| //! 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 crate::clone; |
| use crate::cmp; |
| use crate::fmt; |
| use crate::hash; |
| use crate::intrinsics; |
| use crate::marker::{Copy, PhantomData, Sized}; |
| use crate::ptr; |
| |
| mod manually_drop; |
| #[stable(feature = "manually_drop", since = "1.20.0")] |
| pub use manually_drop::ManuallyDrop; |
| |
| mod maybe_uninit; |
| #[stable(feature = "maybe_uninit", since = "1.36.0")] |
| pub use maybe_uninit::MaybeUninit; |
| |
| #[stable(feature = "rust1", since = "1.0.0")] |
| #[doc(inline)] |
| pub use crate::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. |
| /// The need comes up in some specialized use cases for FFI or unsafe code, but even |
| /// then, [`ManuallyDrop`] is typically preferred. |
| /// |
| /// 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. However, [`ManuallyDrop`] is usually preferred |
| /// for such cases, e.g.: |
| /// |
| /// ``` |
| /// use std::mem::ManuallyDrop; |
| /// |
| /// let v = vec![65, 122]; |
| /// // Before we disassemble `v` into its raw parts, make sure it |
| /// // does not get dropped! |
| /// let mut v = ManuallyDrop::new(v); |
| /// // Now disassemble `v`. These operations cannot panic, so there cannot be a leak. |
| /// let ptr = v.as_mut_ptr(); |
| /// let cap = v.capacity(); |
| /// // Finally, build a `String`. |
| /// let s = unsafe { String::from_raw_parts(ptr, 2, cap) }; |
| /// assert_eq!(s, "Az"); |
| /// // `s` is implicitly dropped and its memory deallocated. |
| /// ``` |
| /// |
| /// Using `ManuallyDrop` here has two advantages: |
| /// |
| /// * We do not "touch" `v` after disassembling it. For some types, operations |
| /// such as passing ownership (to a funcion like `mem::forget`) requires them to actually |
| /// be fully owned right now; that is a promise we do not want to make here as we are |
| /// in the process of transferring ownership to the new `String` we are building. |
| /// * In case of an unexpected panic, `ManuallyDrop` is not dropped, but if the panic |
| /// occurs before `mem::forget` was called we might end up dropping invalid data, |
| /// or double-dropping. In other words, `ManuallyDrop` errs on the side of leaking |
| /// instead of erring on the side of dropping. |
| /// |
| /// [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 |
| /// [`ManuallyDrop`]: struct.ManuallyDrop.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 = "none")] |
| pub fn forget_unsized<T: ?Sized>(t: T) { |
| // SAFETY: the forget intrinsic could be safe, but there's no point in making it safe since |
| // we'll be implementing this function soon via `ManuallyDrop` |
| 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(always)] |
| #[stable(feature = "rust1", since = "1.0.0")] |
| #[rustc_promotable] |
| #[rustc_const_stable(feature = "const_size_of", since = "1.32.0")] |
| 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/ch17-02-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 { |
| 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 { |
| 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(always)] |
| #[stable(feature = "rust1", since = "1.0.0")] |
| #[rustc_promotable] |
| #[rustc_const_stable(feature = "const_align_of", since = "1.32.0")] |
| 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")] |
| #[allow(deprecated)] |
| pub fn align_of_val<T: ?Sized>(val: &T) -> usize { |
| min_align_of_val(val) |
| } |
| |
| /// Returns `true` if 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 [`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. |
| /// |
| /// [`drop_in_place`]: ../ptr/fn.drop_in_place.html |
| /// [`HashMap`]: ../../std/collections/struct.HashMap.html |
| /// |
