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;Run
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;Run
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)); }Run
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); } }Run
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
1.0.0
Returns true if the pointer is null.
Examples
Basic usage:
fn main() { let s: &str = "Follow the rabbit"; let ptr: *const u8 = s.as_ptr(); assert!(!ptr.is_null()); }let s: &str = "Follow the rabbit"; let ptr: *const u8 = s.as_ptr(); assert!(!ptr.is_null());Run
unsafe fn as_ref(self) -> Option<&'a T>
1.9.0
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.
Additionally, the lifetime 'a
returned is arbitrarily chosen and does
not necessarily reflect the actual lifetime of the data.
Examples
Basic usage:
fn main() { let val: *const u8 = &10u8 as *const u8; unsafe { if let Some(val_back) = val.as_ref() { println!("We got back the value: {}!", val_back); } } }let val: *const u8 = &10u8 as *const u8; unsafe { if let Some(val_back) = val.as_ref() { println!("We got back the value: {}!", val_back); } }Run
unsafe fn offset(self, count: isize) -> *const T
1.0.0
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.
Examples
Basic usage:
fn main() { let s: &str = "123"; let ptr: *const u8 = s.as_ptr(); unsafe { println!("{}", *ptr.offset(1) as char); println!("{}", *ptr.offset(2) as char); } }let s: &str = "123"; let ptr: *const u8 = s.as_ptr(); unsafe { println!("{}", *ptr.offset(1) as char); println!("{}", *ptr.offset(2) as char); }Run
Trait Implementations
impl<T> !Sync for *const T where T: ?Sized
1.0.0
impl<T> !Sync for *mut T where T: ?Sized
1.0.0
impl<T> PartialEq<*const T> for *const T where T: ?Sized
1.0.0
fn eq(&self, other: &*const T) -> bool
This method tests for self
and other
values to be equal, and is used by ==
. Read more
fn ne(&self, other: &Rhs) -> bool
1.0.0
This method tests for !=
.
impl<T> PartialEq<*mut T> for *mut T where T: ?Sized
1.0.0
fn eq(&self, other: &*mut T) -> bool
This method tests for self
and other
values to be equal, and is used by ==
. Read more
fn ne(&self, other: &Rhs) -> bool
1.0.0
This method tests for !=
.
impl<T> Clone for *const T where T: ?Sized
1.0.0
fn clone(&self) -> *const T
Returns a copy of the value. Read more
fn clone_from(&mut self, source: &Self)
1.0.0
Performs copy-assignment from source
. Read more
impl<T> Clone for *mut T where T: ?Sized
1.0.0
fn clone(&self) -> *mut T
Returns a copy of the value. Read more
fn clone_from(&mut self, source: &Self)
1.0.0
Performs copy-assignment from source
. Read more
impl<T> Zeroable for *const T where T: ?Sized
impl<T> Zeroable for *mut T where T: ?Sized
impl<T, U> CoerceUnsized<*mut U> for *mut T where T: Unsize<U> + ?Sized, U: ?Sized
impl<T, U> CoerceUnsized<*const U> for *mut T where T: Unsize<U> + ?Sized, U: ?Sized
impl<T, U> CoerceUnsized<*const U> for *const T where T: Unsize<U> + ?Sized, U: ?Sized
impl<T> Ord for *const T where T: ?Sized
1.0.0
fn cmp(&self, other: &*const T) -> Ordering
This method returns an Ordering
between self
and other
. Read more
impl<T> Ord for *mut T where T: ?Sized
1.0.0
fn cmp(&self, other: &*mut T) -> Ordering
This method returns an Ordering
between self
and other
. Read more
impl<T> Debug for *const T
1.0.0
impl<T> Debug for *mut T
1.0.0
impl<T> !Send for *const T where T: ?Sized
1.0.0
impl<T> !Send for *mut T where T: ?Sized
1.0.0
impl<T> Eq for *const T where T: ?Sized
1.0.0
impl<T> Eq for *mut T where T: ?Sized
1.0.0
impl<T> Pointer for *const T where T: ?Sized
1.0.0
impl<T> Pointer for *mut T where T: ?Sized
1.0.0
impl<T> Hash for *const T
1.0.0
fn hash<H>(&self, state: &mut H) where H: Hasher
Feeds this value into the state given, updating the hasher as necessary.
fn hash_slice<H>(data: &[Self], state: &mut H) where H: Hasher
1.3.0
Feeds a slice of this type into the state provided.
impl<T> Hash for *mut T
1.0.0
fn hash<H>(&self, state: &mut H) where H: Hasher
Feeds this value into the state given, updating the hasher as necessary.
fn hash_slice<H>(data: &[Self], state: &mut H) where H: Hasher
1.3.0
Feeds a slice of this type into the state provided.
impl<T> PartialOrd<*const T> for *const T where T: ?Sized
1.0.0
fn partial_cmp(&self, other: &*const T) -> Option<Ordering>
This method returns an ordering between self
and other
values if one exists. Read more
fn lt(&self, other: &*const T) -> bool
This method tests less than (for self
and other
) and is used by the <
operator. Read more
fn le(&self, other: &*const T) -> bool
This method tests less than or equal to (for self
and other
) and is used by the <=
operator. Read more
fn gt(&self, other: &*const T) -> bool
This method tests greater than (for self
and other
) and is used by the >
operator. Read more
fn ge(&self, other: &*const T) -> bool
This method tests greater than or equal to (for self
and other
) and is used by the >=
operator. Read more
impl<T> PartialOrd<*mut T> for *mut T where T: ?Sized
1.0.0
fn partial_cmp(&self, other: &*mut T) -> Option<Ordering>
This method returns an ordering between self
and other
values if one exists. Read more
fn lt(&self, other: &*mut T) -> bool
This method tests less than (for self
and other
) and is used by the <
operator. Read more
fn le(&self, other: &*mut T) -> bool
This method tests less than or equal to (for self
and other
) and is used by the <=
operator. Read more
fn gt(&self, other: &*mut T) -> bool
This method tests greater than (for self
and other
) and is used by the >
operator. Read more
fn ge(&self, other: &*mut T) -> bool
This method tests greater than or equal to (for self
and other
) and is used by the >=
operator. Read more