Editing Rust (section)

==Using Box's==
Using boxes is simple and allows for generics to be avoided.

But it requires dynamic dispatch adding overhead.

When Boxing a member variable in a struct, the struct's constructor must be updated to take a box and any setter methods. Also the at the constructor call site must now Box it's value. Users/consumers/owners of the struct however are unaffected.

 extern crate rand;
 use rand::{Rng, StdRng};

 pub struct Context {
    rng: Box<Rng>, // <- This is changed
 }

 impl Context {
    pub fn new(rng: Box<Rng>) -> Context {  // <- Now this line must be changed
        Context {rng: rng}
    }
    pub fn set_rng(&self, rng: Box<Rng>) {  // <- ...this line must be changed
        self.rng = rng;
    }
 }

 impl SomeTrait for Context {
     //...Blah
 }

 fn use_context(ctx: &Context) {
     //...
 }

 fn main() {
    let seed: &[_] = &[1, 2, 3, 4];
    let mut rng = StdRng::from_seed(seed);
    let mut ctx = Context::new(Box::new(rng)); // <- ...this line must be changed
    use_context(&ctx)
 }