1 // SPDX-License-Identifier: Apache-2.0 OR MIT
3 //! The `Box<T>` type for heap allocation.
5 //! [`Box<T>`], casually referred to as a 'box', provides the simplest form of
6 //! heap allocation in Rust. Boxes provide ownership for this allocation, and
7 //! drop their contents when they go out of scope. Boxes also ensure that they
8 //! never allocate more than `isize::MAX` bytes.
12 //! Move a value from the stack to the heap by creating a [`Box`]:
16 //! let boxed: Box<u8> = Box::new(val);
19 //! Move a value from a [`Box`] back to the stack by [dereferencing]:
22 //! let boxed: Box<u8> = Box::new(5);
23 //! let val: u8 = *boxed;
26 //! Creating a recursive data structure:
31 //! Cons(T, Box<List<T>>),
35 //! let list: List<i32> = List::Cons(1, Box::new(List::Cons(2, Box::new(List::Nil))));
36 //! println!("{list:?}");
39 //! This will print `Cons(1, Cons(2, Nil))`.
41 //! Recursive structures must be boxed, because if the definition of `Cons`
44 //! ```compile_fail,E0072
50 //! It wouldn't work. This is because the size of a `List` depends on how many
51 //! elements are in the list, and so we don't know how much memory to allocate
52 //! for a `Cons`. By introducing a [`Box<T>`], which has a defined size, we know how
53 //! big `Cons` needs to be.
57 //! For non-zero-sized values, a [`Box`] will use the [`Global`] allocator for
58 //! its allocation. It is valid to convert both ways between a [`Box`] and a
59 //! raw pointer allocated with the [`Global`] allocator, given that the
60 //! [`Layout`] used with the allocator is correct for the type. More precisely,
61 //! a `value: *mut T` that has been allocated with the [`Global`] allocator
62 //! with `Layout::for_value(&*value)` may be converted into a box using
63 //! [`Box::<T>::from_raw(value)`]. Conversely, the memory backing a `value: *mut
64 //! T` obtained from [`Box::<T>::into_raw`] may be deallocated using the
65 //! [`Global`] allocator with [`Layout::for_value(&*value)`].
67 //! For zero-sized values, the `Box` pointer still has to be [valid] for reads
68 //! and writes and sufficiently aligned. In particular, casting any aligned
69 //! non-zero integer literal to a raw pointer produces a valid pointer, but a
70 //! pointer pointing into previously allocated memory that since got freed is
71 //! not valid. The recommended way to build a Box to a ZST if `Box::new` cannot
72 //! be used is to use [`ptr::NonNull::dangling`].
74 //! So long as `T: Sized`, a `Box<T>` is guaranteed to be represented
75 //! as a single pointer and is also ABI-compatible with C pointers
76 //! (i.e. the C type `T*`). This means that if you have extern "C"
77 //! Rust functions that will be called from C, you can define those
78 //! Rust functions using `Box<T>` types, and use `T*` as corresponding
79 //! type on the C side. As an example, consider this C header which
80 //! declares functions that create and destroy some kind of `Foo`
86 //! /* Returns ownership to the caller */
87 //! struct Foo* foo_new(void);
89 //! /* Takes ownership from the caller; no-op when invoked with null */
90 //! void foo_delete(struct Foo*);
93 //! These two functions might be implemented in Rust as follows. Here, the
94 //! `struct Foo*` type from C is translated to `Box<Foo>`, which captures
95 //! the ownership constraints. Note also that the nullable argument to
96 //! `foo_delete` is represented in Rust as `Option<Box<Foo>>`, since `Box<Foo>`
104 //! pub extern "C" fn foo_new() -> Box<Foo> {
109 //! pub extern "C" fn foo_delete(_: Option<Box<Foo>>) {}
112 //! Even though `Box<T>` has the same representation and C ABI as a C pointer,
113 //! this does not mean that you can convert an arbitrary `T*` into a `Box<T>`
114 //! and expect things to work. `Box<T>` values will always be fully aligned,
115 //! non-null pointers. Moreover, the destructor for `Box<T>` will attempt to
116 //! free the value with the global allocator. In general, the best practice
117 //! is to only use `Box<T>` for pointers that originated from the global
120 //! **Important.** At least at present, you should avoid using
121 //! `Box<T>` types for functions that are defined in C but invoked
122 //! from Rust. In those cases, you should directly mirror the C types
123 //! as closely as possible. Using types like `Box<T>` where the C
124 //! definition is just using `T*` can lead to undefined behavior, as
125 //! described in [rust-lang/unsafe-code-guidelines#198][ucg#198].
127 //! # Considerations for unsafe code
129 //! **Warning: This section is not normative and is subject to change, possibly
130 //! being relaxed in the future! It is a simplified summary of the rules
131 //! currently implemented in the compiler.**
133 //! The aliasing rules for `Box<T>` are the same as for `&mut T`. `Box<T>`
134 //! asserts uniqueness over its content. Using raw pointers derived from a box
135 //! after that box has been mutated through, moved or borrowed as `&mut T`
136 //! is not allowed. For more guidance on working with box from unsafe code, see
137 //! [rust-lang/unsafe-code-guidelines#326][ucg#326].
140 //! [ucg#198]: https://github.com/rust-lang/unsafe-code-guidelines/issues/198
141 //! [ucg#326]: https://github.com/rust-lang/unsafe-code-guidelines/issues/326
142 //! [dereferencing]: core::ops::Deref
143 //! [`Box::<T>::from_raw(value)`]: Box::from_raw
144 //! [`Global`]: crate::alloc::Global
145 //! [`Layout`]: crate::alloc::Layout
146 //! [`Layout::for_value(&*value)`]: crate::alloc::Layout::for_value
147 //! [valid]: ptr#safety
149 #![stable(feature = "rust1", since = "1.0.0")]
152 use core::async_iter::AsyncIterator;
154 use core::cmp::Ordering;
155 use core::error::Error;
157 use core::future::Future;
158 use core::hash::{Hash, Hasher};
159 use core::iter::FusedIterator;
160 use core::marker::Tuple;
161 use core::marker::Unsize;
164 CoerceUnsized, Deref, DerefMut, DispatchFromDyn, Generator, GeneratorState, Receiver,
167 use core::ptr::{self, Unique};
168 use core::task::{Context, Poll};
170 #[cfg(not(no_global_oom_handling))]
171 use crate::alloc::{handle_alloc_error, WriteCloneIntoRaw};
172 use crate::alloc::{AllocError, Allocator, Global, Layout};
173 #[cfg(not(no_global_oom_handling))]
174 use crate::borrow::Cow;
175 use crate::raw_vec::RawVec;
176 #[cfg(not(no_global_oom_handling))]
177 use crate::str::from_boxed_utf8_unchecked;
178 #[cfg(not(no_global_oom_handling))]
179 use crate::string::String;
180 #[cfg(not(no_global_oom_handling))]
184 #[unstable(feature = "thin_box", issue = "92791")]
185 pub use thin::ThinBox;
190 /// A pointer type that uniquely owns a heap allocation of type `T`.
192 /// See the [module-level documentation](../../std/boxed/index.html) for more.
193 #[lang = "owned_box"]
195 #[stable(feature = "rust1", since = "1.0.0")]
196 // The declaration of the `Box` struct must be kept in sync with the
197 // `alloc::alloc::box_free` function or ICEs will happen. See the comment
198 // on `box_free` for more details.
201 #[unstable(feature = "allocator_api", issue = "32838")] A: Allocator = Global,
205 /// Allocates memory on the heap and then places `x` into it.
207 /// This doesn't actually allocate if `T` is zero-sized.
212 /// let five = Box::new(5);
214 #[cfg(all(not(no_global_oom_handling)))]
216 #[stable(feature = "rust1", since = "1.0.0")]
218 #[rustc_diagnostic_item = "box_new"]
219 pub fn new(x: T) -> Self {
224 /// Constructs a new box with uninitialized contents.
229 /// #![feature(new_uninit)]
231 /// let mut five = Box::<u32>::new_uninit();
233 /// let five = unsafe {
234 /// // Deferred initialization:
235 /// five.as_mut_ptr().write(5);
237 /// five.assume_init()
240 /// assert_eq!(*five, 5)
242 #[cfg(not(no_global_oom_handling))]
243 #[unstable(feature = "new_uninit", issue = "63291")]
246 pub fn new_uninit() -> Box<mem::MaybeUninit<T>> {
247 Self::new_uninit_in(Global)
250 /// Constructs a new `Box` with uninitialized contents, with the memory
251 /// being filled with `0` bytes.
253 /// See [`MaybeUninit::zeroed`][zeroed] for examples of correct and incorrect usage
259 /// #![feature(new_uninit)]
261 /// let zero = Box::<u32>::new_zeroed();
262 /// let zero = unsafe { zero.assume_init() };
264 /// assert_eq!(*zero, 0)
267 /// [zeroed]: mem::MaybeUninit::zeroed
268 #[cfg(not(no_global_oom_handling))]
270 #[unstable(feature = "new_uninit", issue = "63291")]
272 pub fn new_zeroed() -> Box<mem::MaybeUninit<T>> {
273 Self::new_zeroed_in(Global)
276 /// Constructs a new `Pin<Box<T>>`. If `T` does not implement [`Unpin`], then
277 /// `x` will be pinned in memory and unable to be moved.
279 /// Constructing and pinning of the `Box` can also be done in two steps: `Box::pin(x)`
280 /// does the same as <code>[Box::into_pin]\([Box::new]\(x))</code>. Consider using
281 /// [`into_pin`](Box::into_pin) if you already have a `Box<T>`, or if you want to
282 /// construct a (pinned) `Box` in a different way than with [`Box::new`].
283 #[cfg(not(no_global_oom_handling))]
284 #[stable(feature = "pin", since = "1.33.0")]
287 pub fn pin(x: T) -> Pin<Box<T>> {
291 /// Allocates memory on the heap then places `x` into it,
292 /// returning an error if the allocation fails
294 /// This doesn't actually allocate if `T` is zero-sized.
