1 // Copyright 2012-2015 The Rust Project Developers. See the COPYRIGHT
2 // file at the top-level directory of this distribution and at
3 // http://rust-lang.org/COPYRIGHT.
5 // Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
6 // http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
7 // <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
8 // option. This file may not be copied, modified, or distributed
9 // except according to those terms.
11 //! A pointer type for heap allocation.
13 //! `Box<T>`, casually referred to as a 'box', provides the simplest form of
14 //! heap allocation in Rust. Boxes provide ownership for this allocation, and
15 //! drop their contents when they go out of scope.
22 //! let x = Box::new(5);
25 //! Creating a recursive data structure:
30 //! 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);
40 //! This will print `Cons(1, Cons(2, Nil))`.
42 //! Recursive structures must be boxed, because if the definition of `Cons`
49 //! It wouldn't work. This is because the size of a `List` depends on how many
50 //! elements are in the list, and so we don't know how much memory to allocate
51 //! for a `Cons`. By introducing a `Box`, which has a defined size, we know how
52 //! big `Cons` needs to be.
54 #![stable(feature = "rust1", since = "1.0.0")]
61 use core::cmp::Ordering;
63 use core::hash::{self, Hash};
64 use core::iter::FusedIterator;
65 use core::marker::{self, Unsize};
67 use core::ops::{CoerceUnsized, Deref, DerefMut};
68 use core::ops::{BoxPlace, Boxed, InPlace, Place, Placer};
69 use core::ptr::{self, Unique};
70 use core::convert::From;
71 use str::from_boxed_utf8_unchecked;
73 /// A value that represents the heap. This is the default place that the `box`
74 /// keyword allocates into when no place is supplied.
76 /// The following two examples are equivalent:
79 /// #![feature(box_heap)]
81 /// #![feature(box_syntax, placement_in_syntax)]
82 /// use std::boxed::HEAP;
85 /// let foo: Box<i32> = in HEAP { 5 };
89 #[unstable(feature = "box_heap",
90 reason = "may be renamed; uncertain about custom allocator design",
92 pub const HEAP: ExchangeHeapSingleton = ExchangeHeapSingleton { _force_singleton: () };
94 /// This the singleton type used solely for `boxed::HEAP`.
95 #[unstable(feature = "box_heap",
96 reason = "may be renamed; uncertain about custom allocator design",
98 #[derive(Copy, Clone)]
99 pub struct ExchangeHeapSingleton {
100 _force_singleton: (),
103 /// A pointer type for heap allocation.
105 /// See the [module-level documentation](../../std/boxed/index.html) for more.
106 #[lang = "owned_box"]
108 #[stable(feature = "rust1", since = "1.0.0")]
109 pub struct Box<T: ?Sized>(Unique<T>);
111 /// `IntermediateBox` represents uninitialized backing storage for `Box`.
113 /// FIXME (pnkfelix): Ideally we would just reuse `Box<T>` instead of
114 /// introducing a separate `IntermediateBox<T>`; but then you hit
115 /// issues when you e.g. attempt to destructure an instance of `Box`,
116 /// since it is a lang item and so it gets special handling by the
117 /// compiler. Easier just to make this parallel type for now.
