Struct NonNull

#[repr(transparent)]
pub struct NonNull<T: PointeeSized> { pub(in ::ptr::non_null) pointer: *const T is !null }

*mut T but non-zero and covariant.

This is often the correct thing to use when building data structures using raw pointers, but is ultimately more dangerous to use because of its additional properties. If you're not sure if you should use NonNull<T>, just use *mut T!

Unlike *mut T, the pointer must always be non-null, even if the pointer is never dereferenced. This is so that enums may use this forbidden value as a discriminant -- Option<NonNull<T>> has the same size as *mut T. However the pointer may still dangle if it isn't dereferenced.

Unlike *mut T, NonNull<T> is covariant over T. This is usually the correct choice for most data structures and safe abstractions, such as Box, Rc, Arc, Vec, and LinkedList.

In rare cases, if your type exposes a way to mutate the value of T through a NonNull<T>, and you need to prevent unsoundness from variance (for example, if T could be a reference with a shorter lifetime), you should add a field to make your type invariant, such as PhantomData<Cell<T>> or PhantomData<&'a mut T>.

Example of a type that must be invariant:

use std::cell::Cell;
use std::marker::PhantomData;
struct Invariant<T> {
    ptr: std::ptr::NonNull<T>,
    _invariant: PhantomData<Cell<T>>,
}

Notice that NonNull<T> has a From instance for &T. However, this does not change the fact that mutating through a (pointer derived from a) shared reference is undefined behavior unless the mutation happens inside an UnsafeCell<T>. The same goes for creating a mutable reference from a shared reference. When using this From instance without an UnsafeCell<T>, it is your responsibility to ensure that as_mut is never called, and as_ptr is never used for mutation.

Layout

NonNull<T> is guaranteed to have the same layout and bit validity as *mut T with the exception that a null pointer is invalid. Option<NonNull<T>> is guaranteed to be ABI-compatible with *mut T, including in FFI.

Thanks to the null pointer optimization, NonNull<T> and Option<NonNull<T>> are guaranteed to have the same size and alignment:

use std::ptr::NonNull;

assert_eq!(size_of::<NonNull<i16>>(), size_of::<Option<NonNull<i16>>>());
assert_eq!(align_of::<NonNull<i16>>(), align_of::<Option<NonNull<i16>>>());

assert_eq!(size_of::<NonNull<str>>(), size_of::<Option<NonNull<str>>>());
assert_eq!(align_of::<NonNull<str>>(), align_of::<Option<NonNull<str>>>());

Fields

pointer: *const T is !null

Implementations

impl<T> NonNull<MaybeUninit<T>>

const fn cast_init(self) -> NonNull<T>

Casts from a maybe-uninitialized type to its initialized version.

This is always safe, since UB can only occur if the pointer is read before being initialized.

impl<T> NonNull<T>

const fn cast_uninit(self) -> NonNull<MaybeUninit<T>>

Casts from a type to its maybe-uninitialized version.

const fn cast_slice(self, len: usize) -> NonNull<[T]>

Creates a non-null raw slice from a thin pointer and a length.

The len argument is the number of elements, not the number of bytes.

This function is safe, but dereferencing the return value is unsafe. See the documentation of slice::from_raw_parts for slice safety requirements.

Examples

#![feature(ptr_cast_slice)]
use std::ptr::NonNull;

// create a slice pointer when starting out with a pointer to the first element
let mut x = [5, 6, 7];
let nonnull_pointer = NonNull::new(x.as_mut_ptr()).unwrap();
let slice = nonnull_pointer.cast_slice(3);
assert_eq!(unsafe { slice.as_ref()[2] }, 7);

(Note that this example artificially demonstrates a use of this method, but let slice = NonNull::from(&x[..]); would be a better way to write code like this.)

impl<T> NonNull<[T]>

const fn slice_from_raw_parts(data: NonNull<T>, len: usize) -> Self

Creates a non-null raw slice from a thin pointer and a length.

The len argument is the number of elements, not the number of bytes.

This function is safe, but dereferencing the return value is unsafe. See the documentation of slice::from_raw_parts for slice safety requirements.

Examples

use std::ptr::NonNull;

// create a slice pointer when starting out with a pointer to the first element
let mut x = [5, 6, 7];
let nonnull_pointer = NonNull::new(x.as_mut_ptr()).unwrap();
let slice = NonNull::slice_from_raw_parts(nonnull_pointer, 3);
assert_eq!(unsafe { slice.as_ref()[2] }, 7);

(Note that this example artificially demonstrates a use of this method, but let slice = NonNull::from(&x[..]); would be a better way to write code like this.)

const fn len(self) -> usize

Returns the length of a non-null raw slice.

The returned value is the number of elements, not the number of bytes.

This function is safe, even when the non-null raw slice cannot be dereferenced to a slice because the pointer does not have a valid address.

Examples

use std::ptr::NonNull;

let slice: NonNull<[i8]> = NonNull::slice_from_raw_parts(NonNull::dangling(), 3);
assert_eq!(slice.len(), 3);

const fn is_empty(self) -> bool

Returns true if the non-null raw slice has a length of 0.

