Re: New Singleton Scheme

Sat, 6 Oct 2007 18:41:04 CST

It has been known for many years that the double-checked locking pattern
is fundamentally broken. It cannot be implemented portably. Google
Groups will yield thread upon thread on this topic.

What about this implementation:

class Singleton
       static Singleton *instance();
       Singleton() { }

       static volatile bool mInitialized;
       static Singleton *mInstance;

volatile bool Singleton::mInitialized = false;
Singleton *Singleton::mInstance = 0;

Singleton *Singleton::instance()
    if (not mInitialized)
       /* scope */
          Lock lock;
          if (mInstance == 0)
             mInstance = new Singleton;
       mInitialized = true;

    return mInstance;

Precisely it is not double check, that's the idea. The scope is
probably not necessary - it causes, that lock will be released before
setting mInitialized - another "sequence point" added. Also mInstance
is not declared as volatile, because protected by Lock.

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