[Ruby] Singular methods and singular classes

Singular methods and singular classes

The Ruby concept of methods and classes just for a particular object.

Singular method

--The singular method build_greeting belongs only to the string object referenced by the variable message at the time of definition.

message = "Hello"  #String object
def message.build_greeting(target)
  "#{self}, #{target}."
message.build_greeting("world")  # => Hello, world.
message2 = "Hello"  #Objects with the same value but different values
message2.build_greeting("world")  # => NoMethodError

Singleton pattern

--A GoF design pattern that guarantees that only one instance of that class will be created.

def CENTRAL_REPOSITORY.register(target)
  @registered_objects ||= []
  unless @resgistered_objects.include? target
    @registered_objects << target
def CENTRAL_REPOSITORY.unregister(target)
  @registered_objects = []

Singularity class

--The definition expression of the singular class is described as "<< (object)" in the class expression. --The object CENTRAL_REPOSITORY now belongs to the singular class(CENTRAL_REPOSITORY). --Instance methods added to the singular class (CENTRAL_REPOSITORY) can be called on the object CENTRAL_REPOSITORY.

  def register(target)
    @registered_objects ||= []
    unless @registered_objects.include? target
      @registered_objects << target
  def unregister(target)
    @registered_objects ||= []

Relationship with singular methods

-** A singular method is an instance method of a singular class **. ――In other words, what is happening in the above two examples is the same.

Class methods and metaclasses

-** A class method is a singular method ** of a Class object.

--The pseudovariable self refers to Duration itself, so a_week_from is a peculiar method of Duration.

class Duration
  def self.a_week_from(from)
    return self.new(from, from+7*24&60*60)
p Duration.a_week_from(Time.now)


-** A metaclass is a singular class ** of a Class object.

--The singular class (Duration) is also called the metaclass of Duration.

class Duration
  class << self  # self(=Duration)Define a singular class for
    def a_week_from(from)
      return self.new(from, from+7*24*60*60)
p Duration.a_week_from(Time.now)


I was impressed by understanding that "the class of the A class is the Class class (= the A class is the Class object)".


-First Ruby -Ruby Metaclass Hierarchy -Ruby beginner's class << self story (or singular and metaclass) -After all, what is a Ruby peculiar method? -[Ruby] About singular methods

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