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nest-open-source / manifest_repos / toolchain / 6143c57c644e0da7c4a10d1b4af322f92e74e6f7 / . / x86_64 / usr / lib / rustlib / src / rust / library / alloc / src / sync.rs
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#![stable(feature = "rust1", since = "1.0.0")]
//! Thread-safe reference-counting pointers.
//!
//! See the [`Arc<T>`][Arc] documentation for more details.
//!
//! **Note**: This module is only available on platforms that support atomic
//! loads and stores of pointers. This may be detected at compile time using
//! `#[cfg(target_has_atomic = "ptr")]`.
use core::any::Any;
use core::borrow;
use core::cmp::Ordering;
use core::convert::{From, TryFrom};
use core::fmt;
use core::hash::{Hash, Hasher};
use core::hint;
use core::intrinsics::abort;
#[cfg(not(no_global_oom_handling))]
use core::iter;
use core::marker::{PhantomData, Unpin, Unsize};
#[cfg(not(no_global_oom_handling))]
use core::mem::size_of_val;
use core::mem::{self, align_of_val_raw};
use core::ops::{CoerceUnsized, Deref, DispatchFromDyn, Receiver};
use core::panic::{RefUnwindSafe, UnwindSafe};
use core::pin::Pin;
use core::ptr::{self, NonNull};
#[cfg(not(no_global_oom_handling))]
use core::slice::from_raw_parts_mut;
use core::sync::atomic;
use core::sync::atomic::Ordering::{Acquire, Relaxed, Release};
#[cfg(not(no_global_oom_handling))]
use crate::alloc::handle_alloc_error;
#[cfg(not(no_global_oom_handling))]
use crate::alloc::{box_free, WriteCloneIntoRaw};
use crate::alloc::{AllocError, Allocator, Global, Layout};
use crate::borrow::{Cow, ToOwned};
use crate::boxed::Box;
use crate::rc::is_dangling;
#[cfg(not(no_global_oom_handling))]
use crate::string::String;
#[cfg(not(no_global_oom_handling))]
use crate::vec::Vec;
#[cfg(test)]
mod tests;
/// A soft limit on the amount of references that may be made to an `Arc`.
///
/// Going above this limit will abort your program (although not
/// necessarily) at _exactly_ `MAX_REFCOUNT + 1` references.
const MAX_REFCOUNT: usize = (isize::MAX) as usize;
#[cfg(not(sanitize = "thread"))]
macro_rules! acquire {
($x:expr) => {
atomic::fence(Acquire)
};
}
// ThreadSanitizer does not support memory fences. To avoid false positive
// reports in Arc / Weak implementation use atomic loads for synchronization
// instead.
#[cfg(sanitize = "thread")]
macro_rules! acquire {
($x:expr) => {
$x.load(Acquire)
};
}
/// A thread-safe reference-counting pointer. 'Arc' stands for 'Atomically
/// Reference Counted'.
///
/// The type `Arc<T>` provides shared ownership of a value of type `T`,
/// allocated in the heap. Invoking [`clone`][clone] on `Arc` produces
/// a new `Arc` instance, which points to the same allocation on the heap as the
/// source `Arc`, while increasing a reference count. When the last `Arc`
/// pointer to a given allocation is destroyed, the value stored in that allocation (often
/// referred to as "inner value") is also dropped.
///
/// Shared references in Rust disallow mutation by default, and `Arc` is no
/// exception: you cannot generally obtain a mutable reference to something
/// inside an `Arc`. If you need to mutate through an `Arc`, use
/// [`Mutex`][mutex], [`RwLock`][rwlock], or one of the [`Atomic`][atomic]
/// types.
///
/// **Note**: This type is only available on platforms that support atomic
/// loads and stores of pointers, which includes all platforms that support
/// the `std` crate but not all those which only support [`alloc`](crate).
/// This may be detected at compile time using `#[cfg(target_has_atomic = "ptr")]`.
///
/// ## Thread Safety
///
/// Unlike [`Rc<T>`], `Arc<T>` uses atomic operations for its reference
/// counting. This means that it is thread-safe. The disadvantage is that
/// atomic operations are more expensive than ordinary memory accesses. If you
/// are not sharing reference-counted allocations between threads, consider using
/// [`Rc<T>`] for lower overhead. [`Rc<T>`] is a safe default, because the
/// compiler will catch any attempt to send an [`Rc<T>`] between threads.
/// However, a library might choose `Arc<T>` in order to give library consumers
/// more flexibility.
///
/// `Arc<T>` will implement [`Send`] and [`Sync`] as long as the `T` implements
/// [`Send`] and [`Sync`]. Why can't you put a non-thread-safe type `T` in an
/// `Arc<T>` to make it thread-safe? This may be a bit counter-intuitive at
/// first: after all, isn't the point of `Arc<T>` thread safety? The key is
/// this: `Arc<T>` makes it thread safe to have multiple ownership of the same
/// data, but it doesn't add thread safety to its data. Consider
/// <code>Arc<[RefCell\<T>]></code>. [`RefCell<T>`] isn't [`Sync`], and if `Arc<T>` was always
/// [`Send`], <code>Arc<[RefCell\<T>]></code> would be as well. But then we'd have a problem:
/// [`RefCell<T>`] is not thread safe; it keeps track of the borrowing count using
/// non-atomic operations.
///
/// In the end, this means that you may need to pair `Arc<T>` with some sort of
/// [`std::sync`] type, usually [`Mutex<T>`][mutex].
///
/// ## Breaking cycles with `Weak`
///
/// The [`downgrade`][downgrade] method can be used to create a non-owning
/// [`Weak`] pointer. A [`Weak`] pointer can be [`upgrade`][upgrade]d
/// to an `Arc`, but this will return [`None`] if the value stored in the allocation has
/// already been dropped. In other words, `Weak` pointers do not keep the value
/// inside the allocation alive; however, they *do* keep the allocation
/// (the backing store for the value) alive.
///
/// A cycle between `Arc` pointers will never be deallocated. For this reason,
/// [`Weak`] is used to break cycles. For example, a tree could have
/// strong `Arc` pointers from parent nodes to children, and [`Weak`]
/// pointers from children back to their parents.
///
/// # Cloning references
///
/// Creating a new reference from an existing reference-counted pointer is done using the
/// `Clone` trait implemented for [`Arc<T>`][Arc] and [`Weak<T>`][Weak].
///
/// ```
/// use std::sync::Arc;
/// let foo = Arc::new(vec![1.0, 2.0, 3.0]);
/// // The two syntaxes below are equivalent.
/// let a = foo.clone();
/// let b = Arc::clone(&foo);
/// // a, b, and foo are all Arcs that point to the same memory location
/// ```
///
/// ## `Deref` behavior
///
/// `Arc<T>` automatically dereferences to `T` (via the [`Deref`][deref] trait),
/// so you can call `T`'s methods on a value of type `Arc<T>`. To avoid name
/// clashes with `T`'s methods, the methods of `Arc<T>` itself are associated
/// functions, called using [fully qualified syntax]:
///
/// ```
/// use std::sync::Arc;
///
/// let my_arc = Arc::new(());
/// let my_weak = Arc::downgrade(&my_arc);
/// ```
///
/// `Arc<T>`'s implementations of traits like `Clone` may also be called using
/// fully qualified syntax. Some people prefer to use fully qualified syntax,
/// while others prefer using method-call syntax.
///
/// ```
/// use std::sync::Arc;
///
/// let arc = Arc::new(());
/// // Method-call syntax
/// let arc2 = arc.clone();
/// // Fully qualified syntax
/// let arc3 = Arc::clone(&arc);
/// ```
///
/// [`Weak<T>`][Weak] does not auto-dereference to `T`, because the inner value may have
/// already been dropped.
///
/// [`Rc<T>`]: crate::rc::Rc
/// [clone]: Clone::clone
/// [mutex]: ../../std/sync/struct.Mutex.html
/// [rwlock]: ../../std/sync/struct.RwLock.html
/// [atomic]: core::sync::atomic
/// [`Send`]: core::marker::Send
/// [`Sync`]: core::marker::Sync
/// [deref]: core::ops::Deref
/// [downgrade]: Arc::downgrade
/// [upgrade]: Weak::upgrade
/// [RefCell\<T>]: core::cell::RefCell
/// [`RefCell<T>`]: core::cell::RefCell
/// [`std::sync`]: ../../std/sync/index.html
/// [`Arc::clone(&from)`]: Arc::clone
/// [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
///
/// # Examples
///
/// Sharing some immutable data between threads:
///
// Note that we **do not** run these tests here. The windows builders get super
// unhappy if a thread outlives the main thread and then exits at the same time
// (something deadlocks) so we just avoid this entirely by not running these
// tests.
/// ```no_run
/// use std::sync::Arc;
/// use std::thread;
///
/// let five = Arc::new(5);
///
/// for _ in 0..10 {
/// let five = Arc::clone(&five);
///
/// thread::spawn(move || {
/// println!("{five:?}");
/// });
/// }
/// ```
///
/// Sharing a mutable [`AtomicUsize`]:
///
/// [`AtomicUsize`]: core::sync::atomic::AtomicUsize "sync::atomic::AtomicUsize"
///
/// ```no_run
/// use std::sync::Arc;
/// use std::sync::atomic::{AtomicUsize, Ordering};
/// use std::thread;
///
/// let val = Arc::new(AtomicUsize::new(5));
///
/// for _ in 0..10 {
/// let val = Arc::clone(&val);
///
/// thread::spawn(move || {
/// let v = val.fetch_add(1, Ordering::SeqCst);
/// println!("{v:?}");
/// });
/// }
/// ```
///
/// See the [`rc` documentation][rc_examples] for more examples of reference
/// counting in general.
///
/// [rc_examples]: crate::rc#examples
#[cfg_attr(not(test), rustc_diagnostic_item = "Arc")]
#[stable(feature = "rust1", since = "1.0.0")]
pub struct Arc<T: ?Sized> {
ptr: NonNull<ArcInner<T>>,
phantom: PhantomData<ArcInner<T>>,
}
#[stable(feature = "rust1", since = "1.0.0")]
unsafe impl<T: ?Sized + Sync + Send> Send for Arc<T> {}
#[stable(feature = "rust1", since = "1.0.0")]
unsafe impl<T: ?Sized + Sync + Send> Sync for Arc<T> {}
#[stable(feature = "catch_unwind", since = "1.9.0")]
impl<T: RefUnwindSafe + ?Sized> UnwindSafe for Arc<T> {}
#[unstable(feature = "coerce_unsized", issue = "27732")]
impl<T: ?Sized + Unsize<U>, U: ?Sized> CoerceUnsized<Arc<U>> for Arc<T> {}
#[unstable(feature = "dispatch_from_dyn", issue = "none")]
impl<T: ?Sized + Unsize<U>, U: ?Sized> DispatchFromDyn<Arc<U>> for Arc<T> {}
impl<T: ?Sized> Arc<T> {
unsafe fn from_inner(ptr: NonNull<ArcInner<T>>) -> Self {
Self { ptr, phantom: PhantomData }
}
unsafe fn from_ptr(ptr: *mut ArcInner<T>) -> Self {
unsafe { Self::from_inner(NonNull::new_unchecked(ptr)) }
}
}
/// `Weak` is a version of [`Arc`] that holds a non-owning reference to the
/// managed allocation. The allocation is accessed by calling [`upgrade`] on the `Weak`
/// pointer, which returns an <code>[Option]<[Arc]\<T>></code>.
///
/// Since a `Weak` reference does not count towards ownership, it will not
/// prevent the value stored in the allocation from being dropped, and `Weak` itself makes no
/// guarantees about the value still being present. Thus it may return [`None`]
/// when [`upgrade`]d. Note however that a `Weak` reference *does* prevent the allocation
/// itself (the backing store) from being deallocated.
///
/// A `Weak` pointer is useful for keeping a temporary reference to the allocation
/// managed by [`Arc`] without preventing its inner value from being dropped. It is also used to
/// prevent circular references between [`Arc`] pointers, since mutual owning references
/// would never allow either [`Arc`] to be dropped. For example, a tree could
/// have strong [`Arc`] pointers from parent nodes to children, and `Weak`
/// pointers from children back to their parents.
