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1// SPDX-License-Identifier: Apache-2.0 OR MIT
2
3//! The `Box<T>` type for heap allocation.
4//!
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.
9//!
10//! # Examples
11//!
12//! Move a value from the stack to the heap by creating a [`Box`]:
13//!
14//! ```
15//! let val: u8 = 5;
16//! let boxed: Box<u8> = Box::new(val);
17//! ```
18//!
19//! Move a value from a [`Box`] back to the stack by [dereferencing]:
20//!
21//! ```
22//! let boxed: Box<u8> = Box::new(5);
23//! let val: u8 = *boxed;
24//! ```
25//!
26//! Creating a recursive data structure:
27//!
28//! ```
29//! #[derive(Debug)]
30//! enum List<T> {
31//! Cons(T, Box<List<T>>),
32//! Nil,
33//! }
34//!
35//! let list: List<i32> = List::Cons(1, Box::new(List::Cons(2, Box::new(List::Nil))));
36//! println!("{list:?}");
37//! ```
38//!
39//! This will print `Cons(1, Cons(2, Nil))`.
40//!
41//! Recursive structures must be boxed, because if the definition of `Cons`
42//! looked like this:
43//!
44//! ```compile_fail,E0072
45//! # enum List<T> {
46//! Cons(T, List<T>),
47//! # }
48//! ```
49//!
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.
54//!
55//! # Memory layout
56//!
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)`].
66//!
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`].
73//!
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`
81//! value:
82//!
83//! ```c
84//! /* C header */
85//!
86//! /* Returns ownership to the caller */
87//! struct Foo* foo_new(void);
88//!
89//! /* Takes ownership from the caller; no-op when invoked with null */
90//! void foo_delete(struct Foo*);
91//! ```
92//!
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>`
97//! cannot be null.
98//!
99//! ```
100//! #[repr(C)]
101//! pub struct Foo;
102//!
103//! #[no_mangle]
104//! pub extern "C" fn foo_new() -> Box<Foo> {
105//! Box::new(Foo)
106//! }
107//!
108//! #[no_mangle]
109//! pub extern "C" fn foo_delete(_: Option<Box<Foo>>) {}
110//! ```
111//!
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
118//! allocator.
119//!
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].
126//!
127//! # Considerations for unsafe code
128//!
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.**
132//!
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].
138//!
139//!
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
148
149#![stable(feature = "rust1", since = "1.0.0")]
150
151use core::any::Any;
152use core::async_iter::AsyncIterator;
153use core::borrow;
154use core::cmp::Ordering;
155use core::error::Error;
156use core::fmt;
157use core::future::Future;
158use core::hash::{Hash, Hasher};
159use core::iter::FusedIterator;
160use core::marker::Tuple;
161use core::marker::Unsize;
162use core::mem::{self, SizedTypeProperties};
163use core::ops::{
164 CoerceUnsized, Deref, DerefMut, DispatchFromDyn, Generator, GeneratorState, Receiver,
165};
166use core::pin::Pin;
167use core::ptr::{self, NonNull, Unique};
168use core::task::{Context, Poll};
169
170#[cfg(not(no_global_oom_handling))]
171use crate::alloc::{handle_alloc_error, WriteCloneIntoRaw};
172use crate::alloc::{AllocError, Allocator, Global, Layout};
173#[cfg(not(no_global_oom_handling))]
174use crate::borrow::Cow;
175use crate::raw_vec::RawVec;
176#[cfg(not(no_global_oom_handling))]
177use crate::str::from_boxed_utf8_unchecked;
178#[cfg(not(no_global_oom_handling))]
179use crate::string::String;
180#[cfg(not(no_global_oom_handling))]
181use crate::vec::Vec;
182
183#[cfg(not(no_thin))]
184#[unstable(feature = "thin_box", issue = "92791")]
185pub use thin::ThinBox;
186
187#[cfg(not(no_thin))]
188mod thin;
189
190/// A pointer type that uniquely owns a heap allocation of type `T`.
191///
192/// See the [module-level documentation](../../std/boxed/index.html) for more.
193#[lang = "owned_box"]
194#[fundamental]
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.
199pub struct Box<
200 T: ?Sized,
201 #[unstable(feature = "allocator_api", issue = "32838")] A: Allocator = Global,
202>(Unique<T>, A);
203
204impl<T> Box<T> {
205 /// Allocates memory on the heap and then places `x` into it.
206 ///
207 /// This doesn't actually allocate if `T` is zero-sized.
208 ///
209 /// # Examples
210 ///
211 /// ```
212 /// let five = Box::new(5);
213 /// ```
214 #[cfg(all(not(no_global_oom_handling)))]
215 #[inline(always)]
216 #[stable(feature = "rust1", since = "1.0.0")]
217 #[must_use]
218 #[rustc_diagnostic_item = "box_new"]
219 pub fn new(x: T) -> Self {
220 #[rustc_box]
221 Box::new(x)
222 }
223
224 /// Constructs a new box with uninitialized contents.
225 ///
226 /// # Examples
227 ///
228 /// ```
229 /// #![feature(new_uninit)]
230 ///
231 /// let mut five = Box::<u32>::new_uninit();
232 ///
233 /// let five = unsafe {
234 /// // Deferred initialization:
235 /// five.as_mut_ptr().write(5);
236 ///
237 /// five.assume_init()
238 /// };
239 ///
240 /// assert_eq!(*five, 5)
241 /// ```
242 #[cfg(not(no_global_oom_handling))]
243 #[unstable(feature = "new_uninit", issue = "63291")]
244 #[must_use]
245 #[inline]
246 pub fn new_uninit() -> Box<mem::MaybeUninit<T>> {
247 Self::new_uninit_in(Global)
248 }
249
250 /// Constructs a new `Box` with uninitialized contents, with the memory
251 /// being filled with `0` bytes.
252 ///
253 /// See [`MaybeUninit::zeroed`][zeroed] for examples of correct and incorrect usage
254 /// of this method.
255 ///
256 /// # Examples
257 ///
258 /// ```
259 /// #![feature(new_uninit)]
260 ///
261 /// let zero = Box::<u32>::new_zeroed();
262 /// let zero = unsafe { zero.assume_init() };
263 ///
264 /// assert_eq!(*zero, 0)
265 /// ```
266 ///
267 /// [zeroed]: mem::MaybeUninit::zeroed
268 #[cfg(not(no_global_oom_handling))]
269 #[inline]
270 #[unstable(feature = "new_uninit", issue = "63291")]
271 #[must_use]
272 pub fn new_zeroed() -> Box<mem::MaybeUninit<T>> {
273 Self::new_zeroed_in(Global)
274 }
275
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.
278 ///
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")]
285 #[must_use]
286 #[inline(always)]
287 pub fn pin(x: T) -> Pin<Box<T>> {
288 Box::new(x).into()
289 }
290
291 /// Allocates memory on the heap then places `x` into it,
292 /// returning an error if the allocation fails
293 ///
294 /// This doesn't actually allocate if `T` is zero-sized.
295 ///
296 /// # Examples
297 ///
298 /// ```
299 /// #![feature(allocator_api)]
300 ///
301 /// let five = Box::try_new(5)?;
302 /// # Ok::<(), std::alloc::AllocError>(())
303 /// ```
304 #[unstable(feature = "allocator_api", issue = "32838")]
305 #[inline]
306 pub fn try_new(x: T) -> Result<Self, AllocError> {
307 Self::try_new_in(x, Global)
308 }
309
310 /// Constructs a new box with uninitialized contents on the heap,
311 /// returning an error if the allocation fails
312 ///
313 /// # Examples
314 ///
315 /// ```
316 /// #![feature(allocator_api, new_uninit)]
317 ///
318 /// let mut five = Box::<u32>::try_new_uninit()?;
319 ///
320 /// let five = unsafe {
321 /// // Deferred initialization:
322 /// five.as_mut_ptr().write(5);
323 ///
324 /// five.assume_init()
325 /// };
326 ///
327 /// assert_eq!(*five, 5);
328 /// # Ok::<(), std::alloc::AllocError>(())
329 /// ```
330 #[unstable(feature = "allocator_api", issue = "32838")]
331 // #[unstable(feature = "new_uninit", issue = "63291")]
332 #[inline]
333 pub fn try_new_uninit() -> Result<Box<mem::MaybeUninit<T>>, AllocError> {
334 Box::try_new_uninit_in(Global)
335 }
336
337 /// Constructs a new `Box` with uninitialized contents, with the memory
338 /// being filled with `0` bytes on the heap
339 ///
340 /// See [`MaybeUninit::zeroed`][zeroed] for examples of correct and incorrect usage
341 /// of this method.
342 ///
343 /// # Examples
344 ///
345 /// ```
346 /// #![feature(allocator_api, new_uninit)]
347 ///
348 /// let zero = Box::<u32>::try_new_zeroed()?;
349 /// let zero = unsafe { zero.assume_init() };
350 ///
351 /// assert_eq!(*zero, 0);
352 /// # Ok::<(), std::alloc::AllocError>(())
353 /// ```
354 ///
355 /// [zeroed]: mem::MaybeUninit::zeroed
356 #[unstable(feature = "allocator_api", issue = "32838")]
357 // #[unstable(feature = "new_uninit", issue = "63291")]
358 #[inline]
359 pub fn try_new_zeroed() -> Result<Box<mem::MaybeUninit<T>>, AllocError> {
360 Box::try_new_zeroed_in(Global)
361 }
362}
363
364impl<T, A: Allocator> Box<T, A> {
365 /// Allocates memory in the given allocator then places `x` into it.
366 ///
367 /// This doesn't actually allocate if `T` is zero-sized.
368 ///
369 /// # Examples
370 ///
371 /// ```
372 /// #![feature(allocator_api)]
373 ///
374 /// use std::alloc::System;
375 ///
376 /// let five = Box::new_in(5, System);
377 /// ```
378 #[cfg(not(no_global_oom_handling))]
379 #[unstable(feature = "allocator_api", issue = "32838")]
380 #[must_use]
381 #[inline]
382 pub fn new_in(x: T, alloc: A) -> Self
383 where
384 A: Allocator,
385 {
386 let mut boxed = Self::new_uninit_in(alloc);
387 unsafe {
388 boxed.as_mut_ptr().write(x);
389 boxed.assume_init()
390 }
391 }
392
393 /// Allocates memory in the given allocator then places `x` into it,
394 /// returning an error if the allocation fails
395 ///
396 /// This doesn't actually allocate if `T` is zero-sized.
397 ///
398 /// # Examples
399 ///
400 /// ```
401 /// #![feature(allocator_api)]
402 ///
403 /// use std::alloc::System;
404 ///
405 /// let five = Box::try_new_in(5, System)?;
406 /// # Ok::<(), std::alloc::AllocError>(())
407 /// ```
408 #[unstable(feature = "allocator_api", issue = "32838")]
409 #[inline]
410 pub fn try_new_in(x: T, alloc: A) -> Result<Self, AllocError>
411 where
412 A: Allocator,
413 {
414 let mut boxed = Self::try_new_uninit_in(alloc)?;
415 unsafe {
416 boxed.as_mut_ptr().write(x);
417 Ok(boxed.assume_init())
418 }
419 }
420
421 /// Constructs a new box with uninitialized contents in the provided allocator.
