[设计模式] 设计模式课程（十一）-- 单例模式

 1 class Singleton{
 2 private:
 3     Singleton();
 4     Singleton(const Singleton& other);
 5 public:
 6     static Singleton* getInstance();
 7     static Singleton* m_instance;
 8 };
 9 
10 Singleton* Singleton::m_instance=nullptr;
11 
12 //线程非安全版本
13 Singleton* Singleton::getInstance() {
14     if (m_instance == nullptr) {
15         m_instance = new Singleton();
16     }
17     return m_instance;
18 }
19 
20 //线程安全版本，但锁的代价过高
21 Singleton* Singleton::getInstance() {
22     Lock lock;
23     if (m_instance == nullptr) {
24         m_instance = new Singleton();
25     }
26     return m_instance;
27 }
28 
29 //双检查锁，但由于内存读写reorder不安全
30 Singleton* Singleton::getInstance() {
31     
32     if(m_instance==nullptr){
33         Lock lock;
34         if (m_instance == nullptr) {
35             m_instance = new Singleton();
36         }
37     }
38     return m_instance;
39 }
40 
41 //C++ 11版本之后的跨平台实现 (volatile)
42 std::atomic<Singleton*> Singleton::m_instance;
43 std::mutex Singleton::m_mutex;
44 
45 Singleton* Singleton::getInstance() {
46     Singleton* tmp = m_instance.load(std::memory_order_relaxed);
47     std::atomic_thread_fence(std::memory_order_acquire);//获取内存fence
48     if (tmp == nullptr) {
49         std::lock_guard<std::mutex> lock(m_mutex);
50         tmp = m_instance.load(std::memory_order_relaxed);
51         if (tmp == nullptr) {
52             tmp = new Singleton;
53             std::atomic_thread_fence(std::memory_order_release);//释放内存fence
54             m_instance.store(tmp, std::memory_order_relaxed);
55         }
56     }
57     return tmp;
58 }