Easy to understand the difference between Ruby instance method and class method.


Until now, I have not clearly understood the important concepts of Ruby, such as instances and classes, so I will explain them with examples. I think it's something you absolutely must know to use Ruby, but it wasn't easy to understand.

Subject of this article

・ For those who are new to Ruby ・ People whose classes and instances are ambiguous

What is an instance class?

As an example, let's assume that users are posting with ids 1 to 3 as shown below under the Tweet class in a service like twitter. Tweet.png

At this time, each post enclosed in a black square is a ** instance **. Instances are said to be objects created from the types of classes. And Tweet is ** class **. An image of an instance belonging to a class. It's an important concept in Ruby! !!

Understand in code

class Tweet
  #Class method
  def self.tweet
    puts "tweet"

  #Instance method
  def post_tweet(user,about)
    puts "#{user}Is#{about}Posted about"


new_tweet = Tweet.new()

After execution

choco posted about programming

Class method => Method performed on the class.

-Call with "class name.method" outside class. (Tweet.tweet in the above example) ・ If you cut yourself, you will get angry. (** declare itself as a class method **) => It can be defined by ** self. method name ~~ end ** in the class. -All, new, etc. are predefined class methods, and you can operate on the class. (All gets all instances of the class, new creates a new instance in the class)

Instance method => Method performed on the instance.

-Call with "instance name.method" outside the class. (In the above example, new_tweet.post_tweet) Since the instance belongs to the class, create an instance with ** class name.new ** and execute the method for that instance. -Basically, I feel that instance methods are often used during implementation.


As I wrote this article, I thought that Ruby was a very easy-to-understand and intuitive design. From now on, I will try to define the method with a strong awareness of the instance class.

Thank you to everyone who read it. If you have any suggestions, please do not hesitate to contact us.

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