1.0.0[−][src]Struct alloc::sync::Arc
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 value on the heap as the
source Arc
, while increasing a reference count. When the last Arc
pointer to a given value is destroyed, the pointed-to value is also
destroyed.
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.
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 values 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 has already been
dropped.
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
The Arc::clone(&from)
syntax is the most idiomatic because it conveys more explicitly
the meaning of the code. In the example above, this syntax makes it easier to see that
this code is creating a new reference rather than copying the whole content of foo.
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 function-like syntax:
use std::sync::Arc; let my_arc = Arc::new(()); Arc::downgrade(&my_arc);
Weak<T>
does not auto-dereference to T
, because the value may have
already been destroyed.
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::SeqCst); println!("{:?}", v); }); }
See the rc
documentation for more examples of reference
counting in general.
Methods
impl<T> Arc<T>
[src]
pub fn new(data: T) -> Arc<T>
[src]
pub fn pin(data: T) -> Pin<Arc<T>>
1.33.0[src]
Constructs a new Pin<Arc<T>>
. If T
does not implement Unpin
, then
data
will be pinned in memory and unable to be moved.
pub fn try_unwrap(this: Self) -> Result<T, Self>
1.4.0[src]
Returns the contained 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);
impl<T: ?Sized> Arc<T>
[src]
pub fn into_raw(this: Self) -> *const T
1.17.0[src]
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(10); let x_ptr = Arc::into_raw(x); assert_eq!(unsafe { *x_ptr }, 10);
pub unsafe fn from_raw(ptr: *const T) -> Self
1.17.0[src]
Constructs an Arc
from a raw pointer.
The raw pointer must have been previously returned by a call to a
Arc::into_raw
.
This function is unsafe because improper use may lead to memory problems. For example, a double-free may occur if the function is called twice on the same raw pointer.
Examples
use std::sync::Arc; let x = Arc::new(10); 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, 10); // 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!
pub fn into_raw_non_null(this: Self) -> NonNull<T>
[src]
Consumes the Arc
, returning the wrapped pointer as NonNull<T>
.
Examples
#![feature(rc_into_raw_non_null)] use std::sync::Arc; let x = Arc::new(10); let ptr = Arc::into_raw_non_null(x); let deref = unsafe { *ptr.as_ref() }; assert_eq!(deref, 10);
pub fn downgrade(this: &Self) -> Weak<T>
1.4.0[src]
Creates a new Weak
pointer to this value.
Examples
use std::sync::Arc; let five = Arc::new(5); let weak_five = Arc::downgrade(&five);
pub fn weak_count(this: &Self) -> usize
1.15.0[src]
Gets the number of Weak
pointers to this value.
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));
pub fn strong_count(this: &Self) -> usize
1.15.0[src]
Gets the number of strong (Arc
) pointers to this value.
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));
pub fn ptr_eq(this: &Self, other: &Self) -> bool
1.17.0[src]
Returns true
if the two Arc
s point to the same value (not
just values that compare as equal).
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));
impl<T: Clone> Arc<T>
[src]
pub fn make_mut(this: &mut Self) -> &mut T
1.4.0[src]
Makes a mutable reference into the given Arc
.
If there are other Arc
or Weak
pointers to the same value,
then make_mut
will invoke clone
on the inner value to
ensure unique ownership. This is also referred to as clone-on-write.
See also get_mut
, which will fail rather than cloning.
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 values. assert_eq!(*data, 8); assert_eq!(*other_data, 12);
impl<T: ?Sized> Arc<T>
[src]
pub fn get_mut(this: &mut Self) -> Option<&mut T>
1.4.0[src]
Returns a mutable reference to the inner value, if there are
no other Arc
or Weak
pointers to the same value.
Returns None
otherwise, because it is not safe to
mutate a shared value.
See also make_mut
, which will clone
the inner value when it's shared.
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());
impl Arc<dyn Any + Send + Sync>
[src]
pub fn downcast<T>(self) -> Result<Arc<T>, Self> where
T: Any + Send + Sync + 'static,
1.29.0[src]
T: Any + Send + Sync + 'static,
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); } } fn main() { let my_string = "Hello World".to_string(); print_if_string(Arc::new(my_string)); print_if_string(Arc::new(0i8)); }
Trait Implementations
impl<T: ?Sized> Deref for Arc<T>
[src]
impl<T: ?Sized + Unsize<U>, U: ?Sized> DispatchFromDyn<Arc<U>> for Arc<T>
[src]
impl<T: ?Sized + PartialEq> PartialEq<Arc<T>> for Arc<T>
[src]
fn eq(&self, other: &Arc<T>) -> bool
[src]
Equality for two Arc
s.
Two Arc
s are equal if their inner values are equal.
If T
also implements Eq
, two Arc
s that point to the same value are
always equal.
Examples
use std::sync::Arc; let five = Arc::new(5); assert!(five == Arc::new(5));
fn ne(&self, other: &Arc<T>) -> bool
[src]
Inequality for two Arc
s.
