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::marker::{self, Unsize};
66 use core::ops::{CoerceUnsized, Deref, DerefMut};
67 use core::ops::{Placer, Boxed, Place, InPlace, BoxPlace};
68 use core::ptr::{self, Unique};
69 use core::raw::{TraitObject};
71 /// A value that represents the heap. This is the default place that the `box`
72 /// keyword allocates into when no place is supplied.
74 /// The following two examples are equivalent:
77 /// #![feature(box_heap)]
79 /// #![feature(box_syntax, placement_in_syntax)]
80 /// use std::boxed::HEAP;
83 /// let foo = box(HEAP) 5;
87 #[lang = "exchange_heap"]
88 #[unstable(feature = "box_heap",
89 reason = "may be renamed; uncertain about custom allocator design",
91 pub const HEAP: ExchangeHeapSingleton =
92 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 { _force_singleton: () }
101 /// A pointer type for heap allocation.
103 /// See the [module-level documentation](../../std/boxed/index.html) for more.
104 #[lang = "owned_box"]
105 #[stable(feature = "rust1", since = "1.0.0")]
107 pub struct Box<T: ?Sized>(Unique<T>);
109 /// `IntermediateBox` represents uninitialized backing storage for `Box`.
111 /// FIXME (pnkfelix): Ideally we would just reuse `Box<T>` instead of
112 /// introducing a separate `IntermediateBox<T>`; but then you hit
113 /// issues when you e.g. attempt to destructure an instance of `Box`,
114 /// since it is a lang item and so it gets special handling by the
115 /// compiler. Easier just to make this parallel type for now.
117 /// FIXME (pnkfelix): Currently the `box` protocol only supports
118 /// creating instances of sized types. This IntermediateBox is
119 /// designed to be forward-compatible with a future protocol that
120 /// supports creating instances of unsized types; that is why the type
121 /// parameter has the `?Sized` generalization marker, and is also why
122 /// this carries an explicit size. However, it probably does not need
123 /// to carry the explicit alignment; that is just a work-around for
124 /// the fact that the `align_of` intrinsic currently requires the
125 /// input type to be Sized (which I do not think is strictly
127 #[unstable(feature = "placement_in",
128 reason = "placement box design is still being worked out.",
130 pub struct IntermediateBox<T: ?Sized>{
134 marker: marker::PhantomData<*mut T>,
137 impl<T> Place<T> for IntermediateBox<T> {
138 fn pointer(&mut self) -> *mut T {
139 unsafe { ::core::mem::transmute(self.ptr) }
143 unsafe fn finalize<T>(b: IntermediateBox<T>) -> Box<T> {
144 let p = b.ptr as *mut T;
149 fn make_place<T>() -> IntermediateBox<T> {
150 let size = mem::size_of::<T>();
151 let align = mem::align_of::<T>();
153 let p = if size == 0 {
154 heap::EMPTY as *mut u8
157 heap::allocate(size, align)
160 panic!("Box make_place allocation failure.");
165 IntermediateBox { ptr: p, size: size, align: align, marker: marker::PhantomData }
168 impl<T> BoxPlace<T> for IntermediateBox<T> {
169 fn make_place() -> IntermediateBox<T> { make_place() }
172 impl<T> InPlace<T> for IntermediateBox<T> {
174 unsafe fn finalize(self) -> Box<T> { finalize(self) }
177 impl<T> Boxed for Box<T> {
179 type Place = IntermediateBox<T>;
180 unsafe fn finalize(b: IntermediateBox<T>) -> Box<T> { finalize(b) }
183 impl<T> Placer<T> for ExchangeHeapSingleton {
184 type Place = IntermediateBox<T>;
186 fn make_place(self) -> IntermediateBox<T> {
191 impl<T: ?Sized> Drop for IntermediateBox<T> {
195 heap::deallocate(self.ptr, self.size, self.align)
202 /// Allocates memory on the heap and then moves `x` into it.
207 /// let x = Box::new(5);
209 #[stable(feature = "rust1", since = "1.0.0")]
211 pub fn new(x: T) -> Box<T> {
216 impl<T : ?Sized> Box<T> {
217 /// Constructs a box from the raw pointer.
219 /// After this function call, pointer is owned by resulting box.
220 /// In particular, it means that `Box` destructor calls destructor
221 /// of `T` and releases memory. Since the way `Box` allocates and
222 /// releases memory is unspecified, the only valid pointer to pass
223 /// to this function is the one taken from another `Box` with
224 /// `Box::into_raw` function.
226 /// Function is unsafe, because improper use of this function may
227 /// lead to memory problems like double-free, for example if the
228 /// function is called twice on the same raw pointer.
229 #[stable(feature = "box_raw", since = "1.4.0")]
231 pub unsafe fn from_raw(raw: *mut T) -> Self {
235 /// Consumes the `Box`, returning the wrapped raw pointer.
