pub struct Arc<T: ?Sized, A: Allocator = Global> { /* private fields */ }
Expand description
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
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
, RwLock
, or one of the 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
.
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
Arc<RefCell<T>>
. RefCell<T>
isn’t Sync
, and if Arc<T>
was always
Send
, Arc<RefCell<T>>
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>
.
§Breaking cycles with Weak
The downgrade
method can be used to create a non-owning
Weak
pointer. A Weak
pointer can be 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>
and Weak<T>
.
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
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:
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>
does not auto-dereference to T
, because the inner value may have
already been dropped.
§Examples
Sharing some immutable data between threads:
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
:
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::Relaxed);
println!("{v:?}");
});
}
See the rc
documentation for more examples of reference
counting in general.
Implementations§
source§impl<T> Arc<T>
impl<T> Arc<T>
1.60.0 · sourcepub fn new_cyclic<F>(data_fn: F) -> Arc<T>
pub fn new_cyclic<F>(data_fn: F) -> Arc<T>
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
use std::sync::{Arc, Weak};
struct Gadget {
me: Weak<Gadget>,
}
impl Gadget {
/// Constructs 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() }
})
}
/// Returns a reference counted pointer to Self.
fn me(&self) -> Arc<Self> {
self.me.upgrade().unwrap()
}
}
1.82.0 · sourcepub fn new_uninit() -> Arc<MaybeUninit<T>>
pub fn new_uninit() -> Arc<MaybeUninit<T>>
Constructs a new Arc
with uninitialized contents.
§Examples
sourcepub fn new_zeroed() -> Arc<MaybeUninit<T>>
🔬This is a nightly-only experimental API. (new_zeroed_alloc
#129396)
pub fn new_zeroed() -> Arc<MaybeUninit<T>>
new_zeroed_alloc
#129396)Constructs a new Arc
with uninitialized contents, with the memory
being filled with 0
bytes.
See MaybeUninit::zeroed
for examples of correct and incorrect usage
of this method.
§Examples
1.33.0 · sourcepub fn pin(data: T) -> Pin<Arc<T>>
pub fn pin(data: T) -> Pin<Arc<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.
sourcepub fn try_pin(data: T) -> Result<Pin<Arc<T>>, AllocError>
🔬This is a nightly-only experimental API. (allocator_api
#32838)
pub fn try_pin(data: T) -> Result<Pin<Arc<T>>, AllocError>
allocator_api
#32838)Constructs a new Pin<Arc<T>>
, return an error if allocation fails.
sourcepub fn try_new(data: T) -> Result<Arc<T>, AllocError>
🔬This is a nightly-only experimental API. (allocator_api
#32838)
pub fn try_new(data: T) -> Result<Arc<T>, AllocError>
allocator_api
#32838)Constructs a new Arc<T>
, returning an error if allocation fails.
§Examples
sourcepub fn try_new_uninit() -> Result<Arc<MaybeUninit<T>>, AllocError>
🔬This is a nightly-only experimental API. (allocator_api
#32838)
pub fn try_new_uninit() -> Result<Arc<MaybeUninit<T>>, AllocError>
allocator_api
#32838)Constructs a new Arc
with uninitialized contents, returning an error
if allocation fails.
§Examples
sourcepub fn try_new_zeroed() -> Result<Arc<MaybeUninit<T>>, AllocError>
🔬This is a nightly-only experimental API. (allocator_api
#32838)
pub fn try_new_zeroed() -> Result<Arc<MaybeUninit<T>>, AllocError>
allocator_api
#32838)Constructs a new Arc
with uninitialized contents, with the memory
being filled with 0
bytes, returning an error if allocation fails.
See MaybeUninit::zeroed
for examples of correct and incorrect usage
of this method.
§Examples
source§impl<T, A: Allocator> Arc<T, A>
impl<T, A: Allocator> Arc<T, A>
sourcepub fn new_in(data: T, alloc: A) -> Arc<T, A>
🔬This is a nightly-only experimental API. (allocator_api
#32838)
pub fn new_in(data: T, alloc: A) -> Arc<T, A>
allocator_api
#32838)Constructs a new Arc<T>
in the provided allocator.
§Examples
sourcepub fn new_uninit_in(alloc: A) -> Arc<MaybeUninit<T>, A>
🔬This is a nightly-only experimental API. (allocator_api
#32838)
pub fn new_uninit_in(alloc: A) -> Arc<MaybeUninit<T>, A>
allocator_api
#32838)Constructs a new Arc
with uninitialized contents in the provided allocator.
