1 files changed, 76 insertions, 30 deletions
diff --git a/ruby.html.markdown b/ruby.html.markdown
index a3bcbbd5..861a94ad 100644
--- a/ruby.html.markdown
+++ b/ruby.html.markdown
@@ -275,36 +275,36 @@ surround { puts 'hello world' }
 # Define a class with the class keyword
 class Human
 
-     # A class variable. It is shared by all instances of this class.
-    @@species = "H. sapiens"
-
-    # Basic initializer
-    def initialize(name, age=0)
-        # Assign the argument to the "name" instance variable for the instance
-        @name = name
-        # If no age given, we will fall back to the default in the arguments list.
-        @age = age
-    end
-
-    # Basic setter method
-    def name=(name)
-        @name = name
-    end
-
-    # Basic getter method
-    def name
-        @name
-    end
-
-    # A class method uses self to distinguish from instance methods.
-    # It can only be called on the class, not an instance.
-    def self.say(msg)
-       puts "#{msg}"
-    end
-
-    def species
-        @@species
-    end
+  # A class variable. It is shared by all instances of this class.
+  @@species = "H. sapiens"
+
+  # Basic initializer
+  def initialize(name, age=0)
+    # Assign the argument to the "name" instance variable for the instance
+    @name = name
+    # If no age given, we will fall back to the default in the arguments list.
+    @age = age
+  end
+
+  # Basic setter method
+  def name=(name)
+    @name = name
+  end
+
+  # Basic getter method
+  def name
+    @name
+  end
+
+  # A class method uses self to distinguish from instance methods.
+  # It can only be called on the class, not an instance.
+  def self.say(msg)
+    puts "#{msg}"
+  end
+
+  def species
+    @@species
+  end
 
 end
 
@@ -325,4 +325,50 @@ dwight.name #=> "Dwight K. Schrute"
 # Call the class method
 Human.say("Hi") #=> "Hi"
 
+# Class also is object in ruby. So class can have instance variables.
+# Class variable is shared among the class and all of its descendants.
+
+# base class
+class Human
+  @@foo = 0
+
+  def self.foo
+    @@foo
+  end
+
+  def self.foo=(value)
+    @@foo = value
+  end
+end
+
+#  derived class
+class Worker < Human
+end
+
+Human.foo # 0
+Worker.foo # 0
+
+Human.foo = 2 # 2
+Worker.foo # 2
+
+# Class instance variable is not shared by the class's descendants.
+
+class Human
+  @bar = 0
+
+  def self.bar
+    @bar
+  end
+
+  def self.bar=(value)
+    @bar = value
+  end
+end
+
+class Doctor < Human
+end
+
+Human.bar # 0
+Doctor.bar # nil
+
 ```