15.5 Setter methods (instance methods)

Once an object has been built, we may modify its data through setter methods. Those methods take a mutable reference to the self instance, and thus can modify the values of the fields. We usually name the setter methods using the set_ prefix.

As an example, let us write a struct representing the weight of a car, with its empty weight and the current volume of petrol:

:clear

struct Car {
  weight: f32, /* Empty weight in kg */
  petrol_vol: f32, /* Volume of petrol inside the tank in L */
}

The constructor accepts only the empty weight, the volume of petrol being set to a default value of 0L:

impl Car {
  fn new(weight: f32) -> Self {
    Self {
      weight,
      petrol_vol: 0.0,
    }
  }
}

We implement a builder setter for the volume of petrol:

impl Car {
  fn with_petrol(mut self, vol: f32) -> Self {
    self.petrol_vol = vol;
    self
  }
}

And also a setter method:

impl Car {
  fn set_petrol(&mut self, vol: f32) {
    self.petrol_vol = vol;
  }
}

We create an instance of Car:

let mut car = Car::new(800.0).with_petrol(60.0);
(car.weight, car.petrol_vol)

And we modify the petrol volume in this instance using the setter method:

car.set_petrol(40.0);
(car.weight, car.petrol_vol)

see solution

see solution