How do you create a Box when T is a trait-object?

Question

I have the following code

extern crate rand;
use rand::Rng;

pub struct Randomizer {
    rand: Box,
}

impl Randomizer {
    fn new() -> Self {         


        

Answer 1

More about the Sized trait and bound - it's a rather special trait, which is implicitly added to every function, which is why you don't see it listed in the prototype for Box::new:

fn new(x: T) -> Box

Notice that it takes x by value (or move), so you need to know how big it is to even call the function.

In contrast, the Box type itself does not require Sized; it uses the (again special) trait bound ?Sized, which means "opt out of the default Sized bound":

pub struct Box where T: ?Sized(_);

If you look through, there is one way to create a Box with an unsized type:

impl Box where T: ?Sized
....
    unsafe fn from_raw(raw: *mut T) -> Box

so from unsafe code, you can create one from a raw pointer. From then on, all the normal things work.