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// SPDX-License-Identifier: Apache-2.0 OR MIT
//! A pointer type for heap allocation.
//!
//! [`Box<T>`], casually referred to as a 'box', provides the simplest form of
//! heap allocation in Rust. Boxes provide ownership for this allocation, and
//! drop their contents when they go out of scope. Boxes also ensure that they
//! never allocate more than `isize::MAX` bytes.
//!
//! # Examples
//!
//! Move a value from the stack to the heap by creating a [`Box`]:
//!
//! ```
//! let val: u8 = 5;
//! let boxed: Box<u8> = Box::new(val);
//! ```
//!
//! Move a value from a [`Box`] back to the stack by [dereferencing]:
//!
//! ```
//! let boxed: Box<u8> = Box::new(5);
//! let val: u8 = *boxed;
//! ```
//!
//! Creating a recursive data structure:
//!
//! ```
//! #[derive(Debug)]
//! enum List<T> {
//! Cons(T, Box<List<T>>),
//! Nil,
//! }
//!
//! let list: List<i32> = List::Cons(1, Box::new(List::Cons(2, Box::new(List::Nil))));
//! println!("{list:?}");
//! ```
//!
//! This will print `Cons(1, Cons(2, Nil))`.
//!
//! Recursive structures must be boxed, because if the definition of `Cons`
//! looked like this:
//!
//! ```compile_fail,E0072
//! # enum List<T> {
//! Cons(T, List<T>),
//! # }
//! ```
//!
//! It wouldn't work. This is because the size of a `List` depends on how many
//! elements are in the list, and so we don't know how much memory to allocate
//! for a `Cons`. By introducing a [`Box<T>`], which has a defined size, we know how
//! big `Cons` needs to be.
//!
//! # Memory layout
//!
//! For non-zero-sized values, a [`Box`] will use the [`Global`] allocator for
//! its allocation. It is valid to convert both ways between a [`Box`] and a
//! raw pointer allocated with the [`Global`] allocator, given that the
//! [`Layout`] used with the allocator is correct for the type. More precisely,
//! a `value: *mut T` that has been allocated with the [`Global`] allocator
//! with `Layout::for_value(&*value)` may be converted into a box using
//! [`Box::<T>::from_raw(value)`]. Conversely, the memory backing a `value: *mut
//! T` obtained from [`Box::<T>::into_raw`] may be deallocated using the
//! [`Global`] allocator with [`Layout::for_value(&*value)`].
//!
//! For zero-sized values, the `Box` pointer still has to be [valid] for reads
//! and writes and sufficiently aligned. In particular, casting any aligned
//! non-zero integer literal to a raw pointer produces a valid pointer, but a
//! pointer pointing into previously allocated memory that since got freed is
//! not valid. The recommended way to build a Box to a ZST if `Box::new` cannot
//! be used is to use [`ptr::NonNull::dangling`].
//!
//! So long as `T: Sized`, a `Box<T>` is guaranteed to be represented
//! as a single pointer and is also ABI-compatible with C pointers
//! (i.e. the C type `T*`). This means that if you have extern "C"
//! Rust functions that will be called from C, you can define those
//! Rust functions using `Box<T>` types, and use `T*` as corresponding
//! type on the C side. As an example, consider this C header which
//! declares functions that create and destroy some kind of `Foo`
//! value:
//!
//! ```c
//! /* C header */
//!
//! /* Returns ownership to the caller */
//! struct Foo* foo_new(void);
//!
//! /* Takes ownership from the caller; no-op when invoked with null */
//! void foo_delete(struct Foo*);
//! ```
//!
//! These two functions might be implemented in Rust as follows. Here, the
//! `struct Foo*` type from C is translated to `Box<Foo>`, which captures
//! the ownership constraints. Note also that the nullable argument to
//! `foo_delete` is represented in Rust as `Option<Box<Foo>>`, since `Box<Foo>`
//! cannot be null.
//!
//! ```
//! #[repr(C)]
//! pub struct Foo;
//!
//! #[no_mangle]
//! pub extern "C" fn foo_new() -> Box<Foo> {
//! Box::new(Foo)
//! }
//!
//! #[no_mangle]
//! pub extern "C" fn foo_delete(_: Option<Box<Foo>>) {}
//! ```
//!