§Examples
#![feature(get_mut_unchecked)]
#![feature(allocator_api)]
use std::sync::Arc;
use std::alloc::System;
let mut five = Arc::<u32, _>::new_uninit_in(System);
let five = unsafe {
// Deferred initialization:
Arc::get_mut_unchecked(&mut five).as_mut_ptr().write(5);
five.assume_init()
};
assert_eq!(*five, 5)
sourcepub fn new_zeroed_in(alloc: A) -> Arc<MaybeUninit<T>, A>
🔬This is a nightly-only experimental API. (allocator_api
#32838)
pub fn new_zeroed_in(alloc: A) -> Arc<MaybeUninit<T>, A>
allocator_api
#32838)Constructs a new Arc
with uninitialized contents, with the memory
being filled with 0
bytes, in the provided allocator.
See MaybeUninit::zeroed
for examples of correct and incorrect usage
of this method.
§Examples
sourcepub fn pin_in(data: T, alloc: A) -> Pin<Arc<T, A>>where
A: 'static,
🔬This is a nightly-only experimental API. (allocator_api
#32838)
pub fn pin_in(data: T, alloc: A) -> Pin<Arc<T, A>>where
A: 'static,
allocator_api
#32838)Constructs a new Pin<Arc<T, A>>
in the provided allocator. If T
does not implement Unpin
,
then data
will be pinned in memory and unable to be moved.
sourcepub fn try_pin_in(data: T, alloc: A) -> Result<Pin<Arc<T, A>>, AllocError>where
A: 'static,
🔬This is a nightly-only experimental API. (allocator_api
#32838)
pub fn try_pin_in(data: T, alloc: A) -> Result<Pin<Arc<T, A>>, AllocError>where
A: 'static,
allocator_api
#32838)Constructs a new Pin<Arc<T, A>>
in the provided allocator, return an error if allocation
fails.
sourcepub fn try_new_in(data: T, alloc: A) -> Result<Arc<T, A>, AllocError>
🔬This is a nightly-only experimental API. (allocator_api
#32838)
pub fn try_new_in(data: T, alloc: A) -> Result<Arc<T, A>, AllocError>
allocator_api
#32838)Constructs a new Arc<T, A>
in the provided allocator, returning an error if allocation fails.
§Examples
sourcepub fn try_new_uninit_in(alloc: A) -> Result<Arc<MaybeUninit<T>, A>, AllocError>
🔬This is a nightly-only experimental API. (allocator_api
#32838)
pub fn try_new_uninit_in(alloc: A) -> Result<Arc<MaybeUninit<T>, A>, AllocError>
allocator_api
#32838)Constructs a new Arc
with uninitialized contents, in the provided allocator, returning an
error if allocation fails.
§Examples
#![feature(allocator_api)]
#![feature(get_mut_unchecked)]
use std::sync::Arc;
use std::alloc::System;
let mut five = Arc::<u32, _>::try_new_uninit_in(System)?;
let five = unsafe {
// Deferred initialization:
Arc::get_mut_unchecked(&mut five).as_mut_ptr().write(5);
five.assume_init()
};
assert_eq!(*five, 5);
sourcepub fn try_new_zeroed_in(alloc: A) -> Result<Arc<MaybeUninit<T>, A>, AllocError>
🔬This is a nightly-only experimental API. (allocator_api
#32838)
pub fn try_new_zeroed_in(alloc: A) -> Result<Arc<MaybeUninit<T>, A>, AllocError>
allocator_api
#32838)Constructs a new Arc
with uninitialized contents, with the memory
being filled with 0
bytes, in the provided allocator, returning an error if allocation
fails.
See MaybeUninit::zeroed
for examples of correct and incorrect usage
of this method.
§Examples
1.4.0 · sourcepub fn try_unwrap(this: Self) -> Result<T, Self>
pub fn try_unwrap(this: Self) -> Result<T, Self>
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.
It is strongly recommended to use Arc::into_inner
instead if you don’t
keep the Arc
in the Err
case.
Immediately dropping the Err
-value, as the expression
Arc::try_unwrap(this).ok()
does, can cause the strong count to
drop to zero and the inner value of the Arc
to be dropped.
