Sometimes a type needs to be mutated while having multiple aliases. In Rust this is achieved using a pattern called interior mutability. A type has interior mutability if its internal state can be changed through a shared reference to it. This goes against the usual requirement that the value pointed to by a shared reference is not mutated.
std::cell::UnsafeCell<T>
type is the only allowed way in Rust to disable this requirement. When UnsafeCell<T>
is immutably aliased, it is still safe to mutate, or obtain a mutable reference to, the T
it contains. As with all other types, it is undefined behavior to have multiple &mut UnsafeCell<T>
aliases.
Other types with interior mutability can be created by using UnsafeCell<T>
as a field. The standard library provides a variety of types that provide safe interior mutability APIs. For example, std::cell::RefCell<T>
uses run-time borrow checks to ensure the usual rules around multiple references. The std::sync::atomic
module contains types that wrap a value that is only accessed with atomic operations, allowing the value to be shared and mutated across threads.
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Licensed under the Apache License, Version 2.0 or the MIT license, at your option.
https://doc.rust-lang.org/reference/interior-mutability.html