| // Copyright 2012-2014 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. |
| |
| //! Basic functions for dealing with memory. |
| //! |
| //! This module contains functions for querying the size and alignment of |
| //! types, initializing and manipulating memory. |
| |
| #![stable(feature = "rust1", since = "1.0.0")] |
| |
| use clone; |
| use cmp; |
| use fmt; |
| use hash; |
| use intrinsics; |
| use marker::{Copy, PhantomData, Sized}; |
| use ptr; |
| use ops::{Deref, DerefMut}; |
| |
| #[stable(feature = "rust1", since = "1.0.0")] |
| pub use intrinsics::transmute; |
| |
| /// Takes ownership and "forgets" about the value **without running its destructor**. |
| /// |
| /// Any resources the value manages, such as heap memory or a file handle, will linger |
| /// forever in an unreachable state. However, it does not guarantee that pointers |
| /// to this memory will remain valid. |
| /// |
| /// * If you want to leak memory, see [`Box::leak`][leak]. |
| /// * If you want to obtain a raw pointer to the memory, see [`Box::into_raw`][into_raw]. |
| /// * If you want to dispose of a value properly, running its destructor, see |
| /// [`mem::drop`][drop]. |
| /// |
| /// # Safety |
| /// |
| /// `forget` is not marked as `unsafe`, because Rust's safety guarantees |
| /// do not include a guarantee that destructors will always run. For example, |
| /// a program can create a reference cycle using [`Rc`][rc], or call |
| /// [`process::exit`][exit] to exit without running destructors. Thus, allowing |
| /// `mem::forget` from safe code does not fundamentally change Rust's safety |
| /// guarantees. |
| /// |
| /// That said, leaking resources such as memory or I/O objects is usually undesirable, |
| /// so `forget` is only recommended for specialized use cases like those shown below. |
| /// |
| /// Because forgetting a value is allowed, any `unsafe` code you write must |
| /// allow for this possibility. You cannot return a value and expect that the |
| /// caller will necessarily run the value's destructor. |
| /// |
| /// [rc]: ../../std/rc/struct.Rc.html |
| /// [exit]: ../../std/process/fn.exit.html |
| /// |
| /// # Examples |
| /// |
| /// Leak an I/O object, never closing the file: |
| /// |
| /// ```no_run |
| /// use std::mem; |
| /// use std::fs::File; |
| /// |
| /// let file = File::open("foo.txt").unwrap(); |
| /// mem::forget(file); |
| /// ``` |
| /// |
| /// The practical use cases for `forget` are rather specialized and mainly come |
| /// up in unsafe or FFI code. |
| /// |
| /// ## Use case 1 |
| /// |
| /// You have created an uninitialized value using [`mem::uninitialized`][uninit]. |
| /// You must either initialize or `forget` it on every computation path before |
| /// Rust drops it automatically, like at the end of a scope or after a panic. |
| /// Running the destructor on an uninitialized value would be [undefined behavior][ub]. |
| /// |
| /// ``` |
| /// use std::mem; |
| /// use std::ptr; |
| /// |
| /// # let some_condition = false; |
| /// unsafe { |
| /// let mut uninit_vec: Vec<u32> = mem::uninitialized(); |
| /// |
| /// if some_condition { |
| /// // Initialize the variable. |
| /// ptr::write(&mut uninit_vec, Vec::new()); |
| /// } else { |
| /// // Forget the uninitialized value so its destructor doesn't run. |
| /// mem::forget(uninit_vec); |
| /// } |
| /// } |
| /// ``` |
| /// |
| /// ## Use case 2 |
| /// |
| /// You have duplicated the bytes making up a value, without doing a proper |
| /// [`Clone`][clone]. You need the value's destructor to run only once, |
| /// because a double `free` is undefined behavior. |
| /// |
| /// An example is a possible implementation of [`mem::swap`][swap]: |
| /// |
| /// ``` |
| /// use std::mem; |
| /// use std::ptr; |
| /// |
| /// # #[allow(dead_code)] |
| /// fn swap<T>(x: &mut T, y: &mut T) { |
| /// unsafe { |
| /// // Give ourselves some scratch space to work with |
| /// let mut t: T = mem::uninitialized(); |
| /// |
| /// // Perform the swap, `&mut` pointers never alias |
| /// ptr::copy_nonoverlapping(&*x, &mut t, 1); |
| /// ptr::copy_nonoverlapping(&*y, x, 1); |
| /// ptr::copy_nonoverlapping(&t, y, 1); |
| /// |
| /// // y and t now point to the same thing, but we need to completely |
| /// // forget `t` because we do not want to run the destructor for `T` |
| /// // on its value, which is still owned somewhere outside this function. |
| /// mem::forget(t); |
| /// } |
| /// } |
| /// ``` |
| /// |
| /// [drop]: fn.drop.html |
| /// [uninit]: fn.uninitialized.html |
| /// [clone]: ../clone/trait.Clone.html |
| /// [swap]: fn.swap.html |
| /// [box]: ../../std/boxed/struct.Box.html |
| /// [leak]: ../../std/boxed/struct.Box.html#method.leak |
| /// [into_raw]: ../../std/boxed/struct.Box.html#method.into_raw |
| /// [ub]: ../../reference/behavior-considered-undefined.html |
| #[inline] |
| #[stable(feature = "rust1", since = "1.0.0")] |
| pub fn forget<T>(t: T) { |
| ManuallyDrop::new(t); |
