如何實現單例模式

class Singleton {
public:
    static Singleton& getInstance (void) {
        if (! s_instance) {
            pthread_mutex_lock (&s_mutex);
            if (! s_instance)
                s_instance = new Singleton;
            pthread_mutex_unlock (&s_mutex);
        }
        ++s_counter;
        return *s_instance;
    }
    void releaseInstance (void) {
        if (s_counter) {
            pthread_mutex_lock (&s_mutex);
            if (s_counter && --s_counter == 0)
                delete this;
            pthread_mutex_unlock (&s_mutex);
        }
    }
private:
    Singleton (void) {
        cout << "Singleton::Singleton()" << endl;
    }
    ~Singleton (void) {
        cout << "Singleton::~Singleton()" << endl;
        s_instance = NULL;
    }
    Singleton (const Singleton&);
    Singleton& operator= (const Singleton&);
    static Singleton* s_instance;
    static size_t s_counter;
    static pthread_mutex_t s_mutex;
};
Singleton* Singleton::s_instance = NULL;
size_t Singleton::s_counter = 0;
pthread_mutex_t Singleton::s_mutex =
    PTHREAD_MUTEX_INITIALIZER;