std::sync::Arc - Rust
Description: API documentation for the Rust `Arc` struct in crate `std`.
Keywords: rust, rustlang, rust-lang, Arc

# Struct std::sync::Arc 1.0.0 [−] [src]

    pub struct Arc<T> where 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);Run

`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);
        });
    }Run

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);
        });
    }Run

See the `rc` documentation for more examples of reference counting in general.

## Methods

### `impl<T> Arc<T>`
[src]

#### `fn new(data: T) -> Arc<T>`

Constructs a new `Arc<T>`.

# Examples

    use std::sync::Arc;

    let five = Arc::new(5);Run

#### `fn try_unwrap(this: Arc<T>) -> Result<T, Arc<T>>`
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);Run

#### `fn into_raw(this: Arc<T>) -> *mut T`

🔬 This is a nightly-only experimental API. (`rc_raw` #37197)

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

    #![feature(rc_raw)]

    use std::sync::Arc;

    let x = Arc::new(10);
    let x_ptr = Arc::into_raw(x);
    assert_eq!(unsafe { *x_ptr }, 10);Run

#### `unsafe fn from_raw(ptr: *mut T) -> Arc<T>`

🔬 This is a nightly-only experimental API. (`rc_raw` #37197)

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

    #![feature(rc_raw)]

    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!Run

### `impl<T> Arc<T> where T: ?Sized`
[src]

#### `fn downgrade(this: &Arc<T>) -> 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);Run

#### `fn weak_count(this: &Arc<T>) -> usize`
1.15.0

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));Run

#### `fn strong_count(this: &Arc<T>) -> usize`
1.15.0

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 = five.clone();

    // This assertion is deterministic because we haven't shared
    // the `Arc` between threads.
    assert_eq!(2, Arc::strong_count(&five));Run

#### `fn ptr_eq(this: &Arc<T>, other: &Arc<T>) -> bool`

🔬 This is a nightly-only experimental API. (`ptr_eq` #36497)

newly added

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));Run

### `impl<T> Arc<T> where T: Clone`
[src]

#### `fn make_mut(this: &mut Arc<T>) -> &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);Run

### `impl<T> Arc<T> where T: ?Sized`
[src]

#### `fn get_mut(this: &mut Arc<T>) -> 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());Run

## Trait Implementations

### `impl<T> Hash for Arc<T> where T: Hash + ?Sized`
[src]

#### `fn hash<H>(&self, state: &mut H) where H: Hasher`

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> Display for Arc<T> where T: Display + ?Sized`
[src]

#### `fn fmt(&self, f: &mut Formatter) -> Result<(), Error>`

Formats the value using the given formatter.

### `impl<T> PartialOrd<Arc<T>> for Arc<T> where T: PartialOrd<T> + ?Sized`
[src]

#### `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)));Run

#### `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));Run

#### `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));Run

#### `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));Run

#### `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));Run

### `impl<T> AsRef<T> for Arc<T> where T: ?Sized`
1.5.0
[src]

#### `fn as_ref(&self) -> &T`

Performs the conversion.

### `impl<T> Sync for Arc<T> where T: Send + Sync + ?Sized`
[src]

### `impl<T> PartialEq<Arc<T>> for Arc<T> where T: PartialEq<T> + ?Sized`
[src]

#### `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));Run

#### `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));Run

### `impl<T> Send for Arc<T> where T: Send + Sync + ?Sized`
[src]

### `impl<T> Ord for Arc<T> where T: Ord + ?Sized`
[src]

#### `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)));Run

### `impl<T> Deref for Arc<T> where T: ?Sized`
[src]

#### `type Target = T`

The resulting type after dereferencing

#### `fn deref(&self) -> &T`

The method called to dereference a value

### `impl<T> Drop for Arc<T> where T: ?Sized`
[src]

#### `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!"Run

### `impl<T> Eq for Arc<T> where T: Eq + ?Sized`
[src]

### `impl<T> From<T> for Arc<T>`
1.6.0
[src]

#### `fn from(t: T) -> Arc<T>`

Performs the conversion.

### `impl<T> Pointer for Arc<T> where T: ?Sized`
[src]

#### `fn fmt(&self, f: &mut Formatter) -> Result<(), Error>`

Formats the value using the given formatter.

### `impl<T> Debug for Arc<T> where T: Debug + ?Sized`
[src]

#### `fn fmt(&self, f: &mut Formatter) -> Result<(), Error>`

Formats the value using the given formatter.

### `impl<T> Borrow<T> for Arc<T> where T: ?Sized`
[src]

#### `fn borrow(&self) -> &T`

Immutably borrows from an owned value. Read more

### `impl<T> Clone for Arc<T> where T: ?Sized`
[src]

#### `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();Run

#### `fn clone_from(&mut self, source: &Self)`

Performs copy-assignment from `source`. Read more

### `impl<T, U> CoerceUnsized<Arc<U>> for Arc<T> where T: Unsize<U> + ?Sized,
U: ?Sized`
[src]

### `impl<T> Default for Arc<T> where T: Default`
[src]

#### `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);Run

### `impl<T: RefUnwindSafe + ?Sized> UnwindSafe for Arc<T>`
1.9.0
[src]

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