Boxed values

Introduction#

Boxes are a very important part of Rust, and every rustacean should know what they are and how to use them

Creating a Box

In stable Rust you create a Box by using the Box::new function.

let boxed_int: Box<i32> = Box::new(1);

Using Boxed Values

Because Boxes implement the Deref<Target=T>, you can use boxed values just like the value they contain.

let boxed_vec = Box::new(vec![1, 2, 3]);
println!("{}", boxed_vec.get(0));

If you want to pattern match on a boxed value, you may have to dereference the box manually.

struct Point {
    x: i32,
    y: i32,
}

let boxed_point = Box::new(Point { x: 0, y: 0});
// Notice the *. That dereferences the boxed value into just the value
match *boxed_point {
    Point {x, y} => println!("Point is at ({}, {})", x, y),
}

Using Boxes to Create Recursive Enums and Structs

If you try and create a recursive enum in Rust without using Box, you will get a compile time error saying that the enum can’t be sized.

// This gives an error!
enum List {
    Nil,
    Cons(i32, List)
}

In order for the enum to have a defined size, the recursively contained value must be in a Box.

// This works!
enum List {
    Nil,
    Cons(i32, Box<List>)
}

This works because Box always has the same size no matter what T is, which allows Rust to give List a size.