Weak in alloc::rc - Rust (original) (raw)
Struct Weak
1.36.0 · Source
pub struct Weak<T: ?Sized, A: Allocator = Global> { /* private fields */ }Expand description
Weak is a version of Rc that holds a non-owning reference to the managed allocation.
The allocation is accessed by calling upgrade on the Weakpointer, which returns an [Option](../../core/option/enum.Option.html "enum core::option::Option")<[Rc](struct.Rc.html "struct alloc::rc::Rc")<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 Nonewhen 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 Rc without preventing its inner value from being dropped. 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 Weakpointers from children back to their parents.
The typical way to obtain a Weak pointer is to call Rc::downgrade.
1.10.0 (const: 1.73.0) · Source
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());🔬This is a nightly-only experimental API. (allocator_api #32838)
Constructs a new Weak<T>, without allocating any memory, technically in the provided allocator. 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());1.45.0 · Source
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 upgradelater) 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 ptr must point to a block of memory allocated by the 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::rc::{Rc, Weak};
let strong = Rc::new("hello".to_owned());
let raw_1 = Rc::downgrade(&strong).into_raw();
let raw_2 = Rc::downgrade(&strong).into_raw();
assert_eq!(2, Rc::weak_count(&strong));
assert_eq!("hello", &*unsafe { Weak::from_raw(raw_1) }.upgrade().unwrap());
assert_eq!(1, Rc::weak_count(&strong));
drop(strong);
// Decrement the last weak count.
assert!(unsafe { Weak::from_raw(raw_2) }.upgrade().is_none());🔬This is a nightly-only experimental API. (allocator_api #32838)
Returns a reference to the underlying allocator.
1.45.0 · Source
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::rc::Rc;
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_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() });1.45.0 · Source
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 withas_ptr apply.
§Examples
use std::rc::{Rc, Weak};
let strong = Rc::new("hello".to_owned());
let weak = Rc::downgrade(&strong);
let raw = weak.into_raw();
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));🔬This is a nightly-only experimental API. (allocator_api #32838)
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 withas_ptr apply.
§Examples
#![feature(allocator_api)]
use std::rc::{Rc, Weak};
use std::alloc::System;
let strong = Rc::new_in("hello".to_owned(), System);
let weak = Rc::downgrade(&strong);
let (raw, alloc) = weak.into_raw_with_allocator();
assert_eq!(1, Rc::weak_count(&strong));
assert_eq!("hello", unsafe { &*raw });
drop(unsafe { Weak::from_raw_in(raw, alloc) });
assert_eq!(0, Rc::weak_count(&strong));🔬This is a nightly-only experimental API. (allocator_api #32838)
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 upgradelater) 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 ptr 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::rc::{Rc, Weak};
let strong = Rc::new("hello".to_owned());
let raw_1 = Rc::downgrade(&strong).into_raw();
let raw_2 = Rc::downgrade(&strong).into_raw();
assert_eq!(2, Rc::weak_count(&strong));
assert_eq!("hello", &*unsafe { Weak::from_raw(raw_1) }.upgrade().unwrap());
assert_eq!(1, Rc::weak_count(&strong));
drop(strong);
// Decrement the last weak count.
assert!(unsafe { Weak::from_raw(raw_2) }.upgrade().is_none());1.4.0 · Source
Attempts to upgrade the Weak pointer to an Rc, delaying dropping of the inner value if successful.
Returns None if the inner 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());1.41.0 · Source
Gets the number of strong (Rc) pointers pointing to this allocation.
If self was created using Weak::new, this will return 0.
1.41.0 · Source
Gets the number of Weak pointers pointing to this allocation.
If no strong pointers remain, this will return zero.
1.39.0 · Source
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::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));Comparing Weak::new.
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));Makes a clone of the Weak pointer that points to the same allocation.
§Examples
use std::rc::{Rc, Weak};
let weak_five = Rc::downgrade(&Rc::new(5));
let _ = Weak::clone(&weak_five);Performs copy-assignment from source. Read more
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> = Default::default();
assert!(empty.upgrade().is_none());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());