# 1.4.0[−][src]Struct std::rc::Weak

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

`Weak` is a version of `Rc` 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``<``Rc``<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 `Rc` without extending its lifetime. It is also used to prevent circular references between `Rc` pointers, since mutual owning references would never allow either `Rc` to be dropped. For example, a tree could have strong `Rc` 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 `Rc::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::rc::Weak;

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

#### `pub fn as_raw(this: &Weak<T>) -> *const T`[src]

🔬 This is a nightly-only experimental API. (`weak_into_raw` #60728)

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

It is up to the caller to ensure that the object is still alive when accessing it through the pointer.

The pointer may be `null` or be dangling in case the object has already been destroyed.

# Examples

    #![feature(weak_into_raw)]

    use std::rc::{Rc, Weak};
    use std::ptr;

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

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

#### `pub fn into_raw(this: Weak<T>) -> *const T`[src]

🔬 This is a nightly-only experimental API. (`weak_into_raw` #60728)

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

This converts the weak pointer into a raw pointer, preserving the original weak count. 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_raw` apply.

# Examples

    #![feature(weak_into_raw)]

    use std::rc::{Rc, Weak};

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

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

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

#### `pub unsafe fn from_raw(ptr: *const T) -> Weak<T>`[src]

🔬 This is a nightly-only experimental API. (`weak_into_raw` #60728)

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 count. In case a `null` is passed, a dangling `Weak` is returned.

# Safety

The pointer must represent one valid weak count. In other words, it must point to `T` which is or was managed by an `Rc` and the weak count of that `Rc` must not have reached 0. It is allowed for the strong count to be 0.

# Examples

    #![feature(weak_into_raw)]

    use std::rc::{Rc, Weak};

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

    let raw_1 = Weak::into_raw(Rc::downgrade(&strong));
    let raw_2 = Weak::into_raw(Rc::downgrade(&strong));

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

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

    drop(strong);

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

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

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

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

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

# Examples

    use std::rc::Rc;

    let five = Rc::new(5);

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

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

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

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

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

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

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

If `self` was created using `Weak::new`, this will return `None`. If not, the returned value is at least 1, since `self` still points to the value.

#### `pub fn ptr_eq(&self, other: &Weak<T>) -> 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::rc::Rc;

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

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

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

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

Comparing `Weak::new`.

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

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

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

## Trait Implementations

### `impl<T> !Send for Weak<T> where
T: ?Sized,` [src]

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

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

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

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

Drops the `Weak` pointer.

# Examples

    use std::rc::{Rc, Weak};

    struct Foo;

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

    let foo = Rc::new(Foo);
    let weak_foo = Rc::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());Run

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

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

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

# Examples

    use std::rc::{Rc, Weak};

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

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

#### `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>`, allocating memory for `T` without initializing it. Calling `upgrade` on the return value always gives `None`.

# Examples

    use std::rc::Weak;

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

### `impl<T> !Sync for Weak<T> where
T: ?Sized,` [src]

## Auto Trait Implementations

### `impl<T> !UnwindSafe for Weak<T>`

### `impl<T> !RefUnwindSafe for Weak<T>`

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

## Blanket Implementations

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

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

Performs the conversion.

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

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

Performs the conversion.

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

####
ⓘImportant traits for &'_ mut F

### Important traits for &'_ mut F

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

Immutably borrows from an owned value. Read more

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

####
ⓘImportant traits for &'_ mut F

### Important traits for &'_ mut F

`impl<'_, F> Future for &'_ mut F where
F: Unpin + Future + ?Sized, type Output = <F as Future>::Output;impl<'_, I> Iterator for &'_ mut I where
I: Iterator + ?Sized, type Item = <I as Iterator>::Item;impl<'_, R: Read + ?Sized> Read for &'_ mut Rimpl<'_, W: Write + ?Sized> Write for &'_ mut W`
`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

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

#### `type Owned = T`

The resulting type after obtaining ownership.

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

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

#### `fn clone_into(&self, target: &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

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