https://rein.kr/posts/2007-11-13-%EC%98%A4%EB%8A%98%EC%9D%98-%EC%82%BD%EC%A7%88-boostshared_ptrt-%EC%9D%B4%EB%A0%87%EA%B2%8C-%EC%93%B0%EB%A9%B4-%EB%A7%9D%ED%95%9C%EB%8B%A4/