//! Even though `Box<T>` has the same representation and C ABI as a C pointer,
//! this does not mean that you can convert an arbitrary `T*` into a `Box<T>`
//! and expect things to work. `Box<T>` values will always be fully aligned,
//! non-null pointers. Moreover, the destructor for `Box<T>` will attempt to
//! free the value with the global allocator. In general, the best practice
//! is to only use `Box<T>` for pointers that originated from the global
//! allocator.
//!
//! **Important.** At least at present, you should avoid using
//! `Box<T>` types for functions that are defined in C but invoked
//! from Rust. In those cases, you should directly mirror the C types
//! as closely as possible. Using types like `Box<T>` where the C
//! definition is just using `T*` can lead to undefined behavior, as
//! described in [rust-lang/unsafe-code-guidelines#198][ucg#198].
//!
//! [ucg#198]: https://github.com/rust-lang/unsafe-code-guidelines/issues/198
//! [dereferencing]: core::ops::Deref
//! [`Box::<T>::from_raw(value)`]: Box::from_raw
//! [`Global`]: crate::alloc::Global
//! [`Layout`]: crate::alloc::Layout
//! [`Layout::for_value(&*value)`]: crate::alloc::Layout::for_value
//! [valid]: ptr#safety
#![stable(feature = "rust1", since = "1.0.0")]
use core::any::Any;
use core::async_iter::AsyncIterator;
use core::borrow;
use core::cmp::Ordering;
use core::convert::{From, TryFrom};
use core::fmt;
use core::future::Future;
use core::hash::{Hash, Hasher};
#[cfg(not(no_global_oom_handling))]
use core::iter::FromIterator;
use core::iter::{FusedIterator, Iterator};
use core::marker::{Destruct, Unpin, Unsize};
use core::mem;
use core::ops::{
CoerceUnsized, Deref, DerefMut, DispatchFromDyn, Generator, GeneratorState, Receiver,
};
use core::pin::Pin;
use core::ptr::{self, Unique};
use core::task::{Context, Poll};
#[cfg(not(no_global_oom_handling))]
use crate::alloc::{handle_alloc_error, WriteCloneIntoRaw};
use crate::alloc::{AllocError, Allocator, Global, Layout};
#[cfg(not(no_global_oom_handling))]
use crate::borrow::Cow;
use crate::raw_vec::RawVec;
#[cfg(not(no_global_oom_handling))]
use crate::str::from_boxed_utf8_unchecked;
#[cfg(not(no_global_oom_handling))]
use crate::vec::Vec;
#[cfg(not(no_thin))]
#[unstable(feature = "thin_box", issue = "92791")]
pub use thin::ThinBox;
#[cfg(not(no_thin))]
mod thin;
/// A pointer type for heap allocation.
///
/// See the [module-level documentation](../../std/boxed/index.html) for more.
#[lang = "owned_box"]
#[fundamental]
#[stable(feature = "rust1", since = "1.0.0")]
// The declaration of the `Box` struct must be kept in sync with the
// `alloc::alloc::box_free` function or ICEs will happen. See the comment
// on `box_free` for more details.
pub struct Box<
T: ?Sized,
#[unstable(feature = "allocator_api", issue = "32838")] A: Allocator = Global,
>(Unique<T>, A);
impl<T> Box<T> {
/// Allocates memory on the heap and then places `x` into it.
///
/// This doesn't actually allocate if `T` is zero-sized.
///
/// # Examples
///
/// ```
/// let five = Box::new(5);
/// ```
#[cfg(not(no_global_oom_handling))]
#[inline(always)]
#[stable(feature = "rust1", since = "1.0.0")]
#[must_use]
pub fn new(x: T) -> Self {
box x
}
/// Constructs a new box with uninitialized contents.
///
/// # Examples
///
/// ```
/// #![feature(new_uninit)]
///
/// let mut five = Box::<u32>::new_uninit();
///
/// let five = unsafe {
/// // Deferred initialization:
/// five.as_mut_ptr().write(5);
///
/// five.assume_init()
/// };
///
/// assert_eq!(*five, 5)
/// ```
#[cfg(not(no_global_oom_handling))]
#[unstable(feature = "new_uninit", issue = "63291")]
#[must_use]
#[inline]
pub fn new_uninit() -> Box<mem::MaybeUninit<T>> {
Self::new_uninit_in(Global)
}
/// Constructs a new `Box` with uninitialized contents, with the memory
/// being filled with `0` bytes.
