What is the difference between Box and &Trait / Box?

Question

When writing code with traits you can put the trait in a trait bound:

use std::fmt::Debug;

fn myfunction1(v: Box) {
    println!(\"         


        

Answer 1

With Box you are using a trait bound to tell the compiler that you want a Box with an instance of some type T which implements Trait, and you will specify T when you use it. The Rust compiler will likely create different, efficient, code for each different T in your code (monomorphization).

With Box you are telling the compiler that you want a Box with a trait object, a pointer to an unknown type which implements Trait, which means that the compiler will use dynamic dispatch.

I've included two examples which makes the difference a bit clearer:

Box, i.e. trait bound:

use std::fmt::Debug;

struct Wrapper {
    contents: Option>,
}

impl Wrapper {
    fn new() -> Wrapper {
        Wrapper { contents: None }
    }

    fn insert(&mut self, val: Box) {
    }
}

fn main() {
    let mut w = Wrapper::new();

    // makes T for w be an integer type, e.g. Box
    w.insert(Box::new(5));

    // type error, &str is not an integer type
    // w.insert(Box::new("hello"));
}

Box, i.e. trait object:

use std::fmt::Debug;

struct Wrapper {
    contents: Option>,
}

impl Wrapper {
    fn new() -> Wrapper {
        Wrapper { contents: None }
    }

    fn insert(&mut self, val: Box) {
    }
}

fn main() {
    let mut w = Wrapper::new();
    w.insert(Box::new(5));
    w.insert(Box::new("hello"));
}

For further details on the difference between trait bounds and trait objects I recommend the section on trait objects in the first edition of the Rust book.