shown in the
4154/// previous example, `UniqueArc` allows for more flexibility in the construction of cyclic data,
4155/// including fallible or async constructors.
4156#[unstable(feature = "unique_rc_arc", issue = "112566")]
4157pub struct UniqueArc<
4158 T: ?Sized,
4159 #[unstable(feature = "allocator_api", issue = "32838")] A: Allocator = Global,
4160> {
4161 ptr: NonNull<ArcInner<T>>,
4162 // Define the ownership of `ArcInner<T>` for drop-check
4163 _marker: PhantomData<ArcInner<T>>,
4164 // Invariance is necessary for soundness: once other `Weak`
4165 // references exist, we already have a form of shared mutability!
4166 _marker2: PhantomData<*mut T>,
4167 alloc: A,
4168}
4169
4170#[unstable(feature = "unique_rc_arc", issue = "112566")]
4171unsafe impl<T: ?Sized + Sync + Send, A: Allocator + Send> Send for UniqueArc<T, A> {}
4172
4173#[unstable(feature = "unique_rc_arc", issue = "112566")]
4174unsafe impl<T: ?Sized + Sync + Send, A: Allocator + Sync> Sync for UniqueArc<T, A> {}
4175
4176#[unstable(feature = "unique_rc_arc", issue = "112566")]
4177// #[unstable(feature = "coerce_unsized", issue = "18598")]
4178impl<T: ?Sized + Unsize<U>, U: ?Sized, A: Allocator> CoerceUnsized<UniqueArc<U, A>>
4179 for UniqueArc<T, A>
4180{
4181}
4182
4183//#[unstable(feature = "unique_rc_arc", issue = "112566")]
4184#[unstable(feature = "dispatch_from_dyn", issue = "none")]
4185impl<T: ?Sized + Unsize<U>, U: ?Sized> DispatchFromDyn<UniqueArc<U>> for UniqueArc<T> {}
4186
4187#[unstable(feature = "unique_rc_arc", issue = "112566")]
4188impl<T: ?Sized + fmt::Display, A: Allocator> fmt::Display for UniqueArc<T, A> {
4189 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
4190 fmt::Display::fmt(&**self, f)
4191 }
4192}
4193
4194#[unstable(feature = "unique_rc_arc", issue = "112566")]
4195impl<T: ?Sized + fmt::Debug, A: Allocator> fmt::Debug for UniqueArc<T, A> {
4196 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
4197 fmt::Debug::fmt(&**self, f)
4198 }
4199}
4200
4201#[unstable(feature = "unique_rc_arc", issue = "112566")]
4202impl<T: ?Sized, A: Allocator> fmt::Pointer for UniqueArc<T, A> {
4203 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
4204 fmt::Pointer::fmt(&(&raw const **self), f)
4205 }
4206}
4207
4208#[unstable(feature = "unique_rc_arc", issue = "112566")]
4209impl<T: ?Sized, A: Allocator> borrow::Borrow<T> for UniqueArc<T, A> {
4210 fn borrow(&self) -> &T {
4211 &**self
4212 }
4213}
4214
4215#[unstable(feature = "unique_rc_arc", issue = "112566")]
4216impl<T: ?Sized, A: Allocator> borrow::BorrowMut<T> for UniqueArc<T, A> {
4217 fn borrow_mut(&mut self) -> &mut T {
4218 &mut **self
4219 }
4220}
4221
4222#[unstable(feature = "unique_rc_arc", issue = "112566")]
4223impl<T: ?Sized, A: Allocator> AsRef<T> for UniqueArc<T, A> {
4224 fn as_ref(&self) -> &T {
4225 &**self
4226 }
4227}
4228
4229#[unstable(feature = "unique_rc_arc", issue = "112566")]
4230impl<T: ?Sized, A: Allocator> AsMut<T> for UniqueArc<T, A> {
4231 fn as_mut(&mut self) -> &mut T {
4232 &mut **self
4233 }
4234}
4235
4236#[unstable(feature = "unique_rc_arc", issue = "112566")]
4237impl<T: ?Sized, A: Allocator> Unpin for UniqueArc<T, A> {}
4238
4239#[unstable(feature = "unique_rc_arc", issue = "112566")]
4240impl<T: ?Sized + PartialEq, A: Allocator> PartialEq for UniqueArc<T, A> {
4241 /// Equality for two `UniqueArc`s.