| /// # 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_stable(feature = "const_needs_drop", since = "1.36.0")] |
| pub const fn needs_drop<T>() -> bool { |
| intrinsics::needs_drop::<T>() |
| } |
| |
| /// Returns the value of type `T` represented by the all-zero byte-pattern. |
| /// |
| /// This means that, for example, the padding byte in `(u8, u16)` is not |
| /// necessarily zeroed. |
| /// |
| /// There is no guarantee that an all-zero byte-pattern represents a valid value of |
| /// some type `T`. For example, the all-zero byte-pattern is not a valid value |
| /// for reference types (`&T` and `&mut T`). Using `zeroed` on such types |
| /// causes immediate [undefined behavior][ub] because [the Rust compiler assumes][inv] |
| /// that there always is a valid value in a variable it considers initialized. |
| /// |
| /// This has the same effect as [`MaybeUninit::zeroed().assume_init()`][zeroed]. |
| /// It is useful for FFI sometimes, but should generally be avoided. |
| /// |
| /// [zeroed]: union.MaybeUninit.html#method.zeroed |
| /// [ub]: ../../reference/behavior-considered-undefined.html |
| /// [inv]: union.MaybeUninit.html#initialization-invariant |
| /// |
| /// # Examples |
| /// |
| /// Correct usage of this function: initializing an integer with zero. |
| /// |
| /// ``` |
| /// use std::mem; |
| /// |
| /// let x: i32 = unsafe { mem::zeroed() }; |
| /// assert_eq!(0, x); |
| /// ``` |
| /// |
| /// *Incorrect* usage of this function: initializing a reference with zero. |
| /// |
| /// ```rust,no_run |
| /// # #![allow(invalid_value)] |
| /// use std::mem; |
| /// |
| /// let _x: &i32 = unsafe { mem::zeroed() }; // Undefined behavior! |
| /// ``` |
| #[inline] |
| #[stable(feature = "rust1", since = "1.0.0")] |
| #[allow(deprecated_in_future)] |
| #[allow(deprecated)] |
| #[rustc_diagnostic_item = "mem_zeroed"] |
| pub unsafe fn zeroed<T>() -> T { |
| intrinsics::panic_if_uninhabited::<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 function is deprecated.** Use [`MaybeUninit<T>`] instead. |
| /// |
| /// The reason for deprecation is that the function basically cannot be used |
| /// correctly: it has the same effect as [`MaybeUninit::uninit().assume_init()`][uninit]. |
| /// As the [`assume_init` documentation][assume_init] explains, |
| /// [the Rust compiler assumes][inv] that values are properly initialized. |
| /// As a consequence, calling e.g. `mem::uninitialized::<bool>()` causes immediate |
| /// undefined behavior for returning a `bool` that is not definitely either `true` |
| /// or `false`. Worse, truly uninitialized memory like what gets returned here |
| /// is special in that the compiler knows that it does not have a fixed value. |
| /// This makes it undefined behavior to have uninitialized data in a variable even |
| /// if that variable has an integer type. |
| /// (Notice that the rules around uninitialized integers are not finalized yet, but |
| /// until they are, it is advisable to avoid them.) |
| /// |
| /// [`MaybeUninit<T>`]: union.MaybeUninit.html |
| /// [uninit]: union.MaybeUninit.html#method.uninit |
| /// [assume_init]: union.MaybeUninit.html#method.assume_init |
| /// [inv]: union.MaybeUninit.html#initialization-invariant |
| #[inline] |
| #[rustc_deprecated(since = "1.39.0", reason = "use `mem::MaybeUninit` instead")] |
| #[stable(feature = "rust1", since = "1.0.0")] |
| #[allow(deprecated_in_future)] |
| #[allow(deprecated)] |
| #[rustc_diagnostic_item = "mem_uninitialized"] |
| pub unsafe fn uninitialized<T>() -> T { |
| intrinsics::panic_if_uninhabited::<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) { |
| // SAFETY: the raw pointers have been created from safe mutable references satisfying all the |
