Singleton

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Singleton

• Christopher Chiaverini
• Software Design Documentation
• September 18, 2003

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What is the Singleton Pattern?

• The Singleton pattern ensures that a class is
  only instantiated once and provides a global
  access point for this instance

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Creational Pattern

• Abstracts the instantiation process
• Object Creational Pattern
• Delegates the instantiation to another object

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Why use the Singleton Pattern?

• It is important for some classes to only have one
  instance
• It is also important that this single instance is
  easily accessible
• Why not use a global object?
• Global variables make objects accessible, but
  they don't prevent the instantiation of multiple
  objects.

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Solution

• Make the class responsible for keeping track of
  its only instance
• The class can ensure that no other instances are
  created
• This provides a useful way to access the instance
• This is the Singleton pattern

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Examples of Singleton Patterns

• Print Spooler
• File Systems
• Window Managers
• Digital Filters
• Accounting Systems

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Sample Class
Class Singleton public static Singleton
Instance() protected Singleton() private
static Singleton _instance
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Sample Implementation
Singleton Singleton_instance 0 Singleton
SingletonInstance () if (_instance 0)
_instance new Singleton
return _instance
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Things to Notice

• Instance() function ensures that only one
  instance is created and that it is initialized
  before use
• Singleton constructor is declared as protected
• Direct instantiation causes errors at compile
  time
• Since _instance is a pointer, Instance() can
  return a pointer to a subclass of Singleton

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Caution

• In C a Singleton cannot be defined as a global
  or static object that relies on automatic
  initialization
• This does not guarentee that only one instance of
  the static object will be created
• There might not be enough information to create
  every Singleton at static initialization time
• C doesn't define the order in which
  constructors for global objects are called
  dependencies between Singletons will cause errors

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Derived Classes

• Code should be able to use a subclass of a
  Singleton without being modified
• What is the best way to do this?
• Specify the Singleton in the Instance() method
• Implement Instance() in the subclasses
• Neither approach is very flexible

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Registry of Singletons

• Singleton classes register their instance with
  the registry
• Registry maps between string names and Singletons
• Singleton name can be specifed at startup by the
  user or an environment variable
• Problem How to register classes?
• Declare a static instance of each subclass

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Benefits of the Singleton Pattern

• Controlled access to sole instance
• Reduced name space
• Allows refinement of operations and
  representation
• Allows a variable number of instances
• More flexible than class operations

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Use the Singleton Pattern when...

• There must be exactly one instance of a class
• This instance must be accessible from a
  well-known access point
• The instance should be extensible by subclassing
• Clients should be able to use a derived class
  without modifying their code