c++11 改进设计模式 Singleton模式

关于学习 《深入应用c++11》的代码笔记:

c++11之前是这么实现的

template<typename T>
class Singleton{
public:
        static T* Instance(){
                if (m_pInstance == nullptr)
                        m_pInstance = new T();
                return m_pInstance;
        }

        template<typename T0>
        static T* Instance(T0 arg0){
                if (m_pInstance == nullptr)
                        m_pInstance = new T(arg0);
                return m_pInstance;
        }
        
        template<typename T0,typename T1>
        static T* Instance(T0 arg0, T1 arg1){
                if (m_pInstance == nullptr)
                        m_pInstance = new T(arg0, arg1);
                return m_pInstance;
        }

        template<typename T0, typename T1,typename T2>
        static T* Instance(T0 arg0, T1 arg1,T2 arg2){
                if (m_pInstance == nullptr)
                        m_pInstance = new T(arg0, arg1,arg2);
                return m_pInstance;
        }

        template<typename T0, typename T1, typename T2,typename T3>
        static T* Instance(T0 arg0, T1 arg1, T2 arg2,T3 arg3){
                if (m_pInstance == nullptr)
                        m_pInstance = new T(arg0, arg1, arg2,arg3);
                return m_pInstance;
        }

        template<typename T0, typename T1, typename T2, typename T3,typename T4>
        static T* Instance(T0 arg0, T1 arg1, T2 arg2, T3 arg3,T4 arg4){
                if (m_pInstance == nullptr)
                        m_pInstance = new T(arg0, arg1, arg2, arg3,arg4);
                return m_pInstance;
        }

        template<typename T0, typename T1, typename T2, typename T3, typename T4,typename T5>
        static T* Instance(T0 arg0, T1 arg1, T2 arg2, T3 arg3, T4 arg4,T5 arg5){
                if (m_pInstance == nullptr)
                        m_pInstance = new T(arg0, arg1, arg2, arg3, arg4,arg5);
                return m_pInstance;
        }

        static T* GetInstance()
        {
                if (m_pInstance == nullptr)
                        throw std::logic_error("the instance is not init,please init the instance first");
                        
                return m_pInstance;
        }

        static void DestroyInstance(){
                delete m_pInstance;
                m_pInstance = nullptr;
        }

private:
        Singleton(void);
        virtual ~Singleton(void);
        Singleton(const Singleton&);
        Singleton& operator = (const Singleton);

        static T* m_pInstance;
};

template<class T> T* Singleton<T>::m_pInstance = nullptr;

//============================================
struct A{
        A(){}
};

struct B{
        B(int x){}
};

struct C{
        C(int x, double y){}
};

int _tmain(int argc, _TCHAR* argv[])
{
        Singleton<A>::Instance();
        Singleton<A>::Instance();
        Singleton<B>::Instance(1);
        Singleton<C>::Instance(1,3.14);

        Singleton<A>::DestroyInstance();
        Singleton<B>::DestroyInstance();
        Singleton<C>::DestroyInstance();

        return 0;
}

　　c++11之后可以简略一点，使用了可变模板参数

template<typename T>
class Singleton{
public:
        template <typename... Args>
        static T* Instance(Args&&... args){
                if (m_pInstance == nullptr)
                        m_pInstance = new T(std::forward<Args>(args)...);
                return m_pInstance;
        }

        static T* GetInstance(){
                if (m_pInstance == nullptr)
                        throw std::logic_error("the instance is not init,please initialize the instance first");
                return m_pInstance;
        }

        static void DestroyInstance()
        {
                delete m_pInstance;
                m_pInstance = nullptr;
        }

private:
        Singleton(void);
        virtual ~Singleton(void);
        Singleton(const Singleton&);
        Singleton& operator=(const Singleton&);
private:
        static T* m_pInstance;
};

template<class T>T* Singleton<T>::m_pInstance = nullptr;

#include <iostream>
#include <string>

using namespace std;

struct A{
        A(const string&){ cout << "lvalue" << endl; }
        A(string&&x){ cout << "rvalue" << endl; }
};

struct B{
        B(const string&){ cout << "lvalue" << endl; }
        B(string&& x){ cout << "rvalue" << endl; }
};

struct C{
        C(int x, double y){}
        void Fun(){ cout << "Test" << endl; }
}; 


int _tmain(int argc, _TCHAR* argv[])
{
        string str = "bb";
        Singleton<A>::Instance(str);
        Singleton<B>::Instance(std::move(str));
        Singleton<C>::Instance(1,3.14);
        Singleton<C>::GetInstance()->Fun();

        Singleton<A>::DestroyInstance();
        Singleton<B>::DestroyInstance();
        Singleton<C>::DestroyInstance();

        return 0;
}