All pointers in Rust are explicit first-class values. They can be moved or copied, stored into data structs, and returned from functions.
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
&'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 (
These also point to memory owned by some other value. A mutable reference type
&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
Raw pointers (
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 (
&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.
The standard library contains additional 'smart pointer' types beyond references and raw pointers.