1 // Copyright 2012-2014 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 //! Basic functions for dealing with memory.
13 //! This module contains functions for querying the size and alignment of
14 //! types, initializing and manipulating memory.
16 #![stable(feature = "rust1", since = "1.0.0")]
23 use marker::{Copy, PhantomData, Sized};
25 use ops::{Deref, DerefMut};
27 #[stable(feature = "rust1", since = "1.0.0")]
28 pub use intrinsics::transmute;
30 /// Takes ownership and "forgets" about the value **without running its destructor**.
32 /// Any resources the value manages, such as heap memory or a file handle, will linger
33 /// forever in an unreachable state. However, it does not guarantee that pointers
34 /// to this memory will remain valid.
36 /// * If you want to leak memory, see [`Box::leak`][leak].
37 /// * If you want to obtain a raw pointer to the memory, see [`Box::into_raw`][into_raw].
38 /// * If you want to dispose of a value properly, running its destructor, see
39 /// [`mem::drop`][drop].
43 /// `forget` is not marked as `unsafe`, because Rust's safety guarantees
44 /// do not include a guarantee that destructors will always run. For example,
45 /// a program can create a reference cycle using [`Rc`][rc], or call
46 /// [`process::exit`][exit] to exit without running destructors. Thus, allowing
47 /// `mem::forget` from safe code does not fundamentally change Rust's safety
50 /// That said, leaking resources such as memory or I/O objects is usually undesirable,
51 /// so `forget` is only recommended for specialized use cases like those shown below.
53 /// Because forgetting a value is allowed, any `unsafe` code you write must
54 /// allow for this possibility. You cannot return a value and expect that the
55 /// caller will necessarily run the value's destructor.
57 /// [rc]: ../../std/rc/struct.Rc.html
58 /// [exit]: ../../std/process/fn.exit.html
62 /// Leak an I/O object, never closing the file:
66 /// use std::fs::File;
68 /// let file = File::open("foo.txt").unwrap();
69 /// mem::forget(file);
72 /// The practical use cases for `forget` are rather specialized and mainly come
73 /// up in unsafe or FFI code.
77 /// You have created an uninitialized value using [`mem::uninitialized`][uninit].
78 /// You must either initialize or `forget` it on every computation path before
79 /// Rust drops it automatically, like at the end of a scope or after a panic.
80 /// Running the destructor on an uninitialized value would be [undefined behavior][ub].
86 /// # let some_condition = false;
88 /// let mut uninit_vec: Vec<u32> = mem::uninitialized();
90 /// if some_condition {
91 /// // Initialize the variable.
92 /// ptr::write(&mut uninit_vec, Vec::new());
94 /// // Forget the uninitialized value so its destructor doesn't run.
95 /// mem::forget(uninit_vec);
102 /// You have duplicated the bytes making up a value, without doing a proper
103 /// [`Clone`][clone]. You need the value's destructor to run only once,
104 /// because a double `free` is undefined behavior.
106 /// An example is a possible implementation of [`mem::swap`][swap]:
112 /// # #[allow(dead_code)]
113 /// fn swap<T>(x: &mut T, y: &mut T) {
115 /// // Give ourselves some scratch space to work with
116 /// let mut t: T = mem::uninitialized();
118 /// // Perform the swap, `&mut` pointers never alias
119 /// ptr::copy_nonoverlapping(&*x, &mut t, 1);
120 /// ptr::copy_nonoverlapping(&*y, x, 1);
121 /// ptr::copy_nonoverlapping(&t, y, 1);
123 /// // y and t now point to the same thing, but we need to completely
124 /// // forget `t` because we do not want to run the destructor for `T`
125 /// // on its value, which is still owned somewhere outside this function.
131 /// [drop]: fn.drop.html
132 /// [uninit]: fn.uninitialized.html
133 /// [clone]: ../clone/trait.Clone.html
134 /// [swap]: fn.swap.html
135 /// [FFI]: ../../book/first-edition/ffi.html
136 /// [box]: ../../std/boxed/struct.Box.html
137 /// [leak]: ../../std/boxed/struct.Box.html#method.leak
138 /// [into_raw]: ../../std/boxed/struct.Box.html#method.into_raw
139 /// [ub]: ../../reference/behavior-considered-undefined.html
141 #[stable(feature = "rust1", since = "1.0.0")]
142 pub fn forget<T>(t: T) {
143 ManuallyDrop::new(t);
146 /// Returns the size of a type in bytes.
