新类型惯用法

newtype 模式在编译时保证了程序接收到正确类型的值。

For example, a function that measures distance in miles, must be given a value of type Miles.

struct Miles(f64);

struct Kilometers(f64);

impl Miles {
    pub fn to_kilometers(&self) -> Kilometers {
        Kilometers(self.0 * 1.609344)
    }
}

impl Kilometers {
    pub fn to_miles(&self) -> Miles {
        Miles(self.0 / 1.609344)
    }
}

fn is_a_marathon(distance: &Miles) -> bool {
    distance.0 >= 26.2
}

fn main() {
    let distance = Miles(30.0);
    let distance_km = distance.to_kilometers();
    println!("Is a marathon? {}", is_a_marathon(&distance));
    println!("Is a marathon? {}", is_a_marathon(&distance_km.to_miles()));
    // println!("Is a marathon? {}", is_a_marathon(&distance_km));
}

Uncomment the last print statement to observe that the type supplied must be Miles.

要获取 newtype 的基本类型值，你可以使用元组语法或解构语法，如下所示：

struct Miles(f64);

fn main() {
    let distance = Miles(42.0);
    let distance_as_primitive_1: f64 = distance.0; // 元组语法
    let Miles(distance_as_primitive_2) = distance; // 解构语法
}

另请参阅：

struct