What is the use of class_eval and instance_eval?

class_eval and instance_eval allows us to define a method of a class from outside of its original definition and without reopening the class with the standard syntax. This could be useful when you want to add method and make it unknown till runtime.

Example of class_eval:

class User
end

User.class_eval do
  def say_hello
   "Hello!"
  end
end

user = User.new
user.say_hello
 => "Hello!"

Example of instance_eval:

class User
end

User.instance_eval do
  def say_hello
   "Hello!"
  end
end

User.say_hello
 => "Hello!"

Summary:

class_eval allow us to add instance method from outside the class definition and instance_eval allow us to add class method from outside the class definition.

Behind this madness:

class_eval is a method of the Module class, meaning that the receiver will be a module or a class. The block you pass to class_eval is evaluated in the context of that class. Defining a method with the standard def keyword within a class defines an instance method.

instance_eval, on the other hand, is a method of the Object class, meaning that the receiver will be an object. The block you pass to instance_eval is evaluated in the context of that object. That means that User.instance_eval is evaluated in the context of the User object.