polymorphism - Vector of objects belonging to a trait -

consider following code:

trait animal {     fn make_sound(&self) -> string; }  struct cat; impl animal cat {     fn make_sound(&self) -> string {         "meow".to_string()     } }  struct dog; impl animal dog {     fn make_sound(&self) -> string {         "woof".to_string()     } }  fn main () {     let dog: dog = dog;     let cat: cat = cat;     let v: vec<animal> = vec::new();     v.push(cat);     v.push(dog);     animal in v.iter() {         println!("{}", animal.make_sound());     } } 

the compiler tells me v vector of animal when try push cat (type mismatch)

so, how can make vector of objects belonging trait , calls corresponding trait method on each element?

vec<animal> not legal, compiler can't tell because type mismatch somehow hides it. if remove calls push, compiler gives following error:

<anon>:22:9: 22:40 error: instantiating type parameter incompatible type `animal`, not fulfill `sized` [e0144] <anon>:22     let mut v: vec<animal> = vec::new();                   ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 

the reason why that's not legal vec<t> stores many t objects consecutively in memory. however, animal trait, , traits have no size (a cat , dog not guaranteed have same size).

to solve problem, need store has size in vec. straightforward solution wrap values in box, i.e. vec<box<animal>>. box<t> has fixed size (a "fat pointer" if t trait, simple pointer otherwise).

here's working main:

fn main () {     let dog: dog = dog;     let cat: cat = cat;     let mut v: vec<box<animal>> = vec::new();     v.push(box::new(cat));     v.push(box::new(dog));     animal in v.iter() {         println!("{}", animal.make_sound());     } }