///
/// The typical way to obtain a `Weak` pointer is to call [`Arc::downgrade`].
///
/// [`upgrade`]: Weak::upgrade
#[stable(feature = "arc_weak", since = "1.4.0")]
pub struct Weak<T: ?Sized> {
// This is a `NonNull` to allow optimizing the size of this type in enums,
// but it is not necessarily a valid pointer.
// `Weak::new` sets this to `usize::MAX` so that it doesn’t need
// to allocate space on the heap. That's not a value a real pointer
// will ever have because RcBox has alignment at least 2.
// This is only possible when `T: Sized`; unsized `T` never dangle.
ptr: NonNull<ArcInner<T>>,
}
#[stable(feature = "arc_weak", since = "1.4.0")]
unsafe impl<T: ?Sized + Sync + Send> Send for Weak<T> {}
#[stable(feature = "arc_weak", since = "1.4.0")]
unsafe impl<T: ?Sized + Sync + Send> Sync for Weak<T> {}
#[unstable(feature = "coerce_unsized", issue = "27732")]
impl<T: ?Sized + Unsize<U>, U: ?Sized> CoerceUnsized<Weak<U>> for Weak<T> {}
#[unstable(feature = "dispatch_from_dyn", issue = "none")]
impl<T: ?Sized + Unsize<U>, U: ?Sized> DispatchFromDyn<Weak<U>> for Weak<T> {}
#[stable(feature = "arc_weak", since = "1.4.0")]
impl<T: ?Sized + fmt::Debug> fmt::Debug for Weak<T> {
fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
write!(f, "(Weak)")
}
}
// This is repr(C) to future-proof against possible field-reordering, which
// would interfere with otherwise safe [into|from]_raw() of transmutable
// inner types.
#[repr(C)]
struct ArcInner<T: ?Sized> {
strong: atomic::AtomicUsize,
// the value usize::MAX acts as a sentinel for temporarily "locking" the
// ability to upgrade weak pointers or downgrade strong ones; this is used
// to avoid races in `make_mut` and `get_mut`.
weak: atomic::AtomicUsize,
data: T,
}
/// Calculate layout for `ArcInner<T>` using the inner value's layout
fn arcinner_layout_for_value_layout(layout: Layout) -> Layout {
// Calculate layout using the given value layout.
// Previously, layout was calculated on the expression
// `&*(ptr as *const ArcInner<T>)`, but this created a misaligned
// reference (see #54908).
Layout::new::<ArcInner<()>>().extend(layout).unwrap().0.pad_to_align()
}
unsafe impl<T: ?Sized + Sync + Send> Send for ArcInner<T> {}
unsafe impl<T: ?Sized + Sync + Send> Sync for ArcInner<T> {}
impl<T> Arc<T> {
/// Constructs a new `Arc<T>`.
///
/// # Examples
///
/// ```
/// use std::sync::Arc;
///
/// let five = Arc::new(5);
/// ```
#[cfg(not(no_global_oom_handling))]
#[inline]
#[stable(feature = "rust1", since = "1.0.0")]
pub fn new(data: T) -> Arc<T> {
// Start the weak pointer count as 1 which is the weak pointer that's
// held by all the strong pointers (kinda), see std/rc.rs for more info
let x: Box<_> = Box::new(ArcInner {
strong: atomic::AtomicUsize::new(1),
weak: atomic::AtomicUsize::new(1),
data,
});
unsafe { Self::from_inner(Box::leak(x).into()) }
}
/// Constructs a new `Arc<T>` while giving you a `Weak<T>` to the allocation,
/// to allow you to construct a `T` which holds a weak pointer to itself.
///
/// Generally, a structure circularly referencing itself, either directly or
/// indirectly, should not hold a strong reference to itself to prevent a memory leak.
/// Using this function, you get access to the weak pointer during the
/// initialization of `T`, before the `Arc<T>` is created, such that you can
/// clone and store it inside the `T`.
///
/// `new_cyclic` first allocates the managed allocation for the `Arc<T>`,
/// then calls your closure, giving it a `Weak<T>` to this allocation,
/// and only afterwards completes the construction of the `Arc<T>` by placing
/// the `T` returned from your closure into the allocation.
///
/// Since the new `Arc<T>` is not fully-constructed until `Arc<T>::new_cyclic`
/// returns, calling [`upgrade`] on the weak reference inside your closure will
/// fail and result in a `None` value.
///
/// # Panics
///
/// If `data_fn` panics, the panic is propagated to the caller, and the
/// temporary [`Weak<T>`] is dropped normally.
///
/// # Example
///
/// ```
/// # #![allow(dead_code)]
/// use std::sync::{Arc, Weak};
///
/// struct Gadget {
/// me: Weak<Gadget>,
/// }
///
/// impl Gadget {
/// /// Construct a reference counted Gadget.
/// fn new() -> Arc<Self> {
/// // `me` is a `Weak<Gadget>` pointing at the new allocation of the
/// // `Arc` we're constructing.
/// Arc::new_cyclic(|me| {
/// // Create the actual struct here.
/// Gadget { me: me.clone() }
/// })
/// }
///
/// /// Return a reference counted pointer to Self.
/// fn me(&self) -> Arc<Self> {
/// self.me.upgrade().unwrap()
/// }
/// }
/// ```
/// [`upgrade`]: Weak::upgrade
#[cfg(not(no_global_oom_handling))]
#[inline]
#[stable(feature = "arc_new_cyclic", since = "1.60.0")]
pub fn new_cyclic<F>(data_fn: F) -> Arc<T>
where
F: FnOnce(&Weak<T>) -> T,
{
// Construct the inner in the "uninitialized" state with a single
// weak reference.
let uninit_ptr: NonNull<_> = Box::leak(Box::new(ArcInner {
strong: atomic::AtomicUsize::new(0),
weak: atomic::AtomicUsize::new(1),
data: mem::MaybeUninit::<T>::uninit(),
}))
.into();
let init_ptr: NonNull<ArcInner<T>> = uninit_ptr.cast();
let weak = Weak { ptr: init_ptr };
// It's important we don't give up ownership of the weak pointer, or
// else the memory might be freed by the time `data_fn` returns. If
// we really wanted to pass ownership, we could create an additional
// weak pointer for ourselves, but this would result in additional
// updates to the weak reference count which might not be necessary
// otherwise.
let data = data_fn(&weak);
// Now we can properly initialize the inner value and turn our weak
// reference into a strong reference.
let strong = unsafe {
let inner = init_ptr.as_ptr();
ptr::write(ptr::addr_of_mut!((*inner).data), data);
// The above write to the data field must be visible to any threads which
// observe a non-zero strong count. Therefore we need at least "Release" ordering
// in order to synchronize with the `compare_exchange_weak` in `Weak::upgrade`.
//
// "Acquire" ordering is not required. When considering the possible behaviours
// of `data_fn` we only need to look at what it could do with a reference to a
// non-upgradeable `Weak`:
// - It can *clone* the `Weak`, increasing the weak reference count.
// - It can drop those clones, decreasing the weak reference count (but never to zero).
//
// These side effects do not impact us in any way, and no other side effects are
// possible with safe code alone.
let prev_value = (*inner).strong.fetch_add(1, Release);
debug_assert_eq!(prev_value, 0, "No prior strong references should exist");
Arc::from_inner(init_ptr)
};
// Strong references should collectively own a shared weak reference,
// so don't run the destructor for our old weak reference.
mem::forget(weak);
strong
}
/// Constructs a new `Arc` with uninitialized contents.
///
/// # Examples
///
/// ```
/// #![feature(new_uninit)]
/// #![feature(get_mut_unchecked)]
///
/// use std::sync::Arc;
///
/// let mut five = Arc::<u32>::new_uninit();
///
/// // Deferred initialization:
/// Arc::get_mut(&mut five).unwrap().write(5);
///
/// let five = unsafe { five.assume_init() };
///
/// assert_eq!(*five, 5)
/// ```
#[cfg(not(no_global_oom_handling))]
#[unstable(feature = "new_uninit", issue = "63291")]
#[must_use]
pub fn new_uninit() -> Arc<mem::MaybeUninit<T>> {
unsafe {
Arc::from_ptr(Arc::allocate_for_layout(
Layout::new::<T>(),
|layout| Global.allocate(layout),
|mem| mem as *mut ArcInner<mem::MaybeUninit<T>>,
))
}
}
/// Constructs a new `Arc` with uninitialized contents, with the memory
/// being filled with `0` bytes.
///
/// See [`MaybeUninit::zeroed`][zeroed] for examples of correct and incorrect usage
/// of this method.
///
/// # Examples
///
/// ```
/// #![feature(new_uninit)]
///
/// use std::sync::Arc;
///
/// let zero = Arc::<u32>::new_zeroed();
/// let zero = unsafe { zero.assume_init() };
///
/// assert_eq!(*zero, 0)
/// ```
///
/// [zeroed]: mem::MaybeUninit::zeroed
#[cfg(not(no_global_oom_handling))]
#[unstable(feature = "new_uninit", issue = "63291")]
#[must_use]
pub fn new_zeroed() -> Arc<mem::MaybeUninit<T>> {
unsafe {
Arc::from_ptr(Arc::allocate_for_layout(
Layout::new::<T>(),
|layout| Global.allocate_zeroed(layout),
|mem| mem as *mut ArcInner<mem::MaybeUninit<T>>,
))
}
}
/// Constructs a new `Pin<Arc<T>>`. If `T` does not implement `Unpin`, then
/// `data` will be pinned in memory and unable to be moved.
#[cfg(not(no_global_oom_handling))]
#[stable(feature = "pin", since = "1.33.0")]
#[must_use]
pub fn pin(data: T) -> Pin<Arc<T>> {
unsafe { Pin::new_unchecked(Arc::new(data)) }
}
/// Constructs a new `Pin<Arc<T>>`, return an error if allocation fails.
#[unstable(feature = "allocator_api", issue = "32838")]
#[inline]
pub fn try_pin(data: T) -> Result<Pin<Arc<T>>, AllocError> {
unsafe { Ok(Pin::new_unchecked(Arc::try_new(data)?)) }
}
/// Constructs a new `Arc<T>`, returning an error if allocation fails.
///
/// # Examples
///
/// ```
/// #![feature(allocator_api)]
/// use std::sync::Arc;
///
/// let five = Arc::try_new(5)?;
/// # Ok::<(), std::alloc::AllocError>(())
/// ```
#[unstable(feature = "allocator_api", issue = "32838")]
#[inline]
pub fn try_new(data: T) -> Result<Arc<T>, AllocError> {
// Start the weak pointer count as 1 which is the weak pointer that's
// held by all the strong pointers (kinda), see std/rc.rs for more info
let x: Box<_> = Box::try_new(ArcInner {
strong: atomic::AtomicUsize::new(1),
weak: atomic::AtomicUsize::new(1),
data,
})?;
unsafe { Ok(Self::from_inner(Box::leak(x).into())) }
}
/// Constructs a new `Arc` with uninitialized contents, returning an error
/// if allocation fails.
///
/// # Examples
///
/// ```
/// #![feature(new_uninit, allocator_api)]
/// #![feature(get_mut_unchecked)]
///
/// use std::sync::Arc;
///
/// let mut five = Arc::<u32>::try_new_uninit()?;
///
/// // Deferred initialization:
/// Arc::get_mut(&mut five).unwrap().write(5);
///
/// let five = unsafe { five.assume_init() };
///
/// assert_eq!(*five, 5);
/// # Ok::<(), std::alloc::AllocError>(())
/// ```
#[unstable(feature = "allocator_api", issue = "32838")]
// #[unstable(feature = "new_uninit", issue = "63291")]
pub fn try_new_uninit() -> Result<Arc<mem::MaybeUninit<T>>, AllocError> {
unsafe {
Ok(Arc::from_ptr(Arc::try_allocate_for_layout(
Layout::new::<T>(),
|layout| Global.allocate(layout),
|mem| mem as *mut ArcInner<mem::MaybeUninit<T>>,
)?))
}
}
/// Constructs a new `Arc` with uninitialized contents, with the memory
/// being filled with `0` bytes, returning an error if allocation fails.
