设计模式-单实例-C++Singleton类模板实现及使用

设计模式-单实例-C++Singleton类模板实现及使用

单实例类设计注意事项:
1. 构造函数私有化,拷贝构造函数及重载=操作符也私有化,暴露新的访问接口如 getInstance();
2. 推荐使用懒汉式即 getInstance()接口中new新实例且使用双重锁定,既解决多线程竟态问题又保证性能,同时还要防止某一用户delete获取到的实例导致其他用户无法正常使用;
3. 良好的设计会考虑资源回收,可以构造内嵌类Garbo及其静态对象,系统自动析构全局变量及静态成员变量时可以实现自动删除单实例对象,使用时不需再关注对象的释放;
4. 静态成员变量要类外初始化,其中 Singleton<T>::Garbo 作为类型名要在前面加上 typename 显式通知编译系统; ms_pInstance 要初始化为NULL;
5. 将单例类模板声明为这个类的友元; friend class Singleton<XXT>; singleton模板类为XXT类的友元,模板类才能new XXT()实例对象;
6. 通过单例类模板中 XXT* s = Singleton<XXT>::getInstance(); 或公开继承 Singleton<XXT> 后 XXT* s = XXT::getInstance();创建/获取唯一实例对象;

参考代码

#ifndef __SINGLETON_H__
#define __SINGLETON_H__

#include <iostream>
#include <sstream>
#include <thread>
#include <mutex>
#include <queue>
#include <atomic>
#include <functional>
#include <chrono>
#include <memory>
#include <condition_variable>
#include <algorithm>

template <class T>
class Singleton
{
private:
    static std::mutex ms_MuxLock_Singleton;
    using MuxGuard = std::lock_guard<std::mutex>;
    static T* ms_pInstance;
    Singleton(const Singleton& src){(void)src;}
    Singleton &operator=(const Singleton& src){(void)src;};

    class Garbo
    {
    public:
        ~Garbo()
        {
            // std::cout<<"Singleton<"<<typeid(T).name()<<">::Garbo::~Garbo()" << std::endl;
            if (Singleton::ms_pInstance)
            {
                delete Singleton::ms_pInstance;
                Singleton::ms_pInstance = NULL;
            }
        }
        void touch() { return; }
    };
    static Garbo ms_garbo;

protected:
    Singleton() {
        ms_garbo.touch(); //prevent optimised and no garbo instance to trigger deconstruct
    }
    ~Singleton() {}

public:
    static T* getInstance()
    {
        if (ms_pInstance == NULL)
        {
            MuxGuard mlk(ms_MuxLock_Singleton);
            if (ms_pInstance == NULL)
            {
                ms_pInstance = new T();
            }
        }
        return ms_pInstance;
    }
};

template <class T> typename Singleton<T>::Garbo Singleton<T>::ms_garbo;
template <class T> std::mutex Singleton<T>::ms_MuxLock_Singleton;
template <class T> T* Singleton<T>::ms_pInstance = NULL;

#endif //__SINGLETON_H__

 应用示例

/* class MsgSequenceID */
class MsgSequenceID:public Singleton<MsgSequenceID>
{
   friend class Singleton<MsgSequenceID>;
public:
    ~MsgSequenceID();
    uint32_t getSequenceID() {
        uint32_t tmp;
        {
            MuxGuard g(mLock);
            tmp = m_sequenceID++;
        }
        return tmp;
    }
    bool initSequenceID() { m_sequenceID = 0; return true;}

private:
    MsgSequenceID();

    using MuxGuard = std::lock_guard<std::mutex>;
    mutable std::mutex mLock;
    uint32_t m_sequenceID;
};