Derive

The compiler is capable of providing basic implementations for some traits via the #[derive] attribute. These traits can still be manually implemented if a more complex behavior is required.

The following is a list of derivable traits:

  • Comparison traits: Eq, PartialEq, Ord, PartialOrd
  • Clone, to create T from &T via a copy.
  • Copy, to give a type 'copy semantics' instead of 'move semantics'
  • Hash, to compute a hash from &T.
  • Default, to create an empty instance of a data type.
  • Zero, to create a zero instance of a numeric data type.
  • Debug, to format a value using the {:?} formatter.
// `Centimeters`, a tuple struct that can be compared
#[derive(PartialEq, PartialOrd)]
struct Centimeters(f64);

// `Inches`, a tuple struct that can be printed
#[derive(Debug)]
struct Inches(i32);

impl Inches {
    fn to_centimeters(&self) -> Centimeters {
        let &Inches(inches) = self;

        Centimeters(inches as f64 * 2.54)
    }
}

// `Seconds`, a tuple struct no additional attributes
struct Seconds(i32);

fn main() {
    let _one_second = Seconds(1);

    // Error: `Seconds` can't be printed; it doesn't implement the `Debug` trait
    //println!("One second looks like: {:?}", _one_second);
    // TODO ^ Try uncommenting this line

    // Error: `Seconds` can't be compared; it doesn't implement the `PartialEq` trait
    //let _this_is_true = (_one_second == _one_second);
    // TODO ^ Try uncommenting this line

    let foot = Inches(12);

    println!("One foot equals {:?}", foot);

    let meter = Centimeters(100.0);

    let cmp =
        if foot.to_centimeters() < meter {
            "smaller"
        } else {
            "bigger"
        };

    println!("One foot is {} than one meter.", cmp);
}

See also:

derive