Usecase for Rc in Raytracers

use-box.dump

error[E0382]: use of moved value: `metal_shiny`
  --> temp.rs:39:38
   |
37 |   let metal_shiny = Metal::new(0.0);
   |       ----------- move occurs because `metal_shiny` has type `Metal`, which does not implement the `Copy` trait
38 |   let sphere1 = Sphere::new(Box::new(metal_shiny));
   |                                      ----------- value moved here
39 |   let sphere2 = Sphere::new(Box::new(metal_shiny));
   |                                      ^^^^^^^^^^^ value used here after move

use-box.rs

// rustc use-box.rs

use std::rc::Rc;

pub trait Material {
  // Scatters incident rays
  fn scatter (&self); // Definition is elided
}

pub struct Metal {
    // Surface fuzziness
    fuzz: f64,
}

impl Metal {
    pub fn new (fuzz: f64) -> Self {
        Self { fuzz }
    }
}

impl Material for Metal {
    fn scatter(&self) { /* ... */ }
}

pub struct Sphere
{
    // Some other fields have been elided
    material: Box<dyn Material>,
}

impl Sphere {
    pub fn new(material: Box<dyn Material>) -> Self {
        Sphere { material }
    }
}

fn main () {
  let metal_shiny = Box::new(Metal::new(0.0));
  let sphere1 = Sphere::new(metal_shiny);
  let sphere1 = Sphere::new(metal_shiny);
}

use-rc.dump

kwannoel@Kwans-MacBook-Pro temp % rustc temp.rs
warning: unused variable: `sphere1`
  --> temp.rs:38:7
   |
38 |   let sphere1 = Sphere::new(metal_shiny.clone());
   |       ^^^^^^^ help: consider prefixing with an underscore: `_sphere1`
   |
   = note: `#[warn(unused_variables)]` on by default

warning: unused variable: `sphere2`
  --> temp.rs:39:7
   |
39 |   let sphere2 = Sphere::new(metal_shiny.clone());
   |       ^^^^^^^ help: consider prefixing with an underscore: `_sphere2`

warning: field is never read: `fuzz`
  --> temp.rs:11:5
   |
11 |     fuzz: f64,
   |     ^^^^^^^^^
   |
   = note: `#[warn(dead_code)]` on by default

warning: field is never read: `material`
  --> temp.rs:27:5
   |
27 |     material: Rc<dyn Material>,
   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^

use-rc.rs

// rustc use-rc.rs
use std::rc::Rc;

pub trait Material {
  // Scatters incident rays
  fn scatter (&self); // Definition is elided
}

pub struct Metal {
    // Surface fuzziness
    fuzz: f64,
}

impl Metal {
    pub fn new (fuzz: f64) -> Self {
        Self { fuzz }
    }
}

impl Material for Metal {
    fn scatter(&self) { /* ... */ }
}

pub struct Sphere
{
    // Some other fields have been elided
    material: Rc<dyn Material>,
}

impl Sphere {
    pub fn new(material: Rc<dyn Material>) -> Self {
        Sphere { material }
    }
}

fn main () {
  let metal_shiny = Rc::new(Metal::new(0.0));
  let sphere1 = Sphere::new(metal_shiny.clone()); // Clone the pointer, incrementing the reference count
  let sphere2 = Sphere::new(metal_shiny.clone()); // As above
}