Ruby Attributes

By design, in Ruby, only methods can access instance variable.

For more talking about variables, please read ruby-variables

it will cause error:

class Audio
    def initialize(song)
        @song = song
    end
end

my_music = Audio.new("Hello")
my_music.song       #==> NoMethodError, undefined method `song'
  

to access @song, another method has to be defined.

# codes 1
class Audio
    def initialize(song)
        @song = song
    end
    def song
        @song
    end
end

# usage
my_music = Audio.new("Hello")
puts my_music.song
  

Why is it?

I would like to say it is ruby's characteristics.

Or, define method in another way:

# codes 2
class Audio
    def song=(song)
        @song = song
    end
end

# usage
my_music = Audio.new
my_music.song = "Hello"
  

if you pay attention to the difference between codes 1 and codes 2, you will find there are two different type of methods to access instance variables

method access, song
method assignment, song=

Simple Solution

it will be very tedious and cumbersome if there are so many instance variables needing to be accessed, for the sake of debugging or inspecting, then it comes to ruby-attribute.

attr_accessor, has both :var, :var=
attr_reader, has only :var
attr_writer, has only :var=

for example

class Audio
    attr_accessor :song
    def initialize(song)
        @song = song
    end
end

# usage
my_music = Audio.new("Hello")
puts my_music.song
  

Things need to know

any undefined instance variable will return nil
initialize method is not a must, but it can help set initial value

Way to peek at attribute

p class === puts class.inspect # instance variable
class.methods.sort # class methods
puts class.to_yaml #<== require 'yaml'