Smart Pointers

RecipeCrates
Boxstd
Rcstd
RefCellstd
  • 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

book-rust-box std

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 std::ops::Deref⮳ trait, which allows Box<T> values to be treated like references. Implementing the std::ops::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

std

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

RefCell

std

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.