第26页 Item 7.如果容器中包含了通过new创建的指针，切记在容器对象析构前将指针delete掉

To avoid resource leaks
when you have containers of pointers that should be deleted, you must either replace
the pointers with smart reference-counting pointer objects (such as Boost's shared_ptr)
or you must manually delete each pointer in the container before the container is
destroyed.