Struct serde::lib::core::sync::Weak

1.4.0 · source · []
pub struct Weak<T> where
    T: ?Sized
{ ptr: NonNull<ArcInner<T>>, }
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 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.

Fields

ptr: NonNull<ArcInner<T>>

Implementations

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());

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() });

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));

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());

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());

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

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

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.

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()).

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));

Trait Implementations

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);

Performs copy-assignment from source. Read more

Formats the value using the given formatter. Read more

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());

This impl requires the "rc" Cargo feature of Serde. The resulting Weak<T> has a reference count of 0 and cannot be upgraded.

Deserialize this value from the given Serde deserializer. Read more

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());

This impl requires the "rc" Cargo feature of Serde.

Serialize this value into the given Serde serializer. Read more

Auto Trait Implementations

Blanket Implementations

Gets the TypeId of self. Read more

Immutably borrows from an owned value. Read more

Mutably borrows from an owned value. Read more

Returns the argument unchanged.

Calls U::from(self).

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

The resulting type after obtaining ownership.

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

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

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

The type returned in the event of a conversion error.

Performs the conversion.

The type returned in the event of a conversion error.

Performs the conversion.