サンプルコードで身につける: メソッド — impl ブロックと &self

struct Square {
    side: u32,
}

impl Square {
    fn area(&self) -> u32 {
        self.side * self.side
    }
}

fn main() {
    let sq = Square { side: 5 };
    println!("面積 = {}", sq.area());
}

struct Rectangle {
    width: u32,
    height: u32,
}

impl Rectangle {
    fn area(&self) -> u32 {
        self.width * self.height
    }
    fn perimeter(&self) -> u32 {
        (self.width + self.height) * 2
    }
}

fn main() {
    let rect = Rectangle { width: 3, height: 4 };
    println!("面積 = {}", rect.area());
    println!("周の長さ = {}", rect.perimeter());
}

struct Counter {
    count: u32,
}

impl Counter {
    fn increment(&mut self) {
        self.count += 1;
    }
}

fn main() {
    let mut counter = Counter { count: 0 };
    counter.increment();
    counter.increment();
    counter.increment();
    println!("count = {}", counter.count);
}

struct Rectangle {
    width: u32,
    height: u32,
}

impl Rectangle {
    fn can_hold(&self, other: &Rectangle) -> bool {
        self.width > other.width && self.height > other.height
    }
}

fn main() {
    let big = Rectangle { width: 8, height: 6 };
    let small = Rectangle { width: 5, height: 4 };
    println!("big は small を覆える? {}", big.can_hold(&small));
    println!("small は big を覆える? {}", small.can_hold(&big));
}

struct Wallet {
    balance: i64,
}
impl Wallet {
    fn withdraw(&mut self, amount: i64) {
        if amount <= self.balance {
            self.balance -= amount;
            println!("{}円を引き出しました", amount);
        } else {
            println!("残高不足: {}円は引き出せません", amount);
        }
    }
}

fn main() {
    let mut wallet = Wallet { balance: 1000 };
    wallet.withdraw(300);
    wallet.withdraw(5000);
    println!("最終残高: {}円", wallet.balance);
}