c++ singleton

singleton.cc

#include <iostream>
#include <memory>
using namespace std;

// http://stackoverflow.com/questions/1008019/c-singleton-design-pattern
class Singleton
{
public:
    static Singleton& instance()
    {
        static Singleton s;
        // initialized on first use, guarentee to be destroyed
        return s;
    }

    void foo()
    {
        cout << "Singleton foo" << endl;
    }

private:

    Singleton() {};    
    Singleton(const Singleton &s);
    void operator = (const Singleton &);
};

// using auto_ptr (or unique_ptr in c++11)
class Zingleton
{
public:
    static Zingleton& instance()
    {
        if (_instance.get() == NULL)
        {
            _instance = auto_ptr<Zingleton>(new Zingleton);
        }

        return *_instance;
    }

    void bar()
    {
        cout << "Zingleton bar" << endl;
    }
private:
    static auto_ptr<Zingleton> _instance;

    Zingleton() {}
    Zingleton(const Zingleton &z);
    Zingleton& operator= (const Zingleton &);
};

// static initialization must be done in source file, or linking will have problem
auto_ptr<Zingleton> Zingleton::_instance = auto_ptr<Zingleton>();

int main()
{
    Singleton::instance().foo();
    Zingleton::instance().bar();
    return 0;
}