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")]
60 use core::cmp::Ordering;
62 use core::hash::{self, Hash};
63 use core::marker::{self, Unsize};
65 use core::ops::{CoerceUnsized, Deref, DerefMut};
66 use core::ops::{Placer, Boxed, Place, InPlace, BoxPlace};
67 use core::ptr::{self, Unique};
68 use core::raw::{TraitObject};
70 /// A value that represents the heap. This is the default place that the `box`
71 /// keyword allocates into when no place is supplied.
73 /// The following two examples are equivalent:
76 /// #![feature(box_heap)]
78 /// #![feature(box_syntax, placement_in_syntax)]
79 /// use std::boxed::HEAP;
82 /// let foo = box(HEAP) 5;
86 #[lang = "exchange_heap"]
87 #[unstable(feature = "box_heap",
88 reason = "may be renamed; uncertain about custom allocator design")]
89 pub const HEAP: ExchangeHeapSingleton =
90 ExchangeHeapSingleton { _force_singleton: () };
92 /// This the singleton type used solely for `boxed::HEAP`.
93 #[unstable(feature = "box_heap",
94 reason = "may be renamed; uncertain about custom allocator design")]
95 #[derive(Copy, Clone)]
96 pub struct ExchangeHeapSingleton { _force_singleton: () }
98 /// A pointer type for heap allocation.
100 /// See the [module-level documentation](../../std/boxed/index.html) for more.
101 #[lang = "owned_box"]
102 #[stable(feature = "rust1", since = "1.0.0")]
104 pub struct Box<T: ?Sized>(Unique<T>);
106 /// `IntermediateBox` represents uninitialized backing storage for `Box`.
108 /// FIXME (pnkfelix): Ideally we would just reuse `Box<T>` instead of
109 /// introducing a separate `IntermediateBox<T>`; but then you hit
110 /// issues when you e.g. attempt to destructure an instance of `Box`,
111 /// since it is a lang item and so it gets special handling by the
112 /// compiler. Easier just to make this parallel type for now.
114 /// FIXME (pnkfelix): Currently the `box` protocol only supports
115 /// creating instances of sized types. This IntermediateBox is
116 /// designed to be forward-compatible with a future protocol that
117 /// supports creating instances of unsized types; that is why the type
118 /// parameter has the `?Sized` generalization marker, and is also why
119 /// this carries an explicit size. However, it probably does not need
120 /// to carry the explicit alignment; that is just a work-around for
121 /// the fact that the `align_of` intrinsic currently requires the
122 /// input type to be Sized (which I do not think is strictly
124 #[unstable(feature = "placement_in", reason = "placement box design is still being worked out.")]
125 pub struct IntermediateBox<T: ?Sized>{
129 marker: marker::PhantomData<*mut T>,
132 impl<T> Place<T> for IntermediateBox<T> {
133 fn pointer(&mut self) -> *mut T {
134 unsafe { ::core::mem::transmute(self.ptr) }
138 unsafe fn finalize<T>(b: IntermediateBox<T>) -> Box<T> {
139 let p = b.ptr as *mut T;
144 fn make_place<T>() -> IntermediateBox<T> {
145 let size = mem::size_of::<T>();
146 let align = mem::align_of::<T>();
148 let p = if size == 0 {
149 heap::EMPTY as *mut u8
152 heap::allocate(size, align)
155 panic!("Box make_place allocation failure.");
160 IntermediateBox { ptr: p, size: size, align: align, marker: marker::PhantomData }
163 impl<T> BoxPlace<T> for IntermediateBox<T> {
164 fn make_place() -> IntermediateBox<T> { make_place() }
167 impl<T> InPlace<T> for IntermediateBox<T> {
169 unsafe fn finalize(self) -> Box<T> { finalize(self) }
172 impl<T> Boxed for Box<T> {
174 type Place = IntermediateBox<T>;
175 unsafe fn finalize(b: IntermediateBox<T>) -> Box<T> { finalize(b) }
178 impl<T> Placer<T> for ExchangeHeapSingleton {
179 type Place = IntermediateBox<T>;
181 fn make_place(self) -> IntermediateBox<T> {
186 impl<T: ?Sized> Drop for IntermediateBox<T> {
190 heap::deallocate(self.ptr, self.size, self.align)
197 /// Allocates memory on the heap and then moves `x` into it.
202 /// let x = Box::new(5);
204 #[stable(feature = "rust1", since = "1.0.0")]
206 pub fn new(x: T) -> Box<T> {
211 impl<T : ?Sized> Box<T> {
212 /// Constructs a box from the raw pointer.
214 /// After this function call, pointer is owned by resulting box.
215 /// In particular, it means that `Box` destructor calls destructor
216 /// of `T` and releases memory. Since the way `Box` allocates and
217 /// releases memory is unspecified, the only valid pointer to pass
218 /// to this function is the one taken from another `Box` with
219 /// `Box::into_raw` function.
221 /// Function is unsafe, because improper use of this function may
222 /// lead to memory problems like double-free, for example if the
223 /// function is called twice on the same raw pointer.
