Struct ptr_eq::rc::Weak

#[repr(transparent)]pub struct Weak<T: PtrEq + ?Sized>(_);

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 Weak pointer, which returns an Option<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 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 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 Weak pointers from children back to their parents.

The typical way to obtain a Weak pointer is to call Rc::downgrade.

Implementations

impl<T: PtrEq> Weak<T>

pub fn new() -> Weak<T>

Constructs a new Weak<T>, without allocating any memory. Calling upgrade on the return value always gives None.

Examples

use ptr_eq::rc::Weak;
use ptr_eq::PtrEq;
 
#[derive(PtrEq)]
struct Test(i64);

let empty: Weak<Test> = Weak::new();
assert!(empty.upgrade().is_none());

impl<T: PtrEq + ?Sized> Weak<T>

pub fn as_ptr(&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 ptr_eq::rc::Rc;
use ptr_eq::PtrEq;
use std::ptr;
 
#[derive(PtrEq)]
struct Test(i64);
 
let strong = Rc::new(Test(42));
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!(42, unsafe { &*weak.as_ptr() }.0);

drop(strong);
// But not any more. We can do weak.as_ptr(), but accessing the pointer would lead to
// undefined behaviour.
// assert_eq!(42, unsafe { &*weak.as_ptr() }.0);

pub fn into_raw(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 ptr_eq::rc::{Rc, Weak};
use ptr_eq::PtrEq;
use std::ptr;
 
#[derive(PtrEq)]
struct Test(i64);
 
let strong = Rc::new(Test(42));
let weak = Rc::downgrade(&strong);
let raw = weak.into_raw();

assert_eq!(1, Rc::weak_count(&strong));
assert_eq!(42, unsafe { &*raw }.0);

drop(unsafe { Weak::from_raw(raw) });
assert_eq!(0, Rc::weak_count(&strong));

pub unsafe fn from_raw(ptr: *const T) -> Weak<T>

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 ptr_eq::rc::{Rc, Weak};
use ptr_eq::PtrEq;
 
#[derive(PtrEq)]
struct Test(i64);

let strong = Rc::new(Test(42));

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!(42, unsafe { Weak::from_raw(raw_1) }.upgrade().unwrap().0);
assert_eq!(1, Rc::weak_count(&strong));

drop(strong);

// Decrement the last weak count.
assert!(unsafe { Weak::from_raw(raw_2) }.upgrade().is_none());

pub fn upgrade(&self) -> Option<Rc<T>>

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 ptr_eq::rc::{Rc, Weak};
use ptr_eq::PtrEq;
 
#[derive(PtrEq)]
struct Test(i64);

let five = Rc::new(Test(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());

pub fn weak_count(this: &Self) -> usize

Gets the number of Weak pointers pointing to this allocation.

If no strong pointers remain, this will return zero.

pub fn strong_count(this: &Self) -> usize

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

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

Trait Implementations

impl<T: PtrEq + ?Sized> Clone for Weak<T>

fn clone(&self) -> Weak<T>

Makes a clone of the Weak pointer that points to the same allocation.

Examples

use ptr_eq::rc::{Rc, Weak};
use ptr_eq::PtrEq;
 
#[derive(PtrEq)]
struct Test(i64);

let weak_five = Rc::downgrade(&Rc::new(Test(5)));

let _ = Weak::clone(&weak_five);

pub fn clone_from(&mut self, source: &Self)

Performs copy-assignment from source.

impl<T: Debug + PtrEq + ?Sized> Debug for Weak<T>

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

Formats the value using the given formatter.

impl<T: PtrEq> Default for Weak<T>

fn default() -> Weak<T>

Constructs a new Weak<T>, allocating memory for T without initializing it. Calling upgrade on the return value always gives None.

Examples

use ptr_eq::rc::Weak;
use ptr_eq::PtrEq;
 
#[derive(PtrEq)]
struct Test(i64);
 
let empty: Weak<Test> = Default::default();
assert!(empty.upgrade().is_none());