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;
72 /// A value that represents the heap. This is the default place that the `box`
73 /// keyword allocates into when no place is supplied.
75 /// The following two examples are equivalent:
78 /// #![feature(box_heap)]
80 /// #![feature(box_syntax, placement_in_syntax)]
81 /// use std::boxed::HEAP;
84 /// let foo: Box<i32> = in HEAP { 5 };
88 #[unstable(feature = "box_heap",
89 reason = "may be renamed; uncertain about custom allocator design",
91 pub const HEAP: ExchangeHeapSingleton = ExchangeHeapSingleton { _force_singleton: () };
93 /// This the singleton type used solely for `boxed::HEAP`.
94 #[unstable(feature = "box_heap",
95 reason = "may be renamed; uncertain about custom allocator design",
97 #[derive(Copy, Clone)]
98 pub struct ExchangeHeapSingleton {
102 /// A pointer type for heap allocation.
104 /// See the [module-level documentation](../../std/boxed/index.html) for more.
105 #[lang = "owned_box"]
107 #[stable(feature = "rust1", since = "1.0.0")]
108 pub struct Box<T: ?Sized>(Unique<T>);
110 /// `IntermediateBox` represents uninitialized backing storage for `Box`.
112 /// FIXME (pnkfelix): Ideally we would just reuse `Box<T>` instead of
113 /// introducing a separate `IntermediateBox<T>`; but then you hit
114 /// issues when you e.g. attempt to destructure an instance of `Box`,
115 /// since it is a lang item and so it gets special handling by the
116 /// compiler. Easier just to make this parallel type for now.
118 /// FIXME (pnkfelix): Currently the `box` protocol only supports
119 /// creating instances of sized types. This IntermediateBox is
120 /// designed to be forward-compatible with a future protocol that
121 /// supports creating instances of unsized types; that is why the type
122 /// parameter has the `?Sized` generalization marker, and is also why
123 /// this carries an explicit size. However, it probably does not need
124 /// to carry the explicit alignment; that is just a work-around for
125 /// the fact that the `align_of` intrinsic currently requires the
126 /// input type to be Sized (which I do not think is strictly
128 #[unstable(feature = "placement_in",
129 reason = "placement box design is still being worked out.",
131 pub struct IntermediateBox<T: ?Sized> {
135 marker: marker::PhantomData<*mut T>,
138 #[unstable(feature = "placement_in",
139 reason = "placement box design is still being worked out.",
141 impl<T> Place<T> for IntermediateBox<T> {
142 fn pointer(&mut self) -> *mut T {
147 unsafe fn finalize<T>(b: IntermediateBox<T>) -> Box<T> {
148 let p = b.ptr as *mut T;
153 fn make_place<T>() -> IntermediateBox<T> {
154 let size = mem::size_of::<T>();
155 let align = mem::align_of::<T>();
157 let p = if size == 0 {
158 heap::EMPTY as *mut u8
160 let p = unsafe { heap::allocate(size, align) };
162 panic!("Box make_place allocation failure.");
171 marker: marker::PhantomData,
175 #[unstable(feature = "placement_in",
176 reason = "placement box design is still being worked out.",
178 impl<T> BoxPlace<T> for IntermediateBox<T> {
179 fn make_place() -> IntermediateBox<T> {
184 #[unstable(feature = "placement_in",
185 reason = "placement box design is still being worked out.",
187 impl<T> InPlace<T> for IntermediateBox<T> {
189 unsafe fn finalize(self) -> Box<T> {
194 #[unstable(feature = "placement_new_protocol", issue = "27779")]
195 impl<T> Boxed for Box<T> {
197 type Place = IntermediateBox<T>;
198 unsafe fn finalize(b: IntermediateBox<T>) -> Box<T> {
203 #[unstable(feature = "placement_in",
204 reason = "placement box design is still being worked out.",
206 impl<T> Placer<T> for ExchangeHeapSingleton {
207 type Place = IntermediateBox<T>;
209 fn make_place(self) -> IntermediateBox<T> {
214 #[unstable(feature = "placement_in",
215 reason = "placement box design is still being worked out.",
217 impl<T: ?Sized> Drop for IntermediateBox<T> {
220 unsafe { heap::deallocate(self.ptr, self.size, self.align) }
226 /// Allocates memory on the heap and then places `x` into it.