299 /// #![feature(allocator_api)]
301 /// let five = Box::try_new(5)?;
302 /// # Ok::<(), std::alloc::AllocError>(())
304 #[unstable(feature = "allocator_api", issue = "32838")]
306 pub fn try_new(x: T) -> Result<Self, AllocError> {
307 Self::try_new_in(x, Global)
310 /// Constructs a new box with uninitialized contents on the heap,
311 /// returning an error if the allocation fails
316 /// #![feature(allocator_api, new_uninit)]
318 /// let mut five = Box::<u32>::try_new_uninit()?;
320 /// let five = unsafe {
321 /// // Deferred initialization:
322 /// five.as_mut_ptr().write(5);
324 /// five.assume_init()
327 /// assert_eq!(*five, 5);
328 /// # Ok::<(), std::alloc::AllocError>(())
330 #[unstable(feature = "allocator_api", issue = "32838")]
331 // #[unstable(feature = "new_uninit", issue = "63291")]
333 pub fn try_new_uninit() -> Result<Box<mem::MaybeUninit<T>>, AllocError> {
334 Box::try_new_uninit_in(Global)
337 /// Constructs a new `Box` with uninitialized contents, with the memory
338 /// being filled with `0` bytes on the heap
340 /// See [`MaybeUninit::zeroed`][zeroed] for examples of correct and incorrect usage
346 /// #![feature(allocator_api, new_uninit)]
348 /// let zero = Box::<u32>::try_new_zeroed()?;
349 /// let zero = unsafe { zero.assume_init() };
351 /// assert_eq!(*zero, 0);
352 /// # Ok::<(), std::alloc::AllocError>(())
355 /// [zeroed]: mem::MaybeUninit::zeroed
356 #[unstable(feature = "allocator_api", issue = "32838")]
357 // #[unstable(feature = "new_uninit", issue = "63291")]
359 pub fn try_new_zeroed() -> Result<Box<mem::MaybeUninit<T>>, AllocError> {
360 Box::try_new_zeroed_in(Global)
364 impl<T, A: Allocator> Box<T, A> {
365 /// Allocates memory in the given allocator then places `x` into it.
367 /// This doesn't actually allocate if `T` is zero-sized.
372 /// #![feature(allocator_api)]
374 /// use std::alloc::System;
376 /// let five = Box::new_in(5, System);
378 #[cfg(not(no_global_oom_handling))]
379 #[unstable(feature = "allocator_api", issue = "32838")]
382 pub fn new_in(x: T, alloc: A) -> Self
386 let mut boxed = Self::new_uninit_in(alloc);
388 boxed.as_mut_ptr().write(x);
393 /// Allocates memory in the given allocator then places `x` into it,
394 /// returning an error if the allocation fails
396 /// This doesn't actually allocate if `T` is zero-sized.
401 /// #![feature(allocator_api)]
403 /// use std::alloc::System;
405 /// let five = Box::try_new_in(5, System)?;
406 /// # Ok::<(), std::alloc::AllocError>(())
408 #[unstable(feature = "allocator_api", issue = "32838")]
410 pub fn try_new_in(x: T, alloc: A) -> Result<Self, AllocError>
414 let mut boxed = Self::try_new_uninit_in(alloc)?;
416 boxed.as_mut_ptr().write(x);
417 Ok(boxed.assume_init())
421 /// Constructs a new box with uninitialized contents in the provided allocator.
426 /// #![feature(allocator_api, new_uninit)]
428 /// use std::alloc::System;
430 /// let mut five = Box::<u32, _>::new_uninit_in(System);
432 /// let five = unsafe {
433 /// // Deferred initialization:
434 /// five.as_mut_ptr().write(5);
436 /// five.assume_init()
439 /// assert_eq!(*five, 5)
441 #[unstable(feature = "allocator_api", issue = "32838")]
442 #[cfg(not(no_global_oom_handling))]
444 // #[unstable(feature = "new_uninit", issue = "63291")]
445 pub fn new_uninit_in(alloc: A) -> Box<mem::MaybeUninit<T>, A>
449 let layout = Layout::new::<mem::MaybeUninit<T>>();
450 // NOTE: Prefer match over unwrap_or_else since closure sometimes not inlineable.
451 // That would make code size bigger.
452 match Box::try_new_uninit_in(alloc) {
454 Err(_) => handle_alloc_error(layout),
458 /// Constructs a new box with uninitialized contents in the provided allocator,
459 /// returning an error if the allocation fails
464 /// #![feature(allocator_api, new_uninit)]
466 /// use std::alloc::System;
468 /// let mut five = Box::<u32, _>::try_new_uninit_in(System)?;
470 /// let five = unsafe {
471 /// // Deferred initialization:
472 /// five.as_mut_ptr().write(5);
474 /// five.assume_init()
477 /// assert_eq!(*five, 5);
478 /// # Ok::<(), std::alloc::AllocError>(())
480 #[unstable(feature = "allocator_api", issue = "32838")]
481 // #[unstable(feature = "new_uninit", issue = "63291")]
482 pub fn try_new_uninit_in(alloc: A) -> Result<Box<mem::MaybeUninit<T>, A>, AllocError>
486 let layout = Layout::new::<mem::MaybeUninit<T>>();
487 let ptr = alloc.allocate(layout)?.cast();
488 unsafe { Ok(Box::from_raw_in(ptr.as_ptr(), alloc)) }
491 /// Constructs a new `Box` with uninitialized contents, with the memory
492 /// being filled with `0` bytes in the provided allocator.
494 /// See [`MaybeUninit::zeroed`][zeroed] for examples of correct and incorrect usage
500 /// #![feature(allocator_api, new_uninit)]
502 /// use std::alloc::System;
504 /// let zero = Box::<u32, _>::new_zeroed_in(System);
505 /// let zero = unsafe { zero.assume_init() };
507 /// assert_eq!(*zero, 0)
510 /// [zeroed]: mem::MaybeUninit::zeroed
511 #[unstable(feature = "allocator_api", issue = "32838")]
512 #[cfg(not(no_global_oom_handling))]
513 // #[unstable(feature = "new_uninit", issue = "63291")]
515 pub fn new_zeroed_in(alloc: A) -> Box<mem::MaybeUninit<T>, A>
519 let layout = Layout::new::<mem::MaybeUninit<T>>();
520 // NOTE: Prefer match over unwrap_or_else since closure sometimes not inlineable.
521 // That would make code size bigger.
522 match Box::try_new_zeroed_in(alloc) {
524 Err(_) => handle_alloc_error(layout),
528 /// Constructs a new `Box` with uninitialized contents, with the memory
529 /// being filled with `0` bytes in the provided allocator,
530 /// returning an error if the allocation fails,
532 /// See [`MaybeUninit::zeroed`][zeroed] for examples of correct and incorrect usage
538 /// #![feature(allocator_api, new_uninit)]
540 /// use std::alloc::System;
542 /// let zero = Box::<u32, _>::try_new_zeroed_in(System)?;
543 /// let zero = unsafe { zero.assume_init() };
545 /// assert_eq!(*zero, 0);
546 /// # Ok::<(), std::alloc::AllocError>(())
549 /// [zeroed]: mem::MaybeUninit::zeroed
550 #[unstable(feature = "allocator_api", issue = "32838")]
551 // #[unstable(feature = "new_uninit", issue = "63291")]
552 pub fn try_new_zeroed_in(alloc: A) -> Result<Box<mem::MaybeUninit<T>, A>, AllocError>
556 let layout = Layout::new::<mem::MaybeUninit<T>>();
557 let ptr = alloc.allocate_zeroed(layout)?.cast();
558 unsafe { Ok(Box::from_raw_in(ptr.as_ptr(), alloc)) }
561 /// Constructs a new `Pin<Box<T, A>>`. If `T` does not implement [`Unpin`], then
562 /// `x` will be pinned in memory and unable to be moved.
564 /// Constructing and pinning of the `Box` can also be done in two steps: `Box::pin_in(x, alloc)`
565 /// does the same as <code>[Box::into_pin]\([Box::new_in]\(x, alloc))</code>. Consider using
566 /// [`into_pin`](Box::into_pin) if you already have a `Box<T, A>`, or if you want to
567 /// construct a (pinned) `Box` in a different way than with [`Box::new_in`].
568 #[cfg(not(no_global_oom_handling))]
569 #[unstable(feature = "allocator_api", issue = "32838")]
572 pub fn pin_in(x: T, alloc: A) -> Pin<Self>
574 A: 'static + Allocator,
576 Self::into_pin(Self::new_in(x, alloc))
579 /// Converts a `Box<T>` into a `Box<[T]>`
581 /// This conversion does not allocate on the heap and happens in place.
582 #[unstable(feature = "box_into_boxed_slice", issue = "71582")]
583 pub fn into_boxed_slice(boxed: Self) -> Box<[T], A> {
584 let (raw, alloc) = Box::into_raw_with_allocator(boxed);
585 unsafe { Box::from_raw_in(raw as *mut [T; 1], alloc) }
588 /// Consumes the `Box`, returning the wrapped value.
593 /// #![feature(box_into_inner)]
595 /// let c = Box::new(5);
597 /// assert_eq!(Box::into_inner(c), 5);
599 #[unstable(feature = "box_into_inner", issue = "80437")]
601 pub fn into_inner(boxed: Self) -> T {
607 /// Constructs a new boxed slice with uninitialized contents.
612 /// #![feature(new_uninit)]
614 /// let mut values = Box::<[u32]>::new_uninit_slice(3);
616 /// let values = unsafe {
617 /// // Deferred initialization:
618 /// values[0].as_mut_ptr().write(1);
619 /// values[1].as_mut_ptr().write(2);
620 /// values[2].as_mut_ptr().write(3);
622 /// values.assume_init()
625 /// assert_eq!(*values, [1, 2, 3])
627 #[cfg(not(no_global_oom_handling))]
628 #[unstable(feature = "new_uninit", issue = "63291")]
630 pub fn new_uninit_slice(len: usize) -> Box<[mem::MaybeUninit<T>]> {
631 unsafe { RawVec::with_capacity(len).into_box(len) }
634 /// Constructs a new boxed slice with uninitialized contents, with the memory
635 /// being filled with `0` bytes.
