Derek Neighbors

Ruby’s object-oriented design is elegant and simple, but handling method visibility—especially for class methods—can sometimes be surprising. In this article, we’ll explore why class methods cannot be directly made private using the private keyword, and how to explicitly declare private class methods in Ruby.

Instance Methods vs. Class Methods

Ruby methods generally fall into two categories:

Instance Methods: Defined without any prefix (e.g., def method_name). These operate on instances of a class.
Class Methods: Defined with self. (e.g., def self.method_name) or by using class << self to open the singleton class. These operate on the class itself rather than its instances.

When we use private in Ruby, it applies to instance methods by default. If we want to restrict the visibility of class methods, we need to take extra steps. Let’s explore why.

Why Can’t Class Methods Be Made Private Automatically?

The answer lies in how Ruby interprets private and its relationship with self:

Instance Context: Within a class definition, private declares that subsequent methods are private for instances of the class. Ruby doesn’t assume that these methods apply to the class itself.
Class Context: Class methods belong to the singleton class of the object, so private doesn’t apply to them directly unless explicitly specified.

If you define a class method using def self.method_name after private, Ruby won’t treat it as private. Instead, it assumes that private was only relevant to instance methods.

How to Define Private Class Methods

Ruby provides two primary ways to define private class methods:

1. Using `private_class_method`

This approach explicitly declares specific class methods as private:

class MyClass
  def self.my_method
    puts "This is a class method"
  end
  private_class_method :my_method
end

MyClass.my_method # NoMethodError: private method `my_method' called

Here, private_class_method takes one or more method names and restricts their visibility at the class level.

2. Using `class << self` with `private`

Opening the singleton class of the class allows private to apply to class methods:

class MyClass
  class << self
    private

    def my_method
      puts "This is a private class method"
    end
  end
end

MyClass.my_method # NoMethodError: private method `my_method' called

This approach groups all private class methods in one place, which can improve readability.

What Happens If You Use `private` with `def self.method_name`?

To demonstrate the behavior, let’s write a quick example:

class MyClass
  private

  def self.my_method
    puts "This is a class method"
  end
end

MyClass.my_method # Works! Output: "This is a class method"

Surprisingly, even though def self.my_method is placed under private, it isn’t treated as private. Ruby interprets private as affecting only subsequent instance methods, not class methods.

Why Is This the Case?

This behavior arises from Ruby’s design. The private keyword modifies the visibility for the current scope—typically, the instance context. Since self.my_method belongs to the class’s singleton context, private doesn’t apply unless explicitly declared using one of the approaches mentioned earlier.

Practical Tips and Best Practices

Use private_class_method for Explicitness: This approach makes your intent clear and keeps your code understandable.

class MyClass
  def self.my_private_method
    puts "I'm private!"
  end
  private_class_method :my_private_method
end

Use class << self for Readability: When dealing with multiple private class methods, grouping them within a class << self block can improve organization:

class MyClass
  class << self
    private

    def method_one
      puts "Private method one"
    end

    def method_two
      puts "Private method two"
    end
  end
end

Avoid Mixing Scopes: To minimize confusion, avoid placing def self.method_name directly under private. Explicitly declare the desired visibility.

Conclusion

The need to explicitly handle the visibility of class methods in Ruby stems from how private operates by default in the instance context. While this behavior might initially seem unintuitive, Ruby provides flexible tools like private_class_method and class << self to manage class method visibility cleanly and effectively.

By understanding these nuances, you can write Ruby code that is not only functional but also adheres to the principle of least surprise, making it easier for others to read and maintain.