boxed.rs.html -- source
Description: Source to the Rust file `src/liballoc/boxed.rs`.
Keywords: rust, rustlang, rust-lang

      1
      2
      3
      4
      5
      6
      7
      8
      9
     10
     11
     12
     13
     14
     15
     16
     17
     18
     19
     20
     21
     22
     23
     24
     25
     26
     27
     28
     29
     30
     31
     32
     33
     34
     35
     36
     37
     38
     39
     40
     41
     42
     43
     44
     45
     46
     47
     48
     49
     50
     51
     52
     53
     54
     55
     56
     57
     58
     59
     60
     61
     62
     63
     64
     65
     66
     67
     68
     69
     70
     71
     72
     73
     74
     75
     76
     77
     78
     79
     80
     81
     82
     83
     84
     85
     86
     87
     88
     89
     90
     91
     92
     93
     94
     95
     96
     97
     98
     99
    100
    101
    102
    103
    104
    105
    106
    107
    108
    109
    110
    111
    112
    113
    114
    115
    116
    117
    118
    119
    120
    121
    122
    123
    124
    125
    126
    127
    128
    129
    130
    131
    132
    133
    134
    135
    136
    137
    138
    139
    140
    141
    142
    143
    144
    145
    146
    147
    148
    149
    150
    151
    152
    153
    154
    155
    156
    157
    158
    159
    160
    161
    162
    163
    164
    165
    166
    167
    168
    169
    170
    171
    172
    173
    174
    175
    176
    177
    178
    179
    180
    181
    182
    183
    184
    185
    186
    187
    188
    189
    190
    191
    192
    193
    194
    195
    196
    197
    198
    199
    200
    201
    202
    203
    204
    205
    206
    207
    208
    209
    210
    211
    212
    213
    214
    215
    216
    217
    218
    219
    220
    221
    222
    223
    224
    225
    226
    227
    228
    229
    230
    231
    232
    233
    234
    235
    236
    237
    238
    239
    240
    241
    242
    243
    244
    245
    246
    247
    248
    249
    250
    251
    252
    253
    254
    255
    256
    257
    258
    259
    260
    261
    262
    263
    264
    265
    266
    267
    268
    269
    270
    271
    272
    273
    274
    275
    276
    277
    278
    279
    280
    281
    282
    283
    284
    285
    286
    287
    288
    289
    290
    291
    292
    293
    294
    295
    296
    297
    298
    299
    300
    301
    302
    303
    304
    305
    306
    307
    308
    309
    310
    311
    312
    313
    314
    315
    316
    317
    318
    319
    320
    321
    322
    323
    324
    325
    326
    327
    328
    329
    330
    331
    332
    333
    334
    335
    336
    337
    338
    339
    340
    341
    342
    343
    344
    345
    346
    347
    348
    349
    350
    351
    352
    353
    354
    355
    356
    357
    358
    359
    360
    361
    362
    363
    364
    365
    366
    367
    368
    369
    370
    371
    372
    373
    374
    375
    376
    377
    378
    379
    380
    381
    382
    383
    384
    385
    386
    387
    388
    389
    390
    391
    392
    393
    394
    395
    396
    397
    398
    399
    400
    401
    402
    403
    404
    405
    406
    407
    408
    409
    410
    411
    412
    413
    414
    415
    416
    417
    418
    419
    420
    421
    422
    423
    424
    425
    426
    427
    428
    429
    430
    431
    432
    433
    434
    435
    436
    437
    438
    439
    440
    441
    442
    443
    444
    445
    446
    447
    448
    449
    450
    451
    452
    453
    454
    455
    456
    457
    458
    459
    460
    461
    462
    463
    464
    465
    466
    467
    468
    469
    470
    471
    472
    473
    474
    475
    476
    477
    478
    479
    480
    481
    482
    483
    484
    485
    486
    487
    488
    489
    490
    491
    492
    493
    494
    495
    496
    497
    498
    499
    500
    501
    502
    503
    504
    505
    506
    507
    508
    509
    510
    511
    512
    513
    514
    515
    516
    517
    518
    519
    520
    521
    522
    523
    524
    525
    526
    527
    528
    529
    530
    531
    532
    533
    534
    535
    536
    537
    538
    539
    540
    541
    542
    543
    544
    545
    546
    547
    548
    549
    550
    551
    552
    553
    554
    555
    556
    557
    558
    559
    560
    561
    562
    563
    564
    565
    566
    567
    568
    569
    570
    571
    572
    573
    574
    575
    576
    577
    578
    579
    580
    581
    582
    583
    584
    585
    586
    587
    588
    589
    590
    591
    592
    593
    594
    595
    596
    597
    598
    599
    600
    601
    602
    603
    604
    605
    606
    607
    608
    609
    610
    611
    612
    613
    614
    615
    616
    617
    618
    619
    620
    621
    622
    623
    624
    625
    626
    627
    628
    629
    630
    631
    632
    633
    634
    635
    636
    637
    638
    639
    640
    641
    642
    643
    644
    645
    646
    647
    648
    649
    650
    651
    652
    653
    654
    655
    656
    657
    658
    659
    660
    661
    662
    663
    664
    665
    666
    667
    668
    669
    670
    671
    672
    673
    674
    675
    676
    677
    678
    679
    680
    681
    682
    683
    684
    685
    686
    687
    688
    689
    690
    691
    692
    693
    694
    695
    696
    697
    698
    699
    700
    701
    702
    703
    704
    705
    706
    707
    708
    709
    710
    711
    712
    713
    714
    715
    716
    717
    718
    719
    720
    721
    722
    723
    724
    725
    726
    727
    728
    729
    730
    731
    732
    733
    734
    735
    736
    737
    738
    739
    740
    741
    742
    743
    744
    745
    746
    747
    748
    749
    750
    751
    752
    753
    754
    755
    756
    757
    758
    759
    760
    761
    762
    763
    764
    765
    766
    767
    768
    769
    770
    771
    772
    773
    774
    775
    776
    777
    778
    779
    780
    781
    782
    783
    784
    785
    786