637 /// See [`MaybeUninit::zeroed`][zeroed] for examples of correct and incorrect usage
643 /// #![feature(new_uninit)]
645 /// let values = Box::<[u32]>::new_zeroed_slice(3);
646 /// let values = unsafe { values.assume_init() };
648 /// assert_eq!(*values, [0, 0, 0])
651 /// [zeroed]: mem::MaybeUninit::zeroed
652 #[cfg(not(no_global_oom_handling))]
653 #[unstable(feature = "new_uninit", issue = "63291")]
655 pub fn new_zeroed_slice(len: usize) -> Box<[mem::MaybeUninit<T>]> {
656 unsafe { RawVec::with_capacity_zeroed(len).into_box(len) }
659 /// Constructs a new boxed slice with uninitialized contents. Returns an error if
660 /// the allocation fails
665 /// #![feature(allocator_api, new_uninit)]
667 /// let mut values = Box::<[u32]>::try_new_uninit_slice(3)?;
668 /// let values = unsafe {
669 /// // Deferred initialization:
670 /// values[0].as_mut_ptr().write(1);
671 /// values[1].as_mut_ptr().write(2);
672 /// values[2].as_mut_ptr().write(3);
673 /// values.assume_init()
676 /// assert_eq!(*values, [1, 2, 3]);
677 /// # Ok::<(), std::alloc::AllocError>(())
679 #[unstable(feature = "allocator_api", issue = "32838")]
681 pub fn try_new_uninit_slice(len: usize) -> Result<Box<[mem::MaybeUninit<T>]>, AllocError> {
683 let layout = match Layout::array::<mem::MaybeUninit<T>>(len) {
685 Err(_) => return Err(AllocError),
687 let ptr = Global.allocate(layout)?;
688 Ok(RawVec::from_raw_parts_in(ptr.as_mut_ptr() as *mut _, len, Global).into_box(len))
692 /// Constructs a new boxed slice with uninitialized contents, with the memory
693 /// being filled with `0` bytes. Returns an error if the allocation fails
695 /// See [`MaybeUninit::zeroed`][zeroed] for examples of correct and incorrect usage
701 /// #![feature(allocator_api, new_uninit)]
703 /// let values = Box::<[u32]>::try_new_zeroed_slice(3)?;
704 /// let values = unsafe { values.assume_init() };
706 /// assert_eq!(*values, [0, 0, 0]);
707 /// # Ok::<(), std::alloc::AllocError>(())
710 /// [zeroed]: mem::MaybeUninit::zeroed
711 #[unstable(feature = "allocator_api", issue = "32838")]
713 pub fn try_new_zeroed_slice(len: usize) -> Result<Box<[mem::MaybeUninit<T>]>, AllocError> {
715 let layout = match Layout::array::<mem::MaybeUninit<T>>(len) {
717 Err(_) => return Err(AllocError),
719 let ptr = Global.allocate_zeroed(layout)?;
720 Ok(RawVec::from_raw_parts_in(ptr.as_mut_ptr() as *mut _, len, Global).into_box(len))
725 impl<T, A: Allocator> Box<[T], A> {
726 /// Constructs a new boxed slice with uninitialized contents in the provided allocator.
731 /// #![feature(allocator_api, new_uninit)]
733 /// use std::alloc::System;
735 /// let mut values = Box::<[u32], _>::new_uninit_slice_in(3, System);
737 /// let values = unsafe {
738 /// // Deferred initialization:
739 /// values[0].as_mut_ptr().write(1);
740 /// values[1].as_mut_ptr().write(2);
741 /// values[2].as_mut_ptr().write(3);
743 /// values.assume_init()
746 /// assert_eq!(*values, [1, 2, 3])
748 #[cfg(not(no_global_oom_handling))]
749 #[unstable(feature = "allocator_api", issue = "32838")]
750 // #[unstable(feature = "new_uninit", issue = "63291")]
752 pub fn new_uninit_slice_in(len: usize, alloc: A) -> Box<[mem::MaybeUninit<T>], A> {
753 unsafe { RawVec::with_capacity_in(len, alloc).into_box(len) }
756 /// Constructs a new boxed slice with uninitialized contents in the provided allocator,
757 /// with the memory being filled with `0` bytes.
759 /// See [`MaybeUninit::zeroed`][zeroed] for examples of correct and incorrect usage
765 /// #![feature(allocator_api, new_uninit)]
767 /// use std::alloc::System;
769 /// let values = Box::<[u32], _>::new_zeroed_slice_in(3, System);
770 /// let values = unsafe { values.assume_init() };
772 /// assert_eq!(*values, [0, 0, 0])
775 /// [zeroed]: mem::MaybeUninit::zeroed
776 #[cfg(not(no_global_oom_handling))]
777 #[unstable(feature = "allocator_api", issue = "32838")]
778 // #[unstable(feature = "new_uninit", issue = "63291")]
780 pub fn new_zeroed_slice_in(len: usize, alloc: A) -> Box<[mem::MaybeUninit<T>], A> {
781 unsafe { RawVec::with_capacity_zeroed_in(len, alloc).into_box(len) }
785 impl<T, A: Allocator> Box<mem::MaybeUninit<T>, A> {
786 /// Converts to `Box<T, A>`.
790 /// As with [`MaybeUninit::assume_init`],
791 /// it is up to the caller to guarantee that the value
792 /// really is in an initialized state.
793 /// Calling this when the content is not yet fully initialized
794 /// causes immediate undefined behavior.
796 /// [`MaybeUninit::assume_init`]: mem::MaybeUninit::assume_init
801 /// #![feature(new_uninit)]
803 /// let mut five = Box::<u32>::new_uninit();
805 /// let five: Box<u32> = unsafe {
806 /// // Deferred initialization:
807 /// five.as_mut_ptr().write(5);
809 /// five.assume_init()
812 /// assert_eq!(*five, 5)
814 #[unstable(feature = "new_uninit", issue = "63291")]
816 pub unsafe fn assume_init(self) -> Box<T, A> {
817 let (raw, alloc) = Box::into_raw_with_allocator(self);
818 unsafe { Box::from_raw_in(raw as *mut T, alloc) }
821 /// Writes the value and converts to `Box<T, A>`.
823 /// This method converts the box similarly to [`Box::assume_init`] but
824 /// writes `value` into it before conversion thus guaranteeing safety.
825 /// In some scenarios use of this method may improve performance because
826 /// the compiler may be able to optimize copying from stack.
831 /// #![feature(new_uninit)]
833 /// let big_box = Box::<[usize; 1024]>::new_uninit();
835 /// let mut array = [0; 1024];
836 /// for (i, place) in array.iter_mut().enumerate() {
840 /// // The optimizer may be able to elide this copy, so previous code writes
841 /// // to heap directly.
842 /// let big_box = Box::write(big_box, array);
844 /// for (i, x) in big_box.iter().enumerate() {
845 /// assert_eq!(*x, i);
848 #[unstable(feature = "new_uninit", issue = "63291")]
850 pub fn write(mut boxed: Self, value: T) -> Box<T, A> {
852 (*boxed).write(value);
858 impl<T, A: Allocator> Box<[mem::MaybeUninit<T>], A> {
859 /// Converts to `Box<[T], A>`.
863 /// As with [`MaybeUninit::assume_init`],
864 /// it is up to the caller to guarantee that the values
865 /// really are in an initialized state.
866 /// Calling this when the content is not yet fully initialized
867 /// causes immediate undefined behavior.
869 /// [`MaybeUninit::assume_init`]: mem::MaybeUninit::assume_init
874 /// #![feature(new_uninit)]
876 /// let mut values = Box::<[u32]>::new_uninit_slice(3);
878 /// let values = unsafe {
879 /// // Deferred initialization:
880 /// values[0].as_mut_ptr().write(1);
881 /// values[1].as_mut_ptr().write(2);
882 /// values[2].as_mut_ptr().write(3);
884 /// values.assume_init()
887 /// assert_eq!(*values, [1, 2, 3])
889 #[unstable(feature = "new_uninit", issue = "63291")]
891 pub unsafe fn assume_init(self) -> Box<[T], A> {
892 let (raw, alloc) = Box::into_raw_with_allocator(self);
893 unsafe { Box::from_raw_in(raw as *mut [T], alloc) }
897 impl<T: ?Sized> Box<T> {
898 /// Constructs a box from a raw pointer.
900 /// After calling this function, the raw pointer is owned by the
901 /// resulting `Box`. Specifically, the `Box` destructor will call
902 /// the destructor of `T` and free the allocated memory. For this
903 /// to be safe, the memory must have been allocated in accordance
904 /// with the [memory layout] used by `Box` .
908 /// This function is unsafe because improper use may lead to
909 /// memory problems. For example, a double-free may occur if the
910 /// function is called twice on the same raw pointer.
912 /// The safety conditions are described in the [memory layout] section.
916 /// Recreate a `Box` which was previously converted to a raw pointer
917 /// using [`Box::into_raw`]:
919 /// let x = Box::new(5);
920 /// let ptr = Box::into_raw(x);
921 /// let x = unsafe { Box::from_raw(ptr) };
923 /// Manually create a `Box` from scratch by using the global allocator:
925 /// use std::alloc::{alloc, Layout};
928 /// let ptr = alloc(Layout::new::<i32>()) as *mut i32;
929 /// // In general .write is required to avoid attempting to destruct
930 /// // the (uninitialized) previous contents of `ptr`, though for this
931 /// // simple example `*ptr = 5` would have worked as well.
933 /// let x = Box::from_raw(ptr);
937 /// [memory layout]: self#memory-layout
938 /// [`Layout`]: crate::Layout
939 #[stable(feature = "box_raw", since = "1.4.0")]
941 #[must_use = "call `drop(Box::from_raw(ptr))` if you intend to drop the `Box`"]
942 pub unsafe fn from_raw(raw: *mut T) -> Self {
943 unsafe { Self::from_raw_in(raw, Global) }
947 impl<T: ?Sized, A: Allocator> Box<T, A> {
948 /// Constructs a box from a raw pointer in the given allocator.