| // constraints on `ptr::swap_nonoverlapping_one` |
| unsafe { |
| ptr::swap_nonoverlapping_one(x, y); |
| } |
| } |
| |
| /// Replaces `dest` with the default value of `T`, returning the previous `dest` value. |
| /// |
| /// # Examples |
| /// |
| /// A simple example: |
| /// |
| /// ``` |
| /// use std::mem; |
| /// |
| /// let mut v: Vec<i32> = vec![1, 2]; |
| /// |
| /// let old_v = mem::take(&mut v); |
| /// assert_eq!(vec![1, 2], old_v); |
| /// assert!(v.is_empty()); |
| /// ``` |
| /// |
| /// `take` allows taking ownership of a struct field by replacing it with an "empty" value. |
| /// Without `take` 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 `take` can be used to disassociate the original value of `self.buf` from |
| /// `self`, allowing it to be returned: |
| /// |
| /// ``` |
| /// use std::mem; |
| /// |
| /// # struct Buffer<T> { buf: Vec<T> } |
| /// impl<T> Buffer<T> { |
| /// fn get_and_reset(&mut self) -> Vec<T> { |
| /// mem::take(&mut self.buf) |
| /// } |
| /// } |
| /// |
| /// let mut buffer = Buffer { buf: vec![0, 1] }; |
| /// assert_eq!(buffer.buf.len(), 2); |
| /// |
| /// assert_eq!(buffer.get_and_reset(), vec![0, 1]); |
| /// assert_eq!(buffer.buf.len(), 0); |
| /// ``` |
| /// |
| /// [`Clone`]: ../../std/clone/trait.Clone.html |
| #[inline] |
| #[stable(feature = "mem_take", since = "1.40.0")] |
| pub fn take<T: Default>(dest: &mut T) -> T { |
| replace(dest, T::default()) |
| } |
| |
| /// 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!(vec![1, 2], old_v); |
| /// assert_eq!(vec![3, 4, 5], v); |
| /// ``` |
| /// |
| /// `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 replace_index(&mut self, i: usize, v: T) -> T { |
| /// // error: cannot move out of dereference of `&mut`-pointer |
| /// let t = self.buf[i]; |
| /// self.buf[i] = v; |
| /// t |
| /// } |
| /// } |
| /// ``` |
| /// |
| /// Note that `T` does not necessarily implement [`Clone`], so we can't even clone `self.buf[i]` to |
| /// avoid the move. But `replace` can be used to disassociate the original value at that index from |
| /// `self`, allowing it to be returned: |
| /// |
| /// ``` |
| /// # #![allow(dead_code)] |
| /// use std::mem; |
| /// |
| /// # struct Buffer<T> { buf: Vec<T> } |
| /// impl<T> Buffer<T> { |
| /// fn replace_index(&mut self, i: usize, v: T) -> T { |
| /// mem::replace(&mut self.buf[i], v) |
| /// } |
| /// } |
| /// |
| /// let mut buffer = Buffer { buf: vec![0, 1] }; |
| /// assert_eq!(buffer.buf[0], 0); |
| /// |
| /// assert_eq!(buffer.replace_index(0, 2), 0); |
| /// assert_eq!(buffer.buf[0], 2); |
| /// ``` |
| /// |
| /// [`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. |
| /// |
| /// This does call the argument's implementation of [`Drop`][drop]. |
| /// |
| /// 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 |
| /// ``` |
| /// |
| /// 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_array = [10u8]; |
| /// |
| /// unsafe { |
| /// // Copy the data from 'foo_array' and treat it as a 'Foo' |
| /// let mut foo_struct: Foo = mem::transmute_copy(&foo_array); |
| /// assert_eq!(foo_struct.bar, 10); |
| /// |
| /// // Modify the copied data |
| /// foo_struct.bar = 20; |
| /// assert_eq!(foo_struct.bar, 20); |
| /// } |
| /// |
| /// // The contents of 'foo_array' should not have changed |
| /// assert_eq!(foo_array, [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_eq!(mem::discriminant(&Foo::A("bar")), mem::discriminant(&Foo::A("baz"))); |
| /// assert_eq!(mem::discriminant(&Foo::B(1)), mem::discriminant(&Foo::B(2))); |
| /// assert_ne!(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> { |
| Discriminant(intrinsics::discriminant_value(v), PhantomData) |
| } |