| //! Shareable mutable containers. |
| //! |
| //! Rust memory safety is based on this rule: Given an object `T`, it is only possible to |
| //! have one of the following: |
| //! |
| //! - Having several immutable references (`&T`) to the object (also known as **aliasing**). |
| //! - Having one mutable reference (`&mut T`) to the object (also known as **mutability**). |
| //! |
| //! This is enforced by the Rust compiler. However, there are situations where this rule is not |
| //! flexible enough. Sometimes it is required to have multiple references to an object and yet |
| //! mutate it. |
| //! |
| //! Shareable mutable containers exist to permit mutability in a controlled manner, even in the |
| //! presence of aliasing. Both `Cell<T>` and `RefCell<T>` allow doing this in a single-threaded |
| //! way. However, neither `Cell<T>` nor `RefCell<T>` are thread safe (they do not implement |
| //! `Sync`). If you need to do aliasing and mutation between multiple threads it is possible to |
| //! use [`Mutex`](../../std/sync/struct.Mutex.html), |
| //! [`RwLock`](../../std/sync/struct.RwLock.html) or |
| //! [`atomic`](../../core/sync/atomic/index.html) types. |
| //! |
| //! Values of the `Cell<T>` and `RefCell<T>` types may be mutated through shared references (i.e. |
| //! the common `&T` type), whereas most Rust types can only be mutated through unique (`&mut T`) |
| //! references. We say that `Cell<T>` and `RefCell<T>` provide 'interior mutability', in contrast |
| //! with typical Rust types that exhibit 'inherited mutability'. |
| //! |
| //! Cell types come in two flavors: `Cell<T>` and `RefCell<T>`. `Cell<T>` implements interior |
| //! mutability by moving values in and out of the `Cell<T>`. To use references instead of values, |
| //! one must use the `RefCell<T>` type, acquiring a write lock before mutating. `Cell<T>` provides |
| //! methods to retrieve and change the current interior value: |
| //! |
| //! - For types that implement `Copy`, the `get` method retrieves the current interior value. |
| //! - For types that implement `Default`, the `take` method replaces the current interior value |
| //! with `Default::default()` and returns the replaced value. |
| //! - For all types, the `replace` method replaces the current interior value and returns the |
| //! replaced value and the `into_inner` method consumes the `Cell<T>` and returns the interior |
| //! value. Additionally, the `set` method replaces the interior value, dropping the replaced |
| //! value. |
| //! |
| //! `RefCell<T>` uses Rust's lifetimes to implement 'dynamic borrowing', a process whereby one can |
| //! claim temporary, exclusive, mutable access to the inner value. Borrows for `RefCell<T>`s are |
| //! tracked 'at runtime', unlike Rust's native reference types which are entirely tracked |
| //! statically, at compile time. Because `RefCell<T>` borrows are dynamic it is possible to attempt |
| //! to borrow a value that is already mutably borrowed; when this happens it results in thread |
| //! panic. |
| //! |
| //! # When to choose interior mutability |
| //! |
| //! The more common inherited mutability, where one must have unique access to mutate a value, is |
| //! one of the key language elements that enables Rust to reason strongly about pointer aliasing, |
| //! statically preventing crash bugs. Because of that, inherited mutability is preferred, and |
| //! interior mutability is something of a last resort. Since cell types enable mutation where it |
| //! would otherwise be disallowed though, there are occasions when interior mutability might be |
| //! appropriate, or even *must* be used, e.g. |
| //! |
| //! * Introducing mutability 'inside' of something immutable |
| //! * Implementation details of logically-immutable methods. |
| //! * Mutating implementations of `Clone`. |
| //! |
| //! ## Introducing mutability 'inside' of something immutable |
| //! |
| //! Many shared smart pointer types, including `Rc<T>` and `Arc<T>`, provide containers that can be |
| //! cloned and shared between multiple parties. Because the contained values may be |
| //! multiply-aliased, they can only be borrowed with `&`, not `&mut`. Without cells it would be |
| //! impossible to mutate data inside of these smart pointers at all. |
| //! |
| //! It's very common then to put a `RefCell<T>` inside shared pointer types to reintroduce |
| //! mutability: |
| //! |
| //! ``` |
| //! use std::cell::{RefCell, RefMut}; |
| //! use std::collections::HashMap; |
| //! use std::rc::Rc; |
| //! |
| //! fn main() { |
| //! let shared_map: Rc<RefCell<_>> = Rc::new(RefCell::new(HashMap::new())); |
| //! // Create a new block to limit the scope of the dynamic borrow |
| //! { |
| //! let mut map: RefMut<_> = shared_map.borrow_mut(); |
| //! map.insert("africa", 92388); |
| //! map.insert("kyoto", 11837); |
| //! map.insert("piccadilly", 11826); |
| //! map.insert("marbles", 38); |
| //! } |
| //! |
| //! // Note that if we had not let the previous borrow of the cache fall out |
| //! // of scope then the subsequent borrow would cause a dynamic thread panic. |
| //! // This is the major hazard of using `RefCell`. |
| //! let total: i32 = shared_map.borrow().values().sum(); |
| //! println!("{}", total); |
| //! } |
| //! ``` |
| //! |
| //! Note that this example uses `Rc<T>` and not `Arc<T>`. `RefCell<T>`s are for single-threaded |
| //! scenarios. Consider using `RwLock<T>` or `Mutex<T>` if you need shared mutability in a |
| //! multi-threaded situation. |
| //! |
| //! ## Implementation details of logically-immutable methods |
| //! |
| //! Occasionally it may be desirable not to expose in an API that there is mutation happening |
| //! "under the hood". This may be because logically the operation is immutable, but e.g., caching |
| //! forces the implementation to perform mutation; or because you must employ mutation to implement |
| //! a trait method that was originally defined to take `&self`. |
| //! |
| //! ``` |
| //! # #![allow(dead_code)] |
| //! use std::cell::RefCell; |
| //! |
| //! struct Graph { |
| //! edges: Vec<(i32, i32)>, |
| //! span_tree_cache: RefCell<Option<Vec<(i32, i32)>>> |
| //! } |