Examples

use std::ptr::NonNull;

let slice: NonNull<[i8]> = NonNull::slice_from_raw_parts(NonNull::dangling(), 3);
assert!(!slice.is_empty());

const fn as_non_null_ptr(self) -> NonNull<T>

Returns a non-null pointer to the slice's buffer.

Examples

#![feature(slice_ptr_get)]
use std::ptr::NonNull;

let slice: NonNull<[i8]> = NonNull::slice_from_raw_parts(NonNull::dangling(), 3);
assert_eq!(slice.as_non_null_ptr(), NonNull::<i8>::dangling());

const fn as_mut_ptr(self) -> *mut T

Returns a raw pointer to the slice's buffer.

Examples

#![feature(slice_ptr_get)]
use std::ptr::NonNull;

let slice: NonNull<[i8]> = NonNull::slice_from_raw_parts(NonNull::dangling(), 3);
assert_eq!(slice.as_mut_ptr(), NonNull::<i8>::dangling().as_ptr());

const unsafe fn as_uninit_slice<'a>(self) -> &'a [MaybeUninit<T>]

Returns a shared reference to a slice of possibly uninitialized values. In contrast to as_ref, this does not require that the value has to be initialized.

For the mutable counterpart see as_uninit_slice_mut.

Safety

When calling this method, you have to ensure that all of the following is true:

  • The pointer must be valid for reads for ptr.len() * size_of::<T>() many bytes, and it must be properly aligned. This means in particular:

    • The entire memory range of this slice must be contained within a single allocation! Slices can never span across multiple allocations.

    • The pointer must be aligned even for zero-length slices. One reason for this is that enum layout optimizations may rely on references (including slices of any length) being aligned and non-null to distinguish them from other data. You can obtain a pointer that is usable as data for zero-length slices using [NonNull::dangling()].

  • The total size ptr.len() * size_of::<T>() of the slice must be no larger than isize::MAX. See the safety documentation of pointer::offset.

  • You must enforce Rust's aliasing rules, since the returned lifetime 'a is arbitrarily chosen and does not necessarily reflect the actual lifetime of the data. In particular, while this reference exists, the memory the pointer points to must not get mutated (except inside UnsafeCell).

This applies even if the result of this method is unused!

See also slice::from_raw_parts.

const unsafe fn as_uninit_slice_mut<'a>(self) -> &'a mut [MaybeUninit<T>]

Returns a unique reference to a slice of possibly uninitialized values. In contrast to as_mut, this does not require that the value has to be initialized.

For the shared counterpart see as_uninit_slice.

Safety

When calling this method, you have to ensure that all of the following is true:

  • The pointer must be valid for reads and writes for ptr.len() * size_of::<T>() many bytes, and it must be properly aligned. This means in particular:

    • The entire memory range of this slice must be contained within a single allocation! Slices can never span across multiple allocations.

    • The pointer must be aligned even for zero-length slices. One reason for this is that enum layout optimizations may rely on references (including slices of any length) being aligned and non-null to distinguish them from other data. You can obtain a pointer that is usable as data for zero-length slices using [NonNull::dangling()].

  • The total size ptr.len() * size_of::<T>() of the slice must be no larger than isize::MAX. See the safety documentation of pointer::offset.

  • You must enforce Rust's aliasing rules, since the returned lifetime 'a is arbitrarily chosen and does not necessarily reflect the actual lifetime of the data. In particular, while this reference exists, the memory the pointer points to must not get accessed (read or written) through any other pointer.

This applies even if the result of this method is unused!

See also slice::from_raw_parts_mut.

Examples

#![feature(allocator_api, ptr_as_uninit)]

use std::alloc::{Allocator, Layout, Global};
use std::mem::MaybeUninit;
use std::ptr::NonNull;

let memory: NonNull<[u8]> = Global.allocate(Layout::new::<[u8; 32]>())?;
// This is safe as `memory` is valid for reads and writes for `memory.len()` many bytes.
// Note that calling `memory.as_mut()` is not allowed here as the content may be uninitialized.
# #[allow(unused_variables)]
let slice: &mut [MaybeUninit<u8>] = unsafe { memory.as_uninit_slice_mut() };
# // Prevent leaks for Miri.
# unsafe { Global.deallocate(memory.cast(), Layout::new::<[u8; 32]>()); }
# Ok::<_, std::alloc::AllocError>(())

const unsafe fn get_unchecked_mut<I>(self, index: I) -> NonNull<I::Output>
where
    I: ~const SliceIndex<[T]>,

Returns a raw pointer to an element or subslice, without doing bounds checking.

Calling this method with an out-of-bounds index or when self is not dereferenceable is undefined behavior even if the resulting pointer is not used.

