# Struct alloc::sync::Weak

1.4.0 · source ·

    pub struct Weak<T: ?Sized, A: Allocator = Global> { /* private fields */ }

Expand description

`Weak` is a version of `Arc` that holds a non-owning reference to the managed allocation. The allocation 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 value stored in the allocation from being dropped, and `Weak` itself makes no guarantees about the value still being present. Thus it may return `None` when `upgrade`d. Note however that a `Weak` reference does prevent the allocation itself (the backing store) from being deallocated.

A `Weak` pointer is useful for keeping a temporary reference to the allocation managed by `Arc` without preventing its inner value from being dropped. 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`.

## Implementations§

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### impl<T> Weak<T>

1.10.0 (const: 1.73.0) · source

#### pub const fn new() -> Weak<T>

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());

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### impl<T, A: Allocator> Weak<T, A>

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#### pub fn new_in(alloc: A) -> Weak<T, A>

🔬This is a nightly-only experimental API. (`allocator_api` #32838)

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

##### Examples

    #![feature(allocator_api)]

    use std::sync::Weak;
    use std::alloc::System;

    let empty: Weak<i64, _> = Weak::new_in(System);
    assert!(empty.upgrade().is_none());

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### impl<T: ?Sized> Weak<T>

1.45.0 · source

#### pub unsafe fn from_raw(ptr: *const T) -> Self

Converts a raw pointer previously created by `into_raw` back into `Weak<T>`.

This can be used to safely get a strong reference (by calling `upgrade` later) or to deallocate the weak count by dropping the `Weak<T>`.

It takes ownership of one weak reference (with the exception of pointers created by `new`, as these don’t own anything; the method still works on them).

##### Safety

The pointer must have originated from the `into_raw` and must still own its potential weak reference.

It is allowed for the strong count to be 0 at the time of calling this. Nevertheless, this takes ownership of one weak reference currently represented as a raw pointer (the weak count is not modified by this operation) and therefore it must be paired with a previous call to `into_raw`.

##### Examples

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

    let strong = Arc::new("hello".to_owned());

    let raw_1 = Arc::downgrade(&strong).into_raw();
    let raw_2 = Arc::downgrade(&strong).into_raw();

    assert_eq!(2, Arc::weak_count(&strong));

    assert_eq!("hello", &*unsafe { Weak::from_raw(raw_1) }.upgrade().unwrap());
    assert_eq!(1, Arc::weak_count(&strong));

    drop(strong);

    // Decrement the last weak count.
    assert!(unsafe { Weak::from_raw(raw_2) }.upgrade().is_none());

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### impl<T: ?Sized, A: Allocator> Weak<T, A>

1.45.0 · source

#### pub fn as_ptr(&self) -> *const T

Returns a raw pointer to the object `T` pointed to by this `Weak<T>`.

The pointer is valid only if there are some strong references. The pointer may be dangling, unaligned or even `null` otherwise.

##### Examples

    use std::sync::Arc;
    use std::ptr;

    let strong = Arc::new("hello".to_owned());
    let weak = Arc::downgrade(&strong);
    // Both point to the same object
    assert!(ptr::eq(&*strong, weak.as_ptr()));
    // The strong here keeps it alive, so we can still access the object.
    assert_eq!("hello", unsafe { &*weak.as_ptr() });

    drop(strong);
    // But not any more. We can do weak.as_ptr(), but accessing the pointer would lead to
    // undefined behaviour.
    // assert_eq!("hello", unsafe { &*weak.as_ptr() });

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1.45.0 · source

#### pub fn into_raw(self) -> *const T

Consumes the `Weak<T>` and turns it into a raw pointer.

This converts the weak pointer into a raw pointer, while still preserving the ownership of one weak reference (the weak count is not modified by this operation). It can be turned back into the `Weak<T>` with `from_raw`.

The same restrictions of accessing the target of the pointer as with `as_ptr` apply.

##### Examples

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

    let strong = Arc::new("hello".to_owned());
    let weak = Arc::downgrade(&strong);
    let raw = weak.into_raw();

    assert_eq!(1, Arc::weak_count(&strong));
    assert_eq!("hello", unsafe { &*raw });

    drop(unsafe { Weak::from_raw(raw) });
    assert_eq!(0, Arc::weak_count(&strong));

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#### pub unsafe fn from_raw_in(ptr: *const T, alloc: A) -> Self

🔬This is a nightly-only experimental API. (`allocator_api` #32838)

Converts a raw pointer previously created by `into_raw` back into `Weak<T>` in the provided allocator.

This can be used to safely get a strong reference (by calling `upgrade` later) or to deallocate the weak count by dropping the `Weak<T>`.

It takes ownership of one weak reference (with the exception of pointers created by `new`, as these don’t own anything; the method still works on them).

##### Safety

The pointer must have originated from the `into_raw` and must still own its potential weak reference, and must point to a block of memory allocated by `alloc`.

It is allowed for the strong count to be 0 at the time of calling this. Nevertheless, this takes ownership of one weak reference currently represented as a raw pointer (the weak count is not modified by this operation) and therefore it must be paired with a previous call to `into_raw`.