| //! |
| //! impl Graph { |
| //! fn minimum_spanning_tree(&self) -> Vec<(i32, i32)> { |
| //! self.span_tree_cache.borrow_mut() |
| //! .get_or_insert_with(|| self.calc_span_tree()) |
| //! .clone() |
| //! } |
| //! |
| //! fn calc_span_tree(&self) -> Vec<(i32, i32)> { |
| //! // Expensive computation goes here |
| //! vec![] |
| //! } |
| //! } |
| //! ``` |
| //! |
| //! ## Mutating implementations of `Clone` |
| //! |
| //! This is simply a special - but common - case of the previous: hiding mutability for operations |
| //! that appear to be immutable. The `clone` method is expected to not change the source value, and |
| //! is declared to take `&self`, not `&mut self`. Therefore, any mutation that happens in the |
| //! `clone` method must use cell types. For example, `Rc<T>` maintains its reference counts within a |
| //! `Cell<T>`. |
| //! |
| //! ``` |
| //! #![feature(core_intrinsics)] |
| //! use std::cell::Cell; |
| //! use std::ptr::NonNull; |
| //! use std::intrinsics::abort; |
| //! |
| //! struct Rc<T: ?Sized> { |
| //! ptr: NonNull<RcBox<T>> |
| //! } |
| //! |
| //! struct RcBox<T: ?Sized> { |
| //! strong: Cell<usize>, |
| //! refcount: Cell<usize>, |
| //! value: T, |
| //! } |
| //! |
| //! impl<T: ?Sized> Clone for Rc<T> { |
| //! fn clone(&self) -> Rc<T> { |
| //! self.inc_strong(); |
| //! Rc { ptr: self.ptr } |
| //! } |
| //! } |
| //! |
| //! trait RcBoxPtr<T: ?Sized> { |
| //! |
| //! fn inner(&self) -> &RcBox<T>; |
| //! |
| //! fn strong(&self) -> usize { |
| //! self.inner().strong.get() |
| //! } |
| //! |
| //! fn inc_strong(&self) { |
| //! self.inner() |
| //! .strong |
| //! .set(self.strong() |
| //! .checked_add(1) |
| //! .unwrap_or_else(|| unsafe { abort() })); |
| //! } |
| //! } |
| //! |
| //! impl<T: ?Sized> RcBoxPtr<T> for Rc<T> { |
| //! fn inner(&self) -> &RcBox<T> { |
| //! unsafe { |
| //! self.ptr.as_ref() |
| //! } |
| //! } |
| //! } |
| //! ``` |
| //! |
| |
| #![stable(feature = "rust1", since = "1.0.0")] |
| |
| use crate::cmp::Ordering; |
| use crate::fmt::{self, Debug, Display}; |
| use crate::marker::Unsize; |
| use crate::mem; |
| use crate::ops::{Deref, DerefMut, CoerceUnsized}; |
| use crate::ptr; |
| |
| /// A mutable memory location. |
| /// |
| /// # Examples |
| /// |
| /// In this example, you can see that `Cell<T>` enables mutation inside an |
| /// immutable struct. In other words, it enables "interior mutability". |
| /// |
| /// ``` |
| /// use std::cell::Cell; |
| /// |
| /// struct SomeStruct { |
| /// regular_field: u8, |
| /// special_field: Cell<u8>, |
| /// } |
| /// |
| /// let my_struct = SomeStruct { |
| /// regular_field: 0, |
| /// special_field: Cell::new(1), |
| /// }; |
| /// |
| /// let new_value = 100; |
| /// |
| /// // ERROR: `my_struct` is immutable |
| /// // my_struct.regular_field = new_value; |
| /// |
| /// // WORKS: although `my_struct` is immutable, `special_field` is a `Cell`, |
| /// // which can always be mutated |
| /// my_struct.special_field.set(new_value); |
| /// assert_eq!(my_struct.special_field.get(), new_value); |
| /// ``` |
| /// |
| /// See the [module-level documentation](index.html) for more. |
| #[stable(feature = "rust1", since = "1.0.0")] |
| #[repr(transparent)] |
| pub struct Cell<T: ?Sized> { |
| value: UnsafeCell<T>, |
| } |
| |
| impl<T:Copy> Cell<T> { |
| /// Returns a copy of the contained value. |
| /// |
| /// # Examples |
| /// |
| /// ``` |
| /// use std::cell::Cell; |
| /// |
| /// let c = Cell::new(5); |
| /// |
| /// let five = c.get(); |
| /// ``` |
| #[inline] |
| #[stable(feature = "rust1", since = "1.0.0")] |
| pub fn get(&self) -> T { |
| unsafe{ *self.value.get() } |
| } |
| |
| /// Updates the contained value using a function and returns the new value. |
| /// |
| /// # Examples |
| /// |
| /// ``` |
| /// #![feature(cell_update)] |
| /// |
| /// use std::cell::Cell; |
| /// |
| /// let c = Cell::new(5); |
| /// let new = c.update(|x| x + 1); |
| /// |
| /// assert_eq!(new, 6); |
| /// assert_eq!(c.get(), 6); |
| /// ``` |
| #[inline] |
| #[unstable(feature = "cell_update", issue = "50186")] |
| pub fn update<F>(&self, f: F) -> T |
| where |
| F: FnOnce(T) -> T, |
| { |
| let old = self.get(); |
| let new = f(old); |
| self.set(new); |
| new |
| } |
| } |
| |
| #[stable(feature = "rust1", since = "1.0.0")] |
| unsafe impl<T: ?Sized> Send for Cell<T> where T: Send {} |
| |
| #[stable(feature = "rust1", since = "1.0.0")] |
| impl<T: ?Sized> !Sync for Cell<T> {} |
| |
| #[stable(feature = "rust1", since = "1.0.0")] |
| impl<T:Copy> Clone for Cell<T> { |
| #[inline] |
| fn clone(&self) -> Cell<T> { |
| Cell::new(self.get()) |
| } |
| } |
| |
| #[stable(feature = "rust1", since = "1.0.0")] |
| impl<T: Default> Default for Cell<T> { |
| /// Creates a `Cell<T>`, with the `Default` value for T. |
| #[inline] |
| fn default() -> Cell<T> { |
| Cell::new(Default::default()) |
| } |
| } |
| |
| #[stable(feature = "rust1", since = "1.0.0")] |
| impl<T: PartialEq + Copy> PartialEq for Cell<T> { |
| #[inline] |
| fn eq(&self, other: &Cell<T>) -> bool { |
| self.get() == other.get() |
| } |
| } |
| |
| #[stable(feature = "cell_eq", since = "1.2.0")] |
| impl<T: Eq + Copy> Eq for Cell<T> {} |
| |
| #[stable(feature = "cell_ord", since = "1.10.0")] |
| impl<T: PartialOrd + Copy> PartialOrd for Cell<T> { |
| #[inline] |
| fn partial_cmp(&self, other: &Cell<T>) -> Option<Ordering> { |
| self.get().partial_cmp(&other.get()) |
| } |
| |
| #[inline] |
| fn lt(&self, other: &Cell<T>) -> bool { |
| self.get() < other.get() |
| } |
| |
| #[inline] |
| fn le(&self, other: &Cell<T>) -> bool { |
| self.get() <= other.get() |
| } |
| |
| #[inline] |
| fn gt(&self, other: &Cell<T>) -> bool { |
| self.get() > other.get() |
| } |
| |
| #[inline] |
| fn ge(&self, other: &Cell<T>) -> bool { |
| self.get() >= other.get() |
| } |
| } |
| |
| #[stable(feature = "cell_ord", since = "1.10.0")] |
| impl<T: Ord + Copy> Ord for Cell<T> { |
| #[inline] |
| fn cmp(&self, other: &Cell<T>) -> Ordering { |
| self.get().cmp(&other.get()) |
| } |
| } |
| |
| #[stable(feature = "cell_from", since = "1.12.0")] |
| impl<T> From<T> for Cell<T> { |
| fn from(t: T) -> Cell<T> { |
| Cell::new(t) |
| } |