Examples

#![feature(slice_ptr_get)]
use std::ptr::NonNull;

let x = &mut [1, 2, 4];
let x = NonNull::slice_from_raw_parts(NonNull::new(x.as_mut_ptr()).unwrap(), x.len());

unsafe {
    assert_eq!(x.get_unchecked_mut(1).as_ptr(), x.as_non_null_ptr().as_ptr().add(1));
}

impl<T: PointeeSized> NonNull<T>

const unsafe fn new_unchecked(ptr: *mut T) -> Self

Creates a new NonNull.

Note that if you have an &mut, you can use the safe from_mut instead.

Safety

ptr must be non-null.

Examples

use std::ptr::NonNull;

let mut x = 0u32;
let ptr = unsafe { NonNull::new_unchecked(&mut x as *mut _) };

Incorrect usage of this function:

use std::ptr::NonNull;

// NEVER DO THAT!!! This is undefined behavior. ⚠️
let ptr = unsafe { NonNull::<u32>::new_unchecked(std::ptr::null_mut()) };

const fn new(ptr: *mut T) -> Option<Self>

Creates a new NonNull if ptr is non-null.

Note that if you have an &mut, you can use from_mut instead to avoid the Option.

Panics during const evaluation

This method will panic during const evaluation if the pointer cannot be determined to be null or not. See is_null for more information.

Examples

use std::ptr::NonNull;

let mut x = 0u32;
let ptr = NonNull::<u32>::new(&mut x as *mut _).expect("pointer should not be null");

if let Some(ptr) = NonNull::<u32>::new(std::ptr::null_mut()) {
    unreachable!();
}

const fn from_ref(r: &T) -> Self

Converts a reference to a NonNull pointer.

const fn from_mut(r: &mut T) -> Self

Converts a mutable reference to a NonNull pointer.

const fn from_raw_parts(data_pointer: NonNull<impl Thin>, metadata: <T as Pointee>::Metadata) -> NonNull<T>

Performs the same functionality as std::ptr::from_raw_parts, except that a NonNull pointer is returned, as opposed to a raw *const pointer.

See the documentation of std::ptr::from_raw_parts for more details.

const fn to_raw_parts(self) -> (NonNull<()>, <T as Pointee>::Metadata)

Decompose a (possibly wide) pointer into its data pointer and metadata components.

The pointer can be later reconstructed with NonNull::from_raw_parts.

fn addr(self) -> NonZero<usize>

Gets the "address" portion of the pointer.

For more details, see the equivalent method on a raw pointer, pointer::addr.

This is a [Strict Provenance][crate::ptr#strict-provenance] API.

fn expose_provenance(self) -> NonZero<usize>

Exposes the ["provenance"][crate::ptr#provenance] part of the pointer for future use in [with_exposed_provenance][NonNull::with_exposed_provenance] and returns the "address" portion.

For more details, see the equivalent method on a raw pointer, pointer::expose_provenance.

This is an [Exposed Provenance][crate::ptr#exposed-provenance] API.

fn with_addr(self, addr: NonZero<usize>) -> Self

Creates a new pointer with the given address and the [provenance][crate::ptr#provenance] of self.

For more details, see the equivalent method on a raw pointer, pointer::with_addr.

This is a [Strict Provenance][crate::ptr#strict-provenance] API.

fn map_addr(self, f: impl FnOnce(NonZero<usize>) -> NonZero<usize>) -> Self

Creates a new pointer by mapping self's address to a new one, preserving the [provenance][crate::ptr#provenance] of self.

For more details, see the equivalent method on a raw pointer, pointer::map_addr.

This is a [Strict Provenance][crate::ptr#strict-provenance] API.

const fn as_ptr(self) -> *mut T

Acquires the underlying *mut pointer.

Examples

use std::ptr::NonNull;

let mut x = 0u32;
let ptr = NonNull::new(&mut x).expect("pointer should not be null");

let x_value = unsafe { *ptr.as_ptr() };
assert_eq!(x_value, 0);

unsafe { *ptr.as_ptr() += 2; }
let x_value = unsafe { *ptr.as_ptr() };
assert_eq!(x_value, 2);

const unsafe fn as_ref<'a>(&self) -> &'a T

Returns a shared reference to the value. If the value may be uninitialized, as_uninit_ref must be used instead.

For the mutable counterpart see as_mut.

Safety

When calling this method, you have to ensure that the pointer is convertible to a reference.

Examples

use std::ptr::NonNull;

let mut x = 0u32;
let ptr = NonNull::new(&mut x as *mut _).expect("pointer should not be null");

let ref_x = unsafe { ptr.as_ref() };
println!("{ref_x}");

const unsafe fn as_mut<'a>(&mut self) -> &'a mut T

Returns a unique reference to the value. If the value may be uninitialized, as_uninit_mut must be used instead.

For the shared counterpart see as_ref.

Safety

When calling this method, you have to ensure that the pointer is convertible to a reference.

Examples

use std::ptr::NonNull;

let mut x = 0u32;
let mut ptr = NonNull::new(&mut x).expect("pointer should not be null");

let x_ref = unsafe { ptr.as_mut() };
assert_eq!(*x_ref, 0);
*x_ref += 2;
assert_eq!(*x_ref, 2);

const fn cast<U>(self) -> NonNull<U>

Casts to a pointer of another type.