For instance, if two threads execute such an expression in parallel,
there is a race condition without the possibility of unsafety:
The threads could first both check whether they own the last instance
in Arc::try_unwrap
, determine that they both do not, and then both
discard and drop their instance in the call to ok
.
In this scenario, the value inside the Arc
is safely destroyed
by exactly one of the threads, but neither thread will ever be able
to use the value.
§Examples
1.70.0 · sourcepub fn into_inner(this: Self) -> Option<T>
pub fn into_inner(this: Self) -> Option<T>
Returns the inner value, if the Arc
has exactly one strong reference.
Otherwise, None
is returned and the Arc
is dropped.
This will succeed even if there are outstanding weak references.
If Arc::into_inner
is called on every clone of this Arc
,
it is guaranteed that exactly one of the calls returns the inner value.
This means in particular that the inner value is not dropped.
Arc::try_unwrap
is conceptually similar to Arc::into_inner
, but it
is meant for different use-cases. If used as a direct replacement
for Arc::into_inner
anyway, such as with the expression
Arc::try_unwrap(this).ok()
, then it does
not give the same guarantee as described in the previous paragraph.
For more information, see the examples below and read the documentation
of Arc::try_unwrap
.
§Examples
Minimal example demonstrating the guarantee that Arc::into_inner
gives.
use std::sync::Arc;
let x = Arc::new(3);
let y = Arc::clone(&x);
// Two threads calling `Arc::into_inner` on both clones of an `Arc`:
let x_thread = std::thread::spawn(|| Arc::into_inner(x));
let y_thread = std::thread::spawn(|| Arc::into_inner(y));
let x_inner_value = x_thread.join().unwrap();
let y_inner_value = y_thread.join().unwrap();
// One of the threads is guaranteed to receive the inner value:
assert!(matches!(
(x_inner_value, y_inner_value),
(None, Some(3)) | (Some(3), None)
));
// The result could also be `(None, None)` if the threads called
// `Arc::try_unwrap(x).ok()` and `Arc::try_unwrap(y).ok()` instead.
A more practical example demonstrating the need for Arc::into_inner
:
use std::sync::Arc;
// Definition of a simple singly linked list using `Arc`:
#[derive(Clone)]
struct LinkedList<T>(Option<Arc<Node<T>>>);
struct Node<T>(T, Option<Arc<Node<T>>>);
// Dropping a long `LinkedList<T>` relying on the destructor of `Arc`
// can cause a stack overflow. To prevent this, we can provide a
// manual `Drop` implementation that does the destruction in a loop:
impl<T> Drop for LinkedList<T> {
fn drop(&mut self) {
let mut link = self.0.take();
while let Some(arc_node) = link.take() {
if let Some(Node(_value, next)) = Arc::into_inner(arc_node) {
link = next;
}
}
}
}
// Implementation of `new` and `push` omitted
impl<T> LinkedList<T> {
/* ... */
}
// The following code could have still caused a stack overflow
// despite the manual `Drop` impl if that `Drop` impl had used
// `Arc::try_unwrap(arc).ok()` instead of `Arc::into_inner(arc)`.
// Create a long list and clone it
let mut x = LinkedList::new();
let size = 100000;
for i in 0..size {
x.push(i); // Adds i to the front of x
}
let y = x.clone();
// Drop the clones in parallel
let x_thread = std::thread::spawn(|| drop(x));
let y_thread = std::thread::spawn(|| drop(y));
x_thread.join().unwrap();
y_thread.join().unwrap();
source§impl<T> Arc<[T]>
impl<T> Arc<[T]>
1.82.0 · sourcepub fn new_uninit_slice(len: usize) -> Arc<[MaybeUninit<T>]>
pub fn new_uninit_slice(len: usize) -> Arc<[MaybeUninit<T>]>
Constructs a new atomically reference-counted slice with uninitialized contents.
§Examples
#![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])
sourcepub fn new_zeroed_slice(len: usize) -> Arc<[MaybeUninit<T>]>
🔬This is a nightly-only experimental API. (new_zeroed_alloc
#129396)
pub fn new_zeroed_slice(len: usize) -> Arc<[MaybeUninit<T>]>
new_zeroed_alloc
#129396)Constructs a new atomically reference-counted slice with uninitialized contents, with the memory being
filled with 0
bytes.
See MaybeUninit::zeroed
for examples of correct and
incorrect usage of this method.
