Struct std::sync::Weak 1.4.0[−][src]
pub struct Weak<T> where
T: ?Sized, { /* fields omitted */ }
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
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() });
RunConsumes 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));
RunConverts 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());
RunAttempts 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());
RunGets 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 Arc
s or
Weak
s pointing to the same allocation.
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()
).
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));
RunComparing 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));
RunTrait Implementations
Performs copy-assignment from source
. 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());
RunAuto Trait Implementations
impl<T: ?Sized> RefUnwindSafe for Weak<T> where
T: RefUnwindSafe,
impl<T: ?Sized> UnwindSafe for Weak<T> where
T: RefUnwindSafe,
Blanket Implementations
Mutably borrows from an owned value. Read more