| } |
| |
| impl<T> Cell<T> { |
| /// Creates a new `Cell` containing the given value. |
| /// |
| /// # Examples |
| /// |
| /// ``` |
| /// use std::cell::Cell; |
| /// |
| /// let c = Cell::new(5); |
| /// ``` |
| #[stable(feature = "rust1", since = "1.0.0")] |
| #[inline] |
| pub const fn new(value: T) -> Cell<T> { |
| Cell { |
| value: UnsafeCell::new(value), |
| } |
| } |
| |
| /// Sets the contained value. |
| /// |
| /// # Examples |
| /// |
| /// ``` |
| /// use std::cell::Cell; |
| /// |
| /// let c = Cell::new(5); |
| /// |
| /// c.set(10); |
| /// ``` |
| #[inline] |
| #[stable(feature = "rust1", since = "1.0.0")] |
| pub fn set(&self, val: T) { |
| let old = self.replace(val); |
| drop(old); |
| } |
| |
| /// Swaps the values of two Cells. |
| /// Difference with `std::mem::swap` is that this function doesn't require `&mut` reference. |
| /// |
| /// # Examples |
| /// |
| /// ``` |
| /// use std::cell::Cell; |
| /// |
| /// let c1 = Cell::new(5i32); |
| /// let c2 = Cell::new(10i32); |
| /// c1.swap(&c2); |
| /// assert_eq!(10, c1.get()); |
| /// assert_eq!(5, c2.get()); |
| /// ``` |
| #[inline] |
| #[stable(feature = "move_cell", since = "1.17.0")] |
| pub fn swap(&self, other: &Self) { |
| if ptr::eq(self, other) { |
| return; |
| } |
| unsafe { |
| ptr::swap(self.value.get(), other.value.get()); |
| } |
| } |
| |
| /// Replaces the contained value, and returns it. |
| /// |
| /// # Examples |
| /// |
| /// ``` |
| /// use std::cell::Cell; |
| /// |
| /// let cell = Cell::new(5); |
| /// assert_eq!(cell.get(), 5); |
| /// assert_eq!(cell.replace(10), 5); |
| /// assert_eq!(cell.get(), 10); |
| /// ``` |
| #[stable(feature = "move_cell", since = "1.17.0")] |
| pub fn replace(&self, val: T) -> T { |
| mem::replace(unsafe { &mut *self.value.get() }, val) |
| } |
| |
| /// Unwraps the value. |
| /// |
| /// # Examples |
| /// |
| /// ``` |
| /// use std::cell::Cell; |
| /// |
| /// let c = Cell::new(5); |
| /// let five = c.into_inner(); |
| /// |
| /// assert_eq!(five, 5); |
| /// ``` |
| #[stable(feature = "move_cell", since = "1.17.0")] |
| pub fn into_inner(self) -> T { |
| self.value.into_inner() |
| } |
| } |
| |
| impl<T: ?Sized> Cell<T> { |
| /// Returns a raw pointer to the underlying data in this cell. |
| /// |
| /// # Examples |
| /// |
| /// ``` |
| /// use std::cell::Cell; |
| /// |
| /// let c = Cell::new(5); |
| /// |
| /// let ptr = c.as_ptr(); |
| /// ``` |
| #[inline] |
| #[stable(feature = "cell_as_ptr", since = "1.12.0")] |
| pub const fn as_ptr(&self) -> *mut T { |
| self.value.get() |
| } |
| |
| /// Returns a mutable reference to the underlying data. |
| /// |
| /// This call borrows `Cell` mutably (at compile-time) which guarantees |
| /// that we possess the only reference. |
| /// |
| /// # Examples |
| /// |
| /// ``` |
| /// use std::cell::Cell; |
| /// |
| /// let mut c = Cell::new(5); |
| /// *c.get_mut() += 1; |
| /// |
| /// assert_eq!(c.get(), 6); |
| /// ``` |
| #[inline] |
| #[stable(feature = "cell_get_mut", since = "1.11.0")] |
| pub fn get_mut(&mut self) -> &mut T { |
| unsafe { |
| &mut *self.value.get() |
| } |
| } |
| |
| /// Returns a `&Cell<T>` from a `&mut T` |
| /// |
| /// # Examples |
| /// |
| /// ``` |
| /// use std::cell::Cell; |
| /// |
| /// let slice: &mut [i32] = &mut [1, 2, 3]; |
| /// let cell_slice: &Cell<[i32]> = Cell::from_mut(slice); |
| /// let slice_cell: &[Cell<i32>] = cell_slice.as_slice_of_cells(); |
| /// |
| /// assert_eq!(slice_cell.len(), 3); |
| /// ``` |
| #[inline] |
| #[stable(feature = "as_cell", since = "1.37.0")] |
| pub fn from_mut(t: &mut T) -> &Cell<T> { |
| unsafe { |
| &*(t as *mut T as *const Cell<T>) |
| } |
| } |
| } |
| |
| impl<T: Default> Cell<T> { |
| /// Takes the value of the cell, leaving `Default::default()` in its place. |
| /// |
| /// # Examples |
| /// |
| /// ``` |
| /// use std::cell::Cell; |
| /// |
| /// let c = Cell::new(5); |
| /// let five = c.take(); |
| /// |
| /// assert_eq!(five, 5); |
| /// assert_eq!(c.into_inner(), 0); |
| /// ``` |
| #[stable(feature = "move_cell", since = "1.17.0")] |
| pub fn take(&self) -> T { |
| self.replace(Default::default()) |
| } |
| } |
| |
| #[unstable(feature = "coerce_unsized", issue = "27732")] |
| impl<T: CoerceUnsized<U>, U> CoerceUnsized<Cell<U>> for Cell<T> {} |
| |
| impl<T> Cell<[T]> { |
| /// Returns a `&[Cell<T>]` from a `&Cell<[T]>` |
| /// |
| /// # Examples |
| /// |
| /// ``` |
| /// use std::cell::Cell; |
| /// |
| /// let slice: &mut [i32] = &mut [1, 2, 3]; |
| /// let cell_slice: &Cell<[i32]> = Cell::from_mut(slice); |
| /// let slice_cell: &[Cell<i32>] = cell_slice.as_slice_of_cells(); |
| /// |
| /// assert_eq!(slice_cell.len(), 3); |
| /// ``` |
| #[stable(feature = "as_cell", since = "1.37.0")] |
| pub fn as_slice_of_cells(&self) -> &[Cell<T>] { |
| unsafe { |
| &*(self as *const Cell<[T]> as *const [Cell<T>]) |
| } |
| } |
| } |
| |
| /// A mutable memory location with dynamically checked borrow rules |
| /// |
| /// See the [module-level documentation](index.html) for more. |
| #[stable(feature = "rust1", since = "1.0.0")] |
| pub struct RefCell<T: ?Sized> { |
| borrow: Cell<BorrowFlag>, |
| value: UnsafeCell<T>, |
| } |
| |
| /// An error returned by [`RefCell::try_borrow`](struct.RefCell.html#method.try_borrow). |
| #[stable(feature = "try_borrow", since = "1.13.0")] |
| pub struct BorrowError { |
| _private: (), |
| } |
| |
| #[stable(feature = "try_borrow", since = "1.13.0")] |
| impl Debug for BorrowError { |
| fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result { |
| f.debug_struct("BorrowError").finish() |
| } |
| } |
| |
| #[stable(feature = "try_borrow", since = "1.13.0")] |
| impl Display for BorrowError { |
| fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result { |
| Display::fmt("already mutably borrowed", f) |
| } |
| } |
| |
| /// An error returned by [`RefCell::try_borrow_mut`](struct.RefCell.html#method.try_borrow_mut). |
| #[stable(feature = "try_borrow", since = "1.13.0")] |
| pub struct BorrowMutError { |
| _private: (), |
| } |
| |
| #[stable(feature = "try_borrow", since = "1.13.0")] |
| impl Debug for BorrowMutError { |
| fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result { |