237 /// After call to this function, caller is responsible for the memory
238 /// previously managed by `Box`, in particular caller should properly
239 /// destroy `T` and release memory. The proper way to do it is to
240 /// convert pointer back to `Box` with `Box::from_raw` function, because
241 /// `Box` does not specify, how memory is allocated.
246 /// let seventeen = Box::new(17u32);
247 /// let raw = Box::into_raw(seventeen);
248 /// let boxed_again = unsafe { Box::from_raw(raw) };
250 #[stable(feature = "box_raw", since = "1.4.0")]
252 pub fn into_raw(b: Box<T>) -> *mut T {
253 unsafe { mem::transmute(b) }
257 #[stable(feature = "rust1", since = "1.0.0")]
258 impl<T: Default> Default for Box<T> {
259 #[stable(feature = "rust1", since = "1.0.0")]
260 fn default() -> Box<T> { box Default::default() }
263 #[stable(feature = "rust1", since = "1.0.0")]
264 impl<T> Default for Box<[T]> {
265 #[stable(feature = "rust1", since = "1.0.0")]
266 fn default() -> Box<[T]> { Box::<[T; 0]>::new([]) }
269 #[stable(feature = "rust1", since = "1.0.0")]
270 impl<T: Clone> Clone for Box<T> {
271 /// Returns a new box with a `clone()` of this box's contents.
276 /// let x = Box::new(5);
277 /// let y = x.clone();
280 fn clone(&self) -> Box<T> { box {(**self).clone()} }
281 /// Copies `source`'s contents into `self` without creating a new allocation.
286 /// let x = Box::new(5);
287 /// let mut y = Box::new(10);
289 /// y.clone_from(&x);
291 /// assert_eq!(*y, 5);
294 fn clone_from(&mut self, source: &Box<T>) {
295 (**self).clone_from(&(**source));
300 #[stable(feature = "box_slice_clone", since = "1.3.0")]
301 impl Clone for Box<str> {
302 fn clone(&self) -> Self {
303 let len = self.len();
304 let buf = RawVec::with_capacity(len);
306 ptr::copy_nonoverlapping(self.as_ptr(), buf.ptr(), len);
307 mem::transmute(buf.into_box()) // bytes to str ~magic
312 #[stable(feature = "rust1", since = "1.0.0")]
313 impl<T: ?Sized + PartialEq> PartialEq for Box<T> {
315 fn eq(&self, other: &Box<T>) -> bool { PartialEq::eq(&**self, &**other) }
317 fn ne(&self, other: &Box<T>) -> bool { PartialEq::ne(&**self, &**other) }
319 #[stable(feature = "rust1", since = "1.0.0")]
320 impl<T: ?Sized + PartialOrd> PartialOrd for Box<T> {
322 fn partial_cmp(&self, other: &Box<T>) -> Option<Ordering> {
323 PartialOrd::partial_cmp(&**self, &**other)
326 fn lt(&self, other: &Box<T>) -> bool { PartialOrd::lt(&**self, &**other) }
328 fn le(&self, other: &Box<T>) -> bool { PartialOrd::le(&**self, &**other) }
330 fn ge(&self, other: &Box<T>) -> bool { PartialOrd::ge(&**self, &**other) }
332 fn gt(&self, other: &Box<T>) -> bool { PartialOrd::gt(&**self, &**other) }
334 #[stable(feature = "rust1", since = "1.0.0")]
335 impl<T: ?Sized + Ord> Ord for Box<T> {
337 fn cmp(&self, other: &Box<T>) -> Ordering {
338 Ord::cmp(&**self, &**other)
341 #[stable(feature = "rust1", since = "1.0.0")]
342 impl<T: ?Sized + Eq> Eq for Box<T> {}
344 #[stable(feature = "rust1", since = "1.0.0")]
345 impl<T: ?Sized + Hash> Hash for Box<T> {
346 fn hash<H: hash::Hasher>(&self, state: &mut H) {
347 (**self).hash(state);
353 #[stable(feature = "rust1", since = "1.0.0")]
354 /// Attempt to downcast the box to a concrete type.
355 pub fn downcast<T: Any>(self) -> Result<Box<T>, Box<Any>> {
358 // Get the raw representation of the trait object
359 let raw = Box::into_raw(self);
360 let to: TraitObject =
361 mem::transmute::<*mut Any, TraitObject>(raw);
363 // Extract the data pointer
364 Ok(Box::from_raw(to.data as *mut T))
372 impl Box<Any + Send> {
374 #[stable(feature = "rust1", since = "1.0.0")]
375 /// Attempt to downcast the box to a concrete type.
