class Singleton {
public:
static Singleton* instance();
   ...
private:
   static Singleton* volatile pInstance; // volatile added
   int x;
   Singleton() : x(5) {}
};
// from the implementation file
Singleton* Singleton::pInstance = 0;
Singleton* Singleton::instance() {
if (pInstance == 0) {
   Lock lock;
   if (pInstance == 0) {
      Singleton*volatile temp = new Singleton; // volatile added
      pInstance = temp;
   }
}
return pInstance;
}

Example 7: Declaring pInstance.