422 ///
423 /// # Examples
424 ///
425 /// ```
426 /// #![feature(allocator_api, new_uninit)]
427 ///
428 /// use std::alloc::System;
429 ///
430 /// let mut five = Box::<u32, _>::new_uninit_in(System);
431 ///
432 /// let five = unsafe {
433 /// // Deferred initialization:
434 /// five.as_mut_ptr().write(5);
435 ///
436 /// five.assume_init()
437 /// };
438 ///
439 /// assert_eq!(*five, 5)
440 /// ```
441 #[unstable(feature = "allocator_api", issue = "32838")]
442 #[cfg(not(no_global_oom_handling))]
443 #[must_use]
444 // #[unstable(feature = "new_uninit", issue = "63291")]
445 pub fn new_uninit_in(alloc: A) -> Box<mem::MaybeUninit<T>, A>
446 where
447 A: Allocator,
448 {
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) {
453 Ok(m) => m,
454 Err(_) => handle_alloc_error(layout),
455 }
456 }
457
458 /// Constructs a new box with uninitialized contents in the provided allocator,
459 /// returning an error if the allocation fails
460 ///
461 /// # Examples
462 ///
463 /// ```
464 /// #![feature(allocator_api, new_uninit)]
465 ///
466 /// use std::alloc::System;
467 ///
468 /// let mut five = Box::<u32, _>::try_new_uninit_in(System)?;
469 ///
470 /// let five = unsafe {
471 /// // Deferred initialization:
472 /// five.as_mut_ptr().write(5);
473 ///
474 /// five.assume_init()
475 /// };
476 ///
477 /// assert_eq!(*five, 5);
478 /// # Ok::<(), std::alloc::AllocError>(())
479 /// ```
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>
483 where
484 A: Allocator,
485 {
486 let ptr = if T::IS_ZST {
487 NonNull::dangling()
488 } else {
489 let layout = Layout::new::<mem::MaybeUninit<T>>();
490 alloc.allocate(layout)?.cast()
491 };
492 unsafe { Ok(Box::from_raw_in(ptr.as_ptr(), alloc)) }
493 }
494
495 /// Constructs a new `Box` with uninitialized contents, with the memory
496 /// being filled with `0` bytes in the provided allocator.
497 ///
498 /// See [`MaybeUninit::zeroed`][zeroed] for examples of correct and incorrect usage
499 /// of this method.
500 ///
501 /// # Examples
502 ///
503 /// ```
504 /// #![feature(allocator_api, new_uninit)]
505 ///
506 /// use std::alloc::System;
507 ///
508 /// let zero = Box::<u32, _>::new_zeroed_in(System);
509 /// let zero = unsafe { zero.assume_init() };
510 ///
511 /// assert_eq!(*zero, 0)
512 /// ```
513 ///
514 /// [zeroed]: mem::MaybeUninit::zeroed
515 #[unstable(feature = "allocator_api", issue = "32838")]
516 #[cfg(not(no_global_oom_handling))]
517 // #[unstable(feature = "new_uninit", issue = "63291")]
518 #[must_use]
519 pub fn new_zeroed_in(alloc: A) -> Box<mem::MaybeUninit<T>, A>
520 where
521 A: Allocator,
522 {
523 let layout = Layout::new::<mem::MaybeUninit<T>>();
524 // NOTE: Prefer match over unwrap_or_else since closure sometimes not inlineable.
525 // That would make code size bigger.
526 match Box::try_new_zeroed_in(alloc) {
527 Ok(m) => m,
528 Err(_) => handle_alloc_error(layout),
529 }
530 }
531
532 /// Constructs a new `Box` with uninitialized contents, with the memory
533 /// being filled with `0` bytes in the provided allocator,
534 /// returning an error if the allocation fails,
535 ///
536 /// See [`MaybeUninit::zeroed`][zeroed] for examples of correct and incorrect usage
537 /// of this method.
538 ///
539 /// # Examples
540 ///
541 /// ```
542 /// #![feature(allocator_api, new_uninit)]
543 ///
544 /// use std::alloc::System;
545 ///
546 /// let zero = Box::<u32, _>::try_new_zeroed_in(System)?;
547 /// let zero = unsafe { zero.assume_init() };
548 ///
549 /// assert_eq!(*zero, 0);
550 /// # Ok::<(), std::alloc::AllocError>(())
551 /// ```
552 ///
553 /// [zeroed]: mem::MaybeUninit::zeroed
554 #[unstable(feature = "allocator_api", issue = "32838")]
555 // #[unstable(feature = "new_uninit", issue = "63291")]
556 pub fn try_new_zeroed_in(alloc: A) -> Result<Box<mem::MaybeUninit<T>, A>, AllocError>
557 where
558 A: Allocator,
559 {
560 let ptr = if T::IS_ZST {
561 NonNull::dangling()
562 } else {
563 let layout = Layout::new::<mem::MaybeUninit<T>>();
564 alloc.allocate_zeroed(layout)?.cast()
565 };
566 unsafe { Ok(Box::from_raw_in(ptr.as_ptr(), alloc)) }
567 }
568
569 /// Constructs a new `Pin<Box<T, A>>`. If `T` does not implement [`Unpin`], then
570 /// `x` will be pinned in memory and unable to be moved.
571 ///
572 /// Constructing and pinning of the `Box` can also be done in two steps: `Box::pin_in(x, alloc)`
573 /// does the same as <code>[Box::into_pin]\([Box::new_in]\(x, alloc))</code>. Consider using
574 /// [`into_pin`](Box::into_pin) if you already have a `Box<T, A>`, or if you want to
575 /// construct a (pinned) `Box` in a different way than with [`Box::new_in`].
576 #[cfg(not(no_global_oom_handling))]
577 #[unstable(feature = "allocator_api", issue = "32838")]
578 #[must_use]
579 #[inline(always)]
580 pub fn pin_in(x: T, alloc: A) -> Pin<Self>
581 where
582 A: 'static + Allocator,
583 {
584 Self::into_pin(Self::new_in(x, alloc))
585 }
586
587 /// Converts a `Box<T>` into a `Box<[T]>`
588 ///
589 /// This conversion does not allocate on the heap and happens in place.
590 #[unstable(feature = "box_into_boxed_slice", issue = "71582")]
591 pub fn into_boxed_slice(boxed: Self) -> Box<[T], A> {
592 let (raw, alloc) = Box::into_raw_with_allocator(boxed);
593 unsafe { Box::from_raw_in(raw as *mut [T; 1], alloc) }
594 }
595
596 /// Consumes the `Box`, returning the wrapped value.
597 ///
598 /// # Examples
599 ///
600 /// ```
601 /// #![feature(box_into_inner)]
602 ///
603 /// let c = Box::new(5);
604 ///
605 /// assert_eq!(Box::into_inner(c), 5);
606 /// ```
607 #[unstable(feature = "box_into_inner", issue = "80437")]
608 #[inline]
609 pub fn into_inner(boxed: Self) -> T {
610 *boxed
611 }
612}
613
614impl<T> Box<[T]> {
615 /// Constructs a new boxed slice with uninitialized contents.
616 ///
617 /// # Examples
618 ///
619 /// ```
620 /// #![feature(new_uninit)]
621 ///
622 /// let mut values = Box::<[u32]>::new_uninit_slice(3);
623 ///
624 /// let values = unsafe {
625 /// // Deferred initialization:
626 /// values[0].as_mut_ptr().write(1);
627 /// values[1].as_mut_ptr().write(2);
628 /// values[2].as_mut_ptr().write(3);
629 ///
630 /// values.assume_init()
631 /// };
632 ///
633 /// assert_eq!(*values, [1, 2, 3])
634 /// ```
635 #[cfg(not(no_global_oom_handling))]
636 #[unstable(feature = "new_uninit", issue = "63291")]
637 #[must_use]
638 pub fn new_uninit_slice(len: usize) -> Box<[mem::MaybeUninit<T>]> {
639 unsafe { RawVec::with_capacity(len).into_box(len) }
640 }
641
642 /// Constructs a new boxed slice with uninitialized contents, with the memory
643 /// being filled with `0` bytes.
644 ///
645 /// See [`MaybeUninit::zeroed`][zeroed] for examples of correct and incorrect usage
646 /// of this method.
647 ///
648 /// # Examples
649 ///
650 /// ```
651 /// #![feature(new_uninit)]
652 ///
653 /// let values = Box::<[u32]>::new_zeroed_slice(3);
654 /// let values = unsafe { values.assume_init() };
655 ///
656 /// assert_eq!(*values, [0, 0, 0])
657 /// ```
658 ///
659 /// [zeroed]: mem::MaybeUninit::zeroed
660 #[cfg(not(no_global_oom_handling))]
661 #[unstable(feature = "new_uninit", issue = "63291")]
662 #[must_use]
663 pub fn new_zeroed_slice(len: usize) -> Box<[mem::MaybeUninit<T>]> {
664 unsafe { RawVec::with_capacity_zeroed(len).into_box(len) }
665 }
666
667 /// Constructs a new boxed slice with uninitialized contents. Returns an error if
668 /// the allocation fails
669 ///
670 /// # Examples
671 ///
672 /// ```
673 /// #![feature(allocator_api, new_uninit)]
674 ///
675 /// let mut values = Box::<[u32]>::try_new_uninit_slice(3)?;
676 /// let values = unsafe {
677 /// // Deferred initialization:
678 /// values[0].as_mut_ptr().write(1);
679 /// values[1].as_mut_ptr().write(2);
680 /// values[2].as_mut_ptr().write(3);
681 /// values.assume_init()
682 /// };
683 ///
684 /// assert_eq!(*values, [1, 2, 3]);
685 /// # Ok::<(), std::alloc::AllocError>(())
686 /// ```
687 #[unstable(feature = "allocator_api", issue = "32838")]
688 #[inline]
689 pub fn try_new_uninit_slice(len: usize) -> Result<Box<[mem::MaybeUninit<T>]>, AllocError> {
690 let ptr = if T::IS_ZST || len == 0 {
691 NonNull::dangling()
692 } else {
693 let layout = match Layout::array::<mem::MaybeUninit<T>>(len) {
694 Ok(l) => l,
695 Err(_) => return Err(AllocError),
696 };
697 Global.allocate(layout)?.cast()
698 };
699 unsafe { Ok(RawVec::from_raw_parts_in(ptr.as_ptr(), len, Global).into_box(len)) }
700 }
701
702 /// Constructs a new boxed slice with uninitialized contents, with the memory
703 /// being filled with `0` bytes. Returns an error if the allocation fails
704 ///
705 /// See [`MaybeUninit::zeroed`][zeroed] for examples of correct and incorrect usage
706 /// of this method.
707 ///
708 /// # Examples
709 ///
710 /// ```
711 /// #![feature(allocator_api, new_uninit)]
712 ///
713 /// let values = Box::<[u32]>::try_new_zeroed_slice(3)?;
714 /// let values = unsafe { values.assume_init() };
715 ///
716 /// assert_eq!(*values, [0, 0, 0]);
717 /// # Ok::<(), std::alloc::AllocError>(())
718 /// ```
719 ///
720 /// [zeroed]: mem::MaybeUninit::zeroed
721 #[unstable(feature = "allocator_api", issue = "32838")]
722 #[inline]
723 pub fn try_new_zeroed_slice(len: usize) -> Result<Box<[mem::MaybeUninit<T>]>, AllocError> {
724 let ptr = if T::IS_ZST || len == 0 {
725 NonNull::dangling()
726 } else {
727 let layout = match Layout::array::<mem::MaybeUninit<T>>(len) {
728 Ok(l) => l,
729 Err(_) => return Err(AllocError),
730 };
731 Global.allocate_zeroed(layout)?.cast()
732 };
733 unsafe { Ok(RawVec::from_raw_parts_in(ptr.as_ptr(), len, Global).into_box(len)) }
734 }
735}
736
737impl<T, A: Allocator> Box<[T], A> {
738 /// Constructs a new boxed slice with uninitialized contents in the provided allocator.
