# 1.0.0[−][src]Struct core::cell::UnsafeCell

    #[lang = "unsafe_cell"]
    #[repr(transparent)]
    #[repr(no_niche)]
    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. `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: 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:

- If you create a safe reference with lifetime `'a` (either a `&T` or `&mut T` reference) that is accessible by safe code (for example, because you returned it), then you must not access the data in any way that contradicts that reference for the remainder of `'a`. For example, this means that if you take the `*mut T` from an `UnsafeCell<T>` and cast it to an `&T`, then the data in `T` must remain immutable (modulo any `UnsafeCell` data found within `T`, of course) until that reference's lifetime expires. Similarly, if you create a `&mut T` reference that is released to safe code, then you must not access the data within the `UnsafeCell` until that reference expires.

- At all times, you must avoid data races. If multiple threads have access to the same `UnsafeCell`, then any writes must have a proper happens-before relation to all other accesses (or use atomics).

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 while mutating or mutably aliasing the contents of an `&UnsafeCell<T>` is ok (provided you enforce the invariants some other way), it is still undefined behavior to have multiple `&mut UnsafeCell<T>` aliases.

# Examples

    use std::cell::UnsafeCell;

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

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

Run

## Methods

### `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: ?Sized> UnsafeCell<T>`[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 const fn raw_get(this: *const Self) -> *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: CoerceUnsized<U>, U> CoerceUnsized<UnsafeCell<U>> for UnsafeCell<T>`[src]

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

#### `fn fmt(&self, f: &mut Formatter) -> Result`[src]

Formats the value using the given formatter. Read more

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

#### `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]

#### `fn from(t: T) -> UnsafeCell<T>`[src]

Performs the conversion.

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

## Auto Trait Implementations

### `impl<T: ?Sized> Send for UnsafeCell<T> where
T: Send,`

### `impl<T: ?Sized> Unpin for UnsafeCell<T> where
T: Unpin,`

## Blanket Implementations

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

#### `fn type_id(&Self) -> TypeId`[src]

Gets the `TypeId` of `self`. Read more

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

#### `fn borrow(&Self) -> &T`[src]

Immutably borrows from an owned value. Read more

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

#### `fn borrow_mut(&mut Self) -> &mut T`[src]

Mutably borrows from an owned value. Read more

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

#### `fn from(!) -> T`[src]

Performs the conversion.

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

#### `fn from(T) -> T`[src]

Performs the conversion.

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

#### `fn into(Self) -> U`[src]

Performs the conversion.

### `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.

#### `fn try_from(U) -> Result<T, <T as TryFrom<U>>::Error>`[src]

Performs the conversion.

### `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.

#### `fn try_into(Self) -> Result<U, <U as TryFrom<T>>::Error>`[src]

Performs the conversion.