Pointer types

All pointers 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 (&)

Shared references point to memory which is 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)

Mutable references point to memory which is 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; 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.

Raw pointers can be created directly using &raw const for *const pointers and &raw mut for *mut pointers.

Smart Pointers

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

Bit validity

Despite pointers and references being similar to usizes in the machine code emitted on most platforms, the semantics of transmuting a reference or pointer type to a non-pointer type is currently undecided. Thus, it may not be valid to transmute a pointer or reference type, P, to a [u8; size_of::<P>()].

For thin raw pointers (i.e., for P = *const T or P = *mut T for T: Sized), the inverse direction (transmuting from an integer or array of integers to P) is always valid. However, the pointer produced via such a transmutation may not be dereferenced (not even if T has size zero).