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. '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 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: 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 and b both point to the same memory location as foo.Run

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);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 = Arc::clone(&five);

        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 = Arc::clone(&val);

        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]

#### `pub fn new(data: T) -> Arc<T>`
[src]

Constructs a new `Arc<T>`.

# Examples

    use std::sync::Arc;

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

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

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

#### `pub fn into_raw(this: Arc<T>) -> *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);Run

#### `pub unsafe fn from_raw(ptr: *const T) -> Arc<T>`
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!Run

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

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

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

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

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

####
ⓘImportant traits for &'a mut I

### Important traits for &'a mut I

`impl<'a, I> Iterator for &'a mut I where
I: Iterator + ?Sized, type Item = <I as Iterator>::Item;impl<'a, R: Read + ?Sized> Read for &'a mut Rimpl<'a, W: Write + ?Sized> Write for &'a mut W`
`pub fn make_mut(this: &mut Arc<T>) -> &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);Run

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

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

## Trait Implementations

### `impl<T> From<Box<T>> for Arc<T> where
T: ?Sized,`
1.21.0
[src]

#### `fn from(v: Box<T>) -> Arc<T>`
[src]

Performs the conversion.

### `impl From<String> for Arc<str>`
1.21.0
[src]

#### `fn from(v: String) -> Arc<str>`
[src]

Performs the conversion.

### `impl<T> From<Vec<T>> for Arc<[T]>`
1.21.0
[src]

#### `fn from(v: Vec<T>) -> Arc<[T]>`
[src]

Performs the conversion.

### `impl<'a> From<&'a str> for Arc<str>`
1.21.0
[src]

#### `fn from(v: &str) -> Arc<str>`
[src]

Performs the conversion.

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

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

Performs the conversion.

### `impl<'a, T> From<&'a [T]> for Arc<[T]> where
T: Clone,`
1.21.0
[src]

#### `fn from(v: &[T]) -> Arc<[T]>`
[src]

Performs the conversion.

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

### `impl<T> PartialOrd<Arc<T>> for Arc<T> where
T: PartialOrd<T> + ?Sized,`
[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)));Run

#### `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));Run

#### `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));Run

#### `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));Run

#### `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));Run

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

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

Formats the value using the given formatter. Read more

### `impl<T> PartialEq<Arc<T>> for Arc<T> where
T: PartialEq<T> + ?Sized,`
[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.

# Examples

    use std::sync::Arc;

    let five = Arc::new(5);

    assert!(five == Arc::new(5));Run

#### `fn ne(&self, other: &Arc<T>) -> bool`
[src]

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> Drop for Arc<T> where
T: ?Sized,`
[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!"Run

### `impl<T> Default for Arc<T> where
T: Default,`
[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);Run

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

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

Feeds this value into the given [`Hasher`]. Read more

#### `fn hash_slice<H>(data: &[Self], state: &mut H) where
H: Hasher,`
1.3.0
[src]

Feeds a slice of this type into the given [`Hasher`]. Read more

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

####
ⓘImportant traits for &'a mut I

### Important traits for &'a mut I

`impl<'a, I> Iterator for &'a mut I where
I: Iterator + ?Sized, type Item = <I as Iterator>::Item;impl<'a, R: Read + ?Sized> Read for &'a mut Rimpl<'a, W: Write + ?Sized> Write for &'a mut W`
`fn borrow(&self) -> &T`
[src]

Immutably borrows from an owned value. Read more

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

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

Formats the value using the given formatter.

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

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

####
ⓘImportant traits for &'a mut I

### Important traits for &'a mut I

`impl<'a, I> Iterator for &'a mut I where
I: Iterator + ?Sized, type Item = <I as Iterator>::Item;impl<'a, R: Read + ?Sized> Read for &'a mut Rimpl<'a, W: Write + ?Sized> Write for &'a mut W`
`fn as_ref(&self) -> &T`
[src]

Performs the conversion.

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

#### `type Target = T`

The resulting type after dereferencing.

####
ⓘImportant traits for &'a mut I

### Important traits for &'a mut I

`impl<'a, I> Iterator for &'a mut I where
I: Iterator + ?Sized, type Item = <I as Iterator>::Item;impl<'a, R: Read + ?Sized> Read for &'a mut Rimpl<'a, W: Write + ?Sized> Write for &'a mut W`
`fn deref(&self) -> &T`
[src]

Dereferences the value.

### `impl<T> Clone for Arc<T> where
T: ?Sized,`
[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);

    Arc::clone(&five);Run

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

Performs copy-assignment from `source`. Read more

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

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

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

Formats the value using the given formatter. Read more

### `impl<T> Ord for Arc<T> where
T: Ord + ?Sized,`
[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)));Run

#### `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

### `impl From<CString> for Arc<CStr>`
1.23.0
[src]

#### `fn from(s: CString) -> Arc<CStr>`
[src]

Performs the conversion.

### `impl<'a> From<&'a CStr> for Arc<CStr>`
1.23.0
[src]

#### `fn from(s: &CStr) -> Arc<CStr>`
[src]

Performs the conversion.

### `impl From<OsString> for Arc<OsStr>`
1.23.0
[src]

#### `fn from(s: OsString) -> Arc<OsStr>`
[src]

Performs the conversion.

### `impl<'a> From<&'a OsStr> for Arc<OsStr>`
1.23.0
[src]

#### `fn from(s: &OsStr) -> Arc<OsStr>`
[src]

Performs the conversion.

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

### `impl From<PathBuf> for Arc<Path>`
1.23.0
[src]

#### `fn from(s: PathBuf) -> Arc<Path>`
[src]

Performs the conversion.

### `impl<'a> From<&'a Path> for Arc<Path>`
1.23.0
[src]

#### `fn from(s: &Path) -> Arc<Path>`
[src]

Performs the conversion.

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