4242 ///
4243 /// Two `UniqueArc`s are equal if their inner values are equal.
4244 ///
4245 /// # Examples
4246 ///
4247 /// ```
4248 /// #![feature(unique_rc_arc)]
4249 /// use std::sync::UniqueArc;
4250 ///
4251 /// let five = UniqueArc::new(5);
4252 ///
4253 /// assert!(five == UniqueArc::new(5));
4254 /// ```
4255 #[inline]
4256 fn eq(&self, other: &Self) -> bool {
4257 PartialEq::eq(&**self, &**other)
4258 }
4259}
4260
4261#[unstable(feature = "unique_rc_arc", issue = "112566")]
4262impl<T: ?Sized + PartialOrd, A: Allocator> PartialOrd for UniqueArc<T, A> {
4263 /// Partial comparison for two `UniqueArc`s.
4264 ///
4265 /// The two are compared by calling `partial_cmp()` on their inner values.
4266 ///
4267 /// # Examples
4268 ///
4269 /// ```
4270 /// #![feature(unique_rc_arc)]
4271 /// use std::sync::UniqueArc;
4272 /// use std::cmp::Ordering;
4273 ///
4274 /// let five = UniqueArc::new(5);
4275 ///
4276 /// assert_eq!(Some(Ordering::Less), five.partial_cmp(&UniqueArc::new(6)));
4277 /// ```
4278 #[inline(always)]
4279 fn partial_cmp(&self, other: &UniqueArc<T, A>) -> Option<Ordering> {
4280 (**self).partial_cmp(&**other)
4281 }
4282
4283 /// Less-than comparison for two `UniqueArc`s.
4284 ///
4285 /// The two are compared by calling `<` on their inner values.
4286 ///
4287 /// # Examples
4288 ///
4289 /// ```
4290 /// #![feature(unique_rc_arc)]
4291 /// use std::sync::UniqueArc;
4292 ///
4293 /// let five = UniqueArc::new(5);
4294 ///
4295 /// assert!(five < UniqueArc::new(6));
4296 /// ```
4297 #[inline(always)]
4298 fn lt(&self, other: &UniqueArc<T, A>) -> bool {
4299 **self < **other
4300 }
4301
4302 /// 'Less than or equal to' comparison for two `UniqueArc`s.
4303 ///
4304 /// The two are compared by calling `<=` on their inner values.
4305 ///
4306 /// # Examples
4307 ///
4308 /// ```
4309 /// #![feature(unique_rc_arc)]
4310 /// use std::sync::UniqueArc;
4311 ///
4312 /// let five = UniqueArc::new(5);
4313 ///
4314 /// assert!(five <= UniqueArc::new(5));
4315 /// ```
4316 #[inline(always)]
4317 fn le(&self, other: &UniqueArc<T, A>) -> bool {
4318 **self <= **other
4319 }
4320
4321 /// Greater-than comparison for two `UniqueArc`s.
4322 ///
4323 /// The two are compared by calling `>` on their inner values.
4324 ///
4325 /// # Examples
4326 ///
4327 /// ```
4328 /// #![feature(unique_rc_arc)]
4329 /// use std::sync::UniqueArc;
4330 ///
4331 /// let five = UniqueArc::new(5);
4332 ///
4333 /// assert!(five > UniqueArc::new(4));
4334 /// ```
4335 #[inline(always)]
4336 fn gt(&self, other: &UniqueArc<T, A>) -> bool {
4337 **self > **other
4338 }
4339
4340 /// 'Greater than or equal to' comparison for two `UniqueArc`s.
4341 ///
4342 /// The two are compared by calling `>=` on their inner values.
4343 ///
4344 /// # Examples
4345 ///
4346 /// ```
4347 /// #![feature(unique_rc_arc)]
4348 /// use std::sync::UniqueArc;
4349 ///
4350 /// let five = UniqueArc::new(5);
4351 ///
4352 /// assert!(five >= UniqueArc::new(5));
4353 /// ```
4354 #[inline(always)]
4355 fn ge(&self, other: &UniqueArc<T, A>) -> bool {
4356 **self >= **other
4357 }
4358}
4359
4360#[unstable(feature = "unique_rc_arc", issue = "112566")]
4361impl<T: ?Sized + Ord, A: Allocator> Ord for UniqueArc<T, A> {
4362 /// Comparison for two `UniqueArc`s.
