The singleton design pattern in D, C++ and Java

[dsimcha]

== Quote from Justin Johansson (no at spam.com)'s article
> Which language out of C++, D and Java does this classical
> "GoF" (gang-of-four***) design pattern best?
> *** http://en.wikipedia.org/wiki/Design_Patterns
> For my take, I prefer synchronization-free implementations.
> Surely this topic has been around before???
> <bystander awareness="clueless" name="Justin"/>

D makes it very easy to create an efficient thread-safe Singleton implementation,
using a method that was inspired by broken double-checked locking, but uses
thread-local storage to be correct.  The following algorithm guarantees that the
synchronized block guarding the Singleton will be entered at most once per thread
throughout the lifetime of the program.  I invented it myself, but it's fairly
simple and might have been independently invented elsewhere:

class Singleton {
private:
    static bool initialized;  // Thread-local
    __gshared static Singleton instance;

    this() {}

public:

    static Singleton getInstance() {
        if(initialized) {
            return instance;
        }

        synchronized(Singleton.classinfo) {
            scope(success) initialized = true;
            if(instance !is null) {
                return instance;
            }

            instance = new Singleton;
            return instance;
       }
    }
}