///
/// See [`MaybeUninit::zeroed`][zeroed] for examples of correct and incorrect usage
/// of this method.
///
/// # Examples
///
/// ```
/// #![feature(new_uninit, allocator_api)]
///
/// use std::sync::Arc;
///
/// let zero = Arc::<u32>::try_new_zeroed()?;
/// let zero = unsafe { zero.assume_init() };
///
/// assert_eq!(*zero, 0);
/// # Ok::<(), std::alloc::AllocError>(())
/// ```
///
/// [zeroed]: mem::MaybeUninit::zeroed
#[unstable(feature = "allocator_api", issue = "32838")]
// #[unstable(feature = "new_uninit", issue = "63291")]
pub fn try_new_zeroed() -> Result<Arc<mem::MaybeUninit<T>>, AllocError> {
unsafe {
Ok(Arc::from_ptr(Arc::try_allocate_for_layout(
Layout::new::<T>(),
|layout| Global.allocate_zeroed(layout),
|mem| mem as *mut ArcInner<mem::MaybeUninit<T>>,
)?))
}
}
/// Returns the inner value, if the `Arc` has exactly one strong reference.
///
/// Otherwise, an [`Err`] is returned with the same `Arc` that was
/// passed in.
///
/// This will succeed even if there are outstanding weak references.
///
/// # Examples
///
/// ```
/// use std::sync::Arc;
///
/// let x = Arc::new(3);
/// assert_eq!(Arc::try_unwrap(x), Ok(3));
///
/// let x = Arc::new(4);
/// let _y = Arc::clone(&x);
/// assert_eq!(*Arc::try_unwrap(x).unwrap_err(), 4);
/// ```
#[inline]
#[stable(feature = "arc_unique", since = "1.4.0")]
pub fn try_unwrap(this: Self) -> Result<T, Self> {
if this.inner().strong.compare_exchange(1, 0, Relaxed, Relaxed).is_err() {
return Err(this);
}
acquire!(this.inner().strong);
unsafe {
let elem = ptr::read(&this.ptr.as_ref().data);
// Make a weak pointer to clean up the implicit strong-weak reference
let _weak = Weak { ptr: this.ptr };
mem::forget(this);
Ok(elem)
}
}
}
impl<T> Arc<[T]> {
/// Constructs a new atomically reference-counted slice with uninitialized contents.
///
/// # Examples
///
/// ```
/// #![feature(new_uninit)]
/// #![feature(get_mut_unchecked)]
///
/// use std::sync::Arc;
///
/// let mut values = Arc::<[u32]>::new_uninit_slice(3);
///
/// // Deferred initialization:
/// let data = Arc::get_mut(&mut values).unwrap();
/// data[0].write(1);
/// data[1].write(2);
/// data[2].write(3);
///
/// let values = unsafe { values.assume_init() };
///
/// assert_eq!(*values, [1, 2, 3])
/// ```
#[cfg(not(no_global_oom_handling))]
#[unstable(feature = "new_uninit", issue = "63291")]
#[must_use]
pub fn new_uninit_slice(len: usize) -> Arc<[mem::MaybeUninit<T>]> {
unsafe { Arc::from_ptr(Arc::allocate_for_slice(len)) }
}
/// Constructs a new atomically reference-counted slice with uninitialized contents, with the memory being
/// filled with `0` bytes.
///
/// See [`MaybeUninit::zeroed`][zeroed] for examples of correct and
/// incorrect usage of this method.
///
/// # Examples
///
/// ```
/// #![feature(new_uninit)]
///
/// use std::sync::Arc;
///
/// let values = Arc::<[u32]>::new_zeroed_slice(3);
/// let values = unsafe { values.assume_init() };
///
/// assert_eq!(*values, [0, 0, 0])
/// ```
///
/// [zeroed]: mem::MaybeUninit::zeroed
#[cfg(not(no_global_oom_handling))]
#[unstable(feature = "new_uninit", issue = "63291")]
#[must_use]
pub fn new_zeroed_slice(len: usize) -> Arc<[mem::MaybeUninit<T>]> {
unsafe {
Arc::from_ptr(Arc::allocate_for_layout(
Layout::array::<T>(len).unwrap(),
|layout| Global.allocate_zeroed(layout),
|mem| {
ptr::slice_from_raw_parts_mut(mem as *mut T, len)
as *mut ArcInner<[mem::MaybeUninit<T>]>
},
))
}
}
}
impl<T> Arc<mem::MaybeUninit<T>> {
/// Converts to `Arc<T>`.
///
/// # Safety
///
/// As with [`MaybeUninit::assume_init`],
/// it is up to the caller to guarantee that the inner value
/// really is in an initialized state.
/// Calling this when the content is not yet fully initialized
/// causes immediate undefined behavior.
///
/// [`MaybeUninit::assume_init`]: mem::MaybeUninit::assume_init
///
/// # Examples
///
/// ```
/// #![feature(new_uninit)]
/// #![feature(get_mut_unchecked)]
///
/// use std::sync::Arc;
///
/// let mut five = Arc::<u32>::new_uninit();
///
/// // Deferred initialization:
/// Arc::get_mut(&mut five).unwrap().write(5);
///
/// let five = unsafe { five.assume_init() };
///
/// assert_eq!(*five, 5)
/// ```
#[unstable(feature = "new_uninit", issue = "63291")]
#[must_use = "`self` will be dropped if the result is not used"]
#[inline]
pub unsafe fn assume_init(self) -> Arc<T> {
unsafe { Arc::from_inner(mem::ManuallyDrop::new(self).ptr.cast()) }
}
}
impl<T> Arc<[mem::MaybeUninit<T>]> {
/// Converts to `Arc<[T]>`.
///
/// # Safety
///
/// As with [`MaybeUninit::assume_init`],
/// it is up to the caller to guarantee that the inner value
/// really is in an initialized state.
/// Calling this when the content is not yet fully initialized
/// causes immediate undefined behavior.
///
/// [`MaybeUninit::assume_init`]: mem::MaybeUninit::assume_init
///
/// # Examples
///
/// ```
/// #![feature(new_uninit)]
/// #![feature(get_mut_unchecked)]
///
/// use std::sync::Arc;
///
/// let mut values = Arc::<[u32]>::new_uninit_slice(3);
///
/// // Deferred initialization:
/// let data = Arc::get_mut(&mut values).unwrap();
/// data[0].write(1);
/// data[1].write(2);
/// data[2].write(3);
///
/// let values = unsafe { values.assume_init() };
///
/// assert_eq!(*values, [1, 2, 3])
/// ```
#[unstable(feature = "new_uninit", issue = "63291")]
#[must_use = "`self` will be dropped if the result is not used"]
#[inline]
pub unsafe fn assume_init(self) -> Arc<[T]> {
unsafe { Arc::from_ptr(mem::ManuallyDrop::new(self).ptr.as_ptr() as _) }
}
}
impl<T: ?Sized> Arc<T> {
/// Consumes the `Arc`, returning the wrapped pointer.
///
/// To avoid a memory leak the pointer must be converted back to an `Arc` using
/// [`Arc::from_raw`].
///
/// # Examples
///
/// ```
/// use std::sync::Arc;
///
/// let x = Arc::new("hello".to_owned());
/// let x_ptr = Arc::into_raw(x);
/// assert_eq!(unsafe { &*x_ptr }, "hello");
/// ```
#[must_use = "losing the pointer will leak memory"]
#[stable(feature = "rc_raw", since = "1.17.0")]
pub fn into_raw(this: Self) -> *const T {
let ptr = Self::as_ptr(&this);
mem::forget(this);
ptr
}
/// Provides a raw pointer to the data.
///
/// The counts are not affected in any way and the `Arc` is not consumed. The pointer is valid for
/// as long as there are strong counts in the `Arc`.
///
/// # Examples
///
/// ```
/// use std::sync::Arc;
///
/// let x = Arc::new("hello".to_owned());
/// let y = Arc::clone(&x);
/// let x_ptr = Arc::as_ptr(&x);
/// assert_eq!(x_ptr, Arc::as_ptr(&y));
/// assert_eq!(unsafe { &*x_ptr }, "hello");
/// ```
#[must_use]
#[stable(feature = "rc_as_ptr", since = "1.45.0")]
pub fn as_ptr(this: &Self) -> *const T {
let ptr: *mut ArcInner<T> = NonNull::as_ptr(this.ptr);
// SAFETY: This cannot go through Deref::deref or RcBoxPtr::inner because
// this is required to retain raw/mut provenance such that e.g. `get_mut` can
// write through the pointer after the Rc is recovered through `from_raw`.
unsafe { ptr::addr_of_mut!((*ptr).data) }
}
/// Constructs an `Arc<T>` from a raw pointer.
///
/// The raw pointer must have been previously returned by a call to
/// [`Arc<U>::into_raw`][into_raw] where `U` must have the same size and
/// alignment as `T`. This is trivially true if `U` is `T`.
/// Note that if `U` is not `T` but has the same size and alignment, this is
/// basically like transmuting references of different types. See
/// [`mem::transmute`][transmute] for more information on what
/// restrictions apply in this case.
///
/// The user of `from_raw` has to make sure a specific value of `T` is only
/// dropped once.
///
/// This function is unsafe because improper use may lead to memory unsafety,
/// even if the returned `Arc<T>` is never accessed.
///
/// [into_raw]: Arc::into_raw
/// [transmute]: core::mem::transmute
///
/// # Examples
///
/// ```
/// use std::sync::Arc;
///
/// let x = Arc::new("hello".to_owned());
/// let x_ptr = Arc::into_raw(x);
///
/// unsafe {
/// // Convert back to an `Arc` to prevent leak.
/// let x = Arc::from_raw(x_ptr);
/// assert_eq!(&*x, "hello");
///
/// // Further calls to `Arc::from_raw(x_ptr)` would be memory-unsafe.
/// }
///
/// // The memory was freed when `x` went out of scope above, so `x_ptr` is now dangling!
/// ```
#[stable(feature = "rc_raw", since = "1.17.0")]
pub unsafe fn from_raw(ptr: *const T) -> Self {
unsafe {
let offset = data_offset(ptr);
// Reverse the offset to find the original ArcInner.
let arc_ptr = ptr.byte_sub(offset) as *mut ArcInner<T>;
Self::from_ptr(arc_ptr)
}
}
/// Creates a new [`Weak`] pointer to this allocation.
///
/// # Examples
///
/// ```
/// use std::sync::Arc;
///
/// let five = Arc::new(5);
///
/// let weak_five = Arc::downgrade(&five);
/// ```
#[must_use = "this returns a new `Weak` pointer, \
without modifying the original `Arc`"]
#[stable(feature = "arc_weak", since = "1.4.0")]
pub fn downgrade(this: &Self) -> Weak<T> {
// This Relaxed is OK because we're checking the value in the CAS
// below.
let mut cur = this.inner().weak.load(Relaxed);
loop {
// check if the weak counter is currently "locked"; if so, spin.
if cur == usize::MAX {
hint::spin_loop();
cur = this.inner().weak.load(Relaxed);
continue;
}
// NOTE: this code currently ignores the possibility of overflow
// into usize::MAX; in general both Rc and Arc need to be adjusted
// to deal with overflow.
// Unlike with Clone(), we need this to be an Acquire read to
// synchronize with the write coming from `is_unique`, so that the
// events prior to that write happen before this read.
match this.inner().weak.compare_exchange_weak(cur, cur + 1, Acquire, Relaxed) {
Ok(_) => {
// Make sure we do not create a dangling Weak
debug_assert!(!is_dangling(this.ptr.as_ptr()));
return Weak { ptr: this.ptr };
}
Err(old) => cur = old,
}
}
}
/// Gets the number of [`Weak`] pointers to this allocation.
///
/// # Safety
///
/// This method by itself is safe, but using it correctly requires extra care.
/// Another thread can change the weak count at any time,
/// including potentially between calling this method and acting on the result.
///
/// # Examples
///
/// ```
/// use std::sync::Arc;
///
/// let five = Arc::new(5);
/// let _weak_five = Arc::downgrade(&five);
///
/// // This assertion is deterministic because we haven't shared
/// // the `Arc` or `Weak` between threads.
