Singleton Creation the Thread-safe Way

Listing 5: The singleton of try1.h (Listing 2) with a critical section added

// try3.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()
{
   Lock_Guard<Critical_Section> gate(_key);

   if(!_instance)
      _instance = new Singleton;

   return *_instance;
}