What does def `self.function` name mean?

Question

Can anyone explain to me what the meaning of adding self to the method definition is? Is it similar to the this keyword in java?

Answer 1

Contrary to other languages, Ruby has no class methods, but it has singleton methods attached to a particular object.

cat = String.new("cat") def cat.speak     'miaow' end cat.speak #=> "miaow"  cat.singleton_methods #=> ["speak"]  

def cat.speak creates a singleton method attached to the object cat.

When you write class A, it is equivalent to A = Class.new :

A = Class.new def A.speak     "I'm class A" end A.speak #=> "I'm class A"  A.singleton_methods #=> ["speak"]  

def A.speak creates a singleton method attached to the object A. We call it a class method of class A.

When you write

class A     def self.c_method         'in A#c_method'     end end 

you create an instance of Class(*). Inside the class definition, Ruby sets self to this new instance of Class, which has been assigned to the constant A. Thus def self.c_method is equivalent to def cat.speak, that is to say you define a singleton method attached to the object self, which is currently the class A.

Now the class A has two singleton methods, that we commonly call class methods.

A.singleton_methods  => ["c_method", "speak"]  

(*) technically, in this case where A has already been created by A = Class.new, class A reopens the existing class. That's why we have two singleton methods at the end. But in the usual case where it is the first definition of a class, it means Class.new.