/// assert_eq!(1, Arc::weak_count(&five));
/// ```
#[inline]
#[must_use]
#[stable(feature = "arc_counts", since = "1.15.0")]
pub fn weak_count(this: &Self) -> usize {
let cnt = this.inner().weak.load(Acquire);
// If the weak count is currently locked, the value of the
// count was 0 just before taking the lock.
if cnt == usize::MAX { 0 } else { cnt - 1 }
}
/// Gets the number of strong (`Arc`) pointers to this allocation.
///
/// # Safety
///
/// This method by itself is safe, but using it correctly requires extra care.
/// Another thread can change the strong count at any time,
/// including potentially between calling this method and acting on the result.
///
/// # Examples
///
/// ```
/// use std::sync::Arc;
///
/// let five = Arc::new(5);
/// let _also_five = Arc::clone(&five);
///
/// // This assertion is deterministic because we haven't shared
/// // the `Arc` between threads.
/// assert_eq!(2, Arc::strong_count(&five));
/// ```
#[inline]
#[must_use]
#[stable(feature = "arc_counts", since = "1.15.0")]
pub fn strong_count(this: &Self) -> usize {
this.inner().strong.load(Acquire)
}
/// Increments the strong reference count on the `Arc<T>` associated with the
/// provided pointer by one.
///
/// # Safety
///
/// The pointer must have been obtained through `Arc::into_raw`, and the
/// associated `Arc` instance must be valid (i.e. the strong count must be at
/// least 1) for the duration of this method.
///
/// # Examples
///
/// ```
/// use std::sync::Arc;
///
/// let five = Arc::new(5);
///
/// unsafe {
/// let ptr = Arc::into_raw(five);
/// Arc::increment_strong_count(ptr);
///
/// // This assertion is deterministic because we haven't shared
/// // the `Arc` between threads.
/// let five = Arc::from_raw(ptr);
/// assert_eq!(2, Arc::strong_count(&five));
/// }
/// ```
#[inline]
#[stable(feature = "arc_mutate_strong_count", since = "1.51.0")]
pub unsafe fn increment_strong_count(ptr: *const T) {
// Retain Arc, but don't touch refcount by wrapping in ManuallyDrop
let arc = unsafe { mem::ManuallyDrop::new(Arc::<T>::from_raw(ptr)) };
// Now increase refcount, but don't drop new refcount either
let _arc_clone: mem::ManuallyDrop<_> = arc.clone();
}
/// Decrements the strong reference count on the `Arc<T>` associated with the
/// provided pointer by one.
///
/// # Safety
///
/// The pointer must have been obtained through `Arc::into_raw`, and the
/// associated `Arc` instance must be valid (i.e. the strong count must be at
/// least 1) when invoking this method. This method can be used to release the final
/// `Arc` and backing storage, but **should not** be called after the final `Arc` has been
/// released.
///
/// # Examples
///
/// ```
/// use std::sync::Arc;
///
/// let five = Arc::new(5);
///
/// unsafe {
/// let ptr = Arc::into_raw(five);
/// Arc::increment_strong_count(ptr);
///
/// // Those assertions are deterministic because we haven't shared
/// // the `Arc` between threads.
/// let five = Arc::from_raw(ptr);
/// assert_eq!(2, Arc::strong_count(&five));
/// Arc::decrement_strong_count(ptr);
/// assert_eq!(1, Arc::strong_count(&five));
/// }
/// ```
#[inline]
#[stable(feature = "arc_mutate_strong_count", since = "1.51.0")]
pub unsafe fn decrement_strong_count(ptr: *const T) {
unsafe { mem::drop(Arc::from_raw(ptr)) };
}
#[inline]
fn inner(&self) -> &ArcInner<T> {
// This unsafety is ok because while this arc is alive we're guaranteed
// that the inner pointer is valid. Furthermore, we know that the
// `ArcInner` structure itself is `Sync` because the inner data is
// `Sync` as well, so we're ok loaning out an immutable pointer to these
// contents.
unsafe { self.ptr.as_ref() }
}
// Non-inlined part of `drop`.
#[inline(never)]
unsafe fn drop_slow(&mut self) {
// Destroy the data at this time, even though we must not free the box
// allocation itself (there might still be weak pointers lying around).
unsafe { ptr::drop_in_place(Self::get_mut_unchecked(self)) };
// Drop the weak ref collectively held by all strong references
drop(Weak { ptr: self.ptr });
}
/// Returns `true` if the two `Arc`s point to the same allocation in a vein similar to
/// [`ptr::eq`]. See [that function][`ptr::eq`] for caveats when comparing `dyn Trait` pointers.
///
/// # Examples
///
/// ```
/// use std::sync::Arc;
///
/// let five = Arc::new(5);
/// let same_five = Arc::clone(&five);
/// let other_five = Arc::new(5);
///
/// assert!(Arc::ptr_eq(&five, &same_five));
/// assert!(!Arc::ptr_eq(&five, &other_five));
/// ```
///
/// [`ptr::eq`]: core::ptr::eq "ptr::eq"
#[inline]
#[must_use]
#[stable(feature = "ptr_eq", since = "1.17.0")]
pub fn ptr_eq(this: &Self, other: &Self) -> bool {
this.ptr.as_ptr() == other.ptr.as_ptr()
}
}
impl<T: ?Sized> Arc<T> {
/// Allocates an `ArcInner<T>` with sufficient space for
/// a possibly-unsized inner value where the value has the layout provided.
///
/// The function `mem_to_arcinner` is called with the data pointer
/// and must return back a (potentially fat)-pointer for the `ArcInner<T>`.
#[cfg(not(no_global_oom_handling))]
unsafe fn allocate_for_layout(
value_layout: Layout,
allocate: impl FnOnce(Layout) -> Result<NonNull<[u8]>, AllocError>,
mem_to_arcinner: impl FnOnce(*mut u8) -> *mut ArcInner<T>,
) -> *mut ArcInner<T> {
let layout = arcinner_layout_for_value_layout(value_layout);
unsafe {
Arc::try_allocate_for_layout(value_layout, allocate, mem_to_arcinner)
.unwrap_or_else(|_| handle_alloc_error(layout))
}
}
/// Allocates an `ArcInner<T>` with sufficient space for
/// a possibly-unsized inner value where the value has the layout provided,
/// returning an error if allocation fails.
///
/// The function `mem_to_arcinner` is called with the data pointer
/// and must return back a (potentially fat)-pointer for the `ArcInner<T>`.
unsafe fn try_allocate_for_layout(
value_layout: Layout,
allocate: impl FnOnce(Layout) -> Result<NonNull<[u8]>, AllocError>,
mem_to_arcinner: impl FnOnce(*mut u8) -> *mut ArcInner<T>,
) -> Result<*mut ArcInner<T>, AllocError> {
let layout = arcinner_layout_for_value_layout(value_layout);
let ptr = allocate(layout)?;
// Initialize the ArcInner
let inner = mem_to_arcinner(ptr.as_non_null_ptr().as_ptr());
debug_assert_eq!(unsafe { Layout::for_value(&*inner) }, layout);
unsafe {
ptr::write(&mut (*inner).strong, atomic::AtomicUsize::new(1));
ptr::write(&mut (*inner).weak, atomic::AtomicUsize::new(1));
}
Ok(inner)
}
/// Allocates an `ArcInner<T>` with sufficient space for an unsized inner value.
#[cfg(not(no_global_oom_handling))]
unsafe fn allocate_for_ptr(ptr: *const T) -> *mut ArcInner<T> {
// Allocate for the `ArcInner<T>` using the given value.
unsafe {
Self::allocate_for_layout(
Layout::for_value(&*ptr),
|layout| Global.allocate(layout),
|mem| mem.with_metadata_of(ptr as *const ArcInner<T>),
)
}
}
#[cfg(not(no_global_oom_handling))]
fn from_box(v: Box<T>) -> Arc<T> {
unsafe {
let (box_unique, alloc) = Box::into_unique(v);
let bptr = box_unique.as_ptr();
let value_size = size_of_val(&*bptr);
let ptr = Self::allocate_for_ptr(bptr);
// Copy value as bytes
ptr::copy_nonoverlapping(
bptr as *const T as *const u8,
&mut (*ptr).data as *mut _ as *mut u8,
value_size,
);
// Free the allocation without dropping its contents
box_free(box_unique, alloc);
Self::from_ptr(ptr)
}
}
}
impl<T> Arc<[T]> {
/// Allocates an `ArcInner<[T]>` with the given length.
#[cfg(not(no_global_oom_handling))]
unsafe fn allocate_for_slice(len: usize) -> *mut ArcInner<[T]> {
unsafe {
Self::allocate_for_layout(
Layout::array::<T>(len).unwrap(),
|layout| Global.allocate(layout),
|mem| ptr::slice_from_raw_parts_mut(mem as *mut T, len) as *mut ArcInner<[T]>,
)
}
}
/// Copy elements from slice into newly allocated `Arc<[T]>`
///
/// Unsafe because the caller must either take ownership or bind `T: Copy`.
#[cfg(not(no_global_oom_handling))]
unsafe fn copy_from_slice(v: &[T]) -> Arc<[T]> {
unsafe {
let ptr = Self::allocate_for_slice(v.len());
ptr::copy_nonoverlapping(v.as_ptr(), &mut (*ptr).data as *mut [T] as *mut T, v.len());
Self::from_ptr(ptr)
}
}
/// Constructs an `Arc<[T]>` from an iterator known to be of a certain size.
///
/// Behavior is undefined should the size be wrong.
#[cfg(not(no_global_oom_handling))]
unsafe fn from_iter_exact(iter: impl iter::Iterator<Item = T>, len: usize) -> Arc<[T]> {
// Panic guard while cloning T elements.
// In the event of a panic, elements that have been written
// into the new ArcInner will be dropped, then the memory freed.
struct Guard<T> {
mem: NonNull<u8>,
elems: *mut T,
layout: Layout,
n_elems: usize,
}
impl<T> Drop for Guard<T> {
fn drop(&mut self) {
unsafe {
let slice = from_raw_parts_mut(self.elems, self.n_elems);
ptr::drop_in_place(slice);
Global.deallocate(self.mem, self.layout);
}
}
}
unsafe {
let ptr = Self::allocate_for_slice(len);
let mem = ptr as *mut _ as *mut u8;
let layout = Layout::for_value(&*ptr);
// Pointer to first element
let elems = &mut (*ptr).data as *mut [T] as *mut T;
let mut guard = Guard { mem: NonNull::new_unchecked(mem), elems, layout, n_elems: 0 };
for (i, item) in iter.enumerate() {
ptr::write(elems.add(i), item);
guard.n_elems += 1;
}
// All clear. Forget the guard so it doesn't free the new ArcInner.
mem::forget(guard);
Self::from_ptr(ptr)
}
}
}
/// Specialization trait used for `From<&[T]>`.
#[cfg(not(no_global_oom_handling))]
trait ArcFromSlice<T> {
fn from_slice(slice: &[T]) -> Self;
}
#[cfg(not(no_global_oom_handling))]
impl<T: Clone> ArcFromSlice<T> for Arc<[T]> {
#[inline]
default fn from_slice(v: &[T]) -> Self {
unsafe { Self::from_iter_exact(v.iter().cloned(), v.len()) }
}
}
#[cfg(not(no_global_oom_handling))]
impl<T: Copy> ArcFromSlice<T> for Arc<[T]> {
#[inline]
fn from_slice(v: &[T]) -> Self {
unsafe { Arc::copy_from_slice(v) }
}
}
#[stable(feature = "rust1", since = "1.0.0")]
impl<T: ?Sized> Clone for Arc<T> {
/// Makes a clone of the `Arc` pointer.
///
/// This creates another pointer to the same allocation, increasing the
/// strong reference count.
///
/// # Examples
///
/// ```
/// use std::sync::Arc;
///
/// let five = Arc::new(5);
///
/// let _ = Arc::clone(&five);
/// ```
#[inline]
fn clone(&self) -> Arc<T> {
// Using a relaxed ordering is alright here, as knowledge of the
// original reference prevents other threads from erroneously deleting
// the object.
