Calling super

Part of the Object Oriented Programming section of Coddy's Ruby journey. Lesson 24 of 57.

Inside an overriding method, super calls the superclass's version of the same method. The subclass can build on the result instead of repeating it:

class Greeter
  def greet(name)
    "Hello, #{name}"
  end
end

class LoudGreeter < Greeter
  def greet(name)
    super.upcase + "!"
  end
end

puts LoudGreeter.new.greet("Ada")

Output:

HELLO, ADA!

The most common use is initialize: the subclass takes an extra value, passes the shared ones to the superclass with super(...) and stores its own:

class Person
  attr_reader :name, :age

  def initialize(name, age)
    @name = name
    @age = age
  end
end

class Student < Person
  attr_reader :school

  def initialize(name, age, school)
    super(name, age)
    @school = school
  end
end

s = Student.new("Ada", 20, "MIT")
puts "#{s.name}, #{s.age}, #{s.school}"

Output:

Ada, 20, MIT

The three forms differ in what they pass. Bare super passes the same arguments the current method received; super() passes none; super(a, b) passes exactly those:

class Base
  def show(*args)
    "Base got #{args.inspect}"
  end
end

class Child < Base
  def show(a, b)
    [super, super(), super(b)].join(" | ")
  end
end

puts Child.new.show(1, 2)

Output:

Base got [1, 2] | Base got [] | Base got [2]

super can run anywhere in the method, or only under a condition. When the subclass's initialize takes other arguments than its parent's, bare super passes the wrong ones, which is a common ArgumentError:

class Logger
  def log(message)
    puts "LOG #{message}"
  end
end

class QuietLogger < Logger
  def log(message)
    super unless message.start_with?("debug")
  end
end

logger = QuietLogger.new
logger.log("started")
logger.log("debug x=1")
logger.log("done")

Output:

LOG started
LOG done

Challenge

Easy

Complete the classes. Person stores name and age, and its to_s returns Ada (36). Student adds a school: its initialize passes the name and age to super and its to_s returns Ada (20), studies at MIT, built from super. Teacher adds a subject in the same way and returns Bo (45), teaches maths.

The supplied code reads lines person Ada 36, student Ada 20 MIT or teacher Bo 45 maths and prints each object.

Your code goes in person.rb, student.rb and teacher.rb. main.rb holds the supplied input/output code and cannot be edited.

Try it yourself

require_relative 'person'
require_relative 'student'
require_relative 'teacher'

# Supplied input/output code: keep it as it is
input = []
while (line = gets)
  input << line.chomp
end
input.each do |line|
  kind, name, age, extra = line.split
  person = case kind
           when "student" then Student.new(name, age.to_i, extra)
           when "teacher" then Teacher.new(name, age.to_i, extra)
           else Person.new(name, age.to_i)
           end
  puts person
end