739 ///
740 /// # Examples
741 ///
742 /// ```
743 /// #![feature(allocator_api, new_uninit)]
744 ///
745 /// use std::alloc::System;
746 ///
747 /// let mut values = Box::<[u32], _>::new_uninit_slice_in(3, System);
748 ///
749 /// let values = unsafe {
750 /// // Deferred initialization:
751 /// values[0].as_mut_ptr().write(1);
752 /// values[1].as_mut_ptr().write(2);
753 /// values[2].as_mut_ptr().write(3);
754 ///
755 /// values.assume_init()
756 /// };
757 ///
758 /// assert_eq!(*values, [1, 2, 3])
759 /// ```
760 #[cfg(not(no_global_oom_handling))]
761 #[unstable(feature = "allocator_api", issue = "32838")]
762 // #[unstable(feature = "new_uninit", issue = "63291")]
763 #[must_use]
764 pub fn new_uninit_slice_in(len: usize, alloc: A) -> Box<[mem::MaybeUninit<T>], A> {
765 unsafe { RawVec::with_capacity_in(len, alloc).into_box(len) }
766 }
767
768 /// Constructs a new boxed slice with uninitialized contents in the provided allocator,
769 /// with the memory being filled with `0` bytes.
770 ///
771 /// See [`MaybeUninit::zeroed`][zeroed] for examples of correct and incorrect usage
772 /// of this method.
773 ///
774 /// # Examples
775 ///
776 /// ```
777 /// #![feature(allocator_api, new_uninit)]
778 ///
779 /// use std::alloc::System;
780 ///
781 /// let values = Box::<[u32], _>::new_zeroed_slice_in(3, System);
782 /// let values = unsafe { values.assume_init() };
783 ///
784 /// assert_eq!(*values, [0, 0, 0])
785 /// ```
786 ///
787 /// [zeroed]: mem::MaybeUninit::zeroed
788 #[cfg(not(no_global_oom_handling))]
789 #[unstable(feature = "allocator_api", issue = "32838")]
790 // #[unstable(feature = "new_uninit", issue = "63291")]
791 #[must_use]
792 pub fn new_zeroed_slice_in(len: usize, alloc: A) -> Box<[mem::MaybeUninit<T>], A> {
793 unsafe { RawVec::with_capacity_zeroed_in(len, alloc).into_box(len) }
794 }
795}
796
797impl<T, A: Allocator> Box<mem::MaybeUninit<T>, A> {
798 /// Converts to `Box<T, A>`.
799 ///
800 /// # Safety
801 ///
802 /// As with [`MaybeUninit::assume_init`],
803 /// it is up to the caller to guarantee that the value
804 /// really is in an initialized state.
805 /// Calling this when the content is not yet fully initialized
806 /// causes immediate undefined behavior.
807 ///
808 /// [`MaybeUninit::assume_init`]: mem::MaybeUninit::assume_init
809 ///
810 /// # Examples
811 ///
812 /// ```
813 /// #![feature(new_uninit)]
814 ///
815 /// let mut five = Box::<u32>::new_uninit();
816 ///
817 /// let five: Box<u32> = unsafe {
818 /// // Deferred initialization:
819 /// five.as_mut_ptr().write(5);
820 ///
821 /// five.assume_init()
822 /// };
823 ///
824 /// assert_eq!(*five, 5)
825 /// ```
826 #[unstable(feature = "new_uninit", issue = "63291")]
827 #[inline]
828 pub unsafe fn assume_init(self) -> Box<T, A> {
829 let (raw, alloc) = Box::into_raw_with_allocator(self);
830 unsafe { Box::from_raw_in(raw as *mut T, alloc) }
831 }
832
833 /// Writes the value and converts to `Box<T, A>`.
834 ///
835 /// This method converts the box similarly to [`Box::assume_init`] but
836 /// writes `value` into it before conversion thus guaranteeing safety.
837 /// In some scenarios use of this method may improve performance because
838 /// the compiler may be able to optimize copying from stack.
839 ///
840 /// # Examples
841 ///
842 /// ```
843 /// #![feature(new_uninit)]
844 ///
845 /// let big_box = Box::<[usize; 1024]>::new_uninit();
846 ///
847 /// let mut array = [0; 1024];
848 /// for (i, place) in array.iter_mut().enumerate() {
849 /// *place = i;
850 /// }
851 ///
852 /// // The optimizer may be able to elide this copy, so previous code writes
853 /// // to heap directly.
854 /// let big_box = Box::write(big_box, array);
855 ///
856 /// for (i, x) in big_box.iter().enumerate() {
857 /// assert_eq!(*x, i);
858 /// }
859 /// ```
860 #[unstable(feature = "new_uninit", issue = "63291")]
861 #[inline]
862 pub fn write(mut boxed: Self, value: T) -> Box<T, A> {
863 unsafe {
864 (*boxed).write(value);
865 boxed.assume_init()
866 }
867 }
868}
869
870impl<T, A: Allocator> Box<[mem::MaybeUninit<T>], A> {
871 /// Converts to `Box<[T], A>`.
872 ///
873 /// # Safety
874 ///
875 /// As with [`MaybeUninit::assume_init`],
876 /// it is up to the caller to guarantee that the values
877 /// really are in an initialized state.
878 /// Calling this when the content is not yet fully initialized
879 /// causes immediate undefined behavior.
880 ///
881 /// [`MaybeUninit::assume_init`]: mem::MaybeUninit::assume_init
882 ///
883 /// # Examples
884 ///
885 /// ```
886 /// #![feature(new_uninit)]
887 ///
888 /// let mut values = Box::<[u32]>::new_uninit_slice(3);
889 ///
890 /// let values = unsafe {
891 /// // Deferred initialization:
892 /// values[0].as_mut_ptr().write(1);
893 /// values[1].as_mut_ptr().write(2);
894 /// values[2].as_mut_ptr().write(3);
895 ///
896 /// values.assume_init()
897 /// };
898 ///
899 /// assert_eq!(*values, [1, 2, 3])
900 /// ```
901 #[unstable(feature = "new_uninit", issue = "63291")]
902 #[inline]
903 pub unsafe fn assume_init(self) -> Box<[T], A> {
904 let (raw, alloc) = Box::into_raw_with_allocator(self);
905 unsafe { Box::from_raw_in(raw as *mut [T], alloc) }
906 }
907}
908
909impl<T: ?Sized> Box<T> {
910 /// Constructs a box from a raw pointer.
911 ///
912 /// After calling this function, the raw pointer is owned by the
913 /// resulting `Box`. Specifically, the `Box` destructor will call
914 /// the destructor of `T` and free the allocated memory. For this
915 /// to be safe, the memory must have been allocated in accordance
916 /// with the [memory layout] used by `Box` .
917 ///
918 /// # Safety
919 ///
920 /// This function is unsafe because improper use may lead to
921 /// memory problems. For example, a double-free may occur if the
922 /// function is called twice on the same raw pointer.
923 ///
924 /// The safety conditions are described in the [memory layout] section.
925 ///
926 /// # Examples
927 ///
928 /// Recreate a `Box` which was previously converted to a raw pointer
929 /// using [`Box::into_raw`]:
930 /// ```
931 /// let x = Box::new(5);
932 /// let ptr = Box::into_raw(x);
933 /// let x = unsafe { Box::from_raw(ptr) };
934 /// ```
935 /// Manually create a `Box` from scratch by using the global allocator:
936 /// ```
937 /// use std::alloc::{alloc, Layout};
938 ///
939 /// unsafe {
940 /// let ptr = alloc(Layout::new::<i32>()) as *mut i32;
941 /// // In general .write is required to avoid attempting to destruct
942 /// // the (uninitialized) previous contents of `ptr`, though for this
943 /// // simple example `*ptr = 5` would have worked as well.
944 /// ptr.write(5);
945 /// let x = Box::from_raw(ptr);
946 /// }
947 /// ```
948 ///
949 /// [memory layout]: self#memory-layout
950 /// [`Layout`]: crate::Layout
951 #[stable(feature = "box_raw", since = "1.4.0")]
952 #[inline]
953 #[must_use = "call `drop(Box::from_raw(ptr))` if you intend to drop the `Box`"]
954 pub unsafe fn from_raw(raw: *mut T) -> Self {
955 unsafe { Self::from_raw_in(raw, Global) }
956 }
957}
958
959impl<T: ?Sized, A: Allocator> Box<T, A> {
960 /// Constructs a box from a raw pointer in the given allocator.
961 ///
962 /// After calling this function, the raw pointer is owned by the
963 /// resulting `Box`. Specifically, the `Box` destructor will call
964 /// the destructor of `T` and free the allocated memory. For this
965 /// to be safe, the memory must have been allocated in accordance
966 /// with the [memory layout] used by `Box` .
967 ///
968 /// # Safety
969 ///
970 /// This function is unsafe because improper use may lead to
971 /// memory problems. For example, a double-free may occur if the
972 /// function is called twice on the same raw pointer.
973 ///
974 ///
975 /// # Examples
976 ///
977 /// Recreate a `Box` which was previously converted to a raw pointer
978 /// using [`Box::into_raw_with_allocator`]:
979 /// ```
980 /// #![feature(allocator_api)]
981 ///
982 /// use std::alloc::System;
983 ///
984 /// let x = Box::new_in(5, System);
985 /// let (ptr, alloc) = Box::into_raw_with_allocator(x);
986 /// let x = unsafe { Box::from_raw_in(ptr, alloc) };
987 /// ```
988 /// Manually create a `Box` from scratch by using the system allocator:
989 /// ```
990 /// #![feature(allocator_api, slice_ptr_get)]
991 ///
992 /// use std::alloc::{Allocator, Layout, System};
993 ///
994 /// unsafe {
995 /// let ptr = System.allocate(Layout::new::<i32>())?.as_mut_ptr() as *mut i32;
996 /// // In general .write is required to avoid attempting to destruct
997 /// // the (uninitialized) previous contents of `ptr`, though for this
998 /// // simple example `*ptr = 5` would have worked as well.
999 /// ptr.write(5);
1000 /// let x = Box::from_raw_in(ptr, System);
1001 /// }
1002 /// # Ok::<(), std::alloc::AllocError>(())
1003 /// ```
1004 ///
1005 /// [memory layout]: self#memory-layout
1006 /// [`Layout`]: crate::Layout
1007 #[unstable(feature = "allocator_api", issue = "32838")]
1008 #[rustc_const_unstable(feature = "const_box", issue = "92521")]
1009 #[inline]
1010 pub const unsafe fn from_raw_in(raw: *mut T, alloc: A) -> Self {
1011 Box(unsafe { Unique::new_unchecked(raw) }, alloc)
1012 }
1013
1014 /// Consumes the `Box`, returning a wrapped raw pointer.
1015 ///
1016 /// The pointer will be properly aligned and non-null.
1017 ///
1018 /// After calling this function, the caller is responsible for the
1019 /// memory previously managed by the `Box`. In particular, the
1020 /// caller should properly destroy `T` and release the memory, taking
1021 /// into account the [memory layout] used by `Box`. The easiest way to
1022 /// do this is to convert the raw pointer back into a `Box` with the
1023 /// [`Box::from_raw`] function, allowing the `Box` destructor to perform
1024 /// the cleanup.
1025 ///
1026 /// Note: this is an associated function, which means that you have
1027 /// to call it as `Box::into_raw(b)` instead of `b.into_raw()`. This
1028 /// is so that there is no conflict with a method on the inner type.