950 /// After calling this function, the raw pointer is owned by the
951 /// resulting `Box`. Specifically, the `Box` destructor will call
952 /// the destructor of `T` and free the allocated memory. For this
953 /// to be safe, the memory must have been allocated in accordance
954 /// with the [memory layout] used by `Box` .
958 /// This function is unsafe because improper use may lead to
959 /// memory problems. For example, a double-free may occur if the
960 /// function is called twice on the same raw pointer.
965 /// Recreate a `Box` which was previously converted to a raw pointer
966 /// using [`Box::into_raw_with_allocator`]:
968 /// #![feature(allocator_api)]
970 /// use std::alloc::System;
972 /// let x = Box::new_in(5, System);
973 /// let (ptr, alloc) = Box::into_raw_with_allocator(x);
974 /// let x = unsafe { Box::from_raw_in(ptr, alloc) };
976 /// Manually create a `Box` from scratch by using the system allocator:
978 /// #![feature(allocator_api, slice_ptr_get)]
980 /// use std::alloc::{Allocator, Layout, System};
983 /// let ptr = System.allocate(Layout::new::<i32>())?.as_mut_ptr() as *mut i32;
984 /// // In general .write is required to avoid attempting to destruct
985 /// // the (uninitialized) previous contents of `ptr`, though for this
986 /// // simple example `*ptr = 5` would have worked as well.
988 /// let x = Box::from_raw_in(ptr, System);
990 /// # Ok::<(), std::alloc::AllocError>(())
993 /// [memory layout]: self#memory-layout
994 /// [`Layout`]: crate::Layout
995 #[unstable(feature = "allocator_api", issue = "32838")]
996 #[rustc_const_unstable(feature = "const_box", issue = "92521")]
998 pub const unsafe fn from_raw_in(raw: *mut T, alloc: A) -> Self {
999 Box(unsafe { Unique::new_unchecked(raw) }, alloc)
1002 /// Consumes the `Box`, returning a wrapped raw pointer.
1004 /// The pointer will be properly aligned and non-null.
1006 /// After calling this function, the caller is responsible for the
1007 /// memory previously managed by the `Box`. In particular, the
1008 /// caller should properly destroy `T` and release the memory, taking
1009 /// into account the [memory layout] used by `Box`. The easiest way to
1010 /// do this is to convert the raw pointer back into a `Box` with the
1011 /// [`Box::from_raw`] function, allowing the `Box` destructor to perform
1014 /// Note: this is an associated function, which means that you have
1015 /// to call it as `Box::into_raw(b)` instead of `b.into_raw()`. This
1016 /// is so that there is no conflict with a method on the inner type.
1019 /// Converting the raw pointer back into a `Box` with [`Box::from_raw`]
1020 /// for automatic cleanup:
1022 /// let x = Box::new(String::from("Hello"));
1023 /// let ptr = Box::into_raw(x);
1024 /// let x = unsafe { Box::from_raw(ptr) };
1026 /// Manual cleanup by explicitly running the destructor and deallocating
1029 /// use std::alloc::{dealloc, Layout};
1032 /// let x = Box::new(String::from("Hello"));
1033 /// let p = Box::into_raw(x);
1035 /// ptr::drop_in_place(p);
1036 /// dealloc(p as *mut u8, Layout::new::<String>());
1040 /// [memory layout]: self#memory-layout
1041 #[stable(feature = "box_raw", since = "1.4.0")]
1043 pub fn into_raw(b: Self) -> *mut T {
1044 Self::into_raw_with_allocator(b).0
1047 /// Consumes the `Box`, returning a wrapped raw pointer and the allocator.
1049 /// The pointer will be properly aligned and non-null.
1051 /// After calling this function, the caller is responsible for the
1052 /// memory previously managed by the `Box`. In particular, the
1053 /// caller should properly destroy `T` and release the memory, taking
1054 /// into account the [memory layout] used by `Box`. The easiest way to
1055 /// do this is to convert the raw pointer back into a `Box` with the
1056 /// [`Box::from_raw_in`] function, allowing the `Box` destructor to perform
1059 /// Note: this is an associated function, which means that you have
1060 /// to call it as `Box::into_raw_with_allocator(b)` instead of `b.into_raw_with_allocator()`. This
1061 /// is so that there is no conflict with a method on the inner type.
1064 /// Converting the raw pointer back into a `Box` with [`Box::from_raw_in`]
1065 /// for automatic cleanup:
1067 /// #![feature(allocator_api)]
1069 /// use std::alloc::System;
1071 /// let x = Box::new_in(String::from("Hello"), System);
1072 /// let (ptr, alloc) = Box::into_raw_with_allocator(x);
1073 /// let x = unsafe { Box::from_raw_in(ptr, alloc) };
1075 /// Manual cleanup by explicitly running the destructor and deallocating
1078 /// #![feature(allocator_api)]
1080 /// use std::alloc::{Allocator, Layout, System};
1081 /// use std::ptr::{self, NonNull};
1083 /// let x = Box::new_in(String::from("Hello"), System);
1084 /// let (ptr, alloc) = Box::into_raw_with_allocator(x);
1086 /// ptr::drop_in_place(ptr);
1087 /// let non_null = NonNull::new_unchecked(ptr);
1088 /// alloc.deallocate(non_null.cast(), Layout::new::<String>());
1092 /// [memory layout]: self#memory-layout
1093 #[unstable(feature = "allocator_api", issue = "32838")]
1095 pub fn into_raw_with_allocator(b: Self) -> (*mut T, A) {
1096 let (leaked, alloc) = Box::into_unique(b);
1097 (leaked.as_ptr(), alloc)
1101 feature = "ptr_internals",
1103 reason = "use `Box::leak(b).into()` or `Unique::from(Box::leak(b))` instead"
1107 pub fn into_unique(b: Self) -> (Unique<T>, A) {
1108 // Box is recognized as a "unique pointer" by Stacked Borrows, but internally it is a
1109 // raw pointer for the type system. Turning it directly into a raw pointer would not be
1110 // recognized as "releasing" the unique pointer to permit aliased raw accesses,
1111 // so all raw pointer methods have to go through `Box::leak`. Turning *that* to a raw pointer
1112 // behaves correctly.
1113 let alloc = unsafe { ptr::read(&b.1) };
1114 (Unique::from(Box::leak(b)), alloc)
1117 /// Returns a reference to the underlying allocator.
1119 /// Note: this is an associated function, which means that you have
1120 /// to call it as `Box::allocator(&b)` instead of `b.allocator()`. This
1121 /// is so that there is no conflict with a method on the inner type.
1122 #[unstable(feature = "allocator_api", issue = "32838")]
1123 #[rustc_const_unstable(feature = "const_box", issue = "92521")]
1125 pub const fn allocator(b: &Self) -> &A {
1129 /// Consumes and leaks the `Box`, returning a mutable reference,
1130 /// `&'a mut T`. Note that the type `T` must outlive the chosen lifetime
1131 /// `'a`. If the type has only static references, or none at all, then this
1132 /// may be chosen to be `'static`.
1134 /// This function is mainly useful for data that lives for the remainder of
1135 /// the program's life. Dropping the returned reference will cause a memory
1136 /// leak. If this is not acceptable, the reference should first be wrapped
1137 /// with the [`Box::from_raw`] function producing a `Box`. This `Box` can
1138 /// then be dropped which will properly destroy `T` and release the
1139 /// allocated memory.
1141 /// Note: this is an associated function, which means that you have
1142 /// to call it as `Box::leak(b)` instead of `b.leak()`. This
1143 /// is so that there is no conflict with a method on the inner type.
1150 /// let x = Box::new(41);
1151 /// let static_ref: &'static mut usize = Box::leak(x);
1152 /// *static_ref += 1;
1153 /// assert_eq!(*static_ref, 42);
1159 /// let x = vec![1, 2, 3].into_boxed_slice();
1160 /// let static_ref = Box::leak(x);
1161 /// static_ref[0] = 4;
1162 /// assert_eq!(*static_ref, [4, 2, 3]);
1164 #[stable(feature = "box_leak", since = "1.26.0")]
1166 pub fn leak<'a>(b: Self) -> &'a mut T
1170 unsafe { &mut *mem::ManuallyDrop::new(b).0.as_ptr() }
1173 /// Converts a `Box<T>` into a `Pin<Box<T>>`. If `T` does not implement [`Unpin`], then
1174 /// `*boxed` will be pinned in memory and unable to be moved.
1176 /// This conversion does not allocate on the heap and happens in place.
1178 /// This is also available via [`From`].
1180 /// Constructing and pinning a `Box` with <code>Box::into_pin([Box::new]\(x))</code>
1181 /// can also be written more concisely using <code>[Box::pin]\(x)</code>.
1182 /// This `into_pin` method is useful if you already have a `Box<T>`, or you are
1183 /// constructing a (pinned) `Box` in a different way than with [`Box::new`].
1187 /// It's not recommended that crates add an impl like `From<Box<T>> for Pin<T>`,
1188 /// as it'll introduce an ambiguity when calling `Pin::from`.
1189 /// A demonstration of such a poor impl is shown below.
1192 /// # use std::pin::Pin;
1193 /// struct Foo; // A type defined in this crate.
1194 /// impl From<Box<()>> for Pin<Foo> {
1195 /// fn from(_: Box<()>) -> Pin<Foo> {
1200 /// let foo = Box::new(());
1201 /// let bar = Pin::from(foo);
1203 #[stable(feature = "box_into_pin", since = "1.63.0")]
1204 #[rustc_const_unstable(feature = "const_box", issue = "92521")]
1205 pub const fn into_pin(boxed: Self) -> Pin<Self>
1209 // It's not possible to move or replace the insides of a `Pin<Box<T>>`
1210 // when `T: !Unpin`, so it's safe to pin it directly without any
1211 // additional requirements.
