Adding an instance variable to a class in Ruby

Question

How can I add an instance variable to a defined class at runtime, and later get and set its value from outside of the class?

I\'m looking for a metaprogramming so

Answer 1

Mike Stone's answer is already quite comprehensive, but I'd like to add a little detail.

You can modify your class at any moment, even after some instance have been created, and get the results you desire. You can try it out in your console:

s1 = 'string 1'
s2 = 'string 2'

class String
  attr_accessor :my_var
end

s1.my_var = 'comment #1'
s2.my_var = 'comment 2'

puts s1.my_var, s2.my_var