为了保证在多线程环境中某个函数仅被调用一次，比如，需要初始化某个对象，而这个对象只能初始化一次，就可以用std::call_once来保证在多线程环境中只被调用一次。使用std::call_once是，需要一个once_flag作为call_once的入参，它的用法比较简单。

#include <iostream>
#include <thread>
#include <mutex>
using namespace std;template <typename T>
class Singleton
{
public:template<typename... Args>static T* Instance(Args&&... args){if (m_pInstance == nullptr)m_pInstance = new T(std::forward<Args>(args)...);return m_pInstance;}static T* GetInstance(){if (m_pInstance == nullptr)throw std::logic_error("the instance is not init, please initialize the instance first");return m_pInstance;}static void DestroyInstance(){delete m_pInstance;m_pInstance = nullptr;}private:Singleton(void);virtual ~Singleton(void);Singleton(const Singleton&);Singleton& operator=(const Singleton&);
private:static T* m_pInstance;
};template <class T> T* Singleton<T>::m_pInstance = nullptr;class CObject
{
public:CObject(){cout << "CObject construct...." << endl;};~CObject(){cout << "CObject destory ..." << endl;};void start(){cout << "....start....." << endl;}
};std::once_flag flag;void do_once()
{std::call_once(flag, [](){Singleton<CObject>::Instance();});
}int main()
{{///先确保单利构造成功thread t1(do_once);thread t2(do_once);thread t3(do_once);t1.join();t2.join();t3.join();Singleton<CObject>::Instance()->start();}return 0;
}