Union std::mem::MaybeUninit

#[lang = "maybe_uninit"]
#[repr(transparent)]
pub union MaybeUninit<T> {
    // some fields omitted
}

A wrapper type to construct uninitialized instances of T.

Initialization invariant

The compiler, in general, assumes that variables are properly initialized at their respective type. For example, a variable of reference type must be aligned and non-NULL. This is an invariant that must always be upheld, even in unsafe code. As a consequence, zero-initializing a variable of reference type causes instantaneous undefined behavior, no matter whether that reference ever gets used to access memory:

use std::mem::{self, MaybeUninit};

let x: &i32 = unsafe { mem::zeroed() }; // undefined behavior!
// The equivalent code with `MaybeUninit<&i32>`:
let x: &i32 = unsafe { MaybeUninit::zeroed().assume_init() }; // undefined behavior!

This is exploited by the compiler for various optimizations, such as eliding run-time checks and optimizing enum layout.

Similarly, entirely uninitialized memory may have any content, while a bool must always be true or false. Hence, creating an uninitialized bool is undefined behavior:

use std::mem::{self, MaybeUninit};

let b: bool = unsafe { mem::uninitialized() }; // undefined behavior!
// The equivalent code with `MaybeUninit<bool>`:
let b: bool = unsafe { MaybeUninit::uninit().assume_init() }; // undefined behavior!

Moreover, uninitialized memory is special in that the compiler knows that it does not have a fixed value. This makes it undefined behavior to have uninitialized data in a variable even if that variable has an integer type, which otherwise can hold any fixed bit pattern:

use std::mem::{self, MaybeUninit};

let x: i32 = unsafe { mem::uninitialized() }; // undefined behavior!
// The equivalent code with `MaybeUninit<i32>`:
let x: i32 = unsafe { MaybeUninit::uninit().assume_init() }; // undefined behavior!

(Notice that the rules around uninitialized integers are not finalized yet, but until they are, it is advisable to avoid them.)

On top of that, remember that most types have additional invariants beyond merely being considered initialized at the type level. For example, a 1-initialized Vec<T> is considered initialized (under the current implementation; this does not constitute a stable guarantee) because the only requirement the compiler knows about it is that the data pointer must be non-null. Creating such a Vec<T> does not cause immediate undefined behavior, but will cause undefined behavior with most safe operations (including dropping it).

Examples

MaybeUninit<T> serves to enable unsafe code to deal with uninitialized data. It is a signal to the compiler indicating that the data here might not be initialized:

use std::mem::MaybeUninit;

// Create an explicitly uninitialized reference. The compiler knows that data inside
// a `MaybeUninit<T>` may be invalid, and hence this is not UB:
let mut x = MaybeUninit::<&i32>::uninit();
// Set it to a valid value.
unsafe { x.as_mut_ptr().write(&0); }
// Extract the initialized data -- this is only allowed *after* properly
// initializing `x`!
let x = unsafe { x.assume_init() };

The compiler then knows to not make any incorrect assumptions or optimizations on this code.

You can think of MaybeUninit<T> as being a bit like Option<T> but without any of the run-time tracking and without any of the safety checks.

out-pointers

You can use MaybeUninit<T> to implement "out-pointers": instead of returning data from a function, pass it a pointer to some (uninitialized) memory to put the result into. This can be useful when it is important for the caller to control how the memory the result is stored in gets allocated, and you want to avoid unnecessary moves.

use std::mem::MaybeUninit;

unsafe fn make_vec(out: *mut Vec<i32>) {
    // `write` does not drop the old contents, which is important.
    out.write(vec![1, 2, 3]);
}

let mut v = MaybeUninit::uninit();
unsafe { make_vec(v.as_mut_ptr()); }
// Now we know `v` is initialized! This also makes sure the vector gets
// properly dropped.
let v = unsafe { v.assume_init() };
assert_eq!(&v, &[1, 2, 3]);

Initializing an array element-by-element

MaybeUninit<T> can be used to initialize a large array element-by-element:

use std::mem::{self, MaybeUninit};

let data = {
    // Create an uninitialized array of `MaybeUninit`. The `assume_init` is
    // safe because the type we are claiming to have initialized here is a
    // bunch of `MaybeUninit`s, which do not require initialization.
    let mut data: [MaybeUninit<Vec<u32>>; 1000] = unsafe {
        MaybeUninit::uninit().assume_init()
    };

