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

It looks like all of the previous answers assume that you know what the name of the class that you want to tweak is when you are writing your code. Well, that isn't always true (at least, not for me). I might be iterating over a pile of classes that I want to bestow some variable on (say, to hold some metadata or something). In that case something like this will do the job,

# example classes that we want to tweak
class Foo;end
class Bar;end
klasses = [Foo, Bar]

# iterating over a collection of klasses
klasses.each do |klass|
  # #class_eval gets it done
  klass.class_eval do
    attr_accessor :baz
  end
end

# it works
f = Foo.new
f.baz # => nil
f.baz = 'it works' # => "it works"
b = Bar.new
b.baz # => nil
b.baz = 'it still works' # => "it still works"