1029 ///
1030 /// # Examples
1031 /// Converting the raw pointer back into a `Box` with [`Box::from_raw`]
1032 /// for automatic cleanup:
1033 /// ```
1034 /// let x = Box::new(String::from("Hello"));
1035 /// let ptr = Box::into_raw(x);
1036 /// let x = unsafe { Box::from_raw(ptr) };
1037 /// ```
1038 /// Manual cleanup by explicitly running the destructor and deallocating
1039 /// the memory:
1040 /// ```
1041 /// use std::alloc::{dealloc, Layout};
1042 /// use std::ptr;
1043 ///
1044 /// let x = Box::new(String::from("Hello"));
1045 /// let p = Box::into_raw(x);
1046 /// unsafe {
1047 /// ptr::drop_in_place(p);
1048 /// dealloc(p as *mut u8, Layout::new::<String>());
1049 /// }
1050 /// ```
1051 ///
1052 /// [memory layout]: self#memory-layout
1053 #[stable(feature = "box_raw", since = "1.4.0")]
1054 #[inline]
1055 pub fn into_raw(b: Self) -> *mut T {
1056 Self::into_raw_with_allocator(b).0
1057 }
1058
1059 /// Consumes the `Box`, returning a wrapped raw pointer and the allocator.
1060 ///
1061 /// The pointer will be properly aligned and non-null.
1062 ///
1063 /// After calling this function, the caller is responsible for the
1064 /// memory previously managed by the `Box`. In particular, the
1065 /// caller should properly destroy `T` and release the memory, taking
1066 /// into account the [memory layout] used by `Box`. The easiest way to
1067 /// do this is to convert the raw pointer back into a `Box` with the
1068 /// [`Box::from_raw_in`] function, allowing the `Box` destructor to perform
1069 /// the cleanup.
1070 ///
1071 /// Note: this is an associated function, which means that you have
1072 /// to call it as `Box::into_raw_with_allocator(b)` instead of `b.into_raw_with_allocator()`. This
1073 /// is so that there is no conflict with a method on the inner type.
1074 ///
1075 /// # Examples
1076 /// Converting the raw pointer back into a `Box` with [`Box::from_raw_in`]
1077 /// for automatic cleanup:
1078 /// ```
1079 /// #![feature(allocator_api)]
1080 ///
1081 /// use std::alloc::System;
1082 ///
1083 /// let x = Box::new_in(String::from("Hello"), System);
1084 /// let (ptr, alloc) = Box::into_raw_with_allocator(x);
1085 /// let x = unsafe { Box::from_raw_in(ptr, alloc) };
1086 /// ```
1087 /// Manual cleanup by explicitly running the destructor and deallocating
1088 /// the memory:
1089 /// ```
1090 /// #![feature(allocator_api)]
1091 ///
1092 /// use std::alloc::{Allocator, Layout, System};
1093 /// use std::ptr::{self, NonNull};
1094 ///
1095 /// let x = Box::new_in(String::from("Hello"), System);
1096 /// let (ptr, alloc) = Box::into_raw_with_allocator(x);
1097 /// unsafe {
1098 /// ptr::drop_in_place(ptr);
1099 /// let non_null = NonNull::new_unchecked(ptr);
1100 /// alloc.deallocate(non_null.cast(), Layout::new::<String>());
1101 /// }
1102 /// ```
1103 ///
1104 /// [memory layout]: self#memory-layout
1105 #[unstable(feature = "allocator_api", issue = "32838")]
1106 #[inline]
1107 pub fn into_raw_with_allocator(b: Self) -> (*mut T, A) {
1108 let (leaked, alloc) = Box::into_unique(b);
1109 (leaked.as_ptr(), alloc)
1110 }
1111
1112 #[unstable(
1113 feature = "ptr_internals",
1114 issue = "none",
1115 reason = "use `Box::leak(b).into()` or `Unique::from(Box::leak(b))` instead"
1116 )]
1117 #[inline]
1118 #[doc(hidden)]
1119 pub fn into_unique(b: Self) -> (Unique<T>, A) {
1120 // Box is recognized as a "unique pointer" by Stacked Borrows, but internally it is a
1121 // raw pointer for the type system. Turning it directly into a raw pointer would not be
1122 // recognized as "releasing" the unique pointer to permit aliased raw accesses,
1123 // so all raw pointer methods have to go through `Box::leak`. Turning *that* to a raw pointer
1124 // behaves correctly.
1125 let alloc = unsafe { ptr::read(&b.1) };
1126 (Unique::from(Box::leak(b)), alloc)
1127 }
1128
1129 /// Returns a reference to the underlying allocator.
1130 ///
1131 /// Note: this is an associated function, which means that you have
1132 /// to call it as `Box::allocator(&b)` instead of `b.allocator()`. This
1133 /// is so that there is no conflict with a method on the inner type.
1134 #[unstable(feature = "allocator_api", issue = "32838")]
1135 #[rustc_const_unstable(feature = "const_box", issue = "92521")]
1136 #[inline]
1137 pub const fn allocator(b: &Self) -> &A {
1138 &b.1
1139 }
1140
1141 /// Consumes and leaks the `Box`, returning a mutable reference,
1142 /// `&'a mut T`. Note that the type `T` must outlive the chosen lifetime
1143 /// `'a`. If the type has only static references, or none at all, then this
1144 /// may be chosen to be `'static`.
1145 ///
1146 /// This function is mainly useful for data that lives for the remainder of
1147 /// the program's life. Dropping the returned reference will cause a memory
1148 /// leak. If this is not acceptable, the reference should first be wrapped
1149 /// with the [`Box::from_raw`] function producing a `Box`. This `Box` can
1150 /// then be dropped which will properly destroy `T` and release the
1151 /// allocated memory.
1152 ///
1153 /// Note: this is an associated function, which means that you have
1154 /// to call it as `Box::leak(b)` instead of `b.leak()`. This
1155 /// is so that there is no conflict with a method on the inner type.
1156 ///
1157 /// # Examples
1158 ///
1159 /// Simple usage:
1160 ///
1161 /// ```
1162 /// let x = Box::new(41);
1163 /// let static_ref: &'static mut usize = Box::leak(x);
1164 /// *static_ref += 1;
1165 /// assert_eq!(*static_ref, 42);
1166 /// ```
1167 ///
1168 /// Unsized data:
1169 ///
1170 /// ```
1171 /// let x = vec![1, 2, 3].into_boxed_slice();
1172 /// let static_ref = Box::leak(x);
1173 /// static_ref[0] = 4;
1174 /// assert_eq!(*static_ref, [4, 2, 3]);
1175 /// ```
1176 #[stable(feature = "box_leak", since = "1.26.0")]
1177 #[inline]
1178 pub fn leak<'a>(b: Self) -> &'a mut T
1179 where
1180 A: 'a,
1181 {
1182 unsafe { &mut *mem::ManuallyDrop::new(b).0.as_ptr() }
1183 }
1184
1185 /// Converts a `Box<T>` into a `Pin<Box<T>>`. If `T` does not implement [`Unpin`], then
1186 /// `*boxed` will be pinned in memory and unable to be moved.
1187 ///
1188 /// This conversion does not allocate on the heap and happens in place.
1189 ///
1190 /// This is also available via [`From`].
1191 ///
1192 /// Constructing and pinning a `Box` with <code>Box::into_pin([Box::new]\(x))</code>
1193 /// can also be written more concisely using <code>[Box::pin]\(x)</code>.
1194 /// This `into_pin` method is useful if you already have a `Box<T>`, or you are
1195 /// constructing a (pinned) `Box` in a different way than with [`Box::new`].
1196 ///
1197 /// # Notes
1198 ///
1199 /// It's not recommended that crates add an impl like `From<Box<T>> for Pin<T>`,
1200 /// as it'll introduce an ambiguity when calling `Pin::from`.
1201 /// A demonstration of such a poor impl is shown below.
1202 ///
1203 /// ```compile_fail
1204 /// # use std::pin::Pin;
1205 /// struct Foo; // A type defined in this crate.
1206 /// impl From<Box<()>> for Pin<Foo> {
1207 /// fn from(_: Box<()>) -> Pin<Foo> {
1208 /// Pin::new(Foo)
1209 /// }
1210 /// }
1211 ///
1212 /// let foo = Box::new(());
1213 /// let bar = Pin::from(foo);
1214 /// ```
1215 #[stable(feature = "box_into_pin", since = "1.63.0")]
1216 #[rustc_const_unstable(feature = "const_box", issue = "92521")]
1217 pub const fn into_pin(boxed: Self) -> Pin<Self>
1218 where
1219 A: 'static,
1220 {
1221 // It's not possible to move or replace the insides of a `Pin<Box<T>>`
1222 // when `T: !Unpin`, so it's safe to pin it directly without any
1223 // additional requirements.
1224 unsafe { Pin::new_unchecked(boxed) }
1225 }
1226}
1227
1228#[stable(feature = "rust1", since = "1.0.0")]
1229unsafe impl<#[may_dangle] T: ?Sized, A: Allocator> Drop for Box<T, A> {
1230 #[inline]
1231 fn drop(&mut self) {
1232 // the T in the Box is dropped by the compiler before the destructor is run
1233
1234 let ptr = self.0;
1235
1236 unsafe {
1237 let layout = Layout::for_value_raw(ptr.as_ptr());
1238 if layout.size() != 0 {
1239 self.1.deallocate(From::from(ptr.cast()), layout);
1240 }
1241 }
1242 }
1243}
1244
1245#[cfg(not(no_global_oom_handling))]
1246#[stable(feature = "rust1", since = "1.0.0")]
1247impl<T: Default> Default for Box<T> {
1248 /// Creates a `Box<T>`, with the `Default` value for T.
1249 #[inline]
1250 fn default() -> Self {
1251 Box::new(T::default())
1252 }
1253}
1254
1255#[cfg(not(no_global_oom_handling))]
1256#[stable(feature = "rust1", since = "1.0.0")]
1257impl<T> Default for Box<[T]> {
1258 #[inline]
1259 fn default() -> Self {
1260 let ptr: Unique<[T]> = Unique::<[T; 0]>::dangling();
1261 Box(ptr, Global)
1262 }
1263}
1264
1265#[cfg(not(no_global_oom_handling))]
1266#[stable(feature = "default_box_extra", since = "1.17.0")]
1267impl Default for Box<str> {
1268 #[inline]
1269 fn default() -> Self {
1270 // SAFETY: This is the same as `Unique::cast<U>` but with an unsized `U = str`.
1271 let ptr: Unique<str> = unsafe {
1272 let bytes: Unique<[u8]> = Unique::<[u8; 0]>::dangling();
1273 Unique::new_unchecked(bytes.as_ptr() as *mut str)
1274 };
1275 Box(ptr, Global)
1276 }
1277}
1278
1279#[cfg(not(no_global_oom_handling))]
1280#[stable(feature = "rust1", since = "1.0.0")]
1281impl<T: Clone, A: Allocator + Clone> Clone for Box<T, A> {
1282 /// Returns a new box with a `clone()` of this box's contents.
1283 ///
1284 /// # Examples
1285 ///
1286 /// ```
1287 /// let x = Box::new(5);
1288 /// let y = x.clone();
1289 ///
1290 /// // The value is the same
1291 /// assert_eq!(x, y);
1292 ///
1293 /// // But they are unique objects
1294 /// assert_ne!(&*x as *const i32, &*y as *const i32);
1295 /// ```
1296 #[inline]
1297 fn clone(&self) -> Self {
1298 // Pre-allocate memory to allow writing the cloned value directly.
