递归模板实现单例模式

 #include <iostream>
    using namespace std;

    template<class T> class Singleton {
        Singleton(const Singleton&);
        Singleton &operator=(const Singleton&);
    protected:
        Singleton() {}
        virtual ~Singleton() {}
    public:
        static T &instance() {
            static T theInstance;
            return theInstance;
        }
    };

    // A sample class to be made into a Singleton
    class MyClass : public Singleton<MyClass> {
        int x;
    protected:
        friend class Singleton<MyClass>;
        MyClass() { x = 0; }
    public:
        void setValue(int n) { x = n; }
        int getValue() const { return x; }
    };

    int main() {
        MyClass &m = MyClass::instance();
        cout << m.getValue() << endl;
        m.setValue(1);
        cout << m.getValue() << endl;
    }