Two Arc
s are unequal if their inner values are unequal.
If T
also implements Eq
, 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));
impl<T: ?Sized + Eq> Eq for Arc<T>
[src]
impl<T: ?Sized + Ord> Ord for Arc<T>
[src]
fn cmp(&self, other: &Arc<T>) -> Ordering
[src]
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 max(self, other: Self) -> Self
1.21.0[src]
Compares and returns the maximum of two values. Read more
fn min(self, other: Self) -> Self
1.21.0[src]
Compares and returns the minimum of two values. Read more
fn clamp(self, min: Self, max: Self) -> Self
[src]
Restrict a value to a certain interval. Read more
impl<T: ?Sized + PartialOrd> PartialOrd<Arc<T>> for Arc<T>
[src]
fn partial_cmp(&self, other: &Arc<T>) -> Option<Ordering>
[src]
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 lt(&self, other: &Arc<T>) -> bool
[src]
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 le(&self, other: &Arc<T>) -> bool
[src]
'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 gt(&self, other: &Arc<T>) -> bool
[src]
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 ge(&self, other: &Arc<T>) -> bool
[src]
'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));
impl<T: ?Sized + Hash> Hash for Arc<T>
[src]
fn hash<H: Hasher>(&self, state: &mut H)
[src]
fn hash_slice<H>(data: &[Self], state: &mut H) where
H: Hasher,
1.3.0[src]
H: Hasher,
Feeds a slice of this type into the given [Hasher
]. Read more
impl<T: ?Sized + Sync + Send> Send for Arc<T>
[src]
impl<T: ?Sized + Sync + Send> Sync for Arc<T>
[src]
impl<T: ?Sized> Unpin for Arc<T>
1.33.0[src]
impl<T: ?Sized + Display> Display for Arc<T>
[src]
impl<T: ?Sized + Debug> Debug for Arc<T>
[src]
impl<T: ?Sized> Receiver for Arc<T>
[src]
impl<T: ?Sized> Drop for Arc<T>
[src]
fn drop(&mut self)
[src]
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!"
impl<T: ?Sized + Unsize<U>, U: ?Sized> CoerceUnsized<Arc<U>> for Arc<T>
[src]
impl<T: ?Sized> AsRef<T> for Arc<T>
1.5.0[src]
impl<T> From<T> for Arc<T>
1.6.0[src]
impl<'_, T: Clone> From<&'_ [T]> for Arc<[T]>
1.21.0[src]
impl<'_> From<&'_ str> for Arc<str>
1.21.0[src]
impl From<String> for Arc<str>
1.21.0[src]
impl<T: ?Sized> From<Box<T>> for Arc<T>
1.21.0[src]
impl<T> From<Vec<T>> for Arc<[T]>
1.21.0[src]
impl<T: ?Sized> Pointer for Arc<T>
[src]
impl<T: ?Sized> Clone for Arc<T>
[src]
fn clone(&self) -> Arc<T>
[src]
Makes a clone of the Arc
pointer.
This creates another pointer to the same inner value, increasing the strong reference count.
Examples
use std::sync::Arc; let five = Arc::new(5); let _ = Arc::clone(&five);
fn clone_from(&mut self, source: &Self)
[src]
Performs copy-assignment from source
. Read more
impl<T: Default> Default for Arc<T>
[src]
fn default() -> Arc<T>
[src]
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);
impl<T: ?Sized> Borrow<T> for Arc<T>
[src]
Blanket Implementations
impl<T> ToOwned for T where
T: Clone,
[src]
T: Clone,
type Owned = T
The resulting type after obtaining ownership.
fn to_owned(&Self) -> T
[src]
fn clone_into(&Self, &mut T)
[src]
impl<T> ToString for T where
T: Display + ?Sized,
[src]
T: Display + ?Sized,
impl<T, U> TryFrom<U> for T where
U: Into<T>,
[src]
U: Into<T>,
type Error = Infallible
The type returned in the event of a conversion error.
fn try_from(value: U) -> Result<T, <T as TryFrom<U>>::Error>
[src]
impl<T, U> Into<U> for T where
U: From<T>,
[src]
U: From<T>,
impl<T> From<T> for T
[src]
impl<T, U> TryInto<U> for T where
U: TryFrom<T>,
[src]
U: TryFrom<T>,
type Error = <U as TryFrom<T>>::Error
The type returned in the event of a conversion error.
fn try_into(self) -> Result<U, <U as TryFrom<T>>::Error>
[src]
impl<T> Borrow<T> for T where
T: ?Sized,
[src]
T: ?Sized,
impl<T> BorrowMut<T> for T where
T: ?Sized,
[src]
T: ?Sized,
fn borrow_mut(&mut self) -> &mut T
[src]
impl<T> Any for T where
T: 'static + ?Sized,
[src]
T: 'static + ?Sized,