Course of Raku / Objects, I/O, and exceptions / Classes and objects / Inheritance / Exercises / An inherited attribute

Solution: An inherited attribute

Here is a possible solution to the task.

Code

class Vehicle {
    has $.wheels;
}

class Bike is Vehicle {
    has $.wheels = 2;
}

class Car is Vehicle {
    has $.wheels = 4;
}

say Bike.new.wheels;
say Car.new.wheels;

🦋 You can find the source code in the file inherited-attribute.raku.

Output

2
4

Comments

1. Bike and Car both inherit the wheels attribute (and its accessor) from Vehicle. Neither redeclares the attribute from scratch — they only give the inherited one a default value, 2 and 4 respectively.

2. Because the default is baked into each class, you can create the objects with a plain Bike.new and Car.new, and the inherited accessor reads the right number of wheels back.

3. You can still pass the value explicitly, for example Car.new(wheels => 3), and an explicit argument overrides the default. But since a bike or a car most likely already comes with its usual number of wheels, the defaults keep the common case simple while leaving room for the occasional exception.

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