1299 let mut boxed = Self::new_uninit_in(self.1.clone());
1300 unsafe {
1301 (**self).write_clone_into_raw(boxed.as_mut_ptr());
1302 boxed.assume_init()
1303 }
1304 }
1305
1306 /// Copies `source`'s contents into `self` without creating a new allocation.
1307 ///
1308 /// # Examples
1309 ///
1310 /// ```
1311 /// let x = Box::new(5);
1312 /// let mut y = Box::new(10);
1313 /// let yp: *const i32 = &*y;
1314 ///
1315 /// y.clone_from(&x);
1316 ///
1317 /// // The value is the same
1318 /// assert_eq!(x, y);
1319 ///
1320 /// // And no allocation occurred
1321 /// assert_eq!(yp, &*y);
1322 /// ```
1323 #[inline]
1324 fn clone_from(&mut self, source: &Self) {
1325 (**self).clone_from(&(**source));
1326 }
1327}
1328
1329#[cfg(not(no_global_oom_handling))]
1330#[stable(feature = "box_slice_clone", since = "1.3.0")]
1331impl Clone for Box<str> {
1332 fn clone(&self) -> Self {
1333 // this makes a copy of the data
1334 let buf: Box<[u8]> = self.as_bytes().into();
1335 unsafe { from_boxed_utf8_unchecked(buf) }
1336 }
1337}
1338
1339#[stable(feature = "rust1", since = "1.0.0")]
1340impl<T: ?Sized + PartialEq, A: Allocator> PartialEq for Box<T, A> {
1341 #[inline]
1342 fn eq(&self, other: &Self) -> bool {
1343 PartialEq::eq(&**self, &**other)
1344 }
1345 #[inline]
1346 fn ne(&self, other: &Self) -> bool {
1347 PartialEq::ne(&**self, &**other)
1348 }
1349}
1350#[stable(feature = "rust1", since = "1.0.0")]
1351impl<T: ?Sized + PartialOrd, A: Allocator> PartialOrd for Box<T, A> {
1352 #[inline]
1353 fn partial_cmp(&self, other: &Self) -> Option<Ordering> {
1354 PartialOrd::partial_cmp(&**self, &**other)
1355 }
1356 #[inline]
1357 fn lt(&self, other: &Self) -> bool {
1358 PartialOrd::lt(&**self, &**other)
1359 }
1360 #[inline]
1361 fn le(&self, other: &Self) -> bool {
1362 PartialOrd::le(&**self, &**other)
1363 }
1364 #[inline]
1365 fn ge(&self, other: &Self) -> bool {
1366 PartialOrd::ge(&**self, &**other)
1367 }
1368 #[inline]
1369 fn gt(&self, other: &Self) -> bool {
1370 PartialOrd::gt(&**self, &**other)
1371 }
1372}
1373#[stable(feature = "rust1", since = "1.0.0")]
1374impl<T: ?Sized + Ord, A: Allocator> Ord for Box<T, A> {
1375 #[inline]
1376 fn cmp(&self, other: &Self) -> Ordering {
1377 Ord::cmp(&**self, &**other)
1378 }
1379}
1380#[stable(feature = "rust1", since = "1.0.0")]
1381impl<T: ?Sized + Eq, A: Allocator> Eq for Box<T, A> {}
1382
1383#[stable(feature = "rust1", since = "1.0.0")]
1384impl<T: ?Sized + Hash, A: Allocator> Hash for Box<T, A> {
1385 fn hash<H: Hasher>(&self, state: &mut H) {
1386 (**self).hash(state);
1387 }
1388}
1389
1390#[stable(feature = "indirect_hasher_impl", since = "1.22.0")]
1391impl<T: ?Sized + Hasher, A: Allocator> Hasher for Box<T, A> {
1392 fn finish(&self) -> u64 {
1393 (**self).finish()
1394 }
1395 fn write(&mut self, bytes: &[u8]) {
1396 (**self).write(bytes)
1397 }
1398 fn write_u8(&mut self, i: u8) {
1399 (**self).write_u8(i)
1400 }
1401 fn write_u16(&mut self, i: u16) {
1402 (**self).write_u16(i)
1403 }
1404 fn write_u32(&mut self, i: u32) {
1405 (**self).write_u32(i)
1406 }
1407 fn write_u64(&mut self, i: u64) {
1408 (**self).write_u64(i)
1409 }
1410 fn write_u128(&mut self, i: u128) {
1411 (**self).write_u128(i)
1412 }
1413 fn write_usize(&mut self, i: usize) {
1414 (**self).write_usize(i)
1415 }
1416 fn write_i8(&mut self, i: i8) {
1417 (**self).write_i8(i)
1418 }
1419 fn write_i16(&mut self, i: i16) {
1420 (**self).write_i16(i)
1421 }
1422 fn write_i32(&mut self, i: i32) {
1423 (**self).write_i32(i)
1424 }
1425 fn write_i64(&mut self, i: i64) {
1426 (**self).write_i64(i)
1427 }
1428 fn write_i128(&mut self, i: i128) {
1429 (**self).write_i128(i)
1430 }
1431 fn write_isize(&mut self, i: isize) {
1432 (**self).write_isize(i)
1433 }
1434 fn write_length_prefix(&mut self, len: usize) {
1435 (**self).write_length_prefix(len)
1436 }
1437 fn write_str(&mut self, s: &str) {
1438 (**self).write_str(s)
1439 }
1440}
1441
1442#[cfg(not(no_global_oom_handling))]
1443#[stable(feature = "from_for_ptrs", since = "1.6.0")]
1444impl<T> From<T> for Box<T> {
1445 /// Converts a `T` into a `Box<T>`
1446 ///
1447 /// The conversion allocates on the heap and moves `t`
1448 /// from the stack into it.
1449 ///
1450 /// # Examples
1451 ///
1452 /// ```rust
1453 /// let x = 5;
1454 /// let boxed = Box::new(5);
1455 ///
1456 /// assert_eq!(Box::from(x), boxed);
1457 /// ```
1458 fn from(t: T) -> Self {
1459 Box::new(t)
1460 }
1461}
1462
1463#[stable(feature = "pin", since = "1.33.0")]
1464impl<T: ?Sized, A: Allocator> From<Box<T, A>> for Pin<Box<T, A>>
1465where
1466 A: 'static,
1467{
1468 /// Converts a `Box<T>` into a `Pin<Box<T>>`. If `T` does not implement [`Unpin`], then
1469 /// `*boxed` will be pinned in memory and unable to be moved.
1470 ///
1471 /// This conversion does not allocate on the heap and happens in place.
1472 ///
1473 /// This is also available via [`Box::into_pin`].
1474 ///
1475 /// Constructing and pinning a `Box` with <code><Pin<Box\<T>>>::from([Box::new]\(x))</code>
1476 /// can also be written more concisely using <code>[Box::pin]\(x)</code>.
1477 /// This `From` implementation is useful if you already have a `Box<T>`, or you are
1478 /// constructing a (pinned) `Box` in a different way than with [`Box::new`].
1479 fn from(boxed: Box<T, A>) -> Self {
1480 Box::into_pin(boxed)
1481 }
1482}
1483
1484/// Specialization trait used for `From<&[T]>`.
1485#[cfg(not(no_global_oom_handling))]
1486trait BoxFromSlice<T> {
1487 fn from_slice(slice: &[T]) -> Self;
1488}
1489
1490#[cfg(not(no_global_oom_handling))]
1491impl<T: Clone> BoxFromSlice<T> for Box<[T]> {
1492 #[inline]
1493 default fn from_slice(slice: &[T]) -> Self {
1494 slice.to_vec().into_boxed_slice()
1495 }
1496}
1497
1498#[cfg(not(no_global_oom_handling))]
1499impl<T: Copy> BoxFromSlice<T> for Box<[T]> {
1500 #[inline]
1501 fn from_slice(slice: &[T]) -> Self {
1502 let len = slice.len();
1503 let buf = RawVec::with_capacity(len);
1504 unsafe {
1505 ptr::copy_nonoverlapping(slice.as_ptr(), buf.ptr(), len);
1506 buf.into_box(slice.len()).assume_init()
1507 }
1508 }
1509}
1510
1511#[cfg(not(no_global_oom_handling))]
1512#[stable(feature = "box_from_slice", since = "1.17.0")]
1513impl<T: Clone> From<&[T]> for Box<[T]> {
1514 /// Converts a `&[T]` into a `Box<[T]>`
1515 ///
1516 /// This conversion allocates on the heap
1517 /// and performs a copy of `slice` and its contents.
1518 ///
1519 /// # Examples
1520 /// ```rust
1521 /// // create a &[u8] which will be used to create a Box<[u8]>
1522 /// let slice: &[u8] = &[104, 101, 108, 108, 111];
1523 /// let boxed_slice: Box<[u8]> = Box::from(slice);
1524 ///
1525 /// println!("{boxed_slice:?}");
1526 /// ```
1527 #[inline]
1528 fn from(slice: &[T]) -> Box<[T]> {
1529 <Self as BoxFromSlice<T>>::from_slice(slice)
1530 }
1531}
1532
1533#[cfg(not(no_global_oom_handling))]
1534#[stable(feature = "box_from_cow", since = "1.45.0")]
1535impl<T: Clone> From<Cow<'_, [T]>> for Box<[T]> {
1536 /// Converts a `Cow<'_, [T]>` into a `Box<[T]>`
1537 ///
1538 /// When `cow` is the `Cow::Borrowed` variant, this
1539 /// conversion allocates on the heap and copies the
1540 /// underlying slice. Otherwise, it will try to reuse the owned
1541 /// `Vec`'s allocation.
1542 #[inline]
1543 fn from(cow: Cow<'_, [T]>) -> Box<[T]> {
1544 match cow {
1545 Cow::Borrowed(slice) => Box::from(slice),
1546 Cow::Owned(slice) => Box::from(slice),
1547 }
1548 }
1549}
1550
1551#[cfg(not(no_global_oom_handling))]
1552#[stable(feature = "box_from_slice", since = "1.17.0")]
1553impl From<&str> for Box<str> {
1554 /// Converts a `&str` into a `Box<str>`
1555 ///
1556 /// This conversion allocates on the heap
1557 /// and performs a copy of `s`.
1558 ///
1559 /// # Examples
1560 ///
1561 /// ```rust
1562 /// let boxed: Box<str> = Box::from("hello");
1563 /// println!("{boxed}");
1564 /// ```
1565 #[inline]
1566 fn from(s: &str) -> Box<str> {
1567 unsafe { from_boxed_utf8_unchecked(Box::from(s.as_bytes())) }
1568 }
1569}
1570
1571#[cfg(not(no_global_oom_handling))]
1572#[stable(feature = "box_from_cow", since = "1.45.0")]
1573impl From<Cow<'_, str>> for Box<str> {
1574 /// Converts a `Cow<'_, str>` into a `Box<str>`
1575 ///
1576 /// When `cow` is the `Cow::Borrowed` variant, this
1577 /// conversion allocates on the heap and copies the
1578 /// underlying `str`. Otherwise, it will try to reuse the owned
1579 /// `String`'s allocation.
