Struct alloc::arc::Arc 1.0.0
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pub struct Arc<T: ?Sized> { /* fields omitted */ }
A thread-safe reference-counting pointer.
The type Arc<T>
provides shared ownership of a value of type T
,
allocated in the heap. Invoking clone
on Arc
produces
a new pointer to the same value in the heap. 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. If you need to mutate through an Arc
, use Mutex
,
RwLock
, or one of the Atomic
types.
Arc
uses atomic operations for reference counting, so Arc
s can be
sent between threads. In other words, Arc<T>
implements Send
as long as T
implements Send
and Sync
. 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::Rc
for lower overhead. Rc
is a safe default, because
the compiler will catch any attempt to send an Rc
between threads.
However, a library might choose Arc
in order to give library consumers
more flexibility.
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.
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 = five.clone(); 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 = val.clone(); 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>
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fn new(data: T) -> Arc<T>
fn try_unwrap(this: Self) -> Result<T, Self>
1.4.0
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 = x.clone(); assert_eq!(*Arc::try_unwrap(x).unwrap_err(), 4);
impl<T: ?Sized> Arc<T>
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fn downgrade(this: &Self) -> Weak<T>
1.4.0
Creates a new Weak
pointer to this value.
Examples
use std::sync::Arc; let five = Arc::new(5); let weak_five = Arc::downgrade(&five);
fn weak_count(this: &Self) -> usize
Gets the number of Weak
pointers to this value.
Be careful how you use this information, because another thread may change the weak count at any time.
Examples
#![feature(arc_counts)] 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));
fn strong_count(this: &Self) -> usize
Gets the number of strong (Arc
) pointers to this value.
Be careful how you use this information, because another thread may change the strong count at any time.
Examples
#![feature(arc_counts)] use std::sync::Arc; let five = Arc::new(5); let _also_five = five.clone(); // This assertion is deterministic because we haven't shared // the `Arc` between threads. assert_eq!(2, Arc::strong_count(&five));
fn ptr_eq(this: &Self, other: &Self) -> bool
Returns true if the two Arc
s point to the same value (not
just values that compare as equal).
Examples
#![feature(ptr_eq)] use std::sync::Arc; let five = Arc::new(5); let same_five = five.clone(); 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>
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fn make_mut(this: &mut Self) -> &mut T
1.4.0
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 = data.clone(); // 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>
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fn get_mut(this: &mut Self) -> Option<&mut T>
1.4.0
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 = x.clone(); assert!(Arc::get_mut(&mut x).is_none());
Trait Implementations
impl<T: ?Sized + Sync + Send> Send for Arc<T>
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impl<T: ?Sized + Sync + Send> Sync for Arc<T>
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impl<T: ?Sized + Unsize<U>, U: ?Sized> CoerceUnsized<Arc<U>> for Arc<T>
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impl<T: ?Sized> Clone for Arc<T>
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fn clone(&self) -> Arc<T>
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); five.clone();
fn clone_from(&mut self, source: &Self)
Performs copy-assignment from source
. Read more
impl<T: ?Sized> Deref for Arc<T>
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type Target = T
The resulting type after dereferencing
fn deref(&self) -> &T
The method called to dereference a value
impl<T: ?Sized> Drop for Arc<T>
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fn drop(&mut self)
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 = foo.clone(); drop(foo); // Doesn't print anything drop(foo2); // Prints "dropped!"
impl<T: ?Sized + PartialEq> PartialEq for Arc<T>
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fn eq(&self, other: &Arc<T>) -> bool
Equality for two Arc
s.
Two Arc
s are equal if their inner values are equal.
Examples
use std::sync::Arc; let five = Arc::new(5); assert!(five == Arc::new(5));
fn ne(&self, other: &Arc<T>) -> bool
Inequality for two Arc
s.
Two Arc
s are unequal if their inner values are unequal.
Examples
use std::sync::Arc; let five = Arc::new(5); assert!(five != Arc::new(6));
impl<T: ?Sized + PartialOrd> PartialOrd for Arc<T>
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fn partial_cmp(&self, other: &Arc<T>) -> Option<Ordering>
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
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
'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
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
'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 + Ord> Ord for Arc<T>
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fn cmp(&self, other: &Arc<T>) -> Ordering
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)));
impl<T: ?Sized + Eq> Eq for Arc<T>
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impl<T: ?Sized + Display> Display for Arc<T>
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impl<T: ?Sized + Debug> Debug for Arc<T>
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impl<T: ?Sized> Pointer for Arc<T>
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impl<T: Default> Default for Arc<T>
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fn default() -> 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);
impl<T: ?Sized + Hash> Hash for Arc<T>
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fn hash<H: Hasher>(&self, state: &mut H)
Feeds this value into the state given, updating the hasher as necessary.
fn hash_slice<H>(data: &[Self], state: &mut H) where H: Hasher
1.3.0
Feeds a slice of this type into the state provided.
impl<T> From<T> for Arc<T>
1.6.0[src]
fn from(t: T) -> Self
Performs the conversion.
impl<T: ?Sized> Borrow<T> for Arc<T>
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impl<T: ?Sized> AsRef<T> for Arc<T>
1.5.0[src]
fn as_ref(&self) -> &T
Performs the conversion.