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Aliasing

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To alias a method or variable name in Ruby is to create a second name for the method or variable. Aliasing can be used either to provide more expressive options to the programmer using the class, or to help override methods and change the behavior of the class or object. Ruby provides this functionality with the alias and alias_method keywords.

Create a Second Name

The alias keyword takes two arguments: the old method name and the new method name. The method names should be passed as labels as opposed to strings. Labels are used to refer to methods and variables without directly referencing them. If you're a new Ruby programmer, the concept of labels may seem odd, but whenever you see a label such as :methodname, just read it as "the thing called methodname." The following example declares a new class and creates an alias for the on method called start.

 #!/usr/bin/env ruby
 
 class Microwave
   def on
     puts "The microwave is on"
   end
 
   alias :start :on
 end
 
 m = Microwave.new
 m.start # same as m.on
 

Change the Behavior of a Class

There may be times when you want to change the behavior of a class after it's been declared. You can alias and add new methods to an existing class by creating second class declaration that has the same name as the existing class declaration. You can also add aliases and methods to individual objects using a syntax similar to the inherited class syntax. The behavior of any class can be changed by creating an alias for any method and then creating a new method (with the original method name) that calls the method with the alias.

In the following example, a Microwave class is declared and an instance is created. The second class declaration uses the alias method to change the behavior of the on method in order to add a warning message. The third class declaration is used to change the behavior of the specific Microwave instance to add an even more stern warning. When aliasing a method multiple times, be sure to use different method names to store the old method.

 #!/usr/bin/env ruby
 
 class Microwave
   def on
     puts "Microwave is on"
   end
 end
 
 m = Microwave.new
 m.on
 
 class Microwave
   alias :old_on1 :on
 
   def on
     puts "Warning: Do not insert metal objects!"
     old_on1
   end
 end
 
 m.on
 
 # Message for this specific microwave
 class <<m
   alias :old_on2 :on
   
   def on
     puts "This microwave is weak, add extra time"
     old_on2
   end
 end
 
 m.on # Displays extra message
 
 m2 = Microwave.new
 m2.on # Does not display extra message
 
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