///
/// See [`MaybeUninit::zeroed`][zeroed] for examples of correct and incorrect usage
/// of this method.
///
/// # Examples
///
/// ```
/// #![feature(new_uninit)]
///
/// let zero = Box::<u32>::new_zeroed();
/// let zero = unsafe { zero.assume_init() };
///
/// assert_eq!(*zero, 0)
/// ```
///
/// [zeroed]: mem::MaybeUninit::zeroed
#[cfg(not(no_global_oom_handling))]
#[inline]
#[unstable(feature = "new_uninit", issue = "63291")]
#[must_use]
pub fn new_zeroed() -> Box<mem::MaybeUninit<T>> {
Self::new_zeroed_in(Global)
}
/// Constructs a new `Pin<Box<T>>`. If `T` does not implement `Unpin`, then
/// `x` will be pinned in memory and unable to be moved.
#[cfg(not(no_global_oom_handling))]
#[stable(feature = "pin", since = "1.33.0")]
#[must_use]
#[inline(always)]
pub fn pin(x: T) -> Pin<Box<T>> {
(box x).into()
}
/// Allocates memory on the heap then places `x` into it,
/// returning an error if the allocation fails
///
/// This doesn't actually allocate if `T` is zero-sized.
///
/// # Examples
///
/// ```
/// #![feature(allocator_api)]
///
/// let five = Box::try_new(5)?;
/// # Ok::<(), std::alloc::AllocError>(())
/// ```
#[unstable(feature = "allocator_api", issue = "32838")]
#[inline]
pub fn try_new(x: T) -> Result<Self, AllocError> {
Self::try_new_in(x, Global)
}
/// Constructs a new box with uninitialized contents on the heap,
/// returning an error if the allocation fails
///
/// # Examples
///
/// ```
/// #![feature(allocator_api, new_uninit)]
///
/// let mut five = Box::<u32>::try_new_uninit()?;
///
/// let five = unsafe {
/// // Deferred initialization:
/// five.as_mut_ptr().write(5);
///
/// five.assume_init()
/// };
///
/// assert_eq!(*five, 5);
/// # Ok::<(), std::alloc::AllocError>(())
/// ```
#[unstable(feature = "allocator_api", issue = "32838")]
// #[unstable(feature = "new_uninit", issue = "63291")]
#[inline]
pub fn try_new_uninit() -> Result<Box<mem::MaybeUninit<T>>, AllocError> {
Box::try_new_uninit_in(Global)
}
/// Constructs a new `Box` with uninitialized contents, with the memory
/// being filled with `0` bytes on the heap
///
/// See [`MaybeUninit::zeroed`][zeroed] for examples of correct and incorrect usage
/// of this method.
///
/// # Examples
///
/// ```
/// #![feature(allocator_api, new_uninit)]
///
/// let zero = Box::<u32>::try_new_zeroed()?;
/// let zero = unsafe { zero.assume_init() };
///
/// assert_eq!(*zero, 0);
/// # Ok::<(), std::alloc::AllocError>(())
/// ```
///
/// [zeroed]: mem::MaybeUninit::zeroed
#[unstable(feature = "allocator_api", issue = "32838")]
// #[unstable(feature = "new_uninit", issue = "63291")]
#[inline]
pub fn try_new_zeroed() -> Result<Box<mem::MaybeUninit<T>>, AllocError> {
Box::try_new_zeroed_in(Global)
}
}
impl<T, A: Allocator> Box<T, A> {
/// Allocates memory in the given allocator then places `x` into it.
///
/// This doesn't actually allocate if `T` is zero-sized.
///
/// # Examples
///
/// ```
/// #![feature(allocator_api)]
///
/// use std::alloc::System;
///
/// let five = Box::new_in(5, System);
/// ```
#[cfg(not(no_global_oom_handling))]
#[unstable(feature = "allocator_api", issue = "32838")]
#[rustc_const_unstable(feature = "const_box", issue = "92521")]
#[must_use]
#[inline]
pub const fn new_in(x: T, alloc: A) -> Self
where
A: ~const Allocator + ~const Destruct,
{
let mut boxed = Self::new_uninit_in(alloc);
unsafe {
boxed.as_mut_ptr().write(x);
boxed.assume_init()
}
}
/// Allocates memory in the given allocator then places `x` into it,
/// returning an error if the allocation fails
///
/// This doesn't actually allocate if `T` is zero-sized.