4363 ///
4364 /// The two are compared by calling `cmp()` on their inner values.
4365 ///
4366 /// # Examples
4367 ///
4368 /// ```
4369 /// #![feature(unique_rc_arc)]
4370 /// use std::sync::UniqueArc;
4371 /// use std::cmp::Ordering;
4372 ///
4373 /// let five = UniqueArc::new(5);
4374 ///
4375 /// assert_eq!(Ordering::Less, five.cmp(&UniqueArc::new(6)));
4376 /// ```
4377 #[inline]
4378 fn cmp(&self, other: &UniqueArc<T, A>) -> Ordering {
4379 (**self).cmp(&**other)
4380 }
4381}
4382
4383#[unstable(feature = "unique_rc_arc", issue = "112566")]
4384impl<T: ?Sized + Eq, A: Allocator> Eq for UniqueArc<T, A> {}
4385
4386#[unstable(feature = "unique_rc_arc", issue = "112566")]
4387impl<T: ?Sized + Hash, A: Allocator> Hash for UniqueArc<T, A> {
4388 fn hash<H: Hasher>(&self, state: &mut H) {
4389 (**self).hash(state);
4390 }
4391}
4392
4393impl<T> UniqueArc<T, Global> {
4394 /// Creates a new `UniqueArc`.
4395 ///
4396 /// Weak references to this `UniqueArc` can be created with [`UniqueArc::downgrade`]. Upgrading
4397 /// these weak references will fail before the `UniqueArc` has been converted into an [`Arc`].
4398 /// After converting the `UniqueArc` into an [`Arc`], any weak references created beforehand will
4399 /// point to the new [`Arc`].
4400 #[cfg(not(no_global_oom_handling))]
4401 #[unstable(feature = "unique_rc_arc", issue = "112566")]
4402 #[must_use]
4403 pub fn new(value: T) -> Self {
4404 Self::new_in(value, Global)
4405 }
4406}
4407
4408impl<T, A: Allocator> UniqueArc<T, A> {
4409 /// Creates a new `UniqueArc` in the provided allocator.
4410 ///
4411 /// Weak references to this `UniqueArc` can be created with [`UniqueArc::downgrade`]. Upgrading
4412 /// these weak references will fail before the `UniqueArc` has been converted into an [`Arc`].
4413 /// After converting the `UniqueArc` into an [`Arc`], any weak references created beforehand will
4414 /// point to the new [`Arc`].
4415 #[cfg(not(no_global_oom_handling))]
4416 #[unstable(feature = "unique_rc_arc", issue = "112566")]
4417 #[must_use]
4418 // #[unstable(feature = "allocator_api", issue = "32838")]
4419 pub fn new_in(data: T, alloc: A) -> Self {
4420 let (ptr, alloc) = Box::into_unique(Box::new_in(
4421 ArcInner {
4422 strong: atomic::AtomicUsize::new(0),
4423 // keep one weak reference so if all the weak pointers that are created are dropped
4424 // the UniqueArc still stays valid.
4425 weak: atomic::AtomicUsize::new(1),
4426 data,
4427 },
4428 alloc,
4429 ));
4430 Self { ptr: ptr.into(), _marker: PhantomData, _marker2: PhantomData, alloc }
4431 }
4432}
4433
4434impl<T: ?Sized, A: Allocator> UniqueArc<T, A> {
4435 /// Converts the `UniqueArc` into a regular [`Arc`].
4436 ///
4437 /// This consumes the `UniqueArc` and returns a regular [`Arc`] that contains the `value` that
4438 /// is passed to `into_arc`.
4439 ///
4440 /// Any weak references created before this method is called can now be upgraded to strong
4441 /// references.
4442 #[unstable(feature = "unique_rc_arc", issue = "112566")]
4443 #[must_use]
4444 pub fn into_arc(this: Self) -> Arc<T, A> {
4445 let this = ManuallyDrop::new(this);
4446
4447 // Move the allocator out.
4448 // SAFETY: `this.alloc` will not be accessed again, nor dropped because it is in
4449 // a `ManuallyDrop`.
