class MyClass
  def defined_method
    'This method is defined.'
  end
  def method_missing(m, *args)
    "Sorry, I don't know about any #{m} method."
  end
end
o = MyClass.new
o.defined_method

o.undefined_method

class Fixnum
  def method_missing(m, *args)
    if args.size > 0
      raise ArgumentError.new("wrong number of arguments (#{args.size} for 0)")
    end
    match = /^plus_([0-9]+)$/.match(m.to_s)
    if match
      self + match.captures[0].to_i
    else
      raise NoMethodError.
	new("undefined method `#{m}' for #{inspect}:#{self.class}")
    end
  end
end
4.plus_5

10.plus_0

-1.plus_2

100.plus_10000

20.send(:plus_25)

100.minus_3

NoMethodError: undefined method `minus_3' for 100:Fixnum
...

NoMethodError: undefined method `minus_3' for 100:Fixnum
	from (irb):22:in `method_missing'
	from (irb):31

100.plus_5(105)

ArgumentError: wrong number of arguments (1 for 0)
...

ArgumentError: wrong number of arguments (1 for 0)
	from (irb):15:in `method_missing'
	from (irb):32

class BackwardsString
  def initialize(s)
   @s = s
  end
  def method_missing(m, *args, &block)
    result = @s.send(m, *args, &block)
    result.respond_to?(:to_str) ? BackwardsString.new(result) : result
  end
  def to_s
    @s.reverse
  end
  def inspect
    to_s
  end
end
s = BackwardsString.new("I'm backwards.")

s.size

s.upcase

s.reverse

s.no_such_method

NoMethodError: undefined method `no_such_method' for "I'm backwards.":String
...

NoMethodError: undefined method `no_such_method' for "I'm backwards.":String
	from (irb):38:in `send'
	from (irb):38:in `method_missing'
	from (irb):52

class Library < Array
  def add_book(author, title)
    self << [author, title]
  end
  def search_by_author(key)
    reject { |b| !match(b, 0, key) }
  end
  def search_by_author_or_title(key)
    reject { |b| !match(b, 0, key) && !match(b, 1, key) }
  end  
  :private
  def match(b, index, key)
    b[index].index(key) != nil
  end
end
l = Library.new
l.add_book("James Joyce", "Ulysses")
l.add_book("James Joyce", "Finnegans Wake")
l.add_book("John le Carre", "The Little Drummer Boy")
l.add_book("John Rawls", "A Theory of Justice")
l.search_by_author("John")

l.search_by_author_or_title("oy")

class Library
  def method_missing(m, *args)
    search_by_author_or_title(m.to_s)
  end
end
l.oy

l.Fin

l.Jo

class StringFactory
  def StringFactory.method_missing(m, *args)
    return String.new(m.to_s, *args)
  end
end
StringFactory.a_string

StringFactory.another_string

StringFactory.superclass

25.respond_to? :plus_20

class Fixnum
  def respond_to?(m)
    super or (m.to_s =~ /^plus_([0-9]+)$/) != nil
  end
end
25.respond_to? :plus_20

25.respond_to? :succ

25.respond_to? :minus_20