119 /// FIXME (pnkfelix): Currently the `box` protocol only supports
120 /// creating instances of sized types. This IntermediateBox is
121 /// designed to be forward-compatible with a future protocol that
122 /// supports creating instances of unsized types; that is why the type
123 /// parameter has the `?Sized` generalization marker, and is also why
124 /// this carries an explicit size. However, it probably does not need
125 /// to carry the explicit alignment; that is just a work-around for
126 /// the fact that the `align_of` intrinsic currently requires the
127 /// input type to be Sized (which I do not think is strictly
129 #[unstable(feature = "placement_in",
130 reason = "placement box design is still being worked out.",
132 pub struct IntermediateBox<T: ?Sized> {
136 marker: marker::PhantomData<*mut T>,
139 #[unstable(feature = "placement_in",
140 reason = "placement box design is still being worked out.",
142 impl<T> Place<T> for IntermediateBox<T> {
143 fn pointer(&mut self) -> *mut T {
148 unsafe fn finalize<T>(b: IntermediateBox<T>) -> Box<T> {
149 let p = b.ptr as *mut T;
154 fn make_place<T>() -> IntermediateBox<T> {
155 let size = mem::size_of::<T>();
156 let align = mem::align_of::<T>();
158 let p = if size == 0 {
159 heap::EMPTY as *mut u8
161 let p = unsafe { heap::allocate(size, align) };
163 panic!("Box make_place allocation failure.");
172 marker: marker::PhantomData,
176 #[unstable(feature = "placement_in",
177 reason = "placement box design is still being worked out.",
179 impl<T> BoxPlace<T> for IntermediateBox<T> {
180 fn make_place() -> IntermediateBox<T> {
185 #[unstable(feature = "placement_in",
186 reason = "placement box design is still being worked out.",
188 impl<T> InPlace<T> for IntermediateBox<T> {
190 unsafe fn finalize(self) -> Box<T> {
195 #[unstable(feature = "placement_new_protocol", issue = "27779")]
196 impl<T> Boxed for Box<T> {
198 type Place = IntermediateBox<T>;
199 unsafe fn finalize(b: IntermediateBox<T>) -> Box<T> {
204 #[unstable(feature = "placement_in",
205 reason = "placement box design is still being worked out.",
207 impl<T> Placer<T> for ExchangeHeapSingleton {
208 type Place = IntermediateBox<T>;
210 fn make_place(self) -> IntermediateBox<T> {
215 #[unstable(feature = "placement_in",
216 reason = "placement box design is still being worked out.",
218 impl<T: ?Sized> Drop for IntermediateBox<T> {
221 unsafe { heap::deallocate(self.ptr, self.size, self.align) }
227 /// Allocates memory on the heap and then places `x` into it.
229 /// This doesn't actually allocate if `T` is zero-sized.
234 /// let five = Box::new(5);
236 #[stable(feature = "rust1", since = "1.0.0")]
238 pub fn new(x: T) -> Box<T> {
243 impl<T: ?Sized> Box<T> {
244 /// Constructs a box from a raw pointer.
246 /// After calling this function, the raw pointer is owned by the
247 /// resulting `Box`. Specifically, the `Box` destructor will call
248 /// the destructor of `T` and free the allocated memory. Since the
249 /// way `Box` allocates and releases memory is unspecified, the
250 /// only valid pointer to pass to this function is the one taken
251 /// from another `Box` via the [`Box::into_raw`] function.
253 /// This function is unsafe because improper use may lead to
254 /// memory problems. For example, a double-free may occur if the
255 /// function is called twice on the same raw pointer.
257 /// [`Box::into_raw`]: struct.Box.html#method.into_raw
262 /// let x = Box::new(5);
263 /// let ptr = Box::into_raw(x);
264 /// let x = unsafe { Box::from_raw(ptr) };
266 #[stable(feature = "box_raw", since = "1.4.0")]
268 pub unsafe fn from_raw(raw: *mut T) -> Self {
272 /// Consumes the `Box`, returning the wrapped raw pointer.
274 /// After calling this function, the caller is responsible for the
275 /// memory previously managed by the `Box`. In particular, the
276 /// caller should properly destroy `T` and release the memory. The
277 /// proper way to do so is to convert the raw pointer back into a
278 /// `Box` with the [`Box::from_raw`] function.
280 /// Note: this is an associated function, which means that you have
281 /// to call it as `Box::into_raw(b)` instead of `b.into_raw()`. This
282 /// is so that there is no conflict with a method on the inner type.
284 /// [`Box::from_raw`]: struct.Box.html#method.from_raw
289 /// let x = Box::new(5);
290 /// let ptr = Box::into_raw(x);
292 #[stable(feature = "box_raw", since = "1.4.0")]
294 pub fn into_raw(b: Box<T>) -> *mut T {
295 unsafe { mem::transmute(b) }
299 #[stable(feature = "rust1", since = "1.0.0")]
300 unsafe impl<#[may_dangle] T: ?Sized> Drop for Box<T> {
302 // FIXME: Do nothing, drop is currently performed by compiler.
