Struct Weak

struct Weak<T: ?Sized, A: Allocator = crate::alloc::Global> { ... }

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 upgraded. 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

impl<T> Weak<T>

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());

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

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

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());

impl<T: ?Sized> Weak<T>

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, and must point to a block of memory allocated by global allocator.

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());

fn into_raw(self: 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));

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

fn upgrade(self: &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());

fn strong_count(self: &Self) -> usize

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

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

fn weak_count(self: &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 Arcs or Weaks pointing to the same allocation.

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

Returns true if the two Weaks 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));

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));

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

fn allocator(self: &Self) -> &A

Returns a reference to the underlying allocator.

fn as_ptr(self: &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 behavior.
// assert_eq!("hello", unsafe { &*weak.as_ptr() });

fn into_raw_with_allocator(self: Self) -> (*const T, A)

Consumes the Weak<T>, returning the wrapped pointer and allocator.

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_in.

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

Examples

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

let strong = Arc::new_in("hello".to_owned(), System);
let weak = Arc::downgrade(&strong);
let (raw, alloc) = weak.into_raw_with_allocator();

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

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

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

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());

impl<T> Any for Weak<T, A>

fn type_id(self: &Self) -> TypeId

impl<T> Borrow for Weak<T, A>

fn borrow(self: &Self) -> &T

impl<T> BorrowMut for Weak<T, A>

fn borrow_mut(self: &mut Self) -> &mut T

impl<T> CloneToUninit for Weak<T, A>

unsafe fn clone_to_uninit(self: &Self, dest: *mut u8)

impl<T> Default for Weak<T>

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());

impl<T> From for Weak<T, A>

fn from(t: T) -> T

Returns the argument unchanged.

impl<T> ToOwned for Weak<T, A>

fn to_owned(self: &Self) -> T
fn clone_into(self: &Self, target: &mut T)

impl<T, A> Freeze for Weak<T, A>

impl<T, A> RefUnwindSafe for Weak<T, A>

impl<T, A> Unpin for Weak<T, A>

impl<T, A> UnwindSafe for Weak<T, A>

impl<T, U> Into for Weak<T, A>

fn into(self: Self) -> U

Calls U::from(self).

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

impl<T, U> TryFrom for Weak<T, A>

fn try_from(value: U) -> Result<T, <T as TryFrom<U>>::Error>

impl<T, U> TryInto for Weak<T, A>

fn try_into(self: Self) -> Result<U, <U as TryFrom<T>>::Error>

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

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

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

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

impl<T: ?Sized> CloneFromCell for Weak<T>

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

fn clone(self: &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);

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

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

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

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

fn drop(self: &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());

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