    // Copyright 2012-2015 The Rust Project Developers. See the COPYRIGHT
    // file at the top-level directory of this distribution and at
    // http://rust-lang.org/COPYRIGHT.
    //
    // Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
    // http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
    // <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
    // option. This file may not be copied, modified, or distributed
    // except according to those terms.

    //! A pointer type for heap allocation.
    //!
    //! `Box<T>`, casually referred to as a 'box', provides the simplest form of
    //! heap allocation in Rust. Boxes provide ownership for this allocation, and
    //! drop their contents when they go out of scope.
    //!
    //! # Examples
    //!
    //! Creating a box:
    //!
    //! ```
    //! let x = Box::new(5);
    //! ```
    //!
    //! Creating a recursive data structure:
    //!
    //! ```
    //! #[derive(Debug)]
    //! enum List<T> {
    //!     Cons(T, Box<List<T>>),
    //!     Nil,
    //! }
    //!
    //! fn main() {
    //!     let list: List<i32> = List::Cons(1, Box::new(List::Cons(2, Box::new(List::Nil))));
    //!     println!("{:?}", list);
    //! }
    //! ```
    //!
    //! This will print `Cons(1, Cons(2, Nil))`.
    //!
    //! Recursive structures must be boxed, because if the definition of `Cons`
    //! looked like this:
    //!
    //! ```compile_fail,E0072
    //! # enum List<T> {
    //! Cons(T, List<T>),
    //! # }
    //! ```
    //!
    //! It wouldn't work. This is because the size of a `List` depends on how many
    //! elements are in the list, and so we don't know how much memory to allocate
    //! for a `Cons`. By introducing a `Box`, which has a defined size, we know how
    //! big `Cons` needs to be.

