Union nom::lib::std::mem::MaybeUninit

pub union MaybeUninit<T> {
    // some fields omitted
}

🔬 This is a nightly-only experimental API. (maybe_uninit)

A wrapper to construct uninitialized instances of T.

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:

#![feature(maybe_uninit)]
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().into_initialized() }; // 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:

#![feature(maybe_uninit)]
use std::mem::{self, MaybeUninit};

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

#![feature(maybe_uninit)]
use std::mem::{self, MaybeUninit};

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

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

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:

#![feature(maybe_uninit)]
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>::uninitialized();
// Set it to a valid value.
x.set(&0);
// Extract the initialized data -- this is only allowed *after* properly
// initializing `x`!
let x = unsafe { x.into_initialized() };

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

Methods

impl<T> MaybeUninit<T>

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

🔬 This is a nightly-only experimental API. (maybe_uninit)

Creates a new MaybeUninit<T> initialized with the given value.

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 uninitialized() -> MaybeUninit<T>

🔬 This is a nightly-only experimental API. (maybe_uninit)

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.

pub fn zeroed() -> MaybeUninit<T>

🔬 This is a nightly-only experimental API. (maybe_uninit)

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.

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

let x = MaybeUninit::<(u8, bool)>::zeroed();
let x = unsafe { x.into_initialized() };
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.

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

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

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

Important traits for &'_ mut I

impl<'_, I> Iterator for &'_ mut I where
    I: Iterator + ?Sized,  type Item = <I as Iterator>::Item;

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

🔬 This is a nightly-only experimental API. (maybe_uninit)

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

🔬 This is a nightly-only experimental API. (maybe_uninit)

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.

Examples

Correct usage of this method:

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

let mut x = MaybeUninit::<Vec<u32>>::uninitialized();
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:

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

let x = MaybeUninit::<Vec<u32>>::uninitialized();
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

🔬 This is a nightly-only experimental API. (maybe_uninit)

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:

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

let mut x = MaybeUninit::<Vec<u32>>::uninitialized();
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:

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

let mut x = MaybeUninit::<Vec<u32>>::uninitialized();
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 into_initialized(self) -> T

🔬 This is a nightly-only experimental API. (maybe_uninit)

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 undefined behavior.

Examples

Correct usage of this method:

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

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

Incorrect usage of this method:

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

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

pub unsafe fn read_initialized(&self) -> T

🔬 This is a nightly-only experimental API. (maybe_uninit)

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

Whenever possible, it is preferrable to use into_initialized 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.

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

Examples

Correct usage of this method:

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

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

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

Incorrect usage of this method:

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

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

Important traits for &'_ mut I

impl<'_, I> Iterator for &'_ mut I where
    I: Iterator + ?Sized,  type Item = <I as Iterator>::Item;

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

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

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 I

impl<'_, I> Iterator for &'_ mut I where
    I: Iterator + ?Sized,  type Item = <I as Iterator>::Item;

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

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

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)

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)

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

Trait Implementations

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

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.