[Design Pattern/C++] Modern C++ Thread-Safe Singleton Template

-Thread-Safe Singleton Template (Modern C++)-

#include <iostream>
#include <thread>
#include <mutex>
 
template <class T>
class Singleton {
protected:
    Singleton() {};
    ~Singleton() {};
    Singleton(const Singleton& other) {};
 
private:
    static std::once_flag chkSingle;
    static std::shared_ptr<T> pInstance;
 
    static void destroy() {
        if (pInstance != nullptr) {
            pInstance.reset();
            pInstance = nullptr;
        }
    }
 
public:
    static T& getInstance() {
        std::call_once(chkSingle, []()
        {
            pInstance.reset(new T());
            std::atexit(destroy);
        });
        return *(pInstance.get());
    }
};
 
template <class T> std::shared_ptr<T> Singleton <T>::pInstance = nullptr;
template <class T> std::once_flag Singleton <T>::chkSingle;