##### Examples

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

    let strong = Arc::new("hello".to_owned());

    let raw_1 = Arc::downgrade(&strong).into_raw();
    let raw_2 = Arc::downgrade(&strong).into_raw();

    assert_eq!(2, Arc::weak_count(&strong));

    assert_eq!("hello", &*unsafe { Weak::from_raw(raw_1) }.upgrade().unwrap());
    assert_eq!(1, Arc::weak_count(&strong));

    drop(strong);

    // Decrement the last weak count.
    assert!(unsafe { Weak::from_raw(raw_2) }.upgrade().is_none());

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### impl<T: ?Sized, A: Allocator> Weak<T, A>

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#### pub fn upgrade(&self) -> Option<Arc<T, A>>where A: Clone,

Attempts to upgrade the `Weak` pointer to an `Arc`, delaying dropping of the inner value if successful.

Returns `None` if the inner 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());

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1.41.0 · source

#### pub fn strong_count(&self) -> usize

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

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

1.41.0 · source

#### pub fn weak_count(&self) -> usize

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

If `self` was created using `Weak::new`, or if there are no remaining strong pointers, this will return 0.

##### 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 allocation.

1.39.0 · source

#### pub fn ptr_eq(&self, other: &Self) -> bool

Returns `true` if the two `Weak`s point to the same allocation similar to `ptr::eq`, or if both don’t point to any allocation (because they were created with `Weak::new()`). However, this function ignores the metadata of `dyn Trait` pointers.

##### Notes

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

##### Examples

    use std::sync::Arc;

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

    assert!(first.ptr_eq(&second));

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

    assert!(!first.ptr_eq(&third));

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Comparing `Weak::new`.

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

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

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

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## Trait Implementations§

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### impl<T: ?Sized, A: Allocator + Clone> Clone for Weak<T, A>

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#### fn clone(&self) -> Weak<T, A>

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

##### Examples

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

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

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

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1.0.0 · source§

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

Performs copy-assignment from `source`. Read more
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### impl<T: ?Sized> Debug for Weak<T>

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

Formats the value using the given formatter. Read more
1.10.0 · source§

### impl<T> Default for Weak<T>

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#### fn default() -> Weak<T>

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());

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### impl<T: ?Sized, A: Allocator> Drop for Weak<T, A>

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#### fn drop(&mut self)

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());

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### impl<T: ?Sized + Unsize<U>, U: ?Sized, A: Allocator> CoerceUnsized<Weak<U, A>> for Weak<T, A>

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### impl<T: ?Sized + Unsize<U>, U: ?Sized> DispatchFromDyn<Weak<U>> for Weak<T>

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### impl<T: ?Sized + Sync + Send, A: Allocator + Send> Send for Weak<T, A>

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### impl<T: ?Sized + Sync + Send, A: Allocator + Sync> Sync for Weak<T, A>

## Auto Trait Implementations§

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### impl<T: ?Sized, A> RefUnwindSafe for Weak<T, A>where A: RefUnwindSafe, T: RefUnwindSafe,

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### impl<T: ?Sized, A> Unpin for Weak<T, A>where A: Unpin,

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### impl<T: ?Sized, A> UnwindSafe for Weak<T, A>where A: UnwindSafe, T: RefUnwindSafe,

## Blanket Implementations§

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### impl<T> Any for Twhere T: 'static + ?Sized,

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#### fn type_id(&self) -> TypeId

Gets the `TypeId` of `self`. Read more
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### impl<T> Borrow<T> for Twhere T: ?Sized,

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

Immutably borrows from an owned value. Read more
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### impl<T> BorrowMut<T> for Twhere T: ?Sized,

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#### fn borrow_mut(&mut self) -> &mut T

Mutably borrows from an owned value. Read more
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### impl<T> From<T> for T

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#### fn from(t: T) -> T

Returns the argument unchanged.

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### impl<T, U> Into<U> for Twhere U: From<T>,

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#### fn into(self) -> U

Calls `U::from(self)`.

That is, this conversion is whatever the implementation of `From<T> for U` chooses to do.

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### impl<T> ToOwned for Twhere T: Clone,

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#### type Owned = T

The resulting type after obtaining ownership.
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#### fn to_owned(&self) -> T

Creates owned data from borrowed data, usually by cloning. Read more
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#### fn clone_into(&self, target: &mut T)

Uses borrowed data to replace owned data, usually by cloning. Read more
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### impl<T, U> TryFrom<U> for Twhere U: Into<T>,

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#### type Error = Infallible

The type returned in the event of a conversion error.
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#### fn try_from(value: U) -> Result<T, <T as TryFrom<U>>::Error>

Performs the conversion.
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### impl<T, U> TryInto<U> for Twhere U: TryFrom<T>,

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#### type Error = <U as TryFrom<T>>::Error

The type returned in the event of a conversion error.
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#### fn try_into(self) -> Result<U, <U as TryFrom<T>>::Error>

Performs the conversion.