rc::Weak::{as,from,into}_raw

Methods on the Weak to access it as a raw pointer to the data.
This commit is contained in:
Michal 'vorner' Vaner 2019-05-19 12:46:37 +02:00
parent f9d328d7ff
commit 4f1dcb34df
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GPG Key ID: F700D0C019E4C66F
1 changed files with 156 additions and 6 deletions

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@ -239,7 +239,7 @@ use core::fmt;
use core::hash::{Hash, Hasher}; use core::hash::{Hash, Hasher};
use core::intrinsics::abort; use core::intrinsics::abort;
use core::marker::{self, Unpin, Unsize, PhantomData}; use core::marker::{self, Unpin, Unsize, PhantomData};
use core::mem::{self, align_of_val, forget, size_of_val}; use core::mem::{self, align_of, align_of_val, forget, size_of_val};
use core::ops::{Deref, Receiver, CoerceUnsized, DispatchFromDyn}; use core::ops::{Deref, Receiver, CoerceUnsized, DispatchFromDyn};
use core::pin::Pin; use core::pin::Pin;
use core::ptr::{self, NonNull}; use core::ptr::{self, NonNull};
@ -416,11 +416,7 @@ impl<T: ?Sized> Rc<T> {
/// ``` /// ```
#[stable(feature = "rc_raw", since = "1.17.0")] #[stable(feature = "rc_raw", since = "1.17.0")]
pub unsafe fn from_raw(ptr: *const T) -> Self { pub unsafe fn from_raw(ptr: *const T) -> Self {
// Align the unsized value to the end of the RcBox. let offset = data_offset(ptr);
// Because it is ?Sized, it will always be the last field in memory.
let align = align_of_val(&*ptr);
let layout = Layout::new::<RcBox<()>>();
let offset = (layout.size() + layout.padding_needed_for(align)) as isize;
// Reverse the offset to find the original RcBox. // Reverse the offset to find the original RcBox.
let fake_ptr = ptr as *mut RcBox<T>; let fake_ptr = ptr as *mut RcBox<T>;
@ -1262,6 +1258,143 @@ impl<T> Weak<T> {
ptr: NonNull::new(usize::MAX as *mut RcBox<T>).expect("MAX is not 0"), ptr: NonNull::new(usize::MAX as *mut RcBox<T>).expect("MAX is not 0"),
} }
} }
/// Returns a raw pointer to the object `T` pointed to by this `Weak<T>`.
///
/// It is up to the caller to ensure that the object is still alive when accessing it through
/// the pointer.
///
/// The pointer may be [`null`] or be dangling in case the object has already been destroyed.
///
/// # Examples
///
/// ```
/// #![feature(weak_into_raw)]
///
/// use std::rc::{Rc, Weak};
/// use std::ptr;
///
/// let strong = Rc::new(42);
/// let weak = Rc::downgrade(&strong);
/// // Both point to the same object
/// assert!(ptr::eq(&*strong, Weak::as_raw(&weak)));
/// // The strong here keeps it alive, so we can still access the object.
/// assert_eq!(42, unsafe { *Weak::as_raw(&weak) });
///
/// drop(strong);
/// // But not any more. We can do Weak::as_raw(&weak), but accessing the pointer would lead to
/// // undefined behaviour.
/// // assert_eq!(42, unsafe { *Weak::as_raw(&weak) });
/// ```
///
/// [`null`]: ../../std/ptr/fn.null.html
#[unstable(feature = "weak_into_raw", issue = "60728")]
pub fn as_raw(this: &Self) -> *const T {
match this.inner() {
None => ptr::null(),
Some(inner) => {
let offset = data_offset_sized::<T>();
let ptr = inner as *const RcBox<T>;
// Note: while the pointer we create may already point to dropped value, the
// allocation still lives (it must hold the weak point as long as we are alive).
// Therefore, the offset is OK to do, it won't get out of the allocation.
let ptr = unsafe { (ptr as *const u8).offset(offset) };
ptr as *const T
}
}
}
/// Consumes the `Weak<T>` and turns it into a raw pointer.
///
/// This converts the weak pointer into a raw pointer, preserving the original weak count. 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_raw`] apply.