    #![stable(feature = "rust1", since = "1.0.0")]

    use heap::{Heap, Layout, Alloc};
    use raw_vec::RawVec;

    use core::any::Any;
    use core::borrow;
    use core::cmp::Ordering;
    use core::fmt;
    use core::hash::{self, Hash};
    use core::iter::FusedIterator;
    use core::marker::{self, Unsize};
    use core::mem;
    use core::ops::{CoerceUnsized, Deref, DerefMut};
    use core::ops::{BoxPlace, Boxed, InPlace, Place, Placer};
    use core::ptr::{self, Unique};
    use core::convert::From;
    use str::from_boxed_utf8_unchecked;

    /// A value that represents the heap. This is the default place that the `box`
    /// keyword allocates into when no place is supplied.
    ///
    /// The following two examples are equivalent:
    ///
    /// ```
    /// #![feature(box_heap)]
    ///
    /// #![feature(box_syntax, placement_in_syntax)]
    /// use std::boxed::HEAP;
    ///
    /// fn main() {
    ///     let foo: Box<i32> = in HEAP { 5 };
    ///     let foo = box 5;
    /// }
    /// ```
    #[unstable(feature = "box_heap",
               reason = "may be renamed; uncertain about custom allocator design",
               issue = "27779")]
    pub const HEAP: ExchangeHeapSingleton = ExchangeHeapSingleton { _force_singleton: () };

    /// This the singleton type used solely for `boxed::HEAP`.
    #[unstable(feature = "box_heap",
               reason = "may be renamed; uncertain about custom allocator design",
               issue = "27779")]
    #[allow(missing_debug_implementations)]
    #[derive(Copy, Clone)]
    pub struct ExchangeHeapSingleton {
        _force_singleton: (),
    }

    /// A pointer type for heap allocation.
    ///
    /// See the [module-level documentation](../../std/boxed/index.html) for more.
    #[lang = "owned_box"]
    #[fundamental]
    #[stable(feature = "rust1", since = "1.0.0")]
    pub struct Box<T: ?Sized>(Unique<T>);

    /// `IntermediateBox` represents uninitialized backing storage for `Box`.
    ///
    /// FIXME (pnkfelix): Ideally we would just reuse `Box<T>` instead of
    /// introducing a separate `IntermediateBox<T>`; but then you hit
    /// issues when you e.g. attempt to destructure an instance of `Box`,
    /// since it is a lang item and so it gets special handling by the
    /// compiler.  Easier just to make this parallel type for now.
    ///
    /// FIXME (pnkfelix): Currently the `box` protocol only supports
    /// creating instances of sized types. This IntermediateBox is
    /// designed to be forward-compatible with a future protocol that
    /// supports creating instances of unsized types; that is why the type
    /// parameter has the `?Sized` generalization marker, and is also why
    /// this carries an explicit size. However, it probably does not need
    /// to carry the explicit alignment; that is just a work-around for
    /// the fact that the `align_of` intrinsic currently requires the
    /// input type to be Sized (which I do not think is strictly
    /// necessary).
    #[unstable(feature = "placement_in",
               reason = "placement box design is still being worked out.",
               issue = "27779")]
    #[allow(missing_debug_implementations)]
    pub struct IntermediateBox<T: ?Sized> {
        ptr: *mut u8,
        layout: Layout,
        marker: marker::PhantomData<*mut T>,
    }

    #[unstable(feature = "placement_in",
               reason = "placement box design is still being worked out.",
               issue = "27779")]
    impl<T> Place<T> for IntermediateBox<T> {
        fn pointer(&mut self) -> *mut T {
            self.ptr as *mut T
        }
    }

    unsafe fn finalize<T>(b: IntermediateBox<T>) -> Box<T> {
        let p = b.ptr as *mut T;
        mem::forget(b);
        mem::transmute(p)
    }

    fn make_place<T>() -> IntermediateBox<T> {
        let layout = Layout::new::<T>();

        let p = if layout.size() == 0 {
            mem::align_of::<T>() as *mut u8
        } else {
            unsafe {
                Heap.alloc(layout.clone()).unwrap_or_else(|err| {
                    Heap.oom(err)
                })
            }
        };