1212 unsafe { Pin::new_unchecked(boxed) }
1216 #[stable(feature = "rust1", since = "1.0.0")]
1217 unsafe impl<#[may_dangle] T: ?Sized, A: Allocator> Drop for Box<T, A> {
1218 fn drop(&mut self) {
1219 // FIXME: Do nothing, drop is currently performed by compiler.
1223 #[cfg(not(no_global_oom_handling))]
1224 #[stable(feature = "rust1", since = "1.0.0")]
1225 impl<T: Default> Default for Box<T> {
1226 /// Creates a `Box<T>`, with the `Default` value for T.
1228 fn default() -> Self {
1229 Box::new(T::default())
1233 #[cfg(not(no_global_oom_handling))]
1234 #[stable(feature = "rust1", since = "1.0.0")]
1235 impl<T> Default for Box<[T]> {
1237 fn default() -> Self {
1238 let ptr: Unique<[T]> = Unique::<[T; 0]>::dangling();
1243 #[cfg(not(no_global_oom_handling))]
1244 #[stable(feature = "default_box_extra", since = "1.17.0")]
1245 impl Default for Box<str> {
1247 fn default() -> Self {
1248 // SAFETY: This is the same as `Unique::cast<U>` but with an unsized `U = str`.
1249 let ptr: Unique<str> = unsafe {
1250 let bytes: Unique<[u8]> = Unique::<[u8; 0]>::dangling();
1251 Unique::new_unchecked(bytes.as_ptr() as *mut str)
1257 #[cfg(not(no_global_oom_handling))]
1258 #[stable(feature = "rust1", since = "1.0.0")]
1259 impl<T: Clone, A: Allocator + Clone> Clone for Box<T, A> {
1260 /// Returns a new box with a `clone()` of this box's contents.
1265 /// let x = Box::new(5);
1266 /// let y = x.clone();
1268 /// // The value is the same
1269 /// assert_eq!(x, y);
1271 /// // But they are unique objects
1272 /// assert_ne!(&*x as *const i32, &*y as *const i32);
1275 fn clone(&self) -> Self {
1276 // Pre-allocate memory to allow writing the cloned value directly.
1277 let mut boxed = Self::new_uninit_in(self.1.clone());
1279 (**self).write_clone_into_raw(boxed.as_mut_ptr());
1284 /// Copies `source`'s contents into `self` without creating a new allocation.
1289 /// let x = Box::new(5);
1290 /// let mut y = Box::new(10);
1291 /// let yp: *const i32 = &*y;
1293 /// y.clone_from(&x);
1295 /// // The value is the same
1296 /// assert_eq!(x, y);
1298 /// // And no allocation occurred
1299 /// assert_eq!(yp, &*y);
1302 fn clone_from(&mut self, source: &Self) {
1303 (**self).clone_from(&(**source));
1307 #[cfg(not(no_global_oom_handling))]
1308 #[stable(feature = "box_slice_clone", since = "1.3.0")]
1309 impl Clone for Box<str> {
1310 fn clone(&self) -> Self {
1311 // this makes a copy of the data
1312 let buf: Box<[u8]> = self.as_bytes().into();
1313 unsafe { from_boxed_utf8_unchecked(buf) }
1317 #[stable(feature = "rust1", since = "1.0.0")]
1318 impl<T: ?Sized + PartialEq, A: Allocator> PartialEq for Box<T, A> {
1320 fn eq(&self, other: &Self) -> bool {
1321 PartialEq::eq(&**self, &**other)
1324 fn ne(&self, other: &Self) -> bool {
1325 PartialEq::ne(&**self, &**other)
1328 #[stable(feature = "rust1", since = "1.0.0")]
1329 impl<T: ?Sized + PartialOrd, A: Allocator> PartialOrd for Box<T, A> {
1331 fn partial_cmp(&self, other: &Self) -> Option<Ordering> {
1332 PartialOrd::partial_cmp(&**self, &**other)
1335 fn lt(&self, other: &Self) -> bool {
1336 PartialOrd::lt(&**self, &**other)
1339 fn le(&self, other: &Self) -> bool {
1340 PartialOrd::le(&**self, &**other)
1343 fn ge(&self, other: &Self) -> bool {
1344 PartialOrd::ge(&**self, &**other)
1347 fn gt(&self, other: &Self) -> bool {
1348 PartialOrd::gt(&**self, &**other)
1351 #[stable(feature = "rust1", since = "1.0.0")]
1352 impl<T: ?Sized + Ord, A: Allocator> Ord for Box<T, A> {
1354 fn cmp(&self, other: &Self) -> Ordering {
1355 Ord::cmp(&**self, &**other)
1358 #[stable(feature = "rust1", since = "1.0.0")]
1359 impl<T: ?Sized + Eq, A: Allocator> Eq for Box<T, A> {}
1361 #[stable(feature = "rust1", since = "1.0.0")]
1362 impl<T: ?Sized + Hash, A: Allocator> Hash for Box<T, A> {
1363 fn hash<H: Hasher>(&self, state: &mut H) {
1364 (**self).hash(state);
1368 #[stable(feature = "indirect_hasher_impl", since = "1.22.0")]
1369 impl<T: ?Sized + Hasher, A: Allocator> Hasher for Box<T, A> {
1370 fn finish(&self) -> u64 {
1373 fn write(&mut self, bytes: &[u8]) {
1374 (**self).write(bytes)
1376 fn write_u8(&mut self, i: u8) {
1377 (**self).write_u8(i)
1379 fn write_u16(&mut self, i: u16) {
1380 (**self).write_u16(i)
1382 fn write_u32(&mut self, i: u32) {
1383 (**self).write_u32(i)
1385 fn write_u64(&mut self, i: u64) {
1386 (**self).write_u64(i)
1388 fn write_u128(&mut self, i: u128) {
1389 (**self).write_u128(i)
1391 fn write_usize(&mut self, i: usize) {
1392 (**self).write_usize(i)
1394 fn write_i8(&mut self, i: i8) {
1395 (**self).write_i8(i)
1397 fn write_i16(&mut self, i: i16) {
1398 (**self).write_i16(i)
1400 fn write_i32(&mut self, i: i32) {
1401 (**self).write_i32(i)
1403 fn write_i64(&mut self, i: i64) {
1404 (**self).write_i64(i)
1406 fn write_i128(&mut self, i: i128) {
1407 (**self).write_i128(i)
1409 fn write_isize(&mut self, i: isize) {
1410 (**self).write_isize(i)
1412 fn write_length_prefix(&mut self, len: usize) {
1413 (**self).write_length_prefix(len)
1415 fn write_str(&mut self, s: &str) {
1416 (**self).write_str(s)
1420 #[cfg(not(no_global_oom_handling))]
1421 #[stable(feature = "from_for_ptrs", since = "1.6.0")]
1422 impl<T> From<T> for Box<T> {
1423 /// Converts a `T` into a `Box<T>`
1425 /// The conversion allocates on the heap and moves `t`
1426 /// from the stack into it.
1432 /// let boxed = Box::new(5);
1434 /// assert_eq!(Box::from(x), boxed);
1436 fn from(t: T) -> Self {
1441 #[stable(feature = "pin", since = "1.33.0")]
1442 impl<T: ?Sized, A: Allocator> From<Box<T, A>> for Pin<Box<T, A>>
1446 /// Converts a `Box<T>` into a `Pin<Box<T>>`. If `T` does not implement [`Unpin`], then
1447 /// `*boxed` will be pinned in memory and unable to be moved.
1449 /// This conversion does not allocate on the heap and happens in place.
1451 /// This is also available via [`Box::into_pin`].
1453 /// Constructing and pinning a `Box` with <code><Pin<Box\<T>>>::from([Box::new]\(x))</code>
1454 /// can also be written more concisely using <code>[Box::pin]\(x)</code>.
1455 /// This `From` implementation is useful if you already have a `Box<T>`, or you are
1456 /// constructing a (pinned) `Box` in a different way than with [`Box::new`].
1457 fn from(boxed: Box<T, A>) -> Self {
1458 Box::into_pin(boxed)
1462 /// Specialization trait used for `From<&[T]>`.
1463 #[cfg(not(no_global_oom_handling))]
1464 trait BoxFromSlice<T> {
1465 fn from_slice(slice: &[T]) -> Self;
1468 #[cfg(not(no_global_oom_handling))]
1469 impl<T: Clone> BoxFromSlice<T> for Box<[T]> {
1471 default fn from_slice(slice: &[T]) -> Self {
1472 slice.to_vec().into_boxed_slice()
1476 #[cfg(not(no_global_oom_handling))]
1477 impl<T: Copy> BoxFromSlice<T> for Box<[T]> {
1479 fn from_slice(slice: &[T]) -> Self {
1480 let len = slice.len();
1481 let buf = RawVec::with_capacity(len);
1483 ptr::copy_nonoverlapping(slice.as_ptr(), buf.ptr(), len);
1484 buf.into_box(slice.len()).assume_init()
1489 #[cfg(not(no_global_oom_handling))]
1490 #[stable(feature = "box_from_slice", since = "1.17.0")]
1491 impl<T: Clone> From<&[T]> for Box<[T]> {
1492 /// Converts a `&[T]` into a `Box<[T]>`
1494 /// This conversion allocates on the heap
1495 /// and performs a copy of `slice` and its contents.
1499 /// // create a &[u8] which will be used to create a Box<[u8]>
1500 /// let slice: &[u8] = &[104, 101, 108, 108, 111];
1501 /// let boxed_slice: Box<[u8]> = Box::from(slice);
1503 /// println!("{boxed_slice:?}");
1506 fn from(slice: &[T]) -> Box<[T]> {
1507 <Self as BoxFromSlice<T>>::from_slice(slice)
1511 #[cfg(not(no_global_oom_handling))]
1512 #[stable(feature = "box_from_cow", since = "1.45.0")]
1513 impl<T: Clone> From<Cow<'_, [T]>> for Box<[T]> {
1514 /// Converts a `Cow<'_, [T]>` into a `Box<[T]>`
1516 /// When `cow` is the `Cow::Borrowed` variant, this
1517 /// conversion allocates on the heap and copies the
1518 /// underlying slice. Otherwise, it will try to reuse the owned
1519 /// `Vec`'s allocation.