228 /// This doesn't actually allocate if `T` is zero-sized.
233 /// let five = Box::new(5);
235 #[stable(feature = "rust1", since = "1.0.0")]
237 pub fn new(x: T) -> Box<T> {
242 impl<T: ?Sized> Box<T> {
243 /// Constructs a box from a raw pointer.
245 /// After calling this function, the raw pointer is owned by the
246 /// resulting `Box`. Specifically, the `Box` destructor will call
247 /// the destructor of `T` and free the allocated memory. Since the
248 /// way `Box` allocates and releases memory is unspecified, the
249 /// only valid pointer to pass to this function is the one taken
250 /// from another `Box` via the [`Box::into_raw`] function.
252 /// This function is unsafe because improper use may lead to
253 /// memory problems. For example, a double-free may occur if the
254 /// function is called twice on the same raw pointer.
256 /// [`Box::into_raw`]: struct.Box.html#method.into_raw
261 /// let x = Box::new(5);
262 /// let ptr = Box::into_raw(x);
263 /// let x = unsafe { Box::from_raw(ptr) };
265 #[stable(feature = "box_raw", since = "1.4.0")]
267 pub unsafe fn from_raw(raw: *mut T) -> Self {
271 /// Consumes the `Box`, returning the wrapped raw pointer.
273 /// After calling this function, the caller is responsible for the
274 /// memory previously managed by the `Box`. In particular, the
275 /// caller should properly destroy `T` and release the memory. The
276 /// proper way to do so is to convert the raw pointer back into a
277 /// `Box` with the [`Box::from_raw`] function.
279 /// Note: this is an associated function, which means that you have
280 /// to call it as `Box::into_raw(b)` instead of `b.into_raw()`. This
281 /// is so that there is no conflict with a method on the inner type.
283 /// [`Box::from_raw`]: struct.Box.html#method.from_raw
288 /// let x = Box::new(5);
289 /// let ptr = Box::into_raw(x);
291 #[stable(feature = "box_raw", since = "1.4.0")]
293 pub fn into_raw(b: Box<T>) -> *mut T {
294 unsafe { mem::transmute(b) }
298 #[stable(feature = "rust1", since = "1.0.0")]
299 unsafe impl<#[may_dangle] T: ?Sized> Drop for Box<T> {
301 // FIXME: Do nothing, drop is currently performed by compiler.
305 #[stable(feature = "rust1", since = "1.0.0")]
306 impl<T: Default> Default for Box<T> {
307 /// Creates a `Box<T>`, with the `Default` value for T.
308 fn default() -> Box<T> {
309 box Default::default()
313 #[stable(feature = "rust1", since = "1.0.0")]
314 impl<T> Default for Box<[T]> {
315 fn default() -> Box<[T]> {
316 Box::<[T; 0]>::new([])
320 #[stable(feature = "default_box_extra", since = "1.17.0")]
321 impl Default for Box<str> {
322 fn default() -> Box<str> {
323 let default: Box<[u8]> = Default::default();
324 unsafe { mem::transmute(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 mem::transmute(buf.into_box()) // bytes to str ~magic
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 let boxed: Box<[u8]> = Box::from(s.as_bytes());
445 unsafe { mem::transmute(boxed) }
451 #[stable(feature = "rust1", since = "1.0.0")]
452 /// Attempt to downcast the box to a concrete type.