        IntermediateBox {
            ptr: p,
            layout,
            marker: marker::PhantomData,
        }
    }

    #[unstable(feature = "placement_in",
               reason = "placement box design is still being worked out.",
               issue = "27779")]
    impl<T> BoxPlace<T> for IntermediateBox<T> {
        fn make_place() -> IntermediateBox<T> {
            make_place()
        }
    }

    #[unstable(feature = "placement_in",
               reason = "placement box design is still being worked out.",
               issue = "27779")]
    impl<T> InPlace<T> for IntermediateBox<T> {
        type Owner = Box<T>;
        unsafe fn finalize(self) -> Box<T> {
            finalize(self)
        }
    }

    #[unstable(feature = "placement_new_protocol", issue = "27779")]
    impl<T> Boxed for Box<T> {
        type Data = T;
        type Place = IntermediateBox<T>;
        unsafe fn finalize(b: IntermediateBox<T>) -> Box<T> {
            finalize(b)
        }
    }

    #[unstable(feature = "placement_in",
               reason = "placement box design is still being worked out.",
               issue = "27779")]
    impl<T> Placer<T> for ExchangeHeapSingleton {
        type Place = IntermediateBox<T>;

        fn make_place(self) -> IntermediateBox<T> {
            make_place()
        }
    }

    #[unstable(feature = "placement_in",
               reason = "placement box design is still being worked out.",
               issue = "27779")]
    impl<T: ?Sized> Drop for IntermediateBox<T> {
        fn drop(&mut self) {
            if self.layout.size() > 0 {
                unsafe {
                    Heap.dealloc(self.ptr, self.layout.clone())
                }
            }
        }
    }

    impl<T> Box<T> {
        /// Allocates memory on the heap and then places `x` into it.
        ///
        /// This doesn't actually allocate if `T` is zero-sized.
        ///
        /// # Examples
        ///
        /// ```
        /// let five = Box::new(5);
        /// ```
        #[stable(feature = "rust1", since = "1.0.0")]
        #[inline(always)]
        pub fn new(x: T) -> Box<T> {
            box x
        }
    }

    impl<T: ?Sized> Box<T> {
        /// Constructs a box from a raw pointer.
        ///
        /// After calling this function, the raw pointer is owned by the
        /// resulting `Box`. Specifically, the `Box` destructor will call
        /// the destructor of `T` and free the allocated memory. Since the
        /// way `Box` allocates and releases memory is unspecified, the
        /// only valid pointer to pass to this function is the one taken
        /// from another `Box` via the [`Box::into_raw`] function.
        ///
        /// This function is unsafe because improper use may lead to
        /// memory problems. For example, a double-free may occur if the
        /// function is called twice on the same raw pointer.
        ///
        /// [`Box::into_raw`]: struct.Box.html#method.into_raw
        ///
        /// # Examples
        ///
        /// ```
        /// let x = Box::new(5);
        /// let ptr = Box::into_raw(x);
        /// let x = unsafe { Box::from_raw(ptr) };
        /// ```
        #[stable(feature = "box_raw", since = "1.4.0")]
        #[inline]
        pub unsafe fn from_raw(raw: *mut T) -> Self {
            mem::transmute(raw)
        }

        /// Consumes the `Box`, returning the wrapped raw pointer.
        ///
        /// After calling this function, the caller is responsible for the
        /// memory previously managed by the `Box`. In particular, the
        /// caller should properly destroy `T` and release the memory. The
        /// proper way to do so is to convert the raw pointer back into a
        /// `Box` with the [`Box::from_raw`] function.
        ///
        /// Note: this is an associated function, which means that you have
        /// to call it as `Box::into_raw(b)` instead of `b.into_raw()`. This
        /// is so that there is no conflict with a method on the inner type.
        ///
        /// [`Box::from_raw`]: struct.Box.html#method.from_raw
        ///
        /// # Examples
        ///
        /// ```
        /// let x = Box::new(5);
        /// let ptr = Box::into_raw(x);
        /// ```
        #[stable(feature = "box_raw", since = "1.4.0")]
        #[inline]
        pub fn into_raw(b: Box<T>) -> *mut T {
            unsafe { mem::transmute(b) }
        }

