Advanced Rails - Building Industrial-Strength Web Apps in Record Time

Ruby Foundations | 15

This should not be a surprise. There is no magic here. Class methods are found in
the exact same way as instance methods—the only difference is whether the receiver
is a class or an instance of a class.

Now that we know how class methods are looked up, it would seem that we could
define class methods on any class by defining instance methods on theClassobject
(to insert them into Class’sm_tbl). Indeed, this works:

class A; end

# from Module#to_s

A.to_s # => "A"

class Class

def to_s; "Class#to_s"; end

end

A.to_s # => "Class#to_s"

That is an interesting trick, but it is of very limited utility. Usually we want to define
unique class methods on each class. This is where singleton classes of class objects
are used. To open up a singleton class on a class, simply pass the class’s name as the
object to the singleton class notation:

class A; end

class B; end

class <<A

def to_s; "Class A"; end

end

A.to_s # => "Class A"

B.to_s # => "B"

The resulting data structures are shown in Figure 1-10. ClassB is omitted for brevity.

Figure 1-9. Full set of data structures for a single class

Object

Module

super

Class

super

A klass

super