306 #[stable(feature = "rust1", since = "1.0.0")]
307 impl<T: Default> Default for Box<T> {
308 /// Creates a `Box<T>`, with the `Default` value for T.
309 fn default() -> Box<T> {
310 box Default::default()
314 #[stable(feature = "rust1", since = "1.0.0")]
315 impl<T> Default for Box<[T]> {
316 fn default() -> Box<[T]> {
317 Box::<[T; 0]>::new([])
321 #[stable(feature = "default_box_extra", since = "1.17.0")]
322 impl Default for Box<str> {
323 fn default() -> Box<str> {
324 unsafe { from_boxed_utf8_unchecked(Default::default()) }
328 #[stable(feature = "rust1", since = "1.0.0")]
329 impl<T: Clone> Clone for Box<T> {
330 /// Returns a new box with a `clone()` of this box's contents.
335 /// let x = Box::new(5);
336 /// let y = x.clone();
340 fn clone(&self) -> Box<T> {
341 box { (**self).clone() }
343 /// Copies `source`'s contents into `self` without creating a new allocation.
348 /// let x = Box::new(5);
349 /// let mut y = Box::new(10);
351 /// y.clone_from(&x);
353 /// assert_eq!(*y, 5);
356 fn clone_from(&mut self, source: &Box<T>) {
357 (**self).clone_from(&(**source));
362 #[stable(feature = "box_slice_clone", since = "1.3.0")]
363 impl Clone for Box<str> {
364 fn clone(&self) -> Self {
365 let len = self.len();
366 let buf = RawVec::with_capacity(len);
368 ptr::copy_nonoverlapping(self.as_ptr(), buf.ptr(), len);
369 from_boxed_utf8_unchecked(buf.into_box())
374 #[stable(feature = "rust1", since = "1.0.0")]
375 impl<T: ?Sized + PartialEq> PartialEq for Box<T> {
377 fn eq(&self, other: &Box<T>) -> bool {
378 PartialEq::eq(&**self, &**other)
381 fn ne(&self, other: &Box<T>) -> bool {
382 PartialEq::ne(&**self, &**other)
385 #[stable(feature = "rust1", since = "1.0.0")]
386 impl<T: ?Sized + PartialOrd> PartialOrd for Box<T> {
388 fn partial_cmp(&self, other: &Box<T>) -> Option<Ordering> {
389 PartialOrd::partial_cmp(&**self, &**other)
392 fn lt(&self, other: &Box<T>) -> bool {
393 PartialOrd::lt(&**self, &**other)
396 fn le(&self, other: &Box<T>) -> bool {
397 PartialOrd::le(&**self, &**other)
400 fn ge(&self, other: &Box<T>) -> bool {
401 PartialOrd::ge(&**self, &**other)
404 fn gt(&self, other: &Box<T>) -> bool {
405 PartialOrd::gt(&**self, &**other)
408 #[stable(feature = "rust1", since = "1.0.0")]
409 impl<T: ?Sized + Ord> Ord for Box<T> {
411 fn cmp(&self, other: &Box<T>) -> Ordering {
412 Ord::cmp(&**self, &**other)
415 #[stable(feature = "rust1", since = "1.0.0")]
416 impl<T: ?Sized + Eq> Eq for Box<T> {}
418 #[stable(feature = "rust1", since = "1.0.0")]
419 impl<T: ?Sized + Hash> Hash for Box<T> {
420 fn hash<H: hash::Hasher>(&self, state: &mut H) {
421 (**self).hash(state);
425 #[stable(feature = "from_for_ptrs", since = "1.6.0")]
426 impl<T> From<T> for Box<T> {
427 fn from(t: T) -> Self {
432 #[stable(feature = "box_from_slice", since = "1.17.0")]
433 impl<'a, T: Copy> From<&'a [T]> for Box<[T]> {
434 fn from(slice: &'a [T]) -> Box<[T]> {
435 let mut boxed = unsafe { RawVec::with_capacity(slice.len()).into_box() };
436 boxed.copy_from_slice(slice);
441 #[stable(feature = "box_from_slice", since = "1.17.0")]
442 impl<'a> From<&'a str> for Box<str> {
443 fn from(s: &'a str) -> Box<str> {
444 unsafe { from_boxed_utf8_unchecked(Box::from(s.as_bytes())) }
448 #[stable(feature = "boxed_str_conv", since = "1.18.0")]
449 impl From<Box<str>> for Box<[u8]> {
450 fn from(s: Box<str>) -> Self {
459 #[stable(feature = "rust1", since = "1.0.0")]
460 /// Attempt to downcast the box to a concrete type.
