Derive Macro Traits

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.
  • Debug, to format a value using the {:?} formatter.

Mix Example

Usage Example of PartialEq, PartialOrd, Debug

 
// `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 with 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);
}

Clone

When dealing with resources, the default behavior is to transfer them during assignments or function calls.

However, sometimes we need to make a copy of the resource as well.

The Clone trait helps us do exactly this. Most commonly, we can use the .clone() method defined by the Clone trait.

 
// A unit struct without resources
#[derive(Debug, Clone, Copy)]
struct Unit;

// A tuple struct with resources that implements the `Clone` trait
#[derive(Clone, Debug)]
struct Pair(Box<i32>, Box<i32>);

fn main() {
    // Instantiate `Unit`
    let unit = Unit;
    // Copy `Unit`, there are no resources to move
    let copied_unit = unit;

    // Both `Unit`s can be used independently
    println!("original: {:?}", unit);
    println!("copy: {:?}", copied_unit);

    // Instantiate `Pair`
    let pair = Pair(Box::new(1), Box::new(2));
    println!("original: {:?}", pair);

    // Move `pair` into `moved_pair`, moves resources
    let moved_pair = pair;
    println!("moved: {:?}", moved_pair);

    // Error! `pair` has lost its resources
    //println!("original: {:?}", pair);
    // TODO ^ Try uncommenting this line

    // Clone `moved_pair` into `cloned_pair` (resources are included)
    let cloned_pair = moved_pair.clone();
    // Drop the original pair using std::mem::drop
    drop(moved_pair);

    // Error! `moved_pair` has been dropped
    //println!("copy: {:?}", moved_pair);
    // TODO ^ Try uncommenting this line

    // The result from .clone() can still be used!
    println!("clone: {:?}", cloned_pair);
}

See also:

derive