//
// As explained in the [Boost documentation][1], Increasing the
// reference counter can always be done with memory_order_relaxed: New
// references to an object can only be formed from an existing
// reference, and passing an existing reference from one thread to
// another must already provide any required synchronization.
//
// [1]: (www.boost.org/doc/libs/1_55_0/doc/html/atomic/usage_examples.html)
let old_size = self.inner().strong.fetch_add(1, Relaxed);
// However we need to guard against massive refcounts in case someone is `mem::forget`ing
// Arcs. If we don't do this the count can overflow and users will use-after free. This
// branch will never be taken in any realistic program. We abort because such a program is
// incredibly degenerate, and we don't care to support it.
//
// This check is not 100% water-proof: we error when the refcount grows beyond `isize::MAX`.
// But we do that check *after* having done the increment, so there is a chance here that
// the worst already happened and we actually do overflow the `usize` counter. However, that
// requires the counter to grow from `isize::MAX` to `usize::MAX` between the increment
// above and the `abort` below, which seems exceedingly unlikely.
if old_size > MAX_REFCOUNT {
abort();
}
unsafe { Self::from_inner(self.ptr) }
}
}
#[stable(feature = "rust1", since = "1.0.0")]
impl<T: ?Sized> Deref for Arc<T> {
type Target = T;
#[inline]
fn deref(&self) -> &T {
&self.inner().data
}
}
#[unstable(feature = "receiver_trait", issue = "none")]
impl<T: ?Sized> Receiver for Arc<T> {}
impl<T: Clone> Arc<T> {
/// Makes a mutable reference into the given `Arc`.
///
/// If there are other `Arc` pointers to the same allocation, then `make_mut` will
/// [`clone`] the inner value to a new allocation to ensure unique ownership. This is also
/// referred to as clone-on-write.
///
/// However, if there are no other `Arc` pointers to this allocation, but some [`Weak`]
/// pointers, then the [`Weak`] pointers will be dissociated and the inner value will not
/// be cloned.
///
/// See also [`get_mut`], which will fail rather than cloning the inner value
/// or dissociating [`Weak`] pointers.
///
/// [`clone`]: Clone::clone
/// [`get_mut`]: Arc::get_mut
///
/// # Examples
///
/// ```
/// use std::sync::Arc;
///
/// let mut data = Arc::new(5);
///
/// *Arc::make_mut(&mut data) += 1; // Won't clone anything
/// let mut other_data = Arc::clone(&data); // Won't clone inner data
/// *Arc::make_mut(&mut data) += 1; // Clones inner data
/// *Arc::make_mut(&mut data) += 1; // Won't clone anything
/// *Arc::make_mut(&mut other_data) *= 2; // Won't clone anything
///
/// // Now `data` and `other_data` point to different allocations.
/// assert_eq!(*data, 8);
/// assert_eq!(*other_data, 12);
/// ```
///
/// [`Weak`] pointers will be dissociated:
///
/// ```
/// use std::sync::Arc;
///
/// let mut data = Arc::new(75);
/// let weak = Arc::downgrade(&data);
///
/// assert!(75 == *data);
/// assert!(75 == *weak.upgrade().unwrap());
///
/// *Arc::make_mut(&mut data) += 1;
///
/// assert!(76 == *data);
/// assert!(weak.upgrade().is_none());
/// ```
#[cfg(not(no_global_oom_handling))]
#[inline]
#[stable(feature = "arc_unique", since = "1.4.0")]
pub fn make_mut(this: &mut Self) -> &mut T {
// Note that we hold both a strong reference and a weak reference.
// Thus, releasing our strong reference only will not, by itself, cause
// the memory to be deallocated.
//
// Use Acquire to ensure that we see any writes to `weak` that happen
// before release writes (i.e., decrements) to `strong`. Since we hold a
// weak count, there's no chance the ArcInner itself could be
// deallocated.
if this.inner().strong.compare_exchange(1, 0, Acquire, Relaxed).is_err() {
// Another strong pointer exists, so we must clone.
// Pre-allocate memory to allow writing the cloned value directly.
let mut arc = Self::new_uninit();
unsafe {
let data = Arc::get_mut_unchecked(&mut arc);
(**this).write_clone_into_raw(data.as_mut_ptr());
*this = arc.assume_init();
}
} else if this.inner().weak.load(Relaxed) != 1 {
// Relaxed suffices in the above because this is fundamentally an
// optimization: we are always racing with weak pointers being
// dropped. Worst case, we end up allocated a new Arc unnecessarily.
// We removed the last strong ref, but there are additional weak
// refs remaining. We'll move the contents to a new Arc, and
// invalidate the other weak refs.
// Note that it is not possible for the read of `weak` to yield
// usize::MAX (i.e., locked), since the weak count can only be
// locked by a thread with a strong reference.
// Materialize our own implicit weak pointer, so that it can clean
// up the ArcInner as needed.
let _weak = Weak { ptr: this.ptr };
// Can just steal the data, all that's left is Weaks
let mut arc = Self::new_uninit();
unsafe {
let data = Arc::get_mut_unchecked(&mut arc);
data.as_mut_ptr().copy_from_nonoverlapping(&**this, 1);
ptr::write(this, arc.assume_init());
}
} else {
// We were the sole reference of either kind; bump back up the
// strong ref count.
this.inner().strong.store(1, Release);
}
// As with `get_mut()`, the unsafety is ok because our reference was
// either unique to begin with, or became one upon cloning the contents.
unsafe { Self::get_mut_unchecked(this) }
}
/// If we have the only reference to `T` then unwrap it. Otherwise, clone `T` and return the
/// clone.
///
/// Assuming `arc_t` is of type `Arc<T>`, this function is functionally equivalent to
/// `(*arc_t).clone()`, but will avoid cloning the inner value where possible.
///
/// # Examples
///
/// ```
/// #![feature(arc_unwrap_or_clone)]
/// # use std::{ptr, sync::Arc};
/// let inner = String::from("test");
/// let ptr = inner.as_ptr();
///
/// let arc = Arc::new(inner);
/// let inner = Arc::unwrap_or_clone(arc);
/// // The inner value was not cloned
/// assert!(ptr::eq(ptr, inner.as_ptr()));
///
/// let arc = Arc::new(inner);
/// let arc2 = arc.clone();
/// let inner = Arc::unwrap_or_clone(arc);
/// // Because there were 2 references, we had to clone the inner value.
/// assert!(!ptr::eq(ptr, inner.as_ptr()));
/// // `arc2` is the last reference, so when we unwrap it we get back
/// // the original `String`.
/// let inner = Arc::unwrap_or_clone(arc2);
/// assert!(ptr::eq(ptr, inner.as_ptr()));
/// ```
#[inline]
#[unstable(feature = "arc_unwrap_or_clone", issue = "93610")]
pub fn unwrap_or_clone(this: Self) -> T {
Arc::try_unwrap(this).unwrap_or_else(|arc| (*arc).clone())
}
}
impl<T: ?Sized> Arc<T> {
/// Returns a mutable reference into the given `Arc`, if there are
/// no other `Arc` or [`Weak`] pointers to the same allocation.
///
/// Returns [`None`] otherwise, because it is not safe to
/// mutate a shared value.
///
/// See also [`make_mut`][make_mut], which will [`clone`][clone]
/// the inner value when there are other `Arc` pointers.
///
/// [make_mut]: Arc::make_mut
/// [clone]: Clone::clone
///
/// # Examples
///
/// ```
/// use std::sync::Arc;
///
/// let mut x = Arc::new(3);
/// *Arc::get_mut(&mut x).unwrap() = 4;
/// assert_eq!(*x, 4);
///
/// let _y = Arc::clone(&x);
/// assert!(Arc::get_mut(&mut x).is_none());
/// ```
#[inline]
#[stable(feature = "arc_unique", since = "1.4.0")]
pub fn get_mut(this: &mut Self) -> Option<&mut T> {
if this.is_unique() {
// This unsafety is ok because we're guaranteed that the pointer
// returned is the *only* pointer that will ever be returned to T. Our
// reference count is guaranteed to be 1 at this point, and we required
// the Arc itself to be `mut`, so we're returning the only possible
// reference to the inner data.
unsafe { Some(Arc::get_mut_unchecked(this)) }
} else {
None
}
}
/// Returns a mutable reference into the given `Arc`,
/// without any check.
///
/// See also [`get_mut`], which is safe and does appropriate checks.
///
/// [`get_mut`]: Arc::get_mut
///
/// # Safety
///
/// If any other `Arc` or [`Weak`] pointers to the same allocation exist, then
/// they must be must not be dereferenced or have active borrows for the duration
/// of the returned borrow, and their inner type must be exactly the same as the
/// inner type of this Rc (including lifetimes). This is trivially the case if no
/// such pointers exist, for example immediately after `Arc::new`.
///
/// # Examples
///
/// ```
/// #![feature(get_mut_unchecked)]
///
/// use std::sync::Arc;
///
/// let mut x = Arc::new(String::new());
/// unsafe {
/// Arc::get_mut_unchecked(&mut x).push_str("foo")
/// }
/// assert_eq!(*x, "foo");
/// ```
/// Other `Arc` pointers to the same allocation must be to the same type.
/// ```no_run
/// #![feature(get_mut_unchecked)]
///
/// use std::sync::Arc;
///
/// let x: Arc<str> = Arc::from("Hello, world!");
/// let mut y: Arc<[u8]> = x.clone().into();
/// unsafe {
/// // this is Undefined Behavior, because x's inner type is str, not [u8]
/// Arc::get_mut_unchecked(&mut y).fill(0xff); // 0xff is invalid in UTF-8
/// }
/// println!("{}", &*x); // Invalid UTF-8 in a str
/// ```
/// Other `Arc` pointers to the same allocation must be to the exact same type, including lifetimes.
/// ```no_run
/// #![feature(get_mut_unchecked)]
///
/// use std::sync::Arc;
///
/// let x: Arc<&str> = Arc::new("Hello, world!");
/// {
/// let s = String::from("Oh, no!");
/// let mut y: Arc<&str> = x.clone().into();
/// unsafe {
/// // this is Undefined Behavior, because x's inner type
/// // is &'long str, not &'short str
/// *Arc::get_mut_unchecked(&mut y) = &s;
/// }
/// }
/// println!("{}", &*x); // Use-after-free
/// ```
#[inline]
#[unstable(feature = "get_mut_unchecked", issue = "63292")]
pub unsafe fn get_mut_unchecked(this: &mut Self) -> &mut T {
// We are careful to *not* create a reference covering the "count" fields, as
// this would alias with concurrent access to the reference counts (e.g. by `Weak`).
unsafe { &mut (*this.ptr.as_ptr()).data }
}
/// Determine whether this is the unique reference (including weak refs) to
/// the underlying data.
///
/// Note that this requires locking the weak ref count.
fn is_unique(&mut self) -> bool {
// lock the weak pointer count if we appear to be the sole weak pointer
// holder.
//
// The acquire label here ensures a happens-before relationship with any
// writes to `strong` (in particular in `Weak::upgrade`) prior to decrements
// of the `weak` count (via `Weak::drop`, which uses release). If the upgraded
// weak ref was never dropped, the CAS here will fail so we do not care to synchronize.
if self.inner().weak.compare_exchange(1, usize::MAX, Acquire, Relaxed).is_ok() {
// This needs to be an `Acquire` to synchronize with the decrement of the `strong`
// counter in `drop` -- the only access that happens when any but the last reference
// is being dropped.
let unique = self.inner().strong.load(Acquire) == 1;
// The release write here synchronizes with a read in `downgrade`,
// effectively preventing the above read of `strong` from happening
// after the write.
self.inner().weak.store(1, Release); // release the lock
unique
} else {
false
}
}
}
#[stable(feature = "rust1", since = "1.0.0")]
unsafe impl<#[may_dangle] T: ?Sized> Drop for Arc<T> {
/// Drops the `Arc`.
///
/// This will decrement the strong reference count. If the strong reference
/// count reaches zero then the only other references (if any) are
/// [`Weak`], so we `drop` the inner value.
