attr_accessor与attr_reader &实例变量

Question

有谁能告诉我(如果有的话)

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

和

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

或者它们实际上是一样的？

Answer 1

attr_accessor定义getter和setter，attr_reader只定义getter。

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

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

使用上面的代码，您只定义了def engine; @engine ;end。

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

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

使用上面的代码，您只定义了def engine; @engine ;end和def engine=(engine) ;@engine = engine ;end。

Answer 2

attr_accessor :engine允许您读写变量@engine。

attr_reader :engine只允许读取@engine的值

self.engine = engine和@engine = engine几乎做了同样的事情。

Answer 3

他们是不一样的。

用代码来解释它

car = Car.new('cool engine')

对读者来说，你只能阅读。

car.engine 
# cool engine

但你做不到

car.engine = 'even cool engine'

对于attr_accessor，您可以同时进行读写

car.engine
# cool engine
car.engine = 'even cool engine'
car.engine
# even cool engine

例如，变量

如果不想将实例变量公开给外部世界，只使用实例变量

class Car
  # You don't need to use attr_reader and attr_accessor here if you don't want to expose them to outside world
  def initialize(engine)
    @engine = engine
  end
end