In member function ‘void PolyLinkNode::Pushback(int, int)’,
    inlined from ‘PolyLinkNode::PolyLinkNode(int)’ at foo.cc:19:12,
    inlined from ‘int main()’ at foo.cc:60:18:
foo.cc:29:35: warning: ‘p.PolyLinkNode::head’ may be used uninitialized [-Wmaybe-uninitialized]
   29 |                         while (p->next != NULL) p = p->next;
      |                                ~~~^~~~
foo.cc: In function ‘int main()’:
foo.cc:60:22: note: ‘p.PolyLinkNode::head’ was declared here
   60 |         PolyLinkNode p(n);
      |                      ^