§Examples
source§impl<T, A: Allocator> Arc<[T], A>
impl<T, A: Allocator> Arc<[T], A>
sourcepub fn new_uninit_slice_in(len: usize, alloc: A) -> Arc<[MaybeUninit<T>], A>
🔬This is a nightly-only experimental API. (allocator_api
#32838)
pub fn new_uninit_slice_in(len: usize, alloc: A) -> Arc<[MaybeUninit<T>], A>
allocator_api
#32838)Constructs a new atomically reference-counted slice with uninitialized contents in the provided allocator.
§Examples
#![feature(get_mut_unchecked)]
#![feature(allocator_api)]
use std::sync::Arc;
use std::alloc::System;
let mut values = Arc::<[u32], _>::new_uninit_slice_in(3, System);
let values = unsafe {
// Deferred initialization:
Arc::get_mut_unchecked(&mut values)[0].as_mut_ptr().write(1);
Arc::get_mut_unchecked(&mut values)[1].as_mut_ptr().write(2);
Arc::get_mut_unchecked(&mut values)[2].as_mut_ptr().write(3);
values.assume_init()
};
assert_eq!(*values, [1, 2, 3])
sourcepub fn new_zeroed_slice_in(len: usize, alloc: A) -> Arc<[MaybeUninit<T>], A>
🔬This is a nightly-only experimental API. (allocator_api
#32838)
pub fn new_zeroed_slice_in(len: usize, alloc: A) -> Arc<[MaybeUninit<T>], A>
allocator_api
#32838)Constructs a new atomically reference-counted slice with uninitialized contents, with the memory being
filled with 0
bytes, in the provided allocator.
See MaybeUninit::zeroed
for examples of correct and
incorrect usage of this method.
§Examples
source§impl<T, A: Allocator> Arc<MaybeUninit<T>, A>
impl<T, A: Allocator> Arc<MaybeUninit<T>, A>
1.82.0 · sourcepub unsafe fn assume_init(self) -> Arc<T, A>
pub unsafe fn assume_init(self) -> Arc<T, A>
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.
§Examples
source§impl<T, A: Allocator> Arc<[MaybeUninit<T>], A>
impl<T, A: Allocator> Arc<[MaybeUninit<T>], A>
1.82.0 · sourcepub unsafe fn assume_init(self) -> Arc<[T], A>
pub unsafe fn assume_init(self) -> Arc<[T], A>
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.
§Examples
#![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])
source§impl<T: ?Sized> Arc<T>
impl<T: ?Sized> Arc<T>
1.17.0 · sourcepub unsafe fn from_raw(ptr: *const T) -> Self
pub unsafe fn from_raw(ptr: *const T) -> Self
Constructs an Arc<T>
from a raw pointer.
The raw pointer must have been previously returned by a call to
Arc<U>::into_raw
with the following requirements:
- If
U
is sized, it must have the same size and alignment asT
. This is trivially true ifU
isT
. - If
U
is unsized, its data pointer must have the same size and alignment asT
. This is trivially true ifArc<U>
was constructed throughArc<T>
and then converted toArc<U>
through an unsized coercion.
Note that if U
or U
’s data pointer is not T
but has the same size
and alignment, this is basically like transmuting references of
different types. See mem::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.
§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!
Convert a slice back into its original array:
1.51.0 · sourcepub unsafe fn increment_strong_count(ptr: *const T)
pub unsafe fn increment_strong_count(ptr: *const T)
1.51.0 · sourcepub unsafe fn decrement_strong_count(ptr: *const T)
pub unsafe fn decrement_strong_count(ptr: *const T)
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));
}
source§impl<T: ?Sized, A: Allocator> Arc<T, A>
impl<T: ?Sized, A: Allocator> Arc<T, A>
sourcepub fn allocator(this: &Self) -> &A
🔬This is a nightly-only experimental API. (allocator_api
#32838)
pub fn allocator(this: &Self) -> &A
allocator_api
#32838)Returns a reference to the underlying allocator.
Note: this is an associated function, which means that you have
to call it as Arc::allocator(&a)
instead of a.allocator()
. This
is so that there is no conflict with a method on the inner type.
1.17.0 · sourcepub fn into_raw(this: Self) -> *const T
pub fn into_raw(this: Self) -> *const 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
sourcepub fn into_raw_with_allocator(this: Self) -> (*const T, A)
🔬This is a nightly-only experimental API. (allocator_api
#32838)
pub fn into_raw_with_allocator(this: Self) -> (*const T, A)
allocator_api
#32838)Consumes the Arc
, returning the wrapped pointer and allocator.
