1.0.0[][src]Struct std::cell::UnsafeCell

#[lang = "unsafe_cell"]
#[repr(transparent)]
#[repr(no_niche)]pub struct UnsafeCell<T> where
    T: ?Sized
{ /* fields omitted */ }

The core primitive for interior mutability in Rust.

UnsafeCell<T> is a type that wraps some T and indicates unsafe interior operations on the wrapped type. Types with an UnsafeCell<T> field are considered to have an 'unsafe interior'. The UnsafeCell<T> type is the only legal way to obtain aliasable data that is considered mutable. In general, transmuting an &T type into an &mut T is considered undefined behavior.

If you have a reference &SomeStruct, then normally in Rust all fields of SomeStruct are immutable. The compiler makes optimizations based on the knowledge that &T is not mutably aliased or mutated, and that &mut T is unique. UnsafeCell<T> is the only core language feature to work around the restriction that &T may not be mutated. All other types that allow internal mutability, such as Cell<T> and RefCell<T>, use UnsafeCell to wrap their internal data. There is no legal way to obtain aliasing &mut, not even with UnsafeCell<T>.

The UnsafeCell API itself is technically very simple: .get() gives you a raw pointer *mut T to its contents. It is up to you as the abstraction designer to use that raw pointer correctly.

The precise Rust aliasing rules are somewhat in flux, but the main points are not contentious:

To assist with proper design, the following scenarios are explicitly declared legal for single-threaded code:

  1. A &T reference can be released to safe code and there it can co-exist with other &T references, but not with a &mut T

  2. A &mut T reference may be released to safe code provided neither other &mut T nor &T co-exist with it. A &mut T must always be unique.

Note that whilst mutating the contents of an &UnsafeCell<T> (even while other &UnsafeCell<T> references alias the cell) is ok (provided you enforce the above invariants some other way), it is still undefined behavior to have multiple &mut UnsafeCell<T> aliases. That is, UnsafeCell is a wrapper designed to have a special interaction with shared accesses (i.e., through an &UnsafeCell<_> reference); there is no magic whatsoever when dealing with exclusive accesses (e.g., through an &mut UnsafeCell<_>): neither the cell nor the wrapped value may be aliased for the duration of that &mut borrow. This is showcased by the .get_mut() accessor, which is a non-unsafe getter that yields a &mut T.

Examples

Here is an example showcasing how to soundly mutate the contents of an UnsafeCell<_> despite there being multiple references aliasing the cell:

use std::cell::UnsafeCell;

let x: UnsafeCell<i32> = 42.into();
// Get multiple / concurrent / shared references to the same `x`.
let (p1, p2): (&UnsafeCell<i32>, &UnsafeCell<i32>) = (&x, &x);

unsafe {
    // SAFETY: within this scope there are no other references to `x`'s contents,
    // so ours is effectively unique.
    let p1_exclusive: &mut i32 = &mut *p1.get(); // -- borrow --+
    *p1_exclusive += 27; //                                     |
} // <---------- cannot go beyond this point -------------------+

unsafe {
    // SAFETY: within this scope nobody expects to have exclusive access to `x`'s contents,
    // so we can have multiple shared accesses concurrently.
    let p2_shared: &i32 = &*p2.get();
    assert_eq!(*p2_shared, 42 + 27);
    let p1_shared: &i32 = &*p1.get();
    assert_eq!(*p1_shared, *p2_shared);
}
Run

The following example showcases the fact that exclusive access to an UnsafeCell<T> implies exclusive access to its T:

#![feature(unsafe_cell_get_mut)]
#![forbid(unsafe_code)] // with exclusive accesses,
                        // `UnsafeCell` is a transparent no-op wrapper,
                        // so no need for `unsafe` here.
use std::cell::UnsafeCell;

let mut x: UnsafeCell<i32> = 42.into();

// Get a compile-time-checked unique reference to `x`.
let p_unique: &mut UnsafeCell<i32> = &mut x;
// With an exclusive reference, we can mutate the contents for free.
*p_unique.get_mut() = 0;
// Or, equivalently:
x = UnsafeCell::new(0);

// When we own the value, we can extract the contents for free.
let contents: i32 = x.into_inner();
assert_eq!(contents, 0);
Run

Implementations

impl<T> UnsafeCell<T>[src]

pub const fn new(value: T) -> UnsafeCell<T>[src]

Constructs a new instance of UnsafeCell which will wrap the specified value.

All access to the inner value through methods is unsafe.

