Class Inheritance

If class B inherits from class A

then instances of B have the behaviors

of both class A and class B

Inheritance Example

@@@ ruby
class Publication
  attr_accessor :publisher
end

class Magazine < Publication
  attr_accessor :editor
end

m = Magazine.new
m.publisher = "Time Inc."

m.is_a? Magazine #=> true
m.is_a? Publication #=> true
m.class == Publication #=> false

* < is pronounced "inherits from" * not to be confused with "extends" which is for modules

Single vs. Multiple Inheritance

• Ruby has single inheritance
  • each class has one and only one parent class
  • [except for BasicObject]
• Ruby can simulate multiple inheritance using Modules
  • more later

Inheritance Example

@@@ ruby
class Rectangle
  def initialize(width, height)
    @width, @height = width, height
  end
  def area
    @width * @height
  end
end
class Square < Rectangle
  def initialize(width)
    super(width, width)
  end
end

Square.new(10).area #=> 100

A Design Note

• inheritance is often more trouble than it's worth
• there are many ways to solve object-oriented design problems
• also try delegation, configuration, etc.