Singleton

Introduction

Singleton pattern Ensure a class only has one instance, and provide a global point of access to it. This is useful when exactly one object is needed to coordinate actions across the system.

Implementation

• Singleton pattern is implemented by creating a class with a method that creates a new instance of the object if one does not exist.
• If an instance already exists, it simply returns a reference to that object.
• To make sure that the object cannot be instantiated any other way, the constructor is made either private or protected.

UML Diagram

Sample Code

class Singleton
{
  private static Singleton instance;

  // Note: Constructor is 'protected' 
  protected Singleton() 
  {
  }

  public static Singleton Instance()
  {
    // Use 'Lazy initialization' 
    if (instance == null)
    {
      instance = new Singleton();
    }

    return instance;
  }
}