///
/// # Examples
///
/// ```
/// #![feature(weak_into_raw)]
///
/// use std::rc::{Rc, Weak};
///
/// let strong = Rc::new(42);
/// let weak = Rc::downgrade(&strong);
/// let raw = Weak::into_raw(weak);
///
/// assert_eq!(1, Rc::weak_count(&strong));
/// assert_eq!(42, unsafe { *raw });
///
/// drop(unsafe { Weak::from_raw(raw) });
/// assert_eq!(0, Rc::weak_count(&strong));
/// ```
///
/// [`from_raw`]: struct.Weak.html#method.from_raw
/// [`as_raw`]: struct.Weak.html#method.as_raw
#[unstable(feature = "weak_into_raw", issue = "60728")]
pub fn into_raw(this: Self) -> *const T {
let result = Self::as_raw(&this);
mem::forget(this);
result
}
/// 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 count. In case a [`null`] is passed, a dangling [`Weak`] is
/// returned.
///
/// # Safety
///
/// The pointer must represent one valid weak count. In other words, it must point to `T` which
/// is or *was* managed by an [`Rc`] and the weak count of that [`Rc`] must not have reached
/// 0. It is allowed for the strong count to be 0.
///
/// # Examples
///
/// ```
/// #![feature(weak_into_raw)]
///
/// use std::rc::{Rc, Weak};
///
/// let strong = Rc::new(42);
///
/// let raw_1 = Weak::into_raw(Rc::downgrade(&strong));
/// let raw_2 = Weak::into_raw(Rc::downgrade(&strong));
///
/// assert_eq!(2, Rc::weak_count(&strong));
///
/// assert_eq!(42, *Weak::upgrade(&unsafe { Weak::from_raw(raw_1) }).unwrap());
/// assert_eq!(1, Rc::weak_count(&strong));
///
/// drop(strong);
///
/// // Decrement the last weak count.
/// assert!(Weak::upgrade(&unsafe { Weak::from_raw(raw_2) }).is_none());
/// ```
///
/// [`null`]: ../../std/ptr/fn.null.html
/// [`into_raw`]: struct.Weak.html#method.into_raw
/// [`upgrade`]: struct.Weak.html#method.upgrade
/// [`Rc`]: struct.Rc.html
/// [`Weak`]: struct.Weak.html
#[unstable(feature = "weak_into_raw", issue = "60728")]
pub unsafe fn from_raw(ptr: *const T) -> Self {
if ptr.is_null() {
Self::new()
} else {
// See Rc::from_raw for details
let offset = data_offset(ptr);
let fake_ptr = ptr as *mut RcBox<T>;
let ptr = set_data_ptr(fake_ptr, (ptr as *mut u8).offset(-offset));
Weak {
ptr: NonNull::new(ptr).expect("Invalid pointer passed to from_raw"),
}
}
}
} }
pub(crate) fn is_dangling<T: ?Sized>(ptr: NonNull<T>) -> bool { pub(crate) fn is_dangling<T: ?Sized>(ptr: NonNull<T>) -> bool {
@ -2007,3 +2140,20 @@ impl<T: ?Sized> AsRef<T> for Rc<T> {
#[stable(feature = "pin", since = "1.33.0")] #[stable(feature = "pin", since = "1.33.0")]
impl<T: ?Sized> Unpin for Rc<T> { } impl<T: ?Sized> Unpin for Rc<T> { }
unsafe fn data_offset<T: ?Sized>(ptr: *const T) -> isize {
// Align the unsized value to the end of the RcBox.
// Because it is ?Sized, it will always be the last field in memory.
let align = align_of_val(&*ptr);
let layout = Layout::new::<RcBox<()>>();
(layout.size() + layout.padding_needed_for(align)) as isize
}
/// Computes the offset of the data field within ArcInner.
///
/// Unlike [`data_offset`], this doesn't need the pointer, but it works only on `T: Sized`.
fn data_offset_sized<T>() -> isize {
let align = align_of::<T>();
let layout = Layout::new::<RcBox<()>>();
(layout.size() + layout.padding_needed_for(align)) as isize
}