| f.debug_struct("BorrowMutError").finish() |
| } |
| } |
| |
| #[stable(feature = "try_borrow", since = "1.13.0")] |
| impl Display for BorrowMutError { |
| fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result { |
| Display::fmt("already borrowed", f) |
| } |
| } |
| |
| // Positive values represent the number of `Ref` active. Negative values |
| // represent the number of `RefMut` active. Multiple `RefMut`s can only be |
| // active at a time if they refer to distinct, nonoverlapping components of a |
| // `RefCell` (e.g., different ranges of a slice). |
| // |
| // `Ref` and `RefMut` are both two words in size, and so there will likely never |
| // be enough `Ref`s or `RefMut`s in existence to overflow half of the `usize` |
| // range. Thus, a `BorrowFlag` will probably never overflow or underflow. |
| // However, this is not a guarantee, as a pathological program could repeatedly |
| // create and then mem::forget `Ref`s or `RefMut`s. Thus, all code must |
| // explicitly check for overflow and underflow in order to avoid unsafety, or at |
| // least behave correctly in the event that overflow or underflow happens (e.g., |
| // see BorrowRef::new). |
| type BorrowFlag = isize; |
| const UNUSED: BorrowFlag = 0; |
| |
| #[inline(always)] |
| fn is_writing(x: BorrowFlag) -> bool { |
| x < UNUSED |
| } |
| |
| #[inline(always)] |
| fn is_reading(x: BorrowFlag) -> bool { |
| x > UNUSED |
| } |
| |
| impl<T> RefCell<T> { |
| /// Creates a new `RefCell` containing `value`. |
| /// |
| /// # Examples |
| /// |
| /// ``` |
| /// use std::cell::RefCell; |
| /// |
| /// let c = RefCell::new(5); |
| /// ``` |
| #[stable(feature = "rust1", since = "1.0.0")] |
| #[inline] |
| pub const fn new(value: T) -> RefCell<T> { |
| RefCell { |
| value: UnsafeCell::new(value), |
| borrow: Cell::new(UNUSED), |
| } |
| } |
| |
| /// Consumes the `RefCell`, returning the wrapped value. |
| /// |
| /// # Examples |
| /// |
| /// ``` |
| /// use std::cell::RefCell; |
| /// |
| /// let c = RefCell::new(5); |
| /// |
| /// let five = c.into_inner(); |
| /// ``` |
| #[stable(feature = "rust1", since = "1.0.0")] |
| #[inline] |
| pub fn into_inner(self) -> T { |
| // Since this function takes `self` (the `RefCell`) by value, the |
| // compiler statically verifies that it is not currently borrowed. |
| // Therefore the following assertion is just a `debug_assert!`. |
| debug_assert!(self.borrow.get() == UNUSED); |
| self.value.into_inner() |
| } |
| |
| /// Replaces the wrapped value with a new one, returning the old value, |
| /// without deinitializing either one. |
| /// |
| /// This function corresponds to [`std::mem::replace`](../mem/fn.replace.html). |
| /// |
| /// # Panics |
| /// |
| /// Panics if the value is currently borrowed. |
| /// |
| /// # Examples |
| /// |
| /// ``` |
| /// use std::cell::RefCell; |
| /// let cell = RefCell::new(5); |
| /// let old_value = cell.replace(6); |
| /// assert_eq!(old_value, 5); |
| /// assert_eq!(cell, RefCell::new(6)); |
| /// ``` |
| #[inline] |
| #[stable(feature = "refcell_replace", since="1.24.0")] |
| pub fn replace(&self, t: T) -> T { |
| mem::replace(&mut *self.borrow_mut(), t) |
| } |
| |
| /// Replaces the wrapped value with a new one computed from `f`, returning |
| /// the old value, without deinitializing either one. |
| /// |
| /// # Panics |
| /// |
| /// Panics if the value is currently borrowed. |
| /// |
| /// # Examples |
| /// |
| /// ``` |
| /// use std::cell::RefCell; |
| /// let cell = RefCell::new(5); |
| /// let old_value = cell.replace_with(|&mut old| old + 1); |
| /// assert_eq!(old_value, 5); |
| /// assert_eq!(cell, RefCell::new(6)); |
| /// ``` |
| #[inline] |
| #[stable(feature = "refcell_replace_swap", since="1.35.0")] |
| pub fn replace_with<F: FnOnce(&mut T) -> T>(&self, f: F) -> T { |
| let mut_borrow = &mut *self.borrow_mut(); |
| let replacement = f(mut_borrow); |
| mem::replace(mut_borrow, replacement) |
| } |
| |
| /// Swaps the wrapped value of `self` with the wrapped value of `other`, |
| /// without deinitializing either one. |
| /// |
| /// This function corresponds to [`std::mem::swap`](../mem/fn.swap.html). |
| /// |
| /// # Panics |
| /// |
| /// Panics if the value in either `RefCell` is currently borrowed. |
| /// |
| /// # Examples |
| /// |
| /// ``` |
| /// use std::cell::RefCell; |
| /// let c = RefCell::new(5); |
| /// let d = RefCell::new(6); |
| /// c.swap(&d); |
| /// assert_eq!(c, RefCell::new(6)); |
| /// assert_eq!(d, RefCell::new(5)); |
| /// ``` |
| #[inline] |
| #[stable(feature = "refcell_swap", since="1.24.0")] |
| pub fn swap(&self, other: &Self) { |
| mem::swap(&mut *self.borrow_mut(), &mut *other.borrow_mut()) |
| } |
| } |
| |
| impl<T: ?Sized> RefCell<T> { |
| /// Immutably borrows the wrapped value. |
| /// |
| /// The borrow lasts until the returned `Ref` exits scope. Multiple |
| /// immutable borrows can be taken out at the same time. |
| /// |
| /// # Panics |
| /// |
| /// Panics if the value is currently mutably borrowed. For a non-panicking variant, use |
| /// [`try_borrow`](#method.try_borrow). |
| /// |
| /// # Examples |
| /// |
| /// ``` |
| /// use std::cell::RefCell; |
| /// |
| /// let c = RefCell::new(5); |
| /// |
| /// let borrowed_five = c.borrow(); |
| /// let borrowed_five2 = c.borrow(); |
| /// ``` |
| /// |
| /// An example of panic: |
| /// |
| /// ``` |
| /// use std::cell::RefCell; |
| /// use std::thread; |
| /// |
| /// let result = thread::spawn(move || { |
| /// let c = RefCell::new(5); |
| /// let m = c.borrow_mut(); |
| /// |
| /// let b = c.borrow(); // this causes a panic |
| /// }).join(); |
| /// |
| /// assert!(result.is_err()); |
| /// ``` |
| #[stable(feature = "rust1", since = "1.0.0")] |
| #[inline] |
| pub fn borrow(&self) -> Ref<'_, T> { |
| self.try_borrow().expect("already mutably borrowed") |
| } |
| |
| /// Immutably borrows the wrapped value, returning an error if the value is currently mutably |
| /// borrowed. |
| /// |
| /// The borrow lasts until the returned `Ref` exits scope. Multiple immutable borrows can be |
| /// taken out at the same time. |
| /// |
| /// This is the non-panicking variant of [`borrow`](#method.borrow). |