| } |
| |
| /// Like [`forget`], but also accepts unsized values. |
| /// |
| /// This function is just a shim intended to be removed when the `unsized_locals` feature gets |
| /// stabilized. |
| /// |
| /// [`forget`]: fn.forget.html |
| #[inline] |
| #[unstable(feature = "forget_unsized", issue = "0")] |
| pub fn forget_unsized<T: ?Sized>(t: T) { |
| unsafe { intrinsics::forget(t) } |
| } |
| |
| /// Returns the size of a type in bytes. |
| /// |
| /// More specifically, this is the offset in bytes between successive elements |
| /// in an array with that item type including alignment padding. Thus, for any |
| /// type `T` and length `n`, `[T; n]` has a size of `n * size_of::<T>()`. |
| /// |
| /// In general, the size of a type is not stable across compilations, but |
| /// specific types such as primitives are. |
| /// |
| /// The following table gives the size for primitives. |
| /// |
| /// Type | size_of::\<Type>() |
| /// ---- | --------------- |
| /// () | 0 |
| /// bool | 1 |
| /// u8 | 1 |
| /// u16 | 2 |
| /// u32 | 4 |
| /// u64 | 8 |
| /// u128 | 16 |
| /// i8 | 1 |
| /// i16 | 2 |
| /// i32 | 4 |
| /// i64 | 8 |
| /// i128 | 16 |
| /// f32 | 4 |
| /// f64 | 8 |
| /// char | 4 |
| /// |
| /// Furthermore, `usize` and `isize` have the same size. |
| /// |
| /// The types `*const T`, `&T`, `Box<T>`, `Option<&T>`, and `Option<Box<T>>` all have |
| /// the same size. If `T` is Sized, all of those types have the same size as `usize`. |
| /// |
| /// The mutability of a pointer does not change its size. As such, `&T` and `&mut T` |
| /// have the same size. Likewise for `*const T` and `*mut T`. |
| /// |
| /// # Size of `#[repr(C)]` items |
| /// |
| /// The `C` representation for items has a defined layout. With this layout, |
| /// the size of items is also stable as long as all fields have a stable size. |
| /// |
| /// ## Size of Structs |
| /// |
| /// For `structs`, the size is determined by the following algorithm. |
| /// |
| /// For each field in the struct ordered by declaration order: |
| /// |
| /// 1. Add the size of the field. |
| /// 2. Round up the current size to the nearest multiple of the next field's [alignment]. |
| /// |
| /// Finally, round the size of the struct to the nearest multiple of its [alignment]. |
| /// The alignment of the struct is usually the largest alignment of all its |
| /// fields; this can be changed with the use of `repr(align(N))`. |
| /// |
| /// Unlike `C`, zero sized structs are not rounded up to one byte in size. |
| /// |
| /// ## Size of Enums |
| /// |
| /// Enums that carry no data other than the discriminant have the same size as C enums |
| /// on the platform they are compiled for. |
| /// |
| /// ## Size of Unions |
| /// |
| /// The size of a union is the size of its largest field. |
| /// |
| /// Unlike `C`, zero sized unions are not rounded up to one byte in size. |
| /// |
| /// # Examples |
| /// |
| /// ``` |
| /// use std::mem; |
| /// |
| /// // Some primitives |
| /// assert_eq!(4, mem::size_of::<i32>()); |
| /// assert_eq!(8, mem::size_of::<f64>()); |
| /// assert_eq!(0, mem::size_of::<()>()); |
| /// |
| /// // Some arrays |
| /// assert_eq!(8, mem::size_of::<[i32; 2]>()); |
| /// assert_eq!(12, mem::size_of::<[i32; 3]>()); |
| /// assert_eq!(0, mem::size_of::<[i32; 0]>()); |
| /// |
| /// |
| /// // Pointer size equality |
| /// assert_eq!(mem::size_of::<&i32>(), mem::size_of::<*const i32>()); |
| /// assert_eq!(mem::size_of::<&i32>(), mem::size_of::<Box<i32>>()); |
| /// assert_eq!(mem::size_of::<&i32>(), mem::size_of::<Option<&i32>>()); |
| /// assert_eq!(mem::size_of::<Box<i32>>(), mem::size_of::<Option<Box<i32>>>()); |
| /// ``` |
| /// |
| /// Using `#[repr(C)]`. |
| /// |
| /// ``` |
| /// use std::mem; |
| /// |
| /// #[repr(C)] |
| /// struct FieldStruct { |
| /// first: u8, |
| /// second: u16, |
| /// third: u8 |
| /// } |
| /// |
| /// // The size of the first field is 1, so add 1 to the size. Size is 1. |
| /// // The alignment of the second field is 2, so add 1 to the size for padding. Size is 2. |
| /// // The size of the second field is 2, so add 2 to the size. Size is 4. |
| /// // The alignment of the third field is 1, so add 0 to the size for padding. Size is 4. |
| /// // The size of the third field is 1, so add 1 to the size. Size is 5. |
| /// // Finally, the alignment of the struct is 2 (because the largest alignment amongst its |
| /// // fields is 2), so add 1 to the size for padding. Size is 6. |
| /// assert_eq!(6, mem::size_of::<FieldStruct>()); |
| /// |
| /// #[repr(C)] |
| /// struct TupleStruct(u8, u16, u8); |
| /// |
| /// // Tuple structs follow the same rules. |
| /// assert_eq!(6, mem::size_of::<TupleStruct>()); |
| /// |
| /// // Note that reordering the fields can lower the size. We can remove both padding bytes |
| /// // by putting `third` before `second`. |
| /// #[repr(C)] |
| /// struct FieldStructOptimized { |
| /// first: u8, |
| /// third: u8, |
