extend in Ruby

1. What Is include in Ruby
2. What Is extend in Ruby
3. Use include and extend in Ruby

Referencing from class to class and function to function is a common task in Object Oriented Programming. If you are looking for a way to import modules from different classes, then this article can help you.

In Ruby, you can do this in two different ways. By using either include or extend.

In this article, we will discuss the include and extend in Ruby and look at a relevant example to make the topic easier.

What Is include in Ruby

include allows you to import a code module. If you try to get a method of importing the module through the class directly, then you will receive an error as it is imported as a subclass for the superclass.

The solution here is to get it by the instance of the class. You can use include by using the keyword include.

What Is extend in Ruby

Another way to import a module is using the keyword extend. Through the extend, you can import a method as a class method.

But if you try to get a method of an imported module through the instance, you will get an error. You must get that method by the class definition to avoid this error.

Use include and extend in Ruby

In our example below, we will illustrate the use of include and extend keywords in Ruby. Have a look at the below example code.

module MyModule
  def MyMethod
    puts 'DelftStack is nice.'
  end
end

class ClassA
  # only can access MyModule methods
  # By the instance.
  include MyModule
end

class ClassB
  # only can access MyModule methods
  # By the class definition.
  extend MyModule
end

# Access from the object
ClassA.new.MyMethod

# Acess from class
ClassB.MyMethod

Each line’s purpose is commented-out and discussed the working mechanism of the keywords include and extend. When you execute the above example code, you will get the below output.

DelftStack is nice.
DelftStack is nice.

Now, if you try to access the method directly from the class like below:

ClassA.MyMethod

You will receive the below error:

# NoMethodError: undefined  method
# `MyMethod' for ClassA:Class

This is because the ClassA includes the module using the keyword include and include doesn’t allow you to call a method directly from the class.