1521 fn from(cow: Cow<'_, [T]>) -> Box<[T]> {
1523 Cow::Borrowed(slice) => Box::from(slice),
1524 Cow::Owned(slice) => Box::from(slice),
1529 #[cfg(not(no_global_oom_handling))]
1530 #[stable(feature = "box_from_slice", since = "1.17.0")]
1531 impl From<&str> for Box<str> {
1532 /// Converts a `&str` into a `Box<str>`
1534 /// This conversion allocates on the heap
1535 /// and performs a copy of `s`.
1540 /// let boxed: Box<str> = Box::from("hello");
1541 /// println!("{boxed}");
1544 fn from(s: &str) -> Box<str> {
1545 unsafe { from_boxed_utf8_unchecked(Box::from(s.as_bytes())) }
1549 #[cfg(not(no_global_oom_handling))]
1550 #[stable(feature = "box_from_cow", since = "1.45.0")]
1551 impl From<Cow<'_, str>> for Box<str> {
1552 /// Converts a `Cow<'_, str>` into a `Box<str>`
1554 /// When `cow` is the `Cow::Borrowed` variant, this
1555 /// conversion allocates on the heap and copies the
1556 /// underlying `str`. Otherwise, it will try to reuse the owned
1557 /// `String`'s allocation.
1562 /// use std::borrow::Cow;
1564 /// let unboxed = Cow::Borrowed("hello");
1565 /// let boxed: Box<str> = Box::from(unboxed);
1566 /// println!("{boxed}");
1570 /// # use std::borrow::Cow;
1571 /// let unboxed = Cow::Owned("hello".to_string());
1572 /// let boxed: Box<str> = Box::from(unboxed);
1573 /// println!("{boxed}");
1576 fn from(cow: Cow<'_, str>) -> Box<str> {
1578 Cow::Borrowed(s) => Box::from(s),
1579 Cow::Owned(s) => Box::from(s),
1584 #[stable(feature = "boxed_str_conv", since = "1.19.0")]
1585 impl<A: Allocator> From<Box<str, A>> for Box<[u8], A> {
1586 /// Converts a `Box<str>` into a `Box<[u8]>`
1588 /// This conversion does not allocate on the heap and happens in place.
1592 /// // create a Box<str> which will be used to create a Box<[u8]>
1593 /// let boxed: Box<str> = Box::from("hello");
1594 /// let boxed_str: Box<[u8]> = Box::from(boxed);
1596 /// // create a &[u8] which will be used to create a Box<[u8]>
1597 /// let slice: &[u8] = &[104, 101, 108, 108, 111];
1598 /// let boxed_slice = Box::from(slice);
1600 /// assert_eq!(boxed_slice, boxed_str);
1603 fn from(s: Box<str, A>) -> Self {
1604 let (raw, alloc) = Box::into_raw_with_allocator(s);
1605 unsafe { Box::from_raw_in(raw as *mut [u8], alloc) }
1609 #[cfg(not(no_global_oom_handling))]
1610 #[stable(feature = "box_from_array", since = "1.45.0")]
1611 impl<T, const N: usize> From<[T; N]> for Box<[T]> {
1612 /// Converts a `[T; N]` into a `Box<[T]>`
1614 /// This conversion moves the array to newly heap-allocated memory.
1619 /// let boxed: Box<[u8]> = Box::from([4, 2]);
1620 /// println!("{boxed:?}");
1622 fn from(array: [T; N]) -> Box<[T]> {
1627 /// Casts a boxed slice to a boxed array.
1631 /// `boxed_slice.len()` must be exactly `N`.
1632 unsafe fn boxed_slice_as_array_unchecked<T, A: Allocator, const N: usize>(
1633 boxed_slice: Box<[T], A>,
1634 ) -> Box<[T; N], A> {
1635 debug_assert_eq!(boxed_slice.len(), N);
1637 let (ptr, alloc) = Box::into_raw_with_allocator(boxed_slice);
1638 // SAFETY: Pointer and allocator came from an existing box,
1639 // and our safety condition requires that the length is exactly `N`
1640 unsafe { Box::from_raw_in(ptr as *mut [T; N], alloc) }
1643 #[stable(feature = "boxed_slice_try_from", since = "1.43.0")]
1644 impl<T, const N: usize> TryFrom<Box<[T]>> for Box<[T; N]> {
1645 type Error = Box<[T]>;
1647 /// Attempts to convert a `Box<[T]>` into a `Box<[T; N]>`.
1649 /// The conversion occurs in-place and does not require a
1650 /// new memory allocation.
1654 /// Returns the old `Box<[T]>` in the `Err` variant if
1655 /// `boxed_slice.len()` does not equal `N`.
1656 fn try_from(boxed_slice: Box<[T]>) -> Result<Self, Self::Error> {
1657 if boxed_slice.len() == N {
1658 Ok(unsafe { boxed_slice_as_array_unchecked(boxed_slice) })
1665 #[cfg(not(no_global_oom_handling))]
1666 #[stable(feature = "boxed_array_try_from_vec", since = "1.66.0")]
1667 impl<T, const N: usize> TryFrom<Vec<T>> for Box<[T; N]> {
1668 type Error = Vec<T>;
1670 /// Attempts to convert a `Vec<T>` into a `Box<[T; N]>`.
1672 /// Like [`Vec::into_boxed_slice`], this is in-place if `vec.capacity() == N`,
1673 /// but will require a reallocation otherwise.
1677 /// Returns the original `Vec<T>` in the `Err` variant if
1678 /// `boxed_slice.len()` does not equal `N`.
1682 /// This can be used with [`vec!`] to create an array on the heap:
1685 /// let state: Box<[f32; 100]> = vec![1.0; 100].try_into().unwrap();
1686 /// assert_eq!(state.len(), 100);
1688 fn try_from(vec: Vec<T>) -> Result<Self, Self::Error> {
1690 let boxed_slice = vec.into_boxed_slice();
1691 Ok(unsafe { boxed_slice_as_array_unchecked(boxed_slice) })
1698 impl<A: Allocator> Box<dyn Any, A> {
1699 /// Attempt to downcast the box to a concrete type.
1704 /// use std::any::Any;
1706 /// fn print_if_string(value: Box<dyn Any>) {
1707 /// if let Ok(string) = value.downcast::<String>() {
1708 /// println!("String ({}): {}", string.len(), string);
1712 /// let my_string = "Hello World".to_string();
1713 /// print_if_string(Box::new(my_string));
1714 /// print_if_string(Box::new(0i8));
1717 #[stable(feature = "rust1", since = "1.0.0")]
1718 pub fn downcast<T: Any>(self) -> Result<Box<T, A>, Self> {
1719 if self.is::<T>() { unsafe { Ok(self.downcast_unchecked::<T>()) } } else { Err(self) }
1722 /// Downcasts the box to a concrete type.
1724 /// For a safe alternative see [`downcast`].
1729 /// #![feature(downcast_unchecked)]
1731 /// use std::any::Any;
1733 /// let x: Box<dyn Any> = Box::new(1_usize);
1736 /// assert_eq!(*x.downcast_unchecked::<usize>(), 1);
1742 /// The contained value must be of type `T`. Calling this method
1743 /// with the incorrect type is *undefined behavior*.
1745 /// [`downcast`]: Self::downcast
1747 #[unstable(feature = "downcast_unchecked", issue = "90850")]
1748 pub unsafe fn downcast_unchecked<T: Any>(self) -> Box<T, A> {
1749 debug_assert!(self.is::<T>());
1751 let (raw, alloc): (*mut dyn Any, _) = Box::into_raw_with_allocator(self);
1752 Box::from_raw_in(raw as *mut T, alloc)
1757 impl<A: Allocator> Box<dyn Any + Send, A> {
1758 /// Attempt to downcast the box to a concrete type.
1763 /// use std::any::Any;
1765 /// fn print_if_string(value: Box<dyn Any + Send>) {
1766 /// if let Ok(string) = value.downcast::<String>() {
1767 /// println!("String ({}): {}", string.len(), string);
1771 /// let my_string = "Hello World".to_string();
1772 /// print_if_string(Box::new(my_string));
1773 /// print_if_string(Box::new(0i8));
1776 #[stable(feature = "rust1", since = "1.0.0")]
1777 pub fn downcast<T: Any>(self) -> Result<Box<T, A>, Self> {
1778 if self.is::<T>() { unsafe { Ok(self.downcast_unchecked::<T>()) } } else { Err(self) }
1781 /// Downcasts the box to a concrete type.
1783 /// For a safe alternative see [`downcast`].
1788 /// #![feature(downcast_unchecked)]
1790 /// use std::any::Any;
1792 /// let x: Box<dyn Any + Send> = Box::new(1_usize);
1795 /// assert_eq!(*x.downcast_unchecked::<usize>(), 1);
1801 /// The contained value must be of type `T`. Calling this method
1802 /// with the incorrect type is *undefined behavior*.
1804 /// [`downcast`]: Self::downcast
1806 #[unstable(feature = "downcast_unchecked", issue = "90850")]
1807 pub unsafe fn downcast_unchecked<T: Any>(self) -> Box<T, A> {
1808 debug_assert!(self.is::<T>());
1810 let (raw, alloc): (*mut (dyn Any + Send), _) = Box::into_raw_with_allocator(self);
1811 Box::from_raw_in(raw as *mut T, alloc)
1816 impl<A: Allocator> Box<dyn Any + Send + Sync, A> {
1817 /// Attempt to downcast the box to a concrete type.
1822 /// use std::any::Any;
1824 /// fn print_if_string(value: Box<dyn Any + Send + Sync>) {
1825 /// if let Ok(string) = value.downcast::<String>() {
1826 /// println!("String ({}): {}", string.len(), string);
1830 /// let my_string = "Hello World".to_string();
1831 /// print_if_string(Box::new(my_string));
1832 /// print_if_string(Box::new(0i8));
1835 #[stable(feature = "box_send_sync_any_downcast", since = "1.51.0")]
1836 pub fn downcast<T: Any>(self) -> Result<Box<T, A>, Self> {
1837 if self.is::<T>() { unsafe { Ok(self.downcast_unchecked::<T>()) } } else { Err(self) }
1840 /// Downcasts the box to a concrete type.