///
/// # Examples
///
/// ```
/// #![feature(allocator_api)]
///
/// use std::alloc::System;
///
/// let five = Box::try_new_in(5, System)?;
/// # Ok::<(), std::alloc::AllocError>(())
/// ```
#[unstable(feature = "allocator_api", issue = "32838")]
#[rustc_const_unstable(feature = "const_box", issue = "92521")]
#[inline]
pub const fn try_new_in(x: T, alloc: A) -> Result<Self, AllocError>
where
T: ~const Destruct,
A: ~const Allocator + ~const Destruct,
{
let mut boxed = Self::try_new_uninit_in(alloc)?;
unsafe {
boxed.as_mut_ptr().write(x);
Ok(boxed.assume_init())
}
}
/// Constructs a new box with uninitialized contents in the provided allocator.
///
/// # Examples
///
/// ```
/// #![feature(allocator_api, new_uninit)]
///
/// use std::alloc::System;
///
/// let mut five = Box::<u32, _>::new_uninit_in(System);
///
/// let five = unsafe {
/// // Deferred initialization:
/// five.as_mut_ptr().write(5);
///
/// five.assume_init()
/// };
///
/// assert_eq!(*five, 5)
/// ```
#[unstable(feature = "allocator_api", issue = "32838")]
#[rustc_const_unstable(feature = "const_box", issue = "92521")]
#[cfg(not(no_global_oom_handling))]
#[must_use]
// #[unstable(feature = "new_uninit", issue = "63291")]
pub const fn new_uninit_in(alloc: A) -> Box<mem::MaybeUninit<T>, A>
where
A: ~const Allocator + ~const Destruct,
{
let layout = Layout::new::<mem::MaybeUninit<T>>();
// NOTE: Prefer match over unwrap_or_else since closure sometimes not inlineable.
// That would make code size bigger.
match Box::try_new_uninit_in(alloc) {
Ok(m) => m,
Err(_) => handle_alloc_error(layout),
}
}
/// Constructs a new box with uninitialized contents in the provided allocator,
/// returning an error if the allocation fails
///
/// # Examples
///
/// ```
/// #![feature(allocator_api, new_uninit)]
///
/// use std::alloc::System;
///
/// let mut five = Box::<u32, _>::try_new_uninit_in(System)?;
///
/// let five = unsafe {
/// // Deferred initialization:
/// five.as_mut_ptr().write(5);
///
/// five.assume_init()
/// };
///
/// assert_eq!(*five, 5);
/// # Ok::<(), std::alloc::AllocError>(())
/// ```
#[unstable(feature = "allocator_api", issue = "32838")]
// #[unstable(feature = "new_uninit", issue = "63291")]
#[rustc_const_unstable(feature = "const_box", issue = "92521")]
pub const fn try_new_uninit_in(alloc: A) -> Result<Box<mem::MaybeUninit<T>, A>, AllocError>
where
A: ~const Allocator + ~const Destruct,
{
let layout = Layout::new::<mem::MaybeUninit<T>>();
let ptr = alloc.allocate(layout)?.cast();
unsafe { Ok(Box::from_raw_in(ptr.as_ptr(), alloc)) }
}
/// Constructs a new `Box` with uninitialized contents, with the memory
/// being filled with `0` bytes in the provided allocator.
///
/// See [`MaybeUninit::zeroed`][zeroed] for examples of correct and incorrect usage
/// of this method.
///
/// # Examples
///
/// ```
/// #![feature(allocator_api, new_uninit)]
///
/// use std::alloc::System;
///
/// let zero = Box::<u32, _>::new_zeroed_in(System);
/// let zero = unsafe { zero.assume_init() };
///
/// assert_eq!(*zero, 0)
/// ```
///
/// [zeroed]: mem::MaybeUninit::zeroed
#[unstable(feature = "allocator_api", issue = "32838")]
#[rustc_const_unstable(feature = "const_box", issue = "92521")]
#[cfg(not(no_global_oom_handling))]
// #[unstable(feature = "new_uninit", issue = "63291")]
#[must_use]
pub const fn new_zeroed_in(alloc: A) -> Box<mem::MaybeUninit<T>, A>
where
A: ~const Allocator + ~const Destruct,
{
let layout = Layout::new::<mem::MaybeUninit<T>>();
// NOTE: Prefer match over unwrap_or_else since closure sometimes not inlineable.
