Singleton

A singleton is intended to provide only one instance of itself while facilitating a global point of access. Implementing a singleton pattern involves creating a class with a method that creates a new instance of the class. In order to implement a singleton pattern, principles of single instance and global access must be satisfied. The singleton class is like a global repository for an instance of itself, making the constructor private. Therefore, an instance outside the class cannot be created at all, and a singleton can contain only one instance. A singleton class instantiates itself and maintains that instance across systems.

Abstract factory, builder and prototype patterns can use singletons. Façade objects and static objects are often singletons. Singleton implementation requires a mechanism through which a class member can be accessed without having to create a class object and hold on to the value of class members among class objects. The steps involved in creating a singleton are as follows:

1. The constructor is made private. This allows only the class to have access to the singleton.

Example:
class testdata
{
Private testdata ()
{
//… no-op for a singleton


2. A single internal instance of the class is created using a method. The method is called an instance in this example. The method “instance” is used to initialize the class to access a single instance. The instance method is marked as static in this example to give all the threads consistent access. Outside the instance creation, the “lock” statement is used to control multithreaded access. This locks the instance creation to a single thread.
   
   Example:
   
   //Lazy creation of singleton internal instance
   Public static testdata Instance
   {
   Get
   {
   Lock(type of (testdata) )
   {
   If (_instance==null)
   _instance = testdata ();
   }
   Return _instance;
   }
   }