Easy Rust

Box

Box is a very convenient type in Rust. When you use a Box, you can put a type on the heap instead of the stack. To make a new Box, just use Box::new() and put the item inside.

fn just_takes_a_variable<T>(item: T) {} // Takes anything and drops it.

fn main() {
    let my_number = 1; // This is an i32
    just_takes_a_variable(my_number);
    just_takes_a_variable(my_number); // Using this function twice is no problem, because it's Copy

    let my_box = Box::new(1); // This is a Box<i32>
    just_takes_a_variable(my_box.clone()); // Without .clone() the second function would make an error
    just_takes_a_variable(my_box); // because Box is not Copy
}

At first it is hard to imagine where to use it, but you use it in Rust a lot. You remember that & is used for str because the compiler doesn't know the size of a str: it can be any length. But the & reference is always the same length, so the compiler can use it. Box is similar. Also, you can use * on a Box to get to the value, just like with &:

fn main() {
    let my_box = Box::new(1); // This is a Box<i32>
    let an_integer = *my_box; // This is an i32
    println!("{:?}", my_box);
    println!("{:?}", an_integer);
}

This is why Box is called a "smart pointer", because it is like a & reference (a kind of pointer) but can do more things.

You can also use a Box to create structs with the same struct inside. These are called recursive, which means that inside Struct A is maybe another Struct A. Sometimes you can use Boxes to create linked lists, although these lists are not very popular in Rust. But if you want to create a recursive struct, you can use a Box. Here's what happens if you try without a Box:

#![allow(unused)]
fn main() {
struct List {
    item: Option<List>, // ⚠️
}
}

This simple List has one item, that may be Some<List> (another list), or None. Because you can choose None, it will not be recursive forever. But the compiler still doesn't know the size:

error[E0072]: recursive type `List` has infinite size
  --> src\main.rs:16:1
   |
16 | struct List {
   | ^^^^^^^^^^^ recursive type has infinite size
17 |     item: Option<List>,
   |     ------------------ recursive without indirection
   |
   = help: insert indirection (e.g., a `Box`, `Rc`, or `&`) at some point to make `List` representable

You can see that it even suggests trying a Box. So let's put a Box around List:

struct List {
    item: Option<Box<List>>,
}
fn main() {}

Now the compiler is fine with the List, because everything is behind a Box, and it knows the size of a Box. Then a very simple list might look like this:

struct List {
    item: Option<Box<List>>,
}

impl List {
    fn new() -> List {
        List {
            item: Some(Box::new(List { item: None })),
        }
    }
}

fn main() {
    let mut my_list = List::new();
}

Even without data it is a bit complicated, and Rust does not use this type of pattern very much. This is because Rust has strict rules on borrowing and ownership, as you know. But if you want to start a list like this (a linked list), Box can help.

A Box also lets you use std::mem::drop on it, because it's on the heap. That can be convenient sometimes.