Hands on Box in Rust

Things on this page are fragmentary and immature notes/thoughts of the author. Please read with your own judgement!

:timing
:sccache 1

Tips and Traps¶

Box<dyn Trait> is a syntax sugar for Box<dyn Trait + 'static>. If the trait inside Box does not have a static life time, you have to specify it manually, e.g., Box<dyn Trait + 'some_life_time>.
You can leak a memory to make it has a static life time using Box::leak.

let b = Box::new(5);
b

5

println!("b = {}", b);

b = 5

*b

5

Use Box to Construct Recursive Types¶

enum List {
    Cons(i32, List),
    Nil,
}

enum List {
^^^^^^^^^ recursive type has infinite size
    Cons(i32, List),
              ^^^^ recursive without indirection
recursive type `List` has infinite size
help: insert some indirection (e.g., a `Box`, `Rc`, or `&`) to make `List` representable

Box<

#[derive(Debug)]
enum List {
    Cons(i32, Box<List>),
    Nil,
}

use List::{Cons, Nil};

let list = Cons(1, Box::new(Cons(2, Box::new(Cons(3, Box::new(Nil))))));
list

Cons(1, Cons(2, Cons(3, Nil)))

match list {
    Cons(val, tail) => {
        println!("{}", val);
    },
    _ => println!("End of the list"),
}

()

Hands on Box in Rust

Tips and Traps¶

Use Box to Construct Recursive Types¶

References¶