Default Methods

Traits can implement behavior in terms of other trait methods:

trait Equals {
    fn equals(&self, other: &Self) -> bool;
    fn not_equals(&self, other: &Self) -> bool {
        !self.equals(other)
    }
}

#[derive(Debug)]
struct Centimeter(i16);

impl Equals for Centimeter {
    fn equals(&self, other: &Centimeter) -> bool {
        self.0 == other.0
    }
}

fn main() {
    let a = Centimeter(10);
    let b = Centimeter(20);
    println!("{a:?} equals {b:?}: {}", a.equals(&b));
    println!("{a:?} not_equals {b:?}: {}", a.not_equals(&b));
}

  • Traits may specify pre-implemented (default) methods and methods that users are required to implement themselves. Methods with default implementations can rely on required methods.

  • Move method not_equals to a new trait NotEquals.

  • Make Equals a super trait for NotEquals.

    trait NotEquals: Equals {
    fn not_equals(&self, other: &Self) -> bool {
        !self.equals(other)
    }
}

  • Provide a blanket implementation of NotEquals for Equals.

    trait NotEquals {
    fn not_equals(&self, other: &Self) -> bool;
}

impl<T> NotEquals for T where T: Equals {
    fn not_equals(&self, other: &Self) -> bool {
        !self.equals(other)
    }
}
    • With the blanket implementation, you no longer need Equals as a super trait for NotEqual.