Question

While looking over some Ruby code I noticed methods declared with self. prepended to the method name. For example:

def self.someMethod
  //...
end

What does prepending self. to the method name change about the method?

Answer 1

def self.something is a class method, called with:

Class.some_method

def something is an instance method, called with:

class = Class.new
class.some_method

The difference is that one is called on the class itself, the other on an instance of the class.

To define a class method, you can also use the class name, however that will make things more difficult to refactor in the future as the class name may change.

Some sample code:

class Foo
  def self.a
    "a class method"
  end

  def b
    "an instance method"
  end

  def Foo.c
    "another class method"
  end
end

Foo.a # "a class method"
Foo.b # NoMethodError
Foo.c # "another class method"
bar = Foo.new 
bar.a # NoMethodError
bar.b # "an instance method"
bar.c # NoMethodError