# Pointer types

All pointers in Rust are explicit first-class values. They can be moved or
copied, stored into data structs, and returned from functions.

## References (`&` and `&mut`)

> **Syntax**\
> _ReferenceType_ :\
>    `&` [_Lifetime_]? `mut`? [_TypeNoBounds_]

### Shared references (`&`)

These point to memory _owned by some other value_. When a shared reference to
a value is created it prevents direct mutation of the value. [Interior
mutability] provides an exception for this in certain circumstances. As the
name suggests, any number of shared references to a value may exist. A shared
reference type is written `&type`, or `&'a type` when you need to specify an
explicit lifetime. Copying a reference is a "shallow" operation: it involves
only copying the pointer itself, that is, pointers are `Copy`. Releasing a
reference has no effect on the value it points to, but referencing of a
[temporary value] will keep it alive during the scope of the reference itself.

### Mutable references (`&mut`)

These also point to memory owned by some other value. A mutable reference type
is written `&mut type` or `&'a mut type`. A mutable reference (that hasn't been
borrowed) is the only way to access the value it points to, so is not `Copy`.

## Raw pointers (`*const` and `*mut`)

> **Syntax**\
> _RawPointerType_ :\
>    `*` ( `mut` | `const` ) [_TypeNoBounds_]

Raw pointers are pointers without safety or liveness guarantees. Raw pointers
are written as `*const T` or `*mut T`, for example `*const i32` means a raw
pointer to a 32-bit integer. Copying or dropping a raw pointer has no effect
on the lifecycle of any other value. Dereferencing a raw pointer is an
[`unsafe` operation], this can also be used to convert a raw pointer to a
reference by reborrowing it (`&*` or `&mut *`). Raw pointers are generally
discouraged in Rust code; they exist to support interoperability with foreign
code, and writing performance-critical or low-level functions.

When comparing raw pointers they are compared by their address, rather than by
what they point to. When comparing raw pointers to [dynamically sized types] they
also have their additional data compared.

## Smart Pointers

The standard library contains additional 'smart pointer' types beyond references
and raw pointers.

[Interior mutability]: ../interior-mutability.md
[_Lifetime_]: ../trait-bounds.md
[_TypeNoBounds_]: ../types.md#type-expressions
[`unsafe` operation]: ../unsafety.md
[dynamically sized types]: ../dynamically-sized-types.md
[temporary value]: ../expressions.md#temporaries
