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 upgraded.
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.
Constructs a new Weak<T>, without allocating any memory.
Calling upgrade on the return value always gives None.
use std::rc::Weak;
let empty: Weak<i64> = Weak::new();
assert!(empty.upgrade().is_none());Run
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.
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());
drop(strong_five);
drop(five);
assert!(weak_five.upgrade().is_none());Run
Formats the value using the given formatter. Read more
Makes a clone of the Weak pointer that points to the same value.
use std::rc::{Rc, Weak};
let weak_five = Rc::downgrade(&Rc::new(5));
Weak::clone(&weak_five);Run
Performs copy-assignment from source. Read more
Drops the Weak pointer.
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);
drop(foo);
assert!(other_weak_foo.upgrade().is_none());Run
Constructs a new Weak<T>, allocating memory for T without initializing
it. Calling upgrade on the return value always gives None.
use std::rc::Weak;
let empty: Weak<i64> = Default::default();
assert!(empty.upgrade().is_none());Run