    // Dropping a `MaybeUninit` does nothing. Thus using raw pointer
    // assignment instead of `ptr::write` does not cause the old
    // uninitialized value to be dropped. Also if there is a panic during
    // this loop, we have a memory leak, but there is no memory safety
    // issue.
    for elem in &mut data[..] {
        *elem = MaybeUninit::new(vec![42]);
    }

    // Everything is initialized. Transmute the array to the
    // initialized type.
    unsafe { mem::transmute::<_, [Vec<u32>; 1000]>(data) }
};

assert_eq!(&data[0], &[42]);

You can also work with partially initialized arrays, which could be found in low-level datastructures.

use std::mem::MaybeUninit;
use std::ptr;

// Create an uninitialized array of `MaybeUninit`. The `assume_init` is
// safe because the type we are claiming to have initialized here is a
// bunch of `MaybeUninit`s, which do not require initialization.
let mut data: [MaybeUninit<String>; 1000] = unsafe { MaybeUninit::uninit().assume_init() };
// Count the number of elements we have assigned.
let mut data_len: usize = 0;

for elem in &mut data[0..500] {
    *elem = MaybeUninit::new(String::from("hello"));
    data_len += 1;
}

// For each item in the array, drop if we allocated it.
for elem in &mut data[0..data_len] {
    unsafe { ptr::drop_in_place(elem.as_mut_ptr()); }
}

Initializing a struct field-by-field

There is currently no supported way to create a raw pointer or reference to a field of a struct inside MaybeUninit<Struct>. That means it is not possible to create a struct by calling MaybeUninit::uninit::<Struct>() and then writing to its fields.

Layout

MaybeUninit<T> is guaranteed to have the same size, alignment, and ABI as T:

use std::mem::{MaybeUninit, size_of, align_of};
assert_eq!(size_of::<MaybeUninit<u64>>(), size_of::<u64>());
assert_eq!(align_of::<MaybeUninit<u64>>(), align_of::<u64>());

However remember that a type containing a MaybeUninit<T> is not necessarily the same layout; Rust does not in general guarantee that the fields of a Foo<T> have the same order as a Foo<U> even if T and U have the same size and alignment. Furthermore because any bit value is valid for a MaybeUninit<T> the compiler can't apply non-zero/niche-filling optimizations, potentially resulting in a larger size:

assert_eq!(size_of::<Option<bool>>(), 1);
assert_eq!(size_of::<Option<MaybeUninit<bool>>>(), 2);

If T is FFI-safe, then so is MaybeUninit<T>.

While MaybeUninit is #[repr(transparent)] (indicating it guarantees the same size, alignment, and ABI as T), this does not change any of the previous caveats. Option<T> and Option<MaybeUninit<T>> may still have different sizes, and types containing a field of type T may be laid out (and sized) differently than if that field were MaybeUninit<T>. MaybeUninit is a union type, and #[repr(transparent)] on unions is unstable (see the tracking issue). Over time, the exact guarantees of #[repr(transparent)] on unions may evolve, and MaybeUninit may or may not remain #[repr(transparent)]. That said, MaybeUninit<T> will always guarantee that it has the same size, alignment, and ABI as T; it's just that the way MaybeUninit implements that guarantee may evolve.

Methods

impl<T> MaybeUninit<T>

pub const fn new(val: T) -> MaybeUninit<T>

Creates a new MaybeUninit<T> initialized with the given value. It is safe to call assume_init on the return value of this function.

Note that dropping a MaybeUninit<T> will never call T's drop code. It is your responsibility to make sure T gets dropped if it got initialized.

pub const fn uninit() -> MaybeUninit<T>

Creates a new MaybeUninit<T> in an uninitialized state.

Note that dropping a MaybeUninit<T> will never call T's drop code. It is your responsibility to make sure T gets dropped if it got initialized.

See the type-level documentation for some examples.

pub const UNINIT: MaybeUninit<T>

🔬 This is a nightly-only experimental API. (internal_uninit_const #0)

hack to work around promotability

A promotable constant, equivalent to uninit().

pub fn zeroed() -> MaybeUninit<T>

Creates a new MaybeUninit<T> in an uninitialized state, with the memory being filled with 0 bytes. It depends on T whether that already makes for proper initialization. For example, MaybeUninit<usize>::zeroed() is initialized, but MaybeUninit<&'static i32>::zeroed() is not because references must not be null.

Note that dropping a MaybeUninit<T> will never call T's drop code. It is your responsibility to make sure T gets dropped if it got initialized.