// That would make code size bigger.
match Box::try_new_zeroed_in(alloc) {
Ok(m) => m,
Err(_) => handle_alloc_error(layout),
}
}
/// Constructs a new `Box` with uninitialized contents, with the memory
/// being filled with `0` bytes in the provided allocator,
/// returning an error if the allocation fails,
///
/// See [`MaybeUninit::zeroed`][zeroed] for examples of correct and incorrect usage
/// of this method.
///
/// # Examples
///
/// ```
/// #![feature(allocator_api, new_uninit)]
///
/// use std::alloc::System;
///
/// let zero = Box::<u32, _>::try_new_zeroed_in(System)?;
/// let zero = unsafe { zero.assume_init() };
///
/// assert_eq!(*zero, 0);
/// # Ok::<(), std::alloc::AllocError>(())
/// ```
///
/// [zeroed]: mem::MaybeUninit::zeroed
#[unstable(feature = "allocator_api", issue = "32838")]
// #[unstable(feature = "new_uninit", issue = "63291")]
#[rustc_const_unstable(feature = "const_box", issue = "92521")]
pub const fn try_new_zeroed_in(alloc: A) -> Result<Box<mem::MaybeUninit<T>, A>, AllocError>
where
A: ~const Allocator + ~const Destruct,
{
let layout = Layout::new::<mem::MaybeUninit<T>>();
let ptr = alloc.allocate_zeroed(layout)?.cast();
unsafe { Ok(Box::from_raw_in(ptr.as_ptr(), alloc)) }
}
/// Constructs a new `Pin<Box<T, A>>`. If `T` does not implement `Unpin`, then
/// `x` will be pinned in memory and unable to be moved.
#[cfg(not(no_global_oom_handling))]
#[unstable(feature = "allocator_api", issue = "32838")]
#[rustc_const_unstable(feature = "const_box", issue = "92521")]
#[must_use]
#[inline(always)]
pub const fn pin_in(x: T, alloc: A) -> Pin<Self>
where
A: 'static + ~const Allocator + ~const Destruct,
{
Self::into_pin(Self::new_in(x, alloc))
}
/// Converts a `Box<T>` into a `Box<[T]>`
///
/// This conversion does not allocate on the heap and happens in place.
#[unstable(feature = "box_into_boxed_slice", issue = "71582")]
#[rustc_const_unstable(feature = "const_box", issue = "92521")]
pub const fn into_boxed_slice(boxed: Self) -> Box<[T], A> {
let (raw, alloc) = Box::into_raw_with_allocator(boxed);
unsafe { Box::from_raw_in(raw as *mut [T; 1], alloc) }
}
/// Consumes the `Box`, returning the wrapped value.
///
/// # Examples
///
/// ```
/// #![feature(box_into_inner)]
///
/// let c = Box::new(5);
///
/// assert_eq!(Box::into_inner(c), 5);
/// ```
#[unstable(feature = "box_into_inner", issue = "80437")]
#[rustc_const_unstable(feature = "const_box", issue = "92521")]
#[inline]
pub const fn into_inner(boxed: Self) -> T
where
Self: ~const Destruct,
{
*boxed
}
}
impl<T> Box<[T]> {
/// Constructs a new boxed slice with uninitialized contents.
///
/// # Examples
///
/// ```
/// #![feature(new_uninit)]
///
/// let mut values = Box::<[u32]>::new_uninit_slice(3);
///
/// let values = unsafe {
/// // Deferred initialization:
/// values[0].as_mut_ptr().write(1);
/// values[1].as_mut_ptr().write(2);
/// values[2].as_mut_ptr().write(3);
///
/// values.assume_init()
/// };
///
/// assert_eq!(*values, [1, 2, 3])
/// ```
#[cfg(not(no_global_oom_handling))]
#[unstable(feature = "new_uninit", issue = "63291")]
#[must_use]
pub fn new_uninit_slice(len: usize) -> Box<[mem::MaybeUninit<T>]> {
unsafe { RawVec::with_capacity(len).into_box(len) }
}
/// Constructs a new boxed slice with uninitialized contents, with the memory
/// being filled with `0` bytes.
///
/// See [`MaybeUninit::zeroed`][zeroed] for examples of correct and incorrect usage
/// of this method.
