Язык программирования Crystal

Inheritance

Every class except Object, the hierarchy root, inherits from another class (its superclass). If you don't specify one it defaults to Reference for classes and Struct for structs.

A class inherits all instance variables and all instance and class methods of a superclass, including its constructors (new and initialize).

class Person
  def initialize(@name)
  end

  def greet
    puts "Hi, I'm #{@name}"
  end
end

class Employee < Person
end

employee = Employee.new "John"
employee.greet # "Hi, I'm John"

If a class defines a new or initialize then its superclass constructors are not inherited:

class Person
  def initialize(@name)
  end
end

class Employee < Person
  def initialize(@name, @company_name)
  end
end

Employee.new "John", "Acme" # OK
Employee.new "Peter" # Error: wrong number of arguments
                     # for 'Employee:Class#new' (1 for 2)

You can override methods in a derived class:

class Person
  def greet(msg)
    puts "Hi, #{msg}"
  end
end

class Employee < Person
  def greet(msg)
    puts "Hello, #{msg}"
  end
end

p = Person.new
p.greet "everyone" # "Hi, everyone"

e = Employee.new
e.greet "everyone" # "Hello, everyone"

Instead of overriding you can define specialized methods by using type restrictions:

class Person
  def greet(msg)
    puts "Hi, #{msg}"
  end
end

class Employee < Person
  def greet(msg : Int32)
    puts "Hi, this is a number: #{msg}"
  end
end

e = Employee.new
e.greet "everyone" # "Hi, everyone"

e.greet 1 # "Hi, this is a number: 1"

super

You can invoke a superclass' method using super:

class Person
  def greet(msg)
    puts "Hello, "#{msg}"
  end
end

class Employee < Person
  def greet(msg)
    super # Same as: super(msg)
    super("another message")
  end
end

Without arguments nor parenthesis, super receives the same arguments as the method's arguments. Otherwise, it receives the arguments you pass to it.