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
Singleton pattern in C#:
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;
}
}
Singleton pattern in PHP 5:
<?php
class Singleton {
// object instance
static $instance;
private function __construct() {}
private function __clone() {}
public static function getInstance() {
if (!Singleton::$instance instanceof self) {
Singleton::$instance = new self();
}
return Singleton::$instance;
}
}
?>