Primitive Type pointer [−]
Raw, unsafe pointers, *const T, and *mut T.
Working with raw pointers in Rust is uncommon, typically limited to a few patterns.
Use the null function to create null pointers, and the is_null method
of the *const T type to check for null. The *const T type also defines
the offset method, for pointer math.
Common ways to create raw pointers
1. Coerce a reference (&T) or mutable reference (&mut T).
fn main() {
let my_num: i32 = 10;
let my_num_ptr: *const i32 = &my_num;
let mut my_speed: i32 = 88;
let my_speed_ptr: *mut i32 = &mut my_speed;
}let my_num: i32 = 10; let my_num_ptr: *const i32 = &my_num; let mut my_speed: i32 = 88; let my_speed_ptr: *mut i32 = &mut my_speed;
To get a pointer to a boxed value, dereference the box:
fn main() { let my_num: Box<i32> = Box::new(10); let my_num_ptr: *const i32 = &*my_num; let mut my_speed: Box<i32> = Box::new(88); let my_speed_ptr: *mut i32 = &mut *my_speed; }let my_num: Box<i32> = Box::new(10); let my_num_ptr: *const i32 = &*my_num; let mut my_speed: Box<i32> = Box::new(88); let my_speed_ptr: *mut i32 = &mut *my_speed;
This does not take ownership of the original allocation and requires no resource management later, but you must not use the pointer after its lifetime.
2. Consume a box (Box<T>).
The into_raw function consumes a box and returns
the raw pointer. It doesn't destroy T or deallocate any memory.
let my_speed: Box<i32> = Box::new(88); let my_speed: *mut i32 = Box::into_raw(my_speed); // By taking ownership of the original `Box<T>` though // we are obligated to put it together later to be destroyed. unsafe { drop(Box::from_raw(my_speed)); }
Note that here the call to drop is for clarity - it indicates
that we are done with the given value and it should be destroyed.
3. Get it from C.
#![feature(libc)] extern crate libc; use std::mem; fn main() { unsafe { let my_num: *mut i32 = libc::malloc(mem::size_of::<i32>() as libc::size_t) as *mut i32; if my_num.is_null() { panic!("failed to allocate memory"); } libc::free(my_num as *mut libc::c_void); } }extern crate libc; use std::mem; fn main() { unsafe { let my_num: *mut i32 = libc::malloc(mem::size_of::<i32>() as libc::size_t) as *mut i32; if my_num.is_null() { panic!("failed to allocate memory"); } libc::free(my_num as *mut libc::c_void); } }
Usually you wouldn't literally use malloc and free from Rust,
but C APIs hand out a lot of pointers generally, so are a common source
of raw pointers in Rust.
Methods
impl<T> *const T where T: ?Sized
fn is_null(self) -> bool
Returns true if the pointer is null.
unsafe fn as_ref(&self) -> Option<&'a T>
ptr_as_ref #27780): Option is not clearly the right return type, and we may want to tie the return lifetime to a borrow of the raw pointer
Returns None if the pointer is null, or else returns a reference to
the value wrapped in Some.
Safety
While this method and its mutable counterpart are useful for null-safety, it is important to note that this is still an unsafe operation because the returned value could be pointing to invalid memory.
unsafe fn offset(self, count: isize) -> *const T
Calculates the offset from a pointer. count is in units of T; e.g. a
count of 3 represents a pointer offset of 3 * sizeof::<T>() bytes.
Safety
Both the starting and resulting pointer must be either in bounds or one byte past the end of an allocated object. If either pointer is out of bounds or arithmetic overflow occurs then any further use of the returned value will result in undefined behavior.
impl<T> *mut T where T: ?Sized
fn is_null(self) -> bool
Returns true if the pointer is null.
unsafe fn as_ref(&self) -> Option<&'a T>
ptr_as_ref #27780): Option is not clearly the right return type, and we may want to tie the return lifetime to a borrow of the raw pointer
Returns None if the pointer is null, or else returns a reference to
the value wrapped in Some.
Safety
While this method and its mutable counterpart are useful for null-safety, it is important to note that this is still an unsafe operation because the returned value could be pointing to invalid memory.
unsafe fn offset(self, count: isize) -> *mut T
Calculates the offset from a pointer. count is in units of T; e.g. a
count of 3 represents a pointer offset of 3 * sizeof::<T>() bytes.
Safety
The offset must be in-bounds of the object, or one-byte-past-the-end.
Otherwise offset invokes Undefined Behavior, regardless of whether
the pointer is used.
unsafe fn as_mut(&self) -> Option<&'a mut T>
Returns None if the pointer is null, or else returns a mutable
reference to the value wrapped in Some.
Safety
As with as_ref, this is unsafe because it cannot verify the validity
of the returned pointer.