| /// |
| /// # Examples |
| /// |
| /// ``` |
| /// use std::cell::RefCell; |
| /// |
| /// let c = RefCell::new(5); |
| /// |
| /// { |
| /// let m = c.borrow_mut(); |
| /// assert!(c.try_borrow().is_err()); |
| /// } |
| /// |
| /// { |
| /// let m = c.borrow(); |
| /// assert!(c.try_borrow().is_ok()); |
| /// } |
| /// ``` |
| #[stable(feature = "try_borrow", since = "1.13.0")] |
| #[inline] |
| pub fn try_borrow(&self) -> Result<Ref<'_, T>, BorrowError> { |
| match BorrowRef::new(&self.borrow) { |
| Some(b) => Ok(Ref { |
| value: unsafe { &*self.value.get() }, |
| borrow: b, |
| }), |
| None => Err(BorrowError { _private: () }), |
| } |
| } |
| |
| /// Mutably borrows the wrapped value. |
| /// |
| /// The borrow lasts until the returned `RefMut` or all `RefMut`s derived |
| /// from it exit scope. The value cannot be borrowed while this borrow is |
| /// active. |
| /// |
| /// # Panics |
| /// |
| /// Panics if the value is currently borrowed. For a non-panicking variant, use |
| /// [`try_borrow_mut`](#method.try_borrow_mut). |
| /// |
| /// # Examples |
| /// |
| /// ``` |
| /// use std::cell::RefCell; |
| /// |
| /// let c = RefCell::new(5); |
| /// |
| /// *c.borrow_mut() = 7; |
| /// |
| /// assert_eq!(*c.borrow(), 7); |
| /// ``` |
| /// |
| /// An example of panic: |
| /// |
| /// ``` |
| /// use std::cell::RefCell; |
| /// use std::thread; |
| /// |
| /// let result = thread::spawn(move || { |
| /// let c = RefCell::new(5); |
| /// let m = c.borrow(); |
| /// |
| /// let b = c.borrow_mut(); // this causes a panic |
| /// }).join(); |
| /// |
| /// assert!(result.is_err()); |
| /// ``` |
| #[stable(feature = "rust1", since = "1.0.0")] |
| #[inline] |
| pub fn borrow_mut(&self) -> RefMut<'_, T> { |
| self.try_borrow_mut().expect("already borrowed") |
| } |
| |
| /// Mutably borrows the wrapped value, returning an error if the value is currently borrowed. |
| /// |
| /// The borrow lasts until the returned `RefMut` or all `RefMut`s derived |
| /// from it exit scope. The value cannot be borrowed while this borrow is |
| /// active. |
| /// |
| /// This is the non-panicking variant of [`borrow_mut`](#method.borrow_mut). |
| /// |
| /// # Examples |
| /// |
| /// ``` |
| /// use std::cell::RefCell; |
| /// |
| /// let c = RefCell::new(5); |
| /// |
| /// { |
| /// let m = c.borrow(); |
| /// assert!(c.try_borrow_mut().is_err()); |
| /// } |
| /// |
| /// assert!(c.try_borrow_mut().is_ok()); |
| /// ``` |
| #[stable(feature = "try_borrow", since = "1.13.0")] |
| #[inline] |
| pub fn try_borrow_mut(&self) -> Result<RefMut<'_, T>, BorrowMutError> { |
| match BorrowRefMut::new(&self.borrow) { |
| Some(b) => Ok(RefMut { |
| value: unsafe { &mut *self.value.get() }, |
| borrow: b, |
| }), |
| None => Err(BorrowMutError { _private: () }), |
| } |
| } |
| |
| /// Returns a raw pointer to the underlying data in this cell. |
| /// |
| /// # Examples |
| /// |
| /// ``` |
| /// use std::cell::RefCell; |
| /// |
| /// let c = RefCell::new(5); |
| /// |
| /// let ptr = c.as_ptr(); |
| /// ``` |
| #[inline] |
| #[stable(feature = "cell_as_ptr", since = "1.12.0")] |
| pub fn as_ptr(&self) -> *mut T { |
| self.value.get() |
| } |
| |
| /// Returns a mutable reference to the underlying data. |
| /// |
| /// This call borrows `RefCell` mutably (at compile-time) so there is no |
| /// need for dynamic checks. |
| /// |
| /// However be cautious: this method expects `self` to be mutable, which is |
| /// generally not the case when using a `RefCell`. Take a look at the |
| /// [`borrow_mut`] method instead if `self` isn't mutable. |
| /// |
| /// Also, please be aware that this method is only for special circumstances and is usually |
| /// not what you want. In case of doubt, use [`borrow_mut`] instead. |
| /// |
| /// [`borrow_mut`]: #method.borrow_mut |
| /// |
| /// # Examples |
| /// |
| /// ``` |
| /// use std::cell::RefCell; |
| /// |
| /// let mut c = RefCell::new(5); |
| /// *c.get_mut() += 1; |
| /// |
| /// assert_eq!(c, RefCell::new(6)); |
| /// ``` |
| #[inline] |
| #[stable(feature = "cell_get_mut", since = "1.11.0")] |
| pub fn get_mut(&mut self) -> &mut T { |
| unsafe { |
| &mut *self.value.get() |
| } |
| } |
| |
| /// Immutably borrows the wrapped value, returning an error if the value is |
| /// currently mutably borrowed. |
| /// |
| /// # Safety |
| /// |
| /// Unlike `RefCell::borrow`, this method is unsafe because it does not |
| /// return a `Ref`, thus leaving the borrow flag untouched. Mutably |
| /// borrowing the `RefCell` while the reference returned by this method |
| /// is alive is undefined behaviour. |
| /// |
| /// # Examples |
| /// |
| /// ``` |
| /// use std::cell::RefCell; |
| /// |
| /// let c = RefCell::new(5); |
| /// |
| /// { |
| /// let m = c.borrow_mut(); |
| /// assert!(unsafe { c.try_borrow_unguarded() }.is_err()); |
| /// } |
| /// |
| /// { |
| /// let m = c.borrow(); |
| /// assert!(unsafe { c.try_borrow_unguarded() }.is_ok()); |
| /// } |
| /// ``` |
| #[stable(feature = "borrow_state", since = "1.37.0")] |
| #[inline] |
| pub unsafe fn try_borrow_unguarded(&self) -> Result<&T, BorrowError> { |
| if !is_writing(self.borrow.get()) { |
| Ok(&*self.value.get()) |
| } else { |
| Err(BorrowError { _private: () }) |
| } |
| } |
| } |
| |
| #[stable(feature = "rust1", since = "1.0.0")] |
| unsafe impl<T: ?Sized> Send for RefCell<T> where T: Send {} |
| |
| #[stable(feature = "rust1", since = "1.0.0")] |
| impl<T: ?Sized> !Sync for RefCell<T> {} |
| |
| #[stable(feature = "rust1", since = "1.0.0")] |
| impl<T: Clone> Clone for RefCell<T> { |
| /// # Panics |
| /// |
| /// Panics if the value is currently mutably borrowed. |
| #[inline] |
| fn clone(&self) -> RefCell<T> { |
| RefCell::new(self.borrow().clone()) |
| } |
| } |
| |
| #[stable(feature = "rust1", since = "1.0.0")] |
| impl<T: Default> Default for RefCell<T> { |
| /// Creates a `RefCell<T>`, with the `Default` value for T. |
| #[inline] |
| fn default() -> RefCell<T> { |
| RefCell::new(Default::default()) |
| } |
| } |
| |
| #[stable(feature = "rust1", since = "1.0.0")] |
| impl<T: ?Sized + PartialEq> PartialEq for RefCell<T> { |
| /// # Panics |
| /// |