1580 ///
1581 /// # Examples
1582 ///
1583 /// ```rust
1584 /// use std::borrow::Cow;
1585 ///
1586 /// let unboxed = Cow::Borrowed("hello");
1587 /// let boxed: Box<str> = Box::from(unboxed);
1588 /// println!("{boxed}");
1589 /// ```
1590 ///
1591 /// ```rust
1592 /// # use std::borrow::Cow;
1593 /// let unboxed = Cow::Owned("hello".to_string());
1594 /// let boxed: Box<str> = Box::from(unboxed);
1595 /// println!("{boxed}");
1596 /// ```
1597 #[inline]
1598 fn from(cow: Cow<'_, str>) -> Box<str> {
1599 match cow {
1600 Cow::Borrowed(s) => Box::from(s),
1601 Cow::Owned(s) => Box::from(s),
1602 }
1603 }
1604}
1605
1606#[stable(feature = "boxed_str_conv", since = "1.19.0")]
1607impl<A: Allocator> From<Box<str, A>> for Box<[u8], A> {
1608 /// Converts a `Box<str>` into a `Box<[u8]>`
1609 ///
1610 /// This conversion does not allocate on the heap and happens in place.
1611 ///
1612 /// # Examples
1613 /// ```rust
1614 /// // create a Box<str> which will be used to create a Box<[u8]>
1615 /// let boxed: Box<str> = Box::from("hello");
1616 /// let boxed_str: Box<[u8]> = Box::from(boxed);
1617 ///
1618 /// // create a &[u8] which will be used to create a Box<[u8]>
1619 /// let slice: &[u8] = &[104, 101, 108, 108, 111];
1620 /// let boxed_slice = Box::from(slice);
1621 ///
1622 /// assert_eq!(boxed_slice, boxed_str);
1623 /// ```
1624 #[inline]
1625 fn from(s: Box<str, A>) -> Self {
1626 let (raw, alloc) = Box::into_raw_with_allocator(s);
1627 unsafe { Box::from_raw_in(raw as *mut [u8], alloc) }
1628 }
1629}
1630
1631#[cfg(not(no_global_oom_handling))]
1632#[stable(feature = "box_from_array", since = "1.45.0")]
1633impl<T, const N: usize> From<[T; N]> for Box<[T]> {
1634 /// Converts a `[T; N]` into a `Box<[T]>`
1635 ///
1636 /// This conversion moves the array to newly heap-allocated memory.
1637 ///
1638 /// # Examples
1639 ///
1640 /// ```rust
1641 /// let boxed: Box<[u8]> = Box::from([4, 2]);
1642 /// println!("{boxed:?}");
1643 /// ```
1644 fn from(array: [T; N]) -> Box<[T]> {
1645 Box::new(array)
1646 }
1647}
1648
1649/// Casts a boxed slice to a boxed array.
1650///
1651/// # Safety
1652///
1653/// `boxed_slice.len()` must be exactly `N`.
1654unsafe fn boxed_slice_as_array_unchecked<T, A: Allocator, const N: usize>(
1655 boxed_slice: Box<[T], A>,
1656) -> Box<[T; N], A> {
1657 debug_assert_eq!(boxed_slice.len(), N);
1658
1659 let (ptr, alloc) = Box::into_raw_with_allocator(boxed_slice);
1660 // SAFETY: Pointer and allocator came from an existing box,
1661 // and our safety condition requires that the length is exactly `N`
1662 unsafe { Box::from_raw_in(ptr as *mut [T; N], alloc) }
1663}
1664
1665#[stable(feature = "boxed_slice_try_from", since = "1.43.0")]
1666impl<T, const N: usize> TryFrom<Box<[T]>> for Box<[T; N]> {
1667 type Error = Box<[T]>;
1668
1669 /// Attempts to convert a `Box<[T]>` into a `Box<[T; N]>`.
1670 ///
1671 /// The conversion occurs in-place and does not require a
1672 /// new memory allocation.
1673 ///
1674 /// # Errors
1675 ///
1676 /// Returns the old `Box<[T]>` in the `Err` variant if
1677 /// `boxed_slice.len()` does not equal `N`.
1678 fn try_from(boxed_slice: Box<[T]>) -> Result<Self, Self::Error> {
1679 if boxed_slice.len() == N {
1680 Ok(unsafe { boxed_slice_as_array_unchecked(boxed_slice) })
1681 } else {
1682 Err(boxed_slice)
1683 }
1684 }
1685}
1686
1687#[cfg(not(no_global_oom_handling))]
1688#[stable(feature = "boxed_array_try_from_vec", since = "1.66.0")]
1689impl<T, const N: usize> TryFrom<Vec<T>> for Box<[T; N]> {
1690 type Error = Vec<T>;
1691
1692 /// Attempts to convert a `Vec<T>` into a `Box<[T; N]>`.
1693 ///
1694 /// Like [`Vec::into_boxed_slice`], this is in-place if `vec.capacity() == N`,
1695 /// but will require a reallocation otherwise.
1696 ///
1697 /// # Errors
1698 ///
1699 /// Returns the original `Vec<T>` in the `Err` variant if
1700 /// `boxed_slice.len()` does not equal `N`.
1701 ///
1702 /// # Examples
1703 ///
1704 /// This can be used with [`vec!`] to create an array on the heap:
1705 ///
1706 /// ```
1707 /// let state: Box<[f32; 100]> = vec![1.0; 100].try_into().unwrap();
1708 /// assert_eq!(state.len(), 100);
1709 /// ```
1710 fn try_from(vec: Vec<T>) -> Result<Self, Self::Error> {
1711 if vec.len() == N {
1712 let boxed_slice = vec.into_boxed_slice();
1713 Ok(unsafe { boxed_slice_as_array_unchecked(boxed_slice) })
1714 } else {
1715 Err(vec)
1716 }
1717 }
1718}
1719
1720impl<A: Allocator> Box<dyn Any, A> {
1721 /// Attempt to downcast the box to a concrete type.
1722 ///
1723 /// # Examples
1724 ///
1725 /// ```
1726 /// use std::any::Any;
1727 ///
1728 /// fn print_if_string(value: Box<dyn Any>) {
1729 /// if let Ok(string) = value.downcast::<String>() {
1730 /// println!("String ({}): {}", string.len(), string);
1731 /// }
1732 /// }
1733 ///
1734 /// let my_string = "Hello World".to_string();
1735 /// print_if_string(Box::new(my_string));
1736 /// print_if_string(Box::new(0i8));
1737 /// ```
1738 #[inline]
1739 #[stable(feature = "rust1", since = "1.0.0")]
1740 pub fn downcast<T: Any>(self) -> Result<Box<T, A>, Self> {
1741 if self.is::<T>() { unsafe { Ok(self.downcast_unchecked::<T>()) } } else { Err(self) }
1742 }
1743
1744 /// Downcasts the box to a concrete type.
1745 ///
1746 /// For a safe alternative see [`downcast`].
1747 ///
1748 /// # Examples
1749 ///
1750 /// ```
1751 /// #![feature(downcast_unchecked)]
1752 ///
1753 /// use std::any::Any;
1754 ///
1755 /// let x: Box<dyn Any> = Box::new(1_usize);
1756 ///
1757 /// unsafe {
1758 /// assert_eq!(*x.downcast_unchecked::<usize>(), 1);
1759 /// }
1760 /// ```
1761 ///
1762 /// # Safety
1763 ///
1764 /// The contained value must be of type `T`. Calling this method
1765 /// with the incorrect type is *undefined behavior*.
1766 ///
1767 /// [`downcast`]: Self::downcast
1768 #[inline]
1769 #[unstable(feature = "downcast_unchecked", issue = "90850")]
1770 pub unsafe fn downcast_unchecked<T: Any>(self) -> Box<T, A> {
1771 debug_assert!(self.is::<T>());
1772 unsafe {
1773 let (raw, alloc): (*mut dyn Any, _) = Box::into_raw_with_allocator(self);
1774 Box::from_raw_in(raw as *mut T, alloc)
1775 }
1776 }
1777}
1778
1779impl<A: Allocator> Box<dyn Any + Send, A> {
1780 /// Attempt to downcast the box to a concrete type.
1781 ///
1782 /// # Examples
1783 ///
1784 /// ```
1785 /// use std::any::Any;
1786 ///
1787 /// fn print_if_string(value: Box<dyn Any + Send>) {
1788 /// if let Ok(string) = value.downcast::<String>() {
1789 /// println!("String ({}): {}", string.len(), string);
1790 /// }
1791 /// }
1792 ///
1793 /// let my_string = "Hello World".to_string();
1794 /// print_if_string(Box::new(my_string));
1795 /// print_if_string(Box::new(0i8));
1796 /// ```
1797 #[inline]
1798 #[stable(feature = "rust1", since = "1.0.0")]
1799 pub fn downcast<T: Any>(self) -> Result<Box<T, A>, Self> {
1800 if self.is::<T>() { unsafe { Ok(self.downcast_unchecked::<T>()) } } else { Err(self) }
1801 }
1802
1803 /// Downcasts the box to a concrete type.
1804 ///
1805 /// For a safe alternative see [`downcast`].
1806 ///
1807 /// # Examples
1808 ///
1809 /// ```
1810 /// #![feature(downcast_unchecked)]
1811 ///
1812 /// use std::any::Any;
1813 ///
1814 /// let x: Box<dyn Any + Send> = Box::new(1_usize);
1815 ///
1816 /// unsafe {
1817 /// assert_eq!(*x.downcast_unchecked::<usize>(), 1);
1818 /// }
1819 /// ```
1820 ///
1821 /// # Safety
1822 ///
1823 /// The contained value must be of type `T`. Calling this method
1824 /// with the incorrect type is *undefined behavior*.
1825 ///
1826 /// [`downcast`]: Self::downcast
1827 #[inline]
1828 #[unstable(feature = "downcast_unchecked", issue = "90850")]
1829 pub unsafe fn downcast_unchecked<T: Any>(self) -> Box<T, A> {
1830 debug_assert!(self.is::<T>());
1831 unsafe {
1832 let (raw, alloc): (*mut (dyn Any + Send), _) = Box::into_raw_with_allocator(self);
1833 Box::from_raw_in(raw as *mut T, alloc)
1834 }
1835 }
1836}
1837
1838impl<A: Allocator> Box<dyn Any + Send + Sync, A> {
1839 /// Attempt to downcast the box to a concrete type.
1840 ///
1841 /// # Examples
1842 ///
1843 /// ```
1844 /// use std::any::Any;
1845 ///
1846 /// fn print_if_string(value: Box<dyn Any + Send + Sync>) {
1847 /// if let Ok(string) = value.downcast::<String>() {
1848 /// println!("String ({}): {}", string.len(), string);
1849 /// }
1850 /// }
1851 ///
1852 /// let my_string = "Hello World".to_string();
1853 /// print_if_string(Box::new(my_string));
1854 /// print_if_string(Box::new(0i8));
1855 /// ```
1856 #[inline]
1857 #[stable(feature = "box_send_sync_any_downcast", since = "1.51.0")]
1858 pub fn downcast<T: Any>(self) -> Result<Box<T, A>, Self> {
1859 if self.is::<T>() { unsafe { Ok(self.downcast_unchecked::<T>()) } } else { Err(self) }
1860 }
1861
1862 /// Downcasts the box to a concrete type.
1863 ///
1864 /// For a safe alternative see [`downcast`].
1865 ///
1866 /// # Examples
1867 ///
1868 /// ```
1869 /// #![feature(downcast_unchecked)]
1870 ///
1871 /// use std::any::Any;
1872 ///
1873 /// let x: Box<dyn Any + Send + Sync> = Box::new(1_usize);
1874 ///
1875 /// unsafe {
1876 /// assert_eq!(*x.downcast_unchecked::<usize>(), 1);
1877 /// }
1878 /// ```
1879 ///
1880 /// # Safety
1881 ///
1882 /// The contained value must be of type `T`. Calling this method
1883 /// with the incorrect type is *undefined behavior*.