| /// second: u16 |
| /// } |
| /// |
| /// assert_eq!(4, mem::size_of::<FieldStructOptimized>()); |
| /// |
| /// // Union size is the size of the largest field. |
| /// #[repr(C)] |
| /// union ExampleUnion { |
| /// smaller: u8, |
| /// larger: u16 |
| /// } |
| /// |
| /// assert_eq!(2, mem::size_of::<ExampleUnion>()); |
| /// ``` |
| /// |
| /// [alignment]: ./fn.align_of.html |
| #[inline] |
| #[stable(feature = "rust1", since = "1.0.0")] |
| #[rustc_promotable] |
| pub const fn size_of<T>() -> usize { |
| intrinsics::size_of::<T>() |
| } |
| |
| /// Returns the size of the pointed-to value in bytes. |
| /// |
| /// This is usually the same as `size_of::<T>()`. However, when `T` *has* no |
| /// statically known size, e.g., a slice [`[T]`][slice] or a [trait object], |
| /// then `size_of_val` can be used to get the dynamically-known size. |
| /// |
| /// [slice]: ../../std/primitive.slice.html |
| /// [trait object]: ../../book/first-edition/trait-objects.html |
| /// |
| /// # Examples |
| /// |
| /// ``` |
| /// use std::mem; |
| /// |
| /// assert_eq!(4, mem::size_of_val(&5i32)); |
| /// |
| /// let x: [u8; 13] = [0; 13]; |
| /// let y: &[u8] = &x; |
| /// assert_eq!(13, mem::size_of_val(y)); |
| /// ``` |
| #[inline] |
| #[stable(feature = "rust1", since = "1.0.0")] |
| pub fn size_of_val<T: ?Sized>(val: &T) -> usize { |
| unsafe { intrinsics::size_of_val(val) } |
| } |
| |
| /// Returns the [ABI]-required minimum alignment of a type. |
| /// |
| /// Every reference to a value of the type `T` must be a multiple of this number. |
| /// |
| /// This is the alignment used for struct fields. It may be smaller than the preferred alignment. |
| /// |
| /// [ABI]: https://en.wikipedia.org/wiki/Application_binary_interface |
| /// |
| /// # Examples |
| /// |
| /// ``` |
| /// # #![allow(deprecated)] |
| /// use std::mem; |
| /// |
| /// assert_eq!(4, mem::min_align_of::<i32>()); |
| /// ``` |
| #[inline] |
| #[stable(feature = "rust1", since = "1.0.0")] |
| #[rustc_deprecated(reason = "use `align_of` instead", since = "1.2.0")] |
| pub fn min_align_of<T>() -> usize { |
| intrinsics::min_align_of::<T>() |
| } |
| |
| /// Returns the [ABI]-required minimum alignment of the type of the value that `val` points to. |
| /// |
| /// Every reference to a value of the type `T` must be a multiple of this number. |
| /// |
| /// [ABI]: https://en.wikipedia.org/wiki/Application_binary_interface |
| /// |
| /// # Examples |
| /// |
| /// ``` |
| /// # #![allow(deprecated)] |
| /// use std::mem; |
| /// |
| /// assert_eq!(4, mem::min_align_of_val(&5i32)); |
| /// ``` |
| #[inline] |
| #[stable(feature = "rust1", since = "1.0.0")] |
| #[rustc_deprecated(reason = "use `align_of_val` instead", since = "1.2.0")] |
| pub fn min_align_of_val<T: ?Sized>(val: &T) -> usize { |
| unsafe { intrinsics::min_align_of_val(val) } |
| } |
| |
| /// Returns the [ABI]-required minimum alignment of a type. |
| /// |
| /// Every reference to a value of the type `T` must be a multiple of this number. |
| /// |
| /// This is the alignment used for struct fields. It may be smaller than the preferred alignment. |
| /// |
| /// [ABI]: https://en.wikipedia.org/wiki/Application_binary_interface |
| /// |
| /// # Examples |
| /// |
| /// ``` |
| /// use std::mem; |
| /// |
| /// assert_eq!(4, mem::align_of::<i32>()); |
| /// ``` |
| #[inline] |
| #[stable(feature = "rust1", since = "1.0.0")] |
| #[rustc_promotable] |
| pub const fn align_of<T>() -> usize { |
| intrinsics::min_align_of::<T>() |
| } |
| |
| /// Returns the [ABI]-required minimum alignment of the type of the value that `val` points to. |
| /// |
| /// Every reference to a value of the type `T` must be a multiple of this number. |
| /// |
| /// [ABI]: https://en.wikipedia.org/wiki/Application_binary_interface |
| /// |
| /// # Examples |
| /// |
| /// ``` |
| /// use std::mem; |
| /// |
| /// assert_eq!(4, mem::align_of_val(&5i32)); |
| /// ``` |
| #[inline] |
| #[stable(feature = "rust1", since = "1.0.0")] |
| pub fn align_of_val<T: ?Sized>(val: &T) -> usize { |
| unsafe { intrinsics::min_align_of_val(val) } |
| } |
| |
| /// Returns whether dropping values of type `T` matters. |
| /// |
| /// This is purely an optimization hint, and may be implemented conservatively: |
| /// it may return `true` for types that don't actually need to be dropped. |
| /// As such always returning `true` would be a valid implementation of |
| /// this function. However if this function actually returns `false`, then you |
| /// can be certain dropping `T` has no side effect. |
| /// |
| /// Low level implementations of things like collections, which need to manually |
| /// drop their data, should use this function to avoid unnecessarily |
| /// trying to drop all their contents when they are destroyed. This might not |
| /// make a difference in release builds (where a loop that has no side-effects |
| /// is easily detected and eliminated), but is often a big win for debug builds. |
