src/trait/dyn.md

# 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<dyn Animal>`.

```rust,editable
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<dyn Animal> {
    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());
}

```