1842 /// For a safe alternative see [`downcast`].
1847 /// #![feature(downcast_unchecked)]
1849 /// use std::any::Any;
1851 /// let x: Box<dyn Any + Send + Sync> = Box::new(1_usize);
1854 /// assert_eq!(*x.downcast_unchecked::<usize>(), 1);
1860 /// The contained value must be of type `T`. Calling this method
1861 /// with the incorrect type is *undefined behavior*.
1863 /// [`downcast`]: Self::downcast
1865 #[unstable(feature = "downcast_unchecked", issue = "90850")]
1866 pub unsafe fn downcast_unchecked<T: Any>(self) -> Box<T, A> {
1867 debug_assert!(self.is::<T>());
1869 let (raw, alloc): (*mut (dyn Any + Send + Sync), _) =
1870 Box::into_raw_with_allocator(self);
1871 Box::from_raw_in(raw as *mut T, alloc)
1876 #[stable(feature = "rust1", since = "1.0.0")]
1877 impl<T: fmt::Display + ?Sized, A: Allocator> fmt::Display for Box<T, A> {
1878 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
1879 fmt::Display::fmt(&**self, f)
1883 #[stable(feature = "rust1", since = "1.0.0")]
1884 impl<T: fmt::Debug + ?Sized, A: Allocator> fmt::Debug for Box<T, A> {
1885 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
1886 fmt::Debug::fmt(&**self, f)
1890 #[stable(feature = "rust1", since = "1.0.0")]
1891 impl<T: ?Sized, A: Allocator> fmt::Pointer for Box<T, A> {
1892 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
1893 // It's not possible to extract the inner Uniq directly from the Box,
1894 // instead we cast it to a *const which aliases the Unique
1895 let ptr: *const T = &**self;
1896 fmt::Pointer::fmt(&ptr, f)
1900 #[stable(feature = "rust1", since = "1.0.0")]
1901 impl<T: ?Sized, A: Allocator> Deref for Box<T, A> {
1904 fn deref(&self) -> &T {
1909 #[stable(feature = "rust1", since = "1.0.0")]
1910 impl<T: ?Sized, A: Allocator> DerefMut for Box<T, A> {
1911 fn deref_mut(&mut self) -> &mut T {
1916 #[unstable(feature = "receiver_trait", issue = "none")]
1917 impl<T: ?Sized, A: Allocator> Receiver for Box<T, A> {}
1919 #[stable(feature = "rust1", since = "1.0.0")]
1920 impl<I: Iterator + ?Sized, A: Allocator> Iterator for Box<I, A> {
1921 type Item = I::Item;
1922 fn next(&mut self) -> Option<I::Item> {
1925 fn size_hint(&self) -> (usize, Option<usize>) {
1926 (**self).size_hint()
1928 fn nth(&mut self, n: usize) -> Option<I::Item> {
1931 fn last(self) -> Option<I::Item> {
1938 fn last(self) -> Option<Self::Item>;
1941 impl<I: Iterator + ?Sized, A: Allocator> BoxIter for Box<I, A> {
1942 type Item = I::Item;
1943 default fn last(self) -> Option<I::Item> {
1945 fn some<T>(_: Option<T>, x: T) -> Option<T> {
1949 self.fold(None, some)
1953 /// Specialization for sized `I`s that uses `I`s implementation of `last()`
1954 /// instead of the default.
1955 #[stable(feature = "rust1", since = "1.0.0")]
1956 impl<I: Iterator, A: Allocator> BoxIter for Box<I, A> {
1957 fn last(self) -> Option<I::Item> {
1962 #[stable(feature = "rust1", since = "1.0.0")]
1963 impl<I: DoubleEndedIterator + ?Sized, A: Allocator> DoubleEndedIterator for Box<I, A> {
1964 fn next_back(&mut self) -> Option<I::Item> {
1965 (**self).next_back()
1967 fn nth_back(&mut self, n: usize) -> Option<I::Item> {
1968 (**self).nth_back(n)
1971 #[stable(feature = "rust1", since = "1.0.0")]
1972 impl<I: ExactSizeIterator + ?Sized, A: Allocator> ExactSizeIterator for Box<I, A> {
1973 fn len(&self) -> usize {
1976 fn is_empty(&self) -> bool {
1981 #[stable(feature = "fused", since = "1.26.0")]
1982 impl<I: FusedIterator + ?Sized, A: Allocator> FusedIterator for Box<I, A> {}
1984 #[stable(feature = "boxed_closure_impls", since = "1.35.0")]
1985 impl<Args: Tuple, F: FnOnce<Args> + ?Sized, A: Allocator> FnOnce<Args> for Box<F, A> {
1986 type Output = <F as FnOnce<Args>>::Output;
1988 extern "rust-call" fn call_once(self, args: Args) -> Self::Output {
1989 <F as FnOnce<Args>>::call_once(*self, args)
1993 #[stable(feature = "boxed_closure_impls", since = "1.35.0")]
1994 impl<Args: Tuple, F: FnMut<Args> + ?Sized, A: Allocator> FnMut<Args> for Box<F, A> {
1995 extern "rust-call" fn call_mut(&mut self, args: Args) -> Self::Output {
1996 <F as FnMut<Args>>::call_mut(self, args)
2000 #[stable(feature = "boxed_closure_impls", since = "1.35.0")]
2001 impl<Args: Tuple, F: Fn<Args> + ?Sized, A: Allocator> Fn<Args> for Box<F, A> {
2002 extern "rust-call" fn call(&self, args: Args) -> Self::Output {
2003 <F as Fn<Args>>::call(self, args)
2007 #[unstable(feature = "coerce_unsized", issue = "18598")]
2008 impl<T: ?Sized + Unsize<U>, U: ?Sized, A: Allocator> CoerceUnsized<Box<U, A>> for Box<T, A> {}
2010 #[unstable(feature = "dispatch_from_dyn", issue = "none")]
2011 impl<T: ?Sized + Unsize<U>, U: ?Sized> DispatchFromDyn<Box<U>> for Box<T, Global> {}
2013 #[cfg(not(no_global_oom_handling))]
2014 #[stable(feature = "boxed_slice_from_iter", since = "1.32.0")]
2015 impl<I> FromIterator<I> for Box<[I]> {
2016 fn from_iter<T: IntoIterator<Item = I>>(iter: T) -> Self {
2017 iter.into_iter().collect::<Vec<_>>().into_boxed_slice()
2021 #[cfg(not(no_global_oom_handling))]
2022 #[stable(feature = "box_slice_clone", since = "1.3.0")]
2023 impl<T: Clone, A: Allocator + Clone> Clone for Box<[T], A> {
2024 fn clone(&self) -> Self {
2025 let alloc = Box::allocator(self).clone();
2026 self.to_vec_in(alloc).into_boxed_slice()
2029 fn clone_from(&mut self, other: &Self) {
2030 if self.len() == other.len() {
2031 self.clone_from_slice(&other);
2033 *self = other.clone();
2038 #[stable(feature = "box_borrow", since = "1.1.0")]
2039 impl<T: ?Sized, A: Allocator> borrow::Borrow<T> for Box<T, A> {
2040 fn borrow(&self) -> &T {
2045 #[stable(feature = "box_borrow", since = "1.1.0")]
2046 impl<T: ?Sized, A: Allocator> borrow::BorrowMut<T> for Box<T, A> {
2047 fn borrow_mut(&mut self) -> &mut T {
2052 #[stable(since = "1.5.0", feature = "smart_ptr_as_ref")]
2053 impl<T: ?Sized, A: Allocator> AsRef<T> for Box<T, A> {
2054 fn as_ref(&self) -> &T {
2059 #[stable(since = "1.5.0", feature = "smart_ptr_as_ref")]
2060 impl<T: ?Sized, A: Allocator> AsMut<T> for Box<T, A> {
2061 fn as_mut(&mut self) -> &mut T {
2068 * We could have chosen not to add this impl, and instead have written a
2069 * function of Pin<Box<T>> to Pin<T>. Such a function would not be sound,
2070 * because Box<T> implements Unpin even when T does not, as a result of
2073 * We chose this API instead of the alternative for a few reasons:
2074 * - Logically, it is helpful to understand pinning in regard to the
2075 * memory region being pointed to. For this reason none of the
2076 * standard library pointer types support projecting through a pin
2077 * (Box<T> is the only pointer type in std for which this would be
2079 * - It is in practice very useful to have Box<T> be unconditionally
2080 * Unpin because of trait objects, for which the structural auto
2081 * trait functionality does not apply (e.g., Box<dyn Foo> would
2082 * otherwise not be Unpin).
2084 * Another type with the same semantics as Box but only a conditional
2085 * implementation of `Unpin` (where `T: Unpin`) would be valid/safe, and
2086 * could have a method to project a Pin<T> from it.
