C++ static 变量和线程安全和单例模式

1.1 全局变量、文件域的static变量和类的static成员变量在main函数执行之前初始化
1.2 局部静态变量在第一次被使用时初始化

2.1 非局部静态变量是线程安全的
2.2 局部静态变量在C++11后也是线程安全的

分为懒汉式和饿汉式，在使用前全局对象已经创建的是饿汉式单例，在使用的时候创建的是懒汉式单例

01. 普通懒汉式单例，线程不安全

// 懒汉式单例模式，线程不安全
class SingleInstance {
 public:
   static SingleInstance *GetInstance() {
     if (m_SingleInstance == nullptr)
       m_SingleInstance = new SingleInstance;
     return m_SingleInstance;
   }
 private:
   SingleInstance();
   ~SingleInstance();
   SingleInstance(const SingleInstance &signal) = delete;
   SingleInstance &operator=(const SingleInstance &signal) = delete;

 private:
    static SingleInstance *m_SingleInstance;
};

SingleInstance *SingleInstance::m_SingleInstance = NULL;

02. 加锁的懒汉式单例，线程安全

//懒汉式单例，加锁，线程安全
class SingleInstance {
 public:
   static SingleInstance *GetInstance() {
     pthread_mutex_lock(&mutex);
     if (m_SingleInstance == nullptr)
       m_SingleInstance = new SingleInstance;
     return m_SingleInstance;
     pthread_mutex_unlock(&mutex);
   }
 private:
   SingleInstance();
   ~SingleInstance();
   SingleInstance(const SingleInstance &signal) = delete;
   SingleInstance &operator=(const SingleInstance &signal) = delete;
   
   static SingleInstance *m_SingleInstance;
   static pthread_mutex_t mutex;
};
SingleInstance *SingleInstance::m_SingleInstance = NULL;

03. 局部静态变量实现的懒汉式单例，C++11后线程安全（推荐使用的方式）

// 局部静态变量实现的懒汉式单例，C++11后线程安全
class SingleInstance {
 public:
   static SingleInstance *GetInstance() {
     static SingleInstance instance;
     return &instance;
   }
 private:
   SingleInstance();
   ~SingleInstance();
   SingleInstance(const SingleInstance &signal) = delete;
   SingleInstance &operator=(const SingleInstance &signal) = delete;
};

04. 饿汉式单例，天生线程安全，因为在main函数前已经初始化

//饿汉式单例，天生线程安全
class SingleInstance {
public:
  static SingleInstance *GetInstance() { return m_SingleInstance; }
private:
  SingleInstance();
  ~SingleInstance();
  SingleInstance(const SingleInstance &signal) = delete;
  SingleInstance &operator=(const SingleInstance &signal) = delete;
private:
  static SingleInstance *m_SingleInstance;
};
SingleInstance *SingleInstance::m_SingleInstance = new SingleInstance;