use core::fmt;
use core::cmp::{self, Ordering};
use core::intrinsics::abort;
-use core::mem::{self, align_of_val, size_of_val};
+use core::mem::{self, align_of, align_of_val, size_of_val};
use core::ops::{Deref, Receiver, CoerceUnsized, DispatchFromDyn};
use core::pin::Pin;
use core::ptr::{self, NonNull};
/// ```
#[stable(feature = "rc_raw", since = "1.17.0")]
pub unsafe fn from_raw(ptr: *const T) -> Self {
- // Align the unsized value to the end of the ArcInner.
- // Because it is ?Sized, it will always be the last field in memory.
- let align = align_of_val(&*ptr);
- let layout = Layout::new::<ArcInner<()>>();
- let offset = (layout.size() + layout.padding_needed_for(align)) as isize;
+ let offset = data_offset(ptr);
// Reverse the offset to find the original ArcInner.
let fake_ptr = ptr as *mut ArcInner<T>;
ptr: NonNull::new(usize::MAX as *mut ArcInner<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::sync::{Arc, Weak};
+ /// use std::ptr;
+ ///
+ /// let strong = Arc::new(42);
+ /// let weak = Arc::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 ArcInner<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::sync::{Arc, Weak};
+ ///
+ /// let strong = Arc::new(42);
+ /// let weak = Arc::downgrade(&strong);
+ /// let raw = Weak::into_raw(weak);
+ ///
+ /// assert_eq!(1, Arc::weak_count(&strong));
+ /// assert_eq!(42, unsafe { *raw });
+ ///
+ /// drop(unsafe { Weak::from_raw(raw) });
+ /// assert_eq!(0, Arc::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 [`Arc`] and the weak count of that [`Arc`] must not have reached
+ /// 0. It is allowed for the strong count to be 0.
+ ///
+ /// # Examples
+ ///
+ /// ```
+ /// #![feature(weak_into_raw)]
+ ///
+ /// use std::sync::{Arc, Weak};
+ ///
+ /// let strong = Arc::new(42);
+ ///
+ /// let raw_1 = Weak::into_raw(Arc::downgrade(&strong));
+ /// let raw_2 = Weak::into_raw(Arc::downgrade(&strong));
+ ///
+ /// assert_eq!(2, Arc::weak_count(&strong));
+ ///
+ /// assert_eq!(42, *Weak::upgrade(&unsafe { Weak::from_raw(raw_1) }).unwrap());
+ /// assert_eq!(1, Arc::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
+ /// [`Weak`]: struct.Weak.html
+ /// [`Arc`]: struct.Arc.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 Arc::from_raw for details
+ let offset = data_offset(ptr);
+ let fake_ptr = ptr as *mut ArcInner<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"),
+ }
+ }
+ }
}
impl<T: ?Sized> Weak<T> {
#[stable(feature = "pin", since = "1.33.0")]
impl<T: ?Sized> Unpin for Arc<T> { }
+
+/// Computes the offset of the data field within ArcInner.
+unsafe fn data_offset<T: ?Sized>(ptr: *const T) -> isize {
+ // Align the unsized value to the end of the ArcInner.
+ // Because it is ?Sized, it will always be the last field in memory.
+ let align = align_of_val(&*ptr);
+ let layout = Layout::new::<ArcInner<()>>();
+ (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::<ArcInner<()>>();
+ (layout.size() + layout.padding_needed_for(align)) as isize
+}
projects / rust.git / commitdiff