| /// |
| /// Note that `ptr::drop_in_place` already performs this check, so if your workload |
| /// can be reduced to some small number of drop_in_place calls, using this is |
| /// unnecessary. In particular note that you can drop_in_place a slice, and that |
| /// will do a single needs_drop check for all the values. |
| /// |
| /// Types like Vec therefore just `drop_in_place(&mut self[..])` without using |
| /// needs_drop explicitly. Types like HashMap, on the other hand, have to drop |
| /// values one at a time and should use this API. |
| /// |
| /// |
| /// # Examples |
| /// |
| /// Here's an example of how a collection might make use of needs_drop: |
| /// |
| /// ``` |
| /// use std::{mem, ptr}; |
| /// |
| /// pub struct MyCollection<T> { |
| /// # data: [T; 1], |
| /// /* ... */ |
| /// } |
| /// # impl<T> MyCollection<T> { |
| /// # fn iter_mut(&mut self) -> &mut [T] { &mut self.data } |
| /// # fn free_buffer(&mut self) {} |
| /// # } |
| /// |
| /// impl<T> Drop for MyCollection<T> { |
| /// fn drop(&mut self) { |
| /// unsafe { |
| /// // drop the data |
| /// if mem::needs_drop::<T>() { |
| /// for x in self.iter_mut() { |
| /// ptr::drop_in_place(x); |
| /// } |
| /// } |
| /// self.free_buffer(); |
| /// } |
| /// } |
| /// } |
| /// ``` |
| #[inline] |
| #[stable(feature = "needs_drop", since = "1.21.0")] |
| #[rustc_const_unstable(feature = "const_needs_drop")] |
| pub const fn needs_drop<T>() -> bool { |
| intrinsics::needs_drop::<T>() |
| } |
| |
| /// Creates a value whose bytes are all zero. |
| /// |
| /// This has the same effect as allocating space with |
| /// [`mem::uninitialized`][uninit] and then zeroing it out. It is useful for |
| /// FFI sometimes, but should generally be avoided. |
| /// |
| /// There is no guarantee that an all-zero byte-pattern represents a valid value of |
| /// some type `T`. If `T` has a destructor and the value is destroyed (due to |
| /// a panic or the end of a scope) before being initialized, then the destructor |
| /// will run on zeroed data, likely leading to [undefined behavior][ub]. |
| /// |
| /// See also the documentation for [`mem::uninitialized`][uninit], which has |
| /// many of the same caveats. |
| /// |
| /// [uninit]: fn.uninitialized.html |
| /// [ub]: ../../reference/behavior-considered-undefined.html |
| /// |
| /// # Examples |
| /// |
| /// ``` |
| /// use std::mem; |
| /// |
| /// let x: i32 = unsafe { mem::zeroed() }; |
| /// assert_eq!(0, x); |
| /// ``` |
| #[inline] |
| #[rustc_deprecated(since = "2.0.0", reason = "use `mem::MaybeUninit::zeroed` instead")] |
| #[stable(feature = "rust1", since = "1.0.0")] |
| pub unsafe fn zeroed<T>() -> T { |
| intrinsics::init() |
| } |
| |
| /// Bypasses Rust's normal memory-initialization checks by pretending to |
| /// produce a value of type `T`, while doing nothing at all. |
| /// |
| /// **This is incredibly dangerous and should not be done lightly. Deeply |
| /// consider initializing your memory with a default value instead.** |
| /// |
| /// This is useful for FFI functions and initializing arrays sometimes, |
| /// but should generally be avoided. |
| /// |
| /// # Undefined behavior |
| /// |
| /// It is [undefined behavior][ub] to read uninitialized memory, even just an |
| /// uninitialized boolean. For instance, if you branch on the value of such |
| /// a boolean, your program may take one, both, or neither of the branches. |
| /// |
| /// Writing to the uninitialized value is similarly dangerous. Rust believes the |
| /// value is initialized, and will therefore try to [`Drop`] the uninitialized |
| /// value and its fields if you try to overwrite it in a normal manner. The only way |
| /// to safely initialize an uninitialized value is with [`ptr::write`][write], |
| /// [`ptr::copy`][copy], or [`ptr::copy_nonoverlapping`][copy_no]. |
| /// |
| /// If the value does implement [`Drop`], it must be initialized before |
| /// it goes out of scope (and therefore would be dropped). Note that this |
| /// includes a `panic` occurring and unwinding the stack suddenly. |
| /// |
| /// # Examples |
| /// |
| /// Here's how to safely initialize an array of [`Vec`]s. |
| /// |
| /// ``` |
| /// use std::mem; |
| /// use std::ptr; |
| /// |
| /// // Only declare the array. This safely leaves it |
| /// // uninitialized in a way that Rust will track for us. |
| /// // However we can't initialize it element-by-element |
| /// // safely, and we can't use the `[value; 1000]` |
| /// // constructor because it only works with `Copy` data. |
| /// let mut data: [Vec<u32>; 1000]; |
| /// |
| /// unsafe { |
| /// // So we need to do this to initialize it. |
| /// data = mem::uninitialized(); |
| /// |
| /// // DANGER ZONE: if anything panics or otherwise |
| /// // incorrectly reads the array here, we will have |
| /// // Undefined Behavior. |
| /// |
| /// // It's ok to mutably iterate the data, since this |
| /// // doesn't involve reading it at all. |
