I ran into an instance of meta programming in Ruby today, in the Exceptional Ruby book I’m reading for work. It seems that the theme this week is “you don’t know Ruby as well as you could”.

I might be wrong in my understanding here, but this is what I understand thus far:

Ruby stores methods for an object in it’s class, not the object itself. Objects only really store their attributes/variables in memory. However there exists some unseen entity known as the metaclass which belongs to each object, and it can possibly store methods which belong to that object, but not necessarily to that objects class.

class Person
  def speak
    puts "Hello There!"
  end
end

john = Person.new
bob = Person.new

class << john
  def bark
    puts "Ruff! Ruff!"
  end
end

>> john.speak
=> Hello There!

>> bob.speak
=> Hello There!

>> john.bark
=> Ruff! Ruff!

>> bob.bark
NoMethodError: undefined method `bark' for #

The reference to ‘class « john’ opens a code block where methods are defined in the metaclass for ‘john’, and not the ‘Person’ class.

A more thorough understanding of this is explored in this blog post - Metaprogramming in Ruby: It’s All About the Self