Class Method vs Instance Method in Ruby: Key Differences and Usage

In Ruby, a class method is called on the class itself, while an instance method is called on an object created from that class. Class methods use self.method_name or def ClassName.method_name, whereas instance methods are defined normally and operate on individual objects.

Quick Comparison

Here is a quick side-by-side comparison of class methods and instance methods in Ruby.

Aspect	Class Method	Instance Method
Called on	The class itself	An instance (object) of the class
Definition syntax	def self.method_name or def ClassName.method_name	def method_name
Access to instance variables	No direct access	Has access to instance variables of the object
Use case	Behaviors related to the class as a whole	Behaviors related to individual objects
Example call	ClassName.method_name	object.method_name

Key Differences

Class methods belong to the class itself, not to any specific object created from that class. They are useful for functionality that applies to the class as a whole, such as factory methods or utility functions. You define them by prefixing the method name with self. inside the class or by defining them directly on the class name.

Instance methods belong to objects created from the class. They operate on the data stored in each object’s instance variables. You define them normally without self.. When you call an instance method, you do so on a specific object, which allows the method to access and modify that object's state.

In summary, use class methods for actions related to the class itself and instance methods for actions related to individual objects.

Code Comparison

ruby

class Car
  def self.honk
    "Beep beep!"
  end
end

puts Car.honk

Output

Beep beep!

Instance Method Equivalent

ruby

class Car
  def honk
    "Beep beep!"
  end
end

my_car = Car.new
puts my_car.honk

Output

Beep beep!

When to Use Which

Choose class methods when you need to perform actions that are related to the class itself, such as creating new instances in a special way, or utility functions that don't depend on object state. Choose instance methods when you want to work with or modify the data inside individual objects. This keeps your code organized and clear about what operates on the class versus what operates on each object.

Key Takeaways

Class methods are called on the class and do not have access to instance variables.

Instance methods are called on objects and can access their instance variables.

Define class methods with self.method_name, instance methods without self.

Use class methods for class-wide behavior and instance methods for object-specific behavior.

Calling a class method uses ClassName.method_name, calling an instance method uses object.method_name.