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

#[lang = "unsafe_cell"]
pub struct UnsafeCell<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. UnsafeCel<T> is the only core language feature to work around this restriction. All other types that allow internal mutability, such as Cell<T> and RefCell<T> use UnsafeCell to wrap their internal data.

The UnsafeCell API itself is technically very simple: it 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-exit 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 while mutating or mutably aliasing the contents of an & UnsafeCell<T> is okay (provided you enforce the invariants some other way), it is still undefined behavior to have multiple &mut UnsafeCell<T> aliases.


use std::cell::UnsafeCell;
use std::marker::Sync;

struct NotThreadSafe<T> {
    value: UnsafeCell<T>,

unsafe impl<T> Sync for NotThreadSafe<T> {}Run


impl<T> UnsafeCell<T>

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);Run

Unwraps the value.


use std::cell::UnsafeCell;

let uc = UnsafeCell::new(5);

let five = uc.into_inner();Run

impl<T: ?Sized> UnsafeCell<T>

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();Run

Trait Implementations

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

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

Creates an UnsafeCell, with the Default value for T.

impl<T> From<T> for UnsafeCell<T>

Performs the conversion.

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

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

Formats the value using the given formatter. Read more

Auto Trait Implementations

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