| /// // (ptr and len are statically known for arrays) |
| /// for elem in &mut data[..] { |
| /// // *elem = Vec::new() would try to drop the |
| /// // uninitialized memory at `elem` -- bad! |
| /// // |
| /// // Vec::new doesn't allocate or do really |
| /// // anything. It's only safe to call here |
| /// // because we know it won't panic. |
| /// ptr::write(elem, Vec::new()); |
| /// } |
| /// |
| /// // SAFE ZONE: everything is initialized. |
| /// } |
| /// |
| /// println!("{:?}", &data[0]); |
| /// ``` |
| /// |
| /// This example emphasizes exactly how delicate and dangerous using `mem::uninitialized` |
| /// can be. Note that the [`vec!`] macro *does* let you initialize every element with a |
| /// value that is only [`Clone`], so the following is semantically equivalent and |
| /// vastly less dangerous, as long as you can live with an extra heap |
| /// allocation: |
| /// |
| /// ``` |
| /// let data: Vec<Vec<u32>> = vec![Vec::new(); 1000]; |
| /// println!("{:?}", &data[0]); |
| /// ``` |
| /// |
| /// [`Vec`]: ../../std/vec/struct.Vec.html |
| /// [`vec!`]: ../../std/macro.vec.html |
| /// [`Clone`]: ../../std/clone/trait.Clone.html |
| /// [ub]: ../../reference/behavior-considered-undefined.html |
| /// [write]: ../ptr/fn.write.html |
| /// [copy]: ../intrinsics/fn.copy.html |
| /// [copy_no]: ../intrinsics/fn.copy_nonoverlapping.html |
| /// [`Drop`]: ../ops/trait.Drop.html |
| #[inline] |
| #[rustc_deprecated(since = "2.0.0", reason = "use `mem::MaybeUninit::uninitialized` instead")] |
| #[stable(feature = "rust1", since = "1.0.0")] |
| pub unsafe fn uninitialized<T>() -> T { |
| intrinsics::uninit() |
| } |
| |
| /// Swaps the values at two mutable locations, without deinitializing either one. |
| /// |
| /// # Examples |
| /// |
| /// ``` |
| /// use std::mem; |
| /// |
| /// let mut x = 5; |
| /// let mut y = 42; |
| /// |
| /// mem::swap(&mut x, &mut y); |
| /// |
| /// assert_eq!(42, x); |
| /// assert_eq!(5, y); |
| /// ``` |
| #[inline] |
| #[stable(feature = "rust1", since = "1.0.0")] |
| pub fn swap<T>(x: &mut T, y: &mut T) { |
| unsafe { |
| ptr::swap_nonoverlapping_one(x, y); |
| } |
| } |
| |
| /// Moves `src` into the referenced `dest`, returning the previous `dest` value. |
| /// |
| /// Neither value is dropped. |
| /// |
| /// # Examples |
| /// |
| /// A simple example: |
| /// |
| /// ``` |
| /// use std::mem; |
| /// |
| /// let mut v: Vec<i32> = vec![1, 2]; |
| /// |
| /// let old_v = mem::replace(&mut v, vec![3, 4, 5]); |
| /// assert_eq!(2, old_v.len()); |
| /// assert_eq!(3, v.len()); |
| /// ``` |
| /// |
| /// `replace` allows consumption of a struct field by replacing it with another value. |
| /// Without `replace` you can run into issues like these: |
| /// |
| /// ```compile_fail,E0507 |
| /// struct Buffer<T> { buf: Vec<T> } |
| /// |
| /// impl<T> Buffer<T> { |
| /// fn get_and_reset(&mut self) -> Vec<T> { |
| /// // error: cannot move out of dereference of `&mut`-pointer |
| /// let buf = self.buf; |
| /// self.buf = Vec::new(); |
| /// buf |
| /// } |
| /// } |
| /// ``` |
| /// |
| /// Note that `T` does not necessarily implement [`Clone`], so it can't even clone and reset |
| /// `self.buf`. But `replace` can be used to disassociate the original value of `self.buf` from |
| /// `self`, allowing it to be returned: |
| /// |
| /// ``` |
| /// # #![allow(dead_code)] |
| /// use std::mem; |
| /// |
| /// # struct Buffer<T> { buf: Vec<T> } |
| /// impl<T> Buffer<T> { |
| /// fn get_and_reset(&mut self) -> Vec<T> { |
| /// mem::replace(&mut self.buf, Vec::new()) |
| /// } |
| /// } |
| /// ``` |
| /// |
| /// [`Clone`]: ../../std/clone/trait.Clone.html |
| #[inline] |
| #[stable(feature = "rust1", since = "1.0.0")] |
| pub fn replace<T>(dest: &mut T, mut src: T) -> T { |
| swap(dest, &mut src); |
| src |
| } |
| |
| /// Disposes of a value. |
| /// |
| /// While this does call the argument's implementation of [`Drop`][drop], |
| /// it will not release any borrows, as borrows are based on lexical scope. |
| /// |
| /// This effectively does nothing for types which implement `Copy`, e.g. |
| /// integers. Such values are copied and _then_ moved into the function, so the |
| /// value persists after this function call. |
| /// |
| /// This function is not magic; it is literally defined as |
| /// |
| /// ``` |
| /// pub fn drop<T>(_x: T) { } |
| /// ``` |
| /// |
| /// Because `_x` is moved into the function, it is automatically dropped before |
| /// the function returns. |
| /// |
| /// [drop]: ../ops/trait.Drop.html |
| /// |
| /// # Examples |
| /// |
| /// Basic usage: |
| /// |
| /// ``` |
| /// let v = vec![1, 2, 3]; |
| /// |
| /// drop(v); // explicitly drop the vector |
| /// ``` |
| /// |
| /// Borrows are based on lexical scope, so this produces an error: |
| /// |
| /// ```compile_fail,E0502 |
| /// let mut v = vec![1, 2, 3]; |
| /// let x = &v[0]; |
| /// |
| /// drop(x); // explicitly drop the reference, but the borrow still exists |
