[Ruby] Basic knowledge of class instance variables, etc.

Introduction

Here, ruby class variables, instance variables, accessors, etc. I will write the basic part.

Class variables

The class itself, the subclass, Variables that can be referenced from class methods and instance methods.

To define it, add @@ at the beginning.

class SuperKlass
  @@super_klass_var = :super_klass_val
end

class Klass < SuperKlass
  @@class_var = :class_val
  def self.class_method
      puts "class_method called"
      puts @@super_klass_var
      puts @@class_var
  end

  def instance_method
      puts "instance_method called"
      puts @@super_klass_var
      puts @@class_var
  end
end

klass_instance = Klass.new
Klass.class_method  
  # => class_method called
  # => super_klass_val
  # => class_val
  
klass_instance.instance_method
  # => instance_method called
  # => super_klass_val
  # => class_val

Instance variables

By holding a value in an instance variable, it is possible to hold an object-specific state. Note that instance variables cannot be accessed directly from outside the object. You need to use the ʻattr_accessor` method to access it.

class Famiry
  attr_accessor :name
end

father = Famiry.new

father.name
# => nil
father.name = "Namihey"
father.name
# => Namihey

If attr_accessor is not used, a NoMethodError will occur.

class Famiry
end

father = Famiry.new

father.name = "Namihey"
# => undefined method `name=' for #<Famiry:0x00007f8a97908bd8> (NoMethodError)

If you define it by prefixing it with @ in the instance method, it becomes an instance variable.

class Geinin
  def initialize(val)
    @val = val
  end

  def tukkomi
    puts @val
  end

end

east = Geinin.new("why")
east.tukkomi

north = Geinin.new("why")
north.tukkomi

attr_accessor This method allows reference and assignment for instance variables. ʻAttr_reader to deny assignments and allow references only ʻAttr_writer to deny references and allow only assignments Etc. are used properly. Such a method is called a accessor.

The following methods are defined by using the above methods.

#Reference method
def name
  @name
end

#Assignment method
def name=(val)
  @name = val
end

Class instance variable

Since a class is an object of the Class class, the class object itself You can have instance variables.

In the context where self points to a class, just like the instance variables above If you define it with @ at the beginning, it becomes a class instance variable.

You can also refer to it in the context where self points to the class.

class klass
  #Definition
  @class_instance_val = :class_instance_val
    
  def self.class_method
    #reference
    @class_instance_val
  end
end

Differences between class variables and class instance variables

Class variables and class instance variables may feel the same, Because class instance variables are instance variables of class objects It cannot be referenced within the instance method defined in that class.

Also, class variables can be referenced and changed from a class that inherits that class. Class instance variables are not allowed. In summary, the following two points are different.

--Reference from instance method --Class variables ... can --Class instance variable ... I can't --Reference and change from inherited class --Class variables ... can --Class instance variable ... I can't

end