Struct std::cell::UnsafeCell1.0.0[][src]

pub struct UnsafeCell<T> where
    T: ?Sized
{ /* fields omitted */ }

The core primitive for interior mutability in Rust.

If you have a reference &T, then normally in Rust the compiler performs optimizations based on the knowledge that &T points to immutable data. Mutating that data, for example through an alias or by transmuting an &T into an &mut T, is considered undefined behavior. UnsafeCell<T> opts-out of the immutability guarantee for &T: a shared reference &UnsafeCell<T> may point to data that is being mutated. This is called “interior mutability”.

All other types that allow internal mutability, such as Cell<T> and RefCell<T>, internally use UnsafeCell to wrap their data.

Note that only the immutability guarantee for shared references is affected by UnsafeCell. The uniqueness guarantee for mutable references is unaffected. 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 safe getter that yields a &mut T.


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);

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

#![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);


impl<T> UnsafeCell<T>[src]

pub const fn new(value: T) -> UnsafeCell<T>1.0.0 (const: 1.32.0)[src]

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

All access to the inner value through methods is unsafe.


use std::cell::UnsafeCell;

let uc = UnsafeCell::new(5);

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

Unwraps the value.


use std::cell::UnsafeCell;

let uc = UnsafeCell::new(5);

let five = uc.into_inner();

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

pub const fn get(&self) -> *mut T1.0.0 (const: 1.32.0)[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


use std::cell::UnsafeCell;

let uc = UnsafeCell::new(5);

let five = uc.get();

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

Notable traits for &'_ mut F

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

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.


use std::cell::UnsafeCell;

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

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

pub const 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.


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

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);

Trait Implementations

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

impl<T> Default for UnsafeCell<T> where
    T: Default

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, U> CoerceUnsized<UnsafeCell<U>> for UnsafeCell<T> where
    T: CoerceUnsized<U>, 

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

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

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

impl<T> Borrow<T> for T where
    T: ?Sized

impl<T> BorrowMut<T> for T where
    T: ?Sized

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

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

impl<T, U> Into<U> for T where
    U: From<T>, 

impl<T, U> TryFrom<U> for T where
    U: Into<T>, 

type Error = Infallible

The type returned in the event of a conversion error.

impl<T, U> TryInto<U> for T where
    U: TryFrom<T>, 

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

The type returned in the event of a conversion error.