224 #[unstable(feature = "box_raw",
225 reason = "may be renamed or moved out of Box scope")]
227 // NB: may want to be called from_ptr, see comments on CStr::from_ptr
228 pub unsafe fn from_raw(raw: *mut T) -> Self {
232 /// Consumes the `Box`, returning the wrapped raw pointer.
234 /// After call to this function, caller is responsible for the memory
235 /// previously managed by `Box`, in particular caller should properly
236 /// destroy `T` and release memory. The proper way to do it is to
237 /// convert pointer back to `Box` with `Box::from_raw` function, because
238 /// `Box` does not specify, how memory is allocated.
242 /// #![feature(box_raw)]
244 /// let seventeen = Box::new(17u32);
245 /// let raw = Box::into_raw(seventeen);
246 /// let boxed_again = unsafe { Box::from_raw(raw) };
248 #[unstable(feature = "box_raw", reason = "may be renamed")]
250 // NB: may want to be called into_ptr, see comments on CStr::from_ptr
251 pub fn into_raw(b: Box<T>) -> *mut T {
252 unsafe { mem::transmute(b) }
256 #[stable(feature = "rust1", since = "1.0.0")]
257 impl<T: Default> Default for Box<T> {
258 #[stable(feature = "rust1", since = "1.0.0")]
259 fn default() -> Box<T> { box Default::default() }
262 #[stable(feature = "rust1", since = "1.0.0")]
263 impl<T> Default for Box<[T]> {
264 #[stable(feature = "rust1", since = "1.0.0")]
265 fn default() -> Box<[T]> { Box::<[T; 0]>::new([]) }
268 #[stable(feature = "rust1", since = "1.0.0")]
269 impl<T: Clone> Clone for Box<T> {
270 /// Returns a new box with a `clone()` of this box's contents.
275 /// let x = Box::new(5);
276 /// let y = x.clone();
279 fn clone(&self) -> Box<T> { box {(**self).clone()} }
280 /// Copies `source`'s contents into `self` without creating a new allocation.
285 /// #![feature(box_raw)]
287 /// let x = Box::new(5);
288 /// let mut y = Box::new(10);
290 /// y.clone_from(&x);
292 /// assert_eq!(*y, 5);
295 fn clone_from(&mut self, source: &Box<T>) {
296 (**self).clone_from(&(**source));
301 #[stable(feature = "box_slice_clone", since = "1.3.0")]
302 impl Clone for Box<str> {
303 fn clone(&self) -> Self {
304 let len = self.len();
305 let buf = RawVec::with_capacity(len);
307 ptr::copy_nonoverlapping(self.as_ptr(), buf.ptr(), len);
308 mem::transmute(buf.into_box()) // bytes to str ~magic
313 #[stable(feature = "rust1", since = "1.0.0")]
314 impl<T: ?Sized + PartialEq> PartialEq for Box<T> {
316 fn eq(&self, other: &Box<T>) -> bool { PartialEq::eq(&**self, &**other) }
318 fn ne(&self, other: &Box<T>) -> bool { PartialEq::ne(&**self, &**other) }
320 #[stable(feature = "rust1", since = "1.0.0")]
321 impl<T: ?Sized + PartialOrd> PartialOrd for Box<T> {
323 fn partial_cmp(&self, other: &Box<T>) -> Option<Ordering> {
324 PartialOrd::partial_cmp(&**self, &**other)
327 fn lt(&self, other: &Box<T>) -> bool { PartialOrd::lt(&**self, &**other) }
329 fn le(&self, other: &Box<T>) -> bool { PartialOrd::le(&**self, &**other) }
331 fn ge(&self, other: &Box<T>) -> bool { PartialOrd::ge(&**self, &**other) }
333 fn gt(&self, other: &Box<T>) -> bool { PartialOrd::gt(&**self, &**other) }
335 #[stable(feature = "rust1", since = "1.0.0")]
336 impl<T: ?Sized + Ord> Ord for Box<T> {
338 fn cmp(&self, other: &Box<T>) -> Ordering {
339 Ord::cmp(&**self, &**other)
342 #[stable(feature = "rust1", since = "1.0.0")]
343 impl<T: ?Sized + Eq> Eq for Box<T> {}
345 #[stable(feature = "rust1", since = "1.0.0")]
346 impl<T: ?Sized + Hash> Hash for Box<T> {
347 fn hash<H: hash::Hasher>(&self, state: &mut H) {
348 (**self).hash(state);
354 #[stable(feature = "rust1", since = "1.0.0")]
355 /// Attempt to downcast the box to a concrete type.
356 pub fn downcast<T: Any>(self) -> Result<Box<T>, Box<Any>> {
359 // Get the raw representation of the trait object
360 let raw = Box::into_raw(self);
361 let to: TraitObject =
362 mem::transmute::<*mut Any, TraitObject>(raw);
364 // Extract the data pointer
365 Ok(Box::from_raw(to.data as *mut T))
373 impl Box<Any + Send> {
375 #[stable(feature = "rust1", since = "1.0.0")]
376 /// Attempt to downcast the box to a concrete type.
