To make a class method private, the intuitive way is to put private before the method definition like the following.

class Foo

  private

  def self.bar
    "bar"
  end
end

But that does not seem to be sufficient, unfortunately.

The method is still public:

pry(main)> Foo.bar
=> "bar" 

Actually, to make it private, we have to call private_class_method like the following.

class Foo

  def self.bar
    "bar"
  end
  private_class_method :bar

end

We can confirm that it works by calling the class method in a Ruby console.

pry(main)> Foo.bar
NoMethodError: private method `bar' called for Foo:Class

Why is that?

Before we jump into the explanation, please take a look at the following code, which is another way to make a class method private.

class Foo
  class << self

    private

    def bar
      "bar"
    end
  end
end

In the example above, the singleton class is opened and the method bar is defined as a private instance method on it.

The syntax looks different but it works as same as the previous example.

pry(main)> Foo.bar
NoMethodError: private method `bar' called for Foo:Class

In Ruby, as we know, classes are just objects. When we define a “class method” on Foo, we are just defining an instance method on the singleton class of the Foo object.

pry(main)> Foo.singleton_class.private_instance_methods(false)
=> [:bar]

And private is not a special keyword but a method on Module. It is available on Class because Class.is_a? Module.

When private gets called, it sets the visibility for subsequently methods defined on the current object to private.

Therefore, to set the visibility of methods defined on the singleton class of the current object, we need another method. That is where Module#private_class_method comes in.

To summarize, there are two styles of syntax to create a private class method bar on a class Foo.

1. Define the method with the syntax def self.bar and then private_class_method :bar to make it private.
2. Or open the singleton class of Foo and define a private method on the singleton class.

p.s. The term “singleton class” referred in this post is also called “metaclass” or “eigenclass”.