单例模式例子

#pragma once
#include <atomic>
#include <mutex>
namespace sinftech {
template<typename T>
class Singleton {
public:
  static T* get() {
    T* tmp = _instance.load(std::memory_order_relaxed);
    std::atomic_thread_fence(std::memory_order_acquire);
    if(tmp == nullptr) {
      std::lock_guard<std::mutex> lock(_mutex);
      tmp = _instance.load(std::memory_order_relaxed);
      if(tmp == nullptr) {
        tmp = new T;
        std::atomic_thread_fence(std::memory_order_release);
        _instance.store(tmp, std::memory_order_relaxed);
      }
    }
    return tmp;
  }
  static void destroy() {
    T* tmp = _instance.exchange(nullptr, std::memory_order_relaxed);
    delete tmp;
  }
private:
  Singleton() {}
  ~Singleton() {}
  static std::atomic<T*> _instance;
  static std::mutex _mutex;
};
template<typename T>
std::atomic<T*> Singleton<T>::_instance;
template<typename T>
std::mutex Singleton<T>::_mutex;
}//namespace sinftech