Rust MaybeUninit用法及代碼示例

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! ⚠️

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! ⚠️

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! ⚠️

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.
x.write(&0);
// Extract the initialized data -- this is only allowed *after* properly
// initializing `x`!
let x = unsafe { x.assume_init() };

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]);

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.write(vec![42]);
    }

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

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

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.write(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()); }
}

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

#[derive(Debug, PartialEq)]
pub struct Foo {
    name: String,
    list: Vec<u8>,
}

let foo = {
    let mut uninit: MaybeUninit<Foo> = MaybeUninit::uninit();
    let ptr = uninit.as_mut_ptr();

    // Initializing the `name` field
    // Using `write` instead of assignment via `=` to not call `drop` on the
    // old, uninitialized value.
    unsafe { addr_of_mut!((*ptr).name).write("Bob".to_string()); }

    // Initializing the `list` field
    // If there is a panic here, then the `String` in the `name` field leaks.
    unsafe { addr_of_mut!((*ptr).list).write(vec![0, 1, 2]); }

    // All the fields are initialized, so we call `assume_init` to get an initialized Foo.
    unsafe { uninit.assume_init() }
};

assert_eq!(
    foo,
    Foo {
        name: "Bob".to_string(),
        list: vec![0, 1, 2]
    }
);

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>());

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