# Interior Mutability

Sometimes a type needs 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.