465 /// use std::any::Any;
467 /// fn print_if_string(value: Box<Any>) {
468 /// if let Ok(string) = value.downcast::<String>() {
469 /// println!("String ({}): {}", string.len(), string);
474 /// let my_string = "Hello World".to_string();
475 /// print_if_string(Box::new(my_string));
476 /// print_if_string(Box::new(0i8));
479 pub fn downcast<T: Any>(self) -> Result<Box<T>, Box<Any>> {
482 let raw: *mut Any = Box::into_raw(self);
483 Ok(Box::from_raw(raw as *mut T))
491 impl Box<Any + Send> {
493 #[stable(feature = "rust1", since = "1.0.0")]
494 /// Attempt to downcast the box to a concrete type.
499 /// use std::any::Any;
501 /// fn print_if_string(value: Box<Any + Send>) {
502 /// if let Ok(string) = value.downcast::<String>() {
503 /// println!("String ({}): {}", string.len(), string);
508 /// let my_string = "Hello World".to_string();
509 /// print_if_string(Box::new(my_string));
510 /// print_if_string(Box::new(0i8));
513 pub fn downcast<T: Any>(self) -> Result<Box<T>, Box<Any + Send>> {
514 <Box<Any>>::downcast(self).map_err(|s| unsafe {
515 // reapply the Send marker
516 mem::transmute::<Box<Any>, Box<Any + Send>>(s)
521 #[stable(feature = "rust1", since = "1.0.0")]
522 impl<T: fmt::Display + ?Sized> fmt::Display for Box<T> {
523 fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
524 fmt::Display::fmt(&**self, f)
528 #[stable(feature = "rust1", since = "1.0.0")]
529 impl<T: fmt::Debug + ?Sized> fmt::Debug for Box<T> {
530 fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
531 fmt::Debug::fmt(&**self, f)
535 #[stable(feature = "rust1", since = "1.0.0")]
536 impl<T: ?Sized> fmt::Pointer for Box<T> {
537 fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
538 // It's not possible to extract the inner Uniq directly from the Box,
539 // instead we cast it to a *const which aliases the Unique
540 let ptr: *const T = &**self;
541 fmt::Pointer::fmt(&ptr, f)
545 #[stable(feature = "rust1", since = "1.0.0")]
546 impl<T: ?Sized> Deref for Box<T> {
549 fn deref(&self) -> &T {
554 #[stable(feature = "rust1", since = "1.0.0")]
555 impl<T: ?Sized> DerefMut for Box<T> {
556 fn deref_mut(&mut self) -> &mut T {
561 #[stable(feature = "rust1", since = "1.0.0")]
562 impl<I: Iterator + ?Sized> Iterator for Box<I> {
564 fn next(&mut self) -> Option<I::Item> {
567 fn size_hint(&self) -> (usize, Option<usize>) {
570 fn nth(&mut self, n: usize) -> Option<I::Item> {
574 #[stable(feature = "rust1", since = "1.0.0")]
575 impl<I: DoubleEndedIterator + ?Sized> DoubleEndedIterator for Box<I> {
576 fn next_back(&mut self) -> Option<I::Item> {
580 #[stable(feature = "rust1", since = "1.0.0")]
581 impl<I: ExactSizeIterator + ?Sized> ExactSizeIterator for Box<I> {
582 fn len(&self) -> usize {
585 fn is_empty(&self) -> bool {
590 #[unstable(feature = "fused", issue = "35602")]
591 impl<I: FusedIterator + ?Sized> FusedIterator for Box<I> {}
594 /// `FnBox` is a version of the `FnOnce` intended for use with boxed
595 /// closure objects. The idea is that where one would normally store a
596 /// `Box<FnOnce()>` in a data structure, you should use
597 /// `Box<FnBox()>`. The two traits behave essentially the same, except
598 /// that a `FnBox` closure can only be called if it is boxed. (Note
599 /// that `FnBox` may be deprecated in the future if `Box<FnOnce()>`
600 /// closures become directly usable.)
