c++ weak_ptr

weak_ptr是用来监视shared_ptr的，不会使引用计数增加。

	std::shared_ptr<int> t(new int(10));
	std::weak_ptr<int>wk(t);
	if(wk.expired())
		std::cout<<"t is expired"<<std::endl;
	else
		std::cout<<"t is not expired"<<std::endl;