Ruby: Differences between class methods and instance methods, class variables and instance variables

Difference between class method and instance method

First, let's look at the difference between Ruby class methods and instance methods.

What is a class method?

It can be used by the class itself that defines the class method. It is used for processing that uses common information in the class. You can call it without an instance.

When defining a class method, write self. method name. It can be called from the class name Fruits as follows. Also, the new method that creates an instance is a typical class method.

fruits.ruby

calss Fruits
  def self.call #self refers to the class itself
    puts "Called a class method"
  end
end

Fruits.new #The new method is also one of the class methods.
Fruits.call

Running the above ruby ​​file in a terminal gives the following result:

Terminal

ruby fruits.rb

Called a class method

The execution result of Fruits.call is displayed. Fruits.new will not be displayed in the execution result as it will only generate an empty instance.

What is an instance method?

A method that can be used by the instance and is defined in the class. You can use the instance generated from "Class that defines the instance method".

fruits.ruby

calss Fruits
  def call
     puts "I called an instance method"
  end
end

fruits = Fruits.new
fruits.call
#.You can also connect them with and call them as follows.
Fruits.new.call

Terminal

ruby fruits.rb

I called an instance method

initialize method

The initialize method is an instance method that can define the process that is executed at the same time that the instance is created by the new method. Actually, the initialize method is executed automatically even if you do not write it in the class, and in this case nothing is processed.

fruits.rb

class Fruits
  def initialize
    puts "Instance was created"
  end
end

Fruits.new

Running the above ruby ​​file in a terminal gives the following result:

Terminal

ruby fruits.rb

Instance was created

You can see that the process defined in the initialize method was executed after the instance was created in Fruits.new.

Differences between class variables and instance variables

Next, let's look at the difference between class variables and instance variables.

What is a class variable?

It is a variable defined in the class and is defined by writing @@ variable name. Class variables are suitable for use when you want to define common variables as a class.

fruits.rb

class Fruits

  @@fruit = "Orange" #Defined before instantiation for use within the initialize method

  def initialize
    puts "I#{@@fruit}I like"
  end

end

Fruits.new

Execution result

Terminal

ruby fruits.rb

I like oranges

A feature of class methods is that once defined, they can be used anywhere in the class.

fruits.rb

class Fruits

  @@price = 0 

  def initialize(fruit, price)
    @fruit = fruit
    @@price += price
  end

  def self.sum
    puts "The total amount is#{@@price}It's a yen" 
  end

end

Fruits.new("Orange", 200)
Fruits.new("Apple", 300)
Fruits.new("Grape", 500)

Fruits.sum

Terminal

ruby fruits.rb

The total amount is 1000 yen

The price of each fruit was added to the class variable @@ price each time an instance of orange, apple, or grape was created, and the total amount was output by thesummethod.

What is an instance variable?

A variable that can be defined in an individual instance, and is defined by writing @variable name. Instance variables can be used in instance methods other than the defined instance methods.

fruits.rb

class Fruits

  def initialize(fruit)
    @fruit = fruit
    puts "I#{@fruit}I like"
  end

  def dislike
    puts "I#{@fruit}I'm not good at"
  end
end

Fruits.new("Orange")
Fruits.new("Apple")
fruit = Fruits.new("Grape")
fruit.dislike

Terminal

ruby fruits.rb

I like oranges
I like apples
I like grapes
I'm not good at grapes

In the initialize method, the value of the instance variable @ fruit is assigned to each created instance and output by puts.

You can also see that the instance variable @ fruit defined in the initialize method can also be used in the dislike method.

Reference material

Active engineers explain how to use Ruby class variables [For beginners](TechAcademy Magazine)