From 5b828a05f78e5ce0c5b59a118c79163d88da6f4a Mon Sep 17 00:00:00 2001
From: pbihler <bihler@cs.uni-bonn.de>
Date: Wed, 17 Sep 2014 10:47:38 +0200
Subject: [PATCH] Instance Variables added

Added section about instance variables (properties), including warning about accidentially adding it to the class prototype
---
 .../class-variables-and-instance-variables.md | 99 +++++++++++++++++++
 .../classes_and_objects/class-variables.md    | 43 --------
 2 files changed, 99 insertions(+), 43 deletions(-)
 create mode 100644 chapters/classes_and_objects/class-variables-and-instance-variables.md
 delete mode 100644 chapters/classes_and_objects/class-variables.md

diff --git a/chapters/classes_and_objects/class-variables-and-instance-variables.md b/chapters/classes_and_objects/class-variables-and-instance-variables.md
new file mode 100644
index 0000000..23acd37
--- /dev/null
+++ b/chapters/classes_and_objects/class-variables-and-instance-variables.md
@@ -0,0 +1,99 @@
+---
+layout: recipe
+title: Class Variables and Instance Variables
+chapter: Classes and Objects
+---
+## Problem
+
+You want to create class variables and instance variables (properties).
+
+## Solution
+
+### Class Variables
+
+{% highlight coffeescript %}
+class Zoo
+  @MAX_ANIMALS: 50
+  MAX_ZOOKEEPERS: 3
+  
+  helpfulInfo: =>
+    "Zoos may contain a maximum of #{@constructor.MAX_ANIMALS} animals and #{@MAX_ZOOKEEPERS} zoo keepers."
+
+Zoo.MAX_ANIMALS
+# => 50
+
+Zoo.MAX_ZOOKEEPERS
+# => undefined (it is a prototype member)
+
+Zoo::MAX_ZOOKEEPERS
+# => 3
+
+zoo = new Zoo
+zoo.MAX_ZOOKEEPERS
+# => 3
+zoo.helpfulInfo()
+# => "Zoos may contain a maximum of 50 animals and 3 zoo keepers."
+
+zoo.MAX_ZOOKEEPERS = "smelly"
+zoo.MAX_ANIMALS = "seventeen"
+zoo.helpfulInfo()
+# => "Zoos may contain a maximum of 50 animals and smelly zoo keepers."
+{% endhighlight %}
+
+
+### Instance Variables
+
+You have to define instance variables (i.e. properties) inside a class' method, initialize your defaults in the constructor.
+
+{% highlight coffeescript %}
+class Zoo
+  constructor: ->
+    @animals = [] # Here the instance variable is defined
+    
+  addAnimal: (name) ->
+    @animals.push name
+
+
+zoo = new Zoo()
+zoo.addAnimal 'elephant'
+
+otherZoo = new Zoo()
+otherZoo.addAnimal 'lion'
+
+zoo.animals
+# => ['elephant']
+
+otherZoo.animals
+# => ['lion']
+{% endhighlight %}
+
+#### WARNING!
+*Do not add the variable accidently to the prototype, by defining it outside the constructor* (Even if mentioned [elsewhere](http://arcturo.github.io/library/coffeescript/03_classes.html#content), this does not work as intended, due to the underlying JavaScript prototype concept).
+
+{% highlight coffeescript %}
+class BadZoo
+  animals: []           # Translates to BadZoo.prototype.animals = []; and is thus shared between instances
+    
+  addAnimal: (name) ->
+    @animals.push name  # Works due to the prototype concept of Javascript
+
+
+zoo = new BadZoo()
+zoo.addAnimal 'elephant'
+
+otherZoo = new BadZoo()
+otherZoo.addAnimal 'lion'
+
+zoo.animals
+# => ['elephant','lion'] # Oops...
+
+otherZoo.animals
+# => ['elephant','lion'] # Oops...
+
+BadZoo::animals
+# => ['elephant','lion'] # The value is stored in the prototype
+{% endhighlight %}
+
+## Discussion
+
+Coffeescript will store the values of class variables on the class itself rather than on the prototype it defines. These are useful for defining variables on classes which can't be overwritten by instance attribute variables.
diff --git a/chapters/classes_and_objects/class-variables.md b/chapters/classes_and_objects/class-variables.md
deleted file mode 100644
index 9d403ef..0000000
--- a/chapters/classes_and_objects/class-variables.md
+++ /dev/null
@@ -1,43 +0,0 @@
----
-layout: recipe
-title: Class Variables
-chapter: Classes and Objects
----
-## Problem
-
-You want to create a class variable.
-
-## Solution
-
-{% highlight coffeescript %}
-class Zoo
-  @MAX_ANIMALS: 50
-  MAX_ZOOKEEPERS: 3
-  
-  helpfulInfo: =>
-    "Zoos may contain a maximum of #{@constructor.MAX_ANIMALS} animals and #{@MAX_ZOOKEEPERS} zoo keepers."
-
-Zoo.MAX_ANIMALS
-# => 50
-
-Zoo.MAX_ZOOKEEPERS
-# => undefined (it is a prototype member)
-
-Zoo::MAX_ZOOKEEPERS
-# => 3
-
-zoo = new Zoo
-zoo.MAX_ZOOKEEPERS
-# => 3
-zoo.helpfulInfo()
-# => "Zoos may contain a maximum of 50 animals and 3 zoo keepers."
-
-zoo.MAX_ZOOKEEPERS = "smelly"
-zoo.MAX_ANIMALS = "seventeen"
-zoo.helpfulInfo()
-# => "Zoos may contain a maximum of 50 animals and smelly zoo keepers."
-{% endhighlight %}
-
-## Discussion
-
-Coffeescript will store these values on the class itself rather than on the prototype it defines.  These are useful for defining variables on classes which can't be overwritten by instance attribute variables.