Question

I’m learning ruby and ROR at the same time and noticed one thing in someone else’s code. Sometimes I see methods being defined in these two apparently slightly different ways:

class SomeClass < SomeInheritance::Base

  def self.myMethod
  end

  def myOtherMethod
  end

end

Does it make any difference? I mean, does the use of self in a method definition affects the way the method works somehow? Any enlightenment is welcome.

Answer 1

def self.method_name will define a class method rather than an instance method – as will

class << self; def foo; end; end

A good post on the topic is this post from Yehuda Katz

for example:

class Foo
    def method_1
       "called from instance"
    end

    def self.method_2
       "called from class"
    end

    class << self
       def method_3
         "also called from class"
       end
    end
end

> Foo.method_1
NoMethodError: undefined method 'method_1' for Foo:Class

> Foo.method_2
 => "called from class" 

> Foo.method_3
 => "also called from class" 

> f = Foo.new
 => #<Foo:0x10dfe3a40> 

> f.method_1
 => "called from instance"  

> f.method_2
NoMethodError: undefined method 'method_2' for #<Foo:0x10dfe3a40>

> f.method_3
NoMethodError: undefined method 'method_3' for #<Foo:0x10dfe3a40>