units

为 Rust 单位。易于使用，类型安全和可自定义。

使用示例

#[macro_use] extern crate units;

// It is recommended to put units inside a separate module
pub mod my_units {
    // Here we define our unit system with three units
    units! {
        MyUnits {
            Meter[m],
            Second[s],
            Mile[mile]
        }
    }
}

// Import the unit constants and also the dimensionless unit `one`
use my_units::f64::{one, m, s, mile};

fn main() {
    let km = 1000.*m; // Define the `km` unit as 1000 meter
    let h = 60.*60.*s; // Define an hour as 60 * 60 seconds
    // These units could also have been defined as separate base units,
    // in order to further reduce the risk to mix them up.

    // Let's calculate how far a car has moved if it has accelerated
    // uniformly from 0 to 100 km/h in 12 seconds
    let initial_speed = 0.*km/h;
    let final_speed = 100.*km/h;
    let time = 12.*s;
    let acceleration = (final_speed - initial_speed)/time;
    let result = 0.5 * acceleration * time * time; // s = a/2 * t^2
    // Here we use debug formatting, which will automatically print the base dimensions
    println!("{:?}", result); // This will print `166.66666666666669 m`

    // Let's convert the result to miles
    let meter_per_mile = 1609.344*m/mile; // This has unit `m/mile`
    let result_in_miles = result / meter_per_mile; // This has unit `mile`
    // Here we get a dimensionless value by eliminating the unit, then use deref (*) to extract the raw f64.
    println!("{} miles", *(result_in_miles/mile)); // This will print `0.103561865372889 miles`

    // Now we want to know how long a ball will fall until it reaches the
    // floor if dropped from a height of 10 meters
    let height = 1.5*m;
    let g = 9.81*m/s/s; // Use a gravitational constant of 9.81 m/s^2
    let time = (2. * height / g).sqrt(); // sqrt() takes care of the units
    // Print the result in milliseconds and round to 2 digits
    let ms = 0.001*s; // 1 ms = 0.001 s
    println!("The ball falls for {:.2} ms until it hits the ground.", *(time/ms));
    // The above will print `The ball falls for 553.00 ms until it hits the ground.`
}

使用 cargo run --example basics 来运行此示例。

^{还有一个用于为值分配单位的安全语法，但此功能目前仅在 Rust nightly 中可用。请参阅 examples/basics.rs 以查看两个版本。}

它是如何工作的？

上述宏调用将生成一个结构体 MyUnits<U,T=f64>，其中第一个类型参数是一个特殊标记，用于获取正确的维度（它包含每个基本单位的指数，编码为 类型级别的整数）。 Meter、Second 和 Mile 将是具有正确指数的标记类型的类型别名。

第二个类型参数表示包装的数值类型（默认为 f64）。然后自动生成的子模块 my_units::f64 和 my_units::f32 都包含常量 m、s 和 mile，每个都具有正确的维度并包装值 1.0。例如，f64::m 的类型为 MyUnits<Meter, f64>。

此外，还提供了类型 One 和常量 one 用于无单位的值，并且只有这些值可以使用 Deref 特性进行解包（这对于方法调用来说自动生效，但有时需要使用 * 前缀运算符来显式表示，参见上面的示例）。