376 pub fn downcast<T: Any>(self) -> Result<Box<T>, Box<Any + Send>> {
377 <Box<Any>>::downcast(self).map_err(|s| unsafe {
378 // reapply the Send marker
379 mem::transmute::<Box<Any>, Box<Any + Send>>(s)
384 #[stable(feature = "rust1", since = "1.0.0")]
385 impl<T: fmt::Display + ?Sized> fmt::Display for Box<T> {
386 fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
387 fmt::Display::fmt(&**self, f)
391 #[stable(feature = "rust1", since = "1.0.0")]
392 impl<T: fmt::Debug + ?Sized> fmt::Debug for Box<T> {
393 fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
394 fmt::Debug::fmt(&**self, f)
398 #[stable(feature = "rust1", since = "1.0.0")]
399 impl<T> fmt::Pointer for Box<T> {
400 fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
401 // It's not possible to extract the inner Uniq directly from the Box,
402 // instead we cast it to a *const which aliases the Unique
403 let ptr: *const T = &**self;
404 fmt::Pointer::fmt(&ptr, f)
408 #[stable(feature = "rust1", since = "1.0.0")]
409 impl<T: ?Sized> Deref for Box<T> {
412 fn deref(&self) -> &T { &**self }
415 #[stable(feature = "rust1", since = "1.0.0")]
416 impl<T: ?Sized> DerefMut for Box<T> {
417 fn deref_mut(&mut self) -> &mut T { &mut **self }
420 #[stable(feature = "rust1", since = "1.0.0")]
421 impl<I: Iterator + ?Sized> Iterator for Box<I> {
423 fn next(&mut self) -> Option<I::Item> { (**self).next() }
424 fn size_hint(&self) -> (usize, Option<usize>) { (**self).size_hint() }
426 #[stable(feature = "rust1", since = "1.0.0")]
427 impl<I: DoubleEndedIterator + ?Sized> DoubleEndedIterator for Box<I> {
428 fn next_back(&mut self) -> Option<I::Item> { (**self).next_back() }
430 #[stable(feature = "rust1", since = "1.0.0")]
431 impl<I: ExactSizeIterator + ?Sized> ExactSizeIterator for Box<I> {}
434 /// `FnBox` is a version of the `FnOnce` intended for use with boxed
435 /// closure objects. The idea is that where one would normally store a
436 /// `Box<FnOnce()>` in a data structure, you should use
437 /// `Box<FnBox()>`. The two traits behave essentially the same, except
438 /// that a `FnBox` closure can only be called if it is boxed. (Note
439 /// that `FnBox` may be deprecated in the future if `Box<FnOnce()>`
440 /// closures become directly usable.)
444 /// Here is a snippet of code which creates a hashmap full of boxed
445 /// once closures and then removes them one by one, calling each
446 /// closure as it is removed. Note that the type of the closures
447 /// stored in the map is `Box<FnBox() -> i32>` and not `Box<FnOnce()
451 /// #![feature(fnbox)]
453 /// use std::boxed::FnBox;
454 /// use std::collections::HashMap;
456 /// fn make_map() -> HashMap<i32, Box<FnBox() -> i32>> {
457 /// let mut map: HashMap<i32, Box<FnBox() -> i32>> = HashMap::new();
458 /// map.insert(1, Box::new(|| 22));
459 /// map.insert(2, Box::new(|| 44));
464 /// let mut map = make_map();
465 /// for i in &[1, 2] {
466 /// let f = map.remove(&i).unwrap();
467 /// assert_eq!(f(), i * 22);
472 #[unstable(feature = "fnbox", reason = "Newly introduced", issue = "0")]
476 fn call_box(self: Box<Self>, args: A) -> Self::Output;
479 impl<A,F> FnBox<A> for F
482 type Output = F::Output;
484 fn call_box(self: Box<F>, args: A) -> F::Output {
489 impl<'a,A,R> FnOnce<A> for Box<FnBox<A,Output=R>+'a> {
492 extern "rust-call" fn call_once(self, args: A) -> R {
497 impl<'a,A,R> FnOnce<A> for Box<FnBox<A,Output=R>+Send+'a> {
500 extern "rust-call" fn call_once(self, args: A) -> R {
505 impl<T: ?Sized+Unsize<U>, U: ?Sized> CoerceUnsized<Box<U>> for Box<T> {}
507 #[stable(feature = "box_slice_clone", since = "1.3.0")]
508 impl<T: Clone> Clone for Box<[T]> {
509 fn clone(&self) -> Self {
510 let mut new = BoxBuilder {
511 data: RawVec::with_capacity(self.len()),
515 let mut target = new.data.ptr();
517 for item in self.iter() {
519 ptr::write(target, item.clone());
520 target = target.offset(1);
526 return unsafe { new.into_box() };
528 // Helper type for responding to panics correctly.
529 struct BoxBuilder<T> {
534 impl<T> BoxBuilder<T> {
535 unsafe fn into_box(self) -> Box<[T]> {
536 let raw = ptr::read(&self.data);
542 impl<T> Drop for BoxBuilder<T> {
544 let mut data = self.data.ptr();
545 let max = unsafe { data.offset(self.len as isize) };
550 data = data.offset(1);
558 impl<T: ?Sized> borrow::Borrow<T> for Box<T> {
559 fn borrow(&self) -> &T { &**self }
562 impl<T: ?Sized> borrow::BorrowMut<T> for Box<T> {
563 fn borrow_mut(&mut self) -> &mut T { &mut **self }