4450 let alloc: A = unsafe { ptr::read(&this.alloc) };
4451
4452 // SAFETY: This pointer was allocated at creation time so we know it is valid.
4453 unsafe {
4454 // Convert our weak reference into a strong reference
4455 (*this.ptr.as_ptr()).strong.store(1, Release);
4456 Arc::from_inner_in(this.ptr, alloc)
4457 }
4458 }
4459}
4460
4461impl<T: ?Sized, A: Allocator + Clone> UniqueArc<T, A> {
4462 /// Creates a new weak reference to the `UniqueArc`.
4463 ///
4464 /// Attempting to upgrade this weak reference will fail before the `UniqueArc` has been converted
4465 /// to a [`Arc`] using [`UniqueArc::into_arc`].
4466 #[unstable(feature = "unique_rc_arc", issue = "112566")]
4467 #[must_use]
4468 pub fn downgrade(this: &Self) -> Weak<T, A> {
4469 // Using a relaxed ordering is alright here, as knowledge of the
4470 // original reference prevents other threads from erroneously deleting
4471 // the object or converting the object to a normal `Arc<T, A>`.
4472 //
4473 // Note that we don't need to test if the weak counter is locked because there
4474 // are no such operations like `Arc::get_mut` or `Arc::make_mut` that will lock
4475 // the weak counter.
4476 //
4477 // SAFETY: This pointer was allocated at creation time so we know it is valid.
4478 let old_size = unsafe { (*this.ptr.as_ptr()).weak.fetch_add(1, Relaxed) };
4479
4480 // See comments in Arc::clone() for why we do this (for mem::forget).
4481 if old_size > MAX_REFCOUNT {
4482 abort();
4483 }
4484
4485 Weak { ptr: this.ptr, alloc: this.alloc.clone() }
4486 }
4487}
4488
4489#[unstable(feature = "unique_rc_arc", issue = "112566")]
4490impl<T: ?Sized, A: Allocator> Deref for UniqueArc<T, A> {
4491 type Target = T;
4492
4493 fn deref(&self) -> &T {
4494 // SAFETY: This pointer was allocated at creation time so we know it is valid.
4495 unsafe { &self.ptr.as_ref().data }
4496 }
4497}
4498
4499// #[unstable(feature = "unique_rc_arc", issue = "112566")]
4500#[unstable(feature = "pin_coerce_unsized_trait", issue = "123430")]
4501unsafe impl<T: ?Sized> PinCoerceUnsized for UniqueArc<T> {}
4502
4503#[unstable(feature = "unique_rc_arc", issue = "112566")]
4504impl<T: ?Sized, A: Allocator> DerefMut for UniqueArc<T, A> {
4505 fn deref_mut(&mut self) -> &mut T {
4506 // SAFETY: This pointer was allocated at creation time so we know it is valid. We know we
4507 // have unique ownership and therefore it's safe to make a mutable reference because
4508 // `UniqueArc` owns the only strong reference to itself.
4509 // We also need to be careful to only create a mutable reference to the `data` field,
4510 // as a mutable reference to the entire `ArcInner` would assert uniqueness over the
4511 // ref count fields too, invalidating any attempt by `Weak`s to access the ref count.
4512 unsafe { &mut (*self.ptr.as_ptr()).data }
4513 }
4514}
4515
4516#[unstable(feature = "unique_rc_arc", issue = "112566")]
4517// #[unstable(feature = "deref_pure_trait", issue = "87121")]
4518unsafe impl<T: ?Sized, A: Allocator> DerefPure for UniqueArc<T, A> {}
4519
4520#[unstable(feature = "unique_rc_arc", issue = "112566")]
4521unsafe impl<#[may_dangle] T: ?Sized, A: Allocator> Drop for UniqueArc<T, A> {
4522 fn drop(&mut self) {
4523 // See `Arc::drop_slow` which drops an `Arc` with a strong count of 0.
4524 // SAFETY: This pointer was allocated at creation time so we know it is valid.
4525 let _weak = Weak { ptr: self.ptr, alloc: &self.alloc };
4526
4527 unsafe { ptr::drop_in_place(&mut (*self.ptr.as_ptr()).data) };
4528 }
4529}