Ruby class inheritance

ruby does not support multi-inheritance, however, this imperfection can be very well bypassed by using mixin.

then it comes to the discussion of include and extend

include

works for modules, to fulfill mixin, like injecting or copying/pasting code-chuck-reference to current place, in a way maximumly reducing the codes repeating.

however, if it is used inside class, it has to be initialized first by using .new

extend

works like include, however, it does not need initialization.

example

module Base
    def hello
        puts "how are you"
    end
end

class Foo
    include Base    # Foo.new.hello
end

class Bar
    extend Base     # Bar.hello
end

More

A very good document class-inheritance

[backup]

module can be overwritten

multiple times methods defining for module will add different methods into it, if those adding methods have different methods' names.

module Foo
  def good
    puts "good"
  end
end

module Foo
  def nice
    puts "nice"
  end
end


# Foo is rewritten multiple times
# However, Bar will have both good and nice methods
class Bar
  include Foo
end

test = Bar.new

test.good       #==> good
test.nice       #==> nice

However, if the methods have the same name, the old methods in method-resolution-order will be overridden.