[Ruby] Class nesting, inheritance, and the basics of self

Introduction

What is the range of influence of inheritance while working? .. And What does self mean in this case? .. And I could only understand the basic part fluffy, so this time

--Class --Inheritance - self --Nest

I learned while actually writing, so I will write it as a record. I'd like to write methods and variables as well, but I'd like to write them separately because it seems to be long.

Class inheritance

You can specify one superclass when defining a class, If not specified at this time, the object class is automatically inherited. Therefore, whenever you define a new class, it always inherits some class. It will always be somewhere in the inheritance tree.

Target to inherit

--Superclass class methods --Instance method --Instance variables described in the method

You can also refer to superclass constants and class variables, although they are not inherited.

class Klass
  VALUE = "I'm VALUE"
  @@class_val = "I'm class_val"
end

class A < Klass
end

class B < A
  def self.display_bal
    puts @@class_val
  end
end

#Superclass constant reference
puts B::VALUE
# => I'm VALUE

#Inheritance of class variables
B.display_val
# => I'm class_val

self Since self refers to the object itself, its contents will change depending on where you use it.

In the example below, we pointed to the instance object only when used within an instance method. Also, since self used when defining a class method refers to a class object You can also specify the self of the self. method name directly in the class name.

class Klass
  # Klass
  p self
  
  # Klass
  def (p self).class_method
      puts "inner_class_method"  
      # Klass
      p self
  end

  def instance_method
    puts "inner_instance_method"
    # <Klass:0x00007fa07e1a47c8>
    p self
  end
end

Klass.class_method
klass_instanse =  Klass.new
klass_instanse.instance_method

Nesting classes, etc.

When you define a class, you can nest it in other classes. There are the following two patterns as the definition method, but some behaviors will change.

#Write directly in
class TopKlass
  class Klass
  end
end

## ------ ##

# ”::Use
class TopKlass
end

class TopKlass::Klass
end

Differences in behavior depending on how it is defined

Constant reference

・ Describe directly in We will refer to the constants in order from ourselves to the top level. The flow is to refer to the top level in order from the closest to itself, and finally to the top level.

VALUE = "Top level"

class TopKlass
  VALUE = "in TopKlass"

  class InnerKlass
    def self.put_value
      puts VALUE
    end
  end

end

TopKlass::InnerKlass.put_value
# => in TopKlass

・ Describe using namespace If it doesn't have a constant, it then references the top level and then in order from there towards its own class.

VALUE = "Top level"

class TopKlass
  VALUE = "in TopKlass"
end

class TopKlass::InnerKlass
  def self.put_value
    puts VALUE
  end
end

TopKlass::InnerKlass.put_value
# => Top level

Clarify whether to nest in class or module

・ Described in Since class or module is specified in each hierarchy, an error will occur if a module defined separately is specified as class.

module TopKlass
end

class TopKlass
  class InnerKlass
  end
end

# => TopKlass is not a class (TypeError)

・ Describe using namespace Since it is only the one described at the end (far right) that specifies whether it is a class or not Other than that, it makes the judgment of class and module flexible.

module TopKlass
end

class TopKlass::InnerKlass
end

#No error

Whether to throw an exception when there is no object to nest from

・ Described in In the example below, if TopKlass was not defined separately No exception will occur because TopKlass is newly defined.

class TopKlass
  class InnerKlass
  end
end

・ Describe using namespace

In the following cases, an exception will be thrown if TopKlass is not defined separately. It doesn't define a new TopKlass.

class TopKlass::InnerKlass
end

# => uninitialized constant TopKlass (NameError)

end