| /// |
| /// v.push(4); // error: cannot borrow `v` as mutable because it is also |
| /// // borrowed as immutable |
| /// ``` |
| /// |
| /// An inner scope is needed to fix this: |
| /// |
| /// ``` |
| /// let mut v = vec![1, 2, 3]; |
| /// |
| /// { |
| /// let x = &v[0]; |
| /// |
| /// drop(x); // this is now redundant, as `x` is going out of scope anyway |
| /// } |
| /// |
| /// v.push(4); // no problems |
| /// ``` |
| /// |
| /// Since [`RefCell`] enforces the borrow rules at runtime, `drop` can |
| /// release a [`RefCell`] borrow: |
| /// |
| /// ``` |
| /// use std::cell::RefCell; |
| /// |
| /// let x = RefCell::new(1); |
| /// |
| /// let mut mutable_borrow = x.borrow_mut(); |
| /// *mutable_borrow = 1; |
| /// |
| /// drop(mutable_borrow); // relinquish the mutable borrow on this slot |
| /// |
| /// let borrow = x.borrow(); |
| /// println!("{}", *borrow); |
| /// ``` |
| /// |
| /// Integers and other types implementing [`Copy`] are unaffected by `drop`. |
| /// |
| /// ``` |
| /// #[derive(Copy, Clone)] |
| /// struct Foo(u8); |
| /// |
| /// let x = 1; |
| /// let y = Foo(2); |
| /// drop(x); // a copy of `x` is moved and dropped |
| /// drop(y); // a copy of `y` is moved and dropped |
| /// |
| /// println!("x: {}, y: {}", x, y.0); // still available |
| /// ``` |
| /// |
| /// [`RefCell`]: ../../std/cell/struct.RefCell.html |
| /// [`Copy`]: ../../std/marker/trait.Copy.html |
| #[inline] |
| #[stable(feature = "rust1", since = "1.0.0")] |
| pub fn drop<T>(_x: T) { } |
| |
| /// Interprets `src` as having type `&U`, and then reads `src` without moving |
| /// the contained value. |
| /// |
| /// This function will unsafely assume the pointer `src` is valid for |
| /// [`size_of::<U>`][size_of] bytes by transmuting `&T` to `&U` and then reading |
| /// the `&U`. It will also unsafely create a copy of the contained value instead of |
| /// moving out of `src`. |
| /// |
| /// It is not a compile-time error if `T` and `U` have different sizes, but it |
| /// is highly encouraged to only invoke this function where `T` and `U` have the |
| /// same size. This function triggers [undefined behavior][ub] if `U` is larger than |
| /// `T`. |
| /// |
| /// [ub]: ../../reference/behavior-considered-undefined.html |
| /// [size_of]: fn.size_of.html |
| /// |
| /// # Examples |
| /// |
| /// ``` |
| /// use std::mem; |
| /// |
| /// #[repr(packed)] |
| /// struct Foo { |
| /// bar: u8, |
| /// } |
| /// |
| /// let foo_slice = [10u8]; |
| /// |
| /// unsafe { |
| /// // Copy the data from 'foo_slice' and treat it as a 'Foo' |
| /// let mut foo_struct: Foo = mem::transmute_copy(&foo_slice); |
| /// assert_eq!(foo_struct.bar, 10); |
| /// |
| /// // Modify the copied data |
| /// foo_struct.bar = 20; |
| /// assert_eq!(foo_struct.bar, 20); |
| /// } |
| /// |
| /// // The contents of 'foo_slice' should not have changed |
| /// assert_eq!(foo_slice, [10]); |
| /// ``` |
| #[inline] |
| #[stable(feature = "rust1", since = "1.0.0")] |
| pub unsafe fn transmute_copy<T, U>(src: &T) -> U { |
| ptr::read_unaligned(src as *const T as *const U) |
| } |
| |
| /// Opaque type representing the discriminant of an enum. |
| /// |
| /// See the [`discriminant`] function in this module for more information. |
| /// |
| /// [`discriminant`]: fn.discriminant.html |
| #[stable(feature = "discriminant_value", since = "1.21.0")] |
| pub struct Discriminant<T>(u64, PhantomData<fn() -> T>); |
| |
| // N.B. These trait implementations cannot be derived because we don't want any bounds on T. |
| |
| #[stable(feature = "discriminant_value", since = "1.21.0")] |
| impl<T> Copy for Discriminant<T> {} |
| |
| #[stable(feature = "discriminant_value", since = "1.21.0")] |
| impl<T> clone::Clone for Discriminant<T> { |
| fn clone(&self) -> Self { |
| *self |
| } |
| } |
| |
| #[stable(feature = "discriminant_value", since = "1.21.0")] |
| impl<T> cmp::PartialEq for Discriminant<T> { |
| fn eq(&self, rhs: &Self) -> bool { |
| self.0 == rhs.0 |
| } |
| } |
| |
| #[stable(feature = "discriminant_value", since = "1.21.0")] |
| impl<T> cmp::Eq for Discriminant<T> {} |
| |
| #[stable(feature = "discriminant_value", since = "1.21.0")] |
| impl<T> hash::Hash for Discriminant<T> { |
| fn hash<H: hash::Hasher>(&self, state: &mut H) { |
| self.0.hash(state); |
| } |
| } |
| |
| #[stable(feature = "discriminant_value", since = "1.21.0")] |
| impl<T> fmt::Debug for Discriminant<T> { |
| fn fmt(&self, fmt: &mut fmt::Formatter) -> fmt::Result { |
| fmt.debug_tuple("Discriminant") |
| .field(&self.0) |
| .finish() |
| } |
| } |
| |
| /// Returns a value uniquely identifying the enum variant in `v`. |
| /// |
| /// If `T` is not an enum, calling this function will not result in undefined behavior, but the |
| /// return value is unspecified. |
| /// |
| /// # Stability |
| /// |
| /// The discriminant of an enum variant may change if the enum definition changes. A discriminant |