Example

Correct usage of this function: initializing a struct with zero, where all fields of the struct can hold the bit-pattern 0 as a valid value.

use std::mem::MaybeUninit;

let x = MaybeUninit::<(u8, bool)>::zeroed();
let x = unsafe { x.assume_init() };
assert_eq!(x, (0, false));

Incorrect usage of this function: initializing a struct with zero, where some fields cannot hold 0 as a valid value.

use std::mem::MaybeUninit;

enum NotZero { One = 1, Two = 2 };

let x = MaybeUninit::<(u8, NotZero)>::zeroed();
let x = unsafe { x.assume_init() };
// Inside a pair, we create a `NotZero` that does not have a valid discriminant.
// This is undefined behavior.

Important traits for &'_ mut F

impl<'_, F> Future for &'_ mut F where
    F: Unpin + Future + ?Sized,  type Output = <F as Future>::Output;impl<'_, I> Iterator for &'_ mut I where
    I: Iterator + ?Sized,  type Item = <I as Iterator>::Item;impl<'_, R: Read + ?Sized> Read for &'_ mut Rimpl<'_, W: Write + ?Sized> Write for &'_ mut W

pub fn write(&mut self, val: T) -> &mut T

🔬 This is a nightly-only experimental API. (maybe_uninit_extra #63567)

Sets the value of the MaybeUninit<T>. This overwrites any previous value without dropping it, so be careful not to use this twice unless you want to skip running the destructor. For your convenience, this also returns a mutable reference to the (now safely initialized) contents of self.

pub fn as_ptr(&self) -> *const T

Gets a pointer to the contained value. Reading from this pointer or turning it into a reference is undefined behavior unless the MaybeUninit<T> is initialized. Writing to memory that this pointer (non-transitively) points to is undefined behavior (except inside an UnsafeCell<T>).

Examples

Correct usage of this method:

use std::mem::MaybeUninit;

let mut x = MaybeUninit::<Vec<u32>>::uninit();
unsafe { x.as_mut_ptr().write(vec![0,1,2]); }
// Create a reference into the `MaybeUninit<T>`. This is okay because we initialized it.
let x_vec = unsafe { &*x.as_ptr() };
assert_eq!(x_vec.len(), 3);

Incorrect usage of this method:

use std::mem::MaybeUninit;

let x = MaybeUninit::<Vec<u32>>::uninit();
let x_vec = unsafe { &*x.as_ptr() };
// We have created a reference to an uninitialized vector! This is undefined behavior.

(Notice that the rules around references to uninitialized data are not finalized yet, but until they are, it is advisable to avoid them.)

pub fn as_mut_ptr(&mut self) -> *mut T

Gets a mutable pointer to the contained value. Reading from this pointer or turning it into a reference is undefined behavior unless the MaybeUninit<T> is initialized.

Examples

Correct usage of this method:

use std::mem::MaybeUninit;

let mut x = MaybeUninit::<Vec<u32>>::uninit();
unsafe { x.as_mut_ptr().write(vec![0,1,2]); }
// Create a reference into the `MaybeUninit<Vec<u32>>`.
// This is okay because we initialized it.
let x_vec = unsafe { &mut *x.as_mut_ptr() };
x_vec.push(3);
assert_eq!(x_vec.len(), 4);

Incorrect usage of this method:

use std::mem::MaybeUninit;

let mut x = MaybeUninit::<Vec<u32>>::uninit();
let x_vec = unsafe { &mut *x.as_mut_ptr() };
// We have created a reference to an uninitialized vector! This is undefined behavior.

(Notice that the rules around references to uninitialized data are not finalized yet, but until they are, it is advisable to avoid them.)

pub unsafe fn assume_init(self) -> T

Extracts the value from the MaybeUninit<T> container. This is a great way to ensure that the data will get dropped, because the resulting T is subject to the usual drop handling.

Safety

It is up to the caller to guarantee that the MaybeUninit<T> really is in an initialized state. Calling this when the content is not yet fully initialized causes immediate undefined behavior. The type-level documentation contains more information about this initialization invariant.

On top of that, remember that most types have additional invariants beyond merely being considered initialized at the type level. For example, a 1-initialized [Vec<T>] is considered initialized (under the current implementation; this does not constitute a stable guarantee) because the only requirement the compiler knows about it is that the data pointer must be non-null. Creating such a Vec<T> does not cause immediate undefined behavior, but will cause undefined behavior with most safe operations (including dropping it).

