Inheritance

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| Ruby user's guide | Inheritance |

Our classification of objects in everyday life is naturally hierarchical. We know that all cats are mammals, and all mammals are animals. Smaller classes inherit characteristics from the larger classes to which they belong. If all mammals breathe, then all cats breathe.

We can express this concept in ruby:

    ruby> class Mammal
        |   def breathe
        |     print "inhale and exhale\n"
        |   end
        | end
       nil
    ruby> class Cat<Mammal
        |   def speak
        |     print "Meow\n"
        |   end
        | end
       nil

Though we didn't specify how a `Cat` should breathe, every cat will inherit that behavior from the `Mammal` class since `Cat` was defined as a subclass of `Mammal`. (In OO terminology, the smaller class is a subclass and the larger class is a superclass.) Hence from a programmer's standpoint, cats get the ability to breathe for free; after we add a `speak` method, our cats can both breathe and speak.

    ruby> tama = Cat.new
       #<Cat:0xbd80e8>
    ruby> tama.breathe
    inhale and exhale
       nil
    ruby> tama.speak
    Meow
       nil

There will be situations where certain properties of the superclass should not be inherited by a particular subclass. Though birds generally know how to fly, penguins are a flightless subclass of birds.

    ruby> class Bird
        |   def preen
        |     print "I am cleaning my feathers."
        |   end
        |   def fly
        |     print "I am flying."
        |   end
        | end
       nil
    ruby> class Penguin<Bird
        |   def fly
        |     fail "Sorry. I'd rather swim."
        |   end
        | end
       nil

Rather than exhaustively define every characteristic of every new class, we need only to append or to redefine the differences between each subclass and its superclass. This use of inheritance is sometimes called differential programming. It is one of the benefits of object-oriented programming.

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