1884 ///
1885 /// [`downcast`]: Self::downcast
1886 #[inline]
1887 #[unstable(feature = "downcast_unchecked", issue = "90850")]
1888 pub unsafe fn downcast_unchecked<T: Any>(self) -> Box<T, A> {
1889 debug_assert!(self.is::<T>());
1890 unsafe {
1891 let (raw, alloc): (*mut (dyn Any + Send + Sync), _) =
1892 Box::into_raw_with_allocator(self);
1893 Box::from_raw_in(raw as *mut T, alloc)
1894 }
1895 }
1896}
1897
1898#[stable(feature = "rust1", since = "1.0.0")]
1899impl<T: fmt::Display + ?Sized, A: Allocator> fmt::Display for Box<T, A> {
1900 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
1901 fmt::Display::fmt(&**self, f)
1902 }
1903}
1904
1905#[stable(feature = "rust1", since = "1.0.0")]
1906impl<T: fmt::Debug + ?Sized, A: Allocator> fmt::Debug for Box<T, A> {
1907 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
1908 fmt::Debug::fmt(&**self, f)
1909 }
1910}
1911
1912#[stable(feature = "rust1", since = "1.0.0")]
1913impl<T: ?Sized, A: Allocator> fmt::Pointer for Box<T, A> {
1914 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
1915 // It's not possible to extract the inner Uniq directly from the Box,
1916 // instead we cast it to a *const which aliases the Unique
1917 let ptr: *const T = &**self;
1918 fmt::Pointer::fmt(&ptr, f)
1919 }
1920}
1921
1922#[stable(feature = "rust1", since = "1.0.0")]
1923impl<T: ?Sized, A: Allocator> Deref for Box<T, A> {
1924 type Target = T;
1925
1926 fn deref(&self) -> &T {
1927 &**self
1928 }
1929}
1930
1931#[stable(feature = "rust1", since = "1.0.0")]
1932impl<T: ?Sized, A: Allocator> DerefMut for Box<T, A> {
1933 fn deref_mut(&mut self) -> &mut T {
1934 &mut **self
1935 }
1936}
1937
1938#[unstable(feature = "receiver_trait", issue = "none")]
1939impl<T: ?Sized, A: Allocator> Receiver for Box<T, A> {}
1940
1941#[stable(feature = "rust1", since = "1.0.0")]
1942impl<I: Iterator + ?Sized, A: Allocator> Iterator for Box<I, A> {
1943 type Item = I::Item;
1944 fn next(&mut self) -> Option<I::Item> {
1945 (**self).next()
1946 }
1947 fn size_hint(&self) -> (usize, Option<usize>) {
1948 (**self).size_hint()
1949 }
1950 fn nth(&mut self, n: usize) -> Option<I::Item> {
1951 (**self).nth(n)
1952 }
1953 fn last(self) -> Option<I::Item> {
1954 BoxIter::last(self)
1955 }
1956}
1957
1958trait BoxIter {
1959 type Item;
1960 fn last(self) -> Option<Self::Item>;
1961}
1962
1963impl<I: Iterator + ?Sized, A: Allocator> BoxIter for Box<I, A> {
1964 type Item = I::Item;
1965 default fn last(self) -> Option<I::Item> {
1966 #[inline]
1967 fn some<T>(_: Option<T>, x: T) -> Option<T> {
1968 Some(x)
1969 }
1970
1971 self.fold(None, some)
1972 }
1973}
1974
1975/// Specialization for sized `I`s that uses `I`s implementation of `last()`
1976/// instead of the default.
1977#[stable(feature = "rust1", since = "1.0.0")]
1978impl<I: Iterator, A: Allocator> BoxIter for Box<I, A> {
1979 fn last(self) -> Option<I::Item> {
1980 (*self).last()
1981 }
1982}
1983
1984#[stable(feature = "rust1", since = "1.0.0")]
1985impl<I: DoubleEndedIterator + ?Sized, A: Allocator> DoubleEndedIterator for Box<I, A> {
1986 fn next_back(&mut self) -> Option<I::Item> {
1987 (**self).next_back()
1988 }
1989 fn nth_back(&mut self, n: usize) -> Option<I::Item> {
1990 (**self).nth_back(n)
1991 }
1992}
1993#[stable(feature = "rust1", since = "1.0.0")]
1994impl<I: ExactSizeIterator + ?Sized, A: Allocator> ExactSizeIterator for Box<I, A> {
1995 fn len(&self) -> usize {
1996 (**self).len()
1997 }
1998 fn is_empty(&self) -> bool {
1999 (**self).is_empty()
2000 }
2001}
2002
2003#[stable(feature = "fused", since = "1.26.0")]
2004impl<I: FusedIterator + ?Sized, A: Allocator> FusedIterator for Box<I, A> {}
2005
2006#[stable(feature = "boxed_closure_impls", since = "1.35.0")]
2007impl<Args: Tuple, F: FnOnce<Args> + ?Sized, A: Allocator> FnOnce<Args> for Box<F, A> {
2008 type Output = <F as FnOnce<Args>>::Output;
2009
2010 extern "rust-call" fn call_once(self, args: Args) -> Self::Output {
2011 <F as FnOnce<Args>>::call_once(*self, args)
2012 }
2013}
2014
2015#[stable(feature = "boxed_closure_impls", since = "1.35.0")]
2016impl<Args: Tuple, F: FnMut<Args> + ?Sized, A: Allocator> FnMut<Args> for Box<F, A> {
2017 extern "rust-call" fn call_mut(&mut self, args: Args) -> Self::Output {
2018 <F as FnMut<Args>>::call_mut(self, args)
2019 }
2020}
2021
2022#[stable(feature = "boxed_closure_impls", since = "1.35.0")]
2023impl<Args: Tuple, F: Fn<Args> + ?Sized, A: Allocator> Fn<Args> for Box<F, A> {
2024 extern "rust-call" fn call(&self, args: Args) -> Self::Output {
2025 <F as Fn<Args>>::call(self, args)
2026 }
2027}
2028
2029#[unstable(feature = "coerce_unsized", issue = "18598")]
2030impl<T: ?Sized + Unsize<U>, U: ?Sized, A: Allocator> CoerceUnsized<Box<U, A>> for Box<T, A> {}
2031
2032#[unstable(feature = "dispatch_from_dyn", issue = "none")]
2033impl<T: ?Sized + Unsize<U>, U: ?Sized> DispatchFromDyn<Box<U>> for Box<T, Global> {}
2034
2035#[cfg(not(no_global_oom_handling))]
2036#[stable(feature = "boxed_slice_from_iter", since = "1.32.0")]
2037impl<I> FromIterator<I> for Box<[I]> {
2038 fn from_iter<T: IntoIterator<Item = I>>(iter: T) -> Self {
2039 iter.into_iter().collect::<Vec<_>>().into_boxed_slice()
2040 }
2041}
2042
2043#[cfg(not(no_global_oom_handling))]
2044#[stable(feature = "box_slice_clone", since = "1.3.0")]
2045impl<T: Clone, A: Allocator + Clone> Clone for Box<[T], A> {
2046 fn clone(&self) -> Self {
2047 let alloc = Box::allocator(self).clone();
2048 self.to_vec_in(alloc).into_boxed_slice()
2049 }
2050
2051 fn clone_from(&mut self, other: &Self) {
2052 if self.len() == other.len() {
2053 self.clone_from_slice(&other);
2054 } else {
2055 *self = other.clone();
2056 }
2057 }
2058}
2059
2060#[stable(feature = "box_borrow", since = "1.1.0")]
2061impl<T: ?Sized, A: Allocator> borrow::Borrow<T> for Box<T, A> {
2062 fn borrow(&self) -> &T {
2063 &**self
2064 }
2065}
2066
2067#[stable(feature = "box_borrow", since = "1.1.0")]
2068impl<T: ?Sized, A: Allocator> borrow::BorrowMut<T> for Box<T, A> {
2069 fn borrow_mut(&mut self) -> &mut T {
2070 &mut **self
2071 }
2072}
2073
2074#[stable(since = "1.5.0", feature = "smart_ptr_as_ref")]
2075impl<T: ?Sized, A: Allocator> AsRef<T> for Box<T, A> {
2076 fn as_ref(&self) -> &T {
2077 &**self
2078 }
2079}
2080
2081#[stable(since = "1.5.0", feature = "smart_ptr_as_ref")]
2082impl<T: ?Sized, A: Allocator> AsMut<T> for Box<T, A> {
2083 fn as_mut(&mut self) -> &mut T {
2084 &mut **self
2085 }
2086}
2087
2088/* Nota bene
2089 *
2090 * We could have chosen not to add this impl, and instead have written a
2091 * function of Pin<Box<T>> to Pin<T>. Such a function would not be sound,
2092 * because Box<T> implements Unpin even when T does not, as a result of
2093 * this impl.
2094 *
2095 * We chose this API instead of the alternative for a few reasons:
2096 * - Logically, it is helpful to understand pinning in regard to the
2097 * memory region being pointed to. For this reason none of the
2098 * standard library pointer types support projecting through a pin
2099 * (Box<T> is the only pointer type in std for which this would be
2100 * safe.)
2101 * - It is in practice very useful to have Box<T> be unconditionally
2102 * Unpin because of trait objects, for which the structural auto
2103 * trait functionality does not apply (e.g., Box<dyn Foo> would
2104 * otherwise not be Unpin).
2105 *
2106 * Another type with the same semantics as Box but only a conditional
2107 * implementation of `Unpin` (where `T: Unpin`) would be valid/safe, and
2108 * could have a method to project a Pin<T> from it.
2109 */
2110#[stable(feature = "pin", since = "1.33.0")]
2111impl<T: ?Sized, A: Allocator> Unpin for Box<T, A> where A: 'static {}
2112
2113#[unstable(feature = "generator_trait", issue = "43122")]
2114impl<G: ?Sized + Generator<R> + Unpin, R, A: Allocator> Generator<R> for Box<G, A>
2115where
2116 A: 'static,
2117{
2118 type Yield = G::Yield;
2119 type Return = G::Return;
2120
2121 fn resume(mut self: Pin<&mut Self>, arg: R) -> GeneratorState<Self::Yield, Self::Return> {
2122 G::resume(Pin::new(&mut *self), arg)
2123 }
2124}
2125
2126#[unstable(feature = "generator_trait", issue = "43122")]
2127impl<G: ?Sized + Generator<R>, R, A: Allocator> Generator<R> for Pin<Box<G, A>>
2128where
2129 A: 'static,
2130{
2131 type Yield = G::Yield;
2132 type Return = G::Return;
2133
2134 fn resume(mut self: Pin<&mut Self>, arg: R) -> GeneratorState<Self::Yield, Self::Return> {
2135 G::resume((*self).as_mut(), arg)
2136 }
2137}
2138
2139#[stable(feature = "futures_api", since = "1.36.0")]
2140impl<F: ?Sized + Future + Unpin, A: Allocator> Future for Box<F, A>
2141where
2142 A: 'static,
2143{
2144 type Output = F::Output;
2145
2146 fn poll(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
2147 F::poll(Pin::new(&mut *self), cx)
2148 }
2149}
2150
2151#[unstable(feature = "async_iterator", issue = "79024")]
2152impl<S: ?Sized + AsyncIterator + Unpin> AsyncIterator for Box<S> {
2153 type Item = S::Item;
2154
2155 fn poll_next(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Option<Self::Item>> {
2156 Pin::new(&mut **self).poll_next(cx)
2157 }
2158
2159 fn size_hint(&self) -> (usize, Option<usize>) {
2160 (**self).size_hint()
2161 }
2162}
2163
2164impl dyn Error {
2165 #[inline]
2166 #[stable(feature = "error_downcast", since = "1.3.0")]
2167 #[rustc_allow_incoherent_impl]
2168 /// Attempts to downcast the box to a concrete type.