        /// Consumes the `Box`, returning the wrapped pointer as `Unique<T>`.
        ///
        /// After calling this function, the caller is responsible for the
        /// memory previously managed by the `Box`. In particular, the
        /// caller should properly destroy `T` and release the memory. The
        /// proper way to do so is to convert the raw pointer back into a
        /// `Box` with the [`Box::from_raw`] function.
        ///
        /// Note: this is an associated function, which means that you have
        /// to call it as `Box::into_unique(b)` instead of `b.into_unique()`. This
        /// is so that there is no conflict with a method on the inner type.
        ///
        /// [`Box::from_raw`]: struct.Box.html#method.from_raw
        ///
        /// # Examples
        ///
        /// ```
        /// #![feature(unique)]
        ///
        /// fn main() {
        ///     let x = Box::new(5);
        ///     let ptr = Box::into_unique(x);
        /// }
        /// ```
        #[unstable(feature = "unique", reason = "needs an RFC to flesh out design",
                   issue = "27730")]
        #[inline]
        pub fn into_unique(b: Box<T>) -> Unique<T> {
            unsafe { mem::transmute(b) }
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    unsafe impl<#[may_dangle] T: ?Sized> Drop for Box<T> {
        fn drop(&mut self) {
            // FIXME: Do nothing, drop is currently performed by compiler.
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<T: Default> Default for Box<T> {
        /// Creates a `Box<T>`, with the `Default` value for T.
        fn default() -> Box<T> {
            box Default::default()
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<T> Default for Box<[T]> {
        fn default() -> Box<[T]> {
            Box::<[T; 0]>::new([])
        }
    }

    #[stable(feature = "default_box_extra", since = "1.17.0")]
    impl Default for Box<str> {
        fn default() -> Box<str> {
            unsafe { from_boxed_utf8_unchecked(Default::default()) }
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<T: Clone> Clone for Box<T> {
        /// Returns a new box with a `clone()` of this box's contents.
        ///
        /// # Examples
        ///
        /// ```
        /// let x = Box::new(5);
        /// let y = x.clone();
        /// ```
        #[rustfmt_skip]
        #[inline]
        fn clone(&self) -> Box<T> {
            box { (**self).clone() }
        }
        /// Copies `source`'s contents into `self` without creating a new allocation.
        ///
        /// # Examples
        ///
        /// ```
        /// let x = Box::new(5);
        /// let mut y = Box::new(10);
        ///
        /// y.clone_from(&x);
        ///
        /// assert_eq!(*y, 5);
        /// ```
        #[inline]
        fn clone_from(&mut self, source: &Box<T>) {
            (**self).clone_from(&(**source));
        }
    }

    #[stable(feature = "box_slice_clone", since = "1.3.0")]
    impl Clone for Box<str> {
        fn clone(&self) -> Self {
            let len = self.len();
            let buf = RawVec::with_capacity(len);
            unsafe {
                ptr::copy_nonoverlapping(self.as_ptr(), buf.ptr(), len);
                from_boxed_utf8_unchecked(buf.into_box())
            }
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<T: ?Sized + PartialEq> PartialEq for Box<T> {
        #[inline]
        fn eq(&self, other: &Box<T>) -> bool {
            PartialEq::eq(&**self, &**other)
        }
        #[inline]
        fn ne(&self, other: &Box<T>) -> bool {
            PartialEq::ne(&**self, &**other)
        }
    }
    #[stable(feature = "rust1", since = "1.0.0")]
    impl<T: ?Sized + PartialOrd> PartialOrd for Box<T> {
        #[inline]
        fn partial_cmp(&self, other: &Box<T>) -> Option<Ordering> {
            PartialOrd::partial_cmp(&**self, &**other)
        }
        #[inline]
        fn lt(&self, other: &Box<T>) -> bool {
            PartialOrd::lt(&**self, &**other)
        }
        #[inline]
        fn le(&self, other: &Box<T>) -> bool {
            PartialOrd::le(&**self, &**other)
        }
        #[inline]
        fn ge(&self, other: &Box<T>) -> bool {
            PartialOrd::ge(&**self, &**other)
        }
        #[inline]
        fn gt(&self, other: &Box<T>) -> bool {
            PartialOrd::gt(&**self, &**other)
        }
    }
    #[stable(feature = "rust1", since = "1.0.0")]
    impl<T: ?Sized + Ord> Ord for Box<T> {
        #[inline]
        fn cmp(&self, other: &Box<T>) -> Ordering {
            Ord::cmp(&**self, &**other)
        }
    }
    #[stable(feature = "rust1", since = "1.0.0")]
    impl<T: ?Sized + Eq> Eq for Box<T> {}

