rust-by-practice/solutions/generics-traits/trait-object.md

1.

```rust
trait Bird {
    fn quack(&self) -> String;
}

struct Duck;
impl Duck {
    fn swim(&self) {
        println!("Look, the duck is swimming")
    }
}
struct Swan;
impl Swan {
    fn fly(&self) {
        println!("Look, the duck.. oh sorry, the swan is flying")
    }
}

impl Bird for Duck {
    fn quack(&self) -> String{
        "duck duck".to_string()
    }
}

impl Bird for Swan {
    fn quack(&self) -> String{
        "swan swan".to_string()
    }
}

fn main() {
    let duck = Duck;
    duck.swim();

    let bird = hatch_a_bird(2);
    // this bird has forgotten how to swim, so below line will cause an error
    // bird.swim();
    // but it can quak
    assert_eq!(bird.quack(), "duck duck");

    let bird = hatch_a_bird(1);
    // this bird has forgotten how to fly, so below line will cause an error
    // bird.fly();
    // but it can quak too
    assert_eq!(bird.quack(), "swan swan");

    println!("Success!")
}   

fn hatch_a_bird(species: u8) ->Box<dyn Bird> {
    if species == 1 {
        Box::new(Swan{})
    } else {
        Box::new(Duck{})
    }
}
```

2.

```rust
trait Bird {
    fn quack(&self);
}

struct Duck;
impl Duck {
    fn fly(&self) {
        println!("Look, the duck is flying")
    }
}
struct Swan;
impl Swan {
    fn fly(&self) {
        println!("Look, the duck.. oh sorry, the swan is flying")
    }
}

impl Bird for Duck {
    fn quack(&self) {
        println!("{}", "duck duck");
    }
}

impl Bird for Swan {
    fn quack(&self) {
        println!("{}", "swan swan");
    }
}

fn main() {
    let birds: [Box<dyn Bird>; 2] = [Box::new(Duck {}), Box::new(Swan {})];

    for bird in birds {
        bird.quack();
        // when duck and swan turns into Bird, they all forgot how to fly, only remeber how to quack
        // so, the below code will cause an error
        // bird.fly();
    }
}
```

3.

```rust
trait Draw {
    fn draw(&self) -> String;
}

impl Draw for u8 {
    fn draw(&self) -> String {
        format!("u8: {}", *self)
    }
}

impl Draw for f64 {
    fn draw(&self) -> String {
        format!("f64: {}", *self)
    }
}

fn main() {
    let x = 1.1f64;
    let y = 8u8;

    // draw x
    draw_with_box(Box::new(x));

    // draw y
    draw_with_ref(&y);
}

fn draw_with_box(x: Box<dyn Draw>) {
    x.draw();
}

fn draw_with_ref(x: &dyn Draw) {
    x.draw();
}
```

4.

```rust
trait Foo {
    fn method(&self) -> String;
}

impl Foo for u8 {
    fn method(&self) -> String { format!("u8: {}", *self) }
}

impl Foo for String {
    fn method(&self) -> String { format!("string: {}", *self) }
}

// implement below with generics
fn static_dispatch<T: Foo>(x: T) {
    x.method();
}

// implement below with trait objects
fn dynamic_dispatch(x: &dyn Foo) {
    x.method();
}

fn main() {
    let x = 5u8;
    let y = "Hello".to_string();

    static_dispatch(x);
    dynamic_dispatch(&y);

    println!("Success!")
}
```

5.

```rust
trait MyTrait {
    fn f(&self) -> Self;
}

impl MyTrait for u32 {
    fn f(&self) -> u32 { 42 }
}

impl MyTrait for String {
    fn f(&self) -> String { self.clone() }
}

fn my_function(x: impl MyTrait) -> impl MyTrait  {
    x.f()
}

fn main() {
    my_function(13_u32);
    my_function(String::from("abc"));
}
```

```rust
trait MyTrait {
    fn f(&self) -> Box<dyn MyTrait>;
}

impl MyTrait for u32 {
    fn f(&self) -> Box<dyn MyTrait> { Box::new(42) }
}

impl MyTrait for String {
    fn f(&self) -> Box<dyn MyTrait> { Box::new(self.clone()) }
}

fn my_function(x: Box<dyn MyTrait>) -> Box<dyn MyTrait> {
    x.f()
}

fn main() {
    my_function(Box::new(13_u32));
    my_function(Box::new(String::from("abc")));
}
```