这是Singleton模式的正确实现吗？

Question

以下代码是我对Singleton Pattern的实现.

 #include <iostream>

template<class T>
class Uncopyable
{
protected:
    Uncopyable(){}
    ~Uncopyable(){}
private:
    Uncopyable(const Uncopyable<T>&);
    Uncopyable& operator=(const Uncopyable<T>&);
};

template <class T>
class Singleton : private Uncopyable<T>
{
public:
    static T* getInstancePtr()
    {
        return instance;
    }
protected:
    Singleton<T>()
    {
        if(instance == 0)
        {
            instance = new T();
        }
    };
    ~Singleton<T>()
    {

    };
private:
    static T* instance;
};
template<class T> T* Singleton<T>::instance = 0;

class Test : public Singleton<Test>
{
public:
    Test(){};
    ~Test(){};
    inline void test() const
    {
        std::cout << "Blah" << std::endl;
    }
private:
    friend class Singleton<Test>;
protected:
};

int main(int argc, char* argv[])
{
    Test* t = Test::getInstancePtr();
    Test* t2 = Test::getInstancePtr();

    t->test();
    t2->test();

    return 0;
}

它以这种形式工作,但我不确定它是否真的是正确的,因为Singleton的构造函数和析构函数受到保护而不是私有.如果我将它们声明为私有,则代码将无法编译,因为类无法访问它们.这个实现是否可以安全使用,或者我可以做些什么来改进它以确保只创建和使用一个实例.

谢谢

Answer 1

这肯定是单身人士的错误实施.该实现存在太多问题.

在C++ 11中,您可以使用std::call_once和std::once_flag实现单例模式.这是一个例子:

//CRTP base singleton class

template<typename TDerived>
class Singleton 
{
    static std::unique_ptr<TDerived> m_instance;
    static std::once_flag            m_once;

protected:     

    Singleton() {}

public:

    ~Singleton() { }

    static TDerived & GetInstance() 
    {
        std::call_once
        ( 
           Singleton::m_once, 
           [] (){ Singleton::m_instance.reset( new TDerived() ); }
        );
        return *m_instance;
    }
};

template<typename TDerived> 
std::unique_ptr<TDerived>  Singleton<TDerived>::m_instance;

template<typename TDerived> 
std::once_flag   Singleton<TDerived>::m_once;

现在你可以从中得到:

class Demo : public Singleton<Demo>
{
     public:
          void HelloWorld() { std::cout << "HelloWorld" << std::endl; }
};

//call HelloWorld() function through singleton instance!
DemoSingleton::GetInstance().HelloWorld();