is_a? and ancestors

Part of the Object Oriented Programming section of Coddy's Ruby journey. Lesson 25 of 57.

is_a?(Class) asks whether an object is an instance of that class or of any subclass of it. kind_of? is another name for the same method:

class Animal
end

class Dog < Animal
end

rex = Dog.new
puts rex.is_a?(Dog)
puts rex.is_a?(Animal)
puts rex.is_a?(Object)
puts rex.kind_of?(String)

Output:

true
true
true
false

instance_of? is stricter: it is true only for the object's own class. obj.class returns that class:

class Animal
end

class Dog < Animal
end

rex = Dog.new
puts rex.instance_of?(Dog)
puts rex.instance_of?(Animal)
puts rex.class

Output:

true
false
Dog

ancestors lists the classes Ruby searches for methods, starting with the class itself; the full list also contains modules such as Kernel, which chapter 6 covers, so take(3) shows the first entries. Classes can be compared: Dog < Animal is true when Dog inherits from Animal:

class Animal
end

class Dog < Animal
end

p Dog.ancestors.take(3)
puts Dog < Animal
puts Animal < Dog

Output:

[Dog, Animal, Object]
true
false

Use is_a? when code must treat a group of classes the same way, for example to count all mammals, whichever subclass they are. Asking for one exact class with instance_of? breaks as soon as someone adds a subclass:

mammals = animals.count { |a| a.is_a?(Mammal) }       # dogs, bats, whales...
exact = animals.count { |a| a.instance_of?(Mammal) }  # misses every subclass

Challenge

Easy

Declare the animal classes: Animal, Mammal < Animal, Bird < Animal, Dog < Mammal, Bat < Mammal and Penguin < Bird. Animal stores a name. Then complete the helper describe(animal) in functions.rb: it returns Rex is a dog: mammal, Pingu is a penguin: bird, with the class name in lower case and the group decided with is_a?.

The supplied code reads lines dog Rex, creates the objects, prints describe for each and then mammals: 2, birds: 1.

Your code goes in animal.rb, mammal.rb, bird.rb, dog.rb, bat.rb, penguin.rb and functions.rb. main.rb holds the supplied input/output code and cannot be edited.

Try it yourself

require_relative 'animal'
require_relative 'mammal'
require_relative 'bird'
require_relative 'dog'
require_relative 'bat'
require_relative 'penguin'
require_relative 'functions'

# Supplied input/output code: keep it as it is
input = []
while (line = gets)
  input << line.chomp
end
kinds = { "dog" => "Dog", "bat" => "Bat", "penguin" => "Penguin" }
animals = input.map do |line|
  kind, name = line.split
  Object.const_get(kinds[kind]).new(name)
end
animals.each { |a| puts describe(a) }
mammals = animals.count { |a| a.is_a?(Mammal) }
birds = animals.count { |a| a.is_a?(Bird) }
puts "mammals: #{mammals}, birds: #{birds}"