Rust by Example

Returning Traits with dyn

The Rust compiler needs to know how much space every function's return type requires. This means all your functions have to return a concrete type. Unlike other languages, if you have a trait like Animal, you can't write a function that returns Animal, because its different implementations will need different amounts of memory.

However, there's an easy workaround. Instead of returning a trait object directly, our functions return a Box which contains some Animal. A box is just a reference to some memory in the heap. Because a reference has a statically-known size, and the compiler can guarantee it points to a heap-allocated Animal, we can return a trait from our function!

Rust tries to be as explicit as possible whenever it allocates memory on the heap. So if your function returns a pointer-to-trait-on-heap in this way, you need to write the return type with the dyn keyword, e.g. Box.

struct Sheep {}

struct Cow {}

trait Animal {

// Instance method signature

fn noise(&self) -> &'static str;

}

// Implement the `Animal` trait for `Sheep`.

impl Animal for Sheep {

fn noise(&self) -> &'static str {

"baaaaah!"

}

}

// Implement the `Animal` trait for `Cow`.

impl Animal for Cow {

fn noise(&self) -> &'static str {

"moooooo!"

}

}

// Returns some struct that implements Animal, but we don't know which one at compile time.

fn random_animal(random_number: f64) -> Box {

if random_number < 0.5 {

Box::new(Sheep {})

} else {

Box::new(Cow {})

}

}

fn main() {

let random_number = 0.234;

let animal = random_animal(random_number);

println!("You've randomly chosen an animal, and it says {}", animal.noise());

}

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