To avoid a memory leak the pointer must be converted back to an Arc
using
Arc::from_raw_in
.
§Examples
1.45.0 · sourcepub fn as_ptr(this: &Self) -> *const T
pub fn as_ptr(this: &Self) -> *const T
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
sourcepub unsafe fn from_raw_in(ptr: *const T, alloc: A) -> Self
🔬This is a nightly-only experimental API. (allocator_api
#32838)
pub unsafe fn from_raw_in(ptr: *const T, alloc: A) -> Self
allocator_api
#32838)Constructs an Arc<T, A>
from a raw pointer.
The raw pointer must have been previously returned by a call to Arc<U, A>::into_raw
with the following requirements:
- If
U
is sized, it must have the same size and alignment asT
. This is trivially true ifU
isT
. - If
U
is unsized, its data pointer must have the same size and alignment asT
. This is trivially true ifArc<U>
was constructed throughArc<T>
and then converted toArc<U>
through an unsized coercion.
Note that if U
or U
’s data pointer is not T
but has the same size
and alignment, this is basically like transmuting references of
different types. See mem::transmute
for more information
on what restrictions apply in this case.
The raw pointer must point to a block of memory allocated by alloc
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.
§Examples
#![feature(allocator_api)]
use std::sync::Arc;
use std::alloc::System;
let x = Arc::new_in("hello".to_owned(), System);
let x_ptr = Arc::into_raw(x);
unsafe {
// Convert back to an `Arc` to prevent leak.
let x = Arc::from_raw_in(x_ptr, System);
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!
Convert a slice back into its original array:
1.15.0 · sourcepub fn weak_count(this: &Self) -> usize
pub fn weak_count(this: &Self) -> usize
1.15.0 · sourcepub fn strong_count(this: &Self) -> usize
pub fn strong_count(this: &Self) -> usize
sourcepub unsafe fn increment_strong_count_in(ptr: *const T, alloc: A)where
A: Clone,
🔬This is a nightly-only experimental API. (allocator_api
#32838)
pub unsafe fn increment_strong_count_in(ptr: *const T, alloc: A)where
A: Clone,
allocator_api
#32838)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,, and ptr
must point to a block of memory
allocated by alloc
.
§Examples
#![feature(allocator_api)]
use std::sync::Arc;
use std::alloc::System;
let five = Arc::new_in(5, System);
unsafe {
let ptr = Arc::into_raw(five);
Arc::increment_strong_count_in(ptr, System);
// This assertion is deterministic because we haven't shared
// the `Arc` between threads.
let five = Arc::from_raw_in(ptr, System);
assert_eq!(2, Arc::strong_count(&five));
}
sourcepub unsafe fn decrement_strong_count_in(ptr: *const T, alloc: A)
🔬This is a nightly-only experimental API. (allocator_api
#32838)
pub unsafe fn decrement_strong_count_in(ptr: *const T, alloc: A)
allocator_api
#32838)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
, the
associated Arc
instance must be valid (i.e. the strong count must be at
least 1) when invoking this method, and ptr
must point to a block of memory
allocated by alloc
. 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
#![feature(allocator_api)]
use std::sync::Arc;
use std::alloc::System;
let five = Arc::new_in(5, System);
unsafe {
let ptr = Arc::into_raw(five);
Arc::increment_strong_count_in(ptr, System);
// Those assertions are deterministic because we haven't shared
// the `Arc` between threads.
let five = Arc::from_raw_in(ptr, System);
assert_eq!(2, Arc::strong_count(&five));
Arc::decrement_strong_count_in(ptr, System);
assert_eq!(1, Arc::strong_count(&five));
}
source§impl<T: ?Sized + CloneToUninit, A: Allocator + Clone> Arc<T, A>
impl<T: ?Sized + CloneToUninit, A: Allocator + Clone> Arc<T, A>
1.4.0 · sourcepub fn make_mut(this: &mut Self) -> &mut T
pub fn make_mut(this: &mut Self) -> &mut 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.
