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

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

- 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

## Trait Implementations

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

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

### `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 + 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

## Auto Trait Implementations

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

## Blanket Implementations

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

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

Performs the conversion.

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

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

Performs the conversion.

### `impl<T, U> TryInto 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.

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

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

Immutably borrows from an owned value. Read more

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

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

Mutably borrows from an owned value. Read more

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

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

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

# Help

## Keyboard Shortcuts

`?`
  Show this help dialog
`S`
  Focus the search field
`↑`
  Move up in search results
`↓`
  Move down in search results
`↹`
  Switch tab
`⏎`
  Go to active search result
`+`
  Expand all sections
`-`
  Collapse all sections

## Search Tricks

Prefix searches with a type followed by a colon (e.g., `fn:`) to restrict the search to a given type.

Accepted types are: `fn`, `mod`, `struct`, `enum`, `trait`, `type`, `macro`, and `const`.

Search functions by type signature (e.g., `vec -> usize` or `* -> vec`)

Search multiple things at once by splitting your query with comma (e.g., `str,u8` or `String,struct:Vec,test`)