1.36.0[−][src]Union std::mem::MaybeUninit
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!Run
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!Run
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!Run
(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 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() };Run
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]);Run
Initializing an array element-by-element
MaybeUninit<T>
can be used to initialize a large array element-by-element:
use std::mem::{self, MaybeUninit}; use std::ptr; 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, so 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[..] { unsafe { ptr::write(elem.as_mut_ptr(), vec![42]); } } // Everything is initialized. Transmute the array to the // initialized type. unsafe { mem::transmute::<_, [Vec<u32>; 1000]>(data) } }; assert_eq!(&data[0], &[42]);Run
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] { unsafe { ptr::write(elem.as_mut_ptr(), 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()); } }Run
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 and alignment 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>());Run
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);Run
Methods
impl<T> MaybeUninit<T>
[src]
pub const fn new(val: T) -> MaybeUninit<T>
[src]
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>
[src]
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 fn zeroed() -> MaybeUninit<T>
[src]
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));Run
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.Run
ⓘImportant traits for &'_ mut Fpub fn write(&mut self, val: T) -> &mut T
[src]
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
[src]
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);Run
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.Run
(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
[src]
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);Run
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.Run
(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
[src]
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.
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);Run
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.Run
pub unsafe fn read(&self) -> T
[src]
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 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);Run
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!Run
ⓘImportant traits for &'_ mut Fpub unsafe fn get_ref(&self) -> &T
[src]
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 Fpub unsafe fn get_mut(&mut self) -> &mut T
[src]
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
[src]
Gets a pointer to the first element of the array.
pub fn first_ptr_mut(this: &mut [MaybeUninit<T>]) -> *mut T
[src]
Gets a mutable pointer to the first element of the array.
Trait Implementations
impl<T> Clone for MaybeUninit<T> where
T: Copy,
[src]
T: Copy,
fn clone(&self) -> MaybeUninit<T>
[src]
fn clone_from(&mut self, source: &Self)
1.0.0[src]
Performs copy-assignment from source
. Read more
impl<T> Copy for MaybeUninit<T> where
T: Copy,
[src]
T: Copy,
Auto Trait Implementations
impl<T> Send for MaybeUninit<T> where
T: Send,
T: Send,
impl<T> Sync for MaybeUninit<T> where
T: Sync,
T: Sync,
Blanket Implementations
impl<T, U> TryFrom<U> for T where
U: Into<T>,
[src]
U: Into<T>,
type Error = Infallible
The type returned in the event of a conversion error.
fn try_from(value: U) -> Result<T, <T as TryFrom<U>>::Error>
[src]
impl<T, U> Into<U> for T where
U: From<T>,
[src]
U: From<T>,
impl<T> From<T> for T
[src]
impl<T, U> TryInto<U> for T where
U: TryFrom<T>,
[src]
U: TryFrom<T>,
type Error = <U as TryFrom<T>>::Error
The type returned in the event of a conversion error.
fn try_into(self) -> Result<U, <U as TryFrom<T>>::Error>
[src]
impl<T> Borrow<T> for T where
T: ?Sized,
[src]
T: ?Sized,
impl<T> BorrowMut<T> for T where
T: ?Sized,
[src]
T: ?Sized,
ⓘImportant traits for &'_ mut Ffn borrow_mut(&mut self) -> &mut T
[src]
impl<T> Any for T where
T: 'static + ?Sized,
[src]
T: 'static + ?Sized,
impl<T> ToOwned for T where
T: Clone,
[src]
T: Clone,