20.3 Default implementation

We may define default implementations for trait methods. These implementations may be overwritten in types that implement the trait.

Here is a Surface trait that provides a header for a area() method and a default implementation for a describe() method that returns a description of the object:

:clear

trait Surface {
  fn area(&self) -> f64;

  fn describe(&self) -> String {
    format!("Area is {:.2}.", self.area())
  }
}

We define a Rectangle structure that implements the Surface trait:

struct Rectangle {
  width: f64,
  height: f64,
}

impl Surface for Rectangle {
  fn area(&self) -> f64 {
    self.width * self.height
  }
}

The Rectangle structure uses the default implementation of the Surface trait:

let rect = Rectangle {width: 7.2, height: 9.5};
rect.describe()

Here is a Circle structure that implements the Surface trait too, but also overwrite its describe() method:

struct Circle {
  radius: f64,
}

impl Surface for Circle {
  fn area(&self) -> f64 {
    std::f64::consts::PI * self.radius * self.radius
  }
  fn describe(&self) -> String {
    format!("Circle area is {:.5}.", self.area())
  }
}

We see that Circle’s version of the describe() methods is indeed called in place of the default implementation:

let circ = Circle {radius: 6.7};
circ.describe()