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

12 | Chapter 1: Foundational Techniques

The singleton class

Singleton classes(alsometaclassesoreigenclasses; see the upcoming sidebar, “Single-
ton Class Terminology”) allow an object’s behavior to be different from that of other
objects of its class. You’ve probably seen the notation to open up a singleton class
before:

class A

end

objA = A.new

objB = A.new

objA.to_s # => "#<A:0x1cd0a0>"

objB.to_s # => "#<A:0x1c4e28>"

class <<objA # Open the singleton class of objA

def to_s; "Object A"; end

end

objA.to_s # => "Object A"

objB.to_s # => "#<A:0x1c4e28>"

Theclass <<objAnotation opensobjA’s singleton class. Instance methods added to
the singleton class function as instance methods in the lookup chain. The resulting
data structures are shown in Figure 1-8.

Figure 1-7. Ruby’s solution for the diamond problem: linearization

A

C

B

super

D

super

klass

klass C

super

A

B

A

super

klass

klass