Is there a way to call a private Class method from an instance in Ruby?

Question

Other than self.class.send :method, args..., of course. I'd like to make a rather complex method available at both the class and instance level without duplicating the code.

Answer 1

Unless I'm misunderstanding, don't you just need something like this:

class Foo
    private
    def Foo.bar
        # Complex logic goes here
        puts "hi"
    end

    public
    def bar
        Foo.bar
    end
end

Of course you could change the second definition to use your self.class.send approach if you wanted to avoid hardcoding the class name...

Answer 2

You don't have to use send, you can just invoke the method directly on self.class. Of course you could also invoke it using ClassName.method but I don't recommend it as it violates the DRY principle and makes it more difficult to rename your class in the future.

Example:

class Foo
  def bar
    self.class.baz
  end

  private
  def self.baz
    puts "baz called"
  end
end

foo = Foo.new
foo.bar

Ouputs: baz called

Answer 3

Here is a code snippet to go along with the question. Using "private" in a class definition does not apply to class methods. You need to use "private_class_method" as in the following example.

class Foo
  def self.private_bar
    # Complex logic goes here
    puts "hi"
  end
  private_class_method :private_bar
  class <<self
    private
    def another_private_bar
      puts "bar"
    end
  end
  public
  def instance_bar
    self.class.private_bar
  end
  def instance_bar2
    self.class.another_private_bar
  end
end

f=Foo.new
f=instance_bar # NoMethodError: private method `private_bar' called for Foo:Class
f=instance_bar2 # NoMethodError: private method `another_private_bar' called for Foo:Class

I don't see a way to get around this. The documentation says that you cannot specify the receive of a private method. Also you can only access a private method from the same instance. The class Foo is a different object than a given instance of Foo.

Don't take my answer as final. I'm certainly not an expert, but I wanted to provide a code snippet so that others who attempt to answer will have properly private class methods.

Answer 4

@Jonathan Branam: that was my assumption, but I wanted to make sure nobody else had found a way around. Visibility in Ruby is very different from that in Java. You're also quite right that private doesn't work on class methods, though this will declare a private class method:

class Foo
  class <<self
    private
    def bar
      puts 'bar'
    end
  end
end

Foo.bar
# => NoMethodError: private method 'bar' called for Foo:Class