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Ruby

Posted by Ronald Jackson
Ronald Jackson
9,229 Points

Difference between a class variable and attr_accessor in Ruby?

I'm progressing along the Ruby/Rails Track. What is the difference, if any, between a "class variable" and an "attr_accessor" in Ruby?

When defining and working with classes & objects in Ruby, can class variables, which are denoted by @@, serve the same purposes as attr_accessors? If so, why use one over the other?

Thanks for any insights!

Ronald Jackson
Ronald Jackson
9,229 Points

Hi Kang Kyu,

Thank you for the clarification. I'm new to Ruby and programming. You mentioned getter & setter methods. I wasn't familiar with those so I did more research. I found a good explanation on rubymonk.com:

http://rubymonk.com/learning/books/4-ruby-primer-ascent/chapters/45-more-classes/lessons/110-instance-variables

Thanks again for helping me to expand my understanding of Ruby.

Best regards,

Ronald

1 Answer

Kang-Kyu Lee
Kang-Kyu Lee
52,045 Points
Kang-Kyu Lee
Kang-Kyu Lee
52,045 Points

Hi Ronald. I heard attr_accessor would be equivalent with getter-setter "instance" methods.

class Student

  # getter method
  def name
    @name
  end

  # setter method
  def name=(str)
    @name = str
  end

end

equivalent with

class Student
  attr_accessor :name
end

which allows us to do something like this:

a_student = Student.new
a_student.name = "Somebody"
a_student.name  # returns "Somebody"

Basically attr_accessor turn instance variable into instance method ( like @ no longer required ) in the class definition. When an object (instance) calls a method it should look at the class of this instance and find instance method defined in.

But class method or class variables are quite different in a sense. 

class Student
  @@total_number_of_students

  def self.count_them    
  end
end

and then it allows you call the variable and method directly from the class.

  Student.total_number_of_student
  Student.count_them

Thanks. I learn a lot trying to answer and discuss on forum