| /// of some variant will not change between compilations with the same compiler. |
| /// |
| /// # Examples |
| /// |
| /// This can be used to compare enums that carry data, while disregarding |
| /// the actual data: |
| /// |
| /// ``` |
| /// use std::mem; |
| /// |
| /// enum Foo { A(&'static str), B(i32), C(i32) } |
| /// |
| /// assert!(mem::discriminant(&Foo::A("bar")) == mem::discriminant(&Foo::A("baz"))); |
| /// assert!(mem::discriminant(&Foo::B(1)) == mem::discriminant(&Foo::B(2))); |
| /// assert!(mem::discriminant(&Foo::B(3)) != mem::discriminant(&Foo::C(3))); |
| /// ``` |
| #[stable(feature = "discriminant_value", since = "1.21.0")] |
| pub fn discriminant<T>(v: &T) -> Discriminant<T> { |
| unsafe { |
| Discriminant(intrinsics::discriminant_value(v), PhantomData) |
| } |
| } |
| |
| /// A wrapper to inhibit compiler from automatically calling `T`’s destructor. |
| /// |
| /// This wrapper is 0-cost. |
| /// |
| /// # Examples |
| /// |
| /// This wrapper helps with explicitly documenting the drop order dependencies between fields of |
| /// the type: |
| /// |
| /// ```rust |
| /// use std::mem::ManuallyDrop; |
| /// struct Peach; |
| /// struct Banana; |
| /// struct Melon; |
| /// struct FruitBox { |
| /// // Immediately clear there’s something non-trivial going on with these fields. |
| /// peach: ManuallyDrop<Peach>, |
| /// melon: Melon, // Field that’s independent of the other two. |
| /// banana: ManuallyDrop<Banana>, |
| /// } |
| /// |
| /// impl Drop for FruitBox { |
| /// fn drop(&mut self) { |
| /// unsafe { |
| /// // Explicit ordering in which field destructors are run specified in the intuitive |
| /// // location – the destructor of the structure containing the fields. |
| /// // Moreover, one can now reorder fields within the struct however much they want. |
| /// ManuallyDrop::drop(&mut self.peach); |
| /// ManuallyDrop::drop(&mut self.banana); |
| /// } |
| /// // After destructor for `FruitBox` runs (this function), the destructor for Melon gets |
| /// // invoked in the usual manner, as it is not wrapped in `ManuallyDrop`. |
| /// } |
| /// } |
| /// ``` |
| #[stable(feature = "manually_drop", since = "1.20.0")] |
| #[lang = "manually_drop"] |
| #[derive(Copy, Clone, Debug, Default, PartialEq, Eq, PartialOrd, Ord, Hash)] |
| #[repr(transparent)] |
| pub struct ManuallyDrop<T: ?Sized> { |
| value: T, |
| } |
| |
| impl<T> ManuallyDrop<T> { |
| /// Wrap a value to be manually dropped. |
| /// |
| /// # Examples |
| /// |
| /// ```rust |
| /// use std::mem::ManuallyDrop; |
| /// ManuallyDrop::new(Box::new(())); |
| /// ``` |
| #[stable(feature = "manually_drop", since = "1.20.0")] |
| #[inline(always)] |
| pub const fn new(value: T) -> ManuallyDrop<T> { |
| ManuallyDrop { value } |
| } |
| |
| /// Extract the value from the `ManuallyDrop` container. |
| /// |
| /// This allows the value to be dropped again. |
| /// |
| /// # Examples |
| /// |
| /// ```rust |
| /// use std::mem::ManuallyDrop; |
| /// let x = ManuallyDrop::new(Box::new(())); |
| /// let _: Box<()> = ManuallyDrop::into_inner(x); // This drops the `Box`. |
| /// ``` |
| #[stable(feature = "manually_drop", since = "1.20.0")] |
| #[inline(always)] |
| pub const fn into_inner(slot: ManuallyDrop<T>) -> T { |
| slot.value |
| } |
| |
| /// Takes the contained value out. |
| /// |
| /// This method is primarily intended for moving out values in drop. |
| /// Instead of using [`ManuallyDrop::drop`] to manually drop the value, |
| /// you can use this method to take the value and use it however desired. |
| /// `Drop` will be invoked on the returned value following normal end-of-scope rules. |
| /// |
| /// If you have ownership of the container, you can use [`ManuallyDrop::into_inner`] instead. |
| /// |
| /// # Safety |
| /// |
| /// This function semantically moves out the contained value without preventing further usage. |
| /// It is up to the user of this method to ensure that this container is not used again. |
| #[must_use = "if you don't need the value, you can use `ManuallyDrop::drop` instead"] |
| #[unstable(feature = "manually_drop_take", issue = "55422")] |
| #[inline] |
| pub unsafe fn take(slot: &mut ManuallyDrop<T>) -> T { |
| ManuallyDrop::into_inner(ptr::read(slot)) |
| } |
| } |
| |
| impl<T: ?Sized> ManuallyDrop<T> { |
| /// Manually drops the contained value. |
| /// |
| /// If you have ownership of the value, you can use [`ManuallyDrop::into_inner`] instead. |
| /// |
| /// # Safety |
| /// |
| /// This function runs the destructor of the contained value and thus the wrapped value |
| /// now represents uninitialized data. It is up to the user of this method to ensure the |
| /// uninitialized data is not actually used. |
| /// |
| /// [`ManuallyDrop::into_inner`]: #method.into_inner |
| #[stable(feature = "manually_drop", since = "1.20.0")] |
| #[inline] |
| pub unsafe fn drop(slot: &mut ManuallyDrop<T>) { |
| ptr::drop_in_place(&mut slot.value) |
| } |
| } |
| |