604 /// Here is a snippet of code which creates a hashmap full of boxed
605 /// once closures and then removes them one by one, calling each
606 /// closure as it is removed. Note that the type of the closures
607 /// stored in the map is `Box<FnBox() -> i32>` and not `Box<FnOnce()
611 /// #![feature(fnbox)]
613 /// use std::boxed::FnBox;
614 /// use std::collections::HashMap;
616 /// fn make_map() -> HashMap<i32, Box<FnBox() -> i32>> {
617 /// let mut map: HashMap<i32, Box<FnBox() -> i32>> = HashMap::new();
618 /// map.insert(1, Box::new(|| 22));
619 /// map.insert(2, Box::new(|| 44));
624 /// let mut map = make_map();
625 /// for i in &[1, 2] {
626 /// let f = map.remove(&i).unwrap();
627 /// assert_eq!(f(), i * 22);
632 #[unstable(feature = "fnbox",
633 reason = "will be deprecated if and when `Box<FnOnce>` becomes usable", issue = "28796")]
637 fn call_box(self: Box<Self>, args: A) -> Self::Output;
640 #[unstable(feature = "fnbox",
641 reason = "will be deprecated if and when `Box<FnOnce>` becomes usable", issue = "28796")]
642 impl<A, F> FnBox<A> for F
645 type Output = F::Output;
647 fn call_box(self: Box<F>, args: A) -> F::Output {
652 #[unstable(feature = "fnbox",
653 reason = "will be deprecated if and when `Box<FnOnce>` becomes usable", issue = "28796")]
654 impl<'a, A, R> FnOnce<A> for Box<FnBox<A, Output = R> + 'a> {
657 extern "rust-call" fn call_once(self, args: A) -> R {
662 #[unstable(feature = "fnbox",
663 reason = "will be deprecated if and when `Box<FnOnce>` becomes usable", issue = "28796")]
664 impl<'a, A, R> FnOnce<A> for Box<FnBox<A, Output = R> + Send + 'a> {
667 extern "rust-call" fn call_once(self, args: A) -> R {
672 #[unstable(feature = "coerce_unsized", issue = "27732")]
673 impl<T: ?Sized + Unsize<U>, U: ?Sized> CoerceUnsized<Box<U>> for Box<T> {}
675 #[stable(feature = "box_slice_clone", since = "1.3.0")]
676 impl<T: Clone> Clone for Box<[T]> {
677 fn clone(&self) -> Self {
678 let mut new = BoxBuilder {
679 data: RawVec::with_capacity(self.len()),
683 let mut target = new.data.ptr();
685 for item in self.iter() {
687 ptr::write(target, item.clone());
688 target = target.offset(1);
694 return unsafe { new.into_box() };
696 // Helper type for responding to panics correctly.
697 struct BoxBuilder<T> {
702 impl<T> BoxBuilder<T> {
703 unsafe fn into_box(self) -> Box<[T]> {
704 let raw = ptr::read(&self.data);
710 impl<T> Drop for BoxBuilder<T> {
712 let mut data = self.data.ptr();
713 let max = unsafe { data.offset(self.len as isize) };
718 data = data.offset(1);
726 #[stable(feature = "rust1", since = "1.0.0")]
727 impl<T: ?Sized> borrow::Borrow<T> for Box<T> {
728 fn borrow(&self) -> &T {
733 #[stable(feature = "rust1", since = "1.0.0")]
734 impl<T: ?Sized> borrow::BorrowMut<T> for Box<T> {
735 fn borrow_mut(&mut self) -> &mut T {
740 #[stable(since = "1.5.0", feature = "smart_ptr_as_ref")]
741 impl<T: ?Sized> AsRef<T> for Box<T> {
742 fn as_ref(&self) -> &T {
747 #[stable(since = "1.5.0", feature = "smart_ptr_as_ref")]
748 impl<T: ?Sized> AsMut<T> for Box<T> {
749 fn as_mut(&mut self) -> &mut T {