148 /// More specifically, this is the offset in bytes between successive elements
149 /// in an array with that item type including alignment padding. Thus, for any
150 /// type `T` and length `n`, `[T; n]` has a size of `n * size_of::<T>()`.
152 /// In general, the size of a type is not stable across compilations, but
153 /// specific types such as primitives are.
155 /// The following table gives the size for primitives.
157 /// Type | size_of::\<Type>()
158 /// ---- | ---------------
175 /// Furthermore, `usize` and `isize` have the same size.
177 /// The types `*const T`, `&T`, `Box<T>`, `Option<&T>`, and `Option<Box<T>>` all have
178 /// the same size. If `T` is Sized, all of those types have the same size as `usize`.
180 /// The mutability of a pointer does not change its size. As such, `&T` and `&mut T`
181 /// have the same size. Likewise for `*const T` and `*mut T`.
183 /// # Size of `#[repr(C)]` items
185 /// The `C` representation for items has a defined layout. With this layout,
186 /// the size of items is also stable as long as all fields have a stable size.
188 /// ## Size of Structs
190 /// For `structs`, the size is determined by the following algorithm.
192 /// For each field in the struct ordered by declaration order:
194 /// 1. Add the size of the field.
195 /// 2. Round up the current size to the nearest multiple of the next field's [alignment].
197 /// Finally, round the size of the struct to the nearest multiple of its [alignment].
198 /// The alignment of the struct is usually the largest alignment of all its
199 /// fields; this can be changed with the use of `repr(align(N))`.
201 /// Unlike `C`, zero sized structs are not rounded up to one byte in size.
205 /// Enums that carry no data other than the descriminant have the same size as C enums
206 /// on the platform they are compiled for.
208 /// ## Size of Unions
210 /// The size of a union is the size of its largest field.
212 /// Unlike `C`, zero sized unions are not rounded up to one byte in size.
219 /// // Some primitives
220 /// assert_eq!(4, mem::size_of::<i32>());
221 /// assert_eq!(8, mem::size_of::<f64>());
222 /// assert_eq!(0, mem::size_of::<()>());
225 /// assert_eq!(8, mem::size_of::<[i32; 2]>());
226 /// assert_eq!(12, mem::size_of::<[i32; 3]>());
227 /// assert_eq!(0, mem::size_of::<[i32; 0]>());
230 /// // Pointer size equality
231 /// assert_eq!(mem::size_of::<&i32>(), mem::size_of::<*const i32>());
232 /// assert_eq!(mem::size_of::<&i32>(), mem::size_of::<Box<i32>>());
233 /// assert_eq!(mem::size_of::<&i32>(), mem::size_of::<Option<&i32>>());
234 /// assert_eq!(mem::size_of::<Box<i32>>(), mem::size_of::<Option<Box<i32>>>());
237 /// Using `#[repr(C)]`.
243 /// struct FieldStruct {
249 /// // The size of the first field is 1, so add 1 to the size. Size is 1.
250 /// // The alignment of the second field is 2, so add 1 to the size for padding. Size is 2.
251 /// // The size of the second field is 2, so add 2 to the size. Size is 4.
252 /// // The alignment of the third field is 1, so add 0 to the size for padding. Size is 4.
253 /// // The size of the third field is 1, so add 1 to the size. Size is 5.
254 /// // Finally, the alignment of the struct is 2 (because the largest alignment amongst its
255 /// // fields is 2), so add 1 to the size for padding. Size is 6.
256 /// assert_eq!(6, mem::size_of::<FieldStruct>());
259 /// struct TupleStruct(u8, u16, u8);
261 /// // Tuple structs follow the same rules.
262 /// assert_eq!(6, mem::size_of::<TupleStruct>());
264 /// // Note that reordering the fields can lower the size. We can remove both padding bytes
265 /// // by putting `third` before `second`.
267 /// struct FieldStructOptimized {
273 /// assert_eq!(4, mem::size_of::<FieldStructOptimized>());
275 /// // Union size is the size of the largest field.
277 /// union ExampleUnion {
282 /// assert_eq!(2, mem::size_of::<ExampleUnion>());
285 /// [alignment]: ./fn.align_of.html
287 #[stable(feature = "rust1", since = "1.0.0")]
289 pub const fn size_of<T>() -> usize {
290 intrinsics::size_of::<T>()
294 #[stable(feature = "rust1", since = "1.0.0")]
296 /// Ceci n'est pas la documentation
297 pub const fn size_of<T>() -> usize {
298 unsafe { intrinsics::size_of::<T>() }
301 /// Returns the size of the pointed-to value in bytes.