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<T: ?Sized + Hash> Hash for Box<T> {
        fn hash<H: hash::Hasher>(&self, state: &mut H) {
            (**self).hash(state);
        }
    }

    #[stable(feature = "from_for_ptrs", since = "1.6.0")]
    impl<T> From<T> for Box<T> {
        fn from(t: T) -> Self {
            Box::new(t)
        }
    }

    #[stable(feature = "box_from_slice", since = "1.17.0")]
    impl<'a, T: Copy> From<&'a [T]> for Box<[T]> {
        fn from(slice: &'a [T]) -> Box<[T]> {
            let mut boxed = unsafe { RawVec::with_capacity(slice.len()).into_box() };
            boxed.copy_from_slice(slice);
            boxed
        }
    }

    #[stable(feature = "box_from_slice", since = "1.17.0")]
    impl<'a> From<&'a str> for Box<str> {
        fn from(s: &'a str) -> Box<str> {
            unsafe { from_boxed_utf8_unchecked(Box::from(s.as_bytes())) }
        }
    }

    #[stable(feature = "boxed_str_conv", since = "1.19.0")]
    impl From<Box<str>> for Box<[u8]> {
        fn from(s: Box<str>) -> Self {
            unsafe {
                mem::transmute(s)
            }
        }
    }

    impl Box<Any> {
        #[inline]
        #[stable(feature = "rust1", since = "1.0.0")]
        /// Attempt to downcast the box to a concrete type.
        ///
        /// # Examples
        ///
        /// ```
        /// use std::any::Any;
        ///
        /// fn print_if_string(value: Box<Any>) {
        ///     if let Ok(string) = value.downcast::<String>() {
        ///         println!("String ({}): {}", string.len(), string);
        ///     }
        /// }
        ///
        /// fn main() {
        ///     let my_string = "Hello World".to_string();
        ///     print_if_string(Box::new(my_string));
        ///     print_if_string(Box::new(0i8));
        /// }
        /// ```
        pub fn downcast<T: Any>(self) -> Result<Box<T>, Box<Any>> {
            if self.is::<T>() {
                unsafe {
                    let raw: *mut Any = Box::into_raw(self);
                    Ok(Box::from_raw(raw as *mut T))
                }
            } else {
                Err(self)
            }
        }
    }

