phantom-data.rs

use std::marker::PhantomData;
use std::ops::Add;

/// Create void enumerations to define unit types.
#[derive(Debug, Clone, Copy)]
enum Inch {}
#[derive(Debug, Clone, Copy)]
enum Mm {}

/// `Length` is a type with phantom type parameter `Unit`,
/// and is not generic over the length type (that is `f64`).
///
/// `f64` already implements the `Clone` and `Copy` traits.
#[derive(Debug, Clone, Copy)]
struct Length<Unit>(f64, PhantomData<Unit>);

/// The `Add` trait defines the behavior of the `+` operator.
impl<Unit> Add for Length<Unit> {
    type Output = Length<Unit>;

    // add() returns a new `Length` struct containing the sum.
    fn add(self, rhs: Length<Unit>) -> Self::Output {
        // `+` calls the `Add` implementation for `f64`.
        Length(self.0 + rhs.0, PhantomData)
    }
}

fn main() {
    // Specifies `one_foot` to have phantom type parameter `Inch`.
    let one_foot: Length<Inch> = Length(12.0, PhantomData);
    // `one_meter` has phantom type parameter `Mm`.
    let one_meter: Length<Mm> = Length(1000.0, PhantomData);

    // `+` calls the `add()` method we implemented for `Length<Unit>`.
    //
    // Since `Length` implements `Copy`, `add()` does not consume
    // `one_foot` and `one_meter` but copies them into `self` and `rhs`.
    let two_feet = one_foot + one_foot;
    let two_meters = one_meter + one_meter;

    // Addition works.
    println!("one foot + one_foot = {:?} in", two_feet.0);
    println!("one meter + one_meter = {:?} mm", two_meters.0);

    // Nonsensical operations fail as they should:
    // Compile-time Error: type mismatch.
    //let one_feter = one_foot + one_meter;
}