eval.rb

# Instance Methods

class Demo
  def say_hi
    puts "hi"
  end
end

Demo.new.say_hi # => 'hi'
# the say_hi is an instance method of the Demo class


# Class Methods
#
class Demo
  def self.say_hi
    puts "hi"
  end
end

Demo.say_hi # => 'hi'
# the say_hi is a class method for the Demo class


# Instance Eval

Demo.instance_eval do
  def say_goodbye
    puts 'goodbye'
  end
end

Demo.say_goodbye # => 'goodbye'
# the say_goodbye is a class method for the Demo class

# Class Eval

Demo.class_eval do
  def say_goodbye
    puts 'goodbye'
  end
end

Demo.new.say_goodbye # => 'goodbye'
# the say_goodbye is an instance method for the Demo class