303 /// This is usually the same as `size_of::<T>()`. However, when `T` *has* no
304 /// statically known size, e.g. a slice [`[T]`][slice] or a [trait object],
305 /// then `size_of_val` can be used to get the dynamically-known size.
307 /// [slice]: ../../std/primitive.slice.html
308 /// [trait object]: ../../book/first-edition/trait-objects.html
315 /// assert_eq!(4, mem::size_of_val(&5i32));
317 /// let x: [u8; 13] = [0; 13];
318 /// let y: &[u8] = &x;
319 /// assert_eq!(13, mem::size_of_val(y));
322 #[stable(feature = "rust1", since = "1.0.0")]
323 pub fn size_of_val<T: ?Sized>(val: &T) -> usize {
324 unsafe { intrinsics::size_of_val(val) }
327 /// Returns the [ABI]-required minimum alignment of a type.
329 /// Every reference to a value of the type `T` must be a multiple of this number.
331 /// This is the alignment used for struct fields. It may be smaller than the preferred alignment.
333 /// [ABI]: https://en.wikipedia.org/wiki/Application_binary_interface
338 /// # #![allow(deprecated)]
341 /// assert_eq!(4, mem::min_align_of::<i32>());
344 #[stable(feature = "rust1", since = "1.0.0")]
345 #[rustc_deprecated(reason = "use `align_of` instead", since = "1.2.0")]
347 pub fn min_align_of<T>() -> usize {
348 intrinsics::min_align_of::<T>()
352 #[stable(feature = "rust1", since = "1.0.0")]
353 #[rustc_deprecated(reason = "use `align_of` instead", since = "1.2.0")]
355 /// Ceci n'est pas la documentation
356 pub fn min_align_of<T>() -> usize {
357 unsafe { intrinsics::min_align_of::<T>() }
360 /// Returns the [ABI]-required minimum alignment of the type of the value that `val` points to.
362 /// Every reference to a value of the type `T` must be a multiple of this number.
364 /// [ABI]: https://en.wikipedia.org/wiki/Application_binary_interface
369 /// # #![allow(deprecated)]
372 /// assert_eq!(4, mem::min_align_of_val(&5i32));
375 #[stable(feature = "rust1", since = "1.0.0")]
376 #[rustc_deprecated(reason = "use `align_of_val` instead", since = "1.2.0")]
377 pub fn min_align_of_val<T: ?Sized>(val: &T) -> usize {
378 unsafe { intrinsics::min_align_of_val(val) }
381 /// Returns the [ABI]-required minimum alignment of a type.
383 /// Every reference to a value of the type `T` must be a multiple of this number.
385 /// This is the alignment used for struct fields. It may be smaller than the preferred alignment.
387 /// [ABI]: https://en.wikipedia.org/wiki/Application_binary_interface
394 /// assert_eq!(4, mem::align_of::<i32>());
397 #[stable(feature = "rust1", since = "1.0.0")]
399 pub const fn align_of<T>() -> usize {
400 intrinsics::min_align_of::<T>()
404 #[stable(feature = "rust1", since = "1.0.0")]
406 /// Ceci n'est pas la documentation
407 pub const fn align_of<T>() -> usize {
408 unsafe { intrinsics::min_align_of::<T>() }
411 /// Returns the [ABI]-required minimum alignment of the type of the value that `val` points to.
413 /// Every reference to a value of the type `T` must be a multiple of this number.
415 /// [ABI]: https://en.wikipedia.org/wiki/Application_binary_interface
422 /// assert_eq!(4, mem::align_of_val(&5i32));
425 #[stable(feature = "rust1", since = "1.0.0")]
426 pub fn align_of_val<T: ?Sized>(val: &T) -> usize {
427 unsafe { intrinsics::min_align_of_val(val) }
430 /// Returns whether dropping values of type `T` matters.
432 /// This is purely an optimization hint, and may be implemented conservatively:
433 /// it may return `true` for types that don't actually need to be dropped.
434 /// As such always returning `true` would be a valid implementation of
435 /// this function. However if this function actually returns `false`, then you
436 /// can be certain dropping `T` has no side effect.
438 /// Low level implementations of things like collections, which need to manually
439 /// drop their data, should use this function to avoid unnecessarily
440 /// trying to drop all their contents when they are destroyed. This might not
441 /// make a difference in release builds (where a loop that has no side-effects
442 /// is easily detected and eliminated), but is often a big win for debug builds.
