# 1.4.0[−][src]Struct alloc::sync::Weak

    pub struct Weak<T: ?Sized> { /* fields omitted */ }

`Weak` is a version of `Arc` that holds a non-owning reference to the managed value. The value is accessed by calling `upgrade` on the `Weak` pointer, which returns an `Option``<``Arc``<T>>`.

Since a `Weak` reference does not count towards ownership, it will not prevent the inner value from being dropped, and `Weak` itself makes no guarantees about the value still being present and may return `None` when `upgrade`d.

A `Weak` pointer is useful for keeping a temporary reference to the value within `Arc` without extending its lifetime. It is also used to prevent circular references between `Arc` pointers, since mutual owning references would never allow either `Arc` to be dropped. For example, a tree could have strong `Arc` pointers from parent nodes to children, and `Weak` pointers from children back to their parents.

The typical way to obtain a `Weak` pointer is to call `Arc::downgrade`.

## Methods

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

#### `pub fn new() -> Weak<T>`
1.10.0
[src]

Constructs a new `Weak<T>`, without allocating any memory. Calling `upgrade` on the return value always gives `None`.

# Examples

    use std::sync::Weak;

    let empty: Weak<i64> = Weak::new();
    assert!(empty.upgrade().is_none());

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

#### `pub fn upgrade(&self) -> Option<Arc<T>>`[src]

Attempts to upgrade the `Weak` pointer to an `Arc`, extending the lifetime of the value if successful.

Returns `None` if the value has since been dropped.

# Examples

    use std::sync::Arc;

    let five = Arc::new(5);

    let weak_five = Arc::downgrade(&five);

    let strong_five: Option<Arc<_>> = weak_five.upgrade();
    assert!(strong_five.is_some());

    // Destroy all strong pointers.
    drop(strong_five);
    drop(five);

    assert!(weak_five.upgrade().is_none());

#### `pub fn strong_count(&self) -> usize`[src]

🔬 This is a nightly-only experimental API. (`weak_counts` #57977)

Gets the number of strong (`Arc`) pointers pointing to this value.

If `self` was created using `Weak::new`, this will return 0.

#### `pub fn weak_count(&self) -> Option<usize>`[src]

🔬 This is a nightly-only experimental API. (`weak_counts` #57977)

Gets an approximation of the number of `Weak` pointers pointing to this value.

If `self` was created using `Weak::new`, this will return 0. If not, the returned value is at least 1, since `self` still points to the value.

# Accuracy

Due to implementation details, the returned value can be off by 1 in either direction when other threads are manipulating any `Arc`s or `Weak`s pointing to the same value.

#### `pub fn ptr_eq(this: &Self, other: &Self) -> bool`[src]

🔬 This is a nightly-only experimental API. (`weak_ptr_eq` #55981)

Returns `true` if the two `Weak`s point to the same value (not just values that compare as equal).

# Notes

Since this compares pointers it means that `Weak::new()` will equal each other, even though they don't point to any value.

# Examples

    #![feature(weak_ptr_eq)]
    use std::sync::{Arc, Weak};

    let first_rc = Arc::new(5);
    let first = Arc::downgrade(&first_rc);
    let second = Arc::downgrade(&first_rc);

    assert!(Weak::ptr_eq(&first, &second));

    let third_rc = Arc::new(5);
    let third = Arc::downgrade(&third_rc);

    assert!(!Weak::ptr_eq(&first, &third));

Comparing `Weak::new`.

    #![feature(weak_ptr_eq)]
    use std::sync::{Arc, Weak};

    let first = Weak::new();
    let second = Weak::new();
    assert!(Weak::ptr_eq(&first, &second));

    let third_rc = Arc::new(());
    let third = Arc::downgrade(&third_rc);
    assert!(!Weak::ptr_eq(&first, &third));

## Trait Implementations

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

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

Formats the value using the given formatter. Read more

### `impl<T: ?Sized + Unsize<U>, U: ?Sized> DispatchFromDyn<Weak<U>> for Weak<T>`[src]

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

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

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

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

#### `fn drop(&mut self)`[src]

Drops the `Weak` pointer.

# Examples

    use std::sync::{Arc, Weak};

    struct Foo;

    impl Drop for Foo {
        fn drop(&mut self) {
            println!("dropped!");
        }
    }

    let foo = Arc::new(Foo);
    let weak_foo = Arc::downgrade(&foo);
    let other_weak_foo = Weak::clone(&weak_foo);

    drop(weak_foo);   // Doesn't print anything
    drop(foo);        // Prints "dropped!"

    assert!(other_weak_foo.upgrade().is_none());

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

#### `fn clone(&self) -> Weak<T>`[src]

Makes a clone of the `Weak` pointer that points to the same value.

# Examples

    use std::sync::{Arc, Weak};

    let weak_five = Arc::downgrade(&Arc::new(5));

    let _ = Weak::clone(&weak_five);

#### `fn clone_from(&mut self, source: &Self)`
1.0.0
[src]

Performs copy-assignment from `source`. Read more

### `impl<T> Default for Weak<T>`
1.10.0
[src]

#### `fn default() -> Weak<T>`[src]

Constructs a new `Weak<T>`, without allocating memory. Calling `upgrade` on the return value always gives `None`.

# Examples

    use std::sync::Weak;

    let empty: Weak<i64> = Default::default();
    assert!(empty.upgrade().is_none());

## Blanket Implementations

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

#### `type Owned = T`

#### `fn to_owned(&Self) -> T`[src]

Creates owned data from borrowed data, usually by cloning. Read more

#### `fn clone_into(&Self, &mut T)`[src]

🔬 This is a nightly-only experimental API. (`toowned_clone_into` #41263)

recently added

Uses borrowed data to replace owned data, usually by cloning. Read more

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

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

Performs the conversion.

### `impl<T, U> TryFrom for T where
U: Into<T>,` [src]

#### `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]

Performs the conversion.

### `impl<T, U> TryInto for T where
U: TryFrom<T>,` [src]

#### `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]

Performs the conversion.

### `impl<T, U> Into for T where
U: From<T>,` [src]

#### `fn into(self) -> U`[src]

Performs the conversion.

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

#### `fn borrow(&self) -> &T`[src]

Immutably borrows from an owned value. Read more

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

#### `fn borrow_mut(&mut self) -> &mut T`[src]

Mutably borrows from an owned value. Read more

### `impl<T> Any for T where
T: 'static + ?Sized,` [src]

#### `fn type_id(&self) -> TypeId`[src]

Gets the `TypeId` of `self`. Read more

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