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1. 意图

        在开发中，若某些模块或功能只需要一个类实例，所有调用地方通过着一个类对象访问功能，单例模式符合这种类实例创建模式，并且通过提供统一类实例接口访问类对象。

2. 适用性

        《Gof 设计模式-可复用面向对象软件的基础》中对此模式的适用性描述如下：

• 当类只能有一个实例且客户可以从一个公众的访问点访问。
•  当这个唯一实例应该是通过子类化可拓展的，并且客户应该无需更改代码就能使用一个扩展的实例时。

3. 实现

• 饿汉模式：类加载时，类对象创建并初始化，调用时直接使用已经创建好的。

#include <iostream>class Singleton {
public:static Singleton *GetInstance() { return m_instance; }void Print() { std::cout << __FUNCTION__ << std::endl; }private:Singleton() = default;~Singleton() = default;Singleton(const Singleton &) = delete;Singleton &operator=(const Singleton &) = delete;private:static Singleton *m_instance;
};Singleton *Singleton::m_instance = new Singleton;void Test() { Singleton::GetInstance()->Print(); }int main() {Test();return 0;
}

• 懒汉模式：类对象创建与初始化被延迟到真正调用的位置。

#include <iostream>class Singleton {
public:static Singleton *GetInstance() {if (m_instance == nullptr)m_instance = new Singleton;return m_instance;}void Print() { std::cout << __FUNCTION__ << std::endl; }private:Singleton() = default;~Singleton() = default;Singleton(const Singleton &) = delete;Singleton &operator=(const Singleton &) = delete;private:static Singleton *m_instance;
};Singleton *Singleton::m_instance = nullptr;void Test() { Singleton::GetInstance()->Print(); }int main() {Test();return 0;
}

        懒汉模式存在多线程并发问题，可以加锁，如下

#include <iostream>
#include <mutex>std::mutex mtx;class Singleton {
public:static Singleton *GetInstance() {std::lock_guard<std::mutex> locker(mtx);if (m_instance == nullptr)m_instance = new Singleton;return m_instance;}void Print() { std::cout << __FUNCTION__ << std::endl; }private:Singleton() = default;~Singleton() = default;Singleton(const Singleton &) = delete;Singleton &operator=(const Singleton &) = delete;private:static Singleton *m_instance;
};

        以上通过加锁保证了数据的并发安全，但若此对象创建好后多个线程频繁调用，每次都加锁访问可读对象，对程序性能影响较大，于是又出现了双层检查机制，优化访问性能。

#include <iostream>
#include <mutex>std::mutex mtx;class Singleton {
public:static Singleton *GetInstance() {if (m_instance == nullptr) {std::lock_guard<std::mutex> locker(mtx);if (m_instance == nullptr)m_instance = new Singleton;}return m_instance;}void Print() { std::cout << __FUNCTION__ << std::endl; }private:Singleton() = default;~Singleton() = default;Singleton(const Singleton &) = delete;Singleton &operator=(const Singleton &) = delete;private:static Singleton *m_instance;
};Singleton *Singleton::m_instance = nullptr;void Test() { Singleton::GetInstance()->Print(); }int main() {Test();return 0;
}

        双重检查机制实际上还存在潜在的问题，内存访问重新排序（重新排列编译器产生的汇编指令）导致双重锁定失效（考虑类对象内存分配和调用构造函数初始化分为两步执行，指令不顺序执行就无法保证多线程有其它指令在这两步之间执行）。所以需要保证指令顺序执行，避免指令重排。

#include <atomic>
#include <iostream>
#include <mutex>class Singleton {
public:static Singleton *GetInstance() {Singleton *tmp = m_instance.load(std::memory_order_relaxed);std::atomic_thread_fence(std::memory_order_acquire);if (tmp == nullptr) {std::lock_guard<std::mutex> locker(m_mtx);tmp = m_instance.load(std::memory_order_relaxed);if (tmp == nullptr) {tmp = new Singleton;std::atomic_thread_fence(std::memory_order_release);m_instance.store(tmp, std::memory_order_relaxed);}}return m_instance;}void Print() { std::cout << __FUNCTION__ << std::endl; }private:Singleton() = default;~Singleton() = default;Singleton(const Singleton &) = delete;Singleton &operator=(const Singleton &) = delete;private:static std::atomic<Singleton *> m_instance;static std::mutex m_mtx;
};std::atomic<Singleton *> Singleton::m_instance = nullptr;
std::mutex Singleton::m_mtx;void Test() { Singleton::GetInstance()->Print(); }int main() {Test();return 0;
}

        以上使用原子变量及内存序约束实现单例类，避免指令重排问题，同时解决并发问题。但实现略微繁琐，c++11以后对静态变量创建的并发安全提供了保证，简化写法如下：

#include <iostream>class Singleton {
public:static Singleton &GetInstance() {static Singleton instance;return instance;}void Print() { std::cout << __FUNCTION__ << std::endl; }private:Singleton() = default;~Singleton() = default;Singleton(const Singleton &) = delete;Singleton &operator=(const Singleton &) = delete;
};void Test() { Singleton::GetInstance().Print(); }int main() {Test();return 0;
}

4. 优缺点

• 控制类实例数量
• 比类操作更灵活，减少命名空间污染
• 隐藏了类之间的依赖关系
• 影响代码的扩展性
• 影响代码的可测试性
• 不支持包含参数的构造函数

5. 模板实现

• 单实例管理

template <typename T> class SingletonManager {
public:template <typename... Args> static T &GetInstance(Args &&...args) {static T instance(std::forward<Args>(args)...);return instance;}private:SingletonManager() = default;virtual ~SingletonManager() = default;SingletonManager(const SingletonManager &) = delete;SingletonManager &operator=(const SingletonManager &) = delete;
};

• 多实例管理

#include <map>
#include <memory>
#include <string>template <typename T, typename K = std::string> class MultitonManager {
public:template <typename... Args>static T &GetInstance(const K &key, Args &&...args) {return AssignInstance(key, std::forward<Args>(args)...);}template <typename... Args> static T &GetInstance(K &&key, Args &&...args) {return AssignInstance(key, std::forward<Args>(args)...);}private:template <typename Key, typename... Args>static T &AssignInstance(Key &&key, Args &&...args) {auto iter = m_map.find(key);if (iter == m_map.end()) {static T instance;m_map.emplace(key, &instance);return instance;}return *(iter->second);}private:MultitonManager() = default;virtual ~MultitonManager() = default;MultitonManager(const MultitonManager &) = delete;MultitonManager &operator=(const MultitonManager &) = delete;private:static std::map<K, T *> m_map;
};template <typename T, typename K> std::map<K, T *> MultitonManager<T, K>::m_map;