444 /// Note that `ptr::drop_in_place` already performs this check, so if your workload
445 /// can be reduced to some small number of drop_in_place calls, using this is
446 /// unnecessary. In particular note that you can drop_in_place a slice, and that
447 /// will do a single needs_drop check for all the values.
449 /// Types like Vec therefore just `drop_in_place(&mut self[..])` without using
450 /// needs_drop explicitly. Types like HashMap, on the other hand, have to drop
451 /// values one at a time and should use this API.
456 /// Here's an example of how a collection might make use of needs_drop:
459 /// use std::{mem, ptr};
461 /// pub struct MyCollection<T> {
465 /// # impl<T> MyCollection<T> {
466 /// # fn iter_mut(&mut self) -> &mut [T] { &mut self.data }
467 /// # fn free_buffer(&mut self) {}
470 /// impl<T> Drop for MyCollection<T> {
471 /// fn drop(&mut self) {
474 /// if mem::needs_drop::<T>() {
475 /// for x in self.iter_mut() {
476 /// ptr::drop_in_place(x);
479 /// self.free_buffer();
485 #[stable(feature = "needs_drop", since = "1.21.0")]
487 pub const fn needs_drop<T>() -> bool {
488 intrinsics::needs_drop::<T>()
492 #[stable(feature = "needs_drop", since = "1.21.0")]
494 /// Ceci n'est pas la documentation
495 pub fn needs_drop<T>() -> bool {
496 unsafe { intrinsics::needs_drop::<T>() }
499 /// Creates a value whose bytes are all zero.
501 /// This has the same effect as allocating space with
502 /// [`mem::uninitialized`][uninit] and then zeroing it out. It is useful for
503 /// [FFI] sometimes, but should generally be avoided.
505 /// There is no guarantee that an all-zero byte-pattern represents a valid value of
506 /// some type `T`. If `T` has a destructor and the value is destroyed (due to
507 /// a panic or the end of a scope) before being initialized, then the destructor
508 /// will run on zeroed data, likely leading to [undefined behavior][ub].
510 /// See also the documentation for [`mem::uninitialized`][uninit], which has
511 /// many of the same caveats.
513 /// [uninit]: fn.uninitialized.html
514 /// [FFI]: ../../book/first-edition/ffi.html
515 /// [ub]: ../../reference/behavior-considered-undefined.html
522 /// let x: i32 = unsafe { mem::zeroed() };
523 /// assert_eq!(0, x);
526 #[rustc_deprecated(since = "2.0.0", reason = "use `mem::MaybeUninit::zeroed` instead")]
527 #[stable(feature = "rust1", since = "1.0.0")]
528 pub unsafe fn zeroed<T>() -> T {
532 /// Bypasses Rust's normal memory-initialization checks by pretending to
533 /// produce a value of type `T`, while doing nothing at all.
535 /// **This is incredibly dangerous and should not be done lightly. Deeply
536 /// consider initializing your memory with a default value instead.**
538 /// This is useful for [FFI] functions and initializing arrays sometimes,
539 /// but should generally be avoided.
541 /// [FFI]: ../../book/first-edition/ffi.html
543 /// # Undefined behavior
545 /// It is [undefined behavior][ub] to read uninitialized memory, even just an
546 /// uninitialized boolean. For instance, if you branch on the value of such
547 /// a boolean, your program may take one, both, or neither of the branches.
549 /// Writing to the uninitialized value is similarly dangerous. Rust believes the
550 /// value is initialized, and will therefore try to [`Drop`] the uninitialized
551 /// value and its fields if you try to overwrite it in a normal manner. The only way
552 /// to safely initialize an uninitialized value is with [`ptr::write`][write],
553 /// [`ptr::copy`][copy], or [`ptr::copy_nonoverlapping`][copy_no].
555 /// If the value does implement [`Drop`], it must be initialized before
556 /// it goes out of scope (and therefore would be dropped). Note that this
557 /// includes a `panic` occurring and unwinding the stack suddenly.
561 /// Here's how to safely initialize an array of [`Vec`]s.
567 /// // Only declare the array. This safely leaves it
568 /// // uninitialized in a way that Rust will track for us.
569 /// // However we can't initialize it element-by-element
570 /// // safely, and we can't use the `[value; 1000]`
571 /// // constructor because it only works with `Copy` data.