    impl Box<Any + Send> {
        #[inline]
        #[stable(feature = "rust1", since = "1.0.0")]
        /// Attempt to downcast the box to a concrete type.
        ///
        /// # Examples
        ///
        /// ```
        /// use std::any::Any;
        ///
        /// fn print_if_string(value: Box<Any + Send>) {
        ///     if let Ok(string) = value.downcast::<String>() {
        ///         println!("String ({}): {}", string.len(), string);
        ///     }
        /// }
        ///
        /// fn main() {
        ///     let my_string = "Hello World".to_string();
        ///     print_if_string(Box::new(my_string));
        ///     print_if_string(Box::new(0i8));
        /// }
        /// ```
        pub fn downcast<T: Any>(self) -> Result<Box<T>, Box<Any + Send>> {
            <Box<Any>>::downcast(self).map_err(|s| unsafe {
                // reapply the Send marker
                mem::transmute::<Box<Any>, Box<Any + Send>>(s)
            })
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<T: fmt::Display + ?Sized> fmt::Display for Box<T> {
        fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
            fmt::Display::fmt(&**self, f)
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<T: fmt::Debug + ?Sized> fmt::Debug for Box<T> {
        fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
            fmt::Debug::fmt(&**self, f)
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<T: ?Sized> fmt::Pointer for Box<T> {
        fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
            // It's not possible to extract the inner Uniq directly from the Box,
            // instead we cast it to a *const which aliases the Unique
            let ptr: *const T = &**self;
            fmt::Pointer::fmt(&ptr, f)
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<T: ?Sized> Deref for Box<T> {
        type Target = T;

        fn deref(&self) -> &T {
            &**self
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<T: ?Sized> DerefMut for Box<T> {
        fn deref_mut(&mut self) -> &mut T {
            &mut **self
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<I: Iterator + ?Sized> Iterator for Box<I> {
        type Item = I::Item;
        fn next(&mut self) -> Option<I::Item> {
            (**self).next()
        }
        fn size_hint(&self) -> (usize, Option<usize>) {
            (**self).size_hint()
        }
        fn nth(&mut self, n: usize) -> Option<I::Item> {
            (**self).nth(n)
        }
    }
    #[stable(feature = "rust1", since = "1.0.0")]
    impl<I: DoubleEndedIterator + ?Sized> DoubleEndedIterator for Box<I> {
        fn next_back(&mut self) -> Option<I::Item> {
            (**self).next_back()
        }
    }
    #[stable(feature = "rust1", since = "1.0.0")]
    impl<I: ExactSizeIterator + ?Sized> ExactSizeIterator for Box<I> {
        fn len(&self) -> usize {
            (**self).len()
        }
        fn is_empty(&self) -> bool {
            (**self).is_empty()
        }
    }

    #[unstable(feature = "fused", issue = "35602")]
    impl<I: FusedIterator + ?Sized> FusedIterator for Box<I> {}

    /// `FnBox` is a version of the `FnOnce` intended for use with boxed
    /// closure objects. The idea is that where one would normally store a
    /// `Box<FnOnce()>` in a data structure, you should use
    /// `Box<FnBox()>`. The two traits behave essentially the same, except
    /// that a `FnBox` closure can only be called if it is boxed. (Note
    /// that `FnBox` may be deprecated in the future if `Box<FnOnce()>`
    /// closures become directly usable.)
    ///
    /// # Examples
    ///
    /// Here is a snippet of code which creates a hashmap full of boxed
    /// once closures and then removes them one by one, calling each
    /// closure as it is removed. Note that the type of the closures
    /// stored in the map is `Box<FnBox() -> i32>` and not `Box<FnOnce()
    /// -> i32>`.
    ///
    /// ```
    /// #![feature(fnbox)]
    ///
    /// use std::boxed::FnBox;
    /// use std::collections::HashMap;
    ///
    /// fn make_map() -> HashMap<i32, Box<FnBox() -> i32>> {
    ///     let mut map: HashMap<i32, Box<FnBox() -> i32>> = HashMap::new();
    ///     map.insert(1, Box::new(|| 22));
    ///     map.insert(2, Box::new(|| 44));
    ///     map
    /// }
    ///
    /// fn main() {
    ///     let mut map = make_map();
    ///     for i in &[1, 2] {
    ///         let f = map.remove(&i).unwrap();
    ///         assert_eq!(f(), i * 22);
    ///     }
    /// }
    /// ```
    #[rustc_paren_sugar]
    #[unstable(feature = "fnbox",
               reason = "will be deprecated if and when `Box<FnOnce>` becomes usable", issue = "28796")]
    pub trait FnBox<A> {
        type Output;

        fn call_box(self: Box<Self>, args: A) -> Self::Output;
    }

    #[unstable(feature = "fnbox",
               reason = "will be deprecated if and when `Box<FnOnce>` becomes usable", issue = "28796")]
    impl<A, F> FnBox<A> for F
        where F: FnOnce<A>
    {
        type Output = F::Output;

        fn call_box(self: Box<F>, args: A) -> F::Output {
            self.call_once(args)
        }
    }

