Dynamic Instance Variables in Ruby

Ruby continues to surprise me.

While reading Metaprogramming Ruby this evening, I learned that there is no connection between an object’s class and its instance variables in Ruby, unlike C#. The interpreter creates instance variables on the fly, when you assign them a value.

For example, a Task object won’t have the state instance variable until I call the run method on it.

class Task
  def run
    @state = :running
  end
  
  def stop
    @finished = true
  end
end

> compile = Task.new
> compile.instance_variables
=> []

> compile.run
=> :running

> compile.instance_variables
=> [:@state]

It’s possible to have objects of the same class with different instance variables.

For example, if I create another Task object and call the stop method, it won’t have a @state instance variable.

> interpret = Task.new
> interpret.instance_variables
=> []

> interpret.stop
=> true

> interpret.instance_variables
=> [:@finished]

Very interesting.