单例

local static对象在c++11之前不是线程安全的。如果一个线程在构造一半时另外一个线程进来了，就会重复构造进而产生内存泄漏（为何重复构造会产生内存泄露？）c++11规定，在一个线程开始local static对象构造的过程时，另外一个线程进来时会等待，直到前一个线程构造完毕。

#include

class Singleton { private: Singleton() {}; ~Singleton() {}; Singleton(const Singleton&); Singleton& operator=(const Singleton&); public: static Singleton& getInstance() { static Singleton s; return s; } };

//带锁的单例 //每次获取单例都得加锁，访问变成串行 std::mutex m; template T& getSingleton() { std::unique_lock lock(m); static T instance; return instance; }

// Double check lock class Singleton { private: Singleton() {}; Singleton(const Singleton&) {}; public: static Singleton& getInstance(); private: static std::mutex m; static Singleton* instance; };

Singleton* Singleton::instance = nullptr; Singleton& Singleton::getInstance() { if (instance == nullptr) { std::unique_lockstd::mutex lock(m); if (instance == nullptr) { instance = new Singleton(); } } return *instance; }

//内存屏障

//在某些内存模型中有可能出现instance不为nullptr但是没有构造完成得情况，此时的锁其实并没有起作用 //https://blog.csdn.net/qq_35280514/article/details/70211845 //此时只需把instance类型修改一下 static std::atomic<Singleton *> m_instance(nullptr);

chenkangcrack/Cpp_Note

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