§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:
source§impl<T: Clone, A: Allocator> Arc<T, A>
impl<T: Clone, A: Allocator> Arc<T, A>
1.76.0 · sourcepub fn unwrap_or_clone(this: Self) -> T
pub fn unwrap_or_clone(this: Self) -> T
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
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()));
source§impl<T: ?Sized, A: Allocator> Arc<T, A>
impl<T: ?Sized, A: Allocator> Arc<T, A>
1.4.0 · sourcepub fn get_mut(this: &mut Self) -> Option<&mut T>
pub fn get_mut(this: &mut Self) -> Option<&mut T>
sourcepub unsafe fn get_mut_unchecked(this: &mut Self) -> &mut T
🔬This is a nightly-only experimental API. (get_mut_unchecked
#63292)
pub unsafe fn get_mut_unchecked(this: &mut Self) -> &mut T
get_mut_unchecked
#63292)Returns a mutable reference into the given Arc
,
without any check.
See also get_mut
, which is safe and does appropriate checks.
§Safety
If any other Arc
or Weak
pointers to the same allocation exist, then
they 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.
#![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.
#![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
source§impl<A: Allocator> Arc<dyn Any + Send + Sync, A>
impl<A: Allocator> Arc<dyn Any + Send + Sync, A>
1.29.0 · sourcepub fn downcast<T>(self) -> Result<Arc<T, A>, Self>
pub fn downcast<T>(self) -> Result<Arc<T, A>, Self>
Attempts 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));
sourcepub unsafe fn downcast_unchecked<T>(self) -> Arc<T, A>
🔬This is a nightly-only experimental API. (downcast_unchecked
#90850)
pub unsafe fn downcast_unchecked<T>(self) -> Arc<T, A>
downcast_unchecked
#90850)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.
Trait Implementations§
1.0.0 · source§impl<T: ?Sized, A: Allocator + Clone> Clone for Arc<T, A>
impl<T: ?Sized, A: Allocator + Clone> Clone for Arc<T, A>
1.0.0 · source§impl<T: ?Sized, A: Allocator> Drop for Arc<T, A>
impl<T: ?Sized, A: Allocator> Drop for Arc<T, A>
1.52.0 · source§impl<T: Error + ?Sized> Error for Arc<T>
impl<T: Error + ?Sized> Error for Arc<T>
source§fn description(&self) -> &str
fn description(&self) -> &str
source§fn cause(&self) -> Option<&dyn Error>
fn cause(&self) -> Option<&dyn Error>
1.37.0 · source§impl<T> FromIterator<T> for Arc<[T]>
impl<T> FromIterator<T> for Arc<[T]>
source§fn from_iter<I: IntoIterator<Item = T>>(iter: I) -> Self
fn from_iter<I: IntoIterator<Item = T>>(iter: I) -> Self
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:
this behaves as if we wrote:
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.
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:
1.0.0 · source§impl<T: ?Sized + Ord, A: Allocator> Ord for Arc<T, A>
impl<T: ?Sized + Ord, A: Allocator> Ord for Arc<T, A>
source§fn cmp(&self, other: &Arc<T, A>) -> Ordering
fn cmp(&self, other: &Arc<T, A>) -> Ordering
1.21.0 · source§fn max(self, other: Self) -> Selfwhere
Self: Sized,
fn max(self, other: Self) -> Selfwhere
Self: Sized,
1.0.0 · source§impl<T: ?Sized + PartialEq, A: Allocator> PartialEq for Arc<T, A>
impl<T: ?Sized + PartialEq, A: Allocator> PartialEq for Arc<T, A>
source§fn eq(&self, other: &Arc<T, A>) -> bool
fn eq(&self, other: &Arc<T, A>) -> bool
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
1.0.0 · source§impl<T: ?Sized + PartialOrd, A: Allocator> PartialOrd for Arc<T, A>
impl<T: ?Sized + PartialOrd, A: Allocator> PartialOrd for Arc<T, A>
source§fn partial_cmp(&self, other: &Arc<T, A>) -> Option<Ordering>
fn partial_cmp(&self, other: &Arc<T, A>) -> Option<Ordering>
Partial comparison for two Arc
s.
The two are compared by calling partial_cmp()
on their inner values.
§Examples
source§fn lt(&self, other: &Arc<T, A>) -> bool
fn lt(&self, other: &Arc<T, A>) -> bool
Less-than comparison for two Arc
s.
The two are compared by calling <
on their inner values.
§Examples
source§fn le(&self, other: &Arc<T, A>) -> bool
fn le(&self, other: &Arc<T, A>) -> bool
‘Less than or equal to’ comparison for two Arc
s.
The two are compared by calling <=
on their inner values.