attr_accessor vs attr_reader & instance variables

Question

could anyone please tell me the difference (if there are any) between

class Car
  attr_accessor :engine
  def initialize(engine)
    self.engine = engine
  end
end

and

class Car
  attr_reader :engine
  def initialize(engine)
    @engine = engine
  end
end

Or are they practically the same?

Answer 1

attr_accessor defines getter and setter.attr_reader defines only getter.

class Car
  attr_reader :engine
  def initialize(engine)
    @engine = engine
  end
end

Car.instance_methods(false) # => [:engine]

With the above code you defined only def engine; @engine ;end.

class Car
  attr_accessor :engine
  def initialize(engine)
    self.engine = engine
  end
end

Car.instance_methods(false) # => [:engine, :engine=]

With the above code you defined only def engine; @engine ;end and def engine=(engine) ;@engine = engine ;end.