2088 #[stable(feature = "pin", since = "1.33.0")]
2089 impl<T: ?Sized, A: Allocator> Unpin for Box<T, A> where A: 'static {}
2091 #[unstable(feature = "generator_trait", issue = "43122")]
2092 impl<G: ?Sized + Generator<R> + Unpin, R, A: Allocator> Generator<R> for Box<G, A>
2096 type Yield = G::Yield;
2097 type Return = G::Return;
2099 fn resume(mut self: Pin<&mut Self>, arg: R) -> GeneratorState<Self::Yield, Self::Return> {
2100 G::resume(Pin::new(&mut *self), arg)
2104 #[unstable(feature = "generator_trait", issue = "43122")]
2105 impl<G: ?Sized + Generator<R>, R, A: Allocator> Generator<R> for Pin<Box<G, A>>
2109 type Yield = G::Yield;
2110 type Return = G::Return;
2112 fn resume(mut self: Pin<&mut Self>, arg: R) -> GeneratorState<Self::Yield, Self::Return> {
2113 G::resume((*self).as_mut(), arg)
2117 #[stable(feature = "futures_api", since = "1.36.0")]
2118 impl<F: ?Sized + Future + Unpin, A: Allocator> Future for Box<F, A>
2122 type Output = F::Output;
2124 fn poll(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
2125 F::poll(Pin::new(&mut *self), cx)
2129 #[unstable(feature = "async_iterator", issue = "79024")]
2130 impl<S: ?Sized + AsyncIterator + Unpin> AsyncIterator for Box<S> {
2131 type Item = S::Item;
2133 fn poll_next(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Option<Self::Item>> {
2134 Pin::new(&mut **self).poll_next(cx)
2137 fn size_hint(&self) -> (usize, Option<usize>) {
2138 (**self).size_hint()
2144 #[stable(feature = "error_downcast", since = "1.3.0")]
2145 #[rustc_allow_incoherent_impl]
2146 /// Attempts to downcast the box to a concrete type.
2147 pub fn downcast<T: Error + 'static>(self: Box<Self>) -> Result<Box<T>, Box<dyn Error>> {
2150 let raw: *mut dyn Error = Box::into_raw(self);
2151 Ok(Box::from_raw(raw as *mut T))
2159 impl dyn Error + Send {
2161 #[stable(feature = "error_downcast", since = "1.3.0")]
2162 #[rustc_allow_incoherent_impl]
2163 /// Attempts to downcast the box to a concrete type.
2164 pub fn downcast<T: Error + 'static>(self: Box<Self>) -> Result<Box<T>, Box<dyn Error + Send>> {
2165 let err: Box<dyn Error> = self;
2166 <dyn Error>::downcast(err).map_err(|s| unsafe {
2167 // Reapply the `Send` marker.
2168 mem::transmute::<Box<dyn Error>, Box<dyn Error + Send>>(s)
2173 impl dyn Error + Send + Sync {
2175 #[stable(feature = "error_downcast", since = "1.3.0")]
2176 #[rustc_allow_incoherent_impl]
2177 /// Attempts to downcast the box to a concrete type.
2178 pub fn downcast<T: Error + 'static>(self: Box<Self>) -> Result<Box<T>, Box<Self>> {
2179 let err: Box<dyn Error> = self;
2180 <dyn Error>::downcast(err).map_err(|s| unsafe {
2181 // Reapply the `Send + Sync` marker.
2182 mem::transmute::<Box<dyn Error>, Box<dyn Error + Send + Sync>>(s)
2187 #[cfg(not(no_global_oom_handling))]
2188 #[stable(feature = "rust1", since = "1.0.0")]
2189 impl<'a, E: Error + 'a> From<E> for Box<dyn Error + 'a> {
2190 /// Converts a type of [`Error`] into a box of dyn [`Error`].
2195 /// use std::error::Error;
2199 /// #[derive(Debug)]
2202 /// impl fmt::Display for AnError {
2203 /// fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
2204 /// write!(f, "An error")
2208 /// impl Error for AnError {}
2210 /// let an_error = AnError;
2211 /// assert!(0 == mem::size_of_val(&an_error));
2212 /// let a_boxed_error = Box::<dyn Error>::from(an_error);
2213 /// assert!(mem::size_of::<Box<dyn Error>>() == mem::size_of_val(&a_boxed_error))
2215 fn from(err: E) -> Box<dyn Error + 'a> {
2220 #[cfg(not(no_global_oom_handling))]
2221 #[stable(feature = "rust1", since = "1.0.0")]
2222 impl<'a, E: Error + Send + Sync + 'a> From<E> for Box<dyn Error + Send + Sync + 'a> {
2223 /// Converts a type of [`Error`] + [`Send`] + [`Sync`] into a box of
2224 /// dyn [`Error`] + [`Send`] + [`Sync`].
2229 /// use std::error::Error;
2233 /// #[derive(Debug)]
2236 /// impl fmt::Display for AnError {
2237 /// fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
2238 /// write!(f, "An error")
2242 /// impl Error for AnError {}
2244 /// unsafe impl Send for AnError {}
2246 /// unsafe impl Sync for AnError {}
2248 /// let an_error = AnError;
2249 /// assert!(0 == mem::size_of_val(&an_error));
2250 /// let a_boxed_error = Box::<dyn Error + Send + Sync>::from(an_error);
2252 /// mem::size_of::<Box<dyn Error + Send + Sync>>() == mem::size_of_val(&a_boxed_error))
2254 fn from(err: E) -> Box<dyn Error + Send + Sync + 'a> {
2259 #[cfg(not(no_global_oom_handling))]
2260 #[stable(feature = "rust1", since = "1.0.0")]
2261 impl From<String> for Box<dyn Error + Send + Sync> {
2262 /// Converts a [`String`] into a box of dyn [`Error`] + [`Send`] + [`Sync`].
2267 /// use std::error::Error;
2270 /// let a_string_error = "a string error".to_string();
2271 /// let a_boxed_error = Box::<dyn Error + Send + Sync>::from(a_string_error);
2273 /// mem::size_of::<Box<dyn Error + Send + Sync>>() == mem::size_of_val(&a_boxed_error))
2276 fn from(err: String) -> Box<dyn Error + Send + Sync> {
2277 struct StringError(String);
2279 impl Error for StringError {
2280 #[allow(deprecated)]
2281 fn description(&self) -> &str {
2286 impl fmt::Display for StringError {
2287 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
2288 fmt::Display::fmt(&self.0, f)
2292 // Purposefully skip printing "StringError(..)"
2293 impl fmt::Debug for StringError {
2294 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
2295 fmt::Debug::fmt(&self.0, f)
2299 Box::new(StringError(err))
2303 #[cfg(not(no_global_oom_handling))]
2304 #[stable(feature = "string_box_error", since = "1.6.0")]
2305 impl From<String> for Box<dyn Error> {
2306 /// Converts a [`String`] into a box of dyn [`Error`].
2311 /// use std::error::Error;
2314 /// let a_string_error = "a string error".to_string();
2315 /// let a_boxed_error = Box::<dyn Error>::from(a_string_error);
2316 /// assert!(mem::size_of::<Box<dyn Error>>() == mem::size_of_val(&a_boxed_error))
2318 fn from(str_err: String) -> Box<dyn Error> {
2319 let err1: Box<dyn Error + Send + Sync> = From::from(str_err);
2320 let err2: Box<dyn Error> = err1;
2325 #[cfg(not(no_global_oom_handling))]
2326 #[stable(feature = "rust1", since = "1.0.0")]
2327 impl<'a> From<&str> for Box<dyn Error + Send + Sync + 'a> {
2328 /// Converts a [`str`] into a box of dyn [`Error`] + [`Send`] + [`Sync`].
2330 /// [`str`]: prim@str
2335 /// use std::error::Error;
2338 /// let a_str_error = "a str error";
2339 /// let a_boxed_error = Box::<dyn Error + Send + Sync>::from(a_str_error);
2341 /// mem::size_of::<Box<dyn Error + Send + Sync>>() == mem::size_of_val(&a_boxed_error))
2344 fn from(err: &str) -> Box<dyn Error + Send + Sync + 'a> {
2345 From::from(String::from(err))
2349 #[cfg(not(no_global_oom_handling))]
2350 #[stable(feature = "string_box_error", since = "1.6.0")]
2351 impl From<&str> for Box<dyn Error> {
2352 /// Converts a [`str`] into a box of dyn [`Error`].
2354 /// [`str`]: prim@str
2359 /// use std::error::Error;
2362 /// let a_str_error = "a str error";
2363 /// let a_boxed_error = Box::<dyn Error>::from(a_str_error);
2364 /// assert!(mem::size_of::<Box<dyn Error>>() == mem::size_of_val(&a_boxed_error))
2366 fn from(err: &str) -> Box<dyn Error> {
2367 From::from(String::from(err))
2371 #[cfg(not(no_global_oom_handling))]
2372 #[stable(feature = "cow_box_error", since = "1.22.0")]
2373 impl<'a, 'b> From<Cow<'b, str>> for Box<dyn Error + Send + Sync + 'a> {
2374 /// Converts a [`Cow`] into a box of dyn [`Error`] + [`Send`] + [`Sync`].
2379 /// use std::error::Error;
2381 /// use std::borrow::Cow;
2383 /// let a_cow_str_error = Cow::from("a str error");
2384 /// let a_boxed_error = Box::<dyn Error + Send + Sync>::from(a_cow_str_error);
2386 /// mem::size_of::<Box<dyn Error + Send + Sync>>() == mem::size_of_val(&a_boxed_error))
2388 fn from(err: Cow<'b, str>) -> Box<dyn Error + Send + Sync + 'a> {
2389 From::from(String::from(err))
2393 #[cfg(not(no_global_oom_handling))]
2394 #[stable(feature = "cow_box_error", since = "1.22.0")]
2395 impl<'a> From<Cow<'a, str>> for Box<dyn Error> {
2396 /// Converts a [`Cow`] into a box of dyn [`Error`].
2401 /// use std::error::Error;
2403 /// use std::borrow::Cow;
2405 /// let a_cow_str_error = Cow::from("a str error");
2406 /// let a_boxed_error = Box::<dyn Error>::from(a_cow_str_error);
2407 /// assert!(mem::size_of::<Box<dyn Error>>() == mem::size_of_val(&a_boxed_error))
2409 fn from(err: Cow<'a, str>) -> Box<dyn Error> {
2410 From::from(String::from(err))
2414 #[stable(feature = "box_error", since = "1.8.0")]
2415 impl<T: core::error::Error> core::error::Error for Box<T> {
2416 #[allow(deprecated, deprecated_in_future)]
2417 fn description(&self) -> &str {
2418 core::error::Error::description(&**self)
2421 #[allow(deprecated)]
2422 fn cause(&self) -> Option<&dyn core::error::Error> {
2423 core::error::Error::cause(&**self)
2426 fn source(&self) -> Option<&(dyn core::error::Error + 'static)> {
2427 core::error::Error::source(&**self)