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Ruby Programming
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Singletons and Other Constructors

Sometimes you want to override the default way in which Ruby creates objects. As an example, let's look at our jukebox. Because we'll have many jukeboxes, spread all over the country, we want to make maintenance as easy as possible. Part of the requirement is to log everything that happens to a jukebox: the songs that are played, the money received, the strange fluids poured into it, and so on. Because we want to reserve the network bandwidth for music, we'll store these logfiles locally. This means we'll need a class that handles logging. However, we want only one logging object per jukebox, and we want that object to be shared among all the other objects that use it.

Enter the Singleton pattern, documented in Design Patterns . We'll arrange things so that the only way to create a logging object is to call Logger.create, and we'll ensure that only one logging object is ever created.

class Logger
  private_class_method :new
  @@logger = nil
  def Logger.create
    @@logger = new unless @@logger
    @@logger
  end
end

By making Logger's method new private, we prevent anyone from creating a logging object using the conventional constructor. Instead, we provide a class method, Logger.create. This uses the class variable @@logger to keep a reference to a single instance of the logger, returning that instance every time it is called.[The implementation of singletons that we present here is not thread-safe; if multiple threads were running, it would be possible to create multiple logger objects. Rather than add thread safety ourselves, however, we'd probably use the Singleton mixin supplied with Ruby, which is documented on page 468.] We can check this by looking at the object identifiers the method returns.

Logger.create.id 537766930
Logger.create.id 537766930

Using class methods as pseudo-constructors can also make life easier for users of your class. As a trivial example, let's look at a class Shape that represents a regular polygon. Instances of Shape are created by giving the constructor the required number of sides and the total perimeter.

class Shape
  def initialize(numSides, perimeter)
    # ...
  end
end

However, a couple of years later, this class is used in a different application, where the programmers are used to creating shapes by name, and by specifying the length of the side, not the perimeter. Simply add some class methods to Shape.

class Shape
  def Shape.triangle(sideLength)
    Shape.new(3, sideLength*3)
  end
  def Shape.square(sideLength)
    Shape.new(4, sideLength*4)
  end
end

There are many interesting and powerful uses of class methods, but exploring them won't get our jukebox finished any sooner, so let's move on.
Ruby Programming
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  Published under the terms of the Open Publication License Design by Interspire