572 /// let mut data: [Vec<u32>; 1000];
575 /// // So we need to do this to initialize it.
576 /// data = mem::uninitialized();
578 /// // DANGER ZONE: if anything panics or otherwise
579 /// // incorrectly reads the array here, we will have
580 /// // Undefined Behavior.
582 /// // It's ok to mutably iterate the data, since this
583 /// // doesn't involve reading it at all.
584 /// // (ptr and len are statically known for arrays)
585 /// for elem in &mut data[..] {
586 /// // *elem = Vec::new() would try to drop the
587 /// // uninitialized memory at `elem` -- bad!
589 /// // Vec::new doesn't allocate or do really
590 /// // anything. It's only safe to call here
591 /// // because we know it won't panic.
592 /// ptr::write(elem, Vec::new());
595 /// // SAFE ZONE: everything is initialized.
598 /// println!("{:?}", &data[0]);
601 /// This example emphasizes exactly how delicate and dangerous using `mem::uninitialized`
602 /// can be. Note that the [`vec!`] macro *does* let you initialize every element with a
603 /// value that is only [`Clone`], so the following is semantically equivalent and
604 /// vastly less dangerous, as long as you can live with an extra heap
608 /// let data: Vec<Vec<u32>> = vec![Vec::new(); 1000];
609 /// println!("{:?}", &data[0]);
612 /// [`Vec`]: ../../std/vec/struct.Vec.html
613 /// [`vec!`]: ../../std/macro.vec.html
614 /// [`Clone`]: ../../std/clone/trait.Clone.html
615 /// [ub]: ../../reference/behavior-considered-undefined.html
616 /// [write]: ../ptr/fn.write.html
617 /// [copy]: ../intrinsics/fn.copy.html
618 /// [copy_no]: ../intrinsics/fn.copy_nonoverlapping.html
619 /// [`Drop`]: ../ops/trait.Drop.html
621 #[rustc_deprecated(since = "2.0.0", reason = "use `mem::MaybeUninit::uninitialized` instead")]
622 #[stable(feature = "rust1", since = "1.0.0")]
623 pub unsafe fn uninitialized<T>() -> T {
627 /// Swaps the values at two mutable locations, without deinitializing either one.
637 /// mem::swap(&mut x, &mut y);
639 /// assert_eq!(42, x);
640 /// assert_eq!(5, y);
643 #[stable(feature = "rust1", since = "1.0.0")]
644 pub fn swap<T>(x: &mut T, y: &mut T) {
646 ptr::swap_nonoverlapping_one(x, y);
650 /// Moves `src` into the referenced `dest`, returning the previous `dest` value.
652 /// Neither value is dropped.
656 /// A simple example:
661 /// let mut v: Vec<i32> = vec![1, 2];
663 /// let old_v = mem::replace(&mut v, vec![3, 4, 5]);
664 /// assert_eq!(2, old_v.len());
665 /// assert_eq!(3, v.len());
668 /// `replace` allows consumption of a struct field by replacing it with another value.
669 /// Without `replace` you can run into issues like these:
671 /// ```compile_fail,E0507
672 /// struct Buffer<T> { buf: Vec<T> }
674 /// impl<T> Buffer<T> {
675 /// fn get_and_reset(&mut self) -> Vec<T> {
676 /// // error: cannot move out of dereference of `&mut`-pointer
677 /// let buf = self.buf;
678 /// self.buf = Vec::new();
684 /// Note that `T` does not necessarily implement [`Clone`], so it can't even clone and reset
685 /// `self.buf`. But `replace` can be used to disassociate the original value of `self.buf` from
686 /// `self`, allowing it to be returned:
689 /// # #![allow(dead_code)]
692 /// # struct Buffer<T> { buf: Vec<T> }
693 /// impl<T> Buffer<T> {
694 /// fn get_and_reset(&mut self) -> Vec<T> {
695 /// mem::replace(&mut self.buf, Vec::new())
700 /// [`Clone`]: ../../std/clone/trait.Clone.html
702 #[stable(feature = "rust1", since = "1.0.0")]
703 pub fn replace<T>(dest: &mut T, mut src: T) -> T {
704 swap(dest, &mut src);
708 /// Disposes of a value.
710 /// While this does call the argument's implementation of [`Drop`][drop],
711 /// it will not release any borrows, as borrows are based on lexical scope.
713 /// This effectively does nothing for
714 /// [types which implement `Copy`](../../book/first-edition/ownership.html#copy-types),
715 /// e.g. integers. Such values are copied and _then_ moved into the function,
716 /// so the value persists after this function call.