Examples

use std::ptr::NonNull;

let mut x = 0u32;
let ptr = NonNull::new(&mut x as *mut _).expect("pointer should not be null");

let casted_ptr = ptr.cast::<i8>();
let raw_ptr: *mut i8 = casted_ptr.as_ptr();

fn try_cast_aligned<U>(self) -> Option<NonNull<U>>

Try to cast to a pointer of another type by checking alignment.

If the pointer is properly aligned to the target type, it will be cast to the target type. Otherwise, None is returned.

Examples

#![feature(pointer_try_cast_aligned)]
use std::ptr::NonNull;

let mut x = 0u64;

let aligned = NonNull::from_mut(&mut x);
let unaligned = unsafe { aligned.byte_add(1) };

assert!(aligned.try_cast_aligned::<u32>().is_some());
assert!(unaligned.try_cast_aligned::<u32>().is_none());

const unsafe fn offset(self, count: isize) -> Self
where
    T: Sized,

Adds a signed offset to a pointer.

count is in units of T; e.g., a count of 3 represents a pointer offset of 3 * size_of::<T>() bytes.

Safety

If any of the following conditions are violated, the result is Undefined Behavior:

  • The offset in bytes, count * size_of::<T>(), computed on mathematical integers (without "wrapping around"), must fit in an isize.

  • Let result be self.addr() + count * size_of::<T>(), computed on mathematical integers. This must fit in a usize.

  • If the computed offset is non-zero, then self must be [derived from][crate::ptr#provenance] a pointer to some allocation, and the entire memory range between self and result (i.e., min(self.addr(), result)..max(self.addr(), result)) must be in bounds of that allocation.

Allocations can never be larger than isize::MAX bytes and they can only contain addresses representable by usize, so technically the last condition implies the first two. This implies, for instance, that vec.as_ptr().offset(vec.len() as isize) (for vec: Vec<T>) is always safe.

Examples

use std::ptr::NonNull;

let mut s = [1, 2, 3];
let ptr: NonNull<u32> = NonNull::new(s.as_mut_ptr()).unwrap();

unsafe {
    println!("{}", ptr.offset(1).read());
    println!("{}", ptr.offset(2).read());
}

const unsafe fn byte_offset(self, count: isize) -> Self

Calculates the offset from a pointer in bytes.

count is in units of bytes.

This is purely a convenience for casting to a u8 pointer and using [offset][pointer::offset] on it. See that method for documentation and safety requirements.

For non-Sized pointees this operation changes only the data pointer, leaving the metadata untouched.

const unsafe fn add(self, count: usize) -> Self
where
    T: Sized,

Adds an unsigned offset to a pointer.

This can only move the pointer forward (or not move it). If you need to move forward or backward depending on the value, then you might want offset instead which takes a signed offset.

count is in units of T; e.g., a count of 3 represents a pointer offset of 3 * size_of::<T>() bytes.

Safety

If any of the following conditions are violated, the result is Undefined Behavior:

  • The offset in bytes, count * size_of::<T>(), computed on mathematical integers (without "wrapping around"), must fit in an isize.

  • Let result be self.addr() + count * size_of::<T>(), computed on mathematical integers. This must fit in a usize.

  • If the computed offset is non-zero, then self must be [derived from][crate::ptr#provenance] a pointer to some allocation, and the entire memory range between self and result (i.e., self.addr()..result) must be in bounds of that allocation.

Allocations can never be larger than isize::MAX bytes and they can only contain addresses representable by usize, so technically the last condition implies the first two. This implies, for instance, that vec.as_ptr().add(vec.len()) (for vec: Vec<T>) is always safe.

Examples

use std::ptr::NonNull;

let s: &str = "123";
let ptr: NonNull<u8> = NonNull::new(s.as_ptr().cast_mut()).unwrap();

unsafe {
    println!("{}", ptr.add(1).read() as char);
    println!("{}", ptr.add(2).read() as char);
}

const unsafe fn byte_add(self, count: usize) -> Self

Calculates the offset from a pointer in bytes (convenience for .byte_offset(count as isize)).

count is in units of bytes.

This is purely a convenience for casting to a u8 pointer and using [add][NonNull::add] on it. See that method for documentation and safety requirements.

For non-Sized pointees this operation changes only the data pointer, leaving the metadata untouched.

const unsafe fn sub(self, count: usize) -> Self
where
    T: Sized,

Subtracts an unsigned offset from a pointer.

This can only move the pointer backward (or not move it). If you need to move forward or backward depending on the value, then you might want offset instead which takes a signed offset.

count is in units of T; e.g., a count of 3 represents a pointer offset of 3 * size_of::<T>() bytes.

Safety

If any of the following conditions are violated, the result is Undefined Behavior:

  • The offset in bytes, count * size_of::<T>(), computed on mathematical integers (without "wrapping around"), must fit in an isize.

  • Let result be self.addr() - count * size_of::<T>(), computed on mathematical integers. This must fit in a usize.