///
/// # Examples
///
/// ```
/// #![feature(new_uninit)]
///
/// let values = Box::<[u32]>::new_zeroed_slice(3);
/// let values = unsafe { values.assume_init() };
///
/// assert_eq!(*values, [0, 0, 0])
/// ```
///
/// [zeroed]: mem::MaybeUninit::zeroed
#[cfg(not(no_global_oom_handling))]
#[unstable(feature = "new_uninit", issue = "63291")]
#[must_use]
pub fn new_zeroed_slice(len: usize) -> Box<[mem::MaybeUninit<T>]> {
unsafe { RawVec::with_capacity_zeroed(len).into_box(len) }
}
/// Constructs a new boxed slice with uninitialized contents. Returns an error if
/// the allocation fails
///
/// # Examples
///
/// ```
/// #![feature(allocator_api, new_uninit)]
///
/// let mut values = Box::<[u32]>::try_new_uninit_slice(3)?;
/// let values = unsafe {
/// // Deferred initialization:
/// values[0].as_mut_ptr().write(1);
/// values[1].as_mut_ptr().write(2);
/// values[2].as_mut_ptr().write(3);
/// values.assume_init()
/// };
///
/// assert_eq!(*values, [1, 2, 3]);
/// # Ok::<(), std::alloc::AllocError>(())
/// ```
#[unstable(feature = "allocator_api", issue = "32838")]
#[inline]
pub fn try_new_uninit_slice(len: usize) -> Result<Box<[mem::MaybeUninit<T>]>, AllocError> {
unsafe {
let layout = match Layout::array::<mem::MaybeUninit<T>>(len) {
Ok(l) => l,
Err(_) => return Err(AllocError),
};
let ptr = Global.allocate(layout)?;
Ok(RawVec::from_raw_parts_in(ptr.as_mut_ptr() as *mut _, len, Global).into_box(len))
}
}
/// Constructs a new boxed slice with uninitialized contents, with the memory
/// being filled with `0` bytes. Returns an error if the allocation fails
///
/// See [`MaybeUninit::zeroed`][zeroed] for examples of correct and incorrect usage
/// of this method.
///
/// # Examples
///
/// ```
/// #![feature(allocator_api, new_uninit)]
///
/// let values = Box::<[u32]>::try_new_zeroed_slice(3)?;
/// let values = unsafe { values.assume_init() };
///
/// assert_eq!(*values, [0, 0, 0]);
/// # Ok::<(), std::alloc::AllocError>(())
/// ```
///
/// [zeroed]: mem::MaybeUninit::zeroed
#[unstable(feature = "allocator_api", issue = "32838")]
#[inline]
pub fn try_new_zeroed_slice(len: usize) -> Result<Box<[mem::MaybeUninit<T>]>, AllocError> {
unsafe {
let layout = match Layout::array::<mem::MaybeUninit<T>>(len) {
Ok(l) => l,
Err(_) => return Err(AllocError),
};
let ptr = Global.allocate_zeroed(layout)?;
Ok(RawVec::from_raw_parts_in(ptr.as_mut_ptr() as *mut _, len, Global).into_box(len))
}
}
}
impl<T, A: Allocator> Box<[T], A> {
/// Constructs a new boxed slice with uninitialized contents in the provided allocator.
///
/// # Examples
///
/// ```
/// #![feature(allocator_api, new_uninit)]
///
/// use std::alloc::System;
///
/// let mut values = Box::<[u32], _>::new_uninit_slice_in(3, System);
///
/// let values = unsafe {
/// // Deferred initialization:
/// values[0].as_mut_ptr().write(1);
/// values[1].as_mut_ptr().write(2);
/// values[2].as_mut_ptr().write(3);
///
/// values.assume_init()
/// };
///
/// assert_eq!(*values, [1, 2, 3])
/// ```
#[cfg(not(no_global_oom_handling))]
#[unstable(feature = "allocator_api", issue = "32838")]
// #[unstable(feature = "new_uninit", issue = "63291")]
#[must_use]
pub fn new_uninit_slice_in(len: usize, alloc: A) -> Box<[mem::MaybeUninit<T>], A> {
unsafe { RawVec::with_capacity_in(len, alloc).into_box(len) }
}
/// Constructs a new boxed slice with uninitialized contents in the provided allocator,
/// with the memory being filled with `0` bytes.
///
/// See [`MaybeUninit::zeroed`][zeroed] for examples of correct and incorrect usage
/// of this method.