377 pub fn downcast<T: Any>(self) -> Result<Box<T>, Box<Any + Send>> {
378 <Box<Any>>::downcast(self).map_err(|s| unsafe {
379 // reapply the Send marker
380 mem::transmute::<Box<Any>, Box<Any + Send>>(s)
385 #[stable(feature = "rust1", since = "1.0.0")]
386 impl<T: fmt::Display + ?Sized> fmt::Display for Box<T> {
387 fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
388 fmt::Display::fmt(&**self, f)
392 #[stable(feature = "rust1", since = "1.0.0")]
393 impl<T: fmt::Debug + ?Sized> fmt::Debug for Box<T> {
394 fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
395 fmt::Debug::fmt(&**self, f)
399 #[stable(feature = "rust1", since = "1.0.0")]
400 impl<T> fmt::Pointer for Box<T> {
401 fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
402 // It's not possible to extract the inner Uniq directly from the Box,
403 // instead we cast it to a *const which aliases the Unique
404 let ptr: *const T = &**self;
405 fmt::Pointer::fmt(&ptr, f)
409 #[stable(feature = "rust1", since = "1.0.0")]
410 impl<T: ?Sized> Deref for Box<T> {
413 fn deref(&self) -> &T { &**self }
416 #[stable(feature = "rust1", since = "1.0.0")]
417 impl<T: ?Sized> DerefMut for Box<T> {
418 fn deref_mut(&mut self) -> &mut T { &mut **self }
421 #[stable(feature = "rust1", since = "1.0.0")]
422 impl<I: Iterator + ?Sized> Iterator for Box<I> {
424 fn next(&mut self) -> Option<I::Item> { (**self).next() }
425 fn size_hint(&self) -> (usize, Option<usize>) { (**self).size_hint() }
427 #[stable(feature = "rust1", since = "1.0.0")]
428 impl<I: DoubleEndedIterator + ?Sized> DoubleEndedIterator for Box<I> {
429 fn next_back(&mut self) -> Option<I::Item> { (**self).next_back() }
431 #[stable(feature = "rust1", since = "1.0.0")]
432 impl<I: ExactSizeIterator + ?Sized> ExactSizeIterator for Box<I> {}
435 /// `FnBox` is a version of the `FnOnce` intended for use with boxed
436 /// closure objects. The idea is that where one would normally store a
437 /// `Box<FnOnce()>` in a data structure, you should use
438 /// `Box<FnBox()>`. The two traits behave essentially the same, except
439 /// that a `FnBox` closure can only be called if it is boxed. (Note
440 /// that `FnBox` may be deprecated in the future if `Box<FnOnce()>`
441 /// closures become directly usable.)
445 /// Here is a snippet of code which creates a hashmap full of boxed
446 /// once closures and then removes them one by one, calling each
447 /// closure as it is removed. Note that the type of the closures
448 /// stored in the map is `Box<FnBox() -> i32>` and not `Box<FnOnce()
452 /// #![feature(fnbox)]
454 /// use std::boxed::FnBox;
455 /// use std::collections::HashMap;
457 /// fn make_map() -> HashMap<i32, Box<FnBox() -> i32>> {
458 /// let mut map: HashMap<i32, Box<FnBox() -> i32>> = HashMap::new();
459 /// map.insert(1, Box::new(|| 22));
460 /// map.insert(2, Box::new(|| 44));
465 /// let mut map = make_map();
466 /// for i in &[1, 2] {
467 /// let f = map.remove(&i).unwrap();
468 /// assert_eq!(f(), i * 22);
473 #[unstable(feature = "fnbox", reason = "Newly introduced")]
477 fn call_box(self: Box<Self>, args: A) -> Self::Output;
480 impl<A,F> FnBox<A> for F
483 type Output = F::Output;
485 fn call_box(self: Box<F>, args: A) -> F::Output {
490 impl<'a,A,R> FnOnce<A> for Box<FnBox<A,Output=R>+'a> {
493 extern "rust-call" fn call_once(self, args: A) -> R {
498 impl<'a,A,R> FnOnce<A> for Box<FnBox<A,Output=R>+Send+'a> {
501 extern "rust-call" fn call_once(self, args: A) -> R {
506 impl<T: ?Sized+Unsize<U>, U: ?Sized> CoerceUnsized<Box<U>> for Box<T> {}
508 #[stable(feature = "box_slice_clone", since = "1.3.0")]
509 impl<T: Clone> Clone for Box<[T]> {
510 fn clone(&self) -> Self {
511 let mut new = BoxBuilder {
512 data: RawVec::with_capacity(self.len()),
516 let mut target = new.data.ptr();
518 for item in self.iter() {
520 ptr::write(target, item.clone());
521 target = target.offset(1);
527 return unsafe { new.into_box() };
529 // Helper type for responding to panics correctly.
530 struct BoxBuilder<T> {
535 impl<T> BoxBuilder<T> {
536 unsafe fn into_box(self) -> Box<[T]> {
537 let raw = ptr::read(&self.data);
543 impl<T> Drop for BoxBuilder<T> {
545 let mut data = self.data.ptr();
546 let max = unsafe { data.offset(self.len as isize) };
551 data = data.offset(1);