Singleton Creation the Thread-safe Way

Listing 6: The final solution — a singleton that uses the Double-Checked Locking Pattern

// solution.h
#include "sync.h"

class Singleton
{
public:
   static Singleton& instance();
   void show_state();

protected:
   Singleton();

private:
   static Singleton* _instance;
   static Critical_Section _key;
   int state;
};

Singleton* Singleton::_instance = NULL;
Critical_Section Singleton::_key;

Singleton& Singleton::instance()
{
   if(!_instance) // Lock-hint check (Race condition here)
   {
      Lock_Guard<Critical_Section> gate(_key);

      if(!_instance) // Double-check (Race condition resolved)
         _instance = new Singleton;
   }

   return *_instance;
}