718 /// This function is not magic; it is literally defined as
721 /// pub fn drop<T>(_x: T) { }
724 /// Because `_x` is moved into the function, it is automatically dropped before
725 /// the function returns.
727 /// [drop]: ../ops/trait.Drop.html
734 /// let v = vec![1, 2, 3];
736 /// drop(v); // explicitly drop the vector
739 /// Borrows are based on lexical scope, so this produces an error:
741 /// ```compile_fail,E0502
742 /// let mut v = vec![1, 2, 3];
745 /// drop(x); // explicitly drop the reference, but the borrow still exists
747 /// v.push(4); // error: cannot borrow `v` as mutable because it is also
748 /// // borrowed as immutable
751 /// An inner scope is needed to fix this:
754 /// let mut v = vec![1, 2, 3];
759 /// drop(x); // this is now redundant, as `x` is going out of scope anyway
762 /// v.push(4); // no problems
765 /// Since [`RefCell`] enforces the borrow rules at runtime, `drop` can
766 /// release a [`RefCell`] borrow:
769 /// use std::cell::RefCell;
771 /// let x = RefCell::new(1);
773 /// let mut mutable_borrow = x.borrow_mut();
774 /// *mutable_borrow = 1;
776 /// drop(mutable_borrow); // relinquish the mutable borrow on this slot
778 /// let borrow = x.borrow();
779 /// println!("{}", *borrow);
782 /// Integers and other types implementing [`Copy`] are unaffected by `drop`.
785 /// #[derive(Copy, Clone)]
790 /// drop(x); // a copy of `x` is moved and dropped
791 /// drop(y); // a copy of `y` is moved and dropped
793 /// println!("x: {}, y: {}", x, y.0); // still available
796 /// [`RefCell`]: ../../std/cell/struct.RefCell.html
797 /// [`Copy`]: ../../std/marker/trait.Copy.html
799 #[stable(feature = "rust1", since = "1.0.0")]
800 pub fn drop<T>(_x: T) { }
802 /// Interprets `src` as having type `&U`, and then reads `src` without moving
803 /// the contained value.
805 /// This function will unsafely assume the pointer `src` is valid for
806 /// [`size_of::<U>`][size_of] bytes by transmuting `&T` to `&U` and then reading
807 /// the `&U`. It will also unsafely create a copy of the contained value instead of
808 /// moving out of `src`.
810 /// It is not a compile-time error if `T` and `U` have different sizes, but it
811 /// is highly encouraged to only invoke this function where `T` and `U` have the
812 /// same size. This function triggers [undefined behavior][ub] if `U` is larger than
815 /// [ub]: ../../reference/behavior-considered-undefined.html
816 /// [size_of]: fn.size_of.html
828 /// let foo_slice = [10u8];
831 /// // Copy the data from 'foo_slice' and treat it as a 'Foo'
832 /// let mut foo_struct: Foo = mem::transmute_copy(&foo_slice);
833 /// assert_eq!(foo_struct.bar, 10);
835 /// // Modify the copied data
836 /// foo_struct.bar = 20;
837 /// assert_eq!(foo_struct.bar, 20);
840 /// // The contents of 'foo_slice' should not have changed
841 /// assert_eq!(foo_slice, [10]);
844 #[stable(feature = "rust1", since = "1.0.0")]
845 pub unsafe fn transmute_copy<T, U>(src: &T) -> U {
846 ptr::read(src as *const T as *const U)
849 /// Opaque type representing the discriminant of an enum.
851 /// See the [`discriminant`] function in this module for more information.
853 /// [`discriminant`]: fn.discriminant.html
854 #[stable(feature = "discriminant_value", since = "1.21.0")]
855 pub struct Discriminant<T>(u64, PhantomData<fn() -> T>);
857 // N.B. These trait implementations cannot be derived because we don't want any bounds on T.
