Add method to an instanced object

Question

obj = SomeObject.new

def obj.new_method
  \"do some things\"
end

puts obj.new_method
> \"do some things\"

This works ok. However, I need to do

Answer 1

class Some
end 

obj = Some.new

class << obj
  def hello 
    puts 'hello'
  end 
end 

obj.hello

obj2 = Some.new
obj2.hello # error

Syntax class << obj means that we are opening definition of the class for an object. As you probably know we can define Ruby class methods using syntax like this:

class Math

    class << self 

        def cos(x)
            ...
        end 

        def sin(x)
            ...
        end 
    end 
end 

Then we can use those methods like this:

Math.cos(1)

In Ruby, everything is an object - even classes. self here is an object of Math class itself (you can access that object with Math.class). So syntax class << self means we are opening class for Math class object. Yes, it means that Math class has class too (Math.class.class).

Answer 2

You can use modules.

module ObjSingletonMethods
  def new_method
    "do some things"
  end
end


obj.extend ObjSingletonMethods

puts obj.new_method # => do some things

Now if you need to add more methods to that object, you just need to implement the methods in the module and you are done.