  • If the computed offset is non-zero, then self must be [derived from][crate::ptr#provenance] a pointer to some allocation, and the entire memory range between self and result (i.e., result..self.addr()) must be in bounds of that allocation.

Allocations can never be larger than isize::MAX bytes and they can only contain addresses representable by usize, so technically the last condition implies the first two.

Examples

use std::ptr::NonNull;

let s: &str = "123";

unsafe {
    let end: NonNull<u8> = NonNull::new(s.as_ptr().cast_mut()).unwrap().add(3);
    println!("{}", end.sub(1).read() as char);
    println!("{}", end.sub(2).read() as char);
}

const unsafe fn byte_sub(self, count: usize) -> Self

Calculates the offset from a pointer in bytes (convenience for .byte_offset((count as isize).wrapping_neg())).

count is in units of bytes.

This is purely a convenience for casting to a u8 pointer and using [sub][NonNull::sub] on it. See that method for documentation and safety requirements.

For non-Sized pointees this operation changes only the data pointer, leaving the metadata untouched.

const unsafe fn offset_from(self, origin: NonNull<T>) -> isize
where
    T: Sized,

Calculates the distance between two pointers within the same allocation. The returned value is in units of T: the distance in bytes divided by size_of::<T>().

This is equivalent to (self as isize - origin as isize) / (size_of::<T>() as isize), except that it has a lot more opportunities for UB, in exchange for the compiler better understanding what you are doing.

The primary motivation of this method is for computing the len of an array/slice of T that you are currently representing as a "start" and "end" pointer (and "end" is "one past the end" of the array). In that case, end.offset_from(start) gets you the length of the array.

All of the following safety requirements are trivially satisfied for this usecase.

Safety

If any of the following conditions are violated, the result is Undefined Behavior:

  • self and origin must either

    • point to the same address, or
    • both be derived from a pointer to the same allocation, and the memory range between the two pointers must be in bounds of that object. (See below for an example.)

  • The distance between the pointers, in bytes, must be an exact multiple of the size of T.

As a consequence, the absolute distance between the pointers, in bytes, computed on mathematical integers (without "wrapping around"), cannot overflow an isize. This is implied by the in-bounds requirement, and the fact that no allocation can be larger than isize::MAX bytes.

The requirement for pointers to be derived from the same allocation is primarily needed for const-compatibility: the distance between pointers into different allocated objects is not known at compile-time. However, the requirement also exists at runtime and may be exploited by optimizations. If you wish to compute the difference between pointers that are not guaranteed to be from the same allocation, use (self.addr() as isize - origin.addr() as isize) / size_of::<T>().

Panics

This function panics if T is a Zero-Sized Type ("ZST").

Examples

Basic usage:

use std::ptr::NonNull;

let a = [0; 5];
let ptr1: NonNull<u32> = NonNull::from(&a[1]);
let ptr2: NonNull<u32> = NonNull::from(&a[3]);
unsafe {
    assert_eq!(ptr2.offset_from(ptr1), 2);
    assert_eq!(ptr1.offset_from(ptr2), -2);
    assert_eq!(ptr1.offset(2), ptr2);
    assert_eq!(ptr2.offset(-2), ptr1);
}

Incorrect usage:

use std::ptr::NonNull;

let ptr1 = NonNull::new(Box::into_raw(Box::new(0u8))).unwrap();
let ptr2 = NonNull::new(Box::into_raw(Box::new(1u8))).unwrap();
let diff = (ptr2.addr().get() as isize).wrapping_sub(ptr1.addr().get() as isize);
// Make ptr2_other an "alias" of ptr2.add(1), but derived from ptr1.
let diff_plus_1 = diff.wrapping_add(1);
let ptr2_other = NonNull::new(ptr1.as_ptr().wrapping_byte_offset(diff_plus_1)).unwrap();
assert_eq!(ptr2.addr(), ptr2_other.addr());
// Since ptr2_other and ptr2 are derived from pointers to different objects,
// computing their offset is undefined behavior, even though
// they point to addresses that are in-bounds of the same object!

let one = unsafe { ptr2_other.offset_from(ptr2) }; // Undefined Behavior! ⚠️

const unsafe fn byte_offset_from<U: ?Sized>(self, origin: NonNull<U>) -> isize

Calculates the distance between two pointers within the same allocation. The returned value is in units of bytes.

This is purely a convenience for casting to a u8 pointer and using [offset_from][NonNull::offset_from] on it. See that method for documentation and safety requirements.

For non-Sized pointees this operation considers only the data pointers, ignoring the metadata.

const unsafe fn offset_from_unsigned(self, subtracted: NonNull<T>) -> usize
where
    T: Sized,

Calculates the distance between two pointers within the same allocation, where it's known that self is equal to or greater than origin. The returned value is in units of T: the distance in bytes is divided by size_of::<T>().

This computes the same value that offset_from would compute, but with the added precondition that the offset is guaranteed to be non-negative. This method is equivalent to usize::try_from(self.offset_from(origin)).unwrap_unchecked(), but it provides slightly more information to the optimizer, which can sometimes allow it to optimize slightly better with some backends.

