What does `def self.function` name mean?

ruby

Solution

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`.

Problem

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