| #[stable(feature = "manually_drop", since = "1.20.0")] |
| impl<T: ?Sized> Deref for ManuallyDrop<T> { |
| type Target = T; |
| #[inline(always)] |
| fn deref(&self) -> &T { |
| &self.value |
| } |
| } |
| |
| #[stable(feature = "manually_drop", since = "1.20.0")] |
| impl<T: ?Sized> DerefMut for ManuallyDrop<T> { |
| #[inline(always)] |
| fn deref_mut(&mut self) -> &mut T { |
| &mut self.value |
| } |
| } |
| |
| /// A newtype to construct uninitialized instances of `T` |
| #[allow(missing_debug_implementations)] |
| #[unstable(feature = "maybe_uninit", issue = "53491")] |
| // NOTE after stabilizing `MaybeUninit` proceed to deprecate `mem::{uninitialized,zeroed}` |
| pub union MaybeUninit<T> { |
| uninit: (), |
| value: ManuallyDrop<T>, |
| } |
| |
| impl<T> MaybeUninit<T> { |
| /// Create a new `MaybeUninit` initialized with the given value. |
| /// |
| /// Note that dropping a `MaybeUninit` will never call `T`'s drop code. |
| /// It is your responsibility to make sure `T` gets dropped if it got initialized. |
| #[unstable(feature = "maybe_uninit", issue = "53491")] |
| #[inline(always)] |
| pub const fn new(val: T) -> MaybeUninit<T> { |
| MaybeUninit { value: ManuallyDrop::new(val) } |
| } |
| |
| /// Create a new `MaybeUninit` in an uninitialized state. |
| /// |
| /// Note that dropping a `MaybeUninit` will never call `T`'s drop code. |
| /// It is your responsibility to make sure `T` gets dropped if it got initialized. |
| #[unstable(feature = "maybe_uninit", issue = "53491")] |
| #[inline(always)] |
| pub const fn uninitialized() -> MaybeUninit<T> { |
| MaybeUninit { uninit: () } |
| } |
| |
| /// Create a new `MaybeUninit` in an uninitialized state, with the memory being |
| /// filled with `0` bytes. It depends on `T` whether that already makes for |
| /// proper initialization. For example, `MaybeUninit<usize>::zeroed()` is initialized, |
| /// but `MaybeUninit<&'static i32>::zeroed()` is not because references must not |
| /// be null. |
| /// |
| /// Note that dropping a `MaybeUninit` will never call `T`'s drop code. |
| /// It is your responsibility to make sure `T` gets dropped if it got initialized. |
| #[unstable(feature = "maybe_uninit", issue = "53491")] |
| #[inline] |
| pub fn zeroed() -> MaybeUninit<T> { |
| let mut u = MaybeUninit::<T>::uninitialized(); |
| unsafe { |
| u.as_mut_ptr().write_bytes(0u8, 1); |
| } |
| u |
| } |
| |
| /// Set the value of the `MaybeUninit`. This overwrites any previous value without dropping it. |
| #[unstable(feature = "maybe_uninit", issue = "53491")] |
| #[inline(always)] |
| pub fn set(&mut self, val: T) { |
| unsafe { |
| self.value = ManuallyDrop::new(val); |
| } |
| } |
| |
| /// Extract the value from the `MaybeUninit` container. This is a great way |
| /// to ensure that the data will get dropped, because the resulting `T` is |
| /// subject to the usual drop handling. |
| /// |
| /// # Unsafety |
| /// |
| /// It is up to the caller to guarantee that the `MaybeUninit` really is in an initialized |
| /// state, otherwise this will immediately cause undefined behavior. |
| #[unstable(feature = "maybe_uninit", issue = "53491")] |
| #[inline(always)] |
| pub unsafe fn into_inner(self) -> T { |
| ManuallyDrop::into_inner(self.value) |
| } |
| |
| /// Get a reference to the contained value. |
| /// |
| /// # Unsafety |
| /// |
| /// It is up to the caller to guarantee that the `MaybeUninit` really is in an initialized |
| /// state, otherwise this will immediately cause undefined behavior. |
| #[unstable(feature = "maybe_uninit", issue = "53491")] |
| #[inline(always)] |
| pub unsafe fn get_ref(&self) -> &T { |
| &*self.value |
| } |
| |
| /// Get a mutable reference to the contained value. |
| /// |
| /// # Unsafety |
| /// |
| /// It is up to the caller to guarantee that the `MaybeUninit` really is in an initialized |
| /// state, otherwise this will immediately cause undefined behavior. |
| // FIXME(#53491): We currently rely on the above being incorrect, i.e., we have references |
| // to uninitialized data (e.g., in `libcore/fmt/float.rs`). We should make |
| // a final decision about the rules before stabilization. |
| #[unstable(feature = "maybe_uninit", issue = "53491")] |
| #[inline(always)] |
| pub unsafe fn get_mut(&mut self) -> &mut T { |
| &mut *self.value |
| } |
| |
| /// Get a pointer to the contained value. Reading from this pointer will be undefined |
| /// behavior unless the `MaybeUninit` is initialized. |
| #[unstable(feature = "maybe_uninit", issue = "53491")] |
| #[inline(always)] |
| pub fn as_ptr(&self) -> *const T { |
| unsafe { &*self.value as *const T } |
| } |
| |
| /// Get a mutable pointer to the contained value. Reading from this pointer will be undefined |
| /// behavior unless the `MaybeUninit` is initialized. |
| #[unstable(feature = "maybe_uninit", issue = "53491")] |
| #[inline(always)] |
| pub fn as_mut_ptr(&mut self) -> *mut T { |
| unsafe { &mut *self.value as *mut T } |
| } |
| } |