    #[unstable(feature = "fnbox",
               reason = "will be deprecated if and when `Box<FnOnce>` becomes usable", issue = "28796")]
    impl<'a, A, R> FnOnce<A> for Box<FnBox<A, Output = R> + 'a> {
        type Output = R;

        extern "rust-call" fn call_once(self, args: A) -> R {
            self.call_box(args)
        }
    }

    #[unstable(feature = "fnbox",
               reason = "will be deprecated if and when `Box<FnOnce>` becomes usable", issue = "28796")]
    impl<'a, A, R> FnOnce<A> for Box<FnBox<A, Output = R> + Send + 'a> {
        type Output = R;

        extern "rust-call" fn call_once(self, args: A) -> R {
            self.call_box(args)
        }
    }

    #[unstable(feature = "coerce_unsized", issue = "27732")]
    impl<T: ?Sized + Unsize<U>, U: ?Sized> CoerceUnsized<Box<U>> for Box<T> {}

    #[stable(feature = "box_slice_clone", since = "1.3.0")]
    impl<T: Clone> Clone for Box<[T]> {
        fn clone(&self) -> Self {
            let mut new = BoxBuilder {
                data: RawVec::with_capacity(self.len()),
                len: 0,
            };

            let mut target = new.data.ptr();

            for item in self.iter() {
                unsafe {
                    ptr::write(target, item.clone());
                    target = target.offset(1);
                };

                new.len += 1;
            }

            return unsafe { new.into_box() };

            // Helper type for responding to panics correctly.
            struct BoxBuilder<T> {
                data: RawVec<T>,
                len: usize,
            }

            impl<T> BoxBuilder<T> {
                unsafe fn into_box(self) -> Box<[T]> {
                    let raw = ptr::read(&self.data);
                    mem::forget(self);
                    raw.into_box()
                }
            }

            impl<T> Drop for BoxBuilder<T> {
                fn drop(&mut self) {
                    let mut data = self.data.ptr();
                    let max = unsafe { data.offset(self.len as isize) };

                    while data != max {
                        unsafe {
                            ptr::read(data);
                            data = data.offset(1);
                        }
                    }
                }
            }
        }
    }

    #[stable(feature = "box_borrow", since = "1.1.0")]
    impl<T: ?Sized> borrow::Borrow<T> for Box<T> {
        fn borrow(&self) -> &T {
            &**self
        }
    }

    #[stable(feature = "box_borrow", since = "1.1.0")]
    impl<T: ?Sized> borrow::BorrowMut<T> for Box<T> {
        fn borrow_mut(&mut self) -> &mut T {
            &mut **self
        }
    }

    #[stable(since = "1.5.0", feature = "smart_ptr_as_ref")]
    impl<T: ?Sized> AsRef<T> for Box<T> {
        fn as_ref(&self) -> &T {
            &**self
        }
    }

    #[stable(since = "1.5.0", feature = "smart_ptr_as_ref")]
    impl<T: ?Sized> AsMut<T> for Box<T> {
        fn as_mut(&mut self) -> &mut T {
            &mut **self
        }
    }

# Help

## Keyboard Shortcuts

?
  Show this help dialog
S
  Focus the search field
⇤
  Move up in search results
⇥
  Move down in search results
⏎
  Go to active search result
+
  Collapse/expand all sections

## Search Tricks

Prefix searches with a type followed by a colon (e.g. `fn:`) to restrict the search to a given type.

Accepted types are: `fn`, `mod`, `struct`, `enum`, `trait`, `type`, `macro`, and `const`.

Search functions by type signature (e.g. `vec -> usize` or `* -> vec`)