///
/// # Examples
///
/// ```
/// use std::sync::Arc;
///
/// struct Foo;
///
/// impl Drop for Foo {
/// fn drop(&mut self) {
/// println!("dropped!");
/// }
/// }
///
/// let foo = Arc::new(Foo);
/// let foo2 = Arc::clone(&foo);
///
/// drop(foo); // Doesn't print anything
/// drop(foo2); // Prints "dropped!"
/// ```
#[inline]
fn drop(&mut self) {
// Because `fetch_sub` is already atomic, we do not need to synchronize
// with other threads unless we are going to delete the object. This
// same logic applies to the below `fetch_sub` to the `weak` count.
if self.inner().strong.fetch_sub(1, Release) != 1 {
return;
}
// This fence is needed to prevent reordering of use of the data and
// deletion of the data. Because it is marked `Release`, the decreasing
// of the reference count synchronizes with this `Acquire` fence. This
// means that use of the data happens before decreasing the reference
// count, which happens before this fence, which happens before the
// deletion of the data.
//
// As explained in the [Boost documentation][1],
//
// > It is important to enforce any possible access to the object in one
// > thread (through an existing reference) to *happen before* deleting
// > the object in a different thread. This is achieved by a "release"
// > operation after dropping a reference (any access to the object
// > through this reference must obviously happened before), and an
// > "acquire" operation before deleting the object.
//
// In particular, while the contents of an Arc are usually immutable, it's
// possible to have interior writes to something like a Mutex<T>. Since a
// Mutex is not acquired when it is deleted, we can't rely on its
// synchronization logic to make writes in thread A visible to a destructor
// running in thread B.
//
// Also note that the Acquire fence here could probably be replaced with an
// Acquire load, which could improve performance in highly-contended
// situations. See [2].
//
// [1]: (www.boost.org/doc/libs/1_55_0/doc/html/atomic/usage_examples.html)
// [2]: (https://github.com/rust-lang/rust/pull/41714)
acquire!(self.inner().strong);
unsafe {
self.drop_slow();
}
}
}
impl Arc<dyn Any + Send + Sync> {
/// Attempt to downcast the `Arc<dyn Any + Send + Sync>` to a concrete type.
///
/// # Examples
///
/// ```
/// use std::any::Any;
/// use std::sync::Arc;
///
/// fn print_if_string(value: Arc<dyn Any + Send + Sync>) {
/// if let Ok(string) = value.downcast::<String>() {
/// println!("String ({}): {}", string.len(), string);
/// }
/// }
///
/// let my_string = "Hello World".to_string();
/// print_if_string(Arc::new(my_string));
/// print_if_string(Arc::new(0i8));
/// ```
#[inline]
#[stable(feature = "rc_downcast", since = "1.29.0")]
pub fn downcast<T>(self) -> Result<Arc<T>, Self>
where
T: Any + Send + Sync,
{
if (*self).is::<T>() {
unsafe {
let ptr = self.ptr.cast::<ArcInner<T>>();
mem::forget(self);
Ok(Arc::from_inner(ptr))
}
} else {
Err(self)
}
}
/// Downcasts the `Arc<dyn Any + Send + Sync>` to a concrete type.
///
/// For a safe alternative see [`downcast`].
///
/// # Examples
///
/// ```
/// #![feature(downcast_unchecked)]
///
/// use std::any::Any;
/// use std::sync::Arc;
///
/// let x: Arc<dyn Any + Send + Sync> = Arc::new(1_usize);
///
/// unsafe {
/// assert_eq!(*x.downcast_unchecked::<usize>(), 1);
/// }
/// ```
///
/// # Safety
///
/// The contained value must be of type `T`. Calling this method
/// with the incorrect type is *undefined behavior*.
///
///
/// [`downcast`]: Self::downcast
#[inline]
#[unstable(feature = "downcast_unchecked", issue = "90850")]
pub unsafe fn downcast_unchecked<T>(self) -> Arc<T>
where
T: Any + Send + Sync,
{
unsafe {
let ptr = self.ptr.cast::<ArcInner<T>>();
mem::forget(self);
Arc::from_inner(ptr)
}
}
}
impl<T> Weak<T> {
/// Constructs a new `Weak<T>`, without allocating any memory.
/// Calling [`upgrade`] on the return value always gives [`None`].
///
/// [`upgrade`]: Weak::upgrade
///
/// # Examples
///
/// ```
/// use std::sync::Weak;
///
/// let empty: Weak<i64> = Weak::new();
/// assert!(empty.upgrade().is_none());
/// ```
#[stable(feature = "downgraded_weak", since = "1.10.0")]
#[rustc_const_unstable(feature = "const_weak_new", issue = "95091", reason = "recently added")]
#[must_use]
pub const fn new() -> Weak<T> {
Weak { ptr: unsafe { NonNull::new_unchecked(ptr::invalid_mut::<ArcInner<T>>(usize::MAX)) } }
}
}
/// Helper type to allow accessing the reference counts without
/// making any assertions about the data field.
struct WeakInner<'a> {
weak: &'a atomic::AtomicUsize,
strong: &'a atomic::AtomicUsize,
}
impl<T: ?Sized> Weak<T> {
/// Returns a raw pointer to the object `T` pointed to by this `Weak<T>`.
///
/// The pointer is valid only if there are some strong references. The pointer may be dangling,
/// unaligned or even [`null`] otherwise.
///
/// # Examples
///
/// ```
/// use std::sync::Arc;
/// use std::ptr;
///
/// let strong = Arc::new("hello".to_owned());
/// let weak = Arc::downgrade(&strong);
/// // Both point to the same object
/// assert!(ptr::eq(&*strong, weak.as_ptr()));
/// // The strong here keeps it alive, so we can still access the object.
/// assert_eq!("hello", unsafe { &*weak.as_ptr() });
///
/// drop(strong);
/// // But not any more. We can do weak.as_ptr(), but accessing the pointer would lead to
/// // undefined behaviour.
/// // assert_eq!("hello", unsafe { &*weak.as_ptr() });
/// ```
///
/// [`null`]: core::ptr::null "ptr::null"
#[must_use]
#[stable(feature = "weak_into_raw", since = "1.45.0")]
pub fn as_ptr(&self) -> *const T {
let ptr: *mut ArcInner<T> = NonNull::as_ptr(self.ptr);
if is_dangling(ptr) {
// If the pointer is dangling, we return the sentinel directly. This cannot be
// a valid payload address, as the payload is at least as aligned as ArcInner (usize).
ptr as *const T
} else {
// SAFETY: if is_dangling returns false, then the pointer is dereferenceable.
// The payload may be dropped at this point, and we have to maintain provenance,
// so use raw pointer manipulation.
unsafe { ptr::addr_of_mut!((*ptr).data) }
}
}
/// Consumes the `Weak<T>` and turns it into a raw pointer.
///
/// This converts the weak pointer into a raw pointer, while still preserving the ownership of
/// one weak reference (the weak count is not modified by this operation). It can be turned
/// back into the `Weak<T>` with [`from_raw`].
///
/// The same restrictions of accessing the target of the pointer as with
/// [`as_ptr`] apply.
///
/// # Examples
///
/// ```
/// use std::sync::{Arc, Weak};
///
/// let strong = Arc::new("hello".to_owned());
/// let weak = Arc::downgrade(&strong);
/// let raw = weak.into_raw();
///
/// assert_eq!(1, Arc::weak_count(&strong));
/// assert_eq!("hello", unsafe { &*raw });
///
/// drop(unsafe { Weak::from_raw(raw) });
/// assert_eq!(0, Arc::weak_count(&strong));
/// ```
///
/// [`from_raw`]: Weak::from_raw
/// [`as_ptr`]: Weak::as_ptr
#[must_use = "`self` will be dropped if the result is not used"]
#[stable(feature = "weak_into_raw", since = "1.45.0")]
pub fn into_raw(self) -> *const T {
let result = self.as_ptr();
mem::forget(self);
result
}
/// Converts a raw pointer previously created by [`into_raw`] back into `Weak<T>`.
///
/// This can be used to safely get a strong reference (by calling [`upgrade`]
/// later) or to deallocate the weak count by dropping the `Weak<T>`.
///
/// It takes ownership of one weak reference (with the exception of pointers created by [`new`],
/// as these don't own anything; the method still works on them).
///
/// # Safety
///
/// The pointer must have originated from the [`into_raw`] and must still own its potential
/// weak reference.
///
/// It is allowed for the strong count to be 0 at the time of calling this. Nevertheless, this
/// takes ownership of one weak reference currently represented as a raw pointer (the weak
/// count is not modified by this operation) and therefore it must be paired with a previous
/// call to [`into_raw`].
/// # Examples
///
/// ```
/// use std::sync::{Arc, Weak};
///
/// let strong = Arc::new("hello".to_owned());
///
/// let raw_1 = Arc::downgrade(&strong).into_raw();
/// let raw_2 = Arc::downgrade(&strong).into_raw();
///
/// assert_eq!(2, Arc::weak_count(&strong));
///
/// assert_eq!("hello", &*unsafe { Weak::from_raw(raw_1) }.upgrade().unwrap());
/// assert_eq!(1, Arc::weak_count(&strong));
///
/// drop(strong);
///
/// // Decrement the last weak count.
/// assert!(unsafe { Weak::from_raw(raw_2) }.upgrade().is_none());
/// ```
///
/// [`new`]: Weak::new
/// [`into_raw`]: Weak::into_raw
/// [`upgrade`]: Weak::upgrade
#[stable(feature = "weak_into_raw", since = "1.45.0")]
pub unsafe fn from_raw(ptr: *const T) -> Self {
// See Weak::as_ptr for context on how the input pointer is derived.
let ptr = if is_dangling(ptr as *mut T) {
// This is a dangling Weak.
ptr as *mut ArcInner<T>
} else {
// Otherwise, we're guaranteed the pointer came from a nondangling Weak.
// SAFETY: data_offset is safe to call, as ptr references a real (potentially dropped) T.
let offset = unsafe { data_offset(ptr) };
// Thus, we reverse the offset to get the whole RcBox.
// SAFETY: the pointer originated from a Weak, so this offset is safe.
unsafe { ptr.byte_sub(offset) as *mut ArcInner<T> }
};
// SAFETY: we now have recovered the original Weak pointer, so can create the Weak.
Weak { ptr: unsafe { NonNull::new_unchecked(ptr) } }
}
}
impl<T: ?Sized> Weak<T> {
/// Attempts to upgrade the `Weak` pointer to an [`Arc`], delaying
/// dropping of the inner value if successful.
///
/// Returns [`None`] if the inner value has since been dropped.
///
/// # Examples
///
/// ```
/// use std::sync::Arc;
///
/// let five = Arc::new(5);
///
/// let weak_five = Arc::downgrade(&five);
///
/// let strong_five: Option<Arc<_>> = weak_five.upgrade();
/// assert!(strong_five.is_some());
///
/// // Destroy all strong pointers.
/// drop(strong_five);
/// drop(five);
///
/// assert!(weak_five.upgrade().is_none());
/// ```
#[must_use = "this returns a new `Arc`, \
without modifying the original weak pointer"]
#[stable(feature = "arc_weak", since = "1.4.0")]
pub fn upgrade(&self) -> Option<Arc<T>> {
// We use a CAS loop to increment the strong count instead of a
// fetch_add as this function should never take the reference count
// from zero to one.
self.inner()?
.strong
// Relaxed is fine for the failure case because we don't have any expectations about the new state.
// Acquire is necessary for the success case to synchronise with `Arc::new_cyclic`, when the inner
// value can be initialized after `Weak` references have already been created. In that case, we
// expect to observe the fully initialized value.
.fetch_update(Acquire, Relaxed, |n| {
// Any write of 0 we can observe leaves the field in permanently zero state.
if n == 0 {
return None;
}
// See comments in `Arc::clone` for why we do this (for `mem::forget`).
if n > MAX_REFCOUNT {
abort();
}
Some(n + 1)
})
.ok()
// null checked above
.map(|_| unsafe { Arc::from_inner(self.ptr) })
}
/// Gets the number of strong (`Arc`) pointers pointing to this allocation.
///
/// If `self` was created using [`Weak::new`], this will return 0.