This method can be though of as recovering the count that was passed to add (or, with the parameters in the other order, to sub). The following are all equivalent, assuming that their safety preconditions are met:

# unsafe fn blah(ptr: std::ptr::NonNull<u32>, origin: std::ptr::NonNull<u32>, count: usize) -> bool { unsafe {
ptr.offset_from_unsigned(origin) == count
# &&
origin.add(count) == ptr
# &&
ptr.sub(count) == origin
# } }

Safety

  • The distance between the pointers must be non-negative (self >= origin)

  • All the safety conditions of offset_from apply to this method as well; see it for the full details.

Importantly, despite the return type of this method being able to represent a larger offset, it's still not permitted to pass pointers which differ by more than isize::MAX bytes. As such, the result of this method will always be less than or equal to isize::MAX as usize.

Panics

This function panics if T is a Zero-Sized Type ("ZST").

Examples

use std::ptr::NonNull;

let a = [0; 5];
let ptr1: NonNull<u32> = NonNull::from(&a[1]);
let ptr2: NonNull<u32> = NonNull::from(&a[3]);
unsafe {
    assert_eq!(ptr2.offset_from_unsigned(ptr1), 2);
    assert_eq!(ptr1.add(2), ptr2);
    assert_eq!(ptr2.sub(2), ptr1);
    assert_eq!(ptr2.offset_from_unsigned(ptr2), 0);
}

// This would be incorrect, as the pointers are not correctly ordered:
// ptr1.offset_from_unsigned(ptr2)

const unsafe fn byte_offset_from_unsigned<U: ?Sized>(self, origin: NonNull<U>) -> usize

Calculates the distance between two pointers within the same allocation, where it's known that self is equal to or greater than origin. The returned value is in units of bytes.

This is purely a convenience for casting to a u8 pointer and using [offset_from_unsigned][NonNull::offset_from_unsigned] on it. See that method for documentation and safety requirements.

For non-Sized pointees this operation considers only the data pointers, ignoring the metadata.

const unsafe fn read(self) -> T
where
    T: Sized,

Reads the value from self without moving it. This leaves the memory in self unchanged.

See ptr::read for safety concerns and examples.

unsafe fn read_volatile(self) -> T
where
    T: Sized,

Performs a volatile read of the value from self without moving it. This leaves the memory in self unchanged.

Volatile operations are intended to act on I/O memory, and are guaranteed to not be elided or reordered by the compiler across other volatile operations.

See ptr::read_volatile for safety concerns and examples.

const unsafe fn read_unaligned(self) -> T
where
    T: Sized,

Reads the value from self without moving it. This leaves the memory in self unchanged.

Unlike read, the pointer may be unaligned.

See ptr::read_unaligned for safety concerns and examples.

const unsafe fn copy_to(self, dest: NonNull<T>, count: usize)
where
    T: Sized,

Copies count * size_of::<T>() bytes from self to dest. The source and destination may overlap.

NOTE: this has the same argument order as ptr::copy.

See ptr::copy for safety concerns and examples.

const unsafe fn copy_to_nonoverlapping(self, dest: NonNull<T>, count: usize)
where
    T: Sized,

Copies count * size_of::<T>() bytes from self to dest. The source and destination may not overlap.

NOTE: this has the same argument order as ptr::copy_nonoverlapping.

See ptr::copy_nonoverlapping for safety concerns and examples.

const unsafe fn copy_from(self, src: NonNull<T>, count: usize)
where
    T: Sized,

Copies count * size_of::<T>() bytes from src to self. The source and destination may overlap.

NOTE: this has the opposite argument order of ptr::copy.

See ptr::copy for safety concerns and examples.

const unsafe fn copy_from_nonoverlapping(self, src: NonNull<T>, count: usize)
where
    T: Sized,

Copies count * size_of::<T>() bytes from src to self. The source and destination may not overlap.

NOTE: this has the opposite argument order of ptr::copy_nonoverlapping.

See ptr::copy_nonoverlapping for safety concerns and examples.

const unsafe fn drop_in_place(self)
where
    T: ,

Executes the destructor (if any) of the pointed-to value.

See ptr::drop_in_place for safety concerns and examples.

const unsafe fn write(self, val: T)
where
    T: Sized,

Overwrites a memory location with the given value without reading or dropping the old value.

See ptr::write for safety concerns and examples.

const unsafe fn write_bytes(self, val: u8, count: usize)
where
    T: Sized,

Invokes memset on the specified pointer, setting count * size_of::<T>() bytes of memory starting at self to val.

See ptr::write_bytes for safety concerns and examples.

unsafe fn write_volatile(self, val: T)
where
    T: Sized,

Performs a volatile write of a memory location with the given value without reading or dropping the old value.

Volatile operations are intended to act on I/O memory, and are guaranteed to not be elided or reordered by the compiler across other volatile operations.

