A template Singleton class

5年前在学校，写了一篇关于单例设计的Blog，最近需要写3个单例对象，如果每个对象都写一遍单例模式，显得很冗余。所以需要应用单例模板类。

基于之前的内容：https://www.cnblogs.com/wangpei0522/p/4460529.html 

很容易扩展为使用模板继承的单例辅助类：

template<typename T>
class Singleton
{
protected:
    Singleton()
    {
        std::cout << "Construct singleton" << std::endl;
    }

    virtual ~Singleton()
    {
        std::cout << "Destruct singleton" << std::endl;
    }

public:
    static T& GetInstance()
    {
        if (!m_instance_) m_instance_ = new T{};
        static auto* ptr = &m_helper_; // To 读者, 尝试注释运行一遍
        return *m_instance_;
    }

private:
    static T* m_instance_;

private:
    class Helper
    {
    public:
        Helper()
        {
            std::cout << "Help construction " << std::endl;
            GetInstance();
        }

        ~Helper()
        {
            std::cout << "Help destruction " << std::endl;

            if (Singleton<T>::m_instance_)
            {
                delete Singleton<T>::m_instance_;
                Singleton<T>::m_instance_ = nullptr;
            }
        }
    };

    static Helper m_helper_;
};

template<typename T>
T* Singleton<T>::m_instance_ = nullptr;

template<typename T>
typename Singleton<T>::Helper Singleton<T>::m_helper_;

写出这个类并不难。但在第一版中，没有第19行。

如果你注释掉第19行，测试很容易发现：

class A : public Singleton<A>
{
    friend class Singleton<A>;

    A()
    {
        std::cout << "Construct A" << std::endl;
    }

    ~A()
    {
        std::cout << "Destruct A" << std::endl;
    }

    A& operator=(const A&) = delete;

public:
    void Print()
    {
        std::cout << "A's Print function " << std::endl;
    }
};

int main()
{
    A& a = A::GetInstance();

    a.Print();

    return 0;
}

打印的结果是：

Construct singleton
Construct A
A's Print function

显然the static member of a template class does not be initialized

typename Singleton<T>::Helper Singleton<T>::m_helper_;

找了几篇资料发现，static member是按需初始化. 

https://stackoverflow.com/a/6959729

"In a class template, when performing implicit instantiation, the members are instantiated on demand. Since the code does not use the static member, it is not even instantiated in the whole application"

那么一切都很清晰了，也符合模板实例化的原则。 加入第19行的代码：

static auto* ptr = &m_helper_;

得到符合预期的结果：

Help construction
Construct singleton
Construct A
A's Print function
Help destruction
Destruct A
Destruct singleton