[Ruby] Singular methods and singular classes

Introduction

I decided to investigate singular classes and singular methods, so I will summarize the contents.

Singular method

A method that is object-specific. It can be defined by writing the object name before the method name to be defined. Note that the object must already exist when you define it.

class Klass
end

apple = Klass.new
orenge = Klass.new

def apple.tokui_method
  puts "I'm apple"
end

apple.tokui_method
# => I'm apple
orenge.tokui_method
# => undefined method `tokui_method'

If the specified object does not exist when defining it, an error will occur.

class Klass
end

def apple.tokui_method
  puts "I'm apple"
end

apple = Klass.new

apple.tokui_method
# => undefined local variable or method `apple' for main:Object (NameError)

Calling super within a singular method will call the method of the same name defined in the class. There is also the convenience of being able to override that method.

class Klass
  def my_name
    puts "I'm fruits"
  end
end

apple = Klass.new
orenge = Klass.new

def apple.my_name
  super
  puts "I'm apple"
end

apple.my_name
# => I'm fruits
# => I'm apple
orenge.my_name
# => I'm fruits

Singularity class

When you define a singular method on an object, the class in which that method is defined is the singular class.

Unlike regular classes, singular classes are classes used only by certain objects. Therefore, it is forbidden to create objects or subclasses from singular classes.

To check the singular class of an object Use Object # singleton_class.

class Klass
end

obj = Klass.new
# => #<Klass:0x00007f8fb584cc08>

TokuiClass = obj.singleton_class
# => #<Class:#<Klass:0x00007f8fb584cc08>>

TokuiClass.new
# => can't create instance of singleton class (TypeError)

class SubTokuiClass < TokuiClass
end
# => can't make subclass of singleton class (TypeError)

Also, the singular methods mentioned above are defined in singular classes. For singular methods You can check it with singleton_class.method_defined?.

class Klass
end

obj = Klass.new

def obj.tokui_method
  :tokui_method
end

p obj.class.method_defined? :tokui_method
# => false
p obj.singleton_class.method_defined? :tokui_method
# => true

When the singular class is created

When an object is created, the singular class for that object does not exist. Singular classes are created in the following cases:

1, Timing to define singular method 2, Timing of evaluating the singular class definition expression 3, Timing to check singular class for object with Object # singleton_class

Timing to define a singular method

obj = Klass.new

def obj.tokui_method
  :tokui_method
end

Timing of evaluating the singular class definition expression

class << obj
end

Timing to check singular class for object with Object # singleton_class

If the singular class does not exist at the timing you tried to check, it will be newly created.

obj.singleton_class

Objects and singular classes

A singular class is a subclass of the object's class.

class Klass
end

obj = Klass.new
obj.singleton_class.supperclass
# => Klass

When a singular class is created for an object, that object becomes an instance of the singular class, but in Ruby Being an instance of a singular class is now ignored and is always treated as an instance of the object's class. You can check if it is a direct instance of the argument with Object # instance_of?, so let's use this.

obj.instance_of? obj.singleton_class
# => false

obj.instance_of? Klass
# => true

Objects that do not have a singular class

The following objects cannot have singular classes in Ruby.

--Numerical value

• Symbol
• true, false, nil

Integers and symbles return an error

1.singleton_class
# => can't define singleton (TypeError)
:symble.singleton_class
# => can't define singleton (TypeError)

true, false, nil will return their class as a singular class

p true.singleton_class
# => TrueClass

p false.singleton_class
# => FalseClass

p nil.singleton_class
# => NilClass