[SOLVED] How to add a method to an existing object instance at runtime in Ruby?

How to add a method to an existing object instance at runtime in Ruby?

In Javascript I could do this.

var a = {};
a.f1 = function(){};

How can I do this in Ruby?

Update.

In JS code, a is an object instantiated without class. And function(){} is an anonymous function, and a.f1 = adds the function to the object instance. So the function is bound to the instance only, and nothing is related to the class or similar definitions.

Solution

Ruby and JS's object models are very different, but a direct translation may look like this:

a = Object.new
def a.f1(x) 
  2*x
end

a.f1(5) #=> 10

You can also use the Ruby's eigenclass:

class Object
  def metaclass
    class << self; self; end
  end
end

a = Object.new    
# Also: (class << a; self; end).send(:define_method, :f1) do |x|
a.metaclass.send(:define_method, :f1) { |x| 2*x }

A warning note: you'll see this kind of code in meta-programming/monkeypatching/... but it's not usual when writing "normal" code, where other techniques (module mix-ins, mainly) apply.