457 /// use std::any::Any;
459 /// fn print_if_string(value: Box<Any>) {
460 /// if let Ok(string) = value.downcast::<String>() {
461 /// println!("String ({}): {}", string.len(), string);
466 /// let my_string = "Hello World".to_string();
467 /// print_if_string(Box::new(my_string));
468 /// print_if_string(Box::new(0i8));
471 pub fn downcast<T: Any>(self) -> Result<Box<T>, Box<Any>> {
474 let raw: *mut Any = Box::into_raw(self);
475 Ok(Box::from_raw(raw as *mut T))
483 impl Box<Any + Send> {
485 #[stable(feature = "rust1", since = "1.0.0")]
486 /// Attempt to downcast the box to a concrete type.
491 /// use std::any::Any;
493 /// fn print_if_string(value: Box<Any + Send>) {
494 /// if let Ok(string) = value.downcast::<String>() {
495 /// println!("String ({}): {}", string.len(), string);
500 /// let my_string = "Hello World".to_string();
501 /// print_if_string(Box::new(my_string));
502 /// print_if_string(Box::new(0i8));
505 pub fn downcast<T: Any>(self) -> Result<Box<T>, Box<Any + Send>> {
506 <Box<Any>>::downcast(self).map_err(|s| unsafe {
507 // reapply the Send marker
508 mem::transmute::<Box<Any>, Box<Any + Send>>(s)
513 #[stable(feature = "rust1", since = "1.0.0")]
514 impl<T: fmt::Display + ?Sized> fmt::Display for Box<T> {
515 fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
516 fmt::Display::fmt(&**self, f)
520 #[stable(feature = "rust1", since = "1.0.0")]
521 impl<T: fmt::Debug + ?Sized> fmt::Debug for Box<T> {
522 fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
523 fmt::Debug::fmt(&**self, f)
527 #[stable(feature = "rust1", since = "1.0.0")]
528 impl<T: ?Sized> fmt::Pointer for Box<T> {
529 fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
530 // It's not possible to extract the inner Uniq directly from the Box,
531 // instead we cast it to a *const which aliases the Unique
532 let ptr: *const T = &**self;
533 fmt::Pointer::fmt(&ptr, f)
537 #[stable(feature = "rust1", since = "1.0.0")]
538 impl<T: ?Sized> Deref for Box<T> {
541 fn deref(&self) -> &T {
546 #[stable(feature = "rust1", since = "1.0.0")]
547 impl<T: ?Sized> DerefMut for Box<T> {
548 fn deref_mut(&mut self) -> &mut T {
553 #[stable(feature = "rust1", since = "1.0.0")]
554 impl<I: Iterator + ?Sized> Iterator for Box<I> {
556 fn next(&mut self) -> Option<I::Item> {
559 fn size_hint(&self) -> (usize, Option<usize>) {
562 fn nth(&mut self, n: usize) -> Option<I::Item> {
566 #[stable(feature = "rust1", since = "1.0.0")]
567 impl<I: DoubleEndedIterator + ?Sized> DoubleEndedIterator for Box<I> {
568 fn next_back(&mut self) -> Option<I::Item> {
572 #[stable(feature = "rust1", since = "1.0.0")]
573 impl<I: ExactSizeIterator + ?Sized> ExactSizeIterator for Box<I> {
574 fn len(&self) -> usize {
577 fn is_empty(&self) -> bool {
582 #[unstable(feature = "fused", issue = "35602")]
583 impl<I: FusedIterator + ?Sized> FusedIterator for Box<I> {}
586 /// `FnBox` is a version of the `FnOnce` intended for use with boxed
587 /// closure objects. The idea is that where one would normally store a
588 /// `Box<FnOnce()>` in a data structure, you should use
589 /// `Box<FnBox()>`. The two traits behave essentially the same, except
590 /// that a `FnBox` closure can only be called if it is boxed. (Note
591 /// that `FnBox` may be deprecated in the future if `Box<FnOnce()>`
592 /// closures become directly usable.)
