Question

I see how to dynamically add a method to an instance in Ruby with def [instance].[methodname]; [...]; end.

However, I’m interested in attaching a method that exists in another location to a given instance. e.g.

def my_meth
  puts self.foo
end

class MyCls
  attr_accessor :foo
end

my_obj = MyCls.new
my_obj.my_meth

How could I simply attach my_meth to my_obj so that the method call in the final line of the foregoing code would work?

Answer 1

You could use include or extend to add a module to your class, eg. extend:

module Foo
  def my_meth
    puts self.foo
  end
end

class MyCls
  attr_accessor :foo
end

my_obj = MyCls.new
my_obj.extend(Foo)
my_obj.foo = "hello"
my_obj.my_meth

Unless you have a need to mix-in a module on the fly like this it’s generally better to include your module like so:

class MyCls
   include Foo
   attr_accessor :foo
end