Examples

Correct usage of this method:

use std::mem::MaybeUninit;

let mut x = MaybeUninit::<bool>::uninit();
unsafe { x.as_mut_ptr().write(true); }
let x_init = unsafe { x.assume_init() };
assert_eq!(x_init, true);

Incorrect usage of this method:

use std::mem::MaybeUninit;

let x = MaybeUninit::<Vec<u32>>::uninit();
let x_init = unsafe { x.assume_init() };
// `x` had not been initialized yet, so this last line caused undefined behavior.

pub unsafe fn read(&self) -> T

🔬 This is a nightly-only experimental API. (maybe_uninit_extra #63567)

Reads the value from the MaybeUninit<T> container. The resulting T is subject to the usual drop handling.

Whenever possible, it is preferable to use assume_init instead, which prevents duplicating the content of the MaybeUninit<T>.

Safety

It is up to the caller to guarantee that the MaybeUninit<T> really is in an initialized state. Calling this when the content is not yet fully initialized causes undefined behavior. The type-level documentation contains more information about this initialization invariant.

Moreover, this leaves a copy of the same data behind in the MaybeUninit<T>. When using multiple copies of the data (by calling read multiple times, or first calling read and then assume_init), it is your responsibility to ensure that that data may indeed be duplicated.

Examples

Correct usage of this method:

#![feature(maybe_uninit_extra)]
use std::mem::MaybeUninit;

let mut x = MaybeUninit::<u32>::uninit();
x.write(13);
let x1 = unsafe { x.read() };
// `u32` is `Copy`, so we may read multiple times.
let x2 = unsafe { x.read() };
assert_eq!(x1, x2);

let mut x = MaybeUninit::<Option<Vec<u32>>>::uninit();
x.write(None);
let x1 = unsafe { x.read() };
// Duplicating a `None` value is okay, so we may read multiple times.
let x2 = unsafe { x.read() };
assert_eq!(x1, x2);

Incorrect usage of this method:

#![feature(maybe_uninit_extra)]
use std::mem::MaybeUninit;

let mut x = MaybeUninit::<Option<Vec<u32>>>::uninit();
x.write(Some(vec![0,1,2]));
let x1 = unsafe { x.read() };
let x2 = unsafe { x.read() };
// We now created two copies of the same vector, leading to a double-free when
// they both get dropped!

Important traits for &'_ mut F

impl<'_, F> Future for &'_ mut F where
    F: Unpin + Future + ?Sized,  type Output = <F as Future>::Output;impl<'_, I> Iterator for &'_ mut I where
    I: Iterator + ?Sized,  type Item = <I as Iterator>::Item;impl<'_, R: Read + ?Sized> Read for &'_ mut Rimpl<'_, W: Write + ?Sized> Write for &'_ mut W

pub unsafe fn get_ref(&self) -> &T

🔬 This is a nightly-only experimental API. (maybe_uninit_ref #63568)

Gets a reference to the contained value.

Safety

It is up to the caller to guarantee that the MaybeUninit<T> really is in an initialized state. Calling this when the content is not yet fully initialized causes undefined behavior.

Important traits for &'_ mut F

impl<'_, F> Future for &'_ mut F where
    F: Unpin + Future + ?Sized,  type Output = <F as Future>::Output;impl<'_, I> Iterator for &'_ mut I where
    I: Iterator + ?Sized,  type Item = <I as Iterator>::Item;impl<'_, R: Read + ?Sized> Read for &'_ mut Rimpl<'_, W: Write + ?Sized> Write for &'_ mut W

pub unsafe fn get_mut(&mut self) -> &mut T

🔬 This is a nightly-only experimental API. (maybe_uninit_ref #63568)

Gets a mutable reference to the contained value.

Safety

It is up to the caller to guarantee that the MaybeUninit<T> really is in an initialized state. Calling this when the content is not yet fully initialized causes undefined behavior.

pub fn first_ptr(this: &[MaybeUninit<T>]) -> *const T

🔬 This is a nightly-only experimental API. (maybe_uninit_slice #63569)

Gets a pointer to the first element of the array.

pub fn first_ptr_mut(this: &mut [MaybeUninit<T>]) -> *mut T

🔬 This is a nightly-only experimental API. (maybe_uninit_slice #63569)

Gets a mutable pointer to the first element of the array.

Trait Implementations

impl<T> Clone for MaybeUninit<T> where
    T: Copy, 

fn clone(&self) -> MaybeUninit<T>

Returns a copy of the value.

fn clone_from(&mut self, source: &Self)

Performs copy-assignment from source.

impl<T> Copy for MaybeUninit<T> where
    T: Copy,