| /// Panics if the value in either `RefCell` is currently borrowed. |
| #[inline] |
| fn eq(&self, other: &RefCell<T>) -> bool { |
| *self.borrow() == *other.borrow() |
| } |
| } |
| |
| #[stable(feature = "cell_eq", since = "1.2.0")] |
| impl<T: ?Sized + Eq> Eq for RefCell<T> {} |
| |
| #[stable(feature = "cell_ord", since = "1.10.0")] |
| impl<T: ?Sized + PartialOrd> PartialOrd for RefCell<T> { |
| /// # Panics |
| /// |
| /// Panics if the value in either `RefCell` is currently borrowed. |
| #[inline] |
| fn partial_cmp(&self, other: &RefCell<T>) -> Option<Ordering> { |
| self.borrow().partial_cmp(&*other.borrow()) |
| } |
| |
| /// # Panics |
| /// |
| /// Panics if the value in either `RefCell` is currently borrowed. |
| #[inline] |
| fn lt(&self, other: &RefCell<T>) -> bool { |
| *self.borrow() < *other.borrow() |
| } |
| |
| /// # Panics |
| /// |
| /// Panics if the value in either `RefCell` is currently borrowed. |
| #[inline] |
| fn le(&self, other: &RefCell<T>) -> bool { |
| *self.borrow() <= *other.borrow() |
| } |
| |
| /// # Panics |
| /// |
| /// Panics if the value in either `RefCell` is currently borrowed. |
| #[inline] |
| fn gt(&self, other: &RefCell<T>) -> bool { |
| *self.borrow() > *other.borrow() |
| } |
| |
| /// # Panics |
| /// |
| /// Panics if the value in either `RefCell` is currently borrowed. |
| #[inline] |
| fn ge(&self, other: &RefCell<T>) -> bool { |
| *self.borrow() >= *other.borrow() |
| } |
| } |
| |
| #[stable(feature = "cell_ord", since = "1.10.0")] |
| impl<T: ?Sized + Ord> Ord for RefCell<T> { |
| /// # Panics |
| /// |
| /// Panics if the value in either `RefCell` is currently borrowed. |
| #[inline] |
| fn cmp(&self, other: &RefCell<T>) -> Ordering { |
| self.borrow().cmp(&*other.borrow()) |
| } |
| } |
| |
| #[stable(feature = "cell_from", since = "1.12.0")] |
| impl<T> From<T> for RefCell<T> { |
| fn from(t: T) -> RefCell<T> { |
| RefCell::new(t) |
| } |
| } |
| |
| #[unstable(feature = "coerce_unsized", issue = "27732")] |
| impl<T: CoerceUnsized<U>, U> CoerceUnsized<RefCell<U>> for RefCell<T> {} |
| |
| struct BorrowRef<'b> { |
| borrow: &'b Cell<BorrowFlag>, |
| } |
| |
| impl<'b> BorrowRef<'b> { |
| #[inline] |
| fn new(borrow: &'b Cell<BorrowFlag>) -> Option<BorrowRef<'b>> { |
| let b = borrow.get().wrapping_add(1); |
| if !is_reading(b) { |
| // Incrementing borrow can result in a non-reading value (<= 0) in these cases: |
| // 1. It was < 0, i.e. there are writing borrows, so we can't allow a read borrow |
| // due to Rust's reference aliasing rules |
| // 2. It was isize::max_value() (the max amount of reading borrows) and it overflowed |
| // into isize::min_value() (the max amount of writing borrows) so we can't allow |
| // an additional read borrow because isize can't represent so many read borrows |
| // (this can only happen if you mem::forget more than a small constant amount of |
| // `Ref`s, which is not good practice) |
| None |
| } else { |
| // Incrementing borrow can result in a reading value (> 0) in these cases: |
| // 1. It was = 0, i.e. it wasn't borrowed, and we are taking the first read borrow |
| // 2. It was > 0 and < isize::max_value(), i.e. there were read borrows, and isize |
| // is large enough to represent having one more read borrow |
| borrow.set(b); |
| Some(BorrowRef { borrow }) |
| } |
| } |
| } |
| |
| impl Drop for BorrowRef<'_> { |
| #[inline] |
| fn drop(&mut self) { |
| let borrow = self.borrow.get(); |
| debug_assert!(is_reading(borrow)); |
| self.borrow.set(borrow - 1); |
| } |
| } |
| |
| impl Clone for BorrowRef<'_> { |
| #[inline] |
| fn clone(&self) -> Self { |
| // Since this Ref exists, we know the borrow flag |
| // is a reading borrow. |
| let borrow = self.borrow.get(); |
| debug_assert!(is_reading(borrow)); |
| // Prevent the borrow counter from overflowing into |
| // a writing borrow. |
| assert!(borrow != isize::max_value()); |
| self.borrow.set(borrow + 1); |
| BorrowRef { borrow: self.borrow } |
| } |
| } |
| |
| /// Wraps a borrowed reference to a value in a `RefCell` box. |
| /// A wrapper type for an immutably borrowed value from a `RefCell<T>`. |
| /// |
| /// See the [module-level documentation](index.html) for more. |
| #[stable(feature = "rust1", since = "1.0.0")] |
| pub struct Ref<'b, T: ?Sized + 'b> { |
| value: &'b T, |
| borrow: BorrowRef<'b>, |
| } |
| |
| #[stable(feature = "rust1", since = "1.0.0")] |
| impl<T: ?Sized> Deref for Ref<'_, T> { |
| type Target = T; |
| |
| #[inline] |
| fn deref(&self) -> &T { |
| self.value |
| } |
| } |
| |
| impl<'b, T: ?Sized> Ref<'b, T> { |
| /// Copies a `Ref`. |
| /// |
| /// The `RefCell` is already immutably borrowed, so this cannot fail. |
| /// |
| /// This is an associated function that needs to be used as |
| /// `Ref::clone(...)`. A `Clone` implementation or a method would interfere |
| /// with the widespread use of `r.borrow().clone()` to clone the contents of |
| /// a `RefCell`. |
| #[stable(feature = "cell_extras", since = "1.15.0")] |
| #[inline] |
| pub fn clone(orig: &Ref<'b, T>) -> Ref<'b, T> { |
| Ref { |
| value: orig.value, |
| borrow: orig.borrow.clone(), |
| } |
| } |
| |
| /// Makes a new `Ref` for a component of the borrowed data. |
| /// |
| /// The `RefCell` is already immutably borrowed, so this cannot fail. |
| /// |
| /// This is an associated function that needs to be used as `Ref::map(...)`. |
| /// A method would interfere with methods of the same name on the contents |
| /// of a `RefCell` used through `Deref`. |
| /// |
| /// # Examples |
| /// |
| /// ``` |
| /// use std::cell::{RefCell, Ref}; |
| /// |
| /// let c = RefCell::new((5, 'b')); |
| /// let b1: Ref<(u32, char)> = c.borrow(); |
| /// let b2: Ref<u32> = Ref::map(b1, |t| &t.0); |
| /// assert_eq!(*b2, 5) |
| /// ``` |
| #[stable(feature = "cell_map", since = "1.8.0")] |
| #[inline] |
| pub fn map<U: ?Sized, F>(orig: Ref<'b, T>, f: F) -> Ref<'b, U> |
| where F: FnOnce(&T) -> &U |
| { |
| Ref { |
| value: f(orig.value), |
| borrow: orig.borrow, |
| } |
| } |
| |
| /// Splits a `Ref` into multiple `Ref`s for different components of the |
| /// borrowed data. |
| /// |
| /// The `RefCell` is already immutably borrowed, so this cannot fail. |
| /// |