#[must_use]
#[stable(feature = "weak_counts", since = "1.41.0")]
pub fn strong_count(&self) -> usize {
if let Some(inner) = self.inner() { inner.strong.load(Acquire) } else { 0 }
}
/// Gets an approximation of the number of `Weak` pointers pointing to this
/// allocation.
///
/// If `self` was created using [`Weak::new`], or if there are no remaining
/// strong pointers, this will return 0.
///
/// # Accuracy
///
/// Due to implementation details, the returned value can be off by 1 in
/// either direction when other threads are manipulating any `Arc`s or
/// `Weak`s pointing to the same allocation.
#[must_use]
#[stable(feature = "weak_counts", since = "1.41.0")]
pub fn weak_count(&self) -> usize {
self.inner()
.map(|inner| {
let weak = inner.weak.load(Acquire);
let strong = inner.strong.load(Acquire);
if strong == 0 {
0
} else {
// Since we observed that there was at least one strong pointer
// after reading the weak count, we know that the implicit weak
// reference (present whenever any strong references are alive)
// was still around when we observed the weak count, and can
// therefore safely subtract it.
weak - 1
}
})
.unwrap_or(0)
}
/// Returns `None` when the pointer is dangling and there is no allocated `ArcInner`,
/// (i.e., when this `Weak` was created by `Weak::new`).
#[inline]
fn inner(&self) -> Option<WeakInner<'_>> {
if is_dangling(self.ptr.as_ptr()) {
None
} else {
// We are careful to *not* create a reference covering the "data" field, as
// the field may be mutated concurrently (for example, if the last `Arc`
// is dropped, the data field will be dropped in-place).
Some(unsafe {
let ptr = self.ptr.as_ptr();
WeakInner { strong: &(*ptr).strong, weak: &(*ptr).weak }
})
}
}
/// Returns `true` if the two `Weak`s point to the same allocation similar to [`ptr::eq`], or if
/// both don't point to any allocation (because they were created with `Weak::new()`). See [that
/// function][`ptr::eq`] for caveats when comparing `dyn Trait` pointers.
///
/// # Notes
///
/// Since this compares pointers it means that `Weak::new()` will equal each
/// other, even though they don't point to any allocation.
///
/// # Examples
///
/// ```
/// use std::sync::Arc;
///
/// let first_rc = Arc::new(5);
/// let first = Arc::downgrade(&first_rc);
/// let second = Arc::downgrade(&first_rc);
///
/// assert!(first.ptr_eq(&second));
///
/// let third_rc = Arc::new(5);
/// let third = Arc::downgrade(&third_rc);
///
/// assert!(!first.ptr_eq(&third));
/// ```
///
/// Comparing `Weak::new`.
///
/// ```
/// use std::sync::{Arc, Weak};
///
/// let first = Weak::new();
/// let second = Weak::new();
/// assert!(first.ptr_eq(&second));
///
/// let third_rc = Arc::new(());
/// let third = Arc::downgrade(&third_rc);
/// assert!(!first.ptr_eq(&third));
/// ```
///
/// [`ptr::eq`]: core::ptr::eq "ptr::eq"
#[inline]
#[must_use]
#[stable(feature = "weak_ptr_eq", since = "1.39.0")]
pub fn ptr_eq(&self, other: &Self) -> bool {
self.ptr.as_ptr() == other.ptr.as_ptr()
}
}
#[stable(feature = "arc_weak", since = "1.4.0")]
impl<T: ?Sized> Clone for Weak<T> {
/// Makes a clone of the `Weak` pointer that points to the same allocation.
///
/// # Examples
///
/// ```
/// use std::sync::{Arc, Weak};
///
/// let weak_five = Arc::downgrade(&Arc::new(5));
///
/// let _ = Weak::clone(&weak_five);
/// ```
#[inline]
fn clone(&self) -> Weak<T> {
let inner = if let Some(inner) = self.inner() {
inner
} else {
return Weak { ptr: self.ptr };
};
// See comments in Arc::clone() for why this is relaxed. This can use a
// fetch_add (ignoring the lock) because the weak count is only locked
// where are *no other* weak pointers in existence. (So we can't be
// running this code in that case).
let old_size = inner.weak.fetch_add(1, Relaxed);
// See comments in Arc::clone() for why we do this (for mem::forget).
if old_size > MAX_REFCOUNT {
abort();
}
Weak { ptr: self.ptr }
}
}
#[stable(feature = "downgraded_weak", since = "1.10.0")]
impl<T> Default for Weak<T> {
/// Constructs a new `Weak<T>`, without allocating memory.
/// Calling [`upgrade`] on the return value always
/// gives [`None`].
///
/// [`upgrade`]: Weak::upgrade
///
/// # Examples
///
/// ```
/// use std::sync::Weak;
///
/// let empty: Weak<i64> = Default::default();
/// assert!(empty.upgrade().is_none());
/// ```
fn default() -> Weak<T> {
Weak::new()
}
}
#[stable(feature = "arc_weak", since = "1.4.0")]
unsafe impl<#[may_dangle] T: ?Sized> Drop for Weak<T> {
/// Drops the `Weak` pointer.
///
/// # Examples
///
/// ```
/// use std::sync::{Arc, Weak};
///
/// struct Foo;
///
/// impl Drop for Foo {
/// fn drop(&mut self) {
/// println!("dropped!");
/// }
/// }
///
/// let foo = Arc::new(Foo);
/// let weak_foo = Arc::downgrade(&foo);
/// let other_weak_foo = Weak::clone(&weak_foo);
///
/// drop(weak_foo); // Doesn't print anything
/// drop(foo); // Prints "dropped!"
///
/// assert!(other_weak_foo.upgrade().is_none());
/// ```
fn drop(&mut self) {
// If we find out that we were the last weak pointer, then its time to
// deallocate the data entirely. See the discussion in Arc::drop() about
// the memory orderings
//
// It's not necessary to check for the locked state here, because the
// weak count can only be locked if there was precisely one weak ref,
// meaning that drop could only subsequently run ON that remaining weak
// ref, which can only happen after the lock is released.
let inner = if let Some(inner) = self.inner() { inner } else { return };
if inner.weak.fetch_sub(1, Release) == 1 {
acquire!(inner.weak);
unsafe { Global.deallocate(self.ptr.cast(), Layout::for_value_raw(self.ptr.as_ptr())) }
}
}
}
#[stable(feature = "rust1", since = "1.0.0")]
trait ArcEqIdent<T: ?Sized + PartialEq> {
fn eq(&self, other: &Arc<T>) -> bool;
fn ne(&self, other: &Arc<T>) -> bool;
}
#[stable(feature = "rust1", since = "1.0.0")]
impl<T: ?Sized + PartialEq> ArcEqIdent<T> for Arc<T> {
#[inline]
default fn eq(&self, other: &Arc<T>) -> bool {
**self == **other
}
#[inline]
default fn ne(&self, other: &Arc<T>) -> bool {
**self != **other
}
}
/// We're doing this specialization here, and not as a more general optimization on `&T`, because it
/// would otherwise add a cost to all equality checks on refs. We assume that `Arc`s are used to
/// store large values, that are slow to clone, but also heavy to check for equality, causing this
/// cost to pay off more easily. It's also more likely to have two `Arc` clones, that point to
/// the same value, than two `&T`s.
///
/// We can only do this when `T: Eq` as a `PartialEq` might be deliberately irreflexive.
#[stable(feature = "rust1", since = "1.0.0")]
impl<T: ?Sized + crate::rc::MarkerEq> ArcEqIdent<T> for Arc<T> {
#[inline]
fn eq(&self, other: &Arc<T>) -> bool {
Arc::ptr_eq(self, other) || **self == **other
}
#[inline]
fn ne(&self, other: &Arc<T>) -> bool {
!Arc::ptr_eq(self, other) && **self != **other
}
}
#[stable(feature = "rust1", since = "1.0.0")]
impl<T: ?Sized + PartialEq> PartialEq for Arc<T> {
/// Equality for two `Arc`s.
///
/// Two `Arc`s are equal if their inner values are equal, even if they are
/// stored in different allocation.
///
/// If `T` also implements `Eq` (implying reflexivity of equality),
/// two `Arc`s that point to the same allocation are always equal.
///
/// # Examples
///
/// ```
/// use std::sync::Arc;
///
/// let five = Arc::new(5);
///
/// assert!(five == Arc::new(5));
/// ```
#[inline]
fn eq(&self, other: &Arc<T>) -> bool {
ArcEqIdent::eq(self, other)
}
/// Inequality for two `Arc`s.
///
/// Two `Arc`s are unequal if their inner values are unequal.
///
/// If `T` also implements `Eq` (implying reflexivity of equality),
/// two `Arc`s that point to the same value are never unequal.
///
/// # Examples
///
/// ```
/// use std::sync::Arc;
///
/// let five = Arc::new(5);
///
/// assert!(five != Arc::new(6));
/// ```
#[inline]
fn ne(&self, other: &Arc<T>) -> bool {
ArcEqIdent::ne(self, other)
}
}
#[stable(feature = "rust1", since = "1.0.0")]
impl<T: ?Sized + PartialOrd> PartialOrd for Arc<T> {
/// Partial comparison for two `Arc`s.
///
/// The two are compared by calling `partial_cmp()` on their inner values.
///
/// # Examples
///
/// ```
/// use std::sync::Arc;
/// use std::cmp::Ordering;
///
/// let five = Arc::new(5);
///
/// assert_eq!(Some(Ordering::Less), five.partial_cmp(&Arc::new(6)));
/// ```
fn partial_cmp(&self, other: &Arc<T>) -> Option<Ordering> {
(**self).partial_cmp(&**other)
}
/// Less-than comparison for two `Arc`s.
///
/// The two are compared by calling `<` on their inner values.
///
/// # Examples
///
/// ```
/// use std::sync::Arc;
///
/// let five = Arc::new(5);
///
/// assert!(five < Arc::new(6));
/// ```
fn lt(&self, other: &Arc<T>) -> bool {
*(*self) < *(*other)
}
/// 'Less than or equal to' comparison for two `Arc`s.
///
/// The two are compared by calling `<=` on their inner values.
///
/// # Examples
///
/// ```
/// use std::sync::Arc;
///
/// let five = Arc::new(5);
///
/// assert!(five <= Arc::new(5));
/// ```
fn le(&self, other: &Arc<T>) -> bool {
*(*self) <= *(*other)
}
/// Greater-than comparison for two `Arc`s.
///
/// The two are compared by calling `>` on their inner values.
///
/// # Examples
///
/// ```
/// use std::sync::Arc;
///
/// let five = Arc::new(5);
///
/// assert!(five > Arc::new(4));
/// ```
fn gt(&self, other: &Arc<T>) -> bool {
*(*self) > *(*other)
}
/// 'Greater than or equal to' comparison for two `Arc`s.
///
/// The two are compared by calling `>=` on their inner values.
///
/// # Examples
///
/// ```
/// use std::sync::Arc;
///
/// let five = Arc::new(5);
///
/// assert!(five >= Arc::new(5));
/// ```
fn ge(&self, other: &Arc<T>) -> bool {
*(*self) >= *(*other)
}
}
#[stable(feature = "rust1", since = "1.0.0")]
impl<T: ?Sized + Ord> Ord for Arc<T> {
/// Comparison for two `Arc`s.
///
/// The two are compared by calling `cmp()` on their inner values.
///
/// # Examples
///
/// ```
/// use std::sync::Arc;
/// use std::cmp::Ordering;
///
/// let five = Arc::new(5);
///
/// assert_eq!(Ordering::Less, five.cmp(&Arc::new(6)));
/// ```
fn cmp(&self, other: &Arc<T>) -> Ordering {
(**self).cmp(&**other)
}
}
#[stable(feature = "rust1", since = "1.0.0")]
impl<T: ?Sized + Eq> Eq for Arc<T> {}
#[stable(feature = "rust1", since = "1.0.0")]
impl<T: ?Sized + fmt::Display> fmt::Display for Arc<T> {
fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
fmt::Display::fmt(&**self, f)
}
}
#[stable(feature = "rust1", since = "1.0.0")]
impl<T: ?Sized + fmt::Debug> fmt::Debug for Arc<T> {
fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
fmt::Debug::fmt(&**self, f)
}
}
#[stable(feature = "rust1", since = "1.0.0")]
impl<T: ?Sized> fmt::Pointer for Arc<T> {
fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
fmt::Pointer::fmt(&(&**self as *const T), f)
}
}
#[cfg(not(no_global_oom_handling))]
#[stable(feature = "rust1", since = "1.0.0")]
impl<T: Default> Default for Arc<T> {
/// Creates a new `Arc<T>`, with the `Default` value for `T`.