859 #[stable(feature = "discriminant_value", since = "1.21.0")]
860 impl<T> Copy for Discriminant<T> {}
862 #[stable(feature = "discriminant_value", since = "1.21.0")]
863 impl<T> clone::Clone for Discriminant<T> {
864 fn clone(&self) -> Self {
869 #[stable(feature = "discriminant_value", since = "1.21.0")]
870 impl<T> cmp::PartialEq for Discriminant<T> {
871 fn eq(&self, rhs: &Self) -> bool {
876 #[stable(feature = "discriminant_value", since = "1.21.0")]
877 impl<T> cmp::Eq for Discriminant<T> {}
879 #[stable(feature = "discriminant_value", since = "1.21.0")]
880 impl<T> hash::Hash for Discriminant<T> {
881 fn hash<H: hash::Hasher>(&self, state: &mut H) {
886 #[stable(feature = "discriminant_value", since = "1.21.0")]
887 impl<T> fmt::Debug for Discriminant<T> {
888 fn fmt(&self, fmt: &mut fmt::Formatter) -> fmt::Result {
889 fmt.debug_tuple("Discriminant")
895 /// Returns a value uniquely identifying the enum variant in `v`.
897 /// If `T` is not an enum, calling this function will not result in undefined behavior, but the
898 /// return value is unspecified.
902 /// The discriminant of an enum variant may change if the enum definition changes. A discriminant
903 /// of some variant will not change between compilations with the same compiler.
907 /// This can be used to compare enums that carry data, while disregarding
913 /// enum Foo { A(&'static str), B(i32), C(i32) }
915 /// assert!(mem::discriminant(&Foo::A("bar")) == mem::discriminant(&Foo::A("baz")));
916 /// assert!(mem::discriminant(&Foo::B(1)) == mem::discriminant(&Foo::B(2)));
917 /// assert!(mem::discriminant(&Foo::B(3)) != mem::discriminant(&Foo::C(3)));
919 #[stable(feature = "discriminant_value", since = "1.21.0")]
920 pub fn discriminant<T>(v: &T) -> Discriminant<T> {
922 Discriminant(intrinsics::discriminant_value(v), PhantomData)
926 /// A wrapper to inhibit compiler from automatically calling `T`’s destructor.
928 /// This wrapper is 0-cost.
932 /// This wrapper helps with explicitly documenting the drop order dependencies between fields of
936 /// use std::mem::ManuallyDrop;
940 /// struct FruitBox {
941 /// // Immediately clear there’s something non-trivial going on with these fields.
942 /// peach: ManuallyDrop<Peach>,
943 /// melon: Melon, // Field that’s independent of the other two.
944 /// banana: ManuallyDrop<Banana>,
947 /// impl Drop for FruitBox {
948 /// fn drop(&mut self) {
950 /// // Explicit ordering in which field destructors are run specified in the intuitive
951 /// // location – the destructor of the structure containing the fields.
952 /// // Moreover, one can now reorder fields within the struct however much they want.
953 /// ManuallyDrop::drop(&mut self.peach);
954 /// ManuallyDrop::drop(&mut self.banana);
956 /// // After destructor for `FruitBox` runs (this function), the destructor for Melon gets
957 /// // invoked in the usual manner, as it is not wrapped in `ManuallyDrop`.
961 #[stable(feature = "manually_drop", since = "1.20.0")]
962 #[lang = "manually_drop"]
963 #[derive(Copy, Clone, Debug, Default, PartialEq, Eq, PartialOrd, Ord, Hash)]
965 pub struct ManuallyDrop<T: ?Sized> {
969 impl<T> ManuallyDrop<T> {
970 /// Wrap a value to be manually dropped.
975 /// use std::mem::ManuallyDrop;
976 /// ManuallyDrop::new(Box::new(()));
978 #[stable(feature = "manually_drop", since = "1.20.0")]
979 #[rustc_const_unstable(feature = "const_manually_drop_new")]
981 pub const fn new(value: T) -> ManuallyDrop<T> {
982 ManuallyDrop { value }
985 /// Extract the value from the `ManuallyDrop` container.
987 /// This allows the value to be dropped again.
992 /// use std::mem::ManuallyDrop;
993 /// let x = ManuallyDrop::new(Box::new(()));
994 /// let _: Box<()> = ManuallyDrop::into_inner(x); // This drops the `Box`.
996 #[stable(feature = "manually_drop", since = "1.20.0")]
998 pub fn into_inner(slot: ManuallyDrop<T>) -> T {
1003 impl<T: ?Sized> ManuallyDrop<T> {
1004 /// Manually drops the contained value.
1006 /// If you have ownership of the value, you can use [`ManuallyDrop::into_inner`] instead.
1010 /// This function runs the destructor of the contained value and thus the wrapped value
1011 /// now represents uninitialized data. It is up to the user of this method to ensure the
1012 /// uninitialized data is not actually used.