///
/// # Examples
///
/// ```
/// #![feature(allocator_api, new_uninit)]
///
/// use std::alloc::System;
///
/// let values = Box::<[u32], _>::new_zeroed_slice_in(3, System);
/// let values = unsafe { values.assume_init() };
///
/// assert_eq!(*values, [0, 0, 0])
/// ```
///
/// [zeroed]: mem::MaybeUninit::zeroed
#[cfg(not(no_global_oom_handling))]
#[unstable(feature = "allocator_api", issue = "32838")]
// #[unstable(feature = "new_uninit", issue = "63291")]
#[must_use]
pub fn new_zeroed_slice_in(len: usize, alloc: A) -> Box<[mem::MaybeUninit<T>], A> {
unsafe { RawVec::with_capacity_zeroed_in(len, alloc).into_box(len) }
}
}
impl<T, A: Allocator> Box<mem::MaybeUninit<T>, A> {
/// Converts to `Box<T, A>`.
///
/// # Safety
///
/// As with [`MaybeUninit::assume_init`],
/// it is up to the caller to guarantee that the value
/// really is in an initialized state.
/// Calling this when the content is not yet fully initialized
/// causes immediate undefined behavior.
///
/// [`MaybeUninit::assume_init`]: mem::MaybeUninit::assume_init
///
/// # Examples
///
/// ```
/// #![feature(new_uninit)]
///
/// let mut five = Box::<u32>::new_uninit();
///
/// let five: Box<u32> = unsafe {
/// // Deferred initialization:
/// five.as_mut_ptr().write(5);
///
/// five.assume_init()
/// };
///
/// assert_eq!(*five, 5)
/// ```
#[unstable(feature = "new_uninit", issue = "63291")]
#[rustc_const_unstable(feature = "const_box", issue = "92521")]
#[inline]
pub const unsafe fn assume_init(self) -> Box<T, A> {
let (raw, alloc) = Box::into_raw_with_allocator(self);
unsafe { Box::from_raw_in(raw as *mut T, alloc) }
}
/// Writes the value and converts to `Box<T, A>`.
///
/// This method converts the box similarly to [`Box::assume_init`] but
/// writes `value` into it before conversion thus guaranteeing safety.
/// In some scenarios use of this method may improve performance because
/// the compiler may be able to optimize copying from stack.
///
/// # Examples
///
/// ```
/// #![feature(new_uninit)]
///
/// let big_box = Box::<[usize; 1024]>::new_uninit();
///
/// let mut array = [0; 1024];
/// for (i, place) in array.iter_mut().enumerate() {
/// *place = i;
/// }
///
/// // The optimizer may be able to elide this copy, so previous code writes
/// // to heap directly.
/// let big_box = Box::write(big_box, array);
///
/// for (i, x) in big_box.iter().enumerate() {
/// assert_eq!(*x, i);
/// }
/// ```
#[unstable(feature = "new_uninit", issue = "63291")]
#[rustc_const_unstable(feature = "const_box", issue = "92521")]
#[inline]
pub const fn write(mut boxed: Self, value: T) -> Box<T, A> {
unsafe {
(*boxed).write(value);
boxed.assume_init()
}
}
}
impl<T, A: Allocator> Box<[mem::MaybeUninit<T>], A> {
/// Converts to `Box<[T], A>`.
///
/// # Safety
///
/// As with [`MaybeUninit::assume_init`],
/// it is up to the caller to guarantee that the values
/// really are in an initialized state.
/// Calling this when the content is not yet fully initialized
/// causes immediate undefined behavior.
///
/// [`MaybeUninit::assume_init`]: mem::MaybeUninit::assume_init
///
/// # Examples
///
/// ```
/// #![feature(new_uninit)]
///
/// let mut values = Box::<[u32]>::new_uninit_slice(3);
///
/// let values = unsafe {
/// // Deferred initialization:
/// values[0].as_mut_ptr().write(1);
/// values[1].as_mut_ptr().write(2);
/// values[2].as_mut_ptr().write(3);
///
/// values.assume_init()
/// };
///
/// assert_eq!(*values, [1, 2, 3])
/// ```
#[unstable(feature = "new_uninit", issue = "63291")]
#[inline]
pub unsafe fn assume_init(self) -> Box<[T], A> {
let (raw, alloc) = Box::into_raw_with_allocator(self);
unsafe { Box::from_raw_in(raw as *mut [T], alloc) }
}
}
impl<T: ?Sized> Box<T> {
/// Constructs a box from a raw pointer.