596 /// Here is a snippet of code which creates a hashmap full of boxed
597 /// once closures and then removes them one by one, calling each
598 /// closure as it is removed. Note that the type of the closures
599 /// stored in the map is `Box<FnBox() -> i32>` and not `Box<FnOnce()
603 /// #![feature(fnbox)]
605 /// use std::boxed::FnBox;
606 /// use std::collections::HashMap;
608 /// fn make_map() -> HashMap<i32, Box<FnBox() -> i32>> {
609 /// let mut map: HashMap<i32, Box<FnBox() -> i32>> = HashMap::new();
610 /// map.insert(1, Box::new(|| 22));
611 /// map.insert(2, Box::new(|| 44));
616 /// let mut map = make_map();
617 /// for i in &[1, 2] {
618 /// let f = map.remove(&i).unwrap();
619 /// assert_eq!(f(), i * 22);
624 #[unstable(feature = "fnbox",
625 reason = "will be deprecated if and when `Box<FnOnce>` becomes usable", issue = "28796")]
629 fn call_box(self: Box<Self>, args: A) -> Self::Output;
632 #[unstable(feature = "fnbox",
633 reason = "will be deprecated if and when `Box<FnOnce>` becomes usable", issue = "28796")]
634 impl<A, F> FnBox<A> for F
637 type Output = F::Output;
639 fn call_box(self: Box<F>, args: A) -> F::Output {
644 #[unstable(feature = "fnbox",
645 reason = "will be deprecated if and when `Box<FnOnce>` becomes usable", issue = "28796")]
646 impl<'a, A, R> FnOnce<A> for Box<FnBox<A, Output = R> + 'a> {
649 extern "rust-call" fn call_once(self, args: A) -> R {
654 #[unstable(feature = "fnbox",
655 reason = "will be deprecated if and when `Box<FnOnce>` becomes usable", issue = "28796")]
656 impl<'a, A, R> FnOnce<A> for Box<FnBox<A, Output = R> + Send + 'a> {
659 extern "rust-call" fn call_once(self, args: A) -> R {
664 #[unstable(feature = "coerce_unsized", issue = "27732")]
665 impl<T: ?Sized + Unsize<U>, U: ?Sized> CoerceUnsized<Box<U>> for Box<T> {}
667 #[stable(feature = "box_slice_clone", since = "1.3.0")]
668 impl<T: Clone> Clone for Box<[T]> {
669 fn clone(&self) -> Self {
670 let mut new = BoxBuilder {
671 data: RawVec::with_capacity(self.len()),
675 let mut target = new.data.ptr();
677 for item in self.iter() {
679 ptr::write(target, item.clone());
680 target = target.offset(1);
686 return unsafe { new.into_box() };
688 // Helper type for responding to panics correctly.
689 struct BoxBuilder<T> {
694 impl<T> BoxBuilder<T> {
695 unsafe fn into_box(self) -> Box<[T]> {
696 let raw = ptr::read(&self.data);
702 impl<T> Drop for BoxBuilder<T> {
704 let mut data = self.data.ptr();
705 let max = unsafe { data.offset(self.len as isize) };
710 data = data.offset(1);
718 #[stable(feature = "rust1", since = "1.0.0")]
719 impl<T: ?Sized> borrow::Borrow<T> for Box<T> {
720 fn borrow(&self) -> &T {
725 #[stable(feature = "rust1", since = "1.0.0")]
726 impl<T: ?Sized> borrow::BorrowMut<T> for Box<T> {
727 fn borrow_mut(&mut self) -> &mut T {
732 #[stable(since = "1.5.0", feature = "smart_ptr_as_ref")]
733 impl<T: ?Sized> AsRef<T> for Box<T> {
734 fn as_ref(&self) -> &T {
739 #[stable(since = "1.5.0", feature = "smart_ptr_as_ref")]
740 impl<T: ?Sized> AsMut<T> for Box<T> {
741 fn as_mut(&mut self) -> &mut T {