Examples

use std::cell::UnsafeCell;

let uc = UnsafeCell::new(5);
Run

pub fn into_inner(self) -> T[src]

Unwraps the value.

Examples

use std::cell::UnsafeCell;

let uc = UnsafeCell::new(5);

let five = uc.into_inner();
Run

impl<T> UnsafeCell<T> where
    T: ?Sized
[src]

pub const fn get(&self) -> *mut T[src]

Gets a mutable pointer to the wrapped value.

This can be cast to a pointer of any kind. Ensure that the access is unique (no active references, mutable or not) when casting to &mut T, and ensure that there are no mutations or mutable aliases going on when casting to &T

Examples

use std::cell::UnsafeCell;

let uc = UnsafeCell::new(5);

let five = uc.get();
Run

pub fn get_mut(&mut self) -> &mut T

Notable traits for &'_ mut I

impl<'_, I> Iterator for &'_ mut I where
    I: Iterator + ?Sized
type Item = <I as Iterator>::Item;impl<'_, F> Future for &'_ mut F where
    F: Unpin + Future + ?Sized
type Output = <F as Future>::Output;impl<R: Read + ?Sized, '_> Read for &'_ mut Rimpl<W: Write + ?Sized, '_> Write for &'_ mut W
[src]

🔬 This is a nightly-only experimental API. (unsafe_cell_get_mut #76943)

Returns a mutable reference to the underlying data.

This call borrows the UnsafeCell mutably (at compile-time) which guarantees that we possess the only reference.

Examples

#![feature(unsafe_cell_get_mut)]
use std::cell::UnsafeCell;

let mut c = UnsafeCell::new(5);
*c.get_mut() += 1;

assert_eq!(*c.get_mut(), 6);
Run

pub fn raw_get(this: *const UnsafeCell<T>) -> *mut T[src]

🔬 This is a nightly-only experimental API. (unsafe_cell_raw_get #66358)

Gets a mutable pointer to the wrapped value. The difference to get is that this function accepts a raw pointer, which is useful to avoid the creation of temporary references.

The result can be cast to a pointer of any kind. Ensure that the access is unique (no active references, mutable or not) when casting to &mut T, and ensure that there are no mutations or mutable aliases going on when casting to &T.

Examples

Gradual initialization of an UnsafeCell requires raw_get, as calling get would require creating a reference to uninitialized data:

#![feature(unsafe_cell_raw_get)]
use std::cell::UnsafeCell;
use std::mem::MaybeUninit;

let m = MaybeUninit::<UnsafeCell<i32>>::uninit();
unsafe { UnsafeCell::raw_get(m.as_ptr()).write(5); }
let uc = unsafe { m.assume_init() };

assert_eq!(uc.into_inner(), 5);
Run

Trait Implementations

impl<T, U> CoerceUnsized<UnsafeCell<U>> for UnsafeCell<T> where
    T: CoerceUnsized<U>, 
[src]

impl<T> Debug for UnsafeCell<T> where
    T: Debug + ?Sized
1.9.0[src]

impl<T> Default for UnsafeCell<T> where
    T: Default
1.10.0[src]

pub fn default() -> UnsafeCell<T>[src]

Creates an UnsafeCell, with the Default value for T.

impl<T> From<T> for UnsafeCell<T>1.12.0[src]

impl<T: ?Sized> !RefUnwindSafe for UnsafeCell<T>1.9.0[src]

impl<T> !Sync for UnsafeCell<T> where
    T: ?Sized
[src]

Auto Trait Implementations

impl<T: ?Sized> Send for UnsafeCell<T> where
    T: Send

impl<T: ?Sized> Unpin for UnsafeCell<T> where
    T: Unpin

impl<T: ?Sized> UnwindSafe for UnsafeCell<T> where
    T: UnwindSafe

Blanket Implementations

impl<T> Any for T where
    T: 'static + ?Sized
[src]

impl<T> Borrow<T> for T where
    T: ?Sized
[src]

impl<T> BorrowMut<T> for T where
    T: ?Sized
[src]

impl<T> From<!> for T[src]

impl<T> From<T> for T[src]

impl<T, U> Into<U> for T where
    U: From<T>, 
[src]

impl<T, U> TryFrom<U> for T where
    U: Into<T>, 
[src]

type Error = Infallible

The type returned in the event of a conversion error.

impl<T, U> TryInto<U> for T where
    U: TryFrom<T>, 
[src]

type Error = <U as TryFrom<T>>::Error

The type returned in the event of a conversion error.