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CHAPTER 6 CLASSES, OBJECTS, AND MODULES
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Object Orientation Basics
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Let s recap our basic knowledge of classes and objects that we learned over the past few chapters: A class is a blueprint for objects. You only have one class called Shape, but with it you can create multiple instances of shapes (shape objects), all of which have the methods and attributes defined by the Shape class. An object is an instance of a class. If Shape is the class, then x = Shape.new creates a new Shape instance and assigns the object to the variable x. You would then say x is a Shape object, or an object of class Shape.
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Local, Global, Object, and Class Variables
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In 2 you created some classes and added methods to them. To recap, here s a simple demonstration of a class with two methods, and how to use it. First, here s the class itself:
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class Square def initialize(side_length) @side_length = side_length end def area @side_length * @side_length end end
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Next, let s create some square objects and use their area methods:
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a = Square.new(10) b = Square.new(5) puts a.area puts b.area
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CHAPTER 6 CLASSES, OBJECTS, AND MODULES
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The first method and when I say first, I mean the first method in our example; the actual order of methods in code is irrelevant in the Square class is initialize. initialize is a special method that s called when a new object based on that class is created. When you call Square.new(10), the Square class creates a new object instance of itself, and then calls initialize upon that object. In this case, initialize accepts side_length as an argument, as passed through from Square.new(10), and assigns the number 10 to a variable called @side_length. The @ symbol before the variable name is important in this case. But why To understand why some variables are prefixed with certain symbols requires understanding that there are multiple types of variables, such as local, global, object, and class variables.
Local Variables
In previous examples you ve created variables simply, like so:
x = 10 puts x
In Ruby, this sort of basic variable is called a local variable. It can only be used in the same place as where it is defined. If you jump to using an object s methods or a separate method of your own, the variable x doesn t come with you. It s considered to be local in scope. That is, it s only present within the local area of code. Here s an example that demonstrates this:
def basic_method puts x end x = 10 basic_method
This example defines x to equal 10, and then jumps to a local method called basic_method. If you ran this code through irb, you would get an error like this:
NameError: undefined local variable or method `x' for main:Object from (irb):2:in `basic_method'
CHAPTER 6 CLASSES, OBJECTS, AND MODULES
What s happening is that when you jump to basic_method, you re no longer in the same scope as the variable x that you created. Because x is a local variable, it only exists where it was defined. To avoid this problem, it s important to remember to use only local variables where they re being directly used, as this is what they re really for. Here s an example where you have two local variables with the same name but in different scopes:
def basic_method x = 50 puts x end x = 10 basic_method puts x
50 10
This demonstrates that local variables live entirely in their original scope. You set x to 10 in the main code, and set x to 50 inside the method, but x is still 10 when you return to the original scope. The x variable inside basic_method is not the same x variable that s outside of the method. They re separate variables, distinct within their own scopes.
Global Variables
In direct opposition to local variables, Ruby can also use global variables. Much as their name suggests, global variables are available from everywhere within an application, including inside classes or objects. Global variables can be useful, but aren t commonly used in Ruby. They don t mesh well with the ideals of object-oriented programming, as once you start using global variables across an application, your code is likely to become dependent on them. Because the ability to separate blocks of logic from one another is a useful aspect of object-oriented programming, global variables are not favored. However, I ll touch on global variables again later in this book, so it s useful to know how they re constructed. You define global variables by putting a dollar sign ($) in front of the variable name, like so:
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