1014 /// [`ManuallyDrop::into_inner`]: #method.into_inner
1015 #[stable(feature = "manually_drop", since = "1.20.0")]
1017 pub unsafe fn drop(slot: &mut ManuallyDrop<T>) {
1018 ptr::drop_in_place(&mut slot.value)
1022 #[stable(feature = "manually_drop", since = "1.20.0")]
1023 impl<T: ?Sized> Deref for ManuallyDrop<T> {
1026 fn deref(&self) -> &Self::Target {
1031 #[stable(feature = "manually_drop", since = "1.20.0")]
1032 impl<T: ?Sized> DerefMut for ManuallyDrop<T> {
1034 fn deref_mut(&mut self) -> &mut Self::Target {
1039 /// A newtype to construct uninitialized instances of `T`
1040 #[allow(missing_debug_implementations)]
1041 #[unstable(feature = "maybe_uninit", issue = "53491")]
1042 // NOTE after stabilizing `MaybeUninit` proceed to deprecate `mem::{uninitialized,zeroed}`
1043 pub union MaybeUninit<T> {
1045 value: ManuallyDrop<T>,
1048 impl<T> MaybeUninit<T> {
1049 /// Create a new `MaybeUninit` in an uninitialized state.
1051 /// Note that dropping a `MaybeUninit` will never call `T`'s drop code.
1052 /// It is your responsibility to make sure `T` gets dropped if it got initialized.
1053 #[unstable(feature = "maybe_uninit", issue = "53491")]
1054 pub const fn uninitialized() -> MaybeUninit<T> {
1055 MaybeUninit { uninit: () }
1058 /// Create a new `MaybeUninit` in an uninitialized state, with the memory being
1059 /// filled with `0` bytes. It depends on `T` whether that already makes for
1060 /// proper initialization. For example, `MaybeUninit<usize>::zeroed()` is initialized,
1061 /// but `MaybeUninit<&'static i32>::zeroed()` is not because references must not
1064 /// Note that dropping a `MaybeUninit` will never call `T`'s drop code.
1065 /// It is your responsibility to make sure `T` gets dropped if it got initialized.
1066 #[unstable(feature = "maybe_uninit", issue = "53491")]
1067 pub fn zeroed() -> MaybeUninit<T> {
1068 let mut u = MaybeUninit::<T>::uninitialized();
1070 u.as_mut_ptr().write_bytes(0u8, 1);
1075 /// Set the value of the `MaybeUninit`. This overwrites any previous value without dropping it.
1076 #[unstable(feature = "maybe_uninit", issue = "53491")]
1077 pub fn set(&mut self, val: T) {
1079 self.value = ManuallyDrop::new(val);
1083 /// Extract the value from the `MaybeUninit` container. This is a great way
1084 /// to ensure that the data will get dropped, because the resulting `T` is
1085 /// subject to the usual drop handling.
1089 /// It is up to the caller to guarantee that the the `MaybeUninit` really is in an initialized
1090 /// state, otherwise this will immediately cause undefined behavior.
1091 #[unstable(feature = "maybe_uninit", issue = "53491")]
1092 pub unsafe fn into_inner(self) -> T {
1093 ManuallyDrop::into_inner(self.value)
1096 /// Get a reference to the contained value.
1100 /// It is up to the caller to guarantee that the the `MaybeUninit` really is in an initialized
1101 /// state, otherwise this will immediately cause undefined behavior.
1102 #[unstable(feature = "maybe_uninit", issue = "53491")]
1103 pub unsafe fn get_ref(&self) -> &T {
1107 /// Get a mutable reference to the contained value.
1111 /// It is up to the caller to guarantee that the the `MaybeUninit` really is in an initialized
1112 /// state, otherwise this will immediately cause undefined behavior.
1113 #[unstable(feature = "maybe_uninit", issue = "53491")]
1114 pub unsafe fn get_mut(&mut self) -> &mut T {
1118 /// Get a pointer to the contained value. Reading from this pointer will be undefined
1119 /// behavior unless the `MaybeUninit` is initialized.
1120 #[unstable(feature = "maybe_uninit", issue = "53491")]
1121 pub fn as_ptr(&self) -> *const T {
1122 unsafe { &*self.value as *const T }
1125 /// Get a mutable pointer to the contained value. Reading from this pointer will be undefined
1126 /// behavior unless the `MaybeUninit` is initialized.
1127 #[unstable(feature = "maybe_uninit", issue = "53491")]
1128 pub fn as_mut_ptr(&mut self) -> *mut T {
1129 unsafe { &mut *self.value as *mut T }