///
/// After calling this function, the raw pointer is owned by the
/// resulting `Box`. Specifically, the `Box` destructor will call
/// the destructor of `T` and free the allocated memory. For this
/// to be safe, the memory must have been allocated in accordance
/// with the [memory layout] used by `Box` .
///
/// # Safety
///
/// This function is unsafe because improper use may lead to
/// memory problems. For example, a double-free may occur if the
/// function is called twice on the same raw pointer.
///
/// The safety conditions are described in the [memory layout] section.
///
/// # Examples
///
/// Recreate a `Box` which was previously converted to a raw pointer
/// using [`Box::into_raw`]:
/// ```
/// let x = Box::new(5);
/// let ptr = Box::into_raw(x);
/// let x = unsafe { Box::from_raw(ptr) };
/// ```
/// Manually create a `Box` from scratch by using the global allocator:
/// ```
/// use std::alloc::{alloc, Layout};
///
/// unsafe {
/// let ptr = alloc(Layout::new::<i32>()) as *mut i32;
/// // In general .write is required to avoid attempting to destruct
/// // the (uninitialized) previous contents of `ptr`, though for this
/// // simple example `*ptr = 5` would have worked as well.
/// ptr.write(5);
/// let x = Box::from_raw(ptr);
/// }
/// ```
///
/// [memory layout]: self#memory-layout
/// [`Layout`]: crate::Layout
#[stable(feature = "box_raw", since = "1.4.0")]
#[inline]
pub unsafe fn from_raw(raw: *mut T) -> Self {
unsafe { Self::from_raw_in(raw, Global) }
}
}
impl<T: ?Sized, A: Allocator> Box<T, A> {
/// Constructs a box from a raw pointer in the given allocator.
///
/// After calling this function, the raw pointer is owned by the
/// resulting `Box`. Specifically, the `Box` destructor will call
/// the destructor of `T` and free the allocated memory. For this
/// to be safe, the memory must have been allocated in accordance
/// with the [memory layout] used by `Box` .
///
/// # Safety
///
/// This function is unsafe because improper use may lead to
/// memory problems. For example, a double-free may occur if the
/// function is called twice on the same raw pointer.
///
///
/// # Examples
///
/// Recreate a `Box` which was previously converted to a raw pointer
/// using [`Box::into_raw_with_allocator`]:
/// ```
/// #![feature(allocator_api)]
///
/// use std::alloc::System;
///
/// let x = Box::new_in(5, System);
/// let (ptr, alloc) = Box::into_raw_with_allocator(x);
/// let x = unsafe { Box::from_raw_in(ptr, alloc) };
/// ```
/// Manually create a `Box` from scratch by using the system allocator:
/// ```
/// #![feature(allocator_api, slice_ptr_get)]
///
/// use std::alloc::{Allocator, Layout, System};
///
/// unsafe {
/// let ptr = System.allocate(Layout::new::<i32>())?.as_mut_ptr() as *mut i32;
/// // In general .write is required to avoid attempting to destruct
/// // the (uninitialized) previous contents of `ptr`, though for this
/// // simple example `*ptr = 5` would have worked as well.
/// ptr.write(5);
/// let x = Box::from_raw_in(ptr, System);
/// }
/// # Ok::<(), std::alloc::AllocError>(())
/// ```
///
/// [memory layout]: self#memory-layout
/// [`Layout`]: crate::Layout
#[unstable(feature = "allocator_api", issue = "32838")]
#[rustc_const_unstable(feature = "const_box", issue = "92521")]
#[inline]
pub const unsafe fn from_raw_in(raw: *mut T, alloc: A) -> Self {
Box(unsafe { Unique::new_unchecked(raw) }, alloc)
}
/// Consumes the `Box`, returning a wrapped raw pointer.
///
/// The pointer will be properly aligned and non-null.
///
/// After calling this function, the caller is responsible for the
/// memory previously managed by the `Box`. In particular, the
/// caller should properly destroy `T` and release the memory, taking
/// into account the [memory layout] used by `Box`. The easiest way to
/// do this is to convert the raw pointer back into a `Box` with the
/// [`Box::from_raw`] function, allowing the `Box` destructor to perform
/// the cleanup.
///
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