Welcome to a new chapter on some of Rust's most powerful memory management features. We'll begin our exploration of smart pointers with the most straightforward one: Box. A Box is a smart pointer that allows you to store data on the heap instead of the stack. It's a fundamental tool for managing memory and enabling certain data structures in Rust.
We've seen how Rc enables multiple owners and how RefCell allows interior mutability. Individually, they solve specific problems. But when combined, they unlock a powerful pattern for managing shared, mutable state in a single-threaded context. The combination Rc> allows a single piece of data to have multiple owners, any of whom can mutate it.
After learning how to allocate data on the heap with Box, we'll now explore the magic that makes smart pointers so convenient. The Deref trait is the key to this convenience, as it allows a smart pointer struct to be treated like a regular reference. This enables you to write code that works with both references and smart pointers seamlessly.
This series has covered a wide range of smart pointers and memory management techniques. Now it's time to put that knowledge into practice with a classic computer science data structure: the singly linked list. This project will demonstrate why Box is essential for recursive data structures and how to handle ownership and memory management correctly.
We've seen how Box enforces single ownership of heap-allocated data. But what happens when a single value needs to have multiple owners? For example, in a graph data structure, several edges might point to the same node, and that node shouldn't be cleaned up until all those edges are gone. For this, Rust provides the Rc smart pointer, which enables Reference Counting for shared ownership.
We've seen how Rust's borrowing rules are enforced at compile time, and how Rc allows shared ownership of immutable data. But what if we need to mutate data that is shared and theoretically immutable? For this, Rust provides the interior mutability pattern, and its primary tool is the RefCell smart pointer.
The Rc and RefCell patterns are incredibly powerful, but they come with a potential danger: reference cycles. A reference cycle is a situation where items reference each other in a loop, causing their reference counts to never drop to zero. This is a form of memory leak, and while Rust's ownership system prevents most memory safety issues, this is one you must handle carefully yourself. In this article, we'll learn how to create a reference cycle and how to prevent it using weak references.
In the last article, we saw how Rc can create reference cycles, leading to memory leaks. To solve this, Rust provides a companion smart pointer:Weak. A Weak is a non-owning reference that allows you to refer to data owned by an Rc without contributing to its strong reference count. This is the primary tool for breaking reference cycles and creating temporary, safe references to data.