| /// This is an associated function that needs to be used as |
| /// `Ref::map_split(...)`. A method would interfere with methods of the same |
| /// name on the contents of a `RefCell` used through `Deref`. |
| /// |
| /// # Examples |
| /// |
| /// ``` |
| /// use std::cell::{Ref, RefCell}; |
| /// |
| /// let cell = RefCell::new([1, 2, 3, 4]); |
| /// let borrow = cell.borrow(); |
| /// let (begin, end) = Ref::map_split(borrow, |slice| slice.split_at(2)); |
| /// assert_eq!(*begin, [1, 2]); |
| /// assert_eq!(*end, [3, 4]); |
| /// ``` |
| #[stable(feature = "refcell_map_split", since = "1.35.0")] |
| #[inline] |
| pub fn map_split<U: ?Sized, V: ?Sized, F>(orig: Ref<'b, T>, f: F) -> (Ref<'b, U>, Ref<'b, V>) |
| where F: FnOnce(&T) -> (&U, &V) |
| { |
| let (a, b) = f(orig.value); |
| let borrow = orig.borrow.clone(); |
| (Ref { value: a, borrow }, Ref { value: b, borrow: orig.borrow }) |
| } |
| } |
| |
| #[unstable(feature = "coerce_unsized", issue = "27732")] |
| impl<'b, T: ?Sized + Unsize<U>, U: ?Sized> CoerceUnsized<Ref<'b, U>> for Ref<'b, T> {} |
| |
| #[stable(feature = "std_guard_impls", since = "1.20.0")] |
| impl<T: ?Sized + fmt::Display> fmt::Display for Ref<'_, T> { |
| fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result { |
| self.value.fmt(f) |
| } |
| } |
| |
| impl<'b, T: ?Sized> RefMut<'b, T> { |
| /// Makes a new `RefMut` for a component of the borrowed data, e.g., an enum |
| /// variant. |
| /// |
| /// The `RefCell` is already mutably borrowed, so this cannot fail. |
| /// |
| /// This is an associated function that needs to be used as |
| /// `RefMut::map(...)`. A method would interfere with methods of the same |
| /// name on the contents of a `RefCell` used through `Deref`. |
| /// |
| /// # Examples |
| /// |
| /// ``` |
| /// use std::cell::{RefCell, RefMut}; |
| /// |
| /// let c = RefCell::new((5, 'b')); |
| /// { |
| /// let b1: RefMut<(u32, char)> = c.borrow_mut(); |
| /// let mut b2: RefMut<u32> = RefMut::map(b1, |t| &mut t.0); |
| /// assert_eq!(*b2, 5); |
| /// *b2 = 42; |
| /// } |
| /// assert_eq!(*c.borrow(), (42, 'b')); |
| /// ``` |
| #[stable(feature = "cell_map", since = "1.8.0")] |
| #[inline] |
| pub fn map<U: ?Sized, F>(orig: RefMut<'b, T>, f: F) -> RefMut<'b, U> |
| where F: FnOnce(&mut T) -> &mut U |
| { |
| // FIXME(nll-rfc#40): fix borrow-check |
| let RefMut { value, borrow } = orig; |
| RefMut { |
| value: f(value), |
| borrow, |
| } |
| } |
| |
| /// Splits a `RefMut` into multiple `RefMut`s for different components of the |
| /// borrowed data. |
| /// |
| /// The underlying `RefCell` will remain mutably borrowed until both |
| /// returned `RefMut`s go out of scope. |
| /// |
| /// The `RefCell` is already mutably borrowed, so this cannot fail. |
| /// |
| /// This is an associated function that needs to be used as |
| /// `RefMut::map_split(...)`. A method would interfere with methods of the |
| /// same name on the contents of a `RefCell` used through `Deref`. |
| /// |
| /// # Examples |
| /// |
| /// ``` |
| /// use std::cell::{RefCell, RefMut}; |
| /// |
| /// let cell = RefCell::new([1, 2, 3, 4]); |
| /// let borrow = cell.borrow_mut(); |
| /// let (mut begin, mut end) = RefMut::map_split(borrow, |slice| slice.split_at_mut(2)); |
| /// assert_eq!(*begin, [1, 2]); |
| /// assert_eq!(*end, [3, 4]); |
| /// begin.copy_from_slice(&[4, 3]); |
| /// end.copy_from_slice(&[2, 1]); |
| /// ``` |
| #[stable(feature = "refcell_map_split", since = "1.35.0")] |
| #[inline] |
| pub fn map_split<U: ?Sized, V: ?Sized, F>( |
| orig: RefMut<'b, T>, f: F |
| ) -> (RefMut<'b, U>, RefMut<'b, V>) |
| where F: FnOnce(&mut T) -> (&mut U, &mut V) |
| { |
| let (a, b) = f(orig.value); |
| let borrow = orig.borrow.clone(); |
| (RefMut { value: a, borrow }, RefMut { value: b, borrow: orig.borrow }) |
| } |
| } |
| |
| struct BorrowRefMut<'b> { |
| borrow: &'b Cell<BorrowFlag>, |
| } |
| |
| impl Drop for BorrowRefMut<'_> { |
| #[inline] |
| fn drop(&mut self) { |
| let borrow = self.borrow.get(); |
| debug_assert!(is_writing(borrow)); |
| self.borrow.set(borrow + 1); |
| } |
| } |
| |
| impl<'b> BorrowRefMut<'b> { |
| #[inline] |
| fn new(borrow: &'b Cell<BorrowFlag>) -> Option<BorrowRefMut<'b>> { |
| // NOTE: Unlike BorrowRefMut::clone, new is called to create the initial |
| // mutable reference, and so there must currently be no existing |
| // references. Thus, while clone increments the mutable refcount, here |
| // we explicitly only allow going from UNUSED to UNUSED - 1. |
| match borrow.get() { |
| UNUSED => { |
| borrow.set(UNUSED - 1); |
| Some(BorrowRefMut { borrow }) |
| }, |
| _ => None, |
| } |
| } |
| |
| // Clones a `BorrowRefMut`. |
| // |
| // This is only valid if each `BorrowRefMut` is used to track a mutable |
| // reference to a distinct, nonoverlapping range of the original object. |
| // This isn't in a Clone impl so that code doesn't call this implicitly. |
| #[inline] |
| fn clone(&self) -> BorrowRefMut<'b> { |
| let borrow = self.borrow.get(); |
| debug_assert!(is_writing(borrow)); |
| // Prevent the borrow counter from underflowing. |
| assert!(borrow != isize::min_value()); |
| self.borrow.set(borrow - 1); |
| BorrowRefMut { borrow: self.borrow } |
| } |
| } |
| |
| /// A wrapper type for a mutably borrowed value from a `RefCell<T>`. |
| /// |
| /// See the [module-level documentation](index.html) for more. |
| #[stable(feature = "rust1", since = "1.0.0")] |
| pub struct RefMut<'b, T: ?Sized + 'b> { |
| value: &'b mut T, |
| borrow: BorrowRefMut<'b>, |
| } |
| |
| #[stable(feature = "rust1", since = "1.0.0")] |
| impl<T: ?Sized> Deref for RefMut<'_, T> { |
| type Target = T; |
| |
| #[inline] |
| fn deref(&self) -> &T { |
| self.value |
| } |
| } |
| |
| #[stable(feature = "rust1", since = "1.0.0")] |
| impl<T: ?Sized> DerefMut for RefMut<'_, T> { |
| #[inline] |
| fn deref_mut(&mut self) -> &mut T { |
| self.value |
| } |
| } |
| |
| #[unstable(feature = "coerce_unsized", issue = "27732")] |
| impl<'b, T: ?Sized + Unsize<U>, U: ?Sized> CoerceUnsized<RefMut<'b, U>> for RefMut<'b, T> {} |
| |
| #[stable(feature = "std_guard_impls", since = "1.20.0")] |
| impl<T: ?Sized + fmt::Display> fmt::Display for RefMut<'_, T> { |
| fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result { |
| self.value.fmt(f) |
| } |
| } |
| |
| /// The core primitive for interior mutability in Rust. |
| /// |
| /// `UnsafeCell<T>` is a type that wraps some `T` and indicates unsafe interior operations on the |
| /// wrapped type. Types with an `UnsafeCell<T>` field are considered to have an 'unsafe interior'. |
| /// The `UnsafeCell<T>` type is the only legal way to obtain aliasable data that is considered |
| /// mutable. In general, transmuting an `&T` type into an `&mut T` is considered undefined behavior. |
| /// |
| /// If you have a reference `&SomeStruct`, then normally in Rust all fields of `SomeStruct` are |
| /// immutable. The compiler makes optimizations based on the knowledge that `&T` is not mutably |
| /// aliased or mutated, and that `&mut T` is unique. `UnsafeCell<T>` is the only core language |
| /// feature to work around the restriction that `&T` may not be mutated. All other types that |
| /// allow internal mutability, such as `Cell<T>` and `RefCell<T>`, use `UnsafeCell` to wrap their |
| /// internal data. There is *no* legal way to obtain aliasing `&mut`, not even with `UnsafeCell<T>`. |
| /// |
| /// The `UnsafeCell` API itself is technically very simple: it gives you a raw pointer `*mut T` to |
| /// its contents. It is up to _you_ as the abstraction designer to use that raw pointer correctly. |
| /// |
| /// The precise Rust aliasing rules are somewhat in flux, but the main points are not contentious: |
| /// |
| /// - If you create a safe reference with lifetime `'a` (either a `&T` or `&mut T` |
| /// reference) that is accessible by safe code (for example, because you returned it), |
| /// then you must not access the data in any way that contradicts that reference for the |
| /// remainder of `'a`. For example, this means that if you take the `*mut T` from an |
| /// `UnsafeCell<T>` and cast it to an `&T`, then the data in `T` must remain immutable |
| /// (modulo any `UnsafeCell` data found within `T`, of course) until that reference's |
| /// lifetime expires. Similarly, if you create a `&mut T` reference that is released to |
| /// safe code, then you must not access the data within the `UnsafeCell` until that |
| /// reference expires. |
| /// |
| /// - At all times, you must avoid data races. If multiple threads have access to |
| /// the same `UnsafeCell`, then any writes must have a proper happens-before relation to all other |
| /// accesses (or use atomics). |
| /// |
| /// To assist with proper design, the following scenarios are explicitly declared legal |
| /// for single-threaded code: |
| /// |
| /// 1. A `&T` reference can be released to safe code and there it can co-exist with other `&T` |
| /// references, but not with a `&mut T` |
| /// |
| /// 2. A `&mut T` reference may be released to safe code provided neither other `&mut T` nor `&T` |
| /// co-exist with it. A `&mut T` must always be unique. |
| /// |
| /// Note that while mutating or mutably aliasing the contents of an `&UnsafeCell<T>` is |
| /// ok (provided you enforce the invariants some other way), it is still undefined behavior |
| /// to have multiple `&mut UnsafeCell<T>` aliases. |
| /// |
| /// # Examples |
| /// |
| /// ``` |
| /// use std::cell::UnsafeCell; |
| /// |
| /// # #[allow(dead_code)] |
| /// struct NotThreadSafe<T> { |
| /// value: UnsafeCell<T>, |
| /// } |
| /// |
| /// unsafe impl<T> Sync for NotThreadSafe<T> {} |
| /// ``` |
| #[lang = "unsafe_cell"] |
| #[stable(feature = "rust1", since = "1.0.0")] |
| #[repr(transparent)] |
| pub struct UnsafeCell<T: ?Sized> { |
| value: T, |
| } |
| |
| #[stable(feature = "rust1", since = "1.0.0")] |
| impl<T: ?Sized> !Sync for UnsafeCell<T> {} |
| |
| impl<T> UnsafeCell<T> { |
| /// Constructs a new instance of `UnsafeCell` which will wrap the specified |
| /// value. |
| /// |
| /// All access to the inner value through methods is `unsafe`. |
| /// |
| /// # Examples |
| /// |
| /// ``` |
| /// use std::cell::UnsafeCell; |
| /// |
| /// let uc = UnsafeCell::new(5); |
| /// ``` |
| #[stable(feature = "rust1", since = "1.0.0")] |
| #[inline] |
| pub const fn new(value: T) -> UnsafeCell<T> { |
| UnsafeCell { value } |
| } |
| |
| /// Unwraps the value. |
| /// |
| /// # Examples |
| /// |
| /// ``` |
| /// use std::cell::UnsafeCell; |
| /// |
| /// let uc = UnsafeCell::new(5); |
| /// |
| /// let five = uc.into_inner(); |
| /// ``` |
| #[inline] |
| #[stable(feature = "rust1", since = "1.0.0")] |
| pub fn into_inner(self) -> T { |
| self.value |
| } |
| } |
| |
| impl<T: ?Sized> UnsafeCell<T> { |
| /// Gets a mutable pointer to the wrapped value. |
| /// |
| /// This can be cast to a pointer of any kind. |
| /// Ensure that the access is unique (no active references, mutable or not) |
| /// when casting to `&mut T`, and ensure that there are no mutations |
| /// or mutable aliases going on when casting to `&T` |
| /// |
| /// # Examples |
| /// |
| /// ``` |
| /// use std::cell::UnsafeCell; |
| /// |
| /// let uc = UnsafeCell::new(5); |
| /// |
| /// let five = uc.get(); |
| /// ``` |
| #[inline] |
| #[stable(feature = "rust1", since = "1.0.0")] |
| pub const fn get(&self) -> *mut T { |
| // We can just cast the pointer from `UnsafeCell<T>` to `T` because of |
| // #[repr(transparent)] |
| self as *const UnsafeCell<T> as *const T as *mut T |
| } |
| } |
| |
| #[stable(feature = "unsafe_cell_default", since = "1.10.0")] |
| impl<T: Default> Default for UnsafeCell<T> { |
| /// Creates an `UnsafeCell`, with the `Default` value for T. |
| fn default() -> UnsafeCell<T> { |
| UnsafeCell::new(Default::default()) |
| } |
| } |
| |
| #[stable(feature = "cell_from", since = "1.12.0")] |
| impl<T> From<T> for UnsafeCell<T> { |
| fn from(t: T) -> UnsafeCell<T> { |
| UnsafeCell::new(t) |
| } |
| } |
| |
| #[unstable(feature = "coerce_unsized", issue = "27732")] |
| impl<T: CoerceUnsized<U>, U> CoerceUnsized<UnsafeCell<U>> for UnsafeCell<T> {} |
| |
| #[allow(unused)] |
| fn assert_coerce_unsized(a: UnsafeCell<&i32>, b: Cell<&i32>, c: RefCell<&i32>) { |
| let _: UnsafeCell<&dyn Send> = a; |
| let _: Cell<&dyn Send> = b; |
| let _: RefCell<&dyn Send> = c; |
| } |