See ptr::write_volatile for safety concerns and examples.

const unsafe fn write_unaligned(self, val: T)
where
    T: Sized,

Overwrites a memory location with the given value without reading or dropping the old value.

Unlike write, the pointer may be unaligned.

See ptr::write_unaligned for safety concerns and examples.

const unsafe fn replace(self, src: T) -> T
where
    T: Sized,

Replaces the value at self with src, returning the old value, without dropping either.

See ptr::replace for safety concerns and examples.

const unsafe fn swap(self, with: NonNull<T>)
where
    T: Sized,

Swaps the values at two mutable locations of the same type, without deinitializing either. They may overlap, unlike mem::swap which is otherwise equivalent.

See ptr::swap for safety concerns and examples.

fn align_offset(self, align: usize) -> usize
where
    T: Sized,

Computes the offset that needs to be applied to the pointer in order to make it aligned to align.

If it is not possible to align the pointer, the implementation returns usize::MAX.

The offset is expressed in number of T elements, and not bytes.

There are no guarantees whatsoever that offsetting the pointer will not overflow or go beyond the allocation that the pointer points into. It is up to the caller to ensure that the returned offset is correct in all terms other than alignment.

When this is called during compile-time evaluation (which is unstable), the implementation may return usize::MAX in cases where that can never happen at runtime. This is because the actual alignment of pointers is not known yet during compile-time, so an offset with guaranteed alignment can sometimes not be computed. For example, a buffer declared as [u8; N] might be allocated at an odd or an even address, but at compile-time this is not yet known, so the execution has to be correct for either choice. It is therefore impossible to find an offset that is guaranteed to be 2-aligned. (This behavior is subject to change, as usual for unstable APIs.)

Panics

The function panics if align is not a power-of-two.

Examples

Accessing adjacent u8 as u16

use std::ptr::NonNull;

# unsafe {
let x = [5_u8, 6, 7, 8, 9];
let ptr = NonNull::new(x.as_ptr() as *mut u8).unwrap();
let offset = ptr.align_offset(align_of::<u16>());

if offset < x.len() - 1 {
    let u16_ptr = ptr.add(offset).cast::<u16>();
    assert!(u16_ptr.read() == u16::from_ne_bytes([5, 6]) || u16_ptr.read() == u16::from_ne_bytes([6, 7]));
} else {
    // while the pointer can be aligned via `offset`, it would point
    // outside the allocation
}
# }

fn is_aligned(self) -> bool
where
    T: Sized,

Returns whether the pointer is properly aligned for T.

Examples

use std::ptr::NonNull;

// On some platforms, the alignment of i32 is less than 4.
#[repr(align(4))]
struct AlignedI32(i32);

let data = AlignedI32(42);
let ptr = NonNull::<AlignedI32>::from(&data);

assert!(ptr.is_aligned());
assert!(!NonNull::new(ptr.as_ptr().wrapping_byte_add(1)).unwrap().is_aligned());

fn is_aligned_to(self, align: usize) -> bool

Returns whether the pointer is aligned to align.

For non-Sized pointees this operation considers only the data pointer, ignoring the metadata.

Panics

The function panics if align is not a power-of-two (this includes 0).

Examples

#![feature(pointer_is_aligned_to)]

// On some platforms, the alignment of i32 is less than 4.
#[repr(align(4))]
struct AlignedI32(i32);

let data = AlignedI32(42);
let ptr = &data as *const AlignedI32;

assert!(ptr.is_aligned_to(1));
assert!(ptr.is_aligned_to(2));
assert!(ptr.is_aligned_to(4));

assert!(ptr.wrapping_byte_add(2).is_aligned_to(2));
assert!(!ptr.wrapping_byte_add(2).is_aligned_to(4));

assert_ne!(ptr.is_aligned_to(8), ptr.wrapping_add(1).is_aligned_to(8));

impl<T: Sized> NonNull<T>

const fn without_provenance(addr: NonZero<usize>) -> Self

Creates a pointer with the given address and no [provenance][crate::ptr#provenance].

For more details, see the equivalent method on a raw pointer, ptr::without_provenance_mut.

This is a [Strict Provenance][crate::ptr#strict-provenance] API.

const fn dangling() -> Self

Creates a new NonNull that is dangling, but well-aligned.

This is useful for initializing types which lazily allocate, like Vec::new does.

Note that the address of the returned pointer may potentially be that of a valid pointer, which means this must not be used as a "not yet initialized" sentinel value. Types that lazily allocate must track initialization by some other means.

Examples

use std::ptr::NonNull;

let ptr = NonNull::<u32>::dangling();
// Important: don't try to access the value of `ptr` without
// initializing it first! The pointer is not null but isn't valid either!

const fn with_exposed_provenance(addr: NonZero<usize>) -> Self

Converts an address back to a mutable pointer, picking up some previously 'exposed' [provenance][crate::ptr#provenance].

For more details, see the equivalent method on a raw pointer, ptr::with_exposed_provenance_mut.

