Smart Pointers

• Rc<T> enables multiple owners of the same data; Box<T> and RefCell<T> have single owners.
• Box<T> allows immutable or mutable borrows checked at compile time; Rc<T> allows only immutable borrows checked at compile time; RefCell<T> allows immutable or mutable borrows checked at runtime.
• Because RefCell<T> allows mutable borrows checked at runtime, you can mutate the value inside the RefCell<T> even when the RefCell<T> is immutable.

Box

Box<T> allow you to store data on the heap rather than the stack. What remains on the stack is the pointer to the heap data.

The Box<T> type is a smart pointer because it implements the Deref trait, which allows Box<T> values to be treated like references. Implementing the Deref trait allows you to customize the behavior of the dereference operator *.

Use when

• you have a type whose size can’t be known at compile time
• you want to own a value and you care only that it’s a type that implements a particular trait rather than being of a specific type.

Rc

The Rc<T> type keeps track of the number of references to data on the heap so that data can have multiple owners.

RefCell

The RefCell<T> type with its interior mutability gives us a type that we can use when we need an immutable type but need to change an inner value of that type; it also enforces the borrowing rules at runtime instead of at compile time.