Singleton Pattern

Part of TutorialDesignPatterns

Description

Examples of implementing the Gang of Four Singleton Pattern in D. A Singleton is a class guaranteed to have only a single instance, with a single global access point to that instance.

For the sake of brevity, this tutorial does not cover subclassing of Singletons.

Example 1: Simple Singleton

class Singleton {

  public static Singleton instance () {
    if (_instance is null)
      _instance = new Singleton;

    return _instance;
  }

  protected this () {
  }

  private static Singleton _instance;

}

void main () {
  auto var = Singleton.instance;
}

Example 2: Using Interfaces and Mixins

/* This is the basic interface of a Singleton.
 */
interface ISingleton {

  public static ISingleton instance ();

}

/* This is the mixin for a quick basic implementation of a Singleton.
 * Note the use of a static variable local to the method, and the special 'typeof(this)'
 * type expression which allows the mixin to work without knowing the class using it.
 */
template MSingleton () {

  public static typeof(this) instance () {
    static typeof(this) _instance;

    if (_instance is null)
      _instance = new typeof(this);

    return _instance;
  }

}

/* This class implements the pattern using the mixin.
 * Note that using this style, class designers are still on their honor to
 * make their constructor protected.
 */
class Foo : ISingleton {

  mixin MSingleton;

  protected this () {
  }

}

/* This class implements the pattern manually.
 * Note this is really no different from Example 1, except for the interface.
 */
class Bar : ISingleton {

  public static Bar instance () {
    if (_instance is null)
      _instance = new Bar;

    return _instance;
  }

  protected this () {
  }

  private static Bar _instance;

}

/* Now let's access our Singletons.
 */
void main () {
  auto foo = Foo.instance;
  auto bar = Bar.instance;
}