C++ Singleton + MultiThread

#include <iostream>
#include <mutex>
using namespace std;

template <class T>
class Singleton {
public:
	static T *instance() {
		if (object == NULL) {
			mtx.lock();
			if (object == NULL) object = new T;
			mtx.unlock();
		}
		return object;
	}

private:
	Singleton() {}  // be here? necessary?

	static T *object;
	static mutex mtx;
};

template <class T>
T* Singleton<T>::object = NULL;

template <class T>
mutex Singleton<T>::mtx;

class Foo {
};

int main() {
	Foo *singletonFoo = Singleton<Foo>::instance();
	return 0;
}