サンプルコードで身につける: dyn Trait — 異なる型を同じ入れ物に

trait Speak {
    fn speak(&self) -> String;
}
struct Dog;
struct Cat;
impl Speak for Dog {
    fn speak(&self) -> String { String::from("わん") }
}
impl Speak for Cat {
    fn speak(&self) -> String { String::from("にゃー") }
}

fn main() {
    // 異なる型を「Speak を実装した何か」として同じ Vec に入れる
    let animals: Vec<Box<dyn Speak>> = vec![Box::new(Dog), Box::new(Cat)];
    for animal in &animals {
        println!("{}", animal.speak());
    }
}

trait Message {
    fn text(&self) -> String;
}
struct Morning;
struct Night;
impl Message for Morning {
    fn text(&self) -> String { String::from("おはようございます") }
}
impl Message for Night {
    fn text(&self) -> String { String::from("おやすみなさい") }
}

fn main() {
    let hour = 21;
    // どの型が入るかは実行時の値しだい。変数の型は Box<dyn Message> のまま
    let msg: Box<dyn Message> = if hour < 12 { Box::new(Morning) } else { Box::new(Night) };
    println!("{}時の挨拶: {}", hour, msg.text());
}

trait Compressor {
    fn compress(&self, data: &str) -> String;
}
struct Zip;
struct Raw;
impl Compressor for Zip {
    fn compress(&self, data: &str) -> String { format!("zip圧縮({}バイト)", data.len()) }
}
impl Compressor for Raw {
    fn compress(&self, data: &str) -> String { String::from(data) }
}
// 分岐によって異なる型を返せるのは Box<dyn Trait> だから
fn create(kind: &str) -> Box<dyn Compressor> {
    if kind == "zip" { Box::new(Zip) } else { Box::new(Raw) }
}

fn main() {
    println!("{}", create("zip").compress("hello world"));
    println!("{}", create("raw").compress("hello world"));
}

trait Formatter {
    fn format(&self, msg: &str) -> String;
}
struct Plain;
struct Bracket;
impl Formatter for Plain {
    fn format(&self, msg: &str) -> String { String::from(msg) }
}
impl Formatter for Bracket {
    fn format(&self, msg: &str) -> String { format!("[{}]", msg) }
}
struct Logger {
    formatter: Box<dyn Formatter>,
}
fn main() {
    let mut logger = Logger { formatter: Box::new(Plain) };
    println!("{}", logger.formatter.format("起動"));
    logger.formatter = Box::new(Bracket); // 整形部品だけ実行時に差し替える
    println!("{}", logger.formatter.format("処理中"));
}

trait Shape {
    fn area(&self) -> f64;
}
struct Circle { radius: f64 }
struct Rect { width: f64, height: f64 }
impl Shape for Circle {
    fn area(&self) -> f64 { 3.14 * self.radius * self.radius }
}
impl Shape for Rect {
    fn area(&self) -> f64 { self.width * self.height }
}
fn main() {
    let shapes: Vec<Box<dyn Shape>> = vec![
        Box::new(Circle { radius: 1.0 }),
        Box::new(Rect { width: 2.0, height: 3.0 }),
        Box::new(Circle { radius: 0.5 }),
    ];
    let total: f64 = shapes.iter().map(|s| s.area()).sum();
    println!("図形は{}個, 面積の合計 = {}", shapes.len(), total);
}