///
/// # Examples
///
/// ```
/// use std::sync::Arc;
///
/// let x: Arc<i32> = Default::default();
/// assert_eq!(*x, 0);
/// ```
fn default() -> Arc<T> {
Arc::new(Default::default())
}
}
#[stable(feature = "rust1", since = "1.0.0")]
impl<T: ?Sized + Hash> Hash for Arc<T> {
fn hash<H: Hasher>(&self, state: &mut H) {
(**self).hash(state)
}
}
#[cfg(not(no_global_oom_handling))]
#[stable(feature = "from_for_ptrs", since = "1.6.0")]
impl<T> From<T> for Arc<T> {
/// Converts a `T` into an `Arc<T>`
///
/// The conversion moves the value into a
/// newly allocated `Arc`. It is equivalent to
/// calling `Arc::new(t)`.
///
/// # Example
/// ```rust
/// # use std::sync::Arc;
/// let x = 5;
/// let arc = Arc::new(5);
///
/// assert_eq!(Arc::from(x), arc);
/// ```
fn from(t: T) -> Self {
Arc::new(t)
}
}
#[cfg(not(no_global_oom_handling))]
#[stable(feature = "shared_from_slice", since = "1.21.0")]
impl<T: Clone> From<&[T]> for Arc<[T]> {
/// Allocate a reference-counted slice and fill it by cloning `v`'s items.
///
/// # Example
///
/// ```
/// # use std::sync::Arc;
/// let original: &[i32] = &[1, 2, 3];
/// let shared: Arc<[i32]> = Arc::from(original);
/// assert_eq!(&[1, 2, 3], &shared[..]);
/// ```
#[inline]
fn from(v: &[T]) -> Arc<[T]> {
<Self as ArcFromSlice<T>>::from_slice(v)
}
}
#[cfg(not(no_global_oom_handling))]
#[stable(feature = "shared_from_slice", since = "1.21.0")]
impl From<&str> for Arc<str> {
/// Allocate a reference-counted `str` and copy `v` into it.
///
/// # Example
///
/// ```
/// # use std::sync::Arc;
/// let shared: Arc<str> = Arc::from("eggplant");
/// assert_eq!("eggplant", &shared[..]);
/// ```
#[inline]
fn from(v: &str) -> Arc<str> {
let arc = Arc::<[u8]>::from(v.as_bytes());
unsafe { Arc::from_raw(Arc::into_raw(arc) as *const str) }
}
}
#[cfg(not(no_global_oom_handling))]
#[stable(feature = "shared_from_slice", since = "1.21.0")]
impl From<String> for Arc<str> {
/// Allocate a reference-counted `str` and copy `v` into it.
///
/// # Example
///
/// ```
/// # use std::sync::Arc;
/// let unique: String = "eggplant".to_owned();
/// let shared: Arc<str> = Arc::from(unique);
/// assert_eq!("eggplant", &shared[..]);
/// ```
#[inline]
fn from(v: String) -> Arc<str> {
Arc::from(&v[..])
}
}
#[cfg(not(no_global_oom_handling))]
#[stable(feature = "shared_from_slice", since = "1.21.0")]
impl<T: ?Sized> From<Box<T>> for Arc<T> {
/// Move a boxed object to a new, reference-counted allocation.
///
/// # Example
///
/// ```
/// # use std::sync::Arc;
/// let unique: Box<str> = Box::from("eggplant");
/// let shared: Arc<str> = Arc::from(unique);
/// assert_eq!("eggplant", &shared[..]);
/// ```
#[inline]
fn from(v: Box<T>) -> Arc<T> {
Arc::from_box(v)
}
}
#[cfg(not(no_global_oom_handling))]
#[stable(feature = "shared_from_slice", since = "1.21.0")]
impl<T> From<Vec<T>> for Arc<[T]> {
/// Allocate a reference-counted slice and move `v`'s items into it.
///
/// # Example
///
/// ```
/// # use std::sync::Arc;
/// let unique: Vec<i32> = vec![1, 2, 3];
/// let shared: Arc<[i32]> = Arc::from(unique);
/// assert_eq!(&[1, 2, 3], &shared[..]);
/// ```
#[inline]
fn from(mut v: Vec<T>) -> Arc<[T]> {
unsafe {
let rc = Arc::copy_from_slice(&v);
// Allow the Vec to free its memory, but not destroy its contents
v.set_len(0);
rc
}
}
}
#[stable(feature = "shared_from_cow", since = "1.45.0")]
impl<'a, B> From<Cow<'a, B>> for Arc<B>
where
B: ToOwned + ?Sized,
Arc<B>: From<&'a B> + From<B::Owned>,
{
/// Create an atomically reference-counted pointer from
/// a clone-on-write pointer by copying its content.
///
/// # Example
///
/// ```rust
/// # use std::sync::Arc;
/// # use std::borrow::Cow;
/// let cow: Cow<str> = Cow::Borrowed("eggplant");
/// let shared: Arc<str> = Arc::from(cow);
/// assert_eq!("eggplant", &shared[..]);
/// ```
#[inline]
fn from(cow: Cow<'a, B>) -> Arc<B> {
match cow {
Cow::Borrowed(s) => Arc::from(s),
Cow::Owned(s) => Arc::from(s),
}
}
}
#[stable(feature = "shared_from_str", since = "1.62.0")]
impl From<Arc<str>> for Arc<[u8]> {
/// Converts an atomically reference-counted string slice into a byte slice.
///
/// # Example
///
/// ```
/// # use std::sync::Arc;
/// let string: Arc<str> = Arc::from("eggplant");
/// let bytes: Arc<[u8]> = Arc::from(string);
/// assert_eq!("eggplant".as_bytes(), bytes.as_ref());
/// ```
#[inline]
fn from(rc: Arc<str>) -> Self {
// SAFETY: `str` has the same layout as `[u8]`.
unsafe { Arc::from_raw(Arc::into_raw(rc) as *const [u8]) }
}
}
#[stable(feature = "boxed_slice_try_from", since = "1.43.0")]
impl<T, const N: usize> TryFrom<Arc<[T]>> for Arc<[T; N]> {
type Error = Arc<[T]>;
fn try_from(boxed_slice: Arc<[T]>) -> Result<Self, Self::Error> {
if boxed_slice.len() == N {
Ok(unsafe { Arc::from_raw(Arc::into_raw(boxed_slice) as *mut [T; N]) })
} else {
Err(boxed_slice)
}
}
}
#[cfg(not(no_global_oom_handling))]
#[stable(feature = "shared_from_iter", since = "1.37.0")]
impl<T> iter::FromIterator<T> for Arc<[T]> {
/// Takes each element in the `Iterator` and collects it into an `Arc<[T]>`.
///
/// # Performance characteristics
///
/// ## The general case
///
/// In the general case, collecting into `Arc<[T]>` is done by first
/// collecting into a `Vec<T>`. That is, when writing the following:
///
/// ```rust
/// # use std::sync::Arc;
/// let evens: Arc<[u8]> = (0..10).filter(|&x| x % 2 == 0).collect();
/// # assert_eq!(&*evens, &[0, 2, 4, 6, 8]);
/// ```
///
/// this behaves as if we wrote:
///
/// ```rust
/// # use std::sync::Arc;
/// let evens: Arc<[u8]> = (0..10).filter(|&x| x % 2 == 0)
/// .collect::<Vec<_>>() // The first set of allocations happens here.
/// .into(); // A second allocation for `Arc<[T]>` happens here.
/// # assert_eq!(&*evens, &[0, 2, 4, 6, 8]);
/// ```
///
/// This will allocate as many times as needed for constructing the `Vec<T>`
/// and then it will allocate once for turning the `Vec<T>` into the `Arc<[T]>`.
///
/// ## Iterators of known length
///
/// When your `Iterator` implements `TrustedLen` and is of an exact size,
/// a single allocation will be made for the `Arc<[T]>`. For example:
///
/// ```rust
/// # use std::sync::Arc;
/// let evens: Arc<[u8]> = (0..10).collect(); // Just a single allocation happens here.
/// # assert_eq!(&*evens, &*(0..10).collect::<Vec<_>>());
/// ```
fn from_iter<I: iter::IntoIterator<Item = T>>(iter: I) -> Self {
ToArcSlice::to_arc_slice(iter.into_iter())
}
}
/// Specialization trait used for collecting into `Arc<[T]>`.
trait ToArcSlice<T>: Iterator<Item = T> + Sized {
fn to_arc_slice(self) -> Arc<[T]>;
}
#[cfg(not(no_global_oom_handling))]
impl<T, I: Iterator<Item = T>> ToArcSlice<T> for I {
default fn to_arc_slice(self) -> Arc<[T]> {
self.collect::<Vec<T>>().into()
}
}
#[cfg(not(no_global_oom_handling))]
impl<T, I: iter::TrustedLen<Item = T>> ToArcSlice<T> for I {
fn to_arc_slice(self) -> Arc<[T]> {
// This is the case for a `TrustedLen` iterator.
let (low, high) = self.size_hint();
if let Some(high) = high {
debug_assert_eq!(
low,
high,
"TrustedLen iterator's size hint is not exact: {:?}",
(low, high)
);
unsafe {
// SAFETY: We need to ensure that the iterator has an exact length and we have.
Arc::from_iter_exact(self, low)
}
} else {
// TrustedLen contract guarantees that `upper_bound == `None` implies an iterator
// length exceeding `usize::MAX`.
// The default implementation would collect into a vec which would panic.
// Thus we panic here immediately without invoking `Vec` code.
panic!("capacity overflow");
}
}
}
#[stable(feature = "rust1", since = "1.0.0")]
impl<T: ?Sized> borrow::Borrow<T> for Arc<T> {
fn borrow(&self) -> &T {
&**self
}
}
#[stable(since = "1.5.0", feature = "smart_ptr_as_ref")]
impl<T: ?Sized> AsRef<T> for Arc<T> {
fn as_ref(&self) -> &T {
&**self
}
}
#[stable(feature = "pin", since = "1.33.0")]
impl<T: ?Sized> Unpin for Arc<T> {}
/// Get the offset within an `ArcInner` for the payload behind a pointer.
///
/// # Safety
///
/// The pointer must point to (and have valid metadata for) a previously
/// valid instance of T, but the T is allowed to be dropped.
unsafe fn data_offset<T: ?Sized>(ptr: *const T) -> usize {
// Align the unsized value to the end of the ArcInner.
// Because RcBox is repr(C), it will always be the last field in memory.
// SAFETY: since the only unsized types possible are slices, trait objects,
// and extern types, the input safety requirement is currently enough to
// satisfy the requirements of align_of_val_raw; this is an implementation
// detail of the language that must not be relied upon outside of std.
unsafe { data_offset_align(align_of_val_raw(ptr)) }
}
#[inline]
fn data_offset_align(align: usize) -> usize {
let layout = Layout::new::<ArcInner<()>>();
layout.size() + layout.padding_needed_for(align)
}
#[stable(feature = "arc_error", since = "1.52.0")]
impl<T: core::error::Error + ?Sized> core::error::Error for Arc<T> {
#[allow(deprecated, deprecated_in_future)]
fn description(&self) -> &str {
core::error::Error::description(&**self)
}
#[allow(deprecated)]
fn cause(&self) -> Option<&dyn core::error::Error> {
core::error::Error::cause(&**self)
}
fn source(&self) -> Option<&(dyn core::error::Error + 'static)> {
core::error::Error::source(&**self)
}
fn provide<'a>(&'a self, req: &mut core::any::Demand<'a>) {
core::error::Error::provide(&**self, req);
}
}