This is an [Exposed Provenance][crate::ptr#exposed-provenance] API.

const unsafe fn as_uninit_ref<'a>(self) -> &'a MaybeUninit<T>

Returns a shared references to the value. In contrast to as_ref, this does not require that the value has to be initialized.

For the mutable counterpart see as_uninit_mut.

Safety

When calling this method, you have to ensure that the pointer is convertible to a reference. Note that because the created reference is to MaybeUninit<T>, the source pointer can point to uninitialized memory.

const unsafe fn as_uninit_mut<'a>(self) -> &'a mut MaybeUninit<T>

Returns a unique references to the value. In contrast to as_mut, this does not require that the value has to be initialized.

For the shared counterpart see as_uninit_ref.

Safety

When calling this method, you have to ensure that the pointer is convertible to a reference. Note that because the created reference is to MaybeUninit<T>, the source pointer can point to uninitialized memory.

const fn cast_array<const N: usize>(self) -> NonNull<[T; N]>

Casts from a pointer-to-T to a pointer-to-[T; N].

Trait Implementations

impl<T, U: PointeeSized> CoerceUnsized<NonNull<U>> for NonNull<T> where T: Unsize<U> + PointeeSized,

impl<T, U: PointeeSized> DispatchFromDyn<NonNull<U>> for NonNull<T> where T: Unsize<U> + PointeeSized,

impl<T: PointeeSized> !Send for NonNull<T>

impl<T: PointeeSized> !Sync for NonNull<T>

impl<T: PointeeSized> Clone for NonNull<T>

fn clone(&self) -> Self

impl<T: PointeeSized> Copy for NonNull<T>

impl<T: PointeeSized> Debug for NonNull<T>

fn fmt(&self, f: &mut Formatter<'_>) -> Result

impl<T: PointeeSized> Eq for NonNull<T>

impl<T: PointeeSized> From<&T> for NonNull<T>

fn from(r: &T) -> Self

Converts a &T to a NonNull<T>.

This conversion is safe and infallible since references cannot be null.

impl<T: PointeeSized> From<&mut T> for NonNull<T>

fn from(r: &mut T) -> Self

Converts a &mut T to a NonNull<T>.

This conversion is safe and infallible since references cannot be null.

impl<T: PointeeSized> From<Unique<T>> for NonNull<T>

fn from(unique: Unique<T>) -> Self

impl<T: PointeeSized> Hash for NonNull<T>

fn hash<H: Hasher>(&self, state: &mut H)

impl<T: PointeeSized> Ord for NonNull<T>

fn cmp(&self, other: &Self) -> Ordering

impl<T: PointeeSized> PartialEq for NonNull<T>

fn eq(&self, other: &Self) -> bool

impl<T: PointeeSized> PartialOrd for NonNull<T>

fn partial_cmp(&self, other: &Self) -> Option<Ordering>

impl<T: PointeeSized> Pointer for NonNull<T>

fn fmt(&self, f: &mut Formatter<'_>) -> Result

impl<T: PointeeSized> TrivialClone for NonNull<T>

impl<T: RefUnwindSafe + PointeeSized> UnwindSafe for NonNull<T>

Auto Trait Implementations

impl<T> Freeze for NonNull<T> where *const T is !null: Freeze, T: ?Sized,

impl<T> RefUnwindSafe for NonNull<T> where *const T is !null: RefUnwindSafe, T: ?Sized,

impl<T> Unpin for NonNull<T> where *const T is !null: Unpin, T: ?Sized,

impl<T> UnsafeUnpin for NonNull<T> where *const T is !null: UnsafeUnpin, T: ?Sized,

Blanket Implementations

impl<T> Any for NonNull<T> where T: 'static + ?Sized,

fn type_id(&self) -> TypeId

impl<T> Borrow<T> for NonNull<T> where T: ?Sized,

fn borrow(&self) -> &T

impl<T> BorrowMut<T> for NonNull<T> where T: ?Sized,

fn borrow_mut(&mut self) -> &mut T

impl<T> CloneToUninit for NonNull<T> where T: Clone,

unsafe fn clone_to_uninit(&self, dest: *mut u8)

impl<T> From<T> for NonNull<T>

fn from(t: T) -> T

Returns the argument unchanged.

impl<T> Printable for NonNull<T> where T: Copy + Debug,

impl<T> SizeHint for NonNull<T> where T: ?Sized,

fn lower_bound(&self) -> usize
fn upper_bound(&self) -> Option<usize>

impl<T> SizedTypeProperties for NonNull<T>

impl<T, U> Into<U> for NonNull<T> where U: From<T>,

fn into(self) -> U

Calls U::from(self).

That is, this conversion is whatever the implementation of [From]<T> for U chooses to do.

impl<T, U> TryFrom<U> for NonNull<T> where U: Into<T>,

type Error = Infallible;
fn try_from(value: U) -> Result<T, <T as TryFrom<U>>::Error>

impl<T, U> TryInto<U> for NonNull<T> where U: TryFrom<T>,

type Error = <U as TryFrom<T>>::Error;
fn try_into(self) -> Result<U, <U as TryFrom<T>>::Error>