2169 pub fn downcast<T: Error + 'static>(self: Box<Self>) -> Result<Box<T>, Box<dyn Error>> {
2170 if self.is::<T>() {
2171 unsafe {
2172 let raw: *mut dyn Error = Box::into_raw(self);
2173 Ok(Box::from_raw(raw as *mut T))
2174 }
2175 } else {
2176 Err(self)
2177 }
2178 }
2179}
2180
2181impl dyn Error + Send {
2182 #[inline]
2183 #[stable(feature = "error_downcast", since = "1.3.0")]
2184 #[rustc_allow_incoherent_impl]
2185 /// Attempts to downcast the box to a concrete type.
2186 pub fn downcast<T: Error + 'static>(self: Box<Self>) -> Result<Box<T>, Box<dyn Error + Send>> {
2187 let err: Box<dyn Error> = self;
2188 <dyn Error>::downcast(err).map_err(|s| unsafe {
2189 // Reapply the `Send` marker.
2190 Box::from_raw(Box::into_raw(s) as *mut (dyn Error + Send))
2191 })
2192 }
2193}
2194
2195impl dyn Error + Send + Sync {
2196 #[inline]
2197 #[stable(feature = "error_downcast", since = "1.3.0")]
2198 #[rustc_allow_incoherent_impl]
2199 /// Attempts to downcast the box to a concrete type.
2200 pub fn downcast<T: Error + 'static>(self: Box<Self>) -> Result<Box<T>, Box<Self>> {
2201 let err: Box<dyn Error> = self;
2202 <dyn Error>::downcast(err).map_err(|s| unsafe {
2203 // Reapply the `Send + Sync` marker.
2204 Box::from_raw(Box::into_raw(s) as *mut (dyn Error + Send + Sync))
2205 })
2206 }
2207}
2208
2209#[cfg(not(no_global_oom_handling))]
2210#[stable(feature = "rust1", since = "1.0.0")]
2211impl<'a, E: Error + 'a> From<E> for Box<dyn Error + 'a> {
2212 /// Converts a type of [`Error`] into a box of dyn [`Error`].
2213 ///
2214 /// # Examples
2215 ///
2216 /// ```
2217 /// use std::error::Error;
2218 /// use std::fmt;
2219 /// use std::mem;
2220 ///
2221 /// #[derive(Debug)]
2222 /// struct AnError;
2223 ///
2224 /// impl fmt::Display for AnError {
2225 /// fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
2226 /// write!(f, "An error")
2227 /// }
2228 /// }
2229 ///
2230 /// impl Error for AnError {}
2231 ///
2232 /// let an_error = AnError;
2233 /// assert!(0 == mem::size_of_val(&an_error));
2234 /// let a_boxed_error = Box::<dyn Error>::from(an_error);
2235 /// assert!(mem::size_of::<Box<dyn Error>>() == mem::size_of_val(&a_boxed_error))
2236 /// ```
2237 fn from(err: E) -> Box<dyn Error + 'a> {
2238 Box::new(err)
2239 }
2240}
2241
2242#[cfg(not(no_global_oom_handling))]
2243#[stable(feature = "rust1", since = "1.0.0")]
2244impl<'a, E: Error + Send + Sync + 'a> From<E> for Box<dyn Error + Send + Sync + 'a> {
2245 /// Converts a type of [`Error`] + [`Send`] + [`Sync`] into a box of
2246 /// dyn [`Error`] + [`Send`] + [`Sync`].
2247 ///
2248 /// # Examples
2249 ///
2250 /// ```
2251 /// use std::error::Error;
2252 /// use std::fmt;
2253 /// use std::mem;
2254 ///
2255 /// #[derive(Debug)]
2256 /// struct AnError;
2257 ///
2258 /// impl fmt::Display for AnError {
2259 /// fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
2260 /// write!(f, "An error")
2261 /// }
2262 /// }
2263 ///
2264 /// impl Error for AnError {}
2265 ///
2266 /// unsafe impl Send for AnError {}
2267 ///
2268 /// unsafe impl Sync for AnError {}
2269 ///
2270 /// let an_error = AnError;
2271 /// assert!(0 == mem::size_of_val(&an_error));
2272 /// let a_boxed_error = Box::<dyn Error + Send + Sync>::from(an_error);
2273 /// assert!(
2274 /// mem::size_of::<Box<dyn Error + Send + Sync>>() == mem::size_of_val(&a_boxed_error))
2275 /// ```
2276 fn from(err: E) -> Box<dyn Error + Send + Sync + 'a> {
2277 Box::new(err)
2278 }
2279}
2280
2281#[cfg(not(no_global_oom_handling))]
2282#[stable(feature = "rust1", since = "1.0.0")]
2283impl From<String> for Box<dyn Error + Send + Sync> {
2284 /// Converts a [`String`] into a box of dyn [`Error`] + [`Send`] + [`Sync`].
2285 ///
2286 /// # Examples
2287 ///
2288 /// ```
2289 /// use std::error::Error;
2290 /// use std::mem;
2291 ///
2292 /// let a_string_error = "a string error".to_string();
2293 /// let a_boxed_error = Box::<dyn Error + Send + Sync>::from(a_string_error);
2294 /// assert!(
2295 /// mem::size_of::<Box<dyn Error + Send + Sync>>() == mem::size_of_val(&a_boxed_error))
2296 /// ```
2297 #[inline]
2298 fn from(err: String) -> Box<dyn Error + Send + Sync> {
2299 struct StringError(String);
2300
2301 impl Error for StringError {
2302 #[allow(deprecated)]
2303 fn description(&self) -> &str {
2304 &self.0
2305 }
2306 }
2307
2308 impl fmt::Display for StringError {
2309 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
2310 fmt::Display::fmt(&self.0, f)
2311 }
2312 }
2313
2314 // Purposefully skip printing "StringError(..)"
2315 impl fmt::Debug for StringError {
2316 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
2317 fmt::Debug::fmt(&self.0, f)
2318 }
2319 }
2320
2321 Box::new(StringError(err))
2322 }
2323}
2324
2325#[cfg(not(no_global_oom_handling))]
2326#[stable(feature = "string_box_error", since = "1.6.0")]
2327impl From<String> for Box<dyn Error> {
2328 /// Converts a [`String`] into a box of dyn [`Error`].
2329 ///
2330 /// # Examples
2331 ///
2332 /// ```
2333 /// use std::error::Error;
2334 /// use std::mem;
2335 ///
2336 /// let a_string_error = "a string error".to_string();
2337 /// let a_boxed_error = Box::<dyn Error>::from(a_string_error);
2338 /// assert!(mem::size_of::<Box<dyn Error>>() == mem::size_of_val(&a_boxed_error))
2339 /// ```
2340 fn from(str_err: String) -> Box<dyn Error> {
2341 let err1: Box<dyn Error + Send + Sync> = From::from(str_err);
2342 let err2: Box<dyn Error> = err1;
2343 err2
2344 }
2345}
2346
2347#[cfg(not(no_global_oom_handling))]
2348#[stable(feature = "rust1", since = "1.0.0")]
2349impl<'a> From<&str> for Box<dyn Error + Send + Sync + 'a> {
2350 /// Converts a [`str`] into a box of dyn [`Error`] + [`Send`] + [`Sync`].
2351 ///
2352 /// [`str`]: prim@str
2353 ///
2354 /// # Examples
2355 ///
2356 /// ```
2357 /// use std::error::Error;
2358 /// use std::mem;
2359 ///
2360 /// let a_str_error = "a str error";
2361 /// let a_boxed_error = Box::<dyn Error + Send + Sync>::from(a_str_error);
2362 /// assert!(
2363 /// mem::size_of::<Box<dyn Error + Send + Sync>>() == mem::size_of_val(&a_boxed_error))
2364 /// ```
2365 #[inline]
2366 fn from(err: &str) -> Box<dyn Error + Send + Sync + 'a> {
2367 From::from(String::from(err))
2368 }
2369}
2370
2371#[cfg(not(no_global_oom_handling))]
2372#[stable(feature = "string_box_error", since = "1.6.0")]
2373impl From<&str> for Box<dyn Error> {
2374 /// Converts a [`str`] into a box of dyn [`Error`].
2375 ///
2376 /// [`str`]: prim@str
2377 ///
2378 /// # Examples
2379 ///
2380 /// ```
2381 /// use std::error::Error;
2382 /// use std::mem;
2383 ///
2384 /// let a_str_error = "a str error";
2385 /// let a_boxed_error = Box::<dyn Error>::from(a_str_error);
2386 /// assert!(mem::size_of::<Box<dyn Error>>() == mem::size_of_val(&a_boxed_error))
2387 /// ```
2388 fn from(err: &str) -> Box<dyn Error> {
2389 From::from(String::from(err))
2390 }
2391}
2392
2393#[cfg(not(no_global_oom_handling))]
2394#[stable(feature = "cow_box_error", since = "1.22.0")]
2395impl<'a, 'b> From<Cow<'b, str>> for Box<dyn Error + Send + Sync + 'a> {
2396 /// Converts a [`Cow`] into a box of dyn [`Error`] + [`Send`] + [`Sync`].
2397 ///
2398 /// # Examples
2399 ///
2400 /// ```
2401 /// use std::error::Error;
2402 /// use std::mem;
2403 /// use std::borrow::Cow;
2404 ///
2405 /// let a_cow_str_error = Cow::from("a str error");
2406 /// let a_boxed_error = Box::<dyn Error + Send + Sync>::from(a_cow_str_error);
2407 /// assert!(
2408 /// mem::size_of::<Box<dyn Error + Send + Sync>>() == mem::size_of_val(&a_boxed_error))
2409 /// ```
2410 fn from(err: Cow<'b, str>) -> Box<dyn Error + Send + Sync + 'a> {
2411 From::from(String::from(err))
2412 }
2413}
2414
2415#[cfg(not(no_global_oom_handling))]
2416#[stable(feature = "cow_box_error", since = "1.22.0")]
2417impl<'a> From<Cow<'a, str>> for Box<dyn Error> {
2418 /// Converts a [`Cow`] into a box of dyn [`Error`].
2419 ///
2420 /// # Examples
2421 ///
2422 /// ```
2423 /// use std::error::Error;
2424 /// use std::mem;
2425 /// use std::borrow::Cow;
2426 ///
2427 /// let a_cow_str_error = Cow::from("a str error");
2428 /// let a_boxed_error = Box::<dyn Error>::from(a_cow_str_error);
2429 /// assert!(mem::size_of::<Box<dyn Error>>() == mem::size_of_val(&a_boxed_error))
2430 /// ```
2431 fn from(err: Cow<'a, str>) -> Box<dyn Error> {
2432 From::from(String::from(err))
2433 }
2434}
2435
2436#[stable(feature = "box_error", since = "1.8.0")]
2437impl<T: core::error::Error> core::error::Error for Box<T> {
2438 #[allow(deprecated, deprecated_in_future)]
2439 fn description(&self) -> &str {
2440 core::error::Error::description(&**self)
2441 }
2442
2443 #[allow(deprecated)]
2444 fn cause(&self) -> Option<&dyn core::error::Error> {
2445 core::error::Error::cause(&**self)
2446 }
2447
2448 fn source(&self) -> Option<&(dyn core::error::Error + 'static)> {
2449 core::error::Error::source(&**self)
2450 }
2451}